Enhancing thermoelectric performance of K-doped polycrystalline SnSe through band engineering tuning and hydrogen reduction

High-efficient and environment-friendly thermoelectric materials have attracted much attention. The single crystal tin selenide (SnSe) has a record high thermoelectric figure of merit ( ZT ) of 2.6 at ~800 K. Polycrystalline SnSe has better mechanical-strength but inferior ZT values than the single crystal, attribute to the paradoxically higher thermal conductivity ( κ ). In this work, we synthesized K x Sn 1−x Se (x = 0, 0.01, 0.02, 0.03 and 0.04) by doping K 2 CO 3 . A large number of point defects and dislocations generated with CO 2 volatilization, which possibly reduces κ . Based on this process, the K x Sn 1−x Se powders were followed by the hydrogen reduction with 4% H 2 − 96% Ar atmosphere to reduce tin oxide and potassium oxide which have high κ . These strategies lead to an ultra-low κ 0.32 Wm −1 K −1 in K 0.03 Sn 0.97 Se at 798 K. The effects of K doping on electronic transport performance of SnSe were studied by density functional theory calculation and experimental measurement. Doing K can narrow the band gap, increase hole concentration and enhance the electrical conductivity ( σ ) of SnSe. At 298 K, the σ of K x Sn 1−x Se increased by more than 2.5 times compared with the pristine SnSe. In high temperature region (623 K<  T  < 823 K), the σ is mainly affected by thermal excitation. High doping level of K (x = 0.03 and 0.04) suppresses the thermal excitation and limits the maximum of ZT . As the result, A high ZT of 1.06 is obtained at 798 K from K 0.01 Sn 0.99 Se perpendicular to the pressing direction, which is 103.8% larger than that of pristine SnSe. In addition, the hot-pressing polycrystalline K x Sn 1−x Se remains obvious lamellar morphology and show anisotropy. The σ and κ of K x Sn 1−x Se samples perpendicular to the pressure direction are much higher than those of the samples parallel to the pressure direction. This anisotropy decreases with the increasing K content.