Electronic Processes in Organic Semiconductors

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 1.4.3 A Classical Picture of Light Absorption 48 1.4.4 Non-Radiative Transitions: Internal Conversion and Intersystem Crossing 56 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 1.5.2 Excited State Absorption Spectra 75 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 2.1.5 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 2.3 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 2.3.3 Brief Overview Over Different Classes of Conjugated Polymers 144 2.4 Charged Molecules 155 2.4.1 The Creation of Charged Molecules by Injection, Absorption and Doping 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 3.3.3 Trapping Effects 235 3.3.4 Transport at Higher Charge Carrier Densities 237 3.3.5 The Impact of Morphology on Transport 239 3.3.6 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 3.6.2 The Dissociation of the Geminate Pair 263 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 3.8 Decay of Excitations 283 3.8.1 Monomolecular Decay 283 3.8.2 Bimolecular Processes 287 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 4.2.3 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 4.3.2 Strategies to Improve the OLED Efficiencies 362 4.3.3 Strategies to Improving the Emission Color of OLEDs 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 4.4.4 Modifying the Polarity of OFETs 378 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