Reineke and his colleagues were able to get such good results because of several design modifications and refinements to their device. One involves reducing its operating voltage by doping the organic material that connects the light-emitting material to its metallic contacts. “The efficiency of the device is highly reduced if it is near a metal contact” because of a phenomenon called quenching, says Reineke.
Another trick was to make the outer surfaces of the device from types of glass that have optical properties that more closely match those of the device substrate. Otherwise, much of the emitted light is reflected and either reabsorbed or lost through heat. “In conventional structures, about 80 percent of the light is lost,” says Reineke.
The most novel aspect of this new OLED, however, is the organization of different light-emitting materials within the device. Three materials are used–one each for emitting blue, green, and red light–along with a host matrix material in between. Reineke’s trick was to choose a matrix material with a high “spin state” that matched that of the blue and to sandwich the blue material between the green and red, as if it were part of the separating host matrix material.
“The matrix and the blue state are nearly identical,” Reineke says. This means that any electron-hole pairs (excitons) escaping the red or green material will have to pass through the blue, increasing the chances that they will be converted into photons.
“They do a nice job of tailoring the LED layers to get good quantum efficiencies,” says Kazlas. “It shows the promise of OLEDs, but from an industry perspective, OLEDs still have a long way to go.”
Indeed, a major drawback of OLEDs is their longevity. Although companies like Philips are able to make devices with life spans equivalent to fluorescent bulbs–in excess of 10,000 hours–materials that yield higher efficiencies tend not to last so long. “Our devices have lifetimes of just a few hours,” says Reineke.