Papers by Author: Shinobu Fujihara

Abstract: t has been attempted to utilize plastic substrates for electrodes, instead of conventional glass substrates, to fabricate flexible, lightweight, and low-cost dye-sensitized solar cells (DSSCs). We examined a pyrolysis method by which preformed layered hydroxide zinc acetate (LHZA: Zn₅(OH)₈(CH₃COO)₂·2H₂O) films were converted into ZnO films at temperatures below 120 oC. Since the pyrolyzed films still contained zinc acetate compounds, they were immersed in hot water aiming at removing such the impurities. This hot water treatment influenced the film morphology to a large extent. The energy conversion efficiency of 3.41% was obtained for the cell using an N719/ZnO electrode fabricated on an indium tin oxide-coated polyethylene naphthalate (ITO-PEN) substrate through the present pyrolysis method.

Abstract: For fabrication of plastic dye-sensitized solar cells, we propose a dissolution and precipitation method to form nanostructured ZnO films on plastic substrates. Layered hydroxide zinc acetate (LHZA) films on plastic substrates were immersed in acetonitrile at 60 oC. They were converted into ZnO films through the dissolution of LHZA and the precipitation of ZnO in acetonitrile. The resultant films contained a large amount of zinc acetate compound impurities, which were attempted to be removed from the ZnO films by dissolving in water. We achieved the energy conversion efficiency of 2.3% for the N719/ZnO electrode on an ITO-PEN substrate.

Abstract: Tungsten trioxide (WO₃) is known as a visible light responsive photocatalyst, but its photocatalytic activity is relatively low, as compared to that of anatase titanium dioxide (TiO₂). To enhance the activity, high specific surface areas are necessary. In this study, WO₃ particles with a hierarchical architecture, which was assemblies of spherical particles 20 – 30 nm in diameter, were synthesized by the solvothermal method. The hierarchical WO₃ particles had high specific surface areas and their photocatalytic activity was found to be 2.5 times higher than that of the commercial WO₃.

Abstract: Nanostructured ZnO films for use in dye-sensitized solar cells were fabricated by utilizing a chemical bath deposition process and a newly developed liquid–liquid process. Layered zinc hydroxides were first deposited on fluorine-doped tin oxide (FTO)-coated substrates and then heated at 450 °C in air to obtain ZnO. Cells were constructed by employing a quasi-solid-state polymer gel electrolyte. The performance of the cells with the ZnO films from the different deposition processes was compared and discussed in terms of the microstructure. Light-to-electricity conversion efficiency reached 2% in all the cells under full sunlight. The cells were kept for 500 h at room temperature, revealing that they maintained 90% of their initial performance.

Abstract: Dye-sensitized solar cells (DSSCs) are one of the promising photovoltaic devices because of their lower manufacturing cost and higher energy conversion efficiency. Wide-gap, porous semiconducting metal oxides are generally used as electrode materials of DSSCs. Previously we utilized ZnO as DSSC electrodes and achieved a high conversion efficiency of 6.58% by improving the structure of ZnO films. However, open-circuit voltage (VOC), one of the factors to determine the performance of DSSCs, was still at a lower level (≈ 0.60 V) than that of common TiO2-based cells. We believe that the lower VOC is due mainly to the occurrence of recombination. In this work, we tried to enhance VOC by controlling the heating process of electrodes and suppressing the recombination for further development of ZnO-based DSSCs. As a result, we have achieved a higher VOC of 0.725 V, confirming that the sintering behavior (grain growth and/or necking of grains) influenced largely the characteristics of DSSCs.

Abstract: Previously, we reported the fabrication of Na0.44MnO2 and LiMn2O4 single crystalline nanowire structure. Moreover, these electrodes showed good high rate property as lithium ion battery, because the nanostructure electrode is suitable for high rate lithium ion battery. Especially, the fabrication of LiMn2O4 single crystalline nanowire was very interesting results because the synthesis of 1-dimesional single crystal structure of LiMn2O4 is very difficult based on cubic crystal structure without anisotropic structure. The LiMn2O4 single crystalline nanowire was obtained thorough the self template method using Na0.44MnO2 nanowire. In this paper, we report the fabrication of Na0.44MnO2 and LiMn2O4 single crystalline nanowire structure and the property of lithium ion battery as review paper.

Abstract: In dye-sensitized solar cells (DSC), large-size particles in photoanodes can act as　light-scattering center to enhance light harvesting efficiency. In this study, macroporous secondary　TiO2 particles were prepared by a spray drying method and were introduced to DSC. A few kinds of　layered structures were fabricated as photoanodes to evaluate effects of macroporous particles. It was　found that open circuit voltage increased due to the presence of macroporous particles. A photoanode　composed of stacked nanoparticles and macroporous particles showed better cell performance.

Abstract: Nanostructured NiO thick films were fabricated for use in dye-sensitized solar cells (DSCs) as photocathodes. Pastes were prepared by using nanocrystalline NiO powders, polyethylene glycol (PEG) and water. The pastes were then printed on FTO glass substrates by a facile doctor-blade printing method. The NiO films were obtained by heating at 500 °C for 30 min in flowing oxygen. It was shown that the nanostructure of the resultant films was largely dependent on the amount of PEG in the pastes. Coumarin 343 (C343) dye was employed for sensitizing p-type NiO photocathodes. DSCs using our NiO/C343 photocathodes yielded a photocurrent density of 1.26 mA cm–2, an open-circuit photovoltage of 88 mV, a fill factor of 0.33, and a solar energy conversion efficiency of 0.037 %.