[SOUND] [SOUND] Wouldn't it be amazing if I could take some piece of material an I just stick it in the sun and electricity comes out? No moving parts, no fuss, no muss, no maintenance needed. Well, that sounded like science fiction long ago. But actually, today it's reality. A solar, photovoltaic panel. Now, while we use these on more and more applications, and in some remote places, this is the only source of their electricity. The key is to understand, how does it actually work? One of the key things is it's made out of a material called a semiconductor. And for most cases, this is silicon. So what's special about silicon? Nothing in of itself. A semiconductor means it's something that can conduct electricity as long as it's not at absolute zero. Metals conduct electricity all the time. Nonmetals, insulators don't conduct electricity at all. To make a solar cell, we don't just need silicon. We need silicon first as a single crystal. There's some more advanced ways to do it but let's start illustrating that. So silicon has four bonds and that means each silicon atom is bonded to four others. So this crystal goes on. Well, silicon all by itself doesn't do anything particular when you shine light on it. But a very interesting observation was made. You can imagine if you were a scientist and you were dealing with materials, you might want to make a collection. Let's put all of the elements, at least all the solid elements, in a box. And maybe a box shaped like the periodic table, but who's got one of those? So at least we'll put them next to each other. And we'll go across and we got your carbon and then we have on the next row, we've got silicon and next to silicon is phosphorous and you go on. And since you're trying to do experiments on these things, you one day pick up your piece of silicon and you attach some electrodes and lights coming in. And lo and behold, even though you use those electrodes to try to put current in and measure its conductivity, after all it's a semi-conductor, kind of an interesting thing to study. You discover that while it's just sitting there, it's making electricity. And then, you comeback and do that experiment. And it's night time, and it's not making electricity. And you say, but wait, I swore during the day, it was doing this. The next day it's doing it again. And you figure out, my God, light is converting something in this rock to electricity. You try another piece so you do this. It doesn't work, it doesn't work. And you think, you know what? I've been storing my silicon next to a piece of phosphorus. You see, to make a solar cell, not only do we have to start with a semiconductor, we next need to dope it. We have to do doping. It means something like 1 out of 10,000 silicon atoms needs to be replaced by something else. By a phosphorus atom. Now, silicon has four bonds but phosphorus has five. And if it replaces one of the silicons in this whole lattice, it doesn't have any place for this extra bond to go and the extra bond is a free electron. So this is now a mobile electron. It increases the conductivity. Don't get me wrong. It is still electrically neutral along with this mobile electron is an extra positive charge that's stuck to the phosphorus. So we haven't added net electrons. What we've done is we have created more mobile electrons that are allowed to move around. Great, we've now made slightly impure silicon, and it has a higher conductivity. In fact, we call this type of silicon n-type because it has negative charge carriers. Still though you might think, interesting science experiment, how does this make light. Imagine you're in a park and you've got a bunch of young parents. So we've got adult here, an adult here, an adult here, an adult here. And they're all sitting around, because maybe there's a keg of beer or a picnic table. And these adults all have young children with them. So when they walk into the park, right? The young children, my little pink dots here, hand in hand with mommy or daddy. Now, what's going to happen? Are a bunch of young children just going to stay there right next to their parent? The positive nuclei in the silicon and the free-spirited mobile electron. No way, they're going to start to move. They're going to diffuse. They're going to wander away. All right, they're going to go play. We are going to have electron diffusion. But you see, those children are not going to end up over here, across the street. Because after all, they're young children and either the parents are going to say, Johnny come back. Or the children are going to get a little nervous and scared, if they get out of sight of the adults. The same thing that happens here. The electrons are more mobile. They're going to diffuse away. But still, these have negative charges. And these parents back here have positive charges. And there is an electric field, just like that tug of a family bond, these electrons will not stray too far from the group of positive charges. How does that give us a solar cell? Say, this is the top of our cell. And what we're going to do is we're going to put n-type silicon, okay, on the very top. In other words, we're going to have some phosphorous diffuse into the silicon. And let's do this on just plain old intrinsic, meaning regular, silicon. There are more advanced ways and we can do it and maybe make it an even better cell. But right now, let's just deal with this. So here you've got your positive charged silicon atoms in some regular lattice. And occasionally