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Characterization of organic electroluminescent devices introducing fluorine-containing polyimide to hole-transporting layer
Materials Science and Engineering B85 (2001) 195– 198 www.elsevier.com/locate/mseb
Characterization of organic electroluminescent devices introducing fluorine-containing polyimide to hole-transporting layer Chang-Sik Ha *, Jung-Ho Shin, Hyuntaek Lim, Won-Jei Cho Department of Polymer Science and Engineering, Pusan National Uni6ersity, Pusan 609 -735, South Korea Received 24 July 2000
Abstract In the present study, we introduced fluorine-containing polyimide as a matrix polymer to a hole-transporting layer (HTL). The devices consisting of two organic layers were fabricated: ITO/TPD-dispersed polyimide– Alq3– Al devices. The polyimide used in this work was poly(hexafluoropropane dianhydride-co-4,4’-oxydiphenylene) (6FDA-ODA PI). The device performances were affected by the composition of triphenylamine derivative (TPD) and polyimide in the HTL. On increasing the concentration of TPD in the HTL, the relative electroluminescence (EL) and power efficiency increased. When the weight ratio of TPD to 6FDA-ODA PI was 70:30, the TPD706FDA30 device exhibited the lowest turn-on voltage (ca. 10 V) and the best relative EL and power efficiency. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Organic electroluminescent device; Polyimide; Fluorine; Triphenylamine derivative; Hole-transporting layer
1. Introduction Since electroluminescent (EL) phenomenon in organic materials was discovered [1], there have been extensive research efforts on organic electroluminescent devices (OELDs) to apply for the flat panel display (FPD) [2–6]. So far, there have been a great number of studies for new emitting materials and device structures to achieve low driving voltage, high efficiency, full color emission, and good durability [7 – 10]. Thermal stability is one of the most important requirements in OELDs, because the joule-heat generated during the device may severely relax and damage organic materials with inherently lower thermal stability than inorganic materials or metals. Thermal stability is directly related to the device properties such as current density –voltage –EL intensity (J – V – L) characteristics, EL spectra, and lifetime. Kido et al. fabricated OELDs introducing a polymer into a hole-transporting layer (HTL) to bind a thermally unstable organic material, such as N,N%-diphenyl-N,N-di(m-tolyl)benzidine (TPD) [11,12]. Similarly, we have reported OELDs using some * Corresponding author. Fax: + 82-51-514-4331. E-mail address: [email protected] (C.-S. Ha).
polymer binders, such as poly(4,4%-oxydiphenylene pyromellitimide) (PMDA-ODA PI) [13], poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-co-HFP) [14], and thermally curable epoxy resin [15]. Especially, PMDA-ODA PI was used for giving a thermal stability to OELDs [13], and the properties of the OELDs using PVdF-co-HFP were affected by the strong electronegativity of fluorine in the HTL [14]. In this work, the fluorine-containing polyimide was introduced to a HTL for binding a small molecule, such as the triphenylamine derivative. The fluorine-containing polyimide was selected as a matrix polymer to a HTL because of both the thermal stability of polyimide and the effect of the strong electronegativity of fluorine on TPD in HTL. OELDs using TPD-dispersed poly(hexafluoropropane dianhydride-co-4,4’-oxydiphenylene) (6FDA-ODA PI) with different compositions were fabricated to investigate the effect of the composition of TPD and 6FDAODA PI on the device characteristics.
2. Experimental The devices used in this work consist of two organic layers, i.e. ITO-coated glass (150 nm thickness, 15 V/
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 6 1 0 - 9
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sheet resistance)– TPD-dispersed polyimide (50 nm)– Alq3 (60–70 nm) – Al devices. 6FDA-ODA PI was used as a matrix polymer for binding TPD. Insoluble 6FDAODA PI was converted via thermal imidization process from its soluble poly(amic acid) (PPA) precursor, 6FDA-ODA PAA. Solution blends of TPD and 6FDA-
Fig. 1. The PL spectra of the neat TPD and TPD-dispersed 6FDAODA PI films with different compositions.
