Small molecular PDI-functionalized 9,9′-bifluorenylidene acceptors for bulk heterojunction organic solar cells

Two novel small molecular non-fullerene acceptors, BF-PDI2 and BF-PDI4, respectively, carrying two and four peripheral perylenediimide moieties, are obtained from appositely functionalized 9,9′-bifluorenylidene and perylenediimide via Suzuki cross-coupling reaction. Owing to their twisted molecular structures, these acceptors possess good solution processability and exhibit relatively weak intermolecular interactions in both solution and the solid state as evidenced from their spectral characteristics. Both display an optical bandgap of 1.94 eV, and their LUMOs as estimated from electrochemical data are -3.83 and -3.92 eV for BF-PDI2 and BF-PDI4, respectively. Both can be utilized as a fullerene acceptor replacement in bulk heterojunction organic solar cells with a polymer donor, PBT7-Th, affording reasonable power conversion efficiencies of 2.46 and 4.43%, respectively.