A C₆F₅-functionalized benzimidazole acceptor enabling supramolecular fluorinated interactions for enhanced photovoltaic performance and thermal stability

A new benzimidazole (BI)-centered A-D_NA′_ND-A-type nonfullerene acceptor, IPF, incorporating a perfluorophenyl (C₆F₅)-functionalized side chain at the C-2 position of the imidazole unit is synthesized. A counterpart with a C₆H₅-terminated side chain, IP, is also prepared to investigate the effects of perfluorination. Single-crystal diffraction analysis reveals that IP adopts a characteristic elliptical 3D packing structure, formed exclusively through π-π stacking intermolecular interactions. IPF features a distinct two-dimensional mesh-like network crystal structure, interwoven by terminal-to-terminal π-π interactions along the a-axis and fluorine-fluorine (F-F) electrostatic interactions between the C₆F₅ moieties and the C₆F₂ groups of end-groups along the b-axis. When blended with PM6, the IPF-based device achieved a significantly improved power conversion efficiency (PCE) of 14.4%, surpassing the 10.6% PCE of the non-fluorinated IP-based counterpart. The C₆F₅ moiety in IPF drives better interactions with PM6 for formation of a well-defined PM6:IPF complex and crystalline domains, as revealed from solution X-ray scattering, consequently leading to better charge transfer with less charge recombination. Moreover, IPF exhibits a significantly higher glass transition temperature (T_g) of 110 °C compared to IP with a T_g of 91 °C, which alleviates molecular diffusion within the blend film for enhanced thermal stability. As a result, the PM6:IPF device retains 90% of its initial PCE after 240 h heating at 85 °C. This research demonstrates the importance of modulating the self-assembly of the acceptor and its association with the donor through supramolecular F-F interactions to improve the device performance and thermal stability of organic photovoltaics.