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Integration of cost effective luminescent materials into organic light emitting diodes (OLEDs) and molecular approaches to improve the device efficiency

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Integration of cost effective luminescent materials into organic light emitting diodes (OLEDs) and molecular approaches to improve the device efficiency

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Content Integration of Cost Effective Luminescent Materials into
Organic Light Emitting Diodes (OLEDs) and Molecular
Approaches to Improve the Device Efficiency
By
Moon Chul Jung
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
(Materials Science)
December 2021
Copyright 2021 Moon Chul Jung
ii

Acknowledgements
I would like to express my sincere gratitude and appreciation, first and foremost to my
research advisor and mentor, Prof. Mark E. Thompson for offering me the opportunity to be a part
of the group and for continuous care and sincere support during my graduate school. Prof. Mark
E. Thompson not only encouraged me to become a creative and independent scientist but also
provide perfect environment for me.
I would also like to extend my appreciation to Prof. Jayakanth Ravichandran, Prof. Wei
Wu, Prof. Andrea Armani and Prof.Paul Daniel Dapkus for their valuable time and guidance while
serving on my defense and my qualifying exam committee. I am also thankful to Prof. Peter
Djurovich, whose vast knowledge of the literature, creative ideas and challenging questions
inspired me to tackle the challenging problems.
I would like to thank to everyone in my research lab who I collaborate with and have helped
me in my graduate career. First, I would like to thank the senior members and former colleagues
to Dr. Muazzam Idris who always helped me on synthesis, instrument and tricky questions; to Dr.
John Facendola for his cooperation in molecular alignment project; to Dr. Shuyang Shi and Dr.
Rasha Hamze for collaboration on copper project; to Dr. Daniel Sylvinson for his intelligent work
on computational modeling studies; to Dr. Narcisse Ukwitegetse and Dr. Anton Razgoniaev for
their sincere advice and warm-hearted mind; to Dr. Karim for collaboration on hybrid white OLED
project; to Dr. Tyler Fleeham, Dr. Patrick Saris. Dr. Jessica Golden and Dr. Piyume
Wickramasinghe for their insightful idea on chemistry and device; Dr. Tian-yi for exchanging
ideas and training of CHNS instrument. I also would like to thank to Savannah Kapper and Abegail
Tadle for being a great team of “graduation in 2021”; to Jie Ma for her co-management of the
iii

chamber and device work; to Konstantin Mallon for his advice on chemistry; to Dr. Sunil Kumar
Kandappa, Dr. Eric T. McClure, Austin Mencke, Collin Muniz, Jonas Schaab, Mattia Di Niro,
Mahsa Reziyan, Megan Cassingham, Gemma Goh and Darius Allen Shariaty for always providing
a perfect and peaceful working environment and building friendships. I would also like to thank
my collaborator at USC, Dr. Dhritiman Bhattacharyya for his interface studies on Poly-3-
hexylthiophene. Lastly at USC, I would also like to extend my appreciation to USC administration,
Judy May Fong and Michelle Dea who provided me with administrative help and support at USC.
I would like to special thank to our collaborator at University of Michigan Ann Arbor: to
Dr. Jongchan Kim for his modeling and device studies in molecular alignment project; to Chan Ho
So for his device work and discussion in hybrid WOLED project. Also, I would like to thank to
Prof. Andrea Hodge and Prof. Andrea Armani for helping me to adapt in graduate school.
Lastly, I would like to acknowledge invaluable support of my family through the long
journey: my loving parents Ki Hwa Jung and Dong Eul Hwang who were always supportive in my
life. Their unconditional support and help motivated me to go through stressful moments. I would
like to thank my relatives, grandparents, my aunt, uncles, Tae won Jung, Moon Joo Jung and
Hyung jae for their mental support. I also would like to thank my friends in Korea: my best friends
who I met when I was in middle and high school friends, Yu Seung Kim for their endless
friendships.
Above all, I would like to thank my fiancé, Emily Yun for being, and will be with me
during and after this long journey.

iv

Table of Contents
Acknowledgements ......................................................................................................................... ii
List of Tables ................................................................................................................................ vii
List of Figures ................................................................................................................................ ix
Abstract ........................................................................................................................................ xiv
Chapter 1. Introduction ................................................................................................................... 1
1.1. Principle of OLEDs .......................................................................................................... 1
1.1.1. OLEDs Structure and Basic Steps ............................................................................ 1
1.1.2. Host materials ........................................................................................................... 4
1.1.3. Fluorescent Compounds............................................................................................ 6
1.1.4. Spin Orbit Coupling .................................................................................................. 8
1.1.5. Phosphorescent Complexes ...................................................................................... 9
1.1.6. Copper complexes ................................................................................................... 10
1.2. Performances of OLEDs ................................................................................................ 12
1.2.1. External Quantum Efficiency ................................................................................. 12
1.2.2. Outcoupling Efficiency ........................................................................................... 14
1.3. Stability of OLEDs ......................................................................................................... 18
1.3.1. Efficiency roll-off ................................................................................................... 18
1.3.2. Triplet-Triplet Annihilation (TTA) and Triplet-Polaron Annihilation (TPA) ........ 19
1.4. Chapter 1 references ....................................................................................................... 22
Chapter 2. Integration of copper and gold complexes into Organic Light Emitting Diodes
(OLEDs) ........................................................................................................................................ 27
2.1. Introduction .................................................................................................................... 27
2.1.1. Solution processed OLEDs with copper complexes as emitters ............................. 28
2.1.2. Difference between solution process and vacuum deposition in OLEDs ............... 29
2.2. Results and Discussion ................................................................................................... 31
2.2.1. Photophysical Properties ......................................................................................... 31
2.2.2. Host Studies ............................................................................................................ 33
2.2.1. OLEDs optimizations.............................................................................................. 35
2.3. Conclusion ...................................................................................................................... 48
2.4. Experimental Methods ................................................................................................... 49
2.5. Chapter 2 references ....................................................................................................... 50
v

Chapter 3. Fabrication of hybrid White Organic Light Emitting Diodes (OLEDs) employing
imidazole-based neat emitter ........................................................................................................ 54
3.1. Introduction .................................................................................................................... 54
3.1.1. Concepts of Hybrid WOLEDs ................................................................................ 54
3.1.2. The limitations of hybrid WOLED with a neat fluorescent emitter ....................... 57
3.2. Results and Discussions ................................................................................................. 58
3.2.1. Photophysical Properties and Film Studies............................................................. 58
3.2.2. Probing Exciton Profiles ......................................................................................... 61
3.2.1. OLEDs Optimization .............................................................................................. 64
3.3. Conclusions .................................................................................................................... 70
3.4. Experimental Methods ................................................................................................... 71
3.5. Chapter 3 references ....................................................................................................... 72
Chapter 4. Fabrication of hybrid Organic Light Emitting Diodes (OLEDs) utilizing corannulene
derivatives as a host ...................................................................................................................... 74
4.1. Introduction .................................................................................................................... 74
4.1.1. Problem of hybrid WOLEDs .................................................................................. 74
4.1.2. Stacked hybrid White OLEDs ................................................................................ 76
4.1.3. Stacked hybrid White OLEDs ................................................................................ 77
4.1.4. New Host Material for Proposed Stacked hybrid WOLEDs .................................. 81
4.2. Results and Discussion ................................................................................................... 82
4.2.1. Synthesis ................................................................................................................. 82
4.2.1. Photophysical Characterizations ............................................................................. 82
4.2.2. Electrochemistry and DFT calculations .................................................................. 83
4.2.3. Quenching studies ................................................................................................... 85
4.2.4. OLEDs optimization ............................................................................................... 88
4.3. Conclusions .................................................................................................................. 100
4.4. Experimental Methods ................................................................................................. 102
4.4.1. Synthesis ............................................................................................................... 102
4.4.2. Photophysical characterization ............................................................................. 103
4.5. Chapter 4 references ..................................................................................................... 103
Chapter 5. Molecular Alignment of Homoleptic Iridium Phosphors in Organic Light Emitting
Diodes (OLEDs) ......................................................................................................................... 105
5.1. Introduction .................................................................................................................. 105
vi

5.2. Results and Discussion ................................................................................................. 107
5.2.1. Synthesis ............................................................................................................... 107
5.2.2. Photophysical Characterization ............................................................................ 108
5.2.3. Electrochemistry ................................................................................................... 110
5.2.4. DFT and TD-DFT Calculations ............................................................................ 113
5.2.5. The role of structure in dopant alignment ............................................................. 115
5.2.6. The role of chemical asymmetry in dopant alignment.......................................... 119
5.2.7. OLEDs characterizations and Simulations ........................................................... 123
5.3. Conclusions .................................................................................................................. 127
5.4. Experimental methods .................................................................................................. 129
5.4.1. Synthesis ............................................................................................................... 129
5.4.2. Photophysical measurements ................................................................................ 134
5.4.3. Molecular computational modeling ...................................................................... 134
5.4.4. Transition dipole moment vector (TDM) alignment measurements..................... 137
5.4.5. OLED fabrication and device simulation ............................................................. 138
5.5. Chapter 5 references ..................................................................................................... 139
Appendix 1. Near-IR emission from heteroleptic tris-cyclometalated Iridium complexes with a
single emitting ligand for OLED application.............................................................................. 144
A.1.1. Introduction .............................................................................................................. 144
A.1.2. Result and Discussion ............................................................................................... 145
A.1.3. Conclusion ................................................................................................................ 147
A.1.4. Experimental Methods .............................................................................................. 147
A.1.5. Appendix 1 references .............................................................................................. 147
Appendix 2. Device optimizations of deep blue and yellow copper emitters. ........................... 148
A.2.1. Results and Discussion ............................................................................................. 148
A.2.2. Experimental Methods .............................................................................................. 158
A.2.3. Appendix 2 references .............................................................................................. 158

vii

List of Tables
Table 1. 1 SOC parameter (ζ1) of common atoms in cm
-1
.
27
......................................................... 9

Table 2. 1 Maximum wavelength (λmax), quantum yield (Φ), lifetime (τRT), radiative lifetime
(𝒌𝒓 ,𝑹𝑻 ) and non-radiative lifetime (𝒌𝒏𝒓 ,𝑹𝑻 ) of 1a, 1b and 1c in 1% doped PS film. .............. 33
Table 2. 2 HOMO, LUMO and triplet energies of 1a and four host materials ............................ 34
Table 2. 3 Summary of device optimization. The following parameters in the EML were varied:
Identity of the host (UGH3, mCBP, mCP, and Cu host), EML thickness (40 nm), and doping
concentration (5%, 10%, 20% and 25%). The device architectures are: TAPC(40)nm/EML(20,
40nm)/TPBi(40nm, 50nm)/LiF(1nm)/Al(100nm).
a)
Turn on voltage was determined as the
voltage at 1cd/m
2
........................................................................................................................... 42
Table 2. 4 Turn on voltage (VT, defined the voltage at 0.1cd/m
2
), EQE, maximum luminance
(Lmax) and maximum emission wavelength (λmax) of OLEDs shown in Figure 2.12 ................... 46

Table 3. 1 HOMO/LUMO of phenanthro[9,10-d]imidazoles derivatives and photoluminescence
quantum yield (PLQY) of the spin-cast films. The films comprise Ir(ppy)3 and PQIr doped at 10
wt% into I1-I6 and CBP and neat or 100% of I1-I6. .................................................................... 60
Table 3. 2 Commission internationale de leclairage (CIE), Color Correlated Temperature (CCT)
and CRI values of optimized WOLEDs at given current densities. ............................................. 69

Table 4. 1 Device architectures to investigate functionality of HATCN/m-MTDATA as a CGL.
....................................................................................................................................................... 78
Table 4. 2 Device structure of non-stacked and stacked device. ................................................. 79
Table 4. 3 DFT calculations of xylyl corannulene and experimental HOMO/LUMO levels. The
values were derived by using the equation shown in reference
8
. ................................................ 84
Table 4. 4 Quantum yields and lifetimes of the four different doped films. ............................... 87
Table 4. 5 Quantum yields and lifetime of four doped films. ..................................................... 88
Table 4. 6 CIE coordinates and ratio of PQIr intensity to α-aD intensity at various current
densities......................................................................................................................................... 99

Table 5. 1 Maximum emission wavelength (λmax), photoluminescence efficiency (ΦPL), lifetime
(τ), HOMO/LUMO and magnitude of permanent dipole moment of the fac-Ir(C^N)3 complexes.
..................................................................................................................................................... 110
Table 5. 2 HOMO, LUMO and triplet density distribution of phenylimidazole-based complexes
..................................................................................................................................................... 111
Table 5. 3 Optical anisotropy factors of iridium complexes. ..................................................... 118
Table 5. 4 Calculated aspect ratio of all compounds studied in the paper ................................. 119
Table 5. 5 Dipole moments of host materials obtained from DFT calculations. ....................... 119
Table 5. 6 Summary of device performance with simulated EQE and outcoupling efficiency . 125

viii

Table A.2. 1 PLQY and lifetime of films when compound 2 are doped into various host at
different doping concentration. ................................................................................................... 151

ix

List of Figures
Figure 1. 1 Basic structure of OLEDs (Left) and mechanism of generating photons in OLEDs
(Right) ............................................................................................................................................. 1
Figure 1. 2 Molecular structures of commonly used HTL materials in OLEDs. .......................... 3
Figure 1. 3 Molecular structures of commonly used ETL materials in OLEDs ........................... 3
Figure 1. 4 Possible routes for holes/electrons recombination (Left). Singlet and triplet states
(Right). ............................................................................................................................................ 4
Figure 1. 5 Molecular structures of common host materials......................................................... 5
Figure 1. 6 Jablonski diagram illustrating photophysical processes ............................................. 7
Figure 1. 7 Graphical representation of nuclear-centric system (a) vs a nuclear-centered (b) an
electron-centered. ............................................................................................................................ 8
Figure 1. 8 Geometrical distortion pattern of a-c. 4-coordinates, 3-coordinate and 2-coordinate
Cu(Ι) complexes respectively. ...................................................................................................... 11
Figure 1. 9 Illustration of the structural behavior of EML in different devices, (a) with a hetero
structured stack, (b) a graded mixed layer, and (c) a uniformly mixed layer. The data is derived
from reference
47
. .......................................................................................................................... 14
Figure 1. 10 Schematic illustration of an OLED showing different optical loss channels (Top).
Examples of amount of power coupled to different optical channels in the device of reference
53

(Bottom). ....................................................................................................................................... 15
Figure 1. 11 . Extrinsic methods for improving outcoupling efficiency. (a) Microlens array, (b)
scattering particles, (c) index grating. Pictures are derived reference
41
. ..................................... 16
Figure 1. 12 Summarized EQE curve as a function of the current density at which EQE drops to
90% of its maximum value reported. Filled square is phosphorescent OLEDs, open square is
fluorescent OLEDs, figure with dot in it is hybrid OLEDs. Reverse triangle is usage of
outcoupling enhancement, circle is TADF emitters and triangle is TTA.
28
................................. 18

Figure 2. 1 (Left) Molecular structure of CAAC
Men
-CuCz (1a) and CAAC
Men
-Cu(C6F5) (Cu
host). (Right) Emission spectra of 1a and Cu host in the microcrystalline powder form (top) and
in MeCy (bottom) at RT and 77K. Also shown is the emission spectrum of a thin film of
CAAC
Men
-CuCz (1a) doped into Cu host (20 wt%, solution-processed, top) and in MeCy
(bottom) for comparison. .............................................................................................................. 31
Figure 2. 2 (Left) Molecular structure of MAC*-CuCz (1b) and MAC*-AuCz (1c). (Right)
Emission spectra of 1b and 1c in 2-Methyltetrahydrofuran at RT (top) and 77K (bottom). ........ 32
Figure 2. 3 Molecular structures of the three hosts (Left) and emission spectra of 1a doped thin
films (40 nm, 20% vol) doped into three four hosts (Right) ......................................................... 34
Figure 2. 4 Photoluminescence of the doped poly styrene (PS) films of 1b and 1c at 1 vol%
doping concentration. The films were prepared by spin-cast technique. Inset shows the photo of
the doped PS film. ......................................................................................................................... 35
Figure 2. 5 Device architectures with three different hosts and doping concentrations and
molecular structures of HTL and ETL. Thickness of EML is 20nm (Top). EQE and J-V-L plot
(Bottom) ........................................................................................................................................ 36
x

Figure 2. 6 Electroluminescence spectra collected from devices with 1a as an emitter in mCBP
(Top), mCP (Middle), Cu host (Bottom) respectively. Left: 5 vol%, Right: 10 vol% doping
concentrations. .............................................................................................................................. 37
Figure 2. 7 (A) Device architecture with 20nm EML and 20 vol% doping concentration. (B)
EQE plot (C) J-V-L curve (D)-(F) Electroluminescence spectrum of the device at different
voltages. ........................................................................................................................................ 38
Figure 2. 8 Performances of devices employing 1a as an emitter doped into Cu host with
different doping concentration (5, 10, 20 and 30 vol%). The architecture of device is
TAPC(40nm)/EML(40nm)/TPBi(40nm)/LiF(1nm)/Al(100nm). C-E are the EL spectra of 5, 10,
20 and 30 vol% devices respectively. ........................................................................................... 39
Figure 2. 9 Performances of devices employing 1a as an emitter in four different hosts with 20
vol% doping concentration. (A) The device structure (B) EQE plot (C) J-V-L characteristics of
devices when 1a is doped into mCBP (black, squares), mCP (red, circles), Cu host (blue
triangles), and UGH3 (green diamonds) in the EML. (D) EL plot ............................................... 40
Figure 2. 10 Device structure with different host materials and J-V-L plot (Top). EQE and
power efficiency as a function of current density and electroluminescence (Bottom). ................ 43
Figure 2. 11 Device structure with 1b as an emitter at different doping conditions and J-V-L plot
(Top). EQE as a function of current density and EL spectra (Bottom). ....................................... 44
Figure 2. 12 a. Device structure with different ETL thickness (45, 55 and 65 nm respectively)
and doping concentrations (10, 40 and 100% respectively). b. Current density as a function of
voltage c. EL spectra ..................................................................................................................... 45
Figure 2. 13 a. Device structure employing 1c as an emitter with different ETL thicknesses. b.
J-V-L plot c. EQE as a function of current density d. EL spectra ................................................ 47

Figure 3. 1 The excited-state density for phosphorescence and fluorescence, respectively (Left).
Eefficiency-brightness curve for phosphorescence (red) and fluorescence(blue). The data is
derived from reference
1
(Right). ................................................................................................... 56
Figure 3. 2 Energetic scheme of device operation in hybrid WOLEDs. The data is derived from
reference
6
. .................................................................................................................................... 57
Figure 3. 3 Hybrid WOLED energy scheme using a fluorescent dopant (left), hybrid WOLED
energy scheme using a blue-fluorescent neat emitter as a host (right) ......................................... 58
Figure 3. 4 Candidates of blue-neat emitters/hosts for hybrid WOLED and green-emissive
phosphor fac-tris(2-phenylpyridyl)iridium (Ir(ppy)3) and red-emissive phosphor (iridium (III)
bis(2-phenylquinolyl-N, C2’) acetylacetonate (PQIr) .................................................................. 59
Figure 3. 5 Normalized emission spectra of Ir(ppy)3 and PQIr doped films at 10 vol% in
phenanthro[9,10-d]imidazoles and CBP hosts (Top). Normalized emission spectra of I1 in
different matrices at RT and 77K (Bottom). ................................................................................. 60
Figure 3. 6 The device structure with I1 as a neat emitter/host with sensing technique (Top). J-
V-L plot and EQE as a function of current density (Bottom). ...................................................... 61
Figure 3. 7 EL spectra of non-sensing, HTL sensing (Top), middle sensing and ETL sensing
devices (Bottom). .......................................................................................................................... 63
Figure 3. 8 Exciton profiles at different current densities............................................................ 64
xi

Figure 3. 9 Monochromatic OLED characteristics of phenanthro[9,10-d]imidazoles as neat
fluorescent emitters. ...................................................................................................................... 65
Figure 3. 10 Progress in device structures for optimization and EL spectra. .............................. 66
Figure 3. 11 Optimized WOLEDs structure utilizing I1, energy level of the devices and J-V-L
plot (Top). EQE as a function of current density and luminance power efficiency (LPE) as a
function of luminance (Middle). EL spectra (Bottom). ................................................................ 68

Figure 4. 1 Performance comparison between conventional and δ-doping WOLEDs. .............. 75
Figure 4. 2 Photophysical properties of blue fluorophore, α-aD in 2-MeTHF and energy
diagram of non-stacked hybrid WOLEDs structure (Top). Stacked hybrid WOLED device
structures and their energy diagram (Bottom). ............................................................................. 76
Figure 4. 3 a. J-V characteristic curve for device 1-4. Inset is the molecular structures of m-
MTDATA and HATCN. b. HOMO/LUMO levels of material used with flow of charges. ......... 78
Figure 4. 4 Device performances of non-stacked and stacked devices. ...................................... 80
Figure 4. 5 a. Molecular structure of corannulene and its photophysical properties in mixture of
methylcyclohexane and isopentane (3:1 v/v, MP). Absorption and emission spectra of
corannulene was obtained from reference.
7
b. HOMO/LUMO of I1, PQIr and new host. .......... 81
Figure 4. 6 Synthetic route of xylyl-corannulene ....................................................................... 82
Figure 4. 7 Absorption spectrum of corannulene (Black) and xylyl-corannulene (Red) and
emission spectrum of corannulene and xylyl-corannulene in MeTHF solution with different
temperatures (Top). Emission spectrum of corannulene and xylyl-corannulene in solid state with
different temperatures and gated emission spectrum of corannulene and xylyl-corannulene in
solid state at 77K (Bottom) ........................................................................................................... 83
Figure 4. 8 Cyclic Voltammetry and Differential Pulse Voltammetry of xylyl-corannulene..... 84
Figure 4. 9 Stun-Volmer studies of xylyl-corannulene. Absorption (Left) and emission (Right)
spectra are provided respectively. ................................................................................................. 86
Figure 4. 10 a. Absorption spectra of films, b. PL of 1 wt% α-aD doped films, c. PL of 10 wt%
PQIr doped films ........................................................................................................................... 87
Figure 4. 11 Absorption and emission spectra of four doped films. Ir(ppy)2acac and Ir(bt)2acac
were used as emitters. ................................................................................................................... 88
Figure 4. 12 Characteristics of monochromatic devices employing α-aD as an emitter with
mCBP and x-cor hosts. ................................................................................................................. 89
Figure 4. 13 Characteristics of monochromatic devices employing PQIr as an emitter with
mCBP and x-cor hosts. ................................................................................................................. 90
Figure 4. 14 Device structure for measuring exciton profile in the EML and EL spectra of non-
sensing (Top), HTL sensing, middle sensing (Middle) and ETL sensing devices. Exciton profile
at given current densities are calculated accordingly (Bottom). ................................................... 92
Figure 4. 15 Device structure of D1, D2, D3 and J-V-L plot (Top). EQE as a function of current
density and unnormalized EL spectra (Bottom). .......................................................................... 94
Figure 4. 16 Hybrid OLEDs structure and EQE as a function of current density (Top).
Thickness of α-aD doped layer varies from 2 to 10 nm. EL spectra of devices (Bottom). ......... 95
Figure 4. 17 Hybrid OLEDs structure and energy diagram of the devices (Top). J-V-L plot and
EQE-J curve (Middle). EL spectra at 1 and 10 mA/cm
2
respectively (Bottom). .......................... 98
xii

Figure 5. 1 The five fac-Ir(C^N)3 complexes studied here. The three C^N ligands are equivalent
in these facial complexes. Illustrations of the three-dimensional structures of these complexes
are shown with the C3 axis lying within and perpendicular to the plane of page. ...................... 107
Figure 5. 2 Synthesis of Ir(mi)3, Ir(miF)3, Ir(mip)3 and Ir(mipp)3 ............................................. 108
Figure 5. 3 Absorption (Left) and Emission (Right) spectra and photophysical parameters for the
fac-Ir(C^N)3 complexes in 2-methyltetrahydrofuran (2-MeTHF) at room temperature. ........... 109
Figure 5. 4 Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) in MeCN of
a.) Ir(mi)3 in DcFc/DcFc
+
, b.) Ir(miF)3 in Fc/Fc+, c.) Ir(mip)3 in DcFc/DcFc
+
, d.) Ir(mipp)3 in
DcFc/DcFc
+
e.) Ir(ppyCF3)3 in Fc/Fc
+
. DPV of Ir(mi)3 was measured in DMF. ....................... 113
Figure 5. 5 Space-filling models of each fac-Ir(C^N)3 complex with side (a) and top (b) views
to illustrate structural differences. ............................................................................................... 113
Figure 5. 6 The TDM (red arrow) of the fac-Ir(C^N)3 complexes is in the Ir(C^N) plane,
subtending an angle δ between the TDM and the Ir-N bond. The C3 axis gives three equivalent
TDMs, with the angle α between the TDMs and the C3. ............................................................ 114
Figure 5. 7 ADPS measurements and simulations for films of TCTA doped with Ir(mi)3 (Top),
Ir(miF)3 (Middle) and Ir(mip)3 (Bottom) at 10 vol% doping ratio. The measured data have been
fitted black (Isotropic) and red (Perfectly horizontal) lines to determine the degree of orientation.
Ir(mi)3 Θ = 0.26; Ir(miF)3 Θ = 0.22; and Ir(mip)3 Θ = 0.15 in TCTA respectively. .................. 116
Figure 5. 8 ADPS measurements and simulations for films of mCBP doped with Ir(mi)3 (Top),
Ir(miF)3 (Middle) and Ir(mip)3 (Bottom) at 10 vol% doping ratio. The measured data have been
fitted (black and red lines) to determine the degree of orientation. Ir(mi)3 Θ = 0.25; Ir(miF)3 Θ =
0.22; and Ir(mip)3 Θ = 0.16 in mCBP respectively. ................................................................... 117
Figure 5. 9 Ir dopants used in Kim, et al.................................................................................... 118
Figure 5. 10 FPIM intensity profiles in the p-polarized dipole plane (pPP) and s-polarized dipole
plane (sPP) for films of TCTA:26DCzPPy 2:1 doped with Ir(ppyCF3)3 (a) and Ir(ppy)3 (b) at 10
vol% doping ratio to determine the degree of orientation. Experimental data and simulated fits
are expressed as points, solid lines respectively. Insets are the molecular structure of Ir(ppyCF3)3
and Ir(ppy)3 respectively. Θ = 0.29 for Ir(ppyCF3)3 and Θ = 0.35 for Ir(ppy)3 in
TCTA:26DCzPPy 2:1 mixed host respectively. ......................................................................... 121
Figure 5. 11 (a) Anisotropy values as a function of aspect ratio for molecules studied here and
the cyano-substituted complexes from reference
22
(Scheme 1). (b) Electrostatic surface
potential plots for Ir(C^N)3. D3 is the complex in Scheme 1 with R = CH3, the closest analog of
Ir(mi)3 and Ir(miF)3. .................................................................................................................... 122
Figure 5. 12 a.) Device architecture for all OLEDs. TCTA and 26DCzPPy are mixed with 2:1
ratio in the emissive layer with 10 vol% doping concentration, b.) Electroluminescence spectra
of Ir(mi)3 (red), Ir(mip)3 (black), Ir(miF)3 (green), Ir(ppy)3 (blue) and Ir(ppyCF3)3 (purple). c.)
Current density-voltage-luminance curve for all iridium complexes. d.) EQE versus current
density for all iridium complexes. Inset is the molecular structures of materials used in the
devices......................................................................................................................................... 124
Figure 5. 13 Simulated outcoupling efficiencies (Air mode) and the probability of light being
dissipated to other modes (Surface Plasmon Polarization mode, waveguided mode and glass
mode) for Ir(ppy)3 (a), Ir(ppyCF3)3 (b), Ir(mi)3 (c), Ir(miF)3 (d) and Ir(mip)3 (e) in
xiii

TCTA:26DCzPPy mixed host, with the device architecture used for the devices represented in
Figure 5.12. ................................................................................................................................. 126
Figure 5. 14 Orientation of permanent dipole moments of dopants relative to the molecular
frame. Ir(ppy)3 (a), Ir(ppyCF3)3 (b), Ir(mi)3 (c), Ir(miF)3 (d) and Ir(mip)3 (e). The length of the
dipole does not represent its magnitude. ..................................................................................... 127

Figure A.1. 1 Molecular structures of Ir(iqbt)3 and Ir(ppz)2iqbt ................................................ 145
Figure A.1. 2 (a) Device structure with Ir(ppz)2iqbt and Ir(iqbt)3 as dopants (b)
photoluminescence of CBP:Ir(ppz)2dqbt (Black) or Ir(iqbt)3 (Red) 10 vol% film, and
electroluminescence of the devices (filled symbols) (c) External Quantum Efficiency as a
function of current density (d) J-V-L plot................................................................................... 146

Figure A.2. 1 Molecular structures of 1-6 and their emission profile in PS at RT and 77K .... 148
Figure A.2. 2 Device structures and characteristics using compound 4 as an emitter. ............. 149
Figure A.2. 3 Molecular structures of host materials used for compound 2. ............................ 150
Figure A.2. 4 Device structures and performances using compound 2 and UGH3. Four different
HTLs were employed to check hole-dominating behavior in the device. .................................. 151
Figure A.2. 5 Device structures and their characteristics with different doping conditions of
compound 2 in UGH3 host. ........................................................................................................ 152
Figure A.2. 6 Device structure and performances using compound 2 in t-CP host. Electron
Blocking Layer (EBL) was absent or present. ............................................................................ 153
Figure A.2. 7 Device structure and performances using compound 2 in CzSi host at different
doping conditions. ....................................................................................................................... 154
Figure A.2. 8 Device structure and performances employing 2 in DPEPO at various doping
concentrations. ............................................................................................................................ 155
Figure A.2. 9 Device structures to probe the exciton profile in DPEPO-devices and EL spectra
of HTL, middle and ETL- sensing devices. ................................................................................ 156
Figure A.2. 10 Device structures to probe the exciton profile in neat emissive layer of
compound 2 and EL spectra of HTL, middle and ETL- sensing devices. .................................. 157
Figure A.2. 11 BZIAuCz-based device structures and their characteristics at different doping
conditions. ................................................................................................................................... 157

xiv

Abstract
Over the past two decades, Organic Light-Emitting Diodes (OLEDs) have attracted
intensive attention from both industry and academia due to their potential applications in panel
displays and solid-state lighting. However, several major deficiencies should be improved for the
growth of the OLED market. First, the fabrication cost of OLEDs needs to be lowered. According
to the department of energy, materials cost takes more than 10 % of total expense in OLEDs.
Adoption of more abundant materials in OLEDs can decrease the material cost because most
commercially available emitters bear iridium, which is one of the rarest elements on the planet
(0.0003 ppm). In other words, application of earth-rich elements such as copper (50 ppm) would
lead to a dramatic financial benefit in OLEDs. The details of this subject will be discussed in
chapter 2. Another scope for improvement in OLEDs is device efficiency. Intrinsically, collecting
all excitons is a prerequisite to achieve high efficiency. In addition, extracting as many photons
as possible in the device is essential, considering a significant portion of light is wasted before it
reaches outside of the device. Indeed, approximately 50 % and 30 % of light are dissipated by a
metal surface and organic layers respectively. The strategies to enhance the device efficiency will
be discussed from chapter 3 to 5.
Chapter 2 introduces highly efficient and cost-friendly OLEDs using cyclic alkyl amino
carbene (CAAC)-Cu(Ι)-Carbazole (Cz), cyclic monoamido-aminocarbene (MAC*)-Cu-Cz and
MAC*(Au)Cz. OLEDs prepared with CAAC(Cu)Cz doped into an ultra-gap host material yield
maximum external quantum efficiency (EQEmax) of 9.0%, which is one the highest efficiencies
considering that EQEmax of previously reported for blue-emitting Cu-OLEDs is less than 6%.
(MAC*)CuCz devices showed small turn-on voltages (VT) of 2.7 V and EQEmax=19.4 %. This
performance is among the best reported for OLEDs based on Cu(I) dopants. On top of that, the
xv

roll-off in efficiency at a high current is considerably gradual than other OLEDs reported using
four- or three-coordinate Cu emitters, possibly due to short radiative lifetime of the complex.
Similar to (MAC*)Cu(Cz), OLEDs with (MAC*)Au(Cz) displayed an EQEmax=18.1 %, which is
one of the highest records reported for OLEDs with an Au(I) dopant. Efficiency drop at high
current density is also low, further suggesting that cost-effective and stable vacuum-deposited
OLEDs are realizable.
Chapter 3 demonstrates the highly efficient hybrid white OLEDs (WOLEDs) employing
phenanthro[9,10-d]imidazoles derivatives as a neat emitter/host. High photoluminescence
quantum yield (0.80), singlet (3.28 eV) and triplet energy (2.60 eV) allowed one of the
phenanthro[9,10-d]imidazoles derivatives to be utilized as a neat fluorescent emitter/host. Indeed,
monochromatic device of neat emitter showed EQEmax = 5 %, confirming the suitability of the
emitter. An ultra-thin layer of the sensitizer was introduced to probe the exciton profile in the
emissive layer and doped layers of green and red phosphors were placed accordingly in order to
obtain superior color rendering index (CRI). The optimized hybrid WOLED showed EQEmax
=14.2% and maximum luminance power efficiency (LPEmax) of 50 lm/W with only a small change
in CRI (CRI=68 at 1mA/cm
2
to CRI=76 at 100 mA/cm
2
).
Chapter 4 establishes the application of corannulene derivatives in stacked hybrid
WOLEDs. To alleviate efficiency-roll off at high current density in hybrid WOLEDs, a new host
material, xylyl corannulene, was synthesized and investigated. Stern-volmer and film quenching
studies verified the availability of xylyl corannulene as a host for blue fluorophore and red
phosphor. Certainly, a monochromatic device of blue fluorophore and red phosphor with xylyl
corannulene as a host exhibited EQEmax = 2 and 13 %, respectively. Optimized hybrid OLED
where blue and red emitters are mixed in a single layer showed an EQEmax = 17.5 %, affirming the
xvi

full usage of excitons. Relative intensity of triplet to singlet emission at various current densities
was constant with the values around 1.5, which was close to the ratio between triplet and singlet
(T1/S1=3.0), implying singlets and triplets are fairly harvested in separate channels. This highly
efficient hybrid device (B, R) with xylyl corannulene is the first OLED reported that employs
corannulene in OLED applications.
Chapter 5 articulates two factors that affect the alignment of molecules during the vacuum
deposition in OLEDs. The degree of orientation of three homoleptic tris-cyclometalated Iridium
complexes with either different physical shapes or electronic configuration was investigated using
angle dependent photoluminescence spectroscopy (ADPS) and Fourier-plane imaging microscopy
(FPIM). Other properties such as photo-physics and position of transition dipole moments (TDMs)
remained constant, allowing a direct correlation between the experimental result and one variable.
Molecules with oblate spheroidal shapes displayed the best horizontal alignment with outcoupling
efficiency of 0.328 and an EQEmax = 30.5 %. Additionally, chemical asymmetry caused by
attaching trifluoromethyl groups promoted in-plane alignment even though molecules became
more spherical in shape. Experimental EQEs of OLEDs correspond nicely with that of the
simulated one, further confirming that geometry and chemical asymmetry contribute to net
alignment of tris-cyclometalated Ir complexes in vacuum deposited films.

