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Device Engineering and Degradation Mechanism Study of All-
Phosphorescent White Organic Light-Emitting Diodes
By
Lisong Xu
Submitted in Partial Fulfillment of the
Requirements for the Degree
Doctor of Philosophy
Supervised by Professor Ching W. Tang & Professor Lewis J. Rothberg
Materials Science
Arts, Sciences and Engineering
Edmund A. Hajim School of Engineering and Applied Sciences
University of Rochester
Rochester, New York
2017
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Biographical Sketch
Lisong Xu was born in Zhejiang, China in 1986. He received a Bachelor of Science
degree in Materials Science and Engineering from Beihang University in 2009. He
continued to pursue his studies at King Abdullah University of Science and Technology,
Saudi Arabia, where he received his Master of Science degree in Materials Science and
Engineering in 2011. In the fall of 2011, he enrolled in the doctoral program in Materials
Science at the University of Rochester, under the joint supervision of Professor Ching W.
Tang and Professor Lewis J. Rothberg. His field of study is physic, materials and devices
related to organic light-emitting diodes.
List of Publications and Papers Submitted for Publication
[1] L. Xu, C.W. Tang, and L.J. Rothberg, “High efficiency phosphorescent white
organic light-emitting diodes with an ultra-thin red and green co-doped layer and
dual blue emitting layers,” Org. Electron. Physics, Mater. Appl. 32, 54 (2016).
[2] J. Li, L. Xu, C.W. Tang, and A.A. Shestopalov, “High-resolution organic light-
emitting diodes patterned via contact printing,” ACS Appl. Mater. Interfaces 8,
16809 (2016).
[3] J. Li, L. Xu, S. Kim, and A.A. Shestopalov, “Urethane–acrylate polymers in high-
resolution contact printing,” J. Mater. Chem. C 4, 4155 (2016).
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[4] S.C. Dong, L. Xu and C.W. Tang, “Chemical degradation mechanism of TAPC as
hole transport layer in blue phosphorescent OLED,” Org. Electron. Physics, Mater.
Accepted Nov. 2016.
[5] L. Xu, C.W. Tang and L.J. Rothberg, “Investigation of phosphorescent blue and
white organic light-emitting diodes with high efficiency and long lifetime,” In
preparation.
[6] L. Xu, J.U. Wallace and C.W. Tang, “Fractionation of nearly osomeric di-
substituted anthracene mixtures upon thermal vacuum deposition,” In preparation.
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Acknowledgments
First and foremost, I would like to sincerely thank my advisor Professor Ching W.
Tang and co-advisor Lewis J. Rothberg for their continuous guidance and support
throughout the course of my pursuing the doctorate degree. Their rigorous attitude of
research and scholarship taught me all the necessary attributes to achieve academic goals
and made my study very enjoyable, exciting, fruitful and ultimately, a rich experience. I
would also like to thank them for providing me with an amazing research environment.
In addition, I would like to thank Professor Alex Shestopalov of the Department of
Chemical Engineering and Professor Yongli Gao of the Department of Physics and
Astronomy for serving as my thesis committee members and providing prompt and
valuable feedback on my research.
Special thanks go to Mr. Joseph Madathil who taught me the many techniques of
high vacuum systems that were proven to be very useful for my research work. Without
his gracious assistance and guidance, my research would have been more challenging. I
would also like to thank Dr. David S. Weiss and Dr. Ralph H. Young for their valuable
feedback upon my thesis writing. My gratitude also goes to Mr. Mike Culver and Mr. John
Miller for their help on equipment-related matters. I also deeply thank Mr. Larry Kuntz,
Mrs. Sandra Willison, Mrs. Gina Eagan and all faculty and staff members in the
Department of Chemical Engineering and Program of Materials Science for their
administrative support and assistance.
I would like to acknowledge my fellow lab-mates and colleagues: Dr. Minlu Zhang,
Dr. Wei Xia, Dr. Hao Lin, Dr. Hui Wang, Dr. Hsiang Ning (Sunny) Wu, Dr. Felipe Angel,
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Charles Chan, Dr. Sang-Min Lee, Dr. Jason Wallace, Prashant Kumar Singh, Laura
Ciammaruchi, Guy Mongelli, Dr. Chris Favaro, Dr. Kevin Klubek, Aanand Thiyagarajan,
Michael Beckley, Thao Nguyen, Sihan (Jonas) Xie, Soyoun Kim and other group alumni,
for their collaboration and insightful discussions throughout my research. Special thanks
go to Dr. Shou-Cheng Dong from Hong Kong University of Science and Technology for
providing the chance of collaboration and for his generous advice, suggestions and
guidance.
Finally, I would like to express deep gratitude to my family for their unconditional
love, understanding and encouragement, not only during my pursuit for higher education,
but throughout my entire life.
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Abstract
As a possible next-generation solid-state lighting source, white organic light-
emitting diodes (WOLEDs) have the advantages in high power efficiency, large area and
flat panel form factor applications. Phosphorescent emitters and multiple emitting layer
structures are typically used in high efficiency WOLEDs. However due to the complexity
of the device structure comprising a stack of multiple layers of organic thin films, ten or
more organic materials are usually required, and each of the layers in the stack has to be
optimized to produce the desired electrical and optical functions such that collectively a
WOLED of the highest possible efficiency can be achieved. Moreover, device degradation
mechanisms are still unclear for most OLED systems, especially blue phosphorescent
OLEDs. Such challenges require a deep understanding of the device operating principles
and materials/device degradation mechanisms.
This thesis will focus on achieving high-efficiency and color-stable all-
phosphorescent WOLEDs through optimization of the device structures and material
compositions. The operating principles and the degradation mechanisms specific to all-
phosphorescent WOLED will be studied.
First, we investigated a WOLED where a blue emitter was based on a doped mix-
host system with the archetypal bis(4,6-difluorophenyl-pyridinato-N,C2) picolinate
iridium(III), FIrpic, as the blue dopant. In forming the WOLED, the red and green
components were incorporated in a single layer adjacent to the blue layer. The WOLED
efficiency and color were optimized through variations of the mixed-host compositions to
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control the electron-hole recombination zone and the dopant concentrations of the green-
red layers to achieve a balanced white emission.
Second, a WOLED structure with two separate blue layers and an ultra-thin red and
green co-doped layer was studied. Through a systematic investigation of the placement of
the co-doped red and green layer between the blue layers and the material compositions of
these layers, we were able to achieve high-efficiency WOLEDs with controllable white
emission characteristics. We showed that we can use the ultra-thin co-doped layer and two
blue emitting layers to manipulate exciton confinement to certain zones and energy transfer
pathways between the various hosts and dopants.
Third, a blue phosphorescent dopant tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-
imidazole]iridium(III) (Ir(iprpmi)3) with a low ionization potential (HOMO 4.8 eV) and
propensity for hole-trapping was studied in WOLEDs. In a bipolar host, 2,6-bis(3-
(carbazol-9-yl)phenyl)-pyridine (DCzPPy), Ir(iprpmi)3 was found to trap holes at low
concentrations but transport holes at higher concentrations. By adjusting the dopant
concentration and thereby the location of the recombination zone, we were able to
demonstrate blue and white OLEDs with external quantum efficiencies over 20%. The
fabricated WOLEDs shows high color stability over a wide range of luminance. Moreover,
the device lifetime has also been improved with Ir(iprpmi)3 as the emitter compared to
FIrpic.
Last, we analyzed OLED degradation using Laser Desorption Time-Of-Flight Mass
Spectrometry (LDI-TOF-MS) technique. By carefully and systematically comparing the
LDI-TOF patterns of electrically/optically stressed and controlled (unstressed) OLED
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devices, we were able to identify some prominent degradation byproducts and trace
possible chemical pathways involving specific host and dopant materials.
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Contributors and Funding Sources
This work was supervised by a dissertation committee consisting of Professor
Ching W. Tang (advisor) and Professor Alexander A. Shestopalov (committee member) of
the Department of Chemical Engineering, Professor Lewis J. Rothberg (co-advisor) of the
Department of Chemistry, Professor Yongli Gao (committee member) of the Department
of Physics and Astronomy, and Professor John C. Lambropoulos (committee chair) of the
Department of Mechanical Engineering.
Throughout the entire thesis, the organic boats used were based on an initial design
by previous fellow lab-member Dr. Sang-min Lee, Mr. Joseph Madathil and Professor
Ching Tang.
For Chapter 4, the data analyses were conducted in part by Professor Ching W.
Tang and Professor Lewis J. Rothberg and were published in 2016, in an article listed in
the Biographical Sketch.
For Chapter 5, the data analyses were conducted in part by Professor Ching W.
Tang and Dr. Shou-Cheng Dong. The results were presented at the 2016 MRS Spring
Meeting & Exhibit in Phoenix, AZ.
For Chapter 6, Dr. Shou-Cheng Dong of HKUST performed TOF/TOF experiment
and DFT calculation of TAPC, which was supported by IAS at HKUST. Part of the the
analyses were conducted in part by Dr. Dong and Professor Tang, and were submitted for
publication in 2016, in an article listed in the Biographical Sketch.
All other work conducted for this dissertation was completed by Lisong Xu
independently.
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Table of Contents
Biographical Sketch ii
Acknowledgements iv
Abstract vi
Contributors and Funding Source ix
List of Tables xiv
List of Figures xvii
Chapter 1 Background and Introduction 1
1.1. Introduction to White Organic Light Emitting Diodes 1
1.2. Basics of OLEDs 2
1.2.1. Basic Device Physics 2
1.2.2. Fluorescence and Phosphorescence from OLEDs 4
1.2.3. Energy Transfer and Quenching in OLEDs 6
1.3. Performance Characterization of WOLEDs 9
1.4. Status of WOLED Development 12
1.4.1. All-Fluorescent WOLEDs 15
1.4.2. All-Phosphorescent WOLEDs 17
1.4.3. Hybrid WOLEDs 20
1.4.4. TADF WOLEDs 23
1.5. Device Stability and Degradation Mechanism of WOLEDs 25
1.5.1. Instability of Blue Phosphorescent Materials 26
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1.5.2. MALDI-TOF-MS 27
1.6. Objectives and Outline of the Thesis 29
References 32
Chapter 2 Experimental Methods and Materials 40
2.1. Vacuum Vapor Deposition Process 40
2.2. Boat Design and Coater Specifications 42
2.3. Device Fabrication Conditions 45
2.4. Device and Material Characterization 48
2.5. Device Lifetime Test 49
2.6. LDI-TOF-MS Analysis 50
2.7. Materials 51
Chapter 3 White Organic Light-Emitting Diodes with FIrpic in a Mixed-Host 56
3.1. Introduction 56
3.2. Results and Discussion 57
3.2.1. Effects of an mCP Buffer Layer 58
3.2.2. Effects of Host Types for FIrpic 60
3.2.3. Effects of Red Dopant Concentration 65
3.2.4. The Role of a Non-Doped Interlayer 68
3.3. Conclusions 70
References 72
Chapter 4 High Efficiency White Organic Light-Emitting Diodes with an Ultra-Thin
Red and Green Co-Doped Layer and Dual Blue Emitting Layers 74
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4.1. Introduction 74
4.2. Results and Discussion 75
4.3. Conclusions 87
References 88
Chapter 5 Investigation of Phosphorescent Blue and White Organic Light-Emitting
Diodes with High Efficiency and Improved Lifetime 90
5.1. Introduction 90
5.2. Results and Discussion 91
5.3. Conclusions 102
References 103
Chapter 6 Investigating Chemical Degradation Mechanism of High-Triplet-Energy
Materials in Blue Phosphorescent OLED Using LDI-TOF 106
6.1. Introduction 106
6.2. Results and Discussion 108
6.2.1. Device Performance and Lifetime Evaluation 108
6.2.2. Overall Stability Assessment of the Blue PhOLED 109
6.2.3. Degradation of Blue Dopant 114
6.2.4. Degradation of TAPC 114
6.2.5. Degradation of TCTA, DCzPPy and TmPyPB 121
6.3. Conclusions 127
References 128
Chapter 7 Summary and Future Work 130
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References 137
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List of Tables
Table 2.1: Materials used throughout this thesis. HOMO/LUMO/triplet energies were
taken from literature. 52
Table 3.1: EL performance of WOLEDs with the mCP buffer layer. ITO (110
nm)/TAPC:MoO3 (40%, 10 nm)/HTL (30nm)/TCTA:TPBi:FIrpic(28%:57%:15%,
4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20
nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). (Measured at a current density of 5
mA/cm2) 59
Table 3.2: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110
nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (27 nm)/mCP (3 nm)/TCTA:TPBi:FIrpic (x:y,
15%, 4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20
nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). (Measured at a current density of 5
mA/cm2) 61
Table 3.3: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110
nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (27 nm)/mCP (3 nm)/TCTA:TmPyPB:FIrpic
(x:y, 15%, 4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20
nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). (Measured at a current density of 5
mA/cm2) 63
Table 3.4: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110
nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (30 nm)/mCP (x nm)/TCTA:DCzPPy:FIrpic
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(y:z, 15%, 4nm)/DCzPPy:/Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20
nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). (Measured at a current density of 5
mA/cm2) 65
Table 3.5: EL performance of WOLEDs with different red dopant concentrations. ITO (110
nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (30 nm)/mCP (3
nm)/TCTA:DCzPPy:FIrpic(28%:57%:15%, 4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3
(x%, 6%, 6 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm).
(Measured at a current density of 5 mA/cm2) 67
Table 3.6: EL performance of WOLEDs with interlayers. ITO (110 nm)/TAPC:MoO3
(40%, 10 nm)/TAPC (30 nm)/TCTA:FIrpic (85%:15%, 4nm)/Interlayer/Host:/Ir(2-
phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%, 10
nm)/Al (100 nm). (Measured at a current density of 5 mA/cm2) 70
Table 4.1: EL Performance of devices with four different ultra-thin layer doping conditions.
ITO (110nm)/HATCN (3 nm)/TAPC(37 nm)/TCTA:FIrpic (15%, 4nm)/TCTA:Ir(2-
phq)2(acac):Ir(ppy)3 (x%:y%, 0.5 nm)/DCzPPy:FIrpic (20%, 3nm)/TmPyPB (10
nm)/TmPyPB:Cs2CO3 (50%, 30 nm)/Al (100 nm). (a: values at 5 mA/cm2; b:
luminance range from 400 to 4000 cd/m2.) 79
Table 4.2: EL Performance of devices with various thicknesses of the ultra-thin red and
green co-doped layer (driven at 5 mA/cm2). ITO (110nm)/HATCN(3 nm)/TAPC (37
nm)/TCTA:FIrpic (15%, 4 nm)/TCTA:Ir(2-phq)2(acac):Ir(ppy)3 (2%:6%, x
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nm)/DCzPPy:FIrpic (20%, 3 nm)/TmPyPB (10 nm)/TmPyPB:Cs2CO3 (50%, 30
nm)/Al (100 nm). 82
Table 4.3: EL Performance of white devices with selectively blue doped emitting layers
(driven at 5 mA/cm2). ITO (110nm)/HATCN (3 nm)/TAPC(37 nm)/TCTA:FIrpic (x%,
4 nm)/TCTA:Ir(2-phq)2(acac):Ir(ppy)3 (2%:6%, 0.5 nm)/DCzPPy:FIrpic (y%, 3
nm)/TmPyPB (10 nm)/TmPyPB:Cs2CO3 (50%, 30 nm)/Al (100 nm). 84
Table 4.4: EL Performance of blue devices with selectively blue doped emitting layers
(driven at 5 mA/cm2). ITO (110nm)/HATCN (3 nm)/TAPC(37 nm)/TCTA:FIrpic (x%,
4 nm)/ /DCzPPy:FIrpic (y%, 3 nm)/TmPyPB (10 nm)/TmPyPB:Cs2CO3 (30 nm)/Al
(100 nm). 86
Table 5.1: EL Performance of WOLEDs with various Ir(iprpmi)3 concentrations.
ITO/HATCN(3 nm)/TAPC(40 nm)/TCTA(4 nm)/TCTA:Ir(2-phq)2(acac):Ir(ppy)3
(2%, 6%, 1 nm)/DCzPPy:Ir(iprpmi)3 (x%, 4 nm)/TmPyPB(10
nm)/TmPyPB:Cs2CO3(50%, 30 nm)/Al (100 nm). (a: values at 5 mA/cm2, b: measured
at current densities from 0.05 mA/cm2 to 20 mA/cm2.) 97
Table 6.1: List of mass peaks and their proposed structures. 112
Table 6.2: List of mass peaks and their proposed structures. 124
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List of Figures
Figure 1.1: (a) Energy level diagram of a single-layer OLED; (b) Modern OLED device
architecture illustration. 3
Figure 1.2: Spin states of electrons. 5
Figure 1.3: Different photon-emitting mechanisms of OLEDs [26]. 6
Figure 1.4: The schematic diagram of Förster resonance energy transfer [26]. 7
Figure 1.5: The schematic diagram of Dexter electron transfer [26]. 8
Figure 1.6: The CIE chromaticity diagram. 11
Figure 1.7: Various device layouts to realize white light emission. (a) vertically stacked
OLEDs, (b) pixelated monochrome OLEDs, (c) single-emitter-based WOLEDs, (d)
blue OLEDs with down-conversion layers, (e) multiple-doped emission layers (EMLs),
and (f) single OLEDs with a sub-layer EML design. (Reprinted with permission from
ref. [7]) 13
Figure 1.8: Characteristics of a WOLED. a) device structure and b) luminance decay curve.
The inset shows the lifetime versus initial luminance relationship. (Reprinted with
permission from ref. [56]) 17
Figure 1.9: Energy level diagram of a WOLED. Solid lines correspond to HOMO and
LUMO energies. The orange color marks intrinsic regions of the emission layer.
(Reprinted with permission from ref. [14]) 19
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Figure 1.10: Device configurations of WOLEDs. The dopants employed are FIrpic for blue
(B), Ir(ppy)2(acac) for green (G), Ir(BT)2(acac) for yellow (Y), and Ir(MDQ)2(acac)
for red (R). (Reprinted with permission from ref. [36]) 20
Figure 1.11: (a) Device architecture and the energy level diagram of the hybrid WOLED.
(b) Lifetime of the device with Bepp2 as the interlayer. (Reprinted with permission
from ref. [68]) 22
Figure 1.12: Energy-level scheme for materials used in the hybrid WOLED, and exciton
energy diagram of the EMLs. R, G, B, and Tm represent Ir(MDQ)2(acac),
Ir(ppy)2(acac), 4P-NPD, and TmPyPB, respectively. (Reprinted with permission from
ref. [69]) 23
Figure 1.13: Materials, energy-level scheme and exciton-energy transfer mechanism of a
hybrid WOLED incorporating a blue TADF material. (Reprinted with permission
from ref. [71]) 25
Figure 1.14: Schematic of a MALDI-TOF-TOF-MS setup [81]. 29
Figure 2.1: Basic design of a vacuum vapor deposition coating system. 41
Figure 2.2: Design, components and boats for organic and inorganic material deposition.
42
Figure 2.3: Boat assembly and sensor configuration. (a) Boats and sensors alignment, (b)
graphical top view of the boats assembly. 44
Figure 2.4: Configuration of ITO pattern on glass substrates. 47
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Figure 3.1: (a) Energy level diagram of all materials used in WOLEDs. (b) Device structure
of a typical WOLED. 58
Figure 3.2: (a) EL spectra of Devices B1, B2, B3 and B4. (b) EQE vs. luminance vs. PE of
Devices B1, B2, B3 and B4. (Measured at a current density of 5 mA/cm2) 61
Figure 3.3: (a) EL spectra of Devices C1, C2, C3 and C4. (b) EQE vs. luminance vs. PE of
Devices C1, C2, C3 and C4. (Measured at a current density of 5 mA/cm2) 63
Figure 3.4: (a) EL spectra of Devices D1, D2 and D3. (b) EQE vs. luminance vs. PE of
Devices D1, D2 and C3. (Measured at a current density of 5 mA/cm2) 65
Figure 3.5: (a) EL spectra of Devices E1, E2 and E3. (b) EQE vs. luminance vs. PE of
Devices E1, E2 and E3. (Measured at a current density of 5 mA/cm2) 66
Figure 3.6: (a) EL spectra of Devices F1, F2 and F3. (b) EQE vs. luminance vs. PE of
Devices F1, F2 and F3. (Measured at a current density of 5 mA/cm2) 70
Figure 4.1: Energy level diagram and device architecture of a WOLED with an ultra-thin
red, green co-doped emitting layer (LUMO and HOMO energy levels are labeled
above and below the rectangles, triplet energy levels are indicated in parentheses). 76
Figure 4.2: EQE vs current density of devices with four different ultra-thin layer doping
conditions. (Embedded are the EL spectra of the four devices driven at 5 mA/cm2.)
