Lead-Free Mixed-Halide-Based Vacancy-Ordered Perovskites: Status and Opportunities in Optoelectronics

Lead halide perovskites have delivered excellent efficiencies in optoelectronic devices owing to their strong light absorption and low-cost fabrication processes. However, toxicity and limited operational stability remain major obstacles to mass-scale commercialization. In this context, vacancy-ordered perovskites (VOPs) based on Bi³⁺/Sb³⁺, typically within the A₃B₂X₉ structure family, have appeared as promising lead-free alternatives with improved chemical robustness and an attractive optical response. Importantly, their optoelectronic properties are highly tunable through compositional modifications, particularly mixed-halide alloying. Halide substitution (Cl/Br/I) not only permits systematic bandgap and band-edge tuning but can also drive structural and dimensionality transformations (e.g., 0D/1D to 2D layered networks) that improve the electronic connectivity and charge transport. This mini-review consolidates recent progress on lead-free mixed-halide VOPs, demonstrating how halide-driven dimensionality control, site-selective halide ordering, and defect chemistry collectively govern optical transitions, carrier dynamics, and device performance. The mini-review also summarizes key synthesis and thin-film fabrication routes and assesses long-term device stability under relevant stressors. Finally, this mini-review also highlights future opportunities in the phase-pure film growth, the halide homogeneity control, the defect passivation, and the interface/band alignment for the performance enhancement to accelerate the deployment of mixed-halide VOPs across photovoltaics, photodetectors, and emerging optoelectronic platforms.