 So, let us start again ok. So, introduction to solar PV semiconductors for PV cells we want to use a semiconductor material and why semiconductor I will come back to that. This we already discussed PVs electricity can be generated in decent flies way it is modular. So, you can add the capacity whenever you want. So, today if you have 1 kilowatt requirement you install 1 kilowatt tomorrow if you have 2 kilowatt you can add one more kilowatt to make it 2 kilowatt there is no other power plant which actually provide you that kind of facility. It can generate a very small power in milli watt, micro watt, 2 megawatt that I told you yesterday and it is feasible to fulfill all our energy requirement using solar and you can actually have the big power plant and stand alone system also I just want to give a glimpse of what is happening in the world in terms of the photovoltaic production and if you look at this almost exponential graph and which is very pleasing to see that in 2010 the worldwide production was more than 20 gigawatt ok, 20 gigawatt, 20,000 megawatt the whole world production that is the red line and you can see what is happening in other part of the world as well. So, which is a very good sign the worldwide PV production is increasing tremendously there is a very good government policies exist and as the secretary was telling yesterday that there is a need for more and more manpower in this area. So, generations of photovoltaic just a brief overview basically people talk about 3 generations of solar photovoltaic generation 1 is basically a crystalline silicon technology mono crystalline and multi crystalline I will come back to the what it is 2nd generation is referred as thin film technologies this 1st generation has a based on silicon crystalline silicon. So, it is abundant non-toxic good efficiencies, but also have relatively high cost based on silicon. The 2nd generation of thin film technology are basically amorphous silicon cadmium telluride CIGS that is copper indium gallium selenide and this are actually low material cost, but also have the lower efficiency as compared to crystalline silicon we will discuss more about that in detail. So, what people are looking is 3rd generations of solar cell which is extremely low production cost extremely high efficiency. So, that your module cost eventually the module cost is more important. So, that your module cost becomes low. So, that is the benefit of the generation 1 that is the good quality material high efficiency and the benefit of generation 2 that is the low cost generation 3 should have the both of them. But let me tell you that in the in the real picture when you see the real commercial picture there is not much difference in terms of the cost that you can get yes thin film technologies can be relatively cheaper and they are relatively cheaper, but the cost margin that we expect is not that much that one can always decide to go for a thin film. So, is still a big chunk of whatever is produced in the world and whatever is consumed in the world is coming from crystalline silicon thin film. So, crystalline silicon is about 80 percent of the market thin film is about 20 percent. Now, the final cost that we pay as a customer which is in terms of rupees per watt peak the solar PV module power is given as a watt peak peak power. Why it is given as a watt peak? So, let me explain you that briefly why it is given as a. So, PV rating is given into watt that is the power, but peak power. Why this p is added p is actually referring to the peak and why p is added to just emphasize that whenever manufacturer gives you module it is giving you for air mass 1.5 spectrum and air mass 1.5 spectrum is corresponding to 1000 watt per meter square 1000 watt per meter square. Now, we know that 1000 watt per meter square solar intensity is not there always in the morning you will have very low intensity 100 watt per meter square then towards after it will increase from 200, 500 it may go to 800, 900 or 1000 if it is the best sunny day and then it will come down right. So, therefore, manufacturer gives the rating at this power and so this is the peak power that the power rating is actually maximum power that you can get from your module and that is the peak power and that is why we should always refer to not watt, but it is watt peak remember that. So, now the money that we pay as a rupees per watt peak or dollar per watt peak depends on two main parameters and very important to understand. It depends on the price of the module that is rupees per watt peak rupees per watt peak can be written as a rupees per meter square and watt peak per meter square right rupees per meter square is basically how much money you spend to manufacture 1 meter square of module. So, it refers to the production cost rupees per meter square is actually refers to the production cost how much money that you spend in manufacturing 1 meter square of module and watt peak per meter square is basically how much watt peak or how much wattage you can get from your module per meter square. Remember, this is under the air mass 1.5 ok. By the way, this air mass 1.5 is part of what is called the SPC standard test condition. The two standard test condition one parameter is referred to the prediction that is 1000 watt per meter square other parameter is a temperature ok. So, other parameter is a temperature and that comes to 25 degree centigrade. So, standard test condition is referring to the 1000 watt per meter square of solar radiation, air mass 1.5 spectrum and 25 degree centigrade. This is referred as STC. So, your watt peak rating is given under standard test condition ok. This is given under standard test condition. So, this is keep that in mind again all the power is rated at a standard test condition of 1000 watt per meter square and 25 