High-performance n-type transistor developed

Researchers at the Tokyo Institute of Technology (Tokyo Tech) have developed a unipolar n-type transistor with a high electron mobility performance of up to 7.16 cm² V⁻¹ s⁻¹, heralding an exciting future for organic electronics including flexible displays and wearable technologies.

Researchers worldwide are on the hunt for novel materials that can improve the performance of basic components required to develop organic electronics. Now, Tokyo Tech researchers have reported a way of increasing the electron mobility of semiconducting polymers, which have previously proven difficult to optimise.

The researchers focused on enhancing the performance of materials known as n-type semiconducting polymers. These n-type (negative) materials are electron dominant, in contrast to p-type (positive) materials that are hole 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,” said Tokyo Tech researcher Tsuyoshi Michinobu.

The research therefore addresses both a fundamental challenge and a practical need, with team member Yang Wang noting that many organic solar cells, for example, are made from p-type semiconducting polymers and n-type fullerene derivatives. The drawback is that the latter are costly, difficult to synthesise and incompatible with flexible devices.

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

The team’s method involved using a series of new poly(benzothiadiazole-naphthalenediimide) derivatives and fine-tuning the material’s backbone conformation. This was made possible by the introduction of vinylene bridges — structures that are known to be effective spacers based on previous studies — capable of forming hydrogen bonds with neighbouring fluorine and oxygen atoms. Introducing these vinylene bridges required a technical feat so as to optimise the reaction conditions.

The resultant material, described in the Journal of the American Chemical Society, had an improved molecular packaging order and greater strength, which contributed to its impressive electron mobility of 7.16 cm² V⁻¹ s⁻¹ — representing more than a 40% increase over previous comparable results. The researchers also confirmed 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 only 3.40 Å.

“This value is among the shortest for high mobility organic semiconducting polymers,” said Michinobu.

Michinobu acknowledged that there are several remaining challenges. “We need to further optimise the backbone structure,” he said. “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.”

Wang added that the lowest unoccupied molecular orbital (LUMO) levels were located 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,” he said.

In future, the researchers will also aim to improve the air stability of n-channel transistors — a crucial issue for realising practical applications that would include complementary metal-oxide-semiconductor (CMOS)-like logic circuits, all-polymer solar cells, organic photodetectors and organic thermoelectrics.

Image caption: Tsuyoshi Michinobu and Yang Wang fabricating thin-film transistors.

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