The interpretation of photoconductivity measurements in hydrogenated amorphous silicon

Abstract A theory of steady state photoconductivity in trap-dominated materials is formulated in terms of a recombination rate that is an integral of Shockley-Read terms and a global charge neutrality condition that involves the occupation statistics of hole and electron traps. Application of the theory to hydrogenated amorphous silicon is complicated by lack of necessary data, especially on transition rates between band and trap, and various assumptions for these are used to derive the recombination rate. Weak assumptions for the transition rates, coupled with asymmetry in the gap density of states in this material, lead to a simple description of the electron trap statistics. This simplicity allows a relationship to be established between the photoconductive response time and the gap density of states. Various experiments on photoconductivity and its response time are analysed in these terms. Possible origins for light-induced changes in photoconductivity in hydrogenated amorphous silicon are considered.