ODA PAA in N-methyl-2-pyrrolidone (NMP) were prepared with different compositions. The weight ratios of TPD to 6FDA-ODA PAA were 30:70, 50:50, and 70:30. Overall solid concentration was 1 wt.%, and the mixture solutions were spin-coated onto an ITO-glass substrate at 3000 rpm for 2 min. TPD-dispersed 6FDAODA PAA thin film was soft-baked at 80°C for above 30 min and subsequently thermally imidized into 6FDA-ODA PI in an electrical furnace at 180°C for 1 h under nitrogen atmosphere. Then the EL material, tris(8-hydoxyquinolinato)aluminum (Alq3), and aluminum cathode were sequentially prepared by thermal evaporation in a vacuum of ca. 1×10 − 5 Torr. Special care was taken to monitor the thickness of TPD-dispersed polyimide films constant before and after thermal imidization of PI by atomic force microscopy (Nanoscope IIIa, Digital Instruments Co.). The current density–voltage–EL intensity (J–V–L) characteristics of the OELDs were measured using a photomultiplier tube (PMT) (Hamamatsu Photonics Co.) and an electrometer (Keithley 6517) controlled with a personal computer via analog-to-digital converter (ADC) and an IEEE488 GP-IB card. The photoluminescent (PL) and EL spectra were obtained by high-sensitivity fiber optic spectrometer (S2000, Ocean Optics Inc.) with 2048-element linear CCD-array silicon detector. All measurements were carried out under air ambient room temperature condition.
3. Results and discussion
Fig. 2. The dependence of: (a) current density and (b) EL intensity with the applied voltage of the OELDs.
The PL emission spectra of the neat TPD and TPDdispersed 6FDA-ODA PI films with different compositions are shown in Fig. 1. TPD306FDA70 denotes that the composition of TPD and 6FDA-ODA PI is 30 versus 70 by weight percentage. One can see that the emission peak of TPD-dispersed 6FDA-ODA PI is observed at longer wavelength than that of neat TPD. Also the PL peaks of the TPD-dispersed 6FDA-ODA PI films were slightly shifted to longer wavelength with increase in the concentration of TPD in the HTL. It can be considered that the slight red shift (bathochromic shift) of the PL is due to the increase of the vibration mode of TPD in the matrix polymer and the effect of the interaction between TPD and 6FDAODA PI by increasing the concentration of TPD. It is considered that the range of the red shift is not very large and thus marginally affects the optical property of the devices. The dependence of current density and EL intensity with the applied voltage of the OELDs is shown in Fig. 2. The shape of charge injection shows typical diode characteristics, meaning the rectification and recombination of holes and electrons injected from anode and cathode, respectively. The charge injection and turn-on
C.-S. Ha et al. / Materials Science and Engineering B85 (2001) 195–198
Fig. 3. (a) EL intensity and (b) relative power efficiency as a function of current density of the devices.
Fig. 4. The EL spectra of the OELDs. All devices were operated at 18 V.
voltage, corresponding to the voltage for the initiation of the charge injection, were reduced with increasing concentration of TPD. It is due to the increase of the hole-transporting ability by increasing the concentration of TPD in the HTL. It was found that, therefore,
197
the TPD706FDA30 device shows the lowest charge injection and turn-on voltage (ca. 10 V). Fig. 3 shows the EL intensity and the relative power efficiency as a function of the current density of the devices. In the plot of the EL intensity versus the current density of the device, the TPD706FDA30 device exhibited the highest slope, meaning the best efficiency of the devices. It should be noted that the slopes of the TPD506FDA50 and the TPD306FDA70 devices are almost identical. The result suggests that the efficiency is almost same, even though the TPD amount of the TPD306FDA70 device is smaller than that of the TPD506FDA50 device. It may be explained in terms of the effect by the strong electronegativity of fluorine of 6FDA-ODA PI in the HTL. It is suggested that the positively charged carbon in a carbon-fluoride dipole of 6FDA-ODA PI may encourage the electron-donating property of tertiary amine in TPD, leading to the activation of the hole-injecting and the hole-transporting ability of the HTL. A similar result on the effect of the electronegativity of a fluorine-containing polymer was reported in our previous study using PVdF-coHFP [14]. However, the result may be also caused by some other factors, such as the quality of organic films and the distribution of TPD in polyimide. The relative power efficiency (p× [EL intensity]/[W]) of the devices is very stable and gradually increased as the current density increased. One can see that the TPD706FDA30 device exhibits the highest relative power efficiency. Thus, it is thought that the hole injection and radiative recombination are more efficient in the TPD706FDA30 device than the others. It is interesting to note that the power efficiency of the TPD306FDA70 device is higher than that of the TPD506FDA50 device. It is also due to the effect by the electronegativity of fluorine of 6FDAODA PI in the HTL, which is similar to the result in the plot of the EL intensity versus the current density of the devices. From the results of the efficiency of the device, it is suggested that the hole-injecting and holetransporting ability of TPD is more dominant than the interaction between fluorine and tertiary amine at high concentration of TPD (TPD706FDA30 device). In the case of low concentration of TPD (TPD306FDA70 device), however, the efficiency of the device may be governed by the interaction between fluorine and tertiary amine as well as the film property such as film formation ability and uniformity. Fig. 4 shows the EL spectra of the devices. The maximum EL peaks of the devices at 18 V are identically positioned at ca. 520 nm, which correspond to those of a typical OELD using Alq3 as an emitting material. The full-width at half-maximum (FWHM) was about 100 nm, and the bright green light was clearly visible and stable under ordinary room light.
