 what is the structure of commercial silicon solar cell that we want to manufacture. So, this lecture is mainly about the fabrication technology, how do we fabricate solar cell. So, what is the cell structure that we want to fabricate? Yesterday, remember in the design, yesterday in the design, we have discussed about it, cell structure to fabricate. So, what do you want to fabricate? We want to have a texturing, we want to have the inter reflective coating, we want to have the junction, we want to have the back surface field, we want to have the back metal contact, we want to have the front metal contact. These are the various things that we require in our solar cell. So, as we discuss, we have, we will have the texture and then we will have the emitter, we will also have our inter reflective coating, then we will have our, so junction is here, then we will have our P side, this is my N side, this is my N plus by the way, this is my A R C and I have P plus, then I will have the metal contact here and metal contact here and I have the metal contact at the back side. Thus, this is the most simplest structure that we can have for a solar cell. This is the most simplest structure that you can have solar cell. The question is how to fabricate? So, when we start, your starting wafer is flat. This is your starting wafer. Typically, your starting wafer is P silicon. That is it. It is a flat surface and you have to process this, you have to process this wafer to get this structure. You have to process this silicon wafer, which is a flat to get this structure. How to do it? That is the question. So, I will tell you a generic process, what happens in industry. So, as the standard process that you start, you start with your wafer cutting. Normally, you buy this wafer. So, people buy this silicon wafer directly from the market. So, this comes like this. So, this is your starting point. Your silicon wafer is your starting point. Doping of your silicon wafer is already predetermined because you buy your wafer as per the doping requirement. The next process that you do is, the next process is normally to texture it. First of all, we have to clean the wafer also because it is coming from the sawing. Remember, when you saw your wood, the surface of the wood will not be very smooth. There will be a lot of chips of the wood that will be sitting there. Similarly, when you saw your silicon, the surface will not be perfectly smooth and clean. There will be a lot of silicon chips will be sitting there. So, you have to get rid of that. Moreover, you need to get a structure. You need to get a texture. That is the pyramidal form that you have to get. That is the very first step that you do. So, once you do this, once you do the texturing, what happens? Your plane surface, which was like this, now gets textured both the front side and the back side like this. This texturing automatically happens. I told you other day that, if you take any silicon wafer and put in the NaOH or KO solution heated at about 80 degree centigrade key for 5 to 10 minutes, you will see that preferential etching of silicon occurs. I will discuss how it occurs. So, what is the next step you want? Next step you want to is to make a junction. Your next step you want to make a diffusion. So, next step is your POCl 3 diffusion. Basically, you are starting wafer of three types. Now, you want to make a junction of n type and this junction is very thin, 300 nanometer, 200 nanometer, 400 nanometer. Your silicon wafer is thick, 180 micron. The doping level of silicon wafer as we discussed in the design is about 10 to 15 to 10 to 16 per centimeter cube. Now, we want junction to be at only one side. But how? What industry people do is they put 200 wafers together, 300 wafers together in the big furnace and expose it to the phosphorous containing gas. You expose it to the phosphorous containing gas. So, what will happen? This phosphorous will containing gas will actually go everywhere. So, when you are doing in the, when you are actually doing in the. So, what you do is take many wafers. You take a board and these are all your wafers and this you put inside a furnace and you pass the gas, you pass the gas which is having phosphorous atoms. What will happen? The phosphorous atom will actually deposit everywhere. So, as a result of that, what will happen? As a result of that, what will happen? Your emitter is everywhere. Your emitter is everywhere, but that is not what we want. What we want? We want emitter. What we want? We want emitter to be here, n plus layer, the heavily doping of n layer to be only at the front surface not at the back surface. So, that is one of the problem that we must overcome or industry must overcome. So, emitter gets here. Another problem, it is actually shunting everything. So, one thing that people do is isolate this. So, you cut this edges. Actually, cut the edges. Again, if you are using 200 wafers, you have to cut the edges at 200 wafers. All the four sides you have to cut. And typically in industry, any given industry, you make 2000 to 3000 wafers per hour. What is the speed of the process? 