Yoshihiro Nakato

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both theoretical and experimental and the applications to solar energy conversion are reviewed, including recent unpublished results.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...

Promoted Photo-oxidation Reactivity of Particulate BiVO4 Photocatalyst Prepared by a Photoassisted Sol-gel Method

Bismuth vanadate (BiVO 4 ) powder with a scheelite-monoclinic (s-m) structure was synthesized by a photoassisted sol-gel method, i.e., a modified sol-gel method including an additional procedure in which a mixed solution of BiCl 3 , VO(OC 3 H 7 ) 3 , and sodium isopropoxide was irradiated with UV light before hydrolysis. The BiVO 4 powder prepared by the photoassisted sol-gel method (called P-sample) exhibited 1.3-3 times higher activity in oxygen photoevolution reaction than that prepared by the conventional nonphotoassisted sol-gel method (called N-sample), indicating that the photoassisted sol-gel method is effective for improving the photocatalytic activity of BiVO 4 . X-ray diffraction patterns and UV-visible absorption spectra were the same between the P- and N-samples, whereas SEM inspection showed that the P-sample was composed of well-separated small-size particles, compared with the N-sample. X-ray photoelectron spectroscopy inspection also exhibited that the P-sample had a thin surface layer containing Bi 5 + and OH - . It is discussed that the high activity of the P-sample is attributed either to the small size of BiVO 4 particles or a certain catalytic action of the surface layer containing Bi 5 + and OH - .

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.

Spectroscopic observation of interface states of ultrathin silicon oxide

Interface states in the Si band gap present at oxide/Si(100) interfaces for ∼3‐nm‐thick Pt/2.1∼3.6‐nm‐thick silicon oxide/n‐Si(100) metal–oxide–semiconductor devices are investigated by measurements of x‐ray photoelectron spectra under biases between the Pt layer and the Si substrate, and their energy distribution is obtained by analyzing the amount of the energy shift of the substrate Si 2p3/2 peak measured as a function of the bias voltage. All the interface states observed using this new technique have discrete energy levels, showing that they are due to defect states. For the oxide layer formed in H2SO4+H2O2, the interface states have three density maxima at ∼0.3, ∼0.5, and ∼0.7 eV above the valence‐band maximum (VBM). For the oxide layer produced in HNO3, two density maxima appear at ∼0.3 and ∼0.7 eV above the VBM. The energy distribution for the oxide layer grown in HCl+H2O2 has one peak at ∼0.5 eV. The 0.5 eV interface state is attributed to the isolated Si dangling bond defect. The 0.3 and 0.7 eV ...

Solvent effects on the fluorescence spectra of some aliphatic amines in solutions

Abstract The fluorescence and phosphorescence of some aliphatic amines have been measured. A remarkable solvent shift of the fluorescence has been found and explained in terms of the large solvation energy for the Rydberg state. The fluorescence is quenched by oxygen, 1,2-dichloroethane and acetonitrile. A new fluorescence band appears with the addition of water, which has been attributed to the hydrogen-bond complex between the amines and water formed in the formers excited state.

OXIDE THICKNESS DEPENDENCE OF ENERGY SHIFTS IN THE SI 2P LEVELS FOR THE SIO2/SI STRUCTURE, AND ITS ELIMINATION BY A PALLADIUM OVERLAYER

The energy difference between the oxide and substrate Si 2p peaks for silicon oxide/Si structures increases with the oxide thickness. The dependence of the energy shift on the oxide thickness almost disappears with the deposition of a thin palladium overlayer, because of the avoidance of the surface charging effect due to photoemission and because of the nearly constant energy shift resulting from extra atomic relaxation. The true chemical shift of silicon oxide layers thicker than 2 nm is determined to be ∼3.8 eV. For the thickness dependence of the oxide Si 2p energy, the extra atomic relaxation and charging effect are dominant for oxide layers thinner than ∼2 nm and thicker than ∼4 nm, respectively. In the intermediate thickness region, both the effects are important.

Photo-oxidation reaction of water on an n-TiO2 electrode. Improvement in efficiency through formation of surface micropores by photo-etching in H2SO4

Abstract The photocurrent-potential curves and the photoluminescence (PL) peak at 840 nm for n-TiO 2 electrodes, in which the n-type semiconductivity is obtained by hydrogen reduction at 550–700°C are investigated in various electrolyte solutions. It is found that a freshly prepared n-TiO 2 electrode shows a weak photocurrent and no PL, but both the photocurrent and the PL intensity increase substantially with time when the electrode is illuminated during cyclic potential scans in 0.05 M H 2 SO 4 . This is accompanied by the formation of a large number of micropores at the electrode surface. No such activation is observed in 0.1 M HClO 4 and 0.5 M Na 2 SO 4 , but an n-TiO 2 electrode activated by illumination in H 2 SO 4 shows an efficient photocurrent and strong PL in these solutions. It is concluded that the increases in the photocurrent and the PL intensity are caused by the production of an active n-TiO 2 surface for the oxygen photoevolution reaction through the formation of surface micropores due to photoetching in H 2 SO 4 .