The influence of generation and transport processes in the constant photocurrent method

Abstract

We investigate the differences observed in the absorption spectrum of a-Si : H measured by d.c. and a.c. constant-photocurrent methods (CPM). D.c. measurement gives a value for the absorption coefficient, α, at photon energies below the Urbach tail, of up to an order of magnitude greater than a.c. measurement. We examine the free-carrier generation processes occurring in a.c. CPM, and the influence of these processes on the photocurrent frequency response to modulated sub-gap illumination. A simple kinetic model is presented and used to explain quantitatively the differences in the photocurrent frequency response for sub- and super-gap excitation, and hence in the CPM α as determined by d.c. and a.c. techniques. The difference between d.c. and a.c. determined α is explained in terms of the relative contribution of phonon-assisted transitions in the generation process. A useful consequence of the analysis is that an a.c. measurement should provide a more accurate means for determining the density of occupied states in the material.