π–π Stacking Modulation via Polymer Adsorption for Elongated Exciton Diffusion in High‐Efficiency Thick‐Film Organic Solar Cells

Abstract Developing efficient organic solar cells (OSCs) with thick active layers is crucial for roll‐to‐roll printing. However, thicker layers often result in lower efficiency. This study tackles this challenge using a polymer adsorption strategy combined with a layer‐by‐layer approach. Incorporating insulator polystyrene (PS) into the PM6:L8‐BO system creates PM6+PS:L8‐BO blends, effectively suppressing trap states and extending exciton diffusion length in the mixed donor domain. Adding insulating polymers with benzene rings to the donor enhances π–π stacking of donors, boosting intermolecular interactions and electron wave function overlap. This results in more orderly molecular stacking, longer exciton lifetimes, and higher diffusion lengths. The promoted long‐range exciton diffusion leads to high power conversion efficiencies of 19.05% and 18.15% for PM6+PS:L8‐BO blend films with 100 and 300 nm thickness, respectively, as well as a respectable 16.00% for 500 nm. These insights guide material selection for better exciton diffusion, and offer a method for thick‐film OSC fabrication, promoting a prosperous future for practical OSC mass production.