This article provides an in-depth look at what it takes to print organic light-emitting diodes successfully.
In the polymer-based OLED structure, the cathode layer traditionally consists of vacuum-evaporated, low- and high-work-function metals such as barium and aluminum, respectively. The development of a cathode ink is needed to realize a fully printed OLED structure. Conductive silver inks are commercially available in a range of products but result in poor OLED performance when used without an additional electron-injection layer. From the electron- injection point of view, lower-work-function metal such as aluminum is expected to result in to higher OLED performance than silver.
Ball milling is used to process aluminum for cathode ink. The resulting metal flakes are dispersed in a solvent/binder mixture to provide an ink of the required viscosity. The surface of a screen-printed layer of aluminum is shown in Figure 4A. The metal flakes, approximately 5 µm in diameter, can be seen in the SEM image. The disadvantage of using low-work-function metal like aluminum is its reactivity to the atmosphere. The metal flakes oxidize readily when in contact with air.
Additionally, a good contact between the underlying organic layer and the cathode is crucial (Figure 4B). Therefore, the solvent of the cathode ink has to be chosen carefully so that it does not dissolve the underlying emissive layer. Also, the curing temperature of the cathode ink has to be lower than the glass-transition temperatures of the organic materials and the substrate. In the case of aluminum ink, the intrinsic oxide layer can also act as a protective layer and prevent further oxidation of the cathode layer. However, for the printed cathode to be conductive, the preparation and printing of the ink have to be done in inert atmosphere, which adds cost because of specialized equipment required for the process.
First applications for printed OLEDs
While most of the attention on printed-OLED technology comes from the lighting and display industry’s perspective, the first applications for high-volume, low-cost, and relatively low-performance printed OLEDs could be in signage. Market research indicates that signage products manufactured using printed and organic electronics are expected to generate a $2.5 billion worldwide market within the next five years. As the efficiency and lifetime continues to develop in the direction required for displays and lighting, a low-end application would open up a window of opportunity for creating new business from novel products that are based on the competitive strengths of high-volume and cost-efficient printed OLED technology.
Riikka Suhonen, Ph.D. is a research scientist at VTT Technical Research Centre of Finland. She focuses on the printability of organic photovoltaic and light-emitting materials. Suhonen holds an M.Sc. degree in organic chemistry from the University of Oulu, Finland, and finished her Ph.D. studies in Germany at Siemens AG.
Markus Tuomikoski is senior research scientist at VTT Technical Research Centre of Finland and team manager of the prototype manufacturing at the printable-optics and electronics knowledge center. He holds an M.Sc. degree in inorganic chemistry from the University of Oulu, Finland, and was technical manager of the EU-funded ROLLED project, which developed the world’s first fully roll-to-roll-printed OLEDs
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