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Electroluminescence of Tb(o-BBA)3(phen) based on an organic–inorganic heterostructure
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 727–729 www.elsevier.com/locate/jlumin
Electroluminescence of Tb(o-BBA)3(phen) based on an organic–inorganic heterostructure Fujun Zhanga, Zheng Xua,, Suling Zhaoa, Ling Liua, Weiwei Jianga, Bo Sunb, Deang Liua, Jinzhao Huanga, Juan Peib a
Key Laboratory of Luminescence and Optical Information, Ministry of Education Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China b Department of Materials Chemistry, College of Chemistry, Nankai University, Tianjin, 300071, PR China Available online 15 March 2006
Abstract An organic–inorganic heterostructure device was designed to improve the electroluminescence intensity of terbium ion. In this heterostructure, a mixed solution of (Tb(o-BBA)3(phen)) and poly(N-vinylcarbazole) (PVK) was prepared with a weight ratio of 3:1 which was acted as a hole transporting and emitting layer. A wide band semiconductor zinc sulfide (ZnS) layer was used as an electron transporting and hole blocking layer. Characteristic emissions of terbium ion from the organic–inorganic heterostructure device were observed and the electroluminescence intensity of terbium ion increased abruptly when the driving voltage went beyond 20.5 V. This may be due to a directly impact excitation of terbium ion by hot electrons, accelerated in ZnS layer, and then a recombination with the injected holes. The electroluminescence mechanism of Tb(o-BBA)3(phen)-doped PVK was the charge-carrier trapping process by the dopant molecule according to the emission spectra of PVK and the excitation spectra of Tb(o-BBA)3(phen). r 2006 Elsevier B.V. All rights reserved. Keywords: Organic–inorganic heterostructure; Rare earth complex; Electroluminescence
1. Introduction Since a bright photoluminescence (PL) of europium complex was first reported by Weissman in 1942 [1], great progress had been achieved in the field of luminescence of lanthanide organic complexes. In the recent years, organic electroluminescence (OEL) and inorganic electroluminescence (IEL) made a distinguished progress. However, each of them has its disadvantages. Although multi-layer OEL devices were often fabricated in order to balance the charge injection and improve the efficiency, photochemical or electrochemical phenomena, (such as exciplex and electroplex) occurred at the organic–organic interface or in a blend of organic materials [2,3]. These phenomena always varied with the applied electric field strength and sometimes were out of control. Therefore, the emission color of OEL changed with the increase in driving voltage. It is a big shortcoming for the flat panel display. In our study, an Corresponding author. Tel.: +86 10 51688605; fax: 86 10 51683933.
E-mail address: [email protected] (Z. Xu). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.272
inorganic semiconductor ZnS layer was introduced as an electron transporting and hole-blocking layer in order to avoid these phenomena. An organic–inorganic heterostructure (ITO\PVK:Tb\ZnS\Al) was fabricated. It has been reported that an organic–inorganic heterostructure device may hold their advantages of OEL and IEL and discard their disadvantages [4,5]. In this paper, emissions of Tb ion were observed from the specific structure and electroluminescence (EL) mechanism of Tb(o-BBA)3 (phen)-doped PVK was studied.
2. Experiments The chemical structures of organic materials are shown in Fig. 1. Here, PVK was dissolved in chloroform with the concentration of 10 mg/ml. Tb(o-BBA)3(phen) was doped into PVK with a weight ratio of 3:1 to improve the performance of Tb(o-BBA)3(phen) thin film. Then the mixed solution was spin coated at 3000 rpm onto the topcleaned ITO-coated glass substrate. And then the ZnS thin
ARTICLE IN PRESS F. Zhang et al. / Journal of Luminescence 122–123 (2007) 727–729
728
12V 14V 16V 18V
1.0
Intensity (a.u.)
0.8
0.6
0.4
0.2 Fig. 1. Chemical structure of PVK and Tb(o-BBA)3(phen).
0.0
film that acted as an electron transporting and hole blocking layer, was prepared by electron beam evaporation with the growth rate of 0.5 A˚/S under a high vacuum of 2 10 6 Torr. Finally the top Al electrode was prepared by thermal evaporation. The devices were driven by Keithley Source Meter 2410 and the EL spectra were measured by SPEX Fluorolog-3 spectrometer under ambient atmosphere.
