- Email: [email protected]
Electroluminescent properties of a derivative of quinquepyridine
3 August 2001
Chemical Physics Letters 343 (2001) 201±204
www.elsevier.com/locate/cplett
Electroluminescent properties of a derivative of quinquepyridine Y.J. Fu a,*, T.K.S. Wong a, G.M. Wang b, X. Hu b, H.X. Zhang a, Z.S. Gao c, M.H. Jiang c a
b
Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Division of Materials Technology, School of Materials Engineering, Nanyang Technological University, Singapore 639798, Singapore c State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Received 23 November 2000
Abstract 0000
0000
Electroluminescent (EL) properties of 6,6 -dimethyl-40 ,4000 -diphenyl-2,20 :60 ; 200 : 600 ; 2000 :6000 ; 2 -quinquepyridine (DMDPQPY) have been examined. The EL results of typical single layer devices indicate that emitting layer thickness can in¯uence the emission wavelength. The introduction of a hole transporting layer in the double layer device enhances the luminance greatly, showing a better charge balance. It is proposed that 2,6-linked poly(oligo) pyridine derivatives are promising to form a new class of EL material both as emitter and as carrier transporter. Ó 2001 Elsevier Science B.V. All rights reserved.
Since the ®rst highly ecient organic electroluminescent devices (OELD) were reported, [1,2] much attention has been paid to the study of organic electroluminescence (EL) because of its potential application in ¯at panel display and lighting. Pyridine-based conjugated polymers, i.e., 2,5-linked poly(parapyridine) (PPy), poly(pyridylvinylene) (PPyV) and 5,50 -linked poly(2,20 -bipyridine) (PBPy) have recently been arousing much interests in this ®eld [3±7]. They are known to have similar rigid conformations to their phenyl-based conjugated polymer counterparts, such as, polyphenylenevinylene (PPV) emitting green [8], and poly(para-phenylene) (PPP) emitting blue light [9].
*
Corresponding author. Fax: +65-7904161. E-mail address: [email protected] (Y.J. Fu).
It has been suggested that the introduction of the heteroatom nitrogen gives several advantages [3]: (1) high electron anity properties encouraging the possibility to use high work function metals as an electrode; (2) good solubility properties making device fabrication more convenient and (3) resistance to oxidation. On the other hand, 2,6-linked oligo (poly) pyridine derivatives have been a focus for more than 10 years in the ®eld of supramolecular chemistry [10,11]. They have a zig-zag instead of linear and rigid conformation in the metal-free state. Compared with PPy, they are ¯exible and are better chelating ligands for metal ions. Many of their complexes, such as Ru tpy2 [12], etc., have proved to be very good luminescent materials. Nevertheless, the metal-free ligands have never been studied as luminescent materials. Even the photoluminescent (PL) properties of
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 6 7 6 - 5
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Y.J. Fu et al. / Chemical Physics Letters 343 (2001) 201±204
2,6-linked oligopyridines are rarely reported. Only PL properties of the simplest ones, 2,6-substituted pyridine, 2,20 -bipyridine and 2; 20 :60 ; 200 -terpyridine, have been investigated in dierent solution conditions [13±15]. In this Letter, we report the EL properties of a 0000 novel 2,6-linked oligopyridine derivative, 6,6 0000 0000 dimethyl- 40 ,4 -diphenyl- 2, 20 :60 , 200 :600 , 2000 :6000 , 2 quinquepyridine (DMDPQPY). DMDPQPY was prepared as previously reported [11], with the structure shown in the inset of Fig. 1. DMDPQPY ®lm (100 nm thickness) on quartz substrate for PL and absorption spectral measurements was prepared by the method of vacuum evaporation. The absorption spectra at room temperature were recorded with a Shimadzu absorption spectrophotometer (model UV2101PC), and the PL spectra were obtained with a SPEX Fluorolog ¯uorometer at room temperature. As shown in Fig. 1, the emission spectrum, which shows two peaks at 365 and 553 nm, respectively, was obtained under the excitation of 344 nm light. In the measurement of the excitation spectra when two dierent detecting emission wavelengths (365 and 553 nm) were used, we got two excitation spectra, both of which are of the same pattern with a maximum at 344 nm. From Fig. 1, it can also be seen that the maximum position of the excitation spectrum coincides with the shoulder in the absorption spectrum. The absorption edge falls at about 354 nm, indicating a larger energy gap than that of PPy [3]. Compared
Fig. 1. Photoluminescence and absorption spectra of DMDPQPY ®lm (approximate thickness: 100 nm) evaporated on a quartz substrate.
