Episode 2 - Solar Cells - Show Notes

Solar Cells –

Hello, and welcome to episode two of the How Things Work podcast. My name is Jamie Aycock, and in each episode we’ll explore the inner workings of things we see and use every day. In this episode we’ll explore how solar cells work.

Before we discuss the details of the solar cell, we need a little background. Silicon has 14 electrons arranged in three shells. The inner two shells are full, but the outer shell has only four of the eight electrons it can hold. Now, our friend silicon is preoccupied with trying to fill up his outer shell – he (or she, I didn’t really look) is always trying to find 4 more electrons. Lucky for silicon, he’s willing to share. Bring two silicon atoms together and each atom will share one of its outer shell electrons with its neighbor– each atom would now have five electrons in its outer shell.  Expand that idea out to having each silicon atom share outer shell electrons with four neighbors and each silicon atom will be full and happy.

Now, let’s cause a little havoc in our peaceful silicon neighborhood. Every once in a while, let’s replace one of the silicon atoms with phosphorus. While silicon has four electrons in its outer shell, phosphorous has five. Phosphorous tries to get with the program and bonds to four adjacent silicon atoms, but that leaves one electron “left over� and not part of a bond with an adjacent silicon atom. The electron doesn’t just run off – the negatively charged electron is held loosely in place by its attraction to the positively charged proton in the atom’s nucleus. Since this now impure silicon has “left over� electrons, and electrons are negatively charged, this is called n-type material.

In a similar fashion, we can take another slab of silicon and replace a few of the silicon atoms with boron. Boron has only three electrons in its outer shell, so when it tries to share an electron with each of its four silicon neighbors, we end up one electron short, creating what is sometimes called an electron “hole�. This material is called p-type material. If we take a chunk of p-type material and a chunk of n-type material and get them up against each other, we get (drum roll please) – a p-n junction.

Now, let’s add a little light to the equation. Light is made of little packets of energy called photons. Photons from the sun travel through space and end up landing on our little solar cell, crashing into the n-type and p-type material. It doesn’t do a whole lot to the p-type material, but since our “left over� electron in the n-type material isn’t tightly bound, the photon has enough energy to knock the extra electron loose.

At the p-n junction things are starting to get interesting. On the n-type side of the p-n junction there’s a bunch of electrons looking for somewhere to be. On the p-type side there’s a bunch of holes for these electrons to reside in. Right at the boundary where the p-type and n-type material meet, electrons from the n-type side start crossing over into the p-type region and get comfortable in their new homes. As this continues, the holes nearest the p-n junction get filled, making it harder for other electrons to cross over into the p-type material. Eventually it gets so hard for the electrons to cross from the n-type material to the p-type that things just settle down and reach equilibrium.

Something pretty cool has happened as the electrons near the p-n junction were doing their little dance. When we started out, both the p-type and n-type materials were electrically neutral – for every negative electron there was a positive proton to balance things out. But now, a bunch of electrons have crossed over from the n-type to the p-type region. What does this give us? Well, now the n-type material has a positive net charge and the p-type has a negative charge. Does this remind you of anything? It’s beginning to sound sort of like a regular battery.

How do we get something useful out of this? A good analogy to give us an understanding of how to use this would be to take two tanks of water, one at higher pressure than the other. If we connect these two tanks with a pipe, water would flow from one tank to the other, until the pressure in the two tanks even out. If we put something along the pipe, like a paddlewheel, we could harness this energy. The same idea applies with the solar cell. Given a chance, electrons will flow through a wire from the p-type region to the n-type region until the electrical charges balance out. But, since the light keeps knocking electrons free in the n-type material, the cycle will continue. As long as there is light, the process continues, allowing us to use this flow of electrons to charge a battery, spin a small motor – whatever we want.

That’ll wrap things up for this episode of How Things Work. A transcript of this episode can be downloaded from howthingswork.libsyn.com, and if you have any questions, comments, or topic ideas, please drop me a line at [e-mail]. Thanks for listening.

The music you heard was How’d You Know That by Scott Brown. It’s available on Random Chance records at www.randomchancerecords.com and is made available through the IODA Promonet.



Citations –
http://www.wisegeek.com/how-do-solar-panels-work.htm
http://www.thesolarplan.com/articles/how-do-solar-panels-work.html
http://www.nooutage.com/howsolar.htm
http://www.howstuffworks.com/solar-cell2.htm
http://www.soton.ac.uk/~solar/intro/tech6.htm
http://www.sailgb.com/sshop/tech_info.asp?ID=164
http://www.gepower.com/prod_serv/products/solar/en/how_solar_work.htm
http://library.thinkquest.org/27754/apphowpv.html
http://en.wikipedia.org/wiki/Solar_cell