Metal Oxide Paste Composition for Photoelectric Conversion Element and Method for Producing the Same

Abstract

To provide a metal oxide paste composition which can be fabricated by a method easy and simple and low in environment load, which is hard to cause a crack in a sintering process, and which hardly becomes nonuniform while being stored, and which enables formation of an electrode high in energy conversion efficiency.SOLUTION: A paste composition for a photoelectric conversion device comprises a metal oxide, a binder and an organic solvent. The binder contains a water-soluble or water-dispersible binder. The organic solvent contains a water-soluble organic solvent having a boiling point 120°C or above. To 100 mass% of a total amount of the paste composition for a photoelectric conversion device, the content of the metal oxide is 5-25 mass%, and the content of the organic solvent is 60-92.5 mass%. The content of the binder is 30-70 mass% to 100 mass% of the metal oxide in total.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a metal oxide paste composition for a photoelectric conversion element and a method for producing the same.

Background Art

[0002] For the negative electrode of a photoelectric conversion element such as a dye-sensitized solar cell or a perovskite solar cell, the most common method is to apply a metal oxide paste composition such as titanium oxide onto a conductive substrate, fire it, and then support a dye.

[0003] Regarding this metal oxide paste composition, those with higher performance (higher conversion efficiency) and those that can obtain a film with better coatability and homogeneity are desired. At this time, as the solvent constituting the metal oxide paste composition, a solvent having an alcoholic OH group such as α-terpineol is usually used (Patent Document 1, etc.).

[0004] However, when using metal oxide nanoparticles with a small particle size to increase the dye loading amount, in the conventional metal oxide paste composition, since a solvent having an alcoholic OH group is used as described above, there is a problem that the paste is easily cracked after firing.

[0005] In order to solve such a crack problem, as a method for producing a simple and low environmental impact metal oxide paste composition, a method of adding 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 to the metal oxide and homogenizing with a ball mill or the like is also known (Non-Patent Document 1).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Non-Patent Documents

[0007]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, in order to produce a water-based metal oxide paste composition with high energy conversion efficiency, metal oxide nanoparticles synthesized by the hydrothermal synthesis method are often used. However, ethyl cellulose, which is most commonly used as a binder, is insoluble in water. Therefore, although the solvent is replaced with α-terpineol, since α-terpineol is a water-insoluble solvent, a method of replacing water with ethanol and then replacing ethanol with α-terpineol is used. Thus, while multiple solvent replacements are performed, since the metal oxide nanoparticles synthesized by the hydrothermal synthesis method are very small and have good dispersibility, it is impossible to perform solvent replacement by filtration, and it is also difficult to scale up solvent replacement by centrifugation.

[0009] On the other hand, the method of producing a water-based metal oxide paste composition described above can directly produce a metal oxide paste composition from those synthesized by the hydrothermal synthesis method. However, due to the low boiling point of water, there are problems such as easy change in state such as viscosity increase during storage and printing, easy non-uniformity during storage, problems of the harmfulness of nitric acid and corrosion of the surroundings, etc., and it cannot be used industrially.

[0010] Therefore, an object of the present invention is to provide a metal oxide paste composition that can be produced by a simple and environmentally friendly method, is less likely to crack during firing, is less likely to become non-uniform during storage, and can form an electrode with high energy conversion efficiency.

Means for Solving the Problems

[0011] In view of the above object, as a result of intensive studies, the present inventors have solved the above problems by using a paste composition containing a metal oxide, a water-soluble or water-dispersible binder, and a specific organic solvent in specific amounts, and can be produced by a simple method with a low environmental load. It has been found that an electrode with high energy conversion efficiency can be formed, which is less likely to crack during firing and less likely to become non-uniform during storage. Then, through further research, the present invention has been completed. That is, the present invention includes the following configurations.

[0012] Item 1. A paste composition for a photoelectric conversion element containing a metal oxide, a binder, and an organic solvent, wherein the binder includes 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 assuming the total amount of the paste composition for the photoelectric conversion element is 100% by mass, the content of the metal oxide is 5 to 25% by mass, and the content of the organic solvent is 60 to 92.5% by mass, assuming the total amount of the metal oxide is 100% by mass, the content of the binder is 30 to 70% by mass, a paste composition.

[0013] Item 2. The paste composition for a photoelectric conversion element according to Item 1, wherein the binder is a water-soluble cellulose compound.

[0014] Item 3. The paste composition for a photoelectric conversion element according to Item 1 or 2, wherein 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.

[0015] Item 4. The paste composition for a photoelectric conversion element according to any one of Items 1 to 3, wherein the number average molecular weight of the binder is 150,000 or less.

[0016] 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 from 0 to 5. When n is 2 or more, a plurality of R 2 may be the same or different.] The paste composition for a photoelectric conversion element according to any one of Items 1 to 4, containing an organic solvent represented by the formula.

[0017] Item 6. The paste composition for a photoelectric conversion element 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.

[0018] Item 7. The paste composition for a photoelectric conversion element according to any one of Items 1 to 6, wherein the metal oxide contains titanium oxide and / or zinc oxide.

[0019] Item 8. The paste composition for a photoelectric conversion element according to any one of Items 1 to 7, which is for a dye-sensitized solar cell or a perovskite solar cell.

