JP7693364B2 - Metal oxide paste composition for photoelectric conversion element and method for producing same - Google Patents

Description

The present invention relates to a metal oxide paste composition for photoelectric conversion elements and a method for producing the same.

The most common method for producing negative electrodes for photoelectric conversion elements such as dye-sensitized solar cells and perovskite solar cells is to apply a metal oxide paste composition such as titanium oxide onto a conductive substrate, sinter it, and then load the dye onto the substrate.

There is a demand for metal oxide paste compositions with higher performance (higher conversion efficiency) and better coatability to produce homogeneous films. In this case, the solvent that constitutes the metal oxide paste composition is usually a solvent having an alcoholic OH group, such as α-terpineol (Patent Document 1, etc.).

However, when small-particle metal oxide nanoparticles are used to increase the amount of dye carried, conventional metal oxide paste compositions use solvents with alcoholic OH groups as described above, which creates the problem that the paste is prone to cracking after firing.

To solve this problem of cracks, a method for producing a metal oxide paste composition that is simple and has a low environmental impact is known in which water, an acid such as nitric acid, a surface modifier such as acetylacetone, a surfactant such as Triton X-100, and a polymer binder such as polyethylene glycol are added to the metal oxide, and the mixture is homogenized using a ball mill or the like (Non-Patent Document 1).

JP 2004-087202 A

Mie Prefectural Science and Technology Promotion Center Industrial Research Department Research Report (29), 24-28, 2004

However, to produce a water-based metal oxide paste composition with high energy conversion efficiency, metal oxide nanoparticles synthesized by hydrothermal synthesis are often used, but ethyl cellulose, which is most often used as a binder, is not soluble in water. Therefore, the solvent is replaced with α-terpineol, but since α-terpineol is a water-insoluble solvent, a method is used in which water is replaced with ethanol, and then ethanol is replaced with α-terpineol. In this way, while multiple solvent replacements are performed, metal oxide nanoparticles synthesized by hydrothermal synthesis have very small particles and good dispersibility, so solvent replacement by filtration is impossible, and solvent replacement by centrifugation is difficult to scale up.

On the other hand, the above-mentioned method of producing a water-based metal oxide paste composition allows the production of a metal oxide paste composition directly from the composition synthesized by hydrothermal synthesis, but it has problems such as the low boiling point of water, which makes the state prone to change during storage or printing, such as an increase in viscosity, the tendency for the composition to become non-uniform during storage, the harmfulness of nitric acid, and corrosion of the surroundings, and therefore cannot be used industrially.

The present invention aims to provide a metal oxide paste composition that can be produced using a simple method with low environmental impact, is less likely to crack during firing, is less likely to become non-uniform during storage, and can form electrodes with high energy conversion efficiency.

As a result of intensive research in consideration of the above object, the inventors have found that the above problem can be solved by using a paste composition using specific amounts of a metal oxide, a water-soluble or water-dispersible binder, and a specific organic solvent, and that it is possible to form an electrode that is easy to prepare with a low environmental impact method, is less likely to crack during firing, is less likely to become non-uniform during storage, and has high energy conversion efficiency. After further research, the present invention has been completed. In other words, the present invention includes the following configurations.

Item 1. A paste composition for a photoelectric conversion element, comprising a metal oxide, a binder, and an organic solvent,
the binder comprises a water-soluble or water-dispersible binder;
The organic solvent includes a water-soluble organic solvent having a boiling point of 120° C. or higher, and
the content of the metal oxide is 5 to 25 mass % and the content of the organic solvent is 60 to 92.5 mass % relative to 100 mass % of the total amount of the paste composition for photoelectric conversion elements;
The paste composition has a binder content of 30 to 70 mass % relative to 100 mass % of the total amount of the metal oxides.

Item 2. The paste composition for photoelectric conversion elements according to Item 1, wherein the binder is a water-soluble cellulose compound.

Item 3. The paste composition for photoelectric conversion elements according to item 1 or 2, wherein the binder is at least one selected from the group consisting of hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, methyl cellulose, carboxymethyl cellulose, sodium salt of carboxymethyl cellulose, potassium salt of carboxymethyl cellulose, ammonium salt of carboxymethyl cellulose, alginic acid, xanthan gum, tamarind seed gum, locust bean gum, gellan gum, pectin, carrageenan, guar gum, agar, karaya gum, succinoglycan, cellulose nanofiber, dextrin, chitosan, curdlan, agarose, dextran, glucan, glucomannan, xylan, and xyloglucan.

Item 4. A paste composition for photoelectric conversion elements according to any one of items 1 to 3, in which the number average molecular weight of the binder is 150,000 or less.

Item 5. The organic solvent is represented by the general formula (1):
R ¹ (OR ² ) _n OH
[In the formula, R ¹ represents a hydrogen atom, an alkyl group, or an acyl group. R ² represents an alkylene group. n represents an integer of 0 to 5. When n is 2 or more, multiple R ^2s may be the same or different.]
Item 5. The paste composition for photoelectric conversion elements according to any one of items 1 to 4, containing an organic solvent represented by the formula:

Item 6. The paste composition for photoelectric conversion elements according to any one of items 1 to 5, wherein the organic solvent contains at least one selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,3-propanediol, propylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, and monoalkyl derivatives and acetate esters thereof.

Item 7. The paste composition for photoelectric conversion elements according to any one of items 1 to 6, wherein the metal oxide contains titanium oxide and/or zinc oxide.

Item 8. A paste composition for photoelectric conversion elements according to any one of items 1 to 7, for use in dye-sensitized solar cells or perovskite solar cells.

Item 9. A paste composition for photoelectric conversion elements according to any one of items 1 to 8, which is for use in the negative electrode of a dye-sensitized solar cell or a perovskite solar cell.

