What is exciton?

 

There are valence and conduction bands in materials, are atoms, the valence bands are filled. While the conduction band is empty at ground state energy between the valence and conduction, the band is a forbidden band, which is also known as the band. When we provide sufficient energy to the electrons, this energy excites the electron, and the electron jumps from the valence band to the conduction band. It leaves a hole in the valence band. Now, this hole in the valence band and the electron in the conduction band are attracted to each other by the electrostatic force of attraction and make an electron-hole pair. This bound state of the electron-hole pair is known as an exciton.

There are two main types of excitons. Wannier-Mott excitons, which is also known as free, excitons and Frankel excitons, also known as bound excitons. Wannier-Mott excitons are mainly observed in semiconductors, They have a sufficiently large radius that encompasses, many atoms and can move freely throughout the crystal. Franklin are bound excitons are mainly observed in insulators. Bound excitons by contrast have a much smaller radius and are comparable to the size of the unit cell. This makes them localized states which are tightly bound to specific atoms or molecules. That is why they are known as Bound excitons.

 

This image was taken from Kéna-Cohen, S., Strong exciton-photon coupling in organic semiconductor microcavities. Princeton University: 2010.