500
550 Wavelength (nm)
600
Fig. 2. The EL spectra of Tb(o-BBA)3(phen) based on organic–inorganic heterostructure under different driving voltage.
3. Results and discussion The EL spectra of Tb(o-BBA)3(phen) under different driving voltages from the organic–inorganic heterostructure device are shown in Fig. 2. The four characteristic emission peaks of terbium ions were 490 nm, 546 nm, 585 nm and 620 nm corresponding to the transition of 5 D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3, respectively. In this specific structure the ZnS layer was acted as an electron transporting and hole-blocking layer. Therefore, the emission zone was limited within the PVK layer due to a big hole barrier (about 1.3 eV) between the HOMO of PVK and the valence band of ZnS. The energy diagram of the heterostructure was shown in Fig. 3. Under a forward voltage, the electrons and holes were injected from the cathode and the anode, respectively. Under the same electric field, the hole mobility in PVK layer was much smaller than the electron mobility in ZnS layer. Therefore, a large part of driving voltage dropped on the PVK layer. As we know, the charge mobility strongly depends on the electric field strength. Consequently, the hole mobility in PVK layer must be enhanced. The EL intensity was strongly controlled by the hole current because the hole was minor charge carrier in the specific devices. As a result, the EL intensity was improved as shown in Fig. 4. The relationship between the EL intensity of 546 nm and the driving voltage was measured by monitoring the emission of 546 nm. When the driving voltage went beyond 20.5 V, the EL intensity increased abruptly. It was reported that electrons injected from the cathode may be accelerated to a high energy in some inorganic semiconductor materials, such as ZnS and SiO2 [6–8]. These hot electrons recombined directly with the injected hole, or bombarded
Fig. 3. Energy diagram of the organic–inorganic structure devices.
the Tb ions and then recombined with the injected holes within PVK layer. The EL mechanisms of rare earth complexes have been extensively studied in OEL. There were two dominant mechanisms, (i) direct trapping of the injected holes and electrons, and subsequently forming excitons occurred on the rare earth ions [9], (ii) energy transferring from the excited host to the dopant and finally sensitizing the rare earth ions [10]. Energy transfer mechanism involved either a dipole-induced coulombic interaction between the host excitons and the doped complex (Forster energy transfer for electro–fluorescence) or an electron-exchange interaction between them (Dexter energy transfer for electrophosphorescence). Substantial overlapping of the host
ARTICLE IN PRESS F. Zhang et al. / Journal of Luminescence 122–123 (2007) 727–729
0.6
mixture were much alike, both of them were different from the excitation spectrum of Tb(o-BBA)3(phen). So the forest energy transfer process from PVK to Tb(o-BBA)3(phen) may not occur effectively in the blend system. The main EL mechanism of Tb(o-BBA)3(phen) doped in PVK may be the charge-carrier trapping process by the dopant molecule.
0.4
4. Conclusions
0.2
Characteristic emissions of Tb(o-BBA)3(phen)-doped PVK were obtained from the organic–inorganic heterostructure (ITO\PVK:Tb\ZnS\Al) devices. They were 490, 546, 585 and 620 nm corresponding to the transition of 5 D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3 of the Tb ion. A large part of driving voltage dropped on the PVK layer because the hole mobility in PVK layer was much smaller than the electron mobility of ZnS under a same electric field strength. As the electric field strength in PVK layer was increased, the EL intensity increased as well. When the driving voltage went beyond 20.5 V, the EL intensity of 546 nm increased abruptly. This may be due to a direct impact excitation of terbium ion by hot electrons, accelerated in ZnS layer, and then a recombination with the injected holes. The main EL mechanism of PVK doped with Tb(o-BBA)3(phen) was the charge-carrier trapping process by the dopant molecule.
1.0
EL Intensity (a.u.)
0.8
0.0 10
12
14
16
18
20
22
Voltage(V) Fig. 4. The EL intensity of 546 nm-driving voltage characteristic of the heterostructure device.
1.0 (a)
(c) 0.8 Intensity/a.u.
