JP5601007B2 - Coating composition for forming porous photoelectric conversion layer and method for forming porous photoelectric conversion layer - Google Patents

Description

  The present invention relates to a coating composition used for forming a porous photoelectric conversion layer on an electrode substrate in a dye-sensitized solar cell and a method for forming a porous photoelectric conversion layer using the coating composition.

  Currently, there is great expectation for photovoltaic power generation from the viewpoint of global environmental problems and fossil energy resource depletion problems, and single crystal and polycrystalline silicon photoelectric conversion elements are put into practical use as solar cells. However, this type of solar cell is expensive and has a problem of supply of silicon raw materials, and the practical application of solar cells using materials other than silicon is desired.

  From the above viewpoint, recently, a dye-sensitized solar cell has attracted attention as a solar cell using a material other than silicon. As a representative of this dye-sensitized solar cell, a transparent electrode substrate in which a transparent conductive film such as ITO is provided on the surface of a glass substrate or a transparent plastic substrate, and a metal electrode substrate are sensitized with a dye. The peripheral portion between the metal electrode substrate and the transparent electrode substrate is sealed so that the electrolyte layer does not leak. Sealed with a stopper. That is, the region where the metal electrode substrate and the transparent electrode substrate are opposed to each other with the porous photoelectric conversion layer and the electrolyte layer interposed therebetween is the power generation region, and the region sealed with the sealing material is the power generation region. It is a sealing area unrelated to the above. This porous photoelectric conversion layer is generally provided on a transparent electrode substrate, but can also be provided on a metal electrode substrate (see Patent Document 1).

  In the dye-sensitized solar cell having the above-described structure, when visible light is irradiated from the transparent electrode substrate side, the dye in the dye-sensitized porous layer is excited, transitioned from the ground state to the excited state, and excited. The electrons of the dye are injected into the conduction band in the semiconductor porous layer, and pass through an external circuit from the transparent electrode substrate or metal electrode substrate on which the semiconductor porous layer is formed. Move to transparent electrode substrate. The electrons that have moved to the counter electrode substrate are carried by the ions in the electrolyte layer and return to the dye. Electric energy is extracted by repeating such a process. The power generation mechanism of such a dye-sensitized solar cell is different from a pn junction photoelectric conversion element, in which light capture and electronic conduction are performed in different places, and is very similar to a plant photoelectric conversion process. Yes.

  In the dye-sensitized solar cell having the above structure, particularly when the porous photoelectric conversion layer is provided on the metal substrate, the porous photoelectric conversion layer carrying the dye is directly on the low-resistance metal substrate. Therefore, there is an advantage that it is possible to avoid a decrease in conversion efficiency and to suppress an increase in internal resistance (curvature factor; FF) when the cell is enlarged.

  However, when the porous photoelectric conversion layer is provided on the metal substrate and power is generated by light irradiation from the transparent electrode substrate side, the porous photoelectric conversion layer exists on the low-resistance metal substrate, so that the rectifying action is In order to achieve incompleteness, reverse current, and sufficiently high conversion efficiency, there is still room for improvement. There is also a problem that the durability is low and the conversion efficiency decreases with time.

  As means for improving the above problems, the present applicant formed a reverse electron prevention layer made of a chemical conversion treatment film on a metal substrate, and sensitized with a dye on the reverse electron prevention layer. A method for forming a porous oxide semiconductor layer has been proposed (see Patent Document 2).

JP 2001-273937 A JP2008-053024

  However, as proposed in Patent Document 2, a reverse electron prevention layer is formed on a metal substrate by chemical conversion treatment, and a semiconductor porous layer sensitized with a dye is formed on the reverse electron prevention layer. Since the reverse electron prevention layer is highly resistant to electrolytes, it can effectively prevent a decrease in conversion efficiency over time, but the reverse current prevention effect is not so high, and thus a high change efficiency is obtained. It is still insufficient.

  On the other hand, the present applicant has proposed a coating solution for forming a reverse electron prevention layer (Japanese Patent Application No. 2008-178341). That is, this coating solution is composed of an organic solvent solution containing a metal compound capable of forming a metal oxide by heat treatment as a solute. The organic solvent solution contains a solute stabilizer and is 10 cP at 25 ° C. It has the above viscosity, and this is applied to the surface of a metal substrate and dried to form a reverse electron prevention layer serving as a base for a semiconductor porous layer sensitized with a dye. is there. Since the reverse electron prevention layer formed using such a coating liquid is formed from a dense layer of metal oxide, it not only exhibits an excellent rectifying action compared to that formed by chemical conversion treatment, It has good resistance to the electrolyte, and therefore has an advantage that the corrosion of the metal substrate can be effectively prevented and the problem of deterioration in conversion efficiency with time can be effectively avoided.

  However, when the reverse electron prevention layer is formed by the coating liquid as described above, there is a problem that local corrosion (pitting corrosion) occurs on the surface of the underlying metal substrate. Such pitting corrosion expands with time, resulting in a decrease in conversion efficiency. In particular, such pitting corrosion frequently occurs when a reverse electron prevention layer is formed on the surface of a metal substrate having a large surface roughness. Therefore, it is necessary to ensure resistance to the electrolyte and to surely prevent a decrease in conversion efficiency over time, and further improvement is required.

  In addition, when forming the reverse electron prevention layer using the coating liquid as described above, after forming the reverse electron prevention layer, coating with a paste coated with a metal oxide is further applied. It is necessary to form a porous oxide semiconductor layer, and in order to form a porous photoelectric conversion layer including a porous oxide semiconductor layer on an electrode metal substrate, the coating must be performed in two stages and produced. There is also a disadvantage that the property is low.

Accordingly, an object of the present invention is to form a porous photoelectric conversion layer that is excellent in reverse electron prevention and excellent in resistance to electrolytes, and stably exhibits high conversion efficiency without decreasing over time. It is to provide a coating composition.
Another object of the present invention is to provide a method capable of forming a porous photoelectric conversion layer having the above-described characteristics by one-step coating.

According to the present invention, a coating composition used for forming a porous photoelectric conversion layer on an electrode substrate, which is a metal oxide, a metal compound capable of forming an oxide by heat treatment, a dispersant, and an organic solvent The metal oxide is present as dispersed particles, the metal compound is present as a solute , the dispersant is a first dispersant for dispersing the metal oxide, and the metal compound is solute-stabilized. A coating composition is provided that contains a second dispersant for conversion into a second dispersion .

