Solar cells - fabrication & material's used

let's explore how solar panels are constructed and how do we decide what materials what semiconductor materials we use to build them now in a previous video we've already talked about the working principle of solar cells just to recap the idea is when light falls on a pn junction semiconductor if the photons have enough energy more than the band gap electrons can absorb that and jump into the conduction band creating an electron hole pair now if this electron hole pair is generated in the depletion region then they don't have any chance before they have a chance to recombine they will get accelerated in the opposite direction that causes a charges to get accumulated generating a voltage this is what we call the photovoltaic effect and now this voltage can be used to power up some device and we've talked and spoken more about this in a previous video so if you need a refresher feel free to go back and check this out but now let's take a look at a real pn junction semiconductor this is what it would look like and notice you there's a problem over here you see a lot of photons we are getting absorbed over here and the top region depletion region is near the junction far away most protons will not reach over there which means a lot of photons are getting absorbed over here in this region which causes a lot of electron hole pairs in this region but important thing is they will just recombine there is no field to accelerate them they will recombine if we want photovoltaic effect electron hole pairs need to happen in the depletion region right which is far away so the question now is how do we ensure that well we need to make sure the depletion region is very close to the top surface and the way to do that is to make sure whichever side is exposed to the sun we ensure that side is very thin so over here inside is exposed to the sun so we make the inside very thin and so notice usually solar cells will have one side the side exposed to the sun very thin and the other side very thick all right the second question is around providing metallic contacts if i have to connect it to any device i need to put some metallic contacts now how do i do that over here i have to put one metallic contact at the top one at the bottom but at the top i have to also make sure there is enough space for absorption of light so one of the ways in which we can do this is we can put a very thin wire at the top and we can put a very thick sheet at the bottom and then we can put attach this to a bulb but the problem with thin wires is that thin wires have very high resistance think about it a lot of electrons are accumulated over here but for them to go through this bulb there's only one thin path over here so having thin wires is great for absorption of light but they will have very high resistance so how do you reduce the resistance well to reduce the resistance you have to make this wire as thick as possible so maybe you make it very thick like this this is great and this reduces the resistance but now notice this decreases the area available for absorption of light and so here's where you see a problem we need to be able to find a nice balance between having large enough thick enough wire thick enough metallic contact but also having high enough area one of the ways we solve for this is by having finger-like projections so instead of having a thick sheet like this what we'll do is uh we will have fingerlike projections so this allows there's sufficient metallic contacts so a lot of electrons can very easily get absorbed through these tiny finger like projections and go through the through the bulb but there are also sufficient gaps in between which allows enough sunlight to be absorbed and if you were to look at this very closely we will be able to see that so let me show you a close-up of one of the cells so this is a single single solar cell right if i were to if you zoom in over here let me let me zoom in you can see this these thin wires are the finger like projections they are the ones that increase the metallic contact area and this thick wire is like the wire over here that collects all the electrons so it's called the bus it's sort of like a bus right collects all the electrons from these thin fingers and if you look at a solar panel you'll see that a lot of solar cells are connected in series and parallels there's a series connection here and there's a parallel connection and that's how you get a very nice voltage from this entire solar module all right the last question we have to think about is what materials we use to construct this how do we decide that the primary factor is the energy of the photon and the band gap the photons energy must be larger than the band gap for the electron to be able to absorb it and go to the conduction band right so for solar panels our question could be hey what's the energy of the photons coming from the sun well it turns out that the photons energy of the the energy of the photons of the sun there is a spectrum over there from very low energies to very high energies well then the question could be okay what about the majority of the photons that we get what is the energy of those photons it turns out that if you do that calculation it turns out that the majority of the photons will have an energy which is somewhere close to one and a half electron volt rough numbers okay this means that there is a very high chance that the photons that you get from the sun has this much energy one and a half electron volt okay with this info let's now take a look at few materials which are semiconductors and see which one is best suited to construct a solar cell so these are a few semiconductor materials german and silicon you might already have heard of them and these are some compound semiconductors we used some of these in our leds and we've talked about them in our led videos so here are the band gaps can you pause the video and think about which of these would be best suited for absorbing sunlight and converting that into electricity can you pause and give this a shot right if you're given this a shot let's see we can see that there are semiconductors which are very high band gap semiconductors which have band gap close to what we want and there are also few which are less than what we want so let's consider all the three cases and work look at them one by one all right if you look at this one then the photons don't have sufficient energy for the electrons to jump the band gap and as a result electrons will not absorb that energy photon will just pass through no photovoltaic effect no electron hole pairs are formed and so high band gap semiconductors are useless for us so we're not going to use them what about over here here the photon has just the sufficient amount of energy to create an electron hole pair and so this works for us so this will jump electrons will barely be able to reach the conduction band and they will jump all the way to the bottom of the conduction band and so electron hole pairs are formed so this is great what about this one well even here photons have more than sufficient energy to propel the electron into the conduction band so electrons over here absorb all of that 1.5 electron volt and as a result they will not jump here they will jump to the same level as before so they will jump somewhere over here because they have absorbed 1.5 electron volts and because there are lower energy levels available immediately the electron will immediately relax and come to the bottom of the conduction band this happens because electrons always like to go to the lower energy level whenever it's available here immediate lower energy levels are not available but here it is and so when it relaxes from here to here that difference in the energy it gives out as heat which means when band gaps are much smaller than the photons energy some of the light energy is converted into heat so where do we have the most efficient conversion of light to electricity that's happening when the photons energy is very close to that of bad gap and in our example that happens to be the case for gallium arsenide so to construct the most efficient solar cell we have to use gallium arsenide so gallium arsenide is definitely used in construction of solar cells but silicon is also useful even though silicon is less efficient as a solar cell it's much cheaper okay it's much cheaper because it's readily available right gallium arsenide is not you have to you have to make it you have to mix gallium and arsenic together and so because of because silicon is cheaper even though it's less efficient it works out to be cost effective germanium is something that is just way too low and we don't use that