Researches aiming at high efficiency-dye-sensitized solar cells (DSC) are reviewed from the view point of photoconversion interfaces. Developments of dyes converting photons to electrons in near infrared regions are essential for increasing short circuit current (Jsc). Hybrid cells consisting of double dye structures and double titania layer structures are proposed. Leak currents at photoconversion interfaces decrease open circuit voltage (Voc) and fill factor (FF). Therefore, strategies for decreasing leak currents are essential, especially for all-solid DSCs and I⁻/I₃⁻ free DSCs. Structures of these interfaces are discussed in terms of surface passivation, self-organization, dye structures, interactions between dyes and I₃⁻ in titania nanopores. Fundamental studies for tandem cells are also reported.

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Plasmonic field localization is normally utilized on irregularly roughened surfaces or nanostructured surfaces of coinage metals. Although localized fields increase photon-molecule interactions, metal-molecule interactions become an uncontrollable variable because atomic surface features are not organized in such nanostructured metals. However, construction of sphere-plane nano-gaps on atomically defined metal surface enables us to manage metal-molecular interactions in addition to control of plasmonic resonance features. In SERS spectroscopy using this system, well-managed electromagnetic and chemical contributions provide detailed information on geometric and electronic structures of molecular adsorbates. Moreover, highly localized fields within the nano-gaps extend the range of SERS application to highly damping catalytic metal surfaces. In photo-energy conversion, incident photon to current conversion efficiency in porphyrin-based molecular monolayer system is largely increased by optical antenna effect of the nano-gaps.

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Orientation of anisotropic Ag nanoparticles (NPs) on a polycrystalline TiO₂ film was controlled by photoelectrochemical means on the basis of localized surface plasmon resonance. Simultaneous irradiation of the TiO₂ film in a AgNO₃ solution with UV and polarized far red light resulted in deposition of rod-like Ag NPs oriented perpendicularly to the polarization angle. UV light induces photocatalytic growth of Ag NPs by excitation of TiO₂, and polarized far red light limits the growth in the direction parallel to the polarization angle.

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Inorganic semiconductor multilayer thin films of CdS nanoparticles and titania nanosheet (TNS) were prepared by an electrostatic layer-by-layer accumulation technique. The absorbance of immobilized CdS and TNS linearly increased in proportion to the number of accumulated films, but the photoluminescence of CdS nanoparticles was dramatically quenched by being tucked into the TNS layer. Accumulation of the TNS/CdS layer remarkably enhanced the photocurrent generation compared to the case of films layer-by-layer-deposited with CdS nanoparticles and polystyrene sulfonate of an inert polymer. These observations can be explained by the efficient photoinduced electron transfer from photoexcited CdS nanoparticles to adjacent TNS layers.

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Effects of dye modification on photocatalytic water splitting were investigated using Zr-doped potassium tantalate (KTa(Zr)O₃). Various types of organic dyes, xanthenes, triphenylmethanes, carotenoids and polycyclic aromatic hydrocarbon, were investigated as modification dye. Among them, rhodamine 6G effectively improved the H₂ and O₂ formation rate of photocatalytic water splitting on KTa(Zr)O₃. It was found that the organic dye, which has a lowest unoccupied molecular orbital (LUMO) energy level at -0.9 V vs. NHE, shows a positive effect on the photocatalytic activity, suggesting that the suitable electron orbital level exists for charge transfer from KTaO₃ to organic dye followed by Pt cocatalyst.

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This study reports that polynuclear charge-transfer complexes consisting of Ce³⁺ and a Lindqvist-type polyoxometalate center, either W₆O₁₉²⁻ or Mo₆O₁₉²⁻, function as an all-inorganic visible-light-absorbing chromophore. The polynuclear W₆O₁₉/Ce and Mo₆O₁₉/Ce complexes showed an intense absorption band from the UV region to 520 and 600 nm, respectively. The FTIR and Ce L_III-edge X-ray absorption measurements confirmed that the visible absorption is due to the oxo-bridged W⁶⁺/Ce³⁺ and Mo⁶⁺/Ce³⁺. As polyoxometalates have efficient redox and catalytic properties, their visible-light sensitization could provide a new building block for fabricating inorganic, molecular-based photosynthetic assemblies.

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The peroxo species on ST-01 anatase TiO₂ particles in contact with FeCl₃ aqueous solution during oxygen photoevolution were investigated by in-situ MIR-IR absorption spectroscopy. The peaks assigned to the peroxo species (TiOOH and bridging O-O) were generated by UV-irradiation, though observed vibrational energy of bridging O-O species was largely shifted from that of rutile TiO₂ particles (TIO-3). The observed difference between both particles was mainly attributable to the difference in the local surface structure of TiO₂. The existence of unknown reaction paths to consume bridging O-O species without reacting with photogenerated holes was suggested from the observation of sequential change of absorption intensity.

