The effect of substrate on the working of n-CdSe0.7Te0.3 thin-film/polysulphide photoelectrochemical (PEC) solar cells has been investigated in detail. The role of substrates like stainless steel and In2O3-predeposited glass slides on the Mott-Schottky plots is investigated at various frequencies for various surface treatments using both types of substrates and the reasons for the dispersion observed have been explained. The variation of space-charge capacitance (at a fixed electrode potential) as a function of frequency has been investigated and also the power law governing the capacitance dependence. The improvement in the power output characteristics on annealing and photoelectrochemical etching occurs due to the reduction of surface states, leading to a decrease in trap states. © 1998 Elsevier Science S.A. All rights reserved.