 Let me start now it is a very simple this lecture should be the most easiest lecture should be for you because I am going to talk about how the cell how to go from cells to module and how the performance of the cell can be varied or different under the different condition right. So, when you are actually using the cells and module in reality your performance will vary depending on the temperature, depending on the light, depending on the type of module and so on. And that is what we must discuss today, but this must be a very simple lecture for all of you because we have discussed all this we look at the cell different parameters technologies and how to connect them together. So, one thing that we already discussed yesterday that when you make a crystalline silicon in goat it is circular in nature and because putting circular cells in a module you are going to use lot of inactive area. And therefore, you have to cut the solar cells from the side and you get this kind of pseudo square. So, the rounded corners you should whenever you see the rounded corner you should understand that it is a mono crystalline silicon. While in the multi crystalline silicon two identification one is it is going to be a rectangle or square perfect square. Second thing is you are going to get the different patches you can see the different patches. So, whenever you see the different patches you will see that this is a polycrystalline or a multi crystalline silicon solar cell. Multi crystalline silicon is more common technology because your grains are very large in polycrystalline we refer to the grains of micrometer size. So, this is one difference and you should be able to see this in your modules. So, solar cell of course, when you put light it gives the electrical output. So, when there is no sunlight no light no electrical output. So, many people ask you know what will happen in the moon light. So, if moon light or any light source which has enough photons which gives which can excite electron hole pair it is going to work. So, even for example, in cloudy condition in the winter in the monsoon season the many part of the country will have the clouds. But even when there are clouds there is a still you get 200 300 watt per meter square of solar radiation. You can see that you can measure that and the 200 watt and 200 100 watt per meter square is still very high intensity to generate 10 20 percent power from your solar cell. So, that happens even the low intensity cloudy conditions is good enough to produce power. The solar cell IV parameter that we have seen already, but again I am just putting it. So, there is a maximum power point where you get the power maximum corresponding parameter I am in V m we have the current I C and open circuit voltage V or C. But the important factor is how the output of the solar cell is going to vary at a different condition. So, right I told you that the performance or the cell efficiency or cell watt peak power peak power is given is given under standard test condition. And the what is standard test condition I am talking about 1000 watt per meter square this 1000 watt per meter square should be corresponding to air mass 1.5 g there should be a 25 degree centigrade. So, these are the main parameters under which the efficiency of the cell or under this condition whatever wattage you get is actually referred as a watt peak, p refers to the peak. But we know that 1000 watt per meter square will not be there I mean there is hardly any time in the year when you get the 1000 watt per meter square air mass 1.5 g can also change and temperature is always going to be different. So, all these parameters actually can be different and particularly this parameter here and this parameter here are of more having more significance. So, this temperature can be very different and this radiation can be very different. So, how does the cell performance will change as a function of these parameters that is what we want to see. And today we are looking at the cell performance as a function of this parameter, but tomorrow when we look at the module itself, when we are going to look at the module and how to make the module and how does the module performance changes in the field then this discussion will be useful and important. So, we want to see that how the efficiency changes if at all, how the amount of what is the performance difference if the light falling is changes, what happens if the solar cell area changes, what happens if the angle of the light falling on the cell and module changes, what happens if the temperature changes. So, these are the various parameters that can affect the output power of the cell and module. Efficiency again simple p out by p in if you are actually using the current densities or if your cell area is your certain cell area. So, input power is always given in terms of density watt per meter square and therefore, you have to multiply by the area. So, therefore, your output power is your input power, efficiency and area. So, remember this is very simple we can always use this. Other important thing efficiency of the solar cell is fixed and it cannot be changed. Once you manufactured your solar cell once it is manufactured then you cannot change the efficiency. Also, the many people make this mistake that when you say that light many people say that when the light intensity changes efficiency changes that is not the case mostly. So, the efficiency is fixed, efficiency will be the same in the morning and in the afternoon and evening. There may be slight difference. So, what