An inexpensive, ultrathin spray-on window coating for blocking heat and conducting electricity could further reduce energy costs for smart windows and heat-repelling glass, making smart window technology more widely accessible. It was developed by a team at RMIT University, which used a process called ultrasonic spray pyrolysis to fabricate smooth, uniform coatings of high optical and electrical quality. Using commercially available technology, the researchers first nebulized a precursor solution to create a spray that formed tiny, uniformly sized droplets. They sprayed the solution on a heated support substrate. When the solution hit the heated layer, it triggered a chemical reaction that caused the precursor solution to decompose into a solid residue that was deposited as an ultrathin coating. Any byproducts of this reaction were eliminated as vapors, leaving a pure coating with the desired composition. The ultrathin clear coatings are made with a new spray-on method that is fast, cost-effective, and scalable. Courtesy of RMIT University. The researchers characterized the coatings using a suite of techniques, including x-ray diffraction, electron microscopy, x-ray and ultraviolet photoelectron spectroscopy, Hall effect measurements, optical spectroscopy in the visible and near infrared, and atomic force microscopy, to clarify the relationship between the synthetic conditions and functional properties. The spray-on coatings showed transmittance values in the visible spectrum between 80% and 90%. Currently, transparent electrode coatings are made through vacuum deposition methods that are slow and reliant on expensive raw materials like indium. The spray-on method from RMIT is fast and scalable, and it uses tin oxide — a relatively inexpensive material — enhanced with chemicals to improve conductivity and transparency. The ultrathin transparent coatings, which are over 100× thinner than a human hair, allow only visible light through, while blocking both harmful ultraviolet light and heat in the form of infrared radiation. This spray-on coating method could simplify the fabrication of smart windows and low-emissivity glass. The team said that the technique could be optimized precisely to produce coatings tailored to the transparency and conductivity requirements of the many different applications of transparent electrodes. Researcher Jaewon Kim said the next steps for the team will be to develop precursors that will decompose at lower temperatures, which will allow the coatings to be deposited on plastics and used in flexible electronics, and to produce larger prototypes by scaling up the deposition. “The spray-coater we use can be automatically controlled and programmed, so fabricating bigger proof-of-concept panels will be relatively simple,” he said. The research was published in Advanced Materials Interfaces (www.doi.org/10.1002/admi.202000655).