New Unipolar N-Type Transistor Has World-Leading Electron Mobility Performance

A unipolar n-type transistor demonstrating a globally leading electron mobility performance of nearly 7.16 cm2 V1 s−1 has been developed by a research team at Tokyo Tech. This accomplishment announces an electrifying future for organic electronics, together with the development of novel flexible displays and wearable technologies.

Tsuyoshi Michinobu, right: Yang Wang. (Image credit: Tokyo Tech)

Researchers from all over the world are on the quest for innovative materials capable of enhancing the performance of basic components needed to produce organic electronics.

A team of researchers from Tokyo Tech’s Department of Materials Science and Engineering, including Tsuyoshi Michinobu and Yang Wang, recently described an approach to increase the electron mobility of semiconducting polymers, which was earlier considered to be complicated to optimize. Their high-performance material has the potential to achieve an electron mobility of 7.16 cm2 V−1 s−1, representing more than a 40% increase over earlier comparable results.

The focus of a study on this recent development featured in the Journal of the American Chemical Society is on improving the performance of materials called n-type semiconducting polymers. In contrast to p-type (positive) materials that are hole-dominant, the n-type (negative) materials are electron-dominant.

As negatively-charged radicals are intrinsically unstable compared to those that are positively charged, producing stable n-type semiconducting polymers has been a major challenge in organic electronics.

Michinobu, Researcher, Department of Materials Science and Engineering, Tokyo Tech.

The research thus deals with both a practical requirement and a fundamental challenge. Wang points out that several organic solar cells, for instance, are produced from p-type semiconducting polymers and n-type fullerene derivatives. In this case, the disadvantage is that the latter ones are expensive, hard to synthesize, and mismatched with devices that are flexible.

To overcome these disadvantages, high-performance n-type semiconducting polymers are highly desired to advance research on all-polymer solar cells.

Michinobu, Researcher, Department of Materials Science and Engineering, Tokyo Tech.

The team’s technique focused on using a series of new poly(benzothiadiazole-naphthalenediimide) derivatives and refining the material’s backbone conformation. This was achieved by the introduction of vinylene bridges that can form hydrogen bonds with adjacent fluorine and oxygen atoms. These vinylene bridges were introduced by a technical feat in order to optimize the reaction conditions. (Vinylene bridges are structures that are known to be effective spacers based on earlier studies. These spacers had never been used in the context of polymers that were the focus of this study.)

On the whole, the resultant material had an enhanced molecular packaging order and higher strength, which supported the increased electron mobility.

The researchers employed techniques like grazing-incidence wide-angle X-ray scattering (GIWAXS) to confirm that they achieved a very short π−π stacking distance (a measure of how far the charge needs to be carried within the material) of just 3.40 Å.

This value is among the shortest for high mobility organic semiconducting polymers.

Michinobu, Researcher, Department of Materials Science and Engineering, Tokyo Tech.

There are a number of challenges that are yet to be achieved.

We need to further optimize the backbone structure. At the same time, side chain groups also play a significant role in determining the crystallinity and packing orientation of semiconducting polymers. We still have room for improvement.

Michinobu, Researcher, Department of Materials Science and Engineering, Tokyo Tech.

Wang goes on to state that the lowest unoccupied molecular orbital (LUMO) levels were found at −3.8 to −3.9 eV for the reported polymers.

As deeper LUMO levels lead to faster and more stable electron transport, further designs that introduce sp2-N, fluorine and chlorine atoms, for example, could help achieve even deeper LUMO levels.

Michinobu, Researcher, Department of Materials Science and Engineering, Tokyo Tech.

Going forward, the researchers aim to enhance the air stability of n-channel transistors—a vital issue for understanding practical applications that will include complementary metal-oxide-semiconductor (CMOS)-like logic circuits, organic photodetectors, organic thermoelectrics, and all-polymer solar cells.

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