Efficacy of 45 lm/W Achieved in White OLED
Universal Display Corporation (UDC) successfully demonstrated an all phosphorescent white organic light emitting diode (WOLED™) with a power efficacy of 45 lm/W at 1,000 cd/m2. This high-efficacy device was enabled by lowering the device operating voltage, increasing the outcoupling efficiency to ~40% from ~20%, and by incorporating highly efficient phosphorescent emitters that are capable of converting nearly all current passing through a WOLED into light.
Low voltage operation is obtained by incorporating highly conductive transport layers and by designing devices to improve charge injection and recombination in the emissive layer. The outcoupling efficiency was enhanced by using OLED luminaires that significantly reduce the absorption losses incurred when light waveguides in the substrate. Near 100% internal quantum efficiency is realized by using phosphorescent OLEDs (PHOLEDs) whereas only 25% internal quantum efficiency is typically obtained from fluorescent devices. In PHOLEDs, the singlet excited state (S1) excitons may be converted into the triplet excited state (T1) through inter-system crossing via the presence of a heavy metal atom. In these devices, the triplet states can emit radiatively (T1 to S0), enabling record high conversion efficiencies.
Warm white emission from the device has a color rendering index of 78 at (0.38, 0.44), and this color was chosen because it more closely resembles the color of incandescent lamps, which WOLEDs are targeted to replace in the illumination market.
The UDC team will continue to work on further improvements. In 2008, UDC expects to deliver a high-power prototype lighting panel with a power efficacy of 40 lm/W at 100 lm. The high lumen output will be achieved by using a large area (~6-in-sq.) panel, and the OLED architecture will be based on the device described above. The team is also targeting a 100 lm/W white OLED by 2010, which is well in line with the DOE multi-year projection for white OLED light sources.