 In the first lecture we have looked at about the, we have looked at about the upper limits of the parameters current voltage. Now, we will go into the details of design, we want to design to get the higher current, higher voltage, higher field factor. Optical losses I told you that there is a loss because of the reflection from the front metal. There is a this metal itself is causing a loss right because the metal is there it is not allowing light to come from this place that is called the shadow losses and the transmission. So, we want to minimize this. First thing you want to do is put what is called anti-reflective coating. We want to minimize the light reflected from the front surface and therefore, we should do what is called anti-reflective coating or ARC. The principle is very simple that if your light is reflected from this surface and for the anti-reflective coating ARC, ARC design. So, what do you want? This is your P N junction solar cell. Now, you want to put another layer called ARC. Why you want to put this ARC? You want to put this ARC so that your light reflected is minimum. So, their light can be reflected from here or it can come down and get reflected from here. The simple way to minimize the reflection is to create a situation in which this two reflected rays one is from the top surface and one is from this surface results in a destructive interference. What is the destructive interference? If you have you put the two ways out of the 180 degrees phase shift. So, way which is reflected from this surface top of the ARC and way which is reflected from the N surface. If these two waves are having 180 degree phase shift you can create a destructive interference. So, when you create a destructive interference you have to create a interference. You have to create a phase shift. What does it mean? If this distance travelled by this ray within this layer is equal to the half of the half of the wavelength. If this much distance is travelled in the ARC let me draw it little bit in detail. So, this is your ARC layer sorry this is your ARC layer and this is your N type. So, if this distance travelled this distance from here to here and here to here if this is equal to half the wavelength then if this is equal to half the wavelength then you have the destructive interference. So, that is first of all that that is what will happen. So, this some of this two paths should be half from here I am sorry. So, some of this two paths should be half right. So, what should be the thickness of air inter reflective coating? Because this is travelling twice the distance I should have one half of the thickness of this distance travelled. So, typically my thickness of anti-reflective coating should be one-fourth. One-fourth of what? Wavelength. Which wavelength at which I want minimum? I want minimum reflection. So, this is one-fourth of lambda if the wavelength outside the material is lambda then when the light enters in the material the wavelength becomes lambda by N. What is the N? N is a refractive index. Therefore, the thickness D is equal to lambda by N 1 by 4 that is what you see. So, the thickness of anti-reflective coating should be lambda 0, lambda 0 is the wavelength of the photon divided by 4 N 1. Is it clear why the concept 4 came here? Because the D is the half of the half of the half wavelength and that is why this one-fourth is coming here. One problem with this design is that you know your anti-reflective coating is a function the thickness of the anti-reflective coating is a function of wavelength and there are many wavelengths. So, therefore, for each wavelength your coating thickness should be different which you cannot afford. So, therefore, you should minimize you should choose a thickness of the anti-reflective coating such that the reflection is minimum where the intensity of higher. Because D is a function of lambda your D is a function of your D is the thickness of A R C thickness of A R C is a function of lambda. So, your thickness is a function of lambda and so, because for every different wavelength your thickness should be different. So, for what wavelength you should minimize? So, minimize for the so, if I look at the spectrum it is like this right. So, about green light or about 550 nanometer your intensity is higher and therefore, therefore, you should design therefore, you should design. So, this is your spectrum which is higher intensity is higher here about 550 nanometer green photon. Therefore, you should design your thickness value for 550 nanometer the one good problem that you will do in tutorial and you can get in the quiz also. So, we will not go into the details of the design. So, I have considered three values here the reflection is minimum. So, N 0 is the refractive index of air N 1 is the refractive index of your A R C layer N 2 is the refractive index of silicon glass this N 0 can be glass right, because since actually your solar cell is encapsulated between glass and A R C. So, in practice you have your solar cell you have the glass glass is used for encapsulation then you have your A R C then you have your semiconductor P injunction. So, if the refractive index of this layer is N 0 if the refractive index of this layer is N 1 and N 2 then what it shows here N 1 should be that is the refractive index of your middle layer should be equal to square root of N 0 N 2. Refractive index of the glass is 1.5 refractive index of silicon is 3.8 and therefore, square root of 1.5 into 3.8 is about 2. So, therefore, refractive