Enhancing power factor and ZT in non-toxic Bi2S3 bulk materials via band engineering and electronic structure modulation

Abstract

Bismuth sulfide-based thermoelectric materials have attracted considerable interest owing to their abundance of raw materials and non-toxicity. However, their poor electrical properties, particularly their conductivity, have limited their practical applications. In this work, a melting method and spark plasma sintering were employed to synthesize bulk Bi2S3 materials. The dopant MoCl5 was used to regulate the electrical properties of Bi2S3. The results showed that Mo can significantly enhance the conductivity of Bi2S3 compared to other transition metals. The doped sample exhibited an almost two-orders-of-magnitude rise in conductivity relative to the Bi2S3 sample in its pure form. Due to its superior conductivity, the Bi2S3+1 wt% MoCl5 sample achieved an excellent power factor of 550 μW m−1K−1 which is the maximum ever reported in the Bi2S3 system. Simultaneously, the ZT value of this sample approached 0.70 at 773 K, which is a six-fold improvement compared to the pristine sample. Density functional theory was applied to understand the electronic structure changes of the Mo-doped sample, which revealed that trace amounts of MoCl5 doping could significantly boost the density of states in the vicinity of the conduction band and shift the Fermi level to the valence band, leading to a reduction in the bandgap and an increase in free electrons. This work offers new insights into the role of Mo elements in thermoelectric materials and provides a good strategy for augmenting the thermoelectric figures of merit of other n-type thermoelectric materials with inadequate electrical conductivity in the future.