78
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Figure 4.3: Absorption and emission spectra of various materials used in this study
(absorption spectra are normalized at 300 nm and emission spectra are normalized to
their maxima). 81
Figure 4.4: EL Spectra of devices with various thicknesses of the red and green co-doped
layer. 82
Figure 4.5: EQE vs luminance of the devices with selectively blue doped emitting layers.
(Embedded are the EL spectra of the three devices driven at 5 mA/cm2.) 85
Figure 4.6: Transient PL decay of two FIrpic doped films. 85
Figure 4.7: Device lifetime of three blue devices with selectively doped blue emitting
layers. 87
Figure 5.1: Schematic energy level diagram of the materials used in this chapter (LUMO
and HOMO energy levels are labeled above and below rectangles, triplet energy levels
are indicated in parentheses). 92
Figure 5.2: J-V curves of hole-only and electron-only devices with various doping
concentrations of Ir(iprpmi)3. 93
Figure 5.3: Device performance of five blue OLEDs with various Ir(iprpmi)3 dopant
concentrations. (a) Current density vs. voltage, (b) EQE vs. current density, (c) EL
spectra at 5 mA/cm2. 95
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Figure 5.4: Device performance of five WOLEDs with various Ir(iprpmi)3 dopant
concentrations. (a) EQE vs. current density; (b) EL spectra at 5 mA/cm2; (c) color shift
of the device with 9% Ir(iprpmi)3 at current densities from 0.05 to 20 mA/cm2. 99
Figure 5.5: Device lifetime tested at 5 mA/cm2 (WOLEDs EL spectra are in the inset).102
Figure 6.1: Schematic energy diagram for blue PhOLEDs. (The triplet energy is in
parentheses, and HOMO/LUMO energies are below and above the rectangles). 108
Figure 6.2: Efficiencies and lifetime performances of Device A1, A2 and A3. 109
Figure 6.3: Normalized LDI-TOF spectra of Device A3 with and without degradation.110
Figure 6.4: Normalized LDI-TOF spectra of the neat Ir(iprpmi)3 film in the linear mode.
114
Figure 6.5: TOF/TOF spectrum of the TAPC cation. 116
Figure 6.6: LDI-TOF spectra of the neat TAPC film and HATCN/TAPC bilayer. 118
Figure 6.7: Dissociation energy of bonds in the neutral TAPC and TAPC cation. 119
Figure 6.8: Dissociation energy of cracking reactions after cyclohexyl is opened in the
TAPC cation. The dissociation of 1 corresponds to fragments at 570 and 591, and that
of 2 corresponds to the peak at 583. 119
Figure 6.9: Resonant structures (up) of the TAPC cation and ring-opened TAPC cation and
HOMO (down) of TAPC and ring-opened TAPC. 121
Figure 6.10: LDI-TOF-MS spectra of device B1 before and after degradation. 123
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Figure 6.11: LDI-TOF-MS spectra of Device B2 before and after degradation. 125
Figure 6.12: LDI-TOF-MS spectra of Device B3 before and after degradation. 126
1
Chapter 1 Background and Introduction
1.1. Introduction to White Organic Light Emitting Diodes
Ever since their discovery [1], great efforts have been made to develop organic light
emitting diodes (OLEDs) for display applications because OLEDs have superior properties
such as high color contrast, high brightness and power efficiency, mechanical flexibility
and light weight [2–7]. Numerous consumer products, including TVs and mobile phones,
with OLED displays have entered the consumer market. Moreover, as a possible next
generation solid-state lighting source, white OLEDs (WOLEDs) [8] have the advantages
in high-power-efficiency, large-area and flat-panel-form-factor applications [9–13].
Today, a WOLED has reportedly achieved a power efficiency of 120 lm/W, which is higher
than that of a typical fluorescent tube [14].
However, WOLEDs are still facing challenges such as high material and fabrication
costs; complex processing procedures, which typically involve high-vacuum deposition
methods; and lack of accurate theories of exciton formation, diffusion and energy transfer
in WOLEDs [15–17]. Moreover, material-degradation and device-operating-stability
issues further hinder the mass manufacturing of WOLEDs and limit their potential to
compete with LCDs for display applications and LEDs [5, 6, 18–20] for lighting
applications. Therefore, intensive studies are ongoing to achieve high-efficiency, color-
stable and long-lifetime WOLEDs for applications in lighting and displays.
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1.2. Basics of OLEDs
1.2.1. Basic Device Physics
In an OLED, light is generated by the recombination of injected electrons and holes
in an active organic layer. A very basic device structure needs only one organic layer.
Transparent indium tin oxide (ITO) is typically used as an anode, and a low-work-function
metal, such as Ca and Al, is used as a cathode. Between the sandwich-like arrangement of
electrodes is the active organic layer, which can be either small molecules or polymers.
When a voltage is applied to the device, charge carriers are injected into the active organic
layer from the electrodes, namely holes from the anode and electrons from the cathode
[21].
As shown in the energy level diagram in Figure 1.1(a), for electron injection from
the metal cathode to the organic layer, the electron from the metal needs to overcome the
energy barrier between the metal’s Fermi level and the lowest unoccupied molecular orbital
(LUMO) level of the organic material. Similarly, for hole injection, the hole from ITO
anode needs to overcome the barrier between the ITO’s Fermi level and the highest
occupied molecular orbital (HOMO) level of the organic material. After injection, the
electrons and holes are transported across the organic layer by hopping through the LUMO
and HOMO levels, respectively, of the organic molecules. The recombination of these
injected holes and electrons in the organic layer results in either heat dissipation (non-
radiative recombination) or light emission (radiative recombination or
electroluminescence) characteristic of the organic material. The recombination region
depends on the magnitude of the injection barriers and the relative mobilities of holes and
3
electrons while the color of the emitted light is governed by the HOMO-LUMO energy
difference of the organic material.
Figure 1.1: (a) Energy level diagram of a single-layer OLED; (b) Modern OLED device architecture illustration.
In almost all OLED devices, a commonly adopted structure is a stack of multi-
functional organic layers (Figure 1.1(b)). Each of the layers performs a specific charge-
transport or light-emission function with the aim of achieving the highest
electroluminescent efficiency and the lowest possible voltage and power. In general, an
OLED device is fabricated by vapor deposition with which the entire stack of layers is
sequentially deposited layer by layer on a substrate. For a typical bottom-emitting OLED
(Figure 1.1(b)), the anode is indium tin oxide (ITO) pre-deposited on a glass substrate. The
organic layer stack comprises in sequence 1) a hole-injecting layer (HIL), 2) a hole-
transport layer (HTL), 3) an emitting layer (EML), 4) an electron-transport layer (ETL), 5)
4
an electron-injecting layer (EIL) and a cathode. The cathode is typically a metal with a low
work function, such as Al, Ca or Mg. The HIL and EIL, which are typically a mixture of
strong electron donors and acceptors, act as “Ohmic” buffer layers to facilitate hole and
electron injection from anode and cathode, respectively. The HTL and ETL, which are
typically weak electron donors and acceptors, respectively, serve as media for transporting
holes and electrons to the EML. Holes and electrons recombine at the light-emitting layer
to form excitons, which can decay radiatively or non-radiatively. To enhance radiative
recombination, the emitting layer is typically a dopant-host matrix in which the dopant,
present in various concentrations, is a highly fluorescent or phosphorescent organic
compound, and the host is an organic compound or mixture capable of transporting both
holes and electrons [22].
1.2.2. Fluorescence and Phosphorescence from OLEDs
Excitons are formed by the recombination of injected electron-hole pairs in an
OLED device. There are two spin states in an exciton: total spin S = 0 (singlet: anti-parallel
spin vectors with magnetic quantum number = 0) and total spin = 1 (triplet: parallel spin
vectors with a magnetic quantum number Î[-1, 0, 1]). When an electron in the ground state
is excited, it can follow two different paths: one leading to the singlet state and another to
the triplet state. In the first path, all of the energy is used for exciting the electron, whereas
in the second path, part of the energy is used to unpair the spin. So the triplet state is at a
lower energy level. Generally, excitons in an OLED are created in a ratio of 3:1, i.e., 75%
triplets and 25% singlets, due to spin statistics.
5
Figure 1.2: Spin states of electrons.
Fluorescence-based OLEDs utilize only singlet excitons for light emission because
the transition from the lowest singlet excited state to the ground state (also a singlet) is spin
allowed. The transition from the triplet excited state to the ground state is forbidden by
symmetry; therefore, all the triplet excitons are wasted. Consequently, the internal quantum
efficiency of fluorescent OLEDs is limited to 25% [23].
Phosphorescence-based OLEDs [24] utilize both singlet and triplet excitons for
light emission. In addition to the spin-allowed singlet-transition fluorescence, the
transition from the triplet excited state to the ground singlet state is allowed through spin-
orbital coupling. This is made possible by the use of heavy-metal complexes as dopants
(such as Ir(ppy)3). Therefore, the theoretical internal quantum efficiency is 100% [25].
6
Figure 1.3: Different photon-emitting mechanisms of OLEDs [26].
A Jablonski energy diagram is shown in Figure 1.3 to explain the light-emitting
mechanism in terms of molecular energy levels. The diagram illustrates various light-
emission and exciton energy-loss pathways. Other than fluorescent and phosphorescent
emission, a third light-emitting mechanism (delayed fluorescence) is also shown. Delayed
florescence occurs when triplet excitons are converted to singlets through reverse inter-
system crossing, a process which is highly dependent on the energy gap between the lowest
singlet excited state and the triplet state.
1.2.3. Energy Transfer and Quenching in OLEDs
Excitons, either singlet or triplet, formed by electron-hole recombination in an
OLED device can suffer non-radiative decay, which results in a loss in electroluminescence
efficiency. A typical loss mechanism is quenching by which the energy of the exciton is
transferred to a quencher in the vicinity of the exciton prior to its emission as fluorescence
7
or phosphorescence. For singlet excitons, the quenching process is long range and well
known as Förster resonance energy transfer (FRET), whereas for triplet excitons, the
energy transfer is short range and known as Dexter transfer [27].
Förster resonance energy transfer refers to the phenomenon that an excited donor
transfers energy (not an electron) to an acceptor through a non-radiative process (Figure
1.4). To allow energy transfer, the absorption spectrum of the acceptor must overlap the
fluorescence spectrum of the donor. Moreover, FRET relies on the distance-dependent
transfer of energy from a donor molecule to an acceptor molecule through dipole-dipole
interaction between donor and acceptor. When the conditions are ideal for FRET to occur,
no photons will be emitted, but rather the energy is transferred from the donor-excited
energy level to the acceptor molecule, thus resulting in a decrease in the density of excited
state donors and an increase in the density of excited state acceptors. A complete FRET
energy transfer would result in fluorescence from the acceptor and complete quenching of
the donor fluorescence.
Figure 1.4: The schematic diagram of Förster resonance energy transfer.
8
Dexter electron transfer is another mechanism though which an excited donor and
an acceptor exchange electrons to accomplish the non-radiative process. It is a process
whereby two molecules bilaterally exchange their electrons. The reaction rate constant of
Dexter electron transfer exponentially decays as the distance between these two parties
increases. The exchange mechanism typically occurs within 1 nm, much shorter than the
dipole-dipole interaction in FRET. Furthermore, the exchanged electron should occupy the
orbital of the other party, which means that the exchange energy transfer needs the overlap
of the donor and acceptor wave functions. By Dexter electron transfer mechanism, triplet-
triplet annihilation can occur when two triplets (D* and A*) react to produce two singlet
states, as indicated in Equation 1.1.
!∗ + $∗®! + $ (1.1)
Figure 1.5: The schematic diagram of Dexter electron transfer.
In WOLEDs, such energy transfer from host to guest molecules and between two
different guest molecules can be utilized to shape the white emission spectrum and improve
the electroluminescence efficiency. As in most WOLEDs, white emission is realized with
dopants capable of emitting different colors (e.g., red, green and blue). These color dopants
9
can be all incorporated in a single emitting layer that comprises one or more host materials.
Alternatively, these color dopants can be individually incorporated in separate emitting
layers in the OLED stack. Most device architectures reported in the literature were
designed to provide a mechanism to control the multiple pathways for energy and charge
transfer from the host to the dopants or from one dopant to another, all within an individual
emitting layer or in adjacent emitting layers. This is usually done by tuning the thicknesses
and dopant concentrations in the individual layers, and also by employing multiple doped
emitting layers with undoped interlayers sandwiched in between. To understand these
transfer processes, it is often necessary to model the exciton density and diffusion length
in OLED systems. Based on Fick’s second law for particle diffusion, the following
equation (Equation 1.2) is often used to relate the exciton density at a specific location in
the emitting layer to the exciton (or excited state) lifetime, where Lx is the diffusion length,
n0 is the exciton density at the interface where the electron-hole recombination occurs, D
is the diffusion constant and & is the excited-state lifetime [28].
' ( = '*+,-
./012ℎ4- = !&(1.2)
1.3. Performance Characterization of WOLEDs
The efficiency of an OLED is characterized by its external quantum efficiency
(hext), current efficiency (hL) and luminous efficiency (hp). The external quantum
efficiency (also known as EQE) is defined by the ratio of the number of photons emitted
by an OLED into the space outside of the OLED to the number of electrons injected. For a
typical planar OLED structure, the EQE is based on the total number of photons emitted
10
through the transmissive electrode into the viewing direction. The intrinsic quantum
efficiency (hint) is the ratio of the total number of photons generated inside the structure to
the number of electrons injected. It is the product of charge balance ratio (ge-h ≤ 1), the
fraction of emissive exciton states (hs-p) and the radiative decay efficiency (Φi). The
external efficiency is decreased by a light outcoupling factor (hout), which is the fraction of
photons that can escape the device and is limited by wave guiding in the device’s layers
and the substrate. The relation of these parameters is shown in Equation (1.3).
:;-< = :=><×:@A< = B;,C×:D,E×Φ=×:@A< (1.3)
The power efficiency (PE) is defined as the overall light output per consumed
electric power and is generally considered as the most important figure of merit for
WOLED performance. In addition, two extra parameters are needed to characterize the
color quality of WOLEDs: Commission Internationale de L’Eclairage (CIE) coordinates
and color rendering index (CRI). The CIE 1931 chromaticity diagram is shown in Figure
1.6. Along the curved boundary are monochromatic colors with wavelengths indicated in
nm. By mixing any two monochromatic colors in different proportions, any color with CIE
coordinates located between the two points can be generated. Point (0.33, 0.33) is
considered to be the “colorless” white light; however, a somewhat broad region around this
point can also be considered to be white. The black line in the diagram is defined as the
Planckian locus, which indicates the CIE coordinates that can be considered to be
variations of white colors. Each point has a correlated color temperature (CCT). The higher
11
the CCT is, the bluer the white color appears to human eyes. For most lighting applications,
the CCTs typically range from 2,700 to 5,000 K. Low-CCT or warm white light is suited
for residential lighting, whereas high-CCT or cold white light is more generally used in the
workplace.
Figure 1.6: The CIE chromaticity diagram.
Color rendering index is a parameter that is commonly used to characterize the
quality of lighting or how well the lighting matches a black-body radiation of a specific
color temperature. Color rendering index values range from 0 to 100: a CRI value above
80 is considered to be somewhat adequate for general lighting, and a CRI value greater
than 90 is considered to be excellent. WOLEDs tend to have high CRIs due to their
12
generally broad emission spectra that can be tailored with multiple emitters to more closely
resemble the black-body spectra, particularly those at low color temperatures.
OLED device lifetime t1/2 is typically defined as the time for the luminance output
from the device to drop to half of its original level. Equation 1.4 was first used by Van
Slyke [29] to provide a relationship between operating lifetime and output luminance. The
parameter L0 is the initial luminance of an OLED, t1/2 is the half-life time (time to ½ L0 at
constant current), which is inversely proportional to L0, and C is a constant.
4@×2G H = I(1.4)
This equation provides a strictly inverse relationship between the initial luminance
and half-life, which may only be valid over a narrow luminance range. More often, the
lifetime of an OLED is much shorter when it is operated at high current densities (i.e. high
luminance levels). For a more accurate lifetime projection, a modified equation (Equation
1.5) has been adopted, where n is the acceleration coefficient to account for a steeper rate
of luminance loss at higher luminance values. [30–32].
4@>×2G H = I (1.5)
1.4. Status of WOLED Development
For WOLEDs, the device structure tends to be much more complex due to the fact
that a single organic-molecule-based emitter generally cannot provide a sufficiently broad
spectral range to produce a white-color emission, which requires red, green and blue (RGB)
color components. Because of their nearly 100% IQEs, phosphorescent emitters are
expected to be used in high-efficiency WOLEDs. Currently, commercial WOLEDs use
13
phosphorescent emitters for green and red colors, and fluorescent emitters for blue owing
to the lack of stable phosphorescent blue emitters. To achieve high performance WOLEDs,
various device structures (Figure 1.7) have been adopted, including 1) a single EML with
multiple dopants [33–35], 2) multiple EMLs to improve color renditions [14, 15, 36–38],
3) hybrid (incorporating both fluorescent and phosphorescent emitters) WOLEDs [39–42]
for better device lifetime, and 4) tandem devices to increase lifetime and luminance output
[43, 44]. Some of these device designs can lead to a very complex light generation process
that involves charge- and energy-transfer processes among various molecular species in
their ground or excited states within an individual layer and/or between separate layers.
Figure 1.7: Various device layouts to realize white light emission. (a) vertically stacked OLEDs, (b) pixelated monochrome OLEDs, (c) single-emitter-based WOLEDs, (d) blue
OLEDs with down-conversion layers, (e) multiple-doped emission layers (EMLs), and (f) single OLEDs with a sub-layer EML design. (Reprinted with permission from ref. [7])
14
To improve the performance of WOLEDs, two [4, 45, 46] or three [43, 47, 48]
phosphorescent emitters are necessary. These phosphorescent emitters can be incorporated
into a single layer or distributed in multiple layers. The latter has the advantage of a wider
scope for optimizing the color quality and efficiency of WOLEDs, although the fabrication
process may be more complicated.
To date, most research interests are focused on WOLEDs with a multiple-emitting-
layer (multi-EML) structure. For multi-EML WOLEDs, one of the biggest challenges is to
manage the distribution of excitons among the two or more emitters to realize white
emission with a desired spectrum [49]. There are several approaches: 1) insert an interlayer
to block the undesirable energy transfers between adjacent emitting layers, 2) tune the
dopant concentrations and the individual layer thicknesses to either facilitate or reduce the
energy transfers, and 3) control the recombination location with a host material or a
combination of host materials with specific transport characteristics. Another issue with
multi-EML WOLEDs is color shift due to a shift in recombination zone with a varying
drive voltage. Hence, a careful design of the device layer architecture is needed to produce
a high-performance multi-EML WOLED with a good color quality and stability.
There are mainly four types of WOLED: all-fluorescent WOLED, all-
phosphorescent WOLED, hybrid fluorescent-phosphorescent WOLED, and thermally
activated delayed fluorescence (TADF) WOLED. These types are briefly described below.
15
1.4.1. All-Fluorescent WOLEDs
All fluorescent WOLEDs can be categorized into single-EML WOLEDs and multi-
EML WOLEDs. In a single-EML WOLED, there are two ways to achieve white-color
emission: 1) The EML is composed of a red/yellow fluorescent guest doped into a blue
fluorescent host. The concentration of the guest is typically below 1%. Due to the low
concentration of the guest, exciton energy transfer from the blue host molecules to the
red/yellow guest molecules is incomplete, which results in partial blue emission from the
host and red/green emission from the guest, thus leading to the realization of white
emission [50–52]. 2) A non-emitting material is used as the host with appropriate color
dopants, including a blue dopant. White emission can be obtained by adjusting the
concentrations of color dopants [53].
Single-EML WOLEDs are generally less efficient than multi-EML WOLEDs.
