CF<sub>3</sub>‑Functionalized\nSide Chains in\nNonfullerene Acceptors Promote Electrostatic Interactions for Highly\nEfficient Organic Solar Cells

The advent of next-generation nonfullerene acceptors\n(NFAs) has\npropelled major advances in organic solar cells (OSCs). Here we report\nan NFA design incorporating CF<sub>3</sub>-terminated side chains\nhaving varying <i>N</i>-(CH<sub>2</sub>)<sub>n</sub>-CF<sub>3</sub> linker lengths (<i>n</i> = 1, 2, and 3) which introduce\nnew intermolecular interactions, hence strong modulation of the photovoltaic\nresponse. We report a systematic comparison and contrast characterization\nof this NFA series with a comprehensive set of chemical/physical techniques\nversus the heavily studied third-generation NFA, Y6, revealing distinctive\nand beneficial properties of this new NFA series. Single-crystal diffraction\nanalyses reveal unusual two-dimensional mesh-like crystal structures,\nfeaturing strong interactions between the side chain CF<sub>3</sub>-terminal and NFA core F substituents. These atomistic and morphological\nfeatures contribute to enhanced charge mobility and significantly\nenhanced photovoltaic performance. We show that varying the CF<sub>3</sub>-terminated side chain linker length strongly modulates light\nharvesting efficiency as well as charge recombination and the photovoltaic\nbandgap. The CF<sub>3</sub>-(CH<sub>2</sub>)<sub>2</sub>-based OSC\ndemonstrates the most balanced performance metrics, achieving a remarkable\n19.08% power conversion efficiency and an exceptional 80.09% fill-factor.\nThese results imply that introducing CF<sub>3</sub>-terminated side\nchains into other OSC conjugated constituents may accelerate next-generation\nsolar cell development.