Spirobifluorene-Cored Tetrameric Acceptors with a Perpendicular Interlocking Toroidal Topology for Efficient and Stable Organic Solar Cells

Small-molecule fused-ring electron acceptors (FREAs) have greatly advanced the efficiency of organic solar cells (OSCs), yet their long-term operational stability remains a critical hurdle for practical applications. Covalently linking FREA units into oligomers offers a promising strategy to address this issue, but the rational design of molecular topology in such systems remains largely unexplored. In this work, we report a novel three-dimensional (3D) oligomeric acceptor with a high glass-transition temperature (192 °C), namely LT-F, constructed from an orthogonally configured spirofluorene core. This unique 3D configuration adopts a perpendicularly interlocking toroidal topology, which not only effectively suppresses excessive aggregation and moderates the crystallinity of the acceptor phase but also promotes a more uniform vertical phase distribution with a favorable gradient. Notably, LT-F enables strong intermolecular interactions with the host L8-BO acceptor, significantly inhibiting its thermal diffusion and thus substantially enhancing morphological stability under operational stress. As a result, the ternary device based on PM6:L8-BO:LT-F delivers champion power conversion efficiencies of 19.49% and 20.10% when using PEDOT:PSS and a self-assembly layer as the hole-transporting layers (HTLs), respectively, both outperforming the PM6:L8-BO binary counterparts. Moreover, the ternary device with a PEDOT:PSS HTL exhibits markedly improved long-term stability, retaining 90.6% of its initial efficiency after 1000 h of thermal aging at 100 °C and 80.1% after 1200 h under maximum power point tracking. These results highlight the critical importance of 3D topological design in oligomeric acceptors for simultaneously achieving high efficiency and robust stability, providing valuable guidelines for future molecular engineering in high-performance organic photovoltaics.