In the coating composition of the present invention,
(1) The metal oxide is an oxide of a metal selected from the group consisting of titanium, zinc and zirconium,
(2) the metal compound is an alkoxide or chloride of a metal selected from the group consisting of titanium, zinc and zirconium;
(3) pre-Symbol first dispersant, acetic, trimethylacetic acid, glycol ether is at least one selected from the group consisting of β- diketones and water,
( 4 ) The second dispersant is a glycol ether,
( 5 ) The organic solvent is at least one selected from the group consisting of lower alcohols having 4 or less carbon atoms, ethyl cellulose and terpineol,
( 6 ) containing the metal compound in an amount of 0.01 to 50% by weight per metal oxide in terms of metal,
( 7 ) In terms of metal, the first dispersant is contained in an amount of 0.01 to 50% by weight per the metal oxide, and the second dispersant is contained in an amount of 0.01 to 50 per the metal compound. Contained in an amount of% by weight,
Is preferred.

  According to the present invention, the above-described coating composition is applied to the electrode substrate, and then subjected to heat treatment, whereby the reverse electron blocking layer formed on the electrode substrate surface and the porous layer formed on the reverse electron blocking layer are formed. There is provided a method for forming a porous photoelectric conversion layer, characterized in that a porous photoelectric conversion layer comprising an oxide semiconductor layer is formed in one step.

  In the coating composition of the present invention, the metal oxide that exists in the form of dispersed particles is a component that forms a porous oxide semiconductor, and the metal compound that exists in the solute state is heated to become porous oxide. It is a component which produces | generates the metal oxide which forms a physical semiconductor layer, and has a function as a binder of metal oxide particle. A porous photoelectric conversion layer containing a porous oxide semiconductor can be formed in one step by applying this coating composition to a surface of a predetermined electrode substrate, for example, a metal substrate, followed by heat treatment. This is extremely advantageous in terms of productivity.

  Moreover, the porous photoelectric conversion layer formed as described above has excellent reverse electron prevention characteristics (rectification characteristics), and thus can ensure high conversion efficiency and also has excellent resistance to electrolytes. For example, even when it is formed on the surface of a metal substrate having a large surface roughness, pitting corrosion does not occur. Therefore, a decrease in conversion efficiency over time is effectively prevented, and high conversion efficiency can be stably achieved. Can be maintained.

It is a partial expanded sectional view which shows the state of the interface of the porous photoelectric converting layer obtained by forming a back-electron prevention layer and a porous oxide semiconductor layer by a two-step coating, and a metal substrate. It is a partial expanded sectional view which shows the state of the interface of the porous photoelectric converting layer and metal substrate which were formed by one-step coating using the coating composition of this invention. It is a schematic sectional drawing which shows schematic structure of the dye-sensitized solar cell which has the porous photoelectric converting layer formed using the coating composition of this invention.

<Principle of the present invention>
Conventionally, after forming a reverse electron blocking layer by coating and further coating to form a porous oxide semiconductor layer, for example, as shown in FIG. A reverse electron preventing layer 53 made of a metal oxide is formed, and further, a porous oxide semiconductor layer 55 is formed thereon, and the porous oxide semiconductor layer 55 is connected with metal oxide semiconductor particles 55a. Such a porous oxide semiconductor layer 55 is sensitized with a dye and exists in contact with the electrolyte.

  When the photoelectric conversion layer composed of the reverse electron prevention layer 53 and the porous oxide semiconductor layer 55 is formed as described above, the reverse electron prevention layer 53 is formed in a state where the porous oxide semiconductor layer 55 is not formed. Completely covers the surface of the metal substrate 50, and no pinholes are generated. However, in the state in which the porous oxide semiconductor layer 55 is formed thereon, as shown in FIG. 1, pinholes or the like are generated in the reverse electron prevention layer 53, and the surface of the metal substrate 50 is locally localized. The metal oxide semiconductor particles 55a are exposed and come into direct contact with the surface of the metal substrate 50. That is, the porous oxide semiconductor layer 55 is formed by applying a paste containing an organic solvent, a binder component and the like together with the metal oxide semiconductor particles on the surface of the reverse electron prevention layer 53 and heating the paste. When the oxide semiconductor layer 55 is formed, the reverse electron prevention layer 53 contracts, and as a result, a pinhole or the like is generated, and the surface of the metal substrate 50 is locally exposed. In particular, when the back-electron prevention layer 53 is formed on the surface of the metal substrate 50 having a large surface roughness, there are many locally protruding portions, so that the surface of the metal substrate 50 is likely to be exposed. ing.

  Therefore, when the photoelectric conversion layer (porous oxide semiconductor layer 55) in the above state comes into contact with the electrolyte, the exposed portion of the metal substrate 50 comes into contact with the electrolyte that has penetrated into the photoelectric conversion layer. As a result, pitting corrosion occurs and the pitting corrosion expands with time, resulting in a decrease in conversion efficiency.

  However, when the coating composition of the present invention is used, the photoelectric conversion layer composed of the reverse electron prevention layer 53 and the porous oxide semiconductor layer 55 is formed by one-step coating. The shrinkage of the layer 53 does not occur. As shown in FIG. 2, the surface of the metal substrate 50 is always covered with the reverse electron prevention layer 53, and the surface of the metal substrate 50 is not exposed.

  That is, when the coating composition of the present invention is applied to the surface of the metal substrate 50, a liquid layer in which the metal compound is dissolved is formed along the surface of the metal substrate 50. A dispersion layer in which metal oxide particles are dispersed is formed on the top. By performing heat treatment in this state, the organic solvent and the dispersant are removed by volatilization or decomposition from the liquid layer in which the metal compound is dissolved, and metal oxide fine particles are formed from the dissolved metal compound, The metal oxide fine particles are sintered with each other, and accordingly, a dense metal oxide layer having no pinholes, that is, a reverse electron prevention layer 53 is formed on the surface of the metal substrate 50. In addition, in the dispersion layer in which the metal oxide particles are dispersed simultaneously with the formation of the reverse electron prevention layer 53, the organic compound and the dispersing agent are removed, and at the same time, the metal compound dissolved in the dispersion layer. Metal oxide fine particles are generated from the metal oxide particles, the metal oxide particles dispersed as a binder are sintered, and the porous oxide semiconductor layer 55 in which the metal oxide particles 55a are continuous is formed. Will be formed. In the porous photoelectric conversion layer formed in this manner, the reverse electron prevention layer 53 and the porous oxide semiconductor layer 55 are formed by one-step coating, and the heat treatment for forming the layer is performed all at once. The metal oxide semiconductor particles 55a forming the oxide semiconductor layer 55 have digged into the lower reverse-electron-preventing layer 53, and as a result, defects such as pinholes due to the shrinkage of the reverse-electron-preventing layer 53 are completely generated. (It seems that the shrinkage of the reverse current prevention layer 53 is suppressed by the biting of the particles 55a). Therefore, even if the metal substrate 50 has a surface with a large surface roughness, The surface is not exposed, and pitting corrosion due to contact with the electrolyte can be surely prevented, and a decrease in conversion efficiency over time can be reliably avoided.

