 Today's lecture, what we will see is the thin film technology. So, if you look at the crystalline or the solar cell technology, there is a wafer based. So, the solar PV modules if you divide the solar PV technology, let us say. So, you can say one technology is a silicon wafer based technology and the other technology group is thin film technology. So, whatever we have seen is mainly so far as silicon wafer based. The thin film technology is also important aspect of the PV technology, because there are many thin film technology. We are trying to reduce the cost and in today's lecture, I will give you idea about the thin film technologies. Again, there are many slides. There will not be enough time to go through all the slides, but I have kept him anyway in case if you want to study more about thin film technologies. So, that is what we will discuss. What are the generic advantages of thin film technology? What is the need to go from silicon, crystalline silicon wafer based technology to thin film technology? Why people are trying to develop thin film technology? What are the materials that are used for thin film technology? You already know many materials, I am sure, but we will discuss that. What are the deposition techniques that people use for thin film and what kind of junction we use? PN junction or PI junction. What kind of substrate confederation we use? Then we will discuss very briefly. As I said, lot to discuss and there is only one hour for the thin film technology. So, very briefly I will discuss about the thin film technologies, amorphous silicon cadmium to write and CIGS. So, first of all, the thin film technologies are deposited. So, one of the major differences is the silicon wafer. The silicon wafer is, we say it is a grown wafer. We actually grow the wafer from silicon melt. We grow the silicon ingot and from that thin films are not grown. They are deposited. Thin films are not grown. Thin films are deposited. So, one of the very major differences. So, silicon wafer is grown. Thin films are deposited and as a result of that, there is also difference in the structure or difference in the cell making. So, for example, when we talk about silicon wafer. So, we take silicon wafer and we make, we use this wafer itself for making a solar cell. So, we make junction within this, we make contacts, etcetera. Now, the wafer being a thick substrate, 180 micron as I said. Wafer being a thick substrate, it has enough mechanical strength to support its weight and therefore, you do not require any other substrate to make a silicon wafer based solar cell. But thin film, as we know by the characteristic that most of the thin film material are direct band gift semiconductor. Their absorption coefficient are very high. Their absorption length is low and therefore, the thickness required for thin film solar cell is only 1 micron or lower than 1 micron, sometime little higher than 1 micron. So, this 1 micron is a very thin layer. Our, the diameter of the hair, if you see, what is the diameter of the hair? You have diameter of the hair, that your hairs is typically 100 micron, 100 micron in that order. So, therefore, the thickness of the material that you are talking about is much, much less than 100 micron. Therefore, you need some substrate to support the film, thin film. You need some substrate. So, normally what you do? Use some kind of substrate and this substrate is just a supporting substrate. It is not playing any role. It is just supporting the weight or supporting the thin films. So, supporting substrate and there are thin films are deposited. Then you deposit your thin film and this thin film can be of any type and then you make your solar cell here. You are making your solar cell within this thin film. So, in this case, this whole area is a active area. In this case, only the thin film which is top is active area. So, this is another difference. In thin film case, the whole area is not active area. Only the top layer is active layer and the substrate, you need substrate to support thin films. Now, your substrate can be of different type. Crystalline silicon is a thick substrate. So, you have no choice. It is always going to be rigid, but your substrate for thin film can be rigid substrate or it can be flexible substrate. Therefore, your thin film technology can be rigid or it can be flexible, but because your thickness is very high, your wafer-based solar cell is always going to be a rigid solar cells. So, I hope this is clear to all of you. The starting difference between the different PV technology wafer and thin film-based rigid substrate and flexible substrate, grown wafer and deposited active layer. So, thickness of these thin films can be nanometers, tens of and micrometers also. What are the advantages of thin film material? First of all, the film thickness is very less, 1 to 5 micron or sometime even less than that. In the wafer cells, thickness is very high and therefore, you are using less material. You are using less material means less cost. So, that is one possible advantage. Shorter energy payback period. Now, because we have discussed here that your substrate can be rigid and flexible and thin films are deposited. Now, this deposition can occur at very low temperature 200 degree centigrade, 300 degree centigrade, but we make a solar cell, you know crystalline silicon solar cell. Remember the temperatures I was talking about, 2000 degree centigrade for making a silicon wafer itself and then 100, 800 degree centigrade for diffusion and all. So, lot of energy is required to make a crystalline silicon solar cell. Less energy is required to make a thin film solar cell and therefore, whatever energy that has gone into making a solar cell can be recovered in a short time, shorter than the crystalline silicon. How much is that time? Less than a year. Another important difference is the monolithic integration of cells. In crystalline silicon wafer technology, what you do? When you make a module, you make many types of, many solar cells, millions of them and then you connect cells in a series in a module. In module there are 36 cells, we will discuss how, but these cells are connected in series, how they are connected either manually or by some automation. So, the module manufacturing in crystalline, so another difference is in crystalline silicon wafer based technology, cell making and module making. Cell making and module making are two different process. While in case of thin film, while in case of thin film technology, cell making and module making is same process. So, by doing one process, you are not only making the cell, but you are also making your module. While in case of crystalline silicon, you have to first make the solar cell and then you put together solar cell and then make the module. So, that is another very important difference which is advantages to thin film that you can make cell and module together. Because, so large area modules are possible in solar cell, the size of the cell determines the size of the module. And size of the cell is determined by what? It is the technology, CZ process, FZ process remember. So, that will determine the size of your solar