Molecular navigation strategy enables fibrillar self-assembly for efficient as-cast organic solar cells.

Fibrillar networks formed during solution processing play a key role in the high performance of modern organic solar cells (OSCs). Yet their formation is intrinsically non-equilibrium and largely stochastic, leading to discontinuous domains and mismatched donor/acceptor interfaces that limit charge transport. Here, we show that the evolution of fibrillar networks can be guided through molecular design. By designing a cycloalkoxy-functionalized acceptor, O6R-4F, we create a molecular navigator that preferentially localizes at donor/acceptor interfaces, suppresses excessive self-aggregation of L8-BO-C5 acceptor, and promotes coordinated donor/acceptor aggregation and crystallization during film formation. This results in finer, more interconnected fibrillar networks with improved phase separation. As-cast D18: L8-BO-C5: O6R-4F devices achieve a power conversion efficiency of 20.9% without post-deposition treatment. The same approach also improves morphology and performance in chemically distinct donor/acceptor systems, demonstrating that controlling intermolecular interactions provides a general strategy for directing fibrillar network formation in solution-processed organic semiconductors.