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Solar cells - working (and difference from photodiodes)

Let's explore the working principle of solar cells (photovoltaic cells), and how it's different than a photodiode. Created by Mahesh Shenoy.

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  • duskpin ultimate style avatar for user autumn
    why don't the electrons moving towards the n region get repelled by the electrons already present there?
    (2 votes)
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  • blobby green style avatar for user explerdummies
    If you keep shining light on the Solar cell without connecting a load, where will the created electron goes? Will it create a sort of capacitor? Because I think they can't go back the other way to recombine with each other.
    (1 vote)
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    • blobby green style avatar for user uniqxlel
      The electrons and 'holes' would just stay on the N side and P side respectively. They can't move due to the Electric field generated by the depletion region.

      I don't think it will create a capacitor as 'holes' aren't exactly protons. They're only similar to protons in the sense that they react to positive and negative Electric fields like a proton but they aren't attracted to electrons. So no, it won't create a capacitor and the Semiconductor will just be a Semiconductor with a lot more 'holes' and electrons.
      (1 vote)
  • blobby green style avatar for user harshalik2005
    Is the diode used for solar cells forward or reverse biased?
    And will it act the same (forward or reverse biased)in presence of sunlight and dark ?
    (1 vote)
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Video transcript

let's explore solar cells and see how they work so solar cells are basically pn junction diodes they are semiconductor devices pn junctions which convert light light into electricity electricity and it can be any light but since usually we tend to use sunlight because it's available in abundance we call them solar cells and you probably know where these are used for example they have a lot of applications for for example you probably use them in calculators these are the solar cells you have big solar panels use generate electricity to power up houses and you also have solar cells in space probes in outer space this is one of the best way to generate electricity there so these panels over here these are all containing solar cells but how do they work well at the heart of a solar cell is a pn junction so let's quickly recap we've seen that pn junction is basically a single semiconducting crystal on which one side it is doped with what is these these are acceptor impurities so they accept electrons and as a result they leave behind holes a lot of holes and on the other side we have donor impurities they donate electrons so they leave give so they leave electrons okay and the region in between is where electrons and holes destroy each other causing a depletion region this depletion region acts like a barrier and it doesn't allow further diffusion of and recombination of electrons and holes and we've spoken about this in great detail in previous videos and pn junctions if needed refresher feel free to go back and check them out okay but the question now is what would happen if you were to shine light over here what happens if we shine light on our solar cell well let's go to our band structure to understand what happens if you look at the band structure of a semiconductor and if you look at it near the depletion region and we'll see why we're talking about in the depletion region because there are no charge carriers over here we know that the valency band must be completely full there are no holes over here and the conduction band must be absolutely empty no electrons over here so there are no charge carriers over here at all in the depletion region but when a light falls over here and if the light photon has sufficient energy okay if it has energy more than the band gap then some of these electrons can absorb that energy and use that to jump into the conduction band now when this happens outside of the depletion region when the electron hole pairs are formed outside of the depletion region immediately the electron will recombine with that hole and we'll release that energy back and so that process would be useless we do not get any electric generation because of that however when that happens inside the depletion region that's when things get interesting so imagine an electron hole pair is formed inside the depletion region let's say this is a electron and this is the hole that is formed due to sunlight due to the photons light photons okay now before they have a chance to recombine look at this electron this electron gets attracted by this positive charge and therefore it gets accelerated in this direction this hole similarly is attracted by this negative charge or you can say it's repelled by this positive charge and gets accelerated this way so before they have a chance to recombine they are swept across due to the field in the depletion region this is the electric field generating depletion region and as a result when electron hole pairs are formed over here the holes will get accumulated they'll get swept and they get accumulated in the p-type and when the holes get accumulated over here a lot of positive gets accumulated over here so the p-type tends to tends to become positively charged and if you want to really think in terms of electrons you might say hey holes are not real things right so what's happening over here well think of it this way holes are absence of electrons so electrons are being removed from this side and when you remove electrons from a side you end up with positive charge similarly electrons are getting accelerated over here and they get piled up over here and as electrons get piled up over here this side becomes negative and so this point gets negative charge and now notice a voltage is generated between these two sides and this is how in solar cells we use light to generate voltage and that's what we call the photovoltaic effect let me write that down this is what we call photo voltaic effect the name makes sense right we're using photons to generate voltage photovoltaic effect and now what would happen if i were to put metallic contacts over here and then connect a device across it say a bulb well then these electrons sorry these electrons will all repel each other and immediately start flowing through this circuit through the bulb and will recombine with the holes so the electrons will continuously recombine with the holes and as that happens more electron hole pairs are generated and more electrons and holes get accumulated over here and so notice there will be a constant there will be a continuous and constant current supplied by this the bulb will glow and notice this circuit does not have a battery the pn junction itself acts like a battery and that's why it's called a solar cell and so just to quickly summarize what's the working principle the photons absorbed in the depletion region cause electron hole pairs and before they have a chance to recombine they get swept due to the electric field and as a result charges get accumulated causing a voltage the photovoltaic effect and when you put an extern when you connect an external circuit that voltage is going to make the electrons go through the whatever device you have over here to continuously recombine with the holes and causing a current that's the whole idea behind this and that's how you power up all these devices now before we conclude one confusion i always had was how are solar cells different than photodiodes something we saw in a previous video they're also having very similar principle right well short answer is they're basically the same things so even there you're using photons to create electron hole pairs which gets swept across by the electric field the difference is in the application so over here we allow the charges to accumulate to generate a voltage because we are using this as a cell but in a photodiode if you remember in a photodiode we do not allow the charges to get accumulated and the way we do that is by reverse biasing we attach this external cell what happens when you reverse bias the moment electron hole pairs are formed electrons are immediately sucked through this battery and then allowed to recombine with the hole almost immediately and that's why there is absolutely no electron hole accumulation over here so there is no voltage generated over here why do we do that because over here the idea is not to uh generate voltage we're not using this as a cell the idea over here is to generate a current that is proportional to the intensity of light if there are 100 photons falling per second i immediately 100 electrons going over here per second if there are 1000 photons from falling per second i immediately want the current to increase 2000 electrons per second you get the idea i want the current to fluctuate with the light that's the application and you can you know we've talked more about this in the previous video so the basic difference is we do not allow the charges to accumulate over here by reverse biasing we allow the charges to accumulate and generate a voltage and of course there will be differences in the construction the material and even the iv characteristics something that we'll talk about in the future video