The world of ‘pervasive computing’ will put a new emphasis on the importance of displays. Jon Excell looks at what we’ll be looking at in the not too distant future.
Wave goodbye to the grey plastic box, because in ten years time we will own dozens or even hundreds of computers. Sewn into our clothes and embedded in domestic appliances these computers will communicate with each other via Bluetooth (or something similar) in a seamless, integrated environment.
In this new world of ‘pervasive computing’ (a term coined by Hewlett-Packard) the user interface (the display and input devices) will be king. Hence, a drive to produce smaller, lighter, cheaper and more environmentally friendly displays.
Organic electroluminescence (EL), a phenomenon first observed in the 1960’s is one of the most likely candidates.
OLEDs (organic light emitting diodes), are flat panel displays in which electric current is converted to luminescence in structures made from organic thin film materials. The emitted light can be red, green, blue or a combination of all three in a full-color, high-resolution image display, an effect which requires patterning of the organic material in the manufacturing process (to get all colors one must have red, green, and blue subpixels at each pixel of the display).
Advantages over conventional LCD technology include lower power consumption, faster response time, higher brightness levels, wide viewing angle and thinner design. Furthermore, as an OLED screen is self-emitting; it doesn’t require backlighting. Hence its low weight, slim profile, and minimal power requirements.
Organic EL is obtained by placing a charge-transporting and light-emitting organic material between two electrodes (one of which is transparent) and applying a suitable bias. The organic material may be either a polymer, deposited by various solution processing techniques, or low molecular weight molecules, deposited by evaporation or sublimation in a vacuum. Total device thickness (excluding the substrate) is usually less than 1 micron.
Displays can even be prepared on flexible, transparent substrates such as plastic, leading to the possibility that we will be able to take computers into the ‘throne room’, download the news onto a flexible screen, and while away the minutes in time-honoured British fashion.Using technology developed by Eastman Kodak company, Pioneer Electronics was first to introduce organic EL displays into its products, launching in 1998 a 64 x 256 pixel carbon-based organic EL monochrome display for automobiles. Several other companies, notably Philips, Seiko-Epson and Sanyo are close behind.
The best OLEDs are achieving in excess of 20 lumens/watt (for green), although many devices are well below, and lifetime of materials depends on the brightness at which the display is operated, in a few cases this has exceeded 50,000 hours.
Kodak pioneered OLED technology in the late 1980s, and its development for practical applications has generated more than 50 OLED patents. For example, the company has patented a technique for enhancing the electroluminescent efficiency and control of colour output by ‘doping’ the emissive layer with a small amount of highly fluorescent molecules. As well as Pioneer’s application, Kodak’s OLED technology can now also be found in full-colour displays on mobile phones manufactured by Motorola and Sanyo .
Kodak estimates that the global market for OEL panels will exceed $200 million in 2002.
Shine on, you crazy polymer
Snapping at Kodak’s heels is a technology which, rather than carbon-based materials, uses conducting plastics known as Light Emitting Polymers
We spoke to Craig Cruickshank, Market Development Manager for Cambridge Display Technology (CDT), the UK company that’s leading research and development in this area.CDT’s display structure 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.
When a thin-film of conducting polymer is placed between two electrodes connected to a battery, the plastic layer emits light. Electric charges are injected from the electrodes and recombination of these charges is what produces the luminescence. The bandgap, or the energy difference between the valence band and conduction band of the semiconducting polymer, determines the colour (wavelength) of the emitted light.
With one of its partners, Seiko Epson, the company recently unveiled a full colour LEP display using red, green and blue polymer materials.
The display, which achieves colour quality equal to current LCD technology, measures 2.5 inch square has a resolution of 200 by 150 pixels, with 16 grey scaling at system level and will be targeted at initial market entry points for LEP displays products such as mobile phones and personal digital assistants (PDAs).
Phillips Components has also joined the race to bring LEP products to market, with the establishment of a manufacturing facility in Holland. It is a race that the electronics giant may well win, ‘we believe Philips will be earliest to market – very soon, that’s all I can say’ comments Cruickshank.
While CDT’s technology may not be in the same commercial position as Kodak’s, the company is confident that LEPs will be the preferred flat screen technology.
The same advantages as Kodak’s technology: wide-viewing angle, no backlighting required, low power requirements, small dimensions, and so on, are backed up, CDT believes, by a far simpler manufacturing process.
Small-molecule OLEDs are made of several layers of thin organic films, requiring vapour sublimation in a vacuum chamber rather than the simple ink-jet printing Process devised by Seiko-Epson
According to Dr James Sheats of silicon valley’s Rolltronics; ‘The physics of light emission in these devices is similar but significant differences arise when one contemplates manufacturing.’
Using a variation on the Ink-Jet printing process, Seiko Epson has figured out how to print small screens quickly and cheaply.
Ink cartridges are loaded with liquid versions of the red, green and blue polymers and a fourth, conductive polymer. The printer spits tiny droplets of the four polymers onto a screen backing.
As with other OLEDs, lifetime is an issue and, says Cruickshank, work is being done in this area. However, he adds that the current lifetime of prototype materials is around 60,000 to 80,000 hours, and for most current applications this will probably be enough.
With flat panel display sales predicted to amount to $20.8 billion by 2002, will LEPs have a strangle hold on displays of the future? Cruickshank takes a meaured view; ‘there are lots of technologies that are different and better for other applications. We won’t take control of the world: there’s an awful lot of investment in current production, process and manufacturing lines that people will be using for a long time yet.’
Seeing carrots in the dark
Years ago, scientists at Digital’s Semiconductor Engineering Group plugged a pickle into the wall and were delighted to discover that it glowed.
Soon after this discovery, researcher’s at Digital’s Western Research Laboratories published a technical report detailing their own experiences with a variety of pickles. Following the pickle experiment, the researchers hooked up a piece of horse radish kimchi which gave off an unpleasant smell and glowed yellow.
Then, one of the team noticed the oscilloscope trace. The lower trace showed the voltage across the kimchi. It was running at about 140 V RMS. Then he saw the upper trace. The upper trace showed current pulses of about 4 amperes in amplitude. The Kimchi was acting as a rectifier. The kimchi only conducted when the voltage across its terminals was greater than 90V. Further, it only conducted in one direction. (Pickles did not produce such an effect.)
Unfortunately, lack of funding forced the team to move on to other projects, since it was not clear at the time that this light emitting vegetable diode technology could be of use in the construction of high performance information processing devices.