Electroluminescence of hole block material caused by electron accumulation and hole penetration

In this study, we investigated the electroluminescence (EL) mechanisms and processes of hole block material in the multilayer devices with Eu(TTA)(3)phen (TTA = thenoyltrifluoroacetone, phen = 1,10-phenanthroline) doped CBP (4,4'-N,N'-dicarbazolebiphenyl) as the light-emitting layer (EML). First, the hole block ability of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) was experimentally confirmed by comparing the EL spectra. With increasing hole injection, BCP emission emerges and increases gradually due to the increasing hole penetration from EML into the hole block layer (HBL). Interestingly, we found that BCP emission decreases gradually with decreasing electron injection, which was attributed to the decreasing electron accumulation in HBL. By analyzing the evolution of carrier distribution in EML and HBL with decreasing electron injection, we concluded that two preconditions are very important for the EL of BCP: one is the accumulation of electrons in HBL, the other is the penetration of holes from EML into HBL. In addition, we further investigated the detailed penetration processes of holes from EML into HBL, and demonstrated that the doping of Eu(TTA)3phen molecules in EML facilitates the penetration of holes because they function as ladders between CBP and BCP molecules.