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Multicolour electroluminescence diodes using oligophenylene and oligophenylenevinylene multilayers
ELSEVIER
SyntheticMetals76 (1996) 113-115
Multicolour electroluminescence diodes using oligophenylene and oligophenylenevinylene multilayers F. Meghdadi a, G. Leising a,W. Fischer b, F. Stelzer b b Institutfiir
a Institutfiir Festkiirperphysik, Technische Universitdt Chemische Technologie Organischer Stoffe, Technische
Graz, Petersgasse 16, A-8010 Graz, Austria Universitiit Graz, Stremayrgasse 16, A-8010
Graz, Austria
Abstract We present electroluminescence (EL) characteristics for single-layer and multilayer thin films with oligophenylenevinylenes and oligophenylenes. Light-emitting diodes (LEDs) were fabricated by vacuum evaporation techniques of both the oligomer films and the negative electrodes. Depending on the substrate temperature, highly ordered or polycrystalline layers were obtained. Our LEDs emit blue light in the range of 425460 nm (oligophenylenes) and in a broad visible range of 400-600 nm (oligophenylene/oligophenylenevinylene multilayers). The current-voltage (I-V) characteristics of our devices show rectifying behaviour with high rectification ratios. We report I-Vcharacteristics and the respective threshold voltages, quantum efficiencies, photoluminescence (PL), EL and optical absorbance of the thin films and the devices. Keywords: Electroluminescence;Diodes; Oligophenylene;Oligophenylenevinylene
1. Introduction Since the first demonstration of electroluminescence (EL) devices using the conjugated polymer poly(para-phenylenevinylene) [ 1,2] considerable progress has been made in the field of polymer EL devices [ 31, Oligophenylenevinylene
(OPPV) [ 41, poly (para-phenylene) (PPP) and oligophenyls [ 5-91 with different energy gaps provide a range of different emission colours throughout the visible spectrum from blue to red and could therefore be used in multicolour devices and flat panel colour displays.
l-l
(4
oligo-PPP
(b)
oligo-PPV
Multi-organic
2. Experimental In this work we used oligophenylenevinylene (OPPV) , highly purified para-hexaphenyl (PHP) powder which has been synthesized either by electro-reduction of monobromoterphenyl in the presence of Ni catalyst [lo] or produced with improved yield by a simple, one-step procedure starting with biphenyl and using aluminium chloridexupric
chloride as a catalyst [ 111. The poly(para-phenylene) (PPP) obtained via the Kovacic (PPPK) [ 121 and Yamamoto (PPPY) routes of synthesis [ 131 and oligophenylenevinylene [ 141 are used as the source material for evaporation. Fig. 1 depicts their molecular structures and the layer 0379-6779/96/S15.000 1996Elsevier ScienceS.A. All rights reserved
Layer
cc>
Fig. 1. Molecularstructures of oligopolyphenylene(a) andoligophenylenevinylene(b) , anda schemeof theEL device(c) .
arrangement in the device. In order to obtain either multicompound or multilayer thin films of oligophenylene and oligophenylenevinylene we used a high vacuum molecular beam deposition technique (p < lo-’ Torr) with two quartz Knudsen cells as sources. Table 1 summarizes the preparation conditions for the different layers. To fabricate EL devices we used an indium-tin oxide (ITO)-coated glass substrate (BALTRACON, Balzers) and aluminium as hole- and electron-injecting electrodes, respectively.
114
F. Meghdadi et al. /Synthetic Metals 76 (1996) 113-115
Table 1 Preparation conditions for different layers: T,,, temperature of source 1; r,,, temperature of source 2; r,,, substrate temperature; d, thickness of oligomer layer; r, deposition rate Type of film
Ts, PC)
OPPVl OPPV2 OPPVIPPPY OPPVIPPPK OPPV/PHP PPPK PPPY PHP
210 450
i-s2
235 280
(“‘3
d (A)
335 270 270
210 285 280-500 265
115 115
1200 1500
25 25 25 245 245 25
2000 1200 1500 1500 2000
The UV-Vis absorption spectra were taken with a PerkinElmer Lambda-9 spectrophotometer. The photoluminescence (PL) spectra were recorded with a high resolution Jobin Yvon HR640 monochromator equipped with a cooled Hamamatsu R943-02 photomultiplier. The excitation light was provided by a Jobin Yvon double monochromator with a 1000 W xenon lamp as a source. The deposition rate and the thicknessof films (thickness varies typically between 100 and 200 nm) were monitored during the evaporation process with a quartz microbalance. The Z-V characteristics of EL diodes were recorded by a Keithley I-Vsystem consisting of voltage and current sources, voltmeters and an electrometer.
