Thermoelectric properties of Janus BiXI (X = S and Se) monolayers: A first-principles study

Janus monolayers, a novel class of two-dimensional materials, have attracted significant interest owing to their asymmetric atomic arrangement. In this work, we systematically investigate the thermoelectric properties of Janus BiXI (X = S and Se) monolayers, using first-principles calculations and semiclassical Boltzmann transport theory. These dynamically stable BiSI and BiSeI monolayers exhibit indirect bandgaps of 0.870 and 0.797 eV, respectively, when accounting for the spin–orbit coupling effects. The Janus BiSeI monolayer exhibits a lower lattice thermal conductivity of 0.168 W/mK at 300 K, attributed to the increased phonon-scattering channels and enhanced anharmonicity introduced by the heavier Se atom. Furthermore, the Janus BiSeI monolayer demonstrates a superior Seebeck coefficient and high electrical conductivity, resulting in a significantly enhanced power factor. Consequently, the Janus BiSeI monolayer exhibits a higher figure of merit (ZT) value, with 0.895 at 300 K and 2.466 at 700 K, compared to the Janus BiSI monolayer, which has ZT values of 0.158 at 300 K and 0.591 at 700 K. These findings establish the Janus BiSeI monolayer as an excellent candidate for thermoelectric conversion applications. The understanding may have broader implications for the exploration of 2D thermoelectric materials.