Most electronic devices we meet in our life are usually made of inorganic materials such as silicon and belong to inorganic semiconductor devices. However, due to many shortcomings such as rigidity, brittleness, high cost, complex process and poor biocompatibility, traditional silicon-based semiconductors are facing severe challenges. In addition, the manufacturing process of silicon-based semiconductors is approaching the physical limit.

Therefore, scientists all over the world are developing all kinds of new electronic devices to overcome these shortcomings, further improve the performance of electronic devices and expand their application scenarios. In recent years, a new type of electronic device is sought after by scientists. It is an organic electronic device made of organic semiconductor materials. Organic electronic devices not only have good flexibility and transparency, but also are ultra-thin, ultra-light and environmentally friendly. These materials can be treated by simple, environmentally friendly and low-cost processes, such as making solutions and printing in large areas.

These more flexible, lightweight, portable and transparent organic electronic products can be used in many fields, such as flexible solar cells, flexible displays, flexible sensors, flexible wearable devices, implantable devices and so on. Among them, organic light emitting diode (OLED) is a typical case of successful commercialization, and the latest generation of smart phones have begun to use OLED display.

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Today, I want to introduce a new progress in the field of organic electronics.

Recently, a research group led by Tsuyoshi Michinobu and Wang Yang of tokyo institute of technology Department of Materials Science and Engineering reported a unipolar N-type transistor with world-leading electron mobility performance. They adopted a new method to improve the electron mobility of semiconductor polymers, which proved to be difficult to optimize in the past. Their high-performance materials reached 7. 16 cm2 V? 1 s？ Compared with the previous comparable results, the electron mobility of 1 is improved by more than 40%.

The paper published in the Journal of American Chemical Society shows that they focus on improving the properties of so-called "N-type semiconductor polymers". N-type materials are mainly conducted by negatively charged electrons; Relatively speaking, P-type materials are mainly conductive by positively charged holes. Michinobu explained: "Because negatively charged atomic groups are inherently unstable compared with positively charged atomic groups, making stable N-type semiconductors has always been an important challenge in the field of organic electronics."

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However, this research not only meets the basic challenges, but also meets the practical needs. Wang said, for example, many organic solar cells are made of P-type semiconductor polymers and N-type fullerene derivatives. The disadvantage is that the latter is expensive, difficult to synthesize and incompatible with flexible devices. He said: "In order to overcome these shortcomings, high-performance N-type semiconductor polymers are very promising to promote the research of all-polymer solar cells."

The team's method includes adopting a series of new benzothiadiazole-naphthalimide derivatives and fine-tuning the main chain conformation of the material. This method can be realized by introducing "1, 2- vinylidene bridge". Previous studies have shown that this structure is considered as an effective spacer, but this spacer has never been used in the polymer involved in this study. It can form hydrogen bonds with adjacent fluorine atoms and oxygen atoms. The introduction of these "1, 2- vinylidene bridges" requires important technologies that can optimize the reaction conditions.

Generally speaking, the generated materials have better molecular assembly order and higher strength, which is beneficial to improve electron mobility.

Using grazing incidence wide-angle X-ray scattering (GIWAXS) and other techniques, the researchers confirmed that they achieved a very short "π? π stacking distance is only 3.40 Amy (one Amy is one tenth of a nanometer). This distance measures how far the charge needs to be carried in the charge. Michinobu said: "For organic semiconductor polymers with high mobility, this distance is the shortest."

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This achievement indicates an exciting future for organic electronics, and scientists will develop innovative flexible displays and wearable technologies.

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In addition, researchers also face several challenges. He said: "We need to further optimize the backbone structure. At the same time, side chain groups also play an important role in determining the crystallinity and packaging direction of semiconductor polymers. We still have room for improvement. "

Wang pointed out that for the reported polymers, the lowest LUMO energy level is in? 3.8 eV to? Between 3.9 eV, he said: "The deeper LUMO energy level, the faster and more stable electron transmission. Therefore, for example, further design by introducing sp2-N, fluorine atoms and chlorine atoms will help to achieve deeper LUMO energy levels. "

In the future, researchers will also plan to improve the air stability of N-channel transistors. Air stability is a key issue for practical applications such as complementary metal oxide semiconductor (CMOS) logic circuits, all-polymer solar cells, organic photodetectors and organic thermoelectric devices.

reference data

1 https://www . titech . AC . jp/English/news/20 19/043699 . html

2 http://dx . doi . org/ 10. 102 1/jacs . 8b 12499