JPS6410033B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for forming a tungsten oxide film, and is particularly applicable to an electrochromic device (so-called electrochromic device) to form a tungsten oxide film. Regarding the method.

(Conventional technology) Electrochromic (hereinafter abbreviated as "EC")
The device usually uses tungsten oxide on a substrate such as glass or plastic with a transparent electrode such as a tin oxide film or a tin oxide-indium oxide (ITO) film, or a metal film such as aluminum, gold, or copper formed on the surface as an electrode. , an EC material layer such as molybdenum oxide is formed, and an electrolyte layer is inserted between the electrode substrate on which the EC material layer is formed and another opposing electrode substrate.

Tungsten oxide is widely used as an EC material, and is usually formed in the form of a film on an electrode substrate by vacuum evaporation. However, as EC devices become larger, thicker EC material films are required.
When a film is formed by vacuum evaporation, the evaporation time must be increased, which is a factor that increases manufacturing costs.

On the other hand, there is a method in which a solution of a tungsten organic compound dissolved in an organic solvent is applied onto a glass substrate having a transparent electrode, and then heated and thermally decomposed to form a tungsten oxide film (Japanese Patent Laid-Open No.
38379) and a method of forming a film by applying a transition metal compound solution onto an electrode substrate and drying it (Japanese Patent Laid-Open No. 11207/1983) has also been reported.

(Problems to be Solved by the Invention) In the conventional method for forming a tungsten oxide film described above, forming a thick tungsten oxide film using the vacuum evaporation method is costly and requires large-sized work. Not suitable for EC equipment. In addition, by applying a solution of a tungsten compound to a substrate and drying it to form a tungsten oxide film, a relatively thick tungsten oxide film (approximately 5000 Å) can be obtained. Tungsten films also have the disadvantage that, when used as an EC device, as the number of times of color development and fading increases, positive ions such as lithium ions accumulate in the tungsten oxide film, degrading the response characteristics.
This is because cations in the electrolyte layer are gradually accumulated in the tungsten oxide film as the color develops and disappears, prolonging the response time. Due to this phenomenon, when using an EC device as part of other devices, if the other devices are designed according to the initial response characteristics, the operation of the entire system will be disrupted due to changes in response time over time. There was a drawback.

The present invention was made in order to eliminate the drawbacks of the conventional tungsten oxide film formation method, and it is possible to form a relatively thick tungsten oxide film at low cost, and it can be used as an EC device. To provide a method for forming a tungsten oxide film capable of forming a tungsten oxide film that does not change its response time even after repeated coloring and fading operations, that is, its response characteristics do not change over time. With the goal.

(Means for Solving the Problems) The method for forming a tungsten oxide film of the present invention involves adding monovalent or divalent cations to a solution of a tungsten compound in which tungsten alkoxide or phenoxide or tungsten hydroxide is dissolved in an organic solvent. A tungsten oxide film containing the monovalent or divalent cations is formed on the substrate by applying a solution of the tungsten compound containing the monovalent or divalent cations to the substrate and drying and/or baking at a predetermined temperature. It is characterized by forming.

Examples of tungsten alkoxides include methoxide [W(CH ₃ O) ₆ ], ethoxide [W
(C ₂ H ₅ O) ₆ ], propoxide [W (C ₃ H ₇ O) ₆ ], butoxide [W (C ₄ H ₉ O) ₆ ], etc., and tungsten phenoxide [W (C ₆ H ₅ O) ₆ ] also forms a good tungsten oxide film.

Such tungsten alkoxide or tungsten phenoxide can be obtained by dissolving a tungsten halogen compound (for example, tungsten chloride [WCl ₆ ]) in an organic solvent containing a hydroxy group as WCl ₆ +6ROH→W(OR) ₆ +HCl. .

Note that R in the above represents an alkyl group or an aryl group. In addition, examples of the organic solvent containing a hydroxy group of the present invention include methyl alcohol,
Monohydric alcohols such as ethyl alcohol and butyl alcohol, polyhydric alcohols such as ethylene glycol and diethylene glycol, phenol,
Phenols such as methylphenol can be used. Note that the organic solvent ROH is present in excess and acts as it is as a solvent for the produced W(OR) ₆ .

