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The photoluminescent and electroluminescent properties of a new Europium complex
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 683–686 www.elsevier.com/locate/jlumin
The photoluminescent and electroluminescent properties of a new Europium complex Dong Guoa, Zhenbo Denga,, Chunjun Lianga, Peng Lina, Yong Lib, Yizhuang Xub a
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, P.R. China b State Key Laboratory of Rare Earth Materials Chemistry and Application, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Available online 20 March 2006
Abstract A new rare earth (RE) complex Eu(BSA)3phen was synthesized. A narrow emission band from a device structure of ITO/PVK: RE complex/LiF/Al was observed, in which poly N-vinylcarbazole (PVK) was used to improve the film-forming and hole-transporting property of the Eu(BSA)3phen. Excitation, photoluminescence (PL) and electroluminescence (EL) characteristics of the device were studied and an energy transfer from PVK to europium complex was proposed. Effects of doping ratios of Eu(BSA)3phen on the device performance were also studied. r 2006 Elsevier B.V. All rights reserved. Keywords: Rare earths; Europium complex; Energy transfer; Electroluminescence
1. Introduction Since Tang and VanSlyke [1,2] developed multilayer organic light-emitting devices (OLEDs), tremendous efforts have been directed toward improving the device performance [3,4]. Compared with the general organic small molecules and polymers, rare earth (RE) complexes exhibit extremely sharp, well-defined spectral lines due to the emission that originates from the lanthanide metal ion. Theoretically, the upper limit of inner quantum efficiency can reach 100%, which is four times higher than that of devices using other fluorescent materials. The electroluminescence (EL) of rare earth complexes have attracted much attention because of the good characteristics of excellent color purity and high internal quantum efficiencies since the organic EL devices using terbium and europium complexes as emitters have been demonstrated by Kido et al. [5]. In this paper, a new europium complex with a second ligand (Eu(BSA)3phen) was used in the EL device of ITO/PVK: RE complex/LiF/Al, where poly N-vinylcarbazole (PVK) [6] was used to improve the film-forming and Corresponding author. Tel.: +86 10 51688675; fax: +86 10 51683933.
E-mail address: [email protected] (Z. Deng). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.259
hole-transporting property of the Eu(BSA)3phen. The photoluminescence (PL) and EL properties of the device were investigated to analyze the energy transfer process from PVK to europium complex. Additionally, effects of doping ratios of Eu(BSA)3phen on device performance were also discussed. 2. Experimental 2.1. Synthesis of Eu(BSA)3phen Eu(BSA)3phen was prepared by adding 1.5 mmol triethylamine to the solution of 1.5 mmol benzoyl salicylic acid(BSA), 0.5 mmol EuCl3 6H2O and 0.5 mmol phenanthroline in 20 ml of absolute ethanol, while stirring until the white precipitate appeared. The complex Eu(BSA)3phen obtained in a powder form was characterized by elemental analysis. The schematic chemical structure of Eu(BSA)3phen is shown in Fig. 1. 2.2. Fabrication of EL devices The detail molecular structure of the complex and device architecture is shown in Fig. 1, where PVK:Eu(BSA)3phen
ARTICLE IN PRESS D. Guo et al. / Journal of Luminescence 122–123 (2007) 683–686
O
(2) --15V
Eu O C
O
3
1.0
(1) --PL
N
C O
0.8
(3) --17V
N
(4) --19V
0.6
(5) --21V
0.4
Intensity (a.u.)
684
0.2
Eu (BSA)3phen
0.0 (1) (2) (3) (4) (5)
A1 (150nm)
LiF (0.3 nm)
PVK Eu (BSA)3phen (60nm)
ITO
300 350 400 450 500 550 600 650 700 Wavelength/nm Fig. 2. PL spectrum of PVKEu(BSA)3phen in thin film state and EL spectra of the device ITO/PVK: Eu(BSA)3phen/LiF/Al at different voltages.
Fig. 1. Molecular structure of the complex and device architecture.
1.0 2 0.8
I (a.u.)
is the emitting layer and LiF/Al is the bilayer cathode. PVK with 20% Eu(BSA)3phen was dissolved in chloroform with the concentration of 3 mg ml1. After the routine cleaning procedure of ultrasonicating the ITO coated glass in organic solvents, deionized water, the light-emitting layer was coated on the clean dry ITO substrate by spin coating. Then LiF and Al was deposited onto the emitting layer by a conventional thermal evaporation method at a chamber pressure about 3 103 Pa. PVK was used as host material for europium complex in order to improve the film-forming characteristic and conductivity. The PL and EL spectra were measured with the Fluolog-3 fluorescent spectrometer of the American SPEX company, and the brightness was measured by PR-650. All the measurements were carried out at room temperature.