index of your anti-reflective coating should be about 2 right. So, N 0 sorry N 1 is square root of N 0 N 2 which is 1.5 times 3.8 and you will do the calculation it comes about 2. So, you should choose an anti-reflective coating which should have a refractive index of about 2 look at here if you have bare silicon your reflection is very high if your silicon under glass reflection is given by the blue light and if your anti-reflective coating then your reflection for a particular wavelength can really be minimized that is what is the role of anti-reflection coating to minimize the reflection not all the wavelength, but for one particular wavelength where intensity is higher. So, what is this anti-reflective coating that we used in practice there is a material called silicon nitride Si 3 N 4. Silicon nitride is the most commonly used anti-reflective coating in commercial silicon solar cell even in the laboratory solar cell people use silicon nitride because it has a it has a refractive index of about 2. So, silicon nitride is the most commonly used anti-reflective coating. In fact, the blue layer of your solar cell is is is basically because of the silicon nitride the blue color when you look at the solar cell is basically because of the silicon nitride. So, one way to minimize the reflection is to put anti-reflective coating you have this anti-reflective coating where else in your sunglasses sometime you have the anti-reflective coating right wherever you want to minimize the reflection you should have anti-reflective coating. Other way to minimize the reflection is texturing you make your surface rough other way to minimize the reflection is texturing and this is how people will do the texturing. There is the advantage of texturing is first of all ray will come here if it is the case some of the ray may get reflected directly, but but it can in this case when the surface is textured or rough it can also go here and there is a one more chance of absorption right. So, therefore, texturing reduces the chances of reflection and people do the texturing very nicely in in the practice and very easy crystalline silicon is very easy to texture anybody can do even you at your center can do take a potassium hydroxide KOH put in a water heat it at about 80 degree centigrade put your silicon crystalline silicon monocrystalline silicon and once you come out and take it it you will get textured within 5 to 10 minutes it happens automatically because we have not discussed the crystal details. So, once we know the crystal details actually it is possible to explain how texturing is occurring, but it is very easy to do and this crystal this size of the crystals that anybody can do depends on the depends on the crystals. So, typically you will get this kind of pyramids after the crystallization. So, this is all silicon this is all silicon and typically this height of the pyramids will be about 5 micrometer and this also be about 5 micrometer typically ok and be smaller and higher. So, this happens automatically very easy to do put your silicon, but some of the texturing like this which is called inverted pyramid this is pyramid and this is this is a pyramid and this is inverted pyramid. So, this kind of pyramids require photolithography will not go, but this is the way to minimize the reflection and you can see because of this because of this the light which come here if it is not absorbed it will actually reflected and if it is it will have second chance to get absorption it will have second chance to get absorb and the reflection reduces. There are many ways you can the other ways called the light trapping suppose fine you minimize the reflection, but what about this light what about this light which is coming here and now passing by this is possible right because actual material thickness may be very high the transmission is taking place, but what you want is if you can somehow make this light also reflected right you make your backside also rough you make your backside also rough. So, that this light is further reflected then what you are doing you are trapping the light what you are doing you are trapping the light light trapping and light trapping is very very very important research area particularly for thin film solar cell why because you want to minimize the thickness of your material for the cost reason whenever you use less thickness your cost of the solar cell is lower fabrication is lower you want to minimize the thickness whenever it is possible and if you want to minimize you do not want to compromise with the absorption you still want to absorb more and the one way to increase the absorption is to the light trapping reflection from the front side and backside. So, that your light keeps on bouncing back in front this concept is called the light trapping and very important for thin film and even for the crystalline silicon solar cell also. As you can see here if you get the total internal reflection you can get the very well nicely trapped light fine. So, you want to minimize the reflection. So, reflection is like this you do the inter reflective coating as well as anti-reflective texturing by the two things we do to minimize the reflection texturing and coating. So, now your emitter that I have was showing all the way the your concept is now upside down right what is the thickness of the n layer I told you what is the thickness of the n layer I told you 300 to 500 nanometer right that is the thickness of emitter what is the thickness of