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Theoretical, experimental, device fabrication, and degradation studies of materials for optoelectronic devices

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THEORETICAL, EXPERIMENTAL, DEVICE FABRICATION, AND
DEGRADATION STUDIES OF MATERIALS FOR OPTOELECTRONIC DEVICES

by

Azad M. Hassan
________________________________________________________________________

A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)

December 2007

Copyright 2007 Azad M. Hassan

ii

Dedication

With love and gratitude I dedicate this thesis to
my mother, my father, my wife, my sons Jacob and Joseph, and my daughter Summehra

iii
Acknowledgements
At first, I would like to thank Professor Mark E. Thompson for his contribution to
my success. Without his guidance and support my work at the University of Southern
California would not have been possible. As a mentor Professor Thompson is extremely
skilled and distinctive. His unique teaching philosophy promotes independent thinking
and self learning. The most important thing that I learned from him in the last six years is
to be critical and question everything.
I would further like to thank Dr. Robert Bau, Dr. Florian Mansfeld, and Dr. Amy
Barrios for their patience, suggestions, and guidance in my graduate studies. My special
thanks go to Dr. Peter Djurovich for his insightful suggestions and Dr. Vadim
Adamovich for his mentorship at the early part of my graduate career.
I would also like to show appreciation to all the past and present members of the
Thompson research group with whom I have shared conversations and insights over the
years. Thanks to glassblower James Merritt and machine shop personnel Victor Jordan
and Donald Wiggins who skillfully constructed many important items for many of the
experiments that I successfully performed.
Lastly, I would like to acknowledge my family from the bottom of my heart.
Without their love, support, and encouragement I would be nothing today. My sincere
appreciation goes to my parents for all the sacrifice they made for me throughout their
lives. I thank my wife and children for their love and support. Thanks to Almightily and
his beloved one.

Table of Contents
Dedication…………………………………………………………...ii
Acknowledgements………………………………………………....iii
List of Tables……………………………………………………......viii
List of Figures………………………………………………….…...xiii
Abstract…………………………………………………………......xxvii
Chapter 1: Introduction to Semiconductors, Organic Light
Emitting Diodes (OLEDs), and Organic
Photovoltaic Cells (OPVs)………………………..........1
1.1 Introduction to semiconductors……………………………………………..1
1.1.1 Principles of semiconductors……………………………………....2
1.1.2 Doped semiconductor……………………………………………...3
1.1.3 The p-n junction and the recombination of carriers…………….....4
1.2 Introduction to Organic Light-Emitting Diodes (OLEDs)………………….8
1.2.1 Structure of OLED…………………………………………………9
1.2.2 Charge Conduction in OLED……………………………..…….....11
1.2.3 Charge Recombination in OLED………………………..………...13
1.2.4 Generation of Excited States in OLED…………………………...16
1.2.4.1 Absorption and Emission Spectroscopy…………………….18
1.2.5 Energy Transfer in OLED………………………………………....21
1.2.5.1 Förster Energy Transfer……………………………………..23
1.2.5.2 Dexter Energy Transfer………………………………….......27
1.2.6 Current-Voltage Characteristics of OLED…………………….......29
1.2.7 Efficiencies of OLED………………………………………….......31
1.3 Introduction to Organic Photovoltaic Cells (OPV)……………………........33
1.3.1 Photon Absorption and Exciton Generation………………….........35
1.3.2 Exciton Diffusion……………………………………………...…..36
1.3.3 Exciton Dissociation and Charge Generation..................................37
1.3.4 Carrier Diffusion……………………………………………...…...39
1.3.5 Carrier Collection and Photocurrent Generation…………….….....39
1.4 Chapter 1 References………………………………………………...…...….42

iv
Chapter 2: Photophysics, Spectroscopy, and Density
Functional Theory (DFT) Studies of
Phosphorescent Phthalimide Materials……………..47
2.1 Introduction……………………………………………………………….....47
2.2 Experimental………………………………………………………………...50
2.2.1 Synthesis and Characterization……………………………………50
2.2.2 Electrochemical Measurements…………………………………...52
2.2.3 Spectroscopic Measurements……………………………………...52
2.2.4 Computational Methods………………………………………..….53
2.3 Results and Discussion…………………………………………….…..……54
2.3.1 Synthesis of the Phthalimides………………………….…..……...54
2.3.2 Electrochemical analysis…………………………….…..…….......55
2.3.3 Singlet ground state (SGS) geometries and electronic structures…57
2.3.4 Lowest triplet excited state (TES) geometries and electronic
structures………………………………………………………......60
2.3.5 Absorption spectroscopy and computational analysis…………......63
2.3.6 Emission spectroscopy and computational analysis……………….69
2.4 Chapter 2 Conclusion……………………………………………………......78
2.5 Chapter 2 References…………………………………………………..….....80

Chapter 3: Efficient Green and Red Electrophosphorescent
Devices Utilizing High Triplet Energy Phthalimide
Blocking Materials….....................................................92
3.1 Introduction………………………………………….…………………..…..92
3.2 Experimental…………………………………………..…………………......97
3.2.1 Synthesis of the Materials……………………..…………………...97
3.2.2 Thermochemical Measurements………………..……………….....97
3.2.3 Thin Film Fabrication……………………………..…………….....98
3.2.4 Spectroscopic Measurements………………………..………..........98
3.2.5 OLED Fabrication and Testing………………………..……...........99
3.3 Results and Discussion…………………………………………….…..........101
3.3.1 Thermal Analysis…………………………………………....….....102
3.3.2 Thin Film Photophysics……………………………………....…...104
3.3.3 Phthalimide Based Undoped OLEDs………………………....…..108
3.3.4 Phthalimide Based Doped Phosphorescent OLEDs………..…......102
3.4 Chapter 3 Conclusion………………………………………………….…....115
3.5 Chapter 3 References……………………………………………………......117

v
Chapter 4: Computational Studies of Spectroscopic and
Photophysical Properties of (Dmappy)-Pt-(acac)…....122
4.1 Introduction……………………………………………………………….......122
4.2 Experimental….……………………………………………………………....127
4.2.1 Parameterization…………………………………………………….127
4.2.2 Computational Details……………………………………………...128
4.3 Results and Discussion……………………………………………………….130
4.3.1 Singlet ground state (SGS) geometries and electronic structures…..130
4.3.2 Ground state PES scan……………………………………………...135
4.3.2.1 HOMO LUMO energies........………………………………...142
4.3.3 TDDFT analysis…………………………..…………………….......144
4.3.3.1 Electronic absorption spectra and singlet excited states….......147
4.3.4 Lowest triplet excited state (TES) geometries and electronic
structures……………………………………………………………157
4.3.5 Triplet state PES scan…………………………………………….....160
4.3.6 Emission spectroscopy and properties of triplet excited states…......167
4.4 Chapter 4 Conclusion…………………………………………………………174
4.5 Chapter 4 References…………………………………………………………177

Chapter 5: Computational Studies of Spectroscopic and
Photophysical Properties of (NO
2
ppy)-Pt-(acac)…….181
5.1 Introduction……………………………………………………………..….....181
5.2 Experimental…………………………………………………………...……..183
5.2.1 Thin Film Fabrication…………….....................................................183
5.2.2 Spectroscopic Measurements…………………………………….....183
5.2.3 Computational Details……………………………………………....184
5.3 Results and Discussion………………………………………………………..185
5.3.1 Singlet ground state (SGS) geometries and electronic structures…..186
5.3.2 Ground state PES scan………………………………………….......190
5.3.2.1 HOMO LUMO energies…………………………………........192
5.3.3 TDDFT analysis: Electronic absorption spectra and singlet
excited states……………………………………………….……….195
5.3.4 Lowest triplet excited state (TES) geometries and electronic
structures…………………………………………………………....205
5.3.5 Triplet state PES scan…………………………………………….....206
5.3.6 Emission spectroscopy and properties of triplet excited states…......210
5.4 Chapter 5 Conclusion…………………………………………………....218
5.5 Chapter 5 References………………………………………………....….221

vi
Chapter 6: Effects of Electron Transporting Layer (ETL) and
Cathode Surface Coating on the Evolution and
Growth of Dark Spots in Organic Light Emitting
Diodes (OLEDs)…………………………………...….223
6.1 Introduction……………………………………………………………….....223
6.2 Experimental………………………………………………………………...227
6.2.1 Materials and Supplies…………………………………………….227
6.2.2 OLED Fabrication and Testing………………………………..…..227
6.2.3 Dark Spot Growth Measurements…………………………..……..229
6.3 Results and Discussion……………………………………………………....230
6.3.1 Correction factor and data treatment……………………………....233
6.3.1.1 The concept of correction…………………………………....233
6.3.1.2 Applying correction…………………………………….........234
6.3.2 Dark spot growth and behavior………………………………....…244
6.3.3 Correlation between dark spot growth and device lifetime….…….248
6.3.4 Effects of electron transporting layer (ETL)……………….……...250
6.3.5 Effects of over-coating materials…………………………..……...256
6.4 Chapter 6 Conclusion………………………………………………..….…...263
6.5 Chapter 6 References………………………………………………...……...265

Bibliography………………………………………………………..268

Appendices……………………………………………………….....298
Appendix A…………………………………………………..298
A.1 Introduction…………………………………...……………………..298
A.2 Experimental………………………………………………………...302
A.2.1 OPV Device Fabrication and Testing………………………....302
A.3 Results and Discussion……………………………………………...304
A.3.1 OPV cells with NPP and tBuTMPP……………………...…....305
A.3.2 OPV cells with ChBP……………………………………….....312
A.4 Appendix A Conclusion……………………………………………..317
A.5 Appendix A References…………………………..............................319
Appendix B…………………………………………………...321
Appendix C.………..…………………………………………335
Appendix D…………………………………………………...341

vii
List of Tables

Table 2.1: Summary of the electrochemical data. Potentials for reference a, b,
and d were converted from vs. SCE to ferrocence…………………….57

Table 2.2: Structural parameters for the optimized ground and triplet state
geometries……………………………………………………..………..69

Table 2.3: Summary of Emission Energies, dipole moment, and the Spin
Densities……………………………………………………..…………63

Table 2.4: Calculated vertical excitation energies (E), dominant MO transitions,
orbital coefficients, and oscillator strengths for tBuTMPP…………….69

Table 2.5: Summary of photophysical data…….…………..……………………...78

Table 3.1: Summary of the thin film photophysics data…………………………..107

Table 3.2: Summary of un-doped and doped device data showing EL spectra,
maximum EL quantum efficiencies (%QE), and maximum brightness.110

Table 4.1: Comparison of the calculated bond lengths and angles of the three
structural isomers of (dmappy)-Pt-(acac) with corresponding X-ray
data……………………………………………………………………..132

Table 4.2: Highest occupied and lowest virtual molecular orbitals of the three
isomers of (dmappy)-Pt-(acac) are shown with % MO character from
various functionalities making contributions to the respective HOMO
and LUMO……………………………………………………………..135

Table 4.3: Ground state PES result showing the SCF energies and the dipole
moments of (dmappy)-Pt-(acac) as a function of φ and δ……………..141

Table 4.4: Results of the HOMO-LUMO energies and HOMO-LUMO gaps
obtained from single-point calculations of the PES coordinates………144

Table 4.5: Results of the HOMO-LUMO energies and HOMO-LUMO gaps
obtained from single-point calculations of the PES coordinates………146

viii
Table 4.6: TDDFT/CPCM vertical excitation energies (E
VT
), dominant MO
transitions, orbital coefficients, and oscillator strengths calculated for
planar and pyramidal isomers of (dmappy)-Pt-(acac) in toluene, THF,
MeOH, and CH
3
CN................................................................................150

Table 4.7: Summary of the calculated TES bond lengths angles of the three
structural isomers of (dmappy)-Pt-(acac)……………………………...159

Table 4.8: Summary of the calculated spin-densities of TES isomers of
(dmappy)-Pt-(acac) showing percent density of the unpaired
electrons…………………………………………………………….....160

Table 4.9: Summary of the TES-PES result of (dmappy)-Pt-(acac) showing the
potential energy surface, triplet energies, and triplet dipole moments
as a function of φ and δ………………………………………………...166

Table 5.1: Comparison of the calculated bond lengths angles of the two
structural isomers of (NO
2
ppy)-Pt-(acac) with corresponding X-ray
data…….................................................................................................187

Table 5.2: Highest occupied and lowest virtual molecular orbitals of the two
isomers of (NO
2
ppy)-Pt-(acac) are shown with % MO character from
various functionalities making contributions to the respective HOMO
and LUMO…………………………………………………………….189

Table 5.3: Summary of the results of the HOMO-LUMO energies and HOMO-
LUMO gaps……………………………………………………………193

Table 5.4: Uncorrected gas phase TDDFT vertical excitation energies (E
VT
),
dominant MO transitions, orbital coefficients, and oscillator strengths
calculated for planar and perpendicular isomers of (NO
2
ppy)-Pt-
(acac)…………………………………………………………………...197

Table 5.5: Uncorrected TDDFT/CPCM vertical excitation energies (E
VT
),
dominant MO transitions, orbital coefficients, and oscillator strengths
calculated for planar and perpendicular isomers of
(NO
2
ppy)-Pt-(acac) in hexanes………………………………………...200

Table 5.6: Blue-shifted (50nm) TDDFT/CPCM vertical excitation energies
(E
VT
), dominant MO transitions, orbital coefficients, and oscillator
strengths calculated for planar and perpendicular isomers of
(NO
2
ppy)-Pt-(acac) in hexanes are shown with the experimental
optical transitions……………………………………………………...203

ix
Table 5.7: Summary of the calculated TES bond lengths and angles of the two
structural isomers of (NO
2
ppy)-Pt-(acac)……………………………...205

Table 5.8: Summary of the Gaussian and Titan calculated spin-densities of TES
isomers of (NO
2
ppy)-Pt-(acac) showing percent density of the
unpaired electrons……………………………………………………...209

Table 5.9: Summary of the photophysical data…………………………..……….213

Table 5.10: Summary of the dipole moment data obtained from DFT and
TDDFT calculations. Total dipole moment for each state is
depicted in red. X, Y, Z are the individual components of the
dipole oriented in the X, Y, Z direction……………………………...216

Table 6.1: Summary of the curve fitting results obtained from linear, quadratic,
exponential, and sigmoidal-Boltzmann curve fitting…………………..244

Table 6.2: Growth rates of the dark spots showing difference between the clean-
room and ambient air devices………………………………………….246

Table 6.3: Summary of the curve fitting results of the ETL devices are shown
with the standard test devices………………………………………….254

Table 6.4: Summary of ETL studies………………………………………...…….256

Table 6.5: Summary of the curve fitting results of the over-coated devices are
shown with the standard test devices……………………………….....258

Table 6.6: Summary of over-coating studies………………...……………………263

Table A.1: Summary of open-circuit voltage (V
OC
), short-circuit current (J
SC
),
fill-factor (FF), and power conversion efficiencies (η
P
%) of OPV
cells fabricated with NPP, tBuTMPP, ChBP, and BCP EBLs under
1 sun AM 1.5G spectral illumination……………………….………...308

Table A.2: Summary of open-circuit voltage (V
OC
), short-circuit current (J
SC
),
fill-factor (FF), and power conversion efficiencies (η
P
%) of OPV
cells fabricated with 50 Å, 100 Å, and 150 Å of ChBP EBL under
1 sun intensity of simulated AM 1.5G solar illumination
respectively………………………………………………………….....315

x
Table B.1: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for ChBP calculated in gas
phase………….......................................................................................321

Table B.2: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for ChBP calculated in THF
(CPCM)………………………………………………………………...322

Table B.3: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for ChBP calculated in
CH
3
CN (CPCM)….................................................................................323

Table B.4: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for ChBP calculated in
cyclohexane (CPCM)………………………………………………….324

Table B.5: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for NPP calculated in the gas
phase………...........................................................................................326

Table B.6: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for phenylphthalimide
calculated in the gas phase……………………………………………..327

Table B.7: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for PBP calculated in the gas
phase………...........................................................................................328

Table B.8: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for TMPP calculated in the gas
phase…………………………………………………………………...329

Table B.9: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for tBuTMPP calculated in the
gas phase.................................................................................................330

Table B.10: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for tBuTMPP calculated in
THF (CPCM)………………………………………………..............331

Table B.11: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for tBuTMPP calculated in
CH
3
CN (CPCM)……………………………………………………..332

xi
Table B.12: Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for tBuTMPP calculated in
cyclohexane (CPCM)……………………………………..…………333

Table D.1: Summary of OPV cell data for the NPP EBL device…………………342

Table D.2: Summary of OPV cell data for the tBuTMPP EBL device…………...343

Table D.3: Summary of OPV cell data for the ChBP EBL device……………......344

Table D.4: Summary of OPV cell data for the BCP EBL device………………....345

Table D.5: Summary of OPV cell data for the ChBP EBL device with 50Å
thicknesses……………………………………………………..……...346

Table D.6: Summary of OPV cell data for the ChBP EBL device with 100Å
thicknesses……………………………………………………..……...347

Table D.7: Summary of OPV cell data for the ChBP EBL device with 150Å
thicknesses……………………………………………………..……...348

xii
List of Figures

Figure 1.1: (a) Energy band diagram showing the half filled and overlapping
bands of metal, almost filled valence band and almost filled
conduction bands of a semiconductor, and a filled band and an
empty band of an insulator. (b) Energy band diagram of a
semiconductor showing the valence band E
v
, Conduction
band E
c
, energy bandgap E
g
, the vacuum level, E
vac
, Fermi
energy E
F
…………………………………………………………….2

Figure 1.2: Ionization of (a) a donor giving off an electron ( ●) to the
conduction band (b) an acceptor giving off a hole ( ○) to the
valence band………………….............................................................4

Figure 1.3: (a) A p-n junction representing a diode. (b) Creation of a depletion
region by the diffusion of electrons ( ●) and holes ( ○). (c) Current-
voltage characteristic of a p-n
junction………………………………………………………………6

Figure 1.4: A p-n junction (a) at equilibrium (b) forward bias (c) reverse bias…..7

Figure 1.5: Structure of an OLED showing the anode, the cathode and the
organic layers………………………………………………………...10

Figure 1.6: A schematic energy-level diagram of a conventional OLED
showing the occupied and virtual molecular orbitals………………...14

Figure 1.7: Electron hole recombination process showing the coulombic charge
pair, motion time for the formation of the charge pair, the capture
time, formation of the emitting state and photon generation…………15

Figure 1.8: (a) A schematic diagram of hole electron recombination, excited
state generation, and light emission followed by relaxation to the
ground state. (b) Schematic diagram of the spin alignment of the
singlet state showing a net cancellation of the two vectors and the
three triplet states showing the direction of the
vectors…………………………………………………….…….........17

xiii
Figure 1.9: (a) Potential energy surfaces of the singlet ground state (
1
S
0
), the
singlet excited state (
1
S
1
), and the triplet excited state (
1
T
1
)
showing the absorption, fluorescence, and emission path ways (b)
Schematic diagram of the absorption, fluorescence, and
phosphorescence spectra showing the vibronic transitions…….........19

Figure 1.10: A schematic representation of the Förster and Dexter energy
transfer processes in host-guest systems showing singlet-to-singlet
energy transfer can proceed through both the Förster or Dexter
mechanisms and the triplet-to-triplet energy transfer can only
proceed through the Förster mechanism…………………………….23

Figure 1.11: (a) A schematic representation of the Förster energy transfer.
(b) The overlap integral (J) is shown as the dark area, which is
created by the intersection of donor emission spectrum and
acceptor absorption spectrum……………………………………….24

Figure 1.12: Geometry of the donor (D) and acceptor (A) vectors in space
showing the orientation of the dipoles and the angles………………25

Figure 1.13: A schematic representation of the Dexter energy transfer showing
the concerted electron exchange mechanism, where the donor and
the acceptor electrons are exchanged simultaneously; and the
charge transfer electron exchange mechanism, where the donor
and the acceptor electrons are exchanged in steps by forming ionic
radicals………………………………………………………………28

Figure 1.14: (a) A schematic representation of energy level diagram of an
OLED showing the trap distribution underneath the LUMO level.
(b) I-V curves for devices with 200Å TPD and 400Å of Mq
3

(M = Al, Ga, In) showing the ohmic, SCLC, and TCL regions
and the I ∝ V
m+1
relationship, where m goes from 0 to 8 ± 1……….30

Figure 1.15: A schematic representation of the geometric orientation of the
photodetectors placed by OLED (a) Measuring the external
quantum efficiency (b) Measuring the internal quantum efficiency...32

Figure 1.16: A schematic diagram of an OLED device (left) and an OPV device
(right) showing the reversible operating modes……………………..33

Figure 1.17: A schematic diagram of photocurrent generation from OPV cells
showing the five step process………………………………………..34

xiv
Figure 1.18: The AM 1.5 solar spectrum showing the ultra-violet (UV),
visible, and the infra-red (IR) regions………………………………35

Figure 1.19: Schematic diagram of photo induced charge separation in an
organic D/A light harvesting system showing the interfacial
gap (I
g
), LUMO interfacial potential (ΔE
LG
), HOMO interfacial
potential (ΔE
HG
), electron affinity (E
EA
), ionization potential (E
IP
),
and the energy of vacuum (E
vac
)……………………….......................38

Figure 1.20: (a) A p-n junction OPV cell with resistive load. (b) The current-
voltage (I-V) curve of an OPV cell showing the dark current,
photocurrent, open circuit voltage (V
oc
), short-circuit current (I
sc
),
maximum power (P
max
), maximum photogenerated current density
(I
ph
)………………………………………………................................40

Figure 2.1: Microwave synthesis of the phthalimide materials……………...……..54

Figure 2.2: Electrochemistry data of the phthalimide materials showing the
oxidation and reduction waves………………………………………..55

Figure 2.3: DFT optimized SGS geometries of the phthalimide materials
showing the HOMO and the LUMO diagrams (Top). Smallest
subunit of the phenyl phthalimide with arbitrarily numbered atoms
showing the bonds and the dihedral angles measured from DFT
calculation (Bottom)……………………………………………………58

Figure 2.4: Optimized triplet state geometries of all the phthalimides showing
the spin densities and the dihedral angles compared with the ground
state dihedral angles……………………………………………………62

Figure 2.5: Absorption spectra of the tBuTMPP and ChBP in 2me-THF with
superimposed TDDFT excitation energies showing the peak
assignments……………………………………………………………65

Figure 2.6: 298K Absorption and emission spectra of the phthalimides in
2me-THF are shown with 77K spectra in 2MTHF……………………71

Figure 2.7: Energy diagram showing the charge transfer (CT) and charge
localized (CL) excited states…………………………………………..74

Figure 2.8: P-type delayed fluorescence (left). E-type delayed fluorescence
(right)………………………………………………………….……….77

xv
Figure 3.1: a) Structure of a typical phosphorescent OLED showing charge
recombination at the EML. b) State diagram showing the energy
transfer pathways in a host-dopant system. c) An HBL with a deep
HOMO in an OLED blocking hole. d) An exciton blocker with
high triplet energy in and OLED blocking excitons…........................93

Figure 3.2: Structures of the phthalimide materials; TMPP, tBuTMPP, NPP,
and ChBP are shown with conventional hole transporter NPD, host
CBP, electron transporter Alq
3
, hole blocker BCP, and
phosphorescent dopants Irppy and PQIr..............................................96

Figure 3.3: Energy diagrams showing HOMO and LUMO energies of all the
molecules used to make OLEDs with………………………………..101

Figure 3.4: DSC scans of the phthalimide materials showing the glass transition
temperature, T
g
the crystalline transition temperature, T
c
and the
melting temperature, T
m
……………………………………………...102

Figure 3.5: Optical micrograph images of neat NPP showing grainy surface and
TMPP showing smooth surface……………………………………....103

Figure 3.6: Emission of neat and doped films of ChBP, TMPP, and tBuTMPP….104

Figure 3.7: Optical micrograph images of neat NPP showing grainy surface and
TMPP showing smooth surface……………………………………....105

Figure 3.8: Phthalimide reduction potentials (left). Thermochemical cycle for
Irppy constructed from the electrochemistry and photophysics data
showing the electron transfer quenching process (Right). Excited
state oxidation potential: E(Irppy
+/*
) = ET - E(Irppy
+/0
)……………107

Figure 3.9: Electroluminescence (EL) spectra of undoped ChBP, TMPP, and
tBuTMPP devices are shown in comparison with the
photoluminescence (PL) spectra of NPD/ChBP and NPD/
tBuTMPP films. The top plot shows the EL of the standard
NPD and Alq
3
devices with and without BCP hole-blocker………....108

Figure 3.10: Structures of the two types of OLEDs fabricated with the
phthalimides……………………………………………..…………..112

xvi
Figure 3.11: Data for the tBuTMPP based type 2 devices made with Irppy and
PQIr phosphors showing quantum efficiency (%QE) vs. current
density (mA/cm
2
), brightness (Cd/m
2
) vs. voltage (V), current
density (mA/cm
2
) vs. voltage (V), and Intensity (a.u.) vs.
wavelength (nm) plots……………………………………………....114

Figure 4.1: Emitting intramolecular charge transfer (ICT) species of DMABN
are shown. (a) TICT: when the dihedral angle α is 90°, PICT:
when the dihedral angle α is 0°, WICT: when the wagging
angle β changes the hybridization of dimethylamino moiety to sp
3
.
(b) RICT: when the cyano carbon bends to sp
2
……………………....123

Figure 4.2: Dimethylamino group with two extreme orientations are shown.
(left) Coplanar dma group with 0° CNCC dihedral angle showing
increased p-π interaction. (right) Perpendicular dma group with
90° CNCC dihedral angle showing no p-π interaction…………..…..125

Figure 4.3: The three structural isomers of (dmappy)Pt(acac) are shown.
(a) When the p orbital on the nitrogen atom rotates to the
orthogonal position with respect to the ppy plane, the geometry
of the dma group becomes trigonal planar (sp
2
). When the
lone-pair-electrons on the nitrogen atom become coplanar with
respect to the ppy plane, the geometry of the dma group becomes
trigonal pyramidal (sp
3
). (b) Lone-pair facing up. (c) Lone-pair
facing down……………………………………….............................126

Figure 4.4: (a) The direction of the lone-pair-electron-vector on the dma moiety
is shown with respect to the ppy plane, the CNCC dihedral angles θ
1

and θ
2
, and the bisector angle, α. (b) The precessing lone-pair
angle, φ. (c) The pyramid cone angle, δ………………………...…..128

Figure 4.5: Optimized SGS structures of the three isomers of
(dmappy)-Pt-(acac). (a) Dimethylamino trigonal planar (CNCC
dihedral angle = 0°). (b) Dimethylamino trigonal pyramidal with
the nitrogen lone-pair facing up (CNCC dihedral angle = 90°). (c)
Dimethylamino trigonal pyramidal with the nitrogen lone-pair
facing down (CNCC dihedral angle = 90°)…………………………..132

xvii
Figure 4.6: HOMO and LUMO orbital diagrams of the three structural isomers
of (dmappy)-Pt-(acac) are shown along with orbital characteristics
making contribution to the HOMO and the LUMO as percent
contribution from each atom. (Top) Trigonal planar (CNCC = 0°).
(Middle) Trigonal pyramidal with the nitrogen lone-pair facing up
(CNCC = 90°). (Bottom) Trigonal pyramidal with the nitrogen
lone-pair facing down (CNCC = 90°)………………………………..134

Figure 4.7: PES scan results of ground state (dmappy)-Pt-(acac) with the
direction of the lone-pair electrons are shown. (top) Scan results

of C36 rotated from -180° to 0° and 0°to +180°. (bottom) Scan
results of C40 rotated from -180° to 0° and 0°to +180°…...…………136

Figure 4.8: Snapshots of the side view of (dmappy)-Pt-(acac) with pyridine
ring on top and phenyl on the bottom taken at each step of the PES
scan showing the orientation of the dma moiety with respect to the
changing CNCC dihedral angle. The red arrows indicate the
direction of the lone-pair electron at each step of the
scan………………………………………………………...………...138

Figure 4.9: Four ground state inversion centers of (dmappy)-Pt-(acac) showing
lone-pair angles and the energies for the inversion barriers at each
point………………………………………………………………….140

Figure 4.10: (left) A plot of the pyramid cone angle, δ vs. the lone-pair angle,
φ. (right) A plot of the three C-N bond lengths vs. the lone-pair
angle, φ………………………………………………………………141

Figure 4.11: A double Y plot of the HOMO-LUMO energies and HOMO-
LUMO gaps vs. the lone-pair angle, φ showing the changes in
orbital energies as a function of dma rotation…………………..…..143

Figure 4.12: Gas phase singlet and triplet vertical excitation energies, E
VT
of
(dmappy)-Pt-(acac) calculated and plotted against the lone-pair
angle is compared with the total energy of the molecule also
plotted against the lone-pair angle…………………………………..145

Figure 4.13: Room temperature absorption spectra of (dmappy)-Pt-(acac) in
toluene, methanol, acetonitrile, and 2-methyl-THF with
superimposed TDDFT/CPCM vertical excitation energies
showing spectral peak assignments…………………………………148

xviii
Figure 4.14: Optimized TES structures of the three isomers of
(dmappy)-Pt-(acac). (a) Dimethylamino trigonal planar (CNCC
dihedral angle = 0°). (b) Dimethylamino trigonal pyramidal with
the nitrogen lone-pair facing up (CNCC dihedral angle = 90°). (c)
Dimethylamino trigonal pyramidal with the nitrogen lone-pair
facing down (CNCC dihedral angle = 90°). (d) Spin-density plot
of the trigonal planar isomer. (e) Spin-density plot of the trigonal
pyramidal isomers…………………………………………………...158

Figure 4.15: Triplet state potential energy surface (PES) scan of
(dmappy)-Pt-(acac) is shown with the direction of the lone-pair
electrons……………………………………………………………..161

Figure 4.16: Four triplet state inversion centers of (dmappy)-Pt-(acac) showing
lone-pair angles and the energies for the inversion barriers at each
point…………………………………………………………………162

Figure 4.17: (left) A plot of the pyramid cone angle, δ vs. the lone-pair angle,
φ. (right) A plot of the three C-N bond lengths vs. the lone-pair
angle, φ………………………………………………………….......163

Figure 4.18: Dipole moment vs. lone-pair angle, φ………………...……………..164

Figure 4.19: A double Y plot of the SGS and TES PES energies and triplet
energies vs. the lone-pair angle, φ showing change in triplet
energies as a function of dma rotation…………………..…………..165

Figure 4.20: 77K emission spectra of (dmappy)-Pt-(acac) in solvents of different
polarity…………………………………………………...…………..168

Figure 4.21: A schematic energy diagram illustrating the solvatochromic shift
from a hypothetical neutral state, where stabilization of the excited
state (T
1
) causes a bathochromic shift (red-shift) and stabilization
of the ground state (S
1
) causes a hypsochromic shift (blue-shift)…..169

Figure 4.22: Vertical excitation energies calculated in toluene are compared
with the experimental RT and 77K spectra showing preferred
orientation of dma moiety in toluene is δ = -90°…………………....170

Figure 4.23: Vertical excitation energies calculated in THF are compared with
the experimental RT and 77K spectra showing preferred orientation
of dma moiety in THF is δ = 0°……………………………………..171

xix
Figure 4.24: Vertical excitation energies calculated in methanol are compared
with the experimental RT and 77K spectra showing preferred
orientation of dma moiety in methanol is δ = 0°……………………172

Figure 4.25: Vertical excitation energies calculated in acetonitrile are compared
with the experimental RT and 77K spectra showing preferred
orientation of dma moiety in acetonitrile is δ = 0° and -90°……….173

Figure 5.1: (a) (NO
2
ppy)-Pt-(acac) with atom numbers from DFT calculations.
(b) Planar isomer: when the CCNO dihedral becomes 0°, the
nitrogen p orbital becomes coplanar to the π orbital of the pyridine
ring. (c) Orthogonal isomer: when the CCNO dihedral becomes
90°, the nitrogen p orbital becomes orthogonal to the π orbital of
the pyridine ring……………………………………………………..182

Figure 5.2: (a) Crystal structure of (NO
2
ppy)-Pt-(acac). (b) Optimized SGS
structures of the planar isomer of (dmappy)-Pt-(acac) (CCNO
dihedral angle = 0°). (c) SGS optimized structures of the
orthogonal isomer of (dmappy)-Pt-(acac) (CCNO dihedral
angle = 90°)…………………………………………………….........186

Figure 5.3: HOMO and LUMO orbital diagrams of the two structural isomers
of (NO
2
ppy)-Pt-(acac) are shown along with orbital characteristics
making contribution to the HOMO and the LUMO as percent
contribution from each atom. (Top) Planar isomer (CCNO = 0°).
(Bottom) Orthogonal isomer (CCNO = 90°)……………...................188

Figure 5.4: Plot total SCF energy vs. CCNO dihedral angle showing the result
of the ground state PES scan of (NO
2
ppy)-Pt-(acac)………………...190

Figure 5.5: Results of SGS PES scan showing: (top) the plot of C
7
-N bond
length vs. dihedral angle, (middle) the plot of O
36
and O
37
-N bond
length vs. dihedral angle, and (bottom) the plot of ∠O
36
NO
37

vs. dihedral angle……………………………………………………..191

Figure 5.6: A double Y plot of the HOMO-LUMO energies and HOMO-LUMO
gaps vs. the dihedral angle, θ showing the changes in orbital
energies as a function of NO
2
rotation………………………..……..192

Figure 5.7: Highest occupied and lowest unoccupied MOs of
(NO
2
ppy)-Pt-(acac) obtained from SGS PES scan are shown for the
dihedral angles from 0° to 90°. The results show HOMOs remain
unchanged as the LUMOs change with NO
2
rotation…………….....194
xx
Figure 5.8: Room temperature absorption spectra of (NO
2
ppy)-Pt-(acac) in
hexanes with superimposed gas phase TDDFT vertical excitation
energies showing spectral peak assignments…………………….…..196

Figure 5.9: Room temperature absorption spectra of (NO
2
ppy)-Pt-(acac) in
hexanes with superimposed TDDFT/CPCM vertical excitation
energies showing spectral peak assignments. (left) Original Vertical
excitation energies. (right) Vertical excitation energies blue-shifted
by 50nm……………………………………………………………….199

Figure 5.10: Triplet state potential energy surface (PES) scan of
(NO
2
ppy)-Pt-(acac) is shown with the ground state PES scan and
the triplet energies on the right Y axis…………………………...…206

Figure 5.11: Gaussian (blue) spin-density plots of TES optimized structures of
(NO
2
ppy)-Pt-(acac) shown for the dihedral angles from 0° to 90°.
Titan (red) spin-density plots were obtained by optimizing the
structures in the triplet state………………………………………...208

Figure 5.12: Room temperature and 77K emission spectra of
(NO
2
ppy)-Pt-(acac) in hexanes, 2M THF, and toluene……………..211

Figure 5.13: Room temperature emission spectra of (NO
2
ppy)-Pt-(acac) in
polystyrene matrix, CBP matrix, and UGH matrix………………....212

Figure 5.14: Gas phase triplet energies of (NO
2
ppy)-Pt-(acac) calculated for the
seven structures between 0° to 90° dihedral angles is showing a
steady increase in the triplet energies going from the 0° to the 90°
isomer………………………….........................................................214

Figure 5.15: A proposed scheme of excitation and de-excitation pathways of
(NO
2
ppy)-Pt-(acac)…………………………………………………215

Figure 6.1: Block diagram of the experimental setup for dark spot growth
measurement……………………………………………...………….229

Figure 6.2: (a) Picture of the high vacuum environmental chamber. (b) Picture
of the vacuum line setup for device storage and
transportation………………………………………………………...231

Figure 6.3: (a) Picture of the I-V and the J-V measurement setup. (b) Picture
of the device under static vacuum being tested……………………...231

xxi
Figure 6.4: (a) Environmental chamber setup. (b) Internal view of the
environmental chamber…………………………………………...…232

Figure 6.5: The plots of dark area vs. time for the devices A and B before and
after correction…………………………………………...…………..234

Figure 6.6: The plots of corrected curves vs. time for devices A and B. Both
polynomial and linear fits gave the same rates……………………....235

Figure 6.7: The plots of individual dark area vs. time for devices A…………......236

Figure 6.8: The plot of individual dark area vs. time for devices B…………...….237

Figure 6.9: The plots of dark area vs. time for the ETL devices before and after
correction………………………………………………...…………...238

Figure 6.10: On top left the plot of corrected curves vs. time showing linear fits
for the three ETL devices. The top right and the bottom plots
show the individual dark area vs. time for the ETL devices………..239

Figure 6.11: Degradation curve fitting of device A with linear, quadratic,
exponential, and sigmoidal-Boltzmann equations………………….242

Figure 6.12: Degradation curve fitting of device B with linear, quadratic,
exponential, and sigmoidal-Boltzmann equation………………..….243

Figure 6.13: Dark spot diameter vs. time are plotted (a) Clean-room devices.
Shows comparatively smaller dark spots with one uniform rate,
which means that these spots perhaps had the same origin. (b)
Ambient air devices shows large dark spots with two very distinct
rates, hence different origins………………………………………..245

Figure 6.14: (a) At t=0, an internal dark spot (IDS) and an external dark spot
(EDS) is shown. (b) At t = 10 min, the EDS and the IDS both
seemed to have grown. (c) At t=24 min, IDS remained the same
but the EDS grew much bigger. (d) At t=26 min, the IDS was
consumed by the EDS. (e) Example of dark areas caused by
surface defects. (f) and (g) % Dark area vs. time showing areas
with more dark spots degrade faster than the areas with fewer
dark spots……………………………………………………………247

Figure 6.15: (a), (b), (c) Clean-room devices. (d), (e), (f) Ambient air devices.
Half life vs. dark spot density showing a liner relationship between
the device half-lives and initial dark spot densities…………………249
xxii
Figure 6.16: t = 0 Snapshots showing the initial dark spot density for all of the
ETL devices……………………………………………..…………..251

Figure 6.17: Degradation curve fitting of mCP device with linear, quadratic,
exponential, and sigmoidal-Boltzmann equations……………….…252
Figure 6.18: Degradation curve fitting of CBP device with linear, quadratic,
and sigmoidal-Boltzmann equations. (bottom right) Curve fitting
of BAlq device with linear equation………………………………..253

Figure 6.19: (left) Degradation curves of the ETL devices. (right) Log-log
degradation curves of the ETL devices are shown with the
standard test devices for visual comparison………………….…….255

Figure 6.20: (left) Log-log degradation curves of the over-coated devices.
(right) Log-log degradation curves of the wax over-coated
devices are shown with the standard test devices for visual
comparison………………………………………………………….257

Figure 6.21: Sigmoidal-Boltzmann curve fitting of devices over-coated with
paraffin oil, Si oil, carbon glassy powder, and graphite powder……258

Figure 6.22: Sigmoidal-Boltzmann curve fitting of devices over-coated with
C
60
/Si oil, C
60
soot/Si oil, paraffin wax, and BCS wax……………...259

Figure 6.23: The plot of impact factor vs. the experimental half life…………......260

Figure 6.24: The plot is a comparison of the brightness and the %QE between
t=0 and t=8days. Device characteristics are shown to remain
unchanged 8 days after encapsulating with wax. Device 1 is
uncoated, and is nearest to the cathode. Device 2, and 3 are
coated with wax…………………………………………………….262

Figure A.1: Structures of the phthalimide based hole/exciton blocking materials;
NPP, tBuTMPP, and ChBP are shown with conventional exciton
blocking material BCP, donor material CuPc, and acceptor material
C
60
……………………………………………………………………301

Figure A.2: General cell structure: ITO/CuPc/C
60
/EBL/Al…………...…………..303

Figure A.3: Energy diagram showing the HOMO and the LUMO energies of
all the materials studied……………………………………………...304

xxiii
Figure A.4: (a) Current density-voltage (J-V) characteristic of the cell with NPP
EBL in the dark and under 1 sun intensity of simulated AM 1.5G
solar illumination (b) Log of current-density vs voltage of the cell
with NPP EBL under various intensities……………………...……..306

Figure A.5: (a) Current density-voltage (J-V) characteristic of the cell with
tBuTMPP EBL in the dark and under 1 sun intensity of simulated
AM 1.5G solar illumination (b) Log of current-density vs voltage
of the cell with tBuTMPP EBL under various intensities…………..307

Figure A.6: Current density-voltage (J-V) characteristic of a cell with BCP EBL
(a) in the dark and under 1 sun intensity of simulated AM 1.5G solar
illumination (b) Log of current-density vs voltage under various
intensities (c) General cell structure: ITO/CuPc/C
60
/EBL/Al………307

Figure A.7: A solar cell circuit diagram showing the series resistance (R
S
),
the shunt resistance (R
SH
), the forward current (I
F
), and the photo
current (I
PH
)………………………………………………………….308

Figure A.8: Organic PV cells with NPP, tBuTMPP, and BCP buffer layers
showing type A and B device structures and the possibility of
crystalline islands formation in the phthalimide based cells.
Type A: CuPc/C
60
/NPP or tBuTMPP/BCP/Al cell (a) Uniform
film of phthalimide (b) Crystalline film of phthalimide. Type B:
CuPc/C
60
/BCP/NPP or tBuTMPP/Al cell (c) Crystalline film of
phthalimide (d) Uniform film of phthalimide……………………….311

Figure A.9: (a) Current density-voltage (J-V) characteristic of the cell with
ChBP EBL in the dark and under 1 sun intensity of simulated AM
1.5G solar illumination (b) Log of current-density vs voltage of
the cell with ChBP EBL under various
intensities………………………………………………...…………..312

Figure A.10: Current density-voltage (J-V) characteristics of the cells with
50 Å, 100 Å, and 150 Å of ChBP EBL under 1 sun intensity of
simulated AM 1.5G solar illumination respectively showing the
thickness dependent photovoltaic response. Inset is the data for
150 Å……………………………………………………………….315

Figure C.1: Optical micrograph images of neat NPP showing grainy surface and
TMPP showing smooth surface……………………………………...335

xxiv
Figure C.2: Optical micrograph images of Irppy and PQIr doped NPP films
showing smooth surfaces and TMPP films showing smooth
surfaces………………………………………………………………336

Figure C.3: (Top) Data for the tBuTMPP based type 1 and 2 devices made
with the Irppy phosphor showing quantum efficiency (%QE) vs.
current density (mA/cm
2
), brightness (Cd/m
2
) vs voltage (V),
current density (mA/cm
2
) vs voltage (V), and Intensity (a.u.) vs
wavelength (nm) plots. (Bottom) Energy diagram for the type 2
device……………………………………………………..…………337

Figure C.4: (Top) Data for the tBuTMPP based type 1 and 2 devices made
with the PQIr phosphor showing quantum efficiency (%QE) vs.
current density (mA/cm
2
), brightness (Cd/m
2
) vs voltage (V),
current density (mA/cm
2
) vs voltage (V), and Intensity (a.u.) vs.
wavelength (nm) plots. (Bottom) Energy diagram for the type 2
device……………………………………………………..…………338

Figure C.5: (Top) Data for the ChBP based type 1 and 2 devices made
with the PQIr phosphor showing quantum efficiency (%QE) vs.
current density (mA/cm
2
), brightness (Cd/m
2
) vs voltage (V),
current density (mA/cm
2
) vs voltage (V), and Intensity (a.u.) vs
wavelength (nm) plots. (Bottom) Energy diagram for the type 2
device……………………………………………………..…………339

Figure C.6: (Top) Data for the TMPP based type 1 and 2 devices made
with the PQIr phosphor showing quantum efficiency (%QE) vs.
current density (mA/cm
2
), brightness (Cd/m
2
) vs voltage (V),
current density (mA/cm
2
) vs voltage (V), and Intensity (a.u.) vs
wavelength (nm) plots. (Bottom) Energy diagram for the type 2
device………………………………………………….……………340

Figure D.1: (a) Current density-voltage (J-V) characteristic in the dark and
under 20%, 40%, 60%, 80%, and 100% intensities of simulated
AM 1.5G solar illumination of OPV cell with NPP EBL. (b) The
responsivity of the cell (J
SC
/P
0
vs P
0
) under 1 sun. (c) The solar
conversion efficiency ηP%, open circuit voltage V
OC
, and fill
factor FF under 1 sun………………………………………………..342

xxv
Figure D.2: (a) Current density-voltage (J-V) characteristic in the dark and
under 20%, 40%, 60%, 80%, and 100% intensities of simulated
AM 1.5G solar illumination of OPV cell with tBuTMPP EBL. (b)
The responsivity of the cell (J
SC
/P
0
vs P
0
) under 1 sun. (c) The
solar conversion efficiency ηP%, open circuit voltage V
OC
, and
fill factor FF under 1 sun……………………………………….......343

Figure D.3: (a) Current density-voltage (J-V) characteristic in the dark and
under 20%, 40%, 60%, 80%, and 100% intensities of simulated
AM 1.5G solar illumination of OPV cell with ChBP EBL. (b)
The responsivity of the cell (J
SC
/P
0
vs P
0
) under 1 sun. (c) The
solar conversion efficiency ηP%, open circuit voltage V
OC
, and
fill factor FF under 1 sun…………………………………………....344

Figure D.4: (a) Current density-voltage (J-V) characteristic in the dark and
under 20%, 40%, 60%, 80%, and 100% intensities of simulated
AM 1.5G solar illumination of OPV cell with BCP EBL. (b)
The responsivity of the cell (J
SC
/P
0
vs P
0
) under 1 sun. (c) The
solar conversion efficiency ηP%, open circuit voltage V
OC
, and
fill factor FF under 1 sun…………………………………………....345

Figure D.5: (a) Current density-voltage (J-V) characteristic in the dark and
under 20%, 40%, 60%, 80%, and 100% intensities of simulated
AM 1.5G solar illumination of OPV cell with 50Å ChBP EBL. (b)
The responsivity of the cell (J
SC
/P
0
vs P
0
) under 1 sun. (c) The
solar conversion efficiency ηP%, open circuit voltage V
OC
, and
fill factor FF under 1 sun…………………………………………….346

Figure D.6: (a) Current density-voltage (J-V) characteristic in the dark and
under 20%, 40%, 60%, 80%, and 100% intensities of simulated
AM 1.5G solar illumination of OPV cell with 100Å ChBP EBL. (b)
The responsivity of the cell (J
SC
/P
0
vs P
0
) under 1 sun. (c) The
solar conversion efficiency ηP%, open circuit voltage V
OC
, and
fill factor FF under 1 sun…………………………………………….347

Figure D.6: (a) Current density-voltage (J-V) characteristic in the dark and
under 20%, 40%, 60%, 80%, and 100% intensities of simulated
AM 1.5G solar illumination of OPV cell with 150Å ChBP EBL. (b)
The responsivity of the cell (J
SC
/P
0
vs P
0
) under 1 sun. (c) The
solar conversion efficiency ηP%, open circuit voltage V
OC
, and
fill factor FF under 1 sun…………………………………………....348
xxvi
Abstract
The work presented in this thesis has two distinct objectives. The first
objective is to design, synthesize, and study new materials for OLEDs and OPV cells
and in the process make more efficient devices. The second objective is to study the
extrinsic degradation mechanism of OLED and find a way to stop or slow down the
degradation, so in the future more air stable devices can be produced.
Development of high triplet energy materials is important for both OLEDs
and OPV cells. In OLEDs these materials can be effectively used either as hosts or
as hole blocking materials and in OPV cells the materials can be used as exciton
blocking layers (Buffer layers). The second chapter of this thesis describes the
design, synthesis, and characterization studies of new phthalimide based high triplet
energy materials for OLEDs and OPV cells. The chapter gives a detailed narrative
of the electrochemical, photophysical, and density functional theory analysis
(Ground and excited states) of all the materials.
The third chapter deals with thermochemistry, thin-film photophysics, and
OLED fabrication studies of the phthalimide materials. The chapter delves into the
OLED fabrication studies and investigates the behaviors of the devices fabricated
with the phthalimides as hosts and as hole blocking layers (HBL) with red and green
dopants. Very efficient green and red devices were obtained with the phthalimide
HBL compared to the conventional BCP hole blocking layer.
Theoretical studies of platinum based broadband emitter, (dmappy)Pt(acac)
are described in chapter four. In room temperature fluid solution, the
xxvii
(dmappy)Pt(acac) has been shown to emit in two different wavelengths, which shifts
from blue to red depending on solvent polarity. Rotation of the dimethyl amino
moiety on the phenyl ring causes the molecule to emit in different wavelengths. In
the planar geometry, increased conjugation between the p orbital of the nitrogen
atom and the π orbitals of the phenyl ring causes the triplet energy of the system to
decrease. When the dimethyl amino group rotates perpendicular to the plane of the
molecule, the geometry of the dimethyl amino moiety becomes trigonal-by-
pyramidal giving rise to two structural isomers: One with the nitrogen lone pairs
facing up and the other with the lone pairs facing down. As a consequence, the
conjugation between the p-π orbitals decreases and the emission shifts to higher
energy. In chapter four we investigate the spectroscopic behavior of this molecule
by comparing the photophysical data with DFT and TDDFT calculations.
Theoretical investigation of (5NO
2
ppy)Pt(acac) is covered in chapter five.
This molecules is important for both OLEDs and OPV cells. At 77K glass in 2me-
THF and at room temperature in hexanes and polystyrene matrix emission from this
molecule is observed around 550nm. In acetonitrile, toluene, and 2me-THF at 298 K
no emission is observed. We believe that the emission of (5NO
2
ppy)Pt(acac) is
controlled by the rotating NO
2
group on the pyridine ring. Rotation of the NO
2

group to the plane or perpendicular to the plane of the molecule may alter the
emission properties of the molecule. Chapter five uses theoretical methods (DFT
and TDDFT) to investigate the spectroscopic properties of (5NO
2
ppy)Pt(acac).
xxviii
In open air degradation of OLEDs proceeds through the formation and
growth of nonemissive regions called dark spots. Since the dark spots form on the
cathodes and grow by reacting with the water and oxygen molecules that tunnels
through the cathode to the ETL, an effective way of stopping the formation and
growth of dark spots would be by overcoating the cathode with hydrophobic
materials and replacing the ETL with materials that can act as desiccants. Chapter
six investigates the role of electron transporting layers (ETLs) and various
overcoating materials on the degradation OLEDs.

xxix
Chapter 1. Introduction to Semiconductors, Organic Light
Emitting Diodes (OLEDs), and Organic Photovoltaic Cells
(OPVs)

1.1 Introduction to Semiconductors
Semiconductors are electronic devices made from materials such as silicon,
germanium, and gallium arsenide. Integrated circuits (IC), light emitting diodes
(LED), and transistors are few of the examples of semiconductor devices that are
widely used in cell phones, televisions, computers and other important amenities in
our every day lives. Even though the semiconductor industry is governed by
inorganic semiconductors, recent developments of organic based semiconductors
have taken the industry to the next level. Small molecule and polymer based
semiconducting materials have brought outstanding potentials to the electronic and
optoelectronic industries. These new classes of organic materials have remarkable
properties and are used to make a wide range of semiconducting electronic devices
such as organic field effect transistors (FETs), organic light emitting diodes
(OLEDs) and organic photovoltaic cells (OPVs). Furthermore, the costs of
manufacturing organic semiconductor devices are cheaper than the costs for
manufacturing conventional inorganic semiconductors.

1
1.1.1 Principles of Semiconductors
Chemical reactions between molecules require exchange of electrons from
the atomic valence shells. Electrons forming the covalent bonds come from the last
ground-state band called the valence band, and the energy band right above it which
permits the conduction of the electrons, is called the conduction band.
1
In Figure
1.1a valence and conduction bands of metal, semiconductor, and insulator are shown.
Metals like Cu, Au, and Ag that have only one electron in the valence shell contain
half filled bands. Metals consisting of two electrons in the valence shell conduct
when the filled band overlaps with the empty band. The insulators do not conduct
because they have a completely filled valence band and a completely empty
conduction band separated by a large gap.
E
vac
E
F
E
V
Conduction
band
Energy
Band-gap
Valence
band
E
g
E
C
E
Metal Semiconductor Insulator
E
g
(a) (b)
E
vac
E
F
E
V
Conduction
band
Energy
Band-gap
Valence
band
E
g
E
C
E
Metal Semiconductor Insulator
E
g
(a) (b)

Figure 1.1 (a) Energy band diagram showing the half filled and overlapping bands
of metal, almost filled valence band and almost filled conduction bands of a
semiconductor, and a filled band and an empty band of an insulator. (b) Energy band
diagram of a semiconductor showing the valence band E
v
, Conduction band E
c
,
energy bandgap E
g
, the vacuum level, E
vac
, Fermi energy E
F
.
2

2
A semiconductor consists of an almost-filled valence band and an almost-
empty conduction band; and electrons making transitions from the valence band to
the conduction band dominates the behavior of a semiconductor.
2
Figure 1.1b
illustrates the energy bands of a semiconductor, where the bottom edge of the
conduction band is labeled E
c
and the top edge of the valence band is labeled E
V
.
The energy band gap is located between E
c
and E
V
. The distance between the
conduction band, E
c
and the energy of a free electron outside the crystal is called the
E
vac
, and is quantified as electron affinity. The Fermi level, E
F
, represents the
maximum energy of an electron at zero degree Kelvin (0K). At that temperature, all
the allowed energy levels below the Fermi level are occupied, and above the Fermi
level are empty.
1

1.1.2 Doped semiconductor
When a semiconductor incorporates foreign atoms or Impurities into its
crystal structure it then becomes a doped semiconductor. The impurities or dopants
generate free carriers by giving off or accepting electrons. If the dopants give off
electrons to the conduction band they are called donors; and if they give off holes to
the valence band they are called acceptors. An n-type semiconductor is an ionized
donor, which provides electrons in a semiconductor; and a p-type semiconductor is
an ionized acceptor, which provides holes in a semiconductor. The ionization of
donors and acceptors are depicted in figure 1.2, which indicates the donor and
acceptor energies as E
d
and E
a
.
1, 2
Ionization causes the donor to be emptied by
3
putting an electron to the conduction band leaving behind a positively charged donor
ion. The energy of the acceptor remains empty prior to ionization. Upon ionization,
the acceptor level gets filled by an electron from the valence band, which is same as
acceptor giving off a hole to the valence band.
2

E
c
E
d
E
F
E
v
E
v
E
F
E
c
E
a
E
X
(a) (b)
E
c
E
d
E
F
E
v
E
v
E
F
E
c
E
a
E
X
(a) (b)

Figure 1.2 Ionization of (a) a donor giving off an electron ( ●)to the conduction band
(b) an acceptor giving off a hole ( ○)to the valence band.
2

1.1.3 The p-n junction and the recombination of carriers
When a p-type and an n-type semiconductor come in contact with each other
they form a p-n junction (Figure 1.3a). If the p and n-type regions are made out of
the same materials, the junction is called a homojunction. If they are made out of
different materials, the junction is called a heterojunction. A diode is a
semiconductor device that has a p-n junction and a non-linear current-voltage
characteristic. The p-n junction creates a rectifying circuit, meaning it allows the
current flow only in one direction. When a p-n junction is formed, electrons from
the n-region and holes from the p-region diffuse across the junction to combine with
each other. Filling a hole creates a negative ion and leaves behind a positive ion on
the n-side. Similarly, removing an electron creates a positive ion, which creates a
4
negative ion on the p-side. The combining of holes and electrons depletes the
electrons in the n-region and holes in the p-region, which builds up a space-charge
around the junction and the junction is called the “space-charge region” or the
“depletion region” (Figure 1.3b).
1, 2
The current inside a p-n junction can be defined
by the following equation:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
⎥
⎦
⎤
⎢
⎣
⎡
= 1 exp
kT
qV
I I
a
S
(1.1)
where I
s
is a constant, V
a
is the voltage applied to the diode, and kT is the thermal
energy. Figure 1.3c shows a typical current-voltage characteristic plot of a p-n
junction. The V
a
is positive when current, I flows through the diode and negative
when it does not. If V
a
> 0 the junction is forward biased, and if V
a
< 0 then the
junction is reversed biased.
1

The ionized charges inside the depletion region create an internal electric
field, which causes a drift of carriers in the opposite direction. Diffusion of the
carriers from the n to p region continues until the drift current balances the diffusion
current and reaches a thermal equilibrium creating a constant Fermi energy. The
potential across the depletion region in thermal equilibrium is called the “built-in
potential” or the “junction potential” of a semiconductor and can be defined by the
following equation:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
= Φ
2
0
ln
i
d a
n
N N
q
kT
(1.2)
5
where n
i
is the intrinsic carrier concentration and N
a
and N
d
are constants that relate
to the doping concentrations in the p-type and the n-type regions respectively. The
drift current produced by the built-in potential has exactly equal and of opposite sign
of the diffusion current caused by the carrier concentration gradient. So the net
current flow, drift + diffusion equals to zero.
1, 2

+
+
+
+
+
+
+
+ +
Depletion
region
-
-
-
-
-
-
-
(b)
-
-
-
-
+
+
+
+
pn
Electrons
Holes
Negative
ions
Positive
ions
I
V
a
0
(a)
- +
V
n-type

p-type
p-n junction
(c)

Figure 1.3 (a) A p-n junction representing a diode. (b) Creation of a depletion
region by the diffusion of electrons ( ●) and holes ( ○). (c) Current-voltage
characteristic of a p-n junction.

Figure 1.4 shows a p-n junction at equilibrium, in forward bias, and in
reverse bias. At equilibrium, the Fermi levels on both sides of the p-n junction line
up. Electrons and holes reach equilibrium at the depletion zone. When the junction
is forward biased, the electrons in the n-region which have reached the conduction
6
band and have diffused across the junction find themselves at a higher energy than
the holes in the p-region. They readily combine with the holes from the other side
and make a continuous forward bias through the junction. At reverse bias, the p-
region becomes more negative, making it more uphill for the electrons to move
across the junction. In other words, at forward bias, the applied electric field assists
the electrons to overcome the coulomb barrier of the space charge of the depletion
region; and at reverse bias, the applied electric field impedes the flow of electron
across the junction.
1, 2

Valence band
Conduction band
Fermi energy lined up
on both sides
Valence band
p n
pn
Recombination
(a) At equilibrium
(b) Forward bias
p-n
junction
Valence band
Conduction band
p-n
junction
p
n
(c) Reverse bias
Conduction band
Valence band
Conduction band
Fermi energy lined up
on both sides
Valence band
p n
pn
Recombination
(a) At equilibrium
(b) Forward bias
p-n
junction
Valence band
Conduction band
p-n
junction
p
n
(c) Reverse bias
Conduction band

Figure 1.4 A p-n junction (a) at equilibrium (b) forward bias (c) reverse bias.

