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All-inorganic halide semiconductors with perovskite or perovskite-like structure have aroused a widespread concern recently for its environmental friendliness and stabilities while possessing excellent optoelectronic properties. Double perovskite Cs2AgBiBr6 single crystal (SC) is one of the most representative materials in the latest research area. To further improve the device response range and decrease its dark current density effectively, functional layers based on the solution-processed epitaxial method are normally fabricated as heterojunctions. Herein, a novel idea of a broadband heterojunction MAPbI3 (MA = CH3NH3)/Cs2AgBiBr6 is proposed in this work to achieve this goal. A MAPbI3 layer is fabricated on Cs2AgBiBr6 SC substrates through immersing Cs2AgBiBr6 SC into MAPbI3 solution at a MAPbI3 crystallization temperature. Ultimately, this heterojunction device expands the absorption limit from 618 to 838 nm, makes responsivity range redshift from 629 to 860 nm, and achieves a responsivity of 16.8 mA W−1 while the detectivity of 1.33 × 1011 Jones under 622 nm 0.55 mW cm−2 illumination at −20 V bias, maintaining excellent optoelectronic properties.