Kiyoshi Yase

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...

Energy transfer and device performance in phosphorescent dye doped polymer light emitting diodes

Singlet and triplet–triplet energy transfer in phosphorescent dye doped polymer light emitting devices were investigated. Poly(N-vinylcarbazol) and poly[9,9′-di-n-hexyl-2,7-fluorene-alt- 1,4-(2,5-di-n-hexyloxy)phenylene] (PFHP) were selected as the host polymer for the phosphorescent dopants fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3] and 2,3,7,8,12,13, 17,18-octaethyl-21H,23H-porphyrin platinum(II) (PtOEP) because of their high triplet energy levels and long phosphorescence lifetimes. In case of PVK film, efficient triplet energy transfers to both PtOEP and Ir(ppy)3 were observed. In contrast, the triplet energy transfer did not occur or was very weak from PFHP to both PtOEP and Ir(ppy)3 although usual requirements for triplet energy transfer were satisfied. Furthermore, the singlet–singlet energy transfer did not take place from PFHP to Ir(ppy)3 in doped films even though the Forster radius is more than 30 A. However, the blended film of Ir(ppy)3 with PFHP and PMMA showed the green emission from ...

Highly sensitive thin-film organic phototransistors: Effect of wavelength of light source on device performance

Organic phototransistors (OPTs) were fabricated from pentacene and copper phthalocyanine (CuPC) based on the geometry of organic field-effect transistors (OFETs); and the effect of the wavelength of the incident light source on their performance was examined. High performance OFETs with pentacene and CuPC were fabricated and the characteristics of the OPTs were examined under UV and visible-light irradiations with top illumination. The CuPC and pentacene OPTs show a high responsivities of 0.5–2 and 10–50A∕W and maximum IPh∕IDark of 3000 and 1.3×105, respectively, under 365nm UV light. However, under visible light, at a wavelength of 650nm, the pentacene OPTs had 100 times less responsivity, 0.15–0.45A∕W, and a IPh∕IDark of 1000, even though an absorption coefficient three times larger was observed at this wavelength than at 365nm. A strong correlation was found between the performance of the OPTs and the incident photon to current conversion efficiency spectra of an organic semiconductor. The strong depen...

High-photosensitivity p-channel organic phototransistors based on a biphenyl end-capped fused bithiophene oligomer

We report highly photosensitive organic phototransistors (OPTs) based on a 2,5-bis-biphenyl-4-yl-thieno[3,2-b]thiophene (BPTT). The measured maximum sensitivity and the ratio of photocurrent to dark current (Iph∕Idark) in BPTT OPTs were 82A∕W and 2.0×105 under 380nm UV light with 1.55mW∕cm2, respectively. The prepared OPTs show a photocurrent response similar to the absorption spectrum of BPTT. The major mechanisms for photocurrent amplification in this device were verified from experimental results as photovoltaic (turn-on) and photocurrent effect (turn-off) by a fitting to theoretic equations.

Solution-processed n-type organic thin-film transistors with high field-effect mobility

We report the performance of solution-processed n-type organic thin-film transistors (OTFTs) based on C60 derivatives. Long-chain alkyl-substituted C60, C60-fused N-methylpyrrolidine-meta-C12 phenyl (C60MC12), was used as a semiconducting layer. The C60MC12-thin-film transistor shows high electron mobility of 0.067cm2∕Vs in saturation regime. From the result of x-ray diffraction analysis, the C60MC12 active layer forms highly ordered crystalline film. We found that self-assemble ability of long alkyl chains plays an important role for fabrication of highly ordered crystalline film, leading to achievement of high electron mobility in solution-processed n-type OTFTs.

Two-dimensional array of silver nanoparticles

Abstract The silver salts of fatty acids were thermally decomposed at 250°C in an atmosphere of nitrogen to form silver nanoparticles with a size of 5 nm. The particles suspended in toluene put on a hydrophilic surface of the plasma-polymerized film of osmium tetraoxide and on a water surface to form a two-dimensional array. The nanoparticles repelled each other with a distance of 2.8–3.5 nm depending on the length of the alkyl chain. The surface pressure–area isotherms were measured to confirm the condensation mechanism of two-dimensional arrangement of nano-particles surrounded by alkyl moieties.

