Tomofumi Kadoya

High performance ambipolar organic field-effect transistors based on indigo derivatives

A bio-inspired organic semiconductor 5,5′-diphenylindigo shows excellent and well-balanced ambipolar transistor properties; its hole and electron mobilities are 0.56 and 0.95 cm2 V−1 s−1, respectively. The enhanced performance is attributed to the extended π–π overlap of the phenyl groups as well as the characteristic packing pattern that is a hybrid of the herringbone and brickwork structures. The ambipolar transistor characteristics are analyzed considering its operating regions, where a large unipolar saturated region appears due to the difference of the electron and hole threshold voltages.

Benzobisthiadiazole-based conjugated donor–acceptor polymers for organic thin film transistors: effects of π-conjugated bridges on ambipolar transport

A new series of benzobisthiadiazole (BBT)-based donor–acceptor copolymers, namely, PBBT-FT, PBBT-T-FT, and PBBT-Tz-FT, with different π-conjugated bridges have been developed for polymer thin film transistors (TFTs). It was found that inserting different π-conjugated bridges into the backbone of the polymer allowed tailoring of opto-electrical properties, molecular organizations, and accordingly, ambipolar transport of TFTs. The UV-vis-NIR spectra of all three polymers were similar with the low band gaps of around 1.1 eV. While the lowest unoccupied molecular orbital (LUMO) energy levels were also similar (around −3.8 eV), the highest occupied molecular orbital (HOMO) energy levels varied from −5.05 to −5.42 eV because of the different π-conjugated bridges; moreover, their TFTs exhibited different ambipolar transport. p-Type dominant TFT performances with the hole mobility (μh) reaching 0.13 cm2 V−1 s−1 were observed for the prototype polymer PBBT-FT. However, the device based on PBBT-T-FT with thiophene bridges displayed lower but more balanced hole (μh) and electron (μe) mobilities of 6.5 × 10−3 and 1.2 × 10−3 cm2 V−1 s−1, respectively. The device based on PBBT-Tz-FT with the thiazole units exhibited more evenly balanced hole and electron mobilities (μh/μe = 0.45) along with a significantly enhanced μe ∼0.02 cm2 V−1 s−1. These different semiconducting features were ascribed to different molecular orientations and film morphologies revealed by wide-angle X-ray scattering (WAXS) and atomic force microscopy (AFM).

Benzothienobenzothiophene-Based Molecular Conductors: High Conductivity, Large Thermoelectric Power Factor, and One-Dimensional Instability

On the basis of an excellent transistor material, [1]benzothieno[3,2-b][1]benzothiophene (BTBT), a series of highly conductive organic metals with the composition of (BTBT)2XF6 (X = P, As, Sb, and Ta) are prepared and the structural and physical properties are investigated. The room-temperature conductivity amounts to 4100 S cm(-1) in the AsF6 salt, corresponding to the drift mobility of 16 cm(2) V(-1) s(-1). Owing to the high conductivity, this salt shows a thermoelectric power factor of 55-88 μW K(-2) m(-1), which is a large value when this compound is regarded as an organic thermoelectric material. The thermoelectric power and the reflectance spectrum indicate a large bandwidth of 1.4 eV. These salts exhibit an abrupt resistivity jump under 200 K, which turns to an insulating state below 60 K. The paramagnetic spin susceptibility, and the Raman and the IR spectra suggest 4kF charge-density waves as an origin of the low-temperature insulating state.

Nanoparticles of organic conductors: synthesis and application as electrode material in organic field effect transistors

Stabilization of TTF·TCNQ nanoparticles is studied by varying the ionic liquid nature and solvent medium. The best dispersion is obtained in an acetonitrile/acetone mixture and the smaller size by using [BMIM][BF4], as a stabilizing ionic liquid. Applications of well-dispersed TTF·TCNQ nanoparticles (mean diameter of about 35 nm) as electrode material in organic field-effect transistors are also reported.

Charge injection from organic charge-transfer salts to organic semiconductors

Highly conducting films of organic charge-transfer (CT) salts are fabricated by a solution process from the dispersions stabilized by poly(vinylpyrrolidone). This method provides a general way to obtain conducting films of nonvolatile organic cation- and anion-radical salts with inorganic counter ions. Carrier injection from organic CT salts to organic semiconductors is investigated by using these films as electrodes in organic field-effect transistors. Efficient hole injection is observed not only from organic cation-radical salts but also from anion-radical salts to pentacene and sexithiophene. Electron injection is dominant from both types of CT salts to C60, but hole injection and ambipolar characteristics are observed for cation-radical salts. The Fermi levels of the CT salts are discussed on the basis of these observations.

Self-contact thin-film organic transistors based on tetramethyltetrathiafulvalene

Carrier injection from organic contacts to tetramethyltetrathiafulvalene (TMTTF) is investigated in the thin-film transistors. When 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) is patterned on a TMTTF film, the resulting (TMTTF)(TCNQ) works as highly conducting source and drain electrodes. Such self-contact transistors, in which the organic material constructing the active layer is selectively transformed to the contacts, have achieved low contact resistance and high performance.

Organic Field-Effect Transistors Based on Small-Molecule Organic Semiconductors Evaporated under Low Vacuum

Basic small-molecule organic electron acceptors and donors such as dicyanoquinonediimine (DCNQI), tetracyanoquinodimethane (TCNQ), and tetramethyltetrathiafulvalene (TMTTF) do not smoothly form thin films by vacuum evaporation owing to the high vapor pressures. The thin films are, however, fabricated by low-vacuum evaporation, and the resulting organic thin-film transistors have exhibited remarkably improved performance.

All-organic self-contact transistors

Organic transistors with chemically doped source/drain electrodes are fabricated by selectively doping tetracyanoquinodimethane to a thin film of hexamethylenetetrathiafulvalene. Using organic materials to create all components, including substrates, gate electrodes, and dielectrics, all-organic self-contact transistors are realized. Due to the smooth charge carrier injection from organic electrodes composed of the same type of molecules, these transistors exhibit excellent mobility exceeding 1 cm2 V−1 s−1.

Thermoelectric power of oriented thin-film organic conductors

The temperature dependence of thermoelectric power is investigated down to low temperatures for oriented thin films of organic conductors. In addition to the evaporated films, highly oriented films consisting of nanoparticles are fabricated by the solution method for tetrathiafulvalene:tetracyanoquinodimethane (TTF)(TCNQ) and (TTF)[Ni(dmit)2]2 (dmit: 1,3-dithiole-2-thione-4,5-dithiolato). The resulting films show n-type thermoelectric power, whose temperature dependence is similar to the single-crystal result along the most conducting axis. Owing to the ordered orientation and nanostructures, the thermoelectric power factor is considerably improved in comparison with the ordinary organic films.

Air-stable ambipolar organic transistors based on charge-transfer complexes containing dibenzopyrrolopyrrole

It has been known that organic charge-transfer complexes with a mix-stacked structure show transistor properties, and particularly complexes containing 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) show air-stable n-channel transistor properties in thin-film transistors, though ambipolar properties have been reported in a few single-crystal transistors. Here we report, when TCNQ is replaced by 2,5-dimethyl-N,N′-dicyano-p-quinonediimine (DMDCNQI), the dibenzopyrrolo[3,2-b]pyrrole (DBPP) complex exhibits ambipolar properties even in the thin-film transistor. The ambipolar operation is stable in air after several weeks. In addition, the transistor shows a very small difference in the electron and hole threshold voltages.