Thread by @realscientists

What is a solar cell? Solar cells convert the energy in light to electricity. Sun light contains lots of energy but it is not in a form we humans can easily use (here plants are at an advantage). Electricity on the other hand is very handy for us (plants don't have a use for it).

The aim of a solar cell is to convert light to electricity. To do that you need to do three things 1st absorb the light and generate opposite charges (electrons and holes) 2nd separate these opposite charges 3rd extract those charges into an external electrical circuit.

Let's focus on the 1st thing: absorption of light and generation of opposite charges. Lot's of materials absorb light (the asphalt on the street, dark jeans, your skin). Some start to change when they absorb too much light, but the are not necessarily generating opposite charges.

To get a generation of opposite charges you need a semiconductor. A material in which there are separated electronic energy levels. Importantly one lower energy level and then a higher energy level which is separated from the first by a gap. This gap is called the band gap.

If the energy of an incoming photon (particle / quantum of light) is larger than the band gap it can be absorbed by an electron in the lower level which gives the electron enough energy to go into the higher energetic level. This is the photovoltaic effect.

So here is 1 less electron in the lower energy level. Now we just call this lack of an electron, where there used to be 1, a hole. Voila, we have generated opposite charges: an excited electron (- charge) in the higher energy level and a hole (+ charge) in the lower energy level.

Now we 'just' need some matching contact materials so we can get the electrons out on one side of the semiconductor and the holes on the other side. And there we have a solar cell. Sadly in practise it is a bit more troublesome, but this is the underlying principle.