JP4562467B2 - Viscous dispersion containing semiconductor nanoparticles - Google Patents

Description

  The present invention relates to a highly viscous paste made only of semiconductor nanoparticles and a dispersion medium substantially free of an insulating organic binder such as a resin.

  Semiconductor nanoparticles represented by titanium dioxide nanoparticles are used as materials for forming ultrathin or porous films in the field of photocatalysts, in the field of electronics elements such as capacitors, capacitors, and batteries, and in the field of energy such as fuel cells and solar cells. Widely used. As a photocatalyst material, a film forming material containing titanium dioxide nanoparticles has been put to practical use in industry in the form of a paste or spray, and is used as a coating material or a surface modifying material. Also in the field of energy, development of power storage elements and dye-sensitized solar cells using semiconductor nanoparticles having a large specific surface area as an electrode material has become active. The dye-sensitized solar cell is a low-cost solar cell that replaces the conventional solid-junction solar cell that uses a silicon pn junction or a compound semiconductor heterojunction as a photovoltaic layer. This is particularly important as a technology used.

  The basic technology of the dye-sensitized solar cell is disclosed in Non-Patent Document 1 and Patent Document 1. This dye-sensitized solar cell has responded to visible light up to 800 nm and has already achieved an energy conversion efficiency of 10% or more, and has an energy conversion efficiency of 15% or more that surpasses that of amorphous silicon solar cells. Energetic research is continued toward the realization of.

  In particular, research on dye-sensitized solar cells that have different characteristics from silicon solar cells and that are colorful and excellent in transparency, especially on film-type dye-sensitized solar cells that use plastic as a substrate, is active. It is. In conventional glass-type dye-sensitized solar cells, a porous semiconductor is formed by applying a viscous semiconductor nanoparticle-containing viscous dispersion containing a thickening binder and burning the binder at a high temperature (450 ° C. or higher). In contrast to forming a film, a film-type dye-sensitized solar cell using a plastic as a substrate requires a film forming step at a low temperature. As a method for forming a semiconductor porous film at a low temperature necessary for manufacturing such a film-type solar cell, formation of a semiconductor porous film using an electrophoresis method has been proposed (see Non-Patent Document 2 and Patent Document 2). . In addition, a so-called pressing method in which a dispersion of semiconductor fine particles is coated on an electrode support and formed into a film by applying pressure has been proposed (see Patent Document 3).

  In these methods, the semiconductor porous film can be formed at a low temperature of 150 ° C. or less, which is within the heat resistance range of the plastic electrode, and the roll production method used in the printing field can be applied. There is an advantage that a solar cell can be manufactured. However, the solar cell using the electrode obtained in this manner has an energy efficiency of about 1 to 3%, and has a drawback that it is lower in efficiency than a glass electrode manufactured by a conventional baking method.

This is because the impurities derived from the raw materials are completely removed in the conventional high-temperature firing step, but these impurities are not completely removed in the press method or other low-temperature film forming methods, and the semiconductor particles are dispersed in the solvent. This is because impurities (mostly organic substances) present in the film and binder organic substances added in a small amount for film formation are mixed in the porous semiconductor film as an insulating substance. Therefore, in the low-temperature film formation, the contamination of the polymer and organic impurities used as the binder material is reduced to a certain level or less, and a high-purity dye-sensitized semiconductor film substantially containing no binder is formed. There is a strong demand to produce a large area film type solar cell.
US Pat. No. 4,927,721 (Claims and others) JP 2002-100416 A (Claims and others) International Publication No. 00/72373 (Claims and others) “Nature”, Volume 353, p737-740, 1991. “Chemistry letters”, 2002, p1250.

  The present invention has been completed on the basis of research on a viscous dispersion liquid containing semiconductor nanoparticles useful for forming a high-purity porous semiconductor film by low-temperature film formation by the present inventor.

  As a result of studying the composition of an optimal viscous liquid composition for forming a semiconductor porous film by low-temperature film formation on a plastic film, the present inventors have found that a solvent (dispersion medium) for dispersing the semiconductor nanoparticles is used. And a semiconductor nanoparticle-containing viscous dispersion capable of forming a porous thin film with good adhesion on the film by selecting the type of the solvent and the mixing ratio of the solvent and the semiconductor-containing sol. .

