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

Abstract 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) ₃ BiI ₆ (NDMA = N , N ‐dimethylanilinium). Centimeter‐sized single crystals with dimensions up to 20 × 12 × 5 mm ³ 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) ₃ BiI ₆ 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.