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We know that solar cells work when a photon hits the n-type the photon's energy drives free the electrons in the n-type to generate a current. But we also know that when a photon hits the atoms it makes the electrons excited. So why doesn't the photon make the electron excited and makes the electron drive out?

OR is it like this that in solar cells the photon gives so much energy to the electron that when it goes to a higher energy state and changes shell it gets out of the atom shells and becomes a free electron carrier?

Thanks,
Bhavesh

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    $\begingroup$ This is complicated. You will have to understand band structure of semiconductors, and then semiconductor device physics to get a full answer. However, you can also describe the operation of a solar cell purely with thermodynamic, but as you are asking about the actual mechanics I don't think this is what you want. I would suggest reading Jenny Nelson's of Peter Würfel's solar cell books. It's too much to cover in a single answer. $\endgroup$ – boyfarrell Apr 15 '15 at 14:57
  • $\begingroup$ @boyfarrell thanks for your suggestion, i will surely take your advice. :) $\endgroup$ – Bhavesh Apr 15 '15 at 15:23
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    $\begingroup$ Is your question about why the excited electron is no longer bound to the atom? In that case, I think your second statement is more or less right; the photon deposits enough energy to free the electron from being localized to one atom. $\endgroup$ – ragnar Apr 17 '15 at 6:59
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I think a simple view is this:

The solar cell must have a PN junction, which is a junction between p-type (many holes, no electrons) and n-type (many electrons, no holes) materials. Right where they meet there is actually a "depletion width" within which there is hardly any of either. Within this region, as photons come in they generate electron-hole pairs, which really just means that an electron has been excited from the valence to conduction band, leaving a hole behind. The electron is then pushed back to the n-side by the "built-in" electric field, while the hole is pushed to the p-side. Think of this is terms of energy: both end up where their energy is lower, so electrons prefer the n-type material, while holes prefer the p-type. Some understanding of semiconductor doping and Fermi statistics helps greatly to understand this.

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