The electroluminescent investigation of double layer Eu-complex organic electronic luminescence diodes
Introduction
Organic electronic luminescence diodes (OLEDs) attracted many researchers attention because of their potential for large-area, flexible light emitting displays, and so on. It has obtained great progress since Tang and VanSlyke demonstrated the high efficiency double layer structure device [1]. Their device consisted of a hole transporting layer and an emitting layer, and the hole transporting layer can both improve the injection of holes and block electrons, thus the efficiency was improved. However, high efficiency pure red emission was still a challenge for OLEDs. Kido [2] first reported the narrow band emission centered at 612 nm of rare earth Eu3+ was suitable for red emission of OLEDs, and it has high photoluminescence efficiency which caused by the inter-crossing, however, the EL luminescence efficiency was rather low, especially, when it was operated at high current density. Many researchers have studied the mechanism of the efficiency decline and favor to improve it by changing the emission material [3], [4], [5] or optimizing the device structure [6], [7], [8], [9]. Our group has successfully fabricated bright pure red emission by mixing with Eu-complex and TPD [6]; at the same time, it was found that the EL efficiency depends on the molecular ratio of the mixing layer and the highest external quantum efficiency was 4.6% [7]. Hu and his coworker [8] improved the device efficiency using the alternate layer of BCP and Eu-complex as emitting layer and obtained an optimal alternate number for obtaining a high EL efficiency, 3 cd/A. Besides, they reported that the efficiency of the emission from Eu(DBM)3TPPO can be improved by increasing the thickness of the Eu(DBM)3TPPO layer. They attributed this effect to the thicker Eu(DBM)3TPPO layer reduced the hole density in this layer, therefore, the quenching of Eu(DBM)3TPPO in the excited state by the holes was lessened [9].
In this paper, we observed the efficiency of the devices with Eu-complexes was improved at low current density with increasing the thickness of TPD layer and the evolution of emission spectra on operating voltage. Using the dependence of EL efficiency on current density and the carrier injection characteristic, we discussed the reason of the improved EL efficiency. Besides, the change of emission spectra was mostly due to the annihilation of excited state of Eu(DBM)3bath.
Section snippets
Experiments
Fig. 1 shows the materials molecular structures used in this experiment. The devices were based on glasses coated by ITO with a sheet resistance of 100 Ω/□, which was used as anode, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) was the hole transport layer, 40 nm europium(dibenzoylmethanato)3monophenanthroline (Eu(DBM)3bath) was both the electron transport layer and the emitting layer, and the cathode was composed of an ultra-thin LiF layer (1.5 nm) and Al layer (200 nm).
Results and discussion
The relationship between the luminescence efficiency and current density with different thickness of hole transporting layer is shown in Fig. 2. The maximum efficiency of the emission from Eu(DBM)3bath can be improved from 1.38 to 10 cd/A, through increasing the TPD layer from 10 to 80 nm (see the insert of Fig. 2). Besides, the maximum luminescence efficiency of each device was obtained at different current density, and with increasing TPD layer, the corresponding current density was decreased.
Conclusion
In summary, we have obtained the high electroluminescence efficiency, about 10 cd/A, from at low current density region by increasing the hole transporting layer, which cause the low current density at a certain voltage. Through the comparing of emission spectra, the decomposing of Eu(DBM)3bath was found, which partially have effect on the drastic decreasing of efficiency at high current density. This indicated that only increasing the thickness of hole transporting layer or other function layer
Acknowledgments
This work is supported by the National Science Research Project (No. 90201012) of China.
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2012, Thin Solid FilmsCitation Excerpt :This can be ascribed to the reaction between β-diketone and the 3-chloropropyltriethoxysilane inorganic matrix, which gave rise to the hybrid material. The shift of the band at 679 cm− 1 in the spectrum of dbm–Cl to 691 cm− 1 in the spectrum of dbm–Si–Eu is assigned to the formation of a chelate between β-diketone [10–14] and Eu3+ [19–23]. Formation of the complex Eu3+–β-diketone is evident from the bands at 1719 cm− 1 and 1663 cm− 1 that are present in the spectrum of dbm–Si–Eu [10,11,24,25].
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