Porous titanium oxide thin film md photoelectric convertor using the film

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention comprises a titanium oxide thin film and a manufacturing method for obtaining high photoelectric conversion efficiency, a metal oxide semiconductor electrode using the titanium oxide thin film, a dye adsorbed on the surface thereof, an electrolyte having a redox pair, and a counter electrode. It relates to a photoelectric conversion element composed of.

【0002】

[Conventional technology]

There are several types of solar cells, but most of them are diode-type solar cells that utilize the junction of silicon semiconductors. Currently, these solar cells are expensive to manufacture, which is a factor that hinders their widespread use. Dye-sensitized wet solar cells have long been studied due to their low cost potential, but recently Graetzel et al. Announced ones with performance comparable to silicon solar cells (J. Am. Chem. Soc. 115). (1993) 6382) Therefore, expectations for practical use are increasing. The basic structure of a dye-sensitized wet solar cell consists of a metal oxide semiconductor electrode, a dye adsorbed on the surface of the metal oxide semiconductor electrode, an electrolyte having a redox pair, and a counter electrode. Graetzel et al. Titanium oxide (TiO₂) Etc. were made porous to increase the surface area, and the ruthenium complex was adsorbed as a single molecule as a dye to significantly improve the photoelectric conversion efficiency.

【0003】

Since then, some proposals have been made to further improve the characteristics. For example, in Japanese Patent Application Laid-Open No. 9-237641, niobium oxide (Nb) is used as a metal oxide semiconductor.₂O₅) Is used to increase the open circuit voltage. Further, in JP-A-8-81222, TiO₂It is said that by etching the surface of the electrode film, lattice defects and impurities are removed and the conversion efficiency is improved.

【0004】

In order to improve the photoelectric conversion efficiency, it is particularly important to make the metal oxide semiconductor electrode porous to increase the surface area. Conventionally, as a method of forming a porous metal oxide thin film, a surfactant or the like is added to a sol obtained by dispersing ultrafine particles of the metal oxide in water or an organic solvent or hydrolyzing a metal alkoxide. There are known methods such as preparing a coating solution, applying it to a substrate, and heating and drying it. However, in order to improve the electrical continuity of these sintered thin films, it is preferable that the sintering temperature is high. However, for example, when the titanium oxide sol is heat-sintered, as reported by Kavan et al. (J. Electrochem. Soc., 143 (1996) 394), the pore diameter may become smaller as the sintering temperature increases. It has been clarified, and it has been difficult to find firing conditions that achieve both porosity and electrical conduction.

【0005】

Further, in Patent No. 2636158, as a method for forming a porous titanium oxide thin film, a coating liquid in which a mixture of titanium oxide sol and an organic substance such as polyethylene glycol is applied onto a substrate and baked at 600 ° C to 700 ° C. We disclose a method for obtaining a porous titanium oxide thin film. However, the invention of Japanese Patent No. 2636158 has a problem that the selection range of the substrate is limited due to the high heating temperature.

【0006】

On the other hand, as another method for forming a titanium oxide thin film, Japanese Patent Application Laid-Open No. 10-53437 discloses a method of heating titanium peroxide sol to 250 ° C. or higher. However, the titanium oxide thin film formed by the method of JP-A-10-53437 is not porous and is not effective for the application of the photoelectric conversion element of the present invention.

【0007】

[Problems to be Solved by the Invention]

An object of the present invention is to solve such a problem of the prior art and to provide a porous titanium oxide thin film having a large specific surface area (the surface area of titanium oxide with respect to the substrate area of the thin film forming portion). Another object of the present invention is to provide a photoelectric conversion element having high photoelectric conversion efficiency using the above-mentioned porous titanium oxide thin film.

【0008】

[Means for solving problems]

As a result of diligent studies to solve the above problems, the present inventor applies a coating liquid containing at least crystalline titanium oxide and amorphous titanium peroxide sol as components to the substrate, and heat-sinters the substrate to achieve photoelectric conversion efficiency. We have found that an excellent porous titanium oxide thin film can be obtained, and have completed the present invention.

