Effects of triplet energy confinement by charge transporting layers on blue phosphorescent organic light emitting diodes

We have studied the effects of hole transporting layers and electron transporting layers on efficiencies of Iridium(III)bis [(4,6-di-fluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) doped 3,5'-N,N'-dicarbazole-benzene (mCP) host blue PHOLEDs. We found that the device efficiency is very sensitive to the hole transporting materials used and both the triplet energy and carrier transport properties affect the device efficiency. On the other hand, there is no apparent correlation between the device efficiency and the triplet energy of the electron transporting material used. Instead, the device efficiency appears to be determined by the electron mobility of the electron transporting layer only.