Recent Developments in Light-Emitting Polymers
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Recent Developments in Light-Emitting Polymers
Ian D. Rees, Kay L. Robinson, Andrew B. Holmes, Carl R.Towns, and Richard O’Dell Abstract Conjugated light-emitting polymers (LEPs) have real potential to serve as the active layer in a new generation of emissive displays. Emerging as lead candidates for first-generation displays are poly(1,4-phenylene vinylene)s (PPVs) and poly(9,9-dialkylfluorene)s, as well as other polyaromatic materials. The poly(fluorene)s are at present the most commercially developed of these LEP materials for red–green–blue (RGB) applications. The low power consumption of LEP devices in general makes them particularly suited to mobile applications. Combining solutionprocessable emissive polymers with direct-patterning methods such as ink-jet printing will lead to the possibility of low-cost, high-resolution displays. The synthesis and properties of PPVs and poly(9,9-dialkylfluorene)s are briefly reviewed in this article, with a major focus on recent developments. Keywords: conjugated polymers, electroactive organic materials, electroluminescence, optoelectronic materials.
In the last decade, there has been considerable research in the field of lightemitting polymers (LEPs).1–7 They have several advantages over the conventional electroluminescent organic small-molecule materials in that a single-polymer-layer device can be fabricated that exhibits low operating voltages, relatively high quantum efficiency, mechanical strength, and color tunability.8–11 In addition, device fabrication has been simplified by the ability to spincoat thin films and by the development of other solution-patterning techniques, such as ink-jet printing, which are possible with solvent-soluble conjugated polymers. There have been several reports of efficient, colortunable, multiple-layer device structures using small organic molecules.12–15 These devices are fabricated by vacuum sublimation of the organic layers, where patterning is achieved through the use of shadow masks. Polymer-based devices may be multilayered, but in principle they can be distinguished from organic small-molecule devices by combining more than one feature (charge transport, emission) in a single layer. MRS BULLETIN/JUNE 2002
The first LEP was reported by Burroughes et al. in 1990.16 Poly(1,4-phenylene vinylene) [PPV, Chart 1, Structure (1)] was used as the emissive layer to produce yellow–green electroluminescence. However, PPV is not soluble in its fully conjugated form, and there have been many attempts to develop organic soluble conjugated derivatives.17–19 In 1991, Heeger and co-workers20 reported a device fabricated using poly[2-methoxy5-(2-ethylhexyloxy)-1,4-phenylene vinylene] [MEH-PPV, Chart 1, Structure (2)], which is soluble in its conjugated form in
common organic solvents such as dichloromethane and tetrahydrofuran. Furthermore, the preparation of a soluble nitrilesubstituted PPV (cyano-PPV) was also reported [Chart 1, Structure (3)]. This polymer was shown to have a high electron affinity and to enhance the light-emitting efficienc
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