A 3D Cycloparaphenylene-Cored Trimeric Electron Acceptor Regulates Acceptor Fibril Morphology for 20.6%-Efficient Ternary Organic Solar Cells.

The application of cycloparaphenylene (CPP) macrocycles in organic photovoltaics has remained largely unexplored despite their unique three-dimensional (3D) topologies. Herein, we expand the functional scope of CPPs by covalently integrating a carbon-nanohoop scaffold with Y-series electron acceptors to develop a novel 3D trimeric acceptor, CPP-3Y. Distinct from conventional planar molecules, CPP-3Y combines radially oriented π-conjugation, a continuously curved π-surface, and sterically hindered 3D topology, resulting in strong aggregation with low crystallinity. When introduced as a third component into the D18:L8-BO-X binary system, CPP-3Y preferentially associates with the L8-BO-X-rich acceptor phase, resulting in large fibrillar structures in blended thin films. This narrows the energetic tail-state distribution, suppresses trap-assisted and non-radiative recombination, and enables more balanced charge transport. Consequently, D18:L8-BO-X:CPP-3Y ternary OSCs achieve a PCE of 20.62%, outperforming binary devices (19.90%). They also retain over 80% of their initial efficiency after 964 h at 70  °C, whereas the binary devices fall below this level after 432 h. This work demonstrates that cyclic electron acceptors with 3D configurations and strong aggregation can act as morphological regulators to tune the phase separation of photoactive layers and hence realize high PCEs in OSCs.