<Coating composition>
The coating composition of the present invention contains a metal oxide, a metal compound capable of forming an oxide by heat treatment, a dispersant and an organic solvent as essential components.

(A) a metal oxide;
The metal oxide is present as dispersed particles in the coating composition, and is a component for forming a porous oxide semiconductor layer sensitized with a dye as semiconductor fine particles. Examples of such metal oxides include oxides of metals such as titanium, zinc, zirconium, hafnium, strontium, tantalum, chromium, molybdenum, tungsten, or composite oxides containing these metals (for example, SrTiO ₃ , CaTiO _3, etc.). Perovskite-type oxides). Among these, an oxide of at least one metal selected from the group consisting of titanium, zinc and zirconium is preferable, and titanium dioxide is more preferable. In addition, anatase type, brookite type and rutile type are known as titanium dioxide. From the viewpoint of obtaining high conversion efficiency as a porous oxide semiconductor layer, anatase type or brookite type titanium dioxide is known. Is the best.

  Further, the dispersed particle diameter is not particularly limited, but generally, it is preferable that the metal oxide powder having a fine particle diameter is dispersed and adjusted to a particle diameter of, for example, 500 nm or less. If the dispersed particle size is excessively large, variations in the transmission of light to the formed porous layer are likely to occur, which may make it difficult to exhibit stable characteristics as a solar cell.

  The metal oxide particles as described above are preferably contained in the coating composition in the range of 5 to 60% by weight, particularly 10 to 40% by weight. When the content of the metal oxide particles is small, it becomes difficult to form a porous oxide semiconductor layer having a certain thickness, and an excessive amount of metal oxide particles is contained in the coating composition. In addition, the amount of other components, for example, metal oxides to be described later, becomes small, and as a result, there is a possibility that the reverse electron prevention characteristics and the resistance to the electrolyte are lowered.

(B) a metal compound;
The metal compound contained in the coating composition is present as a solute, forms a metal oxide by heat treatment (firing) described later, and functions as a binder for the metal oxide particles described above, and a reverse electron prevention layer. And has a forming function. The metal element of such a metal compound is not particularly limited as long as an oxide can be formed, and may be Ti, Al, Zn, Ni, Fe, Cu, or the like, for example. The form of the compound is not particularly limited as long as it can form an oxide by heat treatment and can be dissolved in an organic solvent. In general, it can be easily obtained, and the oxide can be quickly obtained by heat treatment. And is preferably an alkoxide, hydroxide, or chloride because of its high solubility in organic solvents. In addition, from the viewpoint of the function of the metal oxide particles as a binder, those that can form the same metal oxide as the metal oxide particles are optimal. For example, when titanium dioxide is used as the metal oxide particles For this, titanium alkoxide, particularly titanium isopropoxide is preferred, and titanium tetraisopropoxide is most preferred.

  In the present invention, the metal compound is an amount of 0.01 to 50% by weight, particularly 0.03 to 30% by weight per metal oxide (in terms of metal, based on the amount of metal in the metal oxide). It is preferably contained in the coating composition at a ratio of the amount of metal in the metal compound). That is, if this amount is too small, the thickness of the reverse electron prevention layer 53 formed under the porous oxide semiconductor layer 55 shown in FIG. 2 described above becomes insufficient, resulting in unsatisfactory rectification characteristics. Alternatively, defects such as pinholes are likely to occur, and conversion efficiency is likely to be reduced.

(C) a dispersant;
The dispersant contained in the coating composition of the present invention is used for stably dispersing the metal oxide (a) particles in an organic solvent and stabilizing the metal compound (b) as a solute.

  The first dispersant used to stably disperse the metal oxide (a) particles varies depending on the organic solvent used, but usually from acetic acid, trimethylacetic acid, glycol ether, β-diketone and water. At least one selected from the group is used. These can not only disperse metal oxides such as titanium dioxide in organic solvents relatively stably, but also can be easily volatilized without adversely affecting the semiconductor properties of the metal oxide particles by heating. Because.

  The second dispersant used to stabilize the metal compound (b) as a solute, that is, to prevent precipitation of the metal compound (b) existing as a solute, is itself an organic solvent. A compound which is soluble in water and has a high affinity for a metal compound is used. For example, glycol ether is preferable. Glycol ether is soluble in organic solvents and can be used not only as the first dispersant, but also easily forms a coordinate bond with the metal element, effectively suppressing the precipitation of the metal compound. And exhibits excellent properties as a solute stabilizer. Moreover, since it is chemically stable, even when it coexists with other components, it stably functions as a solute stabilizer for the metal compound (b). When such a second dispersant is not used, the metal compound may be precipitated when it coexists with other components. As a result, the dense metal oxide derived from the metal compound may be precipitated. It becomes difficult to form the layer (that is, the reverse electron prevention layer in FIG. 2), the rectification characteristics are deteriorated, or defects such as pinholes are likely to occur, and the surface of the metal substrate is corroded by contact with the electrolyte. It becomes easy to produce.

In the dispersant described above, the glycol ether that can be used for both the first dispersant and the second dispersant is represented by the following formula:
HOCH ₂ CH ₂ OR
In the formula, R is an alkyl group, an aryl group, or an aralkyl group.
The alkyl group is typically a lower alkyl group having 8 or less carbon atoms such as a methyl group, an ethyl group, an isopropyl group, an n-propyl group, an isobutyl group, an n-butyl group, and an isoamyl group. Examples of the aryl group include a phenyl group, and examples of the aralkyl group include a benzyl group. Among these, glycol ethers in which R is an alkyl group are suitable, and in particular, they have excellent functions as compatibilizing components, prevent precipitation of the titanium compound (b) present as a solute, and are themselves organic. Butyl cellosolve (ethylene glycol monobutyl ether, R = isobutyl group or n-butyl group) and propyl cellosolve (ethylene glycol monopropyl ether, R = propyl) are soluble in solvents and have high affinity for titanium compounds. Group) is preferred, and butyl cellosolve is most preferred.

