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1、Reliability and mechanical durability tests of exible OLED with ALD coatingYasuhiro Jimbo (SID Member)Yuki Tamatsukuri Minato ItoKohei Yokoyama (SID Member)Yoshiharu HirakataShunpei Yamazaki (SID Member)Abstract To improve the reliability and mechanical durability of a exible organic light-emitting

2、di- ode display, the entire exible display is coated with an aluminum oxide lm by atomic layer deposition (ALD). Because the step coverage of ALD is excellent, the AlOx lm was deposited not only on the front and back surfaces but also on the side surfaces of the display. A high-temperature and high-

3、humidity preservation test, repetitive bending tests, and a pencil hardness test were conducted on the exible dis- play with ALD-AlOx coating. The display survived 500 h of a 65°C, 95% preservation test, endured a 100,000-time repetitive bending test with a curvature radius of 4 mm, and was fou

4、nd to have a pencil hardness of 4H.Keywords flexible OLED, high-temperature test, high-humidity test, passivation, atomic layer deposi- tion, crystalline-OS, CAAC.DOI # 10.1002/jsid.3771IntroductionBecause this method allowssivation layer to beformed on a glass substrate, the temperature at which th

5、e passivation layer is formed is not limited by the heatCompared with liquid crystal displays, active matrix organic light-emitting diode (AMOLED) displays are superior in contrast, viewing angle, moving-image response speed, and so on, have been attracting attention, and are being actively develope

6、d.1,2 Organic light-emitting diode (OLED) displays have the additional advantage of offering a stereoscopic effect without using binocular parallax. We refer to such a feature as natural 3D®.3 Ease of fabrication of exible displays is an added feature of AMOLEDs. We have previously reported the

7、 synthesis of extremely high-resolution exible AMOLED displays employing a top-emission or bottom-emission white OLED and a color lter.48of an OLED. Thus, passivation layers with high-barrier prop- erties could be obtained. The WVTR of our passivation layerwas as low as 7 × 10 6 g/m2 day at 38&

8、#176;C and 90% (relativehumidity) RH.11Passivation layers with high-barrier properties can prevent the entry of moisture from the front or back surface of exible OLED displays. However, moisture entering from the sides of the display causes growth of a non-emission area at the periphery.Because the

9、latest mobile devices need to be provided with a touch panel, tolerance for surface scratches is required for exible displays. A human ngernail is said to have a pencil hardness of 2H; accordingly, exible displays need to have a pencil hardness of at least 3H.Thus, we considered coating the entire p

10、anel with alumi- num oxide by atomic layer deposition (ALD) as a method for protecting the sides from moisture and the surface from a scratch.Atomic layer deposition can form lms with excellent step coverage and fewer pinholes; accordingly, it has been studiedChallenges to exible OLED displays arefr

11、ommoisture in the air and prevention of breakage by bending.Use of a plastic substrate rather than a glass substrate allows such displays to be exible; however, plastic transmits mois-ture. Therefore, an OLED must be coated withsivationlayer having low water vapor transmission rate (WVTR). Toobtains

12、ivation layer with a sufciently low WVTR, aninorganic lm of silicon oxide, silicon nitride, aluminum oxide,and the like, or stacking organic and inorganic lms alternately, has been considered.9 However, avoiding adverse effects on the OLED requires a lm encapsulation method that forms passivation la

13、yers on an OLED at low temperature. Therefore, the passivation layer must be thick, or multiple passivation layers must be stacked, which increases formation time and reduces productivity. However, a thick passivation layer is likely to crack when a exible OLED display is bent.as a method for forlms

14、.1214gas barrier lms on OLEDs andFor perforALD, rst, a precursor is added to thechamber and is chemisorbed on the sample surface. Generally, the precursor chemically reacts with an OH group on the sur-face. Because ligands remain on the chemisorbed precursor, only one layer of the precursor is chemi

15、sorbed on the surface. After purging the precursor out of the chamber, an oxidant such as H2O and O3 is added. The precursor adsorbed on the sample surface reacts with the oxidant. When H2O is used, an OH group is formed on the surface. For use of O3, a theoryWe have formedsivation layer on a glass

16、substrate andtransferred the layer onto a plastic lm using our transfertechnology. A exible OLED display fabricated using this method can endure 100,000-time repetitive bending tests.10Received; 07/15/15; accepted 08/30/15.The authors are with Semiconductor Energy Laboratory Co.,., Kanagawa, Japan;:

17、 yj0612sel.co.jp© Copyright 2015 Society for Information Display 1071-0922/15/2307-0377$1.00.Journal of the SID 23/7, 2015313that an OH group is formed via an OR group15 and a theory that O-radical group is formed16 have been proposed. In either case, the surface can react with a precursor.Afte

18、r the oxids purged out of the chamber, the precur-sor is added to the chamber again to be chemisorbed on the surface. In this manner, the cycle of adding the precursorand the oxid s repeated, which forms a metal oxide thinlm with a desired thickness on the sample surface (Fig. 1).Atomic layer deposi