1

Chapter 1. Introduction
1.1. Principle of OLEDs
1.1.1. OLEDs Structure and Basic Steps
Ever since Thomas A. Edison invented the incandescent bulb, the breakthrough that
brought convenience and comfort into peoples lives, mandkind’s demand for energy has increased
exponentially: This demand is fulfilled by limited natural source, which is not only utilized in
staggering amounts, but also not utilized as efficiently as possible. For example, a Planckian
radiator (light bulb) possesses a poor efficiency with only 5% of the power converted to light.
Therefore, efforts need to be made to find alternative light sources with high power efficiency
(lm/W) in this modern society.
1
A promising alternative such as organic light emitting diodes
(OLEDs) has been intensively investigated for several decades due to their higher efficiency
compared to an incandescent bulb, superior color qualities and diverse application potentials such
as flexible displays, television and phone etc.
2-5

Figure 1. 1 Basic structure of OLEDs (Left) and mechanism of generating photons in OLEDs (Right)

2

The basic structure and principle of OLEDs is illustrated in Figure 1.1. OLEDs are
composed of multiple organic layers with the total thicknesses less than 200 nm, sandwiched
between the cathode and the anode. Under the electric field, molecules next to the anode generate
“holes” because they lack of electrons whereas molecules near cathode create “electrons”.
Therefore, frontier orbital highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) should be considered to understand the description of the holes and the
electrons. UV-photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES)
determine the HOMO/LUMO energies of materials, respectively.
6
On top of that, they can be
estimated with indirect methods such as electrochemistry.
6
Upon generation of charges at the
electrodes, they experience four major steps to fulfill the emission: (ⅰ) injection, (ⅱ) migration (ⅲ)
recombination (ⅳ) emission. First, holes and electrons are injected from the anode and the cathode
to hole transporting (HTL) and electron transporting layer (ETL), respectively. Typically,
materials for the anode are metal oxides such as indium-tin-oxide (ITO), because the work function
of the ITO matches well with the HOMO energy of the HTL, promoting an easy hole injection. In
the same context, aluminum/lithium fluoride is widely chosen for the cathode material due to its
low work function, matching well with the LUMO energy of the ETL. After the injection, holes
move away from the anode in the HTL while electrons migrate away from the cathode in the ETL
under electric field. Efficiency of the carrier migration in the HTL and the ETL is quantified by
the charge mobility (cm
2
V
-1
s
-1
). The migration of the carriers in a sold thin film (HTL, ETL) can
be explained by a hopping mechanism and Marcus theory states that the migration of the charges
accompanies with the change in geometric structures of the neutral and charged species.
7

Therefore, the materials with a high reorganization energy between a polaron and a neutral
molecule would lead to a slow charge mobility. The most common HTL materials are triarylamine
3

derivatives such as N,N-di(1-naphthyl)-N,N-diphenyl-(1,1-biphenyl)-4,4-diamine (NPD) (Figure
1.2). Triarylamine group in neutral and cationic form is known as almost isostructural, lowering

Figure 1. 2 Molecular structures of commonly used HTL materials in OLEDs.
the reorganization energies. Indeed, the high hole mobility of NPD (10
-4
cm
2
V
-1
s
-1
) has been
reported.
8
The most widely used ETL materials are heterocyclic aromatic compounds as shown in

Figure 1. 3 Molecular structures of commonly used ETL materials in OLEDs
Figure 1.3. They are either metal coordination compounds or pure organic materials. Among them,
Alq3 is intensively investigated and used in OLEDs due to its high electron mobility (~10
-5
cm
2
V
-
1
s
-1
). Charge migration also occurs in the emissive layer (EML), where light is generated, after
holes and electrons are transferred from the transporting layer to the EML. In general, the EML is
comprised of hosts and dopants where dopants are sparsely doped into host. This is because a neat
4

material normally undergoes self-quenching, limiting the device efficiency. Depending on the
relative energies of HOMO/LUMOs of host and dopant, either host or dopant or both carry charges
in the EML. However, emitters carry both charges in the EML in most OLEDs. When holes and
electrons encounter each other in the EML after the migration, they form a Frenkel exciton, which
is called “recombination”. Upon recombination, holes and electrons can possess either the same
or opposite spin, leading to the formation of singlets and triplets. According to spin statistics in
quantum mechanics, singlets and triplets are formed with a 1:3 ratio as shown in Figure 1.4.
9
This
branching of the excitons impacts on the device efficiencies immensely because fluorescent
dopants use only singlet exciton, implying that only one quarter of the light source can be used
whereas phosphorescent dopants utilize not only singlet but also triplet.

Figure 1. 4 Possible routes for recombination of holes/electrons (Left). Singlet and triplet states (Right).
1.1.2. Host materials
EL characteristics in OLEDs are determined by the choice of materials used in the EML.
As mentioned previously, dopants are generally doped into the host the material in the EML rather
than deposited as a neat layer due to self-quenching in the solid state. Host materials in the EML
must have the excited-state energy higher than that of the emitters, such that the emitter will trap
5

all the excitons efficiently in the EML. If the dopant is co-deposited with appropriate host
materials, EL will produce exclusively a dopant emission even at a low dye concentration.
10
On
the contrary, if the host material has a triplet energy close to that of the dopant, the exciton trapping
by the dopant is incomplete, leading to a poor device efficiency. For example, when red-emissive
platinum octaethylporphyrin (PtOEP) was doped into Tris(8-hydroxyquinoline)aluminum (Alq3)
in the EML, the device showed only a maximum EQE of 3% (EQEmax = 3 %).
11
This is because
the triplet energy of Alq3 (ET1=1.90 eV)
12
is comparable to that of PtOEP (ET1=1.91 eV), leading
to an ineffective exciton harvesting on PtOEP.
3
Similarly, PtOEP-OLEDs with conjugated
polymers as hosts show a low efficiency (EQEmax = 3%) due to low triplet energies of polymers.
13,
14
However, when 4,4’-di(N-carbazolyl)biphenyl (CBP) is used as a host, the efficiency increased
to 6% due to a higher triplet energy of CBP (E T1=2.60 eV).
11, 15
By optimizing the host material
and the device architecture, OLEDs incorporating PtOEP showed the external efficiency as high
as 9 %
16
, 22 %.
17

Figure 1. 5 Molecular structures of common host materials
Figure 1.5 provides the several common host materials used in OLEDs. These materials
can be synthesized in bulk quantities at a relatively low cost and their properties have been well
established since early days. Carbazole derivatives have been widely employed as hosts due to
their high triplet energies and good hole-transporting abilities. CBP is a frequently adapted host
6

material for green-emissive dopants such as fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)3) due
to its high energy (ET1=2.60 eV). For example, Liehm et al., reported a highly efficient device with
an EQE=18.3 % at 400cd/m
2
by employing CBP as a host for Ir(ppy)3 phosphor.
18
High triplet
energy and ambipolar charge characteristics allow CBP to be widely used as a host.
19
This is
because similar charge mobility of holes and electrons in the EML is preferred for the fabrication
of efficient and stable devices. Considering most of materials have unbalanced charge
transportation abilities, either hole or electron dominating, CBP is one of the rare-ambipolar
materials. Indeed, the balanced charge distributions in the CBP-based devices are reported in
elsewhere.
5
Another carbazole-based host material is 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl
(mCBP). The carbazole and the phenyl are linked in a meta position, breaking the conjugation with
the higher triplet energy (ET1=2.87 eV) than CBP. Therefore, mCBP is ordinarily employed as a
host for blue-emissive phosphors such as bis[(4’,6’-difluorophenyl)-pyridinato-N,C
2
]iridium(III)
picolinate (Firpic). For instance, blue-device using Firpic and mCBP accomplished a maximum
EQE as high as 17%.
20
For dopants with the triplet energy higher than 2.80 eV, phosphine-oxide
based host such as bis-(2-(diphenylphosphino)phenyl)ether oxide (DPEPO) is generally utilized
thanks to its high triplet energy (ET1=3.0 eV) and great electron transportation ability.
21
However,
it suffers from major drawbacks such as poor stability because of phosphine oxide moiety,
reducing the device lifetime. In these perspectives, the choice of suitable hosts with high energy
and stability is imperative to fabricate a highly efficient and stable device, especially with deep-
blue emitters. Chapter 2 encompasses the significance of choice of host materials in OLEDs.
1.1.3. Fluorescent Compounds
Understanding how luminescent materials generate light is crucial in tuning the
electroluminescence properties. Photo-physical properties occur through the transitions between
7

ground and excited states of the material. The relation between the excited and the ground states
of molecules is well established in Jablonski diagram shown in Figure 1.6.
22
Upon absorption of
photon, the ground state (S0) of the molecules promotes to a high-lying singlet excited state (Sn).
Sn can undergo a rapid internal conversion (IC) through a vibrational relaxation, resulting in the
lowest excited state (S1). Kasha states that the IC rate ranges from 10
11
-10
14
s
-1
.
23
The electron in
S1 state can decay in three main pathways. It can radiatively decay to S0 (fluorescence), non-

Figure 1. 6 Jablonski diagram illustrating photophysical processes
radiatively relax through IC (thermal deactivation) or inter system crossing (ISC) to the triplet
state, Tn. Typically, the fluorescence transitions occur with a lifetime of several nanoseconds. It is
notable that the photoluminescent quantum yield of fluorophore can be measured as
𝑘 𝑟 𝑘 𝑟 +𝑘 𝑛𝑟
where
𝑘 𝑟 is the radiative rate of fluorescence and 𝑘 𝑛𝑟
is the non-radiative rate. The ISC from Sn to Tn
involves spin flip, which requires the change in angular momentum. Therefore, this transition is
usually forbidden and slow for a pure organic material.
2
Examples of fluorescent materials are
N,N′-di-1-naphthalenyl-N,N′-diphenyl [1,1′:4′,1″:4″,1‴-quaterphenyl]-4,4‴-diamine (4P-NPD)
8

and 4,4 0 -bis(9-ethyl-3-carbazovinylene)-1,1 0-biphenyl (BCzVBi). OLEDs incorporating 4P-
NPD and BCzVBi showed a maximum EQE of 3.4 and 2.4 % respectively due to their high
fluorescent quantum yields.
24, 25
However, the efficiencies are still limited by the 25% of the
excitons usage in fluorophore.
1.1.4. Spin Orbit Coupling
Spin-orbit coupling (SOC) is the key phenomenon to access the triplet energy state. It refers
to a relativistic phenomenon results from the combination of orbital angular momentum and
electronic spin angular momentum. A typical picture of an atom can be described as electrons
revolving around the nucleus as shown in Figure 1.7a. Alternatively, it can be seen as the electrons
are at the center while positively charged nucleus is orbiting around the electrons (Figure 1.7b).
As the positively charged species orbit around the electrons, magnetic field B is formed. In addition,
when the electrons spin themselves, they create a dipole M, because the movement of charged

Figure 1. 7 Graphical representation of nuclear-centric system (a) vs a nuclear-centered (b) an electron-
centered.
species results in magnetic dipole. Therefore, these two magnetic dipole moments can interact
each other, which is termed “spin-orbing coupling”. The relative orientation between spin axis and
orbital angular momentum axis determines the energy level of the system.
15
Given that interaction
9

between magnetic fields follows the columbic nature, the magnitude of the spin-orbit coupling is
dependent on the magnitude of nuclear charge, Z. In other words, heavier atoms produce stronger
spin-orbit coupling, which is known as “the heavy atom effect”.
26
As mentioned previously, the
transitions between spin states (i.g., S1→T1) accompany with the change in angular momentum,
which are the forbidden transitions.
2
However, strong SOC caused by the heavy atom effect
increases the possibility of this forbidden transition, accessing to the triplet state. Table 1.1
summarizes the SOC parameters (ζ1) of common atoms. As atomic number increases, the values
of ζ1 increase, suggesting that the population of triplets will be efficient for heavy metals such as
Ir, Os and Pt etc. On the contrary, pure organic materials exhibit weak SOC and in turn, slow ISC,
leading to slow phosphorescence.
Table 1. 1 SOC parameter (ζ 1) of common atoms in cm
-1
.
27

1.1.5. Phosphorescent Complexes
Electrons in Tn states behave in similar manner as Sn (Figure 1.6). Tn can relax to the lowest
excited state (T1) via vibration coupling and then decay radiatively to the ground state (S0), which
is called phosphorescence. For pure organic material, Sn to Tn transitions are slow (kr,ISC =10
7
s
-1
)
with a lifetime of phosphorescence of second to minutes due to a weak SOC. However,
organometallic complexes with heavy metals such as Os, Pt, and Ir experience quick ISC from Sn
10

to Tn (kr,ISC =10
13
~10
14
s
-1
), followed by fast radiative decay to the ground state (S0) with the
lifetimes of several microseconds (< 100 μs). As mentioned previously, the utilization of the triplet
states is critical to achieve a high efficiency in OLEDs because it allows 100% usage of the
excitons. Indeed, numerous OLEDs with high efficiencies ( >15 %) employ the phosphorescent
materials.
28
In chapter 3, we demonstrated the hybrid OLEDs structure where singlets and triplets
are collected independently, achieving the complete exciton management with high efficiency.
1.1.6. Copper complexes
Again, numerous light-emitting dyes in OLEDs field use heavy atoms such as Ir, Pt and Ru
due to their strong SOC ability enhanced by heavy atoms, allowing the transition of a singlet (S1)
to a triplet (T1) through ISC:S1→ T1.
26
Since singlets and triplets are formed with a 1:3 ratio in the
excitons,
9
harvesting the triplets allows for 100% usage of the excitons, and consequently high
efficiency in OLEDs. With that in consideration, it has been a remarkable challenge to realize
luminescence with relatively light metals such as Cu due to the lack of strong SOC. However, it is
worth the effort to employ more copper in OLEDs as the financial benefit and economic effect
would be quite substantial. Copper is much more abundant (50 ppm) compared to Iridium (0.0003
ppm) in the Earth’s crust, leading to the dramatic cost differences between these two metals.
29

According to Solid-State Lighting R&D Plan, published in 2016 by the Department of Energy
(DOE), approximately 12% of total costs comprises of purchasing organic materials. This
percentage can be dramatically reduced if the technology of copper-based OLEDs reaches the
industry level. Nevertheless, two main challenges exist in terms of fabricating efficient Cu-based
OLEDs. Firstly, synthesizing highly emissive and stable Cu(Ι) complexes is demanding. A strong
SOC is crucial to harvest triplets, which allows for the high quantum efficiency. Cu, Ir and Pt
possess SOC parameter ξ of 857 cm
-1
, 3909 cm
-1
and 4481 cm
-1
, respectively.
30
Consequently, the
11

rates of ISC from the singlets to the triplets (S1-T1) are 10
10
~10
11
s
-1
and 10
13
~10
14
s
-1
for Cu and
Ir complexes, respectively. On top of that, the triplet to ground state (T1-S0) ISC rates are 10
3
~10
4

s
-1
and >10
5
for Cu and Ir compounds, respectively. These parameters obviously suggest that the
triplets are hardly harvested if the copper contributes exceedingly to the charge transfer (CT) state
of the molecule. Secondly, the lowest energy metal to ligand charge transfer (MLCT) transition in
Cu(Ι) complexes is accompanied with geometrical distortions, which lead to increases in non-
radiative decay rates. This geometrical distortion stems from the d
10
to d
9
transition of Cu (Ι)
complexes and is associated with a large reorganization energy. The most common geometrical
distortions in Cu(Ι) complexes are shown Figure 1.8. Four, three and two-coordinates Cu (Ι)
complexes typically suffer from tetrahedral to square planar (Figure 1.8 a), Y-shaped to T-shaped
(Figure 1.8 b) and linear to bent (Figure 1.8 c) geometrical distortions, which in turn increase non-

Figure 1. 8 Geometrical distortion pattern of a-c. 4-coordinates, 3-coordinate and 2-coordinate Cu(Ι)
complexes respectively.
radiative decay rates. Recently, excellent strategies have been introduced to overcome the
shortcomings of Cu (Ι) complexes.
30-32
To avoid the structural reorganization energy associated
with d
10
to d
9
transition of Cu complexes, donor-Cu-acceptor type of molecules have been
suggested. In this molecular structure, the contribution of copper to the charge transfer (CT) state
is minimized to prevent a weak SOC and a slow radiative rate. Rather, donor and acceptor ligands
are simply connected through Cu as a bridge to maintain some degree of the metal involvements
to increase the radiative rate. That is, HOMO and LUMO must be spatially isolated without the
orthogonality, but connected by the least involvement of copper. On top of that, bulky groups are
12

attached to the metal to reduce the reorganization energy associated with the structure distortion
as well. Consequently, highly emissive copper complexes can be synthesized according to the
molecular design of copper complexes mentioned above. However, the fabrication of highly
efficient Cu-based devices with emissive copper complexes is still unsured: Cu(Ι) complexes are
usually thermally unstable due to monodentate ligands and open coordinating sites, which are
challenges in the device fabrication. As a result, several solution-processed devices have been
reported instead of vapor-deposited Cu-based OLEDs.
33-37
Including this work, only a few efficient
vacuum-deposited Cu-based OLEDs have been reported.
30, 32, 38-40
Chapter 2 focuses on the
fabrications of exceedingly efficient Cu-based OLEDs using cyclic alkyl amino carbene (CAAC)
and cyclic monoamido-aminocarbene (MAC*) - Cu (Ι) - Carbazole (Cz) complexes. We
demonstrated that OLEDs with not only exceeding external quantum efficiency (19.4%) but also
small efficiency-roll off can be made utilizing Cu complexes.
1.2. Performances of OLEDs
1.2.1. External Quantum Efficiency
External quantum efficiency (EQE), which measures the ratio of the number of the
outcoupled photons to the number of the injected charges is one of the significant parameters in
characterizing the performance of OLEDs. Several factors influence on EQE such as internal
quantum efficiency (𝛷 𝑃𝐿
), charge balance (𝑛 𝑟 ), the ratio between excitons and usable excitons (𝜒 ),
and outcoupling efficiency (𝑛 𝑜𝑢𝑡 ) as shown in equation 1.1
41
. The values of 𝜒 are 0.25 and 1 for
fluorophores and phosphors, respectively. Internal quantum efficiency (𝛷 𝑃𝐿
), which is the ratio of
𝑛 𝐸𝑄𝐸 = 𝛷 𝑃𝐿
𝑛 𝑟 𝜒 𝑛 𝑜𝑢𝑡 (Equation 1.1)
13

the number of the photons generated to the number of the photon absorbed by materials, can be
estimated by measuring the photoluminescence quantum yield (PLQY) of the EML. If the energy
transfer from the host to the dopant is complete and the dopant is fully emissive, 𝛷 𝑃𝐿
=1. The
definition of the charge balance (𝑛 𝑟 ) is the percentage of the injected charges to be recombined
before they reach the opposite electrode and discharge non-radiatively. In other words, the charge
balance is referred to the recombine efficiency. The charge balance values range from 0 for a non-
emissive device to 1 for a perfectly optimized device. An optimized charge balance is crucial to
achieve a high efficiency, stability and an excellent color coordinates in the lighting application.
5,
42
One of the ways to promote the charge balance is to improve the charge injection from the
electrodes and this can be approached in different manners: the choice of the cathode,
43
the
transport layer
44
and so on. Another commonly used, but powerful method to reach 100% charge
balance is controlling the recombination zone in the EML. The charge injection enhancement
mentioned previously does not completely control the location of recombination zone. Typically,
OLEDs show either hole or electron-dominating charges behavior due to the lack of bipolar
materials, resulting in high exciton densities at the narrow interface of either ETL/EML or
HTL/EML.
45, 46
This confinement of the recombination zone diminishes the charge balance,
leading to a poor device efficiency and stability. To distribute the recombination zone in the EML,
in turn improve the charge balance, several device architectures such as conventional
heterostructure, graded mixed layer and uniformly mixed layer have been reported (Figure 1.9).
47,
48
For example, Zhange el al., reported highly efficient and stable devices via a gradient doping
system in the EML.
49
They controlled the doping ratios in the EML in a such a way that the region
near the HTL is heavily doped whereas the ETL side is slightly doped. The gradient doping in the
EML slowed down the holes, accomplishing a superior charge balance in the device, in turn, a
14

significant efficiency (18.0 ± 0.2 % at J=2.9 mA/cm
2
). Consequently, broadening the charges and
excitons distribution in the emissive layer by using bipolar materials, mixed or gradient doping
technique etc is essential to achieve a high efficiency in OLEDs.

Figure 1. 9 Illustration of the structural behavior of EML in different devices, (a) with a hetero structured
stack, (b) a graded mixed layer, and (c) a uniformly mixed layer. The data is derived from reference
47
.
1.2.2. Outcoupling Efficiency
Outcoupling efficiency, which measures the number of photons extracted from the device
out of the number of photons generated is one of the critical factors in fabricating highly efficient
OLEDs. Large portion of light is trapped in the device due to waveguided and surface plasmon
modes and so on
41, 50, 51
. Figure 1.10 shows the different modes where light is wasted internally
within an OLEDs. Approximately, the light penetrating through the device within the opening
angle of 30° with respect to the substrate normal has chance to escape from the device. However,
not all light infiltrating through organic layers succeeds in being outside of the device. A partial
loss of light occurs in the substrate by the total internal reflection, which is called substrate mode.
41, 52
The light with a higher angle than 30° cannot even arrive the glass substrate because of the
total internal reflection on the organic-substrate interface and eventually is depleted in the organic
layers, called wave-guided modes.
53
Residual emission trapped in the wave-guided modes can also
be dissipated by the absorption of materials and edge emission. Approximately, only 20% of light
produced in the device can be extracted without any enhancement in outcoupling efficiency. For
15

example, the chart in Fig. 1.10 shows the contribution of various optical channels for the reference-
Alq3 OLED presented in the literature.
53
Given the small percentage of outcoupled light (15.3 %),
it is no wonder that numerous approaches to improve the light extraction from the device have
been extensively made over couple of decades.
54-56
Several engineering-strategies have been
attempted to retrieve more light from the device (Figure 1.11). For example, microlens arrays have
been inserted on the glass substrate, widening the light escape cone for total internal reflection.
54,
55
Resultantly, the portion of light residing in the waveguided and substrate mode has been

Figure 1. 10 Schematic illustration of an OLED showing different optical loss channels (Top). Examples
of amount of power coupled to different optical channels in the device of reference
53
(Bottom).
diminished considerably, leading to the increase in outcoupling and EQE by a factor of 1.5.
Another group reported that an ordered array of silica spheres can take out waveguided light,
leading to an enhancement in outcoupling efficiency.
56
In addition, surface roughening conducted
by three different methods such as abrasion by sandblasting, grinding paste and wet etching
16

emancipated the imprisoned light inside the organic layer and substrate.
57
These all extrinsic
engineering methods primarily help to alleviate the light dissipation via waveguided and substrate
mode. However, preventing the surface plasmon resonance with the metal is critical to boost the
outcoupling efficiency because more than half of light dissipation occurs when light couples with
the surface plasmon. In this context, controlling the direction of light in molecular perspective is
essential since the surface plasmon resonance can be effectively avoided if light emits parallel to
the surface normal.

Figure 1. 11 Extrinsic methods for improving outcoupling efficiency. (a) Microlens array, (b) scattering
particles, (c) index grating. Pictures are derived reference
41
.
The intrinsic enhancement in outcoupling with certain molecules was first observed in
green-OLEDs with bis(2-phenylpyridine)(acetylacetonate)iridium(III) (Ir(ppy)2acac), marking a
new record of 29 % EQE.
58
Helander et al., argued that exceeding efficiency of the device is
attributed to the fine tuning of the energy levels and the work function by chlorinating the ITO
anode. However, this high value (> 20 %) is not accomplishable with optically isotropic emitters.
For example, OLEDs with a typical isotropic dopant, Ir(ppy)3 only shows the best efficiency of
~20%. That being said, Ir(ppy)2acac is a non-isotropic emitter, implying the horizontal alignment
of transition dipole moments (TDMs) with respect to substrate. It is noteworthy that light emits
perpendicular to the direction of TDM, meaning that the outcoupling efficiency is proportional to
the degree of horizontal alignment of TDMs. The structural difference between those two
17

compounds was that one phenylpyridine was replaced with an ancillary acetylacetonate ligand
while maintaining overall optical properties, leaving the clue that the molecular structure of
heteroleptic compounds may contribute to the alignment. Indeed, several diketonate complexes
have been reported with the similar degree of horizontal orientation and high device efficiencies
59,
60
whereas only a few homoleptic Iridium complexes such as tris[2-(1-cyclo-
hexenyl)pyridine]iridium(III) (Ir(chpy)3), tris(1-phenylisoquinoline)iridium(III) (Ir(piq)3)
59

showed an optical anisotropy. However, main mechanism of the molecular alignment is yet fully
elucidated. First of all, the correlation between permanent dipole-moments and alignments of
molecules
59
has been suggested. They argued that optical-anisotropic molecules possess a small
permanent dipole moment while the isotropic molecules such as Ir(ppy)3 has a relatively high
permanent dipole moment. However, Ir(piq)3 shows non-isotropic behavior with a large value of
permanent dipole moment, which is the counterexample of the theory. Also, Jurrow et al.,
discovered TDM of heteroleptic diketonate complex, (bppo)2Ir(acac) prefers to align horizontally
with respect to the substrate with a large permanent dipole moment.
61
Secondly, Kim et al., argued
that electrostatic interaction between the dopant and the host dictates the alignment of molecules.
Electron-deficient moiety of the dopants and electron-sufficient region of the aromatic ring in the
host interact favorably, creating “binding sites”, which in turn, lead to the alignment of molecules.
However, Jurrow et al., found that only (bppo)2Ir(acac) shows horizontal alignment behavior
despite the similarity in electrostatic surfaces of both (bppo)2Ir(acac) and (bppo)2Ir(ppy).
61

Besides, other hypotheses stating that organic/vacuum interface
61
or host-dopant interaction
creates alignment of the molecule
62, 63
have been suggested, but further investigation has to be
conducted. Chapter 5 focuses on manipulation of the direction of light in OLEDs by changing the
molecular structure the electronic configurations. We demonstrated that outcoupling efficiency
18

can reach to 33% in the absence of engineering technique, performing a significant maximum EQE
of 30.5 %. It is a surprising achievement, given that only a few blue-emitting OLEDs exceeding
30% of EQE have been reported.
64-66

1.3. Stability of OLEDs
1.3.1. Efficiency roll-off
Efficiency roll-off describes the phenomenon of decreases in the efficiency in OLEDs at
high current densities. It prevents OLEDs from being applied to various applications such as
organic lasers, bio-medical light sources and general lighting, which typically require aa high
power efficacy at a high luminance. For example, organic laser requires the brightness to be above
5000 cd m
-2
,
67
which already passes the level to trigger the efficiency roll-off in OLEDs. The

Figure 1. 12 Summarized EQE curve as a function of the current density at which EQE drops to 90% of
its maximum value reported. Filled square is phosphorescent OLEDs, open square is fluorescent OLEDs,
figure with dot in it is hybrid OLEDs. Reverse triangle is usage of outcoupling enhancement, circle is
TADF emitters and triangle is TTA.
28

19

current status of the efficiency roll-off for the phosphorescent, fluorescent and the hybrid OLEDs
is summarized in Figure 1.12.
28
The data shows that the majority of highly efficient OLEDs are
derived from phosphorescent dopants but J90 (The current density at which EQE drops to 90% of
its maximum value) falls into the range of 1-30 mA/cm
2
. More importantly, this current density
corresponds to the luminance between 1000 to 10000 cd m
-2
, which is within the required
luminance values for OLEDs lighting application. Improving the efficiency roll-off not only
increases the efficacy, but also increases the stability of the device because the efficiency roll-off
and the lifetime of the device is correlated. Consequently, mitigating efficiency roll-off is one of
the hot topics for the future direction in OLEDs.
1.3.2. Triplet-Triplet Annihilation (TTA) and Triplet-Polaron Annihilation
(TPA)
The efficiency roll-off can be attributed to the interactions between many processes such
as bimolecular annihilations, field-induced annihilations and so on. Herein, bimolecular exciton
quenching processes will be primarily discussed as the source of the efficiency roll-off. Among
the numerous annihilation processes in OLEDs, TTA is the most well studied. TTA is the energy
transfer mechanism that occurs between two excited triplet states: one excited molecule (T 1)
transfers its energy to the other excited one (T1), resulting in one ground state (S0) and one excited
state (T1) as shown in scheme 1.1. In other words, one triplet exciton, which is the source of light,
is wasted via TTA, causing a drop in the efficiency. In addition, it is noteworthy that annihilation
occurs when two triplet excitons encounter each other. That is, the probability of TTA happening
𝑇 1
+ 𝑇 1
→ 𝑇 𝑛 + 𝑆 0
→ 𝑇 1
+ 𝑆 0
(Scheme 1.1)
is much higher than that of a Singlet-Singlet Annihilation (SSA), because the lifetime of triplets
and singlets are ~µs and ~ns, respectively. Principally, two triplets can meet each other through
20

two energy transfers: Föster and Dexter energy transfer. Föster energy transfer happens by the
dipole interactions and is proportional to R
-6
where R is the Föster radius.
68
Föster energy transfer
is also sensitive to the overlap between the emission of donor and the absorption of acceptor.
According to the literatures,
69, 70
the TTA rate constant is not influenced by the distance between
two excited species, which is contrary to the result via Dexter energy transfer. Dexter energy
transfer occurs when two triplets are physically close each other (~2nm) since the principle
mechanism is based on the exchange interaction.
71
Therefore, Dexter energy transfer can be
expressed as a diffusive random walk movement, which is proportional to the diffusion constant
and the distance between two excited molecules. This discrepancy in anticipation is explained by
the aggregation of dyes,
15
but the important common fact is that the concentration of the triplets
induces TTA in both models. TTA can be divided into three big categories depending on which
molecules participate in the annihilation: dopant-dopant, dopant-host and host-host annihilation.
For all of cases, density of the excitons can be mathematically expressed as shown in equation
1.2. 𝑛 𝑇 is the exciton density, 𝑘 is the TTA rate constant, τT is the lifetime of the triplets.
15
The
second term describes the loss of excitons from the annihilation, which is non-linear 2
nd
order
𝑑 𝑛 𝑇 𝑑𝑡
= −
𝑛 𝑇 𝜏 𝑇 −
1
2
𝑘 𝑛 𝑇 2
(Equation 1.2)
process. Annihilation between the triplets of dyes is the most commonly observed annihilation in
the conventional host-dyes OLED systems since the triplets are efficiently confined at the dopant
by the energetics of the system. This is based on the assumption that the triplet density of the host
is negligible because the triplet formed on the host will be favorably transferred to the dopant.
However, if the doping system where the triplet energies of the host and dopant are comparable,
TTA between the host and dopant should be counted as shown in scheme 1.2.
15
For example, S.
21

𝑇 1,ℎ𝑜𝑠𝑡 + 𝑇 1,𝑑𝑜𝑝𝑎𝑛𝑡 → 𝑇 1,ℎ𝑜𝑠𝑡 + 𝑆 0,𝑑𝑜𝑝𝑎𝑛𝑡 or 𝑇 1,𝑑𝑜𝑝𝑎𝑛𝑡 + 𝑆 0,ℎ𝑜𝑠𝑡 (Scheme 1.2)
Reineke et al., observed that the 1 mol% Ir(ppy)3-doped into TCTA film shows more intense
emission relative to that of 1 mol% Ir(ppy)3 doped into CBP film.
72
The authors attribute the
increasing annihilation constant in the CBP film to the lower triplet energy of CBP (2.59 eV)
compared to that of TCTA (2.83 eV). That is, not only dopant-dopant annihilation but also dopant-
host annihilation occurs in 1 mol% of Ir(ppy)3:CBP film because the triplet excitons are partially
residing in CBP. Lastly, TTA between the host molecules should be also considered even though
it has been rarely studied in the field. In many OLEDs, triplets are less likely to dwell on the host
owing to the efficient Dexter energy transfer from the host to the dopant by the energetics.
However, the hybrid WOLED structures suggested in chapter 3 are accompanied by inherent TTA
between the host molecules since the devices aim for the exciton formation on the host.
Unfortunately, experimental proofs of TTA within hosts are not sufficient in the field, which might
be worth investing in. Besides the annihilation between the triplet states, charges can also
contribute to the annihilation by interacting with triplets called triplet-polaron annihilation
(TPA).
15, 73
In principle, charges (electrons or holes) can be excited by receiving the energy when
triplets are relaxed to the ground state as shown in the scheme 1.3. The TPA process can be
𝑇 1
+ 𝑃 → 𝑆 0
+ 𝑃 ∗
(P is either electrons or holes and * indicates excited state) (Scheme 1.3)
𝑑 𝑛 𝑇 𝑑𝑡
= −
𝑛 𝑇 𝜏 𝑇 − 𝑘 𝑛 𝑇 𝑛 𝑝 (Equation 1.3)
quantified as shown in the equation 1.3. 𝑛 𝑇 is the exciton density, 𝑛 𝑝 is the polaron density, 𝑘 is
the TPA rate constant and τT is the lifetime of the triplet. Thus, TPA is dependent on both polaron
and exciton density in the device. In the case of dyes with long lifetimes (~30 µs), TTA is typically
dominant over TPA. However, in the devices with the dopants possessing a relatively short
22

lifetime, TPA proves to be non-negligible factor.
15
No matter what, the key is to minimize the
polaron density to prevent TPA which requires the balanced-charge densities in the EML. Chapter
4 focuses on the new host materials for alleviating the efficiency roll-off in hybrid White OLEDs
(WOLEDs) and their application in the devices.

