Modulating Donor Fiber Size via Enhanced Aggregation With an Insulating Polymer Additive Enables Efficient Bilayer Organic Solar Cells

Through precise control of the morphology of nonfullerene acceptors (NFAs), the power conversion efficiency (PCE) of bilayer organic solar cells (OSCs) based on polymer donor/NFAs has surpassed 20%. However, the long polymer chains of polymer donors pose a significant challenge for the regulation of donor morphology. In this study, we employed an insulating polymer, polystyrene (PS), as an additive to modulate the donor morphology. When dissolved in chloroform, PS exhibits strong adhesion properties, which enhance the J‐aggregation of the donor and increase the fiber size. Consequently, devices treated with PS demonstrate prolonged carrier lifetimes, reduced trap density, and lower energetic disorder. As a result, the PCE of devices based on PM6/L8‐BO reaches 19.72%, a substantial improvement over the 18.31% efficiency of the control devices. Moreover, PS demonstrates its universal applicability by substituting PM6 with PBDB‐T or PTQ10. In bilayer OSCs based on PBQ10/L8‐BO and PBDB‐T/L8‐BO, we achieved PCEs of 15.78% and 8.10%, respectively, and higher than the corresponding as‐cast devices, which were 14.83% and 5.98%. Our findings shed light on the critical role that polymer additives play in modulating the donor aggregation morphology, thereby facilitating the solution‐processable fabrication of highly efficient bilayer OSCs.