 So, now let us look at the cell design for the high short circuit current, fine. So, we have found the upper limit of parameters. Now, let us see what we need to do to actually what we need to do to get that particular parameter. So, we want high value of short circuit current and current is what? Number of electrons and number of electrons is what? Number of electrons is basically if you want high current, if you want high ISC value means if this is my solar cell, what you should do? Reflection should be low, transmission should be low, low transmission. So, every photon which is going in should give me one electron hole there and this electron hole should be. So, recombination should be low, low recombination. So, the design is the design of a solar cell to get higher value of short circuit current is nothing but minimize reflection, minimize transmission, get a minimize recombination. So, that is what? Short circuit current first of all it will depend on the area, larger the area of the solar cell, larger the short circuit current, it will depend on the number of photons basically spectrum which is not in our control, it will depend on the intensity of the light, it will depend on the what is the collection probability, what is optical property. Collection probability is how we are minimizing the recombination, we will discuss that. So, cell design motto as per the number of area is concerned, you basically have the larger area, larger area will have more light, more light means more photon, more photon means more electron, more electron is more current, very simple. So, large area should be giving me large current. What about the air mass? Air mass basically spectrum of the sun and we have discussed that air mass 1.5 g, g is strength of the global is the spectrum that we should be considering, but actually it means that different spectrum will have different composition and therefore, will have different types of photon and remember photon energy will affect because some of the photon, high energy photon will get close to the, absorbs close to the surface, some of the low energy photon will transmit. So, if your spectrum is changing, your behavior or the performance of solar cell will also change and mainly the current will also change. So, we will not discuss this in detail. Absorption coefficient we have discussed already in detail that high absorption coefficient you require, high absorption coefficient is good because you require thinner material because it gets absorbed very close to the surface and a penetration with absorption length is very large if absorption coefficient is low. As you have seen many times that also clear from this graph, this is the absorption coefficient for silicon that the low wavelength photon or high energy photons have the high absorption coefficient and therefore, we have seen that blue light gets absorbed very close to the surface, green light absorbed close to the surface and little deeper and red light absorbed close to the surface and almost everywhere in the solar cell. So, this is important to know right, this is very important to know. If I am talking about red light because your absorption coefficient is less, your absorption length is higher, so it requires thicker material. So, therefore, absorption will take place almost everywhere. So, this is very important to understand that blue light is absorbing very close to the surface from where light is entering. What does it mean that all the electron hole pair generated because of the blue light are getting generated close to the surface not deep. So, what does it mean? So, if I have a blue if I my junction is like this my junction is like this this my N and P if my light is coming here and if it is a blue light blue light means what high energy photon may be 3 electron rule photon. So, all the electron hole pair is generated very close to the surface very close to the surface this is a blue light that is what will happen. If I have my same P n junction if I have same P n junction if I have you know red light let us say energy of the red light it is just little above the band gap let us say 1.3 electron. Because it is a lower the red light will have the lower absorption coefficient and because higher absorption length and therefore, generation will occur here, here, here, here, here, here, here, here, here, till the absorption length this is the absorption length. And if I have even lower light then my absorption actually can go all the way through then. So, this is again important to understand that the blue light carriers are generated very close to the surface, but the higher wavelength light or lower energy light will result in little electron hole pair generated deeper and deeper. So, that is what is shown here the red light will do result in generation mainly. And if I look at the complete, if I look at the complete how many electron hole pair gets generated. So, it is given by this. So, if you look at this the generation rate in number per centimeter cube per second is typically for the air mass 1.5 spectrum is in the range of transform 21 photon transform 21 to transform 22 electron hole pairs at the surface huge number right transform 21 to transform 22 no per unit volume per unit time. But as you go deeper and deeper for that particular thickness it decreases. So, what does it tell you that lot of generation occurs, lot of generation occurs very close to the surface keep in mind lot of generation occurs very close to the surface. Because blue photon is giving generation here red photon is giving generation also close to the surface. So, lot of carriers are generated close to the surface with surface from where light is entering into the solar cell. So, if I am light is entering from this side this is my friend surface if light entering from that side this is my friend. So, therefore, now that question is if lot of generation takes place close to the surface where should be my junction. Now, what is the junction? Junction is a place where there is electric field right junction is a place where there is electric field. When there is electric field the chances of recombination is less by because under the electric field the electron hole