High thermoelectric performance in (Ga, Yb) Co-doped Bi2S3 bulks achieved by band structure and carrier concentration modulation

Due to their high abundance, low cost, and low toxicity, Bi₂S₃-based thermoelectric materials without Te are significantly promising for eco-friendly and scalable applications. However, the ZT value of Bi₂S₃-based is not high enough and must be further improved. The rare-earth element doping is a promising strategy for synergistically enhancing the electrical and thermal transport properties. This study selected GaTe and YbBr₃ for co-doping in Bi₂S₃ polycrystals using a solid-state reaction and spark plasma sintering, which enabled simultaneous modulation of the band structure and carrier concentration for significantly enhanced thermoelectric and mechanical properties. Specifically, the synergistic optimization of electrical and phonon transport was initially achieved through GaTe doping in Bi₂S₃, which effectively reduced the lattice thermal conductivity while enhancing electrical conductivity. However, the improvement in thermoelectric performance with GaTe doping alone remained limited. Br substituted S to introduce extra electrons, while Yb replaced Bi to reduce the bandgap from 1.38 to 1.05 eV. The two effects synergistically optimized the electrical transport properties of the matrix. As a result, the optimized Bi₂S₃ + 0.5 wt% GaTe +1.0 wt% YbBr₃ sample achieved an exceptional average power factor of 475.4 μW m⁻¹ K⁻² over 323–773 K. Additionally, point defects, dislocations, and micropores induced by the second phase contributed to strong phonon scattering, leading to a low lattice thermal conductivity of 0.56 W m⁻¹ K⁻¹ at 773 K. Consequently, a peak ZT value of 0.67 at 773 K and a high average ZT value of 0.45 over 323–773 K were achieved, which were relatively higher than other values reported in the literature. Furthermore, the mechanical properties were improved, with hardness increasing from 2.55 to 2.93 GPa, making Bi₂S₃ more deformation-resistant and durable. This study showed that co-doping rare earth elements was an effective approach for enhancing the thermoelectric and mechanical properties of the Bi₂S₃ material.