Enhanced Thermoelectric Properties of Cu₂SnSe₃-Based Materials with Ag₂Se Addition

In this work, (Ag, In)-co-doped Cu₂SnSe₃-based compounds are prepared using a self-propagating high-temperature synthesis process. Ag₂Se and as-synthesized (Ag, In)-co-doped Cu₂SnSe₃-based powders are mixed in a proportion according to the formula of Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/x Ag₂Se (x = 0, 3, 4, and 5%), which is followed by a subsequent plasma-activated sintering (PAS) to obtain consolidated composite bulks. A sandwich experiment is designed to reveal the evolution of the microstructure and phase composition of the composite samples during the PAS process. We investigate the reaction mechanism between Ag₂Se and Cu₂SnSe₃-based matrix as well as the influence of Ag₂Se on the phase composition, microstructure, and thermoelectric transport properties of the composites. Ag₂Se addition is proven to be effective to improve Ag solubility in the Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃ matrix and introduce a CuSe secondary phase and an Ag-rich phase at grain boundaries. The electrical conductivity of Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/x Ag₂Se (x = 0, 3, 4, and 5%) composites decreases while the Seebeck coefficient increases with increasing Ag₂Se addition, resulting in an optimized power factor. Moreover, benefiting from the collective phonon scattering at various defects induced by Ag₂Se addition, the composite samples exhibit significantly suppressed lattice thermal conductivity, which reaches as low as 0.11 W m^-1 K^-1 at 700 K for the x = 5% sample. A peak figure-of-merit (ZT) of 1.26 at 750 K and an average ZT of 0.75 at 300-800 K are obtained for Cu_1.85Ag_0.15Sn_0.91In_0.09Se₃/5% Ag₂Se. This work provides an efficient way to improve average ZT values of Cu₂SnSe₃-based compounds for promising power generation at intermediate temperatures.