TU Darmstadt coats fibres with organic semiconductors

But now Darmstadt’s material scientists have developed a machine with which electronically active materials can be vacuum deposited onto threads.

“In theory, reproducible rotational coating with semiconductor components opens up a whole new world of possibilities in smart textiles,” reports Prof. Heinz von Seggern, director of the field of Electronic Material Properties at the TU Darmstadt. In other words, from the engineering point of view, this is an important step forward weaving electronic components into clothing.

Working towards smart textiles

The materials scientist Tobias Könyves-Toth, who had the idea for the machine as part of the LUMOLED joint project of the Federal Ministry for Education and Research (BMBF), has been able to vacuum deposit organic semiconductor elements onto glass fibres.

“We turned to organic light emitting diodes known as OLEDs because they require the highest standards concerning the substrates. Today, we are able to apply functioning OLEDs to a thread and make it glow. Applying other devices such as transistors or solar cellsphotovoltaics includes other problems, but is comparatively less complex when it comes to the actual coating.”

One problem in small molecule fibre coating is that it has to be carried out under vacuum conditions. “With our rotational coating process we have found a possibility to evenly coat the fibre and then being able to remove it from the vacuum without it coming into contact with air, because OLEDs are highly sensitive to oxygen and water,” explains Tobias Könyves-Toth.

The components are applied to the thread by heating the materials in the vacuum until they sublime or evaporate. The materials condensate on the fibre in exactly the same way as water does when it is heated on a cooker, rises as steam and condenses on a window. A total of seven layers have to be applied to the fibre, some of them just a few atoms thick.

“The layers that are applied to the fibres are only about 200 nanometres thick; in other words, fine dust particles are 50 times bigger than the layer thickness of the OLEDs.” And this is where another problem arises: textile fibres have a rough surface. However, the electronic components only work on a smooth surface; even scratches of just a few nanometres’ thickness can cause problems such as short circuiting.