Lead‐Free CsAgBr2/Cs2AgBiBr6 Heterocrystal‐Based Dual‐Band Self‐Powered Photodetector for Deep Ultraviolet and Visible Light Detection in Secure Optical Communication

Abstract Dual‐band photodetectors are gaining increasing attention for their potential in diverse applications, particularly in optical wireless communication (OWC). In this context, photodetectors capable of simultaneously detecting visible (VIS) and deep ultraviolet (DUV) bands show great promise for enhancing data security by concealing secret key signals. However, the construction of such devices remains a significant challenge. In this study, a novel one‐step synthesis of CsAgBr 2 /Cs 2 AgBiBr 6 heterocrystals is reported, via a partial chemical transformation strategy, enabling the construction of a self‐powered dual‐band photodetector with excellent performance. The device exhibits remarkable responsivities of 0.16 A W −1 at 265 nm (DUV region) and 0.06 A W −1 at 440 nm (VIS region) under zero external bias, with high external quantum efficiencies of 74.9% and 17.8%, respectively. These outstanding optoelectronic properties stem from the strong built‐in electric field within the heterocrystal structure. Furthermore, it is demonstrated that this dual‐band functionality significantly improves OWC security by effectively filtering interfering signals and capturing secret keys and passwords, highlighting the combined role of DUV and VIS in synergistically enabling secure communication. This work not only provides a new approach for the synthesis of lead‐free perovskite heterocrystals but also contributes to the development of next‐generation secure OWC technologies based on dual‐band photodetectors.