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Influence of ZnO nanorod on the luminescent and electrical properties of fluorescent dye-doped polymer nanocomposite
Optical Materials 29 (2006) 216–219 www.elsevier.com/locate/optmat
Influence of ZnO nanorod on the luminescent and electrical properties of fluorescent dye-doped polymer nanocomposite T. Zhang a
a,*
, Z. Xu a, L. Qian a, D.L. Tao b, F. Teng a, X.R. Xu
a
Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044, China b Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 19 May 2005; accepted 29 August 2005 Available online 10 October 2005
Abstract The luminescent properties of fluorescent dye-doped polymer dispersed with ZnO nanorods were investigated. Embedding ZnO nanorods in blend film results in a blue-shifted emission of fluorescent dye. It is accounted for in terms of the difference in permittivity between inorganic oxide nano-material and dye-doped polymer. Moreover, polymer light-emitting diodes with the addition of ZnO nanorods showed the lower threshold voltage and the higher charge current and electroluminescence efficiency. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 78.60.Fi; 72.80.Le Keywords: Nanorod; PLEDs; Fluorescent dye
1. Introduction Blending is a technique known in polymer technology that takes advantage of the processibility of polymers to produce new solid materials or composites with specific structural and physical properties, distinct from the ones of their components. Light-emitting diodes (LEDs) made of polymer blends have shown strongly enhanced electroluminescence (EL) efficiencies, as compared to pure homopolymers. Color conversion, white light emission, polarized light emission, emission line narrowing, and voltage-tunable colors are other effects that have been observed in blends containing light-emitting polymers [1–5]. However, fabricating high efficient devices depends not only on the electronic and the optical properties of the pure organic materials components, but also on the control of charge transport. Carrier trap effect caused by dopant could make carriers transport ability decreasing and device threshold voltages increasing [6]. *
Corresponding author. Tel.: +86 10 51684908; fax: +86 10 51683933. E-mail address: [email protected] (T. Zhang).
0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.08.030
In recent years, the incorporation of nanometer-sized materials into the conducting polymers has been suggested as an efficient scheme to improve the luminescent efficiency of the device. Especially, one-dimensional nano-materials such as carbon or TiO2 nanotube attract tremendous interests of researchers due to their unique and outstanding electric and optoelectronic properties [7–11]. In our previous work, the properties of organic light-emitting diodes (OLEDs) were improved greatly due to doping one-dimensional nano-materials TiO2 nanotube into charge transfer layer [12]. Up to the present, few other nano-materials were applied to this fabrication. In this article, we dispersed ZnO nanorods in fluorescent dye-doped polymer as emitting layer in double layer heterojunction OLEDs. Meanwhile, the influence of ZnO nanorods on optical and charge transfer properties of dye-doped polymer nanocomposite was investigated. 2. Experimental In our experiment, ZnO nanorods were prepared by a simple, low-temperature and tractable method reported
T. Zhang et al. / Optical Materials 29 (2006) 216–219
D0: ITO/PVK: DCJTB (1 wt%) (150 nm)/Alq3(50 nm)/ Al D1: ITO/PVK: DCJTB (1 wt%): ZnO nanorod (5 wt%) (150 nm)/Alq3(50 nm)/Al D2: ITO/PVK: DCJTB (1 wt%): ZnO nanorod (10 wt%) (150 nm)/Alq3(50 nm)/Al The PL and EL spectra were measured by a Spex Fluorolg-3 spectrophotometer. Current- applied voltages curves were obtained by a computer-controlled Keithley 2410. All of the measurements were performed at room temperature under ambient pressure conditions.