Item 10. A method for producing a paste composition for photoelectric conversion elements according to any one of items 1 to 9, comprising:
(1) A production method comprising a step of simultaneously or sequentially mixing the aqueous dispersion containing the metal oxide, the binder, and the organic solvent.

Item 11. The manufacturing method according to item 10, in which the step (1) is a step of mixing the aqueous dispersion containing the metal oxide with the organic solvent and then mixing the mixture with the binder.

Item 12. Furthermore,
(2) The method according to item 10 or 11, further comprising a step of concentrating the dispersion obtained in the step (1).

Item 13. A negative electrode for a photoelectric conversion element, in which a porous coating film made of a dried product of the paste composition for a photoelectric conversion element described in any one of items 1 to 9 is formed on a conductive substrate.

Item 14. The negative electrode for a photoelectric conversion element according to Item 13, which is a negative electrode for a dye-sensitized solar cell or a perovskite solar cell.

Item 15. A photoelectric conversion element comprising the negative electrode for a photoelectric conversion element according to item 13 or 14.

Item 16. The photoelectric conversion element according to item 15, which is a dye-sensitized solar cell or a perovskite solar cell.

The present invention provides a metal oxide paste composition that can be produced using a simple method with low environmental impact, is less likely to crack during firing, is less likely to become non-uniform during storage, and can form electrodes with high energy conversion efficiency.

In this specification, "containing" is a concept that encompasses all of "comprise," "consist essentially of," and "consist of."

In addition, in this specification, when a numerical range is indicated as "A to B," it means A or more and B or less.

The following describes an embodiment of the present invention, but various changes in form and details are possible without departing from the spirit and scope of the claims.

1. Metal oxide paste composition for photoelectric conversion element The metal oxide paste composition for photoelectric conversion element of the present invention is a paste composition for photoelectric conversion element containing a metal oxide, a binder, and an organic solvent, wherein the binder contains a water-soluble or water-dispersible binder, the organic solvent contains a water-soluble organic solvent having a boiling point of 120°C or higher, and the content of the metal compound is 5 to 25 mass%, the content of the binder is 2 to 15 mass%, and the content of the solvent is 60 to 92.5 mass%, relative to 100 mass% of the total amount of the paste composition.

(1-1) Metal oxide The metal oxide is preferably an optical semiconductor. Specific examples include oxides of titanium, zinc, tin, etc., and oxides of titanium, zinc, etc. are preferred. In other words, titanium oxide, zinc oxide, tin oxide, etc. are preferred, titanium oxide, zinc oxide, etc. are more preferred, and titanium oxide is even more preferred.

In the present invention, "titanium oxide" does not only refer to titanium dioxide (TiO ₂ ), but also includes titanium trioxide (Ti ₂ O ₃ ); titanium monoxide (TiO); and titanium dioxide with oxygen deficiency, such as Ti ₄ O ₇ and Ti ₅ O _9. In addition, titanium oxide may contain groups other than Ti-O-Ti, such as terminal OH groups, which are partially derived from the synthesis of titanium oxide. Similarly, "zinc oxide" is a concept that includes not only ZnO but also zinc antimonate. Similarly, "tin oxide" is a concept that includes not only tin dioxide (SnO ₂ ) but also tin monoxide (SnO).

The above metal oxides can be used alone or in combination of two or more. In addition, the above metal oxides can be publicly known or commercially available products.

When titanium oxide is used, it is preferable that it contains anatase-type titanium oxide, which has high activity. More specifically, it is preferable that 50% or more, and particularly 70% or more of the titanium oxide is anatase-type titanium oxide. However, in addition to titanium oxide crystals such as rutile type and brookite type, amorphous titanium oxide, etc. may also be contained.

The size of the metal oxide is preferably a nano-size with an average particle diameter of 5 to 100 nm, i.e., metal oxide nanoparticles, and more preferably an average particle diameter of 10 to 50 nm. However, in order to scatter light, in addition to the metal oxide nanoparticles with an average particle diameter of 5 to 100 nm, metal oxide particles with an average particle diameter of 100 nm or more and metal oxides with an aspect ratio (fibrous or tubular metal oxides such as titanium oxide nanotubes and titanium oxide nanowires) may be contained. The size (average particle diameter) of the metal oxide is calculated from the specific surface area measured by the BET method.

The present invention is useful in that it can suppress cracking even when using small metal oxides (average particle size of about 5 to 100 nm) that tend to crack when the paste composition is dried or fired. For this reason, the present invention makes it possible to reduce the average particle size of the metal oxide in order to increase the amount of dye carried.

The content of the metal oxide in the metal oxide paste composition for photoelectric conversion elements of the present invention is 5 to 25% by mass, preferably 7 to 23% by mass, and more preferably 10 to 20% by mass, based on 100% by mass of the total amount of the paste composition for photoelectric conversion elements. If the content of the metal oxide is less than 5% by mass, the number of printing operations required to ensure the required film thickness increases, which is not efficient. If the content of the metal oxide exceeds 25% by mass, the viscosity increases too much, making printing difficult. When multiple metal oxides are used, it is preferable to adjust the total content so that it is within the above range. When diluting the paste for perovskite solar cells, the concentration of the metal oxide after dilution is not limited to the above and may be low.

(1-2) Binder The metal oxide paste composition for photoelectric conversion elements of the present invention contains a binder that increases the viscosity of the metal oxide paste composition for photoelectric conversion elements to impart thixotropy, and that promotes porosity by providing voids between particles after firing, thereby suppressing cracking during firing.

