Junichi Takeya

Electronic functionalization of solid-to-liquid interfaces between organic semiconductors and ionic liquids: Realization of very high performance organic single-crystal transistors

High-performance electronic function of current amplification is realized with the use of solid-to-liquid interfaces between organic semiconductors and ionic liquid. To hold in place the ionic liquid of 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide known for low viscosity and high ionic conductivity, an elastomeric well structure is fabricated with polydimethylsiloxane on which organic single crystals of rubrene are electrostatically attached. As the result of rapid formation of electric double layers in the ionic liquid interfacing, the high-mobility organic semiconductor crystals’ fast-switching transistor function is demonstrated with the application of gate voltage, realizing the highest sheet transconductance, namely, amplifying performance, ever achieved.

High-power and high-speed organic three-dimensional transistors with submicrometer channels

Three-dimensional organic field-effect transistors with high current density and high switching speed are developed with multiple submicrometer channels arranged perpendicularly to substrates. The short channel length is defined by the height of a multicolumnar structure without an electron-beam-lithography process. For devices using dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene, extremely high current density exceeding 10 A/cm2 and fast switching within 0.2 μs are realized with an on-off ratio of 105. The unprecedented performance is beyond general requirements to control organic light-emitting diodes, so that even more extensive applications to higher-speed active-matrices and display-driving circuits can be realized with organic semiconductors.

Polymorphs of Rubrene Crystal Grown from Solution

Single crystals of rubrene were grown by slow cooling of solutions in various solvents. Hexagonal single crystals were obtained from p-xylene, whereas parallelogram-shaped crystals were grown from aniline. Both types of crystal were obtained from propan-1-ol. Single-crystal X-ray diffraction analyses showed that the hexagonal and parallelogram-shaped crystals belonged to the orthorhombic system and the triclinic system, respectively. The triclinic crystals showed much poorer carrier mobilities than did the orthorhombic crystals.

Solution Growth of Rubrene Single Crystals Using Various Organic Solvents

To fabricate organic field-effect transistors (OFETs) with high carrier mobility, we attempted to grow 5,6,11,12-tetraphenylnaphthacene (rubrene) single crystals from solution and to improve their quality. Investigations into solvents in which rubrene was highly soluble proved that its solubility depended on the presence or absence of aromatic rings and chloro groups rather than on the polarity of the solvents. Rubrene crystals were grown from aromatic solvents, specifically from toluene, p-xylene, and aniline solvents, as well as from 1,2-dichloroethane (DCE) solvent. As a result, rubrene single crystals larger than 1 mm were obtained. Powder X-ray diffraction (XRD) analysis showed that the crystals obtained from the p-xylene and toluene solvents were rubrene, and 1H-nuclear magnetic resonance (1H-NMR) measurement proved that the crystals had not incorporated the solvent at the detection level. In addition, atomic force microscopy (AFM) confirmed that the rubrene crystals grown from the p-xylene and aniline solvents had flat faces and that the crystal from the p-xylene solvent had monomolecular steps on parts of the surfaces. Rubrene single crystal OFETs with graphite electrodes and parylene as an insulator showed carrier mobilities of ~0.75 cm2 V-1 s-1.

High-mobility organic single crystal transistors with submicrometer channels

We demonstrate high-performance electric-field effects in submicrometer-channel organic transistors with rubrene single crystals. Platinum source and drain electrodes are embedded in silicon dioxide gate insulators to reduce thickness of the dielectrics and to minimize the short-channel effect. The miniaturized devices exhibit typical output characteristics with Ohmic linear region, well-defined current saturation, and on-off ratio of 106. Mobility values are in the range of 0.1–0.3cm2∕Vs, which is comparable to those of the best submicrometer organic transistors. Anisotropy in the mobility is detected, indicating that bandlike transport is responsible for the high transistor performance of the short-channel devices.