Screens are becoming smaller and sharper. This can be seen, for example, in augmented reality glasses and the electronic viewfinders of cameras, some of which have a screen diagonal of less than 1 centimeter (0.4 inches).

But the smaller the screen pixels, the more difficult they are to manufacture. Organic light-emitting diodes (OLEDs) show promise for particularly small pixels. Unlike conventional light-emitting diodes, they are not made of crystals but of carbon-based compounds. While conventional light-emitting diodes become dimmer as the crystals are cut smaller, organic light-emitting materials can be used to produce tiny yet bright pixels.

Until now, organic light-emitting materials were not suitable for the high-precision manufacturing methods used in the semiconductor industry. Photolithography, a method used in the industry, involves solvents and other chemicals that attack and degrade organic molecules.

Photoresists for lithography

Researchers at ETH Zurich have now made organic luminescent molecules suitable for photolithography. They produced luminescent polymers in various colors that serve as photoresists while withstanding harsh chemical conditions. The polymers can be applied to a semiconductor chip. The photoresists cross-link when exposed to UV light, enabling the creation of fine-scale geometric structures.

“We’ve developed a new class of long-chain molecules that can be manufactured into OLED pixels using direct light exposure,” says Yinyin Bao, who conducted research at ETH Zurich and is now a professor at the University of Helsinki. He led the research with ETH professor Chih-Jen Shih.

They published their findings in the journal Nature.

Protective shell, luminescent core

To protect the sensitive luminescent molecules from aggressive chemicals, the researchers developed a molecular complex based on the core-shell principle. At its center lies the color-emitting molecule. Surrounding it are arms arranged in a star-like pattern, with outer ends that react to UV light. When exposed to light, they cross-link with the arms of other stars. This renders the material insoluble in that area, a property used in photolithography.

The inner part of the arms also serves an important function: It keeps the reactive ends at a distance from the luminescent molecule, shielding it. A co-author of the study from RMIT University in Melbourne further investigated this protective effect using computer simulations.

“We separate the two functions spatially,” Shih explains. “The light-emitting molecule is protected inside, while the reactive cross-linking groups are on the outside. This allows the photoresist to react during lithography without causing significant damage to the light-emitting core.”

Highest-resolution multicolor fluorescent image to date

The researchers demonstrated how precisely the method can pattern differently colored luminescent materials by producing a high-resolution image of a macaw parrot. It measures 300 by 430 micrometers and consists of 250 by 350 pixels.

The static image is made up of fluorescent colors; it is not yet a display. Its pixels are excited by external light, causing them to fluoresce, rather than powered electrically. It is the highest-resolution multicolor fluorescent image produced using photolithography to date.

The researchers showed that the method can also produce electrically powered light-emitting diodes using another test image: a glowing ETH logo measuring 1 by 2.4 millimeters. This was developed in collaboration with the group led by Hua Wang, a professor of electronics at ETH Zurich.

Next, the researchers aim to further reduce the pixel size of the light-emitting diodes. Producing a functioning screen will also require electronics that allow the individual pixels to be controlled independently.

Shih sees potential applications not only in small screens but also in tiny light sources for medical technology and devices for biological and neuroscientific research. “We can use this to generate light on a small scale and with high precision exactly where it is needed,” Shih says.

Tiny OLEDs could, for example, be used in research equipment to examine individual biological cells or to stimulate nerve cells in a petri dish with light. Very small, precisely controllable light sources could thus be produced directly on microchips and could also be useful in microscopy and sensors.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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