Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.039
Publication date: 18th July 2019
Organic photovoltaics (OPV) offer unique advantages over conventional photovoltaic technologies, but their shorter operational lifetimes limited their application in the real world. In case of fullerene based devices it’s well established that the photofastness of the donor polymer and dimerization of fullerene are focal reasons for the limited operational lifetime. Recently, the development of non-fullerene acceptors (NFA) have pushed the efficiencies over 17%. However, the photostability of these devices are still are not fully explored. We show that the photobleaching of the NFA limits the operational lifetime in these state-of-the-art devices. Here, we demonstrate that lowering of the excited state energies of NFA’s is a way to improve the photo-stability of the OPVs. To test our hypothesis, high efficiency solar cells were fabricated by blending the benchmark donor polymers, PTB7-Th and PBDB-T with ITIC and its halogenated derivatives (ITIC, ITIC-4F and ITIC-4Cl). Upon constant illumination, the roll-off of short-circuit current density (Jsc) is the main cause of the drop in power conversion efficiencies (PCE) in these devices. We found that the chlorination of the peripheral units (ITIC-4Cl) leads to improved photostability of the devices when compared to the un-substituted NFA (ITIC). This is related to the different photo-bleaching kinetics of the neat acceptor films. Nuclear magnetic resonance (NMR) spectroscopy studies reveal that the photochemical reactions involve the chemical substitution on the peripheral bonds (C-H/F) of the dicyanomethylene-indanone (IC) moiety on respective acceptors. These chemical substitution’s is directly related to the excited state energies of the acceptors rather than the aforementioned bond energies.
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