 So, good morning. So, all of you again that you are waiting is solar cell finally. So, so far what we have discussed is lot of theory behind the semiconductor and that is the background I think person who is talking about solar cell must know and there are lot of parameters and once we understand that parameters it is easier for us to see how and why particular solar cell performs well and why particular solar cell does not perform well. So, solar cell is nothing but a p n junction under elimination. So, whenever you put light on a solar on a p n junction it becomes a solar cell and may be in that next lecture I will bring a couple of samples with me and I will show you various types of solar cell. So, solar cell is nothing but a p n junction under elimination. So, the solar cell we will discuss three things have to fall the light must fall on solar cell the light must get absorbed in the solar cell and the carriers that are generated must be separated in a solar cell. So, now we see here that this is the curve that you already know this is the that you already know solar cell under equilibrium condition and that everything is happening and now in this case what we will do is we will put a some light on a solar cell. So, that is what when spectrum falls we are putting some light here I am. So, in this band diagram I am not showing the electrons in holes otherwise which is shown in this band diagrams we are only showing the extra electron hole pair that is generated because of the light only. So, this red dots here and the holes here is actually are generated because of the light only. So, this is situation at time t equal to 0 now what will happen what will happen to this electrons which are generated here and let me show you this in a in a white board and it is better. So, in my p n junction diagram which is like this. So, I am only showing the carriers which are now generated because of the light. So, light is falling hole is generated it is a it electron is generated. So, this is happening almost everywhere and and a result of that electrons are generated electron and hole pairs are generated everywhere by the these are the electrons and hole which are generated because of the light I am not showing the electrons which are there because of the doping I am not showing the electron in hole which is there because of the thermal generation. These are the electrons and holes which are there because of the light falling on the on the solar cell ok. Now, what happens when a light falls on a p-n junction, it becomes a solar cell and what a solar cell should do? Solar cell because of the light falling on it results in a generation of voltage and that voltage can be used to drive the current. What is the voltage? Whenever a positive charge is separated by a negative charge physically, then you generate a voltage. Now, this electron which is here which is generated because of the light generated because of the light, can this electron go to the other side? Can this electron climb the hill and go other side automatically? If my pen is sitting here like this, can this pen automatically lift itself and go to higher energy level? Answer is no right. So, this electron cannot go this side. What about this electron? Can it come here? It is going down in energy right. So, this is my energy axis again and again I am reminding you. This is my energy axis right. So, it is easier for any particle to go come from high energy level to the low energy level. So, therefore, this particle, this electron can easily come down in energy. It can come down in energy. So, therefore, this electrons which are generated and what side is this side? This is n side and this is p side right. This electrons which are generated n side cannot move, but the electrons which are generated p side and this electrons are minority right because they are generated in the p side. They are minority. They will come down here. So, what you will see? There are more electrons which are coming here. By the I am not showing any electron which is because of the doping other. All this electrons is because of the light and light only. Similarly, what about the holes? What I told you? This is the energy of electron and hole energy increases this way. So, all the holes which are generated here cannot go here because they have to increase in energy, but the holes which are minority at the n side actually can come down. The holes which are there in minority can come down. So, what you will see is? All the holes will actually pile up here and once the hole comes here, it cannot go back. Once the electron come here, it cannot go back. What you see? This side you are getting lot of electrons. This side you are getting lot of holes and once these are separated and these are separated, what is this x is? x. So, once they are separated physically, they cannot actually go back. So, what is happening? The p side is automatically getting positive, n side is automatically getting negative and this is what is called the photovoltaic effect. What is it called? It is called the photovoltaic effect. So, when light falls on a p n junction diode, a p side becomes positive, n side becomes negative, an voltage is generated, a photovoltaic is generated and this whole generation of a photovoltaic because of the light falling on a p n junction diode is referred as a photovoltaic effect. Is that clear to everybody? Very important that you make this concept clear that how the voltage is generated and all these carriers are because of the light only, because of the falling of the light. P side becomes positive, n side becomes negative. That is what is shown here. That when the carrier, when the false electron goes this side, goes this side, this side becomes negative, this side becomes positive and you generate. P side becomes positive, n side becomes negative. This is not applied by this is automatically generated voltage in the solar cell. P side is becoming positive and n side is becoming negative. What is this corresponding to forward bias? This condition is corresponding to forward bias. That is why I wrote here a forward voltage is generated. Very nice. So, what is so great about it? When a forward voltage is generated, what happens in a p n junction diode? So, basically because of the light falling on a p n junction diode, p side becomes automatically positive, n side becomes automatically negative and we create what is called the forward bias. Isn't it a magic? And because of this forward bias, what will happen? Because of the forward bias, forward bias current will flow. That is anyway should flow because physics is physics, when you get forward bias current will flow. The question then we should try to ask is why then the solar cell actually generate power? Your solar, your p n junction diode is actually consume power. Why does the solar cell generate power? And good thing is this, look at this diagram. So, within this diagram, you see the electrons are coming from p side to n side. What is