Chuen and Tao [50] reported a single-EML WOLED using 4-{4-[N-(1-naphthyl)-N-
phenylaminophenyl]}-1,7-diphenyl-3,5-dimethyl-1,7-dihydro-dipyrazolo [3,4-b;4’3-e]
pyridine (PAP-NPA) as a blue host and rubrene as a yellow/red dopant. The rubrene
concentration is only 0.5%. The detailed device structure is as follows: ITO/4,4'-bis[N-
(1naphthyl)-N-phenyl- amino]-biphenyl (NPB) (40 nm)/PAP-NPA:rubrene (20
nm)/TPBi(40 nm)/Mg:Ag. The WOLED has a maximum luminance efficacy of 2.92 lm/W
at 6.5 V and a current efficiency of 6.11 cd/A at 7.0 V with a CIE of (0.33, 0.33). Kim et
al. [53] used 9,10-Di(naphth-2-yl)anthracene (ADN) as a host, 4,4'-Bis(9-ethyl-3-
carbazovinylene)-1,1’-biphenyl (BCzVBi), 2,3,6,7-Tetrahydro-1,1,7,7,-tetramethyl-1H
,5H ,11H -10-(2-benzothiazolyl)quinolizino[9,9a ,1gh]coumarin (C545T), and 4-
16
(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran
(DCJTB) for blue, green, and red emission, respectively, in a single-EML WOLED. The
device structure is as follows: ITO/NPB (70 nm)/ADN:7% BCzVBi:0.05% C545T:0.1%
DCJTB (30 nm)/Bphen (30 nm)/Liq (2 nm)/Al (1,200 Å). Optimizing energy transfer
between the guest emitters resulted in a maximum current efficiency of 9.08 cd/A and a
CRI of 82.
For multi-EML WOLEDs, the device architectures are more complex, and
performance optimization involves many factors, including material composition for each
EML layer, its placement with respect to other EML layers, and layer thicknesses. Ho et
al. reported a highly efficient all-fluorescent WOLED in 2007 [54]. They used a dual EML
comprised of 1) 1,4-di-[4-(N,N-diphenyl)amino]styryl-benzene (DSA-ph) as a blue-
emitting host and rubrene as a yellow-emitting guest and 2) a pristine DSA-ph layer
without any dopant. White emission with CIE of (0.32, 0.43) was obtained. Furthermore,
by adopting a p-i-n device architecture (ITO/p-HTL/1,1-bis((di-4-
tolylamino)phenyl)cyclohexane (TAPC) (50 nm)/MADN:0.2% Rubrene:3% DSA-ph:5%
NPB (10 nm)/MADN:3% DSA-Ph:5% NPB (5 nm)/BPhen (10 nm)/n-ETL/LiF (1 nm)/Al
(150 nm)) with TAPC as the hole-transport layer, the device operating voltage was greatly
reduced and a power efficiency of 9.3 lm/W at 1000 cd/m2 and 3.4 V was achieved.
Long WOLED lifetime has been demonstrated in all-fluorescent WOLEDs. Duan
et al. [55] reported a half- lifetime of 150,000 h at an initial brightness of 1,000 cd/m2.
Figure 9 shows the detailed device structure and lifetime performance. The unique feature
of this WOLED is that two blue EMLs are positioned adjacent to each other. Both EMLs
17
use ENPN (6,6�-(1,2-ethenediyl) bis(N- 2-naphthalenyl-N-phenyl-2-naphthalenamine) as
a blue emitter. The blue EML adjacent to the ETL (Alq3) utilizes neat α, β-ADN as the
host. The other blue EML contains a mixture of α, β-ADN and NPB as a host, where NPB
is intended to broaden the recombination region. The third EML layer comprises a yellow
emitter DDAF (3,11-Diphenylamino-7,14-diphenylacenaphtho[1,2-k] fluoranthene) in a
mixed α, β-ADN and NPB host. This WOLED with three EMLs reportedly has a lifetime
that is almost 40 times longer than that of a conventional WOLED.
Figure 1.8: Characteristics of a WOLED. a) device structure and b) luminance decay curve. The inset shows the lifetime versus initial luminance relationship. (Reprinted with
permission from ref. [56])
1.4.2. All-Phosphorescent WOLEDs
In 2009, Wang et al. [56] presented a high-efficiency WOLED that incorporated
two phosphorescent dyes in a single-EML WOLED with a device structure as follows:
ITO/NPB (40 nm)/4,4',4''-Tris(N-carbazolyl)triphenylamine (TCTA) (5 nm)/mCP:6.5%
(a) (b)
18
FIrpic:0.75% (fbi)2Ir(acac) (20 nm)/TAZ (40 nm)/LiF/Al. This study shows that the blue
emission originates from energy transfer (mCP to FIrpic), whereas the orange emission is
a result of direct exciton formation on (fbi)2Ir(acac) due to its low HUMO level that traps
holes. Such a WOLED yields a peak power efficiency of 42.5 lm/W and EQE of 19.3%.
Unipolar host materials such as the hole-transporting material mCP and electron-
transporting material 9,9’-spiro-bisilaanthracene (UGH4) have been used as host materials
of the emitter layers in WOLEDs. Because of their unipolar nature, the recombination
region is confined at the interface adjacent to the HTL or EML, which can lead to more
severe TTA and triplet-polaron quenching [57]. To overcome such a shortcoming, bipolar
host materials or mixed-host systems are used to widen the recombination region [58–61];
2,6-bis(3-(9H-Carbazol-9-yl)phenyl)pyridine (DCzPPy) is a typical bipolar host. The hole
and electron mobilities of DCzPPy are both around 10-5 cm2/V•s. Liu et al. [64] developed
a high-efficiency WOLED with a configuration of ITO/MeO-TPD:F4-TCNQ (100 nm,
4%)/TAPC (20 nm)/DCzPPy:FIrpic:(MPPZ)2Ir(acac) (8 nm, 25%:1%)/TmPyPB (45
nm)/LiF (1 nm)/Al (200 nm) where DCzPPy is the bipolar host. Such a WOLED shows a
power efficiency of 37.1 lm/W at 100 cd/m2 and 31.3 lm/W at 1,000 cd/m2.
Reineke et al. [14] reported a much improved WOLED based on a multi-EML
structure presented in Figure 1.9. Two blue EMLs (FIrpic in TCTA and FIrpic in TPBi, 2
nm each) are sandwiched between a red EML (Ir(MDQ)2(acac)-doped TCTA) and green
EML (Ir(ppy)3-doped TPBi). TCTA and TPBi are hole-transport and electron-transport
materials, respectively. In this WOLED, the recombination zone is confined at the
interface between two blue EMLs. By sandwiching the two blue EMLs with a red EML
19
and a green EML, more excitons can be harvested. Together with an outcoupling structure,
a power efficiency of 90 lm/W at 1,000 cd/m2 was obtained for the WOLED.
Figure 1.9: Energy level diagram of a WOLED. Solid lines correspond to HOMO and LUMO energies. The orange color marks intrinsic regions of the emission layer.
(Reprinted with permission from ref. [14])
Chang et al. [36] fabricated a WOLED with 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl
(CBP) as a common host material for four individual EMLs (red, orange, green and blue).
The device structure is illustrated in Figure 1.10. The exciton recombination region is
located at the CBP/TPBi interface. By utilizing triplet energy conversion, FIrpic excitons
can efficiently transfer energy to green, orange and red dopants sequentially. Such Förster-
type energy transfer was found to have an efficiency of 90%, thus resulting in a WOLED
with an EQE of 20.4% at 5,000 cd/m2.
20
Figure 1.10: Device configurations of WOLEDs. The dopants employed are FIrpic for blue (B), Ir(ppy)2(acac) for green (G), Ir(BT)2(acac) for yellow (Y), and Ir(MDQ)2(acac)
for red (R). (Reprinted with permission from ref. [36])
1.4.3. Hybrid WOLEDs
Although a WOLED with phosphorescent blue emitters typically yields high device
efficiency, the device lifetime is relatively short. One main reason for the short lifetime is
the fast degradation of the blue phosphorescent materials. Moreover, the high triplet energy
level of such blue emitters requires host and transport materials with a large band gap and
a high triplet energy level. Hence, the device operating voltage would increase, which may
lead to more severe electrochemical and thermal degradation of the materials.
To overcome such shortcomings, hybrid WOLEDs with fluorescent blue and
phosphorescent red/green emitters have been widely studied. Using device-structure
engineering and materials selection, it has been proven that triplet excitons of fluorescent
emitters can transfer their energy to triplet states of red and green phosphorescent materials
[62–65]. Such triplet harvesting mechanisms can enhance device efficiency and prolong
device lifetime [66, 67].
21
The biggest challenge to realizing high-efficiency hybrid WOLEDs is separating
the two exciton-harvesting pathways (blue singlet and red/green triplet). This requires the
triplet energy level of the blue fluorescent material to be higher than the triplet energy level
of the red and green phosphorescent materials. If this condition is not met, a specifically
designed device structure (such as one in which a spacer is inserted to block Förster energy
transfer) is needed to alleviate triplet quenching by the blue fluorescent materials.
Liu et al. [68] reported a hybrid WOLED structure with an extremely long lifetime.
The detailed device structure is as follows: ITO/MeO-TPD:F4-TCNQ (100 nm, 4%)/NPB
(20 nm)/MADN:DSA-ph(20 nm, 7%)/Interlayer (3 nm)/Bebq2 :Ir(MDQ)2(acac) (9 nm,
5%)/Bebq2 (25 nm)/LiF (1 nm)/Al (200 nm). The blue fluorescent emitter DSA-ph was
doped into MADN, and the red phosphorescent emitter Ir(MDQ)2(acac) was doped into a
Bebq2. An n-type interlayer was inserted between the blue and red layers. By varying the
types of the interlayer material, it was found that when Bepp2 was used as the interlayer,
the hybrid WOLED had a half-lifetime of 30,000 h (Figure 1.11). However, the device
power efficiency is reported to be only 16.0 lm/W at 100 cd/m2.
22
Figure 1.11: (a) Device architecture and the energy level diagram of the hybrid WOLED. (b) Lifetime of the device with Bepp2 as the interlayer. (Reprinted with permission from
ref. [68])
To further improve device efficiency, high triplet fluorescent materials have been
studied to avoid triplet quenching and the use of an interlayer. N,N�-di-1-naphthalenyl-
N,N-diphenyl-[1,1�:4�,1�:4�,1�-quaterphenyl]-4,4�-diamine (4P-NPD) was first
used by Schwartz et al. [39] to fabricate a hybrid WOLED. 4P-NPD has a triplet energy of
2.3 eV (higher than red/yellow phosphorescent emitters) and a photoluminescence
quantum yield of 92%. Blue triplet harvesting can be realized with proper device
engineering. Sun et al. [69] reported a high-performance hybrid WOLED structure without
(a)
(b)
23
an interlayer between the fluorescent and phosphorescent EMLs. The detailed EML
structure is as follows: TCTA:4% Ir(MDQ)2(acac) (3.5 nm)/TCTA:8%Ir(ppy)2(acac) (5
nm)/TCTA:TmPyPB:4P-NPD(73%:25%:2%, 7 nm). Figure 1.12 illustrates the complete
device stack and device performance characteristics. 4P-NPD was doped into a mixed-host
of TCTA and TmPyPB. The mixed-host broadens the fluorescent blue emission region,
while the blue triplet energy can be transferred to the red and green emitters. The low
concentration of 4P-NPD also minimizes formation of non-luminescent triplet excited
states of 4P-NPD. As a result, such a WOLED has an EQE of 17.0% and a power efficiency
of 34.3 lm/W at 1,000 cd/m2.
Figure 1.12: Energy-level scheme for materials used in the hybrid WOLED, and exciton energy diagram of the EMLs. R, G, B, and Tm represent Ir(MDQ)2(acac), Ir(ppy)2(acac),
4P-NPD, and TmPyPB, respectively. (Reprinted with permission from ref. [69])
1.4.4. TADF WOLEDs
After a breakthrough research reported by Adachi et al. [69] in 2012, thermally
assisted delayed fluorescent (TADF) materials have been actively studied. Such materials
24
have a small singlet and triplet energy split (< 0.1 eV). Therefore, triplets can be thermally
activated and form a reverse intersystem crossing that gives rise to delayed fluorescence.
TADF materials provide a possible alternative method of fabricating high-efficiency and
long-lifetime WOLEDs. A great amount of TADF research [70–75] has been conducted
recently.
Zhang et al. [71] reported a high-efficiency and color-stable hybrid WOLED with
a blue TADF material 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN) and an orange
phosphorescent material (acetylacetonato)bis[2- (thieno[3,2-c]pyridin-4-
yl)phenyl]iridium(III) (PO-01). The device architecture is demonstrated in Figure 1.12.
2CzPN has a triplet energy level of 2.5 eV, which is higher than that of PO-01 (2.2 eV).
When the exciton recombination zone is designed to be located at the interface of the blue
and orange EMLs, blue triplets can efficiently transfer energy to the orange triplet states,
thus yielding a maximum EQE of 22.5% and a power efficiency of 47.6 lm/W.
25
Figure 1.13: Materials, energy-level scheme and exciton-energy transfer mechanism of a hybrid WOLED incorporating a blue TADF material. (Reprinted with permission from
ref. [71])
1.5. Device Stability and Degradation Mechanism of WOLEDs
Device degradation refers to drive-voltage increase and luminance reduction over
a device’s operation time. In general, there are two pathways of device degradation over
time: 1) extrinsic causes, which include material impurities, poor device encapsulation, etc.
and 2) intrinsic causes, which include device architecture and physical and chemical
degradation of the device. Device architecture usually determines the charge balance of
OLED devices. High concentrations of charges in a thin interface can typically cause
instabilities. On the other hand, material properties (such as glass transition temperature,
energy band gap, bond energy, etc.) can also greatly affect device stability. Compared to
monochrome OLEDs, WOLEDs suffer from two more device stability issues: 1) color shift
26
caused by different lifetimes of emissive materials and 2) possible recombination region
shift and exciton distribution change.
1.5.1. Instability of Blue Phosphorescent Materials
According to Universal Display Corporation (UDC, a leading OLED research
company), red, green and blue phosphorescent OLEDs have half-lifetimes of 900,000,
400,000 and 20,000 h, respectively. Phosphorescent WOLED panels have a half-lifetime
of 30,000 h [76]. The lifetimes of blue OLEDs are one order-of-magnitude shorter
compared to that of their red/green counterparts. Phosphorescent WOLEDs’ lifetime is thus
limited by blue phosphorescent materials. For example, the most commonly studied and
commercially available FIrpic blue phosphorescent material is very unstable. It has been
reported that FIrpic-based OLEDs have lifetimes ranging from minutes to approximately
100 hours [77–81]. In addition, FIrpic is unstable for hole transport [82].
To improve device lifetimes of high-efficiency phosphorescent WOLEDs, new
classes of blue phosphorescent materials with stable chemical and electrochemical
properties are in great need of development. Based on the structure–property relationship
of materials, a new series of Ir complexes with phenyl-imidazole ligands have recently
been studied. With such materials, device lifetimes reaching 10,000 h have been reported
[83–88]. For instance, in a recent report, a blue dopant tris[1-(2,6-diisopropylphenyl)-2-
phenyl-1H-imidazole]iridium(III) (Ir(iprpmi)3) has been studied and found to have a
significantly longer device lifetime compared to FIrpic [79].
27
1.5.2. MALDI-TOF-MS
It has been a difficult task to pinpoint the primary degradation pathways in OLED
systems. Nevertheless, it is now widely accepted that material degradation caused by
chemical reactions during device operation is one of the main reasons for device
degradation. There have been a few techniques reported for chemical analysis of OLEDs
to provide an insight into certain material degradation pathways. Such methods include
optical techniques such as infrared and Raman spectroscopy, surface analysis techniques
such as atomic force microscopy (AFM), depth-profiling techniques such as X-ray
photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) and
chemical analysis tools such as high-performance liquid chromatography (HPLC) coupled
with mass spectrometry (MS).
One of the most powerful and successful tools for chemical analysis of OLEDs is
matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-
TOF-MS), which is a technique used to analyze polymers and biomolecules in general [89,
90]. This technique can detect chemical compounds through their mass-to-charge ratio
even at very low concentrations. It has been demonstrated that MALDI-TOF-MS can be
utilized to analyze solid-state thin-film organic semiconductors, with the ability to study
positively or negatively charged ions of the materials and their fragments at high
resolutions.
Figure 1.14 illustrates a schematic of a MALDI apparatus. Samples are typically
dispersed over a large excess of matrix material and applied onto a metal plate. Short-
pulsed laser light (nitrogen laser light, wavelength: 337 nm) then irradiates the sample and
28
triggers desorption and ionization of the target and matrix materials. The ions formed are
then accelerated by a high voltage and enter into a flight tube where they are separated
according to their masses. The time-of-flight detector records the time-of-flight of the ions,
which are converted to mass-to-charge ratio after external or internal calibration. From the
mass-to-charge ratio, the molecular structures of the parent molecule and its fragments can
be deduced. Two types of a TOF detector are used in general: linear and reflectron. In the
linear mode, ions travel directly towards the linear detector; in the reflectron mode, ions
travel through an ion mirror (which is a series of evenly spaced electrodes onto which a
single, linear, electric field is applied) and reach the reflectron detector. A reflectron
corrects for the energy dispersion of ions leaving the source (ions of the same m/z ratio
with different starting kinetic energies), because ions with more kinetic energy penetrate
the reflectron more deeply and spend more time in it, thus compensating for the spread in
kinetic energy. This gives a substantial increase in the resolution of the TOF analyzer.
When analyzing OLED devices, matrix materials are not necessary due to the
abundance of host materials. By using LDI-TOF-MS technique, it has been reported that
in the FIrpic molecular structure, the ancillary picolinate ligand and fluorine-substituted
phenyl-pyridyl ligands are susceptible to photo-induced dissociation [81, 91].
29
Figure 1.14: Schematic of a MALDI-TOF-TOF-MS setup [81].
1.6. Objectives and Outline of the Thesis
Due to the complexity of multi-EML WOLEDs, 10 or more organic materials
(transport, host and guest materials) are typically required to form the layers, and each of
these layers has to be optimized to produce a desired function such that they can
collectively achieve a WOLED with the highest possible quantum efficiency and power
efficiency. Therefore, a deep understanding of device operating principles and mechanisms
is required. Moreover, device degradation mechanisms are still unclear for most OLED
systems. Chemical degradation can be found in almost every organic material used in an
OLED, including charge-carrier-transporting materials, emitters, host materials, etc.
Therefore, it is crucial to understand certain degradation pathways of modern OLED
devices (especially high-performance blue phosphorescent OLEDs) in order to develop
more stable materials and device architectures.
In Chapter 2, the experimental methods for the fabrication and characterization of
various WOLED devices are described.
30
In Chapter 3, a series of multi-EML WOLEDs based on FIrpic in a mixed-host are
investigated. A mixed-host system can help broaden an exciton recombination region and
improve device performance. By varying host types and device structures, device operating
principles in all-phosphorescent multi-EML WOLEDs will be discussed.
Chapter 4 is based on a published paper in Organic Electronics, in which a multi-
EML WOLED comprising two separate blue layers and an ultra-thin red and green co-
doped layer sandwiched in between was studied. Through a systematic investigation of
exciton confinement and various pathways for energy transfer among the hosts and
dopants, it was found that both the ultra-thin co-doped layer and two blue EMLs play a
vital role in achieving high device efficiency and controllable white emission.
In Chapter 5, charge carrier properties of a blue phosphorescent dopant Ir(iprpmi)3
is first studied. Ir(iprpmi)3 is found to be trapping holes when doped at a low concentration
and transporting holes when doped at a high concentration in the bipolar host material
DCzPPy. By varying the blue dopant concentration and controlling the recombination
region, blue and white OLEDs with EQEs over 20% have been achieved. The WOLED
exhibits high color stability over a wide range of luminance. Moreover, device lifetime has
also been improved compared to the common blue dopant FIrpic.
In Chapter 6, we investigate device degradation mechanisms of Ir(iprpmi)3-based
blue OLEDs using the LDI-TOF-MS technique. Materials with high triplet energy (> 2.7
eV) (TAPC, TCTA, TmPyPB and DCzPPy) were selected as host or transport materials.
By carefully and systematically comparing the LDI-TOF patterns of electrically/optically
31
stressed and controlled (unstressed) OLED devices, possible degradation pathways of each
material are proposed and discussed.
