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Effects of chalcogen composition on the thermoelectric properties in Cu2ZnSn(S1−xSex)4 single crystals

The thermoelectric properties of high-quality p-type kesterite Cu2ZnSn(S1−xSex)4 single crystals grown by the traveling heater method were investigated by alloying with S and Se in varying compositions. These materials exhibited low thermal conductivity because of lattice scattering that was caused by the Cu/Zn disordered structure, and high conductivity because of high carrier concentration from changes to the composition. Cu-poor composition is effective to improve the conductivity because of high density of shallow acceptor Cu vacancy. The figure of merit ZT for Cu-poor Cu2ZnSnSe4 single crystal reached 0.3 at 400 K, which was 4 times higher than that of Cu-rich Cu2ZnSnSe4 polycrystal. We discuss the improvement of ZT by the variation of chalcogen composition from aspects of thermoelectric and electrical properties, comprehensively.