Title Page

Contents

Abstract 20

Chapter 1. INTRODUCTION 22

1.1. Organic light-emitting diodes 22

1.2. Emission mechanism 24

1.3. Host-dopant system 27

1.4. Host materials for PhOLEDs 30

1.5. Simple degradation mechanism in PhOLEDs 36

Chapter 2. Boron- and Silane-Based Electron Transport-type Host Materials for Long-lifetime Blue Phosphorescent Organic Light-emitting Diodes 38

2.1. Introduction 38

2.2. Experimenta 42

2.2.1. General information 42

2.2.2. Synthesis 42

2.2.3. Device fabrication 47

2.3. Result and discussion 49

2.3.1. Molecular design and electronic calculations 49

2.3.2. Photophysical properties 54

2.3.3. Thermal properties and energy level 67

2.3.4. MR-TADF emitter evaluation 70

2.3.5. N-type host for blue PhOLEDs 81

2.4. Conclusion 89

Chapter 3. Multiple Resonance Effect-induced Host Materials for Long-lifetime Phosphorescent Organic Light-emitting Diodes 90

3.1. Introduction 90

3.2. Experimental 94

3.2.1. General information 94

3.2.2. Synthesis 95

3.2.3. Device fabrication 101

3.3. Result and discussion 103

3.3.1. Molecular design strategy 103

3.3.2. Density functional theoretical calculations 105

3.3.3. Photophysical and electrochemical properties. 108

3.3.4. MR-type TADF emitter evaluation 130

3.3.5. N-type host for PhOLEDs 139

3.4. Conclusion 145

Chapter 4. Boron-based Thermally Activated Delayed Fluorescence Host Materials as Universal Hosts for Blue Phosphorescent Organic Light-emitting Diodes 146

4.1. Introduction 146

4.2. Experimental 150

4.2.1. General information 150

4.2.2. Synthesis 151

4.2.3. Device fabrication 158

4.3. Result and discussion 160

4.3.1. Molecular design and electronic calculations 160

4.3.2. Density functional theory calculations 164

4.3.3. Material characterization 168

4.3.4. Electroluminescence properties 187

4.4. Conclusion 205

References 206

논문요약 220

Chapter 2. 10

Table 2.1. Photophysical and electrochemical properties of DOBNA, BO-pSi, and BO-2mSi. 57

Table 2.2. Triplet energy values were estimated from the onset wavelength of the corresponding phosphorescence spectra. 60

Table 2.3. Absolute PLQY and rate constant of 2 wt% BO-pSi and BO-2mSi doped in DPEPO. 64

Table 2.4. Solid PL properties of 2, 10, and 50 wt% of BO-pSi and BO-2mSi doped in DPEPO. 73

Table 2.5. Absolute PLQY and rate constant of 2, 10, and 50 wt% of BO-2mSi doped in DPEPO. 75

Table 2.6. Device performance of BO-2mSi doped in DPEPO. 80

Table 2.7. Device performance of the blue PhOLEDs using the DOBNA derivatives as n-type hosts with other blue PhOLEDs reported in recent papers. 88

Chapter 3. 10

Table 3.1. Basic photophysical and electrochemical properties of DOBNA, B-2OCz and B-2OCz-Si. 113

Table 3.2. Summarized emission peaks and FWHM measured at room temperature and 77 K for DOBNA, B-2OCz and B-2OCz-Si. 115

Table 3.3. Solid PL properties of B-2OCz and B-2OCz-Si doped in mCBP and DPEPO. 120

Table 3.4. Absolute PLQY and rate constant of B-2OCz and B-2OCz-Si doped in DPEPO. 124

Table 3.5. Summary of the TADF device performances of B-2OCz and B-2OCz-Si. 134

Table 3.6. Summary of the device performances of B-2OCz-Si doped in mCBP:TSPO1 mixed host. 137

Table 3.7. Device performances of previously reported violet emitters. The data were collected below the emission peak of 420 nm. 138

Table 3.8. Summary of the green PhOLEDs characteristics, employing Ir(ppy)₃ as a dopant. 144

Chapter 4. 11

Table 4.1. Photophysical and electrochemical properties of BO-Cz-Si-1 and BO-Cz-Si-2. 171

Table 4.2. Absolute PLQY and rate constant of mCBP:BO-Cz-Si-1 and mCBP:BO-Cz-Si-2 films. 176

Table 4.3. Absolute PLQY and rate constant of SiCzCz:BO-Cz-Si-1 and SiCzCz:BO-Cz-Si-2 films. 179

Table 4.4. The SCF energies of BO-Cz-Si-1 calculated from the neutral, cationic and anionic geometries in neutral, cationic and anionic states. 186

Table 4.5. The SCF energies of BO-Cz-Si-2 calculated from the neutral, cationic and anionic geometries in neutral, cationic and anionic states. 186

Table 4.6. The reorganization energies for hole transport and electron transport of BO-Cz-Si-1 and BO-Cz-Si-2. 186

