Defects play a key role in the physical properties of semiconductors and devices, and their identification is essential in assessing the reliability of electronic devices.
Defects in Organic Semiconductors and Devices introduces the fundamental aspects of defects in organic semiconductors and devices in relation to the structure of materials and architecture of electronic components. It covers the topics of defect formation and evolution, defect measurement techniques and their adaption to organic devices, the effects of defects on the physical properties of materials and their effects on the performance and lifetime of organic devices. Identifying defects and determining their characteristics in the structure of organic devices such as OLEDs, OFETs and OPVs make it possible to better understand degradation processes and develop solutions to improve the reliability of such devices.
This book is intended for researchers and students in university programs or engineering schools who are specializing in electronics, energy and materials.
Table of Contents
Abbreviations ix
Introduction xvii
Chapter 1. Overview of Organic Semiconductors 1
1.1. Organic semiconductors 2
1.2. Doping of organic semiconductors 4
1.3. Organic electronic devices 6
1.3.1. Architectures of organic devices 6
1.3.2. Organic light-emitting diodes (OLEDs) 11
1.3.3. Organic solar cells (OSCs or OPVs) 13
1.3.4. Organic field-effect transistors (OFETs) 15
Chapter 2. Defects in Materials 19
2.1. Order and disorder 19
2.2. Crystalline semiconductors 20
2.2.1. Localized states 21
2.2.2. Density of states (DOS) 22
2.3. Amorphous semiconductors 22
2.3.1. Localized states 23
2.3.2. Density of states (DOS) 24
2.4. Organic semiconductors 25
2.4.1. Polymer structure 26
2.4.2. Polymer crystallinity 27
2.4.3. Defects in conjugated polymers 28
2.4.4. Defects in small-molecule crystals 29
2.4.5. Localized states 30
2.4.6. Density of states 31
2.5. Distribution of the energetic states 32
Chapter 3. Defects and Physical Properties of Semiconductors 35
3.1. Carrier transport in organic semiconductors 35
3.1.1. Hopping conduction 37
3.1.2. Uniform density of states model 38
3.1.3. Non-uniform density of states models 39
3.2. Effects of defects on the carrier transport 44
3.2.1. Traps and recombination centers 44
3.2.2. Trapping mechanisms and trap parameters 46
3.3. Optical properties of semiconductors and defects 56
3.3.1. Defects and absorption 56
3.3.2. Defects and luminescence 59
Chapter 4. Techniques for Studying Defects in Semiconductors 65
4.1. Electron spin resonance (ESR) 65
4.1.1. Basic concepts of ESR 66
4.1.2. Interpretation of ESR line 67
4.1.3. Electron nuclear double resonance (ENDOR) 68
4.1.4. Investigation of defects using the ESR technique 69
4.2. Optical techniques 72
4.2.1. Fluorescence spectroscopy (FL) 72
4.2.2. Thermally stimulated luminescence (TSL) spectroscopy 75
4.3. Electrical techniques 86
4.3.1. Thermally stimulated current (TSC) technique 87
4.3.2. Current-voltage measurements: space charge-limited current (SCLC) 96
4.3.3. Impedance spectroscopy (IS) 110
4.3.4. Deep-level transient spectroscopy (DLTS) 134
4.3.5. Time of flight (TOF) and charge carrier extraction by linearly increasing voltage (CELIV) techniques 152
Chapter 5. Defect Origins 159
5.1. Defects in organic semiconductors 159
5.1.1. Structural defects 160
5.1.2. Impurity defects 160
5.2. Defects in organic devices 162
5.2.1. Defects from the semiconductor 163
5.2.2. Defects from the surface and the interface 164
5.2.3. Defects from diffused impurities 168
Chapter 6. Defects, Performance and Reliability of Organic Devices 173
6.1. Impact of defects on the performance of organic devices 174
6.1.1. Defects and efficiency of OLEDs 174
6.1.2. Defects and efficiency of OPVs 176
6.1.3. Defects and performance of OFETs 181
6.2. Impact of defects on the stability of organic devices 183
6.2.1. Overview of degradation mechanisms in organic semiconductors and devices 184
6.2.2. Defects and degradation of organic semiconductor and devices 188
Future Prospects 217
References 223
Index 249