happens in the morning and evening your intensity is different your p in is changing p in can be lower or higher p in can be as low as like you know 100 watt per meter square or it can be as high as 1000 watt per meter square. So, because your p in is changing your p out is changing. So, in the morning and evening and afternoon your output power is different it is different because of your input power it is not different because of your efficiency. So, some people make this mistake that efficiency is low in the morning or efficiency is high in the afternoon not the case. It is the p in that is changing and that as a result of that the p out that is changing. So, p out is p in times efficiency times area that does not change area of the cell does not change it is the p in that changes and therefore, your output power. So, keep that in mind now standard test condition is 1000 watt per meter square let us let us take a cell area of 100 centimeter square. So, we will do the quick calculation what calculation we will do we will do this calculation how does this parameter varies. So, we are assuming that we are actually discussing with the cell area of 100 centimeter square 100 centimeter square is not the standard cell area these days people are making larger cells. So, 12.5 by 12.5 millimeter square is the sorry 12.5 by 12.5 centimeter square area is the cell which is commonly used in the industry and many people are actually going for even bigger cell of 15 by 15 centimeter square. So, 10 by 10 that is 100 centimeter square is not the common area, but for the simplification of the calculation we are used it. So, converter efficiency is given for the 1000 watt per meter square. So, if your cell efficiency is 16 percent if your cell efficiency is 16 percent and if your area is 100 centimeter square your power output will be 1.6 watt peak same thing. So, I am talking about 1000 watt per meter square my efficiency is 16 percent and my area is 100 centimeter square if you convert this you are going to get if you are going to get 1.6 watt and because it is at this standard test condition I have to use p here remember when I am saying p I am putting p here I am saying 1000 watt per meter square as well as I am saying 25 degree centigrade. So, this is watt peak so when you are 16 percent solar cell of 100 centimeter square it will give 1.6 watt peak power if you are 12 percent solar cell it will give 1.2 watt peak power assuming the cell temperature is fixed. So, these are the standard test condition we are talking about. So, basically when the efficiency is higher your cell output is higher what happens to the amount of light amount of light as I said will actually affect the output again same consider the 16 percent efficient cell 100 centimeter square area of the cell in two different condition if your input is 1000 watt per meter square and if your temperature is constant 25 degree centigrade you will get 1.6 watt peak same solar cell efficiency 16 percent but your input is 600 watt per meter square your intensity is lower then actually as compared to this is 60 percent. So, as compared to this you will get a 60 percent lower output power. So, you will get 0.96 watt only remember we are only talking about the variation in the light intensity we are not talking about variation in temperature temperature has a separate effect in the reduction of the performance that we will discuss later fine So, when more light falls more output you get and this relationship is almost linear that is why the solar cell that we have given you for your laboratory experiment you must be getting you must got a small solar cell which we are using as a pyronometer. So, the light output power so, this power output power because in this expression efficiency is fixed area is fixed. So, therefore, your output is proportional to the input your output is proportional to the input and we can actually put the expression for the power and we will find out that I output the current output is proportional to the p input and therefore, by measuring the current in the short circuit mode when the short circuit mode s c you can actually find out the power which is proportional to the current because of this relationship therefore, you can use approximately a solar cell to measure the intensity of the light because your power output is proportional to the current and mainly the sorry power output is proportional to the power input density and mainly the short circuit current is directly proportional because your voltage change is having different relationship and this is one example of how things will change for example, I have taken a solar cell which is having 35 milli ampere per centimeter square area. This is the current density j I am talking about the current density. So, what is the current typically as I told you the current density short circuit current density for crystalline silicon solar cell is normally between 30 to 35 milli ampere per centimeter square. So, that is normally what occurs so, once you know the current density for crystalline silicon technology then any solar cell size if you see you can estimate how much current it will produce. So, right now we are considering a cell of 100 centimeter square area. So, how much current it will produce I s c now this is density and this is current. So, let us say I am in the last case I have taken example of 35 milli ampere per centimeter square area is 100 centimeter square. So, your current is going to be 35 milli ampere into 100. So, you will get about 3.5 ampere