3. Results and discussion Photoluminescence (PL) spectra of double layer diodes with and without dispersing ZnO nanorod (D0-D2) are presented in Fig. 2. The PL spectra at the excitation wavelength of 340 nm presented that the fluorescence emission occurs in DCJTB (590 nm) and Alq3 (510 nm) layers. It is seen that, the emission peak of DCJTB shifts from 606 nm to 582 nm with the increasing concentration of ZnO nanorod. Additionally, the relative intensity of DCJTB decreased with the increasing concentration of ZnO nanorod while that of Alq3 at 515 nm increased. Similar to PL spectra, we drew a comparison on electroluminescence (EL) spectra of our samples at the same voltage since the relevant layer thickness of all devices is same and all measure is under the same condition. It is observed that the DCJTB emission is also blue-shifted with the
700000
D0 D1
600000
Intensity (a.u.)
previously [13], in which the introduction of a water-soluble linear polymer polyvinyl pyrrolidone (PVP) is effiective to make ZnO nanorods from a zinc acetate precursor at a low-temperature. Fig. 1a shows the scanning electron micrograph (SEM) image of ZnO nanorods. The diameter and length of the ZnO nanorods are 50 nm and 1 lm, respectively. Poly(vinylcarbazole) (PVK) doped with 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)4H-pyran (DCJTB) at 1 wt% and ZnO nanorods were prepared by sonicating in chloroform respectively. And appropriate volumes were mixed to create a mixture of ZnO nanorods and PVK: DCJTB 1 wt% at two concentrations of 5 wt% and 10 wt%. Quartz wafer and indium tin oxide (ITO) glass were used as substrates for optical and electrical measurements, respectively. The polymer lightemitting diodes (PLEDs) preparation procedure was as follows: The ITO-glass substrate was cleaned and dried. After spin-cast nanocomposites films with thickness of 150 nm, Alq3 (50 nm) and Al cathode were deposited onto the nanocomposite layer by thermal evaporation method under a working pressure (10 6 mbar). A summary of the device structures prepared in this work is as follows (Fig. 1b) [14]:
217
D2
500000 400000 300000 200000 100000 0 350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 2. PL spectra for device D0(PVK:DCJTB 1 wt%/Alq3), D1(PVK: DCJTB 1 wt%: ZnO nanorod 5 wt%/Alq3) and D2(PVK:DCJTB 1 wt%: ZnO nanorod 10 wt%/Alq3), kex = 340 nm.
Fig. 1. (a) SEM photograph of ZnO nanorod (b) device structure and energy level schematics.
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T. Zhang et al. / Optical Materials 29 (2006) 216–219
1.0 mA
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D0
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Current (A)
Intensity (a.u.)
1.5x10-2
1.0x10-2
(b) 5.0x10-3
0.0 0 400
500
600
5
Wavelength (nm) Fig. 3. (a) EL spectra at same current normalized EL spectra at the voltage of 17 V for device D0(PVK:DCJTB 1 wt%/Alq3) (b) D1(PVK:DCJTB 1 wt%: ZnO nanorod 5 wt%/Alq3) and D2(PVK: DCJTB 1 wt%: ZnO nanorod 10 wt%/Alq3).
increasing concentration of ZnO nanorod (shown as Fig. 3b). The above influences of ZnO nanorod on the composite spectra may be accounted for in terms of the difference in permittivity between inorganic oxide nano-material and dye-doped polymer. If the permittivity of host is bigger than that of dopant, excitons emission of dopant would blue-shifted due to screen effect [15]. The bigger permittivity of host is, the higher emission energy of dopant exciton is. Additionally, PVK molecular may wrap round surface of ZnO nanorod, which made the chain conformation of PVK molecular more unwindable. This facilitated energy transfer from PVK to Alq3 and enhanced Alq3 intensity in PL process. The most important experimental fact consists in the increase of electroluminescence efficiency for a given injected current density with the increasing ZnO nanorod shown in Fig. 3(a). Doping ZnO nanorods in polymer enhance holes current density, which made current injection more balance. Fig. 4 shows that the current–voltages curve of PVK: DCJTB 1 wt% with different ratios of ZnO nanorod. It is obviously that device current increased with increasing nanorod. This is related to energy level and mobility of all materials. The available values of band gap Eg for the used materials allow us to find relative positions of the band edges in the nanocomposite (see Fig. 1b). The HOMO and LUMO level of PVK is 2.3 eV and 5.8 eV, DCJTB is 3.2 eV and 5.3 eV. Since ZnO conducting band level is 4.3 eV between HOMO and LUMO level of organic materials, and as mentioned above, PVK molecular chain was made more unwindable due to the dispersing of ZnO nanorod, which induced efficient charge transport along the chain. The connection between aligned molecular chains of PVK on ZnO nanorod also can increase carrier transport in nanocomposites. Thus the current would not be
10
15
20
25
30
Voltage (V)
700
Fig. 4. Current–voltage curves for devices D0(PVK:DCJTB 1 wt%/Alq3), D1(PVK:DCJTB 1 wt%: ZnO nanorod 5 wt%/Alq3) and D2(PVK: DCJTB 1 wt%: ZnO nanorod 10 wt%/Alq3).
