Scientists borrow solar panel tech to create new ultrahigh-res OLED show
Mapping of the meta-OLED display and the underlying metaphotonic layer, which improves the overall brightness and color of the display while keeping it thin and energy efficient. Photo credit: Samsung Advanced Institute of Technology
By expanding existing designs for electrodes for ultra-thin solar modules, Stanford researchers and employees in Korea have developed a new architecture for OLED displays (Organic Light Emitting Diode) that enable televisions, smartphones and virtual or augmented reality devices with resolutions from to to 10,000 pixels per inch (PPI). (For comparison: the resolutions of new smartphones are 400 to 500 PPI.)
Such high pixel density displays can deliver stunning images with detailed details – which is even more important for headset displays that are inches from our faces.
The progress is based on research by materials scientist Mark Brongersma of Stanford University in collaboration with the Samsung Advanced Institute of Technology (SAIT). Brongersma was originally introduced to this research path because he wanted to develop an ultra-thin solar panel design.
“We took advantage of the fact that, at the nano-scale, light can flow around objects like water,” said Brongersma, professor of materials science and engineering and senior author of the October 22nd science paper that detailed this research. “The field of nanoscale photonics keeps bringing new surprises and now we are starting to influence real technologies. Our designs have worked very well for solar cells and now we have the chance to influence next generation displays.”
The new “metaphotonic” OLED displays not only have a record-breaking pixel density, but are also brighter and more color-accurate than existing versions. They are also much easier and cheaper to manufacture.
The heart of an OLED are organic, light-emitting materials. These are located between highly reflective and semi-transparent electrodes, which enable current to be injected into the device. When current flows through an OLED, the emitters emit red, green, or blue light. Each pixel in an OLED display is made up of smaller sub-pixels that create these primary colors. If the resolution is sufficiently high, the pixels will be perceived as one color by the human eye. OLEDs are an attractive technology because they are thin, light and flexible, and produce brighter and more colorful images than other types of displays.
This research aims to offer an alternative to the two types of OLED displays that are currently commercially available. One type – called red-green-blue OLED – has individual subpixels that each contain only one emitter color. These OLEDs are made by spraying each layer of material through a fine metal mesh to control the composition of each pixel. However, they can only be made on a small scale, like a smartphone would.
Larger devices like televisions use 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 easier to manufacture. As the filters reduce the overall light output, white OLED displays are more power hungry and tend to burn images into the screen.
WED-Jae Joo, a SAIT scientist, thought about OLED displays when visiting Stanford from 2016 to 2018. During this time, Joo heard a presentation from Stanford graduate student Majid Esfandyarpour on ultra-thin solar cell technology, in which he developed Brongersma’s lab and realized that it had applications beyond renewable energy.
“Professor Brongersma’s research topics were all very academically profound and like hidden gems to me as an engineer and researcher at Samsung Electronics,” said Joo, lead author of the science paper.
After the presentation, Joo turned to Esfandyarpour with his idea, which led to a collaboration between researchers from Stanford, SAI and Hanyang University in Korea.
“It was very exciting to see that an issue that we have been thinking about in a different context can have such an important impact on OLED displays,” said Esfandyarpour.
A basic foundation
The decisive innovation behind the solar panel and the new OLED is a base layer made of reflective metal with nanoscale (smaller than microscopic) waves, which is known as the optical meta-surface. The meta-surface can manipulate the reflective properties of light and thereby enable the different colors in the pixels to resonate. These resonances are key to facilitating effective light extraction from the OLEDs.
“This is similar to the way musical instruments use acoustic resonance to create beautiful and easily audible sounds,” said Brongersma, who conducted the research at the Geballe Laboratory for Advanced Materials at Stanford.
For example, red emitters have a longer light wavelength than blue emitters, which in conventional RGB OLEDs leads to subpixels of different heights. In order to create a flat screen as a whole, the materials deposited over the emitters must be deposited in unequal thicknesses. In contrast, in the proposed OLEDs, the corrugations of the base layer allow each pixel to have the same height, and this facilitates a simpler process for large-scale as well as micro-scale manufacture.
In laboratory tests, the researchers successfully produced miniature proof-of-concept pixels. Compared to color-filtered white OLEDs (used in OLED televisions), these pixels had higher color purity and a two-fold increase in luminescence efficiency – a measure of how bright the screen is compared to how much energy it uses. They also enable an ultra-high pixel density of 10,000 pixels per inch.
The next steps in integrating this work into a full-size display are being followed by Samsung and Brongersma is eagerly awaiting the results in hopes of being among the first to see the meta-OLED display in action.
Light from rare earths: New possibilities for organic light-emitting diodes
Won-Jae Joo et al., Metasurface-controlled OLED displays over 10,000 pixels per inch, Science 23 Oct 2020: Vol. 3, No. 370, edition 6515, pp. 459-463 DOI: 10.1126 / science.abc8530, science.sciencemag.org/cgi/doi… 1126 / science.abc8530 Provided by Stanford University
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