3
0.6 1 0.4
0.2
0.0 200
3. Results and discussion The PL spectrum of PVK doped with Eu(BSA)3phen film is shown in Fig. 2. It is obvious that there are three emission peaks. The emission peak at 595, 615 nm responds to 5D0-7F1, 5D0-7F2 transition of Eu3+ ion, respectively. There is another PVK emission peak at 410 nm. It was observed that the emission from the Eu3+ ion is more intensive than that from PVK. Fig. 2 also shows the EL spectra of device ITO/PVK: Eu(BSA)3phen/LiF/Al at different driven voltages. It can be seen that the emission mainly comes from europium complex. By comparing to the PL spectrum of pure Eu(BSA)3phen, it was seen that the device had no emission from PVK. It is because of the different mechanism of EL and PL. The excitation spectra of europium complex, PVK and their mixture are shown in Fig. 3, and the emission spectrum of PVK is also shown in Fig. 3. It is clear that the excitation spectra of PVK and their mixture (PVK doped with europium complex) are much alike, and both of them are different from the excitation spectrum of pure europium complex. This shows
4
250
300
350 400 450 500 Wavelength (nm)
550
600
650
Fig. 3. Excitation spectra of PVK (1) (lem ¼ 410 nm), Eu complex (2) (lem ¼ 616 nm), their mixture (3) (lem ¼ 616 nm) and emission spectrum of PVK (4) in thin film state.
that emissions of Eu3+ ions originate from the excitation of PVK [7]. We think there must be some kind of energy transfer between PVK and europium complex. However, the overlap of the PVK emission spectrum and the excitation spectrum of europium complex are very little [8]. So different from terbium complex of same ligand [9], the occurrence possibility of Forster energy transfer process from PVK to Eu(BSA)3phen is also very little. To analyze the energy transfer process from PVK to europium complex, device with the structure of ITO/PVK: RE complex/LiF/Al was fabricated. EL starts at forward bias of 12 V and with the increase of the driven voltage, the luminescence brightness of the monolayer device increases too (Fig. 2). The emissions also come from the characteristic emission of Eu3+ ion. As the reasons mentioned
ARTICLE IN PRESS D. Guo et al. / Journal of Luminescence 122–123 (2007) 683–686
685
1.0 50
0.6
5:1
600
650
40
100 80
30
60
20
40 10
20
10:1 3:1 1:1
0
0 0
0.4
EL brightness/ (cd/m2)
Intensity (a.u.)
0.8
Current density/ (mA/cm2)
120
2
4
6
8 10 12 14 16 18 20 22 24 26 Voltage/V
0.2
0.0 300
350
400
450
500 550 Wavelength/nm
700
Fig. 4. The EL spectra of Eu complex and PVK at different weight ratio under the same current density 0.1 mA/cm2; Inset: current–voltage–luminance characteristic.
above, emissions of Eu3+ ions root in the energy transfer from PVK to europium complex. The EL process of the rare earth complex can be understood as follows. The holes and electrons were injected into the organic layer from the ITO and the Al electrode, respectively. Electron-hole pairs are created on PVK chains firstly. Then the excited electrons and holes are trapped by the ligand. This process results in an electron transition from the S0 state to the first singlet S1 and then triplet T1 excited states of the organic ligand. Then the energy is transferred from the triplet state of the ligand to the nearest resonance level of the Eu3+ ions. Finally, most of the energy is given out by the radiation of 5D0-7F2 transition of the Eu3+ ion. The luminance–voltage characteristics were measured by the PR-650 spectrometer at room temperature. The current density-voltage-brightness behavior of the device is shown in inset of Fig. 4. The current density of the device is almost zero under low voltage. With the increase of the driven voltage, the current density of the monolayer device increases at nonlinear exponentially. Thus, by doping the europium complex into the conjugated polymer PVK, we got narrow band emission of red light and improve the film-forming property and carrier-transporting property. Fig. 4 shows the EL spectra of device at different weight ratio under the same current density 0.1 mA/cm2. The weight ratios of PVK to Eu-complex were 1:1, 3:1, 5:1 and 10:1. It can be seen that the brightness reaches the maximum when weight ratio of PVK: Eu(BSA)3phen was
5:1. It is because that the probability of the electron-hole pair trapping in europium complex increases with the rising of doping ratio before the effect becomes saturated. Moreover the film-forming property and carrier-transporting property are poor when the weight ratio was lower than 5:1. 4. Conclusions In summary, a new europium complex Eu(BSA)3phen was synthesized and used as the emission material for organic electroluminescence. The polymer PVK improved the complex film-forming and carrier-transporting property. We got narrow band emission from a device structure of ITO/PVK: RE complex/LiF/Al. The energy transfer process from PVK to europium complex was discussed by investigate the excitation, PL and EL characteristics. Effects of doping ratios of Eu(BSA)3phen on the device performance were compared and the optimized weight ratio was 5:1 (PVK: RE). Acknowledgements The funding for this research is provided by National Natural Science Foundation of China under contract no. 90201004 and by Beijing Science Foundation under contract no. H0304300204.
