Imaging Nanoscopic Domains in PM6:Y6 Organic Bulk Heterojunctions using PEEM

Organic donor:acceptor (D:A) bulk heterostructures (BHJs) have been widely used in organic photovoltaics (OPVs). With the recent development of Y6-based non fullerene acceptors, state-of-the-art OPV efficiency is reaching ~ 20 %. For Y6-based BHJs, it is unclear why charge separation (CS) can occur efficiently with a very small energy loss. Nanoscopic morphology on how donor polymer and NFAs are arranged can play an important role in determining the CS mechanism. Here, steady-state photoemission electron microscopy (PEEM) is used to resolve the nanoscopic domains in the prototype PM6:Y6 BHJ. A nest-like network of PM6 polymers with a periodicity of around 250 nm is revealed. These results compare well with the morphology captured by conventional atomic force microscopy (AFM). Our results show the feasibility in using time-resolved PEEM to unveil the ultrafast CS dynamics in these nanoscopic domains.