Overview

An exciton is a bound state of an electron and an associated positive charge carrier known as an electron hole. Because the negative electron and the positive hole attract one another by the Coulomb force, they can form a neutral quasiparticle that moves through a solid as a single entity. Excitons are characteristic of insulating and semiconducting materials and appear when the system is excited by light or an electrical stimulus.

Types and key properties

Two common idealized categories are distinguished by their spatial extent and binding strength. Frenkel excitons are tightly bound and localized on a single molecule or lattice site, typical of organic crystals and molecular solids. Wannier–Mott excitons have a radius that spans many unit cells and are more weakly bound; these appear in conventional inorganic semiconductors and in many two-dimensional materials. Binding energies and characteristic radii vary widely: Frenkel excitons tend to have larger binding energies and smaller radii, while Wannier–Mott excitons have smaller binding energies and larger radii, often comparable to the material’s dielectric screening length.

Formation, dynamics and observation

Excitons form when photons promote electrons across a band gap, or by electrical injection in devices. Their lifetime depends on radiative recombination (emission of a photon) and nonradiative processes, and on the material environment. Exciton motion is described by diffusion and can be influenced by disorder, interfaces and temperature. Experimental signatures include distinct absorption peaks, photoluminescence lines and changes in reflectance or scattering spectra; time-resolved optical methods reveal lifetimes and transport.

Applications and advanced phenomena

Excitons are central to the operation and optimization of solar cells, organic light-emitting diodes, photodetectors and emerging excitonic circuits. In strongly coupled optical cavities, excitons hybridize with photons to form exciton–polaritons, which can exhibit macroscopic coherence and condensation-like behavior at relatively high temperatures. Indirect excitons in layered or bilayer structures can have long lifetimes and are studied for excitonic transport and collective phases.

  • Trion: a charged complex made of an exciton plus an extra electron or hole.
  • Biexciton: a bound state of two excitons, analogous to a molecule.
  • Free carriers: unbound electrons and holes that do not form a bound state.

Historically the concepts of localized and extended excitons were developed in early solid-state physics to explain optical spectra of crystals. Modern research continues to explore excitons in low-dimensional materials, their manipulation with light and fields, and their role in optoelectronic technologies.