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Module 1: Solar Cell

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Discussion about The p-n Junction

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So, here we have a p-n junction. So, we will start our discussion on the p-n junction with this image here. So, what do we have, as we saw we you can get a p-type semiconductor by taking silicon and doping it with 3 A group of elements. So, in this case, for example, I am just saying that we are doping it with boron, which is in the 3 A group of elements and then. So, that is this side; that is this side of this sample which I have coloured green for easy identification on our right side we have silicon again silicon, and it is now doped with phosphorus which is a group 5 A element. So, this is what we have done and. So, this side is p and this side is n, the p-type semiconductor is on our left side and n-type semiconductor is on our right side. Now even before I proceed further on this one fundamental material aspect I should alert you to and that is that when we speak of a p-n junction. So, there are 3 things here there is a p material there is an n material and there is a junction. So, that is the point that we should keep in mind, and it is a very important point to keep in mind that junction is essentially here okay. So, that junction is here now the junction is a very important part of this unit the p-n junction. So, when I say important the point that we have to keep in mind is that notionally you can actually take a p material that is separately made, and an n material that is separately made and then you can bring them together okay and then press them against each other. When you do that you do have a p material, you do have an n material and at the place where they have come in contact, you have tried to create a junction. Now that junction is highly imperfect at an atomic level okay, because a surface even though it looks you may polish it and it looks very nice and shiny when you look at it, if you look at it under the microscope at the atomic level it is extremely rough it is up and down in all different ways. So, you will have a junction where let’s say you have a sample that looks some surface that looks like that and another surface that also looks like that. So, then you will have. So, let’s say this is p and this is n, then you have a region here which looks like the junction or where you have tried to create the junction, but that junction is extremely poor. You find a lot of gaps here that will plenty of gaps in that junction so the junction exists only on specific points, where those 2 materials are in very good contact at an atomic level. For the rest of it the junction even though it is there for us in from a visible from our eye perspective the resolution of our eye perspective, in reality, it doesn’t exist and we will have extremely poor characteristics it will try to mimic the characteristics that we are going to describe here in this class. But it will do a poor job of mimicking those characteristics, primarily because the junction is in a very poor state okay. So, it is very important to keep that in mind and so, even though we conceptually talk of a p material and an n material and n junction, and it may become easy to think of it as 2 separate materials that have been put together even descriptively when we are trying to understand their behaviour, it may make sense to describe it that way. In reality, if that’s how you make it, it will do a poor job of functioning so. What they do is they take say a single crystal of silicon and then in that same sample. So, it is already a single crystal, there is no boundary in it, there is no grain boundary in it, it’s a single crystal of silicon we dope it from one side with the p-type of dopant, and we dope it from the other side with the n-type of dopant. Those dopants diffuse and in the middle, they form a boundary. So, at this point, you create a p-n junction, where you have already got atomic level contact atomic-level order and therefore, the junction is very well defined. And so, this junction, if you build it that way, is very well defined okay. So, that’s a very important part of this p-n junction coming together to create a p-n junction. The other thing I will alert you to 2 more points that I think are very important before we move forward from this graph from this plot or the schematic, is that on your left-hand side I have put a lot of positive signs here okay. So, a lot of pluses plus I have put on the left side, and minus minus minus on the right side. So, I think we have to take a step back and understand what exactly we have done here. I want to alert you to the fact that this does not mean that the p side is positively charged, and the n side is negatively charged that is not the case. The p side is independently charged neutral, the n side is independently charged neutral. So, both these materials are charge-neutral, both these sides that you see are charge-neutral okay. So, both are charge-neutral both this site as well as this side, they are essentially charged neutral because what you have done is you have taken silicon and you have doped it with boron. So, silicon has the same number of an equal number of protons and electrons, and it is being doped which means specific silicon atoms are being replaced by boron atoms, which are also having an equal number of protons and electrons. The number of protons and electrons in boron will be not less than that in silicon and so they will that will be different, but within boron, the number of protons and electrons is the same corresponding to its atomic number, and again within silicon the number of protons and electrons is the same correct corresponding to its atomic number. So, given this situation overall this silicon doped with boron is going to be charged neutral, there is not going to be any either deficit of electrons or excess of electrons that is not going to be there. Similarly, when you take silicon and dope it with phosphorus, even though phosphorus actually has one extra valence electron overall it has the same number of protons as it has electrons and therefore, phosphorus by itself is charge neutral, and you are using it to replace silicon atom which is also charged neutral and therefore, the overall silicon doped with phosphorus continues to be charge neutral. So, what is the meaning of putting these minus signs and signs? In that sense, this is similar to what we are discussing here in a metal, where we said that you can have some free electrons that are moving around. So, this charge carrier we have indicated with a sign here. So, similarly here you can think of the p-n junction, the p side of the material to be consisting of a material wherein the bond structure you are short of one electron