OLED Panels: The Way to Larger Screens

 

he market for organic LED panels continues to grow, for small panels in mobile phones and similar devices, as well as in lighting. The trend is accelerating as the possibilities for large OLED panels open up, and a new market for 20-inch and larger OLED TVs may emerge in 2010 or beyond. Development of technologies for larger screens at lower cost is surging ahead. This article probes the shape of the next generation of flat screen TVs. The OLED panel boasts phenomenal performance, including displays that surpass those of CRT, superlative blacks, and ultra-thin bodies only a few millimeters thick. They have been called the ultimate display panel for flat screen TVs. Sony Corp. became the first in the world to volume-produce an 11-inch OLED TV in October 2007, but the technology never took off. At the time, television manufacturers like Toshiba Corp. and Samsung Electronics Co., Ltd. announced plans to volume-produce OLED TVs, but as of the end of 2009, at least, neither company shows any signs of shipping.

LG Electronics Aggressively Pursuing Larger Screens

Development in large-size OLED panels has seemed stagnant for some time, but all of a sudden it has picked up speed again. One of the most active firms is the LG Group: LG Electronics, Inc. announced a 15-inch OLED TV in August 2009, releasing it to the Korean market in December of the same year for three million won. The OLED panel was manufactured by LG Display Co., Ltd., another group firm, and delivers a peak brightness of 450cd/m2, a contrast ratio of at least 100,000:1 and a color reproducibility range of 98% of the NTSC standard. The TFT drive device uses poly-silicon crystallized in a high-temperature process known as solid-phase crystallization (SPC). Each of the red, green, and blue OLED-emitting films is created by vacuum-depositing material via a shadow mask. A cavity structure (multiple reflection interference) is used to expand the range of color reproducibility. LG Display is already working on a small OLED panel for mobile phones. The company announced plans to ramp up a new OLED panel manufacturing line in the first quarter of 2010, and has continues to boost production scale. In June 2009 it entered into a tie-up with Idemitsu Kosan Co. Ltd., securing OLED material supplies, and in December 2009 it announced the acquisition of the OLED business of OLED panel pioneer Eastman Kodak Co.: LG Display is clearly strengthening its R&D capabilities. Building on this foundation, LG Display announced it would volume-produce 20-inch class large-size OLED panels in 2010, 30-inch class in 2011, and 40-inch class in 2012. Vice President Won Kim, in charge of OLED Sales & Marketing at the firm, is confident: “They may be expensive, but it will be possible to buy a 40-inch class OLED TV in 2012.”

 Healthy Growth in Small Panel Market

Part of the reason behind the LG Group’s eagerness is strong growth in small OLED panels. OLED panels are becoming the display of choice in many portable electronic products, including mobile phones, smartphones, media players, and digital cameras (Fig. 1). For mobile products like these that handle imagery, according to Hiroshi Hayase, Director of DisplaySearch in Japan, “The excellent display performance of OLED panels is something that can be immediately appreciated by the user.” More and more manufacturers are entering the market, such as Casio Computer Co., Ltd. and Toppan Printing Co., Ltd. Toppan announced the establishment of a new company to volume-produce small OLED panels for digital cameras and similar applications in November 2009.
Fig. 1 OLEDs Gradually Spread in the Market
OLEDs are achieving full-scale adoption in several markets, primarily mobile phones and smartphones. They are having a tough time penetrating the home electronics market, but larger displays are expected in 2010 or beyond. A forecast of OLED shipment value broken down by application type. Mobile phone (and smartphone) main displays will be the primary growth driver through 2011, but TVs are expected to show gradual growth from 2011. Diagram by Nikkei Electronics based on material courtesy of DisplaySearch. Display market research firm DisplaySearch forecasts that total OLED panel shipment value in 2009 will reach about US$845 million (Fig. 2). Of this, says the firm, mobile phone main screen displays will account for about US$521 million or about 61% of the total. Main screen display shipment value is expected to rise at least five times by 2016, hitting about US$2,820 million. As volume production rises, manufacturing yield at industry leader Samsung Mobile Display Co., Ltd. (SMD) is improving, reaching 60% for VGA (640 pixels × 480 pixels), and 80% for quarter VGA (QVGA; 320 pixels × 240 pixels) screens, according to DisplaySearch’s Hayase. It seems clear that the firm is steadily accumulating expertise in volume production.

