Harnessing sp2-Hybridized Branched Side Chains to Control Pre-Aggregation in Organic Solar Cells

Precise control over molecular assembly during solution processing is essential for optimizing the morphology and photovoltaic performance of organic solar cells (OSCs). Despite its importance, deliberate manipulation of solution-state organization remains largely unexplored in non-fullerene acceptors (NFAs). In this work, we introduce YS1, a NFA incorporating sp²-hybridized branched side chains that fundamentally alter intermolecular interactions in solution. This molecular feature promotes the formation of aggregate-rich precursor states, which subsequently guide film growth and facilitate the development of highly ordered solid-state microstructures. Consequently, YS1 films exhibit enhanced molecular ordering, stronger crystalline coherence, and improved packing characteristics. When incorporated into ternary OSCs, these structural advantages translate into high power conversion efficiency. Our findings reveal that sp²-hybridized branched side chains can serve as an effective molecular tool for directing assembly pathways from solution to solid state, providing new opportunities for morphology engineering in high-performance OSCs.