7
1.2 Introduction to Organic Light-Emitting Diodes (OLEDs)
Organic Light Emitting Diodes (OLEDs) are electroluminescent display
devices made out of stacked organic thin films capped with metal electrodes. The
total thickness of an OLED is less than 500 nm (0.5 thousandths of a millimeter),
making it the most compact flat panel display in the market. Unlike LCD displays,
OLED displays do not require any back lighting and can operate in broader
temperature ranges. OLED displays are lighter, brighter, have wider viewing angles
(160º), faster response time, and low turn on voltages compared to the LCD
displays.
3
A major advantage of OLED display over LCD or CRT displays is light
emission from extremely thin films, which gives OLEDs the foundation for flat and
flexible display technologies. Flexible television and computer screens that can be
rolled up like news papers can be manufactured by vapor depositing small organic
molecules or printing polymers on flexible substrates.
Research and development of OLED are carried out by many companies
throughout the world. Companies like IBM, Philips, Eastman Kodak, Dupont,
Pioneer, Cambridge Display, Universal Display, Plextronics, Air Products and
Chemicals, Starcks, and Sumitomo are working on next generation OLEDs.
4, 5

Plastic Logic, working on animated posters and electronic newspapers, recently
reported success in flexible electronic paper-based displays.
6
Cambridge Display
Technology recently reported a 14-inch OLED screen from polymer light-emitting
diodes using ink-jet printing process.
3
Sony just reported its first 27-inch prototype
OLED HDTV with a contrast ratio greater than 1,000,000:1.
7
And very recently, for
8
the first time near infra-red (NIR) OLED was reported by our Borek et al.
8
This type
of OLEDs can be used in night vision equipments like NIR sensors, NIR goggles,
portable NIR spectrometers, and even NIR computer displays.
The operation of OLED relies on molecular excitation and de-excitation of
organic chromophores. When voltage is applied, luminescent organic molecules
inside the emissive layer gets promoted to the excited states by the recombination of
opposite charge carriers. De-excitation of the molecules back to the ground state
results in generation of photons. Since the energies of the photons depend on the
chromophores used, emissions of different wavelengths are easily obtained by using
the right energy band-gap dopants. An effective way of obtaining pure colored
emission in OLED is by good host dopant interaction, where a proper energy match
between the host and the dopant allows pure RGB, white, and IR emissions.

1.2.1 Structure of OLED
Organic light-emitting diode (OLED)
9
is an electroluminescent
semiconductor device composed of organic dielectrics and metal electrodes. The
simplest form of an OLED includes a hole transporting layer (HTL)
10-12
, which is a
p-type semiconductor, an electron transporting layer (ETL)
13-15
, which is an n-type
semiconductor, and a spacer layer all sandwiched between an anode and a cathode.
16,
17
Figure 1.5 illustrates the overall structure of an OLED composed of an anode, an
HTL, an ETL, an HIL, the spacer layer, and the cathode.
9
HIL
ETL
HTL
Cathode

Spacer

+
-
Anode

Figure 1.5 Structure of an OLED showing the anode, the cathode and the organic
layers.

All OLEDs have at least one of the electrodes transparent for the photons to
exit the device. In conventional OLEDs, the cathode is opaque and the anode is
transparent. Indium Tin Oxide (ITO) (In
2
O
3
: SnO
2
) is the most widely used material
for anode in OLEDs.
18
This material is a transparent conductor and is usually
deposited on glass sheets by sputtering.
19
OLED is fabricated by vapor depositing
(10-
6
torr) or spin-coating organic layers on an ITO coated glass substrate, followed
by vapor deposition of the cathode. Most commonly used HTL and ETL materials
used in OLEDs are N,N’-di(naphthalene-1-yl)-N,N’-diphenyl-benzidine ( α-NPD)
12

and aluminum tris(8-hydroxyquinoline) (Alq
3
)
13, 14
respectively. Some devices also
incorporate copper phthalocyanines (CuPc) as a hole injecting layer (HIL) between
the anode and the HTL layers for balanced charge recombination.
20-22

Most commonly used cathodes in OLEDs are Ca,
23
Mg,
24
or Al.
25
The use of
low work function metals like Ca and Mg in OLEDs improves the electron injection
from the cathode to the ETL. However, OLEDs fabricated with these electrodes
10
actually give decreased device efficiency due to highly reactive nature of these
metals. On the other hand Aluminum, due to its comparatively higher work function
decreases device efficiency by creating a barrier for electron injection.
23
Insertion of
a spacer layer between the ETL and the cathode has been demonstrated to
significantly increase device performance. The presence of LiF layer at the Al-Alq
3

interface lowers the electron injection barrier (Schottky barrier) height by causing
band bending of Alq
3
.
25

1.2.2 Charge Conduction in OLED
Charge conduction in OLED involves the hopping of electrons or holes from
one molecule to another. The organic materials used in OLEDs are insulators in the
ground state. These materials have large tails in their density of states suggesting no
free charges in room temperature. Charge injection causes these molecules to
conduct. When the holes and electrons are injected from the anode and cathode, the
charges move through the organic molecules by hopping or tunneling.
26
In small
molecules, the charges hop between the neighboring molecules. In the conjugated
polymer materials, charge conduction involves both tunneling through the
delocalized molecular orbitals and hopping between different polymer chains. The
mobilities of holes and electrons in organic molecules depend on the properties of
the materials and vary with the applied electric field.
27, 28
Since the electric field
decreases the hopping barrier, the carrier mobility depends on the applied electric
field. Experimentally it has been shown that charge conduction in both small
11
molecules and conjugated polymers follows the Poole-Frenkel (PF) model. Equation
1.3 shows the relationship between applied electric field and carrier mobility defined
by the PF model
() ( )
2 / 1
exp E E
o
γ μ μ = (1.3)
where E is the applied electric field, μ
0
is the zero-field mobility, and γ is the
mobility field activation parameter.
26, 29

The mobilities of charge carriers in organic materials are measured by two
methods; the time of flight (TOF)
30
method and the space-charge limited current
(SCLC)
29
method. In the TOF method, the carrier mobility is determined from the
transit time or the time taken by the carriers to move across the organic layer. This is
achieved by applying voltage to a device composed of a thin semitransparent
electrode, a thick organic layer, and a thick reflecting electrode. In the presence of
electric field, the holes or electrons are driven to the opposite electrode and collected,
and the mobility is measured from the following equation,
tV
d
2
= μ (1.4)
where V is the applied voltage, t is the transit time, and d is the film thickness. Since
the TOF requires the preparation of thick films and the morphology of the thick films
can vary significantly from the thin-films, one disadvantage of the TOF method is
that it cannot exactly give the charge transport characteristics of thin films.
26, 30

Carrier mobility measurements in thin films are conducted by the SCLC
method. This is achieved by measuring current-voltage (IV) characteristics of hole
12
only or electron only devices. The hole only device can be prepared by replacing the
cathode by high work function metals (Au, Pd) and the electron only device can be
prepared by replacing the anode with low work function metals (Al, Ca). When the
barriers of charge injection from the electrodes to the organic layers are negligibly
small (Meaning low injection barrier or high electric field), the mobility of the
carriers limits the current passing through the organic thin films. For single carrier
devices, the field dependent mobility of SCLC in most organic materials follows the
Mott-Gurney law described by the following equation:
()
()
2 / 1
2
0
89 . 0 exp
8
9
E
L
E
J
o
γ μ εε = (1.5)
where J is the current, ε is the permittivity of free space, and ε
0
is the relative
dielectric constant.

1.2.3 Charge Recombination in OLED
When current is applied to an OLED, holes from the anode and electrons
from the cathode diffuse through the organic layers and meet at the HTL/ETL
interface (p-n junction). The recombination of the holes and electrons at the
HTL/ETL interface generates excitons, which emit photons upon radiative decay.
16,
26, 31-34
The band-gap diagram of an inorganic semiconductor shown in figure 1.4
cannot be applied to OLED because the occupied and virtual molecular orbitals
(MOs) in organic semiconductors are not closely packed like inorganic
semiconductors. A correct representation of the energy level diagram of an OLED is
13
shown in Figure 1.6. Here the energy levels of the occupied and the virtual
molecular orbitals (MOs) for HTL and ETL layers are shown along with the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels. Hole-electron recombination in OLEDs occur in the
forward-bias direction when the holes injected from the anode travel through the
HOMO of the HTL and electrons injected from the cathode travel through the
LUMO of the ETL and collect at HTL/ETL interface. This process occurs in two
steps, the charges are first injected into the metal-semiconductor interface region and
then to the hole or electron transporting layers.
h+
e-
ETL
HTL
h+
e-
hν
HOMO (E
V
)
E
FP
E
FN
HOMO (E
V
)
LUMO (E
C
)
LUMO (E
C
)
Occupied MO
Virtual MO
h+
e-
ETL
HTL
h+
e-
hν
HOMO (E
V
)
E
FP
E
FN
HOMO (E
V
)
LUMO (E
C
)
LUMO (E
C
)
Occupied MO
Virtual MO

Figure 1.6 A schematic energy-level diagram of a conventional OLED showing the
occupied and virtual molecular orbitals.

In organic materials hole-electron recombination follows the Langevin
26
or
Thompson
35
bimolecular recombination processes (Figure 1.7). The models treat the
14
recombination process classically by relating to the carrier motion time (τ
m
), which is
the time to get the carriers within the coulombic capture radius (r
c
), the elementary
capture time (τ
c
), which is the time to get to the emitting state, and the average mean
free path (λ) or the hopping distance of the carriers. The ultimate recombination
time (τ
rec
), which is the time the charges take to annihilate each other, is inversely
related to the motion time and the capture time (1/τ
rec
= 1/τ
m
+ 1/τ
c
). The Langevin

h
+
e
-
+ -
Coulombic Charge Pair
Emitting State Ground State
hν
Light
τ
c
τ
m
τ
m r
h
+
e
-
+ -
Coulombic Charge Pair
Emitting State Ground State
hν
Light
τ
c
τ
m
τ
m r

Figure 1.7 Electron hole recombination process showing the coulombic charge pair,
motion time for the formation of the charge pair, the capture time, formation of the
emitting state and photon generation.
35

model holds when the capture time is much smaller than the motion time (τ
c
<< τ
m
)
and the hopping distance is much smaller than the Coulomb Capture radius (λ << r
c
).
15
The Thompson model holds when the capture time is much larger than the motion
time (τ
c
>> τ
m
) and the hopping distance is much larger than the Coulomb Capture
radius (λ >> r
c
). These models are generally valid in low carrier mobility small
molecules or polymer materials.

1.2.4 Generation of Excited States in OLED
Recombination of holes and electrons in OLED involves the generation of the
singlet and the triplet excitonic states. The multiplicities (S) of these excited states
depend on the spin quantum numbers, s = 0, ½, and 1 (S= 2s+1). When an electron
and a hole in the ground state recombine in an organic solid, an excited state of either
singlet or triplet character is formed. Based on spin statistics for each recombination,
one singlet (S
1
) and three triplet states (T
1
) are generated.
35, 36
Figure 1.8a shows the
schematic diagram of the hole electron recombination, generation of the singlet
excited state followed by relaxation and photon emission. The spin alignment of the
singlet and the triplet states are shown in Figure 1.8b. In the pure singlet state (
1
S
1
: s
= 0, Ms = 0) the spins of two electrons remain opposite to each other and the vectors
precess at 180º out of phase resulting in a cancellation of the net vector. Magnetic
torque causes one of the electrons to flip spin, which gives rise to the three triplet
states. The vectors in the triplet state precess in-phase, which causes the magnitudes
of the net vectors for the three triplet states:
3
T
+1
(s = 1, Ms = +1),
3
T
0
(s = 1, Ms =
0). and
3
T-
1
(s = 1, Ms = -1) to be finite.
26

16
Radiative relaxation of the singlet state results in fluorescence. Since the
fluorescence is observed from the singlet state, the statistical upper limit of
electrofluorescence (EF) quantum efficiency (Φ
F
) in OLED is 25%. The triplet
states are nonradiative because the transition to the triplet state is forbidden based on
the selection rule. Radiative relaxation from the triplet states result in
phosphorescence and have much longer radiative lifetime than the fluorescence.
Maximum electrophosphorescence (EP) quantum efficiency (Φ
P
) for a triplet emitter
in OLED is 75%.
35, 36

Singlet Triplet
S = 1, Ms = +1 S = 1, Ms = 0 S = 1, Ms = -1 S = 0, Ms = 0
b)
Ground State Excited State
(Exciton)
Electron-hole
recombination
Light emission
De-excitation Excitation
Ground State
a)
1
S
1
3
T
+1
3
T
0
3
T
-1
Singlet Triplet
S = 1, Ms = +1 S = 1, Ms = 0 S = 1, Ms = -1 S = 0, Ms = 0
b)
Ground State Excited State
(Exciton)
Electron-hole
recombination
Light emission
De-excitation Excitation
Ground State
a)
Ground State Excited State
(Exciton)
Electron-hole
recombination
Light emission
De-excitation Excitation
Ground State
a)
1
S
1
3
T
+1
3
T
0
3
T
-1

Figure 1.8 (a) A schematic diagram of hole electron recombination, excited state
generation, and light emission followed by relaxation to the ground state. (b)
Schematic diagram of the spin alignment of the singlet state showing a net
cancellation of the two vectors and the three triplet states showing the direction of
the vectors.
26

17
Frequency of the light emitted form OLED is determined by the energy band-
gap of the luminescent semiconductor material. This gap can also be correlated to
the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular
orbital (LUMO) gaps of the organic materials. The HOMO energy of a molecule is
related to its oxidation potential and can be compared to the work function or the gas
phase ionization energy (Energy required to remove an electron from an atom).
Experimentally HOMO energies of organic molecules are estimated from cyclic
voltametry and Ultra Violet Photo Electron Spectroscopy (UPS).
37
The LUMO
energy relates to the electron affinity (The energy required to put an additional to a
molecule) or the reduction potential and can be estimated from cyclic voltametry and
Inverse Photo Electron Spectroscopy (IPES)
38
. The HOMO-LUMO gaps can also be
estimated from the intersection of the absorption and emission spectra.

1.2.4.1 Absorption and Emission Spectroscopy
Interaction with light can promote a molecule to its excited state. A common
method of studying the excited state characteristics is by absorption spectroscopy
also known as Ultra-violet (UV) visible spectroscopy. When a molecule comes in
contact with an oscillating electromagnetic wave of E = hν, the energy of the wave
gets transferred to the molecule. This phenomenon is known as resonance, where the
oscillating dipole of light perturbs the electron cloud of the molecule and the
molecule gets promoted to the excited state. The energy gap between the two
electronic states (ΔE) equals the energy of the electromagnetic wave, ΔE = hν.
39

18
ab
0-2
0-1
0-0 0-0
2-0
1-0
0
1
2
0
1
2
0
1
2
4
3
0
1
2
4
3
0
1
2
3
4
0
1
2
3
4
0
1
2
5
6
1
S
0
Absorption
Fluorescence
Phosphorescence
IC
1
S
1
ISC
Vibrational levels
3
T
1
r
E
5
6
7
ab
0
1
2
3
4
0
1
2
3
4
0
1
2
5
6
1
S
0
Absorption
Fluorescence
Phosphorescence
IC
1
S
1
ISC
Vibrational levels
0-2
0-1
0-0 0-0
2-0
1-0
0
1
2
0
1
2
0
1
2
4
3
0
1
2
4
3
0-2
0-1
0-0 0-0
2-0
1-0
0
1
2
0
1
2
0
1
2
4
3
0
1
2
4
3
3
T
1
r
E
5
6
7
0
1
2
3
4
0
1
2
3
4
0
1
2
5
6
1
S
0
Absorption
Fluorescence
Phosphorescence
IC
1
S
1
ISC
Vibrational levels
3
T
1
r
E
5
6
7

Figure 1.9 (a) Potential energy surfaces of the singlet ground state (
1
S
0
), the singlet
excited state (
1
S
1
), and the triplet excited state (
1
T
1
) showing the absorption,
fluorescence, and emission path ways (b) Schematic diagram of the absorption,
fluorescence, and phosphorescence spectra showing the vibronic transitions.

Electronic excitation of a molecule results in a series of vibrational transitions
from a number of vibrational levels in the ground state to a number of different
vibrational levels in the excited states. Progression to the new vibrational states in
the electronically excited states is known as the Frank-Condon (FC) progression. In
which, vertical excitation of a molecule from the ground state to the excited state is
possible where the wavefunction overlap between the initial and the final vibrational
states is maximized (Figure 1.9a). This also relates to the equilibrium geometries of
the molecule, meaning if the two potential curves are similar or vertically less
displaced, then the equilibrium separation (r) should be the same between the ground
19
and the excited state. As a result, the equilibrium geometries of the ground and the
excited state should be similar. On the other hand, if the excited state curve is more
displaced relative to the ground state curve, then the equilibrium geometries of the
ground and the excited state would be different.
39, 40

Since the rates of vibrational and electronic energy relaxation among the
excited states are much faster than the rate of emission, emissions only occur from
the v = 0 vibrational level of the lowest singlet (
1
S
1
) or triplet (
3
T
1
) states (Kasha’s
rule). Figure 1.9a and b shows the ground and excited state potential energy surfaces
of a hypothetical molecule. From which, generation of the singlet-singlet and
singlet-triplet absorptions (
1
S
0
→
1
S
1
,
1
S
0
→
3
T
1
) and singlet-singlet and triplet-singlet
emissions (
1
S
1
→
1
S
0
,
3
T
1
→
1
S
0
) are shown. For
1
S
0
→
1
S
1
and
1
S
1
→
1
S
0
transitions,
vibrational peak assignments for only three vibrational progressions are shown.
Upon electronic excitation, 0-2, 0-1, and 0-0 vibrational transitions may occur, which
upon de-excitation can make 0-0, 1-0, 2-0 transitions. In the absorption spectrum,
the 0-0 transition is the lowest energy transition and shows up at the longest
wavelength. In the emission spectrum, the 0-0 transition is the highest energy
transition and shows up at the shortest wavelength. The singlet energy, E
s
and the
triplet energy, E
T
are defined as the 0-0 energy gaps for the
1
S
1
(v = 0) →
1
S
0
(v = 0)
and
3
T
1
(v = 0) →
1
S
0
(v = 0) transitions respectively. When the vibrational bands in
the absorption and emission spectra are not well resolved, the intersection of the
absorption and emission bands can be used to estimate the singlet and triplet
energies.
39

20
Phosphorescence emission requires spin-orbit coupling (SOC), which
initiates intersystem crossing (ISC) from the singlet manifold (
1
S) to the triplet (
3
T)
manifold through spin flip. Phosphorescence from organic molecules is not very
common because the magnitude of SOC for the first row atoms like C, N, and O is
smaller than the energy of the vibrational couplings. However, molecules with
carbonyl functionalities do show phosphorescence because n →π* transitions in these
molecules initiates SOC. This is caused by a jump from the p orbital in the
molecular plane to a p orbital orthogonal to the molecular plane, which initiates a
change in the angular momentum resulting in a spin flip. The SOC is roughly
proportional to Z
4
, where Z is the atomic number. The magnitude of SOC for heavy
atoms is almost as large as the electronic energy gaps. Therefore, the rate of ISC
enhances significantly in the presence of heavy atom, whether the heavy atom is a
part of the molecule or not (External heavy atom effect). Thermally activated
chemical reactions and bimolecular diffusional quenching processes in room
temperature decrease the triplet lifetimes by competing with nonradiative decay
pathways. In the rigid media, like solid matrix or 77K glass these quenching
pathways are deactivated and the lifetime of phosphorescence increases
significantly.
39, 40

1.2.5 Energy Transfer in OLED
The color of the emitted light in OLED can be fine-tuned by changing the
frequency of the band-gap energy (E
g
) of the emissive material. Most common
21
emissive material used in a fluorescent OLED is Alq
3
, which produces
electrofluorescence at 510 nm. Many materials producing electrofluorescence
throughout the visible spectrum have already been reported; however, low EL
efficiency and line broadening have always been a problem for a lot of these
materials. Since the statistical upper limit of triplet formation is 75%, materials that
can efficiently intersystem cross and emit from the triplet state are more desirable.
Phosphorescent emitters with heavy metal centers have been widely used in OLEDs.
These materials, due to internal heavy atom effect increase SOC and efficiently emit
from the triplet state. Efficient phosphorescent OLEDs with triplet emitters covering
the entire electromagnetic spectrum have been reported. These types of devices use
the strategy of color tuning by host-guest interaction and have significantly high EL
quantum efficiencies and power efficiencies than the undoped devices.
26, 41

When a photoluminescent dye is codeposited with a high energy host, the
dopant can act as a carrier trap and undergo radiative recombination. Furthermore,
energy transfer from the host can also excite the dopant, which can then emit by
radiative decay. In the EL spectrum, both carrier trapping and energy transfer can
contribute to the dopant emission.
41-46
Energy transfer in dye doped OLEDs can
proceed through Förster
39
or Dexter
39, 47
processes. Figure 1.10 gives a pictorial
description of the Förster and Dexter energy transfer pathways in fluorescent and
phosphorescent transitions for a host-guest system. For singlets, energy transfer can
proceed through either Förster or Dexter pathways; and for triplets, since the Förster
process is not dipole allowed energy can transfer only via the Dexter pathway.
26, 39

22

1
S
1
S
3
T
Host
Guest
1
S
1
S
3
T
Förster & Dexter
Dexter
1
S
1
S
3
T
Host
Guest
1
S
1
S
3
T
Förster & Dexter Förster & Dexter
Dexter Dexter

Figure 1.10 A schematic representation of the Förster and Dexter energy transfer
processes in host-guest systems showing singlet-to-singlet energy transfer can
proceed through both the Förster or Dexter mechanisms and the triplet-to-triplet
energy transfer can only proceed through the Förster mechanism.

1.2.5.1 Förster Energy Transfer
The Förster energy transfer process relies on the electrostatic interaction
between a donor and an acceptor molecule. In this mechanism, an excited donor
molecule with oscillating dipoles creates electromagnetic disturbance in the space
around it. When an acceptor molecule in the ground state comes in the vicinity of
the electromagnetic disturbance, due to coulombic interaction it starts oscillating at
the same frequency as the oscillating dipole of the donor. This energy transfer from
the donor to the acceptor occurs through dipole-dipole coupling between the
23
transition dipole moments of the excited donor and the ground state acceptor. In the
process, as the excited donor relaxes to the ground state, it transfers the energy to the
dopant via coulombic interaction (Figure 1.11a). The Förster energy transfer process
occurs in long distances (Up to 100Å) and does not require any physical contact
between the donor and the acceptor molecules.
39, 41

1
(Donor)* + Acceptor Donor +
1
(Acceptor)* + hν
a)
b)
Intensity
E
OIverlap Integral
I
D
(ν)
ε
A
(ν)
∫
∞
≡
0
~
)
~
( )
~
( ν ν ε ν d I J
A D
1
(Donor)* + Acceptor Donor +
1
(Acceptor)* + hν
1
(Donor)* + Acceptor Donor +
1
(Acceptor)* + hν
a)
b)
Intensity
E
OIverlap Integral
I
D
(ν)
ε
A
(ν)
∫
∞
≡
0
~
)
~
( )
~
( ν ν ε ν d I J
A D

Figure 1.11 (a) A schematic representation of the Förster energy transfer. (b) The
overlap integral (J) is shown as the dark area, which is created by the intersection of
donor emission spectrum and acceptor absorption spectrum.

Efficiency of Förster energy transfer depends on the rate of energy transfer of
the donor relative to radiative relaxation. A high rate of energy transfer from the
donor to the acceptor relative to the radiative relaxation of the donor can lead to
24
efficient emission exclusively from the dopant. The relationship in equation 1.6 is an
analytically derived expression for the rate of Förster energy transfer, where n is the
refractive index of the medium, N
a
is the Avogadro’s number, r is the donor acceptor
separation, τ
D
is the fluorescent lifetime of the donor, and κ
2
is the dipole orientation
factor. The term κ
2
considers the fact that the interaction between the two oscillating
dipoles depends on the orientation of the dipoles in space, and for randomly
distributed interacting dipoles it is equal to 2/3. Equation 1.7 gives the definition of
κ
2
, where θ
T
is the angle between the donor (D) and the acceptor (A) dipole moments
(Figure 1.12). The value of θ
T
is defined in equation 1.8, in which θ
D
and θ
A
are the
angles between the separation vector R, and D and A respectively and φ is the
azimuthal angle defined by the planes (D,R) and (A,R).
48

Figure 1.12 Geometry of the donor (D) and acceptor (A) vectors in space showing
the orientation of the dipoles and the angles.
48

The degree of spectral overlap (J) between the donor and the acceptor is
given by the integral part of the equation 1.6 and is shown in Equation 1.9, where I
D

is the fluorescent spectrum of the donor defined in equation 1.10, (Φ
D
is the
25
fluorescent quantum yield), ν is the energy in wavenumber, and ε
A
is the molar
extinction coefficient spectrum of the acceptor. The value of J is a measurement of
the area under the curve formed by the emission spectrum of the donor and the
absorption spectrum of the acceptor, which is directly proportional to the rate of
energy transfer (Figure 1.11b).
4
~
0
6 4
2
~
~
) ( )
~
(
5291 . 0
ν
ν
ν ε ν
τ
κ d
I
r N n
P
dt
d
k
A D
D a
n ET
∫
∞ /
= = (1.6)
()
2 2
cos cos 3 cos
A D T
θ θ θ κ − = (1.7)
A D A D T
θ θ φ θ θ θ cos cos cos sin sin sin cos + = (1.8)
∫
∞
=
0
~
)
~
( )
~
( ν ν ε ν d I J
A D
(1.9)
∫
∞
= Φ
0
~
)
~
( ν ν d I
D D
(1.10)
In OLEDs, random doping causes a large distribution of distances between
the donor and the acceptors, which results in a large distribution of rates. Therefore,
more practical approach for evaluating the Förster energy transfer in OLEDs is to
calculate the Förster radius (R
0
) defined in equation 1.11. In a donor-acceptor
system, the Förster radius is defined as the center-to-center separation between the
donor and the acceptor, where the probability of energy transfer from the donor to
the acceptor is equal to the probability of radiative or nonradiative relaxation of the
donor. A large R
0
means efficient energy transfer from the donor to the acceptor.
39,
41

26
4
~
0
4
2
6
0
~
~
) ( )
~
(
5291 . 0
ν
ν
ν ε ν
κ d
I
N n
R
A D
a
∫
∞ /
= (1.11)

1.2.5.2 Dexter Energy Transfer
An energy transfer mechanism that requires physical interactions between the
donor and the acceptor is called the Dexter energy transfer mechanism. The Dexter
energy transfer process requires that the donor and the acceptor have physical
overlap of electron densities in space. Exchange of electrons occurs in the region of
overlap as the two molecules form a collision complex (D*A). The rate of energy
transfer depends on the orbital overlap and the distance between the donor and the
acceptor defined in equation 1.12.
ν ν ε ν
~
) ( )
~
(
2
exp
~
0
d I
L
R
K P
dt
d
k
A D
DA
n ET
∫
∞ /
⎭
⎬
⎫
⎩
⎨
⎧−
= = (1.12)
⎟
⎠
⎞
⎜
⎝
⎛
=
h
π 2
K (1.13)
where K is a constant related to specific orbital interaction defined in equation 1.13.
J is the spectral overlap (Equation 1.7) between the donor and the acceptor species
normalized for the PL spectrum of the donor and the extinction coefficient spectrum
of the acceptor. A maximum overlap between the donor and the acceptor would
correlate to J = 1. The exponential term relates to the donor-acceptor separation, R
DA

relative to their van der Waals radii, L. The energy transfer is fastest for very short
donor-acceptor separation. As the distance between the donor and the acceptor
27
grows, the electron densities decay exponentially. As a consequence, the electron
exchange also decays exponentially.

1
(D)* A D
+
.
A
-
.
1
(D)* A D
-
.
A
+
.
1
(D)* A D
1
(A)*
hν
CT
Step 1
Step 1
Step 2
Step 2
Concerted
CT
1
(D)* A D
+
.
A
-
.
1
(D)* A D
-
.
A
+
.
1
(D)* A D
1
(A)*
hν
CT
Step 1
Step 1
Step 2
Step 2
Concerted
CT

Figure 1.13 A schematic representation of the Dexter energy transfer showing the
concerted electron exchange mechanism, where the donor and the acceptor electrons
are exchanged simultaneously; and the charge transfer electron exchange
mechanism, where the donor and the acceptor electrons are exchanged in steps by
forming ionic radicals.

28
The electron exchange in the Dexter energy transfer process may occur in
one step or multiple steps. Figure 1.13 depicts the two possible pathways of electron
exchange mechanism. In the concerted pathway, the electrons of the donor and the
acceptor are exchanged simultaneously. In the charge transfer (CT) pathways, the
electrons of the donor and the acceptor are transferred in different steps. In the
process, a radical cation and a radical anion is formed, which in the next step
produces the relaxed donor and the excited acceptor.
39, 41

1.2.6 Current-Voltage Characteristics of OLED
The current-voltage (I-V) characteristic of an OLED follows the typical non-
liner pattern of a p-n junction diode (Section 1.1.4). When charges are injected, a
power-law dependence of the current on voltage (I ∝ V
2
) is observed, which follows
the theory of space-charge-limited-current (SCLC).
49
In OLEDs a high density of
charge traps lie beneath the LUMO level and the conduction of current depends on
the trapped-charge-limited (TCL) current controlled by the bulk properties of the
materials (Figure 1.14a).
49
At low current density, the carrier mobility decreases
because charges get captured by the traps and ohmic conduction dominates the I-V
curve. As the current density increases, more charges get injected into the system
filling the number of limited traps. As a result, the number of traps decreases and the
carrier mobility increases, which rapidly increases the power-law dependence (I ∝
V
m
). At high enough injection levels, the traps get filled completely and no longer
29
influence the transport of electrons. The I-V curve again behaves like an SCL
conductor.
49

Figure 1.14b displays the I-V plots of three test devices fabricated with TPD
HTL and Al, Ga, and In quinolate ETLs. The I-V characteristics of all the materials
follow the power-law dependence of current on voltage and the results agree really
well for bulk-limited conduction in the presence of traps. The solid line is defined
by I ∝ V
m+1
, where m goes from 0 (at low current consistent with ohmic conduction)
to 8 ± 1 (At high current consistent with TCL conduction).
49

(b) (a)
Ohmic
SCLC
TCL
Ohmic
SCLC
TCL

Figure 1.14 (a) A schematic representation of energy level diagram of an OLED
showing the trap distribution underneath the LUMO level. (b) I-V curves for devices
with 200Å TPD and 400Å of Mq
3
(M = Al, Ga, In) showing the ohmic, SCLC, and
TCL regions and the I ∝ V
m+1
relationship, where m goes from 0 to 8 ± 1.
49

30
1.2.7 Efficiencies of OLED
Measuring the efficiency of OLED requires understanding the difference
between the internal and external quantum efficiencies. The internal quantum
efficiency (η
int
) is defined as the ratio of total number of generated photons to the
total number of injected electrons. The external quantum efficiency (η
ext
) is defined
as the ratio of the number of photons emitted by the OLED into the viewing direction
to the number of injected electrons. Equation 1.14 shows the relationship between
the internal and the external quantum efficiencies, where η
c
is the fraction of light
coupled out of the structure into the viewing direction.
50
Since the light output from
the detector is expressed in watts, an analytical expression for measuring the external
quantum efficiency defined in equation 1.15 can be used to convert the light output
from the detector to external quantum efficiency.
() () ( ) λ η λ η λ η
c ext int
= (1.14)
) (
) ( ) ( 10 8
) (
max
10
mA Current
nW t LightOutpu nm
ext
× × ×
=
−
λ
λ η (1.15)
Measuring the external quantum efficiency of OLED requires the use of a
calibrated photodetector that has been corrected for the losses caused by the lenses.
The detector is placed in front of the device so all the photons emitted in the forward
direction can be collected. The internal quantum efficiency of OLED is measured
by placing the OLED inside an integrating sphere containing a calibrated detector,
and then measuring the photon output from the device. Figure 1.15 shows the
31
preferred experimental geometries for measuring the internal and external quantum
efficiencies of OLED.
50

Figure 1.15 A schematic representation of the geometric orientation of the
photodetectors placed by OLED (a) Measuring the external quantum efficiency (b)
Measuring the internal quantum efficiency.
50

32
1.3 Introduction to Organic Photovoltaic Cells (OPV)
Organic photovoltaic cells (OPV) are thin film semiconductor devices that
harvest photons from the solar spectrum and generate photocurrent. The principle of
OPV operation is the reverse of the principle of OLED operation. Meaning, in
OLEDs the photons are generated when current is applied to the device and in OPV
cells current is generated when incident photons get absorbed by the device (Figure
1.16). Like OLEDs, OPV cells are also constructed on a transparent electrode,
which in most cases are ITO. Organic materials are either vapor deposited or spin
coated onto the ITO surface which is then capped by a metallic electrode (Ag, Al).
A simple organic solar cell is composed of an anode, a donor layer (D), an acceptor
layer (A), and a cathode. Most common donor and acceptor materials used in OPV
cells are Copper phthalocyanine (CuPc) and Fullerene (C
60
) respectively.
51

ITO
HTL
ETL
Al
ITO
Donor
Acceptor
Al
Input output
LED mode PV mode
ITO
HTL
ETL
Al
ITO
Donor
Acceptor
Al
Input output
LED mode PV mode

Figure 1.16 A schematic diagram of an OLED device (left) and an OPV device
(right) showing the reversible operating modes.

33
The basic mechanism of photocurrent generation in OPV cells involves five
major steps: (1) Photon absorption and exciton generation, (2) Exciton diffusion, (3)
Charge separation, (4) Charge conduction, and (5) Charge collection, and
photocurrent generation (Figure 1.17). In an OPV cell, both the donor and the
acceptor are capable of absorbing photons. When light is shone on an OPV cell,
electrons get promoted from the HOMO to the LUMO resulting in the formation of
excitons. Once generated, the excitons either migrate to the donor acceptor interface
or decay to the ground state via radiative or nonradiative processes. At the D/A
interface the exciton dissociates into holes and electrons, which then migrate towards
the respective electrodes and generate photocurrent.
1. Photon absorption
and exciton generation
2. Exciton diffusion
hν
D-HOMO
A-HOMO
D-LUMO
A-LUMO
h+
e-
h+
e-
D-HOMO
A-HOMO
D-LUMO
A-LUMO
h+
e-
h+
e-
h+
e-
h+
e-
3. Exciton dissociation
and carrier separation
on the acceptor
A-HOMO
D-LUMO
h+
e-
h+
e-
h+
3. Exciton dissociation
and carrier separation
on the donor
A-HOMO
D-LUMO
e-
h+
e-
e-
h+
+
D-HOMO
A-LUMO
D-HOMO
A-HOMO
D-LUMO
A-LUMO
e-
h+
e-
h+
D-HOMO
A-HOMO
D-LUMO
A-LUMO
e-
h+
e-
h+
4. Carrier diffusion 5. Carrier collection and
photocurrent generation
1. Photon absorption
and exciton generation
2. Exciton diffusion
hν
D-HOMO
A-HOMO
D-LUMO
A-LUMO
h+
e-
h+
e-
hν
D-HOMO
A-HOMO
D-LUMO
A-LUMO
h+
e-
h+
e-
D-HOMO
A-HOMO
D-LUMO
A-LUMO
h+
e-
h+
e-
h+
e-
h+
e-
3. Exciton dissociation
and carrier separation
on the acceptor
A-HOMO
D-LUMO
h+
e-
h+
e-
h+
3. Exciton dissociation
and carrier separation
on the donor
A-HOMO
D-LUMO
e-
h+
e-
e-
h+
+
D-HOMO
A-LUMO
3. Exciton dissociation
and carrier separation
on the acceptor
A-HOMO
D-LUMO
h+
e-
h+
e-
h+
3. Exciton dissociation
and carrier separation
on the donor
A-HOMO
D-LUMO
e-
h+
e-
e-
h+
+
D-HOMO
A-LUMO
D-HOMO
A-HOMO
D-LUMO
A-LUMO
e-
h+
e-
h+
D-HOMO
A-HOMO
D-LUMO
A-LUMO
e-
h+
e-
h+
4. Carrier diffusion 5. Carrier collection and
photocurrent generation

Figure 1.17 A schematic diagram of photocurrent generation from OPV cells
showing the five step process.
34
1.3.1 Photon Absorption and Exciton Generation
The solar light spans a wide range and consists of different colors. On the
surface of the earth, largest photo-flux of solar radiation is between 600-1000 nm
(1.3-2.0 eV) (Figure 1.18). This is also known as the reference terrestrial solar
spectral irradiance: Air Mass 1.5 (AM 1.5), which is the spectral irradiance
distribution on earth’s surface and was developed by the American society of testing
and materials (ASTM) in conjunction with the PV industries. In space, the largest
photo flux is between 400-700 nm (1.8-3.0 eV) and the reference extraterrestrial
solar spectral irradiance is AM 0. For terrestrial applications, energy gaps for PV
materials between 1.3-2.0 is desirable.
52, 53

Figure 1.18 The AM 1.5 solar spectrum showing the ultra-violet (UV), visible, and
the infra-red (IR) regions.
54

35
Absorption of a photon in an OPV cell depends on the kind of semiconductor
materials used. One of the basic requirements of these materials is to have high
extinction through the entire visible spectrum. The second requirement is that the
optical energy gaps of these materials should be equal or close to the energies of the
incident photons. In an ideal case, only a photon with an energy hν > E
og

contributes to photocurrent generation. In organic materials the optical energy gap
(E
og
) is defined as the HOMO-LUMO energy gap and excitation of electrons from
HOMO to LUMO in these materials forms tightly bound excitons instead of a free
electron-hole pair. In inorganic semiconductor materials, the energy gap is known as
the electronic energy gap (E
eg
) defined by the gap formed between the free holes at
the valence band and free electrons at the conduction band. In organics, the
relationship between the optical gap (E
og
) and electronic gap (E
eg
) can be
approximated from the following equation
B og eg
E E E + = (1.16)
where E
B
is the exciton binding energy that represents the minimum energy needed
to separate an intra-molecular excitation into an inter-molecular excitation.
B 52, 55

1.3.2 Exciton Diffusion
Once generated, the excitons either diffuse to the donor-acceptor interface via
intra or inter-chain energy transfer processes or decay to the ground state via
radiative or nonradiative pathways. In organics, the diffusion length of the excitons
depends on the types of excitons formed and the morphology of the materials used.
36
If the morphology of the material is not smooth and has too many defect sites, then
the excitons may get trapped in those defect sites and decay without contributing to
the generation of photocurrent.
52

Absorption of photons can create singlet and triplet excitons. The singlet
excitons usually have short exciton diffusion length (EDL) because of short singlet
lifetimes (EDL of Alq
3
is 10 nm, lifetime 15 ns) and the triplet excitons usually have
longer exciton diffusion length (EDL of Irpp
3
is 60 nm, lifetime 2 μs) because of
long triplet lifetimes.
56
The thickness of the donor and the acceptor layers therefore
significantly contribute to the exciton loss mechanisms. Donor or acceptor layers
thicker than the EDL can cause exciton loss by increasing the number of exciton
decay. Alternatively, donor or the acceptor layers thinner than the EDL can cause
exciton loss due to poor light absorption.
52

1.3.3 Exciton Dissociation and Charge Generation
Excitons can be generated anywhere on the surface of the donor or the
acceptor layers. Once an exciton diffuses to the D/A interface, the interface potential
field then separates the exciton into a free electron at acceptor LUMO and a free hole
at donor HOMO (Figure 1.19). For the donor exciton, once dissociated the electron
migrates to the acceptor LUMO but the hole remains on the donor HOMO.
Alternatively, the electron from the acceptor exciton remains on the acceptor LUMO
but the hole migrates to the Donor HOMO. The interfacial potential field (ΔE)
responsible for exciton dissociation is formed by the energy offset between the
37
frontier orbitals of the donor and the acceptor materials. A donor material with low
ionization potential (E
IP
) and an acceptor material with a high electron affinity (E
EA
)
form a heterojunction. The interfacial potential field (ΔE
LG
) created by the LUMOs
of the donor and the acceptor materials dissociates the donor exciton and the
interfacial potential field (ΔE
HG
) created by the HOMOs of the donor and the
acceptor materials dissociates the Acceptor excitons.
52, 57

D-HOMO
A-HOMO
D-LUMO
A-LUMO
e-
h+
e-
I
g
ΔE
LG
ΔE
HG
D/A Interface
e-
h+
E
EA
E
IP
E
Vac
D-HOMO
A-HOMO
D-LUMO
A-LUMO
e-
h+
e-
I
g
ΔE
LG
ΔE
HG
D/A Interface
e-
h+
E
EA
E
IP
E
Vac

Figure 1.19 Schematic diagram of photo induced charge separation in an organic
D/A light harvesting system showing the interfacial gap (I
g
), LUMO interfacial
potential (ΔE
LG
), HOMO interfacial potential (ΔE
HG
), electron affinity (E
EA
),
ionization potential (E
IP
), and the energy of vacuum (E
vac
).

38
1.3.4 Carrier Diffusion
Once the exciton dissociates into free hole and electron, the hole diffuses
towards the positively charged anode and the electron diffuses towards the
negatively charged cathode. The potentials that drive the holes and electrons to the
respective electrodes are generated by the dissimilarities between the workfunctions
of the electrodes and chemical potentials of the ionic donor and acceptor
molecules.
52
Carriers diffuse from molecule to molecule either by hopping or
tunneling (section 1.1.2).

1.3.5 Carrier Collection and Photocurrent Generation
Carrier collection at the electrodes depends on the energy levels alignment.
When the donor HOMO matches the energy level of the anode and the acceptor
LUMO matches the energy level of the cathode, the contacts between the metals and
the organics become Ohmic and the charges freely collect at the electrodes.
52
The
generated photocurrent (I
L
) in the reverse bias direction then produces a voltage drop
across the resistive load and forward biases the cell generating the forward-bias
current (I
F
) (Figure 1.20 a). The net current of the cell in the reverse-bias direction
can be obtained from the ideal diode equation described in equation 1.1,
⎥
⎦
⎤
⎢
⎣
⎡
− ⎟
⎠
⎞
⎜
⎝
⎛
− = − = 1 exp
kT
qV
I I I I I
a
S L F L
(1.17)
39
As the photocurrent increases the cell becomes forward biased. However, the
photocurrent and the net solar cell currents always remain in the reverse-bias
direction.
58

(a)
-
+
n-type p-type
p-n junction
I
L
I
I
F
hν
R
I
V
V
OC
P
max
I
SC
I
ph
Dark current
Photo current
(b)
I
max
V
max
(a)
-
+
n-type p-type
p-n junction
I
L
I
I
F
hν
R
-
+
n-type p-type
p-n junction
I
L
I
I
F
hν
R
I
V
V
OC
P
max
I
SC
I
ph
Dark current
Photo current
(b)
I
max
V
max
I
V
V
OC
P
max
I
SC
I
ph
Dark current
Photo current
(b)
I
max
V
max

Figure 1.20 (a) A p-n junction OPV cell with resistive load. (b) The current-voltage
(I-V) curve of an OPV cell showing the dark current, photocurrent, open circuit
voltage (V
oc
), short-circuit current (I
sc
), maximum power (P
max
), maximum
photogenerated current density (I
ph
).

A typical I-V curve of an OPV cell is shown in figure 1.20 b. The dark
current represents the current when the cell is not illuminated. When illuminated, the
I-V curve shifts downward by the amount of short-circuit current (I
sc
), which is the
measurement of the maximum current that can run through a cell. The short-circuit
condition occurs when the value of R goes to zero so that the value of V goes to zero.
The value of I
sc
is determined by connecting the two electrodes and by setting the
40
potential across the cell to zero and by measuring the current flow of the cell under
illumination. When the value of R goes to infinity and net current goes to zero, the
open-circuit condition occurs. The open-circuit voltage (V
OC
) is the maximum
voltage difference that can be attained between the two electrodes and can be
described by the following equation,
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+ =
S
L
OC
I
I
V V 1 ln (1.18)
The maximum work that can be obtained from a cell is defined by P
max
, which is the
shaded region on the I-V curve and can be defined by the following equation,
max max max
V I P × = (1.19)
The fill factor of an OPV cell is the measurement of its goodness, which can be
obtained by the following equation,
OC SC
V I
V I
FF
×
×
=
max max
(1.20)
The power conversion efficiency (η) of an OPV cell can be obtained from the
following equation,
In
OC SC
P
V I FF ) ( × ×
= η (1.21)
where P
In
is the incident power.
51, 55, 58, 59

41
1.4 Chapter 1 References
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29. Takeshi Yasuda, Y. Y., De-Chun Zou and Tetsuo Tsutsui Carrier Mobilities
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33. Stephen R. Forrest, P. B., and Mark E. Thompson, The dawn of organic
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34. D. V. Khramtchenkov H. Ba¨ ssler, a. V. I. A., A model of
electroluminescence in organic double-layer
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and Processes. Marcel Dekker: New York, 2005.
36. M. A. Baldo and D. F. O’Brien, M. E. T., S. R. Forrest, Excitonic singlet-
triplet ratio in a semiconducting organic thin film Physical Review B 1999, 60, 14
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Polikarpov, Eugene; Thompson, Mark E., Relationship between the ionization and
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6, (1), 11-20.
38. Mayo, E. D., P.; Thompson, M. E., Manuscript in progress.
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40. McHale, J. L., Molecular Spectroscopy. Prentice Hall: New Jersey, 1999.
41. Andrei A. Shoustikov, Y. Y., and Mark E. Thompson, Electroluminescence
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43. Arnold B. Tamayo, B. D. A., Peter I. Djurovich, Sergey Lamansky,; Irina
Tsyba, N. N. H., Robert Bau, and Mark E. Thompson*, Synthesis and
Characterization of Facial and Meridional Tris-cyclometalated Iridium(III)
Complexes. J. AM. CHEM. SOC. 2003, 125, 7377-7387.

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Electroluminescent Devices. Inorg. Chem. 2005, 44, 8723-8732.

46. Tissa Sajoto, P. I. D., Arnold Tamayo, Muhammed Yousufuddin, Robert
Bau, and; Thompson*, M. E., Blue and Near-UV Phosphorescence from Iridium
Complexes with Cyclometalated Pyrazolyl or N-Heterocyclic Carbene Ligands.
Inorg. Chem. 2005, 44, 7992-8003.

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Chemical Physics 1953, 21, (5), 836-850
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49. Forrest, P. E. B. a. S. R., Electroluminescence from trap-limited current
transport in vacuum deposited organic light emitting devices. Appl. Phys. Lett. 1994,
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Organic Light-Emitting Devices. Advanced Materials 2003, 15, (13), 043-1048.
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Journal of Materials Science 2005, 40, 1429-1443.
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Technology: The Case for Thin-Film Solar Cells Science 1999, 285, 692-698.
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Mark E. Thompson*,†, Enhanced Open-Circuit Voltage in Subphthalocyanine/C60
Organic Photovoltaic Cells. J. AM. CHEM. SOC. 2006, 128, 8108-8109.

46
Chapter 2. Photophysics, Spectroscopy, and Density
Functional Theory (DFT) Studies of Phosphorescent
Phthalimide Materials

2.1 Introduction
An integral part of organic electronic research is to continually develop novel
materials with new properties. Since the invention of the organic light emitting
devices (OLEDs) in the Eighties,
1
significant improvements in the device
performances have been made by incorporating new materials into the system. As a
consequence, a major goal of the OLED community has been to design materials
with characteristics that would yield more efficient devices.
2-14
To date, syntheses of
numerous compounds have been reported. Materials with Iridium,
15-23
Platinum,
24-26

and gallium
27, 28
cyclometallates have been synthesized and studied in great depth for
their structural and luminescent properties. Carbazole and fluorine based materials
29-
31
have been looked at for their charge transporting,
5, 7, 32-41
and host forming
28-30, 41-
48
properties. Naphthalimide based materials have been studied for their high
electron affinities,
49
large electron mobilities,
50
reversible reduction potentials,
46, 51

large HOMO-LUMO gaps,
52-55
high fluorescent quantum yields,
56, 57
and room
temperature dual fluorescence properties.
52, 54, 58, 59
Although many materials with
various desirable properties already exist, charge transporting materials with high
triplet energies are still scant. Phthalimides are known to have high singlet and
47
triplet energies. These molecules have less conjugated aromatic system and
therefore have wider energy gaps than the naphthalimides.
Photophysics
36, 54, 57, 58, 60-74
and electrochemistry
75-84
of the phthalimides
have been studied in detail. Triplet energies of various phthalimide derivatives in
different solvents have been reported to be between 440-453 nm,
54, 60, 71-73
which are
higher than both naphthalimide
46
and fluorine
48
derivatives. Phthalimide derivatives
also have reduction potentials around -2.0 V,
75, 78, 82, 83
which is higher than the
naphthalimides derivatives. Because of high triplet energies and large reduction
potentials, phthalimides can have a wide variety of usage in organic semiconductor
devices. The purpose of this paper is to investigate the properties of different bis-
phthalimide derivatives for potential use in OLEDs and organic photo-voltaic (OPV)
cells.
Materials for organic semiconductors need to have molecular weights high
enough to be sublimed at low pressure. Since the phthalimides are low molecular
weight organic molecules, our strategy for obtaining high triplet energy and high
molecular weight phthalimides involved designing bis-phthalimide compounds with
two phthalimides connected by a spacer. The purpose of the spacer is to add extra
mass and create steric repulsion, which would twist the two phthalimide moieties out
of the plane of the spacer and increase the triplet energy of the entire system by
decreasing conjugation. In this paper we examined the properties of phenyl-
phthalimide and five different bis-phthalimide compounds: N-phthalimido-
phthalimide (NPP), Phenyl-1,4-bis-phthalimide (PBP) (6), 2,3,5,6-tetramethyl-
48
phenyl-1,4-(bis-phthalimide) (TMPP) (7), 2,3,5,6-tetramethyl-phenyl-1,4-bis-(4-t-
butylphthalimide) (tBuTMPP) (8), and 1,2-bis-(4-t-butylphthalimide)-cyclohexane
(ChBP) (9). In NPP the two phthalimide moieties have no spacer and are connected
by an N-N bond. The triplet energy of this molecule should be high because the two
phthalimide moieties should be almost orthogonal to each other. Furthermore, the
triplet energy of NPP should be similar to the phthalimide triplet energy. The triplet
energy of ChBP should also be high due to lack of conjugation through the
cyclohexyl spacer. In addition to that, the triplet energy of ChBP should be similar
to the triplet energy of Methyl-phthalimide. The triplet energies of phenyl-
phthalimide and PBP should be comparatively low because of increased conjugation
between the phenyl ring and the phthalimide moiety. The triplet energies of TMPP,
and tBuTMPP should be higher than phenyl-phthalimide and PBP because the tetra
methyl-phenyl spacer would create more steric hindrance than the phenyl spacer and
twist the two phthalimide moieties out of the plane of the spacer.
In order to completely understand the electrochemical and photophysical
behaviors of the phthalimide materials, detailed theoretical and experimental studies
of all the molecules were conducted. Electrochemistry and absorption/emission
spectroscopy were carried out in different solvents and compared with density
functional theory (DFT)
85
studies. DFT calculations of the molecular and electronic
properties of the molecules in the ground and the excited states, as well as the time
dependent density functional theory (TDDFT)
85-96
studies of the key features of the
UV-vis spectra in the gas phase and in THF, acetonitrile, and cyclohexane were
49
performed. Vertical excitation energies, E
VT
in the solvents were carried out by
incorporating conductor type polarizable continuum model (CPCM)
89, 92-94, 96, 97

model into the TDDFT routine.

2.2 Experimental
2.2.1 Synthesis and Characterization
Unless otherwise noted, all reagents and solvents were obtained from Aldrich
and used without any further purification. Reagents were either dry mixed or mixed
in solvent and dried before synthesis. Syntheses of the phthalimides were conducted
inside a model 300 W microwave reactor manufactured by the CEM corporation.
Reaction crudes were primarily purified by column chromatography. Device grade
chemicals were obtained by subliming the materials at pressures between the 10
-6
to
10
-7
torr in a thermal gradient sublimator manufactured by Lindberg.
The anhydrides and the amines were mixed in two different ways before
synthesis. The reactants were either dry mixed and ground to fine powders in a
mortar and pestle or mixed with dichloromethane, stirred for ten minutes, and then
concentrated in vacuo.
Phenyl-1,4-bis-phthalimide (PBP) (6). A mixture of phenyl-1,2-diamine 3
(1 equiv) and phthalic anhydride 1 (3 equiv) was subjected to microwave (300 W)
irradiation at 250°C for 40 minutes. The dark colored insoluble material was then
sublimed at 265°C to give off-white crystals of 6 in 80% yield.
1
H NMR (250 MHz,
50
CDCl3): δ 8.01 (dd, 4H), δ 7.80 (dd, 4H). Anal. Calcd for C
26
H
20
N
2
O
4
: C, 71.74; H,
3.28; N, 7.61 Found: C, 71.55; H, 3.20; N, 7.61.
2,3,5,6-tetramethyl-phenyl-1,4-(bis-phthalimide) (TMPP) (7). A solid
solution of 2,3,5,6-tetramethyl phenyl-1,2-diamine 4 (1 equiv) and phthalic
anhydride 1 (3 equiv) was placed inside the 300 W microwave reactor and heated to
250°C for 40 minutes. The dark brown material was sublimed at 285°C to afford
light yellow crystals of 7 in 80% yield. Mp (DSC) 462°C.
1
H NMR (250 MHz,
CDCl3): δ 8.01 (dd, 4H), δ 7.80 (dd, 4H), δ 1.57 (s, 12H). Anal. Calcd for
C
26
H
20
N
2
O
4
: C, 73.57; H, 4.75; N, 6.60 Found: C, 73.70; H, 4.66; N, 6.60.
2,3,5,6-tetramethyl-phenyl-1,4-bis-(4-t-butylphthalimide) (tBuTMPP)
(8). A mixture of 2,3,5,6-tetramethyl phenyl-1,2-diamine 4 (1 equiv) and 4-t-butyl
phthalic anhydride 2 (3 equiv) was placed inside the microwave reactor (300 W) and
irradiated for 40 minutes at 120°C. The yellow colored crude was passed through a
short column of silica gel in dichloromethane. Concentration of this elute followed
by a flash chromatographic purification of the crude (SiO
2
, dichloromethane) gave 3
as white powder in 85% yield. Mp (DSC) 413°C. T
g
(DSC) 88°C. T
C
(DSC)
189°C.
1
H NMR (360 MHz, CDCl3): δ 8.01 (d, J = 0.003 Hz, 2H), δ 7.90 (dd, J =
0.043 Hz, 4H), δ 2.09 (s, 12H), δ 1.43 (s, 18H). Anal. Calcd for C
34
H
36
N
2
O
4
: C,
76.09; H, 6.76; N, 5.22. Found: C, 76.31; H, 6.75; N, 5.31.
1,2-bis-(4-t-butylphthalimide)-cyclohexane (ChBP) (9). Cyclohexane-1,4-
diamine 5 (1 equiv) was mixed with 4-t-butyl phthalic anhydride 2 (3 equiv) in
dichloromethane (20 mL). The reaction mixture was then stirred for 10 minutes,
51
concentrated in vacuo, and subjected to microwave (300 W) reaction for 30 minutes
at 250°C. The dark yellow crude was then passed through a silica gel filter,
concentrated, and subjected to flash chromatography (silica gel) in dichloromethane
to give pure yellow crystals of 4 in 60% yield. Mp (DSC) 388°C. T
g
(DSC) 88°C.
1
H NMR (360 MHz, CDCl3): δ 7.70 (br m, 6H), δ 5.07 (m, J = 0.008 Hz, 2H), δ 2.37
(br m, 2H), δ 1.88 (br m, 4H), δ 1.57 (br m, 2H), δ 1.31 (s, 18H). Anal. Calcd for
C
30
H
34
N
2
O
4
: C, 74.05; H, 7.04; N, 5.76. Found: C, 74.11; H, 6.99; N, 5.78.

2.2.2 Electrochemical Measurements.
All the electrochemical measurements were recorded in room temperature
with a model 283 EG&G potentiostat/galvanostat. Measurements were taken at 50
mV/s scanning rate in dry acetonitrile containing 0.1 M tetrabutylammonium
hexafluorophosphate. Ag wire, Pt wire, and glassy carbon electrodes were used as
reference, counter, and working electrodes respectively.

2.2.3 Spectroscopic Measurements.
Absorption spectra were recorded with an Agilent 8453 UV-Visible
spectrometer and corrected for solvent absorption in the background. Emission
spectra were recorded on a PTI QuantaMaster Model C-60SE spectrofluorometer
with a 928 PMT detector corrected for detector sensitivity. Fluorescence quantum
yields were determined in 2me-THF at ~ 0.1 optical density. Lifetimes were
recorded with an IBH photon timing instrument with an IBH model TBX-04 photon
52
detection module. Fluorescence lifetimes were measured with time correlated single
photon counting method (TCSPC) in IBH with a 331nm model N-16 LED.
Lifetimes of the triplet excitons were recorded at 77K on the PTI spectrofluorometer.
The emission decay curves were obtained by manually closing the shutter (~50 ms)
of the excitation source.
The singlet energies E
S
of all the phthalimides were obtained as a reference
from the 20% height of the normalized lowest energy absorption bands. The triplet
energies E
T
were obtained from the highest energy band of the 77K emission spectra.

2.2.4 Computational Methods.
All preliminary geometry optimizations were performed at PM3 level of
theory using the Titan software package.
98
The singlet ground state (SGS) and the
triplet excited state (TES) electronic structure calculations were performed using
DFT method in Gaussian-03 software package.
90
The functional used in this study
was B3LYP, which consists of Becke’s three-parameter equation
99
and Lee, Yang,
and Parr’s non-local hybrid functional.
100
All calculations were performed using
Pople’s double-zeta basis set, 6-31G** containing two polarization functions.
101

The SGS and the TES geometries of all the phthalimides were optimized in
the gas phase. TDDFT calculations were performed on the SGS geometries. Total
of ten singlet and ten triplet states were calculated for each structure. Gas phase
vertical transition energies (E
VT
) were computed for all the phthalimides. TDDFT on
ChBP was performed without the t-butyl groups. Vertical transition energies (E
VT
)
53
in the solvents were calculated for tBuTMPP and ChBP only. Solvent calculations
were performed in THF, acetonitrile, and cyclohexane using the TDDFT/CPCM
method. All Gaussian calculations were performed at the high performance
computing facility (HPCC) of University of Southern California (USC).
102

2.3 Results and Discussion
2.3.1 Synthesis of the Phthalimides
PBP (6): R = H, R
1
= H
TMPP (7): R = CH
3
, R
1
= H
tBuTMPP (8): R = CH
3
, R
1
= t-Bu
N
R
1
O
O R
1
R
1
N
R
1
O
O
O
O
O
H
2
N
R
1
R
1
R
1
NH
2
R
1
+
R R
R
Microwave Irradiation @ 300W
No Solvent
N N
O
O
O
O
t-Bu
NH
2
NH
2
O
O
O
t-Bu
t-Bu
+
Microwave Irradiation @ 300W
No Solvent
ChBP (9)
1: R = H
2: R = t-Bu
3: R
1
= H
4: R
1
= CH
3
2: R = t-Bu 5

Figure 2.1 Microwave synthesis of the phthalimide materials

Synthesis of the target compounds (6-9) was achieved in one step from
commercially available phthalic anhydride 1, 4-t-butyl phthalic anhydride 2, phenyl-
1,4-diamine 3, 2,3,5,6-tetramethyl phenyl-1,4-diamine 4, and cyclohexane-1,4-
diamine 5 (Figure 2.1). All reactions were conducted inside a microwave
103-105

54
reactor in a solvent free environment. Phthalimides 6 and 7 were synthesized by
irradiating the mixtures of anhydride 1 and amine 3 and 1 and amine 4 in two
different reactions respectively. The t-butyl phthalimide compounds 8 and 9 were
made in a similar fashion by reacting the t-butyl anhydride 2 with amine 4 and 5
under microwave conditions separately.