with bipy and terpy, the intense ¯uorescence from DMDPQPY is signi®cant. The factors that make DMDPQPY dierent from simpler oligopyridines are: (1) Extended conjugation. In DMDPQPY the conjugated system is extended by the 2,6 linked pyridyl ring as well as the two phenyl groups linked at position 40 and 4000 . While n ! p is lower than p ! p in energy in bipy and terpy [13±15], the extended conjugation favors the lowering of p ! p energy more than n ! p energy, and as a result a reversed sequence of the energy level is obtained, i.e., the p band dispersion in DMDPQPY makes the singlet p ! p state lower in energy than the singlet n ! p state. The strong luminance of PPy provides another evidence for this point [6]; (2) Substituent eects. It was previously reported that the existence of methyl substituents in the Me2 bipy is essential to its ¯uorescence [15]. In DMDPQPY, the two methyl 0000 groups at 6,6 positions have the same eects. In addition, as was previously illustrated in the formation of a helical complex [11], the extended conjugation caused by the two phenyl groups gives a more rigid structure for DMDPQPY than that 0000 0000 of 2,6 -dimethyl-2,20 :60 ; 200 :600 ; 2000 : 6000 :2 -quinquepyridine (DMQPY) with no phenyl substituents. A rigid structure has been recognized to be conducive to luminescence. Single layer EL devices based on DMDPQPY were fabricated through vacuum evaporation using ITO-coated glass (60 X=square) as the anode and evaporated Al metal as the cathode. EL spectra were measured on a Perkin±Elmer MPF 44A Spectrometer. In the EL spectrum (Fig. 2) of the ®rst device ITO/DMDPQPY(40 nm)/Al (Device 1), a broad emission peak around 492 nm can be observed with a shoulder at about 365 nm. Compared with the PL emission of DMDPQPY at 553 nm, the visible EL emission has a large blue shift of about 61 nm. The EL spectra of the second device (Fig. 2) with a thicker emitting layer, ITO/ DMDPQPY(60 nm)/Al (Device 2), show a visible emission at 538 nm, a 15 nm blue shift from the PL emission, and another at 365 nm. So, in both EL spectra, the UV emission remains at the same position as in the PL spectrum, but the emission in the visible range is blue shifted to a dierent degree. A similar blue shift was previously reported
203
Brightness (a.u.)
Y.J. Fu et al. / Chemical Physics Letters 343 (2001) 201±204
Fig. 2. EL spectra of single layer devices ITO/DMDPQPY(40 nm)/Al (Device 1) and ITO/DMDPQPY(60 nm)/Al (Device 2).
for the EL and PL of PPy, [3,16] in which case the PL emission at 550 nm shifts in the blue direction to 497 nm in the EL spectrum. The reason has not been well understood so far, although morphology was reported to have some eects on the emission position in the PL spectrum [7]. From the above EL results, the emission wavelength in the EL process of the single layer devices changes with the thickness of the organic layer. We have also fabricated devices with emitting layers thicker than 60 nm and have found that they show similar EL spectra to Device 2, i.e., the visible light wavelength remains at 538 nm. The low signal±to±noise ratios in Fig. 2 were caused by the low eciency. Fig. 3 shows the current±voltage and brightness±voltage characteristics of the two devices. Device 1 has a turn-on voltage of about 4 V for both current and luminance, but it seems that a terrace exists in the range of 4±6 V before an intense increase occurs. For Device 2, the turn-on voltage is about 8 V, after which both current and brightness increase steadily. So, with an increase in the layer thickness the turn-on voltage increases and the brightness decreases. The low brightness of the single layer devices indicates that though DMDPQPY can be ecient as an electron transporting material given the large electron anity introduced by the heteroatom nitrogen, it is poor as a hole transporter. Carrier recombination can occur only at the interface of DMDPQPY/ITO, and a thicker emitting layer does not cause much
Fig. 3. Current±voltage (open circles) and brightness±voltage (solid circles) characteristics of Device 1 (dash line) and Device 2 (solid line).