729
0.6 (d)
Acknowledgments 0.4
0.2
The authors express their thanks to the support from NSFC (10374001, 10434030 and 60576016), State key project of basic research (2003CB314707), the Excellent Doctor’s Science and Technology Innovation Foundation of Beijing Jiaotong University (48011) and PD (295).
(b)
0.0 240
280
320
360
400
440
480
References
wavelength/nm Fig. 5. (a) Emission spectrum of PVK under 346 nm excitation, (b) excitation spectra of PVK monitoring 410 nm, (c) excitation spectra of Tb(o-BBA)3(phen) monitoring 546 nm and (d) excitation spectra of their mixture (PVK:Tb) monitoring 546 nm.
emission and the dopant excitation spectra was desirable for an efficient energy transfer. It is shown in Fig. 5 that there existed very little overlapping between the excitation spectrum of Tb(o-BBA)3(phen) and the emission spectrum of PVK. Although the excitation spectra of PVK and their
[1] S.I. Weissman, J. Chem. Phys. 10 (1942) 214. [2] D.D. Gebler, Y.Z. Wang, J.W. Blatchford, Appl. Phys. Lett. 70 (1997) 1644. [3] C.L. Chao, S.A. Chen, Appl. Phys. Lett. 73 (1998) 426. [4] Z. Xu, F. Teng, C. Qu, J. Lumin. 102 (2003) 291. [5] W.G. Yu, Z. Xu, F. Teng, Phys. Lett. A 338 (2005) 402. [6] F.J. Zhang, Z. Xu, F. Teng, Mat. Sci. Eng. B 123 (2005) 84. [7] X.R. Xu, G. Lei, M.Y. Shen, J. Cryst. Growth 101 (1990) 1004. [8] F.J. Zhang, Z. Xu, F. Teng, J. Lumin. 117 (1) (2006) 90. [9] C. Adschi, M.A. Baldo, S.R. Forrest, J. Appl. Phys. 87 (2000) 8049. [10] M.D. McGehee, T. Bergstedt, C. Zhang, et al., Adv. Mater. 11 (1999) 1349.
Journal of Luminescence 122–123 (2007) 727–729 www.elsevier.com/locate/jlumin
Electroluminescence of Tb(o-BBA)3(phen) based on an organic–inorganic heterostructure Fujun Zhanga, Zheng Xua,, Suling Zhaoa, Ling Liua, Weiwei Jianga, Bo Sunb, Deang Liua, Jinzhao Huanga, Juan Peib a
Key Laboratory of Luminescence and Optical Information, Ministry of Education Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China b Department of Materials Chemistry, College of Chemistry, Nankai University, Tianjin, 300071, PR China Available online 15 March 2006
Abstract An organic–inorganic heterostructure device was designed to improve the electroluminescence intensity of terbium ion. In this heterostructure, a mixed solution of (Tb(o-BBA)3(phen)) and poly(N-vinylcarbazole) (PVK) was prepared with a weight ratio of 3:1 which was acted as a hole transporting and emitting layer. A wide band semiconductor zinc sulfide (ZnS) layer was used as an electron transporting and hole blocking layer. Characteristic emissions of terbium ion from the organic–inorganic heterostructure device were observed and the electroluminescence intensity of terbium ion increased abruptly when the driving voltage went beyond 20.5 V. This may be due to a directly impact excitation of terbium ion by hot electrons, accelerated in ZnS layer, and then a recombination with the injected holes. The electroluminescence mechanism of Tb(o-BBA)3(phen)-doped PVK was the charge-carrier trapping process by the dopant molecule according to the emission spectra of PVK and the excitation spectra of Tb(o-BBA)3(phen). r 2006 Elsevier B.V. All rights reserved. Keywords: Organic–inorganic heterostructure; Rare earth complex; Electroluminescence
1. Introduction Since a bright photoluminescence (PL) of europium complex was first reported by Weissman in 1942 [1], great progress had been achieved in the field of luminescence of lanthanide organic complexes. In the recent years, organic electroluminescence (OEL) and inorganic electroluminescence (IEL) made a distinguished progress. However, each of them has its disadvantages. Although multi-layer OEL devices were often fabricated in order to balance the charge injection and improve the efficiency, photochemical or electrochemical phenomena, (such as exciplex and electroplex) occurred at the organic–organic interface or in a blend of organic materials [2,3]. These phenomena always varied with the applied electric field strength and sometimes were out of control. Therefore, the emission color of OEL changed with the increase in driving voltage. It is a big shortcoming for the flat panel display. In our study, an Corresponding author. Tel.: +86 10 51688605; fax: 86 10 51683933.
