Coupled effects of relativistic interactions and defect chemistry on thermoelectric and optical properties

The combined influence of spin-orbit coupling (SOC) and dopant-induced electronic modification is critical for optimising thermoelectric materials based on Bi₂Te₃. Here, we employ HSE06 hybrid functional calculations to investigate pristine and Cu-doped Bi₂Te₃, focusing on structural stability, electronic structure, and transport properties. Cu incorporation induces a slight lattice expansion (∼1.2%) while maintaining dynamical, thermal, and mechanical stability. SOC is found to play a decisive role in shaping the electronic structure, reducing the band gap to ∼0.12-0.18 eV and driving band reordering near the Fermi level. Cu doping preserves the narrow-gap semiconducting character but alters band-edge dispersion and redistributes electronic states, thereby enhancing carrier transport. Consequently, the Seebeck coefficient increases (up to ∼230 µV K^-1) with only a modest decrease in electrical conductivity, yielding an improved power factor. Optical analysis shows enhanced absorption upon doping, while SOC refines spectral features through band splitting. These results demonstrate that the interplay between SOC and Cu doping offers an effective means to tune the electronic structure and improve thermoelectric performance in Bi₂Te₃-based materials.