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Light-emitting diodes made with firefly-mimicking structures could improve efficiency, new research suggests.

The new type of LED light bulb could one day light homes while reducing power bills, according to the findings in Optik.

“LED light bulbs play a key role in clean energy,” says Stuart (Shizhuo) Yin, professor of electrical engineering at Penn State. “Overall commercial LED efficiency is currently only about 50 percent. One of the major concerns is how to improve the so-called light extraction efficiency of the LEDs. Our research focuses on how to get light out of the LED.”

Fireflies and LEDs face similar challenges in releasing the light they produce because the light can reflect backwards and get lost. One solution for LEDs is to texture the surface with microstructures—microscopic projections—that allow more light to escape. In most LEDs these projections are symmetrical, with identical slopes on each side.

Fireflies’ lanterns also have these microstructures, but with asymmetric sides that slant at different angles, giving a lopsided appearance.

“Later I noticed not only do fireflies have these asymmetric microstructures on their lanterns, but a kind of glowing cockroach was also reported to have similar structures on their glowing spots,” says Chang-Jiang Chen, doctoral student in electrical engineering and lead author of the study. “This is where I tried to go a little deeper into the study of light extraction efficiency using asymmetric structures.”

Using asymmetrical pyramids to create microstructured surfaces, the team found that they could improve light extraction efficiency to around 90 percent.

According to Yin, asymmetrical microstructures increase light extraction in two ways. First, the greater surface area of the asymmetric pyramids allows greater interaction of light with the surface, so it traps less light. Second, when light hits the two different slopes of the asymmetric pyramids there is a greater randomization effect of the reflections, which gives light a second chance to escape.

After the researchers used computer-based simulations to show that the asymmetric surface could theoretically improve light extraction, they next demonstrated this experimentally. Using nanoscale 3D printing, the team created symmetric and asymmetric surfaces and measured the amount of light emitted. As expected, the asymmetric surface allowed the release of more light.

The LED-based lighting market is growing rapidly as the demand for clean energy increases, and is estimated to reach $85 billion by 2024.

“Ten years ago, you go to Walmart or Lowes, LEDs are only a small portion (of their lighting stock),” says Yin. “Now, when people buy light bulbs, most people buy LEDs.”

LEDs are more environmentally friendly than traditional incandescent or fluorescent light bulbs because they are longer-lasting and more energy efficient.

Two processes contribute to the overall efficiency of LEDs. The first is the production of light—the quantum efficiency—which scientists measure using how many electrons convert to light when energy passes through the LED material. Scientists have already optimized this part in commercial LEDs. The second process is getting the light out of the LED—called the light extraction efficiency.

“The remaining things we can improve in quantum efficiency are limited,” says Yin. “But there is a lot of space to further improve the light extraction efficiency.”

In commercial LEDs, scientists make the textured surfaces on sapphire wafers. First, scientists use UV light to create a masked pattern on the sapphire surface that provides protection against chemicals. Then when scientists apply chemicals, they dissolve the sapphire around the pattern, creating the pyramid array.

“You can think about it this way, if I protect a circular area and at the same time attack the entire substrate, I should get a volcano-like structure,” explains Chen.

In conventional LEDs, the production process usually produces symmetrical pyramids because of the orientation of the sapphire crystals. According to Chen, the team discovered that if they cut the block of sapphire at a tilted angle, the same process would create the lopsided pyramids. The researchers altered just one part of the production process, suggesting they could easily apply the approach to commercial manufacture of LEDs.

The researchers have filed for a patent on this research.

“Once we obtain the patent, we are considering collaborating with manufacturers in the field to commercialize this technology,” says Yin.

Source: Penn State