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C.-S. Ha et al. / Materials Science and Engineering B85 (2001) 195–198
4. Conclusions OELDs using TPD-dispersed 6FDA-ODA PI with different compositions were successfully fabricated to investigate the effect of the composition of TPD and 6FDA-ODA PI on the device properties, such as J –V– L characteristics, EL spectra, and EL efficiency. The charge injection and the turn-on voltage were reduced with increasing concentration of TPD, due to the increase of the hole-transporting ability by increasing concentration of TPD in the HTL. In the plot of the EL intensity versus the current density of the device, the TPD706FDA30 device exhibited the highest slope, meaning the best efficiency of the device. However, the slopes of the TPD506FDA50 and the TPD306FDA70 devices were almost identical, suggesting that the efficiency is almost same. Also the power efficiency of the TPD306FDA70 device was higher than that of the TPD506FDA50 devices. The results were explained in terms of the strong electronegativity of fluorine of 6FDA-ODA PI in HTL. It was suggested that the efficiency of the devices is explained in a different way depending on the concentration of TPD in the devices; i.e. the hole-injecting and hole-transporting ability of TPD is more dominant in the efficiency of the devices than the interaction between fluorine and tertiary amine at high concentration of TPD (TPD706FDA30 device), while in the case of low concentration of TPD (TPD306FDA70 device), the efficiency of the device is governed by the interaction between fluorine and tertiary amine as well as film properties such as film formation ability and uniformity. The TPD706FDA30 device exhibited the best relative power efficiency, as well as the lowest turn-on voltage (ca. 10 V). The
.
maximum EL peaks of the devices were identically positioned at ca. 520 nm, and the bright green light was clearly visible and stable under ordinary room light.
Acknowledgements The work was supported by the Brain Korea 21 project in 2001 and the Center of Integrated Molecular Systems, POSTECH, South Korea.
References [1] W. Helflich, W.G. Schneider, J. Chem. Phys. 44 (1966) 2902. [2] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 21. [3] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Macky, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [4] D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982. [5] C.H. Chen, J. Shi, C.W. Tang, Macromol. Symp. 125 (1997) 1. [6] T. Tsutsui, MRS Bull. June (1997) 39. [7] C.W. Tang, SID Int. Symp. Dig. Tech. Papers 27 (1996) 181. [8] Y. Hamada, Trans. Electron Devices 44 (1997) 1208. [9] T. Wakimoto, R. Murayama, K. Nagamiya, Y. Okuda, H. Nakada, T. Tohma, SID 96 Dig., 1996, p. 849. [10] G. Grem, G. Leditzky, B. Ullrich, G. Leising, Adv. Mater. 4 (1992) 32. [11] J. Kido, M. Kohda, Appl. Phys. Lett. 61 (1992) 761. [12] J. Kido, K. Hongawa, M. Kohda, Jpn. J. Appl. Phys. 31 (1992) L960. [13] H. Lim, H. Park, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, SPIE Proc. 3281 (1998) 345. [14] E.J. Kang, J.H. Keum, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, SPIE Proc. 3281 (1998) 366. [15] J.H. Keum, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, IUMRSICEM Fourth Int. Conf. on Electronic Materials, August 1998.