Table 4.7. Absolute PLQY and rate constant of mSiTrz. 201

Table 4.8. Device performance of the blue PhOLEDs using the DOBNA derivatives as n-type hosts with other blue PhOLEDs reported in recent papers. 204

Chapter 1. 13

Figure 1.1. The basic structure and emitting mechanism of the OLED device. 23

Figure 1.2. Emission mechanisms of a) fluorescence, b) Phosphorescence, c) TADF process. 26

Figure 1.3. a) Host-Dopant system in emitting layer, and b) The schematic of the Förster energy transfer and Dexter energy transfer in host-dopant system. 28

Figure 1.4. P-type host and n-type host materials for long-lifetime blue PhOLEDs. 35

Chapter 2. 13

Figure 2.1. Molecular design strategy for BO-pSi and BO-2mSi. 50

Figure 2.2. Density functional theory calculations for of DOBNA, BO-pSi, and BO-2mSi. 53

Figure 2.3. UV-vis absorption and PL spectra of (a) DOBNA, (b) BO-pSi, and (c) BO-2mSi. 56

Figure 2.4. Phosphorescence spectra of the solid film state of (a) DOBNA, (b) BO-pSi, and (c) BO-2mSi. 59

Figure 2.5. Transient PL decay curves of 2 wt% BO-pSi and BO-2mSi doped in DPEPO. (a) Prompt component, and (b) delayed component. 63

Figure 2.6. Transient PL decay curves of 50 wt% BO-pSi and BO-2mSi doped in mCBP. (a) Prompt component, and (b) delayed component. 65

Figure 2.7. Solid PL spectra of (a) mCBP:BO-pSi and (b) mCBP:BO-2mSi mixed film. 66

Figure 2.8. (a) Thermogravimetric analysis and (b) differential scanning calorimetry data of DOBNA, BO-pSi, and BO-2mSi. 68

Figure 2.9. Oxidation curves of DOBNA, BO-pSi, and BO-2mSi. 69

Figure 2.10. Solid PL spectra of 2, 10, and 50 wt% (a) BO-pSi and (b) BO-2mSi doped in DPEPO. 72

Figure 2.11. Transient PL decay curves of 2, 10, and 50 wt% BO-2mSi doped in DPEPO. (a) Prompt component and (b) delayed component. 74

Figure 2.12. Device structure, energy diagrams, and molecular structures of violet TADF-OLEDs. 78

Figure 2.13. Device performance of the violet TADF OLEDs using BO-2mSi. (a) EL spectra and (b) EQE-luminance, (c) current density-voltage, and (d)... 79

Figure 2.14. Current density (J) - voltage (V) characteristics of the single charge devices for BO-pSi and BO-2mSi. Open symbols represent hole-only... 81

Figure 2.15. Device structure, energy diagrams, and molecular structures of blue PhOLEDs. 84

Figure 2.16. Device performance of the blue PhOLEDs using the DOBNA derivatives as n-type hosts. (a) EL spectra, (b) current density-voltage-... 85

Figure 2.17. Relative change of PL intensity of mCBP:BO-Si derivatives:CNIr films according to UV exposure time. 86

Figure 2.18. (a) Stability of the hole only devices and (b) electron only device for BO-pSi and BO-2mSi. 87

Chapter 3. 15

Figure 3.1. Design strategy for the pure MR-TADF materials. 104

Figure 3.2. Density functional theory (DFT) calculations for DOBNA, B-2OCz and B-2OCz-Si molecules. 107

Figure 3.3. The UV-vis absorption, fluorescence, and phosphorescence spectra of (a) DOBNA, (b) B-2OCz, and (c) B-2OCz-Si. 111

Figure 3.4. Photophysical properties of DOBNA, B-2OCz and B-2OCz-Si. (a) UV-vis absorption, (b) fluorescence and (c) Phosphorescence spectra. 112

Figure 3.5. PL spectra measured at room temperature and 77K for (a) DOBNA, (b) B-2OCz and (c) B-2OCz-Si. 114

Figure 3.6. Calculated total reorganization energies of (a) DOBNA, (b) B-2OCz and (c) B-2OCz-Si. 116

Figure 3.7. Solid PL spectra of 1 and 10 wt% (a) B-2OCz and (b) B-2OCz-Si doped in mCBP and DPEPO. 119

Figure 3.8. Transient PL decay curves of 1 and 10 wt% (a) B-2OCz and (b) B-2OCz-Si doped in DPEPO. 123

Figure 3.9. Oxidation curves of DOBNA, B-2OCz and B-2OCz-Si. 127

Figure 3.10. (a) Thermogravimetric analysis (TGA) and (b) differential scanning calorimetry (DSC) of DOBNA, B-2OCz and B-2OCz-Si. 128

Figure 3.11. Current density (J)- voltage (V) characteristics of the single charge devices of B-2OCz and B-2OCz-Si neat films. Open and closed symbols... 129

Figure 3.12. Device structure, energy diagrams and molecular structures of TADF device. 132

