First-principles predictions of enhanced thermoelectric properties for Cs2SnI2Cl2 and Cs2PbI2Cl2 monolayers with spin-orbit coupling.

Inspired by the exceptional charge transport properties and ultra-low thermal conductivity of halide perovskite, we investigate the electronic nature, thermal transport, and thermoelectric properties for Ruddlesden-Popper (RP) all-inorganic perovskite, Cs2SnI2Cl2 and Cs2PbI2Cl2 monolayers, using first-principles calculations. During the calculations, spin-orbit coupling has been considered for electronic transport as well as thermoelectric properties. The results show that the Cs2SnI2Cl2 and Cs2PbI2Cl2 monolayers exhibit high carrier mobility and low thermal conductivity. Stronger phonon-phonon interaction is responsible for Cs2SnI2Cl2 monolayer, and results in the fact that thermal conductivity of Cs2SnI2Cl2 monolayer is much lower than that of Cs2PbI2Cl2 monolayer. At 700 K, the values of the figure of merit (ZT) for the n-type doped Cs2SnI2Cl2 and Cs2PbI2Cl2 monolayers are about 1.05 and 0.32 at the optimized carrier concentrations 5.42 × 1012 cm-2 and 9.84 × 1012 cm-2. Moreover, when the spin-orbit coupling is considered, the corresponding ZT values are enhanced to 2.73 and 1.98 at 5.27 × 1011 cm-2 and 6.16 × 1011 cm-2. These results signify that Cs2SnI2Cl2 and Cs2PbI2Cl2 monolayers are promising thermoelectric candidates.