Yasukiyo Ueda

Molecular orientation and anisotropic carrier mobility in poorly soluble polythiophene thin films

Oriented thin films of a poorly soluble polythiophene, poly[benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl(3′,4,4″,4″′-tetradodecyl[2,2′:5′,2″:5″,2″′-quaterthiophene]-5,5″′-diyl)], were fabricated using the friction-transfer technique. The detail of the film structure was investigated using polarized UV-vis spectroscopy, transmission electron microscopy, and grazing-incidence x-ray diffraction observations. The friction-transfer technique enables control of the anisotropic carrier transport according to the face-on and/or edge-on molecular orientations in films with one-directionally aligned polymer backbones.

Pyroelectric Response of Submicron Free-Standing Poly(vinylidene fluoride/trifluoroethylene) Copolymer Thin Films

Submicron-thick free-standing films of ferroelectric poly(vinylidene fluoride/trifluoroethylene) copolymer, P(VDF/TrFE), were fabricated and characterized for application in lead-free pyroelectric infrared sensors. The pyroelectric current and its responsivity were improved by the free-standing structure of the films owing to the reduced thermal capacity and suppression of thermal diffusion in the sensor element. The pyroelectric current for the 500-nm-thick free-standing film was about 20 times higher than that for a conventional on-substrate P(VDF/TrFE) film element, and the thermal time constant for the 500-nm-thick free-standing film was shortened by 1/6 of that for a conventional on-substrate P(VDF/TrFE) film element.

Photovoltaic properties of organic solar cell with octafluorophthalocyanine as electron acceptors

Organic photovoltaic (OPV) cells were fabricated using copper-2,3,9,10,16,17,23,24-octafluorophthalocyanine (F8CuPc) as an electron acceptor. Normal and inverted OPV cells using F8CuPc were showed rectification in dark and photovoltaic characteristics under illumination. The inverted-type cell had good durability. The power conversion efficiency (η) of the inverted-type cell was 0.11%, approximately 6.5 times higher than that of the normal-type cell. The cell using F8CuPc was more stable than that using fullerene in air. These results indicate the possibility of using F8CuPc as an electron acceptor for air-stable OPV cells.

Pillarlike Crystals of Pentacene Prepared from Soluble Precursor

Pillarlike crystals of pentacene were fabricated by heat treatment of 13,6-N-sulfinylacetamidopentacene (SAP) thin films prepared from a soluble precursor. As-spun SAP films were composed of a continuous amorphous layer. SAP molecules were converted to pentacene by an elimination reaction by heat treatment at 110 °C. When the SAP film was heat-treated at 170 °C, the pentacene molecules formed pillarlike crystals of uniform height, wherein they took an edge-on orientation with respect to the substrate.

Enhanced Photovoltaic Performance of Polymer Solar Device by Inserting Nanocrystal Titanium Oxide Interlayer

To improve the power conversion efficiency (PCE) of polymer solar devices, we introduced a highly crystalline nanocrystal titanium oxide layer (nc-TiO2 layer) that was formed by spin-coating a dispersed liquid of precrystallized titanium oxide nanoparticles. The nc-TiO2 layer can be formed more easily and with a higher reproducibility than the TiOx layer formed by a conventional low-temperature sol–gel method. By inserting the nc-TiO2 layer, the PCE was significantly improved from 2.70 to 3.91%. The nc-TiO2 layer enabled a larger decrease in internal resistance than the TiOx layer; thus, the nc-TiO2 layer acts as an effective hole-blocking layer and optical spacer.

Structural and Ferroelectric Characterization of Uniaxially Oriented Vinylidene Fluoride Oligomer Thin Films

A uniaxially oriented film of ferroelectric vinylidene fluoride (VDF) oligomer was fabricated by vacuum deposition on a friction-transferred vinylidene fluoride–trifluoroethylene copolymer; i.e., P(VDF/TrFE), film as a structural template. Fourier transform infrared spectroscopy and scanning electron microscopy measurements revealed that the ultra thin P(VDF/TrFE) film induced the formation of the ferroelectric form I crystal phase and the uniaxial orientation of what to the VDF oligomer by epitaxial growth. The VDF oligomer/P(VDF/TrFE) films exhibit ferroelectric hysteresis loops with a coercive field and a remanent polarization of 150 MV/m and 77 mC/m2, respectively; however, VDF oligomer films without a structural template showed no ferroelectric properties.

Fabrication of One-Dimensionally Oriented Fluorene–Thiophene Copolymer Thin Films and Anisotropic Transistor Characteristics

One-dimensionally oriented poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) thin films with the thickness of approximately 50 nm were deposited by friction-transfer technique. Films were investigated using polarized optical spectroscopy. The dichroic ratio of absorption was estimated to be approximately eight. By means of transmission electron microscope (TEM), we revealed that the main-chains of F8T2 were aligned along the friction direction. Atomic force microscope (AFM) images of as-prepared films showed a roughened surface, whereas the film after subsequent thermal treatment showed remarkably smooth surface. Field-effect transistors utilizing the friction-transferred F8T2 films exhibited a field-effect mobility of 1.88×10-4 cm2 V-1 s-1 along the friction direction, which is about six times higher than that perpendicular to the friction direction.