Titanium dioxide dispersion and its manufacturing method, titanium dioxide film, and titanium dioxide electron transport layer

Abstract

The present invention provides a titanium dioxide dispersion, a method for producing the same, a titanium dioxide film, and a titanium dioxide electron transport layer, all of which have a low environmental impact and can reduce manufacturing costs. [Solution] The present invention provides a method for producing a titanium dioxide dispersion, comprising: a first step of obtaining titanium hydroxide by hydrolysis of a titanium compound; a second step of preparing a peroxotitanic acid complex solution with the titanium hydroxide using hydrogen peroxide and ammonia; a third step of adding a primary hydroxycarboxylic acid to the peroxotitanic acid complex solution to obtain a water-soluble titanium complex solution; and a fourth step of hydrothermally treating the water-soluble titanium complex solution to obtain a primary titanium dioxide dispersion.

Description

Technical Field

[0001] The present invention relates to a titanium oxide dispersion liquid, a method for producing the same, a titanium oxide film, and a titanium oxide electron transport layer.

Background Art

[0002] The titanium oxide film is effective as an electron transport layer of a perovskite solar cell. Conventionally, a method of dispersing high-temperature fired titanium oxide powder in a solvent and applying it has been used to produce a titanium oxide film. However, with this method, only a titanium oxide thin film with a high-temperature firing of 400°C or higher and a thickness exceeding several 100 nm can be produced. Therefore, a heat-resistant material such as glass is required as the material of the substrate. It is difficult to use a thin film resin material (for example, PET, etc.) required for a roll-to-roll manufacturing process. As a result, there have been problems in the low-cost and mass production of perovskite solar cells. On the other hand, in Non-Patent Document 1, it is disclosed that a dispersion liquid in which crystallized titanium oxide fine particles are dispersed in an aqueous solution can be prepared, and a thin film with a thickness of several 10 nm can be formed by heating at about 100°C, and roll-to-roll production becomes possible.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, Non-Patent Document 1 uses metallic titanium as the starting titanium source, which involves a step of dissolving metallic titanium to create a dispersion as a water-soluble titanium complex. This is time-consuming, and because high-purity metallic titanium is expensive, it is difficult to reduce costs.

[0005] Therefore, the object of the present invention is to provide a method for producing a titanium oxide dispersion that uses a titanium compound other than metallic Ti as the starting titanium source, thereby shortening the production time and reducing the production cost, as well as the titanium oxide dispersion, titanium oxide film, and titanium oxide electron transport layer. Another object of the present invention is to provide a titanium dioxide dispersion, a titanium dioxide film, and a titanium dioxide electron transport layer, which can be obtained by adding a hydroxycarboxylic acid to the titanium dioxide dispersion obtained in the present invention to obtain a more uniform titanium dioxide thin film. [Means for solving the problem]

[0006] The gist of the present invention is as follows: [1] A first step of obtaining titanium hydroxide by hydrolysis of a titanium compound, The second step involves preparing a peroxotitanic acid complex solution using the aforementioned titanium hydroxide with hydrogen peroxide and ammonia, A third step involves adding a primary hydroxycarboxylic acid to the peroxotitanic acid complex solution to obtain a water-soluble titanium complex solution. A fourth step of hydrothermally treating the aforementioned water-soluble titanium complex solution to obtain a first titanium oxide dispersion, and a method for producing the titanium oxide dispersion containing the above. [2] The method for producing a titanium oxide dispersion according to [1], wherein the titanium compound is at least one selected from the group consisting of titanium tetrachloride, titanium trichloride, titanium sulfate, and titanium alkoxide. [3] A method for producing a titanium dioxide dispersion according to claim [1] or [2], further comprising a fifth step of adding a second hydroxycarboxylic acid to the first titanium dioxide dispersion to obtain a second titanium dioxide dispersion. [4] The method for producing the titanium dioxide dispersion according to [3], wherein the content of the second hydroxycarboxylic acid in the second titanium dioxide dispersion is 0.1% by mass or more and 10% by mass or less. [5] The first step involves dissolving titanium metal powder in a hydrogen peroxide solution and an ammonia solution to prepare a peroxotitanic acid complex solution, A second step involves adding a hydroxycarboxylic acid to the peroxotitanic acid complex solution to obtain a water-soluble titanium complex solution, A third step involves hydrothermally treating the aforementioned water-soluble titanium complex solution to obtain a first titanium oxide dispersion, A method for producing a titanium dioxide dispersion, comprising a fourth step of adding a second hydroxycarboxylic acid to the first titanium dioxide dispersion to obtain a second titanium dioxide dispersion. [6] The method for producing the titanium dioxide dispersion according to [5], wherein the content of the second hydroxycarboxylic acid in the second titanium dioxide dispersion is 0.1% by mass or more and 10% by mass or less. A titanium dioxide dispersion produced by the method for producing a titanium dioxide dispersion described in any of [7] [1] to [6]. [8] A titanium dioxide dispersion comprising a hydroxycarboxylic acid and titanium dioxide, The titanium dioxide particles in the titanium dioxide dispersion have a dispersibility of 50 nm or less. A titanium dioxide dispersion having a hydroxycarboxylic acid content of 0.1% by mass or more and 10% by mass or less. [9] A step of synthesizing a titanium dioxide dispersion using the method for producing a titanium dioxide dispersion described in any of [1] to [6], The process involves forming a film on a resin substrate using a synthesized titanium dioxide dispersion, and then removing the solvent at a temperature of 300°C or lower to obtain a titanium dioxide (TiOx) film. A method for producing a titanium oxide film, including the following:

[10] The method for producing a titanium oxide film according to [8], wherein the thickness of the titanium oxide film is 100 nm or less.

[11] A titanium oxide film obtained by removing the solvent from the titanium oxide dispersion described in [7].

[12]

[11] comprising the titanium oxide film described above, The titanium oxide electron transport layer, wherein the titanium oxide dispersion liquid contains titanium oxide particles having at least one crystal structure selected from the group consisting of rutile, brookite, bronze, and anatase.

[13] The titanium oxide electron transport layer according to

[12] , which constitutes a perovskite solar cell or a dye-sensitized solar cell.