current sometime what happens you see the smaller solar cell area. For example, solar lanterns are the actually the solar cell that you have with you have I think much smaller area. So, the solar cell that you have with you is about having a 25 centimeter I think it is even less, but let me take example of 25 centimeter square. If this is your solar cell area how much current you are expecting again this is the current under all these parameters is under standard test condition which is 1000 watt per meter square. So, if your light is less your current is going to be less. So, 25 centimeter square into 25 centimeter square area into 35 milli ampere how much you will get 25 into 35.87 ampere. So, you are going to get about 0.87 ampere. So, your current output under standard test condition is directly proportional to the area. What I also told you your ISE is also proportional to the PIN that is another relationship we will use. So, under 1000 watt per meter square you will get from this solar cell area under 1000 watt per meter square this solar cell of crystalline silicon 25 centimeter square having a current density of 35 milli ampere per centimeter square will give 0.87 ampere. Under 500 watt per meter square how much current it will give same solar cell I am talking about under 500 watt per meter how much current will give half of 0.87. So, it will become about 0.4 I am sorry it will become 0.435 ampere right. So, in this way if you know the current density you can find out the current of a solar cell of any given technology and using this kind of relationship you can find out what will be the current at any other given input radiation as well clear. So, here is the graph different IV curve of a same solar cell under different condition. What are those conditions 1000 watt per meter square 800 watt per meter square 600 400 200 in all this we have assumed that temperature is constant. We are we are only discussing what will happen if the intensity of the light is changing. So, fine. So, mainly the current is a linear function you see here 200 400 600 800 1000 watt your current almost increases linearly. So, current is a linear function of the input power density or input light density. Look at the voltage here what is happening to the open circuit voltage what is happening to the open circuit voltage open circuit voltage is not changing linearly why it is not changing linearly because we we know it you know it. The open circuit voltage is k T by q log of I L what is I L light generated current divided by I 0 plus 1. So, I L is a linear function of P in proportional to P in. So, in a way VOC is a log function of input power density right in a way VOC is a log function of a input power density. So, current is a linear function voltage is a log function. So, remember this. So, as we go from morning to evening or morning to afternoon to evening your voltage of a module does not change significantly. As you go from morning condition of let us 100 watt 200 watt 500 watt 800 watt 1000 watt per meter square your voltage does not change significantly, but your current is going to change significantly. It is very simple fact and I hope you all understand this. What about the area we have I told you smaller area means smaller. So, once you know the current density of your technology once you know your P in efficiency you can actually find out how much different solar cell area will produce different current. Current density of crystalline silicon by the way is highest and I will show you the some of the numbers may be tomorrow. Current density of amorphous silicon technology depending on the efficiency will be like 10 milliampere 12 milliampere per centimeter square current density of cadmium toluid and CIGS can be about 15, 20 milliampere per centimeter square is standard test condition because efficiencies are low. So, current density that you can get from silicon solar cell is the highest as of today. So, this is example if the single solar cell of 100 centimeter square is giving you 3.5 amperes is that what you calculated yes that is what you calculated in the previous slide. It will give about 3.5 amperes, but if you take half of the solar cell you get 1.75 amperes because your areas are higher. So, many times in the modules which are used for solar lanterns and small power module have a what people do people make a big solar cell and they cut into smaller pieces. So, that your current can be lower. So, that eventually the power output of a module will be lower if you want to lower voltage module. What happens if the light falls in a different angle? So, that is the question of a light that is the question of the sun tracking right. We have discussed in the very first or second lecture that if you want your module to be always perpendicular to the sun rays you want your module always to be perpendicular to the sun rays. So, if your light falls on some other angle two things happen first of all the power density falling on the module at the same condition is different right. So, if your module is perpendicular to the sun rays, if your module is perpendicular to the sun rays this is L is the width length of a module and width can be anything. Then in this case suppose your power density is x watt per meter square and in the same case if your module is lying flat on the ground same angle incidence theta here theta also here. If that is the case then the same power density is actually distributed to the longer distance. And because of that the available power density to this same module area is now x