Realizing tremendous electrical transport properties of polycrystalline SnSe₂ by Cl-doped and anisotropy

SnSe₂ is regarded as an attractive thermoelectric material for its structural and chemical analogy to SnSe that is claimed with the highest ZT in single crystal. In this study, the pure and Cl-doped SnSe₂ polycrystals (3%, 6%, 9% and 12% molar Cl content) were fabricated in four steps that are hydrothermal synthesis, heating purification, diffusion doping, and spark plasma sintering. The phase structure, lamellar morphology and crystallite orientation were studied for the synthesized SnSe₂ powder and the sintered pellets. The structural evolution was traced from the SnSe₂ hexagonal plates of powders to the (001) oriented grains in pellets. The Cl doping into SnSe₂ was verified by phase composition, lattice parameter, element distribution, and chemical valance. The doped Cl increased both the carrier concentration and the mobility. The anisotropic thermoelectric properties of SnSe₂ bulk materials were investigated as functions of temperature from 50 °C to 300 °C and the doping amount, respectively. The Seebeck coefficient was less anisotropic than the electrical and thermal conduction. The grain orientation influenced the anisotropy of the electrical and thermal conductivity at a similar ratio. The power factors were less dependent on temperature with an optimum in-plane 1.06 mW m⁻¹ K⁻² and out-of-plane 0.41 mW m⁻¹ K⁻². The highest ZTs of 0.3 were attained at 300 °C in both directions.