Perforated Cathode Improves Organic LED Efficiency

Hank Hogan

Organic LEDs may have a bright future, but it would be brighter still if they were more efficient. Now researchers from the University of Utah in Salt Lake City have increased the electrolu-minescence efficiency of an organic LED sevenfold by substituting a holey piece of aluminum for the slab usually found in the devices.

The perforations in the aluminum were 150 nm in size and spaced 300 nm apart, measured from center to center. Those dimensions, said research team leader and physics professor Z. Valy Vardeny, were chosen based on the optical characteristics of the polymer at the heart of the light source.

“The perforated aluminum has resonances in the spectral range of the electroluminescent spectrum from the polymer active layer,” Vardeny said. “If we change the hole pattern, we lose the matching with the electroluminescent spectrum.”

That matching was key to the increased efficiency. In the simplest organic LED, an active organic semiconductor is sandwiched between two metallic electrodes. The electrodes supply the holes and electrons that combine inside the polymer to produce light. However, they also act as a barrier to surface emission and thereby reduce the LED’s efficiency.

Using a partially transparent material such as indium tin oxide to make the anode can lessen that problem. On the other hand, the cathode is typically fabricated of a thick, opaque material such as aluminum. As a result, much of the emitted light never makes it out of the organic LED structure, instead being converted into surface plasmons that will fade away.

The researchers noted that surface plasmons disappear only from a smooth metallic surface. So they had Nanonex Corp. of Princeton, N.J., fabricate an 80-nm-thick aluminum film with 150-nm holes in a 300-nm lattice over a 5 × 5-mm glass substrate. They also created a solid film of the same thickness as a control. They deposited an organic LED on both and compared the performance of the devices.

They found that the perforated cathode had seven times the electroluminescent efficiency because the holes in the aluminum converted some of the surface plasmons into luminescence.

Vardeny said that more research must be done but that there are plans to commercialize the technique. “We are in the process of patenting it,” he said. “Our patent office is in contact with several companies to buy the rights.”

Applied Physics Letters, April 4, 2005, 143501.