Advantages of the Invention

[0007] According to the present invention, there are provided a method for producing a titanium oxide dispersion liquid that uses a titanium compound other than metallic Ti as a starting titanium source and realizes shortening of production time and reduction of production cost, the titanium oxide dispersion liquid, a titanium oxide film, and a titanium oxide electron transport layer. In addition, by adding a hydroxycarboxylic acid to the titanium oxide dispersion liquid obtained in the present invention, there are provided a titanium oxide dispersion liquid, a titanium oxide film, and a titanium oxide electron transport layer that can obtain a more uniform titanium oxide thin film.

Brief Description of the Drawings

[0008] [Figure 1] It is a production flowchart showing a method for producing titanium oxide in Example 1. [Figure 2] It is a production flowchart showing a method for producing titanium oxide in Example 4. [Figure 3] It is a production flowchart showing a method for producing titanium oxide in Example 7. [Figure 4] It is a Raman spectrum of a brookite-type titanium oxide dispersion liquid synthesized in Examples 1 to 3 and Reference 1. [Figure 5] It is a diagram showing the measurement result of the dynamic light scattering particle size distribution of the brookite-type titanium oxide dispersion liquid synthesized in Example 1. [Figure 6] It is a surface SEM image of a bronze-type titanium oxide film produced using the titanium oxide dispersion liquid synthesized in Example 4. [Figure 7] It is a Raman spectrum of anatase-type titanium oxide dispersion liquid synthesized in Example 8. [Figure 8]It is the Raman spectrum of the rutile-type titanium oxide dispersion synthesized in Example 9. [Figure 9] It is the fluorescent X-ray analysis of the bronze-type titanium oxide synthesized in Example 10. [Figure 10] It is the fluorescent X-ray analysis of the bronze-type titanium oxide synthesized in Example 11. [Figure 11] In Example 12, it is a diagram showing the AFM measurement results of a titanium oxide film produced using the perovskite-type titanium oxide dispersion produced in Example 1. [Figure 12] In Example 13, it is a diagram showing the AFM measurement results of a titanium oxide film produced using the perovskite-type titanium oxide dispersion produced in Example 4. [Figure 13] It is a diagram showing the measurement results of the dynamic light scattering particle size distribution of the perovskite-type titanium oxide dispersion adjusted in Comparative Example 1. A, B, and C in the figure represent the data of the 1st, 2nd, and 3rd measurements, respectively. [Figure 14] It is a schematic diagram showing a perovskite solar cell using a titanium oxide electron transport layer according to an embodiment of the present invention.

Modes for Carrying Out the Invention

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show, for the sake of clarity, the characteristic parts enlarged for convenience. Therefore, the dimensional ratios of each component may be different from the actual ones.

[0010] (Method for Producing Titanium Oxide Dispersion) It will be described in detail using the production methods of the following First Embodiment to Third Embodiment. The method for producing a titanium oxide dispersion according to the first embodiment includes a first step of obtaining an aqueous titanium hydroxide solution by hydrolyzing a titanium compound a second step of preparing a peroxotitanic acid complex solution from the titanium hydroxide using hydrogen peroxide and ammonia, and A third step involves adding a hydroxycarboxylic acid to the peroxotitanic acid complex solution to obtain a water-soluble titanium complex solution, The fourth step includes hydrothermally treating the aforementioned water-soluble titanium complex solution to obtain a first titanium oxide dispersion.

[0011] The method for producing a titanium dioxide dispersion of the second embodiment involves a first step of obtaining an aqueous titanium hydroxide solution by hydrolysis of a titanium compound, and The second step involves preparing a peroxotitanic acid complex solution using the aforementioned titanium hydroxide with hydrogen peroxide and ammonia, A third step involves adding a primary hydroxycarboxylic acid to the peroxotitanic acid complex solution to obtain a water-soluble titanium complex solution. A fourth step involves hydrothermally treating the aforementioned water-soluble titanium complex solution to obtain a first titanium oxide dispersion, The method includes a fifth step of adding a second hydroxycarboxylic acid to the first titanium dioxide dispersion to obtain a second titanium dioxide dispersion. Preferably, the content of the second hydroxycarboxylic acid in the second titanium dioxide dispersion is 0.1% by mass or more and 10% by mass or less.

[0012] (Method for producing titanium dioxide dispersion according to the third embodiment) The third embodiment of the method for producing a titanium oxide dispersion comprises a first step of dissolving titanium metal powder in a hydrogen peroxide solution and an ammonia solution to prepare a peroxotitanic acid complex solution, A second step involves adding a primary hydroxycarboxylic acid to the peroxotitanic acid complex solution to obtain a water-soluble titanium complex solution. A third step involves hydrothermally treating the aforementioned water-soluble titanium complex solution to obtain a first titanium oxide dispersion, The method includes a fourth step of adding a second hydroxycarboxylic acid to the first titanium dioxide dispersion to obtain a second titanium dioxide dispersion. Preferably, the content of the second hydroxycarboxylic acid in the second titanium dioxide dispersion is 0.1% by mass or more and 10% by mass or less.

[0013] (First Embodiment) <1st process> The titanium compound is not particularly limited, as long as it is a compound that can be hydrolyzed to synthesize titanium hydroxide (or titanium hydroxide containing hydroxide). The "titanium compound" in this disclosure does not contain metallic titanium. Examples of the titanium compound include trivalent titanium compounds and tetravalent titanium compounds. Examples of trivalent titanium compounds include titanium trihalides such as titanium trichloride and titanium tribromide. Examples of tetravalent titanium compounds include tetrahalides such as TiCl4, TiBr4, and TiI4; trihalogenated alkoxytitaniums such as Ti(OCH3)Cl3, Ti(OC2H5)Cl3, Ti(OC4H9)Cl3, Ti(OC2H5)Br3, and Ti(OC4H9)Br3; dihalogenated dialoxytitaniums such as Ti(OCH3)2Cl2, Ti(OC2H5)2Cl2, Ti(OC4H9)2Cl2, and Ti(OC2H5)2Br2; monohalogenated trialkoxytitaniums such as Ti(OCH3)3Cl, Ti(OC2H5)3Cl, Ti(OC4H9)3Cl, and Ti(OC2H5)3Br; titanium sulfates such as Ti(SO4)2 and Ti2(SO4)3; and alkoxytitaniums such as Ti(OCH3)4, Ti(OC2H5)4, Ti(OC4H9)4, Ti(OC2H5)4, and Ti(OC4H9)4.

