Solution-processed organic photodetectors (OPDs) simultaneously integrating high sensitivity, ultrafast response and broadband detection have rarely been achieved so far. Herein, we demonstrate OPDs based on a semi-tandem structure with remarkable performance by solution-processability. The semi-tandem structure directly superimposes two active layers with complementary absorption spectra, achieving a broad spectral response of 300-1000 nm. It provides a detection covering from ultraviolet to near-infrared range, while the external quantum efficiency in the spectral range of 550-950 nm retains 70%. The high electron and hole injection barriers enable a dark current density as low as 6.51 × 10-5 mA cm-2 at -0.1 V, resulting in a noise current of 3.91 × 10-13 A Hz-1/2 at 70 Hz, which is nearly three times lower than single-junction photodetectors. Encouragingly, the device response speed is improved by suppressing the resistance-capacitance time constant of the device employing semi-tandem structure induced capacitance decreasing. The state-of-the-art OPDs contribute to the response time of 26.27 ns, which is the fastest one in OPDs to the best of our knowledge. We believe that the semi-tandem structures provide a new approach to achieving high-performance photodetectors integrating fast, sensitive and broadband response.