[0020] Item 9. The paste composition for a photoelectric conversion element according to any one of Items 1 to 8, which is for a negative electrode of a dye-sensitized solar cell or a perovskite solar cell.

[0021] Item 10. A method for producing the paste composition for a photoelectric conversion element according to any one of Items 1 to 9, comprising: (1) A step of mixing the aqueous dispersion containing the metal oxide, the binder, and the organic solvent simultaneously or sequentially. The production method comprising this step.

[0022] Item 11. The production method according to Item 10, wherein the step (1) is a step of mixing the binder after mixing the aqueous dispersion containing the metal oxide and the organic solvent.

[0023] Item 12. Further comprising: (2) A step of concentrating the dispersion obtained in the step (1). The production method according to Item 10 or 11, comprising this step.

[0024] Item 13. A negative electrode for a photoelectric conversion element, on which a porous coating film made of a dried product of the paste composition for a photoelectric conversion element according to any one of Items 1 to 9 is formed, on a conductive substrate.

[0025] 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.

[0026] Item 15. A photoelectric conversion element including the negative electrode for a photoelectric conversion element according to Item 13 or 14.

[0027] Item 16. The photoelectric conversion element according to Item 15, which is a dye-sensitized solar cell or a perovskite solar cell.

Effect of the Invention

[0028] According to the present invention, it is possible to provide a metal oxide paste composition that can be produced by a simple method with a low environmental load, is less likely to crack during firing, is less likely to become non-uniform during storage, and can form an electrode with high energy conversion efficiency.

Mode for Carrying Out the Invention

[0029] In this specification, "containing" is a concept that includes any of "comprise", "consist essentially of", and "consist of".

[0030] Also, in this specification, when a numerical range is indicated as "A to B", it means A or more and B or less.

[0031] Hereinafter, embodiments of the present invention will be described, but various changes in form and details are possible without departing from the spirit and scope of the claims.

[0032] 1. Metal Oxide Paste Composition for Photoelectric Conversion Element The metal oxide paste composition for a photoelectric conversion element of the present invention is a paste composition for a photoelectric conversion element containing a metal oxide, a binder, and an organic solvent, wherein the binder includes 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 with the total amount of the paste composition being 100% by mass, the content of the metal compound is 5 to 25% by mass, the content of the binder is 2 to 15% by mass, and the content of the solvent is 60 to 92.5% by mass.

[0033] (1-1) Metal oxide The metal oxide is preferably a photo-semiconductor. Specifically, oxides such as titanium, zinc, and tin are mentioned, and oxides such as titanium and zinc are preferred. That is, titanium oxide, zinc oxide, tin oxide, etc. are preferred, titanium oxide, zinc oxide, etc. are more preferred, and titanium oxide is even more preferred.

[0034] In the present invention, "titanium oxide" does not only refer to titanium dioxide (TiO2), but also includes titanium sesquioxide (Ti2O3); titanium monoxide (TiO); compositions with oxygen deficiency from titanium dioxide typified by Ti4O7, Ti5O9, etc. Further, it may contain groups other than Ti-O-Ti resulting from the synthesis of partially titanium oxide, typified by terminal OH groups. Similarly, "zinc oxide" is a concept that includes not only ZnO but also zinc antimonate, etc. Furthermore, regarding "tin oxide" as well, it is a concept that includes not only tin dioxide (SnO2) but also tin monoxide (SnO), etc.

[0035] The above metal oxides can be used alone or in combination of two or more. Also, known or commercially available products can be used as the above metal oxides.

[0036] When using titanium oxide, it is preferably included anatase-type titanium oxide with high activity. More specifically, it is preferable that 50% or more, 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 included.

[0037] The size of the metal oxide is preferably a nano size with an average particle diameter of 5 to 100 nm, that is, metal oxide nanoparticles, and more preferably the average particle diameter is 10 to 50 nm. However, in order to scatter light, in addition to the metal oxide nanoparticles with the above average particle diameter of 5 to 100 nm, metal oxide particles with an average particle diameter of 100 nm or more and metal oxides having 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 by calculating the average particle diameter from the specific surface area measured by the BET method.

[0038] In the present invention, when the paste composition is dried or fired, it is useful in that cracking can be suppressed even when a small metal oxide (with an average particle diameter of about 5 to 100 nm) that is prone to cracking is used. For this reason, in the present invention, it is possible to reduce the average particle diameter of the metal oxide in order to increase the dye loading amount.

[0039] The content of the metal oxide in the metal oxide paste composition for a photoelectric conversion element of the present invention is 5 to 25% by mass, preferably 7 to 23% by mass, more preferably 10 to 20% by mass, with the total amount of the paste composition for a photoelectric conversion element being 100% by mass. If the content of the metal oxide is less than 5% by mass, the number of printing times for ensuring the required film thickness increases, which is not efficient. Also, if the content of the metal oxide exceeds 25% by mass, the viscosity becomes too high and printing becomes difficult. When using a plurality of metal oxides, it is preferable to adjust so that the total content is within the above range. Also, when diluting the paste for a perovskite solar cell, the concentration of the metal oxide after dilution is not limited to the above and may be thinner.