this pyramids 5 micrometer huge as compared to 500 nanometer 5 micrometer 1000 times higher what does it mean that you are actually what does it mean is that your emitter is not flat. So, if you have if you have wow isn't it interesting and then your backside. So, actually your n reason is like this and your p reason is like this and reason is very thin and because of the texturing you get this and what else is there on the top. So, this is your n region what else is there on the top on the top you also have anti-reflective coating you also have what is called A R C what is the thickness of A R C let us find out thickness of A R C layer should be lambda 0 by 4 n what is lambda 0 where you want to minimize the reflection let us say at 600 nanometer 4 and the refractive index is for the silicon nitride is 2. So, 600 divided by 8 what you get you will get about 18 nanometer. So, your anti-reflective coating is even thinner how much is this anti-reflective coating thickness is 18 nanometer only in industry you get anti-reflective coating of silicon nitride which is only 70 to 80 nanometer your A R C is only your A R C is only 70 to 80 nanometer what is this material silicon nitride what is this refractive index 2 how we calculated the thickness we calculate the thickness using the wavelength where we want minimum reflection where we want minimum reflection where there is a highest intensity where is the highest intensity about 550 to 600 nanometer. So, if you want to minimize reflection at 900 nanometer you can do that you can choose it. So, this is how your anti-reflective coating thickness is only about 70 to 80 nanometer and your solar cell structure is like this your solar cell structure is like this ok. So, your junction is not anymore flat your junction is in commercial silicon solar cell your junction is not flat interesting wow. So, I will I will give some more concepts about the design of solar cell now we want to minimize the recombination how to minimize the recombination first of all where so far so good reflection we have to study now recombination is another thing remember what are the two main things in a solar cell design increase absorption reduce recombination two mantras increase absorption reduce recombination increase absorption means lower reflection increase recombination means lower recombination where recombination can occur everywhere on the front surface in the emitter in the junction in the base in the backside everywhere recombination can occur and we want to minimize this where whether the recombination is taking place or not who will tell us quantum efficiency quantum efficiency tell me where whether the recombination is taking place or not. For example, if the quantum efficiency is low at 400 nanometer because the 400 nanometer is the high energy photon they get absorbed very close to the surface indicates that the recombination is taking place if a quantum efficiency is high indicates that recombination is not taking place and same thing with other we have discussed in the last lecture. If the quantum efficiency high at given location means no recombination less recombination taking place if the quantum efficiency is low at a given place indicates high recombination is taking place. Limit for circuit current we already discussed 46 milli ampere per centimeter square in practice in silicon solar cell we get in practice in crystalline silicon solar cell commercially we get about 30 to 38 milli ampere per centimeter square a number to remember a number to remember that in crystalline silicon solar cell efficiencies that we get is about 30 to 38 milli ampere I am sorry in crystalline silicon solar cell the current density short circuit current density we get is about 30 to 38 milli ampere per centimeter square design for high open circuit. So, if you want to minimize the if you want to minimize the recombination at the surfaces if you want to minimize the recombination at the surfaces. So, minimize recombination at surface we do something what is called the surface passivation passivate the surface passivation means make it calm quiet that is called the passivation passify right. If person is very violent how do you passify charlots which are called it. So, if the surface is very active lot of recombination is taking place how to passify you put some other material and silicon nitride which is a anti reflective coating you actually put a silicon nitride which is a anti reflective coating also act as a surface passivation layer it also passifies. How to get high open circuit voltage again minimize the recombination as we discussed earlier because your open circuit voltage your open circuit voltage depends on log of i l divided by i 0 plus 1 ok i 0 is a reverse saturation current. Reverse saturation current depends on many many parameters. So, one way to minimize the recombination is to minimize the reverse saturation current and then many people were asking you know how to minimize the reverse saturation current how to minimize the recombination and many people were asking us today about the doping. What is the optimum doping? If doping can be tends for 14, 18, 15, 18, 16, 17 what is the optimum doping. So, open circuit voltage sets the limit for the optimum doping ok and then we have to actually go into discuss detail in theory and find out why the doping level has to again there is a compromise within doping level ok. We want in some for some cases we want load high doping high doping basically will result in a ok. So, I