This work systematically investigated the thermoelectric properties of p-type Na and M (M = K, Li, Ag) codoped polycrystalline SnSe. It is found that the electrical properties of polycrystalline SnSe can be improved significantly for (Na, Ag) codoped samples, contributed by the enhanced carrier concentration. Specifically, a carrier concentration of 6.23 × 1019 cm-3 was obtained in Sn0.98Na0.016Ag0.004Se sample at 335 K, an increase of 18% compared with that of the Na single-doped sample (5.22 × 1019 cm-3). The power factor reached ∼0.73 mW m-1 K-2 for the Sn0.98Na0.016Ag0.004Se sample at 785 K, enhanced by ∼26% compared with Na single-doped one. In addition, Sn-rich and Ag-rich particles/areas observed in the matrix of Sn0.98Na0.016Ag0.004Se contribute to the reduction of lattice thermal conductivity from 0.61 W m-1 K-1 for Sn0.98Ag0.02Se to 0.47 W m-1 K-1 at 785 K. The combination of simultaneously enhanced power factor and depressed thermal conductivity leads to a maximum ZT ≈ 1.2 at 785 K and a high average ZT ≈ 0.74 at 335-785 K for Sn0.98Na0.016Ag0.004Se, and generating a high theoretical conversion efficiency of ∼11%. These illuminating discoveries could provide routes to enhance the thermoelectric performance in p-type polycrystalline SnSe.
Keywords: Sn/Ag-rich particles; SnSe; codoping; conversion afficiency; thermoelectric performance.