cos theta and higher the theta I am sorry the smaller is the theta your power density will be is it correct. So, if it is perpendicular you get the highest power if you if the angle is changing less than 90 degree no it is not correct. So, it should be sin theta should not be cos theta it should be sin theta. So, this should be sin theta. So, smaller is the angle theta or the light is coming perpendicular you will have smaller and smaller power. So, this angle this should not be cos theta should be sin theta or what also will happen if your sun rays are coming at some other angle your reflection will also increases. Your reflection from the glass will also increases and that is also the light that is getting inside the solar cell or inside the module from the glass into the solar cell will also reduce and that will also reduce the power output. So, maximum electricity is possible when theta equal to 90 and therefore, therefore, we will discuss the sun tracking is required. So, these are the these are the ways and now we will discuss the effect of temperature for example. So, the temperature also affects the power generated and first of all we are going to discuss why it affects the power generated these are the various ways. Now, the short circuit current is given by this expression or I total current is given by this expression for the diode and we if we can bring the approximation. So, people have done I 0 can be represented by this and this term here first term here the first term here is actually much higher than 1. So, we can neglect this. So, basically your short circuit current can be represented by this. Now, we when you take a derivative of this what we want to see how does the how does the voltage and current changes as a function of time right. So, how what we want what we are interested in is how how the DOC changes with the time how the short circuit current changes with the time or most importantly how the power itself changes with the I am sorry I am sorry sorry not time it is temperature. How the power how the open circuit voltage how the short circuit current and how the fill factor changes with the temperature. So, in the earlier slide we have looked at the performance of the cell and output as a function of light. But now we should look at how this parameters open short circuit voltage short circuit current fill factor and peak power point changes as a function of temperature. So, this is the diode equation and we have approximately can be represented like this and again we do not have time to go into detail. But if you look at if you do the derivative now change in the short circuit current as a function of temperature is very small and we will come back to that again. But change in the open circuit voltage as a function of temperature is significant. So, once you put the values here once you put the values here we can actually get this. So, open circuit voltage with respect to temperature for silicon here the parameter of silicon the band voltage equivalent to band gap that is 1.12 volt V G O it is a not electrons old V O C about 0.6 gamma is 3 and if you put this parameter here you will get about point 2.3 milliold per degree centigrade. So, change in the open circuit voltage for silicon solar cell per degree centigrade rise in temperature is 2.3 milliold. So, basically as the temperature increases there is a negative symbol as the temperature increases the open circuit voltage is open circuit voltage decreases. And why it happens because of the as we discuss again and again because of the temperature there is more generation of electron hole pair intrinsic carrier concentration increases the recombination increases and you get the lower open circuit. Now, so because of this now if you are how to calculate the new open circuit voltage. So, open circuit voltage of a cell at a temperature T is equal to open circuit voltage of a cell at standard test condition minus there is going to be a voltage drop how much voltage drop 2.3 milliold per degree centigrade. And therefore, you should multiply by the change in the temperature as compared to a standard test condition change in the temperature as compared to a standard test condition. So, suppose you have solar cell which is having V O C of 0.6 volt at 25 degree centigrade. Now, in the new condition the temperature has become 50 degree centigrade how much is the voltage at 50 degrees. So, your V O C at 50 degree centigrade is equal to 0.6 under standard test condition minus 2.3 milliold per degree centigrade and how much is the temperature difference. Now, new temperature condition is 50, but my earlier cell characterized at 25 degree standard test condition right. So, delta T is actually 25 50 is the new temperature condition and my old standard test condition is 25 degree. So, this is what you get. So, you get 0.6 minus 2.3 milliold into degree centigrade degree centigrade will cancel 25. So, how much you get? So, 0.6 minus 0.6 minus. So, 0.6 2.3 you multiply by 25 you get the difference. So, roughly it will come about 0.4 0.54. How much is 2.3 milliold into 25? 