degree centigrade ok. So, coming back to this. So, your price of your module in rupees per watt peak depends on the production cost and the efficiency. What do you want? We want low production cost and we want high efficiency. So, that is what is all the scientists keep on doing all over the world that you try to minimize the production cost, try to increase the efficiency I want to mind you again that only increase in efficiency is not the target right. When it comes to the actual commercial module only increase in the efficiency not is not the target which is clearly described here ok. So, suppose if you have a technology with a 10 percent efficiency and by doubling the production cost you are doubling the efficiency in making 20 percent cell. Will it result in the decrease in the price of the module? Let me repeat if you are having a 10 percent technology today by some advancement you are actually increasing the efficiency to 20 percent that is double, but you are also increasing your production cost to double. Will it reduce the price of your module? Answer is no right because both the numerator and denominator is doubling and therefore, price of module is not decreasing and one most important parameter for the user is how much money you pay per watt peak of module. So, therefore, very important to note that it is the production cost as well as efficiency both are important when it comes to the PV module. So, let us look at what are the various technology and then we will come back to the materials ok. The various commercially available technologies are. So, there is two distinction one is called the crystalline silicon wafer based technology wafer is basically a thin sheet of silicon or 10 disc of silicon or you can say thin plate of silicon which is used for making a solar cells in module. When you say thin film actually use a glass substrate or within the glass substrate you deposit your material and that deposited material it become your solar cell ok. So, main difference is in the one case the solar cell itself is a the material or a plate in which the wafer itself is a material plate in which you make solar cell in other case you deposit the material and you make your solar cell. So, the crystalline silicon cells are actually thick cells 180 micron thick current standard and therefore, the cells are made in the wafer or we can say crystalline silicon wafer. So, there are three different crystalline silicon wafer technology one is mono crystalline multi crystalline ribbon silicon and I will come back to that what is mono what is multi crystalline ribbon and this are the various companies ok. So, one category of technologies are crystalline silicon wafer based technology then the other technologies are thin film technology and remember what I told thin film technology thin film technology is the one where you take some substrate which is not actually doing a job of solar cell it is actually only there to help the thin film that you are going to deposit. You take your substrate you deposit your thin film and the deposited thin film is your solar cell the substrate is not substrate is only helping it. So, those kind of technology are referred as a thin film technology and as the name suggests thin basically means that your material which is acting as a solar cell is very thin less than a micron for crystalline silicon the wafer is 180 micron 200 micron thick, but crystalline for a thin film the material is less than a micron or sometime 2 or 3 micron, but much lower than the crystalline silicon. So, what are the thin film technology? They are silicon based I hope you can see this silicon based you have amorphous silicon and other thin film which is nano crystalline silicon that is silicon based and then non silicon based. So, material like cadmium telluride CDT or CIGS copper indium gallium selenide or CIS copper indium selenide or disensitized solar cell. So, these are the non silicon based solar cell. Now, the thin film technologies whether it is amorphous silicon or or CIGS can be flexible it can be deposited at a low temperature and therefore, at low temperature you can use plastics to deposit that or you can use metal foil like very thin sheet of stainless steel can or actually use. So, the modules can be flexible or it can be rigid also, but thin film does not always mean flexible module thin film modules can be flexible cadmium telluride modules are not flexible at all they are always rigid. So, these are so depending on this it can be flexible or rigid and the various thin film technologies are amorphous silicon cadmium telluride and CIGS that are commercially available some disensitized solar cells are also commercially available right. So, now let us look at the challenges that PV technology have for example, the high cost per unit watt is a is a is a big challenge that scientists are trying to reduce it, but let me tell you that when solar cell production started in 70s or when solar cell were used for the space application in 1960s the cost of solar cell were 1000 dollars per watt and what is the cost of solar modules today? Cost of modules you can get as low as dollar 1 dollar or 1.5 dollar per watt. So, significant reduction has already occurred it is very exciting time and more reduction is expected in future. Efficiencies are moderate I will I will give the numbers efficiency of crystalline silicon is about 15 percent module for thin film it is about 8, 9, 10 percent, 11 percent. So, we can further improve the efficiency material because when we are talking about production of solar modules for a very large quantity gigawatt, terawatt ok 1 terawatt is 12. So, when we are talking about terawatt production we require huge amount of material and therefore, we should use the material which is available in large quantity it should also