Effect of secondary phases on thermoelectric properties of Cu2SnSe3

Cu2SnSe3 samples with different secondary phases are prepared from compositions of CuxSnSe3 (x=1.8-2.2) and Cu2SnySe3 (y=0.8-1.2), and their thermoelectric properties are investigated. For CuxSnSe3 samples, the secondary phase is SnSe2 for x=1.8 and 1.9 and SnSe for x=2.2, and nearly single-phase Cu2SnSe3 is obtained for x=2.0 and 2.1. For Cu2SnySe3 samples, the secondary phase is CuSe for y=0.8 and SnSe for x=1.1 and 1.2, and single-phase Cu2SnSe3 is produced for y=0.9 and 1.0. The lattice structure of synthesized Cu2SnSe3 depends on the x and y values, which is cubic for x≤2.0 or y≤1.0 and monoclinic for x>2.0 or y>1.0. Compared with the single-phase Cu2SnSe3 sample (ZT=0.42 at 773 K), the samples with SnSe2 secondary phase show a greatly-enhanced ZT of 0.84 at 773 K, which is likely the best result for un-doped Cu2SnSe3 up to now. The samples with SnSe secondary phase exhibit poor thermoelectric properties.