want what about the doping I want high doping high doping will give me low r s will give me low series resistance ok, but high doping will give me high doping will be give me what more defects right more defects means more recombination ok high doping will give me more recombination and also give me lower diffusion length lower diffusion length. So, therefore, contradiction right I cannot have I want high doping for lower series resistance, but I cannot have high doping for because my recombinational increase and the diffusion length will also decrease and lower diffusion means lower collection I want diffusion length to be as long as possible right. So, there then you have to make a compromise between the diffusion length ok. So, this theory explains and we are not going to the detail why we should have not very high doping level and not very low doping level. So, typically the doping level in p type base doping level in p type base is normally about 10s for 15 to 10s for 16 boron atoms per centimeter cube typically typically in commercial silicon solar cell base doping level is 10s for 15 to 10s for 16 n type emitter have very high doping level right 10s for 18 to 10s for 19 phosphorus atom per centimeter cube why why why like this emitter doping is much higher than the base doping base doping is lower because of the the higher open circuit voltage emitter doping has to be higher because we really want lower lower resistance I will discuss about that ok. So, high open circuit voltage you means minimize the surface recombination how to minimize the surface recombination you put a anti-flict coatings at the front side I will take another slide. So, surface recombination front side I put silicon nitride by the I am showing the flat surface, but you know now that surfaces in reality are not flat they are textured, but in only for the simplicity, but the back side we do very nice thing ok. So, this is the front side this is the front side at the back side we do what is called back surface field at the back side what we do back surface field or we call BSF BSF to minimize the recombination at the back side and the passivation surface passivation is to minimize the reflect recombination at the back front side passivation ok. So, we want to minimize the recombination both the front side and back side at the back side we do what is called the back surface field we create a another layer which is called the P P plus layer ok we create at the back side P plus layer P plus is moldoping ok. Look at your structure now your solar cell is actually I told you emitter is highly doped. So, N plus not N high when you in our in our terminology when we say P and P plus remains high doping ok. So, normal terminologies P is doping and then P plus is heavy doping N is a medium doping and N plus is heavy doping that is the terminology. So, what is your solar cell becoming now your solar cell is N plus P and P plus your solar cell is what N plus P and P plus N plus is your emitter P is your base and you have smaller P plus at the back side that is the front surface ok. This is our solar cell I am showing you this is where the light will enter this is my emitter this is my base and this is my D S F back surface field and then I will make a contact here and I will make a contact here ok. I am showing a solar cell in a different way just to show you what are the different reason. Emitter is clear that it is N type only thing is I am telling you that it is heavily doped ok to minimize the when we do more doping there are more electrons when there are more electrons the conductivity is higher and resistivity is lower. So, when we heavy doping is there to minimize the resistance and I will come back to why P doping cannot be very high because my open circuit voltage will reduce because the recombination will increase. So, P is normal, but the back side my doping is again higher P plus why it is higher because I want to create a back surface field and the back surface field is created very nicely when doping is higher your Fermi level is closer to the valence when doping is lower. So, P layer and P plus layer when doping is lower your Fermi level is little away from the valence band and now you see here the bands are flat here the bands are flat and in this region band is not flat. So, whenever there is a change in potential energy means what potential energy is not constant change in potential energy means electric field ok. So, therefore, there is electric field and therefore, this electric field for electron will actually act as a repellent it will not have electron and which is my electron here which is the important carrier minority because in P type minority carrier is more important in P type which is the minority electron. So, the avoiding recombination of electron is more important and therefore, back surface will field will minimize the recombination at the back surface. So, then we have created the back surface field. So, now look at how we are going we are designing a solar cell which is now having a texturing which is having anti-reflective coating and it is also having a back surface field ok fine. So, now we are actually trying to do the minimize the recombination and the last important third and last important thing is anti-reflective coating and texturing is for increasing the current we want more electron photon to go in. Surface persuasion is there to minimize the recombination because we want higher open circuit voltage from it and residue losses we