Title Page
ABSTRACT
국문 초록
Contents
NOMENCLATURE 21
CHAPTER 1. INTRODUCTION 23
1.1. Introduction 23
1.2. Theoretical Background 28
1.2.1. Principle of OLEDs 28
1.2.2. Efficiency of OLEDs 32
1.2.3. Generation of OLEDs 37
1.2.4. Doping System in OLEDs 40
1.3. References 42
CHAPTER 2. FABRICATION AND CHARACTERIZATION 46
2.1. Thermal Evaporation System 46
2.2. Device Design and Fabrication Procedures 49
2.3. Device Measurement and Characterization 51
2.5. References 55
CHAPTER 3. WHITE PHOSPHORESCENT OLEDs WITH UNDOPED ULTRA-THIN EML 56
3.1. Introduction 57
3.2. Methods 59
3.3. Results 61
3.4. Conclusion 70
3.5. Supplementary materials 71
3.5.1. EL characteristics of fabricated sky-blue and yellow phosphorescence OLEDs. 71
3.5.2. EL characteristics of fabricated WOLEDs with two-color ultra-thin EML-based systems (sky-blue and yellow dopants) 77
3.6. References 81
CHAPTER 4. MECHANISM INVESTIGATION OF PHOSPHORESCENT OLEDs WITH UNDOPED ULTRA-THIN EML 85
4.1. Introduction 86
4.2. Methods 88
4.3. Results 94
4.4. Conclusion 104
4.5. References 105
CHAPTER 5. HIGHLY EFFICIENT PHOSPHORESCENT OLEDs WITH UNDOPED ULTRA-THIN EML 108
5.1. Introduction 109
5.2. Methods 111
5.3. Results 113
5.4. Conclusion 130
5.5. Supplementary materials 131
5.5.1. Calculation of the dopant intermolecular distance 131
5.5.2. Calculation of optical path length-thickness of HTL and ETL 138
5.5.3. EL characteristics of ultra-thin OLEDs for various HTL and ETL thicknesses 142
5.6. References 147
CHAPTER 6. HIGHLY EFFICIENT ULTRA-THIN EML PHOLEDs WITH EXTERNAL LIGHT EXTRACTION LAYER 151
6.1. Introduction 152
6.2. Methods 155
6.3. Results 158
6.4. Conclusion 180
6.5. References 182
CHAPTER 7. CONCLUSION 185
LIST OF PUBLICATIONS 188
International Journals 188
International Conferences 190
Domestic Conferences 192
Table 2.1. Summary of thermal evaporation system 48
Table 3.1. Summary of peak EQE for different ETL and HTL thickness 66
Table 3.2. Summary of WOLEDs characteristics. 69
Table S3.1 FWHM data of sky-blue and yellow OLEDs according to ultra-thin EML thickness. 76
Table 4.1. Device configuration for Set-1 90
Table 4.2. Device configuration for Set-2 92
Table 4.3. Summarized EL characteristics of fabricated Set-2 OLEDs. 100
Table 5.1. Summary of peak EQE for different ETL and HTL thickness 126
Table 5.2. Summary of peak EQE in this study and in previous works. 128
Table S5.1. Summary of average dopant intermolecular distance and efficiency characteristics of doping method in this work and references. 136
Table S5.2. Calculated thicknesses of ETL. 141
Table S5.3. Calculated thicknesses of all layers. 141
Table S5.4. Calculated thicknesses of HTL. 141
Table S5.5. Summary of driving voltage at 10 mA/cm² and driving voltage increase. 145
Table 6.1. Summarized characteristics of diffusers specification. 164
Table 6.2. Summary of device characteristics with various diffusers applied: peak EQE and viewing angle improvement. 168
Table 6.3. Summary of green fluorescent OLEDs device characteristics with various diffusers applied: peak EQE and viewing angle improvement. 172
Figure 1.1. Schematic of the OLEDs basic structure 30
Figure 1.2. Schematic energy level diagram of OLEDs 31
Figure 1.3. Schematic of multi-layer OLED structure and optical ray diagram of light propagation. 36
Figure 1.4. Schematic of electroluminescent exciton dynamic processes of the three generations OLEDs and a summary of pros and cons of each generation 39
Figure 2.1. The DHT thermal evaporation system with glove box 47
Figure 2.2. Schematic of device fabrication process and mask layouts for bottom emission OLEDs. 50
Figure 2.3. The viewing angle variable IVL measurement system schematic. 54
Figure 3.1. (a) Energy level diagram of OLED device with ultra-thin EML structure. (b) Fabricated OLED device structure. (c) Molecular structures of HIL, HTL and ETL. 62
Figure 3.2. Peak EQE versus thickness of dopants (a) FIrpic and (b) Ir(BT)₂acac from 0.0375 nm to 0.3 nm. (c) EL spectra of sky-blue and yellow OLEDs with 0.15 nm ultra-... 65
Figure 3.3. (a) Current density–EQE (J–EQE) characteristics of WOLEDs with sky-blue dopant (0.15 nm) and various yellow dopant thicknesses. (b) EL spectra of white,... 68
Figure S3.1. (a) Current density–voltage–luminance (J–V–L) curve of (a) sky-blue OLEDs and (b) yellow OLEDs. 72
Figure S3.2. (a) Current density–EQE (J–EQE) characteristics of (a) sky-blue OLEDs and (b) yellow OLEDs, current efficiency–luminance–power efficiency (CE–L–PE)... 74
