Toshihide Kamata

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...

Self-aligned self-assembly process for fabricating organic thin-film transistors

A process for fabricating alignment-free, printable, organic thin-film transistors is presented. This process utilizes a self-assembly phenomenon in which soluble nanomaterials such as metallic nanoparticles and organic molecules are self-assembled into a device structure. To demonstrate this process, solution-processed source∕drain electrodes were self-aligned to a gate electrode using a hydrophobic self-assembled monolayer (SAM) optically patterned onto the gate electrode using a backsubstrate exposure technique. An organic semiconductor film deposited on the patterned SAM was selectively ordered and substantially self-aligned to the gate electrode. This process is called self-aligned self-assembly. A field-effect mobility of 0.15cm2∕Vs and potential minimum channel length of 3μm were experimentally demonstrated when pentacene molecules and silver nanoparticles were used as the semiconductor and electrode materials, respectively.

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.

Polariton emission from polysilane-based organic microcavities

We report the observation of strong coupling between exciton and photon modes in a conjugated polymer-based semiconductor microcavity. Thin films of the σ-conjugated poly[bis(p-butylphenyl)silane] (PBPS) were inserted between metal and dielectric mirrors to form the microcavity structures. Variation of the PBPS film thickness between 80 and 140 nm allowed the cavity photon resonance to be tuned in the vicinity of the free exciton energy. The expected anticrossing behavior, with intensity and linewidth averaging, was observed at room temperature in the cavity reflection spectra and the vacuum Rabi splitting was found to be ⩽430 meV. This large value is consistent with the expectations of transfer matrix reflectivity calculations performed with optical constants data derived from a Kramers–Kronig analysis of the PBPS absorption spectrum. Angle-dependent photoluminescence measurements were performed for the microcavity with a 120 nm thickness PBPS layer. Unlike the emission from a standard, weakly coupled, c...

Effect of active layer thickness on device properties of organic thin-film transistors based on Cu(II) phthalocyanine

We investigated the device properties of Cu(II) phthalocyanine (CuPc)-based bottom-contact thin-film transistors (TFTs) with various active layer thicknesses. The operating performance of the TFTs depended on the thickness, and a device with a CuPc layer of 80 nm showed the maximum field-effect mobility. We found that the field-effect mobility of the thinnest and thickest devices exhibited a marked source–drain bias dependence. The characteristics of these devices were not as good as those of the other TFTs. We attributed the source–drain bias dependence to a significantly trap-affected carrier transport process in a conduction channel formed in the vicinity of the gate insulator. We concluded that, in these TFTs, the conduction channel was formed at a different position in the CuPc layer depending on the thickness, and the thickness dependent operating performance originated from variations in the effect of carrier trapping on the carrier transport process in these conduction channels.

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.

Light up-conversion from near-infrared to blue using a photoresponsive organic light-emitting device

A photoresponsive organic light-emitting device combining blue-emitting organic electroluminescent (EL) diode with titanyl phthalocyanine as a near-infrared (IR) sensitive layer was fabricated. By irradiating near-IR light to the device, blue emission occurred in the lower drive voltage (between 5 and 12 V). The result indicates that the device acts as a light switch and/or an up-converter from near-IR light (1.6 eV) to blue (2.6 eV). The EL response times of rise and decay using a near-IR light trigger were 260 and 330 μs, respectively. At a higher voltage (above 12 V), enhancement of blue emission was observed with near-IR light irradiation. The ON/OFF ratio reached a maximum of 103.

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-resolution full-color LCD driven by OTFTs using novel passivation film

A full-color twisted-nematic type liquid crystal display (TN-LCD) of 1.4-in diagonal size driven by organic thin-film transistors (TFTs) has been fabricated. This TN-LCD has 80/spl times/80/spl times/3 (RGB) pixel arrays addressed by pentacene TFTs with a channel width of 50 /spl mu/m. The contact resistance between the pentacene film and the source/drain electrodes has been reduced by selecting the exposure condition of the photoresist in patterning the electrodes. In addition, a solution-processed passivation film with a novel structure, consisting of photosensitive polyvinylalcohol and organosiloxane glass resin, has been developed to protect the TFTs against degradation induced by integration with TN-LCD devices. Consequently, it has been confirmed that the organic-TFT-driven TN-LCD fabricated in this paper is capable of displaying full-color moving images at a resolution of 80 pixels per inch.

Third-order non-linear optical properties of one-dimensional metal complexes

Abstract The third-order non-linear optical properties of evaporated thin films of d 8 transition metal complexes with dionedioxime have been estimated by third harmonic generation. They formed one-dimensional structures and showed large optical non-linearities. Resonance effects were observed for respective metal complexes. The maximum χ (3) value of 5.6 × 10 −11 esu was obtained at the near three-photon resonance wavelength for the platinum complex. The delocalized electrons in linear metal chains may be essential for exhibiting large non-linear optical effects.