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

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 is found that the capping layer effectively suppresses the Ge loss and significantly enhances the film crystallinity by a sufficient annealing. A wide range tuning of bandgap is realized by adjusting the Ge and Se compositions simultaneously, especially the optimum composition to obtain 1.3 eV is 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 is 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.