Nesting-like band gap in bismuth sulfide BiS

The van der Waals material Bi 2 S 3 is a potential solar absorber, but its optoelectronic properties are not fully explored and understood. Here, using theoretical calculations and various experimental techniques under different temperature and hydrostatic pressure conditions, the optoelectronic properties of Bi 2 S 3 are determined. The fundamental absorption edge and photoreflectance transition value has been found to be ∼1.30 eV at room temperature, which is the optimum value, giving a maximum solar cell power conversion efficiency according to the Shockley-Queisser limit for a single-junction solar cell. Temperature-dependent measurements reveal that the total energy change of the fundamental gap between 20 and 300 K is significant at ∼0.16 eV, compared to conventional semiconductors. Theoretical predictions show that Bi 2 S 3 possesses an indirect band gap with the band nesting surrounding. The combination of experimental methods such as photoreflectance, absorption, and photoluminescence clearly shows that the direct optical transition dominates above the indirect one. Therefore, we reveal Bi 2 S 3 as a nesting-like band gap semiconductor with a strong absorption edge and excitonic emission. Bi 2 S 3 is a nesting-like band gap semiconductor, where direct optical transition dominates above the indirect one, resulting in a strong absorption edge and excitonic emission.