Photovoltaic cell question?
A photon of light hits an electron in the valence shell and excites it into the conduction band, creating an electron/hole pair. The electron will move into the n-type layer and the hole to the p-type layer (under the influence of the electric field where the n-type is positive and the p-type negative). But here is the question, I have read that the electron can move from the n-type layer, through an external circuit and back into the p-type layer where it fills a hole, doing work on its way through the external circuit.
Why does the electron do this? My understanding is that the p-type layer was negative (which was the reason the electron moved to the n-type in the first place) so why would the electron move towards the negative p-type through the external circuit???
I may have developed a slight misunderstanding of how solar cells work or even something more basic at some point in the past but I a struggling to understand this. Help would be great! Thanks!
Actually p-type is the side more number of holes and thus has a tendency to accept electrons, and similarly the n-type has more electrons so tends to accept holes.
So the p-type layer is not negative. I think you are referring to the case of a p-n junction diode. The p-type layer has a tendency to accept electrons and the n-type repels electrons(as its high on electrons) so the electron travels from outer circuit to the p-side. The reason why electron goes from the outer circuit and not directly is because in between the 2 layers an immobile non conducting layer is present.
And as per the question of why the electron moves to n-type in the first place is that, we create a deficiency of holes and not excess of electrons. The n-type is usually produced by adding a pentavalent atom(like phosphorous) to tetravalent atom(like Silicon). This cause bonding of 4 electron-hole pairs and leaves 1 electron free and hence the n-type nature.
Similarly for p-type we use a trivalent atom which leaves 1 hole free.
So we never introduce extra electrons instead they are made free.
How photovoltaic cells work
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