Cation Engineering Enhanced Moisture Resistance in Centimeter‐Sized Lead‐Free Halide Perovskite Crystal for Ultrastable X‐Ray Detection

Halide perovskites, particularly those that are lead-free, have garnered significant attention owing to their eco-friendly nature and exceptional performance in X-ray detection. Nevertheless, despite their promising potential, halide perovskites suffer from structural instability in moist environments, which reduces device reliability. Herein, a cation engineering strategy involving the substitution of N-acidic protons with methyl groups of aniline is employed to synthesize a moisture-resistant lead-free halide perovskite (NDMA)₃BiI₆ (NDMA = N,N-dimethylanilinium). Centimeter-sized single crystals with dimensions up to 20 × 12 × 5 mm³ have been grown via a controlled solution cooling method. Benefiting from the cation modification, exceptional moisture resistance has been achieved with a water contact angle of 98.38°, along with retained phase integrity after 7-day water immersion and 1-year ambient storage (72-77% relative humidity). Meanwhile, the X-ray detectors fabricated with (NDMA)₃BiI₆ crystals exhibit a high sensitivity up to 2162 µC Gy_air ^-1 cm^-2, which is comparable to most reported lead-free perovskite detectors. Strikingly, the device retains 82% of its initial sensitivity (1780 µC Gy_air ^-1 cm^-2) following 5 h water immersion, confirming its long-term operational stability. This study provides a pathway for the development of eco-friendly lead-free perovskites that integrate high detection performance with humidity resistance, thereby enabling stable X-ray detection.