Photoresist technology enables sub-μm OLED patterning

Organic EL displays are increasingly used for televisions, mobile devices including smartphones as well as wearable devices. Since they can be made thin and flexible, while also offering excellent response time and contrast ratio. It is said that today’s products require organic EL displays of high pixel density, i.e. around 200ppi for 4K televisions, 500ppi for full HD mobile devices and even higher density for compact displays for wearable devices.

There has been active R&D for organic semiconductors to develop a high-resolution patterning method for organic EL materials to be used in these products. In 2013, Fujifilm and imec jointly developed photoresist technology for organic semiconductors that enables sub-μm patterning without damaging the organic semiconductor materials, based on photolithography, capable of high-resolution patterning on large substrates.

There is no need for additional capital investment since an existing i-line exposure system can be used for the new technology. This is why the technology has attracted wide attention since the development announcement with anticipation of a cost-effective way of manufacturing high-resolution organic semiconductor devices.

In the latest achievement, Fujifilm and imec produced full-colour OLEDs with the photoresist technology for organic semiconductors and successfully verified their performance.  Red, green and blue organic EL materials were patterned, each in the subpixel pitch of 20μm, to create full-colour OLEDs. An OLED array of 40x40 dots at the resolution of 640ppi was realised and illuminated with UV rays to confirm that red, green and blue dots separately emitted light.

In the leftmost image, the OLED array forms a pattern with subpixel pitch of 20μms to achieve a high resolution of 640ppi. The centre image was taken with UV illumination on the OLED array, confirming that red, green and blue dots emit light separately, and the image on the right was taken with voltage applied, instead of UV illumination. All the red, green and blue dots were confirmed to emit light, verifying its correct performance.

The emission of red, green and blue lights was also confirmed in a test involving the application of voltage rather than illumination, confirming its correct performance. These results open up opportunities, such as using the novel photolithography in a multiple patterning process. An example would be creating an OLED array that adds a fourth colour to red, green and blue, as well as developing previously-unseen devices such as a sensor that integrates OLED with the organic photodetector.

This research result is to be presented at the SID Display Week, one of the world’s largest international exhibitions for information displays, held in San Jose, California. Since the commencement of joint research in November 2012, Fujifilm and imec have broken through the boundary of conventional technology to contribute to the progress of technology associated with organic semiconductors, e.g., developing the photoresist technology for organic semiconductors that enables the realisation of high-resolution sub-μm patterns. The two companies will continue to undertake cutting-edge R&D involving semiconductor materials, process technology and system integration, thereby contributing to resolving challenges faced by the organic electronics industry.