High Thermoelectric Performance in PbSe-NaSbSe2 Alloys from Valence Band Convergence and Low Thermal Conductivity

PbSe is an attractive thermoelectric material due to its favorable electronic structure, high melting point, and lower cost compared to PbTe. Herein, the hitherto unexplored alloys of PbSe with NaSbSe2 (NaPbmSbSem+2) are described and the most promising p-type PbSe-based thermoelectrics are found among them. Surprisingly, it is observed that below 500 K, NaPbmSbSem+2 exhibits unorthodox semiconducting-like electrical conductivity, despite possessing degenerate carrier densities of approximate to 10(20) cm(-3). It is shown that the peculiar behavior derives from carrier scattering by the grain boundaries. It is further demonstrated that the high solubility of NaSbSe2 in PbSe augments both the thermoelectric properties while maintaining a rock salt structure. Namely, density functional theory calculations and photoemission spectroscopy demonstrate that introduction of NaSbSe2 lowers the energy separation between the L- and sigma-valence bands and enhances the power factors under 700 K. The crystallographic disorder of Na+, Pb2+, and Sb3+ moreover provides exceptionally strong point defect phonon scattering yielding low lattice thermal conductivities of 1-0.55 W m(-1) K-1 between 400 and 873 K without nanostructures. As a consequence, NaPb10SbSe12 achieves maximum ZT approximate to 1.4 near 900 K when optimally doped. More importantly, NaPb10SbSe12 maintains high ZT across a broad temperature range, giving an estimated record ZT(avg) of approximate to 0.64 between 400 and 873 K, a significant improvement over existing p-type PbSe thermoelectrics.