Grain boundary scattering effects on mobilities in p-type polycrystalline SnSe
The extremely high ZTs of p-type SnSe single crystals have attracted much attention. However, due to the high cost of preparation, SnSe single crystals are difficult to be commercialized. On the other hand, the biggest challenge facing more cost-effective polycrystalline SnSe samples are their inferior electronic properties compared to single crystals. It has been proposed that the crystal orientation is responsible for the difference between the electronic properties of polycrystalline and single crystalline SnSe. To explore the role of the crystal orientation, we synthesized textured pure and Ag-doped polycrystalline SnSe and found that the electronic properties of our most highly oriented polycrystalline SnSe are still not higher than single crystals of SnSe oriented along the a-axis (the least favorable orientation). In this study, we compared the temperature-dependent mobility of Ag-doped polycrystalline samples with Ag-doped single crystals of SnSe. We found that grain boundary scattering is the dominant scattering mechanism in polycrystalline SnSe, and this mechanism is substantially absent in single crystals of SnSe. We conclude that grain boundary scattering, and not an averaging effect of the random grain distribution, is the major reason for the poor performance of polycrystalline SnSe compared to single crystals. Based on our results, improving the thermoelectric performance of polycrystalline SnSe will require identifying a synthesis process that minimizes grain boundary scattering.