Better Micropatterning for OLEDs

Probe station with patterned OLEDs in the clean room of Fraunhofer FEP. (Source: Fh.-FEP)

OLED microdisplays are in­creasingly estab­lishing them­selves in wearables and data glasses. In order to meet the require­ments for higher effi­ciency, higher contrast, and higher resolutions in these appli­cations, Fraunhofer FEP scientists have developed a new micro­patterning approach for OLEDs on silicon substrates. This might eliminate the use of color filters and shadow masks in the future and allow full-color displays to be developed by means of a new process. An increase in effi­ciency and consi­derably broader color gamut have already been demons­trated in first experiments.

Thanks to the shallow overall depth that results from the self-luminous proper­ties of OLEDs and their excellent contrast ratios, manu­facturers are increa­singly turning to OLED microdisplays for AR/VR glasses. Fraunhofer FEP has been conti­nuously immersed in the advanced development of this tech­nology for several years. Nevertheless, there are still some techno­logical challenges that need to be mastered in order to exploit the full potential of OLED tech­nology for use in consumer-ready data glasses and other AR/VR appli­cations. Very high brightness and efficiency, good yields for large chip areas, curved surfaces, integrated eye tracking, and trans­parent sub­strates are some of the tasks still on the researchers’ agenda.

Currently, OLED tech­nology faces the hurdle that full-color displays can only be realized by using color filters or shadow masks, which limit OLED effi­ciency and resolution. Researchers are working intensely on new approaches to fabricate micro­displays characterized by high reso­lution while at the same time offering high efficiency and long operating life time. The patterning of the organic layers of OLEDs is one of the greatest challenges, since conven­tional methods such as photo­lithography cannot be utilized with organic semi­conductor materials. The use of electron beam technology for micro­structuring was successfully demons­trated at the Fraunhofer FEP two years ago. Using its patented process, FEP was able to modify the emission of an OLED through the existing encap­sulation layer to create any feature imaginable and even produce high-resolution grayscale images.

Further develop­ment of the electron beam process has now achieved full-color OLED without using color filters or shadow masks. To create red, green, and blue pixels, an organic layer of the OLED itself is ablated by a thermal electron beam process. This patter­ning causes a change to the thickness of the layer stack, which makes the emission of different colors possible. This is the first major step towards the develop­ment of full-color displays without the use of restric­tive color filters in the process. Elisabeth Boden­stein, developer in the Fraunhofer FEP project team, explains the advantages: “With our electron-beam process it is possible to thermally structure even these sensitive organic materials without damaging the underlying layer.”

The results were obtained by simu­lating and initially estimating the HTL (hole transport layer) thick­nesses that are produced by the electron beam. The researchers actually achieved the decoupling of red, green, and blue emissions from the white OLED. Following proof of concept at Fraunhofer FEP, these colors were demons­trated on the first test substrates, exhibiting com­parable OLED perfor­mance. In addition to using this new process for OLEDs, electron beam processing can also be used for other applications in organic elec­tronics and inorganic layers. Moreover, the electron-beam patterning process is very adaptable and can also be employed in the areas of photo­voltaics, MEMS, and thin-film tech­nology.

Now, Fraunhofer FEP scientists are approaching the next mile­stones, having completed the promising preparatory work. The main goal in the coming years is to use this new method in jointly developing fabri­cation of OLED micro­displays with partners and establish it in the industry through licensing. To do this, the features will be further minia­turized and the process optimized by working together with interested partners from industry. The next step planned is to integrate the micro­patterning into existing processes in order to gain further know-how jointly with industrial partners. This should enable future transfer of the test results into an existing process line to be worked out in order to faci­litate establishing the tech­nology at the industrial level at a later date.

In parallel, the scientists are also planning an enhanced simu­lation of OLEDs. The OLED color spectrum will be broadened by suitably modifying the materials and layer thicknesses. In this way, the prospective incor­poration the displays in data glasses for special appli­cations such as in industrial manu­facturing and medicine should be opened up with the new process. Now, the scientists are ready for specific advanced develop­ment of the tech­nology through partner­ships and joint projects with display, plant, material and system manu­facturers. (Source: Fh.-FEP)

Reference: E. Bodenstein et al.: Realization of RGB colors from top‐emitting white OLED by electron beam patterning, J. SID, online 1. Juni 2018; DOI: 10.1002/jsid.674

Link: Displays & Wearables, Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, Dresden, Germany

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