Effect of Br substitution on thermoelectric transport properties in layered SnSe2

Abstract

The effect of Br substitution on enhancing the thermoelectric properties of n-type layered SnSe2 is investigated characterizing SnSe2-xBrx synthesized with various Br contents (x = 0, 0.004, 0.008, 0.01, and 0.02). The electrical conductivity systematically increases with an increase in x owing to the simultaneous increase in both the carrier concentration (n) and mobility. As x increases, n increases by ~100 times; however, the magnitude of the Seebeck coefficient (|S|) decreases from 600 to 229 μV/K, resulting in an enhanced power factor at room temperature. The density functional theory calculations support that the valley degeneracy at the conduction band minimum increases with the Br substitution in SnSe2, while the conduction band minimum becomes flatter. This finding indicates that the increased density-of-state effective mass leads to the relatively large |S| even under a considerably increased n. Combined with the slight decrease in the thermal conductivity with an increase in x without significant additional phonon scattering, a thermoelectric figure of merit (zT) of 0.62 at 750 K is obtained for SnSe1.98Br0.02 (x = 0.02), which is 50% higher than that for non-doped SnSe2.