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White OLED achieves color stability

23 Oct 2007

A white organic light-emitting diode that exploits stacked phosphorescent materials maintains the same color output at both high and low luminance values.

Researchers in Korea claim to have improved the color stability of white organic light-emitting diodes (WOLEDs) over a wide luminance range. The team says that stable color was observed between 100 cd/m2 and 10,000 cd/m2 without any change of color index or spectrum shape. (Applied Physics Letters 91 123509)

"The color stability we achieved in this work is at least comparable to other light emitting devices such as xenon arc lamps or inorganic LEDs," Jun Yeob Lee, a researcher at Dankook University, told optics.org.

Color coordinates were kept at (0.33, 0.35) over the entire luminance range. The team obtained a quantum efficiency of 8.2% at 500 cd/m2 and a high current efficiency of 12.7 cd/A.

WOLEDs that exhibit color stability, both at high and low luminance, hold promise for general lighting, as backlighting for LCD screens and as OLED displays combined with a color filter. WOLEDs also have the added advantages of flexibility, distributed light emission and low-cost manufacturing.

Lee believes that the key to improved color stability lies in using phosphorescent emitting materials combined with a charge confining structure. By sandwiching a 12 nm thick red phosphorescent emitting layer between two 8 nm thick blue phosphorescent emitting layers, he believes higher quantum efficiency is obtained.

"Phosphorescent emitting materials can theoretically achieve four times higher quantum efficiency compared with fluorescent WOLEDs," said Lee. "The current power efficiency of WOLEDs is around 65 lm/W, but this can be increased to over 100 lm/W by combining OLED technology with light out-coupling technology."

However, in this work the Korean team focused on stability rather than efficiency. "We obtained a quantum efficiency of 8.2% at 500 cd/m2 using conventional materials but I think it is possible to get high quantum efficiency over 20% with an optimized material system."

The team will expand their work to include fabricating three-color WOLEDs. "We will also try other host and dopant materials which can provide high quantum efficiency as well as color stability," concluded Lee.

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