High Thermoelectric Performance in Chalcopyrite Cu_{1- x}Ag_xGaTe₂-ZnTe: Nontrivial Band Structure and Dynamic Doping Effect

The understanding of thermoelectric properties of ternary I-III-VI₂type (I = Cu, Ag; III = Ga, In; and VI = Te) chalcopyrites is less well developed. Although their thermal transport properties are relatively well studied, the relationship between the electronic band structure and charge transport properties of chalcopyrites has been rarely discussed. In this study, we reveal the unusual electronic band structure and the dynamic doping effect that could underpin the promising thermoelectric properties of Cu_1-xAg_xGaTe₂compounds. Density functional theory (DFT) calculations and electronic transport measurements suggest that the Cu_1-xAg_xGaTe₂compounds possess an unusual non-parabolic band structure, which is important for obtaining a high Seebeck coefficient. Moreover, a mid-gap impurity level was also observed in Cu_1-xAg_xGaTe₂, which leads to a strong temperature-dependent carrier concentration and is able to regulate the carrier density at the optimized value for a wide temperature region and thus is beneficial to obtaining the high power factor and high average ZT of Cu_1-xAg_xGaTe₂compounds. We also demonstrate a great improvement in the thermoelectric performance of Cu_1-xAg_xGaTe₂by introducing Cu vacancies and ZnTe alloying. The Cu vacancies are effective in increasing the hole density and the electrical conductivity, while ZnTe alloying reduces the thermal conductivity. As a result, a maximum ZT of 1.43 at 850 K and a record-high average ZT of 0.81 for the Cu_0.68Ag_0.3GaTe₂-0.5%ZnTe compound are achieved.