SnSe: a remarkable new thermoelectric material
The deceptively simple material SnSe has surprised the scientific community by showing an unexpectedly low thermal conductivity and high power factor and it has become a very promising thermoelectric material. Both the electrical and thermal transport properties of SnSe are outstanding. It is remarkable that a binary compound exhibits strong anharmonic and anisotropic bonding, and after hole doping it shows an exceptionally high power factor because of a high electrical conductivity and a strongly enhanced Seebeck coefficient. The latter is enabled by the contribution of multiple electronic valence bands. In this perspective, we discuss the natural features of SnSe, including crystal structures, electronic band structures, and physical and chemical properties. We also compare the electrical transport properties of single crystals and polycrystalline SnSe. The thermal conductivities of polycrystalline samples show wide variation from laboratory to laboratory, with some values being higher than those of single crystals and some lower, which has caused confusion and controversy. To address the issues regarding the thermal transport properties of SnSe, we systematically summarize the reports for SnSe variants, discuss them along with some of our own new results, and offer possible explanations. Finally, some possible strategies are proposed toward future enhancements of the thermoelectric figure of merit of SnSe.