Enhanced Efficiency Roll-Off and Driving Voltage in Hybrid Blue Organic Light-Emitting Diodes with Triplet Harvesting and Mixed Host System

We present the systematic study of a triplet harvesting (TH) mechanism and a mixed host system in the emission layer (EML), which affects the enhanced efficiency roll-off and driving voltage of hybrid blue organic light-emitting diodes (HB OLEDs). The HB OLEDs are fabricated with 1,6- Bis(N-phenyl-p-CN-phenylamino)-pyrenes and bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate as blue fluorescent and phosphorescent emitters within the EML. The TH mechanism transfers triplet excitons from fluorescence to phosphorescence, where they can radiatively decay. In addition, a mixed host system achieves better control of the N,N′-dicarbazolyl-3,5-benzene and 2,2′,2″(1,3,5- tris)tris(1-phenyl)-1H-benzimidazol as hole and electron transport characteristics in the recombination zone through controlling the thickness of the mixed host in the EML. By varying the thickness of the mixed host layer with fluorescent and phosphorescent EML, we show that the TH and mixed host architecture improve electroluminescent efficiency and reduced driving voltage by triplet exciton diffusion into the phosphorescence also reducing the triplet quenching at the EML/electron transport layer interface.