Space charge effects on the electroluminescence efficiency and stability of organic light-emitting devices with mixed emitting layers

In organic light-emitting devices (OLEDs), the decay rate of triplet state population in the electron/hole recombination zone is found to be highly sensitive to space charge densities, providing an avenue for inferring variations in their formation. In OLEDs containing mixtures of N,N-'-Bis(naphthalen-1-yl)-N-'-bis(phenyl)benzidine (NPB) and tris(8-hydroxyquinoline) aluminum (AlQ(3)) in the emitting layer, optimizing the NPB/AlQ(3) is found to reduce hole space charges, and leads to an increase in electroluminescence stability. Conversely, electroluminescence efficiency is found to be only weakly dependent on the mixture composition, suggesting that hole space charges are not effective quenchers of AlQ(3) singlet excitons in mixed emitting layer OLEDs.