Dear Huda M.H

The exciton is elementary excitation of condensed matter. It exists in insulators and semiconductors, and simply is a bound state of electron and hole [binding energy is the amount of energy required to disassociate them] generated when the material absorbs photons, without disassociation it doesn't result in net electrical current. In solar cells, we use semiconductors at where we can generate these excitons by illuminating the device with light. However, to extract a net current from the device we need to disassociate these excitons, so we use the selective contacts [ Electron transport layer and hole transport layers]. At these layers, the excitons are disassociated and can flow towards the corresponding electrodes. So the importance of the binding energy of the excitons comes from this fact.

For more details, you can read in this book:

https://www.amazon.com/Theory-excitons-Solid-physics-Supplements/dp/B0000CMEHE

Or short reading in this article :

Chapter Introduction to Exciton Physics

Hope that help

Greetings.

M.Elshorbagy

Dear Huda M.H

The exciton is elementary excitation of condensed matter. It exists in insulators and semiconductors, and simply is a bound state of electron and hole [binding energy is the amount of energy required to disassociate them] generated when the material absorbs photons, without disassociation it doesn't result in net electrical current. In solar cells, we use semiconductors at where we can generate these excitons by illuminating the device with light. However, to extract a net current from the device we need to disassociate these excitons, so we use the selective contacts [ Electron transport layer and hole transport layers]. At these layers, the excitons are disassociated and can flow towards the corresponding electrodes. So the importance of the binding energy of the excitons comes from this fact.

For more details, you can read in this book:

https://www.amazon.com/Theory-excitons-Solid-physics-Supplements/dp/B0000CMEHE

Or short reading in this article :

Chapter Introduction to Exciton Physics

Hope that help

Greetings.

M.Elshorbagy

Following M.H. Elshorbagy's explanations and simply speaking, excitons are the electrically charged pariticles which could be electrons or holes on the excited states. The binding energy refers to the minimum energy of separating a pair of hole and electron. The importance of a binding energy can be directly related to the current flow within a solar cell.

Look exciton is the combination of a electron and a hole; therefore exciton is a neutral species. That's why you need to provide energy higher than the binding energy of this exciton so that electron and hole can separate and can contribute to the current of the solar cells.

However, in an organic solar cell, the absorption of a photon creates a strongly bound exciton.

Thank you all

When a semiconductor is excited by photon, then the electron from the valence band jumps to conduction band leaving behind a positively-charged electron-hole in the valence band. Then the electron and hole thus generated are bound together by Coulomb force. This bound state together acts as a neutral particle called an exciton. Whenever you supply energy greater than the binding energy of the excitons, the excitons dissociate into electrons and holes which are separately moved towards the electrodes to generate the electric current in the solar cells

Bibek S Dhami thanks a lot to you and all the colleagues

In organic solar cells such as dye-cells, due to existence of high concentration of positive and negative ions adjacent generated e/h pairs, it's important that is considered the electrostatic interaction between the particles. Once is the coulombic force between the e/h pairs which is the annoying factor for e/h completely separation. This force (binding energy) as much as be lower, it's better for separation and transporting of e/h pairs. This case doesn't matter in high quality crystalline cells such as Si solar cells.

Agree with M.H. Elshorbagy

Agree with M.H. Elshorbagy

Once a photon is absorbed from the incident light an electron gets excited from the valence band to the conduction band ( for organic semiconductors : HOMO to the LUMO level). This process results in coulombically bound electron-hole pairs called excitons at ambient conditions.

For inorganic semiconductors the energy required to dissociate these excitons into charge carriers is quite small (which can be achieved at room temperature).

For organic semiconductors photogenerated excitons are strongly bound due to the low dielectric constant, which cannot be dissociated with thermal energy at room temperature.

When an electron of the active layer in solar cells receives energy from the incident photon, it moves to a higher level of energy (concepts of valence and conduction bands or HOMO-LUMO), leaving a hole at its origin level. This electron-hole pair is called an exciton. After separating the electron and the hole (formation of electron-hole pair or exciton), we want to direct each of them (electron and hole) to the corresponding electrode and collect them, for which we need to breakdown the pair of electron-hole which is called exciton breakdown or exciton dissociation. However, in an exciton before breaking down, the electron and hole are not free of each other that means the electron cannot move independently from its related hole and vice versa, because the separated electron and hole are bound together with an energy which is called binding energy of exciton or the strength of coupling. To dissociation of the exciton to the free electron and the free hole, we need some energy to overcome the binding energy of exciton (breaking down the exciton). After breaking down the exciton, they can still be a pair of electron and hole but not a bounded pair because they (electron and hole) are now two independent charges. In general, if the amount of binding energy is high (strong exciton binding energy or more stable exciton), exciton is interested in resisting against dissociation and retaining its electron-hole pair. Electrons and holes are inherently interested in recombination, so if they cannot dissociate, the probability of recombination of electron and holes (exciton decay) increases. It depends on the type of work we do, sometimes we need to make the dissociation at the exciton’s formation origin site, and sometimes we need to take the exciton to the interface and do the dissociation there.