Side‐Chain Terminal Engineering Enables 18% Efficient Non‐Fused Ring Electron Acceptor Organic Solar Cells via Crystallization Control

Non‐fused ring electron acceptors (NFREAs) provide a low‐cost alternative to fused‐ring electron acceptors for organic solar cells (OSCs). Nevertheless, how terminal substituents on alkyl side chains modulate the crystallization behavior of NFREAs remains largely underexplored. Herein, two NFREAs, namely TTF‐F (fluorine‐terminated side chains) and TTO‐F (methoxy‐terminated side chains), were constructed with identical intramolecular conformational locks and terminal IC acceptor units. Although the two molecules possess nearly identical electronic structures, in situ UV–vis absorption spectroscopy confirms that TTF‐F exhibits much faster crystallization kinetics (completing aggregation within 206 ms) relative to TTO‐F (336 ms). Femtosecond transient absorption measurements demonstrate that the hole‐transfer process in D18: TTF‐F blends proceeds fourfold more rapidly (τ 1 = 0.26 ps) than that in D18: TTO‐F (τ 1 = 1.14 ps), consistent with the more ordered π – π stacking revealed by grazing‐incidence wide‐angle x‐ray scattering (GIWAXS). When paired with the polymer donor D18, TTF‐F delivers an outstanding power conversion efficiency (PCE) of 18.02%, far exceeding the 13.35% efficiency of TTO‐F ‐based devices. This substantial performance discrepancy mainly stems from distinct crystallization dynamics and resultant thin‐film microstructures. This work establishes a structure–kinetics–morphology–performance relationship for NFREAs by tuning side‐chain terminal groups and offers rational molecular design strategies to fabricate high‐efficiency NFREA‐based OSCs.