High thermoelectric figure of merit ZT > 1 in SnS polycrystals

Eco-friendly tin sulfide (SnS) has attracted increasing attention in the thermoelectric community because of its elemental abundance and analogous crystal structure to SnSe as a new thermoelectric material. However, so far no high dimensionless thermoelectric figure of merit ZT > 1 was reported in SnS polycrystals. This work found an effective strategy for enhancing the thermoelectric performance of p-type polycrystalline SnS by Ag doping and vacancy engineering, leading to three orders of magnitude increase in carrier concentration and optimized effective mass and carrier mobility. As a result of the enhanced electrical conductivity, three times higher power factor ∼3.85 μW/cm K2 at 877 K is realized in Sn0.995Ag0.005S sample. Interestingly, nanostructuring with Ag nano-precipitates were formed in the Ag-doped SnS sample. Moreover, with introducing Sn vacancies in the crystal structure of Sn0.995-vacAg0.005S, the power factor further enhanced to ∼4.25 μW/cm K2. In addition to the low-frequency phonons scattering by Ag nano-precipitates, dislocations strengthens the scattering of mid-frequency phonon, leading to an ultralow lattice thermal conductivity < 0.5 W/m K above 800 K and a record high ZT up to 1.1 at 877 K in Sn0.99Ag0.005S polycrystals.