0.05 same roughly ok you get about 0.54 something. So, this is your new open circuit voltage. So, because the temperature has increased your open circuit voltage has decreased you can do this calculation and this is very important to do this for a different technology. So, you will find that this 2.3 milliold per degree centigrade this is for silicon ok this is for silicon and this number will be different for different technology. So, this will be different for different technology I will show you tomorrow for the cadmium tilleride and other. So, the short circuit current also changes actually increases ok see the positive sign here, but the increase is very very small in terms of percentage it will be 0.06 percent ok why the short circuit current increases because the band gap decreases as the function of temperature ok the band gap decreases as a function of temperature. So, as the temperature is increasing your B like band gap decreases and because of that what will happen because of the lower band gap more and more photon will have enough energy to excite. So, your I S C will increase. So, that is what it shown here and what happened as the as the temperature increases I am talking about the current here and I am talking about the voltage here as the temperature increases your I 0 increases and because of that your B O C decreases ok. Now similarly you can actually get the other parameters your fill factor look at the minus sign here. So, your actually fill factor decreases as the temperature increases and your peak power will also decrease. So, here we are interested in the what happens to the numbers. So, this numbers are very small, but look at the percentage. So, your fill factor decrease is about 0.15 percent per degree centigrade percent of what percent of standard test condition. Similarly, standard test condition similarly your peak power decreases by 0.0042, 0.005 which is about 0.42, 0.5 percent per degree centigrade rise in temperature. So, this numbers are important and you should remember. So, main thing is your peak power change in your peak power with respect to temperature is minus 0.4 to 0.5 percent per degree centigrade percentage of what percentage of what peak at percent of what peak at STC standard test condition. So, as compared to standard test condition your percentage of power decreases as the temperature increases and this you can do as one of the tutorial. If your what peak power of a let us say cell is we have calculated let us say 1.5 watt under at STC condition what will be the output power at 50 degree centigrade. So, that you can calculate. So, percentage of the peak power this calculation you can do. So, that is what as a temperature increases open circuit voltage decreases that you can see early the change because of the increase in temperature the current increases slightly, but voltage decreases significantly. So, the main result of increase in the output a result increase in temperature is the decrease in the output power. The various parameters that you can see here the efficiency the multi junction solar cell efficiency is very high what are the other efficiency of commercial modules they can see. So, the amorphous silicon is 6 to 9 percent 8 to 11 percent for cadmium to lower end CIGS. These are the parameters that you can get. The typical voltages and current just look at it carefully of various technology. So, the output current for monocrystalline can be very high for polycrystalline will be lower fell factor is about 70 70 80 look at the amorphous silicon the current density the current density is 8 to 15 milli ampere per centimeter square cadmium to write 15 to 25 milli ampere per centimeter square. These are the current densities, but the voltages can be higher than the crystalline silicon. The crystalline silicon voltages 0.5 to 0.6, but in amorphous silicon cadmium to write and gallium arsenate why the voltages higher because of the higher band gap higher band gap results in a higher open circuit voltage. So, that is why you can see the higher voltages here. Fill factors are lower because conductivity is less resistive losses are higher. So, as compared to crystalline silicon the fill factor of thin film modules are lower because of the higher resistive losses that happens. Fine I will stop here that the next thing is about the module and we will discuss about the module. So, this is in this summary what I have discussed is how the performance of the solar cell will be different as a result of the change in area as a result of the temperature as a result of the light falling on it as a result of the tilt and so on. So, the various parameters and what are the various parameters of the different technology. So, in thin film technology because of the higher band gap your current densities are lower, but your open circuit voltage is higher typically thin film technology has a lower fill factor because of the higher resistive losses. And how much is the percentage change in the various parameter. So, the open circuit voltage of crystalline silicon changes by 2.3 milliole per day degree centigrade. The current changes by a very small portion 0.05 percent per degree centigrade rise in temperature. The important thing is the peak power decreases by almost 0.4 to 0.5 percent of the peak power every degree centigrade rise in temperature. So, these are the very important parameters and it affects the performance of the device performance of the solar cell in the real life conditions. And it is very important for us to know how does a given cell or and a module perform in the real life conditions. Right now I will go to. So, there is a time for question answer. Gandhi Nagar. The question is if we do not keep TCO layer at the backside then what will happen? If you do not use TCO layer at the backside what will happen? Actually the backside layer if your light is coming from the top your backside layer can be the question is if you do not use the TCO layer at the backside what will happen? So, if I have my substrate