The Role of Mesoscopic PCBM Crystallites in Solvent Vapor Annealed Copolymer Solar Cells

Solution processable methanofullerene-based solar cells are the most widely studied class of organic photovoltaics (OPVs). The evolution of the electronic properties with solvent vapor annealing (SVA) in polyfluorene-copolymer and [6,6]phenyl-C61-butyric acid methyl ester (PCBM) blended OPVs is studied using various scanning probe techniques: light beam induced current spectroscopy (LBIC), conductive atomic force microscopy (c-AFM), and photoconductive AFM (pc-AFM). We demonstrate that SVA improves the power conversion efficiency by 40% while forming mesoscopic PCBM crystallites and a similar to 3 nm copolymer-rich overlayer at the cathode interface. We find that the large crystallites created during annealing do not directly improve the local performance of the device, but instead attribute the performance improvement to the ripened blend morphology and an increase in the hole mobility of the copolymer in comparison to the unannealed blend. The PCBM-rich aggregates act as a sink for excess PCBM, although excess PCBM is initially required to form the appropriate structural features prior to the annealing process.