Ryohsuke Yamanaka

Kyoto Institute of Technology, Kyoto, Kyoto-fu, Japan

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Publications (15)35.94 Total impact

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    ABSTRACT: AbstractA series of panchromatic ruthenium sensitizers (MJ sensitizers) with attached thiophene and phenyl units bearing alkyl chains was synthesized. A new synthetic route was used to examine all possible positions for the alkyl chains. The absorption spectra showed the sum of a ruthenium complex and peripheral organic chromophore units. The hypochromic effect and blueshift of the metal‐to‐ligand charge‐transfer band observed in the modified ruthenium sensitizers were suppressed by changing the positions of the alkyl chains on the attached thiophene ring. Changing only one alkyl chain also influenced the performance of dye‐sensitized solar cells. Ruthenium sensitizer MJ‐10 with bulky substituent harvests visible and near‐infrared light, and solar cells sensitized by MJ‐10 exhibit an efficiency of 9.1 % under 1 sun irradiation.
    Chemistry 01/2013; 19(3). · 5.83 Impact Factor
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    ABSTRACT: A series of panchromatic ruthenium sensitizers (MJ sensitizers) with attached thiophene and phenyl units bearing alkyl chains was synthesized. A new synthetic route was used to examine all possible positions for the alkyl chains. The absorption spectra showed the sum of a ruthenium complex and peripheral organic chromophore units. The hypochromic effect and blueshift of the metal-to-ligand charge-transfer band observed in the modified ruthenium sensitizers were suppressed by changing the positions of the alkyl chains on the attached thiophene ring. Changing only one alkyl chain also influenced the performance of dye-sensitized solar cells. Ruthenium sensitizer MJ-10 with bulky substituent harvests visible and near-infrared light, and solar cells sensitized by MJ-10 exhibit an efficiency of 9.1 % under 1 sun irradiation.
    Chemistry 11/2012; · 5.93 Impact Factor
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    ABSTRACT: To better understand why titanium tetrachloride (TiCl4) treatment improves short circuit current, we studied its effects on back contact dye-sensitized solar cells sensitized with black dye[tri(thiocyanato)(4,4′,4″-tricarboxy-2,2′:6′,2″-terpyridine)ruthenium(II), Ru(tcterpy)(NCS)3] using transient absorption spectroscopy and electrochemical impedance spectroscopy. We found that the TiCl4 treatment improved short circuit current and achieved an overall energy conversion efficiency of 8.9%. The transient absorption signals did not change as a result of the treatment, suggesting that electron injection efficiency is not affected by the treatment. The impedance related to electron transport between TiO2 particles decreased and the peak frequency of the imaginary part of the electrochemical impedance spectra assigned to electron transfer from TiO2 to the redox couple was shifted to lower frequency by the treatment. This clearly indicates that TiCl4 treatment improved electron transport in the nanocrystalline TiO2 film in back contact dye-sensitized solar cells.
    Energy & Environmental Science 11/2009; 2(11):1205-1209. · 11.65 Impact Factor
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    ABSTRACT: In this paper we investigate a dye-sensitized photovoltaic module (DSM) composed of many rectangular cells connected in series, known as a W-connected type module, where neighboring cells are processed in reverse. The characteristics of the serial connection of silicon solar cells have been investigated extensively, while, with dye-sensitized photovoltaic modules, the influence of each rectangular cell performance on the serially connected module has not been investigated as thoroughly. Here we investigate the performance of the rectangular cell in dye-sensitized photovoltaic modules, and we clarify the correlation between the conversion efficiency of the module and the uniformity of the TiO2 layer thicknesses. With these improvements, we achieved an overall energy conversion efficiency of 8.4% (aperture area: 26.47 cm2) as measured by a public test center under AM-1.5 irradiation. This sets a record for the confirmed efficiency of the DSM.
    Applied Physics Express 07/2009; 2(8):2202. · 2.73 Impact Factor
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    ABSTRACT: We investigated an integrated dye-sensitized solar cell (DSC) module composed of a number of rectangular cells connected in series. Because neighboring cells are processed in reverse, the module is known as the W-contact module. It achieves a high active area of 85% by eliminating the interconnection between neighboring cells. By adjusting the transmittance of the Pt-coated counterelectrode, the TiO2 thickness, and the composition of the electrolyte, we achieved an overall energy conversion efficiency of 8.2% (aperture area of 25.45 cm2) as measured by a public test center under AM 1.5 irradiation. This sets a record for the confirmed efficiency of a DSC submodule.
