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ID: past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2017/paper-2/section-a-iii/19/ii/solution

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Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2017 / Paper 2 / Section A(iii) / 19 / ii / Solution by

Codex  0 2026-10-20

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 $L_{D} = D_{X} τ_{X}$ 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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