Programming Film‐Formation Kinetics via Photoinduced Acceptor Oligomerization for Efficient and Stable Organic Solar Cells

Precise control over the solution‐to‐solid transition is essential for high‐performance organic solar cells (OSCs), yet the rapid crystallization of Y‐series acceptors severely narrows the morphology‐optimization window. Here, we develop a photoresponsive thioctic acid‐functionalized acceptor, L8‐TA, whose photoinduced pre‐oligomerization was systematically optimized to generate Oli‐L8‐TA prior to film formation. When used as a low‐content third component in D18:L8‐BO, Oli‐L8‐TA regulates host crystallization through favorable compatibility with L8‐BO and steric confinement from the oligomeric structure, extending the crystallization window from 152 to 273 ms while enhancing molecular ordering. The optimized blend microstructure exhibits a favorable vertical phase distribution, contributing to improved charge transport and reduced recombination. Consequently, ternary devices achieve a PCE of 20.12% with a fill factor (FF) of 81.31%. Device stability is also enhanced, with extrapolated T 80 lifetimes extended from 490 to 905 h under continuous illumination and from 448 to 750 h at 65°C. These results demonstrate that photoinduced pre‐oligomerization provides a direct molecular‐state handle for programming host crystallization kinetics and optimizing active‐layer morphology during the solution‐to‐solid transition in OSCs.