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Preparation and performance of a new type of blue light-emitting material δ-Alq3
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 663–666 www.elsevier.com/locate/jlumin
Preparation and performance of a new type of blue light-emitting material d-Alq3 Bingshe Xua,, Hua Wanga, Yuying Haob, Zhixiang Gaob, Hefeng Zhoua a
College of Materials Science and Engineering, Taiyuan University of Technology, 79, West Yingze Street, Taiyuan Shanxi 030024, China b College of Science, Taiyuan University of Technology, 79, West Yingze Street, Taiyuan Shanxi 030024, China Available online 29 March 2006
Abstract In this article, d-Alq3 was synthesized by a novel method, which can act as a new type of blue light-emitting material. Performance of d-Alq3 was investigated by SEM, UV–vis absorption spectra, photoluminescence (PL) spectra and electroluminescence (EL) spectra. Finally, it was concluded that grain shape of d-Alq3 is close to column, absorption band of electron transition from HOMO orbit in phenoxide ring to LUMO orbit in pyridine ringis at 432 nm in UV–vis absorption spectra of d-Alq3, optical band gap of d-Alq3 is 2.71 eV, the maximum emission peak (lmax) of d-Alq3 is at 480 nm in PL spectra, lmax of d-Alq3 is 520 nm in EL spectra. r 2006 Elsevier B.V. All rights reserved. Keywords: d-Alq3; a-Alq3; SEM; UV–vis absorption spectra
1. Introduction
2. Experimental
Alq3 is a kind of excellent organic electroluminescent material for its excellent performance. Early research fruits showed that there are two different Alq3 isomers, i.e., merAlq3 and fac-Alq3 [1]. Along with broad application of density functional theory (DFT) in luminescent material design, researches on phases of Alq3 are being hot. In 2000, Brinkmann [2] reported three phases of Alq3, which were aAlq3, b-Alq3 and g-Alq3, and studied the configurations of a-Alq3 and b-Alq3. In 2001, Co¨lle [3] research group reported a novel blue luminescent phase of Alq3, which was named d-Alq3, and simulated its spatial structure. But till 2003, only thermal performance and photoluminescent (PL) performance of d-Alq3 had been reported [4]. In this article, a novel preparation method of d-Alq3 is introduced, and its performance was investigated by scanning electron microscope (SEM), UV–vis absorption spectra, PL spectra and electroluminescent (EL) spectra.
Initiative Alq3 was prepared following the preparation method of Ref. [5] in laboratory. High purity a-Alq3 was obtained by vacuum sublimating. a-Alq3 powder was vacuum heated at about 390 1C in a round-bottom flask, which was connected with a vacuum pump. After vacuum heat treatment for nearly 30 min, powder in flask was washed in DMF and pyridine for several times, and filtered. Finally, gray–green d-Alq3 powder was dried in oven. SEM images were obtained on JEM-6700F SEM. UV–vis absorption spectra were measured on UV–vis scanning spectrophotometer (Lambda Bio40). PL spectra and EL spectra were measured on fluorescence spectrometer (SPR-920D). EL spectra were measured based on OLEDs with structures ITO/CuPc(6 nm)/d-Alq3(30 nm)/ Al(30 nm) and ITO/CuPc(6 nm)/a-Alq3(30 nm)/Al(30 nm), and the emitting area was 5 mm 5 mm. 3. Results and discussions 3.1. SEM images
Corresponding author. Tel./fax: +86 -0351 6010 311.
