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By adopting the alkali metal sodium ion (Na+) doping strategy, the grain size of Cs 3 Bi 2 I 9 (CBI) thin film is significantly increased and the non-radiative recombination defects are effectively passivated. Therefore, the self-powered broadband detection performance of photoelectrochemical detector based on the high-quality CBIN x thin films is greatly enhanced. [Display omitted] • Defect passivated lead-free Cs 3 Bi 2 I 9 polycrystalline perovskite thin film was prepared by alkali metal sodium cation doping. • A simple self-powered broadband photoelectrochemical detector based on Cs 3 Bi 2 I 9 thin film was fabricated. • The detector exhibits excellent photodetection performance without bias at UV, visible and near-infrared region. All-inorganic lead-free bismuth-based perovskites have attracted great interest in the optoelectronic applications due to their low toxicity and excellent stability. However, the inherent polycrystalline (PC) of perovskite films suffers severe charge recombination due to the large numbers of trap density. Herein, high-quality Cs 3 Bi 2 I 9 (CBI) PC thin films are prepared by solution process with alkali sodium (Na+) cation doping strategy, resulting large grain size and lower trap density. Afterward, the photoelectrochemical-type detectors based on Na+ doped CBI thin film is constructed for the first time. The photodetector displays a broadband photoresponse covering ultraviolet (UV), visible and near-infrared (NIR) ranges. Significantly, by precisely controlling Na+ doping concentration, the performance of photodetector can be largely enhanced with the highest responsivity of 53.38 mA/W and detectivity of 5.99 × 109 Jones without bias, and these results exceed the performance of most reported CBI PC thin film based photodetectors. In addition, the photodetector exhibits an obvious increase in photoresponse and outstanding photo-stability after more than 150 on/off cycles. Finally, the mechanism of Na+ doping is further discussed. These results provide an outstanding example for the preparing high-quality CBI PC thin film and also demonstrate the great potential of CBI in the application of self-powered broadband photodetectors. [ABSTRACT FROM AUTHOR]