Liquid-Solid Interface Reactions Drive Enhanced Thermoelectric Performance in Ag₂Se

Ag₂Se is a promising n-type thermoelectric material, but its performance is limited by excessive carrier concentration, compositional inhomogeneity, and phase instability, challenges rooted in a narrow homogeneity range and uncontrolled Ag⁺ diffusion in the superionic phase. Here, we address these issues by exploiting liquid-solid interface reactions using CdSe complexes that remove surface excess Ag to yield stoichiometric Ag₂Se and generate CdSe nanodomains that inhibit Ag⁺ diffusion and constrain grain growth. The resulting Ag₂Se-CdSe nanocomposites exhibit a reproducible, stable figure of merit (zT) of 1.04 between 300 and 390 K. Beyond demonstrating high performance, we elucidate the interfacial chemical reactions that give rise to the observed microstructure and transport properties, providing a foundation for rationally engineering interfacial chemistry to tailor transport properties across diverse thermoelectric material systems.