Organics gear up for full colour - Brief Article

Electronics Times, Nov 6, 2000

The organic light emitting display (OLED) is one of the youngest technologies on offer, although early work demonstrated that it could be viable in 1987.

Independently, a group of research institutes and companies have tried to find organic chemicals that would emit light reliably and continuously when fed with an electric current.

A typical OLED uses a sandwich structure. At the bottom is a metal cathode and the top is a transparent anode array, typically made from indium tin oxide (ITO). In between is an array of organic-material pixel elements selected to generate red, green or blue between a hole and an electron transport layer. When a current is applied and the circuit completed between the cathode and an anode, the organic material in between emits photons.

Until the past year, most displays demonstrated that could use organic emitters were passively addressed, low-resolution displays suitable for low-end designs such as entry-level cellphones or simple point-of-sale displays. That does not mean that the technology cannot ultimately match or even surpass the quality of active-matrix LCDs.

Earlier this year, Kodak demonstrated and explained how the company, with Sanyo, designed and made a small active-matrix display with a pixel density of 150pixel/in. Measuring 2.4in on the diagonal, the full- colour display had a resolution of 284 x 222pixel.

It was constructed on a glass substrate using similar techniques to those needed for building LCDs. The displays were constructed four at a time on a 150mm sq panel. At the Dappcon conference earlier in the year, David Williams, general manager of OLED technology at Kodak, told Electronics Times that the Kodak-Sanyo team was working to quadruple the size of the display and use larger panels to keep the cost of production relatively low.

The main difference between the Kodak-Sanyo OLED and an LED lies in the shadow mask used to separate pixel elements. The type of mask needed is similar to that of a CRT but demands much closer tolerances. When scaled up to 300 x 400mm mother glass, the masks would need millions of precisely placed apertures.

At the Society of Information Displays conference in May, the team said they had made several hundred of the 2.4in displays since the first demonstration models and that "surprisingly, the yields are encouraging". They claimed the display had a similar colour gamut to that of the NTSC standard. The range was not quite as high as NTSC but was close enough to be acceptable.

For a peak brightness of 150cd/sq m, the power needed by the display was 350 to 400mW, about half that of a comparable active-matrix LCD. But a significant proportion of the power is consumed by the red and blue emitters, which have a lower conversion efficiency of about a factor of two than the readily available green. With more research into alternative red and blue emitting organic materials, the consumption of the display could be reduced further.

A company that has crossed over from designing field emission displays to OLEDs has made the first OLED on silicon. Designed for microdisplay- based designs, such as projection monitors and electronic viewfinders, Emagin's design has more than 1.3 million 12 micro m sub-pixels on a chip.

The display uses a white-light OLED technology with colour filters built directly on top of the display. The technology will be applied to the SVGA+, a microdisplay designed for consumer applications that is expected to be ready for sampling early next year.

Copyright: United Business Media Ltd.