[0040] (1-2) Binder The metal oxide paste composition for a photoelectric conversion element of the present invention contains a binder that increases the viscosity of the metal oxide paste composition for a photoelectric conversion element to give thixotropy, and promotes porosity by providing voids between particles after firing, thereby suppressing cracking during firing.

[0041] As the binder, as described above, when using a binder that does not dissolve in water such as ethyl cellulose, it is necessary to perform solvent replacement multiple times to replace the solvent with α-terpineol. On the other hand, in the metal oxide paste composition, it is impossible to perform solvent replacement by filtration, and it is also difficult to scale up solvent replacement by centrifugation. Therefore, in the present invention, a water-soluble or water-dispersible binder is used. In particular, since the metal oxide paste composition for a photoelectric conversion element of the present invention also contains an organic solvent (a water-soluble organic solvent having a boiling point of 120°C or higher), the binder used in the present invention is preferably soluble or dispersible in the organic solvent (organic solvent solubility or organic solvent dispersibility).

[0042] In this specification, "water-soluble" or "organic solvent-soluble" each means the property of dissolving in water or an organic solvent. Specifically, it means that when 10 g / L or more is added to water or an organic solvent at room temperature (25°C), a uniform and transparent state can be maintained. However, in that case, it does not mean that it is uniform down to the molecular level. For example, a cellulose derivative generally regarded as "water-soluble" becomes transparent and uniform when added to water and stirred, but in some cases, it is nanofiber-like when observed with an electron microscope. Also, the water solubility of the organic solvent means that it can be uniformly mixed with water at an arbitrary ratio.

[0043] In addition, in this specification, "water-dispersible" or "organic solvent-dispersible" each means the property of being able to be emulsified or dispersed in water or an organic solvent and stabilized in a particulate state. Specifically, it means that when 1 part by mass of a binder is added to 100 parts by mass of water at room temperature (25°C), no precipitation can be confirmed even after standing for 1 hour or more. This includes the above-mentioned water solubility and organic solvent solubility, and does not necessarily have to be uniform at the molecular level and may be turbid.

[0044] Among such binders, from the viewpoint of affinity with metal oxides and the like, binders having a hydroxyl group (preferably, polymer compounds having a hydroxyl group, particularly polysaccharides having a hydroxyl group) are preferable.

[0045] Specific examples of binders having such properties include polysaccharides having a hydroxyl group such as hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, methyl cellulose, 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, xyloglucan, etc.

[0046] 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. Also, for the above-mentioned metal oxides, known or commercially available products can be used.

[0047] Among these, from the viewpoints of affinity with metal oxides, ease of decomposition during firing, thixotropy, etc., water-soluble cellulose compounds such as hydroxypropyl cellulose, hydroxypropyl methylcellulose, hydroxyethyl cellulose, hydroxyethyl methylcellulose, methylcellulose, carboxymethyl cellulose, sodium carboxymethylcellulose, potassium carboxymethylcellulose, ammonium carboxymethylcellulose are preferred, hydroxypropyl cellulose, hydroxypropyl methylcellulose, hydroxyethyl cellulose, hydroxyethyl methylcellulose, methylcellulose, etc. are more preferred, hydroxypropyl cellulose, hydroxypropyl methylcellulose, hydroxyethyl cellulose, hydroxyethyl methylcellulose, etc. are even more preferred, and hydroxypropyl cellulose, hydroxypropyl methylcellulose, etc. are particularly preferred from the viewpoint of affinity with the organic solvents described later.

[0048] When using these water-soluble cellulose compounds, the degree of substitution is not particularly limited, but from the viewpoints of water solubility, solubility in organic solvents, affinity with metal oxides, ease of decomposition during firing (difficulty of crack generation), thixotropy, energy conversion efficiency, etc., 0.3 to 5 is preferred, and 0.5 to 4 is more preferred.

[0049] The number average molecular weight of the above binder is not particularly limited, and the optimal number average molecular weight varies depending on the type of binder and the required viscosity, but from the viewpoint of easily suppressing the residue after firing, 150,000 or less is preferred, and 120,000 or less is more preferred. The lower limit value of the number average molecular weight of the binder is not particularly limited and is usually about 10,000.

[0050] In the metal oxide paste composition for a photoelectric conversion element of the present invention, the content of the binder is 30 to 70% by mass, preferably 40 to 65% by mass, more preferably 45 to 60% by mass, with the total amount of the metal oxide being 100% by mass. When the ratio of the binder to the metal oxide is less than 30% by mass, pores are not sufficiently formed, and the viscosity and thixotropy required for printing cannot be ensured. Further, when the ratio of the binder to the metal oxide exceeds 70% by mass, the pores are too large, the porous structure after firing becomes brittle, the strength cannot be maintained, the surface area of titania per unit area of the electrode cannot be ensured, and the current during power generation decreases. When a plurality of binders are used, it is preferable to adjust so that the total content with respect to the metal oxide is within the above range.

[0051] (1-3) Organic solvent The organic solvent used in the paste composition for a photoelectric conversion element is preferably a water-soluble solvent having a high affinity with a metal oxide (such as titanium oxide) and capable of dissolving the additives described below.

