Organic LEDs go white with success

Tom Shelley reports on the latest development in advanced materials for displays and lighting



British-developed light emitting materials originally revealed in Eureka in 2001 have found their first major customer for use in full colour displays - and are being developed into high efficiency sources of white light.

Now that organic light emitting diodes (OLEDs) have gone mainstream, Enfield company ELAM-T has raised more funds, changed its name to OLED-T and found its first major customer.

OLED materials, unlike light emitting polymers (LEPs), use relatively small molecules based on complexes of rare earth and other metals. A number of major companies have concluded that OLEDs are the way forward for colour displays because they emit light, so are more efficient than light-absorbing LCDs. They can also be made very thin, have a good viewing angle, and now have acceptable lifetimes.

OLED-T was founded by its chief technology officer, Professor Poopathy Kathirgamanathan, in 1999, who says the basic idea is fairly simple.

"The trick is to get electrons to fall into holes in the right places," he says.

Holes, he explains, are places where structures are one electron short. Achieving this, however, has proved to be a major challenge. The present technology is covered by 66 filed patents - of which 38 have been published and seven granted.

While Professor Kathir and chief executive Myrddin Jones will not reveal the identity of the major customer, it is no secret to say that its manufacturing business is in the Far East, that it currently makes large numbers of LCD displays, and that it intends to go into full production of the OLED displays 6 to 9 months from now.

OLED-T has just raised $7 million (£3.8m) venture capital funding led by E-Synergy. Aberdeen Asset Management, Foresight Venture Partners and Gartmore Investment Management have also invested as part of the funding round.

The company is still working to improve lifetimes, increase efficiency and reduce voltage requirements down to 3V. State of the art is presently a 160,000 hour lifetime for red and green and 10,000 hours for blue. There is also continuing development of device structures to improved efficiency.

"It's not just the materials, but how you put them together," says Prof Kathir.

The team must establish optimum process parameters for the display manufacturers. Materials are already being sold, but not enough to yet enable the business to break even. The chemicals are made in the UK under sub-contract agreement.

OLEDs made by other companies are already in use in displays for mobile phones, MP3 players and some car dashboards. Compared to LCDs, they offer a number of advantages. They are half the thickness and 70 per cent of the weight. They use 20 per cent less power and are visible from every direction with no colour shift. A 10 times faster switching speed provides perfect moving image performance. A 40 per cent bigger colour range results in stronger and more vivid colours. A contrast ratio of more than 1000 to 1 results in vibrant display images with the potential for millions of colours.

Most OLED displays are passive, but the OLED-T team is developing active matrix display technologies with silicon backplanes, similar to those used for active matrix LCDs. These are suitable for full colour, larger displays for mobile phones, PDAs and GPS devices. It is expected that while only 10 per cent of the OLED market is active today, this will increase to 90 per cent by 2010. The OLED-T products boast good electron transport layer technology with up to three times the lifetime and 80 per cent higher efficiency than their leading competitors.

The company is also developing a unique white OLED technology for lighting, which may eventually become a bigger market than displays. Prof Kathir explained that the different colour-emitting chemicals making up the display are placed on top of each other in very thin layers, each no more than 10nm to 15nm thick. This is different to a conventional colour display, where red, green and blue coloured dots are placed beside each other. The sequence of layers is: indium tin oxide, hole injector, hole transporter, blue, red, green, blue, electron transporter. Electrons may fall into holes anywhere in the coloured layers. Both fluorescent and phosphorescent materials are used in combination, which further improves overall electronic efficiency.

The alternative Japanese technology is to create white light by mixing blue and yellow light in the emissive layer. However, there is then not then enough green light to produce a good white light, so it is necessary to add filters, which reduces overall efficiency. The OLED-T team is also working on a single-layer white emitter based on novel chemistry.

The OLED market for displays is expected to reach $4.6 billion (£2.5bn) by 2010. The market for flat emissive white lights for lighting is hard to estimate but likely to be very much greater. Flat lights coated onto substrates offer many opportunities in lighting design not possible with other technologies. Light intensities can be greater than those achieved by fluorescent tubes without any opportunities for dead insects to accumulate behind light diffusers. Efficiencies are potentially greater than for any other kind of white lighting. Low working voltages should mean that shocks from improperly installed lighting circuits become history. And hang-on-the-wall displays will be no heavier or more difficult to move around than framed pictures. In fact, being programmable, they could replace framed pictures in most homes.

OLED-T

Eureka says: The future of UK developed displays and lighting technologies looks bright

Pointers

* High efficiency because light emissive not light absorptive

* Wide viewing angle and no colour change in any direction

* Very fast switching response

* Red, green and blue colour are very intense and saturated

* Contrast ratio better than 1000:1

Author
Tom Shelley

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