Research

Development of semiconducting polymers | Development of new synthetic methodology for the building units

Development of small molecular organic semiconductors

p-Type organic semiconductors

π-Extended acene-based molecules such as pentacene have been widely studied as high-mobility p-type organic semiconductors. However, a drawback of these molecules is the instability in air due to their high-lying HOMO energy levels. Sulfur containing π-extended heteroaromatic compounds, so-called thienoacenes, possess relatively low-lying HOMO energy levels despite having linearly fused molecular structure similar to pentacene. We have developed a number of thienoacenes such as benzothienobenzothiophene (BTBT), dinaphthothienothiophene (DNTT), dianthrathienothiophene (DATT), etc, including their derivatives. In particular, DNTT-based molecules exhibit high stability in air as well as high hole mobilities in OFET devices (~8 cm2 V-1 s-1). Alkyl-substituted BTBTs can be solution-processed, and their OFET devices demonstrate one of the highest mobilities reported to date in this field. We have also found that there are clear correlations between the molecular structure, electronic structure in the solid state, and electrical properties, which give clear guidelines in designing high-performance materials. These organic semiconductors are now commercially available, and are used by a number of research groups all over the world.

Chem. Mater. 27, 5049-5057 (2015). © 2015 American Chemical Society

ACS Appl. Mater. Interfaces 8, 3810-3824 (2016). © 2016 American Chemical Society

Thiophene-fused naphthalene diimides for n-Type organic semiconductors

Core-extended naphthodithiophene diimides (c-NDI) are key structure for electron deficient pi-systems for optelectronic materials. We have been successfully synthesized one of such c-NDIs with one or two fused-thiophene ring(s), namely naphtho[2,3-b]thiophene imide (NTI) and naphtho[2,3-b:6,7-b']dithiophene diimide (NDTI) via nucleophilic hydrogen substitution reaction effected by sodium sulfide reagent. These two novel c-NDIs are useful and versatile building blocks for developing new n-type organic semiconductors and ambipolar semiconductors for organic filed-effect transistors and organic photovoltaics..

J. Mater. Chem. C, in press

J. Am. Chem. Soc. 135, 11445-11448 (2013). © 2013 American Chemical Society

Org. Lett. 18, 68-67 (2016). © 2016 American Chemical Society

Thienoquinoidal-based n-Type organic semiconductors

The number of the research in developing n-type organic semiconductors, specifically solution-processable ones that work in OFET devices under ambient atmosphere, is limited as compared to p-types. Thienoquinoid compounds with electron-withdrawing dicyanomethylene substituents possess low-lying LUMO energy levels sufficient for ambient n-channel OFETs. Our group has reported on a series of oligothienoquinoids with different solubilizing groups, which show high electron mobilities in excess of 0.1 cm2 V-1 s-1. We also developed new electron withdrawing terminal groups for oligothienoquinoids, such as the alkyl or alkoxy carbonylcyanomethylene group. These semiconductors can be solution-processed and exhibit high electron mobilities.

Org. Lett. 16, 240-243 (2014). © 2014 Royal Society of Chemistry

Org. Lett. 16, 1334-1337 (2014). © 2014 American Chemical Society

Development of small molecular organic semiconductor | Development of new synthetic methodology for the building units

Development of semiconducting polymers

n-Type and ambipolar semiconducting polymers

Semiconducting polymers based on strong acceptor units work as n-type or ambipolar semiconductors thanks to their low-lying LUMO energy levels. As TTD and NDTI are strong acceptor units, polymers incorporating these building units exhibit high hole and electron mobilities over 0.1 cm2/Vs. Furthermore, polarity of charge carriers can be tuned by the choice of the co-building unit. We have also successfully fabricated CMOS-like inverters with a single semiconductor layer by using these ambipolar semiconducting polymers.

Macromolecules 48, 576-584 (2015). © 2015 American Chemical Society

Macromolecules 49, 1752-1750 (2016). © 2016 American Chemical Society

Control of major carriers in ambipolar polymer semiconductor by self-assembled monolayers

We have found selective unipolarization of an ambipolar polymer semiconductor, PNDTI-BT, by using different self-assembled monolayers (SAMs). For the p-unipolarization, 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FDTS) was most effective, whereas for the n-unipolarization, 3-(N,N'-dimethylamino)propyltriethoxysilane (MAPS) was the best. Using these selective uniporlarization effects, the complementary inverters based on the ambipolar polymer fabricated by simple solution process showed greatly improved switching behaviors with low power consumption.

Adv. Mater. in press

Development of small molecular organic semiconductors | Development of semiconducting polymers

Development of new synthetic methodology for the building units

Synthetic methodology for thienoacenes

Benzo[b]thiophene is an important substructure of thienoacenes. Although a lot of synthetic methodologies of benzo[b]thiophene and its derivatives have been reported to date, most of them are difficult to be applied to the synthesis of π-extended thienoacenes. We have developed a number of new methodologies for the synthesis of benzo[b]thiophenes, in particular, the annulation of thiophene on a benzene ring. These new methodologies offer not only the accessibility to various thienoacene-based small molecular semiconductors but also the scalability of the thienoacene syntheses. For example, one of the methodologies allowed us to develop a series of the NDT building units for the first time, which are now utilized by a number of research groups in this field.

Eur. J. Org. Chem. 217-227 (2013). © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

J. Am. Chem. Soc. 135, 13900-13913 (2013). © 2013 American Chemical Society

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