Structural evolution-driven enhancement of thermoelectric performance in Bi–S–Se solid solutions

Bismuth sulfide (Bi₂S₃) has garnered extensive attention for thermoelectric (TE) applications due to its earth abundance, non-toxicity, and ultralow lattice thermal conductivity. However, simultaneously improving its electrical and thermal properties remains challenging due to the strong coupling between these parameters. Herein, we propose a structural evolution strategy to synergistically optimize the electrical and thermal transport properties of Bi₂S₃-based TE compounds. By leveraging the distinct crystal structures of Bi₂S₃ and Bi₂Se₃, alloying Se at S sites successfully reconstructs the carrier transport channels within the Bi₂S₃ matrix, thereby facilitating carrier mobility. Additionally, the weakened chemical bonding leads to a significant increase in carrier concentration, approaching the optimum range. Furthermore, the structural evolution from particle-like to lamellar grains, coupled with the large mass difference between Se and S, induces lattice distortion that effectively reduces lattice thermal conductivity. Combining the dramatically enhanced power factor and ultralow thermal conductivity, an optimized figure of merit (<em>ZT</em>) of 0.38 at 623 K is achieved in the Bi₂SSe₂ solid solution. This structural evolution strategy offers a promising avenue for enhancing the TE performance of binary compounds.