The transient localization scenario for charge transport in crystalline organic materials

TL;DR

This paper discusses the transient localization regime in crystalline organic semiconductors, caused by molecular motions, and presents theoretical approaches that explain experimental observations and suggest strategies to enhance charge mobility.

Contribution

It introduces a modern theoretical framework capturing quantum localization effects due to molecular motions, linking transient localization to Anderson localization, and proposes mobility improvement strategies.

Findings

Transient localization explains charge transport limitations.

Quantum localization occurs at short timescales.

Strategies to enhance mobility are proposed.

Abstract

Charge transport in crystalline organic semiconductors is intrinsically limited by the presence of large thermal molecular motions, which are a direct consequence of the weak van der Waals inter-molecular interactions. These lead to an original regime of transport called \textit{transient localization}, sharing features of both localized and itinerant electron systems. After a brief review of experimental observations that pose a challenge to the theory, we concentrate on a commonly studied model which describes the interaction of the charge carriers with inter-molecular vibrations. We present different theoretical approaches that have been applied to the problem in the past, and then turn to more modern approaches that are able to capture the key microscopic phenomenon at the origin of the puzzling experimental observations, i.e. the quantum localization of the electronic wavefuntion at timescales shorter than the typical molecular motions. We describe in particular a relaxation time approximation which clarifies how the transient localization due to dynamical molecular motions relates to the Anderson localization realized for static disorder, and allows us to devise strategies to improve the mobility of actual compounds. The relevance of the transient localization scenario to other classes of systems is briefly discussed.