As described above, when using a binder that is not soluble in water, such as ethyl cellulose, it is necessary to perform solvent replacement multiple times to replace the solvent with α-terpineol. However, in the case of a metal oxide paste composition, solvent replacement by filtration is impossible, and solvent replacement by centrifugation is difficult to scale up. Therefore, in the present invention, a water-soluble or water-dispersible binder is used. In particular, since the metal oxide paste composition for photoelectric conversion elements of the present invention also contains an organic solvent (a water-soluble organic solvent with a boiling point of 120°C or higher), it is preferable that the binder used in the present invention is soluble or dispersible in an organic solvent (organic solvent soluble or organic solvent dispersible).

In this specification, "water-soluble" or "soluble in organic solvent" means the property of dissolving in water or organic solvent, respectively, and specifically means that a homogeneous and transparent state can be maintained when added to water or organic solvent at room temperature (25°C) at 10 g/L or more. However, in this case, it does not mean that it is homogeneous at the molecular level. For example, cellulose derivatives that are generally considered to be "water-soluble" become transparent and homogeneous when added to water and stirred, but in some cases they are nanofiber-like when observed under an electron microscope.
The water solubility of an organic solvent means that it is uniformly mixed with water in any ratio.

In addition, in this specification, "water-dispersible" or "organic solvent-dispersible" means the property of being able to be emulsified or dispersed in water or an organic solvent and stabilized in a fine particle state, respectively. Specifically, this means that when 1 part by mass of the binder is added to 100 parts by mass of water at room temperature (25°C), no precipitation is observed even after leaving to stand for 1 hour or more. This includes the above-mentioned water solubility and organic solvent solubility, but does not necessarily have to be uniform at the molecular level and may be cloudy.

Among such binders, from the viewpoint of affinity with metal oxides, etc., binders having hydroxyl groups (preferably polymer compounds having hydroxyl groups, in particular polysaccharides having hydroxyl groups) are preferred.

Specific examples of binders having such properties include polysaccharides having hydroxyl groups, such as hydroxypropyl cellulose, hydroxypropyl methylcellulose, hydroxyethyl cellulose, hydroxyethyl methylcellulose, methylcellulose, carboxymethyl cellulose, sodium salt of carboxymethyl cellulose, potassium salt of carboxymethyl cellulose, ammonium salt of carboxymethyl cellulose, alginic acid, xanthan gum, tamarind seed gum, locust bean gum, gellan gum, pectin, carrageenan, guar gum, agar, karaya gum, succinoglycan, cellulose nanofiber, dextrin, chitosan, curdlan, agarose, dextran, glucan, glucomannan, xylan, and xyloglucan.

The above-mentioned binders can be used alone or in combination of two or more. The above-mentioned binders can be used alone or in combination of two or more. The above-mentioned metal oxides can be publicly known or commercially available products.

Among these, in terms of affinity with metal oxides, ease of decomposition during firing, thixotropy, and the like, water-soluble cellulose compounds such as hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, methyl cellulose, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, and ammonium carboxymethyl cellulose are preferred, with hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, and methyl cellulose being more preferred, with hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, and hydroxyethyl methyl cellulose being even more preferred, and with hydroxypropyl cellulose, hydroxypropyl methyl cellulose, and hydroxypropyl methyl cellulose being particularly preferred in terms of affinity with organic solvents described below.

When using these water-soluble cellulose compounds, the degree of substitution is not particularly limited, but is preferably 0.3 to 5, and more preferably 0.5 to 4, from the viewpoints of water solubility, solubility in organic solvents, affinity with metal oxides, ease of decomposition during firing (resistance to cracking), thixotropy, energy conversion efficiency, etc.

The number average molecular weight of the binder is not particularly limited, and the optimal number average molecular weight varies depending on the type of binder and the required viscosity. However, from the viewpoint of easily suppressing unburned residue during firing, it is preferably 150,000 or less, and more preferably 120,000 or less. There is no particular lower limit for the number average molecular weight of the binder, and it is usually about 10,000.

The content of the binder in the metal oxide paste composition for photoelectric conversion elements of the present invention is 30 to 70% by mass, preferably 40 to 65% by mass, and more preferably 45 to 60% by mass, based on 100% by mass of the total amount of the metal oxide. If the ratio of the binder to the metal oxide is less than 30% by mass, the pores are not sufficiently formed, and the viscosity and thixotropy required for printing cannot be ensured. If the ratio of the binder to the metal oxide exceeds 70% by mass, the pores are too large, the porous structure after firing becomes weak, and the strength cannot be maintained, the surface area of titania per electrode area cannot be ensured, and the current during power generation decreases. When multiple binders are used, it is preferable to adjust the total content relative to the metal oxide to be within the above range.

(1-3) Organic Solvent The organic solvent used in the paste composition for photoelectric conversion elements is preferably a solvent that is water-soluble and has a high affinity with metal oxides (titanium oxide, etc.) and that can dissolve the additives described below.

From this perspective, in the present invention, a water-soluble organic solvent with a boiling point of 120°C or higher is used as the organic solvent.

The boiling point of the organic solvent is 120°C or higher to prevent drying, viscosity increase, and non-uniformity during use and storage, preferably 140°C or higher from the viewpoint of facilitating solvent replacement from water to organic solvent without filtration or centrifugation, and more preferably 160°C or higher from the viewpoint of facilitating solvent replacement by removing water without losing organic solvent. The upper limit of the boiling point of the organic solvent is not particularly limited, but is usually 450°C.

The lower the vapor pressure of the organic solvent, the less likely it is to volatilize, and the less change over time in the metal oxide paste composition, which is preferable. Specifically, the vapor pressure at 20°C is preferably 0.05 mmHg or less, and more preferably 0.01 mmHg or less. There is no particular lower limit for the vapor pressure of the organic solvent at 20°C, but it is usually about 0.00001 mmHg.