increase in the recombination probability between holes and electrons, but it does cause higher resistance and higher turn-on voltage as a result. In order to improve the EL eciency of the material, double layer devices were fabricated using polyvinylcarbazole (PVK) as hole transporting layer. In the fabrication of the double layer devices, the PVK layer was ®rst spin-coated from its CHCl3 solution (10 mg/ml) onto the ITO-coated glass substrates before the evaporation of DMDPQPY. During the EL spectra measurement, green coloured light can clearly be observed in a dimly lit room. As shown in the inset of Fig. 4, a turn-on voltage of about 14 V is obtained for both current± voltage and brightness±voltage relations. From the EL spectra, two distinctive peaks can be recognized, one at 431 nm and the other at 538 nm. The latter is obviously from DMDPQPY considering that it coincides well with the emission of the single layer device in the visible range. But the former should be from PVK instead of DMDPQPY. The disappearance of the UV emission from DMDPQPY is caused perhaps by the absorption of PVK at 365 nm, which falls on the tail of the absorption spectrum of PVK. The blue emission at 431 nm is somewhat longer in wavelength than the usual PVK emission (420 nm). This is perhaps caused by the interaction of DMDPQPY and PVK at the interface of the two layers to form some kind of exciplex [5] in the recombination process.
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Y.J. Fu et al. / Chemical Physics Letters 343 (2001) 201±204
double layer device, the emission in the UV range disappears and a blue emission from PVK appears instead together with a green emission. The better charge balance makes the EL bright enough to be observed with the naked eye.
References
Fig. 4. EL Spectrum of a typical double layer device, ITO/ PVK(40 nm)/DMDPQPY (60 nm)/Al. Inset: Current±voltage (open circles) and brightness±voltage (solid circles) characteristics of the double layer device.
From the luminance data shown in Figs. 2 and 4 measured under the same conditions, the brightness increased by more than 20 times with the introduction of the hole transporting layer. The large increase in the emission intensity of the double layer device compared with that of the single layer devices suggests a better charge balance. With a typical sample, maximum emission energy was measured as 10 cd=m2 at 24 V and a current density of 27:3 mA=cm2 . The device quantum eciency is about 10 4 photons/electron. The lifetime was measured at half of the maximum 2 luminance at 20 V as 25 h from 5 to 2:5 cd=m . At higher voltages, the luminance decreases more quickly, e.g., at 24 V, lifetime is only a few minutes. In summary, we have reported here the ®rst example of EL devices based on a derivative of 2,6-linked oligopyridines. Two emission peaks, one at 365 nm and another in the visible range, can be obtained from the single layer devices. In the
[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] D.D. Gebler, Y.Z. Wang, J.W. Blatchford, S.W. Jessen, L.B. Lin, T.L. Gustafson, H.L. Wang, T.M. Swager, A.G. MacDiarmid, A.J. Epstein, J. Appl. Phys. 78 (1995) 4264. [4] Y.Z. Wang, A.J. Epstein, Acc. Chem. Res. 32 (1999) 217. [5] D.D. Gebler, Y.Z. Wang, D.-K. Fu, T.M. Swager, A.J. Epstein, J. Chem. Phys. 108 (1998) 7842. [6] Y.Z. Wang, D.D. Gebler, D.J. Spry, D.K. Fu, T.M. Swager, A.G. MacDiarmid, A.J. Epstein, IEEE Trans. Electron Devices 44 (1997) 1263. [7] J.W. Blatchford, S.W. Jessen, L.-B. Lin, T.L. Gustafson, D.K. Fu, H.-L. Wang, T.M. Swager, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 54 (1996) 9180. [8] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [9] G. Grem, G. Leditzky, B. Ullrich, G. Leising, Synth. Met. 51 (1992) 383. [10] C. Piguet, G. Bernardinelli, G. Hopfgartner, Chem. Rev. 97 (1997) 2005. [11] Y.J. Fu, J. Sun, Q. Li, W.N. Dai, Z.Y. Zhou, T.C.W. Mak, W.X. Tang, H.W. Hu, J. Chem. Soc.: Dalton Trans. 11 (1996) 2309. [12] W.Y. Ng, X. Gong, W.K. Chan, Chem. Mater. 11 (1999) 1165. [13] A. Sarkar, S. Chakravorti, J. Lumin. 63 (1995) 143. [14] A. Sarkar, S. Chakravorti, J. Lumin. 65 (1995) 163. [15] M. Yagi, T. Kaneshima, Y. Wada, K. Takemura, Y. Yokoyama, J. Photochem. Photobiol. A: Chem. 84 (1994) 27. [16] T. Yamamoto, T. Maruyama, Z. Zhou, T. Ito, T. Fukuda, Y. Yoneda, F. Begum, T. Ikeda, S. Sasaki, H. Takezoe, A. Fukuda, K. Kubota, J. Am. Chem. Soc. 116 (1994) 4832.
