Study on Luminescence Properties of a Novel Rare Earth Complex Eu(TTA)2 (N-HPA)Phen

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JOURNAL OF RARE EARTHS 25 (2007) 143 - 147

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Study on Luminescence Properties of a Novel Rare Earth Complex Eu( TTA)2(N-HPA)Phen Zhang Yanfei (%%%)’, Xu Zheng (8% $A)’* Lii Yuguang ( 5$5)2p3Zhang Fujun (%%&)’ Wang Yong ( 3. 3 ) I , Zhang Jingchang ( %#k%)’ ( I . Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology , Beijing Jiaotong University , Beijing 100044 China ; 2 . Institute of Modern Catalysis , Beijing University of Chemical Technology Beijing 100029 , China ; 3 . Chemical and Pharmaceutical College of Jiamusi University, Jiamusi 154007 China) Received 15 October 2006; revised 3 December 2006

Abstract: A novel rare earth complex Eu ( TTA)2(N-HPA) Phen (TTA = thenoyltrifluoroacetone, N-HPA = N-phenylanthranilic acid, and phen = 1,lO-phenathroline) , which contains three different ligands , was synthesized. The Eu complex was blended with poly N-vinylcarbazole (PVK) in different weight ratios and spin coated into films. The luminescence properties of films were investigated and energy transfer between PVK and the complex was discussed. Multilayer structural devices consisting of ITO/PVK: Eu (TTA)2(N-HPA) phen/BCP/Alq3/Al were fabricated with PVK: Eu (TTA),(N-HPA) as light-emitting layer. Increasing the concentration of Eu in the PVK thin film would inhibit the emission of PVK to different degrees. Finally, the pure red luminescence of europium( ) was observed when the doping weight ratio was approximately 1:5, which indicated an effective energy transfer from PVK to rare earth complex.

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Key words : energy transfer; ligand ; electroluminescence; rare earth complex Article ID: 1002 - 0721(2007)OZ - 0143 - 05 CLC number : 0482.31 Document code : A

In recent years, organic thin film-based electroluminescent (EL) devices have attracted considerable interest in the research field of display devices because of their merits such as active light emission and easy manufacturing process for large-area flat panel display devices. However, organic micromolecules and polymer cannot meet the requirement of full color display, owing to their poor color purity. Although placing light-filtering or insulation layers onto the surface of OLED will produce narrowband emission“’, it obviously raises the complexity and cost of the technique. Another choice is rare earth comx

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plex with emission peak of typical bandwidths of 5 10 nm, which is in sharp contrast to the dull broadband emission of bandwidths of 100 200 nm for pure organics or their transition metal complex-based EL devices. Among three primary colors, red light emission is considered the weakest one[*’. The transition corresponding to red light occurs at a comparatively small energy band gap, which is hard to match the energy level of carrier transmission layer. At present, research activity involving the design and construction of novel Eu3+complexes for the light-emitting device

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Corresponding author (E-mail : zhengxu @center. njtu. edu .cn) Foundation item: Project supported by the National Natural Science Foundation of China (60576016, 10374001); Natural Science Foundation of Beijing (2073030) ; “973” National Key Basic Research Foundation of China (2003CB314707), National Natural Science Foundation of China ( 10434030) Biography : Zhang Yanfei ( 1982 - ) , Female, Master ; Research field : 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

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is prevalent because of their sharp emission peak at 1 . 2 Preparation and measurement of 612 nm. appliances According to known facts, rare earth ions are Owing to the fine thermal stability and filmexcited upon energy transfer from negative ions of liforming property of PVK, rare earth complex was gands. EL efficiency is very responsive to energy doped into PVK and films were fabricated using a level of anion of ligands, both singlet and triplet spin coating method as usua1[6-81. states. For example, Eu3+ can only accept the transTo avoid the aggregation of molecules, PVK ferred energy from triplet state, which is close to 'D, and Eu ( TTA), ( N-HPA) Phen powders were sepaenergy level of Eu3+13]. Certainly, adding approprirately dissolved in chloroform solution, and they ate ligands will enrich energy levels of ligands, were mixed in different blend mass ratios. The drops thereby strengthening the efficiency of both energy of mixed solution were then spun onto I T 0 glass to transfer and excitation. In contrast to the common rare earth complex with one or two l i g a n d ~ [ ~ * ~ ] ,form films. The I T 0 glass was separately ultrasonithree-ligand material was used in this study, which cally cleaned using ethanol, acetone, and deionized water for 10 min. Before spin coating was carried satisfies the demand of saturated coordination number out, the IT0 glass substrates were treated in ozone of Eu3+, thereby resulting in the formation of a more atmosphere for 8 min to improve the working potenstable compound with more various energy levels of tial of the anode electrode. Organic layers and alumiligands . In this article, the EL of a novel rare earth num electrode were successively fabricated using vaccomplex Eu ( TTA)2(N-HPA) Phen was reported. It uum deposition method. The thickness of BCP and was doped into Poly N-vinylcarbazole (PVK) fabriAlq, are approximately 10 nm, respectively. The cated films by spin coating method and tested using rare earth complex-based electroluminescent ( EL ) structure of EL device is ITO/PVK : Eu ( TTA ), ( Ndevices. HPA) Phen/BCP/Alq,/Al , as shown in Fig. 2 . The blend system PVK/Eu (TTA), (N-HPA) Phen is de1 Experimental signed to be a light-emitting layer. The PL and EL spectra were recorded on a SPEX Fluorolog-3 spec1 . 1 Synthesis of material trometer. All the measurements were carried out at A total of 1 mmol of EuC13*6H20,2 mmol of room temperature. HTTA, 1 mmol of N-HPA, and 1 mmol of 1,102 Results and Discussion Phen were dissolved in 50 ml of ethanol. Ammonia was added to adjust the pH value to 6 7 . A white 2 . 1 Analysis of energy transfer precipitate was produced as 1 mmol of 1 , 10-Phen was added in drops to the solution. The solution was In the emitting layer, two processes compete then stirred for 2 h at room temperature. Finally, the with each other: the light emission of Eu(7TA ), (Nwhite precipitate was filtered for future use. The HPA) Phen and PVK. To suppress the emission of chemical structure of Eu ( TTA), ( N-HPA) Phen is PVK and promote emission of Eu ( TTA )2 (N-HPA ) shown in Fig. 1. Phen simultaneously, it is desired that efficient energy transfer occur from PVK to Eu ( TTA > 2 (N-HPA ) Phen .