limited by hopping of carriers between chains. Secondly, ZnO can also improve the hole-transporting ability of the PVK layer because of higher mobility of ZnO nanorod. Additional investigations aimed to elucidate the changes of luminescence spectra and maximize the growth of electroluminescence in nanocomposites are now in progress. 4. Conclusions Experimental investigations of fluorescent dye-doped polymer nanocomposites show that dispersing of ZnO nanorod may noticeably change the spectrum and intensity of photo- and electroluminescence of nanocomposite. EL efficiency for a given injected current density and charge current of dye-doped PLEDs were also improved by embedding with ZnO nanorod, which caused excitons recombination region shifted. Meanwhile, high permittivity of ZnO nanorod made the emission of DCJTB excitons blue-shifted by screen effect. Acknowledgements The authors express the thanks to the NSFC (10374001, 90301004), state key project of basic research (2003CB314707), China Postdoctoral Science Foundation (2003034324), RFDP (20020004004), NSFB (2032015). References [1] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [2] H. Aziz, Z.D. Popovic, N.X. Hu, A.M. Hor, G. Xu, Science 283 (1999) 1900. [3] M.A. Baldo, D.F. OÕBrien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [4] G. Sakamoto, C. Adachi, T. Koyama, Y. Taniguchi, C.D. Merritt, H. Murata, Z.H. Kafafi, Appl. Phys. Lett. 75 (1999) 766. [5] Y.G. Ma, T.S. Lai, Y. Wu, Adv. Mater. 12 (2000) 433. [6] P.A. Lane, L.C. Palilis, D.F. OÕBrien, C. Giebeler, A.J. Cadby, D.G. Lidzey, A.J. Campbell, W. Blau, D.D.C. Bradley, Phys. Rev. B 63 (2001) 235206.
T. Zhang et al. / Optical Materials 29 (2006) 216–219 [7] P. Fournet, J.N. Coleman, B. Lahr, A. Drury, W.J. Blau, D.F. OÕBrien, H.-H. Ho¨rhold, J. Appl. Phys. 90 (2001) 969. [8] S.A. Curran, P.M. Ajayan, W.J. Blau, D.L. Carroll, J.N. Coleman, A.B. Dalton, A.P. Davey, A. Drury, B. McCarthy, S. Maier, A. Strevens, Adv. Mater. 10 (1998) 1091. [9] J.N. Coleman, S. Curran, A.B. Holme, A.P. Davey, B. McCarthy, W. Blau, R.C. Barklie, Phys. Rev. B 58 (1998) 7492. [10] J.Y. Kim, M. Kim, H.M. Kim, J. Joo, J.H. Choi, Opt. Mater. 21 (2002) 147.
219
[11] T. Yamamoto, A. Taniguchi, Inorg. Chim. Acta 97 (1985) L11. [12] L. Qian, F. Teng, Z.-S. Jin, Z.-J. Zhang, T. Zhang, Y.-B. Hou, S.-Y. Yang, X.-R. Xu, J. Phys. Chem. B 108 (2004) 13928. [13] D.L. Tao, W.Z. Qian, Y. Huang, F. Wei, J. Cryst. Growth 27 (2004) 353. [14] L. Bakueva, S. Musikhin, E.H. Sargent, A. Shik, Surf. Sci. 532–535 (2003) 1051. [15] H.A. Lopez, X.L. Chen, S.A. Jenekhe, P.M. Fauchet, J. Lumin. 80 (1999) 115.
