Luminescent Properties of A Novel Terbium Complex Tb(o-BBA)3(phen)

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Luminescent Properties of A Novel Terbium Complex Tb( o-BBA), ( phen) Liu Ling ( 3d @)’”, Xu Zheng (& @ ) ‘ ’ 2 * , Lou Zhidong (&,& & )”*, Zhang Fujun ( f k s j k )1’2, Sun Bo ($1. ik)3,Pei Juan 48)3 ( 1 . Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044 , China ; 2 . Key Laboratory of Luminescence and Optical Information , Ministry of Education ; 3 . Department of Materials Chemistry, College of Chemistry, Nankai University, Tianjin 300071 , China )

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Received 5 June 2005; revised 5 December 2005

Abstract: A novel rare earth complex of terbium ion with 2-benzoylbenzoic acid and 1,lO-phenathroline (Tb( o-BBA)~ (phen) , o-BBA = 2-benzoylbenzoic acid, phen = 1,lO-phenathroline) was used as an electroluminescent material for the first time. The Tb complex was blended with poly( N-vinylcarbazole) (PVK) in different weight ratios and spinn to coated into films (noted as PVK :Tb films) . The photoluminescence (PL) properties of films were investigated and the optimum weight ratio between PVK and Tb(o-BBA)3(phen) was found to be 3 : 1 . Monolayer devices with the structure ITO/PVK: %/A1 were fabricated and emitted green light, which was characteristic of Tb3 emission. The results show that mechanisms for PL and EL are different. The PL is considered to be caused because of energy transfer and direct excitation to the Tb(o-BBA)3(phen) molecule, while EL is mainly on charging trapping. +

Key words : Tb ccmplex ; energy transfer; charge trapping ; electroluminescence ; rare earths CLC number : 0482.3 Document code : A Article ID : 1002 - 0721 (2OO6)02 - 0253 - 04

Organic light-emitting diodes ( LEDs) are of great interest because they efficiently emit light in the visible region and show promising applications in backlights and flat panel displays. Full color displays require three fundamental colors ( red, blue, and green) . Organolanthanide compounds are satisfactory with this requirement compared to conjugated polymers or small organic molecules because they show very narrow emissions of rare earths. In addition, electroluminescence (EL) efficiency based on lanthanide compounds is not limited by spin ruling, since their photoluminescence (PL) results from the formation of both singlets and triplets of the ligands, and then transferring energy to lanthanide cores”-31. Most investiga-

tions have focused on P-diketonate complexes of rare earths because this kind of complexes has high flnorescent brightness. However, P-diletonate complexes are easily oxided . Therefore, researchers have made great efforts to synthesize more stable lanthanide complexs instead of P-diletonate terbium. In this article, the EL of a novel aromatic carboxylate rare earth complex Tb ( o-BBA)~(phen) was reported. Carboxylate rare earth complex is not suitable for vacuum heat evaporation since it has low thermal decomposition temperature. Additionally it has poor conductivity. Therefore we doped it into PVK and fabricated films by spin coating method. The photoluminescent and electroluminescent properties of this cam-

* Corresponding author (E-mail : zhengxu @ center. njtu .edu .cn ) Foundation item: Project supported by the National Natural Science Foundation of China (60576016, 10374001, 10434030) and “973” National Key Basic Research Foundation of China (2003CB314707)

Biography : Liu Ling ( 1982 - ) , Female, Master candidate ; Engaging in organic electroluminescence Copyight 0 2 0 0 6 , by Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Eisevier B . V . All rights reserved.

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plex were studied and the luminescent mechanisms were discussed.

1 Experimental Tb( o-BBA)~(phen) and PVK powder were dissolved in chloroform solution respectively to avoid the aggregate of molecules, and then mixed in different weight ratios. The concentration of PVK was 10 m g - m l - ' , while Tb(o-BBA)3(phen) was 2 m g - m l - ' . The Tb( o-BBA),( phen) solutions and the mixed solutions (PVK :Tb) were spin coated onto clean quartz to measure the PL properties. The PL spectra of films were recorded on a SPEX Fluorolog-3 spectrometer. The PVK :Tb light-emitting layers were spin coated onto IT0 glass. The IT0 glass was ultrasonically cleaned in ethanol, acetone, and deionized water for 10 min , re~pectively[~'.Before spin coating, the IT0 glass substrates were treated in ozone atmosphere for 8 min to improve the work function of the anode electrode. The thickness of the films was controlled through adjusting the rotating speed. The aluminum electrode and the organic layer were made by vacuum deposition method. The dependence of EL intensity on voltage for the devices was investigated with a Keithley programmable electrometer and a SPEX Fluorolog-3 spectrometer. The current-voltage ( I- V ) characteristics were also measured with a Keithley power. All the measurements were carried out at room temperature.