Figure 3.13. TADF device performances of the 1, 5, 10 and 20 wt% B-2OCz and B-2OCz-Si doped in DPEPO. (a) Current density-voltage, (b)... 133

Figure 3.14. Device performances of 1, 3, 5 and 10 wt% B-2OCz and B-2OCz-Si doped in mCBP:TSPO1 mixed host. (a) Current density-voltage-luminance... 136

Figure 3.15. Device structure, energy diagrams and molecular structures of green PhOLEDs. 141

Figure 3.16. PhOLEDs performances of the B-2OCz-Si and mSiTrz as n-type host. (a) Current density-Voltage-Luminance, (b) External quantum... 142

Figure 3.17. (a) Current density (J)- voltage (V) characteristics of the hole only and electron only devices of B-2OCz-Si and mSiTrz. (b) Stability of the... 143

Chapter 4. 17

Figure 4.1. Molecular design strategy for BO-Cz-Si-1 and BO-Cz-Si-2. 162

Figure 4.2. Optimized geometry and electron density distribution of frontier molecular orbital for BO-Cz-Si-1 and BO-Cz-Si-2. 166

Figure 4.3. Highest occupied natural transition orbital (HONTO) and lowest unoccupied natural transition orbital (LUNTO) distribution at the lowest singlet... 167

Figure 4.4. UV-vis absorption and PL spectra for (a) BO-Cz-Si-1 and (b) BO-Cz-Si-2. 170

Figure 4.5. Solid PL spectra of (a) mCBP:BO-Cz-Si-1 and (b) mCBP:BO-Cz-Si-2 mixed with mCBP film. 174

Figure 4.6. Transient PL decay curves of BO-Cz-Si-1 and BO-Cz-Si-2 doped in mCBP. (a) Prompt component and (b) delayed component. 175

Figure 4.7. Solid PL spectra of (a) SiCzCz:BO-Cz-Si-1 and (b) SiCzCz:BO-Cz-Si-2 mixed film. 177

Figure 4.8. Transient PL decay curves of BO-Cz-Si-1 and BO-Cz-Si-2 doped in SiCzCz. (a) Prompt component and (b) delayed component. 178

Figure 4.9. (a) Thermogravimetric analysis and (b) differential scanning calorimetry data of BO-Cz-Si-1 and BO-Cz-Si-2. 182

Figure 4.10. Oxidation curves of BO-Cz-Si-1 and BO-Cz-Si-2. 183

Figure 4.11. Current density (J) - voltage (V) characteristics of the single charge devices for BO-Cz-Si-1 and BO-Cz-Si-2. (a) Hole only and (b)... 184

Figure 4.12. (a) Hole and (b) electron mobility curves for BO-Cz-Si-1 and BO-Cz-Si-2. 185

Figure 4.13. Device structure, energy diagrams, and molecular structures of sky-blue PhOLEDs. 190

Figure 4.14. The device performances of the BO-Cz-Si-1, BO-Cz-Si-2 and mSitrz as n-type host doped with CNIr. (a) EL spectra, (b) current-voltage- 191

Figure 4.15. Current density (J) - voltage (V) characteristics of the single charge devices for BO-Cz-Si-1, BO-Cz-Si-2 and mSiTrz. 192

Figure 4.16. Transient PL decay curves of mCBP:mSiTrz film. 192

Figure 4.17. Solid PL spectra of BO-Cz-Si-1, BO-Cz-Si-2, and mSiTrz doped with mCBP film. 193

Figure 4.18. (a) Stability of the hole only devices and (b) electron only devices for BO-Cz-Si-1 and BO-Cz-Si-2 194

Figure 4.19. Transient EL decay curves of the device with BO-Cz-Si-1, BO-Cz-Si-2 and mSiTRz as the n-type host for CNIr devices. 195

Figure 4.20. Device structure, energy diagrams, and molecular structures of deep-blue PhOLEDs. 198

Figure 4.21. Solid PL spectra of SiCzCz:mSiTrz mixed film. 199

Figure 4.22. Transient PL decay curves of mSiTrz doped in SiCzCz. (a) Prompt component and (b) delayed component. 200

Figure 4.23. The device performances of the BO-Cz-Si-1, BO-Cz-Si-2 and mSiTrz n-type host doped with BD-02. (a) EL spectra, (b) current-voltage-... 202

Figure 4.24. Transient EL decay curves of the device with BO-Cz-Si-1, BO-Cz-Si-2 and m-SiTrz as the n-type host for BD-02 devices. 203

Chapter 2. 9

Scheme 2.1. Synthetic scheme of BO-pSi and BO-2mSi. 51

Chapter 3. 9

Scheme 3.1. Synthetic scheme of B-2OCz and B-2OCz-Si. 104

Chapter 4. 9

Scheme 4.1. Synthetic scheme of BO-Cz-Si-1. 163

Scheme 4.2. Synthetic scheme of BO-Cz-Si-2. 163