Tiny OLED pixels can be made like microchips
Organic LEDs (OLEDs) show great promise for making small pixels. Whereas conventional LEDs become dimmer the smaller the crystals are cut, organic light-emitting materials can be used to produce tiny yet bright pixels.
Until now, the organic light-emitting materials were not suitable for the high-precision manufacturing methods used in the semiconductor industry. The photolithography used in this industry involves solvents and other chemicals that chemically attack and degrade the organic molecules.
Researchers at ETH Zurich have now succeeded in making organic luminescent molecules suitable for photolithography: they have produced luminescent polymers in various colours 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’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 together with ETH Professor Chih-Jen Shih. They have now published their findings 'Electroluminescent photoresists extending lithographic scaling to OLEDs' in the journal Nature.
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 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.”
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 µms 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 LEDs using another test image: a glowing ETH logo measuring 1 by 2.4 mm. 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 LEDs. 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 thus be produced directly on microchips and could also be useful in microscopy and sensors.
