Influence of doping profile on the efficiency of blue phosphorescent organic light-emitting diodes

We demonstrate that electroluminescent efficiency of blue phosphorescent organic light-emitting diodes could be improved by changing doping profile in the emissive layer (EML) of the device containing iridium(III)bis(4,6-difluorophenyl)-pyridinato-N,C2′) picolinate (FIrpic) and N,N′-dicarbazolyl-3,5-benzene (mCP). By examining the changes of recombination zone and current density with a partially doped EML, we found that most of the recombination occurred at the EML close to the electron transporting layer (ETL). Electron injection and transport were enhanced by increasing doping ratio at the EML close to the ETL and such a stepwise doping profile resulted in almost 40 % improved power efficiency compared to that of a conventional device.