Synergistic Optimization of Mobility and Effective Mass by Sn Alloying Leads to Enhanced Thermoelectric Performance in Cu₂GeSe₃

Cu-based materials, renowned for their environmentally friendly characteristics, have become highly promising candidates for thermoelectric applications because of their excellent thermoelectric properties. However, their intrinsically low carrier mobility restricts further enhancement of thermoelectric performance. In this work, Sn alloying not only enhanced the carrier mobility of Cu₂GeSe₃ but also increased its effective mass, leading to markedly improved thermoelectric properties. The incorporation of Sn effectively increases the formation energy of Cu_Ge defects and V_Cu vacancies, thereby substantially increasing carrier mobility. Moreover, the greater difference in electronegativity of the Sn-alloyed material results in an increased effective mass (m*), which contributes to a remarkable improvement in the Seebeck coefficient. Consequently, these two effects result in a notable increase in PF, reaching 10.22 μW·cm⁻¹·K⁻² at 723 K, representing an increase of approximately 80% compared with that of pristine Cu₂GeSe₃. Additionally, Sn alloying introduces point defect scattering that further decreases the lattice thermal conductivity (κ_lat). Combined with the enhanced power factor (PF), this reduction yields a maximum ZT of ~0.66 at 723 K. Our findings highlight Sn alloying as an effective approach for enhancing thermoelectric performance, providing a rational pathway for developing high-performance copper-based thermoelectric materials.