want to minimize because we want to get the higher field factor from it ok. So, very interesting this is a model of a solar cell and I will show you how to get a model of a solar cell. So, what is your solar cell? Your solar cell is the P n junction diode very nice you put a light on it and because of light it gets positive and you get negative ok. You get a positive and negative and then the current flows in this direction reverse direction this is your solar cell. Look at how I make a model of it I want to make a model of this I want to represent this model. Remember one thing this is current due to the light this is the current due to the light because of my P side is positive N side is negative it is also getting forward bias and if P n junction is forward bias will the forward bias current will flow? If my P n junction is forward bias will the current in forward direction flow or not? Yes it will flow why because if it is a junction it is a P n junction it is getting forward bias it should go flow the forward bias current only thing is forward bias current is smaller. So, this is my current what current forward bias current other than this because my because my device will have finite resistance it will have finite resistance right. So, I should add somewhere the resistance also what is this? This is my series resistance. So, now how do I have to make. So, what are the things I have one current source which is function of light this current I have a current which is function of bias and I have resistance. So, if I draw a current source and this is because of the light and my diode current is flowing in the opposite direction of this current right my diode current forward bias current flows in P to N my light current flows from N to P. So, therefore, my diode current should be in the opposite direction this is my current and there is a resistance in the path. So, this is my series resistance and sometime there can be what is called the shunt resistance also shunt resistance R S n. So, now this is my diode my I V. So, V and here I have the I this is my I L this is my I L and this is my diode current got it. So, in a very nice way whatever we understood from what we understood from the this is the cell model what we understood from the theory. So, far using that theory I can very easily draw the model of a solar cell model of a solar cell it should be the easy very easy job for all of you now right it should be the very easy job for all of you now to draw the model forward current of a diode and light generated current series resistance shunt resistance ok. Based on this you can actually modify your I V characteristic of a solar cell ok what is I V characteristic. Now, I have taken I L as a positive current ok you can take anything because of this I have taken I L light generated current as a positive current ok. So, normally you have I equal to I L minus I 0 e raise to power T V by k T minus 1 light generated current forward bias current and I V is the bias that is getting ok this is the normal thing. This equation do not take care of the series and shunt into account series and shunt into account, but in this equation you can do that how we can do that your voltage appearing here will be voltage V plus I R S ok V plus I R S loss here. So, here the V term will get modified to the V plus I R S. Now, you another current component here if this voltage is V plus I R S and then the current in this divided by V plus I R S by divided by R SH right as we have seen yesterday here. So, you can actually modify your current I L minus I 0 your voltage term will modify because of the series resistance drop that will occur by k T minus 1, but you are adding one more current component which is because of the shunt resistance possibly ok. So, voltage drop is V plus I R S divided by the R SH shunt resistance. So, now, this is your new modified current voltage equation of a real solar cell which has a finite series resistance and it which has a finite shunt resistance. And what is our job? Our job is to make R S as small as possible your resistance of the device should be as small as possible and we want we do not want this current component why because it is in opposite direction of light generated. So, this current component should be as small as possible and therefore, R SH the shunt resistance should be as high as possible typically ideally 0 and they should be infinity simple simple. So, this is your new characteristic of the I V characteristic. Now, the series resistance is a function of many parameters. So, the resistance of the base resistance of the emitter look at first of all how the current is flowing this is another important point you should note and in fact, you will realize that everything that I am teaching is important ok. So, another point that you should note how the current flows where is my contact my contact is here my contact is here metal contact and my back contact is everywhere front surface is actually textured and you know that very well. So, if my electron is generated here and this is my P and this is my N electron is generated here how it will travel it will travel like this because the perpendicular to the cross section here it will face minimum resistance. If it has to travel here then its resistance is higher unnecessarily. So, actually it will go here once it goes here it has to either go to this contact or this contact. So, it will either go like this and like this it will go like this and like this. So, very important