Device Engineering and Degradation Mechanism Study of All- Phosphorescent White Organic Light-Emitting Diodes

Transcription

1 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

2 ii Biographical Sketch Lisong Xu was born in Zhejiang, China in He received a Bachelor of Science degree in Materials Science and Engineering from Beihang University in 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 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 lightemitting diodes patterned via contact printing, ACS Appl. Mater. Interfaces 8, (2016). [3] J. Li, L. Xu, S. Kim, and A.A. Shestopalov, Urethane acrylate polymers in highresolution contact printing, J. Mater. Chem. C 4, 4155 (2016).

3 iii [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 [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 disubstituted anthracene mixtures upon thermal vacuum deposition, In preparation.

4 iv 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,

5 v 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.

6 vi Abstract As a possible next-generation solid-state lighting source, white organic lightemitting 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 allphosphorescent WOLEDs through optimization of the device structures and material compositions. The operating principles and the degradation mechanisms specific to allphosphorescent WOLED will be studied. First, we investigated a WOLED where a blue emitter was based on a doped mixhost 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

7 vii control the electron-hole recombination zone and the dopant concentrations of the greenred 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-1himidazole]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

8 viii devices, we were able to identify some prominent degradation byproducts and trace possible chemical pathways involving specific host and dopant materials.

9 ix 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.

10 x Table of Contents Biographical Sketch Acknowledgements Abstract Contributors and Funding Source List of Tables List of Figures ii iv vi ix xiv xvii Chapter 1 Background and Introduction Introduction to White Organic Light Emitting Diodes Basics of OLEDs Basic Device Physics Fluorescence and Phosphorescence from OLEDs Energy Transfer and Quenching in OLEDs Performance Characterization of WOLEDs Status of WOLED Development All-Fluorescent WOLEDs All-Phosphorescent WOLEDs Hybrid WOLEDs TADF WOLEDs Device Stability and Degradation Mechanism of WOLEDs Instability of Blue Phosphorescent Materials 26

11 xi MALDI-TOF-MS Objectives and Outline of the Thesis 29 References 32 Chapter 2 Experimental Methods and Materials Vacuum Vapor Deposition Process Boat Design and Coater Specifications Device Fabrication Conditions Device and Material Characterization Device Lifetime Test LDI-TOF-MS Analysis Materials 51 Chapter 3 White Organic Light-Emitting Diodes with FIrpic in a Mixed-Host Introduction Results and Discussion Effects of an mcp Buffer Layer Effects of Host Types for FIrpic Effects of Red Dopant Concentration The Role of a Non-Doped Interlayer 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

12 xii 4.1. Introduction Results and Discussion Conclusions 87 References 88 Chapter 5 Investigation of Phosphorescent Blue and White Organic Light-Emitting Diodes with High Efficiency and Improved Lifetime Introduction Results and Discussion Conclusions 102 References 103 Chapter 6 Investigating Chemical Degradation Mechanism of High-Triplet-Energy Materials in Blue Phosphorescent OLED Using LDI-TOF Introduction Results and Discussion Device Performance and Lifetime Evaluation Overall Stability Assessment of the Blue PhOLED Degradation of Blue Dopant Degradation of TAPC Degradation of TCTA, DCzPPy and TmPyPB Conclusions 127 References 128 Chapter 7 Summary and Future Work 130

13 References 137 xiii

14 xiv 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:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) 59 Table 3.2: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110 nm)/tapc:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) 61 Table 3.3: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110 nm)/tapc:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) 63 Table 3.4: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110 nm)/tapc:moo 3 (40%, 10 nm)/tapc (30 nm)/mcp (x nm)/tcta:dczppy:firpic

15 xv (y:z, 15%, 4nm)/DCzPPy:/Ir(2-phq) 2 (acac):ir(ppy) 3 (1.5%, 5%, 4 nm)/bphen (20 nm)/bphen:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) 65 Table 3.5: EL performance of WOLEDs with different red dopant concentrations. ITO (110 nm)/tapc:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) 67 Table 3.6: EL performance of WOLEDs with interlayers. ITO (110 nm)/tapc:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) 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:cs 2 CO 3 (50%, 30 nm)/al (100 nm). (a: values at 5 ma/cm 2 ; b: luminance range from 400 to 4000 cd/m 2.) 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/cm 2 ). 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

