The effect of ZnCl2 and CdCl2 treatment on ZnS/CdS junction partner on CdTe cell performance

Abstract

In this study, the impacts of ZnCl2 and CdCl2 treatments on the structural and optical properties of ZnS/CdS bilayers and on the parameters of CdTe solar cells with ZnS/CdS junction partners were investigated. CdS and ZnS thin films were grown by chemical bath deposition. In the as deposited ZnS/CdS sample, hexagonal CdS and Zn(S,O) phases were formed. After the application of CdCl2 treatment to the sample, ZnO and CdZnS alloys also appeared. In the ZnCl2 treated bilayer, it was observed that the crystal structure of the CdZnS alloy changed from hexagonal to cubic phase. While similar grain structure was observed in the as deposited and the ZnCl2 treated samples; it was seen that CdCl2 treatment significantly affected the grain form and size. CdCl2 treatment resulted in a large increase of Cd-ratio and a more balanced increase Zn-ratio in ZnCl2 treatment. It was determined that ZnCl2 and CdCl2 treatments caused a decrease in the transmittance of the samples. PL spectroscopy revealed the presence of many structural defects such as interstitial zinc, sulfur vacancies, surface states, cadmium vacancies in all bilayer samples. Solar cells with ZnS/CdS, ZnS/CdS (CdCl2-treated) and ZnS/CdS (ZnCl2-treated) junction partners achieved efficiencies of 4.56%, 5.64% and 5.20%, respectively. Solar cell parameters showed that ZnCl2 treatment increased the FF value, while CdCl2 treatment improved the Voc value. An efficiency of 6.01% was obtained from the ZnS/CdS (CdCl2-treated)/CdTe/CdCl2 cell produced by obtaining the highest efficiency with the ZnS/CdS/CdCl2 junction partner. This cell revealed that applying CdCl2 treatment on CdTe significantly increased Voc and Jsc while deteriorating FF.