Enhancing the thermoelectric performance of Sb1.85In0.15Te3 alloys by effective mass increase via in situ ZnTe formation and resultant Te-excess by Zn addition

Sb₂Te₃-based alloys exhibit decent thermoelectric transport properties in mid-temperature range above 550 K, with In-doped Sb₂Te₃ compositions reported to exhibit the higher performance. Doping is known to enhance the thermoelectric efficiency at elevated temperatures by constraining bipolar conduction via bandgap enlargement. In this study, the thermoelectric properties of Sb_1.85In_0.15Te₃ polycrystalline alloys are enhanced by Zn addition, which provides an alternative approach. The in situ formation of ZnTe inclusions during the cooling of nominal Sb_1.85−xIn_0.15Zn_xTe₃ compositions (x = 0, 0.01, 0.02, 0.03, and 0.04) in conventional solid-state reaction induces an excess of Te, which suppresses the intrinsic Te vacancies in Sb_1.85In_0.15Te₃ and thus increases the density-of-state effective mass. Consequently, the power factor increases evenly in the measured temperature range of 300–650 K, reaching 1.17 mW/mK² at 600 K for x = 0.04, representing 19 % improvement over that of the pristine sample. Furthermore, the excess Te and ZnTe inclusions serve as zero- and three-dimensional phonon scattering centers, respectively, effectively reducing lattice thermal conductivity (κ_latt) by effectively scattering both high- and low-frequency phonons. At 300 K, κ_latt of the x = 0.04 sample is 15 % lower than that of the pristine sample, resulting in an enhanced thermoelectric figure of merit, zT, of 0.75 at 600 K, which is 25 % higher than that of pristine Sb_1.85In_0.15Te₃. Since the effective mass increase and lattice thermal conductivity reduction are independent of the known bandgap widening strategies of doping, further enhancement of the thermoelectric efficiency of Sb_1.85In_0.15Te₃ at elevated temperatures can be anticipated through further combined doping.