Enhanced Thermoelectric Properties of Sb_1.85In_0.15Te₃Thermoelectric Alloys via Large Effective Mass Increase Induced by Cd Doping

Sb2Te3-based alloys exhibit robust thermoelectric transport properties in the midtemperature range above 550 K, and In-doped Sb2Te3 compositions are well-known to exhibits the highest efficiency. In this study, Cd was further doped systematically to Sb1.85In0.15Te3 up to 2.5% at the Sb site. It was shown that the Cd2+ substitution for Sb3+ site generates holes very effectively and thus significantly increases the carrier concentration and electrical conductivity. Meanwhile, the Seebeck coefficient decreases rather moderately owing to large increase in density-of-state effective mass, resulting in increase of power factor especially for the temperature higher than 500 K. Total thermal conductivity increases with the doping originated from large increase of electrical conductivity, while the lattice thermal conductivity is gradually reduced with the doping owing to the additional phonon scattering. Consequently, a maximum zT of 1.03 at 650 K is achieved, representing a 69% increase compared with that of pristine Sb1.85In0.15Te3. In addition, the expected zT for various carrier concentrations is calculated by using a single parabolic band model. It was found that the increase of zT is limited by the largely increased carrier concentration, and the zT could be further enhanced by maintaining the carrier concentration, which can be achieved by further doping of electrons.