2.3.2 Electrochemical analysis.
-4 -3 -2 -1 0 1234
-5
0
5
10
15
20
25
30
-1.98
-2.00
-1.62
-2.06

F
c
Current (a.u.)
E (V, vs. F
+
C
/F
C
)
Phenylphthalimide
PBP
NPP
TMPP
tBuTMPP
ChBP
-1.68
-2.01

Figure 2.2 Electrochemistry data of the phthalimide materials showing the
oxidation and reduction waves.

Cyclic voltametry (CV) scans of all the phthalimide materials were studied in
acetonitrile (due to low solubility in acetonitrile, the CV of PBP was measured in
dichloromethane) versus ferrocene/ferrocenium as an internal redox standard (Figure
55
2.2). Electrochemical oxidation and reduction potentials of phthalimide and
methylphthalimide were obtained from the literature references and are reported in
table 2.1 with the results obtained from our studies. No oxidation waves were
observed for any of the phthalimides in any solvent because the oxidation windows
of these molecules fall outside the window of the solvent. CV scans of the
phthalimides showed reduction in every solvent. The values for the reduction
potentials of ChBP, NPP, phenylphthalimide, PBP, TMPP, and tBuTMPP in
acetonitrile were -1.98, -1.68, -2.06,-2.01, -1.62, and -2.00 V vs. ferrocene
respectively.
The CV traces of phenylphthalimide (Pp) and NPP in acetonitrile showed two
reduction waves indicating the formation of the radical anions and dianions of the
forms [Pp]
•-
, [Pp]
2-
[NPP]
•-
, and [NPP]
2-
respectively. For the phenyl-phthalimide,
the first wave was fully reversible and the second wave was pseudo reversible and
for NPP both of the waves were fully reversible. CV scans of ChBP, PBP, TMPP,
and tBuTMPP showed only one reduction wave in acetonitrile suggesting the
formation of the [ChBP]
•-
, [PBP]
.-
, [TMPP]
•-
, and [tBuTMPP]
•-
anions. The traces
for PBP, ChBP and tBuTMPP were quasi reversal in acetonitrile. TMPP showed
complete reversibility in DCM and DMF but in acetonitrile it was irreversible. The
reduction potential of tBuTMPP was measured to be 0.28V higher than the reduction
potential of TMPP. The value of which can be attributed to the electron donating
ability of the t-butyl groups on tBuTMPP, which by donating electron density
increases the reduction potential of the molecule.
56
Table 2.1 Summary of the electrochemical data. Potentials for reference a, b, and d
were converted from vs. SCE to ferrocence.
106

E
o
ox
E
o
red
(1) E
o
red
(2)
Compound
V vs Fc/Fc
+
V vs Fc/Fc
+
V vs Fc/Fc
+
Phthalimide ---
(-1.90)
a
, (-1.87)
b
, (-1.20)
c
, (-1.88)
d
(-2.70)
b
, (-1.43)
c
Methylphthalimide --- (-1.82)
a
, (-1.77)
b
, (-1.21)
c
(-2.61)
a,
(-2.60)
b
, (-1.45)
c
ChBP 1.92 -1.98
NPP 2.28 -1.68 -1.93
Phenylphthalimide 1.79
(-2.06), (-1.76)
a
, (-1.70)
b
, (-1.87)
d
(-2.84), (-2.54)
a
, (-1.90)
b
PBP 1.95 -2.01
e
TMPP 2.34 -1.62
tBuTMPP 1.90 -2.00
a = Ref 81 (In DMF vs sce), b = Ref 77 (In DMF vs. Hg pool)
c = Ref 82 (In CH3CN, H2O, and H2SO4 mixture vs. sce)
d = Ref 74 (In DMF vs sce), e = Data obtained in DCM
All E
o
ox
were obtained by subtracting the E
o
red
from the optical gap (E
s
)

2.3.3 Singlet ground state (SGS) geometries and electronic structures.
The geometries of the singlet ground state were optimized with B3LYP/6-
31G** parameters. The top portion of Figure 2.3 displays the pictures of the HOMO
and LUMO orbitals of all the phthalimide materials obtained from the optimized
SGS geometries. The bottom portion of Figure 2.3 displays the smallest subunits of
the phthalimide molecules with atom numbers corresponding to the DFT
calculations. Table 2.2 gives a summary of the important structural parameters of
the ground and excited state geometries of all the phthalimides. The HOMOs and
LUMOs of NPP and ChBP were found to be delocalized and were located on the
same atoms in both of the molecules. For the phenylphthalimide, major contribution
57

LUMO
Phthalimide
PBP
TMPP
tBuTMPP
ChBP
NPP
Phthalimide compounds HOMO LUMO
Phthalimide
PBP
TMPP
tBuTMPP
ChBP
NPP
Phthalimide compounds HOMO

C
2
C
3
C
1
N
C
4
C
5
C
6
C
7
O
1
O
2
C
2
C
3
C
1
N
C
4
C
5
O
1
O
2
C
6
C
7

Figure 2.3 DFT optimized SGS geometries of the phthalimide materials showing
the HOMO and the LUMO diagrams (Top). Smallest subunit of the phenyl
phthalimide with arbitrarily numbered atoms showing the bonds and the dihedral
angles measured from DFT calculation (Bottom).

58
to the HOMO came from all the atoms, but contribution for the LUMO came only
from the phthalimide moiety. The same behavior was observed for PBP, and as
expected there were no significant structural differences between phenylphthalimide
and PBP in the ground state. The CCNC dihedral angles for both of the molecules
were 40º.

Table 2.2 Structural parameters for the optimized ground and triplet state
geometries.

Dihedral angle C5-N C1-N C4-N
C6C5NC1 (deg)
(Å)(Å)(Å)
ChBP 58 1.469 1.409 1.409
Phenylphthalimide 40 1.428 1.419 1.419
PBP 40 1.426 1.420 1.420
TMPP 68 1.434 1.415 1.415
tBuTMPP 68 1.433 1.417 1.415
NPP 90 --- 1.419 1.419
ChBP 57 1.459 1.431 1.430
Phenylphthalimide 12 1.366 1.505 1.505
PBP 9 1.362 1.505 1.505
TMPP 65 1.383 1.481 1.481
tBuTMPP 55 1.395 1.467 1.465
NPP 90 --- 1.366 1.366
ChBP 58 1.467 1.410 1.409
PBP 38 1.408 1.436 1.436
TMPP 65 1.426 1.421 1.421
tBuTMPP 65 1.428 1.422 1.419
NPP 90 --- 1.366 1.366
Optimized Triplet state geometries (Phthalimide moiety with spin density)
Optimized Triplet state geometries (Phthalimide moiety without spin density)
Optimized ground state geometries

A comparison between tBUTMPP and TMPP SGS geometries showed
similarities as predicted. The C5-N, C1-N, and C4-N bonds in these molecules were
found to be 1.434 Å, 1.415 Å, 1.415 Å respectively. The HOMO and the LUMO of
59
these molecules showed no special overlaps. Since the steric repulsion from the
methyl group forces the phthalimides to twist out of the plane of the phenyl ring, it
causes the CCNC dihedral angles in tBuTMPP and TMPP to increase to 68º from
PBP. As a result, the HOMOs on these molecules localize on the central phenyl and
the LUMOs on the phthalimide moieties. Where in PBP, the HOMO stays
delocalized throughout the molecule. This happens because decreased overlap
increases the HOMO energies in tBUTMPP and TMPP and causes the C5-N bond to
elongate by 0.008 Å and the C1-N and C4-N bonds to shrink by 0.008 Å compared
to PBP.

2.3.4 Lowest triplet excited state (TES) geometries and electronic
structures.
Geometries of the lowest triplet states for the ChBP, NPP,
phenylphthalimide, PBP, TMPP, and tBuTMPP were optimized using the SCF
unrestricted method (uB3LYP/6-31G**). Figure 2.4 shows the spin densities and the
ground and excited state dihedral angles of all the phthalimides. The T
1
emission
energies were obtained by subtracting the optimized ground state SCF energies from
the optimized triplet state SCF energies
96
(Equation 2.1). The calculated T
1
emission
energies, the triplet spin densities, and excited state dipole moments are summarized
in table 3.
0
SCF
1
SCF T
E E SCF E
S T
− = Δ = (2.1)
60
Investigation of the TES geometry of phenylphthalimide showed a significant
variation from the SGS geometry. Increased orbital overlap decreased the C5-N
bond by 0.062 Å in the triplet state (Table 2.3). As a consequence, the CCNC
Dihedral angle decreased to 12º in the triplet state from 40º in the ground state.
Excited state geometry of PBP also showed a large deviation from the ground state.
In the ground state the CCNC dihedral angles of PBP for both of the phthalimides
stayed at 40º, one of which in the excited state changed to 9º. The dihedral angle of
the moiety without any unpaired spin remained almost the same (38º). This caused
the C5-N bond to contract to 1.36Å in the triplet state. The excited state geometries
of NPP, TMPP, and tBuTMPP did not deviate significantly from the ground state
geometries.
The electronic structure of the optimized triplet phenyl-phthalimide showed
20% (0.4) unpaired electron density on the central phenyl ring and 80% (1.6)
unpaired electron density on the phthalimide ring. This suggests that
phosphorescence in this molecule originates from the phthalimide moiety with a
small contribution from the benzene triplet. Similar behaviors were observed in
PBP, TMPP, and tBuTMPP molecules. Table 2.3 gives a summary of the spin
densities of all the phthalimides, which clearly shows that majority of the density lies
on the phthalimide moieties.
The change in dipole moment of phenyl-phthalimide going from the ground
state to the triplet state showed small variation suggesting a small charge separation
in the triplet state. The ground state dipole moments of the PBP, TMPP, and
61
tBuTMPP were calculated to be nearly zero because of the symmetry of the
molecules neutralizes the dipoles in the ground state. In the triplet excited state
however, the loss of symmetry causes the dipoles of these molecules to increase
indicating charge separation in the excited state.

40º
68º
68º
Triplet state dihedral
58º
112º
38º
65º
65º
Ground state dihedral
55º
40º
68º
68º
57º
111ºz
Phenylphthalimide
PBP
TMPP
tBuTMPP
ChBP
NPP
90º 90º
12º
40º
9º
65º
40º
68º
68º
Triplet state dihedral
58º
112º
38º
65º
65º
Ground state dihedral
55º
40º
68º
68º
57º
111ºz
Phenylphthalimide
PBP
TMPP
tBuTMPP
ChBP
NPP
90º 90º
12º
40º
9º
65º

Figure 2.4 Optimized triplet state geometries of all the phthalimides showing the
spin densities and the dihedral angles compared with the ground state dihedral
angles.
62
Table 2.3 Summary of Emission Energies, dipole moment, and the Spin Densities
Exp Calc Exp SGS TES
Compound E
S
(eV) E
T
(eV) E
T
(eV) dipole (D) dipole (D) Phenyl ring Phthalimide 1 Phthalimide 2
ChBP 4.04 3.09 2.90 3.42 4.29 0.03 1.97 0.00
Phenylphthalimide 4.07 2.73 2.87 2.55 3.18 0.44 1.56 ---
PBP 4.07 2.64 2.87 0.00 9.21 0.44 1.49 0.07
TMPP 4.05 2.98 2.91 0.00 5.68 0.34 1.65 0.01
tBuTMPP 4.04 2.99 2.90 0.42 3.69 0.26 1.73 0.01
NPP 4.05 --- 3.02 0.00 0.56 --- 1.99 0.01
E
T
= Triplet energy, E
S
= Singlet energy
SGS = singlet ground state, TES = triplet excited state
TES Spin densities

2.3.5 Absorption spectroscopy and computational analysis.
Summary of the absorption data are provided in Table 2.5. Due to solubility
reasons the molar extinction coefficients were calculated only for tBuTMPP and
ChBP compounds. Absorption spectra of all the phthalimides displayed two distinct
regions; a low energy region of low intensity between 260 – 330 nm and a high
energy region of strong intensity around 260 nm and above. The intense high energy
UV transitions in the phthalimides are caused by polarizations along the long C
2
axis
and the partially structured low energy transitions are caused mostly by out of plane
polarization perpendicular to the C
2
axis.
107
Gawronski et al.
108
studied N-methyl
phthalimide and observed the same transitions in the absorption spectrum. They
assigned the weak tail at 340 nm as n-π* transition, the peak at 320 nm as π-π*
transition, and the high energy peaks at 245 nm and 205 nm as π-π* transitions. Our
assignments are based on the TDDFT calculations, which are state transitions
involving one electron excitation from HOMO to LUMO and molecular orbitals
(MOs) from below the HOMO to LUMO and above the LUMO. For all the
phthalimides, the HOMO-LUMO transitions are n-π* in character and the high
63
energy transitions are π-π* in character. Figure 2.5 shows the stick plot
representation of the calculated oscillator strengths and transition energies of
tBuTMPP and ChBP superimposed on the experimental absorption spectra, where
each vertical bar represents a particular transition for a given wavelength.
TDDFT calculated gas phase vertical transition energies of all the
phthalimides were predicted to be lower than the experimental energies. Since the
experiment probes the optical response of the phthalimides as a function of solvent
perturbation in the ground and the excited state, disagreement between the gas phase
calculations and the optical transition can then be partially attributed to the fact that
the gas phase approximation does not consider the solvent-stabilization effect. Our
investigation of the TDDFT/CPCM results also showed a red-shift from the
experimental absorption transitions. Even though, the vertical excitation energies
calculated in the solvent shells were blue-shifted from the gas phase energies by
approximately 10 nm, the approximations were still lower in energy than the
experimental values. Disagreements between the experimental electronic transition
energies and the calculated TDDFT energies are widespread in the literatures. Many
authors have reported applying statistical correction factors to the TDDFT energies
in order to obtain a good fit with the experimental transition energies.
88, 92-96
In most
cases the effects of functionals
88, 93, 94
and basis sets
96
have been attributed to the
errors in the calculations.
64
225 250 275 300 325
0
2000
4000
6000
8000
10000
12000
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
Molar Absorptivity, ε (M
-1
cm
-1
)
Wavelength, λ (nm)
tBuTMPP Absorption
225 250 275 300 325
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
0.00
0.02
0.04
0.06
0.08
Molar Absorptivity, ε (M
-1
cm
-1
)
Wavelength, λ (nm)
ChBP Absorption

Oscillator Strength
TDDFT THF

Oscillator Strength
TDDFT THF

Figure 2.5 Absorption spectra of the tBuTMPP and ChBP in 2me-THF with
superimposed TDDFT excitation energies showing the peak assignments.

Masunov et al.
95
recently published a paper where they reported TDDFT
studies of organic chromophores with diamines, ciano, thio, and ether functionalities.
The authors in this paper claimed that the B3LYP optimized geometries
systematically underestimate the calculated transition energies. They reasoned that
in the case of conjugated organic molecules the bond length alteration parameter
(BLA: defined as the difference in length between the single and double bonds)
reflects the degree of uneven distribution of π electrons over the bonds. The BLA
parameter is immensely underestimated by the B3LYP functional, which results in
overestimated electronic delocalization giving red-shifted excited state energies.
Bauernschmitt et al.
92
performed TDDFT studies of highly conjugated fullerene
molecule with the B3LYP functional and they too observed excitation energies lower
than the experimental values. Authors in this paper blue-shifted all the vertical
65
transit ion energies by 0.35 eV to match with the proper wavelengths of the
absorption spectrum. In a very recent paper Denis and coworkers
96
reported detailed
TDDFT/CPCM studies of various coumarin molecules with numerous basis sets. In
their studies, the authors acknowledged that the TDDFT significantly underestimates
the excitation energies when charge separation takes place between distant moieties
of a conjugated system.
Since the calculated E
VT
profiles matched really well with the absorption
profiles of all the phthalimides, the most intense E
VT
can therefore be justifiably
correlated to the most intense optical transition (Figure 2.5). However, a general
shift of the transition energies cannot be justified because a linear relationship
between the energy-axis shift and the transitions energies cannot be rationalized for
conjugated and nonconjugated systems. The vertical excitation energies of
conjugated and nonconjugated organic molecules calculated in the same solvent
dielectric (TDDFT/CPCM) cannot completely take the solvatochromic effects in
consideration because of different BLA parameters. Since polarities of the ground
and excited state of a conjugated chromophore would be different than a
nonconjugated chromophore, a solvent system with same dielectric would lead to
differential stablization of the ground and excited states for conjugated and
nonconjugated systems. Consequently, position and intensity of the vertical
transition energies would differ from the transitions of the experimental absorption
spectra. Solvatochromic shifts in tBuTMPP result from solvent induced stabilization
of the electronically delocalized charge transfer (CT) state, which in ChBP comes
66
from a charge-localized (CL) sate. Since the B3LYP functional underestimates the
BLA parameters and overestimates the electronic delocalization, calculated transition
energies for CT conjugated systems and CL non-conjugated systems would not be
the same, and thus would not account for liner solvatochromic shifts. An example of
this can be seen in Figure 4, where a constant shift of 0.80 eV (which was obtained
from the average values by aligning the E
VT
of the highest oscillator strengths with
the optical transitions of the highest absorptivity coefficients) of the vertical
transition energies calculated for tBuTMPP correlates really well with its absorption
spectrum. On the other hand, the long wavelength transition energies calculated for
ChBP ends up being blue shifted from the corresponding transition energies of the
absorption spectrum.
Table 2.4 illustrates the vertical transition energies, the MOs involved in the
major transitions, the orbital coefficients (Defined as the absolute values of the
coefficients of the wavefunction involved in each excitation, which are directly
proportional to the contribution of the given excitation to the transition.)
97
, and the
oscillator strengths of 5 singlet and 5 triplet states of tBuTMPP and ChBP calculated
in THF. The TDDFT/CPCM results for tBuTMPP and ChBP in acetonitrile and
cyclohexane and the gas phase TDDFT results of all the phthalimide derivatives are
reported in appendix B. The S
0
-S
1
excitation energies of tBuTMPP in the gas phase
and in THF, acetonitrile, and cyclohexane were calculated to be 307, 303, 300, and
308 nm respectively, which compared well with the experimental value of 302 nm.
In all four cases major contribution for this electronic transition involved HOMO to
67
LUMO (MO-143 to MO-144) transition with very small contributions from other
MOs. Electronic transitions giving rise to S
2
, S
3
, and S
4
sates involved major
contribution from HOMO to LUMO+1, HOMO-1 to LUMO and HOMO-1 to
LUMO+1 respectively. All the higher state transitions involved equal magnitude of
charge transfer from occupied MOs below the HOMO to LUMO and LUMO+1. In
other words, these are all charge transfer states. Oscillator strengths for the solvent
approximations were higher than the gas phase approximations. The S
0
-S
1
excitation
energies of ChBP were very different from tBuTMPP. Blue shifting calculated
TDDFT energies caused the lowest energy optical transitions to be offset by
approximately 0.30 eV. In all four cases the MOs involved in the transition had
almost equal contributions from HOMO-5 to LUMO and HOMO-2 to LUMO, which
for S
0
-S
2
were from HOMO-5 to LUMO+1 and HOMO-2 to LUMO+1. The
oscillator strengths for these transitions were however smaller than the S
0
-S
3

transition. And contribution for this state had much stronger HOMO (MO-98) to
LUMO (MO-99) character.
The T
0
-T
1
excitation energies of tBuTMPP calculated in the gas phase, THF,
acetonitrile, and cyclohexane were calculated to be 348, 352, 353, and 347 nm
respectively. Contribution to this state transition came from one electron excitation
from HOMO-1 to LUMO and HOMO-2 to LUMO+1. Orbital coefficients for both
of these MO transitions were almost equal in both cases suggesting the triplet state
was a charge transfer in character. These results correlate really well with the results
obtained from the TES geometry analysis
68
Table 2.4 Calculated vertical excitation energies (E), dominant MO transitions,
orbital coefficients, and oscillator strengths for tBuTMPP.

States E
exp
E
theo
ψ
oc
→ ψ
ver
Orbital Oscillator
λ (nm) λ (nm) MO Composition Coefficient Strengths ( f )
S
1
302 348.41 143 → 144 0.7 0.0038
S
2
--- 348.03 143 → 145 0.7 0.0000
S
3
287 338.51 142 → 144 0.6 0.0004
S
4
--- 338.19 142 → 145 0.6 0.0020
S
5
277 311.82 137 → 144, 137 → 145 0.3, 0.3 0.0072
T
1
--- 396.71 141 → 144, 141 → 145 0.3, 0.3 ---
T
2
--- 396.48 141 → 144, 141 → 145 0.3, 0.3 ---
T
3
--- 358.98 142 → 144, 137 → 145 0.3, 0.3 ---
T
4
--- 358.62 137 → 144, 142 → 145 0.3, 0.3 ---
T
5
--- 355.6 142 → 148, 143 → 144 0.3, 0.5 ---
S
1
302 319.39 93 → 99, 96 → 99 0.4, 0.5 0.0002
S
2
--- 318.44 93 → 100, 96 → 100 0.4, 0.5 0.0002
S
3
287 311.03 98 → 99 0.6 0.0005
S
4
--- 308.05 97 → 100, 98 → 100 0.3, 0.6 0.0008
S
5
--- 288.52 89 → 99, 90 → 99 0.3, 0.3 0.0003
T
1
--- 395.85 97 → 99 98 → 99 0.4, 0.5 ---
T
2
--- 392.47 97 → 100 98 → 100 0.5, 0.4 ---
T
3
--- 354.81 93 → 99, 96 → 99 0.4, 0.5 ---
T
4
--- 353.89 93 → 100, 96 → 100 0.4, 0.5 ---
T
5
--- 335.76 91 → 99, 92 → 99 0.4, 0.4 ---
E
exp
= Experimental UV-vis data, E
Theo
= TDDFT vertical transition energy
ψ
oc
→ ψ
ver
= defines one electron excitation from the occupied to the virtual orbitals
Orbital coefficients of 0.3 and above are shown only
tBuTMPP in THF
ChBP in THF

2.3.6 Emission spectroscopy and computational analysis.
Room temperature and 77K emission spectra of all the phthalimide
compounds are shown in Figure 2.6. At 77K in 2me-THF, ChBP and NPP
phosphorescence were observed at λ
max
= 452 and 440 nm with lifetimes, τ = 0.59
69
and 0.67 seconds respectively. The results were consistent with the phthalimide
emissions and lifetimes reported in the literature (Table 2.5). In ChBP and NPP the
phthalimide moieties are not conjugated through a phenyl spacer like in
phenylphthalimide and the HOMOs and LUMOs in these molecules localize on the
phthalimide moieties. In ChBP however, a small contribution to the LUMO seems
to come from the cyclohexyl moiety, which does not effect the emission and the
phosphorescence is only observed from the phthalimide groups. Photoluminescence
spectra of the NPP, PBP, and TMPP molecules in room temperature fluid solution
showed no fluorescence. Photoluminescence of ChBP in room temperature showed
a weak fluorescence at λ
max
= 410 nm with τ = 7.0 ns lifetime consistent with the
methyl and propyl phthalimide fluorescence reported in the literature.
77K spectrum of the phenyl-phthalimide showed a very distinct
phosphorescence band at λ
max
= 449 nm, which matched with the 77K phthalimide
emission from the literature. In addition to that, a very low intensity peak at 359 nm
was also observed in the low temperature spectra. This peak is the fluorescent band
of phenyl-phthalimide at 77K. PBP, TMPP, and tBuTMPP all behaved similar to
phenyl-phthalimide. In low temperature glass the λ
max
of phosphorescence and
fluorescence of these molecules were observed around 450 nm and 360 nm
respectively. Since the 77K emission profile of tBuTMPP showed similarities to the
emission profile of ChBP and NPP, it can be suggested that the tBuTMPP
phosphorescence originates from the phthalimide moiety only. However, the spin
density pictures showed the densities of the unpaired electrons in the
70
phenylphthalimides contain minor contributions from the phenyl spacer too. This
fact indicates that the key component of the triplet emission of all the phthalimide
derivatives comes from the phthalimide functionalities with a mixture of a small
contribution from the phenyl triplet.
TMPP: 77 K Emis s ion
TMPP: Abs orption
NPP: 77 K Emis s ion
NPP: Abs orption
Normalized Intensity
PBP: 77 K Emis s ion
PBP: Abs orption
Phenylphthalimide: 298 K Emis s ion
Phenylphthalimide: 77 K Emis s ion
Phenylphthalimide: Abs orption
250 300 350 400 450 500 550 600
Wavelength, l (nm)
tBuTMPP: 298 K Emis s ion
tBuTMPP: 77 K Emis s ion
tBuTMPP: Abs orption

ChBP: 298 K Emis s ion
ChBP: 77 K Emis s ion
ChBP: Abs orption

Figure 2.6 298K Absorption and emission spectra of the phthalimides in 2me-THF
are shown with 77K spectra in 2MTHF.

71
In room temperature fluid solution, both phenyl-phthalimide and tBuTMPP
showed two very weak fluorescence bands: A short-wavelength (SW) band at 360
nm and a long-wavelength (LW) band at 495 nm for phenylphthalimide and an LW
band at 450 nm for tBuTMPP. Lifetimes of the SW and the LW peaks for all the
phenylphthalimide and tBuTMPP were in nanosecond range (Table 2.5). PBP and
TMPP did not show any room temperature fluorescence probably due to solubility
reasons. However, the 77K SW emissions of these molecules showed the low
intensity fluorescence at λ
max
= 360 nm along with phosphorescence emission. The
topic of dual fluorescence is not new and phenyl-naphthalimides have been studied
in the past for their dual fluorescent properties. These molecules have two
conformationally different excited states, which are generated by the rotating phenyl
group on the nitrogen atom: A short-wavelength SW* state, where the phenyl group
stays perpendicular to the naphthalimide moiety and gives high energy fluorescence
and a long wavelength LW* state, where the phenyl group stays co-planar to the
naphthalimide moiety and gives low energy fluorescence.
52, 54, 58, 59

Based on the model for the phenyl naphthalimide dual fluorescence by Valat
and coworkers
59
, we can propose a similar model for the phenyl-phthalimide
molecules. According to the model: A phenyl-phthalimide molecule, A
0
in its
ground state can absorb a photon and get excited to its lowest singlet excited state,
which upon immediate relaxation can form the Frank-Condon (FC) state,
1
(A)*
(Figure 2.7). Solvent reorganization and geometrical rearrangement of the FC state
can then generate the SW* and the LW* excited states. The geometry of the SW*
72
state is where the phenyl group lies perpendicular to the phthalimide moiety giving
rise to a charge localized
1
(CL-SW)* state. Since the SW fluorescence of the
phenyl-phthalimides have negligible stoke-shift, the geometries of the ground states,
A
0
and the singlet excited states,
1
(CL-SW)* would be similar. The geometries of
the LW* states are very different from the ground state geometries, A
0
. A large
stoke-shift is caused by the formation of the charge transfer state,
1
(CT-LW)*. The
CT state can be either formed by the rotation of the phthalimide moiety to the
coplanar geometry or by stabilization of the charge separated dipole by solvent
molecules that would not require any rotation. Coplanarization of the phthalimide
moiety is not necessary for the formation of the
1
(CT-LW)* state. Extended
conjugation between the phenyl group and the phthalimide moiety in the
1
(CT-LW)*
state favors the CT, which is further stabilized by solvent molecules. Recent studies
of 4-(dimethylamino) benzonitriles (DMABN) and relative molecules have shown
that intramolecular charge transfer in these molecules does not need any twist and
dual fluorescence can be observed from untwisted geometries.
109-111
Gomez
109
et al.
conducted theoretical studies of 4-(dimethylamino)benzonitriles (DMABN) and
relative molecules. They used the complete active space self-consistent field
(CASSCF) method to study the singlet excited state geometries and found that
intramolecular charge transfer in 4-aminobenzonitriles does not require any twist.
Cogan et al.
111
also used the same level of theory in conjunction with TDDFT
(B3LYP/cc-pVDZ) method and came up with the same conclusion. Zachariasse
110

and coworkers recently published dual fluorescence from 1-tert-butyl-6-cyano-
73
1,2,3,4-tetrahydroquinoline (NTC6) in various polar and non-polar solvents. The
authors here concluded that efficient and fast intramolecular CT is possible with
molecules that cannot undergo twist.

A
0
1
(A)*
1
(CL-SW)*
1
(CT-LW)* 3
(CT)*
A
0
1
(A)*
1
(CL-SW)*
1
(CT-LW)* 3
(CT)*

Figure 2.7 Energy diagram showing the charge transfer (CT) and charge localized
(CL) excited states.

Formation of the triplet state,
3
(CT)* in the phthalimides depends on how
conjugated the system is because that can lead to the formation of the CL or CT
states. Therefore, the triplet state can be generated from
1
(A)*,
1
(CL-SW)* or
1
(CT-
LW)* states. Based on the calculated TES geometries, TDDFT results, and the
relative spacing of the energy levels it can be suggested that greatest contribution to
the formation of the charge transfer triplet state,
3
(CT)* in the phenylphthalimides
may come from the
1
(CT-LW)* state.
74
The LW room temperature emission of tBuTMPP was redshifted from ChBP.
In ChBP the HOMO and LUMO localize on the same atoms but in PBP, TMPP, and
tBuTMPP the HOMO on the phenyl and LUMO on the phthalimides creates a CT
state that is stabilized by solvent molecules. Increased wavefunction overlap
between the phenyl ring and the phthalimide moieties red-shifts the fluorescence of
the phenylphthalimide derivatives compared to NPP and ChBP. This fact was
supported well by the TDDFT and TDDFT/CPCM calculations. The vertical
transition energies of tBuTMPP in the gas phase and solvent shells were
approximately 30 nm red-shifted from ChBP, which correlated well with the
experimental shift of 40nm. The 298K spectrum of phenylphthalimide was
redshifted from the phenyl-bis-phthalimide derivatives by almost 30nm. This trend
was also observed in the TDDFT calculations where the phenylphthalimide was
redshfited by 18 nm from its symmetric counterpart PBP. We believe that in
phenylphthalimde this further redshifting of emission is perhaps caused by the
stabilization of the larger dipole in the singlet excited state. In PBP, TMPP, and
tBuTMPP the dipole is cancelled by the two phthalimide moieties. In the
phenylphthalimide however, the lack of symmetry generates the dipole, which is
further stabilized by the solvents giving a red-shifted emission. (geometry
optimization of the singlet excited state of phenylphthalimide was performed by
configuration interaction of the singles (CIS) method. The dipole moment was
calculated to be 5.1 D, which is two times larger than the dipole of the ground state
dipole)
75
The LW emission of the tBuTMPP was found to lie very close to the
phosphorescence emission band. The gap between the room temperature LW
emission and the 77K emission was only 13-15 nm. Carbonyl compounds are well
known to have very small singlet-triplet gap due to n,π* configuration. Intersystem
crossing in these molecules involve n,π* to π,π* transition, where electrons move
between the n to π orbitals. This is caused by the out of plane polarization of the
molecule and initiates the spin-orbit coupling. Since the n and the π* orbitals are
orthogonal to each other, there exists no overlap between the orbitals in space. This
decreases the electrostatic repulsion between the electrons in the n orbital and the π*
orbital, which then decreases singlet-triplet gap.
107
Phthalimides with carbonyl
groups have small singlet triplet gap due to n,π* to π,π* transition.
The small energy gap between the room temperature and the 77K emission
created two dilemmas. One, where the LW emission could be a weak room
temperature phosphorescence and two, where the 77K emission could originate from
the delayed fluorescence caused by aggregation. Aggregate emission has already
been reported by Wintgens and coworkers.
54
This type of emission is caused by the
p-type delayed fluorescence mechanism
112
and is hard to distinguish from a real
phosphorescence. There are two types of delayed fluorescence: An E-type delayed
fluorescence and a P-type delayed fluorescence. E-type delayed fluorescence results
from the repopulation of the singlet state by thermal activation of the triplet state.
The P-type delayed fluorescence is caused by the self annihilation of the two close
lying triplets which gives rise to a singlet state and a ground state. The lifetime of
76
the singlet is in the order of the triplet lifetime. In order to confirm the fact that the
room temperature emission of tBuTMPP is a real fluorescence, and not a weak
phosphorescence and the 77K emission is a real phosphorescence and not a delayed
fluorescence, we tried several experiments. We bubble degassed the tBuTMPP
sample in N
2
for half hour and re-measured its emission in room temperature
immediately after. No change in emission intensity or shift in emission maxima
indicated the emission was indeed from the singlet state. Emission of the tBuTMPP
in 2me-THF and CH
3
I solution also showed no new band at lower energy. 77K
spectra in ethanol, butyronitrile, and 95% butyronitrile/5% butyl acetate (v/v) also
confirmed the same conclusion. The fluorescence efficiency is poor because of rapid
intersystem crossing.

+ +
T
1
T
1
S
1
S
0
+ +
T
1
T
1
S
1
S
0
S
1
S
0
T
1
Delayed fluorescence
Phosphorescence
k
isc
k
isc
-1
S
1
S
0
T
1
Delayed fluorescence
Phosphorescence
k
isc
k
isc
-1

Figure 2.8 P-type delayed fluorescence (left). E-type delayed fluorescence (right)

77
Table 2.5 Summary of photophysical data.

Absorption Emission (298K) Φ
f
τ
f
Emission (77K) τ
ph
Compound Solvent λ
max
(nm) {ε (cm
-1
M
-1
)} λ
max
(nm) (ns) λ
max
(nm) (s)
Phthalimide CH
3
CN
a
--- ---
< 10
-4
--- 440 0.69
Ether
d
--- --- --- --- 443 0.2
DCM
b
--- 520 --- --- --- ---
Methylphthalimide CH3CN
a
290 390 8.0X10
-4
0.185 453 0.8
CH
3
CN
c
--- <420
6.0X10
-4
--- --- ---
EtOH --- <405 5.0X10
-4
--- 450 0.75
H
2
O --- 425
6.0X10
-3
--- --- ---
Propylphthalimides
c
EtOH 292 <400 5.0X10
-4
--- 450 0.75
H
2
O 297 420
5.0X10
-4
--- --- ---
ChBP 2m-THF 294 (1239), 301 (1202) 410 5.2X10
-5
5 381, 452 0.59
EtOH --- Non-emissive --- --- 457 0.59
95%BN/5%BA --- 423 --- --- 460 0.66
CH
3
CN 246, 292, 302 358, 430 --- 9 - 14 --- ---
ButCN 248, 295, 300 349, 425 --- 4 - 10 439, 460 ---
Hexanes 245, 292, 301 non-emissive --- --- --- ---
Cyclohexanes 245, 292, 301 394, 523
e
--- --- --- ---
NPP 2m-THF 303, 294 Non-emissive --- --- 410, 420, 442 0.667
DCM 245, 290, 300 Non-emissive --- --- --- ---
CH
3
CN 240, 296, 303 Non-emissive --- --- --- ---
Hexanes Non-absorptive Non-emissive --- --- --- ---
Phenylphthalimide 2m-THF 257, 291, 300 359, 495
e
, 495
b
--- --- 360, 449 0.224
DCM
e
290, 300 Non-emissive --- --- --- ---
CH
3
CN 292, 301 Non-emissive --- --- --- ---
Hexanes
e
210, 238, 261, 298 475
e
--- 98 --- ---
PBP 2m-THF 250, 292, 302 Non-emissive --- --- 355, 432, 448 0.349
DCM 247, 293, 305 Non-emissive --- --- --- ---
CH
3
CN 247, 294, 303 Non-emissive --- --- --- ---
Hexanes Non-absorptive Non-emissive --- --- --- ---
TMPP 2m-THF 302, 294, 280 Non-emissive --- --- 426, 448 0.36
DCM 245, 280, 295 Non-emissive --- --- --- ---
CH
3
CN 238, 280, 294 Non-emissive --- --- --- ---
Hexanes 290, 300 Non-emissive --- --- --- ---
tBuTMPP 2m-THF 280 (1455), 294 (1353), 302 (1307) 359, 450 --- 2.47 316, 445, 380, 456 0.45
95%BN/5%BA 245, 271, 280, 297, 302 353, 459 --- --- 464 0.84
2m-THF/CH
3
I 280, 294, 302 450 --- --- 456 ---
CH
3
CN 245, 280, 296, 302 463 --- 2 - 11 --- ---
ButCN 242, 280, 296, 304 353, 458 --- 2 - 10 463 ---
Hexanes 240, 282, 292, 301 405
e
--- --- ---
Cyclohexane 243, 280, 302 428 --- 1.6 --- ---
a = ref 54, b = ref 60, c = ref 72, d = ref 74, e = very weak emission, BN = butyronitrile, BA = butyl acetate
Φ
f
= Fluorescence quantum yield, τ
f
= luorescence lifetime, τ
f
= phoephorescence lifetime

2.4 Chapter 2 Conclusion
Electrochemistry, photophysics, and excited state characteristics of new class
of phthalimide materials were studied. Solvent free microwave synthesis was used
to obtain the phthalimides in high yield. Electrochemical analysis showed large
reduction potentials and wide HOMO-LUMO gaps. Electronic spectroscopy showed
weak dual fluorescence in room temperature fluid solution suggesting the formation
78
of
1
(CL-SW)* and
1
(CT-LW)* states upon excitation. Photoluminescence from
these states show up as short wavelength (SW) and long wavelength (LW) emissions
respectively. The geometry of
1
(CL-SW)* state is where the phthalimide groups lie
perpendicular to the phenyl plane giving a CL state. The
1
(CT-LW)* state is a CT
state and the geometry of this state can be either planer or perpendicular.
In low temperature glass the phthalimides showed phosphorescence around
450nm with millisecond lifetimes. Triplet energies of the molecules were between
2.9 to 3.0 eV. The triplet
3
(CT-LW)* state can be formed from the singlet
1
(CL-
SW)* and
1
(CT-LW)* states. However, a probability of direct the formation of the
3
(CT-LW)* state from the
1
(CT-LW)* state is high because the triplet state is also a
CT state and the energy gap between the
3
(CT-LW)* state and the
1
(CT-LW)* state
is very small. These results were supported well by the TDDFT data, which also
showed the formation of charge transfer excited states.
Finally, the electrochemical and Photophysical properties of the phthalimides
suggest that these molecules can be used in OLEDs and solar cells as hosts or
hole/exciton blocking materials. Large HOMO-LUMO gaps will allow the
phthalimides to be good hosts with high energy dopants. Deep HOMO will make
these molecules good candidates for hole blocking materials. High triplet energies
will prevent endothermic energy transfer from the dopant to the host and also make
an efficient exciton blocker.

79
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91
Chapter 3. Efficient Green and Red Electrophosphorescent
Devices Utilizing High Triplet Energy Phthalimide Blocking
Materials

3.1 Introduction
Structure of a typical phosphorescent organic light emitting diode (OLED)
contains a hole transporting layer (HTL)
1-3
, a host-dopant-emissive layer (EML)
4-6
, a
hole blocking layer (HBL)
7-9
, and an electron transporting layer (ETL)
10-12
stacked
sequentially between an anode and a cathode.
7, 8
Under forward electrical bias holes
and electrons from the anode and the cathode diffuse through the organic layers and
recombine in the EML layer to form excitons, which upon radiative decay produce
electroluminescence.
7, 13-17
Since each layer of an OLED has a very specific
purpose, design of an OLED requires choosing materials that fulfills specific
requirements. The material of choice therefore depends on the function of the layer
where it is going to be used in. In this chapter we will explore the use of high triplet
energy phthalimide materials as host and hole/exciton-blocking materials in OLEDs.
The roles of hosts and hole/exciton-blockers are vital in phosphorescent
OLEDs and choosing the right material requires consideration of several factors. An
efficient host must have large highest occupied molecular orbitals (HOMO)-lowest
unoccupied molecular orbitals (LUMO) gap so the charges cannot escape prior to

92
+
-
h+
e-
HT L
ET L
HB L
HB L
+
-
h+
e-
HB L
HB L
ET L
HT L
+
-
h+
e-
HTL
ET L
Host
Dopant
S
1
S
0
T
1
ISC
S
0
T
1
Dopant Host
e-
h+
h ν
E
T
ab
d c
+
-
h+
e-
HT L
ET L
HB L
HB L
+
-
h+
e-
HB L
HB L
ET L
HT L
+
-
h+
e-
HTL
ET L
Host
Dopant
S
1
S
0
T
1
ISC
S
0
T
1
Dopant Host
e-
h+
h ν
E
T
S
1
S
0
T
1
ISC
S
0
T
1
Dopant Host
e-
h+
h ν
E
T
ab
d c

Figure 3.1 a) Structure of a typical phosphorescent OLED showing charge
recombination at the EML. b) State diagram showing the energy transfer pathways
in a host-dopant system. c) An HBL with a deep HOMO in an OLED blocking hole.
d) An exciton blocker with high triplet energy in and OLED blocking excitons.

radiative recombination (Figure 3.1a). Its HOMO energy level must be deeper than
the HOMO of the dopant and the triplet energy must be higher than the triplet energy
of the dopant to prevent endothermic energy transfer from the dopant to the host
(Figure 3.1b).
18
An efficient hole/exciton-blocker must be a good electron
transporter. Its LUMO has to be aligned with the LUMO of the ETL so electrons
93
can easily migrate from the cathode to the recombination zone (Figure 3.1c). It must
also have a very deep HOMO and high triplet energy so the holes can be efficiently
blocked from leaking to the electron transporting layers and the triplet excitons
formed in the recombination zone can be effectively blocked from diffusing to the
cathode (Figure 3.1c and d).
Carbazole derivatives have been widely used as hosts in OLEDs. Most
common carbazole based host used in OLED is 4,4’-N,N’-dicarbazole biphenyl
(CBP).
19-21
Fluorine,
22
triazine,
23
organosilicon,
24
and naphthalimide
25
based
compounds have also been studied as hosts in OLEDs. Kolosov et al. reported
OLEDs with N-22,6-dibromophenyl-1,8-naphthalimide (niBr) as a host and bis(2-
(2’-benzo[4,5-a]thienyl)-pyridinato-N,C3’)iridium(acetyl-acetonate) (btpIr), bis(2-
phenylbenzothiozolato-N,C
2
’)iridium(acetyl-acetonate) (btIr), and fac-tris-(2-
phenylpyredine) Iridium Ir(ppy)
3
as phosphorescent dopants. The authors in this
paper reported poor performance of the niBr Ir(ppy)
3
device because the triplet
energy of niBr (2.3 eV) was lower than the triplet energy of Ir(ppy)
3
(2.4 eV),
Among the hole-blockers, bathocuproine (BCP)
6, 26, 27
and (4-biphenyloxolato
aluminum(III)bis(2-methyl-8- quinolinato)4-phenylphenolate) (BAlq)
18
are the most
commonly used materials in OLED. Triplet energies of these molecules however are
less than 2.4 eV, which limits their usage in OLEDs with high energy emitters.
Among other organic hole-blockers, oxadiazole
28
and trazol
29
based materials have
also been looked at. Adamovich
7
et al. reported highly efficient green devices with
Iridium based hole-blocking material (bis(2-(4,6-difluorophenyl)pyridyl-N,C20)
94
iridium(III) picolinate (FIrpic). The triplet energy of this material was reported to be
2.6 eV. Organosilicon
30
based hole-blocker was recently reported by Yu et al.
Authors in this paper reported blue fluorescent OLEDs with tetra( β-napthyl)silane
(TNS) hole-blocker. The device with TNS reportedly had higher current efficiency
than the one with BCP, but no external quantum efficiencies of the devices were
either reported or discussed. Okumoto and Shirota recently reported devices with
N,N-bis(9,9-dimethylfluoren-2-yl)aniline (F2PA) blue-violet emitter and 1,3,5-tris(4-
fluorophenyl-4-yl)benzene (F-TBB) hole blocker. Maximum external quantum
efficiencies of these devices were less than 2%. Authors also discussed device with
F-TBB hole-blocker and Irppy emitter, but the external quantum efficiency of the
device was only 5.4%.
31, 32
The naphthalimide based host niBr, reported by Kolosov
and coworkers was also tested as a hole blocker in OLEDs. Devices with this hole-
blocker showed weak Alq
3
emission suggesting poor hole blocking ability of
naphthalimides.
One of the most common problems with the existing host and hole-blocking
materials is that their triplet energies are not high enough to be used with blue and
green emitters. Therefore, development of high triplet energy host and hole-blocking
materials is important. We focused on the phthalimide compounds because the
triplet energies of these materials are between 2.9 to 3.0 eV. In the preceding
chapter we characterized various phthalimide materials and concluded that these
molecules have high singlet and triplet energies and can be used as hosts and
95
N
O
O
N
O
O
N
O
O
N
O
O
TMPP
N
O
O
N
O
O
tBuTMPP
N N
O
O
O
O
CH-2p
N N N N
N
Ir
3
N
Ir
2
O
O
Irppy
PQIr
O
N
Al
Alq
3
3
N N
NPP
CBP NPD
BCP

Figure 3.2 Structures of the phthalimide materials; TMPP, tBuTMPP, NPP, and
ChBP are shown with conventional hole transporter NPD, host CBP, electron
transporter Alq
3
, hole blocker BCP, and phosphorescent dopants Irppy and PQIr.

hole/exciton-blockers in OLEDs. To examine the potential of the phthalimides as
hosts devices were fabricated with phthalimide hosts doped with fac-tris-(2-
phenylpyredine) Iridium (Irppy)
33
and bis(1-phenylisoquinolinato-N,C2’)iridium
(acetylacetonate) (PQIr)
34
phosphorescent dopants (Structure:
ITO/NPD/Phthalimide:Dopant (6-8%)/BCP/Alq
3
/Cathode). Figure 3.2 shows the
96
structures of the Iridium phosphors and the phthalimide materials studied in this
chapter. As a host the phthalimides turned out to be inefficient because of exciplex
formation between the host and the HTL and electron transfer quenching of the
dopant by the host. As a hole/exciton-blocker the phthalimides turned out to be very
efficient. Devices with tBuTMPP hole-blocker were highly efficient and performed
better than the ones with the BCP control (Structure: ITO/NPD/CBP:Dopant (6-
8%)/Phthalimide/Alq
3
/Cathode).

3.2 Experimental
3.2.1 Synthesis of the Materials
Synthesis of 1,2-bis-(4-t-butylphthalimide)-cyclohexane (ChBP), 2,3,5,6-
tetramethyl-phenyl-1,4-(bis-phthalimide) (TMPP), and 2,3,5,6-tetramethyl-phenyl-
1,4-bis-(4-t-butylphthalimide) (tBuTMPP) were reported and discussed in chapter 2.
N-Phthalimidophthalimide (NPP) was purchased from Aldrich chemical company.
Phosphorescent green and red emitters were synthesized by the procedures
published by Lamansky, Tamayo, and coworkesrs.
5, 6, 35

3.2.2 Thermochemical Measurements
Thermal properties of the phthalimides were recorded with a differential
scanning calorimeter DSC Q10. Purified samples of the phthalimides (8 -10 mg)
were placed inside the hermetic aluminum pans and heated between 400 to 500 ºC at
the rate of 10 ºC/min. The experiments were carried out under N
2
gas at flow rate of
97
50 mL/min. The samples were then cooled very rapidly by placing a liquid N
2
dewar
on top of the sample holder and re heated beyond the melting points.

3.2.3 Thin Film Fabrication
Quartz substrates and Silicon wafers were rinsed with soap-water, deionized
water, and acetone and blow dried with nitrogen. Thin films of the phthalimides
were grown on quartz and Silicon wafers at pressures between 3-4 μtorr. Materials
were thermally evaporated from tantalum boats at rates between 2-4 Å/s. Film
thicknesses were maintained at 500Å. Silicon wafer films were used to calculate the
density of the phthalimide materials. Densities were calculated from the known
density of NPD and the ratio of the films thicknesses measured by the quartz crystal
monitor and contact angle ellipsometer. Both doped and neat thin films were
prepared with NPP, ChBP, TMPP and tBuTMPP materials. Doped films were
prepared by co-depositing the phthalimide hosts with dopants at concentrations
between 6-8%. Surface morphology of the thin films were examined with an optical
microscope (Nikon Eclipse ME 600) attached to a CCD camera model: Olympus
LKH028884.

3.2.4 Spectroscopic Measurements
Absorption spectra of the thin films were recorded with an Agilent 8453 UV-
Vis spectrometer. Emission spectra were recorded with a QuantaMaster C-60SE
98
spectrofluorometer. Lifetimes of the thin films were recorded in vacuum with an
IBH photon timing instrument connected to an IBH model TBX-04 PMT detector.

3.2.5 OLED Fabrication and Testing
All the ITO coated glass substrates were obtained from Universal Display
Corporation (UDC). Circuit patterns were photolythographically imprinted on the
substrates as 2mm wide stripes with 1mm spacings. Surface resistivity of the ITOs
was measured to be approximately 20 Ω□
-1
.
26
The ITOs were then rinsed with
acetone, sonicated in soap-water solution, washed in de-ionized water, and blow
dried in N
2
. They were then boiled in trichloroethylene, acetone, and ethanol for 5
minutes each. After that, the substrates were treated for ten minutes in the UV-ozone
cleaning chamber.
OLEDS were fabricated inside a high vacuum chamber (Kurt J. Lesker)
equipped with a cryo pump, two crystal monitors, and two power sources. Organic
films were thermally evaporated onto the ITO substrates from tantalum boats at
pressures between 3-4 μtorr. Deposition rates for all the organic materials were
maintained to be between 2-4 Å/s at all time. Prior to the deposition of the cathode,
the chamber was vented with nitrogen and shadow masks consisting 2mm stripes
were placed onto the substrates. Once the pressure reached 3.0 μtorr, 10Å of
Lithium fluoride (LiF) was deposited at 0.2 Å/s rate followed by a 1200Å layer of
Aluminum at rates between 4-5 Å/s.
99
Two types of OLEDs were fabricated with the phthalimide materials.
Devices were made with the phthalimides as hosts or hole blockers in combination
with 1,4-bis(1-naphthylphenylamino)biphenyl (NPD) as the hole transporting layer,
4,4’-N,N’-dicarbazolebiphenyl (CBP) as the control host, 2,9-dimethyl-4,7-diphenyl-
[1,10]phenanthroline (BCP) as the control hole-blocking layer, and Aluminum tris(8-
hydroxyquinoline) (Alq
3
) as the electron transporting layer. Iridium(III) fac-tris(2-
phenylpyridinato-N,C
2’
) (Irppy) and Iridium(III) bis(2-phenylquinolyl-N,C
2’
) (PQIr)
were used as the green and red dopants (Figure 3.1). The device architectures were
as follows: Type 1device: NPD (400Å)/Phthalimide:6-8% dopant (250Å)/BCP
(150Å)/(Alq
3
250Å)/LiF(10 Å)/Al(1200Å) and type 2 device: NPD (400Å)/CBP:6-
8% dopant (250Å)/phthalimide (150Å)/(Alq
3
250Å)/LiF(10 Å)/Al(1200Å) (Figure
7).
All OLEDs were tested in room temperature and pressure in open
atmosphere. LabVIEW program was used to measure the brightness and current-
voltage (I-V) characteristics of the devices. A Keithley 2400 source meter was used
to power up the OLEDs and light coming form the front of the devices was collected
through a UV-818 Si photocathode equipped with a Newport 1835-C optical meter.
Electroluminescence spectra of the devices were recorded using a
spectrofluorometer, model C-60SE.

100
3.3 Results and Discussion
The HOMO-LUMO energies of the phthalimides and all other materials
studied are shown in Figure 3.3. The LUMO energies of the phthalimides were
calculated from equation 3.1, which was obtained by correlating the Inverse
photoelectron spectroscopy data (IPES) with the reduction potentials of various
known compounds.
36
E
LUMO
of NPP, ChBP, TMPP, tBuTMPP were 2.79, 2.42,
2.86, and 2.40 eV respectively. The HOMO energies of the phthalimides were
estimated from the LUMO energies by adding the optical gap (E
S
) to the absolute
values of the reduction potential E
red
(Equation 3.2). E
HOMO
of NPP, ChBP, TMPP,
and tBuTMPP were 6.83, 6.46, 6.91, and 6.44 eVs respectively
E
LUMO
= (1.2±0.09)× (qV
CV
)-(4.8±0.20)eV (3.1)
E
HOMO
= E
S
+ │-E
red
│ (3.2)
NPD CBP ChBP TMPP tBuTMP Irppy PQIr BCP AlQ3
7
6
5
4
3
2
1
6.44
2.40

Energy (eV)
OLED Materials
1.52
5.40
2.00
6.10
2.86
6.46
2.42
6.91
1.56
5.03
2.19
5.20
1.56
6.40
1.96
5.70

Figure 3.3 Energy diagrams showing HOMO and LUMO energies of all the
molecules used to make OLEDs with.

101
3.3.1 Thermal Analysis
0 100 200 300 400 500
-40
-35
-30
-25
-20
-15
-10
-5
0
5
0 100 200 300
1.0
1.5
2.0
T
g
= 75
o
C
T
g
= 88
o
C
T
c
= 189
o
C
317
o
C
388
o
C
413
o
C
Heat flow (w/G)
Temperature (
o
C)
NPP
TMPP
tBuTMPP
ChBP
462
o
C

Figure 3.4 DSC scans of the phthalimide materials showing the glass transition
temperature, T
g
the crystalline transition temperature, T
c
and the melting
temperature, T
m
.

Figure 3.4 shows the differential scanning calorimetry (DSC) scans of the
phthalimide materials. The first heating cycle of all the phthalimides showed only a
single endothermic melting transition, T
m
. The melting temperatures, T
m
of NPP,
ChBP, TMPP, and tBuTMPP were 317ºC, 388ºC, 462ºC, and 413ºC respectively.
The T
m
of the phthalimide materials were sufficiently high suggesting high thermal
stability. The second heating cycle for ChBP and tBuTMPP exhibited the glass
transition temperature, T
g
the crystalline transition temperature, T
c
followed by the
melting temperature, T
m
. Glass transition temperatures, T
g
of tBuTMPP and ChBp
were 88ºC and 75ºC respectively. Appearance of T
g
shows that these molecules
102
form glass. High T
g
also suggests that the molecules form stable amorphous glasses
above room temperature. Since the T
g
of tBuTMPP is higher than the T
g
of ChBP, it
can be suggested that tBuTMPP is a better OLED candidate ChBP.

10 μ 10 μ
NPP neat film TMPP neat film
10 μ 10 μ
NPP neat film TMPP neat film

Figure 3.5 Optical micrograph images of neat NPP showing grainy surface and
TMPP showing smooth surface.

In the second run, no T
g
or T
c
was observed for NPP or TMPP, suggesting
that the samples probably stay crystalline all the time. For NPP and TMPP thin films
it can be suggested that the lattices become crystalline soon after deposition. Figure
3.5 shows the optical micrograph images of NPP and TMPP, which shows rough
surface morphology of the NPP films confirming the fact that NPP forms crystalline
islands immediately after deposition. The microscope image of TMPP on the other
hand does not show any evidence of crystal formation. Therefore, it can be
suggested that perhaps the size of the TMPP crystals was too small to be observed by
optical microscope.
103
3.3.2 Thin Film Photophysics
300 350 400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2

Normalized Intensity (a.u.)
Wavelength, λ (nm)
NPP: Neat film
NPP: Irppy 9%
NPP:PQIr 9%
300 350 400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2

Normalized Intensity (a.u.)
Wavelength, λ (nm)
TMPP: Neat film
TMPP: Irppy 9%
TMPP: PQIr 9%
300 350 400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2

Normalized Intensity (a.u.)
Wavelength (nm)
tBuTMPP: Neat film
tBuTMPP: Irppy 6-8%
tBuTMPP: PQIr 6-8%
300 350 400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2

Normalized Intensity (a.u.)
Wavelength (nm)
ChBP: Neat film
ChBP: Irppy 6-8%
ChBP: PQIr 6-8%

Figure 3.6 Emission of neat and doped films of ChBP, TMPP, and tBuTMPP.

Figure 3.6 shows the photoluminescent (PL) spectra of NPP, ChBP, TMPP,
and tBuTMPP. Photoluminescence of neat NPP film showed very weak and
structured emission with vibronic features at 362 and 424 nm and a maxima at λ
max
=
387 nm. When the NPP was doped with 6-8% PQIr, photoluminescence was
observed with emission maxima at 607 nm suggesting energy transfer from the NPP
104
to PQIr. However, NPP doped with Irppy film only showed a weak band at 418 nm,
characteristic of the pure NPP emission. No Irppy emission was observed. There
could be two reasons for why the PL of Irrpy doped NPP film did not show Irppy
emission. Optical microscope images of the doped NPP films are showed in figure
3.7, which shows rough surfaces for both of the doped films. It is possible that upon
deposition the NPP molecules form crystalline films, or the surface of the film makes
an amorphous to crystalline transition quickly after deposition. Crystalline islands
then phase segregate the NPP molecules from the Irrpy molecules. If the Irppy
molecules fall outside the Förster radius of NPP, then a lack of energy transfer from
NPP to Irppy can quench Irppy emission.
37, 38

NPP: Irppy 9% NPP: PQIr 9%

Figure 3.7 Optical micrograph images of neat NPP showing grainy surface and
TMPP showing smooth surface.