In Chapter 7, an overall conclusion of this study will be provided. Finally, future
work for further improving WOLED performance and probing device degradation
mechanisms is proposed.
32
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Chapter 2 Experimental Methods and Materials
2.1. Vacuum Vapor Deposition Process
Vacuum vapor deposition is a method for coating a thin film or multiple layers of
thin films on a substrate in a vacuum chamber. A large range of materials can be processed
with this method, including metals, high-temperature inorganic materials such as
semiconductors, and low-temperature organic compounds such as low-molecular-weight
or small molecules. For deposition of OLED materials, the temperature required is
generally low, typically below 500 oC, and resistive heating with a suitable crucible is
commonly used.
Figure 2.1 shows a typical vacuum vapor deposition system. The chamber pressure
is on the order of 10-6 torr for vapor deposition, which can be readily achieved with a turbo
or cryo pump, backed by a rotary pump. For deposition of organic materials, the crucible
or boat can be made of pyrex glass or quartz, which can be electrically heated with a
tungsten or nichrome wire. For high-temperature materials, including metals, boats made
of thin tungsten, molybdenum or tantalum foils and crucibles made of graphite, alumina,
or boron nitride are commonly used. Common substrates for OLED devices are glass or
plastic plates, and the substrates are usually kept at ambient temperature to avoid growth
of crystalline films.
41
Figure 2.1: Basic design of a vacuum vapor deposition coating system.
The thin film deposition rate is monitored by a quartz crystal microbalance (QCM)
sensor along with the necessary deposition controller. For OLED materials, the rate is
typically on the order of a few Å/s for the deposition of a single component film. For
deposition of multicomponent films, multiple QCM sensors with independent controllers
are required, and the deposition rate for each material component, which can vary from
below a tenth to several tens of Å /s, must be controlled precisely to produce a film of pre-
determined composition. The deposition rate is dependent on the source temperature which
can be manually or automatically controlled with the deposition controller. A shutter in
between the source and the substrate provides a convenient means of controlling precisely
the thickness of the film deposited on the substrate.
42
2.2. Boat Design and Coater Specifications
WOLED is a multi-layer device comprising a stack of thin organic films, some of
which contain two or more material components, such as the host and dopant in the emitter
layer. Co-deposition of two or more organic materials of various concentrations are needed,
where the concentration can be from as low as less than 1% to over 50%. Therefore,
independent rate control for all material components must be as precise as possible. To
facilitate deposition of multicomponent films in a single deposition chamber of confined
space, a customized boat configuration is required.
Figure 2.2: Design, components and boats for organic and inorganic material deposition.
43
The boat design for the deposition of organic materials was previously developed
by S. Lee of our laboratory. The design, as shown in Figure 2.2, has four parts: a glass test
tube, a coil of nichrome wire, a Macor base, and a pair of electrical contact pins. The glass
test tube (10.3 mm diameter) is cut by a Dremel rotary tool to form an open-sided cylinder
of 4.3 cm. A small side hole (~1 mm diameter) is drilled on the test tube body. The
nichrome wire (22 BNC, 0.0253” nominal diameter) is coiled to a diameter about 10 mm
and tightly fit into the glass test tube. Two holes are drilled in the Macor ceramic base in
order to hold two ends of the nichrome wire. The two ends are clamped with contact pins.
An epoxy resin is applied around the Macor base and the contact pins to bind the four parts
together. An aluminum foil is wrapped around the glass test tube to better conserve heat
and improve deposition rate stability. In general, only about 10 W electrical power is
sufficient to evaporate most organic materials.
The temperature required for most inorganic materials used in OLED devices such
as LiF, MoOx and Cs2CO3 can be as high as above 1000 ºC. Instead of pyrex glass tubes,
Macor, a ceramic material with temperature tolerance up to 1000C, was used for
constructing the boat. A 7/8” diameter Macor rod is machined into a hollowed cylinder of
a dimension similar to the glass cylinder. The bottom of the boat is not drilled through (~
5 mm) in order to hold source materials. Two holes are drilled on the bottom of the tube
for insertion of the nichrome wire, and one hole (~1 mm diameter) is drilled on the side of
boat body for rate monitoring. An aluminum foil is also wrapped around the boat body to
better conserve heat.
44
Figure 2.3: Boat assembly and sensor configuration. (a) Boats and sensors alignment, (b) graphical top view of the boats assembly.
A compact and multi-boat assembly is used for the fabrication of WOLEDs. Figure
2.3(a) shows the arrangement of the boats and the sensor positions. Because of the side
hole on the boats, QCM sensors can be placed on the side of the boats rather than on top,
providing a convenient way of monitoring the individual deposition rates of multiple
materials during co-deposition without cross-talks. Four QCM sensors (two facing back
(a)
(b)
45
and two facing front), are fixed in positions for monitoring up to four material depositions
at the same time. All of the boats (up to 16 boats) are mounted onto a movable aluminum
stage, which can slide from side to side. The boats are evenly distributed on the stage. With
this arrangement, multiple co-depositions can be done without the need to break vacuum
or rearrange the boat positions. The QCM sensors are water cooled to minimize possible
errors caused by radiant heating from the boats.
Figure 2.3(b) is a schematic top view of the boat arrangement. As an example, with
the positions of the four sensors as shown, a thin film with a composition of a blue emitter
doped into a mixed-host of TCTA and DCzPPy can be made by co-evaporation of the three
components. Moving the stage laterally to the right, a red and green co-doped TCTA film
can be achieved. Likewise, MoOx can be doped into TAPC and Cs2CO3 can be doped into
BPhen. It can be seen that this translational boat assembly is quite flexible and well-suited
for fabrication of complex OLED devices, including WOLEDs.
A cryo-pump (Cryo-torr 8) is used with the vacuum chamber. It provides a base
pressure of 5 *10-6 torr within 1.5 hours. The QCM sensors and the power supplies for the
boat sources are controlled using a computer with installed deposition software (INFICON
SQS-242).
2.3. Device Fabrication Conditions
All OLED devices were fabricated on patterned indium-tin-oxide (ITO) coated
glass substrates purchased from Tinwell Electronic Technology Company. The substrate
size is 2 inch by 2 inch. The ITO thickness is 110 nm with a sheet resistance of 15 Ω/sq.
46
The ITO pattern is shown in Figure 2.4. There are 12 narrow ITO stripes and 4 wide ITO
stripes on each substrate. Up to 6 OLEDs with different layer configurations can be
fabricated on one substrate (2 identical OLEDs per layer configuration). This is achieved
by two movable metal shutters beneath the substrate, which control the area of the substrate
that is open to film deposition. All vapor depositions were carried out at a base pressure of
10−6 torr (without breaking vacuum). For host and transport materials, the rate was set to
be around 4 Å/s. For organic dopants, the rate was generally below 1 Å/s depending on the
actual doping percentage. For inorganic materials, both MoOx and LiF rates were set to be
0.5 Å/s. Cs2CO3 deposition rate was below 0.5 Å/s depending on doping percentage.
Aluminum deposition was done using an Alumina coated tungsten boat (from R.D.
Mathies) and was manually controlled with a rate of 10~20 Å/s, up to a total thickness of
1000 Å.
Prior to film deposition, the glass substrates were cleaned in deionized water and
organic (acetone and ethanol with volume proportion of 2:1) baths with ultra-sonication
sequentially. The cleaned substrates were then dried with N2, followed by an O2 plasma
treatment before loading into the vacuum chamber.
47
Figure 2.4: Configuration of ITO pattern on glass substrates.
Figure 2.5(a) shows the organic layers and the aluminum cathode deposited onto a
pre-patterned ITO glass substrate. The overlap of the narrow ITO stripe and aluminum
cathode is the active device area (0.2 cm * 0.5 cm, 0.1 cm2). Figure 2.5(b) shows the two
movable metal shutters that can slide left/right to control the open area of the substrate for
deposition. Such a design allows a maximum of six different layer configurations to be
completed on one substrate, thus offering high throughput productivity in device
fabrication as well as reproducibility in device characteristics by minimizing fabrication
process variables such as chamber conditions and substrate differences. As shown in
48
Figure 2.5(c), by varying just one parameter (blue dopant concentration), multiple WOLED
devices (different color emission) can be fabricated on a single substrate.
Figure 2.5: (a) Photo of fabricated OLED on a ITO coated glass substrate. (b) Two pieces of metal shutters that are movable to fine control deposition conditions on one
substrate. (c) Illuminated devices with different colors on one substrates.
2.4. Device and Material Characterization
Current density-voltage (J-V) data of the OLED devices were obtained using a
Keithley sourcemeter (Keithley 2400). A Photoresearch PR650 was employed to measure
the radiometric and photometric characteristics, including electroluminescent spectra, CIE
co-ordinates, power efficacy (lm/W), current efficiency in candela per ampere (cd/A),
external quantum efficiency in photon per electron (EQE) and other parameters.
Acquisition software was provided by Eastman Kodak Company. A typical device
characterization included stepping up the current density incrementally from 0.01 to 20
mA/cm2 and collection of EL data per each current step. The EL output was measured
normal to the substrate plane with the assumption that angular distribution was Lambertian.
Thanks to the precise deposition control that the coating system provides, devices
(b) (a) (c)
49
fabricated with the same device structure showed good performance reproducibility and
only yielded EQE/PE variations of less than 5%.
In Chapter 4, the UV-Vis absorption spectra were obtained using a Perkin Elmer
Lambda 900 spectrophotometer. Photoluminescence (PL) spectra were recorded on a
Hitachi F-4600 fluorescence spectrophotometer. For exciton transient measurements, a
Quanta-ray GCR-150-10 pulsed Nd:YAG laser with THG (third harmonic generation, 355
nm) output was used to excite the films. Transient PL signals were directed through a
monochromator at 450 nm and detected using a photomultiplier and a Tektronix TDS 3052
oscilloscope.
In Chapter 6, Gauss09 was used for density function theory (DFT) calculations at
b3lyp level with 6-31g(d) as basic set. The sum of the electronic and thermal enthalpies
was used to estimate bond dissociation energy. The calculation was done by Shou-Cheng
Dong at HKUST.
2.5. Device Lifetime Test
For device lifetime evaluation, a completed OLED device (with top electrode) was
transferred to a vacuum assembly after fabrication. In this process the device was exposed
to ambient atmosphere for about 30 s. The assembly was kept at a base pressure of 50
mTorr with a mechanical pump. Such a simple encapsulation method is suitable for OLED
with relatively short lifetime. The devices were driven with a constant current density of 5
mA/cm2 at room temperature.
50
2.6. LDI-TOF-MS Analysis
In Chapter 6, laser desorption/ionization time-of-flight mass spectrometry (LDI-
TOF-MS) analysis was carried out in order to get information about the possible
degradation mechanisms in OLEDs. The spectrometer is a Brüker Autoflex III MALDI-
TOF system. Aged and unaged (control) samples were analyzed under conditions that were
kept as constant as possible. Before loading the samples on a LDI sample plate, the
aluminum cathodes were removed by Kapton tape. The N2 laser frequency was set to be
100 Hz. A total of 500 spectra were acquired at each spot position. The detected mass range
was set to be between 30 and 1,500 Dalton. All data were obtained in positive reflector
mode. To reduce material degradation induced by the laser, the laser power was increased
from 30% of the built-in power step by step (usually by an increment of 5%). Below a
certain laser power threshold, ionization of the sample material could not occur and there
was no signal in the MS spectra. Above the threshold, the MS signals increased with laser
power, usually non-linearly. The MS signal intensities (counts of ions), of the prominent
species were typically adjusted to the order of 104 counts. LDI Mass Spectra were post-
calibrated using molecular mass peaks of materials used in devices as internal standards.
The TOF/TOF experiments were performed by Shou-Cheng Dong on a Brüker
UltrafleXtreme mass spectrometer at Hong Kong University of Science and Technology
(HKUST).
51
2.7. Materials
Table 2.1 lists the acronyms, chemical names, molecular structures, device
functions, HOMO and LUMO levels, and the triplet energy levels for all of the materials
used in this thesis. They are grouped according to the function of the materials.
52
Table 2.1: Materials used throughout this thesis. HOMO/LUMO/triplet energies were taken from literature.
Acronym Chemical Name Molecular Structure Function HOMO
(eV)
LUMO
(eV)
E(T1)
(eV)
MoOx
molybdenum(VI) oxide
- HIL - - -
HATCN
1,4,5,8,9,11-
hexaazatriphenylene-
hexanitrile
HIL - - -
TAPC
1,1-bis((di-4-
tolylamino)phenyl)cyclo
hexane
HTL 5.5 2.3 2.9
mCP 1,3-Bis(N-
carbazolyl)benzene
HTL 5.9 2.4 2.9
53
Acronym Chemical Name Molecular Structure Function HOMO
(eV)
LUMO
(eV)
E(T1)
(eV)
TCTA
4,4,4-tris(N-
carbazolyl)triphenylami
ne
HTM 5.7 2.3 2.9
TmPyPB 1,3,5-tri(m-pyrid-3-yl-
phenyl)-benzene
ETM 6.7 2.8 2.8
TPBi
1,3,5-tris(2-N-
phenylbenzimidazolyl)
benzene
ETM 6.2 2.7 2.74
BPhen 4,7-diphenyl-1,10-
phenanthroline ETL 6.4 3.0 2.6
54
Acronym Chemical Name Molecular Structure Function HOMO
(eV)
LUMO
(eV)
E(T1)
(eV)
DCzPPy 2,6-bis(3-(carbazol-9-
yl)phenyl)-pyridine
Bipolar host 6.68 2.78 2.78
Cs2CO3
cesium carbonate
- EIL - - -
FIrpic
bis(4,6-difluorophenyl-
pyridinato-N,C2)
picolinate- iridium(III)
Blue emitter 5.6 2.5 2.6
Ir(iprpmi)3
tris[1-(2,6-
diisopropylphenyl)-2-
phenyl-1H-
imidazole]iridium(III)
Blue emitter 4.8 2.2 2.66
55
Acronym Chemical Name Molecular Structure Function HOMO
(eV)
LUMO
(eV)
E(T1)
(eV)
Ir(ppy)3 fac-tris(2-phenyl-
pyridinato)-iridium(III)
Green
emitter 5.3 2.9 2.4
Ir(phq)2(acac)
bis(2-phenylquinoline)-
(acetylacetonate)-
iridium(III)
Red emitter 5.3 3.1 2.0
56
Chapter 3 White Organic Light-Emitting Diodes with
FIrpic in a Mixed-Host
3.1. Introduction
Traditional OLEDs and WOLEDs are typically composed of multiple organic
layers, including an HTL, an EML and an ETL [1, 2]. However, in such device structures,
excitons are generally confined in a thin interface in which charge carriers recombine.
Accumulated excitons and charge carriers at such a thin interface can lead to exciton
quenching and induce possible photochemical reactions, and thus reduce device
performance and lifetime [3–6].
With the intent of broadening the recombination region and eliminating interfaces
where charge carriers and excitons are densely accumulated, mixed-host systems have been
reported with improved charge balance, device efficiency and lifetime (especially for blue
phosphorescent OLEDs) [7–12]. Moreover, mixed-host blue EMLs also have been
incorporated into WOLEDs to achieve better device efficiency and stability [13–17].
In this chapter, we present a dual-EML WOLED with one mixed-host blue layer
and one red and green co-doped phosphorescent layer to achieve a warm white color with
a high power efficiency and low drive voltage. The effects of host material types, co-host
composition and dopant concentrations on device performance are studied to better
understand working mechanisms of WOLEDs with a mixed-host blue EML.
57
3.2. Results and Discussion
A p-i-n structure is adopted to help reduce the device operating voltage. A typical
structure of the WOLED is ITO (110 nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (30 nm)/mCP
(3 nm)/blue EML (4 nm)/red and green EML (4 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%,
10nm)/Al(100nm). TAPC and BPhen can help confine excitons due to their relatively
higher triplet energy levels (TAPC has ET1 = 2.9 eV, which is higher than that of FIrpic
(2.62 eV); BPhen has ET1 = 2.5 eV, which is higher than that of Ir(ppy)3 (2.4 eV) and Ir(2-
phq)2(acac) (2.0 eV)). Figure 3.1(a) shows the energy level diagram of a dual-EML
WOLED device (numbers in parentheses indicate the triplet energy of each corresponding
material). Figure 3.1(b) shows a typical WOLED stack structure. Noticeably, the light blue
phosphorescent emitter (FIrpic) is doped into a mixed-host composed of a hole-
transporting material (TCTA) and an electron-transporting material (TPBi).
58
Figure 3.1: (a) Energy level diagram of all materials used in WOLEDs. (b) Device structure of a typical WOLED.
3.2.1. Effects of an mCP Buffer Layer
It is known that in OLED devices, there is a charge imbalance due to different
charge-carrier mobilities (hole mobility at the level of 10-3 cm2/(V•s), and electron mobility
in the range of 10-6~10-5 cm2/(V•s)). Such imbalanced charge carriers can limit device
efficiency as discussed in Chapter 1; therefore, we introduced a bi-layered hole-
transporting structure (TAPC + mCP) to improve the charge balance. The total thickness
of HTL is 30 nm. Device A1 had a neat TAPC as the HTL, and Device A4 had a neat mCP
as the HTL. A thin mCP layer was inserted between TAPC and EML1 for Device A2 and
A3 (3 nm and 5 nm respectively). The device structure was as follows: ITO (110
nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (30-x nm)/mCP (x nm)/EML1/EML2/BPhen (20
nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). A 10 nm TAPC was doped with 40%
MoO3 as an HIL, whereas a 10 nm BPhen was doped with Cs2CO3 as an EIL. Fifteen
59
percent FIrpic was doped into a mixed-host of TCTA and TPBi with a weight ratio of 1:2
as EML1, and 1.5% Ir(2-phq)2(acac) and 5% Ir(ppy)3 were doped into a 4 nm TPBi as
EML2. Table 3.1 summarizes the device performance measured at a current density of 5
mA/cm2.
Table 3.1: EL performance of WOLEDs with the mCP buffer layer. ITO (110 nm)/TAPC:MoO3 (40%, 10 nm)/HTL (30nm)/TCTA:TPBi:FIrpic(28%:57%:15%, 4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). (Measured at a current density of 5 mA/cm2)
Device TAPC (nm)
mCP (nm)
Voltage (V)
EQE (%)
PE (lm/W)
Luminance (cd/m2)
1931 CIE x
1931 CIE y
A1 30 0 3.73 10.6 20.3 1209 0.417 0.436 A2 27 3 3.81 13.9 26.0 1907 0.346 0.473 A3 25 5 4.27 15.4 27.8 1933 0.372 0.456 A4 0 30 9.56 12.7 9.5 1452 0.359 0.429
With the introduction of mCP, due to its lower mobility (10-4 cm2/(V•s)) [18]
compared to TAPC (10-2 cm2/(V•s)) [19] and the 0.4 eV lower HOMO level, device drive
voltage increased from 3.73 V for Device A1 to 9.56 for Device A4. Device EQEs also
increased from 10.6% for Device A1 to 15.4% for Device A3, thus achieving an overall
improvement of power efficiency from 20.3 lm/W to 27.8 lm/W. For Device A4, due to
the much higher drive voltage, PE dropped to only 9.5 lm/W. From CIE values, the four
devices exhibited different white color (a blue shift from A1 to A3). Such a color shift
indicated that the recombination region had shifted towards EML1 when mCP was inserted
between TAPC and the blue EML1. Thus, it can be concluded that the mCP layer works
as a buffer layer for holes transporting. With a proper thickness (such as 3 nm), better
60
charge-carrier balance throughout the devices and, hence, higher device efficiencies can be
achieved.
