It is possible to double the efficiency of colour OLED displays based on white-emitting material, and achieve 10,000 pixel-per-inch density, by replacing the rear reflector with a meta-material, according to Stanford University researchers.

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Generally there are two types of colour OLED display: displays with side-by-side patterns of red, green and blue emissive materials, and those with a white-emitting material (made from a mix of red, green and blue emitters) that require per-pixel red green and blue front filters to define the viewed colour.
The separate side-by-side type is more efficient, but available deposition technology limits both the minimum achievable pixel size and the maximum achievable display size, according to Stanford, essentially limiting use to phones, tablets and small TVs.
Meanwhile the white-emitting type can be made with smaller pixels and over larger areas, but always wastes at least two thirds of emitted light, demanding higher drive power for a given display and potentially shortening phosphor life.

A note on wording – pixel is being used here to describe one patch of green, for example. In an RGB display, these might be referred to as ‘sub-pixels’, with a ‘pixel’ being two, three or more sub-pixels.
In a twist on white emitting technology, the Stanford team has built tiny per-pixel resonant cavities that, for example, only excite the green phosphor within the white emitting mix at green pixel locations – reducing energy waste in the locally un-needed phosphors within the mix, and possibly removing the need for per-pixel front filters.
The cavities are defined by reflection between the transparent front gold electrode and the rear reflector (see diagram).
By building stumpy pillars (80nm high) on the rear mirror surface, the mirror can be turned into a frequency-selective phase-shifter, allowing the optical length of the cavity to be adjusted while physical cavity length remains constant.
The Stanford team has shown that if a cavity is built to sharply resonate at a blue wavelength when it has a plain rear mirror, then patches of appropriately-spaced pillars can cause well-defined green emission or well-defined red emission depending on the pillar spacing. Other colours are also possible.
A finding is that the amount of light produced at the chosen colour can be twice that produced from optically filtering the same white phosphor in a standard display – regaining some of that lost two-thirds – and it can have a narrower bandwidth.
Pillar diameter, inter-pillar pitch and pillar height can all be varied (~100nm, ~200nm and ~60nm median sizes respectively) to alter the characteristics of the cavities. “Pixels with larger pillar diameters clearly offer stronger colour tuning with pitch,” according to the paper ‘Metasurface-driven OLED displays beyond 10,000 pixels per inch’, which describes the work in Science. “The larger-diameter pillars produce smaller gap spacings at the same pitch, and the higher degree of tunability can therefore be linked to the higher sensitivity of the mode index of gap plasmons to the gap size for the smaller gaps.”
Such is the nature of the meta-material, that a 3 x 3 array of pillars can be sufficient to clearly define a red pixel with the same behaviour as an infinite array, according to the researchers.
The work started with quite large pixels, but as they were scaled down, “surprisingly, even with down-scaling of the sub-pixel size to 2.0 and 1.2μm, no appreciable colour distortion or luminance reduction was observed in the meta-cavity pixel”, according to the paper.
1.2μm sub-pixels would mean 10,000pixel/inch in an RGBG pixel, or 15,000pixel/inch in a ‘pentile’ (two sub-pixel per pixel) display.
Nano-imprint fabrication – pressing a hard object into the mirror to form nano-scale pillars, it being tried as a fabrication technique.
Stanford worked with the Samsung Advanced Institute of Technology (SAIT) and Hanyang University in Korea in this research, which was sparked by serendipity.
SAIT scientist Won-Jae Joo visited Stanford from 2016 to 2018 with OLEDs on his mind, and heard Stanford graduate student Majid Esfandyarpour describe solar cell technology he was developing in the lab of Mark Brongersma.
“Professor Brongersma’s research themes were all very academically profound and were like hidden gems for me as an engineer and researcher at Samsung Electronics,” said Joo, who approached Esfandyarpour after the presentation.
“It was quite exciting to see that a problem that we have already thought about in a different context can have such an important impact on OLED displays,” said Esfandyarpour.
Samsung is now attempting to produce a full-size display.