Identification of the Origin of Ultralow Dark Currents in Organic Photodiodes.

Organic 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, we report the temperature dependence of J d in state-of-the-art OPDs with J d down to 10 -9  mA cm -2 at -0.5 V bias. For a variety of donor-acceptor bulk-heterojunction blends we find 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. This article is protected by copyright. All rights reserved.