Mar 01, 2019 | |
Organic electronics: A high-performance unipolar n-type thin-film transistor(Nanowerk News) Researchers at Tokyo Institute of Technology (Tokyo Tech) report a unipolar n-type transistor with a world-leading electron mobility performance of up to 7.16 cm2 V-1 s-1. This achievement heralds an exciting future for organic electronics, including the development of innovative flexible displays and wearable technologies. |
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Researchers worldwide are on the hunt for novel materials that can improve the performance of basic components required to develop organic electronics. | |
Now, a research team at Tokyo Tech's Department of Materials Science and Engineering including Tsuyoshi Michinobu and Yang Wang report a way of increasing the electron mobility of semiconducting polymers, which have previously proven difficult to optimize. Their high-performance material achieves an electron mobility of 7.16 cm2 V-1 s-1, representing more than a 40 percent increase over previous comparable results. | |
In their study published in the Journal of the American Chemical Society ("Significant Improvement of Unipolar n-Type Transistor Performances by Manipulating the Coplanar Backbone Conformation of Electron-Deficient Polymers via Hydrogen Bonding"), they 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," Michinobu explains. | |
The research therefore addresses both a fundamental challenge and a practical need. Wang notes 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 synthesize and incompatible with flexible devices. "To overcome these disadvantages," he says, "high-performance n-type semiconducting polymers are highly desired to advance research on all-polymer solar cells." | |
Rational design of electron-transporting organic semiconducting polymers and their thin film analysis and transistor performances. (© JACS) | |
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[1] capable of forming hydrogen bonds with neighboring fluorine and oxygen atoms. Introducing these vinylene bridges required a technical feat so as to optimize the reaction conditions. | |
Overall, the resultant material had an improved molecular packaging order and greater strength, which contributed to the increased electron mobility. | |
Using techniques such as grazing-incidence wide-angle X-ray scattering (GIWAXS), the researchers confirmed that they achieved an extremely short π-π stacking distance[2] of only 3.40 angstrom. "This value is among the shortest for high mobility organic semiconducting polymers," says Michinobu. | |
There are several remaining challenges. "We need to further optimize the backbone structure," he continues. "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 points out 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 sp1-N, fluorine and chlorine atoms, for example, could help achieve even deeper LUMO levels," he says. | |
In future, the researchers will also aim to improve the air stability of n-channel transistors -- a crucial issue for realizing practical applications that would include complementary metal-oxide-semiconductor (CMOS)-like logic circuits, all-polymer solar cells, organic photodetectors and organic thermoelectrics. | |
Technical terms |
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[1] Vinylene bridges: Structures that are known to be effective spacers based on previous studies. These spacers had never been used in the context of polymers that were the focus of this study. | |
[2] π-π stacking distance: A measure of how far the charge needs to be carried within the material. |
Source: Tokyo Institute of Technology | |
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