E-mail address: [email protected] (Z. Xu). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.272
inorganic semiconductor ZnS layer was introduced as an electron transporting and hole-blocking layer in order to avoid these phenomena. An organic–inorganic heterostructure (ITO\PVK:Tb\ZnS\Al) was fabricated. It has been reported that an organic–inorganic heterostructure device may hold their advantages of OEL and IEL and discard their disadvantages [4,5]. In this paper, emissions of Tb ion were observed from the specific structure and electroluminescence (EL) mechanism of Tb(o-BBA)3 (phen)-doped PVK was studied.
2. Experiments The chemical structures of organic materials are shown in Fig. 1. Here, PVK was dissolved in chloroform with the concentration of 10 mg/ml. Tb(o-BBA)3(phen) was doped into PVK with a weight ratio of 3:1 to improve the performance of Tb(o-BBA)3(phen) thin film. Then the mixed solution was spin coated at 3000 rpm onto the topcleaned ITO-coated glass substrate. And then the ZnS thin
ARTICLE IN PRESS F. Zhang et al. / Journal of Luminescence 122–123 (2007) 727–729
728
12V 14V 16V 18V
1.0
Intensity (a.u.)
0.8
0.6
0.4
0.2 Fig. 1. Chemical structure of PVK and Tb(o-BBA)3(phen).
0.0
film that acted as an electron transporting and hole blocking layer, was prepared by electron beam evaporation with the growth rate of 0.5 A˚/S under a high vacuum of 2 10 6 Torr. Finally the top Al electrode was prepared by thermal evaporation. The devices were driven by Keithley Source Meter 2410 and the EL spectra were measured by SPEX Fluorolog-3 spectrometer under ambient atmosphere.
500
550 Wavelength (nm)
600
Fig. 2. The EL spectra of Tb(o-BBA)3(phen) based on organic–inorganic heterostructure under different driving voltage.
3. Results and discussion The EL spectra of Tb(o-BBA)3(phen) under different driving voltages from the organic–inorganic heterostructure device are shown in Fig. 2. The four characteristic emission peaks of terbium ions were 490 nm, 546 nm, 585 nm and 620 nm corresponding to the transition of 5 D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3, respectively. In this specific structure the ZnS layer was acted as an electron transporting and hole-blocking layer. Therefore, the emission zone was limited within the PVK layer due to a big hole barrier (about 1.3 eV) between the HOMO of PVK and the valence band of ZnS. The energy diagram of the heterostructure was shown in Fig. 3. Under a forward voltage, the electrons and holes were injected from the cathode and the anode, respectively. Under the same electric field, the hole mobility in PVK layer was much smaller than the electron mobility in ZnS layer. Therefore, a large part of driving voltage dropped on the PVK layer. As we know, the charge mobility strongly depends on the electric field strength. Consequently, the hole mobility in PVK layer must be enhanced. The EL intensity was strongly controlled by the hole current because the hole was minor charge carrier in the specific devices. As a result, the EL intensity was improved as shown in Fig. 4. The relationship between the EL intensity of 546 nm and the driving voltage was measured by monitoring the emission of 546 nm. When the driving voltage went beyond 20.5 V, the EL intensity increased abruptly. It was reported that electrons injected from the cathode may be accelerated to a high energy in some inorganic semiconductor materials, such as ZnS and SiO2 [6–8]. These hot electrons recombined directly with the injected hole, or bombarded
Fig. 3. Energy diagram of the organic–inorganic structure devices.
