This book is based on the results of many years of experimental work by the author and his colleagues, dealing with the electronic properties of organic crystals. E. Silinsh has played a leading role in pOinting out the importance of the polarization energy by an excess carrier, in determining not only the character of the carrier mobility in organic crystals, but in determining the band gap and the nature of the all-important trapping site in these crystals. The one-electron model of electronic conductivity that has been so successful in dealing with inorganic semiconductors is singular­ ly unsuccessful in rationalizing the unusual physical properties of organic crystals. A many-body theory is required, and the experimental manifestation of this is the central role played by the crystal polarization enerqies in transferring the results obtained with the isolated molecule, to the solid. The careful studies of E. Silinsh in this field have shown tn detail how this polarization energy develops around the excess carrier (and also the hole-electron pair) sitting on a molecular site in the crystal. As with all insulators, trapping sites playa dominant role in reducing the magnitude of ~he current that can theoretically pass through the organic crystal. It is usually the case that these trapping sites are energetically distributed within the forbidden band of the crystal. For many years, an exponential distribution has shown itself to be useful and reasonably correct: However,' E.

Inhalt

1. Introduction: Characteristic Features of Organic Molecular Crystals.- 1.1 Interaction Forces in Molecular Crystals.- 1.2 The Atom-Atom Potential Method.- 1.3 Aromatic Hydrocarbons - Model Compounds of Organic Molecular Crystals.- 1.3.1 Anthracene.- Anthracene as Model Compound.- Molecular Structure.- Basic Molecular Parameters.- Crystal Structure.- Elastic and Optical Properties.- Metastable Phases in Anthracene.- 1.3.2 Naphthalene.- Molecular Structure.- Basic Molecular Parameters.- Crystal Structure.- Elastic and Optical Properties.- 1.3.3 Higher Linear Polyacenes.- Tetracene and Pentacene.- Hexacene.- 1.3.4 Other Model Aromatic Compounds.- 1.4 Specific Properties of Electronic States in a Molecular Crystal.- 1.5 Basic Characteristics of Electronic Conduction States in Molecular Crystals.- 1.5.1 Band Theory Approach.- 1.5.2 Hopping Versus Band Model.- 1.5.3 Band-to-Hopping Transition.- 1.5.4 Electronic Polarization and Charge Carrier Self-Energy.- 1.5.5 Other Types of Interaction.- 2. Electronic States of an Ideal Molecular Crystal.- 2.1 Neutral Excited States in a Molecular Crystal.- 2.2 Ionized States in a Molecular Crystal.- 2.2.1 The Lyons Model of Ionized States.- 2.2.2 A Modified Lyons Model.- 2.3 Electronic Polarization of a Molecular Crystal by a Charge Carrier.- 2.3.1 Some General Considerations.- 2.3.2 Dynamic and Microelectrostatic Approaches to Electronic Polarization in Molecular Crystals.- 2.4 Electrostatic Methods of Electronic Polarization Energy Calculation in Molecular Crystals.- 2.4.1 Microelectrostatic Methods of Zero-Order Approximation.- 2.4.2 Method of Self-Consistent Polarization Field.- 2.5 Determination of Molecular Polarizability Tensor.- 2.5.1 Experimental Methods.- 2.5.2 Theoretical Methods.- 2.6 Selection of Molecular Polarizability Components bi. for Electronic Polarization Energy Calculations.- 2.7 Extended Polarization Model of Ionized States in Molecular Crystal s.- 2.7.1 Intrinsic Electronic Polarization of a Molecule by a Localized Charge Carrier.- 2.7.2 Vibronic Relaxation and Ionic State Formation.- 2.7.3 Extended Polarization Model Including Ionic States of Electronic Conductivity.- 2.7.4 Dynamic Electronic Polaron States in a Molecular Crystal.- 2.8 Charge Transfer (CT) States in Molecular Crystals.- 2.8.1 General on CT States.- 2.8.2 Evaluation of CT-State Energies in Anthracene and Naphthalene Crystals.- 2.8.3 CT States in Photogeneration Processes.- 2.8.4 CT States in Recombination Processes.