the direction of the current flow? What is the direction of current flow? Direction of current flow is in this direction. So, this is the electron current because of the light. Very nice. What about the holes? The holes are coming from n side to p side. And what is the direction? Hole current is the same direction of the hole flow and therefore, the hole current is flowing in this direction. What do you see? Normally, so what do you see that both current due to the electrons and current due to the holes. So, this is hole current and this is the electron current. Both hole current and electron current is flowing from n to p. In normal p n junction diode, what happens? In normal p n junction diode, what happens? In normal p n junction diode, if I make p side positive n side negative, if I do this, my current flows from p to n. This is a diode. In a solar cell, I have p n. I am making this positive. I am putting a light. This is automatically become positive. This is automatically become a negative. And interestingly, where is the current flowing in this direction? What is this? This is a solar cell. Again, a magic. When you put a forward by a same device, when this was positive, this was negative, current was flowing in this direction. Same device. Now, this is positive happening automatically in this negative. Current is flowing in the opposite direction. And that is how it is possible to generate power. We will look it. We will also look it more closely. So, I hope this is clear to all. We will also look it more closely. We will look it in this way that this is my current axis. This is my voltage axis. What is power? Power is I into V. The convention is that when power is positive, the power is positive, the power is consumed by the device. So, when my device operates in this quadrant, I is positive, V is positive. What about the power? Power is positive. What will happen? If any device which is working in this quadrant, will it generate power or consume power? It will consume power. Look at this curve. This is minus V, minus I, negative axis. What about the power here? Power is equal to minus I times minus V. That is again positive, V I. And because power is positive, any device which is working in this quadrant will it generate power or consume power? Consume power. So, here power is consumed. Here power is consumed. What about what is happening to the power here? I is positive, V is negative. So, power is negative. And therefore, any device which works in this quadrant, power will be generated. In this quadrant also, I is negative, V is positive. So, therefore, power is negative. And because power is negative, power will be any device which is working in this quadrant, power will be generated. So, the device, if it works on the first quadrant, power is consumed. Second quadrant, power is generated. Third quadrant, power is consumed. Fourth quadrant, power is generated. So, where should my solar cell work? Definitely not in the first quadrant. Definitely not in the third quadrant. It should work either here or here. It should work either here or here. What is happening? Where it is working? So, look at this. Because of this charge carrier flow, this side is becoming positive, this side is becoming negative. And where the current is flowing? N to P, negative current. So, my current is negative with respect to convention. Current is negative means my voltage is positive. Current is negative means where is my solar cell operating? My solar cell is operating in this quadrant, fourth quadrant. My solar cell must operate in fourth quadrant and that is what is happening. Voltage is getting positive and the current is becoming negative and that is how the solar cell generates power and your diode consumes power. Is it clear? So, coming back here, solar cell generates power and again I will not go to the derivation. The carrier concentration definitely, this is the graph of the carrier concentration and this side is the majority side and the lower is the minority side. The majority side, what is happening? One important concept that you have to learn here, sorry. Other important concept is that can any carrier which is generated anywhere in the device can come to this junction. There are two competing process that takes place. Suppose this is my junction again. If a electron is generated here because of the light, a light is falling, electron is generated, this electron will have to come here to contribute to the current. So, this is one path. One path for electron is actually to go here, go here and then come here. But this electron can also come down here. Recombination, who stops it? The electron can also recombine. Therefore, and what is the average length that electron, what is the diffusion length? Diffusion length is the average length that a carrier travels before it recombines. So, suppose this is my depletion region edge and if my average length of the diffusion length is l n, if every length is l n, if electron is generated here, if electron is generated here, the probability that this electron will go here is less than the probability that this electron will recombine. Therefore, not all the carriers which are generated everywhere in the device are contributing to the current. Which are the carriers which will contribute to the current? It is the carriers which are generated within the diffusion length, will only contribute to the current. It is very important. So, similarly for the holes, you will have the diffusion length of the hole. So, all the holes at the inside must get generated within this diffusion length. And this is my depletion region width w, very important. Therefore, only the carriers which are generated within this length, l n plus w plus l p are actually collected by a solar cell. Otherwise, carriers generated carriers will recombine. Same thing also happened. If the electron is generated here, it will basically recombine rather than contribute to the current. Very interesting. That is what? What do we want in solar cell now? We want l n to be small or large. We want l p diffusion length of holes to be small or large, as large as possible. Large diffusion length means high quality material. High quality material means more energy to be spent in making a good solar cell. So, these are all compromises that you have to make. So, I can actually find out how much is the current I will generate because of the, because of what? Because of the light. So, first let us say generation rate is g. What is the unit of g? Generation rate is g. What is the unit of g? Number. Let me write it cleanly here. So, I want to find out the generation. I want to find out how much is the current that flows because of the generation. So, generation current. Everybody is now clear. What is the significance of l n, w and l p? So, only carriers which are generated within this are actually contribute to the current. And actually, we can find out how much is the current flowing. So, first of all what is generation rate? g. What