The blend ratio and thickness of the PM6:PY-IT active layer are optimized to realize highly efficient all-polymer organic photodetectors (OPDs). Ultralow dark current densities of less than 5.92 × 10−10 A cm−2 (at −1 V) appear when the PM6:PY-IT active layers have thicknesses in the range of 250–400 nm and blend ratios between 1:1 and 1:1.4, suggesting a wide fabrication window. Fast rise/fall times of 560/250 ns are measured with a cut-off frequency of 530 kHz for the OPD incorporating a 250-nm-thick 1:1 blend. This OPD provides superior performance, with a high detectivity of 2.98 × 1013 Jones and a wide linear dynamic range of 138 dB (at 530 nm (−1 V)). In addition, the ultralow dark current of the OPD retains excellent long-term stability over 500 h of aging testing (thermal stress, ambient conditions, and continuous illumination). With the integration of specific light filters, a system is realized for the efficient detection of indoor plant-growth lighting, allowing the rapid and accurate detection (>97%) of photosynthetic photon flux at various wavelengths of light that directly determine the photosynthesis and photomorphogenesis of indoor plants, supervised by machine learning. The artificial intelligence-trained OPD effectively improves detection accuracy.