【0009】

That is, according to the present invention, there is provided a porous titanium oxide thin film formed by applying a coating liquid containing at least crystalline titanium oxide and an amorphous titanium peroxide sol as components to a substrate and heat sintering. Further, according to the present invention, there is provided a porous titanium oxide thin film characterized in that the crystal form of crystalline titanium oxide is anatase in the above configuration. Further, according to the present invention, there is provided a method for producing a porous titanium oxide thin film, which comprises applying a coating liquid containing at least crystalline titanium oxide and an amorphous titanium peroxide sol as components to a substrate and heat-sintering the substrate. Further, according to the present invention, there is provided a method for producing a porous titanium oxide thin film, characterized in that the crystal form of crystalline titanium oxide is anatase in the above method. Further, according to the present invention, there is provided a method for producing a porous titanium oxide thin film, characterized in that the heating temperature is 250 ° C to 500 ° C in the above method. Further, according to the present invention, there is provided a titanium oxide semiconductor electrode characterized in that a porous titanium oxide thin film having the above structure is formed on a conductive substrate. Further, according to the present invention, in a photoelectric conversion element composed of a metal oxide semiconductor electrode, a dye adsorbed on the surface thereof, an electrolyte having a redox pair, and a counter electrode, the titanium oxide semiconductor electrode is used as the metal oxide semiconductor electrode. Provided is a photoelectric conversion element characterized by having as a component.

【0010】

The porous titanium oxide thin film in the present invention can be obtained, for example, as follows. Water or an organic solvent is added to the crystalline titanium oxide powder or sol and the amorphous titanium peroxide sol, mixed well and uniformly dispersed to prepare a coating liquid. It is obtained by applying this coating liquid to a substrate, drying it, and then heating and firing it at 250 ° C to 500 ° C. The amount of crystalline titanium oxide powder or sol used in the coating liquid is 5 to 40% by weight, preferably 10 to 25% by weight. The amount of the amorphous titanium peroxide sol used in the coating liquid is 5 to 100%, preferably 20 to 80% of the total solid content. Further, as the organic solvent, for example, alcohols such as ethanol, methanol and isopropyl alcohol can be used.

【0011】

The titanium oxide thin film thus obtained is sintered in a porous state in which individual titanium oxide particles are partially in contact with each other, and the surface area of titanium oxide is extremely large with respect to the substrate area of the portion where the titanium oxide thin film is formed. It becomes a large porous thin film. In addition, by using anatase-type crystalline titanium oxide and heating and firing at 250 ° C to 500 ° C, the crystal form of the obtained titanium oxide thin film becomes anatase-type, and the porous titanium oxide thin film with high photocatalytic activity becomes. It becomes.

【0012】

The reason why the porous titanium oxide thin film can be obtained by the present invention is that the titanium peroxide sol changes to anatase-type titanium oxide at a temperature lower than the sintering temperature when the porous titanium oxide is formed only with crystalline titanium oxide. There is. When sintering titanium oxide to form a porous titanium oxide thin film, the temperature must be at least 400 ° C, and even higher temperatures are required to improve the contact between particles and improve electrical conduction. You will need it. However, as the temperature rises, the pore diameter becomes smaller and the porosity is lost. Therefore, it is difficult to select heating and firing conditions that satisfy both electrical conduction and porosity.

【0013】

However, in the case of the porous titanium oxide thin film in the present invention, the amorphous titanium peroxide is changed to anataze type titanium oxide by heating at 250 ° C. or higher. At the same time, it forms a state of good contact with the adjacent crystalline titanium oxide. At this time, since the crystalline titanium oxide has hardly changed, the pores are not blocked.

【0014】

When heated at 250 ° C or lower, amorphous titanium peroxide does not change to anatase-type titanium oxide, so photocatalytic activity cannot be obtained. Further, when heated at 500 ° C. or higher, a part of the anatase-type titanium oxide is changed to rutile-type titanium oxide, so that the photocatalytic activity is remarkably lowered, and it is not suitable as a semiconductor electrode for a photoelectric conversion element.

【0015】

In order to control the size and distribution of the pores of the porous titanium oxide thin film, either the ratio of crystalline titanium oxide to amorphous titanium oxide is controlled at the time of preparing the coating liquid, or the particle size of crystalline titanium oxide is controlled. Or you can do both.

