.2. Theory of solar cell
When the energy of an incoming photon is equal to or greater than the band gap of the material, the photon is absorbed by the material and it excites an electron into the conduction band. A hole is left behind and another electron from valence band moves to this position leaving behind a hole. In this way hole moves through the lattice. Thus it can be said that absorbed photon creates electron–hole pair. When p and n type materials are brought together, diffusion of carriers take place and charges build up on either side of junction and create an electric field. The p–n junction formed helps in separating these electron–hole pair. It acts like a semipermeable membrane and allows only one type of carrier to pass through it. On the n-type semiconductor side it allows only electrons to pass through it, while transport of holes (which are minority carriers) is due to recombination. Similarly p-type semiconductor side allows only holes to pass through it and transport of electron takes place by recombination.