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[Cover Story] Market Growth Hoped for Organic EL, E-Paper Displays

Displays employing totally new operating methods are appearing on the flat panel display market, and prototypes are attracting considerable attention in the engineering community.

New types of flat panel display (FPD) are being pioneered, namely organic electroluminscence (EL) and electronic paper (E-paper) displays. The organic EL is expected to achieve a market for full-color panels in 2002, followed by monochromatic E-paper in 2003 and color E-paper in 2004.

The markets aren't expected to become large-scale until about 2005 for organic EL, and 2010 for E-paper, according to many liquid crystal display (LCD) engineers. This is due to the many technical issues that remain to be resolved; however, there may be major breakthroughs at any time by engineers and researchers around the globe (Fig 1, Fig 2a, Fig 2b).

Superior Features

Organic EL and E-paper displays offer characteristics not found in LCDs. Compared to transmissive-type LCDs using backlights, organic EL displays are said to be superior in terms of thinness, lightness, power consumption, response speed and viewing angle. While the drive voltage is fairly high (about 100V), an inorganic EL display offers better color purity together with all of the fore-mentioned benefits, with the exception of low power consumption. Currently, inorganic displays are only available under a partner license from iFire Technology, Inc of Canada, and are generally expected to have little industrial impact.

According to one manager at a firm involved with both LCD and EL panels, "The mobile market, like cell phones and PDAs [personal digital assistants], is not where the real potential lies for the organic EL display." Organic EL just can't compete with the reflective LCD panels currently used in mobile equipment when it comes to low-power operation or easy viewing in bright environments. Reflective LCDs do not need backlights, which means they are thin and their power consumption is low. For the organic EL display, the major targets are expected to be applications for which transmissive-type LCDs aren't suitable: ultra-thin, ultra-low power notebook personal computers (PC), and monitors, televisions and automotive displays with an ultra-high speed response and ultra-wide viewing angle.

The other newcomer, E-paper, is considered to be the ultimate in reflective displays. It offers display characteristics close to those of paper, and reduced power consumption through a memory effect - neither of which can be implemented in existing reflective LCD panels. The technology is expected to be used first in public bulletin boards and advertising, and later in PDAs and electronic books (E-books).

Mass Interest

The high expectations of organic EL and E-paper displays are evident at society meetings and exhibitions. At the symposium and exhibition of the Society for Information Displays (SID) 2001, held in the US in June, these two display technologies attracted the most attention, and there was standing room only at the sessions on these technologies.

Two of the designs that generated the most interest were a 260,000-color, 100 pixels per inch (ppi) polymer organic EL display, jointly developed by Epson Corp of Japan and Cambridge Display Technology (CDT) of the UK, and an 8-inch (20cm) super video graphics array (SVGA) organic EL panel announced unexpectedly by Samsung SDI Co, Ltd, at a session on low-temperature poly-Si thin film transistor (TFT) displays. The crowds swelled rapidly as people tried to get a view of the displays, and engineers bombarded authors with questions.

At the exhibition, organic EL and E-paper were in the spotlight; the E-paper exhibition by E Ink Corp of the US received a great deal of attention. The firm showed the first prototype panel for portable equipment to utilize the electrophoresis phenomenon. In the field of organic EL, Sony Corp showed a 13-inch (33cm) SVGA panel, Toshiba Corp revealed a 2.85-inch (7.2cm) polymer panel, and Tohoku Pioneer Corp of Japan displayed a 3-inch(7.6cm) panel for PDAs. All were the first of their kind, and aroused considerable interest.

Blue Color, Inkjet

There has been phenomenal development in the area of materials and manufacturing techniques used for organic EL (Fig 3).

Blue low-molecular material with excellent color purity now has a longer service life than ever. As a result, organic EL panels can be used in applications demanding 65,536 colors, such as cell phone screens, for which LCDs are currently used. Idemitsu Kosan Co, Ltd has announced a new blue material that combines long life and high color purity. While conventional materials only achieve a brightness of about 100cd/m2 with a 5,000- to 6,000-hour life, a newly-developed dopant and hole injection layer double the brightness to 200cd/m2, and increase the service life to 10,000 hours.

The color purity of the blue material is defined as x=0.146, y=0.166 by the Commission Internationale de I'ツlairage (CIE) color coordinate standard, and can support full-color images with 16.77 million colors. Idemitsu has apparently been developing the material in secret. Its performance has already been confirmed, and it would seem that technical issues have already been resolved to achieve a blue organic EL capable of handling full color.