you've got the phosphorus atom, right, that has an even extra positive charge. And you have some mobile electrons. Initially, they're all just fine but remember, there are more mobile electrons in an n-type layer than down here in the intrinsic silicon. If I have more things that are free to fuse here than here, I have a density gradient. It means some of these electrons will diffuse down into this direction. Some will just move a little ways. When this happens, just like our children at the park, we now have a charge imbalance. And we have an electric field. An electric field that is trying to draw those charges back. Electrons are negative, so the electric field actually points this direction. But we have now created an electric field at this junction. And we haven't done this by doing anything more than doping only the top layer of the silicon. The guy in his lab, the phosphorous was next to the silicon on the very surface of the silicon. Some phosphorous atoms got in there, replaced the silicons in the lattice over time and voila, you now have a system that gives us a narrow band that has an electric field. Still, we haven't made electricity. Next step. So here, again, is our n-type layer. And then, here is our regular silicon. But notice we're going to have kind of a region in here that has this big electric field pointing through it. This region will not have mobile charge carriers. It will be depleted, why? Because since there's an electric field present, if I did have a mobile charge carrier in this region, it would respond to the electric field and move. This is set up all by itself, electron diffusion leads to a depletion region. So now, let's introduce the sun. Here we go, here is my sun, the sun is happy is wearing sun glasses and we've got a bright sunny day and we've got sunshine coming down to our solar cell. Photons have energy. In fact, they have enough energy to break the bond of an electron to its nucleus. Let me say that again. If I have the sunlight come down, I can produce an electron ion pair. This happens all the time. Right now, the sunshine, if it's shining on you, cells in your body, the atoms in those bodies, the photon comes down and it can break a bond between the electron and the ion. Ultraviolet radiation has higher energy, it breaks more bonds. That's a good thing why you use sunblock or why we have the atmosphere that stops lots of our UV radiation from coming through. But here, in the solar cell, we have the photon come down, and you know what, let's say it gets absorbed there. It will now make an electron ion bond, but in this region, back here in the n-type region, there's no electric field. These things just combine back together. Well, let's say we had a photon that gets absorbed in the depletion region. Now, I have an electron and I have an ion but they are in an electric field. So what will happen is the electron will feel a force this direction. And the ion will feel a force this way. So if I produce an electron-ion pair in the depletion region. I will get movement, Of charge, and movement of charge is equal to electricity. Simply by putting a little bit of phosphorous on top of the silicon, sticking it in the sun, I can separate out charge carriers. Gotta do a little bit more, we have to have something to collect this. We need to put some kind of electrode, basically a wire on the top. And maybe on the very bottom of this whole solar cell, let's put a really thick pad. Maybe cover it all with some kind of metal, right? So this is also going to be some conductor down here, metal. And if we connect this wire to this wire, we could make, A lightbulb. We get movement of charge. Movement of charge is electricity. A simple, wonderful application of solid state physics. Discovered predominantly by accident. All right, here's something very, very bright, sorry about that. Here is really truly amazing. A flat piece of material, has no gears, no water going into it, no nothing. It sits there, light hits it and electricity comes off, enough electricity to run this little fan. So a solar cell, note a couple of things about this. First, remember I had drew that little wire that had to be on top to collect the charge carriers coming to the surface? Those are the wires. See them going across and they blend into these large wires. Those large wires is what would be connected to an external wire to give you the voltage. Why is it blue? Well, we want to absorb all of the light. So we want to make sure to get the highest energy photons, and the lower energy photons, just about all of the photons that come in, we don't want them to bounce off. It would be a terrible solar cell if it was white. Ideally, it should look black. No light should be returning whatsoever. So if you texture the surface or do other things, you can improve its light collection and therefore improve its efficiency. You can take a single solar cell and you can join it to more, making a solar module. Put the solar modules, you get a solar panel. Put a bunch of solar panels, you get what's called a photovoltaic system. And if you live in a sunny climate, you can put lots of these and you can actually generate significant amounts of electricity. Here's another array in the field, clearly you want them to be pointing to where the sun is. And even you can have something so enormous that you can take a picture from space and still see it. That's how solar cells work. In the next segment, we'll talk about what they cost. [MUSIC]
