Ultralow Thermal Conductivity in Diamondoid Structures and High Thermoelectric Performance in (Cu_1-Ag)(In_1-Ga)Te₂

Owing to the diversity of composition and excellent transport properties, the ternary I-III-VI₂ type diamond-like chalcopyrite compounds are attractive functional semiconductors, including as thermoelectric materials. In this family, CuInTe₂ and CuGaTe₂ are well investigated and achieve maximum ZT values of ∼1.4 at 950 K and an average ZT of 0.43. However, both compounds have poor electrical conductivity at low temperature, resulting in low ZT below 450 K. In this work, we have greatly improved the thermoelectric performance in the quinary diamondoid compound (Cu_0.8Ag_0.2)(In_0.2Ga_0.8)Te₂ by understanding and controlling the effects of different constituent elements on the thermoelectric transport properties. Our combined theoretical and experimental effort indicates that Ga in the In site of the lattice decreases the carrier effective mass and improves the electrical conductivity and power factor of Cu_0.8Ag_0.2In_1-GaTe₂. Furthermore, Ag in the Cu site strongly suppresses the heat transport via the enhanced acoustic phonon-optical phonon coupling effects, leading to the ultralow thermal conductivity of ∼0.49 W m^-1 K^-1 at 850 K in Cu_0.8Ag_0.2In_0.2Ga_0.8Te₂. Defect formation energy calculations suggest intrinsic Cu vacancies introduce defect levels that are important to the temperature-dependent hole density and electrical conductivity. Therefore, we introduced extra Cu vacancies to optimize the hole carrier density and improve the power factor of Cu_0.8Ag_0.2In_0.2Ga_0.8Te₂. As a result, a maximum ZT of ∼1.5 at 850 K and an average ZT of 0.78 in the temperature range of 400-850 K are obtained, which is among the highest in the diamond-like compound family.