I\'m a bit confused about how solar cells work. My understanding is that there i

Question

I'm a bit confused about how solar cells work.

My understanding is that there is a p-n junction. A photon is absorbed which creates an electron-hole pair, and the idea is to separate the electron and hole before they recombine in order to produce current.

n-type materials are doped with extra electrons and p-type materials are doped with holes. So why is that in all the diagrams I see, when charge separation occurs, electrons are conducted through the n-type material and holes are conducted through the p-type material? Shouldn't it be the other way round?

Explanation / Answer

It's not quite correct to say that n-type materials are doped with "extra" electrons. To be sure, n-type material is charge neutral. It's more correct to say that n-type material is doped with atoms that "donate" an electron to the conduction band; n-type material has excess mobile electrons versus intrinsic material. So, naturally, n-type material is a good conductor because there are plenty of mobile electrons to participate in an electron current.

p-material is doped with atoms that "accept" an electron from the conduction band so there is a deficit of mobile electrons and thus you would not expect there to be much mobile electron current. However, there are plenty of "holes" that can participate in a "hole" current.

Now, with respect to your question. When an EHP is generated in either type of material, the material remains charge neutral. However, if the EHP is separated by the intrinsic E-field of the PN junction, there is no longer charge neutrality. When an EHP is generated, it is the minority carrier that will be swept across the junction to become a majority carrier on the "other side". Since there is no longer charge neutrality, charge will flow in an external circuit to restore it.

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