be stable in the long term. So, if you are making a module it is expected that module should work for 25 years that is a guaranteed lifetime and therefore, we should use the material which is having long stability. The energy payback period should be low. So, whatever energy has gone into making a solar cell should itself be low and for crystalline silicon the energy payback period is 2 to 3 years and for thin film technology the energy payback period is about 1 year or less than 1 year and whatever money you should spend should be recovered back. So, these are the various challenges that any solar cell technology must overcome whether it is a crystalline silicon whether it is a thin film whether it is a second generation or third generation whatever generation you are talking these are the challenges that must be overcome in order to be a successful solar PV technology ok. So, now the question which material can be used for solar cells can I use metals for solar cells can I use semiconductors for solar cells can I use insulators for solar cells ok. So, these are the three questions we must answer and before answering that we should understand what is the role of a solar cell. What solar cell does? I am sure everybody knows that what solar cell does is you put a light on solar cell it generates electricity ok. Then the next question what is electricity? The electricity flows whenever you have the potential difference ok whenever there is a potential difference you connect a wire to it electricity will flow. Then the next question is what is potential difference? The potential difference occurs or the voltage is generated whenever a positive charge is separated by a negative charge right. So, there is a line of thought we want electricity and therefore, we want voltage we want voltage to be generated in a solar cell. So, therefore, we want positive charge to be separated by a negative charge. So, that is the job solar cell must do it must separate a positive charge from negative charge. So, that they can measure a voltage across the terminal p side and n side or positive side negative side many of you must have done it yesterday measure the voltage when light falls on a solar cell. So, that is the job a solar cell must perform generation of voltage right. Now, can a metal do this job can a metal do this job the answer is no. I will get come back to this, but in a brief basically a metal semiconductor or insulator can be described in terms of the energy level that they represent within the material energy level that represent and we will come back to that again this point. But if you look at the metal and metal. So, basically conduction band and valence band this energy level are all mixed right and we want to we want to create that positive and negative charge, but here all this energy levels are continuous and therefore, the separation is not possible. And therefore, metals cannot be used metals are very good in converting the light energy into heat energy. So, that metals can be used for a solar water heater it can be used for the solar concentrator thermal powered, but it cannot be used for the solar photovoltaic power But look at this diagram this is a diagram of this is a diagram for semiconductor semiconductor have a conduction band and valence band and there is a in between there is no gap there is a gap of energy which means there is no energy there is no house which a carrier can occupy there are no levels there and I will come back to that, but let me quickly. So, when you do that when you have this kind of material then basically you can actually separate a positive charge and a negative charge across this energy gap or band gap and when you can do this you can actually generate voltage. Then now somebody can tell me that insulators also have similar band gap. So, why not to use insulators any answer to that any answer to that why not to use insulator. So, the so my semiconductor will have this kind of band gap. So, this is my valence band this is my conduction band and I will come back to that again and this is a gap this is the energy gap. My insulator will have a normally many times this diagram can be shown only by this line that is the age of the conduction band and this line which is the age of the valence band. So, in a simplified I can show only this diagram that this is the conduction band age this is the valence band age and this is the energy gap. Now, what are the energies of the photon that I receive in my spectrum in our solar spectrum in our galaxy whatever sun is there with us what are the photon energy that we have done the calculation yesterday right. We receive ultraviolet photon we receive violet photon we receive green photon yellow photon red photon and infrared photons right. You have done the calculation of energy what is the range of energy we receive normally the photon energy range in solar spectrum what is your answer we receive about 3.5 electron volt at the higher side and lower side we receive about 3 electron ok. Remember these numbers are very important again photon energy at the higher side is about 3.5 electron volt in the spectrum at the lower side infrared side it is about 0.3 electron. Now, so this is a semiconductor SC now insulator will have very high band gap ok. So, semiconductor band gap you know can vary from let us say 0.3 electron volt to let us say 1 it can go actually 2.5 electron volt something like this insulators have this is the conduction band for insulator valence band for insulator and this band gap for insulator this is insulator. This band gap for insulator is very large it can be more than 5 electron volt. So, because my photons do not have enough energy to excite electron in this gap because the gap is so high it cannot give the energy and jump the electron from lower energy level