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02 Jun 2009
Organic light-emitting devices (OLEDs) are making waves in the displays and lighting markets. Stefan Grabowski tells Marie Freebody about the milestones that need to be reached before the technology finds widespread commercialization.
Stefan Grabowski works at Philips Research Laboratories, Germany, where his focus is on device physics and OLED stack development. Grabowski is also the project manager of OLED100.eu, an EU-funded R&D collaboration that aims to realize efficient OLED products for the European lighting industry. Funded under the seventh Framework programme, the project comprises 14 partners from six countries.
Can you explain how OLEDs work?
OLEDs are solid-state lighting devices that work in a similar way to their inorganic cousins. A typical OLED is composed of two layers of organic material deposited on a transparent substrate, sandwiched between an anode and a cathode terminal. One of the layers is emissive and transports electrons from the cathode; the other layer is conductive and transports holes from the anode. When a current is passed through these layers, light is emitted via electron-hole pair recombination.
Unlike their inorganic counterparts, OLEDs can be easily structured to show patterns of colour or homogeneous white light and the layers that make up the device can be deposited on large areas.
Why is OLED research important?
OLEDs have the potential to become even more efficient than energy-saving bulbs. More than a quarter of electricity consumption in the EU is due to lighting, so an energy-efficient substitute is an important area of research. One of the targets of the OLED100.eu project is to build OLEDs with an efficacy value of 100 lm/W.
When efficient OLEDs are available at mass-production costs, they can be used for a multitude of applications in lighting. OLEDs offer new design options and integration possibilities because they could be built to cover large areas, they are extremely thin and they can be made transparent or flexible. The brightness and colour of OLEDs are fully adjustable, which creates a new way of decorating and personalizing people's surroundings.
What are the main applications and on what timescales will they occur?
There are two main application areas: displays and lighting. Small OLED displays already enjoy widespread application in portable devices such as mobile phones, MP3 players and personal digital assistants. Larger display devices are beginning to emerge on the market. For example, Sony has announced a 27 inch OLED television scheduled to hit the market in 2009.
OLEDs for lighting applications have completely different requirements. These OLEDs do not consist of many small pixels, but of one large emitting area. As a result, the device efficiency must be much higher. At first OLED lighting will appear in niche applications, but more sophisticated products will appear over time.
The vision is to have OLED tiles mounted on walls or ceilings to give a large uniform emitting area. Transparent OLEDs could also be incorporated into windows, which allow daylight to pass through during the day and emit light at night.
How well do you expect OLEDs to penetrate the lighting market?
OLED lighting is on the verge of commercialization. There is an OLED lamp available from the designer Ingo Maurer in Germany (it's based on Osram OLEDs) and Philips offers a technology kit that contains several OLEDs in various shapes and colours along with an electrical driver for a plug-and-play solution. These are not yet mass products but they indicate that OLED lighting is ready for the market. As the technology progresses, so too will the range of applications: from room and office lighting through to decorative and design-oriented lighting.
What is the most important recent advance and what hurdles remain?
The increase in efficiency of white OLEDs to a level that is comparable to that of a compact fluorescent lamp means that OLEDs are now much more efficient than halogen lamps and incandescent bulbs. The lifetime of OLEDs has also increased drastically to values of greater than 10,000 hours, making OLEDs appealing candidates for certain niche lighting applications.
But for general lighting purposes, OLED efficiency as well as lifetime must be improved further. In addition, the size of the emitting area has to be increased in order to fully utilize this unique feature. Therefore, production processes and device architectures are needed to enable uniform current distribution over the complete area as well as high reliability. From a commercialization point of view, mass-manufacturing equipment and processes have to be installed and developed.
What will the next breakthrough be?
A breakthrough is hard to predict. The OLED100.eu project is working to develop OLED technology further so that we will reach efficacy values of 100 lm/W, lifetime values of 100,000 hours and an OLED size of 100 × 100 cm. A breakthrough in the field of blue efficient phosphorescent emitters with a long lifetime would help us on that journey.
For more information about the OLED100.eu project, visit www.oled100.eu/homepage.asp.
• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.
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