Dependence of charge trapping of fluorescent and phosphorescent dopants in organic light-emitting diodes on the dye species and current density

This paper utilizes multilayer organic light-emittingdiodes with a thin layer of dye molecules to study the mechanism ofcharge trapping under different electric regimes. It demonstratesthat the carrier trapping was independent of the current density indevices using fluorescent material as the emitting molecule while thisprocess was exactly opposite when phosphorescent material was used.The triplet--triplet annihilation and dissociation of excitons intofree charge carriers was considered to contribute to the decrease inphosphorescent emission under high electric fields. Moreover, thefluorescent dye molecule with a lower energy gap and ionizedpotential than the host emitter was observed to facilitate thecarrier trapping mechanism, and it would produce photon emission.