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Nanoparticles of ZnS-AgInS₂ solid solution (ZAIS) were prepared in 3-amino-1-propanol (APP) solution by thermolysis of (AgIn)_xZn_2(1−x)(S₂CN(C₂H₅)₂)₄ (x = 0.4−1.0). The resulting APP-modified ZAIS nanoparticles of ca. 3 nm in diameter were dissolved in water. The band gap energy was enlarged by an increase in the x value of the precursor used. An intense photoluminescence was observed, the color of which was tunable from green to red by changing the kind of precursor used. The present nanoparticles with x = 1.0 exhibited an optimal quantum yield of ca. 20%, being higher than that of the conventional thiol-modified ZAIS (x = 1.0) in water prepared by a ligand exchange technique.

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We investigated the photocatalytic activity and catalytic oxygen reduction ability by holes in the valence band (VB) of TiO₂ and reduced Cu(II) species, respectively, produced by the interfacial charge transfer from the VB of TiO₂ to grafted Cu(II). Irradiating Cu(II)-grafted TiO₂ photocatalyst with visible-light completely decomposed acetaldehyde gas (1 µmol) into CO₂ (2 µmol). In situ spectroscopic and electrochemical measurements indicated that Cu species of a lower oxidation state were generated in the absence of O₂. Notably, the generated Cu species converted back to Cu(II) when exposed to O₂, clearly indicating the O₂ reduction activity of such Cu species.

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CaFe₂O₄ (p-type) and ZnO (n-type) electrodes were prepared for photoelectrochemical hydrogen production from water. The CaFe₂O₄ electrode showed photo-reduction current in 0.1 M NaOH aqueous solution at potentials more negative than +0.31 V vs. Ag/AgCl, while ZnO showed photo-oxidation at potentials more positive than −0.71 V vs. Ag/AgCl. Connecting the both electrodes under UV-light illumination, the open-circuit voltage between the electrodes was 0.82 V, and hydrogen gas was generated from the side of the CaFe₂O₄ electrode without applying an external voltage. The gas generation continued for 4 days at least. In addition, the hydrogen was generated even under a reverse bias (−0.1 V) vs. CaFe₂O₄ electrode.

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Dye-sensitized solar cells using two types of clay films with high thermal resistance, flexibility and gas barrier properties were prepared. These films have a microstructure of highly oriented layer-by-layer stacking of 1nm thick smectite clay crystal and are new materials as substrates for dye-sensitized solar-cell electrodes. A solar-toelectricity conversion efficiency of 1.8 % (AM 1.5 illumination) was observed with calcination procedure at 450°C.

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The phototoelectrochemical performance of copper indium sulfide (CuInS₂) nanocrystalline film electrodes prepared by a facile wet process was investigated using an electrolyte solution containing methyl viologen (MV²⁺) as a redox mediator. Coating of their surface with a cadmium sulfide (CdS) layer was found to be necessary to induce photocathodic reduction of MV²⁺. Hydrogen evolution from a buffered solution of pH 6.5 containing MV²⁺ and platinum nanoparticles was observed during the photoelectrochemical reaction with ca. 80% current efficiency.

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The deposition of TiO₂ particles on transparent conducting glass of fluorine doped SnO₂ (FTO) film in a solution containing Ti precursors such as Ti oxyhydroxide, titanyl, and peroxo Ti complexe was performed. UV irradiation of the solution led to linking between the TiO₂ particles and adhesion to the FTO film at room temperature. We applied this process for the improvement of dye-sensitized solar cell performance. The dark currents were suppressed by the deposited TiO₂ all over the mesoporous TiO₂ layers and FTO films. The electron life time measurements indicated a decrease in recombination reactions in the electrode-electrolyte interface.

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Dye-sensitized solar cells based on low-viscosity phosphonium ionic liquids having a methoxymethyl group, triethyl(methoxymethyl)phosphonium bis(trifluoromethylsulfonyl)amide and triethyl(methoxymethyl)phosphonium dicyanamide, as their electrolytes have been successfully fabricated. The photo-energy conversion efficiency was optimized up to 5.8 % under AM 1.5 full sunlight illumination.

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Long term optical properties of fluorescent I-III-VI semiconductor nanoparticles and its solid solution dispersed in the methacrylate resin were tested under various conditions aiming at their use as wavelength converters. The ZnS-AgInS₂ solid solution nanoparticles, which we developed recently, have less stability against a thermal and a photonic stimulus, while the AgInS₂-AgGaS₂ solid solution nanoparticles maintain an acceptable fluorescence property even after 1000 h of a thermal stimulus. Differences in the durability between the two types of semiconductors are discussed based on the solid solubility. The deterioration of the fluorescence property is considered to be due to the phase separation of crystals of ZnS-AgInS₂ system.

