Cover Picture: Solution‐Processed Full‐Color Polymer Organic Light‐Emitting Diode Displays Fabricated by Direct Photolithography (Adv. Funct. Mater. 2/2007)

The first high-resolution full-color OLED display based on a direct photolithographic process is presented by Meerholz and co-workers on p. 191. The cover shows a schematic illustration of the fabrication process, a fluorescence microscope picture demonstrating the micrometer resolution capability of the process, and a picture of the first prototype displaying a test image. Electroluminescent polymers with photoresist-like properties are the basis of the new process (chemical structure shown in the background). The first full-color polymer organic light-emitting diode (OLED) display is reported, fabricated by a direct photolithography process, that is, a process that allows direct structuring of the electroluminescent layer of the OLED by exposure to UV light. The required photosensitivity is introduced by attaching oxetane side groups to the backbone of red-, green-, and blue-light-emitting polymers. This allows for the use of photolithography to selectively crosslink thin films of these polymers. Hence the solution-based process requires neither an additional etching step, as is the case for conventional photoresist lithography, nor does it rely on the use of prestructured substrates, which are required if ink-jet printing is used to pixilate the emissive layer. The process allows for low-cost display fabrication without sacrificing resolution: Structures with features in the range of 2 μm are obtained by patterning the emitting polymers via UV illumination through an ultrafine shadow mask. Compared to state-of-the-art fluorescent OLEDs, the display prototype (pixel size 200 μm × 600 μm) presented here shows very good efficiency as well as good color saturation for all three colors. The application in solid-state lighting is also possible: Pure white light [Commision Internationale de l'Eclairage (CIE) values of 0.33, 0.33 and color rendering index (CRI) of 76] is obtained at an efficiency of 5 cd A–1 by mixing the three colors in the appropriate ratio. For further enhancement of the device efficiency, an additional hole-transport layer (HTL), which is also photo-crosslinkable and therefore suitable to fabricate multilayer devices from solution, is embedded between the anode and the electroluminescent layer.