2000 wafers to 3000 wafers per hour. So, you have to actually cut the wafers of that many numbers. So, there is very interesting way in which people do that, but you have to cut it. So, I am just showing you that you have to remove the, what is called the parasitic junction. So, you have to remove the parasitic junction. So, you actually get cut from the sides. So, your junction is removed from the side now. Still your backside junction is a problem. So, what is the structure right now? Top layer is n plus, middle is p, backside is again n plus. What is the structure that we want? n plus p p plus. The structure we want is we want top layer n plus, then we want p and then we want p plus. That is what we want. What is the right now? Top layer is n plus, middle layer is p and the bottom layer is again n plus. So, we have to actually go from here and get this. How to do that? So, this happens automatically. Now, what next you want? We want to actually deposit anti-reflective coating. We want to deposit anti-reflective coating. So, you do what is anti-reflective coating that we use normally silicon nitride. Si 3 and 4 in this stoichiometric form are normally Si n x. Silicon nitride is what we want to deposit. That can be deposited only on one side. So, typically it is a blue in nature. So, you can see here the silicon nitride is now blue in nature. So, silicon nitride is deposited next. What is the next step? You want to have the metal. One problem to be solved is this right. Backside you have n plus, but we want p plus. So, that has to still be solved. The next step is actually to print metal contact, front metal and the back metal. So, actually do the screen printing. You put your metal at the front side. You put your metal at the back side, but there is another problem here. Another problem is that your anti-reflective coating which is silicon nitride is a dielectric layer. Dielectric layer means it does not conduct. So, your metal contact is not in contact with semiconductor and that is a problem. So, what is the situation right now? You have, this is your A R C layer. This is your n plus layer and then you have p here. Where is your metal sitting? Your metal is sitting on the top of A R C, but this is your semiconductor. This is your metal and in between you have the dielectric layer, non-conductive layer. So, which is not, it will not work because of this your series resistance will be very high and your performance will be very poor. So, what we want? We want metal to come down and make a contact with, we want metal to come down and make a contact with semiconductor with n plus layer. How we can do that in industry? How we can do that in industry? There is a process called contact firing. By the, what are this metal contacts which you are using? What are this metal contact which you are using? This metal on the top is actually silver plus aluminum. There is a small percentage of silver, mainly it is aluminum and the back side, the back contact, the back side we use, the back metal contact is aluminum. There is a reason for that aluminum makes, aluminum makes very good contact with P surface. So, you get good contact. Aluminium does not make contact with n surface, no good contact, but silver makes good contact with n type surface, good contact. When it is a good contact, we are talking about omic contact. So, silver n is good, aluminum P is not, is also good, but aluminum n is not good. So, on the top you have n surface and if you actually only use aluminum, you will not have a good omic contact. So, then you have to use silver, but you cannot use 100 percent silver. It will be very expensive, solar cell will become expensive. So, therefore, people mix silver aluminum and silver does the interfacing job and then aluminum can take care. So, that is why the front surface contact is basically this, the back surface contact is this. So, now one of our problem is that this metal contact, there is a dielectric layer sitting in between, what to do, what to take care, how to solve this problem. So, what people do is what is called the contact firing or co-firing. In the contact firing, what will happen this metal at the front side, actually when you heat it up, when you do the heating, when you do the heating of this contact this, because this layer is very thin. How thin is this layer? Only about 18 nanometer, because this layer is very thin, when you heat it up the metal gets actually diffused inside the dielectric layer and makes a contact with semiconductor. Another nice thing happen is aluminum at the back side, aluminum comes from the which part of the periodic table, aluminum is a trivalent impurity, trivalent impurity. What does it mean? For silicon, if any impurity atom is a trivalent, what does it mean? It will be acting as a P type or N type dopant, it will be acting as a P type dopant. So, when we do this heating process, when you do the heating process, aluminum also diffuses from the back side, aluminum will diffuse from the back side. So, you know back surface is our N plus surface, because of this emitter, back surface the yellow one is N plus surface, but now aluminum will diffuse and aluminum is acting as a P type. So, it will actually overcome that N surface and it will make it P surface. So, this N surface that you can see here at the back side, when you do the contact firing, when you do this high temperature process, the N surface at the back side actually gets automatically converted into the P