Japanese Journal of Applied Physics Vol. 45, No. 3A, 2006, pp. 1829–1831
Brief Communication
#2006 The Japan Society of Applied Physics
Top-Emission Organic Light-Emitting Diodes with Ink-Jet Printed Self-Aligned Emission Zones Ryu-ichi SATOH1 , Shigeki N AKA1;2 , Miki SHIBATA1;2 , Hiroyuki OKADA1;2 , Hiroyoshi O NNAGAWA1;2 , Takeshi M IYABAYASHI2;3 and Toyokazu I NOUE2;3 1
Faculty of Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan Innovation Plaza Tokai, Japan Science and Technology Agency, 23-1 Ahara-cho, Minami-ku, Nagoya 457-0063, Japan 3 Brother Industries, Ltd., 1-1-1 Kawagishi, Mizuho-ku, Nagoya 467-8562, Japan 2
(Received November 15, 2005; revised November 25, 2005; accepted November 26, 2005; published online March 8, 2006)
We have investigated a method of fabricating ink-jet printed self-aligned emission zones for top-emission organic lightemitting diodes. To obtain a bright emission, the number of ink-jet printed shots was varied. From the viewpoint of surface roughness of the bottom electrode, AlNd was used. To reduce sputtering damage, a buffer layer with the hole transport ability of MoO3 was also employed. As a result, the maximum luminance obtained was 1,000 cd/m2 . By application of this fabrication technique, we have achieved a printed display panel 30 30 mm2 resolution of 300 pixels per inch. [DOI: 10.1143/JJAP.45.1829] KEYWORDS: organic electroluminescent device, ink-jet printing, self-alignment, top emission, organic electrophosphorescent material
Organic light-emitting diodes (OLEDs) have excellent properties of low driving voltage, bright emissions, ultrathin films, and flexibility. Many studies of OLEDs using ink-jet printing (IJP) have been carried out, and meter-sized displays have been achieved.1) IJP requires bank formation, entails a high cost, and has difficulty with precise alignment control. Therefore, a new fabrication process not beset by these problems is desired. From the aspect of simplification, we have studied self-aligned bank formation of bottomemission (BE) OLEDs using IJP.2,3) In addition, topemission (TE) OLEDs are interesting for use in active matrix displays with higher aperture ratios. In this study, we investigated the self-aligned TE OLEDs, and high-resolution emission of 300 pixels per inch (ppi) was demonstrated. Figure 1 shows the concept of the self-aligned bank formation technique3) for the TE OLED. First, an insulating organic film is formed on a cathode. Then, a solution of organic electroluminescent materials is ink-jet printed. The insulating layer is dissolved in a solvent and an emission layer is formed alternately on an ink-jet printed dot, i.e., selfalignment OLED fabrication is carried out. Bank formation is not necessary before ink-jet printing of the emission material. Particularly in TE OLEDs, a smoother electrode surface, higher electron-injection efficiency, and lower sputtering damage during sputtering of the upper electrode were issues in our investigation. Details of our solutions for these problems are described. The experimental procedure is as follows: We employed AlNd (Kobelco Research Institute) as the cathode, because AlNd is commonly used for semiconductor devices to prevent hillocks. We sometimes observed short-circuiting of the TE OLED with Al as the bottom electrode. This is possibly caused by grain growth due to aluminum evaporation. From atomic force microscopy (AFM; DI Nanoscope III), the mean roughness of the AlNd film was 1.24 nm, whereas that of Al was 2.09 nm. The short-circuiting problem can be solved using AlNd as the bottom electrode. Next, a hole-blocking layer (HBL) of 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP) or bis(2-methyl-8-quinolinato)(p-phenylphenolato) aluminum (BAlq) was evaporated onto
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IJP head
Ink-jet-printed material
Insulator
Cathode Glass substrate
Anode
Cathode Glass substrate
Fig. 1. Schematic of self-aligned bank formation.
the cathode. Then, an insulating material of poly(methyl methacrylate) (PMMA) was spin-coated on the glass substrate. The solvent for PMMA was tetrahydrofuran (THF), and the concentration was 0.5 wt %. Here, HBL was not dissolved into THF. The solution mixed with the electron transport material 2-(4-biphenyl)-5-(p-tert-butylphenyl)1,3,4-oxadiazole (tBu-PBD) and phosphorescent material fac-tris(2-phenylpyridine) iridium [Ir(ppy)3 ] was ink-jet printed. The mixing ratio of organic materials was tBu-PBD : Ir(ppy)3 ¼ 95 : 5. The solvent used was chloroform, and the concentration of the solution was 1.0 wt %. The BCP and BAlq were difficult to dissolve in chloroform. Therefore, the HBL also unchanged as in the initial film condition. After that, baking was carried out at 60 C for 1 h. Then, a hole transport layer of bis[N-(1-naphthyl)-N-phenyl] benzidine (-NPD) and a buffer layer of MoO3 4,5) for reduction of sputtering damage were thermally evaporated. For a common OLED structure, MoO3 showed excellent hole-injection properties and identical current density (J) versus voltage (V) characteristics were obtained even in a thicker layer of 80 nm. Finally, an anode of indium–zinc oxide (IZO; Idemitsu) was sputtered. The IZO film was deposited at room temperature using a radio-frequency (RF) magnetron sputtering system with a power density of 0.13 W/cm2 . The sheet resistance of the IZO was 100 /sq. The thickness of the IZO was 100 nm. To improve brightness, TE OLEDs are promis-
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Fig. 3. Atomic force microscope image of self-aligned emission zone.
