Masayuki Yahiro

Enhanced figure of merit of a porous thin film of bismuth antimony telluride

A porous thin film of Bi0.4Te3Sb1.6 with an enhanced figure of merit of 1.8 at room temperature was fabricated by flash evaporation on an alumina substrate containing hexagonally arranged nanopores with an average diameter of 20 nm, separated by an average distance of 50 nm. The thermal conductivity was significantly reduced compared with standard Bi0.4Te3Sb1.6 films to 0.25 W/(m⋅K) with no major decrease in either the electrical conductivity (398 S/cm) or the Seebeck coefficient (198 μV/K). The reduction in thermal conductivity was rationalized using a model for the full distribution of the phonon mean free path in the film.

Improvement of electroluminescence performance of organic light-emitting diodes with a liquid-emitting layer by introduction of electrolyte and a hole-blocking layer.

Research on organic light-emitting diodes (OLEDs) containing solid state organic semiconductors has advanced signifi cantly in the past twenty years, resulting in indispensible technology for fl at panel displays [ 1 ] and lighting applications. [ 2 ] This is because OLEDs exhibit not only high-performance light-emitting characteristics [ 3 , 4 ] but also provide unique benefi ts such as light weight and fl exibility, [ 5 , 6 ] which are diffi cult to realize using inorganic LEDs. Taking into account the advantages of organic materials, we developed a unique OLED containing a neat liquid semiconductor, i.e., a liquid OLED that can be used for lighting and display applications. [ 7 ] It is expected that liquid OLEDs will allow the realization of truly fl exible displays because detachment between the liquid emitting layer and electrodes does not occur even when the devices are bent signifi cantly. Furthermore, liquid emitters degraded by long-term use in an OLED can be replaced by a fl ow of fresh organic liquid emitters, removing the problem of OLED degradation resulting from the decomposition of organic materials (see Figure S1 of Supporting Information). Figure 1 demonstrates electroluminescence (EL) from a liquid OLED. Here, a liquid emitter was passed from the top to the bottom of the device by capillary action between two patterned indium tin oxide (ITO) glass substrates separated by a gap of 5.0 ± 0.5 μ m. After injection of the liquid emitter at the top, it penetrates into the gap between the substrates by capillary action and fl ows downward (Figure 1(i)) . The location of the liquid emitter can be detected by photoluminescence as shown in Figure 1(i)–(iv) . EL was not observed before the liquid emitter reaches the area where the two ITO electrodes overlap, as shown in Figure 1(v) . In contrast, intense EL was observed when the liquid emitter reached the area between the electrodes (Figure 1(vi)) . The area exhibiting EL then expanded gradually

Air stable, high performance pentacene thin-film transistor fabricated on SiO2 gate insulator treated with β-phenethyltrichlorosilane

A pentacene thin-film transistor (TFT) was fabricated on a SiO2 gate insulator treated with β-phenethyltrichlorosilane (β-PTS). Employing β-PTS for the surface treatment of SiO2, large grains were present in the initial stage of pentacene crystal growth. The field effect mobility was as high as 1.5cm2∕Vs and the on/off ratio was over 106. The surface treatment dramatically improved the stability in air of the pentacene-TFT’s electrical characteristics. A field effect mobility of over 1cm2∕Vs and on/off ratio of over 105 were maintained after scanning the gate voltage 2000 times in air. This result indicates that the surface treatment with β-PTS not only improved TFT performance but also significantly suppressed the device’s degradation.

IMPROVEMENT OF CURRENT-VOLTAGE CHARACTERISTICS IN ORGANIC LIGHT EMITTING DIODES BY APPLICATION OF REVERSED-BIAS VOLTAGE

The effects of reversed-bias application on current-voltage and luminance-voltage characteristics of standard-type double-layer organic light emitting diodes [OLEDs], ITO/TPD(50 nm)/Alq3(50 nm)/Mg:Ag, were investigated. Both the magnitude of reversed-bias and the duration of reversed-bias application were systematically changed. Evident voltage shifts towards the lower voltage side in current-voltage and luminance-voltage characteristics were observed in the diodes which were treated under various reversed-bias condition. The longer the duration of reversed-bias application, the larger the voltage shift was. A maximum voltage shift of 0.7 V was observed in the diode treated under a -10 V bias for 3 h. Little change in luminance-current density relationships was observed for diodes which were treated under various reversed-bias conditions. The results were interpreted in terms of the movement of ionic impurities and orientational rearrangements of permanent dipoles in organic layers.

