Spray coating is a simple technique of thin and uniform film deposition. The manufactured layers may be organic or inorganic (usually oxide) functional coatings. Compared to other techniques, such as slow and expensive ALD, PVD, or CVD, it is cheap and fast. What is more, it allows the deposition of layers not only on flat surfaces but also on 3D structures of various, often irregular, shapes. This property is used especially in electronics, but also in medicine, communication, and automotive. Despite many advantages the spray coating method is not widely used in the PV industry so far. However, many scientists point out that this trend will change soon.
The classic spray coating method is based on three basic steps. In the first stage, the substrate is selected, and then it should be properly prepared. In the second step, the substrate is covered with
a liquid precursor, often at an elevated temperature. In the third step, the coating is heated, which may be accompanied by a chemical reaction.
Figure 1. The mechanism of classical spray-coating method
In the DMOPV project, the substrate for testing is quartz glass, which is thermally stable. The use of a different type of glass may result in the diffusion of atoms from the glass into the layer, which we had the opportunity to see at the initial stage of our research. It was found that sodium contained in borosilicate glass migrates to the copper oxide layer and is detected there. In the second stage, the glass is coated with the liquid precursor. The process is carried out at an elevated temperature. In the third step, the coating is annealed and a chemical reaction takes place in it, leading to the formation of metallic copper, which is then oxidized in the air to copper oxide. The obtained oxide is finely crystalline and, depending on the applied thermal process, consists of CuO, Cu2O, or a mixture of the two.
Figure 2. The TEM cross section of copper oxide thin film manufactured by spray coating technique in the project.
The video below shows the rapid chemical reaction that takes place during annealing of the deposited precursor as well as fast metallic copper oxidation which leads to the formation of copper oxide.