University of Southern California Dissertations and Theses

Organic photovoltaics: from materials development to device application

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Organic photovoltaics: from materials development to device application

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Content ORGANIC PHOTOVOLTAICS: FROM MATERIALS DEVELOPMENT TO DEVICE
APPLICATION

by
Cong Trinh

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

August 2013

Copyright 2013 Cong Trinh
ii

DEDICATION

Dedicated to My Family:
My Parents (Mr. Duc V. Trinh and Ms. Dinh T. Lai) and Sister (Mrs. Anh M. Trinh)

iii

ACKNOWLEDGEMENTS
I would like to acknowledge my PhD research advisor Prof. Mark E. Thompson
for his mentorship, assistance and guidance throughout my time at graduate school. I
really appreciate the way he mentored me to be an independent scientist and the freedom
he provided me in different research projects. Beside Prof. Thompson, I would like to
thank Prof. Peter I. Djurovich for his enormous help and very helpful discussions in
research. I would like to thank all my qualifying exam and dissertation committee
members: Prof. Dan Dapkus, Prof. Richard Brutchey, Prof. Hanna Reisler and Prof Barry
Thompson for their comments, support and encouragement during the many steps toward
earning my degree.
I am thankful to my family, especially my parents Mr. Duc V. Trinh, Mrs. Dinh
T. Lai and my sister Mrs. Anh M. Trinh for their spiritual support and encouragement
during ten years that I have been living abroad. Kendy, I am so thankful that you spent
five years with me, raising me up every time I failed in research or felt tired with science.
Quang, Phuong, Ngoc, Trung, Huong, Nguyen, Jaden and co Le: I really appreciate the
friendship and the holiday time that I have with you guys, I would be so lonely during the
Christmas, Thanksgiving and New Year without you.
I would like to express my appreciation to Drs. Slava Diev, Cody Schlenker, Kent
Hanson, Rui Zhang, Zhiwei Liu for their help during the first two years when I started my
research at USC. I would like to thank my collaborators at USC: Prof. Stephen Bradforth,
Saptaparna Das for their help in transient absorption measurements; Andrew Bartynski
iv

and Francisco Navarro for their effort in continuing my research projects. My research
would not be successful without the collaboration from Stanford: Prof. Michael Toney,
Dr. Christopher Tassone, Prof. Michael McGehee, Dr. George Burkhard, Prof. Stacey
Bent, Dr. Jonathan Bakke and Thomas Brennan; from Caltech: Prof. Harry Gray and Maraia
Ener; from Michigan: Prof. Steve Forrest, Dr. Jeramy Zimmerman and Dr. Brian
Lassiter.
Last but not least, I would like to thank Judy Hom and Michele Dea for their help
in organization and administration. I also acknowledge my friends at USC, all present
and past MET group members with whom I have had the pleasure to work during 5 years.

v

TABLE OF CONTENTS
Dedication .......................................................................................................................... ii
Acknowledgement ............................................................................................................ iii
List of tables...................................................................................................................... ix
List of figures ......................................................................................................................x
Abstract ........................................................................................................................... xix
Chapter 1. Introduction to organic photovoltaics ...............................................................1
1.1. The need for renewable energy sources and solar energy .........................................1
1.1.1. The concerns over fossil fuels consumption .......................................................1
1.1.2. Renewable energy sources and solar energy .......................................................4
1.1.3. Current PV technologies and their limitations .....................................................6
1.2. Introduction to organic phototoltaics .......................................................................10
1.2.1. Device structures and constituent materials in organic photovoltaics ...............10
1.2.2. General working principles of organic photovoltaics devices ...........................15
1.3. Fundamental processes in OPV ...............................................................................18
1.3.1. Exciton formation ..............................................................................................18
1.3.2. Exciton diffusion ................................................................................................20
1.3.2.1. Energy transfer mechanisms ........................................................................20
1.3.2.2. Exciton diffusion length ...............................................................................23
1.3.3. Charge generation at the donor/acceptor interface ............................................24
1.3.3.1. Thermodynamics of charge generation ........................................................25
1.3.3.2. Kinetics of charge generation ......................................................................26
1.3.3.2.1. Marcus theory ........................................................................................26
1.3.3.2.2. Onsager theory .......................................................................................28
vi

1.3.3.3. General picture fore charge separation process ...........................................31
1.3.4. Charge transport in organic materials ................................................................35
1.3.5. Charge collection ...............................................................................................35
1.4. Characterization of OPV devices .............................................................................36
1.4.1. Electrical and optical characterization of OPV devices .....................................36
1.4.2. Generalized Shockley equation for OPV ...........................................................37
1.4.3. Ideal diode equation for organic heterojunctions ...............................................40
1.5. Characteristics of OPV devices ...............................................................................42
1.5.1. Open voltage circuit ...........................................................................................42
1.5.1.1. Energy offsets between donor’s HOMO and acceptor’s LUMO .................45
1.5.1.2. Analysis based on electrical measurements .................................................46
1.5.2. Short circuit current ...........................................................................................47
1.5.3. Fill factor ............................................................................................................48
1.6. Summary of topics ...................................................................................................49
Chapter 1 Endnote ...........................................................................................................51
Chapter 2. Instrumentation and device fabrication ..........................................................57
2.1. Instrumentation ........................................................................................................57
2.2. Device fabrication ....................................................................................................61
2.2.1. Substrate cleaning ..............................................................................................61
2.2.2. Vacuum vapor deposition ..................................................................................61
2.2.3. Device stesting ...................................................................................................61
Chapter 2 Endnote ...........................................................................................................63
Chapter 3. Chemical annealing and application to organic photovoltaics .......................64
3.1. Introduction ..............................................................................................................64
vii

3.2. Optical properties .....................................................................................................66
3.3. Thin film composition..............................................................................................68
3.4. Thin film structure and morphology ........................................................................69
3.5. Effect of chemical annealing on OPV devices ........................................................73
3.6. Conclusion and outlook ...........................................................................................82
Chapter 3 Endnote ..........................................................................................................84
Chapter 4. Symmetry breaking charge transfer and application to OPV .........................87
4.1. Introduction ..............................................................................................................87
4.2. Synthesis and characterization .................................................................................90
4.3. Crystal structure .......................................................................................................98
4.4. Electronic structure ..................................................................................................99
4.5. Photophyscal properties in defferent solvents .......................................................101
4.6. Electrochemistry ....................................................................................................110
4.7. Transient absorption spectroscopy study ...............................................................112
4.8. Application to OPV ................................................................................................120
4.9. Conclusion .............................................................................................................122
Chapter 4 Endnote .........................................................................................................124
Chapter 5. Energy sensitization of C
60
and application to OPV ....................................127
5.1. Introduction ............................................................................................................127
5.2. Molecular design for sensitizers ............................................................................130
5.3. Synthesis and characterization of sensitizers .........................................................133
5.3.1. Zinc dipyrrins ...................................................................................................133
5.3.1.1 Synthesis of zinc chlorodipyrrin (ZCl) .......................................................134
5.3.1.2. Characterization .........................................................................................137
5.3.2. Iridium dipyrrin complexes ..............................................................................140
viii

5.3.2.1 Synthesis .....................................................................................................141
5.3.2.2. Characterization .........................................................................................143
5.3.3. Subphthalocyanines .........................................................................................146
5.4. Photosensitization study.........................................................................................148
5.4.1. Stern-Volmmer quenching experiements ........................................................148
5.4.2. Photoluminescence study on thin films ...........................................................148
5.5. Morphology of the blended films ..........................................................................152
5.6. Effect of blending on C
60
absorption .....................................................................154
5.7. Application to OPV devices ...................................................................................156
5.7.1. Screening sensitizers ........................................................................................157
5.7.2. Optimization of ZCl sensitized devices ...........................................................160
5.7.3. Sensitization using multiple sensitizers ...........................................................162
5.7.4. Application to red-NIR absorbing donor materials .........................................165
5.8. Conclusion .............................................................................................................167
Chapter 5 Endnote ........................................................................................................169
Chapter 6. Inverted organic photovoltaics with lamellar structure ................................172
6.1. Introduction ............................................................................................................172
6.2. Fabrication and optimization of inverted OPVs ....................................................173
6.3. Comparison of the conventional vs. inverted OPVs ..............................................176
6.4. Origin of photocurrent loss ....................................................................................177
6.5. Origin of photovoltage loss ....................................................................................181
6.6. Conclusion .............................................................................................................182
Chapter 6 Endnote .........................................................................................................184
Bibliography ...................................................................................................................187

ix

LIST OF TABLES
Table 4.1. Dihedral angles (degrees) between planes in zinc dipyrrins Z1-6 ...................99
Table 4.2. Photophysical properties of homoleptic zinc dipyrrin complexes in
different solvents at room temperature ............................................................................105
Table 4.3. Photophysical properties of heteroleptic zinc dipyrrin complexes in
different solvents at room temperature ............................................................................106
Table 4.3. Electrochemical and optical properties of homoleptic complexes.
Electrochemical values were determined by differential pulsed voltammetry (DPV)
vs. Fc
+
/Fc; optical gap E
00
is defined by the midpoint between absorption and
emission spectra in THF; triplet energies were measured in 2-methyl
tetrahydrofuran at 77K .....................................................................................................111
Table 4.4. Kinetic rates of different processes of Z1-3 in different solvents
determined by femtosecond transient absorption measurements .....................................118
Table 4.5. Device performance characteristics under AM1.5G 1 sun illumination.
General device structure is ITO/MoO
3
(8 nm)/donor (10 nm)/C
60
(40 nm)/BCP
(10 nm)/ Al, where D1 (Z1 = donor) and D2 (Z2 = donor) .............................................122
Table 5.1. Photophysical and electrochemical properties of sensitizers (Z1, ZCl,
IrDP, CTrinh, Cl
6
SubPc, F
12
SubPc) and C
60
in solution and thin films .........................132
Table 5.2. Characteristics of co-sensitized devices (D21 and D22) under 1sun
AM1.5G illumination ......................................................................................................164
Table 6.1. Characteristics of conventional and inverted solar cells ...............................177

x

LIST OF FIGURES
Figure 1.1. Concentration of CO
2
based on analysis of atmospheric sample
contained in ice cores and more recent direct measurements. (source:
http://www.ncdc.noaa.gov/; graph is from http://climate.nasa.gov ) ..................................2
Figure 1.2. The total energy sources (republished from IEA 2008b) (upper), and the
photovoltaic solar resources of the United States (from NREL) (lower) ...........................3
Figure 1.3. Global cumulative PV capacities by 2010 ......................................................5
Figure 1.4. Best research cell efficiencies certified by NREL. (from
http://www.nrel.gov) ...........................................................................................................7
Figure 1.5. The cost of PV module. Cost of solar cells is decreasing faster than
predicted. The figure shows the trend of fabrication cost for PV modules ........................9
Figure 1.6. (a) General OPV device structure, and different architectures of the
active layer (b) planar (lamellar) heterojunction (PHJ), (c) Bulk Heterojunction
(BHJ) .................................................................................................................................11
Figure 1.7. Some common organic materials used in OPVs 15
Figure 1.8. Charge photogeneration processes in OPV devices, illustrated using (a)
planar heterojunction of donor and acceptor layers and (b) energy diagrams ..................16
Figure 1.9. The absorption coefficient of organic (C
60
, SubPc and CuPc) and
inorganic (c-Si and GaAs) materials. Solar photon flux derived from ASTM
G173-03 AM 1.5G spectral irradiance ..............................................................................18
Figure 1.10. Schematic presentations of the ground state S
0
, the singlet and triplet
excited states S
1
and T1 in terms of (a) molecular orbital configurations and (b)
state energy diagram, the solid and dotted lines represent the radiative and
nonradiative decay pathways of the excited states ...........................................................19
Figure 1.11. Mechanisms of energy transfer: (a) Forster or dipole – dipole
interaction mechanism, the blue doubled-headed arrows represent electric dipole
generated by the excited state D*; (b) Dexter or electron exchange mechanism. The
spin of the electrons exchanged must obey the spin conservation rules ...........................21
Figure 1.12. Potential energy surface for a donor/acceptor system in OPVs, where
photoexcitation generates
1
D*/A and subsequent electron transfer generates [D
+
A
-
].
xi

ΔG° is the energy difference between the two surface minima; λ is the
reorganization energy ........................................................................................................27
Figure 1.13. Potential energy diagram summarizing Onsager theory. The bold
curves illustrates the potential energy resulting from Coulomb attraction as a
function of electron – hole separation (e - h). Photoexcitation results in generation
of a hot, mobile electron. This electron subsequently relaxes to a particular distance
from the hole, a. If a < r
c
, then the CT state can either undergo germinate
recombination or dissociate into free charges ...................................................................30
Figure 1.14. Electronic state diagram describing the photo-induced charge
generation mechanism in OPVs: S
0
, S
1
and T
1
denote the ground state, singlet and
triplet excited states of the donor, respectively. At the DA interface, electron
transfer from excited state of the donor to acceptor forms CT state: CT
1
is the
lowest (or relaxed) CT state, CT* represents the “hot” CT state. k
IC
and k
RC
are the
relaxation and geminate recombination rate of the CT sates. The final state is the
charge separated (CS) state, where the hole and electron locates separately in the
donor and acceptor layers, respectively 32
Figure 1.15. Typical J – V curves of a solar cell in the dark (dotted traces) and
under light (solid traces) in the log scale (up) and linear scale (down) ............................ 37
Figure 1.16. Single diode equivalent circuit model for modeling solar cells .................38
Figure 1.17. (a) Energy-level diagram of OPV devices. Current is unipolar in the
donor (J
p
) and acceptor (J
n
) layers. (b) Processes occurring within the HJ region ..........40
Figure 3.1. Scheme illustrating the chemical annealing process ....................................64
Figure 3.2. Digital images of the ZnTPP film (1300 Å) treated with pyz over time:
(a) ZnTPP; (b) ZnTPP-pz, 1 min; (c) ZnTPP-pz, 2 min; (d) ZnTPP-pz, 5 min; (e)
ZnTPP-pz, 8 min; (f) ZnTPP-pz, 10 min ..........................................................................66
Figure 3.3. In situ UV-vis spectra of ZnTPP film during chemical annealing
process with different ligands and solvent: (a) pyridine (py), (b) pyrazine (pz), (c)
triazine (tz) and (d) dichloromethane. Insets are Q-band regions ....................................67
Figure 3.4. AFM images of 10 nm ZnTPP film on glass substrates: a) as deposited
(rms = 0.6 nm), and after 3 min treatment with b) pz (rms = 6.7 nm) and c) tz
(rms = 2.1 nm). d) shows the AFM image of a bilayer film composed of
xii

glass/10nm NPD/ 10 nm ZnTPP after annealing with pz for 3 min minutes.
(rms = 4.7 nm) ..................................................................................................................69
Figure 3.5. XRD patterns of 100 nm ZnTPP films (a) before (blue) and after (red)
annealing with pz, with simulated powder pattern shown as black bars (from single
crystal structure of ZnTPP•2pz
2
), and (b) after annealing with tz (blue), with the
simulated powder pattern for ZnTPP•tz shown as black bars. Also shown are the
Packing diagrams of single crystals of (c) ZnTPP•2pz,
2
and (d) ZnTPP•tz, with the
(1,-1,3) and (2,0,2) planes, respectively, shown in green. The H atoms and phenyl
rings are omitted for clarity ...............................................................................................70
Figure 3.6. 2D-GIXD chemically annealed films, a) 50 nm of ZnTPP•pz and b)
NPD (10 nm)/ZnTPP•pz (50 nm). Integrating a cake slice from 1.26 ≥ q ≥ 1.37 Å
-1

vs. polar angle, χ, shows the angular dependence of the (1,-1,3), star, and
superposition of the (2,-1,2) and (1,-2,2), dagger, diffraction planes ...............................71
Figure 3.7. OPV device structures and performance characteristics under AM1.5G
illumination. ZnTPP·pz corresponds to ZnTPP after chemical annealing with pz.
a
Could not be fitted ..........................................................................................................74
Figure 3.8. Current – Voltage characteristics under the dark (open symbols) and
under AM 1.5 illumination (filled symbols) of devices D1 – D8 .....................................75
Figure 3.9. Cyclic voltammetry diagrams of ZnTPP (E
1/2
ox
= 0.38 V) and ZnTPP:pz
(1:10) (E
1/2
ox
= 0.33 V) in dichloromethane solution under N
2
, vs Fc
+
/Fc. Scan rate
100 mV s
-1
. The signal at 0.0 V is the Fc
+
/Fc reference ...................................................76
Figure 3.10. Characteristics of device ITO/C
60
(40 nm)/BCP (10 nm)/Al under
AM 1.5G illumination. Inset is characteristics of the devices with J
SC
of
± 0.02 mA/cm
2
, V
OC
of ± 0.01 V, FF of ± 0.01 and η of ± 0.01 % .................................77
Figure 3.11. Current – Voltage characteristics under the dark (open symbols) and
under AM 1.5 illumination (filled symbols) chemically annealed with triazine. The
donor structures are: D1 - ZnTPP (15 nm); D9 - ZnTPP·tz (15 nm); D10 - ZnTPP·tz
(15 nm)/ZnTPP (5 nm); D11 - ZnTPP·tz (10 nm)/ZnTPP (10 nm) ..................................82
Figure 4.1. Structure of homoleptic (Z1-Z4) and heteroleptic (Z5-6) zinc dipyrrin
complexes .........................................................................................................................88
Figure 4.2. Synthesis of (a) homoleptic (Z2) and (b) heteroleptic (Z5) ........................... 90
xiii

Figure 4.3.
1
H NMR of Z5 in CDCl
3
over time: Freshly prepared solution (upper)
and solution in NMR tube after 4 hours (lower), estimated molar ratio of
heteroleptic:homoleptic complexes (Z5:Z2 = 15:1) .........................................................96
Figure 4.4.
1
H NMR of Z6 in CDCl
3
over time: Freshly prepared solution (upper)
and solution in NMR tube after 8 hours (lower), estimated molar ratio of complexes
Z6:Z3 = 13:1 .....................................................................................................................97
Figure 4.5. ORTEP diagrams of (a) Z3, (b) Z4, (c) Z6 and (d) Z5 at 50%
probability level. H atoms are omitted for clarity. Planes containing different
groups of atoms are indicated by the colored lines ...........................................................98
Figure 4.6. Theoretical study on zinc dipyrrin complexes carried out at
B3LYP/LACVP** level of theory. LUMOs (mesh) and energies of (a) Z1,
degenerate E states and (b) heteroleptic analog of Z1; HOMOs (transparent) of (c)
Z1 and (d) heteroleptic analog of Z1; Electron density surface of (e) Z1 and (f) Z2.
Mesityl group is omitted for simplicity of calculation, Z1 and Z2 have D
2d

symmetry and heteroleptic analog of Z1 has C
2v
symmetry ...........................................100
Figure 4.7. Absorption spectra of (a) Z1-4 in cyclohexane, (b) Z2 in different
solvents, (c) Z2 and Z5 in cyclohexane and (d) Z3 and Z6 in toluene. Cychex –
cyclohexane, THF – tetrahydrofuran, DCM – dichloromethane and ACN –
acetonitrile .......................................................................................................................102
Figure 4.8. Emission spectra of (a) Z1, (b) Z2, (c) Z3, (d) Z4, (e) Z5 and (f) Z6 in
different solvents at room temperature. Cychex – cyclohexane, THF –
tetrahydrofuran and DCM – dichloromethane ................................................................103
Figure 4.9. Emission spectra of Z1-6 in 2-MeTHF at 77K. Insets are
phosphorescent peaks .......................................................................................................104
Figure 4.10. Emission intensities are plotted vs. absorption intensities of Z2
solution in dichloromethane at various concentrations ...................................................107
Figure 4.11. Quantum yield of (a) homoleptic complexes Z1-Z4 (inset is the QY of
Z1-Z4 plotted vs. full width half max of emission peak in CycHex), and (b)
heteroleptic complexes Z5-Z6 plotted vs. solvent polarity index E
T
(30) for
cyclohexane, toluene, THF, CHCl
3
, DCM, DMF and ACN ............................................108
Figure 4.12. Electrochemical measurements of (a) Z1, (b) Z2, (c) Z3 and (4) Z4 in
dry THF under N
2
with a scan rate of 100 mV/s vs Fc/Fc
+
. Arrows indicate the
direction of the scan ........................................................................................................110
xiv

Figure 4.13. Femtosecond transient absorption of Z1 in (a) cyclohexane; (b)
toluene; (c) dichloromethane, inset is the spectrum of Z1 anion defined by
spectroelectrochemical measurement; and (d) acetonitrile. Samples were excited at
500 nm at 160 µJ/cm
2
for (a), (b), (d), and 70 µJ/cm
2
for (c) .........................................113
Figure 4.14. Femtosecond transient absorption of (a) Z2 in cyclohexane, (b) Z2 in
toluene, (c) Z2 in acetonitrile, (d) Z3 in cyclohexane, (e) Z3 in toluene and (f) Z3 in
acetonitrile .......................................................................................................................115
Figure 4.15. Simplified Jablonski diagram illustrating symmetry breaking charge
transfer process. k
rec
is the total recombination rate to either the triplet state or the
ground state .....................................................................................................................116
Figure 4.16. Comparison of dynamics of CT state (blue) and ground state bleach
(red) between Z1 (closed) and Z2 (open) in acetonitrile ................................................117
Figure 4.17. Nanosecond transient absorption spectra of Z1 in (a) acetonitrile, (b)
different solvents at 0.5 µs, and (c) femtosecond transient absorption spectra of Z1
in DCM:MeI (1:4) ...........................................................................................................120
Figure 4.18. Characteristics of the OPV devices using Z1 and Z2 as donor layers:
J-V characteristics under 1 sun AM 1.5G illumination (left) and EQE measurements
(right) .............................................................................................................................. 121
Figure 5.1. Absorption spectra of C
60
in neat films

(black circles), toluene solution
(blue squares) and AM 1.5G solar photon flux (red) ......................................................128
Figure 5.2. Energy diagram showing sensitization pathways in two cases: (a)
represents a sensitizer (e.g. PDI or DCV3T) having higher singlet energy but lower
triplet energy than C
60
. (b) represents energetic depiction of a sensitizer having both
singlet and triplet energies higher than that C
60
, possible pathways for sensitization:
A: singlet transfer, B: triplet transfer and C: electron transfer to form a charge
transfer (CT) state, which decay to the triplet state of C
60
if the CT state energy is
higher than the C
60
triplet. If the energy of the CT state is lower than C
60
triplet, it
will act as a trap state (CT trap) ......................................................................................130
Figure 5.3. Synthesis of ZCl .........................................................................................133
Figure 5.4. NOESY
1
H NMR of ZCl in CD
3
Cl and peak assignment ..........................136
Figure 5.5. (a) ORTEP diagram of ZCl and (b) space filling models of ZCl (upper)
and C
60
(lower) with the proportional sizes as determined from X-ray single crystal
xv

data. The atom marked in red can be either H or Cl with an occupancy ratio H:Cl =
1:3 ...................................................................................................................................137
Figure 5.6. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
diagrams of ZCl in dichloromethane (vs. Fc
+
/Fc). Scan rate 100 mV/s .........................138
Figure 5.7. Calculated energy levels and molecular orbitals of ZCl
11
and ZCl
12
at
B3LYP/LACVP** level of theory using Titan software ................................................138
Figure 5.8. Absorption (blue) and emission (red) spectra of (a) solution of ZCl in
cyclohexane at room temperature, inset is the phosphorescence of ZCl measured in
2-MeTHF at 77K, excitation = 460 nm and (b) ZCl film at room temperature,
excitation = 460 nm. The thin film absorption spectrum of C
60
is also shown for
comparison ......................................................................................................................139
Figure 5.9. Synthesis scheme of IrDP and CTrinh. Atom marked in red can be Cl
or H with molar ratio (H:Cl = 1:9) ..................................................................................140
Figure 5.10.
1
H NMR of CTrinh and peak assignment. Insets are structures of
three compounds in the mixture ......................................................................................142
Figure 5.11.
19
F NMR of CTrinh mixture and peak assignment ....................................143
Figure 5.12. Absorption and emission spectra of (a) IrDP and (b) CTrinh in
solution. Spectra in toluene were measured at room temperature (RT) under N
2
,
emission spectra in 2-MeTHF were measured at 77K .................................................... 143
Figure 5.13. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
diagrams of CTrinh in dichloromethane (vs. Fc
+
/Fc). Scan rate 100 mV/s ....................144
Figure 5.14. Calculated energy levels and molecular orbitals of CTrinhCl
5
and
CTrinhCl
6
at B3LYP/LACVP** level of theory using Titan software: (a) HOMO of
CTrinhCl
5
, (b) LUMO of CTrinhCl
5
, (c) HOMO of CTrinhCl
6
, (d) LUMO of
CTrinhCl
6
, (e) optimized ground state structure of CTrinhCl
6
and (f) calculated spin
density of CTrinhCl
6
.......................................................................................................145
Figure 5.15. Absorption and emission spectra of CTrinh and IrDP in thin films ......... 146
Figure 5.16. Synthetic route to F
12
SubPc and Cl
6
SubPc ............................................... 147
Figure 5.17. Absorption (filled symbols) and emission (open symbols) spectra of
Cl
6
SubPc (blue triangles) and F
12
SubPc (red squares) ................................................... 147
xvi

Figure 5.18. Stern-Volmer quenching experiments of ZCl (a, b), Cl
6
SubPc (c, d)
and F
12
SubPc (e, f) using C
60
as quencher in toluene ..................................................... 149
Figure 5.19. Absorption (solid symbols) and emission under excitation at 514 nm
(open symbols) of 50 nm C
60
film (black circle) and 59 nm C
60
:ZCl film (blue
triangle, 15% ZCl by volume) and 75 nm C
60
:ZCl film (red square, 35% ZCl by
volume). Also shown is an energy level diagram (eV) for energy transfer from ZCl
to C
60
...............................................................................................................................150
Figure 5.20. GIXD data of neat and blended C
60
films on Si substrates: (a) neat C
60

(50 nm), (b) C
60
:ZCl (56 nm, 10%), (c) C
60
:ZCl (67 nm, 33%), (d) C
60
:ZCl
(100 nm, 50%), (e) neat ZCl (50 nm), (f) integrated diffraction intensity of neat and
doped C
60
films at different concentrations. Blending concentration is in % of ZCl
by volume ........................................................................................................................152
Figure 5.21. AFM images (5x5 µm) of (a) neat C
60
, (b) mixed C
60
:ZCl (50% ZCl
by volume) and (c) neat ZCl films on Si substrates ........................................................153
Figure 5.22. Absorption coefficient of BCP:C
60
blend films calculated from optical
constants determined by variable angle spectroscopic ellipsometry. Inset:
Extinction as a function of C
60
fraction for wavelengths of 360 nm and 450 nm,
corresponding to Frenkel and CT absorption features, respectively. Linear and
power law (y = x
2.7
) fits are shown for the 340 and 450 nm data, receptively ...............155
Figure 5.23. Structure and characteristics of OPV devices using an NPD donor
layer with and without ZCl. (a) plot of external quantum efficiency, inset is the
device structure, and (b) J–V curves of devices under one sun AM1.5G
illumination, inset is characteristics of the devices with J
SC
of ± 0.06 mA/cm
2
, V
OC

of ± 0.01 V, FF of ± 0.02 and η of ± 0.1 % ...................................................................157
Figure 5.24. Structure and characteristics of OPV devices using F
12
SubPc
sensitizer: (a) and (b) and Cl6SubPc sensitizer (c) and (d). (a) and (c) show EQE
curves, insets are the device structures; (b) and (d) present J–V curves one sun
AM1.5G illumination, insets are characteristics of the devices with J
SC
of ±
0.06 mA/cm
2
, V
OC
of ± 0.01 V, FF of ± 0.02 and η of ± 0.1 % ....................................159
Figure 5.25. EQE curves of devices using blended C
60
:BCP in the acceptor layers.
Inset is the device structures with the various concentration of BCP: D1 (neat C
60
),
D8 (33% BCP), D9 (50% BCP) and D10 (67% BCP) ................................................... 160
xvii

Figure 5.26. EQE measurements of sensitized devices using ZCl with varied C
60

layer thicknesses at the D/A interface .............................................................................161
Figure 5.27. (a) Characteristics of sensitized devices with a neat layer of C
60
(x =
15 nm) and varied thicknesses (y nm) of the C
60
:ZCl doped layers; y = 0 represents
the reference device with 40 nm of neat C
60
as the acceptor layer; and (b) plot of
external quantum efficiency of D1 and optimized device using sensitizer (D14),
inset is the device structure .............................................................................................162
Figure 5.28. Structure and characteristics of OPV devices using
subphthalocyanines sensitizers. (a) EQE curves, inset is the device structure, and (b)
J–V curves of devices under one sun AM1.5G illumination, inset is characteristics
of the devices with J
SC
of ± 0.06 mA/cm
2
, V
OC
of ± 0.01 V, FF of ± 0.02 and η of
± 0.1 ................................................................................................................................163
Figure 5.29. (a) Structures of co-sensitized devices and (b) EQE curves of the
devices .............................................................................................................................164
Figure 5.30. Structure and characteristics of OPV devices using a squaraine (SQ)
donor layer with and without a sensitizer. (a) J–V curves of devices under one sun
AM1.5G illumination, inset is the SQ structure (b) plot of external quantum
efficiency, inset is the device structure and characteristics of the devices with J
SC
of
± 0.09 mA/cm
2
, V
OC
of ± 0.01 V, FF of ± 0.01, and η of ± 0.09 % ...............................165
Figure 5.31. Structure and characterization of OPV devices using ZnPc donor with
and without sensitizer. (a) J-V curves of devices under 1 sun AM1.5G illumination,
inset is characteristics of the devices with J
SC
of ± 0.09 mA/cm
2
, V
OC
of ± 0.01 V,
FF of ± 0.01, and η of ± 0.09 % and (b) external quantum efficiency measurements,
inset is the device structure .............................................................................................166
Figure 5.32. Structure and characteristics of OPV devices using a squaraine (SQ)
donor layer with and without a sensitizer. (a) Device and DPSQ structures, (b) plot
of external quantum efficiency and (c) J–V curves of devices under one sun
AM1.5G illumination, inset is the device structure and characteristics of the devices
with J
SC
of ± 0.09 mA/cm
2
, V
OC
of ± 0.01 V, FF of ± 0.01, and η of ± 0.09 % ............167
Figure 6.1. Thickness optimization of different layers in devices with the structure:
(a) General i-OPV structure; (b) t = 30 nm, u is varied and v = 20 nm; (c) t is varied,
u = 40 nm and v = 20 nm; (d) t = 30 nm, u = 40 nm and v is varied ..............................175
xviii

Figure 6.2. Structure and characterization of conventional (D1) and inverted solar
cell (D2). (a) Device structures; (b) J-V curves in the dark and under illumination;
(c) External quantum yield (EQE) measurements .......................................................... 176
Figure 6.3. Optical modeling of c-OPV and i-OPV: optical E-field in a) c-OPV
(D1), b) i-OPV (D2); absorbed power Q
j
of c) D1 (integrated Q
j
= 0.19); d) D2
(integrated Q
j
= 0.28). The y-axis presents distances from the glass surface. White
lines were inserted for clarity between layers of the devices .......................................... 179
Figure 6.4. AFM images and section analysis of films on ITO. (a) ITO/ZnO
(20 nm)/C
60
(40 nm) (rms = 3.3 nm); (b) ITO/CuPc (30 nm) (rms = 0.3 nm); (c)
ITO/ZnO (20 nm) (rms = 1.5 nm) ...................................................................................181

xix

ABSTRACT
Organic photovoltaics (OPV) are a promising energy source for the future
because they have potential advantages over the conventional inorganic PV (silicon based
PV), including lightweight, flexibility, and low cost roll-to-roll production. These
advanced properties of organic materials enable future applications with great features,
such as OPV on flexible plastic substrates for mobile devices, or solar paints for cars and
homes. However, the efficiency of OPV is still low (the best reported efficiency of OPV
devices is 9%) compared to inorganic PV (Si-based PV with 15-20% efficiency). The
low efficiency of OPV is due to limits of current organic materials and the lack of
understanding fundamental processes in OPV under operation. In addition, the OPV
devices suffer low stability.
The research presented in this dissertation utilizes both materials design and
device optimization to address the limitations of OPVs. In Chapter 3, we present a
chemical annealing method used to convert the amorphous films to polycrystalline films
and study the effect of morphology change on device performance. In Chapter 4, the
synthesis and characterization of organic dyes that undergo symmetry breaking charge
transfer are shown. These compounds are expected to have lower exciton binding
energies, thus lowering the energy cost for charge separation in OPVs. Chapter 5 covers
the energy sensitization of C
60
and application in OPVs to improve absorption efficiency.
While OPVs with inverted structures have been shown to have improved lifetime, the
efficiency of inverted OPVs are often lower than the conventional OPVs. Study on
xx

device physics of OPVs with inverted structures in Chapter 6 shed light on the origin of
power loss and serves as guidance to design new materials for inverted OPVs.

1

CHAPTER 1. INTRODUCTION TO ORGANIC PHOTOVOLTAICS

1.1. The need for renewable energy sources and solar energy
1.1.1. The concerns over fossil fuels consumption
After centuries of relying on fossil fuels as a primary source of energy, human
being is facing some major problems. First, as the foreseeable depletion of fossil sources
approaches while energy consumption increases, countries are competing for limited
resources, causing wars and global instability. Large oil and gas reserves create conflicts
in the Middle East that results in the deaths of hundreds of thousands of people. Recent
upsurge tension over ocean territories in the South China Sea between rival countries
(China with South East Asian countries, including Vietnam, Philippines, Malaysia and
Brunei) has sparked concern that the area is becoming a flash point with global
consequences. The reason behind the dispute over the ocean areas is a prediction of huge
natural resources, which is estimated as much as 213 billion barrels of oil reserves – 10
times of the proven reserves of the US and 900 trillion cubic feet of natural gas – the
same as proven reserves of Qatar, according to the figures quoted by the US Energy
Information Administration.
The second issue is the environmental effects caused by fossil fuel consumption.
Despite the debates whether recent climate changes are related to human activities, it is
believed that the emission of greenhouse gases such as carbon dioxide causes global
warming. Carbon dioxide concentration shown in Figure 1.1, which is based on
2

comparison of atmospheric samples contained in ice cores and more recent direct
measurements, provides evidence that the concentration of CO
2
has increased
dramatically since the Industrial Revolution. According to a recent report from NASA,
9

the Earth has warmed since 1880 based on three global surface temperature
reconstructions. Even though the 2000s witnessed a solar output decline resulting in an
unusually deep solar minimum in 2007-2009, surface temperatures continue to increase.
The global warming is believed to cause other climate changes: rising sea level, shrinking
ice sheets, warming oceans and other extreme events such as floods, tornados and intense
rain falls.
2,9

Figure 1.1. Concentration of CO
2
based on analysis of atmospheric sample contained
in ice cores and more recent direct measurements. (source:
http://www.ncdc.noaa.gov/; graph is from http://climate.nasa.gov )
3

Figure 1.2. The total energy sources (republished from IEA 2008b) (upper), and the
photovoltaic solar resources of the United States (from NREL) (lower).
4

It is still controversial whether there exists direct link between the disastrous
floods and tornados with the global warming. The global change in weather cannot be
experimentally verified in the laboratory but the coincidence between temperature rise in
recent decade and the detrimental weather should alert us to make efforts to reduce the
global warming. Therefore, replacing fossil fuels with renewable energy sources that do
not emit green house gases should be seriously considered.
1.1.2. Renewable energy sources and solar energy
Figure 1.2 represents all energy sources available on the Earth and the total annual
energy consumption by humans.
1
The renewable energies include solar, hydropower,
ocean wave, tide, biomass, wind and geothermal energies. Although the hydropower had
been well developed and currently generates 15% of global electricity, it can only be built
in regions that have large rivers with steady water streams, and not much more
hydropower can be developed.
2
Among other renewable energies, wind and solar are the
two most developed technologies that have the potential to generate significant portions
of electricity worldwide.
2
The potential wind power is about 1300 TW (1 TW = 10
12
W),
which is about 850 times of global energy consumption, 15-16 TW. However wind
power has two major drawbacks: it is not stable and is strongly influenced by regional
geography.
The annual amount of energy received from the sun far more surpasses the total
estimated fossil resources and Uranium fission. In addition, solar energy has much more
equal distribution over the Earth’s surface compared to wind power. One of the most
5

common ways to utilize the solar energy is to convert light into electricity using solar
cells (or photovoltaics – PV). The solar intensity is approximately 1000 W/m
2
, and the
power demand of human being is 15-16 TW. With a 15% efficiency of solar panels, a
square meter generates 150 W. Thus, the area that is required to generate the total energy
needed by human beings is 1.07 x 10
5
km
2
, which is 0.0723% of the total land area on
earth. If the solar cells with an efficiency of 15% are used on the roof of a house with
100 m
2
of roof area, using the average photovoltaic solar resource of 4.5 kWh/m
2
/day
(see Figure 1.2) in the US, the power capability generated from solar energy will be
67.5 kWh/day. Solar cells will generate 2025 kWh monthly, which is well above the
average electricity power consumption of 940 kWh/month in the US (source The U.S.
Energy Information Administration at www.eia.gov). Thus, using solar cells for homes is
sufficient for a regular household with usual power consumption in the US.

Figure 1.3. Global cumulative PV capacities by 2010. (from ref. 1)
6

1.1.3. Current PV technologies and their limitations
It seems that solar energy is very promising solution for energy problem on the
Earth; however, the solar energy is still not popular. Despite the steep growth of global
cumulative PV capacities (Figure 1.3), this portion of energy is still far less than the
global energy demand (15-16 TW per year). The obstacles for solar energy production
and possible PV technologies that can be used to increase the solar electricity production
will be discussed in this section.
There are four important aspects of solar cells: cost, efficiency, lifetime and
productivity. In the past, efficiency has been thought of as the key factor that indicates
the advancements of solar cells. However, the practical applications have shown that the
cells having best efficiency may not become popular because of high production cost.
Thus, cost is the core issue of solar cells. In addition, the technology of solar cells has to
be compatible with mass-production techniques for the cost and the deployment of solar
panels to be practical.
There are several types of solar cells: single crystalline Si solar cells,
multicrystalline Si solar cells, single junction III-V solar cells, multijunction III-V solar
cells, and thin film solar cells (amorphous Si solar cells, nano- or microcrystalline Si
solar cells, CdTe solar cells, Cu(In, Ga)Se
2
– CIGS solar cells, dye-sensitized solar cells,
organic solar cells and organic-inorganic hybrid solar cells). The efficiencies of solar
cells and the PV module cost are presented in Figures 1.4 and 1.5, respectively.
7

The III-V single junction and multijunction GaAs solar cells are the most efficient
cells, but the production cost is much more expensive than other cells. Thus, highly
efficient III-V solar cells are limited to aerospace applications. The crystalline Si cells
are the most popular commercial PV; however, the cost is still a major concern despite
the significant drop of price in recent years (Figure 1.5). The cost of multicrystalline Si
solar cells is still too high and cannot compete with fossil fuels for electricity generation.
Government subsidies are still necessary to keep this industry alive. On the other hand,
thin film solar cells that consume much less materials are considered to offer the hope of
future development of PV industry without the necessity of the government subsidies.

Figure 1.4. Best research cell efficiencies certified by NREL. (from
http://www.nrel.gov)
8

Currently, CIGS, a-Si and CdTe thin film solar cells have attracted significant
attention due to their high efficiencies and low production cost. On the other hand, if
solar cells will be used as a major energy supply, the total PV area required will be very
large - 1.07 x 10
5
km
2
as mentioned above, so the production capability and material
consumption have to be considered. There are two foreseeable issues with CIGS and
CdTe thin film PVs. Firstly, they are fabricated in vacuum chambers, which will prevent
the future large scale production, thus alternative inkjet printing technologies for
roll-to-roll productions need to be developed. The material shortages are the second issue
because In and Te are rare-earth elements. The industry will face the challenge of
material supply as the solar power production exceeds 100 GW.
2

The emerging third generation of solar cells, including dye-sensitized solar cells
(DSSCs) and organic solar cells or organic photovoltaics (OPV) offers a great potential
for low production cost with a large scale. In addition, DSSCs and OPVs work under low
light intensity, so they are suitable for cloudy weather. The DSSCs have proved to
exhibit high efficiency (more than 10%) and low cost. Unfortunately, current DSSCs
have three major drawbacks. First, they use liquid electrolytes (acetonitrile), which has
low boiling point (80 C) and thus will not be suitable because the cells can be heated up
to 70 – 80 C under sun light. Second, the electrolyte in DSSCs contains iodine, which is
a toxic substance. These two disadvantages lead to a low operation temperature and
require rigorous device encapsulation, resulting in increased cost. Third, the most
efficient DSSCs still use dyes that contain Ru, a rare-earth element. Recent
9

developments on solid state electrolyte
10
and novel organic dyes not containing Ru
11
open
great promise for DSSCs.

OPVs typically use thin films of organic materials as light-absorbing layers, thus
exhibiting several advantages over the inorganic ones and DSSCs. Organic dyes used in
OPV are made from the earth-abundant elements, thus excluding the material limitation
for large scale productions. The band gap and energy levels are highly tunable, thus
offering the flexibility to adjust the system. The thin film fabrication of organic materials
can be done by various techniques, including vapor thermal evaporation (VTE), organic
physical vapor deposition (OPVD) or solution processing under ambient conditions. One
of the greatest potentials of OPV is the possibility to employ cheap roll-to-roll device
fabrication for mass-production.
Despite above advantages, efficiencies of OPVs are currently low (see Figure
1.4). This thesis will overview the working principles and disadvantages of OPVs, and

Figure 1.5. The cost of PV module. (ref. 2). Cost of solar cells is decreasing faster
than predicted. The figure shows the trend of fabrication cost for PV modules.
10

present studies on novel materials and device structure to further understand the working
principles of OPVs, opening the potential to improve efficiency and stability of OPV
devices.
1.2. Introduction to organic photovoltaics
1.2.1. Device structures and constituent materials in organic photovoltaics
A typical OPV device consists of a transparent substrate, a transparent electrode, a
light-absorbing organic active layer and a counter electrode as depicted in Figure 1.6.
Additional interfacial layers on either side of the electrodes are often used to improve
charge collection efficiency of OPV devices. The light-absorbing layer in OPVs
generally consists of two materials, an electron donor and an electron acceptor. There are
two architectures for the photoactive layer: planar (or lamellar) heterojunction (PHJ), in
which the donor and acceptor materials are deposited in separated layers, and bulk
heterojunction (BHJ), which is constructed from a blended mixture of donor and acceptor
materials (Figure 1.6).
Generally, the BHJ OPVs have higher efficiencies than the PHJ ones due to larger
donor/acceptor (DA) interface area. BHJ structure requires the donor and acceptor
materials to form continuous phases for efficient charge conduction; however, controlling
morphology of BHJ is very complicated and not well understood. On the other hand,
simple PHJ structure allows independent control of donor and acceptor structures, thus
offering a great advantage over the BHJ for studying complicated fundamental processes,
especially at the DA interface in OPVs.
11

The electrode definitions used in OPVs are for a cell being irradiated and thus
acting as a galvanic cell. In this case, the cathode is the electrode in contact with the
electron donor and thus the source of holes, and the anode is the source of electrons. In
OPVs, high work function (WF) electrodes are used as cathode to collect holes from the
donor, and the low WF ones as anode to collect electrons from the acceptor.
Electrode materials. The most frequently used substrate in the scientific
literature is a glass substrate coated with a transparent conducting oxide (TCO), tin doped
indium oxide (ITO). Glass substrates are cheap and provide a good barrier against
oxygen and water diffusion into devices. It is also possible to use other flexible
substrates such as poly(ethylene terphthalate), polycarbonate, or polyethersulfone.
ITO coated on glass or plastic substrates are commercially available products that
come with wide variety of thicknesses, conductivities and surface roughnesses. The

Figure 1.6. (a) General OPV device structure, and different architectures of the
active layer (b) planar (lamellar) heterojunction (PHJ), (c) Bulk Heterojunction
(BHJ). Adapted from ref. 5
Active Layer Architectures
Bulk Heterojunction
Planar Heterojunction
Donor
Acceptor
Donor/Acceptor
Blend
a)
b)
c)
12

transparency of ITO, a wide band gap semiconductor with E
g
= 3.2 - 4.0 eV,
12,13
is
typically > 85% through the visible region of the spectrum, which allows efficient
light-harvesting by the active layer of the OPV devices. The conductivity of ITO on
glass ranges from 3000 – 6000 S/cm and is lower on plastic (<1500 S/cm).
14
The ITO
WF lies in a broad range 3.7 – 5.1 eV, often considered to be ~ 4.8-5.0 eV, depending on
the surface pretreatment method used for cleaning, as well as on the In/O and In/Sn
atomic ratios on the surface, and on the surface composition.
15-19
The most commonly
used method for surface cleaning, UV ozone or oxygen plasma tend to raise the WF,
while surface modification with various phosphonic acids offer a great way to tune the
work function of ITO.
19-21

Despite the standard use of ITO as a transparent cathode in OPVs, there are some
limitations of ITO for the future large-scale applications. First, the resistivity of ITO is
still too high and adversely affects OPV performance. For larger active areas, a TCO
with a higher conductivity will be beneficial. Another factor that precludes ITO from
being an ideal cathode for OPV is that ITO exhibit n-type conductivity. A p-type TCO
would be preferable for this role but only few p-type TCOs exist; none of which currently
surpasses ITO performance. Finally, the cost of ITO is high due to rising price of In,
which is currently at $650 – 700$/kg. The price of In may go up if thin film solar
industries continue to use the metal, significantly increasing the demand.
The high cost of ITO has spurred researchers to seek for alternative transparent
electrodes. Current research in applying other TCOs, such as fluorine doped tin oxide
(FTO), aluminum doped zinc oxide (AZO) and TiO
2
, in OPVs has been rapidly growing.
13

Apart from TCOs, carbon nanotubes (CNT),
22-24
graphene
25,26
and metal nanowires
27-29

have been shown to have potential for OPV applications.
Active organic materials. Organic semiconductors, including polymers and
small molecules, have been studied for decades. Of these, only a small fraction has been
successfully used in OPV devices due to the various electrical, optical and stability
requirements demanded of the chosen materials. In this work, the main attention is
focused on small molecule organic semiconductors. Recently, Mishra has published a
comprehensive review on small molecule semiconductors used in OPVs.
30
Some of
common donors, acceptors and anode buffer organic materials used in OPV are presented
in Figure 1.7.
Research in OPV materials mainly focuses on donor materials. Since the first DA
heterojunction OPV device employing a vapor deposited CuPc/PTCBI with the efficiency
of 1%,
31
many donor materials have been developed to achieve highly efficient OPV
devices (up to 7% for a single junction device).
30
It is interesting to note that among
those donor materials, organic dyes with a push-pull structure displays highest
efficiencies. Vapor-deposited BHJ OPV devices using organic dyes with simple donor-
acceptor-acceptor (D-A-A) structure, such as DTDCPB
32-34
in conjunction with C
70
have
been shown to have efficiencies ranging from 5.8 to 6.6%. The BHJ DTDCPB/C
70

device displays high photocurrent (J
SC
= 13.5 mA/cm
2
) and high open voltage circuit
(V
OC
) of 0.93 V.
32
However, the low fill factor (FF) suggests that further device
morphology optimization would lead to even higher efficiency. Another example of
push-pull dyes with a A-D-A structure is dicyanovinyl-capped quinquethiophenes
14

(DCV5T),
35
which was used in p-i-n BHJ OPV devices. The optimized OPV devices
exhibit a high V
OC
(0.95 V) and an excellent FF (0.63) owing to favorable phase
separation and highly crystalline DCV5T phase during deposition on a heated substrate.
35

Even though the photocurrent in DCV5T/C
60
device (11.5 mA/cm
2
) is lower than the
DTDCPB/C
70
device, the high FF results in comparable device efficiency of 6.9%.
Squaraines (SQ)
36-44
dyes, which have a D-A-D structure, have recently gained increasing
attention due to their beneficial properties, such as high extinction coefficients, tunable
electronic structures and band gaps. PHJ OPV devices using SQ have been significantly
improved over the last 3 years from 3%
37
to 6%.
41,44
The classes of push-pull organic
dyes are promising candidates for further development to improve device performance.
Much less research attention has been paid to acceptor materials.
45
Fullerenes
including C
60
, C
70
and their derivatives are the most widely used and the most efficient
electron acceptor material in OPV devices.
30
As for nonfullerene acceptor, peryllene
diimide derivatives such as PDI and PTCBI are the most popular materials. Despite
better absorption than fullerenes, PDI and PTCBI OPV devices suffer from much lower
efficiencies.
46,47
The performance of fullerenes as the acceptor is excellent; however,
their low absorption in the visible spectrum is a big disadvantage. Therefore, it is
necessary to improve the absorption efficiency of the acceptor layer by developing the
new n-type organic dyes or by sensitization of fullerenes. Improving light absorption
efficiency for fullerene-based acceptor will be discussed in details in Chapter 5.
15

1.2.2. General working principles of organic photovoltaics devices
Achieving efficient charge photogeneration has long been recognized as a vital
challenge for molecular-based solar cells. The first OPV device with Schottky diode
structure consisting of an organic dye layer sandwiched between two electrodes with
different WF, resulted in a very poor photocurrent generation efficiencies.
48
Chin Tang
reported the first PHJ devices employing vapor deposited CuPc/PTCBI (see Figure 1.7)
that exhibited relatively efficient photocurrent generation.
31
The benefit of having a DA
heterojunction over the Schottky diode is that the differences in electron affinities (and/or
ionization potentials) between the two materials create an energy offset at the DA
interface, thereby driving efficient exciton dissociation.