cell and your solar cell size cannot or your wafer size cannot be very large and therefore, your cell size cannot be very large. But in thin film technology, you are depositing the layers and your deposition can occur in a very large area. And therefore, the size of the module or the cell can be anything that you want, anything that you want. This flexibility is not there in crystalline silicon. Next property is tunable material properties because you are depositing your thin film material. So, if you are depositing a thin film, for example, when you are depositing a thin film, you can deposit, I will show you what is the meaning of tunable material properties. You can tunable material properties. What does it mean? You can tune the properties. What properties you want to tune? You want to tune the band gap, you want to tune the lifetime, you want to tune the diffusion length, you want to take the best of that. You want to tune the conductivity. These are the properties you want to tune and you want to optimize it for the lower temperature how you can get. Now, in crystalline silicon everything is fixed, your band gap is fixed when you do the doping, your lifetime is fixed. In thin film technology, for example, when you are making, when you are making, let us say gallium arsenide. So, what you can do? You can make gallium arsenide. You can make gallium arsenide such a way that the gallium is 50 percent, arsenic is 50 percent. So, you can make gallium 0.5, arsenic 0.5. So, contribution of a gallium and arsenic atom is equal. You can also make gallium 0.3 and arsenic 0.7. So, there are 70 percent arsenic atom, 30 percent gallium atom. You can also make, you can also actually put aluminium into. So, you can make aluminium, aluminium, gallium, arsenic. So, you are putting another component and you are controlling the ratio of 3. Then you can actually put indium, gallium and instead of arsenic you can put phosphide. By doing all this, so you know with the same kind of material. You are talking about gallium and arsenic, but by doing same kind of material, you can play with the composition of the material. You can actually play with the composition and this is possible in almost all thin film technology. The composition is controllable. The composition is controllable and because the composition is controllable, properties of the thin film technology or thin film material is controllable up to some level and that is one flexibility you have. So, when you want to minimize the losses, transmission losses and thermalization losses, you want to make a multi-junction solar cell. When we want to multi-junction solar cell, what is required? Different band gaps are required. In multi-junction cells, you require different band gaps. Now, crystalline silicon, you cannot change the band gap. Crystalline silicon band gap is fixed. Even if it is a monocrystalline or multi-crystalline, band gap is fixed. Therefore, band gap is fixed. So, therefore, you cannot make a multi-junction crystalline silicon solar cell, but in thin film technology, you can change the band gap. For example, I will show you in this lecture that amorphous silicon by appropriately making amorphous silicon into amorphous silicon germanium. You can make amorphous silicon carbon and in this way, you can actually change the band gap of amorphous silicon and because you can change the band gap, you can make multi-junction solar cell because you can change the band gap. And therefore, fabrication of multi-junction solar cell is possible when you are using thin film technologies, but not in a monocrystalline and multi-crystalline silicon. Is that clear to everybody? Other thing I told you already low temperature processors are possible, 500 degree centigrade or less or even less than 200 degree centigrade sometime and therefore, your energy that you are spending in making solar cell is less and therefore, cost is less and the energy fabric is less and it is also possible to make a transparent module. It is also possible to make a transparent module. So, these are the many advantages of thin film technology. We will see some of some more things. So, the band gap of material typically what you require is 1 to 1.5. Our cadmium tolualride is 1.45 or CIGI, CIGI is about 1, gallium arsenide is about 1.45, 1.5, amorphous silicon is about 1.7. So, they are very good materials in terms of the band gap. The absorption coefficient tends for 4 to 6 per centimeter that is what we require and most of the thin film material have very high absorption coefficient. Just so, 2 point you note from the slide and many of the material thin film materials are very good material because their band gap look at here gallium arsenide, cadmium tolualride, amorphous silicon, indium phosphide, CIGIs or CIS they are all sitting close to the peak where the peak efficiency is possible. All this material have the properties or their properties can be tuned where the peak efficiency is possible and therefore, they are good materials. So, now, I told you that thin film materials are deposited, they are deposited. So, I will quickly tell you the names of the techniques so that you just keep in your ears. You do not have to really understand how does it work, but there are many techniques that are used for deposition and they are very commonly used techniques. All these techniques are available at IIT Bombay in our national center for photovoltaic research and education. We actually are going to offer all our equipment to anybody else who is willing to do the experiment and we will fund the project. There is a scheme which is already going right now that our center of excellence in nano electronic center can be used by people like you if you have wonderful idea and good thing about that is that you do not have to spend any money all the funding is provided by IIT Bombay because we have received the funding in advance and we want to do the same thing. So, therefore, I thought it is good idea just to for me to tell you the names of the deposition techniques and how they work, but I am not going to spend lot of time with this. So, materials are deposited. There are two types of technique, but what is called the physical vapor deposition. So, physically you actually transport the vapor like you operation process is a physical vapor deposition. You heat the material to it is almost close to boiling point the metal gets you operated and it is get transferred from one place to other place. So, if you want to deposit silicon and you operate silicon if you want to deposit silicon oxide you should you operate silicon oxide. So, that is physically you transfer the material from one side to other. In chemical vapor deposition you actually first cause the chemical reaction to occur like as we have discussed yesterday we wanted to deposit silicon nitrate. What are the guesses we used? If you want to deposit silicon nitrate we use silane SiH4 and ammonia NH3 and this two guesses chemically interact with each other in high temperature that result in a formation of silicon nitrate. So, some chemical process is involved in the deposition. So, therefore, two major techniques physical vapor deposition