Physics of Organic Semiconductors

The field of organic electronics has seen a steady growth over the last 20 years. At the same time, our scientific understanding of how to achieve optimum device performance has grown, and this book gives an overview of our present–day knowledge of the physics behind organic semiconductor devices. Based on the very successful first edition, the editors have invited top scientists from the US, Asia, and Europe to include the developments from recent years, covering such fundamental issues as: growth and characterization of thin films of organic semiconductors charge transport and photophysical properties of the materials as well as their electronic structure at interfaces, and analysis and modeling of devices like organic light–emitting diodes, photovoltaic cells or field–effect transistors. The result is an overview of the field for both readers with basic knowledge and for an application–oriented audience. It thus bridges the gap between textbook knowledge largely based on crystalline molecular solids and those books focusing more on device applications.

Foreword V Preface VII List of Contributors XIX Part One Film Growth, Electronic Structure, and Interfaces 1 1 Organic Molecular Beam Deposition 3 Frank Schreiber 1.1 Introduction 3 1.2 Organic Molecular Beam Deposition 5 1.2.1 General Concepts of Thin Film Growth 5 1.2.2 Issues Specific to Organic Thin Film Growth 6 1.2.3 Overview of Popular OMBD Systems 8 1.2.3.1 PTCDA 8 1.2.3.2 DIP 8 1.2.3.3 Phthalocyanines 9 1.2.3.4 Oligoacenes (Anthracene, Tetracene, and Pentacene) 10 1.3 Films on Oxidized Silicon 10 1.3.1 PTCDA 10 1.3.2 DIP 11 1.3.3 Phthalocyanines 13 1.3.4 Pentacene 14 1.4 Films on Aluminum Oxide 14 1.4.1 PTCDA 16 1.4.2 DIP 16 1.4.3 Phthalocyanines 16 1.4.4 Pentacene 17 1.5 Films on Metals 17 1.5.1 PTCDA 18 1.5.1.1 Structure and Epitaxy of PTCDA/Ag(111) 18 1.5.1.2 Comparison with Other Substrates 18 1.5.1.3 Dewetting and Thermal Properties 19 1.5.1.4 Real–Time Growth 19 1.5.2 DIP 21 1.5.3 Phthalocyanines 21 1.5.4 Pentacene 22 1.6 Films on Other Substrates 22 1.7 More Complex Heterostructures and Technical Interfaces 23 1.7.1 Inorganic–Organic Heterostructures 23 1.7.2 Organic–Organic Heterostructures 24 1.8 Summary and Conclusions 28 References 29 2 Electronic Structure of Interfaces with Conjugated Organic Materials 35 Norbert Koch 2.1 Introduction 35 2.2 Energy Levels of Organic Semiconductors 37 2.3 Interfaces between Organic Semiconductors and Electrodes 40 2.3.1 Atomically Clean Metal Electrodes 41 2.3.2 Application–Relevant Electrodes 45 2.3.2.1 Low Work Function Electrodes 47 2.3.2.2 Conducting Polymer Electrodes 49 2.3.2.3 Adjusting the Energy Level Alignment at Electrodes 51 2.4 Energy Levels at Organic Semiconductor Heterojunctions 54 2.4.1 Molecular Orientation Dependence 54 2.4.2 Interfacial Charge Transfer 56 2.4.3 Electrode–Induced Pinning of Energy Levels 56 2.4.4 Molecular Dipoles for Energy Level Tuning 57 2.5 Conclusions 59 References 59 3 Electronic Structure of Molecular Solids: Bridge to the Electrical Conduction 65 Nobuo Ueno 3.1 Introduction 65 3.2 General View of Electronic States of Organic Solids 66 3.2.1 From Single Molecule to Molecular Solid 66 3.2.2 Polaron and Charge Transport 69 3.2.3 Requirement from Thermodynamic Equilibrium 69 3.3 Electronic Structure in Relation to Charge Transport 70 3.3.1 Ultraviolet Photoemission Spectroscopy 70 3.3.2 Energy Band Dispersion and Band Transport Mobility 73 3.3.3 Density–of–States Effects in Polycrystalline Film 