In this case, the by-product hydrochloric acid (HCl) may be left in the tungsten compound solution and included in the tungsten oxide film to be formed, but when the tungsten oxide film is formed on an ITO-coated substrate, There is a risk that HCl will corrode ITO. Therefore, alkali metal alkoxide or phenoxide (MOR) or ammonia gas (NH ₃ ) is added to a solution containing HCl to neutralize part or most of the HCl as HCl + MOR → ROH + MCl or HCl + NH ₃ → NH ₄ Cl. Let's do it.

Alternatively, if a thin film of about 500 Å of an oxide of a group element such as titanium oxide (TiO ₂ ) or silicon oxide (SiO ₂ ) is formed on ITO as the substrate on which the tungsten oxide film is formed, a thin film of about 500 Å can be used. ,
This oxide film protects the ITO from HCl attack without interfering with its conductivity, eliminating the need for neutralization.

When water is added to the solution of tungsten alkoxide or tungsten phenoxide produced as described above, it is hydrolyzed as follows: W(OR) ₆ +3H ₂ O→W(OH) ₆ +6ROH. The water added at this time has the function of hydrolyzing alkoxides or phenoxides to form higher-dimensional polymers, so W
(OR) ₆ plays a role in stabilizing the monomer and easing drying and calcination conditions.

Monovalent or divalent cations (described later) are added to this tungsten hydroxide [W(OH) ₆ ] solution, and the mixture is coated on a substrate at a temperature of 35% to 95% from room temperature of about 10°C or higher to about 100°C. When dried _{in a humidity of}
A film is formed.

The substrate on which this WO ₃ film was formed was heated to 100℃ or
When fired at 300° C., if there is an additive added for porosity control (described later), the additive evaporates and a porous WO ₃ film is formed. In addition, WO ₃ formed in this way
Although the above-mentioned cations are contained in the membrane, their description is omitted in the above reaction formula.

Monovalent or divalent cations added to the above-mentioned W(OH) ₆ solution include lithium ions (Li ⁺ ), sodium ions (Na ⁺ ), potassium ions (K ⁺ ),
Alkali metal ions such as rubidium ion (Rb ⁺ ), cesium ion (Cs ⁺ ), beryllium ion (Be ²⁺ ), magnesium ion (Mg ²⁺ ),
Alkaline earth metal ions such as calcium ions (Ca ²⁺ ), strontium ions (Sr ²⁺ ), and barium ions (Ba ²⁺ ), ammonium ions (NH ₄ ⁺ ), and quaternary alkyl ammonium (NR ₄ ⁺ )
etc. may be added as salts (halides, nitrates, carbonates, perchlorates, sulfates, etc.), hydroxides, alkoxides, and phenoxides. In addition, as a method for adding these cations, there is also a method in which cations are attached to a strong acid type ion exchange resin and a solution of the tungsten compound is passed through the column.

The concentration of the cations added in this way is preferably about 0.05 to 1 in terms of atomic ratio (M ⁺ /W ratio or M ²⁺ /W ratio) to tungsten in the tungsten compound. This is because the atomic ratio is 0.05.
If the tungsten oxide film is smaller, the formed tungsten oxide film often becomes cloudy and does not become transparent, and if the atomic ratio is larger than 1, the formed tungsten oxide film may not change color or fade when used as an EC device. This is because the response speed becomes too slow. Note that even more stable response characteristics can be obtained by setting the concentration to about 0.3 to 0.7 in terms of atomic ratio.

Furthermore, various additives that help form a tungsten oxide film can also be added to the tungsten compound solution.

Such additives include surfactants to improve the wettability of the tungsten compound solution to the substrate or electrodes on the substrate, and to adjust the pH and degree of hydrolysis and condensation of the tungsten compound solution. Acids such as hydrofluoric acid (HF), hydrochloric acid (HCl), nitric acid (HNO ₃ ), acetic acid (CH ₃ COOH), ammonium hydroxide (NH ₄ OH), lithium hydroxide (LiOH), caustic soda ( Alkali such as NaOH) and halogen ions such as iodine ions (I ^- ) and bromine ions (Br ^- ) are used.