and chemical vapor deposition. Look at the examples of the physical vapor deposition your operation is a very simple example, but there is a process called sputtering molecular beam epitaxy and in keeping chemical vapor deposition there are many techniques. So, CVD and there are many prefix to CVD. So, LP is a low pressure, AP is a atmospheric pressure, PE, CVD is plasma enhanced, HW, CVD is hardware CVD, molecular CVD, LP is liquid phase epitaxy. There are so many ways actually you can modify and there are many many others I have given some techniques. Whatever is required if you want to deposit film energy must be provided to the film, energy must be provided substrate. So, your operation if you want to operate something you must provide energy to your substrate or the material which you want to deposit and this energy can be provided either in the form of heat, radiation, electrical, magnetic and there are many ways. So, as I said I am not spending time and quickly go through this. So, physical vapor deposition one of the techniques is your operation. So, you any material that you want to you operate or deposit you keep you keep it here in holder you provide the heat energy whatever way you can do the electrically or electron beam or whatever and you create a vacuum. So, that your vapors can go straight forward and so you have to create a vacuum inside the chamber and you keep your substrate where you want to deposit. So, material like this I have shown you earlier the silicon I showed you earlier this silicon can be used to deposit a morphosilicon. So, you heat the silicon keep it in the chamber use you heat the silicon. So, that vapors of silicon is formed and this vapors this vapors will actually go back this vapors will actually go back and deposited on the substrate this is this substrate can be glass substrate. So, this is one technique for doing the simple deposition that is the operation that another technique is sputtering in this case you have to create some kind of plasma. For example, in argon is one case molecules you ionize the argon. So, some argon positive ions are created you accelerate the positive ions towards the target. Target is the substrate which you want to deposit. So, if you want to deposit silicon then your target is of silicon or silicon nitride or silicon oxide or any other material. So, when this argon is actually accelerated and bombard the target some of the molecules of the target atom will actually get kind of displaced and they will get transferred to the substrate where you want to deposit. So, basically the positive charge ion will bombard the substrate and they will replace some of the atoms of the molecules from the target and then those atoms will get deposited on the substrate. So, this process is called sputtering again you are physically transporting the material that you want to deposit and therefore, this is a PVD technique physical vapor deposition technique. Then you have chemical vapor deposition technique one of the chemical vapor deposition technique I have already given example like in diffusion process we first deposit the phosphorus oxide we have the POCl 3 we actually make a phosphorus oxide on the wafer and then phosphorus diffuses. So, that is one of the chemical vapor deposition similarly, silicon nitride if you want to deposit you insert silane and ammonia the chemical reaction will take place and it will get deposited. So, these are the chemical vapor deposition processes kind of temperatures that can occur 300 2000 degree centigrade and then you have plasma enhance chemical vapor deposition. So, one of the problem with the atmospheric or LPCVD or low pressure CVD is the temperatures that you require is very high and therefore, people have discovered plasma enhance CVD where plasma actually enhances the chemical reaction. What does the plasma do it enhances the chemical reaction and therefore, it is called plasma enhanced chemical vapor deposition technique and temperatures can be really low 200 to 400 degree centigrade sometime people have also demonstrated less than 100 degree centigrade for deposit. So, plasma is created and plasma enhance the chemical process and you insert your reactant gases like silane ammonia reaction will take place and silicon nitride will get deposited. So, this is one technique. So, these are the some techniques just I wanted you put to put in your ears. So, that you know that physical vapor deposition chemical vapor deposition your operations for trying chemical vapor deposition and plasma enhance chemical vapor deposition. These are the some of the technique that can be used in making thin films. I hope everybody is with me so far. So, let me move further and after half an hour we will ask I will give time to ask questions. So, thin film technology some of the common features let me discuss and this is the most important aspect of thin film which is also different from the crystalline silicon. One major difference between crystalline silicon and thin film technology is the is a context that you make. We have discussed in crystalline silicon yesterday that there are metal contacts current flows like this and then it flows like this and it is possible to have the metal contact which are separated from each other by about couple of millimeter 1 to 3 millimeter. Now, same thing is not possible in thin film technology. The reason is that the conductivity of the silicon emitter for example, here is 0.1 to 100 cements per centimeter. Conductivity is very high. Sheet resistance is low, but if you look at the sheet resistance of the thin film material very high tends for 3 to 10 for 7 ohm per square as compared to 30 to 70 ohm per square for emitter. The sheet resistance of the emitter is 30 to 70 ohms per square while the sheet resistance of the thin film is way too high 10 for 3 10 for 7. Therefore, if we have this kind of arrangement in thin film we are going to have the lot of losses. I will I will explain you why I am saying is if you have crystalline silicon your emitter is here here one contact is here other contact is here this is a crystalline silicon. If you want to have similar arrangement in thin film you are going to not get anything. So, if I make this arrangement current I will have to flow like this current flows like this. Now, because my resistivity here the resistivity or you can say there is sheet resistance of the emitter was 30 to 50 70 ohm per square. We have not discussed, but just take it from me that we give ohm per square. In this case the sheet resistance in the range of 10 for 3 to 10 for 7 ohm per square extremely high resistance. Because the resistance of the thin films are very high if I keep the fingers 1 to 3 meters a millimeter apart then there is going to be lot of resistive losses in this emitter. So, if it is not possible I cannot. So, what is the solution? You bring the fingers closer you bring the fingers closer, but when you are bringing the fingers closer what is the problem? When you are bringing the closer finger the problem is that the shadow losses increases. So, lot of light will actually will actually get blocked because of the matter. So, therefore, this is