Description
UNDERSTAND OLED DEVICE PHYSICS FROM CARRIER INJECTION TO LIGHT EMISSION
Physics and Technology of Organic Light-Emitting Diodes presents the first textbook focused solely on OLEDs built from amorphous organic semiconductors. Two veteran researchers with decades of combined expertise detail device operation mechanisms, from carrier injection through light emission, emphasizing the structure and behavior of multilayer thin-film OLEDs that power modern smartphones, televisions, and AR/VR displays.
This book combines the latest theoretical and experimental research with rigorous analysis and practical applications, examining exciplexes, tandem OLED devices, carrier pair generation, and molecular orientation effects. Readers explore degradation mechanisms and device lifetime from a physical perspective, along with ultra-stable glass formation via vacuum deposition. Numerical examples and illustrations throughout support deeper understanding of these concepts.
Readers will also explore:
- Theoretical foundations paired with practical data connecting academic research to industrial OLED development and manufacturing requirements
- Device operation mechanisms specific to amorphous glass organic semiconductors aligned with current technological mainstream applications
- Physical analysis of degradation pathways and device lifetime factors critical for improving OLED reliability and performance
- Tandem OLED architectures and carrier pair generation concepts essential for next-generation high-efficiency display designs
- Vacuum deposition techniques for ultra-stable glass formation enabling superior thin-film quality and device characteristics
Engineers and lab scientists working in OLED development will find authoritative guidance on device physics principles. Graduate students in materials science, applied physics, or electrical engineering gain focused instruction on amorphous organic semiconductor behavior directly applicable to display technology research and development.
Table of Contents
Series Editor’s Foreword xiii
Preface xv
Acknowledgments xvii
Part I General Conception 1
1 Introduction 3
1.1 Operating Mechanism of Organic Light-Emitting Diodes: Device Physics and Molecular Chemistry Pictures 3
1.2 High-Performance Multilayer OLEDs 6
1.3 Overview of Each Chapter 9
References 10
2 Amorphous Glass Organic Semiconductors Used in OLEDs 11
2.1 Three Categories of Organic Semiconductors 11
2.2 Inorganic Semiconductors and Amorphous Glass Organic Semiconductors 16
2.3 p-Doping and n-Doping 18
2.4 Large Currents Flowing Through Amorphous Glass Organic Semiconductors 21
References 25
Part II Physics of Carriers 27
3 Carrier Recombination as Space-Charge-Limited Current and Device Operation Characteristics 29
3.1 Langevin Recombination Model and Its Extensions 30
3.2 Interface-Recombination-Type Device Operation Model for Two-Layer Devices 31
3.3 Double Injection/Recombination Model in Single-Layer Devices 37
3.3.1 Voltage–Current Density Characteristics of Single-Layer Devices 39
3.3.2 Extension to Multilayer Devices 40
3.4 The Concept of Carrier Balance and Emission Efficiency 43
3.4.1 Carrier Balance in Single-Layer Devices 44
3.4.2 Toward Advanced Understanding of Carrier Balance Concept 46
3.4.3 Carrier Balance in Multilayer Devices 51
References 52
4 Carrier Transport in Amorphous Glass Organic Semiconductors 57
4.1 The Role of Carrier Mobility in OLED Performance 57
