Watching Paint Dry: Operando Solvent Vapor Annealing of Organic Solar Cells

The commercialization of organic solar cell (OSC) technology will require highly reproducible techniques for controlling the morphology of bulk heterojunction blends. Variable-pressure solvent vapor annealing (VP-SVA) is one method for postprocessing organic solar cells with high precision; it can prevent the overannealing of cells that plagues conventional SVA processes. To gain insight into the dynamics of the VP-SVA process, we carried out operando measurements on OSCs with correlated in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. We show that the partial pressure of solvent vapor controls the length scale of film reordering, with optimal restructuring taking place below the saturation vapor pressure of the solvent. The experiments reveal how the film crystallinity, domain sizes, and percolation pathways evolve over the course of the VP-SVA process and how subtle differences in these morphological parameters differentiate good OSCs from champion cells.