Figure S3.3. EL spectra of blue and yellow OLEDs with ultra-thin EML according to ultra-thin EML thickness. 75
Figure S3.4. Current density–voltage–luminance (J–V–L) curve of WOLEDs with specific sky-blue dopant and various yellow dopant thicknesses. The thicknesses of sky-... 78
Figure S3.5. Current density–EQE (J–EQE) characteristics of WOLEDs with specific sky-blue dopant and various yellow dopant thicknesses. The thicknesses of sky-blue... 79
Figure S3.6. Current efficiency–luminance–power efficiency (CE–L–PE) characteristics of WOLEDs with specific sky-blue dopant and various yellow dopant... 80
Figure 4.1. Device structure and energy band diagram of fabricated OLEDs 89
Figure 4.2. Insertion structures for fabricated OLEDs of Set-1 (Device-1 to Device-3) 91
Figure 4.3. EL characteristics of fabricated Set-1 OLEDs. (a) Current density with respect to the applied voltage, (b) current efficiency and PE with respect to the luminance,... 95
Figure 4.4. The EL spectra in arbitrary units are presented in two different scales: (left) the normal scale (1X) and (right) the magnified y-scale (10X); (a-c) EL spectra of... 97
Figure 4.5. EL characteristics of fabricated Set-2 OLEDs. (a) CE and PE with respect to the luminance, (b), external quantum efficiency with respect to the luminance and (c)... 99
Figure 4.6. Exciton distribution from the TCTA/TmPyPB interface. The x-axis value is negative for the TCTA side and positive for the TmPyPB side. 102
Figure 5.1. (a) Energy level diagram of OLED device with ultra-thin EML structure. (b) Fabricated OLED device structure. (c) Molecular structures of the organic materials... 114
Figure 5.2. Peak EQE versus thickness of Ir(ppy)₃ from 0.05 nm to 0.2 nm. The inset shows the EQE characteristics of the fabricated OLEDs, as a function of current density. 116
Figure 5.3. The relation between quantum efficiency and average dopant intermolecular distance according to the thickness of ultra-thin EML and doping concentration (a) this... 119
Figure 5.4. Normalized EQE characteristics as a function of current density: (a) HIL (HAT-CN) thickness changed, (b) EIL (LiF) thickness changed. 122
Figure 5.5. Characteristics of six representative devices: (a) current density–EQE (J–EQE) characteristics, (b) current efficiency–luminance (CE–L) characteristics, and (c) power... 124
Figure S5.1. Schematic of assumption-based dopant molecular arrangement in (a) 2-dimensional ultra-thin EML structure, and (b) 3-dimensional conventional doping structure. 132
Figure S5.2. The relation between quantum efficiency and average dopant intermolecular distance according to the thickness of ultra-thin EML and doping concentration (a)this... 135
Figure S5.3. Schematic diagram of constructive interference mechanism in the OLED structure. 139
Figure S5.4. EL characteristics of fabricated OLEDs: (a) Current density–voltage–luminance (J–V–L) characteristics, (b) external quantum efficiency–current density (EQE–... 143
Figure S5.5. EL spectra data of the fabricated OLEDs devices with different HTL and ETL thickness combinations 144
Figure 6.1. (a) Schematic of ultra-thin EML OLEDs device structure with diffuser. (b) Energy level diagram of blue PHOLEDs device with ultra-thin EML structure. (c)... 157
Figure 6.2. (a) Energy level diagram of OLED device with ultra-thin EML structure. (b) Fabricated OLED device structure. (c) Molecular structures of the organic materials... 159
Figure 6.3. EL characteristics of optimized reference device: (a) Current density–voltage–luminance (J–V–L) characteristics, (b) current density–external quantum... 161
Figure 6.4. The SEM images of diffusers and the real image of blue PHOLEDs device. 163
Figure 6.5. The optical characteristics of various diffusers: (a)total transmittance, (b)perpendicular transmittance, (c) haze, and the (d) viewing angle characteristics of... 166
Figure 6.6. (a) The current density–external quantum efficiency (J–EQE) characteristics, and (b) viewing angle characteristics of fluorescent OLEDs with various diffusers. 171
Figure 6.7. FDTD simulation model of fluorescent OLEDs structure with different dipole orientation as (a) X axis, (b) Y axis, and (c) Z axis direction. The far-field... 174
Figure 6.8. The light extraction enhancement characteristics to wavelength according to (a) dipole orientation, and (b) OLEDs device structure. 177