Extraordinary n ‐type Tellurium‐Free Thermoelectric Performance of Y‐Doped BiSe via Synergistic Effect of Vacancies and Band Engineering

Bismuth selenide (BiSe) is an n ‐type thermoelectrics with promising room‐temperature potential as a tellurium‐free alternative to Bi 2 Te 3 ‐based materials. It features ultralow lattice thermal conductivity ( κ lat ), but suffers from excessively high carrier concentration, yielding a low Seebeck coefficient. This remains a fundamental challenge in BiSe. Here, we introduce yttrium (Y) doping in Sb‐alloyed BiSe (Bi 0.7 Sb 0.3 Se), inducing substantial Bi vacancies to reduce carrier concentration. Experimental characterization combined with theoretical calculations demonstrates that Y preferentially substitutes Bi at the Bi 2 bilayer sites, which opens the bandgap and flattens the valence/conduction bands, thereby enhancing the band‐edge density of states (DOS) effective mass and increasing the Seebeck coefficient. This enhances power factor across a broad temperature range. Defect regulation and multiscale nanopores further amplify phonon scattering, reducing κ lat by 50% at 393 K. Consequently, Bi 0.64 Sb 0.3 Y 0.06 Se achieves a peak zT of 0.91 at 393 K and average zT of 0.76 (300 – 523 K), representing significant progress within the emerging BiSe‐based material. Furthermore, an 8‐pair BiSe‐based module achieves a conversion efficiency of 4.6% and a normalized power density of 10.8 µWcm −2 K −2 , which is expected to pave the way for BiSe‐based materials to become a realistic n ‐type thermoelectric material for sustainable applications.