Electronic polarization in pentacene crystals and thin films

TL;DR

This paper evaluates electronic polarization in pentacene crystals and thin films, revealing how polarization energies influence charge transport and screening effects at surfaces and interfaces.

Contribution

It introduces a self-consistent method to compute charge redistribution and polarization energies in pentacene, including at surfaces and interfaces, providing new insights into screening effects.

Findings

Polarization energies are similar in bulk and at surfaces/interfaces, within ~10%.

A single pentacene layer approaches bulk screening properties.

Electronic polarization significantly impacts charge transport in organic systems.

Abstract

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.