1000
Emission colour
r (A/s)
green green green/blue blue blue/green green blue
0.11 0.07 0.18 0.11 1.25 1.25 0.01 0.18
3 400
450
500 Wavelength
550
600
(nm)
Fig. 3. PL spectra of multicompound layers: (a) OPPV/PPPY (A,,, 330 nm); (b) OPPV/PPPK (A,,, 330 nm); (c) OPPV/PHP (A,,390 nm)
3. Results and discussion Fig. 2 shows the room-temperature PL and absorption spectra of OPPV thin films produced at two different source temperatures (210 and 450 “C) and identical substrate temperature (115 “C). The difference between the absorption spectra in the range of 4.1-3.1 eV (300-400 nm) can be attributed to ordering effects of the evaporated molecules on the surface, which have been investigated thoroughly in the case of PHP [ 151, The compound layer consisting of oligophenylenes and oligophenylenevinylenes (with the composition varying from 0.1 to 0.5 molar ratio of OPPV/OPPP)
300
400 Wavelength
500 (nm)
600
Fig. 2. PL (A,,, 330 nm) and absorbance spectra of OPPVl and OPPV2 produced at two different source temperatures (210 and 450 “C); for comparison we show the PL spectrum of poly(para-phenylenevinylene) (PPV)
[Ia.
200
300
400
Wavelength
500
600
(nm)
Fig. 4. EL spectra of oligophenylene devices with PPPY, PPPK, PHP, and absorbance spectra of polycrystalline PHP(25 “C) and highly ordered PHP(17O”C).
allows us to obtain the desired colour, or spectral distribution of the emitted light (Fig. 3). In Fig. 4 we depict the EL spectra of our PHP, PPPK and PPPY devices under forward driving conditions with a current of about 0.2 mA. At elevated substrate temperatures the oligomer molecules grow with a high degree of order, arranging their long axis nearly perpendicular to the surface plane. This order dominates the optical absorbance spectra (Fig. 4) [ 151 but has little influence on the shape of the PL and EL emission spectra. The electrical characterization reveals a good diode behaviour for our homolayer and multicompound layer devices with high rectification ratios of about lo4 at maximum. In Fig. 5 we show two typical Z-V curves for our devices with the same layer thickness of about 2000 A. The onset field for the homo device (a) is around 1.106 V/cm and increases
F. Meghdadi
et al. /Synthetic
0.25
0.15
0;
0.05
76 (1996)
113-115
115
pure PHP, and Wilhelm Graupner for helpful discussions. This research work is supported by the Jubiltiumsfonds der ijsterreichischen Nationalbank (Project No. 5 165).
0.2 a z. 2 E
Metals
0.1
References
0
-0.054 40
-30
-20
-10
0
IO
20
30
40
Voltage (v)
Fig. 5. I-V characteristics of two EL devices: (a) ITO/OPPV/Al ITO/ (OPPV-PPPY compound layer) /Al.
and (b)
slightly for the compound layer with about the same thickness, which we attribute to ordering effects within the multicompound layer. The brightness of the homo and multicompound EL devices is comparable to that of devices made from PHP- and PPP-type ladder polymers [ 61, which have external EL quantum efficiencies between 0.05 and 0.1%.
[l] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, A.P.L. Bum and A.B. Holmes, Nature, 374 (1990) 539. [2] D. Braun and A.J. Heeger, Appl. Phys. Lett., 58 (1991) 1982. [3] G. Leising, G. Kbpping-Gvem. F. Meghdadi, A. Niko, S. Tasch, W. Fischer, L. Piu, M.W. Wagner, R.h. Grubbs, L. Athouel, G.Froyer, U. Scherf and J. Huber, Proc. SPIE, 1910 (1993). [4] H.S. Woo, J.G. Lee, H.K. Min, E.J. Oh, S.I. Park, K.W. Lee, I.H. Lee, S.H. Cho, T.W. Kim and C.H. Park, Synth. Met., 69-71 (1994) 2173. [ 51 G. Grem, G. Leditzky, B. Ullrichand G. Leising,Adv. Mater., 4 (1992) 36.
