So how do silicon solar cells work? Well, as we've learned in the last week we need a PN junction, so we need this junction so that we can create electron hall pass and we can separate them, we already know this from last week. So let's go into what specifically makes silicon solar cells unique. So here's a drawing of how a silicon solar cell is built up. So we have an emitter on top, and L plus doped silicon layer, and we have the P base at the bottom, so slightly doped base material. There's a few things that is unique about silicon. First of all, the wafers are quite thick. They're quite thick because silicon is an indirect band gap material. So we need a lot of thickness in order to have enough absorption. And for that reason there's some specific requirements. So in the case of silicon solar cells we, of course, have the space charge region like with any PN junction. This is, of course, the layout where we have an in built field, that ensures that we can separate electrons and holes. We also have something we need to consider because as I mentioned, the layer is really, really thick and the diffusion length of the charge carriers, the electrons and holes, is not infinite. So there'll be some sort of diffusion length, leave it. And you can see I sort of drawn it our here, that there basically is a limit to how far the electrons, or host, will travel in the material, and this has a lot of consequences. So let's just go through what happens when we absorb photons in this material, and see what happens when we do it at various places inside the layers. So first of all, let's see what happens when we have absorption in the space charge region. Well, the space charge region, of course, up here and in this case we create an electron hold pair as we know this, of course, optimum because that means that the electron hold pair is within the space charge region. And we have the strong electric field that can disassociate this electron hold there so basically all of these absorption events will contribute to the photo current in the end. Let's move onto another situation. Let's look at a situation where the absorption within. The region that's within the diffusion length. Within the diffusion length there's a strong probability that the electron will generate, so that the electron will diffuse in the space region and be carried out. This, of course, is not an infinite probability of this but it has a high probability that this electron whole pair will contribute to the folder current. So the next situation we can look at is absorption within the emitter. So here, it becomes a little bit more tricky. The diffusion rates within the emitter is really, really, low because of the high doping. That means an electron whole keratin will adhere, has a smaller likelihood of contributing to the photocurrent. This, of course, depends a little bit on exactly where in the emitter the absorption takes place. But generally, these photos, which are absorbed in the emitter, can also contribute and does also contribute highly to the photocurrent. Then, we have the last situation. The last situation is where we have an absorption outside the diffusion length. This is really problematic, because now we are in a region of the solar cell where there simply isn't enough time for the electron to diffuse. So they'll typically end up being lost and photons absorbed here will not contribute to the photocurrent. So just to iterate, we can have absorption within this base task region. We can have absorption outside this diffusion length, inside the diffusion length, and inside the emitter. And depending on where the absorption takes place, there's different probabilities of the photons aiding or contributing to the photocurrent.