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.

• Part I General ConceptionChapter 1 Introduction1.1 Operating Mechanism of OLEDs: Device Physics Picture and Molecular Chemistry Picture1.2 High-Performance Multilayer OLEDs1.3 Overview of Each ChapterChapter 2 Amorphous Glass Organic Semiconductors Used in OLEDs2.1 Three Categories of Organic Semiconductors2.2 Inorganic Semiconductors and Amorphous Glass Organic Semiconductors2.3 p-Doping and n-Doping2.4 Large Currents Flowing Through Amorphous Glass Organic SemiconductorsPart II Physics of CarriersChapter 3 Carrier Recombination as Space-Charge-Limited Current and Device Operation Characteristics3.1 Langevin Recombination Model and Its Extensions3.2 Interface-Recombination-Type Device Operation Model for Two-Layer Devices3.3 Double Injection/Recombination Model in Single-Layer Devices3.3.1 Voltage–Current Density Characteristics of Single-Layer Devices3.3.2 Extension to Multilayer Devices3.4 The Concept of Carrier Balance and Emission Efficiency3.4.1 Carrier Balance in Single-Layer Devices3.4.2 Towards Advanced Understanding of Carrier Balance Concept3.4.3 Carrier Balance in Multilayer DevicesChapter 4 Carrier Transport in Amorphous Glass Organic Semiconductors  4.1 The Role of Carrier Mobility in OLED Performance4.2 Mechanism of Carrier Hopping Transport in Amorphous Glass Organic Semiconductor Thin Films4.2.1 Origins of Temperature and Electric Field Dependence of Carrier Mobility4.2.2 Gill’s Empirical Formula for Carrier Mobility4.2.3 Understanding Hopping Transport Process via Bässler Formalism4.2.4 Molecular-Level Understanding Using Marcus Theory4.2.5 Fusion of Molecular-Scale Picture and Macroscopic Physical Picture4.2.6 Dispersive Carrier Transport and Influence of Traps4.3 Methods for Measuring Carrier Mobility4.3.1 Time-Of-Flight Method4.3.2 Dark-Injection Method and Charge Extraction by Linearly Increasing Voltage Method4.3.3 Impedance Spectroscopy (IS) Method4.3.4 SCLC Method4.4 Carrier Mobilities of Carrier Transport Materials for OLEDs4.4.1 Reliability of Measured Mobilities: The Case of NPB4.4.2 Mobilities of Typical Hole- and Electron-Transport Materials4.4.3 What Is Bipolar Carrier Transport?Chapter 5 Carrier Injection from Electrodes in Amorphous Glass Organic Thin Films5.1 Energy Levels of Amorphous Glass Organic Semiconductors5.1.1 Semiconductor Physics-based and Molecular Orbital-based Depiction5.1.2 Ionization Energy and Electron Affinity of Amorphous Glass Organic Semiconductors5.1.3 Relationship Between Driving Voltage and Energy Levels in OLEDs5.1.4 Energy Levels for Electron and Hole Transport5.2 Energy Levels at Metal/Organic Semiconductor and Organic Semiconductor/Organic Semiconductor Interfaces5.2.1 Metal/Semiconductor Contact: Depiction Using Band Structure5.2.2 Metal/Amorphous Glass Organic Semiconductor Contacts5.2.3 Contacts Between Different Amorphous Glass Organic Semiconductors5.3 Mechanisms of Carrier Injection5.3.1 Tunnel Injection Model and Thermionic Emission Model5.3.2 Carrier Injection from Metal Electrodes to Localized Levels of Molecules5.3.3 Carrier Injection Limited Current and Bulk Limited Current5.4 Ohmic Carrier Injection from Electrodes to Amorphous Glass Organic Semiconductors5.4.1 Mechanisms of Ohmic Carrier Injection5.4.2 Ohmic Carrier Injection Using Doped Carrier Transport Layers5.4.3 Ohmic Carrier Injection Using Interfacial Electric Dipole Barrier Layers5.4.4 Effects of Inserting an Insulating Layer at the InterfacePart III Physics of ExcitonsChapter 6 From Exciton Generation to Emission6.1 Generation of Excitons by Carrier Recombination6.2 Singlet and Triplet Excitons6.3 Room-Temperature Phosphorescence6.4 Utilization of Triplet-Triplet Annihilation (TTA)6.4.1 Upper Limit of Singlet Exciton Generation Yield6.4.2 External Quantum Efficiency of OLEDs Utilizing TTA6.4.3 