Hiroshi Tsubomura

Dye sensitised zinc oxide: aqueous electrolyte: platinum photocell

TITANIUM oxide photocells, which are photochemically stable and decompose water into hydrogen and oxygen1,2, have a low efficiency3 and their spectral response is mostly in the ultraviolet, where the Suns output is low. Other semiconductors such as cadmium sulphide or gallium phosphide, having smaller band gaps, are destroyed by light and their photocurrents deteriorate rather rapidly4. Photocurrents in dye sensitised photocells composed of aqueous electrolyte solutions and various semiconductor electrodes have also been studied5–7. All results obtained so far show that the dye sensitised photo-currents are very small, ∼ 10−6–10−9 A cm−2, corresponding to a quantum efficiency ≲ 10−2. In this paper we report the successful construction of cells with a high power efficiency.

A study on a palladium-titanium oxide Schottky diode as a detector for gaseous components

Abstract It has been found that the current through a Schottky barrier formed at the interface between palladium film and n-type titanium oxide (TiO 2 ) single crystal is sensitive to hydrogen or other reducing gases in the ambient. This effect is explained by taking account of the diminished barrier height at the Pd and TiO 2 interface caused by the action of gases, changing the work function of Pd metal. The change in the Pd work function estimated from the result is confirmed by the direct measurements of the metal surface potentials by use of a vibrating capacitor method. Similar electrical properties have been studied for junctions of TiO 2 with Pt, Au, Ni, Al, Cu, Mg and Zn, and of ZnO, CdS, GaP and Si w ith Pd.

Mechanism of carrier transport in highly efficient solar cells having indium tin oxide/Si junctions

The carrier transport mechanism of the Si solar cells having n‐Si/indium tin oxide (ITO) junctions has been studied by use of the current‐voltage and capacitance‐voltage measurements and x‐ray photoelectron spectroscopy. An 11‐A‐thick nonstoichiometric Si oxide layer is formed when ITO is deposited by spray pyrolysis on a Si electrode etched with hydrofluoric acid. In this case, the tunneling probability of majority carriers through the oxide layer is high, and the thermionic emission current over the energy barrier in Si takes a dominant part of the dark current. On the other hand, for a Si electrode where a Si oxide layer is intentionally interposed between ITO and Si, the thermionic emission current is suppressed, and trap‐assisted multistep tunneling through the depletion layer becomes dominant. By making a mat‐structure treatment on the Si surface, a solar energy conversion efficiency of 13% and the photocurrent density of 42.5 mA cm−2 were attained under AM 1 100 mW cm−2 illumination.

Silicon photoelectrodes modified with ultrafine metal islands

Abstract Photoelectrochemical (PEC) cells, equipped with an n-Si electrode modified with ultrafine platinum islands, generate very high photovoltages of 0.66 to 0.68 V. This shows that electrodes of this type form a junction of an ideal nature, and opens a new approach to developing high-efficiency and low-cost solar cells. Our studies on the properties of such electrodes, both theoretical and experimental and the applications to solar energy conversion are reviewed, including recent unpublished results. It is also shown that the principle of the PEC cell can be extended to solid-state cells having a new junction. Some interesting properties of semiconductor electrodes reported by other workers are discussed in relation to the present theoretical model.

Properties of indium tin oxide films prepared by the electron beam evaporation method in relation to characteristics of indium tin oxide/silicon oxide/silicon junction solar cells

Indium tin oxide (ITO)/silicon oxide/silicon (Si) junction solar cells were produced by depositing ITO on a thin silicon oxide‐covered single‐crystal Si substrate using the electron‐beam evaporation method. The current‐voltage (I‐V) characteristics strongly depended on the incident angle (θi) of the evaporated ITO vapor to the Si substrate during the ITO deposition, as well as the post‐deposition heating temperature (Th) and the kind of the ambient gases during post‐deposition heat treatment. The ITO films deposited at θi=0° and treated at Th=380 °C in air formed a high‐energy barrier with p‐Si, and formed ohmic contact with n‐Si. X‐ray diffraction analysis showed that the ITO films deposited at θi=0° contained metal indium. The amount of the metal indium decreased either by reducing the deposition rate of the ITO film or by raising the substrate temperature during the ITO deposition. The ITO films deposited at θi=45° and treated at Th=350∼450 °C in hydrogen, on the other hand, formed a high‐energy barrie...

