High Thermoelectric Performance in n-Type Polycrystalline SnSe via Dual Incorporation of Cl and PbSe and Dense Nanostructures

Despite extensive studies on emerging thermoelectric material SnSe, its n-type form is largely underdeveloped mainly due to the difficulty in stabilizing the carrier concentration at the optimal level. Here, we dually introduce Cl and PbSe to induce n-type conduction in intrinsic p-type SnSe. PbSe alloying enhances the power factor and suppresses lattice thermal conductivity at the same time, giving a highest thermoelectric figure of merit ZT of 1.2 at 823 K for n-type polycrystalline SnSe materials. The best composition is Sn0.90Pb0.15Se0.95Cl0.05. Samples prepared by the solid-state reaction show a high maximum ZT (ZT(max)) similar to 1.1 and similar to 0.8 parallel and perpendicular to the press direction of spark plasma sintering, respectively. Remarkably, post-ball milling and annealing processes considerably reduce structural anisotropy, thereby leading to a ZT(max)similar to 1.2 along both the directions. Hence, the direction giving a ZT(max) is controllable for this system using the specialized preparation methods for specimens. Spherical aberration-corrected scanning transmission electron microscopic analyses reveal the presence of heavily dense edge dislocations and strain fields, not observed in the p-type counterparts, which contribute to decreasing lattice thermal conductivity. Our theoretical calculations employing a Callaway-Debye model support the experimental results for thermal transport and microscopic structures.