How does a solar cell turn sunlight into electricity?

      In a solar cell we have SiO2 which is a crystal, composed of 14 electrons. A silicon atom looks for ways to full up its last shell so it will share electrons with 4 atoms nearby.

See diagram below.

·       However, pure silicon is a bad conductor of electricity since most of its electrons are not free to move around. In order to fix this, the silicon in a solar cell must have impurities. 

N - Type

The process of adding impurities is called doping and when done with phosphorous is called N - type because of the great amount of electrons and it carries a negative charge. N- Typed dope silicon is much better than pure silicon.

When energy is added to the pure silicon, it causes some electrons to break free of their bonds and leave the atom. This leaves behind a hole. These electrons that are free to move about are called free carriers and they carry a current. They move around the lattice looking for another place to fall into. However, since there are such few free carriers in pure silicon, it is not very efficient in terms of the amount of current being produced.

Phosphorous has 5 electrons in its outer shell. So it bonds with the silicon atoms to full up its last shell. However, it leaves behind one phosphorous electron which does not bond. But, there is a positive proton which keeps it in place. 

For silicon mixed with phosphorus; it requires less energy to get rid of electrons in the phosphorous atom. So most of the electrons break free in this case and there are much more free carriers and therefore more current. 

P - Type

In other cases, the solar cell can be doped with boron, which has 3 electrons in its outer shell alone. Instead of having free electrons, the P type has free openings and carries a positive charge. 

So how does it work?

A solar cell has 2 semiconductor layers: A bottom layer which is P type and a top layer which is N type. When these two come together, they form a PN Junction. Electrons from the N side can move across the junction to the P side, causing it to become negatively charged. When these electrons move over to the P side, they can pair up with a hole, creating an electron - hole pair. The reverse applies; holes from the P side can move across the junction and cause the N side to be positively charged. The holes can pair up with electrons to create an electron - hole pair. This charge imbalance on the P and N side creates an electric field. 

When light hits the solar cell in packets of energy, called photons, it breaks apart electron hole pairs, sending the electron to the N side and the hole to the P side. These charges are now separated so there is a difference in potential and a voltage created. 

If we connect the solar cell to an external circuit, this allows the electrons and holes to move around. When the charges move around the circuit, they create and electric current. 

With both a potential difference / voltage and current, we now have power. 

For a more visual understanding, check out the following videos below:

How solar panels work

PN junction solar cells