Figure 1.7. Some common organic materials used in OPVs.
16

Figure 1.8. Charge photogeneration processes in OPV devices, illustrated using (a)
planar heterojunction of donor and acceptor layers and (b) energy diagrams.
1. h
2
3
4
5
a)
1. D + h  D* absorption
2. D* + D  D + D* exciton migration
3. D* + A  [D
+
A
-
] charge transfer
4. [D
+
A
-
]  D
+
+ A
-
charge separation
5. D
+
+ A
-
 current charge conduction
exciton charge transfer state
free electron
free hole
b)
LUMO
HOMO
vacant
orbitals
vacant
orbitals
filled
orbitals
filled
orbitals
1 2
3
+
-
4
5
+
-
+
-
E
exc
E
exc
= exciton energy
17

Despite extensive research effort spent on studying device physics, fundamental
processes in OPV devices are still not fully understood. The general processes of charge
photogeneration in a DA heterojunction device are summarized in Figure 1.8.
Absorption of a photon by the donor as illustrated in Figure 1.8 generates an exciton, an
electrically neutral quasi-particle consisting of a strongly bound electron-hole pair
(step 1). The exciton subsequently diffuses until it reaches the DA interface (step 2),
where electron-transfer from the donor to the acceptor forms a charge transfer (CT) state
(step 3). This CT state is comprised of a partially charge separated state, where the hole
is primarily localized on the highest occupied molecular orbital (HOMO) of the donor,
and the electron on the lowest unoccupied molecular orbital (LUMO) of the acceptor.
Despite being located on different molecules, this electron-hole pair of the CT state still
experiences a significant Coulomb extraction (of the range 0.1-0.5 eV), which is
significantly larger than k
B
T.
7
Note that there is a variety of nomenclatures for the CT
state that are currently used in literature, such as bound polaron (or electron-hole or
radical) pair, charge transfer exciton, and exciplex.
7
The overall process of charge
photogeneration requires charge separation of these initially generated CT states into free
charges (step 4), which can then be transported (step 5) and collected by the
corresponding electrodes. Each step in photovoltaic process will be reviewed in much
greater detail in the following section.

18

1.3. Fundamental processes in OPV
1.3.1. Exciton formation

Photoexcitation process of conjugated organic dyes involves transition between
molecular orbitals with substantial wave function overlap. As a result, the extinction
coefficients, (or the optical density for films, ) are very high, in order of =
10
4
-10
5
M
-1
cm
-1
(or = 10
4
– 10
5
cm
-1
). For many -conjugated organic molecules, the
absorption coefficient is very high compared to conventional inorganic semiconductors,
such as crystalline Si and GaAs as shown in Figure 1.9. Thus, an extremely thin film can
still absorb a substantial fraction of the incident solar photons. For example, a typical
400 600 800 1000
0
1
2
3
4
0
2
4
Si
GaAs
C
60
SubPc
CuPc

Absorption coefficient (10
5
cm
-1
)
Wavelength (nm)
Photon flux (10
18
photons/m
2
)

Figure 1.9. The absorption coefficient of organic (C
60
, SubPc and CuPc) and
inorganic (c-Si and GaAs) materials. Solar photon flux derived from ASTM
G173-03 AM 1.5G spectral irradiance.
19

CuPc/C
60
device with a thickness of 100 nm can capture about 90% of incident light at
630 nm, which is close to the peak of solar spectral irradiance.
As compared to inorganic semiconductors such as Si and GaAs, organic materials
have much lower static dielectric constants, . The typical dielectric constants of organic
materials ranges from 3.5 to 5.5, while in crystalline Si = 11.9.
5
Upon absorption of a
photon, a strongly bound electron-hole pair (or exciton) is formed. The Coulomb
interaction of the electron-hole pair, or the binding energy of the exciton (E
b
exc
= 0.1-
0.5 eV)
5
in organic semiconductors is significantly greater than the thermal energy due to
the low dielectric constants of materials. Other contributors to the appreciable binding
energy are electron - lattice and electron - electron interactions.
49

Figure 1.10. Schematic presentations of the ground state S
0
, the singlet and triplet
excited states S
1
and T1 in terms of (a) molecular orbital configurations and (b) state
energy diagram, the solid and dotted lines represent the radiative and nonradiative
decay pathways of the excited states.
....
....
....
....
....
....
Vacuum level
Energy
0
Energy
S
0
S
1
T
1
S
0
0
Intersystem
crossing
S
1
Fluorescence
Phosphorescence
T
1
a) b)
20

In terms of the molecular orbitals, an exciton (or an excited state) can be
considered as a state that contains two unpaired electrons, which form two different spin
configurations, singlet and triplet as presented in Figure 1.10. The singlet state (S
0
and
S
1
) is the state exhibiting asymmetric spin configuration with regard to exchange, while
the triplet state has symmetric spin configuration. The singlet and triplet excited states
can radiatively recombined to the ground state, producing fluorescence and
phosphorescence, respectively. The typical singlet state lifetime is several nanoseconds,
while the triplet lifetime is orders of magnitude longer ranging from microsecond to
milliseconds.
1.3.2. Exciton diffusion
1.3.2.1. Energy transfer mechanisms
The most commonly encountered electronic energy transfer processes between
the excited state donor d* and the energy acceptor a in organic photochemistry are
presented in Figure 1.11.
3
Note that we use d and a as energy donor and energy acceptor
in this section. The dipole – dipole mechanism (also termed in literature as “Forster”
50
or
“Coulombic” or “resonance mechanism”) describes the energy transfer process by the
overlap of dipolar electric field of d* with a. For the electron exchange mechanism (also
termed in literature as “Dexter”
51
or “orbital overlap mechanism”), the interaction
between d* and a is made through the overlap of the orbitals of d* and a. Therefore, the
dipole – dipole interaction operates via through-space interaction of oscillating electric
21

field produced by d* with a, and does not require a Van der Waals contact or an overlap
of the orbitals of d* and a.

Forster energy transfer. Forster
50
described the rate of resonance energy
transfer using a classical model for the electronic interaction between two electric dipoles
using equation 1.1.
(1.1)
From Eq. 1.1 we see that the key parameters determining the strength of dipole –
dipole interaction are the size of the interacting dipoles µ
d
and µ
a
, which correspond to

Figure 1.11. Mechanisms of energy transfer: (a) Forster or dipole – dipole interaction
mechanism, the blue doubled-headed arrows represent electric dipole generated by the
excited state D*; (b) Dexter or electron exchange mechanism. The spin of the
electrons exchanged must obey the spin conservation rules. Adapted from ref. 3
1
2 1
2
a) Dipole – dipole
1
2 2
1
b) Electron exchange
b)
HOMO
HOMO
LUMO
LUMO
Electron transfer
Electron transfer
Hole transfer Hole transfer
*d *a a d
22

respective transitions d* → d and a → a*, and the distance between the donor and
acceptor molecules R
da
. Forster related the theoretical quantities of oscillator strength (f)
and transition dipoles (µ) to experimental quantities, such as extinction coefficient ( ) or
inherent radiative rate (k
r
) as shown in Eq. 1.2 and 1.3.
3,50
Thus, the rate for dipole –
dipole energy transfer can be expressed as in Eq. 1.4.
(1.2)
(1.3)
(1.4)
The term in Eq. 1.4 is proportionality constant determined by experimental
conditions, such as concentration and solvent polarity index of refraction; 2/3 is a
constant accounting for a random distribution of dipole orientations; J(
a
) is the spectral
density integral, which is the overlap integral between emission spectrum of d and
absorption of a including the extinction coefficient of a (
a
). Thus, the efficiency of
energy transfer by dipole mechanism directly relates to the quantum yield of d, the
extinction coefficient of a and spectral overlap. Since singlet – singlet transitions have
much larger oscillator strength compared to singlet – triplet transitions, the dipole –
dipole mechanism is generally plausible to describe the singlet – singlet energy transfer.
Dexter energy transfer. The degree of electron exchange for energy transfer is
directly related to the orbital overlap of d* and a, the rate of energy transfer is expected
23

to fall off exponentially with the distance separating d* and a. Dexter expressed the rate
constant of energy transfer by electron exchange by Eq. 1.5:
51

(1.5)
Where K is a parameter related to the specific orbital overlap interactions; J is the
normalized spectral overlap interaction, which does not depend on the actual magnitude
of
a
; R
da
is the actual separation between d* and a; is the separation of d* and a
when they are in Van de Waals contact. Both singlet – singlet and triplet – triplet energy
transfers can be described by the electron exchange mechanism. Differing from the
dipole mechanism, the electron transfer mechanism cannot be directly related to
experimental quantities.
1.3.2.2. Exciton diffusion length
In order to generate electrical current in OPV, excitons (singlet or triplet) need to
diffuse to the DA interface within their lifetimes. The length over which excitation can
propagate prior to decay of its population to 1/e (~35%) of its initial value is the exciton
diffusion length (L
D
). Even though the optical densities of organic dyes used in OPVs are
very high, short exciton diffusion lengths (L
D
= 5 – 30 nm)
52-57
severely limit the amount
of absorbed light that may be converted to photocurrent. As a result, researchers have
attempted to circumnavigate this problem by developing BHJ architectures (Figure 1.6),
consisting of nanometer scale phase-segregated donor and acceptor regions to ensure
excitons are formed within L
D
from DA interface. While such BHJ structures are
24

attractive for materials with short L
D
, characterization and control of such structures are
very challenging and are a major limitation for BHJ OPVs.
58,59

Since intermolecular interactions in amorphous organic films are very weak,
exciton migration is often described by hopping via either dipolar or electron exchange
mechanisms. In practice, predicting exciton dynamics using either of these mechanisms
is complicated by conformational distortions, molecular disorders and the presence of
trap states in thin films.
60-62
One of the key challenges for OPV community is to
understand exciton dynamics and to improve exciton diffusion length of organic
semiconductors. Conversion of amorphous films with disordered orientation into
polycrystalline films with well organized molecular packing has been demonstrated to be
an effective method for improving exciton diffusion lengths in organic semiconductor
films. This topic will be considered in detail in Chapter 3.
1.3.3. Charge generation at the donor/acceptor interface
Following exciton diffusion to the DA interface, efficient charge transfer (CT)
quenching of the excited state and the consequent charge separation (CS) must occur in
order to yield the desired photoresponse (steps 3 and 4 in Figure 1.8). It is generally hard
to experimentally distinguish these steps. In the following sections, the thermodynamic
and kinetic factors of the charge generation process are considered in detail to illustrate
the general picture for charge generation at the DA interface.

25

1.3.3.1. Thermodynamics of charge generation
The thermodynamic requirements for the charge generation at the DA interface
are straightforward; the energy of an exciton interacting with an electron acceptor must
be greater than the total energy required for the formation of free charged D
+
and A
-

species, i.e. the sum of ionization energy (E
i
) of the donor and electron affinity of the
acceptor (E
a
). The change in enthalpy for the photoinduced charge generation,
D* + A → D
+
+ A
-
, can be calculated by H = – E
i
+ E
a
– E
00
. Ionization energy of the
donor, E
i
and electron affinity of the acceptor, E
a
can be directly measured by ultraviolet
photoelectron spectroscopy (UPS) or inversed photoelectron spectroscopy (IPES),
respectively; E
i
and E
a
can also be estimated from oxidation and reduction potentials of
the electrochemical measurements in solution using proper correlations.
63,64
E
00
is the
energy of exciton, which can be singlet or triplet. Application of this approach allows us
to separately consider possibility of charge separation from singlet and triplet excitons.
Schlenker et al introduced the ionization energy of the excitonic state, sometime referred
to as the “optical LUMO” (see Figure 1.8b), E
i
* = E
i
+ E
00
, which is a very useful
quantity for assessing whether the charge generation is exothermic.
4
If the value of E
i
* is
larger than E
a
, then the photoinduced exciton dissociation will be exothermic. A similar
calculation of an acceptor exciton’s electron affinity (E
a
*) or “optical HOMO” may be
made to assess whether acceptor exciton dissociation will be exothermic.

26

1.3.3.2. Kinetics of charge generation
Beside thermodynamic consideration, kinetics of electron transfer and charge
separation plays crucial role in determining the rates of the overall charge generation
process. In this section, we will review Marcus and Onsager theories, which are often
used to describe the electron transfer and charge separation processes in OPVs.
1.3.3.2.1. Marcus theory
Marcus’s theory of semiclassical nonadiabatic electron transfer, first developed in
1956,
65
has been successfully applied to a number of chemical systems and also been
extended to photoinduced charge transfer in OPVs. Marcus theory considers the reactant
and product potential energy surfaces as two intersecting harmonic oscillators (parabolas)
with the horizontal axis as the reaction coordinate, representing the motion of all nuclei in
the system, as illustrated in Figure 1.12. Electron transfer must occur at the intersection
point in order to satisfy both energy conservation requirements and the Frank-Condon
principle, which states that electron transfer takes place so rapidly compared to nuclear
motion that no change in nuclear configuration effectively occurs during the transfer.
The intersection point therefore represents the energy level and nuclear configuration that
the reactant state must achieve (through vibrational motion) in order for isoenergetic
electron transfer to occur. The reorganization energy refers to the energy required to
bring the reactant and its surrounding medium to the equilibrium geometry of the product
state.
27

The rate constant for electron transfer, k
ET
, can therefore be determined in term of
a Fermi’s Golden rule type analysis as in Eq. 1.6.
(1.6)
The matrix element term V refers to electronic coupling between the reactant and product
states and thus depends on the overlap of the electronic wave functions of the electron
donor and acceptor. Strong coupling denotes the adiabatic limit, where the two potential
energy surfaces “split” into a lower and an upper surfaces; the electron transfer proceeds
along the lower surface. In case of a DA pair, the electronic coupling is relatively weak,

Figure 1.12. Potential energy surface for a donor/acceptor system in OPVs, where
photoexcitation generates
1
D*/A and subsequent electron transfer generates [D
+
A
-
].
G is the energy difference between the two surface minima; is the reorganization
energy. Adapted from ref. 4

λ
28

such splitting is small, and the electron transfer occurs nonadiabatically, as described in
Eq. 1.6.
The exponential term in Eq. 1.6, corresponding to the Frank-Condon factor,
predicts that as - G increases, so does the electron transfer rate, until the maximum rate
is reached when = - G . At this point, the reaction has no activation barrier. Further
increase in - G will decrease the reaction rate; this is called the Marcus inverted
region. The Marcus theory also predicts that small reorganization energy will facilitate
the electron transfer rate. For solid films, the nonadiabatic theory detailed above might
need to be modified to reflect the presence of a band of the donor or acceptor states rather
than discrete molecular levels.
1.3.3.2.2. Onsager theory
To generate fully dissociated charges, the formed CT state must overcome a
strong Coulombic atraction force within its lifetime. If the dissociation does not take
place, then geminate recombination, which is known to be a significant loss mechanism
in OPVs, will occur. Germinate recombination was first described qualitatively by
Onsager.
66
The proposed model calculates the probability that a Coulombically bound
electron – hole pair in a weak electrolyte undergoing Brownian random motion will
escape its Coulomb attraction and generate free charges. In Onsager model, photon
absorption generates a localized hole and a hot electron; the later, by virtue of its excess
thermal energy, then undergoes rapid motion until it relaxes at distance a (the
thermalization length) from the localized hole, as illustrated in Figure 1.13. The
29

competition between dissociation and back recombination of this CT state depends on the
magnitude of the Coulombic attraction. In particular, Onsager proposed a definition for a
Coulomb capture radius (alternatively called the Onsager radius), r
c
, defined as the
distance at which the Coulomb attraction energy equals the thermal energy k
B
T:
(1.7)
Where e is the charge of electron,
r
is the dielectric constant of the surrounding medium,
0
is the permittivity of vacuum, k
B
is Boltzmann’s constant and T is temperature. If the
thermalization length a is greater than the Coulomb capture radius r
c
, the charge carriers
are considered to be fully dissociated. However, if a < r
c
, the dissociation of the CT state
into free charges occurs with an escape probability of P(E), while the probability for
germinate recombination is 1 – P(E).
The escape probability, P(E) depends on the strength of any applied electric
field E. For low field strengths, the escape probability is given by:
(1.8)
Where a is the initial separation of the two thermalized ions, r
c
is Coulomb capture
radius, k
B
is Boltzmann’s constant and T is temperature.
30

While Onsager theory has proven very effective in predicting experimentally
observed variations in charge photogeneration in certain homogeneous systems, there
remain significant challenges in application of this theory to predict absolute yields of
charge separation at a heterogeneous DA interface. The current implementations of
Onsager theory do not include explicit consideration of dynamic lattice distortions or
relaxations, which present in molecular films. In addition, it is difficult to obtain reliable
estimations for the thermalization and capture lengths (a and r
c
, respectively) due to the
disordered nature of organic films. A further complication is determination of the electric
fields at the organic/organic interface. These may be influenced by not only macroscopic

Figure 1.13. Potential energy diagram summarizing Onsager theory. The bold curves
illustrates the potential energy resulting from Coulomb attraction as a function of
electron – hole separation (e - h). Photoexcitation results in generation of a hot, mobile
electron. This electron subsequently relaxes to a particular distance from the hole, a.
If a < r
c
, then the CT state can either undergo germinate recombination or dissociate
into free charges. Adapted from ref. 7
31

electric fields generated by charges on the electrodes but also by the presence of the
interface dipoles.
56,67,68
Finally, we note that Onsager theory assumes that the initial
generation of the CT state is electric-field independent. However, this assumption might
be not correct as indicated by a number of studies on different systems.
69,70
It is therefore
desirable to develop a better theoretical model to better describe the photoinduced charge
separation at the DA interface.
1.3.3.3. General picture for charge separation process
Building upon the frameworks of Marcus and Onsager theories, the state energy
diagram shown in Figure 1.14 illustrates the charge generation processes at the DA
interface. The photogenerated singlet exciton S
1
of the donor can undergo electron
transfer to the acceptor at the DA interface to form CT state, whose rate of formation is
governed by the Marcus theory described in 1.3.3.2.1. Since the hole and the electron are
located in different molecules, the exchange energy or the energy difference between the
singlet and triplet CT states are small. The CT state, which initially has excess of thermal
energy or “hot” (corresponding to the “crossing point” in Marcus electron transfer theory)
due to the energy offset between LUMOs of the donor and the acceptor. It is generally
accepted that an offset of about 0.3 - 0.5 eV is required for an efficient charge
separation.
5,7
The situation after the electron transfer follows the Onsager picture, the
efficiency of the overall dissociation process is likely to depend on the distance and the
strength of interaction between the hole and the electron.
32

If the CT state successfully dissociates into free charges, the energy of the final
CS state, corresponding to the completely unbound electron-hole pair, is defined by the
(adiabatic) ionization potential (E
i
or E
HOMO
) of the donor and electron affinity of the
acceptor (E
a
or E
LUMO
) of the acceptor, E
DA
= |E
i
– E
a
|. To a large extent, E
DA
defines
the upper limit for the open voltage circuit. We will discuss the open voltage circuit in
much greater detail in the following section.

Figure 1.14. Electronic state diagram describing the photo-induced charge generation
mechanism in OPVs: S
0
, S
1
and T
1
denote the ground state, singlet and triplet excited
states of the donor, respectively. At the DA interface, electron transfer from excited
state of the donor to acceptor forms CT state: CT
1
is the lowest (or relaxed) CT state,
CT* represents the “hot” CT state. k
IC
and k
RC
are the relaxation and geminate
recombination rate of the CT sates. The final state is the charge separated (CS) state,
where the hole and electron locates separately in the donor and acceptor layers,
respectively. Adapted from Ref. 8.

33

The picture described above for photoinduced charge separation is widely
accepted; however, the exact mechanism for free charge generation from the CT state at
the DA interface is a hot topic of debate among OPV community. Depending on the
relative time scales of internal conversion (or relaxation) of the CT state and CS
processes, two scenarios for exciton dissociation can be discussed.
i. k
IC
>> k
CS*
. The hot CT state undergoes a fast relaxation to its lowest
electronic/vibrational state.
8
Loosely speaking, the lowest CT state consists of a bound
pair of a hole and an electron located in HOMO and LUMO levels of adjacent donor and
acceptor molecules, respectively. Since the excess of energy for CS is lost through
thermal relaxation, the probability of CT state dissociation via this route is not likely to
be high. The relaxed CT excitons can recombine to the triplet of the donor (or acceptor)
or the ground state, causing a loss channel.
71,72

ii. k
IC
<< k
CS*
. In this case, the CT exciton dissociation can take place via excited
(hot) levels.
8
The formation of free charges through hot CT excitons has been
experimentally confirmed in both small molecule and polymer OPVs.
71,73-75
Balkulin et
al. have demonstrated that CT excitons localized across the DA interface can be “pushed”
into a delocalized state by infrared photoexcitation, resulting in increased photocurrent.
A question then arises concerning the time scales during which the hot CT state must
dissociate after initial electron transfer to prevent cooling and localization. The two
works recently published in Nature Materials address this issue.
73,74
Jailaubekov and
colleagues
73
use transient second-harmonic generation spectroscopy to probe the
dynamics of CT excitons at the heterojunction between copper phthalocyanine (CuPc)
34

and C
60
. The authors find that the charge generation is very fast (~100 fs), and
independent of the pump photon energy if the bulk CuPc is excited directly, indicating
that hot CT excitons dissociate on such ultrafast time scales. The energy of the hot CT
excitons was found to be ~ 0.3 eV higher than the relaxed ones, and the relaxation of the
hot CT states was estimated ~ 1 ps. Using state-of-the-art simulations, Jailaubekov et al.
were able to show that the upper CT states (for example CT
4
) with longer electron-hole
distance are less bound and dissociate more readily, in agreement with the work of
Bakulin and coworkers.
71
Similar results have been obtained in a blend of the PCPDTBT
polymer and the fullerene derivative PCBM.
74

Most considerations of charge photogeneration do not include the change in
entropy associated with changing from a single species (the exciton) to two separated
charges with random positions relative to each other. Clarke and Durrant noted that the
contribution of entropy factor to CS process can be quite significant.
7
The role of entropy
in charge separation processes has been discussed with respect to the dimensionality of
the organic semiconductor.
76
In one dimensional (1D) materials, the change in entropy,
S, plays no role, but at higher dimensions (2D or 3D), it leads to a substantial decrease
in the Coulomb barrier for charge separation. The effects of S are highest in
equilibrium systems but decrease and become time-dependent in illuminated OPV cells.
Higher-dimensional semiconductors have inherent advantages for charge separation, and
this may be one reason that fullerene and its derivatives, the only truly three-dimensional
organic semiconductors yet known, play such an important role in OPV cells.

35

1.3.4. Charge transport in organic materials.
Once the charges have separated, they move toward their respective electrodes
with an efficiency depending upon their mobility. The weak intermolecular electronic
coupling as well as disorder effects, that exist in conjugated organic materials, cause the
localization of charge carriers and formation of polarons. In such cases, transport relies
on the hopping of polarons from site to site. The rate of this hopping process may be
expressed in terms of nonadiabatic Marcus theory for electron transfer to account for the
electronic coupling of the polarized neutral ground state species to the radical ion and the
internal reorganization energy required for this pair to reach the same geometry.
77,78

Furthermore, the inhomogeneity of amorphous organic films can lead to the formation of
carrier trap sites, which also decreases the charge transfer rate given by a Marcus-type
treatment. As a result, the charge carrier mobility strongly depends on morphology and
can vary over several orders of magnitude when going from highly disordered amorphous
films (typically, 10
-6
– 10
-3
cm
2
V
-1
s
-1
) to highly ordered materials (> 1 cm
2
V
-1
s
-1
).
Fabrication of crystalline films in OPVs is desirable to eliminate carrier traps as well as
to improve the charge hoping rate. This issue will be addressed in Chapter 3.
1.3.5. Charge collection.
Photovoltaic solar energy requires efficient delivery of photogenerated charges to
an electrical load resistance in the external circuit. The electrical contacts employed in
device preparation can strongly impact device performance. The efficiency of the charge
collection cannot be simply determined from the difference between the work functions
36

of the isolated electrodes and the energies of the donor’s HOMO or acceptor’s LUMO.
The properties of this electrode/organic interface are an area of substantial interest due to
complicated interfacial processes, including charge density redistribution, geometry
modifications and chemical reactions. Modification of the electrodes with various
substances, such as oxides, polymer and phosphonic acids, to make better electrical
contact with organic materials has been proven to be a good method to improve charge
collection efficiency in OPVs.
21,79-85

1.4. Characterization of OPV devices
1.4.1. Electrical and optical characterizations of OPV devices
In this section we briefly address the electrical measurements and analysis of
OPV devices. The basic electrical measurement consists of connecting the device to an
external power source, sweeping a DC voltage (V) and measuring the electrical current
density (J). The electrical measurement of each device is done in the dark and under
white light illumination with 1 sun intensity. Typical J – V curves of OPV devices are
shown in Figure 1.15.
There are three important characteristics of a solar cell: short circuit current (J
SC
),
open voltage circuit (V
OC
) and fill factor (FF). J
SC
is the photocurrent when the applied
voltage equals 0 V. V
OC
is the required applied voltage to shut off the current in the
device under white light illumination. A quantity for comparing the maximum output
power relative to the photocurrent and photovoltage produced by any particular device is
the fill factor, given by FF = P
max
/J
SC
V
OC
. The FF is represented in Figure 1.15 as the
37

fractional area of the unfilled gray rectangle (i.e. peak values for J and V, but available
power at these points = 0) occupied by the filled gray rectangle. Finally, the power
conversion efficiency ( η
P
) is calculated by η
P
= P
max
/E
total
or η
P
= (J
SC
V
OC
FF)/E
total
.

Another useful characterization for OPV devices is the external quantum
efficiency (EQE) measurement, which presents how efficiently the device converts
incident photons at a particular wavelength into electricity. The EQE is given by the
number of incident photons divided by number of generated charges.
1.4.2. Generalized Shockley equation for OPV
Working principles of a solar cell in the dark are similar to a diode. The
generalized Schockley equation for a single diode equivalent circuit model, which has

Figure 1.15. Typical J – V curves of a solar cell in the dark (dotted traces) and under
light (solid traces) in the log scale (upper part) and linear scale (lower part). Adapted
from ref. 4
38

been developed to describe conventional inorganic p-n junction solar cells, is applied to
OPVs.
86
The equivalent circuit shown in Figure 1.16 is comprised of: (i) a diode with
saturation current density (J
S
) and ideality factor n, which is close to 2 for organic
semiconductors; (ii) a current source (J
ph
), which corresponds to photocurrent upon
illumination; (iii) a series resistance (R
S
), which has to be minimized and takes into
account the finite conductivity of the organic semiconductors, the contact resistance
between the semiconductors and the adjacent electrodes, and the resistance associated
with electrodes and interconnections; and (iv) a shunt resistance (R
p
), which needs to be
maximized and takes into account the loss of carriers via possible leakage paths; the later
include structural defects such as pinholes in the film, or recombination centers caused by
impurities.
87

Thus, the generalized Shockley equation (Eq. 1.9), describes the current density
(J) vs. voltage (V) characteristics of organic solar cells:
4,86,88

(1.9)

Figure 1.16. Single diode equivalent circuit model for modeling solar cells.
Adapted from Ref. 4
39

where q is the elementary charge, k is the Boltzmann constant, and J
ph
(V) is the
voltage-dependent photocurrent density. For the solar cells with minimal leakage current
R
P
>> R
S
, Eq. 1.9 can be simplified to:
88

(1.10)
Important device characteristics of solar cells such as J
S
, R
S
and n can be obtained
by fitting the dark current-voltage values into Eq. 1.10. The saturation current due to
interface charge generation has been shown to vary exponentially with E
DA
, which is
represented in Eq. 1.11 for systems where J
S
is dominated by recombination, n 2.
86,88-90

(1.11)
The factor of 2 accounts for thermal generation of both an electron and a hole at
the DA interface which requires activation energy of E
DA
/2. Material properties
affecting the magnitude of J
S0
include the reorganization energy for the electron transfer
process, the intermolecular overlap at the DA interface, the layer conductivities, the area
of the DA interface, and the HOMO and LUMO energies of the donor and acceptor
materials.
4,88

Schlenker et al. proposed another expression for J
S
based on Marcus theory for
electron transfer in inverted region (see 1.3.3.2.1):
4

(1.12)
40

where k
rec
is the total rate constant (radiative plus nonradiative) for the recombination
reaction (D
+
A
-
) → D
0
+ A
0
, and [(D
+
A
-
)] is the CT state concentration at the DA
interface (see 1.3.3.2.1).
1.4.3. Ideal diode equation for organic heterojunctions

The generalized Shockley equation gives a reasonably accurate description;
however, it obscures the inherently different physics of organic semiconductors. For
example, the Shockley equation was initially derived for inorganic semiconductors with
energy band structure and loosely bound excitons, while organic semiconductors are
generally characterized by hopping transport and tightly bound excitons. Giebink et al.

Figure 1.17. (a) Energy-level diagram of OPV devices. Current is unipolar in the
donor (J
p
) and acceptor (J
n
) layers. (b) Processes occurring within the HJ region.
Adapted from ref. 6
∆E
DA
41

have developed an ideal diode equation for OPVs using the well-known physics of
organic materials and junctions.
6,91
In their model, it is assumed that the current is solely
governed by charge generation and recombination at the DA heterojunction and that both
processes proceed through the polaron pair (PP) intermediate state (or the D
+
A
-
CT state
described in section 1.2.2).
The schematic in Figure 1.17a defines the energetic of processes in OPV,
6
where
the interfacial gap, ∆E
DA
, is the difference between the HOMO of the donor and the
LUMO of the acceptor, along with any shift due to formation of an interface dipole.
Figure 1.17b exhibits the processes that occur within the HJ volume, a
0
. Excitons diffuse
with current density J
X
to the HJ and undergo charge transfer to form PPs. These may
recombine, at rate k
PPr
, or dissociate, with rate k
PPd
, as determined by the Onsager-Braun
model.
92
The current density, J, contributes to the interfacial free electron (n
I
) and hole
(p
I
) densities, which bimolecularly recombine to form PPs at rate k
rec
.
The ideal diode equation for an organic HJ in the simplest case (i.e. in the absence
of traps) is presented in Eq. 1.13:

(1.13)
42

where η
PPd
= k
PPd
/(k
PPd
+ k
PPr
) is the PP dissociation probability, V
a
is applied voltages,
and N
LU
and N
HO
are the densities of state at the acceptor LUMO and donor HOMO,
respectively. The last term in Eq. 1.13 represents the photocurrent generated by OPV
devices. Compared to the generalized Shockley Equation (Eq. 1.9), the ideal diode
equation (Eq. 1.13) is in similar form; however, Eq. 1.13 carries parameters that better
describe photophysical nature and electrical processes at the organic HJs.
1.5. Characteristics of OPV devices
In the previous sections, fundamental processes and characterization of OPVs
have been reviewed. In this section, we will consider key factors that affect the
characteristics that determine OPV efficiencies. In order to obtain efficient OPVs, it is
desirable to maximize all three device characteristics – V
OC
, J
SC
and FF.
1.5.1. Open voltage circuit
Open voltage circuit is the least understood parameter among three important
characteristics of OPVs. While the optical gaps of organic materials commonly used in
OPVs fall between 1.5 – 3.0 eV, the V
OC
of OPVs are significantly lower than the optical
gaps, e.g. 0.5 – 1.1 V.
30
The origin of photovoltage loss as well as factors determining
the V
OC
are very active area in OPV research.
4,6,7,91,93
In this section, V
OC
will be
considered in light of the generalized Shockley and the ideal diode equations.
The expression for open voltage circuit can be derived from the generalized
Shockley equation (see 1.4.2) assuming that R
P
→ ∞, R
S
→ 0 and J
ph
>> J
S
:
86,88-90

43

(1.14)
Inserting Eq. 1.11 into Eq. 1.14:
(1.15)
In order to obtain high V
OC
, the energy offset between the donor’s HOMO and
acceptor’s LUMO, ∆E
DA
, needs to be maximized while the J
S0
should be minimized (see
Eq. 1.15). Both ∆E
DA
and J
S0
can be extracted from experimental data and will be
discussed in the following sections.
Following Schlenker’s approach, V
OC
can be calculated by inserting Eq. 1.12 into
Eq. 1.14:
4

(1.16)
From Eq. 1.16, it is apparent that the V
OC
is reversely proportional to the
logarithm of both recombination rate, k
rec
, as well as the concentration of the CT states at
the DA interface, [(D
+
A
-
)]. The magnitude of [(D
+
A
-
)] is defined by the rate of charge
injection from the electrodes and the charge conductivities (both hole and electron) of
organic materials.
4
The rate of recombination is proportional to the square of the
electronic coupling of the CT state to the ground state V, and decreases exponentially
with increasing reorganization energy λ (see Eq. 1.6). Thus, device architectures,
fabrication conditions, molecular orientations, film morphologies and materials properties
that minimize V and [(D
+
A
-
)], as well as maximize λ will maximize V
OC
.
4
While
44

electronic coupling and reorganization energy values are hard to determine
experimentally, theoretical calculations offer great potential for studying molecular and
morphological influences on λ, V and consequently on V
OC
.
94

Open voltage circuit can be also determined from the ideal diode equation for
organic HJs (Eq. 1.13):
6

(1.17)
where E
B
is the PP binding energy,
max
is the maximum PP density (or max [(D
+
A
-
)])
that can be sustained at the interface. The logarithm of PP recombination rate, ln k
PPr
has
an inverse relationship with V
OC
. It was suggested that high performance devices with
minimal voltage losses can be achieved by minimizing k
PPr
, which maximizes V
OC
.
95
It
has been previously demonstrated that film morphology, including molecular structure
and orientation at the DA interface, significantly influences the kinetics of PP
dissociation and recombination and thus impacts V
OC
.
88,96,97
Adding steric bulk to the
donor
88
or acceptor
96
has been used to decrease orbital overlap between the donor and
acceptor at DA interface, which slows recombination, thus increasing V
OC
. Moreover,
decreasing orbital overlap at the DA interface through steric or structural control of the
donor and/or acceptor is also expected to decrease the PP binding energy, E
B
, further
contributing to the enhancement of V
OC
.
Both the generalized Shockley and ideal diode equations described above give a
qualitative analysis of V
OC
; however, understanding of how experimentally determined
variables affect V
OC
is desirable in order to design new materials and device architectures
45

for efficient OPVs with high V
OC
. In the following sections we will consider what
experimental variables can be used to predict V
OC
.
1.5.1.1. Energy offsets between donor’s HOMO and acceptor’s LUMO
The V
OC
expressions (Eq. 1.15 and 1.17) derived from the generalized Shockley
and ideal diode equations predict a linear relationship between photovoltage and the DA
energy offsets, ∆E
DA
. This ∆E
DA
can be experimentally determined from the HOMO and
LUMO energy levels of the donor and the acceptor, respectively. In agreement with this
prediction, a linear correlation of V
OC
to ∆E
DA
has been previously reported for several
materials systems.
90,98-100
Thus, for a given class of materials, tuning the electronic
structure of the donor or acceptor material to increase ∆E
DA
is an effective way to
increase V
OC
. For example, Meiss et al. have successfully utilized this strategy by using
F
4
ZnPc instead of ZnPc as a donor material in OPVs. Fluorination of the ZnPc molecule
leads to an increase of ionization energy, which increases ∆E
DA
subsequently resulting in
170 mV improvement of V
OC
in the F
4
ZnPc/C
60
device, while other characteristics such
as FF and J
SC
do not change.
101

On the other hand, increase of ∆E
DA
leads to a decrease in energy offset between
donor and acceptor LUMOs and/or HOMOs, which will subsequently result in inefficient
charge separation or increase of geminate charge recombination.
102,103
Thus, the balance
between increasing ∆E
DA
to achieve high V
OC
and ensuring enough thermodynamic
driving force for charge separation needs to be taken into account while optimizing
materials for OPVs.
46

1.5.1.2. Analysis based on electrical measurements
While ∆E
DA
provides a good thermodynamic consideration of V
OC
, the kinetics of
charge separation and charge recombination at the DA interface have been shown to have
a great impact on the photovoltage (see Eq. 1.15 – 1.17). The kinetic data can be
obtained by analyzing the dark current – voltage characteristics. Dark saturation current
J
S
, ideality factor n and series resistance R
S
are often obtained by fitting the dark J – V
curve to Eq. 1.10. To separate the effect of ∆E
DA
on V
OC
, the J
S0
values have been
calculated by Eq. 1.11 using fitted J
S
and n values.
88
Adding bulky substituents to
organic molecules is an effective way to minimize electronic coupling of the CT state to
the ground state, suppressing J
S0
and consequently increasing V
OC
. Perez
88
and Erwin
96

have shown that for a set of organic materials with a given ∆E
DA
, materials that contain
bulky groups or form amorphous films exhibit low J
S0
, which consequently leads to high
V
OC
. The marked difference in J
S0
for CuPc/C
60
and platinum tetraphenylbenzoporphyrin
(PtTPBP)/C
60
devices (1.5 10
4
and 12 mA/cm
2
, respectively) is due to their molecular
shape dissimilarity. While the flat molecule CuPc is expected to exhibit strong
intermolecular interactions with C
60
, the highly distorted saddle-shaped conformation of
PtTPBP decreases the degree of interaction with C
60
, leading to diminished charge
recombination, and hence a lower J
S0
. As the result, the PtTPBP device displays 0.2 V
higher V
OC
despite the lower ∆E
DA
than that of the CuPc device.
88
A similar result was
observed when perylendiimide derivatives (PDIs) were used as electron acceptor
materials in conjunction with CuPc donor; the device using PDI with bulkier substituents
exhibited a higher V
OC
.
96

47

Generally, devices having low J
S0
are found for materials with bulky substituents
and weak intermolecular interactions in thin films.
88,96
On the other hand, materials that
show weak intermolecular interactions often show poor carrier and exciton transport in
the thin films, ultimately leading to low J
SC
and FF. These contradictory characteristics
need to be taken into account while optimizing device performances. Two experimental
approaches have been recently developed to achieve optimally high V
OC
as well as high
J
SC
and FF.
43,104
In the first approach, a series of donor materials are constructed in a
cascade energy structure, where a thin layer of a donor material with low optical gap and
low J
S0
is placed at the DA interface to attain high V
OC
, and thicker layers of highly
ordered materials are used to obtain high J
SC
and FF.
104
In the second method, a donor
squaraine layer capped with C
60
layer was subjected to solvent annealing. As a result, the
bulk squaraine layer is crystalline with strong intermolecular interactions, while
molecules at the DA interface are largely amorphous due to the interaction with the top
C
60
layer. That particular design leads to significantly improved J
SC
and FF while V
OC

remains the same compared to unannealed devices.
43
Hence, similar approaches can be
used to optimize future devices.
1.5.2. Short circuit current
The short circuit current J
SC
of OPV devices is given by:
86

(1.18)
where q is the elementary charge, η
EQE
( λ ) is external quantum efficiency (EQE) and
Ф
ph
( λ ) is the photon flux density in the incident AM 1.5G spectrum (see Figure 1.9). For
48

convenient discussion, EQE is further broken into different components according to the
fundamental processes discussed in section 1.3:
(1.19)
where η
exc
( λ) is the exciton formation efficiency, η
CS
is the charge separation efficiency
(i.e. efficiency of the free charge formations from excitons) and η
CC
is the charge
collection efficiency, which involves charge conductivity and charge collection at
electrodes. η
exc
( λ) presents how efficiently the device absorbs light to form excitons that
successfully diffuse to the DA interface. Thus, to obtain high photocurrents, a device
should have the following characteristics: (i) broad and efficient light absorption, (ii) long
exciton diffusion length, (iii) efficient charge separation, (iv) high charge mobility and
(v) efficient charge collection at electrodes. The details of the processes (ii) – (v) have
been addressed in section 1.3. Efficient photon collection in a broad range of the solar
spectrum remains one of the big challenges for OPVs due to narrow absorption bands of
organic semiconductors. OPVs with tandem structure have been utilized to effectively
address the absorption problem.
105,106
In chapter 5, we will demonstrate that energy
sensitization is another effective way to broaden the absorption of a device.
1.5.3. Fill factor
The fill factor (FF) describes the fraction of photogenerated charge carriers that
reach the electrodes. Thus, the FF depends on the competition between charge carrier
recombination and collection as a function of the electrical field in the device, including
the applied voltage and the internal built-in field.
6,91,107,108
Among many factors that have
49

been shown to affect FF, the series resistances (R
S
) of the devices have a significant
influence. R
S
should be minimized in order to achieve high FF. Experimental data have
demonstrated that OPVs using polycrystalline organic films with highly ordered
structures exhibit good charge conductivity and high FF.
38,41,43,97,109-112

In addition, balance needs to be maintained between hole and electron mobility of
the donor and the acceptor layers, respectively, in order to obtain high FF.
108
Since the
conductivity of fullerene, the most commonly-used acceptor material, is high, the limited
hole mobility of some donor materials in OPV devices can cause S-shaped J - V curves,
which lowers FF.
113-115

1.6. Summary of topics
Despite impressive improvements in the field over the past decade, organic solar
cells still have the following limitations: (i) low exciton and charge conductivity of the
amorphous organic films. As a consequence, the films need to be thin, which leads to
low light capture efficiency. Thus, fabrication of polycrystalline films of organic
semiconductors with low defect density is desirable to improve both exciton diffusion
length and charge mobility. (ii) High exciton binding energy requires large energy offsets
(0.3 – 0.6 eV) between LUMOs (or HOMOs) of the donor and the acceptor in order to
achieve efficient charge separation. This energy cost for charge separation is substantial
lost in OPVs and can be lowered by designing new organic materials with low exciton
binding energies. (iii) Typical OPVs consist of two light-absorbing materials, in which
C
60
is used as the acceptor. One major drawback of C
60
is low absorption in the visible
50

solar spectrum, resulting in limited absorption efficiency of OPVs. It is thus desirable to
synthesize new electron acceptor materials that have high extinction coefficients or to
find new strategies to improve the absorption of the C
60
-based acceptor layer. (iv)
Stability of OPVs is often low. New device architectures as well as new materials need
to be developed to further improve the lifetime of OPVs.
This work will utilize both materials design and device optimization to address
the above limitation of OPVs. In Chapter 3, we present a chemical annealing method
used to convert the amorphous films to polycrystalline films and study the effect of
morphology change on device performance. In Chapter 4, the synthesis and
characterization of organic dyes that undergo symmetry breaking charge transfer are
shown. These compounds are expected to have lower exciton binding energies, thus
lowering the energy cost for charge separation in OPVs. Chapter 5 covers the energy
sensitization of C
60
and application in OPVs to improve absorption efficiency. While
OPVs with inverted structures have been shown to have improved lifetime, the efficiency
of inverted OPVs are often lower than the conventional OPVs. Study on device physics
of OPVs with inverted structures in Chapter 6 shed light on the origin of power loss and
serves as guidance to design new materials for inverted OPVs.