16 xvi nm)/dczppy:firpic (20%, 3 nm)/tmpypb (10 nm)/tmpypb:cs 2 CO 3 (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/cm 2 ). 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:cs 2 CO 3 (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/cm 2 ). ITO (110nm)/HATCN (3 nm)/tapc(37 nm)/tcta:firpic (x%, 4 nm)/ /DCzPPy:FIrpic (y%, 3 nm)/tmpypb (10 nm)/tmpypb:cs 2 CO 3 (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:cs 2 CO 3 (50%, 30 nm)/al (100 nm). (a: values at 5 ma/cm 2, b: measured at current densities from 0.05 ma/cm 2 to 20 ma/cm 2.) 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

17 xvii 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

18 xviii 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 Bepp 2 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

19 xix 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/cm 2 ) 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/cm 2 ) 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/cm 2 ) 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/cm 2 ) 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/cm 2 ) 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/cm 2.) 78

20 xx 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/cm 2.) 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) 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/cm 2. 95

21 xxi 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/cm 2 ; (c) color shift of the device with 9% Ir(iprpmi) 3 at current densities from 0.05 to 20 ma/cm Figure 5.5: Device lifetime tested at 5 ma/cm 2 (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 A 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 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

22 xxii 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

23 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, colorstable and long-lifetime WOLEDs for applications in lighting and displays.

24 Basics of OLEDs 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 (nonradiative 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

25 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 multifunctional organic layers (Figure 1.1(b)). Each of the layers performs a specific chargetransport 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 holetransport layer (HTL), 3) an emitting layer (EML), 4) an electron-transport layer (ETL), 5)

26 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] 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.

27 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 spinorbital 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].

28 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 lightemission 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 intersystem crossing, a process which is highly dependent on the energy gap between the lowest singlet excited state and the triplet state 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

29 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.

30 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, triplettriplet 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

31 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 L x is the diffusion length, n 0 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]. ' ( = ' * +,-. / 012h 4 - =!& (1.2) 1.3. Performance Characterization of WOLEDs The efficiency of an OLED is characterized by its external quantum efficiency (h ext ), current efficiency (h L ) and luminous efficiency (h p ). 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

32 10 through the transmissive electrode into the viewing direction. The intrinsic quantum efficiency (h int ) 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 (g e-h 1), the fraction of emissive exciton states (h s-p ) and the radiative decay efficiency (Φ i ). The external efficiency is decreased by a light outcoupling factor (h out ), 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). : ;-< = : =>< = B ;,C : D,E Φ = (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

33 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

34 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 t 1/2 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 L 0 is the initial luminance of an OLED, t 1/2 is the half-life time (time to ½ L 0 at constant current), which is inversely proportional to L 0, and C is a constant. 2 G 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 2 G 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

35 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])

36 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-emittinglayer (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, allphosphorescent WOLED, hybrid fluorescent-phosphorescent WOLED, and thermally activated delayed fluorescence (TADF) WOLED. These types are briefly described below.

37 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)-Nphenylaminophenyl]}-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-

38 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 blueemitting 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/m 2 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/m 2. 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

39 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 (Alq 3 ) 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. (a) (b) 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]) 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%

40 18 FIrpic:0.75% (fbi) 2 Ir(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) 2 Ir(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 electrontransporting 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 cm 2 /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) 2 Ir(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/m 2 and 31.3 lm/w at 1,000 cd/m 2. 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

41 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/m 2 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 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 Förstertype 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/m 2.

42 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]) 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].

43 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)/bebq 2 :Ir(MDQ) 2 (acac) (9 nm, 5%)/Bebq 2 (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 Bebq 2. 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 Bepp 2 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/m 2.

44 22 (a) (b) Figure 1.11: (a) Device architecture and the energy level diagram of the hybrid WOLED. (b) Lifetime of the device with Bepp 2 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

45 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/m 2. 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]) 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

46 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 CzPN 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.

47 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

48 26 caused by different lifetimes of emissive materials and 2) possible recombination region shift and exciton distribution change 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].

49 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. Shortpulsed laser light (nitrogen laser light, wavelength: 337 nm) then irradiates the sample and

50 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].

51 29 Figure 1.14: Schematic of a MALDI-TOF-TOF-MS setup [81] 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.

52 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 codoped 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

53 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.

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62 40 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 o C, 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.

63 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 predetermined 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.

64 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.

65 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, 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, MoO x and Cs 2 CO 3 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.

66 44 (a) (b) 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

67 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, MoO x can be doped into TAPC and Cs 2 CO 3 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) 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.

68 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 MoO x and LiF rates were set to be 0.5 Å/s. Cs 2 CO 3 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 N 2, followed by an O 2 plasma treatment before loading into the vacuum chamber.

