Grain boundary decoration by electroless plating Ag enhances the thermoelectric performance of polycrystalline SnSe

Tin selenide (SnSe) is a rising-start thermoelectric compound that holds record-high ZT values for bulk thermoelectrics. Yet, the performance of polycrystalline SnSe is significantly lower than that of monocrystalline SnSe due to the presence of grain boundaries (GBs). Thus, tailoring the microstructures and properties at GBs provides a route to tuning the overall thermoelectric performance. In this work, different GB phases associated with different properties are introduced in polycrystalline SnSe through chemical plating Ag. Two different phases, i.e., AgSnSe₂ and Ag₈SnSe₆ are formed at the GB by controlling the content of Ag. While AgSnSe₂ dominates with the Ag content below 0.08 wt%, Ag₈SnSe₆ is more prominent in the sample with 0.1 wt% Ag. Both phases improve the electrical conductivity and power factor of the material due to the reduced GB resistance. However, the thermal conductivity is also increased with the presence of AgSnSe₂. In contrast, the formation of Ag₈SnSe₆ further lowers the thermal conductivity of polycrystalline SnSe owing to the intrinsically ultralow thermal conductivity of Ag₈SnSe₆. As a result, a more significant enhancement of ZT is achieved with the presence of Ag₈SnSe₆ at GBs. Our work provides a facile way to tune the transport properties at GBs by controlling the chemical plating process, which can be easily adapted to other polycrystalline materials.