Central Unit Fluorination of Non‐Fullerene Acceptors Enables Highly Efficient Organic Solar Cells with Over 18 % Efficiency

Halogenation of terminal of acceptors has been shown to give dramatic improvements in power conversion efficiencies (PCEs) of organic solar cells (OSCs). Similar significant results could be expected from the halogenation of the central units of state-of-the-art Y-series acceptors. Herein, a pair of acceptors, termed CH6 and CH4, featuring a conjugation-extended phenazine central unit with and without fluorination, have been synthesized. The fluorinated CH6 has enhanced molecular interactions and crystallinity, superior fibrillar network morphology and improved charge generation and transport in blend films, thus affording a higher PCE of 18.33 % for CH6-based binary OSCs compared to 16.49 % for the non-fluorinated CH4. The new central site offers further opportunities for structural optimization of Y-series molecules to afford better-performed OSCs and reveals the effectiveness of fluorination on central units.