From the viewpoint of water solubility, it is preferable for such organic solvents to have a hydroxyl group, and from the viewpoint of preventing excessively strong binding to the metal oxide surface and making it easier to improve energy conversion efficiency, it is preferable for the organic solvents to have only one hydroxyl group.

Such an organic solvent is represented by the general formula (1):
R ¹ (OR ² ) _n OH
[In the formula, R ¹ represents a hydrogen atom, an alkyl group, or an acyl group. R ² represents an alkylene group. n represents an integer of 0 to 5. When n is 2 or more, multiple R ^2s may be the same or different.]
Preferred is an organic solvent represented by the formula:

In general formula (1), examples of the alkyl group represented by R ¹ include linear or branched alkyl groups having 1 to 6 carbon atoms (particularly 1 to 4 carbon atoms), such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group.

In the general formula (1), examples of the acyl group represented by R ¹ include acyl groups having 1 to 6 carbon atoms (particularly 1 to 4 carbon atoms), such as formyl, acetyl and propionyl groups.

In general formula (1), from the viewpoints of water solubility, affinity with metal oxides, ease of decomposition during firing (resistance to cracking), thixotropy, energy conversion efficiency, and the like, R ¹ is preferably a hydrogen atom, a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an acetyl group, or the like, and more preferably a hydrogen atom, a methyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an acetyl group, or the like.

In the general formula (1), examples of the alkylene group represented by R ² include linear or branched alkylene groups having 2 to 10 carbon atoms (particularly 2 to 6), such as ethylene, trimethylene, propylene, propylidene, isopropylidene, tetramethylene, 1-methyltrimethylene, 2-methyltrimethylene, 3-methyltrimethylene, 1,1-dimethylethylene, 1,2-dimethylethylene, 1-ethylethylene, 2-ethylethylene, and pentamethylene. Among these, linear alkylene groups having 2 to 10 carbon atoms (particularly 2 to 6), such as ethylene, trimethylene, tetramethylene, and pentamethylene, are preferred from the viewpoints of water solubility, affinity with metal oxides, ease of decomposition during firing (resistance to cracking), thixotropy, and energy conversion efficiency.

When n described below is an integer of 2 or more, a plurality of R ² may be the same or different, but it is convenient that a plurality of R ² are the same.

In general formula (1), from the viewpoints of water solubility, affinity with metal oxides, ease of decomposition during firing (resistance to cracking), thixotropy, energy conversion efficiency, etc., n is preferably an integer from 0 to 5, more preferably an integer from 1 to 4, and even more preferably an integer from 1 to 3.

Specific examples of such organic solvents include alcohol compounds having 1 to 6 carbon atoms (particularly 1 to 5, and further 1 to 4), such as methanol, ethanol, and isopropyl alcohol; glycol compounds such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, butanediol, and pentanediol, or monoalkyl derivatives (monoalkyl ethers) or acetates thereof.

In the monoalkyl derivatives (monoalkyl ethers) of glycol compounds, examples of the alkyl group include those mentioned above. In other words, examples of the monoalkyl derivatives (monoalkyl ethers) of glycol compounds include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, tetraethylene glycol monopropyl ether, tetraethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, Examples of the propylene glycol monobutyl ether include propylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monoethyl ether, tripropylene glycol monopropyl ether, tripropylene glycol monobutyl ether, tetrapropylene glycol monomethyl ether, tetrapropylene glycol monoethyl ether, tetrapropylene glycol monopropyl ether, tetrapropylene glycol monobutyl ether, butanediol monomethyl ether, butanediol monoethyl ether, butanediol monopropyl ether, butanediol monobutyl ether, pentanediol monomethyl ether, pentanediol monoethyl ether, pentanediol monopropyl ether, and pentanediol monobutyl ether.

Examples of acetate esters of glycol compounds include ethylene glycol acetate, diethylene glycol acetate, triethylene glycol acetate, tetraethylene glycol acetate, propylene glycol acetate, dipropylene glycol acetate, tripropylene glycol acetate, tetrapropylene glycol acetate, butanediol acetate, and pentanediol acetate.

As described above, the organic solvent represented by the general formula (1) is preferred, but there are also organic solvents that have sufficient water solubility even without a hydroxyl group, such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and these organic solvents can also be preferably used.

The above organic solvents can be used alone or in combination of two or more. In addition, the above organic solvents can be publicly known or commercially available products.

Among these, from the viewpoints of water solubility, affinity with metal oxides, ease of decomposition during firing (resistance to cracking), thixotropy, energy conversion efficiency, etc., preferred organic solvents are diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, triethylene glycol monobutyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, butanediol, pentanediol, etc., and more preferred are diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, etc.

In the present invention, the content of the organic solvent is 60 to 92.5% by mass, preferably 65 to 90% by mass, based on 100% by mass of the total amount of the metal oxide paste composition for photoelectric conversion elements of the present invention. If the concentration of the organic solvent is less than 60% by mass, the viscosity may be too high and the composition may not be suitable for screen printing, and the pores may become small, resulting in a decrease in energy conversion efficiency. If the concentration of the organic solvent exceeds 92.5% by mass, the solid concentration is too low, resulting in poor film formation efficiency, and the pores may become large and prone to cracking. However, when forming a film with a thickness of several hundred nm by spin coating as the negative electrode of a perovskite solar cell, the content of the organic solvent can be reduced.

In the present invention, the inclusion of a small amount of water as a solvent is not excluded, but from the viewpoint of suppressing changes in performance over time due to evaporation during drying or firing, it is preferable that the water content is as small as possible. For this reason, the content of the above-mentioned organic solvent is preferably 95 to 100% by mass, and more preferably 98 to 100% by mass, with the total amount of the solvent being 100% by mass.

