Paintable electronics: Bringing polymers into line

Amenable to alignment

Engineers at the University of Michigan and electronics company Samsung in Korea have devised a method for bringing otherwise unruly semiconducting polymers into line as verified by X-ray diffraction studies, which they suggest might one day pave the way for cheaper, greener, "paint-on" plastic electronics.

"This is the first thin-layer, conducting, highly aligned film for high-performance, paintable, directly writeable plastic electronics," explains materials scientist Jinsang Kim who publishes details of the work with colleagues, Bong-Gi Kim, Sungbaek Seo, Eun Jeong Jeong, and Jong Won Chung and Bonwon Koo of  Samsung Advanced Institute of Technology, in Youngin, Korea, in the journal Nature Materials. "We developed a molecular design principle to make lyotropic liquid-crystalling conjugated polymers, which are different from conventional conjugated polymers," Kim told SpectroscopyNOW. "Due to their lyotropic liquid crystalline property these conjugated polymers have good assembling properties and nice mobility at the same time."

Semiconductors are, of course, the vital ingredient for computer processors, photovoltaic solar panels, light-emitting diode displays and countless other applications and devices. But they are pricey, delicate, and inorganic semiconductors such as silicon require processing at temperatures well in excess of 1000 Celsius in expensive vacuum systems. Organic and polymer semiconductors can be prepared almost in any standard chemical laboratory, the starting materials are much cheaper too and the end products can be flexible, robust and much more amenable to processing into novel form factors for modern applications.

Spaghetti vs Rotini

Unfortunately, organic semiconductors do not represent a trouble-free starting point, they are not as efficient at transporting charge carriers, electrons and holes, as inorganic semiconductors. This is certainly the case for polymer semiconductors in which each polymer molecule might be viewed as a separate wire within the thread, randomly arranged and each with its own start and end. In contrast, inorganic semiconductors are usually continuous crystalline materials, there are no strands, no ends and rather than being randomly arranged the materials are highly ordered.

However, there is a phenomenon associated with polymer semiconductors that might be exploited to improve their abilities greatly. "Charge mobility along the polymer chains is much faster than between the polymers," Kim explains. To exploit this good conduction along the polymers, researchers have tried to align polymers to lay down what some have referred to as a charge-carrying freeway, but this in itself is not without its challenges, which on a much larger scale one might envision as being rather like trying to line up strands of cooked spaghetti without spattering Bolognese sauce everywhere and without a splash of olive oil to prevent aggregation.

Higher chargers

Kim's group think they have found a way around the problem of how to make the polymers smarter so that they can be brushed on to a surface and so automatically align the polymers in a single direction to create high-performance semiconducting thin-layer films. First, they made their polymers slippery rather than "tacky" like those strands of spaghetti. They also opted for polymers that have a natural twist, akin to helical Rotini pasta rather than linear spaghetti. This reduces adhesion between the polymer molecules. However, a phenomenon not possible with Rotini pasta that occurs with the team's polymers is that their design means that they untwist during the process of brushing them on to the surface so that they subtly attract each other leading them to line up as their solvent evaporates.

The presence of bulky side chains and the ability to partake of sulfur-fluorine interactions in the polymers also ensures that they don't get too close during the alignment process as well as preventing tangling and snagging. Fundamentally, the polymers with these structures will line up in the direction of an applied force, such as the tug of a paintbrush over a glass or flexible plastic substrate. "It's a big breakthrough," adds Kim. "We established a complete molecular design principle for semiconducting polymers with directed alignment capability."

The team has used their aligned thin films to create a simple but working transistor. The measured charge carrier rates of movement of 1,600 times faster in the direction parallel to alignment than across the polymers. "We are currently developing a versatile fabrication method in order to realize high-performance and paintable plastic electronics in various length scales from nanometres to metres," adds Kim.