51

Chapter 1 Endnote
(1) Philibert, C. Solar Energy Perspectives; International Energy Agency (IEA):
Paris, France, 2011.
(2) Lin, C.-F.; Su, W.-F.; Wu, C.-I.; Cheng, I.-C. Organic, Inorganic and Hybrid
Solar Cells. Principles and Practice.; Willey: Hoboken, New Jersey, 2012.
(3) Turro, J. N.; Ramamurthy, V.; Scaiano, J. C. Principles of Molecular
Photochemistry: An introduction; University Science Book: Sausalito, California, 2009.
(4) Schlenker, C. W.; Thompson, M. E. Chemical Communications 2011, 47, 3702-
3716.
(5) Hains, A. W.; Liang, Z.; Woodhouse, M. A.; Gregg, B. A. Chemical Reviews
2010, 110, 6689-6735.
(6) Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R. Physical
Review B 2010, 82.
(7) Clarke, T. M.; Durrant, J. R. Chemical Reviews 2010, 110, 6736-6767.
(8) Bredas, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Accounts of Chemical
Research 2009, 42, 1691-1699.
(9) NASA Global Climate Change report, http://climate.nasa.gov, 2011.
(10) Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G. Nature 2012, 485,
486-U94.
(11) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.;
Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Graetzel, M. Science 2011, 334, 629-
634.
(12) Han, H.; Mayer, J. W.; Alford, T. L. Journal of Applied Physics 2006, 100.
(13) Antony, A.; Nisha, M.; Manoj, R.; Jayaraj, M. K. Applied Surface Science 2004,
225, 294-301.
(14) Yang, Y.; Huang, Q. L.; Metz, A. W.; Ni, J.; Jin, S.; Marks, T. J.; Madsen, M. E.;
DiVenere, A.; Ho, S. T. Advanced Materials 2004, 16, 321-324.
(15) Xue, J. G.; Forrest, S. R. Journal of Applied Physics 2004, 95, 1869-1877.
(16) Rider, D. A.; Harris, K. D.; Wang, D.; Bruce, J.; Fleischauer, M. D.; Tucker, R.
T.; Brett, M. J.; Buriak, J. M. Acs Applied Materials & Interfaces 2009, 1, 279-288.
(17) Chen, C.-P.; Tien, T.-C.; Ko, B.-T.; Chen, Y.-D.; Ting, C. Acs Applied Materials
& Interfaces 2009, 1, 741-745.
(18) Brumbach, M.; Veneman, P. A.; Marrikar, F. S.; Schulmeyer, T.; Simmonds, A.;
Xia, W.; Lee, P.; Armstrong, N. R. Langmuir 2007, 23, 11089-11099.
(19) Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Marder, S. R.; Mudalige, A.;
Marrikar, F. S.; Pemberton, J. E.; Armstrong, N. R. Journal of Physical Chemistry C
2008, 112, 7809-7817.
(20) Paramonov, P. B.; Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Armstrong, N.
R.; Marder, S. R.; Bredas, J.-L. Chemistry of Materials 2008, 20, 5131-5133.
(21) Hotchkiss, P. J.; Jones, S. C.; Paniagua, S. A.; Sharma, A.; Kippelen, B.;
Armstrong, N. R.; Marder, S. R. Accounts of Chemical Research 2012, 45, 337-346.
(22) Dabera, G. D. M. R.; Jayawardena, K. D. G. I.; Prabhath, M. R. R.; Yahya, I.;
Tan, Y. Y.; Nismy, N. A.; Shiozawa, H.; Sauer, M.; Ruiz-Soria, G.; Ayala, P.; Stolojan,
52

V.; Adikaari, A. A. D. T.; Jarowski, P. D.; Pichler, T.; Silva, S. R. P. Acs Nano 2013, 7,
556-565.
(23) Barnes, T. M.; Bergeson, J. D.; Tenent, R. C.; Larsen, B. A.; Teeter, G.; Jones, K.
M.; Blackburn, J. L.; van de Lagemaat, J. Applied Physics Letters 2010, 96.
(24) Pasquier, A. D.; Unalan, H. E.; Kanwal, A.; Miller, S.; Chhowalla, M. Applied
Physics Letters 2005, 87.
(25) De Arco, L. G.; Zhang, Y.; Schlenker, C. W.; Ryu, K.; Thompson, M. E.; Zhou,
C. Acs Nano 2010, 4, 2865-2873.
(26) Yang, D.; Zhou, L.; Chen, L.; Zhao, B.; Zhang, J.; Li, C. Chemical
Communications 2012, 48, 8078-8080.
(27) Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P. Nano Letters 2010, 10, 1276-
1279.
(28) Gaynor, W.; Lee, J.-Y.; Peumans, P. Acs Nano 2010, 4, 30-34.
(29) Yang, L.; Zhang, T.; Zhou, H.; Price, S. C.; Wiley, B. J.; You, W. Acs Applied
Materials & Interfaces 2011, 3, 4075-4084.
(30) Mishra, A.; Baeuerle, P. Angewandte Chemie-International Edition 2012, 51,
2020-2067.
(31) Tang, C. W. Applied Physics Letters 1986, 48, 183-185.
(32) Chen, Y.-H.; Lin, L.-Y.; Lu, C.-W.; Lin, F.; Huang, Z.-Y.; Lin, H.-W.; Wang, P.-
H.; Liu, Y.-H.; Wong, K.-T.; Wen, J.; Miller, D. J.; Darling, S. B. Journal of the
American Chemical Society 2012, 134, 13616-13623.
(33) Chiu, S.-W.; Lin, L.-Y.; Lin, H.-W.; Chen, Y.-H.; Huang, Z.-Y.; Lin, Y.-T.; Lin,
F.; Liu, Y.-H.; Wong, K.-T. Chemical Communications 2012, 48, 1857-1859.
(34) Lin, L.-Y.; Chen, Y.-H.; Huang, Z.-Y.; Lin, H.-W.; Chou, S.-H.; Lin, F.; Chen,
C.-W.; Liu, Y.-H.; Wong, K.-T. Journal of the American Chemical Society 2011, 133,
15822-15825.
(35) Fitzner, R.; Mena-Osteritz, E.; Mishra, A.; Schulz, G.; Reinold, E.; Weil, M.;
Koerner, C.; Ziehlke, H.; Elschner, C.; Leo, K.; Riede, M.; Pfeiffer, M.; Uhrich, C.;
Baeuerle, P. Journal of the American Chemical Society 2012, 134, 11064-11067.
(36) Wang, S.; Hall, L.; Diev, V. V.; Haiges, R.; Wei, G.; Xiao, X.; Djurovich, P. I.;
Forrest, S. R.; Thompson, M. E. Chemistry of Materials 2011, 23, 4789-4798.
(37) Wang, S.; Mayo, E. I.; Perez, M. D.; Griffe, L.; Wei, G.; Djurovich, P. I.; Forrest,
S. R.; Thompson, M. E. Applied Physics Letters 2009, 94, 233304.
(38) Wei, G.; Lunt, R. R.; Sun, K.; Wang, S.; Thompson, M. E.; Forrest, S. R. Nano
Letters 2010, 10, 3555-3559.
(39) Wei, G.; Wang, S.; Renshaw, K.; Thompson, M. E.; Forrest, S. R. Acs Nano
2010, 4, 1927-1934.
(40) Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R. Advanced Energy
Materials 2011, 1, 184-187.
(41) Wei, G.; Xiao, X.; Wang, S.; Sun, K.; Bergemann, K. J.; Thompson, M. E.;
Forrest, S. R. Acs Nano 2012, 6, 972-978.
(42) Wei, G.; Xiao, X.; Wang, S.; Zimmerman, J. D.; Sun, K.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Nano Letters 2011, 11, 4261-4264.
53

(43) Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Nano Letters 2012, 12, 4366-4371.
(44) Xiao, X.; Wei, G.; Wang, S.; Zimmerman, J. D.; Renshaw, C. K.; Thompson, M.
E.; Forrest, S. R. Advanced Materials 2012, 24, 1956-1960.
(45) Sonar, P.; Lim, J. P. F.; Chan, K. L. Energy & Environmental Science 2011, 4,
1558-1574.
(46) Erwin, P.; Thompson, M. E. Applied Physics Letters 2011, 98.
(47) Breeze, A. J.; Salomon, A.; Ginley, D. S.; Tillmann, H.; Horhold, H.; Gregg, B.
A. In Organic Photovoltaics Iii; Kafafi, Z. H., Ed. 2003; Vol. 4801, p 34-39.
(48) Kearns, D.; Calvin, M. Journal of Chemical Physics 1958, 29, 950-951.
(49) Bredas, J. L.; Cornil, J.; Heeger, A. J. Advanced Materials 1996, 8, 447-&.
(50) Forster, T. Annalen Der Physik 1948, 2, 55-75.
(51) Dexter, D. L. Journal of Chemical Physics 1953, 21, 836-850.
(52) Forrest, S. R. Chemical Reviews 1997, 97, 1793-1896.
(53) Haugeneder, A.; Neges, M.; Kallinger, C.; Spirkl, W.; Lemmer, U.; Feldmann, J.;
Scherf, U.; Harth, E.; Gugel, A.; Mullen, K. Physical Review B 1999, 59, 15346-15351.
(54) Guenes, S.; Neugebauer, H.; Sariciftci, N. S. Chemical Reviews 2007, 107, 1324-
1338.
(55) Huijser, A.; Suijkerbuijk, B. M. J. M.; Gebbink, R. J. M. K.; Savenije, T. J.;
Siebbeles, L. D. A. Journal of the American Chemical Society 2008, 130, 2485-2492.
(56) Armstrong, N. R.; Wang, W.; Alloway, D. M.; Placencia, D.; Ratcliff, E.;
Brumbach, M. Macromolecular Rapid Communications 2009, 30, 717-731.
(57) Lunt, R. R.; Giebink, N. C.; Belak, A. A.; Benziger, J. B.; Forrest, S. R. Journal
of Applied Physics 2009, 105.
(58) Giridharagopal, R.; Ginger, D. S. Journal of Physical Chemistry Letters 2010, 1,
1160-1169.
(59) Groves, C.; Reid, O. G.; Ginger, D. S. Accounts of Chemical Research 2010, 43,
612-620.
(60) Saini, S.; Bagchi, B. Physical Chemistry Chemical Physics 2010, 12, 7427-7433.
(61) Jackson, B.; Silbey, R. Journal of Chemical Physics 1983, 78, 4193-4195.
(62) Hennebicq, E.; Pourtois, G.; Scholes, G. D.; Herz, L. M.; Russell, D. M.; Silva,
C.; Setayesh, S.; Grimsdale, A. C.; Mullen, K.; Bredas, J. L.; Beljonne, D. Journal of the
American Chemical Society 2005, 127, 4744-4762.
(63) D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.;
Thompson, M. E. Organic Electronics 2005, 6, 11-20.
(64) Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E. Organic
Electronics 2009, 10, 515-520.
(65) Marcus, R. A. Journal of Chemical Physics 1956, 24, 966-978.
(66) Onsager, L. Physical Review 1938, 54, 554-557.
(67) Arkhipov, V. I.; Heremans, P.; Bassler, H. Applied Physics Letters 2003, 82,
4605-4607.
(68) Koehler, M.; Santos, M. C.; da Luz, M. G. E. Journal of Applied Physics 2006,
99.
54

(69) Arkhipov, V.; Bassler, H.; Emelyanova, E.; Hertel, D.; Gulbinas, V.; Rothberg, L.
Macromolecular Symposia 2004, 212, 13-24.
(70) Hertel, D.; Soh, E. V.; Bassler, H.; Rothberg, L. J. Chemical Physics Letters 2002,
361, 99-105.
(71) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.;
Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. Science
2012, 335, 1340-1344.
(72) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. Advanced Functional Materials
2009, 19, 1939-1948.
(73) Jailaubekov, A. E.; Willard, A. P.; Tritsch, J. R.; Chan, W.-L.; Sai, N.; Gearba,
R.; Kaake, L. G.; Williams, K. J.; Leung, K.; Rossky, P. J.; Zhu, X. Y. Nature Materials
2013, 12, 66-73.
(74) Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H. J.; Brida, D.;
Cerullo, G.; Lanzani, G. Nature Materials 2013, 12, 29-33.
(75) Bakulin, A. A.; Dimitrov, S. D.; Rao, A.; Chow, P. C. Y.; Nielsen, C. B.;
Schroeder, B. C.; McCulloch, I.; Bakker, H. J.; Durrant, J. R.; Friend, R. H. Journal of
Physical Chemistry Letters 2013, 4, 209-215.
(76) Gregg, B. A. Journal of Physical Chemistry Letters 2011, 2, 3013-3015.
(77) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bredas,
J.-L. Chemical Reviews 2007, 107, 926-952.
(78) Nelson, J.; Kwiatkowski, J. J.; Kirkpatrick, J.; Frost, J. M. Accounts of Chemical
Research 2009, 42, 1768-1778.
(79) Steirer, K. X.; Ndione, P. F.; Widjonarko, N. E.; Lloyd, M. T.; Meyer, J.; Ratcliff,
E. L.; Kahn, A.; Armstrong, N. R.; Curtis, C. J.; Ginley, D. S.; Berry, J. J.; Olson, D. C.
Advanced Energy Materials 2011, 1, 813-820.
(80) Steim, R.; Kogler, F. R.; Brabec, C. J. Journal of Materials Chemistry 2010, 20,
2499-2512.
(81) Ratcliff, E. L.; Zacher, B.; Armstrong, N. R. Journal of Physical Chemistry
Letters 2011, 2, 1337-1350.
(82) Ratcliff, E. L.; Meyer, J.; Steirer, K. X.; Garcia, A.; Berry, J. J.; Ginley, D. S.;
Olson, D. C.; Kahn, A.; Armstrong, N. R. Chemistry of Materials 2011, 23, 4988-5000.
(83) Ratcliff, E. L.; Lee, P. A.; Armstrong, N. R. Journal of Materials Chemistry 2010,
20, 2672-2679.
(84) MacDonald, G. A.; Veneman, R. A.; Placencia, D.; Armstrong, N. R. Acs Nano
2012, 6, 9623-9636.
(85) Armstrong, N. R.; Veneman, P. A.; Ratcliff, E.; Placencia, D.; Brumbach, M.
Accounts of Chemical Research 2009, 42, 1748-1757.
(86) Kippelen, B.; Bredas, J.-L. Energy & Environmental Science 2009, 2, 251-261.
(87) Dongaonkar, S.; Servaites, J. D.; Ford, G. M.; Loser, S.; Moore, J.; Gelfand, R.
M.; Mohseni, H.; Hillhouse, H. W.; Agrawal, R.; Ratner, M. A.; Marks, T. J.; Lundstrom,
M. S.; Alam, M. A. Journal of Applied Physics 2010, 108.
(88) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. Journal of the American
Chemical Society 2009, 131, 9281-9286.
55

(89) Li, N.; Lassiter, B. E.; Lunt, R. R.; Wei, G.; Forrest, S. R. Applied Physics Letters
2009, 94.
(90) Rand, B. P.; Burk, D. P.; Forrest, S. R. Physical Review B 2007, 75.
(91) Giebink, N. C.; Lassiter, B. E.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest,
S. R. Physical Review B 2010, 82.
(92) Braun, C. L. Journal of Chemical Physics 1984, 80, 4157-4161.
(93) Clarke, T. M.; Ballantyne, A.; Shoaee, S.; Soon, Y. W.; Duffy, W.; Heeney, M.;
McCulloch, I.; Nelson, J.; Durrant, J. R. Advanced Materials 2010, 22, 5287-5291.
(94) Yi, Y.; Coropceanu, V.; Bredas, J.-L. Journal of the American Chemical Society
2009, 131, 15777-15783.
(95) Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R. Physical
Review B 2010, 82.
(96) Erwin, P.; Thompson, M. E. Applied Physics Letters 2011, 98, 223305.
(97) Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Advanced Materials 2012, Submitted.
(98) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A.
J.; Brabec, C. L. Advanced Materials 2006, 18, 789-+.
(99) Vandewal, K.; Gadisa, A.; Oosterbaan, W. D.; Bertho, S.; Banishoeib, F.; Van
Severen, I.; Lutsen, L.; Cleij, T. J.; Vanderzande, D.; Manca, J. V. Advanced Functional
Materials 2008, 18, 2064-2070.
(100) Janssen, R. A. J.; Nelson, J. Advanced Materials 2012, 25, 1847-1858.
(101) Meiss, J.; Merten, A.; Hein, M.; Schuenemann, C.; Schaefer, S.; Tietze, M.;
Uhrich, C.; Pfeiffer, M.; Leo, K.; Riede, M. Advanced Functional Materials 2012, 22,
405-414.
(102) Servaites, J. D.; Savoie, B. M.; Brink, J. B.; Marks, T. J.; Ratner, M. A. Energy &
Environmental Science 2012, 5, 8343-8350.
(103) Servaites, J. D.; Ratner, M. A.; Marks, T. J. Energy & Environmental Science
2011, 4, 4410-4422.
(104) Schlenker, C. W.; Barlier, V. S.; Chin, S. W.; Whited, M. T.; McAnally, R. E.;
Forrest, S. R.; Thompson, M. E. Chemistry of Materials 2011, 23, 4132-4140.
(105) Lassiter, B. E.; Zimmerman, J. D.; Panda, A.; Xiao, X.; Forrest, S. R. Applied
Physics Letters 2012, 101, 063303.
(106) Riede, M.; Uhrich, C.; Widmer, J.; Timmreck, R.; Wynands, D.; Schwartz, G.;
Gnehr, W.-M.; Hildebrandt, D.; Weiss, A.; Hwang, J.; Sundarraj, S.; Erk, P.; Pfeiffer, M.;
Leo, K. Advanced Functional Materials 2011, 21, 3019-3028.
(107) Dibb, G. F. A.; Jamieson, F. C.; Maurano, A.; Nelson, J.; Durrant, J. R. Journal of
Physical Chemistry Letters 2013, 4, 803-808.
(108) Tress, W.; Merten, A.; Furno, M.; Hein, M.; Leo, K.; Riede, M. Advanced Energy
Materials 2013, Early view online.
(109) Placencia, D.; Wang, W. N.; Shallcross, R. C.; Nebesny, K. W.; Brumbach, M.;
Armstrong, N. R. Adv Funct Mater 2009, 19, 1913-1921.
(110) Placencia, D.; Wang, W. N.; Gantz, J.; Jenkins, J. L.; Armstrong, N. R. J Phys
Chem C 2011, 115, 18873-18884.
(111) Wang, W. N.; Placencia, D.; Armstrong, N. R. Org Electron 2011, 12, 383-393.
56

(112) Lee, O. P.; Yiu, A. T.; Beaujuge, P. M.; Woo, C. H.; Holcombe, T. W.; Millstone,
J. E.; Douglas, J. D.; Chen, M. S.; Frechet, J. M. J. Advanced Materials 2011, 23, 5359-+.
(113) Tress, W.; Petrich, A.; Hummert, M.; Hein, M.; Leo, K.; Riede, M. Applied
Physics Letters 2011, 98.
(114) Tress, W.; Pfuetzner, S.; Leo, K.; Riede, M. Journal of Photonics for Energy
2011, 1.
(115) Zhang, M.; Wang, H.; Tang, C. W. Applied Physics Letters 2011, 99.

57

CHAPTER 2. INSTRUMENTATION AND DEVICE FABRICATION

2.1 Instrumentation
Nuclear magnetic resonance (NMR) spectroscopy. NMR measurements were
performed on a Varian Mercury 400 MHz and Varian Mercury 500 MHz spectrometer.
Absorption. UV-vis spectra were recorded on a Hewlett-Packard 4853 diode
array spectrometer.
Photoluminescence. Steady state emission measurements of thin films and
solutions at room temperature and 77 K were performed using a Photon Technology
International QuantaMaster Model C-60SE spectrofluorimeter.
Photoluminescence measurements of C
60
and C
60
:ZCl films. No emission was
detected from neat C
60
and mixed C
60
:ZCl films using the Photon Technology
International QuantaMaster Model C-60SE spectrofluorimeter. The samples under
nitrogen atmosphere were excited by 514 nm emission lines from a Spectra-Physics
Stability 2017 argon ion laser. Photoluminescence was measured by imaging the excited
area of the sample onto the entrance slit of an Acton Research Spectrapro 500i
spectrograph coupled to a cooled Hamamatsu CCD detector. The acquired data were
corrected for the responses of the CCD array and the diffraction grating.
Luminescence lifetime. Luminescence lifetime measurements were performed by
a time-correlated single-photon counting method using an IBH fluorocube lifetime
58

instrument by equipped with a 405 nm LED excitation source with the IRF value of
0.4 ns.
Luminescence quantum efficiency. Quantum efficiency measurements were
carried out using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated
integrating sphere and model C10027 photonic multichannel analyzer.
Electrochemistry. Cyclic voltammetry (CV) and differential pulsed voltammetry
(DPV) measurements were performed using an EG&G Potentiostat/Galvanostat model
283. Dry dichloromethane from VWR or freshly distilled tetrahydrotetrafuran (THF)
were used as solvent under a N
2
atmosphere with 0.1 M Tetra(n-butyl)ammonium
hexafluorophosphate (Aldrich) as the supporting electrolyte. A glassy carbon working
electrode, a platinum counter electrode were used, with a silver wire as the pseudo
reference electrode. The oxidation potential was measured relative to a
Ferrocenium/Ferrocene (Fc
+
/Fc) redox couple as an internal standard.
Femtosecond transient absorption. Pump and probe pulses were obtained from
the output of a Ti:Sapphire regenerative amplifier (Coherent Legend, 1kHz, 4 mJ, 35 fs).
The excitation pulses centered at 500 nm were generated by pumping a type-II OPA
(Spectra Physics OPA-800C) with ~10 % of the amplifier 800 nm output and mixing the
resulting OPA signal output with the residual 800 nm pump in a type-II BBO crystal.
White light supercontinuum probe pulses, spanning the visible (320-950 nm) were
obtained by focusing a small amount of the amplifier output into a rotating CaF
2
disk.
The supercontinuum probe was collimated and focused with a pair of off-axis parabolic
59

mirrors into sample whereas the pump was focused before the sample position using a 25
cm CaF
2
lens. To avoid any contribution to the observed dynamics from orientational
relaxation, the polarization of the supercontinuum was set at the magic angle (54.7º) with
respect to the pump polarization. The cross correlation between pump and probe in a thin
1mm quartz substrate gave a FWHM of 180 fs for 500 nm excitation. The
supercontinuum probe was dispersed using a spectrograph (Oriel MS127I) onto a 256-
pixel silicon diode array (Hamamatsu) for multiplexed detection of the probe.
Nanosecond-to-millisecond transient absorption. These samples were sealed in 1-cm
x 1-cm quartz cuvettes with kontes valves to keep the solution air-free. The third harmonic of a
10 Hz Q-switched Nd: YAG laser (Spectra-Physics Quanta-Ray PRO-Series, pulse width: 8 ns)
was used to pump an optical parametric oscillator (Spectra-Physics Quanta-Ray MOPO-700),
tunable in the visible region. The excitation wavelength for each sample was chosen such that OD
(at λ excitation) = 0.3-0.4, and the laser power was attenuated to 3 mJ/pulse using a half-wave
plate and polarizer combination. For single-wavelength transient absorption kinetics
measurements, probe light was provided by a 75-W arc lamp operated in either continuous or
pulsed mode. Single wavelengths were selected by a double monochromator with 1 mm slits,
detected by a photomultiplier tube, and amplified and recorded with a transient digitizer.
Atomic force microscopy (AFM). AFM measurements were performed using a
Digital Instruments Nanoscope® Dimension 3100 atomic force microscope under the
tapping mode.
60

X-ray diffraction measurements (XRD). Thin film X-ray diffraction (XRD)
analyses were performed using a Rigaku Ultima IV diffractometer using Cu Kα radiation
source (λ = 1.54 Å).
Grazing incident X-ray diffraction (GIXD). GIXD measurements were
performed at the Stanford Synchrotron Radiation Lightsource (SSRL) on beamline 11-3
with a photon wavelength of 0.0974 nm. The diffraction intensity was detected on a 2-D
image plate (MAR 345) with a pixel size of 150 μm (2300 x 2300 pixels). GIXD images
were analyzed using the software package WxDiff, provided by Dr. Stefan Mannsfeld.
The samples were 20 mm long in the direction of the beam path, and the detector was
located 398.8 mm from the sample center. The beam incidence angle was 0.12° and the
beam size was 50 μm x 150 μm. The data presented here were corrected for the grazing
incidence geometry; further, polar angle plots were background subtracted and
constructed using both grazing incidence and local specular diffraction images.
1

Single crystal crystallography. Diffraction data for ZnTPP•tz in Chapter 3 were
collected on a Bruker SMART APEX CCD diffractometer with graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å). The cell parameters for the complexes were obtained
using the SMART program. The intensity data were processed using the Saint Plus
program. All of the calculations for the structure determination were carried out using
the SHELXTL package (Version 5.1). Absorption corrections were applied by using
SADABS. Hydrogen positions were input and refined in a riding manner along with the
attached carbons.
61

The X-ray intensity data in Chapters 4 and 5 were measured on a Bruker APEX II
CCD system equipped with a TRIUMPH curved-crystal monochromator and a MoKα
fine-focus tube (λ = 0.71073 Å). The frames were integrated with the Bruker SAINT
software package using a SAINT V7.68A algorithm. Data were corrected for absorption
effects using the multi-scan method (SADABS). The structure was solved and refined
using the Bruker SHELXTL Software Package.
2.2 Device Fabrication
2.2.1 Substrate cleaning
Glass substrates coated with indium doped tin oxide (ITO, thickness = 150 ±
10 nm; sheet resistance = 20 ± 5 Ω cm
−2
; transmission 84% at 550 nm; courtesy of Thin
Film Devices, Inc.) were cleaned with soap and boiled in tetrachloroethylene, acetone
and propanol (5 min each). ITO substrates were exposed to ozone atmosphere (UVOCS
T10X10/OES) for 10 min immediately before loading into the high vacuum chamber.
2.2.2 Vacuum vapor deposition
Thin films on glass/ITO substrates were fabricated by vapor thermal evaporation in
high vacuum (1–3 x 10
−6
torr) chambers (Kurt J. Lesker Co. or Angstrom). Evaporation rates
range from 0.1 – 0.2 nm/s for organic materials, 0.05 nm/s for MoO
3
and 0.2 – 0.4 nm/s for Al.
2.3 Device testing
Current-density dependence on applied test voltage J(V) measurements were performed
in air at 25 °C using a Keithley 2420 Sourcemeter (sensitivity = 100 pA) in the dark and under
62

ASTM G173-03 spectral mismatch corrected 100 mW/cm
2
white light illumination from an
AM1.5G filtered 300-W xenon arc lamp (Newport Oriel). Routine spectral mismatch correction
was performed using a silicon photodiode (Hamamatsu S1787-04,8RA filter) calibrated at the
National Renewable Energy Laboratory (NREL). Chopped and filtered monochromatic light
(250 Hz, 10 nm FWHM) from a Cornerstone 260 1/4 M double grating monochromator (Newport
74125) was used in conjunction with an EG&G 7220 lock-in amplifier to perform all spectral
responsivity and spectral mismatch correction measurements.
2

63

Chapter 2 Endnote
(1) Baker, J. L.; Jimison, L. H.; Mannsfeld, S.; Volkman, S.; Yin, S.; Subramanian,
V.; Salleo, A.; Alivisatos, A. P.; Toney, M. F. Langmuir 2010, 26, 9146-9151.
(2) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Adv Funct
Mater 2006, 16, 2016-2023.

64

CHAPTER 3. CHEMICAL ANNEALING AND APPLICATION TO ORGANIC
PHOTOVOLTAICS

3.1. Introduction
While performance of organic solar cells has been significantly improved,
approaching 9% in bulk heterojunction (BHJ) device structures,
1
there is still ample room
for improvement in OPV materials and device performance, as efficiencies as high as
25% may be possible for single junction OPVs.
3
As mentioned in Chapter 1, key
limitations of organic semiconductors include short exciton diffusion length and poor
charge conductivity. Production of crystalline films of molecular semiconductors with
ordered architectures is desirable for improving both exciton diffusion and charge
conductivity.
4-9

Solvent annealing
5,6,8,10-18
and thermal annealing,
4,9,16,17,19-23
are widely used to
prepare highly ordered, crystalline thin-films. In the solvent annealing method, organic
films are exposed to solvent vapors that permeate into the films, imparting sufficient

Figure 3.1. Scheme illustrating the chemical annealing process.

65

mobility to the molecules to promote crystallization.
4,6,8,10-17
Thermal annealing, in
which structural change of thin films is induced by heating, has been successfully
employed in small molecule and polymer bulk heterojunction OPVs.
4,16,17,19-21
However,
high temperature conditions are not desirable for devices employing thermally vulnerable
plastic substrates. Both solvent and thermal annealing methods are physical treatments,
changing the thin film morphology, but not affecting the chemical composition of the
film.
In contrast to solvent and thermal annealing methods for inducing crystallinity in
organic thin films, we present herein a chemical annealing method that leads to changes
in both morphology and chemical composition of organic films. The method relies on a
reaction between the thin film of an organic compound A with a vapor of ligand B to
form an A •B complex in the film, as depicted in Figure 3.1 for Zinc
Tetraphenylporphyrin (ZnTPP) and pyrazine. We have examined the effects of chemical
annealing on morphology, optical properties and chemical composition of the films of
ZnTPP. We see crystalline order develop in ZnTPP films upon chemical annealing with
pyrazine (pz) and triazine (tz). The coordination chemistry of ZnTPP with
nitrogen-based ligands has been studied in solution for many years,
2,24-35
with similar
reactions taking place with ZnTPP supported on oxide surfaces.
36
In the present case, the
coordination reaction promotes a solid-to-solid transformation in the ZnTPP film, which
affects both the morphology and the chemical composition simultaneously. The effects
of the chemical annealing process on the properties of ZnTPP donor layer in OPVs were
examined and are discussed below.
66

3.2. Optical properties

Exposure of ZnTPP films to nitrogen-based ligands leads to a rapid color change
and uptake of the ligands (see Figure 3.2). For example, the chemical annealing of a
100 nm film of ZnTPP with pyrazine vapor is complete within 10 minutes at room
temperature. Timed UV-vis absorption measurements for 15 nm thick ZnTPP films
exposed to vapors of pz and tz are presented in Figure 3.3, showing that the process for
the thinner film goes to completion in 2 min to 4 min. Similar bathochromic shifts are
observed for the Soret and Q-bands of ZnTPP as it reacts with py,
24,32
pz or tz in
solution
34
and in the chemical annealing process. However, the changes in line shape in
chemical annealing differ from what is observed for the solution reaction. The line
widths and shapes of the Soret and Q bands are unchanged on coordination of ligands to

Figure 3.2. Digital images of the ZnTPP film (1300 Å) treated with pyz over time: (a)
ZnTPP; (b) ZnTPP-pz, 1 min; (c) ZnTPP-pz, 2 min; (d) ZnTPP-pz, 5 min; (e) ZnTPP-
pz, 8 min; (f) ZnTPP-pz, 10 min.

d) e) f)
a)
b) c)
a) b) c)
d)
e) f)
67

ZnTPP in solution.
34
In contrast, the Soret and Q bands sharpen, and the lower energy
feature of the Q band intensifies as the reaction progresses in thin films. Similar changes
in optical properties are observed when ZnTPP films are annealed with other nitrogen
basic ligands, such as substituted pyridines, imidazole and trialkyl amine. These changes
in peak shape after chemical annealing suggest a structural change in the ZnTPP films.

Changes in absorption spectra of ZnTPP films upon exposure to vapors of the
noncoordinating solvent dichloromethane have been investigated for comparison with
coordinating ligands (Figure 1d). Unlike coordinating ligands, noncoordinating solvent
vapor causes slight blue shift of absorption maximum and broadening of the spectral

Figure 3.3. In situ UV-vis spectra of ZnTPP film during chemical annealing process
with different ligands and solvent: (a) pyridine (py), (b) pyrazine (pz), (c) triazine (tz)
and (d) dichloromethane. Insets are Q-band regions.

400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
540 560 580 600 620 640
0.00
0.04
0.08
0.12
0.16

Wavelength (nm)
Absorbance

t = 0
5 s
10 s
20 s
1min
a)
400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
540 560 580 600 620 640
0.00
0.04
0.08
0.12
0.16

Absorbance
Wavelength (nm)
t = 0
10 s
20 s
40 s
1 min
2 min
4 min
b)
400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
540 560 580 600 620 640
0.00
0.04
0.08
0.12
0.16

Absorbance

Wavelength (nm)

t = 0
20 s
40 s
1 min
2 min
4 min
c)
400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
520 540 560 580 600 620 640
0.00
0.04
0.08
0.12
0.16

Absorbance

Wavelength (nm)
t = 0
2 min
8 min
17 min
35 min
60 min
d)
68

features. There was only small change in absorbance intensity during solvent annealing
process.
3.3. Thin film composition
Films of ZnTPP (130 nm) were deposited on clean glass substrates. Then the
films were exposed to vapors of pz or tz for 5 hours at room temperature (26-30
o
C). A
control substrate was measured by XRD and UV-vis absorption to make sure that ligands
had completed coordination to the films. The films were then placed back to the
chamber, which was pumped down to 3×10
-6
torr to remove ligands that physically
absorb on the films. After that, the films were rinsed off using CD
3
Cl,
1
H and
13
C NMR
spectra were recorded.
ZnTPP•tz:
1
H NMR (400 MHz, CDCl
3
): δ 7.718 – 7.795 (m, 12H), 8.112
(broaden peak, 3H, Triazine), 8.204 – 8.227 (m, 8H), 8.933 (s, 8H).
13
C NMR (400
MHz, CDCl
3
): δ 121.045, 126.503, 127.448, 131.444, 142.857, 150.150, 164.936.
ZnTPP•pz:
1
H NMR (400 MHz, CDCl
3
): δ 7.023 (broaden peak, 4H, Pyrazine),
7.705 – 7.785 (m, 12H), 8.175 – 8.198 (m, 8H), 8.896 (s, 8H).
13
C NMR (400 MHz,
CDCl
3
): δ 120.871, 126.431, 127.362, 131.837, 134.462, 143.001, 143.376, 150.058.
Measurements of
1
H NMR spectra of solutions prepared by dissolving the films
after annealing with pz and tz show that the films contain ZnTPP and the ligand in a 1:1
ratio. An earlier study also showed that a 1:1 ZnTPP•pz complex is formed in solution
even in the presence of a 200-fold excess of pz.
34
Crystallization of ZnTPP•pz in the
presence of excess pz, however, leads to the formation of crystals consisting of
69

ZnTPP(pz)
2
, in which one pz is coordinated to Zn and the other is present in the
crystalline lattice in a well-defined site, but is not coordinated to Zn.
2

3.4. Thin film structure and morphology

ZnTPP films show signs of aggregation upon chemical annealing with the
nitrogen-based ligands examined here, as indicated by AFM (Figure 3.4). Before
chemical annealing, ZnTPP films are uniform, with an rms roughness of 0.6 nm. The
roughness of the film increases markedly as ZnTPP is chemically annealed. After 3 min
of chemical annealing with tz and pz, 10 nm films of ZnTPP show rms roughnesses of
2.1 nm and 6.7 nm, respectively. The rms roughness of the films continues to increase
over time during chemical annealing, growing to 3.3 nm and 14.4 nm after 10 min
treatment with tz and pz, respectively. Changes in surface roughness are graphically
illustrated by section analyses in Figure 3.4. An increase in roughness of organic films is

Figure 3.4. AFM images of 10 nm ZnTPP film on glass substrates: a) as deposited
(rms = 0.6 nm), and after 3 min treatment with b) pz (rms = 6.7 nm) and c) tz
(rms = 2.1 nm). d) shows the AFM image of a bilayer film composed of glass/10nm
NPD/ 10 nm ZnTPP after annealing with pz for 3 min minutes. (rms = 4.7 nm).

0 nm
7.5 nm
15 nm
0
-7
7
nm
0 μm 1 2 3
Section analysis
0 nm
25 nm
50 nm
0 μm 1 2 3
0
-20
20
nm
Section analysis
0 nm
25 nm
50 nm
0 μm 1 2 3
0
-20
20
nm
Section analysis
0 nm
5 nm
10 nm
0 μm 1 2 3
0
-5
5
nm
Section analysis
c) d)
a)
b)
70

also commonly observed after thermal or solvent annealing processes. For example, the
rms roughnesses of metal–phthalocyanine (TiOPc and InClPc) and squaraine films
increased by a factor of 2 as a result of solvent or thermal annealing.
4-8

Changes in film morphology have been further characterized by XRD studies on
100 nm ZnTPP films before and after chemical annealing with pz and tz. The diffraction
patterns of ZnTPP, ZnTPP•pz and ZnTPP•tz films are shown in Figure 3.5. Before

Figure 3.5. XRD patterns of 100 nm ZnTPP films (a) before (blue) and after (red)
annealing with pz, with simulated powder pattern shown as black bars (from single
crystal structure of ZnTPP•2pz
2
), and (b) after annealing with tz (blue), with the
simulated powder pattern for ZnTPP•tz shown as black bars. Also shown are the
Packing diagrams of single crystals of (c) ZnTPP•2pz,
2
and (d) ZnTPP•tz, with the
(1,-1,3) and (2,0,2) planes, respectively, shown in green. The H atoms and phenyl
rings are omitted for clarity.
71

chemical annealing, the ZnTPP film is amorphous with no diffraction peaks visible.
After chemical annealing, the films become structurally ordered with clearly resolved
diffraction peaks.

It is useful to compare the simulated diffraction patterns from single crystals of
ZnTPP•tz and ZnTPP•2pz
2
with data for the corresponding chemically annealed films
(Figure 3.5). The most intense peaks in XRD patterns of ZnTPP•pz and ZnTPP•tz films
result from X-ray diffraction from the (1,-1,3) and (2,0,2) planes, respectively, which

Figure 3.6. 2D-GIXD chemically annealed films, a) 50 nm of ZnTPP•pz and b) NPD
(10 nm)/ZnTPP•pz (50 nm). Integrating a cake slice from 1.26 ≥ q ≥ 1.37 Å
-1
vs.
polar angle, χ, shows the angular dependence of the (1,-1,3), star, and superposition of
the (2,-1,2) and (1,-2,2), dagger, diffraction planes.
72

contain the porphyrin rings, as illustrated in Figure 3.5, c) and d). The interplanar
distances corresponding to those diffraction peaks are 0.48 nm and 0.47 nm, respectively.
The fact that these lattice planes are the ones enhanced in the XRD pattern suggests that
these lattice planes are parallel to the substrate and diffraction from adjacent crystals by
these lattice planes is reinforced by a similar orientation of adjacent crystals relative to
the substrate.
This was confirmed by 2D-GIXD measurements on ZnTPP film after chemical
annealing with pz (Figure 3.6). Since films with a N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-
1,1′-biphenyl)-4,4′-diamine (NPD) layer between the metal oxide electrode and ZnTPP
layer were used in the OPV studies (see below), measurements were carried out on
chemically annealed ZnTPP films without (Figure 3.6 a) and with (Figure 3.6 b) the NPD
under-layer. The strong diffraction arc centered at q = 1.31 Å
-1
seen for both samples
corresponds to diffraction from the (1,-1,3), (2,-1,2) and (1,-2,2) planes. By integrating a
cake slice over q from 1.26 ≥ q ≥1.37 Å
-1
the intensity vs. polar angle, χ, for these lattice
planes was determined (Figure 3.6 c). The peak at 81.7° corresponds to the angular
orientation of the (1,-1,3) planes while the peak centered at 61° corresponds to the
angular position of a superposition of the (2,-1,2) and (1,-2,2) diffraction peaks. Due to
the fact that the (1,-1,3) lattice plane contains the porphyrin ring (figure 3.5 d) we have
used the angular dependence of this diffraction spot to determine the molecular
orientation within the film and have found that the average orientation of the ZnTPP
molecular plane is 8.2° off parallel to the plane of the substrate after chemical annealing
of both films (with and without NPD layer).
73

Furthermore, the addition of the NPD under-layer suppresses the overall
crystallinity of the ZnTPP film during chemical annealing. The relative degree of
crystallinity was determined by integration of the intensity arising from the (1,-1,3) peak
with respect the chi, indicating that films treated on the NPD layer are roughly half as
crystalline as films without the under-layer. These experimental results clearly show
reorganization of disordered ZnTPP films upon chemical annealing to an ordered
structure with nearly parallel porphyrin macrocycles. The combined effects of increased
crystallinity and ligation from pz and tz lead to significant changes in the UV-vis spectra
of chemically annealed ZnTPP films.
3.5. Effect of chemical annealing on OPV devices.
To explore the effect of chemical annealing on OPV performance, bilayer devices
D1-D8 were prepared with different donor structures and a common device architecture,
as shown in Figure 3.7. After deposition of ZnTPP thin films, the substrates were
chemically annealed with ligands for 4 min under nitrogen atmosphere due to the
sensitivity of ZnTPP to oxygen. After chemical annealing, substrates were loaded back
into the chamber, and C
60
(40 nm) and BCP (10 nm) were deposited. Finally, masks with
1 mm diameter circles were placed on substrates under air, and 100 nm of Al cathode was
deposited. The thicknesses of donor layers given in Figure 3.7 correspond to the
thicknesses of the films after deposition, but before chemical annealing. Current
density-voltage (J-V) curves measured in the dark and under illumination are shown in
Figure 3.8, with the device characteristics given in Figure 3.7.
74

The AM 1.5G based efficiencies of these OPV devices ( ) are low due to poor
overlap of the ZnTPP and ZnTPP•pz absorption spectra with the solar spectrum (see
Figure 3.3). The purpose of this work is to examine effect of chemical annealing on OPV
performance, however, not to achieve high efficiency under solar illumination. The use

Device
J
SC
(mA/cm
2
)
V
OC
(V) FF (%)
R
S

( /cm
2
)
Js
(nA/cm
2
)
n
D1 1.97±0.02 0.853±0.002 0.37±0.00 0.62 ±0.01 94.0±35.0 50±20 2.1±0.1
D2 1.94±0.04 0.334±0.019 0.51±0.02 0.33±0.03 0.9±0.2 2600±600 1.5±0.1
D3 2.44±0.03 0.637±0.005 0.48±0.00 0.74 ±0.01 0.4±0.1 6±3 1.4±0.1
D4 1.90±0.03 0.810±0.003 0.37±0.00 0.57 ±0.01 58.0± 10 80±40 2.1±0.1
D5 2.37±0.03 0.573±0.029 0.48±0.01 0.65± 0.01 0.4±0.2 3±1 1.5±0.2
D6

1.92±0.05 0.860±0.003 0.37±0.01 0.62 ±0.01 -
a
-
a
-
a

D7 1.95±0.01 0.380±0.005 0.52±0.01 0.38 ±0.00 0.4±0.1 1300±100 1.4±0.1
D8 2.44±0.02 0.786±0.005 0.40±0.01 0.70 ±0.01 1.7±0.2 25±3 1.9±0.1

Figure 3.7. OPV device structures and performance characteristics under AM1.5G
illumination. ZnTPP·pz corresponds to ZnTPP after chemical annealing with pz.
a
Could not be fitted.
ITO
ZnTPP(150A)
C
60
40nm
BCP 10nm
Al
donor layer (D1-D8)
Device ITO/Donor layer(s)
D1 ZnTPP (15 nm)
D2 ZnTPP·pz (15 nm)
D3 ZnTPP·pz (10 nm)/ ZnTPP (5 nm)

D4 NPD (10 nm)/ ZnTPP (15 nm)
D5 NPD (10 nm)/ ZnTPP·pz (15 nm)

D6 ZnTPP (20 nm)
D7 ZnTPP·pz (20 nm)
D8 ZnTPP·pz (10 nm)/ ZnTPP (10 nm)


75

of the chemical annealing method with other metal complexes,
37-39
which have better
overlap with the solar spectrum, may improve on the efficiencies reported here.

Devices with chemically annealed ZnTPP show significant changes in fill factor
(FF) and open circuit voltage (V
OC
) when compared with the related un-annealed devices.
Comparing D1 to D2 shows that chemically annealed devices give increased FF, but
lower V
OC
, while the J
sc
is largely unaffected. A similar trend is observed for D4 and D5,
however, J
sc
is higher in the chemically annealed device. The improvement of FF in the
ZnTPP•pz based devices is attributed to improved charge conductivity in the chemically
annealed devices. D2 and D5 show series resistance values, R
S
, that are two orders of
magnitude lower than D1 or D4, due to the increased crystallinity after chemical
annealing with pz. It was generally shown that increased crystallinity of active layers in
organic electronic devices resulted in better charge conductivity.
40-42
Charge mobility of

Figure 3.8. Current – Voltage characteristics under the dark (open symbols) and
under AM 1.5 illumination (filled symbols) of devices D1 – D8.
-1.0 -0.5 0.0 0.5 1.0
-3
-2
-1
0
1
2
3

Current density (mA/cm
2
)
Voltage (V)
D1
D2
D3
D4
D5
-1.0 -0.5 0.0 0.5 1.0
-3
-2
-1
0
1
2
3

Current density (mA/cm
2
)
Voltage (V)
D6
D7
D8
a) b)
76

organic thin film transistors was increased up to several orders of magnitude due to
improved crystallinity of the active layer as results of post annealing processes.
41-45

The decrease of V
OC
upon chemical annealing is significant, dropping by 60% in
D2 compared to the un-annealed device, D1. The V
OC
is related to the difference in
energy between the donor’s Highest Occupied Molecular Orbital (HOMO) and the
acceptor’s Lowest Unoccupied Molecular Orbital (LUMO) ( E
HL
).
46-48
This energy
separation sets the maximum V
OC
achievable for a given donor/acceptor pair, and the
measured V
OC
is often correlated with this value. If coordination of pz to ZnTPP
decreases the zinc porphyrin ionization potential relative to ZnTPP alone, the E
HL
and
thus the V
OC
would be expected to decrease in the ZnTPP•pz based OPVs. A good
approximation of the change in the HOMO energy of ZnTPP films before and after
chemical annealing with pz is the difference in oxidation potentials of ZnTPP and

Figure 3.9. Cyclic voltammetry diagrams of ZnTPP (E
1/2
ox
= 0.38 V) and ZnTPP:pz
(1:10) (E
1/2
ox
= 0.33 V) in dichloromethane solution under N
2
, vs Fc
+
/Fc. Scan rate
100 mV s
-1
. The signal at 0.0 V is the Fc
+
/Fc reference.
-1.0 -0.5 0.0 0.5 1.0
-50.0µ
0.0
50.0µ
100.0µ
150.0µ

Current (A)
Voltage (V)
ZnTPP
ZnTPP:pz 1:10
77

ZnTPP•pz measured in solution. Cyclic voltammetric (CV) measurements show that
coordination of pz to ZnTPP decreases the first oxidation potential by 50 mV (see
Figure 3.9). This decrease in E
HL
is too small to account for the 500 mV drop in V
OC

observed for D2. Thus, while the decrease in E
HL
for the chemically annealed film
contributes to the decrease in V
OC
, it is not the major source of the decrease.

There are two other logical explanations for the decrease in V
OC
observed for D2,
i.e. that the low V
OC
in D2 is the result of direct contact between C
60
and ITO surface
and/or enhanced charge recombination at the donor/acceptor (D/A) heterojunction; both
of which can increase the dark current. A single layer device ITO/C
60
(40 nm)/BCP
(10 nm)/Al has open circuit of 0.46 V (Figure 3.10), so gaps in the donor layer that lead
to ITO-C
60
contact will result in marked lowering of the V
OC
. Significant drops in V
OC

-1.0 -0.5 0.0 0.5 1.0
-3
-2
-1
0
1
2
3
0.87 0.46 0.41 0.17
J
SC
V
OC
FF 
Light
Dark

Current density (mA/cm
2
)
Voltage (V)

Figure 3.10. Characteristics of device ITO/C
60
(40 nm)/BCP (10 nm)/Al under
AM 1.5G illumination. Inset is characteristics of the devices with J
SC
of
± 0.02 mA/cm
2
, V
OC
of ± 0.01 V, FF of ± 0.01 and  of ± 0.01 %.
78

have been observed for OPV devices with solvent or thermally annealed donor layers,
which have excessively rough surfaces.
4,7
The donor layers in these devices have
aggregated sufficiently that the anode is exposed, leading to direct contact of vapor
deposited C
60
to ITO, giving rise to a shunt leakage path. Shunt leakage in annealed
devices is expected to give a marked increase in the dark current and a concomitant
decrease in the V
OC
. There is a well-established inverse relationship between the dark
current and V
OC
(V
OC
 ln(J
SC
/J
S
), J
S
= saturation dark current).
49-54
The role of direct
ITO-C
60
contact and the resulting shunt leakage in lowering V
OC
were explored with
devices D2, D4 and D5. To prevent direct contact between C
60
and ITO, a 10 nm layer of
NPD was placed between ZnTPP•pz and ITO. NPD films are not affected by treatment
with pz vapor, so we expect them to form a continuous layer under ZnTPP•pz, preventing
direct contact of C
60
with the ITO electrode. The NPD layer also leads to less roughening
of the ZnTPP•pz film, as seen in the AFM (Figures 3.4 b and 3.4 d). The presence of
NPD partially suppresses crystallization of ZnTPP, but does not affect molecular
orientation as indicated by 2D-XRD measurements (Figure 3.6). The NPD layer is a
hole-transporting material, widely used in organic light emitting diodes, and does not
significantly affect the performance of devices before chemical annealing (compare D4 to
D1). After chemical annealing with pz, device D5 has a reduced dark current (J
S
), and a
concomitant increase in V
OC
to 570 mV. This is a marked increase relative to D2, but is
still well short of the V
OC
values observed for the unannealed devices, D1 and D4. We
attribute the increase in J
SC
(2.37 mA/cm
2
) of the chemically annealed device D5 to a
longer exciton diffusion length in the ordered ZnTPP•pz film and/or to a more efficient
79

charge collection, expected for the more crystalline film. Attempt to measure exciton
diffusion length by the spectral resolved photoluminescence quenching technique
55
was
not successful due to the increasingly rough ZnTPP•pz film. It is generally observed that
conversion of amorphous organic films into crystalline films improves exciton diffusion
length. Lunt et al.
55
reported a strong relationship between exciton diffusion length and
degree of crystallinity of an organic semiconductor, 3,4,9,10-perylenetetracarboxylic
dianhydride (PTCDA). Exciton diffusion length of the crystalline PTCDA film was
shown to be fourfold higher than that of the amorphous film.
55
The increase of exciton
diffusion length and charge mobility due to highly crystalline organic films have been
shown to improve short circuit current and fill factor in OPV devices.
4,56-58
Thus, placing
a layer of NPD beneath the ZnTPP•pz film prevents direct contact between ITO/C
60

while allowing the benefits of a highly ordered crystalline film to be realized. However,
the dark current of D5 is still larger than that of D1, resulting in a smaller V
OC
.
While the decrease in E
HL
and direct ITO-C
60
contact can partially account for
the decreased V
OC
values observed on chemical annealing, the kinetics of carrier
recombination at the D/A interface has to be considered to completely understand the
origin of low V
OC
in these devices (see 1.3.3.3 and 1.5.1 for detailed discussion). Giebink
et al. have recently developed an ideal diode equation for organic heterojuncitons, using
the physics of organic materials and junctions, which provides an excellent framework to
consider how chemical annealing may affect V
OC
values of the devices presented here.
59

The expression of V
OC
in the simplest case (i.e. no interfacial traps) is given in Eq. 1.17
(Chapter 1).
80

After chemical annealing, the ZnTPP molecules are reorganized, and form a
crystalline film with porphyrin macrocycles oriented nearly parallel to the substrate
(Figures 3.5 and 3.6). The molecular order induced by chemical annealing of ZnTPP
exposes the molecular -orbitals of the porphyrin directly to the C
60
acceptor layer. This
molecular orientation enhances orbital overlap between donor and acceptor, which is
expected to increase k
PPr
and thus decrease V
OC
.
51
Moreover, better orbital overlap
between donor and acceptor will likely stabilize the PP, thus leading to higher PP binding
energy. Bredas et al. estimated that the greater orbital overlap of Pentacene
+
/C
60
-
PP in a
configuration with C
60
interacting with the -face of pentacene leads to 0.2 eV
stabilization of the PP energy compared with a structure that involves C
60
interacting end-
on with the pentacene molecule.
60
Chemical annealing of ZnTPP with pz gives a highly
ordered film; however, the obtained orientation enhances k
PPr
and E
B
, thus leads to the
observed lowering of the V
OC
relative to un-annealed devices. To attain high V
OC
, a
disordered ZnTPP film is preferable to an ordered one with an orientation that enhances
k
PPr
, as some faction of the disordered film will have ZnTPP oriented in a way to reduce
k
PPr
. Thus, it was expected that placing a thin layer of amorphous ZnTPP film on the
chemically annealed film would reduce orbital overlap between donor and acceptor
molecules, leading to a reduction in the PP recombination rate and increase in V
OC
.
Insertion of 5 nm ZnTPP on 10 nm ZnTPP•pz in D3 indeed shows improvement in V
OC

relative to D5, while retaining a good FF (0.48) and J
SC
(2.44 mA/cm
2
). Note that
placing a layer of ZnTPP on top of a chemically annealed ZnTPP•pz film coats exposed
ITO, eliminating shunt leakage paths through direct C
60
-ITO contact, as well as providing
81

a largely disordered ZnTPP interface with C
60
. Even though D3 has a V
OC
value 200 mV
lower than for D1, morphology improvements resulting in better J
SC
and FF have led to a
20% increase of the power conversion efficiency compared with the device without
chemical annealing (D1).
The relationship between molecular ordering at D/A interface and V
OC
values is
observed for D3-D5. These three devices are compared to eliminate the effect of shunt
leakage in the comparison. We expect D4 to have the least ordered ZnTPP surface,
followed by D3 (some order may be passed from the ZnTPP•pz through the 5 nm ZnTPP)
and lastly D5 is expected to have the most order, albeit half that of ZnTPP•pz in D2.
This is the ordering observed for the V
OC
values as well: D4 > D3 > D5.
The thin 5 nm layer of ZnTPP in D3 may not be thick enough to fully cover the
rough surface of ZnTPP•pz film (see Figure 3.4), or completely eliminate structure
directing effects of the underlying ZnTPP•pz layer. Both of these problems may be
solved by deposition of a thicker amorphous ZnTPP film on ZnTPP•pz. To test this
hypothesis, we have fabricated 3 devices with 20 nm donor layers of ZnTPP: a device
without chemical annealing (D6), a chemically annealed device (D7) and a device with a
10 nm layer of ZnTPP deposited on a chemically annealed ZnTPP•pz film (10 nm) (D8)
(Figure 3.7). As expected, D6 and D7 give parameters very similar to their 15 nm
ZnTPP counterparts, D1 and D2, respectively. The 10 nm amorphous ZnTPP layer in
D8, however, gives a significant shift in performance relative to D3. The V
OC
of D8 is
within 60 mV of the value for D6, while J
SC
remains as high as in D5 and D3.
82

Unfortunately, adding a thicker amorphous ZnTPP film results in higher resistivity of D8,
giving rise to a somewhat lower FF (Figure 3.8) and thus efficiency close to that of D3.
The effects of chemical annealing with triazine have been considered in a series
of devices D9 – D11 (Figure 3.11). Since tz induces similar crystallinity of the films,
results are similar to devices chemically annealed with pz.

