1. Introduction ................................................. 1
1.1. What are Organic Solids? ................................ 1
1.2. What are the Special Characteristics of Organic
Solids? ................................................. 9
1.3. Goals and Future Outlook ............................... 15
Problems for Chapter 1 ................................. 16
Literature ............................................. 24
2. Forces and Structures ....................................... 25
2.1. Forces ................................................. 25
2.1.1. Inductive Forces ................................ 26
2.1.2. Van der Waals Forces ............................ 27
2.1.3. Repulsive Forces ................................ 29
2.1.4. Intermolecular Potentials ....................... 30
2.1.5. Coulomb Forces .................................. 33
2.2. Structures ............................................. 34
2.2.1. Crystals of Nonpolar Molecules .................. 34
2.2.2. Crystals of Molecules with Polar Substituents ... 39
2.2.3. Crystals with a Low Packing Density,
Clathrates ...................................... 40
2.2.4. Crystals of Molecules with Charge Transfer,
Radical-ion Salts ............................... 42
2.3. Polymer Single Crystals: Diacetylenes .................. 43
2.4. Thin Films ............................................. 47
2.5. Inorganic-Organic Hybrid Crystals ...................... 51
Problems for Chapter 2 ................................. 52
Literature .................................................. 54
3. Purification of Materials, Crystal Growth and Preparation
of Thin Films ............................................... 57
3.1. Purification ........................................... 57
3.2. Highest Purity ......................................... 61
3.3. Crystal Growth ......................................... 63
3.4. Mixed Crystals ......................................... 70
3.5. Epitaxy, Ultrathin Films 71 Problems for Chapter 3 ..... 72
References .................................................. 73
4. Impurities and Defects ...................................... 75
4.1. Foreign Molecules, Impurities, and X traps ............. 75
4.2. Structural Defects ..................................... 78
4.2.1. Point Defects ................................... 78
4.2.2. Dislocations .................................... 79
4.2.3. Grain Boundaries ................................ 82
4.2.4. Dipolar Disorder ................................ 83
4.3. Characterisation and Analysis of Impurities ............ 84
4.4. Characterisation of Defects ............................ 84
Literature ............................................. 86
5. Molecular and Lattice Dynamics in Organic Molecular
Crystals .................................................... 89
5.1. Introduction ........................................... 89
5.2. Intramolecular Vibrations .............................. 91
5.3. Phonons ................................................ 93
5.3.1. The Eigenvector ................................. 94
5.3.2. The Wavevector .................................. 95
5.3.3. The Frequencies Ω (K)............................ 96
5.3.4. Excitations ..................................... 97
5.4. Experimental Methods ................................... 97
5.4.1. Inelastic Neutron Scattering .................... 97
5.4.2. Raman Scattering and Infrared Absorption ........ 99
5.5. The 12 External Phonons of the Naphthalene Crystal .... 100
5.5.1. Dispersion relations ........................... 100
5.5.2. Pressure and Temperature Dependencies .......... 104
5.6. Analytic Formulation of the Lattice Dynamics in
Molecular Crystals .................................... 107
5.7. Phonons in other Molecular Crystals ................... 109
5.8. Hindered Rotation and Diffusion ....................... 113
5.8.1. Nuclear Magnetic Resonance ..................... 113
5.8.2. Benzene Crystals ............................... 116
5.8.3. Methyl Groups .................................. 118
5.8.4. Diffusion ...................................... 120
Problems for Chapter 5 ................................ 122
References ............................................ 123
6. Electronic Excited States, Excitons, Energy Transfer ....... 125
6.1. Introduction .......................................... 125
6.2. Some historical remarks ............................... 126
6.3. Optical Excited States in Crystals .................... 127
6.4. Davydov Splitting and Mini-Excitons ................... 134
6.5. Frenkel Excitons ...................................... 139
6.5.1. Excitonic States, Fundamental Equations ........ 140
6.5.2. Polarisation and Band Structure ................ 143
6.5.3. Coherence ...................................... 147
6.6. Charge Transfer (CT) Excitons ......................... 149
6.7. Surface Excitons ...................................... 153
6.8. Excimers .............................................. 154
6.9. Exciton Processes, Energy Conduction .................. 156
6.9.1. Sensitised Fluorescence ........................ 157
6.9.2. Delayed Fluorescence by Triplet Excitons ....... 160
6.9.3. Excitonic Processes ............................ 163
6.10.Excitonic Processes in other Systems .................. 171
6.11.Future Developments ................................... 173
Problems for Chapter 6 ................................ 173
Literature ............................................ 174
7. Structure and Dynamics of Triplet States ................... 177
7.1. Introduction and Historical Remarks ................... 177
7.2. Spin Quantisation in Triplet States ................... 182
7.3. The Dipole-Dipole Interaction, Fine Structure ......... 183