The present invention is a dispersion liquid in which crystalline semiconductor nanoparticles made of a rutile, anatase, or brookite type metal oxide having an average particle diameter of 10 nm or more and less than 150 nm are dispersed in a dispersion medium, dispersion media, including the parent aqueous organic solvent from 53 to 92 wt% and the aqueous 8-47 wt%, a possible free solvent mixture comprising organic binder 2 mass% or more, the hydrophilic organic solvent is tert- The viscous dispersion is characterized in that it contains 70% by volume or more of butanol, and the crystalline semiconductor nanoparticles are dispersed in an amount of 15 to 38 % by mass based on the entire dispersion.

  By using the semiconductor nanoparticle-containing viscous dispersion of the present invention, it is possible to form a semiconductor porous film on a general-purpose film and a conductive film by a coating film-forming process at a low temperature. Therefore, by using the semiconductor nanoparticle-containing viscous dispersion of the present invention, it is possible to assemble a film type dye-sensitized photocell excellent in energy conversion efficiency and storage durability.

  The semiconductor nanoparticle-containing viscous dispersion liquid (hereinafter also referred to as “paste”) of the present invention forms a porous semiconductor thin film by forming a porous semiconductor layer material typified by titanium dioxide on a substrate at a low temperature. It is particularly effectively used for the purpose, and in particular, it can be used effectively for the production of a plastic film electrode requiring film formation at a low temperature. This paste is a viscous white opaque liquid in which crystalline semiconductor nanoparticles are dispersed as the main component, and is a resin that is usually added for the purpose of increasing the viscosity or for increasing the adhesion to the substrate during film formation. Since the binder material such as latex and latex is substantially not contained or contained only in a small amount below a limited content, the conductivity of the formed porous semiconductor thin film is maintained at a high level.

  The crystalline nanoparticles contained in the paste of the present invention can be produced using a known method. As the production method, for example, the sol-gel method described in “Science of Sol-Gel Method”, Agne Jofu Co., Ltd. (1998), and oxides are produced by high-temperature hydrolysis of metal chlorides in oxyhydrogen salts. Or a so-called spray pyrolysis method in the gas phase, in which the metal compound is pyrolyzed in a gas phase at a high temperature to form ultrafine particles. The ultrafine particles and nanoparticles of titanium dioxide produced by these methods are described in the supervision of Hiroaki Yanagida, Volume II of Fine Particle Engineering, Volume II, Applied Technology, Fuji Techno System (2002).

The paste of the present invention contains nanoparticles of a crystalline semiconductor material as a main component. As the semiconductor material, a metal oxide can be used. Examples of metal elements of these oxides include titanium, tin, zinc, iron, tungsten, zirconium, strontium, indium, cerium, vanadium, niobium, tantalum, cadmium, zinc, lead, antimony, bismuth, cadmium, lead, and the like. . Preferred examples of the metal compound having a perovskite structure include strontium titanate, calcium titanate, sodium titanate, barium titanate, and potassium niobate.

Preferred semiconductor materials include n-type inorganic semiconductor materials such as TiO ₂ , TiSrO ₃ , ZnO, Nb ₂ O ₃ , SnO ₂ , WO ₃ , Si, CdS, CdSe, V ₂ O ₅ , ZnS, ZnSe, SnSe, There are KTaO ₃ , FeS ₂ , PbS, and the like. Among these, a more preferable semiconductor is a semiconductor material made of a metal oxide. Preferred materials include TiO ₂ , ZnO, SnO ₂ , WO ₃ and Nb ₂ O ₃ . Of these, titanium dioxide (TiO ₂ ) is particularly preferable.

  When crystalline titanium dioxide nanoparticles are used in the paste of the present invention, the titanium dioxide nanoparticles have any one of the rutile, anatase, and brookite crystal structures. Among them, preferred particles for the paste of the present invention are anatase type crystals and brookite type crystals, and the paste of the present invention preferably contains at least a mixture of particles of anatase type crystals and brookite type crystals. The crystal structure can be verified by measuring a diffraction pattern by an X-ray diffraction method or detecting a crystal lattice image by observation with a transmission electron microscope. The crystal structure can be determined by an X-ray diffraction pattern. The shape of the titanium dioxide particles may be various shapes such as amorphous, sphere, polyhedron, fiber, and nanotube, but polyhedron and nanotube are preferred, and polyhedron is particularly preferred.