not the solution this is not the solution you can use. You cannot use the metal contacts because the resistance of this layer is very very high. Why resistance is high? Because the materials are very defected you know they are very amorphous polycrystalline in nature that is why. Therefore, what I need is instead of having arrangement like this if I can collect the current at every point if my contact can be at every point. So, therefore, my current flows always vertically there is no chance for horizontal transfer and because my thicknesses are low my thicknesses are 1 to let us say couple of micron let us say 1 to 5 micrometer because my thickness is low then the resistance of this path can be lower, but if I if I go horizontal my cell can be very large right my cell can be in the order of 100s of centimeters. So, in this path horizontal current flow can be problematic vertical current flow can be fine, but then I need to collect the current at every point. What does it mean? My contact to the thin film solar cell should be continuous. Like here the contact is discrete you have one contact line here then there is a gap of couple of millimeter then you another contact here. In thin film solar cell I cannot afford that because my resistance of the thin film layer is very high and therefore, I need a continuous contact. So, I need a continuous contact, but if I put a continuous contact it is going to block the light. Therefore, what is the property I require of this contact? So, I need a continuous contact that is for sure, but if I use continuous contact it is going to block the light, but light must go inside the solar cell then only I will get any electron hole per generated and therefore, what is the property of this continuous contact? This continuous contact should be should be transparent. This continuous contact should be transparent in nature. So, that is one of the major, major, major difference between the crystalline silicon and thin film technology that thin film technology use context which is transparent in nature. So, this is called this is called the TCO transparent conductive oxide. This has the oxide in nature, but they are transparent and conductive. Normally oxides are not transparent. So, normally oxides are not conductive, but people have developed the transparent conductive oxide which kind of oxide you have. The common term is TCO transparent conductive oxide and there are many oxides like indium tin oxide ITO. So, ITO is commonly used TCO. What is ITO? Tin oxide is a commonly used TCO. Other oxides are zinc oxide. So, I guess it is clear because the resistivity is very high of the thin film. You need to use TCO on the top of thin film if you want to collect the current. This is the same message I am going again. The TCOs are now also doing the other function. In silicon solar cell you use silicon nitride for what? Two functions. One is anti-reflective coating. So, you have silicon nitride for anti-reflective coating. So, same thing should also happen in thin film. The light reflected from your thin film material should be as minimum as possible. And because of that your TCO is having one more function. It is should be using as a anti-reflective coating. And if you are using as anti-reflective coating what rule it should, what condition should satisfy? The condition that TCO, the condition that TCO should satisfy is its thickness of the TCO should be equal to our same lambda by 4 rule and n is the refractive index of TCO. So, the thickness of TCO should be lambda by 4, n is the refractive index. So, very simple the same thing our lambda where we want to minimize the reflection, we want to minimize the reflection where the intensity of the sunlight is highest. Where is the intensity of the sunlight is highest our spectrum for the sun is like this and this highest point occurs at about 550 nanometer. So, my lambda should be 550 nanometer and the refractive index of the material that you are using. So, for example, if you are using ITO indium tin oxide it has a refractive index above 2 or actually tune the refractive index. And therefore, again the thickness that you will get from this calculation will come to about 70, 80, 100 nanometer that is the range of other thicknesses. So, TCO is used, use of TCO is uses first of all contacts. Second is anti-reflective coating and also it is used as a material for light trapping. Remember we want to get each and every possible light that is actually which is going. So, therefore, TCOs are also use a anti-reflective coating. So, look at this schematic arrangement. The TCO is actually made rough, TCO is actually made rough. Look at here close the TCO is made rough. So, that once the light comes in because of the different angle the light will get trapped inside. Though the thickness of the materials that you use for thin film is less and less even people are trying to actually make it thinner and thinner material. When you want to make thinner material and when and you do not want to transmit any light you should have what you should use, you should use concept of light trapping. So, TCO plays a very, very, very important role. The research of the TCO is one of the most important research area in thin film technology. The research on TCO is one of the most important research area in thin film technologies. Some of the properties of the TCO first of all TCOs should be transparent. So, look at the requirement we want more than 90 percent transparency. So, look at the ITO very nice 95 percent transparency, tin oxide, fluorine doped, 90 percent zinc oxide, boron doped, zinc oxide, aluminum doped. These are the various TCOs look at the band gap they are very high band gap materials which means they do not absorb the spectrum of the sun because they are transparent. So, they should be transparent in transmitting the light and they do it nicely because their band gaps are high. Reflective index we need about 2. Sheet resistance should be low basically the resistance of this should be low. So, these are the properties of various TCOs. Now, one of the one of the other important question about again the difference between thin film and the wafer based technology could be what kind of solar cells junction you can use. So, there are 4 different possibilities of junction one is called the homo junction. This is what is used in the solar cell technology crystalline silicon. So, your P type and N type both are same material that is why the junction between the same material is called homo junction. The junction between the 2 different type of material is called hetero junction. I will give some examples. So, for example, if I use P silicon and N silicon crystalline silicon then it is homo junction. So, that is typically the case for the crystalline silicon technology. In cadmium telleride technology I told you earlier also that in cadmium telleride technology I use P type as cadmium telleride where N is my cadmium sulphide. So, this is junction between 2 different materials and therefore, it is called hetero junction similarly in CIGS I have CIGS or CIS copper indium selenide or CIGS I use. So, this is P type in nature and then N type is cadmium