Additionally, if a solvent with a large molecular size is added to the tungsten compound solution, as described above, the solvent will evaporate during the formation of the tungsten oxide film, leaving pores in the formed tungsten oxide film. . Therefore, the porosity of the tungsten oxide film can be controlled by controlling the molecular weight of the solvent added. When a tungsten oxide film is used as an EC device, the larger the porosity, the faster the diffusion rate of ion exchange in the tungsten oxide film when a voltage is applied, and the faster the response speed of the coloring/decoloring reaction. Suitable additives for such porosity control include solvents such as alcohols, glycols, and cellosolves, which have a boiling point higher than the drying temperature and have a large molecular weight.

Methods for applying the tungsten compound solution prepared in this way onto a substrate include the dip method, in which the substrate is immersed in a tank containing the solution, and the solution is sprayed onto a single point on the substrate while rotating the substrate. spin coating method, in which the solution is applied uniformly onto the substrate; spray method, in which the substrate is fixed and the solution ejecting source is moved; A method is used in which a solution is cross-linked with a source and the solution is coated with a constant thickness by moving the source or the substrate while utilizing the surface tension of the solution.

The substrate coated with a solution of tungsten compound is
Next, as mentioned above, after drying at an appropriate temperature, 100℃
or bake at 350℃. In this case, if the tungsten oxide film is fired at a temperature lower than 100°C, crystals will precipitate on the formed tungsten oxide film and a pattern will appear on the film surface, and if the tungsten oxide film is fired at a higher temperature than 350°C, the entire tungsten oxide film will become crystalline. Therefore, when used as an EC device, the tungsten oxide film is difficult to develop and fade, and the response time becomes slow, so neither of the firing conditions is preferable.

The WO ₃ film formed as described above is relatively thick (about 5000 Å), and the amount of tungsten in the film is 200 μm.
g/cm ² can be easily obtained and can be sufficiently applied to EC devices.

(Function) When a substrate on which a tungsten oxide film is formed is fired using an EC device, the phenomenon of color development and fading of the tungsten oxide film can be explained as a reaction of WO ₃ +x(M ⁺ +e ⁻ )MxWO ₃ . In the formula, M is a monovalent or divalent cation, and the formula typically shows the case of a monovalent cation. When the monovalent cation is a lithium ion, MxWO ₃ becomes lithium tungsten bronze (LixWO ₃ ) and exhibits a clear blue color.

However, if the coloring/decoloring operation is repeated many times using this tungsten oxide film, the response time of the coloring/decoloring operation gradually slows down as described above. This is because monovalent or divalent cations supplied from the electrolyte layer are accumulated in the tungsten oxide film, and the electromotive force shifts to the negative side.

Let's consider this in more detail.

FIG. 1 shows the relationship between the amount of cations accumulated in the tungsten oxide film and the amount of charge injected into the tungsten oxide film within a certain period of time when a certain voltage is applied. In the figure, the horizontal axis indicates the number of color development/decolorization drive cycles. Due to the property of the tungsten oxide film that cations accumulate in the film as the color development/decolorization drive is repeated, the accumulated cations in the film eventually It indicates the amount. As shown in the figure, the amount of charge injected into the membrane at a predetermined applied voltage and for a predetermined time decreases as the amount of cations accumulated in the membrane increases. At this time, we have found that if a certain number of cations are accumulated in the tungsten oxide film in advance, the predetermined applied voltage and the amount of charge injected within a predetermined period of time for the number of color development/decolorization cycles will be approximately constant. That is, in FIGS. 1 and 2, graphs 1, 2, and 3 show differences in the injection charge amount curves due to differences in the initial amount of cations mixed into the tungsten oxide film, but graph 2 The initial amount of cations mixed in is larger in Graph 3, and the initial cations mixed in is larger in Graph 3 than in Graph 2. Graph 3
At the stage , the amount of charge injected with respect to the number of cycles is almost constant. As mentioned above, the amount of cations mixed in which almost constant characteristics can be obtained, that is, the concentration of monovalent or divalent cations in the solution of the tungsten compound forming the tungsten oxide film, is as follows:
The atomic ratio with tungsten is about 0.05 to 1.

