Organic semiconductors have found applications in many areas such as OLEDs, mobile phone displays, lighting, photovoltaics and much more.The understanding of the underlying photophysics as well as the evolution of device technology has come to a mature stage and as such a book is required that provides a useful introduction in a brief, coherent and comprehensive way, with a focus on the fundamentals.
Based on a successful and well-proven lecture course given by one of the authors for many years, this book is clearly structured into four chapters:
electronic structure of organic semiconductors,
charges and excited states inorganic semiconductors,
electronic and optical properties of organic semiconductors, and
fundamentals of organic semiconductor devices
Each chapter is complemented by boxes which explore a particular aspect in greater depth or brief introduce a feature that may be familiar to one group of readers yet not to another group.Where figures show original spectra, they are redrawn to be presented uniformly on an energy scale in electron-volt throughout this book for ease of comparison.

Over the past 40 years, the field of organic semiconductors has developed from an a thriving yet small community into a rather large research area. Both, the understanding of the underlying photophysics and device physics as well as the evolution of device technology have come to amature stage. Despite this progress in science and technology, PhD students in this research feld often experience a gap between the knowledge they acquired in the course of their undergraduate studies in physics, engi-neering, chemistry, or materialscience, and the knowledge that is needed to tackle a PhD in this subject area. Similarly, scientists entering the field of organic semiconductors with different back-grounds require reading material that familiarizes them with some of the underlying concepts before embarking on their own in-depth study.
Our book aims to bridge this gap by giving a careful introduction into the field, with a focus on the fundamentals. It is based on a lecture course that AK has given for a number of years at the University of Bayreuth to final year undergraduates and Master/PhD Students with a background in physics or chemistry. After having studied our book, the reader should be well-equipped to explore expert reviews and specialized book chapters that discuss a particular aspect of a material, a mechanism, or a device design in greater depth. While it is primarily intended for Master and PhD students requiring a basis for understanding organic semiconductors, we also delineate current routes in research with a view to stimulate scientific curiosity.
The book is structured in four chapters. In Chapter l, we introduce the reader to basic concepts of molecular photophysics. Thus, we discuss the different types of organic semiconductors, their elec-tronic states, radiative and nonradiative transitions between their states, and howto detect them by spectroscopic means. The way how interactions between chromophores affect charges and excited states is studied in Chapter 2. In particular, we highlight the differences between molecules in the gas phase and in the condensed phase, as well as the effect of structural order on the electronic proper-ties. Dimers, excimers, excitons, and the particular electronic structure of n-conjugated polymers are introduced. For the benefit of those with a background in inorganic semiconductors, we also briefly compare the physics of organic versus inorganic semiconductors. Chapter 3 delineates the processes that are relevant to optoelectronic devices. In particular, the sections on charge carrier transport and on exciton dissociation include recent developments. How these processes can be employed to the fabrication of organic semiconductor devices is presented in Chapter 4.
The text is complemented by boxes. They explore a particular aspect in greater depth or briefly introduce a feature that maybe familiar to one group of readers yet not to another group. Where figures show original spectra, they are redrawn to be presented uniformly on an energy scale in electron-volt throughout this book for ease of comparison. A list of chemical structures is given in the appendix for reference. While this textbook is meant to be self-contained, we have included many references to original work as well as a list of further reading at the end of each chapter. Our objective in including original references is to sensitize PhD students to the notion (i) that conceived knowledge is based on experiments and their interpretation, both of which are done by humans, (ii) that there maybe some discussion as to the most appropriate interpretation until a canonical view is formed in the course of time, and (iii) that it is a good idea for them to use their own judgment about the original literature to for man opinion, in particular in a field where there still is ongoing development. Our intention was to write an experimentally based, reasonably slim textbook for those new to the field rather than a comprehensive review on the state of the art. Thus, not every piece of work that maybe of importance to the field could be included here, and we ask our colleagues to bear with this
Finally, we take the freedom to highlight here two aspects of science that feel important to us. One is perhaps well expressed in a little anecdote about the German philosopher Georg Wilhelm Friedrich Hegel, who lived from 1770 to 1831. Reportedly, he submitted a thesis for habilitation, "De orbit is planet arum" claiming that on logical grounds there can not be more than seven planets. His defense was due on 27 August 1801, yet on 1 January 1801, an eighth planet, the Ceres, had been observed. When the issue was raised to Hegel that his model was in contradiction to the facts, he is said to have replied "too bad for the facts." We hope our readers will keep a healthy critical attitude to perceived scientific ideas and an openness to question their own views in the light of new results that may come up.
The second aspect relates to our opinion that an interdisciplinary field such as organic semicon-ductor requires researchers to take a wide variety of approaches, experimental, theoretical computa-tional, technological, and soon, and that there is no order of priority between them. Obtaining good understanding requires all of these approaches. This is, by the way, not a new phenomenon. When William Shockley gave his Nobel lecture in11December1956on "Transistor technology evokes new physics, " here marked."I would like to express some viewpoints about words often used to classify types of research in physics； for example, pure, applied, unrestricted, fundamental, basic, academic, industrial, and practical. It seems to me that all too frequently some of these words are used in a derogatory sense, on the one hand to be little the practical objectives of producing something use-ful and, on the other hand, to brush off the possible long-range value of explorations into new areas where a useful outcome can not before seen. Frequently, I have been asked if an experiment I have planned is pure or applied research； to me it is more important to know if the experiment will yield new and probably enduring knowledge about nature. If it is likely to yield such knowledge, it is, in my opinion, good fundamental research." We want to encourage in particular young researchers to be open to any kind of scientific approach that seems promising for their investigations.
This book would not exist without the support of many people. We are indebted to our colleagues in Bayreuth, in particular Stephan Kummel, Jurgen Kohler, and Mukundan Thelakkat, who generously took over administrative and grant-writing tasks thus giving AK the space and time to actually write the book, and to Peter Strohriegl and Markus Schwoerer for proof-reading and suggestions. We also thank Paul Blom, Frank Spano, Stavros Athanasopoulos, and Dieter Neher for proof-reading and suggestions. All members of AK's research group contributed in one way or another to this book. Al of the figures and the draft for the book cover were prepared by Katja Huber, who, together with Julian Kahle also edited the references. Chapters were proof-read mainly by Julian Kahle, Christian Schwarz, Sebastian Hoffmann, Tobias Hahn, and Markus Reichenberger. Steffen Tscheuschner, Fabian Panzer, Christina Scharsich, Alexander Rudnick, and Philipp Knauer provided occasional data, calculations, chemical structures, and technical assistance. We are extremely grateful to the entire group for their help in preparing this book. HB thanks the Bayreuth Institute of Macromolecular Science at the University of Bayreuth for providing office space and infrastructural support and he acknowledges his former research group in Marburg for more than 30 years of research that yielded many fruitful ideas and insight.
Bayreuth, March 2015
Anna Kohler and Heinz Bassler

