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Want a White Light? Print It.

Photonics Spectra
Apr 2008
Hank Hogan

It someday may be possible to produce white organic LEDs via printing, courtesy of researchers from the University of Michigan in Ann Arbor who have described using organic vapor jet printing to directly pattern devices on a substrate. The technique combines three individually controllable color components into a white-light device.

The method offers the potential for high manufacturing throughput at a low cost, important attributes for a commodity such as a light source, observed research team member Stephen R. Forrest.

LEDwhite.jpg

A new technique may enable production of white light from printed LEDs. Shown here, an organic vapor jet emerges from a nozzle and deposits organic LED components onto a cold substrate. The resulting stripes glow red, green and blue, giving rise to a white light. Courtesy of Stephen R. Forrest, University of Michigan.


Forrest, a professor of electrical engineering and computer science, said that the new technique is similar to the liquid ink-jet technology found in many desktop printers. However, their organic vapor jet technique uses no liquid solvents. Instead, the investigators heated organic sources to produce a vapor. They then flowed an inert gas carrying the vapor through a hot-walled mixing chamber and a heated nozzle that directed the stream onto a cooled substrate, where the vapor condensed. The nozzle allowed deposition in desired patterns while control over the mixture of organics enabled the tuning of de-vice characteristics.

To demonstrate the concept, the researchers built a machine with five source cells. Three contained dopants, and two contained host materials that produced electrophosphorescent organic LEDs of red, green and blue when doped properly. The 1-mm-diameter nozzle was placed 0.7 mm from a substrate of glass coated with indium tin oxide. Mounted on a movable stage, the substrate traveled beneath the fixed nozzle, allowing the creation of various patterns.

The scientists heated the dopants and host materials in turn, mixed them and deposited them in strips across the substrate. Capping the structures with patterned aluminum completed the device connection. “We worked a very long time to get the machine and all of the dopants to work properly,” Forrest said.

The individual red, green and blue components had turn-on voltages of 3.5 V, with external quantum and luminous power efficiencies comparable to those of devices produced with vacuum thermal evaporation. The white-light devices had a peak efficiency of 11.6 percent at a brightness of 400 cd/m2.

Although these results are promising, further improvements are necessary before volume production can begin. The group is working on one improvement: a machine with multiple nozzles. Estimates are that a 1000-nozzle system could print a 100-cm2 emissive layer in 1.5 s. Even before a multiple-nozzle machine is complete, there is commercial interest from organic LED maker Universal Display Corp. of Ewing, N.J., which now is working with the researchers.

Applied Physics Letters, Feb. 4, 2008, Vol. 92, 053301.


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