Beyond effective mass increase: Optimal range of carrier concentration for thermoelectric efficiency enhancement in Pb-doped Sb_1.85In_0.15Te₃ alloys

Sb₂Te₃-based alloys have excellent thermoelectric transport properties in the medium-temperature range of 500–700 K, and In-doped Sb₂Te₃ compositions are widely recognized as having high thermoelectric-transport efficiencies. This study investigated the thermoelectric properties of systematically Pb-doped Sb_1.85In_0.15Te₃ (Sb_1.85−xPb_xIn_0.15Te₃, where x = 0, 0.01, 0.02, 0.03, 0.04, or 0.05). It was found that Pb²⁺ substitution at Sb³⁺ sites generated holes very effectively; thus, significantly large increases in the carrier concentration and electrical conductivity were observed. Meanwhile, the Seebeck coefficient decreased moderately owing to a large increase in the density-of-state effective mass, resulting in an increase in the power factor, especially for temperatures over 500 K. The total thermal conductivity increased with the doping as a result of a large increase in electrical conductivity, while the lattice thermal conductivity gradually decreased with an increase in doping owing to the additional point defect scattering. Consequently, a high maximum zT of 0.87 at 600 K was achieved for the Sb_1.84Pb_0.01In_0.15Te₃ (x = 0.01) composition, representing a 45% increase compared with that of pristine Sb_1.85In_0.15Te₃, while a decrease in zT was seen for x ≥ 0.02 at temperatures lower than 500 K, even though the thermoelectric quality factor increased for all of the Pb-doped compositions. Further analysis using a single parabolic band model demonstrated that the significant increase in carrier concentration constrained any possible further increase in zT by deoptimizing the power factor and total thermal conductivity for compositions where x ≥ 0.02