pair you know for experience of force which can separate them immediately under the electric field there is a there is a force which can be separate. So, if my again p n junction I have drawn some many times now. So, in this electric field if electron hole pair is generated here this is a flat band flat band means what? Flat band means no electric field as I said earlier this is quasi electric field in this region quasi electric field, but here electric field is not equal to 0. So, if any electron hole pair is generating here there is a electric field and immediately the electron will go hole will go this side and they will get separated. So, recombination is nearly 0 recombination is nearly 0 where in junction, but as you go away from the junction this carrier if it is not within the diffusion length if it is not within the diffusion length of electron l n then they may recombine. So, the chances of separation of carrier is highest in the junction, but as you go away from the junction the chances of separation decreases that is what is shown here. The collection probability of the carrier remember what are the thing that should happen charge absorption separation and collection. The separation probability this is more like a separation probability actually is highest in junction and as you go away from the junction your separation probability decreases fine. Now, I will come back to the there are three different curves and I will come back to the, but just keep that one point that collection or separation chances is highest in the junction as you go away from the junction suppress chances decreases which means recombination chances increases. So, this is my generation profile now most of the generation takes place close to the surface and this is my collection probability profile. So, where should be my junction now? As we have seen in the all throughout this course so far that normally I draw a p n junction like this p n n junction in the middle if I put light like this this if I somehow achieve this my solar cell will have very poor efficiencies why poor efficiencies in this case this will give me poor efficiency why because my generation is taking place here lot of generation is taking place here my junction is here. So, carrier will have to travel all the way near the junction to get separated and therefore, many of them will recombine in this process, but instead of that instead of that if I have my junction here. So, my very normally I have n here normally p here you can consider the same thing here also you can say this is p and this is your n it does not matter and if I light is coming like this now because my generation is happening close to this surface and therefore, this kind of arrangement will give me higher efficiency. So, here we will get high efficiency right and therefore, most of the time in commercial solar cell your n layer which is normally emitter and this is your base and this is your emitter normally your emitter is very very thin the junction is very close to the surface from where light is entering emitter and typically you know in the current case it is about 300 to 500 nanometer only 300 to 500 nanometer. So, what is the thing we are learning that junction the p n junction should be close to the surface from where light is entering p n junction should be the close to the surface from where light is entering because of this reason because of this reason that collection probability is higher at the junctions you should bring your junction where the generation is higher and generation is higher close to the surface. One last concept in this regard is basically how to find out how so each photon should give me one electron whole pair. How do we know each photon is giving one electron whole pair or not then the way to find out is by using what is called the quantum efficiency. What is the efficiency of each quantum? What is one quantum one photon? What is the efficiency of each photon? Photon has a wavelength. So, this quantum efficiency is plotted as a function of wavelength what is the efficiency of each quanta of energy that is what is the efficiency of each photon or basically can is each photon is giving me one electron in the external circuit or not. So, basically what you do you measure the short circuit current you measure your short circuit current per unit spectrum per unit flux of the photon that is your quantum efficiency of each photon. Now, one of the thing that we have shown here is one of the thing that we have shown here let me anyway discuss here is that in a p n junction. So, normally my as we discussed that my emitter is very thin typically light is falling from this side this is my n region this is my p region this is typically what you get and this is this is emitter which is very thin emitter and this is my base p is normally base n is emitter it can be opposite also. Now, my surfaces are very defective place. So, there is a lot of recombination that can take place at the surface there is a lot of recombination that can take place at the surface. So, surface recombination could be high could be high and that is why if you go back to this slide which I have shown you I will show you later this slide that there is a if your surface recombination is less your collection probability from the surface will also be some value, but if your surface is very poor then nothing will be collected from the surface anything that is generated close to the surface will become will recombine. So, therefore, surfaces are the one place in solar cell that must be well passivated or passivation means you have to minimize the recombination at the surface otherwise you will be in a problem your efficiency will be lower. Efficiency by the way the quantum efficiency one thing which is very nicely tells me what is happening about about each solar cell. So, look at this graph. So, this graph tells me that this is the quantum efficiency by the way quantum efficiency is maximum of course, 100 percent means one photon is giving you one electron at the surface quantum efficiencies are low why low because of the surface recombination I told you surface are very defective places in the solar cell and it is and it is a quantum