Hideki Minoura

Photoconductivity of Ultrathin Zinc Oxide Films

Electrical and photoelectrical properties of nondoped and doped zinc oxide films coated on glass plates by the dip-coating method are investigated at room temperature in various ambient atmospheres. The dark conductivity of the nondoped films exponentially decreased with decreasing film thickness while the conductivity under illumination of 350 nm light was almost constant at 100 Scm-1 irrespective of the film thickness. Consequently thinner films showed larger photoresponse than thicker films. This thickness dependence is explained by the variation of ZnO particle size with the film thickness (fine particle model) and the additional effect of the Schottky barrier generated between the film and gold electrodes.

Cathodic electrodeposition of oxide semiconductor thin films and their application to dye-sensitized solar cells

Cathodic electrodeposition of titanium dioxide (TiO2) and zinc oxide (ZnO) thin films has been studied in the aim of developing cost-effective alternative routes to the photoelectrode materials for dye-sensitized solar cells (DSCs). Preparation of porous anatase TiO2 thin film modified by cis-dithiocyanato bis(4,4V-dicarboxylic acid-2,2V-bipyridine)ruthenium(II) (N3) dye has been achieved by a three-step process: cathodic electrodeposition of a Ti hydroxide thin film from an acidic aqueous solution containing TiOSO4 ,H 2O2 and KNO3, heat treatment of the film at 400 jC and chemical adsorption of dyes from solution. The photocurrent action spectrum measured at the N3-modified TiO2 thin film electrode in contact with I � /I3 � redox electrolyte solution revealed incident photon to current conversion efficiency (IPCE) of 37% in the visible range. While TiO2 needed heat treatment for crystallization, direct electrodeposition of crystalline ZnO was possible from an aqueous solution of Zn(NO3)2. Addition of N3 to the deposition bath made it possible to synthesize porous ZnO/N3 hybrid thin film in one step. IPCE of 24% has been achieved for this film. A sandwich cell using the electrodeposited ZnO/N3 hybrid thin film photoelectrode measured Isc=0.61 mA/cm 2 , Voc=0.46 V, F.F.=0.46 and g=0.13% under illumination by an artificial light source (500-W Xe lamp equipped with a <420-nm and an IR cutoff filters, intensity=100 mW cm � 2 ), being the first example of a real working DSC fabricated without any heat treatment. D 2002 Elsevier Science B.V. All rights reserved.

Electrochemical Self-Assembly of Dye-Modified Zinc Oxide Thin Films

One-step electrochemical self-assembly is an excellent new method for the construction of hybrid inorganic/organic films for dye-sensitized solar cells (DSSCs). The promising oxide semiconductor zinc oxide can be electrodeposited in the presence of organic dye molecules to give porous thin films with varying morphology suitable for DSSCs (the Figure shows a ZnO film grown in the presence of a tetrasulfophthalocyanine).

Cathodic Electrodeposition of ZnO/Eosin Y Hybrid Thin Films from Oxygen-Saturated Aqueous Solution of ZnCl2 and Eosin Y

Electrodeposition of ZnO/eosin Y hybrid thin films from aqueous mixed solution of zinc chloride and eosin Y as promoted by reduction of oxygen has been studied. Highly oriented crystalline hybrid thin films with two distinctive structures have been obtained depending on the redox state of eosin Y. Deposition at potentials more positive than that of eosin Y reduction resulted in a formation of compact ZnO crystals into which eosin Y molecules are entrapped, while that accompanied with the reduction of eosin Y yielded a film consisting of sponge-like ZnO crystals with internal nanoporous structure to which eosin Y molecules are adsorbed. The addition of eosin Y accelerated the film growth both in oxidized and reduced forms due to its catalysis toward the reduction of oxygen. Photoelectrochemical measurement in an I - /I 3 redox electrolyte solution resealed that the hybrid thin film in the latter structure performed as an efficient sensitized photoelectrode because of its porous nature.

Novel thiophene -conjugated indoline dyes for zinc oxide solar cells

The application of a series of thiophene-conjugated indoline dyes for zinc oxide solar cells, prepared by the one-step cathode deposition template method, was examined. The introduction of thiophene ring(s) into D131-type indoline dye improved the cell performance due to their appropriate energy levels and bathochromic shift in the UV-vis absorption band on zinc oxide. It is important for the oxidation potential (Eox) of dyes to have a more positive value than ca. 0.25 V vs. Fc/Fc+ in acetonitrile in order to show a high (>70%) incident photon-to-current efficiency.

