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Electroconversion of CdS to Cu x S for thin film solar cells

Electroconversion of vacuum-evaporated CdS films to cuprous sulphide (Cu x S) for the fabrication of thin film CdS/Cu x S solar cells was investigated. The Cu x S films so formed were characterized by electrochemical and electron diffraction methods. The dependence of the stoichiometry x of the Cu x S on conversion parameters such as the concentration, pH and temperature of the electrolyte (CuSO 4 ) and the cathode (CdS) current density was established. A 0.4 M concentration bath with the pH value maintained at 2.5 was found to be most suitable. For this bath, optimum combinations of the bath temperature and cathode current density yielding the most stoichiometric Cu x S (x≥1.99) were found to be in the ranges 55-65°C and 2.5-5.0 mA cm -2 respectively. The films converted under the optimum conditions exhibit predominantly a chalcocite structure. The observed kinetics of the conversion process was understood in terms of a proposed two-step reaction at the cathode, involving a chemical ion exchange and an electrochemical reduction reaction.