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The screens on today’s electronic doodads come in all shapes, sizes, and resolutions, but they have one important thing in common: the red, blue, and green sub-pixels are arranged side-by-side. Even with smaller pixels packed as tightly as possible, this flat arrangement is reaching its theoretical density limit. The next step may be the stackable LED technology developed by a team at MIT. Using a novel ultrathin membrane fabrication process, the researchers have created a display with red, blue, and green pixels in a vertical stack. This could push pixel density an order of magnitude higher, making virtual reality visually indistinguishable from the real world.
Today’s highest-quality displays use organic light-emitting diodes (OLEDs), which produce light when fed an electric current. Even with clever pentile sub-pixel arrangements, the density can’t go much higher. As the pixels get smaller, so too does the amount of light they can emit. The next step is probably microLED, which uses diodes 100 times smaller and composed of inorganic materials. The problem is that microLEDs are fabricated separately as red, green, and blue matrices, which then have to be overlaid with extreme accuracy, leading to higher costs and more waste — if even one pixel is misaligned, the entire panel might need to be discarded.
As detailed in the newly published study in Nature, the technology developed at MIT is essentially a better, less wasteful way to make microLEDs. The research was led by Jeehwan Kim, an engineering professor specializing in fabricating ultrathin, high-performance membranes. The team used past work on 2D membrane materials to manufacture membranes of red, green, and blue sub-pixels. They peeled the membranes away from the rigid base layers and stacked them on top of each other, creating vertically full-color pixels just 4 micrometers wide.
“This is the smallest microLED pixel, and the highest pixel density reported in the journals,” says Jeehwan Kim. These color stacks are just the first step — the team did show that by altering the input voltage, they were able to produce multiple colors in each stacked pixel. However, we would still need a system to control 25 million separate LEDs. Developing that active matrix is something Kim’s group will work on in the future.
Simply being able to fabricate these stacked ultrathin displays is a big step, though. The effective pixel density of the team’s microLED matrix is 5,000 pixels per inch — the highest ever reported in the literature. By comparison, the latest OLED displays from Samsung are about 500 pixels per inch, and even the sharpest VR headsets are under 1,000. If commercialized, vertical pixels would represent a huge increase in clarity, particularly for VR and AR applications. Powering an ultra-high-resolution display is an entirely different challenge, though.
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