In this work, the detailed conversion process of the dominant electroluminescence (EL) mechanism in a device with Eu(TTA)3phen (TTA=thenoyltrifluoroacetone, phen=1,10-phenanthroline) doped CBP (4,4′-N,N′-dicarbazole-biphenyl) film as the emitting layer was investigated by analyzing the evolution of carrier distribution on dye and host molecules with increasing voltage. Firstly, it was confirmed that only electrons can be trapped in Eu(TTA)3phen doped CBP. As a result, holes and electrons would be situated on CBP and Eu(TTA)3phen molecules, respectively, and thus creates an unbalanced carrier distribution on both dye and host molecules. With the help of EL and photoluminescence spectra, the distribution of holes and electrons on both Eu(TTA)3phen and CBP molecules was demonstrated to change gradually with increasing voltage. Therefore, the dominant EL mechanism in this device changes gradually from carrier trapping at relatively low voltage to Förster energy transfer at relatively high voltage.
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 Eu(TTA)3phen (TTA=thenoyltrifluoroacetone, phen=1,10-phenanthroline) doped 4,4′-N,N′-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 Eu(TTA)3phen molecules, thus enhancing the EL efficiency. To further improve the balance of holes and electrons on Eu(TTA)3phen 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.
Due to unique photo-physical characteristics, rare earth (RE) complexes play important roles in many fields, for example, telecommunications, life science, and organic light-emitting diodes (OLEDs). Especially, thanks to narrow emission bandwidth and 100% theoretical internal quantum efficiency (IQE), the study of RE complexes in the electroluminescence field has been a hot research topic in recent 30 years. As a leading technology in solid-state light source fields, OLEDs have attracted great interest from academic researchers and commercial endeavors. In the last decades, OLED-based products have trickled into the commercial market and developed quickly into portable display devices. Here, we briefly introduce the luminescent characteristics and electroluminescent (EL) study of RE complexes in material synthesis and device design. Moreover, we emphatically reveal the innovative application of RE complexes as sensitizers in OLEDs. Through experimental validation, the application of RE complexes as sensitizers can realize the complementary advantages of RE complexes and transition metal complexes, leading to significantly improved performances of OLEDs. The application of RE complexes as sensitizers provides a new strategy for designing and developing novel high performances OLEDs.
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