Phosphors: the black art of displays

17 Jun 2002

Light-emitting phosphors are a vital part of many future-generation displays. However, according to a UK scientist, they are under-researched and under-resourced. Phillip Hill looks at the problems that still need to be solved.

From Opto & Laser Europe January/February 2001

Phosphors: a 'black art'

"Phosphors are probably the most used and least studied component of electronic displays." So said Jack Silver, head of the Centre for Phosphor and Display Materials at the University of Greenwich in the UK.

"The complexity of phosphor preparation is often referred to as a 'black art'," he told the recent Electronic Information Displays conference in London.

He believes that the manufacturers of next-generation television and desktop monitors are neglecting the fundamental research into the materials on which the entire displays industry is based.

This belief is echoed by Chris Williams, director of the UK Chapter of the Society for Information Displays. He said: "It is surprising to see that more than 85% of the total displays market is based on phosphors. To have such a major dependency on a component technology that is identified as poorly understood is a concern."

Silver questions much of the received wisdom current in phosphor development for display devices. The usual practice for CRT manufacturers is to buy industrially available phosphors and assume that the phosphor can be used for new or extended applications. Many companies believe that phosphor optimization can be left to the later stages of the development of the new-technology displays that use these materials. However, Silver is adamant: "History shows that, in nearly all cases, the reverse has been true and industrial exploitation has had to wait for the development of suitable phosphors." He cites two examples: the advent of colour television and thin-film electroluminescent devices.

More importantly now, the choice of light emitter, its quality and reproducibility are crucial to two emerging technologies - organic light-emitting diodes and field-emission displays (FEDs) - which have the potential to replace established liquid-crystal displays (LCDs). The conclusion is that manufacturers of these devices neglect at their peril the all-important light emitters.

In high-definition television, phosphor brightness becomes a problem as the size of the screen increases. The larger the screen the faster the scanning rate required.

Phosphors have a certain build-up time, so those on a 13-inch screen are four times as bright as phosphors on a 30-inch display. The screen brightness can be increased by focusing the electron beam and increasing the anode potential; and by using larger phosphor particles. Both of these present the phosphor chemist with problems that have not yet been overcome, says Silver.

With the development of miniature CRTs, Silver says that there is no correlation between particle size and brightness in powder phosphors as previously thought. In fact, his group at Greenwich has found that the crystallinity - brought about by various synthetic processes - affects the brightness more than the particle size.

LCDs can also make use of phosphor emission. They constitute the most efficient display system, but this is not the case when backlighting is needed. It has been estimated that 96% of the emitted light is lost.

British company Screen Technology has demonstrated that considerable gain in efficiencies can be obtained by using a fluorescent lamp in the near-ultraviolet region.

With FEDs it was previously thought that the addition of conducting powders to standard phosphors merely made the deposited screen more conductive. This is now being questioned, says Silver.

He points out that there is little agreement on which phosphors can be used for FEDs. Some manufacturers claim that even minute traces of alkali metals cause instabilities, while others state that sulphides contaminate the microtip emitters. A few firms actually use sulphide phosphors in microtip devices and report the improved performance of low-voltage phosphors. However, Silver says that the improvements reported so far have not yielded a practical low-voltage FED. Zinc oxide still remains the most efficient phosphor at low voltages, but few quantitative data are available on the maintenance characteristics of low-voltage phosphors under the long-term operating conditions in a real display.

Silver stresses that, although blue, green and red gallium nitride LEDs together make cheap, efficient white light, there are problems. The three LEDs age at different rates so that the light, although it begins as white, could deteriorate quickly. Silver suggests that a combination of LEDs and phosphors could be a long-term solution.

Silver believes that the need for more fundamental research is urgent. "The demand for stable saturated RGB high-resolution phosphors operating over a range of excitation is far from satisfied."

Williams concurs: "The UK is a major source of commercial-production phosphors. If we work to develop a better understanding of the physics of operation and can improve the production processes to reflect this newly acquired knowledge, we will have an opportunity to lead the global market in supplying higher-efficiency raw materials to OEMs."