E-mail address: [email protected] (B. Xu). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.254
Fig. 1(a) and (b) show the SEM images of d-Alq3. Grain shape of d-Alq3 is close to column, whose length is about
ARTICLE IN PRESS B. Xu et al. / Journal of Luminescence 122–123 (2007) 663–666
664
7 mm and diameter is about 2 mm. In Fig. 1(c) and (d), it can also be seen that grain shape of a-Alq3 is sphere, whose diameter is 0.7 mm. It indicates that a-Alq3 grain grows remarkably during vacuum heat treatment, axial growing speed is faster than radial growing speed, and d-Alq3 possesses a new crystal structure. It will be studied in detail in future research work. 3.2. UV–vis absorption spectra Fig. 2 shows UV–vis absorption spectra of d-Alq3 powder, where an absorption band is at 432 nm. This absorption band is assigned to electron transition from HOMO orbit in phenoxide ring to LUMO orbit in pyridine ring, and the corresponding band of a-Alq3 is at 451 nm [6]. Optical band gap of d-Alq3 can be calculated based on UV–vis absorption spectra data, and the calculating
formula is hnA ¼ Bðhn E g Þn . In the formula, A is absorption coefficient, h is Planck coefficient, n (Greek niu) is light wave frequency, Eg is optical band gap, n usually is 0.5 and B is a coefficient relevant to material [7]. Ref. [7] introduced calculating method in detail. Fig. 3 shows the (hnA)2hn curves of d-Alq3 and a-Alq3, where the optical band gap of d-Alq3 is 2.71 eV, and that of aAlq3 is 2.53 eV. In molecular spatial structure of a-Alq3, there is a quasiC3 symmetry axis [2]. In the case of a-Alq3 converting into d-Alq3 by vacuum heat treatment at about 390 1C, three quinoline planes of a-Alq3 molecule rotate around quasi-C3 symmetry axis. In molecular spatial structure of a-Alq3, it induces Al–N bond to shorten, Al–O bond to lengthen, and
Fig. 1. SEM images: (a) d-Alq3 ( 8000), (b) d-Alq3 ( 15000), (c) a-Alq3 ( 2700) and (d) a-Alq3 ( 23000).
ARTICLE IN PRESS B. Xu et al. / Journal of Luminescence 122–123 (2007) 663–666
480
432
515
451 PL Intensity(a.u.)
A(a.u.)
257
254
200
665
300
400 500 600 Wavelength(nm)
700
800 300
Fig. 2. UV–vis absorption spectra : d-Alq3 (solid line) and a-Alq3 (dashed line).
400
500 600 Wavelength(nm)
700
800
Fig. 4. PL spectra:d-Alq3 (dashed line) and a-Alq3 (solid line).
520
0.1 (hvA)2
Optical band gap
2.2
2.3
2.4
2.5 2.6 hv/eV
2.7
2.8
EL Intensity(a.u.)
520
2.9
Fig. 3. (hnA)2 hn curves: d-Alq3 (solid line) and a-Alq3 (dashed line).
300
400
500
600
700
800
Wavelength(nm) Fig. 5. EL spectra: d-Alq3 (dashed line) and a-Alq3 (solid line).
each quinoline plane to distort. It indicates that d-Alq3 lacks symmetry relative to a-Alq3 in molecule spatial structure. The transformation in molecular spatial structure of a-Alq3 reduces electron cloud density on phenoxide and weakens intermolecular conjugated interaction between adjacent Alq3 molecules [8]. Therefore, it can be seen in Fig. 2 that absorption band of electron transition from HOMO to LUMO of d-Alq3 blue shifts 19 nm relative to aAlq3, and optical band gap of d-Alq3 is 0.18 eV wider than it of a-Alq3.
3.3. PL spectra Fig. 4 shows PL spectra of d-Alq3, where the wavelength of maximum emission peak (lmax) of d-Alq3 is 480 nm, dAlq3 exhibits blue-PL. For wider optical band gap of d-
Alq3, it can be seen that its lmax blue shifts 35 nm relative to a-Alq3 in Fig. 4, and lmax of a-Alq3 is 515 nm.
