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$zT$-factor enhancement in SnSe: predictions from first principles calculations

The electronic structure and thermoelectric properties of SnSe are studied by first-principles methods. The inclusion of van der Waals dispersive corrections improves the agreement of structural parameters with experiments. The bands structure and projected density of states justify the macroscopic anisotropy exhibited by this system. An original methodology is used to estimate the chemical potential and the relaxation time for the electrical and thermal conductivities. Following this methodology, the Seebeck coefficient and thermal conductivity for single crystals and polycrystals are described in good agreement with experimental data. As for the electrical conductivity, values calculated with a temperature-dependent relaxation time compare well with available measurements, especially for single crystals, polycrystals are better described by a constant relaxation time. Finally, the figure of merit of SnSe single crystals and polycrystals is calculated. It is found to exhibit a maximum for some "ideal" carrier concentration, and might be noticeably enhanced by using carrier concentrations higher than the experimental ones. From these findings, possible strategies to increase the figure of merit in practise are suggested.