Grain size optimization for high-performance polycrystalline SnSe thermoelectrics

Abstract

To identify the origin of the performance discrepancy between the single crystal and polycrystalline SnSe, here we report the first attempt to investigate the thermoelectric properties of polycrystalline Sn_0.97Na_0.03Se directly made from Sn_0.97Na_0.03Se single crystals. It is found that the oxidization during the powder processing or after SPS greatly decreases the carrier concentration, and the increased oxidized/amorphous grain boundaries reduce the carrier mobility, leading to an order of magnitude reduction in the electrical conductivity compared to that of the single crystals. For the SnSe polycrystals synthesized in this study, a relatively high electrical conductivity for the ball milled sample with an average grain size around 4 μm is well preserved in the whole measured temperature range because of the favorable grain size as well as the grain-boundary potential, yielding a high average zT value of 0.44 perpendicular to the pressing direction.