Novel OLED architecture could allow resolution of 10,000ppi

Such high pixel-density displays could appear to show true-to-life detail even when placed just centimetres from the eye, such as in VR headsets.

The unique design takes advantage of the fact that – on the nanoscale – light can flow around objects like water. “The field of nanoscale photonics keeps bringing new surprises and now we’re starting to impact real technologies,” said Professor Mark Brongersma, who led the project. “Our designs worked really well for solar cells and now we have a chance to impact next-generation displays.”

OLED displays are made up of organic, light-emitting materials sandwiched between reflective electrodes which allow current to flow into the device, causing the emitters to release red, green or blue light. Each pixel is composed of sub-pixels which produce these primary colours and, when the resolution of the display is sufficiently high, the pixels appear as one colour.

There are two types of OLED display commercially available: an RGB-OLED has individual sub-pixels which each contain one colour and which are used for small devices, while a white-OLED has sub-pixels containing a stack of all three coloured emitters and uses filters to determine the final colour (used for larger displays).

The new architecture has a base layer of reflective metal with nanoscale corrugations; this metasurface can manipulate the reflective properties of light and therefore allow different wavelengths (corresponding to colours) to resonate in the pixels. These resonances are key to facilitating effective light extraction from the OLEDs.

“This is akin to the way musical instruments use acoustic resonances to produce beautiful and easily audible tones,” said Brongersma.

Red emitters have a longer wavelength than blue emitters, which corresponds to sub-pixels of different heights in conventional RGB-OLEDs. Creating a flat screen requires the materials deposited above the emitters to be laid down in various thicknesses. In the new architecture, however, the metasurface allows each pixel to be the same height. This facilitates a far simpler process for large-scale and micro-scale fabrication.

The researchers produced miniature proof-of-concept pixels, which had a higher colour purity than white-OLEDs while doubling luminescence efficiency. They also allow for a pixel density of up to 10,000ppi. For comparison, the latest smartphones tend to have a resolution of around 400-500ppi.

Samsung is now pursuing the Stanford team’s work, in an effort to create a full-size display based on this new OLED architecture.

Among other applications, a display with ultra-high resolution could eventually allow for a far more comfortable and immersive VR experience. VR users often complain of the “screen door” effect; the fine gaps between pixels which appear when a display is placed so close to the eyes.