Toshiki Higashino

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.

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.

Air-stable n-channel organic field-effect transistors based on a sulfur rich π-electron acceptor

Thin-film and single-crystal n-channel organic field-effect transistors are built from the sulfur rich π-electron acceptor, (E)-3,3′-diethyl-5,5′-bithiazolidinylidene-2,4,2′,4′-tetrathione (DEBTTT). Different source and drain electrode materials are investigated: gold, the conducting charge transfer salt (tetrathiafulvalene)(tetracyanoquinodimethane), and carbon paste. Regardless of the nature of the electrodes, air-stable n-channel transistors have been obtained. Single crystals exhibit a higher performance than the thin-film transistors with a mobility of up to 0.22 cm2 V−1 s−1. These thin-film and single-crystal devices exhibit excellent long-term stability as demonstrated by the mobility measured during several weeks. The high mobility and air stability are ascribed to the characteristic three-dimensional S–S network coming from the thioketone sulfur atoms.

Band-like transport down to 20 K in organic single-crystal transistors based on dioctylbenzothienobenzothiophene

Band-like transport has been realized down to 20 K in solution-processed single-crystal transistors based on dioctylbenzothienobenzothiophene. The mobility increases from 16 to 52 cm2/V s as the temperature is lowered from 300 to 80 K. An abrupt mobility drop is observed around 80 K, but even below 80 K, gradually increasing mobility is restored again down to 20 K instead of thermally activated transport. From the observation of a shoulder structure in the transfer curve, the mobility drop is attributed to a discrete trap state.

A Single-Component Conductor Based on a Radical Gold Dithiolene Complex with Alkyl-Substituted Thiophene-2,3-dithiolate Ligand

Alkyl-substituted thiophene-2,3-dithiolate ligands are prepared through a Thio-Claisen rearrangement of 4,5-bis(propargylthio)-1,3-dithiole-2-thione derivatives. The two novel dithiolate ligands, namely, 4,5-dimethyl-thiophene-2,3-dithiolate (α-Me2tpdt) and 4-ethyl-5-methyl-thiophene-2,3-dithiolate (α-EtMetpdt), are engaged in anionic Au(III) square planar complexes formulated as [Au(α-Me2tpdt)2](-) and [Au(α-EtMetpdt)2](-), isolated as Ph4P(+) salts. Monoelectronic oxidation gives the neutral radical complexes [Au(α-Me2tpdt)2](•) and [Au(α-EtMetpdt)2](•). The latter crystallizes into uniform stacks with limited interstack interactions, giving rise to a calculated half-filled band structure. It exhibits a semiconducting behavior with room temperature conductivity of 3 × 10(-3) S cm(-1), indicating that this single-component conductor can be described as a Mott insulator. The different structures observed in [Au(α-EtMetpdt)2](•) and the known [Au(Et-thiazdt)2](•) complex (Et-thiazdt: N-ethyl-thiazoline-2-thione-4,5-dithiolate), despite their very similar shapes, are tentatively attributed to differences in the electronic structures of the ligand skeleton.

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.

Air-stable n-channel organic field-effect transistors based on charge-transfer complexes including dimethoxybenzothienobenzothiophene and tetracyanoquinodimethane derivatives

A new donor molecule 3,8-dimethoxy-[1]benzothieno[3,2-b][1]benzothiophene (DMeO-BTBT) is synthesized and the charge-transfer complexes with fluorinated 7,7,8,8-tetracyanoquinodimethane (Fn-TCNQ; n = 0, 2, and 4) are prepared. All complexes (DMeO-BTBT)(Fn-TCNQ) have mixed stack structures, and the thin-film and single-crystal organic transistors show n-channel transistor performance both in vacuum and in air even after one-year storage. Although the performance and stability of the thin-film transistors are improved according to the acceptor ability of Fn-TCNQ, all single-crystal transistors exhibit similar performance and excellent air stability, among which the (DMeO-BTBT)(F2-TCNQ) transistor exhibits the highest mobility of 0.097 cm2 V−1 s−1.

Ambipolar transistor properties of 2,2′-binaphthosemiquinones

Binaphthosemiquinones are proved to show ambipolar transistor properties. These compounds have characteristic blue colors owing to the small energy gaps, because the quinone part acts as an electron acceptor and the alkoxy group acts as an electron donor. Accordingly, these molecules have an analogous electronic structure to indigo. The crystal structure changes depending on the alkoxy groups, though these compounds generally have stacking structures.

Extracting parameters in ambipolar organic transistors based on dicyanomethylene terthiophene

Ambipolar organic transistors based on dicyanomethylene terthiophene show electron and hole mobilities of up to 0.6 and 0.3 cm2 V?1 s?1, respectively, on tetratetracontane, which are thousands of times larger than the reported value. The intrinsic electron and hole threshold voltages are determined by analyzing the output and transfer characteristics, from which the ambipolar characteristics are quantitatively reproduced. Since the transistor is operated mostly in the unipolar region, the trap density of states is estimated from the temperature dependence of the transfer characteristics.

Birhodanines and their sulfur analogues for air-stable n-channel organic transistors

A series of thin-film n-channel organic field-effect transistors based on various birhodanines, 3,3′-dialkyl-5,5′-bithiazolidinylidene-2,2′-dione-4,4′-dithiones (OS-R) and their sulfur analogues, 3,3′-dialkyl-5,5′-bithiazolidinylidene-2,4,2′,4′-tetrathiones (SS-R) are studied. The SS-R compounds have tilted stacking crystal structures, whereas the OS-R compounds show basically herringbone structures. The alkyl chain R length and the intermolecular S–S interactions influence the molecular packing to realize excellent long-term air stability in the thin-film transistors.