Printable electroluminescent sheets promise cost-effective large-format luminous surfaces
At present organic semiconducting light-emitting materials (OLEDs) can be incorporated in thin layers and used on curved surfaces but OLEDs for large-area illumination have until now proved to be cost-intensive owing to their low efficiency and short lifetime.
“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,” explained Peter William de Oliveira, head of the program division Optical Materials. The result is that ‘luminous surfaces’ could now be produced 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,” explained 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,” explained 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.
The researchers from the INM will be presenting their results from 13 to 17 April 2015 in Hall 2 at the stand B46 of the Hannover Messe in Germany.
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