JP2003301283A - Method for manufacturing thin film of porous metal- oxide semiconductor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To develop a method for forming a thin film of a porous metal-oxide semiconductor on a substrate, which has pore sizes of a nano-scale, a controlled fine structure, a large specific surface area, and has a uniform and desired thickness even with a large area. <P>SOLUTION: This method for manufacturing the thin film of the porous metal-oxide semiconductor like a replica of alternately adsorbed films with a nano-pore-size structure on the substrate, comprises forming the alternately adsorbed films having the porous nano-pore-size structure consisting of composite organic substances on the substrate, restructuring it to a porous structure having macropores by pure water treatment, forming metal-oxide particles on the alternately adsorbed films having the porous structure and in the pore structure penetrating to the surface of the substrate with a chemical deposition method from a solution, and then burning down the alternately adsorbed films. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a porous oxide semiconductor thin film, particularly a porous oxide semiconductor thin film suitable for a photocatalyst, an oxide semiconductor dye-bonding electrode having an organic dye bound thereto, a gas sensor and the like.

[0002]

2. Description of the Related Art Porous metal oxide semiconductor materials include oxides of transition metals such as Ti, Nb, Zn, Sn, Zr, Y, La and Ta, and perovskite oxides such as SrTiO ₃ and CaTiO _3. Things are known.

As methods for forming these porous metal oxide semiconductor layers, squeegee method, screen printing method, spin coating method, sputtering method, sol-gel method,
Or a method of applying fine particle slurry and sintering the particles,
A chemical solution deposition method or the like is used.

The porous metal oxide semiconductor thin film has been used as a gas sensor for a long time, but recently, a titanium oxide photocatalyst has attracted attention and a titanium oxide porous thin film photocatalyst has been developed. A wet solar cell including a metal oxide semiconductor electrode sensitized with an organic dye (referred to as an organic dye-sensitized oxide semiconductor electrode) is also known.

In this wet type solar cell, a porous titanium dioxide film is formed on a transparent conductive film, a Ru dipyridyl complex is adsorbed on this surface as a sensitizing dye, and iodine is used as an electron mediator. Wet solar cells are known as typical ones (Graztel et.al. Nature, 3
53, (1991) p737).

In such a dye-sensitized solar cell, a sol in which particles of a metal oxide such as titanium oxide are dispersed on the surface of a transparent glass substrate coated with tin oxide (FTO) to which fluorine is added is doctor blade. Method, etc., 500
A typical method is to form an electrode by firing at a temperature of about ° C.

Others, for example, Japanese Patent Laid-Open No. 2000-3190.
Japanese Unexamined Patent Publication No. 18 discloses a photoelectric conversion element in which a coating solution containing crystalline titanium oxide and amorphous titanium peroxide sol as components is applied to a substrate and a porous titanium oxide thin film is formed by heating and sintering. Japanese Unexamined Patent Publication No. 2001-283944 discloses an organic dye-sensitized oxide semiconductor electrode in which a porous metal oxide semiconductor layer is formed by a method of sintering particles. Further, in JP-A-2001-325998, a sol in which particles are dispersed is coated on the surface of a base material to form a particle layer, and then a metal fluoride solution or a fluoride complex solution is brought into contact with a particle film. A method of forming a metal oxide film having a fine structure on the surface of particles is disclosed.

[0008]

Among the conventional methods of forming a porous metal oxide semiconductor layer, the squeegee method,
With the screen printing method and the spin coating method, it is difficult to uniformly coat a large area. Since the sputtering method requires a vacuum device, it has drawbacks such as difficulty in increasing the area and high manufacturing cost. Further, the method of directly producing an electrode by the sol-gel method has a problem in that it is made porous. Furthermore, when applying a fine particle slurry, it is difficult to increase the surface area. The chemical solution deposition method is a wet process and is a method capable of uniformly forming a thin film on a large area.
However, it is difficult to satisfy the film thickness condition of about 10 μm, which is suitable for the Gratzel cell.