Photoluminescence of neat ChBP showed a structured emission at λ
max
= 380
nm with a vibrational side-band at 413 nm. When ChBP was doped with 6-8% Irppy
and PQIr, the PL spectra showed emissions at λ
max
= 510 and 595 nm respectively
105
corresponding to emissions from Irppy and PQIr respectively. Emissions from the
neat TMPP and tBuTMPP were broad and featureless, with λ
max
= 440 and 458 nm
respectively. The emission spectra of Irppy and PQIr doped films of TMPP and
tBuTMPP were similar to ChBP. The small bumps around 430 nm are due to the
monomer emissions from the pure TMPP and tBuTMPP compounds.
Table 3.1 gives a summary of the photophysical data obtained for all the neat
and doped phthalimide thin films studied in this chapter. Lifetimes of Irppy doped
ChBP, TMPP and tBuTMPP films were measured to be 0.6, 0.2 and 1.0 μs
respectively. These lifetimes were considerably shorter than the CBP doped Irppy
films suggesting emission quenching of Irppy in the ChBP, TMPP and tBuTMPP
matrices. Irppy in its excited state is a sufficiently strong reducing agent and can
readily transfer an electron to the host.
39
Since the reduction potentials of ChBP,
TMPP, and tBuTMPP are larger than the excited state oxidation potential of Irppy, it
is probable that Irppy can get oxidized and decay nonradiatively inside these
matrices. In a host-dopant system, this type of quenching of the dopant is known as
oxidative electron transfer quenching.
40
Figure 3.8 shows the thermochemical cycle
of Irppy constructed from the electrochemistry and photophysics data, which shows
that the reduction potentials of all the phthalimide materials are high enough to be
quenched by excited state Irppy making this class of molecules inefficient hosts for
OLED. Lifetime of PQIr doped tBuTMPP film was same as the CBP film and
showed no quenching of the phosphor. The lifetime of PQIr doped TMPP film
showed a small amount of quenching. Since the reduction potential of TMPP is 0.09
106
eV larger than the excited state reduction potential of PQIr, this quenching could be
due to electron transfer from excited state PQIr to the TMPP host.
Irppy *
Irppy
Irppy
+
E
T
-E(Irppy
+/0
)
0.31 eV
-E(Irppy
+/*
)
-2.12 eV E(TMPP*
0/-
) = -1.62 eV
E(tBuTMPP*
0/-
) = -2.00 eV
E(ChBP*
0/-
) = -1.98 eV
e
-
Irppy *
Irppy
Irppy
+
E
T
-E(Irppy
+/0
)
0.31 eV
-E(Irppy
+/*
)
-2.12 eV E(TMPP*
0/-
) = -1.62 eV
E(tBuTMPP*
0/-
) = -2.00 eV
E(ChBP*
0/-
) = -1.98 eV
e
-

Figure 3.8 Phthalimide reduction potentials (left). Thermochemical cycle for Irppy
constructed from the electrochemistry and photophysics data showing the electron
transfer quenching process (Right). Excited state oxidation potential: E(Irppy
+/*
) =
E
T
- E(Irppy
+/0
).

Table 3.1 Summary of the thin film photophysics data.

E
o
ox
E*
ox
λ
max
τ
Dopant V vs Fc/Fc
+
V vs Fc/Fc
+
Host (nm) ( μs)
CBP 510 2.0
ChBP 507 0.6
TMPP 425, 510 0.2
tBuTMPP 425, 510 1.0
CBP 600 1.1
ChBP 595 0.9
TMPP 425, 600 0.8
tBuTMPP 600 1.1
Eº
ox
: Ground state oxidation potential, E*
ox
: Excited state oxidation potential,
τ = Lifetime
PQIr 0.43 -1.71
Irppy 0.31 -2.12

107
3.3.3 Phthalimide Based Undoped OLEDs
350 400 450 500 550 600 650 700

NPD tBuTMPP AlQ3
NPD tBuTMPP BCP AlQ3
Pl: NPD/tBuTMPP
Wavelength (nm)

NPD TMPP AlQ3
NPD TMPP BCP AlQ3
Normalized Intensity (a.u.)
350 400 450 500 550 600 650 700

NPD AlQ3
NPD BCP AlQ3

NPD ChBP AlQ3
NPD ChBP BCP AlQ3
Pl: NPD/ChBP

Figure 3.9 Electroluminescence (EL) spectra of undoped ChBP, TMPP, and
tBuTMPP devices are shown in comparison with the photoluminescence (PL)
spectra of NPD/ChBP and NPD/tBuTMPP films. The top plot shows the EL of the
standard NPD and Alq
3
devices with and without BCP hole-blocker.

OLEDs were fabricated with ChBP, TMPP, and tBuTMPP materials.
Undoped devices were made initially to understand the behavior of the phthalimides
in OLED. Two sets of devices were made to examine the electron transporting and
108
hole/exciton-blocking ability of the phthalimides. Undoped devices of the following
general structures: 1) ITO/NPD/Phthalimide/Alq
3
/Cathode 2) ITO/ NPD
/Phthalimide/BCP/ Alq
3
/Cathode were investigated and compared with the control
devices of the following structures: 1) ITO/NPD/Alq
3
/Cathode and 2)
ITO/NPD/BCP/Alq
3
/Cathode. Figure 3.9 shows the electroluminescence spectra of
all the undoped phthalimide devices, which were collected between 12-15V to obtain
the maximum resolution with minimum signal-to-noise ratio. The control device
with NPD hole-transporting layer and Alq
3
electron transporting layer showed
characteristic emission of Alq
3
at 520 nm. Because of the hole-blocking ability of
BCP, emission from the control device with BCP hole-blocking layer gave emission
from NPD at 440 nm.
Summary of the maximum brightness, EL λ
max
, and EL quantum efficiencies
of all the devices are presented in table 3.2. The brightness and the EL quantum
efficiency of the unblocked TMPP device were in the same order of magnitude as the
control NPD/Alq
3
device. We believe that the TMPP device behaves like the
NPD/Alq
3
device and the EL from this device originates from Alq
3
. The reason
being, if the TMPP molecules form crystalline films upon deposition, or if the
surface of the film makes an amorphous to crystalline transition quickly after
deposition, then crystalline islands of TMPP could phase segregate on the NPD
surface. As a consequence, Alq
3
could come in direct contact with NPD and
emission from Alq
3
would be observed.

109
Table 3.2 Summary of un-doped and doped device data showing EL spectra,
maximum EL quantum efficiencies (%QE), and maximum brightness.

EL EL efficiency Brightness
Type Host Dopant HBL λ
max
(nm) %QE
(Cd/m
2
)
--- --- --- 520 0.90 1418
--- --- BCP 440 0.91 1260
ChBP --- BCP 515 0.01 0.25
ChBP --- --- 530 0.02 100
TMPP --- --- 512 0.16 1495
TMPP --- BCP 530 0.03 24
tBuTMPP --- --- 537 0.02 4
tBuTMPP --- BCP 520 0.04 16
Test CBP Irppy BCP 512 5.72 16006
1 tBuTMPP Irppy BCP 516 0.04 11
2 CBP Irppy tBuTMPP 510 13.21 3843
Test CBP PQIr BCP 595 5.10 3326
1 tBuTMPP PQIr BCP 595 2.73 518
2 CBP PQIr tBuTMPP 595 8.90 10959
1 TMPP PQIr BCP 590 0.98 8733
2 CBP PQIr TMPP 593 0.43 5509
Undoped devices
Type 1 and Type 2 Doped devices
All the undoped devices are of the NPD/Host/HBL/AlQ
3
structure

EL from the blocked TMPP and all the blocked and unblocked ChBP and
tBuTMPP devices showed very weak and broad emissions in the ranges between
515-540nm. The EL quantum efficiencies of these devices were also an order of
magnitude lower than the standard NPD/Alq
3
and the NPD/BCP/Alq
3
devices
suggesting that these new low energy emissions could not be from the Alq
3

molecules. The low intensities and EL efficiencies of these broad and featureless
emission bands suggested that the emitting species of these devices were probably
exciplexes. This fact was confirmed from the photoluminescence studies of
110
NPD/ChBP and NPD/tBuTMPP films (Figure 3.9), which showed emissions at 530
and 540 nm respectively confirming the exciplex emission. An exciplex is formed
when an electron donating species of one molecule and an electron accepting species
of another molecule form an emitting complex in the excited state.
25
Since the holes
and electrons reside on two different molecules, the total excited state wave function
encompasses the entire emitting complex. Emissions from the exciplexes are usually
observed as broad and featureless bands to the low energy side of the monomer
emission.
37

Exciplex formation between the naphthalimide based blue emitters and N,N’-
bis(3-methylphenyl)-N,N-’diphenylbenzidine (TPD) HTL has been previously
reported by Adachi and coworkers.
41
Authors in this paper reported exciplex
emissions in the ranges between 520-530nm for all the naphthalimide based devices.
The EL spectra of their devices coincided with the PL spectra obtained from the
films and were red-shifted from the monomer emission by 95 nm. The authors
concluded that the exciplexes were formed at the TPD/naphthalimide interfaces and
were very weak in intensity. Hiroshi et al. recently reported exciplex formation
between N-methyphthalimide and phenanthrene.
42
Authors showed that electron
transfer from the relaxed excited state phenanthrene to the N-methyphthalimide
forms a donor-acceptor (D*…A) pair. The radical ion pair D+…A- is then produced
by electron transfer from the D*…A species, which further produces the exciplex
species (D+…A-)*. In our system, NPD is the donor and the phthalimide is the
acceptor and the emissive exciplex is (NPD
+
.phthalimide
-
)*
111
3.3.4 Phthalimide Based Doped Phosphorescent OLEDs

CBP:Dopant(250Å)
Phth (150Å)
ITO
LiF/Al
Phth:Dopant (250Å)
BCP(150Å)
ITO
Type 2 Type 1
AlQ
3
(150Å) AlQ
3
(150Å)
NPD (400Å) NPD (400Å)
LiF/Al
CBP:Dopant(250Å)
Phth (150Å)
ITO
LiF/Al
Phth:Dopant (250Å)
BCP(150Å)
ITO
Type 2 Type 1
AlQ
3
(150Å) AlQ
3
(150Å)
NPD (400Å) NPD (400Å)
LiF/Al

Figure 3.10 Structures of the two types of OLEDs fabricated with the phthalimides

Figure 3.10 shows the architectures of the two types of doped devices that
were fabricated with ChBP, TMPP, and tBuTMPP materials. To investigate the
performance of the phthalimides as hosts, type 1 OLEDs were constructed. The type
2 devices were constructed to evaluate the hole-exciton blocking ability of the
phthalimides in OLEDs. In type 1 OLEDs, 250Å of phthalimides doped with either
Irppy or PQIr phosphors were inserted between the NPD and the BCP layers. The
type 2 devices incorporated 100Å of the phthalimides as hole-exciton blocking layer
placed between the doped CBP layer and the Alq
3
layer. Data for all the TMPP,
ChBP, and tBuTMPP based type 1 and type 2 devices are provided in appendix B.
Table 3.2 summarizes all the device data for the undoped and the doped type 1 and
type 2 devices. All of the type 1 devices performed poorly compared to the test
112
devices. The EL efficiencies of these devices were less than 1% confirming the fact
that the phthalimides are not good hosts for Irppy. The efficiencies of the type 1
PQIr devices were worse than the test devices but better than the type 1 Irppy
devices. The reason for this is that in the type 1 Irppy devices two processes
simultaneously contribute to the lowering of device efficiencies; 1) Exciplex
formation and 2) quenching of the Irppy emission. In the type 1 PQIr devices
however, only the exciplex formation causes the efficiency to go down.
The first set of the type 2 devices were made with the low energy PQIr
emitter to eliminate any energy transfer related problems. All the devices gave
emission from the dopant; however the efficiencies of the ChBP and TMPP devices
were not as high as the tByTMPP devices. Superior performance of tBuTMPP can
be attributed to its high quality film forming characteristics. Data for the type 2
tBuTMPP devices with Irppy and PQIr phosphorescent emitters are shown in figure
3.11. The EL spectra of these devices showed emissions at 510 nm and 595 nm
respectively. External quantum efficiencies (%QE) of the Irppy and PQIr devices
were 13.2 % and 8.2 % respectively. I-V shape and the turn on voltage of the type 2
device were comparable to the control devices. The turn-on voltages (Turn-on
voltage is the voltage at which the brightness is 1Cd/m
2
) of both of the devices were
2.7V approximately. The devices with tBuTMPP hole/exciton-blocker were more
efficient than the control devices made with BCP hole/exciton-blocker. The hole-
blocking ability of tBuTMPP is the same as BCP because the HOMO energies of
tBuTMPP and BCP are the same. However, since the triplet energy of tBuTMPP
113
(2.9 eV) is higher than the triplet energy of BCP (2.6 eV), the molecule performs
better as an exciton blocker than BCP.
400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0

Intensity (A.U.)
Wavelength (nm)
Irppy: Type 2
PQIr: Type 2
Irppy: Control
PQIr: Control
1E-3 0.01 0.1 1 10 100 1000
1E-3
0.01
0.1
1
10
100
tBuTMPP/Irppy: Type 2
tBuTMPP/PQIr: Type 2 Quantum Efficiency, %
Current Density, mA/cm
2
02 4 6 8 10 12
1E-3
0.01
0.1
1
10
100
1000
10000
100000
tBuTMPP/Irppy: Type 2
tBuTMPP.PQIr: Type 2
Brightness, Cd/m
2
Voltage, V
0.1 1 10
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
1000
tBuTMPP/Irppy: Type 2
tBuTMPP/PQIr: Type 2
Current Density, mA/cm
2
Voltage, V

Figure 3.11 Data for the tBuTMPP based type 2 devices made with Irppy and PQIr
phosphors showing quantum efficiency (%QE) vs. current density (mA/cm
2
),
brightness (Cd/m
2
) vs. voltage (V), current density (mA/cm
2
) vs. voltage (V), and
Intensity (a.u.) vs. wavelength (nm) plots.

An efficient hole-blocker can balance the hole-electron recombination by
preventing the diffusion of the holes to the ETL and there by increase the device
efficiency.
16
An efficient exciton blocker however, can increase the device
efficiency by preventing the excitons from migrating to the cathode and decay non-
radiatively. Excitons do not stay spatially confined in the emissive layer after they
114
are generated. Three degrees of freedom allow the excitons to move indiscriminately
in any direction till they decay radiatively or non-radiatively.
38, 43
Excitons that
radiatively decay, generate photons and exit through the front of the devices and get
collected or get waveguided and loose energy as heat.
16, 44
Excitons that diffuse
through the ETL layer, either produce photons or migrate to the cathode and couple
with the surface plasmons of the metal electrode and produce heat.
45, 46
High triplet
energy exciton blocking materials like phthalimides can prevent such loss of
efficiency by preventing the diffusion of the excitons towards the cathode. The
devices with the tBuTMPP hole/exciton blockers were more efficient than the BCP
blockers because the high triplet energy tBuTMPP is a better exciton blocker than
the BCP.

3.4 Chapter 3 Conclusion
Thin film photophysics and device characteristics of various phthalimides
were investigated. Four different phthalimide derivatives; NPP, ChBP, TMPP, and
tBuTMPP were examined in detail. Studies showed that all the phthalimides have
high triplet energies and are thermally stable at high temperatures. The glass
transition temperature (T
g
) of tBuTMPP was well above the room temperature and
vapor deposited film of the material was amorphous. Photoluminescence spectra of
the phthalimides showed emission from the Irppy and PQIr dopants with shorter
lifetimes due to oxidative electron transfer quenching of the dopants. Undoped
devices of the structures NPD/tBuTMPP/Alq
3
and NPD/tBuTMPP/BCP/Alq
3
gave
115
very weak long wavelength emissions at 515 nm. These emissions were not
characteristic of the tBuTMPP monomer emission suggesting exciplex emission
from the (NPD
+
.phthalimide
-
)* species. Both the PL and the EL data suggested that
the phthalimides are inefficient hosts for NPD hole-transporter and Irppy emitter and
the fact was confirmed from the type 1 devices, which gave very dim and low
efficiency devices. Extremely efficient devices were obtained from the type 2 sets.
Irppy doped tBuTMPP and PQIr doped tBuTMPP devices were bright and more
efficient than the control devices. External quantum efficiencies of these devices
were 13.2% and 8.2% respectively. The phthalimides are better exciton blockers
than the most commonly used BCP hole/exciton blocker. The phthalimides have
high triplet energies than the BCP, which allow these molecules to prevent the
diffusion of the dopant excitons to the cathode and loose energy to the surrounding
as heat. This process increases the device efficiency considerably by harvesting
more excitons.
116
3.5 Chapter 3 References
1. Shizuo Tokito, H. T., Akane Okada, and Yasunori Taga, High-temperature
operation of an electroluminescent device fabricated using a novel triphenylamine
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121
Chapter 4. Computational Studies of Spectroscopic and
Photophysical Properties of (Dmappy)-Pt-(acac)

4.1 Introduction
Intramolecular charge transfer in the excited state occurs through the
separation of electronic charges caused by the shift in electron density from a donor
(D) part of the molecule to an acceptor (A) part of the molecule. Such internal
charge transfer leads to the charge transfer (CT) excited states with positive and
negative charges localized on different parts of the molecule. The change in charge
distribution leads to an increase in the magnitude and sometimes in the direction of
the excited state dipole moment as well as a change in the electronic structure and
the geometry of the molecule.
The N,N-dimethylaniline with an acceptor group in the para position serves
as an excellent example of a system that forms CT excited state. The most studied
molecule in this class is the 4-(dimethylamino)benzonitrile (DMABN), in which the
roles of a donor and an acceptor are played by the para substituted dimethylamino
(dma) and the nitrile (CN) moieties respectively.
1-5
Twisting of the dma group
around the benzene ring in this molecule creates an intramolecular charge transfer
(ICT) excited state, which upon relaxation exhibits dual fluorescence. When the
nitrogen lone pairs and the benzene π system are mutually conjugated ICT state is
observed; and when they are orbitally decoupled no ICT state is observed.
2

122

N C N
Wagging
angle, β
Twist
angle, α
CH
3
NCH
3
plane
Benzene plane
(a) (b)
N(CH
3
)
2
C
N
+
-

Figure 4.1 Emitting intramolecular charge transfer (ICT) species of DMABN are
shown. (a) TICT: when the dihedral angle α is 90°, PICT: when the dihedral angle α
is 0°, WICT: when the wagging angle β changes the hybridization of dimethylamino
moiety to sp
3
. (b) RICT: when the cyano carbon bends to sp
2
.

Four different models have been proposed to explain the molecular and
electronic structures of the emitting ICT species of the N,N-dimethylaniline
derivatives (Figure 4.1).
1-12
The first one was proposed by Garbowski et al
13
and it is
called the twisted intramolecular charge transfer (TICT) model. In this model the
emitting species is assumed to have a twisted geometry, with the dihedral angle
between the dma group and the benzene ring being close to 90°.
2, 5, 7
The second one
is the planar intramolecular charge transfer (PICT) model proposed by Zachariasse et
al.
9
, in which the dma group lies in the benzene plane with the dihedral angle being
close to 0°.
4, 5, 7
The third one is called the wagged intramolecular charge transfer
(WICT) model also proposed by Zachariasse and involves the wagging of the dma
group, which causes the hybridization of the dma nitrogen to change from the planar
sp
2
hybridized structure to the pyramidal sp
3
hybridized structure.
2, 5, 7, 14
The final
one is the rehybridized intramolecular charge transfer (RICT) model suggested by
123
Sobolewski et al.
5, 15-17
, which involves the rehybridization of the linear cyano carbon
atom from sp hybridized structure to the bent sp
2
hybridized structure.
7

To date numerous theoretical and experimental studies on various N,N-
dimethylaniline derivatives have been performed to understand their dual
fluorescence properties. Studies of N,N-dimethylaniline derivatives of heavy metal
compounds however have not been performed to a great extent. These materials, due
to heavy atom effect can readily undergo spin orbit coupling (SOC) and emit
efficiently from the triplet state. The (N,N-dimethylamino-phenylpyridine)-Pt-(acac)
or (dmappy)-Pt-(acac) is one such compound which is of considerable interest
because of its dual phosphorescence properties. This molecule is a low triplet energy
broad band emitter that shows phosphorescence between the blue to the red region of
the visible spectrum and can be potentially used as a dopant to make white OLEDs.
In room temperature fluid solutions and at 77K glass, both the ground and the
excited state electronic structures of (dmappy)-Pt-(acac) vary as a function of the
solvent polarity and the spatial orientation of the dimethylamino (dma) moiety. The
lone-pair electrons on the nitrogen atom of the rotating dma moiety interacts with the
benzene π system and in turn influence the overall photophysical properties of the
molecule. When the CNCC dihedral angle becomes 0°, the dma group becomes
coplanar with respect to the phenylpyridine (ppy) ligand and electron density from
the p orbital of the nitrogen atom shifts to the π system of the benzene ring (Figure
4.2). The increased p-π interaction then lowers the triplet energy of the molecule.
Alternatively, when the dma moiety rotates perpendicular to the plane of the
124
molecule (CNCC dihedral angle 90°), the p-π interaction diminishes and the triplet
energy of the molecule goes up.
No interaction p-π interaction
N
N
O
O
O
O
Pt
N
N
Pt

Figure 4.2 Dimethylamino group with two extreme orientations are shown. (left)
Coplanar dma group with 0° CNCC dihedral angle showing increased p-π
interaction. (right) Perpendicular dma group with 90° CNCC dihedral angle showing
no p-π interaction.

The nature and the magnitude of the electronic coupling between the donor
dma group and the acceptor ppy ligand in (dmappy)-Pt-(acac) requires some
clarification. It is assumed that like DMABN the (dmappy)-Pt-(acac) is orbitally
coupled in the planar conformation and the molecule emits from low energy PICT
state because of strong molecular orbital (MO) interactions between the dma moiety
and the ppy ligand. Furthermore, in the twisted conformation the system becomes
orbitally decoupled and the molecule emits from high energy TICT state because of
decreased MO overlap between the dma moiety and the ppy ligand. The nature of
the TICT state in (dmappy)-Pt-(acac) is however a little bit more complicated than
the DMABN. Due to lack of symmetry, the rotation of the dma group around ppy
ligand yields three unique structural isomers in (dmappy)-Pt-(acac) molecule (Figure
4.3). One is the trigonal planar structure with sp
2
hybridization, which occurs at 0°
125
CNCC dihedral angle. When the dma group rotates perpendicular to the plane of the
molecule, hybridization of the dimethylamino nitrogen changes to sp
3
and two
separate trigonal pyramidal structures are formed; one with the lone-pair electrons
facing the pyridine ring and the other with the lone-pair electrons facing away from

Trigonal Pyramidal
Lone Pair 0º
sp
3
Trigonal Planar
Lone Pair 90º
sp
2
Trigonal Pyramidal
Lone Pair 0º
sp
3
N
Pt
O
O
O
O
O
O
N
N
Pt
N
N
Pt
N
R
1
R
2
N
R
1
R
2
N
R
1
R
2
R
1
N
R
2
(b)
(c) (a)

Figure 4.3 The three structural isomers of (dmappy)Pt(acac) are shown. (a) When
the p orbital on the nitrogen atom rotates to the orthogonal position with respect to
the ppy plane, the geometry of the dma group becomes trigonal planar (sp
2
). When
the lone-pair-electrons on the nitrogen atom become coplanar with respect to the ppy
plane, the geometry of the dma group becomes trigonal pyramidal (sp
3
). (b) Lone-
pair facing up. (c) Lone-pair facing down.

the pyridine ring. In this chapter both the ground state and the excited state
electronic structures of the three isomers of (dmappy)-Pt-(acac) will be investigated
126
with the density functional theory (DFT) method. Photophysical properties in the
gas phase and in solvent dielectrics will be studied with Time-dependent (TD) DFT
method. Since the molecule emits at different wavelengths in solvents with different
polarities, TDDFT will be used to understand the spectral transitions and structural
orientations of the molecule in room temperature fluid solution, low temperature
glass, and solid matrices.

4.2 Experimental

4.2.1 Parameterization

The geometry of the dma moiety with respect to the ppy plane and partial
atom numbering of the dma-phenyl section are shown in figure 4.4. The angles θ
1

and θ
2
are the individual dihedral angles created by the R
1
and the R
2
methyl groups.
The lone-pair angle (φ) is defined as the angle between the ppy plane and the orbital
of the lone-pair electrons created by the lone-pair-electron vector as the dma group
precesses around the ppy plane. The bisector angle, α can be calculated from the
angle ∠R
1
NR
2
(Equation 4.1). The lone-pair angle, φ can be calculated from α
through Equation 4.2. The pyramid cone angle (δ) is defined by the sum of the three
angles that defines the cone of the pyramid and should equal to 360° when the
hybridization is sp
2
(Equation 4.3).
127
α
θ θ
θ θ
=
+
= ∠
+ = ∠
2 2
2 1 2 1
2 1 2 1
NR R
NR R
(4.1)
⎩
⎨
⎧
− =
− =
α θ φ
θ α φ
2
1
(4.2)
40 36 40 12 36 12
NC C NC C NC C ∠ + ∠ + ∠ = δ (4.3)
(b)
(c) (a)
N
R
2
R
1
φ
θ
2
θ
1
α
α
ppy plane
N
R
1
R
2
φ
δ C
12
N
C
40
C
36
C
16
C
13
C
11
C
14
C
15

Figure 4.4 (a) The direction of the lone-pair-electron-vector on the dma moiety is
shown with respect to the ppy plane, the CNCC dihedral angles θ
1
and θ
2
, and the
bisector angle, α. (b) The precessing lone-pair angle, φ. (c) The pyramid cone
angle, δ.

4.2.2 Computational Details.
All preliminary geometry optimizations were performed at PM3 level of
theory using the Titan software package.
18
All quantum mechanical calculations
were performed using the Gaussian-03 software package.
19
Electronic structure
calculations of the singlet ground states (SGS) and the triplet excited states (TES)
128
were performed using the density functional theory (DFT) method. Potential energy
surface (PES) scans of the singlet ground states (SGS) and the triplet excited states
were also calculated using the DFT method. For all calculations the B3LYP
functional consisting of Becke’s three-parameter equation
20
and Lee, Yang, and
Parr’s non-local hybrid functional
21
was used with the LANL2DZ basis set, which
uses the Dunning-Hay split valence double-ζ for C,H,N atoms (D95) and Hay-Wadt
double-ζ with Los Alamos National Laboratories relativistic effective core potential
(ECP) for heavy atoms.
22-25

The lowest SGS structure was obtained by optimizing the geometry in the gas
phase without any dihedral constraints. The SGS geometries for the structures with
dihedral angles from 0-180° at every 15° increments were obtained by specifying the
dihedral angles in the Z-matrix and freezing them to a particular value during
geometry optimization. PES scans for the SGS structures were obtained in the gas
phase by rotating the dma group at 5° increments. The lowest TES structures were
calculated at the unrestricted DFT level with a spin multiplicity of 3
(uB3LYP/LANL2DZ). Geometries for the structures with dihedral angles from 0-
180° and the triplet state PES scans were obtained as described before.
Time-dependent (TD) DFT calculations were performed on the gas phase
SGS geometries. Total of ten singlet and ten triplet states were calculated for each
structure. Singlet and triplet vertical transition energies (E
VT
) were calculated in the
gas phase and in solvent shells. Solvent calculations were performed in THF,
acetonitrile, toluene, and methanol using the conductor-like polarizable continuum
129
model (CPCM)
26
in conjunction with TDDFT routine. All Gaussian calculations
were performed at the high performance computing facility (HPCC) of University of
Southern California (USC).
27

4.2 Results and Discussion
In the following sections the results of the DFT and TDDFT calculations will
be discussed and compared with the absorption and emission data. The electronic
structures of the SGS and TES geometries of all the three structural isomers will be
analyzed in terms of bond lengths, dihedral angles, orbital energies, and orbital
populations. The nature of the low-lying singlet and triplet excited states will be
explored with TDDFT method. The vertical excitation energies derived from the
TDDFT method will be used to assign the peaks of the absorption spectra.

4.3.1 Singlet ground state (SGS) geometries and electronic structures.
The calculations of the singlet ground state (SGS) geometries and ground
state potential energy surface (PES) scans were performed using DFT method
consisting of B3LYP/LANL2DZ parameters. Optimized geometries of all three
structural isomers of (dmappy)-Pt-(acac) had C
1
symmetry (Figure 4.5). The lowest
energy structure was calculated to be trigonal planar. In Table 4.1 the calculated C-
N bond distances and angles of the dma moiety and the metal-ligand bond distances
of the coordination sphere are reported in comparison with the X-ray
28
data. The C-
N bond lengths of the dma moiety of the trigonal planar isomer compared well with
130
the crystal data. The C
12
-N bond distance of this isomer was calculated to be almost
the same as the experimental C
12
-N bond distance, which was also found to be 0.05Å
shorter than the other two isomers suggesting a double bond type character in the
trigonal planar form. The C
36
-N and C
40
-N bond distances of the planar isomer were
0.03Å and 0.04Å longer than the experimental values and 0.01Å shorter than the
other two isomers. The pyramid cone angle (δ) of the trigonal planar isomer was
found to be 360°, which was close to the value of δ obtained from the crystal data
(357°) suggesting the fact that the dimethylamino group in the relaxed ground state
exists in the sp
2
-hybridized trigonal planar form. The δ of the trigonal pyramidal
isomers were approximately 342° suggesting sp
3
-hybridized state. The bond lengths
and angles of the dma moiety of the three isomers were further analyzed with PES
results and will be discussed in detail in the subsequent sections.
The calculated Pt-N and Pt-C bond lengths of the trigonal planar isomer were
found to be 0.02Å longer than the experimental value. The Pt-O
20
and Pt-O
21
bond
lengths were approximately 0.04Å and 0.08Å longer than the experimental values.
Deviations from the trigonal planar isomer to the two trigonal pyramidal isomers
were insignificant. This fact suggests that rotation of the dma moiety has none to
negligible effect on the coordination sphere.

131
(a) (b) (c)

Figure 4.5 Optimized SGS structures of the three isomers of (dmappy)-Pt-(acac).
(a) Dimethylamino trigonal planar (CNCC dihedral angle = 0°). (b) Dimethylamino
trigonal pyramidal with the nitrogen lone-pair facing up (CNCC dihedral angle =
90°). (c) Dimethylamino trigonal pyramidal with the nitrogen lone-pair facing down
(CNCC dihedral angle = 90°)

Table 4.1. Comparison of the calculated bond lengths and angles of the three
structural isomers of (dmappy)-Pt-(acac) with corresponding X-ray data.

Bond type Trigonal planar Trigonnal pyramidal Trigonnal pyramidal X-Ray
Lone-pair up Lone-pair down
Pt-N 2.012 2.014 2.013 1.994
Pt-C 1.992 1.990 1.991 1.968
Pt-O
20 2.047 2.046 2.046 2.008
Pt-O
21 2.149 2.144 2.146 2.071
C
12
-N 1.405 1.454 1.454 1.406
C
36
-N 1.464 1.476 1.475 1.434
C
40
-N 1.462 1.477 1.476 1.413
Angle type
C
12
NC
36 120.36 114.95 115.21 121.61
C
12
NC
40 119.89 114.15 114.3 117.65
C
36
NC
40 119.75 113.1 113.21 117.4
Bond distances (Å)
Angles in degrees

132
The highest occupied molecular orbitals (HOMO) and the lowest unoccupied
molecular orbitals (LUMO) of the three structural isomers were obtained from the
SGS geometries. In Figure 4.6 the pictorial description of the HO and LU MOs of
the three isomers along with the orbital framework making percent contribution from
each atom to the occupied and virtual orbitals are depicted. Percent contribution
from the platinum metal, the pyridine ring, the phenyl ring, the dma moiety, and the
acac group to their respective HO and LU MOs are reported in Table 4.2. In each of
the three complexes the three HO molecular orbitals are mostly composed of Pt,
phenyl, and dma occupied MOs and the LU molecular orbitals are composed of
mainly pyridine and some phenyl virtual MOs. In the trigonal planar isomer, major
contribution to the HOMO comes from the platinum d orbital (16%), phenyl π
orbitals (40%), and the nitrogen Pz orbital (30%). Contribution to the LUMO comes
from pyridine π∗ orbitals (75%), phenyl π∗ orbitals (16%) and some platinum d*
orbital (5%).
For the trigonal pyramidal isomers, the overall makeup of the HOMO and the
LUMO in terms of percent contribution remains the same. However, the MO
composition of the dma moiety changes a lot. In the pyramidal isomers percent
contribution from the p orbitals of the dma nitrogen atom switches from all p
z
to a
mixture of p
x
, p
y
, and p
z
orbitals. In each case major contribution comes from the p
y

and p
x
orbitals suggesting decreased MO interaction with the phenyl π system. This
switch of the LUMO character going from the planar to the pyramidal structures
133
plays an important role in the nature of the electronic excited states of (dmappy)-Pt-
(acac) and will be discussed in the next sections.
HOMO LUMO HOMO LUMO
N
Pt 4.73
4.69
0.18
6.09
0.50
3.69
1.24
17.9
0.19
22.4
10.9
3.12
20.9
O
O
0.94
0.29
0.39
1.51
0.01
N
0.07
0.03
0.03
0.03
0.01
N
Pt 16.1
9.07
6.05
6.54
3.03
10.8
4.56
0.10
0.82
0.02
0.88
0.02
1.1
O
0.69
0.43
0.65
0.19
0.58
N
30.03
0.12
0.15
0.03
0.01
O
N
Pt 4.78
4.40
0.43
6.70
0.46
4.07
1.37
17.7
0.14
21.9
10.9
2.99
20.6
O
O
0.88
0.23
0.40
1.38
0.01
N
0.05
0.12
0.13
0.03
0.01
N
Pt
O
N
14.0
6.94
7.84
5.03
3.67
6.3
4.04
0.16
0.51
0.03
0.55
0.07
0.65
O
0.93
0.27
0.86
0.21
0.63
35.0
2.07
0.58
0.03
0.01
N
Pt
17.2
5.34
9.04
4.88
3.63
5.93
5.61
0.96
0.82
0.06
0.85
0.16
0.97
O
O
1.39
0.29
1.29
0.30
0.85
N
29.3
2.38
0.54
0.04
0.01
N
Pt
4.84
4.56
0.44
6.46
0.46
4.27
1.52
17.7
0.17
21.9
10.6
3.15
20.7
O
O
0.83
0.20
0.39
1.28
0.01
N
0.04
0.04
0.07
0.03
0.01
N
Pt 4.73
4.69
0.18
6.09
0.50
3.69
1.24
17.9
0.19
22.4
10.9
3.12
20.9
O
O
0.94
0.29
0.39
1.51
0.01
N
0.07
0.03
0.03
0.03
0.01
N
Pt
O
16.1
9.07
6.05
6.54
3.03
10.8
4.56
0.10
0.82
0.02
0.88
0.02
1.1
O
0.69
0.43
0.65
0.19
0.58
N
30.03
0.12
0.15
0.03
0.01
N
Pt 4.78
4.40
0.43
6.70
0.46
4.07
1.37
17.7
0.14
21.9
10.9
2.99
20.6
O
O
0.88
0.23
0.40
1.38
0.01
N
0.05
0.12
0.13
0.03
0.01
N
Pt
O
N
14.0
6.94
7.84
5.03
3.67
6.3
4.04
0.16
0.51
0.03
0.55
0.07
0.65
O
0.93
0.27
0.86
0.21
0.63
35.0
2.07
0.58
0.03
0.01
N
Pt
17.2
5.34
9.04
4.88
3.63
5.93
5.61
0.96
0.82
0.06
0.85
0.16
0.97
O
O
1.39
0.29
1.29
0.30
0.85
N
29.3
2.38
0.54
0.04
0.01
N
Pt
4.84
4.56
0.44
6.46
0.46
4.27
1.52
17.7
0.17
21.9
10.6
3.15
20.7
O
O
0.83
0.20
0.39
1.28
0.01
N
0.04
0.04
0.07
0.03
0.01

Figure 4.6 HOMO and LUMO orbital diagrams of the three structural isomers of
(dmappy)-Pt-(acac) are shown along with orbital characteristics making contribution
to the HOMO and the LUMO as percent contribution from each atom. (Top)
Trigonal planar (CNCC = 0°). (Middle) Trigonal pyramidal with the nitrogen lone-
pair facing up (CNCC = 90°). (Bottom) Trigonal pyramidal with the nitrogen lone-
pair facing down (CNCC = 90°).

.
134
Table 4.2. Highest occupied and lowest virtual molecular orbitals of the three
isomers of (dmappy)-Pt-(acac) are shown with % MO character from various
functionalities making contributions to the respective HOMO and LUMO.

Functionalities % Character of MO Designation
HOMO LUMO
Trigonal planar (CNCC = 0°)
Pt 16.1% (d) 4.7% (d*) d --> d*
Pyridine 2.9% (π) 75.2% (π*) π --> π*
Phenyl 40.1% (π) 16.4% (π*) π --> π*
dma 30.3% (100% nitrogen Pz) 0.1% (100% nitrogen Pz)
acac 2.6% 3.2%
Trigonal pyramidal (Up CNCC = 90°)
Pt 14% (d) 4.8% (d*) d --> d*
Pyridine 2% (π) 74.2% (π*) π --> π*
Phenyl 33.8% (π) 17.4% (π*) π --> π*
dma 37.6% (nitrogen Px (8%),
Py (67%), Pz (17%))
0.3% (nitrogen Px (16%),
Py (70%), Pz (8%))
acac 2.9% 2.9%
Trigonal pyramidal (Down CNCC = 90°)
Pt 17.2% (d) 4.8% (d*) d --> d*
Pyridine 3.8% (π) 74.2% (π*) π --> π*
Phenyl 34.4% (π) 17.7% (π*) π --> π*
dma 32.3% (nitrogen Px (1%),
Py (81%), Pz (10%))
0.2% (nitrogen Px (18%),
Py (24%), Pz (53%))
acac 4.2% 2.7%

4.3.2 Ground state PES scan
The potential energy surface (PES) scans were conducted on the trigonal
planar isomer by rotating the dimethylamino moiety 360° with 15° increments. In
order to eliminate any computational artifacts that might be caused as a result of
angular constraint during rotation, two 180° scans were performed with each
methylamino group and compared with each other for consistency. For each
methylamino group, scans with 5° increments were performed by rotating the CNCC
dihedral angle once in the +180° direction and once in the -180° direction. The
135
results of total four ground state PES scans of the (dmappy)-Pt-(acac) are depicted in
Figure 4.7. Summary of the PES results are reported in Table 4.3.
-60 0 60 120 180 240 300 360
-2
-1
0
1
2
3
4
5
6
7
Energy (kcal/mol)
Dihedral angle (θ)
PES scan of C36
N
P
N
P
N
P
N
P
N
P
N
P
N
P
N
P
N
P
-90 0 90 180 270
-2
-1
0
1
2
3
4
5
6
7
Energy (kcal/mol)
Lone-pair angle (φ)
PES scan of C36
N
P
N
P
N
P
N
P
N
P
N
P
N
P
N
P
N
P
-60 0 60 120 180 240 300 360
-2
-1
0
1
2
3
4
5
6
7
Energy (kcal/mol)
Dihedral angle (θ)
PES scan of C40
N
P
N
P
N
P
N
P
N
P
N
P
N
P
N
P
N
P
-90 0 90 180 270
-2
-1
0
1
2
3
4
5
6
7
Energy (kcal/mol)
Lone-pair angle (φ)
PES scan of C40
N
P
N
P
N
P
N
P
N
P
N
P
N
P
N
P
N
P

Figure 4.7 PES scan results of ground state (dmappy)-Pt-(acac) with the direction of
the lone-pair electrons are shown. (top) Scan results of C36 rotated from -180° to 0°
and 0°to +180°. (bottom) Scan results of C40 rotated from -180° to 0° and 0°to
+180°.

The potential energy surface scans of the C
36
NC
12
C
16
dihedral angle (θ) is
shown on the top left and the C
40
NC
12
C
16
dihedral angle (θ) is shown on bottom left
136
of the diagram. The corresponding plots of the surfaces plotted against the lone-pair
angle (φ) are shown on the right side of the diagram. The two peaks at
105°(θ)/40°(φ) and 285°(θ)/170°(φ) positions correspond to the highest energy states
where the dma moiety becomes sp
3
hybridized and the lone-pair electrons either face
up or down. The three valleys correspond to the lowest energy structures which
occur when the dihedral/lone-pair angles rotate to 0°(θ)/-90°(φ), 180°(θ)/+90°(φ),
and 360°(θ)/270°(φ) positions. Barrier to rotation was calculated to be 4.25
kcal/mol, which matched well with the published data of 4.44 kcal/mol for N,N-
dimethylaniline.
29
In addition to the three global minimums, two shallow minimums
were also observed at 40°(θ)/-71°(φ) and 215°(θ)/108°(φ) positions. The snapshots
of the orientation of the dma group for every 15° intervals are shown in Figure 4.8.
The red arrows correspond to the direction vector of the lone-pair electrons.
Four inversion centers were also observed on the potential energy surface at
135°(θ)/68°(φ), 140°(θ)/72°(φ), 315°(θ)/207°(φ), and 320°(θ)/246°(φ) angles. These
are the inversion of the dimethylamino group and are characteristic of umbrella type
inversion, which occur at four different points during rotation. In room temperature
fluid solutions the inversion centers can rapidly equilibrate between one another
during rotation. If the barrier to rotation is higher than the inversion barrier then
inversion would be predominant at any time. Inversion of N,N-dimethylaniline has
been studied with microwave spectroscopy and the barrier for inversion for this
molecule has been reported to be as low as 0.2-0.7 kcal/mol.
30
Recently Novikov
29

et al. published a paper on electron diffraction studies and DFT (B3LYP/ccpVTZ)
137
calculations of N,N-dimethylaniline and reported inversion of the amino center to be
as low as 0.091 kcal/mol.

0° 15° 30° 45° 60° 75° 90° 105° 120°
135° 150° 165° 180° 195° 210° 225° 240°
255° 270° 285° 300° 315° 330° 345° 360°
0° 15° 30° 45° 60° 75° 90° 105° 120°
135° 150° 165° 180° 195° 210° 225° 240°
255° 270° 285° 300° 315° 330° 345° 360°

Figure 4.8 Snapshots of the side view of (dmappy)-Pt-(acac) with pyridine ring on
top and phenyl on the bottom taken at each step of the PES scan showing the
orientation of the dma moiety with respect to the changing CNCC dihedral angle.
The red arrows indicate the direction of the lone-pair electron at each step of the
scan.
138
The (dmappy-Pt-(acac) is a much larger and more complex system than N,N-
dimethylaniline and has multiple inversion centers that can stay in equilibrium at any
time. Figure 4.9 shows the four inversion centers of (dmappy-Pt-(acac), where the
vectors indicate the direction of the lone-pair electrons for each inversion center.
Going from (a) to be (b) the inversion barrier was calculated to be approximately 2.2
kcal/mol. This occurs when the dma inverts and rotates to 72° from 68°. The
inversion from (a) to (c) occurs when the dma flips from 68° to 207°. Barrier to this
inversion is only 0.39 kcal/mol. Going from (b) to (d) the dma flips between 72° to
246°. The barrier to this inversion is only 0.10 kcal/mol. The barrier between (c)
and (d) was calculated to be 2.5 kcal/mol, which occurs when the dma inverts to 72°
and then rotates to 207°.
The geometry of the dimethylamino moiety was also probed as a function of
CNCC rotation. For this purpose the data from the 360° (15° increments) PES scan
were further analyzed in detail. In Figure 4.10 the plot of the pyramid cone angle, δ
vs. φ and C-N bond lengths vs. φ are shown. In the δ vs. φ plot, the two minimums at
0° and 180° correspond to the pyramid cone angle being less than 360° and the three
maximums at -90°, +90°, and +270° correspond to the pyramid cone angle being
almost 360°. This data clearly suggest that during rotation the dma group goes from
being sp
2
hybridized trigonal planar form to the sp
3
hybridized trigonal pyramidal
form every 90° intervals.

139
68º 72º
207º 246º
Inversion + rotation
Inversion
Inversion + rotation
Inversion
(a) (b)
(c) (d)
2.2 kcal/mol
2.5 kcal/mol
0.39 kcal/mol
0.10 kcal/mol
68º 72º
207º 246º
Inversion + rotation
Inversion
Inversion + rotation
Inversion
(a) (b)
(c) (d)
2.2 kcal/mol
2.5 kcal/mol
0.39 kcal/mol
0.10 kcal/mol

Figure 4.9 Four ground state inversion centers of (dmappy)-Pt-(acac) showing lone-
pair angles and the energies for the inversion barriers at each point.

The plot of the C-N bond lengths vs. the lone-pair angle also corroborated
well with these results. The C
12
-N bond distance changed from 1.40Å at -90°, +90°,
and +270° to 1.45Å at 0° and 180°. This 0.05Å increase in bond length going from
the trigonal planar form to the pyramidal form suggests that in the trigonal planar
form due to increased MO overlap (higher p-π conjugation), the C
12
-N bond distance
decreases and becomes more double bond in character. As the dma moiety rotates to
the trigonal pyramidal form, the MO over lap decreases and the bond distance
increases giving rise to a more single bond type character. The two methylamino C-
140
N bonds also showed the same trend. However, the variations in bond lengths in
these bonds were approximately 0.02 Å.

-90 0 90 180 270
1.40
1.42
1.44
1.46
1.48
1.50
Bond Length, Α
Lone-pair Angle, φ
C12-N
C36-N
C40-N
-90 0 90 180 270
340
344
348
352
356
360
Ph N
Ph N
Ph N
Cone Angle, δ
Lone-pair Angle, φ

Figure 4.10 (left) A plot of the pyramid cone angle, δ vs. the lone-pair angle, φ.
(right) A plot of the three C-N bond lengths vs. the lone-pair angle, φ.

Table 4.3 Ground state PES result showing the SCF energies and the dipole
moments of (dmappy)-Pt-(acac) as a function of φ and δ.

PES Lone-pair dihedral SCF Dipole
Region angle (φ) angle (θ) SCF (kcal/mol) Moment (D)
Planar -89.67 0 0.00
-81.81 15 0.21
-73.81 30 0.88
-62.21 45 1.52
-50.10 60 1.97
-37.22 75 2.84
-23.35 90 3.73
Pyramidal (up) -8.46 105 4.20 2.03
7.29 120 4.20 1.91
Inversion center 25.82 135 3.65 2.11
Inversion center 74.10 150 0.82 2.08
83.14 165 0.21 2.44
Planar 90.03 180 0.03 2.47
96.68 195 0.22 2.53
106.03 210 0.85 2.55
141
Table 4.3 Continued
117.35 225 1.29 2.62
129.50 240 1.88 2.74
142.58 255 2.88 2.98
156.63 270 3.97 3.31
Pyramidal (down) 171.84 285 4.67 3.56
188.35 300 4.66 3.70
Inversion center 208.41 315 3.77 3.43
Inversion center 253.88 330 0.88 2.84
261.73 345 0.20
Planar 270.00 360 0.01

4.3.2.1 HOMO LUMO energies
All the HOMO and LUMO energies were obtained form the single-point
energy calculations of the optimized coordinates obtained from the ground state PES
scan data. In order to obtain the orbital energies of the trigonal planar isomer, the
two trigonal pyramidal isomer, and all four inversion centers, coordinates of the
geometries were chosen with the dihedral and lone-pair angles starting from 90°(θ)/-
23°(φ) up to 330°(θ)/+254°(φ). All the results of the single-point calculations are
summarized In Table 4.4.
In Figure 4.11 the plot of HOMO-LUMO energies and optical gap (HOMO-
LUMO gap) vs. lone-pair angle of the structures from the single-point energy
calculations is shown. Change in HOMO was found to be larger than the change in
LUMO. This data is consistent with HOMO-LUMO orbital pictures shown in Figure
4.6, which clearly shows that LUMOs remain almost unchanged and only the
HOMOs change with rotation of the dma group. The change in HOMO is more
142
predominant because of electron donating ability of the dma moiety. When φ
becomes 90° increased MO overlap destabilizes the HOMO, which decreases the
optical gap and the singlet energy of the molecule decreases. Alternatively, when the
φ rotates to 0° or 180°, decreased MO overlap stabilizes HOMO, which then
increases the optical gap and increases the singlet energy of the molecule.

-30 0 30 60 90 120 150 180 210 240
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-30 0 30 60 90 120 150 180 210 240
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
LUMO
HOMO
HOMO-LUMO Energies (eV)
Lone-pair angle (φ)
Optical gap (eV)

Figure 4.11 A double Y plot of the HOMO-LUMO energies and HOMO-LUMO
gaps vs. the lone-pair angle, φ showing the changes in orbital energies as a function
of dma rotation.

143
Table 4.4 Results of the HOMO-LUMO energies and HOMO-LUMO gaps obtained
from single-point calculations of the PES coordinates.

PES Lone-pair angle dihedral angle HOMO LUMO HOMO LUMO Gap Gap
Region (φ)(θ) (a.u.) (a.u.) (eV) (eV) (eV) (nm)
-23.35 90 -0.19 -0.07 -5.11 -1.78 3.33 372.03
Pyramidal (up) -8.46 105 -0.19 -0.07 -5.28 -1.80 3.49 355.79
7.29 120 -0.20 -0.07 -5.40 -1.84 3.56 348.37
Inversion center 25.82 135 -0.19 -0.07 -5.26 -1.80 3.46 358.08
Inversion center 74.10 150 -0.18 -0.06 -4.97 -1.76 3.20 386.94
83.14 165 -0.17 -0.06 -4.63 -1.65 2.97 417.11
Planar 90.03 180 -0.17 -0.06 -4.58 -1.64 2.94 421.51
96.68 195 -0.17 -0.06 -4.57 -1.64 2.93 423.79
106.03 210 -0.17 -0.06 -4.62 -1.66 2.96 419.46
117.35 225 -0.17 -0.06 -4.71 -1.69 3.01 411.73
129.50 240 -0.18 -0.06 -4.82 -1.73 3.09 401.71
142.58 255 -0.18 -0.06 -4.95 -1.76 3.20 387.96
156.63 270 -0.19 -0.07 -5.12 -1.79 3.33 372.67
Pyramidal (down) 171.84 285 -0.20 -0.07 -5.31 -1.82 3.49 355.24
188.35 300 -0.20 -0.07 -5.46 -1.82 3.64 340.73
Inversion center 208.41 315 -0.19 -0.07 -5.24 -1.82 3.42 362.27
Inversion center 253.88 330 -0.18 -0.06 -4.90 -1.75 3.15 394.13

4.3.3 TDDFT analysis
Time dependent DFT (TDDFT) calculations were employed to investigate
the low-lying singlet and triplet states of (dmappy)-Pt-(acac) molecule as a function
of dma rotation. Both singlet and triple states were calculated from the optimized
singlet ground state geometries. The outputs of the calculations contained the
vertical excitation energies (E
VT
), orbitals involved in each transition, the orbital
coefficients, and oscillator strengths for each transition. The orbital coefficients are
the coefficients of the wavefunctions for each excitation, which are directly
proportional to the contribution of the given excitation of the transition. Optimized
coordinates from the ground state PES scan were used to calculate the TDDFT
energies in gas phase. In order to understand the excited state properties as a
function of dma rotation, geometries of lone-pair/dihedral angle starting from -
9°(φ)/105°(δ) to 253°(φ)/330°(δ) were used to calculate the TDDFT energies.
144
0 30 60 90 120 150 180 210 240 270
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
0 30 60 90 120 150 180 210 240 270
0
1
2
3
4
5
Singlet
Triplet
Singlet/Triplet Energies (eV)
Lone-pair angle (φ)

Total Energy (kcal/mol)
Total energy

Figure 4.12 Gas phase singlet and triplet vertical excitation energies, E
VT
of
(dmappy)-Pt-(acac) calculated and plotted against the lone-pair angle is compared
with the total energy of the molecule also plotted against the lone-pair angle.

In Figure 4.12 vertical excitation energies of the lowest singlet and triplet
excited states are plotted as a function of the lone-pair angle along with the total
energy of the molecule. The colored regions on the plots correspond to the dma
orientations. Green represents the pyramidal regions, orange represents the inversion
centers, and the blue represents the planar region. In Table 4.5 the vertical transition
energies for the lowest energy singlet and triplet states calculated in the gas phase are
tabulated as a function of the lone-pair angle and dihedral angle. Both the singlet
(S
0
-S
1
) and the triplet (S
0
-T
1
) excitation energies were seemed to increase and
decrease as a function of the dma orientation. As expected at -9°(φ)/105°(δ) and
145
170°(φ)/285°(δ) the singlet and the triplet excitation energies reached maximum
values of 2.9 eV and 2.5 eV respectively. With change in hybridization from sp
3
to
sp
2
the energies decreased in value to 2.4 eV and 2.0 eV respectively. Excitation
energies at the four inversion centers remained approximately 2.4 eV for the singlet
state and 2.0 eV for the triplet state.

Table 4.5 Results of the HOMO-LUMO energies and HOMO-LUMO gaps obtained
from single-point calculations of the PES coordinates.

PES Lone-pair dihedral
Region angle (φ) angle (θ) Energy (eV) Energy (nm) Energy (eV) Energy (nm)
Planar -89.67 0
-81.81 15
-73.81 30
-62.21 45
-50.10 60
-37.22 75
-23.35 90
Pyramidal (up) -8.46 105 2.88 430.65 2.52 492.00
7.29 120 2.98 416.17 2.55 486.44
Inversion center 25.82 135 2.43 509.46 2.03 611.31
Inversion center 74.10 150 2.40 517.62 1.99 622.72
83.14 165 2.38 520.81 1.98 627.14
Planar 90.03 180 2.35 527.27 1.95 635.49
96.68 195 2.37 522.38 2.01 617.89
106.03 210 2.40 517.64 1.99 622.76
117.35 225 2.42 511.8 2.03 612.16
129.50 240 2.49 497.58 2.10 590.33
142.58 255 2.59 479.11 2.21 560.55
156.63 270 2.70 458.6 2.35 528.77
Pyramidal (down) 171.84 285 2.84 436.14 2.48 499.17
188.35 300 2.96 418.22 2.56 485.07
Inversion center 208.41 315 2.41 515.57 2.04 607.52
Inversion center 253.88 330 2.38 520.43 2.00 620.83
261.73 345
Planar 270.00 360
Singlet TDDFT energy Triplet TDDFT energy

Contribution to these lowest energy singlet (S
0
-S
1
) and triplet (S
0
-T
1
) excited
states obtained from the gas phase calculations came from HOMO to LUMO
146
transitions. Further examination of the singlet and triplet excited states of (dmappy)-
Pt(acac) were performed with TDDFT/CPCM model in solvents of various polarities
and will be discussed in detail in the following section.

4.3.3.1 Electronic absorption spectra and singlet excited states
Since the energies of the two trigonal pyramidal structures were almost the
same, TDDFT/CPCM calculations were only performed on one of the pyramidal
(up) isomer. Unless otherwise stated, from this point in this section the isomers will
be referred to as planar and pyramidal isomers only.
Room temperature absorption spectra of (dmappy)-Pt-(acac) in toluene,
methanol, acetonitrile, and 2-methyl-THF display three distinct regions between 300
nm to 600 nm wavelengths. The lowest energy band in the above solvents shows up
at 465 nm, 425 nm, 460 nm, and 460 nm respectively. These broad low energy
bands between 400-480 nm have been previously assigned as metal-to-ligand-
charge-transfer (
1
MLCT) transition and the higher energy bands around 300 to 350
nm have been assigned as π-π* transitions on the phenylpyridine ligand.
28
This
chapter will further examine these bands with the help of TDDFT calculations.
In Figure 4.13 the absorption spectra obtained in all four solvents are plotted
with vertical transition energies, E
VT
obtained from TDDFT/CPCM calculation. The
stick plots represent the E
VT
vs. oscillator strengths (f), where each bar corresponds
to a particular transition for a particular wavelength. The green bars represent the
trigonal planar isomers and the blue bars represent the trigonal pyramidal isomers.
147
Summary of the results for all the TDDFT calculations are provided in Table 4.6.
Overall results of the calculation showed lower excitation energies for the planar
isomer and relatively higher excitation energies for the pyramidal isomers.
Excitation energies for the S
0
-S
1
transition in the planar isomers were approximately
70 nm red-shifted from the pyramidal isomers. For the S
0
-T
1
transition, the
excitation energies for the planar isomers were red-shifted by 120 nm from the
pyramidal isomers.
300 350 400 450 500 550 600
0.0
0.1
0.2
0.3
0.4
0.5
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Intensity (a.u.)
Wavelength (nm)
Toluene

TDDFT (planar) Toluene
TDDFT (pyramidal) Toluene
300 350 400 450 500 550 600
0.0
0.1
0.2
0.3
0.4
0.5
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Intensity (a.u.)
Wavelength (nm)
CH3CN

TDDFT (planar) CH3CN
TDDFT (pyramidal) CH3CN
300 350 400 450 500 550 600
0.0
0.1
0.2
0.3
0.4
0.5
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Intensity (a.u.)
Wavelength (nm)
MeOH

TDDFT (planar) MeOH
TDDFT (pyramidal) MeOH
300 350 400 450 500 550 600
0.0
0.1
0.2
0.3
0.4
0.5
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Intensity (a.u.)
Wavelength (nm)
2M THF

TDDFT (planar) THF
TDDFT (pyramidal) THF

Figure 4.13 Room temperature absorption spectra of (dmappy)-Pt-(acac) in toluene,
methanol, acetonitrile, and 2-methyl-THF with superimposed TDDFT/CPCM
vertical excitation energies showing spectral peak assignments.

148
From the results of the TDDFT calculations of both isomers it is difficult to
assign the dma orientation in each solvent system. Since the band at the lowest
energy is broad and featureless, it is possible that a large distribution of population of
the planar and the pyramidal structures can exist in room temperature fluid solutions.
This can be further verified by examining the oscillator strengths for the S
0
-S
1

transition, which shows very small increase (Δf = 0.001) in oscillator strengths going
from the planar to the pyramidal structures. Therefore, it can be suggested that both
of the isomers can freely exist in room temperature fluid solutions with the
probability of the planar being slightly higher than the pyramidal one.
Largest contribution to the lowest energy S
1
state came from HOMO (MO-
88) to LUMO (MO-89) transition, for which the orbital coefficients and oscillator
strengths in all four solvents were approximately 0.68 and 0.04 respectively. Based
on the orbital population analysis HOMO-LUMO transition for the S
0
-S
1
excitation
can be assigned to have 16% metal-to-metal-and-ligand-charge transfer
1
MMLCT
character and 76% ligand-to-ligand-and-metal-charge transfer (
1
LLMCT) character.
The
1
MMLCT transition involves electronic delocalization from the platinum d
orbital to the platinum d* orbital and π* orbitals of the phenyl (Ph) and pyridine (Py)
rings: 16% Pt (d) → [5% Pt (d*) + 16% Ph (π*) + 75% Py (π*)]. The
1
LLMCT
transition involves electronic delocalization from the phenyl π orbital and dma p
z

orbital to the platinum d* orbital and π* orbitals of the phenyl and pyridine rings:
[40% Ph (π) + 30% dma (p
z
) + 3% Py (π)] → [5% Pt (d*) + 16% Ph (π*) + 75% Py
(π*)].
149
The
1
MMLCT transition in the pyramidal isomer was found to be almost
similar to the planar isomer. The
1
LLMCT transition however showed some
variation. In the pyramidal isomers, the p orbital in the dma moiety switches from
Np
z
to a mixture of Np
x
and Np
y
: [34% Ph (π) + 38% dma (Np
x
+ Np
y
) + 2% Py (π)]
→ [5% Pt (d)* + 17% Ph (π*) + 74% Py (π*) + 0.3% dma (Np
x
* + Np
y
*+ Np
z
*)].
Contributions to the higher states came from occupied and virtual MOs above and
below the HOMO and LUMO. Higher energy states are complex mixtures of MOs
and are charge transfer in character.

