Influence of Substituent Groups on Chemical Reactivity Kinetics of Nonfullerene Acceptors

Substitution (such as fluorine substitution, F) of terminal groups or atoms in nonfullerene acceptors (NFAs) is an effective strategy to tune the optoelectronic properties of nonfullerene electron acceptors and enhance their photovoltaic performance. However, how the substitution influences chemical reactivity or stability has not been reported. Here, we report that the terminal fluorine substitution would increase the chemical reactivity. This is supported by three different pieces of evidence: Frist, a device with fluorine-substituted NFA shows more severe "S" shape in the current density-voltage (J-V) characteristics under illumination when polyethylenimine is used as the interfacial layer and larger efficiency difference compared with that of the reference device with ZnO as an interfacial layer. Second, the fluorine-substituted NFA shows a higher reaction rate with monoethanolamine model compound, monitored by nuclear magnetic resonance (NMR) spectroscopy. Third, mass spectrometry of the products of the NFAs added with monoethanolamine indicates that the reaction is more complete compared with that of the NFAs without F substitution. In addition, we find that Y6 show good chemical stability with amines that suggests that proper design of the donor moiety of the NFA could also improve its chemical stability.