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properties. J. Mater. Chem. C 2014, 2 (48), 10298-10304.
60. Schmidt, T. D.; Lampe, T.; Sylvinson, D. M. R.; Djurovich, P. I.; Thompson, M. E.; Brutting,
W., Emitter Orientation as a Key Parameter in Organic Light-Emitting Diodes. Physical Review Applied
2017, 8 (3).
61. Jurow, M. J.; Mayr, C.; Schmidt, T. D.; Lampe, T.; Djurovich, P. I.; Brutting, W.; Thompson,
M. E., Understanding and predicting the orientation of heteroleptic phosphors in organic light-emitting
materials. Nature Materials 2016, 15 (1), 85-+.
62. Moon, C. K.; Kim, K. H.; Kim, J. J., Unraveling the orientation of phosphors doped in organic
semiconducting layers. Nat. Commun. 2017, 8.
63. Kim, K. H.; Ahn, E. S.; Huh, J. S.; Kim, Y. H.; Kim, J. J., Design of Heteroleptic Ir Complexes
with Horizontal Emitting Dipoles for Highly Efficient Organic Light-Emitting Diodes with an External
Quantum Efficiency of 38%. Chemistry of Materials 2016, 28 (20), 7505-7510.
64. Shin, H.; Lee, S.; Kim, K. H.; Moon, C. K.; Yoo, S. J.; Lee, J. H.; Kim, J. J., Blue
Phosphorescent Organic Light-Emitting Diodes Using an Exciplex Forming Co-host with the External
Quantum Efficiency of Theoretical Limit. Advanced Materials 2014, 26 (27), 4730-+.
65. Shin, H.; Lee, J. H.; Moon, C. K.; Huh, J. S.; Sim, B.; Kim, J. J., Sky-Blue Phosphorescent
OLEDs with 34.1% External Quantum Efficiency Using a Low Refractive Index Electron Transporting
Layer. Advanced Materials 2016, 28 (24), 4920-4925.
66. Wu, T. L.; Huang, M. J.; Lin, C. C.; Huang, P. Y.; Chou, T. Y.; Chen-Cheng, R. W.; Lin, H.
W.; Liu, R. S.; Cheng, C. H., Diboron compound-based organic light-emitting diodes with high
efficiency and reduced efficiency roll-off. Nature Photonics 2018, 12 (4), 235-+.
67. Gather, M. C.; Kohnen, A.; Meerholz, K., White Organic Light-Emitting Diodes. Advanced
Materials 2011, 23 (2), 233-248.
68. Forster, T., *ZWISCHENMOLEKULARE ENERGIEWANDERUNG UND FLUORESZENZ.
Annalen Der Physik 1948, 2 (1-2), 55-75.
69. Engel, E.; Leo, K.; Hoffmann, M., Ultrafast relaxation and exciton-exciton annihilation in
PTCDA thin films at high excitation densities. Chemical Physics 2006, 325 (1), 170-177.
26

70. Staroske, W.; Pfeiffer, M.; Leo, K.; Hoffmann, M., Single-step triplet-triplet annihilation: An
intrinsic limit for the high brightness efficiency of phosphorescent organic light emitting diodes. Physical
Review Letters 2007, 98 (19).
71. Furno, M.; Rosenow, T. C.; Gather, M. C.; Lussem, B.; Leo, K., Analysis of the external and
internal quantum efficiency of multi-emitter, white organic light emitting diodes. Applied Physics Letters
2012, 101 (14).
72. Reineke, S.; Schwartz, G.; Walzer, K.; Leo, K., Direct observation of host-guest triplet-triplet
annihilation in phosphorescent solid mixed films. Physica Status Solidi-Rapid Research Letters 2009, 3
(2-3), 67-69.
73. Reineke, S.; Walzer, K.; Leo, K., Triplet-exciton quenching in organic phosphorescent light-
emitting diodes with Ir-based emitters. Physical Review B 2007, 75 (12).
27

Chapter 2. Integration of copper and gold complexes into Organic
Light Emitting Diodes (OLEDs)
2.1. Introduction
As mentioned previously, OLEDs employing copper complexes as emitters are mainly
solution processed owing to their poor thermal stability.
1-3
Solution process techniques have been
applied in OLEDs due to that fact that they provide a low-cost approach to fabricate the device.
4

In addition, the production of OLEDs via solution process is relatively simple, which is more
practical for large-area devices.
5
Therefore, numerous solution processed OLEDs have been
reported: Early solution-processed OLEDs focused on a simple single hetero device architecture
6-
8
while recent attempts concentrate on more complicated double heterostructure OLEDs.
9-11

However, fabricating solution-processed multilayer OLEDs accompanies with the major issue of
intermixing between layers.
12
Upon depositing subsequent layers, underlying layers can be readily
dissolved or disrupted by the solvent used for deposition. Furthermore, depending on the solubility
of the solute, film formation ability and morphology can vary significantly, affecting the electrical
and the photophysical properties of OLEDs.
13
Thus, a relatively small number of highly efficient
and stable solution-processed OLEDs have been reported compared to vapor-deposited OLEDs.
12,
14
This trend holds for the Cu-based OLEDs. That being said, vacuum deposition techniques are
essential in order to fabricate efficient and stable device employing highly efficient copper
complexes. In this chapter, we demonstrated the fabrication of efficient and stable devices with
mitigated the efficiency roll-off using copper complexes, which is unprecedented for Cu-based
OLEDs.
28

2.1.1. Solution processed OLEDs with copper complexes as emitters
In 2004, Wang et al., reported the first solution processed mononuclear, four-coordinate Cu
OLEDs.
15
They employed [Cu(dnbp)(DPEphos)]BF4 (dnbp=2,9-di-𝑛 -nutyl-1,10-phenanthroline,
DPEphos=bis[2-(diphenylphosphino)phenyl]) as an emitter, which was doped into poly(9-
vinylcarbazole) (PVK). The device structure with the highest performances was
ITO/PEDOT:PSS/PVK:Cu(dnbp)(DPEphos)]BF4 16 wt%/BCP/Alq3/LiF/Al where ITO = indium
tin oxide; PEDOT:PSS = poly(3,4-ethylenedioxythiophene):polystyrene sulfonate; PVK = poly(9-
vinylcarbazole); BCP = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; Alq3 = tris(8-
hydroxyquinolinato) aluminum(III); LiF = lithium fluoride; Al = aluminum. This device reached
the maximum brightness of 1663 cd/m
2
at 28 V, a current efficiency of 10.5 cd/A at 1.0 mA/cm
2

and a power efficiency of 1.6 lm/W at 105 cd/m
2
. However, the turn on voltage (defined as the
voltage at 1cd/m
2
) was significantly larger (14 V) than typical efficient OLEDs, which is not
beneficial for the device operation. This high turn on voltage may result from the usage of
PEDOT:PSS as a HIL considering the its working function (-5.2 eV).
16

Not all solution processed Cu-OLEDs have been reported as inefficient devices. In 2017,
Dawei et al., provided the efficient OLEDs using two-coordinate CAAC-Cu(I) complexes
(CAAC=(alkyl)(amino)carbene, Cz=Carbazole).
3
The device structure with the highest
performance was ITO/PEDOT:PSS/TFB/PVK:CMA2/Bphen/LiF/Al (TFB = poly[9,9-
dioctylfluorene-co-N-(4-butylphenyl)diphenylamine], Bphen= 4,7-Diphenyl-1,10-
phenanthroline). It achieved a maximum EQE of 9.4 %, a maximum current efficiency of 30.4 cd
m
-2
and a low turn on voltage (3.4 V). However, the efficiency roll-off was steep even though the
lifetime of the emitter was considerably shorter (sub-microseconds) than other efficient copper-
based TADF emitters (>3.3 μs). Thus, the poor device stability might be attributed not only to the
29

stability of the dopant but also the device fabrication technique. It is noteworthy that the solution
processed PEDOT:PSS as a hole injection layer can corrode an ITO anode considering its acidic
nature (pH 1.0 – 2.5), leading to the device degradation. Additionally, the absorption of moistures
by PEDOT:PSS can accelerate the device degradation, inducing the significant efficiency roll-off
and inferior device stability.
16
More importantly, the consistency of the device quality was absent
when the solution process technique was adopted. For example, they made 184 devices via
solution-process and their performances varied significantly each time. It is well-known
phenomenon that film morphologies can be different if the layers are deposited by spin-cast,
causing the irregularity in the device performances.
13
As a result, alternative device fabrication
methods such as vacuum deposition needs to be introduced for highly efficient and stable Cu-
based OLEDs.
2.1.2. Difference between solution process and vacuum deposition in OLEDs
It is important to discern the differences between the devices produced by the solution-
processing and the vacuum deposition. In general, the films prepared by the different fabrication
methods can yield the differences in morphological, photophysical and electrical properties.
4
Kim
et al., prepared the films 5 wt% Ir(piq)3: 47.5 wt% NPD: 47.5 wt% 1,3,5-tris(N-
phenylbenzimidazol-2,yl) benzene (TPBi) through the spin-casting and the vacuum deposition to
observe the differences.
13
Interestingly they discovered analogous surface morphologies with
similar average roughness values between the two films, suggesting that high quality, amorphous
films can be made using either method. They attributed the excellent film quality of the solution
processed films to the good film forming ability of TPBi, which is caused by its molecular structure
and high glass transition temperature (124 °C). However, the packing densities of two films were
different: higher packing densities with larger value of refractive indices were observed for the
30

films prepared by the vapor-deposition compared to the solution-processed films. As a result,
OLEDs with the emissive layers prepared by the solution process showed higher a driving voltage
due to the lower packing density and mobility of the film.
Lee et al., explored the correlation between the solvent and morphology of the solution
processed films.
17
They prepared 5 wt% 4,40-bis(2-(4-(N,N-diphenylamino)
phenyl)vinyl)biphenyl (DPAVBi):95wt% 2-(t-butyl)-9,10-bis(20-naphthyl) anthracene (TBADN)
films using toluene and chlorobenzene as solvents. The root-mean-square (RMS) roughness values
of both solution processed films were similar to each other and also to the vapor-deposited film,
corresponding to the result of Kim. However atomic force microscope (AFM) revealed that the
aggregation and the phase separation had occurred, which can negatively affect the quantum
efficiency of emitters, exist only in the film obtained from chlorobenzene solution. This evinces
that the emission characteristic of the emissive layer in the solution processed OLEDs can be
misinterpreted depending on the identity of the solvents whereas the chance of misinterpretation
is less likely to occur in the vacuum deposited OLEDs. Furthermore, they found that the device
lifetimes of the vapor deposited OLEDs are undoubtedly longer than those of the solution
processed OLEDs. Since the molecules in the films attained by the solution processing are not as
compactly packed compared to those in the vacuum deposited layers, solvent impurities remain
inside the films. As a result, it is more likely for oxygen to diffuse into the film, leading to a short
device lifetime.
All these findings mentioned above explain why the standard fabrication method for OLEDs
should be a vapor deposition under vacuum; Consistency, efficiency and stability are superior for
vapor-deposited OLEDs. Therefore, the optimization of vapor deposited Cu-based OLEDs will be
discussed in this chapter.
31

2.2. Results and Discussion
2.2.1. Photophysical Properties

400 500 600 700
0.0
0.5
1.0
400 500 600 700
0.0
0.5
1.0
microcrystalline powder

Normalized Intensity (a. u.)
Cu host, RT
Cu host, 77K
1a in Cu host, 20 wt%

Wavelength (nm)
Cu host, RT
Cu host, 77K
1a, RT
1a, 77K
MeCy

Figure 2. 1 (Left) Molecular structure of CAAC
Men
-CuCz (1a) and CAAC
Men
-Cu(C 6F 5) (Cu host).
(Right) Emission spectra of 1a and Cu host in the microcrystalline powder form (top) and in MeCy
(bottom) at RT and 77K. Also shown is the emission spectrum of a thin film of CAAC
Men
-CuCz (1a)
doped into Cu host (20 wt%, solution-processed, top) and in MeCy (bottom) for comparison.
The molecular structure and the emission spectra of CAAC
Men
-CuCz (1a, CAAC=cyclic
alkyl amino carbene, Cz=carbazole), CAAC
Men
-Cu(C6F5) (Cu host), MAC*-CuCz (MAC*=
monoamido-aminocarbene) and MAC*-AuCz are shown in Figure 2.1, 2.2 respectively. All these
emitters were synthesized by Hamze, Shi et al., and the details of the syntheses are reported in the
literature.
18, 19
Unlike typical blue-emitting iridium phosphors, 1a shows a broader emission
profile. The onset of the emission starts near 400 nm. Therefore, the host material for 1a requires
a higher triplet energy than that of 1a, which is challenging to find. As shown in Figure 2.1, the
triplet energy of the Cu host (ET1 = 2.96 eV) in methylcyclohexane is higher than that (ET1 = 2.88
32

eV) of 1a, carbazolyl-based material, indicating that the energy transfer from the Cu host to 1a
might be efficient. The film comprises 1a doped at 20 wt% into Cu host does not exhibit the host
emission, suggesting that the triplet energy of Cu host in power form is also higher than that of 1a.
As depicted in Figure 2.2, 1b and 1c exhibit broad green-emission in 2-Methyltetrahydrofuran with
the onset emission of 430 nm. Thus CBP, which is widely used host material for green-emissive
emitters with ET= 2.58 eV
20
cannot be used for OLEDs utilizing1b and 1c. Table 2.1 summarizes

400 500 600 700 800
0.0
0.5
1.0
400 500 600 700 800
0.0
0.5
1.0
2-MeTHF

Normalized Intensity (a. u.)
1b
1c

Wavelength (nm)
1b
1c
RT
77 K

Figure 2. 2 (Left) Molecular structure of MAC*-CuCz (1b) and MAC*-AuCz (1c). (Right) Emission
spectra of 1b and 1c in 2-Methyltetrahydrofuran at RT (top) and 77K (bottom).
the photophysical properties of 1a-c in poly styrene (PS) films at 1 wt% doping condition where
non-radiative decay is suppressed. The quantum yields of three films are close to unity with several
micro-seconds lifetimes, making them good luminescent candidates for highly efficient OLEDs.
33

The emission characteristics and quantum yields of 1b and 1c are nearly identical while the
radiative rate of 1c is approximately twice that of analogous 1b due to a smaller singlet and triplet
gap (ΔEST).
21
Considering that the device lifetime is typically proportional to the radiative rate of
the emitters, OLEDs employing gold complexes might be more beneficial for the reduced
efficiency roll-off.
Table 2. 1 Maximum wavelength (λmax), quantum yield (Φ), lifetime (τRT), radiative lifetime (𝒌 𝒓 ,
𝑹𝑻
) and
non-radiative lifetime (𝒌 𝒏𝒓
,
𝑹𝑻
) of 1a, 1b and 1c in 1% doped PS film.

λmax, RT (nm) Φ τRT (μs) 𝒌 𝒓 ,
𝑹𝑻
(s
-1
) 𝒌 𝒏𝒓
,
𝑹𝑻
(s
-1
)
1a 470 1.0 2.8 3.5 × 10
5
3.6× 10
3

1b 506 0.9 1.4 6.4× 10
5
0.71 × 10
5

1c 512 0.85 0.83 10 × 10
5
1.8 × 10
5

2.2.2. Host Studies
One of the requirements to realize highly efficient OLEDs is to choose suitable host materials
because energy transfer from the host to the dopant should be complete. The choice of hosts is
especially important for copper and gold complexes since as mentioned previously, all three
complexes display broad emission features with deeper onset of emission than analogous state-of-
art iridium-based emitters used in OLEDs. For example, 1a possess higher triplet (2.88 eV) than
that of normal blue-emitting Ir complex, requiring ultra-gap host material for the complete energy
transfer. To find the suitable host material for complex 1a, films with 20 vol% of 1a doped into
four different hosts 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) (ET1=2.87 eV), 1,3-Bis(N-
carbazolyl)benzene (mCP) (ET1=2.91 eV), Cu host (ET1=2.96 eV) and 1,3- Bis(trisphenylsilyl)-
34

350 400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
Wavelength (nm)
mCBP
mCP
Cu host
UGH3

Figure 2. 3 Molecular structures of the three hosts (Left) and emission spectra of 1a doped thin films (40
nm, 20% vol) doped into three four hosts (Right)
benzene (UGH3) (ET1=3.5 eV) were made via spin-cast technique. The HOMO/LUMO and triplet
energies of the hosts are summarized in Table 2.2. PLQY of the doped films were 0.9, 0.9, 0.60
Table 2. 2 HOMO, LUMO and triplet energies of 1a and four host materials
HOMO (eV) LUMO (eV) 𝑬 𝑻 𝟏 (eV)
1a -5.1 -1.5 2.88
Cu host -5.8 1.6 2.96
mCBP -6.0 -1.5 2.87
mCP -6.1 -1.5 2.91
UGH3 -7.1 -1.0 3.5

and 0.34 for UGH3, Cu host, mCBP and mCP, respectively. These results clearly show that either
Cu host or UGH3 should be incorporated into OLEDs instead of the commonly used host materials
for blue-emitting emitters, mCBP and mCP. The situation is similar for green emissive dopants.
As shown in Figure 2.4, even though λmax of 1b and 1c in 1 vol% doped PS films are 506 and 512
nm, respectively, their onsets are close to 430 nm. Consequently, the triplet energy of the 1b and
35

1c are 2.56 eV (484 nm), 2.61 eV (470 nm) respectively, which are close to the triplet energy of
typical blue-emitting phosphor. Therefore, mCBP (ET1=2.87 eV) was introduced in OLEDs to
incur the efficient energy transfer from the host to 1b and 1c.
500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
1b

Normalized intensity (a.u.)
Wavelength (nm)
1c

Figure 2. 4 Photoluminescence of the doped poly styrene (PS) films of 1b and 1c at 1 vol% doping
concentration. The films were prepared by spin-cast technique. Inset shows the photo of the doped PS
film.
2.2.1. OLEDs optimizations
CAAC
Men
-CuCz (1a) Device
Devices with two doping concentrations (5, 10 vol%) in three different hosts (mCBP, mCP,
Cu host) were fabricated to verify the suitability of Cu host. The device structure and their
characteristics are provided in Figure 2.5. 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC)
and 2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) were used as a HTL and
an ETL, respectively due to their higher triplet energies.
22
Cu host-based device showed the highest
EQE (4.0%) while mCBP and mCP-based devices displayed a poor efficiency (<1%), implying
that Cu host is a promising candidate. As shown in Figure 2.6, mCBP and mCP- based devices
luminesce the undesired emission below 400 nm, whereas the Cu host-devices show only the
36

10
-1
10
0
10
1
10
2
10
3
0
1
2
3
4
5
6

mCBP:1a 10 vol%
mCBP:1a 5 vol%
mCP:1a 10 vol%
mCP:1a 5 vol%
Cu host:1a 10 vol%
Cu host:1a 5 vol%
EQE (%)
Current density (mA/cm
2
)
0 2 4 6 8 10 12
10
-1
10
0
10
1
10
2
10
3
10
4
mCBP:1a10 vol%
mCBP:1a 5 vol%
mCP:1a 10 vol%
mCP:1a 5 vol%
Cu host:1a 10 vol%
Cu host:1a 5 vol%

Luminescence (cd/m
2
)

Voltage (V)
0
200
400
600
Current density (mA/cm
2
)

Figure 2. 5 Device architectures with three different hosts and doping concentrations and molecular
structures of HTL and ETL. Thickness of EML is 20nm (Top). EQE and J-V-L plot (Bottom)
emission of the dopant, further confirming that mCBP and mCP do not confine the triple excitons
on the Cu emitter as efficiently as the Cu host. However, the device with the Cu host showed the
unexpected emission near 550 nm, corresponding to the degradation upon operation (Figure 2.6).
The degradation of the Cu host-device was confirmed by observing a decrease in EQE after the 1
st

device operation. Unfortunately, the exact mechanism of the degradation is unknown since
numerous factors can contribute to the degradation; dopants, hosts and neighboring layers can
deteriorate in the devices. However, one of the common solutions of the degradation in OLEDs is
37

350 400 450 500 550 600 650
0.0
0.5
1.0
Intensity (a.u.)
Wavelength (nm)
4V
5V
6V
7V
8V
9V
350 400 450 500 550 600 650
0.0
0.5
1.0 4.4V
5V
6V
7V
8V
Intensity (a.u.)
Wavelength (nm)

350 400 450 500 550 600 650
0.0
0.5
1.0 3.8V
4V
4.2V
4.4V
5V
8V
Intensity (a.u.)
Wavelength (nm)

350 400 450 500 550 600 650
0.0
0.5
1.0
3.5V
4V
4.5V
5V
6V
7V
8V
Intensity (a.u.)
Wavelength (nm)

350 400 450 500 550 600 650
0.0
0.5
1.0 3.5V
4V
4.5V

Intensity (a.u.)
Wavelength (nm)
350 400 450 500 550 600 650
0.0
0.5
1.0
Intensity (a.u.)
Wavelength (nm)
3.5V
4V
5V
6V
7V
8V

Figure 2. 6 Electroluminescence spectra collected from devices with 1a as an emitter in mCBP (Top),
mCP (Middle), Cu host (Bottom) respectively. Left: 5 vol%, Right: 10 vol% doping concentrations.
to distribute the exciton densities in the emissive layer.
23
This is because concentrated excitons in
the confined zone will increase the probability of 2
nd
-order annihilation processes such as triplet-
triplet annihilation (TTA) and triplet-polaron-annihilation (TPA), leading to a severe
38

degradation.
24
It is noteworthy that this device stability issue becomes more severe as the energy
of the dopants becomes higher (Blue > Green > Red). That being said, degradation of the device
with 1a as an emitter is likely to happen considering its extraordinarily high triplet energy. To
resolve the degradation issue, 1a was highly doped (20 vol%) into Cu host densities in the EML

Figure 2. 7 (A) Device architecture with 20nm EML and 20 vol% doping concentration. (B) EQE plot
(C) J-V-L curve (D)-(F) Electroluminescence spectrum of the device at different voltages.
(Figure 2.7). The variation in doping concentrations can change the charge balance, considering
that the HOMO/LUMO energies of the host and 1a (Table 2.2). As described in Figure 2.7 the
change in the EL at higher voltages becomes less severe but the device efficiency dropped,
implying that a charge balance, a quantum yield of emissive layer and the device stability are all
correlated. Therefore, finding a suitable doping concentration that maintains the high quantum
yields of the EML with a superior charge balance is critical to fabricate highly efficient stable
OLEDs. As a next step, thicker EML (40nm) with various doping conditions (5, 10, 20, 30 vol%)
39

were introduced in the devices and their characteristics are shown in Figure 2.8. The rationale of
thickening the EML is to spread out the triplet densities in the emissive layer, inducing a

Figure 2. 8 Performances of devices employing 1a as an emitter doped into Cu host with different
doping concentration (5, 10, 20 and 30 vol%). The architecture of device is
TAPC(40nm)/EML(40nm)/TPBi(40nm)/LiF(1nm)/Al(100nm). C-E are the EL spectra of 5, 10, 20 and 30
vol% devices respectively.
suppression of TTA and TPA. As shown in Figure 2.8D (20 vol%), EL spectra of the 20 vol%-
devices are consistent at various voltages, confirming the absence of severe degradation. On top
40

of that, the device yields a maximum EQE of 9.2% at 0.3 mA/cm
2
, demonstrating that the extended
emissive layer contributes to not only the stability but also the efficiency of the device. Even
though the TAPC (40 nm)/Cu host:1a (40 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) device
generates a reasonable EQEmax (9.5 %) at 0.2 mA/cm
2
, it shows a irreversible steep efficiency roll-
off at high current densities (Figure 2.8A), possibly due to the instability of the host. Thus, an
ultra-band gap material, UGH3 (ET=3.5eV), was replaced with Cu host in the same device structure

Figure 2. 9 Performances of devices employing 1a as an emitter in four different hosts with 20 vol%
doping concentration. (A) The device structure (B) EQE plot (C) J-V-L characteristics of devices when 1a
is doped into mCBP (black, squares), mCP (red, circles), Cu host (blue triangles), and UGH3 (green
diamonds) in the EML. (D) EL plot
for Cu host-device, expecting to provide the improved efficiency roll-off. Overall performance of
the devices with the same doping conditions (20 vol%) but different four hosts (mCBP, mCP, Cu
host and UGH3) are summarized in Figure 2.9. All devices exhibit blue ELs, similar to the PLs
41

for 1a in a PS film, which is in contrast to the green OLEDs reported by Di et al.
3
The maximum
EQE shows the similar trend with ΦPL of the 40 nm-thick films of 1a doped into the various hosts:
UGH3=Cu host > mCBP > mCP (0.9, 0.6, 0.3 respectively, at 20 vol%). OLEDs prepared with 1a
doped into UGH3 at 20 vol % give an EQEmax of 9.0% and 16 cd/A at 2 mA/cm
2
(Figure 2.9 B),
consistent with the high triplet energy of the UGH3 host (ET1 = 3.5 eV). Although the EQE values
for green and yellow, or orange-emitting Cu-based OLEDs have been reported to be > 20 %
25
, the
highest efficiencies previously reported for blue-emitting (λmax < 500 nm) Cu-based OLEDs are <
6%.
26, 27
The efficiency roll-off at high current densities is gradual for the UGH3 host-based device
compare to the Cu host-based device (Figure 2.9 B). These results support our hypothesis on the
instability of the Cu host during the device operation. It is notable that the efficiency roll-off for
the UGH3-based device is comparable to the roll-off observed for the UGH3-based OLEDs with
an iridium-based emitter.
28
Interestingly, the lowest EQE was reported with the mCP-based device
although the triplet energy of mCP is slightly higher than that of mCBP. Considering ΦPL of the
40 nm-thick films of 1a doped into the mCBP (0.6) is higher than that of mCP (0.3), it appears that
mCP is simply not well suited as a host for 1a, likely due to the poor solvation in the solid state.
The parameters and the measurements of all the 1a-based devices are summarized in Table 2.3.

42

Table 2. 3 Summary of device optimization. The following parameters in the EML were varied: Identity
of the host (UGH3, mCBP, mCP, and Cu host), EML thickness (40 nm), and doping concentration (5%,
10%, 20% and 25%). The device architectures are: TAPC(40)nm/EML(20, 40nm)/TPBi(40nm,
50nm)/LiF(1nm)/Al(100nm).
a)
Turn on voltage was determined as the voltage at 1cd/m
2
.
Host
EML
thickness
(nm)
Doping
concentration
(vol %)
EQEmax
(%)
Current
density
at EQEmax
(mA/cm
2
)
a)
Vturn on
(V)
Max cd/A Max cd/m
2

UGH3
40 5 7.4 0.1 4.2 10.8 976.1
40 10 5.0 2.6 3.6 7.6 2396
40 20 (ETL=50nm) 8.4 1.8 4.0 15.6 3797
40 25 2.6 35.3 3.1 4.3 4517
mCBP
20 5 0.7 11.7 3.1 1.2 845.7
20 10 0.6 25.9 2.8 1.1 1281
40 20 4.0 10.3 2.8 1.7 2365
mCP
20 5 0.5 11.5 3.5 0.4 418.9
20 10 0.4 17.7 3.4 0.3 286.6
40 20 1.1 69.1 3.3 1.6 1304
Cu host
20 5 4.2 0.008 3.5 3.1 123.2
20 10 3.2 0.1 3.2 3.5 196.5
20 20 2.6 1.9 3.1 3.4 296.9
40 5 0.8 0.009 4.5 0.8 15.4
40 10 1.1 0.02 4.1 1.2 44.7
40 20
9.2 0.3 4.4 10.3 391.7
9.5 0.2 4.4 12.5 433.8
40 30 1.2 10.6 3.1 1.8 515.7

43

MAC*-CuCz (1b), MAC*-AuCz(1c) Device
In order to prove the hypothesis that CBP, widely used host for green-emitting Ir complexes
is not suitable for 1b due to its high triplet energy, two devices were fabricated and their device
structure and characteristics are summarized in Figure 2.10. Both mCBP and CBP were selected
as host materials for the comparison while Bphen was used as an ETL. The EQE-J plot in Figure
2.10 shows that an EQEmax diminishes more than as a factor of two from mCBP (11 %) to CBP
(5%) device, demonstrating that the energy transfer from the CBP to 1b is incomplete. Even though

0 2 4 6
0
5
10
15
20
mCBP
CBP
Voltage (Volts)
Current density (mA/cm
2
)
1
10
100
1000
Luminance (cd/m
2
)
0.1 1 10
0
2
4
6
8
10
12
14 mCBP
CBP
Current density (mA/cm
2
)
EQE (%)
0
5
10
15
20
25
30
Power Efficiency (Lm/W)

300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0

Normalized intensity (AU)
Wavelength (nm)
1% PS film
10% CBP OLED
10% mCBP OLED

Figure 2. 10 Device structure with different host materials and J-V-L plot (Top). EQE and power
efficiency as a function of current density and electroluminescence (Bottom).
All the triplets are captured by 1b in the mCBP mixed film, EQEmax of mCBP-based device (12%)
is still lower than 20%, which is a theoretically achievable EQEmax value for the device without
44

any enhancement in the outcoupling efficiency.
29-31
Therefore, the devices with different doping
conditions (10, 20, 40, 80 and 100%) have been studied to investigate how they impact on the
efficiency of the devices. The device structure and characteristics are summarized in Figure 2.11.

0 1 2 3 4 5 6 7 8 9 10
0
100
200
300
400
500
600
10%
20%
40%
80%
Neat
Voltage (V)
Current density (mA/cm
2
)
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
Luminance (cd/m
2
)
0.01 0.1 1 10 100
0
2
4
6
8
10
12
14
16

EQE (%)
Current density (mA/cm
2
)
10%
20%
40%
80%
Neat

400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0

Normalized intensity (a.u.)
Wavelength (nm)
10%
20%
40%
80%
Neat

Figure 2. 11 Device structure with 1b as an emitter at different doping conditions and J-V-L plot (Top).
EQE as a function of current density and EL spectra (Bottom).
Here, varying doping concentrations have manly two effects on the EQE: Changes in Φ
𝑃𝐿
and 𝑛 𝑟 .
The PLQY generally drops as doping concentrations increase due to the aggregation of dopants
but the charge balance might be improved depending on the transporting properties in the EML.
Hence, finding an optimized doping ratio is one of the keys to fabricate highly efficient devices.
As described in Figure 2.11, the HOMO/LUMO energies of 1b (-5.0/-1.9 eV) are entirely nested
within the host (-6.0/-1.5 eV), implying that the charge transportation in the EML will vary
45

depending on the doping conditions. Additionally, the quantum yields of 10, 20, 40, 100% of 1b
into mCBP films dropped: 100%, 90%, 84%, 72%, respectively. This aggregation of the dopant
corresponds to the bathochromic shift of EL upon a heavy doping (Figure 2.11). However, unlike
the trend of quantum yields of films, the device with 40% doping concentration shows the highest
EQEmax (~16 %). This might be due to the improved charge balance in the EML even though the
quantum yield slightly drops. One interesting feature is that the neat emitter device shows an

Figure 2. 12 a. Device structure with different ETL thickness (45, 55 and 65 nm respectively) and
doping concentrations (10, 40 and 100% respectively). b. Current density as a function of voltage c. EL
spectra
EQEmax =12 %, which is rare case for Ir complexes since they usually become non-emissive upon
a heavy doping. In order to maximize the efficiency, further optimization has been conducted by
changing the ETL thickness, expecting to see the change in the outcoupling efficiency (Figure
2.12). As mentioned previously, the outcoupling efficiency is one of the important factors affecting
the EQE because not all of the generated light can escape in the device. It is also well-known that
46

the ETL thickness is typically sensitive to the outcoupling efficiency.
32
The optimized device
structure is glass substrate/70 nm ITO/5 nm hexaazatriphenylene hexacarbonitrile (HATCN)/40
nm TAPC/10 nm (mCP)/25 nm EML/45 or 55 or 65 nm (TPBi)/1.5 nm 8-hydroxyquinolinato
lithium (LiQ)/100 nm Al. Here, the EML is compound 1b doped into mCBP at 10 or 40 vol %, or
100%. The performances of the devices are summarized in Table 2.4. The devices emit bright
Table 2. 4 Turn on voltage (V T, defined the voltage at 0.1cd/m
2
), EQE, maximum luminance (L max) and
maximum emission wavelength (λ max) of OLEDs shown in Figure 2.12
Doping concentration (%) VT (V) EQEmax (%) Lmax (cd/m
2
) λmax (nm)
10 3.0 17.0 41,000 537
40 2.5 19.4 54,000 543
100 2.5 16.3 41,000 555

green light (543 < λmax<555 nm), consistent with the photoluminescence spectra of the dopant.
The slight red shift in the emission peak maximum at higher doping concentrations is likely due
to a combination of the aggregation and solvation effects. As shown in Figure 2.12, approximately
2% of EQE has been increased when the ETL layer changes from 45 nm to 65 nm, which might
be due to the difference in the outcoupling efficiency. Turn-on voltages (VT, defined at brightness
of 0.1 cd/m
2
) of all devices are low, and decrease from 3.5 to 2.7 V as the doping concentration
increases from 10% to 100%, suggesting that the charges are primarily carried by the
HOMO/LUMO of the emitter. In addition, the device resistance also decreases as dopant
concentration increases. The maximum EQEs of the devices range from 15.4% to 19.4% with the
highest value achieved at a doping concentration of 40% and 65 nm of ETL (Figure 2.12).
Considering the PL efficiency of the EML is 100%, EQEmax= 19.4% is close to the optimized
value, assuming approximately 20% of light is extracted from the device. The device efficiency is
among the highest reported for OLEDs based on Cu(I) dopants.
33-37
Interestingly, the device using
47

a neat emissive layer also demonstrates a high efficiency (EQE =16.3%), which can be attributed
to the high photoluminescent (PL) efficiencies of the vapor-deposited 10% doped (ΦPL = 100%)
and the neat (ΦPL =74%) films of 1b. More importantly, the efficiency roll-offs at high drive
currents for all the devices are significantly smaller than reported for OLEDs using four- and three-
coordinate Cu dopants where the EQE dropped to < 2% at 100 mA/cm
2
in most cases,
26, 33, 38, 39

and to 7% in one exceptional case.
37
The small roll-off here is likely due to the short emission
decay lifetime of 1b, limiting the TTA and the TPA at high brightness current.
40

MAC*-AuCz (1c) Device

Figure 2. 13 a. Device structure employing 1c as an emitter with different ETL thicknesses. b. J-V-L plot
c. EQE as a function of current density d. EL spectra