Influence of ZnO nanorod on the luminescent and electrical properties of fluorescent dye-doped polymer nanocomposite T. Zhang a
a,*
, Z. Xu a, L. Qian a, D.L. Tao b, F. Teng a, X.R. Xu
a
Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044, China b Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 19 May 2005; accepted 29 August 2005 Available online 10 October 2005
Abstract The luminescent properties of fluorescent dye-doped polymer dispersed with ZnO nanorods were investigated. Embedding ZnO nanorods in blend film results in a blue-shifted emission of fluorescent dye. It is accounted for in terms of the difference in permittivity between inorganic oxide nano-material and dye-doped polymer. Moreover, polymer light-emitting diodes with the addition of ZnO nanorods showed the lower threshold voltage and the higher charge current and electroluminescence efficiency. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 78.60.Fi; 72.80.Le Keywords: Nanorod; PLEDs; Fluorescent dye
1. Introduction Blending is a technique known in polymer technology that takes advantage of the processibility of polymers to produce new solid materials or composites with specific structural and physical properties, distinct from the ones of their components. Light-emitting diodes (LEDs) made of polymer blends have shown strongly enhanced electroluminescence (EL) efficiencies, as compared to pure homopolymers. Color conversion, white light emission, polarized light emission, emission line narrowing, and voltage-tunable colors are other effects that have been observed in blends containing light-emitting polymers [1–5]. However, fabricating high efficient devices depends not only on the electronic and the optical properties of the pure organic materials components, but also on the control of charge transport. Carrier trap effect caused by dopant could make carriers transport ability decreasing and device threshold voltages increasing [6]. *
Corresponding author. Tel.: +86 10 51684908; fax: +86 10 51683933. E-mail address: [email protected] (T. Zhang).
0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.08.030
In recent years, the incorporation of nanometer-sized materials into the conducting polymers has been suggested as an efficient scheme to improve the luminescent efficiency of the device. Especially, one-dimensional nano-materials such as carbon or TiO2 nanotube attract tremendous interests of researchers due to their unique and outstanding electric and optoelectronic properties [7–11]. In our previous work, the properties of organic light-emitting diodes (OLEDs) were improved greatly due to doping one-dimensional nano-materials TiO2 nanotube into charge transfer layer [12]. Up to the present, few other nano-materials were applied to this fabrication. In this article, we dispersed ZnO nanorods in fluorescent dye-doped polymer as emitting layer in double layer heterojunction OLEDs. Meanwhile, the influence of ZnO nanorods on optical and charge transfer properties of dye-doped polymer nanocomposite was investigated. 2. Experimental In our experiment, ZnO nanorods were prepared by a simple, low-temperature and tractable method reported
T. Zhang et al. / Optical Materials 29 (2006) 216–219
D0: ITO/PVK: DCJTB (1 wt%) (150 nm)/Alq3(50 nm)/ Al D1: ITO/PVK: DCJTB (1 wt%): ZnO nanorod (5 wt%) (150 nm)/Alq3(50 nm)/Al D2: ITO/PVK: DCJTB (1 wt%): ZnO nanorod (10 wt%) (150 nm)/Alq3(50 nm)/Al The PL and EL spectra were measured by a Spex Fluorolg-3 spectrophotometer. Current- applied voltages curves were obtained by a computer-controlled Keithley 2410. All of the measurements were performed at room temperature under ambient pressure conditions.
