The Technological Trends of Future AMOLED

Invited Paper

The Technological Trends of Future AMOLED Jong hyuk Lee*, Hye Dong Kim, Chang Ho Lee, Hyun-Joong Chung, Sung Chul Kim, and Sang Soo Kim Technology Center, Samsung Mobile Display Co., LTD San#24 Nongseo-Dong, Kiheung-Gu, Yongin-Si, Gyeonggi-Do 446-711, Korea ABSTRACT The authors review the technological trends for the future AMOLED, especially for unique applications to small- and medium-sized displays as well as large-sized AMOLED TV. The unique characteristics of AMOLED enable paper-thin, foldable, bendable and transparent displays which the other display technology can’t easily realize. For large-sized AMOLED TV, TFT backplane, color patterning and encapsulation are the key technological issues and the new technologies should be developed for the mass production of AMOLED TV. The issues and some candidate technologies which can pave the way for mass production of AMOLED TV are also briefly reviewed. Keywords: AMOLED, AMOLED TV, Back Plane, Color Patterning, Encapsulation, OLED

1. INTRODUCTION Since the first commercial product of OLED for car audio display by Pioneer, the evolution of OLED technology has been accelerated with the increasing demands for better image quality and novel applications. Indeed, AMOLED has been rapidly expanding its market share for small-sized mobile applications since the launch of mass production in 2007. The vivid color image, low power consumption and novel design pushed industries dive into the market of AMOLED for mobile phones and the other potable displays. In a mass production point of view, small-sized AMOLED almost attained a stage of technological maturity. However, it still needs some more improvements in power consumption, life time, image sticking and so on. In order to meet those stringent requirements, new materials with high efficiency and optimization of OLED device structure is necessary. Apart from applications to mobile phones, AMOLED can open the new area of applications that the other display devices can’t easily realize. The unique characteristics of AMOLED enable paper-thin, foldable, bendable and transparent displays. Since the AMOLED is self-emitting, light emission can be controlled for each pixel at extremely high speed. Therefore, it is intrinsically possible for AMOLED to express high contrast, blur-less motion features, vivid colors, and wide viewing angle. Samsung already exhibited 31” [1] and 40” [2] AMOLED TV prototypes in the conferences and exhibitions and many people sympathize that AMOLED could be an ultimate solution for future TV. In this article, the technological trends for the AMOLED are reviewed, especially for the unique applications for smalland medium-sized displays, excluding normal applications to mobile phones or other portable displays. And the authors also report the technological issues of large-area AMOLED TV, including TFT backplane, color patterning and encapsulation technologies.

2. THE UNIQUE APPLICATIONS OF AMOLED

Organic Light Emitting Materials and Devices XIII, edited by Franky So, Chihaya Adachi, Proc. of SPIE Vol. 7415, 74150O · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.829021

Proc. of SPIE Vol. 7415 74150O-1

2.1 Paper-thin, foldable, flexible AMOLED For future display technologies, there is significant interest in providing display devices with mechanical flexibility and transparency. And also, there are continuous requirements for ultra-thin display and extendable or foldable display without seam line between the two panels. Different from LCD which, in principle, needs two glass panels, AMOLED needs not upper glass panel if the organic materials and metal cathode layer in OLED device are protected from environmental water vapors or other gases. The single glass device gives many unique characteristics to AMOLED. Owing to these characteristics, paper-thin and bendable card display or foldable display can be realized, as is shown in figure 1.

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Fig. 1. (a) 6.5” flexible and (b) foldable AMOLED display by Samsung.

A number of technology developments are pointing towards flexible display surfaces that can be rolled, like paper. AMOLED rather than the other displays is considered as optimum solution for that purpose. Many different types of applications are envisaged such as roll-up displays incorporated into mobile phones, or handheld navigation systems that provide larger information screens to be carried on the move, and there may also be a considerable market in smart cards and ticketing. Paper-thin, foldable, flexible even flapping AMOLEDs can’t easily made by normal way of encapsulation method which seals two glass panels; instead, thin film encapsulation technology should be employed that protects the organic device but leaves it thin and flexible. Thin film encapsulation is very powerful solution for obtaining unique characteristics of AMOLED[3]. Instead of using upper encapsulation glass, TFE employs layer-by-layer deposition of thick films with compensating diffusion barrier properties. The biggest merit of TFE is that it enables single glass display, which makes extremely slim and flexible panels possible. The challenges for TFE include material optimization, minimization of stacking layers, and applicability for large size mother glasses. Figure 2 shows a new and promising application of electronic passport which AMOLED is included in the passport and credit card, Electronic passport, which Bundesdruckerei and Samsung co-developed, has been displayed in the Cebit 09 and drew much attention by the public. 2.2 Transparent AMOLED If you've seen the movie "Minority Report" and amazed with the transparent display controlled by Tom Cruise, you can expect to see transparent OLED pixels sooner or later. Transparent OLED is divided into two types of both-direction emitting type and see-though type. Both direction emitting type of transparent OLEDs have only transparent components (substrate, cathode and anode) and, when turned off, are up to 85 percent as transparent as their substrate and electrode. When a transparent OLED display is turned on, it allows light to pass in both directions. Most approach for this type of transparent OLED is to use transparent TFTs (thin-film transistors) made of a 100-nanometer-thick layer of zinc-tinoxide, which transmits more than 90 percent of visible light.


