NHK Laboratories Note No. 470

Novel Selective Dye-diffusion Technique Using Laser Irradiation
-for Organic Polymer Electroluminescent Devices-


by
Isao TANAKA, Youji INOUE, Norihiko ISHII*,
Katsu TANAKA, Yoshitaka IZUMI, and Shinji OKAMOTO
(Display and Optical Devices)
(Recording Technology and Mechanical Engineering*)
ABSTRACT
    The heat-transfer process described in this paper will be useful for controllably and selectively doping polymer films to make hyperfine full- color polymer electroluminescent displays. A polymer receiver film is placed in direct contact with a dye-dispersed polymer film coated onto an efficient photo-absorbing substrate to permit heat-transfer dye-diffusion process. The widely utilized magneto-optical storage-material, TbFeCo was used as the photo-absorbing material. We irradiated it with a laser in order to generate heat. This method can be used to control the lateral and vertical dopant distributions in polymer films for electroluminescent devices. We demonstrated that coumarin 6 and Nile red can be diffuse into the rather thermally stable polymer poly(N- vinylcarbazole) containing the electron-transporting 2-(4-biphenylyl)-5-(4-tert- butyl-pheny1)-1,3,4-oxadiazole. The diffusion power and time for about 1-mm2 doping area were 1.2 W and about 10s.
1. INTRODUCTION
    The advances in information technology are increasing the need for high-performance electronic displays. Flat panel display, which is typically liquid crystal display (LCD), has attracted much interest in recent years. Furthermore, plasma display panel (PDP), field emission display (FED), and electroluminescent (EL) display are also important, and are being developed actively. The organic EL displays are especially attractive not only because they are (1) very thin, (2) very light, and (3) self-emitting, but also because (4) they respond quickly and (5) can be driven at low voltages. They would thus be useful in personal digital assistants and in digital-broadcasting receivers.
    The current-injection type of EL in an organic semiconductor, single crystals of anthracene, was first reported in 1965 [1], and by 1987 Tang and VanSlyke had demonstrated extremely efficient EL in double-layer thin-film devices [2]. The hole- transporting layer in these devices was an aromatic diamine, and the emissive electron- transporting layer was an 8-hydroxyquinoline aluminum (Alq3). The total thickness of the organic layer was about 135 nm. Many other charge-transporting and emissive materials have been investigated, particularly in Japan, where a great deal of activity has been devoted to developing devices of this type. Monochrome and area-color organic EL displays using small-molecule materials are already available commercially. Full-color organic EL displays are expected to appear soon.
    When we make EL devices using small molecules, however, to integrate differently color pixels on a substrate, we have to control the mechanical shadow mask precisely, and deposit thin films under ultrahigh vacuum. On the other hand, since EL in conjugated polymers was reported in 1990, by a group of Burroughes using poly(p-phenylene vinylene), PPV as the single semiconducting layer [3], polymer EL devices have also been investigated extensively in Europe and the U. S. They are attractive because they generate light efficiently, and can be manufactured inexpensively because the solution-processing for forming thin polymer films is easy. The biggest problem in making full-color displays by this way is that it is too difficult to pattern the fine RGB color pixels precisely. Some of the methods have been developed in attempts to solve this problem; ink-jet printing the polymer solution [4,5], photo-oxidizing dopant molecules locally [6], and heat-transfer dye-diffusion [7-9]. In this paper, we describe a novel dye-diffusion technique that uses laser irradiation and is based on the spin-coating process, for patterning the emission colors.

2. Polymer EL device structure
    The cross section and energy diagram of a typical single-layer polymer EL device are shown in Fig.1. The transparent anode, a layer of indium tin oxide (ITO), allows the light generated within the polymer layer to leave the device. The top electrode is conveniently formed by thermal evaporation of a metal. When the polymer layer is biased in such a way that positive and negative charge carriers are injected from the electrodes, the capture of oppositely charged carriers results in the emission of photons, so-called electroluminescence.

(a) Cross section

(b) energy diagram of a typical single-layer polymer EL device

Fig. 1. (a) Cross section and (b) energy diagram of a typical single-layer polymer EL device.