69 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 cm 2 ). 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

70 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. (a) (b) (c) 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 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/cm 2 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

71 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 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 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/cm 2 at room temperature.

72 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 N 2 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 10 4 counts. LDI Mass Spectra were postcalibrated 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).

73 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.

74 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) MoO x molybdenum(vi) oxide - HIL HATCN HIL TAPC tolylamino)phenyl)cyclo hexane HTL mcp 1,4,5,8,9,11- hexaazatriphenylenehexanitrile 1,1-bis((di-4-1,3-Bis(Ncarbazolyl)benzene HTL

75 Acronym Chemical Name Molecular Structure Function HOMO (ev) LUMO (ev) E( T1 ) (ev) TCTA ne HTM TmPyPB ETM TPBi benzene ETM BPhen 4,4,4-tris(Ncarbazolyl)triphenylami 1,3,5-tri(m-pyrid-3-ylphenyl)-benzene 1,3,5-tris(2-Nphenylbenzimidazolyl) 4,7-diphenyl-1,10- phenanthroline ETL

76 Acronym Chemical Name Molecular Structure Function HOMO (ev) LUMO (ev) E( T1 ) (ev) DCzPPy yl)phenyl)-pyridine Bipolar host Cs 2 CO 3 cesium carbonate - EIL FIrpic 2,6-bis(3-(carbazol-9- bis(4,6-difluorophenylpyridinato-n,c 2 ) picolinate- iridium(iii) Blue emitter Ir(iprpmi) 3 tris[1-(2,6- diisopropylphenyl)-2- phenyl-1himidazole]iridium(iii) Blue emitter

77 Acronym Chemical Name Molecular Structure Function HOMO (ev) LUMO (ev) E( T1 ) (ev) Ir(ppy) 3 fac-tris(2-phenylpyridinato)-iridium(iii) Green emitter Ir(phq) 2 (acac) bis(2-phenylquinoline)- (acetylacetonate)- iridium(iii) Red emitter

78 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.

79 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:moo 3 (40%, 10 nm)/tapc (30 nm)/mcp (3 nm)/blue EML (4 nm)/red and green EML (4 nm)/bphen (20 nm)/bphen:cs 2 CO 3 (50%, 10nm)/Al(100nm). TAPC and BPhen can help confine excitons due to their relatively higher triplet energy levels (TAPC has E T1 = 2.9 ev, which is higher than that of FIrpic (2.62 ev); BPhen has E T1 = 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 holetransporting material (TCTA) and an electron-transporting material (TPBi).

80 58 Figure 3.1: (a) Energy level diagram of all materials used in WOLEDs. (b) Device structure of a typical WOLED 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 cm 2 /(V s), and electron mobility in the range of 10-6 ~10-5 cm 2 /(V s)). Such imbalanced charge carriers can limit device efficiency as discussed in Chapter 1; therefore, we introduced a bi-layered holetransporting 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:moo 3 (40%, 10 nm)/tapc (30-x nm)/mcp (x nm)/eml1/eml2/bphen (20 nm)/bphen:cs 2 CO 3 (50%, 10 nm)/al (100 nm). A 10 nm TAPC was doped with 40% MoO 3 as an HIL, whereas a 10 nm BPhen was doped with Cs 2 CO 3 as an EIL. Fifteen

81 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/cm 2. Table 3.1: EL performance of WOLEDs with the mcp buffer layer. ITO (110 nm)/tapc:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) Device TAPC (nm) mcp (nm) Voltage (V) EQE (%) PE (lm/w) Luminance (cd/m 2 ) 1931 CIE x 1931 CIE y A A A A With the introduction of mcp, due to its lower mobility (10-4 cm 2 /(V s)) [18] compared to TAPC (10-2 cm 2 /(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

82 60 charge-carrier balance throughout the devices and, hence, higher device efficiencies can be achieved 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:moo 3 (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:cs 2 CO 3 (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/cm 2 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.

83 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/cm 2 ) 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:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) Device TCTA: TPBi Voltage (V) EQE (%) PE (lm/w) Luminance (cd/m 2 ) 1931 CIE x 1931 CIE y B1 0: B2 1: B3 1: B4 1:

84 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:MoO 3 (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:cs 2 CO 3 (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/cm 2 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.

