M-Series Nonfullerene Acceptors with Varied Fluorinated End Groups: Crystal Structure, Intermolecular Interaction, Charge Transport, and Photovoltaic Performance

Intermolecular interaction of nonfullerene acceptors (NFAs) is essential for controlling their photovoltaic performance. Fluorinated substituents attached at the end groups of NFAs can significantly affect their frontier molecular orbitals and intermolecular interactions. Herein, four heteroheptacene-based NFAs (ML-1F, ML-2FO, ML-2FM, and ML-2FP) with different fluorinated end groups are designed, synthesized, and characterized. The impact of the end group on the crystal structures, optical, electrochemical, charge transport, and photovoltaic properties of these NFAs are systematically studied. In comparison with the acceptor with monofluorinated end groups, the acceptors with difluorinated end groups show reduced bandgaps and downshifted energy levels. Single-crystal results demonstrate that the intermolecular interactions including the pi-pi stacking distance and molecular aggregation behavior of these acceptors are also affected by the variation of end groups thereby leading to the acceptors with varied charge transport properties. In combination with the polymer donor of PM6, ML-2FM exhibits the highest power conversion efficiency (PCE) of 15.33% with a short-circuit current density of 23.73 mA cm(-2) and a fill factor of 0.734. However, ML-2FP displays an inferior PCE of 10.48% which is lower than that of ML-1F (11.41% PCE). The results show that the fluorine substituent number and position of end group are of vital importance in determining their photovoltaic performance.