 Good afternoon, if you already do not know I am Chetan Singh Solanki and faculty from Department of Energy Science and Engineering at IIT Bombay this is only IIT to have such department and we are focusing lot on energy both conventional as well as non-conventional energy. I have done my master's in microelectronics from IIT Bombay itself 97 and then I did my PhD from IMAIC that is Inter-University Microelectronics Center in Belgium that I graduated in 2004 and since 2004 I am with IIT Bombay so as such I have been researching on solar photovoltaic technology for about 11 years now and I have seen the development of PV research particularly in the country from the very beginning and lot of things now happening but early in 1999 when I decided to work on PV people thought you know it was really stupid decision to work on it because being in microelectronics you have all kind of access to the companies like Texas Instruments Center where I actually work for some time before going for PhD. So now it is a very good time to be in photovoltaics and lot of activities happening in the country and in the world and I am sure in the future as well there will be more and more requirement of the manpower and researchers and therefore I think this is the perfect time to start such activity on photovoltaics. So one thing I would like to request is that please ask question when and whenever you ask whenever you feel that there is a need for question, Mike will be available please ask them without any hesitation and let us make it very interactive and so that you will learn out of it and you utilize your time better and I utilize my time better. The whole idea of this five days is that we will not have enough time to cover the entire syllabus that is planned for December workshop. The idea of this workshop is to get a glimpse of it how we are going to conduct the courses so that the back at home you can actually do the better coordination. Together with the way we have planned you must have the timetable now with you right now. What we have planned is together with the several theory lectures there are laboratories and tutorials and we will take help of our other students for conducting tutorials. What also I have written a book on solar photovoltaics it is called Solar Photovoltaics Fundamentals, Technologies and Applications. Right now this book is not available in the market otherwise the plan was to give one book to each of you right now but I am sure this book is going under review and going for the second edition which will be out in about a month's time. So when during the December workshop this book will be given to all thousand participants so we will ask who are the how many people have registered in your centre and then this book will be sent to all of you. We will try to send one book to you in advance for that you know so quite a significant part of the course is actually being taught from this particular book. So let me begin again another thing is that while going through this I am sure many of you are already aware of some of the issues or the things that we are going to talk about here but the whole idea is to conduct this workshop in a manner as we are going to conduct in December for the other teachers. Also I have seen from the morning session is that not all of you are having full-fledged course on Solar Photovoltaics but it may happen as there is a lot of demand on photovoltaics may happen that in future you actually conduct this kind of course. So this whole course is planned as if it is a one full course which not only takes care of the fundamentals but also talks little bit about the technologies and the power electronic side as well as the application. So as a result what we have tried is actually to put together a good overview about the PV technologies and applications so that a student going through such program or such course would have enough knowledge about the photovoltaic technology and depending on the need or requirement he may pursue M-Tech and PhD programs or he may go to the industry directly. So this kind of knowledge will be sufficient for having that kind of background. Any questions so far? So let me start. So electric city scenario of India generation what you can see here in you might be knowing already that total we have about 170,000 megawatt of which coal is significant, hydro is about 25, 26 percent, nuclear very small percentage, gas is not that big, renewable is now getting bigger and bigger share of our installed electricity. So this comes to about 13,000 megawatt and out of which majority of is wind turbines, the contribution of solar photovoltaic and thermal is very small right now. But because of the Jawaharlal Nehru National Solar Mission we expect that this number, this share of the pie is to go bigger and bigger. One other important thing we must take always notice that there is a difference in the installed capacity of conventional grid connected power plant and the capacity of the renewable energy system. So for example, even if you have let us say 10,000 megawatt of solar photovoltaic module but 10,000 megawatt does not operate for 24 hours. So there is a capacity factor which is much smaller than the coal based power plant. So coal based power plant of let us say 10 megawatt would operate for 80 to 90 percent of the time. While same 10 megawatt power plant of photovoltaic will not operate for 80 to 90 percent of time. So therefore, the amount of energy generated from renewable energy system for the same installed capacity is normally much lower than the amount of energy generated from the conventional technology. So that we should keep in mind. So in a way this share actually is little bit misleading ok. So if you look at the share of energy provided by the renewable energy it is much lower. But it is not, what you can see here is about 10 percent it will be like 2.5 percent ok. So finally, the goal is not to put not to only increase the share in terms of the megawatt capacity but to increase the share in terms of total electricity generated that is more important. As I told in the morning that right now our per capita per year energy consumption is only 650 while world average is 2000 while the many developed countries have more than 10,000 and about 400 million people do not access to electricity and as for the ministry of power 80,000 villages are not connected with the grid right now ok. So there is a huge demand for energy and solar energy solution can be one appropriate solution to supply that energy requirement. Jawaharlal Nehru National Solar Mission