3.2.2. Effects of Host Types for FIrpic
To better study the charge-carrier recombination region, different compositions of
HTM and ETM were used as a mixed-host for FIrpic. FIrpic concentration was fixed at
15%. TCTA and TPBi were chosen as a mixed-host for FIrpic, with the following device
structure: ITO (110 nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (27 nm)/mCP (3 nm)/
TCTA:TPBi:FIrpic (x:y, 15%, 4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3 (1.5%:5%, 4
nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). From Device B1 to Device
B4, the TCTA:TPBi ratio in EML1 was 0:1, 1:5, 1:2 and 1:1, respectively. Detailed device
performance at 5 mA/cm2 is summarized in Table 3.2. In the absence of TCTA (Device
B1), the recombination region is confined to the HTL/EML1 interface. Excitons are
severely quenched, leading to a much lower EQE of 9.9%. In the spectrum (Figure 3.2),
the blue peak dominates, with a low red emission being observed, which mainly comes
from exciton diffusion and energy transfer from FIrpic to Ir(2-phq)2(acac). As the TCTA
concentration in the TCTA:TPBi mixture increases, the device drive voltage decreases;
EQE increases for Devices B2, B3 and B4. More green and red emissions are also observed
in the white spectra. This can be attributed to the shallow HOMO level (5.7 eV) and high
mobility of TCTA that enable the transfer of holes to the EML1/EML2 interface, where
holes can be easily captured by Ir(2-phq)2(acac) and Ir(ppy)3 and form excitons due to their
low energy HOMO levels.
61
Figure 3.2: (a) EL spectra of Devices B1, B2, B3 and B4. (b) EQE vs. luminance vs. PE of Devices B1, B2, B3 and B4. (Measured at a current density of 5 mA/cm2)
A well-balanced mixed-host composition in EML1 can evidently broaden the
exciton generation region and alleviate quenching due to charge accumulation in thin
interfaces. With TCTA as a component of EML1, holes are more readily transported to the
EML1/EML2 interface to directly form green and red excitons. Hence, excitons are more
efficiently used for light output, leading to higher EQEs of WOLEDs.
Table 3.2: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110 nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (27 nm)/mCP (3 nm)/TCTA:TPBi:FIrpic (x:y, 15%, 4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). (Measured at a current density of 5 mA/cm2)
Device TCTA: TPBi
Voltage (V)
EQE (%)
PE (lm/W)
Luminance (cd/m2)
1931 CIE x
1931 CIE y
B1 0:1 4.16 9.89 17.1 1138 0.353 0.439 B2 1:5 4.14 11.8 21.5 1355 0.384 0.431 B3 1:2 3.94 13.9 26.1 1633 0.375 0.438 B4 1:1 3.85 15.2 29.1 1784 0.431 0.446
62
We then employed TmPyPB as another ETM to replace TPBi and studied charge
transport of the mixed-host layer. The detailed device structures are as follows: ITO
(110nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (27 nm)/mCP (3 nm)/TCTA:TmPyPB: FIrpic
(x:y, 15%, 4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3(1.5%:5%, 4 nm)/BPhen (20
nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). From Device C1 to Device C4, the
TCTA:TmPyPB ratio in EML1 ranges was 0:1, 1:5, 1:2 to 1:1, respectively. The device
performance at a current density of 5 mA/cm2 is summarized in Table 3.3. EQEs were
greatly improved with the increase in TCTA composition, with more red and green
emissions being observed in EL spectra (Figure 3.3). This finding is in agreement with the
previous study in which TPBi was used as the ETM.
Noticeably, when only TPBi was used as the host for FIrpic (Device B1), the EQE
was 9.9%. With TmPyPB as the only host material for FIrpic (Device C1), the EQE
dropped to 5.9%. No TCTA was present in both devices; therefore, the highly concentrated
FIrpic (15%) transported holes to some extent due to its HOMO level being the same as
that of mCP (5.9 eV). Electrons had only one path which was to be transported by TPBi or
TmPyPB in each device configuration. Thus, the HTL/EML1 interface was the
recombination region. The formed TmPyPB anions (TmPyPB-) or TPBi anions (TPBi-)
have a probability of quenching FIrpic, thus leading to a decreased EQE. Comparing the
performances of Device B1 and C1, TmPyPB anions appear to be to a more efficient
exciton quencher than TPBi anions.
63
Figure 3.3: (a) EL spectra of Devices C1, C2, C3 and C4. (b) EQE vs. luminance vs. PE of Devices C1, C2, C3 and C4. (Measured at a current density of 5 mA/cm2)
Table 3.3: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110 nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (27 nm)/mCP (3 nm)/TCTA:TmPyPB:FIrpic (x:y, 15%, 4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%,10 nm)/Al (100 nm). (Measured at a current density of 5 mA/cm2)
Device TCTA:
TmPyPB Voltage
(V) EQE (%)
PE (lm/W)
Luminance (cd/m2)
1931 CIE x
1931 CIE y
C1 0:1 4.18 5.9 9.2 610 0.306 0.383 C2 1:5 4.10 8.8 13.9 908 0.357 0.386 C3 1:2 3.74 12.8 23.4 1414 0.406 0.422 C4 1:1 3.63 14.9 28.6 1654 0.440 0.433
The benefit of mixing HTM and ETM as a host for FIrpic to alleviate exciton
quenching can be realized by adopting a single bipolar host, such as DCzPPy. We
fabricated WOLEDs D1, D2 and D3 with DCzPPy as the universal host for all three
primary dopants. The device structure is as follows: ITO (110 nm)/ TAPC:MoO3 (40%, 10
nm)/TAPC (30 nm)/mCP (x nm)/TCTA:DCzPPy:FIrpic (y:z, 15%, 4 nm)/DCzPPy:Ir(2-
phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al
(100 nm). For Devices D1 and D2, a thin mCP layer was inserted between TAPC and
64
EML1 to balance charge carriers. For Devices D2 and D3, only DCzPPy was used as the
host material for FIrpic, whereas for device D1, mixed TCTA:DCzPPy host (1:2) was used
to control the EL spectra while reducing the drive voltage. Table 3.4 summarizes device
performance at a current density of 5 mA/cm2. For devices with the mCP buffer layer, EQE
was about 20% higher than that of those without mCP (see Devices D2 and D3). Although
the device drive voltage went up by 0.15 V at 5 mA/cm2, the overall PE improved from
21.9 lm/W (Device D3) to 25.3 lm/W (Device D2). With the introduction of TCTA in
Device D1, more holes pass through EML1 and get trapped by red and green dopants.
Therefore, red and green emission peaks appear more prominent in the EL spectra. Due to
the reduced drive voltage, the PE reaches 29.4 lm/W at 5 mA/cm2 with a warm white color.
Compared with the mixed-host of TCTA and TPBi/TmPyPB, the mixed-host of TCTA and
bipolar material DCzPPy balanced devices’ charge carriers more effectively, and a higher
EQE was achieved (~20% EQE). However, there was almost a 1 V drive voltage increase
at 5 mA/cm2 for Device D1, which inhibited further improvement of the PE. The increased
voltage can be contributed to two factors: 1) DCzPPy has a higher LUMO level (2.56 eV)
compared to that of TmPyPB, and a deeper HOMO level (6.05 eV) compared to that of
TCTA. Therefore, electrons and holes both experience higher energy barriers. 2) Charge
carrier mobilities of DCzPPy (hole and electron mobilities in the order of 10-5 cm2/(V•s)
[19]) are lower than those of typical unipolar hosts (such as TCTA and TmPyPB). Figure
3.4(b) illustrates the luminance-EQE-PE curves of Devices D1, D2 and D3. The
TCTA:DCzPPy mixed-host in Device D1 reduced efficiency roll-off at higher current
65
density, mainly due to an improved charge-carrier balance and a broadened recombination
region.
Figure 3.4: (a) EL spectra of Devices D1, D2 and D3. (b) EQE vs. luminance vs. PE of Devices D1, D2 and C3. (Measured at a current density of 5 mA/cm2)
Table 3.4: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110 nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (30 nm)/mCP (x nm)/TCTA:DCzPPy:FIrpic (y:z, 15%, 4nm)/DCzPPy:/Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). (Measured at a current density of 5 mA/cm2)
Device mCP (nm)
TCTA: DCzPPy
Voltage (V)
EQE (%)
PE (lm/W)
Luminance (cd/m2)
1931 CIE x
1931 CIE y
D1 3 1:2 4.76 18.5 29.4 2224 0.427 0.461 D2 3 0:1 5.20 18.1 25.3 2098 0.346 0.433 D3 0 0:1 5.05 15.1 21.9 1760 0.350 0.436
3.2.3. Effects of Red Dopant Concentration
Layer EML2 seems to be critical not only for the overall EL spectra, but also device
EQEs. The concentrations of red and green dopants determine the efficiency of energy
transfer from FIrpic to Ir(ppy)3 and Ir(2-phq)2(acac), and also from Ir(ppy)3 to Ir(2-
66
phq)2(acac). Moreover, the low-lying HOMO levels of these two dopants can efficiently
trap holes at increased dopant concentrations. We fixed the green dopant’s concentration
at 6% and varied the red dopant’s concentration for Devices E1, E2 and E3 (1%, 1.5% and
2%, respectively) to study the energy transfer among the three emitters. The thickness of
EML2 (6 nm) was made slightly larger than that of EML1to achieve better control of the
low dopant concentration. The detailed device structures are as follows: ITO (110 nm)/
TAPC:MoO3 (40%, 10 nm)/TAPC (30 nm)/mCP (3 nm)/ TCTA:DCzPPy:FIrpic
(28%:57%:15%, 4 nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3 (x%, 6%, 6 nm)/BPhen (20
nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm).
Figure 3.5: (a) EL spectra of Devices E1, E2 and E3. (b) EQE vs. luminance vs. PE of Devices E1, E2 and E3. (Measured at a current density of 5 mA/cm2)
As demonstrated in Figure 3.5(a), the devices’ EL spectra differed significantly
even with a small concentration variation of the red dopant. Device E3 had the smallest
amount of the red dopant; hence, the green emission dominates the EL spectrum. With the
67
slightly higher red dopant concentration of 1.5% for E2, the red emission surpassed the
green emission, indicating a more efficient energy transfer from the green dopant to the red
dopant. The blue emission at 474 nm remains almost unchanged. With the red dopant’s
concentration further increased to 2% for E1, the blue emission is suppressed, and the EL
spectrum is predominantly red. Moreover, a low green emission could be observed. This
finding indicates that with a higher red dopant concentration, energy transfer from both
FIrpic and Ir(ppy)3 to Ir(2-phq)2(acac) becomes more efficient.
Table 3.5 summarizes device performance at a current density of 5 mA/cm2. With
increased red dopant concentration, device drive voltage slightly increases from 4.46 V to
4.53 V. Device EQEs decrease from 14.4% to 12.2%. The combined effect of these two
factors is a significant drop in power efficiency from 26.1 lm/W (Device E3) to 16.2 lm/W
(Device E1). The change in drive voltage and EQE indicates that direct trapping of charge
that occurs at red dopant sites becomes stronger at higher red dopant concentrations.
Table 3.5: EL performance of WOLEDs with different red dopant concentrations. ITO (110 nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (30 nm)/mCP (3 nm)/TCTA:DCzPPy:FIrpic(28%:57%:15%, 4nm)/TPBi:Ir(2-phq)2(acac):Ir(ppy)3 (x%, 6%, 6 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). (Measured at a current density of 5 mA/cm2)
Device Red:Green
Voltage (V)
EQE (%)
PE (lm/W)
Luminance (cd/m2)
1931 CIE x
1931 CIE y
E1 2%:6% 4.53 12.2 16.2 1167 0.504 0.394 E2 1.5%:6% 4.52 14.7 23.7 1705 0.415 0.442 E3 1%:6% 4.46 14.4 26.1 1852 0.372 0.478
68
The competition between energy transfer from FIrpic excitons to green/red dopants
and the internal energy transfer between green and red dopants largely depends on dopant
concentration in EML2. An optimal dopant concentration would not only improve the
white color quality but also increase a device’s efficiency.
3.2.4. The Role of a Non-Doped Interlayer
As discussed in Chapter 1, the concept of inserting interlayers between EMLs to
control triplet energy transfer has been utilized in multi-EML WOLEDs. Typically, the
introduction of an extra layer would cause an increase in device drive voltage and a
decrease in EQE. The extent of voltage increase and EQE drop depends on the thickness
and the charge transport property of the interlayer.
To study the effects of interlayers on device performance, we fabricated three
WOLEDs (F1, F2 and F3) with an interlayer inserted between EML1 and EML2. To
minimize a possible drive voltage increase, the interlayer material is also used as the host
material for the red and green dopants. Two materials were investigated as the interlayer,
namely TPBi (for Device F1) and DCzPPy (for Device F2). To further study the charge-
carrier transport property of DCzPPy, Device F3 was fabricated in which DCzPPy doped
with 15% FIrpic was the interlayer. For EML1, only TCTA was used as the host to control
charge in carrier recombination region. The thicknesses of both EML1 and EML2 were
fixed at 4 nm. The interlayer thickness was fixed at 2 nm. The device structures were as
follows: ITO (110 nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (30 nm)/ TCTA: FIrpic
69
(85%:15%, 4 nm)/Interlayer/Host:Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20
nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm).
Table 3.6 summarizes device performance at 5 mA/cm2. Devices F2 and F3 have
approximately 1 V higher drive voltage than Device F1 in which TPBi is used as the host
and interlayer instead of DCzPPy. Device EQEs of F2 and F3 are almost 30% higher than
that of Device F1. In Figure 3.6(a), EL spectra of the three devices indicate that the
intensities of red and green emissions are almost identical, while the blue emission is
weaker for F2 and F3, indicating that fewer excitons were generated in EML1. DCzPPy
and TPBi have triplet energies of 2.70 eV and 2.74 eV, respectively, which are both higher
than that of FIrpic (2.62 eV); hence, the exciton quenching effect should not be severe. The
change in exciton difference in EML1 for F1 and F2 could be attributed to the bipolar
transporting property of DCzPPy, in which holes can travel through the interlayer and get
trapped by red and green dopants, thus leading to more efficient use of excitons by direct
recombination. However, for Device F1, electron transporting material TPBi was used as
the interlayer, which led to carrier recombination being confined to the EML1/interlayer
interface. Therefore, more FIrpic excitons could be generated. Compared to F2, the
introduction of FIrpic to the interlayer for Device F3 does not affect the overall device
performance (EQE, EL spectra and PE remain almost unchanged). Such findings indicate
that the exciton recombination region is located at the EML1/interlayer interface rather at
the interlayer/EML2 interface for both device F2 and F3. Figure 3.6(b) illustrates an EQE
roll-off at high luminance. The EQE of Device F1 exhibits stronger roll-off. A possible
70
reason is that triplet-polaron quenching is more severe at the EML1/TPBi interface
compared to the EML1/DCzPPy interface.
Figure 3.6: (a) EL spectra of Devices F1, F2 and F3. (b) EQE vs. luminance vs. PE of Devices F1, F2 and F3. (Measured at a current density of 5 mA/cm2)
Table 3.6: EL performance of WOLEDs with interlayers. ITO (110 nm)/TAPC:MoO3 (40%, 10 nm)/TAPC (30 nm)/TCTA:FIrpic (85%:15%, 4nm)/Interlayer/Host:/Ir(2-phq)2(acac):Ir(ppy)3 (1.5%, 5%, 4 nm)/BPhen (20 nm)/BPhen:Cs2CO3 (50%, 10 nm)/Al (100 nm). (Measured at a current density of 5 mA/cm2)
Device Interlayer 2 nm
Voltage (V)
EQE (%)
PE (lm/W)
Luminance (cd/m2)
1931 CIE x
1931 CIE y
F1 TPBi 3.80 11.9 22.9 1389 0.387 0.442 F2 DCzPPy 4.73 15.4 25.4 1907 0.393 0.469 F3 FIrpic:DCzPPy 4.67 15.4 25.7 1907 0.390 0.478
3.3. Conclusions
A systematic and layer-by-layer study was conducted to optimize the performance
of a dual-EML WOLED structure. The hole-buffer layer mCP was found to balance charge
carriers and improve device efficiency. By varying the mixed-host compositions of the blue
layer and dopant concentrations of the green-red layer, the electron-hole recombination
71
zone could be controlled, and a balanced white emission was achieved. The optimized
WOLED structure exhibits a PE of 33 lm/W and an EQE of 18% at 1,000 cd/m2.
72
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74
Chapter 4 High Efficiency White Organic Light-
Emitting Diodes with an Ultra-Thin Red and Green Co-
Doped Layer and Dual Blue Emitting Layers
4.1. Introduction
WOLEDs are currently being utilized for both display and lighting applications.
Ever since their first demonstration, the research focus has been on improving the WOLED
efficiency, brightness, and lifetime. To produce high efficiency WOLEDs, phosphorescent
emitters are indispensable, as they provide a pathway of achieving emission with a nearly
100% internal quantum efficiency. Significant enhancement in efficiency has also been
realized in various device layer architectures, including a single-layer emitter with multiple
color dopants [1, 2], a multiple-layer emitter consisting of two or more adjoining EMLs [3-
7], and hybrid WOLEDs [8-10]. To obtain multi-fold improvements in both lifetime and
brightness, tandem structures are often implemented in WOLEDs at the expense of layer
complexity [11-12].
To date, most research interest in WOLEDs is focused on multi-EML structures
because they provide better control of the recombination and emission processes, enabling
a higher efficiency. From the perspective of device fabrication, it is much easier to adopt
an emitter structure in a WOLED consisting of two broadband EMLs producing
complementary blue-green and orange-red color layers. In contrast, WOLEDs with three
primary colors tend to produce white color with a better color rendering index [13-15].
75
Introduction of an extra layer to accommodate three emitters, however, makes it
challenging to manage interlayer charge-transport and energy-transfer between the various
hosts and dopants. Those processes not only control the emission efficiency and the color
balance, but also affect color-stability at various drive voltages [7, 16-21].
In this chapter, we describe a WOLED with a triple-layer emitter structure
consisting of an ultra-thin co-doped red and green layer sandwiched in between two blue
EMLs. By tailoring the doping concentration and layer thicknesses, we can control the
exciton energy transfer amongst the hosts and dopants. Our device structure produces
WOLEDs with an extremely high EQE (over 20%) and a power efficiency of 40 lm/W at
1000 cd/m2 and 3.7 V. At the same time, the color variation is minimal over a wide range
of emission intensities.
4.2. Results and Discussion
The WOLED structure for this study is as follows: ITO (110nm)/HATCN (3
nm)/TAPC (37 nm)/TCTA:FIrpic (4 nm)/red-green co-doped layer (0.5 nm)/ DCzPPy (4
nm)/TmPyPB:FIrpic (10 nm)/TmPyPB:Cs2CO3 (50%, 30 nm)/Al (100 nm). The thickness
of each individual layer was optimized to achieve the highest external quantum yield
possible without necessarily increasing the drive voltage.
To reduce the operating voltage, HATCN was deposited on top of pre-cleaned ITO
substrates as the hole injection layer. TAPC was chosen as the hole transporting material.
The electron transporting material was doped with cesium carbonate (Cs2CO3) to help
increase electron injection efficiency from the cathode. Hole-transporting material TCTA
76
and bipolar-transporting materials DCzPPy were chosen as the two host materials for the
blue emitter FIrpic. In between the two blue EMLs, a red emitter Ir(2-phq)2(acac) and a
green emitter Ir(ppy)3 were doped into an ultra-thin TCTA layer. The deposition rate of
each organic layer was monitored by quartz crystal sensors via a side aperture on the boats.
Selected because of their relatively high triplet energy levels, TAPC (2.9 eV) and TmPyPB
(2.78 eV) serve to confine the triplet excitons generated in FIrpic (2.62 eV), Ir(ppy)3 (2.4
eV) and Ir(2-phq)2(acac) (2.0 eV) within the EMLs. These triplet energy levels are
indicated (in parentheses) in the energy level diagram as shown in Figure 4.1(a), along with
the LUMO and HOMO energy levels (labeled above and below the rectangles) for the
sequence of layers from TAPC to TmPyPB. For clarity, the corresponding WOLED
configuration including the layer thicknesses and dopant concentrations is shown in Figure
4.1(b).
Figure 4.1: Energy level diagram and device architecture of a WOLED with an ultra-thin red, green co-doped emitting layer (LUMO and HOMO energy levels are labeled above
and below the rectangles, triplet energy levels are indicated in parentheses).
77
The injected holes enter the EMLs first through the blue (TCTA:FIrpic) layer and
then the red-green (TCTA:Ir(2-phq)2(acac):Ir(ppy)3) layer. Since both of these layers use
TCTA, a hole-transporting material, as the host, the majority of holes are expected to
traverse these two layers. The injected electrons enter the EMLs through the blue
(DCzPPy:FIrpic) layer, where DCzPPy, a bipolar-transporting material, is the host. As
shown in Figure1(a), the energy offsets between TCTA and DCzPPy are substantial (0.35
eV for HOMO and 0.16 eV for LUMO), providing a suitable interface to localize electron-
hole recombination. Due to this specific arrangement for the EMLs, the long-lived triplet
excitons formed as a result of these recombination events can effectively diffuse in the
TCTA and DCzPPy hosts and are subsequently redistributed between the blue, green and
red dopants commensurate with their concentrations in these hosts and their distance from
the TCTA/DCzPPy interface.