is the unit of g? Number per centimeter cube per second. So, many carriers are generated per centimeter cube per second. Are we interested in number? We are interested in charge. So, therefore, I must multiply by q. So, this is the charge generated per unit volume per unit time. So, now I have coulomb. Charge generated per unit volume per unit time. What I am looking for? I want to find out the current. Now, this I must multiply by the area which is receiving the light. So, within what area the current is generated? So, I must multiply by the area and it is per unit volume. So, basically I should multiply by the volume. So, I have multiplied by the area terms which means I have multiplied by the centimeter square. I must multiply by one more length term. And what is this length term? Area which is the cross section area where light is falling and how much is the length from where the carriers are effectively collected? What is the length from where carriers are collected? l n plus w plus l p. This is equal to the current because of the generation. Is that okay? Current due to the generation. A q, g is the generation rate. So, what is the unit of length? If I multiply by centimeter here, this is here. This centimeter is corresponding to this centimeter square is corresponding to area. Coulomb is corresponding to q and g is number per unit volume per unit time. So, that gives me unit of m p s, Coulomb per second. Very nice. So, this I got the generation current. Now, you know the IV equation of a diode. Can we make an IV equation of a? Can you write an IV equation of a solar cell? What is the difference in the IV equation of solar cell? Extra current component. What is the direction of current component? Opposite to the bias. The direction of current component opposite to the bias. If you are making p positive forward bias, the current flows from n to p. In the normal diode, current flows from p to n. But because bias is happening, the normal current components are anyway heavy. Because of the bias, forward bias is happening. Everything is normal current components happening. So, the diode equation is this. I equal to I 0 e raise to the power q v by k t minus 1. This is the diode equation. I want to write, convert into a solar cell equation. Any help? Any help? Solar cell equation. How can I convert this into solar cell equation? What am I telling? There is one extra current component, which is this one. I have to add that current component. That is it. So, if I write I equal to I 0 e raise to the power q v by k t minus 1. Because voltage is generated, how much voltage v is generated? Therefore, my forward bias current flows. Therefore, if I generate I l, normally this is called the light generated current I l. So, light generated current is here. Now, this equation becomes a solar cell equation. Very simple, same. So, I 0 takes care of all material parameters. Mobility, aerial lifetime, diffusion length, these, that, etcetera. L takes care of diffusion length here and generation rate. So, generation rate takes care of light. It takes care of diffusion length. It takes care of the area of the solar cell and depletion weight everything. So, now this equation takes care of all the parameters of the material that will come. Now, you can make a solar cell of amorphous silicon, cadmium telluride, copper indium, gallium selenide, gallium arsenide, chrysoline silicon. Any solar cell that you make, you can actually bring the corresponding parameter in this equation and actually you can plot a current voltage equation or a current voltage graph for a solar cell. You always run short of time. So, again I am not going to the derivation, but the derivation is same. Then, finally, you get the equation of a solar cell. I equal to I 0 e raise to the power q by k t minus 1 minus I l and I l is a light generated current. Expression for I 0, I have told you very detailed and expression for I l also I have told you. So, this is what we have seen. The power is positive here and power will be generated if the solar cell work in the fourth quadrant or in the second quadrant. That we have seen, this is the derivation I am not going for the details. Now, so this is what happens. What is the black curve here? The black curve is the curve for p-enjunction diode. What is the red curve? What is the red curve? Red curve is the curve for the solar cell. The black curve is the curve for the p-enjunction diode and the red curve is the curve for the solar cell. The same curve actually shifted down because of the light falling on it. There, if your solar cell operates in this region, here your current is negative, voltage is positive, power is negative. Therefore, your solar cell generates power in this area. In practice, I know that my solar cell current is negative because it flows in the direction opposite to the normal current. But in practice, just for our convenience, only for our convenience, how do we plot the solar cell I v curve like this? We plot current to the positive axis. What you should know as a participant of this course very well that this axis is actually a negative current axis. You all will remember this axis is actually a negative current axis. Now, these are the parameters of solar cell and many of you have already done the experiment. So, I do not have to explain in detail. But the parameters that we are interested in open circuit voltage, short circuit current, maximum power point, current and voltage at maximum power point. So, if this is your maximum power point, current and voltage at maximum power point, fill factor, efficiency, series resistance, shunt resistance. These are all the parameters that we should know about the solar cell. And we should also know the typical value of this parameter for a different types of solar cell. So, that we can quantify it and we can compare it. So, if you want to compare open circuit voltage of amorphous silicon, open circuit voltage of cadmium chloride, open circuit voltage of crystalline silicon. Similarly, we should be able to compare the short circuit current and the maximum power and so on. So, I will quickly take you to the parameters and as I said many of you have already been doing the experiment on the laboratory kit. And therefore, you may know it already. First of all, this is my solar cell equation. I total is I 0 e raise power q v by k t minus 1 minus I L. This is my solar cell equation. And this is my negative current axis. Remember, remember, remember. This is my negative current axis. Short circuit current is then the maximum current that you actually can get from your solar cell, maximum current. And it happens when you short circuit. So, definitely when you are doing measurement with a multimeter and the current mode directly putting your multimeter under the with the context of the solar module that is provided to you or the solar cell that is provided to you in the kit, that is the you are short circuiting your solar cell or module when you are doing that. And