of Multi Color Polymer EL Devices the Photo-bleaching Method

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1 'I Journal of Photopolymer Science and Technology Volume 14,Number2(2001) TAPJ Fabrication using of Multi Color Polymer EL Devices the Photo-bleaching Method Satoshi Shirai and Junji Kido Graduate School of Science and Engineering, Yamagata University Yonezawa, Yamagata , JAPAN Several kinds of multi color polymer EL devices were fabricated by means of the photo-bleaching method Poly(N-vinylcarbazole)(PVK) was aspersed with photo-bleaching cbpants having green or yellow fluorescent colors. In order to tune the emission Dolor and pattern the device, the PVK emitter layer was exposed to specific lights after formed by spin coating so that a specific dye is photo-bleached and becomes non-emissive. Green and blue (G-B), two color and yellow, green and blue (Y-G-B), three color devices were fabricated successfully. Keywords: polymer EL devices, multicolor, the photo-bleaching method 1. Introduction Organic electroluminescent (EL) devices are a class of light-emitting devices composed of organic materials as active layers, and they are expected to be the flat panel displays of the next generation [1 ]. Two types of organic materials are used in devices: small molecules and polymers. Recently, the device life time and efficiencies have been improved and dot matrix displays using the small molecules have been commercialized [ 2 ]. Since all the three primary colors of red (R), green (G) and blue (B) can be obtained in the hue of emitted light of these devices, enthusiastic research efforts have been made to realize R-G-B multicolor and full color displays. For the fabrication of arrays of R-G-B pixels, several methods have been proposed. As an alternative approach, we developed white EL devices and propose multicolor displays based on white EL devices combined with color filters [3 ] [4]. In this device, white light generated in the organic layers is filtered to give three primary colors. A similar method using fluorescent color conversion layers was also proposed [ 5 ], in which blue-light-emitting devices are combined with fluorescent color conversion layers inserted between transparent electrode and substrate, and blue light is used to excite green and red fluorescent layers, thus converting blue light to green and red light. These two approaches can be applicable to polymer EL devices, in which polymers are difficult to be pixelated. However, the use of color filters or color conversion layers always decreases the device efficiencies because of the absorption loss or the conversion loss. The most straight forward way is so-called " side -by-side" method. This configuration provides the most efficient devices. In the device based on small molecules, R G-B pixels are independently constructed by vacuum deposition using a shadow mask II 6 ] Therefore, the device fabrication process is time consuming and costfull. Also the pixel size in micrometer scale is technically difficult to achieve. On the other hand, arrays of RGB pixels fabricate by inkjet printing in the polymer EL devices have been reported [ 7 ]. Thus, the device fabrication process of the polymer devices is easier than that of small molecules. In this study, we developed a novel and simple method called " the photo-bleaching method", for the fabrication of micro patterned multicolor EL devices, which particularly useful for polymer-based devices [ 8 ]. In this approach, we take advantage of the instability of organic fluorescent molecules upon photo-irradiation in the presence oxygen; that is, organic dyes can be photo-oxidized or photo-bleached. By exposing a polymer layer Received Accepted April 6, 2001 May 25,

2 doped with fluorescent dyes to ultraviolet (UV) light, light-sensitive dyes can be photo-oxidized and become non-emissive. This process changes the hue of the emitted light of the UV-exposed area. We have already shown that, using rubrene-doped polymers, we can fabricate two color devices having green and yellow emission, and three color devices having blue, white and yellow emission. Therefore, the fluorescent dyes having anthracene or naphthacene backbone is very useful for this method. Here we report green-blue two color devices and yellow-green-blue three color devices using photo-bleachable green and yellow dyes. 2. Method Indium-tin oxide (ITO)-coated glass substrates were cleaned in a ultrasonic bath using several solvents and were treated in a UV-ozone chamber just before device fabrication. For single layer devices, a polymer layer was formed by spin coating from the solution, and magnesium (Mg)-silver (Ag) (10:1) top electrode was formed by vacuum deposition at 5 x 10 6 Torr. UV-irradiation was done in an ambient atmosphere using a high pressure mercury lamp USHIO USH-250 D with a filter just after polymer layer formed by spin coating. Luminance was measured using a Topcon BM-8 luminance meter and electroluminescence spectra were taken on a Hamamatsu photonics PMA-10 optical multichannel analyzer and Photoluminescence (PL) spectra were taken on a fluorescence spectrometer, Instruments SA Fluoro Max Results and Discussion 3.1. Green-blue two color devices Two color devices using 5,12-Diphenylnaphthacene (DPN) Figure 1 shows the materials used. 5,12-Diphenylnaphthacene (DPN) is a green fluorescent dye and photo-bleached in this study. Photoluminescence (PL) spectra of the thin films of PVK doped with DPN (1.Omol%) are displayed in Figure 2, which were formed by spin coating using a 1,2-dichloroethane solution containing the polymer and DPN. The spectrum (a) in Figure 2 shows a peak at 500 nm originating from the DPN. Figure 2-b, c, d, display the decrease in the intensity of the peak from DPN upon UV irradiation, demonstrating that the concentration of the emissive form of DPN in the polymer layer can be lowered by UV irradiation while the emission peak from the polymer remains relatively unaffected. Single layer devices were fabricated using dyes doped PVK as an emitter layer. The polymer layer was formed by spin coated on ITO substrates. On top of the polymer layer, Mg: Ag electrode was deposited. Since single layer devices require hole injection and electron injection to the emitter layer. Hole transporting polymer, PVK was molecularly dispersed with electron-transporting additive, 1,3,4-oxadiazole derivative, PBD at a concentration of 3Owt%a. When 3mol% of 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) was doped to the polymer, blue emission from TPB peaking at 450nm was obtained from the device Fig.l The structure of material used. Fig.2 PL spectra of DPN (lmol%) doped PVK films: (spectrum a) without UV irradiation, (spectrum b) after UV irradiation of 90 mj/cm'`, (spectrum c) after UV irradiation of 900 mj/cm`, (spectrum d) after UV irradiation of 2700 mj/cm2. 318