Semiconductor nanoparticles (for example, titanium dioxide nanoparticles) in the paste for the size of an average particle size, 10 nm or more, Ru der less than 150 nm. The average particle size is more preferably 15 nm or more and 100 nm or less. The average particle diameter is most preferably 20 nm or more and 80 nm or less. The average particle diameter of the nanoparticles is calculated from a particle size distribution measured by, for example, an optical correlation method using a laser light scattering method or observation with a scanning electron microscope.

  The semiconductor nanoparticles (eg, titanium dioxide nanoparticles) contained in the paste may include two or more kinds of fine particles having different average particle diameters and particle size distributions. That is, in addition to nanoparticles, fine particles having a large average particle diameter can be mixed. In this case, it is preferable that crystalline titanium dioxide particles having an average particle diameter of 150 nm or more and 600 nm or less are included as large particles. Such large particles can be added to the nanoparticles at a mass ratio of 5 to 80%, and preferably added at a mass ratio of 10 to 50%.

Semiconductor nanoparticles, based on the weight of the total composition of the paste, Ru 15-38% by mass.

  In the paste of the present invention, various inorganic compounds other than semiconductors can be mixed as additives. Examples of inorganic compounds include various oxides, semiconductor materials, and conductive materials. Inorganic oxides include metals, alkali metals, alkaline earth metals, transition metals, rare earth oxides, lanthanoids, and non-metallic oxides such as Si, P, and Se. Here, examples of metals include Al, Ge, Sn, In, Sb, Tl, Pb, Bi, and the like. Examples of alkali metals and alkaline earth metals include Li, Mg, Ca, Sr, Ba, and the like. Can be mentioned. Examples of transition metals include Ti, V, Cr, Mn, Fe, Ni, Zn, Zr, Nb, Mo, Ru, Pd, W, Os, Ir, Pt, Au, and the like. Examples of the conductive material include metals, noble metals, and carbon-based materials.

  The paste of the present invention is a viscous liquid composition having a sufficiently high viscosity necessary for coating, and the viscosity is preferably 800 mPa · s or more. Here, 1 mPa · s corresponds to 1 centipoise. The viscosity of the paste can be measured by a capillary tube viscosity measurement method, a rotational viscosity measurement method, or the like. The viscosity of the paste of the present invention is more preferably 1000 mPa · s or more. Further, the viscosity of the paste is preferably 3000 mPa · s or more and 15000 mPa · s or less.

In the viscous liquid composition of the present invention, a mixed solvent of a hydrophilic organic solvent mainly composed of tert-butanol and water is used as a dispersion medium . Na us, the hydrophilic organic solvent, the number of carbon atom is 3-5 alcohol, and other lower alcohols such as methanol or ethanol, a small amount other hydrophilic organic solvents such as acetone and ether (30 wt% or less , Preferably 20% by mass or less, particularly preferably 10% by mass or less, and further 5% by mass or less).

  In the paste of the present invention, water is mixed as a dispersion solvent in addition to alcohol. Water is added for the purpose of improving the dispersion of the nanoparticles and maintaining the viscosity of the paste at an appropriate level. The water content is 8% by mass or more and 43% by mass or less in the mixed solvent. The water content is preferably 15% by mass or more and 35% by mass or less. Especially, it is preferable that they are 15 mass% or more and 25 mass% or less. Moreover, it is preferable that the volume ratio of the water and alcohol which comprise this paste exists in the range of 1: 7-1: 1.7. The addition of water is particularly effective for pastes using oxide semiconductor nanoparticles such as titanium dioxide nanoparticles.

