Dual‐Grain‐Size Microcrystal Architectures for Stress‐Regulated Lead‐Free Perovskite Thick‐Film X‐Ray Detectors

ABSTRACT Polycrystalline lead‐free Cs 2 AgBiBr 6 perovskites are attractive for large‐area direct x‐ray detection owing to their low toxicity and high chemical stability. However, optimizing the thick films required for efficient x‐ray absorption remains challenging under low‐temperature wet‐processing conditions, where microstructural densification is strongly coupled to internal residual stress, leading to structural discontinuities, ion migration signatures, and degraded charge transport. Here, we demonstrate a stress‐regulated dual‐grain‐size microcrystal strategy for polycrystalline Cs 2 AgBiBr 6 thick films. By rationally balancing large and small microcrystals, dense and crack‐free 1 mm‐thick films with a near stress‐relaxed state are achieved, resulting in reduced defect density, suppressed ion migration, and improved charge transport. Direct x‐ray detectors fabricated from the optimized films exhibit low dark current drift, reduced noise, and enhanced carrier transport, delivering a high sensitivity of 3500 µC Gy air −1 cm −2 and a low detection limit of 9.7 nGy air s −1 . The devices further show stable x‐ray responses under continuous irradiation and repeated bias cycling. This work establishes stress regulation through grain‐size engineering as an effective design principle for high‐performance lead‐free polycrystalline x‐ray detectors and provides a viable materials framework for scalable direct x‐ray detection.