Furthermore, since the porous metal oxide semiconductor layer is composed of a semiconductor material such as titanium oxide,
There is a problem that the conductivity is insufficient, and if the electrode area increases, the internal resistance increases and it becomes difficult to extract a large current. In particular, in the case of an organic dye-sensitized oxide semiconductor electrode, even if electrons are rapidly injected from the excited dye into the metal oxide semiconductor layer, the metal oxide semiconductor layer hinders the movement of electrons, resulting in a transparent conductive film. There is a problem that it acts as an internal resistance until reaching.

In view of the above problems, a porous metal oxide semiconductor thin film having a pore size of nanoscale and a controlled fine structure and a large specific surface area can be formed on a substrate to solve the above-mentioned problems evenly over a large area. There is a demand for the development of a method that can be used for manufacturing.

[0011]

Means for Solving the Problems The present inventors have solved the above problems by utilizing a method for forming an alternating adsorption film having a porous nanopore diameter structure composed of a composite organic substance, and thus solving the above problems. It has been found that can be manufactured.

That is, according to the present invention, an alternating adsorption film having a porous nanopore diameter structure composed of a composite organic substance is formed on a substrate,
After reorganization into a porous structure having macropores by pure water treatment, metal oxide particles are formed by a chemical solution deposition method in the pore structure penetrating on the alternate adsorption film of the porous structure and to the substrate surface. A porous metal oxide semiconductor thin film, which is formed and then burned off to form a replica-like porous metal oxide semiconductor thin film of the alternate adsorption film having a nanopore diameter structure on the substrate. Is a manufacturing method.

Further, according to the present invention, an alternating adsorption film having a porous nanopore diameter structure composed of a composite organic substance is formed on a transparent conductive substrate, and after reorganization into a porous structure having macropores by pure water treatment, Nanoparticles are formed on the substrate by forming metal oxide particles on the alternating adsorption film having the porous structure and in the pore structure penetrating to the surface of the substrate by the chemical solution deposition method, and then burning the alternating adsorption film. A method for producing an oxide semiconductor dye-bonded electrode, which comprises forming a replica-like porous metal oxide semiconductor thin film of an alternate adsorption film having a pore size structure.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have previously proposed that the nano-scale pore size and microstructure-controlled alternate adsorption (Laye).
We have developed a manufacturing method for r-by-Layer Electrostatic Self-Assembly film (Applied Physics, Vol. 69, No. 5, 553-5).
57, 2000, JP 2001-62286 A). According to this method, an aqueous solution of a positive electrolyte polymer (cation) and an aqueous solution of a negative electrolyte polymer (anion) are prepared in separate containers, and these containers are provided with a substrate (deposition film) to which an initial surface charge is applied. By alternately dipping the film material), a composite organic ultrathin film (alternating adsorption film) having a multilayer structure can be obtained on the substrate.

For example, when a glass substrate is used as the film-forming material, the surface of the glass substrate is subjected to hydrophilic treatment to introduce an OH ⁻ group to the surface to give a negative charge as an initial surface charge. Then, when this substrate whose surface is negatively charged is dipped in a positive electrolyte polymer aqueous solution, the positive electrolyte polymer is adsorbed on the surface by Coulomb force until at least the surface charge is neutralized, and a single layer of ultrathin film is formed. It is formed. The surface portion of the ultrathin film thus formed is positively charged. Therefore, this time, if this substrate is dipped in the negative electrolyte polymer aqueous solution, the negative electrolyte polymer is adsorbed by the Coulomb force, and one layer of ultrathin film is formed.

In this manner, by alternately immersing the substrate in the aqueous solution in the two containers, the ultrathin film layer made of the positive electrolyte polymer and the ultrathin film layer made of the negative electrolyte polymer can be alternately formed. As a positive charge, PPV precursor, polypyrrole / polyaniline (doped state), PA
It is possible to form a composite organic thin film having a multilayer structure in which H, etc., such as PAA, TPA, SPS, etc. are combined as negative charges. By this method, for example, an organic light-emitting element with a combination of PPV / PAA and a chemical filter with a combination of PAH / PPA can be produced.

FIG. 1 is a conceptual diagram showing the principle of manufacturing a general alternating adsorption film. Here, a basic principle of preparing a substrate 10 such as glass or silicon as a film forming material and forming an alternate adsorption film on the surface thereof will be described. First, the first
An aqueous solution of a positive electrolyte polymer (cation) is prepared in the container 20, and an aqueous solution of a negative electrolyte polymer (anion) is prepared in the second container 30.

