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
Answer
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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