G. Grem, V. Martin, F. Meghdadi, C. Paar, J. Stampfl, J. Sturm, S. Tasch and G. Leising, Synth. Met., 69-71 (1995) 2193. [7] W. Graupner, G. Grem, F. Meghdadi, C. Paar, G. Leising, W. Fischer, F. Stelzer, U. Scherfand K. Miillen, Mol. Crysf.Liq. Cryst.,256 ( 1994) [6]
54.
4. Conclusions Homolayer and multicompound layers based on oligophenylenevinylene and oligophenylene have been fabricated by molecular beam deposition and applied as the active layers for EL devices. By controlling the composition of the oligomer layers, we are able to obtain EL emission colours in the whole visible range. Acknowledgements The authors would like to thank G. Froyer and L. Athouel, Institut de MatBriaux de Nantes, for the synthesis of highly
[8] T. Kanbara, C. Mori, H. Wakayama, T. Fukuda, Z.H. Zhou, T. Maruyama, K. Osakada and T. Yamamoto, Solid State Commun., 83 (1992) 771. [9] K. Miyashita and M. Kaneko, Macromol. Chern., Rapid Cormnun., 15 (1994) 511. [IO] G. Froyer, Y. PIOUS,E. Dall’arche, C. Chevrot and A. Sivoe, Fr. Patent No. 90 08091 ( 1990). [ 1l] P. Kovacic and R.M. Lange, J. Org. Chem., 29 (1964) 2416. [ 121 P. Kovacic and A. Kyryakis, J. Am. Chem. SOL, 85 (1963) 454. [ 131 T. Yamamoto and A. Yamamoto, Chern. I.&t., (1977) 353. [ 141 M. Krakovyak, T. Ananieva, E. Anufrieva, T. Nekrasova and G. Sinitsina, Makromol. Chem., 186 (1986) 1075. [ 151 A. Niko, F. Meghdadi, C. Ambrosch-Draxl, P. Vogl and G. Leising, Synth. Met., 76 (1996) 177. [ 161 D.D.C. Bradley, R.H. Friend, KS. Wong, W. Hayes, H. Lindenberger and S. Roth, Springer Series in SolidState Sciences, Vol. 76, Springer, Berlin, 1987, p. 107.
SyntheticMetals76 (1996) 113-115
Multicolour electroluminescence diodes using oligophenylene and oligophenylenevinylene multilayers F. Meghdadi a, G. Leising a,W. Fischer b, F. Stelzer b b Institutfiir
a Institutfiir Festkiirperphysik, Technische Universitdt Chemische Technologie Organischer Stoffe, Technische
Graz, Petersgasse 16, A-8010 Graz, Austria Universitiit Graz, Stremayrgasse 16, A-8010
Graz, Austria
Abstract We present electroluminescence (EL) characteristics for single-layer and multilayer thin films with oligophenylenevinylenes and oligophenylenes. Light-emitting diodes (LEDs) were fabricated by vacuum evaporation techniques of both the oligomer films and the negative electrodes. Depending on the substrate temperature, highly ordered or polycrystalline layers were obtained. Our LEDs emit blue light in the range of 425460 nm (oligophenylenes) and in a broad visible range of 400-600 nm (oligophenylene/oligophenylenevinylene multilayers). The current-voltage (I-V) characteristics of our devices show rectifying behaviour with high rectification ratios. We report I-Vcharacteristics and the respective threshold voltages, quantum efficiencies, photoluminescence (PL), EL and optical absorbance of the thin films and the devices. Keywords: Electroluminescence;Diodes; Oligophenylene;Oligophenylenevinylene
1. Introduction Since the first demonstration of electroluminescence (EL) devices using the conjugated polymer poly(para-phenylenevinylene) [ 1,2] considerable progress has been made in the field of polymer EL devices [ 31, Oligophenylenevinylene
(OPPV) [ 41, poly (para-phenylene) (PPP) and oligophenyls [ 5-91 with different energy gaps provide a range of different emission colours throughout the visible spectrum from blue to red and could therefore be used in multicolour devices and flat panel colour displays.
