Confinement of holes and electrons in blue organic light-emitting diodes with additional red emissive layers

We used various emissive layer (EML) structures with ultrathin red EMLs to enhance the charge carrier balance and carrier recombination rate in blue PHOLED devices. These EML materials have different energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The ultrathin red EMLs, which were inserted in between the blue EMLs, effectively confined the charge carriers in EML, and increased the carrier recombination rate. The thickness of the individual EML was optimized, under 30nm of the total thickness of EML. The blue PHOLEDs with ultrathin red EMLs achieved a luminous efficiency of 19.24cd/A, which was 28.7% higher than those without ultrathin red EMLs, and the maximum external quantum efficiency was 11.81% at 500cd/m2.