Chemical Physics Letters 343 (2001) 201±204
www.elsevier.com/locate/cplett
Electroluminescent properties of a derivative of quinquepyridine Y.J. Fu a,*, T.K.S. Wong a, G.M. Wang b, X. Hu b, H.X. Zhang a, Z.S. Gao c, M.H. Jiang c a
b
Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Division of Materials Technology, School of Materials Engineering, Nanyang Technological University, Singapore 639798, Singapore c State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Received 23 November 2000
Abstract 0000
0000
Electroluminescent (EL) properties of 6,6 -dimethyl-40 ,4000 -diphenyl-2,20 :60 ; 200 : 600 ; 2000 :6000 ; 2 -quinquepyridine (DMDPQPY) have been examined. The EL results of typical single layer devices indicate that emitting layer thickness can in¯uence the emission wavelength. The introduction of a hole transporting layer in the double layer device enhances the luminance greatly, showing a better charge balance. It is proposed that 2,6-linked poly(oligo) pyridine derivatives are promising to form a new class of EL material both as emitter and as carrier transporter. Ó 2001 Elsevier Science B.V. All rights reserved.
Since the ®rst highly ecient organic electroluminescent devices (OELD) were reported, [1,2] much attention has been paid to the study of organic electroluminescence (EL) because of its potential application in ¯at panel display and lighting. Pyridine-based conjugated polymers, i.e., 2,5-linked poly(parapyridine) (PPy), poly(pyridylvinylene) (PPyV) and 5,50 -linked poly(2,20 -bipyridine) (PBPy) have recently been arousing much interests in this ®eld [3±7]. They are known to have similar rigid conformations to their phenyl-based conjugated polymer counterparts, such as, polyphenylenevinylene (PPV) emitting green [8], and poly(para-phenylene) (PPP) emitting blue light [9].
*
Corresponding author. Fax: +65-7904161. E-mail address: [email protected] (Y.J. Fu).
It has been suggested that the introduction of the heteroatom nitrogen gives several advantages [3]: (1) high electron anity properties encouraging the possibility to use high work function metals as an electrode; (2) good solubility properties making device fabrication more convenient and (3) resistance to oxidation. On the other hand, 2,6-linked oligo (poly) pyridine derivatives have been a focus for more than 10 years in the ®eld of supramolecular chemistry [10,11]. They have a zig-zag instead of linear and rigid conformation in the metal-free state. Compared with PPy, they are ¯exible and are better chelating ligands for metal ions. Many of their complexes, such as Ru tpy2 [12], etc., have proved to be very good luminescent materials. Nevertheless, the metal-free ligands have never been studied as luminescent materials. Even the photoluminescent (PL) properties of
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 6 7 6 - 5
202
Y.J. Fu et al. / Chemical Physics Letters 343 (2001) 201±204
2,6-linked oligopyridines are rarely reported. Only PL properties of the simplest ones, 2,6-substituted pyridine, 2,20 -bipyridine and 2; 20 :60 ; 200 -terpyridine, have been investigated in dierent solution conditions [13±15]. In this Letter, we report the EL properties of a 0000 novel 2,6-linked oligopyridine derivative, 6,6 0000 0000 dimethyl- 40 ,4 -diphenyl- 2, 20 :60 , 200 :600 , 2000 :6000 , 2 quinquepyridine (DMDPQPY). DMDPQPY was prepared as previously reported [11], with the structure shown in the inset of Fig. 1. DMDPQPY ®lm (100 nm thickness) on quartz substrate for PL and absorption spectral measurements was prepared by the method of vacuum evaporation. The absorption spectra at room temperature were recorded with a Shimadzu absorption spectrophotometer (model UV2101PC), and the PL spectra were obtained with a SPEX Fluorolog ¯uorometer at room temperature. As shown in Fig. 1, the emission spectrum, which shows two peaks at 365 and 553 nm, respectively, was obtained under the excitation of 344 nm light. In the measurement of the excitation spectra when two dierent detecting emission wavelengths (365 and 553 nm) were used, we got two excitation spectra, both of which are of the same pattern with a maximum at 344 nm. From Fig. 1, it can also be seen that the maximum position of the excitation spectrum coincides with the shoulder in the absorption spectrum. The absorption edge falls at about 354 nm, indicating a larger energy gap than that of PPy [3]. Compared
Fig. 1. Photoluminescence and absorption spectra of DMDPQPY ®lm (approximate thickness: 100 nm) evaporated on a quartz substrate.