    The polymer we used in this work consisted primarily of poly(N-vinylcalbazole), PVK, which transports holes, containing 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4- oxadiazole, PBD, which transpolts electrons. The emission color was adjusted by doping the polymer with dye molecules: coumarin 6 (C6) or Nile red. The chemical structures of these materials are shown in Fig. 2. Devices of this type, with a polymer film about 100- nm tuck, were readily fabricated by solution-processing the semiconducting polymer onto lTO-coated glass. Spin-coating from solution has been demonstrated to be capable of producing layers whose thickness varies no more than a few angstroms spread over several square centimeters. As shown in Fig. 1(b), ITO has a relatively high work function, and is therefore suitable for use as a hole-injecting electrode. Holes were injected from the anode into the highest occupied molecular orbitals (HOMO), corresponding to the valence band, of the hole-transporting PVK. On the other hand, low-work-function metals such as Al, Mg, Li, and Ca are suitable materials for the electron-injecting electrode. We therefore injected electrons from a LiF/Al cathode into the lowest unoccupied molecular orbitals (LUMO), corresponding to conduction band, of the electron-transporting PBD. As a result of the hopping of carriers, the recombination of electrons and holes of the C6 or Nile red dopants causes strong EL emission.

Fig. 2. Structure of materials used in polymer thin-film EL devices.
Fig. 2. Structure of materials used in polymer thin-film EL devices.


3. Dye-diffusion technique using laser irradiation
    The setup for the heat-transfer dye-diffusion process using laser irradiation [10] is shown schematically in Fig. 3. We prepared two films in advance: a source plate of polymer film in which dye was dispersed, and a receiver plate of undoped polymer film. The receiver film consisting of PVK (70% by weight in the final solution) as the hole- transporting host material and PBD (30% by weight) as the electron-transporting material was spin-coated from a chloroform solution onto a prepatterned ITO-coated glass plate, The film on the source plate was spin-coated, from a chloroform solution of PVK containing a dye molecule at a concentration between 10% and 50% by weight, onto a photo-absorbing substrate (described in the next paragraph). All chemicals were purchased from Aldrich, and were used as received.

Fig. 3. Experimental arrangement for heat-transfer dye-diffusion using laser irradiation.
Fig. 3. Experimental arrangement for heat-transfer dye-diffusion using laser irradiation.

     As shown in Fig. 3, the receiver polymer was placed in direct contact with the dye- dispersed source polymer in order to permit heat-transfer dye-diffusion. After a pressure of about 20 g/cm2 was applied on top of the receiver plate to ensure intimate contact with the source film, the photo-absorbing source substrate was irradiated with the 532-nm beam, about 1 mm in diameter, from a frequency-doubled Nd:YVO4 laser with a power range of 0.5 to 5 W (Coherent, Verdi). The source substrate was the widely used heat-mode mageneto-optical (MO) storage media, TbFeCo deposited on glass, which efficiently converts light to heat. The 50-nm SiNx film was evaporated on TbFeCo layer with 100-nm thickness because of the protection of oxidation of the TbFeCo layer. Both layers were deposited by conventional rf-sputtering. We controlled the diffusion temperature by adjusting the laser power and the exposure time. After the source plate was removed, a LiF electron injection layer with 0.5-nm thickness and an Al electrode with 150-nm thickness were vacuum-deposited onto the dye-diffused receiver polymer film. If a source plate containing other dye-dispersed polymer is change, we can perform selective doping of polymers for multicolor EL devices or color pixels in polymer EL displays for large areas. It is well known that in a guest-host system the doping level of guest dyes should be controlled carefully to obtain the optimum performance. First, to get some idea of how the transfer process may proceed, we evaluated the temperature distribution on the source substrate during the irradiation. For each fllm, we assumed the following three- dimensional thermal diffusion equation: [11]


