"LED lightbulbs play a key role in clean energy," said Stuart (Shizhuo) Yin, professor of electrical engineering. "Overall commercial LED efficiency is currently only about 50 per cent. 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 that they produce because the light can reflect backwards and is 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 the researchers noticed that the microstructures on firefly lanterns were asymmetric: the sides slanted at different angles, giving a lopsided appearance.
Using asymmetrical pyramids to create microstructured surfaces, the team found that they could improve light extraction efficiency to around 90 per cent.
According to prof. 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 that less light is trapped. Second, when light hits the two different slopes of the asymmetric pyramids there is a greater randomisation effect of the reflections and light is given 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 more light to be released.
The LED-based lighting market is growing rapidly as the demand for clean energy increases, and is estimated to reach $85 billion by 2024.
Two processes contribute to the overall efficiency of LEDs. The first is the production of light - the quantum efficiency - which is measured by how many electrons are converted to light when energy passes through the LED material. This part has already been optimised 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," said prof. Yin. "But there is a lot of space to further improve the light extraction efficiency."
In commercial LEDs, the textured surfaces are made on sapphire wafers. First, UV light is used to create a masked pattern on the sapphire surface that provides protection against chemicals. Then when chemicals are applied, 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," explained Chang-Jiang Chen, doctoral student in electrical engineering and lead author in the study.
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 their approach could easily be applied to commercial manufacture of LEDs.
"Once we obtain the patent, we are considering collaborating with manufacturers in the field to commercialise this technology," said prof. Yin.