7.3.1. Zero Field (B0 = 0) ............................ 183
7.3.2. Zeeman Splitting (B0 ≠ 0) ...................... 289
7.3.3. Powder Spectra ................................. 192
7.4. Mini-Excitons ......................................... 192
7.5. Triplet Excitons ...................................... 199
7.5.1. Anthracene and Naphthalene Crystals: Two-
dimensional Triplet Excitons ................... 199
7.5.2. Dibromonaphthalene Crystals: coherent, one-
dimensional Triplet Excitons ................... 203
7.6. Optical Spin Polarisation (OEP) ....................... 204
7.7. Optical Nuclear-Spin Polarisation (ONP) ............... 222
7.8. Perspectives .......................................... 214
Problems for Chapter 7 ................................ 214
Literature ............................................ 215
8. Organic Semiconductors ..................................... 217
8.1. Preliminary Historical Remarks ........................ 220
8.2. Conductivity and Mobility of nearly-free Charge
Carriers .............................................. 223
8.3. Charge Carriers in Organic Semiconductors: Polarons,
Shallow Traps and Deep Traps .......................... 228
8.4. Generation of Charge Carriers and Charge Transport:
Experimental Methods .................................. 234
8.4.1. The TOF Method: Gaussian Transport ............. 234
8.4.2. Photogeneration of Charge Carriers ............. 238
8.4.3. Contacts, Injection, Ejection, and Dark
Currents ....................................... 244
8.4.4. Space-Charge Limited Currents .................. 255
8.5. Charge-Carrier Mobilities in Organic Molecular
Crystals .............................................. 263
8.5.1. Band- or Hopping Conductivity? ................. 263
8.5.2. Temperature Dependence and Anisotropy of
the Mobilities ................................. 265
8.5.3. Electric-field Dependence ...................... 269
8.5.4. Band Structures ................................ 272
8.5.5. Charge-Carrier Traps ........................... 277
8.6. Charge Transport in Disordered Organic
Semiconductors ........................................ 279
8.6.1. The Bässler Model .............................. 282
8.6.2. Mobilities in High-Purity Films:
Temperature, Electric-Field, and Time
Dependence ..................................... 284
8.6.3. Binary Systems ................................. 289
8.6.4. Discotic Liquid Crystals ....................... 290
8.6.5. Stationary Dark Currents ....................... 292
Problems for Chapter 8 ................................ 303
Literature ............................................ 303
9. Organic Crystals of High Conductivity ...................... 307
9.1. Donor-Acceptor Systems ................................ 307
9.2. Strong CT Complexes, Radical-ion Salts ................ 308
9.3. The Organic Metal TTF-TCNQ - Peierls Transition and
Charge-Density Waves .................................. 314
9.4. Other Radical-ion Salts and CT Complexes .............. 322
9.5. Radical-Anion Salts of DCNQI .......................... 323
9.6. Radical-Cation Salts of the Arenes .................... 330
9.6.1. Direct-current Conductivity .................... 330
9.6.2. X-Ray Scattering ............................... 334
9.6.3. Optical Reflection Spectrum .................... 335
9.6.4. Magnetic Susceptibility ........................ 337
9.6.5. Spin Resonance of the Conduction Electrons
(ESR) .......................................... 339
9.6.6. Charge-Density-Wave Transport .................. 343
Problems for Chapter 9 ................................ 346
Literature ............................................ 347
10.Organic Superconductors .................................... 351
10.1.Introduction .......................................... 351
10.2.Mainly One-dimensional Charge-Transfer Salts as
Superconductors; Bechgaard Salts ...................... 353
10.3.Quasi-Two-dimensional Charge-Transfer Systems as
Superconductors ....................................... 356
10.4.The Nature of the Superconducting State in Organic
Salts ................................................. 359
10.5.Three-dimensional Superconductivity in Fullerene
Compounds ............................................. 361
Literature ............................................ 363
11.Electroluminescence and the Photovoltaic Effect ............ 365
11.1.Electroluminescence: Organic Light-Emitting Diodes
(OLEDs) ............................................... 366
11.1.1.Historical Remarks ............................. 366
11.1.2.The Principle of the ОLED ...................... 368
11.1.3.Multilayer OLEDs ............................... 373
11.1.4. Electro-optical Properties .................... 377
11.2.Photovoltaic Effect: Organic Photovoltaic Cells ....... 381
11.2.1. Exciton Dissociation .......................... 382
11.2.2. Photovoltaic Characteristics .................. 384
11.2.3. CuPc/C60 Solar Cells .......................... 386
Literature ............................................ 389
12.Towards a Molecular Electronics ............................ 391
12.1.What is Molecular Electronics and What Will it Do? .... 391
12.2.Molecules as Switches, Photochromic Effects ........... 392
12.3.Molecular Wires ....................................... 395
12.4.Light-Induced Phase Transitions ....................... 396
12.5.Molecular Rectifiers .................................. 400
12.6.Molecular Transistors ................................. 401
12.7.Molecular Storage Units ............................... 406
Appendix: Coloured Plates ..................................... 411
Index ......................................................... 417
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