Kenzo Kojima

Performance of poly(3-hexylthiophene) organic field-effect transistors on cross-linked poly(4-vinyl phenol) dielectric layer and solvent effects

Bottom-contact organic field-effect transistors (OFETs) were fabricated using a polymer gate insulator cross-linked poly(4-vinyl phenol) with regioregular poly(3-hexylthiophene) (RR-P3HT) as an active layer from different organic solvents. With this polymer dielectric, a field-effect mobility of 0.084±0.006cm2V−1s−1 was obtained. Solvents and interfacial properties have pronounced effects in determining the crystallinity and device performance of RR-P3HT on the polymer gate layer. Morphology correlation with the charge carrier mobility of RR-P3HT OFETs is investigated. Large nanoscale crystalline island densities of this polymer play an important role in the high charge carrier mobility of devices.

Electronic conduction in polyethylene naphthalate at high electric fields

The electric conduction in polyethylene naphthalate film was investigated under high electric fields. The current strongly depended on the electrode materials, especially on the cathode metal. Schottky‐injection current was observed in the specimen with an Al cathode at the fields less than 106 V/cm, while at higher fields (>106 V/cm) tunneling‐injection current and electroluminescence were observed simultaneously, suggesting the impact ionization by electrons injected from the electrode.

Luminescence of dyes doped in a sol–gel coating film

Abstract A sol–gel coating film doped with coumarin 6 as a host and rhodamine 6G as a guest was prepared by a sol–gel technique. A coumarin 6-doped sol–gel coating film showed the spectral changes of absorption and fluorescence due to the change of molecular structure originating from the acid–base equilibrium of immobilized dyes. A rhodamine 6G-doped sol–gel coating film did not show these spectral changes. However, when rhodamine 6G was doped together with coumarin 6 in a sol–gel coating film, its emission showed increase in intensity with the increase of pH, irrespective of a constant absorption of irradiated photons. Based on these observations, energy transfer between coumarin 6 and rhodamine 6G was examined.

Electroluminescence in Polyethylene Terephthalate (PET). II. : AC Voltage

Electroluminescence in polyethylene terephthalate (PET) was observed under AC voltages. The electroluminescence was found to be controlled by the injection of electrons from the Al electrode, together with the space charge accumulation. The occurrence of the injection and accumulation of electrons was demonstrated by thermally-stimulated current analysis. The significant modification of the internal field by the space charge resulted in an asymmetrical waveform of the electroluminescence in each half-cycle of the AC voltage.

Thermally Stimulated Currents in Polyethylene Terephthalate due to Injected Charge

Thermally stimulated currents (TSC) from polyethylene terephthalate (PET) electrets have been investigated in the temperature range from -190 to +120°C. PET electrets were formed by applying a high field in a short time at various temperatures. Both dependencies of TSC on electrode metals (Au, Al) and the polarity of an applied field were remarkable in the temperature range above -40°C. The results indicate that electrons injected from the Al cathode greatly contribute to the TSC above -40°C and that the TSCs below -40°C for Al electrodes and at all temperatures for Au electrodes are mainly induced by the depolarization of dipoles.

Performance of Organic Field-Effect Transistors with Poly(3-hexylthiophene) as the Semiconductor Layer and Poly(4-vinylphenol) Thin Film Untreated and Treated by Hexamethyldisilazane as the Gate Insulator

The relationship between the carrier mobility and the structure/morphology of polymer semiconductor molecules on a substrate remains a very important issue in organic electronics. In this paper, the relationship between the crystallinity of drop-cast regioregular poly(3-hexylthiophene-2,5-diyl) (RR-P3HT) thin films and polymer substrates was discussed by using heat-crosslinkable poly(4-vinylphenol) (PVP) as the gate insulator in organic field-effect transistors (OFETs). The effect of a self-assembled monolayer (SAM) of 1,1,1,3,3,3-hexamethyldisilazane (HMDS) on crosslinked PVP was also examined and the performance of the OFET was investigated. Results show that not only the orientation and crystallinity of the polymer semiconductor are very important in determining the OFET performance, but also the gate dielectric properties, surface roughness, and the interface properties between the gate and semiconductor layers are important factors.

Electroluminescenee in Polyethylene Terephthalate (PET) I. Impulse Voltage

There have been several reports concerning the electroluminescence (EL) in polymers using ac(1,2) or impulse voltages(2,3). Several mechanisms have been considered to explain the EL in polymers. Generally, the emission will occur in two ways, that is, the emission resulted from a recombination of an electron and a hole injected from electrodes and the emission from the molecules excited, or ionized in an extreme case, by the electron with high energy, injected from electrodes or ejected from localized states and accelerated by a high electric field. Accumulation of space charges in polymers will modify the field intensity in the bulk or the surface layer of polymers and seriously affect the EL intensity.

Performance of Organic Field-Effect Transistor Based on Poly(3-hexylthiophene) as a Semiconductor and Titanium Dioxide Gate Dielectrics by the Solution Process

Organic field-effect transistors (OFETs) were fabricated with a high-dielectric-constant and high-permittivity titanium dioxide (TiO2) as a gate insulator and regioregular poly(3-hexylthiophene-2,5-diyl) (RR-P3HT) as the electronically active semiconductor. Positive OFET characteristics were obtained with a low threshold voltage (+3 V) and high field-effect mobility (3.73 ×10-3 cm2 V-1 s-1). The dielectric material (TiO2) was prepared by the sol–gel technique and the gate insulator layer was fabricated by spin coating. The RR-P3HT thin films were fabricated by drop casting with different solution concentrations. The thickness of the thin films was measured using the surface profile measuring system. The fabricated thin film structure was analyzed by atomic force microscopy (AFM), X-ray diffraction (XRD), and UV–visible absorption spectra. The X-ray result shows that the drop-cast RR-P3HT thin film has a high crystallinity on the TiO2 surface, which leads to the high field-effect mobility of the OFET. The results show that the OFET performances are not only dependent on the orientation and crystallinity of the polymer semiconductor, but that also the gate dielectric properties, surface roughness, and interface properties between the gate and semiconductor layers are very important for the efficient performance of the OFET.

Carrier Traps in Polyethylene Naphthalate (PEN): Photoelectret

Electronic carrier traps in polyethylene naphthalate (PEN) were investigated by TSC and TL analysis in the temperature range from -196°C to 160°C. Two broad peaks were observed in the TSC and TL from PEN films illuminated with light of wavelength 360 nm at -196°C. The detrapping process was closely related to the molecular motions, the so-called γ and β relaxations. The apparent trap depths ranged from 0.1–0.5 eV.

Organic field-effect transistors with crosslinkable poly(vinyl alcohol) insulator and spin-coated/drop-cast poly(3-hexylthiophene-2,5-diyl) semiconductor

Organic field-effect transistors (OFETs) with water-soluble and crosslinkable poly(vinyl alcohol) (PVA) as a gate insulator and poly(3-hexylthiophene) (P3HT) as an active semiconductor were fabricated, and the device performances were investigated. Heat-crosslinked PVA displays better dielectric properties because it contains less water and the OFETs show enhanced on/off ratios and improved stability. P3HT films were fabricated by spin-coating and drop-casting methods and characterized by atomic force microscopy (AFM) and UV–visible absorption spectra. The mobilities of drop-cast P3HT films are one order higher than those of the spin-coated films, which is ascribed to the slower drying of the solvent and more-ordered structure in the films. However, OFETs with drop-cast P3HT show inferior saturation characteristics and lower on/off ratios. This is probably due to higher roughness and thickness of the drop-cast film and nonnegligible carrier transportation in the bulk, which can be demonstrated by the conductance measurements.