Synthetic and electroluminescent properties of two novel europium complexes with benzimidazole derivatives as second ligands

Synthetic and electroluminescent properties of two novel europium complexes with benzimidazole derivatives as second ligands

Synthetic Metals 128 (2002) 241–245 Synthetic and electroluminescent properties of two novel europium complexes with benzimidazole derivatives as sec...

116KB Sizes 0 Downloads 15 Views

Synthetic Metals 128 (2002) 241–245

Synthetic and electroluminescent properties of two novel europium complexes with benzimidazole derivatives as second ligands Ling Huanga, Ke-Zhi Wanga,b, Chun-Hui Huanga,*, De-Qing Gaoa, Lin-Pei Jinb a

State Key Laboratory of Rare Earth Materials Chemistry and Applications and the University of Hong Kong Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry, Peking University, Beijing 100871, PR China b Department of Chemistry, Beijing Normal University, Beijing 100875, PR China Received 3 November 2000; received in revised form 10 August 2001; accepted 2 October 2001

Abstract With two second ligands, 2-(2-pyridyl)benzimidazole (HPBM) and 1-ethyl-2-(2-pyridyl)benzimidazole (EPBM), two novel europium complexes, Eu(TTA)3HPBM and Eu(TTA)3EPBM (TTA, thenoyltrifluoroacetonato) were synthesized and applied to organic electroluminescent (EL) devices as the emitting materials. The devices of ITO/TPD/Eu(TTA)3HPBM (or Eu(TTA)3EPBM)/AlQ/Al were fabricated. The EL devices emit red light originating from these two europium complexes. The EL brightness of Eu(TTA)3EPBM is much higher than that of Eu(TTA)3HPBM. For Eu(TTA)3EPBM, a maximum luminance of 36.6 cd/m2 was achieved at 21 V. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Europium complex; The second ligand; 2-(2-pyridyl)benzimidazole; 1-ethyl-2-(2-pyridyl)benzimidazole; Organic electroluminescent devices; Red photoluminescence

1. Introduction Efficient organic electroluminescent devices (OELDs) are promising candidates for the new generation of full-color flat panel displays. Three primary colors blue, green and red are required for commercial application. Many efforts have been spent on developing pure red emitting materials with high quantum and luminous efficiencies. Fluorescent and phosphorescent dyes, such as DCM2 [1], PtOEP [2] and Bptp2Ir (acac) [3] are under investigations. However, they emit about 100 nm broad spectra bandwidth. Europium complexes are suitable luminescent materials for red primary color because they show very strong red Eu3þ ion emission at around 614 nm with very sharp spectral bandwidth (full width at half maximum less than 5 nm). Furthermore, they exhibit high internal quantum efficiency. Hence, they have been applied in OELDs by synthesizing different b-diketones to coordinate with Eu(III) ions [4–7] or using the technique of co-deposition with another material to avoid the selfquenching of the europium complexes [8–11]. However, * Corresponding author. Tel.: þ86-10-6275-7156/2772; fax: þ86-10-6275-1708. E-mail addresses: [email protected], [email protected] (C.-H. Huang).

the luminance and electroluminescent (EL) efficiency based on the Eu complexes are low due to their poor carrier transporting properties. One of the ways to solve this problem is to introduce a second ligand. For example, using bathophenanthroline (Bath) as the second ligand, electron-transporting characteristics and light-emitting properties of the europium complex Eu(DBM)3Bath (DBM, benzoylmethide) apparently can be improved. Hu et al. [12,13] reported that the europium complex Eu(DBM)3TPPO has hole conductivity with triphenylphosphine oxide (TPPO) as the second ligand. We have reported that Eu(DBM)3HPBM and Eu(DBM)3EPBM (HPBM, 2-(2-pyridyl)benzimidazole; EPBM, 1-ethyl-2-(2pyridyl)benzimidazole) can be easily deposited as a transparent and homogeneous thin film by vacuum deposition [14]. Pure red EL emission was obtained using devices of ITO/TPD/Eu(DBM)3EPBM/AlQ/Al with the maximum brightness of 180 cd/m2 at 16 V, where TPD and AlQ are abbreviations of N,N0 -diphenyl-N,N0 -bis(3-methylphenyl)1,10 -biphenyl-4,40 -diamine and tris(8-hydroxyquinoline). In this paper, thenoyltrifluoroacetonato (TTA) was chosen as the first ligand due to its high photoluminescent efficiency in Eu(TTA)3Phen. Eu(TTA)3HPBM and Eu(TTA)3EPBM were synthesized and applied in the triple-layer EL device as the emitting layer.

