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Illumination & Displays

From displays to lighting

As displays based on active-matrix OLEDs begin to be included in electronic products, the lessons learned are opening up new opportunities in lighting.
9 July 2007, SPIE Newsroom. DOI: 10.1117/2.2200707.0001

It all started with displays. Despite their benefits, liquid crystal displays (LCDs) have their downsides. One of the main ones is their requirement for backlighting, which makes up around 30 percent of an LCD panel. These add to the complexity of the device and therefore increase the number of things that could go wrong.

Many in the industry have predicted that the solution lies with organic light emitting diodes (OLEDs), an emerging alternative based on organic self-emitting compounds. As the pixels of these devices emit light themselves, there is no need for backlighting. OLED displays can be much simpler and thinner, and they also use less energy, extending device battery life. What's more, they promise performance improvements over LCDs.

The past decade has already seen passive-matrix OLED displays find their way into commercial products and within the past year the first active-matrix OLED displays for applications such as cellphones and MP3 players have been announced from the likes of Samsung and Sony. "The reviews that I've read have been extremely positive about OLED performance compared with LCDs," says Janice Mahon, who is responsible for the chemical supply business and corporate marketing for Universal Display ( Ewing, NJ). "OLEDs are much less materials-intensive than LCDs so there are intrinsic advantages with OLEDs," she adds.

Despite these announcements, OLED displays are still in their early stages, and the level of market take-up remains to be seen. Cost is still an issue: over recent years price wars have driven down the cost of LCDs, meaning that the gap between the established technology and the new alternative is widening. And Sven Murano, who leads the project for white OLED light development at Novaled AG (Dresden, Germany) points out that the performance of OLED displays may not be a sufficient enough improvement on their LCD counterparts to tempt manufacturers or customers to switch. "Companies are pushing on the market and starting production. We just need to see if they have the stamina to maintain production," he explains.

However, displays are not the limit of the potential for OLEDs. Indeed if industry hopes are realized, this application area could be far surpassed in volume by the lighting market. It's no secret that the traditional light bulbs that have long been essential parts of modern homes are hugely energy-inefficient, while the much more efficient fluorescent tubes give very harsh white lighting and often require screens, mirrors or reflective ribs to reduce their glare.

Figure 1: An OLED light research sample from Philips.

OLED lights would be very different, according to those working on their technology. OLED lighting promises to be the first technology to offer a flat, area source rather a point source, so they emit light uniformly from their entire surface. This paves the way for glare-free lighting panels on office ceilings or stylish lights incorporated into furniture. And this aspect of OLED lights could be extended in the future because the technology for building OLED stacks could potentially be modified to give flexible or even transparent lights, a property that will differentiate them from LEDs.

Such lighting technology is still at an R&D stage but it is already making significant strides forward. The headline-grabbing figure is energy efficiency and in this OLEDs have already outstripped the 10-15 lm/W of traditional light bulbs and is about level with the 20 lm/W of halogen lamps. Late last year Konica Minolta of Japan even reported an OLED light at 64 lm/W, although few details of the experimental conditions were revealed.

The pan-European OLED project, OLLA, which involves 24 commercial and academic partners from eight countries, has recently announced a prototype white OLED light source with an efficiency of 25 lm/W and a lifetime of 5,000 hours from an initial brightness of 1,000 cd/m². This is good progress toward the group's goal of 50 lm/W combined with a lifetime of 10,000 hours, with a minimum tile size of 15x15 cm2by mid-2008, according to Peter Visser of Philips Lighting, who is project manager of OLLA. He also points out that this announcement relates to an OLED light measured under standard conditions that OLEDs might be operated under. OLLA is urging that OLED manufacturers all adopt the same standard approach so that performance announcements can be easily compared.

Visser sees the combination of specifications of OLLA's prototype as an important breakthrough for the lighting industry. "We're hunting for a combination of extreme lifetime and efficiency, while not compromising on the white color experience of traditional light sources," he explains. Mahon of Universal Display agrees with this goal. "We need to get the color rendering, lifetime, efficiency and brightness all together in one OLED," she says.

Such advances in lighting can benefit from research done in developing OLED displays. "A great deal of technology is transferable from displays to lighting and that is going to be a great benefit," comments Mahon.

Figure 2: An OLLA OLED light.

Nonetheless, there are some significant differences between the display and lighting markets. The most obvious of these is cost. Despite the price wars in the displays market, people will be prepared to pay far more to replace their TV screen than they would to replace a light bulb. "The displays industry is starting to produce OLED displays at costs of around $100 per square foot, but lighting demands prices that are orders of magnitude lower," says Mahon.

This difference in cost has a knock-on effect on the development of OLED lighting. Some things are intrinsically cheaper because the technical requirements are less complicated. For example, OLED lights are typically monopixelated, unlike displays based on a combination of red, green and blue OLEDs, which require advanced lithographic patterning steps and pixelation. In addition, OLED lights do not require a back plane, whereas displays do.

But there are other ways that costs must be addressed to bring display technology down to an affordable lighting level. For example, the cost of the encapsulation scheme must be considered. OLEDs are sensitive to air and moisture, so would only last a few hours without encapsulation. Any improvements to the encapsulation will prolong the lifetime of the product but will bring additional costs that are easier to meet within the cost of a display than within the cost of a light.

Another issue is scale of production. OLEDs for lighting will need to be built on much larger substrates and in much higher volumes to meet the anticipated demands. There is plenty of research going on at the moment into different substrates and processing technologies and, in the long term, OLED lighting could be produced cheaply by means of roll-to-roll processing. "Most equipment we use now is custom-built research equipment so it doesn't give high throughput. It gives a couple of OLED samples per day where really we might want a couple of square meters of OLEDs per minute or more," points out OLLA's Visser. "We need to produce them in the kinds of quantities that light bulbs are currently produced in, around 5 billion per year," he adds.

Brightness requirements also differ between OLED lights and displays. As Visser explains, displays will not be any brighter than about 200-300 cd/m2

In addition, there are differences in the color requirements between the two application areas, as Novaled's Murano points out. "Displays based on white light with color filters require very white light whereas lighting needs a warmer, yellowish light, although as OLED lights become widely used, the whole range of white colors will be required."

Despite the challenges ahead, the industry is upbeat about the potential of OLEDs. As display companies watch for the reaction to the new wave of active-matrix OLED displays, the first smaller OLED lighting products could begin to emerge as early as next year. The first lighting applications are likely to be fairly small, niche applications such as signage. Signs for emergency exits, for example, do not require particularly high powers or a specific color of white light, although they do face stringent rules governing their lifetime. OLED lights could also end up competing with their display cousins by providing backlights for LCDs in portable devices. And within the next decade, as volumes and performance improve, other applications such as general home and office lighting could all turn their attentions to OLED lighting.

Meanwhile research will continue into improving efficiency, color rendering, lifetime and searching for the elusive long-living blue OLED emitter materials. And both the display and lighting industries will be closely watching each other's OLED work to take advantage of any developments that could boost the two markets.

Siân Harris
Siân Harris is a science and technology journalist based in the UK.