Takehito Kodzasa

Influence of moisture on device characteristics of polythiophene-based field-effect transistors

We investigated a field-effect transistor (FET) based on a poly(3-n-hexylthiophene) (P3HT) to determine the influence of moisture on device characteristics and thus gain a deep understanding of the mechanism underlying the susceptibility to air of the operation of FETs of this kind. The fundamental output characteristics, which include effective field-effect modulation and saturation behavior in the output current, remained almost the same for every current–voltage profile in a vacuum, N2 and O2. By contrast, operation in N2 humidified with water resulted in enlarged off-state conduction and deterioration in the saturation behavior, in the same manner as that experienced with exposure to room air. We concluded that atmospheric water had a greater effect on the susceptibility of the device operation to air than O2, whose p-type doping activity as regards P3HT caused only a small increase in the conductivity of the active layer and a slight decrease in the field-effect mobility with exposure at ambient pres...

Surface Potential Control of an Insulator Layer for the High Performance Organic FET

It has been indicated that the pentacene crystal growth near the channel region is the most essential factor for determining pentacene FET properties and strongly depends on the surface energy of the substrate. In this study, surface modification effect of the SiO 2 /Si substrate by several kinds of polymers was investigated for the purpose of controlling surface potential and improve the FET properties. Matching the surface energy between the insulator and semiconductor layers using a polymer surface modifier gave improvement of the pentacene crystal growth and its FET properties. It was revealed that the molecular diffusion on a solid substrate and crystal growth mechanism were essential to give the effects.

Threshold voltage stability of organic field-effect transistors for various chemical species in the insulator surface

The relationship between the threshold voltage (Vt) stability and the chemical species of the insulator surface was investigated by using organic field-effect transistors with different types of self-assembled monolayers on a SiO2 insulator. The Vt shift induced by gate bias stressing was considerably increased by the introduction of long-chain chemical species to the SiO2 surface. In order to obtain high-performance and high-stability organic transistors, insulator surfaces with short-chain chemical species that can improve transistor performance without degrading stability are required.

Investigation for surface modification of polymer as an insulator layer of organic FET

We have investigated a double layer structured polymer gate dielectric for the organic field-effect transistor (FET) with the purpose of improving the performance of the polymer gate insulator. A polymer gate dielectric often causes a large hysteresis in the transfer characteristics of the organic FET. In this study, a water-soluble clay mineral layer was inserted between the PMMA and pentacene layer. It brought about improvement of the drain current and disappearance of the hysteresis.

Influence of fine roughness of insulator surface on threshold voltage stability of organic field-effect transistors

We have investigated the influence of the surface roughness of an insulator on the threshold voltage shift caused by gate bias stressing in organic field-effect transistors (OFETs). Our investigation was conducted for OFETs with SiO2 insulators. We observed that the threshold voltage shift is extremely sensitive to changes in the fine roughness of the SiO2 surface; the shift increased with the roughness. The large shift in OFETs with rough SiO2 insulators can be attributed to lattice distortion in pentacene layers deposited on rough SiO2 surfaces.

High Performance Organic FET with Double-Semiconductor Layers

The over-coating effect of a second semiconductor layer on a pentacene field-effect transistor (FET) was investigated. Me-PTC (n-type) coating gave improvements of the field-effect mobility and I DS for pentacene (p-type) FET. Such remarkable improvements were not observed in the TiOPC (p-type) coated pentacene (p-type) FET. These phenomena indicate that n-type semiconductor coating on a p-type semiconductor layer reduce the effect of trapped electrons at the crystal grain boundary. Besides, the over-coated layer also prevents the effect of water molecules adsorbed on pentacene crystals. Consequently, it was revealed that the p/n type hetero double-layered structure is effective to improve FET properties.

Development of Field-Effect Transistor-Type Photorewritable Memory Using Photochromic Interface Layer

We developed a novel field-effect transistor (FET) type photorewritable memory using a photochromic interface layer between the active layer and the gate insulator layer. A diarylethene (DAE) derivative was employed as a photochromic material and pentacene was employed as an active layer. DAE has two types of photoisomer, i.e., the closed- and open-ring isomers. In this study, it was clarified that the highest occupied molecular orbital (HOMO) level of the closed-ring DAE worked as an interfacial deep trap level, and that the generation of the interfacial deep trap level by photoisomerization induced the photoswitching and photomemory behaviors of transistor properties.

Temporal Changes in Source--Drain Current for Organic Field-Effect Transistors Caused by Dipole on Insulator Surface

The influence of the dipole of an insulator surface on temporal changes in the source–drain current was investigated by using organic field-effect transistors with a surface-modified SiO2 insulator. The source–drain current decreased drastically with respect to time when the dipoles of the insulator surface displaced slightly. In order to obtain highly stable organic transistors, it is thus necessary to remove the mobile dipoles from the insulator surface.

Effects of The Substituents on the Nonlinear Optical Properties of Bis(1,2-Diaryl-1,2-Ethylenedithiolato)Metal Complexes

Abstract Electrochemical properties and third harmonic generation (THG) properties of bis(1,2-disubstituted-1,2-ethylenedithiolato)metal complexes were examined. This is the first time to report the THG properties of evaporated thin films of these complexes. Nonlinear optical (NLO) properties of these complexes in solutions were already reported by other research groups. It is notable that the title complexes show relatively large THG properties in both solid state and solution. It seems that the electronic state of these complexes by itself has some influences upon THG properties. The effects of the substituents in the dithiolato ligands were larger than those in the phenyl groups in the dithiolato ligands on both the reduction half-wave potentials and the THG properties. The reported molecular structure of bis(1,2-diphenyl-1,2-ethylene-dithiolato)nickel showed the planes of phenyl groups were not on the same plane as the metaladithiolene rings. These results suggest that the electronic state of the meta...

Electrode Effects of Organic Thin-Film Transistor with Top and Bottom Contact Configuration

Organic semiconductors have several unique properties that are different from those of inorganic semiconductors. Therefore, in order to develop an organic transistor, it is important to investigate structural factors of a device that govern the operation mechanism. Recently, we have designed a new device structure of an organic field-effect transistor (FET) with a diagonal configuration of source and drain electrodes, named as the top and bottom contact (TBC) configuration. In this device structure, the source and drain electrodes are prepared individually; therefore, it is convenient to independently study electrode effects on FET properties. In this study, we have prepared a pentacene TBC-FET, and studied electric field effects on FET properties in relation to the electrode configuration. By controlling several factors for the bottom electrode, we have observed marked changes in FET properties. It was revealed that the observed phenomena were mainly due to a difference in carrier injection efficiency.