Progress in Electroluminescent Devices Using Molecular Thin Films
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during the past 10 years, mainly in Japan's private-sector laboratories. These activities are reflected by the tremendous increase in the number of publications in academic spring and fall meetings in Japan in Figure 1. Independent of the widespread, intense research activities in Japan, polymer light-emitting diodes (LEDs) using fully 7r-conjugated polymers have appeared in the United Kingdom, and interest in the field has spread rapidly throughout the world.4'5 Polymer EL devices are reviewed by Yang in another article of this issue. Here we will
focus our attention on vacuum-sublimed dye systems. We would like to stress that no essential difference exists between vacuum-sublimed dye systems and Trconjugated polymer systems from the viewpoints of both working mechanism and material design. Because the designs of both device structures and constituent dye molecules in multilayer-dye EL devices have high flexibility, studies on the devices are expected to be important for the establishment of material-design concepts. Thus we have investigated the role of multilayer structures in multilayer EL devices composed of sublimed dye films. Through our systematic studies on devices made of vacuum-sublimed dye films, we have established a key understanding that the basic EL mechanism from charge injection to emission is common in all molecular materials since emissions in EL devices are via neutral excitations.6"8 According to this basic understanding, we have proposed that the concept for materials design of sublimed-dye multilayer films can be extended to fully Tr-conjugated polymer films, polymer films with chromophores on main chains or side chains, and even polymer-dispersed dye systems. For example, dyes used for multilayer sublimed-dye EL devices have found usage in charge transport and emissive dyes for polymer-dispersed dye EL devices, and the roles of dyes as charge transport, charge recombination,
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• JSAP • CSJ
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DSPSJ
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10 Spring Fall Spring Fall Spring Fall Spring Fall Spring Fall Spring Fall Spring Fall Spring Fall Spring Fall 1988 1988 1989 1989 1990 1990 1991 1991 1992 1992 1993 1993 1994 1994 1995 199S 1996 1996
Year Figure 1. A summary giving the numbers of papers on electroluminescence (EL) of molecular materials at annual spring and fall meetings of the Japan Society of Applied Physics (JSAP), the Society of Polymer Science, Japan (SPSJ); and the Chemical Society of Japan (CSJ) in the past nine years.
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Progress in Electroluminescent Devices Using Molecular Thin Films
and emission in multilayer-dye films and polymer-dispersed dye systems have been almost the same.910 The situation has been very similar in the design of polymers with chromophores in skeletal or side chains. We have demonstrated some examples for systematic molecular design of polymers with charge transport and emissive chromophores in their skeletons.1112 In this article, first we show our basic understanding of the mechanism for charge-injection-type EL through the derivation of some basic formulae that quantita
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