Screens of Plastic Could Be Fantastic

Last week, Lee Dye of the Los Angeles Times reported on a research project by Ghassan Jabbour, an assistant research professor in the optical sciences department at the University of Arizona in Tucson, whose team is developing thin computer display screens so flexible that they can be folded and tucked away in your pocket.

Prof. Jabbour of the Center for Advanced Multifunctional Nonlinear Optical Polymers and Molecular Assemblies (CAMP) is working on technology that would enable the fabrication of display screens 1,000 times thinner than a human hair using organic light-emitting diodes that can be printed on a sheet of plastic.

While these printed plastic screens would be similar in some ways to the familiar LCD screens like the one I'm typing these words on right now, they also will have some significant differences. For one thing, they emit their own light and will be usable in in any ambient lighting from pitch dark to direct sunlight. They will also be viewable from virtually any angle and should be cheaper to manufacture.

Jabbour's technology uses organic (carbon-based) diodes that emit light in different colors when stimulated by electric current. Three diodes - one green, one blue, and one red - make each screen pixel.

Lee Dye reports that Prof. Jabbour and his team of colleagues at the University of Arizona, directed by Professor N. Peyghambarian, whose research is partially funded by the Department of Defense, have adapted the technology of screen printing using a frame, a fabric, design stencil, ink, and a squeegee to deposit the carbon-based molecules that make up the diodes directly on a thin film of plastic.

Prof Jabbour and his team are not the only ones working on flexible printed plastic screen technology. Similar plastic screen technology is also being developed by Britain's Cambridge Display Technology (CDT). CDT claims that it, along with Japanese partner Seiko Epson, has successfully made plastic display screens just by printing them on an ink-jet printer.

CDT calls the plastic screens light-emitting polymers (LEPs), a type of plastic that can be charged to change color speck by speck. LEPs are bright and sturdy. They need little power, no backlight, can be read at almost any angle, and will be cheap to manufacture, costing only an estimated 60 percent as much as LCDs to produce.

Production is projected to begin by 2002, concentrating first on the mobile phone-sized screens, but the company is hoping to eventually wipe out the demand for LCD technology.

CDT, which is partly owned by Cambridge University, hopes that LEP screens can eventually be made from soft plastics that will allow them to be rolled up. They also hope to replace the CRT.

The prototype colour display was made using CDT's red, green, and blue polymer materials and the industry first ink-jet printing process developed for the project. The developers claim that the LEP color display achieves color quality equal to current liquid crystal display (LCD) technology and is comparable to displays found in many notebook computers.

LEPs do not require the inefficient colour filter required for conventional LCDs since color is generated directly on the front face using phosphors. Contrast, brightness, and colour are the same from all angles of view.

Because LEP technology eliminates the viewing angle dependence of conventional LCDs, benefits include:

• more lines addressable (higher level of multiplexability)
• higher contrast
• less critical operating margins
• reduced temperature sensitivity
• larger displays possible

In 1998, CDT and Seiko-Epson demonstrated a monochrome plastic television display based on LEP technology and announced their intention to develop a colour LEP display.

CDT's prototype colour display measures 2.5 inch square, has a resolution of 200 by 150 pixels, with 16 grey shades at system level. It will be targeted at initial market entry points for LEP displays products such as mobile phones and PDAs. Beyond this, CDT and Seiko-Epson expect this technology eventually to penetrate all other display markets.

David E. Mentley, Vice-President, Stanford Resources, Inc. has published a paper on "The Market Potential for Organic Light-Emitting Diode Displays," which is available for download here, along with several other papers on LEP development.

Mr. Mentley notes that "OLED (Organic Light Emitting Diode) technology has some very attractive features that are likely to give it a prominent place in the flat panel display market of the future. Driving its future success is the continuing and accelerating development of both manufacturing process technology and new materials systems. OLED technology brings a new set of attributes to the display world and display pioneers have long sought many of these characteristics."

Based on a breakthrough manufacturing technique which uses ink-jet printing to deposit individual pixels made up of the red, green, and blue LEP materials directly onto the substrate, potential display size is limited only by the size of the available wafer with no impact on the overall throughput when deployed in existing manufacturing lines.

"The pre-production color light emitting polymer display being shown by CDT and Seiko-Epson has a color density similar to current LCDs. The techniques being jointly developed by the two companies means that the manufacturing cost of an LEP display will be significantly less than the cost of producing conventional LCD or cathode ray tube displays," said Dr. Shimoda, general manager of basic research with Seiko-Epson.

"CDT is very excited about the market opportunity for a full color light emitting polymer display. The market needs a video-capable display for emerging mobile applications that is light-emissive, color, and low-power. CDT and Seiko-Epson are working towards the commercial availability of a product with precisely these characteristics. Over time we expect this technology eventually to penetrate all other display markets," comments Dr Daniel McCaughan, president and chief operating officer (COO), CDT.