[0052] From such a viewpoint, in the present invention, as the organic solvent, a water-soluble organic solvent having a boiling point of 120 °C or higher is used.

[0053] The boiling point of the organic solvent is 120 °C or higher for preventing drying, viscosity increase, and non-uniformity during use and storage, preferably 140 °C or higher from the viewpoint of easily performing solvent substitution from water to the organic solvent without passing through filtration or centrifugation, and more preferably 160 °C or higher from the viewpoint of easily performing solvent substitution by removing water without losing the organic solvent. The upper limit value of the boiling point of the organic solvent is not particularly limited, but is usually 450 °C.

[0054] Further, the lower the vapor pressure of the organic solvent, the less likely it is to volatilize, and the less the change over time of the metal oxide paste composition, which is preferable. Specifically, the vapor pressure at 20 °C is preferably 0.05 mmHg or less, more preferably 0.01 mmHg or less. The lower limit value of the vapor pressure of the organic solvent at 20 °C is not particularly limited, but is usually about 0.00001 mmHg.

[0055] As such an organic solvent, from the viewpoint of water solubility, it preferably has a hydroxyl group. Further, from the viewpoint of being less likely to bind too strongly to the surface of the metal oxide and being likely to improve the energy conversion efficiency, it preferably has only one hydroxyl group.

[0056] As such an organic solvent, 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, a plurality of R 2 may be the same or different.] The organic solvent represented by is preferred.

[0057] In the general formula (1), examples of the alkyl group represented by R 1 include linear or branched alkyl groups having 1 to 6 (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.

[0058] In the general formula (1), examples of the acyl group represented by R 1 include acyl groups having 1 to 6 (particularly 1 to 4) carbon atoms such as a formyl group, an acetyl group, and a propionyl group.

[0059] In the general formula (1), as R 1 , among others, from the viewpoints of water solubility, affinity with the metal oxide, ease of decomposition during firing (resistance to crack generation), thixotropy, energy conversion efficiency, etc., 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, etc. are preferred, and a hydrogen atom, a methyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an acetyl group, etc. are more preferred.

[0060] In general formula (1), R 2 Examples of the alkylene group represented by 2 include linear or branched alkylene groups having 2 to 10 (especially 2 to 6) carbon atoms such as ethylene group, trimethylene group, propylene group, propylidene group, isopropylidene group, tetramethylene group, 1-methyltrimethylene group, 2-methyltrimethylene group, 3-methyltrimethylene group, 1,1-dimethylethylene group, 1,2-dimethylethylene group, 1-ethylethylene group, 2-ethylethylene group, pentamethylene group, etc. Among them, from the viewpoints of water solubility, affinity with metal oxides, ease of decomposition during firing (resistance to crack generation), thixotropy, energy conversion efficiency, etc., linear alkylene groups having 2 to 10 (especially 2 to 6) carbon atoms such as ethylene group, trimethylene group, tetramethylene group, pentamethylene group are preferable.

[0061] In addition, when n described later 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.

[0062] In general formula (1), from the viewpoints of water solubility, affinity with metal oxides, ease of decomposition during firing (resistance to crack generation), thixotropy, energy conversion efficiency, etc., n is preferably an integer of 0 to 5, more preferably an integer of 1 to 4, and even more preferably an integer of 1 to 3.

[0063] Specific examples of such organic solvents include, for example, alcohol compounds having 1 to 6 (especially 1 to 5, and further 1 to 4) carbon atoms such as methanol, ethanol, isopropyl alcohol; glycol compounds such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, butanediol, pentanediol or their monoalkyl derivatives (monoalkyl ethers) or acetate esters, etc.

[0064] In the monoalkyl derivative (monoalkyl ether) of a glycol compound, examples of the alkyl group include those described above. That is, examples of the monoalkyl derivative (monoalkyl ether) of a glycol compound include, for example, 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, propylene glycol monobutyl ether, dipropylene 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, pentanediol monobutyl ether, and the like.

[0065] Examples of the acetate of the glycol compound 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, pentanediol acetate, and the like.

[0066] As described above, the organic solvent represented by the general formula (1) is preferred. However, there are also organic solvents that have sufficient water solubility even without having a hydroxyl group, such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc. These organic solvents can also be preferably used.

[0067] The above organic solvents can be used alone or in combination of two or more. In addition, known or commercially available products can be used as the above organic solvents.

[0068] Among these, as the organic solvent, from the viewpoints of water solubility, affinity with metal oxides, ease of decomposition during firing (resistance to crack generation), thixotropy, energy conversion efficiency, etc., 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. are preferable, and diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, etc. are more preferable.

[0069] In the present invention, the content of the organic solvent is 60 to 92.5% by mass, preferably 65 to 90% by mass, with the total amount of the metal oxide paste composition for a photoelectric conversion element of the present invention being 100% by mass. If the concentration of the organic solvent is less than 60% by mass, the viscosity may be too high and unsuitable for screen printing, the pores may become small, and the energy conversion efficiency may decrease. Also, if the concentration of the organic solvent exceeds 92.5% by mass, the solid content concentration may be too low, the film-forming efficiency may be poor, the pores may become large, and it may be prone to cracking. However, when forming a film with a thickness of several 100 nm by spin coating as the negative electrode of a perovskite solar cell, the content of the organic solvent can be reduced.

