Exploring the (Bi,Sb)₂(S,Se)₃ system for photovoltaics and SWIR sensors

(Bi,Sb)₂(S,Se)₃ chalcogenides form a versatile class of semiconductors that have recently gained attention for photovoltaic (PV) energy conversion and short-wavelength (SWIR) to mid-infrared (MIR) sensors. These materials have a number of favourable properties, including suitable and tunable band gaps, the use of non-toxic and non-scarce elements, and compatibility with low-temperature fabrication routes. An important feature of these materials is their anisotropic opto-electrical behaviour, which makes crystallographic orientation a critical parameter for device performance. In this contribution, we first review the various synthesis routes and alloying strategies used for this material family, and we discuss the relationship between directional growth and device efficiency. A full range of Bi-Sb-S-Se compositions with band gaps targeted for both short-SWIR sensing and PV applications has been prepared by thermal evaporation of Sb₂Se₃, Bi₂Se₃ and Sb₂S₃ powders, followed by post-annealing under different temperatures and atmospheres. Transmission and photoluminescence measurements were used to determine the band gaps, while X-ray diffraction analysis provided insight into crystalline phases, alloy formation, and potential secondary phases. The high-band-gap Sb₂S₃ was alloyed with small amounts of Ag, resulting in a slight decrease in band gap and modifications to the microstructure. For SWIR applications, Sb₂Se₃ was alloyed with Bi₂Se₃ to reduce the band gap of pure Sb₂Se₃ (≈1.17 eV). However, the band gap could not be decreased beyond approximately 0.9 eV, due to the limited solubility of Bi in the orthorhombic Sb₂Se₃ lattice. At higher Bi concentrations, rhombohedral Bi₂Se₃ phases were formed, preventing further band-gap tuning. Initial device measurements showed diode behaviour and a measurable photoresponse, providing a promising starting point for optimization.