A silicon cell is like a four-part sandwich. The bread on either side consists of thin strips of metallic electrodes. They extract the power generated within the solar cell and conduct it to an external circuit.
Just like a sandwich, it’s the filling which is the most interesting part – this is where photons from the sun are converted into usable electricity. The filling of a solar cell consists of two different layers of silicon: negative and positive silicon, or n- and p-type silicon.
Creating positive or negative types of silicon is relatively easy. The silicon is impregnated with elements known as dopants. Dopants replace some of the silicon atoms in the crystal structure, allowing the number of electrons present in each layer to be manipulated.
For instance, phosphorus is used to create n-type silicon while boron is used to create p-type silicon. Phosphorus has one more electron than silicon. When substituted into the silicon structure, the electron is so weakly bound to the phosphorus that it can move freely within the crystal, creating a negative charge.
On the other hand, boron has fewer electrons than silicon and sucks up silicon’s electrons. This creates “electron holes” – regions of mobile positive charge in the crystal structure.
At the interface of the p- and n- type silicon, the positive electron holes and the electrons combine. It’s not a simple electrostatic interaction, but the upshot is that you get a slightly positive charge in the n-type silicon and a slightly negative charge in the p-type silicon at the interface of the n- and p- type silicon – the opposite of what you might expect.
Photons from the sun pass between the strips of the top electrode and strike silicon atoms in the crystal structure. Like the strike of a cue ball, the colliding photon gives some of the silicon electrons enough energy to escape from their parent silicon atom.
The “free” electrons move to and accumulate within the n-type silicon.
Once free electrons have accumulated in the n-type silicon, it’s time to put all the free electrons to work. In order to use their energy, the electrodes must be connected via an external circuit. Electrons flow through the electrodes and the external electric circuit from the n-type to the p-type. The p-type silicon acts as an electron sink. Without it, the electron flow would clog up.
It is this flow of electrons that creates the electrical current we can use to power appliances or charge batteries for when the sun isn’t shining.
Andrew Stapleton is a scientist, science writer and podcaster based in Adelaide.
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