48

Since the photophysical properties of 1b and 1c are nearly identical, the optimized device
structure for 1b was applied to the 1c-based devices. The device structure is the same as the one
shown in Figure 2.12, except for 1b being replaced with 1c. The device characteristics are
summarized in Figure 2.13. The devices emit bright green light (λmax = 512 nm), consistent with
the photoluminescence spectra of the dopant. As illustrated in Figure 2.13, approximately 2% of
EQE has been increased when the thickness of the ETL layer changes from 45 nm to 65 nm which
might be due to the difference in the outcoupling efficiency. The turn on voltage (VT, defined as
the voltage at 1cd/m
2
) of all devices is 2.5 V, which is as low as state-of-art Ir-based OLEDs. The
maximum EQEs of the devices are 18.1, 16.8 and 14.5 %, respectively for 65, 55 and 45 nm of the
ETL. Considering the PL efficiency of the EML is 89%, EQEmax=18.1 % is close to the theoretical
maximum value for the device without any enhancement in outcoupling. Therefore, the device
performance is well-optimized and this efficiency is among the highest reported for OLEDs with
Au(I) as a dopant.
3, 41
Similar to the 1b-based device, the efficiency roll-off at high drive currents
for all the devices is comparable to the roll-off observed for mCBP-based OLEDs with an iridium-
based emitter. It is noteworthy that efficiency roll-offs in OLEDs with 1b and 1c are similar even
though the radiative lifetime of 1c is faster than that of 1b, suggesting that not only the radiative
lifetime of the emitters but also other factors can impact on device operation lifetime.
2.3. Conclusion
The fabrication of OLEDs with more abundant earth materials can bring an enormous
economic impact, given that nearly 12% of the total costs in OLEDs comprises of purchasing
organic materials. The majority of Cu-based OLEDs are made via solution-processed technique
due to the weak thermal stability of Cu complexes. However, vapor deposited Cu-OLEDs should
be a long-term goal because solution-processed OLEDs are typically inferior in the device
49

stability. Here, we demonstrated that highly efficient Cu, Au-based OLEDs with reasonable
efficiency roll-off can be fabricated by the vacuum deposition techniques.
CAAC
Men
-CuCz (1a), MAC*-CuCz (1b) and MAC*-AuCz (1c) luminesce with an unity PL
efficiency in the PS mixed film, and several microseconds of radiative lifetimes, making them
good candidates for efficient OLEDs. The selection of the appropriate host materials is crucial for
these types of Cu and Au complexes because of their broad emission profiles. This is more
challenging for blue-emitting emitter such as 1a because stable host materials are rarely available.
An ultra-gap host material, UGH3 was employed to avoid the any exciton loss through the host in
the EML. The optimized device of 1a doped into UGH3 at 20 vol % gives an EQEmax = 9.0% and
16 cd/A at 2 mA/cm
2
with the reasonable efficiency that is observed for UGH3-based OLEDs with
an iridium-based emitter. This is one the highest efficiencies reported for blue-emitting (λmax < 500
nm) Cu-based OLEDs.
The host materials for MAC*-CuCz (1b) and MAC*-AuCz (1c) were adopted according to
their broad emission profile. mCBP with a higher triplet energy than that of 1b and 1c was
introduced in the device. The doping conditions varied from 10% to 100%, expecting to observe
the change in the internal quantum efficiency and the charge balance in the EML, eventually
altering the device efficiency. Moreover, the thickness of the ETL was controlled to induce the
change in the outcoupling efficiency. EQEmax = 19.4 %, 18.1 % were achieved for 1b and 1c-
devices respectively with an unprecedented efficiency roll-offs for Cu- and Au-OLEDs. These
performances are among the best values recorded for Cu-based OLEDs.
2.4. Experimental Methods
Photophysical characterization
50

Quantum yields at room temperature were measured using a Hamamatsu C9920 system
equipped with a xenon lamp, calibrated integrating sphere and model C10027 photonic
multichannel analyzer (PMA)
OLED fabrications and characterization
OLED devices were fabricated on pre-patterned ITO-coated glass substrates (20 ± 5 Ω cm
-
2
, Thin Film Devices, Inc.). Prior to deposition, the substrates were cleaned with soap, rinsed with
deionized water and sonicated for 15 minutes. Afterwards, two subsequent rinses and 15-minute
sonication baths were performed in acetone and isopropyl alcohol sequentially. All organic layers
as well as the Al cathode were deposited in a vacuum thermal evaporator, EVO Vac 800 deposition
system from Angstrom Engineering, at 6 x 10
-7
Torr.
Current-voltage-luminescence (J-V-L) curves were using a Keithley power source meter
model 2400 and a Newport multifunction optical model 1835-C, PIN-220DP/SB blue-enhanced
silicon photodiodes (OSI optoelectronics Ltd.). The sensor was set to measure power at an energy
of 520 nm, followed by correcting to the average electroluminescence wavelength for each
individual device during data process. Electroluminescence (EL) spectra of OLEDs were measured
using the fluorimeter (model C-60 Photon Technology International QuantaMaster) at several
voltages.
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54

Chapter 3. Fabrication of hybrid White Organic Light Emitting
Diodes (OLEDs) employing imidazole-based neat emitter
3.1. Introduction
3.1.1. Concepts of Hybrid WOLEDs
White organic light-emitting diodes (WOLEDs) have been intensively developed for
decades due to their promising future solid-state lighting applications and displays. Numerous
factors such as low driving voltage, efficiency and stability are important in WOLEDs fields. In
general, the two most important factors are color quality and efficiency. The quality of the white
color needs be consistent during operation, while revealing the true color of the objects. Color
Rendering Index (CRI) is the one of the quantitative indicators describing how well a light source
shows the genuine color of objects scaling from 0 to 100. Generally, WOLEDs with CRI values
higher than 80 are considered good at color rendering.
1
In order to create a white color luminance
with a reasonable CRI, mainly two color (blue and yellow) or three color (blue, green and red)-
WOLEDs have been fabricated in OLED fields. Second, WOLEDs are required to be efficient
with high EQE and power efficiency (lm/W). The first main task to achieve a high EQE in the
device is to utilize all excitons (both singlet and triplet) generated in the emissive layer (EML)
because the excitons are the source of light. As mentioned previously, according to spin-statistics
in quantum mechanics, excitons are formed as singlets and triplets with a 1:3 ratio.
1
Therefore,
devices with fluorescent dyes, which utilize only singlets, possess inherent limitation from
reaching high efficiency. On the contrary, quadruple efficiencies in OLEDs with phosphorescent
dyes have been reported, proving that 100% usage of the excitons by triplet harvesting is essential
for a highly efficient device. As a result, 100% of internal quantum efficiency has been
demonstrated in WOLEDs by employing solely phosphorescent dopants, providing high EQEs
55

and luminous power efficiencies.
2
However, all-phosphors-doped WOLEDs experience severe
efficiency roll-off at high luminance with a short operational device lifetime. This is because
phosphors have longer lifetime (µs) compared to that (ns) of fluorophores. In principle, the excited
state density (n) of dyes increases the brightness of OLEDs while lifetime of dyes (𝜏 ) is inversely
proportional to the brightness of OLEDs.
3
Considering a 1:3 spin branching ratio of a singlet and
triplet state, the brightness of OLEDs (L) can be expressed as follows:
L ~
𝑛 4𝜏 (For fluorescence), L ~
𝑛 𝜏 (For phosphorescence) (Equation 3.1)
L is the brightness of OLED; 𝜏 is the lifetime of dyes
Equation 3.1 describes that if the lifetimes of fluorophore and phosphor are 10 ns and 1 µs,
respectively, the exciton density of phosphor-based device requires it to be higher by 25-fold
relative to that of a fluorophore-based device to achieve the same brightness. In other words,
exciton concentration in the case of phosphors is significantly higher compared to fluorophores at
high luminance, leading to an increase in probability of 2
nd
-order annihilation processes such as
triplet-triplet annihilation (TTA) and triplet-polaron-annihilation (TPA) (Figure 3.1).
4
In addition,
stable blue-emitting phosphors hardly exist
5
and host materials for blue-phosphors require even
higher energy, which is another challenge for phosphors-WOLEDs. All of these demands and
drawbacks of phosphors-WOLEDs contribute to a change in the WOLED structure: Hybrid
WOLED replacing blue-phosphorescence with blue-fluorescence. People started to introduce
blue-fluorophores in WOLED due to several advantages. First, if appropriate singlet and triplet
energies are tuned by fluorophores, 100% internal quantum efficiency is still feasible since blue-
dopant captures singlets while green and red-phosphors harness the remainder of lower-energy
triplets.
6
Device operation lifetime can also be enhanced because of reduced exciton densities in
56

Figure 3. 1 The excited-state density for phosphorescence and fluorescence, respectively (Left).
Eefficiency-brightness curve for phosphorescence (red) and fluorescence(blue). The data is derived from
reference
1
(Right).
the EML when fluorophores are introduced. Besides, synthetical challenge in making stable host
material for blue-emitting phosphor can be deleted. Moreover, a superior CRI can be derived by
harvesting singlets from blue-fluorescent dyes and triplets from green and red-phosphors,
separately
1
because intensities of blue to green and red emission become close to a 1:3 ratio. More
importantly, energy losses caused by the energy transfer from a singlet to a triplet in phosphors
can be omitted, maximizing the device power efficiency (lm/W). In 2006, Sun et al., demonstrated
a hybrid WOLED concept with superior performance (CRI=85, EQEmax=18.7 %, PEmax=37.6
lm/W). The principle of the energy transfer within the device is shown in Figure 3.2. Once
excitons (singlet and triplet) are formed on the host, a singlet is transferred to a neighboring
fluorescent dopant by Förster energy transfer due to the singlet energy of the dopant being lower
than that of host. Contrarily, triplets formed on the host cannot be efficiently transferred to a triplet
of a blue dopant by neither Förster nor Dexter energy transfer due to its low concentration (<5%).
Rather, a triplet diffuses into red and green-phosphors in the EML due to its long diffusion length
(~100nm).
7
In this scheme, it is crucial that excitons are formed onto the host and not on any of
the dopants. If a large fraction of the charges is trapped by phosphors and excitons are formed
57

directly on phosphors, then the blue emission becomes too weak to make a white color.
Additionally, if charges are dominantly trapped by fluorescent dyes, non-radiative triplet of the
dopant becomes activated due to its low energy, resulting in loss of excitons. The latter scenario
can be omitted with the hybrid WOLED structure utilizing a neat fluorescent blue-emitter as a
host.

Figure 3. 2 Energetic scheme of device operation in hybrid WOLEDs. The data is derived from
reference
6
.
3.1.2. The limitations of hybrid WOLED with a neat fluorescent emitter
The hybrid WOLED structure with a neat fluorescent blue-emitter as mentioned above requires
a host material with a high quantum yield in its solid state and higher triplet energy than that of
green, red- emitting phosphors. One of the well-known blue neat emitter is a 4P-NPD.
8-10
Hybrid
WOLED with 4P-NPD as a neat blue emitter/host provided EQEmax= 20.3 % with 57.6 lm/W
power efficacy at 100 cd m
-2
brightness.
8
Even though the efficiency of the device is high,
constraints still exist: triplet energy of 4P-NPD is 2.31 eV, which is slightly lower than that of
Ir(ppy)3 (2.40 eV), leading to a partial loss of triplet excitons. Thus, Ir(ppy)3 was introduced right
next to the recombination zone to prevent further loss of excitons caused by 4P-NPD. In this work,
new hybrid WOLED energetic scheme has been suggested to eliminate the waste of triplets by the
S
S
T
HOST
Exciton
formation
zone
RED and
GREEN
phosphorescent
dopants
T
S
T
χ
s
= 0.25
χ
t
= 0.75
BLUE
fluorescent
dopant
Förster transfer
Diffusive transfer
T
Energy
58

host (Figure 3.3). A new blue neat emitter/host material with higher triplets than that of green
phosphors is used in the device, resulting in a spontaneous triplet energy transfer from the host to
the dopants. As a result, unwanted dissipation of excitons is no longer available, achieving 100%
internal quantum efficiency. This new blue-emitter/host material requires several properties: a high
quantum yield in its solid state, higher triplet energy than that of phosphors and superior
transporting ability of charges and so on. Details of the compound are presented in the following
section.

Figure 3. 3 Hybrid WOLED energy scheme using a fluorescent dopant (left), hybrid WOLED energy
scheme using a blue-fluorescent neat emitter as a host (right)

3.2. Results and Discussions
3.2.1. Photophysical Properties and Film Studies.
Several phenanthro[9,10-d]imidazoles derivatives (I1-I6) are suggested as candidates for
blue-neat fluorophores/hosts and their molecular structures are shown in Figure 3.4. To investigate
the suitability of I1-I6 as host materials for Ir(ppy)3, Iridium (III) bis(2-phenylquinolyl-N, C2’)
acetylacetonate (PQIr) and a neat emitter, PLQYs of several films were measured. Ir(ppy)3 and
PQIr were doped into I1-I6 and CBP at 10 wt% doping concentration and neat films of I1-I6 were
made. HOMO/LUMOs, PLQYs and PL spectra are shown in Figure 3.4 and Figure 3.5,
respectively. First, PLQYs of neat films of I1, I5 and I6 are 0.95, 0.81 and 0.83, respectively,
59

indicating that they can function as neat blue emitters. However, when Ir(ppy)3 is introduced into
I5 and I6, PLQYs become low (0.30 and 0.36 respectively). Contrarily, the triplet energy of I1 is
2.64 eV in its solid state, which is higher than that (2.40eV) of Ir(ppy)3, implying the triplet would
not be wasted by I1 (Figure 3.5). Indeed, Ir(ppy)3-doped film at 10 wt% into I1 displays moderately
high PLQY (0.65), which is comparable to the value (0.98) of the reference film, CBP:Ir(ppy)3. In
addition, PQIr-doped film at 10 wt% into I1 shows a high PLQY (0.70), suggesting that I1 can be
a host for PQIr. Complete energy transfer from I1 to Ir(ppy)3 or PQIr can be further confirmed by
the absence of fluorescent emissions in the PL spectra (Figure 3.5). Consequently, among I1-I6,
only I1 displays deep blue emission (λmax=400 nm) with high quantum yield (0.95) in its solid state
and reasonable PLQYs when Ir(ppy)3 and PQIr are doped, satisfying the requirements for the host
mentioned above.

Figure 3. 4 Candidates of blue-neat emitters/hosts for hybrid WOLED and green-emissive phosphor fac-
tris(2-phenylpyridyl)iridium (Ir(ppy) 3) and red-emissive phosphor (iridium (III) bis(2-phenylquinolyl-N,
C2’) acetylacetonate (PQIr)

60

Table 3. 1 HOMO/LUMO of phenanthro[9,10-d]imidazoles derivatives and photoluminescence quantum
yield (PLQY) of the spin-cast films. The films comprise Ir(ppy) 3 and PQIr doped at 10 wt% into I1-I6 and
CBP and neat or 100% of I1-I6.
PLQY I1 I2 I3 I4 I5 I6 CBP
Ir(ppy)3 10 wt%
doped film
0.65 0.49 0.56 0.39 0.30 0.36 0.98
PQIr 10 wt%
doped film
0.70 0.69 0.67 0.64 0.69 0.66 0.40
Neat film 0.95 0.10 0.10 0.55 0.81 0.83 0.40
HOMO (eV) -5.8 -5.8 -5.9 -5.9 -5.9 -5.9 -6.1
LUMO (eV) -1.7 -1.9 -1.9 -2.0 -2.0 -2.0 -1.9

Figure 3. 5 Normalized emission spectra of Ir(ppy) 3 and PQIr doped films at 10 vol% in
phenanthro[9,10-d]imidazoles and CBP hosts (Top). Normalized emission spectra of I1 in different
matrices at RT and 77K (Bottom).

61

3.2.2. Probing Exciton Profiles

0 2 4 6 8 10
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
None
HTL
Middle
ETL

Luminescence (cd/m
2
)

Voltage (V)
0
100
200
300
400
500
Current density (mA/cm
2
)
10
-2
10
-1
10
0
10
1
10
2
0
2
4
6
8
10
12
14
16

None
HTL
Middle
ETL
EQE (%)
Current density (mA/cm
2
)

Figure 3. 6 The device structure with I1 as a neat emitter/host with sensing technique (Top). J-V-L plot
and EQE as a function of current density (Bottom).
Prior to WOLED fabrication, estimating exciton profiles in the EML is imperative because
phosphors need to be positioned at the right position to obtain a superior CRI. Projection of exciton
profiles can be anticipated by utilizing a through sensing technique whereby ultra-thin layers of
lower-lying triplet materials such as PQIr are inserted in the EML at different locations.
11-13
It is
noteworthy that the layer of the sensitizer needs to be thin enough so that there is no change in
charge distribution in the EML. The principle of sensing technique is simple; triplets residing on
the host will be transferred to the low-lying triplets of the PQIr if they are within the Föster radius
of PQIr, leading to PQIr emission. That is, the relative PQIr intensity is an indicator of local exciton
concentration in the EML. Details of the sensing technique is established in the experimental
62

section. Figure 3.6 shows device structures for probing exciton profile with their characteristics.
EL spectra of the devices at different current densities are summarized in Figure 3.7. Device
structures for sensitizing excitons are TAPC 60 nm/ mCP 10nnm/ EMLs 20 nm /Bphen 40 nm/
LiF 1 nm/ Al 100 nm. EMLs are composed of either neat I1 (Non-sensing) or 1Å of PQIr doped
into I1 at the HTL/EML interface, middle of the EML and at the ETL/EML interface. As shown
in J-V-L plot in Figure 3.6, current densities of non-sensing and sensing devices are nearly
identical, illustrating that the ultra-thin layer of the PQIr does not change in charge distribution in
the EML. This is a surprising result considering the HOMO/LUMO (-5.3/-2.3 eV) of PQIr are
nested by that (-5.8/-1.7 eV) of I1. HOMO and LUMO of the sensor material are shallower and
deeper than that of that of the host, respectively. This means that holes and electrons will be trapped
by the sensor. However, PQIr does not catch the charges because the doped layer is extraordinarily
thin, but it can still capture triplet excitons. As shown in Figure 3.7, PQIr emission is tremendously
intense at the ETL/EML interface at low current density, whereas weak at the HTL/EML interface.
However, as current density increases, the emission trend behaves the opposite way. The
recombination zone shifts from the EML/ETL to the HTL/EML interface as the current density
rises. In other words, this device is hole-dominating in general but the recombination zone expands
to the HTL side as current density increases. This hole-dominating characteristic is also proven
by EQE-J curve; EQEmax= 15 % is achieved for the ETL sensing device while EQEmax is only 10
% for the HTL sensing device. Triplet harvesting by way of the diffusion process becomes less
efficient as the sensitizer is located far from the recombination zone. Relative exciton
concentrations at given current density were calculated with the method described in the
experimental section and summarized in Figure 3.8. Corresponding to the EQE-J plot,
recombination occurs exclusively at the ETL/EML interface at low current densities. However,
63

400 500 600
0.0
0.2
0.4
0.6
0.8
1.0
No sensing
0.1 mA/cm
2
1 mA/cm
2
10mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)

400 500 600 700
0.0
0.5
1.0
1.5
2.0
0.1 mA/cm
2
1 mA/cm
2
10mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
HTL sensing

400 500 600 700
0
1
2
3
4
5
6
0.1 mA/cm
2
1 mA/cm
2
10mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
Middle sensing

400 500 600 700
0
20
40
60
80
100
120
140
0.1 mA/cm
2
1 mA/cm
2
10mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
ETL sensing

Figure 3. 7 EL spectra of non-sensing, HTL sensing (Top), middle sensing and ETL sensing devices
(Bottom).
excitons are distributed over the EML as current densities increase. This change in exciton profiles
depending on current densities implies that EL spectrum of the WOLED might not be consistent
at different current densities. To compensate this behavior and acquire a high CRI, phosphors
(PQIr and Ir(ppy)3) should be inserted at appropriate positions. As commented previously, 100%
internal quantum efficiency can be achieved if the recombination zone is formed on the host and
triplets are captured by the phosphors through the diffusion process. Trapping charges on
phosphors needs to be averted and thus conventional co-doping systems might not be
recommended, considering nested HOMO/LUMOs of phosphors by the host. In order to avoid
64

intensive exciton formation on the phosphors, a δ-doping layer of PQIr (~1Å ) is selected instead
of a conventional doping method.
14
In this technique, PQIr might be just scattered on the surface
without trapping charges but may efficiently capture triplets by the diffusion process.

Normalized exciton profile (a.u.)
0.1 mA/cm
2
1 mA/cm
2
10 mA/cm
2
0 5 10 15 20
Singlet + Triplet sensing (PQIr)
100 mA/cm
2
Position x (nm)

Figure 3. 8 Exciton profiles at different current densities.
3.2.1. OLEDs Optimization
Monochromatic devices utilizing I1, I4-6 as neat emitters have been fabricated to validate the
high quantum yields of these materials in the device. The device structures are TAPC 40 nm/ mCP
10nnm/ I1, I4-I6 40 nm /Bphen 40 nm/ LiF 1 nm/ Al 100 nm and characteristics of the devices are
shown in Figure 3.9. As shown in the EL spectra of I1 and I4-I6, only the fluorescence of the
dopant is observed, indicating effective singlet harvesting by the emitters. This effective singlet
65

0 2 4 6 8 10
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
I1
I4
I5
I6
Luminescence (cd/m
2
)

Voltage (V)

0
100
200
300
400
500
Current density (mA/cm
2
)

10
-2
10
-1
10
0
10
1
10
2
10
3
0
1
2
3
4
5
6

I1
I4
I5
I6
EQE (%)
Current density (mA/cm
2
)

400 500 600
0.0
0.2
0.4
0.6
0.8
1.0 I1
I4
I5
I6
Intensity (a.u.)
Wavelength (nm)

Figure 3. 9 Monochromatic OLED characteristics of phenanthro[9,10-d]imidazoles as neat fluorescent
emitters.
confinement on the emitters is also reflected by high EQEs (~5%) in the devices. Assuming PLQY
of the EML is 1.0 and outcoupling efficiency is 20%, which is a typical value for OLEDs without
any enhancement in outcoupling, achievable theoretical maximum EQEs for fluorophore-based
OLEDs are approximately 5%. In other words, internal quantum yields of I1 and I4-I6 as neat
emitters are close to unity in the devices, corresponding to the results of films summarized in Table
3.1. However, I1 was further investigated for three-color hybrid WOLED fabrication due to its
high triplet compared to that of Ir(ppy)3. Based on the exciton profiles in the EMLs (Figure 3.8),
relative positions of Ir(ppy)3 and PQIr layer have been determined. Modification of device
structures and their EL are shown in Figure 3.10. Optimization of devices was focused on
66

Figure 3. 10 Progress in device structures for optimization and EL spectra.
achieving constant EL spectrum with high efficiency. The structure of D1 is TAPC 60 nm/ mCP
10nnm/ I1 10 nm/ PQIr 1 Å / I15 nm/ I1:Ir(ppy)3 4 vol% 5 nm/ Bphen 40 nm/ LiF 1 nm/ Al 100
nm. 5 nm of Ir(ppy)3 doped layer at 4 vol% was located at the EML/ETL interface to obtain green
emission. An ultra-thin layer of PQIr (1 Å ) was separated from an Ir(ppy)3-doped layer by 5 nm
I1 layer to avoid a triplet transfer from Ir(ppy)3 to PQIr. At low current densities, mostly emissions
of PQIr and Ir(ppy)3 are present due to exclusive exciton formation on the EML/ETL interface as
revealed in exciton profile. Considering singlet, triplet energies of I1(3.28 eV, 2.60 eV) and
Ir(ppy)3 (2.58 eV, 2.40 eV), all excitons formed on I1 are transferred to triplet of Ir(ppy)3, resulting
in a strong green emission but no blue emission. As current densities increase, the relative intensity
of I1 becomes stronger because the recombination zone shifts to the HTL/EML interface.
However, emission of Ir(ppy)3 does not grow from 1 to 100 mA/cm
2
since Ir(ppy)3 doped layer is
only located at the EML/ETL interface. To compensate the lack of blue emission at low current
67

densities, 5 nm of I1 layer was introduced at the EML/ETL interface while keeping the total
thickness of the EML constant (D2). As a result, relative intensity of green emission diminished
at low current densities while blue emission increased. Unfortunately, intensity of the green
emission is exceedingly reduced at a high current density since the recombination zone is far away
from the Ir(ppy)3-doped layer, which also corresponds to the exciton profile. Therefore, an ultra-
thin (0.5 Å ) layer of Ir(ppy)3 is embedded at the HTL/EML interface to recover the green emission
at a high current density (D3). δ-doping layer of Ir(ppy)3 is selected instead of conventional doping
to avoid charge trapping, maintaining the charge distribution in the EML. In addition, doping
condition of Ir(ppy)3 increased from 4 to 6 vol% to acquire more intense green emission at low
current densities. Consequently, intensity of I1 emission in D3 has been improved significantly
compared to that of D1 and D2 at 0.1 mA/cm
2
. More importantly, intensity of the green emission
keeps increasing as current densities increase, contributing to a superior CRI. Indeed, the relative
intensity ratio of fluorescence to phosphorescence becomes close to 1:3 as current rises, which is
ideal for white-light with a high CRI. This 1:3 ratio is also consistent with the singlet to triplet
exciton formation ratio in quantum mechanics, suggesting that both singlet and triplet are collected
by fluorophore and phosphor effectively.
Optimized device structures and characteristics are shown in Figure 3.11 and EL properties
are summarized in Table 3.2. An ultra-thin (0.4 Å ) layer of Ir(ppy)3 is embedded at the HTL/EML
interface to slightly reduce green emission at high current densities. To reduce red emission at low
current densities, δ-doping layer of PQIr was varied as 0.4 and 0.3 Å , respectively. As depicted in
the J-V-L plot, an ultra-thin layer of PQIr does not change current densities in the devices.
However, intensity of PQIr differs depending on the thickness of PQIr, δ-doped layer ; 0.3 Å -
68

0 2 4 6 8 10
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
0.4A
0.3A
Luminescence (cd/m
2
)

Voltage (V)

0
100
200
300
400
500
600
Current density (mA/cm
2
)

10
-2
10
-1
10
0
10
1
10
2
0
2
4
6
8
10
12
14
16

0.4A
0.3A
EQE (%)
Current density (mA/cm
2
)

1 10 100 1000 10000
0
10
20
30
40
50
LPE(lm/W)
Luminance (cd/m
2
)
0.4 A
0.3 A

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
0.4 A PQIr
0.1 mA/cm
2
1 mA/cm
2
10 mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0 0.1 mA/cm
2
1 mA/cm
2
10mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
0.3 A PQIr

Figure 3. 11 Optimized WOLEDs structure utilizing I1, energy level of the devices and J-V-L plot (Top).
EQE as a function of current density and luminance power efficiency (LPE) as a function of luminance
(Middle). EL spectra (Bottom).
69

device has a higher CRI value than that of the 0.4 Å -device due to reduced red emission (Table
3.2). In general, PQIr emission is dominant at low current densities, leading to a low CRI value
and warm-white color. However, the CRI value keeps rising as current densities increase with the
highest value of CRI being76 for the 0.3 Å - device. Considering the nature of the change in exciton
profile of this device structure, the change in color quality (CRI=68 at 1mA/cm
2
to CRI=76 at 100
mA/cm
2
) is not significant while maintaining reasonably high CRI values. EQEmax =14.2% and
LPEmax =50 lm/W have been achieved with 0.3 Å -device, which are higher values than the ones
reported in other groups with phenanthro[9,10-d]imidazoles derivatives as emitters in the hybrid
WOLED application.
15-18
However, it is noteworthy that EQEmax and LPEmax values are not highest
Table 3. 2 Commission internationale de leclairage (CIE), Color Correlated Temperature (CCT) and CRI
values of optimized WOLEDs at given current densities.
Current density CIE CCT CRI
0.4 Å
0.1 mA/cm
2

(0.50, 0.47) 2612 K 57
0.3 Å (0.49, 0.47) 2802 K 62
0.4 Å
1 mA/cm
2

(0.48, 0.46) 2732 K 65
0.3 Å (0.47, 0.47) 2977 K 68
0.4 Å
10 mA/cm
2

(0.45, 0.44) 3049 K 71
0.3 Å (0.43, 0.45) 3471 K 73
0.4 Å
100 mA/cm
2

(0.43, 0.41) 3271 K 73
0.3 Å (0.39, 0.40) 3831 K 76

among the hybrid WOLEDs reported,
6, 19
possibly due to the usage of δ-doping technique. The
ultra-thin layer of PQIr (0.3 Å ) may have a higher concentrated triplet density compared to the one
in the conventional doping-PHOLED where the excitons spread out within the host materials.
Thus, the δ-doped device may have a higher probability of the 2
nd
order process such as TTA and
TPA, causing steep efficiency roll-off compared to conventional OLEDs.
20
Typically,
70

conventionally doped-PHOLEDs have relatively reduced efficiency roll-off. In other words, even
though the devices shown in Figure 3.11 demonstrate reasonable EQE and LPE, improvement in
efficiency roll-off should be followed and this subject will be discussed in the following chapter.
3.3.Conclusions
In this work, we demonstrated the hybrid WOLED concept utilizing phenanthro[9,10-
d]imidazoles derivative (I1) as a neat blue-emitter and Ir(ppy)3 and PQIr as phosphors.
Requirements for neat blue-emitters/host in hybrid WOLED are possessing high quantum yield as
solid state and higher triplet energy than that of green phosphor. Among I1-I6, I1 and I4-6
displayed high quantum yields (> 0.8) in their solid states, making them good candidates for the
host material. However, only the film comprised of Ir(ppy)3 doped at 10 wt% into I1 provided high
PL quantum yield (0.65) while the rest of the films showed lower values (< 0.35). Thus, I1 was
chosen for hybrid WOLED fabrication.
Color quantity of the white light, which can be quantified by CRI is also an important factor
in hybrid WOLED. Typically, WOLEDs with CRI values higher than 80 are considered good at
color rendering. Probing exciton profiles in the EML is essential to achieve a high CRI value
during the device operation and also significant device efficiency. The ultra-thin layer (~1 Å ) of
the sensitizer, PQIr was scattered at the HTL/EML, middle of the EML and the ETL/EML interface
to probe the exciton profile. Relative intensity of PQIr revealed that recombination zone tends to
be formed on the EML/ETL interface at low voltage while it shifts to the HTL/EML side as voltage
rises. Based on this profile, optimized hybrid WOLED devices achieved great performances
including moderate CRI (68), warm white emission (CCT = 2900K), high LPE (50 lm/W) and
high EQE (14.2%) at low current densities. As current densities increased, CRI increased up to 76
with a higher CCT (3831K). However, efficiency roll-off at high current density is steep
71

presumably due to the drastic saturation of excitons occurring on the ultra-thin layers of PQIr and
Ir(ppy)3. Since HOMO/LUMO energy levels of both Ir(ppy)3 and PQIr are nested by I1,
conventional doping could not be employed because it would capture a majority of the charges,
leading to an inferior CRI. Therefore, a new type of host materials that do not capture charges may
be needed to adapt the conventional doping technique and in turn, solve the efficiency roll-off.
This subject will be further discussed in the following section.
3.4. Experimental Methods
Refer to the experimental methods section of chapter 2 for photophysical characterizations,
OLEDs fabrication and characterizations.
Measurements of exciton profiles

An ultra-thin layer (0.5-1Å ) of (iridium (III) bis(2-phenylquinolyl-N, C2’) acetylacetonate
(PQIr) (i.e., δ-doped PQIr) was inserted in the emissive layer region to estimate the relative exciton
profile. Emission spectrum and EQEs at different current densities were used to calculate exciton
profile. Exciton profile at a given position is proportional to EQE and the integrated region of PQIr
emission, but inversely proportional to the integrated area of the entire EL.
72

𝑁 (𝑥 )~𝐸𝑄𝐸 (𝑥 )∙
𝐼 𝑃𝑄 𝐼𝑟
(𝑥 )
𝐼𝑎𝑙𝑙 (𝑥 )

N(𝑥 ) is a relative exciton concentration at position 𝑥 , IPQIr is an integrated region of PQIr
emission, Iall is an integrated area of entire EL. It is worth noting that outcoupling factor should be
also considered to calculate accurate exciton profile since it affects intensities of electro-
luminescence but it was omitted in this study for the simplicity.
3.5. Chapter 3 references
1. Reineke, S.; Thomschke, M.; Lussem, B.; Leo, K., White organic light-emitting diodes: Status
and perspective. Reviews of Modern Physics 2013, 85 (3), 1245-1293.
2. Adamovich, V.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. I.; D'Andrade, B.
W.; Adachi, C.; Forrest, S. R.; Thompson, M. E., High efficiency single dopant white
electrophosphorescent light emitting diodes. New Journal of Chemistry 2002, 26 (9), 1171-1178.
3. Reineke, S.; Walzer, K.; Leo, K., Triplet-exciton quenching in organic phosphorescent light-
emitting diodes with Ir-based emitters. Physical Review B 2007, 75 (12).
4. Baldo, M. A.; Adachi, C.; Forrest, S. R., Transient analysis of organic electrophosphorescence.
II. Transient analysis of triplet-triplet annihilation. Physical Review B 2000, 62 (16), 10967-10977.
5. Dai, Y. F.; Zhang, H. M.; Zhang, Z. Q.; Liu, Y. P.; Chen, J. S.; Ma, D. G., Highly efficient and
stable tandem organic light-emitting devices based on HAT-CN/HAT-CN:TAPC/TAPC as a charge
generation layer. Journal of Materials Chemistry C 2015, 3 (26), 6809-6814.
6. Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R., Management of
singlet and triplet excitons for efficient white organic light-emitting devices. Nature 2006, 440 (7086),
908-912.
7. Baldo, M. A.; O'Brien, D. F.; Thompson, M. E.; Forrest, S. R., Excitonic singlet-triplet ratio in a
semiconducting organic thin film. Physical Review B 1999, 60 (20), 14422-14428.
8. Schwartz, G.; Pfeiffer, M.; Reineke, S.; Walzer, K.; Leo, K., Harvesting triplet excitons from
fluorescent blue emitters in white organic light-emitting diodes. Advanced Materials 2007, 19 (21), 3672-
+.
9. Rosenow, T. C.; Furno, M.; Reineke, S.; Olthof, S.; Lussem, B.; Leo, K., Highly efficient
white organic light-emitting diodes based on fluorescent blue emitters. Journal of Applied Physics 2010,
108 (11).
10. Hofmann, S.; Hummert, M.; Scholz, R.; Luschtinetz, R.; Murawski, C.; Will, P. A.;
Hintschich, S. I.; Alex, J.; Jankus, V.; Monkman, A. P.; Lussem, B.; Leo, K.; Gather, M. C.,
Engineering Blue Fluorescent Bulk Emitters for OLEDs: Triplet Harvesting by Green Phosphors.
Chemistry of Materials 2014, 26 (7), 2414-2426.
11. Zhang, Y. F.; Lee, J.; Forrest, S. R., Tenfold increase in the lifetime of blue phosphorescent
organic light-emitting diodes. Nature Communications 2014, 5.
12. Lee, J.; Jeong, C.; Batagoda, T.; Coburn, C.; Thompson, M. E.; Forrest, S. R., Hot excited state
management for long-lived blue phosphorescent organic light-emitting diodes. Nature Communications
2017, 8.
73

13. Coburn, C.; Forrest, S. R., Effects of Charge Balance and Exciton Confinement on the
Operational Lifetime of Blue Phosphorescent Organic Light-Emitting Diodes. Phys. Rev. Appl. 2017, 7
(4), 041002.
14. Wu, S. F.; Li, S. H.; Sun, Q.; Huang, C. C.; Fung, M. K., Highly Efficient White Organic Light-
Emitting Diodes with Ultrathin Emissive Layers and a Spacer-Free Structure. Scientific Reports 2016, 6.
15. Qin, W.; Yang, Z.; Jiang, Y.; Lam, J. W. Y.; Liang, G.; Kwok, H. S.; Tang, B. Z.,
Construction of Efficient Deep Blue Aggregation-Induced Emission Luminogen from Triphenylethene for
Nondoped Organic Light-Emitting Diodes. Chem. Mater. 2015, 27 (11), 3892-3901.
16. Chen, W.-C.; Zhu, Z.-L.; Lee, C.-S., Organic Light-Emitting Diodes Based on Imidazole
Semiconductors. Adv. Opt. Mater. 2018, 6 (18), 1800258.
17. Song, W.; Shi, L.; Gao, L.; Hu, P.; Mu, H.; Xia, Z.; Huang, J.; Su, J., [1,2,4]Triazolo[1,5-
a]pyridine as Building Blocks for Universal Host Materials for High-Performance Red, Green, Blue and
White Phosphorescent Organic Light-Emitting Devices. ACS Applied Materials & Interfaces 2018, 10
(6), 5714-5722.
18. Li, X.-L.; Ouyang, X.; Chen, D.; Cai, X.; Liu, M.; Ge, Z.; Cao, Y.; Su, S.-J., Highly efficient
blue and warm white organic light-emitting diodes with a simplified structure. Nanotechnology 2016, 27
(12), 124001.
19. Sun, N.; Wang, Q.; Zhao, Y.; Chen, Y.; Yang, D.; Zhao, F.; Chen, J.; Ma, D., High-
Performance Hybrid White Organic Light-Emitting Devices without Interlayer between Fluorescent and
Phosphorescent Emissive Regions. Adv. Mater. 2014, 26 (10), 1617-1621.
20. Wu, S.; Li, S.; Sun, Q.; Huang, C.; Fung, M.-K., Highly Efficient White Organic Light-
Emitting Diodes with Ultrathin Emissive Layers and a Spacer-Free Structure. Scientific Reports 2016, 6
(1), 25821.

