Insights Into Preaggregation Control of Y‐Series Nonfullerene Acceptors in Liquid State for Highly Efficient Binary Organic Solar Cells

Abstract Leveraging breakthroughs in Y‐series nonfullerene acceptors (NFAs), organic solar cells (OSCs) have achieved impressive power conversion efficiencies (PCEs) exceeding 19%. However, progress in advancing OSCs has decelerated due to constraints in realizing the full potential of the Y‐series NFAs. Herein, a simple yet effective solid additive‐induced preaggregation control method employing 2‐chloro‐5‐iodopyridine (PDCI) is reported to unlock the full potential of the Y‐series NFAs. Specifically, PDCI interacts predominantly with Y‐series NFAs enabling enhanced and ordered phase‐aggregation in solution. This method leads to a notable improvement and a redshifted absorption of the acceptor phase during film formation, along with improved crystallinity. Moreover, the PDCI‐induced preaggregation of NFAs in the solution enables ordered molecule packing during the film‐formation process through delicate intermediate states transition. Consequently, the PDCI‐induced preaggregated significantly improves the PCE of PM6:Y6 OSCs from 16.12% to 18.12%, among the best values reported for PM6:Y6 OSCs. Importantly, this approach is universally applicable to other Y‐series NFA‐based OSCs, achieving a champion PCE of 19.02% for the PM6:BTP‐eC9 system. Thus, the preaggregation control strategy further unlocks the potential of Y‐series NFAs, offering a promising avenue for enhancing the photovoltaic performance of Y‐series NFA‐based OSCs.