Study of Electroplex Emission from Organic Electroluminescent Devices with Rare Earth Complex

Available online at www.sciencedirect.com JOURNAL OF

ScienceDirect JOURNAL OF RARE EARTHS 25 (2007) 148 - 151

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Study of Electroplex Emission from Organic Electroluminescent Devices with Rare Earth Complex Wang Yong ( 5 % ) , Zhao Suling (&ZX@) ++ , Xu Zheng (-% @) , Huang Jinzhao (*&@ ) , Yuan Guangcai (bi$ ) , Zhao Dewei (&&I&,) (Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology , Beijing Jiaotong University , Beijing 100044 , China ) Received 15 June 2006; revised 30 August 2006

Abstract : A novel organic electroluminescent device was made with the structure of ITO/PVK: Tb0.5EuO.5( TTA),Dipy/ BCP/Alq,/Al(a) which utilized the rare earth complex Tbo.5Euo.5(TTA)3Dipy as the emitting layer. When it was driven under a direct electric field, 612 nm emission from Eu3+ and 410 nm emission from PVK were observed. In addition, in the EL spectrum a new peak at 490 nm appeared. From the analysis of different devices, the mechanism of the new emission was studied. It was concluded that the new emission was the electroplex originating from the interface between the ligand (TTA),Dipy and BCP. Key words : electroluminescence( EL) ; ligand ; electroplex; rare earth complex Article ID : 1002 - 0721(2oO7)02 - 0148 - 04 CLC number : 0482.31 Document code : A

Much progress has been made on the electroluminescence of rare earth materials ever since the Kid0 group reported the OLED that utilizes the rare earth materials as the light-emitting material in 1 9 9 0 ~ “ ~ . The rare earth materials have many properties that are useful in the fabrication of the electroluminescent devices. The electroluminescence of the rare earth materials has a sharp spectral bandwidth, high internal quantum efficiency, and good monocolor property, which facilitates its use in flat panel displays. However, the electroplex and exciplex emission are important optical phenomena that often occur at an organidorganic interface or a blend of organic material inside an OLED, and as a result, the purity of the rare earth emission is influenced. In this article, a novel rare earth complex Tb,,5Eu,.5( TTA),Dipy ,

which is used as an emitting material in an OLED, was synthesized. The electroplex emission was observed and its mechanism was studied.

1 Experimental 1.1 Synthesis of material A homogeneous mixture of 1 mmol TbCl, * 6H20 and 1 mmol EuC13.6H20 (the mole ratios of the Ed’ to Tb3+ is 0.5 : 0.5) was prepared. The mixture was heated at 60 “c and was whisked. A total of 10 ml of 0 . 1 mo1.L-’ a-a’-dipyridyl was slowly added in to the mixture. The pH value of this mixture was adjusted to 6 7 by adding 1 mmol of triethylamine. The mixture was kept at room temperature for 24 h , and the white sediment produced in large quantity was distilled and

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* Corresponding author (E-mail : slzhao @ center. njtu .edu .cn) Foundation item: Project supported by the National Natural Science Foundation of China (60576016, 10374001, 10434030) ; “973” National Key Basic Research Foundation of China (2003CB314707) Biography : Wang Yong ( 1982 - ) , Male, Master, Engaging in organic electroluminescence Copyright 0 2 0 0 7 , by Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B . V . All rights reserved

Wang Y et a1 . Electroplex Emission from Organic Electroluminescent Devices with RE Complex

washed using alcohol. Then, the white sediment was dried at 70 "c in air. The chemical structure of the white material is shown in Fig. 1.

1.2 Fabrication and measurement of devices A total of 10 mg-ml-' of PVK and 5 mg.rn1-l of Tb, Eu, ( TTA),Dipy were dissolved in chloroform solution and were blended together at the weight ratio of 5 : 1. The PVK : Tb, sEuo (TTA),Dipy mixture was spin-coated onto a cleaned I T 0 glass substrate. Subsequently, the ITO-coated glass substrate was cleaned consecutively in an ultrasonic bath containing deionized water, ethanol and acetone. BCP , Alq,, and A1 were deposited onto the emitting layer at a chamber pressure of about 2 x 10-' Pa at a rate of 0.02 nm s-' . The thickness of BCP and Alq, were 12 and 8 nm, respectively. Five devices were fabricated with the structures as follows : ITO/PVK : Tb, sEuo (TTA),Dipy/BCP/Alq,/Al( a ) , ITO/PVK : Tb, ,Eu, s( TTA) ,Dipy/BCP/Al( b ) , ITOlPVK: Tb, Eu, ( TTA ),Dipy/Alq,/Al ( c ) , ITO/PVK/BCP/Al (d ) , and ITO/PVK : Gd ( TTA ),Dipy/BCP/Al ( e ) . The PL and EL spectra were measured on a Fluolog3 fluorescent spectrometer at room temperature.