Then, the surface of the substrate 10 to be the film forming material is subjected to hydrophilic treatment to introduce an OH ⁻ group into the surface to prepare a substrate having a negative charge as an initial surface charge. The circle in FIG. 1 (a) is a conceptual diagram showing the state where the surface of the substrate 10 is negatively charged in this way. Subsequently, when the negatively charged substrate 10 is put into the first container 20, the positive electrolyte polymer comes into contact with the surface of the substrate 10 and is adsorbed by the Coulomb force. The circle in FIG. 1 (b) is a conceptual diagram showing a state in which a positive electrolyte polymer is adsorbed.

When the substrate 10 is placed in the second container 30, the negative electrolyte polymer comes into contact with the surface of the substrate 10 and is adsorbed by the Coulomb force.
The circle in FIG. 1 (c) is a conceptual diagram showing a state in which the negative electrolyte polymer is adsorbed.

In this way, if the substrate 10 is alternately immersed in the first container 20 and the second container 30, the substrate 10
On the surface of, the layers made of positive electrolyte polymer and the layers made of negative electrolyte polymer are alternately deposited and accumulated, and finally an alternate adsorption film having a multilayer structure is formed. To be done.

In the method of the present invention, first, the alternate adsorption film is formed on the substrate by using the above-described alternate adsorption film forming method. In the case of an organic dye-sensitized oxide semiconductor electrode, I
A glass substrate having a transparent conductive film such as TO and FTO (SnO ₂ : F) is used.

Next, the alternating adsorption film thus formed is subjected to a dilute hydrochloric acid immersion treatment to change the structure of the film to make it porous. The pH of dilute hydrochloric acid is preferably 2.4 to 2.6, and 2.5.
Is the best. Dilute nitric acid and dilute acetic acid do not make it porous.

After that, when it is further immersed in pure water, the film thickness, pore diameter, surface structure, refractive index, etc. are changed to form a porous membrane, and a syringe-like macropore having a pore diameter of about 50 to 400 nm is formed. Are reorganized into a porous structure having. The dissociation constant is closely related to this structural change. PAA is protonated by dilute hydrochloric acid to form a bond (COO ^-- N
H ₃ ⁺ ) is broken to make it porous. afterwards,
It is considered that when it is immersed in pure water (pH = 5 to 6), it is deprotonated again, recombined and structurally changed again. The temperature may be room temperature, and the immersion time may be about 20 seconds or less. It is important that the nanopores penetrate to the surface of the substrate in order to bond the oxide film and the substrate, and it is desirable that the pores exist at a high density.

Next, the substrate on which the alternating adsorption film reorganized into a porous structure is formed is immersed in a chemical deposition solution. Using a chemical deposition solution, TiO ₂ , SiO ₂ , SnO ₂ , V ₂
O _{5 and the} like can be deposited. When TiO ₂ is deposited, it is left standing in a TiF ₄ precursor solution adjusted to pH 1.8 to 3.1 (<pH 2.2 for obtaining a TiO ₂ thin film having good crystallinity) with NH ₄ OH for 3 to 24 hours. Place. Pure water was used as a solvent, TiF _{4 was} 0.04 M, and NH ₄ O was used.
A few drops of H can be added to form a film up to about 2 μm at a temperature of 60 ° C.

After removing the substrate from the chemical deposition solution, it is dried. FIG. 2 shows a state (a) in which metal oxide particles are formed on the alternate adsorption film and in the pore structure thereof and a state (b) in which the alternate adsorption film is burned down to form a porous metal oxide thin film. It is a schematic diagram. According to the chemical solution deposition method,
The solution sufficiently penetrates into the pores that penetrate to the surface of the substrate of the alternate adsorption film 11 having a porous structure, and metal oxide particles are formed inside the pores. As shown in FIG. Metal oxide particles 12 that have penetrated into the surface of the substrate 10 from above the film of the adsorbed film 11 and within the pore structure thereof and joined to the surface of the substrate
A layered structure of is formed. Next, the metal oxide particles are fired within a temperature range in which the alternate adsorption film 11 is burned out.