(1-4) Other Components In addition, additives that are usually blended in metal oxide paste compositions for photoelectric conversion elements such as dye-sensitized solar cells and perovskite solar cells, such as dispersants for dispersing metal oxides, thickeners for increasing the viscosity of the metal oxide paste composition for photoelectric conversion elements to provide thixotropy, leveling agents for increasing the smoothness of the coating film, and other organic solvents compatible with the organic solvents described above can be added to the metal oxide paste composition for photoelectric conversion elements of the present invention, within a range that does not impair the effects of the present invention.

2. Manufacturing Method The manufacturing method of the metal oxide paste composition for photoelectric conversion elements of the present invention is not particularly limited, but may be
(1) It is preferable to include a step of simultaneously or sequentially mixing the aqueous dispersion containing the metal oxide, the binder, and the organic solvent.

In step (1), the process method is not particularly limited and can be carried out according to conventional methods.

There is no particular restriction on the order of mixing, and they may be mixed simultaneously or sequentially. However, it is preferable to add the binder later, since the viscosity increases after the binder is added. In other words, it is preferable that step (1) is a step in which the aqueous dispersion containing the metal oxide is mixed with the organic solvent, and then mixed with the binder.

In order to prevent the binder from forming lumps, it is preferable to dissolve the binder in a solvent beforehand. Solvents that can be used in this case include water, the organic solvents listed above, and solvents that are compatible with both water and the organic solvents listed above. However, it is also possible to add the binder without dissolving it in a solvent beforehand.

In addition, in consideration of workability, it is also preferable to prepare a dispersion liquid having a low concentration of the metal oxide once and then concentrate it.
(2) It is preferable to include a step of concentrating the dispersion obtained in the step (1).

The concentration method is not particularly limited, and can be carried out, for example, using an evaporator.

3. Negative Electrode When forming an electrode (particularly a negative electrode) for a photoelectric conversion element such as a dye-sensitized solar cell or a perovskite solar cell, it is preferable to form a porous coating film obtained by using the metal oxide paste composition for a photoelectric conversion element of the present invention on a resin substrate or a glass substrate.

The resin substrate is not particularly limited as long as it is a conductive resin substrate, but examples include polyesters such as polyethylene naphthalate resin substrate (PEN resin substrate) and polyethylene terephthalate resin substrate (PET resin substrate); polyamide; polysulfone; polyethersulfone; polyetheretherketone; polyphenylene sulfide; polycarbonate; polyimide; polymethyl methacrylate; polystyrene; cellulose triacetate; polymethylpentene, etc.

There are no particular limitations on the glass substrate, and any known or commercially available glass can be used, including colorless or colored glass, wired glass, glass blocks, etc.

The resin or glass substrate can have a thickness of about 0.05 to 10 mm.

In the present invention, the porous coating film may be formed directly on the surface of the resin substrate or the glass substrate, or may be formed via a transparent conductive film.

Examples of transparent conductive films include tin-doped indium oxide film (ITO film), fluorine-doped tin oxide film (FTO film), antimony-doped tin oxide film (ATO film), aluminum-doped zinc oxide film (AZO film), and gallium-doped zinc oxide film (GZO film). By passing through these transparent conductive films, it becomes easy to extract the generated current to the outside. The film thickness of these transparent conductive films is preferably about 0.02 to 10 μm.

The electrode (particularly the negative electrode) of the present invention may have the following two forms, for example:

(3-1) Aspect 1
A porous coating film obtained by using the metal oxide paste composition for photoelectric conversion elements of the present invention is formed on a resin substrate or a glass substrate via a transparent conductive film, and can be used as an electrode for photoelectric conversion elements of the present invention, such as dye-sensitized solar cells and perovskite solar cells. The resin substrate, glass substrate, and transparent conductive film are as described above.

Specifically, the electrodes for photoelectric conversion elements such as dye-sensitized solar cells and perovskite solar cells of the present invention can be formed as follows.

First, a transparent conductive film is formed on a resin or glass substrate by vacuum deposition, ion plating, CVD, sputtering, sol-gel, nanoparticle composite, or the like. The surface resistance of the substrate thus obtained is preferably 50 Ω/□ or less.

Then, it is preferable to apply the metal oxide paste composition for photoelectric conversion elements of the present invention thereon, and dry or bake it. When using a resin substrate, the heating conditions are preferably 150°C or less.

In this case, it is preferable to apply the coating so that the resulting coating film has a thickness of about 2 to 40 μm from the viewpoints of preventing cracking and adhesion to the substrate.

(3-2) Aspect 2
A porous coating film obtained by using the metal oxide paste composition for photoelectric conversion element of the present invention may be directly formed on a resin substrate or a glass substrate, and a porous metal film may be further formed thereon to form an electrode for photoelectric conversion element of the present invention, such as a dye-sensitized solar cell or a perovskite solar cell. The resin substrate and the glass substrate are as described above. When forming a porous coating film on a resin substrate or a glass substrate, the same method as that of the above-mentioned embodiment 1 may be adopted.

The porous metal film that can be used in aspect 2 is not particularly limited as long as it is a metal that is not corroded (does not react) with ions contained in the electrolyte, such as iodine ions and bromine ions, and examples include titanium, tungsten, platinum, and gold. By forming these porous metal films, it becomes easier to extract the generated current to the outside. The surface resistivity of these porous metal films is not particularly limited, but is preferably 10 Ω/□ or less, and the film thickness is also not particularly limited, but is preferably 150 nm or more.

A porous metal film can be formed on top of the porous coating film formed on a resin substrate or glass substrate by a thin film formation method such as sputtering.

4. Photoelectric conversion elements (dye-sensitized solar cells, perovskite solar cells, etc.)
The photoelectric conversion element of the present invention (particularly the dye-sensitized solar cell) can be obtained by forming a counter electrode (counter electrode) on the porous coating film of the electrode for a photoelectric conversion element such as the dye-sensitized solar cell or perovskite solar cell of the present invention, and filling the space between these electrodes with an electrolyte.