sulphide. So, this is another example of hetero junction. Junction between 2 different materials 2 different materials because their band gaps are already different their materials itself is different. So, there are various examples of homo net relation. So, C D T in CIGS is our example of this. Amorphous silicon is a another kind of junction. Amorphous silicon I will come back and explain to you. Amorphous silicon has what is called not P N junction. Amorphous silicon has P I N junction. So, you one type of junction is P N junction other type of junction is P I N junction. I refers for you know I refers for what remember when we look at when we are describing the concentration of electrons and hole we talked about N I and P I. What is the I referring for? I is referring for intrinsic no doping case. So, here I layer is referred to the layer which is having no doping fine. So, junctions can be P I N type and I will discuss in more detail why there is a need for P I N type and their junction can be multi junction cell 1 of 2 and 3 of different band gap. So, top cells have higher band gap, the lower middle cell has lower band gap and the bottom cell has the lowest band gap. So, these are the possible ways you can make the junction. What is the need for making a P I N junction? So, for that you need to understand that we have discussed the diffusion length. We have discussed about the diffusion length, diffusion length. What is the diffusion length? We have L diffusion is equal to square root of D tau, D is the diffusion coefficient T is the tau. This is you can have diffusion length for L electrons and you can have diffusion length for hole. Now, your D your k T by q is equal to D by mu Einstein relationship and if I replace in this above equation you get L diffusion length is equal to I will get mu tau into k T by q. So, this is the product mobility and life time. Now, similar to diffusion length I can also have drift length when carriers are moving under electric field that motion is called drift motion, when carriers are moving under concentration gradient that motion is called diffusion motion. So, similar to diffusion length I can also have drift length. What is the drift length? You can define you know the what is your mobility is defined mobility is how much the drift velocity your carriers can achieve per unit electric field. The carriers can achieve per unit electric field. Now, so I can have drift velocity equal to mu into electric field. Your drift what is drift velocity the distance travelled by drift mechanism within the carrier life time within the carrier life time because within the carrier life time carrier will recombine. So, this can be drift distance travelled can be referred as L drift divided by tau distance over time is a speed. So, you have mu in electric field. So, your L drift or drift length is equal to mu tau and electric field. So, now, mu tau products look at here mu tau products comes in the drift length also and the mu tau products come into the diffusion length also. Mu tau product in a way defines the quality of the material. If your life time is higher your quality of the layer is higher. If your mobility is higher quality if your of your layer is higher. So, the mu tau product defines the quality electrical quality of your material. Now, for the same mu tau which length will be higher for the same. So, for the same product of mu and tau which length will be high drift diffusion length will be higher or drift length will be higher. So, for same value of mu and tau your diffusion your drift length will be greater than your diffusion length. Why? Because the product comes here the product comes in the square of the root. Normally your drift length is 10 times higher than the diffusion length normally normally this is 10 times I am sorry normally 10 times higher. And therefore, when the mu tau product is not high especially when the very defective material like amorphous silicon mu tau product is very bad. And therefore, instead of relying on a diffusion mechanism we should rely on a drift mechanism. So, basically separation of the charge under electric field separation of charge under electric field that is what I shown here. So, therefore, if we create a PIN junction in a solar cell in amorphous silicon cell by the all other film technology like CIGS and CDTE or polycrystalline silicon they are not so defective as amorphous silicon. Amorphous silicon is completely random arrangement of atoms. Polycrystalline as I discussed earlier will have some arrangement of atoms and monocrystalline is the best arrangement of atoms. And because both CDTE and CIGS are polycrystalline in nature their mu tau product is much better than the amorphous silicon. So, amorphous silicon is only technology where charge separation is dependent on the drift mechanism rather than diffusion and every other process you use diffusion. So, what we do? We use PIN substrate. So, we have junctions P, I and N. If I want to draw the band diagram of the PIN what should I do? I is intrinsic layer which means the band gap is the middle the intrinsic energy level is the middle of the two. So, I must draw the band Fermi level flat this is how we draw the band diagram. Then I must draw the P type amorphous silicon which is having like this valence band is close to the Fermi level and conduction means apart and this band gap should be equal to the band gap of amorphous silicon how much 1.7. Here I should draw the conduction band like this band gap should go to band amorphous silicon. Now, I need to connect this two and whenever our bands are not flat what does it mean electric fail? Whenever our bands are not flat means potential energy is not constant and the change in the potential energy with respect to x indicates what it indicates electric field? It indicates electric field. So, here now this is my P side this is my intrinsic layer and this is my N type and here if my photo generation takes place at this point electron in hole if the generation takes place because of the electric field this electron will immediately go this side this will will immediately goes. So, the chances of separation is higher because of the electric field and therefore, for P i N type solar cell it is only the P i N junction that will work P N junction solar cell for amorphous silicon will not work P N junction for amorphous silicon will not work it is only the P i N junction because the electric field in the intrinsic layer helps the separation of a generated electron hole pair. Remember amorphous silicon is very defected material and the lifetime is so low that if this will not happen if the electric field is not there there will be a immediate recombination of the carriers. So, there are two types of arrangement we need not to discuss substrate and super straight sometime the light comes from the side from where you are using to deposit sometime the light comes from the top. So, this is what is called the super straight arrangement which is the substrate where you are depositing is a transparent. But in substrate arrangement your substrate where you are depositing can be metal and therefore, light can come from the