Light-emitting diodes (LEDs) are the modern lighting devices used in lamps, signals, signs or displays. By contrast, organic semiconducting light-emitting materials (OLEDs) can be incorporated in thin layers and used on curved surfaces. However, OLEDs for large-area illumination are cost-intensive at present owing to their low efficiency and short lifetime.
One promising alternative for modern lighting is electroluminescence. Special nanoparticles, so-called phosphors, are excited in an electric field to emit light. Researchers at the INM – Leibniz Institute for New Materials have now developed a new method that enables electroluminescence on large, curved surfaces in a cost-effective way: in this case, the light-emitting layer and all other components are produced by means of wet-chemical, printable methods.
The researchers from the INM will be presenting their results from 13 to 17 April 2015 in the Hannover Messe.
“For processing we only need temperatures below 200 degrees Celsius. This means that we can apply all the required partial layers even to films or other flexible substrates,” explains Peter William de Oliveira, head of the program division Optical Materials. Hence, “luminous surfaces” could be produced very cost-effectively and even in large formats.
The luminous unit consists of two electrically conductive layers, between which the light-emitting particles are sandwiched in a dielectric binder layer. At least one of the conductive layers is also transparent. Due to the insulating layer, the absorbed energy is efficiently converted into light and appreciable heating is avoided. On application of an AC voltage, light is emitted from the electroluminescent layer. “We embed luminous particles in the form of functionalized zinc sulphide nanoparticles as phosphors into the binder layer,” explains de Oliveira “these are doped with copper or manganese. At present this allows the generation of green and blue-green light.”
The electroluminescent light sheets developed at the INM can be directly connected to the customary mains voltage of 230 volts. Rectifiers, ballast capacitors or other switching units that first adapt the voltage can be omitted.
The researchers are currently working on further functionalization of the phosphor nanoparticles. “Our goal is to generate white light by means of an altered doping or by introducing coloured pigments into the luminous layer,” says physicist de Oliveira. At the same time the developers want to alter the materials in such a way that the light sheets can be used even at a lower mains voltage.
INM conducts research and development to create new materials – for today, tomorrow and beyond. Chemists, physicists, biologists, materials scientists and engineers team up to focus on these essential questions: Which material properties are new, how can they be investigated and how can they be tailored for industrial applications in the future? Four research thrusts determine the current developments at INM: New materials for energy application, new concepts for medical surfaces, new surface materials for tribological systems and nano safety and nano bio. Research at INM is performed in three fields: Nanocomposite Technology, Interface Materials, and Bio Interfaces.
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