Exciton

An exciton is a bound state of an electron and a hole in a solid, held together by Coulomb attraction. It forms when a semiconductor or insulator absorbs a photon and promotes an electron from the valence band to the conduction band, leaving a positively charged hole. The exciton is a neutral quasiparticle that can transfer energy without net charge movement.

Two main classes are Frenkel and Wannier-Mott excitons. Frenkel excitons are tightly bound with radii on the order of a lattice constant and occur mainly in organic crystals. Wannier-Mott excitons have larger radii spanning several lattice constants and are common in inorganic semiconductors with strong dielectric screening.

Binding energies vary: Frenkel excitons are typically tens to hundreds of meV; Wannier-Mott excitons are typically a few to tens of meV in bulk materials, though two-dimensional materials often exhibit binding energies in the hundreds of meV. Excitons can diffuse and have lifetimes from picoseconds to nanoseconds; they may dissociate into free carriers under thermal energy, electric fields, or at heterojunctions.

Observation and theory: Excitons are studied by absorption and photoluminescence spectroscopy. The simplest descriptions use the hydrogenic model or the effective mass approximation for Wannier-Mott excitons, with more accurate treatments based on solving the Bethe-Salpeter equation.

Applications: Excitons influence the performance of solar cells, light-emitting diodes, and photodetectors. They also enable exciton-polaritons in microcavities, a platform for exploring light–matter interactions.