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DISPLAY TECHNOLOGY
A solution-processing method developed by scientists in Germany patterns red, green, and blue electroluminescent polymers with the resolution required for flat-screen, full-color organic light-emitting diode (OLED) displays.
The work was carried out by University of Munich researcher Klaus Meerholz, who is now professor of physical chemistry at the University of Cologne; Heinrich Becker, research manager at Covion Organic Semiconductors, Frankfurt; Oskar Nuyken, professor of polymer chemistry at the Technical University of Munich; and coworkers [Nature, 421, 829 (2003)]. The process combines the ease of polymer-based OLED device manufacturing by solution processing with the well-established photolithographic patterning process, the researchers point out. They believe the technology to be highly promising for fabrication of true-color matrix displays offering patterning of colors at high resolution. "We can achieve a resolution of a few micrometers with our materials, which is much better than the 50-mm resolution of the pixels in current displays," Meerholz tells C&EN. The technique uses spin-coating to deposit each polymer from solution onto a transparent substrate in the presence of a photoacid. The polymer film is then irradiated with ultraviolet light through a shadow mask. The illumination causes the polymer to cross-link and form an insoluble material. The still-soluble material in the areas of the film that have not been irradiated is washed away with solvent. The two other polymers are then deposited in the same way to fabricate pixilated devices with three individually addressable colors. The researchers used Suzuki polycondensation to prepare the materials: three oxetane-functionalized spirobifluorene-co-fluorene polymers emitting blue, green, and red light. The Suzuki reaction typically employs a palladium catalyst to synthesize biaryl compounds by cross-coupling functionalized aryl halides with aryl boronates. The group employed a triphenylphosphine palladium compound as its catalyst and a diboronic ester compound, an oxetane-functionalized hole-conducting aromatic dibromide, and several other dibromo-aromatic compounds as monomers. "This work is an exciting demonstration of the possibilities that can emerge from collaboration between polymer chemists and device physicists, chemists, and engineers," comments University of Cambridge professor of organic and polymer chemistry Andrew Holmes in the same issue of Nature (page 800). "But it remains to be seen whether devices patterned in this way can match the operating lifetimes of more than 10,000 hours exhibited by their ink-jet-printed counterparts, and whether their operation will be affected by the residual photoacid residing in the polymer films." Meerholz and colleagues note that, except for the blue-emitting polymer, the efficiencies of the electroluminescent polymers do not reach the best state-of-the-art values yet. These values can certainly be achieved by optimizing the synthetic route, they suggest.
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