ARTICLE IN PRESS 686
D. Guo et al. / Journal of Luminescence 122–123 (2007) 683–686
References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [3] C.H. Yang, G.F. He, R.Q. Wang, Y.F. Li, Thin Solid Films 363 (2000) 218. [4] Y. Cao, I.D. Parker, G. Yu, C. Zhang, A.J. Heeger, Nature 397 (1999) 414. [5] J. Kido, H. Hayase, K. Hongawa, K. Nagai, K. Okuyama, Appl. Phys. Lett. 65 (1994) 2124.
[6] X. Gao, Z.B. Deng, D.L. Tao, F. Bai, N. Duan, Y.Z. Xu, J.G. Wu, J. Chin. Rare Earth Soc. 19 (2001) 536. [7] C.J. Liang, W.L. Li, Z.R. Hong, X.Y. Liu, J.B. Peng, L. Liu, Z.Y. Lu, M.G. Xie, Z.B. Liu, J.Q. Yu, D.X. Zhao, Synt. Meta. 91 (1997) 151. [8] C.J. Liang, Z.R. Hong, X.Y. Liu, D.X. Zhao, D. Zhao, W.L. Li, J.B. Peng, J.Q. Yu, C.S. Lee, S.T. Lee, Thin Solid Films 359 (2000) 14. [9] F. Bai, Z.B. Deng, X. Gao, Y. Li, Y.Z. Xu, J.G. Wu, J. Chin. Rare Earth Soc. 20 (2002) 652.
Journal of Luminescence 122–123 (2007) 683–686 www.elsevier.com/locate/jlumin
The photoluminescent and electroluminescent properties of a new Europium complex Dong Guoa, Zhenbo Denga,, Chunjun Lianga, Peng Lina, Yong Lib, Yizhuang Xub a
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, P.R. China b State Key Laboratory of Rare Earth Materials Chemistry and Application, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Available online 20 March 2006
Abstract A new rare earth (RE) complex Eu(BSA)3phen was synthesized. A narrow emission band from a device structure of ITO/PVK: RE complex/LiF/Al was observed, in which poly N-vinylcarbazole (PVK) was used to improve the film-forming and hole-transporting property of the Eu(BSA)3phen. Excitation, photoluminescence (PL) and electroluminescence (EL) characteristics of the device were studied and an energy transfer from PVK to europium complex was proposed. Effects of doping ratios of Eu(BSA)3phen on the device performance were also studied. r 2006 Elsevier B.V. All rights reserved. Keywords: Rare earths; Europium complex; Energy transfer; Electroluminescence
1. Introduction Since Tang and VanSlyke [1,2] developed multilayer organic light-emitting devices (OLEDs), tremendous efforts have been directed toward improving the device performance [3,4]. Compared with the general organic small molecules and polymers, rare earth (RE) complexes exhibit extremely sharp, well-defined spectral lines due to the emission that originates from the lanthanide metal ion. Theoretically, the upper limit of inner quantum efficiency can reach 100%, which is four times higher than that of devices using other fluorescent materials. The electroluminescence (EL) of rare earth complexes have attracted much attention because of the good characteristics of excellent color purity and high internal quantum efficiencies since the organic EL devices using terbium and europium complexes as emitters have been demonstrated by Kido et al. [5]. In this paper, a new europium complex with a second ligand (Eu(BSA)3phen) was used in the EL device of ITO/PVK: RE complex/LiF/Al, where poly N-vinylcarbazole (PVK) [6] was used to improve the film-forming and Corresponding author. Tel.: +86 10 51688675; fax: +86 10 51683933.
