Solar cells - IV characteristics

let's explore the vi characteristics of a solar cell we've already seen solar cells are pn junctions that convert light into electricity and the way it works is when you shine light on a pn junction the photons that are absorbed in the depletion region cause electron hole pairs to be formed and before they have time to recombine electrons get accelerated towards the n side holes get accelerated towards the p side as a result we have electrons being accumulated on this side causing a negative charge on this side holes get accumulated over here causing a positive charge on this side and as a result a potential difference is created this is how solar cells convert light into voltage photovoltaic effect and we have talked about this in detail in our previous videos on the introduction to solar cells working principles so if you need more clarity feel free to go back and check those out but what we're going to do over here is draw an iv characteristics so let's go ahead and do that so i'm going to do i along the y axis draw v along the x axis and let's try and figure this out now because over here solar cells themselves act like a battery we need to be a little bit careful i used to find this very tricky so let's do this slowly the way i like to start is by thinking about open circuit let's start by not attaching anything okay what's going to happen to the voltage as i keep shining light on it that's what we want to think about we know that as we shine light more and more electron hole pairs are formed and they get accumulated more and more over here and as a result the voltage keeps increasing but my question is if i don't attach anything over here and i keep on shining light would the voltage keep on increasing forever because more and more and more electron hole holes will get accumulated will it never stop will it keep on increasing i want you to pause the video and think a little bit about it what do you think is going to happen eventually all right now as we shine light more and more electron hole pairs are continuously generated and continuously swept and so they are continuously accumulated but what's important to understand is that as more and more holes get accumulated on this side it becomes harder for further more holes to come over here because these holes will start repelling these holes and the same thing is going to happen over here as more and more electrons get accumulated it becomes harder and harder for even more electrons to get accumulated it becomes they go slower think of it as they become slower and slower and slower slower and slower and slower eventually there will come a point where so many holes are accumulated over here that when this hole tries to go over here it gets repelled completely and comes back similarly when this electron tries to go over there it gets repellent and turns back and as a result these electron holes will come back to the deflection region they will destroy each other and so we'll reach a point where there are so much of accumulated charges that electron holes are no longer able to sw you know get swept and so they basically will come back and keep destroying and so we've now reached a point another equilibrium point where the voltage will no longer increase we have now reached a maximum voltage right and so we're going to start plotting let's plot that over here so where should i plot this i know at this stage current is 0. so it's on the current axis we're going to get zero and i have some voltage in fact maximum voltage but is that voltage positive or negative well remember p is positive when p is connected to positive we call this forward bias and forward bias is taken as a positive voltage all right so our graph is going to be positive voltage but zero current so it's going to be somewhere over here right and we call this voltage voltage of open circuit oc because there is no circuit over here this open and this is the maximum voltage you can ever get and what does this voltage really depend on does it depend upon the intensity of light well sure it does but not much you can see even if i increase the intensity of light or decrease the intensity of light after that saturation has reached no more electron hole pairs will be able to be swept across right with more light i will get more electron hole pairs sure but they will still just come back and recombine so this value doesn't depend much on light but this depends on the construction actually if i make this longer i will be able to accumulate even more charges and have even more voltage so anyways this is our open circuit voltage now let's think about what happens when we you know close this circuit and here what i like to do is start by thinking about a short circuit so no resistance so let's say we completely short it okay again what do you think is going to happen there's a path now and there is no resistance can you pause the video and think about what's going to happen and try to plot this point again on this circuit on this graph so pause the video and give this a try all right so now because there's a path available and there's absolutely no resistance all these accumulated electrons there's no reason for them to stay accumulated all of those accumulated electrons will start flowing through this wire and they'll start destroying or not not destroying think of it destroying yeah and recombining with the holes over here and so all the holes will be gone and will now reach a point where for every electron and hole that is formed the moment an electron hole is formed they experience no repulsion anymore because there are no accumulated charges anymore and as a result they're freely you know they're able to freely go through and so i'll now have a very high current and guess what because no more accumulated charges no more voltage so there'll be no potential difference across the two ends does that make sense well if you think about it kind of makes sense right because we know in a short circuit because there is no resistance there cannot be a potential difference across it but logically also now hopefully it makes sense that because there are no accumulated charges there will be no potential difference