current flows vertically in the base vertically in the base and horizontally in the emitter current flows vertically in the base and horizontally in the emitter. So, this explains why your emitter has to be heavily doped. What I told you what is the thickness of the emitter? I am repeating again and again 300 to 500 nanometer your thickness of emitter is 300 to 500 nanometer very thin layer very thin layer. And therefore, therefore, if this current flows lot of current flows in that direction lot of current flows in this direction resistance will be higher. And therefore, in order to minimize the resistance you should increase the doping level and therefore, normally the emitter doping that is why I told you the emitter doping is tens for 18 to about tens for 19 phosphorus type and base doping is about tens for 15 to tens for 16 boron and this is phosphorus per centimeter cube boron per centimeter cube. So, that is one reason why your emitter should be heavily doped. Other reason is that if you make heavy doping your defects will increase and your diffusion length will decrease no problem because your thickness of emitter is only 300 nanometers even if your diffusion length comes from 100 micrometer to 2 micrometer 5 micrometer you do not care because your emitter is very very thin. So, current flows like this then it flows like this and then it gets collected by the fingers and then all the fingers are connected to what is called the bus bar and your bus bar carries the current, your bus bus carries the current. So, because the time is over let me stop here some of this design about the series resistance and the fill factor I will discuss tomorrow we are running out of the time some of the slides I will discuss tomorrow before we starting the next lecture. So, before I stop let me summarize what we have discussed in this design of a solar cell. We particularly looked at to increase the short circuit current by minimizing the reflection. So, we have looked at the anti-reflective coating and actually you can calculate the thickness of the anti-reflective coating and anti-reflective coating is based on the concept of destructive interference and then we looked at about the texturing and the parameters that is formed. Then we have looked at about the minimizing open circuit voltage by minimizing the recombination at the front surface and the bake surface. Front surface recombination is minimized by putting a passivation layer and I told you that anti-reflective coating silicon nitride also not only acts as a anti-reflective coating, but it is also acts as a passivation layer and the bake side recombination is reduced by what is called the bake surface field. I also told you that emitter has to be heavily doped. So, the symbol for heavy doping is called n plus n is a medium doping tends for 15, 16, n plus is a heavy doping tends for 18, 19. Then you have the base which is p type and the bake side is again p plus layer which is heavily doped because of the bake surface formation. And then we started discussing about the series resistance and shunt resistance. We have looked at the model of a solar cell very interesting model of a solar cell from whatever discussed we have discussed very easily simply. We can actually make the model of the solar cell considering that solar cell is the current source and the forward bias current series and shunt resistance based on that you can modify the IV equation of a solar cell and get the right equation. And then we started looking at the series resistance minimization. One way to minimize the series resistance is increase the doping of the emitter because emitter is very thin. Again the current flows vertically in the base and horizontally in the emitter. So, thank you very much. We will stop here. I see there are lot of questions. So, I will take as many questions possible. So, first question goes to Jaipur College, Kukas. Hello, sir. Sorry, what part of time in your discussion you mentioned? You mentioned that the wind speed also affect the performance of a battery or a solar cell. Could you please explain how does the wind speed? Wind speed affects the efficiency of a solar cell. Sorry, what affects the efficiency of a solar cell? Wind speed. Wind speed, okay. Wind speed actually will not affect the efficiency of the solar cell. Wind speed will result in a cooling of a solar cell in a module. And when the wind is higher, the cooling will be higher and temperature will be little bit lower. And that because the temperature will be lower, it can affect the efficiency. Otherwise it will not. It will have effect of cooling. Amritha, Koyamchur. Can we plot to the optical losses to 0 level? In the practical, anything doing 0 and 100% are you know very difficult, right? Anything doing 0 and 100% is very difficult. So, bringing to 0 is nearly impossible. From a previous chapter, so our light causes the both the currents, diffusion and drift currents. So, the diffusion current causes by our photon. So, and the phonon acts some role. So, what is that phonon? Phonon, in a indirect band gets semiconductor. Phonon, without phonon absorption, the photon cannot get absorbed, you know, because in silicon like material, if a electron is going from valence band to conduction band, it has to change its energy level and it has to change its momentum. Then only absorption is complete. So, for photon is coming, it is giving energy and phonon is coming, it is. Thank you sir. S.D.N.I.T. Suraj. This terminology, emitter and base, though