  In addition, since the first dispersant other than the glycol ether, that is, acetic acid, trimethylacetic acid, β-diketone and water do not function as the second dispersant, the second dispersant (glycol ether) is used. ), And an appropriate one is used depending on the type of organic solvent used.

  Examples of the β-diketone used as the first dispersant include acetylacetone, 1,3-cyclohexadione, methylenebis-1,3-cyclohexadione, 2-benzyl-1,3-cyclohexadione. , Acetyltetralone, palmitoyltetralone, stearoyltetralone, benzoyltetralone, 2-acetylcyclohexanone, 2-benzoylcyclohexanone, 2-acetyl-1,3-cyclohexanedione, bis (benzoyl) methane, benzoyl-p-chlorobenzoyl Methane, bis (4-methylbenzoyl) methane, bis (2-hydroxybenzoyl) methane, benzoylacetone, tribenzoylmethane, diacetylbenzoylmethane, stearoylbenzoylmethane, palmitoylbenzoylmethane, lauro Rubenzoylmethane, dibenzoylmethane, bis (4-chlorobenzoyl) methane, bis (methylene-3,4-dioxybenzoyl) methane, benzoylacetylphenylmethane, stearoyl (4-methoxybenzoyl) methane, butanoylacetone, di Examples include stearoylmethane, stearoylacetone, bis (cyclohexanoyl) -methane, and dipivaloylmethane, and among these, acetylacetone is preferred.

  The above-mentioned first dispersant is preferably used in an amount of 0.01 to 50% by weight, particularly 0.02 to 20% by weight, in terms of metal, per metal oxide (a).

The second dispersant described above is preferably used in an amount of 0.01 to 50% by weight, particularly 0.02 to 30% by weight, in terms of metal, per metal compound (b).
In addition, when glycol ether is used as both the first dispersant and the second dispersant, both the amount of the metal oxide (a) and the amount of the metal compound (b) in terms of metal are as described above. It should be used to satisfy the range.

(D) an organic solvent;
In the present invention, the organic solvent can be used as a dispersion medium for the metal oxide particles (a) described above, and further dissolves the metal compound (b) described above and has a high affinity for the dispersant (c). If it is, various kinds of materials can be used without any particular limitation, but it forms a viscous coating solution particularly suitable for screen printing and is volatilized by heating without adversely affecting the electrical properties of the metal oxide. From the viewpoint of being able to be produced, at least one selected from the group consisting of lower alcohols having 4 or less carbon atoms, ethyl cellulose and terpineol is preferable.

  In particular, examples of the lower alcohol include methanol, ethanol, isopropanol, and butanol, which are particularly suitable as a dispersion medium for metal oxide particles and a solvent for the metal compound (b), such as screen printing. In consideration of suitability for high-viscosity coating, it is preferably used as a mixed solvent with terpineol and ethyl cellulose.

Terpineol (C ₁₀ H ₁₈ O) is an unsaturated alcohol generated by dehydrating one molecule of water from 1,8-terbin, and three types of α, β and γ are known. Although the type can also be used, in general, α-terpineol (Bp: 219 to 221 ° C.) or a mixture containing α-terpineol as a main component and other types such as β-terpineol mixed (generally commercially available) What is being done is a mixture).

  Terpineol is a viscous liquid, but has good affinity with the metal oxide fine particles and metal compound dispersed in the above-mentioned lower alcohol. Similarly to the lower alcohol, terpineol is a metal oxide produced by heating ( For example, it can be easily volatilized without adversely affecting the electrical properties of titanium dioxide).

  Furthermore, ethyl cellulose, like terpineol, can be easily decomposed and removed by heat treatment without adversely affecting the electrical properties of the metal oxide produced from the metal compound. It has the function of. Therefore, ethyl cellulose is optimally used in combination with other organic solvents.For example, when only a lower alcohol or terpineol is used as an organic solvent and a metal compound solution is prepared, the viscosity of the coating composition becomes extremely low, Although dripping or the like tends to occur during coating, the viscosity of the coating composition can be adjusted to a range suitable for coating by using ethyl cellulose in combination.

  In addition, although ethyl cellulose having various molecular weights is commercially available, from the viewpoint of adjusting the coating liquid to a viscosity particularly suitable for screen printing, toluene is used as a solvent, and a solid ethylcellulose concentration 10% solution is used. What has a viscosity (25 degreeC) in the range of 30-50 cP is suitable.

  From the above viewpoint, the mixed solvent used as the organic solvent in the present invention is generally in the range of ethyl cellulose / terpineol (weight ratio) = 0.1 / 99.9 to 20/80, particularly 3/97. In addition, an appropriate amount of lower alcohol is used as a dispersion medium for the metal oxide particles so that the viscosity of the coating composition is in a range suitable for coating (for example, 15 to 500 cP at 25 ° C.). It is good to use.

<Preparation of coating composition>
The coating composition of the present invention containing each of the above-described components has a dispersion in which metal oxide particles are dispersed and a metal oxide (b) dissolved, particularly in order to allow the metal compound (b) to exist stably as a solute. The solution is preferably prepared separately and then mixed by mixing these dispersions with the solution. When the components are mixed all at once, the metal oxide (b) may be precipitated in an aggregated state. In such a case, the reverse electron blocking layer (dense This is because it becomes difficult to form a (metal oxide layer).

  In preparing the coating composition as described above, the dispersion in which the metal oxide particles are dispersed is obtained by adding the metal oxide particles and the first dispersant to a part of the organic solvent, particularly the lower alcohol. It is obtained by mixing and stirring in the amount ratio. In addition, the solution in which the metal oxide (b) is dissolved is obtained by mixing the remaining organic solvent, particularly the ethyl cellulose / terpineol mixed solvent, with the metal compound and the second dispersant in a predetermined amount ratio and stirring. can get. At this time, it can be heated to an appropriate temperature. The amount of the organic solvent used in these dispersions or solutions is such that the viscosity of the coating composition is suitable for coating when the coating composition is prepared by mixing the two (for example, the viscosity at 25 ° C. The amount may be 10 cP or more, particularly 50 to 2000 cP), and the amount is such that the dissolved metal compound does not precipitate.

