Scientists stack sub-pixels for sharper MicroLED displays

The sharpness of images on a MicroLED screen is limited by how tightly the pixels that make up the display are packed. MIT scientists have taken a unique approach to packing them much tighter, by vertically stacking the pixels' components.

On a regular OLED TV (or computer screen), each pixel is actually made up of three OLED sub-pixels – one red, one green, one blue – that are arranged side by side. By illuminating those tiny OLEDs in different combinations, the pixels are able to produce a wide range of colors.

On some newer TVs, micro-LEDs serve as the sub-pixels, instead of OLEDs. Such MicroLED TVs are claimed to combine the brilliant colors and deep blacks of OLED screens with the brightness of LCD screens.

That said, micro-LED pixels can't be packed as densely as OLED pixels. While this might not matter much on a TV screen, the lower resolution could be noticeable in devices such as VR headsets.

If each micro-LED pixel was only one sub-pixel wide (instead of three), it would be possible to squeeze three times as many pixels into a given amount of screen space, greatly improving image resolution. The MIT team has done just that, by creating pixels in which the micro-LEDs are stacked vertically, not laid out laterally.

Led by Assoc. Prof. Jeehwan Kim, the scientists developed a technique which begins with ultra-thin red, green and blue LED membranes being stacked one on top of the other, forming a layer-cake-like arrangement. That "cake" is then finely sliced in a grid pattern, dividing it up into a multitude of individual pixels – each one is just 4 microns wide.

In lab tests, by altering the voltage applied to each micro-LED within one such pixel, the researchers were able to produce a rainbow of colors, as is possible with an OLED pixel. One might wonder, though, wouldn't the blue micro-LED at the top of the stack always show up the most, with the red micro-LED at the bottom always showing up the least?

"The color (energy) of light emitted by an LED depends on the LED's band gap – blue LEDs have the widest band gap (hence blue light has the highest energy) and red LEDs have the smallest (lowest energy)," study co-author Jiho Shin explained to us. "A material will not absorb light (photons) with energy smaller than the band gap, so red and green light will penetrate through the blue LED layer unabsorbed, which is why we stack these layers vertically in the order of R (bottom), G (middle), B (top)."

The scientists are now working on methods of controlling millions of micro-LED pixels simultaneously, as would be required in devices such as VR headsets, TVs or computer screens.

A paper on the research was recently published in the journal Nature.

Source: MIT