Thermoelectric performance of n-type polycrystalline SnSe with surface depletion by pressureless sintering

Highlights: • Polycrystalline SnSe samples were prepared by pressureless sintering. • Depending on the sintering time, the SnSe samples show the p-n transition. • Impurities from oxidation and the deficiency of Se result in the p-n transition. • Demonstration of high n-type thermopower of SnSe by inducing Se defects. In this study, polycrystalline SnSe samples were prepared by a series of mechanical alloying (MA), uniaxial compaction, and subsequently pressureless sintering. It was found that the grains in the resultant SnSe samples grow along the (1 1 1) plane direction, resulting in forming a layered structure. Depending on the sintering time, the SnSe samples show the conversion of p-type conduction with the Seebeck coefficient value of +503 μV/K to n-type conduction with the Seebeck coefficient value of −1500 μV/K. The samples were characterized using X-ray diffraction and Raman spectroscopy to reveal that the transition of the Seebeck properties is due to the imbalanced composition of SnSe caused by impurities (SnSe₂, SnSe_1-x) formed during pressureless sintering. Also, the experiments and characterization results demonstrated a possibility to enhance thermoelectric properties if the pressureless sintering conditions would be well-controlled.