Stacking LEDs instead of placing them side by side could enable fully immersive virtual reality displays
Take apart your laptop screen and you'll find a plate at its center with a pattern of red, green and blue LED pixels, arranged end to end like a detailed Lite Brite display. When powered, the LEDs together can produce every shade in the rainbow, resulting in a full-color display. Over the years, the size of individual pixels has shrunk, allowing more pixels to be packed into devices to produce sharper, higher-resolution digital displays.
But, like computer transistors, LEDs are reaching the limits of how small they can be while operating efficiently. This limitation is particularly evident in close-range displays such as augmented reality and virtual reality devices, where limited pixel density can lead to a "screen door effect" where users perceive streaks in the space between pixels.
Now, MIT engineers have developed a new way to create sharper, defect-free displays. Rather than replacing red, green, and blue light-emitting diodes side by side in a horizontal patchwork, the team invented a way to stack diodes to create vertical, multicolor pixels.
Each stacked pixel can generate the full commercial color range and is approximately 4 microns wide. Micropixels or "microLEDs" can be packed into densities of 5,000 pixels per inch.
"This is the smallest LED pixel, and this is the pixel density reported in the journal," said Jeehwan Kim, associate professor of mechanical engineering at MIT. "We show that vertical pixelation is a way to achieve higher resolution displays in a smaller space."
"With virtual reality, right now there's a limit to how real they can look," adds Jiho Shin, a postdoc in Kim's research group. "Using our vertical micro-LEDs, you can have a completely immersive experience and be unable to distinguish virtual from reality."
The team's results were published in the journal Nature. Kim and Shin's co-authors include members of Kim's lab, researchers at MIT, and collaborators from Georgia Tech Europe, Sejong University, and universities in the United States, France, and South Korea.
Today's digital displays are powered by organic light-emitting diodes (OLEDs) - plastic diodes that emit light in response to an electrical current. OLED is the latest digital display technology, but the diodes degrade over time, causing the screen to age. The technology also reaches the limits of how small diodes can be reduced, limiting their sharpness and resolution.
For next-generation display technology, researchers are exploring inorganic micro-LEDs—diodes that are one-hundredth the size of traditional LEDs and made from inorganic single-crystal semiconductor materials. Compared to OLED, Micro-LED performs better, consumes less energy and lasts longer.
But micro-LED manufacturing requires extreme precision because the tiny red, green and blue pixels need to be grown individually on a wafer and then placed on a board, aligned with each other in order to properly reflect and produce various colors and shades. Achieving this kind of microscopic precision is a difficult task, requiring the entire device to be scrapped if a pixel is found to be out of place.
"This kind of pick-and-place manufacturing is likely to misplace pixels on a very small scale," Kim said. "If you have a misalignment, you have to throw away that material or it will ruin a display."
An MIT team has come up with a way to potentially reduce waste in making micro-LEDs that are not needed to be aligned pixel-by-pixel. This technology is a completely different approach to vertical LEDs compared to traditional horizontal pixel arrangements.
Kim's team focuses on developing technologies to create pure, ultrathin, high-performance membranes with the goal of designing smaller, thinner, more flexible and more practical electronics. The team previously developed a method to grow and exfoliate perfect two-dimensional single crystal materials from silicon wafers and other surfaces - a method they call two-dimensional material-based layer transfer, or 2DLT.
In the current study, the researchers used the same method to grow ultrathin films of red, green and blue LEDs. They then peeled the entire LED film off its base wafer and stacked them on top of each other to create a layer cake of red, green and blue films. They can then carve the cake into a pattern of tiny vertical pixels, each just 4 microns across.
"In a traditional display, each R, G and B pixel is arranged horizontally, which limits the size of each pixel you can create," Shin points out. "Because we stack all three pixels vertically, we can theoretically reduce the pixel area by a third."
As a demonstration, the team built a vertical LED pixel and showed that they could produce a variety of colors in a single pixel by varying the voltage applied to each pixel's red, green, and blue films.
"If you have a higher red current and a weaker blue current, the pixel will appear pink, and so on," Shin said. "We are able to create all mixed colors and our displays can cover close to the available commercial color space."
The team plans to improve the operation of vertical pixels. So far, they have shown that they can stimulate a single structure to produce a full spectrum of colors. They will work on making arrays of many vertical micro-LED pixels.
"You need a system to control 25 million LEDs individually," Shin said. "Here we have only partially demonstrated this. Active matrix operation is something we need to develop further."
"So far, we've shown the community that we can grow, peel and stack ultra-thin LEDs," Kim said. “This is a solution for small displays like smart watches and virtual reality devices, where you need a high density of pixels to produce vivid, vivid images.”
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