 Hi, I'm Zor. Welcome to a new Zor education. We continue talking about transformation of certain types of energy into electricity and in this case in this lecture we will talk about transformation of solar energy into electricity. The way how it's done is very different from the previous explanations of how for instance kinetic energy is transformed into electricity or chemical energy. So it's different and what's important is that in case of sun the energy of sun is free and if we are able to convert it into electricity, well great free energy. Well, it's not simple, but it is possible and that's where the solar panels are coming from. Now this lecture will not be about technical details of how solar panels are actually made. It's more about the principles of the functionality of the solar panels. These are purely theoretical principles and they are related to semiconductors. So I will have to talk about how semiconductors working. So basically again, this is not a technical lecture. It's a theoretical lecture. And I think it's very interesting and it's very important to understand actually what are the basic principles behind generation of electricity from sunlight. This lecture is part of the course. The course is called Physics for Teens. It's presented on Unisor.com. If you found this lecture on Unisor or anywhere else on sorry on YouTube or anywhere else, I suggest you to use rather the website Unisor.com which links to YouTube lecture because every lecture on Unisor has textual notes, which are basically like a textbook. So it's very useful not only to listen but also to read exactly the same thing. Well, almost exactly the same thing. And there are some nice pictures, which probably are much better than whatever I will try to picture here. Maybe some more detail formulas or something else. Plus the website contains exams for those who are interested in challenging themselves. And the website is completely free. Unisor has no advertising or anything else. Access is completely free. No strings attached. Okay, so let's talk about solar panels. Now, the main component of the solar panel is silicon. This is chemical formula for silicon. Now, what is silicon? It's an element. By the way, don't mix silicon and silicon with E letter at the end. This is an element, an elementary element which occurs in nature. The silicon with E is basically an artificially made chemical compound with silicon as a component, but it's a completely different thing. So we are not talking about silicon with E. We are talking about silicon like this one. Okay, and the chemical formula is Si. Well, it's interesting that silicon is probably one of the most frequently occurring element on Earth. Basically, it's part of the silica and it's basically sand. So there is a lot of silicon which we can actually use. So this is the main component of solar battery. Now what's important about silicon is it's not a metal, but it's not a dielectric either. So it doesn't conduct electricity as well as metals, but it doesn't really serve as a dielectric, which means completely inactive electrically. It's in between. Well, that's why it's called semiconductor, and that's why silicon is the main component in all the electronics which we have. It's all based on the principle of semi-conductivity, which I'm going to explain right now in some details. More details probably will be in a separate lecture about semiconductors. This is an application of semiconductors to solar energy usage. So structure of the atom of silicon is. It has 14 protons, certain number of neutrons. We are not talking about neutrons because we are interested only in electrical component and around this you have 14 electrons, but 14 electrons are not on the same orbit. They are actually on three orbits. One, two and three. You have two electrons here. You have eight electrons here, and you have four electrons here. So the most, the outermost orbit, which is actually the orbit where so-called valence electrons are located. Those which are related to chemical and other bonds with atom, with other atoms. It has only four. Now, on the website, I do have a picture of the structure of the crystalline structure of silicon. You see, these are four electrons. So there are four connections and other atoms also have four electrons on their outermost orbits. And electrons are actually connected. It's called covalent bond. So when many atoms are present, they are connected through these covalent bonds. And I will depict it as this. We have four electrons. Each one has four electrons. So that would be my flat representation of the structure of the crystalline structure of silicon. In reality, it's supposed to be a three-dimensional structure. And that's why I'm actually referring you to Unisor.com, where the notes for this same lecture contain a nice picture. The idea is, if you consider tetrahedron, which is basically like a triangle and this triangle, a pyramid. So in the center, you have an atom. And in four other angles, you have atoms. And that's how this represents a three-dimensional structure of the crystalline silicon. But on the flat surface, I will just for completely educational purposes. I will depict it as this. So each atom has four connections. So this is the inner structure of the crystalline silicon. So every crystal has a three-dimensional structure, which I will represent on the flat surface as this. And four connections means that each atom has four balance electrons, and they are connected among themselves, these electrons on the outermost orbits. They are connected into covalent bonds. And that's what makes the crystal. These covalent bonds are rather strong. Considering we are talking about electrons on the outermost orbits, probably their relationship between the atoms, I mean between the electrons, which are actually making these covalent bonds, is probably stronger than the relationship with the protons inside the nuclei. So again, the existence of these covalent bonds is very, very important for the whole semiconductor industry, if you wish. Now these atoms are neutral because the number of protons is 14 and the number of electrons is also 14. Two on the inner, eight on the middle, and four on the outer orbit. Okay, so it's neutral, which means if you will just, you know, connect, if you will try to measure its resistance. So you put some kind of probe with some voltage and you will try to measure the electricity, the current. You will not see