As the electrolyte, a high dielectric constant is preferable so that a larger amount of electrolytes such as iodine ions can be dissolved, and a low viscosity is preferable so that the dissolved ions can move easily. Such a solvent is not particularly limited, but one or more of the following can be used: nitrile compounds such as acetonitrile, propionitrile, glutarodinitrile, methoxyacetonitrile, and benzonitrile; carbonate compounds such as ethylene carbonate and propylene carbonate; ether compounds such as dioxane and diethyl ether; chain ethers such as ethylene glycol dialkyl ether, propylene glycol dialkyl ether, polyethylene glycol dialkyl ether, and polypropylene glycol dialkyl ether; alcohols such as methanol, ethanol, ethylene glycol monoalkyl ether, propylene glycol monoalkyl ether, polyethylene glycol monoalkyl ether, and polypropylene glycol monoalkyl ether; polyhydric alcohols such as ethylene glycol, propylene glycol, polyethylene glycol, polypropylene glycol, and glycerin; heterocyclic compounds such as 3-methyl-2-oxazolidinone; aprotic polar substances such as dimethyl sulfoxide and sulfolane; and water.

The counter electrode may be a single layer structure made of a conductive material, or may be composed of a conductive layer and a substrate. The substrate is not particularly limited, and the material, thickness, dimensions, shape, etc. can be appropriately selected according to the purpose. For example, metal, colorless or colored glass, wire glass, glass block, etc. can be used, and resin may also be used. Examples of such resins include polyesters such as polyethylene terephthalate, polyamides, polysulfones, polyethersulfones, polyetheretherketones, polyphenylene sulfide, polycarbonates, polyimides, polymethyl methacrylates, polystyrenes, cellulose triacetate, polymethylpentenes, etc. Alternatively, the counter electrode may be formed by applying, plating, or vapor-depositing (PVD, CVD) a conductive material directly onto the charge transport layer.

Conductive materials include metals such as platinum, gold, nickel, titanium, aluminum, copper, silver, and tungsten, as well as materials with low resistivity such as carbon materials and conductive organic substances.

A metal lead may also be used to reduce the resistance of the counter electrode. The metal lead is preferably made of a metal such as platinum, gold, nickel, titanium, aluminum, copper, silver, or tungsten, and is particularly preferably made of aluminum or silver.

In the present invention, it is preferable to support (adsorb, contain, etc.) a dye in the porous coating film before forming the counter electrode, for the purpose of improving the light absorption efficiency of the electrode of the present invention, etc.

The dye is not particularly limited as long as it has absorption characteristics in the visible or near infrared range and improves (sensitizes) the light absorption efficiency of the semiconductor layer, but metal complex dyes, organic dyes, natural dyes, semiconductors, etc. are preferred. In addition, in order to impart adsorption to the porous coating film, dyes having functional groups in the molecule such as carboxyl groups, hydroxyl groups, sulfonyl groups, phosphonyl groups, carboxylalkyl groups, hydroxyalkyl groups, sulfonylalkyl groups, and phosphonylalkyl groups are preferably used.

Examples of metal complex dyes that can be used include complexes of ruthenium, osmium, iron, cobalt, zinc, and mercury (e.g., mercury chrome, etc.), metal phthalocyanines, and chlorophyll. Examples of organic dyes include, but are not limited to, cyanine dyes, hemicyanine dyes, merocyanine dyes, xanthene dyes, triphenylmethane dyes, and metal-free phthalocyanine dyes. Semiconductors that can be used as dyes are preferably amorphous semiconductors with a large i-type light absorption coefficient, direct transition semiconductors, and fine particle semiconductors that exhibit quantum size effects and efficiently absorb visible light. Usually, various semiconductors, metal complex dyes, organic dyes, or two or more dyes can be mixed to widen the wavelength range of photoelectric conversion as much as possible and increase the conversion efficiency. In addition, the dyes to be mixed and their ratio can be selected to match the wavelength range and intensity distribution of the desired light source.

For example, the dye can be adsorbed onto the porous coating film by spraying or spin-coating a solution of the dye dissolved in a solvent onto the porous coating film, followed by drying. In this case, the substrate may be heated to an appropriate temperature. Alternatively, the porous coating film may be immersed in the solution to adsorb the dye. The immersion time is not particularly limited as long as the dye is sufficiently adsorbed, but is preferably 10 minutes to 30 hours, more preferably 1 to 20 hours. If necessary, the solvent or substrate may be heated during immersion. The dye concentration in the solution is about 1 to 1000 mmol/L, preferably about 10 to 500 mmol/L.

The solvent used is not particularly limited, but water and organic solvents are preferably used. Examples of organic solvents include alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and t-butanol; nitriles such as acetonitrile, propionitrile, methoxypropionitrile, and glutaronitrile; aromatic hydrocarbons such as benzene, toluene, o-xylene, m-xylene, and p-xylene; aliphatic hydrocarbons such as pentane, hexane, and heptane; alicyclic hydrocarbons such as cyclohexane; ketones such as acetone, methyl ethyl ketone, diethyl ketone, and 2-butanone; ethers such as diethyl ether and tetrahydrofuran; ethylene carbonate, propylene carbonate, nitromethane, dimethylformamide, and dimethylsulfate. sulfoxide, hexamethylphosphoamide, dimethoxyethane, gamma-butyrolactone, gamma-valerolactone, sulfolane, dimethoxyethane, adiponitrile, methoxyacetonitrile, dimethylacetamide, methylpyrrolidinone, dimethylsulfoxide, dioxolane, sulfolane, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, ethyl dimethyl phosphate, tributyl phosphate, tripentyl phosphate, trihexyl phosphate, triheptyl phosphate, trioctyl phosphate, trinonyl phosphate, tridecyl phosphate, tris(trifluoromethyl) phosphate, tris(pentafluoroethyl) phosphate, triphenyl phosphate, polyethylene glycol, polyethylene glycol, etc.

