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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?

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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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  • $\begingroup$ I've been doing a bit of reading and it's my understanding that there are two competing forces for driving electrons and holes across a p-n junction, diffusion and drift. Diffusion makes electrons travel from the n-type to the p-type material, leaving behind positive ions. A depletion region occurs, and over this region there is an electric field due to the electrons in the p-type and positive ions in the n-type. This electric field balances out the diffusion so there's no net current. So how is it then when a electron hole pair is created, it creates current? $\endgroup$
    – Eddy
    Jul 2 '12 at 14:15
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    $\begingroup$ The electric field in the depletion region "sweeps" out mobile charge carriers so there is no current through this region "depleted" of mobile charge carriers. When EHPs are created in or around the depletion region, mobile charge carries now exist where there were none. We say carriers are "injected" into the depletion region where they may now participate in an electric current. $\endgroup$
    – Alfred Centauri
    Jul 2 '12 at 14:30

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