and therefore, it is ready to accept an electron from somewhere okay. So, it is ready to accept an electron from somewhere, and effectively release this lack of electron there or release this hole. And so, this ability to release that hole is what we are saying, what we are referring to by is indicating that it is it can release these positive charges or essentially the positively charged holes. The same way the same thing would be true with the n side of this material, it is essentially charged neutral, but then if you look at the bonds that are present there, it essentially has one additional because the phosphorus has one additional valence electron, that valence electron is easy to release and is ready to move around the system. So, when you create this material this n-type semiconductor, you have all the ionic cores sitting around and you have this free electron running through that material. You can think of it that way at room temperature, it is in a position to do that. Similarly, on the p side, it is equivalent of saying that you have this relatively neutral material, in which you have this negatively charged ions sitting at some location, with this positive set of charge carriers that are running uniformly throughout that system at very low with a very little amount of energy. So, the positive that you see here is the availability of those positive charge carriers that can move across. So, that is positive that you see here, and the negative that you see here is the availability of those electrons on this side which can move anywhere if they are given the opportunity. So, that is what we are referring to as this p-n junction, and what we are referring to here is the signs that you see there I will also point out that one additional aspect you should keep in mind is the orientation of this sample. So, throughout this class, we will sort of stick to this horizontal orientation. So, you are going to see the p on your left-hand side, and n on your right-hand side. So, many diagrams the few diagrams that you are going to see here are all going to follow this kind of a layout. When you use it as a solar cell, this is not the orientation in which it will sit, it will sort of be 90 degrees in the other way. So, you are going to have one type of semiconductor on top, and another type of semiconductor of the bottom and the junction is going to be in the middle. So, you are going to have something like this and you will have a junction okay. So, this will be the junction, this will be your let’s say the n-type and this will be the p-type okay. So, this is a junction. So, in a solar cell, the orientation is the other way, it is not the way you are seeing in the screen or as you will see through the rest of the slides that you will see in this class, in any case when we talk about solar cells as we come up to that discussion I will again alert you to it. So, some of the diagrams we will draw here you have to at least visually in your mind imagine them to be turned rotated at 90 degrees vertically, and then you can understand how it relates to what is going on in the solar cell. So, anyway. So, this is the diagram; now starting here we will add some more detail. So, what happens is once you bring these 2 together once you bring this p-n junction together initially it looks something like this. So, all you know positive holes which are sitting on the p side which are freely roaming around on the free side are on the p side, and you can think of it that way conceptually, and then on the n side, you have all these electrons that are freely roaming around. But once you know to create the situation where conceptually you suddenly brought them together, like I said that is not how it happens you build it that way, but let’s say you brought it together and you created this situation. (Refer Slide Time: 29:58). What happens is from a material perspective the electrons suddenly have access to the entire sample okay. So, although originally they started only here the Electrons, and now notionally they have this I mean they have access at least physical access is there to this entire extent of the sample, I mean there is a sample ahead of them and given that they are freely roaming around, and there is thermal energy available to them they are free to try to access that volume as well. So, they will start diffusing so they start diffusing into a region which is different from the region from where they originated, all right. So, this happens, and that is how some of these electrons are now have moved into this region. So, electrons have moved this way, I started they started in the n region and they moved into the p region. Similarly, the holes which were freely roaming around in you know p side of the sample, suddenly get the access they see that there is space available on the n side of the sample and they start roaming into that region. So, you have holes moving the same. So, I will put as h plus. So, what happened? These electrons and holes came from a region that was close to this. So, I will just remove this mark here and we will get back to it. So, these electrons came from a region that was relatively close to the boundary and then these are the electrons that moved across. Similarly, the holes also came from a region that was relatively close to the boundary and these are the holes that moved across okay. So, in a region close to the boundary or close to the junction, you are suddenly short or you are missing holes on the p side of the sample, and you are missing electrons on the n side of the sample because they started diffusing into each other all right. The ionic cores itself that are the silicon atoms as well as the boron atoms or the phosphorus atoms that are present, they are not moving at room temperature that is not we don’t have sufficient energy to break the bonds, these are all covalent bonds to break those bonds and to get those atoms to diffuse across, there will be some minuscule diffusion, but that is that will be completely negligible in this scale. So, but in general, they are going to just stay stuck. So, the core structure which is the p structure as well as the n structure we will remain, that basic framework will remain the p structure will remain on our left-hand side and the n structure will remain on our right-hand side. So, that basic structural remain, only the charge carriers suddenly have this freedom they start moving and so, suddenly on the n side we have lost some electrons because they went off into the p side, and on the p side we have lost some holes which have gone on to the n side. As a result of the there is a variation in the charge you suddenly see on the p side of the sample negative charge building up, and on the n side of the sample positive charge building up okay and so, that is what I have sort of indicated by the fact