Monolayer square-Ag2X (X = S, Se): Excellent n-type thermoelectric materials with high power factors

Abstract

Thermoelectric devices based on thermoelectric materials, which can be utilized to transform heat to electric energy, are regarded as the clean and reliable energy-harvestings that reduce greenhouse gas and noxious gas missions. In this work, thermoelectric properties of square-Ag2X (X = S, Se) monolayers have been systematically studied through uniting Boltzmann transport theory and first-principles. The results show that square-Ag2X (X = S, Se) monolayers possess ultrahigh electrical conductivities due to their ultralight electron effective masses, together with high Seebeck coefficients, resulting in ultrahigh power factors (0.014 and 0.018 W/m K2). Moreover, phonon transport calculations unveil that square-Ag2X (X = S, Se) monolayers exhibit low lattice thermal conductivities (3.46 and 2.33 W/m K) at 300 K, which can be traced back to their small Debye temperature, low acoustic group velocities, short phonon relaxation times and large Grüneisen parameters. Accordingly, the obtained figure of merit of monolayer square-Ag2Se for optimal n-type doping at 700 K can approach 2.81, which is larger than the well-known n-type thermoelectric monolayer SnSe (2.32) and Mg3Sb2 (2.21). Therefore, the excellent thermoelectric properties of square-Ag2Se monolayer along with its all-round stability verified by the phonon dispersion, ab initio molecular dynamic simulation and accordance of Born–Huang criteria highlight its prominent potential for thermoelectric applications.