Another Push from Lighting Applications

The emergence of the OLED lighting market is adding more wind to the sails. OLED panels offer a variety of characteristics including surface emission, transparency, thinness, and light weight, opening up potential in a host of markets that existing light sources (fluorescent, incandescent, white LED) can’t touch. And that is luring a host of new companies into the market. Manufacturers are ramping up volume production of OLED lighting. In Japan, Lumiotec Inc. plans to begin volume production in January 2010, while Konica Minolta Holdings, Inc. announced in November 2009 that it would invest 3.5 billion yen into a prototype line. Overseas, Royal Philips Electronics NV, General Electric Co., and others are planning volume production. DisplaySearch believes that the OLED lighting market will begin to take off in 2010, growing to about US$2.838 billion in 2016. Expanding production levels for small OLED panels and increasing adoption of OLED lighting will no doubt further accelerate the development of practical large-size OLED panels. They have a number of points in common, including materials and device architecture, not to mention that expertise is being accumulated now on how to boost manufacturing yield in volume production.

Tapping into the Power of the “Eco Boom” ?

Characteristics such as saving energy and ecologically sound design emerged as major dimensions for competition in flat screen TVs at the start of 2009, and this trend as well is contributing to the early commercialization of large OLED panels. In January 2009, for example, Sony announced the BRAVIA VE5 series of LCD TVs, trumpeting a 40% reduction in energy consumption from the prior model. In September 2009 Sharp Corp. released the LED AQUOS LX series with white LED backlights, emphasizing image quality and low energy consumption. Considering rising interest in the environment, it is likely this trend will continue. One Japanese LCD engineer explains that one reason OLED panels are so hot is that “energy consumption in LCD panels has been cut about as low as it can go, due to limitations imposed by the structure itself.” LCD panels use voltage to control the liquid crystal polymer, blocking the backlight light or allowing it to shine through, and making tonal display possible. The LCD has a complex structure, consisting of TFT, liquid crystal layer, color filters, two polarizer sheets, and two glass sheets, just to name the larger components, and each one increases total backlight light loss. The LCD engineer quoted above adds, “only about 5% of the backlight light actually passes through an LCD panel, and it is incredibly difficult to increase that number.” OLED panels are self-emitting devices, and have low loss because of their principle of operation. Many engineers working with them agree that a material delivering higher light emission efficiency would make it easier to reduce OLED panel power consumption, and that OLEDs will at least match the low power consumption of LCD panels, if not beat it. Recognizing this situation, manufacturers are once again developing large OLED panels for televisions. In addition to the above-mentioned LG Display, Sony revealed in its business overview in November 2009 that it will continue to invest into developing its own displays, including OLEDs. Samsung Electronics, the largest TV manufacturer in the world, has not disclosed plans to volume-produce large OLED panels, but subsidiary SMD is the largest company in manufacturing small OLED panels, and essentially monopolizes the active matrix OLED panel manufacturing field. In short, it probably has more knowledge about manufacturing OLED panels than anyone in the industry. Several analysts familiar with the TV field predict Samsung Electronics will modify its existing fifth-generation LCD panel line to make OLED panels for TVs.

