Masayuki Okuya

Thermal decomposition of thiourea complexes of Cu(I), Zn(II), and Sn(II) chlorides as precursors for the spray pyrolysis deposition of sulfide thin films

Abstract Comparative study on thermal decomposition of four thiourea–metal complexes, solid precursors for spray pyrolysis deposition (SPD) methods, had been carried out by using thermoanalytical and X-ray diffraction (XRD) methods in combination. [Cu2(tu)6]Cl2·2H2O (1), ZnCl2(tu)2 (2), Sn2Cl4(tu)5·2H2O (3), and SnCl2(tu) (4) prepared from aqueous thiourea (tu) solutions, had been decomposed in a simultaneous thermogravimetric/differential thermoanalytical (TG/DTA) apparatus, while XRD phase analysis of solid residues obtained at various temperatures had been performed. In static air, a main degradation step between 200°C and 300°C led to the formation of Cu9S5 (digenite), ZnS (zinc blende), and SnS2 (berndtite), respectively. Unfortunately, some chemical species with low volatility still remained in condensed phase, while above 400°C, primary oxidation products of sulfides such as Cu2O(SO4) (dolerophanite), ZnO, and SnO2 also occurred. Nevertheless, intermittent spray pyrolysis depositions of Cu2SnS3 and Cu2ZnSnS4 films from some thiourea solutions with Cu(I), Zn(II), and Sn(II) onto glass substrates, with temperature ranging between 225°C and 300°C, were successful.

Suppression of recombinations in a dye-sensitized photoelectrochemical cell made from a film of tin IV oxide crystallites coated with a thin layer of aluminium oxide

A dye-sensitized photoelectrochemical cell consisting of a film of SnO2 crystallites coated with ultrafine particles of Al2O3 generates an exceptionally high open-circuit voltage as compared to a cell made only from SnO2. Al2O3 coating on SnO2 improves the efficiency and the fill factor while delivering reasonably high photocurrents. Photoexcited dye molecules on Al2O3 injects electrons into the conduction band of SnO2 via tunnelling through the Al2O3 barrier. Suppression of recombinations of electrons with the dye cations and the acceptors at the electrolytic interface build up the quasi-Fermi level in SnO2 with an impressive increase of the open-circuit voltage.

Low temperature deposition of SnO2 thin films as transparent electrodes by spray pyrolysis of tetra-n-butyltin(IV)

Abstract Low temperature deposition of SnO 2 thin films from tetra- n -butyltin(IV) was investigated by a spray pyrolysis deposition (SPD) technique. The substrate temperature was successfully reduced as low as 340°C by adding H 2 O 2 to the source solution. The role of H 2 O 2 is to promote SnO 2 formation not only by inducing an oxygen atmosphere during the pyrolysis of tetra- n -butyltin(IV) but also by producing tin peroxide complexes in the source solution. The average transmittance of the film exceeded 80% in the visible light region, and the electrical resistivity was lowered to 1.5×10 −3 Ω cm, which is attributed to the high crystallinity. These findings lead to the possibility that a new substrate with lower melting point would be applicable to this SPD technique.

Spray pyrolysis deposition of zinc ferrite films from metal nitrates solutions

Abstract Well-crystallized spinel ZnFe2O4 thin films with good adhesion have been successfully prepared on glass substrates heated above 350°C by a spray pyrolysis deposition from mixed zinc and iron nitrates aqueous solutions. The solution pH has an influence on the surface morphology of ZnFe2O4 films. A smooth and dense surface was obtained from the mixed solution with a pH of 2.0, while a rough and inhomogeneous surface could be observed from the solution with a pH of 8.0. The crystallite size of ZnFe2O4 films was found to be small, ranging from 10 to 21 nm as calculated from X-ray diffraction patterns, which is of interest for magnetic material applications. The indirect and direct band gap energies of the sprayed films were estimated to be 1.78 and 2.70 eV, respectively. A schematic illustration is proposed to show the possible deposition processes of ZnFe2O4 thin films by spray pyrolysis.

