High ZT >1.4 Realized in Bi₂Se₂S via Amorphous–Crystalline Coexistence Structure of Secondary Phase and Bilayer Rotation

TeCl₄ doping induces an amorphous–crystalline coexistence and bilayer rotation in layered Bi₂Se₂S, synergistically optimizing electrical and thermal transport. The unique structure shifts the CBM/VBM, converting Bi₂Se₂S 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 Bi₂Se₂S toward mid temperature thermoelectric applications.