Yoshimasa Bando

Stabilization of organic field-effect transistors in hexamethylenetetrathiafulvalene derivatives substituted by bulky alkyl groups

Hexamethylenetetrathiafulvalene (HMTTF) derivatives substituted by tert-butyl, n-pentyl, and 1,1-dimethylpropyl groups are prepared, and the transistor properties are investigated. The compounds substituted by bulky tertiary groups exhibit high mobilities up to 0.98 cm2/Vs in the thin-film transistors and 2.3 cm2/Vs in the single-crystal transistors. At the same time these compounds realize low threshold voltages close to zero and large on–off ratios. The high mobility and the low threshold voltage are maintained more than one month in air. The single-crystal X-ray structure analyses reveal uniform stacking structures. These observations demonstrate that the low threshold voltage and the stable device performance are not solely determined by the energy levels, but are remarkably improved by the closely packed structures derived from the bulky alkyl groups.

Contact resistance and electrode material dependence of air-stable n-channel organic field-effect transistors using dimethyldicyanoquinonediimine (DMDCNQI)

N-channel organic field-effect transistors with stable performance at ambient conditions are fabricated on the basis of an electron-accepting molecule, dimethyldicyanoquinone diimine (DMDCNQI). The transistors are investigated by varying source and drain electrode materials: Au, Ag, Cu, and a highly conducting organic charge-transfer salt, (tetrathiafulvalene)(tetracyanoquinodimethane) [(TTF)(TCNQ)]. The devices with the Au electrode show lowest contact resistance and highest electron mobility (0.011 cm2V−1 s−1 for bottom-contact configuration), and the performance decreases in the order of Au > (TTF)(TCNQ) > Ag > Cu. This order does not seem related to the metal work functions, but is attributed to the organic–metal interfacial potentials. DMDCNQI forms highly conducting charge-transfer complexes with Ag and Cu, but the complex layer increases the interfacial potential as well as the electron-injection barrier and also increases the off-current for short channel devices. The air stability is not determined solely by the organic semiconductor but is considerably influenced by the electrode materials.

Dielectric Response and Electric-Field-Induced Metastable State in an Organic Conductor β-(meso-DMBEDT-TTF)2PF6

Dielectric response and nonlinear conduction are studied in an organic conductor β-( meso -DMBEDT-TTF) 2 PF 6 , which undergoes the metal–insulator transition caused by the checkerboard-type charge ordering (CCO). The formation of the long-range CCO (LR-CCO) was observed as a sudden increase in the dielectric constant below 70 K. Below 70 K, the I – V curve shows a remarkable negative differential resistance with a non-monotonous voltage drop, and its nonlinearity survives up to about 100 K. Time-resolved sample voltage data shows a steep drop followed by a transient plateau and a further drop into the most conductive state. The former steep voltage drop may cause the negative differential resistance in the I – V curve, while the transient plateau implies a metastable state induced by the electric field.

Voltage oscillation associated with nonlinear conductivity in the organic conductor α-(BEDT-TTF)2I3

Characteristic voltage oscillation phenomenon is observed in an organic conductor α-[bis(ethylenedithio)tetrathiafulvalene]2I3, in the nonlinear conductivity region below the metal-insulator transition at 135 K. The oscillation, which is clearly visible in the wave form, appears only when the current is applied in the direction of the charge alternation in the two-dimensional stripe charge order. The frequency of order 10 kHz increases linearly with the applied current. These aspects are interpreted from the viewpoint of collective motion of the two-dimensional charge order.

Current orientation and contact distance dependence of rapid voltage oscillations in the organic conductor β″-[bis(ethylenedithio)tetrathiafulvalene]3(HSO4)2

In an organic conductor β″-[bis(ethylenedithio)tetrathiafulvalene]3(HSO4)2, characteristic voltage oscillation is observed in the negative differential resistance region of the nonlinear conductivity below the metal–insulator transition at 125 K. The observed frequency f is 4–25 kHz and increases linearly with the collective current Jco. The oscillation appears in the two crystal directions of the conducting layer in agreement with the two-dimensional nonstripe charge order, where the anisotropy of the Jco/f slope is about two. The voltage oscillation disappears when the contact distance is larger than 0.02 cm, and at the same time the current-voltage characteristics loses a sharp negative resistance region. Since this critical length corresponds to the characteristic domain size of the charge order, the observed oscillation is interpreted by coherent transport of charge order which can move in different two directions.

Nonlinear Conductivity in Dicyanoquinonediimine Complexes

Nonlinear conductivity is observed below the metal–insulator (M–I) transitions of molecular conductors, halogen-substituted ( R 1 , R 2 -DCNQI) 2 Cu (DCNQI: dicyanoquinonediimine, R 1 , R 2 : methyl or halogen). Despite the difference of the M–I transition temperatures depending on the halogens, these compounds show nonlinear properties at similar low temperatures (<80 K), and the characteristics are regarded as “activation” type. The complex of deuterated dimethyl-DCNQI ( d 2 -DMeDCNQI) 2 Cu, which shows reentrant M–I–M transitions, exhibits irreversible switching from a low-conducting state to a high-conducting state in the intermediate I state. Since the Peierls distortion is irreversibly erased by the electric field, this phenomenon is called “Peierls memory”. In addition, “inverse” nonlinear conductivity from a high-conducting state to a low-conducting state is observed at the low-temperature M state, which is not only entirely reversible but also accompanied by a new kind of rapid current oscillatio...

Organic Field-Effect Transistor Materials Based on Cycloalkane-Capped Tetrathiapentalene Derivatives

Following the previous preparation of the cyclopentane-capped tetrathiapentalene (TTP) molecule (1), we have prepared new TTP-based organic semiconductors capped with cyclobutane (2) and cyclohexane (3) units, and investigated their crystal and thin-film structures together with the field-effect transistor (FET) characteristics. The molecule capped with cyclobutane units (2) forms a uniform stacking structure extending in different directions, leading to a three-dimensional intermolecular interaction. In the crystal of the cyclohexane-capped compound (3), the S-shaped molecules stack uniformly along the crystallographic b- and c-axes, and the estimated intermolecular interaction is two dimensional. The organic FETs (OFETs) prepared by vapor deposition show markedly different mobilities, 1.3×10-2 and 0.1 cm2 V-1 s-1 for 2 and 3, respectively, although the electrochemical properties are almost the same. The striking difference between the FET properties is attributed to the very different crystal structures of the TTP molecules depending on the terminal groups.

Isotropic Uniaxial Strain Effect on the Incommensurate Organic Superconductor : (MDT-TS)(AuI2)0.441

Uniaxial strain is applied to the incommensurate organic superconductor, (MDT-TS)(AuI 2 ) 0.441 , along the three crystallographic axes, where MDT-TS is 5 H -2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene. The metal–insulator transition temperature ( T MI ) exhibits almost the same uniaxial strain dependence in all directions, but the tight-binding calculation cannot explain the isotropic uniaxal strain effect. The superconducting critical pressure ( P c ) decreases in the order of a > b > c . The isotropic uniaxial strain effect for T MI is quite unusual in highly anisotropic molecular conductors.