Efficient dye-sensitized photoelectrochemical cells made from nanocrystalline tin(IV) oxide–zinc oxide composite films

Dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO2 yield energy conversion efficiencies ~10%. The efficiencies of similar cells with films of other oxide materials (SnO2, ZnO) are well below the above value. However, the cells made from SnO2–ZnO composite films give efficiencies comparable to TiO2 cells. Two types of composite systems with SnO2 and ZnO are possible. In the first type, SnO2 crystallites are covered with an ultra-thin (<1 nm) outer shell of ZnO2 and in the second type, the film comprises SnO2 crystallites (~10 nm) with a thin ZnO outer shell and larger ZnO particles (~100 nm). The short-circuit photocurrent and efficiency of these cells are ~17 mA cm−2, 19 mA cm−2 and 7%, 8% respectively. This paper explains in detail how a thin shell of ZnO on SnO2 could effectively counteract recombinations of electrons with acceptors in the electrolyte (e.g., I3−) and increase the efficiency although SnO2 and ZnO are individually not good materials for dye-sensitized photoelectrochemical cells. In the second type, larger ZnO crystallites reduce the rate of geminate recombinations, in addition to the effect of the outer shell.

Sensitization of Nanocrystalline SnO2 Films with Indoline Dyes

Using newly synthesized indoline dyes as sensitizers for photoelectrochemical solar cells based on SnO2 nanocrystalline films, an energy conversion efficiency of 2.8% is achieved, compared with 1.2% for ruthenium bipyridyl dye (N-719) under the same experimental conditions. It is suggested that the formation of nonquenching aggregates and the better passivation of surface states on SnO2 by indoline dyes are instrumental factors in giving a higher efficiency for the cell sensitized with this dye.

Preparation of covellite and digenite thin films by an intermittent spray pyrolysis deposition method

Abstract Intermittent spray pyrolysis of 0.01–0.20 M aqueous solutions of CuCl (dissolved in the presence of 3-fold molar amount of thiourea) onto a heated alkali-free glass substrate in air already at 225–275°C resulted in various thin films of CuS (covellite) and/or Cu1.8S (Cu9S5, digenite). A combination of low pyrolysis temperatures (225–250°C) and low solution concentrations (0.01–0.02 M) has resulted in the formation of thin solid CuS films. At a pyrolysis temperature of 275°C Cu1.8S films have been obtained. The higher concentration range (0.05–0.20 M) has been found favorable for simultaneous formation of Cu1.8S and CuS. In the two-phase layers the crystal growth of CuS has seemed to be limited, whilst the growth of Cu1.8S has been preferentially favored especially on its (0 0 15) lattice plane.

Thermal decomposition of di-n-butyltin(IV) diacetate as a precursor for the spray pyrolysis deposition of oriented SnO2 thin films

Abstract [100]-Oriented SnO2 thin films were obtained on glass substrates from di-n-butyltin(IV) diacetate at 480°C by a spray pyrolysis deposition technique. The thermal decomposition processes of the starting materials were studied by vapor pressure measurement, gas chromatography, and mass spectroscopy for the elucidation of the formation mechanism of the films. Consequently, a growth model concerning the [100]-oriented SnO2 thin films was proposed on the basis of crystallographic considerations, in which the chemical species O–Sn–O appears to be the pyrolytic product of the starting material.

Fluorine doped tin oxide film with high haze and transmittance prepared for dye-sensitized solar cells

Fluorine doped tin oxide (FTO) transparent conductive oxide (TCO) film for dye-sensitized solar cell (DSSC) was investigated. Haze of the incident light through TCO film was easily tuned by controlling the surface morphology of FTO deposited on tin doped indium oxide (ITO) nano-particle seed layer pre-coated on a glass substrate, and the light harvest within the cell was effectively enhanced with high haze TCO film. The conversion efficiency of DSSC fabricated with TCO film with the haze of 30.1% reached as high as 7.7%, attributing to the consequence of the effective light harvest with the scattering within the cell.

Thin Film Formation of the Polyvinylpyrrolidone-Added Europium Tetrakis (Dibenzoylmethide)-Triethylammonium and Its Mechanoluminescent Properties

Mechanoluminescence (ML) is a light emission from materials induced by any mechanical stress. We have synthesized europium tetrakis (dibenzoylmethide)-triethylammonium (EuD4TEA) and investigated its mechanoluminescent properties. The study has revealed that the ML intensity of EuD4TEA is enhanced by an addition of polyvinylpyrrolidone (PVP). Recently we have succeeded to form a thin film of the PVP-added EuD4TEA. The thin films with different amounts of the PVP addition are formed on an Al2O3 buffer layer prepared on Ni substrate. The present study has investigated a relationship between both the ML and the photoluminescence properties of thin films and the addition amount. The results suggest that the properties are strongly related to a crystallinity of the thin films.