Composition‐Engineered (CsI) x (BiI 3 ) y Perovskites for Lead‐Free Ultraviolet Photodetection and Memristive Applications

Abstract All‐inorganic bismuth‐based perovskites are promising candidates for photodetectors and memristors owing to their wide bandgap, long carrier diffusion length, low toxicity, and excellent environmental stability. However, their practical applications are hindered by suboptimal detection performance and inadequate resistive switching (RS) robustness. Herein, a series of bismuth‐based perovskites with tunable CsI‐to‐BiI 3 molar ratios, denoted as (CsI) x (BiI 3 ) y (CBI), are fabricated via a simple drop‐casting method onto zinc oxide (ZnO) to form ZnO/CBI heterojunctions for investigating their optoelectronic and memristive properties. The results demonstrate that compositional engineering of (CsI) x (BiI 3 ) y profoundly influences the crystal quality, optoelectronic characteristics, and charge transport dynamics. When CBI composition is ideal stoichiometrically balanced (3:2), the photodetector exhibits optimal ultraviolet (UV) detection performance, achieving a maximum detectivity of 2.23 × 10 11 Jones. Furthermore, BiI 3 ‐rich compositions exhibit pronounced RS behavior at low‐bias voltages. Upon expanding the voltage sweep range, the device manifests concurrent RS and negative differential resistance under reverse‐bias conditions. Furthermore, both the photodetector and memristor devices exhibit excellent thermal and humidity stability. These results demonstrate that CBI composition tuning enables ZnO/CBI heterojunctions to realize both UV detection and memory functions with long‐term stability and environmental friendliness, advancing lead‐free multifunctional optoelectronic devices.