Spatially dispersive transport: A mesoscopic phenomenon in disordered organic semiconductors

We present time-of-flight-type calculations of the transport master equation applied to thin film disordered materials. We show that the energetic disorder in conjunction with a thin film results in electronic inhomogeneity. This inhomogeneity manifests itself as dispersive transport which can be described as a linear sum of close to normal-transport paths. Namely, in thin films of disordered materials the transport parameters do not converge to the infinite sample parameters but present a dispersive mesoscopic phenomenon. By defining a spatial distribution function of the charge velocity (mobility) we are able to examine the effect of the degree of disorder and film thickness on the electronic inhomogeneity. We postulate that in a given sample the spatial distribution characteristic of holes and the one characteristic of electrons are most likely nonidentical. Hence, in organic thin-film light-emitting diodes the energetic disorder is a limiting factor concerning charge recombination and efficiency.