Negative field dependence of mobility in disordered organic thin films due to non-equilibrium charge transport

Monte Carlo simulation is carried out to understand the influence of non-equilibrium charge transport on field dependence of mobility in disordered organic thin films. A negative field dependence of mobility (NFDM) at low field regime is observed when the sample thickness is small. The strength of the observed NFDM vanishes gradually with increase in the sample thickness leading to the saturation of mobility for the sample thickness of the order of few microns. The strength of the observed NFDM is found to increase with decrease of energetic disorder and increase of temperature. The observed results are explained on the basis of carrier relaxing towards the bottom of the density of states in the process of attaining the quasi-equilibrium.