Bimolecular excited species in optical emission from organic electroluminescent devices

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.