[0014] Among these, titanium tetrachloride, titanium trichloride, titanium sulfate, and titanium alkoxide are preferred. Titanium tetrachloride (TiCl4) is more preferred because it is inexpensive and readily available.

[0015] Examples of conditions for the hydrolysis of the titanium compound include the following combinations of conditions.

[0016] Temperature conditions: 0 to 100°C is preferred.

[0017] The mixing ratio (mass ratio, molar ratio, or concentration) of the titanium compound to water is preferably 1 to 10000, more preferably 10 to 1000, and even more preferably 20 to 500.

[0018] Solvent type: Aqueous organic solvents are preferred, and water is more preferred. Examples of aqueous organic solvents include water or a mixture of water and a water-soluble organic solvent. Examples of the water-soluble organic solvents include alcohols such as methanol and ethanol; ethers such as ethylene glycol dimethyl ether; ketones such as acetone; nitriles such as acetonitrile; and carboxylic acids such as acetic acid. The mixing ratio of water to water-soluble organic solvent is approximately 10 / 90 to 99.9 / 0.1 by weight, preferably 50 / 50 to 99 / 1. In the present invention, it is preferable to use water in particular because it eliminates the need for organic solvent recovery work.

[0019] Method for mixing titanium compounds with aqueous solvents: It is preferable to add the titanium compound to the aqueous solvent. The addition rate should be controlled so that the temperature of the solution due to hydrolysis does not exceed a certain range (for example, 5°C to 95°C).

[0020] pH: Preferably between 1 and 14, more preferably between 4 and 13, and even more preferably between 7 and 12. When the above titanium compound is titanium chloride or titanium sulfate, it is preferable to adjust the pH to a basic pH. For example, it is preferable to add aqueous ammonia, as shown in Example 1 in Figure 1 and described later. For example, the volume mixing ratio (NH3 water / water) with 30% by mass aqueous ammonia is preferably 1 / 5 to 1 / 2, and more preferably 1 / 5 to 1 / 3.

[0021] In the first step, it is preferable to precipitate the titanium compound as titanium hydroxide in basic water. The basicity is preferably prepared using ammonia water or the like. The precipitated titanium hydroxide can be isolated from the solution by known methods. For example, it can be isolated by standing overnight and then filtration by suction. When isolating the titanium hydroxide precipitate, it is preferable to wash it with water.

[0022] <Second process> <Peroxotitanic acid complex> Peroxotitanic acid complexes can be obtained, for example, by adding hydrogen peroxide and ammonia to a titanium compound TiX, as shown in the reaction formula (1) below. TiX + 3H2O2 + NH3 → [Ti(OH)3O2] - +H2O+NH4 + (1)

[0023] In the second step, it is preferable to use an aqueous solution of hydrogen peroxide or ammonia with a concentration of, for example, 3% to 80%. It is more preferable to use an aqueous solution of 30% by mass of either. For example, it is preferable to use a solution obtained by mixing 30% hydrogen peroxide and 30% ammonia in a volume ratio of 4:1. The precipitate of titanium hydroxide, isolated and washed in the first step, is dissolved in a mixed solution of hydrogen peroxide and ammonia to obtain an aqueous solution of the peroxotitanium complex. For every 100 moles of titanium hydroxide based on the element Ti, the number of moles of hydrogen peroxide is preferably 1 to 200, more preferably 5 to 100, and even more preferably 10 to 50. For every 100 moles of titanium hydroxide (based on the element Ti), the number of moles of ammonia is preferably 2 to 80, more preferably 5 to 40, and even more preferably 10 to 20.

[0024] The peroxotitanic acid complex can be produced by adding hydrogen peroxide and ammonia in the above ranges to a titanium compound and stirring. Excess hydrogen peroxide and ammonia can be decomposed, for example, by adding a hydroxycarboxylic acid in a later step and then heating and stirring overnight at 30-100°C (e.g., 60°C).

[0025] <3rd process> <First hydroxycarboxylic acid> The first hydroxycarboxylic acid is represented, for example, by the following formula (2). HO-R'-COOH (3) (In the formula, R' represents a divalent aliphatic hydrocarbon group, a divalent alicyclic hydrocarbon group, or a divalent aromatic hydrocarbon group, which may have substituents.)

[0026] Examples of divalent aliphatic hydrocarbon groups in R' include linear or branched alkylene groups having 1 to 18 carbon atoms, such as methylene, ethylene, methylethylene, trimethylene, 1-methyltrimethylene, 2-methyltrimethylene, 2,2-dimethyltrimethylene, tetramethylene, pentamethylene, hexamethylene, heptamethylene, octamethylene, nonamethylene, 2,2,4-trimethylhexamethylene, decamethylene, and dodecamethylene; alkenylene groups having approximately 2 to 6 carbon atoms, such as propenylene; and alkylene groups having approximately 2 to 6 carbon atoms, such as propynylene.

[0027] Examples of divalent alicyclic hydrocarbon groups in R' include cycloalkylene groups such as cyclohexylidene, 1,2-cyclopentylene, 1,2-cyclohexylene, 1,3-cyclohexylene, and 1,4-cyclohexylene; cycloalkenylene groups such as 2-cyclohexene-1,4-diyl; and divalent bridged cyclic groups such as adamantane-1,3-diyl.

[0028] Examples of divalent aromatic hydrocarbon groups in R' include 1,2-phenylene, 1,3-phenylene, 1,4-phenylene, 1,4-naphthylene, and biphenylene groups.

[0029] Examples of substituents that R' may have include a carboxyl group, a hydroxyl group, an amino group, a phenyl group, and so on.

[0030] Specific examples of primary hydroxycarboxylic acids include glycolic acid, lactic acid, 2-hydroxy-n-butyric acid, 2-hydroxyisobutyric acid, tartaric acid, malic acid, citric acid, isocitric acid, glyceric acid, mevalonic acid, salicylic acid, gallic acid, shikimic acid, and the like. Among these, citric acid, lactic acid, glycolic acid, tartaric acid, malic acid, etc. are preferred.

[0031] As for the amount of the first hydroxycarboxylic acid added, for example, the amount of the first hydroxycarboxylic acid added (in moles) per 100 moles of the peroxotitanium complex obtained in the second step, based on the number of Ti elements, is preferably 10 to 1000, more preferably 50 to 500, and even more preferably 100 to 500. For example, in Example 1 described later, the same number of moles of glycolic acid was used. By adding the primary hydroxycarboxylic acid within the above range, it is possible to prepare an aqueous titanium solution that does not precipitate in a pH range from strongly acidic to weakly basic.

