Kodai Iijima

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.

An iodine effect in ambipolar organic field-effect transistors based on indigo derivatives

5,5′-Diiodoindigo (4) exhibits excellent ambipolar transistor properties with hole/electron mobilities of μh/μe = 0.42/0.85 cm2 V−1 s−1. The halogen substituted indigos show decreasing tilt angles from F to I in the crystals. In addition, the iodine–iodine interaction provides extraordinarily large interchain interaction. However, the X-ray diffraction suggests that the indigo molecules are arranged approximately perpendicular to the substrate in the thin films, probably due to the extra iodine–iodine interaction. The remarkable performance is ascribed to this characteristic supramolecular interaction.

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.

Impact of bulky phenylalkyl substituents on the air-stable n-channel transistors of birhodanine analogues

By introducing bulky 2-phenylethyl groups into sulfur-rich electron acceptors, 5,5′-bithiazolidinylidene-2,2′-dione-4,4′-dithione and 5,5′-bithiazolidinylidene-2,4,2′,4′-tetrathione, electron transport with the mobility of 0.27 cm2 V−1 s−1 with ambient and long-term stability is achieved in thin-film transistors. Bulky groups destroy the intermolecular S–S network, but the long-term transistor stability is maintained. Here, benzyl groups realize one-dimensional stacking structures, whereas 2-phenylethyl groups lead to herringbone structures.