Effect of Cu-Zn-Sn capping on the preparation of Cu₂Zn(Sn,Ge)(S,Se)₄ absorber layers from Cu-Zn-Sn-Ge alloy precursors

Abstract This study reports on the fabrication of Cu 2 Zn(Sn,Ge)(S,Se) 4 (CZTGSSe) alloy thin films via sulfo-selenization of Cu-Zn-Sn-Ge alloy precursors in a vacuum-sealed quartz tube. To mitigate Ge re-evaporation during annealing, a Cu-Zn-Sn alloy capping layer was introduced, and its effects were systematically investigated. It was found that the capping layer effectively suppresses the Ge loss and significantly enhances the film crystallinity by sufficient annealing. A wide range tuning of the bandgap was realized by adjusting the Ge and Se compositions simultaneously, especially the optimum composition to obtain 1.3 eV was achieved by tuning the Ge/(Sn+Ge) ratio within 0.25–0.3 and that of the Se/(S+Se) ratio within 0.4–0.7 ranges properly. It was observed that the solar cells employing these absorber layers demonstrate substantial improvements in the open-circuit voltage, short-circuit current, and fill factor. Furthermore, external quantum efficiency spectra indicate the enhancement of carrier transport and p–n junction quality.