 So, solar cell structure looks like this, by the way we spent two full course on solar cell, one course on fundamentals other is on advanced features. So, there is a lot of material which I do not have time to teach, but I am just giving a brief overview of it. So, by the in the earlier drawing your solar cell, how does it look like your P type and in type? Normally solar this type of solar cell will never work, this is only the drawing this type of solar cell will never work or will give very bad efficiency. What will work is this, the junction is very thin, the wave the thickness of the silicon wafer in which you start, now a day the industry is standard is 180 micron, when I was doing PHD it was 400 micron and before that it was like 600, 700 micron. So, people are going for thinner and thinner and thinner wafer to save the cost because silicon is expensive. In that 180 micron, yes your junction should not sit at 90 micron, not in the middle, by the way it is nearly impossible to have the 90 micron junction thick, it will take one month time to make a solar cell, to have a junction diffusion at 90 microns is really difficult. What is what is people are doing is right now making a junction of 300 nanometer, 400 nanometer at best and people are going for below 300 nanometer, 150, 175, 200 nanometer ok. So, that is the junction depth. Out of the whole 180 micron thick material, your junction is sitting very thin and the top of the layer. That is one of the design parameters people have done it. Then you have, so you have the starting materials called the base, then you make your emitter that is a junction ok. The current in the industry the base is of P type and emitter is N type ok, base is of P type material emitter is N type. When the solar cell started in early 50s, the people were actually doing opposite, the base was N type junctions P type, then people have realized that no P type base and N type emitter is better. Now, people are again realizing that N type base and P type emitter is better ok. And a lot of people are actually trying to make the same thing which people have tried in 1950s for some reason, for some good reason ok. But most of the industry right now is P type base and N type emitter. So, there is a anti-reflection texturing ok, so reduce it because if you do not do this what happens? Absorption will be less. What is your first job? To absorb everything possible. So, what does it mean? You should not reflect and it should not transmit, it should not reflect, it should not transmit, third possible thing is absorption, it should absorb everything and for that people do the texturing. Then next job is there is also anti-reflective coating ok, texturing alone is not enough or good enough to reduce the reflection, then people also do the anti-reflective coating ARC. Then there is a bus bar and fingers, so fingers are actually, so solar current is generated everywhere in your device and the little fingers collects the current and then pass the current to the thicker line which is called bus bar and this bus bar is connected to the other solar cell and that there is a rear side contact ok. So, this is how the structure of the solar cell looks like. So, when you talk about the design of the solar cell, we need to worry about each and everything. So, what is the base material? How much it is doped? What is the junction? What is the depth of the junction? What is the doping level of the junction? How you are doing anti-reflective texturing? How you are doing coating? Which material you are using for coating? What is the thickness of it? What is the composition of the material? Are you doing surface specification or not? What is the, how many fingers you have to use? What is the width of the finger? What is the, what is the height of the finger? What is the width of the bus bar and all this, these are all design thing. One need to design this depending on the requirement or depending on the constraint that you have. So, this is how your junction should look like right. Is that clear? I do not have the actual solar cell with me, maybe you will get there is a live visit and you will get the chance to look at the various solar cell model. I am sure many of you already looked at it. Actually, we have the p-type semiconductor which looks like this, n-type semiconductor looks like this. So, we have the Fermi level in the p-type close to the valence band, Fermi level close to the conduction band. In actually, in the December course, we will go through the details of this also. So, that right now we do not have time, but this is just introduction to PV, right. I am just going through fast and then you create a spatial region. So, there are like holes, so this is n side. These are the fixed charges, mobile charges and because it is n side, n-type of semiconductor. So, negative charges are mobile charges, you have the positive mobile charges and the fixed negative charges is a p-type semiconductor. And as soon as I connect a p-type and n-type semiconductor, what happens? There is a diffusion, right, a carrier motion of the carriers or particles in this case from high density to low density, that always happens, right. So, the electrons are high in this number, they will diffuse at the other side. If the holes are high in number, they will diffuse to this side and so there is a electron going that side, hole coming this side, but this process does not happen in finite time. It cannot happen in finite time. If it happens in finite time, you know what will happen? All the electrons will get killed and all the holes will get killed. We do not want