Tokiyoshi Umeda

Surface-energy-dependent field-effect mobilities up to 1 cm2/V s for polymer thin-film transistor

The field-effect mobility of a liquid-crystalline semiconducting polymer, poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (PB16TTT), has depended significantly on the surface energies of self-assembled monolayers (SAMs) formed on insulating layers. Using a SAM with fluoroalkyl groups, with a low surface energy of 13.3 mN/m, the mobility of PB16TTT reached as high as 1 cm2/V s. These results indicate that an edge-on orientation of the polymer chains progresses more favorably on the surfaces with low surface energies via the liquid-crystalline phase.

Reducing the contact resistance of bottom-contact pentacene thin-film transistors by employing a MoOx carrier injection layer

We report on the reduced contact resistance in bottom-contact (BC) pentacene thin-film transistors (TFTs) with a molybdenum oxide (MoOx) carrier injection layer. MoOx layers were placed between the gate insulator and the source-drain (S-D) electrodes instead of the conventional adhesive layer such as Cr or Ti. The performance of the BC pentacene-TFT with the MoOx injection layer was significantly improved at low operating voltages. The contact resistance of the MoOx∕Au S-D electrodes, estimated using the gated-transmission line method, was nearly two orders of magnitude smaller than that of conventional Cr∕Au electrodes at the gate voltage of −10V. The highest performance was obtained with a MoOx injection layer a few nanometers thick, which was comparable to the effective channel thickness of the pentacene-TFT on the gate insulator. This result indicated the importance of the direct connection between the MoOx injection layer and the effective channel to reduce the contact resistance.

Significant improvement of electron mobility in organic thin-film transistors based on thiazolothiazole derivative by employing self-assembled monolayer

n-type organic thin-film transistors based on a thiazolothiazole derivative were fabricated on a SiO2 gate insulator treated with n-alkyl self-assembled monolayers (SAMs), which were composed of various alkyl chain lengths. The field-effect electron mobility increased depending on the alkyl chain length of the SAMs. A long alkyl chain significantly improved the on current and electron mobility. The highest electron mobility of 1.2cm2∕Vs and on/off ratio of 107 were achieved with an alkyl chain longer than that of tetradecyl-trichlorosilane. This result is attributed to the suppression of the influence of electron trap sites on the SiO2 gate insulator by employing the SAM with the long alkyl chain.

Influence of H2O and O2 on threshold voltage shift in organic thin-film transistors: Deprotonation of SiOH on SiO2 gate-insulator surface

The influence of H2O and O2 on the transistor characteristics in p- and n-type organic thin-film transistors (OTFTs) fabricated on the SiO2 gate insulator was investigated. In both p- and n-type OTFTs, the threshold voltage (Vth) shifted drastically to positive direction after exposure to ambient air and dry air, although the field-effect mobilities in saturation regime were almost unchanged before and after the Vth shift. The Vth shifts to the positive direction indicate that negative charges are generated on the SiO2 gate-insulator surface by exposure to ambient air and dry air. The influence of SiO− on the gate-insulator surface and deprotonation processes of SiOH caused by H2O and O2 were discussed as the origin of the significantly positive Vth shift.

High-mobility and air-stable organic thin-film transistors with highly ordered semiconducting polymer films

We report on high crystalline thin films of liquid-crystalline polythiophene derivative, poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (PB16TTT) that exhibit terrace structures and molecular steps of its polymer chains by annealing in its liquid-crystalline phase. The crystallinity of the PB16TTT polymer films formed on SiO2 gate insulating layers with smooth self-assembled monolayer was improved by changing the octyltrichlorosilane treatment time for the SiO2, which led to reproducible high field-effect mobilities of the polymer thin-film transistors up to 0.44cm2∕Vs. High stability of the transistor for repeated stressing in ambient air was also demonstrated.

Effect of ZnO layer on characteristics of conducting polymer/C60 photovoltaic cell

Photovoltaic properties of a photovoltaic cell with structure indium–tin oxide (ITO)/ZnO/C60/poly(3-hexylthiophene) (PAT6)/Au have been investigated. The C60/PAT6 heterojunction of this cell was fabricated by spin-coating a chloroform solution of PAT6 onto the C60 thin film formed on ZnO-coated ITO. Fabricated by this method, the photovoltaic cell has demonstrated a high efficiency for solar cells based on organic polymers, i.e. a monochromic external quantum efficiency of over 70% at the peak wavelength and a power conversion efficiency of 1.0% under AM1.5 spectral illumination of 100 mW cm−2.

