Optimizing thermoelectric efficiency in Sn-doped Sb₂Te₃ alloys through Se alloying by modulating carrier and phonon transport

Sb₂Te₃-based alloys have attracted considerable attention owing to their favorable thermoelectric transport properties in the mid-temperature range. Herein, the thermoelectric transport properties of pristine Sb₂Te₃ and Sn_0.03Sb_1.97Te_3-ySe_y (y = 0, 0.05, 0.10, 0.15, 0.20, and 0.25) alloys were systematically investigated by combining Sn doping with Se alloying. The 1.5 at. % Sn doping in Sb₂Te₃ as Sn_0.03Sb_1.97Te₃ increased the density-of-states effective mass (m_d*) and reduced lattice thermal conductivity (κ_latt) compared with those of Sb₂Te₃, thereby enhancing the maximum thermoelectric figure of merit to 0.58 from 0.46. However, it also significantly increased the carrier concentration (n_H) to 1.46 × 10²⁰ cm⁻³, thereby limiting further enhancement of the thermoelectric figure of merit (zT). Se alloying was introduced to reduce n_H by suppressing the native antisite defects. With Se alloying, n_H gradually decreased, as expected; however, a concurrent slight reduction in m_d* was observed and, therefore, further increase in power factor was limited. Nevertheless, Se alloying led to a rather large reduction in total thermal conductivity owing to reductions in both electronic and lattice thermal conductivities induced by n_H decrease and additional phonon scattering, respectively. Consequently, the overall zT increased with a small amount of Se alloying up to y = 0.10, with a maximum zT of 0.67 at 650 K, compared with 0.46 of Sb₂Te₃ and 0.59 of Sn_0.03Sb_1.97Te₃. This study demonstrated that the dual doping or further alloying of the doped compositions could be effective, as it leads to a rather large reduction in thermal conductivity despite a potential slight decrease in enhanced m_d* owing to mobility degradation.