Non-fullerene acceptors with nitrogen-containing six-membered heterocycle cores for the applications in organic solar cells

Abstract Constructing unfused-core non-fullerene acceptors (NFAs) via intramolecular noncovalent interactions has been recognized as an effective route to obtain high-efficiency and low-cost concurrently for organic solar cells (OSCs). Herein, we report two new unfused-core NFAs, NCIC and NOCIC, with pyrazine and pyridazine as the cores, respectively. In contrast to traditional unfused-cores with fluorine or alkyloxy substituents, the nitrogen-containing six-membered heterocycle cores of NCIC and NOCIC have no substituents, which simplifies further synthetic steps of unfused-core NFAs. In the meantime, utilizing N⋯S and/or N⋯H interactions between the cores and adjacent cyclopentadithiophene (CPDT) units, NCIC and NOCIC own good π-conjugated backbone planarity, consequently, show intense absorptions from 550 nm to 800 nm. Moreover, the electronegativity of aromatic N atoms in the cores pulls down the highest occupied molecular orbital (HOMO) energy levels of the two NFAs, with -5.65 eV and -5.59 eV for NCIC and NOCIC, respectively, which is favorable for them to pair with the polymer donor with a deep HOMO level, PM6, to fabricate bulk-heterojunction (BHJ) OSCs. Finally, the PM6:NCIC based devices yield the best power conversion efficiency (PCE) of 10.32%, while those based on PM6:NOCIC blends provide a maximum PCE of 9.89%. This work demonstrates that the nitrogen-containing six-membered heterocycle is a kind of potential building block for high-performance and easy-synthesis unfused-core NFAs.