to higher energy level. By the way this all this band diagrams the this is the this is the energy excess remember this excess is the energy excess and what is this excess? This excess is a distance or a space ok. So, because in insulator my band gap is so high that no photon in my solar spectrum can excite the electron, but on the other end semiconductor have the band gap about 0.3 to 2.5 electron ok. For example, silicon the most commonly used material for solar cell have the band gap of 1.12 electron volt ok. The silicon band gap the band gap of silicon is 1.12 electron volt. Look at my photons there are enough photons in the spectrum which will have enough energy to excite which will have enough energy to excite electron in silicon and therefore, insulator cannot be used metal cannot be used what I can use is only semiconductors is that clear to everybody ok. So, what are the semiconductors we can use many. So, if you look at the periodic table column 4 group 4 5 6 2 3 silicon is sitting here which is the most commonly material used in solar cell, but we can use germanium we can scar burn boron aluminium gallium indium zinc cadmium phosphorus arsenic antimony sulphur selenium tellurium all these materials either in one way or other way are being used for solar cell application all these materials ok. So, silicon is used in elemental form. So, we can use elemental form like silicon and germanium we can actually make the compound. So, we can make two materials together like gallium and arsenic together indium and phosphorus together make indium phosphate or we can put three of them together aluminium gallium arsenide mercury cadmium telluride or we can put four of them together five of them together whatever it is ok. So, the question is why do we put many semiconductor material together why do we put many semiconductor material together to make a solar cell and the answer is that we want best properties for our solar cells and what are those base properties we will come back to that ok, but basically the idea here is that yes you can combine more than one you can combine more than one material and make your solar cells. So, look at what is commercially available today silicon solar cells are commercially available cadmium telluride solar cells are commercially available gallium arsenide solar cells are commercially available, but a very expensive use for the space application and you can also actually put other materials and make a solar cell ok. So, these are about the semiconductor that can be used now what are the properties that we are interested in ok. So, we are interested in the structure of the material. So, how the atoms in the materials are arranged we are interested in the band gap of the material as I said band gap is very important if it is too high not many photons will be able to excite electrons if it is too low then there is a problem in the voltage I will come to that and we want a semiconductor which has a good absorption right. For example, glass glass is a transparent to the solar spectrum right we get the light through the glass because glass does not absorb anything, but we want our solar cell to absorb the light and convert it into the electricity. So, absorption is also very important. So, these are the important parameters we look for, but from the commercial perspective we must also look for ease of fabrication. So, you may have a very wonderful technology, but if it is difficult to fabricate then it is of no use. Temperature of fabrication is also important. So, normally we want low temperature. So, that your budget or your electricity spent in making the solar cells low and we want high throughput we should be able to make 2000 solar cells to 1 hour 3000 solar cell 1 hour. So, these are the various properties we try to look into. So, let us look at the structural properties of the material. The structure of the material are basically we are talking about how the atoms are arranged in a given material. Atoms can be arranged in a very systematic manner like this what I shown here. Look at in any direction all the atoms are arranged by a rule. There is some order and this order is extended in all directions. So, this order if your material is 1 centimeter long this order is extended 1 centimeter long. If your material is 10 centimeter long your order is extended 10 centimeter or if your material is infinitely long the order is extended infinite. And therefore, this kind of material is called mono crystalline. This kind of material is called mono crystalline fine. Mono crystalline materials are of very high quality they are very high quality because very very well order is maintained, but it requires lot of energy. We will come back to that. Look at the multi crystalline. So, multi crystalline are similar to mono there is a order, but this order is available in some page of the material and different order is available in a different page different order is available in different page. And the in between this two order there is a disorder in between the two orders there is a disorder at the boundary there is a disorder at the boundary. And these orders are not good these are the defects in semiconductors and they are not good for the solar cell operation. Nevertheless the cost of this kind of material is lower and multi crystalline silicon this grains can be very large this grain can be 1 millimeter or it can be 1 centimeter also. So, this is referred as a multi crystalline many different crystal therefore, multi crystalline. This is also what is called the micro crystalline silicon micro crystalline silicon you have small order. So, this is a there is only small area where there is some order and this small area may be you know 100 nanometer 500 nanometer only that range there is atomic arrangement in order format in a