Thermoelectric Ag₂Se: Imperfection, Homogeneity, and Reproducibility

Ag₂Se is a narrow band gap n-type semiconductor with high carrier mobility and low lattice thermal conductivity. It has high thermoelectric performance near room temperature. However, there is a noticeable data discrepancy for thermoelectric performance in the reported literature studies, which greatly hinders the rational understanding and potential application of this material. In this work, we comprehensively studied the homogeneity, reproducibility, and thermal stability of bulk Ag₂Se prepared by melting and mechanical alloying methods followed by spark plasma sintering. By virtue of the atom probe topology technique, we revealed nanosized Ag- or Se-rich precipitates and micropores with Se-aggregated interfaces that have not been detected previously. The samples prepared by melting and spark plasma sintering exhibit the best homogeneity and repeatability in thermoelectric properties despite abundant nanoprecipitates. Moreover, the thermoelectric performance of Ag₂Se is greatly improved by introducing a slight amount of excess selenium. The average zT can steadily reach 0.8-0.9 in the range of 300-380 K, which is among the highest values reported for Ag₂Se-based materials. This work will rationalize the evaluation of the thermoelectric performance of Ag₂Se.