Thermoelectric Properties of Single-Phase n-Type Bi₁₄Te₁₃S₈

Bismuth telluride (Bi₂Te₃) and its alloys are among the best thermoelectric materials at room temperature. Bi₁₄Te₁₃S₈, a material with a similar crystal structure, contains sulfur that can potentially improve the thermoelectric performance through widening the band gap and reducing the lattice thermal conductivity. This compound forms in sulfur-added Bi₂Te₃ alloys. Here, a polycrystalline iodine-doped Bi₁₄Te₁₃S₈ sample is investigated; an optimum iodine concentration of 1 at. % resulted in the power factor of 3.5 mW²·m^-1·K^-1 at room temperature. Iodine doping reduced the lattice thermal conductivity by more than 30% by enhancing the phonon scattering. An improved thermoelectric figure of merit zT of ∼0.29 at 520 K was obtained for 1-1.5 at. % iodine-doped Bi₁₄Te₁₃S₈. First-principles calculations indicate that Bi₁₄Te₁₃S₈ has a larger band gap compared to bismuth telluride, which allows for a reduction in the bipolar effect; however, a lower effective mass reduced the thermopower for a similar carrier concentration. This study demonstrates that tuned iodine doping can effectively optimize the thermoelectric performance of Bi₁₄Te₁₃S₈, highlighting its contribution in multiphase sulfur-alloyed Bi₂Te₃-based materials.