We have grown thin films of a-Si:H and a-Ge:H by reactive magnetron sputtering and have studied their photoelectronic properties in surface cell structures. The dark and photocurrents do not obey the expected scaling laws with respect to changes in the sample thickness. This is explained in terms of a model which assumes depletion regions at both surfaces of the thin-film structure. We have analyzed the properties of such structures and have used the thickness dependence of the dark current and activation energy to estimate: (1) the defect concentrations near the Fermi level and (2) the amount of surface band bending. Defect densities so obtained are in agreement with those determined from space-charge limited currents (SCLC’s) and photodeflection spectroscopy (PDS).