Characteristics of thin films fabricated by spray coating on rough and permeable paper substrates

This work presents the results of simple but effective experiments designed and performed to study the characteristics of coatings made by air-assist spray deposition of a food-dye solution on regular printer and glossy papers, with different surface roughness and porosity. The objective of this work is twofold: First, the colorful nature of coatings made it possible to visualize spray patterns and coverage using an optical microscope and image analysis software, although detailed analysis of film roughness, thickness, and morphology was also performed using sophisticated characterization tools, such as confocal laser scanning microscopy, scanning electron microscopy, and atomic force microscopy. These results help in understanding the effect of processing parameters on spray coating on rough surfaces, such as already spray-coated solar cell films in a stacked thin film solar cell device. The underneath spray-on layers (such as a buffer layer underneath the active layer in a polymer solar cell) are usually rough. Paper substrate resembles such rough surfaces. Second, our results can provide insight into the feasibility of using paper substrates to fabricate paper-based photovoltaic solar cells. Effects of substrate type (wetting, porosity, and roughness), substrate temperature, spray nozzle travel speed over the substrate, precursor solution flow rate, and the number of spray passes are studied on coating characteristics. A novel approach is also employed to improve the coating characteristics via imposing ultrasonic vibration to the paper substrates, where the vibration results in a significant improvement in all coating indices.