Preface XI

Table of Boxes XIII

1 The Electronic Structure of Organic Semiconductors 1

1.1 Introduction 1

      1.1.1 What Are "Organic Semiconductors"? 1

      1.1.2 Historical Context 3

1.2 Different Organic Semiconductor Materials 5

      1.2.1 Molecular Crystals 5

      1.2.2 Amorphous Molecular Films 7

      1.2.3 Polymer Films 9

      1.2.4 Further Related Compounds 14

      1.2.5 A Comment on Synthetic Approaches 15

1.3 Electronic States of a Molecule 17

      1.3.1 Atomic Orbitals in Carbon 17

      1.3.2 From Atomic Orbitals to Molecular Orbitals 19

      1.3.3 From Orbitals to States 25

      1.3.4 Singlet and Triplet States 28

1.4 Transitions between Molecular States 31

      1.4.1 The Potential Energy Curve 31

      1.4.2 Radiative Transitions: Absorption and Emission 37

      1.4.2.1 The Electronic Factor 38

      1.4.2.2 The Vibrational Factor 41

      1.4.2.3 The Spin Factor 45

      1.4.3 A Classical Picture of Light Absorption 48

      1.4.3.1 The Lorentz Oscillator Model and the Complex Refractive Index 48

      1.4.3.2 Relating Experimental and Quantum Mechanical Quantities: The Einstein Coefficients, the Strickler-Berg Expression, and the Oscillator Strength 52