74

Chapter 4. Fabrication of hybrid Organic Light Emitting Diodes
(OLEDs) utilizing corannulene derivatives as a host
4.1. Introduction
4.1.1. Problem of hybrid WOLEDs
Even though an efficient hybrid WOLED is successfully fabricated using an imidazole-
based material, a few constraints still exist: PLQY of I1:Ir(ppy)3-doped film is not unity,
suggesting fractional exciton losses. Moreover, severe efficiency roll-off happens at high current
density supposedly due to the δ-doped layer of phosphors. As explained in chapter 1, heavily
concentrated triplets within an ultra-thin layer are prone to cause TTA and TPA, which induce
severe efficiency roll-off at high current density. Therefore, conventional doping of phosphors
throughout the EML in hybrid WOLEDs is more beneficial to lighten the efficiency-roll off caused
by TTA and TPA. However, a hybrid WOLED with I1 as a neat emitter cannot introduce the
conventional doping system because excitons are directly formed on the phosphor, saturating the
phosphor emission. For example, Figure 4.1 illustrates the comparisons between the δ-doping and
the conventional doping system in the hybrid I1-based WOLEDs. The device structure with the δ-
doping technique is the same as the optimized one in the chapter 3, whereas PQIr and Ir(ppy)3 are
lightly doped into I1 for the conventional doping device architecture. As shown in Figure 4.1,
relative intensity of the PQIr emissions at given current densities are more intense for the
conventionally doped OLEDs even though PQIr is mildly doped (2 vol%). This is because holes
and electrons are carried by PQIr instead of I1, considering the HOMO/LUMOs are nested by that
of I1. That is, the emission caused by direct charge trapping on the phosphor is not avoidable in
this device structure. If PQIr is heavily doped for reducing the efficiency roll-off, then the red
75

0 2 4 6 8 10
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
Conventional doping
d-doping
Luminescence (cd/m
2
)

Voltage (V)

0
100
200
300
400
Current density (mA/cm
2
)
10
-2
10
-1
10
0
10
1
10
2
10
3
0
2
4
6
8
10
12
14
16
Conventional doping
d-doping

EQE (%)
Current density (mA/cm
2
)

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Conventional doping
0.1 mA/cm
2
1 mA/cm
2
10 mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0 0.1 mA/cm
2
1 mA/cm
2
10mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
d-doping

Figure 4. 1 Performance comparison between conventional and δ-doping WOLEDs.
emission will become stronger, deteriorating the color quality of the white light. That being said,
a new red-emissive dopant with a shallower LUMO and a deeper HOMO than that of the host’s is
required. Alternatively, the host with a deeper LUMO and a shallower HOMO than that of the red
emissive phosphor’s is in need. In this chapter, the latter scenario will be discussed.
76

4.1.2. Stacked hybrid White OLEDs
Not only a new host material, but also a new device architecture has been applied to
eliminate the difficulty in synthesizing host materials for green, red emissive phosphors. Figure
4.2 illustrates the energy levels of the previous (top) and a new (bottom) hybrid WOLED structure.
Aza-boron-dipyridylmethenes (α-aD) was introduced as a blue emissive fluorophore due to its
high PLQY (>80 %) with the appropriate emission wavelength (λmax= 430 nm). The previous
hybrid WOLED structure using α-aD requires the host material with a higher triplet energy than
that of the green phosphor, i.g., Ir(ppy)3 (ET1=2.50 eV) and a low triplet energy than that of α-aD

Figure 4. 2 Photophysical properties of blue fluorophore, α-aD in 2-MeTHF and energy diagram of non-
stacked hybrid WOLEDs structure (Top). Stacked hybrid WOLED device structures and their energy
diagram (Bottom).
77

(ET1=2.56 eV) to promote the efficient triplet transfer from the host to the phosphor without the
exciton loss. This small triplet energy window for the host (2.50 < ET1, host< 2.56 eV) makes
synthesis challenging especially considering a deeper LUMO/a shallower HOMO than that of the
red emissive phosphor is in need. In the new structure however, the blue-emitting fluorophore and
the red-emitting phosphor are in the same layer while the green-emitting phosphor is in another
layer. These two emissive layers are separated by a charge generation layer (CGL), which makes
this structure as a “stacked WOLED”. This new structure erases the synthetical challenges of
making a blue fluorophore, satisfying all tricky requirements. This is because the designing of new
host material with a higher singlet energy than that of the blue-fluorophore’s and a triplet energy
than that of the red-phosphor’s is relatively easy. Furthermore, a triplet quenching by the host is
not a concern anymore because the energy on the host will be efficiently transferred to a low-lying
triplet energy of the red-emitting phosphor.
4.1.3. Stacked hybrid White OLEDs
A charge generation layer is the unit that creates charges in the stacked hybrid WOLEDs.
Several benefits of employing the CGL in the Stacked OLEDs are known.
1
First, tuning the color
of the white emission is relatively easy because emission layers are separated. Second, thanks to
the extra charges generated from the CGL, higher luminance at a given current density can be
achievable compared to the non-stacked device. In turn, an efficiency roll-off, which occurs at
high current density can be diminished. Diverse CGLs including metal-metal, metal oxide,
organic-metal and p/n doping organic bilayer types have been reported in the OLED field.
2, 3

Metal-related CGLs are not ideal due to a high thermal evaporation temperature of metals, which
is inimical to organic layers. On the other hand, p-type/n-type CGLs are widely investigated in the
OLED field without causing a damaging issue to the organic layer. Among them, HATCN is a
78

well-known n-type material due to the deep LUMO energy. The molecular structure of HATCN
is shown in Figure 4.3. A combination with p-type materials such as TAPC has been well studied
as a working CGL in the field.
2, 4
Herein, the functionality of HATCN/m- 4,4',4''-Tris[phenyl(m-
tolyl)amino]triphenylamine (MTDATA) as a CGL is investigated to apply into the new hybrid
stacked WOLED structure. Four different device architecture and characteristics of the CGL are
presented in Table 4.1 and Figure 4.3. Device 1 did not show any current flow under forward bias
because TAPC (HTL), Bphen (ETL) were inserted next to the cathode and anode, respectively, to
Table 4. 1 Device architectures to investigate functionality of HATCN/m-MTDATA as a CGL.
Devices Layer structures
Device 1 ITO/Bphen(40 nm)/TAPC(60 nm)/Al(100 nm)
Device 2 ITO/Bphen(40 nm)/HATCN(15 nm)/m-MTDATA(15 nm)/TAPC(30
nm)/Al(100 nm)
Device 3 ITO/Bphen(40 nm)/LiF(1 nm)/Al(1 nm)/HATCN(15 nm)/m-MTDATA(15
nm)/TAPC(30 nm)/Al(100 nm)
Device 4 ITO/Bphen(40 nm)/LiF(1 nm)/Al(1 nm)/HATCN(15 nm)/HATCN:m-
MTDATA 2:1 mixing(15 nm)/TAPC(30 nm)/Al(100 nm)

Figure 4. 3 a. J-V characteristic curve for device 1-4. Inset is the molecular structures of m-MTDATA
and HATCN. b. HOMO/LUMO levels of material used with flow of charges.
prevent the injection of the charges from the electrodes. The energy barrier between the HOMO
of Bphen and ITO is significant (~2 eV), impeding the hole injection from an ITO to the Bphen
layer. Likewise, the difference between the work function of an aluminum and a LUMO of TAPC
79

is compelling, preventing the electron injection to the TAPC layer. HATCN/m-MTDATA as a
CGL is inserted in device 2 but no current flow is detected, indicating the absence of the charge
generation in device 2. This can be attributed to the large energy gap between HATCN and Bphen,
suggesting that generated electrons in HATCN/m-MTDATA CGL cannot be efficiently injected
from HATCN to Bphen. A current flow is detected when a LiF/Al is introduced between Bphen
and the CGL layer due to the improvement in electron injections (Device 3). Furthermore, a mixed
layer of HATCN:m-MTDATA creates charges more efficiently than the HATCN/m-MTDATA
layer, showing the intense current flow in device 4. The HATCN:m-MTDATA unit is further
tested in the actual stacked WOLED to verify the functionality of the CGL. As shown in Table
4.2, monochromatic devices with Ir(ppy)3 and PQIr are fabricated to compare the non-stacked
devices to the stacked device. Characteristics of the three devices are summarized in Figure 4.4.
Figure 4.4 indicates that EQEs of the stacked devices are proportional to the sum of the non-
Table 4. 2 Device structure of non-stacked and stacked device.
Device Device structure with thickness (nm)
Non-stacked Ir(ppy)3
device
TAPC(20)/CBP:Ir(ppy)3 10%
(15)/Bphen(30)/LiF(1)/Al(100)
Non-stacked PQIr
device
TAPC(20)/CBP:PQIr 10% (15)/Bphen(30)/LiF(1)/Al(100)
Stacked device
TAPC(20)/CBP:Ir(ppy)3 10% (15)/Bphen(30)/LiF(1) /Al(1)
/HATCN:m-TDATA 2:1 (15)/TAPC(20)/
Bphen(30)/LiF(1)/Al(100)

stacked devices, proving that the CGL functions normally. Driving voltages of the stacked devices
are double the non-stacked devices, corresponding to the characteristics of the stacked-devices.
The EQE-L plot shows that EQE of the stacked device is equal to the sum of the non-stacked PQIr
and Ir(ppy)3 devices, suggesting that charges are generated in the CGL. A luminance of the stacked
device at a given current density is higher than that of the non-stacked devices, also suggesting
80

that charges are created in the CGL. The current efficiency (cd/A) of the stacked device is similar
to the sum of the non-stacked devices because light emits from the two layers in the stacked
devices. It is noteworthy that the non-stacked monochromatic devices have extremely low EQEs,
even though PLQYs of Ir(ppy)3 and PQIr are typically close to unity, possibly due to impurities.
However, the brightness and EQE of the stacked device are proportional to that of the individual
monochromatic devices, demonstrating the functionality of HATCN:m-MTDATA as a CGL.
0 2 4 6 8 10
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
Ir(ppy)
3
PQIr
Stacked
Luminescence (cd/m
2
)

Voltage (V)

0
100
200
300
400
500
600
700
800
Current density (mA/cm
2
)

10 100 1000
0
2
4
6
8
10
Open:Current efficiency
Closed:EQE
EQE (%)
Luminance (cd/m
2
)
Ir(ppy)
3
PQIr
Stacked
1
10
100
Current efficiency (cd/A)
1 10 100
0
2
4
6
8
Voltage (V)
Current density (mA/cm
2
)
1
10
100
1000
10000
100000
Ir(ppy)
3
PQIr
Stacked
Luminance (cd/m
2
)

500 600 700
0.0
0.2
0.4
0.6
0.8
1.0 0.1 mA/cm
2
1 mA/cm
2
10mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)

Figure 4. 4 Device performances of non-stacked and stacked devices.

81

4.1.4. New Host Material for Proposed Stacked hybrid WOLEDs
One of the candidates meeting the requirements for the new host material is corannulene
derivatives. Corannulene has been proposed for many promising applications such as nanotubes
5,6

and so on, but there have been no reports on the usage of corannulene derivatives in OLEDs. The
molecular structure of corannulene with its absorption and emission spectra in the mixture of
methylcyclohexane and isopentane (MP, 3:1 v/v) is shown in Figure 4.5. Corannulene is the
smallest fragment of C60 that exhibits the appropriate singlet (3.1 eV) and triplet (2.5 eV) energy
to be a host material for both the fluorescent and green-emissive phosphorescent dyes.
7

Additionally, the LUMO energy of the corannulene is also low-lying, accepting up to four
electrons. As shown in Figure 4.5b, a new host material should possess a deep LUMO, carrying
the electrons to prevent a direct charge trapping on the PQIr. However, corannulene has been
barely used in OLEDs because corannulene itself is too volatile, which is detrimental to the device
application. In order to increase a molecular weight, the xylyl moiety is attached to the corannulene
(Xylyl-corannulene), expecting no change in photo-physical properties.

Figure 4. 5 a. Molecular structure of corannulene and its photophysical properties in mixture of
methylcyclohexane and isopentane (3:1 v/v, MP). Absorption and emission spectra of corannulene was
obtained from reference.
7
b. HOMO/LUMO of I1, PQIr and new host.
82

4.2. Results and Discussion
4.2.1. Synthesis
Synthetic route of xylyl-corannulene (X-cor) is provided in Figure 4.6.

Figure 4. 6 Synthetic route of xylyl-corannulene
4.2.1. Photophysical Characterizations
The photophysical properties of corannulene and x-cor are presented in Figure 4.7. The
absorption and emission spectra of two materials in MeTHF at RT and 77K are nearly identical,
supporting that attaching a xylyl group on corannulene does change photophysical properties.
PLQYs of corannulene and x-cor were 0.05 (τ=7.6 ns) and 0.07 (τ=10 ns), respectively. The
emission spectra of x-cor in its solid state, which is close to the environment of the film in OLEDs,
reveal that the singlet energy of x-cor is 3.2 eV. The gated PL spectra of corannulene at 77K
suggests that the triplet energy of x-cor is 2.5 eV. In other words, x-cor can be a host material for
both the blue-emitting fluorescent and the red-emitting phosphorescent dyes at the same time.

83

300 350 400 450
0.0
0.2
0.4
0.6
In MeTHF
Corannulene
Xylyl-Corannulene
Absorbance (a.u.)
Wavelength (nm)

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Corannulene @298K
Xylyl-Corannulene @298K
Corannulene @77K
Xylyl-Corannulene @77K
Intensity (a.u.)
Wavelength (nm)
In MeTHF
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
476 nm
492 nm
Corannulene
Xylyl-Corannuelene
Intensity (a.u.)
Wavelength (nm)
Gated, in MeTHF @77K

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Corannulene
Xylyl-Corannulene
Corannulene Gated
Xylyl-Corannulene Gated
Intensity (a.u.)
Wavelength (nm)
Solid @77K

Figure 4. 7 Absorption spectrum of corannulene (Black) and xylyl-corannulene (Red) and emission
spectrum of corannulene and xylyl-corannulene in MeTHF solution with different temperatures (Top).
Emission spectrum of corannulene and xylyl-corannulene in solid state with different temperatures and
gated emission spectrum of corannulene and xylyl-corannulene in solid state at 77K (Bottom)
4.2.2. Electrochemistry and DFT calculations
To calculate the HOMO/LUMO levels of x-cor, cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) of x-cor were measured in acetonitrile (Figure 4.8) The oxidation and
reduction potentials of x-cor were 1.4, -2.3 V (vs ferrocene), respectively. The HOMO/LUMOs
were estimated as -6.4 eV and -2.2 eV, respectively using the equation reported in the literature
(Table 4.3).
8
These experimental values are moderately far from their DFT calculation values (-
84

5.9/-1.6 eV). The LUMO of x-cor and PQIr are very close (-2.2 and -2.3 eV respectively),
encouraging the possibility of the electron transportation via xylyl-corannulene.
-2 -1 0 1 2
-100
-50
0
50
100
150
Current (mA)
Potential vs DcFc/DcFc
+
(V)
Fc/Fc
+
ox
1
red
1
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-120
-100
-80
-60
-40
-20
0
20
40
60
DI(F-R) (mA)
Potential vs Fc/Fc
+
(V)
red
1
ox
1
Fc/Fc
+

Figure 4. 8 Cyclic Voltammetry and Differential Pulse Voltammetry of xylyl-corannulene

Table 4. 3 DFT calculations of xylyl corannulene and experimental HOMO/LUMO levels. The values
were derived by using the equation shown in reference
8
.
DFT calculation Experimental values
HOMO

-5.9 eV
-6.4 eV
LUMO

-1.6 eV
-2.2 eV

85

4.2.3. Quenching studies
In order to figure out whether corannulene quenches the singlet of α-aD and the triplet of
PQIr, stern-volmer
9
studies have been conducted as shown in Figure 4.9. First, the absorptions
and photoluminescence of corannulene, α-aD and α-aD:corannulene 1:8 mixed solution have been
measured. α-aD has a characteristic absorption of around 410 and 420 nm, which is absent in
corannulene. The intensities of the absorptions α-aD only and α-aD:corannulene 1:8 mixture at
410 and 420 nm are identical, indicating that the amount of α-aD is the same for both solutions.
Photoluminescence intensities for both α-aD only and α-aD:corannulene 1:8 mixture are identical,
proving that the fluorescence of α-aD is not quenched upon adding corannulene. In other words,
the singlet energy of corannulene is higher than the singlet energy of α-aD. Lifetime (3 ns) and
quantum yield (58%) of the two solutions remain constant, further validating that corannulene can
be a host for α-aD. Corannulene also does not quench the PQIr emission because the lifetime (1.7
µs) and quantum yield (67 %) are consistent for PQIr-only, and PQIr:corannulene 1:4 mixed
solution. Additionally, the PL intensities of two solutions are the same, further demonstrating that
the triplet energy of corannulene is higher than the triplet energy of PQIr. These results confirm
that the xylyl moiety attached to the corannulene increases the molecular weight while maintaining
the photo-physical properties of the corannulene.
It is noteworthy that photo-physics of materials varies depending on the matrices (i.g.,
solution vs solid state). To verify that the singlet and triplet energy of xylyl corannulene are higher
that of α-aD and PQIr in device application, four films were prepared as shown in Figure 4.10. α-
aD and PQIr were doped into poly (methyl methacrylate) (PMMA) and x-cor at 1 wt% for α-aD
and 10 wt% for PQIr. Overall, the absorption and emission in the xylyl-corannulene matrix show
86

a bathochromic shift compared to a PMMA film. This might be because xylyl-corannulene is more
polar than PMMA. As summarized in Table 4.4, PLQYs of the PQIr-doped films in PMMA

Figure 4. 9 Stun-Volmer studies of xylyl-corannulene. Absorption (Left) and emission (Right) spectra
are provided respectively.
(0.26) and xylyl-corannulene (0.27) are constant, indicating that PQIr is not quenched by the host.
However, PLQY of the PQIr 10wt%-doped film should be higher than 0.26, because PQIr is highly
emissive. This low PLQY might result from the usage of contaminated PQIr instead of a pure one.
However, we can still infer from the constant PLQY values that corannulene does not quench the
triplet of PQIr. In the same context, α-aD-doped films at 1 wt% in PMMA and xylyl corannulene
show PLQY of 0.65 and 0.55, respectively, which is within the experimental error. These stern-
volmer and films studies corroborate that x-cor can act as a host for α-aD and PQIr in OLEDs.
87

The further usages of x-cor has been expedited by doping green-phosphor bis[2-(2-
pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) (Ir(ppy)2acac) and yellow-phoshpor Bis(2-
benzo[b]thiophen-2-ylpyridine)(acetylacetonate)iridium(III) (Ir(bt)2acac). Ir(ppy)2acac and
Ir(bt)2acac were doped into mCBP and x-cor at a 10 vol% doping concentration and their
photophysical properties are summarized in Figure 4.11 and Table 4.5. The PLQYs and lifetimes

Figure 4. 10 a. Absorption spectra of films, b. PL of 1 wt% α-aD doped films, c. PL of 10 wt% PQIr
doped films
Table 4. 4 Quantum yields and lifetimes of the four different doped films.
Φ τ
1 wt% α-aD
in PMMA 0.65 3.2 ns
in xylyl-corannulene 0.54 2.9 ns
10 wt% PQIr
in PMMA 0.26 1.7 μs
in xylyl-corannulene 0.27 1.7 μs

are 0.90 and 1.3 μs, respectively when Ir(ppy)2acac was embedded in mCBP (ET1,mCBP=2.87 eV).
On the contrary, the PLQY of Ir(ppy)2acac dropped significantly (0.1) in x-cor, suggesting that the
triplet excitons are lost through x-cor. However, PLQYs and lifetimes remain constant when
Ir(bt)2acac are doped into mCBP and x-cor, verifying that the triplet energy of x-cor is higher than
that of Ir(bt)2acac. These results suggest that not only a stacked WOLED but also a non-stacked
88

2-color hybrid WOLED (B, Y) can be fabricated employing x-cor and Ir(bt)2acac as the host and
the dopant, respectively.

300 400 500 600 700
0.0
0.2
0.4
0.6
0.8
Ir(ppy)
2
acac
mCBP
X-corannulene
Absorbance (a.u.)
Wavelength (nm)
mCBP x-cor
0.0
0.2
0.4
0.6
0.8
1.0
Normalized intensity (a.u.)

300 400 500 600 700
0.0
0.2
0.4
0.6
0.8
mCBP
Ir(bt)
2
acac
mCBP
X-corannulene
Absorbance (a.u.)
Wavelength (nm)
x-cor
0.0
0.2
0.4
0.6
0.8
1.0
Normalized intensity (a.u.)

Figure 4. 11 Absorption and emission spectra of four doped films. Ir(ppy) 2acac and Ir(bt) 2acac were used
as emitters.
Table 4. 5 Quantum yields and lifetime of four doped films.
Films Φ τ (μs)
mCBP:Ir(ppy)2acac 10% 0.9 1.3
X-corannulene:Ir(ppy)2acac 10% 0.1 Long component
mCBP:Ir(bt)2acac 10% 0.87 1.2
X-corannulene:Ir(bt)2acac 10% 0.80 1.2

4.2.4. OLEDs optimization
After checking the validity of x-cor as a host material in the solution and the film,
monochromatic devices have been fabricated. The device structures are molybdenum oxide
(MoOx) 2 nm/ TAPC 40 nm/ EMLs 25 nm/ Bphen 40 nm/ LiQ 1 nm /Al 100nm. The EMLs consist
of α-aD doped into mCBP and x-cor at 10 vol%. The energy levels and characteristics of the
89

devices are presented in Figure 4.12. A current density as a function of voltages demonstrates
that a x-cor-based devices is more conductive than a mCBP-based device at given voltages. The
relative energy levels in the emissive layer shows that the HOMO energy of α-aD (-5.9 eV) is
shallower than the HOMO energy of both mCBP (-6.0 eV) and x-cor (-6.4 eV), implying α-aD

0 5 10 15
1E-7
1E-5
0.001
0.1
10
1000
mCBP
Xylyl-corannulene
J
OLED
(mA/cm
2
)
V
OLED
(V)

0.01 0.1 1 10 100
0
1
2
3
4
mCBP
Xylyl-corannulene
EQE (%)
Current Density (mA/cm
2
)

400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2
mCBP
Xylyl-corannulene
Normalized Intensity (a.u.)
Wavelength (nm)

Figure 4. 12 Characteristics of monochromatic devices employing α-aD as an emitter with mCBP and x-
cor hosts.
primarily carries the holes in the EML. On the contrary, the LUMO of α-aD (-2.0 eV) is deeper
than that of mCBP (-1.5 eV), but shallower than that of x-cor (-2.1 eV). Consequently, α-aD mainly
carries the electrons in a mCBP-based device, while x-cor transports the majority of electrons in
the x-cor-based device because of the deeper LUMO energy. On top of that, α-aD is lightly doped
90

in the EML (1 vol%), further supporting that the electrons move along the x-cor. Thus, the
conductivity change in these monochromatic devices may result from the different identities of the
electron carriers. EQEmax are 2.8 and 1.8 % for the mCBP- and the x-cor-devices, respectively.
The PLQYs of α-aD in the PMMA and x-cor matrices at 1 wt% are 0.65 and 0.54 (Table 4.4),
respectively. Thus, achievable theoretical EQEmax are 3.25 and 2.7%, respectively, assuming the
charge balance is 1 and the outcoupling efficiency is 0.2. The small deviation between the
experimental and the calculated-EQEmax values in x-cor-device can be attributed to the
employment of the unoptimized device structure, possibly leading to the low charge balance (<1)

0 5 10 15
1E-7
1E-5
0.001
0.1
10
1000
mCBP
Xylyl-corannulene
J
OLED
(mA/cm
2
)
V
OLED
(V)

0.01 0.1 1 10 100
0
5
10
15
20
mCBP
Xylyl-corannulene
EQE (%)
Current Density (mA/cm
2
)

400 450 500 550 600 650 700 750 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
mCBP
Xylyl-corannulene
Normalized Intensity (a.u.)
Wavelength (nm)

Figure 4. 13 Characteristics of monochromatic devices employing PQIr as an emitter with mCBP and x-
cor hosts.
91

and the outcoupling efficiency (< 0.20). Nonetheless, a reasonable EQEmax (~1.8%) and the EL
spectrum without the host emission confirm that x-cor is an appropriate host material for α-aD.
The same device structures except for an α-aD being replaced with a PQIr were created and their
characteristics are summarized in Figure 4.13. A current density at given voltage is higher for a
x-cor-based device, suggesting that the charges move differently in x-cor and mCBP. In both
devices, holes are highly likely to move along the PQIr because of its shallower HOMO level (-
5.3 eV). However, the LUMO of PQIr (-2.2 eV) is significantly deeper than that of mCBP (-1.5
eV), whereas relatively similar to that of x-cor (-2.2 eV). Therefore, PQIr chiefly carries the
electrons in the mCBP-based devices while both PQIr and x-cor can contribute to the
transportation of the electrons in the x-cor based device, considering the comparable LUMO
energies of the host and the dopant. Thus, different charge behaviors in these monochromatic
devices can be attributed to the different identities of the main electron carriers. EQEmax are 14.6
and 12.6 % for mCBP and x-cor-based devices, respectively, implying that the triplet energy
transfer from the hosts to the PQIr is complete. The EL spectra of both devices display a sole PQIr
emission, further proving the efficient energy transfer. Prior to the fabrication of hybrid 2-color
devices (B, R), the exciton profile in the EML has been estimated using the sensing technique
mentioned in chapter 3. Figure 4.14 shows the device structure, EL spectra of the devices and the
exciton profiles at various current densities. The device structures for sensitizing excitons are
TAPC 40 nm/ x-cor 20 nm /Bphen 40 nm/ LiF 1 nm/ Al 100 nm. The EMLs are composed of
either a neat x-cor (Non-sensing) or a 0.5 Å -thick PQIr layer into the x-cor at the HTL/EML
interface, the middle of the EML and the EML/ETL interface. As shown in Figure 4.14, the PQIr
emission is the most intense at the middle of the EML at low current densities, whereas it is the
92

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
No sensing
1 mA/cm
2
10 mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)

400 500 600 700
0
1
2
3
4
1 mA/cm
2
10 mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
HTL sensing

400 500 600 700
0
1
2
3
4
5
6
7
Middle sensing
1 mA/cm
2
10 mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
1 mA/cm
2
10 mA/cm
2
100 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
ETL sensing

0.1 mA/cm
2

Normalized exciton profile (a.u.)
1 mA/cm
2

10 mA/cm
2
0 5 10 15 20

Position x (nm)
100 mA/cm
2
Singlet + Triplet sensing (PQIr)

Figure 4. 14 Device structure for measuring exciton profile in the EML and EL spectra of non-sensing
(Top), HTL sensing, middle sensing (Middle) and ETL sensing devices. Exciton profile at given current
densities are calculated accordingly (Bottom).
93

weakest at the EML/ETL interface. This EL trend remains constant as the current density increases
unlike in the case of the I1-based device in chapter 3. In other words, the recombination zone tends
to form from the HTL/EML interface to the middle of the EML at all current densities (Figure
4.14). The triplet harvesting occurred by the diffusion process becomes less efficient as the
sensitizer is located far from the recombination zone. The weak PQIr emission at the EML/ETL
interface is surprising because the triplet diffusion length is typically known to be ~100 nm.
10
The
sensing results indicate that the triplet diffusion of the x-cor is not as efficient as other materials
with a long triplet diffusion length. The inefficient triplet diffusion of x-cor may be correlated with
its interesting property of dynamic bowl-to-bowl inversion. There has been numerous reports of
the bowl-to-bowl inversion in corannulene derivatives with various bowl depths, affecting their
the radiative decay behavior.
11, 12
The rate of this inversion for corannulene ranges in microsecond
regime, which is the same time scale as the radiative decays for several phosphorescent
materials.
13,14,15
Therefore, it is feasible that the excited triplet of x-cor is dissipated non-radiatively
before the energy transfer into the low-lying triplet occurs. On top of that, other secondary
molecular distortions besides bowl-to-bowl inversion, may participate in the fluxional process.
To confirm that the location of the exciton formation zone is predominantly from the
HTL/EML interface to the middle of the EML, three devices (D1-D3) were fabricated. The device
structure and characteristics are shown in Figure 4.15. D1 has a 4 nm thick x-cor spacer between
a 5 nm thick 6 vol% x-cor:PQIr layer and a 13 nm thick 1 vol% x-cor:α-aD layer. D2 has a 7 nm
neat x-cor spacer between a Bphen and a 13 nm thick 1 vol% x-cor:α-aD layer, whereas D3 has a
20 nm thick uniformly doped 1 vol% x-cor:α-aD layer. Inserting an undoped x-cor between the
fluorescent and the phosphorescent doped layer with a thickness larger than a Föster radius
(typically ~3 nm) prevents the direct energy transfer from the fluorophore to the phosphor. Both
94

D2 and D3 show the similar unnormalized EL spectra and EQEmax of 0.8%, indicating that the
majority of the exciton formation occurs within 13 nm distance from the HTL/EML interface.
Additionally, the absence of the x-cor emission in D2 implies that the charge density in the
ETL/EML interface, available for the direct exciton formation on the host, is negligible,
corresponding to the result of the exciton profile in the EML. The current densities of D1-D3 at
given voltages are the same, further suggesting that the direct charge trapping on the PQIr is
mitigated. However, an EQEmax of D1 has increased to 1.4% with the additional 0.6% emission

0 2 4 6 8 10
10
-1
10
0
10
1
10
2
10
3
10
4
D1
D2
D3
Luminescence (cd/m
2
)

Voltage (V)

0
100
200
300
400
Current density (mA/cm
2
)

10
-1
10
0
10
1
10
2
10
3
0.0
0.5
1.0
1.5
2.0

D1
D2
D3
EQE (%)
Current density (mA/cm
2
)
400 500 600 700
0
1000000
2000000
3000000
Intensity (a.u.)
Wavelength (nm)
D1
D2
D3

100 mA/cm
2

Figure 4. 15 Device structure of D1, D2, D3 and J-V-L plot (Top). EQE as a function of current density
and unnormalized EL spectra (Bottom).
95

10
-2
10
-1
10
0
10
1
10
2
0
2
4
6
8
10
12
14
16

10 nm
5 nm
2 nm
EQE (%)
Current density (mA/cm
2
)
400 500 600 700
0.0
0.4
0.8
1.2
1.6
2.0
1 mA/cm
2
Intensity (a.u.)
Wavelength (nm)
10 nm
5 nm
2 nm
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
Wavelength (nm)
1 mA/cm
2
10 mA/cm
2
100 mA/cm
2

2 nm

Figure 4. 16 Hybrid OLEDs structure and EQE as a function of current density (Top). Thickness of α-aD
doped layer varies from 2 to 10 nm. EL spectra of devices (Bottom).
coming from PQIr. The intensity of α-aD is the same for D1 and D3, whereas D3 shows the
additional PQIr emission. These results demonstrate that the triplet diffusion process dominates
over the direct charge trapping/the exciton formation on the PQIr, because the charges that are
trapped by PQIr would lead to a distinct decrease in the α-aD emission. Even though these findings
confirm that a conventional doping technique can be applied using x-cor as a host, while avoiding
the direct charge trapping on the PQIr, the device efficiency is considerably low due to the
inefficient triplet harvesting. The intensity of the PQIr emission is even weaker than that of the
emission of α-aD, further indicating that a collection of more triplets is in need to improve the
efficiency.
96

On the basis of these results, the hybrid OLEDs were fabricated by doping α-aD near the
HTL/EML interface and PQIr at the EML/ETL interface. The thickness of x-cor:α-aD 1 vol%
doped layer varied from 2 to 10 nm while the total thickness of the EML was fixed at 20 nm. The
details of the device structure and characteristics along with the energy levels are shown in Figure
4.16. NPD was used as a HTL while mCP was inserted next to the NPD layer to slow down the
hole transportation, expecting that the exciton formation occurs at the HTL/EML interface. The
normalized EL spectra at 1 mA/cm
2
shows that the relative intensity of α-aD increases as the x-
cor: α-aD doped layer becomes thinner. An EQEmax also increases from 4 to 14 % as the α-aD-
doped layer changes from 10 to 2 nm. Consequently, the advance in the device efficiency can be
attributed to the improvement in triplet harvesting by placing the phosphor near the recombination
zone. The EL spectra of 2 nm thick x-cor:α-aD-device maintains the same emission profile at
various current densities, consistent with the current density-independent exciton profile in the
EML. This constant EL differs from the observations of an all-phosphor-doped WOLED, where
the blue emission becomes bigger at a high current density due to the requirement for a high energy
excitation of the blue phosphor.
16
A fitting of the EL spectra of the 2 nm thick x-cor:α-aD-doped
layer with the individual dopants’ spectra suggests that the ratio of the emission from the
phosphorescent to the fluorescent dopants approaches 4.0, which is close to the value with the
intrinsic triplet-to-singlet exciton formation ratio (3.0) in the emissive organic materials.
17
The
intrinsic triplet-to-singlet exciton ratio and the separation of the singlet and triplet harvesting
channel lead to the high efficiency (~14%) and constant EL. However, the emission from 400 to
430 nm, which is absent in the α-aD emission, starts to grow as the current density increases,
indicating that the charges are leaking to the neighboring layers in the device. This charge leakage
may also lead to the rapid drop in EQE at low current density as shown in Figure 4.16. To
97

determine the origin of the unexpected emission in the device, two HTL/electron blocking layer
(EBL) combination has been utilized while other layers remain the same: TAPC only, TAPC/mCP.
First of all, when NPD is replaced with TAPC (TAPC/mCP), the undesired emission still existed,
meaning that the emission from 400 to 430 nm was not the fluorescence of the NPD. Interestingly,
pure emission of the α-aD was observed in the EL when TAPC was solely used as a HTL,
suggesting that mCP was the origin of the high energy emission.
Based on these observations, the optimized hybrid OLEDs were fabricated by using mCBP
and a neat layer of x-cor as an EBL and spacer, respectively. The 2 nm thick-undoped x-cor layer
between the α-aD and the PQIr was inserted to see whether it prohibits the direct energy transfer
from the fluorophore to the phosphor. The device structure and characteristics are shown in Figure
4.17. The CIE coordinates of the EL and the intensity ratio of PQIr to α-aD are summarized in
Table 4.6. As illustrated in Figure 4.17, the objective is to create the exciton formation on x-cor
located at the HTL/EML interface. Once exciton is formed on x-cor, the singlet exciton will be
Föster transferred to the α-aD due to its lower singlet energy (2.86 eV), leading to a blue
fluorescence. It is noteworthy that the 2 nm-thick neat x-cor layer would keep the direct singlet
energy transfer from the α-aD to the PQIr. The triplets residing on x-cor would not be dissipated
via the triplets of the α-aD because of its higher triplet energy (2.56 eV). Rather, it will diffuse into
the low-lying triplets of the PQIr (2.30 eV), resulting in a phosphorescence. In this scheme, full
usage of the excitons is fulfilled while maintaining the constant EL profile at various voltages, and
while minimizing the exchange energy loss. The J-V-L plot shows that the conductivity of the
devices slightly decreases when the spacer is introduced because the total thickness of the device
increases. EQEmax = 17.5, 19.5 % are accomplished for the spacer- and the non-spacer devices,
respectively, advocating that approximately 100% of the excitons contributed to the emission.
98

0 2 4 6 8 10
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
No spacer
Spacer
Luminescence (cd/m
2
)

Voltage (V)

0
50
100
Current density (mA/cm
2
)
10
-2
10
-1
10
0
10
1
10
2
0
5
10
15
20

No spacer
Spacer
EQE (%)
Current density (mA/cm
2
)
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
Wavelength (nm)
1 mA/cm
2
10 mA/cm
2
No spacer
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
Wavelength (nm)
1 mA/cm
2
10 mA/cm
2
Spacer

Figure 4. 17 Hybrid OLEDs structure and energy diagram of the devices (Top). J-V-L plot and EQE-J
curve (Middle). EL spectra at 1 and 10 mA/cm
2
respectively (Bottom).
More importantly, a displeasing emission from 400 to 430 nm no longer exists, reflecting the
absence of the charge leakage in the device. Indeed, the sharp drop in the EQE at a low current
99

density disappeared when mCP was replaced with mCBP, accompanied with a distinct EQE rise
from 14 % to 19.5 % (No spacer-devices). The EL in Figure 4.7 obviously shows the functionality
of the spacer. As shown in Table 4.6, the ratio of the phosphorescence to the fluorescence ranges
from 5.7 to 7.5 for the spacer-free device whereas it decreases to 1.5 for the device with a spacer.
That being said, the PQIr emission becomes stronger by sacrificing the α-aD emission if these two
Table 4. 6 CIE coordinates and ratio of PQIr intensity to α-aD intensity at various current densities.

