Tech Guide: Organic LEDs: The future of displays

A decade after the effect was first discovered, organic light emitting diodes (OLEDs) have finally made it into commercial use. Three recent significant announcements illustrate the range of applications of this new technology: Kodak has included an OLED display in a new camera, Samsung in a wristwatch GPRS mobile phone, and Taiwanese company IDTech--a joint operation between IBM Japan and Chi Mei Optoelectronics--has announced a 20 inch monitor. These announcements are significant because OLEDs have some potential advantages over other display options that could make them the technology of choice in the next two to six years.

Like the very familiar light emitting diodes in use today, OLEDs are small lumps of material that glow when a voltage is applied. Again like ordinary LEDs, they produce light of various colours, don't make much waste heat, and can be made very small. However, while traditional LEDs are made using semiconducting elements such as silicon, gallium and so on, with normal semiconductor production techniques, OLEDs are made from plastic compounds originally investigated for making amplifiers or switches. The light emitting effect was discovered almost by accident. Because plastics are much easier to work with in production, OLEDs have the potential to be used in many more ways than other displays.

There are two main classes of OLED, small molecule and polymer. Small molecule OLEDs are built up by depositing molecules of the compound onto the display itself under very low pressures, analogous to the way layers of silicon circuits are applied. Polymer OLEDs have the active molecules suspended in a liquid like pigments in paint, and can be printed onto displays using ink jets, screen printing or any of the various contact techniques used for ordinary inks. While small molecule OLED displays are limited in their size by the vacuum chambers used to make them and have the same form as most conventional displays, polymer OLEDs can be huge--Canon has talked about 500 inch displays or greater--printed onto flexible substrates and created very quickly. Resolution approaching 300 dpi is also possible, approaching the quality of ink on paper.

However, both approaches suffer from some of OLED's weaknesses. The compounds degrade quickly on contact with oxygen or water, making the production process tricky and requiring very good encapsulation technology. Also, the compounds degrade over time, limiting the maximum life of a display, while different colours degrade at different rates making the colour balance change.

None of this has dissuaded companies from investing heavily in the technology, because the upside of OLEDs is significant. As well as having the potential for many different forms of display, the technology can be much brighter, thinner and more efficient than any other type of flat screen. The overwhelming majority of flat monitors and laptop displays these days use thin film transistor liquid crystal devices, TFTLCD, which are made from arrays of pixels comprised of three switched elements letting light through red, green and blue filters. These need a very bright light behind them, as even when switched on each filter blocks around two thirds of the light passing through--and the backlight is still on even when the screen is black.

OLED displays have the same array of pixels with three elements for red, green and blue, but because each element generates its own glow at the right frequency, no light is ever made that doesn't contribute to the image. This leads to massive efficiency savings--the IDTech screen claims to use 25 watts to create 300 candela per square metre brightness. Equivalent LCDs use three times as much power. Because OLEDs don't need the backlight or nearly as many layers as TFTLCDs, they can be made much thinner, and because they don't use polarised light filters they have a much wider viewing angle. They also have a much wider working temperature range.

Despite the appearance of commercial products, these are very early days for OLEDs. Expect them first as backlights for LCDs, and in small consumer products where thinness and power consumption are at a premium, while short lifetimes aren't so important. Mobile phones rarely last more than eighteen months in use, for example. To some extent, the same considerations apply to laptops, especially now that low-power processors are leaving the screen as the major drain on battery life. The more esoteric applications--household lighting, TV screens on cornflake packets and glowing clothes--are still some way away, but few researchers in the field think that they won't happen in ten to twenty years. The future's bright--the future's OLED.