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Journal of Luminescence 122–123 (2007) 671–673

www.elsevier.com/locate/jlumin

Electroluminescent devices based on rare-earthcomplex TbY(p-MBA)6(phen)2

Zheng Chena, Zhenbo Denga,�, Yumeng Shia, Ying Xua, Jing Xiaoa,Yuanyuan Zhanga, Ruifen Wangb

aKey Laboratory of Luminescene and Optical Information, Ministry of Education, Institute of Optoelectronic Technology,

Beijing Jiaotong University, Beijing 100044, P.R. ChinabDepartment of Chemistry, Hebei Normal University, Shijiazhuang 050091, China

Available online 20 March 2006

Abstract

A new rare-earth complex TbY(p-MBA)6(phen)2 was synthesized and studied. Pure green emission was observed from the devices that

were fabricated with the structure as follows: Device 1: ITO/PVK: TbY(p-MBA)6(phen)2/LiF/Al; Device 2: ITO/PVK: TbY(p-

MBA)6(phen)2/AlQ/LiF/Al. PVK was used to improve the film-forming and hole-transporting properties of the TbY(p-MBA)6(phen)2.

The characteristics of these devices were investigated and compared with Tb(p-MBA)3phen, which had been reported. The results show

that TbY(p-MBA)6(phen)2 has much better performance than Tb(p-MBA)3phen. This improvement is attributed to the introduction of

yttrium. The highest EL brightness of Device 1 is 37.5 cd/cm2 at a fixed bias of 19V, and the highest EL brightness of Device 2 reaches

333.7 cd/cm2 at 23V.

r 2006 Elsevier B.V. All rights reserved.

Keywords: Terbium complex; Electroluminescence (EL); Exciton; Rare earth

1. Introduction

Since the first double-layer organic light-emitting devices(OLEDs) reported by Tang and Vanslyke [1], OLEDsattracted much attention due to their potential applica-tions as large area flat-panel displays. Great endeavorswere made into the development of pure colors. But thefull-width at half maximum (FWHM) of the normalmaterials used in OLEDs as generally about 50–100 nm.In 1990s, Kido’s team firstly fabricated the electrolumines-cent devices based on rare-earth complexes [2], theirFWHM were about 10 nm. Because of its narrow spectralemission, OLEDs based on rare-earth complexes attractedmore and more attention. In this paper, we synthesized aterbium complex TbY(p-MBA)6(phen)2 and fabricatedOLEDs based on it. The properties of single- and double-

e front matter r 2006 Elsevier B.V. All rights reserved.

min.2006.01.256

ing author. Tel: (+86) 10 51688675;

683933.

ess: zbdeng@center.njtu.edu.cn (Z. Deng).

layer devices, both utilizing PVK doped with complex asthe light-emitting layer, were studied.

2. Experimental

2.1. Synthesis of TbY(p-MBA)6(phen)2

TbCl3 � 6H2O, YCl3 � 6H2O, p-methyl phenylformicacid(p-MBA) and 1,10-phenanthroline (phen) were dis-solved in 95%C2H5OH while the molar ratio of Tb3+,Y3+, p-MBA and phen was 1:1:6:2 respectively. The pHvalue of the p-methyl phenylformic acid solution wasadjusted to 6–7 with 1mol L�1NaOH solution. The grainalcohol solutions of two ligands were mixed and then thismixture was added to another mixed solution made up ofethanol solutions of TbCl3 and YCl3 drop by drop, while awhite precipitate formed. The schematic chemical structureof the white precipitate is TbY(p-MBA)6(phen)2 which wasapproved by elementary and crystal structure analysis.

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(1)

(2)

(4)

(3)

Fig. 2. The emission spectra of the PVK (1) and the excitation spectra of

the PVK (2), TbY(p-MBA)6(phen)2 (3) and their mixture (4).

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Z. Chen et al. / Journal of Luminescence 122–123 (2007) 671–673672

2.2. Fabrication of OLEDs

Two kinds of devices were fabricated with the structureas follows:

Device1: ITO/PVK: TbY(p-MBA)6(phen)2 /LiF/Al,Device2: ITO/PVK: TbY(p-MBA)6(phen)2 /AlQ/LiF/Al.The TbY(p-MBA)6(phen)2 and the PVK were dissolved

in chloroform by the mass concentration of 1.10mgml�1,mixed the complex and the PVK chloroform solution whilethe mass ratio of the TbY(p-MBA)6(phen)2 and the PVKwas 1:5. Preparing the light-emitting layer by spin coating,the mixed solution onto a glass substrate coated with ITOwith a sheet resistance of 20O/&. Then LiF, aluminum(for Device 1) or AlQ, LiF, aluminum (for Device 2) weredeposited on the emitting layer by vacuum depositionunder a pressure of about 3� 10�3 Pa. Before fabricatingthe organic layers, the substrate had been cleaned withultra-purified water and organic solvents, and treated witha UV-ozone ambient. LiF was used as electron injectionmaterial. PVK was used as host material for terbiumcomplex. AlQ was used as electron transfer layer and PVK:TbY(p-MBA)6(phen)2 was used as light-emitting layer.Because the complex should decompose at the hightemperature, the light-emitting layer could not be preparedby vacuum deposition. The PL, EL and excitation spectrawere measured by the Fluolog-3 fluorescent spectrometer,and the brightness was measured by PR-650. All of thesemeasurements were carried out under room temperature.

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Fig. 3. The EL spectra of the single layer device at different voltages and

their B–V curve (in the insert).

