ETH Zurich develops new organic material for color-emitting OLEDs

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

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

They have published their findings in 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 centre lies the colour-emitting molecule. Surrounding it are arms arranged in a star-like pattern, the outer ends of which 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 utilised in photolithography.

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

“We separate the two functions spatially,” explains ETH Professor Shih. “The light-emitting molecule is protected inside, whilst 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.”

High-res multicolor fluorescent image

The researchers demonstrated just how precisely different coloured luminescent materials can be patterned using this method with a high-resolution test image: they produced an image of a macaw parrot measuring 300 by 430 micrometres and consisting of 250 by 350 pixels. This is a static image made up of fluorescent colours – in other words, it is not yet a display. In this image, the pixels are not made to glow electrically, but are excited by external light, causing them to fluoresce. The image is the highest-resolution multicolour image made up of fluorescent colours to date that has been produced using photolithography. 

The researchers demonstrated that the method can also be used to produce electrically powered light-emitting diodes using another test image: a glowing ETH logo measuring 1 by 2.4 millimetres. 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. To ultimately produce a functioning screen, electronics are still required that allow the individual pixels to be controlled independently of one another. 

Shih sees potential applications not only in small screens, but also in tiny light sources for medical technology and in 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,” says Shih. Tiny OLEDs could, for example, be used in research equipment to examine individual biological cells in a targeted manner or to stimulate nerve cells in a Petri dish with light. Very small and precisely controllable light sources could be produced directly on microchips using this new approach and could also be useful in microscopy and sensors.