Influence of band gap enlargement on thermoelectric properties of Bi₂Se₃ by solid-solution alloying with In₂Se₃

Bi₂Se₃ alloys are considered promising thermoelectric materials for application at ambient to moderately high temperatures. However, because of the narrow band gap (E_g) of Bi₂Se₃-based materials, bipolar conduction dominates and impairs their thermoelectric performance at elevated temperatures. In this study, we investigated the electrical and thermal transport properties of Bi₂Se₃ alloyed with indium (In) by incorporating as much as 50% In in the ((Bi_1-xIn_x)₂Se₃, where x = 0, 0.125, 0.25, 0.375, and 0.5) compositions, which widen the E_g of the Bi₂Se₃. Analysis of the activation energy, the Goldsmid−Sharp E_g, and optical E_g revealed that E_g might gradually become wider with increasing In content. This gradual widening of E_g led to a corresponding gradual decrease in the carrier concentration. However, the heaving doping of In induces an exponential decrease in the mobility, which resulted in an significant decrease in the electrical conductivity and power factor. The lattice thermal conductivity decreased significantly owing to strong phonon scattering by the heavy In doping and the bipolar thermal conductivity appeared to be reduced. Overall, a thermoelectric figure of merit is significantly reduced for the heavily In-doped Bi₂Se₃ mainly due to significant reduction of the mobility. Although the significant reduction in κ_latt and κ_bp was observed. Therefore, further theoretical analysis based on a two-band model was performed to further investigate the estimated correlation on the electrical and thermal transport properties of Bi₂Se₃ and its E_g.