Synergistically Enhanced Thermoelectric Performance of Cu₂SnSe₃-Based Composites via Ag Doping Balance

As an ecofriendly and low-cost thermoelectric material, Cu₂SnSe₃ has recently drawn much attention. In this work, the thermoelectric properties of Cu₂SnSe₃-based materials have been synergistically optimized. Ag doping at the Cu site enables strong phonon scattering via large strain field fluctuations and increases the effective mass of carriers through band engineering, which results in a reduced lattice thermal conductivity and enhanced Seebeck coefficient. Thus, a peak ZT value of 1.04 at 800 K is obtained for Cu_1.85Ag_0.15SnSe₃. Then, In is further doped at the Sn site to further increase the carrier concentration and power factor; the ZT values of Cu_1.85Ag_0.15Sn_1-InSe₃ samples are highly improved in the temperature range of 300-800 K, and a peak ZT value of 1.12 is obtained at 800 K for the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃ sample. Considering that Ag₂S decomposes to Ag and S completely under vacuum and high current field, the sulfur vapor would be pumped away by a vacuum pump, and the generated Ag not only enriches at the grain boundary of the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃ bulk material but also enters the matrix and occupies the Cu site, leading to the extruding Cu and Se forming the second phase of CuSe nanoparticles. Thus, the power factor greatly improves to 13.8 μW cm^-1 K^-2 at 700 K, and the lattice thermal conductivity is as low as 0.12 W m^-1 K^-1 at 800 K for the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/4% Ag₂S composite. Finally, a high ZT value of 1.58 is obtained at 800 K for the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/4% Ag₂S composite, which is nearly an increase of 204% compared to that of Cu₂SnSe₃. This work provides an effective solution to optimize the conflicting material properties for Cu₂SnSe₃-based thermoelectric materials.