19、tion forms a lm through repetition of surface reaction of a gas and a solid as described earlier, re- quiring no gas phase reaction. Therefore, a lm is formed uni- formly on the entire surface contacting the atmosphere in the chamber. Thus, ALD is a deposition method excellent in step coverage.We fa

20、bricated a exible OLED display using our transfer technology and formed an AlOx coating on the display by ALD. This paper reports the results of a high-temperature and high-humidity preservation test, repetitive bending tests, and a pencil hardness test conducted on the display.2Flexible AMOLED fabr

21、ication methodWe employed a white tandem OLED + top emission + color lter structure, which can easily achieve high resolutions. We fabricated a exible AMOLED using our transfer technol-ogy. TheFirst, ans are as follows.inorganic separation layer of tungsten (W) wasformed on a glass substrate; then,

22、a 1.2-m-thick passivationlayer of silicon oxide, silicon nitride, and the like, a eld-effect transistor (FET) layer, and an OLED layer were stacked in this order. The surface of W was oxidized to form about 10-nm-thick WOx layer,17 which became brittle because of baking. This leads to physical separ

23、ation of the stack with a trigger. A separation layer of W and sivation layer were formed on another glass substrate in a similar manner, and then a color lter was formed thereon.We used a c-axis-aligned a-b-plane-anchored crystal oxide semiconductor (CAAC-OS) for the FET. CAAC-OS is an oxide semico

24、nductor (OS) that we developed, which hascrystalliand a c-axis aligned perpendicular to the lm sur-face. In-Ga-Zn oxide (IGZO) can be used as one of theFIGURE 1 Proposed mechanism of ALD. ALD, atomic layer deposi- tion; TMA, trimethylaluminum.OSs.1820 The formation of a CAAC-IGZO thin lm over aFIGUR

25、E 2 Fabrication ow of a exible AMOLED using our transfer technology. AMOLED, active matrix organic light-emitting diode; FET, eld-effect transistor; OLED, organic light-emitting diode.314Jimbo et al. / Reliability of exible OLED with ALD coatingglass substrate requires annealing at less than 500

26、6;C. We succeeded in for a channel-etched OS-FET that uses a CAAC-IGZO for an active layer and has improved character- istics and reliability caused by lowering the gap level.21Figure 2 shows a method for fabricating a exible AMOLED using our transfer technology.An OLED, which is a white tandem devi

27、ce, was formed on the FET substrate by vacuum evaporation. An epoxy resin was applied on the color lter substrate, and the FET substrate and color lter substrate were bonded in nitrogen atmo- sphere. The cell gap between the two substrates was approx- imately 5 m.The W layer on the FET substrate sid

28、e was cut with a laser light or glass scriber, and the two glass substrates were pulled apart, which caused separation at the interface between the W layer and the passivation layer. Then, a plastic lm in place of the glass substrate was bonded to the passivation layer with an adhesive. The W layer

29、and glass substrate on the color lter substrate side were separated in the same manner, and a plas- tic lm was bonded. Thus, a exible display is obtained.3ALD coatingFIGURE 3 ALD coating of exible OLED display. ALD, atomic layer deposition; OLED, organic light-emitting diode; TMA, trimethylaluminum;

30、 FET, eld-effect transistor.We formed an aluminum oxide lm of 100 nm on the entire surface of a exible OLED display, that is, not only on the front and back surfaces but also on the side surfaces (Fig. 3). For the formation of the aluminum oxide lm, thermal ALD using trimethylaluminum and ozone as t

31、he materials was employed.Figure 4 shows the transmittance of an ALD-AlOx lm. The transmittance of the lm is high enough to allow the lm to be provided on the display surface.We observed the sides of the panel coated with AlOx by ALD by cross-sectional scanning transmission electron micro- scope (ST

32、EM).Figure 5 shows the STEM image and the energy dispersiveX-ray analysis mapimage. As can be seen from these im-FIGURE 4 Transmittance of ALD-AlOx (with a thickness of 100 nm). ALD, atomic layer deposition.ages, an AlOx lm is deposited on the side surface of the panel.FIGURE 5 STEM image and an EDX

33、-mapimage of the side of the panel. STEM, scan-ning transmission electron microscope; EDX, energy dispersive X-ray.Journal of the SID 23/7, 2015315The repetitive bending tests were conducted using a book- type tester as shown in Fig. 8. After being bent 100,000 times at the rate of 43 times per minu

34、te, the display was pin a 65°C, 95% RH environment for more than 24 h, and whether a defect was generated in display was observed.4High-temperatureandhigh-humiditypreservation testPreserving the exible OLED displays fabricated in the afore- mentioned manner in a high-temperature and high-humidi

35、ty environment (65°C, 95%) for 800 h, we conducted observa-tions of teration and growth of dark spots or a non-emission area at the periphery after 0, 100, 240, 500, and 800 h, having the displays emit white light from the entire dis-play surfaces. Table 1 lists the specications of the exible O

36、LED displays used for the test.Figure 6 shows the fabricated exible OLED display. As the width of the frame decreases, the length of time required for moisture entering from the sides of the display to reach the display region also decreases. In each of the samples used for this test, the narrowest

