Bimolecular excited species in optical emission from organic electroluminescent devices

Abstract The article provides an overview of emissive states generated in emitting layers (EMLs) of organic electroluminescent devices, e.g. organic light-emitting-diodes (LEDs). In EMLs of organic LEDs the oppositely charged carriers (usually holes and electrons) recombine (combine) releasing a considerable amount of energy sufficient to generate various excited states. These are either (i) molecular excited states (localized excitons), or (ii) bi-molecular (B-M) species: excimers, electromers, exciplexes and electroplexes. In contrast to localized excitons, B-M excited states, being rather hardly populated by exciting light, are efficiently generated under electrical excitation and lead often to dramatic differences between photo- and electro-luminescence spectra of the same molecular solid. The consequences of the formation of B-M excited species for optical and electrical characteristics of organic LEDs are discussed and illustrated by various examples.