Simulation of Hydrogenated Amorphous Silicon: Temperature Dependence of Nonequilibrium Distribution Functions for Trap States

Nonequilibrium distribution functions (NDFs) for trap states in the mobility gap under photoillumination and zero bias voltage are derived by a constructed self-consistent drift–diffusion simulator consisting of the Poisson equation and current continuity equations for hydrogenated amorphous silicon (a-Si:H). Regarding the temperature dependence of the NDF, we find that the values of the NDF decrease with increasing temperature (negative temperature dependence) in the energy region near the conduction band for p-type a-Si:H. This is the reverse of the temperature dependence of the equilibrium distribution functions (EDFs) for the trap states in the mobility gap. Furthermore, we show that this new physical characteristic can be applied in explaining the temperature characteristic of the photoconductivity caused by electron hopping in the conduction band tail for a-Si:H. The photoconductivity of a-Si:H decreases with increasing temperature, which is called thermal quenching (TQ). We show that the TQ observed at low temperatures of approximately 200 K for p-type a-Si:H can be explained by the electron hopping model, with the p-type NDF having a negative temperature dependence.