Effects of ZnSO4 concentration on the high-rate deposition of CBD-Zn(O,S) buffer layer for Cu(In,Ga)Se2 thin-film solar cells

Abstract

To utilize eco-friendly materials in Cu(In,Ga)Se2 (CIGS) thin-film solar cells, a Zn-based buffer layer has been adopted to replace the conventional CdS buffer layer. Buffer layers can be grown using a variety of techniques, with chemical bath deposition (CBD) generally used for the preparation of the Zn(O,S) buffer layer. The growth rate of the CBD-Zn(O,S) buffer layer is determined by the dissociation rate of S during chemical reaction between Zn and S ions. Thiourea (TU) is a conventional precursor of S, but it suffers from poor dissociation rate of S, leading to prolonged layer deposition times. In this study, we replaced TU with thioacetamide (TAA), thereby drastically shortening the deposition time from 80 min (TU) to 7 min (TAA). Furthermore, incorporating Zn with S ions is known to be difficult owing to the high-speed release of S ions from TAA. However, by adjusting the molar concentration of ZnSO4, we discovered a process to effectively form the Zn–S bonds for the CBD-Zn(O,S) buffer layer. The efficiency of the CIGS thin-film solar cell shows the best performance at a ZnSO4 concentration of 0.05 M, which rivals the performance of the TU base process. The physical and chemical properties of CBD-Zn(O,S) grown at a high deposition rate using TAA were examined as a function of ZnSO4 molar concentration. We proposed a growth model that accounts for the changes in film thickness and surface morphology evolution, both of which depend on the ZnSO4 concentration. Additionally, to investigate the p–n junction properties in terms of device operation, we elucidated the band alignment at the CIGS/CBD-Zn(O,S) interface, concentrating on the interfacial barrier height. It is empirically demonstrated that a lower ΔEC value improves the solar cell operation.