Polymer Electroluminescent Devices

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tion by Braun and Heeger at the University of California of light-emitting diodes (LEDs) using soluble conjugated PPP DO-PPP

PPV

MEH-PPV

CN-PPV

BCHA-PPV

Figure 1. Illustrations of some commonly used conjugated polymers and their soluble derivatives. PPP = poly(p-phenylene), DO-PPP = dihexyloxyfphenylene vinylene), PPV= poly(p-phenylene vinylene, MEH-PPV = poly(2-methoxy-5-2'ethyl-hexyloxy) -1,4 -phenylene vinylene), CN-PPV = cyanosubstituted phenylene vinylene, BCHA-PPV = poly(2,5bis(cheolestranoxy)-1.4phenylene vinylene).

polymers,7 and by Gustafsson et al. at UNIAX Corporation of flexible LEDs8 further spurred interest in polymer LEDs. Here are just a few examples illustrating the rapid progress of polymer LEDs within the past few years: whiteemission electroluminescence demonstrated by Kido et al.;9 the development of blue-emission LEDs made of poly(paraphenylene) (PPP) by Leising et al.10 of polyalkylfluorene by Uchida et al.,11 and of poly(p-phenylene vinylene) (PPV) copolymer by Sokolik et al.;12 the pioneer work in applying pyridine-based and pyridyl vinylene-based polymers to light-emitting devices as alternatives to PPVs by Epstein et al.;13 the invention of symmetrically configured alternatingcurrent light-emitting diodes by Wang et al.;14 the demonstration of block copolymers made of an alternating conjugated and nonconjugated sequence, which mimic the semiconductor quantum wells by Burn et al.,15 as well as similar work done by Yang et al.;16 the illustration of polarized electroluminescence by Dyreklev et al.;17 the application of self-assembled multilayer technology to the fabrication of LEDs by Fou et al.;18 and the deep understanding of the photochemical decay mechanism of PPV by Yan et al.19 A more comprehensive review of polymer LEDs appears in Reference 20. An alternative approach to polymer LEDs was disclosed by Pei et al. at UNIAX in 1995. They demonstrated the in situ creation of a light-emitting p-n junction in thin films of conjugated polymers admixed with a solid electrolyte.21 The so-called polymer light-emitting electrochemical cells (LECs) emitted bright light at less than 4 V with high efficiency. Recently the performance of some polymeric EL devices has achieved levels suitable for some practical applications. In the following, we will briefly describe the fundamental properties of conjugated polymers that are involved in the operation of both polymer LEDs and LECs. We also discuss the various techniques that have been developed to improve the performance of these devices. Conjugated Polymers Conjugated polymers are a novel class of semiconducting materials that combine the electronic and optical properties of semiconductors and the processability of conventional polymers.22 The semiconducting properties of conjugated polymers originate from the delocalized 77 orbitals formed in carbon-containing compounds, such as poly(phenylene vinylene), polythiophene, and poly(p31

Polymer Electroluminescent Devices

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phenylene). Figure 1 shows some of the commonly used

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