We fabricated four devices, B1/B2, B1/R&G/B2, B1/R/B2, and B1/G/B2, having
different composite EMLs. B1/B2 is a blue device with two different blue EMLs as the
composite emitter comprised of 15% FIrpic doped TCTA (B1) and 20% FIrpic doped
DCzPPy (B2). B1/R/B2, B1/G/B2 and B1/R&G/B2, are devices with three EMLs as the
composite emitter where an ultra-thin red, green or red and green (co-doped) EML is
inserted between the blue EMLs B1 and B2, respectively. The thickness of this interlayer is
only 0.5 nm and the dopant concentrations were adjusted to produce a balanced white
emission with high efficiency.
Figure 4.2 shows the plot of external quantum efficiency (EQE) versus current
density for the four devices. Table 1 summarizes the performance data at 5 mA/cm2. It can
78
be seen that all four devices exhibit high EQE ranging from 17.5% for the blue device
B1/B2 to 20.3% for predominately green device B1/G/B2. The drive voltages for these
devices are also very similar, approximately 3.8 ± 0.1 V (at 5 mA/cm2). The power
efficiency varies substantially due to a large variation in emission colors from these
devices, ranging from 31 lm/W for B1/B2 to 52 lm/W for B1/G/B2. The B1/R&G/B2 device
provides a warm white emission with color co-ordinates of (0.458, 0.448) that shift only
slightly over a luminance range of 400-4000 cd/m2. In contrast, device B1/R/B2 shows a
cool white emission with color ordinates of (0.382, 0.400) that vary marginally over the
same luminance range.
Figure 4.2: EQE vs current density of devices with four different ultra-thin layer doping conditions. (Embedded are the EL spectra of the four devices driven at 5 mA/cm2.)
79
The inset in Figure 4.2 shows the spectral response of these four devices driven at
a current density of 5 mA/cm2. The blue B1/B2 device exhibits only FIrpic emission with a
peak at 474 nm. Device B1/R/B2 with a red-doped interlayer shows a cold white color due
to the lack of green emission whereas the B1/G/B2 device with a green-doped interlayer
exhibits mostly green emission with a peak at 510 nm. Both devices retain blue FIrpic
emission due to incomplete energy transfer. It is worth noting that the FIrpic emission is
suppressed in the B1/G/B2 device compared to the B1/R/B1 device. This feature simply
indicates that energy transfer from blue FIrpic is more efficient to Ir(ppy)3 at a higher
concentration compared to Ir(2-phq)2(acac) at a much lower concentration. However, with
a red and green co-doped layer, the red emission from the B1/R&G/B2 device is enhanced
as a result of triplet energy transfer from the green to red dopants. This transfer is in
addition to the direct channel from FIrpic to the red dopant.
Table 4.1: EL Performance of devices with four different ultra-thin layer doping conditions. ITO (110nm)/HATCN (3 nm)/TAPC(37 nm)/TCTA:FIrpic (15%, 4nm)/TCTA:Ir(2-phq)2(acac):Ir(ppy)3 (x%:y%, 0.5 nm)/DCzPPy:FIrpic (20%, 3nm)/TmPyPB (10 nm)/TmPyPB:Cs2CO3 (50%, 30 nm)/Al (100 nm). (a: values at 5 mA/cm2; b: luminance range from 400 to 4000 cd/m2.)
Device Voltage (V)a
EQE (%)a
PE (lm/W)a
Luminance (cd/m2)a
1931 CIE xa
1931 CIE ya
1931 CIE Dxb
1931 CIE Dyb
B1/R&G/B2 3.91 18.5 34.80 2170 0.458 0.448 ±0.018 ±0.012 B1/R/B2 3.88 19.2 33.73 2082 0.382 0.400 ±0.028 ±0.003 B1/G/B2 3.84 20.3 51.71 3158 0.250 0.563 ±0.007 ±0.016 B1/B2 3.77 17.5 31.67 1899 0.155 0.373 ±0.001 ±0.002
To further understand the underlying sequential exciton energy transfer mechanism
from host to dopant and dopant to dopant, we measured the photoluminescence and
80
absorption of the hosts and dopants. Figure 4.3 shows the PL and absorption spectra of the
host and dopant materials. Both TCTA and DCzPPy have a PL peak centered around 400
nm, which overlaps with the absorption of FIrpic and Ir(ppy)3, indicating that energy
transfer from these two hosts to the blue and green dopants should be efficient. In contrast,
the energy transfer to the red dopant is inefficient as the absorption of Ir(2-phq)2(acac),
which centers at 440 nm and 520 nm, has little overlap with the host PL. The PL of FIrpic
and Ir(ppy)3 peaks at 470 nm and 520 nm, respectively and overlaps well with the
absorption of Ir(2-phq)2(acac), therefore the energy transfer from the blue and green
dopants to the red dopant can be efficient. Moreover, the overlap between FIrpic emission
and Ir(2-phq)2(acac) absorption is larger than that between FIrpic emission and Ir(ppy)3
absorption. Hence, the weaker blue emission in the B1/G/B2 device compared to B1/R/B2
device can mostly be attributed to the higher Ir(ppy)3 doping concentration (6%) compared
to the rather low Ir(2-phq)2(acac) doping concentration (2%).
81
Figure 4.3: Absorption and emission spectra of various materials used in this study (absorption spectra are normalized at 300 nm and emission spectra are normalized to
their maxima).
To investigate the effects of the red and green co-doped layer on the white emission
spectrum, we fabricated three devices where the thickness of the co-doped layer is 0.5, 0.75
and 1 nm respectively. As shown in Figure 4.4, it was found that EQE is practically
identical for all three devices (see Table 4.2 for detailed EL performance). Nonetheless,
increasing the thickness of the co-doped layer causes the red emission to increase relative
to the blue emission. This indicates increased energy transfer from FIrpic and TCTA
excitons to red and green dopants. Since the blue emission from FIrpic exciton formed at
the TCTA/DCzPPy interface must be balanced with the red and green emission from the
co-doped EML, the tri-layer design presented here where we can control both composition
82
and thicknesses is an extremely flexible architecture to engineer white emission with
specific color temperatures.
Table 4.2: EL Performance of devices with various thicknesses of the ultra-thin red and green co-doped layer (driven at 5 mA/cm2). ITO (110nm)/HATCN(3 nm)/TAPC (37 nm)/TCTA:FIrpic (15%, 4 nm)/TCTA:Ir(2-phq)2(acac):Ir(ppy)3 (2%:6%, x nm)/DCzPPy:FIrpic (20%, 3 nm)/TmPyPB (10 nm)/TmPyPB:Cs2CO3 (50%, 30 nm)/Al (100 nm).
Co-doped layer thickness (nm)
Voltage (V)
EQE (%)
PE (lm/W)
Luminance (cd/m2)
1931 CIE x
1931 CIE y
0.5 3.86 17.2 33.7 2074 0.414 0.454 0.75 3.88 17.4 33.9 2091 0.468 0.458 1.0 3.86 18.5 35.4 2170 0.484 0.452
Figure 4.4: EL Spectra of devices with various thicknesses of the red and green co-doped layer.
83
To understand the function of the dual blue EMLs in the exciton formation and
energy transfer processes, we examined three white devices with the following emitter
structures:
W1: TCTA:FIrpic / TCTA:Ir(2-phq)2(acac):Ir(ppy)3 / DCzPPy,
W2: TCTA:FIrpic / TCTA:Ir(2-phq)2(acac):Ir(ppy)3 / DCzPPy:FIrpic and
W3: TCTA / TCTA:Ir(2-phq)2(acac):Ir(ppy)3 / DCzPPy:FIrpic.
For Device W1, only the TCTA layer was doped with FIrpic whereas the DCzPPy
layer was undoped; for Device W2, both the TCTA and DCzPPy layers were doped; and
for Device W3, only the DCzPPy layer was doped. The thickness of the doped or undoped
layer was 4 nm and the FIrpic concentration in TCTA and DCzPPy was 15% and 20%,
respectively. The detailed EL performance of the devices is summarized in Table 4.3. As
shown in Figure 4.5, the EQEs are almost identical regardless of the variation of the emitter
structures. However, it can be seen that the spectral responses (inset of Figure 4.5) are quite
different, especially in the blue region. Relative to the red and green emissions, Device W1
shows the strongest blue emission whereas it is the weakest in Device W3. Emissions from
the green and red dopants can come from three different pathways: 1) direct energy transfer
from host DCzPPy or TCTA to the dopants, 2) direct exciton formation at the dopants due
to hole trapping, 3) indirect energy transfer from host DCzPPy or TCTA to the dopants via
FIrpic. Pathways 1) and 2) should contribute to the red and green emission irrespective of
the emitter structures, while pathway 3) would lead to different blue emission intensity if
FIrpic excitons were to have a different lifetime in the TCTA and DCzPPy hosts. The
84
lower blue emission intensity observed in the FIrpic doped DCzPPy (Device W3) suggests
that FIrpic excitons are longer lived in DCzPPy than in TCTA.
The photoluminescence lifetime of FIrpic in TCTA and DCzPPy hosts was
measured on films (50 nm) of compositions identical to those of FIrpic doped layers used
in the devices. FIrpic in DCzPPy was found to have a lifetime of 0.94 µs compared to 0.61
µs for FIrpic in TCTA (see Figure 4.6). These lifetime results are in agreement with the
device data that the longer-lived FIrpic excitons in DCzPPy more likely undergo energy
transfer to the adjacent red and green dopants than FIrpic in TCTA.
Table 4.3: EL Performance of white devices with selectively blue doped emitting layers (driven at 5 mA/cm2). ITO (110nm)/HATCN (3 nm)/TAPC(37 nm)/TCTA:FIrpic (x%, 4 nm)/TCTA:Ir(2-phq)2(acac):Ir(ppy)3 (2%:6%, 0.5 nm)/DCzPPy:FIrpic (y%, 3 nm)/TmPyPB (10 nm)/TmPyPB:Cs2CO3 (50%, 30 nm)/Al (100 nm).
Device Voltage (V)
EQE (%)
PE (lm/W)
Luminance (cd/m2)
1931 CIE x
1931 CIE y
W1 4.04 18.6 32.1 2135 0.419 0.428 W2 3.91 18.5 34.8 2170 0.459 0.453 W3 4.05 19.2 35.2 2265 0.491 0.453
85
Figure 4.5: EQE vs luminance of the devices with selectively blue doped emitting layers. (Embedded are the EL spectra of the three devices driven at 5 mA/cm2.)
Figure 4.6: Transient PL decay of two FIrpic doped films.
86
Furthermore, we fabricated pure blue devices (without the green and red co-doped
layer) for device lifetime studies. It is known that FIrpic molecules are susceptible to
excited state dissociation [22-24] releasing the ancillary picolinate ligand. We therefore
expect a device with FIrpic doped DCzPPy as the blue emitting layer to be less stable than
one with FIrpic doped TCTA due to the longer excited state lifetime in the former. This
conjecture is consistent with the relative device lifetimes observed in three blue devices of
the following emitter structures:
B1: TCTA:FIrpic (15%) / DCzPPy,
B2: TCTA:FIrpic (15%) / DCzPPy:FIrpic (20%), and
B3: TCTA / DCzPPy:FIrpic (20%).
All three devices show blue emissions from FIrpic with relatively similar EQE of
16.8%, 15.8%, and 17.7%, respectively. However, when tested at 5 mA/cm2 with an initial
luminance of about 1800 cd/m2, Device B3 with FIrpic doped DCzPPy exhibited a
considerably lower half-life of only about 8 minutes compared to about 28 minutes for
Devices B1 and B2 where FIrpic was doped in TCTA. (Table 4.4 includes device EL
performance and Figure 4.7 shows the lifetime test data.)
Table 4.4: EL Performance of blue devices with selectively blue doped emitting layers (driven at 5 mA/cm2). ITO (110nm)/HATCN (3 nm)/TAPC(37 nm)/TCTA:FIrpic (x%, 4 nm)/ /DCzPPy:FIrpic (y%, 3 nm)/TmPyPB (10 nm)/TmPyPB:Cs2CO3 (30 nm)/Al (100 nm).
Device Voltage (V)
EQE (%)
PE (lm/W)
Luminance (cd/m2)
1931 CIE x
1931 CIE y
B1 4.60 17.7 26.7 1959 0.161 0.384 B2 4.62 16.8 25.6 1882 0.166 0.389 B3 4.95 15.8 21.8 1717 0.157 0.375
87
Figure 4.7: Device lifetime of three blue devices with selectively doped blue emitting layers.
4.3. Conclusions
We have successfully demonstrated high-efficiency WOLEDs with an emitter
structure consisting of an ultra-thin red and green co-doped layer sandwiched in between
two blue layers. Using this flexible architecture, we were able to adjust the compositions
and thicknesses of the individual layers to realize WOLEDs with EQE of nearly 20% and
luminance efficacy of over 40 lm/W (at 1000 cd/m2) and minimal color shift over a large
range of intensities (400-4000 cd/m2). We also found that the device degradation is related
to the lifetime of FIrpic excited states, which is dependent on the host materials.
88
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[23] I.R. De Moraes, S. Scholz, B. Lüssem, and K. Leo, Org. Electron. 12, 341 (2011).
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90
Chapter 5 Investigation of Phosphorescent Blue and
White Organic Light-Emitting Diodes with High
Efficiency and Improved Lifetime
5.1. Introduction
After decades of development, OLEDs have successfully been used for display
applications in mobile phones and TVs. However, as a potential solid-state lighting source
to replace traditional light sources such as LEDs, incandescent light bulbs and fluorescent
tubes [1–5], WOLEDs are still in the development stage. To achieve high-efficiency
WOLEDs, phosphorescent emitters based on heavy metals such as iridium(III) and
platinum are inevitable, thanks to their nearly 100% IQEs [6–8]. Although a number of red
and green phosphorescent materials with a high efficiency and long lifetime (> 100,000 h)
have been developed [9, 10], blue phosphorescent materials are still suffering from much
shorter device lifetimes [12, 13]. Such device instability can be attributed to various forms
of material degradation caused by chemical reactions and bond cleavage in the excited state
of the molecules (including wide band gap dopant, host and transport materials) [14–17].
Therefore, the lifetime of a high-performance WOLED is greatly restricted by the choice
of blue phosphorescent materials.
Typical iridium-based blue phosphorescent dopants, such as FIrpic and FIr6, have
been intensely studied and devices reported with EQEs above 20% [18–21] and lifetimes
ranging from minutes [22, 23] to hours [24, 25] (depending on device structures and stress
91
conditions). It has been shown that the fluorine ligand in these two materials (FIrpic and
FIr6), the ancillary picolinate ligand in FIrpic [14, 26] and the pyrazolyl-borates ligand in
FIr6 [14, 27] are susceptible to dissociation. Moreover, FIrpic has been reported to be
unstable with respect to hole transport [17].
More stable blue dopants such as Ir(iprpmi)3 with an imidazole-phenol ligand, were
first reported by Lin et al. [28]. OLED lifetimes of up to 1,000 h have been achieved [28–
30]. A recent report from our laboratory showed that the efficiency and lifetime of
Ir(iprpmi)3-based OLEDs were highly dependent on the choice of HTM and ETM [31]. In
this chapter, we describe in more detail how the properties of the blue phosphorescent
dopant (Ir(iprpmi)3) affect the performance of the blue and white OLEDs, including
improvements in lifetime over the FIrpic-based devices.
5.2. Results and Discussion
As illustrated in Figure 5.1, HATCN was used as an HIL, and TAPC as an HTL.
The EML was either Ir(iprpmi)3 or FIrpic doped into a bipolar host DCzPPy. Adjacent to
the EML was an undoped TmPyPB layer for electron transport. Cesium carbonate
(Cs2CO3) doped TmPyPB (50%) was the EIL, and aluminum was the cathode. For
WOLEDs, TCTA was the host for the red dopant (Ir(2-phq)2(acac)) and green dopant
(Ir(ppy)3). The HOMO and LUMO levels (labeled above and below the rectangles) and the
triplet energy levels (in parentheses) for the materials used are also indicated in Figure 5.1.
92
Figure 5.1: Schematic energy level diagram of the materials used in this chapter (LUMO and HOMO energy levels are labeled above and below rectangles, triplet energy levels
are indicated in parentheses).
For the study of the charge-carrier transport properties of Ir(iprpmi)3, hole-only (A)
and electron-only (B) devices with the following layer structures were fabricated: (A)
ITO/HATCN(3 nm)/TAPC(40 nm)/DCzPPy:Ir(iprpmi)3 (x%, 30 nm)/HATCN(3 nm)/Al,
and (B) ITO/TmPyPB(10 nm)/DCzPPy:Ir(iprpmi)3(x%, 30 nm)/TmPyPB(40 nm)/LiF(1
nm)/Al. For both devices, a bipolar host DCzPPy was used and the concentration of the
dopant Ir(iprpmi)3 was varied from 0% to 20% (0%, 3%, 6%, 9%, 15% and 20% namely).
Figure 5.2 summarizes the current density-drive voltage (J-V) characteristics. For
the hole-only device without Ir(iprpmi)3, the drive voltage was the lowest. With 3%
Ir(iprpmi)3, the drive voltage was substantially increased to 11 V at 5 mA/cm2. With further
increase in Ir(iprpmi)3 concentration, the drive voltage began to decrease, but only to a
level above that of the device without Ir(iprpmi)3. This J-V behavior shows that Ir(iprpmi)3
93
is an effective hole trap when doped at a low concentration in DCzPPy, which has a high
HOMO level at 6.05 eV. At higher concentrations of Ir(iprpmi)3, the hole transport took
place in Ir(iprpmi)3 in conjunction with the transport in the host, although with a lower
mobility. In contrast, for the electron-only devices (as illustrated in Figure 5.2(b)), the drive
voltage increased proportionally with increasing Ir(iprpmi)3 concentration over the entire
range of 0% to 20%. This indicates that Ir(iprpmi)3 does not support electron transport in
DCzPPy owing to the relatively higher LUMO level of Ir(iprpmi)3 (2.2 eV) compared to
that of DCzPPy (2.56 eV).
Figure 5.2: J-V curves of hole-only and electron-only devices with various doping concentrations of Ir(iprpmi)3.
A set of five blue OLEDs were fabricated with the following device structures:
ITO/HATCN(3 nm)/TAPC(40 nm)/TCTA(4 nm)/DCzPPy:Ir(iprpmi)3 (x%, 10
nm)/TmPyPB(10 nm)/TmPyPB:Cs2CO3(50%, 30 nm)/Al(100 nm), where the dopant
Ir(iprpmi)3 concentration in host DCzPPy was varied from 3% to 20%. A 4 nm TCTA
(a)
(b)
94
layer was the buffer between TAPC and the EML to provide balanced recombination.
Figure 5.3(a) shows the J-V characteristics of the five blue OLEDs. As expected, the drive
voltage decreased proportionally with an increasing Ir(iprpmi)3 concentration. At 5
mA/cm2, the drive voltage for the device with 20% Ir(iprpmi)3 was approximately 0.6 V
lower than that of the device with 3% Ir(iprpmi)3. This modest decrease in voltage was in
part due to a thinner EML (10 nm). With such a thin EML, the device’s EQE is expected
to be more sensitive to the dopant concentration. As demonstrated in Figure 5.3(b), at
5mA/cm2, devices with 15% and 20% Ir(iprpmi)3 exhibited lower EQEs compared to the
moderately doped devices (6% and 9% Ir(iprpmi)3). This can be attributed to increased
self-quenching at high dopant concentrations. The emission spectra (Figure 5.2(c)) indicate
a slight red shift with increasing Ir(iprpmi)3 concentration, which can be an indication of a
shift of the recombination region towards the EML/ETL interface.
95
Figure 5.3: Device performance of five blue OLEDs with various Ir(iprpmi)3 dopant concentrations. (a) Current density vs. voltage, (b) EQE vs. current density, (c) EL
spectra at 5 mA/cm2.