it is a diode, we feel that when emitter base is there, whether there is a collector too. Is there any significance of in using this terminology? Yes, the terminology of emitter base actually comes from the transistor only. That is why emitter and base phenomenology is erupted from that from the transistor, but there is no collector here. S.D.N.I.T. Suraj. More question, sir, whether I do not know it is relevant or not, in less transmission losses and resistive losses, the reason for which are basically the difference in the band gap energy and the energy of the light waves, wave length rather. S.D.N.I.T. Suraj. So, whether there is anything like modulation or conversion of this energy range, any device, whether it is available or whether there is an area of research, because I mean using which we can convert all these to around 1.45 electron holes and get the maximum, be it any wavelength. S.D.N.I.T. Suraj. Very nice question, very nice question that if the whole problem of this transmission and distribution losses is about the difference in the photon energy and the band gap energy, why not to convert all the photons to a band gap energy. And this kind of change, this kind of change in energy, photon energy is what some people are looking at and this terminology is called up conversion and down conversion. So, people are trying to convert high energy photons into low energy photons and low energy photons into high energy photons, you can use single material to do that. There is not lot of success so far, but yes what you are saying is a very correct idea that if you can convert all the photons, all the spectrum into one particular photon it can be useful. Partly it is done in what is called the thermo photovoltaics. In thermo photovoltaics, solar energy is first converted into thermal energy of higher temperature, let us say 2000 Kelvin and then whatever the spectrum is generated may be close to the solo cell band gap and can do more efficient job. Bharamati. Terminal voltage is positive and electric field is opposite to that in the band that how this is, how this happens that terminal voltage is positive, p is positive and negative, then also in the band gap sorry in the band gap in the w electric field is in opposite direction. No actually make sure that you are understanding correctly you know the potential energy of the electron and the potential is minus, you know there is a minus sign involved. If you look at the band diagram carefully you will see that the whatever the generated voltage is actually what is appearing at the terminal. So, look at the band diagram carefully, look at the signs of energy of electrons carefully and you will find out the answer. Amrita school column. You have given two expression for finding field factor and you said that when VLC increases field factor increases, but when we consider the expression when VLC increases it is evident that field factor decreases. No, so that VLC by the way VLC in the expression is not VLC it is VLC normalize to this small VLC, small VLC normalize to KT by Q that is what you should plot ok. So, in the expression let me tell you in the expression you have you should use not VLC you should have used VLC normalize to KT by Q and that is small VLC. So, this is what you should use in the expression then I think you will not have this problem. Sir, if the light is made to fall on p region p region of the solar cell then will the direction of current reverses. No, direction of current will not reverse wherever light is falling direction of current is because of the whatever the energy arrangement of the p and n region. Wherever light comes from the direction of the current will always be will always be in the negative direction that is n to p. Always the direction of light generated current will always be from n to p irrespective of the direction from where the current light is falling. PVPS Institute, Vijay Vada. Hello sir. Hello. Yes. Sir, why we are taking always n type as a top layer sir. I think I have explained you that it is a more like for a technological reason when we take n type as a top layer you know top surface we have to minimize the recombination we have to passivate the surface and people have found that it is easier to passivate the n layer as compared to p layer and that is why we use top layer as a n layer. Second reason is you know p layer is very thick and my minority carriers in the base is electron and electrons have a higher mobility as compared to holes and electron will have to travel higher distances. Therefore, it is better that electrons are used in the as a conducting medium in the p layer minority carriers and for this reason we always use n as a top, but this is not the only possibility there are researchers who are trying to use otherwise also you can use p layer as a top n layer as a bottom also, but commercially n is on the top p is on the bottom. Sir, my other question is we know photoelectric effects. So, when our light falls on some metal surface electrons emits from that surface. So, is it possible in case of this photovoltaic cells that means when our electrons comes out the number of charging areas number also decreases that also affects the efficiency is it possible sir. Photoelectric effect is not possible considering of solar spectrum right solar spectrum has certain energy photoelectric effect is when high energy electrons when high energy electrons falls on a material electrons emits out ok. That happens when you have high energy x rays