0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 6 5 6 - 7

242

L. Huang et al. / Synthetic Metals 128 (2002) 241–245

2. Experiment

3. Results and discussion

HPBM and EPBM were synthesized according to the procedure in [14]. Eu(TTA)3HPBM and Eu(TTA)3EPBM were synthesized by the conventional method [15]. Ethanol solution of 1 mmol HPBM (or EPBM) and 3 mmol HTTA was neutralized by 2 mol/dm3 NaOH aqueous solution 1.5 ml, then added dropwise with 1 mmol Eu(NO3)3 aqueous solution while stirring. The resulting solution was heated at 60 8C for 0.5 h. The crude product was collected by filtration and washed with hot ethanol. Calc. for Eu(TTA)3HPBM (C36H21O6F9S3N3Eu): C, 42.76; H, 1.98; N, 4.05. Found: C, 42.77; H, 2.08; N, 4.16. Calc. for Eu(TTA)3EPBM (C38H25O6F9S3N3Eu): C, 43.20; H, 2.36; N, 3.84. Found: C, 43.93; H, 2.41; N, 4.05. The organic materials and the cell configurations of the EL devices are show in Fig. 1. In the devices, TPD was used as the hole-transporting layer, Eu(TTA)3HPBM (or Eu(TTA)3EPBM) as emitting layer and AlQ as the electron-transporting layer. The thickness of TPD is 40 nm while the total thickness of Eu complex and AlQ layer is 80 nm with different thickness ratios of Eu complex and AlQ. The devices were fabricated by sequential thermal deposition of the materials onto an indium–tin oxide (ITO) substrate below a pressure of 1  103 Pa in one time. The ITO layer, supplied by Chinese South Glass Group, is about 150 nm thick with a sheet resistance of 15 O/&. The cleaning procedure included sonication in detergent solution, pure water, acetone, toluene and ethanol. The deposition rates for the organic materials and aluminum were 0.2 and 1.2 nm/s. PL and EL were measured with a Hitachi F-4500 fluorescence spectrophotometer. The brightness was measured with a ST-86LA spot photometer. The layer thickness was controlled in vacuo with an IL-1000 quartz crystal monitor and was also calibrated by a Dektak [16] surface profile measuring system. All measurements were carried out at room temperature under ambient atmosphere.

The EL spectra with different thickness ratios of Eu(TTA)3HPBM and AlQ are shown in Fig. 2. It can be seen that the spectral feature is sensitive to the thickness ratio of the emitting and electron-transporting layers. The EL spectrum of the ITO/TPD (40 nm)/Eu(TTA)3HPBM (60 nm)/AlQ (20 nm)/Al device is consistent with the PL spectrum of Eu(TTA)3HPBM in vacuum-deposited films on quartz substrates (Fig. 3), indicating the device emission only from the complex of Eu(TTA)3HPBM and without the contribution of AlQ. However, while the thickness of the emitter layer get reduced, a broad band around 530 nm appears and increases, which is due to the emission from the electron-transporting layer AlQ. To the naked eyes, the color of the devices changes from red to yellowish orange. This can be attributed to exciton migration from Eu(TTA)3HPBM to AlQ at their interface. A red brightness was obtained with the maximum luminance of 0.43 cd/m2 at 26 V and 1.58 mA/cm2 in the device of ITO/TPD (40 nm)/ Eu(TTA)3HPBM (60 nm)/AlQ (20 nm)/Al (100 nm), while yellowish orange brightness of 98.3 cd/m2 was achieved at 26 V and 122 mA/cm2 in the device of ITO/TPD (40 nm)/ Eu(TTA)3HPBM (20 nm)/AlQ (60 nm)/Al (100 nm). Fig. 4 shows the EL spectra with different thickness ratios of Eu(TTA)3EPBM and AlQ. The spectral feature hardly varies with the thickness ratios of the emitting and electrontransporting layers. The brightness of the devices increases with the increasing thickness of AlQ. This indicates that the hole–electron recombination is almost complete in the emitting layer in spite of the different thickness ratios of Eu(TTA)3EPBM/AlQ. In the device of ITO/TPD (40 nm)/ Eu(TTA)3EPBM (40 nm)/AlQ (40 nm)/Al (100 nm), a pure sharp red light emission was obtained, which can be contributed to the characteristic transition of Eu3þ of 5 D0 ! 7 Fj ðj ¼ 04Þ. The maximum brightness of 36.6 cd/m2 was obtained at 21 V and 201 mA/cm2.