and if I have this my absorber and the top is TCO because light is coming from here. So, this electrode has to be transparent. This is one electrode and this is another electrode. Now, this electrode can be TCO as I said earlier or it can be metal also. So, typically people also use silver silver here at the backside very thin layer of silver aluminum you can use. So, you can use TCO or the metal. So, normally you have to provide two metal contacts at the front and one is at the back. So, some contact has to be there. So, it can be TCO or it can be metal contact, but some metal contact some contact has to be there. There has to be two electrodes front electrode and the back electrode. If we use a crystalline silicon and another option is thin film. So, the only disadvantage of thin film or silicon cells are means lower current density or anything. Well, as compared to the thin film and crystalline silicon the what you are saying the only disadvantage of thin film is the lower current density you are. So, lower current density results in lower efficiency. So, one of the main disadvantage of thin film technology is the lower efficiency. So, now, if I use a crystalline silicon module it will give me efficiency of 14 percent, but if I use amorphous silicon module it may give only efficiency of 7 percent. So, when your efficiency are lower your for the same amount of power you have to install double module right. So, if I am putting 1 megawatt power plant my module area for amorphous silicon will be much much larger and therefore, all the associated cost of the plant that is you have to put more modules you have to make more connections you have to use larger land area all those cost increases because of the lower efficiency of thin film technology. So, that is one of the disadvantage that can happen. Also the cost of the thin film technology is supposed to be very low as compared to crystalline silicon, but commercially that has not been observed the cost of the thin film is not normally much lower than the crystalline silicon. And therefore, and also the performance of the thin film technology over a period of 25 years is not observed. People have not installed thin film module in the field for 25 years because the technology is new as compared to crystalline silicon. So, therefore, there are some doubts as compared to thin film and that is why the world market is mainly crystalline silicon more than 80 percent of modules are crystalline silicon. Why TCO is not used in crystalline vapor based technology for front contact instead of fingers? Okay, question is why TCO is not used in the as a front contact instead of metal fingers as I told you that the front contact the front side in the crystalline silicon solar cell is the emitter which is heavily doped n-type silicon. And heavily doped n-type silicon has higher conductivity as compared to TCO. So, if I put a TCO first of all I am going to lose some light because the transparency of TCO is not 100 percent. And because my emitter which is heavily doped and having higher conductivity than my TCO can do the job better than TCO. And therefore, in crystalline silicon solar cell use of TCO will not provide you any benefit and the finger arrangement, finger metal contact arrangement is the best arrangement that you can have. Okay, question from NIT Pritchie. Sir, which metals are used as a substrates? And also what is the thickness of the substrate? Substrates as I told you earlier normally the glass is used and typically the thickness of the glass is about 3 millimeter. Okay, thickness of the glass is 3 millimeter. When we use metal foil I am not sure about the thickness, but again thickness will be less than a mic less than a millimeter for a thin metal sheets. Which type of metals sir? Which metals we are using? For the metal thin metal it is a stainless steel. For the metal sheet we use stainless steel. Sir one more question, one more question sir. For large area coating which technology is involved sir? For 2 meter, 2 meter. For large area coating people use PECVD plasma enhanced chemical vapor deposition even for the silicon nitride that conventionally people deposit as an inter reflective coating in crystalline silicon solar cell most of the technique use PECVD plasma enhanced chemical vapor deposition. Jabalpur. My question is that photo cell acts as a transducer converts light energy into electrical energy. Similarly in optical communication we use photo detector it also converts it also does the same. To increase the sensitivity of in optical photo detector hetero junction structure is used. So, does it happen in the solar cell also? Can solar cell also as I told you hetero junctions are used actually your cadmium telluride and cadmium sulphide. So, cadmium telluride is a junction with cadmium sulphide. So, cadmium telluride solar cells are actually hetero junction amorphous silicon cell structures are actually hetero junction your CIG solar cells are hetero junctions. So, yes hetero junctions are used in solar cells also. Government call is Salim. Sir, what are the factors that are affecting the life time of the PV? What are the factors that affects the lifetime of the PV you know the. So, when PV modules are installed in the field the temperature of the module keeps on changing right every day every night in the day time the temperature of the cell will be very high because of the glass cover in the night time the temperature will go low because of this temperature cycle your metal semiconductor contact gets degrades and therefore, the