Simple Upsizing Not the Solution

There are a number of problems that will have to be resolved before large OLED panels can be made, though, one of which is establishing volume-production technology capable of producing large panels cheaply. The lack of such a technology is the major reason why manufacturers worldwide have never shown more than prototypes when it comes to OLED panels or TVs of over 20 inches (Fig. 3). The TFTs, film growth process, etc., used in small and medium OLED panels cannot be used directly in manufacturing large OLED panels, although (as mentioned below) a few ways around the problem are emerging.
Fig. 3 Prototypes Only for 20-Inch and Larger OLED TVs
TV, panel and other manufacturers have shown a variety of prototypes for 20-inch and larger TVs at society meetings, exhibitions, etc., but thus far the only volume-production models on the market are the 11-inch from Sony and the 15-inch from LG Electronics.The biggest obstacle to large OLED panels is the rapid improvement in the performance of competing LCD panels, along with dropping cost and a few other factors. An engineer in the OLED field explains, “That’s why it is so difficult for OLED panels to demonstrate superiority over LCD TVs with their strong points alone: image quality, thinness, etc.”  Since 2007, when the push to develop large screens and high resolution (1920 pixels × 1080 pixels) tapered off, LCD panels have made significant progress in image quality, thinness, and other characteristics. Conventional cold-cathode fluorescent lamp (CCFL) backlights are being replaced by LEDs, bringing display performance up to par with that of OLED panels. For example, most LCD TVs with direct-illumination LED backlights offer a contrast ratio of one million-to-one and color reproducibility of 100% or better of NTSC. This performance beats the 15-inch OLED TV from LG Electronics mentioned above. Using an edge light LED backlight, the thinnest part is no more than 20mm thick. While OLED TVs could offer even better specs (image quality, thinness, etc.), it is difficult to make the difference significant to potential buyers. This is why it seems likely large OLED panels will, for now, aim at applications where they can avoid competing with LCD panels. At FPD International  in October 2009, a number of such panel prototypes were exhibited (Fig. 4). LG Display and SMD jointly developed a “transparent display” that can be seen through. Transparent materials were used for both cathode and anode, and light is emitted from both sides of the panel. A staffer at LG Display explained it was intended for public applications such as digital signage.
Fig. 4 LG Display and SMD Pioneering New Applications Just for OLEDs
LG Display and SMD are developing bendable OLED panels, transparent designs, and more. Such applications are based on the unique advantages of OLED panels and are difficult to handle with LCD panels. LG Display also showed a display designed for medical applications, with high readability and an excellent contrast ratio. SMD had a smart card display, only 50µm thick and bendable. Volume production dates were not announced for either display, but considering the unique advantages of OLED panels, they seem almost certain to be commercialized.

Steady Development of Technologies for Larger Displays at Lower Cost

Technology development for larger OLED panels is also forging ahead. According to Takatoshi Tsujimura, Senior Director, OLED Systems at Kodak Japan, Ltd., panel manufacturers are “not only pursuing display performance, but also selecting technologies that are expected to achieve the highest manufacturing yield in volume production.”The keys to larger displays are TFTs that can be used to drive OLEDs and can handle the larger display area, and film growth technology capable of forming organic electroluminescent light-emitting layers at low cost and over large areas.
Fig. 5 Two Major Technological Obstacles to Larger Size, Lower Cost
There are a number of technical obstacles to achieving larger OLED panel size at lower cost. In particular, TFT materials that can be used with larger glass substrates and OLED device multi-layer process technologies will have to be developed. LG Display, already aiming to volume-produce a 40-inch display in 2012, has provided some indices for OLED panel manufacturing, relative to LCD panels of the same size. The company’s Kim explains, “We plan to use fifth- and sixth-generation glass substrate in 2012, yielding two to four 40-inch panel sheets each. That should mean material costs hit 150%, with a yield of about 70%. In 2016 we’ll probably use 10th-generation substrates, taking 18 40-inch sheets apiece, just like LCD panels, dropping our material costs to 70% or 80% for about the same yield.”

Wide Variety of TFT Candidates

Let’s take a closer look at possible ways to improve TFTs. In order to achieve reduced cost, TFTs will have to be manufactured on glass substrates the same size as the liquid crystal panels used for TVs. It is difficult to use the amorphous silicon TFTs of large LCD panels, or the low-temperature poly-silicon (LTPS) of small and medium LCD panels without some changes, though. The manufacturing process for large OLED panels is therefore being reviewed, with the most promising solutions including polycrystalline silicon TFTs made without laser annealing, and amorphous oxide semiconductor TFTs. There are two reasons why amorphous silicon TFTs cannot be used―first, carrier mobility is only about 1cm2/Vs, making it impossible to attain satisfactory brightness. The second is that threshold voltage drifts over time, causing display variations. LTPS TFTs, on the other hand, have a high carrier mobility of about 100cm2/Vs, and a threshold voltage drift only about one-tenth that of amorphous silicon TFTs. This is why SMD uses them in the small and medium OLED panels now in volume production, and in its 31-inch prototype. The problem with LTPS TFTs is the difficulty in making larger glass substrates. After the amorphous silicon film is formed, it must be crystallized using laser annealing, and that can increase the variation in transistor characteristics. In LCD panels, third-generation glass is the largest used. This situation has forced panel manufacturers to move ahead with the development of new TFT materials and manufacturing processes. Concretely, they are improving the silicon TFT manufacturing process and adopting amorphous oxide semiconductors, among other things.

