Field diffusion in disordered organic materials under conditions of occupied deep states

A simple analytical model of the field-diffusion coefficient is developed for moderate carrier concentrations. Hopping transport is described by the multiple-trapping model based on the transport-level concept. A continuity equation with a diffusion coefficient depending on carrier concentration is obtained, the time dependence of the field-diffusion coefficient under non-steady-state conditions is found. The time intervals in which deep state population affects the mobility and diffusion coefficient under conditions of time-of-flight experiments are estimated. It is shown that the field-diffusion coefficient increases in a long time interval while the mobility is unchanged, which is reminiscent of a similar case of nonequilibrium initial carrier generation at the low-concentration limit.