Effect of Electric Field on Coulomb‐Stabilized Excitons in Host/Guest Systems for Deep‐Blue Electrophosphorescence

Here, a study of the electric field induced quenching on the phosphorescence intensity of a deep‐blue triplet emitter dispersed in different host materials is presented. The hosts are characterized by a higher triplet excitonic level with respect to the emitter, ensuring efficient energy transfer and exciton confinement, whereas they differ in the highest occupied molecular orbital (HOMO) alignment, forming type I and type II host/guest heterostructures. While the type I structure shows negligible electric field induced quenching, a quenching up to 25% for the type II at a field of 2 MV/cm is reported. A similar quenching behaviour is also reported for thin films of the pure emitter, revealing an important luminescence loss mechanism for aggregated emitter molecules. These results are interpreted by considering Coulomb stabilized excitons in the type II heterostructure and in the pure emitter, that become very sensitive to dissociation upon application of the field. These results clarify the role of external electric field quenching on the phosphorescence of triplet emitters and provide useful insights for the design of deep‐blue electrophosphorescent devices with a reduced efficiency roll‐off. The effect of electric field on the phosphorescence of type I and II emitting structures for deep‐blue OLEDs is investigated. Among the different effects influencing the efficiency of OLEDs based on phosphors, electric field induced quenching of phosphorescence is very sensitive to the emitter‐host combination. The design rules for obtaining material combinations capable of reduced quenching are outlined and discussed.