surface, remember. So, now it has become the green layer. Earlier it was like this, now it has become the green layer and what has happened? Remember the metal contact were not in contact with semiconductor, after the firing two things are happening, the metal contact is coming closer, if you can see it clearly. Now the metal contact is making through and it is connected with semiconductor and back side where there was a yellow layer, which was the N plus layer has now become a P plus layer, it has become a P plus layer. So, what is this now? You have the N plus here, you have the P here and you have the P plus here and that is what is the structure we want and we also got the anterior flictive coating that is a blue layer and then we got the metal contact at the front and metal contact at the back, that is the cell structure we want. So, this is the typical solar cell process that happens in industry and finally, you get like this, you have the back side textured, you have the P plus at the back which is very thin, your P is very thick, your N plus at the top, you have the metal which is normally having aspect ratio of less than 1. So, width is higher than the height, typical spacing 1, 2 millimeter, 3 millimeter. So, let me show you this whole process again, just I will go very quickly through it. So, you start with the silicon wafer, you do the texturing, you do your junction formation, then you cut from the sides, then you actually put your anterior flictive coating, then you put your metal contacts, then you do the contact firing and your solar cell is ready. So, this is what is the industrial process and if you do this process, well very easily you can get a solar cell of about 15 percent efficiency. If you do this well very easily you can get a solar cell of about 15 to 16 percent efficiency, but I told you that laboratory solar cells demonstrated is very high efficiency. So, in the laboratory, laboratory silicon cell, people have demonstrated efficiency of about 25 percent. Commercial silicon cells, people have obtained the efficiency about 15, 16 percent. Why so much is the difference between the laboratory cell and the commercial cell? There are many processes which from the design perspective you want. There are many processes which from the design perspective you want, but we cannot implement in the industry because of the cost reason. We cannot implement that in industry because of the cost reason and therefore, the efficiency of the commercial silicon solar cell is always lower than the laboratory efficiency, the best laboratory efficiency that has been demonstrated. So, there are many features we want. We do not want high recombination at the surface. So, in laboratory you can take special care for that. We do not want high recombination at the back surface. You can take special care for that. We actually want two anti-reflective coating so that we can minimize the reflection at two different wavelengths and things like we want high aspect ratio. So, we want finger width to be small and height to be higher. We cannot do that in industry and so on. So, there are many compromises that industry we make and therefore, our efficiencies are always lower in industry as compared to the lab. So, now, I will take you through the processes one by one so that you get a feel of it, how it is done and what temperature it is done and I will come to the question answer soon. Because the first process is choice of wafer and it comes from the design. So, our choice is about 10 for 16 atom per centimeter cube. So, that is your starting wafer. Then you do the texturing. We have not discussed the orientation about the silicon atoms in a solar cell, but actually when you look at the crystalline silicon wafer, the atoms are nicely arranged. So, atoms are. So, when you look at the silicon wafer, atoms are nicely arranged and based on the arrangement you can actually define the directions in the silicon wafer. So, if you look at from one side it looks like this. So, these are the how silicon atoms. So, if you look. So, normally the starting point is the direction called 1 0 0. We do not have time to discuss it that, but if you look in this direction you get the direction 1 1 1. So, when your starting point is 1 0 0, the density of atoms in one direction is different than the density of atoms in other direction. Same thing you can see if you have a systematic arrangement of chairs. If you look at the systematic arrangement of chairs in the classroom and if you look from different directions, you get the different, you get to see the different views of arrangement. Same thing is true with the silicon atoms. So, when you do the etching of silicon, normally because the density is lower in 1 0 0 direction as compared to 1 1 1 direction, the etching is faster in 1 0 0 direction. Etching is faster in 1 0 0 direction as compared to 1 1 1 direction. Just imagine right now that 1 0 0 is some kind of direction in the crystal, 1 1 is another direction. In 1 0 0 direction the density of atoms is lower and therefore, etching is faster and in 1 1 direction the density is higher and etching is slower. That what happens when I expose in 1 0 0 direction in this direction the etching becomes faster in this direction, etching is slower and therefore, you automatically