Hi,
For amorphous Si, this intrinsic layer will balance electron and holes mobility. Photons should be absorbed in this intrinsic layer, which is about 500 nm, while n and p layers are in the rage of 20 nm. Whereas in crystalline Si, photons are absorbed in the thick p layer.
Though the built in potential is same in p- i- n junction as in p- n junction, the electric field extends over a wider region due to intrinsic layer in between and hence depletion region is elongated.This is useful for materials having short minority carrier diffusion length as minority carriers produced by light will not contribute to the photocurrent.
Intrinsic layer was found to increase the contact area of P-N junction so that enabling more excitons to dissociate into free charges. This is believed to increase the efficiency of the solar cell.
If we look up to the a-Si crystalline structure, we can notice the higher disorder of molecules comparing to monocrystalline silicon, thus, the charge carrier lifetime is shorter (more interactions, resistive losses, fast recombination) in a-Si. Therefore, having an ntrinsic layer between pn junction will not only strengthen the cherge carrie lifetime, but also the stability of a-Si.
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