Ordered J‐Aggregation Directed by Inner Alkyl Chains Enables Additive‐ and Annealing‐Free Organic Solar Cells With 20.73% Efficiency

Molecular aggregation behavior of non-fullerene acceptors (NFAs) plays a pivotal role in the photovoltaic performance of organic solar cells (OSCs). In Y-series NFAs, excessive H-like core contacts may increase packing disorder, while terminal group-mediated J-like interactions favor electronic coupling, luminescence, and reduced nonradiative loss. Controlling the H/J aggregation balance therefore remains a key challenge. Herein, three novel NFAs-C8C8, C8-7R, and C8EH-featuring identical conjugated backbones but systematically varied inner alkyl chains were designed to finely modulate aggregation states. Theoretical calculations and single-crystal analysis show that increased inner-chain steric hindrance suppresses core-involved contacts (H-aggregation) while favoring terminal group-mediated (J-aggregation) interactions. Accordingly, C8EH exhibits the strongest J-preferred packing tendency and enhanced molecular ordering, supported by compact terminal contacts, redshifted absorption, faster aggregation, and refined fibrillar morphology. When fabricated with the donor D18, C8EH-based OSC devices delivered an outstanding power conversion efficiency (PCE) of 20.73%, representing one of the highest values reported for additive- and annealing-free OSCs with higher current and reduced energy loss. This work demonstrates that inner-chain steric modulation can promote J-aggregated packing and ordered molecular organization, providing an effective strategy for high-performance NFAs and process-simplified OSCs.