[0032] In the third step, it is preferable to heat the water-soluble titanium complex solution to evaporate it to dryness and remove excess hydrogen peroxide and ammonia. The temperature conditions for the third step are the same as those for the first step.

[0033] <4th process> The water-soluble titanium complex solution used for hydrothermal treatment may be the aqueous solution of the water-soluble titanium complex obtained in the third step, or it may be the solid water-soluble titanium complex obtained by heating the aqueous solution of the water-soluble titanium complex to evaporate to dryness and removing excess hydrogen peroxide and ammonia. When using the solid water-soluble titanium complex, the water-soluble titanium complex solution may be prepared by adding a predetermined amount of aqueous solvent. The concentration based on the element Ti (e.g., moles / L) is preferably 0.01 to 10, more preferably 0.1 to 8, and even more preferably 0.5 to 6. For example, in Example 1, a 1M water-soluble titanium complex solution was used for hydrothermal treatment.

[0034] The hydrothermal treatment conditions can be carried out using known apparatus and methods. For example, it can be carried out using a Teflon® / stainless steel pressure-resistant sealed container with a volume of 10 mL to 500 mL. The hydrothermal treatment temperature is more preferably 100 to 250°C, 120 to 240°C, or 150 to 230°C. The hydrothermal treatment time is preferably 5 minutes to 72 hours, more preferably 10 minutes to 48 hours, and even more preferably 30 minutes to 24 hours. The aqueous organic solvent used during the hydrothermal treatment may be the same as the "aqueous organic solvent" described in the first step, or it may be different.

[0035] <Hydrothermal treatment process> In the hydrothermal treatment step, the reaction temperature of the aqueous solution containing the water-soluble titanium complex obtained in the third step is preferably 100 to 374°C, and more preferably 120 to 240°C. Since the hydrothermal reaction takes place in water at 100°C or higher, the lower limit is 100°C and the upper limit is the supercritical temperature of 374°C. When using a relatively simple Teflon® container, a temperature of 240°C or lower, which is below the softening point of Teflon®, is preferred. It is preferable to perform the hydrothermal treatment under a pressure equal to or greater than the saturated vapor pressure at that reaction temperature for 10 minutes to 48 hours (preferably 30 minutes to 24 hours). During the hydrothermal treatment process, the desired crystalline polymorph can be produced by adjusting the pH range within the reaction system. For example, to produce brookite, the pH may be 7 to 13. To produce bronze, the pH may be 3 or lower. To produce anatase, the pH may be 0 to 13. To produce rutile, the pH may be 4 to 10. The hydrothermal treatment is carried out at a pressure equal to or greater than the saturated vapor pressure at the reaction temperature (i.e., in a closed system). The reaction pressure (absolute pressure) during the hydrothermal treatment is, for example, 0.2 MPa to 3.3 MPa (preferably 0.4 MPa to 3 MPa).

[0036] Furthermore, the time for hydrothermal treatment is between 5 minutes and 72 hours (for example, between 10 minutes and 48 hours, preferably between 30 minutes and 24 hours).

[0037] After hydrothermal treatment, the titanium dioxide dispersion can be used as is. Furthermore, after hydrothermal treatment, titanium oxide can be obtained by centrifuging and washing as needed.

[0038] <Reaction conditions and the crystal structure of titanium dioxide> Using the aforementioned water-soluble titanium complex, hydrothermal treatment can be performed according to the required crystalline polymorphism, particle size, and shape of titanium oxide. Four types of titanium dioxide—anatase, rutile, brookite, and bronze—can be obtained by hydrothermal treatment of the water-soluble titanium complex described above.

[0039] <<Rutile>> Hydrothermal treatment using glycolic acid or lactic acid at near-neutral temperatures yields large rutile particles with a particle size of 30 nm or more. When using glycolic acid, large rutile particles can also be obtained at temperatures between 120 and 230°C. When using lactic acid, large rutile particles with a particle size of 30 nm or more can be obtained at temperatures above 200°C, while anatase particles can be obtained at temperatures below 200°C. Small rutile crystals can be obtained by hydrothermal treatment using tartaric acid in the presence of hydrogen peroxide at a near-neutral pH. Using conventional methods, it is difficult to produce rutile nanoparticles. For example, due to its high-temperature phase, coarse particles are usually obtained. On the other hand, a dispersion of nanoparticles can be obtained by using the method of the present invention.

[0040] <<Burkite>> Brookite can be obtained by hydrothermal treatment using glycolic acid or lactic acid in a basic environment with a pH of 7 or higher. Using conventional methods, brookite is difficult to synthesize. This is because brookite is a catastrophic phase that does not appear during calcination. On the other hand, the method of the present invention yields a dispersion with extremely good crystallinity and high dispersibility.

[0041] <<Bronze>> Bronze can be obtained using glycolic acid in a strongly acidic environment with a pH of around 1. pH can be adjusted using oxoacids such as sulfuric acid or nitric acid, but the use of hydrochloric acid is not preferred. Conventionally, alkali titanate is generally produced by proton exchange of the alkali and then calcining. On the other hand, since the method of the present invention does not involve firing, it is possible to create a dispersion in which bronze is well dispersed in water.

[0042] <<Anatase>> Under conditions other than those for which rutile, brookite, and bronze are obtained, anatase is produced. For example, using citric acid or malic acid yields anatase regardless of pH. Using glycolic acid yields anatase even in acidic conditions. Tartaric acid also yields anatase if it does not contain hydrogen peroxide. In other words, rutile, brookite, and bronze are obtained under the specific conditions mentioned above, and anatase is obtained when those conditions are not met. It is possible to produce something similar to the present invention using conventional methods. On the other hand, the method of the present invention offers high dispersibility because it is produced without drying. Furthermore, the particle size can be controlled by the type of complex and temperature. For example, currently, using a malic acid complex yields a titanium dioxide dispersion with a desirable particle size of about 10 nm (neither too small nor too large). A titanium dioxide thin film prepared using this dispersion exhibits high power generation efficiency.

[0043] <<Method for evaluating the crystal structure of titanium dioxide>> The crystalline structure of titanium dioxide can be evaluated using the X-ray diffraction measurement method or Raman spectroscopy measurement method described in the following Non-Patent Documents A to C.