is giving lot of push to the solar photovoltaic technology. And as you can see here it actually plans to go up to the 20,000 megawatts of solar power. So what I was talking is that there is a huge push provided by the national Nehru Solar Mission and you can see the face wise manner it is going from the 1000 megawatt 4000 and 20000 by year 2022. And lot of this power is actually will be in the form of the upgrade system which is about 2000 megawatt which is particularly the case that you will be talking that the street lighting small home lighting system solar length and everything comes in this. And my personal guess is this number will be actually bigger than the grid connected power because there is a lot of areas where there is no power and solar energy can immediately supply the power. Not only the national, not only the government of India but there are many state government that is now taking active part in the not mission but they have created their own policy. For example, Gujarat is one of them. Gujarat has policy of how it is something like about 500 megawatts of solar power and it is actually moving very fast. So these are the other states where solar power is being kind of planned and going this is the what is being applied so far. So this is the not capacity installed but under the Jawaharlal Nehru National Solar Mission this much is the planned capacity. And as you can see from the slide that this is the phase one where it is ending in 2013 where by which time we should have about 1000 megawatt of installed capacity. And I see that is happening very much there is lot of people who are interested in it. And there are many state governments that are actually doing it as you can see the distance is lot of involved capacity but you find other parts of also like Maharashtra the Mahajinko Maharashtra state government is actually planning for 120 megawatt of the photovoltaic power plant. If it is installed before then other power plant this will be the world's biggest solar PV plant in it is being planned in Dhule a place called Sakray. Some of you are aware of Sakray in Dhule but as you can see there are other states Andhra Pradesh, Odisha, Tamil Nadu, Karnataka, Punjab who are actually taking initiative to install solar PV plant. And under the Nehru Solar Mission there are lot of initiatives that are being taken actually to promote the PV technology and installation of the PV technology. And the initiatives are taken at the all friends. So for example in terms of the technology there is a push for all the technology not only crystalline silicon we will see what are the various technologies. The feed-in tariff is being provided. The feed-in tariff is 17.91 rupees per unit of electricity so that is the government promises to buy that electricity at that cost which is very attractive business proposition. And that is why there is lot of interest in the country about the feed-in tariff. This policy is new to India but it has been under operation for years for many countries and Germany is one of the leading example. Germany, Japan, Spain, these are the leading countries and now some of the states of US also for example California is the first state where such policies are being executed. There is also what is called renewable energy purchase obligation for the state electricity generation boards. So, I think right now the plan is that state electricity board should install 0.25 percent of their capacity with solar energy. And the 0.25 percent by the way by all means is very large number though it is in terms of percent very small but installing that much amount of renewable energy technology or the photovoltaic or solar thermal is a big number. Main power generation as I said many of the programs are happening around the country. There is a promotion for the domestic production so the people are encouraged to install the manufacturing in the country and various soft loans are available. Scalability because there are several phases in which this national mission is planned going from the phase 1 to phase 3 and the application is focused on both grid and off grid system. So, there is a all kind of things are being taken care in this national mission. This is the current installation in terms of the production of the cells and modules as you can see now there are many many companies which are doing the production of the cells and modules. So, there are about 15 companies who are doing solar cell production and our total capacity is now 675 megawatt number which is small but still growing very fast in terms of the photovoltaic module our capacity is about 1.1 gigawatt which is again a significant number. So, one thing to be noticed that however the large capacity is both in terms of megawatt 675 megawatt and how much I said our installed capacity right now for the PV the installed capacity the power plants are only 10 to 15 megawatt while our production capacity is 675 for solar cell and 1000 megawatt for solar PV module. So, what happens to those modules they are all exported so what does it mean there is a very good manufacturing capacity exist in the country but there is not enough market and that is why traditionally all these cells and modules have been actually been actually exported to the other countries like Germany and Spain being one of them. But now we see that because of the national mission there is a more and more deployment coming. So, now significant number significant means still we are only about using 20-25 percent of our total production capacity but in future it is likely to improve. So, there is a significant growth in industry going from 0.06 megawatt into the 5 to 1.1 gigawatt I am sorry 60 megawatt to 1.1 gigawatt in 2010 the huge growth and this growth is about compound annual growth rate AGR is about 75 percent very very high growth rate. I do not think there is any other industry in the country which is growing with such a high rate which is good sign and we see that more and more of the deployment will occur and the important thing for all of us as a teacher is that there is going to be more demand for the students both for the researchers technical level deployment level and all kind of thing. And that is where our role is actually to create that kind of manpower which will very much be required to for the nation to be succeed. Just to give an example the for example in 2004-5 China was nowhere in the scene when we looking at the worldwide production of the solar cells and modules China was nowhere when you look at the data from 2004-5 you know what is the current