How Do Solar Panels Work? (Physics of Solar Cells)

How the heck does that work? Thanks to Brilliant.org for sponsoring this
episode. Hey Crazies. It’s time for another episode of
“How the heck does that work?” I’m sure many of you have wondered over the years: How does solar power work? Well, today’s your lucky day! To help out with this, I’ve designed a special clone just for this video: Solar-Powered Clone 20% OK, maybe we’ll come back to him later. First, let’s get some lingo out of the way. When I say solar panel, you're probably
picturing something like this, or maybe even this.

But these are called solar arrays. They’re collections of solar panels. A single panel or module looks like this and that panel is made of even smaller parts called solar cells. These cells are where the solar power comes from. Technically speaking though, power sources don’t actually provide power. They provide energy in the form of voltage,
but we’ll get to that later. That reminds me: Hey Solar-Powered Clone, how are you doing? 30% Hmm, maybe he could have been designed a little better. Anyway, let’s get into the structure of
one of those solar cells. They might look like one thing, but there
are several layers. A metal plate on the back, two different types
of semi-conductors, a metal grid, an anti-reflection coating,
and piece of glass. Now, that’s a lot of layers, but this is all
about turning light energy into electrical energy. So let’s follow the light.

The first layer the light encounters is glass. Glass is an insulator, so it’s not going
to conduct any noticeable electricity. It’s also transparent, so most of the light
just passes through. The reason it’s there is to keep everything else out. The other layers a kind of fragile, so the
glass protects them. Next is the anti-reflection layer. That’s the layer that makes the solar cell look dark. Doesn’t that mean the light never gets through? Oh, no, it lets the light in. It just doesn’t let it back out. Really? How does that work? This coating is so cool. Let me explain. The semi-conductors underneath are a bit too shiny. If they were exposed, over 30% of the light would just reflect away. That simply won’t do. If we want to use something like this on the large-scale power grid, we need it to be as efficient as possible. The glass already reflected away about 5% of the light and we’re going to lose a bunch of it to heating.

We can’t really afford to lose much more. The anti-reflection coating helps us hold
onto what we’ve got left and it works like this. You can’t just coat it in black; otherwise,
all the light would heat the cell and you wouldn’t get any electricity. This coating has to be transparent. It must let the light pass through. But what happens is you get a reflection off
the top and the bottom. If the coating is just the right thickness,
the two will cancel and the reflected light disappears,
leaving only the incoming light.

All of it? Unfortunately, no, but most of it. The amount of cancellation is wavelength dependent, but it’s the best we can do. OK, so both the glass and the anti-reflection
coating solve some practical issues, but you’re not here for practical issues. You probably want to hear how a solar cell
actually generates electricity. That’s where the semi-conductors come in, Terry Lee. The best one we’ve got is silicon, right
in the middle of the chart. Full insulators won’t work because the jump to conduction band is too big. Conductors won’t work because they’re already conductive. We want the incoming light to make it conductive. Semi-conductors need a boost to become conductive,
but only a small one. A little visible light will be just enough. There’s a slight itty bitty obstacle for us though. Silicon is four from the left on the periodic table. One. Two. Three. Four. That means each silicon atom only has four electrons in the valence band. All four of which get used up when they bond
to each other in the semi-conductor.

The incoming light might break some of those loose, but it’s not enough. Solar-Powered Clone knows what I mean. 65% Man, I should have at least pre-charged him for this. Anyway, pure silicon isn’t going to be enough. We need to enhance it using a process called doping. Well, yeah, kind of like that. We are technically injecting something that
doesn’t belong to enhance performance. But, unlike sports doping, this is totally legal. Where was I? Right! Enhancements. We need extra electrons that aren’t part of a bond. The number of valence electrons is equal to how many columns over we are, so we just need to step over one more column to phosphorus. That’s four electrons for bonding and one extra for us to move around. We still want this to be mostly silicon, but
if we mix in some phosphorus, we get some spare unbonded electrons to work with. The incoming light can excite those electrons up to the conduction level, but that’s not going to do us any good if we don’t have anywhere for them to go.

One step to the left of silicon, there are only three valence electrons. That gives us a hole or opening that our extra electrons will want to fill. Unfortunately, aluminum atoms are a little too big to fit inside the silicon, so we take one step up to boron instead. They’ll fit quite nicely in the silicon
and leave us some electron holes to work with. The two of them together are the key to how a solar cell works.

Separate, the two types of silicon are neutral. There’s no net charge on either one. But the moment the phosphorus-doped silicon touches the boron-doped silicon, there’s a mad rush of electrons from the phosphorus to the boron at the boundary. This creates an imbalance of charge inside the solar cell. Some of the phosphorus are now positively charged because they’re missing electrons. and some of the boron are negatively charged because they have extras. Any imbalance of charge will give us an electric field or, more importantly for circuits, it will give us a voltage, which is just an amount of energy per unit charge. When more electrons are ready to move, the voltage tells them which way to go. Pretty quickly, that initial rush of electrons
forms a barrier between the sides. They reach an equilibrium and stop flowing. They’ll only move again if we give them
energy to move.

Energy from incoming light. We just need a couple conductors to connect
the silicon to a circuit and, BAM!, you’ve got yourself a solar cell. Why does the top one have that grid shape? Oh, yeah, that makes perfect sense. I promise. The back conductor is a full plate, but the
front conductor has to leave some space. Not enough space and the light can’t get
through to the silicon. Too much space and the electrons have to travel
too far along the silicon. The grid pattern is a happy medium between the two. A happy little conductor. I’m such a dork. So the silicon-pair separates charge and gives us a voltage, but the voltage of an individual solar cell isn’t actually that high. It’s only about half of a volt. For comparison, a double-A battery provides 1.5 volts, and a wall socket in the U.S. provides 120 volts give or take. If we want to use solar cells in the power grid, we need a lot of cells in a panel
and a lot of panels in an array.

Of course, there are other obstacles to consider too. Solar cells only provide DC, so we have to convert it to AC. But that’s not really a big problem with one of these. Sunlight can vary from moment-to-moment
or place-to-place, so we need an adequate storage device. But we have those. They’re called batteries and capacitors. Why use carbon fuels, which mess up our atmosphere and ocean, when we have a giant nuclear furnace releasing a seemingly endless supply of light energy. It’s 1000 W of power for every square meter of Earth. We should be taking advantage! So, are you ever going to look at solar panels the same way again? Let us know in the comments.

Thanks for liking and sharing this video. Don’t forget to subscribe if you’d like to keep up with us. And until next time, remember, it’s OK to be a little crazy. Okay, I’m ready! Dude, I just finished. Are you interested in Solar Energy? Then check out the Solar Energy course on brilliant.org. Brilliant a problem-solving website that teaches you to think like scientist. Knowing physics as well as I know it takes
more than just watching videos. It takes loads of practice. Brilliant gets you to solidify concepts by
giving you fun and challenging problems. They even have hints and solutions to help out along the way. It's a service you don't really see anywhere else. If you're interested, visit brilliant.org/ScienceAsylum and the first 200 people to sign up will get 20% off an annual premium subscription. It's a really good offer if you want to strengthen your knowledge. Taral Patil and George Charney pointed out
that my current was going the wrong way.

That’s not actually true because I wasn’t ever showing the current. I was showing the direction of electron flow, which is opposite the current. Ben Franklin made everything so complicated. Anyway, I’ll address this again when we get to circuits. Thanks for watching!.

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