nature photonics | VOL 1 | JANUARY 2007 | www.nature.com/naturephotonics 33

INDUSTRY PERSPECTIVE | TECHNOLOGY FOCUS

Brian D’Andrade is a senior scientist at the Universal Display Corporation in Ewing, New Jersey, USA.

e-mail: BDAndrade@universaldisplay.com

A s lighting now accounts for over 8% of all the energy consumed in the United States each year, new

highly effi cient sources of white light are urgently required. Th e crux of the problem is that today’s incandescent and fl uorescent lamps have a limited effi cacy (conversion effi ciency from electricity to useful light) of only around 4% (about 12 lm W–1) and 25% (90 lm W–1) respectively. Conventional LED technology based on compound semiconductors has made great progress in helping address the problem and has now demonstrated effi cacies of 70–100 lm W–1, but organic LEDs (OLEDs) off er a very promising alternative.

The chief attraction of OLED technology is that it is not only potentially very efficient but can also be fabricated on large, thin, flexible substrates making it ideal for ‘wallpaper style’ diffuse sources of light for general lighting, backlights for portable electronics or flat-panel displays.

In contrast to their semiconductor cousins, OLEDs rely on electroluminescence from organic molecules to generate light. By tailoring the composition of the organic material, it is possible to create devices that emit red, green, blue or collectively white light when electrically driven.

Th e very fi rst white OLED devices were based on fl uorescent emission and suff ered from a poor effi cacy of just 1 lm W–1, but today’s state-of-the-art phosphorescent devices (PHOLEDs) from the Universal Display Corporation (UDC) are able to off er 60 lm W–1 at 1,000 cd m–2. What’s more, it is anticipated that PHOLED technology will be able to break the 100 lm W–1 value by 2010.

By exploiting phosphorescence, PHOLEDs are able to convert 100% of the

electrons that are injected into the device into photons; whereas this fi gure is limited to 25% for OLEDs relying on fl uorescence. Phosphorescence in combination with other technologies enables electrical-to-optical conversion effi ciencies for PHOLEDs to exceed 50%.

FABRICATION

PHOLEDs are commonly fabricated by vacuum thermal evaporation because this tends to give the best performance characteristics. Metals (for example silver) and organic materials, such as

White organic LED technology is rapidly coming of age thanks to the latest research in phosphorescent designs that offer record-breaking effi ciency.

LIGHTING

White phosphorescent LEDs offer effi cient answer

The effi cacy of PHOLEDs is expected to break 100 lm W–1 by 2010. The best devices so far offer around 60 lm W–1.

A PHOLED panel from UDC that emits high-quality white light with suffi cient colour rendering to clearly distinguish red, green yellow and blue objects.

An optical fi xture that is used to double PHOLED power effi cacy.

UDC

UDC

UDC

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34 nature photonics | VOL 1 | JANUARY 2007 | www.nature.com/naturephotonics

INDUSTRY PERSPECTIVE | TECHNOLOGY FOCUS

tris(8-hydroxy-quinoline)aluminium, are sublimed in vacuum and condense into thin fi lms on rigid or fl exible substrates, such as glass or plastic.

Several thin-film layers (each 10–50 nm thick) are deposited on each other and each layer performs a defined function such as generation of a specific colour or the transportation of electronic charge away from the electrodes towards the organic dopants. The collective aim is to maximize the efficiency at which electrons and holes recombine and cause organic molecules to emit photons.

PHOLEDs can also be fabricated using several other methods: organic vapour phase deposition, spin coating, laser-induced thermal imaging, ink-jet printing, and organic vapour jet printing. For low-cost manufacturing of high-effi cacy PHOLEDs, the choice of fabrication method depends on operational costs, length of processing time, reproducibility of device performance, and the organic material’s compatibility with the process. PHOLEDs may one day be manufactured using roll-to-roll processes similar to those used to create magazines and books.

So far UDC has succeeded in making PHOLEDs with active areas that are up to 25 cm2 and just 0.7 mm thick.

PERFORMANCE

Th e performance characteristics of white PHOLEDs have considerably improved over the past fi ve years thanks to a combination of factors (see Table 1).

For example, high-conductivity organic materials now allow the operating voltage of PHOLEDs to be kept below 4 V; optical fi xtures help boost the effi ciency of the extraction of photons into free space from the organic layers where they are generated; and device designs have been optimized to ensure effi cient electron and hole recombination.

Owing to the broad nature of molecular emission (with a typical full-width at half-maximum of 60 nm), PHOLEDs can achieve a colour-rendering index (CRI) of 70 or greater at colour temperatures ranging from the warm white colour of incandescent lamps to the cool white colours of LEDs. For example, a white PHOLED can have sufficient colour rendering to adequately illuminate red, blue and yellow flowers.

Combining two molecular emitters with blue–green and yellow–orange emission produces white with a CRI in the 70s; whereas combining three emitters with blue, green and red emission can produce white with a CRI in the upper 80s. A CRI in the 90s may be achieved but the current PHOLED development is focusing on effi cacy and lifespan instead.

Th e lifespan or lifetime of an OLED is oft en defi ned as the time it takes light output to decrease by 50%, when the device is driven at a constant current. Th is fi gure is known as the LT50. Laboratory scale, 2 mm2, warm-white PHOLEDs with an initial luminance of 1,000 cd m–2 have achieved an LT50 of 10,000 hours, and this level of performance makes PHOLEDs very attractive for niche lighting applications such as night lights. However, the display industry will require LT50 exceeding 50,000 hours for cooler white colours, so signifi cant development is still required before the widespread use of this technology in displays.

FUTURE OUTLOOK

Despite all this recent progress there is still a signifi cant amount of work

to be done before high-effi cacy white PHOLEDs become a commercial reality. Many of these challenges relate to taking laboratory devices into mass production.

One issue is simply scaling the size of the technology. PHOLEDs are required to have active areas in excess of 25 cm2 to generate 600 lumens — the equivalent lumen output of a 50 W incandescent bulb. Th e effi cacy, lifespan and colour uniformity for such large-area devices are quite diff erent from those of 1 mm2 devices, which are oft en made in the lab. A key challenge is ensuring that the conductivity of the transparent electrode in a large device is high enough to avoid increased operating voltages, resistive heating, non-uniform emission, and diff erential ageing across the active area.

Although the commercial introduction of white PHOLEDs as large-area general lighting sources is

still several years away, major lamp manufacturers and the display industry are expressing keen interest in the technology and its development.

In the meantime, recent improvements in PHOLED performance have inspired developers and designers to consider exploiting the remarkable qualities of this unique diff use light source in other areas. Initial products, where cost- and power-effi ciency requirements may be less demanding include low-to-medium brightness backlights for portable electronics (mobile phones, watches and personal digital assistants (PDAs)), automotive instrument panels, emergency-exit signs, novelty clothing, headwear and footwear lighting, decorations and safety and festive lighting. Given the trends in PHOLED development, the outlook for PHOLED technology is bright.

300 nmVoltage

Glass (or plastic or thin film)

Glass, plastic or foil...

Organic stack

Top electrode

Bottom electrode

–

+

The typical structure of a PHOLED: By fabricating the device on a plastic substrate, thin, fl exible light sources can be created.

The PHOLED fl ower garden has 25 cm2 devices that emit blue, green, orange, yellow and red light.

Table 1 Performance for a 25 cm2 white PHOLED

Maximum size 25 cm2

Thickness 0.7 mm (without optical fixtures)1.2 cm (with optical fixtures)

Drive voltage 5.6 V

Colour-rendering　Index

71

Efficacy 30 lm W–1 at 1,000 cd m–2

UDC

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