A team of researchers at the Leibniz Institute for New Materials (INM) has found a cost-effective way to enable electroluminescence on large, curved surfaces. According to the team, the technique allows the light-emitting layer and all other components to be produced by means of wet-chemical, printable methods.

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.

"For processing we only need temperatures below 200°C. This means that we can apply all the required partial layers even to films or other flexible substrates," said Peter William de Oliveira, head of the programme division Optical Materials, INM. 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 functionalised zinc sulphide nanoparticles as phosphors into the binder layer," noted 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 230V. Rectifiers, ballast capacitors or other switching units that first adapt the voltage can be omitted.

The researchers are working on further functionalisation 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," added 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.

By: DocMemory
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