Sixth-Generation Glass with Silicon TFTs

The manufacturing process is being improved by finding a way to crystallize amorphous silicon without using laser annealing. For example, the 15-inch volume-production design from LG Display, introduced above, uses a special process called solid-phase crystallization (SPC). Amorphous silicon is heated to about 700°C to create polycrystalline silicon. Carrier mobility is about 20cm2/Vs, and the threshold voltage drift is about the same as LTPS. Heat treatment does present a problem in the form of glass substrate shrinkage, but LG Display’s Kim reveals they can already handle up to sixth-generation glass. This means that the firm has established volume-production technology up through about 30-inch displays. The problem now is finding a way to use eighth-generation substrates: Kim states the only way will be to develop new manufacturing equipment. SMD, meanwhile, is developing a polycrystalline silicon TFT called super-grain silicon (SGS). A trace amount of nickel is coated onto the amorphous silicon substrate to serve as nuclei for crystal growth, and then polycrystalline silicon is formed at elevated temperatures. The company used this SGS process in a 40-inch prototype first disclosed in October 2008.

Repeatability Issues in Oxide Semiconductors

Of the amorphous semiconductors, In-Ga-Zn-O (IGZO) is thought to be the most promising TFT material for large OLEDs. IGZO TFT carrier mobility is about 10cm2/Vs, and threshold voltage drift about equal. It can be manufactured by sputtering, which is attractive because it means no major changes would be needed to make it on existing LCD panel lines. In the future, it could well be made through a coating process rather than sputtering, which would lower costs even more. Manufacturers in Korea and Taiwan are especially interested in developing oxide semiconductor TFTs, and a number of firms showed prototype OLED panels, LCD panels, and other items at FPD International. The largest screen was a 19-inch design from SMD. LG Display and AU Optronics Corp. have disclosed that they are using amorphous IGZO. The Samsung Group has not revealed what type of oxide semiconductors it is using, but has announced a number of IGZO TFT prototypes in the past and is likely to still be using IGZO. The biggest problem with oxide semiconductors is the poor repeatability of the manufacturing process. LG Display’s Kim, however, points out, “We expect the situation to improve a bit when we use heat treatment after film deposition.”

White OLEDs, Front and Center

Another key to larger displays is the deposition process used for light-emitting devices. In general, there are two possibilities here: coating phosphors independently emitting red, green, and blue (RGB), or using white material with three (again, RGB) color filters.In the small and medium OLED panels in volume production now, low-molecular phosphors are formed by vacuum deposition using a shadow mask. However, it is extremely difficult to ensure sub-pixel alignment with shadow masks. As a result, most people in the industry feel the approach cannot be used on substrates larger than fourth-generation. In addition, material utilization efficiency is low, raising costs. Developers are now eyeing a combination of white light emitters and color filters as a way to easily obtain larger panels. Eastman Kodak Co. has been using this approach for some time now, and LG Display is also making prototypes as it prepares to acquire the firm. In general, color filters make it unnecessary to coat phosphors, so it is much easier to increase substrate size. The only way to expand the color reproducibility range, however, is by increasing the thickness of each of the RGB layers. Color filters increase the amount of light absorbed, stealing some of the panel brightness that is such an advantage for OLEDs. The only way to ensure brightness on a par with coated designs is to boost the brightness of the white light emitted, which means higher power consumption and shorter service life. There is a trade-off between a wider color reproduction range and lower power consumption. The 8.1-inch prototype produced by Eastman Kodak claims to have found a way around this problem while achieving a wider color reproducibility range: 100% of NTSC. Power consumption is, according to Kodak Japan’s Tsujimura, “no more than 2W for the 8.1-inch display, using the latest white emitters. An LCD panel of the same size would consume about 2W to 4W, making this quite competitive.” The device is a bottom emission design, with light emitted from the TFT side, so the color filters must be formed above the TFT substrate. These performance improvements are due to two major reasons. The first is that the emission efficiency of the white material has been improved dramatically, supported by development for lighting applications. The material used by Eastman Kodak has a current conversion efficiency of about 50cd/A, which Kodak Japan’s Tsujimura says represents an annual rise of 50% since 2007. The other reason is the use of the firm’s proprietary W-RGBW sub-pixel array, coupled with a modified drive scheme. Compared to the conventional design using three color filters (RGB), it delivers superior color reproducibility at lower power.
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