Development of π-Conjugated polymers (π共役系ポリマーの開発)

 The simplest structure of π-conjugated polymer is polyacetylene, which led to the Nobel Prize in Chemistry 2000 by Professors Shirakawa, Heeger, and MacDiarmid, while our research focuses on π-conjugated polymers based on thiophenes, benzene, and/or fused heteroaromatic rings. These π-conjugated polymers would exhibit various functions such as conductivity, luminescence, and photovoltaic properties, on the basis of their π-electron systems. One of the important properties for π-conjugated polymers is to have a highly crystalline structure, which allows us to have greater device performances. We design and synthesize new π-conjugated heteroaromatics, and π-conjugated polymers with high crystallinity using these heteroaromatics as the building units.

Thiazole-based building units (チアゾール系骨格)

 Thiazole-based heteroaromatic rings possess electron deficient nature due to the “pyridine-type” nitrogen atom. Therefore, incorporating them into π-conjugated polymer backbones results in deep HOMO energy levels, which would improve the air stability of the polymers. Typically, π-conjugated polymers having thiazole-based rings possess relatively large (wide) bandgaps. Thiazole-based rings can reduce steric hindrance between the neighboring units compared to thiophene-based rings, which is expected to enhance the backbone coplanarity and rigidity. This should lead to high crystallinity and thereby high charge carrier mobility. 


Chem. Mater.201022, 4191–4196. Adv. Mater.201022, 4993–4997. Adv. Mater., 201224, 425–430. Adv. Mater., 201426, 331–338.

Naphthobischalcogenadiazoles (NXz) (ナフトビスカルコゲナジアゾール

 Owing to the large π-electron system of naphthobischalcogena diazoles (NXz), incorporation of NXz into the polymer backbone brings strong interchain interactions and thereby high crystallinity to the resulting polymers. In addition, the high electron-deficiency of NXz results in donor (D)-acceptor (A) polymer backbones, which further enhances interchain interaction. In fact, polymers based on NXz exhibit high carrier transport mobilities comparable to amorphous silicon in transistors, and show one of the highest power conversion efficiencies in polymer/fullerene solar cells.


J. Am. Chem. Soc.2012134, 3498–3507. Chem. Mater. 201527, 6558–6570. Nat. Commun., 20156, 10085. J. Am. Chem. Soc. 2016138, 10265–10275. Adv. Mater201729, 1605218. Chem. Eur.J. 2018, 24, 19228 –19235. Macromolecules 2019, 52, 3909−3917. Adv. Energy Mater202010, 1903278.

Imide-functionalized building unit (イミド基を有する骨格

 Due to the strong electron withdrawing nature of imide groups, imide-functionalized π-electron systems can deepen LUMO energy levels as well as HOMO energy levels of π-conjugated polymers. Since deep LUMO energy levels enable electron transport, the use of imide-functionalized systems is a highly effective way to create π-conjugated polymers with an n-type semiconducting character. Our group has developed original imide-functionalized building units with large π-system, and revealed that the polymers with these building units demonstrate great electron transport properties.


Adv. Mater201628, 6921-6925. ACS Appl. Mater. Interfaces 2019, 11, 23410−23416.

Thienoquinoids (チエノキノイド

 Thienoquinoids (TQs) are useful building units for electron-transporting materials due to their deep frontier energy levels. Furthermore, incorporation of thienoquinoids into the polymer backbone provides small bandgaps due to the reduction of bond length alternation along the polymer backbone. For example, a polymer based on benzodithiophendione (BDTD), which we have developed, gives a colorless and transparent thin film owing to the extremely small bandgap, and exhibits ambipolar behavior with high hole and electron mobilities.


J. Mater. Chem. C, 20142, 2307–2312. Adv. Electron. Mater., 20151, 1500039. J. Am. Chem. Soc. 2016138, 7725–7732.

Manipulation of polymer crystallinity and orientation (ポリマーの結晶性と配向性の制御)

 In transistors, because charge carriers are transported horizontally to the substrate, “edge-on” orientation in which polymer backbones stand on the substrate, is favorable (figure left side). In solar cells, on the other hand, because charge carriers are transported perpendicularly to the substrate, “face-on orientation” in which the backbones lie flat on the substrate is favorable (figure right side). As such, the orientation of π-conjugated polymers is also crucial, in addition to crystallinity, to improve the device performance. Our group has found that the polymer orientation can be controlled by carefully designing the alkyl side chains.


J. Am. Chem. Soc., 2013135, 24, 8834–8837. Adv. Mater., 201426, 331–338. Nat. Photon. 20159, 403–408. ACS Appl. Mater. Interfaces 2018, 10, 32420−32425. ACS Appl. Polym. Mater. 2019, 1, 1257−1262.

Application to organic devices (有機デバイスへの応用)

 Organic devices can be made by “coating” or “printing” an ink of a π-conjugated polymer and can be lightweight, very thin, and flexible, which are not the case in conventional inorganic devices, thus attract much attention as next-generation devices. We fabricate and measure organic devices such as organic field-effect transistors (OFETs, left in the figure) and organic solar cells (OPV, right in the figure, nanotechnology) in addition to the synthesis of π-conjugated polymers. To date, we have developed a number of π-conjugated polymers and found that these polymers demonstrate world class performance in both OFETs and OPVs.