Concept of excitons

The concept of Wannier-Mott excitons is valid for inorganic semiconductors such as Si, Ge, and GaAs, because, in these materials, the large overlap of interatomic electronic wave functions enables electrons and holes to be far apart but bound in an excitonic state. The excitonic concept from the theory point of view was initially developed only for crystalline solids, and it used to be believed that excitons cannot be formed in amorphous semiconductors. This chapter presents concepts of excitons in crystalline and amorphous solids, both inorganic and organic. In organic semiconductors, mainly Frenkel excitons are created, and the absorption of photons creating Frenkel excitons, both singlet and triplet, is described in detail. Two processes of exciting triplet excitons are described: direct excitation to the triplet through the exciton-spin-orbit-photon interaction; and indirect excitation through the intersystem crossing caused by the exciton-spin-orbit-molecular vibration interactio.