Foreword ..................................................... V
Preface ..................................................... VU
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
1.5.1.1 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 σ and ∑ ................ 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 Brütting
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
Hetero junctions ......................................... 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 Microstracture 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
12.3.1 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 Rafaі 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 Lüssem, 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-Туре Doping ..................................... 438
14.2.2.1 n-Туре Doping Using Alkali Metals ........ 438
14.2.2.2 n-Туре Doping Using Molecular Compounds
with Very High HOMO Levels ............... 440
14.2.2.3 n-Туре 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 ηq ........... 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 Brütting and Jörg 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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