【0016】

The particle size of the crystalline titanium oxide used in the present invention is preferably 1 to 100 nm, particularly preferably 10 to 30 nm. The crystalline form of crystalline titanium oxide is preferably anatase type from the viewpoint of photocatalytic activity. The anatase-type titanium oxide may be a commercially available powder, sol, or slurry, or may have a desired particle size by a known method such as hydrolyzing titanium oxide alkoxide. When using a commercially available powder, it is preferable to eliminate the secondary agglutination of the particles, and it is preferable to pulverize the particles using a mortar, a ball mill, or the like at the time of preparing the coating liquid. At this time, acetylacetone, an acid, or an alkali can be added in order to prevent the particles from which the secondary agglutination has been released from agglutinating again.

【0017】

Ceramics, glass, heat-resistant plastics, etc. that can withstand the heating and firing temperature can be applied to the substrate. Especially when manufacturing semiconductor electrodes, metal, ITO or SnO₂Transparent electrodes such as, etc. can be applied, [0018]

In order to apply a coating liquid composed of at least crystalline titanium oxide and amorphous titanium peroxide sol to the substrate, known methods such as wire bar, blade, dipping, and spray coating can be used.

【0019】

A surfactant can be added to the coating liquid in order to improve the film-forming property on the substrate. Further, the viscosity of the coating liquid can be controlled by adding glycols such as ethylene glycol or a water-soluble polymer.

【0020】

By forming such a porous titanium oxide thin film, a good photocatalyst can be formed, and the choice of the substrate is wide. Furthermore, when this porous titanium oxide thin film is applied to the electrodes of a photoelectric conversion element such as a dye-sensitized solar cell, since there are many dye adsorption sites of titanium oxide, more light can be absorbed, so that photoelectric conversion A highly efficient photoelectric conversion element can be obtained.

【0021】

[Example]

Hereinafter, the present invention will be specifically described with reference to Examples, but the embodiments of the present invention are not limited to these Examples.

【0022】

Embodiments of the present invention will be described below with reference to FIG. 1 is a substrate such as glass, 2 is ITO, SnO₂A transparent conductive film consisting of: F, ZnO: Al, etc., 3 is a porous titanium oxide thin film, 4 is a dye such as ruthenium bipyricyl, zinc porphyrin, copper phthalocyanine, chlorophyll, rose bengal, eosin, etc., 5 is I^-/ I₃^-, Br^-/ Br₃^-Electrolyte having a redox pair such as, 6 is a counter electrode made of Pt or the like. Light enters from the top of the figure.

【0023】

Next, an example of the method for manufacturing the solar cell will be described. First, for example, SnO is applied onto the glass substrate 1 by a sputtering method, a CVD method, a sol-gel method, or the like.₂: Prepare two sheets with F film 2 formed, SnO₂: Since the F film functions as a current collector, it is desirable that the sheet resistance is 50 Ω / or less, preferably 10 Ω / or less. After forming the above-mentioned porous titanium oxide thin film 3 on one of these, a sensitizing dye, for example, ruthenium bipyridi 5 is adsorbed. The film thickness of the titanium oxide thin film is preferably about 1 to 50 nm. In order to adsorb the dye on the titanium oxide thin film, the titanium oxide semiconductor electrode may be immersed in a solution in which the dye is dissolved in a solvent such as water, alcohol or toluene. It is preferable to have a functional group such as a carboxyl group, a hydroxyl group, or a sulfone group in the element molecule because the dye is chemically fixed on the surface of titanium oxide. A typical example is [Ruthenium (4,4'-dicarboxy-2,2'-bipyridine).₂(Isothiocyanato)₂] There is a ruthenium complex represented by. The other SnO mentioned above₂A Pt (fine particle) layer 6 is formed on the F film by a sputtering method, a vapor deposition method, an electrochemical method, or the like. The film thickness is preferably about 1 to 50 nm. After superimposing the pair of substrates formed as described above via the spacer, for example, I^-/ I₃^-The electrolyte solution 5 having a redox pair is injected and sealed with a sealant. As the electrolyte solution, a solution obtained by adding iodine and tetrapropylammonium iodide to a mixed solvent of ethylene carbonate and acetonitrile can be preferably used. The cell formed in this way has, for example, CeO as a member that absorbs ultraviolet rays.₂A lead glass containing the above (a commercially available sharp cut filter such as L-40 or L-42 may be used) may be attached to the incident side of the light.

