= Solution
An \b[<exciton>] is a Coulomb-correlated electron-hole pair, electrically neutral overall. Three useful properties are:
* \b[<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.
* \b[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>.
* \b[Lifetime:] the characteristic time before radiative or nonradiative decay. Together with its <diffusion coefficient> it determines a transport scale, often expressed as $L_D=\sqrt{D_X\tau_X}$ 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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