Physics of bandgap formation in Cu-Sb-Se based novel thermoelectrics: The role of Sb valency and Cu d levels

In this paper we discuss the results of ab initio electronic structure calculations for Cu ₃SbSe ₄ (Se4) and Cu ₃SbSe ₃ (Se3), two narrow bandgap semiconductors of thermoelectric interest. We find that Sb is trivalent in both the compounds, in contrast to a simple nominal valence (ionic) picture which suggests that Sb should be 5+ in Se4. The gap formation in Se4 is quite subtle, with hybridization between Sb 5s and the neighboring Se 4s, 4p orbitals, position of Cu d states, and non-local exchange interaction, each playing significant roles. Thermopower calculations show that Se4 is a better p-type system. Our theoretical results for Se4 agree very well with recent experimental results obtained by Skoug etal (2011 Sci. Adv. Mater. 3 602).