Science Questions

How do solar panels turn sunlight into electricity?

Sun, 29th Nov 2009

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Question

Chris Cattaneo, via twitter asked:

How do solar panels turn sunlight into electricity?

Answer

A solar panel is a semi-conductor device which will produce an electrical voltage and current if light falls on it. It is actually a giant diode - a one way valve for electricity, and when light falls on it this gives some electrons enough energy to jump through the valve, and the only way they can get back is to flow around the circuit doing useful work.

What is going on in more detail?

Normally each atom in the semi-conductor has 4 electrons and there are none free, but if you add a few atoms with 5 electrons there are some extra electrons which can move easily and carry a current (an N-type semi-conductor). Similarly if you add some atoms with only 3 electrons and then you get some gaps which other electrons can move into (a P-type semi-conductor). The electrons can now move a bit like the tiles in a sliding tile puzzle, but it is easier to think of a positively charged hole moving through the semiconductor.

N-type Semiconductor

P-type semi-conductor

N-type with a few extra electrons to carry current

P-type which has a few electrons missing which can be thought of as positively charged 'holes' and can carry current.

A diode is made if you join a lump of P-type semi-conductor to a lump of N-type. When you do this there are lots of extra electrons in the n-type region which will tend to diffuse into the p-type region and holes from the p-type region will diffuse into the N-type. So you end up wit the p-type region being negatively charged and the n-type region being positively charged.

Diode drift

Connecting cell to a circuit

Electrons and holes diffuse over the border.

This means that if you connect the diode to a circuit this charge flows around the circuit very quickly.

If you connect the diode to a circuit, this charge will flow around the circuit evening out the voltage on the two sides, and you will be left with a region in the centre with no charge carriers.

Shorted solar cell

Light hitting the solar cell

You are left with an uncharged diode with an area in the centre with no charge carriers.

If light hits this central region it knocks electrons off an atom creating electrons and holes which can flow around the circuit.

Light can hit this non-conducting region in the middle with no free electrons , and knock an electron off an atom.  This creates a free electron and a hole. These can then flow through the circuit as electricity and do useful work. The more light which hits it the more current and therefore power it will produce.

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