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We present an approach to electronic polarization in molecular solids treated as a set of quantum systems interacting classically. Individual molecules are dealt with rigorously as quantummechanical systems subject to classical external fields created by all other molecules and, possibly, external sources. Self-consistent equations are derived for induced dipoles and for atomic charges whose redistribution in external fields is given explicitly by an atom-atom polarizability tensor. Electronic polarization is studied in two representative organic molecular crystals, anthracene and perylenetetracarboxylic acid dianhydride (PTCDA), and contrasted to previous results for systems of polarizable points. The stabilization energies of the neutral lattice, of isolated anions and cations, and of cation-anion pairs are found. Charge redistribution on ions is included. The dielectric tensors of crystals are successfully related to gas-phase properties and provide consistency checks on polarization energies. The procedure is generally applicable to organic crystals in the limit of no intermolecular overlap.

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This review summarizes the current understanding of electrostatic phenomena in ordered and disordered organic semiconductors, outlines numerical schemes developed for quantitative evaluation of electrostatic and induction contributions to ionization potentials and electron affinities of organic molecules in a solid state, and illustrates two applications of these techniques: interpretation of photoelectron spectroscopy of thin films and energetics of heterointerfaces in organic solar cells.

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The electronic polarization energies, P = P+ + P−, of a PTCDA (perylenetetracarboxylic acid dianhydride) cation and anion in a crystalline thin film on a metallic substrate are computed and compared with measurements of the PTCDA transport gap on gold and silver. Both experiments and theory show that P is 500 meV larger in a PTCDA monolayer than in 50Å films. Electronic polarization in systems with surfaces and interfaces are obtained self-consistently in terms of charge redistribution within molecules. I. TRANSPORT GAPThe electronic structure of organic molecular crystals is strikingly different from the conventional inorganic semiconductors, such as Si, in that the electronic polarization of the dielectric medium by charge carriers constitutes a major effect, with energy scale greater than transfer integrals or temperature [1,2]. The transport gap E t for creating a separated electron-hole pair has a substantial (1-2 eV) polarization energy contribution [3] and exceeds the optical gap by ∼ 1 eV. Limited overlap rationalizes the modest mobilities of organic molecular solids. Devices such as light-emitting diodes, thin film transistors, or photovoltaic cells require charge transport and are consequently based on thin films, quite often deposited on metallic substrates [4,5]. Organic electronics relies heavily on controlling films with monolayer precision, on forming structures with several thin films, and on characterizing the interfaces. The positions of transport states and mechanisms for charge injection are among the outstanding issues for exploiting organic devices. We focus here on the electronic polarization of crystalline thin films near surfaces and interfaces. We find that electronic polarization, and hence E t , in a prototypical organic crystal is significantly different at a free surface, near a metal-organic interface, in thin organic layers, and in the bulk.Weak intermolecular forces characterize organic molecular crystals, whose electronic and vibrational spectra are readily related to gas-phase transitions [1,2]. Due to small transfer integrals, charge carriers are molecular ions embedded in the lattice of neutral molecules. The transport gap E t in the crystal is derived from the charge gap for electron transfer in the gas phase, I(g) − A(g), which is the difference between the ionization potential and the electron affinity. But crystals have electrostatic interactions even in the limit of no overlap, and charge carriers are surrounded by self-consistent polarization clouds. In contrast to polaronic effects, electronic polarization is instantaneous and directly affects the positions of energy levels. Formation of polarization clouds is associated with stabilization energy P + for cations (the "holes") and P − for anions (the "electrons"). The combination P = P + + P − occurs in E t = I(g) − A(g) − P . Since Coulomb interactions are long-ranged, polarization clouds extend over many lattice constants and P depends on the proximity to surfaces and interfaces.

We investigate the optical absorption spectrum in a Holstein model for a molecular chain with Frenkel excitons and linear coupling to one internal vibration. The model is extended for nearest-neighbor chargetransfer excitons that mix with the Frenkel excitons. We represent the Hamiltonian in a displaced oscillator ͑Lang-Firsov͒ basis and employ a problem-adapted scheme for the truncation of the phonon basis. For weak and intermediate electronic coupling, the complete absorption spectrum and the structure of the relevant eigenstates become accessible by direct numerical diagonalization. We discuss the structure of the phonon clouds and the applicability of the molecular vibron model, in which only joint exciton-phonon configurations are included. As examples, we model absorption spectra of PTCDA ͑3,4,9,10-perylenetetracarboxylic dianhy-dride͒ and MePTCDI (N-NЈ-dimethylperylene-3,4,9,10-dicarboximide͒.

Electronic polarization is evaluated in pentacene crystals and in thin films on a metallic substrate using a self-consistent method for computing charge redistribution in non-overlapping molecules. The optical dielectric constant and its principal axes are reported for a neutral crystal. The polarization energies P+ and P- of a cation and anion at infinite separation are found for both molecules in the crystal's unit cell in the bulk, at the surface, and at the organic-metal interface of a film of N molecular layers. We find that a single pentacene layer with herring-bone packing provides a screening environment approaching the bulk. The polarization contribution to the transport gap P=(P+)+(P-), which is 2.01 eV in the bulk, decreases and increases by only ~ 10% at surfaces and interfaces, respectively. We also compute the polarization energy of charge-transfer (CT) states with fixed separation between anion and cation, and compare to electroabsorption data and to submolecular calculations. Electronic polarization of ~ 1 eV per charge has a major role for transport in organic molecular systems with limited overlap.Comment: 10 revtex pages, 6 PS figures embedde