03 Dec 2009
Researchers at the Fraunhofer Institute for Laser Technology (ILT) in Aachen, Germany, are working with industrial partner Philips on a laser-based fabrication process that promises to cut the cost of manufacturing organic LEDs (OLEDs) while also enhancing device luminosity.
OLEDs are highly efficient, high-luminous-intensity light sources based on organic materials. The devices consist of one or several active organic layers that are energized by two large-area electrodes.
The initiated current flow leads to electron-hole recombinations in the organic layer. This in turn produces photons, which radiate into the half-space through the conductive, transparent anode – consisting of indium tin oxide (ITO) or similar materials.
To distribute the electrical energy evenly over the entire surface of the OLEDs, metallic conductor paths are applied to the ITO layer. The size of the conductor paths is critical: too wide and they can adversely affect the luminous homogeneity of the light source.
Until now, the metallic conductor material has been applied to the surface of the OLEDs in an energy-intensive sputtering process. Put simply, an atomic layer is deposited over the entire surface of the substrate in a high vacuum and removed again using a photolithographic method in the areas where conductor paths are not required.
There are problems with this subtractive process, however. For starters, it's expensive, owing to the effort involved in applying and then removing the metal layer not required (which involves a material loss of up to 90%).
The photolithographic removal step is also an environmental headache (the etching solution contains metals that must be disposed of after use). Finally, conventionally produced conductor paths have a width of up to 120 µm, which is not optimum for homogeneous luminosity.
The Fraunhofer ILT team is now developing an additive laser technique to apply micro-scale conductor paths for its industrial partner Philips.
In the first step, a mask foil is placed on the surface of the conductor, representing the negative to the conductor path geometry later required. This is then covered by a donor foil whose material will constitute the conductor path (e.g. aluminium or copper). The assembly is fixed in place and hit with laser radiation travelling at a speed of up to 2.5 m/s along the mask geometry.
A mixture of melt drops and vapour forms, which is then transferred from the donor foil to the substrate. The solidified mixture produces the conductor path, whose geometry is determined by the mask.
"This enables us to produce narrow metallic paths with adjustable widths between 40 and 100 µm," explained Christian Vedder, project leader at Fraunhofer ILT. "They [the paths] exhibit variable thicknesses between 3 and 15 µm and a resistance of <0.05 Ω/sq, so that the electrons are distributed optimally over the entire surface. Our aim is to produce homogeneous luminosity over the entire surface with the new technique."
Equally significantly, the process takes place in an ambient atmosphere (no expensive vacuum chamber) and there is no material loss because the residual material of the donor foil can be reused.
The use of the laser also ensures greater flexibility in how the layer is configured, says Holger Schwab, project manager for OLED lighting at Philips.
"This process can considerably reduce the cost of producing OLEDs," he added. "The key factors are that almost 100% of the material is utilized and no structuring processes are needed."
• OLEDs are being billed as a compelling growth technology in large-area and decorative illumination. Analysts at NanoMarkets forecast a worldwide market volume of over $2.9bn for 2012, with sales increasing to around $5.9bn by 2014.
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