[0070] In the present invention, although it does not exclude the inclusion of a small amount of water as the solvent, from the viewpoint of suppressing the change in performance over time due to volatilization during drying or firing, etc., the water content is preferably as small as possible. For this reason, with the total amount of the solvent being 100% by mass, the content of the above-described organic solvent is preferably 95 to 100% by mass, and more preferably 98 to 100% by mass.

[0071] (1-4) Other components In addition to the above, the metal oxide paste composition for a photoelectric conversion element of the present invention usually contains additives blended in a metal oxide paste composition for a photoelectric conversion element such as a dye-sensitized solar cell or a perovskite solar cell. For example, a dispersant for dispersing a metal oxide, a thickener for increasing the viscosity of the metal oxide paste composition for a photoelectric conversion element and imparting thixotropy, a leveling agent for increasing the smoothness of a coating film, and other organic solvents compatible with the above-described organic solvent can be added within a range not impairing the effects of the present invention.

[0072] 2. Manufacturing Method The method for producing the metal oxide paste composition for a photoelectric conversion element of the present invention is not particularly limited, but (1) A step of simultaneously or sequentially mixing an aqueous dispersion containing the metal oxide, the binder, and the organic solvent is preferably provided.

[0073] In step (1), the process method is not particularly limited and can be carried out according to a conventional method.

[0074] The mixing order is not particularly limited and may be simultaneous or sequential. However, since the viscosity increases after the binder is added, it is preferable to add the binder later. That is, step (1) is preferably a step of mixing the aqueous dispersion containing the metal oxide and the organic solvent and then mixing with the binder.

[0075] In addition, in order to easily prevent the binder from forming lumps, it is preferably dissolved in a solvent in advance. As the solvent that can be used at this time, water, the above-described organic solvent, a solvent having compatibility with both water and the above-described organic solvent, etc. can be used. However, it is also possible to add the binder without dissolving it in a solvent in advance.

[0076] Also, in consideration of workability, it is also preferably carried out to prepare a dispersion once with a low concentration of the metal oxide and then concentrate it. That is, after the step (1), (2) The step of concentrating the dispersion obtained in the step (1) is preferably provided.

[0077] The method of concentration is not particularly limited, and for example, it can be carried out using an evaporator or the like.

[0078] 3. Negative Electrode When forming an electrode (especially 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.

[0079] The resin substrate is not particularly limited as long as it is a conductive resin substrate. For example, 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; triacetate cellulose; polymethylpentene and the like can be mentioned.

[0080] The glass substrate is also not particularly limited, and known or commercially available ones can be used, and any of colorless or colored glass, wired glass, glass blocks, etc. may be used.

[0081] As this resin substrate or glass substrate, those having a plate thickness of about 0.05 to 10 mm can be used.

[0082] 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.

[0083] Examples of the transparent conductive film include, for example, 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), gallium-doped zinc oxide film (GZO film), and the like. By using these transparent conductive films, it becomes easy to take out the generated current to the outside. The film thickness of these transparent conductive films is preferably about 0.02 to 10 μm.

[0084] Examples of the electrode (particularly the negative electrode) of the present invention include, for example, the following two embodiments.

[0085] (3-1) Embodiment 1 On a resin substrate or a glass substrate, a porous coating film obtained by using the metal oxide paste composition for a photoelectric conversion element of the present invention can be formed via a transparent conductive film, and used as an electrode for a photoelectric conversion element such as a dye-sensitized solar cell or a perovskite solar cell of the present invention. The resin substrate, the glass substrate, and the transparent conductive film are as described above.

[0086] Specifically, as follows, an electrode for a photoelectric conversion element such as a dye-sensitized solar cell or a perovskite solar cell of the present invention can be formed.

[0087] First, a transparent conductive film is formed on a resin substrate or a glass substrate by a vacuum evaporation method, an ion plating method, a CVD method, a sputtering method, a sol-gel method, a nanoparticle composite, or the like. The surface resistance of the obtained substrate is preferably 50 Ω / sq or less.

[0088] Then, the metal oxide paste composition for a photoelectric conversion element of the present invention is applied thereon, and it is preferably dried or fired. When using a resin substrate, the heating condition is preferably 150°C or less.

[0089] At this time, it is preferable to apply so that the film thickness of the obtained coating film is about 2 to 40 μm from the viewpoints of crack suppression and adhesion to the substrate.

[0090] (3-2) Embodiment 2 On a resin substrate or a glass substrate, a porous coating film obtained by using the metal oxide paste composition for a photoelectric conversion element of the present invention may be directly formed, and further, a porous metal film may be formed thereon to be used as an electrode for a photoelectric conversion element such as a dye-sensitized solar cell or a perovskite solar cell of the present invention. The resin substrate and the glass substrate are the same as those described above. Further, when forming a porous coating film on the resin substrate or the glass substrate, the same method as in the above Embodiment 1 can be adopted.

[0091] The porous metal film that can be used in Embodiment 2 is not particularly limited as long as it is a metal that is not attacked (does not react) by ions contained in an electrolytic solution such as iodine ions and bromine ions. For example, titanium, tungsten, platinum, gold, etc. can be mentioned. By forming these porous metal films, it becomes easier to take out the generated current to the outside. The surface resistivity of these porous metal films is not particularly limited, but is preferably 10 Ω / square or less, and the film thickness is also not particularly limited, but is preferably 150 nm or more.