3.6. Conclusions and Outlook
We have presented a chemical annealing process wherein exposure of ZnTPP
films to vapors of different ligands can proceed quantitatively and under mild conditions
(room temperature and ambient pressure) to form ZnTPP•ligand films, causing marked
changes in the optical properties and morphology of the films. The conversion of
-1.0 -0.5 0.0 0.5 1.0
-3
-2
-1
0
1
2
3

Current density (mA/cm
2
)
Voltage (V)
D1
D9
D10
D11

Figure 3.11. Current – Voltage characteristics under the dark (open symbols) and
under AM 1.5 illumination (filled symbols) chemically annealed with triazine. The
structures of the donor layers are: D1 - ZnTPP (15 nm); D9 - ZnTPP·tz (15 nm);
D10 - ZnTPP·tz (15 nm)/ZnTPP (5 nm); D11 - ZnTPP·tz (10 nm)/ZnTPP (10 nm)
83

amorphous films to highly crystalline films containing porphyrin macrocycles nearly
parallel to the substrate, has been observed and characterized by XRD techniques. These
morphological changes give rise to good conductivity, longer exciton diffusion length,
and therefore to improved J
SC
and FF of OPV devices. At the same time, the structural
change observed for ZnTPP•pz results in a low V
OC
, due to increased PP recombination
rate at the D/A heterojunction. Proper placement of an amorphous or poorly ordered
organic layer reduced the dark saturation current, J
S
while maintaining the benefits of
enhanced photocurrent J
SC
and FF, compensating for the loss in V
OC
and resulting in a
15-20% improvement of device efficiency.
While the structural change induced on chemical annealing ZnTPP with nitrogen-
basic ligands led to a crystal orientation in the thin film that resulted in a reduced V
OC
,
this may not be the case for all ligands and metal complexes that are subjected to
chemical annealing. It is possible that chemical annealing could lead to crystal
orientations in the thin film that enhance V
OC
. An edge on orientation of the donor
molecules at the D/A interface is expected to lead to slow PP recombination and high V
OC

for OPVs with such a structure.
60-64
We are currently exploring the effects of chemical
annealing on other organic, metalorganic and organometallic compounds to explore the
generality of this approach to forming highly ordered thin films and its utility for
enhancing the performance of OPVs.

84

Chapter 3 Endnote
(1) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y.
Adv Mater 2011, 23, 4636-4643.
(2) Byrn, M. P.; Curtis, C. J.; Hsiou, Y.; Khan, S. I.; Sawin, P. A.; Tendick, S. K.;
Terzis, A.; Strouse, C. E. J Am Chem Soc 1993, 115, 9480-9497.
(3) Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R. Physical
Review B 2011, 83.
(4) Wei, G.; Lunt, R. R.; Sun, K.; Wang, S.; Thompson, M. E.; Forrest, S. R. Nano
Lett 2010, 10, 3555-3559.
(5) Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Advanced Materials 2012, Submitted.
(6) Placencia, D.; Wang, W. N.; Shallcross, R. C.; Nebesny, K. W.; Brumbach, M.;
Armstrong, N. R. Adv Funct Mater 2009, 19, 1913-1921.
(7) Placencia, D.; Wang, W. N.; Gantz, J.; Jenkins, J. L.; Armstrong, N. R. J Phys
Chem C 2011, 115, 18873-18884.
(8) Wang, W. N.; Placencia, D.; Armstrong, N. R. Org Electron 2011, 12, 383-393.
(9) Wei, G.; Xiao, X.; Wang, S.; Sun, K.; Bergemann, K. J.; Thompson, M. E.;
Forrest, S. R. Acs Nano 2012, 6, 972-978.
(10) Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R. Advanced Energy
Materials 2011, 1, 184-187.
(11) Adams, D. M.; Kerimo, J.; Olson, E. J. C.; Zaban, A.; Gregg, B. A.; Barbara, P. F.
J Am Chem Soc 1997, 119, 10608-10619.
(12) Chen, F. C.; Ko, C. J.; Wu, J. L.; Chen, W. C. Sol Energ Mat Sol C 2010, 94,
2426-2430.
(13) Conboy, J. C.; Olson, E. J. C.; Adams, D. M.; Kerimo, J.; Zaban, A.; Gregg, B.
A.; Barbara, P. F. J Phys Chem B 1998, 102, 4516-4525.
(14) Del Cano, T.; Parra, V.; Rodriguez-Mendez, M. L.; Aroca, R. F.; De Saja, J. A.
Appl Surf Sci 2005, 246, 327-333.
(15) Gregg, B. A. J Phys Chem-Us 1996, 100, 852-859.
(16) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y.
Nat Mater 2005, 4, 864-868.
(17) Peet, J.; Senatore, M. L.; Heeger, A. J.; Bazan, G. C. Advanced Materials 2009,
21, 1521-1527.
(18) De Luca, G.; Treossi, E.; Liscio, A.; Mativetsky, J. M.; Scolaro, L. M.; Palermo,
V.; Samori, P. Journal of Materials Chemistry 2010, 20, 2493-2498.
(19) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv Funct Mater 2005,
15, 1617-1622.
(20) Walker, B.; Tomayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garcia, A.;
Tantiwiwat, M.; Nguyen, T. Q. Adv Funct Mater 2009, 19, 3063-3069.
(21) Wei, G.; Wang, S.; Renshaw, K.; Thompson, M. E.; Forrest, S. R. Acs Nano
2010, 4, 1927-1934.
85

(22) Wei, G.; Xiao, X.; Wang, S.; Zimmerman, J. D.; Sun, K.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Nano Lett 2011, 11, 4261-4264.
(23) Wei, G.; Xiao, X.; Wang, S.; Sun, K.; Bergemann, K. J.; Thompson, M. E.;
Forrest, S. R. Acs Nano 2012, 6, 972-976.
(24) Miller, J. R.; Dorough, G. D. J Am Chem Soc 1952, 74, 3977-3981.
(25) Dorough, G. D.; Miller, J. R.; Huennekens, F. M. J Am Chem Soc 1951, 73, 4315-
4320.
(26) Kadish, K. M.; Shiue, L. R. Inorganic Chemistry 1982, 21, 3623-3630.
(27) Kadish, K. M.; Shiue, L. R.; Rhodes, R. K.; Bottomley, L. A. Inorganic
Chemistry 1981, 20, 1274-1277.
(28) Vogel, G. C.; Beckmann, B. A. Inorganic Chemistry 1976, 15, 483-484.
(29) Kolling, O. W. Inorganic Chemistry 1979, 18, 1175-1176.
(30) Drago, R. S.; Kroeger, M. K.; Stahlbush, J. R. Inorganic Chemistry 1981, 20,
306-308.
(31) Kirksey, C. H.; Hambrigh.P; Storm, C. B. Inorganic Chemistry 1969, 8, 2141-&.
(32) Nappa, M.; Valentine, J. S. J Am Chem Soc 1978, 100, 5075-5080.
(33) Lin, C. L.; Fang, M. Y.; Cheng, S. H. Journal of Electroanalytical Chemistry
2002, 531, 155-162.
(34) Danger, B. R.; Bedient, K.; Maiti, M.; Burgess, I. J.; Steer, R. P. Journal of
Physical Chemistry A 2010, 114, 10960-10968.
(35) Humphrybaker, R.; Kalyanasundaram, K. Journal of Photochemistry 1985, 31,
105-112.
(36) Crouch, A. M.; Langford, C. H. Journal of Electroanalytical Chemistry 1987,
221, 83-93.
(37) Diev, V. V.; Hanson, K.; Zimmerman, J. D.; Forrest, S. R.; Thompson, M. E.
Angewandte Chemie (International Ed.) 2010, 49, 5523-5526.
(38) Diev, V. V.; Schlenker, C. W.; Hanson, K.; Zhong, Q.; Zimmerman, J. D.;
Forrest, S. R.; Thompson, M. E. The Journal of Organic Chemistry 2012, 77, 143-159.
(39) Zimmerman, J. D.; Yu, E. K.; Diev, V. V.; Hanson, K.; Thompson, M. E.;
Forrest, S. R. Org Electron 2011, 12, 869-873.
(40) Karl, N. Synthetic Metals 2003, 133, 649-657.
(41) Shtein, M.; Mapel, J.; Benziger, J. B.; Forrest, S. R. Appl Phys Lett 2002, 81, 268-
270.
(42) Lee, S. S.; Loo, Y.-L. In Annual Review of Chemical and Biomolecular
Engineering, Vol 1; Prausnitz, J. M. D. M. F. S. M. A., Ed. 2010; Vol. 1, p 59-78.
(43) Chou, W. Y.; Cheng, H. L. Adv Funct Mater 2004, 14, 811-815.
(44) Dickey, K. C.; Anthony, J. E.; Loo, Y.-L. Adv Mater 2006, 18, 1721-+.
(45) Kang, S. J.; Noh, M.; Park, D. S.; Kim, H. J.; Whang, C. N.; Chang, C. H. Journal
of Applied Physics 2004, 95, 2293-2296.
(46) Brumbach, M.; Placencia, D.; Armstrong, N. R. J Phys Chem C 2008, 112, 3142-
3151.
(47) Rand, B. P.; Burk, D. P.; Forrest, S. R. Physical Review B 2007, 75.
86

(48) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A.
J.; Brabec, C. L. Advanced Materials 2006, 18, 789-794.
(49) Schlenker, C. W.; Thompson, M. E. Chemical Communications 2011, 47, 3702-
3716.
(50) Potscavage, W. J.; Yoo, S.; Kippelen, B. Appl Phys Lett 2008, 93, 193308.
(51) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. J Am Chem Soc 2009,
131, 9281-9286.
(52) Li, N.; Lassiter, B. E.; Lunt, R. R.; Wei, G.; Forrest, S. R. Appl Phys Lett 2009,
94, 023307.
(53) Kippelen, B.; Bredas, J.-L. Energy & Environmental Science 2009, 2, 251-261.
(54) Erwin, P.; Thompson, M. E. Appl Phys Lett 2011, 98, 223305.
(55) Lunt, R. R.; Benziger, J. B.; Forrest, S. R. Adv Mater 2010, 22, 1233.
(56) Vasseur, K.; Rand, B. P.; Cheyns, D.; Froyen, L.; Heremans, P. Chemistry of
Materials 2010, 23, 886-895.
(57) Yang, J.; Zhu, F.; Yu, B.; Wang, H.; Yan, D. Appl Phys Lett 2012, 100.
(58) Chen, W.; Huang, L.; Qiao, X.; Yang, J.; Yu, B.; Yan, D. Org Electron 2012, 13,
1086-1091.
(59) Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R. Physical
Review B 2010, 82.
(60) Yi, Y. P.; Coropceanu, V.; Bredas, J. L. J Am Chem Soc 2009, 131, 15777-15783.
(61) Hoermann, U.; Wagner, J.; Gruber, M.; Opitz, A.; Bruetting, W. Physica Status
Solidi-Rapid Research Letters 2011, 5, 241-243.
(62) Ojala, A.; Petersen, A.; Fuchs, A.; Lovrincic, R.; Poelking, C.; Trollmann, J.;
Hwang, J.; Lennartz, C.; Reichelt, H.; Hoeffken, H. W.; Pucci, A.; Erk, P.; Kirchartz, T.;
Wuerthner, F. Adv Funct Mater 2012, 22, 86-96.
(63) Wagner, J.; Gruber, M.; Hinderhofer, A.; Wilke, A.; Broeker, B.; Frisch, J.;
Amsalem, P.; Vollmer, A.; Opitz, A.; Koch, N.; Schreiber, F.; Bruetting, W. Adv Funct
Mater 2010, 20, 4295-4303.
(64) Yokoyama, D.; Wang, Z. Q.; Pu, Y.-J.; Kobayashi, K.; Kido, J.; Hong, Z. Sol
Energ Mat Sol C 2012, 98, 472-475.

87

CHAPTER 4. SYMMETRY BREAKING CHARGE TRANSFER AND
APPLICATION TO ORGANIC PHOTOVOLTAICS

4.1. Introduction
Photoinduced charge transfer (CT) via symmetry breaking (SB) plays a crucial
role in photosynthetic reaction centers in living systems.
1-8
In such systems, which
contain two or more virtually identical and symmetric chromophores, CT from one
chromophore to another occurs upon electronic-excitation, producing SB charge
separation. Of great potential interest, but less well explored, are SBCT processes in
organic photovoltaics (OPV) and related systems,
9-17
where absorption in the visible
spectrum is desirable. Among well-documented compounds exhibiting SB phenomena
are bianthryl derivatives,
18-26
but they do not absorb visible light. Indeed, very few
organic dyes that absorb in the visible region undergo SBCT processes.
8,27-31
We have
investigated the photophysics of zinc dipyrrins (Figure 4.1) that exhibit intense visible
absorptions in different organic solvents. Of interest is that these compounds undergo
photoinduced SBCT processes in weakly polar to polar solvents. Zinc dipyrrins and
analogous compounds are attractive because, in addition to strong absorption in the
visible region of the spectrum, their syntheses are easy and scalable, and the large body
of work on boron dipyrromethene (BODIPY) can be exploited.
32,33

As introduced in Chapter 1, the low dielectric constants of organic materials used
in OPVs lead to high exciton binding energies, and a large energy offset between donor
88

and acceptor is required to promote charge separation-generation at the donor/acceptor
interface (D/A).
34
Use of organic dyes that undergo SBCT processes might be a potential
solution to reduce the energy cost for exciton dissociation to free charges at D/A. The
polar environment at D/A may be sufficient to induce SBCT in the chromophore, leading
to spontaneous formation of internal CT excitons. Charge separation between electron
donor and acceptor materials from these CT excitons is expected to proceed with lower
energetic requirements than for typical Frenkel excitons found in organic materials.

Fluorescent metallodipyrrins have attracted considerable attention due to their
potential use as probes for sensing metal ions (in particular, Zn
2+
) in living systems.
35,36

However, in spite of the increasing number of reported metallodipyrrins,
33,37-47
their
photophysics are not well understood. Unlike the highly fluorescent BODIPYs,
32,33

homoleptic complexes MD
n
(D = dipyrrin or -extended dipyrrin ligands)
39,40,42,43,45

generally exhibit low to moderate fluorescent quantum yield (QY). In contrast, the

Figure 4.1. Structure of homoleptic (Z1-Z4) and heteroleptic (Z5-6) zinc dipyrrin
complexes.
89

heteroleptic complexes MDX
n
(X is an ancillary ligand) were shown to have quite high
QY (up to 80%).
37-41,46,47

Several nonradiative deactivation pathways have been suggested for the
photoinduced excited state of homoleptic zinc dipyrrins and -extended dipyrrins.
Lindsey and coworkers
43
reported that rotation of the phenyl ring at the meso-position on
the dipyrrin ligand is a source of nonradiative deactivation of the excited state.
Replacement of phenyl with the more bulky mesityl substituent hinders this rotation,
leading to an improved quantum efficiency of 36% in toluene.
43
A later study on other
mesityl-substituted zinc dipyrrins showed that the QY of the homoleptic complexes was
strongly dependent on solvent polarity, dropping from 20-30 % in toluene to 0-5 % in
dichloromethane (DCM).
40
The authors assumed that non-emissive charge separated
states (i.e. D
+
-Zn-D
-
) formed in polar solvents, decreasing the fluorescence QY;
40

however, no experimental data was provided to support this assumption. Strong excitonic
coupling between ligands has been rationalized as another nonradiative deactivation of
zinc -extended dipyrrin complexes, which contain nonorthogonal ligands.
39
However,
excitonic coupling
48,49
between the nearly orthogonal ligands in homoleptic zinc dipyrrin
compounds Z1 and Z2 is negligible,
50,51
and thus cannot be used to explain the lower QY
compared to their heteroleptic counterparts. We have determined that SBCT in polar
solvents is an effectively nonradiative decay pathway for the electronic excited states of
zinc dipyrrins. In nonpolar solvents such as cyclohexane, the homoleptic zinc dipyrrins
do not undergo SBCT, thus exhibiting even higher QY than their heteroleptic analogs.
90

4.2. Synthesis and characterization
Homoleptic complexes Z1-4. Heteroleptic and homoleptic complexes of 5-
mesityl dipyrrin zinc complexes were chosen for study to eliminate substituent rotation as
a nonradiative deactivation pathway.
43
Representative synthetic schemes for preparing
homoleptic (Z1-Z4) and heteroleptic (Z5, Z6) complexes are presented in Figure 4.2.
The homoleptic complexes Z2-4 have been successfully synthesized in a one pot reaction
starting from the corresponding pyrrole and mesitaldehyde with total yields from 8-13%.

2,4-dimethylpyrrole (98%) and 3-ethyl-2,4-dimethylpyrrole were purchased from
TCI America; 2-methylpyrrole was synthesized by published procedure; synthesis of Z1
followed the procedures in literature.
42
The obtained Z1 was further purified by
sublimation under vacuum. Syntheses of Z2-3 were modified from the literature
procedures for Z1. The synthetic procedure for preparing Z2 is given below; the same
synthesis was used for Z3 and Z4.

Figure 4.2. Synthesis of (a) homoleptic (Z2) and (b) heteroleptic (Z5) complexes
91

Bis(1,9-dimethyl-5-mesityldipyrrinato) zinc (Z2). A solution of mesitaldehyde
(4.6 g, 30.9 mmol) and 2-methylpyrrole (5 g, 61.7 mmol) in 200 ml dichloromethane was
prepared and 3 drops of trifluoroacetic acid (TFA) were added to the rapidly stirred
solution under nitrogen. After stirring for 6 hours, the reaction was quenched with 3 ml
of triethylamine. The reaction mixture was washed with a saturated solution of Na
2
CO
3
in
water (100 ml, 3 times), and brine (100 ml, 1 time), and dried over anhydrous Na
2
SO
4
.
The solvent was then removed under reduced pressure to obtain the viscous pale yellow
liquid which solidifies upon standing at room temperature; the yield was quantitative.
The product 1,9-dimethyl-5-mesityldipyrromethane was used without further purification.
A solution of 1,9-dimethyl-5-mesityldipyrromethane in 250 ml freshly distilled
tetrahydrofuran (THF) was prepared, and a solution of 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) (7.02 g, 30.9 mmol) in 35 ml of THF was added slowly to the
stirred solution under nitrogen. After stirring for 1 hour, the reaction was quenched with
10 ml of triethylamine. The solvent was removed under reduced pressure. The product
mixture was dissolved in 500 ml of dichloromethane, and the solution was washed with
saturated NaHCO
3
solution (250 ml, 3 times) and brine (250 ml, 1 time). The solution
was then dried over anhydrous Na
2
SO
4
and filtered. The product 1,9-dimethyl-5-
mesityldipyrromethene was used for the next step without further purification.
A solution of zinc acetate dihydrate (Zn(OAc)
2
•2H
2
O) (20 g, 91 mmol) in 100 ml
of methanol was added to the solution of 1,9-dimethyl-5-mesityldipyrromethene in
dichloromethane under air. The reaction mixture was stirred overnight. After that, the
reaction mixture was filtered using filter paper, and the solvents were removed under
92

reduced pressure yielding a dark red solid. The obtained solid was passed through a short
neutral alumina plug using hexanes/dichloromethane (7/3) mixture as eluent; the orange-
red portion was collected. The solvents were removed, and the solid was recrystallized
from DCM/MeOH, yielding 2.5 g of dark green solid Z2 (12.3% total yield). The
obtained Z2 product was further purified by gradient sublimation under ultra high
vacuum (10
-5
torr) at 220 C – 160 C – 120 C gradient temperature zones.
1
H NMR
(400 MHz, CDCl
3
): ppm 6.92 (s, 4H), 6.46-6.43 (m, J = 4.25 Hz, 4H), 6.13 (d, J = 3.94
Hz, 4H), 2.37 (s, 6H), 2.14 (s, 12H), 2.11 (s, 12H) matches well with published data.
50

Bis(1,3,7,9-tetramethyl-5-mesityldipyrrinato) zinc (Z3). 3.0 g of orange-red solid
(13% total yield). The obtained Z3 was further purified by gradient sublimation under
ultra high vacuum (10
-5
torr) at 230 C – 160 C – 120 C gradient temperature zones.
1
H
NMR (500 MHz, CDCl
3
): ppm 6.93 (s, 4H), 5.91 (s, 4H), 2.35 (s, 6H), 2.12 (s, 12H),
2.04 (s, 12H), 1.31 (s, 12H).
13
C NMR (500 MHz, CDCl
3
): ppm 155.90, 143.63,
143.15, 137.35, 136.22, 135.57, 134.54, 128.73, 119.56, 21.21, 19.26, 16.12, 14.83.
HSMS: calcd for C
44
H
51
N
4
Zn (MH
+
): 699.3400, found: 699.3407. C, H, N elemental
analysis for C
44
H
51
N
4
Zn: calcd (%) C (75.47), H (7.20), N (8.00); found (%) C (75.84),
H (7.27), N (8.06).
Bis(2,8-diethyl-1,3,7,9-tetramethyl-5-mesityldipyrrinato) zinc (Z4). 0.8 g of
orange-red solid (8% total yield). The obtained Z4 was further purified by gradient
sublimation under ultra high vacuum (10
-5
torr) at 250 C – 160 C – 120 C gradient
temperature zones.
1
H NMR (500 MHz, CDCl
3
): ppm 6.92 (s, 4H), 2.36 (s, 6H), 2.25 (q,
J = 7.40 Hz, 8H), 2.11 (s, 12H), 1.97 (s, 12H), 1.19 (s, 12H), 0.91 (t, J = 7.49 Hz, 12H).
93

13
C NMR (500 MHz, CDCl
3
): ppm 154.68, 142.03, 137.19, 137.10, 137.03, 136.00,
134.17, 130.66, 128.55, 21.24, 19.47, 17.92, 15.25, 14.35, 11.75. HSMS: calcd for
C
52
H
67
N
4
Zn (MH
+
) 811.4652, found 811.4658. CNH analysis for C
52
H
66
N
4
Zn: calcd (%)
C (76.87), H (8.19), N (6.90); found C (76.98), H (8.35), N (6.97).
Synthesis of heteroleptic complexes Z5-6. The heteroleptic complexes
disproportionate in solution to form homoleptic complexes, i.e. Zn(dipyrrin)
2
and
Zn( -diketonate)
2
, as seen for other heteroleptic Zn complexes.
39,52
Of the three ancillary
ligands we examined, 2,2,6,6-tetramethyl-3,5-heptanedione (DPM), pentane-2,4-dione
and 1,1,1,5,5,5-hexafluoro-pentane-2,4-dione, pure heteroleptic complexes were
successfully isolated only with the DPM ligand. Representative procedure for Z5 is the
following.
(2,2,6,6-tetramethyl-3,5-heptanedionato)-(1,9-dimethyl-5-mesityldipyrrinato) zinc
(Z5). Concentrated HCl (4 drops) was added to a solution of Z2 (96 mg, 0.149 mmol) in
15 ml dichloromethane. The reaction mixture was rigorously shaken for 30 seconds, then
washed with brine (2 times), and Na
2
CO
3
solution (2 times). The obtained solution of
ligand in dichloromethane was dried over anhydrous Na
2
CO
3
and filtered. The dried
dipyrrin ligand was used without further purification.
A solution of bis-(2,2,6,6-tetramethyl-3,5-heptanedionato)-zinc, Zn(DPM)
2

(150 mg, 0.347 mmol) in a mixture of dichloromethane/methanol (40/40 ml) was
prepared, and slowly added to a solution of the dipyrrin ligand in 15 ml dichloromethane
over the course of 5 minutes while stirring. The reaction mixture became brightly
94

emissive upon addition of Zn(DPM)
2
. To prevent disproportionation of Z5, the solvents
were immediately removed under reduced pressure after addition of Zn(DPM)
2,
yielding
an orange red solid. The obtained orange solid was dissolved in 10 ml dichloromethane,
and 15 ml of methanol was layered on top. This solution was stored for 2 days at -5 C.
After 2 days, two distinct types of red crystals formed: large orange red crystals
(heteroleptic complex) and small cubic dark red crystals (homoleptic complex). The
large orange red crystals were isolated under microscope, yielding 18 mg of heteroleptic
complex Z5. The obtained crystal was suitable for crystal structure determination by X-
ray diffraction. Unlike the homoleptic complex Z2, the heteroleptic product Z5 in
dichloromethane is highly emissive under UV-lamp illumination.
1
H NMR (400 MHz,
CDCl
3
, ppm): 6.89 (s, 2H), 6.42-6.35 (m, 2H), 6.15-6.10 (m, 2H), 5.83-5.80 (m, 1H),
2.35 (s, 3H), 2.23 (s, 6H), 2.14 (s, 6H), 1.19 (s, 18H).
13
C NMR (400 MHz, CDCl3,
ppm): 203.52, 158.63, 143.85, 138.78, 136.86, 136.81, 135.34, 131.87, 127.41, 116.86,
89.60, 41.73, 28.40, 21.09, 20.06, 16.79. HSMS: calcd for C
31
H
41
N
2
O
2
Zn (MH
+
):
537.2454, found: 527.2463.
(2,2,6,6-tetramethyl-3,5-heptanedionato)-(1,3,7,9-tetramethyl-5-mesityl
dipyrrinato) zinc (Z6). Similar procedure was used for synthesis of Z6. An orange solid
mixture of heteroleptic and homoleptic complexes was obtained. Unlike the weakly
emissive solid of other zinc dipyrrins, this orange solid mixture has very bright yellow
emission under UV-illumination. The heteroleptic complex was separated from the
homoleptic one by gradient sublimation under ultrahigh vacuum (10
-5
torr) at 170 C –
120 C – 50 C gradient temperature zones (note that Z3 sublimes at higher temperature –
95

230 C, see above), yielding of Z6 (50% yield). A suitable crystal from the sublimed
solid was used for crystal structure determination.
1
H NMR (400 MHz, CDCl
3
, ppm):
6.91 (s, 2H), 5.92 (s, 2H), 5.79 (s, 1H), 2.33 (s, 3H), 2.13 (s, 12H), 1.28 (s, 6H), 1.18 (s,
18H).
13
C NMR (400 MHz, CDCl3, ppm): 203.25, 155.90, 144.19, 144.06, 137.46,
135.83, 135.71, 134.87, 128.75, 119.62, 89.46, 41.65, 28.38, 21.19, 19.68, 16.48, 14.81.
HSMS: calcd for C
33
H
45
N
2
O
2
Zn (MH
+
): 565.2767, found: 565.2764.
Heteroleptic Z5 and Z6 disproportionate to homoleptic complexes in chloroform
over the course of several hours as observed by NMR measurements (see Figures 4.3 and
4.4). However, Z5 and Z6 are stable in the solid state and can be sublimed in vacuum,
without disproportionation. Formation of Z5 and Z6 has been confirmed by crystal
structure analysis (see below).
96

Figure 4.3.
1
H NMR of Z5 in CDCl
3
over time: Freshly prepared solution (upper) and
solution in NMR tube after 4 hours (lower), estimated molar ratio of
heteroleptic:homoleptic complexes (Z5:Z2 = 15:1)
ppm (f1)
1.50 2.00
0
1000
2000
3000
4000
5000
6000
2.345
2.227
2.142
1.545
1.196
1.193
2.97
5.98
5.98
18.18
7.15
ppm (f1)
6.00 6.50 7.00
0
500
1000
1500
2000
2500
6.888
6.393
6.383
6.127
6.117
5.817
5.814
2.00
1.95
1.99
0.98
//
ppm (f1)
1.50 2.00
0
1000
2000
3000
4000
5000
6000
2.345
2.227
2.142
1.545
1.196
1.193
2.97
5.98
5.98
18.18
7.15
//
ppm (f1)
6.00 6.50
0
500
6.922
6.890
6.448
6.438
6.395
6.386
6.130
6.120
5.817
5.730
2.00
0.28
2.00
1.00
2.25
0.27
0.10
ppm (f1)
1.50 2.00
0
500
1000
1500
2000
2500
3000
2.369
2.347
2.230
2.144
2.111
1.549
1.196
3.09
0.39
6.21
6.65
0.82
8.18
21.02
ppm (f1)
1.50 2.00
0
500
1000
1500
2000
2500
3000
2.369
2.347
2.230
2.144
2.111
1.549
1.196
3.09
0.39
6.21
6.65
0.82
8.18
21.02
// //
Homoleptic Z2
97

Figure 4.4.
1
H NMR of Z6 in CDCl
3
over time: Freshly prepared solution (upper) and
solution in NMR tube after 8 hours (lower), estimated molar ratio of complexes Z6:Z3
= 13:1
ppm (t1)
6.00 6.50
0
1000
2000
3000
4000
5000
6000
7000
6.905
5.924
5.791
2.05
1.01
2.00
ppm (t1)
1.50 2.00
0
1000
2000
3000
4000
5000
6000
7000
2.327
2.133
1.551
1.276
1.179
3.39
18.14
5.96
12.17
ppm (t1)
6.00 6.50
0
1000
2000
3000
4000
5000
6000
7000
6.905
5.924
5.791
2.05
1.01
2.00
// //
ppm (t1)
6.00 6.50
0
1000
2000
3000
4000
6.928
6.905
5.924
5.905
5.791
5.731
2.00
0.31
2.00
0.25
0.99
0.11
ppm (t1)
6.00 6.50
0
1000
2000
3000
4000
6.928
6.905
5.924
5.905
5.791
5.731
2.00
0.31
2.00
0.25
0.99
0.11
ppm (t1)
1.50 2.00
0
1000
2000
3000
4000
2.343
2.326
2.133
2.112
2.031
1.553
1.303
1.276
1.179
3.04
12.31
0.71
0.84
0.54
20.87
6.54
1.04
3.33
// //
Homoleptic Z3
98

4.3. Crystal structure

Structures of representative homoleptic (Z3 and Z4) and heteroleptic (Z5 and Z6)
complexes are shown in Figure 4.5. The structures of Z3 and Z4 are similar to published
structures of Z1 and Z2.
50
The Zn-N bond lengths are similar in Z1-6 complexes, ranging
from 1.95 to 1.99 Å. To evaluate the degree of distortion of the complexes, dihedral
angles between the planes encompassing different groups of atoms have been measured
as shown in Figure 4.5 and Table 1. Compared to BODIPYs, which are essentially flat,
53

the distortion of the dipyrrin-zinc framework is more significant with dihedral angles
between the pyrrole planes 1 and 2 varying from 3 to 18 degrees. The zinc center in each
complex adopts a tetrahedral configuration with the two ligands held nearly
perpendicular to each other. The angles between two dipyrrin ligands are 83, 88, 82 and
86 degrees for Z1-4, respectively. However, there is no clear correlation between

Figure 4.5. ORTEP diagrams of (a) Z3, (b) Z4, (c) Z6 and (d) Z5 at 50% probability
level. H atoms are omitted for clarity. Planes containing different groups of atoms are
indicated by the colored lines
2
3
4
1
5
b)
1
2
3
4
5
2
3
4
1
5
a)
c)
a) b)
d)
99

alkylation and distortion of the dipyrrin framework in Z1-4, and the distortion may be the
result of crystal packing forces.
Table 4.1. Dihedral angles (degrees) between planes in zinc dipyrrins Z1-6.
Planes Z1
(a)
Z2
(a)
Z3 Z4 Z5 Z6
1,2 5.3
3.1
10.4
8.5
9.6
6.5
9.2
8.8
4.4
-
12.1
-
3,4 7.3
3.9
2.4
2.1
9.8
3.3
6.7
2.8
1.2
-
11.9
-
5,1 18.0
5.9
7.3
4.5
8.5
7.4
14.1
6.3
5.7
-
8.5
-
5,2 14.4
2.9
6.3
4.4
10.9
3.5
5.2
7.3
5.1
-
8.5
-
5,3 17.8
6.1
6.2
1.7
7.1
6.0
11.8
5.3
5.4
-
10.2
-
5,4 14.7
4.3
3.8
1.7
10.9
4.5
5.2
7.3
4.7
-
10.2
-
5,5

83.4 87.9 82.1 86.4 89.1 86.5
(a)
From Ref.
50

4.4. Electronic structure
In order to gain insight into the electronic structure of heteroleptic and homoleptic
complexes, theoretical calculations in the gas phase have been performed at the
B3LYP/LACVP** level of theory using Titan program. Structures of the complexes
have been fully optimized; highest occupied molecular orbital (HOMO), lowest
unoccupied molecular orbital (LUMO) and corresponding energies are shown in
Figure 4.6. The HOMO of the homoleptic complex localizes predominantly on the two
100

dipyrrin ligands without participation of the orbitals of the Zn metal, while the LUMOs
with E symmetry are doubly degenerate and localize on separate ligands (Figure 4.6).

Figure 4.6. Theoretical study on zinc dipyrrin complexes carried out at
B3LYP/LACVP** level of theory. LUMOs (mesh) and energies of (a) Z1, degenerate
E states and (b) heteroleptic analog of Z1; HOMOs (transparent) of (c) Z1 and (d)
heteroleptic analog of Z1; Electron density surface of (e) Z1 and (f) Z2. Mesityl group
is omitted for simplicity of calculation, Z1 and Z2 have D
2d
symmetry and heteroleptic
analog of Z1 has C
2v
symmetry.
- 5.38 eV
- 2.15 eV
- 5.36 eV
- 2.14 eV - 2.14 eV
a) b)
c)
d)
e)
f)
101

The calculation suggests that there is no electronic communication between the
two dipyrrin ligands in the homoleptic complex. Frontier orbitals of the heteroleptic
complex solely populate the dipyrrin ligand, excluding any participation of the ancillary
ligand. In consistence with MO analysis, the calculated energies of HOMO and LUMO
of the two complexes are similar.
4.5. Photophysical properties in different solvents
Absorption spectra of Z1-6 in different solvents are presented in Figure 4.7, and
the photophysical properties of Z1-6 at room temperature are summarized in Tables 4.2
and 4.3. It should be noted that freshly prepared Z5-6 solutions were used for each step
of photophysical characterization to minimize disproportionation. Zinc dipyrrins Z1-6
absorb strongly from 400 to 550 nm; the absorption spectra are relatively independent of
solvent polarity, indicating little dipole change upon excitation. The similarity between
the absorption spectra of homoleptic complex Z2 and heteroleptic analog Z5 suggests
little to no excitonic or electronic couplings between the two dipyrrin ligands (Figure 4.7c
and 4.7d), in agreement with crystallographic and computational results.
102

300 350 400 450 500 550
0.0
0.5
1.0

Normalized intensity
Wavelength (nm)
Z1
Z2
Z3
Z4
a)
300 350 400 450 500 550
0.0
0.5
1.0
b)

Intensity (a.u.)
Wavelength (nm)
CycHex
Toluene
THF
CHCl
3
DCM
ACN

300 350 400 450 500 550
0.0
0.5
1.0
c)

Normalized intensity
Wavelength (nm)
Z2
Z5
300 350 400 450 500 550
0.0
0.5
1.0
d)

Normalized intensity
Wavelength (nm)
Z3
Z6

Figure 4.7. Absorption spectra of (a) Z1-4 in cyclohexane, (b) Z2 in different
solvents, (c) Z2 and Z5 in cyclohexane and (d) Z3 and Z6 in toluene. Cychex –
cyclohexane, THF – tetrahydrofuran, DCM – dichloromethane and ACN –
acetonitrile.
103

500 550 600 650 700 750
0.0
0.5
1.0
a)

Normalized intensity
Wavelength (nm)
CycHex
Toluene
THF
CHCl
3
DCM
500 550 600 650 700 750
0.0
0.5
1.0
b)

Normalized intensity
Wavelength (nm)
CycHex
Toluene
THF
CHCl
3
DCM
500 550 600 650 700 750
0.0
0.5
1.0

Intensity (a.u.)
Wavelength (nm)
CycHex
Toluene
THF
CHCl
3
DCM
c)
500 550 600 650 700 750
0.0
0.5
1.0

Intensity (a.u.)
Wavelength (nm)
CycHex
Toluene
THF
CHCl
3
DCM
d)
500 550 600 650 700 750
0.0
0.5
1.0
e)

Intensity (a.u.)
Wavelength (nm)
CycHex
Toluene
THF
CHCl
3
DCM
500 550 600 650 700 750
0.0
0.5
1.0

Intensity (a.u.)
Wavelength (nm)
CycHex
Toluene
THF
CHCl
3
DCM
f)

Figure 4.8. Emission spectra of (a) Z1, (b) Z2, (c) Z3, (d) Z4, (e) Z5 and (f) Z6 in
different solvents at room temperature. Cychex – cyclohexane, THF – tetrahydrofuran
and DCM – dichloromethane.

104

500 550 600 650 700 750 800
0.0
0.5
1.0
650 700 750 800
0.000
0.002
0.004

Intensity (a.u.)
Wavelength (nm)
Z1
a)
500 550 600 650 700 750 800
0.0
0.5
1.0
b)
700 750 800
0.00
0.01
0.02

Intensity (a.u.)
Wavelength (nm)
Z2
500 550 600 650 700 750 800
0.0
0.5
1.0
c)
650 700 750 800
0.000
0.005
0.010

Intensity (a.u.)
Wavelength (nm)
Z3
500 550 600 650 700 750 800
0.0
0.5
1.0
d)
650 700 750 800
0.00
0.01
0.02

Intensity (a.u.)
Wavelength (nm)
Z4

500 550 600 650 700 750 800
0.0
0.5
1.0
700 750 800
0.00
0.01
0.02

Intensity (a.u.)
Wavelength (nm)
Z5
e)
500 550 600 650 700 750 800
0.0
0.5
1.0
f)
650 700 750
0.0000
0.0001
0.0002

Intensity (a.u.)
Wavelength (nm)
Z6

Figure 4.9. Emission spectra of Z1-6 in 2-MeTHF at 77K. Insets are phosphorescent
peaks.
105

Table 4.2. Photophysical properties of homoleptic zinc dipyrrin complexes in different
solvents at room temperature.

Solvents
ab

(nm)
fwhm
ab

(cm
-1
)
em

(nm)
fwhm
em

(cm
-1
)
ab-em

(cm
-1
)
QY
(%)

(ns)
k
r

(ns
-1
)
k
nr

(ns
-1
)
Z1 CycHex 484 1530 501 1260 638 46.5 4.1 0.11 0.13
Toluene 486 1443 503 1256 659 33.3 3.0

0.11 0.22
THF 484 1437 494 1231 406 2.7 2.6
(a)
0.01 0.37
CHCl
3
485 1483 496 1218 426 2.2 -
(a)
- -
DCM 485 1471 495 1131 429 1.7 2.1
(b)
- -
ACN 481 1423 490 1180 396 < 0.1 1.7
(b)
- -
Z2 CycHex 493 983 506 944 501 65.0 4.9 0.13 0.07
Toluene 495 1022 509 959 567 19.0 3.9 0.05 0.21
THF 493 1016 507 1013 586 0.9 2.9
(a)
0.003 0.34
CHCl
3
494 1083 509 1013 592 0.5 - - -
DCM 493 1074 508
(650)
1108 706 < 0.1 2.5
(b)

(2.2
(c)
)
- -
ACN 490 1056 508 1494 736 - 1.4
(b)
- -
Z3 CycHex 489 1036 507 1473 676 16.3 1.5 0.11 0.56
Toluene 491 1102 509 1523 736 13.8 2.1 0.07 0.41
THF 489 1092 511 1576 902 2.0 2.8
(a)
0.007 0.35
CHCl
3
490 1057 513 1494 915 0.5 - - -
DCM 488 1127 509
(653)
1591 819 < 0.1 2.4
(b)

(2.5
(c)
)
- -
Z4 CycHex 506 1393 533 1511 1026 17.2 2.0 0.09 0.44
Toluene 508 1323 533 1565 920 13.4 2.4 0.06 0.36
THF 505 1432 532 1604 982 2.6 2.6
(a)
0.01 0.37
CHCl
3
507 1296 532 1632 917 0.7 - - -
DCM 506 1403 528
(674)
1503 831 < 0.1 - - -
106

Table 4.3. Photophysical properties of heteroleptic zinc dipyrrin complexes in different
solvents at room temperature.
(a)
detected at 520 nm, there is a faster component than instrument resolution of 0.4 ns,
(b)

detected at 520 nm, there is a faster component than instrument resolution of 22 ps,
(c)

detected at 645 nm.
While the absorption spectra of Z1-Z6 are quite similar, the emission spectra of
the homoleptic and heteroleptic complexes exhibit a distinct difference with respect to
solvent dependence (Figure 4.8). The emission spectra of Z5 and Z6 (Figure 4.8e and f)
remain unchanged from non-polar to polar solvents. In contrast, as solvent polarity
increases, the fluorescence intensities of Z1-4 markedly decrease, and additional broad
emission peaks emerge for Z2-4 (Figure 4.8a-d); these emission peaks red shift with
increased solvent polarity. The broad emission band at lower energy is distinctly
Solvents
ab

(nm)
fwhm
ab

(cm
-1
)
em

(nm)
fwhm
em

(cm
-1
)
ab-em

(cm
-1
)
QY
(%)

(ns)
k
r

(ns
-1
)
k
nr

(ns
-1
)
Z5 CycHex 495 733 503 863 305 51.6 3.4 0.15 0.14
Toluene 497 739 506 861 350 48.4 2.8 0.17 0.18
THF 495 675 504 849 337 41.2 2.3 0.18 0.26
CHCl
3
496 714 505 866 343 42.2 2.2 0.19 0.26
DCM 495 777 505 885 388 30.3 1.8 0.17 0.39
DMF 495 781 505 882 386 26.0 -
(a)
- -
ACN 492 713 501 885 367 17.3 -
(a)
- -
Z6 CycHex 491 757 503 1311 467 8.5 0.8 0.11 1.14
Toluene 493 739 504 1240 463 8.5 0.8 0.11 1.14
THF 490 723 503 1290 516 4.9 -
(a)
- -
CHCl
3
491 755 503 1220 485 5.0 -
(a)
- -
DCM 490 869 502 1290 485 3.7 -
(a)
- -
107

different from the phosphorescence spectrum of Z2 recorded at 77 K in
2-methyltetrahydofuran (2-MeTHF) (Figure 4.9).
To determine if the broad emission at 650 nm originates from excimers, emission
intensities of Z2 at 508 nm and 650 nm were measured at a range of concentrations (see
Figure 4.10). The intensities of the two bands vary linearly with concentration,
suggesting that excimer emission is not responsible for the weak red emission in these
compounds. Thus, we believe that the emission band at 650 nm is due to SBCT emission,
as observed for meso-coupled BODIPYs.
31

The quantum efficiencies of zinc dipyrrins Z1-6 have been measured in different
solvents, and the results are presented in Tables 4.2, 4.3 and Figure 4.11. In nonpolar
cyclohexane, the homoleptic complexes Z1-4 are quite emissive with QY varying from
0.17 to 0.70, revealing an interesting effect of alkylation on QY: Z2 > Z1 > Z3 Z4
0.04 0.08 0.12 0.16
0.0
0.5
1.0
1.5
2.0 emission at 508 nm
emission at 650 nm
Emission internsity (a.u.)
Absorbance

Figure 4.10. Emission intensities are plotted vs. absorption intensities of Z2 solution in
dichloromethane at various concentrations.
108

(Table 4.2). Similar alkylation effects on QY were observed in the heteroleptic
complexes: Z5 > Z6. There is also a correlation between the full width at half maximum
height (fwhm) of emission peaks and QY of Z1-4 in cyclohexane (Figure 4.11a, inset);
broader emission peaks are associated with lower QY, suggesting that structural
distortion of the excited states increases in the order Z2 < Z1 < Z3 < Z4.

The QY of homoleptic complexes Z1-4 sharply decrease as the solvent polarity
increases, in agreement with previous reports
40
(QY vs. solvent polarity index E
T
(30)
54
is
plotted in Figure 4.11). In contrast, the QY values of heteroleptic complexes display
much less solvent dependence than their homoleptic analogs (Figure 4.11b). It is noted
that the QY of Z2 and Z4 in toluene (0.19 and 0.13, respectively) are smaller than the
previously reported values (0.28 and 0.20, respectively).
40

32 36 40 44
0.0
0.2
0.4
0.6
0.8
a)
1000 1200 1400
0.0
0.2
0.4
0.6
0.8
Z4
Z3
Z1

QE
Emission fwhm (cm
-1
)
Z2
In CyHex

QY
Solvent polarity index E
T
(30) (kcal/mol)
Z1
Z2
Z3
Z4
32 36 40 44
0.0
0.2
0.4
0.6
0.8 b)

QY
Solvent polarity index E
T
(30) (kcal/mol)
Z5
Z6

Figure 4.11. Quantum yield of (a) homoleptic complexes Z1-Z4 (inset is the QY of
Z1-Z4 plotted vs. full width half max of emission peak in CycHex), and (b)
heteroleptic complexes Z5-Z6 plotted vs. solvent polarity index E
T
(30) for
cyclohexane, toluene, THF, CHCl
3
, DCM, DMF and ACN.
109

The decrease in QY and appearance of an additional broad peak in the emission
spectra of Z1-4 at longer wavelengths in polar solvents suggest a deactivation pathway to
a weakly emissive state, most probably to a CT state via an SBCT mechanism similar to
what was observed in other related systems.
19-21,27,29
The smaller dependence on solvent
polarity of the heteroleptic complexes further supports the hypothesis that the observed
photophysical properties of Z1-4 are associated with SBCT, which is not possible for Z5
and Z6.
The weakly emissive nature of the CT state of zinc dipyrrins is in contrast to the
efficient CT emission observed for bianthryl
19,22,55
and biperylenyl.
27
In the latter
complexes, significant electronic coupling exists between the two SBCT units.
55
For Z1-
4, poor molecular orbital overlap and weak dipolar coupling between the two nearly
orthogonal dipyrrin ligands, as seen in the computational studies and crystal structure,
can be used to explain the decreased emissivity. It is interesting to note that the emission
from the CT state of nonalkylated zinc dipyrrin Z1 was not detected in polar solvent in
contrast to Z2-4. Our electron density calculation of Z2-4 reveals the overlap of the
methyl group at R1 positions (see Figure 4.1) and the dipyrrin ligands, which is absent in
Z1. This weak electronic interaction might be responsible for the weak emission from
the CT state observed in Z2-4 in polar solvents.
In nonpolar cyclohexane, Z1-4 display single exponential fluorescence decays
with lifetimes ( varying from 1.4 to 4.4 ns. In polar solvents, the fluorescence decays
are multi-exponential with a sub-nanosecond component which is faster than our
110

instrument’s response. These observations also indicate deactivation of the locally
excited state to a nonemissive or weakly emissive state in polar solvents.
It is interesting to note that the heteroleptic complexes exhibit lower QY
compared to homoleptic ones. This may be due to enhanced disproportionation rates in
the excited compounds, leading to nonradiative deactivation. Z5 and Z6 possess similar
k
r
to Z2 and Z3, but the heteroleptic ones exhibit larger k
nr
by factor of 2 (Tables 4.2).
4.6. Electrochemistry.

