Exploring ways to produce white light from organic LEDs (OLEDs), researchers at National Chiao Tung University in Hsinchu, Taiwan, have found that stacking two emitters can yield a device with three times the luminance efficiency of a single unit. High-efficiency and high-reliability white OLEDs could be used as LCD backlights, as paper-thin and flexible light sources, or for other applications. However, current devices aren’t up to the task. A white organic LED stacked atop another forms a tandem emitter that is three times as efficient as a single device. Courtesy National Chiao Tung University. “The efficiency and life span of white OLEDs have to be improved further,” said Shiao-Wen Hwang, a postdoctoral fellow at the university’s Microelectronics and Information Research Center. Based on previous work, the investigators believed that stacking two OLEDs would lead to a more efficient and more stable device. White OLEDs are constructed by mixing two complementary colors, such as yellow and blue. Typically, these are found in two light-emitting layers 20 to 40 nm in thickness that butt against each other. The layers are doped with an appropriate fluorescent material, and the device sits on glass, with the light-emitting layers enclosed by conductors that control the OLEDs’ operation. The researchers constructed two stacked devices. In one, a yellow OLED sat atop a blue one. In the other, they placed one standard white emitter atop another. In both cases, they used a 15-nm-thick, two-layer interconnect between the devices that was composed of magnesium-doped aluminum tris(8-hydroxyquinoline) and tungsten oxide. That arrangement, Hwang said, was carefully chosen. “The efficiency must be more than two times that of one-unit white OLEDs,” he explained. “If not, fabricating tandem OLEDs is meaningless. So we optimized the interconnect thickness.” Both the yellow-on-blue and the white-on-white arrangements resulted in white light. However, the latter tandem approach was more reliable and more efficient. The researchers projected that the half-life of a tandem OLED with an initial luminance of 100 cd/m2 would be more than 80,000 hours. The maximum efficiency, they found, was 22 cd/A, three times that of a single white OLED. These results are encouraging, but Hwang said ongoing research could lead to an even better device. “The next step is to decrease the driving voltage of tandem white OLEDs and further improve the power efficiency.” Applied Physics Letters, Dec. 19, 2005, 253501.