 Good afternoon, all remote centers. Good afternoon to all of you. If there are any questions, please ask me or put it on the chat. All remote centers, Bharamati. My question is that the potential across PN junction, external potential developed and the field inside the gap seem to be opposite to each other. How is this? So, potential outside is potential generated and the gap and the energy field inside is opposite to each other. I think it will be clear if you draw the energy band diagram. This is P side, this is N side and we know that when light falls, this terminal gets positive, this terminal gets negative. This is the external potential generated. Now, what about the electric field? Electric field occurs whenever the bands are not flat. Whenever you draw the band diagram and if the band is not flat, it decays electric field. Higher is the slope of band, higher is the electric field. So, between these two there will be electric field and this electric field occurs because of what reason? Because of the recombination of the free electron and holes at the interface when you bring a P side and N side together. So, N side there are lot of holes. So, you get this as a positive charge because the electrons which are mobile goes to the other side. The electrons goes to the other side, it leaves behind a positive charge. Similarly, the P side, at the P side you will have lot of holes which are mobile, but once the hole goes away, once the hole leaves this side and recombine with the electron, what it leaves behind is a negative charge. Now, this positive and negative charge are fixed. These are the charge on the ion on the atom itself. Now, this positive charge is fixed, negative charge is fixed and because of this, the field, now direction of the field is in this direction while this positive, this charge and this charge, this positive and this negative is in the opposite direction. So, whenever we extract higher voltage, so this voltage which is generated because of light actually opposes the internal electric field, opposes the internal electric field. Is that the answer to your question? That is clear, but how this happens and when we take, when terminal voltage becomes 0, inside electric field increases and when terminal voltage increases, inside field decreases, how it happens? Because they are opposite to each other. So, when you are getting higher voltage outside, your opposition to internal voltage is also higher and therefore, the net effect is that you have the lower field inside, but when you are 0 outside voltage means you are expecting less voltage. So, therefore, your internal field is not affected. So, the maximum internal field is what you can get as a built in electric field when no light is falling. As soon as light falls, the light results in a generation of voltage which is opposite of electric field inside and therefore, the net effect on internal field will become lower and lower as your voltage appearing outside the outer terminal is higher and higher. There is a question on chat which type of contact is used most either buried or a screen. Now, I did not discuss this in detail. The buried, there is a concept called buried contact and a, so basically what we want is, let me use it. So, for example, you have one structure which is our simple p n junction structure, your contacts are here and these are screen printed contact. In another structure what people do is, you actually make a contact like this. So, this is your emitter is now like this. This is your n region here, n plus region and your n plus region is here also and here also and then you make your, then you make your contact here which is actually also buried. So, this is your n plus region, this is your metal contact, it goes all the way, your metal contact goes all the way and then your n plus region. So, in this case the carrier which are collected here can also go to the junction, while in this case carrier which is collected here will have to travel longer distance. So, in this is, in the buried contact, this is case for the buried contact and the terminal is such as contact is buried inside and this is the normal contact. So, the advantage of buried contact is your collection is better because your metal is going inside the semiconductor also. Now, as a result of that the efficiency of this is higher, but the cost of fabrication is also higher because you are doing something which is not normal. So, by some mechanism you have to drill a hole inside the, drill a trench inside the semiconductor normally done by the laser and then you actually use electroplating to do it. So, there are many more additional steps that are added in the processing. Therefore, efficiency increases, cost also increases in the process becomes more complicated. While in the normal contact screen printing, screen printed contacts are used and screen printed contacts are very simple, right. Screen printing process itself is very simple, very optimized. You can do screen printing of large number of efforts per hour. Already automation exists in the screen printing and therefore, screen printed contact is simpler and because of simpler, simplicity cost is also low and because of the cost low, this is the most commonly, most commonly used contact. Screen printed contact are most commonly used contacts. I hope that answers your question. Another question chaired from NIT Calicut is if the PV is floating in the sunlight, what will happen to its life and efficiency? PV is floating in the sunlight, I mean, do you mean that PV is actually in the sunlight and not connected? The what will happen is the efficiency definitely will degrade because the efficiency degradation mainly occurs because of the metal semiconductor contact. This contact degrades. Why degrades? Because you know metal has a different expansion coefficient, your semiconductor has a different expansion coefficient and we will discuss in the module that actually your cell is at much higher temperature than the ambient temperature because of that and in the night the temperature goes low in the day time it goes high. So, it goes to the various temperature cycles and because of that the addition between the contact and the temperature met semiconductor decreases and as a result of that series resistance increases and as a result of that efficiency decreases. That is one effect. The anti-reflective coating and other layers also degrades and therefore, if you putting it in light, definitely efficiency are going to go down. Any other questions? There is cloudy day weather panel works or not. If there is a cloudy day weather panel work or not, panel do not know whether there is a cloudy day or not cloudy day. What panel know is only whether light is falling or not falling. If light falls on your panel even though it is a diffused light. So, under the cloudy condition the light that will fall on your module