Ultra‐Long 1D Double Perovskite CsAg _0.4 Cu _1.6 I ₃ Single Crystal for UV Imaging Memory

Abstract Emerging copper‐based lead‐free perovskites have garnered significant attention for ultraviolet (UV) photodetection due to their atmospheric stability and optoelectronic properties. However, their practical applications are hindered by low carrier density and limited device performance. In this work, a breakthrough is reported in synthesizing ultra‐long 1D double perovskite CsAg _x Cu _2‐x I ₃ (0≤x≤2) single crystals (SCs) via an anti‐solvent recrystallization method. The SCs achieve unprecedented lengths of 2.2 cm for CsAg _0.4 Cu _1.6 I ₃ and 3 cm for CsAg ₂ I ₃ (CAI123), setting new benchmarks for Cu/Ag‐based perovskites. By tuning the Ag‐doping ratio, the bandgap is continuously modulated from 3.60 to 3.24 eV, enabling tailored optoelectronic responses. The optimized CsAg _0.4 Cu _1.6 I ₃ ‐based photodetector exhibits exceptional performance at 360 nm, with a peak responsivity of 82.5 mA W ⁻¹ and a high detectivity of 1.3 × 10 ¹² Jones (@ 3V; light intensity: 2.8 mW cm ⁻² ), superior than CsCu ₂ I ₃ (20.7 mA W ⁻¹ ) and CAI123 (15.8 mA W ⁻¹ ). Notably, the detector array demonstrates pioneering UV imaging memory capabilities, retaining patterns for over 10 s via slow carrier release from self‐trapped and defect‐related states. This work not only advances the synthesis of eco‐friendly perovskites but also unlocks novel functionalities for next‐generation optoelectronics in UV sensing, imaging, and memory devices.