efficiency is a function of wavelength right. So, short wavelength means high energy photons high energy photons absorbs very close to the surface right as your energy of the photon decreases they absorb deeper and deeper and deeper. So, therefore, by quantum efficiency I can find out which of the photons are efficiently working in a solar cell and which are the photons are not efficiently working right. So, if I if I look at the the the performance or the quantum efficiency for a 1000 nanometer 1000 nanometer is a low energy photon it will absorb almost everywhere. So, this if my efficiency are low which means what my 1000 this photon is getting absorbed almost everywhere long wavelength photon. So, this mainly will tell me that what is happening in my back side of the material rear side of the solar cell my high energy photons will tell me what is happening at the front side and my medium energy photons will tell me what is happening near the junction ok. So, by quantum efficiency I can actually probe the solar cell depth how the front surface is performing how the junction is performing how the bulk of the material is performing and how the rear side is performing ok. I I will tell you I will try to explain this with the with the on the whiteboard in the last slide ok. I told you that I told you that high energy photon or let us say blue photon where it will get absorbed here blue photon will give me electron here ok. So, if I am plotting the quantum efficiency quantum efficiency and if I am plotting the plot for 400 nanometer if this and this is my junction let us say this is my junction this is my if for 400 nanometer which have photons are electron hole pair are generating close to the surface if all this electrons are actually recombining then my efficiency that is the number of electron per photon number of electron per photon right that is my quantum efficiency will be low right. So, then my quantum efficiency low, but assume that all this photons which are generating all the electrons which are generating here is not recombining every all all electrons are actually separated then my quantum efficiency will be high my highest value of quantum efficiency will be 100 percent ok that is the blue photon and now let us say I have I am putting green photon. So, how is my front surface will the blue photon will tell me how is my front surface where the recombination with the front surface is taking place or not taking place the what about the green photon then green photon will have little lower energy and will go little deeper here also here also at the front surface where will also go deeper then my green photon will tell me what is happening my junction. If my junction is very good and no recombination is taking place at the junction then the quantum efficiency of green photon will be high ok. So, therefore, green photon will tell me what is happening at about 550 nanometer then I am putting the red photon red photon will have even lower energy and they will go deeper and will give me the generation here also, but they will give me generation here also ok. So, if my bulk of the material is very bad and lot of recombination is taking place the corresponding to 700 nanometer my efficiency quantum efficiency will be low, but if my material is very good quantum the crystal quality is very good less recombination is taking place in the bulk my efficiency will be high and therefore, I will get high efficiency here also. Now, I get a infrared photon infrared IR photon ok IR photon will have even lower energy and it will get generated here here here and it will also get generated close to the back surface also ok. Now, if the if my infrared if the back surface is also lot of recombination taking place at the back surface the efficiency of the let us say 1000 nanometer photon will be low quantum efficiency will be low, but if the back surface recombination is also low and therefore, all this carrier also contributing to the current then efficiency of 1000 nanometer photon will also be high quantum efficiency. And therefore, by the quantum efficiency I can find out what is happening at the front surface what is happening at the junction what is happening in the middle of the material and what is happening in the rear side. So, typically I would expect quantum efficiency graphs to be like this and beyond if my band gap is a if the if a photon of energy less than the band gap is coming it is not absorbing that the if quantum efficiency will become 0 ok. In this way my quantum efficiency is a very good tool quantum efficiency is a very good tool to probe the solar cell ok fine. So, let me stop here I am already crossed the limit already crossed the limit for this. So, let me stop. So, let me summarize before I stop that that first of all what are the upper limit of parameters upper limit of short circuit current depends on the band gap and spectrum upper limit of open circuit voltage depends on the band gap and recombination upper limit of field factor depends on the series resistance and the open circuit voltage and all this determines a bit band gap and combination of will this determine the what is the optimum band gap for the obtaining highest efficiency. Low band gap material will give me high short circuit current, but lower open circuit voltage high band gap material will give me higher open circuit voltage, but low short circuit current and therefore, I have to find out the optimum value and optimum value of the band gap is 1.45 electron volt. Then we started looking for how to design a solar cell for highest short circuit current. So, minimize reflection and most important thing is where to place your junction your junction should be very close to the surface from where light is entering because most of the generation takes place close to the surface and therefore, junction should be close to the surface and that is why today's commercial world your junction is located only about 300 to 500 nanometer from the top surface. So, your n layer is very thin and your p layer is very thick remember crystalline silicon solar cell is 180 micron your n layer is only half a micron or less and your p layer is 180 micron. Then I most importantly we