Semiconducting polymers could be used in organic electronics

Tuesday, 20 August, 2019

Semiconducting polymers could be used in organic electronics

Scientists at the Tokyo Institute of Technology (Tokyo Tech) have modified a previous synthesis method to create a new semiconducting polymer with remarkable properties that could be used in organic electronic devices such as thin film transistors.

Semiconducting polymers — very large chain-like molecules made from repeating sub-units — are increasingly drawing the attention of researchers because of their potential applications in organic electronic devices. Like most semiconducting materials, semiconducting polymers can be classified as p-type or n-type according to their conducting properties. Although p-type semiconducting polymers have seen dramatic improvements thanks to recent advances, the same cannot be said about their n-type counterparts, whose electron-conducting characteristics (also known as ‘electron mobility’) are still poor.

Unfortunately, high-performance n-type semiconducting polymers are necessary for many green applications, such as various types of solar cells. The main challenges holding back the development of n-type semiconducting polymers are the limited molecular design strategies and synthesis procedures available. Among the existing synthesis methods, DArP (direct arylation polycondensation) has shown promising results for producing n-type semiconducting polymers in an environmentally friendly and efficient way. However, until now, the building blocks (monomers) used in the DArP method were required to have an orienting group in order to produce polymers reliably, and this severely limited the applicability of DArP to make high-performance semiconducting polymers.

Tokyo Tech researchers led by Professor Tsuyoshi Michinobu have now found a way around this. They managed to reliably produce two long n-type semiconducting polymers (referred to as P1 and P2) through the DArP method by using palladium and copper as catalysts, which are materials or substances that can be used to promote or inhibit specific reactions. Their work has been published in the journal Angewandte Chemie.

The two polymers were almost identical and contained two thiazole rings — pentagonal organic molecules that contain a nitrogen atom and a sulfur atom. However, the position of the nitrogen atom of the thiazole rings was slightly different between P1 and P2 and, as the researchers found out, this led to significant and unexpected changes in their semiconducting properties and structure.

Even though P1 had a more planar structure and was expected to have a higher electron mobility, it was P2 that stole the show. The backbone of this polymer is twisted and looks similar to alternating chain links. More importantly, the researchers were surprised to find that the electron mobility of P2 was 40 times higher than that of P1 and even higher than that of the current benchmark n-type semiconducting polymer.

“Our results suggest the possibility of P2 being the new benchmark among n-type semiconducting materials for organic electronics,” Prof Michinobu said.

In addition, semiconducting devices made using P2 were also remarkably stable, even when stored in air for a long time, which is known to be a weakness of n-type semiconducting polymers. The researchers believe that the promising properties of P2 are because of its more crystalline (ordered) structure compared with P1, which changes the previous notion that semiconducting polymers should have a very planar structure to have better semiconducting properties.

“Our new DArP method opens a door for synthesising various promising n-type semiconducting polymers which cannot be obtained via traditional methods,” Prof Michinobu said. The work should thus be another step in the direction towards a greener future with sustainable organic electronics.

Image caption: Professor Tsuyoshi Michinobu and Dr Yang Wang.

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