Non-equilibrium transport of charge carriers in disordered organic materials

Abstract

An analytic theory of non-equilibrium hopping charge transport in disordered organicmaterials is developed. It rests on the concept of effective transport energy and includesquasi-equilibrium (normal) and extremely non-equilibrium (dispersive) regimes of hoppingtransport as limiting cases at long and short times, respectively. Special attention is paid tothe regime of moderately weak non-equilibrium transport. In this regime thequasi-equilibrium value of the mobility is nearly established, whereas the coefficient offield-assisted diffusion continues to increase at long times. Analytic expressions forrelaxation times in the context of field-assisted diffusion and carrier drift havebeen obtained. The results of the theory are in agreement both with the data oftime-of-flight experiments for molecularly doped polymers and the results of numericalsimulations of the Gaussian disorder model. The impact of non-equilibrium effects onthe transit time of charge carriers in thin organic films with a thickness of theorder of 100 nm, which is typical for organic light-emitting diodes, is outlined.