Dear Colleagues,
If we pass through the photon energy equal or greater than the bandgap of the materila, then only to crate the electron hole pair. Bandgap means distance between the lowest point of the CB to highest point of the VB. But electron transition not only the VB to CB. Because below the CB many defect level are created from the defects. So The transition possible to VB to LUMO 1 or VB to LUMO 2, etc.
Normally UV spectrum w got one absorption peak and this is use ful for measuring the bandgap. But this maximum comes from VB to LUMO 1 or VB to LUMO 2 or VB to LUMO N...?
The above question i made some spelling mistake. Sorry for this trouble.
My doubt is, how to prove the excited electron goes to LUMO 1(lowest point of the CB). Because it go to LUMO 2, LUMO 3, etc.
Suppose it goes to LUMO 2,there also we got absorption or it goes other lumo levels. From this absorb spectrum we calculate band gap. But I think it wrong.
Because band gap means distance between the highest VB to lowest CB (LUMO 1) not LUMO 2.
Dear John,
The absorption of photon is accomplished by a valence electron which is transferred from an filled electronic level in the valence band to an empty level in the conduction band. The organic semiconductors are characterized by narrow valence band and conduction band because of their molecular nature. Therefore The absorption band will be narrow peaking at the maximum density of states at the valence band and conduction band. Accordingly the position of the peak is more or less equal to the energy gap. Really it is slightly greater than the band gap.
Best wishes
This link may be helpful for you
https://www.researchgate.net/post/How_can_I_calculate_the_band_gap_from_UV- Vis_absorption_spectra_of_thin_films_consisting_of_TiO2_and_other_metal_complexes