Upconversion-Type High-Efficiency OLEDs6.5 Utilization of Thermally Activated Delayed Fluorescence6.5.1 Analysis of TADF Process6.5.2 Factors Governing Reverse Intersystem CrossingChapter 7 Diffusion, Transfer, and Annihilation of Excitons7.1 Elementary Processes of Intermolecular Energy Transfer7.1.1 Förster-Type Resonant Energy Transfer (FRET)7.1.2 Dexter-Type Electron Exchange Energy Transfer7.2 Exciton Diffusion7.2.1 Diffusion Length of Singlet Excitons7.2.2 Diffusion Length of Triplet Excitons7.3 Exciton Transfer7.4 Nonradiative Decay Processes of Excitons7.4.1 Nonradiative Thermal Deactivation and Deactivation by Impurities7.4.2 Annihilation through Collisions of Excitons7.4.3 Deactivation of Excitons by Collision with Carriers7.5 Kinetics from Exciton Generation to AnnihilationPart IV Physics of Advanced OLEDsChapter 8 Utilization of Exciplexes8.1 From Discovery of Exciplex to Its Utilization in High-Performance OLEDs8.2 Charge-Transfer Complexes Composed of Donor and Acceptor Molecules8.3 Mechanism of Exciplex Formation8.4 OLEDs Utilizing Exciplexes8.5 OutlookChapter 9 Tandem OLEDs and the Concept of Carrier-Pair Generation9.1 Evolution of Tandem OLEDs9.2 Various Types of Intermediate Connecting Layers Used in Tandem OLEDs9.3 Mechanisms of Carrier-Pair Generation in the Intermediate Connecting Layer9.4 OutlookChapter 10 Molecular Orientation in Amorphous Glass Organic Thin Films10.1 How Was the Usefulness of the Molecular Orientation Effect Discovered?10.1.1 Single Crystal and Polymer Thin Films10.1.2 Organic Amorphous Glass Thin Films10.2 Analytical Evaluation of Molecular Orientation in Amorphous Glass Organic Semiconductor Thin Films10.2.1 Orientation Distribution Function in a Uniaxially Oriented System10.2.2 Methods for Evaluating Orientation Order Parameter10.3 Generation Mechanism of Molecular Orientation in Amorphous Glass Organic Semiconductors10.4 Spontaneous Orientation Polarization of Permanent Electric Dipoles in Amorphous Glass Organic Thin Films10.4.1 Discovery of Spontaneous Orientation Polarization in Vacuum-Deposited Thin Films10.4.2 Spontaneous Orientation Polarization Expressed by Orientation Distribution Function10.4.3 Spontaneous Orientation Polarization in OLED Materials10.4.4 Spontaneous Orientation Polarization and Device Characteristics10.5 OutlookChapter 11 Ultra-Stable Glass via Vacuum Deposition11.1 What Is Ultra-Stable Glass?11.1.1 Consideration in Terms of Energy Landscape11.1.2 Consideration in Terms of Temperature Dependence of Thermodynamic Quantities11.1.3 Consideration in Terms of Local Molecular Motions11.2 Formation of Ultra-Stable Glass via Vacuum Deposition11.2.1 Indicators of Ultra-Stable Glass Formation11.2.2 Relationship Between Ultra-Stable Glass Formation and Molecular Orientation11.3 Enhancing Device Performance by Using Ultra-Stable Glass11.3.1 Improvement in Thermal and Mechanical Properties11.3.2 Suppression of Impurity Diffusion and Chemical Reactions11.3.3 Improvements in Electronic Properties and Device Performance11.4 OutlookPart V Reliability Issue of OLEDsChapter 12 Degradation Mechanisms and Operational Lifetime12.1 What Is Driving-Induced Degradation of OLEDs?12.1.1 Extrinsic Factors and Intrinsic Factors12.1.2 Initial Degradation and Long-Term Degradation12.2 Description of Luminance Decay Curves Using a Simple Degradation Model12.2.1 Non-Emissive Recombination Site Generation Model12.2.2 Exciton-Quenching Site Generation Model12.3 Phenomenological Analytical Formulation for Describing Luminance Decay Curves12.3.1 Exponential Decay Curves12.3.2 Stretched Exponential Decay Curves12.3.3 Becquerel-Type Decay Curves12.4 Molecular-Level Considerations of Device Degradation12.4.1 Elementary Processes of Degradation Reactions12.4.2 Bond Strength and Degradation Reactions12.4.3 Challenges for Achieving Long Lifetimes in Blue-Emitting OLEDs