the Tb ions and then recombined with the injected holes within PVK layer. The EL mechanisms of rare earth complexes have been extensively studied in OEL. There were two dominant mechanisms, (i) direct trapping of the injected holes and electrons, and subsequently forming excitons occurred on the rare earth ions [9], (ii) energy transferring from the excited host to the dopant and finally sensitizing the rare earth ions [10]. Energy transfer mechanism involved either a dipole-induced coulombic interaction between the host excitons and the doped complex (Forster energy transfer for electro–fluorescence) or an electron-exchange interaction between them (Dexter energy transfer for electrophosphorescence). Substantial overlapping of the host
ARTICLE IN PRESS F. Zhang et al. / Journal of Luminescence 122–123 (2007) 727–729
0.6
mixture were much alike, both of them were different from the excitation spectrum of Tb(o-BBA)3(phen). So the forest energy transfer process from PVK to Tb(o-BBA)3(phen) may not occur effectively in the blend system. The main EL mechanism of Tb(o-BBA)3(phen) doped in PVK may be the charge-carrier trapping process by the dopant molecule.
0.4
4. Conclusions
0.2
Characteristic emissions of Tb(o-BBA)3(phen)-doped PVK were obtained from the organic–inorganic heterostructure (ITO\PVK:Tb\ZnS\Al) devices. They were 490, 546, 585 and 620 nm corresponding to the transition of 5 D4–7F6, 5D4–7F5, 5D4–7F4 and 5D4–7F3 of the Tb ion. A large part of driving voltage dropped on the PVK layer because the hole mobility in PVK layer was much smaller than the electron mobility of ZnS under a same electric field strength. As the electric field strength in PVK layer was increased, the EL intensity increased as well. When the driving voltage went beyond 20.5 V, the EL intensity of 546 nm increased abruptly. This may be due to a direct impact excitation of terbium ion by hot electrons, accelerated in ZnS layer, and then a recombination with the injected holes. The main EL mechanism of PVK doped with Tb(o-BBA)3(phen) was the charge-carrier trapping process by the dopant molecule.
1.0
EL Intensity (a.u.)
0.8
0.0 10
12
14
16
18
20
22
Voltage(V) Fig. 4. The EL intensity of 546 nm-driving voltage characteristic of the heterostructure device.
1.0 (a)
(c) 0.8 Intensity/a.u.
729
0.6 (d)
Acknowledgments 0.4
0.2
The authors express their thanks to the support from NSFC (10374001, 10434030 and 60576016), State key project of basic research (2003CB314707), the Excellent Doctor’s Science and Technology Innovation Foundation of Beijing Jiaotong University (48011) and PD (295).
(b)
0.0 240
280
320
360
400
440
480
References
wavelength/nm Fig. 5. (a) Emission spectrum of PVK under 346 nm excitation, (b) excitation spectra of PVK monitoring 410 nm, (c) excitation spectra of Tb(o-BBA)3(phen) monitoring 546 nm and (d) excitation spectra of their mixture (PVK:Tb) monitoring 546 nm.
emission and the dopant excitation spectra was desirable for an efficient energy transfer. It is shown in Fig. 5 that there existed very little overlapping between the excitation spectrum of Tb(o-BBA)3(phen) and the emission spectrum of PVK. Although the excitation spectra of PVK and their
[1] S.I. Weissman, J. Chem. Phys. 10 (1942) 214. [2] D.D. Gebler, Y.Z. Wang, J.W. Blatchford, Appl. Phys. Lett. 70 (1997) 1644. [3] C.L. Chao, S.A. Chen, Appl. Phys. Lett. 73 (1998) 426. [4] Z. Xu, F. Teng, C. Qu, J. Lumin. 102 (2003) 291. [5] W.G. Yu, Z. Xu, F. Teng, Phys. Lett. A 338 (2005) 402. [6] F.J. Zhang, Z. Xu, F. Teng, Mat. Sci. Eng. B 123 (2005) 84. [7] X.R. Xu, G. Lei, M.Y. Shen, J. Cryst. Growth 101 (1990) 1004. [8] F.J. Zhang, Z. Xu, F. Teng, J. Lumin. 117 (1) (2006) 90. [9] C. Adschi, M.A. Baldo, S.R. Forrest, J. Appl. Phys. 87 (2000) 8049. [10] M.D. McGehee, T. Bergstedt, C. Zhang, et al., Adv. Mater. 11 (1999) 1349.