Organolead halide perovskites have emerged as the most promising materials for various optoelectronic devices,especially solar cells,because of their excellent optoelectronic preperties.Here,we present the first report of low-voltage high-gain phototransistors based on perovskite/organic-semiconductor vertical heterojunctions,which show ultrahigh responsivities of ～ 109 A W-1 and specific detectivities of ～ 1014 Jones in a broadband region from the ultraviolet to the near infrared.The high sensitivity of the devices is attributed to a pronounced photogating effect that is mainly due to the long carrier lifetimes and strong light absorption in the perovskite material.In addition,flexible perovskite photodetectors have been successfully prepared via a solution process and show high sensitivity as well as excellent flexibility and bending durability.The high performance and facile solution-based fabrication of the perovskite/organic-semiconductor phototransistors indicate their promise for potential application for ultrasensitive broadband photodetection.