It was recalled that the constant photo-current method was widely applied to amorphous hydrogenated material as being an efficient means for determining the optical absorption coefficient in the low-absorption region, and for estimating the defect density in the band-gap. However, such use was based upon the assumption that the material parameters were uniform throughout the sample, and it was possible that practical situations might not fulfil that assumption. The effect of an inhomogeneous spatial distribution of defects, and of surface recombination, upon the resolved defect density was therefore estimated here by using a steady-state diffusion equation for photo-excited carriers. Differences of as much as a factor of 2 were observed within the range of possible experimental conditions. It was concluded that the practical effect of this discrepancy was likely to be an under-estimation with respect to the average defect density within a film.

Q.Zhang, G.J.Adriaenssens: Journal of Applied Physics, 1997, 81[10], 6795-9