where C is specific heat, is density, T is temperature, t is time, and k is thermal conductivity. Q is the heat due to the laser irradiation_ These values were estimated from bulk samples data. The profile of the laser beam was assumed Gaussian-like. It is obvious from the simulated temperature distribution on the film surface of the source substrate of SiNx/TbFeCo/glass that the localized thermal-energy pattern is formed by the laser irradiation, and that the temperature at the laser spot is more than 100 degree. According to the analysis of diffusion temperature in Ref. 8, the heat transfer occurs at the temperature above 100 degree. It is noted that the highest temperature produced by the laser irradiation is much lower than the glass transition temperature of PVK, 210 degree. It is well kown that the crystallization of polymer causes the worse EL performance. The dye diffusion from the source into receiver films at an elevated temperature obeys Fick's diffusion theory, [12] under the boundary conditions at two film-substrate interfaces. Thus, it is expected that the area of the heat-transfer dye-diffusion, so-called doping area, can be controlled by scanning of the laser spot.
     Photoluminescence (PL) and EL spectra were measured using an Ocean Optics USB2000 spectrometer with a resolution of 4 nm. Excitation light for PL measurements was produced by combining a xenon lamp with a power of 300 W and a single monochromator (Jobin Ybon, TRIAX-190) with 3.5-nm resolution. The driving source for EL measurements was a digital source meter (Keithley, 2400).

4. Results and Discussion
Fig. 4
Fig. 4. PL spectrum of PVK/PBD film and PL and EL spectra of the films doped with C6
and Nile red by heat-transfer diffusion technique with laser irradiation.

    Figure 4 shows the PL spectrum ofa PVK/PBD blend film, which is the receiver film before dye diffusion, and the PL and EL spectra of films doped with C6 and Nile red by the heat-transfer dye-diffusion. The wavelength for PL excitation was 300 nm, the source dye concentration was about 50% by weight, and the laser power and irradiation time were 1.2 W and about 10 s. As shown in Fig. 4(a), since the polymer blend contains PBD, the 425nm PL emission from PBD is detected. Focusing on PL spectra in Figs. 4(b) and 4(c), we can see that the deep-blue emission of PBD is strongly quenched, and the green and red emissions from C6 and Nile red dominate the spectrum. Again, in EL spectra, the emission is only observed from the low band gap dye. Note that our diffusion time was 10 s, while the diffusion time in the conventional heat-transfer dye-diffusion process is the order of minutes [7-9]. Thus, this transfer technique using laser irradiation is more efficient for the formation ofthe localized color pixels.

Fig. 5. PL intensity versus laser power during heat-transfer diffusion using source films with several C6 concentrations.
Fig. 5. PL intensity versus laser power during heat-transfer diffusion
using source films with several C6 concentrations.

     To find the optimum condition for dye traveling from source into receiver films, we measured the PL intensity from the doped dye under several diffusion conditions. Figure 5 shows the PL intensity as a function of laser power for heat-transfer diffusion using source films with several C6 concentrations. The diameter of the probe light beam for PL was about 1 mm, which equals to the diameter of the laser beam for heat-transfer diffusion. It is obvious from Fig. 5 that, for different dye concentrations, the PL intensity has a maximum at a different laser power. In the lower power region, the doping concentration becomes very high with increasing the laser power, which results in the bright PL. On the other hand, in the higher power region, the PL intensity is remarkably reduced with increasing the laser power. This is due to the concentration quenching or the dye re-evaporation, which originate from overdoping and overheating under the high-laser-power condition, respectively. The emissive doping area is observed at the outside of the probe beam area, which results in spreading of thermal diffusion at high temperature. Furthermore, the laser power at which the PL intensity is the maximum increases with increasing dye concentration of the source plate. The higher source dye concentrations lead to efficient dye traveling. Although this diffusion process may produce graded doping profiles in the polymer layer, the consequent feature may be useful in different device designs with high performances such as enhancement in efficiency [9]. Using another photo-absorbing material, a garnet film on glass, [13] which is the MO material for blue laser, we were also able to use this technique for dye doping.
    Figure 6 shows the PL intensity from C6 and Nile red as a function of the diffusion time at a laser power of 1.2 W for the source dye concentrations of about 50 % by weight.

Fig. 6. PL intensity from C6 and Nile red as a function of the diffusion time at laser power of 1.2W.
Fig. 6. PL intensity from C6 and Nile red as a function of the diffusion time at laser power of 1.2W.

Fig. 7. Photograph of polymer EL devices under operation fabricated by dye-diffusion technique using laser irradiation.
Fig. 7. Photograph of polymer EL devices under operation fabricated by dye-diffusion technique using laser irradiation.