77 3.4 Electron–Phonon Coupling, Hopping Mobility, and Polaron Binding Energy 79 3.4.1 Basic Background 79 3.4.2 Experimental Reorganization Energy and Polaron Binding Energy 82 3.5 Summary 86 References 87 4 Interfacial Doping for Efficient Charge Injection in Organic Semiconductors 91 Jae–Hyun Lee and Jang–Joo Kim 4.1 Introduction 91 4.2 Insertion of an Interfacial Layer in the Organic/Electrode Junction 92 4.2.1 Electron Injection 92 4.2.2 Hole Injection 95 4.3 Doped Organic/Electrode Junctions 99 4.3.1 “Doping” in Organic Semiconductors 99 4.3.2 Dopants in Organic Semiconductors 100 4.3.3 Charge Generation Efficiencies of Dopants 101 4.3.4 Hole Injection through the p–Doped Organic Layer/Anode Junction 104 4.3.5 Electron Injection through the n–Doped Organic Layer/Cathode Junction 108 4.4 Doped Organic/Undoped Organic Junction 109 4.5 Applications 112 4.5.1 OLEDs 112 4.5.2 OPVs 112 4.5.3 OFETs 114 4.6 Conclusions 115 References 115 5 Displacement Current Measurement for Exploring Charge Carrier Dynamics in Organic Semiconductor Devices 119 Yutaka Noguchi, Yuya Tanaka, Yukimasa Miyazaki, Naoki Sato, Yasuo Nakayama, and Hisao Ishii 5.1 Introduction 119 5.2 Displacement Current Measurement 123 5.2.1 DCM for Quasi–Static States 124 5.2.1.1 Basic Concepts of DCM 124 5.2.1.2 Trapped Charges and Injection Voltage 125 5.2.1.3 Intermediate State between Depletion and Accumulation 127 5.2.2 DCM for Transient States 129 5.2.2.1 Sweep Rate Dependence in DCM Curves 130 5.3 Charge Accumulation at Organic Heterointerfaces 135 5.3.1 Elements of Charge Accumulation at Organic Heterointerfaces 135 5.3.2 Interface Charges and Orientation Polarization 137 5.3.3 Light–Induced Space Charges in Alq3 Film 144 5.4 Conclusions 147 References 149 Part Two Charge Transport 155 6 Effects of Gaussian Disorder on Charge–Carrier Transport and Recombination in Organic Semiconductors 157 Reinder Coehoorn and Peter A. Bobbert 6.1 Introduction 157 6.2 Mobility Models for Hopping in a Disordered Gaussian DOS 161 6.2.1 The Extended Gaussian Disorder Model 161 6.2.2 The Extended Correlated Disorder Model 165 6.2.3 Mobility in Host–Guest Systems 166 6.3 Modeling of the Recombination Rate 169 6.3.1 Recombination in Systems with a Gaussian DOS 169 6.3.2 Recombination in Host–Guest Systems with a Gaussian Host DOS 172 6.4 OLED Device Modeling 173 6.4.1 Single–Layer OLEDs: Analytical Drift–Only Theory 173 6.4.2 The Role of Charge–Carrier Diffusion 176 6.4.3 The Role of Gaussian Disorder: One–Dimensional Device Simulations 179 6.4.4 The Role of Gaussian Disorder: Three–Dimensional Device Simulations 182 6.5 Experimental Studies 186 6.5.1 Overview 186 6.5.2 PF–TAA–Based Polymer OLEDs 189 6.6 Conclusions and Outlook 194 References 196 7 Charge Transport Physics of High–Mobility Molecular Semiconductors 201 Henning Sirringhaus, Tomo Sakanoue, and Jui–Fen Chang 7.1 Introduction 201 7.2 Review of Recent High–Mobility Small–Molecule and Polymer Organic Semiconductors 202 7.3 General Discussion of Transport Physics/Transport Models of Organic Semiconductors 208 7.3.1 Static Disorder Parameters s and S 219 7.4 Transport Physics of High–Mobility Molecular Semiconductors 221 7.5 Conclusions 234 References 234 8 Ambipolar Charge–Carrier Transport in Molecular Field–Effect Transistors 239 Andreas Opitz and Wolfgang Br€utting 8.1 Introduction 239 8.2 Ambipolar Charge–Carrier Transport in Blends of Molecular Hole– and Electron–Conducting Materials 244 8.3 Ambipolar Charge–Carrier Transport in Molecular Semiconductors by Applying a