By expanding on existing designs for electrodes of ultra-thin solar panels, researchers have developed a new architecture for organic light-emitting diode (OLED) displays that could enable televisions, smartphones, and virtual or augmented reality devices with resolutions of up to 10,000 pixels per inch (PPI). (For comparison, the resolutions of new smartphones are around 400 to 500 PPI.) The “metaphotonic” OLED displays would also be brighter and have better color accuracy than existing versions and would be much easier and cost-effective to produce.
At the heart of an OLED are organic light-emitting materials sandwiched between highly reflective and semi-transparent electrodes that enable current injection into the device. When electricity flows through an OLED, the emitters give off red, green, or blue light. Each pixel in an OLED display is composed of smaller sub-pixels that produce these primary colors. When the resolution is sufficiently high, the pixels are perceived as one color by the human eye. OLEDs are thin, light, and flexible and produce brighter and more colorful images than other kinds of displays.
This research aims to offer an alternative to the two types of OLED displays currently commercially available. One type — a red-green-blue OLED — has individual sub-pixels that each contain only one color of emitter. These OLEDs are fabricated by spraying each layer of materials through a fine metal mesh to control the composition of each pixel. They can only be produced on a small scale, however, like what would be used for a smartphone.
Larger devices like TVs employ white OLED displays. Each of these sub-pixels contains a stack of all three emitters and then relies on filters to determine the final sub-pixel color, which is simpler to fabricate. Since the filters reduce the overall output of light, white OLED displays are more power-hungry and prone to having images burn into the screen.
The innovation behind both a solar panel and the new OLED is a base layer of reflective metal with nanoscale corrugations called an optical metasurface that can manipulate the reflective properties of light and thereby allow the different colors to resonate in the pixels. These resonances are key to facilitating effective light extraction from the OLEDs. Red emitters, for example, have a longer wavelength of light than blue emitters, which, in conventional RGB-OLEDs, translates to sub-pixels of different heights. In order to create a flat screen, the materials deposited above the emitters have to be laid down in unequal thicknesses. By contrast, in the proposed OLEDs, the base layer corrugations allow each pixel to be the same height and this facilitates a simpler process for large-scale as well as microscale fabrication.
The researchers produced miniature proof-of-concept pixels that, when compared with color-filtered white-OLEDs, had a higher color purity and a twofold increase in luminescence efficiency — a measure of how bright the screen is compared to how much energy it uses. They also allow for an ultrahigh pixel density of 10,000 pixels per inch.