85 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/cm 2 ) Table 3.3: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110 nm)/tapc:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) Device TCTA: TmPyPB Voltage (V) EQE (%) PE (lm/w) Luminance (cd/m 2 ) 1931 CIE x 1931 CIE y C1 0: C2 1: C3 1: C4 1: 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:MoO 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). For Devices D1 and D2, a thin mcp layer was inserted between TAPC and

86 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/cm 2. 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/cm 2, 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/cm 2 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/cm 2 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 cm 2 /(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

87 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/cm 2 ) Table 3.4: EL performance of WOLEDs with a mixed-host for FIrpic. ITO (110 nm)/tapc:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) Device mcp TCTA: Voltage EQE PE Luminance (nm) DCzPPy (V) (%) (lm/w) (cd/m 2 ) CIE x CIE y D1 3 1: D2 3 0: D3 0 0: 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-

88 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:MoO 3 (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:cs 2 CO 3 (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/cm 2 ) 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

89 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/cm 2. 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:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) Device Red:Green Voltage (V) EQE (%) PE (lm/w) Luminance (cd/m 2 ) 1931 CIE x 1931 CIE y E1 2%:6% E2 1.5%:6% E3 1%:6%

90 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 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 chargecarrier 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:moo 3 (40%, 10 nm)/tapc (30 nm)/ TCTA: FIrpic

91 69 (85%:15%, 4 nm)/interlayer/host:ir(2-phq) 2 (acac):ir(ppy) 3 (1.5%, 5%, 4 nm)/bphen (20 nm)/bphen:cs 2 CO 3 (50%, 10 nm)/al (100 nm). Table 3.6 summarizes device performance at 5 ma/cm 2. 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

92 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/cm 2 ) Table 3.6: EL performance of WOLEDs with interlayers. ITO (110 nm)/tapc:moo 3 (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:cs 2 CO 3 (50%, 10 nm)/al (100 nm). (Measured at a current density of 5 ma/cm 2 ) Device Interlayer 2 nm Voltage (V) EQE (%) PE (lm/w) Luminance (cd/m 2 ) 1931 CIE x 1931 CIE y F1 TPBi F2 DCzPPy F3 FIrpic:DCzPPy 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

93 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/m 2.

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96 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].

97 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/m 2 and 3.7 V. At the same time, the color variation is minimal over a wide range of emission intensities 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:cs 2 CO 3 (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 (Cs 2 CO 3 ) to help increase electron injection efficiency from the cathode. Hole-transporting material TCTA

98 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).

99 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 electronhole 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, B 1 /B 2, B 1 /R&G/B 2, B 1 /R/B 2, and B 1 /G/B 2, having different composite EMLs. B 1 /B 2 is a blue device with two different blue EMLs as the composite emitter comprised of 15% FIrpic doped TCTA (B 1 ) and 20% FIrpic doped DCzPPy (B 2 ). B 1 /R/B 2, B 1 /G/B 2 and B 1 /R&G/B 2, 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 B 1 and B 2, 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/cm 2. It can

100 78 be seen that all four devices exhibit high EQE ranging from 17.5% for the blue device B 1 /B 2 to 20.3% for predominately green device B 1 /G/B 2. The drive voltages for these devices are also very similar, approximately 3.8 ± 0.1 V (at 5 ma/cm 2 ). The power efficiency varies substantially due to a large variation in emission colors from these devices, ranging from 31 lm/w for B 1 /B 2 to 52 lm/w for B 1 /G/B 2. The B 1 /R&G/B 2 device provides a warm white emission with color co-ordinates of (0.458, 0.448) that shift only slightly over a luminance range of cd/m 2. In contrast, device B 1 /R/B 2 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/cm 2.)

101 79 The inset in Figure 4.2 shows the spectral response of these four devices driven at a current density of 5 ma/cm 2. The blue B 1 /B 2 device exhibits only FIrpic emission with a peak at 474 nm. Device B 1 /R/B 2 with a red-doped interlayer shows a cold white color due to the lack of green emission whereas the B 1 /G/B 2 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 B 1 /G/B 2 device compared to the B 1 /R/B 1 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 B 1 /R&G/B 2 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:cs 2 CO 3 (50%, 30 nm)/al (100 nm). (a: values at 5 ma/cm 2 ; b: luminance range from 400 to 4000 cd/m 2.) Device Voltage (V) a EQE PE (%) a (lm/w) a Luminance 1931 (cd/m 2 ) a CIE x a 1931 CIE y a 1931 CIE Dx b 1931 CIE Dy b B 1 /R&G/B ±0.018 ±0.012 B 1 /R/B ±0.028 ±0.003 B 1 /G/B ±0.007 ±0.016 B 1 /B ±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

102 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 B 1 /G/B 2 device compared to B 1 /R/B 2 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%).