status China has surpassed every other country in the world they are the number one producer of the solar photo and the role model and cells and the raw material so such a huge difference in just 5 years time and of course we are nowhere close to it but we cannot ignore the solar energy there has to be enough deployment of solar energy and therefore both research as well as the deployment level we need large amount of manpower and also money. So, I think there is no doubt about this slide that solar PV, Voltec technology can use solar radiation as a resource and produce lot of energy and the resources is tremendous only small percentage of the solar radiation if you convert into the electricity you will have enough electricity for the whole country I think when we do the solar radiation we will discuss this point that only about 60 kilometer by 40 kilometer area if you put the solar PV module it will good enough to generate electricity for the whole country all over electricity requirement and I think our country is very big 3000 kilometer by some 2500 kilometers our area is huge so very small percentage of area will be good enough we will do the calculation on that. So, however there are challenges to be overcome if you really want to deploy the large area high cost per unit watt and that is basically the cost of the material is very high right now. But I have seen tremendous changes from even my research times in but over the period it has really changed significantly. So, solar photovoltaic module the cost was something like 1000 dollar per watt that was the cost in 73 75 when the solar photovoltaic technology came into the picture first time and it has really come down significantly particularly over the last 2 years you know what is the cost of module current current cost of PV module 3 to 6 to 70 rupees per watt any other case 120. So, this are kind of number you will see. So, in terms of the price what you see in the market today particularly the crystalline silicon solar cell technology you will get it at about when you are going for the large scale hundreds of kilowatts you will get at the rate about 95 to 100 rupees per watt. Some of the thin film technology you may get lower than that you may get at 80 85 rupees or if you buy in a more retail manner 1 kilowatt 500 watt then you will get it may be 100 and 100 and 10 rupees per watt. This number just 2 years before was 220 250 rupees 280 rupees per watt. So, within last 2 years the drop in the PV module price is significant and it has come down to below 100 rupees about 2.2 dollars per watt which is very fascinating. I do not know if you are following the research for me it is really fascinating that now for many applications the solar photovoltaic has already become the cost effective as compared to the grid and connected system. But still that is a challenge even the 2.2 dollar or about 100 rupees per watt is still a large number and we would like to see this cost as low as 40 rupees 50 rupees per watt. So, there is still a challenge in terms of the material. The efficiencies are moderate, but not too bad. The efficiencies that we get the module today is at the rate of 14 percent, 15 percent. There are people who are also producing modules at 19, 20 percent, 21 percent and the solar cell as efficiency for crystalline silicon cell has been demonstrated over 24, 25 percent and for the multi junction over 42 percent or so. So, there has been large kind of improvement in the efficiency and further improvement is required and if there is a time I will tell you that efficiency is not the only number to look for it is the cost per unit watt is that is more important. So, there are technology which can make the solar cell at 25 percent efficiency, but not everybody makes the solar cell at 25 percent efficiency because the cost of doing that is too high and therefore, efficiency is not the only number to look for it is the cost of production for a given efficiency eventually converted into the cost in terms of rupees per watt of PV module that is the more important number. What we what is also a challenge is the availability of material. So, should not actually develop a technology which is which is using material which is not available abundantly right now that worldwide in worldwide production of solar PV modules is about any idea any guess worldwide production of solar PV module. No guess just guess any number yeah 9 gigawatt it has already crossed almost about 12 gigawatt which is a big number right and 12,000 megawatts of solar PV module and putting this together this is still a very small percentage of worldwide installed grid electricity installed capacity is in terawatt. So, the 10 gigawatt at 12 gigawatt is still a very small number what does it mean that if you really want if you really see that photo tech modules are going to be a solution for the future electricity generation we are going to we are talking about the module manufacturing in terawatt level and when you go into terawatt level your material requirements are huge and if you are using any material which is not abundantly available you are in trouble. For example, the cadmium telluride or CIGS for the copper indium gallium selenide the indium is a rare the indium is being used for all LCD displays indium is being used as a TCO and many other application. So, the indium cost used to be free this era used to be something like 40 to 50 dollar per kilogram you know what is it any guess for the indium cost right now 1500 to 2000 dollars per kilogram just because it is so rare that if you try to use more and more of it the cost is going to escalate. And therefore, when you talk about solar PV technology we should really look for the materials which are abundantly available. So, that we do not you know get into the limitation silicon which is commonly used materials not of that category it is really abundantly available and therefore, not a problem you can produce as much as cells in modules as you want. Long term stability is another challenge for the PV technology right now the life of the PV module is supposed to be 25 years. 