Fig. 1. The organic materials and the cell configurations of the EL devices.

L. Huang et al. / Synthetic Metals 128 (2002) 241–245

243

Fig. 2. The normalized EL spectra of the EL devices with different Eu(TTA)3HPBM and AlQ thickness at 20 or 21 V. The total of Eu(TTA)3HPBM and AlQ thickness is fixed at 80 nm.

Fig. 3. PL spectra of Eu(TTA)3HPBM (dashed line) and Eu(TTA)3EPBM (solid line) in vacuum-deposition films on quartz substrates.

From the data above, the EL property of Eu(TTA)3EPBM is much higher than that of Eu(TTA)3HPBM. The similar results was obtained using Eu(DBM)3HPBM and Eu(DBM)3EPBM as the emitters [14]. This can be explained by the different chemical structures of these two second ligands. It is well know that the high vibration frequency of N–H bond can contribute to the quenching of the lanthanide emission [17], which may not be desirable for EL operation. The substitution of ethyl can reduce the role of quenching. Furthermore, tertiary amine is more stable in the high electric field than the binary amine [1]. A typical example is the remarkable improvement in EL stability using the N,N-dimethyl substituted quinacridones instead of the quinacridones as the emitters. In addition, the introduction of

ethyl group can help the hole–electron recombination almost in the emitting layer and the film formation is improved [11]. Therefore, Eu(TTA)3EPBM is a better red emitting materials than Eu(TTA)3HPBM. Compared with the brightness of previously reported Eu(TTA)3Phen (7 cd/m2, 16 V, 125 mA/cm2) [13], the luminance level of Eu(TTA)3EPBM is also improved because the second ligand of EPBM has one additional N atom and greater p-electron density than that of Phen. But the luminance level of Eu(TTA)3HPBM is lower than that of Eu(TTA)3Phen. This can be partly consequent on the relatively poor electron-transporting property. Among the three europium complexes, the EL efficiency of the europium complex with EPBM as the second ligand is the highest.

244

L. Huang et al. / Synthetic Metals 128 (2002) 241–245

Fig. 4. The normalized EL spectra of the EL devices with different Eu(TTA)3EPBM and AlQ thickness at 20 or 21 V. The total of Eu(TTA)3HPBM and AlQ thickness is fixed at 80 nm.

Fig. 5. The luminance (open square)–current density (solid circle)–voltage characteristics of an ITO/TPD (40 nm)/Eu(TTA)3EPBM (40 nm)/AlQ (40 nm)/Al (100 nm) triple-layer EL device.

