Efficient White OLED Employs Down-Conversion
Daniel S. Burgess
A white organic LED (OLED) incorporating a blue phosphorescent dye and a down-conversion phosphor has achieved a luminous efficacy of 25 lm/W. Reported by investigators at Osram Opto Semiconductors Inc. in San Jose, Calif., and at Osram GmbH in Munich, Germany, the device not only furthers the development of OLEDs for lighting applications, but also offers insights regarding models of conversion efficiency in such down-conversion emitters.
Although there are issues with the lifetime and environmental sensitivity of the materials involved, explained Benjamin C. Krummacher, who is now based at the company’s facility in Regensburg, Germany, the researchers opted for the down-conversion approach because it eases fabrication and offers color stability. For the 70-nm-thick blue-emitting layer, they employed polyvinyl-carbazole as the host, iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2'](picolinato) as the phosphorescent dopant and an electron-transport material. The down-conversion phosphor was an Osram product based on a nitridosilicate phosphor and was mixed in a silicone matrix, applied to the outer surface of the glass substrate and cured for 24 h.
An OLED with a 90-µm-thick down-conversion layer had CIE color coordinates of (0.26, 0.40). The thickness of the layer determines the X-coordinate but does not affect the Y-coordinate. The luminous efficacy was 25 lm/W at an efficiency of nearly 39 cd/A.
The scientists found that the experimental conversion efficiency of the down-conversion phosphor was 180 percent, compared with the 94 percent predicted from theoretical models. To eliminate the contribution of volumetric scattering effects in the down-conversion layer to light extraction, they measured the enhancement of an orange OLED to which the layer was added and to which the phosphor would not respond. The results indicated that scattering alone was not responsible for the higher-than-expected conversion efficiency, and the investigators suggest that the isotropic radiation pattern of the excited phosphors accounts for the phenomenon.
Krummacher said that the team plans to focus on the lifetime of the OLED materials and their sensitivity to humidity and oxygen. Currently, he said, the devices must be sandwiched between glass, limiting applications.
Applied Physics Letters, March 13, 2006, 113506.
MORE FROM PHOTONICS MEDIA