Prof. Sven Stafström, Linköping University

Charge transport in organic materials

Electric conduction is controlled by the density of charge carriers and the mobility of these carriers. In their pristine form, organic materials normally possess a low charge carrier density and the molecular nature of organic systems often leads to electron localization and a low mobility hopping type of transport. However, despite the fact that the interactions that favor an ordered crystalline phase of molecular based materials are relatively weak, it is possible to reduce the disorder to such an extent that delocalized states can exist. This introduces a possibility to increase the mobility and go from a hopping type of transport to band-like transport. This transition (but going in the opposite direction) is referred to as the metal-insulator-transition (MIT). In the first part of this presentation I will discuss the problem of electron localization as a result of disorder as well as models for charge transport below and above the MIT. Another type of localization is due to electron-phonon coupling. This coupling results in a charge carrier which is accompanied by a reorganization of the atomic positions. Depending on the relation between the strengths of the inter-molecular electronic couplings and the intra-molecular electron-phonon coupling, charge transport is as a nonadiabatic polaron hopping process or an adiabatic polaron drift process. In my presentation I will describe both these processes. I will also make a comparison between this onadiabatic-to-adiabatic transition and the MIT discussed above.

For more information contact Dr. Jean-Luc Brédas (404-385-4986).