P-type quaternary chalcogenides of Cu 2 ZnSn(S,Se) 4 nanocrystals: Large-scale synthesis, bandgap engineering and their thermoelectric performances

Cu2ZnSn(S,Se)4 nanocrystals up to 10 grams per batch were acquired by a large-scale hydrothermal synthetic method. The bandgap engineering of Cu2ZnSn(S,Se)4 nanocrystals was achieved by controlling the ratios of sulfur and selenium atoms in the solid solution nanocrystals. The bandgaps of Cu2ZnSn(S,Se)4 nanocrystals were tuned approximately linearly from 1.29, 1.34, 1.39, 1.44 to 1.52 eV with the increase of sulfur and selenium atomic ratios. Finally, the thermoelectric properties of Cu2ZnSn(S,Se)4 nanocrystals with the various ratios of Se/(S+Se) (0, 0.25, 0.5, 0.75, 1) from 300 to 700 K are elaborated in detail, indicating that the thermal conductivity of Cu2ZnSn(S,Se)4 nanocrystals is lower than that of their bulk counterparts and other binary materials. Besides, the Seebeck coefficients and figure of merit for Cu2ZnSn(S,Se)4 nanocrystals (x = 0.25, 0.5, 0.75) is higher than those of Cu2ZnSnS4 and Cu2ZnSnSe4 nanocrystals. It is indicated that transferring from bulk materials to the solid solution nanomaterials is an effective way to further enhance the TE performance of CZTS NCs.