A new study led by MIT researchers could drive the development of more energy-efficient digital displays — such as flat-screen TVs, augmented and virtual reality headsets, smartphone screens, medical imaging devices, and even large-area ambient lighting surfaces — that also generate richer, brighter colors.
The MIT scientists, in collaboration with researchers at Samsung, studied the microscopic changes that occur inside LEDs that utilize electrically excited quantum dots, which are precisely shaped nanoscale semiconductor particles that emit extremely pure colored light.
Quantum dots are currently used in some of the computer and television displays with the best picture quality available. The efficiency of these displays could be further improved, and their manufacturing process further simplified, if the quantum dots could be electrically excited, as was first demonstrated in the quantum dot LED (QD-LED) structures over 20 years ago.
But limitations on the operating lifespans of these QD-LEDs have prevented their widespread use in commercial applications.
The new study shows how encapsulating QD-LEDs in an acrylate-based resin can extend their lifespan by minimizing the physical degradation that would otherwise occur during QD-LED operation.
The researchers demonstrated that encapsulating QD-LEDs with a resin layer using a simple, scalable process boosts stability and performance. In some devices, resin encapsulation enabled a 5,000-fold lifespan improvement. Importantly, their study reveals the fundamental reasons resin encapsulation is effective.
“The insights into how and why quantum dot LEDs get modified during their operation open the possibility of fixing everything that holds back commercialization of QD-LED displays. This technology can provide a light source like never before — pure in color, paper thin, and of large area, transforming how we produce both displays and general lighting,” says Vladimir Bulović, the Fariborz Maseeh (1990) Professor of Emerging Technology, principal investigator in the Research Laboratory of Electronics (RLE), director of MIT.nano, and senior author of this study.
He is joined on the paper by lead author Ruiqi Zhang, an electrical engineering and computer science graduate student; Moungi Bawendi, the Lester Wolfe Professor of Chemistry; and other colleagues at MIT and Samsung SAIT. The research appears today in Science Advances.
A blue bottleneck
This paper draws on foundational work by Bawendi, who shared the Nobel Prize in Chemistry in 2023 for discovering and synthesizing quantum dots, and engineering work by Bulović, who joined MIT in 2000, when he began collaborating with Bawendi to make efficient LED displays using quantum dots.
Conventional LED displays utilize thousands of tiny lightbulbs that generate the red, green, and blue light needed to create the perception of any color on the visible spectrum. More advanced OLED screens, which Bulović was developing through his graduate work at Princeton University, utilize electrically excited, glowing organic molecules instead of light bulbs.
Bulović, Bawendi, and others at MIT sought to replace the organic molecules with quantum dots, which emit purer red, green, and blue light in a more energy-efficient manner.
“With quantum dots, the color quality of the screen would be more visually appealing and more optically flexible. One can mix and match those quantum dot colors more precisely to generate any color that is needed,” says Bulović.
Their collaboration generated a series of inventions on quantum dot LED technologies, leading to the launch of the startup QD Vision, which successfully commercialized the first-ever displays containing quantum dots. In 2016, QD Vision was acquired by Samsung, which incorporated a less efficient form of quantum dot technology into their “QLED” displays.
Although they are more energy-efficient, electrically excited QD-LEDs have still not been commercialized, particularly since the limited lifetime of the blue QD-LED does not meet the requirements of commercial displays.
“The blue quantum dot LEDs are 50 to 100 times less stable than their red and green counterparts. If you use them in an LED display, your TV might last for just a few months before it stops working. We wanted to understand what is different about the blue quantum dot LEDs,” Zhang says.
A nanoscale investigation
He and his collaborators developed a technique to slice a tiny QD-LED in nanoscale-thin slivers, revealing the device cross-section. They examined these cross-sections under extremely powerful microscopes at MIT.nano. This precise method allowed them to see what happens at the nanoscale to the ultrathin layers of materials stacked inside the QD-LED.
They explored the structural and chemical changes that occurred in each layer of red and blue QD-LEDs by comparing cross-sections of freshly made devices to cross-sections of devices that were operated on overdrive. The researchers found that during operation, the three core functional layers that enable blue QD-LEDs to glow are degraded, with modified morphology and reduced thickness.
The distinct quantum dots also get merged together, losing their shape. This layer thinning and coarsening is caused, in part, by the release of extra hydrogen and oxygen during operation.
“We don’t yet know exactly where these extra elements are coming from — there are so many possibilities. But we definitely don’t want extra hydrogen and oxygen in the device,” Zhang says.
To prevent this degradation, they utilized a technique sometimes adopted by industry. They encapsulated the QD-LEDs with an acrylate-based resin.
They discovered that this encapsulation technique suppresses the release of the hydrogen and oxygen and inhibits some of the degradation that changes the morphology of the layers of the blue QD-LED.
“For the first time, we have insights into the details of what happens inside these structures of many mixed and layered materials that form the QD-LED. No one knew this before,” Bulović says.
This encapsulation strategy, which is a cost-effective and scalable technique, led to an eightfold improvement in the lifetime of red QD-LEDs and more than a 5,000-fold lifetime improvement in blue QD-LEDs.
The researchers believe the resin prevents the formation of moisture in the cloud of gases that surrounds the quantum dot. That moisture likely causes the QD-LED to degrade.
However, their experiments revealed that resin encapsulation does not eliminate all sources of degradation.
The researchers are now exploring the addition of extra layers to QD-LEDs that could further improve efficiency and lifespan. They also plan to build on the lessons learned in this study to increase the stability of QD-LEDs for other applications.
“This version of quantum dot LEDs would be better than anything that exists now — simpler to make, more efficient, and higher performing. This could open vistas into many more ways of thinking about this technology, not just for the sake of displays or lighting, but also for sensors, lasers, and so on,” says Bulović.
This work was funded by the Samsung Advanced Institute of Technology. The research was carried out, in part, using MIT.nano facilities.
de MIT News https://ift.tt/9thzAoW
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