Characterization of organic electroluminescent devices introducing fluorine-containing polyimide to hole-transporting layer Chang-Sik Ha *, Jung-Ho Shin, Hyuntaek Lim, Won-Jei Cho Department of Polymer Science and Engineering, Pusan National Uni6ersity, Pusan 609 -735, South Korea Received 24 July 2000
Abstract In the present study, we introduced fluorine-containing polyimide as a matrix polymer to a hole-transporting layer (HTL). The devices consisting of two organic layers were fabricated: ITO/TPD-dispersed polyimide– Alq3– Al devices. The polyimide used in this work was poly(hexafluoropropane dianhydride-co-4,4’-oxydiphenylene) (6FDA-ODA PI). The device performances were affected by the composition of triphenylamine derivative (TPD) and polyimide in the HTL. On increasing the concentration of TPD in the HTL, the relative electroluminescence (EL) and power efficiency increased. When the weight ratio of TPD to 6FDA-ODA PI was 70:30, the TPD706FDA30 device exhibited the lowest turn-on voltage (ca. 10 V) and the best relative EL and power efficiency. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Organic electroluminescent device; Polyimide; Fluorine; Triphenylamine derivative; Hole-transporting layer
1. Introduction Since electroluminescent (EL) phenomenon in organic materials was discovered [1], there have been extensive research efforts on organic electroluminescent devices (OELDs) to apply for the flat panel display (FPD) [2–6]. So far, there have been a great number of studies for new emitting materials and device structures to achieve low driving voltage, high efficiency, full color emission, and good durability [7 – 10]. Thermal stability is one of the most important requirements in OELDs, because the joule-heat generated during the device may severely relax and damage organic materials with inherently lower thermal stability than inorganic materials or metals. Thermal stability is directly related to the device properties such as current density –voltage –EL intensity (J – V – L) characteristics, EL spectra, and lifetime. Kido et al. fabricated OELDs introducing a polymer into a hole-transporting layer (HTL) to bind a thermally unstable organic material, such as N,N%-diphenyl-N,N-di(m-tolyl)benzidine (TPD) [11,12]. Similarly, we have reported OELDs using some * Corresponding author. Fax: + 82-51-514-4331. E-mail address: [email protected] (C.-S. Ha).
polymer binders, such as poly(4,4%-oxydiphenylene pyromellitimide) (PMDA-ODA PI) [13], poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-co-HFP) [14], and thermally curable epoxy resin [15]. Especially, PMDA-ODA PI was used for giving a thermal stability to OELDs [13], and the properties of the OELDs using PVdF-co-HFP were affected by the strong electronegativity of fluorine in the HTL [14]. In this work, the fluorine-containing polyimide was introduced to a HTL for binding a small molecule, such as the triphenylamine derivative. The fluorine-containing polyimide was selected as a matrix polymer to a HTL because of both the thermal stability of polyimide and the effect of the strong electronegativity of fluorine on TPD in HTL. OELDs using TPD-dispersed poly(hexafluoropropane dianhydride-co-4,4’-oxydiphenylene) (6FDA-ODA PI) with different compositions were fabricated to investigate the effect of the composition of TPD and 6FDAODA PI on the device characteristics.
2. Experimental The devices used in this work consist of two organic layers, i.e. ITO-coated glass (150 nm thickness, 15 V/
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 6 1 0 - 9
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C.-S. Ha et al. / Materials Science and Engineering B85 (2001) 195–198
sheet resistance)– TPD-dispersed polyimide (50 nm)– Alq3 (60–70 nm) – Al devices. 6FDA-ODA PI was used as a matrix polymer for binding TPD. Insoluble 6FDAODA PI was converted via thermal imidization process from its soluble poly(amic acid) (PPA) precursor, 6FDA-ODA PAA. Solution blends of TPD and 6FDA-
Fig. 1. The PL spectra of the neat TPD and TPD-dispersed 6FDAODA PI films with different compositions.