To reduce interactions such as aggregation between dyes, a colorless compound with surfactant properties may be added to the dye adsorption solution and co-adsorbed onto the porous coating film. Examples of such colorless compounds include steroid compounds and sulfonates, such as cholic acid, deoxycholic acid, chenodeoxycholic acid, and taurodeoxycholic acid, which have carboxyl or sulfo groups.

It is preferable to remove any unadsorbed dye by washing immediately after the adsorption step. Washing is preferably performed in a wet washing tank using acetonitrile, an alcohol-based solvent, or the like.

After the dye is adsorbed, the surface of the porous coating film may be treated with amines, quaternary ammonium salts, ureido compounds having at least one ureido group, silyl compounds having at least one silyl group, alkali metal salts, alkaline earth metal salts, etc. Preferred examples of amines include pyridine, 4-t-butylpyridine, polyvinylpyridine, etc. Preferred examples of quaternary ammonium salts include tetrabutylammonium iodide, tetrahexylammonium iodide, etc. These may be used by dissolving them in an organic solvent, or may be used as is if they are liquid.

Dye-sensitized solar cells can be manufactured by modularizing the photoelectric conversion elements obtained as described above and providing the required electrical wiring.

The above describes the application to dye-sensitized solar cells, but when applied to perovskite solar cells, the electrodes for photoelectric conversion elements of the dye-sensitized solar cells, perovskite solar cells, etc. of the present invention can also be formed in the usual manner, except that they are used as, for example, negative electrodes.

The present invention will be specifically explained based on examples, but the present invention is not limited to these.

Example 1
120 g (2 mol) of acetic acid was added to 142.1 g (0.5 mol) of titanium tetraisopropoxide, and the mixture was stirred for 15 minutes, and 550 g of water was added. The pH of this dispersion was 2.5. A large amount of translucent precipitate was generated, but when the mixture was stirred for 60 minutes and then heated at 60°C, all of the precipitate was dissolved.

After that, the reaction mixture was stirred at normal pressure (0.1 MPa) at 80°C for 5 hours, and water was added to the reaction mixture to make a total of 800 g, resulting in a translucent dispersion. This dispersion contained 40 g of titanium oxide.

200 g of this dispersion was placed in a titanium autoclave and reacted for 3 hours in a hot air oven at 220° C. The liquid was heated at 200° C. to recover titania nanoparticles, and the average particle size was calculated from the BET specific surface area (93 m ² /g) to be 16 nm. In addition, crystallinity was analyzed by X-ray diffraction and found to be 100% anatase.

40.6 g of 1,5-pentanediol was added to 200 g of this dispersion liquid, and ultrasonic dispersion was performed. Furthermore, 5 g of hydroxypropyl cellulose (molecular weight 100,000: hydroxypropyl cellulose 3.0-5.9 manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and ultrasonic dispersion was performed.

Then, water was distilled off at 60°C and 50 hPa to obtain 55 g of a white paste composition.

The appearance of the paste composition remained unchanged after one month.

This paste composition was applied onto a glass substrate (10Ω/□) on which a fluorine-doped tin oxide film (FTO film) had been formed, so that the thickness after drying would be 10 μm, and after baking at 550° C., it was immersed in 5×10 ⁻⁴ mol/L of Ru dye (N719) at 25° C. for 20 hours to prepare a negative electrode for a dye-sensitized solar cell. Note that after baking, it was confirmed by optical microscope that no cracks had occurred in the paste composition after baking.

This negative electrode was attached to a glass substrate (positive electrode) on which a tin-doped indium oxide film (ITO film) was formed by sputtering platinum, and an electrolyte (3-methoxypropionitrile containing 0.15 M iodine, 0.1 M lithium iodide, 0.5 M ethylmethylimidazolium iodide, and 0.5 M tert-butylpyridine) was sealed between the electrodes to produce a photoelectric conversion element (dye-sensitized solar cell).

The photoelectric conversion element (dye-sensitized solar cell) thus produced was irradiated with light having an intensity of 100 mW/ ^cm2 under the condition of AM1.5 using a solar simulator, and the photoelectric conversion characteristics were evaluated. As a result, the photoelectric conversion efficiency was 6.3%.

The same crack and photoelectric conversion efficiency measurements were performed in the following examples and comparative examples.

Example 2
The experiment was carried out in the same manner as in Example 1, except that 40.6 g of diethylene glycol monobutyl ether was used instead of 40.6 g of 1,5-pentanediol.

As a result, no cracks were generated after firing the paste composition, and a transparent coating film was obtained. In addition, the photoelectric conversion efficiency was 7.3%.

In addition, the appearance of the paste composition remained unchanged after one month.

Example 3
The experiment was carried out in the same manner as in Example 2, except that hydroxypropyl cellulose (molecular weight 100,000) was replaced with hydroxypropyl cellulose (molecular weight 60,000: hydroxypropyl cellulose 2.0-2.9 manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

As a result, no cracks were generated after firing the paste composition, and a transparent coating film was obtained. In addition, the photoelectric conversion efficiency was 6.7%.

In addition, the appearance of the paste composition remained unchanged after one month.

Example 4
The experiment was carried out in the same manner as in Example 2, except that hydroxypropyl cellulose (molecular weight 100,000) was replaced with hydroxypropyl cellulose (molecular weight 140,000: hydroxypropyl cellulose 6.0-10.0 manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

As a result, no cracks were generated after firing the paste composition, and a transparent coating film was obtained. In addition, the photoelectric conversion efficiency was 6.2%.

