Photodetectors with high photoelectronic gain generally require a high negative workingvoltage and a very low environment temperature. They also exhibit low response speedand narrow linear dynamic range (LDR). Here, an organic photodiode is demonstrated,which shows a large amount of photon to electron multiplication at room temperaturewith highest external quantum efficiency (EQE) from ultraviolet (UV) to near-infraredregion of 5.02 × 103% (29.55 A W−1) under a very low positive voltage of 1.0 V,accompanied with a fast response speed and a high LDR from 10−7 to 101 mW cm−2.At a relatively high positive bias of 10 V, the EQE is up to 1.59 × 105% (936.05 A W−1).Inversely, no gain is found at negative bias. The gain behavior is exactly similar to abipolar phototransistor, which is attributed to the photoinduced release of accumulatedcarriers. The devices at a low voltage exhibit a normalized detectivity (D*) over 1014Jones by actual measurements, which is about two or three order of magnitudes higherthan that of the highest existing photodetectors. These pave a new way for realization ofhigh sensitive detectors with fast response toward the single photon detection.