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 Sn0.03Sb1.97Te3-ySey (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 Sn0.03Sb1.97Te3 increased the density-of-states effective mass (md*) and reduced lattice thermal conductivity (κlatt) 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 (nH) to 1.46 × 1020 cm-3, thereby limiting further enhancement of the thermoelectric figure of merit (zT). Se alloying was introduced to reduce nH by suppressing the native antisite defects. With Se alloying, nH gradually decreased, as expected; however, a concurrent slight reduction in md* 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 nH 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 Sn0.03Sb1.97Te3. 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 md* owing to mobility degradation.