Highly Ordered Polymorphism of Small Molecule Acceptor Delivering Efficient and Stable Binary Organic Solar Cells.

Advancements in narrow bandgap organic small molecule acceptors (SMAs) has promoted organic solar cell (OSC) efficiencies beyond 20%. Achieving this milestone necessitates precise control over the active layer morphology, particularly its crystallinity and phase distribution, to optimize light absorption, charge transport, and suppress charge recombination. However, controlling SMA morphology remains a significant challenge due to their strong aggregation tendency. Existing methods, including high-temperature annealing, and introducing high boiling point additives, frequently yield disordered polymorphs with limited scalability. Here, we report a novel approach of utilizing 4-bromochlorobenzene as a volatile solid additive to induce the formation of a highly ordered polymorph of BTP-eC9 through mild annealing at 60 °C. This marks the first demonstration of such an ordered SMA polymorph, exhibiting optical properties comparable to ideal crystals, including enhanced anisotropy, refractive index, and extinction coefficients. The specific polymorph further enables the formation of a well-organized PM6 donor arrangement, establishing an optimal bicontinuous network morphology. Consequently, the OSCs based on PM6:BTP-eC9 achieve a power conversion efficiency of 19.53%, which further increases to 20.32% with the addition of an antireflection layer. This work provides a scalable and effective strategy for enhancing OSC performance and highlights the critical role of polymorphism in optimizing photovoltaic performance.