Unified model of ballistic and diffusive carrier transport: Application to photovoltaic materials

A unified model of one-dimensional ballistic and diffusive carrier transport in semiconductors, valid for arbitrary mean free path and arbitrary shape of the band edge profiles, is applied in a study of the effect of grain boundaries on the transport properties of photovoltaic materials. Adopting the trapping model to describe the grain boundaries, dark conductivities and photoconductivities are calculated as a function of donor density for samples consisting of chains of grains with length comparable to the mean free path. The results of the unified model are found to deviate substantially from those of the purely ballistic and purely diffusive limits.