therefore, you measure the short circuit current of your solar cell. What is the short circuit current of what is. So, short circuit current is what is given by this. What is the short circuit current density that you measured in your solar cell? Question to all of you. What is the density of the short circuit current that you measured? We will come back to this numbers in detail, but this is the maximum current that will go. Maximum voltage that you can get from your solar cell is the open circuit voltage. Maximum voltage that you can get is the open circuit voltage when the terminals are open. And you can modify this equation put open terminal means i total equal to 0. When you i total is equal to 0 you can rearrange this equation in the form of when i total is 0 v equal to what is v? v equal to v o c when i equal to 0 v equal to v o c. So, when you do that you get the expression for the open circuit voltage fine. Then you have the maximum power point. So, at this point and you know that well, but let me put in the graph. So, if you in this point if I have this is my i v curve now, this is my negative current axis and write positive always for the convenience. So, at this point because my current is 0 power is 0 at this point my voltage is 0 power is 0. So, if I draw power versus voltage my power is 0 here my power is corresponding to this also 0. If I increase if I work on this point now my volt current is high by my voltage is low. If I go further my current is still high my voltage is even higher. So, power will increase and then I go here my voltage is increasing current is high. So, my power is high if I operate here I reach to maximum if I operate here my current starts decreasing and voltage increasing. So, my power decreases. So, actually I get a curve like this actually same curve I can plot here. So, somewhere near the knee we call it knee of the curve somewhere near the knee of the curve we get the maximum power something like this. So, this is my maximum power point and so, maximum power point or the P m will be product of current and voltage. What current and voltage? Voltage at the maximum power point is called V m and the current at maximum power point is called I m. So, this is my product of I m into V m. So, this is what I get the P m that is the maximum power that I can get. So, important thing is that not every time you will get the maximum power that is important that only there is one point in the whole I v characteristic your solar cell can operate here your solar cell can operate here everywhere in this curve, but there is only one point there is only one point when the power is maximum that is important thing to remember that your solar cell or your solar module does not give you the maximum power all the time. The important parameter some other important parameter is called the fill factor somebody was asking the fill factor is simply the factor that gives the feeling the factor that gives the feeling. Fill factor is this. So, if I have if I have a solar cell which is having a short circuit current I s c and V o c you know. So, ideally area under the curve should be my power is not it power is a product of current into voltage. So, if I multiply I s c by V o c that should be the power ideally I should have got, but my curve is not square my curve is not square how is my curve my curve is like this because of this curve the actual power that you get is only area under this curve area under this curve not even this. So, actual my operating point is this I am sorry the area is this rectangle I am sorry this not this let me cut it here. So, my if this is my I s c and this is my V o c then this is the rectangle which is my actual power, but normally because my curve is like this not square my maximum power point is here. So, area is here. So, not the not the complete area of the ideal curve is filled that is what is called the fill factor. So, normally the power you get is V m into I m ideally what you should have got V o c into I s c that is what you should have got what you get is V m into I m and therefore, your fill factor is this V m into I m divided V o c I s c how much of the total rectangle ideal rectangle is filled by the actual one. So, that is called your fill factor and that is what is the ratio of the two. So, V m by I m divided by V o c by I s c ok. Now what is the next parameters next parameter is your efficiency how much efficiency you get from your solar cell and typically maximum power divided by power input from here it is clear that what is clear that your V m into I m from here it is clear that your V m into I m which is nothing, but your P m can also be given in terms of V o c I s c and fill factor. So, here V m into I m which is equal to P m which is equal to V o c time I s c time fill factor. So, when you are trying to find out the efficiency it is the P m divided by P in P m is the maximum power and what is maximum power can be given in terms of the this parameter which are the important solar cell parameter open circuit voltage of a solar cell short circuit current of a solar cell fill factor of the solar cell is important. So, you can actually write V o c times I s c times fill factor divided by P n ok. So, I am sure you are doing this all the time in the experiment. So, that is your efficiency the important thing is what is your P in what is your P input power what is input power whatever you get in the standard test condition standard test condition is your input power and that we have seen air mass and your standard test condition is air mass 1.5 g. So, your input power that you should consider in this expression input power that you consider this expression should be corresponding to air mass how air mass is defined we have seen already in the earlier lecture 1 over cos theta theta is the angle between overhead position of the sun to any other position of the sun. So, under the air mass 1.5 under the air mass 1.5 we get 1000 watt per meter square radiation. So, therefore, when you are solving the problem you should take P in as 1000 watt per meter square. Now, 1000 watt of power is available in 1 square meter. So, therefore, what is required how much area you are receiving. So, you require a solar cell area also. So, eventually when you when you want to write the efficiency it should be V o c times I s c times fill factor divided with P in. Now, many times what we give how we give we do not give I s c what we give J s c we give current density. So, what is J s c it is I s c divided by area. So, when you are calculating the I s c you must take area into account you must take area into account. So, if you are taking I s c this is current not the current density t. So, if you are talking about 1 centimeter square area solar cell or 1 meter square area you must take area into account here. So, I s c will be J s c into area or the other way people write the expression for efficiencies V o c you can write J s c and fill factor and you can say input area input power