Formation of Dual Vacancies Leads to a High Thermoelectric Performance in Cu₂GeSe_3+x

Recently, ternary Cu 2 -IV-Se 3 (IV = Sb, Ge, Sn) compounds have received intensive attention for their thermoelectric potential. Among them, Cu 2 GeSe 3 has been less studied due to its low doping efficiency and poor electrical performance compared to Cu 2 SnSe 3 . In this study, the thermoelectric properties of Cu 2 GeSe 3+ x samples are investigated. It is found that excess Se leads to a rise in both carrier concentration and mobility due to dual vacancy level excitation and valence band sharpening, resulting in significantly enhanced electrical performance. The power factor of Cu 2 GeSe 3.03 increases by 270% compared to the pristine sample at 723 K. By further alloying Ag on the Cu site in Cu 2 GeSe 3.03, the lattice thermal conductivity is markedly reduced while a relatively high power factor in the high-temperature region is maintained. Finally, the peak ZT of the Cu 1.85 Ag 0.15 GeSe 3.03qaz sample reaches 1.17 at 773 K. These results suggest that creating dual cation vacancies can be an effective route to improve the electrical performance of Cu 2 -IV-Se 3 compounds.