Enhanced thermoelectric properties of AgBi3S5 through grain boundary and band engineering via Te alloying and Cd doping

AgBi3S5-based thermoelectric materials, characterized by their unique monoclinic crystal structure, exhibit intrinsically low lattice thermal conductivity. The presence of earth-friendly elements further enhances their viability for practical thermoelectric applications. Nevertheless, the electrical transport properties and thermal conductivity remain suboptimal, necessitating further refinement to achieve superior performance. This study demonstrates a synergistic enhancement in the thermoelectric performance of AgBi3S5 through simultaneous reduction of lattice thermal conductivity and optimization of carrier concentration and mobility. Te alloying introduces a high density of grain boundaries and dislocations, which significantly enhances phonons scattering and thereby effectively suppresses the lattice thermal conductivity. Additionally, Cd doping effectively increases the electron carrier concentration, driving the Fermi level into the conduction band. Multiple conduction bands contribute to electron transport due to band convergence, thereby significantly enhancing carrier mobility. Through the synergistic optimization of carrier concentration and mobility, both the electrical conductivity and power factor of AgBi3S5 are substantially improved. As a result, a maximum ZT value of ∼0.44 is achieved at 773 K in Ag0.95Cd0.05Bi3(S0.97Te0.03)5, representing an impressive 83% improvement compared to pristine AgBi3S5. This work underscores the effectiveness of combined compositional engineering strategies in advancing the thermoelectric performance of polycrystalline AgBi3S5 and provides valuable guidance for optimizing other emerging thermoelectric systems.