all the electrons are going to get killed. So, what happens? As soon as you, some of the electrons goes, it leaves behind a positive charge. Positive charge is basically what atoms, which are sitting there in space, which cannot move. Electrons can move, holes can move, but the fixed atoms cannot move. So, they are sitting in the space, there, the electrons will go there, hole will come from the other side and they will recombine, ok, so they neutralize each other. So, what is left behind is a positive, a line of positive fixed charges and a line of a negative fixed charges and by definition, this will create an electric field, right, as separation of a positive charge from the separation of a negative charge is electric field. The direction of electric field is from positive to negative, ok. As a result, what will happen? If this is the direction of the electric field, if any hole want to come this side again, it will feel a repulsion. So, holes are not allowed to come here. If this is the direction field, the force on the electron is in this direction, right, the force on the electron is opposite to the direction of electric field, because of the negative charge of the electron. So, if electrons are going this direction, it will feel a strong force from that side. So, after some adjustment, the electron cannot further flow, so the other side in holes cannot further flow to this side and the motion will start and you create a electric field in between. So, that is our property of the junction that you create an electric field and of course, you can do a lot of calculation to measure what is the electric field, how much is the width of the depletion region, if you are certain doping at the n side and p side, how much is the depletion region width at this side and that is a lot of calculation you can do at this point, we are not doing right now. But the important thing is to look at the junction itself. The junction looks like this, what side is the p side or n side, those who are sleeping please come back to the live mode, p side or n side, p side, this is n side, these are the holes, these are the electrons, yellows are the electrons, empty are the holes. There are four things happening, the four things happen to your semiconductor all the time, whether you are putting light or not, whether you are biasing it or not, nothing happens all the time, what are those, it is very important for you to understand again. So, this is p side, there are lots of holes and there are very small number of electrons right, from where these electrons are coming? No, you are doping, you are not doping, p type you are doping to create a hole, you are not creating electrons, but these are the generated electrons from the thermal energy, some of the electrons have enough energy to go there, thermal excitation, minority carrier. This is n type, there are lots of electrons because you doped it and there are some holes also, this hole is again created because of thermal excitation, some of the electron actually gone there, left behind a hole. So, there is a minority electron, majority holes at p side, majority electrons and minority holes at the n side, what can happen to this particle, what is this axis, energy and this is space. This electron if you want to come down, can it come down, can you go to the other side, can this electron go to the other side or not, yes or no? Raise your hands, let us do the polling, if you think it can go other side, raise your hands, anybody? No, right now I am not giving any other condition, yeah, just think it can go other side, how many other things it cannot go other side, you have to do some action, if you think it cannot go to other side raise your hands, except that everybody should raise your hands now, you have to do some activity, right? You cannot be neutral in this case, you have to do the voting, you have to choose what happens, if it goes it does not go, so raise your hands if it does not go, you have to really say that it does not go, ok, most of you do not want to decide whether it goes or forget our electron, ok, look at this plane, ok, it goes up and it is come down, what is happening? What is happening? So, it goes up, why it goes up? Because I am throwing it, I am giving it energy, why it comes down? It always comes down to energy, right, anybody, right? One common example is if I ask you to sit here in this room for another 20 hours, what will happen? You will go down in, so soon you will lie down on the floor, sit down on the floor and after some 20 hours you will lie down on the floor. So, that is the exact example what will happen to this electron? Anybody which is at higher energy level, you always like to come down to the lower energy level, ok. So, for this electron it is easy to come down always, right, because coming down in energy, right, this is my energy excess, is not it? This is my energy excess. So, this electron can always come down because going down the energy, unless without additional support it can come down. What about this electron? Can they go down? Go up this side? Most of them cannot, why? Because they have to go up in energy, somebody has to give that extra energy to them, right, but some of the electron which are at high energy level possibly can go, possibly can go, right. Same thing is true for the whole, for the whole energy increase or excess is in this direction. So, this whole, you know, because whole is having opposite charge in the electron and therefore, the energy excess for the whole is this direction. So, this whole cannot go there, this whole can always come down here. So, four