<Formation of porous photoelectric conversion layer>
In the present invention, by using the above coating composition, a porous photoelectric conversion layer having a reverse electron prevention property can be formed by a single-stage coating. That is, with reference to FIG. 2, a porous photoelectric conversion layer is formed by applying this coating composition to the surface to be a power generation region of a metal substrate 50 used as an electrode substrate, followed by heat treatment (firing). Is done. That is, by this heat treatment, the organic solvent and the like are removed by volatilization, thermal decomposition, etc., and an oxide of the metal compound is generated, and the metal oxide particles and dispersed particles generated according to the principle described above are present. Oxide particles are sintered, and a porous photoelectric conversion layer comprising a dense reverse electron prevention layer 53 existing on the surface of the metal substrate 50 and a porous oxide semiconductor layer 55 existing on the reverse electron prevention layer 53 is formed. That is why.

  The coating composition can be applied by known application means such as screen printing, spray spraying, brush coating, spin coating, dipping, etc., but screen printing is effective in that it can be applied efficiently and continuously. Is preferred.

  The heat treatment conditions after coating vary depending on the type of metal compound or metal oxide used, but are generally performed by heating and holding the coating layer at a high temperature of 300 to 600 ° C. for 10 to 180 minutes.

  In addition, the coating amount of the coating composition is usually such that the thickness of the reverse electron prevention layer 53 under the porous photoelectric conversion layer to be formed is 10 to 500 nm, particularly 30 to 200 nm, and the porous oxide semiconductor layer thereon The thickness of 55 is set to be about 5 to 20 μm.

  In the porous photoelectric conversion layer (specifically, the porous oxide semiconductor layer 55) obtained as described above, a dye is adsorbed and supported according to a conventional method, and the porous photoelectric layer sensitized with such a dye is used. The metal substrate provided with the conversion layer is used as a negative electrode substrate for use as an electrode substrate of a solar cell. Accordingly, the porous oxide semiconductor layer 55 in the porous photoelectric conversion layer needs to be porous in order to support the dye. For example, the relative density by the Archimedes method is 50 to 90%, particularly 50 to It is preferably about 70%, so that a large surface area can be secured and an effective amount of the dye can be supported.

  The dye adsorbed on the porous photoelectric conversion layer is adsorbed and supported by bringing the dye solution into contact with the porous layer. The contact of the dye solution is usually performed by dipping, and the adsorption treatment time (immersion time) is usually about 30 minutes to 24 hours, and after adsorption, the solvent of the dye solution is removed by drying, A sensitizing dye is adsorbed and supported on the surface and inside of the porous oxide semiconductor layer 55.

The dye used can function as a sensitizing dye and has a linking group such as a carboxylate group, a cyano group, a phosphate group, an oxime group, a dioxime group, a hydroxyquinoline group, a salicylate group, and an α-keto-enol group. Those known per se are used. For example, a ruthenium complex, an osmium complex, an iron complex, etc. can be used without any limitation. Ruthenium-tris (2,2′-bispyridyl-4,4′-dicarboxylate), ruthenium-cis-diaqua-bis (2,2′-bispyridyl-4,4) in that it has a particularly broad absorption band. Ruthenium-based complexes such as' -dicarboxylate) are preferred. Such a dye solution of a sensitizing dye is prepared using an alcohol-based organic solvent such as ethanol or butanol as a solvent, and the dye concentration is usually about 3 × 10 ^{−4 to} 5 × 10 ⁻⁴ mol / l. It is good to do.

<Dye-sensitized solar cell>
The metal substrate having a porous photoelectric conversion layer sensitized with a dye as described above on its surface is used as, for example, a negative electrode substrate of a dye-sensitized solar cell having a structure shown in FIG.

  Referring to FIG. 3, in the solar cell having this structure, a porous photoelectric conversion layer 13 sensitized with a dye is formed on the surface of metal substrate 11, and this is used as negative electrode substrate 10 to form electrolyte layer 20. The porous photoelectric conversion layer 13 has a structure in which the transparent electrode substrate (positive electrode substrate) 1 is sandwiched between the transparent electrode substrate 1 and the periphery thereof is sealed with a sealant 30. It is formed by one-step coating using the composition. That is, the porous photoelectric conversion layer 13 includes a porous oxide semiconductor layer 13a on which a dye is adsorbed and supported, and a dense and thin reverse electron prevention layer 13b formed on the surface side of the metal substrate 11. .

The metal substrate 11 that holds the porous photoelectric conversion layer 13 is not particularly limited as long as it is formed from a metal material having a low electrical resistance, but generally, a ratio of 6 × 10 ⁻⁶ Ω · m or less. A metal or alloy having resistance, such as aluminum, iron (steel), stainless steel, copper, nickel or the like is used. Further, the thickness of the metal substrate 11 is not particularly limited as long as it has a thickness enough to maintain an appropriate mechanical strength. If productivity is not taken into consideration, the metal substrate 11 may be formed on a resin film or the like, for example, by vapor deposition. Of course, the substrate such as the resin film does not need to be transparent.

  In the present invention, a chemical conversion treatment film (having a reverse electron prevention function) disclosed in Japanese Patent Application Laid-Open No. 2008-53165 or the like is formed on the surface of the metal substrate 11 which is the base of the porous conversion photoelectric layer 13. It is also possible to form the porous conversion photoelectric layer 13 on this using the above-described coating composition of the present invention.

  As understood from FIG. 3, the porous photoelectric conversion layer 13 is formed in a portion that becomes the power generation region X, and the periphery thereof is a sealed region Y that does not participate in power generation.

  The transparent electrode substrate 1 arranged to face the negative electrode substrate 10 composed of the metal substrate 11 and the porous photoelectric conversion layer 13 sensitized with the dye as described above has a transparent conductive film 5 and an electron on the surface of the transparent substrate 3. The reducing conductive layer 7 is formed.

  The transparent substrate 3 only needs to have high light transmittance, and is formed of, for example, transparent glass or a transparent resin film. The thickness and size are appropriately determined according to the intended use of the dye-sensitized solar cell to be finally formed.

