Dual Conformational Locks Enable Stable and Efficient Nonfullerene Acceptors without Exocyclic Vinyl Linkages

The intrinsic instability of photovoltaic materials remains a major barrier to the commercial viability of organic solar cells (OSCs). Among various degradation pathways, exocyclic vinylene linkages in nonfullerene acceptors (NFAs) have been identified as a primary source of photochemical vulnerability. While replacing these double bonds with carbon-carbon (C-C) single bonds can enhance stability, the low rotational energy barrier of C-C single bonds induces potential conformational disorder, disrupting molecular planarity and degrading device performance. To overcome this efficiency-stability trade-off, we design and synthesize three A-D-A type NFAs in which the electron-rich central core (D) is connected to electron-deficient terminals (A) via carbon-carbon single bonds. To restrict rotation around these single bonds, heteroatom incorporation is strategically applied to both the core and terminal units. Specifically, fluorine atoms are introduced into the terminal groups, and alkoxyl chains are introduced into the central core, fostering concurrent F···S and S···O noncovalent interactions that lock the molecular conformation. The resulting acceptor F6OF, featuring dual conformational locks, adopts a highly planar geometry, which facilitates intramolecular charge transfer and promotes ordered intermolecular packing. This structural modification leads to a bathochromically shifted and intensified absorption, enabling greater photon harvest, as well as improved crystallinity that refines the morphology of the bulk heterojunction. Consequently, organic solar cells based on F6OF achieve a power conversion efficiency of 10.45%─one of the highest reported values for organic solar cells utilizing C-C single-bond-linked NFAs. Furthermore, F6OF exhibits exceptional photostability and thermal stability, retaining 94.0% of its initial absorption after 72 h of continuous illumination and 96.2% after 144 h of prolonged thermal annealing at 100 °C. These combined merits establish the dual conformational locking strategy as a promising design paradigm for simultaneously achieving high efficiency and long-term operational stability in next-generation organic photovoltaics.