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Fig. 2. Molecular structure of organic materials studied.
130 µm Fig. 4. Optical micrograph of emission pattern.
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ing because they suppress the waveguiding modes.6) This issue of improvement of the brightness is now under investigation. By applying a thinner IZO layer, an improvement in the brightness was confirmed for the device made by evaporation. For durability, the IZO film is inherently stable in the presence of moisture and oxygen compared with conventional transparent top-electrodes with low work function, such as an ultra thin Ca/Ag stack.7) Specifications for the ink-jet printing apparatus (Brother Industries) are as follows: ceramic ink-jet head; 128 nozzles; piezodriving; 150 ppi resolution; and 50 pl droplet volume. The number droplets ejected was 100 shots/s, the velocity of the substrate was 20 mm/s, and the accuracy of positioning was 15 mm. The ink-jet head had two lines of alternating ejection holes; therefore, a pattern resolution of 300 ppi can be achieved for back-and-forth motion and for precise movements of the ink-jet head. Current density versus voltage (J–V) and luminance versus current density (L–J) characteristics were measured using a semiconductor parameter analyzer (HP, 4155B) and a luminance meter (TOPCON, BM-3). The device area was estimated from a computed image of the emission dots. We used AFM to evaluate the evenness of the film surface. Figure 3 shows an AFM image of a typical ink-jet printed dot on insulating material. A clear circular dot with a diameter of 130 mm was formed on the insulating material. The periphery of the dot was thicker than the center of the dot. In our previous investigation,2) clear segregation of printed dots scale between the polymer insulating material and the emission material of small organic molecules occurred. We could estimate that the residual concentration of PMMA was at most 10%.3) Therefore, similar dot-sized segregation occurs at self-aligned dots, whereas the mean roughness of the center of the dot was as small as 2.0 nm. The emission region was 100 nm thick. Figure 4 shows an optical micrograph of the emission pattern. The dot size of the emission was 75 mm. At the
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: 50 mA/cm2 : 10 mA/cm2 2 3 4 Number of shots
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Fig. 5. Luminance at 10 and 50 mA/cm2 as function of number of shots.
periphery of the dot, emission was not observed because the periphery of the organic layer was thick. Thickness of the emission layer can be controlled by overprinting shots. Figures 5 and 6 show luminance and optical micrographs of emission patterns as a function of the number of shots. In the device on which three dots were overprinted, the maximum luminance was obtained because of the optimum thickness of the organic layer. However, by increasing shots to five, the ink was driven away and the thinner organic material remained at the center. Figure 7 also shows a cross-sectional view of the AFM data. By increasing the number of ink-jet shots, thinner and thicker organic layers at the center and edge, respectively, were confirmed. Figures 8 and 9 show a comparison of J–V and L–J characteristics, respectively. The device structure was glass/ AlNd (50 nm)/hole-blocking layer (BAlq or BCP) (50 nm)/ emission layer/-NPD (50 nm)/MoO3 (30 nm)/IZO. The
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(a) 1 shot
Luminance L (cd/m2 )
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(b) 3 shots
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BAlq BCP
102 101 100
10-1 100 101 102 103 Current density J (mA/cm2 )
Fig. 9. Luminance vs voltage characteristics of self-aligned TE-OLED.
(c) 4 shots
(d) 5 shots
Height (µm)
Fig. 6. Optical micrograph of emission pattern as function of number of shots.
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Fig. 10. Emission pattern of self-aligned TE-OLED with 300 ppi resolution.
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Fig. 7. Cross-sectional view of atomic force microscope data as function of number of shots.
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BAlq BCP
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Fig. 8. Current density vs voltage characteristics of self-aligned TEOLED.
maximum luminance obtained was 1,000 cd/m2 . From J–V characteristics, lower-voltage operation was confirmed using BAlq as HBL. Electron injection from the cathode to BAlq occurred efficiently. However, at higher current densities, brightness was saturated. This phenomenon was caused by partial dissolution of the electron injection layer into ink-jet ink. On the other hand, for L–J characteristics, brighter emission was obtained with BCP than with HBL at the same current densities. It is assumed that BCP exhibits better holeblocking ability. By combining these results, stacking layer
of the cathode/BAlq/BCP is expected to better characteristics at lower-voltage operation and brighter emission. Using this fabrication technique, we have demonstrated a prototype display panel. The panel size was 30 30 mm2 , and its resolution was 300 ppi. Figure 10 shows an example of a pattern emission, the Toyama Universitys insignia, at a luminance of 100 cd/m2 . Clear emission of the pattern was achieved. Therefore, a fine emission pattern of 300 ppi without bank formation and short-circuiting was demonstrated. We applied a method of fabricating a top-emission organic light emitting diode with an ink-jet printed selfaligned emission zone. The maximum luminance obtained was 1,000 cd/m2 . Applying this technique, we demonstrated a printed display panel 30 30 mm2 with a resolution of 300 ppi. We thank Kobelco Research Institute Inc. for supplying the AlNd alloy.
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