top. So, this is the difference between substrate and super straight arrangement. Now, one of the important difference between the thin film and crystalline silicon technology I told you is that cell making process and module making process is same cell making process and module making process is same how it is same I will show you how to make a module of crystalline silicon will show later. We have discussed only how to make a cells of crystalline silicon in this slide I am discussing with you how to make a cell and module together of a crystalline silicon. Before I go I want to tell you the terminologies that we are using. So, in thin film you use substrate this is a substrate where you want to deposit your layer then you have to use what is called TCO transparent conductive oxide or some metal contact your substrate can be glass or it can be silver also because it is the back side. Then you use your main layer your main layer is your what is called absorber layer absorber layer is the layer which absorbs the light and it is the main p n junction absorber layer is your junction, but it is referred absorber layer. So, if you absorber layer can be p i n for amorphous silicon for cadmium telluride absorber layer will be cadmium telluride and cadmium sulfide for C i s or C i g s absorber layer is cadmium C i g s and cadmium sulfide and then you use your TCO on the top. Top layer cannot be metal contact because the light is coming from here light is coming from here. So, it has to be transparent. So, this is the terminologies. So, when we start we start with the substrate we deposit the back metal contact or back contact we deposit the absorber layer and we deposit the front contact three back contact absorber layer front contact. So, these are the three main layers that normally we deposit. So, that is what I will show you now that how the cells and modules are fabricated. So, our starting point is a glass which is already coated with the TCO that is a standard practice in the commercial world to make a solar module thin film solar module where you are using a glass which is already deposited with a TCO you can buy such kind of glass TCO coated glass are commercially available people use also for the research purpose. So, one metal contact or one electrode is already available fine. Now, here this glass can be huge glass it can be 2 meter by 2 meter, but within this glass you have to make your cells within this glass substrate on the top of this glass substrate you have to make your cells as well as module, but you have to define somewhere what is the size of your cell. So, which means you have to make cuts into the TCO. So, you have to make a cut why you have to make a cut we have to make discontinuity otherwise it will be one single cell I do not want one single cell of 1 meter by 1 meter I want module of 1 meter by 1 meter, but within the module there has to be many solar cells together right one solar cell will not give you high voltage. And therefore, normally when you make a module many cells are put together in series to get a high voltage. So, I have to make a disconnection between this metal contact or this contact this transparent contact which is conductive nature I have to make a disconnection. So, that I can create a different solar cells everybody is getting that you what you what you need to do is you make a if this is your glass substrate and this is your TCO you have to make a disconnection between the TCO. So, that you can make different solar cell. So, how do you do this you do it by laser. So, there are lasers available remember look at the animation this is my first slide and this is my next slide. So, by laser I have actually made the cuts and each of this individual strip now each of this individual strip is actually a solar cell. And in practice this individual strip will look like this. So, what you are doing you are taking your substrate and there is a TCO and this TCO is like this. So, this is your 3D view of the substrate fine. Now, these are the this lines are a laser cut this lines are this line this line this line are laser cuts. So, basically you are cutting the TCO. So, that you are making the discontinuity you are making the discontinuity and if this strip will be a different cell this strip will be a different cell and this is strip will be a different cell. So, now in this way you can create a different cell. So, that is a laser cut here next is you deposit your absorber layer what is absorber layer it is a p-n junction or p-i-n junction if it is a morphosilicon you deposit p-i-n junction. Again the problem here is this your absorber layer is continuous. So, you have to again make a disconnections between the this so that you can create again the disconnection the connection it can be made by laser. So, you again make a laser cut here. So, you make one laser cut here make laser cut here. So, again you are making a laser cut. So, in order to separate the absorber layer again in the size of the solar cell fine. So, this is your next step what is the final step you have to put a top contact top contact can be TCO. You put another top contact or it can be a metal contact as I said depending on from where the light is entering if this contact can be TCO or it can be a metal. If light is glass if your substrate is glass light can enter from here also and this can be a metal. But again the problem is that this metal is continuous layer right the problem is that this metal is a continuous layer and again you have to separate this. So, how you are going to separate again you are going to use laser cut. So, you make a laser cut like this make a laser cut like this and this is how your module you can encapsulate this module is continuous. So, how the current is going to flow from the back contact it will go to the back contact it will go to the absorber layer it will come like this it will flow like this it will go here it will flow like this to go here and flow like this. So, in this way by using appropriate laser cuts we actually separated the cells from each other and by doing. So, we are also connected them together in series we have separated the cells and we have connected them in series. So, now in this way you are not only getting your cell fabricated at the end of the process you are also getting your module fabricated at the end of the process. You are also getting your module fabricated at the end of process you can actually do the encapsulation and your cell is ready. Size of the thin film solar cells some people are asking thin film solar cell can be of any possible size it depends on the laser cuts. So, defined by the laser cuts if you are making a laser cut you can make it 1 centimeter the size of the solar cell as we discuss here. This width can be 1 centimeter and this length can be as much as you want this length can be as much as you want. So, this length can be 1 feet 2 feet 1 meter and even 2 meter. So, this width is important. So, typically this width of the solar cell width of one solar cell is 1 centimeter why for the same reason that your your TCO is not conductive. So, current flow in this direction should be minimum the length of the current