Thermoelectric performance in Ag2Se nanocomposites: The role of interstitial Ag and Pb orbital hybridization

Abstract

Ag₂Se has emerged as a promising thermoelectric (TE) material for room-temperature applications. However, its TE performance is limited by the low carrier effective mass (m*) of only 0.1 m₀, where m₀ represents the free electron mass. In this study, we employ a microwave-assisted method to synthesize nanostructured Ag_2-xSe with Pb doping that is found to increase m* to 0.4 m₀. Accordingly, Seebeck coefficient is significantly enhanced, which together with the maintained high electrical conductivity, leads to enhanced electronic transport. The increase in m* is systematically investigated by density functional theory calculations and linked to the enhanced electronic by modeling simulations. The calculated band structures reveal that the hybridization of heavy Pb-6p orbitals flattens the conduction band edges, and thereby enhances m*. Furthermore, Pb doping significantly reduces lattice thermal conductivity due to the high-density point defects, dislocations, and grain boundaries, as revealed by detailed electron microscopy characterizations. The synergy from both enhanced electronic transport and reduced phonon propagation yielded a maximum figure of merit of 1.04 at 376 K, and an average figure of merit of 1.0 for Pb-doped Ag_1.9Se. The optimized TE performance is further validated in a flexible TE generator, which produced a maximum output power of 0.6 μW at a temperature difference of 45 K. These findings demonstrate that enhancing m* and increasing phonon-scattering using vacancy tuning and aliovalent doping effectively boosts the TE performance of Ag₂Se, a strategy that can be extended to other TE materials to maximize their potentials for power generation and thermoelectric cooling applications.