Shohei Sakaguchi

Probing TiO2/Dye Interface in Dye Sensitized Solar Cells Using Surface Potential Measurement

Surface potential measurement using scanning Kelvin-probe microscopy was conducted to investigate the relationship between dye characteristics and open circuit voltage (Voc). It was observed that Voc increased with the increase in surface potential of dye as compared to that of bare TiO2 giving increased efficiency of dye sensitized solar cells. Addition of chenodeoxycholic acid (DCA) as co-adsorbent caused positive shift of surface potential, which explained the increase of Voc upon DCA incorporation. It was further explained by the negative shift of TiO2 conduction band edge in the presence of DCA.

Additives for Increased Photoenergy Conversion Efficiencies of Quasi-Solid, Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are solidified with gelators containing polyvinylpyridine and 1,2,4,5-tetra(bromomethyl)benzene. The photoconversion efficiencies are improved by new additives. Lil and t-butylpyridine are commonly added in electrolytes for increasing short-circuit current (Jsc) and open-circuit voltage (Voc). These additives inhibit our gel electrolyte precursors from solidifying. We found that new additives, combinations of acetic acid, and methylpyrimidine or methylbenzimidazole, do not inhibit the solidification and are effective for increasing both Jsc and Voc. These mechanisms are discussed in terms of electron diffusion coefficients, 13 ion diffusion coefficients, and charge-transfer resistances between counter electrodes and gel electrolytes.

Gait recognition using spectral features of foot motion

Gait as a motion-based biometric has the merit of being non-contact and unobtrusive. In this paper, we proposed a gait recognition approach using spectral features of horizontal and vertical movement of ankles in a normal walk. Gait recognition experiments using the spectral features in term of the magnitude, phase and phase-weighted magnitude show that both magnitude and phase spectra are effective gait signatures, but magnitude spectra are slightly superior. We also proposed the use of geometrical mean based spectral features for gait recognition. Experimental results with 9 subjects show encouraging results in the same-day test, while the effect of time covariate is confirmed in the cross-month test.

Dye Sensitized Solar Cells with High Photo-Energy Conversion-Controll of Nano-Particle Surfaces

Some of factors affecting photo-conversion efficiency of dye sensitized solar cells (DSCs) are discussed in terms of TiO2 electrodes. The first topic is on the surface modification of TiO2 nano-particles, which is associated with electron traps on the surface of TiO2 nano-particles. The surface is modified with dye molecules under pressurized CO2 atmosphere to increase the surface coverage of TiO2 nano-particles with dye molecules. This increases Jsc because of an increase in the amount of dye molecules and a decrease in the amount of trapping sites on TiO2 nano-particles. In addition, the decrease in the amount of trap sites increases Voc because decreases in Voc are brought about by the recombination of I2 molecules with electrons trapped on the TiO2 surfaces. Selective staining for tandem cells is proposed. The second topic is on the contact between a SnO2/F transparent conductive layer (TCL) and nano-particles. Polishing the TCL surfaces with silica nano-particles increases the contact, resulting in Jsc increases

Quasi-solid dye-sensitized solar cells: control of TiO2-gel electrolyte interfaces

Quasi-solid dye sensitized solar cells (Q-DSSC) were fabricated by employing gel electrolytes containing ionic liquids and gelators. Sufficient physical contacts between nano-crystalline TiO2 particles and gel electrolytes in nano-porous TiO2 layers were achieved by solidifying gel electrolyte precursors after the cells are filled with the electrolytes. Photo-currents increased largely by embedding carboxylic acids among dye molecules on TiO2 crystals. The nano-porous TiO2 electrolytes were fabricated by dipping the dye anchored TiO2 substrates in dilute solutions of carboxylic acids. It was found that resistances in the TiO2 layers decreased by these treatments.

Dye-sensitized solar cells with high efficiency: from interface control to solidification

Some items to increase photovoltaic performances of dye sensitized solar cells (DSC) are reported. Focus is put on the fabrication of the electron collection path and ionic path. In order to increase the surface coverage of TiO2 nano-particles with dye molecules, the dye adsorption on TiO2 layers was carried out under a pressurized CO2 atmosphere (CO2 process). The CO2 process promoted the dye adsorption and shortened the dye adsorption time to 1/10 - 1/100. In addition, solar cells prepared by the CO2 process had higher Voc and Jsc than those prepared by the conventional dipping process. The increase in the photovoltaic performance was explained by the large electron diffusion coefficient in TiO2 layers and by longer electron life time in TiO2 layers. Thermally stimulated current measurement (TSC) implied that the surface electron trap on the TiO2 nano-particle was passivated by the sufficient dye adsorptions on the TiO2 surfaces. In addition, it was found that the dye aggregation was prevented by the CO2 process, which increased the photovoltaic performances. Hybrid dye sensitized solar cells having two-dye-layer-structures were fabricated by the CO2 process for the first time in order to absorb the light having wide range of wavelength. In addition, three-dimensional W electrodes were fabricated on the thick TiO2 layer in order to collect electrons in the TiO2 layer effectively. Solid type DSCs are reported. High performance quasi-solid DSCs were fabricated by preparing ionic paths in the quasi-solid electrolyte layers. The ionic path was fabricated by the surface modification of the straight nano-pore walls in a porous Al2O3 membrane, where, I-/I3 - ion species were concentrated and were expected to diffuse by Grötthuss mechanism.