Influence of defects on the thermoelectricity in SnSe: A comprehensive theoretical study

SnSe has emerged as an efficient and fascinating thermoelectric material. A fundamental understanding of the effects and nature of intrinsic defects and dopants in SnSe is crucial to optimize its thermoelectric performance. In this paper, we perform first-principles calculations to examine the native and extrinsic point-defect properties in SnSe. We show that the easy formation of acceptorlike Sn vacancy (${\mathrm{V}}_{\mathrm{Sn}}$) is responsible for the $p$-type conductivity in intrinsic SnSe. We also propose a mechanism and explain the anomalous temperature dependence of the carrier concentration in intrinsic SnSe crystals. Concerning the extrinsic defects, we focus on the dopants used in experiments. We find that Na (Ag) substitution on Sn site, ${\mathrm{Na}}_{\mathrm{Sn}}$ (${\mathrm{Ag}}_{\mathrm{Sn}}$), acts as acceptor, whereas, substitutional ${\mathrm{Br}}_{\text{Se}}, {\mathrm{I}}_{\text{Se}}$, and ${\mathrm{Bi}}_{\text{Sn}}$ dopants act as donor. It is shown that for Ag doping, its carrier concentration will be saturated with increasing doping concentration due to the coexistence of compensated defects (${\mathrm{Ag}}_{\mathrm{i}}$ and ${\mathrm{Ag}}_{\text{Sn}}$). Furthermore, we analyze how this doping introduced carrier impact on their thermoelectric characteristics. It is found that the more efficient doping of Na, Br, and I can realize higher $ZT$.