Passivated Insulator Surface 246 8.4 Electrode Variation for Ambipolar and Bipolar Transport 252 8.5 Applications of Bipolar Transport for Ambipolar and Complementary Inverters 256 8.6 Realization of Light–Emitting Transistors with Combined Al and TTF–TCNQ Electrodes 260 8.7 Conclusion 261 References 262 9 Organic Magnetoresistance and Spin Diffusion in Organic Semiconductor Thin–Film Devices 267 Markus Wohlgenannt 9.1 Introduction 267 9.1.1 Organization of This Chapter 268 9.2 Organic Magnetoresistance 270 9.2.1 Dependence of Organic Magnetoresistance on Hyperfine Coupling Strength 271 9.2.2 Organic Magnetoresistance in a Material with Strong Spin–Orbit Coupling 272 9.2.3 Organic Magnetoresistance in Doped Devices 275 9.2.4 Conclusions for Organic Spintronics 277 9.3 Theory of Spin–Orbit Coupling in Singly Charged Polymer Chains 277 9.4 Theory of Spin Diffusion in Disordered Organic Semiconductors 280 9.5 Distinguishing between Tunneling and Injection Regimes of Ferromagnet/Organic Semiconductor/Ferromagnet Junctions 284 9.6 Conclusion 289 References 290 Part Three Photophysics 295 10 Excitons at Polymer Interfaces 297 Neil Greenham 10.1 Introduction 297 10.2 Fabrication and Structural Characterization of Polymer Heterojunctions 298 10.3 Electronic Structure at Polymer/Polymer Interfaces 305 10.4 Excitons at Homointerfaces 307 10.5 Type–I Heterojunctions 312 10.6 Type–II Heterojunctions 314 10.7 CT State Recombination 319 10.8 Charge Separation and Photovoltaic Devices based on Polymer: Polymer Blends 322 10.9 Future Directions 327 References 328 11 Electronic Processes at Organic Semiconductor Heterojunctions: The Mechanism of Exciton Dissociation in Semicrystalline Solid–State Microstructures 333 Francis Paquin, Gianluca Latini, Maciej Sakowicz, Paul–Ludovic Karsenti, Linjun Wang, David Beljonne, Natalie Stingelin, and Carlos Silva 11.1 Introduction 333 11.2 Experimental Methods 334 11.3 Results and Analysis 334 11.3.1 Photophysics of Charge Photogeneration and Recombination Probed by Time–Resolved PL Spectroscopy 334 11.3.1.1 Absorption and Steady–State PL 334 11.3.1.2 Time–Resolved PL Measurements 335 11.3.1.3 Quantum Chemical Calculations 341 11.3.2 Solid–State Microstructure Dependence 342 11.3.2.1 Polymer Microstructure 342 11.3.2.2 Dependence of Time–Resolved PL on Molecular Weight 344 11.4 Conclusions 345 References 345 12 Recent Progress in the Understanding of Exciton Dynamics within Phosphorescent OLEDs 349 Sebastian Reineke and Marc A. Baldo 12.1 Introduction 349 12.2 Exciton Formation 349 12.2.1 Background 349 12.2.2 Spin Mixing for Higher Efficiency 351 12.2.2.1 Exciton Mixing and Phosphorescence 351 12.2.2.2 CT State Mixing and Enhanced Fluorescence 352 12.2.2.3 Thermally Activated Delayed Fluorescence 355 12.2.2.4 Summary: Comparison between Phosphorescence, Extrafluorescence, and TADF 357 12.3 Distributing Excitons in the Organic Layer(s) 357 12.3.1.1 Excitonic Confinement: Host–Guest Systems 357 12.3.1.2 Exciton Generation Zone 358 12.3.1.3 Exciton Migration 359 12.3.1.4 Triplet Harvesting 361 12.4 High Brightness Effects in Phosphorescent Devices 362 References 367 13 Organometallic Emitters for OLEDs: Triplet Harvesting, Singlet Harvesting, Case Structures, and Trends 371 Hartmut Yersin, Andreas F. Rausch, and Rafa» Czerwieniec 13.1 Introduction 371 13.2 Electroluminescence 372 13.2.1 Triplet Harvesting 372 13.2.2 Singlet Harvesting 374 13.3 Triplet Emitters: Basic Understanding and Trends 375 13.3.1 Energy States 376 13.3.2 The Triplet State and Spin–Orbit