Thermoelectric performance of materials with Cu$Ch_4$ ($Ch=$ S, Se) tetrahedra: Similarities and differences among their low-dimensional electronic structure from first principles

In this study, we perform a comparative theoretical study on the thermoelectric performance of materials with Cu$Ch_4$ ($Ch=$ S, Se) tetrahedra, including famous thermoelectric materials BiCuSeO and tetrahedrite Cu$_{12}$Sb$_4$S$_{13}$, by means of first-principles calculations. By comparing these electronic band structures, we find that many of these materials possess a Cu-$t_{2g}$ band structure consisting of quasi-one-dimensional band dispersions and the isotropic (two-dimensional for layered compounds) band dispersion near the valence-band edge. Therefore, the key factors for the thermoelectric performance are the anisotropy of the former band dispersion and the degeneracy of these two kinds of band dispersions. We also find that a large extension of the chalcogen orbitals often improves their thermoelectric performance by improving these two factors or by going beyond such a basic band structure through a large alternation of its shape. Such a large extension of the chalcogen orbitals might partially originate from the anisotropic Cu-$Ch$ bond geometry of a tetrahedron. Our study reveals interesting similarities and differences of materials with Cu$Ch_4$, which provides important knowledge for a future search of high-performance thermoelectric materials.