High Thermoelectric Performance in Chalcopyrite Cu1-xAgxGaTe2-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 welldeveloped. Although their thermal transport properties are relatively wellstudied, the relationship between the electronic band structure and chargetransport properties of chalcopyrites has been rarely discussed. In this study, wereveal the unusual electronic band structure and the dynamic doping effect thatcould underpin the promising thermoelectric properties of Cu1-xAgxGaTe2compounds. Density functional theory (DFT) calculations and electronictransport measurements suggest that the Cu1-xAgxGaTe2compounds possess anunusual non-parabolic band structure, which is important for obtaining a highSeebeck coefficient. Moreover, a mid-gap impurity level was also observed inCu1-xAgxGaTe2, which leads to a strong temperature-dependent carrier concentration and is able to regulate the carrier density atthe optimized value for a wide temperature region and thus is beneficial to obtaining the high power factor and high averageZTofCu1-xAgxGaTe2compounds. We also demonstrate a great improvement in the thermoelectric performance of Cu1-xAgxGaTe2byintroducing Cu vacancies and ZnTe alloying. The Cu vacancies are effective in increasing the hole density and the electricalconductivity, while ZnTe alloying reduces the thermal conductivity. As a result, a maximumZTof 1.43 at 850 K and a record-highaverageZTof 0.81 for the Cu0.68Ag0.3GaTe2-0.5%ZnTe compound are achieved