  Typical examples of the transparent conductive film 5 formed on the transparent substrate 3 include a film made of an indium oxide-tin oxide alloy (ITO film), a film in which tin oxide is doped with fluorine (FTO film), and the like. An ITO film is preferable because of its high electron reducing property and particularly desirable characteristics as a cathode. These are formed on the transparent substrate 3 by vapor deposition, and the thickness is usually about 500 nm to 700 nm.

  The electron reduction conductive layer 7 formed on the transparent conductive film 5 is generally made of a thin platinum layer, and has a function of quickly transferring electrons flowing into the transparent conductive film 5 to the electrolyte layer 20. is there. Such an electron reduction conductive layer 20 is thinly formed by vapor deposition so that the average thickness thereof is about 0.1 to 1.5 nm so as not to impair the light transmittance.

  The negative electrode substrate 10 and the transparent electrode substrate (positive electrode substrate) 1 formed as described above are opposed to each other with the electrolyte layer 20 therebetween, and the porous photoelectric conversion layer 13 (sensitized with the electrolyte layer 20 and the dye) ( In particular, the power generation region X is formed by the semiconductor porous layer 13a). Such an electrolyte layer 20 is formed of various electrolyte solutions containing cations such as lithium ions and anions such as chlorine ions as in the case of known solar cells. Further, it is preferable that an oxidation-reduction pair capable of reversibly taking an oxidized structure and a reduced structure is present in the electrolyte 20. Examples of such an oxidized-reduced pair include iodine-iodine compounds, bromine- Examples thereof include bromine compounds and quinone-hydroquinone. The electrolyte layer 20 is sealed by the sealing material 30 provided in the sealing region Y located at the periphery of the power generation region X, and liquid leakage from between the electrodes is prevented. In general, the thickness of the electrolyte layer 20 is generally about 10 to 50 μm, although it varies depending on the size of the battery finally formed.

  As the sealing material 30, various heat-sealable thermoplastic resins or thermoplastic elastomers such as low-density polyethylene, high-density polyethylene, polypropylene, poly-1-butene, poly-4-methyl-1-pentene, or ethylene, Polyolefin resins such as random or block copolymers of α-olefins such as propylene, 1-butene and 4-methyl-1-pentene; ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, ethylene- Ethylene-vinyl compound copolymer resin such as vinyl chloride copolymer; Styrenic resin such as polystyrene, acrylonitrile-styrene copolymer, ABS, α-methylstyrene-styrene copolymer; polyvinyl alcohol, polyvinyl pyrrolidone, polychlorinated Vinyl, polyvinylidene chloride, vinyl chloride Nyl-vinylidene chloride copolymer, vinyl resins such as polyacrylic acid, polymethacrylic acid, polymethyl acrylate, polymethyl methacrylate; nylon 6, nylon 6-6, nylon 6-10, nylon 11, nylon 12, etc. Polyamide resin; Polyester resin such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate; Polycarbonate; Polyphenylene oxide; Cellulose derivatives such as carboxymethyl cellulose and hydroxyethyl cellulose; Starch such as oxidized starch, etherified starch, dextrin; and mixtures thereof A resin comprising, for example, is used.

  That is, the sealing material 30 is obtained by molding into a ring shape having a width corresponding to the sealing region Y, for example, by extrusion molding, injection molding, or the like using the above-described thermoplastic resin. By heat sealing (thermocompression bonding) in a state where the material 30 is sandwiched between the negative electrode substrate 10 and the transparent electrode substrate 1 arranged to face each other, the negative electrode substrate 10 and the transparent electrode substrate 1 are joined. Next, an injection tube is inserted into the sealing material 30, and an electrolyte solution for forming the electrolyte layer 20 is injected into the space between the two electrode substrates through the injection tube, whereby the structure shown in FIG. The dye-sensitized solar cell can be obtained.

  When a transparent resin film or the like is used as the transparent substrate 3, for example, the negative electrode substrate 10 and the transparent electrode substrate 1 are sealed with a sealing agent 30, and then filled with an electrolyte solution from an unsealed opening, Finally, the dye-sensitized solar cell having the structure shown in FIG. 3 can also be produced by completely sealing the opening with the sealant 30.

  In the dye-sensitized solar cell thus formed, the dye carried on the porous photoelectric conversion layer 13 (semiconductor porous layer 13a) is excited by irradiating visible light from the transparent electrode substrate 1 side. , The transition from the ground state to the excited state, the excited dye electrons are injected into the conduction band in the photoelectric conversion layer 13, and an external circuit (not shown) through the metal electrode substrate 10 (metal substrate 11). It moves to the transparent electrode substrate 1 through. The electrons that have moved to the transparent electrode substrate 1 are carried by the ions in the electrolyte layer 20 and return to the pigment. By repeating such a process, electric energy is extracted and electric power is generated. That is, in such a solar cell, since the reverse electron prevention layer 13b that functions as a rectifying barrier is formed in a region facing the metal substrate 11 in the porous photoelectric conversion layer 13, a reverse current is effectively prevented and high conversion is achieved. Efficiency can be obtained.

  Moreover, the porous photoelectric conversion layer 13 having the two-layer structure as described above is formed by one-step coating. That is, the reverse electron blocking layer 13b existing on the surface of the metal substrate 11 and the porous oxide semiconductor layer 13a thereon are integrally formed by one-step coating, and therefore, when forming the porous oxide semiconductor layer 13a. Defects such as pinholes due to heat treatment do not occur in the reverse electron prevention layer 13b. For example, even if the surface of the metal substrate 11 is a rough surface having a large surface roughness, in the power generation region X, the surface is It is completely covered with the reverse electron prevention layer 13b, so that the surface of the metal substrate 11 is not exposed and does not directly contact the porous oxide semiconductor layer 13a. In addition, the reverse electron prevention layer 13b is formed of a metal oxide having resistance to the electrolyte. Therefore, corrosion of the metal substrate 11 by the electrolyte solution from the electrolyte layer 20 can be surely prevented, and this dye-sensitized solar cell exhibits extremely high durability and effectively prevents a decrease in conversion efficiency over time. ing.

  Furthermore, in the above solar cell, since the porous photoelectric conversion layer 13 composed of the reverse electron prevention layer 13b and the porous oxide semiconductor layer 13a can be formed by a single coating, the productivity is extremely high.

