Shamjid,

Actually, it is related:) The concept of the electron-hole pair from semiconductor physics refers to one electron and one hole somewhere in semiconductor and in the appropriate bands that results after promoting an electron from the valence band to the conduction band by any means (thermal, optical, electrical etc.) Once you have the electron in the conduction band, it becomes a free electron (and you can say the same thing about the hole). They don't remember their place of origin and the electron is not bound to the hole - they can move independently and conduct the current. So, electron-hole pair in this case is just a concept but not the entity.

For the exciton, the electron and the hole are bound by the binding energy (read about this in the question I suggested). That is, they are not free, the electron can't move independently from that particular hole (and vice versa). The exciton is the entity consisting from two charges bound together if you prefer the plane simple language.

The strength of the coupling or the binding energy determines the stability of the exciton. In a process of dissociation, the exciton breaks down to the free electron and the free hole (which are not attached to each other and can be considered as two independent charges but the general concept of the electron-hole pair still can applied to it). Perhaps, a point of confusion for you is that it's called the pair (i.e., it's ONE electron and ONE hole) but not the BOUND PAIR!

You've got it. In fact these are bound electron hole pairs forming their own hydrogenic energy levels in the excited materials. Binding energy is stronger for materials having low dielectric constant for example in most organic semiconductors. They are stable as long as the binding energy is higher than the thermal energy.

Hi,

You may want to look at the following question that is related to yours.

https://www.researchgate.net/post/How_excitons_get_dissociated_into_free_electron_and_holes

Actually my question is not related as both of you answered, now I try to give what I'm seeking to know....

I have heard hole is a vacancy created by electron in basics of electronics. But come to semiconductor physics (specifically Solar cells), there is a different concept related to hole; that is electron-hole pair. I couldn't relate these two. So please help me to get the basic basic concept of exciton and exciton formation.....

Shamjid,

Actually, it is related:) The concept of the electron-hole pair from semiconductor physics refers to one electron and one hole somewhere in semiconductor and in the appropriate bands that results after promoting an electron from the valence band to the conduction band by any means (thermal, optical, electrical etc.) Once you have the electron in the conduction band, it becomes a free electron (and you can say the same thing about the hole). They don't remember their place of origin and the electron is not bound to the hole - they can move independently and conduct the current. So, electron-hole pair in this case is just a concept but not the entity.

For the exciton, the electron and the hole are bound by the binding energy (read about this in the question I suggested). That is, they are not free, the electron can't move independently from that particular hole (and vice versa). The exciton is the entity consisting from two charges bound together if you prefer the plane simple language.

The strength of the coupling or the binding energy determines the stability of the exciton. In a process of dissociation, the exciton breaks down to the free electron and the free hole (which are not attached to each other and can be considered as two independent charges but the general concept of the electron-hole pair still can applied to it). Perhaps, a point of confusion for you is that it's called the pair (i.e., it's ONE electron and ONE hole) but not the BOUND PAIR!

Dear Serge ,

' Electron-hole pair in this case is just a concept but not the entity '

Why it is so?

Even it is a concept, how it is introduced?

As you said, when electron excited to conduction band it leaves one hole in the valence band, that means free electrons in conduction band and free holes in the valence band. But , where the exciton will form, in valence band or conduction band or else where?

Two aspects to consider - the band structure of semiconductors doesn't really have a spatial dimension, right? Second, the energy of the exciton formation is LOWER than the Eg (energy gap) by the binding energy of the exciton (can be from 1 meV to 1 eV). You can consider the electron and the hole orbiting around the common center of gravity (the Bohr model is applied with the electron mass replaced by the reduced mass of the electron and hole).

When doing optical studies, one would see the exciton peak at energies right below Eg. So, in the band diagram exciton levels would be below the conduction band edge AND spatially localized.

Dear Serge,

Thanks a lot for giving very good response to my question... Really great...

Could you please give me some good references for the above mentioned concept of exciton?

Absolutely! I'd recommend to start with a common textbook of Kittel "Solid State Physics" (Wiley, any edition). There is a chapter on excitons in the book. It might give you all the answers. If you would need more sources, just post it and I'd offer few more books. Good luck!

Serge

Dear Serge,

Thanks a lot......

Dear colleagues,

welcome,

You can read the sections of the generation and recombination of electron and holes in semiconductors in the easy to understated book in the link:Book Electronic Devices

Best wishes

In inorganic materials light absorption occurs in the form of electron-hole pair recombination but in organic materials this occurs in the form of excitons. Excitons are electrically neutral and tightly bound coulombic electron hole pairs and have strong binding energies. In general, these materials have low dielectric constants, strong coulomb interaction and very weak non-coulomb interactions. Due to these reasons electron wavefunction spatially resricted to induced potential well .

Whenever a material absorbs a photon it produces an exciton which is a state of bound electron hole pair. This binding energy deepens on the dielectric constant of the material. As the dielectric constant increases the exciton binding energy decreases such that it becomes so small to be directly dissociated by the thermal energy in the metallic semiconductors. In organic semiconductors with small dielectric constant the exciton binding energy may amount to about 0.3 eV- 0.7eV.

Best wishes

In semiconductor electrons absorve photons of energy and jump to Valence band to conduction band ,so conductivity of material increases due to availablity of electrons in conduction band