Solvent selection in physical supercritical fluid deposition

Abstract

Physical supercritical fluid deposition is a method of thin film deposition that does not rely on in-situ chemical reactions. Instead, the technique relies on a maximum in the isobaric saturation solubility to deposit material onto a heated substrate. As such, a strong understanding of the saturation solubility is crucial to designing a successful deposition procedure. Here, we report the first study on the problem of solvent selection in physical supercritical fluid deposition, developing qualitative categories of a solvent-solute systems by studying poly(3-hexyl thiophene) (P3HT) solutions at supercritical pressures. Overpowered solvents exhibit high saturation solubilities and do not exhibit a maximum in the isobaric solubility as exemplified by pentane. Underpowered solvents exhibit extremely low saturation solubilities and as such are not suitable for this deposition technique as demonstrated by a fluorocarbon solvent. Optimum solvents are characterized by moderate saturation solubilities, showing a maximum in the isobaric saturation solubility with respect to temperature. This behavior is demonstrated with a chlorofluorolefin solvent. We also demonstrate the properties of mixtures formed by these three classes of solvents. The mixture of an optimum solvent with an overpowered solvent appears effective for physical supercritical fluid deposition, whereas the mixture of an overpowered and underpowered solvent does not. In sum, the work demonstrates guidelines to solvent selection that will allow for the deposition of a wider range of materials.