ITRI Developing World Leading Flexible Display Technologies
Display Industry Background and Trends
Taiwan's TFT-LCD industry in 2007 became number one in the world in production value of Flat Panel Displays (FPDs). According to Dr. Janglin (John) Chen, General Director of ITRI's Display Technology Center (DTC), the domestic TFT-LCD industry is relatively weak in both upstream (materials/equipment) and downstream (branding) compared to its main rivals, Korea and Japan. About 70% of the cost of TFT-LCD is in the equipment and materials, so because Taiwanese companies were latecomers, they have mainly played a follower role, focusing on high volume, low margin production.
In 2008 Taiwan's FPD industry production value actually declined by 0.6%, according to ITRI's Industrial Economics & Knowledge Center (IEK), due to the economic slowdown and the weakening of competitors' currencies. However, while most companies have cut production, in general they have continued new technology R&D in order to lay a foundation for growth once the economy picks up again. According to Dr. Chen, "Flexible display is coming in three to five years. Taiwan companies had better get ahead of the technology curve if they want to survive."
Several broader trends favor flexible displays, perhaps foremost being the demand for power saving devices with low environmental impact from production to use. Current FPDs – mainly TFT-LCDs on glass – have complex structure with many components, which not only raises production costs but also reduces light- and energy-efficiency. LCD TVs are huge home power consumers, while for laptops they are the main reason for short battery life. Another potential benefit of E-books would be to reduce the environmental and other costs associated with the harvesting, production, distribution, and disposal of paper.
Flexible displays have other advantages such as durability – LCDs on glass substrate are not only thicker and heavier than displays on plastic but also relatively easier to break. Perhaps the greatest advantage is that flexible displays can be manufactured by Roll-to-Roll process. As opposed to Sheet-to-sheet (Batch) processing, which produces ICs one sheet at a time, Roll-to-roll production lines resemble old-fashioned reel-to-reel movie projectors. Roll-to-roll allows for fast and low-cost production, and also removes limits on display length, making it ideal for producing large-scale displays. Today's biggest displays on glass substrates are less than 3 meters, because of the difficulty of working with larger glass sheets. In addition, a rollable display can be bigger than the device itself – i.e., it can scroll up inside a cell phone or PDA and be easily unrolled for a larger display.
The range of applications for flexible displays is sometimes divided into low-information content and high-information content. The former category includes easily updatable shelf labels in stores; reusable, rewritable guest badges for visitors; smart cards, labels and tickets, signs large and small, as well as information displays (such as airport timetables). These simpler applications can provide the foothold to allow further technology development, improve performance and lower costs so that computer/notebook monitors, TVs, digital camera and digital picture frames, and other high-information applications can enter the market.
A major challenge for achieving flexible displays is producing the electronics on plastic, which generally cannot handle high processing temperatures and may suffer thermal expansion. Another big issue is that the technologies that flexible display could replace – LCDs, or paper – are quite good as far as resolution, contrast, viewing angle, and so on. For flexible display to take off, it must at least match the competition on image quality.
There are dozens of technological approaches to flexible display, each of which has its strengths and weaknesses for particular applications. ITRI does basic research (such as materials and equipment) for most technical areas, while specializing in a few which, based on its strengths and on market needs, the greatest impact can be achieved. Current R&D is focused in two main areas: energy saving applications and mobile devices.
Energy-saving Bistable Ch-LCD for E-paper Applications
Cholesteric LCD (Ch-LCD) is a reflective type display, meaning that like a book, it is easy to read in bright light but less readable in dim light. Ch-LCD has high brightness, high contrast, wide viewing angle, and minimal glare. Because it is reflective it does not need a TFT driver or a backlight, reducing thickness and substrate cost. Its biggest advantage is on power consumption. Because Ch-LCD is Bistable, it only uses power when the image changes, so that on average it uses about 1/50 the power of transmissive LCD. This makes it very suitable for E-books and other portable devices, large outdoor signs, retail and supermarket shelf labels, ID badges, and more.
ITRI is working on several different types of Ch-LCD, but broadly speaking they can be divided into single color (monochrome) and full color. For monochrome Ch-LCD, ITRI has already developed the world's largest area bistable Ch-LCD, a 24 cm by 300 cm display with reflectance over 30% and resolution of 30dpi (dots per inch). Moreover, this display can be manufactured Roll-to-roll, which means faster, lower-cost production, and also removes limits on display size. The display width of 24 cm is limited by the production equipment, but there is theoretically no upper limit on display length.