which is watt per meter square and you should multiply by area here same thing J s c by a either way. So, either you can give your expression in like this or you can give your expression for efficiency like this. So, if you are either take I s c here or if you are taking current density here in the numerator you take area into account. So, I hope that is it problem will give. So, so far what we have discussed we have discussed the semiconductor fundamentals you have discussed the P n junction we have discussed the P n junction diode under illumination that is a solar cell. Now, the next step will discuss is how to design a solar cell, how to make a solar cell, how to put more light into the solar cell, how to collect more carriers into the solar cell, how to fabricate a solar cell and things like that. And eventually towards the end of it we will see how to use solar cell, how to make solar PV modules, how to design solar PV modules and eventually use those solar PV modules in a photovoltaic system. So, you are half way through I hope you already learned a lot and know about lot about solar cell. So, now we are we will started discussing the solar cell design, solar cell fabrication and eventually solar cell applications. So, I will stop here in this lecture what we have discussed is how the P n junction becomes a solar cell just by putting light on it because you put light on it you get a automatic generation of voltage which forward bias is junction which is P side becomes positive and side become negative. But, important is the current due to the light flows in the reverse direction and because voltage is positive forward bias current is negative the power is negative and therefore, a P n junction under the light act as a solar cell and that can be used to generate the power. So, either your device is working in this fourth quadrant or second quadrant then only you can generate power if your device works in the first quadrant or the third quadrant your solar cell or your junction consume power and then later on we have looked at the expression for the generated current. If your generation intensity is given generation rate is given a diffusion lens are given l n l p then you can find out the how much current is generated. I have also shown you that how it is important to have a longer diffusion length for electron in whole. If the diffusion lengths are not long then the electron which is generated very away from the junction will not be contributing to the current it will recombine rather than contributing to the current. So, the long generation long diffusion length is important for both electron and whole and finally, we have looked at the device parameters short circuit current open circuit voltage cell factor efficiency maximum power point and current and voltage at the maximum power point. I have also made a remark when you are taking P n in order to find the efficiency calculation then P n is equal to air mass 1.5 spectrum 1.5 that is 1000 watt per meter square and I have made sure that you have to take a either total current generated or the if you are taking current density do not forget to multiply by the area. I am ready to take the questions. Kolam Guaid. How will you determine the value of shunt resistance and series resistance for a solar cell? How we determine the value of the shunt and series resistance? You can determine the value of shunt and series resistance from the slope of the curve itself and I will show you in the next lecture how to do that. Amritha, Coimbatore. As you know our function of voltage generated is based upon our mobility and diffusion coefficient and length and our N A and D N D. So, but these parameters are determined by our N A and D N D. So, what is the maximum level of N A and N D? The maximum yes you are correct that the doping level determines many parameters. So, what is the maximum level of doping? Now, again when we discuss the design of a solar cell and how to design a solar cell, we will discuss what is the optimum doping level. So, normally we do not want to have a very low doping because of the low doping your resistance will be higher and your losses will be higher, I square or losses will be higher. So, we do not want low doping, we also we do not want high doping. So, how we determine how much is the doping to be done? That we will discuss when we will discuss the design of solar cells. Baramati. Sir, whether photo current is due to diffusion or drift of charge carriers generated by light? Photo current whether it is due to diffusion or drift very nice. So, if you look at into the details of if you plot the carrier concentration profile, you will see that large portion of the carrier motion of the light generated current happens randomly, there is no drift no diffusion, but when the carrier come close to the junction, when the carrier come close to the junction. So, let me go to the wide board and explain you. So, normally what happens? Now, this is your P N junction, the light is falling here and you are generating electrons here and these are the light generated electrons. Now, once this electron goes to the other side and this electron also goes to the other side, this electron going to the other side is because of the drift, there is electric field at ghost, but because now this electron is gone to the other side, there is a shortage of electron here. So, this shortage of electron will result in a diffusion like condition. So, this electron will actually come here because here there are more electrons, here there are less electrons. So, before the junction just near the junction, you have some kind of diffusion taking place, at the junction you have some kind of drift taking place. So, the carrier motion due to the light generated carriers is kind of a combination, it is some part of the diffusion take place, some part of the drift take place, is that correct? Are you clear? So, when I plot the carrier concentration profile, so this is the concentration and this is the x, normally my concentration in zero current condition is like this constant. But when the light is falling, when light is falling the concentration profile becomes like this. So, very close to the junction there is a difference in the concentration and because of this you create what is called concentration gradient. So, this is your junction. So, within this area there is a diffusion taking place, but within the junction there is a drift taking place. So, it is a combination of diffusion and drift. Sir, my opinion is that at the junction photo current is surely a diffusion current and not drift, because electric field at the junction is exactly in reverse position that of the terminal voltage, electric field. No, so if you look at the look at the direction of electric field, look at the direction of electric field at the junction, what is the direction of electric field? So, if I go to the junction, if I go to the wide board the junction is like this, right. Now, the direction of electric