Y6-Derived Non-fullerene Acceptors with Unsaturated Alkyl Side Chains Enabling Improved Molecular Packing for Highly Efficient Additive-Free Organic Solar Cells

Achieving optimal morphology and photovoltaic performance without additives remains a prominent challenge in contemporary organic solar cells (OSCs). In this work, we developed three new Y6-derived nonfullerene acceptors (NFAs), namely, BTP-H15, BTP-H13, and BTP-H17, by introducing saturated octyl chains and unsaturated octenyl or octynyl chains to the outer positions of the DA′D core. Compared with the saturated chains, the incorporation of unsaturated chains effectively reduces the spatial steric hindrance between the outer side chains and the fused skeletons, thus enhancing the planarity of the related NFAs. Although BTP-H13 exhibited stronger aggregation than the other two NFAs, the lack of an efficient co-oriented half-skeleton stacking mode resulted in a relatively loose three-dimensional (3D) network structure and poor charge transport characteristics. In contrast, BTP-H15 showcased comprehensive advantages, including optimal framework planarity, close π–π stacking distances, and a robust 3D network, leading to superior morphology and enhanced charge transport. Consequently, the additive-free OSCs based on BTP-H15 realized impressive efficiencies of 18.46% in the binary devices and 19.36% in the ternary devices.