      1.4.4 Non-Radiative Transitions: Internal Conversion and Intersystem Crossing 56

      1.4.4.1 The Franck-Condon Factor F and the Energy Gap Law 57

      1.4.4.2 The Electronic Coupling J 58

      1.4.4.3 Accepting Modes, Promoting Modes, and the Isotope Rule 59

      1.4.4.4 Implications of the Energy Gap Law 60

      1.4.4.5 The Strong Coupling Limit 61

      1.4.5 Basic Photophysical Parameters: Lifetimes and Quantum Yields 62

1.5 Spectroscopic Methods 64

      1.5.1 Photoluminescence Spectra, Lifetimes, and Quantum Yields 67

      1.5.1.1 Steady State Spectra and Quantum Yields 68

      1.5.1.2 Spectra and Lifetimes in the Nanosecond to Second Range 72

      1.5.1.3 Spectra and Lifetimes in the Picosecond to Nanosecond Range 73

      1.5.1.4 Spectra and Time Scales below the Picosecond Range 74

      1.5.2 Excited State Absorption Spectra 75

      1.5.2.1 Steady State Spectra (Photoinduced Absorption) 75

      1.5.2.2 Spectra in the Nanosecond Range (Flash Photolysis) 77

      1.5.2.3 Spectra in the Femtosecond Range (fs Pump-Probe Measurements) 78

      1.5.3 Fluorescence Excitation Spectroscopy 79

1.6 Further Reading 80

References 81

2 Charges and Excited States in Organic Semiconductors 87

2.1 Excited Molecules from the Gas Phase to the Amorphous Film 87

      2.1.1 Effects due to Polarization 87

      2.1.2 Effects due to Statistical Averaging 91

      2.1.3 Effects due to Environmental Dynamics 94

      2.1.4 Effects due to Electronic Coupling between Identical Molecules – Dimers and Excimers 99

      2.1.4.1 Electronic Interaction in the Ground State 99

      2.1.4.2 Electronic Interaction in the Excited State 99

      2.1.4.3 Oscillator Strength of Dimer and Excimer Transitions 105

      2.1.4.4 Singlet and Triplet Dimers/Excimers 107

      2.1.5 Effects due to Electronic Coupling between Dissimilar Molecules – Complexes and Exciplexes 111

      2.1.6 Electromers and Electroplexes 113

2.2 Excited Molecules in Crystalline Phases – The Frenkel Exciton 114

      2.2.1 The Frenkel Exciton Concept for One Molecule per Unit Cell 114

      2.2.2 The Frenkel Exciton Concept for Two Molecules per Unit Cell 117

      2.2.3 Coherent and Incoherent Motion of Frenkel Excitons 118

      2.2.4 Förster and Dexter Type Energy Transfer 119

      2.2.5 Experimental Examples for Frenkel Excitons in Ordered Molecular Arrays 123

      2.2.5.1 Molecular Crystals: Anthracene and Tetracene 123

      2.2.5.2 Cyclic Arrays of Chromophores: Light-Harvesting Proteins 124

      2.2.5.3 Molecular J and H Aggregates: Cyanine Dyes and Carotenes 126

      2.2.5.4 Weakly Interacting H and J Aggregates with Vibronic Coupling 127

2.3 Excited States in π-Conjugated Polymers 133

      2.3.1 Crystalline Polymers: Poly (diacetylene)s (PDAs) 133

      2.3.2 Concepts for Noncrystalline Polymers 136

      2.3.2.1 The Basic Idea 136

      2.3.2.2 Quantitative Approaches: Exciton Models 141

      2.3.2.3 Comparison Against Experimental Data 143

      2.3.3 Brief Overview Over Different Classes of Conjugated Polymers 144

      2.3.3.1 Poly (ene)s/Poly (acetylene)s 144

      2.3.3.2 Poly (p-phenylene vinylenes) 147

      2.3.3.3 Poly (p-phenylene)s 150

      2.3.3.4 Poly (thiophene)s 152

      2.3.3.5 Poly (silane)s/Poly (silylene)s 153

      2.3.3.6 Low-Gap Donor-Acceptor Polymers 154

2.4 Charged Molecules 155

