Spray Coating Vs. Immersion for Self-Assembly of Gemini Perfluorinated Phosphonic Acids on Indium Tin Oxide

Self-assembled monolayers (SAMs) are widely used to engineer the surface properties of metals and metal oxide layers. This article reports the fabrication of gemini perfluorinated phosphonic acids SAMs on indium tin oxide (ITO). The self-assembled monolayers are to be used for patterning nickel lines by electroplating on ITO in order to obtain low-cost conductor grids on heterojunction solar cells. The aim is to develop large-scale, low-cost processes that will both reduce production times and produce strong, covalently bonded monolayers of phosphonic acids on metal oxide surfaces along with uniform coverage. The gemini perfluorinated phosphonic acid provides extremely high hydrophobicity at the surface of ITO with a static contact angle of water above 115 degrees and a remarkably low hysteresis of approximately 15 degrees between advancing and receding contact angles. Well-known octadecylphosphonic acid is used as a reference material. Two deposition methods, by immersion and by spray-coating, are compared for the self-assembly of the molecules. The effect of the solvents used during the immersion, spray-coating, and rinsing steps are investigated. The surface properties are characterized by atomic force microscopy, contact angle measurements, and attenuated total reflectance Fourier-transform infrared spectroscopy.