An exciton is a Coulomb-correlated electron-hole pair, electrically neutral overall. Three useful properties are:
- Exciton binding energy: the energy required to separate the correlated pair into unbound carriers. A larger value makes thermal dissociation less effective; dielectric screening and electronic delocalization influence it.
- Spatial extent: the characteristic electron-hole separation or exciton radius. A localized Frenkel-like exciton is small compared with a Wannier-Mott exciton spread over many unit cells.
- Lifetime: the characteristic time before radiative or nonradiative decay. Together with its diffusion coefficient it determines a transport scale, often expressed as in a specified diffusion-length convention.
In many molecular organic semiconductors, limited screening and localization make exciton binding important, so absorption need not immediately produce collectable free charges.
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