This can be understood from FIG.
That is, the electromotive force when the tungsten oxide film performs the coloring/discoloring operation decreases in accordance with the amount of charge injected into the tungsten oxide film, as shown in FIG. 2A. On the other hand, as shown in the figure, if monovalent or divalent cations are mixed into the tungsten oxide film in advance, the curves of the amount of injected charge versus the electromotive force move almost in parallel in the negative direction. Figure 2 B, C].

As explained above, when a tungsten oxide film is formed by pre-mixing predetermined cations into the tungsten oxide film, the predetermined applied voltage and time to the film can be maintained even if the number of cycles of color development/decolorization drive is repeated. Since the amount of charge injected into the film remains constant, the electromotive force involved in the coloring and fading phenomenon of the film remains constant regardless of the number of driving cycles. Therefore, the response time for color development and fading of a tungsten oxide film formed by pre-mixing cations does not change over time and remains constant even when driven at a constant voltage.

(Example) Example 1 Lithium chloride (LiCl) was added to an ethyl alcohol solution of 0.5 mol (M) of WCl ₆ at the atomic ratio of Li/W.
0.3, and this solution was applied by spraying onto a glass substrate with an ITO electrode coating formed on the surface, and the substrate was kept at room temperature and humidity of 95%, stored once, and then baked at 150°C for 1 hour to coat the substrate. A WO ₃ film with a thickness of 1 μm was obtained.

The WO ₃ film prepared in this way was treated with lithium perchlorate (LiClO ₄ ) in γ-butyrolactone solvent.
The electrode was immersed in a 1M solution, a platinum (Pt) electrode was used as a counter electrode, and a saturated calomel electrode (SCE) was used as a reference electrode, and the electrode was swept with a triangular wave at 50 mv/sec to measure the voltage-current characteristics shown in Figure 3. In the figure, the graph drawn with a solid line is the characteristic of the WO ₃ film of this example,
The graph drawn with a broken line is the characteristic measured under the same conditions for a WO ₃ film of the same thickness (1 μm) that was created by vacuum evaporation and did not contain cations in advance. As is clear from the figure, the electromotive force for obtaining the same current value is negatively shifted in the WO ₃ film of this example compared to the vacuum-deposited film, which is the same tendency as shown in Fig. 2. is shown.

Figure 4 shows an EC device constructed using the WO ₃ membrane of this example and an electrolyte solution containing 1M LiClO ₄ dissolved in a γ-butyrolactone solvent, and a voltage of ±1.25V applied to the electrodes of the EC device. This is a graph plotting the data of a so-called cycle test in which the amount of charge injected into the WO ₃ film was measured when the coloring/decoloring operation was switched back and forth by applying the same at a frequency of 1 Hz. In the figure, the solid line represents the data of the EC device using the WO ₃ film of this example, and the broken line represents the WO ₃ film obtained by the vacuum evaporation method shown above.
Data for an EC device using a membrane is shown. As is clear from the figure, in the EC device using a vacuum-deposited film, there is a change in the amount of injected charge over time, whereas in this example
In the EC device using the WO ₃ membrane, almost no change over time was observed even after more than 600,000 cycle tests.

Example 2 Sodium ethoxide (C ₂ H ₅ ONa) was added to the same tungsten alkoxide solution as in Example 1.
A solution obtained by adding 0.5 Na/W atomic ratio and further adding 10% propylene glycol was coated on the ITO electrode substrate using a spin coating method.
_The mixture was fired at 200° C. for 1 hour to obtain a porous WO ₃ film having a WO 3 density of 3.2 g/cm ³ and a thickness of 1 μm.

As a result of measuring the voltage-current characteristics and cycle test of this WO ₃ film, almost the same data as the data of Example 1 shown in FIGS. 3 and 4 were obtained. This is due to the delay in response time due to the shift in the negative direction of the electromotive force due to the large amount of cations.
This is because compensation was achieved by making the WO ₃ film porous.

