Compensation of increased carrier concentration and thermal conductivity in enhancing thermoelectric efficiency in Sn-doped Sb-In-Te alloys

Sb₂Te₃-based alloys exhibit promising thermoelectric transport properties in the mid-temperature range, and In doping can suppress the lattice thermal conductivity while maintaining favorable electrical performance. In this study, Sn was systematically doped into Sb_1.85In_0.15Te₃ up to 2% at the Sb site (Sn_xSb_1.85−xIn_0.15Te₃, x = 0–0.04) to examine its effect on the crystal structure and interrelated electrical and thermal transport properties. X-ray photoelectron spectroscopy measurements showed that the concurrent substitution of Sn²⁺ and Sn⁴⁺ for Sb³⁺ and Sb⁵⁺ sites in Sb_1.85In_0.15Te₃ effectively introduced holes, thereby increasing the carrier concentration and electrical conductivity. Although the Seebeck coefficient decreased with doping, the increase in the effective mass of the density of states significantly mitigated this decrease, thereby contributing to an overall enhancement in the power factor. The total thermal conductivity increased with Sn doping because of the enhanced electrical conductivity, while the lattice thermal conductivity gradually decreased because of the increased phonon scattering arising from Sn doping. Consequently, the enhancement of the thermoelectric figure of merit (zT) was only observed for the low-doped sample (x = 0.01), which peaked at 0.84 at 600 K. For the highly doped samples (x = 0.03 and 0.04), the enhancement of zT was limited owing to the increase in the total thermal conductivity. Based on further single parabolic band model analysis, the increase in carrier concentration induced by Sn doping was observed to eventually limit the further enhancement of zT, because of the compensation for the increasing carrier concentration and the resulting increase in the total thermal conductivity.