First-principles study of anisotropic thermoelectric transport properties of IV-VI semiconductor compounds SnSe and SnS

Tin selenide (SnSe) and tin sulfide (SnS) have recently attracted particular interest due to their great potential for large-scale thermoelectric applications. A complete prediction of the thermoelectric performance and the understanding of underlying heat and charge transport details are the key to further improvement of their thermoelectric efficiency. We conduct comprehensive investigations of both thermal and electrical transport properties of SnSe and SnS using first-principles calculations combined with the Boltzmann transport theory. Due to the distinct layered lattice structure, SnSe and SnS exhibit similarly anisotropic thermal and electrical behaviors. The cross-plane lattice thermal conductivity kappa(L) is 40-60% lower than the in-plane values. Extremely low kappa(L) is found for both materials because of high anharmonicity, while the average kappa(L) of SnS is similar to 8% higher than that of SnSe from 300 to 750 K. It is suggested that nanostructuring would be difficult to further decrease kappa(L) because of the short mean free paths of dominant phonon modes (1-30 nm at 300 K), while alloying would be efficient in reducing kappa(L) considering that the relative kappa(L) contribution (similar to 65%) of optical phonons is remarkably large. On the electrical side, the anisotropic electrical conductivities are mainly due to the different effective masses of holes and electrons along the a, b, and c axes. This leads to the highest optimal ZT values along the b axis and lowest ones along the a axis in both p-type materials. However, the n-type ones exhibit the highest ZTs along the a axis due to the enhancement of power factor when the chemical potential gradually approaches the secondary conduction band valley that causes significant increase in electron mobility and density of states. Owing to the larger mobility and smaller kappa(L) along the given direction, SnSe exhibits larger optimal ZTs compared with SnS in both p-and n-type materials. For both materials, the peak ZTs of n-type materials are much higher than those of p-type ones along the same direction. The predicted highest ZT values at 750 K are 1.0 in SnSe and 0.6 in SnS along the b axis for the p-type doping, while those for the n-type doping reach 2.7 in SnSe and 1.5 in SnS along the a axis, rendering them among the best bulk thermoelectric materials for large-scale applications. Our calculations show reasonable agreements with the experimental results and quantitatively predict the great potential in further enhancing the thermoelectric performance of SnSe and SnS, especially for the n-type materials.