field is up hill the potential energy, potential energy is decreasing like this. So, my electric field is opposite direction, because my electric field is like the direction like this, the force on electron is in this direction. So, any electron which is here will actually experience this force in this direction. Therefore, this electron will go here. So, therefore, this current is because of the drift current which is because of the electric field which is because of the force on electron. So, therefore, at the junction, the entry to the junction it is the drift current that is flowing that is causing you on the electron force is negative on the electron that of the motion of the electron, force is in opposite direction. So, it is a retarding force. No, it is not retarding force, the force the electron will only flow in the direction where there is a force, ok. The force is in the direction from the end to P sorry P 2 N here and therefore, force this is the direction of electric field, the direction of electric field is this I am plotting here and the force on electron is in opposite direction, ok. So, force is in the direction this direction therefore, electron must also move in this direction that is what is called the drift motion, ok. Just think about think carefully about it and you will understand that, ok. V N i T in Akpur. Hello sir. In the experience the efficiency of this solar cell is coming out to be around 30 percent. So, is it right or so we have. Please send that 30, please send the 30 percent solar cell to me. So, 30 percent is definitely not the correct answer. Ok sir, what should be the maximum efficiency of that? The theoretical efficiency is about 29 percent when I take the highest possible parameters which has never been achieved, ok. Efficiency also depends on many other structure and everything, but single junction solar cell under the normal standard taste condition should have efficiency of about 29 percent ideal. Nobody in the world has ever demonstrated that. S V N i T Surat. Yeah, good afternoon sir. If I short circuit the solar cell in a full light for 4, 5 days then what will be the results after 4, 5 days I am starting using it normally. Nothing will happen, you can short circuit it for 4, 5 days or as long as you want. There will be no difference in efficiency and no, no, no, nothing. Further working patterns? Nothing, nothing. So, solar cell is supposed to work for 25 years and if you look at the maximum power point, the maximum power point current is very close to the short circuit current. So, solar cell actually can handle that much current very easily. So, even if you short circuit it will not affect the solar cell in any other way. What about the B N sanctioned non equilibrium condition? So, solar cell is actually a P N junction under the non equilibrium condition, right. Equilibrium condition is one when there is no light, there is no bias, there is no temperature, there is no electric field, there is no magnetic field, nothing. Solar cell actually you are putting a light on it means you are disturbing the equilibrium. So, actually your solar cell is nothing but a P N junction under non equilibrium condition. One more questions. Yeah, go ahead. The electrons and holes which are present in the diffusion length only will take part in the current flow as you said. Is there any chance of increasing the diffusion length in the N side and the P side? Yes, it is possible to, yeah, you are correct that electrons and holes which are generated within the diffusion length only contributes to the current in a solar cell. And it ideally we want diffusion length to be as long as possible, but high diffusion length means high quality of material, low defect density, low impurity level and when you are really making high quality material which means that you are really spending lot of money. So, therefore, normally we have to find the optimum value, right. What is the best performance you can get from a given material? For example, amorphous silicon, the diffusion length in amorphous silicon may be only 20 nanometer, 15 nanometer while the diffusion length in a crystalline silicon is 200 micrometer, 300 micrometer, right. So, it is the compromise between the quality of the material and the cost, but amorphous silicon is cheaper to produce and crystalline silicon is very difficult to produce. So, all that optimization is required. Sir, one more question. Sir, you have to say in a nice manner the function of the PN junction, I have one doubt sir. That is from solar radiation in the electromagnetic spectrum, visible, invisible, ultraviolet, X-ray gamma rays, so many radiations are there available. If we place our PN junction towards the sun's radiation, which electromagnetic spectrum radiation will attract or generate more minority charge carriers in the PN junction? Can you explain me sir? Yes. So, ideally one photon generates one electron-hole pair, ok, not more than that. Either it generates or does not generate. So, 0 and 1. So, one photon, either it is ultraviolet or visible or infrared, one photon will give you only one electron-hole pair, ok. So, both all of them work. All of them work simultaneously, all of them only give, yeah, all of them only give one electron. So, ideally we want to, you know, for a very high energy photon, we want two electrons to be generated. That, you know, that is the, that is what people are trying to do. But in practice, all photons, whether it is ultraviolet or visible or infrared, if it is absorbed in the material, it gives only one electron-hole pair, ok. I will take some question on the chat. As increase in temperature causes generation of electron-hole pair, then why it reduces the efficiency? This generation of electron-hole pair, which is because of the increase in temperature, works to reduce the potential, you know. Earlier, if you remember, if you discuss that, if you increase the temperature, the potential decreases. So, this electron-hole pair, which is there because of the temperature, actually works in reduction of the potential that is generated, ok. And or actually it results in a increase in the carriers, which will actually enhance the recombination also. So, your recombination also increases with the temperature. And because your recombination increases, why recombination increases? Because it provides more partner carriers. And because your recombination increases, as a function of temperature, your voltage decreases and your efficiency decreases. So, the carriers, which are there because of the temperature, do not help. Why should diode current flow in the absence of forward bias? The diode current should not flow in the absence of forward bias. But in the zero bias, net current is zero, but there is a small drift current and small diffusion current, which is flowing inside the diode all the time. What happens all the time? One last question from the Bhopal. Sir, how the drift current is not depending on biasing? Why the drift current does