that you know with this movement of charge, you are suddenly having positive charges sitting here because the electrons left that place and negative charges sitting here because the holes left that place right. So, originally those 2 regions were completely neutral, but suddenly now you have this situation, where there is a charge that has been built up. So, that is why when you create a p-n junction, we and this situation begins to build, we refer to this as the space charge region okay. So, that is this is one term that is used it is called a space charge region. So, this is the region of space inside that material, where a charge buildup has been created. It is also referred to as a depletion region because the majority charge carriers in each side of the Junction have been lost. So, they have been depleted on the n side the electrons were the majority charge carriers they left that region and so, you have been depleted of electrons, on the p side holes where the majority charge carrier they also left that region and therefore, you have been depleted of holes. So, that is why it is also referred to as the depletion region. So, if you look at the diagram below, I took the same thing as what is above just and highlighted the fact that this continues to remain to charge neutral this also continues to remain to charge neutral. So, this effect is limited to some region, and it is a relatively sharply defined region because they start diffusing and then it comes to a halt why does it come to a halt and why doesn’t this continue indefinitely? It has got to do with the fact that the electrons and the holes are charged particles right and the framework from which they arrive to start with, is oppositely charged right. So, whereas, in metal in the sample in the original example that we looked at, all the ions the ionic core from one end of the sample to the other end of the sample the entire ionic core is all uniformly positively charged, and the electrons are uniformly negatively charged, therefore, the electrons can go from one end to the other and nothing happens it’s all charged neutral. Here is a p-n junction, the ionic cores, if you keep aside the charge carriers if you detach the charge carriers if you look at the ionic cores are going to be positive on the n side and negative on the p side. So, therefore, it is the same ionic core is not there from n to n, it’s a different ionic core on either side of that junction, therefore, the electrons cannot freely roam everywhere in the sample because they see a different ionic core on one side of the junction, relative to the other side of the junction and so, as they move it is no longer being charged neutral you are building up within the sample and. So, charges built up and therefore, a field is built-up electric field E, which goes from the positive charge to the negative charge, because this field is where it begins to oppose the flow of further flow of electrons or holes ok. So, it this field is building up it is now trying to prevent further buildup of the field and so, as the diffusion process continues slowly the diffusion process begins to slow down because the electrons are forced to now go up this barrier and they are unable to do. So, because already some electrons have gone and that repulsion is now sending them back. So, you have electrons moving due to diffusion which is a thermal process. So, they are continuing to do so as much as possible, because they just randomly moving around including across the junction, and then because the build there has been a field built up some of the electrons are being sent back, if you want to look at this discussion from the perspective of electrons. So, some of the electrons are being sent back. So, the field sends back electrons and the diffusion sends the electrons forward, diffusion tries to push the electrons from the n side across the junction into the p side, the field pushes the electrons from the p side back into the n side across the junction. So, when the 2 rates reach equilibrium, you arrive at this structure that you see which is the p-n junction with its depletion region or space charge region. So, that is how this junction behaves after it has been created. I would also add that in this figure the way we have drawn it and in many of the figures that we will see you would see a sense of symmetry here. In the sense that whatever I am showing you on the left-hand side of your image, I am giving you an analogous image on the right-hand side of the image, but is not necessary that has to be symmetric the way I have shown you. (Refer Slide Time: 37:19) So, for example, I can have space charge region and depletion or the depletion region looking very asymmetric across the boundary. So, if you look at the top figure here, the region is symmetric on both sides, you see the same extent of the region on both sides of this junction right that is because the p-type material and the n-type material were in this sample, in this example that we have been discussing although we didn’t explicitly say it and now I am going to say that explicitly the p-type sample, as well as the n-type sample which came together for this p-n junction both of them, were doped to the same level or in other words the doping concentration on the p side of the sample, was the same as the doping concentration on the n side of the sample. That is the reason why the concentration of charges on or the concentration of charges or the concentration of the availability of those charge carriers and charge carrier concentration is the same on both sides. And if it is same on both sides then you see this symmetric behaviour. Then for some number of electrons that cross over, since charge overall charge neutrality is there, you will have a same number of holes cross over to the other side and you will also have the region over which the holes come and the region over which the electrons come being similar okay because the doping concentration is the same. So, if you are going to access you know one million Dopant atoms, for example, you have to go to the same depth on the n side of the junction, as you have to go to the p side of the junction to get one million charge carriers on either side of the junction. So, that is how it happens if the Dopant concentration is the same. Supposing on the other hand as you see in the sample below the this is doped to a less low concentration and you have a higher concentration. So, when you have a higher concentration of Dopant on the n side, to get the same number of charge carriers you have to go to a certain depth to get those charge carriers and then send them across to this site. But on the p side because you are now having a lower concentration you have to go much deeper into the sample to get the same number of charge carriers and send them across right.