will have some intensity. It will have some photons which is having energy higher than the band gap energy and as long as that is true solar cell is going to work. Only thing is because your intensity of your light which is falling under the cloudy day condition will be low and as a result of that the amount of electron hole pair generation will be low and as a result of that power generation will be low. But your solar cell can definitely work under the cloudy condition also. Another question from Bharamati. Hello sir. If we are not extracting energy from solar cell, that is if solar cell is open circuited, if solar cell is open circuited then it will get heated as forward current increases and it is better to keep solar cell extracting the energy. Otherwise solar cell temperature will go up. Is it there sir like this? True if you are not extracting the amount of electricity generated it will be dissipated inside the solar cell. But when solar cell is inside the module and as I am saying we will discuss it tomorrow. When solar cell is inside the module because the cell is covered by the glass and this glass cover of the cell acts as a greenhouse effect. And because of that greenhouse effect the cell temperature is normally 15, 20 to 25 degree centigrade higher than the ambient temperature. And as a result of that whatever generation that or whatever the temperature that increases because of not extracting is not much. And normally in the field you are never going to let the cell in a open circuit condition. You are always going to extract the power. So, that condition is normally not occur does not occur. And I T C another question what is the difference between the conventional photovoltaic and in actual practice we never keep cell short circuited or open circuited they are always loaded. But when you are doing the short circuit again the power is actually dissipated. When you are keeping short circuited again the power is dissipated in the cell itself. Where is the power going when it is short circuited? When it is short circuited power is again dissipated. Now the current will flow and I square R losses will also occur. Where the I square R losses will go it will go within the cell itself. Optimum is peak power. Optimum is peak power right operate your cell at peak power. So, actually I was asking a question I was trying to answer a question somebody asked when I T C what is the difference between the conventional photovoltaic and satellite photovoltaic cell or the solar cell which is used in space. Now the conventional solar cell which are here the major difference between the conventional cell and the cell which is used in satellite is the efficiency right. So, the conventional cell I have lower efficiency conventional cell and cell used in space right used in space. So, here the efficiency is low and cost is low. Here the efficiency is high and cost is also high. When you are using the solar cell for the ground application or terrestrial application most important concern for us is the cost right. But when you are using the solar cell for a space application the most important concept is the concern is not cost, but the weight right. Because the cost of putting 1 kilogram of space vehicle is actually the main cost. So, the weight becomes more important in putting the cells in a space and therefore, if you are using a higher efficiency cell means you are generating higher power for the same weight right for the same weight will generate higher power and therefore, the advantage will be there in terms of the cost because you have to put the less weight into the space and that is the many times cells like cells which are used in space are gallium arsenide based triple junction solar cell. This kind of solar cell have demonstrated efficiency of over 30 percent for single junction and over 42 percent more than 42 percent for three junctions under light concentration. So, these are much higher efficiency as compared to what typically you can get as a 15 to 16 percent conventional crystalline silicon cell. So, the material is different the material is different the cost of making this kind of solar cell is very high and therefore, it is not affordable to generate the power in the terrestrial application, but if you are putting in space the weight is more important and therefore, cost is affordable. If there is any other question Professor Fernandes is also here if you have any question you can ask question either by chat or by the audio question from NITC again fine. So, let me answer your question what about the tilt angle used in space. Now, I am not sure, but I guess that the panels at the space are always on a tracking mode panels and space are always in a tracking mode not the fixed angle because it is very easy to implement the tracking mode and if you are doing the tracking and if you are in space because your intensity of light is always going to be high your gain in the space because of the tracking will be much higher. So, I guess the panels are continuously tracked and kept perpendicular to the sun rays all the time. Question from Bharamati again. My question is that we design solar cell on the basis of p n junction. So, why not to use transistor topology to avoid recombination in the base region. Can you elaborate more how the transition topology will avoid the recombination? Can you throw some light on that? Yes sir that when reverse bias collector junction there is a more drift current is possible in reverse bias is given more drift current is possible and recombination are avoided in base region. So, to avoid the carrier produced carrier generated and then they recombine to avoid that process can we use that reverse bias collector junction topology such that recombination can be kept at minimum that is my idea. So, why not to go to a higher level topology then the junction simply ok. Yeah let me think about it first of all whenever we want to generate power using solar photovoltaic in no situation we would like to bias any junction right. We want to automatically create that voltages. Second thing is when you are making a diode when you are making a p n junction it is a very large area device. You remember if I showed you that when you do the diffusion it is a process done at 800 to 1000 degree centigrade. If you want to create one more region there may be another diffusion state or the implantation state. So, solar photovoltaic is very sensitive to the cost of fabrication and that has to be kept in mind. So, in a transistor topology because of the bias control over the base you can actually control the depletion and you can control the drift and collection in the collector, but in photovoltaic we do not want to use any kind of bias that can that is not possible right. In order to generate power you should not be applying the power to the cell and therefore, that may not work fine there are no questions.