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Title Page

Contents

List of abbreviation 16

Abstract 22

Chapter 1. Introduction 24

1.1. Basic structure of organic light-emitting diodes 24

1.2. Emitter types and emission mechanisms in OLEDs 27

1.3. Current and future OLED applications 30

1.4. Current issues with red emitters in OLEDs 33

1.5. Approach methods for highly efficient red dopant 36

1.5.1. Various efficiency factors of OLEDs 36

1.5.2. Relationship between efficiency and radiative decay rate 41

Chapter 2. Design Strategy for Highly Efficient and Stable Pt(II) Red Dopants by Singlet-Triplet Energy Gap Control 46

2.1. Introduction 46

2.1.1. History and issues of Pt(II) red phosphorescent dopants 46

2.1.2. Design strategy for highly efficient Pt(II) red dopants 51

2.2. Experimental 54

2.2.1. General information 54

2.2.2. Synthesis 56

2.2.3. Device fabrication 67

2.2.4. Computational methods 69

2.3. Results & Discussion 70

2.3.1. Study for efficiency improvement 70

2.3.2. Study for device lifetime improvement 87

2.3.3. Molecular optimization for highly efficient and stable Pt(II) red-dopants 100

2.4. Conclusions 115

Chapter 3. Development of Novel Chromophore for Highly Efficient Ir(III) Red Dopants Using Transition Dipole Moment 117

3.1. Introduction 117

3.1.1. Issues of Ir(III) red phosphorescent dopant 117

3.1.2. Design strategy for highly efficient Ir(III) red dopants 120

3.2. Experimental 125

3.2.1. General information 125

3.2.2. Synthesis 126

3.2.3. Device fabrication 137

3.3. Results and Discussion 138

3.4. Conclusions 163

References 164

논문요약 173

List of Tables

Chapter 2 10

Table 2-1. Summary of the molecular caculations for RD01 to RD05. 72

Table 2-2. Summary of orbital composition distribution using the NTO plot. 75

Table 2-3. Summary of material property data for RD01, RD02, and RD03. 80

Table 2-4. Summary of OLED device performance for RD01 and RD02. 83

Table 2-5. Summary of molecular calculations for RD06, RD07 and comparison with RD01. 89

Table 2-6. Summary of material property data for RD06, RD07, and comparison with RD01. 95

Table 2-7. Summary of OLED device performance for RD06, RD07, and comparison with RD01. 99

Table 2-8. Summary of molecular calculations for RD08, RD09, and comparison with RD01. 103

Table 2-9. Summary of material property data for RD09 and comparison with RD01. 106

Table 2-10. Summary of OLED device performance for RD09, RD09R', and comparison with RD01. 109

Chapter 3 11

Table 3-1. C-C bond lengths of the specific benzene ring in RD-IQ, RD-BIQ-A, and RD-BIQ-B. 143

Table 3-2. Summary of molecular calculations for RD-IQ, RD-BIQ-A, and RD-BIQ-B. 145

Table 3-3. Summary of material property data for RD-IQ, RD-BIQ-A, and and RD-BIQ-B. 151

Table 3-4. Summary of OLED device performance for RD-IQ, RD-BIQ-A, and RD-BIQ-B. 161

List of Figures

Chapter 1 12

Figure 1-1. Basic OLED structure and component layer functions. 25

Figure 1-2. Mechanisms of fluorescence and phosphorescence in the Jablonski diagram. 27

Figure 1-3. OLED technology roadmap to the future. 30

Figure 1-4. (a) Comparison of BT2020(or Rec2020) and Adobe RGB in CIE index. (b) S₁(or T₁)-S₀ vibrational coupling by small energy bandgap. 34

Chapter 2 12

Figure 2-1. Development history of red phosphorescent dopants. 48

Figure 2-2. S₁ energy control for reducing △EST.[이미지참조] 53

Figure 2-3. Molecular design concept for reducing △EST.[이미지참조] 71

Figure 2-4. Chemical structures and calculated distributions of HOMOs and LUMOs at lowest triplet excited state. 74

Figure 2-5. (a) PL emission spectra, and (b) transient PL decay curve for RD01, RD02, and RD03. 79

Figure 2-6. (a) Energy level diagram and (b) chemical structure of materials used in OLED devices. 83

Figure 2-7. (a) Current density-Voltage-Luminance curves, (b) external quantum efficiency-luminance, and (c) EL spectra of RD01, RD02 devices. 85

Figure 2-8. Device lifetime for RD01 and RD02 at 25 mA/cm². 86

Figure 2-9. (a) Conceptual molecular design for reducing intermolecular interactions and (b) resulting new dopant structure. 88

Figure 2-10. (a) The three-dimensional structure and (b) accessible ratio for RD01, RD06, and RD07. 91

Figure 2-11. Comparison of PL emission spectra for RD01, RD06, and RD07. 94

Figure 2-12. (a) EL spectrum and (b) external quantum efficiency-luminance of RD01, RD06, and RD07 devices. 97

Figure 2-13. Device lifetime comparison for RD01, RD06, and RD07 at 3,000 cd/m². 98

