Y6-Derived\nNon-fullerene Acceptors with Unsaturated\nAlkyl Side Chains Enabling Improved Molecular Packing for Highly Efficient\nAdditive-Free Organic Solar Cells

Achieving optimal morphology and photovoltaic performance\nwithout\nadditives remains a prominent challenge in contemporary organic solar\ncells (OSCs). In this work, we developed three new Y6-derived nonfullerene\nacceptors (NFAs), namely, BTP-H15, BTP-H13, and BTP-H17, by introducing\nsaturated octyl chains and unsaturated octenyl or octynyl chains to\nthe outer positions of the DA′D core. Compared with the saturated\nchains, the incorporation of unsaturated chains effectively reduces\nthe spatial steric hindrance between the outer side chains and the\nfused skeletons, thus enhancing the planarity of the related NFAs.\nAlthough BTP-H13 exhibited stronger aggregation than the other two\nNFAs, the lack of an efficient co-oriented half-skeleton stacking\nmode resulted in a relatively loose three-dimensional (3D) network\nstructure and poor charge transport characteristics. In contrast,\nBTP-H15 showcased comprehensive advantages, including optimal framework\nplanarity, close π–π stacking distances, and a\nrobust 3D network, leading to superior morphology and enhanced charge\ntransport. Consequently, the additive-free OSCs based on BTP-H15 realized\nimpressive efficiencies of 18.46% in the binary devices and 19.36%\nin the ternary devices.