Thereby, as conceptually shown in FIG. 2B, the alternate adsorption film 11 is burned down to form the pores 13, and the metal oxide is used as a replica of the alternate adsorption film 11 having the porous structure. The thin film 14 is formed and a high surface area structure can be constructed. This makes it possible to produce a metal oxide thin film having a desired film thickness uniformly in a large area as a replica using nano-scale fine pores and an alternating adsorption film having a controlled fine structure as a template. The metal oxide thin film obtained by this method is
When it is used as an organic dye-sensitized oxide semiconductor electrode, it has a large specific surface area and electrically acts like a parallel connection, and the internal resistance does not increase.

[0027]

Example 1 As a polycation of an electrolyte polymer, polyallylamine hydrochloride (PAH, Aldrich, Mw =
70,000), polyacrylic acid as a polyanion (PAA, Poly Science, Mw = 90,0)
00) was used. PAA pH 3.5, PAH pH
The solution concentration was adjusted to 7.5 and the solution concentration was 10 ^-2 M on a transparent glass substrate coated with tin oxide (FTO) to which fluorine was added.
Layer accumulated. A cross-sectional TEM image of the obtained alternate adsorption film is shown in FIG.
Shown in.

This alternate adsorption film was diluted with dilute hydrochloric acid having a pH of 2.5.
Soaked for a minute. FIG. 4 shows a TEM image of a cross-sectional structure after the dilute hydrochloric acid immersion treatment. It can be seen from FIG. 4 that the structure of the film has changed. Next, it was immersed in pure water for 20 seconds. FIG. 5 shows the TE of the cross-sectional structure of the alternate adsorption film after being immersed in pure water.
The M image is shown. From FIG. 5, it can be seen that the particles are reorganized into a porous structure having syringe-like macropores having a diameter of 50 to 400 nm.

On the thin film thus prepared, TiF
₄ TiO ₂ thin films were accumulated from the precursor solution. As the TiF ₄ precursor solution, 0.248 g of TiF ₄ was dissolved in 50 ml of pure water, stirred for 1 hour, and immersed in a solution adjusted to pH 2.2 with NH ₄ OH at 60 ° C. for 10 hours to precipitate TiO ₂ particles. . FIG. 6 shows an SEM image of the multilayer structure of the surface on which the TiO ₂ thin film is formed. Next, the substrate was taken out of the solution, and heated at 650 ° C. to be baked to burn off the polymer of the alternate adsorption film. FIG. 7 shows an SEM image of the surface structure in which the alternate adsorption film was burned down.

As shown in FIGS. 6 and 7, on the surface of the substrate, a replica-like porous TiO ₂ thin film tracing the porous structure of the alternate adsorption film was prepared. It was also confirmed that the structure of TiO ₂ was maintained before and after firing. FIG. 8 shows SEM images (a) and (b) of a cylindrical structure and SEM images (c) and (d) of a layered structure inside the thin film obtained by observing the TiO ₂ thin film in more detail. Figure 8
As shown in (a) and FIG. 8 (b), a characteristic columnar structure was confirmed inside the TiO ₂ thin film. Also, FIG.
As shown in (c) and FIG. 8 (d), a layered structure was confirmed.

The substrate on which the porous TiO ₂ thin film was formed by the above method was immersed in the Ru dye for 12 hours. Then, after washing with ethanol, the electrolytic solution was dropped and sandwiched between Pt plates to prepare a dye-sensitized solar cell, and current-voltage (IV) characteristics and open-circuit voltage (Voc) due to light irradiation were measured. . FIG. 9 shows current-voltage characteristics in comparison with the measured values of Comparative Example 1 described later. Further, FIG. 10 also shows the open circuit voltage by light irradiation. The current density (Isc) is about twice as high as that of Comparative Example 1, and the open circuit voltage is 740 m at the highest value as compared with Comparative Example 1.
V, which was about 60 mV higher than that of Comparative Example 1.