  When the organic paste is mixed with the paste of the present invention, it is preferable to use the binder under a condition where the binder content is below a certain level. The binder means a binding aid having an effect on bonding between particles and adhesion between the particles and the substrate, and includes a resin material, a polymer material, and wax. About the paste of this invention, content of the binder which consists of organic substances needs to be less than 2% with respect to the total mass of a paste. Moreover, it is preferable that the paste of this invention does not contain a binder substantially. Here, the phrase “substantially free of binder” means that the content of the organic binder in the mass of the entire composition is 1% by mass or less. The most preferable condition is that the binder content is adjusted to 0.5% by mass or less. Examples of the binder resin include polyethylene glycol, methyl cellulose, ethyl cellulose, polyvinylidene fluoride, polymethyl methacrylate, polyacrylonitrile, and the like.

  The paste of the present invention is preferably an acidic liquid for the purpose of preventing aggregation of oxide semiconductor nanoparticles, and is preferably an acidic liquid having a pH of 1 to 6. Further, it is more preferably an acidic liquid having a pH of 3 to 5.

  An electrode coated with a porous metal oxide semiconductor layer can be produced by applying the paste of the present invention on an electrode substrate and subjecting it to a low-temperature heat treatment. In other words, the paste of the present invention was applied to a substrate with a thickness of 50 μm to 200 μm, and the obtained liquid film was dried, and then subjected to heat treatment at a low temperature of room temperature to 150 ° C., thereby being closely adhered to the substrate. A metal oxide semiconductor layer is obtained. The heat treatment at a low temperature is preferably performed in the range of 120 to 150 ° C. The porous layer thus produced is a mesoporous film having nano-sized pores. For coating with the pace of the present invention, a doctor blade method, a squeegee method, a screen printing method, or the like can be used.

  As the substrate on which the paste is applied, a substrate or an electrode substrate made of a material such as glass, metal, or plastic can be used, but a substrate or an electrode made of a flexible plastic support is preferable. Particularly preferable is a transparent conductive plastic film useful for use as an electrode, and it is particularly preferable to use a transparent conductive plastic film having a surface resistance of 20Ω / □ or less. A preferable electrode prepared using the paste of the present invention is a plastic electrode in which a porous semiconductor layer is coated on the surface of a transparent conductive plastic film having a surface resistance of 20Ω / □ or less. The thickness of the plastic electrode including the porous semiconductor layer is preferably in the range of 150 to 700 μm, and particularly preferably in the range of 200 to 450 μm. The thickness of the plastic support alone is preferably from 140 to 650 μm, particularly preferably from 180 to 400 μm.

  The transparent conductive plastic film used for coating the paste of the present invention is composed of a conductive layer and a plastic support that carries the conductive layer. For the plastic support of the transparent conductive plastic film, a material that is uncolored and has high transparency, high heat resistance, excellent chemical resistance and gas barrier properties, and low cost is preferably selected. From this viewpoint, preferable plastic materials include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), syndiotactic polystyrene (SPS), polyphenylene sulfide (PPS), polycarbonate (PC), polyarylate (PAr), polysulfone. (PSF), polyester sulfone (PES), polyetherimide (PEI), transparent polyimide (PI), etc. are used. Among these, polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are particularly preferable in terms of chemical resistance and cost.

  For the conductive layer of the transparent conductive plastic support, as a conductive material, metals such as platinum, gold, silver, copper, aluminum and indium, carbon or conductive metal oxides such as indium-tin composite oxide, tin oxide, etc. Can be used. Of these, conductive metal oxides are preferable from the viewpoint of optical transparency, and indium-tin composite oxide (ITO) and zinc oxide are particularly preferable. The conductive layer needs to have a low surface resistance (or sheet resistance), and the surface resistance value is preferably 20 Ω / □ or less. The surface resistance value is preferably 10Ω / □ or less, more preferably 3Ω / □ or less. Auxiliary leads for collecting current can be arranged on the conductive layer by patterning or the like. Such auxiliary leads are usually formed of a low-resistance metal material such as copper, silver, aluminum, platinum, gold, titanium, or nickel. In such a transparent conductive layer patterned with auxiliary leads, the surface resistance value is measured as the resistance value of the surface including the auxiliary leads, and the value is preferably 10Ω / □ or less, more preferably 3Ω / □ or less. is there. Such an auxiliary lead pattern is preferably formed on a transparent plastic substrate, and a transparent conductive layer made of an ITO film or the like is further provided thereon.