According to Dr. Tim Tsai, Deputy General Director of DTC, in order for Ch-LCD to be the future of E-paper, current resolution of 30dpi must continue its march toward 300dpi. ITRI has already begun to establish a strong patent position in key Ch-LCD technologies, from materials and production processes to display design and applications.
World Leading Color Ch-LCD
Most current E-books use black/white/gray displays, but many labs are hard at work on color Ch-LCD. According to Frank Shiu, a Deputy Division Director in DTC, most color Ch-LCDs being developed around the world use three layers (Red-Green-Blue, or RGB), but ITRI is the only one that can achieve color Ch-LCD in a single layer. ITRI's single layer color Ch-LCD is made by dividing the single layer into three vertical sub-pixels, then using Pixelized Vacuum Filling or Ink-Jet Printing to put all 3 colors in each pixel. Thus the colors are all in a single layer and do not mix. The advantages of this single layer structure include no pixel alignment problem, reliable production, low cost driver system, increased flexibility, and it does not need color filter, raising light reflection efficiency.
In 2008 ITRI has developed a 10.4 inch QVGA single layer color Ch-LCD that features thickness reduced by 60% compared to traditional 3-layer designs, resolution of 40 pixel-per-inch, and reflectivity of 30%, which already meets the global standard for 3-layer color Ch-LCD reflectivity of 25-30%. According to Dr. Shiu, ITRI's goal for 2009 is to get the resolution up to 100 ppi (current notebooks are 70-100ppi). If these color flexible screens can get up to notebook standards, then E-books can really become a killer application that will drive further technology evolution.
Rollable AMOLED for Mobile Devices
AMOLED (Active Matrix Organic Light-Emitting Diode) is an emissive display technology, or one in which illumination comes from within. AMOLEDs have the advantage that, unlike TFT-LCDs, they are self-emitting and thus do not require a backlight. Because they also have higher brightness and visibility, are energy-saving, and have fast reaction time, AMOLEDs are already widely produced on glass substrates for use in cell phones and other mobile devices. In addition, because OLED has a simpler structure compared to LCD, it is lighter, thinner, and more suitable for flexible substrates.
According to Dr. C.C. Lee, a Division Director in DTC responsible for AMOLED development, "Taiwan has some world-class display producers that are already making AMOLED on glass, so shifting to AMOLED on flexible substrates is building strength on strength. It is a big opportunity." To reduce the barrier of entry costs for domestic TFT-LCD makers to produce AMOLED on flexible substrate, ITRI worked across departments to develop Separation Technology for Polyimide (PI) Plastic Substrates. This technology utilizes a De-Bonding Layer to successfully detach the PI Plastic Substrate from glass panels without jeopardizing delicate devices such as electronic transistors and circuits (for more detail please see ITRI Today #55, p.10). This will allow panel manufacturers to produce transistor arrays for flexible panels without having to procure new equipment or change their current processes.
ITRI has also developed a world-leading rollable 4.1" single color AMOLED prototype with thickness less than 0.2 cm, bending radius of 1.5 cm or less, brightness 100 nits, and resolution 320 x 240. The production process is relatively simple, and in durability tests so far, even after rolling 1000 times the display quality is unaffected. ITRI plans to continue improving durability and display quality, working towards the goal of full color rollable displays of LCD quality – or better.
Electro-wetting display (EWD) is an emerging technology that produces images by applying a voltage to manipulate the movement of colored oil film. EWD has simple structure and does not need a backlight, liquid crystal or color filter to produce color images. The materials used in EWD – water and a harmless oil dye – are not only environmentally friendly but also very low cost. Another advantage of EWD is its fast response time (color-change time can reach 10ms), which makes it suitable for moving images. The fact that it uses water also opens up the possibility of a transparent display.
While current EWD technology development worldwide is around 0.8 to 2.5 inches, ITRI has already produced a 6" EWD display with transmission rate up to 75%, using self-developed Ink-jet printing technologies to dose red, green and blue oils into the unit pixels individually. It has a simple structure and can integrate existing manufacturing processes for glass substrate, and use existing color dyes, so production costs are relatively low, allowing local manufacturers to get into mass production quickly. ITRI has also developed complete alignment packaging technology for large area substrate production. It should be noted that ITRI's prototype EWD is on a glass substrate and therefore not flexible. The goal for next stage is to produce EWD color display that can do moving images, and a flexible EWD, to provide another breakthrough technology for industry.