Native Defects‐Induced Fermi‐Level Pinning and Diffuson‐Mediated Thermal Transport in BiSbSe 3 Thermoelectrics

ABSTRACT 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 BiSbSe 3 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 BiSbSe 3 . Our calculations predict that eliminating Fermi‐level pinning could enable p ‐type BiSbSe 3 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.