In this video we look at two important aspects that ends up affecting the efficiency of the solar cell. One is reflection and the other is absorption. So if we start out with reflection, any material ultimately reflect light, and this is mostly determined by the band gap of the material. When light comes in, it'll reflect some light back. And if the difference between the refractive index of the material and air, the medium from which light is coming is is big, then the reflection loss will be quite big. Silicon, for example, has a really high index of refraction. Meaning we get a lot of reflection. >> Look at silicon. We don't do anything to the surface, it actually reflects a lot of light back. Silicon can be for polishing, it's a quite good mirror. If you work with a polished silicon wafer it's an excellent mirror. That means it reflects a lot of light. That's not ideal, so we need to do as much as we can to reduce the reflection from the front surface. Reflection loss is a very direct loss you can see. If the light that comes from the sun is reflected away from the cell before it even reaches the bulk there it's lost. It's not like you get a second chance with that light unless you do something to get that. So we want to minimize reflections across the spectrum of wavelengths that's relevant. So there are many ways we can reduce reflection losses. One is we can introduce an anti-reflective coating in between the solar cell and air. And this coating should ideally have an index of refraction that's in between the solar cell and air. In that case, we'll reduce the reflection losses. There's also a lot of other techniques we can use, for example, texturing of the surface or nanotexturing. But all of this is outside the scope of this course. So we won't go much deeper into it, but I'd encourage you to watch the full interview with Rasmus where he discusses different tactics to decrease reflection losses. It's important to notice that not all types of solar cells have the same requirements for anti-reflective coatings. So you've just heard silicon is a really good mirror on its own, so there we really need to look into optimizing the reflection losses. You can see I have a silicon wafer here. And sort of the bluish appearance you get from it is because of this anti-reflective coating. And as you can see, the silicon solar cell is not a good mirror at all, so it has a quite efficient anti-reflective coating. Another type of solar cell can show you is this polymer solar cell. This polymer solar cell has no anti-reflective coating, and in fact it doesn't really need it because the index of a fraction of the polymer material is much closer to air than silicon. So there's much less to gain by introducing an anti-corrective coating in this instance. Another important aspect we need to talk about in regards to solar cells is the absorption of light. So we already know that if we have a photon coming in with the correct energy, we can absorb it in the semiconductor. However, we forgot to mention something that's really important, and that is that there's a distinction between what we can call a direct band gap material and an indirect band gap material. A direct band gap material behaves like we would imagine. If a voltron is coming in and interacts with a crystal lattice, it'll get absorbed and promote an electron to the conduction band if there's sufficient energy. However, if we have an indirect band gap material, the situation is a little bit different. And the reason for this is that in our previous simplification, we just looked at the energy bands of the semiconductors. And what we forgot to mention is that this energy gap is depending on the crystal direction, or more precisely, the crystal momentum. And the reason for this is we didn't discuss the band diagram of semiconductors. Here we won't go into much further details except to say that for certain semiconductors, the crystal momentum is important in connection to absorption. And this is what is being represented here. So in some semiconductors, it's not enough to have enough energy to overcome the band gap. We'll also need a crystal vibration in order to get an absorption of it. And in that case absorption becomes much less likely. So that means we need thicker materials. And the reason I'm introducing this topic now is that silicon which is the most used material for solar cells is an indirect backup material. And that means at really low temperatures, silicon is actually transparent to red light. And you can imagine that's because at really low temperatures, close to absolute zero, there's not a lot of crystal vibrations. So therefore absorption events are really unlikely. This also has the consequence that silicon solar cells need to be quite thick in order to work properly. >> Launch with silicon as the substrate for affordable types is that silicon is an indirect band gap material. So that means we need a lot of material to absorb a sufficient amount of sunlight, a sufficient amount of photons, to create the power we want from the solar cells. We need a lot of material. We need much more than thin-film solar cells, for example, that are made from direct semiconductors. So a typical silicon solar cell is about 180 microns thick. So that's actually much thicker than a typical thin-film solar cell, only being a few microns, or even less. >> So in the remainder of the course, we'll talk about different types of solar cells. In the next week, we'll talk about silicon solar cells and thin-film solar cells. And the main difference between these two types of solar cells are exactly that silicon solar cells are indirect band gap materials, while all the thin-film solar cells are direct band gap materials. So this means they can absorb light even if they are a lot thinner.