get the formation of the pyramidical structure. You can use actually both acetic and alkaline solution, typically industry use alkaline solution. Potassium hydroxide and sodium hydroxide is commonly used for doing the texturing. Very easy process you can do and this is why finally, what you get right. I showed you yesterday also when you are doing the etching you get the pyramids formed. You get the pyramids formed like this. Then next process is junction formation. How do you make a junction formation? Junction formation occurs by the diffusion process called diffusion and as I showed you put many wafers together in a furnace if I have the picture. Many wafers together and the atoms of the dopant has to go physically inside the wafer. Atoms of the dopant has to go physically inside the wafer. So, when you are doing the doping. So, your phosphorus atom is sitting here. So, phosphorus atom must go inside. It must travel inside and occupy some space. So, this is the physical diffusion of atoms and therefore, somebody was asking why to choose only phosphorus, why not other and we have to worry about the size of the phosphorus atom. If the size of the phosphorus atom is too big it will have difficulty in going through. If it is too small it will also create defects. Now, because this motion is occurring physically you require high temperature. So, doping you require high temperature and this temperature is normally between 800 to 1000 degree centigrade. 800 to 1000 degree centigrade you need to dope then only carriers will move. Second problem is the doping is not uniform. Why it is not uniform? Because when you are starting point your p surface is like this is the doping level, but when you are doing the diffusion your and first of all emitter is heavily doped. So, when you are doing the diffusion you have this motion. So, in time and temperature process your emitter is like this and I will show you again how what we mean. So, when we start the doping I am I am plotting the concentration. So, this is my concentration and this is my depth. So, my starting is a p silicon. This silicon is normally transfer 15 to transfer 16 per centimeter cube. When I dope my emitter is heavily doped right we have seen already because we want low resistance from the emitter. We want low resistance from the emitter and therefore doping level has to be higher. So, that conductivity is higher. So, your doping is like this, but atoms are actually intering from the surface. So, they will inters most of the atom will appear on the surface and then slowly they will go in. So, doping will look like this your n layer your n layer or n plus layer actually will have the doping of transfer 18 to transfer 19 atoms per centimeter cube. So, this is your p layer and this is your n layer and wherever you have the n layer and p layer meeting concentration meeting that is your junction. So, this becomes your junction. So, this is how it looks like and this is how you get and it happens at high temperature 800 to 1000 degree centigrade and the time normally takes is 10 to 60 minutes. And the depth of the junction what is the depth of the junction your junction forms here a junction forms here. So, this is your depth of the junction. So, junction depth is as I said 200 to 500 nanometer and this thickness of your wafer is very large this thickness is 180 micrometer. So, as compared to 180 micrometer the junction depth is very very small nanometer fine. There is a theory behind the diffusion how it takes place and there are whether the source is constant or it is limited and what will be the profile of the diffusion will depend on the temperature and time 2 of the important parameter at what temperature you are doing your diffusion and for what time you are doing your diffusion right. So, if I if I am doing the diffusion if I do for 10 minutes it profile will like this if I do for 20 minutes it like this for 30 minutes like this. So, as I increase the time the more and more dopants go deeper and deeper into the silicon wafer. So, with respect to time the profile keeps changing this is the depth. So, time is important temperature is important temperature is required to do the diffusion normally as I said the temperature requirement is 800 to 1000 degree centigrade. If you do your diffusion at 400 degree centigrade you may take some couple of years actually to get your junction. So, temperature is also very important. So, therefore, this graph is plotted for D and T D is the diffusion coefficient which is the function of temperature and T is the time and various impurity diffuse at the different rates various impurity diffuse at the different rates. For example, you see the diffusivity of Goron is less aluminum is higher gold which is not a desired impurity is very high. Sodium, potassium, iron, copper these are all all the problematic impurities right these are all the problems for silicon solar cells. By any chance if you have this impurities and if you are doing your diffusion at high temperature you are going to get a you are guaranteed to get a low solar cell efficiency. So, for potassium, sodium, gold, iron, copper, silver these are undesired impurity they should not be there in silicon if it is there then you are in good trouble. So, this is how you do the diffusion you