Coplanar Dimeric Acceptors with Bathochromic Absorption and Torsion-Free Backbones through Precise Fluorination Enabling Efficient Organic Photovoltaics with 18.63% Efficiency.

Giant dimeric acceptors (GDAs), a sub-type of acceptor materials for organic solar cells (OSCs), have garnered much attention due to the synergistic advantages of their monomeric and polymeric acceptors, forming a well-defined molecular structure with a giant molecular weight for high efficiency and stability. In this study, for the first time, two new GDAs, DYF-V and DY2F-V are designed and synthesized for OSC operation, by connecting one vinylene linker with the mono-/di-fluorinated end group on two Y-series monomers, respectively. After fluorination, both DYF-V and DY2F-V exhibit bathochromic absorption and denser packing modes due to the stronger intramolecular charge transfer effect and torsion-free backbones. Through precise fluorination, the DYF-V-based devices exhibit the highest performance of 18.63% among the GDA-based OSCs, outperforming its non-fluorinated counterpart, DY-V-based ones (16.53%). Theoretical and morphological results demonstrate that proper fluorination in DYF-V-based devices strengthens intra/intermolecular interactions for enhanced crystallinity, superior phase segregation, and less energy disorder, which is beneficial for fast exciton dissociation, rapid carrier transport, and suppressed charge recombination. The work demonstrates that proper fluorination on GDAs with rigid coplanar backbones is effective for broader photon harvesting, stronger packing, and robust stability in GDA-based OSCs.