Tl2XY (X, Y = S, Se) monolayers with low lattice thermal conductivity and high thermoelectric performance by full-band approach with four scattering mechanisms

Abstract The low lattice thermal conductivity and high thermoelectric performance of the Janus Tl 2 SSe monolayer in the temperature region of 300–700 K are identified based on the thermoelectric properties of the Tl 2 S 2 , Tl 2 SSe, and Tl 2 Se 2 monolayers. The transport coefficients for carrier concentrations and temperatures are obtained by solving the linearized Boltzmann transport equation in a full-band electronic structure. Four scattering mechanisms of acoustic deformation potential, optical deformation potential, polar optical phonon, and ionized impurity scatterings are considered. The ionized impurity scattering is recognized as the most important. The lattice thermal conductivity of the Janus Tl 2 SSe monolayer is substantially smaller than those of the Tl 2 S 2 , Tl 2 Se 2 monolayers with higher symmetry. Moreover, we find that the Janus structures of the Tl 2 SSe monolayer increase the dielectronic constants and enhance the polar optical phonon scattering, then reduce the power factor to some extent. Therefore, the lattice thermal conductivity actually couples with the transport coefficient and cannot be individually regulated as is usually assumed. However, the ZT value of the Tl 2 SSe monolayer can still reach 1.77 at 700 K even if the intrinsic concentration and the bipolar effect are included. Therefore, the Tl 2 SSe monolayer is expected to be a promising candidate for thermoelectric materials.