As illustrated in Figure 5.3(b), the EQE of the devices with various Ir(iprpmi)3
concentrations exhibits very different behaviors at low current densities. With low
concentrations (3%), the EQE is only about 12% at the current density of 0.01 mA/cm2. At
high Ir(iprpmi)3 dopant concentrations (15% and 20%), the EQE is even lower (at about
5%). However, with medium Ir(iprpmi)3 concentrations (6% and 9%), the EQE is the
highest at 20%. Such phenomena can be explained as follows. At a low Ir(iprpmi)3
(a) (b)
(c)
96
concentration, holes are predominantly trapped at the HTL/EML interface, and electron
dominates as the transport in the EML. Therefore, the recombination zone is confined to
the HTL/EML interface, which leads to possible polaron-triplet quenching and triplet-
triplet annihilations and, therefore, a low EQE. As the current density increases, holes
transport starts in the EML, thus resulting in the broadening of the recombination region
and, consequently, a gradual increase in EQE. For the highly doped devices (15% and 20%
Ir(iprpmi)3), holes are readily transported across the EML via the Ir(iprpmi)3 dopant in the
DCzPPy host, whereas electron injection from TmPyPB to DCzPPy is impeded by a 0.2
eV energy barrier. As the current density increases at a higher bias voltage, electrons are
more easily injected and transported through the EML, thus causing the recombination
region to shift towards the HTL/EML interface and an increased EQE. With moderate
Ir(iprpmi)3 concentrations (6% and 9%), the recombination zone is more extended inside
the EML as the holes do not get trapped at the HTL/EML interface because of assisted hole
transport via Ir(iprpmi)3), and they provide a more balanced recombination with the
injected electrons. EQEs above 20% are therefore achieved as a consequence.
Based on the structures of the preceding blue devices, a set of five WOLEDs were
fabricated with an addition of a red and green co-doped thin layer. The detailed layer
structure is as follows: ITO/HATCN(3 nm)/TAPC(40 nm)/TCTA(4 nm)/TCTA:Ir(2-
phq)2(acac):Ir(ppy)3 (2%, 6%, 1 nm)/DCzPPy:Ir(iprpmi)3 (x%, 4 nm)/TmPyPB(10
nm)/TmPyPB:Cs2CO3(50%, 30 nm)/Al(100 nm), where the Ir(iprpmi)3 concentration is
varied from 3% to 20% in the blue EML. The thickness (1nm) and doping concentration
97
for the red (2%) and green (6%) co-doped layer were fixed at optimized values based on
our previous study [32].
Table 5.1 summarizes the performance of the WOLEDs. Among them, the drive
voltage was the highest for 3% Ir(iprpmi)3 at 4.98 V and lowest for 20% Ir(iprpmi)3 at 4.07
V. This voltage trend is in agreement with the blue devices described earlier. The EQEs of
the low (3% Ir(iprpmi)3) and moderately doped (6% and 9%) devices were all above 20%,
indicating a highly effective exciton confinement with minimal exciton quenching. On the
contrary, for higher-doped devices (15% and 20% Ir(iprpmi)3), the EQEs were down to
17.3% and 14.4%, respectively. This can be partially attributed to an increase in self-
quenching at higher dopant concentration, as in the blue devices. However, with the red
and green co-doped layer between TCTA and the blue EML, the triplet energy transfer and
exciton distribution among the three dopants are more complicated and, therefore, can
strongly affect the dependence of the device’s EQEs and colors on drive conditions.
Table 5.1: EL Performance of WOLEDs with various Ir(iprpmi)3 concentrations. ITO/HATCN(3 nm)/TAPC(40 nm)/TCTA(4 nm)/TCTA:Ir(2-phq)2(acac):Ir(ppy)3 (2%, 6%, 1 nm)/DCzPPy:Ir(iprpmi)3 (x%, 4 nm)/TmPyPB(10 nm)/TmPyPB:Cs2CO3(50%, 30 nm)/Al (100 nm). (a: values at 5 mA/cm2, b: measured at current densities from 0.05 mA/cm2 to 20 mA/cm2.)
Ir(iprpmi)3 (%)
Voltage (V)a
EQE (%)a
PE (lm/W)a
Luminance (cd/m2)a
CIE xa
CIE ya
CIE Dxb
CIE Dyb
3 4.98 20.4 29.0 2299 0.494 0.428 ±0.016 ±0.010 6 4.65 21.7 33.6 2481 0.462 0.453 ±0.006 ±0.008 9 4.57 20.7 33.5 2438 0.407 0.434 ±0.001 ±0.005
15 4.39 17.3 30.3 2118 0.342 0.444 ±0.024 ±0.003 20 4.07 14.4 27.9 1805 0.309 0.449 ±0.015 ±0.005
98
Figure 5.4(a) illustrates EQE’s dependence on the current density. WOLEDs with
3%, 6% and 9% Ir(iprpmi)3 all exhibit very high EQE (about 25%) at 0.01 mA/cm2. Such
a phenomena can be explained as follows: Due to hole trapping by Ir(iprpmi)3 at low
concentrations, the recombination region is located near the red and green co-doped
layer/blue layer interface, resulting in efficient triplet energy transfer from Ir(iprpmi)3 to
Ir(ppy)3 and Ir(2-phq)2(acac), and effectively little loss of excitons generated by the
recombination processes. EQEs for these three WOLEDs only slightly roll off to 18% at
high current densities, mainly due to the usual charge quenching. For the highly doped
WOLEDs (15% and 20%), the hole current is expected to dominate at low bias due to
assisted hole transport via Ir(iprpmi)3. Hence, the carrier recombination region in these
WOLEDs is mainly located near the blue layer/ETL interface, which leads to reduced
efficiency of energy transfer from Ir(iprpmi)3 to Ir(ppy)3 and Ir(2-phq)2(acac). Since blue
excitons cannot be efficiently utilized, the EQEs are, therefore, lower at low current
densities. As the bias increases, electrons can be more easily injected and transported in
the blue layer, thus resulting in a broad recombination region and a gradual increase in
EQEs.
99
Figure 5.4: Device performance of five WOLEDs with various Ir(iprpmi)3 dopant concentrations. (a) EQE vs. current density; (b) EL spectra at 5 mA/cm2; (c) color shift of
the device with 9% Ir(iprpmi)3 at current densities from 0.05 to 20 mA/cm2.
The electroluminescence spectra of the WOLEDs are illustrated in Figure 5.4(b).
At low Ir(iprpmi)3 concentrations (3% and 6%), the red emission from Ir(2-phq)2(acac) is
predominant, whereas at higher Ir(iprpmi)3 concentrations, the blue emission gains
intensity and eventually dominates the spectrum. These spectral behaviors are a clear
indication of the shift of the recombination region from the HTL side towards the ETL side
as Ir(iprpmi)3 concentration increases.
(a) (b)
(c)
100
The details of WOLED performance are summarized in Table 5.1. The power
efficacy at 5 mA/cm2 for all five WOLEDs are remarkably high as it peaks at about 33
lm/W for 6% and 9% Ir(iprpmi)3. Table 5.1 also illustrates the CIE shifts (Dx and Dy) at
various current densities (from 0.05 to 20 mA/cm2). Surprisingly, the moderately
Ir(iprpmi)3-doped devices (6% and 9%) exhibit very small color shift (< 0.01, < 0.01) while
the low Ir(iprpmi)3-doped device (3%) and high-doped devices (15% and 20%) indicate a
marginal color shift (±0.02 and ±0.01, respectively). The color shift of WOLEDs originates
from the shift of the recombination region and redistribution of excitons among the red,
green and blue emitters at various bias values. Due to the fact that Ir(iprpmi)3 in DCzPPy
can play the role of hole trapping and transport depending on the concentration, the shift
of the recombination region can be minimized by optimizing the concentration to achieve
a WOLED with a stable white emission at various current densities. Figure 5.4(c) illustrates
the normalized spectra of a WOLED with 9% Ir(iprpmi)3 biased from 0.05 mA/cm2 to 20
mA/cm2. It can be seen that the white emission is extremely stable.
To examine the devices lifetimes, we fabricated both blue and white OLEDs with
FIrpic or Ir(iprpmi)3 as the blue dopant. The blue device layer structure is as follows:
ITO/HATCN(3 nm)/TAPC(40 nm)/TCTA(4 nm)/DCzPPy:Blue Dopant (x%, 30
nm)/TmPyPB(10 nm)/TmPyPB:Cs2CO3(50%, 30 nm)/Al(100 nm). The dopant
concentration was optimized to be 20% for FIrpic and 9% for Ir(iprpmi)3, with DCzPPy
being the host. The EQEs of the FIrpic- and Ir(iprpmi)3-based blue OLEDs were 15.8%
and 18.6% at 5 mA/cm2, respectively. For WOLEDs, in order to achieve a similar white
spectrum (see the inset of Figure 5.5), the FIrpic-based device structure was as follows:
101
ITO/HATCN(3 nm)/TAPC(40 nm)/TCTA:FIrpic(15%, 4 nm)/TCTA:Ir(2-
phq)2(acac):Ir(ppy)3 (2%, 6%, 1 nm)/DCzPPy:FIrpic(20%, 4 nm)/TmPyPB(10
nm)/TmPyPB+Cs2CO3(50%, 30 nm)/Al (100 nm), whereas the Ir(iprpmi)3-based device
structure had an Ir(iprpmi)3 concentration of 9%. The EL spectra of the two WOLEDs at 5
mA/cm2 are illustrated in the inset of Figure 5.5. The EQEs of the Flrpic- and Ir(iprpmi)3-
based WOLEDs were 16.7% and 19.2%, respectively. For the lifetime test, all devices were
driven at a constant current density of 5 mA/cm2. Figure 5.5 illustrates that the FIrpic-based
blue device is the shortest-lived with a half-lifetime of only 10 min, whereas the
Ir(iprpmi)3-based blue device had a much improved half-lifetime of 2.5 h. For the
WOLEDs, the half-lifetime is considerably better compared to the blue devices. The
FIrpic-based WOLED had a half-lifetime of about 5 h, and the Ir(iprpmi)3-based WOLED
had a half-lifetime approximately 20 h. FIrpic is highly unstable due to the loss of fluorine
substituents and the breakdown of the picolinate ligand during device operation [14, 15].
On the contrary, the phenyl-imidazole ligands in Ir(iprpmi)3 are believed to be more
electro-chemically stable, which leads to an order-of-magnitude improvement in device
lifetime. However, due to the instability of the wide band gaps of the hole transport material
(TAPC) [33, 34] and electron transport material (TmPyPB) [31], the WOLED lifetime may
also be limited by transport layers.
102
Figure 5.5: Device lifetime tested at 5 mA/cm2 (WOLEDs EL spectra are in the inset).
5.3. Conclusions
By investigating the charge-carrier transporting properties of Ir(iprpmi)3 in hole-
only and electron-only devices, we have shown that the Ir(iprpmi)3 dopant at a low
concentration traps holes, and it transports holes at a high concentration in the DCzPPy
bipolar host material. Based on this property, we varied the Ir(iprpmi)3 concentration to
control the recombination region and thereby achieved high-efficiency blue and white
OLEDs with EQEs over 20%. The optimized WOLEDs showed power efficiency close to
40 lm/W at 1,000 cd/m2 and exhibited a minimal color shift (±0.001, ±0.005) over a large
current density range of 0.05–20 mA/cm2. Compared to devices with FIrpic as the blue
dopant, Ir(iprpmi)3-based blue and white OLEDs had significantly improved device
lifetimes under the same stress conditions.
103
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106
Chapter 6 Investigating Chemical Degradation
Mechanism of High-Triplet-Energy Materials in Blue
Phosphorescent OLED Using LDI-TOF
6.1. Introduction
Phosphorescent OLED (PhOLED) has already been adopted in commercial OLED
panels for green and red pixels because of its superior device efficiency and lifetime. Blue
PhOLED, however, is yet to deliver sufficient device lifetime to replace conventional
fluorescent OLED that is currently being used in commercial products [1–4].
The longevity of blue PhOLED is highly dependent on the phosphorescent emitters
and host and transport materials used. In previous studies, it has been concluded that FIrpic,
which is an efficient blue phosphorescent dopant that is widely used in PhOLED research,
is chemically unstable during device operation [5]. The device lifetime of FIrpic is typically
within several hours. Disassociation of the picolinate auxiliary ligand has been identified
as the main degradation pathway of FIrpic through the LDI-TOF technique [6, 7]. In our
recent study, we demonstrated that using homoleptic iridium complex Ir(iprpmi)3 as the
blue dopant can significantly improve device lifetime. We also found out that transport
materials with lower triplet energies tend to result in longer lifetimes and lower efficiencies
for devices, while transport materials with higher triplet energies, i.e. TAPC and TmPyPB,
result in short-lived devices, although with higher efficiencies and lower drive voltages [8].
To make blue PhOLED useful in practice, both high efficiency and long lifetime need to
107
be achieved simultaneously. Understanding the degradation mechanisms of a high-triplet-
energy host and transport materials is crucial to realizing an applicable blue PhOLED. LDI-
TOF, first adopted in OLED degradation analysis by Leo K. et al. [9], has proven to be an
effective tool in chemical degradation analysis. Besides FIrpic, degradation patterns of
various OLED materials have been studied in situ using LDI-TOF [10, 11].
In this work, we investigate the degradation pattern of high-efficiency Ir(iprpmi)3-
based blue phosphorescent OLED comprising materials with high triplet energy (> 2.7 eV).
TAPC, TCTA, TmPyPB and DCzPPy were selected as a host or transport material. We
started with evaluating device performance and lifetime by varying host types for
Ir(iprpmi)3. Degradation products of each material (transport, host and emitter) have been
systematically analyzed by LDI-TOF. The chemical compositions of aged devices were
probed in situ using the LDI-TOF technique. The results suggest that chemical degradation
mainly occurs at the HTL/EML or EML/ETL interface, where the exciton density is high.
From fragment structures and theoretically calculated bond dissociation energies, a cation-
induced ring-open mechanism was deduced as an alternative chemical degradation
pathway of TAPC. TCTA as a host degrades by means of C-N dissociation. The TmPyPB
electron transport material mainly undergoes protonation at recombination interfaces.
Bipolar transporting material DCzPPy, when used as a host, exhibits fewer tendencies of
C-N bond cleavage and is relatively stable.
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6.2. Results and Discussion
6.2.1. Device Performance and Lifetime Evaluation
Figure 6.1: Schematic energy diagram for blue PhOLEDs. (The triplet energy is in parentheses, and HOMO/LUMO energies are below and above the rectangles).
Three blue PhOLEDs with different host materials were fabricated side by side with
a device structure of ITO/HATCN (3 nm)/TAPC (40 nm)/TCTA (4 nm)/Host:Ir(iprpmi)3
(9%, 10 nm)/TmPyPB (10 nm)/TmPyPB:Cs2CO3 (50%, 30 nm)/Al. The three host
materials were TmPyPB, TCTA, DCzPPy. The molecular structure and triplet energy level
of materials used are illustrated in Figure 6.1. HATCN was the HIL, and TAPC was the
HTL. TCTA was used as an exciton blocking layer that isolated TAPC from the EML. The
DCzPPy bipolar host [12] was used to sensitize Ir(iprpmi)3, which is a relatively stable blue
dopant, as we have previously reported [8]. Neat and heavily doped TmPyPB were utilized
as the ETL and EIL, respectively.
The device EQE and lifetime performance are illustrated in Figure 6.2, in which
Device A3 exhibits an EQE of over 20%, while Devices A1 and A2 only have EQEs of
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approximately 15%. This can be partially attributed to the bipolar carrier transport property
of DCzPPy, which leads to a broadened recombination region throughout the EML. With
Devices A1 and A2, the recombination region is confined to the TCTA/TmPyPB interface,
where self-quenching might happen. Moreover, from the device lifetime test (illustrated in
Figure 6.2(b)), it is evident that the brightness of all three devices drops to under 30% in
less than 20 h operation at 5 mA/cm2. Nevertheless, Device A3, in which DCzPPy is the
host, exhibits a relatively longer lifetime compared to Devices A1 and A2.
Figure 6.2: Efficiencies and lifetime performances of Device A1, A2 and A3.
6.2.2. Overall Stability Assessment of the Blue PhOLED
To investigate the cause of the blue PhOLED’s short device lifetime, an aged
Device A3 was subjected to LDI-TOF analysis along with an unaged device as reference.
The aged sample was driven at a constant current density of 5 mA/cm2 for 24 h in a vacuum
assembly. Since the laser (wavelength: 337 nm) can induce photo-fragmentation of OLED
110
materials, both samples were analyzed in one run at the same laser intensity and mode.
Figure 6.3 illustrates the LDI-TOF spectra of the two samples.
Figure 6.3: Normalized LDI-TOF spectra of Device A3 with and without degradation.
Based on the LDI-TOF and TOF/TOF spectra of the individual materials, most of
the peaks in Figure 6.3 are assigned to proposed molecular structures. Mass peaks at 626,
740 and 1103 correspond to molecular masses of TAPC, TCTA and Ir(iprpmi)3,
respectively. HATCN was not detected because of its high ionization potential. DCzPPy
was absent due to its lower ionization potential (IP) compared to TAPC. TmPyPB was also
absent due to its lack of absorption at the laser wavelength (337 nm). All other peaks in the
111
spectra are fragments and adducts, the proposed structures that are summarized in Table
6.1.
112
Table 6.1: List of mass peaks and their proposed structures.
Mass (m/z) Origin Nature Proposed structure
431 TAPC Fragment
465 TAPC Fragment
499 TCTA Fragment
536 TAPC Fragment
557 TAPC Fragment
570 TAPC Fragment
583 TAPC Fragment
591 TAPC Adduct
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Mass (m/z) Origin Nature Proposed structure
626 TAPC Molecular mass
717 TAPC Adduct
740 TCTA Molecular mass
799 Ir(iprpmi)3 Fragment
855 Ir(iprpmi)3 Fragment
Product ion formed by dissociation of meta-stable Ir(iprpmi)3
+ in post-source decay (PSD); it possibly has the same structure as 799.
1057 TAPC Adduct
1103 Ir(iprpmi)3 Molecular mass
a. Peaks at 855 have abnormally low resolutions (large FWHM) and is absent in the linear
mode, which indicates that it is a product ion from a precursor dissociation in the PSD.
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6.2.3. Degradation of Blue Dopant
Ir(iprpmi)3 undergoes simple Ir-C and Ir-N dissociations, which result in
Ir(iprpmi)2+ at 799 m/z. The peak at 855 m/z also corresponds to the Ir species, possibly
still Ir(iprpmi)2+, formed by meta-stable parent ions dissociating in the reflectron. This can
be supported by data from the linear-mode test (which does not involve a reflectron) of a
neat Ir(iprpmi)3 film. Figure 6.4 illustrates the LDI-TOF results of UV aged (254 nm, 24 h)
Ir(iprpmi)3 films tested at reflectron mode and linear mode respectively. As shown in
Figure 6.4, the peak at 855 m/z disappeared in the linear mode.
Figure 6.4: Normalized LDI-TOF spectra of the neat Ir(iprpmi)3 film in the linear mode.
6.2.4. Degradation of TAPC
TAPC is one of the earliest HTMs used in OLEDs. Its high ionization potential and
hole mobility ensure good hole injection and transport. Its wide band gap is also beneficial
to exciton confinement, particularly in blue phosphorescent and TADF OLEDs. To date,
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TAPC is still widely used in OLED research to produce some of the highest device
efficiencies [13]. However, devices using TAPC had much shorter lifetimes compared to
devices using other HTMs, e.g., NPB. Kondakov et al. conducted an extensive degradation
study on TAPC- and NPB-based OLED devices [14]. They found that TAPC chemically
degraded more than NPB did both at the recombination interface and in the bulk. From
identified byproducts, C-N bond dissociation was reconstructed as the main degradation
pathway with C-C bond cleavage between phenyl group and cyclohexyl ring as a minor
pathway. The high exciton energy of TAPC was attributed as the driven force of these
degradation pathways. According to this mechanism, the initial degradation started at the
interface with homolytic bond dissociation of neutral TAPC and the degradation in the bulk
was suggested to be caused by radical chain reactions as evidenced by the formation of
high molecular weight byproducts, other potential pathways such as rupture of cyclohexyl
ring, or simply a higher degree of interface deterioration.
