Cosolvent Effects When Blade-Coating a Low-Solubility Conjugated Polymer for Bulk Heterojunction Organic...

The adoption of solution-processed active layers in the production of thin-film photovoltaics is hampered by the transition from research fabrication techniques to scalable processing. We report a detailed study of the role of processing in determining the morphology and performance of organic photovoltaic devices using a commercially available, low-solubility, high-molar mass diketopyrrolopyrrole-based polymer donor. Ambient blade coating of thick layers in an inverted architecture was performed to best model scalable processing. Device performance was strongly dependent on the introduction of either o-dichlorobenzene (DCB), 1,8-diiodooctane, or diphenyl ether cosolvent into the chloroform (CHCl₃) solution, which were all shown to drastically improve the morphology. To understand the origin of these morphological changes as a result of the addition of the cosolvent, in situ studies with grazing-incidence X-ray scattering and optical reflection interferometry were performed. Use of any of the cosolvents decreases the domain size relative to the single solvent system and moved the drying mechanism away from what is likely liquid-liquid phase separation to solid-liquid phase separation driven by polymer aggregation. Comparing the CHCl₃ + DCB cast films to the CHCl₃-only cast films, we observed both the formation of small domains and an increase in crystallinity during the evaporation of DCB due to a high nucleation rate from supersaturation. This resulted in percolated bulk heterojunction networks that performed similarly well with a wide range of film thicknesses from 180 to 440 nm, making this system amenable to continuous roll-to-roll processing methods.