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490 nm still existed. But in the EL spectrum (Fig. 4) of the device ITO/PVK :Tb,.sEuo.S(TTA),Dipy/Alq,/ Al, the peak at 490 nm disappeared, and only the emission from Alq3 (510 nm) and Eu3+(610 nm) were detected. It is concluded that the new emission at 490 nm is related to the layer of BCP. But it is not clear whether the new peak corresponds to the interaction between PVK and BCP. In 1.0

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EL spectra of device ( a ) ITOIPVK : Tb, Euo ('lTA),DipylBCP/Alq,/Al at different voltages 1.0-

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2 Results and Discussion 2.1 Electroluminescence

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First, the EL spectra of device ( a ) were measured, as shown in Fig.2. Besides the emission of Eu3+ at 612 nm and 'I, a new peak at 490 nm PVK emission at 410 nmL3, was observed. The emission is neither from the pure PVK and BCP nor from the ligand of the rare earth complex. In order to eliminate the possibility of spectrum overlap between the emission of PVK and Alq, , another device with the structure of ITO/PVK : Tb,,,Eu,,, (TTA),Dipy/BCP/Al was fabricated. The EL spectrum is shown in Fig.3. The emission at

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Fig. 4 Fig. 1 Chemical structure of TbosEuo.s(TTA)3Dipy

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EL spectra of device ( c ) ITO/PVK : Tbo.sE u ~ . ~ ('lTA)3Dipy/Alq3/Al at different voltages

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order to make it clear, a bi-layer device with the structure of ITO/PVK/BCP/Al was fabricated. The EL spectrum is shown in Fig. 5. Only the emission at 410 nm from PVK was observed. Thus the new emission at 490 nm does not result from the interaction between PVK and BCP. But it may originate from the interface between BCP and the ligand of the rare earth complex. According to the Jan Kalinowski Model['], when D + and A- (D denotes donor molecules and A denotes acceptor molecules ) encounter at the interface, two states may be formed: the local excited configuration ( A * D > ) and the charge transfer configuration ( I A-D' > ) . The wave function is as follows : ! P E x = C I I A * D >+ C , I A - D ' > C1 and C2 are two coefficients. They express the ability of these two states to mix with each other. I A - D + > represents the electron transition from A to D . Electrons at the LUMO energy level of A recombine directly with holes at the HOMO energy level of D to form excitons, which lead to the emission of electroplex. Electroplex occurs only under a high electric field. Fig. 6 shows the PL spectrum of the bi - layer PVK : Tb, Euo ( TTA ), Dipy / BCP . The 1

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Fig. 5 EL spectrum of device (d) : ITO/PVK/BCP/Al

emission at 490 nm was not observed. It is reasonable to conclude that it is the emission from electroplex originating from the interface between BCP and the ligand of the rare earth complex.

2.2 Mechanism of electroplex emission The LUMO and HOMO energy level of the singlet state of ligand TTA are 3 . 72 and 7 . 2 6 eV'6' 7 1 respectively. The LUMO and HOMO energy levels of BCP are 3 . 2 and 6 . 5 eV , respectively. It is possible that the electrons transit from the LUMO energy level of ligand TTA to the HOMO energy level of BCP under an electric field. This transition results in an electroplex emission at 490 nm, as shown in Fig. 7 . Under an electric field, holes and electrons are ejected from anode and cathode, respectively. Holes are transported in PVK and electrons are transported in BCP. When they get to the interface between PVK and BCP, because of the barrier (AHOM0 = 0 . 4 eV, ALUM0 = 0. 7 e V ) , holes and electrons are accumulated in the HOMO energy level of PVK and LUMO energy level of BCP separately. With the increase in voltage , some holes tunnel to the HOMO of BCP, and some relax to the LUMO of the ligand TTA. On the other hand, there are two possibilities for electrons in BCP: one possibility is that electrons leave BCP layer and form excitons in PVK, leading to the emission of PVK. The other possibility is that electrons tunnel directly to the LUMO of ligand TTA . At the PVK/BCP interface, they recombine with the holes tunneling into the HOMO energy level of BCP. As a result, the electroplex emission occurs. In order to study the influence of rare earth ions on the electroplex emission, Eu3+ and Tb3+ were replaced with Gd3+. Because the excited state of Gd3+ is very high, it mismatches greatly with the triplet state of the ligand and the ground state of BCP;

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Fig. 6 PL spectrum of Tb, s E ,(TTA),Dipy/BCP ~

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Energy level diagram of device ITO/PVK: Tbo.sEuo (TTA),Dipy/BCP/Al

Wang Y et a1 . Electroplex Emission from Organic Electroluminescent Devices with RE Complex

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recombined and led to the emission of electroplex. The study of the mechanism of electroplex emission in OLED based on rare earth complex was advantageous for the optimization of the structure of devices and obtaining the pure light from rare earth ions.

References :

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Fig. 8 EL spectrum of device (e) : ITO/PVK: Gd (TTA),Dipy/BCP/Al

therefore, it the emission from the excited state of Gd3+ to the ground state of BCP is impossible[81.The device with the structure of ITOlPVK : Gd ( TTA ),Dipy/BCP/Al was fabricated and its electroluminescence was detected, as shown in Fig. 8. The emission at 490 nm was also observed. It confirms that the electroplex emission originates from the interface between ligand of the rare earth complex and BCP.

3 Conclusion The emission of electroplex was observed in the . ~ ),Dipy/BCP/Alq,/ device ITO/PVK : Tb,,,E U ~( TTA A1 , in which PVK : Tb,,,Eu,,, (TTA),Dipy acted as the emitting layer. From the analysis of different devices, it was concluded that emission of electroplex originates from the interaction between the ligand of the rare earth complex and BCP. At the interface between the ligand and BCP , holes and electrons

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