The relationship between transient and steady-state photoconductivity in amorphous semiconductors

Abstract

Dispersive transport experiments in a-Si:H and a-As 2Se 3 have shown that current resulting from excess carriers is limited by multiple trapping in an exponential density of states. In this paper, the relationship between steady-state photoconductivity and the transient phenomena is explored. Because of the exponential density of states, the well-known G γ dependence is expected for steady state, where G is the excitation density. The power γ is related to the dispersion parameter, α, by γ = (1 + α) −1. It is predicted that, in the approach to steady state, the current rises as t α . When the recombination coefficient, b r, is much greater than the trapping coefficient b t, the current is predicted to exceed the steady-state value by a factor, which depends on b r b t and α, before settling down to the steady level. The time of the maximum and the time to reach steady state are predicted to vary as G − γ . After reaching steady state, when the excitation is turned off the current is predicted to decay as t − α for the same amount of time as is required to reach steady state when the light is turned on. After this time, the current is predicted to decay as t −1. A number of important parameters characterizing transport and recombination in a multiple trapping system can be obtained from the approach to and decay from steady state.