C-shaped ortho-benzodipyrrole-based acceptors with different electronic effects of top substituents for as-cast green-solvent processed high-performance organic solar cells

The milestone achievement of organic solar cells (OSCs) in recent years relies on the rapid development of A-DA ' D-A type small molecule acceptors (SMAs) with C-shaped molecular geometry. Recently, C-shaped ortho-benzodipyrrole (o-BDP)-based SMAs without fused electron-withdrawing triazole or thiadiazole units exhibit strong near-infrared absorption. However, the effects of the top substituents on the photovoltaic performance of this type of SMA is still unclear. In this work, three SMAs, namely, FF24-Cl, FM24-Cl and MM24-Cl, with the top substituents of difluorine, fluorine and methyl, and dimethyl, respectively, were synthesized based on C-shaped o-BDP. From FF24-Cl to FM24-Cl and then to MM24-Cl, the gradually decreased electron-withdrawing ability of the top substituents leads to an orderly enhanced intramolecular charge transfer (ICT) effect, resulting in red-shifted absorption and a reduced bandgap (E-g). In addition, the nature of the top substituents has a significant effect on the homo-molecular interaction between the SMAs and the hetero-molecular interaction of the SMAs with polymer donors. Finally, due to the suitable molecular packing and appropriate phase separation modulated by the top substituents, an FM24-Cl-based OSC processed by non-halogenated solvent without any extra treatment exhibits elevated exciton dissociation, enhanced charge transport and suppressed carrier recombination, achieving a higher power conversion efficiency (PCE) of 18.30%. As far as we know, this efficiency is the highest value for the OSCs fabricated by applying the simplest processing method, which is important for the future application of OSCs. In particular, the energy loss (E-loss) of SMA-based OSCs gradually decreases with the increased electron-donating ability of the top substituents, and an MM24-Cl-based OSC exhibits the lowest E-loss of 0.47 eV among the OSCs with PCE exceeding 17%. This work indicates that the top substituents play an important role in regulating the physicochemical and photovoltaic properties of the C-shaped o-BDP-based SMAs and may provide new opportunities for obtaining OSCs with lower E-loss and higher PCE.