Zinc(II) complexes of azadipyrromethene have shown promise as non-fullerene electron acceptor for solution-processed organic solar cells. To increase electron affinity and further tune the properties, we studied the effect of fluorinationof bis[2,8-diphenylethynyl-1,3,7,9-tetraphenylazadipyrromethene]zinc(II) (Zn(WS3)2) on its optical, electrochemical and device properties. A fluorine atom was installed either at the para position of the proximal phenyls, the distal phenyls or the pyrrolic phenylethynyls, refered to as the proximal, distal and pyrrolic positions, respectively. In order to study the degree of fluorination we also added –CF3 to the pyrrolic positions. All compexes had similar absorption spectra, 600-800nm in films, complementing the absorption of the well-known donor poly(3-hexylthiophene) (P3HT). The chelates were tested in bulk heterojunction organic photovoltaic (OPV) devices using P3HT as the electron donor. Compared to the unfluorinated acceptor Zn(WS3)2, fluorination generally increased power conversion efficiencies (PCEs). The best results were obtained when either F or CF3 were added to the pyrrolic positions, with a PCE of 3.3% and 3.7%, respectively. Atomic force microscopy images revealed a favorable phase separation and showed no sign of large-scale aggregation for all blends. Light intensity measurements revealed that bimolecular recombination limits performance in these fullerene-free devices, and that the addition of fluorine suppressed bimolecular recombination, with the largest suppression seen with the pyrrolic substitutions F and CF3. Electron mobility increased with fluorination, again with the largest increase when adding fluorines to the pyrrolic positions, reaching mobilities as high as ~10-3 cm2V-1s-1, on par with electron mobility of the ubiquitous phenyl-C61-butyric acid methyl ester (PCBM) acceptor in blends. These results point to the importance of the pyrrolic phenylethylnyls chemical composition for optimizing charge transport and device performance for these type of complexes.

Fluorination increases the electron mobility of zinc azadipyrromethene-based electron acceptors and enhances performance of organic solar cells

256th National Meeting (2018)

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