Solubility‐Driven Acceptor Fibrilization Enables Bilayer Organic Solar Cells with Efficiency Approaching 20%

Abstract Modulating the fibril size of the non‐fullerene acceptors has emerged as a promising approach to improve exciton dissociation and charge transport in organic solar cells. While processing solvent additives can promote acceptor aggregation, achieving controlled fibrilization remains challenging due to the complex donor–acceptor interactions in bulk heterojunctions. Herein, a solubility‐driven strategy is proposed to fibril the acceptors and evaluate their impact on the photovoltaic performance in bilayer organic solar cells, circumventing the complexities of donor–acceptor mixtures. By tuning the acceptor mass ratio, their solubility in dichloromethane is modulated, leading to acceptor fiber formation at different length scales during film deposition. Specifically, incorporating BTP‐eC9 into L8‐BO results in larger fiber sizes with increasing BTP‐eC9 content, enhancing molecular packing and thin‐film crystallinity. These structural improvements boost electron mobility and charge extraction efficiency, yielding a power conversion efficiency approaching 20% in bilayer devices with PM6. This solubility‐driven acceptor fibrilization strategy is effective across various acceptor combinations. These findings underscore the critical role of solubility in fine‐tuning acceptor morphology, offering a promising pathway for advancing solution‐processed bilayer organic solar cells.