AbstractOrganic bulk heterojunction photodiodes (OPDs) attract attention for sensing and imaging. Their detectivity is typically limited by a substantial reverse bias dark current density (J_d). Recently, using thermal admittance or spectral photocurrent measurements, J_d has been attributed to thermal charge generation mediated by mid‐gap states. Here, the temperature dependence of J_d in state‐of‐the‐art OPDs is reported with J_d down to 10⁻⁹ mA cm⁻² at −0.5 V bias. For a variety of donor‐acceptor bulk‐heterojunction blends it is found that the thermal activation energy of J_d is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5 eV, but higher than expected for mid‐gap states. Ultra‐sensitive sub‐bandgap photocurrent spectroscopy reveals that the minimum photon energy for optical charge generation in OPDs correlates with the dark current thermal activation energy. The dark current in OPDs is attributed to thermal charge generation at the donor‐acceptor interface mediated by intra‐gap states near the band edges.