E-mail address: [email protected] (Z. Deng). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.259
hole-transporting property of the Eu(BSA)3phen. The photoluminescence (PL) and EL properties of the device were investigated to analyze the energy transfer process from PVK to europium complex. Additionally, effects of doping ratios of Eu(BSA)3phen on device performance were also discussed. 2. Experimental 2.1. Synthesis of Eu(BSA)3phen Eu(BSA)3phen was prepared by adding 1.5 mmol triethylamine to the solution of 1.5 mmol benzoyl salicylic acid(BSA), 0.5 mmol EuCl3 6H2O and 0.5 mmol phenanthroline in 20 ml of absolute ethanol, while stirring until the white precipitate appeared. The complex Eu(BSA)3phen obtained in a powder form was characterized by elemental analysis. The schematic chemical structure of Eu(BSA)3phen is shown in Fig. 1. 2.2. Fabrication of EL devices The detail molecular structure of the complex and device architecture is shown in Fig. 1, where PVK:Eu(BSA)3phen
ARTICLE IN PRESS D. Guo et al. / Journal of Luminescence 122–123 (2007) 683–686
O
(2) --15V
Eu O C
O
3
1.0
(1) --PL
N
C O
0.8
(3) --17V
N
(4) --19V
0.6
(5) --21V
0.4
Intensity (a.u.)
684
0.2
Eu (BSA)3phen
0.0 (1) (2) (3) (4) (5)
A1 (150nm)
LiF (0.3 nm)
PVK Eu (BSA)3phen (60nm)
ITO
300 350 400 450 500 550 600 650 700 Wavelength/nm Fig. 2. PL spectrum of PVKEu(BSA)3phen in thin film state and EL spectra of the device ITO/PVK: Eu(BSA)3phen/LiF/Al at different voltages.
Fig. 1. Molecular structure of the complex and device architecture.
1.0 2 0.8
I (a.u.)
is the emitting layer and LiF/Al is the bilayer cathode. PVK with 20% Eu(BSA)3phen was dissolved in chloroform with the concentration of 3 mg ml1. After the routine cleaning procedure of ultrasonicating the ITO coated glass in organic solvents, deionized water, the light-emitting layer was coated on the clean dry ITO substrate by spin coating. Then LiF and Al was deposited onto the emitting layer by a conventional thermal evaporation method at a chamber pressure about 3 103 Pa. PVK was used as host material for europium complex in order to improve the film-forming characteristic and conductivity. The PL and EL spectra were measured with the Fluolog-3 fluorescent spectrometer of the American SPEX company, and the brightness was measured by PR-650. All the measurements were carried out at room temperature.
3
0.6 1 0.4
0.2
0.0 200
3. Results and discussion The PL spectrum of PVK doped with Eu(BSA)3phen film is shown in Fig. 2. It is obvious that there are three emission peaks. The emission peak at 595, 615 nm responds to 5D0-7F1, 5D0-7F2 transition of Eu3+ ion, respectively. There is another PVK emission peak at 410 nm. It was observed that the emission from the Eu3+ ion is more intensive than that from PVK. Fig. 2 also shows the EL spectra of device ITO/PVK: Eu(BSA)3phen/LiF/Al at different driven voltages. It can be seen that the emission mainly comes from europium complex. By comparing to the PL spectrum of pure Eu(BSA)3phen, it was seen that the device had no emission from PVK. It is because of the different mechanism of EL and PL. The excitation spectra of europium complex, PVK and their mixture are shown in Fig. 3, and the emission spectrum of PVK is also shown in Fig. 3. It is clear that the excitation spectra of PVK and their mixture (PVK doped with europium complex) are much alike, and both of them are different from the excitation spectrum of pure europium complex. This shows
4
250
300
350 400 450 500 Wavelength (nm)
550
600
650
Fig. 3. Excitation spectra of PVK (1) (lem ¼ 410 nm), Eu complex (2) (lem ¼ 616 nm), their mixture (3) (lem ¼ 616 nm) and emission spectrum of PVK (4) in thin film state.
that emissions of Eu3+ ions originate from the excitation of PVK [7]. We think there must be some kind of energy transfer between PVK and europium complex. However, the overlap of the PVK emission spectrum and the excitation spectrum of europium complex are very little [8]. So different from terbium complex of same ligand [9], the occurrence possibility of Forster energy transfer process from PVK to Eu(BSA)3phen is also very little. To analyze the energy transfer process from PVK to europium complex, device with the structure of ITO/PVK: RE complex/LiF/Al was fabricated. EL starts at forward bias of 12 V and with the increase of the driven voltage, the luminescence brightness of the monolayer device increases too (Fig. 2). The emissions also come from the characteristic emission of Eu3+ ion. As the reasons mentioned
ARTICLE IN PRESS D. Guo et al. / Journal of Luminescence 122–123 (2007) 683–686
685
1.0 50
0.6
5:1
600
650
40
100 80
30
60
20
40 10
20
10:1 3:1 1:1
0
0 0
0.4
EL brightness/ (cd/m2)
Intensity (a.u.)
