Largely Optimized Thermoelectric Performance in (Ag, S) Coalloyed Cu₂Te Thermoelectric Materials

Cu₂Te-based compounds are expected to exhibit superior thermoelectric performance compared to Cu₂S and Cu₂Se due to the larger atomic mass and more covalent bonding of Te atoms, such as chalcogenide selenide and chalcogenide lead. However, the currently reported figure of merit (ZT) for Cu₂Te is still lower than that of Cu₂S and Cu₂Se. In this work, the thermoelectric transport properties are studied and optimized by two-step coalloying of Cu₂Te-based materials. An improved average power factor of 7.21 μW cm^-1 K^-2 is obtained between room temperature and 773 K because of the largely optimized carrier concentration and Seebeck coefficient. Additionally, the total thermal conductivity decreases to 0.55 W m^-1 K^-1 for Ag_0.7Cu_1.3Te_0.94S_0.06 W m^-1 K^-1 at room temperature, and the lowest thermal conductivity of 0.49 W m^-1 K^-1 is obtained in Ag_0.7Cu_1.3Te_0.92S_0.08 at 523 K because of the tuned carrier concentration and increased entropy. Combined with the simultaneous optimization of electrical transport properties and thermal transport properties, (Ag, S) coalloyed Cu₂Te shows a maximum ZT of 1.4 at 773 K, which is 460% higher than that of Cu₂Te at the corresponding temperature.