3.4. EL spectra Fig. 5 shows EL spectra of d-Alq3. It is interesting to find that lmax of d-Alq3 and a-Alq3 are all 520 nm. This phenomenon is identical with what Co¨lle [4] reported in 2003. In the case of forming film by vacuum heating, it was found that d-Alq3 phase would convert into a-Alq3 phase again in early research work [8]. It might be the consequence that tree quinoline planes keep rotating around quasi-C3 axis and resume original position of
ARTICLE IN PRESS 666
B. Xu et al. / Journal of Luminescence 122–123 (2007) 663–666
a-Alq3. The detail reason will be studied in future research work. 4. Conclusions In summary, grain shape of d-Alq3 is nearly column, whose length is about 7 mm and diameter is about 2 mm, absorption band of electron transition from HOMO orbit in phenoxide ring to LUMO orbit in pyridine ring shifts towards short wavelength in UV–vis absorption spectra of d-Alq3 relative to a-Alq3, optical band gap of d-Alq3 is 2.71 eV, lmax of d-Alq3 is 480 nm in PL spectra, and lmax of d-Alq3 is 520 nm in EL spectra. Acknowledgements This work was financially supported by National Basic Research Program of China (2004CB217808), National Natural Science Foundation of China (20271037, 20471041), Major Project of National Natural Science
Foundation of China (90306014), Natural Science Foundation of Shanxi (20041066) and Scientific Research Foundation for The Returned Chinese Overseas Scholars of Shanxi (200523). References [1] A. Curioni, M. Boero, W. Andreoni. Chem. Phys. Lett. 294 (1998) 263. [2] M. Brinkmann, G. Gadret, M. Muccini, C. Taliani, N. Masciocchi, A. Sironi. J. Am. Chem. Soc. 122 (2000) 5147. [3] M. Brauna, J. Gmeiner, M. Tzolov, M. Co¨lle, F.D. Meyer, W. Milius, O. Wendland, J.U. von.Schu¨tz, W. Bru¨tting, J. Chem. Phys. 114 (2001) 9625. [4] M. Co¨lle, J. Gmeiner, W. Milius, H. Hillebrechtand, W. Bru¨tting, Adv. Funct. Mater. 13 (2003) 108. [5] B.S. Xu, H. Wang, H.F. Zhou, Y.Y. Hao, J. Li, Y.L. Lu, Chinese Patent: ZL02135615.7. [6] C.H. Chen, J. Shi, Coord. Chem. Rev. 171 (1998) 161. [7] Y.Y. Hao, H.T. Hao, H. Wang, H.F. Zhou, X.G. Liu, B.S. Xu, Spectrosc. Spect. Anal. 124 (2004) 1524. [8] H. Wang, Y.Y. Hao, Z.X. Gao, H.F. Zhou, B.S. Xu, Spectrosc. Spect. Anal., to be published.
Journal of Luminescence 122–123 (2007) 663–666 www.elsevier.com/locate/jlumin
Preparation and performance of a new type of blue light-emitting material d-Alq3 Bingshe Xua,, Hua Wanga, Yuying Haob, Zhixiang Gaob, Hefeng Zhoua a
College of Materials Science and Engineering, Taiyuan University of Technology, 79, West Yingze Street, Taiyuan Shanxi 030024, China b College of Science, Taiyuan University of Technology, 79, West Yingze Street, Taiyuan Shanxi 030024, China Available online 29 March 2006
Abstract In this article, d-Alq3 was synthesized by a novel method, which can act as a new type of blue light-emitting material. Performance of d-Alq3 was investigated by SEM, UV–vis absorption spectra, photoluminescence (PL) spectra and electroluminescence (EL) spectra. Finally, it was concluded that grain shape of d-Alq3 is close to column, absorption band of electron transition from HOMO orbit in phenoxide ring to LUMO orbit in pyridine ringis at 432 nm in UV–vis absorption spectra of d-Alq3, optical band gap of d-Alq3 is 2.71 eV, the maximum emission peak (lmax) of d-Alq3 is at 480 nm in PL spectra, lmax of d-Alq3 is 520 nm in EL spectra. r 2006 Elsevier B.V. All rights reserved. Keywords: d-Alq3; a-Alq3; SEM; UV–vis absorption spectra
1. Introduction
2. Experimental
Alq3 is a kind of excellent organic electroluminescent material for its excellent performance. Early research fruits showed that there are two different Alq3 isomers, i.e., merAlq3 and fac-Alq3 [1]. Along with broad application of density functional theory (DFT) in luminescent material design, researches on phases of Alq3 are being hot. In 2000, Brinkmann [2] reported three phases of Alq3, which were aAlq3, b-Alq3 and g-Alq3, and studied the configurations of a-Alq3 and b-Alq3. In 2001, Co¨lle [3] research group reported a novel blue luminescent phase of Alq3, which was named d-Alq3, and simulated its spatial structure. But till 2003, only thermal performance and photoluminescent (PL) performance of d-Alq3 had been reported [4]. In this article, a novel preparation method of d-Alq3 is introduced, and its performance was investigated by scanning electron microscope (SEM), UV–vis absorption spectra, PL spectra and electroluminescent (EL) spectra.