0.8
Current density/ (mA/cm2)
120
2
4
6
8 10 12 14 16 18 20 22 24 26 Voltage/V
0.2
0.0 300
350
400
450
500 550 Wavelength/nm
700
Fig. 4. The EL spectra of Eu complex and PVK at different weight ratio under the same current density 0.1 mA/cm2; Inset: current–voltage–luminance characteristic.
above, emissions of Eu3+ ions root in the energy transfer from PVK to europium complex. The EL process of the rare earth complex can be understood as follows. The holes and electrons were injected into the organic layer from the ITO and the Al electrode, respectively. Electron-hole pairs are created on PVK chains firstly. Then the excited electrons and holes are trapped by the ligand. This process results in an electron transition from the S0 state to the first singlet S1 and then triplet T1 excited states of the organic ligand. Then the energy is transferred from the triplet state of the ligand to the nearest resonance level of the Eu3+ ions. Finally, most of the energy is given out by the radiation of 5D0-7F2 transition of the Eu3+ ion. The luminance–voltage characteristics were measured by the PR-650 spectrometer at room temperature. The current density-voltage-brightness behavior of the device is shown in inset of Fig. 4. The current density of the device is almost zero under low voltage. With the increase of the driven voltage, the current density of the monolayer device increases at nonlinear exponentially. Thus, by doping the europium complex into the conjugated polymer PVK, we got narrow band emission of red light and improve the film-forming property and carrier-transporting property. Fig. 4 shows the EL spectra of device at different weight ratio under the same current density 0.1 mA/cm2. The weight ratios of PVK to Eu-complex were 1:1, 3:1, 5:1 and 10:1. It can be seen that the brightness reaches the maximum when weight ratio of PVK: Eu(BSA)3phen was
5:1. It is because that the probability of the electron-hole pair trapping in europium complex increases with the rising of doping ratio before the effect becomes saturated. Moreover the film-forming property and carrier-transporting property are poor when the weight ratio was lower than 5:1. 4. Conclusions In summary, a new europium complex Eu(BSA)3phen was synthesized and used as the emission material for organic electroluminescence. The polymer PVK improved the complex film-forming and carrier-transporting property. We got narrow band emission from a device structure of ITO/PVK: RE complex/LiF/Al. The energy transfer process from PVK to europium complex was discussed by investigate the excitation, PL and EL characteristics. Effects of doping ratios of Eu(BSA)3phen on the device performance were compared and the optimized weight ratio was 5:1 (PVK: RE). Acknowledgements The funding for this research is provided by National Natural Science Foundation of China under contract no. 90201004 and by Beijing Science Foundation under contract no. H0304300204.
ARTICLE IN PRESS 686
D. Guo et al. / Journal of Luminescence 122–123 (2007) 683–686
References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [3] C.H. Yang, G.F. He, R.Q. Wang, Y.F. Li, Thin Solid Films 363 (2000) 218. [4] Y. Cao, I.D. Parker, G. Yu, C. Zhang, A.J. Heeger, Nature 397 (1999) 414. [5] J. Kido, H. Hayase, K. Hongawa, K. Nagai, K. Okuyama, Appl. Phys. Lett. 65 (1994) 2124.
[6] X. Gao, Z.B. Deng, D.L. Tao, F. Bai, N. Duan, Y.Z. Xu, J.G. Wu, J. Chin. Rare Earth Soc. 19 (2001) 536. [7] C.J. Liang, W.L. Li, Z.R. Hong, X.Y. Liu, J.B. Peng, L. Liu, Z.Y. Lu, M.G. Xie, Z.B. Liu, J.Q. Yu, D.X. Zhao, Synt. Meta. 91 (1997) 151. [8] C.J. Liang, Z.R. Hong, X.Y. Liu, D.X. Zhao, D. Zhao, W.L. Li, J.B. Peng, J.Q. Yu, C.S. Lee, S.T. Lee, Thin Solid Films 359 (2000) 14. [9] F. Bai, Z.B. Deng, X. Gao, Y. Li, Y.Z. Xu, J.G. Wu, J. Chin. Rare Earth Soc. 20 (2002) 652.