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

April 22, 2020

How the heck does that work? I am sure many of you have wondered over the years: How does solar power work? Well, today’s your lucky day! I’ve designed a special clone just for this case: Solar-Powered Clone 20%. OK, we’ll come back to him later. First, let us get some lingo out of the way. When I say solar panel, you are probably picturing something like this, or maybe even this. But these are called solar arrays. They are 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 do not 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 us 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 is a lot of layers, but this is all about turning light energy into electrical energy. So let us follow the light.

The first layer the light encounters is glass. Glass is an insulator, so it is not going to conduct any noticeable electricity. It is also transparent, so most of the light just passes through. The reason it is 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 is 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 will not 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 cannot 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 cannot just coat it in black; otherwise, all the light would heat the cell and you would not 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 is the best we can do. OK, so both the glass and the anti-reflection coating solve some practical issues, but you are not here for practical issues. You probably want to hear how a solar cell actually generates electricity. That is where the semi-conductors come in, Terry Lee. The best one we have got is silicon, right in the middle of the chart. Full insulators will not work because the jump to conduction band is too big. Conductors will not work because they are 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 is 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 does not belong to enhance performance. But, unlike sports doping, this is totally legal.

We need extra electrons that are not 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 is 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 is not going to do us any good if we do not 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, aluminium atoms are a little too big to fit inside the silicon, so we take one step up to boron instead. They will 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 is no net charge on either one.

But the moment the phosphorus-doped silicon touches the boron-doped silicon, there is 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 are 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 will 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 has 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 cannot 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 am 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 is 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 is 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 are 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 is 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?


 

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