[0092] The porous metal film can be formed on the porous coating film formed on the resin substrate or the glass substrate by a thin film forming method such as a sputtering method.

[0093] 4. Photoelectric Conversion Element (Dye-Sensitized Solar Cell, Perovskite Solar Cell, etc.) The photoelectric conversion element (particularly a dye-sensitized solar cell) of the present invention is 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 the perovskite solar cell of the present invention and filling the space between these electrodes with an electrolytic solution.

[0094] As the electrolyte, a solvent having a high dielectric constant is preferred so that more electrolytes such as iodine ions can be dissolved, and a solvent having a low viscosity is preferred so that the dissolved ions can move easily. Such solvents are not particularly limited. For example, nitrile compounds such as acetonitrile, propionitrile, glutaronitrile, 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 and the like can be used alone or in combination of two or more.

[0095] The counter electrode may have 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, metals, colorless or colored glass, wired glass, glass blocks, etc. can be used, and resins can also be used. Such resins include polyesters such as polyethylene terephthalate, polyamides, polysulfones, polyethersulfones, polyetheretherketones, polyphenylene sulfides, polycarbonates, polyimides, polymethyl methacrylates, polystyrenes, cellulose triacetate, polymethylpentene, and the like. Further, a counter electrode may be formed by applying, plating, or vapor depositing (PVD, CVD) a conductive material directly on the charge transport layer.

[0096] As the conductive material, metals such as platinum, gold, nickel, titanium, aluminum, copper, silver, tungsten, etc., and materials with low specific resistance such as carbon materials and conductive organic substances are used.

[0097] Also, a metal lead may be used for the purpose of reducing the resistance of the counter electrode. The metal lead is preferably made of a metal such as platinum, gold, nickel, titanium, aluminum, copper, silver, tungsten, etc., and particularly preferably made of aluminum or silver.

[0098] In the present invention, before forming the counter electrode, for the purpose of improving the light absorption efficiency of the electrode of the present invention, etc., it is preferable to support (adsorb, contain, etc.) a dye on the porous coating film.

[0099] The dye is not particularly limited as long as it has absorption characteristics in the visible region or near-infrared region and improves (sensitizes) the light absorption efficiency of the semiconductor layer, but metal complex dyes, organic dyes, natural dyes, semiconductors, etc. are preferable. Further, in order to impart adsorptivity to the porous coating film, those having functional groups such as carboxyl group, hydroxyl group, sulfonyl group, phosphonyl group, carboxyalkyl group, hydroxyalkyl group, sulfonylalkyl group, phosphonylalkyl group, etc. in the molecule of the dye are preferably used.

[0100] Examples of metal complex pigments include complexes of ruthenium, osmium, iron, cobalt, zinc, mercury (such as mericrumechrome, etc.), metal phthalocyanine, chlorophyll, etc. Examples of organic pigments include cyanine-based pigments, hemicyanine-based pigments, merocyanine-based pigments, xanthene-based pigments, triphenylmethane-based pigments, metal-free phthalocyanine-based pigments, etc., but are not limited thereto. As the semiconductor that can be used as a pigment, an amorphous semiconductor or a direct transition type semiconductor having a large i-type light absorption coefficient, or a fine particle semiconductor that exhibits a quantum size effect and efficiently absorbs visible light is preferable. Usually, in order to make the wavelength range of photoelectric conversion as wide as possible and increase the conversion efficiency, one kind of various semiconductors, metal complex pigments, and organic pigments, or two or more kinds of pigments can be mixed. Further, the pigments to be mixed and their ratios can be selected according to the wavelength range and intensity distribution of the target light source.

[0101] As a method for adsorbing a pigment onto a porous coating film, for example, it can be formed by a method in which a solution obtained by dissolving a pigment in a solvent is applied onto the porous coating film by spray coating, spin coating, or the like and then dried. In this case, the substrate may be heated to an appropriate temperature. Further, a method in which the porous coating film is immersed in a solution for adsorption can also be used. The immersion time is not particularly limited as long as the pigment is sufficiently adsorbed, but is preferably 10 minutes to 30 hours, more preferably 1 to 20 hours. Further, the solvent or the substrate may be heated during immersion if necessary. The concentration of the pigment in the case of making a solution is about 1 to 1000 mmol / L, preferably about 10 to 500 mmol / L.

[0102] The solvent to be 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, t-butanol; nitriles such as acetonitrile, propionitrile, methoxypropionitrile, glutaronitrile; aromatic hydrocarbons such as benzene, toluene, o-xylene, m-xylene, p-xylene; aliphatic hydrocarbons such as pentane, hexane, heptane; alicyclic hydrocarbons such as cyclohexane; ketones such as acetone, methyl ethyl ketone, diethyl ketone, 2-butanone; ethers such as diethyl ether, tetrahydrofuran; ethylene carbonate, propylene carbonate, nitromethane, dimethylformamide, dimethyl sulfoxide, hexamethylphosphoramide, dimethoxyethane, γ-butyrolactone, γ-valerolactone, sulfolane, dimethoxyethane, adiponitrile, methoxyacetonitrile, dimethylacetamide, methylpyrrolidinone, dimethyl sulfoxide, dioxolane, sulfolane, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, ethyldimethyl phosphate, tributyl phosphate, tripentyl phosphate, trihexyl phosphate, triheptyl phosphate, trioctyl phosphate, trinonyl phosphate, tridecyl phosphate, tris(trifluoromethyl) phosphate, tris(pentafluoroethyl) phosphate, triphenyl polyethylene glycol, polyethylene glycol, etc.

