Thermal Narrowing of the Electronic Bandwidths in Organic Molecular Semiconductors: Impact of the Crystal Thermal Expansion

TL;DR

This study uses electronic-structure calculations to show that thermal expansion causes band narrowing in organic semiconductors like pentacene and rubrene, affecting their charge transport properties.

Contribution

It demonstrates that thermal expansion, not polaron effects, primarily causes bandwidth narrowing at higher temperatures in these materials.

Findings

Bandwidth narrows with increasing temperature.

Thermal expansion is the main cause of band narrowing.

Charge transport properties are influenced by thermal expansion.

Abstract

We report on electronic-structure calculations for the pentacene and rubrene crystals, based on experimental crystal geometries measured at different temperatures. The results are in very good agreement with angle-resolved photoelectron spectroscopy data that indicate that the widths of the valence and conduction bands in both materials become narrower at higher temperatures. Our findings strongly suggest that the thermal bandwidth narrowing in the pentacene and rubrene crystals is primarily caused by the thermal expansion of the lattice rather than by a renormalization of the transfer integrals induced by a polaron effect. The effect of thermal expansion on the charge-transport properties is also discussed.