not depend on the bias? Because drift current is coming because of the minority carriers, ok. So, let me go to the one last slide. So, if I draw this energy band diagram again, ok. This is direction of electric field and the drift current is the current, which is because of the electron, which is moving in this direction, right. So, where this electron comes here in the P side? This is P side and N side. Where this electron is coming at the P side? It comes because of the generation. What generation? Thermal generation, ok. In a P N junction diode, this electrons come because of the thermal generation. So, once your temperature is fixed, the number of your carriers are fixed. When the number of carriers is fixed, the current flowing because of those carriers are fixed and therefore, drift current is not a function of bias. Sir, my question is from your textbook from page number 104. Equation number 5.9 sir. The equation says that field factor is equal to F F naught times 1 minus R s and second equation is field factor is equal to F F naught times 1 minus 1 over R shunt, both R s and R shunt in per unit values. Yes, so basically, no, it does not. It does not mean. What it means is that your field factor depends, your field factor depends on series resistance, small R s, N shunt R s and R sh. So, the value that you see the R s is a characteristic resistance, characteristic series resistance and R sh is characteristic shunt resistance. So, basically, what that equation is telling that how your field factor will vary as a function of series resistance. So, your field factor is F F 0 means this is the field factor when there is no series resistance or series resistance is 0 like this. So, if your series resistance increases, your field factor is going to decrease. So, this is the equation is giving you how the field factor varies as a series resistance. Ideally, we want series resistance to be as low as possible close to 0 and we want shunt resistance to be as high as possible close to infinity. Therefore, when we want to give your impact of shunt resistance on field factor, we use the inverse relationship, so R sh. So, both are different thing. One is giving you how the shunt resistance will affect the field factor, other is saying how the series resistance will affect the field factor. The left hand side is same, field factor and field factor. So, equating the both right hand sides. So, we have F F 0 cancelled and 1 minus. No, you cannot do that. Amal Jyothi. Sir, actually when we place a for PV and depends on the English of the sun, there will be intensity of the light will be varying and we are looking for maximum power or peak power. So, depending on the light intensity, the peak power also will be varying and should we have to change the series resistance or all resistance to get maximum power? Yes, you are correct that as the intensity of the light changes, as the intensity of the light changes, the maximum power point also changes. So, for example, this is if this is the curve for the afternoon time, your curve for the n o clock, 9 o clock will be different. Now, all of them we will have the different maximum power point. So, it can be here, it can be here, it can be here. So, now in order to get the maximum power, you should operate your load at this resistance or this resistance or this resistance or this resistance. So, at a different time, the optimum load is also different and because your maximum power point is changing, your load should also be changing and this job is done by what is called MPPT, maximum power point raking. So, these are the various devices that is available, which actually keeps track of the maximum power point at various condition of solar radiation, so that your load can get the maximum power out of it. Yes, this MPPT technique is used, it is called maximum power point raking devices. There are devices available commercially. So, when we install a solar system, so we should actually use the MPPT in between, so that you can extract maximum power. Otherwise, we will discuss later also that if your load is not matching with the load requirement or the resistance requirement at the maximum power condition, you will not expect the high power. Hello sir, my question is that, can we use a composite material made of different layers of silicon, germanium, gallium arsenide, so as to absorb maximum possible spectrum, solar spectrum because different materials have different absorption then. Yeah, but when we combine the, so the question is can we use different material to absorb maximum spectrum? In the one solar cell, answer is no because when you are putting the different materials of a different band gap, eventually your material will have one effective band gap, right. Same material will not respond to the all the wavelengths, even if it responds, we will learn later that your voltage, open circuit voltage will also reduce. So, within one single junction solar cell, it will not be useful to put materials of different band gap. Sir, can we use different layers one over the other? We can actually make different solar cells of different materials. So, what we can do is this, we can make you know one p-n junction of silicon and then we make another p-n junction of germanium. So, this is one solar cell, cell number one, this is another solar cell, cell number two. So, when the light falls and here on the top of it, you can also make a gallium arsenate solar cell for example, cell number zero. So, when light falls on it, the high energy photon will get absorbed here and whatever is passing by will get absorbed here and whatever is passing by will get absorbed here. In this way, we can actually absorb the spectrum from all wavelengths, but these are the three different solar cell. So, you can use different material, but you can use, you have to use each material as a different solar cell, you cannot combine in this one thing. College of Engineering Pune. My question is in p-n junction diode under a forward condition, the current is because of diffusion current, but under forward condition if we increase the temperature of that device, then whether if current will be changed and whether it will affect the overall current of the device. So, in the forward, you are correct that in the forward bias condition, the main current is because of the diffusion current right. Now, when we are changing the, when we change the temperature, it will definitely affect the current, but because the forward bias current is so large in quantity as compared to the reverse saturation current which will get affected. So, therefore, the main change will, when the current change in the current due to the temperature will not be significant. So, the percentage of drift current is very, very less as compared to diffusion current in forward condition. Yes, you can see that you know the, because your drift current is flowing in the reverse direction from n to p, your forward current is flowing in the forward direction p to