3 Fig.3 EL spectra for ITO/PVK (100 nm) doped with PBD (30wt%), TPB, and DPN /Mg:Ag devices: (spectrum a) TPB (3 mol%), and no DPN (0 mal%), (spectrum b) TPB (3 mol%), and DPN (0.1 mol%), (spectrum c) TPB (3 mol%), and DPN (1 mol%). Fig.4 EL spectra for ITa/PVK (100 nm) doped with PBD (30wt%), TPB (3 mol%a), and DPN (0.4 mol%) / Mg:Ag devices: (spectrum a) without UV (365 inn) irradiation, (spectrum b) after UV (365 nm) irradiation of 90 mjlcmz. as shown in Figure 3 (a). In addition to TPB, 0.1 mol% of DPN was added to the system, the hue of emitted light from blue to green as shown in Figure 3 (b). In this case, the excited energy transferred effectively from TPB to DPN by the resonance type energy transfer. Two color, green and blue emitting devices having dyes doped PVK were fabricated by the photo-bleaching method. Electroluminescence (EL) spectra of these devices were displayed in Figure 4. Green emission originating from DPN was observed from the device without irradiation of UV light to the PVK layer (Figure 4 (a)). In contrast, when polymer layer was irradiated with UV light, the hue of the emitted light from the device become blue, originating from TPB in the polymer (Figure 4 (b)). This result indicates that the concentration of emissive DPN is lowered by UV irradiation and the energy transfer from TPB to DSA becomes inefficient Two color devices using 9,10-Diphenylanthacene (DS A) 9,10-Diphenylanthacene (DSA) is also a green fluorescent dye and photo-bleached in this study. Photoluminescence (PL) spectra of the thin films of PVK doped with DSA (1.Omol%) are displayed in Figure 5. The spectrum (a) in Figure 5 shows a peak at 500 nm originating from the DSA. Figure 5-b, c, d, displays the decrease in the intensity of the peak from DSA upon UV irradiation, demonstrating that DSA can also be photobleached. As demonstrated with DPN, two color, green and blue emitting devices having DSA doped PVK were fabricated by the photo-bleaching method. EL spectra of these devices were displayed in Figure 6. Green emission originating from DSA was observed from the device without UV irradiation (Figure 6 (a)). In contrast, when polymer layer was irradiated with UV light, the hue of the emitted light was changed from green to blue (Figure 6 (b)). This result indicates that the concentration of emissive DSA is lowered by UV irradiation and the energy transfer from TPB to DSA becomes inefficient Yellow-green-blue three color device Three color device using DPN First, we investigated the dependence of photo-degradation rate of DPN upon wavelength of irradiating light. Figure 7 shows relationship between relative PL intensity of DPN and irradiation time. When polymer films containing 319

