Color-tunable LEDs could reduce screens' harsh light and enhance image resolution

April 20, 2021

Color-tunable LED aims to reduce screens' blue light, improve image quality. (Unsplash/Victoria Heath)

Physicists have invented a novel LED chip that could effectively glow in any tone, in contrast with other models that require mixing together a trinity of colors — red, green and blue — to form a spectrum of shades.

By developing a single chip that can switch between colors, the inventors say they addressed two major problems with traditional LED: the blue light's suspected disruption of human sleep, and a trade-off between high resolution and low energy consumption in small-screen devices. A patent application for the international collaboration's color-tunable LED was published by the U.S. Patent and Trademark Office on March 25.

LED, or light-emitting diode, displays typically have three color chips per pixel: red, green and blue. For a chip to emit color, the material inside must be pulsed by a voltage. That induces an energy state of the light-producing material, which then takes on an associated tone. Different materials reach different energy states to make different colors.

However, the researchers found that after adding a specific ion to gallium nitride, the material used for blue, and pulsing it, there was a time lag between the induced energy state and the consequent emission of color. Catching the material within that lag and pulsing it a second time induces another energy state, and a new color — and this can be done a third time, and theoretically, a fourth or more. 

Like the 2010 film Inception's dream-within-a-dream concept, the material could enter excited state within excited state, with each state manifesting a separate color.

Volkmar Dierolf, chair of the physics department at Lehigh University and one of the researchers behind the novel LED, told The Academic Times he had two motivations for inventing it. 

First, the existing need for three color chips of varying energy efficiency per pixel, with each based on a separate material, makes it more difficult to manufacture small displays, such as Apple Watches, that call for high resolution and low energy consumption. 

Additionally, the blue chip emits the brightest light, with the highest efficiency. So the digital blue light, notorious for its presumed impacts on human sleep cycles, tends to remain within a screen as a sort of backlight, even if the display requires nonblue colors. Otherwise, a device has to use extra energy for a warm tone that's equally bright. 

"It's much easier to make it that very cold, bluish light," Dierolf said. "Making it similarly bright but making it a little bit more reddish [with] warmer colors and moving around there — that is the challenge."

For instance, LCD, or liquid-crystal displays, which are used in some iPhones, start with all three colors turned on. "Typically, the display technology — mostly the LCD-type ones — are just taking away colors," Dierolf said. "They start from that very bluish white, and then they take things away."

Still, there's usually a limit to how much blue one can truly remove, because of its role in effective screen illumination. That limitation is apparent when one looks at the difference between the outer black rim of an iPhone and the device's black screen while the phone is turned on: The screen will never be quite as dark as the rim.

Frequent exposure to blue light is widely speculated to negatively affect human sleep patterns and contribute to eye strain. That's why some technology companies are promoting forms of "night mode," which reduces a screen's blue light and turns it toward a warmer tone. 

"Medical research suggests to move away from the bluish light because there are studies that show [it contributes to] insomnia and related issues," Dierolf said. But according to the researcher, the downside of night mode is that it drains phone batteries because of warmer colors' lack of energy efficiency.

Dierolf's team addressed both these issues — of sleep disturbance and small-screen image resolution — by altering the blue-prone gallium nitride for the new LED. But first the researchers had a hurdle to overcome, because this material is traditionally troublesome when one attempts to make it emit a color other than blue.

"The general problem that exists with the gallium nitride technology for lighting applications," Dierolf explained, "is that the LEDs are pretty good for blue lights, and they are OK for green light, but creating red light is very, very difficult."

Conventionally, Dierolf continued, "We have a blue LED, and then we put an extra piece of material that absorbs part of the blue and re-emits in a wider range of spectrum to give us a [different] color."

Moving gallium nitride from blue to green and then to red typically involves adding a material called indium. But adding too much can lead to consequences, such as the indium reacting with the nitride directly and "segregating," Dierolf said, meaning it pulls nitride away from the other material. In this way, the emission efficiency is reduced as the gallium nitride LED approaches red or yellow tones.

Instead of indium, Dierolf's invention uses a rare-earth ion dopant in the gallium nitride LED. When pulsed, these ions changed the blue light straight into red with high efficiency. With this development, the team achieved its initial goal of making a red LED on par with the blue LED.

But along the way, Dierolf stumbled upon a surprising aspect of the ion. 

"We obtained a very deep understanding of the processes that are involved," Dierolf said. "And what you find is that the ion, it gets excited very effectively with high efficiency, but the ion is very slow."

He continued, "It stays in that excited state for, like, 200 or so microseconds, and then it will emit the red light. But while it's in that excited state, that gives you an opportunity to modify it to excited again."

If the ion is excited again within that 200-microsecond gap, Dierolf explained, it can result in a different energy state, another — albeit smaller — time lag, and a new color. And, he says, that process can be repeated.

"You could imagine, and we can also demonstrate," Dierolf said, "that you can create yet another reservoir that is probably left for just two microseconds, then, from there, you can still go into a third system, which then would probably emit in the blue."

While his research demonstrated it's possible to produce the three major colors, Dierolf believes the technology could be manipulated to produce many other colors and shades, as well.

"By timing these pulses, you can manipulate from where the emission would come from," he continued. "We can shuffle this thing around by a smart way of exciting it."

The application for the patent, "Color tunable light emission diode and micro LED display," was filed April 14, 2020 with the U.S. Patent and Trademark Office. It was published March 25 with the application number 16/848175. The earliest priority date was Sep 23, 2019. The inventors of the pending patent are Volkmar Dierolf, Brandon Mitchell, Ruoqiao Wei, Yasufumi Fujiwara, Tomasz Gregorkiewicz, Shuhei Ichikawa, Jun Tatebayashi and Dolf Timmerman. The applicants listed are Lehigh University and Osaka University. 

Parola Analytics provided technical research for this story. 

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