Coupling 378 13.3.3 Emission Decay Time and Zero–Field Splitting: A General Trend 382 13.4 Case Studies: Blue Light Emitting Pt(II) and Ir(III) Compounds 386 13.4.1 Pt(II) Compounds 388 13.4.1.1 Photophysical Properties at Ambient Temperature 388 13.4.1.2 High–Resolution Spectroscopy: Triplet Substates and Vibrational Satellite Structures 391 13.4.2 Ir(III) Compounds 400 13.4.2.1 Photophysical Properties at Ambient Temperature 400 13.4.2.2 Electronic 0–0 Transitions and Energy Level Diagrams of the Emitting Triplet States 402 13.4.2.3 Vibrational Satellite Structures Exemplified on Ir(4,6–dFppy)2(acac) 404 13.4.2.4 Effects of the Nonchromophoric Ligands 405 13.4.3 Comparison of Photophysical Properties of Pt(II) and Ir(III) Compounds 407 13.5 Case Studies: Singlet Harvesting and Blue Light Emitting Cu(I) Complexes 408 13.5.1 Photophysical Properties at Ambient Temperature 408 13.5.2 Triplet State Emission and Thermally Activated Fluorescence 411 13.5.3 Singlet Harvesting: Cu(I) Complexes as OLED–Emitters 415 13.6 Conclusion 417 References 420 Part Four Device Physics 425 14 Doping of Organic Semiconductors 427 Björn L€ussem, Moritz Riede, and Karl Leo 14.1 Introduction 427 14.2 Doping Fundamentals 430 14.2.1 p–Type Doping 433 14.2.1.1 Control of the Position of the Fermi Level by Doping 433 14.2.1.2 Doping Efficiency 436 14.2.2 n–Type Doping 438 14.2.2.1 n–Type Doping Using Alkali Metals 438 14.2.2.2 n–Type Doping Using Molecular Compounds with Very High HOMO Levels 440 14.2.2.3 n–Type Doping Using Precursors 442 14.2.3 Contacts with Doped Semiconductors 446 14.3 Organic p–n Junctions 447 14.3.1 p–n–Homojunctions 447 14.3.1.1 Experiments 448 14.3.2 Reverse Currents in p–n–Junctions 452 14.4 OLEDs with Doped Transport Layers 454 14.4.1 Efficiency of OLEDs 454 14.4.1.1 External Quantum Efficiency hq 455 14.4.1.2 Power Efficiency or Luminous Efficacy 457 14.4.2 p–i–n OLEDs 457 14.4.2.1 Highly Efficient Monochrome Devices 459 14.4.2.2 p–i–n Devices: White OLEDs 463 14.4.2.3 Triplet Harvesting OLEDs 466 14.4.2.4 Conclusion 468 14.5 Organic Solar Cells with Doped Transport Layers 468 14.5.1 Solar Cell Characteristics 472 14.5.2 Organic p–i–n Solar Cells 474 14.5.2.1 Brief History of Vacuum–Deposited Organic Solar Cells 474 14.5.2.2 Advantages of Molecular Doping in OSC 476 14.5.2.3 Optical Optimization 478 14.5.2.4 Tandem Devices 479 14.6 Conclusion 486 14.7 Summary and Outlook 486 References 488 15 Device Efficiency of Organic Light–Emitting Diodes 497 Wolfgang Br€utting and J€org Frischeisen 15.1 Introduction 497 15.2 OLED Operation 498 15.2.1 OLED Architecture and Stack Layout 498 15.2.2 Working Principles of OLEDs 499 15.2.3 OLED Materials 500 15.2.4 White OLEDs 502 15.3 Electroluminescence Quantum Efficiency 503 15.3.1 Factors Determining the EQE 503 15.3.2 Luminous Efficacy 505 15.4 Fundamentals of Light Outcoupling in OLEDs 506 15.4.1 Optical Loss Channels 506 15.4.2 Optical Modeling of OLEDs 508 15.4.3 Simulation–Based Optimization of OLED Layer Stacks 513 15.4.4 Influence of the Emitter Quantum Efficiency 515 15.4.5 Comprehensive Efficiency Analysis of OLEDs 516 15.5 Approaches to Improved Light Outcoupling 520 15.5.1 Overview of Different Techniques 520 15.5.2 Reduction of Surface Plasmon Losses 522 15.5.2.1 Basic Properties of SPPs 522 15.5.2.2 Scattering Approaches 523 15.5.2.3 Index Coupling 524 15.5.2.4 Emitter Orientation 529 15.6 Conclusion 533 References 534 16 Light Outcoupling in Organic Light–Emitting Devices 541 Chih–Hung Tsai and Chung–Chih Wu 16.1 Introduction 