much. Primarily because it's not a metal. So these electrons are really kind of strongly connected to each other. They're not like really free electrons. I mean they're relatively free, but not as free as in the metals. So basically you will not see a lot of conductivity. On the other hand, if you will heat it, which means you will excite electrons more, obviously, right? The heat energy is transferred into kinetic energy of the electrons. They will be a little bit more inclined to leave their place and go somewhere else. So the resistance would be diminishing and the conductivity will be increasing of these semiconductors with increase of the temperature. Or any other form of exciting, like for instance, sunlight. Sunlight also excites electrons, right? It's energy. Well, it's basically heat in some way or another. And that would be actually the principle. But again, right now, if you will just measure resistance, it would be very high. Even if you will, you know, increase the temperature or sunlight, it will increase a little bit the conductivity. But it will not be a source of electricity, obviously, because it's completely neutral. How can we make it a source of energy? How can we make sunlight? Even if it just, even if we will heat it or put sunlight, how can we make produce electricity? And that's the next topic. Okay. So the first thing which we do is we will slightly change this particular structure. We will introduce some impurity, some add-on. How, let's not talk about how, but there is a way to replace atom of silicon with atom of, let's say, phosphorus. The phosphorus chemical formula is P. What's interesting about phosphorus? Phosphorus has five electrons on its outer orbit. Actually, some other elements will do as well. So if it has five, it has one more electron here. So these are silica, silica. These are all silica. And a little bit of phosphorus somewhere here. Here you might have another silica and another silica and then another phosphorus, something like this. So whenever we introduce certain amount of phosphorus inside the silicon, I will have these electrons, extra electrons, which are not bonded to anything. So they're also on the outer orbit of atom, which means they are, well, might be a little freer than the others. But what's different between this free electron and the outer orbit of phosphorus and these four electrons, which are on the outer orbit of the silicon, these are related to other atoms and they're bonded through covalent bonds. This one doesn't have any covalent bond. So it's freer, so to speak. Well, what happens in this particular case? Well, it's relatively free. So you can just go somewhere. Where can it go? Well, for instance, it can go to another atom of silicon, replace it in the covalent bond. But that would free electron from silicon. So in this particular case, we will have certain number of electrons, but as many electrons as many atoms of phosphorus are introduced, right? So we will have these electrons, which are kind of floating around. They might replace electrons in in silica, which frees that electron, which they have replaced, and that one will go further. And all these wandering of electrons will definitely occur in the mixture of silicon and phosphorus. Why? Because phosphorus has one extra electron on its orbit. Now, the whole thing is still electrically neutral because phosphorus has one more proton, obviously, and one more electron. So the whole thing is electrically neutral. However, it more resembles like metals, for instance, which are electrically neutral, but there are many electrons which are relatively free, those from the outer orbits. They are not exactly, you know, strongly connected to to nuclear, and they can just jump to another atom, replace those electrons on the outer orbit, push them further, et cetera, et cetera. So this wandering of electrons occurs in this particular thing a little bit more like in the metals. OK. Now, this thing is called n-type, n stands for negative and negative because electrons are negative and we are having these floating wandering electrons. OK, let's consider a slightly different situation. Let's consider, instead of atom of phosphorus, we will add atoms of boron. Now, what's different here? Boron has three electrons on its outer orbit, less than four, and that's why I did not really put this connection. So this connection from this atom remains basically unattended. From the boron side, there is nothing. Well, they call it a hole, basically. So instead of extra electron, which is negatively charged in case we add phosphorus, we had we have an absence of an electron or again it's called a hole. Well, and what happens with this hole? Basically, it plays the role of positively charged particle. And it's also wandering around, it's floating, because, for instance, this electron becomes unbonded so it can jump to here. And that actually has extra electron in the boron. But extra hole here. And then this hole can be filled from, let's say, this electron and the hole will go here. So as soon as we have one missing electron, which is basically not connected, which really makes a hole in the covalent connection, this hole can wander around, can float around because in its place, some other electron can actually jump. So in this case, we have a different type of kind of an interesting material. It basically resembles metal, but not in a negative part, negative sense, but in a positive sense, because this hole actually means absence of electrons and the hole is floating around. The whole thing still, again, is electrically neutral, but there are positive holes floating around and that's why it's called p-type. So now we have not just a silicon, which is electrically neutral and stable without anything floating around inside. We have two kinds of semiconductors. We have a p-type semiconductor, which is silicon with some kind of impurity embedded in it atoms of boron. And some others, not only boron, some others as well can be edited to introduce the same effect. And also we have an n-type where extra electrons, in both cases, in p-type semiconductors and n-type semiconductors, we have certain particles. Well, in case of a hole, it's not really a particle, it's an absence of particle, but it plays exactly the same role. Something is floating around and basically introduces some carriers which we can use in some kind of an electrical scheme. And now, obviously, we are approaching how we do this particular