Since their discovery in the 1950s, organic light emitting diodes (OLEDs) have been regarded as holding significant potential. OLEDs are light emitters, which means displays featuring them don't need a backlight. Hence battery operated products which feature an OLED display should have longer operating lifetimes than their LCD equipped equivalents.
A variation on the OLED theme was pursued in the late 1980s at Cambridge University. The work by Dr Richard Friend and his colleagues led to the development of polymer OLEDs and the establishment of Cambridge Display Technology.
OLEDs are seen to have great potential in the displays market – including rollable displays – but another potential application for OLEDs is as a lighting technology. While solid state LEDs are being developed as point light sources, OLEDs hold the potential to be used for general space illumination.
OLED technology is seen as a means of creating flat, thin and lightweight light sources. Using this approach, say researchers, diffuse non glaring illumination can be created. Because of its relatively high efficiency, OLED lighting can reduce energy consumption.
The topic has been the subject of European research for some time. Work started with the OLLA – Organic LEDs for Lighting Applications – Project in 2004.
The OLLA Project, as its name suggests, worked on developing high efficiency white LEDs for use in lighting applications. The project brought together 24 partners from eight European countries.
When the project came to the end of its four year life in June 2008, it had achieved its original goal and was able to demonstrate the basic technology for a white OLED light source. The source, which had an efficiency of 50.7lumen/W, had an initial brightness of 1000cd/m2 and was thought to have a lifetime well in excess of 10,000hr.
In recognition of its achievements, OLLA received the 2008 Organic Semiconductor Industry Award for Research and Development. Receiving the award, OLLA project manager Peter Visser said: "It's a clear recognition for all the work we did together, both within Philips and within a large group of European OLLA partners in an Open Innovation environment, to make this project a great success."
Visser believes OLLA has kick started similar work around the world.
"OLLA was the first and largest European research project on OLED lighting. It has initiated global momentum for research on OLED lighting. Now, several similar research projects on OLED lighting have started in Japan, Korea, China, the UK, Germany and in the US."
OLLA has been succeeded by another European initiative called OLED100.eu and backed by €12.5million of EU funding. The project will build on OLLA, again focusing on general lighting applications. However, the goalposts have been moved to provide a greater challenge. OLED100.eu is aiming to create an OLED light source measuring 1 x 1m which has an efficiency of 100lumen/W and a life expectancy of 100,000hr. A further target has been set in order to commercialise the technology. That is producing such a device for €100/m2.
Project manager Dr Stefan Grabowski of Philips Research noted: "The European Council has agreed to cut CO2 emission by at least 20% by 2020 and OLED100.eu is an important initiative to advance the development of energy efficient lighting solutions. Building on the success of OLLA, OLED100.eu will deliver OLEDs with twice the efficiency, 10 times the operational lifetime and 10 times the substrate size. The participation of leading lighting manufacturers like Philips and Osram will ensure a rapid transfer of any result into real products."
But Dr Grabowski pointed out the OLED technology being pursued for lighting purposes is significantly different to that being used to create displays. "Although there are similarities – thickness, for example – the properties will be different. We are looking for energy efficiency, which is not so important for the display sector."
Another difference is scale. "Display companies are looking to develop OLED technology to produce small pixels. We're working at the other extreme, looking to develop OLED tiles – effectively one pixel measuring 33 x 33cm."
OLED100.eu is basing its development program on small molecule OLEDs. These are multilayer thin film devices created on a substrate by vacuum evaporation. The polymer OLEDS used in displays feature, by contrast, one or two layers.
OLED100.eu is, of course, continuing the development work done by OLLA. "OLLA laid the foundation," Dr Grabowski noted. "OLED100.eu is moving towards creating a specification for general lighting; including making devices with higher efficiencies, larger areas and cheaper to manufacture."
In particular, the project is looking to double the technology's efficiency. "OLLA demonstrated an efficiency of 50lumen/W, which is comparable to a compact fluorescent lamp. OLED100 wants to double that efficiency to 100lumen/W, which is comparable to a fluorescent tube."
Asked why the project had picked targets featuring the number 100, Dr Grabowski noted that it was important to increase targets. "In particular, improving efficiency is important. Around 25% of domestic energy in Europe is consumed by lighting. While 100lumen/W is the target, we think that is a reasonable target and one that can be reached within the next three years."
Manufacturing the 33 x 33cm OLED tiles will also be a challenge. "We have to make large tiles which retain the same efficiency across their surface," said Dr Grabowski. "For the moment, the tiles will be constructed using a glass substrate, but the technique could be adapted to work with a plastic substrate."