fill factor decreases and the performance decreases also. Also the inter reflective coating because of the because of the ambient condition though it is very well encapsulated, but there is a chance that your inter reflective coating also degrades with the refractive index changes and etcetera that will also result in decrease in performance. The encapsulant layer that is used in the module is some kind of polymer sheet and that polymer when exposed to the ultraviolet radiation in that ambient for a very long period can also result in a degradation in performance. So, there are many parameters that can result in the degradation in the performance of a module over a period of life time and how much how fast it degrades determines the life time. So, typically as we I told you several time that in the life time of 25 years 20 percent degradation is allowed in the life time of 25 years 20 percent degradation is allowed, which means after 25 year lifetime your module will still perform 80 percent of its initial wattage. What does it mean? If your module is 100 watt in the beginning after 25 years in standard test condition it should still give you 80 watts. Sir, is this because of the electron wall pair after 25 years will there be a decrease in electron hole pair created? No, that decrease in electron hole pair creation can only occur when your light going inside is low. When your light going inside the cell is not low and that low can happen because your polymer which is used as in kept in encapsulant can become darker. So, your light getting inside will become low, but once the light goes in the generation is as same as the 25 years ago. So, semiconductor itself does not change. The other things which are associated semiconductor the metal contact the encapsulant the inter reflective coating those materials can change, but a semiconductor itself does not change. Amritha, Koyamchur. Sir, I have one doubt. You have said in one place that crystalline silicon produce no output under low illumination condition when compare with the cadmium telluride, but in some other place you have said the performance of the cadmium telluride is very low. So, I have a confusion can you please explain me. So, the out did I say the output is low the output of the crystalline output of the thin film technology can be low under the low light condition because the all thin film materials are high band gap material and under the low light condition the percentage of high energy photon decreases right that is what the curve I showed you under low light condition the percentage of high energy photon decreases and because the thin film modules are the high band gap material their generation will decrease more as compared to the generation of a in crystalline silicon solar cell. And therefore, performance of thin film will be lower in the low light condition as compared to crystalline silicon, but there are many other factors that comes into picture. The temperature the spectral content etcetera will also become very important. So, that is one parameter, but overall performance which module will perform will depend on many many other parameters. So, good morning sir. What is nano crystalline or micro crystalline? What is the size of the grains? In the nano crystalline silicon the size of the grains you talk about 20, 30, 40, nanometers ok. In micro crystalline it is actually bit of confusing micro crystalline sometime grains about 1 micrometer. So, 500 nanometer, 400 nanometer, 800 nanometer that you will call as a micro crystalline, but nano crystalline is typically less than 100 nanometer or so ok MNIT Bhopal. Sir you in the first lecture you told about TCO is a very very important area of research. Could you throw some light on the areas means people are working on TCO? Ok. So, the TCO is very important because it can be used as a front contact and the back contact it is also used as anti-reflective coating the whole performance of and TCO is also used as a light trapping right. In the properties that we want TCO is it has to be transparent, it has to be as transparent as possible, it has to be as conductive as possible and deposition of TCO in the large area uniformly is also very important. So, what and also the TCO once you deposit after the TCO deposition you are depositing your absorber layer right. So, in cadmium toluide for example, if you are depositing TCO and your absorber layer your absorber layer is deposition takes place at high temperature. Divering that subsequent processing after deposition of TCO, the TCO property should not get affected ok. So, these are all the parameters which are important for the performance of the thin film technology and TCO plays a very important role. Amal Jyothi College, Kerala. Do we ever connect two types of PV cells in parallel because the voltage output will be different it will cause some intra current. Yeah, do we ever connect I mean of course crazy people can connect the two different kind of modules together, but normal people do not connect, but I mean yeah I mean if we can match the current voltage is always a old when you are putting the modules in series voltage is not a problem right. So, if we can connect one module of 6 volt, other module of 12 volt absolutely no problem because in series connection voltage are just aided up, but the current matching is a problem right. So, if your current is matched of the two different kind of modules then I do not think there is a issue of connecting two modules in series, but normally people do not do it there. College of