The emerging light absorber material bismuth oxide iodide BiOI, possesses convenient solution processibility and excellent chemical stability under ambient conditions along with a high light absorption coefficient reaching 5·10⁴ cm⁻¹. Classified as “defect-tolerant,” BiOI is considered a green and low-cost alternative to lead-halide perovskites in optoelectronic devices. Its investigation in photoresponsive electronic devices, however, is limited due to its anisotropic carrier mobility and unique morphology in thin films. To utilize the advantageous properties of BiOI, in this work, it is integrated into a phototransistor as a bilayer heterojunction with the organic semiconductor DPPDTT. The smooth interfaces and higher carrier mobility of DPPDTT compared to BiOI and its hydrophobic nature enable their synergistic hybridization in a heterojunction that is optically active from the UV to the NIR region. The unencapsulated heterojunction phototransistors are stable for at least three months under atmospheric conditions. They show a high I_light/I_dark current ratio of over 10⁴ at only 0.7 mW·cm⁻² irradiation intensity at all investigated wavelengths, and a specific detectivity up to 5·10¹² Jones. Initial synaptic measurements additionally reveal a neuromorphic behavior in the devices. This work charts a course towards the realization of cost-effective high-performance photoresponsive electronics for diverse applications.