Transition-metal oxides are widely used to improve the power conversion efficiencies of organic or perovskite solar cells because their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of semiconductor materials. In this work, the atomic-layer-deposition of ultrathin Co3O4 anode interlayers which are used as hole transporting/electron blocking layers in organic photodetectors is investigated. Incident light loss and carrier transport loss could be minimized due to the ultrashort transport path. It is found that the highly smooth Co3O4 interlayer with a deep valence band of 5.3 eV and a shallow conduction band of 1.6 eV effectively promotes photogenerated charge extraction and suppresses the electron injection under reverse bias, resulting in a significantly improved photodetection performance. At an optimal Co3O4 thickness of 1 nm, the dark current of the Co3O4 device is almost 1 order of magnitude lower than that of the PEDOT:PSS device. As a result, we have demonstrated a P3HT:PC61BM-based device with a low dark current of 2.5 nA cm−2 and a high detectivity of 1013 Jones at −1 V bias, which are higher than those of the commercial silicon-based photodiodes. Such ultrathin charge blocking layers are proved to be versatile in reducing the dark current for devices with NIR-absorbing NFA.