Full-color OLEDs are manufactured with color filters or shadow masks. This drives production costs and limits brightness and resolution. Researchers at Fraunhofer FEP have developed a process that does not require a color filter.
OLED microdisplays are often the first choice for data glasses. They are thin and do without backlighting. Although the pixels are luminescent, color filters are still required for full-color displays. For microdisplays, this is particularly noticeable in the form of limited resolution and brightness.
Companies have been looking for new production methods for some time. The structuring of organic layers in OLEDs is a problem, since conventional methods such as photolithography cannot be applied to organic semiconductor materials.
Two years ago, researchers at the Fraunhofer Institute for Organic Electronics, Electron Beam, and Plasma Technology demonstrated a promising approach. They use electron beam technology for microstructuring the organic layers. With the now patented process, a complete OLED was structured through its encapsulation. With this method, any structures and even high-resolution images in grayscale could already be created at that time.
The process is now so mature that full-color OLED displays can also be produced without the need for color filters. Red, green, and blue emitting pixels are created by structuring an organic layer of a white emitting OLED with a thermal electron beam process. This structuring changes the thickness of the layer stack, which leads to the decoupling of red, green, and blue.
The electron beam process makes it possible to thermally structure the »sensitive organic materials without damaging underlying layers,« says the developer in the project team, Elisabeth Bodenstein, pointing out the new possibilities.
The results were obtained by simulating and initially estimating the HTL (hole transport layer) thicknesses that are structured with the electron beam. With principle proofs on first test substrates (Figure 1), the colors could be demonstrated with comparable optical performance and appearance of the OLED. The Fraunhofer researchers see further areas of application in the processing of inorganic layers, in photovoltaics, for the production of MEMS, and in the field of thin-film technology.
In the coming years, a manufacturing process for OLED microdisplays suitable for industrial use is to be developed together with partners and established in the industry through licensing. Industrial partners are sought for the necessary miniaturization of structures and process optimization.
Then, in a further step, microstructuring is integrated into existing processes in order to build up the necessary production knowledge for and with industrial partners. The future transfer of the test results into an existing process line is to be developed in order to enable the manufacturing process to be established at industrial level at a later date.