thing. What if we will have the n-type semiconductor and connect it to this p-type? n-type has floating negative electrons. p-type has floating holes. I will use plus as a positive thing. Now, what happens next? Well, if they are really close together, I mean really close, like contact, very nice contact, it's basically two relatively small disks, let's say, or squares of silicon with added impurities, impurity of producing n-type in one case and impurity of producing p-type in another case. And these two plates of silicon are just bonded together somehow. This area, now, electrons are floating, basically, right, and holes are floating around all the time, by themselves. Now, what happens in the borderline? Well, the borderline happens very interesting thing. When electron approaches here, hole approaches from here, well, electron is a negatively charged particle, hole is absence of this, so there is some kind of atom here with a missing bond, a missing link. It needs, well, it will bond together, basically. This electron will replace absent link of the atom here, and as I was saying, covalent links are very strong. They are probably stronger than the relationship between electron and nuclei. And that's why this bonding together happens. Now, as soon as we have this type of bonding together, what happens is this thing used to be electrically neutral, but now this electron comes here. It replaces absent link, bond together, but now we have one electron greater on the bottom part and one electron less on the top part. So this diffusion on the border between n-type and p-type causes the whole thing charged with minus here and plus here, because the electron comes here, used to be neutral, now it has a negative sign, and this one used to be neutral, but now it lost the electron, so it becomes positive. So in the immediate neighborhood of this borderline, there is this p-n junction, as they're saying, p-n junction where the electric charge actually exists. The part of the p-type which is close to the border becomes negative and part of the n-type which is close to the border becomes positive. So it's really built up up to a certain extent and then when the negative electricity in the bottom part becomes relatively strong, it prevents electrons from diffusion down and obviously the whole thing, the whole process stops. However, it stops only after certain electromotive force or voltage has been accumulated between these two plates. So the electrons which are accumulated in the borderline are probably again, can probably float around a little bit, maybe not, maybe yes, but these are very, very thin plates, so basically it doesn't really matter how deep it goes. What important is that the bottom plate is actually becomes negative and the top one, which is n-type, becomes positive. And there is certain voltage between them, not the big one, really very, very small one, but there is one. Now, what happens if we will start exciting electrons on the top? Well, we will increase their energy and if the layer of negative electrons was sufficient before to prevent other electrons to diffuse with excited electrons, it will probably go a little bit deeper and the more excitement we will introduce into electrons above, the bigger difference in the potential, electric potential between top and bottom plates will be, the more voltage will be really observed. And finally, if we will connect them with a wire, with some kind of a load or whatever, what happens then? Well, the accumulated electricity, negative electricity here will go back here and it will basically compensate this plus. But then again, it diminishes the negative potential of the bottom part and the diffusion again can start. So if we just have this particular thing, we will have electricity circulating. Now, if we will introduce the heat or sunlight or whatever else which excites from the top, from the n-type, we will have a more intense electricity and the more excited electrons are, the more intense diffusion will be there and therefore, the more voltage we will observe on this battery. Basically, that's the principle. As long as we have two different n-types and p-types, semi-conductor connected to each other, we will have this electricity flowing from this to this. But electrons will flow from here to here. The flow of electricity for whatever reasons is from positive to negative. So it's opposite to electrons and so the arrow will be here. Okay, so this is the main principle. Now, how do we actually make it? Well, again, as I was saying, we have two different silicon plates. One with introduced impurity which makes it n-type. Another introduced impurity which makes it p-type. Let's say phosphorus and boron. And then, if we will introduce sunlight as an exciting factor for electrons, well, then we will have basically the electricity. Now, in practice, usually there are not very big ones, these silicon plates, but then you can connect them in series, one after another after another, which will increase their combined voltage. So usually they are making, let's say, squares, something like this, and they combine a certain number of squares into big flat surface, which is basically called solar panels. And then you introduce basically a certain number of solar panels, you put it on a roof or whatever, and they will produce certain electricity. Now, I did not discuss any technical aspects. How do we connect it to the electrical grid, to any kind of devices, etc. Deliberately, this is not the purpose of the lecture. The purpose of the lecture is to explain this particular principle, the PN junction and its role in creating the electricity. And this PN junction mechanism is used like everywhere we see electronics, in all the televisions, computers, etc., etc. It's all built on this principle. That's why it's very important to understand it. I'll probably talk about this more whenever I will talk about electronics and semiconductors. But I just wanted to, and I'll probably repeat more or less what I just said, but in this particular case, this structure, the structure of the atoms of silicon with certain impurities and the PN junction, that's something which you have to really remember and understand. That's it for today. Thank you very much. And good luck. By the way, I do suggest you to read on the Unisor.com. I do suggest you to read the notes for this lecture and look at the nice pictures, much better than whatever I put here. Okay, take care. Thank you.