Ultralow‐Noise Blue‐Green‐Light CsPbBr 3 Photodetectors Enabled by CuSCN Selective Coverage Toward Long‐Distance Underwater Wireless Optical Communication

ABSTRACT The excellent thermal stability and blue‐green spectral response window of all‐inorganic CsPbBr 3 perovskite make it highly suitable for underwater wireless optical communication (UWOC). However, in complex ocean environments, severe optical attenuation limits long‐distance UWOC, necessitating photodetectors with ultralow noise and high signal‐to‐noise ratio (SNR). Here, we introduce a CuSCN selective covering structure to achieve ultralow‐noise CsPbBr 3 photodetectors with exceptionally high SNR. The intrinsically rough surface of polycrystalline CsPbBr 3 films allows partial CuSCN coverage, forming the selective microstructure in which grain boundaries are selectively coated while grain tops remain exposed. The structure enhances upward energy‐band bending near grain boundaries, suppressing leakage current channels, while simultaneously passivating defect‐rich grain boundaries and improving interfacial charge separation. Consequently, the photodetectors exhibit a pronounced reduction in dark current and a slight enhancement in photocurrent. Optimized devices exhibit an ultralow noise current of 8.74 fA Hz −1/2 and an exceptionally high SNR of up to 183 dB, representing a leading value among reported CsPbBr 3 photodetectors. Furthermore, when implemented as receivers in prototype UWOC systems, photodetectors enable a long transmission distance of up to 605 m. This work demonstrates an effective microstructural engineering strategy to enhance the SNR of perovskite photodetectors and advances their potential for long‐distance UWOC.