25 years means even after 25 years the module is supposed to give you 80 percent of its performance that is how we define the life 80 percent of its initial performance should still be giving at the end of 25 years. So, therefore, it is important to choose a material we can where you can get a stable performance for the long long period of time. Long energy payback period is another issue. So, you should not use a material where lot of energy goes in making a solar cell right if you put lot of energy in making a solar cell how you can recover that energy from the solar cell if that will not happen then there is no point in making a solar cell itself right now the payback energy for crystalline silicon solar cell is about 2 to 3 years which is still very high lot of energy goes in making a solar cell ok. So, what does it mean the solar cell the crystalline silicon solar cell itself takes 2 to 3 years to generate energy which is gone into making silicon solar cell. So, out of 25 years lifetime 3 years or 2 to 3 years is of no use. So, therefore, we should try to minimize the energy that goes in making a solar cell and some of the thin film technology are good at it and their energy payback period is less than a year and the most important thing the money payback period it should give us money back right whatever we are putting it if that is not the case no industry would come forward and actually invest money into it, but so as the cost goes down money payback period is also goes down and again it is a challenge. So, these are all of these are the challenge for any solar PV technology this is common to all of them you take it crystalline silicon amorphous silicon CIGS organic solar any technology you call it the main challenge of this if you can fulfill all this wonderful I tell my student that if you can fulfill all this this is the route to become billionaire you solve this problem and I guarantee that you will become billionaire very next day because there is a huge demand right lot of people are actually I am coming from as I said US and Santa Clara California California the Bay Area I do not know if you heard of it it is it is a silicon valley it is also called silicon valley and silicon valley is the place where all the new technology comes in. So, if a very good idea go to silicon valley people will give millions of dollars immediately and you can make the difference ok, but what is required is solution to this problem or at least one of this problem if not all of this problem even at one of this is also ok fine. So, here is the my lecture starts now. So, these are the introduction any questions so far I know all of you are many of you are feeling sleepy and it is really difficult task to keep people awake just after the lunch. So, if anybody yeah China modules are they important in India and if yes what guaranteed warranty it comes well China modules are being important in India yes, but many of them do not believe them. The thing is that as long as there is a certification called IEC 61215 International Electro-Tactical Commission certifies the module I mean they have actually come up with this code and any manufacturer before selling it to the Indian market at least or outside also should actually pass this module this test IEC 61215 and once they are passed then it is kind of guaranteed that they will actually perform under condition for that long time yeah. So, I would not like to come in more than that ok. So, which material to be used for solar cell application and if you understand this you understand half of the solar photo type ok any idea. So, which material we can use for solar photo type semiconductor very good and why semiconductor why not aluminum it should be p-n junction why not silicon oxide why not silicon nitride and one ok. So, we should actually let us start looking at the electron flow in a coal based power plant can you draw the electron how what happens to the electron when it goes to cycle right electron must be going through a cycle right it at power plant something happens to it then it is transmitted into the transmission and something happens to it then it goes through the load like you know tube light and all it something happens to it and then it goes back to the power and that is the how it cycle is completed right and we want our solar cell to do the same job right should also be doing same thing as the electricity from the coal is going. So, what happens to the electron is if I if I draw the if I draw the let us say potential of the electron and at the power plant what we are doing is we are raising its potential because this what happens is the power plant right I am drawing very simple. So, some arbitrary reference number it is it is raised that potential is raised to some level how much level what is the output voltage at the power plant level some thousands of kv. 11 kv 33 kv 11 kv 33 kv even higher also depending ok. So, it is the thousands of kv. So, we actually increase its potential to some certain number and then it goes to the substations and substation the voltage is dropped and dropped right and eventually when it is delivered to the home it is at certain voltage 230 volt right. So, it comes to 230 volt and then finally, we actually use it in the load it comes down to the same potential where of course, there is a drop in the lines and all we are not considering it comes down to the same level and then it goes to the cycle right this is what happens in terms of potential it increase it potential is the power plant and then it goes to the various step down voltages eventually the load and it comes back and I want same thing to happen to electron in a solar cell or I want same thing to happen to a material which is used for solar cell application right. So, what does it mean? Somewhere in a material its potential should increase ok and in knowing the material properties it is not possible to use any metals for this kind of application you cannot use any metals for this kind of application because why not metals are the continuous band gap material band gap is continuous this is not no way we can create a potential difference even if electron gets energy in the in the in the metal it will lose its energy in the in terms of the heat you will not get this kind of enhancement in the potential fine. So, why not to use insulators insulators have also certain kind of band gap right you can use insulator it is also certain kind of band gap why do not we use insulator then it is a large compared to what the insulators have large band gap agreed compared to what right good point. So, compared to energy available to the photon it is very large right you know we need to have we need to conduct this operation in solar cell this operation is done by. So, I am now coming to the solar cell this operation is done by putting a photon on it. So, the photon should actually do this job of increasing the potential energy ok, but if this level is very high as compared to the energy of the photon in a spectrum then this will never