| Research presented on July 8 at the International Conference onScience and Technology of Synthetic Metals in Brazil providesinsight into factors that influence the injection efficiency. Abalanced injection of positive and negative charge carriers intothe organic layer is important to achieve high quantum efficiency,but the interface between the metallic coating and organic layerwhere the injection occurs is poorly understood.
Placing an organic layer on top of the conductive layer modifieseach layer's individual work function, or the minimum energy neededto extract the first electron from the metal.
"Measuring the work functions independently for each layerdoes not provide an indication of how their energy levels matchwhen they touch each other," explained Jean-Luc Brédas,a computational materials chemist, professor in the GeorgiaInstitute of Technology's School of Chemistry and Biochemistry andGeorgia Research Alliance Eminent Scholar.
The energy levels for each layer should align when attached;otherwise, a barrier will form and a higher voltage will berequired to send current in.
With funding from the Office of Naval Research, Brédas firstdeveloped a theoretical model of the interface between conventionalmetals and a single layer of organic molecules forming aself-assembled monolayer on the metal. His goal was to determinehow the metal work function could be modified by depositing theself-assembled monolayer.
Brédas and postdoctoral research fellow Georg Heimel, who isnow at the Humboldt University in Berlin, looked for changes in thework function of gold when they modified the chemical nature of thehead group of the organic molecules in the self-assembled monolayerand the nature of the docking group, which directly connected theorganic layer and metal.
The study, published in the April 2007 issue of Nano Letters,showed that changing the head group of the organic moleculeslocated far from the surface and changing the docking groupprovided two nearly independent ways to modify the metal workfunction.
While studying two metal substrates -- gold and silver -- theresearchers found that even though the chemical interface betweenthe metal and thiol-based self-assembled monolayer were different,the organic-covered metals had virtually identical work functions.
Postdoctoral research fellow Pavel Paramonov, who is now anassistant research professor at the University of Akron, expandedthe original work to model the interface between a self-assembledmonolayer and indium tin oxide, the conducting material commonlyused as the transparent electrode in liquid crystal displays andorganic light-emitting diodes.
"Researchers frequently cover the hydrophilic indium tin oxidesurface with a self-assembled monolayer containing a hydrophobicsubgroup pointing away from the surface, providing much betteradherence and compatibility with the active organic layer thatcomes on top," said Brédas.
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