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We found that anatase TiO₂ nanocrystals were formed in the titania gel by treating with hot-water at 90°C under ambient pressure. The amount of the precipitated anatase nanocrystals increased with hot-water treatment time. Crystalline TiO₂ composite films were successfully prepared from the film composed of TiO₂ (P25) nanoparticles and titania gel by subsequent hot-water treatment at 90°C. TiO₂ nanoparticles were interconnected by the sol-gel derived, partially crystallized titania by the continuing hot-water treatment. A dye-sensitized solar cell (DSSC) using the hot-water treated TiO₂ composite film showed 1.9 mA cm⁻² of short-circuit current density and 0.6 V of open-circuit voltage. The fill factor (ff) and conversion efficiency (η) were 0.63 and 0.71%, respectively.

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The surface of a p-Cu₂O semiconductor photoelectrode was modified by electrodeposition of Pt nanoparticles and analyzed by XRD, SEM, XPS, and EIS (electrochemical impedance spectrometry) methods besides photocurrent measurements. The XRD, SEM, and XPS analyses showed the fabrication of Cu₂O film and the deposition of Pt particles. On the electrodeposition of Pt nanoparticles, cathodic photocurrent was enhanced. The EIS analysis suggested that Pt nanoparticles enhance the charge transfer process to the solution.

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Sonophotocatalysis of malonic acid solution was carried out in an argon atmosphere. The coupling effects of the combination of photocatalysis and sonolysis were observed. The decomposition rate of malonic acid was improved; that is, the reaction rate for a combined system was higher than the sum of the rates of photocatalysis and sonolysis. Hydrogen peroxide formed from solvent (water) during sonolysis plays an important role for the synergistic effect. A small amount of succinic acid was detected by sonophotocatalysis. It is a characteristic product of the combined system and was not observed for each individual irradiation system. In this process, the carbon numbers of dicarboxylic acid increased from C = 3 (malonic acid) to C = 4 (succinic acid), namely increasing molecular weight or alkyl carbon chain elongation. Therefore, sonophotocatalysis has the potential to make a valuable contribution to organic reactions in water.

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Pulse electrodeposition of p-type CuO semiconductor films for the light absorption layer of photovoltaic cells was conducted. Shapes of the potential pulse affected the crystallinity of CuO films. X-ray diffraction (XRD) spectra of all the CuO films among this study showed (002) orientation through the heteroepitaxial growth on the (111)-oriented Au substrates. The CuO layer formed with the most favorable pulse condition showed a sharp diffraction peak assigned to (002) plane of CuO with 0.20° of full width at half maximum (FWHM) value, in contrast to that of the CuO film potentiostatically deposited at 850 mV vs. Ag/AgCl being 0.46°. The FWHM values indicated that the pulse-deposited CuO film had much better crystallinity and contained less residual stress induced by electrodeposition than the potentiostatically deposited CuO. Photocurrent of the pulse-deposited CuO film in NaOH solution was greater than that of the potentiostatically deposited CuO. This enhancement in photoresponse was considered to be due to the high crystallinity of pulse-deposited CuO.

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The novel photocatalysis-plasma hybrid air-purification unit using a titanium-mesh sheet modified with TiO₂ (TMiP^TM) and a surface discharge-induced plasma (SPCP) was investigated. The SPCP unit generates air-plasma on wide area of its surface and TMiP sheet was successfully irradiated by air-plasma without any destruction. Significant decomposition of the ammonia gas has been achieved by the photocatalysis-plasma hybrid reactor. In the present experimental condition, the SPCP unit without TMiP and UV-lamp with TMiP was not effective on the decomposition of the ammonia gas. Therefore, synergistic effects of catalytic activation and plasma excitation on the hybrid reactor with large surface area is useful for significant oxidative decomposition of gaseous pollutants.

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Metal ions doping into ZnO formed different energy states such as shallow, middle, or deep impurity states between the band gap and these states induce the visible-light absorption. M_xZn_1−xO without co-catalyst showed negligible visible-light activity. Surprisingly, Cu²⁺-modification induced visible-light activity over M_xZn_1−xO and they became an active visible-light photocatalyst. Experimental results conclude that visible-light-activities strongly depend on their reduction potentials of dopants' impurity states.

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Photo-excitation of photosystem (PS) I complexes at an SnO₂/electrolyte interface yielded an anodic photocurrent in the presence of methyl or benzyl viologen. In sharp contrast to ordinary spectral sensitization of semiconductor electrodes, the photocurrent appeared at potentials far negative of the flatband potential of SnO₂, the photocurrent formed a bell-shaped curve against the potential, and the peak potential shifted in a Nernstian manner as a function of the viologen concentration. These results suggest an isoenergetic electron transfer from the reduced form of mediator (viologen) bound just beneath the PS I complex surface.

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