Proceedings of MATSUS23 & Sustainable Technology Forum València (STECH23) (MATSUS23)
DOI: https://doi.org/10.29363/nanoge.matsus.2023.030
Publication date: 22nd December 2022
Organic bulk heterojunction photodiodes (OPDs) attract wide attention for light sensing and imaging but their detectivity is typically limited by a substantial reverse bias dark current density Jd. Recently, the Jd in OPDs was attributed to thermal charge generation mediated by mid-gap states. The evidence of such trap states, however, comes from rather indirect techniques such as thermal admittance and/or photocurrent measurements. In this work, we study the temperature dependence of Jd in state-of-the-art OPDs that have Jd values down to 10−9 mA cm−2. For a variety of donor-acceptor bulk-heterojunction (BHJ) blends we find that the activation energy of Jd is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5 eV. Ultra-sensitive sub-bandgap photocurrent spectroscopy on these diodes show evidence for sub-bandgap states. The minimum energy for optical charge generation from these sub-bandgap states correlates very well with the dark current thermal activation energy. From this, we conclude that the dark current in these OPDs is caused by thermal charge generation at the donor-acceptor interface mediated by sub-bandgap states that create an activation energy that is 0.3-0.5 eV below the bandgap energy. By studying donor- and acceptor-only diodes, we find that such sub-bandgap states are prevalent in polymer (donor) semiconductor materials, and in the fullerene acceptor PCBM, but not in the non-fullerene acceptor materials studied.
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