are using in solar spectrum photoelectric effect will not occur it is only photovoltaic effect that will occur. Sastra University. Good afternoon sir. It is possible to make a modeling of a solar cell by using your SQL sir. Is it possible to model the solar cell using SQL? I think yes it is possible to model use so is SQL to model the solar cell yes it is possible ok. Let me take some chat questions why pentavalent or trivalent atom are used for doping rather than using bivalent and exavalent atom etcetera. So, when you want to do the doping first of all we want to create extra electron in extra hole that is the most important thing, but other than that you have to make sure that what is how the doping is happening. When the doping is happening this atom is actually going inside the crystal right think you are you are already having a silicon material intrinsic ok. Now, you have to put one atom there. So, this atom has to physically go. So, the size of the dopant if it is very small or very big as compared to your lattice spacing then it will create a problem. So, there are many other parameters that you have to see what is the diffusion coefficient at what temperature it diffuses, what is the size of the atom, if the size of the atom is different than the size of your bulk atom then that will create a problem and therefore, other things have to taken into account. So, my question is like a photon of single light like say red, all the photon of red light will be having the same energy. Then why it is why it is absorbed in a different length in a semiconductor? Ok good question because all the photons of red light if they have the same energy why they get absorbed at a different length ok. So, imagine that one this photon once it is entering inside the material with entering semiconductor it is going through the probability right. So, it will the photon will go it will have some interaction if this interaction is strong enough it will get the energy from the photon and give it to the electron ok. So, because this is statistical kind of phenomena so, because and it is probability right. So, when that interaction take place can happen within certain distance and that is why it is not only absorbed at one location, but it can absorb at different locations. So, like by passing from front to end layer there may be a chance that it will be colliding with other items. Right. There is a sound that red photon will give it energy to another electron. Sir is it possible to a single photon can excite more than one electron? Normally yes in principle it is possible to that single photon excite more than one electron. Some cases it has been observed and that is what the phenomena we look forward. But in practice if you look at the quantum efficiency analysis and other normally it does not happen ok, but if the photon energy is twice the energy of the band gap it is possible that it can actually give more than excitation to more than one electron, but normally does not happen ok. Question on chat in p type material why the current moving in vertical direction and n type material why the current is moving in the horizontal direction. You know I told you that because in the horizontal direction if you look at our, if you look at this explanation here that now this current is moving in this direction because this is the shortest path and low resistance path for this photon or this sorry this electron. This is the lowest resistance path the vertical path right, but once it goes there because my contact is only sitting here and here my contact is not everywhere. So, once it goes here the then it has to move this direction. It has to move in this direction in order to get collected at the context right. So, for electron in the base the lowest resistance path is like this, but in emitter the resistance path is like this. And therefore, here it will move vertically and here it will move horizontally. Imagine that the thickness of the emitter is very thin it is only 300 to 500 nanometer. It is very thin as compared to the distance. The distance between these two metal contact is of the order of millimeter. 1000 1000 micrometer or it is 1000 1000 nanometer. So, as compared to this thickness this width between the two contact is very very high and therefore, and therefore, the current will have to flow horizontally in the emitter. Shivaji Nagar. Good afternoon sir. Sir, my question is regarding anti-reflecting coating. Thickness of anti-reflecting coating is such that it should use rise to destructive interference for reflected rays. So, my question is what is happening with the energy of reflected rays after destructive interference because energy should get conserved. Definitely yes. So, the destructive interference is occurring and destructive interference film means 0 energy reflected right. So, nothing is reflected. So, everything is going in. Another question from GSITS is the current generate is the current generated is negative if not how it is operated in the fourth quadrant. Of course, current generated is negative with respect to our convention. What is our convention? In a p-n junction diode we say that current flows from p side to n side that we call as a positive current. In the solar cell the current flows from n side to p side and therefore, with respect to our convention the current is negative. So, current will always flow negative and then only you will have the power generation operation from a solar cell. So, let me stop here. Thank you very much.