flow should be minimum otherwise your resistive losses will be very. So, size is no matter 1 centimeter is normally used the area sometime can be 5 meter square there are there was applied material tools which can actually make 5 by 7 meter square module huge module 5 by 7 meter square. One single module is 5 by 7 meter square you can sleep on it the dimensions 2 by 3 meter 2.3 meter by 2.1 meter. So, some people make of long modules also 1 kilometer long some flexible modules can be as long as 1 kilometer they are continuous roll to roll process. So, you are substrate in that is a flexible substrate it. So, you substrate is a metal thin metal foil. So, use your metal foil you deposit your back contact absorber layer front contact and this keeps on going. So, you can make really 1 kilometer long substrate also. One another difference that that advantage of the thin film module that as I told you that thin film module the cells are actually a cells are thin strips, but very long. But in crystalline silicon cells are like this. So, if there is a shadow caused. So, look at in the crystalline silicon what will have if there is a shadow the whole cell is completely blocked and because the whole cell is completely blocked the current flowing in the whole module is completely blocked 0 right. Because in the module all cells are series connected will see the design of the modules tomorrow will see the design of the modules tomorrow cells are completely series connected. But in the module of amorphous silicon the cells are a module of crystalline silicon also I am sorry module of cadmium tellerite also or CIS also the cells are a long strips. So, even if a same shading causes on this module only some portion of the cell is shaded and therefore, other portion of the cell is still active and therefore, though the current will reduce because of the shading, but it will not become 0. So, that is another advantage of the thin film technology. One important thing and I will stop here there are many many slides, but I said I will not go into all the slide one important thing is all many of the thin film modules are high band gate materials and what you see here is the spectrum of different types. So, your air mass 1.5 g is yellow yellow curve then your air mass 1 air mass 0 is green curve what you see here is as the intensity decreases the high energy photons are also decreasing I will zoom it. So, that you can have a closer look at this as the intensity decreasing as I am going from the high value to the low value of the air mass what you see is this is the high energy photon. So, the contribution of high energy photon in your spectrum is decreasing contribution of high energy photon in your spectrum is decreasing means which material will get affected most you have you have the so contribution due to spectrum when light intensity is low when light intensity is low what is happening high energy photon percentage I would say is also low high energy photon percentage is also low right. This is what I am showing here when the intensity is low 207 watt per meter square 600 watt per meter square high energy photon is also low the percentage of high energy photon in your spectrum is also low. So, which this will affect what so if I have crystalline silicon solar cell and if I have cadmium tilleride solar cell little higher band gap this band gap is 1.45 electron volt I am sorry this is 1.12 electron volt and this is 1.45 electron volt. So, when my intensity is lower when it will occur it will occur early in the morning late in the evening when that will occur high energy percentage of high energy photon will decrease which one which module this is going to affect most this module right cadmium tilleride because cadmium tilleride high band gap it mainly survives on the high energy photon as compared to crystalline silicon. Therefore, the performance of this this module will be lower as compared to this module. So, this is one way right, but there are many other factors which affects the performance of the module and we should discuss that in detail there are many other factor for example, temperature because of the temperature your crystalline silicon will perform less but your cadmium tilleride will perform better. So, there are many parameters which affects the module performance how much power you get out of the module at a given condition and this is a very kind of high value question which module should I use in the field. If I am putting my power plant in Karnataka if I am putting my power plant in Madhya Pradesh if I am putting my power plant in Jaipur which type of module should I use and that is a very very big question and there is no easy simple answer to that ok. With this there are many many other things to discuss one last slide probably that thin film solar cell have the either it can be silicon based or non-silicon based. Silicon based you have merphous silicon micro crystalline silicon in non-silicon based you have CDT, CIGS and organic dye sensitize solar cell. Solar thin film solar module can be rigid and flexible cadmium tilleride can only be rigid because of high temperature process. So, thin film metals are not used metals are not used as a substrate, but amorphous silicon modules can be done at low temperature. So, you can use flexible metals CIGS can be done at low temperature. So, again you can module can be flexible or rigid. I will stop here as I said there are many many other slides are there I just uploaded the slides. So, that if anybody is interested you can go more into details about the thin film technology, but whatever I have discussed let me summarize and I think that is a good summary of how first of all we have looked at the various aspects various aspects in which thin film technologies are different than the crystalline silicon technology. What are these aspects? The aspects are includes kind of the thin film materials are deposited silicon wafer is grown. Then we have looked at the different band gap that is tuned or the material properties can be tuned because you can make compound semiconductor. We also looked at that crystalline silicon can only be rigid, but thin film can be flexible. We also looked at the various properties in which metal contact cannot be has to be transparent in thin film otherwise it will not work. So, therefore, we looked at the TCO transparent conductive oxide and what are the various oxides that are being used. We also looked at the crystalline silicon the cell making technology and module making technology are different, but in thin film technology both cell making and module making is same. We eventually looked at when shadow occurs thin film technology may be performing better than the crystalline silicon because the cells are very long strip continuous. Thin film modules can be very large size meter square and so on crystalline silicon modules cannot be that large size and the thin film modules can be flexible and it can be very long even 1 kilometer long is possible. So, these are the various differences between thin film and thick film or crystalline silicon and I hope it answers many of your questions that people have in mind. Let me take