541 16.2 Theories and Properties of OLED Optics 542 16.3 A Few Techniques and Device Structures to Enhance Light Outcoupling of OLEDs 544 16.3.1 Second–Antinode OLED 544 16.3.2 Top–Emitting OLEDs Capped with Microlens or Scattering Layers 549 16.3.3 OLED with Internal Scattering 554 16.3.4 OLED Utilizing Surface Plasmon Polariton–Mediated Energy Transfer 561 16.4 Summary 571 References 571 17 Photogeneration and Recombination in Polymer Solar Cells 575 Carsten Deibel, Andreas Baumann, and Vladimir Dyakonov 17.1 Introduction 575 17.2 Photogeneration of Charge Carriers 577 17.3 Charge Carrier Transport in Disordered Organic Semiconductors 583 17.4 Recombination of Photogenerated Charge Carriers 588 17.5 Open–Circuit Voltage 593 17.6 Summary 595 References 595 18 Light–Emitting Organic Crystal Field–Effect Transistors for Future Organic Injection Lasers 603 Hajime Nakanotani and Chihaya Adachi 18.1 Introduction 603 18.2 Highly Photoluminescent Oligo(p–phenylenevinylene) Derivatives 608 18.3 Ambipolar Light–Emitting Field–Effect Transistors Based on Organic Single Crystals 610 18.3.1 Ambipolar Carrier Transport Characteristics of Single Crystals of OPV Derivatives 610 18.3.2 EL Characteristics of LE–OFETs Based on Organic Single Crystals 611 18.3.3 Tuning of Carrier Density by Interfacial Carrier Doping in Single Crystals of OPV Derivatives 613 18.3.3.1 Interfacial Carrier Doping Based on Electron Transfer from an Organic Single Crystal into a MoOx Layer 613 18.3.3.2 Application of Interfacial Carrier Doping for Ambipolar LE–OFETs 614 18.3.3.3 Estimation of Singlet Exciton Density in the Recombination Zone 616 18.4 Summary and the Outlook for Future Organic Injection Lasers 617 References 619 Index 623

Wolfgang Brütting , University of Augsburg, Germany. Professor Brütting received his PhD in Physics from the University of Bayreuth in 1995 with a work on charge–density wave systems. Thereafter he moved to the field of organic semiconductors where he could take part in the development of organic light–emitting devices for display applications, inter alia as a visiting scientist at Kyushu University and IBM Zurich Research Laboratory. In 2003 he became Professor for Experimental Physics at the University of Augsburg. His current research activities include thin fi lm growth, photophysics and electrical transport in organic semiconductor devices. Chihaya Adachi , received his PhD from Kyushu University in 1991. In 2005, he was appointed Full Professor at the Center for Future Chemistry in Kyushu Univ. and since 2010 he is director of the Center for Organic Photonics and Electronics Research (OPERA). He is serving on the editorial board of Organic Electronics (Elsevier). His current research interests are organic opto–electronics such as OLED, organic FET,organic solar cells, organic laser diode and fundamental photo–physical and electronic processes in organic solidstate thin films.

“The book is well organized and clearly written. Each chapter has its own reference list and there is a comprehensive index at the end. It is a must for anyone working in organic optoelectronics.”  ( Optics & Photonics , 22 August 2013) "... is a useful contribution to the field and well worth buying." ChemPhysChem on the first edition "There is no doubt this book will be a useful companion to current researchers of whichever strand – physicists, chemists, materials scientists, and electrical engineers alike, as well as researchers about to enter the field." Advanced Materials on the first edition