well order, but other there is no atomic arrangement there is all random. So, then this is referred as a micro crystalline or some people call it also nano crystalline because the range of the order is in range of hundreds of nanometers. Then there is this material which is amorphous absolutely no order all items are randomly arranged absolutely it is like a crowd when you go to the when you go to the mela the crowd or the people will be randomly distributed there is no order then it is referred as a amorphous material. And because there is no order there is a lot of defects and because there is a lot of defects it is not good for solar cell I will come back to why it is not good it is not good for solar cell, but still people have found a way to minimize the defect density. So, in this case the defect density is in the range of you know tens for 18 tens for 19 defects per centimeter square people have per centimeter cube sorry people have reduced this by hydrogen passivation. So, all the amorphous silicon commercial technology that you see today in the market has actually amorphous in nature. So, all the items are randomly arranged, but it is the defect density is reduced by putting hydrogen inside it. So, it is called as the passivated amorphous. These are the 5 categories of the structures material structure mono crystalline is best multicrystalline is little lower grade micro crystalline even lower grade passivated amorphous silicon is even lower grade and amorphous silicon is really worth the material. But as you go from amorphous to mono crystalline your cost of making the material increases. Also as you go from amorphous silicon towards the mono crystalline your efficiency is also increasing. So, then as I told you earlier that the cost of the module both things are important production cost is important efficiency is also important it is not the only efficiency that is the matter and that is why you see in the market that is the both amorphous silicon technology is also there and mono crystalline silicon technology is also there. So, this is a fine balance of the cost and the efficiency. So, material structure whether it is amorphous or mono crystalline or micro crystalline affects the optical properties. Optical properties are band gap and how much it absorbs it also affects the electrical properties which is the conductivity and the mobility. So, these are the parameter we should discuss now how it affects optical properties conduct and what are the optical properties what is the value of the band gap absorption how do we define absorption how do you define mobility etcetera. Let us look at the band gap of the material and one question if you can answer what is the ideal band gap that should that can be used for the solar cell uplift. Is there any ideal band gap or any band gap is ok we know that insulators have very high band gap not good metals have very low band gap or 0 band gap they are not good for solar cell. So, definitely somewhere in between there is a number which is the most ideally suited for a solar cell and we should try to answer that question what is the ideal band gap of the material that can be used for solar cell. So, I will I will give a little bit kind of a basics why the band gaps are created. So, I think most of you know probably that when you look at the single atom within the single atom the electrons are orbiting there is a nucleus in the atom and then the electrons are orbiting around the nucleus and the energy of this electron can be different and depending on that the orbit of the electron will also be different. So, for example, if you have the nucleus if you have the nucleus here the electron can orbit at one level or it can orbit at other level or it can orbit other level and also there is a this quantum theory we says that in the first orbit you know there are quantum number there can be only two electrons and the next there can be eight electrons and there can be eighteen electrons and so on and this continues within this orbit there are various energy levels. So, basic idea is that in one orbit there is one fixed energy level in between energy is not possible in between energy is not possible. So, this is what is the this is what happens and this is example of a hydrogen atom for example, that the energy levels are either here or here or here or here or here or here. This are given as a negative energy because this is measured with respect to the nucleus, but the basic concept that here is that yes within an atom if you look at there are various energy levels that electron can occupy it cannot take any energy level and therefore, it is referred as a quantization of energy. When I talk about the speed of a motorcycle it can be 10 kilometer per hour 10.1 kilometer per hour it can be 10.11 kilometer per hour or it can be 12.1 kilometer per hour, but when I talk about energy of a electron it can be either one. So, either it can be this level or it can be that level it cannot be in between. So, that is referred as a quantization. So, I have try to produce it in a simple way. So, if you look at one atom there is one energy level there is another energy level there is another energy level. Then what happens if you put two atoms together when two atoms together. So, one atom has its own energy level the other atom will have its own energy level and this energy level as per the there is a principle called the exclusion principle that both the energy level at cannot be at the same level. So, there has to be some speed in energy level that occurs as per the Pauli's exclusion principle. So, when two atoms comes this happens when many atoms comes this happens. So, energy level cannot be exactly the same because no two electron as per the Pauli's exclusion principle no not more than two electron can occupy the same