One alternative to OLEDs is electroluminescence, in which phosphors are excited in an electric field and researchers at the INM–Leibniz Institute for New Materials have developed a method that enables electroluminescence to be deployed cost effectively on large, curved surfaces. The team says the light emitting layer and all other components can be produced using wet chemical printable methods.
"We only need temperatures of less than 200°C," said Peter William de Oliveira, head of the optical materials program division. "This means we can apply all the required partial layers, even to films or other flexible substrates."
The luminous unit consists of two electrically conductive layers, with the light emitting particles sandwiched in a dielectric binder layer. At least one of the conductive layers is also transparent. On application of an AC voltage, light is emitted from the electroluminescent layer.
"We embed functionalised zinc sulphide nanoparticles as phosphors into the binder layer," de Oliveira explained. "These are doped with copper or manganese. This allows the generation of green and blue-green light."
The electroluminescent light sheets can be connected directly to the 230V mains supply, with no need for rectifiers, ballast capacitors or other switching units.
The researchers are now working on different phosphors. "Our goal is to generate white light by means of an altered doping or by introducing coloured pigments into the luminous layer," said de Oliveira. The team also wants to alter the materials so the displays can operate from a lower voltage.