[0103] In order to reduce interactions such as aggregation between dyes, a colorless compound having the properties of a surfactant may be added to the dye adsorption liquid and co-adsorbed on the porous coating film. Examples of such colorless compounds include steroid compounds such as cholic acid, deoxycholic acid, chenodeoxycholic acid, taurodeoxycholic acid having a carboxyl group or a sulfo group, and sulfonates.

[0104] The unadsorbed dye is preferably removed promptly by washing after the adsorption step. The washing is preferably carried out using acetonitrile, an alcohol-based solvent, etc. in a wet washing tank.

[0105] After adsorbing the pigment, 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. Examples of preferred amines include pyridine, 4-t-butylpyridine, polyvinylpyridine, etc. Examples of preferred quaternary ammonium salts include tetrabutylammonium iodide, tetrahexylammonium iodide, etc. These may be used after being dissolved in an organic solvent, or may be used as they are in the case of liquids.

[0106] The dye-sensitized solar cell can be manufactured by modularizing the photoelectric conversion element obtained as described above and providing predetermined electrical wiring.

[0107] In the above description, the case of applying to a dye-sensitized solar cell has been explained. However, also in the case of applying to a perovskite solar cell, other than using the electrode for a photoelectric conversion element such as the dye-sensitized solar cell and perovskite solar cell of the present invention as, for example, a negative electrode, it can be formed according to a conventional method.

Examples

[0108] The present invention will be specifically described based on examples, but the present invention is not limited only to these.

[0109] Example 1 142.1 g (0.5 mol) of titanium tetraisopropoxide was added with 120 g (2 mol) of acetic acid and stirred for 15 minutes, and then 550 g of water was added. The pH of this dispersion was 2.5. A large amount of translucent precipitate was generated, but after stirring for 60 minutes and then heating, all the precipitate dissolved at 60 °C.

[0110] Thereafter, after stirring at normal pressure (0.1 MPa) and 80 °C for 5 hours, water was added to the reaction solution and adjusted to a total of 800 g to obtain a translucent dispersion. This dispersion contained 40 g of titanium oxide.

[0111] 200 g of this dispersion was placed in a titanium autoclave and reacted in a hot air furnace at 220 °C for 3 hours. This solution was heated at 200 °C to recover titania nanoparticles, and when the average particle size was calculated from the BET specific surface area (93 m 2 / g), it was 16 nm. Also, when the crystallinity was analyzed by X-ray diffraction, it was 100% anatase.

[0112] To 200 g of this dispersion, 40.6 g of 1,5-pentanediol was added and ultrasonic dispersion was performed. Further, 5 g of hydroxypropyl cellulose (molecular weight 100,000: hydroxypropyl cellulose manufactured by Fujifilm Wako Pure Chemical Corporation 3.0 - 5.9) was added and ultrasonic dispersion was performed.

[0113] Thereafter, water was distilled off at 60 °C and 50 hPa to obtain 55 g of a white paste composition.

[0114] Note that the appearance of the paste composition did not change even after one month.

[0115] This paste composition was applied onto a glass substrate (10 Ω / □) on which a fluorine-doped tin oxide film (FTO film) was formed so that the thickness after drying would be 10 μm, fired at 550 °C, and then immersed in 5 × 10 -4 mol / L of Ru dye (N719) at 25 °C for 20 hours to fabricate the negative electrode of a dye-sensitized solar cell. Note that after firing, it was confirmed by an optical microscope that no cracks occurred in the paste composition after firing.

[0116] This negative electrode was bonded to a glass substrate (positive electrode) on which a tin-doped indium oxide film (ITO film) sputtered with platinum was formed, and an electrolyte solution (a solution in which 0.15 M iodine, 0.1 M lithium iodide, 0.5 M ethylmethylimidazolium iodide, and 0.5 M tert-butylpyridine were dissolved in 3-methoxypropionitrile) was sealed between the electrodes to fabricate a photoelectric conversion element (dye-sensitized solar cell).

[0117] The fabricated photoelectric conversion element (dye-sensitized solar cell) was irradiated with light having an intensity of 100 mW / cm under the conditions of AM1.5 using a solar simulator, and as a result of evaluating the photoelectric conversion characteristics, the photoelectric conversion efficiency was 6.3%. 2 The experiment was conducted 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.

[0118] Note that the measurement of cracks and photoelectric conversion efficiency was also carried out in the same manner in the following Examples and Comparative Examples.

[0119] Example 2 As a result, no cracks occurred after firing the paste composition, and a transparent coating film was obtained. Also, the photoelectric conversion efficiency was 7.3%.

[0120] Moreover, the appearance of the paste composition did not change even after one month.