happen and insulator band gap is very high. How much high silicon oxide has a band gap of about 5 electron volt some insulator will the band gap 8 electron volt. What is the highest energy of the photon in the solar spectrum? We can do the calculation later, but let me tell you right now the highest energy is about 3.5 electron volt ok. So, what does it mean immediately now makes me clear that I cannot use any material first of all which is more than 3.5 electron volt right I cannot use any material where the band gap is overlapping or 0 like in metals. So, what are the materials I can use all those materials which is having band gap energy between 0 to 3.5 electron volt I can use that and this materials in our definition are called semiconductors. So, remember that in solar cell I need to do this operation and we will get back. So, in solar cell I need to do this operation so that we can electron to go through the same cycle again ok. So, that is what we in metals we have this kind of arrangement the bands are overlapping and therefore, you cannot create that kind of potential difference which you want to create and you cannot create therefore, the potential voltage or therefore, you cannot use it for electricity generation, but you can use this kind of material for heat generation. So, this is this kind of material are the perfect materials for solar thermal technology right. You want to actually generate in heat in solar thermal and you want to generate voltage in solar photo type that is the fundamental difference right. So, when you want to generate heat it is this kind of material and when you want to generate the voltage it is this kind of material. What is this kind of material? There is a one band there is another band there is a band gap and this is where we would do the operation which is done in the power plant. This is where the electron is actually given the extra energy to get in to increase its potential energy and that is what we want. So, we need a material with some kind of band gap and therefore, semiconductors are the material which can use for solar cell and metals are the material which can be used for solar thermal technology. What are the materials that we can use? Many for example, silicon is most commonly used material almost 80 percent of the modules being produced worldwide today are from silicon, but then you have many other semiconductors, gallium arsenide, cadmium telluride, cadmium and telluride. You have combination of the indium, gallium, zinc is being used, poron, phosphorus, arsenic, antimony, sulfur everything is used and not only this we also use in combination of the two. So, not only the elemental semiconductor we also use combination of the two for example, compounds semiconductor like gallium arsenide the world's highest solar cells are actually highest efficiency solar cells are made in gallium arsenide and its compound. Ternary compounds algase, alum, gallium and arsenide and quaternary compound alum, gallium arsenic, arsenic, phosphide ok. So, people try to mix various kinds of material to actually make the solar cell and we actually have the various compounds that that are being used for solar cell applications. Why do people mix and match? Why do people make the compound semiconductor and actually many kind of material? Try to get the best properties of the material to the fine-tuning and what is the best property we are looking for? We are looking for the best brand gap and the two important properties that we are looking for one is the best optical property which is the appropriate band gap and for solar cell application the best or the highest efficiency is possible when your band gap is 1.45 electron volt ok. So, that is the material we look for, but that is not all we are also looking for the material which is having band gap of 1.45 electron volt, but also having good electrical properties in high conductivity, high lifetime, high diffusion length, higher minority carrier lifetime and so on. So, that two of electrical properties and to combine it put together we are looking to have the best optical and best electron property at the lowest possible cost. So, that is a major concern we want to. So, that is why people are trying to actually use many many materials. So, the next question is fine we need to use semiconductor of the suitable band gap and electrical properties. So, the next question is can semiconductor alone work as a solar cell? Can we use just a p type solar semiconductor and say give me electricity out of it can you do that? As you know we cannot do that right what is what is need as a junction is required ok and why we need a junction. So, eventually want a potential difference ok and again biophysics you know the definition of potential difference there has to be a positive charge and negative charge physically separated from each other right they have to physical separation and this physical separation when physical separation exists then only potential increase ok. You can increase the potential energy by you know changes by injecting electron or exciting electron from low energy level which is a valence into high energy level which is conduction band, but that is increase in the potential energy only that does not alone necessarily create a potential difference. For the potential difference what is required is excited electron should not be at the same physical space, but there should be a separate physical space right you are getting point. So, if you talk about the two different energy level. So, what my photon is doing is taking a electron here creating a hole and you are generating a electron in the valence in the conduction band is a conduction band energy level and the valence band energy level, but this what is the I am sure everybody is aware of this terminologies energy band diagram what is the y axis? This axis is energy what is the x axis space or position x axis is space ok. So, now what I am saying is in the in the earlier diagram there has if when you want to create a potential difference there is should be a positive charge at one place negative charge at other place. What is happening in this case? The hole is created the electron is created, but they are at the same space. So, does it mean that you created the potential? No it does not mean it does not you not created the potential what you have done you only created the difference in the potential energy ok. So, therefore, if you choose one semiconductor which is shown here you will not create the potential difference you need something else to do the potential