Comparative Example 1 A TiO ₂ thin film was formed under the same conditions as in Example 1 on a transparent glass substrate coated with tin oxide (FTO) to which fluorine was added without forming an alternate adsorption film, and dye sensitization was performed. Type solar cell was prepared, and an electric current was changed in the same manner as in Example 1.
The voltage (IV) characteristic and the open circuit voltage by light irradiation were measured. The results are shown in FIGS. 9 and 10.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing the manufacturing principle of a general alternating adsorption film.

FIG. 2 shows a state (a) in which metal oxide particles are formed on the alternate adsorption film and in the pore structure thereof and a state (b) in which the alternate adsorption film is burned down to form a porous metal oxide thin film. It is a schematic diagram.

3 is a drawing-substituting photograph showing a TEM image of a cross-sectional structure of an alternate adsorption film formed in Example 1. FIG.

4 is a drawing-substituting photograph showing a TEM image of a cross-sectional structure after dipping the alternating adsorption film formed in Example 1 in dilute hydrochloric acid.

5 is a drawing-substituting photograph showing a TEM image of the cross-sectional structure of the alternate adsorption film formed by dipping the alternate adsorption film formed in Example 1 in dilute hydrochloric acid and further immersing it in pure water.

FIG. 6 is a SEM having a multilayer structure of a surface on which the alternate adsorption film formed in Example 1 was immersed in dilute hydrochloric acid, immersed in pure water, and then a TiO ₂ thin film was formed by a chemical solution deposition method.
It is a drawing substitute photograph which shows an image.

FIG. 7 is a surface structure in which the alternate adsorption film formed in Example 1 is immersed in dilute hydrochloric acid, immersed in pure water, and then a TiO ₂ thin film is formed by a chemical solution deposition method, and the alternate adsorption film is burned off. 3 is a drawing-substituting photograph showing an SEM image of FIG.

[8] The alternate adsorption film formed in Example 1 and dilute hydrochloric acid immersion treatment, was dipped in pure water, a TiO ₂ thin film formed by a chemical solution deposition method, TiO ₂ obtained by burned by further firing the alternate adsorption film SEM images (a) and (b) of the columnar structure inside the thin film and SEM images (c) of the layered structure,
It is a drawing substitute photograph which shows (d).

FIG. 9 is a graph showing the measurement results of IV characteristics of dye-sensitized solar cells manufactured by the methods of Example 1 and Comparative Example 1.

FIG. 10 is a graph showing the measurement results of open circuit voltage of a dye-sensitized solar cell manufactured by the method of Example 1 and Comparative Example 1 by light irradiation.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shinobu Takenaka             3-14-1, Hiyoshi, Kohoku Ward, Yokohama City Keio University             Faculty of Science and Engineering F term (reference) 2G046 AA01 BA01 BA09 BB02 BC05                       DC14 EA02 EA04 EA09 FE15                       FE31 FE35 FE38 FE39 FE44                       FE45                 4K044 AA12 BA12 BA21 BB02 BB13                       BC14 CA53 CA62                 5F051 AA07 AA14 BA12 BA13 CB11                       FA03 FA06 GA03                 5H032 AA06 AS06 AS16 BB02 BB05                       EE02 EE16

Claims (2)

[Claims] 1. An alternating adsorption film having a porous nanopore diameter structure composed of a composite organic substance is formed on a substrate, and after being reorganized into a porous structure having macropores by a pure water treatment, the alternating porous structure is formed. Alternating adsorption film of nanopore size structure on the substrate by forming metal oxide particles by chemical solution deposition method on the adsorption film and in its pore structure penetrating to the surface of the substrate, and then burning off the alternate adsorption film 1. A method for producing a porous metal oxide semiconductor thin film, which comprises forming a replica-shaped porous metal oxide semiconductor thin film according to claim 1. 2. An alternating adsorption film having a porous nanopore diameter structure composed of a composite organic substance is formed on a transparent conductive substrate, and the pure structure is reorganized into a porous structure having macropores, and then the porous structure is formed. The metal oxide particles are formed by chemical solution deposition on the alternate adsorption film and in the pore structure that penetrates to the surface of the substrate, and then the alternate adsorption film is burned down to form the alternate nanopore size structure on the substrate. A method for producing an oxide semiconductor dye-bonded electrode, comprising forming a replica-like porous metal oxide semiconductor thin film of an adsorption film.