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Absorption of Light

Absorption of Light explanation

Incident photons on the semiconductor’s surface will either be reflected from the surface, or will be absorbed into the material, if neither of the above two processes happen, light will pass through the material. In photovoltaic devices, both reflection and transmission are naturally considered loss factors, because the photons that are not absorbed cannot be used to generate power. When a photon is absorbed, it has the opportunity of exciting an electron to the conduction band from the valence band. A major factor in the determination of if a photon will be absorbed or transmitted is the photon’s energy. Consequently, the electron will only be excited into the conduction band from the valence band if the photon has enough energy If the photon energy is equal to or greater than the material’s band gap, the photon will be absorbed by the material and electron is excited into the conduction band. When the photon is absorbed both of the minority and majority carrier are generated. The charge carriers generation by photons is the base of the production of energy by photovoltaic cells.  

Photon Energy and absorption of light

Photons striking a semiconductor material fall into one of three groups based on their energy in comparison to the energy of the semiconductor band gap: Photons with energy (Eph ) less than the band gap energy EG   (Eph < EG ) interact weakly with the semiconductor, only passing through it as if the semiconductor is transparent. When Eph = EG  , photons have only enough energy for creating an electron hole pair and photons are efficiently absorbed. Photons with energy quiet larger than the band gap ( Eph > EG ) are absorbed strongly. But, in solar applications, when the photon energy is greater than the band gap, the excess energy is wasted because electrons thermalize  quickly back down to the edges of the conduction band.  

Absorption of light and Solar applications

The photons generate both a majority and a minority carrier. In multiple solar applications, as a result of doping, the number of majority carriers already present are of an order of magnitude more than the number of light generated carriers in the solar cell . Consequently, the number of majority carriers in an illuminated semiconductor does not change significantly. But, the contrary is also true for the number of minority carriers. The photo-generated minority carriers number is outweighed by the minority carriers number present in the doped pv cells in the dark (because the minority carrier concentration is so minor in doping), and as a result the number of light generated carriers can be used to approximate the number of minority carriers in an illuminated solar cell.    
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