Ambipolar organic field-effect transistors based on a low band gap semiconductor with balanced hole and electron mobilities

The authors have demonstrated the thin-film properties and the ambipolar transport of a delocalized singlet biradical hydrocarbon with two phenalenyl radical moieties (Ph2-IDPL). The organic field-effect transistors (OFETs) based on Ph2-IDPL exhibit ambipolar transport with balanced hole and electron mobilities in the order of 10−3cm2∕Vs. The Ph2-IDPL film is an organic semiconductor with a low band gap of 0.8eV and has small injection barriers from gold electrodes to both the highest occupied molecular orbital and the lowest unoccupied molecular orbital. A complementary metal-oxide-semiconductor-like inverter using two identical Ph2-IDPL based ambipolar OFETs shows a sharp inversion of the input voltage with high gain.

Highly polarized polymer light-emitting diodes utilizing friction-transferred poly(9,9-dioctylfluorene) thin films

Polarized polymer light-emitting diodes (PLEDs) have been constructed utilizing friction-transferred poly(9,9-dioctylfluorene) (PFO) thin films. The friction transfer technique allows oriented PFO to be deposited directly onto an indium tin oxide anode without an alignment layer such as polyimide. Polarized absorption and photoluminescence spectra revealed that the polymer backbones are highly aligned in the friction direction. We fabricated PLEDs consisting of friction-transferred PFO as an emissive layer, vacuum-deposited bathocuproine as an electron transport and hole-blocking layer, and a vacuum-deposited LiF∕Al cathode. Highly polarized blue emission with an integrated polarization ratio of 31 and a luminance of up to 300cd∕m2 was observed from the PLEDs.

Compatibilizing effect of maleic anhydride functionalized SEBS triblock elastomer through a reaction induced phase formation in the blends of polyamide6 and polycarbonate: 2. Mechanical properties

Abstract The relationship between mechanical properties and phase morphology of blends of polyamide6 (PA6) and polycarbonate (PC) compatibilized with a maleic anhydride functionalized poly[styrene-b-(ethylene-co-butylene)-b-styrene] triblock copolymer (SEBS-gMA) were explored. The in situ chemical reaction between the maleic anhydride of SEBS and the amine end group of PA6 during melt mixing induces the encapsulation of SEBS-gMA on the PC domains in PA6 rich blends. Through this phase formation, the adhesion on the domain boundary between PA6 and PC are improved and thus mechanical properties are improved. The use of the combination of SEBS-gMA and unfunctionalized SEBS as compatibilizers has been found to provide remarkable improvement of mechanical properties in the PA6/PC blends. A transmission electron microscope (TEM) study has revealed that the encapsulation of SEBS around the PC domains becomes gradually incomplete by the use of both SEBS-gMA and unfunctionalized SEBS in the PA6 rich blends and at the same time the dispersed SEBS domains in the PA6 matrix enlarge with increasing the ratio of unfunctionalized SEBS to SEBS-gMA. In addition, the SEBS phase encapsulating the PC domains become thicker accompanying with the incompletion of the encapsulation. Although the encapsulation is incomplete, maximum impact strength and maximum strain at break in tensile test can be obtained when certain combinations of SEBS-gMA and SEBS are used. The observation of the domain boundary TEM has revealed that the thicker SEBS phase on the domain boundary contains the micro domain structure of SEBS, where the polystyrene phase forms a cylinder in a hexagonal arrangement in the poly(ethylene-co-butylene) matrix. Through this micro domain structure, SEBS is assumed to perform as a thermoplastic elastomer and toughen the domain boundary between PA6 and PC.

Effect of light irradiation on the characteristics of organic field-effect transistors

The effect of light irradiation on the characteristics of organic field-effect transistors containing sexithiophene (6-T) and pentacene was examined. Organic phototransistors (OPTs) in which 6-T and pentacene were incorporated were fabricated. Their response behaviors were investigated under conditions of irradiation by either modulated or continuous ultraviolet light with various intensities. Both devices showed two distinguishable responses, i.e., fast and slow responses from photoconductive and photovoltaic effects, respectively. The fast response is mainly the result of the generation of mobile carriers by the absorption of a photon energy higher than the band gap energy of the semiconductor and, subsequently, an increase in conductance via a greater flow of photogenerated mobile carriers into the channel layer. On the other hand, the slow response, which was confirmed by a light induced shift in the threshold voltage (Vth) or the switch-on voltage (VO), is the result of a slow release of accumulated ...