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

Abstract

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 KxSn1−xSe (x = 0, 0.01, 0.02, 0.03 and 0.04) by doping K2CO3. A large number of point defects and dislocations generated with CO2 volatilization, which possibly reduces κ. Based on this process, the KxSn1−xSe powders were followed by the hydrogen reduction with 4% H2 − 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−1K−1 in K0.03Sn0.97Se 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 KxSn1−xSe 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 K0.01Sn0.99Se perpendicular to the pressing direction, which is 103.8% larger than that of pristine SnSe. In addition, the hot-pressing polycrystalline KxSn1−xSe remains obvious lamellar morphology and show anisotropy. The σ and κ of KxSn1−xSe 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.