The field of organic electronics has seen a steady growth over the last 15 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, Japan, 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 or organic lasers. 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.

Wolfgang Bruetting, University of Augsburg. Professor Br?tting received his Ph.D. in Physics from the University of Bayreuth in 1995 with a work on charge transport in one-dimensional charge-density wave systems. He then moved to the field of organic semiconductors where he could take part in the development of organic light-emitting devices for display applications. In 2003 he became Professor for Experimental Physics at the University of Augsburg. His current research activities include thin film growth, photophysics and electrical transport in organic semiconductor devices.

Chihaya Adachi, Kyushu University in Japan. Professor Adachi worked at Chitose Institute of Science and Technology until he joined the Center for Future Chemistry as a Professor. He is heading the Division for Optical Functional Materials which research activities on organic optical electronics range from the synthesis of organic semiconductor materials to the development of devices and the clarification of properties.

Russell J. Holmes is Assistant Professor at the Dept. of Chemical Engineering and Materials Science at the University of Minnesota. His research is primarily focused on the study of thin films and single crystals of organic materials. Russel Holmes is specifically interested in how these materials behave from a growth standpoint, the characterization of their optical and electrical properties, and their performance in optoelectronic devices.

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

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