103 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

104 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/cm 2 ). 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:cs 2 CO 3 (50%, 30 nm)/al (100 nm). Co-doped layer thickness (nm) Voltage (V) EQE (%) PE (lm/w) Luminance (cd/m 2 ) 1931 CIE x 1931 CIE y Figure 4.4: EL Spectra of devices with various thicknesses of the red and green co-doped layer.

105 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

106 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/cm 2 ). 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:cs 2 CO 3 (50%, 30 nm)/al (100 nm). Device Voltage EQE PE Luminance (V) (%) (lm/w) (cd/m 2 ) CIE x CIE y W W W

107 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/cm 2.) Figure 4.6: Transient PL decay of two FIrpic doped films.

108 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/cm 2 with an initial luminance of about 1800 cd/m 2, 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/cm 2 ). ITO (110nm)/HATCN (3 nm)/tapc(37 nm)/tcta:firpic (x%, 4 nm)/ /DCzPPy:FIrpic (y%, 3 nm)/tmpypb (10 nm)/tmpypb:cs 2 CO 3 (30 nm)/al (100 nm). Device Voltage EQE PE Luminance (V) (%) (lm/w) (cd/m 2 ) CIE x CIE y B B B

109 87 Figure 4.7: Device lifetime of three blue devices with selectively doped blue emitting layers 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/m 2 ) and minimal color shift over a large range of intensities ( cd/m 2 ). We also found that the device degradation is related to the lifetime of FIrpic excited states, which is dependent on the host materials.

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112 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

113 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 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 (Cs 2 CO 3 ) 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.

114 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/cm 2. 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

115 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). (a) (b) 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:cs 2 CO 3 (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

116 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/cm 2, 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/cm 2, 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.

117 95 (a) (b) (c) 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/cm 2. 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/cm 2. 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

118 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 triplettriplet 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:cs 2 CO 3 (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

119 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 selfquenching 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:cs 2 CO 3 (50%, 30 nm)/al (100 nm). (a: values at 5 ma/cm 2, b: measured at current densities from 0.05 ma/cm 2 to 20 ma/cm 2.) Ir(iprpmi) 3 (%) Voltage (V) a EQE (%) a PE (lm/w) a Luminance (cd/m 2 ) a CIE x a CIE y a CIE Dx b CIE Dy b ±0.016 ± ±0.006 ± ±0.001 ± ±0.024 ± ±0.015 ±0.005

120 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/cm 2. 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.

121 99 (a) (b) (c) 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/cm 2 ; (c) color shift of the device with 9% Ir(iprpmi) 3 at current densities from 0.05 to 20 ma/cm 2. 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.

122 100 The details of WOLED performance are summarized in Table 5.1. The power efficacy at 5 ma/cm 2 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/cm 2 ). 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/cm 2 to 20 ma/cm 2. 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:cs 2 CO 3 (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/cm 2, 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:

123 101 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+cs 2 CO 3 (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/cm 2 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/cm 2. 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.

124 102 Figure 5.5: Device lifetime tested at 5 ma/cm 2 (WOLEDs EL spectra are in the inset) Conclusions By investigating the charge-carrier transporting properties of Ir(iprpmi) 3 in holeonly 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/m 2 and exhibited a minimal color shift (±0.001, ±0.005) over a large current density range of ma/cm 2. 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.

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128 106 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

129 107 be achieved simultaneously. Understanding the degradation mechanisms of a high-tripletenergy 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 cationinduced 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.

130 Results and Discussion 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:cs 2 CO 3 (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

131 109 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/cm 2. 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 A 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/cm 2 for 24 h in a vacuum assembly. Since the laser (wavelength: 337 nm) can induce photo-fragmentation of OLED

132 110 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

133 111 spectra are fragments and adducts, the proposed structures that are summarized in Table 6.1.

134 112 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

135 113 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 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.

136 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 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,

137 115 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 Cyclohexyl rupture results in fragments at 465, 557, 570 and 583. A combination of both

138 116 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

139 117 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 cationinduced 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.

140 118 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.

141 119 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

142 120 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.

143 121 Figure 6.9: Resonant structures (up) of the TAPC cation and ring-opened TAPC cation and HOMO (down) of TAPC and ring-opened TAPC 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.

144 122 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/cm 2 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].

145 123 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 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

146 124 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