Volatile Solid‐Assisted Molecular Assembly Enables Eco‐Friendly Processed Organic Photovoltaic Cells with High Efficiency and Photostability

Abstract Achieving environmentally friendly solvent‐processed high‐performance organic photovoltaic cells (OPVs) is a crucial step toward their commercialization. Currently, OPVs with competitive efficiencies rely heavily on harmful halogenated solvent additives. Herein, the green and low‐cost 9‐fluorenone (9‐FL) is employed as a solid additive. By using the o ‐xylene/9‐FL solvent system, the PM6:BTP‐eC9‐based devices deliver power‐conversion efficiencies of 18.6% and 17.9% via spin‐coating and blade‐coating respectively, outperforming all PM6:Y‐series binary devices with green solvents. It is found that the addition of 9‐FL can regulate the molecular assembly of both PM6 and BTP‐eC9 in film‐formation (molecule‐level mixing) and post‐annealing (thermal‐assisted molecular reorganization with additive volatilization) stages, so as to optimize the blend morphology. As a result, the charge transport ability of donor and acceptor phases are simultaneously enhanced, and the trap‐assisted recombination is reduced, which contributes to the higher short‐circuit current density and fill factor. Moreover, the generation of photo‐induced traps is significantly suppressed, resulting in improved stability under illumination. It is further demonstrated the excellent universality of 9‐FL in various photoactive systems, making it a promising strategy to advance the development of eco‐friendly OPVs.