4 Fig.5 PL spectra of DSA (lmol%) doped PVK films: (spectrum a) without UV irradiation, (spectrum b) after UV irradiation of 90 mj/cm2, (spectrum c) after UV irradiation of 900 mj/cm'`, (spectrum d) after UV irradiation of 2700 mj/cm2. Fig.6 EL spectra for ITOIPVK (100 nm) doped with PBD (30wt%), TPB (3 mol%a), and DSA (0.4 mol%) I Mg:Ag devices: (spectrum a) without UV (365 am) irradiation, (spectrum b) after UV (365 nm) irradiation of 4050 mj/cm mol% DPN are irradiated with 546 or 578 nm light (shown in Figure 7, closed circles and closed triangles), the degradation rate of DPN was slower than that with 365, 406 or 436 nm (Figure 7, open circles, open triangles and open squares). Single layer devices containing two photo-bleaching dyes, DPN and rubrene, were fabricated. The device structure is ITO I PVK doped with PBD (30 wt%), TPB (3 mol%), DPN (0.3 mol%) and rubrene (0.15 mol%) / Mg:Ag. From this device, yellow emission originating from rubrene was observed (shown in Figure 8 (a)). Photo-bleaching of rubrene was carried out with specific light (578nm) after the formation of the polymer film by spin coating. After light irradiation, the peak intensity of rubrene decreased and an emission peak originating from DPN at 500 nm appeared. The device after light irradiation for 30 minutes emitted green light as shown in Figure 8 (b). Then, the polymer layer was further irradiated with UV (365nm) light for photo-bleaching of DPN. The devices emitted blue to green light depending on the exposure dose. These results indicate that the concentration of emissive dyes can be decreases by light irradiation and the energy transfer from TPB to DPN and to rubrene becomes inefficient. Finally, blue emission was obtained from the device after UV irradiation of 8.1 JIcm2 (shown in Figure 8 (c)). Thus, the hue of emitted light from this device was changed from yellow to green and to blue by the photo-bleaching process Three color device wing DSA In the same way, the PVK based single layer devices containing two photo-bleaching dyes, DSA and rubrene were fabricated. The device structure is ITO I PVK doped with PBD (30 wt%), TPB (3 mol%), DSA (0.15 mol%) and rubrene (0.075 mol%) 1 Mg:Ag. From this device, yellow emission originating from rubrene was observed (shown in Figure 9 (a)). Photo-bleaching of rubrene was carried out with irradiation of light at 578nm after the formation of the polymer film by spin coating. After light irradiation, the peak intensity of rubrene decreased and an emission peak originating from DSA at 500 nm appeared. The device after light irradiation for 30 minutes emitted green light as shown in Figure 9 (b). Then, the polymer layer was furher irradiated with UV light at 365nm just after photobleaching rubrene for photobleaching 320

5 J. Photopolym. Sc1. Technol., Vol.14, No.2, 2001 Fig.7 Relationship between relative PL intensity of DPN and light irradiation time: (open circles) 365 nm light irradiation, (open triangles) 406 nm light irradiation, (open squares) 436 nm light irradiation, (closed circles) 546 nm light irradiation, triangles) 578 nm light irradiation. DSA. The devices emitted blue to green light depending on the exposure dose. These results also indicate that the concentration of emissive dyes decreases by light irradiation and the energy transfer from TPB to DSA and to rubrene becomes inefficient. Finally, blue emission was obtained from the device after W irradiation of 8.1 Jlcm2 (shown in Figure 9 (c)). Thus, the hue of emitted light from this device was changed from yellow to green and to blue by the photobleaching process. Three color EL devices were constructed by partial light irradiation of the polymer emitter layer through photo-masks, and the photograph of the device is shown in Figure 10. This device (26 mm x 31 mm) was fabricated to be different in emission colors, yellow, green and blue. 4. Conclusion In conclusion, we successfully developed two color, green and blue, and three color, yellow, green and blue, polymer EL devices using photobleaching dyes by means of the photobleaching method. (closed Fig.8 EL spectra for ITO/PVK (100 nm) doped with PBD (30wt%), TPB (3 mol%), DPN (0.3 mol%) and Rubrene (0.15 nnol%) I Mg:Ag devices: (spectrum a) without light irradiation, (spectrum b) with 578 nm irradiation for 30minutes just after PVK layer spin coated, (spectrum c) with UV (365 nrn) irradiation of 8.1 J/cm2 just after 578 nm light irradiation for 3Ominutes. Fig.9 EL spectra for 1TO/PVK (100 nm) doped with PBD (30wt%), TPB (3 mol%o), DSA (0.15 mol%) and Rubrene (0.075 mol%) I Mg:Ag devices: (spectrum a) without light irradiation, (spectrum b) with 578 nm irradiation for 30minutes just after PVK layer spin coated, (spectrum c) with UV (365 nm) irradiation of 8.1 J/cm2 just after 578 nm light irradiation for 30minutes. 321

6 Fig.10 A photograph of three color Polymer EL device. Reference 1) Service, R. F. Science (1996) 2) Nakada, H. & Tohma, T. The 8th International Workshop on Electroluminescence, Extended Abstract, (1996) 3) Kido, J. & Nagai, K. Qyo Butsuri 63, (1994) 4) Kido, J. & Kimura, M. & Nagai, K. Science 267, (1995) 5) Hosokawa, C., Matsuura, M., Eida, M., Hironaka, Y.& Kusumoto, T. IS & T's 49th Annual Conference, Proceedings, (1996) 6) Kijima, Y., Asai, N., Kishii, N. & Tamura, S. IEEE Trans. Electron Devices 44, (1997) 7) Shimoda, T., Kanbe, S., Kobayashi, H., Seki, S., Kiguchi, H., Yudasaka, I., Kimura, M., Miyashita, S., Friend, R. H., Burroughes, J. H. and Towns, C. R., 1999 SID Int. Symo. Dig. Tech. Papers. 376, (1999) 8) Kido, J. Yamagata, Y.& Harada, G. Extended Abstracts (The44th Spring Meeting) The Japan Society of Applied Physics and Related Societies 3,1156 (1997) 322