Investigation on degraded thermoelectric performance of SnSe₂ polycrystalline alloys by Bi doping

SnSe₂ layered alloys can be considered promising thermoelectric materials with enhancement in their electrical transport properties. Several studies have shown that electron doping via anion doping could effectively enhance the thermoelectric properties of SnSe₂ polycrystalline alloys. In this study, the influence of Bi doping, which is cation doping, on the electric and thermoelectric transport properties of SnSe₂ polycrystalline alloys was investigated systematically by synthesizing a series of Sn_1−xBi_xSe₂ (x = 0.02, 0.04, 0.06, and 0.08) compositions. It was shown that Bi ion in SnSe₂ exhibited an oxidation state of + 3 by X-ray photoelectron spectroscopy and Bi behaved as an acceptor in the Sn⁴⁺ site of SnSe₂, thereby greatly decreasing the electron concentration of SnSe₂ from 2.2 × 10¹⁹ to 2.3 × 10¹⁵ cm⁻³ at x = 0.02. Therefore, a significant decrease in electrical conductivity was observed, and the Seebeck coefficient was also decreased owing to the severe decrease in the density of state effective mass, resulting in severe power factor decrease by the Bi doping. Total and lattice thermal conductivity did not show decrease with the Bi doping. Consequently, the maximum thermoelectric figure of merit zT for the doped samples decreased to ~ 0.01 from 0.10 for pristine SnSe₂. Additionally, analysis of the thermoelectric performance was performed based on single parabolic band model, suggesting that increasing carrier concentration to ~ 10²⁰ cm⁻³ could maximize the zT value of SnSe₂.