Power/Performance Bits: Dec. 28

Shrinking LEDs
Researchers from King Abdullah University of Science and Technology (KAUST) are working to make LEDs smaller.

Micrometer-scale light-emitting diodes (μLEDs) could be an ideal building block for future microLED displays, but devices based on nitride-based alloys used to achieve a broad color range become poor emitters of light when shrunk to micrometer scales.

“The main obstacle to reducing the size of the devices is the damage to the sidewalls of the LED structure generated during the fabrication process,” said Martin Velazquez-Rizo, a Ph.D. student at KAUST. “Defects provide an electrical path for a leakage current that does not contribute to the light emission.” This effect gets worse as the size of the LED shrinks, which has limited the LED size to approximately 400 by 400 micrometers. Blue and green indium gallium nitride (InGaN) µLEDs have shown good performance even down to 1 μm, but red has proven challenging.

The researchers constructed bright red InGaN µLEDs measuring 17 × 17 micrometers, which were placed in a 10 by 10 array, using atomic deposition. A chemical treatment applied to the array eliminated the damage to the µLED sidewalls. “We confirmed with atomic-scale observations that the sidewalls had high crystallinity after the treatment,” said Velazquez-Rizo. “Performing this type of observation requires specialized tools and sample preparation.”

Each square millimeter on the device’s surface had high output power of 1.76 milliwatts, compared to previous devices that reported an output power of less than 1 milliwatt per millimeter square. The team then demonstrated their red μLEDs with green and blue InGaN μLEDs to create a wide color-range device. “The next step in our research is to further improve the efficiency of our μLEDs and decrease their lateral dimensions below 10 micrometers,” said Velazquez-Rizo.

Foldable QLEDs
Researchers from the Institute for Basic Science in Korea, Daegu Gyeongbuk Institute of Science and Technology (DGIST), and Pusan National University developed ultra-thin quantum dot light-emitting diodes (QLEDs) that are foldable.

The foldable QLEDs could be freely transformed into various user-customized 3D structures, such as butterflies, airplanes, and pyramids, as demonstrated in a video produced by the researchers.

Previously, the team created a flexible QLED display 3 micrometers thick. To make it truly foldable, a new fabrication process was deployed that can partially etch the epoxy film deposited on the QLED surface without damaging the underlying QLED. Using a power-controllable carbon dioxide pulsed laser and silver-aluminum alloy-based etch-stop layers, the etching depth can be precisely controlled. As the laser-etched part of the device is relatively thinner than the surrounding region, it is possible to etch out deformation lines along which the device can be folded like origami paper.

With the technique, the researchers were able to control the radius of curvature down to less than 50 micrometers. The entire QLED including the fold line was able to maintain a stable light-emitting performance even when after it was repeatedly folded 500 times.

“We were able to build a 3D foldable QLED that can be freely folded just like a paper artwork”, said Kim Dae-Hyeong, the vice-director of the Center for Nanoparticle Research at IBS. He also said, “By fabricating the passively driven, 3D foldable QLED arrays composed of 64 individual pixels, we have shown the possibility of developing displays with greater complexity in the future.”

Battery-free DIY
Researchers from Northwestern University and Delft University of Technology are offering a platform that allows hobbyists to build battery-free electronic devices that use intermittent harvested energy.

The platform includes the energy-harvesting BFree Shield as well as a power-failure-resistant version of Python. They said it can be applied to many kinds of do-it-yourself battery powered electronics projects, such as motion sensors and other smart home devices. Currently, a solar module can be attached to the BFree Shield.

“Right now, it’s virtually impossible for hobbyists to develop devices with battery-free hardware, so we wanted to democratize our battery-free platform,” said Josiah Hester, an assistant professor of electrical and computer engineering and computer science at Northwestern. “Makers all over the internet are asking how to extend their devices’ battery life. They are asking the wrong question. We want them to forget about the battery and instead think about more sustainable ways to generate energy.”

The BFree hardware. (Credit: TU Delft/Northwestern University)

“The maker community is typically more interested in rapidly deploying their devices, and that quickness doesn’t always go well with sustainability,” added Przemyslaw Pawelczak, an assistant professor in the Embedded and Network Systems Group at TU Delft. “We wanted to design a viable product that can connect these two worlds.”

To enable devices to run on intermittent energy, BFree pauses calculations when power is interrupted. When power returns, it automatically resumes where it left off without losing memory or needing to run through a long list of operations before restarting.

To make the process user friendly, the researchers coded BFree with software to interpret Python programs for battery-free devices. A user can attach the BFree Shield onto the Adafruit Metro M0 maker platform (or slightly modify it to work with other CircuitPython-based platforms) and then program the device as they typically would.

“We wanted to make it totally invisible for the final user,” said Vito Kortbeek, a Ph.D. candidate at TU Delft. “So, we tried to keep the original experience of the device the same without the user seeing how we changed the software to interpret the Python files for battery-free technology.”