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

Recently, ternary Cu₂-IV-Se₃ (IV = Sb, Ge, Sn) compounds have received intensive attention for their thermoelectric potential. Among them, Cu₂GeSe₃ has been less studied due to its low doping efficiency and poor electrical performance compared to Cu₂SnSe₃. In this study, the thermoelectric properties of Cu₂GeSe_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 Cu₂GeSe_3.03 increases by 270% compared to the pristine sample at 723 K. By further alloying Ag on the Cu site in Cu₂GeSe_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_1.85Ag_0.15GeSe_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 Cu₂-IV-Se₃ compounds.