Physics and Technology of Organic Light-Emitting Diodes

TETSUO TSUTSUI, Doctor of Engineering, is Professor Emeritus at Kyushu University, Japan, with over 40 years of experience in OLED research. A veteran researcher who authored pioneering papers and advised industrial OLED development, he received the SID Jan Rajchman Prize in 2011 for his contributions to display technology. TAKESHI YASUDA, Doctor of Engineering, is Principal Researcher at the Research Center for Macromolecules and Biomaterials, National Institute for Materials Science (NIMS), Japan. His work centers on developing new organic semiconductors through the fabrication and evaluation of organic thin-film devices.

• Series Editor’s Foreword xiiiPreface xvAcknowledgments xviiPart I General Conception 11 Introduction 31.1 Operating Mechanism of Organic Light-Emitting Diodes: Device Physics and Molecular Chemistry Pictures 31.2 High-Performance Multilayer OLEDs 61.3 Overview of Each Chapter 9References 102 Amorphous Glass Organic Semiconductors Used in OLEDs 112.1 Three Categories of Organic Semiconductors 112.2 Inorganic Semiconductors and Amorphous Glass Organic Semiconductors 162.3 p-Doping and n-Doping 182.4 Large Currents Flowing Through Amorphous Glass Organic Semiconductors 21References 25Part II Physics of Carriers 273 Carrier Recombination as Space-Charge-Limited Current and Device Operation Characteristics 293.1 Langevin Recombination Model and Its Extensions 303.2 Interface-Recombination-Type Device Operation Model for Two-Layer Devices 313.3 Double Injection/Recombination Model in Single-Layer Devices 373.3.1 Voltage–Current Density Characteristics of Single-Layer Devices 393.3.2 Extension to Multilayer Devices 403.4 The Concept of Carrier Balance and Emission Efficiency 433.4.1 Carrier Balance in Single-Layer Devices 443.4.2 Toward Advanced Understanding of Carrier Balance Concept 463.4.3 Carrier Balance in Multilayer Devices 51References 524 Carrier Transport in Amorphous Glass Organic Semiconductors 574.1 The Role of Carrier Mobility in OLED Performance 574.2 Mechanism of Carrier Hopping Transport in Amorphous Glass Organic Semiconductor Thin Films 584.2.1 Origins of Temperature and Electric Field Dependence of Carrier Mobility 584.2.2 Gill’s Empirical Formula for Carrier Mobility 594.2.3 Understanding Hopping Transport Process via Bässler Formalism 624.2.4 Molecular-Level Understanding Using Marcus Theory 644.2.5 Fusion of Molecular-Scale Picture and Macroscopic Physical Picture 664.2.6 Dispersive Carrier Transport and Influence of Traps 704.3 Methods for Measuring Carrier Mobility 724.3.1 Time-of-Flight Method 734.3.2 Dark-Injection Transient SCLC Method and Charge Extraction by Linearly Increasing Voltage Method 744.3.3 Impedance Spectroscopy Method 764.3.4 SCLC Method 774.4 Carrier Mobilities of Carrier Transport Materials for OLEDs 784.4.1 Reliability of Measured Mobilities: The Case of NPB 784.4.2 Mobilities of Typical Hole- and Electron-Transport Materials 804.4.3 What Is Bipolar Carrier Transport? 83References 875 Carrier Injection from Electrodes in Amorphous Glass Organic Thin Films 955.1 Energy Levels of Amorphous Glass Organic Semiconductors 955.1.1 Semiconductor Physics-Based and Molecular Orbital-Based Depiction 955.1.2 Ionization Energy and Electron Affinity of Amorphous Glass Organic Semiconductors 985.1.3 Relationship Between Driving Voltage and Energy Levels in OLEDs 1015.1.4 Energy Levels for Electron and Hole Transport 1035.2 Energy Levels at Metal/Organic Semiconductor and Organic Semiconductor/Organic Semiconductor Interfaces 1075.2.1 Metal/Semiconductor Contact: Depiction Using Band Structure 1085.2.2 Metal/Amorphous Glass Organic Semiconductor Contacts 1095.2.3 Contacts Between Different Amorphous Glass Organic Semiconductors 1145.3 Mechanisms of Carrier Injection 1155.3.1 Tunnel Injection Model and Thermionic Emission Model 1155.3.2 Carrier Injection from Metal Electrodes to Localized Levels of Molecules 1165.3.3 Carrier Injection Limited Current and Bulk Limited Current 1185.4 Ohmic Carrier Injection from Electrodes to Amorphous Glass Organic Semiconductors 1195.4.1 Mechanisms of Ohmic Carrier Injection 1195.4.2 Ohmic Carrier Injection Using Doped Carrier Transport Layers 1215.4.3 Ohmic Carrier Injection Using Interfacial Electric Dipole Barrier Layers 1235.4.4 Effects of Inserting an Insulating Layer at the Interface 126References 