  A plastic electrode carrying a semiconductor porous film produced as described above using the paste of the present invention can be used as a dye-sensitized electrode. In order to produce a dye-sensitized electrode, it is necessary to sensitize the surface of the metal oxide semiconductor layer by adsorption of the dye. As the dye molecules used for sensitization, known sensitizing materials that have been used so far in dye-sensitized semiconductors are widely used. Examples of such dyes include organic dyes such as cyanine, merocyanine, oxonol, xanthene, squarylium, polymethine, coumarin, riboflavin, and perylene, Ru complexes, metal phthalocyanine derivatives, metal porphyrin derivatives, There are complex dyes such as chlorophyll derivatives. In addition, synthetic dyes and natural dyes described in “Functional Materials”, June 2003, pages 5 to 18 and “Journal of Chemical Physics” (J. Chem. Phys.), B.C. An organic dye mainly composed of coumarin described in Vol. 107, page 597 (2003) can also be used.

  The porous semiconductor layer formed on the plastic electrode substrate is substantially composed only of an inorganic compound material containing a semiconductor and a dye. The term “consisting essentially of an inorganic compound material containing a semiconductor and a pigment only” means that the inorganic compound material containing a semiconductor and the pigment are the main components of the particle layer, and the total mass of these components is the particle layer. Means approximately equal to the mass of the total solid content. Examples of solid components other than the two types of inorganic compound materials including semiconductors and pigments that may be mixed in the particle layer include a small amount of a polymer resin binder and a carbon material. The ratio of the solid content excluding the inorganic compound material containing the semiconductor and the pigment to the total mass of the porous semiconductor layer is preferably 1% by mass or less.

  In the present invention, the ratio of the inorganic compound containing a semiconductor to the total mass of the porous particle layer can be measured, for example, by the following method. That is, the porous particle layer is peeled from the plastic support, and as a substance other than the particle layer constituent component contained in the porous particle layer, a liquid component or a solid component derived from a constituent component such as an electrolytic solution is used. The solvent is used to wash away only the particle layer, and then the particle layer is dried and weighed. This mass is the mass of the total solid content.

  Next, the total solid content is thoroughly washed with a polar organic solvent such as alcohol and acetonitrile and a nonpolar organic solvent such as toluene and chloroform to remove the organic substance, and then the particle layer is heated to 400 ° C. in an oxygen atmosphere or in air. Heat for 1 hour or longer, and weigh the residue after heating. The dry mass divided by the mass of the total solid is the ratio of the mass of the inorganic compound containing the semiconductor to the mass of the total solid.

  The proportion of the dye mass in the dye-sensitized electrode can be measured, for example, by the following method. That is, after peeling off the particle layer from the plastic support and weighing the total mass, the particle layer is sufficiently washed with an organic solvent effective for elution of the dye such as water, methanol, acetonitrile, and the dye is removed from the particle layer. Detach and wash the particle layer to such an extent that the color of the dye hardly remains. After removing the solvent from the washing solution containing the dye by evaporation, the dry mass of the remaining dye is weighed. A value obtained by dividing the dry mass of the pigment by the total solid mass is the ratio of the target pigment.

  Moreover, what subtracted the sum total of the ratio of the inorganic compound calculated | required in this way and the ratio of a pigment | dye from 1 is calculated | required as a ratio which the mass of solid content except an inorganic compound and a pigment | dye accounts for the total mass of this particle layer. What is contained as solid content except an inorganic compound and a pigment | dye includes binders, such as a polymer resin, for example.

  In the porous semiconductor layer formed by applying the paste of the present invention, the porosity shown by the volume fraction occupied by the pores in the layer is preferably in the range of 40% to 85%. The porosity is particularly preferably in the range of 50% to 75%.

  A dye-sensitized solar cell and a photovoltaic cell can be produced using a porous metal oxide semiconductor electrode produced by applying the paste of the present invention on a plastic electrode. As the porous metal oxide semiconductor layer, a titanium dioxide layer exhibits particularly excellent performance. A mechanically flexible film composed of a multilayer body in which a dye-sensitized plastic electrode obtained by adsorbing a dye to a porous titanium dioxide layer is used as a photoelectrode, and an ion conductive layer and a counter electrode are laminated thereon. Type solar cells and photoelectric conversion elements can be produced.