      2.4.1 The Creation of Charged Molecules by Injection, Absorption and Doping 157

      2.4.1.1 By Injection 157

      2.4.2 Charged Molecules in Disordered Films 161

      2.4.3 Charged Molecules in Crystals 164

      2.4.4 Determining the Energy Levels of Charged Molecules by Cyclovoltammetry and Photoemission Spectroscopy 167

2.5 A Comparison between Inorganic and Organic Semiconductors 171

      2.5.1 Crystals 171

      2.5.2 Amorphous Solids 174

      2.5.3 The Su–Schrieffer–Heeger (SSH) Model for Conjugated Polymers 175

2.6 Further Reading 181

References 182

3 Electronic and Optical Processes of Organic Semiconductors 193

3.1 Basic Aspects of Electrical Current in a Device 194

      3.1.1 Injection Limited Currents 195

      3.1.2 Unipolar Space Charge Limited (SCL) Current 196

      3.1.3 Bipolar Space Charge Limited Current 200

3.2 Charge Injection Mechanisms 201

      3.2.1 Fowler–Nordheim Tunneling Injection 202

      3.2.2 Richardson–SchottkyThermionic Injection 203

      3.2.3 Thermally Activated Injection into a Disordered Organic Semiconductor 204

3.3 Charge Carrier Transport 208

      3.3.1 Experimental Techniques to Measure Charge Carrier Mobility 208

      3.3.2 Carrier Transport in the Band Regime and in the Hopping Regime 213

      3.3.2.1 Band Transport 215

      3.3.2.2 Hopping Transport 217

      3.3.2.3 Polaronic Transport 217

      3.3.2.4 Disorder-Controlled Transport 223

      3.3.2.5 Superposition of Polaron and Disorder Effects 233

      3.3.3 Trapping Effects 235

      3.3.4 Transport at Higher Charge Carrier Densities 237

      3.3.5 The Impact of Morphology on Transport 239

      3.3.5.1 The Influence of Excimers and Traps 239

      3.3.5.2 The Role of Aggregates and Crystallites 240

      3.3.5.3 Self-Ordering in Discotic Liquid Crystals 241

      3.3.5.4 Polycrystalline Films 243

      3.3.6 Charge Transport on Short Lengths Scales and Time Scales 244

3.4 Non-Geminate Charge Carrier Recombination 246

      3.4.1 Recombination without Traps (Langevin-Type Recombination) 246

      3.4.2 Recombination with Traps (Shockley–Read–Hall-Like Recombination) 247

3.5 Generation of Excitations 249

      3.5.1 Optical Generation 249

      3.5.2 Electrical Generation 251

      3.5.3 Secondary Processes 252

3.6 Dissociation of Excitations 254

      3.6.1 Geminate Pair Creation 254

      3.6.1.1 The Timescale of Charge Transfer 254

      3.6.1.2 Properties of Geminate Pairs in Single-Compound Materials 256

      3.6.1.3 Geminate Pairs in Material satan Interface or Containing Traps 259

      3.6.1.4 Geminate Pairs in Donor-Acceptor Systems 260

      3.6.2 The Dissociation of the Geminate Pair 263

      3.6.2.1 The Onsager (1938) Model 263

      3.6.2.2 The Onsager-Braun Model 265

      3.6.2.3 Hong and No ol and is Time-Dependent Formalism 266

      3.6.2.4 Pump-Push-Probe Experiments to Monitor the Geminate Pair Population 267

      3.6.2.5 Contemporary Models Considering the Effects of Conjugation Lengths 269

      3.6.2.6 The Influence of Disorder on Geminate Pair Dissociation 272

3.7 Diffusion of Excitations 274

      3.7.1 Exciton Diffusion in a Molecular Crystal 274

      3.7.2 Diffusion of Excitations in Amorphous Condensed Phases 276

      3.7.3 Experimental Techniques to Measure Exciton Diffusion 276

      3.7.3.1 Measuring Diffusion by Luminescence Quenching 276

      3.7.3.2 Monitoring Spectral Diffusion 279