The ink-jet technique allows the LEP material to be printed from a liquid solution and brings advantages when compared to existing and emerging display manufacturing processes by eliminating the need for backlights, colour filters and polarisers used in LCD displays, and complex multi-shadowing techniques for depositing small molecules.

The structure of the display consists of two polymer layers, a conducting polymer layer covering all the pixels and a light emitting colour polymer layer where each pixel consists of a third of each of the red, green, and blue colour LEP materials.

Based on a poly-silicon active matrix driver (whatever that is) the color display uses a digital drive scheme based on time and area variables addressing each pixel separately. This is in contrast to the Thin Film Transistor (TFT) analogue drive system of the previous monochrome display, which was more susceptible to the variance of the TFT threshold voltage.

CDT is a world leader in research and commercial development of Light Emitting Polymers (LEPs) and expects that LEP technology will be used in areas such as mobile communications, computers, consumer electronics, and, ultimately, as an alternative to the cathode ray tube (CRT) - the display in conventional televisions and computer monitors. Moving into the 21st century, the company predicts that LEPs will become the basis for new products such as virtual reality headsets and flexible or formable displays.

Based in Cambridge, UK, CDT was founded by Cambridge University and a seed venture capitalist in 1992 and has subsequently been through a number of investment rounds. In July 1999 the company moved premises in order to support a rapidly growing number of staff and to provide new chemistry facilities.

CDT was founded after initial work at the Cavendish Laboratory, led by Richard Friend and at the Melville Laboratory led by Andrew Holmes, discovered that Light Emitting Diodes (LEDs) could be made from polymers as well as from traditional semiconductors.

The team found that the polymer poly p-phenylenevinylene (PPV), emitted yellow-green light when sandwiched between a pair of electrodes. Initially this proved to be of little practical value, as it produced an efficiency of less than 0.01%. However, by changing the chemical composition of the polymer and the structure of the device, an efficiency of 5% was achieved, bringing it well into the range of conventional LEDs.

In the past year, CDT researchers have synthesised polymers which emit light in the red, green, and blue regions of the visible spectrum at low voltage and high efficiency. Work is underway to develop driving schemes which would allow these to be used to construct full colour graphic displays without the need for a complex active array of electronic switches. Research and development is currently focused on extending lifetime and reliability of the displays, developing more efficient light emitting structures, designing manufacturable processes, and devising effective drive schemes for graphic displays.

The research team realised the enormous potential of this discovery, and, after taking out key patents on the polymer technology, they established CDT to commercially exploit their findings. These patents, including a fundamental one on light emission from conjugated polymers, have now started to grant in the USA and Europe, and are in process in the rest of the world.

CDT's exploitation route for LEP technology is through licensing and technology transfer via corporate partnerships and joint developments. The company recognises that, although it is currently at the forefront of LEP research and development, it needs to leverage developed manufacturing and marketing skills which are also essential to be a world class display manufacturer. Through licensing patents and performing technology transfer, CDT aims to bring display manufacturers up to the state of the art in light emitting polymers as quickly as possible. The graphics display market worldwide was valued at over $30 billion in 1999.

CDT has already announced commercial agreements with Philips, Uniax, Hoechst, DuPont, Seiko Epson, Delta Electronics, and Agilent Laboratories to date, with more under development.

LEP displays are constructed by applying a thin film of light emitting polymer onto a glass or plastic substrate coated with a transparent, indium tin oxide electrode. An aluminium electrode is sputtered or evaporated on top of the polymer. Application of an electric field between the two electrodes results in emission of light from the polymer.

The LEP display has a number of very attractive features. The response time is fast (sub-microsecond), switching occurs at low voltage (5V), and the intensity of light is proportional to current. If the electrodes are patterned, for example in orthogonal X and Y lines, light will be emitted from the area at the intersection of these lines.

The technology therefore combines the low voltage DC benefits of traditional LEDs with large area patternability associated with non-emissive display technologies such as LCDs.

Unlike liquid crystal or plasma displays, which require thin film processing on two glass plates, LEPs can be fabricated on one sheet of glass or plastic, which greatly simplifies processing and reduces cost. Additionally, the ability to manufacture devices on flexible plastic substrates introduces new form factor opportunities and, for example, allows displays which conform to unique shapes to be produced.

For more information, on LEP display technology visit:

• http://www.cdtltd.co.uk
• http://www.dta.ihost.com/newsletter/jan_focus.html
• http://www.covion.com/NY_pres/ny-slides/sld005.htm
• http://www.cheng.cam.ac.uk/groups/polymer/RMP/RPLEP.html
• http://www.jamac.demon.co.uk/FutureWatch/AdvancedDisplays.htm