[0044] [Non-patent Document A] Koji Tomita, Valery Petrykin, Makoto Kobayashi, Motoo Shiro, Masahiro Yoshimura, Masato Kakihana; “A water-soluble titanium complex for the selective synthesis of nanocrystalline brookite, rutile, and anatase by a hydrothermal method”, Angewandte Chemie 118(15) 2438-2441 (2006.3). [Non-patent document B] K. Yamamoto, K. Tomita, K. Fujita, M. Kobayashi, V. Petrykin, M. Kakihana “Synthesis of TiO2(B) using glycolato titanium complex and post-synthetic hydrothermal crystal growth of TiO2(B)” Journal of Crystal growth, 311 619-622 (2008.9). [Non-patent Document C] Makoto Kobayashi, Valery V. Petrykin, Masato Kakihana, Koji Tomita, Masahiro Yoshimura; “One-Step Synthesis of TiO2(B) Nanoparticles from a Water-Soluble Titanium Complex” Chemistry of Materials 19(22), 5373-5376 (2007.9).

[0045] The resulting titanium dioxide dispersion is a state in which titanium dioxide is dispersed in solution. To remove unwanted impurities, the particles are settled by centrifugation, water is added to redisperse them, and this centrifugation and redispersion process is repeated to produce a titanium dioxide dispersion.

[0046] (Second Embodiment) In the method for producing a titanium dioxide dispersion of the second embodiment, the first to fourth steps are the same as the first to fourth steps of the first embodiment described above. <5th process> The second hydroxycarboxylic acid added in step 5 has the same meaning as the "first hydroxycarboxylic acid" used in step 3 of the first embodiment. In the method for producing the titanium dioxide dispersion of the second embodiment, the second hydroxycarboxylic acid may be the same type of compound as the first hydroxycarboxylic acid, or a different type of compound may be used. Furthermore, the second hydroxycarboxylic acid may be the same compound as the first hydroxycarboxylic acid, or a different compound may be used. The content of the second hydroxycarboxylic acid in the titanium dispersion (second titanium oxide dispersion) obtained in the fifth step is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 5% by mass or less. For example, in Example 4, the citric acid content was 3% by mass.

[0047] (Third embodiment) In the method for producing a titanium dioxide dispersion of the third embodiment, steps 2 to 4 are the same as steps 3 to 5 of the second embodiment described above. <1st process> Examples of titanium powder used as a starting titanium source include those with a particle size of 45 μm. For example, titanium 3N45 mesh manufactured by High Purity Chemicals is one such example. In the first step of the third embodiment, it is preferable to use an aqueous solution of hydrogen peroxide or ammonia with a concentration of, for example, 10% to 80%. It is more preferable to use an aqueous solution of 30% by mass of either. For example, it is preferable to use a solution obtained by mixing 30% hydrogen peroxide and 30% ammonia in a volume ratio of 4:1. After dissolving metallic titanium powder in hydrogen peroxide solution and ammonia solution, the peroxotitanic acid complex can be obtained by, for example, allowing it to stand for 2 hours. For every 100 moles of titanium metal, the number of moles of hydrogen peroxide is preferably between 100 and 20,000, more preferably between 500 and 10,000, and even more preferably between 1,000 and 5,000. For every 100 moles of titanium metal, the number of moles of ammonia is preferably between 200 and 8000, more preferably between 500 and 4000, and even more preferably between 1000 and 2000.

[0048] For every 100 parts by mass of metallic titanium, the amount of hydrogen peroxide added (parts by mass) is preferably 1,000 to 30,000, more preferably 2,000 to 20,000, and even more preferably 4,000 to 10,000. For every 100 parts by mass of metallic titanium, the amount of ammonia added (parts by mass) is preferably 200 to 10,000, more preferably 500 to 6,000, and even more preferably 1,000 to 3,000.

[0049] The peroxotitanic acid complex can be produced by mixing metallic titanium with amounts of hydrogen peroxide and ammonia within the above range and stirring. Excess hydrogen peroxide and ammonia can be decomposed, for example, by adding a hydroxycarboxylic acid and then heating and stirring overnight at 30-80°C (e.g., 60°C).

[0050] (Titanium dioxide dispersion) An example of a titanium dioxide dispersion according to one embodiment of the present invention is a titanium dioxide dispersion obtained by the manufacturing method according to the first to third embodiments. In this embodiment, the titanium dioxide dispersion obtained by the manufacturing method of the second and third embodiments is preferred. Because the second hydroxycarboxylic acid is added to the first titanium dioxide dispersion obtained by the manufacturing method of the first embodiment, a more uniform titanium dioxide film can be formed. The titanium dioxide dispersion of this embodiment preferably contains a secondary hydroxycarboxylic acid and titanium dioxide. The secondary hydroxycarboxylic acid has the same meaning as, for example, the "secondary hydroxycarboxylic acid" described in the second embodiment. The titanium dioxide is, for example, the titanium dioxide contained in the titanium dioxide dispersion (first titanium dioxide dispersion or second titanium dioxide dispersion) obtained by the manufacturing method of the first to third embodiments.

[0051] The titanium oxide dispersion obtained by the manufacturing methods of the first to third embodiments described above preferably contains substantially no vanadium compounds and substantially no tin compounds. "Substantially no" means, for example, containing 1% by mass or less, 0.5% by mass or less, or 0.1% by mass or less. For example, it is preferable that the content of the tin component in the titanium dioxide dispersion is greater than 1,000 and 1,100 or more in molar ratio (Ti / Sn) with titanium dioxide. Furthermore, it is preferable that the content of the vanadium component in the titanium dioxide dispersion is greater than 10,000 and 12,000 or more in molar ratio (Ti / V) with titanium dioxide.

[0052] The titanium dioxide dispersion of this embodiment preferably has at least one feature selected from the group consisting of, for example, (i) to (v) below. (i) The titanium dioxide (TiO2) particles in the aqueous solution have a dispersibility of 50 nm or less. In this invention, "dispersibility of 50 nm or less" means that the size of titanium dioxide particles in an aggregated state in an aqueous solution is 50 nm or less. Furthermore, one method for evaluating particle size in a dispersion is the dynamic light scattering particle size distribution measurement method using the Horiba SZ-100 nanoparticle analyzer. It is preferable that the particle size is the volume-average particle size. (ii) The concentration of titanium dioxide (TiO2) particles in the dispersion is 1% by mass or more. (iii) The primary particle size of titanium dioxide (TiO2) in the dispersion is between 1 nm and 30 nm. (iv) The titanium dioxide (TiO2) in the dispersion contains two or more elements selected from the group consisting of anatase, rutile, brookite, and bronze. (v) In the dispersion, the molar ratio of titanium dioxide (TiO2) to organic acid is 1:10 to 10:1.

