Competing influence of Cd doping on thermoelectric properties of Sb2Te3 alloys between carrier generation and phonon scattering

Abstract

Sb2Te3-based alloys are known for their promising thermoelectric transport properties in the mid-temperature range above 550 K. In this study, we systematically investigated the effects of Cd doping on Sb2Te3 by synthesizing CdxSb2-xTe3 alloys with doping levels up to x = 0.04. Substitution of Cd2+ at Sb3+ lattice sites introduced additional holes, leading to a marked increase in carrier concentration and, consequently, electrical conductivity. However, this increase was accompanied by a reduction in the Seebeck coefficient, resulting in a significant decrease in the power factor from 3.7 mW/mK2 to 1.0 mW/mK2 for x = 0.04 with increasing Cd content x. Nevertheless, the lattice thermal conductivity was substantially reduced upon Cd doping, primarily due to enhanced phonon scattering arising from the size and mass difference between Cd2+ and Sb3+ from 1.4 W/mK to 0.41 W/mK (x = 0.04). Despite the lowered lattice contribution, the total thermal conductivity increased from 3.0 W/mK to 3.8 W/mK with doping of x = 0.04, driven by the large increase of electrical conductivity to 4900 S/cm. As a result, the thermoelectric figure of merit (zT) was decreased by Cd doping due to decrease in power factor and increase of total thermal conductivity. According to Boltzmann transport calculations based on the single parabolic band model, it was revealed that the increase carrier concentration was responsible for the lower power factor and higher total thermal conductivity, which resulted in the lower zT.