difference. So, a semiconductor of p type or n type or any other type alone cannot do the job you need that physical separation and that the physical separation is possible only if there is a junction that we need to understand how right. Just take it right now that a physical separation is possible only if there is a junction how we will say and that is how we need to generate is that point clear you need to teach to your students. So, make sure that it is clear. So, I will I will come back to that how that how the separation is possible that normally is it takes quite some time to come to the point, but take it for the right now that you need to have separate and therefore, you need p n junction. What kind of energy we have in the spectrum? In the whole electromagnetic spectrum the the solar spectrum is very very narrow and the energy and the wavelength of the photons in spectrum are connected by this equation 1.125. So, what we do we will find out what is there in the solar spectrum right now very quickly. So, one thing we talked about is the energy of the photons in the solar spectrum. So, just notice the solar spectrum ranges from where to where solar spectrum is starts at about 200 at about 300 electron volt I am sorry 300 nanometer and it goes all the way up to 5 micrometer 5000 nanometer. The wavelength is connected with the energy of the photon with this equation which is nothing but e equal to h nu nu is a frequency you divide you convert that to c by lambda is a lambda is in micrometer. So, let us find out what is the highest energy of the photon in solar spectrum I am saying the photon spectrum starts at near u v in the solar spectrum you have visible range which is 400 to 700 380 to some 700 nanometer small portion of the ultraviolet is also there in the spectrum about 8 percent of our energy comes in ultraviolet range and then you have some infrared also infrared all the way till about 45 micrometer wavelength. So, that is the wavelength and frequency is also there what I need to find out from you is what is the energy range what is the range of the photon energy which can be given by this. So, please do the calculation quickly my shortest wavelength which is ultraviolet wavelength is small when I go from the ultraviolet to visible to infrared wavelength increases normally this is radiation is called short wavelength and this is called infrared long wavelength. So, the short shorter wavelength starts at about let us say 380 nanometer. So, what is the energy corresponding to 380 nanometer photon when I say 380 nanometer I am talking about wavelength and make sure that this is in micrometer. So, you have I want to find out what is the energy in electron volt of a short wavelength photon of the highest energy that is 1 to 4 and wavelength in micrometer how much is the wavelength 0.380 because it has to be in micrometer. So, that is the energy at the higher side then I can also calculate energy in electron volt at the infrared side at infrared side our spectrum actually goes all the way up to 4 micrometer. So, this is in 5 micrometer. So, this is 4 and what you get is in terms of the electron volt how much you are getting here 2.96 about 3 electron volt how much you are getting here 0.28 0.28 electron. So, you have the band so, you have the photon energy going from 2.96 volt that is about 3 electron volt which is the highest energy of the photon and the lowest energy is 0.28 electron volt is that fine keep this in mind. So, all our material now we are now more narrow in terms of the choice of the material. So, your material has to be less than 3 electron volt and more than 0.2 electron volt 0.28 electron volt. So, when we have energy of the incoming photon from ranging from very small 0.28 to 3 electron volt what it can do to my material. So, one possible interaction when your energy is very very small one possible interaction is that it does not do anything it only get absorbing the material and only results in the vibration of the bonds between atom and atom various kinds of vibration can happen that is the case when your photon energy is very small. Another case is that when the photon is falling on it actually excites electron in this case no electron is getting excited in this case electron is excited from one energy level to other energy level. What are the circles that you see here orbitals it is a Bohr atomic model. So, this is a nucleus and then you have the various orbitals. So, if the energy is between this energy level up to 3.5 electron volt energy can be transferred from to the electron such that it goes from one orbital to orbital of lower energy to orbital of higher energy. And the other interaction could be that if the energies are very high in the x rays and e v greater than 3.5 electron volt the electron may get out of the where the electron is going it is becoming free it is going out of the material. What is this effect called? It is called photoelectric effect here the ionization is happening when this effect itself is called photoelectric effect discovered by Einstein. What do you think which one is happening in a solar cell? Last one we do not have energy of this much right we have just do that where does the calculation what is the highest energy we got? So, we do not have 3.5 electron volt energy. So, this cannot happen if this happens we are still in the problem why this electron is getting out of our control we want this electron to do the job we want the electron to go to the load give me the light or run my fan or do anything and then get out of it. If this is going we are still in a problem if this is happening there is still a problem. So, what happens this is what is the most important interaction that we want. So, the electron or the photon interacting with the material should excite an electron such that it goes to the higher energy level in the same material. So, this kind of interaction is what is of our interest. So, this is then finally, the generation that we have the photon coming in if the photon is of energy less than the band gap energy. What happens? If photon coming in is less than the band energy what happens? Nothing happens nothing happens ok. And the good example of this is our glass windows nothing happens right everything comes in the glass is transparent because the glass is what is one of the oxide and it is having a very high band gap all the photons in the spectrum is having lower energy nothing get absorbed it is everything is transparent everything comes through and therefore, glass is transparent also another good example is your mobile