Functional polymers light up
Microscopic pixels formed by laser bombardment
There is an art in producing organic light-emitting diodes. First of all, ultra-thin layers only 100 nanometres «thick» have to be deposited on top of each other. Furthermore, the coloured luminescent polymers have to be distributed with extreme accuracy and a defined structure to form a pixel matrix. Until now, a microprinting technique has been used to achieve this. Empa researchers from the Functional polymers laboratory working with colleagues from the Materials Group of the Paul Scherrer Institute (PSI) have now found an alternative. Their newly developed method makes do without solvents for the transfer process, allowing organic polymers to be applied in pixel form without the microstructures merging into one another.
The laser catapulting method has already been used for some time to transfer heat-resistant metals and ceramic powders onto surfaces. This is done by applying a layer of the donor material on a transparent substrate. A precisely collimated laser beam is then directed at the layer from behind, causing a tiny piece the size of the beam to be ejected from the layer. This flies at the speed of sound towards the recipient layer and impacts on it.
Instinctive feel for material and structure
Part of getting the particles to «fly» is a matter of having the right instinctive feeling. This applies equally at Empa when it comes to producing the thin film preparations for donor and recipient substrates in an oxygen-free atmosphere or at PSI where transfer experiments are conducted on the ultraviolet laser system and a pulsed laser beam is fired at the prepared wafers. And finally it also takes a deft touch to supply the individual pixels with an electric contact and to apply a voltage. All this effort is rewarded when the microscopic organic LEDs finally light up.
International interest in laser transfer development
Experts from all over the world are following the developments, which are being funded by the Swiss National Science Foundation, with interest. An international research group recently reported on its own laser transfer experiments with living nerve cells using the triazene polymers synthesized at Empa. The controlled transport of the cells onto a biological substrate was so gentle that new nerve cells began growing shortly afterwards. This technique will quite possibly be used in the near future to apply living cells to microchips with pinpoint precision to produce biosensors.