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

ABSTRACT 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.