04 Sep 2007
A low power consumption white organic light emitting device could be a promising alternative to conventional lighting devices.
Researchers in Hong Kong claim to have developed the highest power efficiency non-doped white organic light emitting device (WOLED) made using a single white-emitting material. They believe that their high purity, color stable device, surpasses existing WOLEDs and is a more efficient and environmentally friendly alternative to inorganic LEDs.( Applied Physics Letters 91 023503)
"We believe our WOLED represents the highest power efficiency ever achieved by using small-molecule fluorescent materials," Shuit-Tong Lee, a professor at City University of Hong Kong, told optics.org. "WOLEDs offer a high luminous efficiency and low power consumption alternative to inorganic LEDs."
Although high brightness WOLEDs have been reported before, Lee believes that there are no practical applications due to lack of color stability. "Our device has a luminance of 10,000 cd/m2 with excellent color stability which remains unchanged over a range of luminance from 100 to 20,000 cd/m2, a high current efficiency of 9.4 cd/A and power efficiency of 9.0 lm/W," explained Lee. "They can be used in full-color displays with color filters and in backlighting for liquid crystal displays as well as in vehicle, advertisement and traffic lights."
WOLEDs are usually based on multiple emission materials which require complicated doping processes to produce high purity white light. This can waste costly organic material and the devices also suffer from color dependence on driving voltage and color sensitivity on emitting layer thickness.
"High efficiency WOLEDs have never been reported because of the low quantum yield of the white-emitting materials," commented Lee. "Using a single white-emitting material overcomes these problems."
Lee's device uses both intrinsic and exciplex emission to produce white light. Intrinsic emission occurs when molecules are excited. This increase in energy leads to light emission. An exciplex is formed when a molecule in an excited state couples with a molecule in the ground state. The intrinsic and exciplex emission occur at two different wavelengths and combine to produce white light.
"The excited part of the molecule emits at 472 nm which is blue emission. The exciplex is lower in energy so emits at a longer wavelength of 572 nm which is yellow emission," explained Lee. "The combination of blue and yellow emission results in a white-emitting device."
The team ultimately hopes to achieve a power efficiency of 100 lm/W by incorporating nanomaterials within the organic materials. "The key problems we need to solve are the lifetime of the device, color temperatures of the white light, color stability and output intensity," concluded Lee.