No spacer Spacer
CIE 𝐼 𝑃𝑄𝐼𝑟 /𝐼 𝛼 −𝑎𝐷
CIE 𝐼 𝑃𝑄𝐼𝑟 /𝐼 𝛼 −𝑎𝐷

1 mA/cm
2
(0.53, 0.35) 7.5 (0.38, 0.23) 1.5
10 mA/cm
2
(0.52, 0.33) 5.7 (0.38, 0.23) 1.5

dopants are placed next to each other. On the contrary, the 2 nm-thick spacer between the two
dopants inhibits the direct energy transfer from the α-aD to the PQIr, retrieving a fluorescent
emission. Unfortunately, 𝐼 𝑃𝑄𝐼𝑟 /𝐼 𝛼 −𝑎𝐷
is fairly deviated from the triplet-to-singlet ratio (3.0) no
matter what the spacer is introduced or not. The partial triplet losses may exist in the device with
the spacer because 𝐼 𝑃𝑄𝐼𝑟 /𝐼 𝛼 −𝑎𝐷
is less than 3.0. This may be attributed to the short triplet diffusion
length of the x-cor, corresponding to the result of the exciton profile measurement. Indeed, an
EQEmax of 17.5 % in the spacer-device is slightly lower than 20 %, possibly due to the partial loss
of the triplets. Further optimization such as reducing the thickness of the spacer and so on would
establish a perfect separation between the singlet and the triplet harvesting channels. Nonetheless,
it is noteworthy that the CIE coordinates are constant in the two devices even though the current
density increases. Additionally, considering that no devices employing corannulene-derivatives
have been reported in literature, except for one patent,
18
this highly efficient hybrid OLED (B, R)
100

with x-cor is the first OLED that fulfill the interest in incorporating corannulene into OLED
applications.
4.3.Conclusions
Utilizing the blue fluorescence and the red/green phosphorescence is a promising strategy
to realize highly efficient WOLEDs. However, the efficiency roll-off observed at high current
densities in the hybrid WOLEDs limits their applications in various fields. The efficiency roll-off
is closely related to the 2
nd
order process such as TTA and TPA. The hybrid WOLED structure
utilized in chapter 3 contains two main issues. First, an ultra-thin layer of PQIr causes a fast
saturation of the triplets, leading to a steep EQE drop starting at a low current density. Second,
synthesizing a host material for both the green- and the red-emissive phosphors is challenging due
to the narrow triplet energy requirement window. Therefore, a new host material with the stacked
hybrid WOLED structure was introduced in this chapter. A new host material requires a higher
singlet energy than that of the blue fluorophore (i.g., α-aD) and a deeper LUMO energy than that
of the PQIr to avoid the direct charge trapping by the PQIr. Conventionally doped PQIr into a new
host would resolve the fast saturation of the triplets on the PQIr while maintaining an excellent
tunability of EL spectra.
Corannulene derivatives are promising candidates for the new host material due to the
appropriate singlet (3.1 eV), triplet (2.5 eV) and low-lying LUMO energy of the corannulene. It is
surprising that there has also been intense interest in incorporating corannulene into organic
electronic applications
19
but no literatures have been reported so far except for one patent.
18
Xylyl
moiety was attached to corannulene to increase the molecular weight while maintaining the
photophysical properties. Electrochemistry measurements reveal that the LUMO of x-cor is -2.2
eV, which is comparable to that of PQIr (-2.3 eV). Stern-volmer studies inform that the emissions
101

of α-aD and PQIr remain intact in the solution upon addition of x-cor, confirming that both α-aD
and PQIr can be hosted by x-cor. Additionally, the quantum yields of the films comprising α-aD
or PQIr doped into PMMA or x-cor are similar, further suggesting that the singlet and triplet
energies of the x-cor are higher than α-aD and PQIr.
The monochromatic devices of α-aD and PQIr in x-cor display a reasonable EQEmax of 1.8
and 12.6 %, respectively, which are comparable to that of the reference devices (2.8 and 14.8 %)
with mCBP as a host. The exciton profile in the TAPC 40 nm/ x-cor 20 nm/ Bphen 40 nm /LiF 1
nm/ Al 100 nm device indicates that the exciton formation occurs mostly from the HTL/EML
interface to the middle of the EML at various current density. The optimized device structure with
TAPC 40 nm/ mCBP 20 nm/x-cor:α-aD 1 vol% 2 nm/ x-cor 0 or 2 nm/ x-cor:PQIr 10 vol% 18
nm/ Bphen 40 nm/ LiF 1 nm/ Al 100 nm shows EQEmax =19.5 and 17.5 %, respectively, affirming
the full utilization of the excitons. In addition, the constant CIE coordinates during the device
operation imply that the singlets/triplets are collected independently to some degree. However,
𝐼 𝑃𝑄𝐼𝑟 /𝐼 𝛼 −𝑎𝐷
is moderately departed from the ideal triplet-to-singlet ratio (3.0) for both devices,
meaning that further optimization such as reducing the thickness of the spacer is required to build
the perfect separate channels for collecting the singlets/triplets. Nonetheless, this highly efficient
hybrid OLED (B, R) with x-cor is the first OLEDs employing corannulene in the OLED
applications.
18

Lastly, the function of the HATCN:m-MTDATA 2:1 mixing (15 nm) layer as a CGL has
been verified by observing an enough current flow under a forward bias in the ITO/ Bphen (40
nm)/ LiF (1 nm)/ Al (1 nm)/ HATCN (15 nm)/ HATCN:m-MTDATA 2:1 mixing (15 nm)/ TAPC
(30 nm)/ Al (100 nm) device. Therefore, the fabrication of the new stacked-hybrid WOLED,
where the CGL separates the blue/red emissive layer from the green emissive layer, is highly
102

realizable by combining the CGL unit and the 2-color (B, R) hybrid OLED with x-cor. This
stacked-hybrid WOLED expects to alleviate not only TTAs within the same phosphors (i.g., red-
red phosphor) but also between other phosphors (i.g., red-green phosphors), showing a small
efficiency roll-off. In addition, the two color non-stacked WOLED (B, Y) can be also fabricated
using x-cor and Ir(bt)2acac due to the high PLQY of x-cor:Ir(bt)2acac 10 vol% film (0.80), opening
up another application of corannulene derivatives into OLEDs.
4.4. Experimental Methods
4.4.1. Synthesis
Chemicals were purchased from commercial sources and used as received. All syntheses
were carried out in inert N2 gas atmosphere. Bromo-corannulene was prepared as reported
previously.
20

Xylyl-corannulene. A three neck flask was charged with bromo-corannulene (2.2 g, 6.68 mmol),
(2,6-dimethylphenyl)boronic acid (2.00 g, 013.37 mmol),
Tris(dibenzylideneacetone)dipalladium(0) (772.27 mg, 0.668 mmol) and 90 mL of toluene mixed
with 45 mL of DI water. A condenser was attached and the reaction was thoroughly degassed, after
which Tris(dibenzylideneacetone)dipalladium(0) was added. The reaction was then heated to
reflux for 48 hrs and then cooled to ambient temperature. The reaction was then filtered and
washed with dichloromethane and resulting filtrate was concentrated in vacuo. The crude mixture
was chromatographed in 100% hexanes to give a bright white-color solid (1.2 g, 50.6 %) Elemental
Analysis: Anal. Cacld. for C28H18: C, 94.88 %; H, 5.12 %; N, 0.00 %. Found: C, 93.77 %; H, 5.30
%; N, 0.00 %
103

4.4.2. Photophysical characterization
Photoluminescence spectra were measured using a QuantaMaster Photon Technology
International phosphorescence/fluorescence spectrofluorometer. Phosphorescent lifetimes were
measured by time-correlated single-photon counting using an IBH Fluorocube instrument
equipped with an LED excitation source. Quantum yield measurements were carried out using a
Hamamatsu C9920 system equipped with a xenon lamp, calibrated integrating sphere and model
C10027 photonic multi-channel analyzer (PMA). UV−vis spectra were recorded on a
Hewlett-Packard 4853 diode array spectrometer. The samples were deaerated by extensive
sparging with N2.
4.5. Chapter 4 references
1. Reineke, S.; Thomschke, M.; Lüssem, B.; Leo, K., White organic light-emitting diodes: Status
and perspective. Reviews of Modern Physics 2013, 85 (3), 1245-1293.
2. Dai, Y. F.; Zhang, H. M.; Zhang, Z. Q.; Liu, Y. P.; Chen, J. S.; Ma, D. G., Highly efficient and
stable tandem organic light-emitting devices based on HAT-CN/HAT-CN:TAPC/TAPC as a charge
generation layer. Journal of Materials Chemistry C 2015, 3 (26), 6809-6814.
3. Chiba, T.; Pu, Y. J.; Miyazaki, R.; Nakayama, K.; Sasabe, H.; Kido, J., Ultra-high efficiency
by multiple emission from stacked organic light-emitting devices. Organic Electronics 2011, 12 (4), 710-
715.
4. Sun, H. D.; Guo, Q. X.; Yang, D. Z.; Chen, Y. H.; Chen, J. S.; Ma, D. G., High Efficiency
Tandem Organic Light Emitting Diode Using an Organic Heterojunction as the Charge Generation Layer:
An Investigation into the Charge Generation Model and Device Performance. Acs Photonics 2015, 2 (2),
271-279.
5. Wong, B. M., Noncovalent Interactions in Supramolecular Complexes: A Study on Corannulene
and the Double Concave Buckycatcher. Journal of Computational Chemistry 2009, 30 (1), 51-56.
6. Sanyal, S.; Manna, A. K.; Pati, S. K., Functional Corannulene: Diverse Structures, Enhanced
Charge Transport, and Tunable Optoelectronic Properties. Chemphyschem 2014, 15 (5), 885-893.
7. Yamaji, M.; Takehira, K.; Mikoshiba, T.; Tojo, S.; Okada, Y.; Fujitsuka, M.; Majima, T.;
Tobita, S.; Nishimura, J., Photophysical and photochemical properties of corannulenes studied by
emission and optoacoustic measurements, laser flash photolysis and pulse radiolysis. Chem. Phys. Lett.
2006, 425 (1), 53-57.
8. Sworakowski, J., How accurate are energies of HOMO and LUMO levels in small-molecule
organic semiconductors determined from cyclic voltammetry or optical spectroscopy? Synth. Met. 2018,
235, 125-130.
9. Boaz, H.; Rollefson, G. K., The Quenching of Fluorescence. Deviations from the Stern-Volmer
Law. Journal of the American Chemical Society 1950, 72 (8), 3435-3443.
10. Baldo, M. A.; O'Brien, D. F.; Thompson, M. E.; Forrest, S. R., Excitonic singlet-triplet ratio in a
semiconducting organic thin film. Physical Review B 1999, 60 (20), 14422-14428.
104

11. Scott, L. T.; Hashemi, M. M.; Bratcher, M. S., Corannulene Bowl-to-Bowl Inversion is Rapid at
Room-Temperature. Journal of the American Chemical Society 1992, 114 (5), 1920-1921.
12. Seiders, T. J.; Baldridge, K. K.; Grube, G. H.; Siegel, J. S., Structure/energy correlation of bowl
depth and inversion barrier in corannulene derivatives: Combined experimental and quantum mechanical
analysis. Journal of the American Chemical Society 2001, 123 (4), 517-525.
13. Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E.,
Synthesis and characterization of phosphorescent cyclometalated platinum complexes. Inorganic
Chemistry 2002, 41 (12), 3055-3066.
14. Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.;
Thompson, M. E., Synthesis and characterization of facial and meridional tris-cyclometalated iridium(III)
complexes. Journal of the American Chemical Society 2003, 125 (24), 7377-7387.
15. Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J.
C.; Bau, R.; Thompson, M. E., Synthetic control of excited-state properties in cyclometalated Ir(III)
complexes using ancillary ligands. Inorg. Chem. 2005, 44 (6), 1713-1727.
16. D'Andrade, B. W.; Holmes, R. J.; Forrest, S. R., Efficient Organic Electrophosphorescent White-
Light-Emitting Device with a Triple Doped Emissive Layer. Adv. Mater. 2004, 16 (7), 624-628.
17. Baldo, M. A.; O’Brien, D. F.; Thompson, M. E.; Forrest, S. R., Excitonic singlet-triplet ratio in a
semiconducting organic thin film. Physical Review B 1999, 60 (20), 14422-14428.
18. Yamada, N.; Ueno, K.; Nishimura, J.; Okada, Y. Corannulene Compound and Organic Light-
Emitting Device. 2007.
19. Mack, J.; Vogel, P.; Jones, D.; Kaval, N.; Sutton, A., The development of corannulene-based
blue emitters. Organic & Biomolecular Chemistry 2007, 5 (15), 2448-2452.
20. Yamada, M.; Tashiro, S.; Miyake, R.; Shionoya, M., A cyclopalladated complex of corannulene
with a pyridine pendant and its columnar self-assembly. Dalton Transactions 2013, 42 (10), 3300-3303.

105

Chapter 5. Molecular Alignment of Homoleptic Iridium Phosphors
in Organic Light Emitting Diodes (OLEDs)
5.1. Introduction
Organic light emitting diodes (OLEDs) have been intensively investigated due to their
capacity to give high luminance efficiency and color quality.
1
Even though 100% internal quantum
efficiency has been achieved using phosphorescent dopants, fabrication of extremely high
efficiency OLEDs has been hampered by relatively inefficient light extraction from the device.
2, 3

Typically, 80% of photons produced by the OLED are lost due to waveguiding, absorption and
coupling to surface plasmon modes.
2, 4, 5
Several extrinsic approaches have been introduced to
reduce waveguiding and absorption losses,
6, 7
including the use of microlens arrays,
8-11
scattering
particles
12, 13
and mechanical roughening of the substrate.
14, 15
By using a combination of these
approaches, the losses of the light generated by electroluminescence drop to ca. 30%.
11
The
outcoupling can be further improved by controlling alignment of transition dipole moments
(TDMs) of the emitting molecules in emissive layer. Considering that light is primarily emitted
perpendicular to the TDM, alignment of the TDM parallel to the substrate can reduce the excitation
of waveguide, surface plasmon and lossy metal modes, while increasing the air and substrate
modes.
16-22
The degree of alignment of the TDM is given by the anisotropy factor, , which
corresponds to the ratio of the emitted power of the projection of the net TDM onto the axis
perpendicular to the substrate (p z
2
), to the sum of the total power of the light emitted:
[Θ =
𝑝 𝑧 2
(𝑝 𝑥 2
+ 𝑝 𝑦 2
+ 𝑝 𝑧 2
)
⁄
]. Thus, a complex with an isotropic TDM orientation gives  = 0.33,
whereas one with all TDMs parallel to the substrate (in the 𝑥 − 𝑦 plane) gives  = 0.
23
Bis-
106

cyclometalated Ir diketonate complexes are common emissive dopants in OLEDs that show
anisotropy factors of 0.22-0.25 in vacuum deposited films, indicating a net in-plane TDM
alignment.
24, 25
In contrast, several homoleptic tris-cyclometalated Ir complexes such as Ir(ppy)3,
are isotropic in doped films ( = 0.33).
20, 25, 26

Molecular alignment in vacuum deposited films requires that diffusion along the surface
be sufficiently rapid for molecules to find a preferred orientation before being overcoated with
additional deposited material.
27
However, the underlying molecular features needed to guide the
alignment of emissive dopants in the films is not yet completely understood. For complexes such
as rigid-rod emitters, association of the molecule with the organic surface on deposition drives a
horizontal arrangement.
3, 20, 28
For square-planar platinum-based emitters, horizontal alignment can
be induced by the orientation of the host
29
or by introducing a flat, ordered templating layer before
depositing the film.
30
For octahedral (C^N)2Ir(L^X) type complexes, Jurow et al., proposed that
the organic/vacuum interface induces reorientation of dopants due to the inherent chemical
asymmetries of the surface. Here, C^N represents a cyclometalated ligand and L^X an auxiliary
ligand.
31
Recently it has been shown that molecular alignment in a series of homoleptic Ir
complexes can be correlated to the effects of geometric anisotropy and electrostatic interactions
with the surface of a growing film.
22, 32
Thus, it should be possible to design features into a
homoleptic Ir complex that favor a specific molecular orientation capable of enhancing the
horizontal alignment of the TDM that improves outcoupling of the emitted light.
In 2014, Udagawa, et al., reported a blue-emitting OLED with an external quantum efficiency
of EQE = 30% that utilized tris(mesityl-2-phenyl-1H-imidazole)iridium [Ir(mi)3, Figure 5.1].
33

The high EQE in these OLEDs suggests that the TDMs of the Ir(mi)3 dopants are horizontally (in-
plane) aligned. Here we use angle dependent-photoluminescence spectroscopy (ADPS)
23, 34
and
107

Fourier-plane imaging microscopy (FPIM)
35
to measure the alignment of the TDMs of
fac-tris(mesityl-2-phenyl-1H-imidazole)iridium (Ir(mi)3), fac-tris(2-phenylpyridyl)iridium
(Ir(ppy)3) and several substituted derivatives of these complexes (Figure 5.1). A net horizontal
alignment of TDMs is observed for all complexes, aside from Ir(ppy)3, leading to OLEDs with
EQEs = 22.3-30.5%. In this family of emitters, both its molecular shape (i.e., deviation from
roughly spheroidal) and non-uniformity in the electrostatic surface potential (ESP) of an emitter
gives rise to a preferred dopant alignment. The non-uniformity in the ESP markedly increases upon
addition of electron withdrawing groups to the aryl groups of the cyclometalating ligands in the
organometallic complexes,
32
and is found to enhance alignment of the dopants. Complexes with
highly non-uniform ESP will hereafter be referred to as having high “chemical asymmetry”.

Figure 5. 1 The five fac-Ir(C^N) 3 complexes studied here. The three C^N ligands are equivalent in these
facial complexes. Illustrations of the three-dimensional structures of these complexes are shown with the
C 3 axis lying within and perpendicular to the plane of page.

5.2. Results and Discussion
5.2.1. Synthesis
Ir(mi)3, fac-tris(1-mesityl-2-(4-(trifluoromethyl)phenyl)-1H-imidazole)iridium (Ir(miF)3),
fac-tris((3,5-dimethyl-[1,1'-biphenyl]-4-yl)-2-phenyl-1H-imidazole)iridium (Ir(mip)3) and fac-
108

tris((3,5-dimethyl-[1,1':4',1''-terphenyl]-4-yl)-2-phenyl-1H-imidazole)iridium (Ir(mipp)3)
complexes were synthesized using modified versions of literature methods
36, 37
and shown in
Figure 5.2. Ir(ppy)3 and Ir(ppyCF3)3 were synthesized using literature moethods.
33, 37-39
All the
complexes were obtained as facial (fac) isomers and details of syntheses are discussed in
experimental section.

Figure 5. 2 Synthesis of Ir(mi) 3, Ir(miF) 3, Ir(mip) 3 and Ir(mipp) 3
5.2.2. Photophysical Characterization
The absorption spectra of Ir(mi)3, Ir(miF)3 and Ir(mip)3 are shown in Figure 5.3. All of the
three complexes show intense high energy bands (λ < 360 nm, ε > 10 mM
-1
cm
-1
), corresponding
109

to π→π* transition on the cyclometalated ligands. Broad and less intense absorption bands at low
energy (λ = 360-440 nm, ε > 2 x 10 mM
-1
cm
-1
) are assigned to Metal-to Ligand-Charge Transfer
transitions (MLCT
1
). MLCT
3
transitions are apparent at lower energy (λ> 450 nm, ε < 0.1 mM
-
1
cm
-1
) in the absorption spectra with weaker intensity due to limited spin-orbit coupling with the
single states by the iridium metal center. Absorption spectra of Ir(ppy)3 and Ir(ppyCF3)3 are
reported in the literature.
38, 40
Emission spectra of the phenylimidazole-based compounds display
sky blue luminescence, with Ir(miF)3 showing a slight bathochromic shift relative to the other two
derivatives (Figure 5.3). The Ir(ppy)3 and Ir(ppyCF3)3 complexes emit in the green, with the CF3
substituents leading to a small blue shift in emission. As summarized in Table 5.1, The complexes
have photoluminescence lifetimes (τ) in the microsecond range at both room temperature and 77K,
and high quantum yields (ΦPL > 90%). The values for τ and Φ PL observed are comparable to those
found in other homoleptic tris-cyclometalated iridium (III) complexes.
37, 41

300 350 400 450 500
0
5
10
15
20

e (mM
-1
cm
-1
)
Wavelength (nm)
Ir(mi)
3
Ir(miF)
3
Ir(mip)
3
450 500 550 600 650 700
-0.5
0.0
0.5
1.0

Normalized Intensity
Wavelength (nm)
Ir(mi)
3
Ir(miF)
3
Ir(mip)
3
Ir(ppy)
3
Ir(ppyCF
3
)
3

Figure 5. 3 Absorption (Left) and Emission (Right) spectra and photophysical parameters for the fac-
Ir(C^N) 3 complexes in 2-methyltetrahydrofuran (2-MeTHF) at room temperature.
110

Table 5. 1 Maximum emission wavelength (λmax), photoluminescence efficiency (ΦPL), lifetime
(τ), HOMO/LUMO and magnitude of permanent dipole moment of the fac-Ir(C^N)3 complexes.

λmax
(nm)
a

Φ PL
a

τ
(μs)
a

HOMO/LUMO
(eV)
b

Permanent
dipole moment
(Debye)
c

Ir(mi)3 464 0.91 2.0 -4.9/
d
6.9
Ir(mip)3 466 0.98 1.8 -4.9/-1.5 6.7
Ir(miF)3 484 0.99 2.5 -5.2/-1.5 12.7
Ir(ppy)3 512 1.0 1.2 -5.2/-1.7 6.4
Ir(ppyCF3)3 506 0.98 1.2 -5.5/-1.9 16.3
(a)
Measured in 2-MeTHF solution.
(b)
HOMO and LUMO were determined using the
electrochemical potentials as reported in reference
42
.
(c)
Calculated using DFT
(B3LYP/LACV3P**).
(d)
The reduction potential for Ir(mi)3 was not observable in
dimethylformamide (DMF) solvent.
The energies for the highest occupied (HOMO) and lowest unoccupied molecular orbital
(LUMO) were determined using solution electrochemical measurements (Table 5.1). The
presence of the electron withdrawing trifluoromethyl groups in Ir(miF)3 and Ir(ppyCF3)3 stabilize
the HOMO energies by 0.30 eV relative to their parent complexes. The permanent dipole moments
(PDM) of the complexes were calculated using density functional theory (DFT) and given in Table
5.1. All of the compounds have their PDM directed along the C3 axis, with the CF3 substitution
leading to a substantial increase in the magnitude of the PDM.
5.2.3. Electrochemistry
The redox properties of the Ir complexes studied here except for Ir(ppy)3 were investigated
by cyclic voltammetry and differential pulse voltammetry in acetonitrile solution with 0.1 M
TBAF (Figure 5.4). Electrochemistry of Ir(ppy)3 is already reported in the literature.
38
Oxidation
potentials of Ir(mi)3, Ir(mip)3 and Ir(mipp)3 are 0.07 V, 0.10 V and 0.10 V, respectively with
reversibility. These values are close to Eox1 = 0.10 V- 0.12 V which are reported with other tris-
cyclometalated phenylimidazole iridium complexes. Ir(miF)3 is more difficult to oxidize (Eox1 =
111

0.40 V) compared to others due to stabilization of HOMO by the presence of electron withdrawing
trifluoromethyl group on aryl moiety as shown in Table 5.2. Reduction potential of Ir(mi)3 could
not be measured while Ir(miF)3 and Ir(mip)3 show reduced reduction wave (Ered1 = -2.84V, -2.86V
respectively). Significant difference in reduction potentials between Ir(mi)3 and the rest can be
explained by considering distribution of LUMO. As shown in Table 5.2, first reduction of Ir(miF)3
can be assigned to phenyl imidazole ligand. Therefore, trifluoromethyl moiety on aryl group
stabilizes not only the HOMO but also the LUMO of Ir(miF)3, leading to a smaller reduction
potential. On the other hand, LUMO of Ir(mi)3 and Ir(mip)3 are mainly located on mesityl and
phenyl xylyl ligand respectively. The second phenyl group attached to mesityl imidazole ligand
allows for the electrons to delocalize across extended xylyl phenyl backbone of the ligand,
resulting in stabilization of the LUMO of Ir(mip)3.
Table 5. 2 HOMO, LUMO and triplet density distribution of phenylimidazole-based complexes
HOMO LUMO Triplet density
Ir(mi)3

Ir(miF)3

Ir(mip)3

Ir(mipp)3

112

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-10
-5
0
5
10
15
20
25
30
ox
1
a.
Current (mA)
Potential vs DcFc/DcFc
+
(V)
DcFc/DcFc
+

-3 -2 -1 0 1
-20
0
20
40
60
80
100
DI(F-R) (mA)
Potential vs DcFc/DcFc
+
(V)
DcFc/DcFc
+
ox
1

-3 -2 -1 0 1
-100
-80
-60
-40
-20
0
20
b.
Current (mA)
Potential vs Fc/Fc
+
(V)
ox
1
Fc/Fc
+
red
1
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-25
0
25
50
75
ox
1
red
1
Fc/Fc
+
DI(F-R) (mA)
Potential vs Fc/Fc
+
(V)

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-25
-20
-15
-10
-5
0
5
10
c.
DcFc/DcFc
+
Current (mA)
Potential vs DcFc/DcFc
+
(V)
ox
1
red
1
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-20
-10
0
10
20
30
40
red
1
DcFc
+
D I(F-R) (mA)
Potential vs DcFc
+
/DcFc (V)
ox
1

-3 -2 -1 0 1 2 3
-80
-60
-40
-20
0
20
40
60
d.
red
1
ox
1
Current (mA)
Potential vs DcFc/DcFc
+
(V)
-2 -1 0 1 2
-60
-40
-20
0
20
40
ox
1
D(F-R) mA
Potential vs DcFc
+
/DcFc (V)
red
1

113

-3 -2 -1 0 1 2
-60
-40
-20
0
20
40
60
80
100
e.
Current (mA)
Potential vs Fc/Fc
+
(V)
Fc/Fc
+
-3 -2 -1 0 1 2
-40
-20
0
20
40
DI(F-R) (mA)
Potential vs Fc/Fc
+
(V)
Fc/Fc
+
-2.5
0.6

Figure 5. 4 Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) in MeCN of a.) Ir(mi) 3
in DcFc/DcFc
+
, b.) Ir(miF) 3 in Fc/Fc+, c.) Ir(mip) 3 in DcFc/DcFc
+
, d.) Ir(mipp) 3 in DcFc/DcFc
+
e.)
Ir(ppyCF 3) 3 in Fc/Fc
+
. DPV of Ir(mi) 3 was measured in DMF.
5.2.4. DFT and TD-DFT Calculations
Here we compare the spatial anisotropies of the five emissive dopants, with shapes ranging
from roughly spherical to oblate spheroidal. Space-filling models of the complexes with views
looking both along and down the C3 axis are shown in Figure 5.5. To quantify the anisotropy, the
Ir(ppCF3)3 Ir(ppy)3 Ir(miF)3 Ir(mi)3 Ir(mip)3

Aspect ratio 1.0 1.2 1.9 2.2 3.0
Figure 5. 5 Space-filling models of each fac-Ir(C^N) 3 complex with side (a) and top (b) views to
illustrate structural differences.
3D moment of inertia matrix of each complex was computed and diagonalized to yield three
eigenvalues corresponding to the dimensions along its three principal axes of an ellipsoid that
encloses the molecule (see experimental section for calculation details). The aspect ratio of the
114

molecule is defined as the ratio of the eigenvalues for the major and minor axes. The Ir(ppy)3
complex has a slightly ellipsoidal shape (aspect ratio of 1.2) due to a compression along the C3
axis. The Ir(mi)3 complex has a more oblate spheroidal shape, with an aspect ratio of 2.2.
Extending the imidazolyl ligand by appending an additional phenyl group in Ir(mip)3 increases the
aspect ratio to 3.0. Addition of CF3 groups decreases the aspect ratio relative to the parent
complexes, giving ratios for Ir(miF)3 of 1.9, and Ir(ppyCF3)3 of 1.0. In all cases, the long axis of
the oblate shape lies perpendicular to the C3 axis.