Table 4.6 TDDFT/CPCM vertical excitation energies (E
VT
), dominant MO
transitions, orbital coefficients, and oscillator strengths calculated for planar and
pyramidal isomers of (dmappy)-Pt-(acac) in toluene, THF, MeOH, and CH
3
CN.

TDDFT in toluene for trigonal planar isomer
States E
VT
E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(eV) (nm) MO Composition Coefficients Strengths (f)
S
1
2.3793 521.1 88 -> 89 0.67693 0.0352
S
2
2.8109 441.09 88 -> 90 0.69291 0.0421
S
3
3.0243 409.96 88 -> 91 0.69031 0.0217
S
4
3.305 375.15 86 -> 89 0.70102 0.0066
S
5
3.4689 357.41 87 -> 89 0.65212 0.1723
S
6
3.6269 341.85 86 -> 90 0.68956 0.0004
S
7
3.763 329.48 85 -> 89 0.57673 0.0169
87 -> 90 0.30475
S
8
3.827 323.97 85 -> 89 0.28366 0.0258
87 -> 90 0.57175
S
9
3.9702 312.28 88 -> 92 0.62761 0.0001
S
10
4.0109 309.12 84 -> 89 0.44412 0.01
87 -> 91 0.49373
T
1
1.9757 627.55 88A -> 89A 0.7104 0
T
2
2.6018 476.53 87A -> 90A 0.25948 0
88A -> 90A 0.61603 0
T
3
2.8054 441.95 88A -> 91A 0.65494 0
150
Table 4.6 Continued
T
4
2.8381 436.85 85A -> 89A 0.31142 0
87A -> 89A 0.55572 0
T
5
3.0104 411.85 87A -> 90A 0.57454 0
88A -> 90A 0.28147 0
T
6
3.2082 386.46 86A -> 89A 0.70619 0
T
7
3.2678 379.41 84A -> 89A 0.39695 0
85A -> 89A 0.2504 0
87A -> 89A 0.30973 0
88A -> 93A 0.27815 0
TDDFT in toluene for trigonal pyramidal isomer
States E
VT
E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(eV) (nm) MO Composition Coefficients Strengths (f)
S
1
2.7707 447.48 88 -> 89 0.67917 0.0338
S
2
3.2281 384.08 88 -> 90 0.67506 0.0282
S
3
3.3393 371.29 85 -> 89 0.69904 0.007
S
4
86 -> 89 0.33345
3.4172 362.83 88 -> 91 0.58806 0.0007
S
5
3.4692 357.39 86 -> 89 0.48323 0.1359
87 -> 89 0.40624
S
6
3.5603 348.24 86 -> 90 0.32282 0.0698
87 -> 89 0.52554
88 -> 91 0.29753
S
7
3.6647 338.32 85 -> 90 0.69103 0.0004
S
8
3.8471 322.28 86 -> 90 0.23857 0.0448
87 -> 90 0.59747
S
9
4.0227 308.21 88 -> 92 0.40306 0.024
84 -> 89 0.30182
86 -> 90
S
10
4.0784 304 86 -> 90 0.54118 0.0548
87 -> 91 0.30026
T
1
2.4384 508.48 86A -> 89A 0.25111 0
88A -> 89A 0.65903 0
T
2
2.7759 446.64 87A -> 90A 0.46402
88A -> 90A 0.3659 0
87A -> 89A 0.2825
T
3
2.8657 432.65 86A -> 89A 0.39463 0
87A -> 89A 0.31129
87A -> 90A 0.29645
T
4
3.1713 390.95 88A -> 90A 0.25807 0
88A -> 91A 0.46292 0
T
5
3.205 386.84 86A -> 90A 0.26453 0
151
Table 4.6 Continued
87A -> 89A 0.31084 0
87A -> 90A 0.25619 0
88A -> 90A 0.41599
T
6
3.2395 eV 382.72 85A -> 89A 0.70439 0
87A -> 89A 0.2873
88A -> 91A 0.45051
TDDFT in THF for trigonal planar isomer
States E
VT
E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(eV) (nm) MO Composition Coefficients Strengths (f)
S
1
2.419 512.55 88 -> 89 0.67646 0.0352
S
2
2.8143 440.55 88 -> 90 0.69386 0.0381
S
3
3.0818 402.31 88 -> 91 0.69112 0.0209
S
4
3.371 367.8 86 -> 89 0.70067 0.0066
S
5
3.5193 352.3 87 -> 89 0.65016 0.1606
S
6
3.6491 339.76 86 -> 90 0.6952 0.0004
S
7
3.7988 326.37 85 -> 89 0.55237 0.0239
87 -> 90 0.33443
S
8
3.8534 321.76 85 -> 89 0.31585 0.0218
87 -> 90 0.55333
S
9
3.9603 313.07 88 -> 92 0.64758 0.0001
S
10
4.0697 304.65 84 -> 89 0.47585 0.0165
87 -> 91 0.46293
T
1
2.0014 619.5 88A -> 89A 0.71083 0
T
2
2.6116 474.74 87A -> 90A 0.27116 0
88A -> 90A 0.6165 0
T
3
2.8603 433.47 85A -> 89A 0.26207 0
87A -> 89A 0.30938
88A -> 91A 0.5331
T
4
2.876 431.1 87A -> 89A 0.43827 0
88A -> 91A 0.38622 0
T
5
3.0185 410.75 87A -> 90A 0.58028 0
88A -> 90A 0.2849 0
T
6
3.2697 379.19 86A -> 89A 0.7054 0
T
7
3.2957 376.21 84A -> 89A 0.36827 0
85A -> 89A 0.25201 0
87A -> 89A 0.33523 0
88A -> 93A 0.28808 0
TDDFT in THF for trigonal pyramidal isomer
States E
VT
E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(eV) (nm) MO Composition Coefficients Strengths (f)
S
1
2.8254 438.83 88 -> 89 0.67918 0.0339
152
Table 4.6 Continued
S
2
3.2509 381.38 88 -> 90 0.67537 0.0267
S
3
3.3926 365.45 85 -> 89 0.69864 0.0069
S
4
3.475 356.79 86 -> 89 0.46704 0.0094
88 -> 91 0.48833
S
5
3.5208 352.15 86 -> 89 0.37886 0.124
87 -> 89 0.45591
88 -> 91 0.31034
S
6
3.6126 343.2 86 -> 89 0.28945 0.0644
87 -> 89 0.49152
88 -> 91 0.37902
S
7
3.6756 337.32 85 -> 90 0.69609 0.0005
S
8
3.8666 320.66 87 -> 90 0.59615 0.0425
S
9
4.0594 305.43 84 -> 89 0.3023 0.0477
86 -> 90 0.49218
S
10
4.1075 301.85 84 -> 89 0.37462 0.0419
86 -> 90 0.39328
87 -> 91 0.32371
T
1
2.4784 500.26 86A -> 89A 0.26793 0
88A -> 89A 0.65151
T
2
2.7934 443.85 87A -> 90A 0.50092
88A -> 90A 0.37441 0
T
3
2.8962 428.1 86A -> 89A 0.42503 0
87A -> 89A 0.29549
87A -> 90A 0.25772
T
4
3.2189 385.18 84A -> 89A 0.25395 0
87A -> 89A 0.3811 0
88A -> 91A 0.40373 0
T
5
3.2271 384.2 86A -> 90A 0.29639 0
87A -> 90A 0.30018 0
88A -> 90A 0.49384
T
6
3.2876 377.13 85A -> 89A 0.70366 0
T
7
3.3792 366.91 86A -> 91A 0.25101
T
8
87A -> 89A 0.28536
88A -> 91A 0.49903
TDDFT in methanol for planar isomer
States E
VT
E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(eV) (nm) MO Composition Coefficients Strengths (f)
S
1
2.4367 508.82 88 -> 89 0.67591 0.0338
S
2
2.8057 441.9 88 -> 90 0.69373 0.0365
S
3
3.1086 398.85 88 -> 91 0.69114 0.0197
S
4
3.3689 368.02 86 -> 89 0.70035 0.006
153
Table 4.6 Continued
S
5
3.5355 350.68 85 -> 89 0.14983 0.1536
87 -> 89 0.64769
S
6
3.619 342.59 86 -> 90 0.69718 0.0005
S
7
3.8091 325.49 85 -> 89 0.49674 0.0275
87 -> 90 0.41086
S
8
3.8515 321.91 85 -> 89 0.39377 0.0173
87 -> 90 0.49843
S
9
3.9535 313.61 88 -> 92 0.65066 0.0001
S
10
4.0918 303.01 84 -> 89 0.48278 0.0168
87 -> 91 0.45376
T
1
2.0125 616.08 88A -> 89A 0.71049 0
T
2
2.606 475.77 87A -> 90A 0.26585
88A -> 90A -0.61912
T
3
2.8745 431.33 85A -> 89A 0.34379
87A -> 89A 0.46138
88A -> 91A 0.31566
T
4
2.8967 428.02 87A -> 89A 0.26481
88A -> 91A -0.57432
T
5
3.0144 411.31 87A -> 90A 0.58223
88A -> 90A 0.27861
T
6
3.2651 379.73 86A -> 89A 0.7051
T
7
3.304 375.26 84A -> 89A 0.36063
85A -> 89A -0.25026
87A -> 89A 0.33964
88A -> 93A -0.29216
TDDFT in methanol for trigonal pyramidal isomer
States E
VT
E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(eV) (nm) MO Composition Coefficients Strengths (f)
S
1
2.847 435.48 88 -> 89 0.67851 0.0325
S
2
3.2502 381.47 88 -> 90 0.67455 0.0258
S
3
3.3814 366.66 85 -> 89 0.69483 0.0062
S
4
3.4936 354.89 86 -> 89 0.52904 0.0209
88 -> 91 0.41754
S
5
3.5383 350.41 86 -> 89 0.27787 0.1108
87 -> 89 0.49227
88 -> 91 0.36399
S
6
3.6354 341.05 86 -> 89 0.28689 0.0578
87 -> 89 0.46042
88 -> 91 0.41261
S
7
3.6394 340.67 85 -> 90 0.69184 0.0011
S
8
3.8596 321.24 87 -> 90 0.59859 0.0412
154
Table 4.6 Continued
S
9
4.0618 305.24 86 -> 90 0.56742 0.0548
S
10
4.1185 301.04 84 -> 89 0.4326 0.0314
86 -> 90 0.27824
87 -> 91 0.34446
T
1
2.4931 497.31 86A -> 89A 0.27165 0
88A -> 89A 0.64832
T
2
2.7948 443.62 87A -> 90A 0.50692
88A -> 90A 0.37912 0
T
3
2.9047 426.84 86A -> 89A 0.42333 0
87A -> 89A 0.30148
T
4
3.2241 384.56 86A -> 90A 0.27466 0
87A -> 89A 0.28532 0
88A -> 90A 0.44255 0
T
5
3.2329 383.51 87A -> 89A 0.32215 0
88A -> 91A 0.36764 0
T
6
3.2734 378.76 85A -> 89A 0.70049 0
T
7
3.3987 364.8 86A -> 91A 0.2504
T
8
87A -> 89A 0.27581
88A -> 91A 0.51633
TDDFT in acetonitrile for planar isomer
States E
VT
E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(eV) (nm) MO Composition Coefficients Strengths (f)
S
1
2.4366 508.85 88 -> 89 0.67608 0.0344
S
2
2.8103 441.18 88 -> 90 0.69392 0.0366
S
3
3.1092 398.76 88 -> 91 0.69122 0.02
S
4
3.3879 365.96 86 -> 89 0.7004 0.0062
S
5
3.5394 350.3 87 -> 89 0.64842 0.1542

S
6
3.6421 340.42 86 -> 90 0.69687 0.0005
S
7
3.8125 325.2 85 -> 89 0.52087 0.0274
87 -> 90 0.37807
S
8
3.8588 321.3 85 -> 89 0.36014 0.0186
87 -> 90 0.52357
S
9
3.9537 313.59 88 -> 92 0.65116 0.0001
S
10
4.0944 302.81 84 -> 89 0.4866 0.018
87 -> 91 0.44921
T
1
2.0124 616.1 88A -> 89A 0.71069 0
T
2
2.6107 474.91 87A -> 90A 0.26977 0
88A -> 90A 0.61811
T
3
2.8763 431.05 85A -> 89A 0.34298 0
155
Table 4.6 Continued
87A -> 89A 0.44326
88A -> 91A 0.34484
T
4
2.8985 427.76 87A -> 89A 0.28607 0
88A -> 91A 0.5574
T
5
3.0182 410.79 87A -> 90A 0.58099 0
88A -> 90A 0.28187
T
6
3.2841 377.53 86A -> 89A 0.70505 0
T
7
3.3064 374.99 84A -> 89A 0.35729 0
85A -> 89A 0.25053
87A -> 89A 0.34314
88A -> 93A 0.29171
TDDFT in acetonitrile for trigonal pyramidal isomer
States E
VT
E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(eV) (nm) MO Composition Coefficients Strengths (f)
S
1
2.8473 435.44 88 -> 89 0.67892 0.0331
S
2
3.2549 380.91 88 -> 90 0.67504 0.0259
S
3
3.4022 364.43 85 -> 89 0.6978 0.0065
S
4
3.4953 354.71 86 -> 89 0.52308 0.0179
88 -> 91 0.42637
S
5
3.5421 350.03 86 -> 89 0.30505 0.1143
87 -> 89 0.48576
88 -> 91 0.35068
S
6
3.6362 340.97 86 -> 89 0.27652 0.0595
87 -> 89 0.46756
88 -> 91 0.41672
S
7
3.6633 338.45 85 -> 90 0.69696 0.0005
S
8
3.8679 320.55 87 -> 90 0.59585 0.0411
S
9
4.0651 305 86 -> 90 0.56382 0.0556
S
10
4.1216 300.81 84 -> 89 0.43599 0.0346
86 -> 90 0.28437
87 -> 91 0.3324
T
1
2.4939 497.15 86A -> 89A 0.27422 0
88A -> 89A 0.64809
T
2
2.7978 443.15 87A -> 90A 0.51052 0
88A -> 90A 0.51052
T
3
2.9073 426.46 86A -> 89A 0.43299 0
87A -> 89A 0.2892
T
4
3.2293 383.94 87A -> 89A 0.35584 0
88A -> 90A 0.37659
T
5
3.2358 383.16 87A -> 89A 0.25215 0
88A -> 90A 0.33695
156
Table 4.6 Continued
88A -> 91A 0.33779
T
6
3.2942 376.37 85A -> 89A 0.70287 0
T
7
3.4014 364.51 86A -> 91A 0.2505
87A -> 89A 0.27985
88A -> 91A 0.51594

4.3.4 Lowest triplet excited state (TES) geometries and electronic
structures.
Geometry optimization and potential energy surface scan of the lowest
energy triplet states were performed using the SCF unrestricted method
(uB3LYP/LANL2DZ). Similar to the ground state the lowest energy optimized
structure in the triplet sate was also a trigonal planar. Geometry optimized structures
of the three triplet state isomers of (dmappy)-Pt-(acac) and their spin-density plots
are shown in Figure 4.14. Summary of the calculated bond lengths and angles of the
three isomers are reported in Table 4.7. Compared to the SGS geometries, in the
TES geometries the Pt-N and the Pt-O
21
bonds in the coordination sphere decreased
in size by approximately 0.01Å and the Pt-O
20
bonds increased in size by about
0.02Å. The Pt-C bond however decreased about 0.04Å in the triplet state.
On the dma moiety the C
12
-N bond also shortened about 0.01Å-0.02Å while
the other two C-N bonds lengthened by approximately 0.01Å. The overall trend in
triplet state going from the trigonal planar to the trigonal pyramidal were the same as
the ground state. Meaning, that the dma moiety in the triplet state also rotates from
the sp
2
hybridized structure to the sp
3
hybridized structure every 90° interval. As a
157
consequence the value of δ changes from 360° in the planar state to 340° in the
pyramidal state. The only difference being however, in the triplet state the
pyramidalization is more intense than the ground state.

(a) (b) (c)
(d) (e)

Figure 4.14 Optimized TES structures of the three isomers of (dmappy)-Pt-(acac).
(a) Dimethylamino trigonal planar (CNCC dihedral angle = 0°). (b) Dimethylamino
trigonal pyramidal with the nitrogen lone-pair facing up (CNCC dihedral angle =
90°). (c) Dimethylamino trigonal pyramidal with the nitrogen lone-pair facing down
(CNCC dihedral angle = 90°). (d) Spin-density plot of the trigonal planar isomer.
(e) Spin-density plot of the trigonal pyramidal isomers.

158
Table 4.7 Summary of the calculated TES bond lengths angles of the three structural
isomers of (dmappy)-Pt-(acac).

Bond type Trigonal planar Trigonnal pyramidal Trigonnal pyramidal
Lone-pair up Lone-pair down
Pt-N 2.007 2.002 2.006
Pt-C 1.952 1.952 1.950
Pt-O
20
2.071 2.068 2.071
Pt-O
21
2.142 2.140 2.140
C
12
-N 1.385 1.449 1.450
C
36
-N 1.475 1.480 1.482
C
40
-N 1.473 1.483 1.483
Angle type
C
12
NC
36
121.304 114.094 114.503
C
12
NC
40
120.252 114.160 113.509
C
36
NC
40
118.444 112.227 111.707
TES Angles in degrees
TES Bond distances (Å)

The Gaussian spin-density plots of the triplet state (dmappy)-Pt-(acac)
showed most of the unpaired electron density on the dmappy ligand and very little on
the acac. In Table 4.8 the density of the unpaired electrons of the triplet state
isomers are summarized as percent density. The trigonal planar isomer showed 9%
unpaired electron density on the platinum metal, 31% on the pyridine ring, 48% on
the phenyl ring, and almost 11% on the dma moiety. In the trigonal pyramidal form,
the percent density changed to 11% unpaired electron density on the platinum metal,
35% on the pyridine ring, 48% on the phenyl ring, and almost 6% on the dma
moiety. In both cases less than 1% spin-density was observed on the acac ligand
suggesting the triplet emission mainly comes from the dmappy ligand with partial
contribution from the platinum metal. However, this behavior changes a little bit in
159
the pyramidal isomers, where decreased MO overlap reduces the density of unpaired
electrons on the dma moiety.
Table 4.8 Summary of the calculated spin-densities of TES isomers of (dmappy)-Pt-
(acac) showing percent density of the unpaired electrons.

Functionalities Trigonal planar Trigonal pyramidal
Pt 9.07% 11.16%
Pyridine 31.39% 34.50%
Phenyl 48.31% 47.65%
dma 10.57% 5.97%
acac 0.66% 0.73%

4.3.5 Triplet state PES scan
In the triplet state the potential energy surface (PES) scan was conducted on
the trigonal planar isomer by rotating the dimethylamino moiety 360° with 15°
increments. Table 4.9 gives a summary of the TES-PES results. In Figure 4.15 the
result of the triplet state PES scan is shown as a function of the lone-pair angle, δ.
Similar to the ground state the triplet state potential energy surface also showed two
minimas and three maximas. The three minimas at 0°(θ)/-90°(φ), 180°(θ)/+90°(φ),
and 360°(θ)/+270°(φ) correspond to the triplet state trigonal planar structures and the
two maximas at 105°(θ)/-10°(φ) and 285°(θ)/-170°(φ) correspond to the trigonal
pyramidal structures. Barrier to rotation was calculated to be 15 kcal/mol
160
-100 -50 0 50 100 150 200 250 300
-6
0
6
12
18
24
Energy (kcal/mol)
Lone-pair angle (φ)
Triplet PES
N
P
N
P
N
P
N
P
N
P
N
P
N
P

Figure 4.15 Triplet state potential energy surface (PES) scan of (dmappy)-Pt-(acac)
is shown with the direction of the lone-pair electrons.

Four inversion centers were observed at 105°(θ)/-10°(φ), 120°(θ)/48°(φ),
285°(θ)/170°(φ), and 300°(θ)/227°(φ) angles. In Figure 4.16 the four triplet state
inversion centers of (dmappy-Pt-(acac) are shown. In the diagram the vectors
indicate the direction of the lone-pair electrons for each inversion center. Inversion
barrier calculated for the step (a) to (b) was 10 kcal/mol. This happens as a result of
inversion of dma followed by rotation to 48° from -10°. When the dma flips from -
10° to 170°, the inversion from (a) to (c) occurs with a low inversion barrier of 0.42
kcal/mol. Barrier to inversion for the step involving a dma flip from 48°(b) to
227°(d) happens at 1.0 kcal/mol. Inversion + rotation between (c) and (d) occurs
when the dma inverts to 170° and then rotates to 227° (2.5 kcal/mol).
161
-10º 48º
170º
227º
Inversion + rotation
Inversion + rotation
10 kcal/mol
11 kcal/mol
Inversion
Inversion
(a) (b)
(c) (d)
0.42 kcal/mol
1.0 kcal/mol

Figure 4.16 Four triplet state inversion centers of (dmappy)-Pt-(acac) showing lone-
pair angles and the energies for the inversion barriers at each point.

The geometries of the various triplet state structures were further analyzed in
terms of dimethylamino bond lengths and angles. In Figure 4.17 pyramid cone angle
vs. φ and C-N bond lengths vs. φ plots are shown, which clearly show the same trend
observed for the ground state geometries.
162
-90 0 90 180 270
340
344
348
352
356
360
Ph N
Ph N
Ph N
Cone Angle, δ
Lone-pair Angle, φ
-90 0 90 180 270
1.38
1.40
1.42
1.44
1.46
1.48
1.50
Bond Length, Α
Lone-pair Angle, φ
C12-N
C36-N
C40-N

Figure 4.17 (left) A plot of the pyramid cone angle, δ vs. the lone-pair angle, φ.
(right) A plot of the three C-N bond lengths vs. the lone-pair angle, φ.

Dipole moments of the triplet state geometries located on various points of
the potential energy surface were compared with their counterparts located on the
ground state potential energy surface. Figure 4.18 shows the graph of the ground
state and triplet state dipole moments vs. lone-pair angle, φ. No radical increase or
decrease going from the ground state to the triplet excited state confirmed the fact
that charge separation in the triplet state is small. Similar trend for both the ground
and the excited state also suggested that the electronic structures and charge
separation in both of the state was same. However, a small increase in dipole
moment going from the pyramidal up form to the planar form was observed.
Furthermore, a larger increase in dipole moment was also observed as the dma
rotated from the planar position to the pyramidal down position.
163
-45 0 45 90 135 180 225 270
1.5
2.0
2.5
3.0
3.5
4.0
Dipole moment (D)
Lone-pair angle (φ)
TES dipole moment
SGS dipole moment

Figure 4.18 Dipole moment vs. lone-pair angle, φ.

The triplet energies (E
T
) of (dmappy)-Pt-(acac) were calculated by
subtracting the ground state SCF energies from the triplet state SCF energies
(Equation 2.1). In Figure 4.19, the SGS and TES PES energies in eV are plotted vs.
the lone-pair angle, δ. The difference between the SGS and TES PES energies
shows the triplet energies of the molecule in eV. A comparison of the triplet
energies in nanometer scale is also shown on the right Y plot. From the plot it is
evident that the triplet energy of the molecule remains the lowest (1.98 eV/625 nm
corresponding to red emission) at δ = -90°. As the dma rotates towards the
pyramidal position, the triplet energy gradually increases from red to yellow to green
emission line eventually peaking out at 2.5 eV (502 nm). The trend is observed as
164
valley, peak, and valley going from -90° to 0° to +90° corresponding to planar,
pyramidal, and planar form.
-90 -60 -30 0 30 60 90 120 150 180 210 240 270
0.0
0.5
1.0
1.5
2.0
2.5
660
640
620
600
580
560
540
520
500
PES Energy (eV)
Lone-pair angle (φ)
TES-PES
SGS-PES
Tiplet energy (nm)
Triplet Energy

Figure 4.19 A double Y plot of the SGS and TES PES energies and triplet energies
vs. the lone-pair angle, φ showing change in triplet energies as a function of dma
rotation.

Triplet energies (E
T
) were also found to be lower than the TDDFT vertical
excitation energies (E
VT
). This was expected and is a result of triplet emission
arising from the relaxed excited state. The TDDFT routine locates the excited state
by using the linear response theory and by calculating the change in density in
response to a perturbation as a function of time. Therefore, TDDFT can be used to
understand the properties of the excited states, which can be obtained in the output as
the oscillator strength of the excitation, the MOs involved in the excitation, and the
orbital coefficients that are involved in the excitation. However, it cannot be used to
obtain the structural information of that particular excited state because TDDFT does
not calculate the geometry of the excited state it locates. When a molecule gets
165
excited it makes a vertical transition (Frank-Condon jump) to the nearest excited
state. However, emission does not come immediately from that excited geometry.
The molecule first relaxes to the lowest energy conformation in that excited state and
emission arises from that geometry. This is the reason the calculated singlet and
triplet energies are lower in energy than the TDDFT excitation energies. The
difference in energy between the TDDFT energy and the emission energy (ΔE = E
VT

- E
T
) is the excited state relaxation energy.

Table 4.9 Summary of the TES-PES result of (dmappy)-Pt-(acac) showing the
potential energy surface, triplet energies, and triplet dipole moments as a function of
φ and δ.

PES Lone-pair dihedral Triplet state T
1
Emission T
1
Emission Dipole
Region angle (φ) angle (θ) SCF (kcal/mol) Energy (eV) Energy (nm) Moment (D)
Planar -89.85 0 0.00 1.98 625.01
-77.42 15 0.32 1.99 623.48
-66.17 30 1.47 2.01 617.14
-57.15 45 3.39 2.07 600.39
-46.75 60 6.08 2.16 573.49
-35.21 75 9.44 2.27 546.14
-22.88 90 12.88 2.38 520.86
Pyramidal (up) +Inversion center -9.55 105 15.37 2.47 502.37 1.65
Inversion center 48.25 120 4.96 2.02 614.81 1.67
59.10 135 2.53 1.94 640.75 2.15
68.49 150 1.07 2.00 621.52 2.15
78.38 165 0.29 1.99 623.90 2.27
Planar 90.13 180 0.01 1.98 625.27 2.39
101.59 195 0.31 1.99 623.82 2.39
111.71 210 1.09 1.99 621.69 2.34
120.93 225 2.54 2.04 608.41 2.25
131.70 240 5.01 2.12 585.05 2.32
143.54 255 8.40 2.22 557.70 2.55
156.09 270 12.33 2.35 528.47 2.82
Pyramidal (down) +Inversion center 169.95 285 15.79 2.47 502.87 3.13
Inversion center 226.76 300 6.07 2.04 606.38 3.26
237.11 315 3.39 1.97 630.20 1.91
246.18 330 1.46 2.01 617.31 2.13
257.43 345 0.30 1.99 623.65
Planar 269.92 360 0.00 1.98 625.27

166
4.3.6 Emission spectroscopy and properties of triplet excited states.
The 77 K mission spectra of (dmappy)-Pt-(acac) in toluene, 3M pentane, 2M
THF, MeOH, and CH
3
CN are presented in Figure 4.20. As expected. The data
shows shifts in emission maxima as a function of solvent polarity. In toluene,
emission was observed as a broad emission band at λ
max
= 595 nm with a shoulder at
640 nm. The lifetime, τ for the λ
max
was measured to be 13.4 μs corresponding to
phosphorescence. In 3M pentane, a broader emission band was observed with the
λ
max
and the shoulder being blue shifted to 560 nm and 590 nm respectively. The
lifetimes for these emissions were measured to be 9.9 μs and 15 μs respectively. In
2-methyl THF, the spectrum was more structured and the λ
max
was further blue
shifted to 470 nm (τ = 9.3 μs). The emission in methanol showed very fine vibronic
structures with three distinct peaks at 470 nm, 500 nm, and 540 nm. Lifetimes for
these peaks were between 8-12 μs range.
Most interesting result was obtained in CH
3
CN. Emission spectrum of
(dmappy)-Pt-(acac) in this solvent showed to two separate regions: A high energy
low intensity region with fine vibronic structures was observed at λ
max
= 470 nm
with two shoulders at 450 nm and 500 nm, and a low energy high intensity peak was
observed at λ
max
= 605 nm with a broad shoulder around 640 nm. The lifetime of the
peak at 470 nm was 8 μs and the peak at 607 nm was 13 μs indicating the presence
of two emitting species. These results correlate really well with the charge transfer
model discussed earlier throughout the chapter. Because of freely rotating
167
dimethylamino moiety, the electronic structure of the entire molecule change in
solvents with different polarity. As a result of this solvent-solute interaction, the
molecule relaxes to different structures in different media with different polarity and
emits from different states with different energies.
300 350 400 450 500 550 600 650 700 750
CH3CN
CH3CN
Wavelength, λ

MeOH
MeOH

3-M pentane
3-M pentane
300 350 400 450 500 550 600 650 700 750

Toluene
Toluene
Normalized Intensity
2M THF
2M THF

Figure 4.20 77K emission spectra of (dmappy)-Pt-(acac) in solvents of different
polarity.
168
Solvent stabilization and relaxation plays an important role in the absorption
and emission spectroscopy of (dmappy)-Pt-(acac). That is why as the polarity of the
solvent increases, hypsochromic shift (blue-shift) is observed both for absorption and
emission spectra. This phenomenon is known as negative solvatochromism and is a
consequence of the ground state stabilization (Figure 4.21).
31
When the excited state
is stabilized, a bathochromic shift (red-shift) causes emission from lower energy
state. In a non-polar solvent like toluene, the molecule exists in a trigonal planar
form. As the polarity of the solvent increases, such as in 2M THF and MeOH,
increased dipole of the solvent then rearranges the dipole of the molecule to
minimize the total energy of the solvent-solute system. As a result, the dma moiety
twists to polar and more stable trigonal pyramidal form and the molecule emits from
that state.
T
1
T
1
S
0
S
0
T
1
S
0
Hypsochromic
shift
Bathochromic
shift
T
1
T
1
S
0
S
0
T
1
S
0
Hypsochromic
shift
Bathochromic
shift

Figure 4.21 A schematic energy diagram illustrating the solvatochromic shift from a
hypothetical neutral state, where stabilization of the excited state (T
1
) causes a
bathochromic shift (red-shift) and stabilization of the ground state (S
1
) causes a
hypsochromic shift (blue-shift).
169
Triplet energies obtained from the 77K emission spectra were compared with
the E
VT
from TDDFT/CPCM calculations. In Figure 4.22-4.25, the calculated
TDDFT/CPCM results are shown in comparison with the experimental emission
spectra obtained in room temperature (RT spectra were not discussed here because of
the lack of lifetime data) and 77K. For toluene, it is evident from the

Figure 4.22 Vertical excitation energies calculated in toluene are compared with the
experimental RT and 77K spectra showing preferred orientation of dma moiety in
toluene is δ = -90°.

calculation that preferred orientation of dma moiety in this solvent is planar (δ = -
90°). E
VT
calculated for the S
0
→T
1
transition in this solvent was 628 nm (orbital
coefficient of 0.71) for the planar isomer, which was in agreement with the 77K and
-90º
Planar
S
0
T
1
S
1
521 nm
628 nm 595 nm
77K
640 nm
RT 0º
Pyramidal
T
1
S
1
447 nm
T
1
T
1
S
1
Toluene
466 nm
508 nm
170
RT emissions occurring at 595 nm and 640 nm respectively. Contribution to this
excitation came from HOMO (MO-88) - LUMO (MO-89) transition, which is 16%
3
MMLCT in character and 76%
3
LLMCT in character. This result also correlated
well with the triplet energy, E
T
(T
1
→S
0
) obtained from ΔSCF values, which was
calculated to be 625 nm for the planar structure. The reason that the value of E
T
is
lower than the value of E
VT
is because E
T
was obtained from gas ΔSCF values and
E
VT
was obtained from TDDFT/CPCM calculations in toluene.
-90º
Planar
S
0
77 RT 0º
Pyramidal
T
1
S
1
439 nm
500 nm
523 nm
T
1
T
1
S
1
513 nm

Figure 4.23 Vertical excitation energies calculated in THF are compared with the
experimental RT and 77K spectra showing preferred orientation of dma moiety in
THF is δ = 0°.

619 nm
S
1
THF
T
1
466 nm
500 nm
171
In THF (Figure 4.23), preferred orientation of the dma moiety was found to
be pyramidal (δ = 0°). E
VT
calculated in this solvent was 500 nm for the S
0
→T
1

transition, which compared well with the 77K emission at 500 nm and RT emission
at 523 nm. Contribution to this transition was associated with HOMO (MO-88) to
LUMO (MO-89) transition (0.65) corresponding to mostly
3
MMLCT and
3
LLMCT
characters and HOMO-2 (MO 86) to LUMO (MO 89) transition (0.28)
corresponding to some charge transfer character. Gas phase triplet energy (T
1
→S
0
)
calculated for this structure was 502 nm and compared well with the experimental
values and TDDFT/CPCM value.
-90º
Planar
S
0
77K RT
0º
Pyramidal
T
1
S
1
435 nm
497 nm
505 nm
T
1
T
1
S
1
509 nm

Figure 4.24 Vertical excitation energies calculated in methanol are compared with
the experimental RT and 77K spectra showing preferred orientation of dma moiety
in methanol is δ = 0°.
616 nm
502nm
S
1
MeOH
435 nm
T
1
172
Most favored dma orientation in methanol was found to be pyramidal (δ =
0°). Vertical transition energies calculated in methanol for the S
0
→T
1
transition was
497 nm for the pyramidal structure, which also compared well with the gas phase
triplet energy of 502 nm and both 77K and RT emissions at 502 nm and 505 nm
respectively. Major contribution to this state transition came from HOMO (MO-88)
to LUMO (MO-89) transition (0.65) and HOMO-2 (MO 86) to LUMO (MO 89)
transition (0.28). Both of the MO transitions were similar to the MO transitions in
THF transitions.
-90º
Planar
S
0
77K RT 0º
Pyramidal
T
1
S
1
435 nm
497
nm
505 nm
T
1
T
1
S
1
509 nm

Figure 4.25 Vertical excitation energies calculated in acetonitrile are compared with
the experimental RT and 77K spectra showing preferred orientation of dma moiety
in acetonitrile is δ = 0° and -90°.
616 nm
T
1
466 nm
S
1
CH
3
CN
607 nm
T
1
470 nm
173
The S
0
→T
1
transition in acetonitrile was a mixture of toluene and
THF/methanol type transition. In this solvent, the both planar and the pyramidal
isomers were found to be emitting at 607 nm and 470 nm respectively, indicating a
large distribution of population existing in the frozen matrix. However, in room
temperature fluid solution only the pyramidal isomer was found to be predominant
(505 nm). Vertical transition energies for the planar (616 nm) and the pyramidal
(497 nm) isomers calculated in acetonitrile and triplet energies for the planar and the
pyramidal isomers calculated in the gas phase were in good agreement with the
experimental results. The planar isomer was found to be associated with HOMO-
LUMO excitation (0.67) and the pyramidal isomer was found to be associated with
(HOMO-2)-LUMO (0.27) and HOMO-LUMO (0.65) excitations corresponding to
3
MMLCT and
3
LLMCT characters.

4.4 Chapter 4 Conclusion
Density functional theory (DFT) and time-dependent (TD) DFT studies were
performed on (dmappy)-Pt-(acac). Preferred orientation of the dimethylamino
moiety in the relaxed ground state was found to be trigonal planar. The ground state
electronic structure of the planar complex as calculated by B3LYP/LANL2DZ
density functional parameters was found to be in good agreement with the X-ray
crystallographic structure. Potential energy surface (PES) scan of the singlet ground
state geometries showed three minimas at 0°(θ)/-90°(φ), 180°(θ)/+90°(φ), and
360°(θ)/270°(φ) positions and two maximas at 105°(θ)/40°(φ) and 285°(θ)/170°(φ)
174
positions. The two maximas were the points on the surface associated with the two
pyramidal isomers: one facing the pyridine ring and the other facing away from the
pyridine ring. The minimas were the points associated with the trigonal planar
isomer. There were also four inversion centers located on the PE surface at
135°(θ)/68°(φ), 140°(θ)/72°(φ), 315°(θ)/207°(φ), and 320°(θ)/246°(φ) angles. The
inversion centers are characteristic of umbrella type inversion of the dimethylamino
group. Due to low energy inversion barriers, in room temperature fluid solutions the
inversion centers rapidly equilibrate between one another during rotation. PES scan
of the lowest triplet excited state also showed similar results as the ground state PES.
Singlet and triplet excited states were examined by the TDDFT method.
Vertical transition energies were calculated in gas phase and in solvents with CPCM
method. In 77K frozen toluene matrix, (dmappy)-Pt-(acac) was found to emit from
the trigonal planar isomer consistent with red emission at 595 nm. In 2M THF, the
77K phosphorescence was observed at 500 nm and room temperature
phosphorescence at 523 nm, which corroborated with E
VT
of 500 nm and E
T
of 502
nm suggesting the emitting species was trigonal pyramidal. Similar behavior was
observed in methanol, which showed green phosphorescence both in RT (505 nm)
and at 77K (502 nm). The E
VT
(497 nm) calculated in methanol and E
T
(502 nm)
calculated in gas phase were in very good agreement with the experimental results.
In acetonitrile, dual phosphorescence was observed at 77 K and was characterized as
emissions being emanating from both the planar and the pyramidal isomers.
175
All of the low lying singlet and triplet transitions were categorized as 16%
MMLCT in character and 76% LLMCT in character corresponding to a significant
mixture of the metal d orbital and ligand π orbitals. MO transitions for these S
0
→S
1

and S
0
→T
1
transitions involved mostly HOMO to LUMO charge transfer character.

176
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efficient shared-exponent basis sets for the first-and second-row atoms. J. Chem.
Phys 1984, 81, (12), 6027.
23. Wadt, P. J. H. a. W. R., Ab Initio effective core potentials for molecular
calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys 1985,
82, (1), 270.
24. Wadt, P. J. H. a. W. R., Ab Initio effective core potentials for molecular
calculations. Potentials for main group elements Na-Bi J. Chem. Phys 1985, 82, (1),
284.

25. Wadt, P. J. H. a. W. R., Ab Initio effective core potentials for molecular
calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys 1985,
82, (1), 299.
26. Stanislav R. Stoyanov, J. M. V., and D. Paul Rillema, Time-Dependent
Density Functional Theory Study of the Spectroscopic Properties Related to
Aggregation in the Platinum(II) Biphenyl Dicarbonyl Complex. Inorganic Chemistry
2003, 42, (24), 7852.

27. HPCC.
28. Jason Brooks, Y. B., Sergey Lamansky, Peter I. Djurovich, Irina Tsyba,;
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Cyclometalated Platinum Complexes. Inorg. Chem. 2002, 41, 3055-3066.
179
29. V. P. Novikov, S. S., and L. V. Vilkov, Molecular Structure and
Conformation of N,N-dimethyaniline by Gas-Phase Electron Diffraction and
Quantum-Chemical Calculations. Russian Journal of General Chemistry 2004, 74,
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161.
31. Reichardt, C., Chem. Rev. 1994, 2319-2358.

180
Chapter 5. Computational Studies of Spectroscopic and
Photophysical Properties of (NO
2
ppy)-Pt-(acac)

5.1 Introduction
(NO
2
ppy)-Pt-(acac) is a low triplet energy phosphorescence emitter that can
be potentially used as a dopant in OLEDs. This molecule is a derivative of
(dmappy)-Pt-(acac), where the rotating dimethylamino moiety on the phenyl ring is
replaced by the rotating nitro group on the pyridine ring. However, unlike the dma
moiety on (dmappy)-Pt-(acac), the nitro moiety on (NO
2
ppy)-Pt-(acac) is an electron
withdrawing group. This electron withdrawing effect gives (NO
2
ppy)-Pt-(acac)
some unique properties that makes it behave differently from (dmappy)-Pt-(acac).
In Figure 5.1 the two possible structural isomers of (NO
2
ppy)-Pt-(acac) are
shown that can form as a consequence of NO
2
rotation. When the NO
2
moiety
rotates parallel to the plane of the molecule, the nitrogen p orbital becomes coplanar
with respect to the π orbital of the pyridine ring. This orientation increases the
interaction between the π system of the pyridine ring and the nitrogen p orbital. As a
result, the triplet energy of the molecule decreases. Conversely, when the NO
2

moiety rotates perpendicular to the plane of the molecule, the p-π interaction
diminishes and the triplet energy increases. Rotation of the NO
2
moiety influences
the ground and the excited state electronic structures of the (NO
2
ppy)-Pt-(acac)
181
molecule, which effects the solvent solute interaction. Therefore, in solvents with
varying polarities, the molecule shows solvent dependent emission.
(b)
Planar isomer
∠CCNO = 0°
p-π interaction
(c)
Orthogonal isomer
∠CCNO = 90°
No interaction
C
3
C
6
C
5
N
2
C
4
C
7
C
13
C
16
C
12
C
15
C
14
C
11
Pt
O
20
C
24
C
23
C
22
O
21
C
26
C
30
N
35
O
36
O
37
N
N
O
O
O O
N
N
O O
O
O
(a)

Figure 5.1 (a) (NO
2
ppy)-Pt-(acac) with atom numbers from DFT calculations. (b)
Planar isomer: when the CCNO dihedral becomes 0°, the nitrogen p orbital becomes
coplanar to the π orbital of the pyridine ring. (c) Orthogonal isomer: when the
CCNO dihedral becomes 90°, the nitrogen p orbital becomes orthogonal to the π
orbital of the pyridine ring.

Like (dmappy)-Pt-(acac), (NO
2
ppy)-Pt-(acac) is also capable of dual
phosphorescence. But interestingly, the emission only comes from one state in this
molecule, which causes it to be emissive in some solvents and completely non-
182
emissive in other. The primary goal of this chapter is to investigate the
photophysical behaviors and understand the spectroscopic properties of (NO
2
ppy)-
Pt-(acac). In order to achieve that goal, density functional theory (DFT) method will
be used to examine the ground and triplet state electronic structures. Singlet and
triplet excited states will be further studied in detail by time-dependent (TD) DFT
method and compared with electronic absorption and emission spectra.

5.2 Experimental

5.2.1 Thin Film Fabrication
Quartz substrates and Silicon wafers were rinsed with soap-water, deionized
water, and acetone and blow dried with nitrogen. Thin films were grown on quartz
and Silicon wafers at pressures between 3-4 μtorr. Materials were thermally
evaporated from tantalum boats at rates between 2-4 Å/s. Film thicknesses were
maintained at 500Å. Doped films were prepared by co-depositing the CBP and
UGH hosts with (NO
2
ppy)-Pt-(acac) at concentrations between 6-8%. Surface
morphology of the thin films were examined with an optical microscope (Nikon
Eclipse ME 600) attached to a CCD camera model: Olympus LKH028884.
5.2.2 Spectroscopic Measurements
Absorption spectra of the thin films were recorded with an Agilent 8453 UV-
Vis spectrometer. Emission spectra were recorded with a QuantaMaster C-60SE
183
spectrofluorometer. Lifetimes of the thin films were recorded in vacuum with an
IBH photon timing instrument connected to an IBH model TBX-04 PMT detector.

5.2.3 Computational Details.
All preliminary geometry optimizations were performed at PM3 level of
theory using the Titan software package.
1
All quantum mechanical calculations were
performed using the Gaussian-03 software package.
2
Electronic structure
calculations of the singlet ground states (SGS) and the triplet excited states (TES)
were performed using the density functional theory (DFT) method. Potential energy
surface (PES) scans of the SGS and TES were also calculated using the DFT method.
For all calculations the B3LYP functional consisting of Becke’s three-parameter
equation
3
and Lee, Yang, and Parr’s non-local hybrid functional
4
was used with the
LANL2DZ basis set, which uses the Dunning-Hay split valence double-ζ for C,H,N
atoms (D95) and Hay-Wadt double-ζ with Los Alamos National Laboratories
relativistic effective core potential (ECP) for heavy atoms.
5-8

The lowest SGS structure was obtained by optimizing the geometry in the gas
phase without any dihedral constraints. The SGS geometries for the structures with
dihedral angles from 0-360° at every 15° increments were obtained by specifying the
dihedral angles in the Z-matrix and by freezing them to a particular value during
geometry optimization. PES scans for the SGS structures were obtained in the gas
phase by rotating the dma group at 15° increments. The lowest TES structures were
calculated at an unrestricted DFT level with a spin multiplicity of 3
184
(uB3LYP/LANL2DZ). Geometries for the structures with dihedral angles from 0-
360° and the triplet state PES scans were obtained as described before.
Time-dependent (TD) DFT calculations were performed on the gas phase
SGS geometries. Total of ten singlet and ten triplet states were calculated for each
structure. Singlet and triplet vertical transition energies (E
VT
) were calculated in the
gas phase and in solvent shells. Solvent calculations were performed in hexanes and
acetonitrile using the conductor-like polarizable continuum model (CPCM)
9
in
conjunction with TDDFT routine. All Gaussian calculations were performed at the
high performance computing facility (HPCC) of University of Southern California
(USC).
10

5.3 Results and Discussion
In the following sections the results of the DFT and TDDFT calculations will
be discussed and compared with the absorption and emission data. The electronic
structures of the SGS and TES geometries of all the three structural isomers will be
analyzed in terms of bond lengths, dihedral angles, orbital energies, and orbital
populations. The nature of the low-lying singlet and triplet excited states will be
explored with TDDFT method. The vertical excitation energies derived from the
TDDFT method will be used to assign the peaks of the absorption spectra.

185
5.3.1 Singlet ground state (SGS) geometries and electronic structures.
Geometry optimization of the singlet ground state (SGS) structures and
ground state potential energy surface (PES) scans were performed using DFT
method with B3LYP functional and LANL2DZ basis set. In Figure 5.2, optimized
structures of the planar and the orthogonal isomers of (NO
2
ppy)-Pt-(acac) are
displayed with the ortep crystal structure. The lowest energy SGS structure was
calculated to be planar with C1 symmetry, which correlated well with the planar
structure obtained by X-ray crystallography.

(a) (b) (c)

Figure 5.2 (a) Crystal structure of (NO
2
ppy)-Pt-(acac). (b) Optimized SGS
structures of the planar isomer of (dmappy)-Pt-(acac) (CCNO dihedral angle = 0°).
(c) SGS optimized structures of the orthogonal isomer of (dmappy)-Pt-(acac)
(CCNO dihedral angle = 90°)

Summary of the calculated and crystallographic bond distances and angles
are tabulated in Table 5.1. All the calculated bond lengths of the planar structure
turned out to be slightly longer than the X-ray bond lengths. The Pt-N and Pt-C
186
bond distances were approximately 0.02 Å longer and the Pt-O bond distances were
approximately 0.04 Å longer than the X-ray bond distances. Variations in bond
distances within the coordination-sphere between the planar and the orthogonal
isomers were very small, suggesting that rotation of NO
2
has very little effect on the
coordination-sphere.
The C
7
-N bond length of the planar isomer was almost the same as the
experimental bond length (0.01 Å longer), which was however 0.02 Å shorter than
C
7
-N bond of the orthogonal isomer indicating a double bond type character in the
planar form. The N-O bonds of the planar isomer were more or less 0.06 Å longer
than the experimental N-O bonds and approximately 0.001 Å longer than the
orthogonal isomer.
Table 5.1. Comparison of the calculated bond lengths angles of the two structural
isomers of (NO
2
ppy)-Pt-(acac) with corresponding X-ray data.

SGS geometries
Bonds and angle Planar isomer Orthogonal isomer X-Ray
Pt-N 2.013 2.010 1.990
Pt-C 1.986 1.990 1.965
Pt-O
20
2.039 2.039 2.000
Pt-O
21
2.127 2.134 2.089
C
7
-N 1.464 1.479 1.459
N
35
-O
36
1.282 1.277 1.23
N
35
-O
37
1.281 1.277 1.213
O
36
-N
35
-O
37
123.93 124.80 124.91

187
HOMO LUMO
N
Pt 40.2
2.36
9.70
0.16
6.74
4.74
6.16
0.44
1.12
0.77
1.28
0.31
0.96
O
O
7.31
1.81
6.29
1.40
4.18
0.01
0.07
0.08
0.38 0.24
N
OO
N
Pt 0.87
2.07
0.13
2.50
0.11
2.13
0.08
10.5
0.15
8.25
4.69
6.02
0.25
O
O
0.09
0.04
0.06
0.03
0.08
0.00
0.00
N
OO
25.1
18.6 18.2
N
Pt
N
O O
40.9
2.18
11.0
0.43
7.01
6.20
7.21
0.16
1.25
0.38
1.41
0.46
1.31
O
O
6.03
1.54
4.85
1.33
3.21
0.01
0.05
0.00
0.16 0.17
N
Pt
0.72
0.09
0.05
0.08
0.08
0.18
0.46
1.11
0.70
4.59
5.29
6.92
0.45
O
O
0.01
0.03
0.02
0.02
0.07
0.01
0.03
N
O O
34.75
21.8 21.6

Figure 5.3 HOMO and LUMO orbital diagrams of the two structural isomers of
(NO
2
ppy)-Pt-(acac) are shown along with orbital characteristics making contribution
to the HOMO and the LUMO as percent contribution from each atom. (Top) Planar
isomer (CCNO = 0°). (Bottom) Orthogonal isomer (CCNO = 90°).

Population analysis of HOMO and LUMO are summarized Table 5.2. In
Figure 5.3, HOMO and LUMO orbital diagrams of the two structural isomers of
(NO
2
ppy)-Pt-(acac) are shown along with orbital framework as percent contribution
from each atom to the respective highest occupied (HO) and the lowest unoccupied
(LU) molecular orbitals. For the planar isomer HOMO was found to be primarily
composed of 40% Pt d orbital, 30% phenyl π orbital, and about 5% pyridine π
orbital. Contribution from the NO
2
moiety was almost negligible. The HOMO also
had about 21% acac character as well. The LUMO was found to be mainly
188
composed of 62% NO
2
, 30% pyridine, and 8% phenyl π∗ orbitals. Contribution to
LUMO was from

Table 5.2 Highest occupied and lowest virtual molecular orbitals of the two isomers
of (NO
2
ppy)-Pt-(acac) are shown with % MO character from various functionalities
making contributions to the respective HOMO and LUMO.

Functionalities % Character of MO Designation
HOMO LUMO
Planar (CCNO = 0°)
Pt 40.2% (d) 0.9% (d*) d --> d*
Pyridine 4.9% (π) 29.8% (π*) π --> π*
Phenyl 29.8% (π) 7.0% (π*) π --> π*
NO
2
0.7% (100% nitrogen p
z

and oxygen p
z
)
61.9% (100% nitrogen p
z

and oxygen p
z
)
acac 21.1% 0.3%
Orthogonal CCNO = 90°)
Pt 41.0% (d) 0.7% (d*) d --> d*
Pyridine 5% (π) 19.1% (π*) π --> π*
Phenyl 34.0% (π) 0.9% (π*) π --> π*
0.3% (nitrogen p
x
(25%),
p
y
(42%), p
z
(26%))
78.2% (nitrogen p
x
(50%),
p
y
(28%), p
z
(15%))
NO
2
(O
36
/O
37
p
x
(30/57%), p
y

(58/27%), p
z
(26/1%))
(O
36
/O
37
p
x
(65/65%), p
y

(34/34%))
acac 2.9% 2.9%

The HOMO of the orthogonal isomer was pretty much the same as the planar
isomer except for the NO
2
moiety. The MO composition of this functionality varied
significantly from the planar isomer. In the planar isomer, contribution from the
nitrogen atom was only from its p
z
orbital, which in the orthogonal isomer switched
to a mixture of p
x
, p
y
, and p
z
orbitals. The over all makeup of the two oxygen atoms
also changed from very small amount of mixed p orbitals to mostly p
x
, and p
y

orbitals. This switch from all p
z
orbitals in the planar to a mixture of p orbitals in the
189
orthogonal isomer significantly changes the electronic structures of the excited
states.

5.3.2 Ground state PES scan
-180 -120 -60 0 60 120 180
0
1
2
3
4
5
6
7
8
9
10
SCF Energy (kcal/mol)
CCNO dihedral angle (θ)
(NO2ppy)-Pt-(acac)

Figure 5.4 Plot total SCF energy vs. CCNO dihedral angle showing the result of the
ground state PES scan of (NO
2
ppy)-Pt-(acac).

The ground state potential energy surface scan (PES) was performed on the
optimized SGS geometry by rotating the NO
2
moiety 360° with 15° increments. The
result of the PES scan is shown as a plot of SCF energy vs. CCNO dihedral angle in
Figure 6.4. The surface showed two peaks and three valleys for the 360° rotation.
The two peaks at -90° and +90° represent the orthogonal isomer and the three peaks
at -180°, 0°, and +180° represent the planar isomer. The energy difference between
the planar and the orthogonal structure was calculated to be 8.38 kcal/mol, which
190
correlated well with the published results of 6-8 kcal/mol calculated for the energy
difference between the planar and the orthogonal structure of nitrobenzene.
11, 12

1.460
1.464
1.468
1.472
1.476
1.480
-180 -135 -90 -45 0 45 90 135 180
C7-N

Bond length (A)
1.2765
1.2788
1.2811
O36-N
O37-N
Bond length (A)
-180 -135 -90 -45 0 45 90 135 180
123.8
124.0
124.2
124.4
124.6
124.8 O36-N-O37
Bond angle (
0
)
CCNO dihedral angle (θ)

Figure 5.5 Results of SGS PES scan showing: (top) the plot of C
7
-N bond length vs.
dihedral angle, (middle) the plot of O
36
and O
37
-N bond length vs. dihedral angle,
and (bottom) the plot of ∠O
36
NO
37
vs. dihedral angle.

The PES scan also showed that the C
7
-N and O
36
-N, and O
37
-N bonds in the
NO
2
moiety changes as a function of NO
2
rotation. Figure 5.5 displays the bond
distances and angles in the NO
2
moiety plotted against the dihedral angle. The top
plot represents the change in C
7
-N bond length, which decreases in size when the
dihedral angle becomes 0° and the NO
2
moiety becomes planar. The bond length
191
changes approximately 0.02Å going from the pyramidal to the planar form. In the
planar form p-π interaction between the N and the pyridine ring increases, this
decreases the C7-N bond and increases the O
36
/O
37
-N bonds. As a result ∠O
36
NO
37

decreases in size. In the pyramidal form, a decrease in conjugation causes the C
7
-N
bond to increase in size and O
36
/O
37
-N bonds to decrease in size, which then causes
∠O
36
NO
37
to increase in size.

5.3.2.1 HOMO LUMO energies
0 2040 6080 100
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
0 2040 6080 100
2.48
2.52
2.56
2.60
2.64
HOMO
LUMO
Orbital Energies (eV)
CCNO Dihedral Angle (Θ)

Optical Gap (eV)

Figure 5.6 A double Y plot of the HOMO-LUMO energies and HOMO-LUMO gaps
vs. the dihedral angle, θ showing the changes in orbital energies as a function of NO
2

rotation.

The orbital energies for the highest occupied (HO) and the lowest unoccupied
(LU) molecular orbitals of (NO
2
ppy)-Pt-(acac) were obtained from the SGS
192
geometries in two different ways. In the first way, the energies were obtained by
frozen-core approximation from the optimized PES coordinates. In the second way,
the orbital energies were obtained by geometry optimization with constrained
dihedral angle (with double and single constraints from 0° to 90°). Results obtained
from both methods showed good agreement.

Table 5.3 Summary of the results of the HOMO-LUMO energies and HOMO-
LUMO gaps.

CCNO HOMO LUMO Gap Gap
θ (eV) (eV) (eV) (eV)
0 -6.04 -3.56 2.48 500.49
15 -6.05 -3.51 2.53 490.90
30 -6.02 -3.48 2.57 482.88
45 -5.98 -3.51 2.61 474.24
60 -5.95 -3.51 2.64 469.26
75 -5.93 -3.50 2.63 470.71
90 -5.94 -3.41 2.62 472.86

In Figure5.6, a double Y plot of the HOMO-LUMO energies and HOMO-
LUMO gaps vs. the dihedral angle, θ is shown. Both HOMO and LUMO were
found to change as a function of NO
2
rotation. However, the change in LUMO was
found to be slightly greater than the change in HOMO (Table 5.3). The optical gap
was found to be the least at 0° and increased steadily up to 90° with some fluctuation
in between. In Figure 5.7, the HOMO and LUMO diagrams of (NO
2
ppy)-Pt-(acac)
obtained from SGS PES scan are shown for the dihedral angles starting from 0° to
193
90°. From the MO pictures it is evident that the HOMOs remain unchanged and the
LUMOs change steadily as a function of NO
2
rotation.
0°
HOMO LUMO
30°
60°
90°
15°
HOMO LUMO
45°
75°

Figure 5.7 Highest occupied and lowest unoccupied MOs of (NO
2
ppy)-Pt-(acac)
obtained from SGS PES scan are shown for the dihedral angles from 0° to 90°. The
results show HOMOs remain unchanged as the LUMOs change with NO
2
rotation.

194
The effect on LUMO is due to the electron withdrawing ability of the NO
2

moiety. At 0°, the LUMO seems to delocalize throughout the ppy ligand due to
increased conjugation between the NO
2
moiety and the ppy ligand. As a result of
this effect the optical gap decreases. At 90°, the LUMO localizes mostly on the ppy
ligand due to decreased conjugation between the NO
2
moiety and the ppy ligand.
This effect destabilizes the LUMO (moves up), which then increases the optical gap
of the system.

5.3.3 TDDFT analysis: Electronic absorption spectra and singlet
excited states
The low-lying singlet and triplet states of (NO
2
ppy)-Pt-(acac) were examined
by time dependent (TD) DFT calculations. Ten singlet and ten triple states were
calculated from the optimized singlet ground state geometries of the planar (CCNO =
0°) and the perpendicular (CCNO = 90°) structures. Calculations were performed in
the gas phase in hexanes and in CH
3
CN. Vertical excitation energies in hexanes and
CH
3
CN were calculated with TDDFT/CPCM method. The outputs of the TDDFT
calculations contained the vertical excitation energies (E
VT
), orbitals involved in each
transition, the orbital coefficients, and oscillator strengths for each transition. The
orbital coefficients are the coefficients of the wavefunctions for each excitation,
which are directly proportional to the contribution of the given excitation of the
transition.
195
The electronic absorption spectrum of (NO
2
ppy)-Pt-(acac) was studied in
hexanes. Two distinct regions were observed on the spectrum: A low intensity
region of low energies showed up between 400nm to 550 nm and a high intensity
region of high energies showed up between 300 nm to 400 nm. Figure 5.8 shows the
absorption spectrum of (NO
2
ppy)-Pt-(acac) with vertical excitation energies and
oscillator strengths calculated in hexanes overlaid on top of it, where the blue bars
represent the planar isomer and the red bars represent the orthogonal isomer. As
expected, the overall results of the calculation showed lower excitation energies for
the planar isomer and relatively higher excitation energies for the orthogonal isomer.
The probable assignments of these optical transitions were made on the basis of the
computational assignment of the singlet excited states. The results are summarized
in tables 5.4, 5.5, and 5.6.
300 350 400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0
0.00
0.05
0.10
0.15
0.20
Intensity (a.u.)
Wavelength (nm)
abs
Oscillator strengths
Singlet Flat
Singlet 90 degree

Figure 5.8 Room temperature absorption spectra of (NO
2
ppy)-Pt-(acac) in hexanes
with superimposed gas phase TDDFT vertical excitation energies showing spectral
peak assignments.

