High ZT >1.4 Realized in Bi ₂ Se ₂ S via Amorphous–Crystalline Coexistence Structure of Secondary Phase and Bilayer Rotation

Abstract TeCl 4 doping induces an amorphous–crystalline coexistence and bilayer rotation in layered Bi 2 Se 2 S, synergistically optimizing electrical and thermal transport. The unique structure shifts the CBM/VBM, converting Bi 2 Se 2 S from a direct to an indirect semiconductor and narrowing the bandgap from 1.07 to 0.88 eV. Band‐edge flattening increases carrier effective mass, while Te‐5p states enhance conduction‐band contribution, raise the Seebeck coefficient, and tune the Fermi level to strengthen n type behavior. A power factor of 800 µW·m −1 ·K −2 is achieved at 773 K. Periodic moiré fringes and lattice distortion intensify mid to high frequency phonon scattering, lowering lattice thermal conductivity to 0.27 W·m −1 ·K −1 . Consequently, ZT reaches 1.44 at 773 K with an average ZT of 0.78 over 423–773 K. We also demonstrate, for the first time, a single leg n type thermoelectric module based on this material, delivering 2.3% efficiency under a 300 K temperature difference. This work establishes an effective route to simultaneously achieve low κ and high electrical performance via amorphous–crystalline coexistence and bilayer rotation, advancing Bi 2 Se 2 S toward mid temperature thermoelectric applications.