Figure 4.12. Electrochemical measurements of (a) Z1, (b) Z2, (c) Z3 and (4) Z4 in
dry THF under N
2
with a scan rate of 100 mV/s vs Fc/Fc
+
. Arrows indicate the
direction of the scan.
-5 -4 -3 -2 -1 0 1 2
-60
-30
0
30
60

Current (a.u.)
Voltage (V)
Z1_CV
Z1_DPV1
Z1_DPV2
a)
-5 -4 -3 -2 -1 0 1 2
-20
0
20
40
60
80

Current (a.u.)
Voltage (V)
Z2_CV
Z2_DPV1
Z2_DPV2
b)
-5 -4 -3 -2 -1 0 1 2
-20
0
20
40
60

Current (a.u.)
Voltage (V)
Z3_CV
Z3_DPV1
Z3_DPV2
c)
-5 -4 -3 -2 -1 0 1 2
-30
0
30
60
90 d)
Z4_CV
Z4_DPV1
Z4_DPV2

Current (a.u.)
Voltage (V)
111

Table 4.3. Electrochemical and optical properties of homoleptic complexes.
Electrochemical values were determined by differential pulsed voltammetry (DPV) vs.
Fc
+
/Fc; optical gap E
00
is defined by the midpoint between absorption and emission
spectra in THF; triplet energies were measured in 2-methyl tetrahydrofuran at 77K.

The possibility for Z1-4 to undergo SBCT was examined by electrochemistry.
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out in
dry THF under N
2
, and the results are presented in Figure 4.12 and Table 4.3. Z1-Z4
exhibit two distinct reversible reduction peaks and one irreversible oxidation peak, with
the exception of Z4, in which two oxidation peaks were detected. Since the difference
between oxidation and reduction values (electrochemical gap, E
redox
) is larger than the
S
1
optical gap, E
00
, Coulombic stabilization of the CT state in polar solvents polar
solvents is required for Z1-4 to undergo photo-induced SBCT. The required stabilization
energies of CT states under SBCT conditions vary from 0.19 to 0.28 eV (see table 3).
8,23

Thus, SBCT is not expected in nonpolar solvents, whereas the CT state stabilization by
Coulombic interaction with polar solvents, such as acetonitrile, is estimated to exceed 0.3
eV
8,23
, enabling SBCT.

(V)

(V)

(V)
E
redox

(V)
E
00

(eV)
E
T

(eV)
E
00
- e E
redox
(eV)
Z1 -2.33 -1.93 0.80 2.73 2.54 1.82 0.19
Z2 -2.42 -2.11 0.60 2.71 2.48 1.75 0.23
Z3 -2.79 -2.35 0.38 2.73 2.49 1.74 0.24
Z4 -2.82 -2.44 0.24
0.37
2.68 2.40 1.72 0.28
112

In contrast to Z1-4, bianthryl,
8
biperylenyl
8
and BODIPY dyads
31
exhibit smaller
electrochemical gaps than optical ones, thus SBCT can be favorable even in nonpolar
solvents. Consistent with this, the SBCT of bianthryl was experimentally observed in
cyclohexane,
19
hexanes
19,22
and heptane.
18

4.7. Transient absorption spectroscopy study
Femtosecond and nanosecond transient absorption (TA) measurements have been
performed to confirm the presence of SBCT, and to study the kientics of such processes.
In order to predict the absorptions of the SBCT state, spectroelectrochemical
measurements were performed. The one-electron reduced Z1 has a broader absorption
profile (Figure 4.13c, inset), with increased absorption from 370-430 nm, and a distinct
peak at 517 nm. Unfortunately, Z1 shows irreversible electrochemical oxidation,
precluding our characterization of the cation directly.
Femtosecond TA spectra of Z1 in cyclohexane, toluene, dichloromethane and
acetonitrile are presented in Figure 4.13. In cyclohexane, excitation at 500 nm populates
the S
1
state, as observed by a ground state bleach from 430-500 nm, stimulated emission
(520-600 nm), and excited state absorption at 345 nm (Figure 4.13a).
In polar solvents such as dichloromethane and acetonitrile, stimulated emission
and excited state absorption at 345 nm from S
1
appear immediately following excitation,
similar to what is observed in cyclohexane. However, over the course of 4 – 6 ps, the
stimulated emission band disappears and new induced absorption bands at 370 nm and
517 nm grow in. The induced absorption at 517 nm is similar to that of the Z1 anion (see
113

inset Figure 4.13c). Since the induced absorption peak at 370 evolves with similar
kinetics to that at 517 nm, we assign the 370 nm peak to the new excited state as well.
This state is assigned as the SBCT species; note that the characteristic SBCT absorptions
are absent in cyclohexane, indicating that stabilization by polar solvents is needed to
favor SBCT the local excited state of zinc dipyrrins, in agreement with the
electrochemical analysis.

Figure 4.13. Femtosecond transient absorption of Z1 in (a) cyclohexane; (b) toluene;
(c) dichloromethane, inset is the spectrum of Z1 anion defined by
spectroelectrochemical measurement; and (d) acetonitrile. Samples were excited at
500 nm with a pump fluence of 160 µJ/cm
2
for (a), (b), (d), and 70 µJ/cm
2
for (c).
114

In a weakly polar solvent, toluene, both S
1
stimulated emission and induced
absorption at 370 nm are observed beyond 1.2 ns time window, suggesting that that the
kinetic evolution of the excited species is different from what was observed in polar
solvents and nonpolar cyclohexane (Figure 4.13b). The induced absorption at 370 nm
indicates SBCT of Z1 in toluene, however the induced absorption at 517 nm is hidden
due to the overlap with the stimulated emission band. Additionally, the stimulated
emission persists longer than 1.3 ns, which is much longer than that in acetonitrile or
dichloromethane. The QY of Z1 in toluene is also much higher than that in acetonitrile
and dichloromethane (Table 4.2). Similar results were seen in the case of Z2-3 in toluene
(Figure 4.14). These observations can be explained by the presence of an equilibrium
between the local excited state S
1
and CT states in toluene. We propose that the solvation
energy by weakly polar toluene lowers the energy of the CT state close to that of the local
excited state. A similar equilibrium between locally excited and CT states of bianthryl
was reported in weakly polar media.
22,25,26
In polar solvents, large solvation energies
stabilize the CT states, shifting the equilibrium to the formation of the charge transfer
species.
115

Figure 4.14. Femtosecond transient absorption of (a) Z2 in cyclohexane, (b) Z2 in
toluene, (c) Z2 in acetonitrile, (d) Z3 in cyclohexane, (e) Z3 in toluene and (f) Z3 in
acetonitrile.
350 400 450 500 550 600
-30
-20
-10
0
b)

Abs
Wavelength (nm)
0.5 ps
5 ps
10 ps
500 ps
1200 ps
350 400 450 500 550 600
-30
-20
-10
0
10
c)

Abs
Wavelength (nm)
0.5 ps
5 ps
10 ps
500 ps
1200 ps
350 400 450 500 550 600
-20
-10
0
d)

Abs
Wavelength (nm)
0.5 ps
5 ps
10 ps
500 ps
1200 ps
350 400 450 500 550 600
-15
-10
-5
0
5 e)

Abs
Wavelength (nm)
0.5 ps
5 ps
10 ps
500 ps
1200 ps
350 400 450 500 550 600
-20
-10
0
f)

Abs
Wavelength (nm)
0.5 ps
5 ps
10 ps
500 ps
1200 ps
350 400 450 500 550 600
-20
-10
0
a)

Abs
Wavelength (nm)
0.5 ps
5 ps
10 ps
500 ps
1200 ps
116

Based on femtosecond TA measurements, a simplified Jablonski diagram is
proposed to explain the obtained results (Figure 4.15). Following the proposed scheme,
global fitting of TA data of Z1-3 in different solvents has been performed, and the results
are presented in Table 4.4. In the nonpolar solvent cyclohexane, SBCT does not occur
and the kinetics of transient species were fit to a mono-exponential decay with lifetimes
of 4.5 ns, 4.8 ns and 1.4 ns for Z1, Z2 and Z3, respectively. These lifetimes are in good
agreement with fluorescence lifetimes of the respective compounds in cyclohexane
(Table 4.2).
TA data of Z1 in toluene were fit with a different model to account for the
equilibrium between the local excited state S
1
and the CT state. The forward and
backward lifetimes (1/k
CT
and 1/k
CR
) between the S
1
and the CT states are 13 and 25 ps,
respectively. Since these lifetimes are two orders of magnitude faster than both the

Figure 4.15. Simplified Jablonski diagram illustrating symmetry breaking charge
transfer process. k
rec
is the total recombination rate to either the triplet state or the
ground state.
S
0
S
1
CT
[DZnD]
*
DZnD
[D
+
ZnD
-
]
*
nonpolar
polar
[DZnD]
*
T
1
k
CT
k
CT
k
CR
weakly polar
k
rec
117

charge recombination lifetime of the CT state (1/k
rec
= 3.5 ns) and the decay lifetime of
the local excited state (3.0 ns), this suggests our assumption about a fast equilibrium is
indeed reasonable.
In polar dichloromethane, the fitting yields lifetimes of 5.5 ps and 3.3 ns for the
formation (1/k
CT
) and the recombination (1/k
rec
) of the CT state of Z1, respectively. In
acetonitrile, the lifetimes of CT formation and recombination (3.6 ps and 2.1 ns) are
faster than in dichloromethane. Generally, the lifetime of CT state formation (1.1 – 5.5
ps) in polar solvents is three orders of magnitude faster than the recombination lifetime
(0.9 – 3.3 ns) (Table 4.4).

Interestingly, compared to Z1, femtosecond TA of Z2 and Z3 in acetonitrile
(Figures 4.13 - 4.16) show faster lifetimesfor both CT state formation and recombination
(1/k
CT
= 1.1, 1.0 ps and 1/k
rec
= 0.9, 1.4 ns for Z2 and Z3, respectively). Z2 and Z3 are

Figure 4.16. Comparison of dynamics of CT state (blue) and ground state bleach (red)
between Z1 (closed) and Z2 (open) in acetonitrile.

118

expected to have less degree of rotational motion of the two dipyrrin ligands around the
Zn metal center compared to Z1 due to the presence of methyl groups at the alpha-
positions. Thus, steric hindrance creates a more restricted orthogonal geometry of the
ground and CT states in Z2 and Z3, leading to the increase of the electron transfer rates
for both CT state formation and recombination. In addition, the electronic interaction
through the methyl group as discussed above might also enhance the charge transfer rates
(i.e. both CT state formation and recombination rates).
Table 4.4. Kinetic rates of different processes of Z1-3 in different solvents determined
by femtosecond transient absorption measurements.

Femtosecond TA measurements show that SBCT of zinc dipyrrin occurs in
weakly polar and polar solvents, however the question about deactivation pathways of the
CT state still remains open. Schmidt and coworkers
22
reported that the CT state of
Solvent 1/k
CT

(ps)
1/k
CR

(ps)
1/k
rec

(ns)
1/(k
r
+ k
nr
)
(ns)
Z1 CycHex - - - 4.5
Toluene 14 22 3.5 3.0
DCM 5.5 - 3.3 -
ACN 3.6 - 2.1 -
Z2 CycHex - - - 4.8
Toluene 8.6 15.5 4.0 3.9
ACN 1.1 - 0.9
Z3 CycHex - - - 1.4
Toluene 2.3 11.2 2.5 2.1
ACN 1.0 - 1.4 -
119

bianthryl recombined radiatively to the ground state or nonradiatively to triplet states in
polar solvents; the nonradiative internal conversion CT→S
0
was negligible. In contrast
to bianthryl, the zinc dipyrrin CT state is only weakly emissive, indicating that the
deactivation pathway is mainly nonradiative via either recombination to the triplet state
T
1
or internal conversion to the ground state S
0
. Internal conversion CT→S
0
is observed
in femtosecond TA as the GS bleach recovers with a concomitant decrease of the CT
induced absorption (Figure 4.16). The triplet energies of zinc dipyrrins measured at 77 K
in 2-MeTHF (1.75, 1.74 and 1.72 eV for Z2-4, respectively, see Table 4.3) are lower than
the CT states (1.92, 1.90 and 1.84 eV for Z2-4, respectively, as estimated from maxima
of the CT emission peaks in dichloromethane); the energies of CT states in less polar
solvents are expected to be even higher. Thus, the charge recombination CT→T
1
is
expected to be thermodynamically favorable.
Nanosecond transient absorption measurements of Z1 in different solvents have
been performed to further study the deactivation of CT states, and the results are
presented in Figure 4.17. In all solvents, the induced absorption peaks from CT species
decay within the instrument response (approx. 20 ns), and new induced absorption bands
at 350 – 450 nm (max peak at 420 nm) and 550 – 600 nm are observed. To elucidate the
origin of the new induced absorption features, femtosecond TA spectra of Z1 in mixed
solvents of dichloromethane/methyl iodide (DCM:MeI 1:4), which is expected to
enhance the intersystem crossing rate of Z1 from the singlet excited state to the triplet
state,
56
have been collected (Figure 4.17c). The fast intersystem crossing rate to the
triplet state yields induced absorption that matches well with microsecond TA spectrum
120

of Z1 in acetonitrile (Figure 4.17a). Thus, only the triplet state was observed in
acetonitrile from 20 ns to milliseconds.

Similar results were obtained in toluene. The triplet state formation in
cyclohexane was also observed; however, the signal intensity is much weaker than that
observed in more polar solvents (Figure 4.17b). We speculate that the triplet formation in
cyclohexane via intersystem crossing (S
1
→T
1
) is less efficient than triplet state formation
via the CT state (S
1
→CT→T
1
), as observed in toluene and acetonitrile. By fitting the
nanosecond TA kinetic traces to a single exponential decay, the triplet decay lifetimes

Figure 4.17. Nanosecond transient absorption spectra of Z1 in (a) acetonitrile, (b)
different solvents at 0.5 µs, and (c) femtosecond transient absorption spectra of Z1 in
DCM:MeI (1:4).
121

(1/k
T1
) of Z1 were determined to be 16, 50, and 33 µs in cyclohexane, toluene, and
acetonitrile, respectively.
4.8. Application to organic photovoltaics

OPV devices using Z1 and Z2 as a donor materials and fullerene C
60
as an
acceptor material have been fabricated using vacuum deposition technique on glass
coated with Indium doped Tin Oxide (ITO) substrate. A MoO
3
layer is used as a hole
conducting/electron blocking layer in OPV devices and is deposited directly on top of the
ITO. Device structures and characteristics are shown in Table 4.5 and Figure 4.18. Both
of the devices have significant photocurrents (3.06 and 3.30 mA/cm
2
). EQE
measurements (Figure 4.18b) confirm the contribution of Z1 and Z2 to the photocurrent
(up to 30% EQE at 500 nm). While the photocurrents are similar in D1 and D2, there is a
-0.5 0.0 0.5 1.0
-3
0
3

Current Density (mA/cm
2
)
Voltage (V)
D1
D2
400 500 600 700 800
0
10
20
30

EQE (%)
Wavelength (nm)
D1
D2

Figure 4.18. Characteristics of the OPV devices using Z1 and Z2 as donor layers: J-V
characteristics under 1 sun AM 1.5G illumination (left) and EQE measurements
(right).
122

significant difference in FF and V
OC
. The 200 mV lower V
OC
in D2 is well correlated
with 200 mV difference in oxidation potentials of Z1 and Z2 (Table 4.3). Higher FF in
D2 might be due to better hole conductivity of the Z2 donor layer.
Table 4.5. Device performance characteristics under AM1.5G 1 sun illumination.
General device structure is ITO/MoO
3
(8 nm)/donor (10 nm)/C
60
(40 nm)/BCP (10 nm)/
Al, where D1 (Z1 = donor) and D2 (Z2 = donor).
Device J
SC
(mA/cm
2
) V
OC
(V) FF (%)
D1 3.06 0.82 0.35 0.88
D2 3.30 0.61 0.55 1.12

4.9. Conclusion
Homoleptic zinc dipyrrins exhibit photophysical properties that are strongly
influenced by solvent polarity. These solvent-dependent properties have been explained
by a deactivation of the local excited state via SBCT assisted by polar solvents. Transient
absorption measurements revealed that SBCT proceeds in polar solvents at a rate two
orders of magnitude faster than charge recombination. This fast charge transfer lifetime
(1.0 – 14 ps) in combination with slower charge recombination lifetime (1.0 – 3.5 ns) is a
desirable property for materials used in OPVs as it allows sufficient time for charge
separation at the donor/acceptor interface.
The SBCT also sheds light on the origin of the low QY exhibited by homoleptic
zinc dipyrrin complexes in polar and weakly polar solvents, compared to their
heteroleptic counterparts. Our results suggest that metallodipyrrin complexes containing
123

only one dipyrrin ligand may be good candidates as highly fluorescent probes in a range
of applications, however, instability to disproportionation is drawback for the Zn
complexes.
This study demonstrates that SBCT can occur in systems that contain
chromophores with weak electronic couplings. It is interesting to note that other systems
that undergo SBCT exhibit some degree of electronic overlap between the two
chromophores, as indicated by calculated frontier orbitals.
31,55
Additionally, while the
previous systems have some degree of rotational freedom leading to the twisted internal
charge transfer mechanism,
23
the Z1-4 dipyrrin complexes are capable of SBCT even
with perpendicular ligands that display very limited rotation.
Finally, Z1 and Z2 have been used as donor materials in prototype OPV devices.
The devices work quite well with distinct contributions from Z1 and Z2 photoresponses.
Small photocurrents of the prototype devices are due to narrow blue-green absorption of
the donor materials, though it is expected that expending -conjugation of the compounds
will red shift the absorption and thus further improves photocurrent.

124

Chapter 4 Endnote
(1) Huang, L.; Ponomarenko, N.; Wiederrecht, G. P.; Tiede, D. M. Proceedings of the
National Academy of Sciences of the United States of America 2012, 109, 4851-4856.
(2) Moore, L. J.; Zhou, H. L.; Boxer, S. G. Biochemistry 1999, 38, 11949-11960.
(3) Mueller, M. G.; Slavov, C.; Luthra, R.; Redding, K. E.; Holzwarth, A. R.
Proceedings of the National Academy of Sciences of the United States of America 2010,
107, 4123-4128.
(4) Xiong, L.; Seibert, M.; Gusev, A. V.; Wasielewski, M. R.; Hemann, C.; Hille, C.
R.; Sayre, R. T. J. Phys. Chem. B 2004, 108, 16904-16911.
(5) Lathrop, E. J. P.; Friesner, R. A. J. Phys. Chem. 1994, 98, 3056-3066.
(6) Lockhart, D. J.; Boxer, S. G. Biochemistry 1987, 26, 664-668.
(7) Meech, S. R.; Hoff, A. J.; Wiersma, D. A. Chem. Phys. Lett. 1985, 121, 287-292.
(8) Vauthey, E. Chemphyschem 2012, 13, 2001-2011.
(9) Barber, J.; Tran, P. D. Journal of the Royal Society Interface 2013, 10.
(10) Bottari, G.; Trukhina, O.; Ince, M.; Torres, T. Coord. Chem. Rev. 2012, 256,
2453-2477.
(11) Cowan, A. J.; Durrant, J. R. Chem. Soc. Rev. 2013, 42, 2281-2293.
(12) Izumi, Y. Coord. Chem. Rev. 2013, 257, 171-186.
(13) Luo, S.-P.; Mejia, E.; Friedrich, A.; Pazidis, A.; Junge, H.; Surkus, A.-E.;
Jackstell, R.; Denurra, S.; Gladiali, S.; Lochbrunner, S.; Beller, M. Angewandte Chemie-
International Edition 2013, 52, 419-423.
(14) Manbeck, G. F.; Brewer, K. J. Coord. Chem. Rev. 2013, 257, 1660-1675.
(15) Pillai, S.; Ravensbergen, J.; Antoniuk-Pablant, A.; Sherman, B. D.; van Grondelle,
R.; Frese, R. N.; Moore, T. A.; Gust, D.; Moore, A. L.; Kennis, J. T. M. PCCP 2013, 15,
4775-4784.
(16) Woller, J. G.; Hannestad, J. K.; Albinsson, B. J. Am. Chem. Soc. 2013, 135, 2759-
2768.
(17) Young, K. J.; Martini, L. A.; Milot, R. L.; Snoeberger, R. C., III; Batista, V. S.;
Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W. Coord. Chem. Rev. 2012, 256,
2503-2520.
(18) Asami, N.; Takaya, T.; Yabumoto, S.; Shigeto, S.; Hamaguchi, H.-o.; Iwata, K. J.
Phys. Chem. A 2010, 114, 6351-6355.
(19) Piet, J. J.; Schuddeboom, W.; Wegewijs, B. R.; Grozema, F. C.; Warman, J. M. J.
Am. Chem. Soc. 2001, 123, 5337-5347.
(20) Schneide.F; Lippert, E. Berichte Der Bunsen-Gesellschaft Fur Physikalische
Chemie 1968, 72, 1155-1160.
(21) Schneide.F; Lippert, E. Berichte Der Bunsen-Gesellschaft Fur Physikalische
Chemie 1970, 74, 624-630.
(22) Schutz, M.; Schmidt, R. J. Phys. Chem. 1996, 100, 2012-2018.
(23) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev. 2003, 103, 3899-4031.
(24) Rettig, W.; Zander, M. Berichte Der Bunsen-Gesellschaft-Physical Chemistry
Chemical Physics 1983, 87, 1143-1149.
125

(25) Zander, M.; Rettig, W. Chem. Phys. Lett. 1984, 110, 602-610.
(26) Fritz, R.; Rettig, W.; Nishiyama, K.; Okada, T.; Muller, U.; Mullen, K. J. Phys.
Chem. A 1997, 101, 2796-2802.
(27) Dobkowski, J.; Grabowski, Z. R.; Paeplow, B.; Rettig, W.; Koch, K. H.; Mullen,
K.; Lapouyade, R. New J. Chem. 1994, 18, 525-533.
(28) Giaimo, J. M.; Lockard, J. V.; Sinks, L. E.; Scott, A. M.; Wilson, T. M.;
Wasielewski, M. R. J. Phys. Chem. A 2008, 112, 2322-2330.
(29) Khandelwal, H.; Mallia, A. R.; Cheriya, R. T.; Hariharan, M. PCCP 2012, 14,
15282-15285.
(30) Markovic, V.; Villamaina, D.; Barabanov, I.; Daku, L. M. L.; Vauthey, E.
Angewandte Chemie-International Edition 2011, 50, 7596-7598.
(31) Whited, M. T.; Patel, N. M.; Roberts, S. T.; Allen, K.; Djurovich, P. I.; Bradforth,
S. E.; Thompson, M. E. Chem. Commun. 2011, 48, 284-286.
(32) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891-4932.
(33) Wood, T. E.; Thompson, A. Chem. Rev. 2007, 107, 1831-1861.
(34) Clarke, T. M.; Durrant, J. R. Chemical Reviews 2010, 110, 6736-6767.
(35) Mbatia, H. W.; Burdette, S. C. Biochemistry 2012, 51, 7212-7224.
(36) Nolan, E. M.; Lippard, S. J. Acc. Chem. Res. 2009, 42, 193-203.
(37) Crawford, S. M.; Ali, A. A.-S.; Cameron, T. S.; Thompson, A. Inorg. Chem. 2011,
50, 8207-8213.
(38) Ding, Y.; Xie, Y.; Li, X.; Hill, J. P.; Zhang, W.; Zhu, W. Chem. Commun. 2011,
47, 5431-5433.
(39) Filatov, M. A.; Lebedev, A. Y.; Mukhin, S. N.; Vinogradov, S. A.; Cheprakov, A.
V. J. Am. Chem. Soc. 2010, 132, 9552-9554.
(40) Kusaka, S.; Sakamoto, R.; Kitagawa, Y.; Okumura, M.; Nishihara, H. Chemistry-
an Asian Journal 2012, 7, 907-910.
(41) Sakamoto, N.; Ikeda, C.; Yamamura, M.; Nabeshima, T. J. Am. Chem. Soc. 2011,
133, 4726-4729.
(42) Yu, L. H.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin, E.; Diers,
J. R.; Boyle, P. D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. Inorg. Chem. 2003, 42, 6629-
6647.
(43) Sazanovich, I. V.; Kirmaier, C.; Hindin, E.; Yu, L. H.; Bocian, D. F.; Lindsey, J.
S.; Holten, D. J. Am. Chem. Soc. 2004, 126, 2664-2665.
(44) Sutton, J. M.; Rogerson, E.; Wilson, C. J.; Sparke, A. E.; Archibald, S. J.; Boyle,
R. W. Chem. Commun. 2004, 1328-1329.
(45) Thoi, V. S.; Stork, J. R.; Magde, D.; Cohen, S. M. Inorg. Chem. 2006, 45, 10688-
10697.
(46) Ikeda, C.; Ueda, S.; Nabeshima, T. Chem. Commun. 2009, 2544-2546.
(47) Kobayashi, J.; Kushida, T.; Kawashima, T. J. Am. Chem. Soc. 2009, 131, 10836-+.
(48) McRae, E. G.; Kasha, M. The molecular exciton model; Academic Press: New
York, 1964.
(49) Kasha, M. Radiat. Res. 1963, 20, 55-70.
(50) Maeda, H.; Hashimoto, T.; Fujii, R.; Hasegawa, M. Journal of Nanoscience and
Nanotechnology 2009, 9, 240-248.
126

(51) Telfer, S. G.; McLean, T. M.; Waterland, M. R. Dalton Transactions 2011, 40,
3097-3108.
(52) Amiri, A.; Comeau, I. M.; Thompson, A. J. Heterocycl. Chem. 2006, 43, 431-435.
(53) Kee, H. L.; Kirmaier, C.; Yu, L. H.; Thamyongkit, P.; Youngblood, W. J.; Calder,
M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W. R.; Birge, R. R.; Lindsey, J. S.;
Holten, D. J. Phys. Chem. B 2005, 109, 20433-20443.
(54) Reichardt, C. Chem. Rev. 1994, 94, 2319-2358.
(55) Grozema, F. C.; Swart, M.; Zijlstra, R. W. J.; Piet, J. J.; Siebbeles, L. D. A.; van
Duijnen, P. T. J. Am. Chem. Soc. 2005, 127, 11019-11028.
(56) Sabatini, R. P.; McCormick, T. M.; Lazarides, T.; Wilson, K. C.; Eisenberg, R.;
McCamant, D. W. Journal of Physical Chemistry Letters 2011, 2, 223-227.

127

CHAPTER 5. ENERGY SENSITIZATION OF C
60
AND APPLICATION TO
ORGANIC PHOTOVOLTAICS

5.1. Introduction
A typical OPV device consists of separate electron donor and acceptor materials,
which are responsible for light absorption and charge generation, as excitons dissociate at
the donor/acceptor (D/A) interface.
1-3
While research has largely focused on the
development of new donor materials, fullerenes remain the most popular acceptor
materials because of their good electron conductivity
4-7
and ability to promote efficient
D/A charge separation.
8,9
Fullerenes have been shown to perform well with variety of
donor materials,
9
and the all of the reported high efficiency OPV devices utilize fullerene
acceptors.
10
A key drawback of the most commonly used fullerene, C
60
, is its low
absorption in the visible part of spectrum due to the symmetry forbidden nature of the
lowest energy electronic transition, = 670 nm. A neat C
60
film shows a high optical
density between 400 – 500 nm assigned to an intermolecular charge transfer absorption.
11

The poor overlap of this C
60
absorption with the AM1.5G solar flux (Figure 5.1) requires
that the donor material(s) collect the majority of the incident photons. As absorption of
the donor material is extended to the near-infrared (NIR), the green-to-orange part of
spectrum is left unabsorbed due to the relatively narrow absorption bands of organic
dyes.
12-20

128

In order to efficiently harvest solar energy, it would thus be desirable to have new
acceptor materials that strongly absorb visible light, while also retaining the favorable
electron mobility and charge separation efficiencies found in fullerenes. In this way,
light absorption could be distributed more equitably between the donor and acceptor
materials, potentially allowing the device’s photoresponse to be fully extended into the
near-infrared (NIR). Unfortunately, the performance of non-fullerene acceptors is
generally inferior to that of fullerenes.
8,9,21,22
Energy sensitization of fullerenes is an
alternative method to improve both the breadth and the efficiency of visible light
absorption in fullerene-based acceptors.
The sensitization of phenyl-C
61
-butyric acid methyl ester (PCBM) using a
perylene diimide (PDI) derivative has been recently reported by Hesse et al.
23
It was
shown that the PCBM-PDI blend worked well with the UV-absorbing

Figure 5.1. Absorption spectra of C
60
in neat films

(black circles), toluene solution
(blue squares) and AM 1.5G solar photon flux (red).

400 600 800 1000
0
1
0
2
4

Absorption intensity (au)
Wavelength (nm)
Photon flux (10
18
photons/m
2
)

129

hexabenzocoronene donor, but not with a red-NIR-absorbing polymer donor.
23
This
dependence on donor material might be the result of dissimilarities between the
molecular shapes of PDI (flat) and PCBM (ball-like), leading to unfavorable phase
separation between the two acceptors. Moreover, while the singlet energy of PDI is
higher than that of PCBM, the triplet energy of PDI (1.2 eV)
24,25
is lower (PCBM triplet
=1.55 eV). Thus, it is possible that singlet energy passed from PDI to PCBM can, after
intersystem crossing (ISC), undergo triplet transfer back to PDI as was previously
observed in a PDI-C
60
dyad.
26,27
From an energetic standpoint, this energy ping-pong
effect (illustrated in Figure 2a) is an undesirable process because the absorbed light
energy is not confined to the fullerene, causing energy loss due to the low energy of
sensitizer’s triplet. In addition, even though a photoresponse from PDI was observed in a
device using a blended PCBM-PDI acceptor layer, it remains unclear whether the
observed photocurrent comes from energy sensitization of PCBM or from direct charge
separation between PDI and the donor at the DA interface. A similar energy ping-pong
effect in a dicyanovinyl-terthiophene (DCV3T)-C
60
acceptor blend has been intentionally
utilized in OPV devices
28,29
where it was postulated that the triplet of DCV3T induced
charge separation at the DA interface.
29
Even though, as stated above, triplet transfer
back to the sensitizer is undesired, in this case the conductivity and fill factor (FF) of
OPV devices using these blended acceptors was improved compared to the devices using
a neat DCV3T acceptor.
To take full advantage of the superior electron transport properties of C
60
while
minimizing possible energy loss by back triplet energy transfer from C
60
to the sensitizer,
130

the sensitized energy is confined to C
60
. We have designed a blended acceptor layer
where C
60
is used as the host material that conducts both excitons and electrons for a
sensitizer that serves as a light absorbing guest. Upon excitation of the sensitizer, energy
is transferred to the C
60
host and subsequently transported to the DA interface where,
after charge separation, electrons are conducted by the C
60
host to the anode.

5.2. Molecular design for sensitizers.
Three possible pathways to sensitize C
60
via photoexcitation of a sensitizer are
shown in Figure 5.2b: (A) singlet energy transfer, (B) triplet energy transfer or

Figure 5.2. Energy diagram showing sensitization pathways in two cases: (a)
represents a sensitizer (e.g. PDI or DCV3T) having higher singlet energy but lower
triplet energy than C
60
. (b) represents energetic depiction of a sensitizer having both
singlet and triplet energies higher than that C
60
, possible pathways for sensitization:
A: singlet transfer, B: triplet transfer and C: electron transfer to form a charge transfer
(CT) state, which decay to the triplet state of C
60
if the CT state energy is higher than
the C
60
triplet. If the energy of the CT state is lower than C
60
triplet, it will act as a
trap state (CT trap).
sensitizer
S
1
T
1
S
1
T
1
CT
C
60
A
C
B
Energy (eV)
0
S
1
T
1
S
1
T
1
C
60
Energy (eV)
0
b) a)
trap
CT
131

(C) electron transfer from sensitizer to C
60
followed by charge recombination to the
triplet state of C
60
. It is important that all three pathways be viable for the sensitization of
C
60
, because the non-viable pathway can act as an exciton or charge trap in the mixed C
60

film. The three pathways impose several requirements for an efficient sensitizer. (1)
Both singlet and triplet energies of the sensitizer need to be greater than C
60
to guarantee
efficient energy transfer to C
60
. (2) The oxidation potential has to be sufficiently high to
ensure that the energy of the charge transfer (CT) state (i.e. Sen
-
C
60
+
), if formed, is
greater than that of the triplet state of C
60
, . To a first approximation, the CT
energy is given by , where q is the elementary charge, and
are the oxidation potential of sensitizer and reduction potential of C
60
, respectively,
and (estimated ~ 0.3 eV)
30
is the Columbic interaction between the sensitizer cation

and C
60
anion. Given a C
60
reduction potential of -1.06 V (vs. Fc/Fc
+
),
31
must be >
0.74 V to satisfy this criterion. If the energy of the charge transfer state Sen
+
C
60
-
is lower
than C
60
triplet state, it will serve as a net electron donor to C
60
,
32
forming a charge trap
state in the host-guest acceptor layer (Figure 5.2b). This type of charge trapping has been
problematic in previous works that involved blending an additional donor material into
C
60
layer.
33,34

(3) In order to maintain good electron conductivity by the C
60
host, the
reduction potential of the sensitizer has to be lower (more negative) than C
60
to ensure
that electrons are conducted by C
60
without being trapped by the sensitizer. (4) In
addition, it is desirable that the molecular size and shape of the sensitizer should be
similar to C
60
so as to not severely disrupt the molecular packing and thus maintain good
132

electron conductivity in the C
60
based acceptor layer. It should be emphasized that our
design strategy is different from the previous works with PDI or DCV3T
23,29
as all energy
absorbed in the acceptor layer will ultimately be located on the C
60
host.
Table 5.1. Photophysical and electrochemical properties of sensitizers (Z1, ZCl, IrDP,
CTrinh, Cl
6
SubPc, F
12
SubPc) and C
60
in solution and thin films.
Solution Thin film
E
S
(eV)
E
T

(eV)
E
ox
(V)
(b)

(V)
(b)

QE
(%)

E
S
(eV)
(a)

E
T
(eV)
Z1 2.54
(a)
1.82
(c)
+0.71 -1.94 41
(f)
3.9
(f)
2.33 -
ZCl 2.37
(a)
1.75
(c)
+1.22 -1.30 18
(f)
2.2
(f)
2.22 1.60
IrDP 2.57

1.86 +0.64 -1.82 8.8
(g)
16.7
(g)
2.49 1.82
CTrinh 2.38
(g)
1.73
(g)
+0.93 -1.54 4.1
(g)
15.8
(g)

F
12
SubPc 2.10 1.45
(k)
+1.00
(h)
-0.93
(h)
27.3
(l)
3.1
(l)
2.07 -
Cl
6
SubPc 2.17 - +0.85
(i)
-1.25
(i)
27.7
(l)
3.0
(l)
2.06 -
C
60
1.86 1.55
(d)
+1.26
(e)
-1.06 - - 1.84 1.50,
1.44
(d)

(a)
Determined by the midpoint between the normalized absorption and emission spectra
recorded in cyclohexane.
(b)
vs. Fc
+
/Fc.
(c)
Measured in 2-MeTHF at 77K.
(d)
Ref. 35,
(e)

Ref. 31,
(f)
(ns), in cyclohexane,
(g)
(µs), in toluene,
(h)
calc. from Ref.36 using
E
OX
(Fc
+
/Fc) = 0.5 V vs. SCE,
(i)
Ref.37,
(k)
Ref. 38,
(l)
(ns), in toluene.

Singlet and triplet energies as well as reduction and oxidation potentials of C
60
are
summarized in Table 5.1. While there are many organic dyes that have strong absorption
in the visible spectrum and higher singlet energy than C
60
, there are few classes of
organic compounds exhibiting higher or similar triplet energy to C
60
as well. Dipyrrins
(E
T
= 1.82 eV)
39
and porphyrins (1.60 - 1.65 eV),
40,41
meet these criteria, however the flat
molecular shape of porphyrins is very different from C
60
. Zinc dipyrrin complexes
42
have
133

a quasi-spherical shape and size similar to C
60
, and are thus better candidates according to
our design rules. Both singlet
42
and triplet
39
energies of dipyrrin complexes are higher
than those of C
60
. Another benefit of the dipyrrin complexes over porphyrins is that the
energy levels of zinc dipyrrins complexes are readily tunable via chemical modification
of the pyrrole rings or substituents at the meso-positions, and they can be prepared by
straightforward synthetic procedures.
43
Zinc dipyrrins have been used as linkers and/or
light absorbers in multichromorphoric light absorbing
42,44
or charge separating systems
45

in solution, but they have not been used in OPV devices.
Another attractive class of compounds that can be used as the C
60
sensitizer is
subpthalocyanines. These cone-shaped molecules have similar size to C
60
, higher singlet
energy than C
60
and slightly lower triplet energy (Table 5.1).
38,46
Efficient singlet energy
transfer from subphthalocyanine to fullerene has been shown in solution.
38,46
The triplet
energy of F
12
SubPc is only 0.05 eV (= 2k
B
T) lower than that of C
60
, so it is expected that
the back triplet energy transfer is not readily accessible as in the case of PDI/C
60
or
DCV3T/C
60
systems.
5.3. Synthesis and characterization of sensitizers
5.3.1. Zinc dipyrrins

Figure 5.3. Synthesis of ZCl.
134

The unsubstituted bis(5-mesityldipyrrinato) zinc (Z1) was synthesized and
characterized following a published procedure.
44
Unfortunately, the oxidation potential
of ZH (+0.71 V vs. Fc
+
/Fc) is not sufficient to satisfy the requirement 2 above, and a low
energy CT state is expected, i.e. ZH
+
C
60
-
. Indeed, ZH can be used as a donor material in
OPV devices using C
60
as the acceptor (see Chapter 4). In order to raise the oxidation
potential of zinc dipyrrin, high electron affinity substituents, i.e. Cl atoms, were added to
the pyrrole rings.
5.3.1.1. Synthesis of zinc chlorodipyrrin (ZCl)
The synthesis of a chlorinated zinc dipyrrin is summarized in Figure 5.3.
Synthesis of 5-mesityldipyrromethane (DPM) was followed published procedure.
47
A
solution of DPM (0.62 g, 2.34 mmol) in 60 ml freshly distilled tetrahydrofuran (THF)
was prepared; the solution was cooled down using dry ice/acetone bath and was bubbled
with nitrogen for 5 min. A solution of N-chlorosuccinimide (NCS) (3.1 g, 23.4 mmol) in
70 ml THF was prepared, and slowly added to the DPM solution under nitrogen. The
reaction mixture was stirred and allowed to warm up to room temperature for 2 h over
which time the reaction mixture turned dark red in color. After stirring for an additional
2 h at room temperature, the reaction was stopped. The solvent was removed under
reduced pressure, and the crude products were dissolved in 300 ml dichloromethane. The
crude products were washed with NaHCO
3
solution and brine twice. The dark red
solution of the products in dichloromethane was used without further purification.
135

A solution of Zn(OAc)
2
.2H
2
O (2.5 g) in 30 ml CH
3
OH was prepared, and added
to the solution of the dark red products. The reaction turned dark green in color and
emitted green-yellow color under UV lamp illumination. The reaction mixture was
stirred over night. The solvents were removed under reduced pressure. The solid
reaction mixture was dissolved in dichloromethane, and inorganic salts were filtered off.
The solution was washed with Na
2
CO
3
solution twice and once with brine. After
removal of the solvent, the crude product was passed through short neutral Al
2
O
3
flash
column using dichloromethane/hexanes (1/9) as eluent, and the orange-red fraction was
collected. After removing the solvent, a dark red product was collected. The product
was dissolved in DCM, and recrystallized by layering MeOH on top. Red-green crystals
(ZCl) were collected (0.2 g, 17% total yield). ZCl was further purified by gradient
sublimation under vacuum (10
-5
torr) at 280 C - 200 C - 140 C gradient temperature
zones. The sublimed crystals were qualified for X-ray single crystal structure
determination.
The product ZCl is a mixture of two compounds C
36
H
22
Cl
12
N
4
Zn (ZCl
12
) and
C
36
H
23
Cl
11
N
4
Zn (ZCl
11
) with molar ratio 3:1 (general formula: C
144
H
89
Cl
47
N
16
Zn
4
).
1
H
NMR (400 MHz, CDCl
3
): ppm 6.95 (s, 4H, mesityl), 6.49 (s, 0.23 H, pyrrole),
2.40-2.35 (m, 6H, mesityl), 2.09-2.04 (m, 12H, mesityl).
13
C NMR (500 MHz, CDCl
3
):
(ppm) 147.00, 143.29, 139.29, 136.04, 135.55, 132.46, 132.33, 130.30, 130.26, 129.74,
129.13, 128.43, 119.36, 118.70, 21.34, 21.23, 19.49, 19.42. HRMS: C
36
H
22
Cl
12
N
4
Zn:
calculated 999.7304, found 999.7333; C
36
H
22
Cl
11
N
4
Zn: calculated 965.7694, found
136

965.7705. Elemental analysis: theoretical C, 43.55%; H, 2.26%; N, 5.64%. Found: C,
43.54%; H, 1.96%; N, 5.50%.

The unique site of the residual pyrrolic hydrogen of the ZCl
11
derivative
(indicated in red in Figure 5.3) was confirmed by standard and 1D-NOESY NMR
techniques (Figure 5.4). Attempts to separate these derivatives using column
chromatography, recrystallization or thermal gradient sublimation were unsuccessful.
The ZCl
12
:ZCl
11
ratio could be increased using either longer reaction times or elevated
temperatures; however, the yields obtained under these conditions were so unacceptably

Figure 5.4. NOESY
1
H NMR of ZCl in CD
3
Cl and peak assignment.
137

low (1-5%) that the 1:3 mixture of ZCl
11
:ZCl
12
(abbreviated ZCl) was used for all
subsequent experiments.

5.3.1.2. Characterization
X-ray analysis was performed on a co-crystal of ZCl
11
and ZCl
12
, and the
molecular structure is shown in Figure 5.5 (the atom marked in red can be either H or Cl
with an occupancy ratio H:Cl = 1:3). The two dipyrrin ligands are held nearly
perpendicular to each other through Zn center with a dihedral angle of 87.6 , forming a
quasi-spherical molecular shape. The molecular volume of ZCl (1000 Å
3
) is similar to
that of C
60
(725 Å
3
) as estimated from the crystal structure.
48

Electrochemical measurements of ZCl carried out in dichloromethane with Fc
+
/Fc
as the internal standard are presented in Figure 5.6. ZCl undergoes irreversible oxidation
at +1.22 V and two reversible reductions at -1.30 and -1.54 V (vs. Fc
+
/Fc). Since no
discernible difference was observed in the redox potentials of ZCl
11
and ZCl
12
, the
electronic properties of the two compounds were evaluated using DFT calculations, and
a) b)

Figure 5.5. (a) ORTEP diagram of ZCl and (b) space filling models of ZCl (upper) and
C
60
(lower) with the proportional sizes as determined from X-ray single crystal data.
The atom marked in red can be either H or Cl with an occupancy ratio H:Cl = 1:3
138

the results are presented in Figure 5.7. In agreement with the experimental data, ZCl
11

and ZCl
12
have similar energies for their respective highest occupied molecular orbitals
(HOMO = -6.13 and -6.25 eV), lowest unoccupied molecular orbitals (LUMO = -3.23
and -3.29 eV) and triplet states (E
T
= 1.70 eV for both).

Figure 5.7. Calculated energy levels and molecular orbitals of ZCl
11
and ZCl
12
at
B3LYP/LACVP** level of theory using Titan software.
LUMO
HOMO
-6.25 eV
-6.13 eV
-3.23 eV
-3.29 eV

Figure 5.6. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
diagrams of ZCl in dichloromethane (vs. Fc
+
/Fc). Scan rate 100 mV/s.
-2 -1 0 1 2
+1.22
-1.54

Current (A)
Voltage vs. Fc
+
/Fc (V)
0.0005 A
-1.30
139

The absorption and emission spectra of ZCl in cyclohexane solution and neat film
are shown in Figure 5.8; the photophysical properties are summarized in Table 5.1. ZCl
displays an intense absorption band between 450–600 nm and is strongly fluorescent in
cyclohexane at room temperature (quantum yield = 18%, = 2.2 ns). Phosphorescence
(λ
max
= 710 nm, lifetime of ~ 0.5 ms) of ZCl was observed in 2-methyltetrahydrofuran (2-
MeTHF) at 77 K (Figure 5.8a). Chlorination slightly lowers both singlet and triplet states
of ZCl compared to ZH by 170 and 70 meV, respectively. However, the singlet and
triplet energies of ZCl are still significantly higher than C
60
, satisfying requirement 1 for
the sensitizer. Chlorination significantly increases the oxidation potential from +0.71
(ZH) to +1.22 V (ZCl). In addition, the reduction potential of ZCl is 240 mV more
300 400 500 600 700 800
0.0
0.5
1.0
0.5 ms
650 700 750

abs ZCl
em ZCl

Normalized intensities
Wavelength (nm)
a)
2.2 ns
300 400 500 600 700 800
0
1
2
3
4
0.0
0.2
0.4
0.6
0.8
1.0 abs ZCl
abs C
60

Abs. coefficient (10
5
cm
-1
)
Wavelength (nm)
b)
em ZCl
Normalized em. intensity
Figure 5.8. Absorption (blue) and emission (red) spectra of (a) solution of ZCl in
cyclohexane at room temperature, inset is the phosphorescence of ZCl measured in
2-MeTHF at 77K, excitation = 460 nm and (b) ZCl film at room temperature,
excitation = 460 nm. The thin film absorption spectrum of C
60
is also shown for
comparison.