4.2 Mechanism of Carrier Hopping Transport in Amorphous Glass Organic Semiconductor Thin Films 58
4.2.1 Origins of Temperature and Electric Field Dependence of Carrier Mobility 58
4.2.2 Gill’s Empirical Formula for Carrier Mobility 59
4.2.3 Understanding Hopping Transport Process via Bässler Formalism 62
4.2.4 Molecular-Level Understanding Using Marcus Theory 64
4.2.5 Fusion of Molecular-Scale Picture and Macroscopic Physical Picture 66
4.2.6 Dispersive Carrier Transport and Influence of Traps 70
4.3 Methods for Measuring Carrier Mobility 72
4.3.1 Time-of-Flight Method 73
4.3.2 Dark-Injection Transient SCLC Method and Charge Extraction by Linearly Increasing Voltage Method 74
4.3.3 Impedance Spectroscopy Method 76
4.3.4 SCLC Method 77
4.4 Carrier Mobilities of Carrier Transport Materials for OLEDs 78
4.4.1 Reliability of Measured Mobilities: The Case of NPB 78
4.4.2 Mobilities of Typical Hole- and Electron-Transport Materials 80
4.4.3 What Is Bipolar Carrier Transport? 83
References 87
5 Carrier Injection from Electrodes in Amorphous Glass Organic Thin Films 95
5.1 Energy Levels of Amorphous Glass Organic Semiconductors 95
5.1.1 Semiconductor Physics-Based and Molecular Orbital-Based Depiction 95
5.1.2 Ionization Energy and Electron Affinity of Amorphous Glass Organic Semiconductors 98
5.1.3 Relationship Between Driving Voltage and Energy Levels in OLEDs 101
5.1.4 Energy Levels for Electron and Hole Transport 103
5.2 Energy Levels at Metal/Organic Semiconductor and Organic Semiconductor/Organic Semiconductor Interfaces 107
5.2.1 Metal/Semiconductor Contact: Depiction Using Band Structure 108
5.2.2 Metal/Amorphous Glass Organic Semiconductor Contacts 109
5.2.3 Contacts Between Different Amorphous Glass Organic Semiconductors 114
5.3 Mechanisms of Carrier Injection 115
5.3.1 Tunnel Injection Model and Thermionic Emission Model 115
5.3.2 Carrier Injection from Metal Electrodes to Localized Levels of Molecules 116
5.3.3 Carrier Injection Limited Current and Bulk Limited Current 118
5.4 Ohmic Carrier Injection from Electrodes to Amorphous Glass Organic Semiconductors 119
5.4.1 Mechanisms of Ohmic Carrier Injection 119
5.4.2 Ohmic Carrier Injection Using Doped Carrier Transport Layers 121
5.4.3 Ohmic Carrier Injection Using Interfacial Electric Dipole Barrier Layers 123
5.4.4 Effects of Inserting an Insulating Layer at the Interface 126
References 126
Part III Physics of Excitons 135
6 From Exciton Generation to Emission 137
6.1 Generation of Excitons by Carrier Recombination 137
6.2 Singlet and Triplet Excitons 139
6.3 Room-Temperature Phosphorescence 142
6.4 Utilization of TTA 143
6.4.1 Upper Limit of Singlet Exciton Generation Yield 144
6.4.1.1 Spin Statistics Theory: Upper Limit of TTA Yield 10% 144
6.4.1.2 Spin Statistics Theory Without Quintet States: Upper Limit of TTA Yield 20% 144
6.4.1.3 Excited-State Level Dominant Theory: Upper Limit of TTA Yield 50% 145
6.4.2 External Quantum Efficiency of OLEDs Using TTA 146
6.4.3 Upconversion-Type High-Efficiency OLEDs 147
6.5 Utilization of TADF 152
6.5.1 Analysis of TADF Process 153
6.5.2 Factors Governing RISC 157
References 161
7 Diffusion, Transfer, and Annihilation of Excitons 167
7.1 Elementary Processes of Intermolecular Energy Transfer 167
7.1.1 Förster-Type Resonant Energy Transfer 168
7.1.2 Dexter-Type Electron Exchange Energy Transfer 170
7.2 Exciton Diffusion 171
7.2.1 Diffusion Length of Singlet Excitons 172
7.2.2 Diffusion Length of Triplet Excitons 173
7.3 Exciton Transfer 175