n and the net current is a forward current that you see is the net current right and the net current is much higher than the reverse bias current. Sir, my another question is, in solar cell module whether overall current is due to a drift current? In a solar cell module as I said, we cannot say overall is drift or diffusion normally you know in the device, the both current components plays important role. As I was explaining in the morning that when the light falls on a solar cell, the current, the charge carriers goes from one side to other side and there is a small diffusion that takes place as well as drift that takes place you know. So, there is a contribution of both the current components. So, the light generated current we cannot say it is only drift or only diffusion when the both the components are playing a role. Sir, my another question is, in solar cell as the light intensity increases, the voltage increases, then whether this voltage changes the value of diffusion current? Yes, the diffusion current even in solar cell you know, solar cell when the light increases voltage increases and then that this also will discuss more in the next lecture, but this voltage which is getting generated across the junction is actually also forward biasing your junction right. So, because the junction is getting forward bias, it is also resulting in increase in the current forward bias current. Sir, we have ok thank you sir, we have performed experiment on PV module. So, in that PV module, two modules we have found out, one is bleach in color and another is grey in color. So, whether thin thin technology strictly follow the grey color? No, not necessarily thin film technology from the different material have little bit different color ok. So, cadmium telluride will have a more darkish color, amorphous silicon depending on the single junction or double junction will have different color, CIGS will have different color, CIGS will be little bit reddish in color. So, the color of thin film module depends on the material and the thickness and many other parameters. Sir, for the PV sale, whether there are limitations for the area? I told earlier in one of the when it comes to crystalline silicon, the area is fixed by the silicon wafer that is produced in the industry and that silicon wafer size is 156 by 156 millimeter square, but when it comes to thin film, there is no limitation on the area. In page number 29 of your textbook, so you have you have told that the lattice of the solar cell unit cell of silicon is made up of two phase centered cubic cell. So, can you please throw some light upon that sir? Yeah. So, actually silicon is also having a kind of diamond structure right, but this diamond is not a phase centered cubic you know in the phase centered cubic you have a cube and there are. So, within the phase centered cubic for example, you will have let me see if I can draw it very nicely ok. So, in phase centered cubic you will have one atom at each corner right. So, you will have one atom each corner of the cube and then we will have one atom at each phase also ok. So, this is one phase, this is another phase, this is another phase and this is another phase and there is one at the front and one at the back ok. So, this is so the circles are actually atoms at the corner and this dots are actually atoms at the phases ok. So, there are six phases and a cube. So, you have six atoms at the phase and one atom each at the corner. So, there are eight atoms at the corner. So, this is one unit cell of a phase centered material. Now, your crystalline silicon is not phase centered cubic. So, crystalline silicon is actually FCC structure plus 1 by 4 FCC, but plus FCC of 1 by 4 X, 1 by 4 Y, 1 by 4 Z. What does it mean? You take one FCC structure and we are talking still about within the limits of this lattice. So, this lattice is A, this is A and this is also A. So, within this lattice if I take 2 FCC atom unit cell sitting exactly in the same location and I keep 1 cube fixed and I move other cube. How much I move? I much move one fourth in X direction, one fourth in Y and one fourth in Z. After doing that find out how many extra, how many atoms are there within the same volume? How many atoms are there within this volume? What volume? A length, A width and A height. So, you will find that 4 extra atoms appear because of the this particular thing. So, we have one of this and then you have another of this. So, you got getting my point 2 FCC sitting exactly at the same location keep one FCC fixed move other FCC by 1 by 4 in X, 1 by 4 in Y, 1 by 4 in Z and see how many extra atoms are coming within the same volume. So, then when you see this you will find that I am sorry 4 extra atoms are coming and that one you get that that shifting then whatever the arrangement of atoms you will get that is your silicon crystal arrangement. Got it? Okay sir and in page number 1 noted there is a collection probability curve of minority carrier sir. Y axis that is collection probability axis should come at the center of the curve sir, curve drawn. Am I right sir? Okay, let me see what is the equation on the left side is e power minus x by L p. So, if I remember correctly the curve is drawn like this right. So, there is a junction here and the collection probability is drawn like this right. Yes sir. Yeah. So, what are you saying now? That X equal to 0 should start at the center of the curve that is at the center of the rectangle. No that is not see you know what is this? This area is a junction okay and this is what is this? X equal to 0. What is X equal to 0? This is the surface front surface from light is entering. So, if I make it draw it like this as a plan. So, my light will actually enter from this direction. So, this point is my surface front surface this point is my back surface and therefore, junction will be somewhere between the front and back surface. Junction cannot be outside the outside the front surface right? Yes sir. Okay. But on the left side the equation is e power minus x by L p and on the right side it is e power minus x by L n. So, to satisfy the equation shouldn't the X equal to 0 start at the center of the axis? I see. So, I will have to look at the equation. So, maybe equation has written considering it is in the center. But whatever is the diagram that I have shown the graphics is correct because in reality this is how the junction appears. Okay sir. Okay. So, it has to be e power x by L p then if we consider this diagram. Yeah. Okay sir. Thank you sir. Thank you very much sir. Okay. Can the collection probability be less at the entrance of the light radiation? That's right. Whenever the light enters the collection probability is lower and that is why it is very important to find a right place for the junction and we will discuss in the design of a solar cell how to optimize so that we do not lose collector we do not lose the carriers which are generated from the right from the surface itself. Okay. Thank you. Have a good day.