things are happening, electron can move here, this is called the drift, because it is going under the electric field, do not worry about that if you do not understand. This electron can go there because of the diffusion, this whole can go there because of the diffusion, concentration gradient, this whole can come here because of the drift. This happens to a semiconductor irrespective of whether you are applying or light or not, you are applying bias or not, whatever it is, it happens all the time, ok. But what we are interested, what happens if addition to this, we put a light on it. In addition to this, we put a light on it. So, what is happening if you put a light on it? Because of the light, what is happening? Extra electrons are generated, right, everywhere, holes in electrons are generated everywhere. This action is in addition to the, this action, is that clear to everybody? This action is taking place in addition to this action. So, therefore, you have a lot of electrons generated in addition to this. Now, we apply the same theory. What about this electron, can get out to the other side or not? All this electron can go to the other side? Yes, they are going down in energy, so they can go. Can this electron come to the other side? No, they are going up in energy, automatically they will not come, right. This holes can they go down here? Yes. Yes. This holes can go there? No, right. So, what is happening eventually? So, this electrons can come down here, this holes can come down here and eventually you see the negative charge being piled up there and positive charge being piled up here and that is what. And this electron, once it is here, it cannot go back, permanently separated. This hole once here cannot go back permanently separated. What you see here now? What is this axis? Space. Space, physical distance. So, what you see now? A negative charge and positive charge being separated from each other and they cannot go back again. They cannot go back and come down here. And therefore, what you have done? You have done the job of solar cell. That is the separation of a positive charge and a negative charge from each other. Yeah. What will happen if you remove right? This will come down here. This voltage is there as long as there is a light because this is the balance in the dynamic. This is a dynamic balance here. And this static nature, this will not happen. As soon as you remove the light, slowly this charges are will come down within no time. And no time can be a microsecond or nanosecond. Is it clear? So, this is what solar cell has to do. It creates a positive charge, sorry negative charge here, positive charge here. And therefore, this side becomes positive, that side become negative. And that is the, what is called the photovoltaic effect. This is photovoltaic effect, right? Is that clear to everybody? Again, very, very, very important to understand this because the whole design of solar cell, everything is this. When you talk about increasing efficiency, you have to worry about this. So, where are the energy losses happening rest of it? 33 percent. This electron going down the energy. It is required that it goes down the energy, loss of energy, right? This electron is going, hole is going down the energy. That is required it will go loss of energy. Eventually, it has to go to the context. So, there is some loss of energy. So, all combine this small, small energy you come from 33 percent to 14 percent. So, what is the, what is the challenge to the scientist or engineer to take care of each and everything? How is the generation happening? Are you reflecting any photons? If the, if the generation happening, is there any recombination? If there is a recombination, how much you are losing here? How much you are losing at the contact? How much light is being transmitted? And things like that. How much is the loss due to the series resistance? How much is the loss due to the shunt resistance? A lot of things to worry about. The most important thing for solar cell is of course, this happens and we know what? P side is getting positive, N side is getting negative. What is this condition? In PN junction diode, P side is getting positive, N side is getting negative. What is this condition? Forward bias. Forward bias PN junction diode. All of you know, right? This is a forward bias PN junction diode. Without looking at the screen, tell me that what is the direction of current in a forward bias PN junction diode? P is positive, N is negative, current flows from positive to negative, everybody knows that, right? Is that right? Right? Ok. Look here, what is happening to current? Where is the electron going? Electron is going P to N, which means the direction of current opposite, N to P. Where is the hole going? Hole is growing N to P, which means the direction of current N to P. So, total current is N to P. What is the bias? P to N. That is strange, isn't it? Isn't that strange? In your PN junction diode, your current always goes from P side to N side, because you PC forward and N is negative bias. Same thing is happening here. P is getting positive, N is getting negative, but the current is going in opposite direction, isn't it strange? For the PN junction, it is consuming power, but we do not want solar cell to consume power. Solar cell should give us power and therefore, it is very important that the direction of current is opposite of the voltage generator, right? And therefore, if my solar cell is operating, if it is here, the current is positive, voltage is positive. That is your PN, the curve of the PN junction diode, right? If current is