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Fig. 1

Schematic chemical structure of Eu (TTA), (N-HPA) Phen

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Structure of device ITO/PVK: EulBCPIAlqJAl

Zhang Y F et a1 . Luminescence Properties of a Novel RE Complex Eu ( TTA ) ( N-HPA ) Phen

According to Forster energy transfer mechanism"] , the efficiency of energy transport is determined by both distance between donor and acceptor and overlap integral between emission spectrum of donor and absorption spectrum of acceptor. To verify such kind of energy transfer, the photoluminescence (PL) and EL properties of this complex were studied and the luminescent mechanisms were discussed. The excitation spectra of the films Eu (TTA), (N-HPA ) Phen , PVK : Eu ( TTA )* ( N-HPA ) Phen , and PVK with monitor wavelengths of 420 nm are shown in Fig. 3 (PVK :Eu (TTA), (N-HPA ) Phen = 5: 1 ) . The excitation peak of blending system is much close to that of PVK . Evidently, it is PVK that initiates the energy transfer in blend system[" lo' . From Fig. 4, it can be seen that there exists an overlap between the excitation spectrum of Eu( TTA), (N-HPA) Phen and the emission spectrum of PVK film. The prerequisite of energy transfer was thus satisfied . Both the PL spectra illustrate that the

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2.2 Study of luminescence properties Increasing applicable contacts of donor ions and acceptor ions by adjusting concentration of dopant in solution promotes energy transfer. And to ensure a correlation between blending proportion and efficiency of energy transport, were the mixtures span and coated withratios3:1, 5 : 1 , 8 : 1 , 10:1, a n d 0 : l i n to films. After recording the PL emission spectra with the excitation wavelength of 345 nm, they were normalized at 612 nm to demonstrate the diverse luminescent intensity of PVK as shown in Fig. 5. In the case of relative increment of Eu ( TTA)z (N-HPA) Phen to PVK, blue shift of the emission peak for PVK appeared, which was believed to be caused by a solid solvent effect and depend on electric dipole moment of both dopant and acceptant"!. Blue shift occurs with the increase of dopant density when electric dipole moment of dopant is larger than that of acceptant. The emission intensity of PVK increased gradually with density. At mass ratio 3 : 1, spectrum is not smooth and reproducibility is poor, which is attributed to the presence of very little PVK to form film. So sample 5 : 1 was chosen to fabricate EL devices. At the same time, 8 : 1 devices were chosen as reference samples for comparison. The emission spectra of 5 : 1 and 8 : 1 devices at 16 V and normalized at 612 nm are shown in Fig. 6 . The emission of PVK disappeared in 5 :1 device, and red light with good monochromaticity was achieved . CIE coordinate was ( 0 . 4 6 , 0 . 3 3 ) . The appropriate blending proportion considerably suppressed the intrinsic luminescence of PVK. 0.14

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Forster energy-transfer process from PVK to E u ( m ), (N-HPA)Phen may occur in the blend system.

