I suggest to read the information from the following link
www.tf.uni-kiel.de/matwis/amat/semi_en/kap_5/.../t5_1_3.htmlCachedSimilar
Normally, elecctron can go back from LUMO to HOMO orbital and emmit light.
I suggest to read the information from the following link
www.tf.uni-kiel.de/matwis/amat/semi_en/kap_5/.../t5_1_3.htmlCachedSimilar
As a result of absorption of photons of energy equal or greater than the band gap energy of the semiconductor, the excitons (electrons and holes) are generated in the conduction and valence band, respectively. Due to the very short life time (~ 10E-8 s) of these charge carriers, they recombine with each other. This is called exciton recombination. The same phenomenon is also present in an organic semiconductor. I hope the concept would be clear now.
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Dear Issam,
The link is not available. Can you please write it properly
An exciton can form when a photon is absorbed by a semiconductor. This excites an electron from the valence band into the conduction band. In turn, this leaves behind a positively-charged electron hole (an abstraction for the location from which an electron was moved). The electron in the conduction band is then effectively attracted to this localized hole by the repulsive Coulomb forces from large numbers of electrons surrounding the hole and excited electron. This attraction provides a stabilizing energy balance. Consequently, the exciton has slightly less energy than the unbound electron and hole.
The recombination of the electron and hole, i.e. the decay of the exciton, is limited by resonance stabilization due to the overlap of the electron and hole wave functions, resulting in an extended lifetime for the exciton.
Excitons are generated when valence band electrons or homo electrons absorb an incident photon on the semicondcutor either metallic or organic. In case of metallic excitons they dissociate promptly as the binding between the electron and hole in the exciton is very weak while this binding energy is strong in the organic material because its low dielectric constant. Af they are not dissociated they diffuse in the material after which it will recombine and disappear. So the excitons diffuse a distance called the diffusion length in their lifetime.
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