l-l
(4
oligo-PPP
(b)
oligo-PPV
Multi-organic
2. Experimental In this work we used oligophenylenevinylene (OPPV) , highly purified para-hexaphenyl (PHP) powder which has been synthesized either by electro-reduction of monobromoterphenyl in the presence of Ni catalyst [lo] or produced with improved yield by a simple, one-step procedure starting with biphenyl and using aluminium chloridexupric
chloride as a catalyst [ 111. The poly(para-phenylene) (PPP) obtained via the Kovacic (PPPK) [ 121 and Yamamoto (PPPY) routes of synthesis [ 131 and oligophenylenevinylene [ 141 are used as the source material for evaporation. Fig. 1 depicts their molecular structures and the layer 0379-6779/96/S15.000 1996Elsevier ScienceS.A. All rights reserved
Layer
cc>
Fig. 1. Molecularstructures of oligopolyphenylene(a) andoligophenylenevinylene(b) , anda schemeof theEL device(c) .
arrangement in the device. In order to obtain either multicompound or multilayer thin films of oligophenylene and oligophenylenevinylene we used a high vacuum molecular beam deposition technique (p < lo-’ Torr) with two quartz Knudsen cells as sources. Table 1 summarizes the preparation conditions for the different layers. To fabricate EL devices we used an indium-tin oxide (ITO)-coated glass substrate (BALTRACON, Balzers) and aluminium as hole- and electron-injecting electrodes, respectively.
114
F. Meghdadi et al. /Synthetic Metals 76 (1996) 113-115
Table 1 Preparation conditions for different layers: T,,, temperature of source 1; r,,, temperature of source 2; r,,, substrate temperature; d, thickness of oligomer layer; r, deposition rate Type of film
Ts, PC)
OPPVl OPPV2 OPPVIPPPY OPPVIPPPK OPPV/PHP PPPK PPPY PHP
210 450
i-s2
235 280
(“‘3
d (A)
335 270 270
210 285 280-500 265
115 115
1200 1500
25 25 25 245 245 25
2000 1200 1500 1500 2000
The UV-Vis absorption spectra were taken with a PerkinElmer Lambda-9 spectrophotometer. The photoluminescence (PL) spectra were recorded with a high resolution Jobin Yvon HR640 monochromator equipped with a cooled Hamamatsu R943-02 photomultiplier. The excitation light was provided by a Jobin Yvon double monochromator with a 1000 W xenon lamp as a source. The deposition rate and the thicknessof films (thickness varies typically between 100 and 200 nm) were monitored during the evaporation process with a quartz microbalance. The Z-V characteristics of EL diodes were recorded by a Keithley I-Vsystem consisting of voltage and current sources, voltmeters and an electrometer.
1000
Emission colour
r (A/s)
green green green/blue blue blue/green green blue
0.11 0.07 0.18 0.11 1.25 1.25 0.01 0.18
3 400
450
500 Wavelength
550
600
(nm)
Fig. 3. PL spectra of multicompound layers: (a) OPPV/PPPY (A,,, 330 nm); (b) OPPV/PPPK (A,,, 330 nm); (c) OPPV/PHP (A,,390 nm)
3. Results and discussion Fig. 2 shows the room-temperature PL and absorption spectra of OPPV thin films produced at two different source temperatures (210 and 450 “C) and identical substrate temperature (115 “C). The difference between the absorption spectra in the range of 4.1-3.1 eV (300-400 nm) can be attributed to ordering effects of the evaporated molecules on the surface, which have been investigated thoroughly in the case of PHP [ 151, The compound layer consisting of oligophenylenes and oligophenylenevinylenes (with the composition varying from 0.1 to 0.5 molar ratio of OPPV/OPPP)
300
400 Wavelength
500 (nm)
600
Fig. 2. PL (A,,, 330 nm) and absorbance spectra of OPPVl and OPPV2 produced at two different source temperatures (210 and 450 “C); for comparison we show the PL spectrum of poly(para-phenylenevinylene) (PPV)
[Ia.
