Bi2X (X = Ge, Sn) monolayers: Promising thermoelectric materials with ultra-low thermal conductivity

Thermoelectric materials can realize the direct conversion between heat and electricity, which provides a new strategy for waste heat treatment and power generation. The dimensionless value ZT determines the efficiency of the device. Here, we report a record ZT maximum of about 6.96 at 700 K by performing first-principles calculations on penta-Bi₂X (X = Ge, Sn) monolayers. We verified the stability of all systems and calculated the thermoelectric transport properties. The multiband energy degeneracy leads to large Seebeck coefficients, while the small phonon group velocity and strong anharmonic phonon scattering result in ultra-low thermal conductivity. Thus, in both systems, the higher ZT values are found. The high performance of Bi₂Ge and Bi₂Sn monolayers provide a new guidance for finding layered thermoelectric materials.