The charge localization deteriorating the thermoelectric properties: The case of kiddcreekite-type Cu6WSnSe8

In principle, thermoelectric materials with superior performance require high lattice symmetry and heavy components integrated in semiconductors. Cu6WSnSe8, isostructural to kiddcreekite Cu6WSnS8, possesses the characteristics for being a promising thermoelectric material, including narrow band-gap, high crystal symmetry, heavy constituent elements, and complex crystal structure. In this work, the thermoelectric transport properties of pristine and copper-deficient Cu6WSnSe8 were investigated. The extremely low hole mobility restricts Cu6WSnSe8 as an excellent thermoelectric parent despite the fact that Cu-deficiency can increase the electrical conductivity and boost the power factor of Cu5.94WSnSe8 by four times than that of pristine Cu6WSnSe8. Combined with density functional theory calculations and X-ray photoelectron spectra analysis, we conclude that the extremely low hole mobility derives from the charge localization around W cations. Due to charge transfer and redistribution, the high valence state W6+ seizes electrons from Se2−, and thus localizing carriers.