[0053] (Titanium oxide film) A titanium oxide film according to one embodiment of the present invention is obtained, for example, by removing the solvent from a titanium oxide dispersion obtained by the manufacturing method of the first to third embodiments. Examples of methods for removing the solvent include heating, light irradiation, blowing air, and vacuuming. For example, a titanium oxide film can be produced by forming the titanium oxide dispersion on a target substrate and drying it at room temperature to about 200°C (preferably 100 to 180°C). A drying temperature of 100°C or higher is preferable to sufficiently remove moisture. The method for forming the film can be a known method and is not particularly limited. Examples of methods for forming the film include dip coating, spin coating, spray coating, bar coating, and inkjet.

[0054] The thickness of the pre-titanium oxide film is preferably 100 nm or less, more preferably 5 to 50 nm, and even more preferably 5 to 20 nm.

[0055] A method for producing a titanium oxide film may include, for example, the following steps 1 and 2. Step 1: A step of synthesizing a titanium dioxide dispersion using the manufacturing method according to the first to third embodiments. Step 2: Using the obtained titanium dioxide dispersion, a film is formed on a resin substrate, and then the solvent is removed at a temperature of 300°C or lower to obtain a titanium dioxide (TiOx) film. In step 2, methods for removing the solvent from the titanium dioxide dispersion include, for example, heating, light irradiation, blowing air, and vacuuming.

[0056] (Titanium oxide electron transport layer) The titanium oxide film of this embodiment can be used, for example, as a titanium oxide electron transport layer in a solar cell. Examples of such solar cells include perovskite solar cells (e.g., Figure 14) and dye-sensitized solar cells. The perovskite solar cell 1 shown in Figure 14 comprises a substrate 2, a transparent electrode 4, an electron transport layer 6, a perovskite layer 8, a hole transport layer (HTL) 10, and a positive electrode 12. Examples of the substrate 2 include a glass substrate or a resin substrate such as PET. Examples of the transparent electrode 4 include FTO or ITO. Examples of the electron transport layer 6 include a titanium oxide electron transport layer made of the titanium oxide film of this embodiment. Examples of the positive electrode 12 include Au. The titanium oxide film of this embodiment can be formed at a low temperature of 200°C or lower, with a thickness of 100 nm or less, and more preferably 50 nm or less, making it preferable to use as a titanium oxide electron transport layer in a perovskite solar cell.

[0057] The titanium oxide film of this embodiment is preferably prepared using the titanium oxide dispersion of this embodiment, which contains titanium oxide particles having at least one crystalline structure selected from the group consisting of rutile, anatase, brookite, and bronze. For example, these four crystal forms have different band gaps, and ideally, the usable wavelength range widens in the order of rutile > anatase > brookite > bronze, while the extractable power increases in the opposite direction. Therefore, compared to the anatase commonly used in the past, brookite, bronze, and rutile allow for increased power generation efficiency by appropriately altering the energy levels of the conduction band (LUMO) of the perovskite layer, thereby controlling the optical absorption wavelength range and extraction voltage. These crystalline forms can only be fabricated as thin films using the method of the present invention. [Examples]

[0058] The following describes embodiments of the present invention. The present invention is not limited to the following embodiments.

[0059] (Evaluation method) <Raman scattering measurement> Equipment: Irix STR150 Sample preparation method: Titanium oxide dispersion was dropped onto a glass substrate and dried at 60°C before measurement.

[0060] <Dynamic light scattering particle size distribution measurement> Equipment: Horiba SZ-100 Sample preparation method: Dilute the titanium dioxide dispersion with deionized water, place it in a glass cell, and measure.

[0061] <sem> Equipment: Hitachi High-Tech S-4800 Sample preparation method: After drying, the sample or substrate is attached to carbon tape, and then observed after Au-Pd deposition.

[0062] <afm> Equipment: Shimadzu Corporation SPM-9700 Sample preparation method: Measurement of the substrate surface after TiO2 film deposition.

[0063] <X-ray fluorescence analysis> Equipment: HITACHI EA1000A III Sample preparation method: Place the dried TiO2 powder on a resin film and measure.

[0064] (Example 1) As shown in the manufacturing flow chart in Figure 1, 10 g of titanium tetrachloride (manufactured by Fujifilm Wako, special grade 99.0%) was measured out as the starting titanium source, hydrolysis was performed using 200 mL of water and 10 g of 30% NH3 water, and the mixture was allowed to stand overnight. After that, it was filtered by suction and washed with distilled water to obtain titanium hydroxide (hydrated titanium oxide). The recovered precipitate of titanium hydroxide was dissolved in a solution of 30% hydrogen peroxide and 30% ammonia mixed in a volume ratio of 4:1 to synthesize a peroxotitanium complex. To the synthesized perooxotitanium complex, glycolic acid was added in the same molar amount as titanium as a complexing agent to form a titanium glycolate complex. After standing for one day, the mixture was heated and evaporated to dryness twice to remove excess hydrogen peroxide and ammonia. The starting titanium source, the titanium glycolate complex, was diluted with water to a concentration of 1 M. The 1 M titanium glycolate complex was hydrothermally treated at 200°C for 5 hours in a pressure-resistant sealed container (Teflon® / stainless steel) to synthesize brookite-type titanium oxide (TiO2). Raman scattering measurements were performed on the obtained titanium dioxide dispersion. The results are shown in Figure 4. Dynamic light scattering particle size distribution measurements were performed on the obtained titanium dioxide dispersion. The results are shown in Figure 5. Figure 5 shows that the particle size of the dispersion was estimated using dynamic light scattering. The particle size distribution was approximately 70-80 nm, and the almost consistent distribution across three measurements confirmed that aggregation did not occur. This confirmed that a highly dispersible titanium dioxide dispersion could be prepared.