Engineering Pune. Sir, is it possible to use the buried contacts to reduce the shadow losses? Yes, the main purpose of the buried contact solar cell that I have shown yesterday in crystalline silicon technology is actually to reduce the losses ok, because we want low resistance and low resistance can be obtained if you use large amount of metal, but we do not want to increase the width of the metal contact, but we want to increase the height of the metal contact. So, when we go for the buried contact structure, the width of the metal contact can be low and height can be high. So, we can actually maintain the good conductivity of the metal contact and therefore, the buried contact technology will reduce the shadow losses and that is the one of the advantage of buried contact solar cells that is an in fact one of the main advantage of buried contact solar cells. With this TCO technology, can the absorption coefficient is improved by 100 percent? Well, the TCO does not help to increase the absorption coefficient of your absorber material ok, your main active layers like cadmium toluoride amorphous silicon CIGS, these are the main absorber layer and this layer absorption coefficient of this layer depends only on their own composition of the material, their own properties of the material they do not depend on the TCO. Remember TCO is only there to make a metal contact at the front and back or to make a contact or at the front and back and to provide you the anti-reflective property and the light trapping. It is not helping you, it is not helping the absorber to increase the absorption, but yeah it helps to increase the light trapping which will help to increase absorption ok. Fine, I think lot of question let me take some question in chat, is it possible to use graphene as a transparent conductive material? I told you many times people have asked this question, yes it is possible, but there is no even there is not good enough research which can convince people that it is possible. So, there are lot of doubts whether it can be used or not, in principle you may try to use it. Does the decrease in temperature has the same effect as increase in the temperature? Yes, if you are going to decrease the temperature below 25 degree centigrade, then actually you are going to gain in terms of the open circuit voltage and the peak power. So, yes decrease in the temperature has the same effect as increase in the temperature. So, when you decrease the temperature below 25 degree centigrade, your performance will be better as compared to the standard test condition performance and if you are going to increase the temperature about 25 degree centigrade, your performance will be poorer as compared to the standard test condition performance ok. So, the GSITS can be concentrate the solar radiation in to increase the optical density. Yes, solar radiation can be concentrated and there are technology like concentrators solar PV technology and which you use optical means of concentrating the light. So, that once you once you concentrate more light you can actually generate more power, but in order to concentrate you require optics, you require centraking, you requires first source solar cell and so on. So, there are lot of other preparation that is required in order to make a in order to concentrate the light and use it for the solar cell application. Coimbatore is it? Yes. Sir, I have one doubt. Yes, go ahead. Can you explain simply the light trapping procedure sir? Light trapping procedure yes, it is a very simple that if you are going to use light trapping means light once it is intered should not escape ok. So, if you are having this and if you are having rough surfaces right like this and they are absorbers. So, once light comes here it may get reflected here and from here it may get reflected back and get reflected back. So, once light inter the solar cell once light inter the solar cell it should not escape it should not go here that is not that we should try to minimize this. Similarly, once light goes back it should not escape from here. So, that also we should minimize. So, this is called light trapping once light inters the material it should be reflected front and back within the layer. So, that effective length that light is travelling within the semiconductor increases light trapping is to increase the effective length that light travels within the semiconductor should be higher and that is what is the light trapping. So, escape of the light from the front side and escape of the light from the back side should be avoided and that can happen if you get the condition of total internal reflection at the back side ok. If you get a condition of T i r or total internal reflection at the front side and back side the light can completely be trapped ok. Actually, people have shown that the light can pass 4 n square time where n is a refractive index. So, it comes to almost 50 times ok. So, if you do the proper layer selection the light can go front and back 50 times in the ideal possible condition and that is what is referred as a and this is what is light trapping you know ok. Is that clear? So, the same principle is followed. Yeah, the same principle is follows in the yeah waveguide for example, waveguide everywhere we want to couple the light for the longer distances you can actually use total internal reflection. So, the question is how to create the condition of total internal reflection in a solar cell in a simpler way so that you can replicate for the large solar cell in a low cost ok. Thank you very much.