200
300
400
Wavelength
500
600
(nm)
Fig. 4. EL spectra of oligophenylene devices with PPPY, PPPK, PHP, and absorbance spectra of polycrystalline PHP(25 “C) and highly ordered PHP(17O”C).
allows us to obtain the desired colour, or spectral distribution of the emitted light (Fig. 3). In Fig. 4 we depict the EL spectra of our PHP, PPPK and PPPY devices under forward driving conditions with a current of about 0.2 mA. At elevated substrate temperatures the oligomer molecules grow with a high degree of order, arranging their long axis nearly perpendicular to the surface plane. This order dominates the optical absorbance spectra (Fig. 4) [ 151 but has little influence on the shape of the PL and EL emission spectra. The electrical characterization reveals a good diode behaviour for our homolayer and multicompound layer devices with high rectification ratios of about lo4 at maximum. In Fig. 5 we show two typical Z-V curves for our devices with the same layer thickness of about 2000 A. The onset field for the homo device (a) is around 1.106 V/cm and increases
F. Meghdadi
et al. /Synthetic
0.25
0.15
0;
0.05
76 (1996)
113-115
115
pure PHP, and Wilhelm Graupner for helpful discussions. This research work is supported by the Jubiltiumsfonds der ijsterreichischen Nationalbank (Project No. 5 165).
0.2 a z. 2 E
Metals
0.1
References
0
-0.054 40
-30
-20
-10
0
IO
20
30
40
Voltage (v)
Fig. 5. I-V characteristics of two EL devices: (a) ITO/OPPV/Al ITO/ (OPPV-PPPY compound layer) /Al.
and (b)
slightly for the compound layer with about the same thickness, which we attribute to ordering effects within the multicompound layer. The brightness of the homo and multicompound EL devices is comparable to that of devices made from PHP- and PPP-type ladder polymers [ 61, which have external EL quantum efficiencies between 0.05 and 0.1%.
[l] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, A.P.L. Bum and A.B. Holmes, Nature, 374 (1990) 539. [2] D. Braun and A.J. Heeger, Appl. Phys. Lett., 58 (1991) 1982. [3] G. Leising, G. Kbpping-Gvem. F. Meghdadi, A. Niko, S. Tasch, W. Fischer, L. Piu, M.W. Wagner, R.h. Grubbs, L. Athouel, G.Froyer, U. Scherf and J. Huber, Proc. SPIE, 1910 (1993). [4] H.S. Woo, J.G. Lee, H.K. Min, E.J. Oh, S.I. Park, K.W. Lee, I.H. Lee, S.H. Cho, T.W. Kim and C.H. Park, Synth. Met., 69-71 (1994) 2173. [ 51 G. Grem, G. Leditzky, B. Ullrichand G. Leising,Adv. Mater., 4 (1992) 36.
G. Grem, V. Martin, F. Meghdadi, C. Paar, J. Stampfl, J. Sturm, S. Tasch and G. Leising, Synth. Met., 69-71 (1995) 2193. [7] W. Graupner, G. Grem, F. Meghdadi, C. Paar, G. Leising, W. Fischer, F. Stelzer, U. Scherfand K. Miillen, Mol. Crysf.Liq. Cryst.,256 ( 1994) [6]
54.
4. Conclusions Homolayer and multicompound layers based on oligophenylenevinylene and oligophenylene have been fabricated by molecular beam deposition and applied as the active layers for EL devices. By controlling the composition of the oligomer layers, we are able to obtain EL emission colours in the whole visible range. Acknowledgements The authors would like to thank G. Froyer and L. Athouel, Institut de MatBriaux de Nantes, for the synthesis of highly
[8] T. Kanbara, C. Mori, H. Wakayama, T. Fukuda, Z.H. Zhou, T. Maruyama, K. Osakada and T. Yamamoto, Solid State Commun., 83 (1992) 771. [9] K. Miyashita and M. Kaneko, Macromol. Chern., Rapid Cormnun., 15 (1994) 511. [IO] G. Froyer, Y. PIOUS,E. Dall’arche, C. Chevrot and A. Sivoe, Fr. Patent No. 90 08091 ( 1990). [ 1l] P. Kovacic and R.M. Lange, J. Org. Chem., 29 (1964) 2416. [ 121 P. Kovacic and A. Kyryakis, J. Am. Chem. SOL, 85 (1963) 454. [ 131 T. Yamamoto and A. Yamamoto, Chern. I.&t., (1977) 353. [ 141 M. Krakovyak, T. Ananieva, E. Anufrieva, T. Nekrasova and G. Sinitsina, Makromol. Chem., 186 (1986) 1075. [ 151 A. Niko, F. Meghdadi, C. Ambrosch-Draxl, P. Vogl and G. Leising, Synth. Met., 76 (1996) 177. [ 161 D.D.C. Bradley, R.H. Friend, KS. Wong, W. Hayes, H. Lindenberger and S. Roth, Springer Series in SolidState Sciences, Vol. 76, Springer, Berlin, 1987, p. 107.