[0065] (Example 2) Brookite-type titanium oxide (TiO2) was synthesized in the same manner as in Example 1, except that titanium sulfate (manufactured by Fujifilm Wako, 30% titanium(IV) sulfate solution) was used as the starting titanium source. Raman scattering measurements were performed on the obtained titanium dioxide dispersion. The results are shown in Figure 4.

[0066] (Example 3) Brookite-type TiO2 was synthesized in the same manner as in Example 1, except that tetraisopropoxytitanium (manufactured by Fujifilm Wako, 95% first-grade) was used as the starting titanium source and hydrolysis was carried out using 200 mL of water. Raman scattering measurements were performed on the obtained titanium dioxide dispersion. The results are shown in Figure 4.

[0067] (Reference example 1) The brookite-type TiO2 was synthesized in the same manner as in Example 1, except that a peroxotitanium complex was synthesized by adding 10 g of 30% NH3 water and 40 g of 30% hydrogen peroxide water to 10 mmol = 0.47 g of metallic titanium, and allowing it to stand for 2 hours while cooling with water. Raman scattering measurements were performed on the obtained titanium dioxide dispersion. The results are shown in Figure 4.

[0068] (Example 4) As shown in the manufacturing flow diagram in Figure 2, a brookite titanium oxide dispersion (first titanium oxide dispersion) was synthesized in the same manner as in Example 1. Citric acid was added to the obtained titanium oxide dispersion to prepare a titanium oxide dispersion containing 3% by mass of citric acid (second titanium oxide dispersion). A brookite-type titanium oxide film was fabricated using the brookite-type titanium oxide dispersion obtained in Example 4, and SEM images of the film were taken. The results are shown in Figure 6.

[0069] (Examples 5-6) Citric acid was added to each of the titanium dioxide dispersions obtained in Examples 2 and 3 to prepare titanium dioxide dispersions containing 3% by mass of citric acid.

[0070] (Example 7) As shown in the manufacturing flow diagram in Figure 3, a brookite titanium oxide dispersion (first titanium oxide dispersion) was synthesized in the same manner as in Reference Example 1. Citric acid was added to the obtained titanium oxide dispersion to prepare a titanium oxide dispersion containing 3% by mass of citric acid (second titanium oxide dispersion).

[0071] (Example 8) "Anatase-type titanium dioxide dispersion" The peroxotitanium complex was synthesized using the same method as in Example 1. To the synthesized perooxotitanium complex, malic acid was added in the same molar amount as titanium as a complexing agent to form a titanium malate complex. After standing for one day, the mixture was heated and evaporated to dryness twice to remove excess hydrogen peroxide and ammonia. The starting titanium source titanium malate complex was diluted with water to a concentration of 1 M. The 1 M titanium malate complex was hydrothermally treated at 200°C for 5 hours in a pressure-resistant sealed container (Teflon® / stainless steel) to synthesize rutile-type titanium dioxide. The Raman spectrum of the obtained anatase-type titanium dioxide dispersion was measured. The results are shown in Figure 7. In comparison with the Raman spectrum of the anatase-type titanium dioxide dispersion synthesized in Reference Example 2 below, 398 cm⁻¹ -1 , 513cm -1 , 631cm -1 A peak was observed, indicating that the titanium dioxide (TiO2) anatase phase had been successfully synthesized.

[0072] (Reference example 2) "Anatase-type titanium dioxide dispersion" The peroxotitanium complex was synthesized using a method similar to that used in Reference Example 1. To the synthesized perooxotitanium complex, malic acid was added in the same molar amount as titanium as a complexing agent to form a titanium malate complex. After standing for one day, the mixture was heated and evaporated to dryness twice to remove excess hydrogen peroxide and ammonia. The starting titanium source titanium malate complex was diluted with water to a concentration of 1 M. The 1 M titanium malate complex was hydrothermally treated at 200°C for 5 hours in a pressure-resistant sealed container (Teflon® / stainless steel) to synthesize anatase-type titanium dioxide. The Raman spectrum of the obtained anatase-type titanium dioxide dispersion was measured. The results are shown in Figure 7 as a reference.

[0073] (Example 9) "Rutile-type titanium dioxide dispersion" The peroxotitanium complex was synthesized using the same method as in Example 1. To the synthesized perooxotitanium complex, tartaric acid was added as a complexing agent in an amount of 1 / 2 molar relative to titanium to form a titanium tartrate complex. After standing for 1 day, heating and evaporation to dryness were performed twice to remove excess hydrogen peroxide and ammonia. The starting titanium source titanium tartrate complex was adjusted in concentration by adding 20% ​​by mass hydrogen peroxide solution to make a total of 1 M. The 1 M titanium tartrate complex was hydrothermally treated at 200°C for 5 hours in a pressure-resistant sealed container (Teflon® / stainless steel) to synthesize rutile-type titanium dioxide. The Raman spectrum of the obtained rutile-type titanium dioxide dispersion was measured. The results are shown in Figure 8. By comparing the Raman spectra of the rutile-type titanium dioxide dispersion synthesized in Reference Example 2 below, it was confirmed that each titanium dioxide (TiO2) polymorph was synthesized without any problems.

[0074] (Reference example 3) "Rutile-type titanium dioxide dispersion" The peroxotitanium complex was synthesized using a method similar to that used in Reference Example 1. Tartaric acid was added to the synthesized perooxotitanium complex in the same molar amount as titanium as a complexing agent to form a titanium tartrate complex. After standing for one day, the mixture was heated and evaporated to dryness twice to remove excess hydrogen peroxide and ammonia. The starting titanium source titanium tartrate complex was diluted with water to a concentration of 1 M. The 1 M titanium tartrate complex was hydrothermally treated at 200°C for 5 hours in a pressure-resistant sealed container (Teflon® / stainless steel) to synthesize rutile-type titanium oxide. The Raman spectrum of the obtained rutile-type titanium oxide dispersion was measured. The results are shown in Figure 8 as a reference.

[0075] (Example 10) "Bronze-type titanium dioxide dispersion" The peroxotitanium complex was synthesized using the same method as in Example 2. To the synthesized peroxotitanium complex, glycolic acid was added as a complexing agent in an amount 1.5 times the molar ratio to titanium to form a titanium glycolate complex. After standing for one day, the mixture was heated and evaporated to dryness twice to remove excess hydrogen peroxide and ammonia. To the starting titanium source titanium glycolate complex, 10 mL of water was added, and 3 mL of concentrated sulfuric acid was added to make it acidic. Water was then added to adjust the concentration to 1 M. The 1 M titanium glycolate complex was hydrothermally treated at 200°C for 5 hours in a pressure-resistant sealed container (Teflon® / stainless steel) to synthesize bronze-type TiO2. After synthesis, the material was washed with water. Fluorescent X-ray analysis was performed on the solid content obtained by evaporating the solvent from the synthesized titanium dioxide dispersion. The results are shown in Figure 9. The Ti content was 96.6% by mass and the S content was 3.4% by mass.

