Modeling thermoelectric properties of monolayer and bilayer WS2 by including intravalley and intervalley scattering mechanisms

Herein, we investigate the impact of all electron and phonon scattering mechanisms on the electrical and thermal transport properties of the monolayer and bilayer transition-metal dichalcogenide WS2. We used the Boltzmann transport equation under the relaxation-time approximation to calculate both the electron and phonon transport properties. Due to multiple valleys near the Fermi energy level, intervalley scattering is seen as the prominent scattering mechanism that critically impacts the electrical transport properties in both of these materials. The power factor reduces by 93% and 83% for monolayer and bilayer WS2, respectively, due to intervalley scattering leading to almost equal values for both materials at room temperature. Earlier theoretical reports on monolayer WS2 overestimated the experimentally observed lattice thermal conductivity values. Experimental observations suggest that when monolayer and bilayer WS2 is formed, defects might be present in these systems, affecting phonon transport. We found that defect scattering significantly contributes to phonon scattering when the relative sulfur vacancy concentration exceeds 0.01 for both monolayer and bilayer WS2, resulting in a remarkable 99% agreement with experimental values. A high ZT similar to 3 is attained for monolayer WS2 as compared to ZT similar to 2 for bilayer WS2 at 800 K because of the higher thermal conductivity of the bilayer due to elevated group velocities of its optical-phonon modes. This work presents a deep insight into the scattering mechanisms controlling the thermoelectric performance of monolayer and bilayer WS2.