    Applied Physics Letters 01/2009; 94(1):013305-013305-3. · 3.79 Impact Factor
  • Naoki Koide, Ryohsuke Yamanaka, Hiroyuki Katayama
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    ABSTRACT: An equivalent circuit for DSCs was studied using electrochemical impedance spectroscopy measurement, and the improvements of conversion efficiency of DSCs for not only single cells but also integrated modules were investigated. Further improvement of cell characteristics of DSCs was also investigated from the view point of modified TiO2 films and series-internal resistance design. The series-internal resistance elements were found to correlate positively with the sheet resistance of the transparent conducting oxide and the thickness of the electrolyte layer and negatively with the roughness factor of the platinum counter-electrode. The short circuit current density (Jsc) of the DSCs was effectively improved by use of a high-haze TiO2 film. In addition, the analysis of TiO2/dye interface by scanning probe microscopy and transient absorption spectroscopy were also useful for the study. As a result, the maximum single cell conversion efficiency of over 11% was obtained. Furthermore, an integrated DSC module composed of many rectangular cells connected in series was fabricated and the efficiency was increased to 8.4% (confirmed by AIST) by realizing high active area and high uniformity.
    MRS Proceedings. 12/2008; 1211.
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    ABSTRACT: a b s t r a c t The influence of the TiO 2 /electrode interface was investigated on electron transport properties at the interface and in TiO 2 porous film in back contact dye-sensitized solar cells. Analysis of dye-sensitized solar cells (DSCs) with Ti and TCO indicated that electron transport properties at TiO 2 /Ti and TiO 2 /TCO interfaces are similar despite the former's lack of a 'built-in potential'. The dependence of short circuit current density on TiO 2 thickness indicated that TiO 2 electron transport is not affected by 'built-in potential' or electrode structure. Electron transport thus appears similar in back contact dye-sensitized solar cells and DSCs. A back contact dye-sensitized solar cell fabricated with a Ti electrode and optimum TiO 2 porous film showed a conversion efficiency of 7.8% with a metal mask under an air mass of 1.5 sunlight.
    Solar Energy Materials and Solar Cells - SOLAR ENERG MATER SOLAR CELLS. 12/2008; 93(6).
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    ABSTRACT: The electron transport properties of a back contact dye-sensitized solar cell (BCDSC) were investigated in comparison with a conventional DSC. It was found that the BCDSC had a lower short circuit current density (J<sub> SC </sub>) at the same thickness of TiO <sub>2</sub> film and that J<sub> SC </sub> was not proportional to the thickness of the TiO <sub>2</sub> film. Calculation of electron transport length in the TiO <sub>2</sub> film suggested that the injected electron travels a longer distance to the electrode in the BCDSC than in the DSC. TiCl <sub>4</sub> treatment of the TiO <sub>2</sub> film produced a marked improvement of the J<sub> SC </sub> value in the BCDSC due to an increase in the electron diffusion coefficient of the TiO <sub>2</sub> film, whereas the J<sub> SC </sub> of the DSC remained almost unchanged. It is clear that the value of J<sub> SC </sub> in the BCDSC is more dependent on the electron transport properties of the TiO <sub>2</sub> film than in the DSC. Under standard AM 1.5 irradiation (100 mW   cm <sup>-2</sup>) , a BCDSC with N719 dye yielded an overall conversion efficiency of 8.0%.
    Journal of Applied Physics 10/2008; · 2.21 Impact Factor
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    ABSTRACT: Much attention has been paid to the development of dye-sensitized solar cells (DSCs) owing to a possibility of low production cost. One of the main factors that has hampered the practical application of DSCs is the use of a transparent conducting oxide (TCO) substrate of very high cost. Here we introduce a newly structured DSC back contact dye-sensitized solar cell (BCDSC) in which the TCO is omitted and a porous Ti electrode for electron collection, called a back contact electrode (BCE), is placed on the side opposite to the side of light irradiation. We succeeded in fabricating on the TiO2 film a BCE with large enough pore size for both efficient dye adsorption onto the film and smooth I−/I3− redox couple transportation through the film. Electrochemical impedance spectroscopy study showed that increase in the BCE thickness causes increase in the I−/I3− redox couple transport resistance at the same time as reduction of BCE resistance. Under standard air mass (AM) 1.5 irradiation (100 mW cm−2), a BCDSC with N719 dye yielded an overall conversion efficiency of 8.4%. An intensity modulated photovoltage spectroscopy (IMVS) study indicated that the lifetime of electrons in the TiO2 film of the BCDSC is long enough for them to reach the BCE. The BCDSC therefore has the potential for high photovoltaic performance even though the TiO2 film contains many interfaces between TiO2 particles such as the grain boundary in the polycrystalline silicon. The high-efficiency BCDSC achieved in the present study brings a low-cost DSC one step closer to practical use.
    Chemistry of Materials - CHEM MATER. 07/2008; 20(15).
