Electronic polarization in pentacene crystals and thin films

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 nonoverlapping 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 states with fixed separation between anion and cation, and compare to electroabsorption data and to submolecular calculations. Electronic polarization of ∼ I eV per charge has a major role for transport in organic molecular systems with limited overlap.