Efficient Deep-Blue Organic LED Created

Michael A. Greenwood

An efficient deep-blue organic LED (OLED) that could be used for display applications has been created by researchers in Taiwan.

Although a number of such devices have been fabricated in recent years, important structural information about how they were created — including dopants and host materials — in many cases has not been fully disclosed because of patents or commercial secrets.

This deep-blue organic LED with an SK-1 doping concentration of 7 percent achieved an electro-luminescence efficiency of 5.0 cd/A at 20 mA/cm².

Researchers at National Chiao Tung University in Hsinshu are reporting that they have fabricated a blue OLED with a 4-(styryl)biphenyl-core-based fluorescent dopant (SK-1) in the wide-bandgap 2-methyl-9,10-di(1-naphthyl)anthracene host system.

This design approach yielded an electroluminescence efficiency of 5.0 cd/A and an external quantum efficiency of 4.2 percent at 20 mA/cm². It had saturated blue CIE_x,y coordinates of X=0.15 and Y=0.14 and a half-decay lifetime of 8000 h with an initial brightness of 100 cd/m². The scientists determined that a doping concentration of 7 percent provided the optimal results.

Other research teams’ efforts, from which all the details of the fabrication technique have not been divulged, have reported efficiencies as high as 7.4 cd/A.

A high-efficiency deep-blue OLED has a number of potential benefits, said researchers Meng-Huan Ho and professor Chin H. Chen. It can reduce the power consumption in a full-color OLED display and generate other colors via an energy cascade.

The investigators said that their fabrication technique eventually could be used for displays, but first they must reduce the high drive voltage and improve the lifetime of the device. To reduce the drive voltage, they plan to introduce the p-i-n structure. Under the same driving voltage, the p-i-n architecture supplies more current density than conventional OLEDs because of the incorporation of the conductive transporting layers. As a result, the brightness can be enhanced, provided that proper charge balance also can be maintained.

Applied Physics Letters, Aug. 20, 2007, Vol. 91, 083515.