discussed the quantum efficiency. Quantum efficiency is efficiency of each quanta each photon. So, what how many electron is given by one photon typically one photon can give one electron. So, therefore, quantum efficiency can be 100 percent, but because of the recombination taking place at the surface at the junction and reflection taking place quantum efficiency is not always 100 percent especially close to the front surface because your surface can also recombine and the back surface quantum efficiency can be lower ok. So, let me stop here and I will take couple of questions we are getting late for the next lecture as always, but I know you are enjoying your lecture. So, you do not mind doing that ok. So, thank you let us start and question answers ok NIT Surat. Sir, I want to ask that solar cell we have seen in which p type material is below and n type material is on a top and light is falling on a just n type material. So, can we made a solar cell said that light is falling on a both p type and n type material means if we consider the solar cell as a cubic and solar cell we have discussed in this the light is falling on a just on a top. So, can you make a solar cell in a said that light is falling on a side. So, that light is falling on a both p type and n type. Yes, yes it is very much possible that you can make a solar cell that lights falls on a both p type n type typically standard solar cell is a cell where you mentioned correctly that light n layer light falls on the side of n layer and the p layer is at the bottom. You can actually make the other type of also solar cell you can make p layer on the top n layer on the bottom or you can make a solar cell where light falls from every direction. In fact, there are solar cell which is called bifacial solar cell which can actually accept the light from all the direction and can perform well. So, yes it is possible to make a solar cell, but making a cubic solar cell will be not a good idea right. Because on a front side your solar cell if you make your junction at one side then you are it is closer to where the maximum generation is taking place, but if a light is coming from the other side it is farthest from the place where maximum generation is taking place. Therefore, making a cubic solar cell is not a good idea. Tejpur University. Sir, when the band gap is reduced that means less band gap then open circuit voltage will less and the short circuit current will increase. So, why it is? Well, when the band gap is reduced open circuit voltage will be lower and short circuit current will be higher right. Now, because of the lower band gap more and more photon have enough energy to excite a electron. Remember, what is the requirement for electron excitation that the energy of the photon should be equal to or greater than the band gap. So, in the band gap is lower more and more photon have enough energy to excite a electron and therefore, if more electrons are excited more separation will occur and more current will flow. And then the open circuit voltage for the lower band gap material will be lower because your exciting electron to only smaller potential energy increase right. And potential energy means higher is the potential that you can get because your excitation is lower your voltage is lower. If you excite electron to higher potential energy you will get higher voltage. I want to ask one question what is the efficient what is the percentage how that we can obtain from silicon cell for band gap of 1.12 electron volt. What is the highest efficiency that can obtain say that 44 percent we can get from 1.45 electron volt. Yeah. So, basically this 29 percent efficiency number that is given is for the crystalline silicon of 1.12 electron volt. 29 percent is the ideally highest possible efficiency that you can get from crystalline silicon. You remember that graph that I have shown you it is actually close to that number only. So, if you look at that efficiency graph and maybe if I can go back to that same graph again. So, this is the band gap versus the efficiency you can see here crystalline silicon the possible efficiency is crystalline silicon is here 1 the 29 percent is about is for the 1.45 electron volt this is the ideal possible efficiencies crystalline silicon will have little lower efficiencies ideally possible ok. Manipal. This design and application of solar cell depends on many parameters for effective utilization and applications any standards are contemplated or available. No, there is no standard for design of solar cell because the design depends on so many different parameters. For example, if you are starting silicon substrate is of a different quality different lifetime ok or more defected then your design has to be different. If your material is amorphous silicon your design has to be different if your material is some other then your design has to be different. So, there is no standard available for design of solar cell you have to do it on your own. So, depending on the market constraint and availability users has to think and use. Well, I mean they are for a standard for example, when for a crystalline silicon solar cell when people are making solar cells in millions and millions in number and when your input wafer is fixed then of course, your design is standard, but if your your material itself is changing your design will also change. And especially when you are trying to get higher in higher efficiency people are looking for many alternative design to get more and more charge carrier absorption and separation. Baramati sir, why N type is placed in front of in front? Good question why? Why N type is on the top and B side? Why N type is on the top and B side is on the bottom? The reason is coming from the technological perspective for example, you know the front surface where lot of generation takes place there can also be lot of recombination ok. People have shown that it is possible to reduce the recombination of the N surface then the P surface ok. Therefore, it is easier for people to make top layer as a N layer and the bottom is the P layer. Other thing is in the P layer the minority carriers are electron and electron have higher mobility as compared to whole. And