140

negative than that of C
60
. Thus, the oxidation and reduction potentials of ZCl satisfy the
sensitizer requirements 2 and 3, respectively.
The optical density of neat ZCl thin films is nearly seven-fold greater than that of
C
60
in the range between 470 to 570 nm (Figure 5.8b). Interestingly, both fluorescence
and phosphorescence were observed in a neat film at room temperature (Figure 5.8b).
Despite a 60 nm red shift of the phosphorescence peak of the thin film compared to that
in 2-MeTHF solution at 77K, the triplet energy of ZCl is still 0.10 eV higher than that of
C
60
. Thus, the energy levels of ZCl in the neat films also fulfill the requirement (1) given
above for the sensitizer. Moreover, significant overlap exists between ZCl emission and
C
60
absorption spectra, assisting singlet energy transfer from the sensitizer to the C
60
host
by the Förster mechanism.
5.3.2. Iridium dipyrrin complexes

Figure 5.9. Synthesis scheme of IrDP and CTrinh. Atom marked in red can be Cl
or H with molar ratio (H:Cl = 1:9)
141

5.3.2.1. Synthesis
Since the triplet sensitization pathway (Figure 5.2b, path B) is of interest, it is
desirable to synthesize compounds that have rapid intersystem crossing (ISC) rate. Thus,
iridium complexes with dipyrrin ligands, which are known to have high ISC rates
(1/k
ISC
< 1 ps), have been synthesized according to the scheme shown in Figure 5.9.
IrDP was synthesized followed the published procedure.
39

IrDP compound was characterized by
1
H and
13
C NMR.
1
H NMR (400 MHz,
CDCl
3
) ppm 5.78 (dd, J = 8.58, 2.37 Hz, 2H), 6.25 (dd, J = 4.30, 1.41 Hz, 2H), 6.44
(ddd, J = 12.51, 9.23, 2.35 Hz, 2H), 6.51 (dd, J = 4.29, 1.30 Hz, 2H), 6.79 (t, J = 1.32
Hz, 2H), 6.96 (ddd, J = 7.32, 5.87, 1.35 Hz, 2H), 7.51-7.34 (m, 6H), 7.68 (dd, J = 12.25,
4.46 Hz, 2H), 7.86-7.78 (m, 2H), 8.24 (d, J = 8.38 Hz, 2H).
13
C NMR (400 MHz,
CDCl
3
) ppm. 165.36, 165.30, 164.52, 164.42, 162.48, 162.39, 162.37, 162.27, 160.78,
160.73, 160.30, 160.20, 152.06, 151.98, 149.58, 148.60, 139.24, 137.12, 137.06, 134.12,
131.73, 131.62, 130.30, 128.32, 128.30, 128.27, 128.07, 127.00, 122.89, 122.73, 122.08,
117.27, 114.00, 113.97, 113.87, 113.85, 97.52, 97.30, 97.09.
CTrinh was synthesized by refluxing chlorodipyrrin ligands with [IrCl(f
2
ppy)
2
]
2

in THF (see Figure 5.9). Similar to the synthesis of ZCl, CTrinh was obtained as an
inseparable mixture of two compounds with five and six chlorine atoms (CTrinhCl
5
and
CTrinhCl
6
with molar ratio 1:9). There are two possible diasteromers for CTrinhCl
5
.
1
H
and
19
F NMR are shown in Figures 5.10 and 5.11, respectively.

142

Figure 5.10.
1
H NMR of CTrinh and peak assignment. Insets are structures of three
compounds in the mixture.
143

5.3.2.2. Characterization

300 400 500 600 700 800
0.0
0.5
1.0 em RT
em 77K

Normalized intensities
Wavelength (nm)
abs RT
a)
300 400 500 600 700 800
0.0
0.5
1.0 em RT
em 77K

Normalized intensities
Wavelength (nm)
abs RT
b)

Figure 5.12. Absorption and emission spectra of (a) IrDP and (b) CTrinh in solution.
Spectra in toluene were measured at room temperature (RT) under N
2
, emission
spectra in 2-MeTHF were measured at 77K.

Figure 5.11.
19
F NMR of CTrinh mixture and peak assignment.
144

Absorption and emission spectra of IrDP and CTrinh in solution at room
temperature and 77K are shown in Figure 5.12. Singlet and triplet energies of iridium
complexes are very similar to zinc compounds (Table 5.1). As expected, emission from
iridium complexes is phosphorescent with µs-scale lifetimes (15.8 and 16.7 µs) and
moderate QY (4.5% and 8.8 %) for CTrinh and IrDP, respectively. The singlet and
triplet state energies of CTrinh and IrDP are similar to those of ZCl and Z1 analogs
(Table 5.1). Electrochemical measurements have been carried out to determine the
reduction and oxidation potentials of CTrinh (Figure 5.13). CTrinh undergoes reversible
reduction and quasi-reversible oxidation at -1.54 and +0.93 V vs. Fc
+
/Fc, respectively.
Similar to ZCl, there is no noticeable difference in redox potentials of CTrinhCl
5
and
CTrinhCl
6
. Compared to ZCl, both reduction and oxidation potentials of CTrinh are
negatively shifted by 300 and 240 mV, respectively (Table 5.1). While the oxidation
-2 -1 0 1
-4
-2
0
2
Current (

Voltage (V)
CV
DPV 1
DPV 2
0.93
-1.54

Figure 5.13. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV)
diagrams of CTrinh in dichloromethane (vs. Fc
+
/Fc). Scan rate 100 mV/s.

145

potential of IrDP (+0.64 V) does not satisfy requirement (2) for the sensitizer (see section
5.2), the chlorination of dipyrrin ligand significantly raises the oxidation potential of
CTrinh making CTrinh a good candidate for C
60
sensitization.

Theoretical calculation at B3LYP/LACVP** level of theory using Titan software
has been performed on CTrinhCl
5
and CTrinhCl
6
to gain insight into electronic structures
as well as energy levels of the two compounds (Figure 5.14). The HOMOs and LUMOs
of CTrinhCl
5
and CTrinhCl
6
mainly localize on chlorodipyrrin ligands; there is only

Figure 5.14. Calculated energy levels and molecular orbitals of CTrinhCl
5
and
CTrinhCl
6
at B3LYP/LACVP** level of theory using Titan software: (a) HOMO of
CTrinhCl
5
, (b) LUMO of CTrinhCl
5
, (c) HOMO of CTrinhCl
6
, (d) LUMO of
CTrinhCl
6
, (e) optimized ground state structure of CTrinhCl
6
and (f) calculated spin
density of CTrinhCl
6
.

-2.78 eV
-5.80 eV
LUMO
-2.65 eV
HOMO
-5.68 eV
a) c)
b)
d)
e)
f)
146

small contribution of iridium’s d-orbitals to the frontier molecular orbitals. Interestingly,
the spin density of the triplet state of CTrinhCl
6
delocalizes over chlorodipyrrin and
f2ppy ligands with minimum contribution from the iridium metal center.
Absorption and emission spectra of IrDP and CTrinh are presented in Figure 5.15.
The optical density of iridium complexes are significantly lower than that of zinc dipyrrin
complexes because only one dipyrrin ligand in iridium complexes is responsible for light
absorption while the molecular volumes of CTrinh and IrDP are larger than that of ZCl
and Z1. However, the optical densities of iridum dipyrrin complexes are noticeably
higher than that of C
60
. Thus, all photophysical and electrochemical properties of CTrinh
satisfy the requirements for a C
60
sensitizer (see section 5.2).

5.3.3. Subphthalocyanines
Synthesis of halogenated subphthalocyanines (Cl
6
SubPc and F
12
SubPc) following
the published procedures is summarized in Figure 5.16.
37,49
Characterization of
300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0.2
0.4
0.6
0.8
1.0
abs. C
60
ab IrDP

Optical density, ' (x10
5
cm
-1
)
Wavelength (nm)
em IrDP
Em. intensity (a.u.)

Figure 5.15. Absorption and emission spectra of CTrinh and IrDP in thin films.
147

halogenated subphthalocyanines is summarized in Table 5.1. After synthesis, Cl
6
SubPc
and F
12
SubPc were further purified by gradient sublimation under vacuum.

Absorption and emission spectra of halogenated subphthalocyanines are presented
in Figure 5.17; the absorption spectra are red-shifted compared to that of ZCl and CTrinh.
Extinction coefficients of SubPcs are 6-8 folds higher than that of C
60
at 580 nm. Singlet
energies of phthalocyanines are higher than that of C
60
, however the triplet energies are
300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0.2
0.4
0.6
0.8
1.0
ab C
60
ab Cl
6
SubPc
ab F
12
SubPc

Optical density, ' (x10
5
cm
-1
)
Wavelength (nm)
em.
em.
Em. intensity (a.u.)

Figure 5.17. Absorption (filled symbols) and emission (open symbols) spectra of
Cl
6
SubPc (blue triangles) and F
12
SubPc (red squares).

Figure 5.16. Synthetic route to F
12
SubPc and Cl
6
SubPc.
148

slightly lower. Since the difference in triplet energies of F
12
SubPc and C
60
is small
(0.05 eV ~ 2k
B
T), thermal equilibrium between the two triplet states are expected at room
temperature or the triplet energy back transfer from C
60
to F
12
SubPc is not an efficient
process. Oxidation potentials of Cl
6
SubPc and F
12
SubPc also satisfy requirement (2) for
a sensitizer.
5.4. Photosensitization study
5.4.1. Stern-Volmer quenching experiments
The photosensitization efficiency was quantified by Stern–Volmer quenching
measurements of ZCl:C
60
mixtures in toluene solution (see Figure 5.18). The
measurements were carried out in toluene under ambient conditions. ZCl, Cl
6
SubPc and
F
12
SubPc emissions are efficiently quenched by C
60
. The rate constant for luminescent
quenching is k
q
= 7.8 x 10
12
, 1.61 x 10
12
and 1.47 x 10
12
M
-1
s
-1
for ZCl, Cl
6
SubPc and
F
12
SubPc, respectively. Higher quenching rate of ZCl than SubPcs might be due to better
spectral overlap between ZCl emission and C
60
absorption spectra that would lead to
more efficient Forster energy transfer from ZCl to C
60
in solution. However, the
quenching experiment does not shed light on the sensitization mechanism, since both
energy and electron transfer are thermodynamically viable.
5.4.2. Photoluminescence study on thin films
To examine whether energy transfer processes from ZCl to C
60
will lead to
sensitization in the solid state, absorption and photoluminescence (PL) spectra were
collected from neat C
60
and mixed C
60
:ZCl films with varied ratios (Figure 5.19).
149

Note that amount of C
60
in these films is kept constant. In all blended samples,
the fluorescence of ZCl was completely quenched and emission bands with spectral
profiles identical to neat C
60
were observed between 700–900 nm. The luminescence

Figure 5.18. Stern-Volmer quenching experiments of ZCl (a, b), Cl
6
SubPc (c, d) and
F
12
SubPc (e, f) using C
60
as quencher in toluene.

0.0 0.2 0.4
1
2
3
Linear fit: Y = A + BX
A = 1.001(13)
B = 2183(71)
goodness of fit: R = 0.998
I
0
/I
Concentration of C
60
(mM)
b)
500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
Em. intensity (a.u.)
Wavelength (nm)
Upon adding C
60
a)
0.0 0.2 0.4
1
2
3
Linear fit: Y = A + BX
A = 1.01(02)
B = 4830(10)
goodness of fit: R = 0.999
I
0
/I
Concentration of C
60
(mM)
d)
550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0 c)

Em. intensity (a.u.)
Wavelength (nm)
Upon adding C
60
550 600 650 700
0.0
0.2
0.4
0.6
0.8
e)

Em. intensity (a.u.)
Wavelength (nm)
Upon adding C
60
0.0 0.2 0.4
1
2
3
f)
Linear fit: Y = A + BX
A = 0.96(03)
B = 4550(14)
goodness of fit: R = 0.998
I
0
/I
Concentration of C
60
(mM)
150

from the thin films is assigned to the excited singlet state of C
60
since the band shape and
peak maximum (
max
= 737 nm) matches well with published fluorescence spectra of
C
60
50,51
and differs markedly in peak position from the phosphorescence of ZCl
(Figure 5.8). The fluorescence intensity increased with higher ZCl content and
consequent rise in absorbance from the blended films.

While the luminescent quenching of ZCl could occur via an electron transfer
mechanism to form a ZCl
+
C
60
-
CT intermediate (pathway C in Figure 5.2b), the energy of
this CT state is estimated to be 2.0 eV. Subsequent charge recombination of the CT state
would be expected to lead to a triplet state localized on C
60
and not give rise to a
fluorescence signal.
32,52
Thus, the PL measurements indicate that photoexcited ZCl
300 400 500 600 700 800 900
0.0
0.2
0.4
0.6
0
6
12
18
T
1
S
1
C
60
1.84
1.50
1.60
2.22
ZCl
Absorbance
Wavelength (nm)
em. intensity (10
4
counts)

Figure 5.19. Absorption (solid symbols) and emission under excitation at 514 nm
(open symbols) of 50 nm C
60
film (black circle) and 59 nm C
60
:ZCl film (blue triangle,
15% ZCl by volume) and 75 nm C
60
:ZCl film (red square, 35% ZCl by volume). Also
shown is an energy level diagram (eV) for energy transfer from ZCl to C
60
.
151

transfers the singlet energy to the C
60
host in the thin films, resulting in increased
fluorescence from C
60
(see pathway A in Figure 5.2b).
Another sensitization pathway in the C
60
:ZCl blend is triplet energy transfer from
ZCl to C
60
(pathway B in Figure 5.2b). Luminescent spectra of ZCl in both solution and
neat solid clearly show competitive ISC to the triplet state. However, the rate of ISC is at
best 3.7 x 10
8
s
-1
, which is an order of magnitude smaller than quenching rate constant by
C
60
determined from Stern-Volmer quenching experiments. Regardless, any triplets
formed on ZCl can be subsequently transferred to the lower triplet state of C
60
. Thus, in
all possible pathways of either singlet, triplet or electron transfer from ZCl to C
60
, the
triplet energy of C
60
is the lowest energy state, assuring that the sensitized energy is
cascaded to the C
60
host according to the design presented in Figure 5.2b.
Dilute concentration of C
60
(5%) blended into Cl
6
SubPc and F
12
SubPc films
results in completely quenched photoluminescence. Since the ISC rates of halogenated
SubPcs (at best 1 x 10
8
s
-1
) are an order of magnitude smaller than the quenching rate
determined in the Stern-Volmer experiment, singlet energy or electron transfers from
SubPcs to C
60
are expected in the solid state. Thus, sensitization pathways A or C
(Figure 5.2b) are expected for halogenated subphthalocyanines.
In the case of iridium dipyrrin complexes, the ISC rate is faster than 200 fs as
determined by ultrafast transient absorption. Hence, the sensitization by B or C
pathways, which would ultimately lead to formation of C
60
triplet state (Figure 5.2b) are
the most plausible.
152

5.5. Morphology of blended films
Since the blended C
60
films are intended for use in OPV devices, it is important to
understand the effect that the presence of a sensitizer has on molecular packing of C
60
in
the solid state. Grazing incident X-ray diffraction (GIXD) measurements were carried
out on thin films of C
60
(50 nm), ZCl (50 nm) and mixed C
60
:ZCl films with various
concentrations, and the results are presented in Figure 5.20. The neat C
60
film shows six
diffraction peaks at 0.77, 1.27, 1.48, 2.01, 2.19 and 2.32 Å
-1
, which are indexed to the fcc

Figure 5.20. GIXD data of neat and blended C
60
films on Si substrates: (a) neat C
60

(50 nm), (b) C
60
:ZCl (56 nm, 10%), (c) C
60
:ZCl (67 nm, 33%), (d) C
60
:ZCl (100 nm,
50%), (e) neat ZCl (50 nm), (f) integrated diffraction intensity of neat and doped C
60

films at different concentrations. Blending concentration is in % of ZCl by volume.

153

crystal phase as the 111, 220, 311, 420, 422, and 333 peaks respectively,
53
whereas the
neat films of ZCl are amorphous. Upon addition of ZCl, the C
60
phase become
substantially more disordered as indicated by the dramatic broadening of the diffraction
rings, reduced peak intensity and the inability to resolve the 220 and 311 peaks. Thus,
the GIXD measurements show that the mixed films become nano-crystalline or
amorphous upon addition of the ZCl sensitizer.

Analysis by AFM techniques presented in Figure 5.21 also supports a decrease in
molecular order in the blended ZCl:C
60
films. AFM images of neat films of C
60
were
found to be rough (rms = 1.85 nm), whereas neat films of ZCl were smooth (rms =
0.59 nm). Interestingly, the AFM image of the 1:1 mixed film shows no sign of phase
separation and is quite smooth (rms = 0.39 nm). Thus, the AFM and GIXD data indicates
that ZCl guest molecules are homogeneously dispersed in the C
60
host, which is likely
due to similar the sizes and shapes of both compounds. Homogeneous dispersion of ZCl
into C
60
is also beneficial for efficient photosensitization as the ZCl molecules will be
located in close proximity to the C
60
molecules.

Figure 5.21. AFM images (5x5 µm) of (a) neat C
60
, (b) mixed C
60
:ZCl (50% ZCl by
volume) and (c) neat ZCl films on Si substrates.
8 nm
0 nm
a) b) c)
154

5.6. Effect of blending on C
60
absorption
The absorption profile of C
60
is particularly rich, and this has led to some
uncertainty in the importance of its various absorption features in the photocurrent
generation process. For example, the C
60
absorption spectrum in solution is dominated
by two features with peaks at wavelengths of = 260 nm and 340 nm that are attributed
to allowed electronic transitions resulting in Frenkel-type (i.e. monomolecular) excited
states, while the absorption at longer wavelengths is low due to a symmetry-forbidden
transition.
5
On transition from solution to the solid state, a significant increase in
absorption between = 400 and 550 nm is observed (Figure 5.1). This transition is
attributed to the emergence of an intermolecular CT state
5
resulting from the excitation
of an electron from the HOMO of one fullerene into the LUMO of its nearest neighbors.
Since the presence of sensitizers in C
60
:sensitizer films disrupt the intermolecular
interaction between C
60
molecules, it is of particular interest to understand how blending
guest molecules into C
60
affects its absorption profile. The relative roles played by direct
absorption into the Frenkel or the CT states have been explored by blending C
60
with the
large energy gap (and hence transparent) material, bathocuproine (BCP). BCP has larger
singlet (3.17 eV)
8
and triplet (2.62 eV)
8
energies than those of C
60
(corresponding
energies of 1.86 eV
5
and 1.55 eV
9
). Additionally, the HOMO and LUMO energies of
BCP are at -6.5 eV
10
and -1.6 eV
8
relative to vacuum, respectively, while for C
60
they are
-6.4 eV
11
and -4.0 eV.
11
Thus, C
60
does not engage in either energy or electron transfer to
BCP.
155

The absorption spectra of the neat and blended BCP:C
60
films are shown in
Figure 5.22. The absorption coefficient, , of the allowed Frenkel transition at = 340
nm is proportional to the C
60
fraction as predicted by Beer’s law, reflecting the
monomolecular nature of this transition. In contrast, absorption of the CT absorption at
= 450 nm exhibits a power-law dependence, viz.: , where x is the C
60
volume
fraction and m = 2.7 ± 0.1 (see inset, Figure 5.22). This implies that the formation of CT
excitons involves two to three molecules, in agreement with previous studies,
5, 12
and
that even modest C
60
dilutions significantly reduces CT absorption.

300 400 500 600 700 800
0
1
2
3
4
0.0 0.5 1.0
0.0
0.5
1.0
C
60
fraction
340 nm
450 nm

a.u.)

C
60
C
60
:BCP 2:1
C
60
:BCP 1:1 C
60
:BCP 1:2
BCP
(10
5
cm
-1
)
Wavelength (nm)

Figure 5.22. Absorption coefficient of BCP:C
60
blend films calculated from optical
constants determined by variable angle spectroscopic ellipsometry. Inset: Extinction
as a function of C
60
fraction for wavelengths of 360 nm and 450 nm, corresponding to
Frenkel and CT absorption features, respectively. Linear and power law (y = x
2.7
) fits
are shown for the 340 and 450 nm data, receptively.
156

5.7. Application to OPV devices
OPVs were fabricated with a common multilayer structure of
ITO/MoO
3
/donor/acceptor/buffer/Al. Since the purpose of this work is to explore the
sensitization process in the visible spectrum, the UV absorbing N, N’-di-[(1-naphthyl)-
N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPD) was chosen as the donor material.
NPD shows negligible absorption at wavelengths longer than 400 nm, which allows the
photoresponse from sensitizers and C
60
to be unambiguously assigned. MoO
3
is used to
improve hole extraction efficiency from the donor layer. The acceptor layer can be neat
C
60
or mixed C
60
:sensitizer with various architectures, and the buffer layer is
bathocuproine (BCP).
There are several ways to construct the sensitized devices; one straight forward
method is to deposit the mixed C
60
:sensitizer film with varied sensitizer concentrations
directly on the donor layer, as was done in previous works.
23,29
Unfortunately, this
simple architecture creates ambiguity as to whether any photoresponse from the sensitizer
originates from photosensitization of C
60
or from direct charge separation between donor
and sensitizer at the DA interface. Thus, sensitized devices with a structure
ITO/MoO
3
(10 nm)/NPD (10 nm)/C
60
/C
60
:sensitizer/BCP (7 nm)/Al were constructed to
prevent any direct contact of the sensitizers with the donor layer. The sensitizer
candidates were initially screened using the device structure ITO/MoO
3
(10 nm)/NPD
(10 nm)/C
60
(5 nm)/C
60
:sensitizer/BCP (7 nm)/Al, followed by optimization of the device
structure in order to achieve the best sensitization efficiency.
157

5.7.1. Screening sensitizers
In this section the OPV devices with various sensitizers are explored using very
thin layer of neat C
60
(5 nm) at the DA interface to ensure that sensitized excitons can
diffuse to the DA interface. Current–voltage (J–V) and external quantum efficiency
(EQE) curves of OPV devices utilizing a C
60
:ZCl layer with 15% (D2) and 50% (D3) of
ZCl by volume on top of a thin C
60
layer (5 nm) are presented in Figure 5.23.

The values for the open circuit voltage (V
OC
) and fill factor (FF) in both sensitized
devices remain unchanged from those for the reference device D1. Seeing as all three
devices have the same NPD/C
60
interface, the thermodynamics and kinetics of the charge
separation/recombination processes at this D/A boundary are identical, and consequently,
the V
OC
is unaffected. The unaltered FF value in devices D2-3 indicates that the charge
400 500 600 700 800
0
10
20
30
40
EQE (%)
Wavelength (nm)
D1: x = 40, y = 0
D2: x = 5, y = 40, z = 15%
D3: x = 5, y = 40, z = 50%
MoO
3
, 10 nm
ITO
NPD, 11 nm
C
60
, x nm
C
60
:ZCl z%, y nm
BCP, 7 nm
Al
C
60
ZCl
a)
-0.5 0.0 0.5 1.0
-4
-2
0

Current density (mA/cm
2
)
Voltage (V)
D1
D2
D3
1.06
1.17
1.19
0.46 0.89 2.51
2.92 0.88 0.46
0.44 0.89 3.04
D3
D2
D1
FF
J
SC
V
OC
b)
Figure 5.23. Structure and characteristics of OPV devices using an NPD donor layer
with and without ZCl. (a) plot of external quantum efficiency, inset is the device
structure, and (b) J –V curves of devices under one sun AM1.5G illumination, inset is
characteristics of the devices with J
SC
of ± 0.06 mA/cm
2
, V
OC
of ± 0.01 V, FF of
± 0.02 and of ± 0.1 %
158

collection efficiency is well maintained despite the lower crystallinity of the mixed
C
60
:ZCl film. The high conductivity of C
60
, along with the similar size and shape of ZCl
and C
60
allows a homogenous C
60
:ZCl layer to form with sufficient percolation pathways
in C
60
to enable good electron conduction.
The photocurrents of the devices drop as the ZCl concentration increases,
D1>D2>D3. The reduction in photocurrent can be understood from EQE measurements
(Figure 8b). EQE clearly indicates the contribution of the sensitizer ZCl (peak maximum
at 550 nm) to the photocurrents in devices D2 and D3, with the ZCl photoresponse
proportional to its concentration. As there is no direct contact between ZCl and the donor
layer, the increased photoresponse between 500–600 nm in D2 and D3 must come from
photoenergy absorbed by ZCl and subsequently transferred to C
60
. In contrast, the
photoresponse from C
60
decreases as ZCl concentration increases in devices D2 and D3.
This decrease is slightly larger than the increase due to ZCl sensitization, resulting in a
net decrease in the photocurrent of D2 and D3.
Similar results were observed for Cl
6
SubPc and F
12
SubPc sensitized devices as
shown in Figure 5.24. The FF and V
OC
of the sensitized devices were unchanged. Clear
photoresponses from subphthalocyanines were observed at around 600 nm, however the
increase from the sensitizers’ responses were compensated by the decrease in C
60

photoresponse, resulting in small changes of photocurrents.
159

In order to clarify the origin of the decrease in the C
60
photoresponse in devices
using sensitizers, a series of devices using mixed C
60
:BCP layer were fabricated. While
there is no change in FF and V
OC
in devices using mixed C
60
:BCP similar to the
sensitized devices, the photocurrent substantially decreases as the BCP concentration
400 500 600 700 800
0
10
20
30
40
C
60
F
12
SubPc
EQE (%)
Wavelength (nm)
D1
D4: 9%
D5: 15%
D6: 33%
MoO
3
, 10 nm
ITO
NPD, 11 nm
C
60
, 5 nm
C
60
:F
12
SubPc, 40
BCP, 7 nm
Al
a)
-0.5 0.0 0.5 1.0
-4
-2
0
2.74 0.88 0.47 1.13
D6

Current density (mA/cm
2
)
Voltage (V)
D1
D4
D5
D6
1.06
1.09
1.19
0.46 0.87 2.67
2.76 0.86 0.47
0.44 0.89 3.04
D5
D4
D1
FF
J
SC
V
OC
b)

400 500 600 700 800
0
10
20
30
40
EQE (%)
Wavelength (nm)
D1
D7: 17% Cl
6
SubPc
C
60
Cl
6
SubPc
MoO
3
, 10 nm
ITO
NPD, 11 nm
C
60
, 5 nm
C
60
:Cl
6
SubPc, 40
BCP, 7 nm
Al
c)
-0.5 0.0 0.5 1.0
-4
-2
0

Current density (mA/cm
2
)
Voltage (V)
D1
D7
1.11
1.19
2.90 0.88 0.44
0.44 0.87 3.04
D7
D1
FF
J
SC
V
OC
d)

Figure 5.24. Structure and characteristics of OPV devices using F
12
SubPc sensitizer:
(a) and (b) and Cl6SubPc sensitizer (c) and (d). (a) and (c) show EQE curves, insets
are the device structures; (b) and (d) present J –V curves one sun AM1.5G
illumination, insets are characteristics of the devices with J
SC
of ± 0.06 mA/cm
2
, V
OC

of ± 0.01 V, FF of ± 0.02 and of ± 0.1 %
160

increases. EQE measurements (Figure 5.25) indicate that the decrease in photocurrents is
due to the decrease in C
60
photoresponse, which is well correlated with absorption profile
(see Figure 5.22). Thus, blending a guest molecule into C
60
films results in the decrease
of the CT absorption, and consequently to the decrease of C
60
photoresponse in 400–
500 nm region.

5.7.2. Optimization of ZCl sensitized devices
It is desirable to maintain the photoresponse from C
60
while maximizing the
photoresponse from the sensitizer ZCl, thus sensitized devices with a high ZCl
concentration (50%) were chosen for further optimization. A series of devices consisting
of a constant thickness of the acceptor layer and varied thicknesses of the neat C
60
layer
at the D/A interface, ITO/MoO
3
/NPD/C
60
(x nm)/C
60
:ZCl (1:1, 45-x nm)/BCP/Al, have

Figure 5.25. EQE curves of devices using blended C
60
:BCP in the acceptor layers.
Inset is the device structures with the various concentration of BCP: D1 (neat C
60
), D8
(33% BCP), D9 (50% BCP) and D10 (67% BCP)
400 500 600 700 800
0
10
20
30
40
EQE (%)
Wavelength (nm)
S1
S2
S3
S4
Al
BCP, 7 nm
C
60
:BCP (A:B), 40 nm
C
60
, 5 nm
NPD, 11 nm
MoO
3
, 10 nm
ITO
D1
D8
D9
D10
161

been fabricated. The EQE measurements of these devices are presented in Figure 5.26.
In this set of OPVs, the total thickness of neat C
60
and blended C
60
:ZCl was kept at
45 nm. As the thickness of the neat layer of C
60
increases from 5 nm (D3) to 15 nm
(D11), the photoresponse of C
60
is fully recovered, while a significant contribution from
ZCl is still observed. A further increase of C
60
thickness to 25 nm (D12) markedly
decreases the ZCl photoresponse along with the same response photoresponse from C
60
.
When 35 nm of C
60
is used (D13), no photoresponse from ZCl is observed, likely due to
the limited diffusion length of the sensitized excitons.

Sensitized devices with 15 nm of C
60
at the D/A interface were further optimized
by varying the thickness of the mixed C
60
:ZCl (1:1) layer. The results are presented in
Figure 5.27a. Similar to other sensitized devices, the FF and V
OC
remains largely
unchanged, and only J
SC
varies as the thickness of mixed C
60
:ZCl layer changes. The J
SC

value reaches a maximum at a mixed layer thickness of 50 nm. Compared to the control
400 500 600 700 800
0
10
20
30
40
EQE (%)
Wavelength (nm)
D1: x = 40, y = 0
D11: x = 15, y = 30
D12: x = 25, y = 20
D13: x = 35, y = 10
MoO
3
, 10 nm
ITO
NPD, 11 nm
C
60
, x nm
C
60
:ZCl 1:1, y nm
BCP, 7 nm
Al
ZCl
C
60

Figure 5.26. EQE measurements of sensitized devices using ZCl with varied C
60
layer
thicknesses at the D/A interface.
162

device, D1, using only neat C
60
as acceptor layer, D14 exhibits a markedly higher
photocurrent ( J
SC
= 1.0 mA/cm
2
). Thus, the improved J
SC
, in combination with
unchanged V
OC
and FF, results in an increase of the power conversion efficiency from
1.16% (D1) to 1.56% (D14).

5.7.3. Sensitization using multiple sensitizers
We have demonstrated that sensitization of ZCl can be used to broaden the
absorption of C
60
-based acceptor layer out to 600 nm. However, this approach can be
applied to multiple sensitizers to further extend the absorption of the acceptor layer.
Since F
12
SubPc and Cl
6
SubPc absorb out to 650 nm (see Figures 5.17 and 5.24), co-
sensitization of either subphthalocyanines and ZCl presents an interesting way to
implement the absorption of the acceptor layer in the red region of solar spectrum.

Figure 5.27. (a) Characteristics of sensitized devices with a neat layer of C
60
(x =
15 nm) and varied thicknesses (y nm) of the C
60
:ZCl doped layers; y = 0 represents the
reference device with 40 nm of neat C
60
as the acceptor layer; and (b) plot of external
quantum efficiency of D1 and optimized device using sensitizer (D14), inset is the
device structure.

0 30 60 90
0.0
0.5
1.0
1.5
0
2
4

FF, V
OC
(V),
p
(%),
Doped layer thickness, y (nm)
FF
J
SC
(mA/cm
2
)
p
V
OC
J
SC
a)
400 500 600 700 800
0
10
20
30
40
C
60
D1: x = 40, y = 0
D14: x = 15, y = 50
MoO
x
, 10 nm
EQE (%)
Wavelength (nm)
ITO
NPD, 11 nm
C
60
, x nm
C
60
:ZCl 1:1, y nm
BCP, 7 nm
Al
ZCl
b)
163

Sensitized solar cells using F
12
SubPc and Cl
6
SubPc were fabricated to evaluate
the sensitizers, and the results are shown in Figure 5.28. As can be seen, the Cl
6
SubPc
leads to higher sensitized photocurrent improvement, therefore Cl
6
SubPc was chosen for
use in co-sensitization with ZCl.

There are two ways to construct co-sensitized devices, codeposition (D21) or
energy cascade (D22) structures of Cl
6
SubPc and ZCl, as shown in Figure 5.29. In the
device D22, the photoresponse of Cl
6
SubPc, which is located near the D/A interface, is
much stronger than that of ZCl. Since the C
60
:ZCl layer is positioned further way from
the D/A interface, low ZCl photoresponse is attributed to limited sensitized exciton
diffusion lengths. In the device, D21, both sensitizers are evenly distributed in the
blended layer, thus leading to clear photoresponses from both sensitizers. Device
400 500 600 700 800
0
10
20
30
40
EQE (%)
Wavelength (nm)
D1: C
60
, 25 nm
D19:F
12
SubPc, 40nm
D20:F
6
SubPc, 30nm
MoO
3
, 10 nm
ITO
NPD, 11 nm
C
60
, 15 nm
C
60
:Sen 1:1, y nm
BCP, 7 nm
Al
a)

-0.4 0.0 0.4 0.8 1.2
-4
-3
-2
-1
0
1
D20
D19
1.32 0.43
0.89 3.43

Current density (mA/cm
2
)
Voltage (V)
D1
D19
D20
1.22
1.14
FF
3.16 0.90 0.43
0.46 0.88 2.83
J
SC
V
OC

D1
b)

Figure 5.28. Structure and characteristics of OPV devices using subphthalocyanines
sensitizers. (a) EQE curves, inset is the device structure, and (b) J –V curves of devices
under one sun AM1.5G illumination, inset is characteristics of the devices with J
SC
of
± 0.06 mA/cm
2
, V
OC
of ± 0.01 V, FF of ± 0.02 and of ± 0.1
164

characteristics are presented in Table 5.2. Similar to what was observed in singly
sensitized devices, co-sensitized devices have the same V
OC
and FF as D1, but the
photocurrent is significantly improved (∆J
SC
= 1.2 mA/cm
2
) because of the extended
absorption of the acceptor layer up to 650 nm, resulting in improvement of efficiency
from 1.16 to 1.77%.
Table 5.2. Characteristics of co-sensitized devices (D21 and D22) under 1sun AM1.5G
illumination.
Devices J
SC

(± 0.06 mA/cm
2
)
V
OC

(± 0.01 V)
FF
(±0.02)

(± 0.1 %)
D1 3.04 0.89 0.44 1.19
D21 4.30 0.87 0.47 1.77
D22 4.10 0.90 0.44 1.60

Figure 5.29. (a) Structures of co-sensitized devices and (b) EQE curves of the devices.
NPD 11nm
C
60
:Cl
6
SubPc,
30nm, 50%
C
60
15 nm
ITO
BCP (7nm)
Al
C
60
:ZCl, 30nm,
50%
MoO
3
11nm
NPD 11nm
C
60
:ZCl:Cl
6
SubPc,
50:25:25%, 70nm
C
60
15 nm
ITO
BCP (7nm)
Al
MoO
3
11nm
D21:codeposition D22:cascade
a)
400 500 600 700
0
10
20
30
Cl
6
SubPc
ZCl
C
60
C
60
EQE (%)
Wavelength (nm)
D1
D21
D22
ZCl
Cl
6
SubPc
Sensitize energy
to C
60
b)
165

5.7.4. Application to red-NIR absorbing donor materials

As the absorption of donor materials is extended to the red-NIR part of spectrum,
the green-to-orange part is left unabsorbed; 2,4-bis[4-(N,N-diisobutylamino)-2,6-
dihydroxyphenyl] squaraine (SQ)
54
and zinc phthalocyanine (ZnPc) donor based OPVs
illustrate this problem (Figure 5.30 and 5.31). There are minima in the EQE curves at
roughly 550 nm in both SQ/C
60
and ZnPc/C
60
devices where neither donor (ZnPc or SQ)
nor C
60
absorb effectively. The absorption of ZCl matches well with the EQE minima,
thus ZCl can potentially be used to fill up the absorption gap in SQ and ZnPc OPV
devices. Sensitized devices with the optimal acceptor structure have been fabricated with
ZnPc and SQ. As indicated by EQE measurements (Figure 5.30), the photoresponses
400 500 600 700 800
0
10
20
30
40
50

SQ
ZCl
C
60
EQE (%)
Wavelength (nm)
D15
D16
MoO
x
, 11 nm
ITO
SQ, 11 nm
C
60
, x nm
C
60
:ZCl, y nm
BCP, 7 nm
Al
C
60
2.78
2.39
7.07 0.78 0.51
0.53 0.76 5.88
D16
D15
FF J
SC
V
OC
b)

Figure 5.30. Structure and characteristics of OPV devices using a squaraine (SQ)
donor layer with and without a sensitizer. (a) J–V curves of devices under one sun
AM1.5G illumination, inset is the SQ structure (b) plot of external quantum efficiency,
inset is the device structure (x = 15 nm, y = 50 nm) and characteristics of the devices
with J
SC
of ± 0.09 mA/cm
2
, V
OC
of ± 0.01 V, FF of ± 0.01, and of ± 0.09 %
.
0.0 0.5 1.0
-8
-4
0

O
-
O
-
OH
OH
HO
HO
N N 2+

Current density (mA/cm
2
)
Voltage (V)
D15
D16
a)
166

from SQ and C
60
remain unchanged while the ZCl response nicely fills up the dip in the
EQE curve, leading to a photocurrent improvement of 1.0 - 1.2 mA/cm
2
without any
other change in the device characteristics. This increase of photocurrent is comparable to
what was observed in the NPD/C
60
device. Similar improvement is observed when ZCl
is used in ZnPc/C
60
OPV devices (Figure 5.31). Thus, the obtained results demonstrate
that the ZCl sensitizer can be used with a variety of donor materials.

The multichromophoric sensitization using ZCl and Cl
6
SubPc were also applied
to 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] squaraine (DPSQ) devices. The
results are presented in Figure 5.32. As can be seen, the photoresponse of the sensitized
device nicely cover the whole spectral range from 350 to 850 nm, resulting in significant
-0.3 0.0 0.3 0.6
-6
-4
-2
0
2

Current density (mA/cm
2
)
Voltage (V)
D17
D18
a)
1.50
1.21
6.27 0.46 0.52
0.50 0.45 5.37
D18
D17
FF
J
SC
V
OC
400 500 600 700 800
0
10
20
30
40

EQE (%)
Wavelength (nm)
D17:x=40, y=0
D18:x=15, y =46
C
60
ITO
ZnPc, 30 nm
C
60
, x nm
C
60
:ZCl 2:1,y nm
BCP, 7 nm
Al
b)

Figure 5.31. Structure and characterization of OPV devices using ZnPc donor with
and without sensitizer. (a) J-V curves of devices under 1 sun AM1.5G illumination,
inset is characteristics of the devices with J
SC
of ± 0.09 mA/cm
2
, V
OC
of ± 0.01 V, FF
of ± 0.01, and of ± 0.09 % and (b) external quantum efficiency measurements, inset
is the device structure.

167

improvements of both photocurrent (from 6.58 to 8.83 mA/cm
2
) and efficiency (from
3.65 to 4.71%).

5.8. Conclusion
In summary, we have shown a set of energetic requirements and molecular design
for a C
60
sensitizer. An ideal sensitizer should have higher singlet and triplet energies

400 500 600 700 800
0
10
20
30
40
50

C
60
Cl
6
SubPc
EQE (%)
Wavelength (nm)
D23: C
60
D24: sensitized C
60
ZCl
DPSQ
b)
0.0 0.5 1.0
-8
-6
-4
-2
0
2

Current density (mA/cm
2
)
Voltage (V)
D23
D24
4.71
3.65
FF
8.83 0.92 0.57
0.61 0.93 6.58
D24
D23
FF
J
SC
V
OC
c)

Figure 5.32. Structure and characteristics of OPV devices using a squaraine (SQ)
donor layer with and without a sensitizer. (a) Device and DPSQ structures, (b) plot of
external quantum efficiency and (c) J –V curves of devices under one sun AM1.5G
illumination, inset is the device structure and characteristics of the devices with J
SC
of
± 0.09 mA/cm
2
, V
OC
of ± 0.01 V, FF of ± 0.01, and of ± 0.09 %

DPSQ 11nm
C
60
40 nm
ITO
BCP (7nm)
Al
MoO
3
11nm
DPSQ 11nm
C
60
:ZCl:Cl
6
SubPc,
50:25:25%, 70nm
C
60
15 nm
ITO
BCP (7nm)
Al
MoO
3
11nm
D23
D24
a)
DPSQ
168

than C
60
in addition to a sufficiently high oxidation potential to produce a Sen
+
C
60
-
CT
state with a higher energy than the C
60
triplet exciton. A chlorinated zinc dipyrrin, ZCl
and halogenated subphthalocyanines (Cl
6
SubPc and F
12
SubPc) were synthesized and
fully characterized. Stern-Volmer quenching experiments and PL measurements of
mixed C
60
:sensitizer films show that sensitizers efficiently transfers absorbed
photoenergy to C
60
.
Several OPV devices employing the C
60
:ZCl host-guest acceptor layer have been
constructed and optimized to demonstrate the sensitization approach. The optimal
acceptor layer consisting of a neat C
60
(15 nm) and a mixed C
60
:ZCl (1:1, 50 nm) films
has been used to maintain the beneficial properties of C
60
while taking advantage of ZCl
absorption. In the sensitized devices with various donor materials, efficient energy
transfer from the photoexcited state of ZCl to C
60
can increase the photoresponse of the
acceptor layer up to 33% without changing other device characteristics. The sensitization
approach presented here could be used for multiple sensitizers to extend the absorption
range of the electron acceptor layer. The co-sensitized devices using both ZCl and
Cl
6
SubPc sensitizers show photoresponses of the C
60
-based acceptor layer up to 650 nm.
While we have focused on the C
60
acceptor layer in the present study, a similar approach
could be used with other fullerenes as well as sensitized donor layers to broaden the
photoresponse of OPVs.

169

Chapter 5 Endnote
(1) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736-6767.
(2) Hains, A. W.; Liang, Z.; Woodhouse, M. A.; Gregg, B. A. Chem. Rev. 2010, 110,
6689-6735.
(3) Schlenker, C. W.; Thompson, M. E. Chemical Communications 2011, 47, 3702-
3716.
(4) Anthopoulos, T. D.; Singh, B.; Marjanovic, N.; Sariciftci, N. S.; Ramil, A. M.;
Sitter, H.; Colle, M.; de Leeuw, D. M. Applied Physics Letters 2006, 89.
(5) Liu, Y.; Zhao, J. Chemical Communications 2012, 48, 3751-3753.
(6) Zhang, X. H.; Kippelen, B. Applied Physics Letters 2008, 93.
(7) Zhang, X.-H.; Kippelen, B. Journal of Applied Physics 2008, 104.
(8) Lin, Y.; Li, Y.; Zhan, X. Chem. Soc. Rev. 2012, 41, 4245-4272.
(9) Mishra, A.; Baeuerle, P. Angewandte Chemie-International Edition 2012, 51,
2020-2067.
(10) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.;
Chen, C.-C.; Gao, J.; Li, G.; Yang, Y. Nature Communications 2013, 4, 1446.
(11) Kazaoui, S.; Minami, N.; Tanabe, Y.; Byrne, H. J.; Eilmes, A.; Petelenz, P.
Physical Review B 1998, 58, 7689-7700.
(12) Schlenker, C. W.; Thompson, M. E. Chemical Communications 2011, 47, 3702–
3716.
(13) Peumans, P.; Uchida, S.; Forrest, S. R. Nature 2003, 425, 158-162.
(14) Peumans, P.; Yakimov, A.; Forrest, S. R. Journal of Applied Physics 2003, 93,
3693-3723.
(15) Lassiter, B. E.; Wei, G.; Wang, S.; Zimmerman, J. D.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Applied Physics Letters 2011, 98, 243307-3.
(16) Wei, G.; Lunt, R. R.; Sun, K.; Wang, S.; Thompson, M. E.; Forrest, S. R. Nano
Letters 2010, 10, 3555-3559.
(17) Wei, G.; Xiao, X.; Wang, S.; Sun, K.; Bergemann, K. J.; Thompson, M. E.;
Forrest, S. R. Acs Nano 2012, 6, 972-978.
(18) Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Nano Letters 2012, 12, 4366-4371.
(19) Wei, G.; Wang, S.; Renshaw, K.; Thompson, M. E.; Forrest, S. R. Acs Nano
2010, 4, 1927-1934.
(20) Wei, G.; Xiao, X.; Wang, S.; Zimmerman, J. D.; Sun, K.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Nano Letters 2011, 11, 4261-4264.
(21) Anthony, J. E. Chemistry of Materials 2011, 23, 583-590.
(22) Sonar, P.; Lim, J. P. F.; Chan, K. L. Energy & Environmental Science 2011, 4,
1558-1574.
(23) Hesse, H. C.; Weickert, J.; Hundschell, C.; Feng, X.; Muellen, K.; Nickel, B.;
Mozer, A. J.; Schmidt-Mende, L. Advanced Energy Materials 2011, 1, 861-869.
(24) Ford, W. E.; Kamat, P. V. Journal of Physical Chemistry 1987, 91, 6373-6380.
170

(25) Kircher, T.; Lohmannsroben, H. G. Physical Chemistry Chemical Physics 1999,
1, 3987-3992.
(26) Baffreau, J.; Leroy-Lhez, S.; Van Anh, N.; Williams, R. M.; Hudhomme, P.
Chemistry-a European Journal 2008, 14, 4974-4992.
(27) Baffreau, J.; Leroy-Lhez, S.; Hudhomme, P.; Groeneveld, M. M.; van Stokkum, I.
H. M.; Williams, R. M. Journal of Physical Chemistry A 2006, 110, 13123-13125.
(28) Schueppel, R.; Uhrich, C.; Pfeiffer, M.; Leo, K.; Brier, E.; Reinold, E.; Baeuerle,
P. Chemphyschem 2007, 8, 1497-1503.
(29) Uhrich, C.; Schueppel, R.; Petrich, A.; Pfeiffer, M.; Leo, K.; Brier, E.; Kilickiran,
P.; Baeuerle, P. Advanced Functional Materials 2007, 17, 2991-2999.
(30) Vauthey, E. Chemphyschem 2012, 13, 2001-2011.
(31) Yang, Y. F.; Arias, F.; Echegoyen, L.; Chibante, L. P. F.; Flanagan, S.;
Robertson, A.; Wilson, L. J. Journal of the American Chemical Society 1995, 117, 7801-
7804.
(32) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. Advanced Functional Materials
2009, 19, 1939-1948.
(33) Dastoor, P. C.; McNeill, C. R.; Frohne, H.; Foster, C. J.; Dean, B.; Fell, C. J.;
Belcher, W. J.; Canipbell, W. M.; Officer, D. L.; Blake, I. M.; Thordarson, P.; Crossley,
M. J.; Hush, N. S.; Reimers, J. R. Journal of Physical Chemistry C 2007, 111, 15415-
15426.
(34) Kubo, Y.; Watanabe, K.; Nishiyabu, R.; Hata, R.; Murakami, A.; Shoda, T.; Ota,
H. Organic Letters 2011, 13, 4574-4577.
(35) Vandenheuvel, D. J.; Chan, I. Y.; Groenen, E. J. J.; Schmidt, J.; Meijer, G.
Chemical Physics Letters 1994, 231, 111-118.
(36) Kipp, R. A.; Simon, J. A.; Beggs, M.; Ensley, H. E.; Schmehl, R. H. J. Phys.
Chem. A 1998, 102, 5659-5664.
(37) Sullivan, P.; Duraud, A.; Hancox, I.; Beaumont, N.; Mirri, G.; Tucker, J. H. R.;
Hatton, R. A.; Shipman, M.; Jones, T. S. Advanced Energy Materials 2011, 1, 352-355.
(38) Gonzalez- Rodriguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.; Herranz, M. A.;
Echegoyen, L. Journal of the American Chemical Society 2004, 126, 6301-6313.
(39) Hanson, K.; Tamayo, A.; Diev, V. V.; Whited, M. T.; Djurovich, P. I.; Thompson,
M. E. Inorg. Chem. 2010, 49, 6077-6084.
(40) Azenha, E. G.; Serra, A. C.; Pineiro, M.; Pereira, M. M.; de Melo, J. S.; Arnaut,
L. G.; Formosinho, S. J.; Gonsalves, A. Chemical Physics 2002, 280, 177-190.
(41) Pineiro, M.; Carvalho, A. L.; Pereira, M. M.; Gonsalves, A.; Arnaut, L. G.;
Formosinho, S. J. Chemistry-a European Journal 1998, 4, 2299-2307.
(42) Sazanovich, I. V.; Kirmaier, C.; Hindin, E.; Yu, L. H.; Bocian, D. F.; Lindsey, J.
S.; Holten, D. Journal of the American Chemical Society 2004, 126, 2664-2665.
(43) Ulrich, G.; Ziessel, R.; Harriman, A. Angewandte Chemie-International Edition
2008, 47, 1184-1201.
(44) Yu, L. H.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin, E.; Diers,
J. R.; Boyle, P. D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. Inorganic Chemistry 2003,
42, 6629-6647.
171

(45) Rio, Y.; Sanchez-Garcia, D.; Seitz, W.; Torres, T.; Sessler, J. L.; Guldi, D. M.
Chemistry-a European Journal 2009, 15, 3956-3959.
(46) Gonzalez-Rodriguez, D.; Torres, T.; Angeles Herranz, M.; Echegoyen, L.;
Carbonell, E.; Guldi, D. M. Chemistry-a European Journal 2008, 14, 7670-7679.
(47) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.; Lindsey, J. S.
Organic Process Research & Development 2003, 7, 799-812.
(48) Dorset, D. L.; McCourt, M. P. Acta Crystallographica Section A 1994, 50, 344-
351.
(49) Claessens, C. G.; Gonzalez-Rodriguez, D.; del Rey, B.; Torres, T.; Mark, G.;
Schuchmann, H. P.; von Sonntag, C.; MacDonald, J. G.; Nohr, R. S. Eur. J. Org. Chem.
2003, 2547-2551.
(50) Capozzi, V.; Celentano, G.; Perna, G.; Lorusso, G. F.; Minafra, A. Journal of
Luminescence 2000, 86, 129-135.
(51) Ichida, M.; Sakai, M.; Yajima, T.; Nakamura, A. Progress in Crystal Growth and
Characterization of Materials 1996, 33, 125-128.
(52) Ziessel, R.; Allen, B. D.; Rewinska, D. B.; Harriman, A. Chemistry-a European
Journal 2009, 15, 7382-7393.
(53) Li, J.; Komiya, S.; Tamura, T.; Nagasaki, C.; Kihara, J.; Kishio, K.; Kitazawa, K.
Physica C 1992, 195, 205-208.
(54) Wang, S.; Mayo, E. I.; Perez, M. D.; Griffe, L.; Wei, G.; Djurovich, P. I.; Forrest,
S. R.; Thompson, M. E. Applied Physics Letters 2009, 94, 233304.

