New Synthesis Technique Paves Way for Organic Electronics

Researchers at Tokyo Institute of Technology (Tokyo Tech) alter an earlier synthesis technique to develop a new semiconducting polymer with extraordinary properties which could be used in organic electronic devices like thin film transistors.

New direct arylation polycondensation method opens the door to synthesize various promising n-type semiconducting polymers. (Image credit: Tokyo Institute of Technology)

Semiconducting polymers, huge chain-like molecules made from repeating sub-units, are progressively gaining the attention of scientists because of their prospective applications in organic electronic devices.

Like the majority of semiconducting materials, semiconducting polymers can be categorized as p-type or n-type based on their conducting properties. Although p-type semiconducting polymers have seen great enhancements thanks to the latest advances, the same cannot be said about their n-type equivalents, whose electron-conducting features (or “electron mobility”) are still weak.

Unfortunately, high-performance n-type semiconducting polymers are essential for a number of green applications, such as different types of solar cells. The key challenges slowing down back the creation of n-type semiconducting polymers are the limited molecular design strategies and synthesis processes available.

Among the current synthesis techniques, DArP (which stands for “direct arylation polycondensation”) has shown favorable results for creating n-type semiconducting polymers in an eco-friendly and efficient way. However, thus far, the building blocks (monomers) used in the DArP technique were required to have an orienting group so as to create polymers reliably, and this greatly restricted the applicability of DArP to make high-performance semiconducting polymers.

Fortunately, a research team from the Tokyo Institute of Technology led by Prof. Tsuyoshi Michinobu discovered a solution. They were successful in reliably producing two long n-type semiconducting polymers (called P1 and P2) through the DArP technique by using copper and palladium as catalysts, which are substances or materials that can be used inhibit or promote specific reactions.

The two polymers were almost identical and had two thiazole rings–pentagonal organic molecules that comprised of a nitrogen atom and a sulfur atom. However, the position of the nitrogen atom of the thiazole rings was marginally different between P1 and P2 and, as the scientists discovered, this resulted in significant and unanticipated changes in their semiconducting properties and structure.

Even though P1 had a more planar structure and was anticipated to possess higher electron mobility, it was P2 which was efficient. The polymer’s backbone is twisted and looks quite like alternating chain links. More notably, the scientists were amazed to learn that the electron mobility of P2 was forty times higher than that of P1 and even higher than that of the existing benchmark n-type semiconducting polymer.

Our results suggest the possibility of P2 being the new benchmark among n-type semiconducting materials for organic electronics.

Tsuyoshi Michinobu, Study Lead and Professor, Tokyo Institute of Technology

Furthermore, semiconducting devices created using P2 were also strikingly stable, even when stored in air for an extended time, which is said to be a weakness of n-type semiconducting polymers. The scientists believe that the favorable properties of P2 are due to its more crystalline (ordered) structure compared with P1, which alters the earlier concept that semiconducting polymers should possess a very planar structure to have better semiconducting properties.

Our new DArP method opens a door for synthesizing various promising n-type semiconducting polymers which cannot be obtained via traditional methods.

Tsuyoshi Michinobu, Study Lead and Professor, Tokyo Institute of Technology

This research is one step closer to a greener future using sustainable organic electronics.


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