Vapour-Induced Demixing in Solution-Processed Ferroelectric Films for Organic Memory Applications

Cost-effective production of organic memory devices requires ambient solution casting of ferroelectric polymers, such as poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)). However, in response to condensation of water vapor from the environment the drying polymer solution phase separates, which yields a fluid state morphology comprising polymer-rich micro-droplets in a solvent–rich medium. The resulting rough polymer films give poor device performance. Whereas this process of vapor-induced phase separation is useful in the production of microporous membranes, it must be avoided in the manufacture of thin-film memory devices. Through experiment and numerical calculation we identify a processing window for smooth films, determined by the evaporation rate, hygroscopicity and the ambient humidity. Modeling of the phase dynamics provides important insight on how microstructure and feature size emerge during simultaneous solvent evaporation and water condensation. Calculated morphologies are consistent with experiments and elucidate the coupling between domain size and ambient humidity. Our findings support a scenario wherein the dominant feature size in the dry polymer film is to a large extent already determined in the early stages of demixing.