take all many of your wafers together if you take many of your wafers together and you insert your gases inside. So, dopant gas if you are doing the phosphorus diffusion then the phosphorus containing gate COCl 3 if you are doing boron diffusion then boron containing gas that you have to do. Then finally, edge isolation you have to cut the edges remember you have to cut the edges to in order to avoid the shunting how do you cut the edges which is which is done by what is called the plasma etching. So, you actually put the stake of wafers together how do you do the edge isolation the process called edge isolation cutting the edges. What you do you put many of your wafers together you make a stake of 400 500 wafers. So, these are all your wafers then you expose to the plasma you expose to the plasma how much material you have to cut how much material you have to cut you have to cut material equivalent to the thickness of the junction or the junction depth what is your junction depth couple of 100 nanometer. So, your junction depth is only 200 to 500 nanometer. So, you have to cut very small distances and therefore, it can be easily be done with the plasma earlier people used to do mechanically also then people also do with the laser with the plasma etching is the most common technique that is used. After that the next step is anti reflective coating deposition which material we want we want silicon nitride about 70 to 80 nanometer the method that is used for silicon nitride deposition is PECVD plasma enhanced chemical vapor deposition plasma enhanced chemical vapor deposition. And this can be done in a one side way. So, normally what you do you have your big chamber and all your wafers are sitting here you want to deposit silicon nitride. So, what you do silicon nitride. So, what you do you put silicon silane into it then you also put ammonia into it silane and ammonia and then you actually ignite your plasma you ignite your plasma. So, what will happen this 2 gases will react and actually will give you silicon nitride SIN and then this silicon nitride gets deposited and this deposition will only occur at one side and this kind of tool is called PECVD plasma enhanced chemical vapor deposition plasma enhanced chemical vapor deposition. So, it is some kind of chemical process that takes between the 2 gases silicon containing gases and nitrogen containing gas. Silane is a silicon containing gas and ammonia normally used is a nitrogen containing gas and the reaction of this 2 will give silicon nitride which gets deposited. And I told you earlier also that the blue color of your solar cell the blue color of your solar cell is actually because of the silicon nitride fine. So, the silicon nitride is next and how to design the silicon nitride we have seen the square the refractive index of silicon nitride should be square root of the layer which is above the silicon nitride and the layer which is below the silicon nitride. So, you get a silicon nitride here remember the schematic I am showing the flat surface, but in reality it is not the flat surface it is textured surface. Another point of caution here is that all the gases that are used in the semiconductor industry are very dangerous gases. What are the gases we use for example, when we want to make diffusion of phosphorus we use phosphine E H 3 or silane Si H 4 or when we want to make a diffusion of boron we use diborane B 2 H 6 or we use arsine for the phosphorus again. Look at the hazards. So, these are toxic and flammable gases toxic and flammable what is the toxicity limit allowed parts per million. So, arsine for example, is one of the most dangerous gas is a allowed limit in the atmosphere is only 0.05 parts per million. So, in almost a billion parts of the air there allowed limit is 1 molecule of the arsine only. If it is more than that what it will do it will kill you because it is all very dangerous gases very toxic gases you can see the allowed limit allowed limit in parts per million and billion. So, if you are using silane if you are using phosphine if you are using diborane if you are using arsine in the laboratory you have to be extremely extremely extremely extremely cautious and this is one of the reason why you cannot have these gases everywhere. Again nobody in India manufactures these gases or whenever we have to do the processing we have to import these gases even for the academic institutions also. So, this is the point of caution then you do the metallization front and back metal and the last step is a contact firing. Metallization is done by screen printing screen printings are simple you actually take your screens with the holes as per the pattern what pattern we want we want a bus bar which is a thicker opening and the finger which is a finer opening and your metal paste is kind of semi solid you do the printing as you do your business cards visiting cards same way you do the metal printing screen printing of contact also. So, I will stop here I think that gives you very clear cut idea about how solar cells are manufactured in the industry starting with the wafer you do the texturing then you do the emitter formation which is everywhere. So, you have to isolate then you to the edge isolation then you deposit your anti-reflective coating then you do your metal