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Excitation spectra of film of Eu(TTA),(N-HPA)Phen with the monitor wavelength of 612 nm and emission spectrum of film of PVK with the excitation wavelength of 345 nm

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Emission spectra of film Eu ( 'ITA)2(N-HPA) Phen/ PVK at different mass proportion 3 : 1 , 5 : 1 , 8 : 1, l O : l , and 0 : 1 with the excitation wavelength of 345 nrn

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The normalized EL spectra of device 5: 1 at different voltages are shown in Fig. 7 . There is little or no emission of PVK . An efficient energy transfer occurred from PVK to Eu ( TTA ), ( N-HPA ) Phen . It means that 10 nm BCP layer blocked holes from effectively infusing into Alq, layer and charge carriers were retained in the emitting layer. A new emission peak appears at 490 nm, which belongs to neither PVK nor Alqa ( at approximately 520 nm>. The LUMO and HOMO of singlet state in ligand TTA are 3.72 and 7.26 eV[2231, respectively, and LUMO and HOMO of BCP are 3 . 2 and 6 . 5 eV, respectively, as shown in Fig.8. Under the function of bias, holes and electrons accumulate at the HOMO of PVK and LUMO of BCP, respectively. Besides generating radiative transition to cause emissions of PVK and rare earth ions, some of the electrons were captured by LUMO of TTA (LUMO of TTA is higher than that of BCP by 0 . 5 2 eV, which enables the electrons to overcome the potential barrier more easil y ) , and electron-hole pairs were formed, with holes at HOMO of BCP just at the interface of PVK/BCP. The electron-hole pairs then recombined to generate electroplex emission'"] . After analyzing the energy

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Normalized EL spectrum of device ITO/PVK : Eu complex (5: l)/BCP/Alg/Al at different voltages

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Fig. 8 Energy level sketch of device ITO/PVK : Eu ( TTA l2 (N-HPA) Phen/BCP/Alq,/Al

level of the device, it is concluded that the wide spectral band at approximately 490 nm is attributed to electroplex emission caused by interaction between ligands of rare earth complex and BCP. Further experiments to support the reasoning are not included in this article.

3 Conclusion A novel rare earth complex Eu(TTA),(N-HPA) Phen , which includes three different ligands , was synthesized and used as a luminescent material to fabricate efficient EL devices for the first time. The best mass ratio of PVK : Eu ( TTA ) 2 ( N-HPA ) Phen was 5 : 1 , at which the energy between PVK and Eu (TTA), (N-HPA) Phen was transferred completely, and unnecessary emission of PVK was suppressed to the largest extent. And hole-blocking layer BCP was thick enough to restrict holes in the blending layer, thereby avoiding emission from Alq, . Finally, stable red light emission of Eu3+was obtained.

References : [ l ] Zhang Ting. Energy and charge transfer in electroluminescence of dopant polymer [ D ] , Beijing: Beijing Jiaotong University Doctor Degree Dissertation, 2005. [21 Zheng Youxuan. Photoluminescence and electroluminescence of rare earth complex [ D 1. Changchun: Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 2002. [3] Li Jianyu. Rare Earth Luminescent Material and Application [ M 1. Beijing: Chemical Industry Press, 2003. 1. [4] Liu Junfeng, Teng Feng, Xu Zheng. Luminescence properties of a new kind of rare earth complexes Tb(mbenzoicacid),[ J ] . Spectroscopy and Spectral Analysis , 2004, 24(5): 519. [S] Bian Zuqiang; Huang Chunhui. Several important

%hang Y F et a1 . Luminescence Properties of a Novel RE Complex Eu ( TTA )t ( N-HPA ) Phen factors influencing electroluminescent efficiency of lanthanide complexes [ J ] . Journal of the Chinese Rare Earth Society (in Chin. ) , 2004, 22( 1) : 7 . [0] Tao Dongliang, Zhang Ting, Xu Yi zhuang, Xu Zheng. Gao Xin, Zhang Yueping, Wu Jingguang, Xu Xurong . Study of energy transfer in luminescent materials containing lanthanide elements [ J] . Journal of the Chinese Rare Earth Society (in Chin. ) , 2001, 19(6 ) : 54.3. [ 71 Liu Ling, Xu Zheng , Lou Zhidong , Zhang Fujun, Sun Bo, Pei Juan. Luminescent mechanism for Tb ( o BBA)?(phen) [ J ] . Journal of the Chinese Rare Earth Sociery (in Chin. ) , 2005, 23(6) : 25. [ 81 Guo Dong, Liang Chunjun, Lin Peng , Deng Zhenbo, Bai Feng , Li Yong , Xu Yizhuang , Wu Jinguang . PL and EL properties of Eu (BSA),phen doped in PVK

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[ J ] . Journal of the Chinese Rare Earth Society (in Chin. ) , 2004, 22(6) : 162. [9] Li Lele, Su Haiquan, Qin Jianfang. Synthesis and characterization of ternary solid complex of europium8HOQ-'lTA [ J ]. Journal of #he Chinese Rare Earth Society (in Chin.), 2006, 24(2): 22. [ 101 Zhang Yuanyuan, Deng Zhenbo, Liang Chunjun, Chen Baomei, Xiao Jing, Xu Denghui, Wang Ruifen. Emission characteristics of PVK doped TbY (0-MBA), (phen), systems [ J ] . J . Rare Earths, 2006, 24( 2) : 150. [ 111 Chao Hong, Gao Xichun, Huang Chunhui. Electroplex emission from a layer of a mixture of a europium complex and Tri ( 8-quinolinolinolato) aluminum [ J ] . Applied Surjke Science , 2000, 16 : 443.