Fabrication of Polymer Light Emitting Diodes by Layer-by-Layer Complexation Technique
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Jaehyun Kim*, Kethinni G. Chittibabu**, Mario J. Cazeca**, Woohong Kim***, Jayant Kumar*, and Sukant K. Tripathy* *Center for Advanced Materials, Department of Chemistry and Physics, University of Massachusetts, Lowell, MA, 01854, U.S.A. ** Molecular Technologies, Inc., Westford, MA, 01886, U.S.A. *** Samsung Central Research Institute of Chemical Technology, Taejeon, 305-380, Korea
ABSTRACT
Multilayer films were fabricated employing a new layer-by-layer complexation technique by alternatively dipping substrates in solutions of macromolecular ligands and Eu3" ions. A luminescent poly[2-(3-thienyl)ethanol hydroxycarbonyl-methyl urethane] (H-PURET) was prepared by hydrolysis of thiophene polymer, poly[2-(3-thienyl)ethanol butoxycarbonyl-methyl urethane] (PURET), which was developed in our laboratory. H-PURET was used as macromolecular ligands. The multilayer deposition was monitored using UV-visible spectroscopy. As the number of bilayers increases, the absorption due to the polymer increases. The multilayer films were characterized by infrared and fluorescence spectroscopic techniques. Electroluminescence brightness easily obtained from the multilayer film of conjugated polymerEu3 +complex was measured to be about 40nW. INTRODUCTION
Electroluminescent devices from organic materials have attracted considerable interest, since Tang and VanSlyke" 2 introduced injection-type electroluminescent devices based on sublimed organic materials, due to their potential application as future flat panel displays.'-' The devices were fabricated with charge transport layers and an emissive layer, which provide high brightness outputs under low driving voltage. Lanthanide metal complexes are known to exhibit photoluminescence with very high quantum efficiencies and are expected to show efficient electroluminescence. However, long-term stability is a serious concern for those kinds of devices. The morphological rearrangement of the sublimed luminescent and charge transport molecules, as well as the diffusion of the electrode metals into the emitting layer 8 during operation, results in the degradation of these devices. Recent discovery of using conjugated polymers such as poly(phenylene vinylene) for electroluminescent devices has provided an interesting approach to realize the potential applications of the devices, leading to intensive investigations to identify new materials with enhanced light emitting properties.9-" However, the conjugated polymers still suffer from relatively low quantum efficiency. It is possible to make self-assembled multilayer thin films from the combinations of conjugated polymers and highly luminescent lanthanide metal ions through layer-by-layer 527 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
complexation technique. This technique may be a promising process for improving the stability of present organic devices without losing their electroluminescence. This improvement can be achieved by incorporation of highly efficient metal complexes into relatively stable polymers. In this
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