Enhanced Electroluminescence Efficiency of Phosphorescent Organic Light-Emitting Diodes by Controlling the Triplet Energy of the Hole-Blocking Layer

This letter reports on the effect of the triplet energy (E _T) of the hole-blocking layer (HBL) on triplet exciton quenching between the emissive layer (EML) host and the HBL of phosphorescent organic light-emitting diodes (PHOLEDs). Using different EML hosts and HBLs having different E _T\´s and electron mobilities, the effects of the E _T\´s of the HBL have been analyzed. When the E _T of the HBL is lower than that of the EML host, the PHOLEDs show significant dependence of reduced device performances. PHOLEDs having a 3-(4-biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole HBL show 65% improved external quantum efficiency (EQE) at 500 cd/m² with a 4, 4\´,4"-tris( N-carbazolyl)-triphenyl-amine host than that with an N,N\´ -dicarbazolyl-4-4^\´\´biphenyl host, while PHOLEDs having a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline HBL show 41% reduced EQE. To solve the remaining key issue of developing highly efficient PHOLEDs, the E _T of matching the EML and the HBL is extremely desirable and is also explored.