phones your mobile phones the radiation coming is what radiation you use in your mobile phone of very longer wavelength very small energy and that photons having so low energy that it comes through the wall also or the absorption is very weak almost behaves as a transparent it does absorbed because sometimes wall is thick, but it transparent the energy is so small. So, it depends on the energy level. So, if the energy level is lower than the the band gap different nothing happens and this is one question to answer why efficiency is only 14 percent because our spectrum consists what are the wavelengths from 3 almost 3 electron volt to 0.28 electron volt and what is the band gap of a silicon 1.12 ok. Silicon is saying band gap of 1.12 electron what does it mean? Many of the photons are just passing by not getting absorbed, but that is not the mistake of sun, sun cannot do anything what you can do you are not able to use it right. So, you are passing it you are losing it. So, we are having lower efficiency. So, that is one of the reason for silicon having a band gap of 1.12 electron volt we lose 23 percent of energy not in our control how much 23 percent of energy just go without having any interaction with the material itself. So, how much is remaining out of 100 23 percent of electron are gone or 23 percent of energy is gone. So, remaining is 77 let us say how come 77 becomes 14 right we will see one by one. So, this is one thing if the green photon comes sometime we also call this photon by the color ok infrared photon is colorless it is out of visible right. Red photon is the photon having 700 nanometer band gap the blue photon is photon having 400 nanometer band gap the green photon is in the green part of the visible spectrum about 550 nanometer. 550 nanometer corresponding. So, how much energy do the calculation 550 nanometer photon means what is the energy of the photon. So, what we need is a green photon I am talking about of having energy 550 nanometer sorry wavelength 550 nanometer I want to know how much is the energy about 2 electron volt. So, the green photon because its energy is higher than the band gap energy because its energy is higher than the band gap energy it can actually excite a electron from the valence band to conduction band right. And if very energy photon comes the blue photon which will having energy of about 3 electron volt will actually excite electron 2 by the what is this here. So, this is the age of the conduction what is about this what is about this about this what is this by the way x is energy what is about this energy levels, but they are in continuous and we call it as a conduction band right about this is the age of the conduction band above this there are many energy level and they form what is called band below this are also energy levels, but there are it is called valence band. What is between this there are no energy levels energy gap or called band gap there are no energy level ok. Now, the blue photon comes in it excites electron the electron goes how much high in energy with respect to this how much energy it will go up corresponding to the energy of the photon. So, if photon energy is 3 electron will go up to 3 electron where is my energy level for silicon 1.12 ok. So, what is happening to the rest of the energy ok. So, this is my valence band level conduction band level the photon energy is coming 3 electron volt this is my silicon. So, band gap is 1.12 electron volt if this gets excited electron get excited go away all the way up to. So, this is my 0 energy level this energy level should be 3 electron volt. So, my electron is here, but what I need is 1.12 and because there are various energy level this electron will actually collide here and there and actually lose its energy. It is actually going in the conduction band with various energy level lose its energy how much energy it loses 1.88 that much energy is lost that much energy it comes down to the same level. So, actually out of 3 electron volt how much is required or how much I can use is only 1.12 and how much I am losing is 1.88. That is another loss that is another loss of energy this loss of energy is significant and it accounts to 33 percent of the losses how much 33 percent. So, what was the last number 77 minus 33 how much is remaining 44 is what in your hand ok. I tell my student that even Lord Shiva come to you and sit next to you and say what is what can I do for you and you say give me a best solar cell using silicon make me a best solar cell what you will say sorry 54 56 percent is not in my hand if that has to happen what you what you have to do you need to change the physics you cannot do anything with that 56 percent losses is that clear. So, what you have to play with is only 44 percent energy you have to play with only 44 percent energy you have to get best utilization of the 44 energy and if engineers and scientists are getting 15 percent is it bad right out of 44 percent if you current engineers and scientists are using 14 percent is it bad I would say not at all you know what is the efficiency of the photosynthesis process which is a natural process happening from billions of millions of years what is the efficiency it also does the same thing right it takes the light converts into the mass. What is the efficiency less than 1 percent for most of the biomass efficiency is less than 1 percent nature uses less than 1 percent of incoming light in the best case it is like bamboo and all they use 4 to 5 percent. So, 14 percent is not bad at all but it is too expensive and therefore we have to do make it further by the 14 percent is a module level at solar cell level people are making 16 17 even there are some industries like and sun power which are making solar cells at more than 20 percent efficiency is this clear again very important from the solar cell perspective to understand this now I had not put the any name, but if you take various materials which are the commercially available today make it crystalline silicon polycrystalline silicon amorphous silicon cadmium chloride CIG whatever it is same kind of argument actually you can put with the other materials also and actually you can find out exactly how much the energy are going to lose because of the photon is just transferring not getting absorbed how much of energy you are going to lose because of the extra energy of the photon. We wish we can use this energy and there are some concept now people are working on it is very theoretical level where we can actually use energy at that you can extract the photon at 3 electron rule