now so at this point in a short circuit we'll have zero voltage and we'll have a very high current so the question now is is this a positive current or a negative current remember our forward current is taken as positive and forward current is from p to n which direction is this current well electrons are flowing from p to n meaning the current is in the opposite direction so the current is this way inside the current direction is from n to p and so this is a negative current so let's plot that on the graph now we have zero voltage but we'll have a negative current so it'll be somewhere over here and this is what we call the short circuit current and again this will be the maximum current i can ever get the reason for that is because this is the only possible case where i have no accumulated charges so no repulsion and because of that i get the maximum you know maximum current over here all right what does this current depend on well this current purely depends on the amount of light that is falling think about it if 100 photons are falling are being absorbed i will have 100 electron pairs being created per second and i'll have 100 electrons flowing over here per second if i double that number automatically this number will double and so the current is completely governed by light if i double the light intensity this light will this current will increase if i get rid of that light the current will go to zero there will be no current makes sense right okay now let's think about what happens in between you can kind of think that the graph has to go from here to here but let's try to do this logically if i want to get a case in between i need to consider a resistance somewhere in between this was open circuit infinite resistance this is short circuit zero resistance so let's try to put in some resistance now non-zero resistance so let's say i put in a very tiny resistance over here very tiny so i'm going to show it this way very tiny resistance what's going to happen again i want you to fast first and think a little bit about this all right let's see now since there is a some resistance over here when current flows through a resistance or at least think of it this way for current to flow through a resistance there needs to be a potential difference and how do we get a potential difference here the potential difference can only happen if there are some accumulated charges right so now it's going to happen to push the current through this resistance some electrons will get accumulated over here some holes will get accumulated over here that will cause the that will provide the required potential difference and now once the potential difference is established current can keep flowing and notice because there are some accumulated charges these electrons will experience some repulsion these holes will experience some repulsion and immediately as a result you can now see the current will reduce a little bit so the current now is going to be little less than the short circuit current and now we have some positive voltage so if you want to plot this where would it be i have some positive voltage very little i have current which is little less than the short circuit current so it's going to be somewhere over here and so my graph is going to look somewhat over here all right what if i increase that resistance a little bit more so i'm going to increase that resistance a little bit more what's going to happen if i have more resistance i require more voltage to pass current through it and so to get more voltage more charges get accumulated the potential difference increases even more and as the potential difference increases the repulsion increases and as a result the charges slow down uh it starts slowing down and so what happens to my current the current reduces even more so you can see where we're going with this as i increase the resistance the voltage starts increasing the current starts reducing and so hopefully you can now see what happens to our graph the graph is going to look somewhat like this so this is the graph of the solar cell before we conclude i just want to go a little bit beyond just this quadrant this fourth quadrant because this will help us look at the you know photodiodes in general bigger picture we'll get to see so let's come back to this point it's the short circuit so let me just get rid of this and put a short circuit over here let's get rid of all of this remember what happened in short circuit in short circuit we said there's not going to be zero voltage there'll be no voltage whatsoever and all the electron holes that are formed immediately start flowing giving us a maximum current right okay what if i want to go beyond this point what if i want to go towards the left how do i do that i need to somehow put a negative voltage there's no way i can get a negative voltage on a solar cell so solar cell can't do that but what if i attach a battery and reverse bias it then i will get a negative voltage right and now if i do that i will still keep getting the same current because the current purely depends upon light and so if i go beyond this i'll keep getting the same current and you may have seen this before this is our photodiode now in this region this is acting like a photodiode i have reverse biased it and i'm getting that same maximum current all right how do i go beyond this point well to go beyond this point i need to get a positive current there's no way my solar cell can give me a positive current to do that i have to forward bias my diode by adding a battery so if i do that i add a positive here negative here and if i do that i will now forward bias and i'll start getting a positive current and if i do that i will get a forward current it looks like this does this graph look familiar to you it should because this looks very similar to the normal pn graph pn junction graph vi characteristics that we have seen before only difference is it would have it was upwards so the normal pn graph was somewhat like this at zero voltage i get zero current and then this is what i this is what we got so notice this means when you shine light on a pn junction you shift that entire graph down the more light you shine the more you shift it down