energy level and that law must always fulfill and therefore, the energy level split little bit. So, what you see if you go from single atom to many atoms you create many an energy level at one level that many energy level at one level and many energy level at one level. So, you are getting a hint what is happening this you are actually resulting in a formation of not energy level, but energy bands resulting in a formation of energy band. So, when you put trillions of atoms together billions and trillions of atoms together what happens is you find an energy band. So, this is for example, when you isolated energy levels or isolated atoms you have isolated energy level when you start pulling many atoms together there is a split in an energy level and you form energy bands. So, there are many energy levels at low energy or the energy level which are close to the nucleus do not take part in all this interaction with the energy level with the outer side takes part. So, typically when we represent the band diagram we are actually only showing you this. So, what you can see there is a one band here there is a energy gap there and there is another band here and this is when you show the energy band diagram this is the picture we are showing this band energy gap and this band and that is how we show here. So, we show this band energy gap and this band. If you look at the crystalline silicon or the silicon atom silicon atom has silicon atom has 14 electrons in silicon there are 14 electrons. Now, this electron are distributed. So, this orbit can have 2, 8 or 18 in order to have stable it requires minimum 8 it requires minimum. So, if I want to distribute electrons of silicon 2 electrons will be in this orbit 8 electrons will be in this orbit. So, 2 plus 8 10 I have left with the 4 electrons. So, the 4 electrons can actually go in here. So, there are 8 let us say there are in there is a sub shell there are 8 locations are available and we have only 4 electrons left. So, out of the 8 4 electrons will occupy the 4 locations and other 4 locations will be available for electron to occupy or we can say empty space those are the locations which are available. So, 4 locations are available. So, when the formation of bands occur that is what says the this band will have all the states occupy this band will have all the states empty and that is why it refers. So, if you look at the simplified energy band diagram this refers the lower band refers to the valence band and it is completely filled all the energy levels available in this bands are completely filled this is referred as a conduction band and it is completely empty. Empty means there are all energy levels are available electron can actually go and live there. So, these are the houses which are available, but this houses do not have any electron all the houses which are available here are filled by the electron. So, this is the typical scenario of silicon at 0 Kelvin this is scenario of and as I showed simplified way we can show it like this the energy band diagram can be shown like this this is the edge of the conduction band this is the edge of the valence band and the energy gap in between. What is the axis keep always note this point never forget this axis is the energy axis and this axis is a distance this is very important to understand the solar cell operation is that clear. So, there is a band gap all the states above the conduction band are empty electron actually can go and occupy the space or occupy the energy level all energy levels below this a valence band age are filled. One more property of semiconductor that this is the diagram of a semiconductor in the space of energy and the distance we can also draw the diagram in the space of energy and the momentum. So, there are two types of semiconductors that are available one is called the direct semiconductor other is called indirect or basically direct band gap semiconductor and indirect band gap semiconductor. What is the meaning of direct band gap? So, when we want to excite electron this is the highest energy in this valence band by the way this is the valence band energy level this are the conduction band energy level. So, when we want to excite electron it will actually go from this minimum energy level is this right minimum energy. So, it can gallery go here from the maximum to minimum because this is the minimum energy gap and this axis is momentum. So, what it means that in a direct band gap semiconductor no change in the momentum is required only a change in the energy is required. If you want to put a photon on a semiconductor the photon will the electron it will give its energy to electron electron will go from lower energy level to higher energy level and this can happen without change in momentum. So, this is a direct band gap semiconductor there is other kind of semiconductor which is called indirect band gap semiconductor look at here what is here the semiconductor the conduction band is actually shifted with respect to the valence band in momentum and because of that if you want to excite electron it will have to go from here to here. So, change its energy and change its momentum. So, two steps are required if you want to excite electron in this kind of semiconductor arrangement the electron will have to increase its energy and it will have to increase its momentum. So, two steps are required change in the energy change in the photon and whenever more than one people are involved in a job what happens it becomes slower or the possibility or the probability of that happening becomes smaller and therefore, absorption of the material absorption of the light or absorption of the photon in indirect band gap semiconductor become weak as compared to the absorption in direct semiconductor ok. So, high absorption probability in a direct semiconductor