Through the past few weeks you've been learning a lot about solar energy, a lot about solar cells, and in particular about solar cell that is, all our studies here are centered around the organic or polymer solar cell. And in this module and the next one, we are going to focus on the actual methods we use to make them. So the printing and coating methods used to form these thin functional multi-layer stacks. Before you start making your solar cells films, you need something. You need an ink, and the ink is a liquid. Typically, most of it is solvent. And in this solvent or it could be water, it could be an organic solvent. It could be a chlorinated organic solvent. In this solvent you've dissolved your material that you need to deposit the layer of. The property of the ink is critical for the success of the application technique, so printing or coding technique. Once you've formed the wet film, from the ink, using the method. You dry it so the material that's dissolved in this ink, this liquid, is left on the surface. And that is what, in the end, constitute a particular layer in your solar cell. So we're actually going to make those thin functional films using an ink, a solution, a liquid that we have to deposit on a thin substrate. Ideally we want to do it in a roll to roll process so the foil isn't moving while we do it. And there are many, many ways of doing it. I should also add at this point that perhaps the functionality of the solar cell in the end or of the particular layer that we are making in the solar cell state depends on both on the, also on the printing or coding process. And therefore, for particular ink, for a particular layer, this particular precision method That is the optimal one. And you never know this in advance. So I'm going to run you through all those and the first distinction you make is whether it's a coating technique or a printing technique. And in this first module, I'm going to go through coating techniques. Now, the coating techniques and the printing techniques, they can be divided according to their dimension areas the solar cell stack is a multi-layer structure. In its functional form, typically it comprises at least 4 or 5, sometimes 8 to 10 or 14 different layers, that each serve a purpose. The coding or printing technique has what we call a dimensionality in the lateral plane. So in the surface plane of the solar cell, but also in horizontally, so most of the film foreign techniques they are in the lateral plane so they can be zero dimensional, one dimensional, or two dimensional. Now for the coding techniques, generally they are all zero dimensional. That means you cover the entire lateral plane. So make a coat, a painting, so the entire surface is coated with this particular technique. Most of the time we want the layer to be smooth and even or a well-defined thickness, but sometimes you also want a pattern etiquette. So, if we focus on the coating techniques that form an even, homogenous layer, they fall in a few different categories. Mostly they are non-contact techniques. That means that we are either spraying the material, the ink, onto the surface. We can just be pouring it over it or forcing it form a narrow gap between a moving web and a knife, thereby creating a liquid film that is of an even web thickness. When this dries, you get an even, ideal at least, an even dry film. There's also a technique called Slot die coating that we use, that has the slight advantage that you can actually control the wick thickness within a given range. There's a coating window. And then we have a technique called Spin coating that is a very, very useful laboratory coating method but that perhaps is not a useful method from an industrial point of view. So in addition to those, the quality of the films formed by this method, the amount of materials they use, the ink properties that are required for them to work successfully, there's also this issue in addition to the film quality of how wasteful is an ink form process and if we can control the wet layer. Now the spray coating is perhaps the one that everybody can relate the most to. It's like spray painting an object. You have some control over thickness. The more you spray, the more liquid you deposit and therefore in the dry film, the more solid you get. The spray coating method is possible to use through a shallow mass, so you can in principle form a path, but generally it is a difficult method to make patterns with, so it's best to make an even overcoat of a surface. The drying to an even film is often a very delicate interplay between the ink and the drying and how the liquid particles or droplets are formed in the nozzle. And how they evaporate during transit between the nozzle and the substrate. How they then coalesce on the surface and dry out to form a film. So this heavily impacts morphology and from this part of the review, when you look at the organic source. So as you've learned, the nano morphology is extremely important and critical. The spray coating method is, while simple. Very difficult to control when it comes to establishing a fixed morphology or the morphology in relation to the application method. Also if you do pattern through a mask, you have quite a significant waste because you have to spray ink over your mask. So the areas that you don't want to be coded on your surface. This ink has to go somewhere, this ends up on your mask of. So there is some loss when you on it that way. If you do that. But generally, it's viewed that inherently dimensional coding technique. Another simple technique is the knife coating technique, where you basically put ink on the surface. You pull it over the surface. And you force it between a gap formed between your substrate, so the surface that you want covered, and a knife. And there's a well defined gap between your surface and the knife. This gives you, ideally, an even wet layer. And this technique is also a non-contact technique. And it only allows for fully coated surfaces. But it has inherently good firm thickness control. So you can make very large areas of very even layer thicknesses with a high degree of leveling. So very smooth surfaces. Spin coating technique. It's a technique often used in a laboratory. It's fantastic because you can make very small substrates, and it's very reproducible. You basically spin your substrate, typically a small object the size of a microscope slide or a few centimeters by a few centimeters. And the thickness of the wet film you form is a complex interplay between the shear rate formed at the surface, the rate at which liquid evaporates or solvent evaporates, and the viscosity of your solvent. And in principle it's a very predictable method so for a given solvent. And a given viscosity for your ink composition and a given locational speed on your object, you can invariably protrusively make films of the same thickness and same evenness or smoothness. One disadvantage of the spin coating technique is of course that, as you see you add perhaps a milliliter or less of ink and in reality most of it is lost as it is ejected from the outskirts of the fastest spinning object. So in reality the majority of your ink is thrown away and ends up inside the spin coat. So from the materials proximity point of view. It is a wasteful technique, but from the point of view of being being a reproducible authority technique, it's very very useful and perhaps that's also why it's been extremely successful in the majority of solar cells reported today have been prepared by spin coating. Now techniques are mentioned, perhaps spin coating is highly scalable. So is knife coating, spin coating is, of course, not. Common to those three method is that they are, we could call it, self-needed from the point of view that we do not really control thickness through. As a piece of software, cannot control the way the thickness, for instance, in a predictable manner, for the program. It's very much inherent to the ink, or the method that you use should have occasion method. So a given ink with a given setup will give a fixed thickness. You don't have to think about it but you cannot control it.