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    ABSTRACT: The improvement of cell performance of dye-sensitized solar cells (DSCs) was investigated by means of haze of TiO<sub>2</sub> electrodes and series-internal resistance. It was found that the high haze of TiO <sub>2</sub> electrodes effectively improves the external quantum efficiency of DSCs. The series-internal resistance is successfully reduced by increasing the surface roughness of platinum counter electrodes and by decreasing the thickness of electrolyte layer. The highest single cell efficiency of 11.1% (aperture area: 0.219 cm<sup>2 </sup>) was achieved and confirmed by a public test center (AIST). Furthermore, large-scaled cell was fabricated using current collecting metal grids, and the efficiency of 6.8% (aperture area: 101 cm<sup>2 </sup>) was obtained. Moreover, integrated DSC module was investigated and the efficiency of 6.3% (aperture area: 26.50 cm<sup>2</sup>) was also confirmed by the public test center
    Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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    ABSTRACT: The effect of the iodide/triiodide redox electrolyte in various organic solvents on the photoelectrochemical properties of bis(tetrabutylammonium) cis-bis(thiocyanato)bis(4-carboxy-2,2′-bipyridine-4′-carboxylato)ruthenium(II)-sensitized nanocrystalline TiO2 solar cells was studied. Solvents with large donor numbers dramatically enhanced the open-circuit voltage (Voc), but usually reduced the short-circuit photocurrent density (Jsc). For a mixed solvent of tetrahydrofuran (THF) and acetonitrile, Voc increased and the fill factor decreased with increasing THF concentration, but Jsc remained relatively constant. As the partial charge of the N or O atom of the solvent molecule increased, Voc increased, but Jsc was unchanged up to a certain value of the partial charge (for THF, −0.46). For cells using 0.3M 4-tert-butylpyridine and 20vol% THF in the electrolyte, a short-circuit photocurrent density of 18.23mAcm−2, an open-circuit voltage of 0.73V, a fill factor of 0.73, and an overall conversion efficiency of 9.74% were obtained.
    Solar Energy Materials and Solar Cells - SOLAR ENERG MATER SOLAR CELLS. 01/2006; 90(5):649-658.
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    ABSTRACT: With the aim of increasing conversion efficiency, the series-internal resistance of dye-sensitized solar cells (DSCs) was investigated with electrochemical impedance spectroscopy measurement based on an equivalent circuit of DSCs. It was found that series-internal resistance correlates positively with the sheet resistance of the transparent conducting oxide and the thickness of the electrolyte layer and negatively with the roughness factor of the platinum counter electrode. A cell sensitized with a black dye with series-internal resistance of 1.8 Ω cm <sup>2</sup> was fabricated and showed conversion efficiency of 10.2% when measured with a metal mask under an air mass of 1.5 sunlight.
    Applied Physics Letters 06/2005; · 3.79 Impact Factor
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    ABSTRACT: In order to elevate conversion efficiency, the series-internal resistance of dye-sensitized solar cells (DSCs) was investigated with electrochemical impedance spectroscopy measurement based on an equivalent circuit of DSCs. It was found that series-internal resistance correlates negatively with the roughness factor of the platinum counter electrode and positively with the thickness of the electrolyte layer. A cell sensitized with a black dye with series-internal resistance of 1.8 Ωcm<sup>2</sup> was fabricated and showed conversion efficiency of 10.2%, which was confirmed at the National Institute of Advanced Industrial Science and Technology (AIST, Japan).
    Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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    ABSTRACT: Quasi-solid state dye-sensitized solar cells were fabricated using an oligomer having three polymerizable reactive groups. Only 7% polymer concentration in the polymer electrolyte is found enough to form a stable quasi-solid structure and a three-dimensional polymer network structure is proposed. Conductivity measurement of the polymer electrolyte in different organic solvents shows that the ionic conductivity increases with decreasing the viscosity of the solvent and a high ionic conductivity of 9 mS/cm is observed for the polymer electrolyte composition of 0.2 M DMPII, 0.5 M LiI, 0.05 M I2 in the mixed solvent of ethylene carbonate and γ-butyrolactone (30:70 v/v). A short circuit photocurrent density of 14.8 mA/cm2, an open circuit voltage of 0.78 V, a fill factor of 0.70 and an overall conversion efficiency of 8.1% under AM1.5 irradiation (100 mW/cm2) was observed when fabricated a quasi-solid state dye-sensitized solar cells using these high conducting polymer electrolyte. The quasi-solid cells with the polymer electrolyte show higher open circuit voltage than that of the liquid cells that may be due to the suppression of the back electron transfer between the conduction band of the TiO2 electrode and the triiodide ion in the electrolyte.
    Journal of Photochemistry and Photobiology A: Chemistry. 01/2004;
  • R. Komiya, Liyuan Han, R. Yamanaka, T. Mitate
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    ABSTRACT: Quasi-solid state dye-sensitized solar cells with high ion conducting polymer electrolyte of three-dimensional network (9.2 mS cm/sup -1/) were constructed. A solar to electrical energy conversion efficiency of 8.1% was obtained under AM 1.5 irradiation by optimization of the polymer contents, electrolyte composition and solvent in polymer electrolyte.
    Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003