Research Progress on All-Inorganic Bismuth-Based Halide Perovskite Cs3Bi2X9 (X = Cl, Br, I) and Its Applications in Optoelectronic Devices

All-inorganic bismuth-based halide perovskites Cs3Bi2X9 (X = Cl, Br, I) have attracted widespread attention in recent years because of their favorable environmental stability, low toxicity, and unique low-dimensional structures and optoelectronic properties. This review systematically surveys the current state of Cs3Bi2X9 nanomaterials, with emphasis on their crystal structures, synthesis methods, and optoelectronic properties, and summarizes their research progress in photodetectors, X-ray and radiation detectors, light-emitting devices, solar cells, and memristors. Strategies for tuning the material properties and device performance, including mixed-halide alloying, elemental doping, defect passivation, and heterostructure construction, are also analyzed. On this basis, the main challenges facing Cs3Bi2X9, namely its indirect band gap, limited carrier transport, defect control, and scalable fabrication, are identified. Future development is discussed in terms of composition and band-gap engineering, defect and interface control, high-quality fabrication, and device integration. Overall, this review draws an outline of bismuth-based halide perovskites as well as their applications in optoelectronic devices, which will help to bring about new possibilities and call for more efforts to unleash the potential of Cs3Bi2X9.