ODA PAA in N-methyl-2-pyrrolidone (NMP) were prepared with different compositions. The weight ratios of TPD to 6FDA-ODA PAA were 30:70, 50:50, and 70:30. Overall solid concentration was 1 wt.%, and the mixture solutions were spin-coated onto an ITO-glass substrate at 3000 rpm for 2 min. TPD-dispersed 6FDAODA PAA thin film was soft-baked at 80°C for above 30 min and subsequently thermally imidized into 6FDA-ODA PI in an electrical furnace at 180°C for 1 h under nitrogen atmosphere. Then the EL material, tris(8-hydoxyquinolinato)aluminum (Alq3), and aluminum cathode were sequentially prepared by thermal evaporation in a vacuum of ca. 1×10 − 5 Torr. Special care was taken to monitor the thickness of TPD-dispersed polyimide films constant before and after thermal imidization of PI by atomic force microscopy (Nanoscope IIIa, Digital Instruments Co.). The current density–voltage–EL intensity (J–V–L) characteristics of the OELDs were measured using a photomultiplier tube (PMT) (Hamamatsu Photonics Co.) and an electrometer (Keithley 6517) controlled with a personal computer via analog-to-digital converter (ADC) and an IEEE488 GP-IB card. The photoluminescent (PL) and EL spectra were obtained by high-sensitivity fiber optic spectrometer (S2000, Ocean Optics Inc.) with 2048-element linear CCD-array silicon detector. All measurements were carried out under air ambient room temperature condition.
3. Results and discussion
Fig. 2. The dependence of: (a) current density and (b) EL intensity with the applied voltage of the OELDs.
The PL emission spectra of the neat TPD and TPDdispersed 6FDA-ODA PI films with different compositions are shown in Fig. 1. TPD306FDA70 denotes that the composition of TPD and 6FDA-ODA PI is 30 versus 70 by weight percentage. One can see that the emission peak of TPD-dispersed 6FDA-ODA PI is observed at longer wavelength than that of neat TPD. Also the PL peaks of the TPD-dispersed 6FDA-ODA PI films were slightly shifted to longer wavelength with increase in the concentration of TPD in the HTL. It can be considered that the slight red shift (bathochromic shift) of the PL is due to the increase of the vibration mode of TPD in the matrix polymer and the effect of the interaction between TPD and 6FDAODA PI by increasing the concentration of TPD. It is considered that the range of the red shift is not very large and thus marginally affects the optical property of the devices. The dependence of current density and EL intensity with the applied voltage of the OELDs is shown in Fig. 2. The shape of charge injection shows typical diode characteristics, meaning the rectification and recombination of holes and electrons injected from anode and cathode, respectively. The charge injection and turn-on
C.-S. Ha et al. / Materials Science and Engineering B85 (2001) 195–198
Fig. 3. (a) EL intensity and (b) relative power efficiency as a function of current density of the devices.
Fig. 4. The EL spectra of the OELDs. All devices were operated at 18 V.
voltage, corresponding to the voltage for the initiation of the charge injection, were reduced with increasing concentration of TPD. It is due to the increase of the hole-transporting ability by increasing the concentration of TPD in the HTL. It was found that, therefore,
197
the TPD706FDA30 device shows the lowest charge injection and turn-on voltage (ca. 10 V). Fig. 3 shows the EL intensity and the relative power efficiency as a function of the current density of the devices. In the plot of the EL intensity versus the current density of the device, the TPD706FDA30 device exhibited the highest slope, meaning the best efficiency of the devices. It should be noted that the slopes of the TPD506FDA50 and the TPD306FDA70 devices are almost identical. The result suggests that the efficiency is almost same, even though the TPD amount of the TPD306FDA70 device is smaller than that of the TPD506FDA50 device. It may be explained in terms of the effect by the strong electronegativity of fluorine of 6FDA-ODA PI in the HTL. It is suggested that the positively charged carbon in a carbon-fluoride dipole of 6FDA-ODA PI may encourage the electron-donating property of tertiary amine in TPD, leading to the activation of the hole-injecting and the hole-transporting ability of the HTL. A similar result on the effect of the electronegativity of a fluorine-containing polymer was reported in our previous study using PVdF-coHFP [14]. However, the result may be also caused by some other factors, such as the quality of organic films and the distribution of TPD in polyimide. The relative power efficiency (p× [EL intensity]/[W]) of the devices is very stable and gradually increased as the current density increased. One can see that the TPD706FDA30 device exhibits the highest relative power efficiency. Thus, it is thought that the hole injection and radiative recombination are more efficient in the TPD706FDA30 device than the others. It is interesting to note that the power efficiency of the TPD306FDA70 device is higher than that of the TPD506FDA50 device. It is also due to the effect by the electronegativity of fluorine of 6FDAODA PI in the HTL, which is similar to the result in the plot of the EL intensity versus the current density of the devices. From the results of the efficiency of the device, it is suggested that the hole-injecting and holetransporting ability of TPD is more dominant than the interaction between fluorine and tertiary amine at high concentration of TPD (TPD706FDA30 device). In the case of low concentration of TPD (TPD306FDA70 device), however, the efficiency of the device may be governed by the interaction between fluorine and tertiary amine as well as the film property such as film formation ability and uniformity. Fig. 4 shows the EL spectra of the devices. The maximum EL peaks of the devices at 18 V are identically positioned at ca. 520 nm, which correspond to those of a typical OELD using Alq3 as an emitting material. The full-width at half-maximum (FWHM) was about 100 nm, and the bright green light was clearly visible and stable under ordinary room light.
