Doping Achieves High Thermoelectric Performance in SnS: A First-Principles Study

Among the thermoelectrics discovered in the past few decades, SnS stands out as a promising candidate for its inexpensive, earth-abundant, and environment-friendly merits. However, with emerging research on optimizing the thermoelectric performance of SnS, there are not many theoretical studies giving explicit analysis about the underlying mechanism of charge and heat transport in the system. In this work, we find an abnormal optical-phonon-dominated κ_L in SnS with heat-carrying optical phonons showing higher group velocity than acoustic phonons. These high-velocity phonon modes are contributed by "antiphase" movements in the adjacent Sn-S sublayers. Meanwhile, we calculate the electrical properties with a nonempirical carrier lifetime and discover that the optical phonon also plays an essential role in the charge transport process, limiting the carrier mobility dominantly. Our calculation results suggest that p-type SnS can achieve a maximal ZT of 1.68 at 850 K and a hole concentration of 5.5 × 10¹⁹ cm^-3 even without band engineering. We further investigate 11 possible dopants and screen out 4 candidates (Na, K, Tl, and Ag) that effectively boost the hole concentration in SnS. Defect calculations reveal that Na is the best dopant for SnS, while we also suggest K and Tl as potential candidates, for they can also help SnS achieve its optimal hole concentration. To ensure that each dopant reaches its best doping effect, we suggest that doped SnS samples be synthesized under sulfur-excess circumstances and the synthesis temperature be higher than 1353 K. Our findings provide insight into the charge and heat transport process of SnS and pave the way for the rational design of high-performance SnS-based thermoelectric materials.