3. Results and discussion

Fig. 1 shows the PL spectra of pure complex and PVKdoped with TbY(p-MBA)6(phen)2 film with differentweight ratios. While the weight ratio of the PVK increases,the emission of the PVK enhances obviously. Butcompared with the emission of the Tb3+ ions, the intensityof PVK is weak.

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pure

Fig. 1. The PL spectra of the pure complex and the complex doped in

PVK with different weight ratios.

Fig. 2 shows that the emission spectra of the PVK (1)and the excitation spectra of the TbY(p-MBA)6(phen)2 (3)have a little overlap. And the excitation spectra of PVKdoped with TbY(p-MBA)6(phen)2 (4) and pure PVK (2)are very similar, especially in the range of 345–400 nm, butmuch different from that of pure TbY(p-MBA)6(phen)2(3). So we considered that there exist Forester energytransfer from the PVK to the TbY(p-MBA)6(phen)2 in themixture film. However, the curve (4) also has a peak atsame location with pure TbY(p-MBA)6(phen)2 (3). So, apart of the luminescence should come from the directexcitation of complex [3].Fig. 3 shows the EL spectra of the single layer device at

different voltages. Compared with the PL spectra of thepure complex, it is clear that the main emission from thedevice comes from the terbium complex. B–V curve isshown in the insert of Fig. 3. The highest brightness of thesingle layer device reached 37.5 cd/cm2 at 19V.

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Fig. 4. The EL spectra of the double layer device at different voltages and

their B–V curve (in the insert).

Z. Chen et al. / Journal of Luminescence 122–123 (2007) 671–673 673

Fig. 4 shows the EL spectra of the double layer device atdifferent voltages. The emission from the complex isenhanced by the increase in fixed bias. The peaks are notmodified with the change of voltage. B–V curve is shown inthe insert of Fig. 4. The highest brightness of this devicereaches 333.7 cd/cm2 at 23V. Compared with the Tb(p-MBA)3phen, which had been reported [4], TbY(p-MBA)6(-phen)2 has much better performance.

Compared with the EL and PL spectra of the PVKdoped with complex, the emission of PVK is observedobviously in the PL spectra, but it can not be observed inthe EL spectra at all. We consider that the mechanism ofthe photoluminescence and electroluminescence in themixture film are very different.

According to the theory of Forester energy transfer, thespeed rate of the energy transfer can be described by thisformula

X ¼

Z1; 2� H 0

�� ��1�; 2� ��� ��2g1 Eð Þg2 Eð ÞdE.

g1(E) and g2(E) are the normalized spectral typefunctions. This formula point out that the speed rate ofthe energy transfer between the host and guest is decidedby the overlap between the emission spectra of the host andthe excitation spectra of the guest; greater the overlap, thefaster the energy transfer [5]. In Fig. 2, the overlap of (1)and (3) is only a little, but the emission of the PVK iscompletely restrained. It means that the luminescence ofthe complex probably comes from two sources. One is theForester energy transfer between PVK and complex, andanother is the carrier trapping by the terbium ligand.

In the state of the Forester energy transfer, carriersinjected from electrode and the excitations are created on

PVK; then the energy transfers from PVK to terbiumligands, the terbium ligands are excited from ground stateto singlet S1 and then to the triplet T1 by transition, andlater the energy transfers from the lowest triplet to 5D4 ofthe Tb3+ by inner-molecule energy transfer. The lumines-cence comes from the transition from 5D4 to 7Fj

ðj ¼ 6; 5; 4; 3Þ. In the state of carrier trapping, PVK is usedas hole transfer material and the complex forms electrontrap. The carriers are trapped directly by the complex, theterbium ligands are excited and finally lead to theluminescence of the Tb3+.In both of these states, the step of the energy transfer

from terbium ligands to the Tb3+ is very important.Compared with Tb(p-MBA)3phen, half position of theluminescence center is occupied by Y3+. The number of theluminescence center is reduced. But the electron orbits ofthe Y3+ (4s24p6) are fully and steadily filled. Their lowestexcited state level is higher than the triple of the ligands,and Y3+ does not participate in the energy transfer. So theefficiency of the energy transfer increases and theperformance of TbY(p-MBA)6(phen)2 is much better thanthat of Tb(p-MBA)3phen.

4. Conclusions

A new rare-earth complex TbY(p-MBA)6(phen)2 wassynthesized to be used in organic electroluminescence. Twokinds of OLEDs using PVK doped with complex as light-emitting layer were fabricated. Compared with Tb(p-MBA)3phen, which had been reported, the introductionof the Y3+ ions makes a great increase in the performanceof the devices. The highest brightness of the single layerdevice reached 37.5 cd/cm2 at 19V and that of the doublelayer device reached 333.7 cd/cm2 at 23V.

Acknowledgements

We gratefully acknowledge the financial support ofNational Natural Science Foundation of China (undercontract No. 90201004), Beijing Science and TechnologyFoundation (under contract No. H030430020410) and HeiBei Natural Science Foundation (under contractNo.203148).

References

[1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913.

[2] J. Kido, K. Nagai, Y. Okamoto, et al., Chem. Ltt. 235 (1991) 1267.

[3] Y. Xu, Z.B. Deng, et al., J. Optoelectron. laser 16 (2005) 319 (in

Chinese).

[4] Y. Xu, Z.B. Deng, et al., J. Chin. Rare Earth Soc. 22 (2004) 883 (in

Chinese).

[5] S.H. Huang, The Principles and Methods of Laser Spectroscopy, Jilin

University Press, Changchun, 2001, p. 31.