Figure 2-14. The design concept for high efficiency and long device lifetime. 101

Figure 2-15. Comparison of the PL emission spectra for RD01 and RD09. 105

Figure 2-16. Current density-Voltage characteristics of (a) electron-only device (EOD) and (b) hole-only device (HOD) with 2 wt% RD01 or RD09... 110

Figure 2-17. The energy level diagram of RD09R' OLED device and chemical structure of R'. 111

Figure 2-18. (a) Current density-Voltage curves, (b) external quantum efficiency-luminance, and (c) EL spectra of RD01, RD09, and RD09R' devices. 113

Figure 2-19. Device lifetime comparison for RD01, RD09, and RD09R' at 3,000 cd/m². 114

Figure 2-20. Novel RD09 dopant with outstanding performance and UHD color gamut. 116

Chapter 3 14

Figure 3-1. Classification of first- and second-generation Ir(III) red phosphorescent dopants according to main ligand structures. 119

Figure 3-2. The attempted design for reducing the △EST factor in an Ir(III) red dopant with a piq ligand, and the expected wavelength of the peak emission.[이미지참조] 124

Figure 3-3. Novel Ir(III) red dopant design concept to enhance the TDM factor without red-shift in the emission wavelength. 141

Figure 3-4. Ground state density functional theory calculations for RD-IQ, RD-BIQ-A, and RD-BIQ-B. 142

Figure 3-5. Molecular orbital calculation results of RD-IQ, RD-BIQ-A, and RD-BIQ-B. 144

Figure 3-6. (a) UV-vis absorption and PL emission spectra, and (b) transient PL decay curve for RD-IQ, RD-BIQ-A, and RD-BIQ-B. 150

Figure 3-7. TGA data for RD-IQ, RD-BIQ-A, and RD-BIQ-B under ambient pressure (1 atm). 153

Figure 3-8. TGA data for (a) RD-IQ, (b) RD-BIQ-A, and (c) RD-BIQ-B under reduced pressure (1 Pa). 155

Figure 3-9. (a) Current density-Voltage-Luminance curves, (b) external quantum efficiency-luminance, and (c) EL spectra of RD-IQ, RD-BIQ-A,... 160

Figure 3-10. Horizontal ratio analysis of RD-IQ, RD-BIA-A, and RD-BIQ-B based on angle-dependent PL measurements. 162

List of Schemes

Chapter 2 9

Scheme 2-1. Synthetic procedure for RD02. 56

Scheme 2-2. Synthetic procedure for RD03. 59

Scheme 2-3. Synthetic procedure for RD06. 61

Scheme 2-4. Synthetic procedure for RD07. 63

Scheme 2-5. Synthetic procedure for RD09. 64

Scheme 2-6. Schematic of the synthesis process for RD01 to RD04. 76

Chapter 3 9

Scheme 3-1. Synthetic procedure for RD-IQ. 126

Scheme 3-2. Synthetic procedure for RD-BIQ-A. 128

Scheme 3-3. Synthetic procedure for RD-BIQ-B. 132

Scheme 3-4. Schematic of the synthesis process for RD-BIQ-A and RD- BIQ-B. 146

초록보기

 본 논문은 차세대 OLED 디스플레이에서 요구되는 높은 효율을 가지는 백금과 이리듐을 기반으로 한 적색 인광 재료의 개발에 관한 연구이다. 에너지 밴드 갭 법칙에 따라 장파장 영역에서의 적색 발광 재료의 효율이 감소하는 근본적인 문제를 해결하는 것이 주요한 연구 목표이다. 본 연구에서 제안된 문제 해결 방식은 발광 재료의 효율을 결정하는 핵심 요소인 복사 감쇠율 상수 (kr)를 향상시키는데 중점을 두었다. 특히, kr의 중요한 구성 인자인 발광 물질의 단일항과 삼중항 상태의 에너지 차이 (△EST)와 전이 쌍극자 모멘트 (TDM)를 제어하는 새로운 리간드 설계에 대한 연구 내용이다.

이 연구의 첫 번째 주제는 적색 발광을 나타내는 백금 기반의 인광 물질에 집중하였다. 연구에서는 △EST를 제어하여 발광 효율을 향상시키는 설계 방법을 제시하고, 이를 통해 기존 물질 보다 최대 외부 양자 효율을 40 % 향상시키는 결과를 도출했다. 또한, 치환체를 도입해 소자 수명을 5배 이상 크게 늘렸다. 두 번째 연구에서는 이리듐 기반의 적색 인광 물질의 효율을 개선하기 위해, kr의 핵심 구성 요소인 TDM을 향상시키는 데 초점을 맞춘 혁신적인 분자 설계 접근법을 제시하였다. 이 방법을 통해 기존 이리듐 기반의 적색 인광 물질에 비해 효율이 25 % 향상된 새로운 물질을 개발하였다. 이러한 연구 결과는 향후 차세대 OLED 디스플레이의 높은 요구 성능을 만족시키는 인광 소재의 지속적인 개발 가능성을 제시하고, 더불어 고효율을 구현하는 적색 인광 물질에 적합한 신규한 리간드를 소개한다.

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