Secondary phases and their influence on the composition of the kesterite phase in CZTS and CZTSe thin films

Secondary phases zinc sulfide selenide and copper sulfide in Cu2ZnSnS4 CZTS and Cu2ZnSnSe4 CZTSe thin film samples are investigated by X ray absorption near edge structure XANES analysis at the chalcogen K edges. Because of the formation of secondary phases the composition of the kesterite phase can deviate significantly from the total sample composition. For a large set of non stoichiometric samples we find that the cation ratios of the kesterite phase never exceed Zn Sn 1 even for Zn rich CZTS and CZTSe, with all excess Zn being contained in secondary phases. For CZTS the cation ratios are found to be additionally constrained by Cu Sn amp; 8804; 2, which means that Cu excess always leads to the formation of CuxS secondary phases. These results give clear bounds on the Cu rich and Zn rich sides of the single phase region in polycrystalline CZTS Se thin film