Effect of pressure on the thermoelectric performance of monolayer Janus MoSSe materials with different native vacancy defects

The presence of vacancy defects in two-dimensional (2D) materials can substantially influence their thermoelectric performance. In this study, we employed first-principles methods combined with the non-equilibrium Green's functional formalisms (NEGF-DFT) to reveal the impact of pressure on the thermoelectric performance of monolayer Janus MoSSe without and with vacancy defects (V_S and V_Se). The application of pressure can enhance the thermoelectric figure of merit (ZT) of a material without vacancy defects by significantly increasing the power factor (PF), and the ZT can increase by nearly twice at room temperature (300 K). Then, varying pressures exert different influences on the ZT of materials with the two types of vacancy defects. The ZT of the material with V_S vacancy defects at room temperature gradually decreases with increasing pressure. This is because pressure not only reduces the PF but also enhances the total thermal conductivity of the material. Lower pressure (0-5 GPa) will lead to a certain increase in the ZT of the material with V_Se vacancy defects, while higher pressure (7.5 GPa) will reduce the ZT. The primary factor contributing to the enhancement of the ZT value is the enhancement of PF, whereas the reduction stems from an increase in total thermal conductivity. Our results reveal the relationship between the thermoelectric performance and pressure of monolayer Janus MoSSe without and with vacancy defects.