low absorption probability in a indirect sense semiconductor because of the high absorption probability the direct band gap semiconductor should be thinner only ok only thin amount of material is good enough and therefore, all thin films cadmium telluride, CIGS, gallium arsenide, amorphous silicon all the materials are actually direct band gap semiconductors are behaves like a direct band gap semiconductor and have very thin material requirements ok. So, cadmium telluride for example, can be only 2 to 3 micron thick amorphous silicon can be only couple of hundreds of nanometers unfortunately silicon is a indirect band gap semiconductor and because of that the absorption probabilities are low and therefore, you need to have thicker material to absorb the more light and because of the thicker material you need to use thick material which is about 180 to 200 micron. So, that is another property of semiconductor that is of importance. So, direct band gap semiconductor and indirect band gap semiconductor I told you and the examples let me write for you here. So, the direct band gap which means thin material right direct band gap is amorphous silicon, you have cadmium telluride, you have CIGS and you have gallium arsenide and these are all commercially available technology and they are all thin film technology ok and there is a indirect band gap semiconductor ok. Indirect band gap is basically a thick material you have to use an example is silicon ok. So, silicon is indirect and therefore, it has to be thicker. These are the various band gaps of the solar materials used for solar cell applications. So, look at crystalline silicon 1.12 electron volt indirect CIGS 1.1 electron volt, but it can vary depending on the composition of the material because of therefore, elements sitting there copper, indium, gallium, selenium. So, because the composition will change the band gap you can have the variation in the band gap. Cadmium telluride typically 1.42 electron volt, but band gap can be sorry different will normally will be about 1.4 not this range. Gallium arsenide you can change the band gap in a huge range it can be by mixing the compound 3, compound 4, compound can be as low as 0.5 electron volt, it can be as high as 2.5 electron volt. Amorphous silicon again you can change the band gap typically it is about 1.7, 1.8 electron volt, but you can change the band gap depending on the composition. Remember these numbers because they are useful to understand the operation of solar cells. What are the typical band gaps 1.12 silicon CIGS 1.1, CDT 1.42, gallium arsenide 1.45, amorphous silicon 1.7. This is what we have seen that if you know the energy of the photon, if you know the wavelength of the photon we can find out the energy or if you know the energy of the photon we can find out the wavelength. Why I am asking that that one question that you should do in your tutorial I have given this as a problem. Find out what is the minimum wavelength or the energy of the photon that will get absorbed. For example, gallium arsenide is having a band gap of 1.45 electron volt. What does it mean that what does it mean that if the band gap of the material is this EG this is the conduction band H, this is the valence band H and if the photon falling on the solar cell is having energy H nu normally it is given H is the constant use of frequency. So, the requirement is that H nu should be either greater than or equal to EG. So, energy of the photon should be equal to or greater than the band gap energy and that must happen. So, for example, if you are talking about gallium arsenide solar cell the photon of energy having less than 1.45 electron volt will not get absorbed will not get absorbed and will just transmit. We will discuss that more in detail when we discuss the solar cell. So, we are almost at the end just want to focus again that finally, there are many material parameters which are important the band gap is important the structural arrangement is important the carrier lifetime is important, but the most important is the price of the module in terms of the rupees per watt which depends on the production cost and the efficiency not only the efficiency. There are some other parameters which are important. For example, how much money that you are paying per kilowatt hour of energy generated. Another parameter is how much kilowatt hour you will generate if you installed 1 kilowatt of module or 1 kilowatt peak of module. What is the price of module? What is the production cost of the module? What is the efficiency? So, these are the various parameter we will discuss over this course. There is sufficient time I will discuss more of this, but the idea of this lecture was to actually discuss what are the material that can be used you know what is the material that can be used for solar cell you know which are the direct band gap which are the indirect band gap you know what are the thin film solar cell and what is the thick or the solar cell you know what are the crystalline silicon technology and what are the thin film technology. You know about the structure of the material that it can be amorphous, it can be micro crystalline, it can be polycrystalline, it can be monocrystalline and you know the various band gaps of the material that is available. In the later lecture we will actually look at the more of these properties. So, what are the optical properties of the material? What are the electrical properties of the material? How they affect the performance? We look at the manufacturing and the main and the performance of the solar cell. So, these are the various parameters we will continue to discuss when we discuss the theory of the solar cell. So, with this thank you for your attention. I hope I have summarized it and I hope this is a good beginning to discuss solar