7.4 Nonradiative Decay Processes of Excitons 177
7.4.1 Nonradiative Thermal Deactivation and Deactivation by Impurities 177
7.4.2 Annihilation Through Collisions of Excitons 178
7.4.3 Deactivation of Excitons by Collision with Carriers 181
7.5 Kinetics from Exciton Generation to Annihilation 183
References 184
Part IV Physics of Advanced OLEDs 189
8 Utilization of Exciplexes 191
8.1 From Discovery of Exciplex to Its Utilization in High-Performance OLEDs 192
8.2 CT Complexes Composed of Donor and Acceptor Molecules 194
8.3 Mechanism of Exciplex Formation 195
8.4 OLEDs Using Exciplexes 202
8.5 Outlook 203
References 205
9 Tandem Organic Light-Emitting Diodes and the Concept of Carrier-Pair Generation 209
9.1 Evolution of Tandem OLEDs 209
9.2 Various Types of Intermediate Connecting Layers Used in Tandem OLEDs 212
9.3 Mechanisms of Carrier-Pair Generation in the Intermediate Connecting Layer 214
9.4 Outlook 221
References 224
10 Molecular Orientation in Amorphous Glass Organic Thin Films 227
10.1 How Was the Usefulness of the Molecular Orientation Effect Discovered? 228
10.1.1 Single Crystal and Polymer Thin Films 228
10.1.2 Organic Amorphous Glass Thin Films 229
10.2 Analytical Evaluation of Molecular Orientation in Amorphous Glass Organic Thin Films 231
10.2.1 Orientation Distribution Function in a Uniaxially Oriented System 232
10.2.2 Method for Evaluating Orientation Order Parameter 233
10.3 Generation Mechanism of Molecular Orientation in ag-OS 237
10.4 SOP of PEDs in Amorphous Glass Organic Thin Films 240
10.4.1 Discovery of SOP in Vacuum-Deposited Thin Films 241
10.4.2 SOP Expressed by Orientation Distribution Function 242
10.4.3 SOP in OLED Materials 244
10.4.4 SOP and Device Characteristics 246
10.5 Outlook 247
References 247
11 Ultrastable Glass via Vacuum Deposition 255
11.1 What Is USG? 256
11.1.1 Consideration in Terms of Energy Landscape 256
11.1.2 Consideration in Terms of Temperature Dependence of Thermodynamic Quantities 258
11.1.3 Consideration in Terms of Local Molecular Motions 262
11.2 Formation of USG via Vacuum Deposition 263
11.2.1 Indicators of USG Formation 263
11.2.2 Relationship Between USG Formation and Molecular Orientation 265
11.3 Enhancing Device Performance by Using USG 265
11.3.1 Improvement in Thermal and Mechanical Properties 266
11.3.2 Suppression of Impurity Diffusion and Chemical Reactions 267
11.3.3 Improvements in Electronic Properties and Device Performance 267
11.4 Outlook 268
References 269
Part V Reliability Issue of OLEDs 273
12 Degradation Mechanisms and Operational Lifetime 275
12.1 What Is Driving-Induced Degradation of OLEDs? 275
12.1.1 Extrinsic Factors and Intrinsic Factors 275
12.1.2 Initial Degradation and Long-Term Degradation 278
12.2 Description of Luminance Decay Curves Using a Simple Degradation Model 280
12.2.1 Nonemissive Recombination Site Generation Model 280
12.2.2 Exciton-Quenching Site Generation Model 282
12.3 Phenomenological Analytical Formulation for Describing Luminance Decay Curves 284
12.3.1 Exponential Decay Curves 285
12.3.2 Stretched Exponential Decay Curves 286
12.3.3 Becquerel-Type Decay Curves 288
12.4 Molecular-Level Considerations of Device Degradation 290
12.4.1 Elementary Processes of Degradation Reactions 291
12.4.2 Bond Strength and Degradation Reactions 293
12.4.3 Challenges for Achieving Long Lifetimes in Blue-Emitting OLEDs 294
References 296
Index 301
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