few questions before we go to the next lecture. Let us start. K. G. Sumaya. Good afternoon sir. Is glass commonly used substrate? Yes, glass is commonly used substrate in thin film technology. Or there are some other materials also used for substrate in thin film technology. So, in thin film technology 2 substrates are used. I will show you. In thin film technology substrate, substrate for thin film 2 substrates are used. One is glass and other is metal foils, thin metal sheet or sometime plastic. Most common is glass. So, most commonly is glass, but sometime you can when you want to make a flexible module, you can use metal foil as well as plastic also. But glass is most commonly used substrate in thin film technology. Baramati. Hello, sir. How oxide with 3.4 in electron hold gap conducts? How oxide with the 3.4 electron hold conducts? Question that I am not in position to answer, but there are special kind of defects level you can say it is a defects level which makes conduction in silicon in oxides possible. But you have to go to the physics of the oxide and honestly I do not understand how exactly the oxides are made conductive. G. S. I. T. S. Indore. Which technology is more durable? Thin film or vapor based? Which technology is more durable? Thin film or crystalline silicon, all the manufacturers that are making crystalline silicon as well as thin film technology, all of them guarantee 25 years lifetime. Though people have we have seen crystalline silicon modules installed in the field for 25 years, nobody has seen a thin film modules installed in the field for 25 years. But in principle all the manufacturers guarantee you 25 years lifetime. Ok, Slampur. Yeah, it is mentioned that physical vapor deposition techniques like we have to vaporize the silicon material. So, it means we need we need to go beyond 2000 degree Celsius. So, it actually will be a disadvantage in comparison with vapor based technology. Ok, yeah it is true that when you want to operate we have to really go close to the melting point. The melting point of silicon is about 1480 degree centigrade, not 2000 you do not have to go to 2000, but yes. But the thing is that your volume of silicon that you want to melt is very small and therefore the energy required will not be as much as the energy required to melt the melt the whole bath for the silicon wafer are in good making. Sirpur. Good morning, sir. As compared to vapor deposition of silicon vapor technology, the thin film technology has the aging effect. So, the life of the thin film solar cell is about 2 to 3 years, but the life of the vapor cell is about 25 to 30 years. So, the then how we can increase the aging effect of the thin film solar cell? You got it absolutely wrong. Life of the crystalline, life of the crystalline silicon solar cell is 25 years, but even the life of thin film cells are also very long. As I said many manufacture almost all manufacturers guarantee that your thin film modules are also going to live for 25 years. So, there is no question of life, low life for thin film technologies. However, there is a lower efficiency of thin film modules and degradation probably can be faster, can be faster, but manufacturers guarantee that it will not degrade faster and they guarantee that the performance degradation of the thin film module will be as good as the degradation in crystalline silicon modules. That is what the manufacturers today guarantee. So, as far as the performance is concerned and degradation is considered, they guarantee the same thing. What manufacturers guarantee in both cases is that first 15 years, 10 percent degradation and the next 10 years another 10 percent degradation. So, total in 25 years, 20 percent degradation both crystalline silicon as well as thin film modules. There are many questions MNIT Bhopal. Hello sir, good afternoon. I just want to ask that in thin film module how the solar cells are connected in series? Although you have explained in 31 number slide, but I did not get that. 31 number slide you want me to go again. Let me try. I am going to try on white board. If my drawing is good, you will get it otherwise you have to read. So, let me try how that is connected. So, for example, what we do is you start with the substrate. Now, you deposit your TCO. Fine, this is a TCO. My drawing is good so far. Now, you have to deposit your absorber layer. So, this is your TCO I am showing is blank, but my absorber layer is deposited like this. Then you have like this and continues. So, this is your absorber layer or main active layer and then you have to deposit your metal contact. So, top metal contact or top TCO, then you will have it like this. Then you will have another. I hope it is clear now. This is your top contact which can be another TCO. This is bottom contact. So, now current will have to flow from one contact to other. So, it has to go like this and it has to come like this. I think I am not getting it right, but if you follow the process, what should happen? What actually you are doing is you have your TCO substrate sorry you have your glass substrate. So, what you are doing is actually the cell is connected like bottom of the cell is connected to the top here, bottom of the cell is connected to top here. So, if you do the right drawing, I think I did not make it right here, but if you do the right drawing, this is what you are going to get. So, your current will flow like this. It will come like this. It will go like this. It will come like this and it will go to the next cell. So, this is how the cells are connected in series. Fine. That is it. I will take maybe one question from the chat. In sputtering deposition technique, can we use DC supply instead of AC supply across the substrate? What is the effect? In the sputtering technique, you can use DC supply, but when you use DC supply, your substrate has to be conducting right, because the whole current has to flow. So, substrate has to be conducting if you are using a DC supply. PC 1D software, can we please explain the, PC 1D software, can you please explain the parameters that we give in steady state and transient register of the base circuit? I hope this is not the time to discuss that. We will see. You will find out the time. Out of the physical and chemical deposition, lifetime is more. I guess the question is out of the physical vapor and chemical vapor deposition, in which case the lifetime is more. It all depends on the composition of the material and we cannot say directly that physical vapor deposition is better or chemical vapor deposition is better. So, it depends on the quality of material, both structural quality as well as electrical quality. So, how much is the recombination center? What is the defect density? That is all will depend. In the thickness is very small of the thin film. How it is protected? First of all, you have to do the encapsulation and your fabrication technique has to be very perfect. Thickness is small. So, that has to be continuous layer. It has to be uniform thickness and these are the challenges of thin film technology. Fine. I think enough of thin film and you got the fair amount of idea about the thin film technology.