Optimization of the nucleation-site density for the electrodeposition of cadmium sulfide on indium-tin-oxide

Cadmium sulfide (CdS) is a preferred heterojunction partner for a number of chalcogenide-based solar cells. In view of this, interest has grown in the use of solution-based deposition techniques as an alternative route for the preparation of uniform ultrathin films of CdS. However, the quality of the electrodeposited CdS films on indium-tin oxide (ITO) remains far from optimal. This is because the ITO surface is electrochemically heterogeneous due to the presence of indium oxide; nucleation and further electrodeposition of CdS does not transpire on the oxided sites. Hence, only coarse-grained coatings, instead of homogeneous ultrathin films, are generated at un-pretreated ITO surfaces. In the present study, a mitigation of the amount of interfacial In oxide was attempted in order to increase the nucleation-site (indium-metal site) density. The procedure consisted of two steps: (i) Mild electrochemical reduction of the ITO to convert surface In(III) to In(0), followed by (ii) surface-limited redox replacement (SLRR) of In(0) by Cu via an aqueous solution of Cu2+. This procedure resulted in the formation of a high density of oxide-free Cu on which CdS nuclei would form; the thickness was such that optical transparency was largely undiminished. A ten-fold increase in CdS site density was observed, and that permitted the epitaxial growth of a second semiconductor, CdTe, atop the CdS film. The influences of applied potential and deposition time on nucleation-site sizes and densities were also studied.