photovoltaic solar cells and the operation of it manufacturing. Spend time on this lecture whenever you get and try to absorb the basic themes about the materials about the band gap structural properties direct versus indirect thin thick film versus thin film etcetera. So, now it is time to take the questions. Government call is Salim. On what basis we have to select the PV model whether we have to monocrystalline or multi crystalline? So, this is a very good question million dollar question everybody is trying to find answer. I will not answer this question now, but we wait for some more time some more lectures and over a period of this course you will get the answer, but in general there is no specific answer. It is a very important question for industry what to do. If anybody knows the answer he can get a million dollars. C O E T Pune. I would like to ask about mechanism design. Can you include in your presentation brief idea of mechanism design for continuous sun trekking? About the continuous sun trekking. Yes actually the slides that I have shown and I have not gone through I have not discussed. Basically the basic idea of the continuous trekking is to make that theta that we have discussed equal to 0 right. So, when so the whole idea of continuous trekking is to make theta equal to 0 this can be done in two way. One is by using the equations that I have given to you in the presentation. Other way is to use the sensors. So, if we use the sensor in a such a way that it can monitor the position of the sun. So, you can actually direct your motors to do that. So, both ways are possible and people actually use both of them together in order to increase their accuracy. So, sensors will actually sense a position of the sun. For example, let me show you that for example, if you have a if you have a if for example, sun is having this position you are having a tube here which is having a sensor here right. So, now if sun ray is like this then the this sensor will actually sense the radiation, but if sun's position becomes like this then the no light is falling. So, this sensor will immediately find out that sun has actually moved and based on this you can actually design electronic circuit so that your panel is also moving. So, either you can sense the position of the sun and do the tracking or you can actually use the equations that I have given and not discussed to find out. Okay sir, thank you. Okay, Jabalpur College. Sir, my question is does the efficiency of PV module includes the life of PV module also? The efficiency of the PV module does not include the life. So, efficiency is an instantaneous value and the efficiency of a module keeps decreasing over a period of time. So, when we say the life of the module is 25 years which means that after 25 years as per the international standard the efficiency decreases by 20 percent. So, after 20 years if today let us say efficiency is 10 percent and after 25 years the efficiency will decrease by 20 percent that is how the life of a module is defined. Okay sir, you have given one question that whether that company will survive or not then for that we could able to determine only price of module. So, how we could comment on that key whether that company will survive or not? Okay, so, good that you pointed that the price of the module we should compare what are the other prices of different companies. Okay, so, the prices of the different company today is as I told you is available at you know 60 rupees per watt, 50 rupees per watt, 70 rupees per watt and if any company is producing module which is higher than that cost will not survive. So, basically you have to compare the price of the standard commercial module with respect to this company. But you have given the given only one company data. So, how we could compare that? Other you should know basically, other you should know what are the current what is currently happening in the market that you should know. Engineering college Pune, go ahead. Sir, I just wanted to ask you said that indirect band gap is has an example of silicon. Okay. Right. So, why are we using silicon even if it is having low bandwidth means low absorption capacity. Okay, good question. So, because silicon is having low absorption capacity, why do we use silicon? And the answer is that silicon has been used being used for the micro electronics industry for a very long time. Okay, now six decades, almost 60 years you are using silicon and there is a lot of knowledge available of silicon, there is a lot of manufacturing capacities available of silicon. Silicon is by the way the single largest material produced in the highest purity on the earth and silicon is also available in the large quantity. Silicon also gives stable efficiency solar cell. So, despite it is being a indirect band gap semiconductor, there are so many other advantages that almost 80 percent of the solar modules in the world are produced using silicon. Can we reduce this means deficiency by doping it with some other thing? No doping does not affect the band gap, it does not affect the indirect nature of the silicon. By the way the silicon when it goes to the amorphous silicon form when the then it actually starts behaving like a direct band gap. So, by doping we cannot do that, but by changing the structural form we can do that, but then it actually reduces the quality of the material etc. But the knowledge about the crystalline silicon is so good and people are actually trying to make it thin and thin. So, earlier crystalline silicon cells were available at 400 micron thick, 500 micron thick and now today commercially it is available at 180 micron thick and people are going to try to as thin as 150 micron. So, there is a lot of work going on to reduce the thickness of silicon. Thank you all. Let me stop here. Thank you.