High thermoelectric performance of monolayer Janus Zn<i>AX</i>Te (A = Ge, Sn; X = S, Se) induced by large band degeneracy and low lattice thermal conductivity

Thermoelectric conversion is a crucial approach to addressing waste heat utilization and energy challenges in the 21st century. Enhancing the thermoelectric figure of merit (ZT) has become a central focus in the research of thermoelectric materials and devices. In this study, using first-principles calculations combined with the Boltzmann transport method, we systematically investigate the thermoelectric properties of monolayer Janus materials ZnAXTe (A= Ge, Sn; X= S, Se). The lowest conduction band of these materials is found to consist of multiple nearly degenerate valleys, resulting in a power factor significantly higher than that of the widely studied thermoelectric material SnSe. Furthermore, the strong interaction between low-frequency optical phonons and acoustic phonons leads to relatively low lattice thermal conductivity. The synergy of a high power factor and low lattice thermal conductivity enables these materials to achieve remarkably high ZT values. For instance, the ZT values of n-type ZnSnSTe and ZnSnSeTe reach 3.07 and 3.14 at 800 K, respectively, demonstrating excellent high-temperature thermoelectric performance. This study highlights their potential in thermoelectric devices and provides valuable theoretical guidance for the design and development of high-performance thermoelectric materials.