because your junction is closer to the top surface and your P layer thickness is 180 micron the minority electrons in the P layer will have to travel longer distances and the higher mobility helps to travel longer distances. And therefore, it is also good to have P layer as a base layer. Shivaji university? The red light generates EHP in the interior area and the blue light or low wavelength photon generates EHP close to the surface. Why it is so? Ok the blue light generates electron hole layer close to the surface and red light generates electron hole pair not in the interior it generates everywhere also close to the surface. Please make sure that you do not misunderstand this what I said that the blue light will generate a blue light will generate EHPs close to the surface ok. A green light green light will generate EHPs in this volume also close to the surface. What is this? This is absorption length and a red light will generate EHPs here, here, here, here and here and deeper also ok. Also close to the surface, but they also generate deeper. Why it happens? Because of the lower absorption coefficient red light will have lower absorption coefficient and therefore, they will actually generate here also here also here also here and infrared and infrared light infrared light will generate everywhere here also and some of them also transmit ok. So, it is like this is that clear please all the participant do not get confused that only red light generates in the middle it generates everywhere including the surface ok. Some question in the chat and we will run late on that again. What is the difference between solar cell and photo, photo detective device later? Next course is the edging effect of solar cell affect the efficiency also what is the average life of a P and N region. Edging effect can definitely affect the solar cell efficiency and as the as the time progresses when the modules are installed in the field solar cell efficiency keeps on decreasing ok. How much it decrease? People also guarantee that that first 15 years the efficiency will decrease only 10 percent of the initial efficiency first 15 years and in the next 10 years efficiency will decrease by another 10 percent. Now, this 10 percent are relative to the initial value ok. So, 10 percent 10 percent 20 percent. So, in the 25 years lifetime people guarantee that efficiency only decrease by 20 percent of the initial value. What does it mean? That even after 20 years your solar module will still perform 80 percent of its initial voltage or efficiency. So, as degradation occurs P and N region normally does not degrade which is a metal contact and the encapsulation and the inter reflective coating in all results in a degradation. We will discuss that in detail. Is there any possibility to minimize the thermal thermalization and transmission losses? Answer is yes. It is possible to minimize the thermalization and transmission losses. How to do that? You can do it by making a multi junction solar cell ok. By the way if I take a material of 1.1 electron ok. I let me anyway discuss it. We are running very very short of time, but I will spend less time later. So, if I have the. So, the question that I am trying to solve is how to minimize thermalization losses and transmission losses ok. So, if I have a if I have a photon of 2 electron old band gate and I have a material also 2 electron old band gate. What is the efficiency? 100 percent. If I have a photon of 1.5 electron old band gate sorry 1.5 electron old energy and my band gate is also 1.5 electron old. What is the transmission distribution and thermalization losses? 0. If I have photon of 1 electron old band gate sorry if I have photon of 1 electron energy. And my band gate is also 1 electron old. What is the thermalization and transmission losses? 0 right. So, whenever my energy of the photon is equal to the band gap of the photon of the material, then I do not lose. I do not lose in terms of the thermalization losses and I do not lose in terms of transmission losses. If my photon is 1.5 electron volt and if I have a band gap of 1.5 electron volt I do not lose anything. If I have photon of 1 electron volt and if I have band gap also 1 electron volt I do not lose anything. So, what I am trying to say if you match photon energy with your band gap for all the photons which means there are infinite photons and therefore, you should have the infinite band gaps. Infinite band gaps are not possible, but what you can do is if you can do the multiple junctions as I said. So, if you make a now you put 3 solar cells on together right. So, now you make sure that your light is coming from this direction. So, this is your cell number 1, this is your cell number 2 and this is your cell number 3. If your light is coming 2 electron volt and above light will get absorbed here and then the rest of the light will get passed 1.5 electron volt and above light will absorb here and then remaining will be passed. So, now in this way by matching your photon energy close to the band gap energy bringing down you are bringing down the thermalization losses and you are reducing the transmission losses. So, by going from a single junction to multiple junction you can minimize the losses in your solar cell thermalization losses and both transmission losses. Last question multi multi junction solar cell design. How many junctions can we create within a low production cost? Very good question. If you can create two junctions with a using the cost of one single solar cell, I will guarantee you that you will get a noble price. I will guarantee the noble price to you. So, anybody everybody is trying to do that they are trying to make more junctions at the lower cost are trying to make a higher cell efficiency without increasing the cost. Anybody who can do that will get a noble price or in fact anybody who can make a solar cell with a low very low thermalization losses and very low transmission losses and not adding too much of the cost can actually get something like 10 noble prices because the whole world want solar cell at very very low cost in high efficiency. So, there is lot of potential to get many many noble prices get ready for that and let us stop here.