with bipy and terpy, the intense ¯uorescence from DMDPQPY is signi®cant. The factors that make DMDPQPY dierent from simpler oligopyridines are: (1) Extended conjugation. In DMDPQPY the conjugated system is extended by the 2,6 linked pyridyl ring as well as the two phenyl groups linked at position 40 and 4000 . While n ! p is lower than p ! p in energy in bipy and terpy [13±15], the extended conjugation favors the lowering of p ! p energy more than n ! p energy, and as a result a reversed sequence of the energy level is obtained, i.e., the p band dispersion in DMDPQPY makes the singlet p ! p state lower in energy than the singlet n ! p state. The strong luminance of PPy provides another evidence for this point [6]; (2) Substituent eects. It was previously reported that the existence of methyl substituents in the Me2 bipy is essential to its ¯uorescence [15]. In DMDPQPY, the two methyl 0000 groups at 6,6 positions have the same eects. In addition, as was previously illustrated in the formation of a helical complex [11], the extended conjugation caused by the two phenyl groups gives a more rigid structure for DMDPQPY than that 0000 0000 of 2,6 -dimethyl-2,20 :60 ; 200 :600 ; 2000 : 6000 :2 -quinquepyridine (DMQPY) with no phenyl substituents. A rigid structure has been recognized to be conducive to luminescence. Single layer EL devices based on DMDPQPY were fabricated through vacuum evaporation using ITO-coated glass (60 X=square) as the anode and evaporated Al metal as the cathode. EL spectra were measured on a Perkin±Elmer MPF 44A Spectrometer. In the EL spectrum (Fig. 2) of the ®rst device ITO/DMDPQPY(40 nm)/Al (Device 1), a broad emission peak around 492 nm can be observed with a shoulder at about 365 nm. Compared with the PL emission of DMDPQPY at 553 nm, the visible EL emission has a large blue shift of about 61 nm. The EL spectra of the second device (Fig. 2) with a thicker emitting layer, ITO/ DMDPQPY(60 nm)/Al (Device 2), show a visible emission at 538 nm, a 15 nm blue shift from the PL emission, and another at 365 nm. So, in both EL spectra, the UV emission remains at the same position as in the PL spectrum, but the emission in the visible range is blue shifted to a dierent degree. A similar blue shift was previously reported
203
Brightness (a.u.)
Y.J. Fu et al. / Chemical Physics Letters 343 (2001) 201±204
Fig. 2. EL spectra of single layer devices ITO/DMDPQPY(40 nm)/Al (Device 1) and ITO/DMDPQPY(60 nm)/Al (Device 2).
for the EL and PL of PPy, [3,16] in which case the PL emission at 550 nm shifts in the blue direction to 497 nm in the EL spectrum. The reason has not been well understood so far, although morphology was reported to have some eects on the emission position in the PL spectrum [7]. From the above EL results, the emission wavelength in the EL process of the single layer devices changes with the thickness of the organic layer. We have also fabricated devices with emitting layers thicker than 60 nm and have found that they show similar EL spectra to Device 2, i.e., the visible light wavelength remains at 538 nm. The low signal±to±noise ratios in Fig. 2 were caused by the low eciency. Fig. 3 shows the current±voltage and brightness±voltage characteristics of the two devices. Device 1 has a turn-on voltage of about 4 V for both current and luminance, but it seems that a terrace exists in the range of 4±6 V before an intense increase occurs. For Device 2, the turn-on voltage is about 8 V, after which both current and brightness increase steadily. So, with an increase in the layer thickness the turn-on voltage increases and the brightness decreases. The low brightness of the single layer devices indicates that though DMDPQPY can be ecient as an electron transporting material given the large electron anity introduced by the heteroatom nitrogen, it is poor as a hole transporter. Carrier recombination can occur only at the interface of DMDPQPY/ITO, and a thicker emitting layer does not cause much
Fig. 3. Current±voltage (open circles) and brightness±voltage (solid circles) characteristics of Device 1 (dash line) and Device 2 (solid line).