These time profiles show maxima at different diffusion times. Long diffusion times are associated with faint luminescence. The different diffusion profiles of C6 and Nile red should result in different traveling processes and different concentrations at which quenching occurs. This diffusion technique with laser irradiation has the advantage of optimizing the diffusion temperature for different dyes.
    By repeatedly diffusing C6 and Nile red into the receiver films, we made pixels with different colors (Fig. 7). The circle-like doping area is clearly formed at the irradiation laser spot for diffusion. It is noted that the dye-diffusion technique described here should be applicable to the fabrication of dye-doped polymer EL devices. Reducing the beam diameter to the order of 1 m will enable us to make hyperfine displays by diffusing dye molecules into extremely small regions of polymer films. This method is applicable not only to the recently reported highly efficient polymer EL devices using electrophosphorecent materials [14] but also to other devices such as polymer photovoltaic cells.

5. CONCLUSION
    We have developed an effective heat-transfer dye-diffusion process using laser irradiation. It can be used for controllably and selectively doping the polymer films for full-color polymer EL displays with small color pixels. In this process, a polymer receiver film is placed in direct contact with a dye-dispersed polymer film coated onto a photo- absorbing TbFeCo substrate. It has been demonstrated that this simple method can be used for diffusing C6 and Nile red into the rather thermally-stable polymer PVK containing electron-transporting PBD. For a 1-mm2 doping area, the diffusion power and time were 1.2 W and about 10 s. A doping area with only 1- m order in diameter could be obtained by reducing the laser spot size. This novel diffusion method can be used to dope small areas enough to serve as pixels in a full-color display, and can be used to control the lateral and vertical dopant distributions in polymer films for EL devices. Furthermore, we can find the optimum conditions of the heat-transfer diffusion for the various dye molecules by adjusting the laser-irradiation condition. This work will be helpful for the fabrication of full-color polymer EL displays.


References

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  3. J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, and A.B. Holmes, "Light-emitting diodes based on conjugated polymers", Nature 347,539 (1990).
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  6. S. Shirai and J. Kido, "Fabrication of three color polymer EL devices using photobleaching method", Proc. 10th Int. Workshop on Inorganic and Organic Electroluminescence (EL'00) p.289 (2000).
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  8. K. Tada and M. Onoda, "Three-color polymer light-emitting devices patterned by maskless dye diffusion onto prepatterned electrode", Jpn. J. Appl. Phys. 38, L1143 (1999).
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Tanaka Dr. Isao Tanaka
He received the Dr. Eng. degree in applied physics from Osaka City University, Osaka, Japan, in 1994. He joined NHK in 1994. Since 1994, he has been with NHK Science and Technical Research Laboratories. He has been engaged in the research on high-density optical storage system using holographic recording materials and EL displays.
Inoue Mr. Youji Inoue
He received the B. Eng. and M. Eng. degrees in electronic engineering from Osaka University, Osaka, Japan, in 1986 and 1988, respectively. He joined NHK in 1988. Since 1991, he has been with NHK Science and Technical Research Laboratories. He has been engaged in the research on LSI and EL displays.
Ishii Mr. Norihiko Ishii
He received the B. Eng. and M. Eng. degrees in instrumentation engineering from Keio University, Yokohama, Japan, in 1991 and 1993, respectively. He joined NHK in 1993. Since 1993, he has been with NHK Science and Technical Research Laboratories. He has been engaged in the research on optical isolators, mangeto-optical materials, and phase change materials.
Tanaka Dr. Katsu Tanaka
He received the Dr. Eng. degree in electrical engineering from Tokyo University of Agriculture and Technology, Tokyo, Japan, in 1996. He joined NHK in 1989. Since 1991, he has been with NHK Science and Technical Research Laboratories. He has been engaged in the research on high sensitive images pick-up devices and EL displays.
Izumi Mr. Yoshitaka Izumi
He received the B. Eng. and M. Eng. degrees in electronic engineering from Ehime University, Matsuyama, Japan, in 1991 and 1993, respectively. He joined NHK in 1993. Since 1996, he has been with NHK Science and Technical Research Laboratories. He has been engaged in the research on EL displays.
Okamoto Dr. Shinji Okamoto
He received the Dr. Eng. degree from Tokyo Institute of Technology, Tokyo, Japan, in 1995. He joined NHK in 1980. Since 1983, he has been with NHK Science and Technical Research Laboratories. He has been engaged in research on EL displays and electron emission devices. He is currently a senior research engineer of NHK Science and Technical Research Laboratories.


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