Example 3 The same solution as in Example 1 was applied and baked in the same manner on an electrode substrate on which a 500 Å thick TiO ₂ film was formed on ITO to obtain a 1 μm thick WO ₃ film on the electrode substrate. When an EC device was constructed using this WO ₃ film and a cycle test was performed, no deterioration in characteristics was shown even after more than 800,000 cycle tests.

This is due to the fact that the TiO ₂ film has more surface hydroxyl groups than the ITO film, and also because the TiO ₂ film blocks HCl in the tungsten ethoxide solution.

(Effects of the Invention) In the method for forming a tungsten oxide film according to the present invention, a solution in which a monovalent or divalent cation is dissolved in a solution of a tungsten compound in which a tungsten alkoxide, phenoxide, or tungsten hydroxide is dissolved in an organic solvent is used. Since the tungsten oxide film is formed by coating and firing the tungsten oxide film on the substrate, a relatively thick tungsten oxide film can be obtained easily and at low cost, which is extremely convenient for producing large EC devices, etc. be. In addition, since the formed tungsten oxide film contains cations in advance, when used as an EC device, the response time for color development and fading does not change over time, making it stable.
An EC device is obtained. Note that the coloring time of the response time appears to be delayed at the initial stage of the coloring/decoloring operation compared to a film that has not been pre-injected with cations, but this delay can be compensated for by increasing the applied voltage. It is fully recoverable. At the same time, the erasing time will be correspondingly shortened.
Furthermore, by mixing additives into the tungsten compound solution, it is possible to form a tungsten oxide film with appropriate porosity, thereby controlling the diffusion rate of MxWO ₃ in the tungsten oxide film. Therefore, from this point of view as well, the response speed of color development and fading can be improved.

[Brief explanation of drawings]

1 and 2 are graphs showing the operating principle of a tungsten oxide film formed by the tungsten oxide film forming method of the present invention, and FIGS. 3 and 4 are graphs showing a tungsten oxide film formed by an embodiment of the present invention. It is a graph showing operating characteristics.

Claims (1)

[Claims] 1. Monovalent or divalent cations are dissolved in a solution of a tungsten compound in which tungsten alkoxide or phenoxide or tungsten hydroxide is dissolved in an organic solvent, and a solution of the tungsten compound containing the cation is prepared. A method for forming a tungsten oxide film, the method comprising applying the tungsten oxide film to a substrate and drying and/or baking at a predetermined temperature to form a tungsten oxide film containing the monovalent or divalent cations on the substrate. 2. The method for forming a tungsten oxide film according to claim 1, wherein the monovalent cation is a hydrogen atom, an alkali metal, an ammonium ion, or a quaternary alkyl ammonium. 3. The method for forming a tungsten oxide film according to claim 1, wherein the divalent cation is an alkaline earth metal. 4. Any one of claims 1 to 3, wherein the substrate is a substrate with electrodes formed on its surface.
The method for forming a tungsten oxide film described in Section 1. 5 Claim 4 in which the electrode is a transparent conductive film
The method for forming a tungsten oxide film described in Section 1. 6. The method for forming a tungsten oxide film according to claim 5, wherein the electrode is a tin oxide film or a tin oxide-indium oxide (ITO) film. 7. The method for forming a tungsten oxide film according to claim 4, wherein the electrode is a metal film. 8. The method for forming a tungsten oxide film according to claim 4, wherein the substrate is a substrate on which a thin film is further formed on the electrode. 9. The method of forming a tungsten oxide film according to claim 8, wherein the thin film is an oxide of a group element. 10. The method for forming a tungsten oxide film according to claim 1, wherein the tungsten compound solution contains water. 11. The method for forming a tungsten oxide film according to claim 1, wherein the tungsten compound solution contains an additive for controlling the porosity of the tungsten oxide film to be formed. 12 The tungsten compound solution has a pH of
A patent containing an additive for adjusting the degree of hydrolysis, an additive for controlling the degree of condensation of tungsten monomer in the solution, or an additive for improving the wettability between the solution and the substrate surface A method for forming a tungsten oxide film according to claim 1. 13. The method for forming a tungsten oxide film according to claim 1, wherein the tungsten alkoxide or tungsten phenoxide is produced by dissolving a tungsten halogen compound in an organic solvent containing a hydroxyl group.