Hi Aung Moe, As you well know it is significant that correlated electron-hole pairs or excitons play a dominant role in the optoelectronic properties of 2D materials systems. The formation times of excitons in these systems are generally fast (approx 0.5 ps ) compared to their radiative and non-radiative lifetimes (e.g 5 ps or more). Of course the exciton lifetime depends on the temperature, exciton delocalization, presence and concentration of defects, type of substrate, if its a Mo-based or W-based material, and also whether its a monolayer or bilayer etc. The implication for a long lived exciton is that it presents you the opportunity to control the dynamics of excitons and harness the power of this quasiparticle system before it dissociates into free electron and hole. In photovoltaics, the longer the exciton lives, recombination is reduced, higher the quantum yield and the conversion efficiency. Hope this is of some help in your study.

Dear Alagu Thilagam, thank you for your insight explanation.

Dear Yan,

Adding to the above said, there is evidence that the exciton lifetime is affected by the energy gap of the organic material. A correlation is found between the exciton lifetime and the energy gap of the material such that the lifetime increases as the bandgap increases. It is found also that the exciton lifetime depends on the size of the crysatllites in the material such that it decreases with larger grain size. This also attributed to the decrease of the bandgap with grain size increase. For mor information please follow the link: https://pdfs.semanticscholar.org/828c/8785a4f92f8e6b69c2126273ba3e71054de1.pdf,

As for the organic solar cells one contact the acceptor material directly to the donor material such that the excitons generated in the light absorbing donor material diffuse smaller distance than their diffusion length to the donor acceptor interface. I mean the mixing between the two materials may be down to molecular level.This will be the best case to dissociate all the formed excitons. This is case for the bulk heterojunction organic solar cells.

Best wishes

@Abdelhalim Zekry

Dear Sir,

Thank you for your valuable information and explanation.