172

CHAPTER 6. INVERTED ORGANIC PHOTOVOLTAICS WITH LAMELLAR
STRUCTURE

6.1. Introduction
Beside the efficiency requirement of OPVs, further stability improvement is
needed for these cells to become practical. OPVs with a “conventional” structure
(c-OPV) are prepared on transparent substrates, with the basic structure
substrate/cathode/donor/acceptor/anode. Note that the electrode definitions used here are
for a cell being irradiated and thus acting as a galvanic cell. In this case, the cathode is
the electrode in contact with the electron donor and thus the source of holes, and the
anode is the source of electrons. In a c-OPV the cathode is a high work function (WF)
material, most commonly Indium Tin Oxide – ITO, or ITO coated with
poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) - PEDOT:PSS, and the anode
is a low work function (WF) anode, e.g. aluminum (Al) or alkali metal modified Al
electrodes – lithium fluoride (LiF/Al), Calcium (Ca/Al), or barium (Ba/Al). c-OPVs
often suffer from low stability,
1-4
and noncapsulated devices tend to degrade within a
matter of hours at ambient conditions.
2-5
The degradation mechanisms are thought to
involve oxidation of the low WF electrode or acceptor/anode interface due to exposure of
this contact to the air and degradation of the ITO/PEDOT:PSS contact.
2,6
Inverted OPV
devices (i-OPVs) with a basic structure of substrate/anode/acceptor/donor/cathode have
173

markedly improved lifetimes over c-OPVs,
5,7-14
due to effective encapsulation of the
anode and anode/acceptor interface by the organic materials and cathode.
Two types of i-OPVs have been studied: bottom-illuminated and top-illuminated,
i.e. illuminating through the substrate/anode or through a transparent cathode,
respectively, which lead to different choices for electrode materials. The most widely
studied is the bottom-illuminated i-OPV, which utilizes ITO modified with a thin coating
of a low WF metal oxide such as titanium oxide (TiO
x
),
15-17
zinc oxide (ZnO),
17-21
and
aluminum oxide (Al
2
O
3
),
22
to form the anode, and a high WF metal such as silver (Ag),
gold (Au) or molybdenum trioxide, MoO
3
/metals used as an opaque cathode.
2,6
In the
top-illuminated i-OPV, organic layers are deposited on a low WF metal anode, and a
transparent cathode is required to allow for illumination through the top electrode, which
is commonly sputtered ITO.
8,9
The majority of the reported studies on i-OPVs have
involved bottom illuminated bulk heterojunction (BHJ) i-OPVs, which exhibit similar or
higher efficiencies than analogous c-OPV BHJ devices.
21-26
The reported i-OPVs with a
bilayer architecture are generally found to have lower efficiencies than their c-OPV
counterparts.
5,7-14
In this work, we investigate the sources of the power losses observed
in bilayer i-OPVs, which will aid in the design of optimal donor and acceptor materials
for use in i-OPVs.
6.2. Fabrication and optimization of inverted OPVs
In our study, glass/ITO substrates were coated with a 20 nm thick layer of ZnO by
atomic layer deposition (ALD),
27
forming the i-OPV anode; MoO
3
/Al was used as the
174

cathode for the i-OPV cells. The use of ALD deposited ZnO is an attractive approach for
converting an ITO electrode to an i-OPV anode, as the ALD method gives a stable,
conformal coating, which can be deposited over large areas.
28
While both of these anode
and cathode systems have been employed in i-OPV devices,
19,20
ALD ZnO anodes have
only been explored in BHJ i-OPV structures. Kippelen, et al., have shown that polymer
based BHJ i-OPVs with ALD ZnO anodes give performance parameters that are very
similar to c-OPVs with the same organic materials.
21

ZnO was grown using a custom-built warm wall ALD reactor. The precursors
diethylzinc (DEZn, Sigma-Aldrich) and H
2
O were maintained at room temperature
(~ 22°C), and needle valves controlled the rate of dosing of each precursor. The
deposition temperature was 150 °C for all depositions by ALD, and the number of cycles
was varied to study the effect of thickness on electrode modification. Nitrogen was used
as the carrier and purge gas at a constant flow rate of 60 sccm. The optimized cycle of
ALD consisted of 0.4 sec DEZn, 10 sec purge, 0.4 sec H
2
O, and a 10 sec purge.
As observed previously for ZnO based anodes, the ITO/ZnO anodes of the
inverted devices require light soaking in simulated 1 sun light for 2 min to improve the
conductivity of ZnO layer.
21
The light soaking process is thought to desorb negatively
charged oxygen molecules trapped at the grain boundaries of ZnO, thus improving the
conductivity of the ZnO film.
21
The general structure of i-OPV is: ITO/ZnO (20 nm)/C
60

(t nm)/CuPc (u nm)/MoO
3
(v nm)/Al. In order to achieve the best performance,
thicknesses of active layers were optimized, and the results are presented in Figure 6.1.
175

MoO
3
thickness shows the greatest effect on the device performance. While ultra thin
layers of MoO
3
(5 and 10 nm) reduced the V
OC
, J
SC
and FF. When the thickness of MoO
3

is thicker than 20 nm, V
OC
does not exhibit strong dependence on CuPc and C
60

thicknesses. The optimal thicknesses of layers in i-OPV are C
60
(40 nm), CuPc (30 nm)
and MoO
3
(30 nm).

Figure 6.1. Thickness optimization of different layers in devices with the structure:
(a) General i-OPV structure; (b) t = 30 nm, u is varied and v = 20 nm; (c) t is varied,
u = 40 nm and v = 20 nm; (d) t = 30 nm, u = 40 nm and v is varied.
30 40 50
0.0
0.2
0.4
0.6
0.8
0
2
4
FF

P

FF, 
p
(%)
C
60
thickness (nm)
b)
J
SC
(mA/cm
2
)
J
SC

20 30 40 50
0.0
0.2
0.4
0.6
0.8
0
2
4
FF

P

FF, 
p
(%)
CuPc thickness (nm)
c)
J
SC
J
SC
(mA/cm
2
)
0 20 40 60
0.0
0.2
0.4
0.6
0.8
0
2
4
FF

P
V
OC

FF, 
p
(%), V
OC
(V)
MoO
3
thickness (nm)
J
SC
(mA/cm
2
)
J
SC

d)
ITO
C
60
(t nm)
CuPc (u nm)
ZnO (20 nm)
MoO
3
(v nm)
Al (100 nm)
+
-
i-OPV
a)
176

6.3. Comparison of the conventional vs. inverted OPVs
Device structures and current density-voltage curves of c-OPV and i-OPV devices
(D1 and D2, respectively) in the dark and under simulated 1 sun (100 mW/cm
2
) AM1.5G
illumination are shown in Figure 6.2. The structure of D1 is ITO/CuPc (30 nm)/C
60

(40 nm)/BCP (10 nm)/Al, and D2 is the related inverted device [ITO/ZnO (20 nm)/C
60

(40 nm)/CuPc (30 nm)/MoO
3
(30 nm)/Al]. Important device characteristics after spectral
mismatch correction are presented in Table 6.1. While the efficiencies for the two
devices differ, the optimized thicknesses of active organic layers in i-OPV and c-OPV
devices are the same. During fabrication, the devices were exposed to the air to place
masks for metal electrode deposition. Thus, the V
OC
of the c-OPV is slightly lower than
the best published values.
29,30
All i-OPV and c-OPV devices were fabricated and tested
under the same conditions. Our i-OPV device performance is similar to the best reported
i-OPV using CuPc donor and C
60
acceptor layers.
5,7,10,12-14

Figure 6.2. Structure and characterization of conventional (D1) and inverted solar
cell (D2). (a) Device structures; (b) J-V curves in the dark and under illumination; (c)
External quantum yield (EQE) measurements.

-0.50 -0.25 0.00 0.25 0.50
-6
-3
0
3
10
-7
10
-5
10
-3
10
-1
(A/cm
2
)
(mA/cm
2
)
Voltage (V)

Current density
D1 light
D2 light
D1 dark
D2 dark

400 500 600 700 800
0
5
10
15
20

EQE (%)
Wavelength (nm)
D1
D2
ITO
C
60
(40 nm)
CuPc (30 nm)
ZnO (20 nm)
MoO
3
(30 nm)
Al (100 nm)
ITO
C
60
(40nm)
BCP (10 nm)
Al (100 nm)
-
+
CuPc(30 nm)
+
-
c-OPV
D1
i-OPV
D2
(a)
(b) (c)
177

Compared with the c-OPV D1, the i-OPV device D2 has a lower short circuit
current (J
SC
) (25 % decrease), open circuit voltage (V
OC
) (15 % decrease) and fill factor
(FF) (5 % decrease), resulting in a 40% decrease in power conversion efficiency. Higher
series resistance R
S
in D2, resulting in slightly lower FF (Table 6.1) is possibly due to
lower intrinsic conductivity of ZnO as compared with ITO. The origins of photocurrent
and voltage losses will be explored in the following sections.
Table 6.1. Characteristics of conventional and inverted solar cells.

J
SC

(mA/cm
2
)
V
OC

(V)
FF

η
(%)
n
a

J
a

(μA/cm
2
)
R
s
a

(Ω/cm
2
)
calc.
V
OC
b

(V)
conventional
D1

4.76(8)

0.38(1)

0.55(1)

1.00(2)

2.1(1)

4.4(2)

0.4(1)

0.37(1)
inverted
D2

3.66(8)

0.33(1)

0.52(1)

0.62(3)

2.1(1)

12(2)

2.2(7)

0.30(1)
D3 3.37(6) 0.35(1) 0.50(1) 0.59(1) 2.2(1) 8.6(8) 6.5(5) 0.33(1)
D4 3.65(9) 0.33(1) 0.51(1) 0.61(3) 2.2(1) 11(2) 1.4(5) 0.32(1)
The standard deviation for each value was determined for 3-4 devices and is listed for the
last digit of that number in parentheses. The errors in measurement are smaller than the
estimated standard deviation values.
a
Values obtained by fitting the dark J-V curve to the
Eq.1.10.
b
Calc. V
OC
is the V
OC
calculated using fitted n, J
S
and measured J
SC
into
Eq.1.14.
6.4. Origin of photocurrent loss
The low J
SC
of D2 can be understood by analyzing the external quantum
efficiencies (EQE) of D1 and D2 (Figure 6.2 c). The reduced photocurrent of D2
compared to D1 is due to the decrease of photoresponse of active layers, especially in the
178

CuPc response (λ = 550-800 nm). The decrease in CuPc response could be due to several
causes, including damage during deposition of MoO
3
, exciton quenching at organic/metal
oxide interfaces or an optical electric field distribution in the device that does not match
the distribution of absorbers in the organic layers.
To rule out damage to the CuPc layer upon MoO
3
deposition as a source of poor
CuPc response, we prepared a device with a thin layer of the hole transporting
N,N ’-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPD) inserted
between CuPc and MoO
3
layers, i-OPV D3 - ITO/ZnO (20 nm)/C
60
(40 nm)/CuPc
(30 nm)/NPD (10 nm)/MoO
3
(20 nm)/Al. Introduction of NPD layer did not lead to any
significant change in device characteristics (Table 6.1), suggesting that damage to the
CuPc on MoO
3
deposition and exciton quenching at the organic/MoO
3
interface are not
responsible for the poor CuPc response. Bathocuproine (BCP) is often used as electron
transporting and exciton blocking layer in c-OPVs,
31,32
whereas in our i-OPV C
60
is in
direct contact with the ZnO layer. To eliminate the possibility of exciton quenching at
the organic/ZnO interface, a thin film of BCP was placed between ZnO and C
60
layers
(i-OPV D4 - ITO/ZnO (20 nm)/BCP (5nm)/C
60
(40 nm)/CuPc (30 nm)/MoO
3

(30 nm)/Al). Again, the device parameters for D4 remained the same as those of D2
(Table 6.1).
Modeling (Figure 6.3) of c-OPV and i-OPV devices was performed using a
transfer-matrix formalism
33,34
to define the optical field distribution in the devices. This
approach has shown good agreement with experimental data of OPVs and is useful in
179

understanding the response of devices with different geometries.
8,33,34
The simulated
optical electric field distributions are similar in i-OPV and c-OPV devices, with a long
wavelength maximum field intensities between 600-800 nm located near the substrate
and a short wavelength maximum between 400-600 nm positioned away from the
substrate (Figure 6.3a and 6.3b).

Active organic layers in the c-OPV have the red absorbing CuPc positioned near
the substrate, and the blue absorbing C
60
located further away. This leads to a maximum
absorbed power in the device (Figure 6.3c). In contrast, the reversed order of CuPc and
C
60
layers in i-OPV results in lower absorbed power Q
j
(Figure 6.3d). Integrated

Figure 6.3. Optical modeling of c-OPV and i-OPV: optical E-field in a) c-OPV
(D1), b) i-OPV (D2); absorbed power Q
j
of c) D1 (integrated Q
j
= 0.19); d) D2
(integrated Q
j
= 0.28). The y-axis presents distances from the glass surface. White
lines were inserted for clarity between layers of the devices.

Wavelength (nm)
Distance (nm)
400 600 800
50
100
150
200
Wavelength (nm)
Distance (nm)
400 600 800
50
100
150
200
Wavelength (nm)
Distance (nm)
400 600 800
50
100
150
200
Wavelength (nm)
Distance (nm)
400 600 800
50
100
150
200
Wavelength (nm)
Distance (nm)

400 600 800
50
100
150
200
Wavelength (nm)
Distance (nm)

400 600 800
50
100
150
200
0.2
0.4
0.6
0.8
1
1.2
0.5
1
1.5
2
x 10
-5
Wavelength (nm)
Distance (nm)

400 600 800
50
100
150
200
Wavelength (nm)
Distance (nm)

400 600 800
50
100
150
200
0.2
0.4
0.6
0.8
1
1.2
0.5
1
1.5
2
x 10
-5
ITO
CuPc
C
60
BCP
ITO
CuPc
C
60
BCP
Wavelength (nm)
Distance (nm)
400 600 800
50
100
150
200
Wavelength (nm)
Distance (nm)
400 600 800
50
100
150
200
Wavelength (nm)
Distance (nm)
400 600 800
50
100
150
200
Wavelength (nm)
Distance (nm)
400 600 800
50
100
150
200
ZnO
ITO
C
60
CuPc
MoO
3
C
60
ZnO
ITO
CuPc
MoO
3
(a)
(b)
(d) (c)
0.1
2
x10
-5
0.1
1.2
180

absorbed power in D2 is 30 % lower than that in D1, which shows good agreement with
measured photocurrent values of D1 and D2 (Table 6.1). Our result implies that the
optimal materials used in bilayer i-OPVs should be a donor that absorbs at short
wavelength range in combination with an acceptor that absorbs at long wavelength range.
While there are a wide range of donors reported for OPVs, the best acceptors are
all based on fullerenes. The fullerene family absorbs in the blue for C
60
and its
derivatives, and in the green for C
70
and its derivatives, but there are no red absorbing
fullerenes, making this family poorly suited for bottom-illuminated i-OPVs. In contrast,
BHJ i-OPVs using ALD ZnO anode and fullerene based acceptor reported by Kippelen,
et al., did not show a significant difference in either the device performance or EQE
curves, relative to the c-OPV with the same organic materials. The explanation for this
apparent discrepancy is that the poor match of the optical electric field distribution of the
donor and acceptor in lamellar i-OPVs is avoided in i-OPVs with bulk heterojunction
structure, since the donor and acceptor materials are evenly distributed throughout the
organic layer.
21-26
It is expected that top-illuminated i-OPV device structure
(substrate/metal-anode/acceptor/donor/oxide-cathode) have an improved optical field
distribution seen for our i-OPVs, as they possess similar optical electric field distribution
to c-OPV structure. Consistent with this, Tong, et al. reported top-illuminated i-OPVs
using two different heterojunctions CuPc/PTCBI
8
and SubPc/C
60
.
9
The first i-OPV
shows comparable J
SC
to c-OPV,
8
while the second i-OPV has only 12% photocurrent
loss as compared to c-OPV.
9

181

6.5. Origin of photovoltage loss
The combination of EQE and simulations demonstrated that current collection
decreases due to a loss of absorption in the red spectrum; however, this analysis does not
address the decreased V
OC
for the i-OPV. It has been well established that the open
circuit voltage can be related to J
SC
and the saturation dark current, J
S
, for OPVs (see
section 1.5.1, Equation 1.10). This inverse relationship between V
OC
and dark saturation
current, J
S
(V
OC
 ln(J
SC
/J
S
) has been demonstrated for a range of donor and acceptor
materials.
35-40

As shown in Table 6.1, J
S
of D2 is larger than that of D1 by a factor of 3.
Calculating V
OC
by Eq. 1.14 using fitted parameters from the dark current-voltage
response and the measured photocurrent J
SC
shows good agreement with the measured

Figure 6.4. AFM images and section analysis of films on ITO. (a) ITO/ZnO
(20 nm)/C
60
(40 nm) (rms = 3.3 nm); (b) ITO/CuPc (30 nm) (rms = 0.3 nm); (c)
ITO/ZnO (20 nm) (rms = 1.5 nm).

10 nm
0 nm
20 nm
0 nm
0 µm 5 µm
5 nm
0 nm
0 µm 5 µm
10 nm
0 nm
10 nm
0 nm
20 nm
0 nm
0 µm 5 µm
a) b) c)
182

V
OC
(table 6.1). The decrease in the V
OC
of D2 relative to D1 is caused by both larger
dark saturation current and lower J
SC
of D2.
Figure 6.4 shows AFM images of 40 nm C
60
film on ITO/ZnO and 30 nm CuPc
film on ITO. ALD grown ZnO films are quite rough with the root mean square
roughness (rms) of 1.5 nm compared to ITO rms of 0.5 - 0.7 nm. The rms roughness of a
C
60
film deposited on ITO/ZnO substrate is 3.3 nm, which is twice that of the initial
ITO/ZnO substrate and ten times larger than the rms value of 0.3 nm observed for a CuPc
film on ITO substrate. Thus, the i-OPV has a larger donor/acceptor (D/A) interface,
leading to higher thermally populated charge recombination probability at the D/A
interface, and thus a larger J
S
, than analogous c-OPVs.
6.6. Conclusion
In summary, we have carried out thorough analysis on bilayer i-OPV and c-OPV
devices using C
60
and CuPc as acceptor and donor materials, respectively. Compared to
conventional devices, inverted devices have lower J
SC
(25%) and lower V
OC
(15%),
resulting in 40% loss of power conversion efficiency. Low V
OC
is due to large dark
saturated current J
S
, presumably caused by increased charge recombination at rough A/D
interface in the inverted device. Our optical modeling on i-OPV and c-OPV indicates
that the lower photocurrent obtained in i-OPV is due to inappropriate optical electric field
distribution in the device. The optimal set of materials for lamellar i-OPV would be blue-
green absorbing donor in combination with red-NIR absorbing acceptor. We are
currently exploring nonfullerene acceptors, absorbing in the red-NIR spectrum to be used
183

in bottom-illuminated bilayer i-OPVs. Moreover, a more uniform cathode contact
(ITO/ZnO) would minimize J
S
and thus increase V
OC
.
184

Chapter 6 Endnote
(1) Hermenau, M.; Riede, M.; Leo, K.; Gevorgyan, S. A.; Krebs, F. C.; Norrman, K.
Solar Energy Materials and Solar Cells 2011, 95, 1268-1277.
(2) Grossiord, N.; Kroon, J. M.; Andriessen, R.; Blom, P. W. M. Org. Electron. 2012,
13, 432-456.
(3) Wu, H. R.; Song, Q. L.; Wang, M. L.; Li, F. Y.; Yang, H.; Wu, Y.; Huang, C. H.;
Ding, X. M.; Hou, X. Y. Thin Solid Films 2007, 515, 8050-8053.
(4) Karak, S.; Pradhan, S.; Dhar, A. Semicond. Sci. Technol. 2011, 26, 095020.
(5) Song, Q. L.; Wang, M. L.; Obbard, E. G.; Sun, X. Y.; Ding, X. M.; Hou, X. Y.;
Li, C. M. Appl. Phys. Lett. 2006, 89, 251118.
(6) Jorgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.;
Krebs, F. C. Adv. Mater. 2012, 24, 580-612.
(7) Zhong, S.; Zhong, J. Q.; Wang, X. Z.; Huang, M. Y.; Qi, D. C.; Chen, Z. K.;
Chen, W. J. Phys. Chem. C 2012, 116, 2521-2526.
(8) Tong, X.; Bailey-Salzman, R. F.; Wei, G.; Forrest, S. R. Appl. Phys. Lett. 2008,
93, 173304.
(9) Tong, X.; Lassiter, B. E.; Forrest, S. R. Org. Electron. 2010, 11, 705-709.
(10) Zhang, F. J.; Zhao, D. W.; Zhuo, Z. L.; Wang, H.; Xu, Z.; Wang, Y. S. Solar
Energy Materials and Solar Cells 2010, 94, 2416-2421.
(11) Liu, J. P.; Wang, S. S.; Bian, Z. Q.; Shan, M. N.; Huang, C. H. Chem. Phys. Lett.
2009, 470, 103-106.
(12) Morsli, M.; Cattin, L.; Bernede, J. C.; Kumar, P.; Chand, S. Journal of Physics D-
Applied Physics 2010, 43, 335103.
(13) Oida, T.; Naito, T.; Miyagawa, Y.; Sasaki, M.; Harafuji, K. Japanese Journal of
Applied Physics 2011, 50, 081601.
(14) Wang, M. L.; Song, Q. L.; Wu, H. R.; Ding, B. F.; Gao, X. D.; Sun, X. Y.; Ding,
X. M.; Hou, X. Y. Org. Electron. 2007, 8, 445-449.
(15) Sasajima, I.; Uesaka, S.; Kuwabara, T.; Yamaguchi, T.; Takahashi, K. Org.
Electron. 2011, 12, 113-118.
(16) Kuwabara, T.; Nakayama, T.; Uozumi, K.; Yamaguchi, T.; Takahashi, K. Solar
Energy Materials and Solar Cells 2008, 92, 1476-1482.
(17) Oh, H.; Krantz, J.; Litzov, I.; Stubhan, T.; Pinna, L.; Brabec, C. J. Solar Energy
Materials and Solar Cells 2011, 95, 2194-2199.
(18) Chang, C.-Y.; Tsai, F.-Y. J. Mater. Chem. 2011, 21, 5710-5715.
(19) Kyaw, A. K. K.; Sun, X. W.; Jiang, C. Y.; Lo, G. Q.; Zhao, D. W.; Kwong, D. L.
Appl. Phys. Lett. 2008, 93, 221107.
(20) Wang, J.-C.; Weng, W.-T.; Tsai, M.-Y.; Lee, M.-K.; Horng, S.-F.; Perng, T.-P.;
Kei, C.-C.; Yu, C.-C.; Meng, H.-F. J. Mater. Chem. 2010, 20, 862-866.
(21) Cheun, H.; Fuentes-Hernandez, C.; Zhou, Y.; Potscavage, W. J., Jr.; Kim, S.-J.;
Shim, J.; Dindar, A.; Kippelen, B. J. Phys. Chem. C 2010, 114, 20713-20718.
(22) Zhou, Y.; Cheun, H.; Potscavage, W. J., Jr.; Fuentes-Hernandez, C.; Kim, S.-J.;
Kippelen, B. J. Mater. Chem. 2010, 20, 6189-6194.
185

(23) Zhang, F.; Xu, X.; Tang, W.; Zhang, J.; Zhuo, Z.; Wang, J.; Wang, J.; Xu, Z.;
Wang, Y. Solar Energy Materials and Solar Cells 2011, 95, 1785-1799.
(24) Cheng, Y.-J.; Hsieh, C.-H.; He, Y.; Hsu, C.-S.; Li, Y. J. Am. Chem. Soc. 2010,
132, 17381-17383.
(25) Chang, C.-Y.; Cheng, Y.-J.; Hung, S.-H.; Wu, J.-S.; Kao, W.-S.; Lee, C.-H.; Hsu,
C.-S. Adv. Mater. 2012, 24, 549-.
(26) Hsieh, C.-H.; Cheng, Y.-J.; Li, P.-J.; Chen, C.-H.; Dubosc, M.; Liang, R.-M.;
Hsu, C.-S. J. Am. Chem. Soc. 2010, 132, 4887-4893.
(27) Tanskanen, J. T.; Bakke, J. R.; Pakkanen, T. A.; Bent, S. F. Journal of Vacuum
Science & Technology A 2011, 29, 031507.
(28) Bakke, J. R.; Pickrahn, K. L.; Brennan, T. P.; Bent, S. F. Nanoscale 2011, 3,
3482-3508.
(29) Jeong, W.-I.; Lee, Y. E.; Shim, H.-S.; Kim, T.-M.; Kim, S.-Y.; Kim, J.-J. Adv.
Funct. Mater. 2012, 22, 3089-3094.
(30) Lo, M. F.; Ng, T. W.; Liu, T. Z.; Roy, V. A. L.; Lai, S. L.; Fung, M. K.; Lee, C.
S.; Lee, S. T. Appl. Phys. Lett. 2010, 96, 113303.
(31) Gommans, H.; Verreet, B.; Rand, B. P.; Muller, R.; Poortmans, J.; Heremans, P.;
Genoe, J. Adv. Funct. Mater. 2008, 18, 3686-3691.
(32) Peumans, P.; Bulovic, V.; Forrest, S. R. Appl. Phys. Lett. 2000, 76, 2650-2652.
(33) Pettersson, L. A. A.; Roman, L. S.; Inganas, O. J. Appl. Phys. 1999, 86, 487-496.
(34) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693-3723.
(35) Li, N.; Lassiter, B. E.; Lunt, R. R.; Wei, G.; Forrest, S. R. Appl. Phys. Lett. 2009,
94, 023307.
(36) Erwin, P.; Thompson, M. E. Appl. Phys. Lett. 2011, 98, 223305.
(37) Kippelen, B.; Bredas, J.-L. Energy & Environmental Science 2009, 2, 251-261.
(38) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2009,
131, 9281-9286.
(39) Potscavage, W. J.; Yoo, S.; Kippelen, B. Appl. Phys. Lett. 2008, 93, 193308.
(40) Schlenker, C. W.; Thompson, M. E. Chem. Commun. 2011, 47, 3702-3716.

187

BIBLIOGRAPHY
(1) Abasto, D. F.; Mohseni, M.; Lloyd, S.; Zanardi, P. Philos T R Soc A 2012, 370,
3750-3770.
(2) Adams, D. M.; Kerimo, J.; Olson, E. J. C.; Zaban, A.; Gregg, B. A.; Barbara, P. F.
Journal of the American Chemical Society 1997, 119, 10608-10619.
(3) Agranovich, V. M.; Basko, D. M.; La Rocca, G. C. Physical Review B 2012, 86.
(4) Amiri, A.; Comeau, I. M.; Thompson, A. J. Heterocycl. Chem. 2006, 43, 431-435.
(5) Anthony, J. E. Chemistry of Materials 2011, 23, 583-590.
(6) Anthopoulos, T. D.; Singh, B.; Marjanovic, N.; Sariciftci, N. S.; Ramil, A. M.;
Sitter, H.; Colle, M.; de Leeuw, D. M. Applied Physics Letters 2006, 89.
(7) Antony, A.; Nisha, M.; Manoj, R.; Jayaraj, M. K. Applied Surface Science 2004,
225, 294-301.
(8) Arkhipov, V.; Bassler, H.; Emelyanova, E.; Hertel, D.; Gulbinas, V.; Rothberg, L.
Macromolecular Symposia 2004, 212, 13-24.
(9) Arkhipov, V. I.; Heremans, P.; Bassler, H. Applied Physics Letters 2003, 82,
4605-4607.
(10) Armstrong, N. R.; Veneman, P. A.; Ratcliff, E.; Placencia, D.; Brumbach, M.
Accounts of Chemical Research 2009, 42, 1748-1757.
(11) Armstrong, N. R.; Wang, W.; Alloway, D. M.; Placencia, D.; Ratcliff, E.;
Brumbach, M. Macromolecular Rapid Communications 2009, 30, 717-731.
(12) Asami, N.; Takaya, T.; Yabumoto, S.; Shigeto, S.; Hamaguchi, H.-o.; Iwata, K. J.
Phys. Chem. A 2010, 114, 6351-6355.
(13) Azenha, E. G.; Serra, A. C.; Pineiro, M.; Pereira, M. M.; de Melo, J. S.; Arnaut,
L. G.; Formosinho, S. J.; Gonsalves, A. Chem. Phys. 2002, 280, 177-190.
(14) Baffreau, J.; Leroy-Lhez, S.; Hudhomme, P.; Groeneveld, M. M.; van Stokkum, I.
H. M.; Williams, R. M. J. Phys. Chem. A 2006, 110, 13123-13125.
(15) Baffreau, J.; Leroy-Lhez, S.; Van Anh, N.; Williams, R. M.; Hudhomme, P.
Chemistry-a European Journal 2008, 14, 4974-4992.
188

(16) Bailey-Salzman, R. F.; Rand, B. P.; Forrest, S. R. Applied Physics Letters 2007,
91, -.
(17) Baker, J. L.; Jimison, L. H.; Mannsfeld, S.; Volkman, S.; Yin, S.; Subramanian,
V.; Salleo, A.; Alivisatos, A. P.; Toney, M. F. Langmuir 2010, 26, 9146-9151.
(18) Bakke, J. R.; Pickrahn, K. L.; Brennan, T. P.; Bent, S. F. Nanoscale 2011, 3,
3482-3508.
(19) Bakulin, A. A.; Dimitrov, S. D.; Rao, A.; Chow, P. C. Y.; Nielsen, C. B.;
Schroeder, B. C.; McCulloch, I.; Bakker, H. J.; Durrant, J. R.; Friend, R. H. Journal of
Physical Chemistry Letters 2013, 4, 209-215.
(20) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.;
Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. Science
2012, 335, 1340-1344.
(21) Barber, J.; Tran, P. D. Journal of the Royal Society Interface 2013, 10.
(22) Barford, W.; Bittner, E. R.; Ward, A. J. Phys. Chem. A 2012, 116, 10319-10327.
(23) Barnes, T. M.; Bergeson, J. D.; Tenent, R. C.; Larsen, B. A.; Teeter, G.; Jones, K.
M.; Blackburn, J. L.; van de Lagemaat, J. Applied Physics Letters 2010, 96.
(24) Baumann, A. S., T.J.; Murthy, D.H.K.; Heeney, M.; Dyakonov, V.; Deibel, C.
Advanced Functional Materials 2011, 21, 1687-1692.
(25) Beljonne, D.; Cornil, J.; Muccioli, L.; Zannoni, C.; Bredas, J.-L.; Castet, F.
Chemistry of Materials 2011, 23, 591-609.
(26) Bottari, G.; Trukhina, O.; Ince, M.; Torres, T. Coord. Chem. Rev. 2012, 256,
2453-2477.
(27) Boucle, J.; Ackermann, J. Polym. Int. 2012, 61, 355-373.
(28) Braun, C. L. Journal of Chemical Physics 1984, 80, 4157-4161.
(29) Bredas, J. L.; Cornil, J.; Heeger, A. J. Advanced Materials 1996, 8, 447-&.
(30) Bredas, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Accounts of Chemical
Research 2009, 42, 1691-1699.
(31) Breeze, A. J.; Salomon, A.; Ginley, D. S.; Tillmann, H.; Horhold, H.; Gregg, B.
A. In Organic Photovoltaics Iii; Kafafi, Z. H., Ed. 2003; Vol. 4801, p 34-39.
189

(32) Brendel, J. C.; Burchardt, H.; Thelakkat, M. Journal of Materials Chemistry 2012,
22, 24386-24393.
(33) Brumbach, M.; Placencia, D.; Armstrong, N. R. Journal of Physical Chemistry C
2008, 112, 3142-3151.
(34) Brumbach, M.; Veneman, P. A.; Marrikar, F. S.; Schulmeyer, T.; Simmonds, A.;
Xia, W.; Lee, P.; Armstrong, N. R. Langmuir 2007, 23, 11089-11099.
(35) Byrn, M. P.; Curtis, C. J.; Hsiou, Y.; Khan, S. I.; Sawin, P. A.; Tendick, S. K.;
Terzis, A.; Strouse, C. E. Journal of the American Chemical Society 1993, 115, 9480-
9497.
(36) Capozzi, V.; Casamassima, G.; Lorusso, G. F.; Minafra, A.; Piccolo, R.; Trovato,
T.; Valentini, A. Solid State Commun. 1996, 98, 853-858.
(37) Capozzi, V.; Celentano, G.; Perna, G.; Lorusso, G. F.; Minafra, A. J. Lumin.
2000, 86, 129-135.
(38) Capozzi, V.; Santoro, M.; Perna, G.; Celentano, G.; Minafra, A.; Casamassima,
G. European Physical Journal-Applied Physics 2001, 14, 3-11.
(39) Casalegno, M.; Carbonera, C.; Luzzati, S.; Raos, G. Organic Electronics 2012, 13,
750-761.
(40) Chang, C.-Y.; Cheng, Y.-J.; Hung, S.-H.; Wu, J.-S.; Kao, W.-S.; Lee, C.-H.; Hsu,
C.-S. Advanced Materials 2012, 24, 549-.
(41) Chang, C.-Y.; Tsai, F.-Y. Journal of Materials Chemistry 2011, 21, 5710-5715.
(42) Chen, C.-P.; Tien, T.-C.; Ko, B.-T.; Chen, Y.-D.; Ting, C. Acs Applied Materials
& Interfaces 2009, 1, 741-745.
(43) Chen, F. C.; Ko, C. J.; Wu, J. L.; Chen, W. C. Sol. Energy Mater. Sol. Cells 2010,
94, 2426-2430.
(44) Chen, G.; Yokoyama, D.; Sasabe, H.; Hong, Z. R.; Yang, Y.; Kido, J. Applied
Physics Letters 2012, 101.
(45) Chen, K. S.; Yip, H. L.; Schlenker, C. W.; Ginger, D. S.; Jen, A. K. Y. Organic
Electronics 2012, 13, 2870-2878.
190

(46) Chen, W. C.; Huang, L. Z.; Qiao, X. L.; Yang, J. B.; Yu, B.; Yan, D. H. Organic
Electronics 2012, 13, 1086-1091.
(47) Chen, Y.-H.; Lin, L.-Y.; Lu, C.-W.; Lin, F.; Huang, Z.-Y.; Lin, H.-W.; Wang, P.-
H.; Liu, Y.-H.; Wong, K.-T.; Wen, J.; Miller, D. J.; Darling, S. B. Journal of the
American Chemical Society 2012, 134, 13616-13623.
(48) Cheng, Y.-J.; Hsieh, C.-H.; He, Y.; Hsu, C.-S.; Li, Y. Journal of the American
Chemical Society 2010, 132, 17381-17383.
(49) Cheun, H.; Fuentes-Hernandez, C.; Zhou, Y.; Potscavage, W. J., Jr.; Kim, S.-J.;
Shim, J.; Dindar, A.; Kippelen, B. Journal of Physical Chemistry C 2010, 114, 20713-
20718.
(50) Chichibu, S. F.; Hazu, K.; Ishikawa, Y.; Tashiro, M.; Namita, H.; Nagao, S.;
Fujito, K.; Uedono, A. Journal of Applied Physics 2012, 111.
(51) Chiu, S.-W.; Lin, L.-Y.; Lin, H.-W.; Chen, Y.-H.; Huang, Z.-Y.; Lin, Y.-T.; Lin,
F.; Liu, Y.-H.; Wong, K.-T. Chemical Communications 2012, 48, 1857-1859.
(52) Chou, W. Y.; Cheng, H. L. Advanced Functional Materials 2004, 14, 811-815.
(53) Chu, T.-Y.; Tsang, S.-W.; Zhou, J.; Verly, P. G.; Lu, J.; Beaupre, S.; Leclerc, M.;
Tao, Y. Sol. Energy Mater. Sol. Cells 2012, 96, 155-159.
(54) Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G. Nature 2012, 485,
486-U94.
(55) Claessens, C. G.; Gonzalez-Rodriguez, D.; del Rey, B.; Torres, T.; Mark, G.;
Schuchmann, H. P.; von Sonntag, C.; MacDonald, J. G.; Nohr, R. S. Eur. J. Org. Chem.
2003, 2547-2551.
(56) Clarke, T. M.; Ballantyne, A.; Shoaee, S.; Soon, Y. W.; Duffy, W.; Heeney, M.;
McCulloch, I.; Nelson, J.; Durrant, J. R. Advanced Materials 2010, 22, 5287-5291.
(57) Clarke, T. M.; Durrant, J. R. Chemical Reviews 2010, 110, 6736-6767.
(58) Conboy, J. C.; Olson, E. J. C.; Adams, D. M.; Kerimo, J.; Zaban, A.; Gregg, B.
A.; Barbara, P. F. J. Phys. Chem. B 1998, 102, 4516-4525.
(59) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bredas,
J.-L. Chemical Reviews 2007, 107, 926-952.
191

(60) Cowan, A. J.; Durrant, J. R. Chem. Soc. Rev. 2013, 42, 2281-2293.
(61) Crawford, S. M.; Ali, A. A.-S.; Cameron, T. S.; Thompson, A. Inorg. Chem.
2011, 50, 8207-8213.
(62) Crochet, J. J.; Duque, J. G.; Werner, J. H.; Lounis, B.; Cognet, L.; Doorn, S. K.
Nano Letters 2012, 12, 5091-5096.
(63) Crouch, A. M.; Langford, C. H. J. Electroanal. Chem. 1987, 221, 83-93.
(64) Dabera, G. D. M. R.; Jayawardena, K. D. G. I.; Prabhath, M. R. R.; Yahya, I.;
Tan, Y. Y.; Nismy, N. A.; Shiozawa, H.; Sauer, M.; Ruiz-Soria, G.; Ayala, P.; Stolojan,
V.; Adikaari, A. A. D. T.; Jarowski, P. D.; Pichler, T.; Silva, S. R. P. Acs Nano 2013, 7,
556-565.
(65) D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.;
Thompson, M. E. Organic Electronics 2005, 6, 11-20.
(66) Danger, B. R.; Bedient, K.; Maiti, M.; Burgess, I. J.; Steer, R. P. J. Phys. Chem. A
2010, 114, 10960-10968.
(67) Dastoor, P. C.; McNeill, C. R.; Frohne, H.; Foster, C. J.; Dean, B.; Fell, C. J.;
Belcher, W. J.; Canipbell, W. M.; Officer, D. L.; Blake, I. M.; Thordarson, P.; Crossley,
M. J.; Hush, N. S.; Reimers, J. R. Journal of Physical Chemistry C 2007, 111, 15415-
15426.
(68) De Arco, L. G.; Zhang, Y.; Schlenker, C. W.; Ryu, K.; Thompson, M. E.; Zhou,
C. Acs Nano 2010, 4, 2865-2873.
(69) De Luca, G.; Treossi, E.; Liscio, A.; Mativetsky, J. M.; Scolaro, L. M.; Palermo,
V.; Samori, P. Journal of Materials Chemistry 2010, 20, 2493-2498.
(70) Del Cano, T.; Parra, V.; Rodriguez-Mendez, M. L.; Aroca, R. F.; De Saja, J. A.
Applied Surface Science 2005, 246, 327-333.
(71) Dexter, D. L. Journal of Chemical Physics 1953, 21, 836-850.
(72) Di Nuzzo, D.; Aguirre, A.; Shahid, M.; Gevaerts, V. S.; Meskers, S. C. J.;
Janssen, R. A. J. Advanced Materials 2010, 22, 4321-+.
(73) Dibb, G. F. A.; Jamieson, F. C.; Maurano, A.; Nelson, J.; Durrant, J. R. Journal of
Physical Chemistry Letters 2013, 4, 803-808.
192

(74) Dickey, K. C.; Anthony, J. E.; Loo, Y.-L. Advanced Materials 2006, 18, 1721-+.
(75) Diev, V. V.; Hanson, K.; Zimmerman, J. D.; Forrest, S. R.; Thompson, M. E.
Angewandte Chemie (International Ed.) 2010, 49, 5523-5526.
(76) Diev, V. V.; Schlenker, C. W.; Hanson, K.; Zhong, Q.; Zimmerman, J. D.;
Forrest, S. R.; Thompson, M. E. The Journal of Organic Chemistry 2012, 77, 143-159.
(77) Ding, Y.; Xie, Y.; Li, X.; Hill, J. P.; Zhang, W.; Zhu, W. Chemical
Communications 2011, 47, 5431-5433.
(78) Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E. Organic Electronics
2009, 10, 515-520.
(79) Dobkowski, J.; Grabowski, Z. R.; Paeplow, B.; Rettig, W.; Koch, K. H.; Mullen,
K.; Lapouyade, R. New J. Chem. 1994, 18, 525-533.
(80) Dongaonkar, S.; Servaites, J. D.; Ford, G. M.; Loser, S.; Moore, J.; Gelfand, R.
M.; Mohseni, H.; Hillhouse, H. W.; Agrawal, R.; Ratner, M. A.; Marks, T. J.; Lundstrom,
M. S.; Alam, M. A. Journal of Applied Physics 2010, 108.
(81) Dorough, G. D.; Miller, J. R.; Huennekens, F. M. Journal of the American
Chemical Society 1951, 73, 4315-4320.
(82) Dorset, D. L.; McCourt, M. P. Acta Crystallographica Section A 1994, 50, 344-
351.
(83) Dyachenko, O. A.; Graja, A. Fullerene Sci. Technol. 1999, 7, 317-385.
(84) El Gemayel, M.; Treier, M.; Musumeci, C.; Li, C.; Muellen, K.; Samori, P.
Journal of the American Chemical Society, 134, 2429-2433.
(85) Erwin, P.; Thompson, M. E. Applied Physics Letters 2011, 98, 223305.
(86) Filatov, M. A.; Lebedev, A. Y.; Mukhin, S. N.; Vinogradov, S. A.; Cheprakov, A.
V. Journal of the American Chemical Society 2010, 132, 9552-9554.
(87) Fitzner, R.; Mena-Osteritz, E.; Mishra, A.; Schulz, G.; Reinold, E.; Weil, M.;
Koerner, C.; Ziehlke, H.; Elschner, C.; Leo, K.; Riede, M.; Pfeiffer, M.; Uhrich, C.;
Baeuerle, P. Journal of the American Chemical Society 2012, 134, 11064-11067.
(88) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373-6380.
(89) Forrest, S. R. Chemical Reviews 1997, 97, 1793-1896.
193

(90) Forster, T. Annalen Der Physik 1948, 2, 55-75.
(91) Fravventura, M. C.; Hwang, J.; Suijkerbuijk, J. W. A.; Erk, P.; Siebbeles, L. D.
A.; Savenije, T. J. Journal of Physical Chemistry Letters 2012, 3, 2367-2373.
(92) Fritz, R.; Rettig, W.; Nishiyama, K.; Okada, T.; Muller, U.; Mullen, K. J. Phys.
Chem. A 1997, 101, 2796-2802.
(93) Fuller, M. J.; Gusev, A. V.; Wasielewski, M. R. Isr. J. Chem. 2004, 44, 101-108.
(94) Gaynor, W.; Lee, J.-Y.; Peumans, P. Acs Nano 2010, 4, 30-34.
(95) Giaimo, J. M.; Gusev, A. V.; Wasielewski, M. R. Journal of the American
Chemical Society 2002, 124, 8530-8531.
(96) Giaimo, J. M.; Lockard, J. V.; Sinks, L. E.; Scott, A. M.; Wilson, T. M.;
Wasielewski, M. R. J. Phys. Chem. A 2008, 112, 2322-2330.
(97) Giebink, N. C.; Lassiter, B. E.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest,
S. R. Physical Review B 2010, 82.
(98) Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R. Physical
Review B 2010, 82.
(99) Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R. Physical
Review B 2011, 83, 195326.
(100) Giridharagopal, R.; Ginger, D. S. Journal of Physical Chemistry Letters 2010, 1,
1160-1169.
(101) Gommans, H.; Verreet, B.; Rand, B. P.; Muller, R.; Poortmans, J.; Heremans, P.;
Genoe, J. Advanced Functional Materials 2008, 18, 3686-3691.
(102) Gonzalez- Rodriguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.; Herranz, M. A.;
Echegoyen, L. Journal of the American Chemical Society 2004, 126, 6301-6313.
(103) Gonzalez-Rodriguez, D.; Torres, T.; Angeles Herranz, M.; Echegoyen, L.;
Carbonell, E.; Guldi, D. M. Chemistry-a European Journal 2008, 14, 7670-7679.
(104) Gorgolis, S.; Giannopoulou, A.; Anastassopoulos, D.; Kounavis, P. Journal of
Applied Physics 2012, 112.
(105) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chemical Reviews 2003, 103, 3899-
4031.
194

(106) Grancini, G.; Maiuri, M.; Fazzi, D.; Petrozza, A.; Egelhaaf, H. J.; Brida, D.;
Cerullo, G.; Lanzani, G. Nature Materials 2013, 12, 29-33.
(107) Gregg, B. A. Molecular Crystals and Liquid Crystals Science and Technology
Section a-Molecular Crystals and Liquid Crystals 1994, 257, 219-&.
(108) Gregg, B. A. J. Phys. Chem. 1996, 100, 852-859.
(109) Gregg, B. A. Journal of Physical Chemistry Letters 2011, 2, 3013-3015.
(110) Groenen, E. J. J.; Poluektov, O. G.; Matsushita, M.; Schmidt, J.; Vanderwaals, J.
H.; Meijer, G. Chemical Physics Letters 1992, 197, 314-318.
(111) Grossiord, N.; Kroon, J. M.; Andriessen, R.; Blom, P. W. M. Organic Electronics
2012, 13, 432-456.
(112) Groves, C.; Reid, O. G.; Ginger, D. S. Accounts of Chemical Research 2010, 43,
612-620.
(113) Grozema, F. C.; Swart, M.; Zijlstra, R. W. J.; Piet, J. J.; Siebbeles, L. D. A.; van
Duijnen, P. T. Journal of the American Chemical Society 2005, 127, 11019-11028.
(114) Guenes, S.; Neugebauer, H.; Sariciftci, N. S. Chemical Reviews 2007, 107, 1324-
1338.
(115) Haegel, N. M.; Chisholm, D. J.; Cole, R. A. J. Cryst. Growth 2012, 352, 218-223.
(116) Hains, A. W.; Liang, Z.; Woodhouse, M. A.; Gregg, B. A. Chemical Reviews
2010, 110, 6689-6735.
(117) Han, H.; Mayer, J. W.; Alford, T. L. Journal of Applied Physics 2006, 100.
(118) Hanson, K.; Tamayo, A.; Diev, V. V.; Whited, M. T.; Djurovich, P. I.; Thompson,
M. E. Inorg. Chem. 2010, 49, 6077-6084.
(119) Haugeneder, A.; Neges, M.; Kallinger, C.; Spirkl, W.; Lemmer, U.; Feldmann, J.;
Scherf, U.; Harth, E.; Gugel, A.; Mullen, K. Physical Review B 1999, 59, 15346-15351.
(120) He, Z.; Zhong, C.; Huang, X.; Wong, W.-Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y.
Advanced Materials 2011, 23, 4636-4643.
(121) Hebard, A. F.; Arthur, J. R.; Allara, D. L. Journal of Applied Physics 1978, 49,
6039-6044.
195

