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

Abstract

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 π–π 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.

Anode interfacial modification for non‐fullerene polymer solar cells: Recent advances and prospects

“…Recently, Zou et al synthesized new NF‐acceptors 24,25 . Further improvement in performance was noted after developing Y18, BTP‐4Cl, BTP‐M, BTP‐eC9, and ML‐2FM NF‐acceptors 26–29 . At present, the state‐of‐the‐art PCE has reached over 20%, which is the highest reported PCE up to date 30 .…”

“…24,25 Further improvement in performance was noted after developing Y18, BTP-4Cl, BTP-M, BTP-eC9, and ML-2FM NF-acceptors. [26][27][28][29] At present, the state-of-the-art PCE has reached over 20%, which is the highest reported PCE up to date. 30 Such impressive improvements in PSCs efficiency are mainly attributed to the high absorption property and structure tunability of NF-acceptors (Figure 1).…”

Crystal structures in state-of-the-art non-fullerene electron acceptors

“…In addition to the above mentioned, Zheng et al developed a series of NFAs (M-series) based on ladder-type heteroheptacene without sp 3 -hybridized bridging atoms. 39 They investigated the crystalline structure change of the heteroheptacene-based NFAs with different uorinated terminal groups. The chemical structures and the crystal structures of the three NFAs are shown in Fig.…”

Enhancing Photovoltaic Performance of Ladder‐Type Heteroarene‐Based Electron Acceptors by Modulating Molecular Packing†

“…The lone pair electrons in the p‐orbital of the nitrogen atom can be delocalized along the whole π‐conjugated system, which not only improves intramolecular charge transfer effect to extend absorption, but also reduces the reorganization energy to achieve a small energy loss. [ 42‐45 ] Thus, PSCs using such nitrogen‐bridged fused‐ring NFAs are expected to achieve both high open‐circuit voltages ( V oc s) and large short‐circuit currents ( J sc s). On the other hand, the sp 3 ‐hybridized carbon atom with a tetrahedral configuration forces the bonded two side‐chains to stick out from the backbone plane of NFAs, and the resulted steric hindrance will inevitably interfere with the compact intermolecular packing.…”