Uncovering the Crystalline Packing Advantages of Asymmetric Y-Series Acceptors for Efficient Additive-Insensitive and Intrinsically Stable Organic Solar Cells

Organic solar cells (OSCs) hold immense potential as renewable energy sources, but the performance-stability conundrum remains a major obstacle on the road to OSC commercialization. To address this, in this study, molecular packing behaviors of representative state-of-the-art Y-series non-fullerene acceptors (NFAs) are studied, focusing on their terminal group symmetries. Utilizing grazing-incidence wide-angle X-ray scattering, the distinct crystalline packing structure of these NFAs is revealed. Remarkably, NFAs with asymmetric terminals exhibited excellent additive insensitivity and thermal stability for their crystalline structure, unlike their symmetric counterparts. Molecular dynamics simulations confirmed the stable and robust crystalline feature of asymmetric NFAs and attributed to their large molecular dipole moments. These findings showed the great potential of asymmetric NFAs in fabricating efficient and intrinsically stable additive-free OSC devices for real applications. This work highlights the crucial role of non-fullerene acceptors (NFA) terminal-asymmetry in the formation of characteristic and robust crystalline packing structures. It is uncovered that in strong contrast to the symmetric molecules, the three asymmetric NFAs (BTP-S1, BTP-S2, and BTP-S9) possess unique additive-insensitive and thermally stable crystalline packing, which further enable efficient, durable, and commercially viable OSC devices. image