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C.-S. Ha et al. / Materials Science and Engineering B85 (2001) 195–198
4. Conclusions OELDs using TPD-dispersed 6FDA-ODA PI with different compositions were successfully fabricated to investigate the effect of the composition of TPD and 6FDA-ODA PI on the device properties, such as J –V– L characteristics, EL spectra, and EL efficiency. The charge injection and the turn-on voltage were reduced with increasing concentration of TPD, due to the increase of the hole-transporting ability by increasing concentration of TPD in the HTL. In the plot of the EL intensity versus the current density of the device, the TPD706FDA30 device exhibited the highest slope, meaning the best efficiency of the device. However, the slopes of the TPD506FDA50 and the TPD306FDA70 devices were almost identical, suggesting that the efficiency is almost same. Also the power efficiency of the TPD306FDA70 device was higher than that of the TPD506FDA50 devices. The results were explained in terms of the strong electronegativity of fluorine of 6FDA-ODA PI in HTL. It was suggested that the efficiency of the devices is explained in a different way depending on the concentration of TPD in the devices; i.e. the hole-injecting and hole-transporting ability of TPD is more dominant in the efficiency of the devices than the interaction between fluorine and tertiary amine at high concentration of TPD (TPD706FDA30 device), while in the case of low concentration of TPD (TPD306FDA70 device), the efficiency of the device is governed by the interaction between fluorine and tertiary amine as well as film properties such as film formation ability and uniformity. The TPD706FDA30 device exhibited the best relative power efficiency, as well as the lowest turn-on voltage (ca. 10 V). The
.
maximum EL peaks of the devices were identically positioned at ca. 520 nm, and the bright green light was clearly visible and stable under ordinary room light.
Acknowledgements The work was supported by the Brain Korea 21 project in 2001 and the Center of Integrated Molecular Systems, POSTECH, South Korea.
References [1] W. Helflich, W.G. Schneider, J. Chem. Phys. 44 (1966) 2902. [2] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 21. [3] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Macky, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [4] D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982. [5] C.H. Chen, J. Shi, C.W. Tang, Macromol. Symp. 125 (1997) 1. [6] T. Tsutsui, MRS Bull. June (1997) 39. [7] C.W. Tang, SID Int. Symp. Dig. Tech. Papers 27 (1996) 181. [8] Y. Hamada, Trans. Electron Devices 44 (1997) 1208. [9] T. Wakimoto, R. Murayama, K. Nagamiya, Y. Okuda, H. Nakada, T. Tohma, SID 96 Dig., 1996, p. 849. [10] G. Grem, G. Leditzky, B. Ullrich, G. Leising, Adv. Mater. 4 (1992) 32. [11] J. Kido, M. Kohda, Appl. Phys. Lett. 61 (1992) 761. [12] J. Kido, K. Hongawa, M. Kohda, Jpn. J. Appl. Phys. 31 (1992) L960. [13] H. Lim, H. Park, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, SPIE Proc. 3281 (1998) 345. [14] E.J. Kang, J.H. Keum, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, SPIE Proc. 3281 (1998) 366. [15] J.H. Keum, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, IUMRSICEM Fourth Int. Conf. on Electronic Materials, August 1998.