196
The gas phase vertical excitation energies of (NO
2
ppy)-Pt-(acac) were
calculated to be different than the experimental optical transitions. The S
0
→S
1

optical transition for the planar isomer in the gas phase (Table 5.4) was calculated to
be 550 nm, which was found to be 58 nm red-shifted from the experimental optical
transition in hexanes (492 nm). The same transition for the gas phase orthogonal
isomer was calculated to be 483 nm, which was approximately 67 nm blue-shifted
from the planar isomer and was much closer to the experimental transition.
However, the oscillator strength for this transition was 0. Therefore, based on the
gas phase calculations, it can be hypothesized that the conformation of (NO
2
ppy)-Pt-
(acac) in the singlet excited state is planar. The result is consistent with the lowest
energy conformation predicted by the DFT method.

Table 5.4 Uncorrected gas phase TDDFT vertical excitation energies (E
VT
),
dominant MO transitions, orbital coefficients, and oscillator strengths calculated for
planar and perpendicular isomers of (NO
2
ppy)-Pt-(acac).

Planar (NO
2
ppy)-Pt-(acac) in the gas phase
States E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(nm) MO Composition Coefficients
strength
(f)
S
1
550.72 87 -> 88 0.68124 0.0494
S
2
491.45 86 -> 88 0.68085 0.0236
S
3
482.4 85 -> 88 0.69936 0.0006
S
4
396.27 87 -> 89 0.64922 0.0055
S
5
372.45 83 -> 88 0.30273 0.0735
84 -> 88 0.58086
S
6
370.41 82 -> 88 0.67759 0.0065
S
7
364 83 -> 88 0.54408 0.0545
86 -> 89 0.35957
S
8
362.16 85 -> 89 0.69702 0.0017
197
Table 5.4 Continued
S
9
349.01 83 -> 88 0.28201 0.2062
86 -> 89 0.50506
S
10
346.68 79 -> 88 0.6292 0.0048
T
1
617.67 87A -> 88A 0.67457 ---
T
2
539.67 86A -> 88A 0.6352 ---
T
3
511.76 77A -> 88A 0.73162 ---
T
4
489.15 85A -> 88A 0.69662 ---
T
5
443.8 84A -> 88A 0.61378 ---
T
6
425.77 87A -> 89A 0.66073 ---
T
7
412.28 79A -> 88A 0.69105 ---
Pyramidal (NO
2
ppy)-Pt-(acac) in the gas phase
States E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(nm) MO Composition Coefficients
strength
(f)
S
1
483.22 87 -> 88 0.69633 0.0000
S
2
435.47 87 -> 89 0.6317 0.0238
S
3
431.05 86 -> 88 0.68986 0.0000
S
4
406.75 85 -> 88 0.70546 0.0005
S
5
395.1 85 -> 89 0.69878 0.0023
S
6
379.47 86 -> 89 0.59053 0.0541
S
7
369.11 84 -> 88 0.61337 0.0000
S
8
361.33 87 -> 90 0.64468 0.0573
S
9
332.21 86 -> 90 0.62208 0.0170
S
10
328.47 85 -> 90 0.70134 0.0002
T
1
515.25 75A -> 88A 0.77509 ---
76A -> 88A 0.56828
T
2
496.01 86A -> 89A 0.3203 ---
87A -> 89A 0.62513
T
3
489.2 87A -> 88A 0.64475 ---
T
4
417.59 86A -> 89A 0.56014 ---
87A -> 89A 0.34379
T
5
440.73 86A -> 88A 0.57822 ---
T
6
410.18 85A -> 89A 0.7014 ---
T
7
409.78 86A -> 91A 0.45377 ---

198

300 350 400 450 500 550
0.0
0.2
0.4
0.6
0.8
1.0
0.00
0.05
0.10
0.15
0.20
0.25
Intensity (a.u.)
Wavelength (nm)
abs
Oscillator strengths
Singlet Flat
Singlet 90 degree
300350 400450 500550
0.0
0.2
0.4
0.6
0.8
1.0
0.00
0.05
0.10
0.15
0.20
0.25
Intensity (a.u.)
Wavelength (nm)
abs
Oscillator strengths
Singlet Flat
Singlet 90 degree

Figure 5.9 Room temperature absorption spectra of (NO
2
ppy)-Pt-(acac) in hexanes
with superimposed TDDFT/CPCM vertical excitation energies showing spectral
peak assignments. (left) Original Vertical excitation energies. (right) Vertical
excitation energies blue-shifted by 50nm.

Further examination of the singlet-to-singlet excited state transitions was
done with TDDFT/CPCM calculations. The S
0
→S
1
optical transition for the planar
isomer in hexanes was calculated to be 565 nm, which was 73 nm red-shifted from
the experimental transition and 15 nm red-shifted from the gas phase transition
(Figure 5.9). However, the transition in hexanes (oscillator strength = 0.07) was
calculated to be much higher in intensity than in the gas phase transition (oscillator
strength = 0.04) suggesting a higher probability of this transition in hexane. For the
orthogonal isomer, the E
VT
in hexane was calculated to be 508 nm, which turned out
be much closer to the experimental energy than the planar isomer. However, the
oscillator strength for this transition was zero just like it was predicted in the gas
phase calculation. Furthermore, for the orthogonal isomer, oscillator strengths
calculated for the S
0
→S
2
and S
0
→S
3
state transitions in hexanes were also calculated
199
to be 0. In the planar isomer, oscillator strengths for the first three states were
calculated to be 0.07, 0.03, and 0.01 suggesting that (NO
2
ppy)-Pt-(acac) exists in the
planar form in the singlet excited state. This result agrees with the DFT calculation
of the ground state, which showed that in the ground state (NO
2
ppy)-Pt-(acac) exists
in the planar form. Therefore, from the calculations it can be suggested that the
S
0
→S
1
transition is a planar →planar transition.

Table 5.5 Uncorrected TDDFT/CPCM vertical excitation energies (E
VT
), dominant
MO transitions, orbital coefficients, and oscillator strengths calculated for planar and
perpendicular isomers of (NO
2
ppy)-Pt-(acac) in hexanes.

Planar (NO
2
ppy)-Pt-(acac) in hexanes
States E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(nm) MO Composition Coefficients
strength
(f)
S
1
565.03 87 -> 88 0.68711 0.0701
S
2
500.93 86 -> 88 0.64709 0.0338
S
3
500.15 85 -> 88 0.6614 0.0052
S
4
391.08 87 -> 89 0.63958 0.0134
S
5
379.79 84 -> 88 0.64748 0.1962
S
6
372.96 82 -> 88 0.68031 0.0065
S
7
370.32 83 -> 88 0.65029 0.0894
S
8
365.23 85 -> 89 0.6987 0.0042
S
9
351.97 86 -> 89 0.62165 0.1711
S
10
346.16 86 -> 89 0.62165 0.0001
T
1
625.05 87A -> 88A 0.67649 ---
T
2
545.28 86A -> 88A 0.64639 ---
T
3
505.53 85A -> 88A 0.69891 ---
T
4
491.81 76A -> 88A 0.77415 ---
77A -> 88A 0.30669 ---
T
5
447.7 84A -> 88A 0.61621 ---
T
6
422.14 87A -> 89A 0.65783 ---
T
7
402.97 78A -> 88A 0.34975 ---
86A -> 89A 0.31891 ---
200
Table 5.5 Continued
Pyramidal (NO
2
ppy)-Pt-(acac) in hexanes
States E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
(nm) MO Composition Coefficients
strength
(f)
S
1
507.87 87 -> 88 0.69749 0.0000
S
2
452.04 86 -> 88 0.69285 0.0000
S
3
433.49 85 -> 88 0.69553 0.0005
S
4
428.2 87 -> 89 0.62907 0.0417
S
5
396.8 85 -> 89 0.70004 0.0054
S
6
380.86 84 -> 88 0.63996 0.0000
S
7
376.29 86 -> 89 0.62062 0.0952
S
8
355.7 87 -> 90 0.67464 0.0620
S
9
337.69 83 -> 88 0.69265 0.0000
S
10
335.05 82 -> 88 0.69369 0.0004
T
1
514.89 87A -> 88A 0.63909 ---
74A -> 88A 0.33703 ---
T
2
500.73 74A -> 88A 0.85033 ---
77A -> 88A 0.29047 ---
T
3
487.1 86A -> 89A 0.33842 ---
87A -> 89A 0.61523 ---
T
4
458.53 86A -> 88A 0.62852 ---
T
5
446.21 86A -> 89A 0.54753 ---
87A -> 89A 0.36231 ---
T
6
434.41 85A -> 88A 0.70506 ---

Mismatch between the experimental electronic transition energies and
TDDFT calculated vertical excitation energies are common in the literatures and
have been shown to be related to the functionals
13-15
and basis sets
16
. It is also
known that B3LYP optimized geometries systematically underestimate the
calculated transition energies.
17
The term bond length alteration parameter (BLA),
defined as the difference in length between the single and the double bonds reflects
the degree of uneven distribution of π electrons over the bonds. The B3LYP
functional significantly underestimates the BLA parameter, which results in
201
overestimated electronic delocalization giving red-shifted excited state energies.
Therefore, to obtain a good fit with the experimental transition energies application
of correction factors is also common in the literatures.
13-18
In our case, since the
TDDFT/CPCM calculated optical transition profile matched well with the absorption
spectral profile. The calculated transition energies were then 50 nm blue-shifted by
correlating the most intense optical transition with the vertical excitation energy of
the highest oscillator strength.
The corrected version of the TDDFT energies is tabulated in comparison with
the experimental optical transitions in Table 5.6. For the lowest energy S
0
→S
1

optical transition calculated energy was 515 nm after correction and was closer to the
experimental transition of 492 nm. Contribution to this state came from HOMO
(MO-87) to LUMO (MO-88) transition, orbital coefficient and oscillator strength for
which was 0.69 and 0.04 respectively. Based on the orbital population analysis
HOMO-LUMO transition for the S
0
-S
1
excitation can be assigned to have 40%
metal-to-metal-and-ligand-charge transfer
1
MMLCT character and 60% ligand-to-
ligand-and-metal-charge transfer (
1
LLMCT) character.
The
1
MMLCT transition involves electronic delocalization from the platinum
d orbital to the platinum d* orbital, π* orbitals of the phenyl (Ph) and the pyridine
(Py) rings, and p
z
* orbital of NO
2
. The
1
LLMCT transition involves electronic
delocalization from the phenyl π orbital, acac p orbitals, and NO
2
Np
z
+ Op
z
orbitals
to the platinum d* orbital, π* orbitals of the phenyl (Ph) and pyridine (Py) rings, and
p
z
* orbital of NO
2
. In the planar isomer, the
1
MMLCT transition corresponds to:
202
40% Pt (d) → [62% NO
2
(p
z
*) + 30% Py (π*) + 7% Ph (π*) + 1% Pt (d*)] and the
the
1
LLMCT transition corresponds to: [30% Ph (π) + 21% acac (p) + 5% Py (π) +
0.7% NO
2
(p
z
)] → [62% NO
2
(p
z
*) + 30% Py (π*) + 7% Ph (π*) + 1% Pt (d*)].
The
1
MMLCT transition in the orthogonal isomer behaves similar to the
planar isomer. The transition corresponds to: 41% Pt (d) → [78% NO
2
(p
z
*) + 19%
Py (π*) + 1% Ph (π*) + 0.7% Pt (d*)]. The
1
LLMCT transition in the orthogonal
isomer however varies more because the Np
z
and Op
z
orbitals in the NO
2
moiety
switch from 100% p
z
to a mixture of Np
x
/Op
x
, Np
y
/Op
y
, and Np
z
/Op
z
in HOMO and
LUMO: [34% Ph (π) + 5% Py (π) + 0.3% NO
2
(Np
x
/Op
x
+ Np
y
/Op
y
+ Np
z
/Op
z
)] →
[78% NO
2
(Np
x
*/Op
x
* + Np
y
*/Op
y
* + Np
z
*/Op
z
*) + 19% Py (π*) + 1% Ph (π*) +
0.7% Pt (d*)]. For the S
0
→S
2
and S
0
→S
3
electronic transitions contributions came
from HOMO-1 (MO-86) to LUMO (MO-88) and HOMO-2 (MO-86) to LUMO
(MO-88) respectively. Contributions to the higher states come from occupied and
virtual MOs above and below the HOMO and LUMO. Higher energy states were
complex mixtures of MOs and are charge transfer in character.

Table 5.6 Blue-shifted (50nm) TDDFT/CPCM vertical excitation energies (E
VT
),
dominant MO transitions, orbital coefficients, and oscillator strengths calculated for
planar and perpendicular isomers of (NO
2
ppy)-Pt-(acac) in hexanes are shown with
the experimental optical transitions.

Planar (NO
2
ppy)-Pt-(acac) in hexanes
States Experimental E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
λ (nm) (nm) MO Composition Coefficients
strength
(f)
S
1
492 515.03 87 -> 88 0.68711 0.0701
S
2
466 450.93 86 -> 88 0.64709 0.0338
203
Table 5.6 Continued
S
3
440 450.15 85 -> 88 0.6614 0.0052
S
4
418 341.08 87 -> 89 0.63958 0.0134
S
5
410 329.79 84 -> 88 0.64748 0.1962
S
6
370 322.96 82 -> 88 0.68031 0.0065
S
7
365 320.32 83 -> 88 0.65029 0.0894
S
8
340 315.23 85 -> 89 0.6987 0.0042
S
9
325 301.97 86 -> 89 0.62165 0.1711
S
10
311 296.16 86 -> 89 0.62165 0.0001
T
1
--- 575.05 87A -> 88A 0.67649 ---
T
2
--- 495.28 86A -> 88A 0.64639 ---
T
3
--- 455.53 85A -> 88A 0.69891 ---
T
4
--- 441.81 76A -> 88A 0.77415 ---
77A -> 88A 0.30669 ---
T
5
--- 397.7 84A -> 88A 0.61621 ---
T
6
--- 372.14 87A -> 89A 0.65783 ---
T
7
--- 352.97 78A -> 88A 0.34975 ---
86A -> 89A 0.31891 ---
Pyramidal (NO
2
ppy)-Pt-(acac) in hexanes
States Experimental E
VT
Ψ
OC
-> Ψ
VER
Orbital Oscillator
λ (nm) (nm) MO Composition Coefficients
strength
(f)
S
1
492 457.87 87 -> 88 0.69749 0.0000
S
2
466 402.04 86 -> 88 0.69285 0.0000
S
3
440 383.49 85 -> 88 0.69553 0.0005
S
4
418 378.2 87 -> 89 0.62907 0.0417
S
5
410 346.8 85 -> 89 0.70004 0.0054
S
6
370 330.86 84 -> 88 0.63996 0.0000
S
7
365 326.29 86 -> 89 0.62062 0.0952
S
8
340 305.7 87 -> 90 0.67464 0.0620
S
9
325 287.69 83 -> 88 0.69265 0.0000
S
10
311 285.05 82 -> 88 0.69369 0.0004
T
1
--- 464.89 87A -> 88A 0.63909 ---
74A -> 88A 0.33703 ---
T
2
--- 450.73 74A -> 88A 0.85033 ---
77A -> 88A 0.29047 ---
T
3
437.1 86A -> 89A 0.33842 ---
87A -> 89A 0.61523 ---
T
4
--- 408.53 86A -> 88A 0.62852 ---
T
5
--- 396.21 86A -> 89A 0.54753 ---
204
Table 5.6 Continued
87A -> 89A 0.36231 ---
T
6
--- 384.41 85A -> 88A 0.70506 ---

5.3.4 Lowest triplet excited state (TES) geometries and electronic
structures.
Lowest triplet excited state geometries and potential energy surface (PES)
scans were calculated using unrestricted SCF method (uB3LYP/LANL2DZ). The
lowest energy optimized structure in the triplet state was also found to be planar.
Gaussian was unable to optimize the orthogonal isomer in the triplet state.
Therefore, optimized structure for the orthogonal isomer was obtained from the
triplet state PES scan. Summary of the calculated bond lengths of the two optimized
triplet state isomers of (NO
2
ppy)-Pt-(acac) are reported in Table 5.7. In the triplet
state, the Pt-N and the Pt-O
20
bonds elongated and the Pt-C and the Pt-O
21
shortened
for both isomers.

Table 5.7 Summary of the calculated TES bond lengths and angles of the two
structural isomers of (NO
2
ppy)-Pt-(acac).

TES geometries
Bonds and
angle
Planar
isomer
Orthogonal
isomer
Pt-N 2.019 2.012
Pt-C 1.949 1.987
Pt-O
20
2.060 2.040
Pt-O
21
2.101 2.132
C
7
-N 1.419 1.447
N
35
-O
36
1.305 1.366
N
35
-O
37
1.309 1.373
205
The C
7
-N bond in the triplet state planar isomer decreased in size by about
0.4Å and the N-O bonds increased in size about 0.02 Å from the ground state planar
isomer, suggesting that in the planar triplet state conjugation between the NO
2

moiety and the ppy ligand increases. Similar trend was observed for the orthogonal
isomer, which also showed a decrease in C
7
-N bond length in the triplet state.
However, the N-O bonds in the triplet orthogonal isomer increased by almost 0.1 Å
compared to the singlet orthogonal isomer. The overall trend in the triplet state were
same as the ground state, meaning the C
7
-N bond in the triplet state orthogonal
isomer increased in size compared to the planar isomer.

5.3.5 Triplet state PES scan
0 204060 80 100 120
640
620
600
580
560
540
520
500
0 204060 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Triplet Energy (nm)

Total Energy (eV)
CCNO Dihedral Angle (Θ)
Ground state
Triplet state

Figure 5.10 Triplet state potential energy surface (PES) scan of (NO
2
ppy)-Pt-(acac)
is shown with the ground state PES scan and the triplet energies on the right Y axis.

206
The triplet state potential energy surface (PES) scan was performed on the
optimized SGS planar isomer by rotating the NO
2
moiety 360° with 15° increments.
In Figure 5.10, the result of the triplet state PES scan from 0° to 90° is shown with
the ground state PES scan and triplet energies (E
T
). The triplet energies of
(NO
2
ppy)-Pt-(acac) were calculated by subtracting the ground state SCF energies
from the triplet state SCF energies (Equation 2.1). The difference between the SGS
and TES PES energies shows the triplet energies of the molecule in eV. A
comparison of the triplet energies in nanometer scale is also shown on the right Y
plot. Similar to the ground state, the triplet state potential energy surface showed a
maxima at 90°. However, unlike the individual DFT optimization structure, which
showed the triplet state minima at 0°, the triplet state minima calculated from the
difference between the SGS and TES PES showed the triplet minima at 45°. Barrier
to 0° → 90° rotation was calculated to be 8.4 kcal/mol. As expected, the triplet
energy increased going from the planar to the perpendicular isomer. The triplet
energies calculated for the planar and the perpendicular isomers were 2.02 eV
(corresponding to red emission) and 2.22 eV (corresponding to yellow emission)
respectively.
The triplet spin density plots of the structures with dihedral angle from
CCNO = 0° to 90° are depicted in Figure 5.11. The blue contour plots were obtained
from Gaussian calculations and the red ones from Titan calculations. The Gaussian
spin densities shows very little change in the density of unpaired electrons going
from 0° to 90°. For the two extreme cases, Gaussian calculated spin densities on the
207
0° 30°
60°
90°
15°
45° 75°
0°
90°
0° 30°
60°
90°
15°
45° 75°
0°
90°

Figure 5.11 Gaussian (blue) spin-density plots of TES optimized structures of
(NO
2
ppy)-Pt-(acac) shown for the dihedral angles from 0° to 90°. Titan (red) spin-
density plots were obtained by optimizing the structures in the triplet state.

NO
2
moiety remain almost the same, suggesting that a large contribution to the
triplet emission in the orthogonal isomer comes from the NO
2
moiety. Since this
result was contradictory to the experimental results, for consistency it was compared
208
with Titan optimized triplet state geometries. The spin density data for the two
triplet isomers calculated both in Gaussian and Titan are summarized in Table 5.8.
For the Gaussian calculated planar isomer, 12% unpaired electron density was
observed on Pt, 39% on pyridine, 32% on phenyl, 15% on NO
2
, and about 1.6% on
acac, indicating that emission from the triplet state planar isomer arises
predominantly from the ppy ligand (71%) with some contribution from the NO
2

moiety and the metal. These results corroborated well with the Titan results.
Table 5.8 Summary of the Gaussian and Titan calculated spin-densities of TES
isomers of (NO
2
ppy)-Pt-(acac) showing percent density of the unpaired electrons.

Gaussian calculation
Functionalities Planar isomer Orthogonal isomer
Pt 12.22% 2.34%
Pyridine 39.27% 7.28%
Phenyl 31.97% 7.10%
NO
2
14.95% 83.02%
acac 1.60% 0.25%
Titan calculation
Functionalities Planar isomer Orthogonal isomer
Pt 13.15% 13.39%
Pyridine 33.58% 39.39%
Phenyl 39.37% 44.09%
NO
2
11.15% 1.29%
acac 2.72% 1.80%

Going from the planar to the perpendicular triplet state isomer, the density
calculated by Titan did not change much for the overall molecule except for the fact
that the percent electron density on the NO
2
moiety decreased from 11% to 1.3%.
This fact suggests that the triplet emission from the orthogonal isomer mostly comes
from the ppy ligand and the metal. The calculation in Gaussian however, disagreed
209
with this result. For the orthogonal isomer, Gaussian-calculated spin density on Pt
decreased to 2.3%, on pyridine to 7.2%, and on phenyl to 7.1%. Furthermore
contrary to Titan calculations, the electron density on the NO
2
moiety increased to
83%, suggesting that emission from the triplet state orthogonal isomer comes
predominantly from the NO
2
moiety. This result is incorrect and it is obtained as a
consequence of Gaussian’s limitation.

5.3.6 Emission spectroscopy and properties of the triplet excited states.
Room temperature and 77K emission spectra of (NO
2
ppy)-Pt-(acac) in
hexanes, 2-methyl THF, and toluene are shown in Figure 5.12. All the photophysical
data are summarized in table 5.9. Room temperature emission of (NO
2
ppy)-Pt-
(acac) in hexanes showed up as an intense narrow band at λ
max
= 547 nm with a
shoulder at 586 nm. Lifetime for this emission was measured to be between 2-3 μs,
consistent with phosphorescence emission. In 77K hexane glass, the
phosphorescence bands blue shifted to 535 nm and 570 nm. This is expected to
happen at very low temperature and is known as rigidochromic shift. The 77K
phosphorescence spectra in toluene and 2M-THF behaved exactly like in hexanes
and showed up around the same regions. The lifetimes for these low temperature
emission maxima were measured to be approximately 10 μs. The narrow intense
emission band around 540 nm indicate that the distribution of one emitting species is
predominant.

210
450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
Toluene: RT emission
Toluene: 77K emission
0.0
0.2
0.4
0.6
0.8
1.0
2M THF: RT emission
2M THF: 77K emission
Intensity (a.u.)
0.0
0.2
0.4
0.6
0.8
1.0
450 500 550 600 650 700 750

Hexanes: RT emission
Hexanes: 77K emission

Figure 5.12 Room temperature and 77K emission spectra of (NO
2
ppy)-Pt-(acac) in
hexanes, 2M THF, and toluene.

The room temperature phosphorescence spectra in these solvents however,
did not behave like in hexanes. The spectra were broad and very weak in intensity
with much shorter lifetimes (2-3μs) suggesting a large distribution of different
emitting species. The shorter phosphorescence lifetimes also suggest that in room
temperature some species are non-emissive or very weakly emissive. This fact was
211
consistent with emission spectroscopy performed in acetonitrile, where no emissions
were observed at RT and at 77K.

475 500 525 550 575 600 625 650 675 700
0.0
0.2
0.4
0.6
0.8
1.0
UGH: RT emission
0.0
0.2
0.4
0.6
0.8
1.0
CBP: RT emission
Intensity (a.u.)
0.0
0.2
0.4
0.6
0.8
1.0
475 500 525 550 575 600 625 650 675 700

Polystyrene: RT emission
Polystyrene: 77K emission

Figure 5.13 Room temperature emission spectra of (NO
2
ppy)-Pt-(acac) in
polystyrene matrix, CBP matrix, and UGH matrix.

Thin film photoluminescence spectra of (NO
2
ppy)-Pt-(acac) in polystyrene,
CBP, and UGH matrices are displayed in Figure 5.13. Emissions from (NO
2
ppy)-Pt-
(acac) inside these solid hosts also showed weak, broad, and short-lived
phosphorescence bands between 550 nm to 560 nm consistent with the fact that the
212
emissive species in orthogonal in the triplet sate. These results corroborate well with
the calculated triplet energy of 558 nm corresponding to the 90° isomer.

Table 5.9 Summary of the photophysical data
Medium Emission Lifetime Emission Lifetime Emission
RT (nm) RT (μs) 77K (nm) 77K (μs) Color
Hexane 547, 586 2-3 535, 570 --- Yellow
Toluene 525, 560
a
2-3 545, 584 10.2 Yellow
2me-THF 480, 525,
560
a,b
2-3 535, 567 10 Yellow
Polystyrene
matrix 553
a,b
4.8 553, 592 9.6 Yellow
UGH matrix 550
a,b
4 --- --- Yellow
CBP matrix 560
a,b
3 --- --- Yellow
a = weak emission, b = broad emission

Since the emission from (NO
2
ppy)-Pt-(acac) always shows up around 550
nm, based on the emission properties and DFT results it can be suggested that
preferred orientation of the triplet emitting species in solid and liquid solutions at any
temperature is perpendicular. Meaning, that the triplet state responsible for the
yellow emission in RT and at 77k is the orthogonal structural isomer of (NO
2
ppy)-
Pt-(acac). Furthermore, if any solvent reorients the molecule to its planar form then
it becomes non-emissive, which was observed in highly polar solvent acetonitrile.
Therefore, in a slightly polar solvent like 2me-THF, the population distribution of
the planar isomer increases giving rise to a weak dual emission, where the intense
peak at 560 nm corresponds to the perpendicular isomer and the broad low energy
tail corresponds to the planar non-perpendicular isomers.
213
In Figure 5.14, the calculated triplet energies of (NO
2
ppy)-Pt-(acac) are
shown as a function of NO
2
rotation. The results of these triplet state DFT
calculations show that the only triplet species that emits yellow is the 90° conformer
and rests are all red. The results of the DFT calculation however, also showed that
the preferred orientation of (NO
2
ppy)-Pt-(acac) in the ground state is planar. In
addition to that, the TDDFT/CPCM calculations showed that S
0
→S
1
transition is a
planar →planar transition. So, the question remains what happens when (NO
2
ppy)-
Pt-(acac) absorbs light?

C-C-N-O dihedral angle
T
1
T
1
S
0
T
1
T
1
558 nm
T
1
595 nm
T
1
Figure 5.14 Gas phase triplet energies of (NO
2
ppy)-Pt-(acac) calculated for the
seven structures between 0° to 90° dihedral angles is showing a steady increase in
the triplet energies going from the 0° to the 90° isomer.

Figure 5.15 depicts one of the proposed scheme of absorption and emission
pathways of (NO
2
ppy)-Pt-(acac). Based on the DFT and TDDFT calculations it can
617 nm
626 nm
619 nm
626 nm
615 nm
T
1
0º 15 30 45 60 75 90
214
be suggested that (NO
2
ppy)-Pt-(acac) in the ground state (S
0
) exists in the planar
form. When it absorbs light, the molecule can either get excited to the planar singlet
excited state S
1
, or to the planar triplet excited state T
1
. Upon direct excitation to the
planar triplet manifold, the molecule cannot rotate from the planar to the orthogonal
form because based on the TDDFT calculation the triplet excitation energy is lower
than the triplet emission energy. Therefore, at 77K this process may not occur. The
possibility of this occurring in room temperature is also very thin because the triplet
vertical excitation energy is 66 meV lower than the triplet emission energy. Since
the room temperature kT is 26 meV, it would take the energy equivalence of more
than 2kT to populate the orthogonal triplet state in room temperature.
S
0
90º
E
T
: 615 nm
T
1
11.24 D
S
1
Relax
ISC
T
1
E
T
: 558 nm
Rotate 0°-90° ? Relax
ISC
3.08 D
?
0º 0º 90º 0º
T
1
S
1
TDDFT
515 nm
(f = 0.07)
3.53 D
Rotate 0°-90° , ISC
?
T
1
T
1
E
T
: 558 nm
TDDFT
575 nm
3.08 D
3.53 D
0º
Relax
S
0
90º
E
T
: 615 nm
T
1
11.24 D
S
1
Relax
ISC
T
1
E
T
: 558 nm
Rotate 0°-90° ? Relax
ISC
3.08 D
?
0º 0º 90º 0º
T
1
S
1
TDDFT
515 nm
(f = 0.07)
3.53 D
Rotate 0°-90° , ISC
?
T
1
T
1
E
T
: 558 nm
TDDFT
575 nm
3.08 D
3.53 D
0º
Relax

Figure 5.15 A proposed scheme of excitation and de-excitation pathways of
(NO
2
ppy)-Pt-(acac).

215
From the S
1
excited state, the molecule can relax to its lowest energy S
1
state
and fluoresce back to the ground state. However, since no fluorescence is observed
either in RT or at 77K, the possibility of fluorescence simply does not exist.
Furthermore, since emission in region around 600-700 nm is not observed, the
possibility of phosphorescence also diminishes. Therefore, the most probable thing
that happens to the planar molecule in the S
1
state is that it rotates to the S
1

orthogonal conformer. Once the 0° to 90° rotation takes place, the molecule can go
down two possible pathways: One, where it can intersystem cross to the triplet
manifold (T
1
), relax to the lowest energy T
1
emitting state and phosphoresce back to
the ground state; Two, where it can first relax to the lowest energy S
1
orthogonal
conformer, then intersystem cross to the T
1
triplet state and phosphoresce back to the
ground state (Equation 5.1).
S
0
(Planar) + hν → S
1
(Planar) → T
1
(Perpendicular) → S
0
(Planar) + hν (5.1)

Table 5.10 Summary of the dipole moment data obtained from DFT and TDDFT
calculations. Total dipole moment for each state is depicted in red. X, Y, Z are the
individual components of the dipole oriented in the X, Y, and Z directions.

States
Isomer S
0
S
1
T
1
X=3.0613 X=2.5667 X=-9.8841
Y=2.6514 Y=2.4229 Y=5.3501
Z=0.0002 Z=0.0921 Z=0.0010
Planar
4.05D 3.53D 11.24D
X=1.4307 X=1.4891 X=2.6273
Y=1.0957 Y=1.8410 Y=1.5970
Z=-0.4317 Z=0.0187 Z=-0.2495
Orthogonal
1.85D 2.37D 3.08D

216
The reason phosphorescence is not observed from the triplet planar isomer is
because the dipole moment change required to go to this state is s really large.
Results of the dipole moment data obtained from the DFT and the TDDFT
calculations are summarized Table 5.10, which shows the dipole moment of the
planar S
1
excited state is only 3.53D and the planar T
1
excited state is 11.2D. The
energy required to adjust the dipole from the planar S
1
to the planar T
1
is larger than
the energy required to rotate from the planar conformer to the orthogonal conformer.
In addition to that, since the dipole moment of the orthogonal T
1
excited state is
3.08D, the energy required to rotate from the planar S
1
conformer to the orthogonal
T
1
conformer is very small.
E
VT
calculated for the S
0
(planar) →T
1
(planar) transition in hexanes was
calculated to be 575 nm (orbital coefficient of 0.68), which was in agreement with
the 77K and RT emissions occurring at 545 nm. Contribution to this excitation came
from HOMO (MO-87) - LUMO (MO-88) transition, which is 41%
3
MMLCT in
character and 59%
3
LLMCT in character. This result also correlated well with the
triplet energy calculated for T
1
(perpendicular) →S
0
(planar) deexcitation obtained
from ΔSCF values, which was calculated to be 558 nm for the planar structure.
One possible explanation of emission quenching in polar solvents is the
formation of a coordination compex between (NO
2
ppy)-Pt-(acac) and the solvent.
Since Pt is an electron deficient metal, it is possible that in a polar media the
(NO
2
ppy)-Pt-(acac) forms a coordination complex with the solvent. As a
consequence, a nonemissive exciplex is formed in the excited state that decays via
217
non-radiative pathway. There are two possible ways the (NO
2
ppy)-Pt-(acac) can
decay non-radiatively: One, where the electron deficient Pt can coordinate with the
solvent in the ground state, which upon optical excitation can form the non-emissive
exciplex and decay by releasing heat (Equation 5.2); Two, where the molecule can
first get excited to the triplet state, form a non-emissive exciplex with the solvent and
then decay non-radiatively (Equation 5.3).
(NO
2
ppy)-Pt-(acac) + solvent ⇋ [(NO
2
ppy)-Pt-(acac).solvent] + hν → [(NO
2
ppy)-
Pt-(acac)
+
.solvent
-
]* → (NO
2
ppy)-Pt-(acac) + solvent + heat (5.2)
(NO
2
ppy)-Pt-(acac) + solvent + hν → [(NO
2
ppy)-Pt-(acac)]* + solvent →
[(NO
2
ppy)-Pt-(acac)
+
.solvent
-
]* → (NO
2
ppy)-Pt-(acac) + solvent + heat (5.3)

5.4 Chapter 5 Conclusion
Computational studies of the spectroscopic properties of (NO
2
ppy)-Pt-(acac)
were performed with density functional theory (DFT) and time-dependent (TD) DFT
methods. DFT method was used to study the ground and excited state electronic
structures and TDDFT method was used to study the properties of the excited states.
Preferred orientation of (NO
2
ppy)-Pt-(acac) in the relaxed ground state was found to
be planar. The ground state electronic structure of the planar complex as calculated
by B3LYP/LANL2DZ DFT parameters was found to be in good agreement with the
X-ray crystallographic structure. Potential energy surface (PES) scan of the singlet
ground state geometries showed three minimas corresponding to the planar isomer
218
and two maximas corresponding to the perpendicular isomer. PES scan of the lowest
triplet excited state also showed similar results as the ground state PES.
At 77K in hexanes, 2me-THF, and toluene emissions from (NO
2
ppy)-Pt-
(acac) were observed as yellow phosphorescence around 550 nm, consistent with
emission from the orthogonal isomer. Room temperature emission spectra and
emissions from the solid matrices also showed up roughly around the same region
confirming the fact that triplet emission from (NO
2
ppy)-Pt-(acac) is only observed
from the orthogonal form. Furthermore, any media that favors the planar isomer
becomes non emissive. Triplet vertical transition energies calculated in gas phase
and in hexanes with CPCM method corroborated with the experimental results. All
of the low lying singlet and triplet transitions were categorized as 41% MMLCT in
character and 59% LLMCT in character corresponding to a significant mixture of the
metal d orbital and ligand π orbitals. MO transitions for these S
0
→S
1
and S
0
→T
1

transitions involved mostly HOMO to LUMO charge transfer character. Triplet
emission energy calculated from the ΔSCF value was 558 nm and also compared
well the experimental values.
Based on the DFT and TDDFT calculations it is speculated that the planar
(NO
2
ppy)-Pt-(acac) in the ground state absorbs light and gets promoted to the planar
singlet or triplet state. From the planar triplet state the molecule cannot rotate to the
orthogonal triplet state because the energy of the triplet planar state is 66 meV lower
than the orthogonal triplet state. From the planar singlet state, the molecule does not
ISC to the planar triplet because this is dipole disallowed. As a result, the molecule
219
rotates to the orthogonal singlet state, jumps to the triplet manifold through
intersystem crossing, and then emit from that state.
Emission quenching of (NO
2
ppy)-Pt-(acac) in toluene and acetonitrile can be
explained through coordination of solvents with (NO
2
ppy)-Pt-(acac). This happens
because electron deficient Pt forms nonemissive exciplex with polar solvent
molecules and decay via nonradiative pathways.

220
5.5 Chapter 5 References

1. Titan. Wavefunction, Inc., Schrodinger, Inc.: Irvine, CA, 1999.
2. Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,; M. A.;
Cheeseman, J. R. Z., V. G.; Montgomery, J. A., Jr.;; Stratmann, R. E. B., J. C.;
Dapprich, S.; Millam, J. M.; Daniels,; A. D.; Kudin, K. N. S., M. C.; Farkas, O.;
Tomasi, J.; Barone,; V.; Cossi, M. C., R.; Mennucci, B.; Pomelli, C.; Adamo, C.;;
Clifford, S. O., J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;; Morokuma, K. M., D. K.;
Rabuck, A. D.; Raghavachari, K.;; Foresman, J. B. C., J.; Ortiz, J. V.; Stefanov, B.
B.; Liu, G.;; Liashenko, A. P., P.; Komaromi, I.; Gomperts, R.; Martin, R.; L.; Fox,
D. J. K., T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,; A.; Gonzalez, C. C., M.;
Gill, P. M. W.; Johnson, B. G.;; Chen, W. W., M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle,; E. S.; Pople, J. A., Gaussian03, revision B.03. Gaussian, Inc:
Pittsburgh, PA, 2003.
3. Becke, A. D., Density-functional thermochemistry. III. The role of exact
exchange. J. Chem. Phys. 1993, 98, (7), 5648-5652.
4. Lee, C. Y., W.; Parr, R.G. , Physical Review B: Condense. Matter Mater.
Phys 1988, 37, 785-789.
5. walter J. Stevens, H. B., Morris Krauss, Compact effective potentials and
efficient shared-exponent basis sets for the first-and second-row atoms. J. Chem.
Phys 1984, 81, (12), 6027.
6. Wadt, P. J. H. a. W. R., Ab Initio effective core potentials for molecular
calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys 1985,
82, (1), 270.
7. Wadt, P. J. H. a. W. R., Ab Initio effective core potentials for molecular
calculations. Potentials for main group elements Na-Bi J. Chem. Phys 1985, 82, (1),
284.

8. Wadt, P. J. H. a. W. R., Ab Initio effective core potentials for molecular
calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys 1985,
82, (1), 299.
9. Stanislav R. Stoyanov, J. M. V., and D. Paul Rillema, Time-Dependent
Density Functional Theory Study of the Spectroscopic Properties Related to
Aggregation in the Platinum(II) Biphenyl Dicarbonyl Complex. Inorganic Chemistry
2003, 42, (24), 7852.

221
10. HPCC.
11. Pople, M. H.-G. a. J. A., Internal rotation in conjugated molecules:
nitroethylene and nitrobenzene. Chem. Phys. Lett. 1990, 173, (5-6), 585-589.
12. Sancho-Garcia, J. C. P.-J., A. J. , Nitrobenzene rotational energy barrier: A
syrvey of several ab initio methods. J. Chem. Phys 2003, 119, (10), 5121-5127.
13. Christodoulos Makedonas, C. A. M., *,† Fernando J. Lahoz,‡ and Ana I.
Balana‡, Synthesis, Characterization, and Crystal Structure of the Pd(phen)(bdt)
Complex. A DFT and TDDFT Study of Its Ground Electronic and Excited States
Compared to Those of Analogous Complexes. Inorg. Chem. 2003, 42, 8853-8865
2003, 42, 8853-8865.
14. Stoyanov, S. R. V., J. M.; Rillema, D. P., Spectroscopic Properties of [Pt2(-
P2O5H2)4]:4- A Time-Dependent Density Functional Theory and Conductor-like
Polarizable Continuum Model Investigation J. Phys. Chem. B. 2004, 108, (32),
12175-12180.
15. Simona Fantacci, F. D. A., and Annabella Selloni, Absorption Spectrum and
Solvatochromism of the [Ru(4,4¢-COOH-2,2¢-bpy)2(NCS)2] Molecular Dye by
Time Dependent Density Functional Theory. J. AM. CHEM. SOC. 2003, 125, 4381-
4387.
16. Denis Jacquemin and Eric A. Perpète, G. S. a. M. J. F., Xavier Assfeld , Ilaria
Ciofini and Carlo Adamo Time-dependent density functional theory investigation of
the absorption, fluorescence, and phosphorescence spectra of solvated coumarins. J.
Chem. Phys. 2006, 125, 164324.
17. Tretiak, A. M. a. S., Prediction of Two-Photon Absorption Properties for
Organic Chromophores Using Time-Dependent Density-Functional Theory. J. Phys.
Chem. B 2004, 108, 899-907 2004, 108, 899-907.
18. Rudiger Bauernschmitt, R. A., Frank H. Hennrich, and; Kappes, M. M.,
Experiment versus Time Dependent Density Functional Theory Prediction of
Fullerene Electronic Absorption. J. Am. Chem. Soc. 1998, 120, 5052-5059.

222
Chapter 6. Effects of Electron Transporting Layer (ETL)
and Cathode Surface Coating on the Evolution and Growth
of Dark Spots in Organic Light Emitting Diodes (OLEDs)

6.1 Introduction
Ever since the invention of organic light emitting diodes (OLEDs) by Tang
and VanSlyke,
1
the demand for low cost flat panel displays has lead the research
efforts to obtain high efficiency devices. Besides focusing on the optimization of
performances, significant efforts have also been made to understand the device
failure mechanisms. The cause of failure has been attributed to the loss of luminance
due to degradation, which in OLEDs proceeds through two independent processes:
An intrinsic process and an extrinsic process. In an intrinsic process, a device under
electrical stress loses the electroluminescence (EL) efficiency over a long period of
time, which ultimately results in a complete failure of the device.
2-5
In an extrinsic
process, formation and growth of non-emissive areas known as dark spots, engulf the
entire device in a relatively short period of time resulting in a premature death of the
device.
6, 7

Dark spots have been the subject of intense studies in the last decade.
Numerous efforts have been made to understand the formation and growth of dark
spots and various mechanisms have been proposed to explain the behaviors of dark
spots. Research showed that OLEDs are extremely sensitive to atmosphere. Water
223
and oxygen molecules from the air can diffuse into the device through the
microscopic pinholes, cracks, and grain boundaries formed during the fabrication
process. Dust particles from the surroundings can also deposit during and after the
deposition and create large defect sites that emerge as dark spots.
6, 8, 9
Delamination
of the organic cathode interface
10, 11
and formation of bubbles or dome like
structures
2, 11
have been suggested to be responsible for the formation and growth of
dark spots. Dark spot occurrence has also been related to crystallization of the
NPD
12
layer induced by joule heating and crystallization of the AlQ
3

2
layer due to
exposure to humid atmosphere. Studies also showed that the dark spots formation
does not depend on any initial defects and a device does not need to be under
electrical stress for the dark spots to form and grow.
6, 13
In small molecule and
polymer
14
based OLEDs bubbles are initiated by electrical stress, which depend on
the applied voltage, and can form in regions free from defects. In these devices
oxidation of reduced AlQ
3
-
can lead to poor electron injection leading to the
formation of the dark spots.
6

In recent years tremendous efforts have also been made to impede the growth
of dark spots in OLEDs. Sun and coworkers demonstrated that fluorocarbon can be
used to protect the cathode layer from water permeation.
15
Chua et al. showed that a
thin layer of parylene can be used to cover the anode to flatten the ITOs to decrease
the growth of dark spots.
16
Ghosh et al. demonstrated superior OLED performance
by depositing Aluminum oxide on cathode followed by parylene and a cover glass.
17

In addition to these methods, plasma treatment of the ITO
18
, thermal treatment of the
224
entire device
19
, encapsulation using thermal chemical-vapor-deposition of polymer
films (TCVDPF),
20
and passivation by dessicants
21
have also been shown to decrease
the occurrence and growth of dark spots.
In this chapter the growth of dark spots and overall degradation patterns of
OLEDs of the architecture; ITO/CuPc/NPD/AlQ
3
/LiF/Al are investigated. There are
three main goals of the chapter: (1) to develop a correction factor so two devices of
the same architecture with different numbers of initial dark spots can be compared,
(2) to examine the various types of EL dark spots that can form and grow under
different conditions and make correlation between their origins, lifetimes, and
growth rates, and (3) to be able to predict the life time of a device based on the initial
dark area or number of dark spots and study the growth of dark spots as a function of
ETL and cathode over-coating materials.
In order to properly understand the behavior of the dark spots, the organic-
cathode and the cathode-air interfaces were specifically considered because the
defect sites such as the pinholes on OLEDs appear on the cathode-air interface as
dark spots and tunnel through the organic layers below. Therefore, OLEDs
fabricated in two extreme environments were examined. Devices made in the clean-
room environment and in the ambient air environment were investigated for this
purpose.
To investigate the occurrence and growth of dark spots as a function of
changing electron transporting layers (ETL) four clean-room devices of the
following structures ITO/CuPc/NPD/AlQ
3
/ETL/LiF/Al were studied. The ETLs in
225
devices 1, 2, 3, and 4 were N,N’-dicarbazolyl-3,5-benzene (mCP), 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthro-line (CBP), (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-
tetramethyljuloli-dyl-9-enyl) -4H-pyran) (BAlq), and none respectively. To study
the effects of various over coating materials and examine if over coating the cathode
can decelerate the dark spot growth, devices of the structure
ITO/CuPc/NPD/AlQ
3
/ETL/LiF/Al were studied.
Cu
N
N
N
N
NN
N N
N
O
Al
N
O
O
N
N N
mCP
CuPc BAlq
CBP

Knowing the role of ETL on dark spot growth will allow us to fabricate more
efficient devices. An efficient over coating material may allow us to come up cost
efficient hermetic encapsulation method, which will allow us to make long lasting
devices.

226
6.2 Experimental

6.2.1 Materials and Supplies
Unless otherwise noted, all reagents and solvents were obtained from Aldrich
and used without any further purification. All the clean-room devices were prepared
and encapsulated with epoxy and glass cover inside a class 10,000 clean room by
Universal Display Corporation (UDC). Devices were fabricated on indium tin oxide
(ITO) coated glass substrates prepared in a class 100 clean room. All the ITO coated
glass substrates were obtained from Universal Display Corporation (UDC).

6.2.2 OLED Fabrication and Testing
The ambient air devices were prepared by photolythographically imprinting
the circuit patterns on the ITO substrates as 2mm wide stripes with 1mm spacings.
Surface resistivity of the ITOs was measured to be approximately 20 Ω□
-1
.
22
The
ITOs were then rinsed with acetone, sonicated in soap-water solution, washed in de-
ionized water, and blow dried in N
2
. They were then boiled in trichloroethylene,
acetone, and ethanol for 5 minutes each. After that, the substrates were treated for
ten minutes in the UV-ozone cleaning chamber.
OLEDS were fabricated inside a high vacuum chamber (Kurt J. Lesker)
equipped with a cryo pump, two crystal monitors, and two power sources. Organic
films were thermally evaporated onto the ITO substrates from tantalum boats at
227
pressures between 3-4 μtorr. Deposition rates for all the organic materials were
maintained to be between 2-4 Å/s at all time. Prior to the deposition of the cathode,
the chamber was vented with nitrogen and shadow masks consisting 2mm stripes
were placed onto the substrates. Once the pressure reached 3.0 μtorr, 10Å of
Lithium fluoride (LiF) was deposited at 0.2 Å/s rate followed by a 1200Å layer of
Aluminum at rates between 4-5 Å/s.
Brightness and the Current- Voltage (I-V) characteristics of the encapsulated
clean-room devices were tested in room temperature and pressure in open
atmosphere. The ambient air devices were tested under vacuum. A Keithley 2400
source meter was used to power up the OLEDs and light coming form the front of
the devices was collected through a UV-818 Si photocathode equipped with a
Newport 1835-C optical meter. General structures of all the devices were,
ITO/CuPc(100Å)/NPD(500Å)/AlQ
3
(500Å)/LiF(10Å)/Al(1000Å). Structures of the
devices used in the ETL studies were ITO/CuPc(100Å)/NPD(500Å)/
AlQ
3
(500Å)/ETL(100Å)LiF(10Å)/Al(1000Å). Materials used for ETL studies were
N,N’-dicarbazolyl-3,5-benzene (mCP), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthro-
line (CBP), and (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljuloli-dyl-9-
enyl)-4H-pyran) (BAlq). Devices used for the over-coating studies had the structure,
ITO/CuPc(100Å)/NPD(500Å)/AlQ
3
(500Å)LiF(10Å)/Al(1000Å). Devices with the
above architecture will be defined as the standard CuPc devices throughout the paper

228
6.2.3 Dark Spot Growth Measurements
Dark spot growths were measured inside an environmental chamber equipped
with an optical microscope, a digital camera model: Magnafire S99800, a DC power
supply, and a humidity controller model: ETS 524 (GP-4303TP) (Figure 6.1). All
dark spot measurements were taken at 80% relative humidity (RH) and under
constant flow of N
2
. The data were obtained at 6.0 V constant voltage, unless
otherwise stated.
CCD Camera
Optical
Microscope
DC Power
Supply
_
+
Humidifier
Humidity
Controller
Vacuum outlet
N
2
inlet
Chamber vent
OLED
CCD Camera
Optical
Microscope
DC Power
Supply
_
+
Humidifier
Humidity
Controller
Vacuum inlet
N
2
inlet
Chamber vent
OLED
CCD Camera
Optical
Microscope
DC Power
Supply
_
+
Humidifier
Humidity
Controller
Vacuum outlet
N
2
inlet
Chamber vent
OLED OLED
CCD Camera
Optical
Microscope
DC Power
Supply
_
+
Humidifier
Humidity
Controller
Vacuum inlet
N
2
inlet
Chamber vent
OLED OLED

Figure 6.1 (a) Block diagram of the experimental setup for dark spot growth
measurement.

Over-coating studies were conducted inside the glove box. Eutectics melted
wax, and all the pure oils were applied by gently pouring them on to the cathode of
229
the devices. All the other materials were made into pastes by mixing them with oils.
All devices were left sitting inside the glove box for at least half hour after the
application of the over-coating materials. The devices were then placed inside the
environmental chamber equipped with a vacuum trap system, which kept the devices
under vacuum until the chamber reached a suitable equilibrium atmosphere. After
half hour the vacuum trap, which is controlled from outside the chamber, was
released and the degradation characteristics of the devices were measured.

6.3 Results and Discussion
Figure 6.2 shows the picture of the high vacuum chamber and the vacuum
line set up used to store and transport devices (out side the clean room). After
depressurization of the chamber, devices were first stored inside a dry vacuum
dessicator (no desiccants inside) equipped with a mechanical pump (10
-3
torr). Each
stored devices were then individually taken out of the dessicator and placed in front
of glass vacuum traps (15 mm inner-diameter) equipped with stopcocks and
connected to a vacuum manifold. After storing the devices on the vacuum traps for 5
minutes in dynamic vacuum, the stopcocks on the traps were closed and the devices
attached to the traps under static vacuum were transported for current-voltage
studies.

230
Vacuum
Manifold
Dessicrator
Vacuum
Trap
High Vacuum
Chamber
Cryogenic
Pump
Chamber
Controls
(a) (b)
Vacuum
Manifold
Dessicrator
Vacuum
Trap
High Vacuum
Chamber
Cryogenic
Pump
Chamber
Controls
(a) (b)

Figure 6.2 (a) Picture of the high vacuum environmental chamber. (b) Picture of the
vacuum line setup for device storage and transportation.

The devices under static vacuum were then placed in front of an optical beam
detector and subjected to I-V characteristics studies under vacuum (Figure 6.3). I-V
and the J-V characteristics were measured by scanning the devices from 0 to 12V
variable voltages.
Device
under vacuum
Vacuum Trap
X-Y-Z Positioning
stage
Optical
beam detector
Device attached
to the vacuum trap
(a) (b)
Device
under vacuum
Vacuum Trap
X-Y-Z Positioning
stage
Optical
beam detector
Device attached
to the vacuum trap
(a) (b)

Figure 6.3 (a) Picture of the I-V and the J-V measurement setup. (b) Picture of the
device under static vacuum being tested.

231
The devices attached to the traps were then placed on a device mount inside
the environmental chamber facing the optical microscope (Figure 6.4). The vacuum
traps were then connected to the vacuum inlet from inside the chamber and the
stopcock opened with the vacuum pump turned on (10
-3
torr). The anode and the
cathode of the devices were connected to the DC power supply placed outside the
chamber. Once the device was set to be under dynamic vacuum, the environmental
chamber was purged with N
2
for five minutes. The chamber was sealed from outside
and relative humidity was set to 80%. The DC power supply was turned on and data
were collected under vacuum. After half hour the vacuum valve from outside the
chamber was closed to release the trap from the device and the degradation data were
collected at 80% RH.
DC Power
Supply Digital
Camera
Temperature
Controller
Humidity
Controller
Vacuum
Line
Optical
Microscope
Device
Mount
Magnetic Mount
Vacuum
Inlet
Sensor
Humidifier
Inlet
Fan
Vacuum Trap
Device
Under Vacuum
Chamber
Vent
(a) (b)
DC Power
Supply Digital
Camera
Temperature
Controller
Humidity
Controller
Vacuum
Line
Optical
Microscope
DC Power
Supply Digital
Camera
Temperature
Controller
Humidity
Controller
Vacuum
Line
Optical
Microscope
Device
Mount
Magnetic Mount
Vacuum
Inlet
Sensor
Humidifier
Inlet
Fan
Vacuum Trap
Device
Under Vacuum
Chamber
Vent
Device
Mount
Magnetic Mount
Vacuum
Inlet
Sensor
Humidifier
Inlet
Fan
Vacuum Trap
Device
Under Vacuum
Chamber
Vent
(a) (b)

Figure 6.4 (a) Environmental chamber setup. (b) Internal view of the environmental
chamber.

232
For the packaged clean-room devices, the glass seals were removed from the
devices and placed inside the environmental chamber facing the microscope and data
were collected as mentioned above.

6.3.1 Correction factor and data treatment.
To be able to compare between dissimilar devices a correction factor needs to
be developed, which can be used as a benchmark throughout the chapter. Two
devices of the same architecture: ITO/CuPc/NPD/Alq3/LiF-Al, were studied for this
purpose. Device A had 10 initial dark spots and device B had 3 initial dark spots.

6.3.1.1 The concept of correction
To begin, it can be assumed that the overall growth rate, R
G
of a device is the
same as the average growth rate, R
A
of the dark spots on the device. For which the
following relationship should hold.
C*R
A
= R
G
(6.1)
Here C is the constant of multiplication that arises from error and can have
contribution from anomalous DS growth such as nonlinear growth or lack of growth.
If there is no error then C should be equal to one. R
G
can be defined as the total dark
area, A on a device divided by the number of initial dark spots, DS. Since the growth
rate of a dark spot, R is dark area (DA)/time, dividing the total dark area (the total
area of a device is the observation window defined by the microscope) of the device
by the number of initial dark spots should yield the corrected growth rate (Equation
233
6.2). Figure 6.5 shows the plots of dark area vs. time of the devices A and B before
and after correction. It is evident from the plots that after correction the total dark
area of the corrected plots are much less than the original total dark area,
corresponding to DA/DS.
DS
A
R
G
= (6.2)
B/3 DS
A/10 DS
3 DS
10 DS
0 100 200 300 400 500 600 700 800
0
5x10
5
1x10
6
2x10
6
2x10
6
3x10
6
Device B
Device A
Device B corrected
Device A corrected
Dark Area ( μ
2
)
Time (min)

Figure 6.5 The plots of dark area vs. time for the devices A and B before and after
correction.

6.3.1.2 Applying correction
Since the dark spots fuse with each other upon growth, to avoid any
anomalies all data need to be fitted up to the point right before the fusion occurs.
Therefore, based on the density of the dark spots, device A was fitted up to 50
minutes and device B fitted up to 135 minutes (Figure 6.6). Device A was fitted
with linear and quadratic equation and device B was fitted with only quadratic
234
equation. For device A, both of the equations gave same rate. The linear curve
fitting was obtained by fitting the curve with Equation 6.3, where m is the slope of
the straight line and b is the y intercept. The quadratic fitting was obtained by fitting
the data with Equation 6.4. Rate from the quadratic equation was obtained by taking
the first derivative of Equation 6.4, which gave Equation 6.5. All linear and non-
linear curve fittings were obtained using Origin 7.0.
b mx y + = (6.3)
C Bx Ax y + + =
2
(6.4)
B Ax
dx
dy
+ = 2 (6.5)
-20 0 20 40 60 80 100 120 140
6.0x10
3
8.0x10
3
1.0x10
4
1.2x10
4
1.4x10
4
1.6x10
4
1.8x10
4 Y =7029.68795+107.41624 X-0.23823 X
2
Device B corrected
Polynomial Fit of Data2_B
Dark Area/Dark Spot ( μ
2
)
Time (min)
010 20 30 40 50
2.0x10
3
4.0x10
3
6.0x10
3
8.0x10
3
1.0x10
4
1.2x10
4
1.4x10
4
Device A corrected
Linear Fit of Data2_D
Y =2778.16696+68.26937 X+2.97511 X
2
Polynomial Fit of Data2_D
Dark Area/Dark Spot ( μ
2
)
Time (min)

Figure 6.6 The plots of corrected curves vs. time for devices A and B. Both
polynomial and linear fits gave the same rates.

The overall growth rate for the devices A and B were calculated to be 202
μ
2
/min and 75 μ
2
/min respectively. Figure 6.7 shows the plots of growth rates of 10
dark spots obtained from device A, from which the average growth rate of the dark
235
spots was calculated. The average growth rate of ten dark spots on device A was
calculated to be 196 μ
2
/min. As expected, the result was in good agreement with the
relationship 6.1, where the constant of error was calculated to be 1.03 suggesting
almost no anomalous growth.
0 102030 40 50
0.0
4.0x10
3
8.0x10
3
1.2x10
4
1.6x10
4
2.0x10
4
2.4x10
4
Spot # 1: 45.95 micr2/min
Spot # 2: 19.83 micr2/min
Spot # 3: 284.65 micr2/min
Spot # 4: 185.81micr2/min
Linear Fit of Data2_E
Linear Fit of Data2_C
Linear Fit of Data2_B
Linear Fit of Data2_D
DS Area ( μ
2
)
Time (min)
0 1020 30 4050
0.0
4.0x10
3
8.0x10
3
1.2x10
4
1.6x10
4
2.0x10
4
2.4x10
4
Spot # 5: 313.33 micr2/min
Spot # 6: 29.22 micr2/min
Spot # 7: 152.89 micr2/min
Spot # 8: 304.18 micr2/min
Linear Fit of Data2_I
Linear Fit of Data2_H
Linear Fit of Data2_G
Linear Fit of Data2_F
DS Area ( μ
2
)
Time (min)
0 1020 30 4050
2.0x10
3
4.0x10
3
6.0x10
3
8.0x10
3
1.0x10
4
1.2x10
4
1.4x10
4
Device A corrected
Linear Fit of Data2_D
Y =2778.16696+68.26937 X+2.97511 X
2
Polynomial Fit of Data2_D
Dark Area/Dark Spot ( μ
2
)
Time (min)
0 10 2030 4050
0.0
4.0x10
3
8.0x10
3
1.2x10
4
1.6x10
4
2.0x10
4
2.4x10
4
Spot # 9: 207.32 micr2/min
Spot # 10: 319.84 micr2/min
Linear Fit of Data2_J
Linear Fit of Data2_K
DS Area ( μ
2
)
Time (min)

Figure 6.7 The plots of individual dark area vs. time for devices A.