As the ionic conductive electrolyte layer used for the film type solar cell, an aqueous electrolyte solution, an organic solvent electrolyte solution, an ionic liquid electrolyte solution (molten salt electrolyte solution), or the like can be used. As an oxidation-reduction agent contained in these electrolyte solutions, a combination of I ₂ and iodide (as iodide, metal iodide such as LiI, NaI, KI, etc., tetraalkylammonium iodide, pyridinium iodide, imidazolium iodide) Electrolyte containing iodine salt of quaternary ammonium compound such as id, etc., Br ₂ and bromide combination (as bromide metal bromide such as LiBr, NaBr, KBr, etc., or quaternary ammonium such as tetraalkylammonium bromide, pyridinium bromide) In addition to electrolyte solutions containing compound bromine salts, etc., metal complexes such as ferrocyanate-ferricyanate and ferrocene-ferricinium ions, sulfur compounds such as sodium polysulfide and alkylthiol-alkyldisulfides are used. Can wear. Among these, an electrolyte in which an iodine salt of a quaternary ammonium compound such as I ₂ and LiI, pyridinium iodide, imidazolium iodide or the like is combined is preferable in terms of providing high performance as a photovoltaic cell.

  For the conductive layer of the counter electrode of the solar cell, metals such as platinum, gold, silver, copper, titanium, aluminum, magnesium, and indium, carbon-based materials or conductive metal oxides, indium-tin composite oxide (ITO), fluorine Conductive metal oxides such as doped tin oxide (FTO) can be used as the conductive material. Of these, platinum, titanium, ITO film, and carbon-based material are preferable in terms of excellent corrosion resistance.

  In addition to the above basic layer structure, various types of layers can be further provided as desired in the film type photovoltaic cell made using the paste of the present invention. For example, a dense semiconductor thin film layer can be provided as an undercoat layer between the conductive plastic support and the porous semiconductor layer.

A metal oxide is preferable as the undercoat layer, and examples thereof include TiO ₂ , SnO ₂ , Fe ₂ O ₃ , WO ₃ , ZnO, and Nb ₂ O ₅ . The undercoat layer is, for example, Electrochim. In addition to the spray pyrolysis method described in Acta 40, 643-352 (1995), it can be applied by sputtering or the like. The preferred thickness of the undercoat layer is 5 to 100 nm. In addition, a functional layer such as a protective layer, an antireflection layer, or a gas barrier layer is provided on the outer surface of one or both of the porous semiconductor electrode and the counter electrode acting as a photoelectrode, between the conductive layer and the substrate, or in the middle of the substrate. May be. These functional layers can be formed by a coating method, a vapor deposition method, a bonding method, or the like depending on the material.

  Next, the best mode for carrying out the present invention will be described as examples.

[Preparation of viscous liquid composition]
Contains rutile / anatase mixed crystalline titanium dioxide nanoparticles (average particle size 60 nm) and rutile crystalline titanium dioxide particles (average particle size about 300 nm, particle size distribution 200-500 nm) in a mass ratio of 5: 1. An acidic sol aqueous solution (concentration: 20% by mass) in which 21 g of particle powder and titanium dioxide nanoparticles (particle size: 10 to 30 nm) containing brookite-type crystal particles were dispersed was mixed with 150 mL of tert-butanol. A white viscous liquid composition (mass: about 220 g) was prepared by uniformly stirring and mixing this mixture with a rotating / revolving mixing conditioner. The content of water in this paste was 25% in volume and 27% in mass with respect to the total composition, and the volume ratio of water to alcohol was 1: 2.7. The content of titanium dioxide in the paste was 16% by mass. This paste consisted of only titanium dioxide and a solvent, and was a viscous binder-free paste containing no binder. The viscosity measured by a rotational viscometer was 2400 mO · s. The paste was an acidic liquid and the pH was 4.

