Wednesday, July 18, 2018

A Bright Future: Quantum Dots and the Quest for Energy Efficient White Lights

From stadium lights to night lights, the modern way of life runs on artificial illumination. This lighting is costly—in both environmental and economic terms—but last week in the journal Optica, a team of researchers from Koç University in Turkey introduced a new kind of white light source, based on blue LEDs and quantum dots, that could lower the costs of lighting up our world.

Containers of red and green quantum dots illuminated under ultraviolet light.
Image Credit: Sedat Nizamoglu, Koç University.
We use three main types of lighting today: incandescent, fluorescent, and LED. All are produced in different ways, require different technologies, and have different spectral characteristics. Among the three, light emitting diodes (LEDs) are the most energy efficient, most versatile, and longest-lasting. For these reasons and more, we’re in the middle of an LED revolution.

Some materials produce light when an electric current or an electric field travels through them; this is called electroluminescence. LEDs are made of semiconductor materials that have been engineered to display electroluminescence in a specific color range, though this engineering is easier said than done: creating a material with just the right properties to light up a certain color is so notoriously difficult that 2014's Nobel prize went to the scientists who finally figured out the blue LED. Electroluminescence is a great method for producing the individual colors of the rainbow, but not for white light: since white is a mixture of all of the colors of light, rather than an actual color itself, you can’t produce white light from electroluminescence alone.

The most common way of making a white LED is to cover a blue LED with a phosphor-based coating. It works like this: The blue LED produces blue light. Some of this blue light is absorbed by the phosphor coating, which in turn produces a yellowish light (“yellow-ish” because it actually produces a broad range of colors centered on yellow). This light mixes with the rest of the blue light to produce white light.

This works well enough, as you can see by the white LED bulbs illuminating homes and office buildings these days, but there are reasons to aim higher. First, we use a ton of white light. Reducing energy consumption is a challenge we have to take seriously. Second, for many applications, it’s important to be able to fine tune the properties of light. Given the broad range of colors emitted by a phosphor-coating, that’s not easy to do with traditional white LEDs.

Enter "quantum dots": nano-sized bits of semiconducting materials that emit light of different colors based on their size—a trait that sets them apart from   They produce a much narrower range of light than phosphor coatings, so they present an appealing alternative to traditional white LEDs. The idea is to replace the phosphor coating on a blue LED with a layer of quantum dots that emit red and green light. The combination of blue, red, and green light would produce high-quality white light whose properties you could easily tune by changing the concentrations and sizes of the dots. It sounds great in theory, but it turns out that such devices have a problem with efficiency.

“Quantum dots hold great promise for efficient lighting applications, and they are still operating below their potential,” says Sedat Nizamoglu, one of the senior researchers behind the new white LED. The good news is that there is a lot of room to generate more efficient technologies.

Two researchers inject a solution of quantum dots into a lens.
Image Credit: Sedat Nizamoglu, Koç University.
With this in mind, the Koç University team took a new approach to creating quantum-dot based white LEDs. First, they systematically synthesized quantum dots under different conditions and measured their fluorescence in response to blue light. This was challenging and time-consuming, but the team identified the quantum dots that most efficiently convert blue light to red and green light. Next, the team explored how the concentrations of green and red quantum dots impact the process, this time with simulations, so they could optimize the concentrations for efficiency as well.

Finally, the team designed a prototype white LED that addressed efficiency from yet another angle. Quantum dots are synthesized in a liquid. When you transfer them to a solid surface, like the top of a blue LED, a lot of the light produced by the quantum dots is absorbed by the surface. To avoid these losses, the researchers created a flexible lens—somewhere between a soft contact lens and a tiny, transparent water balloon—that covers the blue LED. They injected a liquid containing the red and green quantum dots right into the lens, allowing them to stay in liquid form. To get a glimpse of the process, check out the video below:

The result was the most efficient quantum-dot based white LED to-date. While it still lags behind phosphor-based white LEDs, the researchers have noted several ways to further optimize their system. They estimate that this approach has the potential to yield a white LED with an efficiency of 200 lumens per electrical watt, a target that researchers worldwide are aiming to reach in the next few years. In addition, this approach is easy, inexpensive, and should be scalable for mass production.

The results are promising, but there’s a lot to do before the commercial potential of this system can really be assessed. “The first step is to further increase the efficiency levels of the LEDs,” explains Nizamoglu, “and the second one is to reach high-efficiency levels with ‘green’ materials. For commercialization, synthesis procedures need to be scaled up to industry level in order to mass produce red- and green-emitting quantum dots.” In addition, the researchers need to study the long-term stability of the liquid-based LED approach in more detail.

Nizamoglu appreciates the fact that he can actually see the outcome of this kind research, at least in the lab. In some areas of research, he says, scientists work on projects they can’t really see or touch. “One of the most interesting part of working on white LEDs is that after you combine all the components, you can see the bright emission generated by the LED, which motivates us further to do better,” he says.

—Kendra Redmond

No comments:

Post a Comment