Control and Characterization of Organic Solar Cell Morphology Through Variable-Pressure Solvent Vapor Annealing

Solvent vapor annealing (SVA) of organic photovoltaics (OPVs) has become an important postdeposition treatment, but current OPV SVA methods are difficult to reproduce and neither tunable nor scalable. Herein, it is shown that a variable-pressure solvent vapor annealing (VP-SVA) system can be used to reproducibly and tunably anneal OPV active layers to produce highly controlled film morphologies. We show that VP-SVA is useful not only for the well-studied P3HT:PC61BM model system, but also for modern OPV active layers based on nonfullerene acceptors. Phase separation and material crystallinity are precisely tuned by controlling the solvent vapor concentration used during the annealing process, as evidenced by photoinduced force microscopy (PiFM) and grazing incidence wide-angle X-ray scattering (GIWAXS). The results show that overannealing occurs at saturated solvent vapor pressures, highlighting the importance of the VP-SVA technique.