Improved photoelectrochemical performance of electrodeposited ZnO/EosinY hybrid thin films by dye re-adsorption

Dye desorption and re-adsorption post treatments on electrochemically self-assembled nanoporous ZnO/eosinY hybrid thin films lead to a large improvement of the dye-sensitized photoelectrochemical performance, achieving an incident photon to current conversion efficiency up to 90%.

Photoelectrochemical sensitisation of ZnO–tetrasulfophthalocyaninatozinc composites prepared by electrochemical self-assembly

Electrodes prepared by a one-step electrochemical synthesis of a dye-loaded ZnO thin film from an aqueous mixture of zinc nitrate and a water soluble phthalocyanine, 2,9,16,23-tetrasulfophthalocyaninatozinc(II) (TSPcZn) have been studied. Homogeneously blue transparent particulate thin films were formed. Monomeric or aggregated TSPcZn species could be obtained dependent on the deposition conditions. Visible light absorbed in TSPcZn led to sensitisation of ZnO. Negative shifts of the rest potential and anodic photocurrents were obtained in contact with an iodide-containing electrolyte in accordance with the n-type conduction of ZnO. Time-resolved measurements indicated charging and discharging of surface states, allowing discussion of the alignment of energy levels in the contact as well as of charge transfer and recombination in the cases of monomeric or aggregated TSPcZn. Electrons were injected into ZnO from the first excited singlet state of TSPcZn. Monomeric TSPcZn showed a considerably larger quantum efficiency in sensitisation of ZnO, whereas a larger rate of charge transfer to the electrolyte was derived for the aggregated form.

Electrophoretically Deposited CdS and CdSe Anodes for Photoelectrochemical Cells

The electrophoretic deposition of CdS, CdSe, and their mixtures has been studied for the preparation of thin film anodes for photoelectrochemical cells. These films were obtained in a homogeneous state on SnO/sub 2//sup -/ coated glass substrates from the suspensions using polar organic dispersion media. Prior to photoelectrochemical experiments, the thin film anodes were thermally treated for 3 hr at 400/sup 0/-600/sup 0/C. An energy conversion efficiency of 2% was observed for the photoelectrochemical cells using these film anodes in S/sup 2 -//S /SUB lx/ /sup 2 -/ electrolytes.

Anodic reactions of several reducing agents on illuminated cadmium sulfide electrode

Anodic reactions of several reducing agents occurring in competition with the electrode dissolution at an illuminated CdS electrode were studied under the potentiostatic condition. S2−, SO2−3, and S2O2−3 ions, which can effectively suppress the photoanodic dissolution of CdS, shift the flatband potential of this electrode to the cathodic direction, leading to the shift of the onset potential for the anodic photocurrent. This shift is attributed to the adsorption of these ions on the electrode surface. Two- or three-step wave appeared in the photoanodic polarization curves in the electrolytes containing low concentration of these ions. The mechanism for these electrodic reactions was discussed in detail in terms of the energy band structure. It can be concluded that the reducing agents which interact so strongly with the CdS electrode surface that the energy band structure at the electrode/electrolyte interface is changed can suppress effectively the photoanodic dissolution of the electrode. The tendency of this interaction was S2− > SO2−3 > S2O2−3.

Photoelectrochemical properties of ZnO/tetrasulfophthalocyanine hybrid thin films prepared by electrochemical self-assembly

Dye-loaded zinc oxide (ZnO) thin films prepared by a one-step electrochemical synthesis from aqueous mixtures of zinc nitrate and water soluble tetrasulfonated metallophthalocyanines (TSPcMt), in which Mt = Zn(II), Al(III)[OH] or Si(IV)[OH]2 , have been studied photoelectrochemically. Visible light absorbed in the TSPcMt led to the sensitization of ZnO. A significantly higher conversion efficiency was found for monomeric as compared to aggregated TSPcMt adsorbed on the ZnO surface. The highest photocurrents were observed for TSPcSi. This could be explained by a strong interaction of the TSPcSi with the ZnO, leading to a highly ordered growth of the ZnO/TSPcSi films with a large fraction of TSPcSi monomers adsorbed to the ZnO surface. The strong adsorption also leads to a more efficient electron injection from the TSPcSi monomers into the conduction band of ZnO as compared to other TSPcMt.