Initiative Alq3 was prepared following the preparation method of Ref. [5] in laboratory. High purity a-Alq3 was obtained by vacuum sublimating. a-Alq3 powder was vacuum heated at about 390 1C in a round-bottom flask, which was connected with a vacuum pump. After vacuum heat treatment for nearly 30 min, powder in flask was washed in DMF and pyridine for several times, and filtered. Finally, gray–green d-Alq3 powder was dried in oven. SEM images were obtained on JEM-6700F SEM. UV–vis absorption spectra were measured on UV–vis scanning spectrophotometer (Lambda Bio40). PL spectra and EL spectra were measured on fluorescence spectrometer (SPR-920D). EL spectra were measured based on OLEDs with structures ITO/CuPc(6 nm)/d-Alq3(30 nm)/ Al(30 nm) and ITO/CuPc(6 nm)/a-Alq3(30 nm)/Al(30 nm), and the emitting area was 5 mm 5 mm. 3. Results and discussions 3.1. SEM images
Corresponding author. Tel./fax: +86 -0351 6010 311.
E-mail address: [email protected] (B. Xu). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.254
Fig. 1(a) and (b) show the SEM images of d-Alq3. Grain shape of d-Alq3 is close to column, whose length is about
ARTICLE IN PRESS B. Xu et al. / Journal of Luminescence 122–123 (2007) 663–666
664
7 mm and diameter is about 2 mm. In Fig. 1(c) and (d), it can also be seen that grain shape of a-Alq3 is sphere, whose diameter is 0.7 mm. It indicates that a-Alq3 grain grows remarkably during vacuum heat treatment, axial growing speed is faster than radial growing speed, and d-Alq3 possesses a new crystal structure. It will be studied in detail in future research work. 3.2. UV–vis absorption spectra Fig. 2 shows UV–vis absorption spectra of d-Alq3 powder, where an absorption band is at 432 nm. This absorption band is assigned to electron transition from HOMO orbit in phenoxide ring to LUMO orbit in pyridine ring, and the corresponding band of a-Alq3 is at 451 nm [6]. Optical band gap of d-Alq3 can be calculated based on UV–vis absorption spectra data, and the calculating
formula is hnA ¼ Bðhn E g Þn . In the formula, A is absorption coefficient, h is Planck coefficient, n (Greek niu) is light wave frequency, Eg is optical band gap, n usually is 0.5 and B is a coefficient relevant to material [7]. Ref. [7] introduced calculating method in detail. Fig. 3 shows the (hnA)2hn curves of d-Alq3 and a-Alq3, where the optical band gap of d-Alq3 is 2.71 eV, and that of aAlq3 is 2.53 eV. In molecular spatial structure of a-Alq3, there is a quasiC3 symmetry axis [2]. In the case of a-Alq3 converting into d-Alq3 by vacuum heat treatment at about 390 1C, three quinoline planes of a-Alq3 molecule rotate around quasi-C3 symmetry axis. In molecular spatial structure of a-Alq3, it induces Al–N bond to shorten, Al–O bond to lengthen, and
Fig. 1. SEM images: (a) d-Alq3 ( 8000), (b) d-Alq3 ( 15000), (c) a-Alq3 ( 2700) and (d) a-Alq3 ( 23000).
ARTICLE IN PRESS B. Xu et al. / Journal of Luminescence 122–123 (2007) 663–666
480
432
515
451 PL Intensity(a.u.)
A(a.u.)
257
254
200
665
300
400 500 600 Wavelength(nm)
700
800 300
Fig. 2. UV–vis absorption spectra : d-Alq3 (solid line) and a-Alq3 (dashed line).
400
500 600 Wavelength(nm)
700
800
Fig. 4. PL spectra:d-Alq3 (dashed line) and a-Alq3 (solid line).
520
0.1 (hvA)2
Optical band gap
2.2
2.3
2.4
2.5 2.6 hv/eV
2.7
2.8
EL Intensity(a.u.)
