Native Defects-Induced Fermi-Level Pinning and Diffuson-Mediated Thermal Transport in BiSbSe3 Thermoelectrics.

Ultralow thermal conductivity is widely recognized as a core characteristic of promising thermoelectrics, yet many such materials still fail to realize high thermoelectric performance. Te-free BiSbSe3 embodies this contradiction: its intrinsically ultralow thermal conductivity makes it promising for medium-temperature thermoelectric applications, while stable p-type transport remains elusive and the optimization limits of both conduction types remain unclear. Here, we reveal that native defects and Sb containing lone-pairs jointly govern its transport behavior. Electron microscopy analysis and first-principles calculations confirm that Se vacancies and cation-on-Se antisite defects possess low formation enthalpies, stabilizing n-type conduction, compensating holes, and pinning the Fermi level away from the valence band maximum. Moreover, strong near-band-edge Sb-Se hybridization softens Sb-dominated low-frequency optical phonons, promoting acoustic-optical coupling and diffuson-like thermal transport at high temperatures. By matching theory and experiment, we identify carrier mobility degradation at high donor concentrations as the main limitation for n-type BiSbSe3. Our calculations predict that eliminating Fermi-level pinning could enable p-type BiSbSe3 to achieve an excellent average ZT of ∼ 1.4 over 300-800 K. These findings not only clarify the long-standing underperformance of ultralow thermal conductivity thermoelectric compounds, but also establish a universal chemical design framework for optimizing Te-free thermoelectrics.