deposition at front and back and you do the contact firing. So, in a step by step process I have shown you how the how the various processes are done. So, now we have thankfully I could stop it 5 minutes before or 10 minutes before the official time allotted for this lecture we will stop here I will take the questions on silicon fabrication as well as solar cell fabrication of silicon type and in the next lecture now we will start when I will come next tomorrow I will actually talk you about the performance of the solar cell and the module. First of all we will see how to design a module how to design a module how many cells to connect in all once we do that we should like at the performance of solar cell module and one of the last lecture will be how to how to design a PV system. So, if you want to design a PV system for your house or your college then how to do that. So, let me start taking questions now. KG Somaya, you have not mentioned about ion implantation in your slides. So, is it used for doping in PV industry? No, ion implantation is not used for the doping in a PV industry it is a expensive process not commercially viable for solar cell, but there are some people who are trying to do the research to get ion implantation in a specific manner when we want higher efficiency, but normally it is not used. VNIT in Akpur? Etching of the texturing of the surface by etching in KOH solution is this possible only for 1 1 1 surface orientation or also possible for 1 0 0 surface orientation? Normally the starting surface orientation is 1 0 0 ok. Now the etching will always occur preferentially in 1 0 0 direction as compared to 1 1 1 direction ok. So, if you are if you are going to have the starting wafer as a 1 1 1 that you will not get the pyramids ok, because you are getting 1 0 0 as a starting you get very nice pyramids. ESG college? You told 1 1 1 0 0 is prepared to 1 1 1 orientation is it common for all type of silicon I mean crystalline polycrystalline or multicrystalline ok. So, when we come to the defining direction the direction is only defined for monocrystalline silicon right, because in multicrystalline there are so many different crystal of different directions it is not possible to give 1 particular direction for a multicrystalline right. Remember the multicrystalline silicon the multicrystalline silicon have all all possible orientations right. So, in multicrystalline silicon if I if I try to draw you will have 1 crystal of this size 1 crystal of other side ok. So, this is how the multicrystalline silicon look ok. Now each of this crystal each of this crystal this part for example, has its own orientation this part will have its own orientation this part will have its own orientation and so on. And therefore, it is not possible to define a direction in a multicrystalline silicon it is only possible to define the direction in monocrystalline silicon only ok H. Poor University. Why monocrystalline looks slightly blackish after the reflective coating and multicrystalline is blueish type. Why monocrystalline looks slightly blackish as compared to the multicrystalline which is more bluish one reason possibly could be because monocrystalline has the same orientation it is possible to actually get the good texturing all over the surface. And multicrystalline silicon because there are multicrystalline there are many many crystals each having different orientation texturing of a multicrystalline silicon is you listen to my answer first the texturing of a multicrystalline silicon is not easy because every every crystal has its own orientation. And therefore, you may not get the best reflection from each of the crystal and therefore, some crystal will probably reflect more and and therefore, it will appear little bit more brightish or more bluish as compared to monocrystalline. Does aging affect the texturing? Aging does not affect the texturing aging is what it is some possibly the moment of moment of atoms, but silicon is is a kind of it requires lot of energy to move any silicon atoms. So, once you have the texturing done I do not think the at the room temperature or even 100 degree 200 degree centigrade the silicon atoms can move. And if silicon atoms are not moving which means it is not going to affect the texturing as a result of a time or age. Government call a Salem? Sir, while monocrystalline crystal pooling in monocrystalline pool air they rotate with some speed rotate the seed with some speed how they fix to the speed. So, when you are when you are pooling the monocrystalline ingot from the melt of silicon first of all rotation is required because you want your impurities to be homogeneously distributed in the material. So, that you know the concentration of impurity in your silicon ingot is uniform imagine the silicon ingot is normally about a meter or one and half meter long. So, impurity concentration at the top of the ingot middle of the ingot bottom of the ingot should be same. So, it should be a uniform the speed of the rotation then will determine the diameter of the ingot. So, if you are rotating with a very slow speed more and more silicon atom can can get attached from the side also. And therefore, the speed will determine the diameter of the ingot also. For for safe in the monocrystalline they are machined with a