[0121] The experiment was conducted in the same manner as in Example 2 except that hydroxypropyl cellulose (molecular weight 100,000) was changed to hydroxypropyl cellulose (molecular weight 60,000: hydroxypropyl cellulose manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., 2.0 - 2.9).

[0122] Example 3 As a result, no cracks occurred after firing the paste composition, and a transparent coating film was obtained. Also, the photoelectric conversion efficiency was 6.7%.

[0123] Moreover, the appearance of the paste composition did not change even after one month.

[0124] The experiment was conducted in the same manner as in Example 2 except that hydroxypropyl cellulose (molecular weight 100,000) was changed to hydroxypropyl cellulose (molecular weight 140,000: hydroxypropyl cellulose manufactured by Fuji Film Wako Pure Chemical Industries, Ltd., 6.0 - 10.0).

[0125] Example 4 As a result, no cracks occurred after firing the paste composition, and a transparent coating film was obtained. Also, the photoelectric conversion efficiency was 6.7%.

[0126] As a result, no cracks occurred after firing the paste composition, and a transparent coating film was obtained. Also, the photoelectric conversion efficiency was 6.2%.

[0127] Also, the appearance of the paste composition did not change even after one month.

[0128] Example 5 An experiment was conducted in the same manner as in Example 2, except that 40.6 g of diethylene glycol monobutyl ether was changed to 36.1 g.

[0129] As a result, no cracks occurred after firing the paste composition, and a transparent coating film was obtained. Also, the photoelectric conversion efficiency was 6.9%.

[0130] Also, the appearance of the paste composition did not change even after one month.

[0131] Example 6 An experiment was conducted in the same manner as in Example 2, except that 40.6 g of diethylene glycol monobutyl ether was changed to 33.4 g.

[0132] As a result, no cracks occurred after firing the paste composition, and a transparent coating film was obtained. Also, the photoelectric conversion efficiency was 6.2%.

[0133] Also, the appearance of the paste composition did not change even after one month.

[0134] Comparative Example 1 To 18 g of titania nanoparticles ST-01 (manufactured by Ishihara Sangyo Co., Ltd.: average particle diameter 7 nm), 60 g of acetic acid, 100 g of ethanol, 73 g of α-terpineol, and 90 g of a 10 mass% ethanol solution of ethyl cellulose were added, and concentration was performed at 40 °C and 70 hPa to obtain 100 g of a paste composition. An experiment was conducted in the same manner as in Example 1 using this paste composition.

[0135] As a result, cracks occurred after firing the paste composition, and it was cloudy. Also, the photoelectric conversion efficiency was 5.8%.

[0136] Also, after one week, the paste composition had increased in viscosity, lost its smoothness, and became non-uniform.

[0137] Comparative Example 2 To 18 g of titania nanoparticles ST-21 (manufactured by Ishihara Sangyo Co., Ltd.: average particle diameter 30 nm), 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 and permeated with a paint shaker, resulting in a paste composition. Using this paste composition, experiments were conducted in the same manner as in Example 1.

[0138] As a result, cracks did not occur after firing the paste, but it was cloudy. Also, the photoelectric conversion efficiency was 5.7%.

[0139] Also, after one month, the paste composition had increased in viscosity, lost its smoothness, and became non-uniform.

[0140] Thus, by selecting a binder and a solvent, a metal oxide paste composition with high energy conversion efficiency could be produced. Also, it had excellent stability over time compared to conventional aqueous paste compositions.

Claims

1. A paste composition used for a porous coating film formed on a conductive substrate in an electrode for a photoelectric conversion element, The paste composition contains a metal oxide, a binder, and an organic solvent, The binder includes 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 Taking the total amount of the paste composition for the photoelectric conversion element as 100% by mass, the content of the metal oxide is 5 to 25% by mass, and the content of the water-soluble organic solvent is 60 to 92.5% by mass, A paste composition in which, taking the total amount of the metal oxide as 100% by mass, the content of the binder is 30 to 70% by mass.

2. The paste composition according to claim 1, wherein the binder is a water-soluble cellulose compound.

3. The paste composition according to claim 1 or 2, wherein 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.

4. The paste composition according to any one of claims 1 to 3, wherein the number average molecular weight of the binder is 150,000 or less.

5. The organic solvent is represented by the general formula (1): R １ (OR ２ ) ｎ OH [wherein, 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, a plurality of R ２ may be the same or different. ] The paste composition according to any one of claims 1 to 4, containing an organic solvent represented by

6. 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.

7. The paste composition according to any one of claims 1 to 6, wherein the metal oxide contains titanium oxide and / or zinc oxide.

8. The paste composition according to any one of claims 1 to 7, which is for a dye-sensitized solar cell or a perovskite solar cell.

9. The paste composition according to any one of claims 1 to 8, which is for a negative electrode of a dye-sensitized solar cell or a perovskite solar cell.

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

11. The production method according to claim 10, wherein the step (1) is a step of mixing the binder after mixing the aqueous dispersion containing the metal oxide and the organic solvent.

12. Further, (2) A step of concentrating the dispersion obtained in the step (1) The manufacturing method according to claim 10 or 11, comprising the above.

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

14. 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.

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

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