direct charge-carrier trapping by the dopant molecule"] . Energy transfer mechanism involves either a dipole-induced Coulombic interaction between the host excitons and the doped dye (Forster energy transfer for electrofluorescence ) or an electron-exchange interaction between them (Dexter energy transfer for electrophosphorescence ) 16' . Substantial overlap of the host emission and the dopant absorption spectra is desirable for an efficient energy transfer'"*'. The emission spectrum of the PVK film ( Aex = 346 nm) and the excitation spectra of PVK (A?, = 410 rim) , Th ( ~ - B B A ) ~ ( p h e n(A,, ) = 546 nm) and the FVK: Tb film ( A = 546 nm ) are shown in Fig. 3. It can he seen that there exists a small overlap between the excitation spectrum of Tb( o - B B A ) ~(phen) and the emission spectrum of PVK . Besides, the excitation spectra of PVK and their mixture are much alike, and both of them are different from the excitation spectrum of 'l'b (0-BBA), ( phen) . So the forest energy transfer process from PVK to Tb ( o - B B A ) ~( phen) may occur in the blend system, but the energy transfer is not com1

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2 Results and Discussion The pure emission of terbium ions is observed in the pure Tb(o-BBA),(phen) film when excited at 346 nm, as shown in Fig.2. The four emission peaks are at 490, 546, 585 and 620 nm, corresponding to the ' D 4 4 7 F 6 , 'D4+7Fs, 'D4p7F4and 'D4+'F3 transition, respectively. However, the blend film exhibits strong emission of both Tb3+and PVK when the ratio between PVK and Tb complex is 5 : 1. The emission mechanism of rare earth doped materials is generally through energy transfer from the excited host to the dopant andlor

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Emission spectrum of PVK( 1 ) and excitation spectra of PVK(21, T b ( ~ - B B A ) ~ ( p h e (n3) ) , and their mixture ( PVK :Tb) (4)

Liu L et a1 . Luminescent Properties Tb ( 0-BBA ) (phen ) plete['-"' . It is also noticed that there is a shoulder peak situated at 271 nm which is just an excitation peak of T b ( ~ - B B A ) ~ ( p h e n )meaning , that the light is partly originated from the direct excitation to the Tb( oBBA)3( phen) molecule. The PL spectra for the PVK:Tb films with different Tb complex concentration are presented in Fig. 3 . By increasing the Tb complex content, the emission of PVK is quenched, and the emission of Tb3+ becomes stronger. When the ratio between PVK and Tb complex is higher than 5 : 1, the blend film exhibits the obvious emission of PVK. When the ratio is 3 : 1, the emission of PVK is very poor. It remains the same even if the ratio is more than 3 : 1 . Therefore the ratio of 3 : 1 is properly chosen for the electroluminescent devices. The electroluminescent spectra of the OLED device with the structure of ITO/PVK :Tb/A1 at 14 V are shown in Fig. 5 . From the intensity-voltage curve which is obtained from the time base spectrum, it can be seen that the brightness of the device increases dra-

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matically when the voltage is higher than 10 V . The electroluminescent device also shows the characteristic emission of Tb3* ions, but the emission of the PVK is completely quenched compared to the PL spectrum of the mixture which is maybe because of the different mechanism of the EL and PL. For further understanding the EL mechanism, the current-voltage characteristics of the monolayer devices with different PVK :Tb ratios were investigated ( see Fig. 6 ) . By increasing the Tb ( o - B B A ) ~( phen ) content, the quantity of the current decreases dramatically while the emission intensity increases greatly at the same time. That is because the increased traps in the light-emitting layer seize a great deal of carriers and lead to the decreasing of the current. The seized holes and electrons recombine and emit light, which correspond to the enhancement of emission intensity (Fig. 7) . The logarithm plot of I-V curve at the ratio of 3 : 1 is studied and is found that it shows approximate linear behavior at certain domain. When the voltage varies from 9 to 16 V , the slope n equals to 2 . 3 , and n value becomes 5 . 2 at

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EL spectrum of device with structure of ITO/PVK : Tb (3:1)/Al at 16 V (The inserted picture is the relative intensity curve at different voltage)

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the high field domain higher than 16 V when the current increases rapidly. These characteristics are in accordance with the space-charge limited current (SCLC ) model for the half-insulator containing trapping predicted by L a m ~ e r t " ~ 'In . the lower field domain, the current is originated from the thermionic emission and the curve versus voltage ( I - V ) takes on an Ohm characteristic, I cc V . With increasing the injecting charge carriers, the current changes into the SCLC . If trapping energy existed in the dielectric, I (or J ) should be in direct proportion to v" and the n value is from 2 . 2 to 4. 7[l4]. The value of 2 . 3 just belongs to this range, so there are trapping centers caused by the Tb ( o-BBA ) 3 ( phen ) molecule in the PVK :Tb emitting layer. The EL mechanism should be that injected holes and electrons trapped by Tb ( o BBA), ( phen) molecules form excitions , subsequently, the excitions are transferred to the central Tb3+ ions, and the Tb3' ions transmit and emit light.

1995, 65: 2124. [2] Campos R A, Kovalev I P , Guo Y , et al. [ J ] . Appl. Phys. Lett., 1996, 80: 7144. [ 31 Liang C J , Zhao D , Hong Z R , et a1 . Improved perfor-

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3 Conclusion A novel rare earth complex Tb (0-BBA )3 (phen) was used as a luminescent material to fabricate efficient EL devices. The PL spectrum shows that this novel complex can emit the characteristic light of Tb3+. By analyzing the PL, EL and I-V characteristic, it is concluded that the electroluminescent mechanism for this novel complex is the direct charge-carrier trapping. The devices are not optimized, and our further work would be to change the devices structure and enhance the brightness .

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