lathe is it correct method? They have to machine what monocrystalline the seed layer the seed layer which is starting point of getting the silicon ingot will determine the monocrystalline probably structure of the well over. Sir could you please tell us how to explain this fixed second law of diffusion means that there is a t and x both. So, it is a three dimension picture. No. So, normally fixed law fixed law it is not three we have we normally take it as a one dimensional only, but there are two parameters one is the space and other is time. So, as I plotted as I plotted earlier that when you draw the fixed second law what will happen? If you if you are considering the concentration of dopants inside the silicon and if it is your depth. So, for some time that doping profile will look like this. So, what is happening concentration and depth is changing with respect to time at some other time you will have this profile at some other time this is this profile. So, in this way the concentration is a function of x and time both. So, that is what is given in the in the in the fixed law that how the depth and time both are changing like when we discuss the diffusion when you discuss the I V equation of a diode we said the concentration of carriers is changing with respect to space as well as with respect to time right. Same thing will also occur in the diffusion of this dopant that concentration of dopant atom will change with respect to space as well as with respect to time. So, as we expose it for more time the density will be more on the the gradient will be less. No, as we expose it the more at the surface concentration remains almost constant, but it the atoms will have more and more time to go deeper and deeper right. It will have more and more time to go deeper and deeper that is what happens ok. If you want to make a junction in the laboratory should we follow the same process of phosphorus diffusion or there are different processes which we can adopt ok. If you want to make a junction in the laboratory one way to do is a phosphorus diffusion there are other ways also there are much simpler ways also for example, if you want you want diffusion there are let us say I will show you phosphorus. So, p diffusion if you want one one way to do is P O C L 3 this is a if you want phosphorus diffusion one way to do is is use a P O C L 3 this is a liquid source by the way you have to kind of bubble it through the furnace other way is called spin on dopants. These are the liquids which are available in the small bottles and you can take your p-type silicon wafer you can spin this dopant on your wafer and actually heat it up. Third way is to take a solid source ok. So, sometime some similar to wafer you get a wafer of phosphorus containing is a solid piece of wafer. So, what you do is you take your silicon wafer and you take your dopant wafer and then you put your silicon wafer and dopant wafer alternatively. So, that and then you heat it up. So, the diffusion takes place in the silicon. So, there are other a couple of simpler ways also that you can actually use spin on dopant which is much easier and you can use the solid source. NIT Warangal. So, my question is you have said that silicon wafer is manufactured using wire sawing operation. So, but the thickness is very less or somewhere around 500 microns as you have said. So, can we get such very thin wafer using wire sawing? Can you throw some light on it sir? Yes, wire sawing by the thickness is not 500 micron. I am repeating again again the commercial thickness right now we get is 180 micron only 1 at 0. Yes, and yes it is possible to get not possible it is actually done in industry to get 180 micron thin silicon wafer sawed by wire sawing. Yes, it is possible very much and people are actually trying to go thinner and thinner, but once you go lower thickness if once you go lower than 180 micron I think somewhere the down the line about 100 micron the less than your silicon wafer start becoming flexible or actually the one problem why people are limited to 180 micron is that if you go to lower thickness like 150 micron or 140 micron the handling of silicon wafer in industry becomes difficult. Why difficult? You know what I am talking about I am talking about 2000 to 3000 wafer per hour production. So, when your silicon wafer is thin it becomes more and more difficult to handle it, but wire sawing technique it is possible to get 180 micron and even less. Sastra University, good morning sir I want to know whether only commercial silicon solar cells are used in PV modules or laboratory solar cells are also used and why? Laboratory solar cells are used in laboratory not in commercial way they are very expensive they cannot be produced in a very large quantity and also on. So, when people do the experiment in laboratory they work with a 5, 10, 20 wafers in commercial way you are talking about again I am repeating 2000 wafers per hour and you produce millions of wafers. So, laboratory solar cells are only for laboratory. Hello sir there is one more question whether we want to select a solar module on which basis we can select either a monocrystalline or polycrystalline which efficiency will be more for monocrystalline or for polycrystalline or it is not considerable. Lecture number 16 tomorrow. So, wait