As far as manufacturing is concerned, the industry is finally recognizing that the inkjet method is viable. Many engineers decided it was a "proven" technology after seeing the 260,000-color polymer organic EL panels made with inkjet technology by Epson and Toshiba. Until now, there have been no prototype color panels made with inkjet printing; there have therefore been certain reservations about inkjet technology, but it has now become clear that it does indeed work.

Prototype Panels

The prototype E-paper panel, exhibited by E Ink, offers a 2- to 4-bit gray scale, a reflectivity of 40%, a contrast of 10:1, and a response time of 150ms, with a service life of 10,000 to 20,000 hours.

E Ink has also developed a 12.1-inch (31cm) SVGA TFT panel jointly with the TJ Watson Research Center of IBM Corp. This panel only supports a monochromatic display, but the resolution is a high 83ppi, which is on a par with LCDs. It also handles a 16-level gray scale, and has a response time of 350ms; because of its memory effect, the power consumption is, in principal, less than that of a reflective LCD.

E Ink also indicated that a higher definition of up to 200ppi could possibly be achieved, which would match the quality of printing, and provide a color display with sufficient flexibility for bending. E Ink's electrophoresis displays express black and white by applying an electric field to the upper and lower plates filled with microcapsules and liquid. Each microcapsule contains electrically-charged black and white pigment chips with, for example, a positive charge for white chips and a negative charge for black chips.

The highest resolution is thought to be limited by the size of the microcapsules, but E Ink showed photographs of the electrophoresis operation on 200-ppi equivalent-sized electrodes to demonstrate the potential for achieving a 200ppi resolution.

Announcements have also been made regarding flexibility, including details about stainless steel substrates and electrophoresis displays combining TFT organic-semiconductors. The TFT organic-semiconductor displays were jointly developed with Bell Laboratories of Lucent Technologies of the US. Color implementation will be through the use of color filters. E Ink also exhibited a prototype under development with Toppan Printing Co, Ltd of Japan (Fig 4).

Obstacles

While much progress has been made with organic EL and E-paper displays, there are still several issues that need to be resolved.

The biggest problem with organic EL is that the materials - whether polymer or low-molecular- deteriorate over time. The pixels are likely to deteriorate if powered on for a long period, causing "scarring" to the screen. This problem must be solved before organic EL panels can be used in ultra-thin, ultra-low power notebook PCs, or in televisions combining a fast response with a wide viewing angle. Circuit technology cannot compensate for the deterioration, which means that new materials must be developed with significantly longer life spans. One of the main problems is with blue polymer, which has a service life of only about 2,500 hours.

Cost is also an issue. Widespread adoption of large-size organic EL panels will require manufacturing costs to be lowered, using low-temperature poly-Si TFT substrates, down to at least the level of amorphous Si TFT LCD panels. This can only be accomplished by improving the production yield of low-temperature poly-Si TFT substrates for organic EL applications. Unfortunately, TFT substrates for organic EL tend to have considerably more transistor characteristic variations than in LCDs; as a result, a number of breakthroughs are needed to sufficiently boost the yield.

With E-paper, one problem lies in the trade-off between response time and drive voltage. The panel has to provide a high-speed response in order to scan through pages quickly in an E-book, but the higher drive voltage would cause driver integrated circuit (IC) costs to soar. E-paper also has poor impact resistance.

Seiichi Iino, senior general manager of the Display Development Center at Seiko Epson's Fujimi Plant, explained that entering markets where these problems are not critical will be "of prime importance in new technologies like organic EL and E-paper." At the same time, manufacturers will continue to push ahead with R&D; and production to overcome these problems. Unless the targets are defined very carefully, though, it is possible that these promising new technologies could just end up as passing fads.

by Naoki Tanaka

  • Websites:
    Bell Laboratories: http://www.belllabs.com
    Cambridge Display Technology: http://www.cdtltd.co.uk
    E Ink: http://www.eink.com
    Idemitsu Kosan: http://www.idemitsu.co.jp
    iFire Technology: http://www.ifire.com
    Lucent Technologies: http://www.lucent.com
    Samsung SDI: http://www.samsungsdi.com
    Seiko Epson Corp: http://www.epson.co.jp/e
    Sony Corp: http://www.sony.co.jp/en
    Tohoku Pioneer Corp: http://www.pioneer.co.jp
    Toppan Printing Co: http://www.toppan.co.jp/english
    Toshiba Corp: http://www.toshiba.co.jp

    (September 2001 Issue, Nikkei Electronics Asia)

  • Copyright (c) 1996-2001 Nikkei Business Publications, Inc. All Rights Reserved.