Takayuki Usui

Liquid crystals for organic thin-film transistors

Crystalline thin films of organic semiconductors are a good candidate for field effect transistor (FET) materials in printed electronics. However, there are currently two main problems, which are associated with inhomogeneity and poor thermal durability of these films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability of films up to 200 °C. In addition, the mobility of FETs is dramatically enhanced by about one order of magnitude (over 10 cm2 V−1 s−1) after thermal annealing at 120 °C in bottom-gate-bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs may help overcome upcoming difficulties with novel technologies for printed electronics.

Self-Directed Orientation of Molecular Columns Based on n-Type Hexaazatrinaphthylenes (HATNAs) for Electron Transport

Self-organizing n-type hexaazatrinaphthylenes (HATNAs) with various bay-located side chains have been synthesized. The HATNA derivatives are able to form long-range molecular columns with self-directed growth directions. In particular, alkyl-substituted HATNAs showed in-plane molecular columns with axes parallel to substrates, whereas the columnar orientation of the HATNAs with alkylethynyl or alkylthio groups strongly depended on the length of the introduced side chains. Interestingly, the derivative with octylthio chains exhibited out-of-plane molecular columns, in which electron mobility of up to 10(-3)  cm(2)  V(-1)  s(-1) was determined through the time-of-flight technique, highlighting the fact that such molecular columns based on bay-substituted HATNAs are promising n-type semiconductors for device applications.

Phase transition, optical and photoconductive properties of bay-substituted benzoporphyrin derivatives

Two benzoporphyrin derivatives substituted with long side chains at the bay positions have been synthesized through a facile route. They were confirmed to show two separate columnar phases upon cooling, i.e., a hexagonal columnar phase and a rectangular columnar phase. In particular, the temperature dependence of the electronic spectra revealed that the Q-band split into two peaks, indicating the existence of typical herringbone-type dimers. Moreover, their photoconductive properties were studied by steady-state and transient photocurrent measurements. It was found that both benzoporphyrins exhibited very fast hole mobility over 0.1 cm2 V−1 s−1 in both columnar phases and a high charge carrier generation efficiency up to 2%, demonstrating that they are potential p-type organic semiconductors for photovoltaic devices.

Synthesis of a novel smectic liquid crystalline glass and characterization of its charge carrier transport properties

A new organic semiconductor material with both a self-organized molecular alignment in smectic liquid crystals and a solid nature in amorphous materials, i.e., a glassy liquid crystalline organic semiconductor has been proposed, and its model compound, 1,3,5-benzenetricarboxylic acid, tris{12-[6′-(4′′-octylphenyl)-2′-naphthyloxy]-1-dodecyl ester} (LCG-triester 8-PNP-O12), was synthesized, and we characterized its phase transition behaviors in slow cooling and rapid cooling. It exhibited smectic A (SmA) and smectic B (SmBhex) phases in slow cooling, and a SmBhex glassy phase with a high Tg close to 60 °C when cooled rapidly from the SmA and SmBhex phases. Its charge carrier transport properties in the smectic mesophases, including the liquid crystal glassy phase, were first studied by the time-of-flight (TOF) technique, which revealed that the hole mobility in the SmBhex glassy phase was about 3.5 × 10−4 cm2 v−1s−1, comparable to that in the SmBhex liquid crystal phase. We concluded that the high charge transport properties of the smectic mesophase are successfully maintained in the solid glassy state.

Bulk mobility of polycrystalline thin films of quaterthiophene derivatives

Although polycrystalline thin films of organic materials are applied for organic field effect transistors, their bulk electrical properties have not been characterized. In this study, we investigated hole transport properties in the polycrystalline thin films of a series of quaterthiophene derivatives by both time-of-flight experiments and field effect measurements. The time-of-flight bulk mobility was almost same or smaller than field effect mobility. Both mobilities showed a similar trend as a function of side chain length. We discussed these results from the view point of the intrinsic nature of quaterthiophenes, which gave a fundamental knowledge of charge transport properties in the organic polycrystalline thin films.

Self-Organization in Bulk Heterojunction Solar Cells with Small Molecules

We fabricated both conventional and inverted bulk heterojunction solar cells by spin-coating with a rod-like liquid crystal of 1,4-diketo-N,N-dimethyl-3,6-bis(4-dodecyloxyphenyl) pyrrolo[3.4-c]pyrrole as a donor and C61PCBM as an acceptor. In both cell structures, we found that molecular reorientation and self-organization of the pyrrolopyrrole derivative occurred so as to form conduction channels after the thermal treatment at 60 °C for 30 min and that the power conversion efficiency was improved by 30 times, up to 1%. These results prove that the bulk nature of rod-like liquid crystals is effective for ordered bulk heterojunction solar cells leading to the improvement of the cell performance.