Significant increase of photocurrent upon UV light exposure is demonstrated in a narrow-bandgap polymer-based photodetector using ZnO nanoparticles as anode interfacial layer. The phenomenon is attributed to the UV light illumination induced oxygen molecules desorption from surface of ZnO nanoparticles, which reduces the electron injection barrier at the anode interface. Ultrahigh external quantum efficiency of 140 000% and extremely low gain threshold voltage of 1.5 mV are achieved in this device with 30 s UV light irradiation. The gain mechanism is explained by the fast transit and replenishment of photogenerated electrons within their lifetime, which is prolonged by the electron-only device structure, and the experiment results fit well with the proposed photoconductive model.