Enhancing power factor of SnSe sheet with grain boundary by doping germanium or silicon

Grain boundaries (GBs) widely exist in two-dimensional (2D) and three-dimensional (3D) materials in experiment, which significantly affect the thermoelectric performance because of the scattering effect on the transport of both electron and phonon. Motivated by the research progress in 3D SnSe, we have systematically studied the GBs in a SnSe monolayer including their stable geometric configurations, the effect of GBs on power factor and Seebeck coefficient, and the strategies to improve the performance by using first principles calculations combined with semiclassical Boltzmann theory. We find that the GBs increase the potential energy barrier of carriers and decrease the valley degeneracy of the conducting bands, leading to the reduction of Seebeck coefficient, as compared to that of the pristine SnSe sheet. We further demonstrate that the trapping gap states are effectively eliminated or reduced by doping germanium or silicon, leading to the enhanced electrical conductivity, power factor, and Seebeck coefficient. These findings shed lights on developing practical strategies for modulating the thermoelectric performance of 2D polycrystalline sheets.