Figure 5. 6 The TDM (red arrow) of the fac-Ir(C^N) 3 complexes is in the Ir(C^N) plane, subtending an
angle δ between the TDM and the Ir-N bond. The C 3 axis gives three equivalent TDMs, with the angle α
between the TDMs and the C 3.
The molecular orientation of the luminescent complex relative to the substrate can be
established from the optical anisotropy of dopant-based films (vide infra). However, to do so the
dopant’s TDM needs to be mapped onto the molecular frame of the compound. This mapping of
the TDM of the triplet excited state for each dopant was carried out using time-dependent density
functional theory (TDDFT) with the zero-order regular approximation (ZORA) that incorporates
spin-orbit coupling.
43-46
The TDM is localized in the plane of a single Ir(C^N) moiety with the
origin on the Ir atom (Figure 5.6). The C3 axis of the Ir(C^N)3 complex leads to three such TDMs
in the molecule whose orientations lie at the angle, δ, relative to the Ir-N bond. Mapping the
115

orientation of the TDM onto the full molecular frame gives its angle, α, with the C3 axis. The
Ir(C^N)3 complexes considered here have TDMs that are nearly orthogonal to the C3 axis (α = 84-
94°). Thus, a horizontal TDM alignment is indicative a dopant oriented with the C3 axis
perpendicular to the substrate. One might speculate that slight deviation from perfect angle (α =
90°) might induce a meaningful change in anisotropy value. However, based on a mathematical
representation that relates the value of  to the angle δ for a facial octahedral complex and relation
between δ and α
31
 changes no more than 0.01 for a decrease in  from 90° to 84° . This is within the
error range of APDS measurement (0.01-0.04), indicating directions of TDMs in all compounds are close
to be in ideal condition.
5.2.5. The role of structure in dopant alignment
The three phenyl-imidazole complexes were investigated using ADPS (see experimental
section for details) in vacuum deposited films doped at 10 wt% in tris(4-carbazoyl-9-
ylphenyl)amine (TCTA) or 3,3-di(9H-carbazol-9-yl)-1,1-biphenyl (mCBP) hosts (Figures 5.7 and
5.8 respectively). Anisotropy factors measured for these films are insensitive to the host (see Table
5.3).
47
Values obtained for films doped with Ir(mi)3 (Θ = 0.26) are similar to those reported for
heteroleptic iridium complexes, and are consistent with the high EQE observed for Ir(mi)3 based
OLEDs reported by Udagawa, et al.
33
Molecular alignment in heteroleptic iridium complexes (i.e.
(ppy)2Ir(acac)) is thought to be driven by the chemical asymmetry induced by the auxiliary
ligand;
19, 26, 47-49
however, this rationale is problematic for explaining alignment of the homoleptic
Ir(mi)3 complex. We postulate instead that the oblate shape of the molecule favors van der Waals
π-π interactions between the dopant and the organic surface when the long axis of the ellipsoid is
parallel to the surface (C3 axis perpendicular).
50
Similar π -π interactions have been used to account
for the alignment of planar
30
and rigid rod-like
28, 47
dopants in vacuum deposited films. This
116

explanation predicts that the spherical shape of Ir(ppy)3 would not favor a particular TDM
orientation, consistent with the optical isotropy in doped thin films.
48, 49

0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60 70 80 90
0.0
0.2
0.4
0.6
0.8
1.0
Ir(mi)
3
:TCTA (10 vol.%)
 = 0.26 0.02
 = 0.00
 = 0.33
Normalized Intensity
Ir(miF)
3
:TCTA (10 vol.%)
 = 0.22 0.02
 = 0.15 0.04
Ir(mip)
3
:TCTA (10 vol.%)

Detector Angle ()

Figure 5. 7 ADPS measurements and simulations for films of TCTA doped with Ir(mi) 3 (Top), Ir(miF) 3
(Middle) and Ir(mip) 3 (Bottom) at 10 vol% doping ratio. The measured data have been fitted black
(Isotropic) and red (Perfectly horizontal) lines to determine the degree of orientation. Ir(mi) 3 Θ = 0.26;
Ir(miF) 3 Θ = 0.22; and Ir(mip) 3 Θ = 0.15 in TCTA respectively.
The anisotropy data for Ir(mip)3 doped films supports the hypothesis that maximizing the
π-π interaction area of the dopant and organic surface, and thus the van der Waals attractive forces,
drives horizontal alignment. The value found for Ir(mip)3 (Θ = 0.15) is one of the lowest among Ir
117

0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60 70 80 90
0.0
0.2
0.4
0.6
0.8
1.0
 = 0.00
Ir(mi)
3
:mCBP (10 vol.%)
 = 0.25 0.01
 = 0.33
Normalized Intensity
Ir(miF)
3
:mCBP (10 vol.%)
 = 0.22 0.01
 = 0.16 0.02

Ir(mip)
3
:mCBP (10 vol.%)
Detector Angle ()

Figure 5. 8 ADPS measurements and simulations for films of mCBP doped with Ir(mi)3 (Top), Ir(miF)3
(Middle) and Ir(mip)3 (Bottom) at 10 vol% doping ratio. The measured data have been fitted (black and
red lines) to determine the degree of orientation. Ir(mi) 3 Θ = 0.25; Ir(miF) 3 Θ = 0.22; and Ir(mip) 3 Θ =
0.16 in mCBP respectively.
complexes.
19, 22, 26, 47-49
Molecular interactions between Ir(mip)3 and the host during deposition is
expected to be strongest when the long axis of the oblate spheroid is parallel to the substrate. With
this in mind we also prepared a larger analog of Ir(mip)3 by replacing the para-methyl group on
each mip ligand with a phenyl ring, thus further increasing the aspect ratio to 3.7 (See Figure 5.2).
Unfortunately, Ir(mipp)3 decomposed upon sublimation precluding the study of vacuum deposited
films using this derivative. A similar correlation between alignment to the substrate plane and
118

Table 5. 3 Optical anisotropy factors of iridium complexes.
Emitter Host 
(ppy)2Ir(acac) CBP 0.23
26

Ir(ppy)3
TCTA:B3PYMPM 0.33
48

CBP 0.32
51

TCTA:26DCzPPy 0.35
Ir(ppyCF3)3 TCTA:26DCzPPy 0.29
Ir(mi)3
TCTA 0.26
mCBP 0.25
Ir(miF)3
TCTA 0.22
mCBP 0.22
Ir(mip)3
TCTA 0.15
mCBP 0.16

dopant shape has been reported for a series of cyano-substituted phenyl-imidazole iridium
compounds by Kim, et al. (Figure 5.9).
22
Aspect ratios for these complexes calculated as described

Figure 5. 9 Ir dopants used in Kim, et al.
above give values ranging from 1.4 to 2.9 (see Table 5.4). The values of Θ for these dopants were
also found to decrease with an increase in the aspect ratio of the dopant. The authors modelled the
dopant alignment using a combination of Coulomb and van der Waals forces between the host and
the dopants. In their model, the Coulomb force exerted by the permanent dipole moment is roughly
equal when the PDM is pointed either at or away from the substrate. Since their modeling predicted
a significantly lower magnitude for Coulomb relative to the van der Waals interactions, the
Coulomb term only becomes relevant for dopants having a small aspect ratio, especially when
deposited in host materials with high PDMs (the host molecules used in the Kim study has a PDM
119

Table 5. 4 Calculated aspect ratio of all compounds studied in the paper
Ir(ppy)3 Ir(ppyCF)3 Ir(mi)3 Ir(miF)3 Ir(mip)3 D1
*
D2
*
D3
*
D4
*
D5
*

Aspect
ratio
1.2 1.0 2.2 1.9 3.0 1.3 1.9 2.1 2.7 2.9
*
Molecules in reference
52

of 5.0 D, Table 5.5).
22, 24
The energetic models by Kim, et al., thus suggest that Coulomb forces
acting on molecules with high dipole moments should favor dopant alignment. Countering this
proposal is the fact that fac-Ir(ppy)3 has a PDM of 6.4 D, and is isotropic in host materials of
varying polarity.
51
In contrast, a model by Jurow, et al.,
31
that incorporates the effects of chemical
asymmetry can also be used to explain the net alignment of dopants of Kim et al., with low aspect
ratios since these tris-chelated phenyl-imidazolyl complexes have nitrile groups situated close
together on the molecular surface.
Table 5. 5 Dipole moments of host materials obtained from DFT calculations.
Compound Dipole moment (S0 in Debye)
TCTA 0.1
mCBP
(a)
0.8
26DCzPPY 2.6
(c)

HT
(b)
0.8
(c)

ET
(b)
4.7
(c)

a) Crystal structure of mCBP from reference
53
is used b) Host materials
from reference
54
c) The value listed is the average of dipole moments
found for the different conformers of the molecule shown above.

5.2.6. The role of chemical asymmetry in dopant alignment
We also investigated the impact of chemical asymmetry in the iridium phenyl-imidazole
complexes by examining a derivative with trifluoromethyl substituents. The CF3 groups in Ir(miF)3
significantly alter the chemical asymmetry of the complex by presenting a fluorinated region on
120

one face of the molecule (Figure 5.5), which increases the PDM from 6.9 D for Ir(mi) 3 to 12.7 D
for Ir(miF)3. While the larger PDM increases Coulomb attraction between host and dopant, the
dipole moments of the host materials chosen for our study are low (PDM = 0.1-2.9 D, Table 5.5).
The CF3 groups decrease the aspect ratio of Ir(miF)3 to 1.9, which is expected to decrease the
degree of horizontal alignment. Nevertheless, the anisotropy factor measured for Ir(miF)3 is lower
than that of Ir(mi)3 (Θ = 0.22  0.02 and 0.26  0.02, respectively). Moreover, the anisotropy
factors for Ir(miF)3 are unaffected by the magnitude of the dipole moment of the host matrix.
Apparently, the electronic asymmetry imparted by the CF3 groups compensates for loss in
alignment due to the lower aspect ratio of Ir(miF)3. Molecular interactions between the
trifluoromethyl moieties and the aromatic π-systems of the host are disfavored,
55, 56
thus it is
expected that the dopants will orient with their fluorine-rich side directed away from the substrate
toward the vacuum during deposition. Consequently, horizontal orientation of the Ir(miF)3
complex is promoted by CF3 groups even though attractive interactions between the π-system of
the host and the dopant are diminished by the substituents.
To further demonstrate the contribution of chemical asymmetry to the molecular
orientation, trifluoromethyl groups were introduced onto the ligands of Ir(ppy)3 to make
Ir(ppyCF3)3 (Figure 5.1). FPIM measurements (see experimental section for details) were
conducted on vapor deposited films consisting of 10% of Ir(ppy)3 or Ir(ppyCF3)3 doped (TCTA) :
2,6-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (26DCzPPY) mixed host materials to determine the
orientation of the TDMs (Figure 5.10).
35
The anisotropy factor found for Ir(ppyCF3)3 is smaller
than that of Ir(ppy)3 (Θ = 0.29 and 0.35, respectively) despite the relatively spherical shape of
Ir(ppyCF3)3 (aspect ratio = 1.0). Thus, the net horizontal alignment of Ir(ppyCF3)3 compared to
121

Ir(ppy)3 is attributed to the CF3 groups, suggesting that chemical asymmetry can increase the
degree of horizontal alignment of the TDMs in the thin film.
-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2
0
1
2
3
4
k
r
 = 0.29
Intensity (a.u.)
sPP
Exp.
pPP
Exp.
a)

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2
0
1
2
3
4
b)
 = 0.35

Intensity (a.u.)
k
r
sPP
Exp.
pPP
Exp.

Figure 5. 10 FPIM intensity profiles in the p-polarized dipole plane (pPP) and s-polarized dipole plane
(sPP) for films of TCTA:26DCzPPy 2:1 doped with Ir(ppyCF 3) 3 (a) and Ir(ppy) 3 (b) at 10 vol% doping
ratio to determine the degree of orientation. Experimental data and simulated fits are expressed as points,
solid lines respectively. Insets are the molecular structure of Ir(ppyCF 3) 3 and Ir(ppy) 3 respectively. Θ =
0.29 for Ir(ppyCF 3) 3 and Θ = 0.35 for Ir(ppy) 3 in TCTA:26DCzPPy 2:1 mixed host respectively.
A plot of the anisotropy factor versus the aspect ratio of the compounds studied here, as
well as the cyano-substituted derivatives of Kim, et al., is shown in Figure 5.11. The compounds
are clustered into three groups depending the type (or absence) of substituent, with each grouping
122

1.0 1.5 2.0 2.5 3.0 3.5
0.1
0.2
0.3
0.4
Ir(ppy)
3
, Ir(mi)
3
, Ir(mip)
3
Ir(miF)
3
, Ir(ppyCF
3
)
3
Complexes from Scheme 1

Aspect ratio
(a)

Figure 5. 11 (a) Anisotropy values as a function of aspect ratio for molecules studied here and the cyano-
substituted complexes from reference
22
(Scheme 1). (b) Electrostatic surface potential plots for Ir(C^N) 3.
D3 is the complex in Scheme 1 with R = CH 3, the closest analog of Ir(mi) 3 and Ir(miF) 3.
having a similar dependence of  on the aspect ratio. The presence of chemical asymmetry can be
illustrated using the electrostatic surface potential of the complex. The ESP calculated for the
unsubstituted complexes is relatively uniform, whereas in both the CF3- and cyano-substituted
derivatives, the ESPs are non-uniform (Figure 5.11b). The color of the ESP surface is based on the
energy of a proton moved across the surface. A large negative energy (red) denotes a high negative
charge at the surface, whereas a large positive energy (blue) indicates a high positive charge at the
surface of the molecule. The most negative ESPs of the acceptor substituted molecules are
symmetrically disposed around the C3 axis, forming a “patch” of high ESP. The patch of high
ESP reinforces an alignment of the molecule that favors low Θ. Dopant alignment appears to be
a cooperative process, with both high aspect ratio and non-uniform ESP contributing to a lower Θ.
Interestingly, the cyano substituents promote a higher degree of dopant alignment than CF3-
substitution for a given dopant aspect ratio. One possible explanation to account for this effect is
a difference in the electrostatic force exerted by the various substituents. The partial charge
calculated for the cyano-nitrogens of D3 is -0.53 versus -0.27 for the fluorine atoms in Ir(miF)3.
123

The larger magnitude of charge on the cyano group results in an increased electrostatic force on
the face of the molecule than one generated by the trifluoromethyl groups, thereby promoting more
effective alignment in the former dopant. Note that D3 and Ir(miF)3 have their substituents on
different positions of the phenyl-imidazole ligand; D3 is para whereas the CF3 in Ir(miF)3 is meta
to Ir. However, when Ir(miF)3 is modelled with the CF3 groups para to Ir the partial charge at the
fluorine atoms is -0.22, close to that of Ir(miF)3. The magnitude of electrostatic charge on the
surface of the molecule depends more on the identity of the functional group than its substitution
site.
5.2.7. OLEDs characterizations and Simulations
OLEDs utilizing the five Ir(C^N)3 compounds investigated here as emissive dopants illustrate how
their enhanced horizontal TDM alignment affects outcoupling, and in turn external quantum
efficiency. Figure 5.12 illustrates the device along with the performance data obtained using the
structure: glass substrate / 70 nm ITO / 50 nm 4,4′-cyclohexylidenebis
[N,N-bis(4-methylphenyl)benzenamine] (TAPC) / 15 nm EML / 50 nm 3,3',5,5'-tetra[(m-pyridyl)-
phen-3-yl]biphenyl (BP4mPy) / 1.5 nm 8-hydroxyquinolinato lithium (LiQ) / 100 nm Al. The
emissive layers (EML) comprise the iridium complexes doped at 10 vol% into a TCTA-
26DCzPPY mixed host (ratio = 2:1). The mixed host system was employed to enhance injection
and transport of charges in the emissive layer, resulting in improved charge balance in the EML
and low drive voltage.
57
The device performance parameters are summarized in Table 5.6. As
shown in Figure 5.12b, the electroluminescence (EL) spectra have distinct vibronic features and
no detectable host emission, consistent with efficient exciton trapping on the dopant. The
maximum efficiency of the device using Ir(ppy)3 (EQE = 22.3  0.6%) is close to the maximum
expected value for an isotropic dopant and no extrinsic outcoupling enhancements from a glass
124

substrate,
20
whereas the efficiency of devices using Ir(mi)3, Ir(miF)3 and Ir(mip)3 (EQE = 26.4 
0.4, 25.5  0.2 and 30.5  0.6 % respectively) due to their horizontal TDM alignment. Note that
OLEDs using dopants with CF3 substituents, despite having low values for  in films, are less
efficient than the analogous devices using the parent dopants, likely due to their low
photoluminescence quantum yields.
450 500 550 600 650 700
-0.5
0.0
0.5
1.0

Normalized Intensity
Wavelength (nm)
Ir(mi)
3
Ir(mip)
3
Ir(miF)
3
Ir(ppy)
3
Ir(ppyCF
3
)
3
b.

0 2 4 6 8 10
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
Ir(mi)
3
Ir(mip)
3
Ir(miF)
3
Ir(ppy)
3
Ir(ppyCF
3
)
3
Luminescence (cd/m
Voltage (V)
0
100
200
300
400
500
c.
Current density (mA/cm
2
)
0.01 0.1 1 10 100
0
5
10
15
20
25
30
35
Ir(mi)
3
Ir(mip)
3
Ir(miF)
3
Ir(ppy)
3
Ir(ppyCF
3
)
3

EQE (%)
Current density (mA/cm
2
)
d.

Figure 5. 12 a.) Device architecture for all OLEDs. TCTA and 26DCzPPy are mixed with 2:1 ratio in the
emissive layer with 10 vol% doping concentration, b.) Electroluminescence spectra of Ir(mi) 3 (red),
Ir(mip) 3 (black), Ir(miF) 3 (green), Ir(ppy) 3 (blue) and Ir(ppyCF 3) 3 (purple). c.) Current density-voltage-
luminance curve for all iridium complexes. d.) EQE versus current density for all iridium complexes.
Inset is the molecular structures of materials used in the devices.

125

Table 5. 6 Summary of device performance with simulated EQE and outcoupling efficiency
VT
a)

Experimental
EQEmax

ΦPL
b)
Outcoupling
efficiency
Simulated
EQE
CIE
Ir(ppy)3 2.9 V 22.3 % 0.91 0.246 22.4 % (0.26, 0.63)
Ir(ppyCF3)3 4.5 V 20.0 % 0.82 0.254 20.8 % (0.26, 0.63)
Ir(mi)3 2.5 V 26.4 % 0.94 0.286 26.9 % (0.23, 0.53)
Ir(miF)3 3.9 V 25.5 % 0.91 0.302 27.5 % (0.22, 0.53)
Ir(mip)3 2.8 V 30.5 % 0.97 0.328 31.8 % (0.25, 0.58)
a)
Turn-on voltage is defined as the voltage at brightness 0.1 cd m
-2
,
b)
Photoluminescence quantum yield
measured for the dopant in a TCTA:26DCzPPy mixed film.
The modal power distributions of the PHOLEDs were calculated based on Green’s function
analysis, using the Θ values determined by ADPS and FPIM (see experimental section for
details).
58, 59
This analysis, with results in Figure 5.13, allows us to estimate the fraction of the
EL emission in the forward direction (air mode) versus waveguided modes in the OLED and in
the glass substrate, and surface plasmon modes in the cathode. As shown in Table 5.6, outcoupling
efficiencies (air mode) are consistent with the degree of dopant alignment from the ADPS and
FPIM measurements: outcoupling is highest for Ir(mip)3 with 32.8% of the light forward scattered,
compared to 24.6 % for Ir(ppy)3. Simulated EQEs were obtained by multiplying ΦPL by the
calculated outcoupling efficiency of each dopant (Table 5.6), showing a close correspondence
between the measured and predicted EQE values for a given value of Θ. The discrepancy between
experimental and simulated EQEs is the largest when Ir(miF) 3 is the dopant. This could be due to a small
difference in the Θ value for Ir(miF) 3 in the mixed host as compared to Θ in mCBP (0.8 D) and TCTA (0.1
D) hosts (Table 5.5), given that the magnitude of PDM of Ir(miF) 3 (12.7 D) is higher than those of Ir(mi) 3
(6.9 D) and Ir(mip) 3 (6.7 D). The turn-on voltage (VT = voltage at 0.1 cd/m
2
) for OLEDs using the
non-fluorinated dopants range from 2.5-2.9 V, whereas the VT of
126

0.246
0.427
0.205
0.122
0.254
0.404
0.216
0.126
0.286
0.355
0.193
0.166
0.302
0.329
0.199
0.17
0.328
0.273
0.204 0.195
0.0
0.1
0.2
0.3
0.4
 = 0.35
Ir(ppy)
3
Air mode
SPP mode
Waveguided
mode Glass mode
d)
0.0
0.1
0.2
0.3
0.4  = 0.29
Ir(ppyCF
3
)
3
Air mode
SPP mode
Waveguided
mode Glass mode
c)
0.0
0.1
0.2
0.3
0.4
a)
Air mode
Ir(mi)
3
SPP mode
Waveguided
mode
Glass mode
 = 0.26
0.170
0.0
0.1
0.2
0.3
0.4
 = 0.22
Ir(miF)
3
Air mode SPP mode
Waveguided
mode
Glass mode
b)
0.0
0.1
0.2
0.3
0.4
 = 0.15
Ir(mip)
3
Air mode
SPP mode
Waveguided
mode
Glass mode
e)

Figure 5. 13 Simulated outcoupling efficiencies (Air mode) and the probability of light being dissipated
to other modes (Surface Plasmon Polarization mode, waveguided mode and glass mode) for Ir(ppy) 3 (a),
Ir(ppyCF 3) 3 (b), Ir(mi) 3 (c), Ir(miF) 3 (d) and Ir(mip) 3 (e) in TCTA:26DCzPPy mixed host, with the device
architecture used for the devices represented in Figure 5.12.
using Ir(miF)3 and Ir(ppyCF3)3 are 3.9 V and 4.5V, OLEDs respectively (Figure 5.12c). The larger
VT for the devices with CF3 substituted dopants is attributed to a large interfacial dipole from
spontaneous ordering of dopant PDMs at the EML/ETL interface.
60, 61
It has been reported that a
polarized interface can strongly affect charge transport and injection.
62-65
The PDMs estimated
from DFT calculations for the non-fluorinated dopants are 6.4-6.9 D, whereas those of the CF3
substituted derivatives are 12.7 and 16.3 D for Ir(miF)3 and Ir(ppyCF3)3, respectively (Table 5.1).
In the emitters studied here, the PDM coincides with the C3 axis of the molecule (Figure 5.14).
Thus, the increase in VT suggests that Ir(miF)3 and Ir(ppyCF3)3 are aligned with the CF3 groups
oriented toward the vacuum interface during the deposition. This ordering polarizes the HTL/EML
interface to hinder injection of holes into the EML, which shifts the J-V curve to higher voltages,
as observed.
127

Figure 5. 14 Orientation of permanent dipole moments of dopants relative to the molecular frame.
Ir(ppy) 3 (a), Ir(ppyCF 3) 3 (b), Ir(mi) 3 (c), Ir(miF) 3 (d) and Ir(mip) 3 (e). The length of the dipole does not
represent its magnitude.
5.3. Conclusions
The relationship between the shapes of three homoleptic tris-cyclometalated Ir complexes
and their degree of alignment when doped into vacuum deposited thin films was investigated using
angle dependent photoluminescence spectroscopy and Fourier-plane imaging microscopy.
Molecules with oblate spheroidal shapes show the highest degree of in-plane alignment, with the
long axis of the electronic density ellipsoid showing a net parallel alignment relative to the
128

substrate. The driving force for this process is likely due to enhanced van der Waals interactions
of the higher surface polar area “face” over the equatorial “edge” of the ellipsoid. Thus, molecules
with spherical, or near spherical shapes are expected to show no preferred orientation in the
vacuum deposited films, as observed for complexes with a low aspect ratio such as Ir(ppy)3.
Addition of trifluoromethyl substituents onto one polar face of the tris-cyclometalated Ir
complexes demonstrate the role of chemical asymmetry in the alignment of molecules. The CF3
groups in Ir(miF)3 and Ir(ppyCF3)3 make their shapes less oblate, which should promote isotropic
orientation in doped thin films. This is not the case, however, as films doped with the fluorinated
derivatives have anisotropy factors that are lower than those using the parent complexes. The
fluorinated “patch” on the surface of the complexes has a lower affinity for the surface of the host
matrix owing to a decrease in van der Waals interactions. The asymmetry in molecular attraction
is expected to favor an orientation of the dopant with the fluorinated face of the complex directed
away from the surface toward the vacuum during deposition.
This work provides two approaches to achieve net alignment of tris-cyclometalated Ir
complexes in vacuum deposited films. Altering the molecular shape to approximate an oblate
spheroid or adding functional groups to one face of the molecule can lead to a net horizontal
alignment of the transition dipole moments in doped films. While the oblate shape may drive the
complex toward a parallel arrangement relative to the substrate, the principal axis of the molecule
can be directed either toward or away from the substrate. The addition of trifluoromethyl
substituents to one face of the molecule will not only promote net alignment relative to the
substrate but can also orient of the principal axis of the complex to be directed either toward or
away from the substrate, depending on the site of substitution. Moreover, these two effects, when
129

acting in concert, can lead to further improvement in the degree of horizontal alignment, and hence
lead to a higher outcoupling efficiency when used in OLEDs.
5.4. Experimental methods
5.4.1. Synthesis
All reagents and solvents were received from commercial sources such as Sigma Aldrich.
All complexation procedures were carried out in inert N2 gas atmosphere despite the air stability
of the complexes, the main concern being the oxidative and thermal stability of intermediates at
the high temperatures of the reactions. Both the [(mi)2IrCl]2 and [(miF)2IrCl]2 dimers were
synthesized by the Nonoyama method which involves heating IrCl3 ·H2O to 110 °C with 2−2.5
equivalents of mi-H and miF-H in a 3:1 mixture of 2-ethoxyethanol and deionized water.
66

Ir(ppy)3, Ir(ppyCF3)3 Ir(mi)3 were prepared according to the literature procedure.
33, 37-39
Except for
Ir(mipp)3, three compounds sublime with the reasonable yields (>50%), allowing for them to be
employed in OLEDs.
1-mesityl-2-phenyl-1H-imidazole (mi-H). A three neck flask was charged with 2,4,6-
trimethylaniline (10.0 g, 74 mmol), glyoxal (10.73 g, 74.0 mmol) and 125 mL of methanol. The
reaction mixture stirred at room temperature for 20 hours, upon which benzaldehyde (7.85 g, 74
mmol) and ammonium chloride (3.96 g, 74 mmol). A condenser was attached and the reaction was
heated to reflux. Phosphoric acid (724 mg, 7.4 mmol) was added after one hour and the reaction
was left to reflux for an additional 24 hrs. The reaction was cooled to ambient temperature and
concentrated in vacuo to remove the methanol solvent. The crude mixture was diluted with ethyl
acetate and treated with 1 M aqueous sodium hydroxide solution. The layers were then extracted
and separated with water three times, and the resultant organic layer was then washed with brine,
dried with sodium sulfate and concentrated in vacuo. The crude mixture was further purified by
130

column chromatography (4:1 hexanes:ethyl acetate) to yield a pale yellow solid (1.67 g, 8.6%).
1
H
NMR (400 MHz, CDCl3 δ) 7.41 – 7.37 (m, 2H), 7.29 (d, J = 1.2 Hz, 1H), 7.22 – 7.17 (m, 3H),
6.94 (d, J = 0.7 Hz, 2H), 6.86 (d, J = 1.2 Hz, 1H), 2.33 (s, 3H), 1.90 (d, J = 0.6 Hz, 6H).
1-mesityl-2-(4-(trifluoromethyl)phenyl)-1H-imidazole (miF-H). A three neck flask was
charged with 2,4,6-trimethylaniline (7.00 g, 51.8 mmol), glyoxal (7.51 g, 51.8 mmol) and 125 mL
of methanol. The reaction mixture stirred at room temperature for 20 hours, upon which
4-(trifluoromethyl)benzaldehyde (9.01 g, 51.8 mmol) and ammonium chloride (2.77 g, 51.8
mmol). A condenser was attached and the reaction was heated to reflux. Phosphoric acid (507 mg,
5.18 mmol) was added after one hour and the reaction was left to reflux for an additional 24 hrs.
The reaction was cooled to ambient temperature and concentrated in vacuo to remove the methanol
solvent. The crude mixture was diluted with ethyl acetate and treated with 1 M aqueous sodium
hydroxide solution. The layers were then extracted and separated with water three times, and the
resultant organic layer was then washed with brine, dried with sodium sulfate and concentrated in
vacuo. The crude mixture was further purified by column chromatography (4:1 hexanes:ethyl
acetate) to yield a pale yellow solid (1.56 g, 9.3%).
1
H NMR (400 MHz, Acetone-d6) δ 7.64 – 7.57
(m, 4H), 7.31 (d, J = 1.2 Hz, 1H), 7.18 (d, J = 1.2 Hz, 1H), 7.08 (m, 2H), 2.35 (s, 3H), 1.90 (s,
6H). Elemental Analysis: Anal. Cacld. for C19H17F3N2: C, 69.08 %; H, 5.19 %; N, 8.48 %. Found:
C, 69.27 %; H, 5.55 %; N, 8.71 %
1-(3,5-dimethyl-[1,1'-biphenyl]-4-yl)-2-phenyl-1H-imidazole (mip-H). A three neck
flask was charged with 3,5-dimethyl-[1,1'-biphenyl]-4-amine (5.00 g, 25.3 mmol), glyoxal (3.68
g, 25.3 mmol) and 100 mL of methanol. The reaction mixture stirred at room temperature for 20
hours, upon which benzaldehyde (2.69 g, 25.3 mmol) and ammonium chloride (1.36 g, 25.3
mmol). A condenser was attached and the reaction was heated to reflux. Phosphoric acid (248 mg,
131

2.53 mmol) was added after one hour and the reaction was left to reflux for an additional 24 hrs.
The reaction was cooled to ambient temperature and concentrated in vacuo to remove the methanol
solvent. The crude mixture was diluted with ethyl acetate and treated with 1 M aqueous sodium
hydroxide solution. The layers were then extracted and separated with water three times, and the
resultant organic layer was then washed with brine, dried with sodium sulfate and concentrated in
vacuo. The crude mixture was further purified by column chromatography (4:1 hexanes:ethyl
acetate) to yield a pale yellow solid (784 mg, 9.5%).
1
H NMR (400 MHz, Acetone-d6 δ) 7.73 (m,
2H), 7.55 (m, 2H), 7.48 (m, 4H), 7.40 (m, 1H), 7.28 (d, J = 1.2 Hz, 1H), 7.25 (m, 3H), 7.17 (d, J
= 1.2 Hz, 1H), 2.03 (m, 6H). Elemental Analysis: Anal. Cacld. for C23H20N2: C, 85.15 %; H, 6.21
%; N, 8.63 %. Found: C, 84.86 %; H, 6.28 %; N, 8.60 %
1-(3,5-dimethyl-[1,1':4',1''-terphenyl]-4-yl)-2-phenyl-1H-imidazole (mipp-H). A three
neck flask was charged with 4-bromo-2,6-dimethylaniline (25.0 g, 125 mmol), glyoxal (18.1 g,
125 mmol) and 230 mL of methanol. The reaction mixture stirred at room temperature for 20
hours, upon which benzaldehyde (13.3 g, 125 mmol) and ammonium chloride (6.68 g, 125 mmol).
A condenser was attached and the reaction was heated to reflux. Phosphoric acid (1.22 g, 12.5
mmol) was added after one hour and the reaction was left to reflux for an additional 24 hrs. The
reaction was cooled to ambient temperature and concentrated in vacuo to remove the methanol
solvent. The crude mixture was diluted with ethyl acetate and treated with 1 M aqueous sodium
hydroxide solution. The layers were then extracted and separated with water three times, and the
resultant organic layer was then washed with brine, dried with sodium sulfate and concentrated in
vacuo. The crude mixture was further purified by column chromatography (4:1 hexanes:ethyl
acetate) to yield 1-(4-bromo-2,6-dimethylphenyl)-2-phenyl-1λ
4
,3λ
2
-imidazole (2.5 g, 6%).. Then,
one neck flask was charged with 1-(4-bromo-2,6-dimethylphenyl)-2-phenyl-1λ
4
,3λ
2
-imidazole
132

(2.50 g, 7.64 mmol), [1,1'-biphenyl]-4-ylboronic acid (3.03 g, 15.3 mmol), potassium carbonate
(10.6 g, 76.4 mmol) and Tetrakis(triphenylphosphine)-palladium(0) (883 mg, 764 μmol). The
flask was degassed and 75 ml of toluene and water (2:1) mixed solutions was added. The reaction
was heated to reflux for 24 hours and was cooled to ambient temperature. The crude mixture was
diluted with ethyl acetate and extracted with water three times, and the resultant organic layer was
then washed with brine, dried with sodium sulfate and concentrated in vacuo. The crude mixture
was further purified by column chromatography (4:1 hexanes:ethyl acetate) to yield a pale yellow
solid (1.1 g, 36 %).
1
H NMR (400 MHz, Acetone-d6) δ 7.88 – 7.83 (m, 2H), 7.82 – 7.78 (m, 2H),
7.76 – 7.72 (m, 2H), 7.62 (p, J = 0.6 Hz, 2H), 7.53 – 7.45 (m, 4H), 7.42 – 7.36 (m, 1H), 7.29 (d, J
= 1.2 Hz, 1H), 7.27 – 7.23 (m, 3H), 7.19 (d, J = 1.2 Hz, 1H), 2.05 (s, 6H). Elemental Analysis:
Anal. Cacld. for C29H24N2: C, 86.97 %; H, 6.04 %; N, 6.99 %. Found: C, 86.18 %; H, 5.97 %; N,
6.79 %
Ir(mi)3. A pressure flask was charged with [(mi)2IrCl]2 dimer (80 mg, 0.053 mmol), mi-H
ligand (70.1 mg, 0.267 mmol) and 7 mL of 50:50 dioxane water. The flask was degassed, sealed
and heated to 130
o
C for six days. The reaction was then cooled to ambient temperature and
filtered, washing the precipitate with water and cold methanol to give a pale yellow emissive solid
(43.2 mg, 83 %).
1
H NMR (400 MHz, Acetone-d6 δ) 7.11 (d, J = 7.7 Hz, 2H), 6.99 (d, J = 1.5 Hz,
1H), 6.84 (ddd, J = 7.6, 1.3, 0.6 Hz, 1H), 6.76 (d, J = 1.5 Hz, 1H), 6.49 (ddd, J = 7.6, 7.2, 1.4 Hz,
1H), 6.35 (ddd, J = 7.7, 7.2, 1.4 Hz, 1H), 6.21 (ddd, J = 7.8, 1.4, 0.6 Hz, 1H), 2.38 (s, 3H), 2.06 (s,
3H), 1.82 (s, 3H).
Ir(miF)3. A pressure flask was charged with [(miF)2IrCl]2 dimer (330 mg, 0.186 mmol),
miF-H ligand (418 mg, 1.27 mmol) and 23 mL of 50:50 dioxane water. The flask was degassed,
sealed and heated to 130 °C for six days. The reaction was then cooled to ambient temperature,
133

diluted in ethyl acetate and extracted with water three times. The organic layers were washed with
brine, dried with sodium sulfate and concentrated in vacuo. The crude mixture was further purified
by column chromatography (4:1 hexanes:ethyl acetate) to yield a bright yellow solid.(65.9 mg, 30
%).
1
H NMR (400 MHz, Acetone-d6) δ 7.21 – 7.15 (m, 3H), 6.97 (d, J = 1.9 Hz, 1H), 6.88 (d, J =
1.9 Hz, 1H), 6.77 – 6.71 (m, 1H), 6.35 (d, J = 8.1 Hz, 1H), 2.41 (s, 3H), 2.13 (s, 3H), 2.09 (s, 3H).
Elemental Analysis: Anal. Cacld. for C57H48F9IrN6: C, 58.01 %; H, 4.10 %; N, 7.12 %. Found: C,
57.64 %; H, 3.94 %; N, 7.28 %
Ir(mip)3. A round bottom was charged with iridium (III) acetylacetonate (175 mg, 0.357
mmol), mip-H ligand (580 mg, 1.79 mmol), and tridecane (135 mg, 0.357 mmol). A condenser
was attached and the reaction was degassed, heated to 240 °C for 48 hours. The reaction was
cooled to ambient temperature and column chromatography on the crude mixture was further
purified by column chromatography (4:1 hexanes:ethyl acetate) to give a pale yellow emissive
solid (104 mg, 25%).
1
H NMR (400 MHz, Acetone-d6 δ) δ 7.79 (m, 2H), 7.65 (m, 2H), 7.52 (m,
2H), 7.43 (m, 1H), 7.13 (d, J = 1.5 Hz, 1H), 6.90 (d, J = 7.0 Hz, 1H), 6.87 (d, J = 1.5 Hz, 1H), 6.54
(ddd, J = 8.4, 7.3, 1.5 Hz, 1H), 6.40 (ddd, J = 8.4, 7.1, 1.3 Hz, 1H), 6.34 (dd, J =7.3, 1.5 Hz, 1H),
2.24 (s, 3H), 1.98 (s, 3H). Elemental Analysis: Anal. Cacld. for C69H57IrN6: C, 71.29 %; H, 4.94
%; N, 7.23 %. Found: C, 70.59 %; H, 4.98 %; N, 6.97 %
Ir(mipp)3. A round bottom was charged with iridium (III) acetylacetonate (175 mg, 0.357
mmol), mipp-H ligand (580 mg, 1.79 mmol), and tridecane (135 mg, 0.357 mmol). A condenser
was attached and the reaction was degassed, heated to 240
°
C for 48 hours. The reaction was cooled
to ambient temperature and column chromatography on the crude mixture was further purified by
column chromatography (4:1 hexanes:ethyl acetate) to yield a pale yellow solid.to give a pale
yellow emissive solid (104 mg, 25%).
1
H NMR (400 MHz, Acetone-d6) δ 7.92 (d, J = 7.6, 2H),
134