(122) Hennebicq, E.; Pourtois, G.; Scholes, G. D.; Herz, L. M.; Russell, D. M.; Silva,
C.; Setayesh, S.; Grimsdale, A. C.; Mullen, K.; Bredas, J. L.; Beljonne, D. Journal of the
American Chemical Society 2005, 127, 4744-4762.
(123) Henson, Z. B.; Welch, G. C.; Poll, T. v.-d.; Bazan, G. C. J. Am. Chem. Soc 2012,
134, 3766-3779.
(124) Hermenau, M.; Riede, M.; Leo, K.; Gevorgyan, S. A.; Krebs, F. C.; Norrman, K.
Sol. Energy Mater. Sol. Cells 2011, 95, 1268-1277.
(125) Hertel, D.; Soh, E. V.; Bassler, H.; Rothberg, L. J. Chemical Physics Letters 2002,
361, 99-105.
(126) Hesse, H. C.; Schaffer, C.; Hundschell, C.; Narita, A.; Feng, X. L.; Mullen, K.;
Nickel, B.; Schmidt-Mende, L. Phys Status Solidi A 2012, 209, 785-789.
(127) Hesse, H. C.; Weickert, J.; Hundschell, C.; Feng, X.; Muellen, K.; Nickel, B.;
Mozer, A. J.; Schmidt-Mende, L. Advanced Energy Materials 2011, 1, 861-869.
(128) Hoermann, U.; Wagner, J.; Gruber, M.; Opitz, A.; Bruetting, W. Physica Status
Solidi-Rapid Research Letters 2011, 5, 241-243.
(129) Hofmann, S.; Rosenow, T. C.; Gather, M. C.; Lussem, B.; Leo, K. Physical
Review B 2012, 85.
(130) Hotchkiss, P. J.; Jones, S. C.; Paniagua, S. A.; Sharma, A.; Kippelen, B.;
Armstrong, N. R.; Marder, S. R. Accounts of Chemical Research 2012, 45, 337-346.
(131) Hotchkiss, P. J.; Jones, S. C.; Paniagua, S. A.; Sharma, A.; Kippelen, B.;
Armstrong, N. R.; Marder, S. R. Accounts of Chemical Research 2012, 45, 337-346.
(132) Hsieh, C.-H.; Cheng, Y.-J.; Li, P.-J.; Chen, C.-H.; Dubosc, M.; Liang, R.-M.;
Hsu, C.-S. Journal of the American Chemical Society 2010, 132, 4887-4893.
(133) Hu, B.; Hinds, B. J. Ieee T Nanotechnol 2012, 11, 1073-1079.
(134) Hu, Z.; Zhang, J.; Liu, Y.; Hao, Z.; Zhang, X.; Zhao, Y. Sol. Energy Mater. Sol.
Cells 2011, 95, 2126-2130.
(135) Huang, L.; Ponomarenko, N.; Wiederrecht, G. P.; Tiede, D. M. Proceedings of the
National Academy of Sciences of the United States of America 2012, 109, 4851-4856.
196

(136) Huijser, A.; Suijkerbuijk, B. M. J. M.; Gebbink, R. J. M. K.; Savenije, T. J.;
Siebbeles, L. D. A. Journal of the American Chemical Society 2008, 130, 2485-2492.
(137) Humphrybaker, R.; Kalyanasundaram, K. Journal of Photochemistry 1985, 31,
105-112.
(138) Ichida, M.; Nakamura, A.; Shinohara, H.; Saitoh, Y. Chemical Physics Letters
1998, 289, 579-585.
(139) Ichida, M.; Sakai, M.; Yajima, T.; Nakamura, A. Prog. Cryst. Growth Charact.
Mater. 1996, 33, 125-128.
(140) Ichikawa, M. Thin Solid Films 2013, 527, 239-243.
(141) Ikeda, C.; Ueda, S.; Nabeshima, T. Chemical Communications 2009, 2544-2546.
(142) Izumi, Y. Coord. Chem. Rev. 2013, 257, 171-186.
(143) Jackson, B.; Silbey, R. Journal of Chemical Physics 1983, 78, 4193-4195.
(144) Jailaubekov, A. E.; Willard, A. P.; Tritsch, J. R.; Chan, W.-L.; Sai, N.; Gearba,
R.; Kaake, L. G.; Williams, K. J.; Leung, K.; Rossky, P. J.; Zhu, X. Y. Nature Materials
2013, 12, 66-73.
(145) Janssen, R. A. J.; Nelson, J. Advanced Materials 2012, 25, 1847-1858.
(146) Jeong, W.-I.; Lee, Y. E.; Shim, H.-S.; Kim, T.-M.; Kim, S.-Y.; Kim, J.-J.
Advanced Functional Materials 2012, 22, 3089-3094.
(147) Jiang, C. Y.; Sun, X. W.; Zhao, D. W.; Kyaw, A. K. K.; Li, Y. N. Sol. Energy
Mater. Sol. Cells 2010, 94, 1618-1621.
(148) Jorgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.;
Krebs, F. C. Advanced Materials 2012, 24, 580-612.
(149) Kadish, K. M.; Shiue, L. R. Inorg. Chem. 1982, 21, 3623-3630.
(150) Kadish, K. M.; Shiue, L. R.; Rhodes, R. K.; Bottomley, L. A. Inorg. Chem. 1981,
20, 1274-1277.
(151) Kakade, S.; Ghosh, R.; Palit, D. K. Journal of Physical Chemistry C 2012, 116,
15155-15166.
(152) Kalimuthu, P.; John, S. A. Langmuir 2009, 25, 12414-12418.
197

(153) Kang, S. J.; Noh, M.; Park, D. S.; Kim, H. J.; Whang, C. N.; Chang, C. H. Journal
of Applied Physics 2004, 95, 2293-2296.
(154) Karak, S.; Pradhan, S.; Dhar, A. Semicond. Sci. Technol. 2011, 26, 095020.
(155) Karl, N. Synth. Met. 2003, 133, 649-657.
(156) Kasha, M. Radiat. Res. 1963, 20, 55-70.
(157) Kazaoui, S.; Minami, N.; Tanabe, Y.; Byrne, H. J.; Eilmes, A.; Petelenz, P.
Physical Review B 1998, 58, 7689-7700.
(158) Kearns, D.; Calvin, M. Journal of Chemical Physics 1958, 29, 950-951.
(159) Kee, H. L.; Kirmaier, C.; Yu, L. H.; Thamyongkit, P.; Youngblood, W. J.; Calder,
M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W. R.; Birge, R. R.; Lindsey, J. S.;
Holten, D. J. Phys. Chem. B 2005, 109, 20433-20443.
(160) Khandelwal, H.; Mallia, A. R.; Cheriya, R. T.; Hariharan, M. Physical Chemistry
Chemical Physics 2012, 14, 15282-15285.
(161) Kipp, R. A.; Simon, J. A.; Beggs, M.; Ensley, H. E.; Schmehl, R. H. J. Phys.
Chem. A 1998, 102, 5659-5664.
(162) Kippelen, B.; Bredas, J.-L. Energy & Environmental Science 2009, 2, 251-261.
(163) Kircher, T.; Lohmannsroben, H. G. Physical Chemistry Chemical Physics 1999,
1, 3987-3992.
(164) Kirksey, C. H.; Hambrigh.P; Storm, C. B. Inorg. Chem. 1969, 8, 2141-&.
(165) Kobayashi, J.; Kushida, T.; Kawashima, T. Journal of the American Chemical
Society 2009, 131, 10836-+.
(166) Koehler, M.; Santos, M. C.; da Luz, M. G. E. Journal of Applied Physics 2006,
99.
(167) Kubo, Y.; Watanabe, K.; Nishiyabu, R.; Hata, R.; Murakami, A.; Shoda, T.; Ota,
H. Org. Lett. 2011, 13, 4574-4577.
(168) Kusaka, S.; Sakamoto, R.; Kitagawa, Y.; Okumura, M.; Nishihara, H. Chemistry-
an Asian Journal 2012, 7, 907-910.
198

(169) Kuwabara, T.; Nakayama, T.; Uozumi, K.; Yamaguchi, T.; Takahashi, K. Sol.
Energy Mater. Sol. Cells 2008, 92, 1476-1482.
(170) Kyaw, A. K. K.; Sun, X. W.; Jiang, C. Y.; Lo, G. Q.; Zhao, D. W.; Kwong, D. L.
Applied Physics Letters 2008, 93, 221107.
(171) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.; Lindsey, J. S.
Organic Process Research & Development 2003, 7, 799-812.
(172) Lassiter, B. E.; Lunt, R. R.; Renshaw, C. K.; Forrest, S. R. Opt. Express, 18,
A444-A450.
(173) Lassiter, B. E.; Wei, G.; Wang, S.; Zimmerman, J. D.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Applied Physics Letters 2011, 98.
(174) Lassiter, B. E.; Zimmerman, J. D.; Panda, A.; Xiao, X.; Forrest, S. R. Applied
Physics Letters 2012, 101, 063303.
(175) Lathrop, E. J. P.; Friesner, R. A. J. Phys. Chem. 1994, 98, 3056-3066.
(176) Lazarides, T.; McCormick, T. M.; Wilson, K. C.; Lee, S.; McCamant, D. W.;
Eisenberg, R. Journal of the American Chemical Society 2011, 133, 350-364.
(177) Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo, M. A.;
Manca, J. V.; Van Voorhis, T. Journal of the American Chemical Society 2010, 132,
11878-11880.
(178) Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P. Nano Letters 2010, 10, 1276-
1279.
(179) Lee, O. P.; Yiu, A. T.; Beaujuge, P. M.; Woo, C. H.; Holcombe, T. W.; Millstone,
J. E.; Douglas, J. D.; Chen, M. S.; Frechet, J. M. J. Advanced Materials 2011, 23, 5359-+.
(180) Lee, S. S.; Loo, Y.-L. In Annual Review of Chemical and Biomolecular
Engineering, Vol 1; Prausnitz, J. M. D. M. F. S. M. A., Ed. 2010; Vol. 1, p 59-78.
(181) Li, G.; Chu, C. W.; Shrotriya, V.; Huang, J.; Yang, Y. Applied Physics Letters
2006, 88, 253503.
(182) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y.
Nature Materials 2005, 4, 864-868.
199

(183) Li, H.; Tee, B. C. K.; Cha, J. J.; Cui, Y.; Chung, J. W.; Lee, S. Y.; Bao, Z. Journal
of the American Chemical Society 2012, 134, 2760-2765.
(184) Li, J.; Komiya, S.; Tamura, T.; Nagasaki, C.; Kihara, J.; Kishio, K.; Kitazawa, K.
Physica C 1992, 195, 205-208.
(185) Li, N.; Lassiter, B. E.; Lunt, R. R.; Wei, G.; Forrest, S. R. Applied Physics Letters
2009, 94.
(186) Li, Y. W.; Chen, L. W.; Chen, Y. J.; Li, C.; Zhang, P.; Gao, L. F.; Yang, X. M.;
Tu, Y. F.; Zhu, X. L. Sol. Energy Mater. Sol. Cells 2013, 108, 136-145.
(187) Liang, Z.; Nardes, A. M.; van de Lagemaat, J.; Gregg, B. A. Advanced Functional
Materials 2012, 22, 1087-1091.
(188) Lim, J. M.; Kim, P.; Yoon, M. C.; Sung, J.; Dehm, V.; Chen, Z. J.; Wurthner, F.;
Kim, D. Chem Sci 2013, 4, 388-397.
(189) Lin, C.-F.; Su, W.-F.; Wu, C.-I.; Cheng, I.-C. Organic, Inorganic and Hybrid
Solar Cells. Principles and Practice.; Willey: Hoboken, New Jersey, 2012.
(190) Lin, C. L.; Fang, M. Y.; Cheng, S. H. J. Electroanal. Chem. 2002, 531, 155-162.
(191) Lin, L.-Y.; Chen, Y.-H.; Huang, Z.-Y.; Lin, H.-W.; Chou, S.-H.; Lin, F.; Chen,
C.-W.; Liu, Y.-H.; Wong, K.-T. Journal of the American Chemical Society 2011, 133,
15822-15825.
(192) Lin, Y.; Li, Y.; Zhan, X. Chem. Soc. Rev. 2012, 41, 4245-4272.
(193) Linares, M.; Beljonne, D.; Cornil, J.; Lancaster, K.; Bredas, J. L.; Verlaak, S.;
Mityashin, A.; Heremans, P.; Fuchs, A.; Lennartz, C.; Ide, J.; Mereau, R.; Aurel, P.;
Ducasse, L.; Castet, F. Journal of Physical Chemistry C 2010, 114, 3215-3224.
(194) Liu, J. P.; Wang, S. S.; Bian, Z. Q.; Shan, M. N.; Huang, C. H. Chemical Physics
Letters 2009, 470, 103-106.
(195) Liu, S. W.; Su, W. C.; Lee, C. C.; Lin, C. F.; Yeh, S. C.; Chen, C. T.; Lee, J. H.
Organic Electronics 2012, 13, 2118-2129.
(196) Liu, Y.; Zhao, J. Chemical Communications 2012, 48, 3751-3753.
200

(197) Liu, Y. D.; Chu, B.; Su, Z. S.; Li, W. L.; Zhuang, T. J.; Jin, F. M.; Yan, X. W.;
Zhao, B.; Zhang, F.; Fan, D.; Wang, J. B.; Gao, Y. Organic Electronics 2012, 13, 2865-
2869.
(198) Lo, M. F.; Ng, T. W.; Liu, T. Z.; Roy, V. A. L.; Lai, S. L.; Fung, M. K.; Lee, C.
S.; Lee, S. T. Applied Physics Letters 2010, 96, 113303.
(199) Lo, M. M. C.; Fu, G. C. Journal of the American Chemical Society 2002, 124,
4572-4573.
(200) Lockhart, D. J.; Boxer, S. G. Biochemistry 1987, 26, 664-668.
(201) Loh, L.; Dunn, S. J Nanosci Nanotechno 2012, 12, 8215-8230.
(202) Loudet, A.; Burgess, K. Chemical Reviews 2007, 107, 4891-4932.
(203) Lunt, R. R.; Benziger, J. B.; Forrest, S. R. Advanced Materials 2010, 22, 1233.
(204) Lunt, R. R.; Giebink, N. C.; Belak, A. A.; Benziger, J. B.; Forrest, S. R. Journal of
Applied Physics 2009, 105.
(205) Luo, S.-P.; Mejia, E.; Friedrich, A.; Pazidis, A.; Junge, H.; Surkus, A.-E.;
Jackstell, R.; Denurra, S.; Gladiali, S.; Lochbrunner, S.; Beller, M. Angewandte Chemie-
International Edition 2013, 52, 419-423.
(206) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Advanced Functional
Materials 2005, 15, 1617-1622.
(207) MacDonald, G. A.; Veneman, R. A.; Placencia, D.; Armstrong, N. R. Acs Nano
2012, 6, 9623-9636.
(208) Maeda, H.; Hashimoto, T.; Fujii, R.; Hasegawa, M. J Nanosci Nanotechno 2009,
9, 240-248.
(209) Manbeck, G. F.; Brewer, K. J. Coord. Chem. Rev. 2013, 257, 1660-1675.
(210) Marcus, R. A. Journal of Chemical Physics 1956, 24, 966-978.
(211) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265-322.
(212) Markovic, V.; Villamaina, D.; Barabanov, I.; Daku, L. M. L.; Vauthey, E.
Angewandte Chemie-International Edition 2011, 50, 7596-7598.
(213) Markovitsi, D.; Lecuyer, I.; Simon, J. J. Phys. Chem. 1991, 95, 3620-3626.
201

(214) Mbatia, H. W.; Burdette, S. C. Biochemistry 2012, 51, 7212-7224.
(215) McLean, T. M.; Moody, J. L.; Waterland, M. R.; Telfer, S. G. Inorg. Chem. 2012,
51, 446-455.
(216) McRae, E. G.; Kasha, M. The molecular exciton model; Academic Press: New
York, 1964.
(217) Meech, S. R.; Hoff, A. J.; Wiersma, D. A. Chemical Physics Letters 1985, 121,
287-292.
(218) Meiss, J.; Merten, A.; Hein, M.; Schuenemann, C.; Schaefer, S.; Tietze, M.;
Uhrich, C.; Pfeiffer, M.; Leo, K.; Riede, M. Advanced Functional Materials 2012, 22,
405-414.
(219) Menke, S. M.; Luhman, W. A.; Holmes, R. J. Nature Materials 2013, 12, 152-
157.
(220) Mikhnenko, O. V.; Azimi, H.; Scharber, M.; Morana, M.; Blom, P. W. M.; Loi,
M. A. Energy & Environmental Science 2012, 5, 6960-6965.
(221) Mikhnenko, O. V.; Ruiter, R.; Blom, P. W. M.; Loi, M. A. Phys. Rev. Lett. 2012,
108.
(222) Miller, J. R.; Dorough, G. D. Journal of the American Chemical Society 1952, 74,
3977-3981.
(223) Mishra, A.; Baeuerle, P. Angewandte Chemie-International Edition 2012, 51,
2020-2067.
(224) Modi, N.; Lockwood, D. J.; Wu, X.; Baribeau, J. M.; Tsybeskov, L. Journal of
Applied Physics 2012, 111.
(225) Moore, L. J.; Zhou, H. L.; Boxer, S. G. Biochemistry 1999, 38, 11949-11960.
(226) Mori, D.; Benten, H.; Ohkita, H.; Ito, S.; Miyake, K. Acs Applied Materials &
Interfaces 2012, 4, 3325-3329.
(227) Morsli, M.; Cattin, L.; Bernede, J. C.; Kumar, P.; Chand, S. Journal of Physics D-
Applied Physics 2010, 43, 335103.
(228) Mouri, S.; Miyauchi, Y.; Matsuda, K. Journal of Physical Chemistry C 2012, 116,
10282-10286.
202

(229) Mueller, M. G.; Slavov, C.; Luthra, R.; Redding, K. E.; Holzwarth, A. R.
Proceedings of the National Academy of Sciences of the United States of America 2010,
107, 4123-4128.
(230) Nappa, M.; Valentine, J. S. Journal of the American Chemical Society 1978, 100,
5075-5080.
(231) NASA Global Climate Change report, http://climate.nasa.gov, 2011.
(232) Nelson, J.; Kwiatkowski, J. J.; Kirkpatrick, J.; Frost, J. M. Accounts of Chemical
Research 2009, 42, 1768-1778.
(233) Nolan, E. M.; Lippard, S. J. Accounts of Chemical Research 2009, 42, 193-203.
(234) Noltemeyer, M.; Bertram, F.; Hempel, T.; Bastek, B.; Polyakov, A.; Christen, J.;
Brandt, M.; Lorenz, M.; Grundmann, M. J. Mater. Res. 2012, 27, 2225-2231.
(235) Oh, H.; Krantz, J.; Litzov, I.; Stubhan, T.; Pinna, L.; Brabec, C. J. Sol. Energy
Mater. Sol. Cells 2011, 95, 2194-2199.
(236) Oida, T.; Naito, T.; Miyagawa, Y.; Sasaki, M.; Harafuji, K. Japanese Journal of
Applied Physics 2011, 50, 081601.
(237) Ojala, A.; Petersen, A.; Fuchs, A.; Lovrincic, R.; Poelking, C.; Trollmann, J.;
Hwang, J.; Lennartz, C.; Reichelt, H.; Hoeffken, H. W.; Pucci, A.; Erk, P.; Kirchartz, T.;
Wuerthner, F. Advanced Functional Materials 2012, 22, 86-96.
(238) Onsager, L. Physical Review 1938, 54, 554-557.
(239) Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Marder, S. R.; Mudalige, A.;
Marrikar, F. S.; Pemberton, J. E.; Armstrong, N. R. Journal of Physical Chemistry C
2008, 112, 7809-7817.
(240) Paramonov, P. B.; Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Armstrong, N.
R.; Marder, S. R.; Bredas, J.-L. Chemistry of Materials 2008, 20, 5131-5133.
(241) Park, B.; Whitham, K.; Cho, J.; Reichmanis, E. Acs Nano 2012, 6, 9466-9474.
(242) Pasquier, A. D.; Unalan, H. E.; Kanwal, A.; Miller, S.; Chhowalla, M. Applied
Physics Letters 2005, 87.
(243) Paulus, G. L. C.; Ham, M. H.; Strano, M. S. Nanotechnology 2012, 23.
203

(244) Peet, J.; Senatore, M. L.; Heeger, A. J.; Bazan, G. C. Advanced Materials 2009,
21, 1521-1527.
(245) Perez, M. D.; Borek, C.; Djurovich, P. I.; Mayo, E. I.; Lunt, R. R.; Forrest, S. R.;
Thompson, M. E. Advanced Materials 2009, 21, 1517-1520.
(246) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. Journal of the American
Chemical Society 2009, 131, 9281-9286.
(247) Petritsch, K.; Friend, R. H.; Lux, A.; Rozenberg, G.; Moratti, S. C.; Holmes, A. B.
Synth. Met. 1999, 102, 1776-1777.
(248) Pettersson, L. A. A.; Roman, L. S.; Inganas, O. Journal of Applied Physics 1999,
86, 487-496.
(249) Peumans, P.; Bulovic, V.; Forrest, S. R. Applied Physics Letters 2000, 76, 2650-
2652.
(250) Peumans, P.; Uchida, S.; Forrest, S. R. Nature 2003, 425, 158-162.
(251) Peumans, P.; Yakimov, A.; Forrest, S. R. Journal of Applied Physics 2003, 93,
3693-3723.
(252) Philibert, C. Solar Energy Perspectives; International Energy Agency (IEA):
Paris, France, 2011.
(253) Piersimoni, F.; Chambon, S.; Vandewal, K.; Mens, R.; Boonen, T.; Gadisa, A.;
Izquierdo, M.; Filippone, S.; Ruttens, B.; D'Haen, J.; Martin, N.; Lutsen, L.;
Vanderzande, D.; Adriaensens, P.; Manca, J. V. Journal of Physical Chemistry C 2011,
115, 10873-10880.
(254) Piersimoni, F.; Cheyns, D.; Vandewal, K.; Manca, J. V.; Rand, B. P. Journal of
Physical Chemistry Letters 2012, 3, 2064-2068.
(255) Piet, J. J.; Schuddeboom, W.; Wegewijs, B. R.; Grozema, F. C.; Warman, J. M.
Journal of the American Chemical Society 2001, 123, 5337-5347.
(256) Pillai, S.; Ravensbergen, J.; Antoniuk-Pablant, A.; Sherman, B. D.; van
Grondelle, R.; Frese, R. N.; Moore, T. A.; Gust, D.; Moore, A. L.; Kennis, J. T. M.
Physical Chemistry Chemical Physics 2013, 15, 4775-4784.
(257) Pineiro, M.; Carvalho, A. L.; Pereira, M. M.; Gonsalves, A.; Arnaut, L. G.;
Formosinho, S. J. Chemistry-a European Journal 1998, 4, 2299-2307.
204

(258) Placencia, D.; Wang, W. N.; Gantz, J.; Jenkins, J. L.; Armstrong, N. R. Journal of
Physical Chemistry C 2011, 115, 18873-18884.
(259) Placencia, D.; Wang, W. N.; Shallcross, R. C.; Nebesny, K. W.; Brumbach, M.;
Armstrong, N. R. Advanced Functional Materials 2009, 19, 1913-1921.
(260) Polyakov, A. N.; Noltemeyer, M.; Hempel, T.; Christen, J.; Stepovich, M. A. J
Surf Invest-X-Ray+ 2012, 6, 901-905.
(261) Poschlad, A.; Meded, V.; Maul, R.; Wenzel, W. Nanoscale Res Lett 2012, 7, 1-5.
(262) Potscavage, W. J., Jr.; Sharma, A.; Kippelen, B. Accounts of Chemical Research
2009, 42, 1758-1767.
(263) Potscavage, W. J.; Yoo, S.; Kippelen, B. Applied Physics Letters 2008, 93,
193308.
(264) Rand, B. P.; Burk, D. P.; Forrest, S. R. Physical Review B 2007, 75.
(265) Ratcliff, E. L.; Lee, P. A.; Armstrong, N. R. Journal of Materials Chemistry 2010,
20, 2672-2679.
(266) Ratcliff, E. L.; Meyer, J.; Steirer, K. X.; Garcia, A.; Berry, J. J.; Ginley, D. S.;
Olson, D. C.; Kahn, A.; Armstrong, N. R. Chemistry of Materials 2011, 23, 4988-5000.
(267) Ratcliff, E. L.; Zacher, B.; Armstrong, N. R. Journal of Physical Chemistry
Letters 2011, 2, 1337-1350.
(268) Reichardt, C. Chemical Reviews 1994, 94, 2319-2358.
(269) Reinhard, M.; Hanisch, J.; Zhang, Z.; Ahlswede, E.; Colsmann, A.; Lemmer, U.
Applied Physics Letters 2011, 98, 053303.
(270) Rettig, W.; Zander, M. Berichte Der Bunsen-Gesellschaft-Physical Chemistry
Chemical Physics 1983, 87, 1143-1149.
(271) Rider, D. A.; Harris, K. D.; Wang, D.; Bruce, J.; Fleischauer, M. D.; Tucker, R.
T.; Brett, M. J.; Buriak, J. M. Acs Applied Materials & Interfaces 2009, 1, 279-288.
(272) Riede, M.; Uhrich, C.; Widmer, J.; Timmreck, R.; Wynands, D.; Schwartz, G.;
Gnehr, W.-M.; Hildebrandt, D.; Weiss, A.; Hwang, J.; Sundarraj, S.; Erk, P.; Pfeiffer, M.;
Leo, K. Advanced Functional Materials 2011, 21, 3019-3028.
205

(273) Rio, Y.; Sanchez-Garcia, D.; Seitz, W.; Torres, T.; Sessler, J. L.; Guldi, D. M.
Chemistry-a European Journal 2009, 15, 3956-3959.
(274) Sabatini, R. P.; McCormick, T. M.; Lazarides, T.; Wilson, K. C.; Eisenberg, R.;
McCamant, D. W. Journal of Physical Chemistry Letters 2011, 2, 223-227.
(275) Saini, S.; Bagchi, B. Physical Chemistry Chemical Physics 2010, 12, 7427-7433.
(276) Sakamoto, N.; Ikeda, C.; Yamamura, M.; Nabeshima, T. Journal of the American
Chemical Society 2011, 133, 4726-4729.
(277) Sasajima, I.; Uesaka, S.; Kuwabara, T.; Yamaguchi, T.; Takahashi, K. Organic
Electronics 2011, 12, 113-118.
(278) Sazanovich, I. V.; Kirmaier, C.; Hindin, E.; Yu, L. H.; Bocian, D. F.; Lindsey, J.
S.; Holten, D. Journal of the American Chemical Society 2004, 126, 2664-2665.
(279) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A.
J.; Brabec, C. L. Advanced Materials 2006, 18, 789-+.
(280) Schlenker, C. W.; Barlier, V. S.; Chin, S. W.; Whited, M. T.; McAnally, R. E.;
Forrest, S. R.; Thompson, M. E. Chemistry of Materials 2011, 23, 4132-4140.
(281) Schlenker, C. W.; Chen, K.-S.; Yip, H.-L.; Li, C.-Z.; Bradshaw, L. R.;
Ochsenbein, S. T.; Ding, F.; Li, X. S.; Gamelin, D. R.; Jen, A. K. Y.; Ginger, D. S.
Journal of the American Chemical Society 2012, 134, 19661-19668.
(282) Schlenker, C. W.; Thompson, M. E. Chemical Communications 2011, 47, 3702-
3716.
(283) Schneide.F; Lippert, E. Berichte Der Bunsen-Gesellschaft Fur Physikalische
Chemie 1968, 72, 1155-1160.
(284) Schneide.F; Lippert, E. Berichte Der Bunsen-Gesellschaft Fur Physikalische
Chemie 1970, 74, 624-630.
(285) Schueppel, R.; Uhrich, C.; Pfeiffer, M.; Leo, K.; Brier, E.; Reinold, E.; Baeuerle,
P. Chemphyschem 2007, 8, 1497-1503.
(286) Schutz, M.; Schmidt, R. J. Phys. Chem. 1996, 100, 2012-2018.
(287) Schwedhelm, R.; Kipp, L.; Dallmeyer, A.; Skibowski, M. Physical Review B
1998, 58, 13176-13180.
206

(288) Seo, H. O.; Park, S.-Y.; Shim, W. H.; Kim, K.-D.; Lee, K. H.; Jo, M. Y.; Kim, J.
H.; Lee, E.; Kim, D.-W.; Kim, Y. D.; Lim, D. C. Journal of Physical Chemistry C 2011,
115, 21517-21520.
(289) Servaites, J. D.; Ratner, M. A.; Marks, T. J. Energy & Environmental Science
2011, 4, 4410-4422.
(290) Servaites, J. D.; Savoie, B. M.; Brink, J. B.; Marks, T. J.; Ratner, M. A. Energy &
Environmental Science 2012, 5, 8343-8350.
(291) Service, R. F. Science 2011, 332, 293.
(292) Service, R. F. Science 2011, 332, 293-293.
(293) Shim, H.-S.; Kim, H. J.; Kim, J. W.; Kim, S.-Y.; Jeong, W.-I.; Kim, T.-M.; Kim,
J.-J. Journal of Materials Chemistry, 22, 9077-9081.
(294) Shoaee, S.; Clarke, T. M.; Huang, C.; Barlow, S.; Marder, S. R.; Heeney, M.;
McCulloch, I.; Durrant, J. R. Journal of the American Chemical Society 2010, 132,
12919-12926.
(295) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Advanced
Functional Materials 2006, 16, 2016-2023.
(296) Shtein, M.; Mapel, J.; Benziger, J. B.; Forrest, S. R. Applied Physics Letters 2002,
81, 268-270.
(297) Sonar, P.; Lim, J. P. F.; Chan, K. L. Energy & Environmental Science 2011, 4,
1558-1574.
(298) Song, Q. L.; Wang, M. L.; Obbard, E. G.; Sun, X. Y.; Ding, X. M.; Hou, X. Y.;
Li, C. M. Applied Physics Letters 2006, 89, 251118.
(299) Steim, R.; Kogler, F. R.; Brabec, C. J. Journal of Materials Chemistry 2010, 20,
2499-2512.
(300) Steirer, K. X.; Ndione, P. F.; Widjonarko, N. E.; Lloyd, M. T.; Meyer, J.; Ratcliff,
E. L.; Kahn, A.; Armstrong, N. R.; Curtis, C. J.; Ginley, D. S.; Berry, J. J.; Olson, D. C.
Advanced Energy Materials 2011, 1, 813-820.
(301) Sullivan, P.; Duraud, A.; Hancox, I.; Beaumont, N.; Mirri, G.; Tucker, J. H. R.;
Hatton, R. A.; Shipman, M.; Jones, T. S. Advanced Energy Materials 2011, 1, 352-355.
207

(302) Sun, H.; Weickert, J.; Hesse, H. C.; Schmidt-Mende, L. Sol. Energy Mater. Sol.
Cells 2011, 95, 3450-3454.
(303) Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J.
Nature Materials 2012, 11, 44-48.
(304) Sutton, J. M.; Rogerson, E.; Wilson, C. J.; Sparke, A. E.; Archibald, S. J.; Boyle,
R. W. Chemical Communications 2004, 1328-1329.
(305) Tang, C. W. Applied Physics Letters 1986, 48, 183-185.
(306) Tang, Z.; Andersson, L. M.; George, Z.; Vandewal, K.; Tvingstedt, K.; Heriksson,
P.; Kroon, R.; Andersson, M. R.; Inganas, O. Advanced Materials 2012, 24, 554-558.
(307) Tanskanen, J. T.; Bakke, J. R.; Pakkanen, T. A.; Bent, S. F. Journal of Vacuum
Science & Technology A 2011, 29, 031507.
(308) Telfer, S. G.; McLean, T. M.; Waterland, M. R. Dalton Transactions 2011, 40,
3097-3108.
(309) Thoi, V. S.; Stork, J. R.; Magde, D.; Cohen, S. M. Inorg. Chem. 2006, 45, 10688-
10697.
(310) Tong, X.; Bailey-Salzman, R. F.; Wei, G.; Forrest, S. R. Applied Physics Letters
2008, 93, 173304.
(311) Tong, X.; Lassiter, B. E.; Forrest, S. R. Organic Electronics 2010, 11, 705-709.
(312) Tousek, J.; Touskova, J.; Remes, Z.; Cermak, J.; Kousal, J.; Kindl, D.; Kuritka, I.
Chemical Physics Letters 2012, 552, 49-52.
(313) Tress, W.; Merten, A.; Furno, M.; Hein, M.; Leo, K.; Riede, M. Advanced Energy
Materials 2013, Early view online.
(314) Tress, W.; Petrich, A.; Hummert, M.; Hein, M.; Leo, K.; Riede, M. Applied
Physics Letters 2011, 98.
(315) Tress, W.; Pfuetzner, S.; Leo, K.; Riede, M. Journal of Photonics for Energy
2011, 1.
(316) Trinh, C.; Whited, M. T.; Steiner, A.; Tassone, C. J.; Toney, M. F.; Thompson,
M. E. Chemistry of Materials 2012, 24, 2583-2591.
208

(317) Tsutsumi, J.; Matsui, H.; Yamada, T.; Kumai, R.; Hasegawa, T. Journal of
Physical Chemistry C 2012, 116, 23957-23964.
(318) Turro, J. N.; Ramamurthy, V.; Scaiano, J. C. Principles of Molecular
Photochemistry: An introduction; University Science Book: Sausalito, California, 2009.
(319) Uhrich, C.; Schueppel, R.; Petrich, A.; Pfeiffer, M.; Leo, K.; Brier, E.; Kilickiran,
P.; Baeuerle, P. Advanced Functional Materials 2007, 17, 2991-2999.
(320) Ulrich, G.; Ziessel, R.; Harriman, A. Angewandte Chemie-International Edition
2008, 47, 1184-1201.
(321) Vakhshouri, K.; Kesava, S. V.; Kozub, D. R.; Gomez, E. D. Mater. Lett. 2013, 90,
97-102.
(322) Valleau, S.; Saikin, S. K.; Yung, M. H.; Guzik, A. A. Journal of Chemical Physics
2012, 137.
(323) Vandenheuvel, D. J.; Chan, I. Y.; Groenen, E. J. J.; Schmidt, J.; Meijer, G.
Chemical Physics Letters 1994, 231, 111-118.
(324) Vandewal, K.; Gadisa, A.; Oosterbaan, W. D.; Bertho, S.; Banishoeib, F.; Van
Severen, I.; Lutsen, L.; Cleij, T. J.; Vanderzande, D.; Manca, J. V. Advanced Functional
Materials 2008, 18, 2064-2070.
(325) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V. Nature
Materials 2009, 8, 904-909.
(326) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V. Physical
Review B 2010, 81.
(327) Vasseur, K.; Rand, B. P.; Cheyns, D.; Froyen, L.; Heremans, P. Chemistry of
Materials 2010, 23, 886-895.
(328) Vauthey, E. Chemphyschem 2012, 13, 2001-2011.
(329) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. Advanced Functional Materials
2009, 19, 1939-1948.
(330) Verreet, B.; Rand, B. P.; Cheyns, D.; Hadipour, A.; Aernouts, T.; Heremans, P.;
Medina, A.; Claessens, C. G.; Torres, T. Advanced Energy Materials 2011, 1, 565-568.
(331) Vogel, G. C.; Beckmann, B. A. Inorg. Chem. 1976, 15, 483-484.
209

(332) Wagner, J.; Gruber, M.; Hinderhofer, A.; Wilke, A.; Broeker, B.; Frisch, J.;
Amsalem, P.; Vollmer, A.; Opitz, A.; Koch, N.; Schreiber, F.; Bruetting, W. Advanced
Functional Materials 2010, 20, 4295-4303.
(333) Walker, B.; Tomayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garcia, A.;
Tantiwiwat, M.; Nguyen, T. Q. Advanced Functional Materials 2009, 19, 3063-3069.
(334) Wang, J.-C.; Weng, W.-T.; Tsai, M.-Y.; Lee, M.-K.; Horng, S.-F.; Perng, T.-P.;
Kei, C.-C.; Yu, C.-C.; Meng, H.-F. Journal of Materials Chemistry 2010, 20, 862-866.
(335) Wang, M. L.; Song, Q. L.; Wu, H. R.; Ding, B. F.; Gao, X. D.; Sun, X. Y.; Ding,
X. M.; Hou, X. Y. Organic Electronics 2007, 8, 445-449.
(336) Wang, R.; Ruzicka, B. A.; Kumar, N.; Bellus, M. Z.; Chiu, H. Y.; Zhao, H.
Physical Review B 2012, 86.
(337) Wang, S.; Hall, L.; Diev, V. V.; Haiges, R.; Wei, G.; Xiao, X.; Djurovich, P. I.;
Forrest, S. R.; Thompson, M. E. Chemistry of Materials 2011, 23, 4789-4798.
(338) Wang, S.; Mayo, E. I.; Perez, M. D.; Griffe, L.; Wei, G.; Djurovich, P. I.; Forrest,
S. R.; Thompson, M. E. Applied Physics Letters 2009, 94, 233304.
(339) Wang, W. N.; Placencia, D.; Armstrong, N. R. Organic Electronics 2011, 12, 383-
393.
(340) Ward, A. J.; Ruseckas, A.; Samuel, I. D. W. Journal of Physical Chemistry C
2012, 116, 23931-23937.
(341) Wei, G.; Lunt, R. R.; Sun, K.; Wang, S.; Thompson, M. E.; Forrest, S. R. Nano
Letters 2010, 10, 3555-3559.
(342) Wei, G.; Wang, S.; Renshaw, K.; Thompson, M. E.; Forrest, S. R. Acs Nano
2010, 4, 1927-1934.
(343) Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R. Advanced Energy
Materials 2011, 1, 184-187.
(344) Wei, G.; Xiao, X.; Wang, S.; Sun, K.; Bergemann, K. J.; Thompson, M. E.;
Forrest, S. R. Acs Nano 2011, 6, 972-978.
(345) Wei, G.; Xiao, X.; Wang, S.; Sun, K.; Bergemann, K. J.; Thompson, M. E.;
Forrest, S. R. Acs Nano 2012, 6, 972-978.
210

(346) Wei, G.; Xiao, X.; Wang, S.; Zimmerman, J. D.; Sun, K.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Nano Letters 2011, 11, 4261-4264.
(347) Wei, G.; Xiao, X.; Wang, S.; Zimmerman, J. D.; Sun, K.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Nano Letters 2012, 11, 4261-4264.
(348) Whited, M. T.; Patel, N. M.; Roberts, S. T.; Allen, K.; Djurovich, P. I.; Bradforth,
S. E.; Thompson, M. E. Chemical Communications 2011, 48, 284-286.
(349) Wilke, A.; Endres, J.; Hoermann, U.; Niederhausen, J.; Schlesinger, R.; Frisch, J.;
Amsalem, P.; Wagner, J.; Gruber, M.; Opitz, A.; Vollmer, A.; Bruetting, W.; Kahn, A.;
Koch, N. Applied Physics Letters 2012, 101.
(350) Wisnet, A.; Thomann, M.; Weickert, J.; Schmidt-Mende, L.; Scheu, C. J Mater
Sci 2012, 47, 6459-6466.
(351) Woller, J. G.; Hannestad, J. K.; Albinsson, B. Journal of the American Chemical
Society 2013, 135, 2759-2768.
(352) Wood, T. E.; Thompson, A. Chemical Reviews 2007, 107, 1831-1861.
(353) Wrobel, D.; Graja, A. Coord. Chem. Rev. 2011, 255, 2555-2577.
(354) Wu, H. R.; Song, Q. L.; Wang, M. L.; Li, F. Y.; Yang, H.; Wu, Y.; Huang, C. H.;
Ding, X. M.; Hou, X. Y. Thin Solid Films 2007, 515, 8050-8053.
(355) Wu, W.; Zhao, J.; Sun, J.; Guo, S. J. Org. Chem. 2012, 77, 5305-5312.
(356) Wu, Y. Z.; Zhang, W. L.; Ni, W. D.; Zhang, C. R.; Zhang, D. J. Acta Phys Sin-Ch
Ed 2012, 61.
(357) Xiao, X.; Wei, G.; Wang, S.; Zimmerman, J. D.; Renshaw, C. K.; Thompson, M.
E.; Forrest, S. R. Advanced Materials 2012, 24, 1956-1960.
(358) Xiong, L.; Seibert, M.; Gusev, A. V.; Wasielewski, M. R.; Hemann, C.; Hille, C.
R.; Sayre, R. T. J. Phys. Chem. B 2004, 108, 16904-16911.
(359) Xue, J. G.; Forrest, S. R. Journal of Applied Physics 2004, 95, 1869-1877.
(360) Yambem, S. D.; Liao, K. S.; Curran, S. A. Sol. Energy Mater. Sol. Cells 2012,
101, 227-231.
(361) Yang, D.; Zhou, L.; Chen, L.; Zhao, B.; Zhang, J.; Li, C. Chemical
Communications 2012, 48, 8078-8080.
211

(362) Yang, J.; Zhu, F.; Yu, B.; Wang, H.; Yan, D. Applied Physics Letters 2012, 100.
(363) Yang, J. B.; Zhu, F.; Yu, B.; Wang, H. B.; Yan, D. H. Applied Physics Letters
2012, 100.
(364) Yang, L.; Zhang, T.; Zhou, H.; Price, S. C.; Wiley, B. J.; You, W. Acs Applied
Materials & Interfaces 2011, 3, 4075-4084.
(365) Yang, Y.; Huang, Q. L.; Metz, A. W.; Ni, J.; Jin, S.; Marks, T. J.; Madsen, M. E.;
DiVenere, A.; Ho, S. T. Advanced Materials 2004, 16, 321-324.
(366) Yang, Y. F.; Arias, F.; Echegoyen, L.; Chibante, L. P. F.; Flanagan, S.;
Robertson, A.; Wilson, L. J. Journal of the American Chemical Society 1995, 117, 7801-
7804.
(367) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.;
Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Graetzel, M. Science 2011, 334, 629-
634.
(368) Yi, Y.; Coropceanu, V.; Bredas, J.-L. Journal of the American Chemical Society
2009, 131, 15777-15783.
(369) Yi, Y. P.; Coropceanu, V.; Bredas, J. L. Journal of the American Chemical
Society 2009, 131, 15777-15783.
(370) Yokoyama, D.; Wang, Z. Q.; Pu, Y. J.; Kobayashi, K.; Kido, J.; Hong, Z. R. Sol.
Energy Mater. Sol. Cells 2012, 98, 472-475.
(371) Yokoyama, D.; Wang, Z. Q.; Pu, Y.-J.; Kobayashi, K.; Kido, J.; Hong, Z. Sol.
Energy Mater. Sol. Cells 2012, 98, 472-475.
(372) Yoo, J.; Chon, B.; Tang, W.; Joo, T.; Dang, L. S.; Yi, G. C. Applied Physics
Letters 2012, 100.
(373) Yook, K. S.; Chin, B. D.; Lee, J. Y.; Lassiter, B. E.; Forrest, S. R. Applied
Physics Letters, 99.
(374) Yost, S. R.; Hontz, E.; Yeganeh, S.; Van Voorhis, T. Journal of Physical
Chemistry C 2012, 116, 17369-17377.
(375) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.;
Chen, C.-C.; Gao, J.; Li, G.; Yang, Y. Nature Communications 2013, 4, 1446.
212

(376) Young, K. J.; Martini, L. A.; Milot, R. L.; Snoeberger, R. C., III; Batista, V. S.;
Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W. Coord. Chem. Rev. 2012, 256,
2503-2520.
(377) Yu, L. H.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin, E.; Diers,
J. R.; Boyle, P. D.; Bocian, D. F.; Holten, D.; Lindsey, J. S. Inorg. Chem. 2003, 42, 6629-
6647.
(378) Zander, M.; Rettig, W. Chemical Physics Letters 1984, 110, 602-610.
(379) Zhang, F.; Xu, X.; Tang, W.; Zhang, J.; Zhuo, Z.; Wang, J.; Wang, J.; Xu, Z.;
Wang, Y. Sol. Energy Mater. Sol. Cells 2011, 95, 1785-1799.
(380) Zhang, F. J.; Zhao, D. W.; Zhuo, Z. L.; Wang, H.; Xu, Z.; Wang, Y. S. Sol.
Energy Mater. Sol. Cells 2010, 94, 2416-2421.
(381) Zhang, H.; Ouyang, J. Applied Physics Letters 2010, 97, 063509.
(382) Zhang, M.; Wang, H.; Tang, C. W. Applied Physics Letters 2011, 99.
(383) Zhang, T.; Birgersson, E.; Ananthanarayanan, K.; Yong, C. H.; Thummalakunta,
L. N. S. A.; Luther, J. Journal of Applied Physics 2012, 112.
(384) Zhang, X. H.; Kippelen, B. Applied Physics Letters 2008, 93.
(385) Zhang, X.-H.; Kippelen, B. Journal of Applied Physics 2008, 104.
(386) Zhong, S.; Zhong, J. Q.; Wang, X. Z.; Huang, M. Y.; Qi, D. C.; Chen, Z. K.;
Chen, W. Journal of Physical Chemistry C 2012, 116, 2521-2526.
(387) Zhou, Y.; Cheun, H.; Potscavage, W. J., Jr.; Fuentes-Hernandez, C.; Kim, S.-J.;
Kippelen, B. Journal of Materials Chemistry 2010, 20, 6189-6194.
(388) Ziessel, R.; Allen, B. D.; Rewinska, D. B.; Harriman, A. Chemistry-a European
Journal 2009, 15, 7382-7393.
(389) Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R. Nano Letters 2012, 12, 4366-4371.
(390) Zimmerman, J. D.; Xiao, X.; Renshaw, K.; Thompson, M. E.; Forrest, S. R.
Advanced Energy Materials 2012, Submitted.
(391) Zimmerman, J. D.; Yu, E. K.; Diev, V. V.; Hanson, K.; Thompson, M. E.;
Forrest, S. R. Organic Electronics 2011, 12, 869-873.

Abstract (if available)

Abstract Organic photovoltaics (OPV) are a promising energy source for the future because they have potential advantages over the conventional inorganic PV (silicon based PV), including lightweight, flexibility, and low cost roll to roll production. These advanced properties of organic materials enable future applications with great features, such as OPV on flexible plastic substrates for mobile devices, or solar paints for cars and homes. However, the efficiency of OPV is still low (the best reported efficiency of OPV devices is 9%) compared to inorganic PV (Si-based PV with 15-20% efficiency). The low efficiency of OPV is due to limits of current organic materials and the lack of understanding fundamental processes in OPV under operation. In addition, the OPV devices suffer low stability. ❧ The research presented in this dissertation utilizes both materials design and device optimization to address the limitations of OPVs. In Chapter 3, we present a chemical annealing method used to convert the amorphous films to polycrystalline films and study the effect of morphology change on device performance. In Chapter 4, the synthesis and characterization of organic dyes that undergo symmetry breaking charge transfer are shown. These compounds are expected to have lower exciton binding energies, thus lowering the energy cost for charge separation in OPVs. Chapter 5 covers the energy sensitization of C60 and application in OPVs to improve absorption efficiency. While OPVs with inverted structures have been shown to have improved lifetime, the efficiency of inverted OPVs are often lower than the conventional OPVs. Study on device physics of OPVs with inverted structures in Chapter 6 shed light on the origin of power loss and serves as guidance to design new materials for inverted OPVs.

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University of Southern California Dissertations and Theses

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Asset Metadata

Creator Trinh, Cong (author)

Core Title Organic photovoltaics: from materials development to device application

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 07/30/2013

Defense Date 06/18/2013

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemical annealing,clean energy,device physics,electron acceptor,electron donor,energy,energy sensitization,fullerene,inverted solar cell,OAI-PMH Harvest,organic dyes,organic electronics,organic materials,organic solar cells,organic thin films,photophysics,solar cells,solar energy,sustainability

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Brutchey, Richard L. (committee member), Dapkus, Daniel P. (committee member)

Creator Email congtrin@usc.edu,congtrinh83@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-307571

Unique identifier UC11294853

Identifier etd-TrinhCong-1888.pdf (filename),usctheses-c3-307571 (legacy record id)

Legacy Identifier etd-TrinhCong-1888.pdf

Dmrecord 307571

Document Type Dissertation

Format application/pdf (imt)

Rights Trinh, Cong

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA