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Flexible Screens: Next Up Front
by Arthur Fryatt

Since the earliest days of television technology, the traditional means of image display has been the glass cathode-ray tube. To a much more limited extent, liquid crystal and plasma displays have been developed but even these require thin film processing on two glass plates.

This is a rather costly process and the finished product still retains the fragility of glass. Now, a British company Cambridge Display Technology - backed by a nubber of investors and information technology companies, is leading research and commercial development into light-emitting polymers (LEPs).

Significant Changes
These flat, light-emitting devices should significantly change a wide range of consumer electronic products in the future. The new technology is expected to be used in areas such as mobile communications, computers, consumer electronics and, ultimately, as an alternative to the cathode-ray tube as the means of display in conventional televisions and computer monitors. Moving into the next century, LEPs are likely to become the basis for improving products such as virtual-reality headsets.

The company was founded in 1 992 by Cambridge University with the help of seed-venture investment and has 16 employees, 13 of whom are dedicated to research. Cambridge Display Technology was founded after initial work at Cambridge University's Cavendish Laboratory led by Richard Friend, Cavendish Professor of Physics, and Andrew Holmes. In 1989 they discovered that light-emitting diodes (LEDs) could be made from polymers instead of the traditional semi-conductors.

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%.

But by changing the chemical composition of the polymer and the structure of the device, an efficiency of 2.5% was achieved, bringing it well into the range of conventional LEDs. It was realised that there was considerable potential in this discovery and after taking out key patents on the polymer technology, the team established the company to exploit their findings commercially.

Wide Funding Sources
Funding came mainly from Cambridge University but additional support has come from diverse investors including the British rock music group, Genesis, and Herman Hauser, a founding director of Acorn Computers, now an Olivetti company.

LEP displays are constructed by applying a thin film of the light-emitting polymer on to 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 the emission of light from the polymer.

The effect of the LEP display has a number of attractive features. The response time is fast (sub-micro second), switching occurs at low voltage (less than 5V) and the intensity of light is proportional to the 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.

High Information Displays
The technology therefore combines the low-voltage DC (direct current) benefits of traditional LEDs with large area patternability associated with non-emissive display technologies such as liquid crystal display. This provides a powerful technology base for building high information content displays.

Unlike liquid crystal or plasma displays - which require thin film processing on two glass plates - light-emitting polymers can be fabricated on one sheet of plastic or glass. This greatly simplifies processing and reduces cost.

Additionally, the ability to make devices on flexible plastic substrates introduces new screen form opportunities and, for example, allows displays to be produced that conform to unique shapes. The richness of organic chemistry allows the fabrication of new materials with different emission properties, including a wide range of colours. The benefits of this are evident in how quickly light emission from LEPs has covered the visible spectrum when compared with traditional semi-conductor LEDs which took some 20 years.

In the past year, the research team at Cambridge Display Technology has synthesised polymers that emit light in the red, green and blue regions of the visible spectrum. Work is underway to develop driving techniques that would allow these to be used to construct full-colour graphic displays without the need for a complex active array of electronic switches.

Graphic Displays
The company is moving the technology ahead on a number of fronts. Research and development is focused on extending the lifetime and reliability of the system; developing more efficient light-emitting structures; designing manufacturable processes, as well as devising the effective drive schemes for graphic displays previously mentioned. The markets accessible to LEPs will increase over time as the technology matures. However, the early target markets (comprising backlights, seven-segment and alpha-numeric displays) are currently estimated at $2.5 billion per year.

The characteristic of light-emitting polymer products will also enable concepts and designs in information display that are not available with current technologies. Composite products (combining backlights, seven-segment, alpha-numeric and dot matrix displays) are examples of this. The longer-term objective is to enter graphics-display markets, currently valued at $20 billion per year.

Market Chances
Because the company is focused on this opportunity alone, it has established and believes it can maintain a technological lead over its competition. As part of its marketing programme, the company has identified a series of market opportunities for LEPs as the technology matures.

The first product will use the technology's current performance for an intermittent-use backlight, the first shipment of which is planned for 1997. Later products will be sevensegment and alpha-numeric displays as well as dot matrix and video displays. In all cases, there are significant market opportunities where LEP products will have benefits over the existing technology solutions.

Cambridge Display Technology will market, design and manufacture leading-edge light-emitting polymer products in these focus markets under its own name. The company will establish a pilot manufacturing line and, working with an independent manufacturer, will transfer production to a full-scale line.

For more information contact:
Cambridge Display Technology Ltd
181a Huntingdon Road, Cambridge, United Kingdom, CB3 ODJ
Tel: +44 1223 276351 / Fax: +44 1223 276402      Copyright ©1999, LLC