Interpenetrating Interface in Organic Photovoltaic Cells with Heterojunction of Poly(3-hexylthiophene) and C60

The optical and photovoltaic properties of a photovoltaic cell with a structure of indium-tin-oxide (ITO)/C60/poly(3-hexylthiophene) (PAT6)/Au have been investigated. The C60/PAT6 heterojunction of this cell was fabricated by spin-coating the chloroform solution of PAT6 onto the C60 thin film formed on ITO. The photovoltaic cell fabricated by this method has demonstrated the high efficiency for a solar cell based on organic polymers. From the optical properties and the direct observation of the C60/PAT6 heterojunction, it has been clarified that the interpenetrating interface of PAT6 and C60 contributes to the high photovoltaic performance.

Improvement of Characteristics of Organic Photovoltaic Devices Composed of Conducting Polymer-Fullerene Systems by Introduction of ZnO Layer

Photovoltaic cells, which consist of a heterojunction of conducting polymer-fullerene systems and a zinc oxide (ZnO) thin film, have been studied. In this structure, an incident photon is used effectively because exciton generation and dissociation, which contribute to photocurrent, mainly occur on the light incident side in a conducting polymer layer. In the layered photovoltaic cells with the structure of indium-tin-oxide (ITO) / ZnO / fullerene / conducting polymer / Au, a monochromatic energy conversion efficiency of 5.2% under 0.6 mW/cm2 illumination at 500 nm and an external quantum efficiency reaching 74% at the peak wavelength have been realized, which can be explained by the interpenetrated fullerene / conducting polymer interface and the short distance between the electron-generation region and electrode enhancing electron collection to the electrode.

Enhancement of Photoresponse by Enlarging the Effective Interface between Conducting Polymer and Titanium Oxide in Photovoltaic Device

The morphology of titanium oxide films, prepared via two different methods, oxidation following the evaporation of titanium and spin-coating a suspension of TiO2 nanoparticles, was investigated by scanning electron micrograph (SEM) and X-ray diffraction (XRD). SEM images indicated that the surface of the TiO2 film is rough and the pore size is large, compared with those of the TiOx film, which results from the connected network of the TiO2 film. Pore filling is likely to be easier in the TiO2 film than in the TiOx film due to its larger pore sizes. Poly(3-hexylthiophene) (PAT6) chains can be interrupted by the TiO2 network structure, as evidenced in UV-Vis spectrum. XRD revealed better crystallinity in the TiO2 film than in the TiOx film. The investigation of the photoluminescence (PL) spectra revealed that PL of PAT6 is quenched when it was filled into the network pores. We prepared the photovoltaic devices using PAT6 as both sensitizer and hole conductor and titanium oxide and PV films as the electron conductor, to reveal the effect of the morphology of the titanium oxide films on the photovoltaic performance. An improvement of conversion efficiency by over twofold in the TiO2 cells compared with that of the TiOx cells was observed. These phenomena suggest that the photovoltaic performances of PAT-TiO2 (or TiOx)/PV cell were dependent on the interfacial morphology between both PAT6 and TiOx and PAT6 and PV.

Dependences of Characteristics of Polymer Solar Cells Based on Bulk Heterojunction of Poly(3-hexylthiophene) and C60 on Composite Ratio and Annealing Temperature

Polymer solar cells based on bulk heterojunction of poly(3-hexylthiophene) (PAT6) and C60 with high efficiencies have been demonstrated. The efficiencies were markedly dependent on composite ratios and annealing temperatures, and energy conversion efficiencies of 2.2% under white light and 6.5% under monochromatic light were obtained in a cell with a PAT6:C60 composite ratio of 1:0.5 annealed at 100 °C. Furthermore, the absorption of PAT6:C60 composite films in a wavelength range related to the absorption of PAT6 depended on the C60 composite ratios, and was changed significantly by annealing. These changes are discussed taking into consideration the hindrance of the crystallization of PAT6 by C60 and the phase segregation between PAT6 and C60 through the aggregation and crystallization of each molecule caused by annealing.