In addition, the appearance of the paste composition remained unchanged after one month.

Example 5
The experiment was carried out in the same manner as in Example 2, except that the amount of diethylene glycol monobutyl ether was changed from 40.6 g to 36.1 g.

As a result, no cracks were generated after firing the paste composition, and a transparent coating film was obtained. In addition, the photoelectric conversion efficiency was 6.9%.

In addition, the appearance of the paste composition remained unchanged after one month.

Example 6
The experiment was carried out in the same manner as in Example 2, except that the amount of diethylene glycol monobutyl ether was changed from 40.6 g to 33.4 g.

As a result, no cracks were generated after firing the paste composition, and a transparent coating film was obtained. In addition, the photoelectric conversion efficiency was 6.2%.

In addition, the appearance of the paste composition remained unchanged after one month.

Comparative Example 1
60 g of acetic acid, 100 g of ethanol, 73 g of α-terpineol, and 90 g of a 10% by mass ethanol solution of ethyl cellulose were added to 18 g of titania nanoparticles ST-01 (manufactured by Ishihara Sangyo Kaisha, Ltd.: average particle size 7 nm), and the mixture was concentrated at 40° C. and 70 hPa to obtain 100 g of a paste composition. An experiment similar to that in Example 1 was carried out using this paste composition.

As a result, cracks occurred and the paste composition became cloudy after firing. The photoelectric conversion efficiency was 5.8%.

After one week, the paste composition had become highly viscous, lost its smoothness, and became non-uniform.

Comparative Example 2
30 g of water, 0.6 g of acetylacetone, 0.1 g of surfactant (Triton X-100), and 9 g of polyethylene glycol (number average molecular weight 20,000: manufactured by Sigma-Aldrich) were added to 18 g of titania nanoparticles ST-21 (manufactured by Ishihara Sangyo Kaisha, Ltd.: average particle size 30 nm), and the mixture was allowed to penetrate using a paint shaker, resulting in a paste composition. Using this paste composition, an experiment similar to that in Example 1 was carried out.

As a result, no cracks were observed after the paste was fired, but it became cloudy. The photoelectric conversion efficiency was 5.7%.

After one month, the paste composition had become highly viscous, lost its smoothness, and became non-uniform.

In this way, by selecting the binder and solvent, it was possible to produce a metal oxide paste composition with high energy conversion efficiency. In addition, the composition also had superior stability over time compared to conventional water-based paste compositions.

Claims (16)

A paste composition used for a porous coating film formed on a conductive substrate in an electrode for a photoelectric conversion element, comprising:
The paste composition contains a metal oxide, a binder, and an organic solvent,
the binder comprises a water-soluble or water-dispersible binder;
The organic solvent includes a water-soluble organic solvent having a boiling point of 120° C. or higher, and
the content of the metal oxide is 5 to 25 mass % and the content of the water-soluble organic solvent is 60 to 92.5 mass % relative to 100 mass % of the total amount of the paste composition for photoelectric conversion elements;
The paste composition has a binder content of 30 to 70 mass % relative to 100 mass % of the total amount of the metal oxides. The paste composition of claim 1 , wherein the binder is a water-soluble cellulose compound. The binder is at least one selected from the group consisting of hydroxypropyl cellulose, hydroxypropyl methylcellulose, hydroxyethyl cellulose, hydroxyethyl methylcellulose, methylcellulose, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, ammonium carboxymethyl cellulose, alginic acid, xanthan gum, tamarind seed gum, locust bean gum, gellan gum, pectin, carrageenan, guar gum, agar, karaya gum, succinoglycan, cellulose nanofiber, dextrin, chitosan, curdlan, agarose, dextran, glucan, glucomannan, xylan and xyloglucan. The paste composition according to claim 1 or 2. The paste composition according to any one of claims 1 to 3, wherein the binder has a number average molecular weight of 150,000 or less. The organic solvent is represented by the general formula (1):
R ¹ (OR ² ) _n OH
[In the formula, R ¹ represents a hydrogen atom, an alkyl group, or an acyl group. R ² represents an alkylene group. n represents an integer of 0 to 5. When n is 2 or more, multiple R ^2s may be the same or different.]
The paste composition according to any one of claims 1 to 4, comprising an organic solvent represented by the formula: The paste composition according to any one of claims 1 to 5, wherein the organic solvent contains at least one selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,3-propanediol, propylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, and monoalkyl derivatives and acetate esters thereof. The paste composition according to any one of claims 1 to 6, wherein the metal oxide contains titanium oxide and/or zinc oxide. The paste composition according to any one of claims 1 to 7, which is for use in a dye-sensitized solar cell or a perovskite solar cell. The paste composition according to any one of claims 1 to 8, which is for use in a negative electrode of a dye-sensitized solar cell or a perovskite solar cell. 10. A method for producing a paste composition according to claim 1, comprising: (1) a step of simultaneously or sequentially mixing the aqueous dispersion containing the metal oxide, the binder, and the organic solvent. The manufacturing method according to claim 10, wherein the step (1) is a step of mixing the aqueous dispersion containing the metal oxide with the organic solvent, and then mixing the mixture with the binder. moreover,
The method according to claim 10 or 11, further comprising: (2) a step of concentrating the dispersion obtained in the step (1). 10. A negative electrode for a photoelectric conversion element, comprising a porous coating film made of a dried product of the paste composition according to claim 1 formed on a conductive substrate. The negative electrode for a photoelectric conversion element according to claim 13, which is a negative electrode for a dye-sensitized solar cell or a perovskite solar cell. A photoelectric conversion element comprising the negative electrode for a photoelectric conversion element according to claim 13 or 14. The photoelectric conversion element according to claim 15, which is a dye-sensitized solar cell or a perovskite solar cell.