Firefly inspired surfaces improve the efficiency of LED bulbs – ScienceDaily

"LED bulbs play a key role in clean energy," said Stuart (Shizhuo) Yin, professor of electrical engineering. "The overall commercial efficiency of LEDs is currently only around 50%. One of the major concerns is how to improve the so-called suction efficiency of LED light. Our research focuses on how to extract light from LEDs. "

Fireflies and light-emitting diodes (LEDs) face similar challenges in releasing the light they produce because the light can be reflected back and lost. One solution for LEDs is to texture the surface with microstructures – microscopic projections – allowing more light to escape. In most LEDs, these projections are symmetrical, with identical slopes on each side.

Firefly lanterns also have these microstructures, but the researchers noted that the microstructures of firefly lanterns were asymmetrical – their sides were slanted at different angles, giving an asymmetrical appearance.

"Later, I noticed that not only did fireflies have these asymmetrical microstructures on their lanterns, but a kind of gloomy cockroach would also have similar structures on their bright spots," said Chang-Jiang Chen, PhD student. in electrical engineering the study. "That's where I tried to deepen a little study of the efficiency of light extraction using asymmetric structures."

Using asymmetric pyramids to create microstructured surfaces, the team found that they could improve the light extraction efficiency up to about 90%. The results were recently published online in Optik and will appear in the print edition of April.

According to Yin, asymmetric microstructures increase the extraction of light in two ways. First, the larger area of ​​the asymmetric pyramids allows greater interaction of light with the surface, so less light is trapped. Second, when the light hits the two different slopes of the asymmetrical pyramids, the effect of randomization of reflections is more important and the light has a second chance to escape.

After using computer simulations to show that the asymmetric surface could theoretically improve light extraction, the researchers then experimentally demonstrated this possibility. Using nanoscale 3D printing, the team created symmetrical and asymmetrical surfaces and measured the amount of light emitted. As expected, the asymmetric surface allowed more light to be released.

The LED – based lighting market is growing rapidly with increasing demand for clean energy and is expected to reach $ 85 billion by 2024.

"Ten years ago you went to Walmart or Lowes, LEDs only represent a small portion (of their lighting stock)," Yin said. "Now, when people buy light bulbs, most people buy LEDs."

LEDs are more environmentally friendly than conventional incandescent or fluorescent bulbs because they last longer and consume less energy.

Two processes contribute to the overall efficiency of LEDs. The first is the light output – the quantum efficiency – which is measured by the number of electrons converted to light when the energy passes through the LED material. This part has already been optimized in commercial LEDs. The second process is to extract the light from the LED, called light extraction efficiency.

"The remaining progress we can improve in terms of quantum efficiency is limited," said Yin. "But there is a lot of space to improve the efficiency of light extraction."

In commercial LEDs, textured surfaces are made on sapphire wafers. First, UV light is used to create a hidden pattern on the sapphire surface that protects chemicals. Then, when chemicals are applied, they dissolve the sapphire around the pattern, creating a pyramid network.

"You can think of it this way: If I protect a circular area while attacking the entire substrate, I should have a volcano-like structure," Chen said.

In conventional LEDs, the production process typically produces symmetrical pyramids due to the orientation of the sapphire crystals. According to Chen, the team discovered that if they cut the sapphire block at an inclined angle, the same process would create asymmetrical pyramids. The researchers only modified part of the production process, suggesting that their approach could easily be applied to the commercial manufacture of LEDs.

The researchers have filed a patent for this research.

"Once the patent is obtained, we plan to work with the industry to market this technology," Yin said. Jimmy Yao, Wenbin Zhu, Ju-Hung Chao, Annan Shang and Yun-Goo Lee, PhD students in Electrical Engineering, also participated in the project.