Compositional engineering of chemical bath deposited (Zn,Cd)S buffer layers for electrodeposited CuIn(S,Se)₂ and coevaporated Cu(In, Ga)Se ₂ solar cells

A comparative study of chemical bath deposition (CBD) of ZnS, CdS, and a mixture of(Cd,Zn)S buffer layers has been carried out on electrodeposited CuIn(S,Se)² (CISSe) and coevaporated Cu(In,Ga)Se₂ (CIGS) absorbers. For an optimal bath composition with the ratio of [Zn]/[Cd]=25, efficiencies higher than those obtained with CdS and ZnS recipes, both on co-evaporated CIGS and electrodeposited CISSe, have been obtained independent of the absorber used. In order to better understand the (Cd,Zn)S system and its impact on the increased efficiency of cells, predictions from the solubility diagrams of CdS and ZnS in aqueous medium were made. This analysis was completed by in situ growth studies with varying bath composition by quartz crystal microbalance (QCM). The morphology and composition of the films were studied using scanning electron microscopy (SEM) and X-ray photoelectron spectra (XPS) techniques. Preliminary XPS studies showed that films are composed of a mixture of CdS and Zn(O,OH) phases and not a pure ternary Cd_1-xZn _xS compound. The effect of the [Zn]/[Cd] molar ratio on properties of the corresponding CISSe and CIGS solar cells was investigated by current voltage [J(V)] and capacitance voltage [C(V)] characterizations. The origin of optimal results is discussed.