for couple of lectures you will find the answer ok NIT Trichy. Sir if nitrogen is useful in fabrication of solar cells why you need to depend on that flammable and toxic gases nitrogen can also be used no sir. Nitrogen ok nitrogen can also be used for a ok. So, question is nitrogen can nitrogen also be used for making the n type instead of phosphine and arsine in other very toxic gases. The answer is no you know it is very important that diffusion takes place it is very important that you get that any atomic form it is very important that you actually affect your crystal lattice as minimum as possible and it is not possible to do the same get the same thing with nitrogen. So, therefore we have no choice than to use any alternative to the toxic gases ok material material of the bus bar is same as the fingers same material only thing the dimensions of the bus bar is thicker the bus bar is thicker as compared to the fingers. Gandhinagar. Regarding loping concentration for boron p type vapors are available. So, is the if we do the grading of the p concentration along the thickness of the vapors. So, is it beneficial for the performance of the silicon solar cell and second question is such kind of vapors are available commercially. So, the point is very nice that normally the boron vapour is doped is available with the constant doping. So, when you take the boron vapour like this and if I if I go from one side to other side the concentration is uniform. So, his question is if we do not if you have non uniform concentration is it useful. Answer is yes it is very much useful because if you create a non uniform concentration you can create a electric field and that electric field can actually be useful in collecting more and more minority carrier. But imagine that I am actually doing a I am doing the whole ingot right if the if this is my ingot and I told you this ingot is 1 meter long actually in your tutorial I have given you problem to find out how many vapors you can get from a 1 silicon ingot. Now, it is it is not possible to you know grade it the vary the doping concentration right you want doping concentration of a particular variation in each vapour it is not possible because you are making a ingot of a 1 meter length. So, in practice that kind of doping concentration is not possible to get, but it is desirable if you if there is any other nice technique which is low cost it can be used. Let me take couple of question from the chat. In solar cell we are connected number of series and shunt resistance for getting the regulated output we have to calculate the current value by Ohm's law, but Ohm's law is applicable only for constant temperature. In solar cell we did not get the constant temperature is it applicable for Ohm's law in our solar cell I am confused what you are writing. We will discuss may be tomorrow in in the following lectures what we are talking about basically how the performance of cells and modules changes. So, that we will discuss. Sastra University question was whether industry started using CNTs for effective carrier collection answer is no no industries using CNTs for effective carrier collection. In fact, there are not many research lab also which are using CNTs for effective carrier collection. So, that is still a good research topic you can work on it. How much energy is spent on producing 1 kilowatt of solar panel? Good question. So, 1 kilowatt of solar panel the cost of this energy that is that is gone into manufacturing a panel is recovered in about 1 to 2 years. Now, 1 kilowatt of panel how much energy 1 kilowatt of panel will produce? Again we will discuss it, but I think roughly it about about 3 to 4 or about 3 to 5 kilowatt hour of energy is spent in making 1 watt peak of module 3 to 5 kilowatt hour of energy spent in making 1 watt peak of module. When we will do the system analysis we can actually take this question. The answer is simple to find you know your 1 kilowatt peak module. If it is working for 5 hours per day per day and if there are 365 days this much is the energy produced. If you multiply this is the energy produced in 1 year. Now, I am saying the let us say if the payback period payback period what payback period I am talking about energy payback period. If energy payback period is 2 years then you can say whatever is the energy comes multiply by 2 that is the energy consumed. So, that is the typical answer, but we can look into the detail more. What are the minimum temperature requirement for converting from silicon to silane and trichlorosilane dichlorosilane chlorosilane respectively? Converting silicon to silane we do not convert silicon to silane we do opposite we convert silane to silicon. We convert dichlorosilane to silicon we convert trichlorosilane to silicon the kind of temperature that is required is again you have to you know dissociate silane. So, silane S i H 4 should get. So, if you are doing it is ever S i H 4 should get converted into S i and some H 2 to H 2. So, this would this process will require some temperature. This process will require some temperature and I guess typically this temperature will be about 1000 degree 1200 degree 800 degree centigrade. So, this temperature will be in that particular range. So, require some temperature to do that. So, thank you.