Solution‐Processed Mixed Halide Wide‐Bandgap Perovskite CsPb(Cl1‐xFx)₃ for Ultrafast Self‐Powered Ultraviolet Photodetector

All‐inorganic wide‐bandgap perovskites have emerged as promising semiconductors for UV photodetectors due to their excellent thermal and UV stability, solution‐processability, and superior optoelectronic properties. Herein, wide‐bandgap F‐substituted perovskites CsPb(Cl 1‐ x F x ) 3 are first uncovered and investigated, which can enlarge the bandgap, improve carrier lifetime, and significantly reduce defect states. The F substitution with a contracted perovskite lattice is in contrast to conventional halide (I/Br) perovskites, in which F is interstitially incorporated. Additionally, an ionic liquid [AMIM]Cl‐assisted two‐step fabrication strategy is demonstrated to regulate perovskite crystallization, resulting in high‐quality polycrystalline films with excellent optical, electronic, and morphological features. It effectively addresses the poor and unbalanced solubility of perovskite precursors encountered in conventional solution‐processed methods for CsPbCl 3. The self‐powered UV photodetectors based on SnO 2 /CsPb(Cl 1‐ x F x ) 3 exhibit excellent device performance with an ultrafast response time as low as 0.49/0.53 µs, which is outstanding compared to the recent reports. The detectivity and responsivity can be up to 1.18 × 10 13 Jones and 0.09 A W −1 , respectively. The device displays a long‐term stability in ambient air. Encouragingly, high‐quality, real‐time self‐powered UV imaging is successfully achieved by employing the photodetector arrays. This work opens up the possibility of obtaining wide‐bandgap all‐inorganic mixed halide perovskites and the promising applications in ultrafast UV photodetectors.