Photo Voltaic Principles
A pair of positive and negative charges called electron-hole pairs are created in the solar cell by absorbed solar radiations. As discussed earlier, the suitable materials for absorbing the energy of photons of sunlight (light is radiant energy whose quantum is called photon) are semi-conductors like silicon, gallium arsenide with dopants. In a semi-conductor the valence band has electrons at lower level of energy which is fully occupied. Comparatively, electrons at conduction band have higher level of energy and they are not fully occupied.
The difference in the minimum energy of electrons in conductor band and the maximum energy of electrons in the valence band is called band gap energy, Eg.
When solar radiation fall, the difference of photon energy of sun light, E and the band gap energy, Eg are absorbed by the cell material. It excites some of the electrons and they jump across the band gap across the valence band to conduction band. It leaves behind holes in the valence band. Therefore, electron hole pairs are created. Thus the electrons in the conduction band and the holes in the valence band can be made to flow in an external circuit if the potential difference exists within the cell.
Max Planck suggested that the energy of photon is proportional to the frequency of radiation i.e.
EP = hv = h . C/λ ….(1)
where
EP = photon energy
h = Planck’s constant = 6.62 x 10-27 ergs-s
= 6.6256 x 10-34 J. s = 4.13576 x 10-15 eV.s
v = frequency of radiation in Hertz (Hz)
C = speed of light = 2.99793 x 108 m/s ≃ 3 x 10 m/s
λ = wave length of radiations in m.
Substituting the values in the above equation.
EP = 4.13576 x 10-15 x 3 x 108 / λ x 10-6 (if λ is in μm)
EP ≃ 1.24 / λ (eV), where λ is in μm …(2)