There is another coating technique that is very scalable, very useful, in the context of organic photovoltaics, at least, and that is slot die coating. I should mention one thing about all the other techniques, is that the ink is always very exposed to the atmosphere. For spray coating, you have droplets flying. For spin coating, you have a wet film exposed while you dry it. And for knife coating, you have the ink that is kept in your bath in front of the knife is also in full air exposure. In the case of knife coating, it is possible to make a chamber around it to a certain extent. But generally, there's a large amount of air contact. The slot die coding method is a completely closed system, so the ink is only exposed, at the very point of application. And in addition to that, it is a pre-metered coding technique, from the point of view that you can control the wet thickness, within a window, of course, you can't make it infinitely thin or infinitely thick. If its very thick it will run off the surface. If it's too thin it simply will not be able to cover the surface. But for any given ink, any given surface that you're coating with a given surface energy, given surface tension for your ink, there is a range of thicknesses at what thickness that you can achieve by controlling a pump. So you pump in your ink into this coating head. As the substrate moves, you can also control the speed of your wet, or your, your substrate. And with those two parameters, you actually control the wet thickness, and thereby, in the end, also, the dry thickness. Slot die coating gives, like knife coating, very, very smooth films. In addition to, making zero-dimensional films, slot die coating actually has the possibility to, to make one-dimensional patterns, so a stripe, a continuous stripe of ink. Now I mentioned in the very beginning of this, lesson, that, that coating methods were zero-dimensional. And, of course, this is the truth but with a little modification. I already said that the spray coating method could be patterned into a shape, a two-dimensional shape, or something else, if you did it through a mask. You can also control the thickness. I mentioned this. So you have a horizontal, sorry, a vertical, control in the coatings or control with the file drive thickness of your, of your layer. I also mentioned that slot die coating could make stripes, and I would like to finish off this lesson by saying that the coating techniques generally apply an even layer in the horizontal plane. Whereas those modifications can include different dimensionality in the vertical plane. For instance with a spray coater, you can just keep spraying. You build up a layer, thicker. With a slot die coater, we've shown here if you, that, a meniscus, guide and a shim, a mast, that define some slot openings where you can make the stripes. So there you have a one-dimensionality in the lateral plane. You can also make, a slot die coating head where you have several slots after the, each other. So you form a wet layer, for instance, of three different inks in the same coating step. So as the web passes, you simultaneously deposit three different liquids on top of each other. Those three different liquids, they lie on top of each other in the wet film, and when they dry out, the solids that are in each of the inks, in this case the black, the red, and the green ink, they end up in the dry film on top of each other. So even though in this case you have a full overcoat, so it's a zero-dimensional film, there is actually a dimensionality in the vertical plane where you have first, material A, and material B, and finally, stacked on top of both A and B, you have material C. So the coating techniques do allow for some sort of dimensionality, but it's not something that is visible to the human eye. As in the traditional printing forms that we will hear about in the next session where you can print text or, it is fully two-dimensional patterning in the lateral plane. In addition to the, coating techniques I mentioned, there are many, many other coating techniques. Some of them less relevant for organic photovoltaics or organic solar cells. But there are techniques such as slide coating, or curtain coating, or bar coating, or coil coating, and I think the best-known example to you, would be curtain coating. And, and this is something if you enjoyed a Mars Bar or something like that, it has been curtain coated. But it's generally for very high-viscous materials and thick inks, and if you want to prepare thin inks using curtain or slide coating, you need exceptionally high speeds. And this might in the future prove useful in the context of organic photovoltaics, but at the moment, even at the pilot scale level, processing speeds are too low, for us to access the regime where those coating techniques become relevant.
