Non-equilibrium transport of charge carriers in disordered organic materials

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