7.86 – 7.80 (d, J = 7.6, 2H), 7.78 – 7.69 (m, 4H), 7.54 – 7.47 (t, 2H), 7.40 (t, J = 9.1, 6.8, 2.2, 1.1
Hz, 1H), 7.16 (d, J = 1.8, 0.9 Hz, 1H), 6.95 – 6.87 (m, 2H), 6.59 – 6.52 (t, 1H), 6.42 (t, J = 7.1, 1.1
Hz, 1H), 6.38 – 6.33 (m, 1H), 2.26 (s, 3H), 2.00 (s, 3H). Elemental Analysis: Anal. Cacld. for
C87H69IrN6: C, 75.14 %; H, 5.00 %; N, 6.04 %. Found: C, 74.30 %; H, 5.00 %; N, 6.07 %
5.4.2. Photophysical measurements
Refer to the experimental methods section of chapter 3 for photophysical characterizations.
5.4.3. Molecular computational modeling
Aspect ratio calculation
To compute the aspect ratios, the 3D moments matrix of each complex was computed by
taking the Ir atom as the center. The moments matrix (𝑀 ) is then computed as:
𝑀 =
[

∑(𝑥 𝑖 − 𝑥 𝐼𝑟
)
2
𝑖 ∑(𝑥 𝑖 − 𝑥 𝐼𝑟
)(𝑦 𝑖 − 𝑦 𝐼𝑟
)
𝑖 ∑(𝑥 𝑖 − 𝑥 𝐼𝑟
)(𝑧 𝑖 − 𝑧 𝐼𝑟
)
𝑖 ∑(𝑥 𝑖 − 𝑥 𝐼𝑟
)(𝑦 𝑖 − 𝑦 𝐼𝑟
)
𝑖 ∑(𝑦 𝑖 − 𝑦 𝐼𝑟
)
2
𝑖 ∑(𝑦 𝑖 − 𝑦 𝐼𝑟
)(𝑧 𝑖 − 𝑧 𝐼𝑟
)
𝑖 ∑(𝑥 𝑖 − 𝑥 𝐼𝑟
)(𝑧 𝑖 − 𝑧 𝐼𝑟
)
𝑖 ∑(𝑦 𝑖 − 𝑦 𝐼𝑟
)(𝑧 𝑖 − 𝑧 𝐼𝑟
)
𝑖 ∑(𝑧 𝑖 − 𝑧 𝐼𝑟
)
2
𝑖 ]

where, 𝑥 𝑖 , 𝑦 𝑖 , 𝑧 𝑖 are the positional coordinates of an atom i in the molecule and 𝑥 𝐼𝑟
, 𝑦 𝐼𝑟
, 𝑧 𝐼𝑟
are
the coordinates of the central Ir atom. The matrix 𝑀 is then diagonalized to obtain the
corresponding eigenvalues, λ1, λ2 and λ3. Then, √λ
1
, √λ
2
and √λ
3
represent the relative length of
the principal semi-axes of a hypothetical ellipsoid hull that represents the molecule (i.e. 𝑎 ∝ √λ
1
,
𝑏 ∝ √λ
2
,𝑐 ∝ √λ
3
).
In our case, since we are dealing with homoleptic octahedral complexes, the lengths of at
least two of the semi-axes are expected to be similar (𝑎 ≈ 𝑏 ). The aspect ratio can then be
135

computed as the ratio between 𝑎 and 𝑐 . The DFT ground state optimized geometries were used to
compute the aspect ratios in all cases.
Energy and permanent dipole moment calculations
The ground (S0) and triplet (T1) state geometries of the complexes reported here were
optimized at the B3LYP/LACV3P** level using the Jaguar (v. 9.4 release 15) program within the
Material Science suite
67
developed by Schrödinger, LLC. To compute the TDMs for
phosphorescent (T1 → S0) emission in these molecules, time-dependent density functional theory
(TDDFT) with the zero order regular approximation (ZORA) approach
43-45
(SOC-TDDFT) as
implemented in Jaguar was utilized. The ZORA Hamiltonian incorporates spin-orbit coupling
(SOC) effects essential to compute TDMs associated with triplet (T1 → S0) emission. The SOC-
TDDFT calculations were performed on structures optimized in the T1 state using the B3LYP
functional and a mixed basis set utilizing the DYALL-2ZCVP-ZORA-J-Pt-Gen set for the Ir atom
and the 6-31G** set for the remaining atoms.
136

Molecular structures of host materials used in the paper and reference
54
along with the structures
of different conformers

137

5.4.4. Transition dipole moment vector (TDM) alignment measurements
All thin films used in PL measurements were deposited at 0.9 Å /s and 0.1 Å /s for the host
and dopant molecules, respectively, on 0.2 mm thick fused silica glass by vacuum thermal
evaporation in a chamber with a base pressure of 1 × 10
-7
torr. The deposition rate and thicknesses
were controlled using a quartz crystal thickness monitor. Following the deposition, devices were
encapsulated using an epoxy seal around the edge of a 1.57 mm thick cover glass in an ultrapure
N2 environment.
Angle dependent p-polarized emission spectroscopy (ADPS)
34, 68
was used to determine the
ordering of Ir(mi)3, Ir(mip)3 and Ir(miF)3 in doped thin films. The substrate was placed
perpendicular to the plane of detection and the emission is outcoupled from the substrate using a
2 cm radius, half-cylindrical lens. The emission along the plane of detection was decomposed into
transverse electric (TE) and magnetic (TM) modes using a polarization analyzer. A motorized stage
was used to position the detector. Simulations of the angular intensity profile are based on the
dyadic Green’s function in a birefringent medium.
59
A least-squares algorithm was used to fit the
experimental data to the simulation. The refractive indices and extinction coefficients of materials
were measured using variable-angle spectroscopic ellipsometry.
Fourier-plane imaging microscopy (FPIM) was used to determine the orientation of Ir(ppy)3
and Ir(ppyCF3)3 in doped films.
69
The Fourier microscope consists of two parts, (i) an inverted
fluorescence microscope (Olympus IX73) with a 325 nm He-Cd continuous-wave laser, and (ii) a
system comprising Fourier lens (Thorlabs), optical filters, a linear polarizer, and a spectrometer
with a 1024×1024 charge coupled device (CCD) array (Princeton Instruments). The
photoluminescence of the sample was coupled through an oil immersion objective (×100,
NA=1.40, Olympus). The Fourier lens (f = 300 mm) was used to reconstruct the Fourier image
138

plane on the CCD. A long-pass filter was used to prevent the laser beam being incident on the
CCD, while a band-pass filter with the pass band near the peak wavelength of the dopant
photoluminescence was also placed in the optical path. A linear polarizer separates the emission
into two the orthogonal planes corresponding to the p- and s- polarized plane modes. The obtained
emission contour was fitted according to the reported method.
69
To suppress imaging artefacts in
the high-k region, the k-space fitting was performed over a limited range of -1.1 < kx/k0 < 1.1.
70-74

5.4.5. OLED fabrication and device simulation

OLED fabrication
All thin films used in PL measurements were deposited at 0.9 Å /s and 0.1 Å /s for the host
and dopant molecules, respectively, on 0.2 mm thick fused silica glass by vacuum thermal
evaporation in a chamber with a base pressure of 1 × 10
-7
torr. The deposition rate and thicknesses
were controlled using a quartz crystal thickness monitor. Following the deposition, devices were
encapsulated using an epoxy seal around the edge of a 1.57 mm thick cover glass in an ultrapure
N2 environment. PhOLEDs were grown by vacuum thermal evaporation (VTE) on pre-cleaned
glass substrates coated with 70 nm thick indium tin oxide (ITO). The device structures were: 70
nm ITO/50 nm 4,4′-cyclohexylidenebis(N,N-bis(4-methylphenyl)benzenamine) (TAPC) /EML,
15 nm Co-host, tris(4-carbazoyl-9-yl-phenyl)amine (TCTA): 2,6-bis(3-(9H-carbazol-9-
yl)phenyl)pyridine (26DCzPPy) 2 mixed with the dopants doped at 10 vol.%/50 nm 3,3',5,5'-
tetra[(m-pyridyl)-phen-3-yl]biphenyl (BP4mPy)/1.5 nm Li quinolate/Al. The current density-
voltage (J-V) characteristics were measured using a parameter analyzer (HP4145, Hewlett-
Packard) and a calibrated photodiode (FDS1010-CAL, Thorlabs, Inc.) following standard
procedures.
75
The emission spectra at J = 100 mA cm
-2
were measured using a calibrated
139

spectrometer (USB4000, Ocean Optics, Inc) connected to the device via an optical fiber (P400-5-
UV-VIS, Ocean Optics, Inc).
Device simulation
The modal power distribution of the PHOLED was calculated based on Green’s function
analysis.
58, 59
The device structure used for the simulation is: ITO 70 nm / 1,1-bis[(di-4-
tolylamino)phenyl]cyclohexane (TAPC) 50 nm / tris(4-carbazoyl-9-ylphenyl)amine (TCTA) :
2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) 2 mixed host 15 nm (Active Layer) /
(BP4mPy) 3,3',5,5'-tetra[(m-pyridyl)-phen-3-yl]biphenyl 50 nm / Al 100 nm. Refractive indices
for all materials were measured using the variable-angle spectroscopic ellipsometry at λ = 470 nm,
484 nm, 472 nm, 530 nm corresponding to the peak wavelength for Ir(mi)3, Ir(miF)3, Ir(mip)3,
Ir(ppy)3 and Ir(ppyCF3)3 respectively. The dipole orientation of each dopant ( = 0.26, 0.22, 0.15,
0.35, 0.29 for Ir(mi)3, Ir(miF)3, Ir(mip)3, Ir(ppy)3, Ir(ppyCF3)3 respectively) measured via angle
dependent p-polarized emission spectrum and Fourier-plane imaging microscopy was used in the
simulations
26
and the emitter location was assumed to be in the cathode (Al) side of the EML. The
radiative efficiencies of each Ir(miX)3 doped into the TCTA : 26DCzPPy 2 mixed host matrix were
measured with an integrating sphere following the previously reported method.
76

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Appendix 1. Near-IR emission from heteroleptic tris-cyclometalated
Iridium complexes with a single emitting ligand for OLED
application
A.1.1. Introduction
Near-infrared (NIR) OLEDs have gained great attentions recently due to their potential
applications in sensors, optical communication, night-vision-readable displays and medical
systems, etc.
1-3
Various classes of NIR-emitters have been exploited including organic small
molecules, boron dipyrromethene dyes, metalloporphyrins, transition-metal complexes and
lanthanide ones.
4, 5
Phosphorescent transition-metal complexes have been the major focus of NIR-
OLEDs due to their efficient NIR emission. Among them, derivatives of Ir(III) have been
intensively investigated. In 2020, solution-processed NIR OLEDs with λmax=730 nm showed a
maximum EQE of 6.9% for heteroleptic iridium complexes with fused-heterocyclic phenazine
derivatives cyclometalated ligands.
6
The synthesis of heteroleptic has been accepted as a useful
way to tune the emission energy, the device is still made by solution-process techniques.
Considering vacuum-deposited devices exhibit higher efficiency and lifetime compared to the
solution-processed devices, it is worth investigating vapor-deposited NIR-OLEDs. Here, we
fabricated the efficient vacuum-deposited NIR-OLEDs with Ir(iqbt)3 and Ir(ppz)2iqbt where (1-
iqbt= benzo[b]thiophen-2-yl)-isoquinolinate, ppz= 1-phenylpyrazole. We demonstrate that the
lower molecular weight of Ir(ppz)2(iqbt) compared to the homoleptic complex is beneficial for a
vacuum thermal processing of emitters into OLEDs.

145

A.1.2. Result and Discussion
OLEDs were prepared with Ir(ppz)2iqbt and Ir(iqbt)3 as emissive dopants. The molecular
structures are shown in Figure A.1.1. Ir(ppz)2iqbt was selected among the heteroleptic complexes

Figure A.1. 1 Molecular structures of Ir(iqbt) 3 and Ir(ppz) 2iqbt
given the comparatively better photophysical properties, absence of an emitting homoleptic
scrambled complex and reversible redox behavior. The thin films’ photoluminescence of the
complexes doped at 10 vol% into CBP was measured. Quantum yields of Ir(ppz)2iqbt and Ir(iqbt)3-
doped films show 22% and 29% with lifetimes of 2.6 μs and 2.1 μs, respectively. Only dopant
emission is observed, indicating that energy transfer from the host to the dopants is efficient.
OLEDs were fabricated by vacuum deposition with the structure shown in Figure A.1.2a. NPD
and Bphen were used as hole-transporting and electron-transporting layers, respectively, and an
emissive layer consisting of CBP doped at 10 vol%. ITO was used as an anode and Al as the
evaporated cathode. Considering approximately 20% of electroluminescence is typically
outcoupled without any enhancement to the outcoupling technique, the measured 3% of maximum
EQE observed for the Ir(ppz)2iqbt based OLED is close to its theoretical maximum EQE (~4%).
In addition, The maximum EQE for Ir(iqbt)3 device is also close to 3%, which is an improved
value compared to that of the solution-processed device reported.
7

146

Both Ir(ppz)2iqbt and Ir(iqbt)3-based OLEDs show gradual efficiency roll-off upon increasing
current density, which is consistent with the short lifetime of the molecules. As illustrated in
A.1.2d, Ir(iqbt)3 device is more conductive than the Ir(ppz)2iqbt counterpart. The difference in
current density between the two devices might be due to different LUMO levels. In particular, the
LUMO of Ir(iqbt)3 is deeper than that of Ir(ppz)2iqbt, allowing electrons to be injected and
transported to the emissive layer more efficiently.

600 700 800
0.0
0.2
0.4
0.6
0.8
1.0

Intensity (a.u.)
Wavelength (nm)
EL, Ir(ppz)
2
iqbt
PL
EL, Ir(iqbt)
3
PL
(b)

10
-1
10
0
10
1
10
2
0
1
2
3
4
Ir(ppz)
2
iqbt
Ir(iqbt)
3

EQE (%)
Current density (mA/cm
2
)
(c)
0 2 4 6 8 10
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3

Voltage (V)
(d)
-50
0
50
100
150
200
250
Current density (mA/cm
2
)
Luminance (cd/m
2
)

Figure A.1. 2 (a) Device structure with Ir(ppz) 2iqbt and Ir(iqbt) 3 as dopants (b) photoluminescence of
CBP:Ir(ppz) 2dqbt (Black) or Ir(iqbt) 3 (Red) 10 vol% film, and electroluminescence of the devices (filled
symbols) (c) External Quantum Efficiency as a function of current density (d) J-V-L plot
147

A.1.3. Conclusion
The vacuum-deposited device with Ir(ppz)2iqbt as a dopant showed a maximum EQE of 3%,
outperforming the efficiencies obtained in solution-processed devices. It demonstrated that
employing a single emissive ligand in heteroleptic Ir(III) complexes is enough to realize the
features of a homoleptic system while maintaining low molecular weight. This result opens up a
new strategy to come up with the NIR Iridium complexes that can be applied to a vacuum-
processed device.
A.1.4. Experimental Methods
Refer to the experimental methods section of chapter 2 for photophysical characterizations,
OLEDs fabrication and characterizations.
A.1.5. Appendix 1 references

1. Guo, Z.; Park, S.; Yoon, J.; Shin, I., Recent progress in the development of near-infrared
fluorescent probes for bioimaging applications. Chem. Soc. Rev. 2014, 43 (1), 16-29.
2. Xiang, H.; Cheng, J.; Ma, X.; Zhou, X.; Chruma, J. J., Near-infrared phosphorescence:
materials and applications. Chem. Soc. Rev. 2013, 42 (14), 6128-6185.
3. Zampetti, A.; Minotto, A.; Cacialli, F., Near-Infrared (NIR) Organic Light-Emitting Diodes
(OLEDs): Challenges and Opportunities. Adv. Funct. Mater. 2019, 29 (21), 1807623.
4. Lu, H.; Mack, J.; Yang, Y.; Shen, Z., Structural modification strategies for the rational design of
red/NIR region BODIPYs. Chem. Soc. Rev. 2014, 43 (13), 4778-4823.
5. Barbieri, A.; Bandini, E.; Monti, F.; Praveen, V. K.; Armaroli, N., The Rise of Near-Infrared
Emitters: Organic Dyes, Porphyrinoids, and Transition Metal Complexes. Top Curr Chem (Cham) 2016,
374 (4), 47.
6. Chen, Z.; Zhang, H.; Wen, D.; Wu, W.; Zeng, Q.; Chen, S.; Wong, W.-Y., A simple and
efficient approach toward deep-red to near-infrared-emitting iridium(iii) complexes for organic light-
emitting diodes with external quantum efficiencies of over 10%. Chemical Science 2020, 11 (9), 2342-
2349.
7. Ikawa, S.; Yagi, S.; Maeda, T.; Nakazumi, H.; Fujiwara, H.; Koseki, S.; Sakurai, Y., Photo-
and electroluminescence from deep-red- and near-infrared-phosphorescent tris-cyclometalated
iridium(III) complexes bearing largely π-extended ligands. Inorg. Chem. Commun. 2013, 38, 14-19.

148

Appendix 2. Device optimizations of deep blue and yellow copper
emitters.
A.2.1. Results and Discussion
Besides (MAC*)Cu(Cz) (3) mentioned in chapter 2, other copper compounds such as
(MAC*)Cu(CzCN2) (1), (MAC*)Cu(CzCN) (2), (DAC*)Cu(CzCN2) (4), (DAC*)Cu(CzCN) (5),
and (DAC*)Cu(Cz)(6) (MAC = cyclic monoamido-aminocarbene, DAC = cyclic diamidocarbene,
“*” indicates that the aryl group bound to N is2,6-diisopropylphenyl, Cz = carbazole) are also
promising in fabricating highly efficient OLEDs. Molecular structures and their emission in PS at
RT and 77K are shown in Figure A.2.1. λmax increases from 1 to 6 and they all show broad
featureless emissions, which is similar to compound 3. The device optimization using complex 3
is discussed in chapter 2. Among these copper complexes, monochromatic devices using complex
2 and 4 were optimized because they display blue and yellow emissions. Therefore, the fabrication
of the three color-Cu-WOLEDs may be feasible with complex 2, 3 and 4 in the future. The PLQYs

400 500 600 700 800
0.0
0.5
1.0
6 5 4 3 2

Normalized emission (AU)
Wavelength (nm)
RT
77 K
1
(a)

Figure A.2. 1 Molecular structures of 1-6 and their emission profile in PS at RT and 77K
of 2 and 4 in PS films at 1 wt% were 1.0 and 0.78, with lifetimes of 1.3 and 2.6, respectively.
First of all, the monochromatic device employing 4 has been fabricated as shown in Figure A.2.
149

Figure A.2. 2 Device structures and characteristics using compound 4 as an emitter.
mCBP was selected as a host due to the broad emission of complex 4, making triplet energy higher
than that of typical iridium yellow emitters. The films comprised of 2 doped into mCBP at 10 and
20 vol % showed PLQYs of 0.68, 0.59, respectively, implying the energy transfer from the host to
the dopant is complete. PLQYs less than unity may be attributed to the consequence of the energy
gap law.
1
Small decreases in PLQYs upon heavier doping result from the aggregation effect,
corresponding to the red-shifted EL spectra (Figure A.2.2). Both 10 and 20 vol% devices show
EQEmax = 17.8 and 16.2%, respectively, which are close to the theoretical maxim EQE values. As
a next step, the device optimization using complex 2 has been processed. Similar to the case of
complex 3, the choice of host materials is critical because the onset of complex 2 is significantly
deeper than that of typical blue-emitting phosphors (λonset< 380 nm). Consequently, several
compounds with high triplet energies have been investigated: UGH3, 1,3,5-Tris(carbazol-9-
yl)benzene (t-CP), 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) and
150

DPEPO. Their molecular structures are shown in Figure A.2.3. PLQYs and lifetimes of the films
comprised of complex 2 doped into the four hosts at different doping concentrations were
measured and summarized in Table A.2.1. When complex 2 was doped into UGH3 at 10 and 20
vol%, PLQY remained constant (0.64), whereas it dropped to 0.42 at a 40 vol% doping condition
due to the aggregation of the dopants. PLQYs of complex 2 in DPEPO showed similar trends:
They dropped from 0.85 to 0.47 as doping condition varies from 10 to 40 %. Interestingly, all four
films at 20 vol% with different hosts showed comparable PLQYs (0.55-0.67) with similar lifetimes
(0.91-1.74 μs), suggesting that the energy transfer from the hosts to complex 2 is efficient for all.
Based on this film study, the monochromatic device with complex 2 and UGH3 as an EML was

Figure A.2. 3 Molecular structures of host materials used for compound 2.
fabricated. The device performances with their architectures are shown in Figure A.2.4. TAPC
and Bphen are introduced as the HTL and ETL, respectively. 5 nm thick layer of UGH3 was
inserted between the EML and Bphen as an exciton blocker, as the triplet energy of Bphen
(ET1=2.58 eV)
2
is lower than that of the dopant. Four different exciton/electron blocker layers
151

Table A.2. 1 PLQY and lifetime of films when compound 2 are doped into various host at different
doping concentration.
Host/Doping concentration Φ τ (μs)
UGH3
10 % 0.64 1.15
20 % 0.64 1.18
40 % 0.42 0.93
DPEPO
10 % 0.85 1.68
20 % 0.66 1.74
40 % 0.47 1.2
t-CP 20 % 0.67
0.58
(58%)/1.1(42%)
CzSi 20 % 0.55 0.91

Figure A.2. 4 Device structures and performances using compound 2 and UGH3. Four different HTLs
were employed to check hole-dominating behavior in the device.
(EBLs) were placed between the HTL and the EML to slow down the hole mobility: mCP, UGH3,
UGH3:TAPC 1:1 and UGH3:TAPC 2:1. As shown in Figure A.2.4, the device with mCP and
UGH3 blocker displays the similar EQEmax of 8.0 % even though the triplet energy of UGH3 is
much higher than that of mCP. Similarity in EQEmax values indicates that the recombination zone
152

may be formed primarily at the ETL/EML interface, avoiding the exciton leakage through mCP.
Indeed, when UGH3:TAPC mixed layer was employed as an EBL, EQEmax dropped by
approximately 2 %, possibly due to the poor charge balance, resulting from the hole-dominating
device characteristics. Given that PLQY of the EML is 0.64, the maximum EQE of 8 % is lower
than the theoretical maximum value (~ 12 %), further suggesting that the charge balance should
be improved in the device. Figure A.2.5 shows the device structures at different doping
concentrations and their respective performances. 5 nm and 10 nm thick UGH3 were employed as
an EBL and a HBL, respectively. As mentioned in chapter 2, the variation in doping ratios can
influence PLQY, the charge balance and the outcoupling of the EML, and therefore, the device
efficiency. 20 %-doped device showed the highest EQE of 8 %. Next, t-CP was employed as a

Figure A.2. 5 Device structures and their characteristics with different doping conditions of compound 2
in UGH3 host.
host in the OLEDs. The triplet of mCBP is located on biphenyl, whereas the triplet of t-CP is
placed on a carbazole. Therefore, the triplet of t-CP was expected to be higher than that of mCBP.
153

However, the t-CP-based device did not improve the device efficiency. The device performances
with their structures are summarized in Figure A.2.6. The doping concentration was fixed as 20
% while UGH3 as an EBL was either absent or present. EQEmax of 8.0% was achieved with the
device using an EBL, which produced the same result as the UGH3-based device. The situation
was similar when CzSi was employed in the EML. Figure A.2.7 shows the structures and
performances of CzSi-devices. The thicknesses of the EML were modified from 20, 30 to 40 nm
to observe any differences in the device efficiency. The J-V-L plot indicates that the devices
become more resistive as the thickness of the EML increases. However, the change in EQEmax is
not distinguishable between 30 and 40 nm- EML devices, because two devices displayed the same
maximum EQE of 8 %. Based on the UGH3, t-CP and CzSi device results, it can be inferred that
other materials with superior triplet energy and balanced charge mobility are required for further

Figure A.2. 6 Device structure and performances using compound 2 in t-CP host. Electron Blocking
Layer (EBL) was absent or present.

154

Figure A.2. 7 Device structure and performances using compound 2 in CzSi host at different doping
conditions.
optimization. In this context, DPEPO was utilized as a host because numerous deep-blue
PHOLEDs with high efficiency were reported while employing DPEPO.
3
Figure A.2.8 shows the
architecture of DPEPO-devices with their characteristics. Doping concentrations varied from 10,
20 to 40 % while UGH3 was introduced as a HBL and an EBL. EL spectra display bathochromic
shift upon heavier doping conditions due to the aggregation effect. The J-V-L plot indicates that
conductivities of the devices increase as the doping ratio increases, because the HOMO/LUMO of
the dopants are nested by that of DPEPO’s. More importantly, maximum EQE reached up to 15
% at 10 and 20 vol% doping concentrations. Considering PLQYs of DPEPO:2 10, 20% film are
0.85 and 0.64, these values are comparable to the theoretical maximum. The significant increase
in EQE for the DPEPO-device compared to devices with other hosts is surprising because the
energy transfer from the hosts to the dopant seems complete for all hosts. This enhancement in
EQEs can be explained by the estimation of the exciton profile. Figure A.2.9 shows the EL spectra
155

Figure A.2. 8 Device structure and performances employing 2 in DPEPO at various doping
concentrations.
of sensing devices. An ultra-thin layer of PQIr was doped at the HTL/EML interface, middle of
the EML and the ETL/EML interface. UGH3:TAPC 3:1 mixed layer was used as an EBL. As
shown in Figure A.2.9, relative intensity of the PQIr emission is intense at the middle of the EML
and HTL/EML interface while negligible at the ETL/EML interface. In addition, this trend holds
for various current densities, indicating that excitons are mainly formed from the HTL/EML
interface to the middle of the EML. This electron-dominating behavior of the DPEPO-device
shows a trait opposite to that of the device with a neat-layer of complex 2. Figure A.2.10 shows
the EL spectra of the sensing devices where the neat layer of complex 2 was used as an EML. PQIr
was inserted at the HTL/EML interface, middle of the EML and the ETL/EML interface. As
illustrated in Figure A.2.10, the PQIr emission is significant at the middle of the EML and at the
156

ETL/EML interface, suggesting that the main recombination zone is formed from the middle to
the EML/ETL interface. Therefore, the increase in EQE for DPEPO-devices compared to other
hosts-devices can be attributed to the enhancement in the charge balance.

Figure A.2. 9 Device structures to probe the exciton profile in DPEPO-devices and EL spectra of HTL,
middle and ETL- sensing devices.
Another deep-blue 2-coordinate coinage copper complex bearing a sterically bulky
benzimidazolyl carbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene (BZI),
and Cz as an anionic ligand was investigated. PS:BZIAuCz 1%-doped film exhibits unity quantum
yield with the lifetime of 1.13 μs, which is promising for the fabrication of highly efficient OELDs.
Figure A.2.11 illustrates the BZIAuCz-device architecture, their performances and EL spectra at
different current densities. As BZIAuCz has a high triplet energy, ultra-gap host material, UGH3
was utilized as a host. The doping concentrations varied from 5, 10 and 15 %. The device with a
15 % doping condition shows the undesired emission starting at 550 nm as current density
157

increased, suggesting the degradation of the device. Indeed, the current density in the was changed
as multiple sweeps of the devices were run, further confirming the degradation effect. However,

Figure A.2. 10 Device structures to probe the exciton profile in neat emissive layer of compound 2 and
EL spectra of HTL, middle and ETL- sensing devices.

Figure A.2. 11 BZIAuCz-based device structures and their characteristics at different doping conditions.

158

this unexpected emission was significantly diminished for 5 and 10%-doped devices. Moreover,
the device with a 5 % doping ratio shows the maximum EQE of 12 %. In conclusion, with
appropriate device structures, doping concentrations and the choice of suitable host materials,
OLED efficiency has the potential to significantly improve with cost-effective materials such as
copper and gold.
A.2.2. Experimental Methods
Refer to the experimental methods section of chapter 2 for photophysical characterizations,
OLEDs fabrication and characterizations.
A.2.3. Appendix 2 references
1. Caspar, J. V.; Meyer, T. J., Application of the energy gap law to nonradiative, excited-state
decay. The Journal of Physical Chemistry 1983, 87 (6), 952-957.
2. Hofmann, S.; Rosenow, T. C.; Gather, M. C.; Lüssem, B.; Leo, K., Singlet exciton diffusion
length in organic light-emitting diodes. Physical Review B 2012, 85 (24), 245209.
3. Wong, M. Y.; Zysman-Colman, E., Purely Organic Thermally Activated Delayed Fluorescence
Materials for Organic Light-Emitting Diodes. Adv. Mater. 2017, 29 (22), 1605444.

Abstract (if available)

Abstract Over the past two decades, Organic Light-Emitting Diodes (OLEDs) have attracted intensive attention from both industry and academia due to their potential applications in panel displays and solid-state lighting. However, several major deficiencies should be improved for the growth of the OLED market. First, the fabrication cost of OLEDs needs to be lowered. According to the department of energy, materials cost takes more than 10% of total expense in OLEDs. Adoption of more abundant materials in OLEDs can decrease the material cost because most commercially available emitters bear iridium, which is one of the rarest elements on the planet (0.0003 ppm). In other words, application of earth-rich elements such as copper (50 ppm) would lead to a dramatic financial benefit in OLEDs. The details of this subject will be discussed in chapter 2. Another scope for improvement in OLEDs is device efficiency. Intrinsically, collecting all excitons is a prerequisite to achieve high efficiency. In addition, extracting as many photons as possible in the device is essential, considering a significant portion of light is wasted before it reaches outside of the device. Indeed, approximately 50% and 30% of light are dissipated by a metal surface and organic layers respectively. The strategies to enhance the device efficiency will be discussed from chapter 3 to 5. ❧ Chapter 2 introduces highly efficient and cost-friendly OLEDs using cyclic alkyl amino carbene (CAAC)-Cu(Ι)-Carbazole (Cz), cyclic monoamido-aminocarbene (MAC*)-Cu-Cz and MAC*(Au)Cz. OLEDs prepared with CAAC(Cu)Cz doped into an ultra-gap host material yield maximum external quantum efficiency (EQEₘₐₓ) of 9.0%, which is one the highest efficiencies considering that EQEₘₐₓ of previously reported for blue-emitting Cu-OLEDs is less than 6%. (MAC*)CuCz devices showed small turn-on voltages (VT) of 2.7 V and EQEₘₐₓ = 19.4%. This performance is among the best reported for OLEDs based on Cu(I) dopants. On top of that, the roll-off in efficiency at a high current is considerably gradual than other OLEDs reported using four- or three-coordinate Cu emitters, possibly due to short radiative lifetime of the complex. Similar to (MAC*)Cu(Cz), OLEDs with (MAC*)Au(Cz) displayed an EQEₘₐₓ = 18.1%, which is one of the highest records reported for OLEDs with an Au(I) dopant. Efficiency drop at high current density is also low, further suggesting that cost-effective and stable vacuum-deposited OLEDs are realizable. ❧ Chapter 3 demonstrates the highly efficient hybrid white OLEDs (WOLEDs) employing phenanthro[9,10-d]imidazoles derivatives as a neat emitter/host. High photoluminescence quantum yield (0.80), singlet (3.28 eV) and triplet energy (2.60 eV) allowed one of the phenanthro[9,10-d]imidazoles derivatives to be utilized as a neat fluorescent emitter/host. Indeed, monochromatic device of neat emitter showed EQEₘₐₓ = 5%, confirming the suitability of the emitter. An ultra-thin layer of the sensitizer was introduced to probe the exciton profile in the emissive layer and doped layers of green and red phosphors were placed accordingly in order to obtain superior color rendering index (CRI). The optimized hybrid WOLED showed EQEₘₐₓ = 14.2% and maximum luminance power efficiency (LPEₘₐₓ) of 50 lm/W with only a small change in CRI (CRI = 68 at 1mA/cm² to CRI = 76 at 100 mA/cm²). ❧ Chapter 4 establishes the application of corannulene derivatives in stacked hybrid WOLEDs. To alleviate efficiency-roll off at high current density in hybrid WOLEDs, a new host material, xylyl corannulene, was synthesized and investigated. Stern-volmer and film quenching studies verified the availability of xylyl corannulene as a host for blue fluorophore and red phosphor. Certainly, a monochromatic device of blue fluorophore and red phosphor with xylyl corannulene as a host exhibited EQEₘₐₓ = 2 and 13%, respectively. Optimized hybrid OLED where blue and red emitters are mixed in a single layer showed an EQEₘₐₓ = 17.5%, affirming the full usage of excitons. Relative intensity of triplet to singlet emission at various current densities was constant with the values around 1.5, which was close to the ratio between triplet and singlet (T₁/S₁ = 3.0), implying singlets and triplets are fairly harvested in separate channels. This highly efficient hybrid device (B, R) with xylyl corannulene is the first OLED reported that employs corannulene in OLED applications. ❧ Chapter 5 articulates two factors that affect the alignment of molecules during the vacuum deposition in OLEDs. The degree of orientation of three homoleptic tris-cyclometalated Iridium complexes with either different physical shapes or electronic configuration was investigated using angle dependent photoluminescence spectroscopy (ADPS) and Fourier-plane imaging microscopy (FPIM). Other properties such as photo-physics and position of transition dipole moments (TDMs) remained constant, allowing a direct correlation between the experimental result and one variable. Molecules with oblate spheroidal shapes displayed the best horizontal alignment with outcoupling efficiency of 0.328 and an EQEₘₐₓ = 30.5%. Additionally, chemical asymmetry caused by attaching trifluoromethyl groups promoted in-plane alignment even though molecules became more spherical in shape. Experimental EQEs of OLEDs correspond nicely with that of the simulated one, further confirming that geometry and chemical asymmetry contribute to net alignment of tris-cyclometalated Ir complexes in vacuum deposited films.

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Asset Metadata

Creator Jung, Moon Chul (author)

Core Title Integration of cost effective luminescent materials into organic light emitting diodes (OLEDs) and molecular approaches to improve the device efficiency

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Degree Conferral Date 2021-12

Publication Date 09/29/2021

Defense Date 08/24/2021

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

Tag copper complexes,cost effective,device,dopants,efficiency,host materials,luminescent materials,OAI-PMH Harvest,OLEDs,outcoupling efficiency

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Ravichandran, Jayakanth (committee member), Wu, Wei (committee member)

Creator Email moonchuj@usc.edu,munchol12@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC16010303

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