37、portion of the frame was 5.5 mm wide. Figure 7 shows photographs of exible OLED displays emitting white light before, during, and after the 65°C, 95% RH preservation test. A dark spot or a non-emission area at the periphery did not appear until 800 h at the 65°C, 95%RH preservation test. H

38、owever, luminance started to decrease after 800 h.FIGURE 7 Results of preservation test.5Mechanical durabilityRepetitive bending tests and a pencil hardness test were con- ducted on exible OLED displays fabricated in the aforemen- tioned manner.TABLE 1 Characteristics of a 3.4-inch exible OLED displ

39、ay.Screen diagonal Driving methodNumber of effective pixels Pixel pitchResolution Aperture ratio Source driver Scan driver3.4 inch Active matrix540 × RGB × 9600.078 × 0.078 mm326 ppi 44.4%Integrated IntegratedOLED, organic light-emitting diode; RGB, red, green, and blue.FIGURE 8 Repet

40、itive bending test of exible OLED display. OLED, or- ganic light-emitting diode.FIGURE 6 Distance from the exible display edge to the display region.316Jimbo et al. / Reliability of exible OLED with ALD coatingThe results are shown in Table 2. For each of outside bending (the display surface faces o

41、utward) and inside bending (the display surface faces inward) tests with a curvature radius of 5 or 4 mm, no display defect was observed after 100,000-time bending and the 65°C, 95% RH preservation test. As for the inside bending test with a curvature radius of 3 mm, cracks in the outermost ALD

42、-AlOx lm were found; however, there was no display defect. It is probably because there was no crack in the passivation layers, and therefore, the OLED did not deteriorate.A display without an ALD coating endured a 100,000-time inside bending test with a curvature radius of 2 mm.10 Be- cause the sur

43、face of a display is subjected to the highest bend- ing stress, securing a bendability of a exible display with a surface coating is a challenge for the future.The pencil hardness test was conducted using the tester shown in Fig. 9. A pencil was set on the tester at an angle of 45°. The lead of

44、 the pencil was sharpened into a column hav- ing a at tip. The tester ran on the display surface such that the pencil was pushed on the display surface with a load of750 gf. In a pencil hardness test, generally, the highest pencil grade that does not scratch the lm surface corresponds to the pencil

45、hardness of the lm. In this test, we dened the pencil hardness of the display as the highest pencil grade that does not scratch the lm surface or cause display defects. The test results are shown in Table 3. The formation of an ALD-AlOx lm on the display surface improved the pencil hardness of the d

46、isplay from 3H to 4H.6We fabricated exible OLED displays with an aluminum ox-ide coating formed by ALD. The display with the ALD-AlOxcoating survived preservation testing at 65°C, 95% RH for more than 500 h without deterioration. No display defect was observed in the display subjected to 100,00

47、0-time repeti-tive bending with a curvature radius of 4 mm. Furthermore, the ALD-AlOx coating improved the pencil hardness of the display from 3H to 4H.TABLE 2 Results of repetitive bending tests.Curvature radius (mm)Outside bendingInside bendingReferences1 M. Noda et al., SID Digest, 43, 998 (2012)

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49、ma et al., AMFPD2013, 223 (2013).9 P. E. Burrows, Displays, 22, No. 2, 6569 (2001).10 R. Komatsu et al., SID Digest, 45, 326 (2014).11 Y. Jimbo et al., SID Digest, 45, 322 (2014).12 J. G. Lee et al., SID Digest, 44, 358 (2013).13 J. A. Bertrand et al., 54th Annual Technical Conference Proceedings, C

50、hicago, IL April 1621, (2011).14 P. F. Carcia et al., J. Appl. Phys., 106, 023533 (2009).15 D. N. Goldstein et al., J. Phys. Chem. C, 112, 1953019539 (2008).16 X. Liu et al., J. Electrochem. Soc., 152, No. 3, G213G219 (2005).17 S. Idojiri et al., SID Digest, 46, 8 (2015).18 N. Kimizuka and T. Mohri,

51、 J. Solid State Chem., 60, 382 (1985).19 M. Nakamura et al., J. Solid State Chem., 93, 298 (1991).20 N. Kimizuka et al., J. Solid State Chem., 116, 170 (1995).21 M. Tsubuku et al., SID Digest, 44, 166 (2013).543OK* OK* OK*OK* OK* Cracks*No defect or crack was observed;*cracks in AlOx were observed,

52、but there was no display defect.FIGURE 9 Pencil hardness test of exible OLED display. OLED, or- ganic light-emitting diode.Yasuhiro Jimbo received his BE and ME degrees in Nagoya University, Japan, in 2001 and 2003, re- spectively. After graduation, he joined Semicon- ductor Energy Laboratory and has been engaged in R&D of OLED.TABLE 3 Results of pencil hardness tests.With ALDWithout ALD4H3HALD, atomic layer deposition.Journal of the SID 23/7, 2015317Yuki Tamatsukuri received his BE degrees in Tokyo University of Science, Japan, in 2010. After gradu- ation, he join