3.8 Decay of Excitations 283

      3.8.1 Monomolecular Decay 283

      3.8.1.1 Fluorescence 283

      3.8.1.2 Phosphorescence 286

      3.8.2 Bimolecular Processes 287

      3.8.2.1 Singlet-Singlet-Annihilation 288

      3.8.2.2 Triplet-Triplet-Annihilation 288

      3.8.2.3 Triplet-Charge-Annihilation 290

      3.8.2.4 Singlet-Triplet-Annihilation 291

3.9 Further Reading 292

References 292

4 Fundamentals of Organic Semiconductor Devices 307

4.1 Basic Solar Cells and Light-Emitting Diode Structures 311

      4.1.1 Basic Fabrication Steps 311

      4.1.2 Electrode Geometries 315

      4.1.3 The Basic Operation of a Single-Layer OLED 317

      4.1.4 Multi-Layer OLED Architectures 322

      4.1.5 The Current–Voltage–Luminance Characteristics of an OLED 324

      4.1.6 The Basic Operation of an OSC 326

      4.1.7 The Current–Voltage Characteristics of an OSC 327

4.2 Solar Cell Performance 331

      4.2.1 Determining Solar Cell Efficiencies 331

      4.2.2 Strategies to Increase the Photocurrent 334

      4.2.2.1 The Bilayer Device 337

      4.2.2.2 The Bulk Heterojunction Device 339

      4.2.2.3 The Multilayer Device 345

      4.2.3 Strategies to Increasing the Open-Circuit Voltage 345

      4.2.4 Strategies to Improve the Fill-Factor 347

      4.2.5 The Thermodynamic Efficiency Limit 349

4.3 Light-Emitting Diode Performance 353

      4.3.1 Determining OLED Efficiencies and Color 353

      4.3.1.1 Photometric and Radiometric Units 353

      4.3.1.2 Defining Efi ciencies 355

      4.3.1.3 Color Coordinates 356

      4.3.1.4 The Color Rendering Index 360

      4.3.1.5 Lifetime Measurements 361

      4.3.2 Strategies to Improve the OLED Efficiencies 362

      4.3.3 Strategies to Improving the Emission Color of OLEDs 366

      4.3.3.1 Single Color OLEDs 366

      4.3.3.2 White Organic Light Emitting Diodes (WOLEDs) 366

4.4 Transistors 368

      4.4.1 The Operational Principle of an OFET 369

      4.4.2 Evaluating OFET Performance 373

      4.4.3 Improving OFET Performance 374

      4.4.3.1 Choosing Source and Drain Electrodes 374

      4.4.3.2 Choosing the Gate Insulator 375

      4.4.3.3 Improving Charge Transport 377

      4.4.4 Modifying the Polarity of OFETs 378

      4.4.4.1 n-Type Transistors 379

      4.4.4.2 Am bipolar Transistors 380

4.5 Further Reading 382

References 382

Appendices 389

Chemical Structures 389

A.1 Selected Polymers 390

      A.1.1 π-Conjugated Homopolymers 390

      A.1.2 π-Conjugated Copolymers 391

      A.1.3 Other Polymers of Interest 392

A.2 Selected π-Conjugated Low-MolecularWeight Compounds 393

A.3 Selected Phosphorescent Compounds 397

A.4 Non-Conjugated Low-MolecularWeight Compounds 397

Index 399

Heinz Bassler is a retired Professor at the Bayreuth Institute of Macromolecular Research (BIMF) at the University of Bayreuth. From 1970 to 2002 he worked as Professor in the Department of Physical Chemistry at the Philipps University in Marburg in Germany, having obtained his PhD degree in Physics from the Tech-nical Univerity in Munich, Germany, in 1963. His research interest concerns the optoelectronics of organic solids with paricular emphasis on charge transport and on the spectroscopy of conjugated polymers. He is widely recognized for his studies on the effects of disorder in organic semiconductors.

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