positive, voltage is positive, I have a power consuming device. In this quadrant, current is negative, voltage is also negative. So, power is again positive, right? Current is negative, voltage is negative, power is again positive, which means again power consuming device. But what do I want to create? I want to create a power generating device. So, power product should be negative. So, either it can operate here, where current is positive, voltage is negative or it can operate here, voltage is positive, current is negative, right? So, where our solar cell is operating, we know the positive voltage because P is getting forward, positive and it is negative. So, our voltage is positive, which means and the current is negative. So, where is the solar cell operating? In the fourth quadrant always. It has to operate in the fourth quadrant. Any device which is not operating in either fourth quadrant or second quadrant will consume power, but our solar cell is a power generating device and therefore, it should operate somewhere here. So, like this. So, same IV, because it is a junction, PN junction, the behavior is remain same exactly, except that this is a large negative current flows and because of the large negative current, the whole IV curve is now shifted down to the fourth quadrant tomorrow also. So, we have the, we have this condition. So, actually this is forget about everything else. This is your solar cell characteristic. Everything else is same as the till here. It is the PN junction diode and you add additional component, which is this negative current. That is it. And this whole equation becomes your solar cell equation. The whole equation becomes your solar cell equation. So, there are few parameters I define. First of all, the solar cell equation, which is there in the fourth quadrant. Now, I have plotted inversely in the first quadrant. So, what you see here is that the current is positive, but actually what is it? In reality, what is this? Negative current. Keep this in mind. In reality, that is the negative current. So, your IV curve now looks this. It is much simpler. Your maximum power point is here. This point is your short circuit current. That is the maximum current that you can get from your device. Typically, the short circuit current for the silicon solar cell is in the range of 30 to 35 milliampere per centimeter square. Then you have the open circuit voltage, which is this point, which is the maximum voltage you can get from your device. And typically for crystalline silicon, it is about 550 millivolt to 600 millivolt and 552.6 volt. Then you have an open circuit voltage from this equation, which is a solar cell equation. You can put total current to be 0. That is the open circuit case and you will get the expression for V, which becomes open circuit voltage this. Then you have done the maximum power point. So, when you take the product of the current and voltage, you will find that the maximum power will occur somewhere near the knee of this curve. So, this is your maximum power point. So, you should always operate your solar cell here. And therefore, electrons you will listen to professor Fernandes sometime how to make sure that your solar cell is operating in this point not here and here. If you make it operate here and here, you lose power or you will not generate as much power as it should be. So, that this is the maximum power point. And there is a corresponding voltage at the maximum power point current and voltage for the maximum power point. And then you have the fill factor. Fill factor is the ratio of I m V m that is the power, the maximum power divided by I s c and V s c. That is ideally what you could have got. And it happens so, that this is given by the fill of this. So, there is one rectangular this and how much of the rectangular is filled by the this actual curve at the maximum power point. And therefore, the name is fill factor how much of that is filled. So, this is the actual curve ideally and there is a light green is the curve you can fill it. So, if this filling is 80 percent of this, then the fill factor is 80 percent. And it can be given by the ratio V m I m by V o c I s c. And finally, the most important parameter is the efficiency which is what the maximum power you can get V m I m divided by the input power that is the efficiency. And input power is given by what is called air mass 1.5 g, we will discuss it while discussing the solar radiation I think on Thursday. What is p in and how we decide about that? So, input power is normally either 1000 watt per meter square. And because of the relationship between fill factor and V m I m, you can also write efficiency in the range by in terms of V o c I s in fill factor ok. So, that is the expression for the efficiency. You can do the quick calculation in the evening today. If you look at the typical number of I s c V o c I have given right. V o c for crystalline silicon is 0.5 to 0.6 I s c is in the range of 30 to 35 milliampere per centimeter square. Fill factor is typical in the range of 70 to 80 percent. If you put this numbers here and input power is 1000 watt per meter square, you will be able to calculate the efficiency. And you will see that the efficiency comes in the range of 14, 15, 16 percent depending on the number you choose ok. So, that is assignment for you homework. I will check before tomorrow start before the lecture. Please do the homework ok. So, let us stop here and I hope that is a quick introduction to the solar cells. Any questions? No? Ok. So, thank you very much again and let us break for the TA and then you will have the next lecture.