Vertical LEDs could triple resolution of displays

Today’s screens use a plate patterned with pixels formed from red, green, and blue LEDs arranged end to end, which shine in different intensities to generate the full spectrum of colours.

Over the years, the size of individual pixels has shrunk, enabling many more of them to be packed into devices to produce sharper, higher-resolution digital displays.

But much like computer transistors, LEDs are reaching a limit to how small they can be while also performing effectively.

This limit is especially noticeable in close-range displays such as augmented and virtual reality devices, where limited pixel density results in a “screen door effect” such that users perceive stripes in the space between pixels.

The new stacked pixels can generate the full range of colours and measure about 4 microns wide. The microscopic pixels, or 'micro-LEDs', can be packed to a density of 5,000 pixels per inch.

“This is the smallest micro-LED pixel, and the highest pixel density reported in the journals,” said Jeehwan Kim, associate professor of mechanical engineering at MIT. “We show that vertical pixelation is the way to go for higher-resolution displays in a smaller footprint.”

“For virtual reality, right now there is a limit to how real they can look,” adds Jiho Shin, a postdoc in Kim’s research group. “With our vertical micro-LEDs, you could have a completely immersive experience and wouldn’t be able to distinguish virtual from reality.”

The MIT team previously developed a technique to fabricate pure, ultrathin, high-performance membranes, with a view toward engineering smaller, thinner, more flexible and functional electronics.

They employed this same approach to grow ultrathin membranes of red, green, and blue LEDs.

They then peeled the entire LED membranes away from their base wafers, and stacked them together to make a layer cake of red, green, and blue membranes. They could then carve the cake into patterns of tiny, vertical pixels, each as small as 4 microns wide.

“In conventional displays, each R, G, and B pixel is arranged laterally, which limits how small you can create each pixel,” Shin added. “Because we are stacking all three pixels vertically, in theory we could reduce the pixel area by a third.”

As a demonstration, the team fabricated a vertical LED pixel, and showed that by altering the voltage applied to each of the pixel’s red, green, and blue membranes, they could produce various colours in a single pixel.

“If you have a higher current to red, and weaker to blue, the pixel would appear pink, and so on,” Shin said. “We’re able to create all the mixed colours, and our display can cover close to the commercial colour space that’s available.”

The team plans to improve the operation of the vertical pixels. So far, they have shown they can stimulate an individual structure to produce the full spectrum of colours. They will work toward making an array of many vertical micro-LED pixels.

“You need a system to control 25 million LEDs separately,” Shin says. “Here, we’ve only partially demonstrated that. The active-matrix operation is something we’ll need to further develop.”