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With all of the talk about green energy, there obviously has been a lot of interest of in solar power as well. But not many people know how solar cells even work. Sure they know that they produce electricity when light hits them, but that’s about the extent of their knowledge of photovoltaics (another name for solar power producing products). Are you one of these people? If so, fear not, keep reading and you’ll understand soon enough!

First of all solar cells are always made of specific materials (usually silicon) because of that material’s electrical conductive properties – they are called semi-conductors. A semi-conductor is a material that doesn’t conduct electricity very well (like metal – a conductor does), but it doesn’t resist it either (like piece of plastic does).  So electricity can pass through them but not super easily. The reason that solar cells are made of semi-conductor material is because that is the only way to take advantage of something called “doping.” But first you need to understand how light transfers energy.

Light can be thought of in two ways, it can be thought of as traveling in waves (like radio waves for instance), or as traveling in little packets of energy called photons (like a bunch of balls flying through the air). Now, hopefully you already know this but if you don’t, sunlight (any white light actually) is really made up of all of th different colors of light put together (like a collapsed rainbow). So If you think of light as waves then the different colors of light are different sized waves. And if you think of light as photons, then the different colors of light are different size balls. The larger the wave or ball, the more energy is contained in the light.

Now I’ll explain what doping is. We have the ability to make materials with a specific amount of one thing or another in it. So the silicon cells that we make for solar cells aren’t really just silicon. There are at least two different materials that make up a solar cell. The two types can both be silicon but they will be “doped” with another element (boron or phosphorous for example) so that they have slightly different electrical properties. The silicon material that is doped with boron makes that material have less electrons in it than normal (p-type), and the silicon material that is doped with phosphorous makes that material have more electrons in it (n-type). A layer of p-type material is placed onto the n-type material. That’s the main part of a solar cell.

This two-layered cell is what produces the electrical current in a solar cell. This is how it works. The n-type material is pointed up and the p-type side is on the bottom. Now I’m going to explain this as if light is made of photons (little balls) because it’s easier to understand that way. So because the n-type side of the cell is pointed up, the sunlight hits that first. Depending on how heavily doped that material is, only certain size photons will actually hit an atom in the material and push the right amount of energy into it that it shoots off one of it’s electrons. So some of the photons go right through the top layer and into the bottom layer where they hit an atom with just the right amount of energy to make it shoot off another electron. That free electron goes from the bottom (p-type) layer into the top (n-type) layer. There are also photons that hit atoms in the top layer and give them enough energy to an atom in that layer to shoot off an electron. Some of them go back to the same atom they came from or to other atom that is missing one, but some of them will just float around for a while until they can go somewhere else. Now since there are free electrons coming from the bottom layer into the top layer, and from the top layer itself, there are a bunch of extra electrons in the top layer that are ready to go somewhere.  So now all you need to do is connect a wire to the top and the other end of that wire to the bottom, and you’ll give the electrons in the top a way to get out of that layer and go into the bottom layer where they came from. There you go, now you have electrons moving from one end of a wire to another, which is an electrical current, which is electrical power!

Now that you unerstand the core process, I’ll explain a few more details about solar cells. First of all putting a wire on one side of a solar cell isn’t very efficient since they are pretty darn big compared to the size of electrons, so the electrons from a corner of the cell would have to travel pretty far through a lot of material in order to get to the wire. That means it has more of an opportunity to simply fall back into an atom that just had an electron shoot off from it and not actually even make it to the wire. So to get around this, two things are done. First of all the top of a solar cell is covered with a layer of a special kind of material that is both transparent so light can pass through it, and conductive so the free electrons will have a place to go to form a current. Also a grid of wires is laid over that transparent conductive layer so the electrons can get from that layer into a wire more quickly and easily and be routed off to whatever device it’s hooked up to. 

Another important thing is that the bottom of the solar cell is covered with a layer of metal so that the electrons can get back from the other side of the wire to the bottom layer of the cell. It’s metal because it’s very conductive, and it’s reflective so any light that gets all the way through both layers of material will bounce off the metal and go back through both layers of material again, giving it more of a chance to hit an atom and knock off an electron.

So now you can think of a solar cells as a peanut butter and jelly sandwich. The bottom piece of bread is a sheet of metal, the peanut butter is the p-type layer, the jelly is the n-type layer, and the top piece of bread is the transparent conductive material/wire grid and layer of glass. 

Now I did skip a crucial part of how solar cells work which is the junction between the n-type and p-type material layers because it’s a little more difficult to understand. But it’s what makes the electrons from the bottom layer go to the top and the free electrons in the top layer not go back through that junction into the bottom layer. If they did, then the solar cell wouldn’t work. Basically when you put the two types of materials together, the bottom of the top layer gets a slighly positive charge and the top of the bottom layer gets a slightly negative charge. That makes electrons drawn towards the bottom of the top layer (and therefore go into the top layer) but repelled from the top of the bottom layer (and therefore not go back into the bottom layer). 

So now that you know how photovoltaic solar cells work! See how smart you are?!  =)