Enhanced Band Convergence and Ultra‐Low Thermal Conductivity Lead to High Thermoelectric Performance in SnTe

SnTe, a structural analogue of champion thermoelectric (TE) material PbTe, has recently attracted wide attention for TE energy conversion. Herein, we demonstrate a co-doping strategy to improve the TE performance of SnTe via simultaneous modulation of electronic structure and phonon transport. The electrical transport is optimized by 3 mol % Ag doping in self-compensated SnTe (i.e., Sn_1.03 Te). Further, Mg doping in SnAg_0.03 Te resulted in highly converged valence bands, which enhanced the Seebeck coefficient markedly. The energy gap between two uppermost valence bands (ΔE_v ) decreases to 0.10 eV in Sn_0.92 Ag_0.03 Mg_0.08 Te compared to 0.35 eV in pristine SnTe. The optimized p-type carrier concentration and highly converged valence bands gave a high power factor of ca. 27 μW cm^-1 K^-2 at 865 K in Sn_0.92 Ag_0.03 Mg_0.08 Te. The lattice thermal conductivity of Sn_0.92 Ag_0.03 Mg_0.08 Te reached to an ultra-low value of ≈0.23 W m^-1 K^-1 at 865 K due to the formation of MgTe nanoprecipitates in SnTe matrix. These combined effects resulted in a high TE figure of merit, zT≈1.55 at 865 K in Sn_0.92 Ag_0.03 Mg_0.08 Te.