The time evolution of surface defect density and width of space charge region in thin layer of amorphous silicon is observed experimentally by Fourier transform photocurrent spectroscopy. This work reviews the assumption that photocurrent is insensitive to surface defects for samples thinner than 1500 nm. We show that correct evaluation based on simple optical model comprising layers representing surface defects and layers representing space charge region with reduced collection allows obtaining the same results as from photothermal deflection spectroscopy. Our main approach is the comparison of photocurrent or photothermal deflection spectra measured in absorptance/transmittance mode from layer and substrate side of the thin film. (C) 2011 Elsevier B.V. All rights reserved.