When the second ligands was same, the EL property of Eu(TTA)3HPBM and Eu(TTA)3EPBM is much lower than that of Eu(DBM)3HPBM and Eu(DBM)3EPBM. This indicates the importance of the role of the first ligands in the europium complexes using in OELD. It is notable that the effect of the first ligands is bigger than that of the second ligands. The luminance–current density–voltage characteristics of an ITO/TPD (40 nm)/Eu(TTA)3EPBM (40 nm)/AlQ (40 nm)/Al (100 nm) triple-layer EL device are shown in Fig. 5. It shows that the luminance increases with increasing injection currents and bias voltages, because the recombination efficiency increases as the more charge carriers are injected from the anode and cathode. Above a bias voltage of

21 V, the luminance decreases despite increased current densities. It can be partly attributed to the quenching of the excited state of the europium complex in high concentration of charge carriers [18].

4. Conclusions Two novel europium complexes with two different second ligands were synthesized and used to prepare multilayer EL devices. A very sharp bright red EL spectral peak originating from Eu(TTA)3EPBM was obtained with the maximum luminance of 36.6 cd/m2 from a triple-layer device. By comparing the EL efficiency and the chemical structures

L. Huang et al. / Synthetic Metals 128 (2002) 241–245

of three europium complexes with different second ligands (Phen, EPBM and HPBM), some helpful information was obtained for the development of red and multicolor EL display application. In order to obtain higher luminance, other methods, such as using LiF/Al or Mg: Ag cathodes or employing a co-deposition technique with other materials will be efficient. Acknowledgements The authors thank to the State Key Program of Basic Research (G1998061308), National Nature Science Foundation of China (59872001, 20023005, 20071004), Doctoral Program Foundation of High Education (99000132) and Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry) to KZW for the financial supports of this work. References [1] C.H. Chen, J. Shi, C.W. Tang, Macromol. Symp. 125 (1997) 1. [2] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature (London) 395 (1998) 151.

245

[3] C. Adachi, M.A. Baldo, S.R. Forrest, S. Lamansky, M.E. Thompson, R.C. Kwong, Appl. Phys. Lett. 78 (2001) 1622. [4] M. Uekawa, Y. Miyamoto, H. Ikeda, K. Kaifu, T. Nakaya, Bull. Chem. Soc. Jpn. 71 (1998) 2253. [5] K. Okada, M. Uekawa, Y.F. Wang, T.M. Chen, T. Nakaya, Chem. Lett. (1998) 801. [6] K. Okada, Y.F. Wang, T. Nakaya, Synth. Met. 97 (1998) 113. [7] K. Okada, Y.F. Wang, T.M. Chen, M. Kitamura, T. Nakaya, H. Inoue, J. Mater. Chem. 9 (1999) 3023. [8] J. Kido, K. Nagai, Y. Okamoto, T. Skothetm, Y. Yamagata, Chem. Lett. (1991) 1267. [9] J. Kido, H. Hayase, K. Hongawa, K. Nagai, K. Okuyama, Appl. Phys. Lett. 65 (1994) 3124. [10] C.J. Liang, D. Zhao, Z.R. Hong, D.X. Zhao, X.Y. Liu, W.L. Li, J.B. Peng, J.Q. Yu, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 76 (2000) 67. [11] G.E. Habbour, J.F. Wang, B. Kippelen, N. Peyghambarian, Jpn. J. Appl. Phys. 38 (1999) L1553. [12] W.P. Hu, M. Matsumura, M.Z. Wang, L.P. Li, Jpn. J. Appl. Phys. 39 (2000) 6445. [13] W.P. Hu, M. Matsumura, M.Z. Wang, L.P. Li, Appl. Phys. Lett. 77 (2000) 4271. [14] L. Huang, K.-Z. Wang, C.-H. Huang, F.Y. Li, Y.Y. Huang, J. Mater. Chem. 11 (2001) 790. [15] L.R. Melby, N.J. Rose, E. Abramson, J.C. Caris, J. Am. Chem. Soc. 86 (1964) 5117. [16] J. Kido, K. Nagai, Y. Okamoto, J. Alloys Compd. 192 (1993) 30. [17] S.W. Magennis, S. Parsons, A. Corval, J.D. Woollins, Z. Pikramenou, Chem. Commun. (1999) 61. [18] C. Adachi, M.A. Baldo, S.R. Forrest, J. Appl. Phys. 87 (2000) 8049.