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(b) Fig. 2. (a) Electronic passport and (b) credit card display

A see-through type of transparent OLED display can be either active or passive-matrix. See-through type of transparent OLED composes of small area emitting part and see-through part like window. Top-emitting OLEDs region have a substrate that is either opaque or reflective. They are best suited to active-matrix design. In this type, the TFTs and the OLED pixels are positioned next to each other. The OLED pixel can be placed on top of the TFT driver circuit without interference. Samsung showcased the "Window Display," an OLED panel with a transparent of 30%. Samsung used four 12.1-inch Window Displays to make a "window". The resolution of the panel is 840×504, and its luminance is 200cd/m2. The color reproduction range is 100% of the NTSC standard. The response time is 0.01ms. Samsung 12.1 inch qFHD transparent display is shown in figure 3.

Fig. 3. Samsung 12.1 inch qFHD transparent display at SID 2009.

In most transparent OLED, transparent ITO anode and semi-transparent metal cathode such as Mg:Ag are used with good hole and electron injection properties. However, resistance and transparence of Mg:Ag semi-transparent metal cathode is not enough to be applied to the Transparent OLEDs. Recently, many efforts have been made to use indium tin oxide (ITO) and zinc oxide (ZnO) doped with impurities as transparent cathode by sputter deposition method [4-7]. However, it has already reported that the high sputtering power and their high work function led to failures of transparent


OLEDs [8]. It is sure that novel transparent cathode material with low resistance, high transparency and no drawbacks on the device stability should be developed for the better transparent display with enough transparency and image quality.

3. TECHNOLOGICAL ISSUES FOR LARGE-SIZED AMOLED The basic and the most important feature of TV is the ability to reproduce real image. With the launch of high-definition digital broadcasting, spectators can now feel the vivid presence from the large flat-panel TV screen. Therefore, the perceptual image quality becomes more important than the simple measures such as contrast, luminance, and color gamut. Since the AMOLED is self-emitting, light emission can be controlled for each pixel at extremely high speed. Therefore, it is intrinsically possible for AMOLED to express high contrast, blur-less motion features, vivid colors, and wide viewing angle. In that sense, many people sympathize that AMOLED could be an ultimate solution for future TV. Figure 4 shows world largest 40” AMOLED TV by Samsung. However, there should be some technological progresses for the mass production, although the prototypes of AMOLED TVs have been displayed in many conferences and exhibitions. In the following sections, the key technological issues of TFT back plane, color patterning, encapsulation and materials for the large-sized AMOLED will be discussed.

Fig. 4. World largest 40” AMOLED TV by Samsung.

3.1 Technological issue of backplane Low temperature poly-Si (LTPS) TFTs fabricated by ELA is currently employed in the mass production of AMOLEDs owing to their excellent TFT performance and device stability. For large area applications, however, uniformity and scalability issues challenge its application. Moreover, ELA-based LTPS TFT requires large number of masks (8~11) compared to that of LCD (4). Amorphous oxide TFTs can be an attractive solution to the scaling up issue. Basically, oxide TFTs can combine the merits of a-Si and LTPS TFTs [9]; good uniformity, large carrier mobility (~10 cm2/V.sec), excellent subthreshold gate swing (down to 0.20 V/dec) and simple sputtering process at low temperature. However, device instability problem should be solved to use oxide TFTs for AMOLEDs. Therefore, proper passivation material and production process are required to oxide TFT fabrication. Figure 5 shows oxide TFT-based AMOLED display for note PC.


Fig. 5. Samsung 12.1” WXGA oxide TFT-based AMOLED display for note PC.

3.2 Technological issue of OLED patterning Shadow mask technology, also known as fine metal mask (FMM), is currently employed in the mass production of AMOLEDs. However, the FMM are prone to confront sagging problems when applied to large-size mother glass because the masks are made by too thin metal films (50μm thick) to sustain large area. In addition, FMM has other issues such as pixel size variation by ±10μm, shadow effect by the metal thickness, and alignment accuracy between the mask and substrate. Therefore, it is considered that direct printing methods, such as ink-jet and nozzle printing, are the most effective for large-size AMOLED because they exploit the complete use of OLED materials. In comparison with evaporation-based materials, however, soluble materials for AMOLED have a serious disadvantage – short lifetime. Therefore, development of good soluble OLED materials with good stability and surface uniformity is the biggest challenge for printing techniques. 3.3 Technological issue of encapsulation For small-sized AMOLED devices, edge sealing encapsulation with inorganic frit material which is locally heated with laser is enough to fabricate reliable panels. However, for large devices, edge sealing with the frit has serious problems such as mechanical strength under external stress. In order to prevent those drawbacks, new techniques such as filling the gap between two glasses are currently under development. The challenges for these techniques include the development of liquid filler material and filler-injecting technology. And also thin film encapsulation (TFE) can also be another interesting solution which enables single glass display.

4. CONCLUSION In this article, the authors have shown the unique advantages of AMOLED and technological issues for large-sized AMOLED. The unique characteristics of AMOLED can open the new area of applications for paper-thin, foldable, bendable and transparent displays. For the large-sized AMOLED TV, new technologies and materials for TFT backplane, color patterning and encapsulation should be developed for the mass production in the near future.


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