that is the best thing you can do, but very far from the practice and may become reality in some 20 30 years time down the line not right now. Anything that goes up must come down that is a law of nature is not it anything that goes up must come down and that also happens in semiconductor in solar cell. So, electron getting excited may come down. So, it may come down directly bend to bend it is called recombination earlier process is called generation this process is called recombination opposite of generation. So, the electron may come down by the way do you want recombination to happen in a solar cell you do not want right because what you want we do not want to let this electron go without doing our job we want to do we want this electron to do the job what is the job it should run our fan or tube later we do not want to do it. So, so this is if this is a generation taking place ok the reverse is the recombination we do not want recombination to take, but eventually what happens electron will come down to the same level as in power plant the electron goes up in potential goes down down at various substation then comes down and comes to them eventually it will electron will come down to the same energy level, but not without doing the job there is a mean problem right. So, what we want we do not want this process to happen what we want this electron to actually get outside our circuit go through the load and then come down then we do not have problem. So, this path is ok this path is not ok right. So, clear we electron eventually will come down to low energy level, but after doing its job. So, so this is what we want without coming through the we just it should go through and recombination should not occur, but it does occur it does occur and that is where the role of engineers and scientists is to actually look for the new material which is low cost easy to fabricated, but still good. So, that this path does not it and there is a band to band recombination does not happen in solar cells silicon solar cells. So, often it happens in a gallium arsenide direct band gap semiconductor there is a larger recombination in which the electron which is coming down gives its energy to some other electron and goes up the the auser is the name of scientist who has discovered this and there is another recombination where the defects some energy level which are in between. Now, normally in a very nice material like single crystal material there should not be any defects or there should be very minimum defects. So, your band is actually there are no continuous no defects, but if your material is defected material can be defected due to two reasons one reason is that the defects because of the crystalline disorder you know in crystalline material all items are sitting very nicely in order if some of the items are missing that is a crystalline disorder that is a one way of disorder. Second is that if there are another impurities in the materials if you are silicon, but there are iron there is a copper there is a magnesium zinc and whatnot. So, that is another disorder. So, two types of disorder can create this kind of energy level and that gives rise to recombination. For solar cell application this is the most common particularly silicon for the silicon solar cell this is the most common way of recombination. So, 44 minus 44 minus recombination you cannot avoid it it never 0 never 0. So, you may count for something like 10 percent losses 10 to 15 percent in the good case or in the worst case it can be really huge everything may recombine. So, 44 minus 10 you are come to 33 percent you will still find out where are the others losses going on ok. So, something like this happens and the recombination you cannot avoid fine, but as I said the solar cell is not just one single semiconductor cannot do that there has to be a junction which is normally solar cell looks like this you have the p type solar cell sorry p type semiconductor n type semiconductor junction between them and then you have the contact at the both the sides right. So, what do you want your solar cell to do? First of all your solar cell should be if light is falling it should if light is falling what should happen? It should absorb it should interact with the light should not choose a material it does not interact with the light it should interact with the light and create a electron hole pair fine. Once it is it is done what do you want your solar cell to do? It should actually separate you know what we want eventually our solar cell we want a electrostatic potential to be generated and electrostatic potential means a physical separation between a positive charge and a negative charge. So, after absorption if there is a electron hole pair they should get separated physically and the physical separation occurs over a p junction and p and n regions. So, between over the junction the physical separation should occur something like this. So, electron should go at one direction hole should go other direction such that they are never coming back to close to each other to get the recombination and that is the job of the junction. Once separated the junction does not allow them to come together and recombine again ok. Once separation is done what you want? You want electron to come out of the circuit and do your job. So, then eventually you want there are context to the front and back you want those electron and hold to be collected at the circuit and go through the external load ok. So, these are the three main function a solar cell should perform. What are those three functions? Absorption one is absorption second is separation and third is transportation or collection. So, these are the three things the solar cell must be doing. Absorption separation and collection any solar cell should be doing this whether solar cell is of germanium or gallium arsenate or silicon or some material which is not existed in the earth or some other combination any solar cell it must do this three function without which you do not get the the job done. Is that clear? And different solar cell technology strives to maximize the efficiency of the above operation in different way ok. Some solar cells have p-n junctions, some have the p-n junctions, some have the multi junction, some have the hetero junction, some have the air sea coating, some have the texturing, some have the back surface field, some have the back reflection what not. Lot of people are trying lot of things to just do these three functions. Every solar cell should do this function. Any questions so far? No? Ok good.