Chemical bonding, conductive network, and thermoelectric performance of the ternary semiconductors Cu2SnX3 (X = Se, S) from first principles

The p-type Cu2SnX3 (X = Se, S) compounds are known experimentally to be good thermoelectric materials, although the reasons for this good performance in an adamantine-derived crystal structure are not well understood. Here, we demonstrate the existence of a three-dimensional (3D) hole conductive network in these ternary diamondlike Cu2SnX3 (X = Se, S) semiconductors using ab initio calculations, and identify the features of the electronic structure responsible for this good performance. We also provide results as a function of doping level to find the regime where the highest performance will be realized and estimate the maximum figure of merit, ZT. Our results clearly show that the strong hybridization between 3d orbitals from copper and p orbitals from selenium or sulfur at the upper valence band leads to the 3D p-type hole transport channel, mainly consisting of Cu-X and X-X networks in Cu2SnX3 (X = Se, S). The resulting heavy, but still conductive, hybridized bands of Cu d-chalcogen p character are highly favorable for thermoelectric performance. The electrical transport properties of these p-type materials are mainly determined by these bands and have been investigated by Boltzmann transport methods. The optimal doping levels of Cu2SnX3 are estimated to be around 0.1 holes per unit cell at 700 K. The theoretical figure of merit ZT has been predicted.