126Part III Physics of Excitons 1356 From Exciton Generation to Emission 1376.1 Generation of Excitons by Carrier Recombination 1376.2 Singlet and Triplet Excitons 1396.3 Room-Temperature Phosphorescence 1426.4 Utilization of TTA 1436.4.1 Upper Limit of Singlet Exciton Generation Yield 1446.4.1.1 Spin Statistics Theory: Upper Limit of TTA Yield 10% 1446.4.1.2 Spin Statistics Theory Without Quintet States: Upper Limit of TTA Yield 20% 1446.4.1.3 Excited-State Level Dominant Theory: Upper Limit of TTA Yield 50% 1456.4.2 External Quantum Efficiency of OLEDs Using TTA 1466.4.3 Upconversion-Type High-Efficiency OLEDs 1476.5 Utilization of TADF 1526.5.1 Analysis of TADF Process 1536.5.2 Factors Governing RISC 157References 1617 Diffusion, Transfer, and Annihilation of Excitons 1677.1 Elementary Processes of Intermolecular Energy Transfer 1677.1.1 Förster-Type Resonant Energy Transfer 1687.1.2 Dexter-Type Electron Exchange Energy Transfer 1707.2 Exciton Diffusion 1717.2.1 Diffusion Length of Singlet Excitons 1727.2.2 Diffusion Length of Triplet Excitons 1737.3 Exciton Transfer 1757.4 Nonradiative Decay Processes of Excitons 1777.4.1 Nonradiative Thermal Deactivation and Deactivation by Impurities 1777.4.2 Annihilation Through Collisions of Excitons 1787.4.3 Deactivation of Excitons by Collision with Carriers 1817.5 Kinetics from Exciton Generation to Annihilation 183References 184Part IV Physics of Advanced OLEDs 1898 Utilization of Exciplexes 1918.1 From Discovery of Exciplex to Its Utilization in High-Performance OLEDs 1928.2 CT Complexes Composed of Donor and Acceptor Molecules 1948.3 Mechanism of Exciplex Formation 1958.4 OLEDs Using Exciplexes 2028.5 Outlook 203References 2059 Tandem Organic Light-Emitting Diodes and the Concept of Carrier-Pair Generation 2099.1 Evolution of Tandem OLEDs 2099.2 Various Types of Intermediate Connecting Layers Used in Tandem OLEDs 2129.3 Mechanisms of Carrier-Pair Generation in the Intermediate Connecting Layer 2149.4 Outlook 221References 22410 Molecular Orientation in Amorphous Glass Organic Thin Films 22710.1 How Was the Usefulness of the Molecular Orientation Effect Discovered? 22810.1.1 Single Crystal and Polymer Thin Films 22810.1.2 Organic Amorphous Glass Thin Films 22910.2 Analytical Evaluation of Molecular Orientation in Amorphous Glass Organic Thin Films 23110.2.1 Orientation Distribution Function in a Uniaxially Oriented System 23210.2.2 Method for Evaluating Orientation Order Parameter 23310.3 Generation Mechanism of Molecular Orientation in ag-OS 23710.4 SOP of PEDs in Amorphous Glass Organic Thin Films 24010.4.1 Discovery of SOP in Vacuum-Deposited Thin Films 24110.4.2 SOP Expressed by Orientation Distribution Function 24210.4.3 SOP in OLED Materials 24410.4.4 SOP and Device Characteristics 24610.5 Outlook 247References 24711 Ultrastable Glass via Vacuum Deposition 25511.1 What Is USG? 25611.1.1 Consideration in Terms of Energy Landscape 25611.1.2 Consideration in Terms of Temperature Dependence of Thermodynamic Quantities 25811.1.3 Consideration in Terms of Local Molecular Motions 26211.2 Formation of USG via Vacuum Deposition 26311.2.1 Indicators of USG Formation 26311.2.2 Relationship Between USG Formation and Molecular Orientation 26511.3 Enhancing Device Performance by Using USG 26511.3.1 Improvement in Thermal and Mechanical Properties 26611.3.2 Suppression of Impurity Diffusion and Chemical Reactions 26711.3.3 Improvements in Electronic Properties and Device Performance 26711.4 Outlook 268References 269Part V Reliability Issue of OLEDs 27312 Degradation Mechanisms and Operational Lifetime 27512.1 What Is Driving-Induced Degradation of OLEDs? 27512.1.1 Extrinsic Factors and Intrinsic Factors 27512.1.2 Initial Degradation and Long-Term Degradation 27812.2 Description of Luminance Decay Curves Using a Simple Degradation Model 28012.2.1 Nonemissive Recombination Site Generation Model 28012.2.2 Exciton-Quenching Site Generation Model 28212.3 Phenomenological Analytical Formulation for Describing Luminance Decay Curves 28412.3.1 Exponential Decay Curves 28512.3.2 Stretched Exponential Decay Curves 28612.3.3 Becquerel-Type Decay Curves 28812.4 Molecular-Level Considerations of Device Degradation 29012.4.1 Elementary Processes of Degradation Reactions 29112.4.2 Bond Strength and Degradation Reactions 29312.4.3 Challenges for Achieving Long Lifetimes in Blue-Emitting OLEDs 294References 296Index 301