increase in the recombination probability between holes and electrons, but it does cause higher resistance and higher turn-on voltage as a result. In order to improve the EL eciency of the material, double layer devices were fabricated using polyvinylcarbazole (PVK) as hole transporting layer. In the fabrication of the double layer devices, the PVK layer was ®rst spin-coated from its CHCl3 solution (10 mg/ml) onto the ITO-coated glass substrates before the evaporation of DMDPQPY. During the EL spectra measurement, green coloured light can clearly be observed in a dimly lit room. As shown in the inset of Fig. 4, a turn-on voltage of about 14 V is obtained for both current± voltage and brightness±voltage relations. From the EL spectra, two distinctive peaks can be recognized, one at 431 nm and the other at 538 nm. The latter is obviously from DMDPQPY considering that it coincides well with the emission of the single layer device in the visible range. But the former should be from PVK instead of DMDPQPY. The disappearance of the UV emission from DMDPQPY is caused perhaps by the absorption of PVK at 365 nm, which falls on the tail of the absorption spectrum of PVK. The blue emission at 431 nm is somewhat longer in wavelength than the usual PVK emission (420 nm). This is perhaps caused by the interaction of DMDPQPY and PVK at the interface of the two layers to form some kind of exciplex [5] in the recombination process.
204
Y.J. Fu et al. / Chemical Physics Letters 343 (2001) 201±204
double layer device, the emission in the UV range disappears and a blue emission from PVK appears instead together with a green emission. The better charge balance makes the EL bright enough to be observed with the naked eye.
References
Fig. 4. EL Spectrum of a typical double layer device, ITO/ PVK(40 nm)/DMDPQPY (60 nm)/Al. Inset: Current±voltage (open circles) and brightness±voltage (solid circles) characteristics of the double layer device.
From the luminance data shown in Figs. 2 and 4 measured under the same conditions, the brightness increased by more than 20 times with the introduction of the hole transporting layer. The large increase in the emission intensity of the double layer device compared with that of the single layer devices suggests a better charge balance. With a typical sample, maximum emission energy was measured as 10 cd=m2 at 24 V and a current density of 27:3 mA=cm2 . The device quantum eciency is about 10 4 photons/electron. The lifetime was measured at half of the maximum 2 luminance at 20 V as 25 h from 5 to 2:5 cd=m . At higher voltages, the luminance decreases more quickly, e.g., at 24 V, lifetime is only a few minutes. In summary, we have reported here the ®rst example of EL devices based on a derivative of 2,6-linked oligopyridines. Two emission peaks, one at 365 nm and another in the visible range, can be obtained from the single layer devices. In the
[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] D.D. Gebler, Y.Z. Wang, J.W. Blatchford, S.W. Jessen, L.B. Lin, T.L. Gustafson, H.L. Wang, T.M. Swager, A.G. MacDiarmid, A.J. Epstein, J. Appl. Phys. 78 (1995) 4264. [4] Y.Z. Wang, A.J. Epstein, Acc. Chem. Res. 32 (1999) 217. [5] D.D. Gebler, Y.Z. Wang, D.-K. Fu, T.M. Swager, A.J. Epstein, J. Chem. Phys. 108 (1998) 7842. [6] Y.Z. Wang, D.D. Gebler, D.J. Spry, D.K. Fu, T.M. Swager, A.G. MacDiarmid, A.J. Epstein, IEEE Trans. Electron Devices 44 (1997) 1263. [7] J.W. Blatchford, S.W. Jessen, L.-B. Lin, T.L. Gustafson, D.K. Fu, H.-L. Wang, T.M. Swager, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B 54 (1996) 9180. [8] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [9] G. Grem, G. Leditzky, B. Ullrich, G. Leising, Synth. Met. 51 (1992) 383. [10] C. Piguet, G. Bernardinelli, G. Hopfgartner, Chem. Rev. 97 (1997) 2005. [11] Y.J. Fu, J. Sun, Q. Li, W.N. Dai, Z.Y. Zhou, T.C.W. Mak, W.X. Tang, H.W. Hu, J. Chem. Soc.: Dalton Trans. 11 (1996) 2309. [12] W.Y. Ng, X. Gong, W.K. Chan, Chem. Mater. 11 (1999) 1165. [13] A. Sarkar, S. Chakravorti, J. Lumin. 63 (1995) 143. [14] A. Sarkar, S. Chakravorti, J. Lumin. 65 (1995) 163. [15] M. Yagi, T. Kaneshima, Y. Wada, K. Takemura, Y. Yokoyama, J. Photochem. Photobiol. A: Chem. 84 (1994) 27. [16] T. Yamamoto, T. Maruyama, Z. Zhou, T. Ito, T. Fukuda, Y. Yoneda, F. Begum, T. Ikeda, S. Sasaki, H. Takezoe, A. Fukuda, K. Kubota, J. Am. Chem. Soc. 116 (1994) 4832.