Geminate Recombination in Some Amorphous Materials

Abstract

Transfer of charge between localized states is believed to play a dominant role in photogeneration and charge transport in many organic and inorganic disordered solid materials. Until recently, however, the main experimental efforts were devoted to inorganic noncrystalline semi-conductors, in particular, amorphous chalcogenides and tetrahedrally bonded solids, where the localized states arise from atomic bond disorder. These studies led to significant progress in our understanding of the many novel aspects of charge transport and generation in disordered materials. The studies employed in this rapidly growing discipline have, in more recent years, also been extended into the realm of disordered organic solids. It already seems apparent that amorphous organic solids exhibit rather fundamental differences compared to the differences in degree that exist between the chalcogenides and tetrahedrally bonded solids. Such differences stem from the fact that organic crystalline solids are typically characterized by very weak van der Waals-type bonding between the molecules, which are the constituent building blocks. The weak intermolecular interactions lead to very narrow energy bands, resulting in low mobilities for which band theories of transport are either questionable or invalid. Almost intuitively, therefore, one expects the noncrystalline organic state to be dominated by the molecules themselves and dynamic charge transfer in both photogeneration and transport to involve charge exchange between neighboring molecules.