Similar to device A, the average growth rate of three dark spots for device B
was calculated to be 81 μ
2
/min (Figure 6.8). This data correlated well with the
overall growth rate of 75 μ
2
/min calculated for device B. However, it is important to
understand that one of the dark spots in device B does not grow at all for the first 140
236
minutes. Therefore, for the first 140 minutes the average growth rate of the dark
spots in device B should be calculated only for the spots that grow in that period of
time. This action yields the average growth rate of the dark spots in device B to be
121 μ
2
/min, which does not satisfy the relationship 6.1 and also does not accurately
represent the average growth rate of the dark spots for the entire life of the device.
Furthermore, the constant of error for device B was also calculated to be 0.62
suggesting a large error due to the lack of growth of one dark spot. Therefore, in
order to better represent the entire device, the R
A
in device B needs to be calculated
for three dark spots.
-20 0 20 40 60 80 100 120 140
6.0x10
3
8.0x10
3
1.0x10
4
1.2x10
4
1.4x10
4
1.6x10
4
1.8x10
4
2.0x10
4
2.2x10
4
2.4x10
4
2.6x10
4
Spot # 1: 139.83 micr2/min
Spot # 2: 0.00 micr2/min
Spot # 3: 102.39 micr2/min
Linear Fit of Data1_B
Linear Fit of Data1_C
Linear Fit of Data1_D
DS Area ( μ
2
)
Time (min)

Figure 6.8 The plot of individual dark area vs. time for devices B.

However, the data for device A suggests that the relationship 6.1 holds well
for a device if all the dark spots in that device behave similarly and have a uniform
growth rate. Therefore, the question remains, in order to decrease the error caused
by non-uniform growth and to better satisfy the relationship 6.1, should the average
237
growth rate of the dark spots be calculated from all the spots in that device? To test
that assumption other devices with multiple dark spots need to be studied.
This assumption was verified by studying a different set of devices with
slightly different architectures. Three devices with mcP, CBP, and BAlq ETLs were
investigated for this study. The device with mCP ETL had 5 initial dark spots, the
device with CBP ETL had 5 initial dark spots, and the device with BAlq ETL had 2
initial dark spots. Correction was applied to the DS growth curves by dividing the
total dark area of the devices by the number of initial dark spots as described in
Equation 6.2. Figure 6.9 displays the dark spot growth curve of the three ETL
devices before and after the application of correction.
0 500 1000 1500 2000 2500
0.0
5.0x10
5
1.0x10
6
1.5x10
6
2.0x10
6
2.5x10
6
mcP
CBP
BAlq
mcP corrected
CBP corrected
BAlq corrected
Dark Area/Dark Spot ( μ
2
)
Time (min)
5 DS
5 DS
2 DS

Figure 6.9 The plots of dark area vs. time for the ETL devices before and after
correction.

All the devices were corrected and fitted up to 135 minutes. Figure 6.10
shows the curve fitting plots of the ETL devices. The overall growth rates of the
238
three devices were calculated to be: mcP = 447 μ
2
/min, CBP = 545 μ
2
/min, and BAlq
= 424 μ
2
/min. The curve fitting of the individual dark spots for the three devices
gave average DS growth rates for mcP = 498 μ
2
/min, CBP = 667 μ
2
/min, and BAlq =
460μ
2
/min. Predictably, for the ETL devices the overall growth rates also correlated
well with the average growth rates of the dark spots for each device. The constants
of error in these devices were calculated to be 0.90, 0.82, and 0.92 respectively,
which were closer to unity and the relationship in 6.1 was satisfied.
-20 0 20 40 60 80 100 120 140
0.0
2.0x10
4
4.0x10
4
6.0x10
4
8.0x10
4
1.0x10
5
1.2x10
5
1.4x10
5
Spot # 1: 373.14 micr2/min
Spot # 2: 856.03 micr2/min
Spot # 3: 566.60 micr2/min
Spot # 4: 37.34 micr2/min
Spot # 5: 655.70 micr2/min
Linear Fit of Data1_B
Linear Fit of Data1_C
Linear Fit of Data1_D
Linear Fit of Data1_E
Linear Fit of Data1_F
DS Area ( μ
2
)
Time (min)
-20 0 20 40 60 80 100 120 140
0.0
2.0x10
4
4.0x10
4
6.0x10
4
8.0x10
4
1.0x10
5
1.2x10
5
1.4x10
5
Spot # 1: 465.53 micr2/min
Spot # 2: 730.12 micr2/min
Spot # 3: 805.42 micr2/min
Linear Fit of Data7_B
Linear Fit of Data7_C
Linear Fit of Data7_D
DS Area ( μ
2
)
Time (min)
-20 0 20 40 60 80 100 120 140
0.0
2.0x10
4
4.0x10
4
6.0x10
4
8.0x10
4
1.0x10
5
1.2x10
5
1.4x10
5
Spot # 1: 510.23 micr2/min
Spot # 2: 410.74 micr2/min
Linear Fit of Data11_B
Linear Fit of Data11_C
DS Area ( μ
2
)
Time (min)
mCP
CBP
BAlq
-20 0 20 40 60 80 100 120 140
0
1x10
4
2x10
4
3x10
4
4x10
4
5x10
4
6x10
4
7x10
4
8x10
4
9x10
4
mcP corrected: 447.16 micr2/min
CBP corrected: 545.35 micr2/min
BAlq corrected: 424.01 micr2/min
Linear Fit of Data2_B
Linear Fit of Data2_C
Linear Fit of Data2_D
Dark Area/Dark Spot ( μ
2
)
Time (min)

Figure 6.10 On top left the plot of corrected curves vs. time showing linear fits for
the three ETL devices. The top right and the bottom plots show the individual dark
area vs. time for the ETL devices

239
The results from the ETL devices showed that C gets closer to unity if all
dark spots, regardless of their growth pattern, are used to calculate the R
A
. For that
reason, the average growth rate of the dark spots for device B was calculated from all
three dark spots. This caused the constant of error to be closer to be 0.92, which is
much closer to 1.0 than 0.62.
The overall growth rates of the two devices A and B after correction were
compared. As predicted, the relationship showed that device A, which had ten dark
spots was degrading at a faster rate than device B, which had three dark spots. The
overall growth rate of device A was 2.5 (±0.25) times faster than the rate of device B
(Equation 6.6). This happens because larger number of dark spots means larger
initial degraded area, which means more moisture penetration per unit time and
faster aerobic oxidation per unit time. As a result, device A degrades faster than
device B in the same period of time.
R
G
(device A) = 2.5 (±0.25)* R
G
(device B) (6.6)
It is important to understand that the relationship defined in Equation 6.6 has
to be true for the entire degradation curve of each device. The same ratio should
apply to fitting the total decay in order to determine why the method is best.
Therefore, to obtain the total dark area growth rates, entire degradation curves for
both of the devices need to be fitted. After correction, if the obtained relationship
between the two devices is still the same as Equation 6.6, then that method can be
used as a standard tool for further data analysis.
240
In order to get the best fit, the degradation curves of both the devices were
analyzed with linear, quadratic, exponential, and sigmoidal-Boltzmann equations.
Figure 6.11 and 6.12 shows the curve fitting plots for device A and B with all four
above mentioned equations. In Table 6.1, the summary of the curve fitting results
are reported for the two devices. The linear curve fitting was obtained by fitting the
linear section of the curves with Equation 6.3. For device A, this region was
between 100 to 180 minutes and for device B the region was between 325 to 550
minutes. The rates obtained from the linear curve fitting for the devices were not in
agreement with the relationship defined in Equation 6.6. The quadratic and the
exponential fittings were obtained by fitting the data up to the half life of each
device. Rates from the quadratic equations were obtained by using Equation 6.5.
Again, the rates were not in agreement with Equation 6.6.
Exponential fitting of the curves were obtained by using Equation 6.7, where
y
0
is the y intercept and k is the rate constant. The rate constants (k) obtained from
exponential fitting were 0.0232 min
-1
and 0.01001 min
-1
for device A and B
respectively (r
2
> 0.99). The results of the exponential fitting showed good
agreement with Equation 6.6 and the relation ship between the devices turned out to
be; R
G
(device A) = 2.3* R
G
(device B).
{} kx y y
a
exp = (6.7)
A very good fit (r
2
> 0.99) to the overall degradation patterns was obtained by
using the sigmoidal Boltzmann function described in Equation 6.8, where A
0
is the
initial dark area (At t = 0), A
f
is the total dark area at the end, t
1/2
is the half life
241
(Corresponding to the inflection point on the graph), and dt is the time constant of
degradation and is the same as the rate constant k. The rate constants (k) obtained
from the sigmoidal-Boltzmann equation fitting were 0.0340 min
-1
and 0.0128 min
-1
for device A and B respectively. The results of the sigmoidal-Boltzmann fitting
showed better agreement with Equation 6.6 and the relation ship between the devices
was; R
G
(device A) = 2.6* R
G
(device B).
()
f
dt
t t
f
A
e
A A
t y +
+
−
=
−
2 / 1
1
0
(6.8)
-20 0 20 40 60 80 100 120 140
0.0
2.0x10
4
4.0x10
4
6.0x10
4
8.0x10
4
1.0x10
5
1.2x10
5
Data: Data2_D
Model: Exp2PMod1
Equation: y = a*exp(b*x)
Weighting:
y No weighting

Chi^2/DoF = 11247483.82367
R^2 = 0.99047

a 5754.95601 ±408.93965
b 0.0232 ±0.00063
Device A corrected
Exp2PMod1 fit of Data2_D
Dark Area/Dark Spot ( μ
2
)
Time (min)
-20 0 20 40 60 80 100 120 140
0.0
2.0x10
4
4.0x10
4
6.0x10
4
8.0x10
4
1.0x10
5
1.2x10
5
1.4x10
5
Y =2738.84625+40.82337 X+3.63482 X
2
+0.01916 X
3
Device A corrected
Polynomial Fit of Data2_D
Dark Area/Dark Spot ( μ
2
)
Time (min)
Rate 843.38
100 120 140 160 180
6.0x10
4
8.0x10
4
1.0x10
5
1.2x10
5
1.4x10
5
1.6x10
5
1.8x10
5
2.0x10
5
2.2x10
5
Dark Area/Dark Spot ( μ
2
)
Time (min)
Device A corrected
Linear Fit of Data2_D
Rate 1922.78
-100 -50 0 50 100 150 200 250
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
2.5x10
5
Data: Data2_D
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
y No weighting

Chi^2/DoF = 10012468.08035
R^2 = 0.99872

A1 2297.21277 ±1314.94713
A2 244205.02605 ±2273.70736
x0 133.24977 ±0.7897
dx 29.09848 ±0.78104
Device A corrected
Boltzmann fit of Data2_D
Dark Area/Dark Spot ( μ
2
)
Time (min)

Figure 6.11 Degradation curve fitting of device A with linear, quadratic,
exponential, and sigmoidal-Boltzmann equations.

242
0 100 200 300 400 500
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
2.5x10
5
3.0x10
5
3.5x10
5
4.0x10
5
Data: Data2_B
Model: Exp2PMod1
Equation: y = a*exp(b*x)
Weighting:
y No weighting

Chi^2/DoF = 70011807.41135
R^2 = 0.99363

a 4888.10625 ±251.64955
b 0.01001 ±0.00013
Device B corrected
Exp2PMod1 fit of Data2_B
Dark Area/Dark Spot ( μ
2
)
Time (min)
0 100 200 300 400 500
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
2.5x10
5
3.0x10
5
3.5x10
5
4.0x10
5
Y =6689.24546+197.88853 X-2.07437 X
2
+0.00811 X
3
Device B corrected
Polynomial Fit of Data2_B
Dark Area/Dark Spot ( μ
2
)
Time (min)
Rate 67.91
300 350 400 450 500 550
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
6x10
5
7x10
5
Device B corrected
Linear Fit of Data2_B
Dark Area/Dark Spot ( μ
2
)
Time (min)
Rate 2184.99
-200 0 200 400 600 800
-1x10
5
0
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
6x10
5
7x10
5
8x10
5
Data: Data2_B
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
yNo weighting

Chi^2/DoF = 57963284.77045
R^2 = 0.99935

A1 2768.67752 ±1367.1923
A2 784318.7487 ±2261.95788
x0 451.51901 ±0.79573
dx 78.06666 ±0.74713
Device B corrected
Boltzmann fit of Data2_B
Dark Area/Dark Spot ( μ
2
)
Time (min)

Figure 6.12 Degradation curve fitting of device B with linear, quadratic,
exponential, and sigmoidal-Boltzmann equation.

Since the rate constants calculated with the sigmoidal-Boltzmann equation
gives almost the same relation as in Equation 6.6, the rate constants of the devices
for the ETL and the over-coating studies can be calculated by the sigmoidal-
Boltzmann equation. The rate constants can then be compared with the average rate
constant, k
ave
= 0.0234 min
-1
obtained from Equation 6.8 for devices A and B.

243
Table 6.1 Summary of the curve fitting results obtained from linear, quadratic,
exponential, and sigmoidal-Boltzmann curve fitting.

Device Linear QuadraticExponential Sigmoidal
(min
-1
) (min
-1
) (min
-1
) (min
-1
)
A (10DS) 1923 843.38 0.0232 0.0340
B (3DS) 2185 67.91 0.0100 0.0128

6.3.2 Dark spot growth and behavior
Optical microscope enhanced electroluminescence images of OLEDs
fabricated inside the clean-room and outside the clean-room were compared. Figure
6.13 shows the t=0 snapshots of the optical images of the devices. The clean-room
device clearly shows fewer numbers of dark spots compared to the ambient air
device, which appeared much denser and larger in size than the ones fabricated
inside the clean room. Exposure to open atmosphere during device fabrication
causes increased number of dark spots in the ambient air devices. Since the dark sots
grow in the present of air, handling in the air also causes the spots to grow in size.
Clean room spots are small because there are very few number of dust particles in
the air. The dark spots in the clean-room devices are usually formed by: diffusion of
oxygen ions from the ITO
16
; substrate defects caused by scratches or spikes on
ITO
16
; delamination of the organics; cracks; or grain boundaries.
6

244
a
0 5 10 15 20
25
50
75
100
125
150
175
200
225
250
275
Spot 1
Spot 2
Spot 3
Spot 4
Spot 5
Darkspot Diameter ( μ)
Time (min)
0 10 203040 5060
25
50
75
100
125
150
175
200
225
250
275
Spot 1
Spot 2
Spot 3
Spot 4
Spot 5
Darkspot Diameter ( μ)
Time (min)
b
1
2
3
4
3
5
500 μ
b
1
2
3
4
3
5
500 μ
1
2
3
4
5
500 μ
1
2
3
4
5
500 μ

Figure 6.13 Dark spot diameter vs. time are plotted (a) Clean-room devices. Shows
comparatively smaller dark spots with one uniform rate, which means that these
spots perhaps had the same origin. (b) Ambient air devices shows large dark spots
with two very distinct rates, hence different origins.

Table 6.2 summarizes the growth rates of the dark spots for the clean-room
and the ambient air devices. The data shows that the rates are linear for all the spots.
This data is consistent with the literature where Lim et al.
23-27
reported linear growth
rates for controlled growth of dark spots that were mechanically initiated by various
sizes of spherical silica particles. It was also evident that the larger spots grow at
245
higher rates than the smaller spots. The rates for the spots 2, 3 and 5 in Figure 6.13a
were higher than the rate of spot 1. Similarly the rate for spot 5 in Figure 6.13b was
higher than the rate of spot 3, which was higher than the rate of spot 4. The drastic
change in rate for spot 4 of the clean-room device is caused by the fusion of two
spots.
The growth rates for the ambient air device showed two very distinct types of
dark spots suggesting that these spots probably had different origins. The rates for
the spots 1 and 2 stayed almost zero till fusion occurred (Figure 6.14b). These spots
are the static dark spots (SDS), which lie sandwiched between the layers underneath
the cathode. Static dark spots are probably generated from small defects during
deposition. Dust particles that are few Å in diameter can create nonconductive
pathways and may appear as dark spots. In Figure 6.14, static and dynamic dark
spots are shown at t=10, 24, and 26 minutes. At t=24 minutes, the size of SDS
remained the same but the dynamic dark spots (DDS) grew bigger. At t=26 minutes,
The IDS was completely consumed by the DDS.

Table 6.2 Growth rates of the dark spots showing difference between the clean-room
and ambient air devices

Spots Diameter Rate Diameter Rate
( μ)( μ/min) ( μ)( μ/min)
1 24 1.9 33 0
2 43 2.5 31 0
3 45 2.35 70 2.65
4 48 59 2.22
5 53 2.39 162 3.96
Ambient air Clean-room

246
0 20 406080 100 120 140
0
20
40
60
80
100

% Selected Area
Time (min)
Area 1
Area 2
(c) T = 26 min (a) T = 10 min
SDS
DDS
SDS
DDS
70 μX56 μ
(b) T = 24 min
SDS
DDS
70 μX56 μ 70 μX56 μ
412 μX516 μ
(d)
77 μX79 μ 77 μX79 μ
Area 1 Area 2
(e) f)
(c) T = 26 min (a) T = 10 min
SDS
DDS
SDS
DDS
70 μX56 μ
(b) T = 24 min
SDS
DDS
70 μX56 μ
(b) T = 24 min
SDS
DDS
70 μX56 μ 70 μX56 μ
412 μX516 μ
(d)
412 μX516 μ
(d)
77 μX79 μ 77 μX79 μ
Area 1 Area 2
(e) f)

Figure 6.14 (a) At t=0, an static dark spot (SDS) and an dynamic dark spot (DDS) is
shown. (b) At t = 10 min, the DDS and the SDS both seemed to have grown. (c) At
t=24 min, SDS remained the same but the DDS grew much bigger. (d) At t=26 min,
the SDS was consumed by the DDS. (e) Example of dark areas caused by surface
defects. (f) and (g) % Dark area vs. time showing areas with more dark spots
degrade faster than the areas with fewer dark spots.

Figure 6.14d shows an example of the dark areas created by a scratch on the
ITO or the glass. Devices with such large defects degrade very fast due to large
areas that can accommodate more water and oxygen molecules at any time. Dark
spots formed on the devices constructed outside of the clean room come from all of
the above, but they mostly come from atmospheric dust particles. Dust particles
deposit on the organic surfaces or ITOs during atmospheric exposure. These
particles are micron size and create deep and wide holes on the devices. These are
the dynamic dark spots (DDS), which create shadow areas during deposition. The
247
shadows are holes on the devices that go from cathode to anode. During deposition,
the shadow areas are responsible for cathode depositing on anode and causing shorts.
Areas with high and low density of dark spots were selected to see the dark
spot growth behavior. Figure 6.14 shows the two areas and the plot of the dark area
vs. time. The data showed that the area with more dark spots degrade at a faster rate
consistent with the previous data. Large areas can accommodate more water and
oxygen molecules at a time than smaller areas, so they degrade at a faster rate.

6.3.3 Correlation between dark spot growth and device lifetime
To examine the relationship between the number of initial dark spots and the
device half lives six devices from two different origins were studied. Three of them
were fabricated inside the clean-room and three in the ambient environment. The
optical EL images of the devices failed to show all the dark spots at t = 0. Evolution
of different dark spots were observed around t = 10%-20% coverage. These are not
new spots. These spots were perhaps always present, which were not visible through
the microscope because of their submicron size. Half lives of these devices were
then compared with the initial number of dark spots at t = 10% coverage.
Figure 6.15 displays the EL optical images of the three clean-room devices
and three ambient air devices examined for this experiment. A correlation between
the device half-lives ( τ
1/2
) and the initial dark spot density ( ρ
ds
) showed a liner fit
(Equation 6.9), suggesting inverse relationship between the two parameters. The
intercept A and the slope B are the proportionality constants with values of 0.51 and
248
20.59 respectively. The non-zero intercept indicates that the devices were partially
degraded prior to data acquisition. The slope shows the change in device half-lives
with respect to the initial numbers of dark spots.
A B
t
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
= % 20
2 / 1
ds
1
ρ
τ (6.9)
ab c
d ef
500 μ 500 μ 500 μ
500 μ 500 μ 500 μ
ab c
d ef
500 μ 500 μ 500 μ
500 μ 500 μ 500 μ
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
0
2
4
6
8
Halflife (hours)
Linear Fit of Data1_B
Half life, τ
1/2
(h)
1/ ρ
DS

Figure 6.15 (a), (b), (c) Clean-room devices. (d), (e), (f) Ambient air devices. Half
life vs. dark spot density showing a liner relationship between the device half-lives
and initial dark spot densities.

The half life obtained from Equation 6.9 can be used as a benchmark half life
with a given number of initial dark spots. The value can be interpreted as the actual
half life of a device without any internal or external perturbations. In the following
sections we will use values obtained from Equation 6.8 as standard half lives and
compare with the experimental ones obtained from the ETL and over coating studies.
To investigate the relationship between dark spot growth and degradation of entire
device, degradation patterns of all the devices were examined. As predicted, all the
devices showed a sigmoidal degradation pattern.
249
To be able to predict the device half-life of any device from the sigmoidal-
Boltzmann equation, the equation needs to be solved for t
1/2
. Equation 6.10 is the
version of the sigmoidal-Boltzmann equation, which was obtained by algebraic
manipulation of Equation 6.8. Equation 6.10 can be used to predict the half lives of
devices if the dark areas at t = 0 are known. Furthermore, it can also be used to
predict the half life of any device and used as a standard half life without any
encapsulation.
dt
A y
A A
t t
f
f
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
−
−
− = 1 ln
0
2 / 1
(6.10)
The prediction of device half lives with Equation 6.10 should have more
weight than equation 6.8 because it takes the exact initial degraded are to calculate
the half lives. Where as, Equation 6.9 only uses the initial number of dark spot
counts to do the same thing. Since the size of the dark spots can vary from device to
device, it needs to be remembered that two devices with the same number of darks
spots can have very different % degraded areas. This issue will be further discussed
in section 6.3.5.

6.3.4 Effects of electron transporting layer (ETL)
The effects of electron transporting layer (ETL) on the growth of dark spots
are important because this layer sits underneath the cathode and can play a
significant role in extrinsic degradation. In order to investigate the role of ETL,
devices with mCP, CBP, and BAlq ETLs inserted between Alq
3
layer and the
250
cathode were studied. Figure 6.16 displays the EL snapshots of the devices, which
shows approximately 0.1% coverage for all the devices at time t=0. Since the
degradation curves of BCP and mCP devices follow pseudo-sigmoidal pattern and
BAlq device follows a linear pattern, for the purpose of comparison and to obtain the
best results, the degradation curves of the ETL devices were fitted with multiple
functions.
Alq
3
Alq
3
/BAlq
Alq
3
/CBP Alq
3
/mCP
933 μ X 790 μ 933 μ X 790 μ
933 μ X 790 μ 933 μ X 790 μ

Figure 6.16 t = 0 Snapshots showing the initial dark spot density for all of the ETL
devices.

The degradation curve of mCP device was analyzed with linear, quadratic,
exponential and sigmoidal-Boltzmann equations (Figure 6.17). The pseudo
sigmoidal CBP curve was only fitted with linear, polynomial and sigmoidal-
Boltzmann equations (Figure 6.18). The degradation curve of BAlq was only
analyzed with linear equation because surprisingly this curve was not sigmoidal in
shape (Figure 6.18). Furthermore, growth of the two dark spot areas also showed
251
linear increase instead of parabolic increase. This strange behavior of the BAlq
device is unexplainable.
0 50 100 150 200 250 300
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
2.5x10
5
Data: Data2_B
Model: Exp2PMod1
Equation: y = a*exp(b*x)
Weighting:
y No weighting

Chi^2/DoF = 14890270.19704
R^2 = 0.99648

a 16462.94112 ±403.24673
b 0.0103 ±0.00011
mCP corrected
Dark Area/Dark Spot ( μ
2
)
Time (min)
0 50 100 150 200 250 300
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
2.5x10
5
Y =4361.42434+476.63468 X-1.80354 X
2
+0.01297 X
3
mcP corrected: 447.16 micr2/min
Polynomial Fit of Data2_B
Dark Area/Dark Spot ( μ
2
)
Time (min)
Rate 465.58
220 240 260 280 300 320 340 360 380
1.5x10
5
2.0x10
5
2.5x10
5
3.0x10
5
3.5x10
5
4.0x10
5
4.5x10
5
mcP corrected: 447.16 micr2/min
Linear Fit of Data2_B
Dark Area/Dark Spot ( μ
2
)
Time (min)
Rate 1684.16
-300 -150 0 150 300 450 600
0
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
Data: Data2_B
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
y No weighting

Chi^2/DoF = 24649521.60484
R^2 = 0.99921

A1 13834.3623 ±1439.39531
A2 467716.59642 ±1168.75393
x0 262.57065 ±0.74256
dx 59.56565 ±0.72305
mcP corrected: 447.16 micr2/min
Dark Area/Dark Spot ( μ
2
)
Time (min)

Figure 6.17 Degradation curve fitting of mCP device with linear, quadratic,
exponential, and sigmoidal-Boltzmann equations.

In Table 6.3 the summary of the curve fitting data of the ETL devices are
reported with the data for the test devices. Figure 6.19 depicts the degradation
curves of the ETL and the test devices for comparison. For the mCP device, linear
and quadratic fittings were not in good agreement and were discarded. The
exponential and the sigmoidal-Boltzmann fittings however, were in good agreement
252
with other and the average rate constant, k
mCP
for the fittings was approximately 0.01
min
-1
. Comparison of the k
mCP
with the average rate constant, k
ave
= 0.0234 min
-1
for
the test devices showed slower rate of DS growth for the mCP device. Comparison
of k
CBP
with k
ave
showed similar rate for the CBP and the test devices. Comparison
of the linear rate for the BAlq device with the test devices showed much slower rate
for the BAlq device.
250 300 350 400 450 500
1.5x10
5
2.0x10
5
2.5x10
5
3.0x10
5
3.5x10
5
4.0x10
5
CBP corrected: 545.35 micr2/min
Linear Fit of Data2_C
Dark Area/Dark Spot ( μ
2
)
Time (min)
Rate 892.65
-50 0 50 100 150 200 250 300 350
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
2.5x10
5
Y =10635.30346+523.53304 X+0.01783 X
2
+0.00137 X
3
CBP corrected: 545.35 micr2/min
Polynomial Fit of Data2_C
Dark Area/Dark Spot ( μ
2
)
Time (min)
Rate 549.44
-300 -150 0 150 300 450 600 750
0
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
Data: Data2_C
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
yNo weighting

Chi^2/DoF = 14826690.41998
R^2 = 0.99912

A1 -55141.97608 ±5906.71424
A2 560007.26396 ±10933.42778
x0 351.17233 ±4.2133
dx 173.51787 ±5.52721
CBP corrected: 545.35 micr2/min
Dark Area/Dark Spot ( μ
2
)
Time (min)
0 200 400 600 800 1000 1200 1400
0
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
6x10
5
7x10
5
BAlq corrected: 424.01 micr2/min
Linear Fit of Data2_D
Dark Area/Dark Spot ( μ
2
)
Time (min)
Rate 490.71

Figure 6.18 Degradation curve fitting of CBP device with linear, quadratic, and
sigmoidal-Boltzmann equations. (bottom right) Curve fitting of BAlq device with
linear equation.

253
Table 6.3 Summary of the curve fitting results of the ETL devices are shown with
the standard test devices.

Device Linear QuadraticExponential Sigmoidal
(min
-1
) (min
-1
) (min
-1
) (min
-1
)
A (10DS) 1923 843.38 0.0232 0.0340
B (3DS) 2185 67.91 0.0100 0.0128
mCP 1684 --- 0.0100 0.0058
CBP 892 --- --- 0.0017
BAlq 491 --- --- ---

Comparison of the sigmoidal-Boltzmann rate constants between the
mCP and the CBP devices showed slightly slower rate for the mCP device. This was
contrary to the actual visual observation, which shows a faster degradation rate for
the mCP device (Figure 6.19). The reason for this discrepancy is the actual shape of
the curves, which clearly shows that both the mCP and the CBP curves are not
completely sigmoidal. CBP is more linear in the beginning and mCP has a longer
saturation period at the end and both of them have large linear component.
Therefore, a better comparison should be obtained from the linear fit. As expected,
the linear rates showed much faster rate for the mCP device. Furthermore, when all
three devices were compared, the rate for the BAlq device turned out to be the
slowest as observed.
The ETL devices were further analyzed in terms of half-lives. Table 6.4
summarizes the initial dark spot densities at t = 0, % dark areas at t = 0, experimental
half lives, and half lives calculated using Equations 6.9 and 6.10 for the devices with
various ETLs. Experimental half life of the standard device showed good agreement
with the two calculated half lives. Furthermore, the experimental half lives of CBP
254
and mCP devices also showed good agreement with the half-lives calculated with
Equation 6.9. However, the half-lives calculated with Equation 6.10 turned out to be
larger than the experimental half-lives. This data for the CBP and the mCP devices
was contradictory to the data obtained from the rate calculation. Since the
degradation curves of both the mCP and CBP devices were not completely
sigmoidal, and both the rates and the half-lives were calculated with sigmoidal-
Boltzmann function, then it can be speculated that these analysis were not
completely correct. Therefore, nothing can be said about these data.

10 100 1000
10
4
10
5
10
6
mcP corrected: 447.16 micr2/min
CBP corrected: 545.35 micr2/min
BAlq corrected: 424.01 micr2/min
Device A corrected
Device B corrected
Dark Area ( μ
2
)
Time (min)
0 500 1000 1500 2000 2500
-1x10
5
0
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
6x10
5
7x10
5
8x10
5
9x10
5
1x10
6
1x10
6
mcP corrected: 447.16 micr2/min
CBP corrected: 545.35 micr2/min
BAlq corrected: 424.01 micr2/min
Dark Area ( μ
2
)
Time (min)

Figure 6.19 (left) Degradation curves of the ETL devices. (right) Log-log
degradation curves of the ETL devices are shown with the standard test devices for
visual comparison.

The half life of BAlq calculated with Equation 6.9 did not agree very well
with the experimental half-life. The reason for this is the linear equation, which has
less weight. However, based on the rate calculation, which agreed well with the
255
longer experimental half-life, it can be suggested that the BALq was best ETL of all.
The device with the BAlq layer was the most stable one. Over all degradation of this
device showed an almost linear growth of dark spot and the half life of this device
was more than two times longer than the standard device. The reason for this could
be that since the BAlq is a molecule with a penta-coordinated Al at the center, it
perhaps acts as a desiccant and traps the atmospheric water molecules with the
available coordination site with empty orbitals. The effect decreases the degradation
process by preventing water penetration into the ETL.

Table 6.4 Summary of ETL studies.

Equation 6.9 Equation 6.10
Dark spots % Dark Area Experimental τ
1/2
Calculated τ
1/2
Calculated τ
1/2
ETL t = 20% t = 0 (hours) (hours) (hours)
mCP 5 0.73 4.33 4.63 7.31
CBP 5 0.97 5.5 4.63 6.65
Balq 2 0.96 17.58 10.81 ---
Alq3 3 0.89 7.50 7.38 6.85

6.3.5 Effects of over-coating materials
The concept of over-coating is very simple and is analogous to filling up a
hole or over-coating its surface with cement so water cannot seep in. In the case of
OLED, over coating materials need to be hydrophobic to prevent water permeation.
The more hydrophobic is the material, the less it interacts with the atmospheric water
molecules, and the less the device degrades. Silicon oil, Silicon-carbon paste, wax,
256
and parylene were used as cathode over-coating materials for OLEDs. Data showed
that the materials can be effectively used to retard the growth of dark spots. These
materials can be effectively used to encapsulate OLEDs or any type of organic
electronic devices. These materials are inexpensive and encapsulation can be done
in open air.
10 100 1000 10000
10
4
10
5
10
6
Paraffin wax
BCS wax
Device A corrected
Device B corrected
Dark Area/Dark Spot ( μ
2
)
Time (min)
10 100 1000
10
3
10
4
10
5
10
6
Si oil
Carbon glassy powder (2-13m)
Graphite powder/Si oil
C60/Si oil
C60 soot/Si oil
Device A corrected
Device B corrected
Paraffin oil
Dark Area/Dark Spot ( μ
2
)
Time (min)

Figure 6.20 (left) Log-log degradation curves of the over-coated devices. (right)
Log-log degradation curves of the wax over-coated devices are shown with the
standard test devices for visual comparison.

Since the degradation curves of all the over-coated devices had sigmoidal
shape, the curves were fitted with sigmoidal-Boltzmann equation (Figure 6.20). The
fitted results are summarized in Table 6.5. Figure 6.21 shows the curve fitting plots
of the devices over-coated with paraffin oil, Si oil, carbon glassy powder, and
graphite powder. Figure 6.22 shows the curve fitting plots of the devices over-
coated with C
60
/Si oil, C
60
soot/Si oil, paraffin wax, and BCS wax. The results of the
curve fittings showed that in average the over-coated devices had slower rate
compared to the test devices. The rate constants for the carbon glassy powder,
257
graphite powder, C
60
/Si oil, and C
60
soot/Si oil, and paraffin wax were an order of
magnitude smaller than the rate constants of the test devices.
-200 0 200 400 600
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
Data: Data5_C
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
yNo weighting

Chi^2/DoF = 2930526.30378
R^2 = 0.99946

A1 23354.16724 ±1338.68982
A2 225026.34087 ±2294.85889
x 0 353.1043 ±2.64702
dx 93.0663 ±2.77671
Si oil
Boltzmann fit of Data5_C
Dark Area/Dark Spot ( μ
2
)
Time (min)
-200 0 200 400 600 800
0.0
2.0x10
5
4.0x10
5
6.0x10
5
8.0x10
5
1.0x10
6
1.2x10
6
Data: Data5_B
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
yNo weighting

Chi^2/DoF = 543618915.78027
R^2 = 0.99718

A1 18395.41081 ±12890.68307
A2 1203901.31072 ±25458.60697
x 0 460.62205 ±5.94149
dx 101.91928 ±5.72185
Paraffin oil
Boltzmann fit of Data5_B
Dark Area/Dark Spot ( μ
2
)
Time (min)
-200 0 200 400 600
0.0
5.0x10
3
1.0x10
4
1.5x10
4
2.0x10
4
2.5x10
4
3.0x10
4
3.5x10
4
4.0x10
4
4.5x10
4
Data: Data5_D
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
yNo weighting

Chi^2/DoF = 110799.16032
R^2 = 0.99946

A1 4541.05533 ±260.2927
A2 43754.90525 ±446.194
x 0 353.10141 ±2.64688
dx 93.06387 ±2.77656
Carbon glassy powder (2-13m)
Boltzmann fit of Data5_D
Dark Area/Dark Spot ( μ
2
)
Time (min)
-300 0 300 600 900 1200 1500
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
Data: Data5_E
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
yNo weighting

Chi^2/DoF = 2062871.65446
R^2 = 0.99944

A1 -17122.46813 ±1709.90631
A2 183224.57642 ±1427.42338
x 0 650.18214 ±4.72459
dx 270.20966 ±6.26707
Graphite powder/Si oil
Boltzmann fit of Data5_E
Dark Area/Dark Spot ( μ
2
)
Time (min)

Figure 6.21 Sigmoidal-Boltzmann curve fitting of devices over-coated with paraffin
oil, Si oil, carbon glassy powder, and graphite powder.

Table 6.5 Summary of the curve fitting results of the over-coated devices are shown
with the standard test devices.

Device Sigmoidal
(min
-1
)
A (10DS) 0.034
B (3DS) 0.013
Paraffin oil 0.010
Si oil 0.011
Carbon glassy powder 0.011
Graphite powder 0.004
258
Table 6.5 Continued
Fullerene + Si oil 0.006
Fullerene soot + Si oil 0.004
Paraffin wax 0.002
BCS wax 0.000
Parylene ---

-200 0 200 400 600 800 1000
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
Data: Data5_F
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
yNo weighting

Chi^2/DoF = 6546454.66156
R^2 = 0.99869

A1 -3626.40182 ±1795.47305
A2 229060.43372 ±5652.34958
x 0 591.49799 ±8.00055
dx 155.33457 ±6.6227
C60/Si oil
Boltzmann fit of Data5_F
Dark Area/Dark Spot ( μ
2
)
Time (min)
-800 -400 0 400 800 1200 1600
0.0
5.0x10
4
1.0x10
5
1.5x10
5
2.0x10
5
Data: Data5_G
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
yNo weighting

Chi^2/DoF = 8934612.7549
R^2 = 0.99814

A1 -7722.22161 ±2090.88782
A2 196425.62233 ±2258.71456
x 0 722.38505 ±6.74933
dx 226.04363 ±8.00523
C60 soot/Si oil
Boltzmann fit of Data5_G
Dark Area/Dark Spot ( μ
2
)
Time (min)
-9000 -4500 0 4500 9000 13500 18000
-1x10
5
0
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
6x10
5
Data: Data5_I
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
y No weighting

Chi^2/DoF = 108688587.48744
R^2 = 0.9973

A1 -68018.6476 ±3921.19723
A2 590998.93552 ±1685.68656
x0 6129.81929 ±35.66639
dx 2706.09937 ±34.07966
BCS wax
Boltzmann fit of Data5_I
Dark Area/Dark Spot ( μ
2
)
Time (min)
-1000 0 1000 2000 3000 4000 5000
0.0
2.0x10
5
4.0x10
5
6.0x10
5
8.0x10
5
1.0x10
6
1.2x10
6
1.4x10
6
Data: Data5_H
Model: Boltzmann
Equation:
y = A2 + (A1-A2)/(1 + exp((x-x0)/dx))
Weighting:
y No weighting

Chi^2/DoF = 436705312.21909
R^2 = 0.99821

A1 -36111.74103 ±5585.92053
A2 1315901.60104 ±5746.30345
x 0 2468.48206 ±9.30315
dx 611.47096 ±9.79065
Paraffin wax
Boltzmann fit of Data5_H
Dark Area/Dark Spot ( μ
2
)
Time (min)

Figure 6.22 Sigmoidal-Boltzmann curve fitting of devices over-coated with C
60
/Si
oil, C
60
soot/Si oil, paraffin wax, and BCS wax.

The over-coating data were further analyzed with half-life calculations.
Table 6.3 summarizes the data for the over-coating studies. The calculated half lives
were obtained from Equations 6.9 and 6.10, which reflect the half lives of uncoated
devices. These half lives correspond to the actual half lives if the devices were not
259
over-coated. Hence all the experimental half lives will be considered as the
increased half lives after over-coating. The impact of the over-coating materials will
be obtained from the impact factor (IF), which is calculated by dividing the
experimental half life with the calculated half life. The higher the IF value the better
the over-coating material is. In Figure 6.23 the plot of impact factor vs. the
experimental half life is shown, which clearly shows the rise in impact factor with
the rise in half life. However, we have to remember that prediction of the device half
life based only on the number of dark spots is crude because it does not give any
information about how much degradation has really occurred.
0 50 100 150 200 250
0
20
40
60
80
100
120
140
IF1
IF3
Impact Factor (IF)
Half life, τ
1/2

Figure 6.23 The plot of impact factor vs. the experimental half life

Examples of these types of devices can be seen in Table 6.6, where device 4
and 5 both have 9 dark spots, but the % initial degraded area of device 4 is 3.4% and
device 5 is 10.7%. The effect can be seen in the calculated half lives, where
Equation 6.9 predicts 2.8 hours half lives for both of the devices and Equation 6.10
predicts 4.5 hour for device 4 and 2.8 hour for device 5. Similarly we can also see
260
for devices 6 and 7, the initial number of dark spots and % initial degraded areas are
same for both of the devices. The half lives predicted by Equation 6.9 are 2.2 and
half lives predicted by Equation 6.10 are 4.6 for both of the devices. That is why the
difference between IF
1
and IF
3
varies a lot when two devices with same numbers of
initial dark spots but very different initial degraded areas are compared.
Application of Ga-In and Ga-In-Sn eutectics showed the device area in
contact with the eutectics became non-emissive immediately after adsorption. Upon
adsorption the eutectic destroys the device and the destruction occurs immediately
(Device optical EL images are provided in the supplementary section).
Application of Silicone oil on the cathode of a device with 11 dark spots and
13% degraded area had a half life of 5.25 hours. The impact factors, IF
1
and IF
3
of
this material were roughly 2.0 suggesting a two time increase of the device life time.
The IFs calculated for the paraffin oil coated device was less than the silicon oil.
The IF for the device with 11% degraded area and 9 dark spots over-coated with
graphite/Si oil paste was 3.9 and performed better than the device with 3.4%
degraded area and 9 dark spots over-coated with carbon (2-13 μ)/Si oil paste. The
device with Fullerene soot paste had higher IF values than the device with Fullerene
paste.
Wax was one of the best over-coating materials of all. The ability of wax to
solidify in room temperature makes it a more efficient over-coating material. Figure
6.24 shows the picture of the devices at t = 0 and 8 days. A substrate with 3 devices
was used for this study, where device 1 was left open, device 2 was coated with
261
bottle-cap-sealing (BCS) wax, and device 3 was coated with paraffin wax. Paraffin
wax over-coated device with 1 initial dark spot and 2% degraded area had a half life
of 41 hours. The impact factors, IF
1
and IF
2
for this device were 2 and 7.5
respectively. The device over coated with BCS wax survived for 105 hours in the
open air. The impact factors, IF
1
and IF
2
for this device were 18.6 and 22.8
respectively, indicating a superiority of BCX wax over the paraffin wax. I-V, J-V,
and %QE of all the devices were measured at time t = 0 and 8 days (Figure 6.24).
Device characteristics after encapsulation with wax remained almost unchanged after
8 days.
23
0
2
4
6
8
10
12
14
Brightnes and %QE
Device
t=0: Brightness X 100
t=0: QE (%)
t=8 days: Brightness X 100
t=8 days: QE (%)
T = 0
T = 8 days
Dead
2mmX2mm 2mmX2mm
2mmX2mm
2mmX2mm
2mmX2mm
Device 1
No Over coating
Device 2
BCS wax
Device 3
Paraffin wax

Figure 6.24 The plot is a comparison of the brightness and the %QE between t=0
and t=8days. Device characteristics are shown to remain unchanged 8 days after
encapsulating with wax. Device 1 is uncoated, and is nearest to the cathode. Device
2, and 3 are coated with wax.

Parylene turned out to be the best over-coating material of all (1.2 μ Parylene
was vapor deposited on a device cathode). This experiment was conducted inside a
class 100 clean room with encapsulated device. The device with 4 initial dark spots
and over 23% degraded area over coated with parylene gave a half life of 216 hours.
262
The impact factors, IF
1
and IF
2
for this device were 38 and 134 respectively. When a
device is badly degraded to begin with, more weight should be given to the IF
calculated from Equation 6.9 because it takes the total initial degraded area to
calculate the half life. In the case of parylene coated device the impact factor was
significantly high for Equation 6.9 because the calculated half life of this device was
1.6 hours, which would be the actual half-life if the device was not over coated at all.

Table 6.6 Summary of over-coating studies.

Over-coating Materials Dark spots % Dark Area Experimental Equation 6.9 IF
1
Equation 6.10 IF
3
t = 10% t = 0 τ
1/2
(hours) τ
1/2
(hours) t
Exp
/t
Th
τ
1/2
(hours) t
Exp
/t
Th
1. Uncoated device 3 0.9 7.5 7.4 1.0 6.9 1.1
2. Paraffin oil 2 5.3 7.5 10.8 0.7 3.9 1.9
3. Si oil 11 12.6 5.3 2.4 2.2 2.6 2.0
4. C powder (2-13 μ) + Si oil 9 3.4 6.8 2.8 2.4 4.5 1.5
5. Graphite powder + Si oil 9 10.7 11.0 2.8 3.9 2.8 3.9
6. Fullerene + Si oil 12 3.3 9.0 2.2 4.0 4.6 2.0
7. Fullerene soot + Si oil 12 3.3 12.0 2.2 5.4 4.6 2.6
8. Paraffin wax 1 1.9 41.0 21.1 1.9 5.5 7.5
9. Bottle cap sealing wax 4 3.2 105.1 5.7 18.6 4.6 22.8
10. Parylene 4 23.3 216.0 5.7 38.2 1.6 134.3
Data collected @ 80% relative humidity and in N
2
atmosphere

6.4 Chapter 6 Conclusion
Correlation between overall degradation rate and average growth rate of all
dark spots were made. A correction method was developed to make comparison
between two dissimilar devices of same architecture. Based on the correction
method created, devices were compared and data were analyzed by fitting the
degradation curves with linear, quadratic, exponential, and Sigmoidal-Boltzmann
equations. Rates for the ETL and over-coated devices were compared with the
average rate of the test devices.
263
The device degradation data were further analyzed by predicting half-lives
with linear and Sigmoidal-Boltzmann equations. The linear equation was developed
based on the fact that the device life times were found to be inversely proportional to
the initial number of dark spots. The slope from the linear fit was used to predict the
device half lives. Since the overall growth of dark spots as a function of time
showed a sigmoidal behavior, Sigmoidal Boltzmann fit of the data was used to
predict the device half lives based on the initial degraded area. Finally both the rates
and the half-lives were used to evaluate all the data.
Effects of ETL were studied as a function of device degradation in the
presence of air. The device with the BAlq ETL had the slowest degradation rate.
Since BAlq is a penta-coordinate Al compound, it can act as a desiccant by binding
to the water molecules from the air hence slowing the degradation process of the
device. Silicon oil and Silicon-carbon paste over coating materials as cathode
encapsulating materials showed deceleration of growth of dark spots. Half lives
were shown to increase two to five times in open air. Wax encapsulation in air
showed a twenty times increase in device half life. Clean room encapsulation of
Parylene showed best performance of all. The half life of the Parylene over coated
device was 130 times longer than uncoated device.

264
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297
Appendix A. Study of the Phthalimides as Exciton Blocker
in Organic Photovoltaic (OPV) Cells

A.1 Introduction
Organic photovoltaic (OPV) cells are light harvesting semiconductor devices
composed of an anode, a donor layer (D), an acceptor layer (A), and a cathode.
Under optical excitation, excitons are generated and dissociated into opposite
charges, which gives rise to photocurrent. In a simple D-A cell, excitons dissociate
into individual charges at the D/A interface. Once generated, the charges diffuse
through the organic films and collect at the respective electrodes. In order for the
excitons to dissociate into charges however they must form either very close to the
D-A interface, or form at a distance which is equal or less than the exciton diffusion
length (L
D
). Excitons that form far from the D-A interface, either decay non-
radiatively, or recombine to generate photons, or migrate towards the cathode and
get quenched.
1, 2

In a conventional D-A cell, direct contact between the photoactive layer and
the cathode leads to significant lowering of the device efficiency. The three major
causes that contribute to this decrease in efficiency are: (1) Cathode induced damage
of the acceptor layer
2, 3
, (2) Injection of electrons from the cathode to the acceptor
layer
4
, and (3) Migration of excitons from the acceptor layer to the cathode.
2
In
order to reduce these problems, double hetero-junction OPV cells with a transparent
298
organic exciton blocking layer (EBL) or a buffer layer (BL) inserted between the
photoactive region and the cathode have been introduced.
1, 4-9
OPV devices with
copper phthalocyanine (CuPc) donor layer, C
60
acceptor layer, and various EBLs or
BLs have been widely studied and shown to have improved device performances
over the simple D-A cells.
1, 5, 8, 10, 11
Device with bathocuproine (BCP) as an EBL
was first reported by Peuman et al. Power conversion efficiency of this
CuPc/C
60
/BCP cell was 3.6% under 1 sun AM1.5 spectral illumination.
1
Authors
claimed that high efficiency of this device was a consequence of exciton blocking
ability of BCP, which by impeding the diffusion of excitons to the cathode increase
the probability of exciton dissociation at the D-A interface and increase the
efficiency of the cell. Similar finding was also reported by Vogel
7
and coworkers
recently, where the authors suggested that in a CuPc/C
60
/BCP device, BCP improves
device efficiency by facilitating electron transport out of the C
60
film to the Al
cathode. Furthermore, insertion of the BCP layer also strongly reduces exciton
quenching at the C
60
-Al interface hence increasing the device efficiency. Contrary to
these studies Song et al. reported that exciton blocking role of the buffer layer
reported by Peuman and Vogel et al. might not be true. They studied CuPc/C
60

device with Alq
3
BL and argued that the buffer layer increases the device efficiency
in OPV cells by inhibiting the transfer of electrons from the cathode to the C
60
layer.
4

The role of a buffer layer has always been a subject of controversy in OPV
cells. To date several explanations have been given to clarify how this spacer
enhances device performance. The five major explanations that have been widely
299
accepted are: (1) BL prevents the damage of the acceptor layer during cathode
deposition, (2) it facilitates electron transfer from the acceptor layer to the cathode,
(3) it inhibits electron transfer from the cathode to the acceptor layer, (4) it inhibits
the transfer of holes from the acceptor layer to the cathode, and (5) it inhibits the
transfer of excitons from the acceptor layer to the cathode. Since most of the above
reasonings rely on the HOMO-LUMO alignment of the buffer layer with the
acceptor layer, and the LUMO of BCP is 1.9 eV higher than the LUMO of C
60
, then
a CuPc/C
60
/BCP device should not work. Yet, a CuPc/C
60
/BCP cell is more efficient
than a CuPc/C
60
cell. This happens because the electrons in a CuPc/C
60
/BCP cell
diffuse through the C
60
film to the cathode by hopping through defect sites on BCP
created during cathode deposition. It is because of this damage mediated charge
transport in a BCP film, the material functions well as an electron transporter in a
CuPc/C
60
/BCP cell.
1, 11

One prerequisite for an efficient exciton blocking material is high triplet
energy so it can effectively block excitons from being quenched by the metal
cathode. In addition to that, the EBL material must also be optically transparent so it
does not absorb photons reflected from the cathode.
8, 11
The phthalimide materials
have triplet energies in the ranges between 2.9-3.0 eV, which are 300-400 meV
higher than the conventional exciton blocker BCP. These materials also have
relatively low absorption coefficients compared to the photoactive materials, CuPc
and C
60
. In OLEDs the phthalimides have already proved to be better exciton
blocker than BCP. Furthermore, since the triplet energies of phthalocyanine (1.24
300
eV) and C
60
(1.57 eV) are much lower than the triplet energies of the phthalimides,
the phthalimides should serve as excellent candidates for EBL or BL in an OPV cell.

N
O
O
N
O
O
N
O
O
N
O
O
tBuTMPP
N
N
O
O
O
O
ChBp
N N
NPP BCP
CuPc
Cu
N N
N N
N
N
N
N
C
60

Figure A.1 Structures of the phthalimide based hole/exciton blocking materials;
NPP, tBuTMPP, and ChBP are shown with conventional exciton blocking material
BCP, donor material CuPc, and acceptor material C
60
.

This chapter explores the use of the phthalimides as buffer layer materials in
the OPV cells. Devices with CuPc donor layer, C
60
acceptor layer, and phthalimide
301
spacer layer were fabricated. Figure 4.1 shows all the materials used in this study.
Three sets of devices were constructed with NPP, tBuTMPP, and ChBP materials
and compared with the control device with BCP EBL. The NPP and tBuTMPP
devices showed no rectification behavior. The device with ChBP EBL performed
very well and showed high rectification ratio in the dark and under light.

A.2 Experimental
A.2.1 OPV Device Fabrication and Testing
ITO coated glass substrates with approximately 20 Ω□
-1
surface resistivity
12

were first cleaned in dilute detergent/water solution (tergitol/water). They were then
rinsed with de-ionized water and dried in pure N
2
. After that, they were boiled in
trichloroethylene for 5 minutes, rinsed in acetone and ethanol and dried in pure N
2

prior to UV-ozone cleaning for 10 minutes. PV cells were fabricated inside a high
vacuum chamber (Kurt J. Lesker) equipped with a cryo pump, two crystal monitors,
and two power sources. Organic films were thermally evaporated onto the ITO
substrates from tantalum boats at pressures between 3-4 μtorr. Deposition rates for
all the organic materials were maintained to be around 2 Å/s at all time. Prior to the
deposition of the cathode, the chamber was vented with nitrogen and shadow masks
with 0.92 mm diameter (6.65×10
-3
cm
2
) holes were placed onto the substrates. Once
the pressure reached 3.0×10
-3
torr, 1000Å of Aluminum was deposited at rates
between 4-5 Å/s.
302
Devices were fabricated with the phthalimides as exciton blocking layer in
combination with copper phthalocyanine (CuPc) as the donor layer and C
60
as the
acceptor layer (Figure A.2). The device architectures were as follows: CuPc
(400Å)/C
60
(200Å)/Phthalimide (100Å)/Al(1000Å). Control devices were made
with BCP as the exciton blocking layer. For all the devices current density-voltage
(J-V) characteristic in the dark and under 20%, 40%, 60%, 80%, and 100%
intensities of simulated AM 1.5G solar illumination, the responsivity of the cell
(J
SC
/P
0
vs P
0
) under 1 sun, the solar conversion efficiency ηP%, open circuit voltage
V
OC
, and fill factor FF under 1 sun are provided in Appendix D.

ITO
CuPc
C
60
EBL
Al
ITO
CuPc
C
60
EBL
Al

Figure A.2 General cell structure: ITO/CuPc/C
60
/EBL/Al.

All OPV cells were tested in room temperature and pressure in open
atmosphere. Photocurrents were generated by illuminating the PV cells with a solar
simulator (Newport lamp model 66902 powered by Thermo Oriel 69911 power
supply) equipped with an AM 1.5G filter (Newport model 62020). Light intensities
303
were measured with a Newport thermopile model 70260. Current-voltage (I-V)
characteristics were measured by scanning the devices from -1.0 V to +1.0 V
(increments of 0.04 V) with a Keithley 2420 3A source meter.

A.3 Results and Discussion

6.44
2.40
tBuTMPP
BCP
6.4
1.56
NPP
6.83
2.78
ChBp
6.46
2.42
4.8
CuPc
C
60
6.2
5.2
3.3
3.5
4.3
6.44
2.40
tBuTMPP
BCP
6.4
1.56
NPP
6.83
2.78
ChBp
6.46
2.42
4.8
CuPc
C
60
6.2
5.2
3.3
3.5
4.3

Figure A.3 Energy diagram showing the HOMO and the LUMO energies of all the
materials studied.

The HOMO-LUMO energy diagrams of all the materials used in this study
are depicted in Figure A.3. The HOMO of NPP is 0.43 eV deeper than the HOMO
of BCP, which allows the HOMO-HOMO interface of C
60
-NPP to have 0.20 eV
extra barrier to holes compared to C
60
-BCP interface. In addition to that, since the
304
LUMO of NPP is 1.22 eV deeper than the LUMO of BCP, the barrier for electron
injection going from C
60
to NPP decreases by almost 1.0 eV compared to BCP. The
hole barrier for the HOMO-HOMO interfaces of C
60
-ChBP and C
60
-tBuTMPP
compared to C
60
-BCP are almost negligible. However, the LUMO-LUMO interface
offsets of C
60
-ChBP and C
60
-tBuTMPP are approximately 0.80 eV deeper than C
60
-
BCP interface, allowing ChBP and tBuTMPP molecules to have smaller electron
injection barrier over BCP. A comparison of the HOMO-LUMO energy level
alignment and triplet energies between the phthalimides and the BCP, it can be
suggested that the phthalimides would serve better as buffers or exciton blockers in
OPV cells than BCP. We examined the potential of NPP as EBL first because it has
the deepest HOMO and LUMO amongst all the phthalimides. tBuTMPP was chosen
because of its large rectification ratio in OLED. Besides that, the material also
performed better than BCP as an exciton blocker in OLED.

A.3.1 OPV cells with NPP and tBuTMPP
Figures A.4, A.5, and A.6 show the current density-voltage (J-V)
characteristics of the cells with NPP, tBuTMPP, and BCP EBLs in the dark and
under 1 sun AM 1.5G solar illumination respectively. Both of the phthalimide
devices gave very poor performance. The open-circuit voltage (V
OC
), short-circuit
current (J
SC
), fill-factor (FF), and power conversion efficiencies (η
P
%) of the devices
turned out to be much lower than the BCP cell (Table A.1). The J-V curve of the
305
BCP cell showed a typical diode characteristic while the cells with NPP and
tBuTMPP EBLs showed s shaped curves.
In figures A.4b, A.5b and A.6b the log of the current-density vs. voltage
(logJ-V) of the NPP, tBuTMPP, and BCP cells are shown respectively. The device
with BCP EBL, which behaves like a real diode, shows a very large rectification
ratio in the dark and under light. At negative bias in this device, almost no current
passes through and the logJ-V curves remain flat, when the resistance goes to infinity
and the voltage reaches an open circuit condition the current goes to zero and then
rises steeply due to large forward bias current. Both the NPP and tBuTMPP cells
behaved very differently than the BCP cell and showed poor rectification ratio. The
logJ-V curves in these devices appeared V shaped due to large current at negative
bias.

Figure A.4 (a) Current density-voltage (J-V) characteristic of the cell with NPP
EBL in the dark and under 1 sun intensity of simulated AM 1.5G solar illumination
(b) Log of current-density vs voltage of the cell with NPP EBL under various
intensities.
306
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-0.050
-0.025
0.000
0.025
0.050
Voltage (V)

Current density (mA/cm
2
)
NPP: Dark
NPP: 1 Sun
(a) (b)
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
10n
100n
1µ
10µ
100µ
1m

Current density (A/cm
2
)
Voltage (V)
Dark
20%
40%
60%
80%
100%

Figure A.5 (a) Current density-voltage (J-V) characteristic of the cell with
tBuTMPP EBL in the dark and under 1 sun intensity of simulated AM 1.5G solar
illumination (b) Log of current-density vs voltage of the cell with tBuTMPP EBL
under various intensities.

Figure A.6 Current density-voltage (J-V) characteristic of a cell with BCP EBL (a)
in the dark and under 1 sun intensity of simulated AM 1.5G solar illumination (b)
Log of current-density vs voltage under various intensities (c) General cell structure:
ITO/CuPc/C
60
/EBL/Al.