Because it has a lower IP than TCTA and DCzPPy, TAPC almost completely
overshadowed other peaks in both aged and unaged samples, except Ir(iprpmi)3, which has
the lowest IP in this set of materials. In the aged device, peaks from TCTA and DCzPPy
were revealed, and the intensity of TAPC fragments increased, thus indicating chemical
degradation of TAPC. From fragment structures listed in Table 6.1, two fragmentation
pathways can be reconstructed for TAPC: 1) C-N bond cleavage and 2) cyclohexyl rupture.
The C-N cleavage gives fragments at 431 and 536, and adducts at 717 and 1056.
Cyclohexyl rupture results in fragments at 465, 557, 570 and 583. A combination of both
116
fragmentation pathways yields an adduct at 591. Some of these fragments are protonated
most likely by interactions with hydrogen-abundant fragments from cyclohexyl.
Figure 6.5: TOF/TOF spectrum of the TAPC cation.
In the structure of Device A3, TAPC is merely used as an HTM isolated from the
recombination interface by TCTA. Excitons formed in the EML can hardly affect the
TAPC layer. For it to degrade chemically, a self-initiated reaction is necessary. During
operation, the TAPC molecule exists in the neutral state and cationic state. Thus, the
stability of the TAPC cation is crucial to device stability. TOF/TOF, which is a tandem
mass technique, provides a useful way to study the intrinsic fragmentation of cations in
vacuum. As illustrated in Figure 6.5, the mass peak at 583, which is the product ion of
117
cyclohexyl rupture process, appears to be the dominating fragment in the TOF/TOF
spectrum of the TAPC cation. It strongly suggests that TAPC mainly follows a cation-
induced ring-rupture chemical degradation pathway, rather than the exciton-provoked C-
N cleavage mechanism proposed by Kondakov [14]. Additionally, the complete
suppression of TAPC precursor peaks indicates a high fragmentation ratio of the TAPC
cation; hence, a higher chance of degradation when TAPC is used as an HTM.
To elucidate the effect of positive charge on the fragmentation of TAPC in solid
films, a comparison LDI-TOF experiment was conducted on a neat TAPC film (20 nm)
and an HATCN (3 nm)/TAPC (20 nm) bilayer structure. HATCN, which is a strong
electron acceptor, can form charge a transfer complex with electron donating TAPC at the
interface, thus producing TAPC cations in situ. When irradiated by a laser with a relatively
lower intensity in LDI-TOF, these two samples showed different fragmentation patterns
(Figure 6.6). The neat TAPC film mainly underwent C-N cleavage fragmentation, whereas
HATCN/TAPC showed more fragments from the cyclohexyl rupture. Further increase of
laser intensity also increased cyclohexyl rupture fragments in the neat TAPC film. But the
fragment intensity is much lower than that in the HATCN/TAPC sample. It can be
concluded that direct laser irradiation on a neutral TAPC causes a C-N bond dissociation,
whereas the irradiation on a TAPC cation induces cyclohexyl ring-open reaction.
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Figure 6.6: LDI-TOF spectra of the neat TAPC film and HATCN/TAPC bilayer.
To explain the different fragmentation pathways of neutral and cationic TAPC, the
density function theory (DFT) method was utilized to calculate the dissociation energy of
each broken bond in both the neat TAPC and the TAPC cation (Figure 6.7). In the neutral
TAPC, the dissociation energies of both C-N bonds, which are comparable to the value
reported by Kondakov [14], are lower than that of the C-C bond in cyclohexyl ring.
However, in the TAPC cation, C-N bond dissociation energies increase while, the
cyclohexyl ring-opening energy drastically decreases to 1.4 eV. Once the cyclohexyl ring
is open, further cracking reactions ensue due to the high reactivity of the radical cation
(Figure 6.8). The low energy barrier significantly increases the chance of degradation of
the TAPC cation, which is in agreement with TOF/TOF and LDI-TOF results.
119
Figure 6.7: Dissociation energy of bonds in the neutral TAPC and TAPC cation.
Figure 6.8: Dissociation energy of cracking reactions after cyclohexyl is opened in the TAPC cation. The dissociation of 1 corresponds to fragments at 570 and 591, and that of
2 corresponds to the peak at 583.
To understand the origin of the low ring-opening energy in the TAPC cation, the
resonant forms of the TAPC cation and ring-opened TAPC cation, together with calculated
highest occupied molecular orbitals (HOMOs) of TAPC and ring-opened TAPC, are
120
illustrated in Figure 6.9. In the TAPC cation, positive charge is located on each of the two
amine groups, separated by the center cyclohexyl ring. But once the ring is open, the central
carbon acts like a bridge, delocalizing the positive charge on both amine groups. The
extended resonance lowers the energy of the reactive radical cation and, therefore, lowers
the energy barrier between the TAPC cation and ring-opened TAPC cation. In TAPC, the
HOMO is separated by the cyclohexyl ring. While in ring-opened TAPC, the HOMO on
two amines is connected by the central carbon radical, thus reasserting the cation-induced
ring-opening mechanism. However, the driving force of this ring opening reaction in
OLED device is still not clear. It is possible that the TAPC radical cations at excited state,
which may be produced by photo-excitation from ambient light, is involved in the
degradation.
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Figure 6.9: Resonant structures (up) of the TAPC cation and ring-opened TAPC cation and HOMO (down) of TAPC and ring-opened TAPC.
6.2.5. Degradation of TCTA, DCzPPy and TmPyPB
Fragmentations of TCTA were observed in the degraded sample at peaks 499 and
799, as illustrated in Figure 6.3. However, it is hard to draw any conclusion about the origin
of these fragmentations due to the matrix effect and IP hierarchy of analytes [15]. The peak
intensity may not necessarily reflect the actual abundance of each species. Signals from
TAPC and its fragments can overshadow other peaks, which makes it hard to analyze
possible degradation of other materials (such as TCTA, DCzPPy and TmPyPB) in the
presence of TAPC.
122
To eliminate the swamping effect of TAPC signals and study the chemical reaction
at the interface of the host materials, a series of bilayer structured OLED devices were
fabricated without the blue phosphorescent dopant. Device B1 had the following structure:
ITO/HATCN (3 nm)/TCTA (40 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al. Excitons are formed
at TCTA/TmPyPB interface where TCTA+ and TmPyPB- are accumulated and
recombined. Device B1 was driven at a constant current density of 5 mA/cm2 for 24 h in a
vacuum assembly and underwent LDI-TOF analysis. Figure 6.10 shows the spectra Device
B1 before and after electrical aging. Without TAPC, a mass peak at 740 from the TCTA
parent is the dominant signal in both samples. Mass peak 499, which corresponds to a
fragment of TCTA ([TCTA-PhCz]+), can also be observed. Moreover, one more peak (at
982 m/z) shows up in the spectra of the two samples. This peak is identified as an adduct
of TCTA and a fragment of TCTA [TCTA+PhCz]+. Such an observation reveals an
obvious TCTA degradation pattern, where the C-N bond located at the central amine
moiety breaks down to produce phenol-carbazole (PhCz) and [TCTA-PhCz] radicals. This
homolytic dissociation mechanism is the main degradation pathway of TCTA, which
agrees with the finding in other reported studies [9, 14, 16].
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Figure 6.10: LDI-TOF-MS spectra of device B1 before and after degradation.
In the aged sample, four more peaks become prominent, namely those at 538, 779,
1073 and 1277. These four peaks are identified as a protonated TmPyPB cation
([TmPyPB+H]+), adduct between TmPyPB and a fragment of TCTA ([TmPyPB+PhCz]+),
protonated TmPyPB dimer ([TmPyPB+TmPyPB]+) and adduct between TCTA and
TmPyPB ([TCTA+TmPyPB]+), respectively. Table 6.2 lists the mass values, fragment
origins and proposed structures of the peaks. The protonation process of TmPyPB is still
unclear. However, it can be concluded that at the TCTA/TmPyPB interface, there exist
various species of degradation products, such as neutral radicals, charged ions and reaction
products. Therefore, non-radiative recombination and exciton quenching would lead to a
drop in device efficiency and a short lifetime. Such phenomena can also explain the shorter
124
device lifetime of Devices A2 and A3 in which the recombination is confined to the
TCTA/TmPyPB interface.
Table 6.2: List of mass peaks and their proposed structures.
Mass (m/z) Origin Nature Proposed structure
539 TmPyPB Adduct
779 TCTA,
TmPyPB Fragment
982 TCTA Adduct
1073 TmPyPB Adduct
1277 TCTA,
TmPyPB Adduct
125
As a comparison, Device B2 (in which TmPyPB is replaced with DCzPPy) was
fabricated with the following structure: ITO/HATCN (3 nm)/TCTA (40 nm)/DCzPPy (40
nm)/LiF (1 nm)/Al. In this structure, hole are transported through TCTA and DCzPPy, and
electrons are transported via DCzPPy. Recombination is extended throughout the DCzPPy
layer. Two samples of Device B2 (before and after electrical aging) were analyzed by LDI-
TOF. The resultant spectra are illustrated in Figure 6.11. Compared to Device B1, Device
B2 exhibited little difference between samples before and after aging. Three major peaks
(499 and 741 belong to TCTA, and 562 belongs to DCzPPy) are observed in the two
samples without noticeable new peaks after degradation. This finding means that the
TCTA/DCzPPy interface is more stable than the TCTA/TmPyPB interface. Relative
intensity of peak 562 (DCzPPy) increases after 24 h of aging, indicating that DCzPPy is
more stable than TCTA in supporting electron and hole transport.
Figure 6.11: LDI-TOF-MS spectra of Device B2 before and after degradation.
126
The LDI-TOF-MS study of Devices B1 and B2 with TmPyPB as a strong electron
acceptor shows that this material is very unstable when in direct contact with a strong
electron donor such as TCTA. Protonated TmPyPB ([TmPyPB+H]+) and reaction products
between TCTA radicals and TmPyPB radicals can be observed after electrical aging. To
solidify this finding, two samples of Device B3 were fabricated with TAPC as the HTL.
The detailed device structure is as follows: ITO/HATCN (3 nm)/TAPC (40 nm)/DCzPPy
(40 nm)/LiF (1 nm)/Al. Peaks 465, 557, 570, 591 and 626 are from TAPC and its
fragments, all of which were observed in both B3 samples. However, after 24 h of aging,
peak 538 (protonated TmPyPB) appears in the spectra, which was also found in aged
Device B1.
Figure 6.12: LDI-TOF-MS spectra of Device B3 before and after degradation.
127
6.3. Conclusions
We have established a methodology to study the degradation of a set of OLED
materials through LDI-TOF-MS, TOF/TOF measurement, and DFT calculations. We
proposed that TAPC as a hole transport material undergoes a cation-induced cyclohexyl
ring-opening reaction as a degradation pathway during device operation. This finding
suggests that TAPC is not a desirable HTM for OLED devices due to the instability of its
radical cation. The Ir(iprpmi)3 blue phosphorescent dopant’s degradation is mainly induced
by metal-organic ligand dissociation. Host material TCTA mainly undergoes C-N bond
breaking at the central amine moiety. TCTA radicals and fragments can react with
TmPyPB radicals when the recombination is located at the TCTA/TmPyPB interface.
TmPyPB undergoes protonation and is likely to also undergo dimerization. DCzPPy is
relatively stable as a transport and host material. These findings are in agreement with the
blue PhOLED device performance and lifetime data.
128
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Chapter 7 Summary and Future Work
This thesis is mainly focused on device engineering of WOLEDs based on
phosphorescent red, green and blue emitters. High-efficiency WOLEDs were achieved by
optimizing the layer structures and material compositions and gaining a better
understanding of the device operation and degradation mechanisms, including the possible
chemical degradation pathways of various material components. A systematic device
degradation study was conducted utilizing the LDI-TOF technique.
In Chapter 3, a dual-EML WOLED structure was studied. The effects of a host
material type, compositions of the mixed-host system, dopant concentration of the
red/green co-doped layer and non-emissive interlayer were investigated. By optimizing all
of these parameters, WOLEDs with high power efficiency (~33 lm/W at 1,000 cd/m2) and
high EQE (~18%) have been achieved.
In Chapter 4, we successfully fabricated high-efficiency WOLEDs with a reduced
color shift. An emitter structure consisting of an ultra-thin red and green co-doped layer
sandwiched in between two blue layers was developed. This emitter structure provides the
flexibility to fine tune the WOLED performance, including the color shift without
compromising the efficiency. By adjusting the compositions and thicknesses of the
individual layers, an EQE of almost 20% and a luminance of over 40 lm/W (at 1,000 cd/m2)
have been achieved with minimal color shift over a large range of luminance (400–4000
cd/m2). The function of all three EMLs and exciton energy-transfer mechanisms among
hosts and guests were systematically studied. We also found that the device degradation
was related to the lifetime of excited FIrpic states and dependent on the host materials.
131
WOLEDs fabricated in Chapters 3 and 4 were based on the common FIrpic blue
phosphorescent dopant. Although high-efficiency WOLEDs were successfully
demonstrated with FIrpic, they were very short-lived. In Chapter 5, we investigated another
iridium emitter with phenyl-imidazole ligands, namely Ir(iprpmi)3, which is known to be
relatively more stable. We conducted studies of the charge-carrier transport properties of
Ir(iprpmi)3 in hole-only and electron-only devices and found that Ir(iprpmi)3 trapped holes
efficiently at low concentrations but transported holes at high concentrations in the bipolar
host material, DCzPPy. With Ir(iprpmi)3/DCzPPy as a blue emitter, we fabricated high-
efficiency blue and white OLEDs with EQEs over 20%. Moreover, the power efficiency
of WOLEDs was close to 40 lm/W at 1,000 cd/m2. By varying the concentration of the
hole-trapping Ir(iprpmi)3 in the bipolar DCzPPy host, high-efficiency WOLEDs with
minimal color shift (±0.001, ±0.005) over a current density range of 0.05–20 mA/cm2 were
fabricated. Compared to FIrpic-based devices, Ir(iprpmi)3-based WOLEDs exhibited
significantly longer lifetimes under similar test conditions.
Although the WOLEDs discussed in Chapter 5 exhibited much improved lifetimes
compared to FIrpic-based devices, their half-lifetimes were still below 100 h. To
understand the causes of device instability, we investigated in Chapter 6 the possible
chemical degradation pathways using LDI-TOF, TOF/TOF techniques and DFT
calculations. From the LDI-TOF fragmentation patterns, we found that Ir(iprpmi)3 was
more stable than FIrpic, possibly due to the lack of fluorine substituents and the ancillary
picolinate ligand in Ir(iprpmi)3. The main degradation of Ir(iprpmi)3 was induced by metal-
organic ligand dissociation, which was caused by the Ir-C and Ir-N bonds’ cleavage. We
132
found that TAPC as a hole transport material was very unstable during device operation.
In OLED devices, TAPC can undergo a cation-induced cyclohexyl ring-opening reaction
as the main degradation pathway, and the C-N bond dissociation appears to be a minor
degradation pathway. By contrast, TCTA mainly undergoes C-N bond dissociation at the
central amine moiety. Bipolar host material DCzPPy is relatively stable. There were hardly
any noticeable differences in the LDI-TOF fragmentation patterns of the aged and unaged
devices. We also found that electron-transporting material TmPyPB was another main
cause of device degradation. The LDI-TOF-MS analysis of TCTA/TmPyPB indicated that
the TCTA radical and its various fragments can react with TmPyPB radicals at the
TCTA/TmPyPB interface. We also found that TmPyPB can undergo protonation and
dimerization.
Although we have successfully demonstrated high-efficiency and color-stable
WOLEDs, the findings of this study indicated that there is a critical need to further improve
the device lifetime while maintaining high efficiency. Proposed future work is discussed
as follows:
1) From a device engineering perspective, it is desirable to broaden the electron-
hole recombination region. As discussed in Chapter 3, a wide recombination
region can improve the overall device efficiency by alleviating charge-carrier
and exciton accumulation at the emitting layer interfaces. The LDI-TOF
analysis in Chapter 6 indicated that exciton quenchers formed at either the
EML/HTL or EML/ETL interface can contribute to a reduced device efficiency
and lifetime. To reduce quenching at these interfaces, a bipolar material or a
133
mixture of electron donors and acceptors should be used as the host of the EML,
especially for the blue layer. Because the mobility of electron-transport
materials is generally much higher than the mobility of hole-transport materials,
it is not easy to find a mixed-host system without an excessive accumulation of
holes near the ETL/EML interface. A bipolar host material [1–5] with proper
hole-transporting and electron-transporting moieties can perform the multiple
functions of balanced injection, transportation and recombination of charge
carriers. It would be beneficial to investigate charge-carrier transporting
properties and material stabilities of bipolar host materials to find useful host-
material candidates for stable WOLEDs. Other than DCzPPy, other noteworthy
bipolar host materials include carbazole/diphenyl phosphine oxide (mCPPO1)
[6], phenyl-carbazole/pyridine (PCz-BFP) [7] and triphenyl
amine/benzimidazole (p-BISiTPA) [8].
2) The choice of a blue phosphorescent dopant is another key factor to achieving
stable WOLEDs. Unfortunately, there have been few reports of stable blue
phosphorescent dopants. As found in this work, Ir(iprpmi)3 exhibits an
improved device lifetime. Compared to FIrpic, Ir(iprpmi)3 has a more rigid and
bulky molecular structure in the phenyl-imidazole ligands, which may be the
reason for the improved stability observed in the Ir(iprpmi)3-based blue and
white OLED devices (Chapter 5). Based on this finding, we suggest that a
detailed structure–property relationship should be studied with a particular
attention on the effect of molecular rigidity on the blue OLED lifetime. There
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have been a few reports of high-efficiency blue OLEDs based on Ir phenyl-
imidazole derivatives [9–12]. Therefore, device-lifetime tests and degradation
analysis can be conducted using these materials to determine the relationships
between molecular structures and device performances. Moreover, LDI-TOF
showed that one of the weakest bonds in iridium-based emitters is the Ir-C or
Ir-N bond. Therefore, non-metal-based dopants such as TADF may provide a
new material class for simultaneously achieving high efficiencies and long
lifetimes in WOLEDs. Although there have been few reports on lifetime studies
based on blue TADF materials [13, 14], WOLEDs incorporating TADF blue
emitters are worth investigating.
3) LDI-TOF analyses, including ours, have found that, in addition to degradation
of dopant materials with electrical stress, there are considerable evidence of
degradation in transport and host materials, including reactions between
dopants and transport materials. We have demonstrated that the prototypical
hole-transport material TAPC, commonly used to achieve high efficiency in
blue and white OLEDs, is unstable because of the C-C bond dissociation of its
radical cation. TCTA is relatively stable compared to TAPC, but it is also
susceptible to C-N bond dissociation. Therefore, the search for hole-transport
materials should go beyond the amine classes. LDI-TOF on HATCN-induced
cations can provide useful information to study the stability of cationic species
of hole-transport materials.
135
4) Most high-efficiency OLEDs utilize materials with aromatic pyridyl nitrogen
moieties for the ETL. These materials tend to produce high triplet energy and,
therefore, can help confine the triplet excitons to the EML. However, these
materials also tend to be unstable when used in blue OLED devices. As shown
in Chapter 6, chemical reaction products of TmPyPB were detected in degraded
devices with a TCTA/TmPyPB bilayer structure. While LDI-TOF can be
readily used for analyzing degradation products from electron donors such as
the hole-transport aromatic amines and Ir dopants, it is not particularly useful
for detecting degradation products from electron acceptors such as TmPyPB.
LDI-TOF may be used to probe the interface degradation between the ETL and
HTL, where the degradation products involving complexation between donor
and acceptor may have sufficiently low ionization potentials to be detectable.
5) LDI typically involves complex chemistry processes, including intramolecular
fragmentation, intermolecular energy and electron transfer, and reactions
between active species. The observed difference in the fragmentation patterns
between aged and unaged OLED samples may account for the presence of a
minute amount of degraded materials. Moreover, due to the high energy of the
nitrogen laser (337 nm) used in LDI, molecular fragmentation will inevitably
occur during the LDI process, making it extraordinarily difficult to assess the
degradation that results from device aging alone. Therefore, it is important to
perform in situ analysis in which control and test samples can be carefully
analyzed under identical conditions. Furthermore, it would be helpful to further
136
refine the LDI techniques in which quantitative results can be obtained. Other
techniques such as liquid chromatography mass spectrometry [15–17] should
also be employed to supplement the LDI analysis in detecting photochemical
and degradation products in OLED devices.
137
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