There's a phenomenon in physics known as the photoelectric effect, which forms the basis for solar photovoltaic technology.
The photoelectric effect states that when a photon (a tiny, massless particle that makes up light) strikes a metal surface, if it has enough energy, it is able to release one electron from being confined to an atom in the metal.
I won't go into the detail of how this works here, but just remember that it's a one to one ratio we're dealing with here: when one photon with sufficient energy strikes a metal surface, one electron is liberated.
If we skip forward a bit and relate the number of freed electrons to electricity, we can see that the amount of electricity produced by this system would be directly proportional to the intensity of the light (i.e. number of photons) striking the metal surface. (The brighter the sunlight, the more power we get).
A solar photovoltaic cell. Photo courtesy of Kevin T. Houle
It's All About Sandwiches
So, how do we get from "freed electrons" to electricity? Solar photovoltaic cells makes use of semiconductors, which have properties of both metals and non metals in terms of the way the electrons behave in the semiconductor.
I'll do my best to explain. Think of a sandwich. That's right, a sandwich; everyone loves sandwiches. You have two slices of bread, and a gap in the middle between them where there is no bread!
Now imagine looking at this from the side: you have a band of bread at the bottom, then a gap, then another band above the gap. This sandwich is your semiconductor.
Imagine that the bottom band of "bread" is filled with electrons, and the top one is relatively empty. Light comes in, excites some electrons in the bottom band, and if the photons have enough energy, the electrons jump up across the gap to the band on top.
If we apply a voltage to this semiconductor now we can get the electrons in the top band ("conduction band") to move either one way or the other. We can also do this by using different materials for the semiconductor.
If we have a continuous supply of light (during the day, say), then a current will flow as the supply of electrons making the jump is sustained. This is also why if you have more light, you get more electrons flowing, so a greater current (more power!)
So that's pretty much it, the basics of how solar photovoltaic technology works. It's all sandwiches!
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