-0.50 -0.25 0.00 0.25 0.50
-4
-3
-2
-1
0
1
2
3
4
-0.5 0.0 0.5 1.0 1.5
-0.0050
-0.0025
0.0000
0.0025
0.0050
Voltage (V)

Current density (mA/cm
2
)
tBuTMPP:Dark
tBuTMPP:1 Sun
(a) (b)
-0.5 0.0 0.5 1.0 1.5
1n
10n
100n
1µ
10µ

Current density (A/cm
2
)
Voltage (V)
Dark
20%
40%
60%
80%
100%
Voltage (V)

Current density (mA/cm
2
)
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
100n
1µ
10µ
100µ
1m
10m
100m
1

Current density (A/cm
2
)
Voltage (V)
BCP:Dark
BCP:1 Sun
Dark
20%
40%
60%
80%
100%
307
Table A.1 Summary of open-circuit voltage (V
OC
), short-circuit current (J
SC
), fill-
factor (FF), and power conversion efficiencies (η
P
%) of OPV cells fabricated with
NPP, tBuTMPP, ChBP, and BCP EBLs under 1 sun AM 1.5G spectral illumination.

EBLs J
SC
(mA/cm
2
) V
OC
(V) FF η
P
(%)
NPP 0.00596 0.3 0.172 3.23E-04
tBuTMPP 0.003 0.398 0.269 2.80E-04
ChBP 1.61 0.423 0.3971 0.282
BCP 2.95 0.398 0.6158 0.758

A decrease in the FF in a solar cell is generally caused by a current leakage
and a poor rectification of the diode. In a typical photovoltaic cell, the shunt
resistance (R
SH
) stays very large to prevent leakage current and the series resistance
(R
S
) stays very low to give a sharp rise in the forward current (Figure A.7).
13
When
the current leakage increases, the shunt resistance (R
SH
) becomes small and when the
series resistance (R
S
) increases, the forward current drops off. And a combination of
both may give rise to an s shaped J-V curve or a V shaped logJ-V curve.

R
S
R
SH
Diode
I
PH
I
F
R
S
R
SH
Diode
I
PH
I
F

Figure A.7 A solar cell circuit diagram showing the series resistance (R
S
), the shunt
resistance (R
SH
), the forward current (I
F
), and the photo current (I
PH
).

308
In the phthalimide based cells, two independent factors may contribute to the
occurrence of such shapes that leads to significant lowering of the cell efficiencies:
(1) where the phthalimide BL acts as an electron barrier and shows resistance to
current flow and (2) where the phthalimide BL acts like it does not exist and the cell
behaves like a simple D-A cell. In the BCP cell the LUMO-LUMO energy offset
between the C
60
and BCP is decreased by the cathode induced defects in BCP. If the
phthalimide BLs act like resistive films and these defect sites do not exist in these
cells then instead of being electron transporters these materials would behave like
electron blockers with high electron barriers. Resistance to current flow between the
photoactive C
60
layer and the cathode in the forward direction would compromise the
rectification in the cell and decrease the FF. That means in the phthalimide cells,
after generation of the free charges, if the electrons cannot overcome the energy
barrier of the BL and migrate toward the cathode, they would either accumulate at
the interface and recombine with the holes or migrate in the opposite direction and
recombine with the holes. As a result the FF and the power conversion efficiency
(η
P
%) would decrease significantly.
The s shaped J-V curves in the NPP and the tBuTMPP devices clearly show
the increased resistance in the forward bias direction. The slow rise of the J-V curve
in the forward direction shows increased series resistance (R
S
) due to poor bulk
transport properties of the phthalimides and less ohmic contact between C
60
layer
and the cathode.
14
In addition to that, in the phthalimide based cells both the dark
and the photocurrent curves show characteristic fall off in the negative bias direction,
309
which in the BCP cell remains flat. The sharp fall off of the J-V curve in the
negative bias direction means a large current leakage in the system due to low shunt
resistance (R
SH
).
14

Poor photovoltaic response and the occurrence of s shaped curve is normal
for D-A cells without any EBL and have been observed in CuPc/C
60
and ZnPc/C
60

devices.
6, 7
In a cell with a thin buffer layer, this could occur if the existence of the
buffer layer is compromised due to non-uniform coverage. Since NPP forms crystals
upon deposition, it is probable that phase segregation after deposition creates direct
contact between the C
60
and the Al and the device behaves like a CuPc/C
60
DA cell.
For the NPP based device this type of behavior was expected because NPP is already
known to form crystals on glass soon after deposition (Figure A.1). For the
tBuTMPP device however, poor rectification ratio and low FF was disconcerting
because in OLEDs this material showed good rectification behavior. Although, there
is a probability that upon deposition on C
60
film the tBuTMPP might also crystallize
yielding a CuPc/C
60
DA type behavior. To elucidate the fact whether the NPP and
tBuTMPP forms crystals or not following experiments need to be conducted.
1. CuPc/C
60
and CuPc/C
60
/BCP devices need to be fabricated at the same time and
their J-V curves need to be compared. These devices would serve as controls for
later experiments.
2. C
60
/NPP, C
60
/tBuTMPP, and C
60
/BCP thin films need to be made at the same
time. The surface morphologies of the films need to be studied under optical
microscope and AFM. If crystals form upon deposition then the films would
310
appear rough under optical microscope. Also at high magnification scattered
islands may be observed. Amorphous films would appear smooth and defect
free.

CuPc
C
60
Al
NPP or tBuTMPP
Type A
CuPc
C
60
Islands of
NPP
or
tBuTMPP
Al
CuPc
C
60
Al
BCP
CuPc
C
60
Al
BCP
NPP or tBuTMPP
Type B
BCP
(a) (b) (c) (d)
CuPc
C
60
Al
NPP or tBuTMPP
Type A
CuPc
C
60
Islands of
NPP
or
tBuTMPP
Al
CuPc
C
60
Al
BCP
CuPc
C
60
Al
BCP
NPP or tBuTMPP
Type B
BCP
(a) (b) (c) (d)

Figure A.8 Organic PV cells with NPP, tBuTMPP, and BCP buffer layers showing
type A and B device structures and the possibility of crystalline islands formation in
the phthalimide based cells. Type A: CuPc/C
60
/NPP or tBuTMPP/BCP/Al cell (a)
Uniform film of phthalimide (b) Crystalline film of phthalimide. Type B:
CuPc/C
60
/BCP/NPP or tBuTMPP/Al cell (c) Crystalline film of phthalimide (d)
Uniform film of phthalimide.

3. Devices of the following structures need to be fabricated and studied: Type A:
CuPc/C
60
/NPP/BCP/Al and CuPc/C
60
/tBuTMPP/BCP/Al and type B:
CuPc/C
60
/BCP/NPP/Al, and CuPc/C
60
/BCP/tBuTMPP/Al. Figure A.8 shows the
structures of the two types of devices that need to be studied. If the NPP or
tBuTMPP crystallizes upon deposition on C
60
film, then the type A device would
behave like a CuPc/C
60
/BCP device. However if no crystallization occurs, then
the cell would either shut off or be extremely inefficient due to LUMO-LUMO
offset barrier of the C
60
-phthalimide interface. Similarly in the type B devices, if
311
the phthalimides crystallizes on BCP then the cells may work and give poor
photovoltaic response due to direct contact points between BCP and the cathode.
However, if the phthalimides form uniform amorphous films then the cell may
not work.

A.3.2 OPV cells with ChBP

Figure A.9 (a) Current density-voltage (J-V) characteristic of the cell with ChBP
EBL in the dark and under 1 sun intensity of simulated AM 1.5G solar illumination
(b) Log of current-density vs voltage of the cell with ChBP EBL under various
intensities.

A substantial improvement in device performance was observed when NPP
and tBuTMPP buffer layers were replaced by ChBP buffer layer. The values for J
SC

(1.61 mA/cm
2
), V
OC
(0.42 V), FF (0.40), and η
P
% (0.30) of the ChBP device were
much higher than the other phthalimide devices. Figure A.9 shows the J-V and logJ-
V curves of the device made with ChBP BL in the dark and under 1 sun AM 1.5G
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-4
-3
-2
-1
0
1
2
3
4
Voltage (V)

Current density (mA/cm
2
)
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
100n
1µ
10µ
100µ
1m
10m
100m

Current density (A/cm
2
)
Voltage (V)
Dark
20%
40%
60%
80%
100%
Dark
20%
40%
60%
80%
100%
312
spectral illumination respectively. The device behaved like a diode and showed high
rectification ratio in the dark and under light. However unlike the BCP device, the J-
V curves of the ChBP device did not remain flat at negative bias. This happens when
current leakage caused by pinholes or shorts reduces the R
SH
. As a consequence,
current passes through the device at negative bias and decreases FF and the power
conversion efficiency. In the ChBP device, this perhaps causes the 35-50% lowering
of the short-circuit current, fill factor, and power conversion efficiency. On the other
hand, since the shape of the dark current of the ChBP device at negative bias was
similar to the BCP device, it can be suggested that some other factors may have
played a role in decreasing the efficiency of the ChBP device.
Although the ChBP device was 40% less efficient than the BCP device, it
behaved like a typical diode, showed much less resistance, and very small current
leakage compared to the other phthalimide devices. In addition to that the open-
circuit voltage of this device was also 30 mV higher than the BCP device. Higher
V
OC
of the ChBP device suggests that the parameter may not be only influenced by
the interfacial gap (Interfacial gap, I
g
: the energy gap between the HOMO of the
donor and the LUMO of the acceptor) of the donor and the acceptor materials.
15
The
open circuit voltage is a sensitive function of the materials and the electrodes. Both
the interfacial gap
16
and the differences in the metal work functions
17
have been
shown to significantly affect V
OC
. Since the bulk charge transport properties of the
materials differ from their interface properties, materials with different HOMO-
LUMO gaps influence the V
OC
. Furthermore, the interfacial dipoles at the metal-
313
insulator interfaces lead to band-bending, a phenomenon is known as the “Fermi
level pinning,” which change the work function of the metals and in turn affect the
V
OC
.
13
It is known that the Fermi level pinning between the fullerene and the cathode
affects the V
OC
of a CuPc/C
60
/metal cell. However, whether the phenomenon affects
the V
OC
of the CuPc/C
60
/BCP/metal cell is not known.
It is important to understand why the ChBP device failed to perform similar
to or better than the BCP device. Its diode like behavior indicates that the bulk and
interface properties of this material must differ from NPP and tBuTMPP. Since the
NPP and tBuTMPP molecules are linear and the ChBP is bent like the BCP, it is
probable that the surface morphology and the thin film properties of ChBP and BCP
are similar. It is also possible that charge transfer in ChBP is also enhanced by
cathode induced mid-gap states like in BCP.
In order to examine if cathode induced damage sites are responsible for
electron transfer in ChBP we constructed OPV cells with ChBP buffer layers of three
different thicknesses. Increase in the photovoltaic response with decreasing ChBP
thicknesses would indicate that perhaps ChBp-Al interface is responsible for better
device performances. Figure A.10 shows the J-V curves of the devices with 50 Å,
100 Å, and 150 Å of ChBP BL under 1 sun intensity of simulated AM 1.5G spectral
illumination respectively. Unfortunately all of the devices including the one with
100 Å performed very poorly suggesting that the experiment failed for some
unknown reason(s) (Table A.2). However, based on the trend of performance we
can say that 100 Å is still the best.
314
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-4
-2
0
2
4
6
8
10
Voltage (V)

Current density (mA/cm
2
)
CH2p 50A
CH2p 100A
CH2p 150A
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-0.0020
-0.0015
-0.0010
-0.0005
0.0000
0.0005
0.0010
0.0015
0.0020
Voltage (V)

Current density (mA/cm
2
)
Dark
20%
40%
60%
80%
100%

Figure A.10 Current density-voltage (J-V) characteristics of the cells with 50 Å, 100
Å, and 150 Å of ChBP EBL under 1 sun intensity of simulated AM 1.5G solar
illumination respectively showing the thickness dependent photovoltaic response.
Inset is the data for 150 Å.

Table A.2 Summary of open-circuit voltage (V
OC
), short-circuit current (J
SC
), fill-
factor (FF), and power conversion efficiencies (η
P
%) of OPV cells fabricated with
50 Å, 100 Å, and 150 Å of ChBP EBL under 1 sun intensity of simulated AM 1.5G
solar illumination respectively.

Thickness J
SC
(mA/cm
2
) V
OC
(V) FF η
P
(%)
50Å 0.826 0.213 0.2726 0.0475
100Å 1.33 0.328 0.3136 0.136
150Å 2.16E-04 0.209 0.2926 1.30E-05
315
Why ChBP is the only phthalimide molecule that works in a solar cell is a
question that is yet to be answered. And to answer that question we need to design
experiments that would address its role as a buffer layer in a solar cell. Therefore,
following experiments need to be conducted to answer the above questions
1. C
60
/ChBP and C
60
/BCP thin films need to be made and their surface
morphologies need to be studied under optical microscope and AFM.
2. For further verification and statistical purposes CuPc/C
60
/ChBP/Al devices need
to be made few more times along with the CuPc/C
60
/BCP/Al control. If the V
OC

of the ChBP device is always higher than the BCP device then new light would
be shed into what affects the change in V
OC
.
3. The above experiment needs to be repeated with Ag cathode.
4. Thickness dependent studies need to be done again. Devices need to be made
with 25 Å, 50 Å, 75 Å, and 100 Å of ChBP layers with Al and Ag cathodes.
5. Devices with cathode doped into ChBP need to be made. For this study the
following devices need to be looked at: CuPc/C
60
/ChBP:Al(x%)/Al and
CuPc/C
60
/ChBP:Ag(x%)/Ag, where x = 5%, 10%, 15% etc. If the devices with
metal doped ChBP performs better than the undoped ones then that would
indicate the mid-gap states effect on photovoltaic response.
6. The cathode layer of a CuPc/C
60
/ChBP/cathode device can be peeled of by
applying a scotch tape on the cathode surface and then removing it. The
Secondary Ion Mass Spectrometry (SIMS) of the surface can then be conducted
316
to see if the particles sputtered during ion bombardment contain any cathode
materials.
7. Charge conductivity experiments need to be studied to compare the conductivity
between ChBP and BCP.

A.4 Appendix A Conclusion
Organic photovoltaic cells were fabricated with phthalimide based materials;
NPP, tBuTMPP, and ChBP buffer layers and compared with the one with BCP
buffer layer. The devices with the NPP and tBuTMPP buffer layers showed no
rectification behavior and performed extremely poorly. These devices showed high
resistance to current flow and large current leakage in the dark and under light. It is
probable that both of these molecules form crystals and phase segregate upon
deposition. As a consequence the devices behave like simple donor-acceptor
devices.
The device with ChBP turned out to be the best of all the phthalimide based
devices. This device behaved like a typical diode and showed large rectification
ratio in the dark and under light. The photocurrent conversion efficiency, fill factor,
and short-circuit current of this device were much higher than the NPP and the
tBuTMPP devices. Although the values were still less than the control, the open-
circuit voltage of the ChBP device however, was approximately 30 mV higher than
the BCP device suggesting that V
OC
may be partially affected by the buffer layer too.
317
Our experiments with the phthalimides showed that good exciton blocking
ability does not necessarily mean a better buffer layer. Also, if a material rectifies in
an OLED will not automatically rectify in a solar cell. The phthalimide materials
used in the solar cell study possessed higher triplet energies than the conventional
solar cell materials; CuPc and C
60
, and BCP. Yet they still performed poorly. ChBP,
which was the best buffer layer material amongst all the phthalimides, still proved to
be less efficient than the conventional buffer layer BCP.
318
A.5 Appendix A References
1. Forrest, P. P. a. S. R., Very-high-efficiency double-heterostructure copper
phthalocyanine/C60 photovoltaic cells. Appl. Phys. Lett. 2001, 79, (1), 126.
2. Peter Peumans, A. Y., and Stephen R. Forrest, Small molecular weight
organic thin-film photodetectors and solar cells. Journal of Applied Physics 2003,
93, (7), 3693-3723
3. Y. Hirose, A. K., V. Aristov, P. Soukiassian, V. Bulovic and S. R. Forrest,
Chemistry and electronic properties of metal-organic semiconductor interfaces: Al,
Ti, In, Sn, Ag, and Au on PTCDA. PHYSICAL REVIEW B 1996, 54, (19), 13748.
4. Q. L. Song and C. M. Lia, M. L. W., X. Y. Sun, and X. Y. Houa, Role of
buffer in organic solar cells using C60 as an acceptor. APPLIED PHYSICS LETTERS
90 2007, 90, 071109.
5. Q.L. Song, F. Y. L., H. Yang, H.R. Wu, X.Z. Wang, W. Zhou, J.M. Zhao,
X.M. Ding, C.H. Huang, X.Y. Hou, Small-molecule organic solar cells with
improved stability. Chemical Physics Letters 2005, 416, 42–46.
6. Hur, S. W. O., Hyun Seok; Oh, Yong Cheul; Chung, Dong Hoe; Lee, Joon
Ung; Park, Jong Wook; Kim, Tae Wan, Organic photovoltaic effects using CuPc and
C60 depending on layer thickness. Synthetic Metals 2005, 154, (1-3), 49-52.
7. M. Vogel, S. D., Ch. Breyer, M. Ch. Lux-Steiner, and K. Fostiropoulos, On
the function of a bathocuproine buffer layer in organic photovoltaic cells. APPLIED
PHYSICS LETTERS 2006, 89, 163501.
8. M. Y. Chan, C. S. L., a S. L. Lai, M. K. Fung, F. L. Wong, H. Y. Sun, K. M.
Lau, and S. T. Lee, Efficient organic photovoltaic devices using a combination of
exciton blocking layer and anodic buffer layer. JOURNAL OF APPLIED PHYSICS
2006, 100, 094506.
9. M. Y. Chan, S. L. L., K. M. Lau, C. S. Lee, and S. T. Lee, Application of
metal-doped organic layer both as exciton blocker and optical spacer for organic
photovoltaic devices. APPLIED PHYSICS LETTERS 2006, 89, 163515.
10. Takahiro Osasa, Y. M., Tadayoshi Matsumura, Michio Matsumura,
Determination of photo-active region in organic thin film solar cells with an organic
heterojunction. Solar Energy Materials & Solar Cells 2006, 90, 3136–3142.

319
11. B. P. Rand, J. L., J. Xue, R. J. Holmes, M. E. Thompson, S. R. Forrest,
Organic Double-Heterostructure Photovoltaic Cells Employing Thick
Tris(acetylacetonato)ruthenium(III) Exciton-Blocking Layers. Advanced Materials
2005, 17, (22), 2714-2718.
12. Chihaya Adachi, M. A. B., Stephen R. Forresta,Sergey Lamansky, Mark E.
Thompson, Raymond C. Kwong, High-efficiency red electrophosphorescence
devices. Appl. Phys. Lett. 2001, 78, (11), 1622-1624.
13. Harald Hoppe, N. S. S., Organic Solar Cells: An Overview. Journal of
Materials Research 2004, 19, (7), 1924-1945.
14. A. Shah, P. T., R. Tscharner, N. Wyrsch, and H. Keppner, Photovoltaic
Technology: The Case for Thin-Film Solar Cells Science 1999, 285, 692-698.
15. Christoph J. Brabec, A. C., Dieter Meissner, N. Serdar Sariciftci, Thomas
Fromherz,; Minze T. Rispens, L. S., and Jan C. Hummelen, Origin of the Open
Circuit Voltage of Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, (5), 374.
16. Kristin L. Mutolo, E. I. M., † Barry P. Rand,‡ Stephen R. Forrest,‡,§ and
Mark E. Thompson*,†, Enhanced Open-Circuit Voltage in Subphthalocyanine/C60
Organic Photovoltaic Cells. J. AM. CHEM. SOC. 2006, 128, 8108-8109.
17. Parker, I. D., Carrier Tunneling and Device Characteristics in Polymer Light-
emitting Diodes. J. Appl. Phys. 1994, 75, 1656.

320
Appendix B
TDDFT excitation energies calculated in the gas phase and in the solvent
dielectrics (CPCM) are tabulated. Absolute values of the orbital coefficients are
reported only for the major components of the excitation.
Table B.1 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for ChBP calculated in gas phase.

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
331.07 97 ->99 0.482 0.0001
S
2
330.24 97 ->100 0.516 0.0002
S
3
303.45 98 ->99 0.619 0.0002
S
4
300.58 98 ->100 0.602 0.0001
S
7
278.91 96 ->99 0.458 0.0003
S
8
277.88 96 ->100 0.403 0.0008
S
9
277.55 95 ->99 0.468 0.0004
S
10
276.37 95 ->100 0.518 0.0007
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
386.92 95 ->99 0.312
--- 98 ->99 0.404
T
2
383.43 95 ->100 0.326
--- 98 ->100 0.356
T
3
370.3 96 ->99 0.359
--- 97 ->99 0.486
T
4
369.49 96 ->100 0.302
--- 97 ->100 0.524
T
5
346.67 91 ->99 0.340
--- 98 ->99 0.385
T
6
343.91 91 ->100 0.358
--- 98 ->100 0.371
T
7
331.07 96 ->99 0.339
--- 97 ->99 0.482
T
8
330.24 97 ->100 0.516
321
Table B.1 Continued
T
9
325.01 89 ->99 0.375
--- 92 ->99 0.360
T
10
324.47 90 ->100 0.330
--- 92 ->100 0.376

Table B.2 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for ChBP calculated in THF (CPCM).

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
319.39 93 -> 99 0.413 0.0002
--- 96 -> 99 0.506 ---
S
2
318.44 93 -> 100 0.400 0.0002
--- 96 -> 100 0.513 ---
S
3
311.03 98 -> 99 0.606 0.0005
S
4
308.05 97 ->100 0.327 0.0008
--- 98 ->100 0.581 ---
S
5
288.52 90 ->99 0.309 0.0003
--- 97 ->99 0.334 ---
S
6
287.77 90 ->100 0.379 0.0004
--- 97 ->100 0.311 ---
S
7
282.24 97 ->100 0.414 0.0020
--- 98 ->100 0.312 ---
S
8
282.07 97 -> 99 0.440 0.0005
S
9
278.03 95 -> 99 0.592 0.0687
S
10
276.36 94 ->100 0.573 0.0835
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
395.85 97 ->99 0.431
--- 98 ->99 0.480
T
2
392.27 97 ->100 0.483
--- 98 ->100 0.425
T
3
354.81 93 -> 99 0.446
--- 96 -> 99 0.507
322
Table B.2 Continued
T
4
353.89 93 -> 100 0.435
--- 96 -> 100 0.517
T
5
335.76 91 ->99 0.367
--- 92 ->99 0.412
T
6
334.54 95 ->99 0.690
T
7
333.16 91 ->100 0.352
--- 92 ->100 0.390
T
8
332.27 94 ->100 0.672
T
9
319.39 93 ->99 0.413
T
10
318.44 93 -> 100 0.400
--- 96 -> 100 0.513

Table B.3 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for ChBP calculated in CH
3
CN (CPCM).

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
317.67 93 -> 99 0.419 0.0001
--- 96 -> 99 0.455 ---
S
2
316.78 93 ->100 0.400 0.0002
--- 96 ->100 0.472 ---
S
3
312.54 98 -> 99 0.602 0.0007
S
4
309.48 97 ->100 0.335 0.0011
--- 98 ->100 0.578 ---
S
5
288.18 97 -> 99 0.424 0.0006
S
6
287.45 90 ->100 0.323 0.0007
--- 97 ->100 0.400 ---
S
7
282.7 90 ->100 0.341 0.0012
--- 97 ->100 0.393 ---
S
8
282.63 97 ->99 0.414 0.0008
S
9
279.21 95 ->99 0.546 0.0692
S
10
277.47 94 ->100 0.480 0.0837

323
Table B.3 Continued
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
397.62 97 ->99 0.438
--- 98 ->99 0.489
T
2
393.93 97 ->100 0.490
--- 98 ->100 0.434
T
3
352.53 93 -> 99 0.453
--- 96 -> 99 0.458
T
4
351.71 93 -> 100 0.435
--- 96 -> 100 0.479
T
5
336.89 95 ->99 0.637
T
6
334.55 94 ->100 0.605
--- 95 ->100 0.304
T
7
334.13 91 ->99 0.371
--- 92 ->99 0.430
T
8
331.53 91 ->100 0.368
--- 92 ->100 0.419
T
9
317.67 93 -> 99 0.419
--- 96 -> 99 0.456
T
10
316.78 93 -> 100 0.400
--- 96 -> 100 0.472

Table B.4 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for ChBP calculated in cyclohexane (CPCM).

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
324.74 95 -> 99 0.417 0.0002
--- 96 -> 99 0.514 ---
S
2
323.93 95 ->100 0.386 0.0002
--- 96 ->100 0.487 ---
S
3
307.45 98 -> 99 0.616 0.0001
S
4
304.42 97 ->100 0.303 0.0001
--- 98 ->100 0.598 ---
324
Table B.4 Continued
S
5
291.24 89 -> 99 0.335 0.0000
--- 90 -> 99 0.339 ---
S
6
290.48 90 ->100 0.405 0.0001
S
7
280.16 97 -> 99 0.535 0.0022
S
8
279.84 97 ->100 0.518 0.0003
S
9
274.76 94 -> 99 0.439 0.0483
--- 95 -> 99 0.302 ---
S
10
273.38 95 -> 99 0.394 0.0130
--- 96 -> 99 0.348 ---
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
391.38 97 ->99 0.389
--- 98 ->99 0.451
T
2
387.77 97 ->100 0.452
--- 98 ->100 0.403
T
3
361.8 95 -> 99 0.447
--- 96 -> 99 0.516
T
4
361.06 95 -> 100 0.418
--- 96 -> 100 0.492
T
5
340.76 91 ->99 0.380
--- 92 ->99 0.37045
T
6
338.14 91 ->100 0.364
--- 92 ->100 0.37289
T
7
328.46 94 ->99 0.646
T
8
326.2 93 ->100 0.643
T
9
324.74 95 -> 99 0.417
--- 96 -> 99 0.514
T
10
--- 95 -> 100 0.38643
--- --- 0.48711

325
Table B.5 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for NPP calculated in the gas phase.

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
325.72 75 -> 76 0.274 0.0003
--- 74 -> 77 0.284 ---
--- 74 -> 76 0.393 ---
S
2
325.53 74 -> 77 0.275 0.0003
--- 75 -> 76 0.282 ---
--- 75 -> 77 0.394 ---
S
5
288.83 74 -> 76 0.319 0.0025
--- 75 -> 77 0.326 ---
--- 75 -> 76 0.459 ---
S
6
288.66 75 -> 77 0.318 0.0025
--- 74 -> 76 0.331 ---
--- 74 -> 77 0.455 ---
S
7
273.15 73 -> 77 0.427 0.1301
--- 72 -> 76 0.446 ---
S
8
272.41 72 -> 77 0.429 0.0009
--- 73 -> 76 0.436 ---
S
9
264.91 71 -> 76 0.601 0.0073
S
10
264.89 70 -> 76 0.610 0.0194
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
386.86 70 -> 77 0.287
--- 70 -> 76 0.400
T
2
386.52 71 -> 76 0.288
--- 72 -> 76 0.400
T
3
366.24 75 -> 76 0.272
--- 74 -> 77 0.288
--- 74 -> 76 0.391
T
4
366 74 -> 77 0.278
--- 75 -> 76 0.279
--- 75 -> 77 0.397
T
5
329.92 73 -> 77 0.310
--- 72 -> 76 0.414
--- 73 -> 76 0.427
326
Table B.5 Continued
T
6
329.72 73 -> 77 0.409
--- 72 -> 77 0.425
T
9
323.82 69 -> 77 0.330
--- 68 -> 76 0.385
--- 69 -> 76 0.406
T
10
323.6 69 -> 76 0.326
--- 68 -> 77 0.389
--- 69 -> 77 0.407

Table B.6 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for phenylphthalimide calculated in the gas
phase.

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
364.4 58 -> 59 0.688 0.0029
S
2
328.93 56 -> 59 0.682 0.0003
S
5
276.3 55 -> 59 0.423 0.0313
--- 58 -> 60 0.527 ---
S
6
260.49 55 -> 59 0.454 0.317
--- 58 -> 60 0.439 ---
S
7
257.78 54 -> 59 0.600 0.0260
S
8
250.89 57 -> 60 0.648 0.0025
S
9
240.38 56 -> 60 0.677 0.0008
S
10
227.84 57 -> 61 0.410 0.0035
--- 58 -> 62 0.49396 ---
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
411.27 58 -> 59 0.663
T
2
369.17 56 -> 59 0.620
T
3
367.64 54 -> 59 0.571
--- 56 -> 59 0.333
T
5
344.57 57 -> 62 0.450
--- 58 -> 60 0.434
--- 58 -> 61 0.498
327
Table B.6 Continued
T
6
332.47 53 -> 59 0.453
--- 57 -> 59 0.525
T
8
321.7 55 -> 59 0.724
T
9
319.78 57 -> 59 0.685
T
10
302.61 53 -> 59 0.50207
--- 57 -> 59 0.47079

Table B.7 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for PBP calculated in the gas phase.

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
382.95 95 ->96 0.690 0.003
S
2
382.75 95 ->97 0.690 0.002
S
3
328.88 92 ->96 0.461 0.000
--- 93 ->97 0.487 ---
S
4
328.87 92 ->97 0.461 0.001
--- 93 ->96 0.487 ---
S
7
291.85 95 ->98 0.672 0.493
S
9
282.39 85 ->96 0.416 0.000
--- 87 ->97 0.486 ---
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
422.66 95 -> 96 0.665
T
2
422.54 95 -> 97 0.665
T
5
371.18 88 -> 96 0.355
--- 92 -> 96 0.319
--- 93 -> 97 0.338
T
6
371.14 88 -> 97 0.356
--- 92 -> 97 0.318
--- 93 -> 96 0.336
T
7
367.72 88 -> 96 0.313
--- 91 -> 97 0.366
--- 88 -> 96 0.355

328
Table B.7 Continued
T
8
367.7 88 -> 97 0.311
--- 91 -> 96 0.367
--- 92 -> 97 0.357
T
9
364.11 94 -> 101 0.345
--- 95 -> 98 0.557
--- 95 -> 100 0.426
T
10
329.57 85 -> 97 0.320
--- 87 -> 96 0.375
--- 94 -> 97 0.483

Table B.8 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for TMPP calculated in the gas phase.

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
364.38 111 ->112 0.706 0.0014
S
2
363.55 111 ->113 0.706 0.0006
S
3
351.91 110 ->112 0.632 0.0007
S
4
351.23 110 ->113 0.631 0.0006
S
5
322.46 108 ->112 0.424 0.0003
--- 109 ->112 0.435 ---
S
6
321.71 108 ->113 0.421 0.0003
--- 109 ->113 0.436 ---
S
7
286.43 101 ->112 0.454 0.0008
--- 102 ->112 0.469 ---
S
8
285.95 101 ->113 0.460 0.0007
--- 102 ->113 0.462 ---
S
9
281.55 106 ->112 0.300 0.0010
--- 107 ->112 0.558 ---
S
10
281.36 106 ->113 0.294 0.0006
--- 107 ->113 0.561 ---

329
Table B.8 Continued
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
386.44 110 ->112 0.45884
T
2
385.58 110 ->113 0.45494
T
3
376.46 108 ->112 0.42579
T
4
375.88 108 ->113 0.42376
T
5
366.63 111 ->112 0.60453
T
7
364.17 111 ->113 0.63092

Table B.9 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for tBuTMPP calculated in the gas phase.

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
356.97 143 ->144 0.706 0.0022
S
2
356.52 143 ->145 0.706 0.0004
S
3
347.32 142 ->144 0.628 0.0006
S
4
346.89 142 ->145 0.627 0.0006
S
5
319.33 139 ->144 0.363 0.0007
--- 140 ->144 0.336 0.0007
S
6
319.22 139->145 0.361 0.0010
--- 140 ->145 0.277 0.0010
S
7
286.13 141 ->144 0.480 0.0341
S
8
285.68 141 ->145 0.477 0.0195
S
9
284.58 133 ->144 0.434 0.0014
--- 134 ->144 0.432 0.0014
S
10
284.27 133 ->145 0.477 0.0004
--- 134 ->145 0.410 0.0004
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
388.53 141 ->144 0.35223
--- 142 ->144 0.3489
T
2
388.25 141 ->145 0.34979
--- 142 ->145 0.34903
330
Table B.9 Continued
T
3
374.05 139 ->144 0.35003
--- 140 ->144 0.37928
T
4
373.68 140 ->145 0.38199
--- 142 ->145 0.29924
T
5
360.84 143 ->144 0.55983
--- 143 ->149 0.33384
T
6
357.03 143->145 0.64819
T
7
356.97 143 ->144 0.70591
T
8
356.52 143 ->145 0.70596
T
9
349.05 143 ->149 0.50561
--- 142 ->148 0.42917
T
10
347.32 142 ->144 0.62804

Table B.10 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for tBuTMPP calculated in THF (CPCM).

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
348.41 143 ->144 0.706 0.0038
S
2
348.03 143 ->145 0.706 0.0000
S
3
338.51 142 ->144 0.647 0.0004
S
4
338.19 142 ->145 0.646 0.0020
S
5
311.82 137 ->144 0.322 0.0072
--- 137 ->145 0.248 ---
S
6
311.67 137 ->144 0.246 0.0063
--- 137 ->145 0.319 ---
S
7
292.27 141 ->144 0.453 0.0830
S
8
291.99 141 ->145 0.450 0.0240
S
9
280.51 138 ->145 0.377 0.1487
--- 139 ->144 0.452 ---
S
10
280.27 138 ->144 0.376 0.0002
--- 139 ->145 0.440 ---

331
Table B.10 Continued
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
396.71 141 ->144 0.32891
--- 141 ->145 0.31681
T
2
396.48 141 ->144 0.31495
--- 141 ->145 0.32557
T
3
358.98 137 ->145 0.32048
--- 142 ->144 0.33365
T
4
358.62 137 ->144 0.3215
--- 142 ->145 0.33147
T
5
355.6 143 ->144 0.47074
--- 143 ->149 0.47243
T
7
348.41 143 ->145 0.70206
T
8
348.03 143 ->145 0.70577
T
9
344.06 143 ->144 0.52056

Table B.11 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for tBuTMPP calculated in CH
3
CN (CPCM).

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
345.47 143 ->144 0.706 0.0032
S
2
344.86 143 ->145 0.706 0.0008
S
3
336.4 142 ->144 0.648 0.0007
S
4
335.92 142 ->145 0.648 0.0026
S
5
310.78 136 ->144 0.305 0.0108
--- 137 ->144 0.378 ---
S
6
310.58 136 ->145 0.296 0.0091
--- 137 ->145 0.385 ---
S
7
292.55 141 ->144 0.465 0.0746
S
8
292.13 141 ->145 0.472 0.0337
S
9
281.72 138 ->145 0.307 0.1505
--- 139 ->144 0.419 ---
S
10
281.45 138 ->144 0.320 0.0024
332
Table B.11 Continued
--- 139 ->145 0.428 ---
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
398.39 140 ->144 0.340
--- 141 ->144 0.426
T
2
398.03 141 ->144 0.325
--- 141 ->145 0.432
T
4
355.50 137 ->145 0.362
--- 142 ->145 0.369
T
9
342.05 143 ->144 0.489
--- 143 ->149 0.333

Table B.12 Vertical excitation energies (E), dominant MO transitions, orbital
coefficients, and oscillator strengths for tBuTMPP calculated in cyclohexane
(CPCM).

Singlet Calculated λ ψ
oc
→ ψ
ver
Orbital Oscillator
states (nm) MO Composition Coefficient Strengths (f)
S
1
353.21 143 ->144 0.706 0.0032
S
2
352.78 143 ->145 0.706 0.0003
S
3
342.68 142 ->144 0.642 0.0007
S
4
342.28 142 ->145 0.641 0.0010
S
5
314.95 139 ->144 0.378 0.0022
S
6
314.79 139 ->145 0.378 0.0024
S
7
289.9 141 ->144 0.486 0.0607
S
8
289.51 141 ->145 0.484 0.0312
S
9
280.18 133 ->144 0.415 0.0009
--- 134 ->144 0.427 ---
S
10
279.84 133 ->145 0.443 0.0004
--- 134 ->145 0.393 ---
Triplet Calculated λ ψ
oc
→ ψ
ver
Orbital
states (nm) MO Composition Coefficient
T
1
392.45 140 ->144 0.32929
--- 141 ->144 0.37802
T
2
392.16 140 ->145 0.32748
333
Table B.12 Continued
--- 141 ->145 0.37608
T
3
366.04 139 ->144 0.34459
--- 142 ->144 0.3455
T
4
365.6 139 ->145 0.34398
--- 142 ->145 0.34418
T
5
358.32 143 ->144 0.5251
T
6
353.23 143 ->145 0.66713
T
9
347.18 143 ->144 0.41154
--- 143 ->149 0.4607

334
Appendix C
Optical microscope images of neat NPP and TMPP thin films (500Å) are
shown with thin films of NPP and TMPP films doped with Irppy and PQIr.

10 μ 10 μ
NPP neat film TMPP neat film
10 μ 10 μ
NPP neat film TMPP neat film

Figure C.1 Optical micrograph images of neat NPP showing grainy surface and
TMPP showing smooth surface.

335
NPP: Irppy 9% NPP: PQIr 9%
TMPP: Irppy 9%
TMPP: PQIr 9%

Figure C.2 Optical micrograph images of Irppy and PQIr doped NPP films showing
smooth surfaces and TMPP films showing smooth surfaces.

Raw data for tBuTMPP, ChBP, and TMPP devices made with Irppy and PQIr
phosphors are presented. Each set of four plots show quantum efficiency (%QE) vs
current density (mA/cm
2
), brightness (Cd/m
2
) vs voltage (V), current density
(mA/cm
2
) vs voltage (V), and Intensity (a.u.) vs wavelength (nm). For each
material, energy diagram for only type 2 devices are shown.

336
1E-3 0.01 0.1 1 10 100 1000
1E-3
0.01
0.1
1
10
100
1000

6.44
tBuTMPP
5.4
6.1
CBP
5.7
+
NPD
-
1.52
1.96
1.56
2.40
AlQ3
PQIr
5.03
2.19
6.44
tBuTMPP
5.4
6.1
CBP
5.7
+
NPD
-
1.52
1.96
1.56
2.40
AlQ3
PQIr
5.03
2.19

Figure C.3 (Top) Data for the tBuTMPP based type 1 and 2 devices made with the
Irppy phosphor showing quantum efficiency (%QE) vs current density (mA/cm
2
),
brightness (Cd/m
2
) vs. voltage (V), current density (mA/cm
2
) vs. voltage (V), and
Intensity (a.u.) vs. wavelength (nm) plots. (Bottom) Energy diagram for the type 2
device.
Quantum Efficiency, %
Current Density, mA/cm
2
CBP/Irppy: Control
tBuTMPP/Irppy: Type 1
tBuTMPP/Irppy: Type 2
0.8
6.0
13.2
02 4 6 8 10 12
1E-3
0.01
0.1
1
10
100
1000
10000
100000
CBP/Irppy: Control
tBuTMPP/Irppy: Type 1
Brightness, Cd/m
2
Voltage, V
tBuTMPP/Irppy: Type 2
0.1 1 10
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
1000
Current Density, mA/cm
2
Voltage, V
CBP/Irppy: Control
tBuTMPP/Irppy: Type 1
tBuTMPP/Irppy: Type 2
400 450 500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0

Intensity (A.U.)
Wavelength (nm)
CBP/Irppy: Control
tBuTMPP/Irppy: Type 1
tBuTMPP/Irppy: Type 2
337
6.44
tBuTMPP
5.4
6.1
CBP
5.7
+
NPD
-
1.52
1.96
2.0
2.40
AlQ3
PQIr
5.20
2.19
6.44
tBuTMPP
5.4
6.1
CBP
5.7
+
NPD
-
1.52
1.96
2.0
2.40
AlQ3
PQIr
5.20
2.19
1 10 100
1
10

Figure C.4 (Top) Data for the tBuTMPP based type 1 and 2 devices made with the
PQIr phosphor showing quantum efficiency (%QE) vs current density (mA/cm
2
),
brightness (Cd/m
2
) vs. voltage (V), current density (mA/cm
2
) vs. voltage (V), and
Intensity (a.u.) vs. wavelength (nm) plots. (Bottom) Energy diagram for the type 2
device.

8.2
Quantum Efficiency, %
Current Density, mA/cm
2
CBP/PQIr: Control
tBuTMPP/PQIr: Type 1
tBuTMPP/PQIr: Type 2
2.2
5.0
0.1 1 10
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
1000
Current Density, m A/cm
2
Voltage, V
CBP/PQIr: Control
tBuTMPP/PQIr: Type 1
tBuTMPP/PQIr: Type 2
02 46 8 10 12
1E-3
0.01
0.1
1
10
100
1000
10000
100000
CBP/PQIr: Control
tBuTMPP/PQIr: Type 1
Brightness, Cd/m
2
Voltage, V
tBuTMPP/PQIr: Type 2
500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0

Intensity (A.U.)
Wavelength (nm)
CBP/PQIr: Control
tBuTMPP/PQIr: Type 1
tBuTMPP/PQIr: Type 2
338
6.46
ChBP
5.4
6.1
CBP
5.7
+
NPD
-
1.52
1.96
2.0
2.42
AlQ3
PQIr
5.20
2.19
6.46
ChBP
5.4
6.1
CBP
5.7
+
NPD
-
1.52
1.96
2.0
2.42
AlQ3
PQIr
5.20
2.19
400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0

Figure C.5 (Top) Data for the ChBP based type 1 and 2 devices made with the PQIr
phosphor showing quantum efficiency (%QE) vs. current density (mA/cm
2
),
brightness (Cd/m
2
) vs. voltage (V), current density (mA/cm
2
) vs. voltage (V), and
Intensity (a.u.) vs. wavelength (nm) plots. (Bottom) Energy diagram for the type 2
device.

Intensity (A.U.)
Wavelength (nm)
CBP/PQIr: Control
ChBP/PQIr: Type 1
ChBP/PQIr: Type 2
1 10 100 1000
0.1
1
10
Quantum Efficiency, %
Current Density, mA/cm
2
CBP/PQIr: Control
ChBP/PQIr: Type 1
ChBP/PQIr: Type 2
02 4 6 8 10 12
0.01
0.1
1
10
100
1000
10000
CBP/PQIr: Control
ChBP/PQIr: Type 1
Brightness, Cd/m
2
Voltage, V
ChBP/PQIr: Type 2
0.1 1 10
1E-4
1E-3
0.01
0.1
1
10
100
1000
Current Density, mA/cm
2
Voltage, V
CBP/PQIr: Control
ChBP/PQIr: Type 1
ChBP/PQIr: Type 2
339
6.91
TMPP
5.4
6.1
CBP
5.7
+
NPD
-
1.52
1.96
2.0
2.86
AlQ3
PQIr
5.20
2.19
6.91
TMPP
5.4
6.1
CBP
5.7
+
NPD
-
1.52
1.96
2.0
2.86
AlQ3
PQIr
5.20
2.19

400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2

Figure C.6 (Top) Data for the TMPP based type 1 and 2 devices made with the
PQIr phosphor showing quantum efficiency (%QE) vs. current density (mA/cm
2
),
brightness (Cd/m
2
) vs voltage (V), current density (mA/cm
2
) vs. voltage (V), and
Intensity (a.u.) vs. wavelength (nm) plots. (Bottom) Energy diagram for the type 2
device.
Normalized intensity
Wavelength, λ (nm)
CBP/PQIr: Control
TMPP/PQIr: Type 1
TMPP/PQIr: Type 2
1 10 100 1000
0.1
1
10
Quantum Efficiency, %
Current Density, mA/cm
2
CBP/PQIr: Control
TMPP/PQIr: Type 1
TMPP/PQIr: Type 2
02468 10 12
0.01
0.1
1
10
100
1000
10000
100000
NPD/TMPP:PQIr/TMPP/AlQ3
NPD/CBP:PQIr/BCP/AlQ3
NPD/CBP:PQIr/TMPP/AlQ3
Brightness, Cd/m
2
Voltage, V
NPD/TMPP:PQIr/BCP/AlQ3
0.1 1 10
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
1000
Current Density, mA/cm
2
Voltage, V
NPD/TMPP:PQIr/TMPP/AlQ3
NPD/CBP:PQIr/BCP/AlQ3
NPD/CBP:PQIr/TMPP/AlQ3
NPD/TMPP:PQIr/BCP/AlQ3
340
Appendix D
Raw data of organic PV cells made with NPP, tBuTMPP, ChBP, and BCP
EBLs are presented. Each set of plots show current density-voltage (J-V)
characteristic in the dark and under 20%, 40%, 60%, 80%, and 100% intensities of
simulated AM 1.5G solar illumination, the responsivity of the cell (J
SC
/P
0
vs P
0
)
under 1 sun, the solar conversion efficiency ηP%, open circuit voltage V
OC
, and fill
factor FF under 1 sun. For each material, summary of all the raw data is also
presented in a tabulated form.

341
Table D.1 Summary of OPV cell data for the NPP EBL device
P
0
(mw/cm
2
)J
SC
(A) J
SC
/P
0
(A/W) V
OC
(V) P
max
(mw/cm
2
)FF η
P
(%)
28.2 0.00848 3.01E-04 0.308 3.86E-04 0.1477 1.37E-03
60.6 0.00731 1.21E-04 0.307 3.73E-04 0.1664 6.16E-04
95.3 0.00596 6.26E-05 0.3 3.08E-04 0.172 3.23E-04
115 0.0047 4.10E-05 0.285 2.54E-04 0.1895 2.21E-04
146 1.91E-04 1.30E-06 0.169 9.93E-06 0.3086 6.80E-06
NPP

Figure D.1 (a) Current density-voltage (J-V) characteristic in the dark and under
20%, 40%, 60%, 80%, and 100% intensities of simulated AM 1.5G solar
illumination of OPV cell with NPP EBL. (b) The responsivity of the cell (J
SC
/P
0
vs
P
0
) under 1 sun. (c) The solar conversion efficiency ηP%, open circuit voltage V
OC
,
and fill factor FF under 1 sun.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
0.01
0.02
0.03
0.04
0.05
Voltage (V)

Current density (mA/cm
2
)
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
10n
100n
1µ
10µ
100µ
1m

Current density (A/cm
2
)
Voltage (V)
Dark
Dark 20%
20% 40%
40% 60%
80% 60%
100% 80%
100%
10
0.00000
0.00005
0.00010
0.00015
0.00020
0.00025
0.00030
0.00035

J
SC
/ P
O
(A / W)
P
O
(mW/cm
2
)
10
-0.0002
0.0000
0.0002
0004
0.0006
0.0008
0.0010
0.0012
0.0014
P
O
(mW/cm
2
)

0.12
0.15
0.18
0.21
0.24
0.27
0.30
0.33
V
OC
(V), FF
η
P
(%)
0.
342
Table D.2 Summary of OPV cell data for the tBuTMPP EBL device
P
0
(mw/cm
2
)J
SC
(A) J
SC
/P
0
(A/W) V
OC
(V) P
max
(mw/cm
2
)FF η
P
(%)
29.4 0.003 1.18E-04 0.353 3.28E-04 0.267 0.001
61.1 0.003 4.90E-05 0.378 2.89E-04 0.255 4.73E-04
95.7 0.003 2.62E-05 0.398 2.68E-04 0.269 2.80E-04
115 0.002 1.78E-05 0.408 2.23E-04 0.267 1.94E-04
146 0.003 1.87E-05 0.421 2.98E-04 0.259 2.03E-04
tBuTMPP

Figure D.2 (a) Current density-voltage (J-V) characteristic in the dark and under
20%, 40%, 60%, 80%, and 100% intensities of simulated AM 1.5G solar
illumination of OPV cell with tBuTMPP EBL. (b) The responsivity of the cell
(J
SC
/P
0
vs P
0
) under 1 sun. (c) The solar conversion efficiency ηP%, open circuit
voltage V
OC
, and fill factor FF under 1 sun.

-0.5 0.0 0.5 1.0 1.5
-0.030
-0.025
-0.020
-0.015
-0.010
-0.005
0.000
0.005
Voltage (V)

Current density (mA/cm
2
)
-0.5 0.0 0.5 1.0 1.5
1n
10n
100n
1µ
10µ

Current density (A/cm
2
)
Voltage (V)
Dark
20%
40%
60%
Dark
80%
20%
100%
40%
60%
80%
100%
10
0.00000
0.00005
0.00010
0.00015

J
SC
/ P
O
(A / W)
P
O
(mW/cm
2
)
10
0.0002
0.0004
0006
0.0008
0.0010
0.0012
P
O
(mW/cm
2
)

0.24
0.27
0.30
0.33
0.36
0.39
0.42
V
OC
(V), FF
0.
η
P
(%)
343
Table D.3 Summary of OPV cell data for the ChBP EBL device
P
0
(mw/cm
2
)J
SC
(A) J
SC
/P
0
(A/W) V
OC
(V) P
max
(mw/cm
2
)FF η
P
(%)
25.8 0.629 0.0244 0.384 0.0937 0.3881 0.363
58.1 1.04 0.0179 0.407 0.1652 0.3914 0.284
95.7 1.61 0.0168 0.423 0.2701 0.3971 0.282
115 1.97 0.0172 0.431 0.3415 0.4019 0.297
147 2.49 0.0169 0.44 0.4391 0.4011 0.298
ChBP

Figure D.3 (a) Current density-voltage (J-V) characteristic in the dark and under
20%, 40%, 60%, 80%, and 100% intensities of simulated AM 1.5G solar
illumination of OPV cell with ChBP EBL. (b) The responsivity of the cell (J
SC
/P
0
vs
P
0
) under 1 sun. (c) The solar conversion efficiency ηP%, open circuit voltage V
OC
,
and fill factor FF under 1 sun.

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-4
-3
-2
-1
0
1
2
3
4
Voltage (V)

Current density (mA/cm
2
)
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
100n
1µ
10µ
100µ
1m
10m
100m

Current density (A/cm
2
)
Voltage (V)
Dark
20%
40%
60%
80%
100%
Dark
20%
40%
60%
80%
100%
10
0.0150
0.0175
0.0200
0.0225
0.0250

J
SC
/ P
O
(A / W)
P
O
(mW/cm
2
)
10
0.28
0.30
32
0.34
0.36
0.38
P
O
(mW/cm
2
)

0.39
0.42
0.45
V
OC
(V), FF
0.
η
P
(%)
344
Table D.4 Summary of OPV cell data for the BCP EBL device
P
0
(mw/cm
2
)J
SC
(A) J
SC
/P
0
(A/W) V
OC
(V) P
max
(mw/cm
2
)FF η
P
(%)
28.2 1.12 0.0397 0.358 0.2419 0.6042 0.858
60.6 1.88 0.0311 0.382 0.4282 0.5947 0.707
95.3 2.95 0.031 0.398 0.7222 0.6158 0.758
115 3.64 0.0318 0.405 0.8948 0.6079 0.781
146 4.6 0.0315 0.414 1.15 0.6037 0.787
BCP

igure D.4 (a) Current density-voltage (J-V) characteristic in the dark and under F
20%, 40%, 60%, 80%, and 100% intensities of simulated AM 1.5G solar
illumination of OPV cell with BCP EBL. (b) The responsivity of the cell (J
SC
/P
0
vs
P
0
) under 1 sun. (c) The solar conversion efficiency ηP%, open circuit voltage V
OC
,
and fill factor FF under 1 sun.

-0.50 -0.25 0.00 0.25 0.50
-5
-4
-3
-2
-1
0
1
2
3
4
5
Voltage (V)

Current density (mA/cm
2
)
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
100n
1µ
10µ
100µ
1m
10m
100m
1

Current density (A/cm
2
)
Voltage (V)
Dark
20%
40%
60%
80%
100%
Dark
20%
40%
60%
80%
100%
10
0.030
0.032
0.034
0.036
0.038
0.040

J
SC
/ P
O
(A / W)
P
O
(mW/cm
2
)
10
0.70
75
0.80
0.85
0.
P
O
(mW/cm
2
)

%)
0.36
0.40
0.44
0.48
0.52
0.56
0.60
0.64
V
OC
(V), FF
η
P
(
345
Thickness Studied of the ChBP EBL

Table D.5 Summary of OPV cell data for the ChBP EBL device with 50Å
thicknesses

P
0
(mw/cm
2
)J
SC
(A) J
SC
/P
0
(A/W) V
OC
(V) P
max
(mw/cm
2
)FF η
P
(%)
31.5 0.384 0.0122 0.12 0.01219 0.2655 0.0387
63.8 0.579 0.00907 0.165 0.02504 0.2628 0.0393
101 0.826 0.00817 0.213 0.04808 0.2726 0.0475
119 0.933 0.00785 0.233 0.06009 0.2757 0.0505
150 1.11 0.00739 0.263 0.08336 0.286 0.0556
ChBP 50Å

Figure D.5 (a) Current density-voltage (J-V) characteristic in the dark and under
20%, 40%, 60%, 80%, and 100% intensities of simulated AM 1.5G solar
illumination of OPV cell with 50Å ChBP EBL. (b) The responsivity of the cell
(J
SC
/P
0
vs P
0
) under 1 sun. (c) The solar conversion efficiency ηP%, open circuit
voltage V
OC
, and fill factor FF under 1 sun.

-0.50 -0.25 0.00 0.25 0.50
-4
-3
-2
-1
0
1
2
3
4
Voltage (V)

Current density (mA/cm
2
)
-0.4 -0.2 0.00.2 0.40.6 0.81.0
10µ
100µ
1m
10m
100m
1

Current density (A/cm
2
)
Voltage (V)
Dark
20%
40%
60%
80%
100%
Dark
20%
40%
60%
80%
100%
10
0.007
0.008
0.009
0.010
0.011
0.012
0.013

J
SC
/ P
O
(A / W)
P
O
(mW/cm
2
)
10
0.04
0.05
0.06
P
O
(mW/cm
2
)

0.12
0.15
0.18
0.21
0.24
0.27
0.30
V
OC
(V), FF
η
P
(%)
346
Table D.6 Summary of OPV cell data for the ChBP EBL device with 100Å
thicknesses

P
0
(mw/cm
2
)J
SC
(A) J
SC
/P
0
(A/W) V
OC
(V) P
max
(mw/cm
2
)FF η
P
(%)
31.5 0.528 0.0168 0.235 0.0353 0.2845 0.112
63.8 0.872 0.0137 0.288 0.07492 0.2984 0.117
101 1.33 0.0132 0.328 0.1371 0.3136 0.136
119 1.63 0.0137 0.346 0.1784 0.3159 0.15
150 2.05 0.0136 0.365 0.2375 0.3181 0.158
ChBP 100Å

Figure D.6 (a) Current density-voltage (J-V) characteristic in the dark and under
20%, 40%, 60%, 80%, and 100% intensities of simulated AM 1.5G solar
illumination of OPV cell with 100Å ChBP EBL. (b) The responsivity of the cell
(J
SC
/P
0
vs P
0
) under 1 sun. (c) The solar conversion efficiency ηP%, open circuit
voltage V
OC
, and fill factor FF under 1 sun.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
10µ
100µ
1m
10m

Current density (A/cm
2
)
Voltage (V)
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-4
-3
-2
-1
0
1
2
3
4
Dark
20%
40%
60%
80%
100%
10
0.013
0.014
0.015
0.016
0.017

J
SC
/ P
O
(A / W)
P
O
(mW/cm
2
)
Voltage (V)

Current density (A/cm
2
)
Dark
20%
40%
60%
80%
100%
10
0.11
0.12
13
0.14
0.15
0.16
P
O
(mW/cm
2
)

0.21
0.24
0.27
0.30
0.33
0.36
0.39
V
OC
(V), FF
0.
η
P
(%)
347
Table D.7 Summary of OPV cell data for the ChBP EBL device with 150Å
thicknesses

P
0
J
SC
J
SC
/P
0
V
OC
P
max
FF η
P
(%)
P
0
(mw/cm
2
)J
SC
(A) J
SC
/P
0
(A/W) V
OC
(V) P
max
(mw/cm
2
)FF η
P
(%)
63.8 1.66E-04 2.60E-06 0.225 1.18E-05 0.315 1.85E-05
101 2.16E-04 2.13E-06 0.209 1.32E-05 0.2926 1.30E-05
119 2.21E-04 1.86E-06 0.273 1.76E-05 0.2914 1.48E-05
150 2.26E-04 1.51E-06 0.206 1.29E-05 0.2768 8.60E-06
ChBP 150Å

Figure D.7 (a) Current density-voltage (J-V) characteristic in the dark and under
20%, 40%, 60%, 80%, and 100% intensities of simulated AM 1.5G solar
illumination of OPV cell with 150Å ChBP EBL. (b) The responsivity of the cell
(J
SC
/P
0
vs P
0
) under 1 sun. (c) The solar conversion efficiency ηP%, open circuit
voltage V
OC
, and fill factor FF under 1 sun.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-0.002
-0.001
0.000
0.001
0.002
Voltage (V)

Current density (mA/cm
2
)
-0.5 0.0 0.5 1.0 1.5
1n
10n
100n
1µ
10µ
100µ

Current density (A/cm
2
)
Voltage (V)
Dark
20%
40%
60%
80%
100%
Dark
20%
40%
60%
80%
100%
10
1.5x10
-6
2.0x10
-6
2.5x10
-6
3.0x10
-6
3.5x10
-6
4.0x10
-6
4.5x10
-6
5.0x10
-6

J
SC
/ P
O
(A / W)
P
O
(mW/cm
2
)
10
5.0x10
-6
1.0x10
-5
5x10
-5
2.0x10
-5
2.5x10
-5
3.0x10
-5
P
O
(mW/cm
2
)

0.18
0.20
0.22
0.24
0.26
0.28
0.30
0.32
V
OC
(V), FF
1.
η
P
(%)
348

Abstract (if available)

Abstract The work presented in this thesis has two distinct objectives. The first objective is to design, synthesize, and study new materials for OLEDs and OPV cells and in the process make more efficient devices. The second objective is to study the extrinsic degradation mechanism of OLED and find a way to stop or slow down the degradation, so in the future more air stable devices can be produced.

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University of Southern California Dissertations and Theses

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Asset Metadata

Creator Hassan, Azad M. (author)

Core Title Theoretical, experimental, device fabrication, and degradation studies of materials for optoelectronic devices

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 10/10/2007

Defense Date 07/31/2007

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag computational studies,dark spots,device,DFT,excited state,OAI-PMH Harvest,OLEDs,optical spectroscopy,OPV cells,photophysics,solar cells,TDDFT

Language English

Advisor Thompson, Mark E. (committee chair), Bau, Robert (committee member), Mansfeld, Florian B. (committee member)

Creator Email mhassan@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m859

Unique identifier UC1170709

Identifier etd-Hassan-20071010 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-585378 (legacy record id),usctheses-m859 (legacy record id)

Legacy Identifier etd-Hassan-20071010.pdf

Dmrecord 585378

Document Type Dissertation

Rights Hassan, Azad M.

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Repository Name Libraries, University of Southern California

Repository Location Los Angeles, California

Repository Email cisadmin@lib.usc.edu