- 2.9 Experimental Determination of Energy Structure Parameters in Molecular Crystals.- 2.10 Energy Structure of an Anthracene Crystal.- 2.11 Energy Structure of Aromatic and Heterocyclic Molecular Crystals.- 3. Role of Structural Defects in the Formation of Local Electronic States in Molecular Crystals.- 3.1 Statistical Aspects of the Formation of Local States of Polarization Origin.- 3.2 General Considerations on the Role of Specific Structural Defects.- 3.3 Point Defects (Vacancies) in Molecular Crystals, Their Crystallographic and Electronic Properties.- 3.4 Dislocation Defects, Their Role in Local State Formation.- 3.5 Energetics of Dislocations in Molecular Crystals.- 3.5.1 Discrete Configuration of Dislocations.- 3.5.2 Basic Elastic Properties of Anthracene and Naphthalene Crystals.- 3.5.3 Energy Estimates for Basal Edge Dislocations in an Anthracene Crystal.- 3.6 Atomic and Molecular Models of the Dislocation Core.- 3.6.1 Models of Spherical Atoms and Molecules.- 3.6.2 Polyatomic Molecular Models.- 3.7 Dislocation Alignments and Aggregations, Their Configurational and Energetic Properties.- 3.7.1 Interaction Between Dislocations.- 3.7.2 Dislocation Alignments.- 3.7.3 Dislocation Ensembles.- 3.8 Grain Boundaries, Their Energetic Characteristics.- 3.8.1 Energy of Grain Boundaries in Molecular Crystals.- 3.8.2 Relative Lattice Compression on Grain Boundaries of an Anthracene Crystal.- 3.9 Stacking Faults in Molecular Crystals.- 3.9.1 General on Stacking Faults.- 3.9.2 Stacking Faults in Anthracene-Type Crystals, Their Energetic Characteristics.- 3.9.3 Calculations of Equilibrium Configuration of Molecules in Stacking Faults of an Anthracene Crystal.- 3.10 Formation of Predimer States in the Regions of Extended Structural Defects of Anthracene-Type Crystals.- 3.11 Some More Complex Two- and Three-Dimensional Lattice Defects in Molecular Crystals.- 3.12 Observation of Structural Defects in Molecular Crystals.- 3.12.1 Optical Low Resolution Technique.- 3.12.2 Electron Microscopy and Diffraction Techniques.- 3.12.3 X-Ray Methods.- 3.13 Main Characteristics of Dislocation Defects in Some Model Molecular Crystals.- 3.13.1 Dominant Types of Dislocatioas in Anthracene Space Group Crystal s.- 3.13.2 Density of Dislocations in Anthracene Crystals, Its Dependence on Crystal Growth and Treatment.- 4. Local Trapping Centers for Excitons in Molecular Crystals.- 4.1 Theory of Exciton States in a Deformed Molecular Crystal.- 4.2 Electron Level Shifts in Hydrostatically Compressed Molecular Crystal s.- 4.3 Formation of Local Exciton Trapping Centers in Structural Defects of a Crystal.- 5. Local Trapping States for Charge Carriers in Molecular Crystals.- 5.1 Electronic Polarization Energy of a Compressed Anthracene Crystal.- 5.2 Formation of Local Trapping Centers for Charge Carriers in Structural Defects of a Real Molecular Crystal.- 5.3 Energy Spectrum of Local States of Polarization Origin in Stacking Faults of an Anthracene Crystal.- 5.4 Local Surface States of Polarization Origin in Molecular Crystals.- 5.5 Local States of Polarization Origin in the Vicinity of a Lattice Vacancy.- 5.6 Local Charge Carrier Trapping in Covalent, Ionic and Molecular Crystal s.- 5.7 Randomizing Factors Determining Gaussian Distribution of Local States of Structural Origin.- 5.8 Investigation of Local Trapping States by Method of Space Charge Limited Currents (SCLC).- 5.8.1 General Considerations.- 5.8.2 Injecting and Blocking Contacts.- 5.8.3 Conventional SCLC Theories of Discrete and Exponential Approximation of Trap Distribution.- SCLC Theory for an Insulator With Discrete Trap Distribution.- SCLC Theory for an Insulator With Exponential Trap Distribution.- Applicability Limits of Diffusion-Free SCLC Theory Approximation.- 5.8.4 Criteria for Validity of SCLC Conditions.- 5.8.5 Difficulties in Interpreting Experimental CV Characteristics in Terms of Discrete and Exponential Trap Distribution Models.- 5.9 Phenomenological SCLC Theory for Molecular C…