3. Results and discussion Photoluminescence (PL) spectra of double layer diodes with and without dispersing ZnO nanorod (D0-D2) are presented in Fig. 2. The PL spectra at the excitation wavelength of 340 nm presented that the fluorescence emission occurs in DCJTB (590 nm) and Alq3 (510 nm) layers. It is seen that, the emission peak of DCJTB shifts from 606 nm to 582 nm with the increasing concentration of ZnO nanorod. Additionally, the relative intensity of DCJTB decreased with the increasing concentration of ZnO nanorod while that of Alq3 at 515 nm increased. Similar to PL spectra, we drew a comparison on electroluminescence (EL) spectra of our samples at the same voltage since the relevant layer thickness of all devices is same and all measure is under the same condition. It is observed that the DCJTB emission is also blue-shifted with the
700000
D0 D1
600000
Intensity (a.u.)
previously [13], in which the introduction of a water-soluble linear polymer polyvinyl pyrrolidone (PVP) is effiective to make ZnO nanorods from a zinc acetate precursor at a low-temperature. Fig. 1a shows the scanning electron micrograph (SEM) image of ZnO nanorods. The diameter and length of the ZnO nanorods are 50 nm and 1 lm, respectively. Poly(vinylcarbazole) (PVK) doped with 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)4H-pyran (DCJTB) at 1 wt% and ZnO nanorods were prepared by sonicating in chloroform respectively. And appropriate volumes were mixed to create a mixture of ZnO nanorods and PVK: DCJTB 1 wt% at two concentrations of 5 wt% and 10 wt%. Quartz wafer and indium tin oxide (ITO) glass were used as substrates for optical and electrical measurements, respectively. The polymer lightemitting diodes (PLEDs) preparation procedure was as follows: The ITO-glass substrate was cleaned and dried. After spin-cast nanocomposites films with thickness of 150 nm, Alq3 (50 nm) and Al cathode were deposited onto the nanocomposite layer by thermal evaporation method under a working pressure (10 6 mbar). A summary of the device structures prepared in this work is as follows (Fig. 1b) [14]:
217
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500000 400000 300000 200000 100000 0 350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 2. PL spectra for device D0(PVK:DCJTB 1 wt%/Alq3), D1(PVK: DCJTB 1 wt%: ZnO nanorod 5 wt%/Alq3) and D2(PVK:DCJTB 1 wt%: ZnO nanorod 10 wt%/Alq3), kex = 340 nm.
Fig. 1. (a) SEM photograph of ZnO nanorod (b) device structure and energy level schematics.
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D0
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500
600
5
Wavelength (nm) Fig. 3. (a) EL spectra at same current normalized EL spectra at the voltage of 17 V for device D0(PVK:DCJTB 1 wt%/Alq3) (b) D1(PVK:DCJTB 1 wt%: ZnO nanorod 5 wt%/Alq3) and D2(PVK: DCJTB 1 wt%: ZnO nanorod 10 wt%/Alq3).
increasing concentration of ZnO nanorod (shown as Fig. 3b). The above influences of ZnO nanorod on the composite spectra may be accounted for in terms of the difference in permittivity between inorganic oxide nano-material and dye-doped polymer. If the permittivity of host is bigger than that of dopant, excitons emission of dopant would blue-shifted due to screen effect [15]. The bigger permittivity of host is, the higher emission energy of dopant exciton is. Additionally, PVK molecular may wrap round surface of ZnO nanorod, which made the chain conformation of PVK molecular more unwindable. This facilitated energy transfer from PVK to Alq3 and enhanced Alq3 intensity in PL process. The most important experimental fact consists in the increase of electroluminescence efficiency for a given injected current density with the increasing ZnO nanorod shown in Fig. 3(a). Doping ZnO nanorods in polymer enhance holes current density, which made current injection more balance. Fig. 4 shows that the current–voltages curve of PVK: DCJTB 1 wt% with different ratios of ZnO nanorod. It is obviously that device current increased with increasing nanorod. This is related to energy level and mobility of all materials. The available values of band gap Eg for the used materials allow us to find relative positions of the band edges in the nanocomposite (see Fig. 1b). The HOMO and LUMO level of PVK is 2.3 eV and 5.8 eV, DCJTB is 3.2 eV and 5.3 eV. Since ZnO conducting band level is 4.3 eV between HOMO and LUMO level of organic materials, and as mentioned above, PVK molecular chain was made more unwindable due to the dispersing of ZnO nanorod, which induced efficient charge transport along the chain. The connection between aligned molecular chains of PVK on ZnO nanorod also can increase carrier transport in nanocomposites. Thus the current would not be
10
15
20
25
30
Voltage (V)
700
Fig. 4. Current–voltage curves for devices D0(PVK:DCJTB 1 wt%/Alq3), D1(PVK:DCJTB 1 wt%: ZnO nanorod 5 wt%/Alq3) and D2(PVK: DCJTB 1 wt%: ZnO nanorod 10 wt%/Alq3).
