Bo-Kun Koo

Influence of Electrolyte Composition on the Photovoltaic Performance and Stability of Dye-Sensitized Solar Cells with Multiwalled Carbon Nanotube Catalysts

Efficient dye-sensitized solar cells (DSCs) were realized by using multiwalled carbon nanotubes (CNTs) as the counter-electrode catalyst. The catalytic layers were produced from an aqueous paste of mass-produced raw CNTs with carboxymethylcellulose polymer by low-temperature (70 degrees C) drying. We found that the highly disordered CNTs played the important role of increasing the fill factor of DSCs with electrolytes including large molecules and that the presence of Li(+) as the counter charges for I(3)(-)/I(-) redox couples reduced the chemical stability when using the CNT catalyst. Our experiments showed that by replacing the conventional Pt catalyst and Li(+)-based electrolyte with the proposed CNT catalyst and an electrolyte containing 1-butyl-3-methylimidazolium cations instead of Li(+), the energy conversion efficiency increased from 6.51% to 7.13%. This result suggests that highly defective CNT catalysts prepared by low-temperature drying are viable cost-effective alternatives for DSCs, as long as the electrolytes composition is optimized.

Effects of Non-Noble Metal Current-Collecting Grids on Internal Resistance for Dye-Sensitized Solar Cells

We studied the effectiveness of various non-noble metals used as current-collecting grids on fluorine-doped Sn0 2 (FTO) for dye-sensitized solar cells (DSCs). Non-noble metal grids were deposited by dc magnetron sputtering, and the mechanical adhesion of non-noble metals was improved by a 300 nm thick Cr interlayer between FTO and a grid. The metals were evaluated on the basis of photocurrent-voltage characteristics and impedance spectra. An analysis using an equivalent circuit model showed that such metal grids affect the internal ohmic resistance, which is inversely related to the fill factor. A 1 μm thick NiCr grid affords a fill factor of 0.639, as much as that of Ag, but with a slightly lower efficiency of 6.4%, which supports the feasibility of using stable non-noble metal grids in DSCs, albeit with a 5% reduction in photovoltaic efficiency.

Photonic decomposition of ultrafine, rutile phased TiO2 powder in aqueous 4-chlorophenol solutions

The redox properties of a homogeneously precipitated TiO2 rutile powder with a BET surface area of ∼200 m 2 g −1 ,c onsisting of acicular primary particles, were characterized usin ga photocatalytic reaction in aqueous 4-chlorophenol, Cu- and Pb-EDTA solutions under ultraviolet irradiation. Results were compared to those of commercial P25 TiO2 powder with spherical primary particles as well as those of home-made anatase TiO2 powder with ∼200 m 2 g −1 BET surface area. Here, the anatase powder also includes mainly the primary particles very similar to the acicular shapes of the rutile TiO2 powder. The rutile powder showed the fastest decomposition rate and the largest amount of product in the photoredox, compared with the anatase or P25 powder, while the anatase powder unexpectedly showed the slowes tr at ea nd the smallest amount in the same experiments regardless of having almost the same surface area. The excellent photoredox abilities of this rutile powder appear to be due to a specific powder preparation method, for example a homogeneous precipitation leading to direct crystallization from the solution, regardless of their crystalline structures, even when the powders have similar particle shapes and surface areas. (Some figures in this article are in colour only in the electronic version)

Electrochemical Properties of Carbon Nano-tube as the Counter Electrode of Dye-sensitized solar cell

Studies on porous oxide electrode, dye and electrolyte for dye-sensitized solar cells have been intensively carried out until now. However, counter electrode have not been much studied so far. Accordingly, it is needed to investigate new counter electrode materials with superior catalyst property and to substitute for Pt electrode. In this case, carbon nano-tubes (CNTs) are one of alternatives for counter electrodes as following merits: low resistivity, excellent electron emission property, large surface area and low cost due to development of mass production technique. Such advantages gave us to select multiwalled CNTs (MWCNT) as counter electrode for dye-sensitized solar cell. Also, cyclic voltammetry and impedance spectroscopy were used to investigate electrochemical properties of both CNT electrode and Pt electrode. It was found that sheet resistance of CNT electrode was similar to that of Pt electrode, also, electrochemical properties of CNT electrode was superior to that of Pt electrode on the basis on the measurement of CV and impedance spectrum. It was found that CNT is likely to be a very promising electrode material for dye solar cells.

Characteristics of Nano-crystalline TiO 2 Dye-sensitized Solar Cells having Counter Electrodes with Different Preparing Process

The Pt counter electrode of a dye-sensitized solar cell (DSSC) plays a role in helping redox reaction of iodine ions in electrolyte, also, transferring electrons into electrolyte. In this case, it is expected that characteristics of Pt electrodes strongly depend on fabrication process and its surface condition. In this study, Pt electrodes were prepared by a electro-deposition and a RF magnetron sputtering. Electrochemical behavior of Pt electrodes was compared using cyclic-voltammetry and impedance spectroscopy. Surface morphology of Pt electrodes was investigated by FE-SEM and AFM. I-V characteristics of DSSC were measured and discussed in association with the surface properties of counter electrode. As a result, electrochemical properties of electro-deposited Pt electrode were superior to that of sputtered Pt electrode. This is likely that enlarged area of surface in electro-deposited Pt electrode in comparison with the case of sputtered Pt electrode playa role in enhancing such electrochemical properties.

Electronic state and down-conversion effect of Y-doped ZnO

The electronic state of Y doped ZnO (YZO) was calculated using the density functional theory. In this study, the program used for the calculation on theoretical structures of ZnO and YZO was Vienna Ab-initio Simulation Package (VASP), which is a sort of pseudo potential method. The detail of electronic structure was obtained by the discrete variational Xα (DV-Xα) method, which is a sort of molecular orbital full potential method. The density of state and energy levels of dopant elements was shown and discussed in association with optical properties, especially related to down-conversion effect. The down-conversion effect of YZO was experimentally investigated by preparing thin films deposited on F doped SnO2 (FTO) glass substrates by sol-gel method using the spin-coating system. A homogeneous and stable solution was prepared by dissolving acetates in the solution added diethanolamine as sol-gel stabilizer. In order to confirm a ultraviolet ray interruption and down-conversion effects, the transmission spectrum and the fluorescent spectrum of YZO films were estimated. The results obtained by experiment were compared with the calculated structure.

Comparison of Electrical Properties and AFM Images of DSSCs with Various Sintering Temperature of TiO 2 Electrodes

In order to improve the efficiency of dye-sensitized solar cell (DSSC), electrode screen-printed on transparent conducting oxide (TCO) substrate was sintered in variation with different temperature. electrode on fluorine doped tin oxide (FTO) glass was assembled with Pt counter electrode on FTO glass. I-V properties of DSSCs were measured under solar simulator. Also, effect of sintering temperature on surface morphology of films was investigated to understand correlation between its surface morphology and sintering temperature. Such surface morphology was observed by atomic force microscopy (AFM). Below sintering temperature of , efficiency of DSSCs was relatively lower due to lower open circuit voltage. Oppositely, above sintering temperature of , efficiency of DSSCs was relatively higher due to higher open circuit voltage. In both cases, lower fill factor (FF) was observed. However, at sintering temperature of , both efficiency and fill factor of DSSCs were mutually complementary, enhancing highest fill factor and efficiency. Such results can be explained in comparison of surface morphology with schematic diagram of energy states on the electrode surface. Consequently, it was considered that optimum sintering temperature of a-terpinol included paste is at .