In this study, we investigated the dependence of electroluminescence (EL) efficiency on carrier distribution in the light-emitting layer (EML) of the device based on $\text{Eu}{\left(\text{TTA}\right)}_3\text{phen}$ ($\text{TTA}=\text{thenoyltrifluoroacetone}$ , $\text{phen}=1,10\text{-phenanthroline}$ ) doped $4,4^{\prime }\text{-}N,N^{\prime }$ -dicarbazole-biphenyl (CBP) system. We found that EL efficiency increases monotonously with increasing hole injection even when holes are the majority carriers. This phenomenon was attributed to the accumulation of holes in EML, which improves the balance of holes and electrons on $\text{Eu}{\left(\text{TTA}\right)}_3\text{phen}$ molecules, thus enhancing the EL efficiency. To further improve the balance of holes and electrons on $\text{Eu}{\left(\text{TTA}\right)}_3\text{phen}$ molecules, the injection of electron was gently decreased by modulating the thickness of Al and LiF layers. Interestingly, EL efficiency increases gradually to a maximum and then decreases rapidly with decreasing electron injection. As a result, the device with 80 nm Al and 1.2 nm LiF obtained the maximal current efficiency of 9.53 cd/A, power efficiency of 5.35 lm/W, and external quantum efficiency of 5.15%. Our experimental results demonstrated that the balance of holes and electrons on dye molecules is the precondition for the doped device to obtain high EL efficiency, when carrier trapping is the dominant EL mechanism.

The electron mobilities of NDI-C14 based thin film transistors get a great improvement by vapor deposition on the surface of three SAMs modified SiO₂ dielectric. This is associated with its close packing arrangement of NDI-C14 film on these SAMs.