Exciton

An exciton (English exciton of excitation, excitation ) is a bound electron -hole pair in an insulator or a semiconductor. It is thus an elementary excitation of the solid and also as a phonon or polaron a quasiparticle. An exciton can move through the crystal and transported while its excitation energy through it without a charge transport takes place, since the exciton is electrically neutral. Excitons have integer spin.

An exciton plays an important role in the absorption of light in semiconductors. It may arise, for example, when a photon enters a semiconductor, and excites an electron to transfer from the valence band into the conduction band. The electron and the valence band resulting oppositely charged holes run through the Coulomb force at each other. This situation is similar to a hydrogen atom and can also describe quantum mechanical analog. The bound electron / hole pair has a slightly lower energy than the unbound state. However, the spatial extent of the binding energy is usually much smaller and much larger than the hydrogen atom, since the Coulomb interaction between electron and hole is partially shielded ( " screening").

Depending on their properties, a distinction between the limiting cases of the Wannier excitons and Frenkel, with quite known intermediate states exist.

Mott - Wannier excitons

Mott - Wannier excitons ( Nevill Francis Mott and according to Gregory Hugh Wannier ) describe phenomenologically the limit of large distances. An electron and a hole, described by effective masses orbiting each. The influence of the surrounding solid is considered in continuum approximation by an effective dielectric constant.

The energy E of a Wannier exciton in a semiconductor is given analogous to the hydrogen atom may be approximated by

With

The mean negative term is herein often referred to as an exciton binding energy

Frenkel excitons

Frenkel excitons, named after Yakov Ilich Frenkel, describe the inverse approximation, are localized in the electron and hole at one lattice site. The energy of interaction is then essentially as the overlap of the charge clouds ( described by the wave function of the electron and the hole ) to be seen.

A Frenkel exciton is observed when the material in which it has been suggested that having high exciton binding energy. In particular, the purely thermal excitation is then not enough at room temperature to separate the electron and hole apart ( dissociate ). In the deep mutual potential funnel electron and hole in a small distance ( order of 1 nm ) are held each other. What has been described is the rule in organic semiconductors and accordingly important for their description. Here, an exciton binding energy typically in the order of 1 eV.

Energy transfer in photosynthesis and in organic solar cells

Energetic exciton transfer plays a role in the antenna complexes of photosystems of plants. The antenna pigments photosystems be brought about by the absorption of light in an excited state. The energy is transferred to the adjacent radiation-free pigments ( Förster resonance energy transfer). Only when the pigment dimer was added by the transmission of excitons in an excited state in the reaction center, an electron transfer occurs. This is one of the molecules of the " special pair " from an electron, which is replaced by an electron from the photolysis of water.

Even in organic solar cells excitons need to be broken to release the energy. This is achieved inter alia by the use of so-called heterojunctions.