Potentiometric investigations of antigen-antibody and enzyme-enzyme inhibitor reactions using chemically modified metal electrodes.

The electrical potential between an immunoreactive electrode and a reference electrode in a buffer solution was studied. The immunoelectrode was made of titanium wire, on which an antigen or an antibody was chemically fixed. The electric potential of the electrode sensitized with anti-hCG gamma-globulin shifted in the positive direction in the presence of a small amount of hCG in the solution. On the other hand, the potential of the hCG-sensitized electrode ran in the negative direction upon addition of anti-hCG to the buffer solution. Similar changes in potential were observed between trypsin and its inhibitor, aprotinin. Kinetic analysis was made for the reactions between these species and the change in potential was explained by a simple charge transfer model.

Hydrogen photoevolution at p-type silicon electrodes coated with discontinuous metal layers

Abstract Hydrogen photoevolution at p-type silicon (p-Si) electrodes coated with platinum or gold layers by various methods has been studied in acid solutions. A p-Si electrode coated uniformly with a Pt layer by vacuum evaporation showed a current-potential curve similar to that for a Pt metal electrode, indicating that the Pt/p-Si contact is nearly ohmic. However, after alkali etching this electrode showed good rectifying character in the dark as well as an efficient photocurrent, indicating that the effective barrier height at the Pt/p-Si contact is much increased. Similar results were obtained for Au-coated p-Si electrodes. When Pt was deposited photoelectrochemically on p-Si, it showed an efficient photocurrent, contrary to the electrodes treated by vacuum deposition. These results are explained well by our recently proposed theory on the mechanism of the generation of photovoltages at semiconductor electrodes coated with discontinuous metal layers.

Organic amino compounds with very low ionization potentials

Abstract The gas-phase ionization potentials of some organic amino compounds have been measured by the photo-ionization method. The values obtained lie in the range 5 to 6 eV, probably the lowest of the ionization potentials of organic molecules reported so far.

Photoelectrolysis of Water under Visible Light with Doped SrTiO3 Electrodes

Etudes du comportement photoelectrochimique des electrodes de SrTiO 3 dopees avec divers oxydes metalliques (RuO 2 , CoO, Ce 2 O 3 , Cr 2 O 3 , etc.), en solution aqueuse

Structures and functions of thin metal layers on semiconductor electrodes

Abstract Metal-coated semiconductor electrodes such as Au/n-TiO 2 and Au/n-GaP show photovoltaic effect that cannot be explained by the conventional potential barrier model for metal—semiconductor contact. From experimental and theoretical investigations it has been concluded that the microscopically discontinuous structure of the metal layer is responsible for the anomalous photovoltaic effect. Several theoretical conclusions which are interesting from the point of view of solar energy conversion are derived. (1) Metal-coated semiconductor electrodes in electrolyte solutions generate high photovoltages at the metal—semiconductor interface in cases where the metal layer forms islands approximately 5 nm in diameter and separated by more than 20 nm from each other provided that the potential of the electrode is controlled so as to give band bending in the metal-free part of the surface. The maximum photovoltage can increase up to the equivalent of the band gap of the semiconductor in ideal cases. (2) The metal—semiconductor contact becomes ohmic when the potential of the electrode approaches the flat-band potential for the bare electrode in cases where the metal layer either has cracks or forms islands with gaps wider than 20 nm, say. Changes in the barrier height at the metal—semiconductor interface are not assumed in the theory, which can be applied to any metal—semiconductor pair. The conclusions provide a thoeretical basis for the explanation of the mechanisms of interesting properties of metal-coated semiconductor photoelectrodes and photocatalysts, such as enhanced hydrogen photoevolution on platinum-coated p-type semiconductor electrodes or on platinum-coated semiconductor particles in solution.