[0076] (Example 11) "Bronze-type titanium dioxide dispersion" Bronze-type TiO2 was synthesized using the same method as in Example 10. After synthesis, the titanium dioxide was washed with 8M ammonia water. X-ray fluorescence analysis was performed on the solid content obtained by evaporating the solvent from the synthesized titanium dioxide dispersion. The results are shown in Figure 10. The Ti content was 98.4% by mass and the S content was 1.6% by mass. The results from Examples 10 and 11 indicate that when titanium sulfate is used as the starting titanium source, washing with ammonia water is effective in reducing the amount of residual sulfur.

[0077] (Example 12) Using the first titanium dioxide dispersion obtained in Example 1, a titanium dioxide film was prepared according to the following procedure. Substrate: FTO glass substrate Application method: Spin coating Heating temperature: 100℃ Heating time: 1 hour The fabricated first titanium oxide film was evaluated using AFM. The results are shown in Figure 11. The arithmetic mean roughness of the film was 20.639 nm, and the root mean square roughness was 26.204 nm.

[0078] (Example 13) A titanium dioxide film was prepared using the second titanium dioxide dispersion obtained in Example 4, in the same manner as in Example 12. It was evaluated in the same manner as in Example 12. The results are shown in Figure 12. The arithmetic mean roughness of the film was 8.861 nm, and the root mean square roughness was 11.232 nm. The results from Examples 12 and 13 show that using citric acid suppresses aggregation and reduces roughness.

[0079] (Comparative Example 1) High-purity brookite-type titanium dioxide (TiO2) manufactured by Kagaku was dispersed in water to prepare a brookite-type titanium dioxide dispersion. The dynamic light scattering particle size distribution was measured for the prepared brookite-type titanium dioxide dispersion. The results are shown in Figure 13. In the figure, A, B, and C represent the data from the 1st, 2nd, and 3rd measurements, respectively. Figure 13 shows that there is variation in the range of several hundred to several thousand nanometers, indicating significant aggregation. Although the primary particles are sufficiently small (tens of nanometers), drying causes the strongly aggregated particles to become larger, leading to sedimentation during measurement and unstable data. [Explanation of Symbols]

[0080] 1: Perovskite solar cells 2: Substrate (for example, glass substrate or resin substrate such as PET) 4: Transparent electrode (e.g., FTO or ITO) 6. Electron transport layer (ETL) (e.g., titanium oxide electron transport layer) 8: Perovskite layer 10: Hole transport layer (HTL) 12: Positive electrode (e.g., Au)< / afm> < / sem>

Claims

1. The first step involves obtaining titanium hydroxide by hydrolysis of a titanium compound, The second step involves preparing a peroxotitanic acid complex solution using the aforementioned titanium hydroxide with hydrogen peroxide and ammonia, A third step involves adding a primary hydroxycarboxylic acid to the peroxotitanic acid complex solution to obtain a water-soluble titanium complex solution. A fourth step of hydrothermally treating the aforementioned water-soluble titanium complex solution to obtain a first titanium oxide dispersion, and a method for producing the titanium oxide dispersion containing the above.

2. The method for producing a titanium oxide dispersion according to claim 1, wherein the titanium compound is at least one selected from the group consisting of titanium tetrachloride, titanium trichloride, titanium sulfate, and titanium alkoxide.

3. A method for producing a titanium dioxide dispersion according to claim 1, further comprising a fifth step of adding a second hydroxycarboxylic acid to the first titanium dioxide dispersion to obtain a second titanium dioxide dispersion.

4. The method for producing a titanium dioxide dispersion according to claim 3, wherein the content of the second hydroxycarboxylic acid in the second titanium dioxide dispersion is 0.1% by mass or more and 10% by mass or less.

5. The first step involves dissolving titanium metal powder in a hydrogen peroxide solution and an ammonia solution to prepare a peroxotitanic acid complex solution, A second step involves adding a primary hydroxycarboxylic acid to the peroxotitanic acid complex solution to obtain a water-soluble titanium complex solution. A third step involves hydrothermally treating the aforementioned water-soluble titanium complex solution to obtain a first titanium oxide dispersion, A method for producing a titanium dioxide dispersion, comprising a fourth step of adding a second hydroxycarboxylic acid to the first titanium dioxide dispersion to obtain a second titanium dioxide dispersion.

6. The method for producing a titanium dioxide dispersion according to claim 5, wherein the content of the second hydroxycarboxylic acid in the second titanium dioxide dispersion is 0.1% by mass or more and 10% by mass or less.

7. A titanium dioxide dispersion obtained by the method for producing a titanium dioxide dispersion according to any one of claims 1 to 6.

8. A titanium dioxide dispersion containing a hydroxycarboxylic acid and titanium dioxide, The titanium dioxide particles in the titanium dioxide dispersion have a dispersibility of 50 nm or less. A titanium dioxide dispersion having a hydroxycarboxylic acid content of 0.1% by mass or more and 10% by mass or less.

9. A step of synthesizing a titanium dioxide dispersion using the method for producing a titanium dioxide dispersion according to any one of claims 1 to 6, The process involves forming a film on a resin substrate using the obtained titanium dioxide dispersion, and then removing the solvent at a temperature of 300°C or lower to obtain a titanium dioxide (TiOx) film. A method for producing a titanium oxide film, including [the specified component].

10. A method for producing a titanium oxide film according to claim 9, wherein the thickness of the titanium oxide film is 100 nm or less.

11. A titanium oxide film obtained by removing the solvent from the titanium oxide dispersion described in claim 7.

12. The titanium oxide film described in claim 11 is included, A titanium oxide electron transport layer wherein the titanium oxide dispersion contains titanium oxide particles having at least one crystalline structure selected from the group consisting of rutile, brookite, bronze, and anatase.

13. The titanium oxide electron transport layer according to claim 12, comprising a perovskite solar cell or a dye-sensitized solar cell.

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