In the example described above, the case where the porous photoelectric conversion layer formed of the coating composition is formed on the surface of the metal substrate has been described as an example. However, such a porous photoelectric conversion layer is limited to the above example. Of course, it may be formed on the surface of the transparent electrode substrate, for example.
Moreover, it is also possible to apply | coat the coating composition of this invention on the reverse electricity prevention layer on the metal substrate currently formed by the well-known means, and to form a porous photoelectric converting layer. Moreover, after forming a porous photoelectric converting layer using the coating composition of this invention, of course, it is also possible to form a porous photoelectric converting layer further on this by a conventionally well-known means.

The excellent effects of the present invention will be described in the following experimental examples.
<Example 1>
(Preparation of oxide semiconductor layer forming paste)
Two types of commercially available TiO ₂ particles having a spherical particle size of 30 nm and a polyhedral (irregular shape) particle size of 15 nm are used as a main agent, ethanol is used in an amount of 70% by weight in the paste, and acetic acid is used as a dispersant in the paste in an amount of 0.0%. A TiO ₂ paste containing 05 wt% was prepared.

(Preparation of reverse electron prevention layer forming paste)
Titanium tetraisopropoxide is the main agent, terpineol and ethylcellulose are mixed in a 2/98 weight ratio as a solvent, and butyl cellosolve is mixed at a concentration of 3% by weight as a stabilizer. %, A paste for a reverse electron prevention layer was prepared.

  The paste for forming an oxide semiconductor and the paste for forming a reverse electron prevention layer were mixed while being stirred to prepare a mixed paste.

  Next, a commercially available aluminum plate (thickness 0.3 mm) is prepared as a metal substrate, and the above-prepared mixed paste is applied to the aluminum plate, and then baked at 450 ° C. for 30 minutes to prevent reverse electrons. A layer and an oxide semiconductor layer were manufactured. When the cross sections of the anti-electrostatic layer and the oxide semiconductor layer thus formed were observed with a HAADF image by a transmission electron microscope (TEM), the thickness of each layer was about 200 nm for the anti-electron prevention layer. The layer was about 10 μm. In addition, it was confirmed that the titanium dioxide particles forming the oxide semiconductor layer dig into the dense back-prevention layer underneath.

Further, the oxide semiconductor layer was immersed in a dye solution composed of a ruthenium complex dye dispersed in ethanol having a purity of 99.5% for 24 hours, and then dried to obtain a negative electrode structure. The ruthenium complex dye used is represented by the following formula.
[Ru (dcbpy) ₂ (NCS) ₂ ] · 2H ₂ O
On the other hand, a counter electrode (positive electrode) structure composed of an ITO / PEN film deposited with platinum was prepared.

A dye-sensitized solar cell was manufactured by sandwiching an electrolyte solution between the counter electrode structure and the negative electrode structure prepared above. As the electrolyte _solution, used was added 4-tert-butylpyridine LiI / I 2 a (0.5 mol / 0.025 mol) to those dissolved in methoxypropionitrile.
The obtained battery was stored in a room temperature environment and checked after 1000 hours. As a result, corrosion was not developed and there was no decrease in conversion efficiency.

<Example 2>
An electrode and a dye-sensitized solar cell were prepared under the same conditions as in Example 1 except that butyl cellosolve was used instead of acetic acid as a dispersant for the oxide semiconductor layer forming paste.
When the cross section of the electrode was observed with TEM, the reverse electron blocking layer was about 150 nm and the oxide semiconductor layer was about 10 μm. In addition, it was confirmed that the titanium dioxide particles forming the oxide semiconductor layer dig into the dense back-prevention layer underneath.
Furthermore, when the obtained battery was stored in a room temperature environment and confirmed after 1000 hours, corrosion was not developed and there was no decrease in conversion efficiency.

<Example 3>
An electrode and a dye-sensitized solar cell were prepared under the same conditions as in Example 2 except that trimethylacetic acid was used in place of butyl cellosolve as a dispersant for the paste for forming the oxide semiconductor layer.
When the cross section of the electrode was observed with TEM, the reverse electron blocking layer was about 150 nm and the oxide semiconductor layer was about 10 μm. In addition, it was confirmed that the titanium dioxide particles forming the oxide semiconductor layer dig into the dense back-prevention layer underneath.
Furthermore, when the obtained battery was stored in a room temperature environment and confirmed after 1000 hours, corrosion was not developed and there was no decrease in conversion efficiency.

<Example 4>
An electrode under the same conditions as in Example 2 except that a mixture of acetic acid and distilled water (acetic acid / distilled water = 50/50 wt%) was used instead of butyl cellosolve as a dispersant for the paste for forming the oxide semiconductor layer. A dye-sensitized solar cell was prepared.
When the cross section of the electrode was observed with TEM, the reverse electron blocking layer was about 150 nm and the oxide semiconductor layer was about 10 μm. In addition, it was confirmed that the titanium dioxide particles forming the oxide semiconductor layer dig into the dense back-prevention layer underneath.
Furthermore, when the obtained battery was stored in a room temperature environment and confirmed after 1000 hours, corrosion was not developed and there was no decrease in conversion efficiency.

<Example 5>
An electrode and a dye-sensitized solar cell were prepared under the same conditions as in Example 1 except that acetylacetone was used in place of butyl cellosolve as a dispersant for the oxide semiconductor layer forming paste.
When the cross section of the electrode was observed with TEM, the reverse electron blocking layer was about 150 nm and the oxide semiconductor layer was about 10 μm. In addition, it was confirmed that the titanium dioxide particles forming the oxide semiconductor layer dig into the dense back-prevention layer underneath.
Furthermore, when the obtained battery was stored in a room temperature environment and confirmed after 1000 hours, corrosion was not developed and there was no decrease in conversion efficiency.

<Example 6>
As a dispersant for the paste for forming the oxide semiconductor layer, a mixture of acetic acid and distilled water (acetic acid / distilled water = 50/50 wt%) was used instead of butyl cellosolve. Moreover, as a reverse electron prevention layer forming paste, a mixed solvent of terpineol and ethyl cellulose in a weight ratio of 2/98 is used as a solvent, and titanium tetraisopropoxide (main agent) and a dispersant (stabilizing component) are used as the mixed solvent. ) Was used as a mixture of propyl cellosolve (propyl cellosolve concentration: 3 wt%, titanium concentration: 0.5 wt%). An electrode and a dye-sensitized solar cell were prepared under the same conditions as in Example 2 except that this oxide semiconductor layer forming paste and the reverse electron blocking layer forming paste were used.
When the cross section of the electrode was observed with TEM, the reverse electron blocking layer was about 150 nm and the oxide semiconductor layer was about 10 μm. In addition, it was confirmed that the titanium dioxide particles forming the oxide semiconductor layer dig into the dense back-prevention layer underneath.
Furthermore, when the obtained battery was stored in a room temperature environment and confirmed after 1000 hours, corrosion was not developed and there was no decrease in conversion efficiency.