limited by hopping of carriers between chains. Secondly, ZnO can also improve the hole-transporting ability of the PVK layer because of higher mobility of ZnO nanorod. Additional investigations aimed to elucidate the changes of luminescence spectra and maximize the growth of electroluminescence in nanocomposites are now in progress. 4. Conclusions Experimental investigations of fluorescent dye-doped polymer nanocomposites show that dispersing of ZnO nanorod may noticeably change the spectrum and intensity of photo- and electroluminescence of nanocomposite. EL efficiency for a given injected current density and charge current of dye-doped PLEDs were also improved by embedding with ZnO nanorod, which caused excitons recombination region shifted. Meanwhile, high permittivity of ZnO nanorod made the emission of DCJTB excitons blue-shifted by screen effect. Acknowledgements The authors express the thanks to the NSFC (10374001, 90301004), state key project of basic research (2003CB314707), China Postdoctoral Science Foundation (2003034324), RFDP (20020004004), NSFB (2032015). References [1] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [2] H. Aziz, Z.D. Popovic, N.X. Hu, A.M. Hor, G. Xu, Science 283 (1999) 1900. [3] M.A. Baldo, D.F. OÕBrien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [4] G. Sakamoto, C. Adachi, T. Koyama, Y. Taniguchi, C.D. Merritt, H. Murata, Z.H. Kafafi, Appl. Phys. Lett. 75 (1999) 766. [5] Y.G. Ma, T.S. Lai, Y. Wu, Adv. Mater. 12 (2000) 433. [6] P.A. Lane, L.C. Palilis, D.F. OÕBrien, C. Giebeler, A.J. Cadby, D.G. Lidzey, A.J. Campbell, W. Blau, D.D.C. Bradley, Phys. Rev. B 63 (2001) 235206.
T. Zhang et al. / Optical Materials 29 (2006) 216–219 [7] P. Fournet, J.N. Coleman, B. Lahr, A. Drury, W.J. Blau, D.F. OÕBrien, H.-H. Ho¨rhold, J. Appl. Phys. 90 (2001) 969. [8] S.A. Curran, P.M. Ajayan, W.J. Blau, D.L. Carroll, J.N. Coleman, A.B. Dalton, A.P. Davey, A. Drury, B. McCarthy, S. Maier, A. Strevens, Adv. Mater. 10 (1998) 1091. [9] J.N. Coleman, S. Curran, A.B. Holme, A.P. Davey, B. McCarthy, W. Blau, R.C. Barklie, Phys. Rev. B 58 (1998) 7492. [10] J.Y. Kim, M. Kim, H.M. Kim, J. Joo, J.H. Choi, Opt. Mater. 21 (2002) 147.
219
[11] T. Yamamoto, A. Taniguchi, Inorg. Chim. Acta 97 (1985) L11. [12] L. Qian, F. Teng, Z.-S. Jin, Z.-J. Zhang, T. Zhang, Y.-B. Hou, S.-Y. Yang, X.-R. Xu, J. Phys. Chem. B 108 (2004) 13928. [13] D.L. Tao, W.Z. Qian, Y. Huang, F. Wei, J. Cryst. Growth 27 (2004) 353. [14] L. Bakueva, S. Musikhin, E.H. Sargent, A. Shik, Surf. Sci. 532–535 (2003) 1051. [15] H.A. Lopez, X.L. Chen, S.A. Jenekhe, P.M. Fauchet, J. Lumin. 80 (1999) 115.