Low lattice thermal conductivity and its role in the remarkable thermoelectric performance of newly predicted SiS<inf>2</inf> and SiSe<inf>2</inf> monolayers

For high-efficiency thermoelectric power conversion, not only improvement of existing materials properties but also prediction and synthesis of new thermoelectric materials are needed. Here, we have carried out a systematic investigation on the thermoelectric performance of newly predicted two dimensional (2D) semiconducting SiS2 and SiSe2 monolayers using density functional theory (DFT) and solving the Boltzmann transport equations (BTEs) for electrons and phonons. Our computed value of lattice thermal conductivity (kph) in SiSe2 monolayer is ultralow, which results in a high thermoelectric figure of merit (zT) value of 0.86 (0.83) for p-type (n-type) at 900 K in SiSe2 monolayer. While in SiS2 monolayer, zT value are 0.77 (p-type) and 0.71 (n-type) at 900 K. The values of kph are attributed to low group velocity, strong anharmonicity and phonon–phonon coupling of acoustic and low-frequency optical branches, leading to larger scattering, smaller mean free path, and shorter lifetime of phonons. It is also found that p-type doping is more effective than n-type doping to get optimal power factor (PF) and zT. Our findings suggest that newly predicted semiconducting SiSe2 and SiS2 monolayers can be very promising thermoelectric materials for the fabrication of high-efficiency thermoelectric power generators to convert waste heat into electricity.