【0024】

Example 1 (Preparation of Porous Titanium Oxide Semiconductor Electrode) Amorphous titanium peroxide sol is produced by the following method. 10 ml of a 16% aqueous solution of titanium tetrachloride (Wako Pure Chemical Industries, Ltd.) diluted 20-fold with ion-exchanged water, adjusted to pH 6.5 with an aqueous solution of ammonium hydroxide, and cooled to 5 ° C to 36% hydrogen peroxide solution. 6 ml was added, and the mixture was stirred for 12 hours to obtain a titanium peroxide sol. To 3 g of anatase-type titanium oxide powder (Ishihara Techno), 10 ml of the above titanium peroxide sol and 0.2 ml of acetylacetone were added and mixed in a mortar so as to disaggregate the titanium oxide powder to prepare a coating solution. SnO this coating liquid on the glass substrate 1 by the sol-gel method.₂: F film 2 is applied on a transparent electrode formed so that the sheet resistance is 10Ω / , air-dried for 30 minutes, then heated and fired at 450 ° C for 30 minutes, and a titanium oxide semiconductor electrode with a film thickness of about 10 μm. Got When this thin film was observed with a scanning electron microscope, it was in a porous state in which titanium oxide having a particle size of about 20 nm was partially connected.

【0025】

Example 2 (Preparation of photoelectric conversion element) The porous titanium oxide semiconductor electrode of Example 1 was replaced with [ruthenium (4,4'-dicarboxy-2,2'-bipyridine).₂(Isothiocyanato)₂] Is immersed in an ethanol solution of the ruthenium complex, refluxed for 10 minutes, and TiO.₂The ruthenium complex was adsorbed on the surface of the electrode. On the other hand, a Pt film was deposited at a film thickness of 20 nm on an ITO glass substrate having a sheet resistance of 10 Ω / by a vacuum deposition method. Both of these substrates are superposed with a gap of about 10 μm via a bead or rod-shaped insulating spacer, and a redox electrolyte solution in which iodine and tetrapropylammonium iodide are added to a mixed solvent of ethylene carbonate and acetonitrile is prepared. After injection, it was sealed with an epoxy adhesive. Under pseudo sunlight irradiation of this photoelectric conversion element (AM1.5, 100mW / cm²The photoelectric conversion efficiency in) was 7.8%.

【0026】

[Effect of the invention]

According to the inventions of claims 1 and 3, a thin film can be formed without closing the pores by heat-sintering the coating film containing at least crystalline titanium oxide and amorphous titanium peroxide sol, so that the porous material has a large specific surface area. A titanium oxide thin film can be obtained. According to the inventions of claims 2 and 4, by using the anatase-type crystalline titanium oxide, the obtained porous titanium oxide thin film becomes an anatase-type titanium oxide thin film, so that a thin film having high photocatalytic activity can be obtained. According to the invention of claim 5, since the heating temperature is 250 ° C to 500 ° C, the heating temperature is lower than that of the conventional heat sintering, so that the applicable substrate options are expanded. According to the invention of claim 6, the semiconductor electrode on which the porous titanium oxide thin film having a large specific surface area is formed has a large electrode reaction site and a large electrode reaction amount. According to the invention of claim 7, since the photoelectric conversion element has a porous titanium oxide semiconductor electrode having a large specific surface area, the amount of light absorbed by the dye adsorbed on titanium oxide increases, so that the amount of generated charge increases. Photoelectric conversion efficiency is improved.

[Simple explanation of drawings]

[Figure 1]

It is sectional drawing which shows typically an example of the photoelectric conversion element by this invention.

[Explanation of symbols]

1 board 2 Transparent conductive film 3 Metal oxide semiconductor electrode 4 pigment Electrolyte with 5 redox pairs 6 Counter electrode