Spontaneous and reverse-bias induced recovery behavior in organic electroluminescent diodes

Double-layer organic electroluminescent diodes composed of a spin-coated polyvinylcarvbazole layer and a vacuum-sublimed tris-(8-hydroxyquinoline)aluminum layer were prepared. The diodes were driven at constant voltage and were also kept under short-circuit or reverse-bias conditions. Observations of luminance-current density-voltage relations at constant voltage driving were repeated. The decrease of both luminance and current density during constant voltage driving was observed. Both spontaneous and reverse-bias assisted recoveries of device performances were observed, and these degradation and recovery phenomena were discussed in terms of the movement of ionic impurities in organic layers.

Formation of organic crystalline nanopillar arrays and their application to organic photovoltaic cells.

To enhance the performance of organic photovoltaic (OPV) cells, preparation of organic nanometer-sized pillar arrays is fascinating because a significantly large area of a donor/acceptor heterointerface having continuous conduction path to both anode and cathode electrodes can be realized. In this study, we grew cupper phthalocyanine (CuPc) crystalline nanopillar arrays by conventional thermal gradient sublimation technique using a few-nanometer-sized trigger seeds composed of a CuPc and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) stacked layer. We optimized the pillar density by tuning crystal growth condition in order to apply it to OPV cells.

Estimation of electron traps in carbon-60 field-effect transistors by a thermally stimulated current technique

The authors investigated the influence of O2 and H2O molecules absorbed in carbon-60 (C60) films on their electron trap and n-type field-effect transistor (FET) characteristics. Electron traps in the C60 films were directly measured using a thermally stimulated current (TSC) technique. The TSC results demonstrate that the absorption of O2 and H2O molecules in the C60 films induced an increase in the electron trap concentration, which degrades C60 FET characteristics. By annealing the C60 films at 100°C for 8h, the electron trap concentrations were markedly lowered, enhancing the C60 FET characteristics. An electron mobility of 0.017cm2∕Vs and a current on/off ratio of 106 were observed from the degassed C60 FETs.

Electrical characteristics of single-component ambipolar organic field-effect transistors and effects of air exposure on them

We investigated the electrical characteristics of single-component ambipolar organic field-effect transistors (OFETs) by controlling the device structure and preparation and the measurement conditions. Six organic semiconductor materials (copper-phthalocyanine, tris-(8-hydroxyquinoline)aluminum (Alq3), alpha-sexithiophene, 4-4′-bis-styrylphenyl, 2,7-diphenyl[1]benzothieno[3,2-b]benzothiophene, or a photopolymerized polydiacethylene derivative (PDA) were used as the active layer, and all were found to transport both holes and electrons. The PDA-based FETs had the highest hole and electron mobilities (0.12 and 0.025 cm2/V s, respectively). We also investigated the effect of air exposure on the OFETs. The hole mobility was barely affected by the exposure while the electron mobility was significantly affected. The threshold voltage for p-channel operation was shifted by the exposure while that for n-channel operation was not, indicating that the hole density in the active layer is increased by air exposure wh...

Blue-Light-Emitting Ambipolar Field-Effect Transistors Using an Organic Single Crystal of 1,4-Bis(4-methylstyryl)benzene

An ambipolar light-emitting organic field-effect transistor (LE-OFET) based on a 1,4-Bis(4-methylstyryl)benzene (BSB-Me) single crystal was developed. The BSB-Me single crystal has very high photoluminescence quantum efficiency (ΦPL) of 89±2%, while ΦPL of the BSB-Me vapor-deposited film is limited to a much lower value of 54±2%. Ambipolar operation with successive blue electroluminescence from the FETs based on the BSB-Me single crystals was demonstrated by realizing nearly equal electron and hole mobilities [about 0.005 cm2/(V s)] with asymmetric gold–calcium contacts. Since BSB-Me single crystals can perform light amplification, the BSB-Me-based ambipolar LE-OFET is a promising candidate for future electrically driven organic blue-emitting solid-state lasers.

Ambipolar light-emitting organic field-effect transistors using a wide-band-gap blue-emitting small molecule

The authors applied a wide-band-gap (2.9eV) molecule of 4,-4′-bis(styryl)biphenyl (BSBP) as an active layer in light-emitting organic field-effect transistors. They found that BSBP provided both relatively high field-effect hole mobility of 0.01cm2∕Vs and photoluminescence efficiency of 20% in thin film. They achieved ambipolar operation by without breaking vacuum through devices’ preparation and measurements, applying aluminum contacts, and inserting a hydroxyl-free poly(methylmethacrylate) layer, and light emission was observed when the device was operated in the ambipolar mode. The results presented here will open the way to fabricating efficient light-emitting transistors with high mobility.