Recent development of polyfluorene-based RGB materials for light emitting diodes
As a continuation of our presentation at IUMRS-2000 in Hong Kong, we report the latest development of polyfluorene-based Red, Green and Blue (RGB) materials for light emitting diodes at The Dow Chemical Company. A modified Suzuki coupling process is used to synthesize RGB fluorene-based homopolymers and copolymers. Optimization of reaction conditions has led to a highly efficient procedure to generate polymers with controlled molecular weight (Mw), ranging from 10,000 to above 500,000 depending on the requirements of the desired applications. The optical and electronic properties of the polymers are tailored through selective incorporation of different aromatic units into the fluorene copolymer systems. By using this methodology, a portfolio of fluorene-based polymers has been designed and synthesized, achieving emissive colors that cover the entire visible spectrum. The performance of fluorene-based polymers in light emitting devices has been optimized by modifying the polymer compositions so as to increase charge mobilities and to improve the carrier injection balance. As a result of these compositional changes, devices based on Dow's green emitters, using bi-layer structures on indium tin oxide substrates and evaporated metal cathodes, have demonstrated unprecedented high efficiencies at high brightness levels and long lifetime performance. A device comprised of a Dow green emitting polymer has a low turn-on voltage of 2.25 V and exhibits a peak efficiency of 10.5Cd/A at 6600Cd/m2 at 4.85 V. These devices maintain an efficiency of greater than 10.0Cd/A up to 50,000Cd/m2and demonstrate very good stability as exemplified by a device half-life of greater than 1500-h starting from 1100Cd/m2. Similar outstanding progress with red and blue emitters has also been made and will be discussed.
light emitting diodes polyfluorenes electroluminescent photoluminescent