<Example 7>
As a dispersant for the paste for forming the oxide semiconductor layer, a mixture of acetic acid and distilled water (acetic acid / distilled water = 50/50 wt%) is used instead of butyl cellosolve. An electrode and a dye-sensitized solar cell were prepared under the same conditions as in Example 2 except that the butyl cellosolve concentration was changed to 0.01 wt% as the (stabilizing component).
When the cross section of the electrode was observed with TEM, the reverse electron blocking layer was about 150 nm and the oxide semiconductor layer was about 10 μm. In addition, it was confirmed that the titanium dioxide particles forming the oxide semiconductor layer dig into the dense back-prevention layer underneath.
Furthermore, when the obtained battery was stored in a room temperature environment and confirmed after 1000 hours, corrosion was not developed and there was no decrease in conversion efficiency.

<Example 8>
As a dispersant for the paste for forming the oxide semiconductor layer, a mixture of acetic acid and distilled water (acetic acid / distilled water = 50/50 wt%) is used instead of butyl cellosolve. An electrode and a dye-sensitized solar cell were prepared under the same conditions as in Example 2 except that the concentration of butyl cellosolve was 20% by weight as (stabilizing component).
When the cross section of the electrode was observed with TEM, the reverse electron blocking layer was about 150 nm and the oxide semiconductor layer was about 10 μm. In addition, it was confirmed that the titanium dioxide particles forming the oxide semiconductor layer dig into the dense back-prevention layer underneath.
Furthermore, when the obtained battery was stored in a room temperature environment and confirmed after 1000 hours, corrosion was not developed and there was no decrease in conversion efficiency.

<Example 9>
As a dispersant for the paste for forming the oxide semiconductor layer, a mixture of acetic acid and distilled water (acetic acid / distilled water = 50/50 wt%) is used instead of butyl cellosolve. An electrode and a dye-sensitized solar cell were prepared under the same conditions as in Example 2 except that the concentration of butyl cellosolve was 50% by weight as (stabilizing component).
When the cross section of the electrode was observed with TEM, the reverse electron blocking layer was about 150 nm and the oxide semiconductor layer was about 10 μm. In addition, it was confirmed that the titanium dioxide particles forming the oxide semiconductor layer dig into the dense back-prevention layer underneath.
Furthermore, when the obtained battery was stored in a room temperature environment and confirmed after 1000 hours, corrosion was not developed and there was no decrease in conversion efficiency.

<Comparative Example 1>
(Oxide semiconductor layer forming paste)
A paste for forming the oxide semiconductor layer was manufactured in the same manner as in Experimental Example 1.
(Reverse electron prevention layer forming paste)
The paste for forming the reverse electron prevention layer was prepared in the same manner as in Experimental Example 1.

Next, on the same aluminum plate as in Experimental Example 1, first, the paste for a reverse electron prevention layer prepared above was applied, dried at 120 ° C., and then a TiO ₂ paste was applied thereon, and then 450 The reverse electron blocking layer and the oxide semiconductor layer were prepared by baking at 30 ° C. for 30 minutes. The thickness of each layer was about 200 nm for the reverse electron blocking layer and about 10 μm for the oxide semiconductor layer. Thereafter, firing was performed in the same manner as in Experimental Example 1 to form a reverse electron prevention layer and an oxide semiconductor layer. When the film thickness of each layer was measured, the reverse electron prevention layer had a non-uniform film thickness of about 20 to 500 nm, and the oxide semiconductor layer had a thickness of about 10 μm.
Then, a battery was produced in the same manner as in Example 1, stored in a room temperature environment, and confirmed after 24 hours. As a result, corrosion occurred and the battery did not function substantially.
From the phenomenon in which corrosion has developed, this is considered to be due to the presence of exposed portions on the aluminum surface.

1: Transparent electrode substrate 3: Transparent substrate 5: Transparent conductive layer 10: Negative electrode substrate 11 (50): Metal substrate 13: Porous photoelectric conversion layer 13a (55): Porous oxide semiconductor layer 13b (53): Reverse electron Prevention layer 20: electrolyte layer

Claims (9)

A coating composition used to form a porous photoelectric conversion layer on an electrode substrate,
Including a metal oxide, a metal compound capable of forming an oxide by heat treatment, a dispersant and an organic solvent,
The metal oxide is present as dispersed particles, the metal compound is present as a solute ,
The dispersant contains a first dispersant for dispersing the metal oxide and a second dispersant for solute stabilization of the metal compound . .   The coating composition according to claim 1, wherein the metal oxide is an oxide of a metal selected from the group consisting of titanium, zinc, and zirconium.   The coating composition according to claim 1 or 2, wherein the metal compound is an alkoxide or chloride of a metal selected from the group consisting of titanium, zinc and zirconium. The coating composition according to claim 1 , wherein the first dispersant is at least one selected from the group consisting of acetic acid, trimethylacetic acid, glycol ether, β-diketone, and water. The coating composition according to claim 1, wherein the second dispersant is a glycol ether. The coating composition according to any one of claims 1 to 5 , wherein the organic solvent is at least one selected from the group consisting of lower alcohols having 4 or less carbon atoms, ethyl cellulose, and terpineol. The coating composition according to any one of claims 1 to 6 , wherein the metal compound is contained in an amount of 0.01 to 50% by weight per metal oxide in terms of metal. In terms of metal, the first dispersant is contained in an amount of 0.01 to 50% by weight per the metal oxide, and the second dispersant is 0.01 to 50% by weight per the metal compound. The coating composition according to claim 1 , which is contained in an amount. A coating composition according to any one of claims 1 to 8 is applied to an electrode substrate, and then heat-treated to form a reverse electron blocking layer formed on the electrode substrate surface and a porous layer formed on the reverse electron blocking layer. A porous photoelectric conversion layer comprising a porous oxide semiconductor layer is formed in a single step.

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