520
2.9
Fig. 3. (hnA)2 hn curves: d-Alq3 (solid line) and a-Alq3 (dashed line).
300
400
500
600
700
800
Wavelength(nm) Fig. 5. EL spectra: d-Alq3 (dashed line) and a-Alq3 (solid line).
each quinoline plane to distort. It indicates that d-Alq3 lacks symmetry relative to a-Alq3 in molecule spatial structure. The transformation in molecular spatial structure of a-Alq3 reduces electron cloud density on phenoxide and weakens intermolecular conjugated interaction between adjacent Alq3 molecules [8]. Therefore, it can be seen in Fig. 2 that absorption band of electron transition from HOMO to LUMO of d-Alq3 blue shifts 19 nm relative to aAlq3, and optical band gap of d-Alq3 is 0.18 eV wider than it of a-Alq3.
3.3. PL spectra Fig. 4 shows PL spectra of d-Alq3, where the wavelength of maximum emission peak (lmax) of d-Alq3 is 480 nm, dAlq3 exhibits blue-PL. For wider optical band gap of d-
Alq3, it can be seen that its lmax blue shifts 35 nm relative to a-Alq3 in Fig. 4, and lmax of a-Alq3 is 515 nm.
3.4. EL spectra Fig. 5 shows EL spectra of d-Alq3. It is interesting to find that lmax of d-Alq3 and a-Alq3 are all 520 nm. This phenomenon is identical with what Co¨lle [4] reported in 2003. In the case of forming film by vacuum heating, it was found that d-Alq3 phase would convert into a-Alq3 phase again in early research work [8]. It might be the consequence that tree quinoline planes keep rotating around quasi-C3 axis and resume original position of
ARTICLE IN PRESS 666
B. Xu et al. / Journal of Luminescence 122–123 (2007) 663–666
a-Alq3. The detail reason will be studied in future research work. 4. Conclusions In summary, grain shape of d-Alq3 is nearly column, whose length is about 7 mm and diameter is about 2 mm, absorption band of electron transition from HOMO orbit in phenoxide ring to LUMO orbit in pyridine ring shifts towards short wavelength in UV–vis absorption spectra of d-Alq3 relative to a-Alq3, optical band gap of d-Alq3 is 2.71 eV, lmax of d-Alq3 is 480 nm in PL spectra, and lmax of d-Alq3 is 520 nm in EL spectra. Acknowledgements This work was financially supported by National Basic Research Program of China (2004CB217808), National Natural Science Foundation of China (20271037, 20471041), Major Project of National Natural Science
Foundation of China (90306014), Natural Science Foundation of Shanxi (20041066) and Scientific Research Foundation for The Returned Chinese Overseas Scholars of Shanxi (200523). References [1] A. Curioni, M. Boero, W. Andreoni. Chem. Phys. Lett. 294 (1998) 263. [2] M. Brinkmann, G. Gadret, M. Muccini, C. Taliani, N. Masciocchi, A. Sironi. J. Am. Chem. Soc. 122 (2000) 5147. [3] M. Brauna, J. Gmeiner, M. Tzolov, M. Co¨lle, F.D. Meyer, W. Milius, O. Wendland, J.U. von.Schu¨tz, W. Bru¨tting, J. Chem. Phys. 114 (2001) 9625. [4] M. Co¨lle, J. Gmeiner, W. Milius, H. Hillebrechtand, W. Bru¨tting, Adv. Funct. Mater. 13 (2003) 108. [5] B.S. Xu, H. Wang, H.F. Zhou, Y.Y. Hao, J. Li, Y.L. Lu, Chinese Patent: ZL02135615.7. [6] C.H. Chen, J. Shi, Coord. Chem. Rev. 171 (1998) 161. [7] Y.Y. Hao, H.T. Hao, H. Wang, H.F. Zhou, X.G. Liu, B.S. Xu, Spectrosc. Spect. Anal. 124 (2004) 1524. [8] H. Wang, Y.Y. Hao, Z.X. Gao, H.F. Zhou, B.S. Xu, Spectrosc. Spect. Anal., to be published.