Investigation of double emissive layer structures on phosphorescent blue organic light-emitting diodes

The performances of blue phosphorescent organic light-emitting diodes (PHOLEDs) at high current densities have been investigated with double emissive layer structures (D-EMLs). The D-EMLs are comprised of two emissive layers with a hole-transport-type host of N,N′-dicarbazolyl-3,5-benzene (mCP) and an electro transport-type ultrawide band-gap host of m-bis-(triphenylsilyl)benzene (UGH3) both doped with a blue electro-phosphorescent dopant of iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C2′) picolinate (FIrpic). The expansion of hole/electron recombination zone in D-EMLs has been successfully achieved by controlling of each EML properties, therefore external quantum efficiency, especially at high current density region was significantly enhanced. Moreover, the blue PHOLED with D-EMLs showed substantially reduced roll-off with the external quantum efficiency of 10.0% at 5000 cd/m2.