Electronic Processes in Organic Semiconductors

Anna Koehler has been Professor and Chair of Experimental Physics II at the University of Bayreuth since 2007. After completing her PhD 1996 with Sir Richard Friend at the University of Cambridge, UK, she held Research Fellowships by Peterhouse, Cambridge, and by the Royal Society, UK. She was appointed Professor at the University of Potsdam, Germany, in 2003. Her research centres on the photophysical properties of organic semiconductors, with a focus on energy and charge transfer processes in singlet and triplet excited states.Heinz Baessler is 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 Technical University in Munich, Germany, in 1963. His research interest concerns the optoelectronics of organic solids with particular 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.

• Preface XITable of Boxes XIII1 The Electronic Structure of Organic Semiconductors 11.1 Introduction 11.1.1 What Are “Organic Semiconductors”? 11.1.2 Historical Context 31.2 Different Organic Semiconductor Materials 51.2.1 Molecular Crystals 51.2.2 Amorphous Molecular Films 71.2.3 Polymer Films 91.2.4 Further Related Compounds 141.2.5 A Comment on Synthetic Approaches 151.3 Electronic States of a Molecule 171.3.1 Atomic Orbitals in Carbon 171.3.2 From Atomic Orbitals to Molecular Orbitals 191.3.3 From Orbitals to States 251.3.4 Singlet and Triplet States 281.4 Transitions between Molecular States 311.4.1 The Potential Energy Curve 311.4.2 Radiative Transitions: Absorption and Emission 371.4.3 A Classical Picture of Light Absorption 481.4.4 Non-Radiative Transitions: Internal Conversion and Intersystem Crossing 561.4.5 Basic Photophysical Parameters: Lifetimes and Quantum Yields 621.5 Spectroscopic Methods 641.5.1 Photoluminescence Spectra, Lifetimes, and Quantum Yields 671.5.2 Excited State Absorption Spectra 751.5.3 Fluorescence Excitation Spectroscopy 791.6 Further Reading 80References 812 Charges and Excited States in Organic Semiconductors 872.1 Excited Molecules from the Gas Phase to the Amorphous Film 872.1.1 Effects due to Polarization 872.1.2 Effects due to Statistical Averaging 912.1.3 Effects due to Environmental Dynamics 942.1.4 Effects due to Electronic Coupling between Identical Molecules – Dimers and Excimers 992.1.5 Effects due to Electronic Coupling between Dissimilar Molecules – Complexes and Exciplexes 1112.1.6 Electromers and Electroplexes 1132.2 Excited Molecules in Crystalline Phases – The Frenkel Exciton 1142.2.1 The Frenkel Exciton Concept for One Molecule per Unit Cell 1142.2.2 The Frenkel Exciton Concept for Two Molecules per Unit Cell 1172.2.3 Coherent and Incoherent Motion of Frenkel Excitons 1182.2.4 Förster and Dexter Type Energy Transfer 1192.2.5 Experimental Examples for Frenkel Excitons in Ordered Molecular Arrays 1232.3 Excited States in π-Conjugated Polymers 1332.3.1 Crystalline Polymers: Poly(diacetylene)s (PDAs) 1332.3.2 Concepts for Noncrystalline Polymers 1362.3.3 Brief Overview Over Different Classes of Conjugated Polymers 1442.4 Charged Molecules 1552.4.1 The Creation of Charged Molecules by Injection, Absorption and Doping 1572.4.2 Charged Molecules in Disordered Films 1612.4.3 Charged Molecules in Crystals 1642.4.4 Determining the Energy Levels of Charged Molecules by Cyclovoltammetry and Photoemission Spectroscopy 1672.5 A Comparison between Inorganic and Organic Semiconductors 1712.5.1 Crystals 1712.5.2 Amorphous Solids 1742.5.3 The Su–Schrieffer–Heeger (SSH) Model for Conjugated Polymers 1752.6 Further Reading 181References 1823 Electronic and Optical Processes of Organic Semiconductors 1933.1 Basic Aspects of Electrical Current in a Device 1943.1.1 Injection Limited Currents 1953.1.2 Unipolar Space Charge Limited (SCL) Current 1963.1.3 Bipolar Space Charge Limited Current 2003.2 Charge Injection Mechanisms 2013.2.1 Fowler–Nordheim Tunneling Injection 2023.2.2 Richardson–SchottkyThermionic Injection 2033.2.3 Thermally Activated Injection into a Disordered Organic Semiconductor 2043.3 Charge Carrier Transport 2083.3.1 Experimental Techniques to Measure Charge Carrier Mobility 2083.3.2 Carrier Transport in the Band Regime and in the Hopping Regime 2133.3.3 Trapping Effects 2353.3.4 Transport at Higher Charge Carrier Densities 2373.3.5 The Impact of Morphology on Transport 2393.3.6 Charge Transport on Short Lengths Scales and Time Scales 2443.4 Non-Geminate Charge Carrier Recombination 2463.4.1 Recombination without Traps (Langevin-Type Recombination) 2463.4.2 Recombination with Traps (Shockley–Read–Hall-Like Recombination) 2473.5 Generation of Excitations 2493.5.1 Optical Generation 2493.5.2 Electrical Generation 2513.5.3 Secondary Processes 2523.6 Dissociation of Excitations 2543.6.1 Geminate Pair Creation 2543.6.2 The Dissociation of the Geminate Pair 2633.7 Diffusion of Excitations 2743.7.1 Exciton Diffusion in a Molecular Crystal 2743.7.2 Diffusion of Excitations in Amorphous Condensed Phases 2763.7.3 Experimental Techniques to Measure Exciton Diffusion 2763.8 Decay of Excitations 2833.8.1 Monomolecular Decay 2833.8.2 Bimolecular Processes 2873.9 Further Reading 292References 2924 Fundamentals of Organic Semiconductor Devices 3074.1 Basic Solar Cells and Light-Emitting Diode Structures 3114.1.1 Basic Fabrication Steps 3114.1.2 Electrode Geometries 3154.1.3 The Basic Operation of a Single-Layer OLED 3174.1.4 Multi-Layer OLED Architectures 3224.1.5 The Current–Voltage–Luminance Characteristics of an OLED 3244.1.6 The Basic Operation of an OSC 3264.1.7 The Current–Voltage Characteristics of an OSC 3274.2 Solar Cell Performance 3314.2.1 Determining Solar Cell Efficiencies 3314.2.2 Strategies to Increase the Photocurrent 3344.2.3 Strategies to Increasing the Open-Circuit Voltage 3454.2.4 Strategies to Improve the Fill-Factor 3474.2.5 The Thermodynamic Efficiency Limit 3494.3 Light-Emitting Diode Performance 3534.3.1 Determining OLED Efficiencies and Color 3534.3.2 Strategies to Improve the OLED Efficiencies 3624.3.3 Strategies to Improving the Emission Color of OLEDs 3664.4 Transistors 3684.4.1 The Operational Principle of an OFET 3694.4.2 Evaluating OFET Performance 3734.4.3 Improving OFET Performance 3744.4.4 Modifying the Polarity of OFETs 3784.5 Further Reading 382References 382Appendices 389Chemical Structures 389A.1 Selected Polymers 390A.1.1 π-Conjugated Homopolymers 390A.1.2 π-Conjugated Copolymers 391A.1.3 Other Polymers of Interest 392A.2 Selected π-Conjugated Low-MolecularWeight Compounds 393A.3 Selected Phosphorescent Compounds 397A.4 Non-Conjugated Low-MolecularWeight Compounds 397Index 399