Research presented on July 8 at the International Conference on Science and Technology of Synthetic Metals in Brazil provides insight into factors that influence the injection efficiency. A balanced injection of positive and negative charge carriers into the organic layer is important to achieve high quantum efficiency, but the interface between the metallic coating and organic layer where the injection occurs is poorly understood.
Placing an organic layer on top of the conductive layer modifies each layer’s individual work function, or the minimum energy needed to extract the first electron from the metal.
“Measuring the work functions independently for each layer does not provide an indication of how their energy levels match when they touch each other,” explained Jean-Luc Brédas, a computational materials chemist, professor in the Georgia Institute of Technology’s School of Chemistry and Biochemistry and Georgia Research Alliance Eminent Scholar.
The energy levels for each layer should align when attached; otherwise, a barrier will form and a higher voltage will be required to send current in.
With funding from the Office of Naval Research, Brédas first developed a theoretical model of the interface between conventional metals and a single layer of organic molecules forming a self-assembled monolayer on the metal. His goal was to determine how the metal work function could be modified by depositing the self-assembled monolayer.
Brédas and postdoctoral research fellow Georg Heimel, who is now at the Humboldt University in Berlin, looked for changes in the work function of gold when they modified the chemical nature of the head group of the organic molecules in the self-assembled monolayer and the nature of the docking group, which directly connected the organic layer and metal.
The study, published in the April 2007 issue of Nano Letters, showed that changing the head group of the organic molecules located far from the surface and changing the docking group provided two nearly independent ways to modify the metal work function.
While studying two metal substrates – gold and silver – the researchers found that even though the chemical interface between the 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 an assistant research professor at the University of Akron, expanded the original work to model the interface between a self-assembled monolayer and indium tin oxide, the conducting material commonly used as the transparent electrode in liquid crystal displays and organic light-emitting diodes.
“Researchers frequently cover the hydrophilic indium tin oxide surface with a self-assembled monolayer containing a hydrophobic subgroup pointing away from the surface, providing much better adherence and compatibility with the active organic layer that comes on top,” said Brédas.
The cover layer also prevents the indium from diffusing into the active organic layer and degrading the device, but adding this layer also provides a way to fine-tune the work function.
With funding from the Solvay Group, Paramonov modeled the indium tin oxide surface, which was a complex task because indium tin oxide is not stoichiometric – every vendor’s indium tin oxide is somewhat different. Then he modeled the binding of a self-assembled monolayer of phosphonic acid to the indium tin oxide surface. Paramonov’s first goal was to determine how the oxygen and phosphorus atoms of the self-assembled monolayer bind to the indium tin oxide surface.
In collaboration with Seth Marder, a professor in the Georgia Tech School of Chemistry and Biochemistry, and Neal Armstrong, a professor in the Department of Chemistry at the University of Arizona, they were able to characterize the main binding modes of the phosphonic acid molecules on indium tin oxide. This work has led to further research characterizing the impact of the self-assembled monolayer on the indium tin oxide work function, according to Brédas.
“More theoretical work needs to be done to study conducting oxides used as transparent electrodes in organic solar cells and organic transistors,” added Brédas. “On the experimental side, the quality of the self-assembled monolayer coverage also needs to be improved.”
Researchers usually design devices with potentially well-aligned energy levels when the layers are measured individually, but they should be examining the layers when they are attached, according to Brédas. This is because the reorganization of the chemical, electronic and geometric structures of the two layers at the interface has a major impact on the overall device characteristics.
Technical Contact: Jean-Luc Brédas (404-385-4986); E-mail: (firstname.lastname@example.org).
Abby Vogel | Newswise Science News
Multi-institutional collaboration uncovers how molecular machines assemble
02.12.2016 | Salk Institute
Fertilized egg cells trigger and monitor loss of sperm’s epigenetic memory
02.12.2016 | IMBA - Institut für Molekulare Biotechnologie der Österreichischen Akademie der Wissenschaften GmbH
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.
The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...
Broadband rotational spectroscopy unravels structural reshaping of isolated molecules in the gas phase to accommodate water
In two recent publications in the Journal of Chemical Physics and in the Journal of Physical Chemistry Letters, researchers around Melanie Schnell from the Max...
The efficiency of power electronic systems is not solely dependent on electrical efficiency but also on weight, for example, in mobile systems. When the weight of relevant components and devices in airplanes, for instance, is reduced, fuel savings can be achieved and correspondingly greenhouse gas emissions decreased. New materials and components based on gallium nitride (GaN) can help to reduce weight and increase the efficiency. With these new materials, power electronic switches can be operated at higher switching frequency, resulting in higher power density and lower material costs.
Researchers at the Fraunhofer Institute for Solar Energy Systems ISE together with partners have investigated how these materials can be used to make power...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
02.12.2016 | Medical Engineering
02.12.2016 | Agricultural and Forestry Science
02.12.2016 | Physics and Astronomy