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

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, , 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 as determined by d.c. and a.c. techniques. The difference between d.c. and a.c. determined 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.