A Lanthanide‐Ion Redox Shuttle Enables Grain‐Boundary Passivation in Copper Halide Films for Ultraviolet Communication and Imaging

ABSTRACT The urgent demand for space satellite communication is driving the development of low‐cost, high‐sensitivity ultraviolet (UV) detectors. Wide‐bandgap halide semiconductors are promising candidates due to their excellent photosensitivity and low‐temperature processability. However, polycrystalline halide thin films fabricated by low‐cost methods typically suffer from severe grain‐boundary defects, such as neutral iodine species (I) 0 and undercoordinated metal ions, which impede carrier transport and induce nonradiative recombination. Here, we employ a lanthanide‐ion redox shuttle to selectively reconstruct the grain‐boundary defect states in Cs 3 Cu 2 I 5 polycrystalline films. Introducing the europium ion pair (Eu 2+ /Eu 3+ ) effectively suppresses deep‐level defects related to I 0 and Cu + at the grain boundaries through a localized charge‐regulation and defect‐reconstruction cycle. Benefiting from this mechanism, carrier trapping and nonradiative recombination at the grain boundaries are significantly reduced. Consequently, the fabricated vertical indium tin oxide （ITO）/Cs 3 Cu 2 I 5 :Eu/Au UV photodetector exhibits excellent photo response under zero bias, with a responsivity of 2.29 mA W −1 (at 330 nm), a specific detectivity of 2.27 × 10 10 Jones, and a fast response time of 34.1 ms. The detector also demonstrates robust environmental stability, and enables low‐power UV optical communication and imaging, showing its great potential for cost‐effective, high‐sensitivity detection applications in space satellite communication.