  As comparative experiments, various pastes having different compositions from the above pastes were prepared. First, various pastes with different water contents were prepared by changing the amounts of the sol aqueous solution and alcohol used. Secondly, a paste was prepared using 2-propanol (isopropyl alcohol), 1-pentanol and 1-hexanol instead of tert-butanol as the dispersing alcohol. Third, the amount of titanium dioxide particle powder used was changed to prepare a paste in which the content of titanium oxide in the paste was changed in the range of 5% by mass to 50% by mass. Fourthly, mixed crystalline particles (average particle size 15 nm) of rutile and anatase not containing brookite crystalline particles were dispersed in a sol aqueous solution in which titanium dioxide nanoparticles containing brookite crystalline particles were dispersed. A paste was prepared in place of the aqueous solution.

[Characteristic evaluation of paste]
The pastes having various compositions described above were evaluated for viscosity, coating performance, and storage stability. The coating performance was evaluated as follows. The paste was applied to a polyethylene terephthalate film having a thickness of 125 μm with a liquid film thickness of 100 μm by a squeegee method, and the coated film was dried at room temperature and further dried at 150 ° C. for 5 minutes. The quality of the obtained dry film was evaluated from two viewpoints. The first point is the uniformity of the surface state of the film, which was visually evaluated. The second point is the strength of adhesion of the semiconductor film. A fatigue test is performed in which the film is mechanically bent 10 times to a curvature of 1.0 cm ⁻¹ , and after this fatigue test, the state of peeling of the porous semiconductor layer is visually observed. Judged by. The results of these evaluations were determined in three stages: A: very good, B: good, inferior to C: B, but practical.

  For storage stability, the paste in a sealed container was stored at 4 ° C. in a refrigerator for 30 days under light shielding for 30 days. After storage, the container is shaken by hand, the paste is stirred, the viscosity is measured again, and the storage stability is evaluated based on the change in viscosity and coating performance. A: very good, B: good, C : It was inferior to B, but it was judged in three steps: practical. Table 1 shows the composition of the paste thus prepared, and Table 2 shows the evaluation results of paste viscosity, coating performance, and storage stability.

[Preparation of various semiconductor nanoparticle pastes]
In place of the above titanium dioxide, tin oxide (average particle size 35 nm), zinc oxide (average particle size 60 nm), cadmium sulfide (particle size 10 to 50 nm) were used as semiconductor nanocrystals in the same manner as in the above examples. A binder-free paste was prepared. These semiconductors were mixed with an aqueous gel solution of titanium dioxide nanoparticles containing brookite-type crystal particles as in the above example, and tert-butanol was used as the alcohol. The water content in the obtained paste was 23 to 30% by mass with respect to the total composition, and the semiconductor content was 15 to 22% by mass. These three types of pastes were evaluated for viscosity, coating performance, and storage stability. The composition of the paste is shown in Table 1, and the evaluation results are shown in Table 2.

[Preparation of dye-sensitized solar cell using paste]
(1) Production of Plastic Film Electrode As a transparent conductive plastic film, polyethylene terephthalate (PET) having a film thickness of 200 μm and a surface resistance of 15 Ω / □ that carries ITO as a conductive film was used. In order to reduce the surface resistance of this conductive film, a silver dispersed paste is screen-printed, and an auxiliary lead wire for collecting silver having a line width of 100 μm and a thickness of 20 μm is formed on the ITO film in a parallel line shape with a gap of 10 mm. Patterned. On these silver patterns, a polyester resin was applied as a protective film with a width of 250 μm to completely protect the silver wire. The practical sheet resistance of the obtained patterned conductive ITO-PET film was 3Ω / □.

  The ITO surface of this ITO-PET film was coated with the tert-butanol-dispersed titanium dioxide dispersion paste (water content: 27% by mass) prepared in Example 1 above by a doctor blade method at a liquid thickness of 100 μm, and room temperature. And dried at 150 ° C. for 5 minutes to produce a film electrode carrying a porous titanium oxide particle layer.

  In order to produce a comparative electrode, first, a polyethylene binder (PEG) powder having an average molecular weight of 50,000 as a solid content other than a semiconductor material was added as a resin binder during the composition of the paste for producing the electrode. Then, a binder-containing paste was prepared. The PEG content was changed in the range of 0.2% by mass to 5% by mass for comparison. In addition, as a comparison of the differences in the semiconductor nanoparticles, the tin oxide-containing paste and zinc oxide-containing paste prepared in the above examples were used to carry a film electrode carrying a porous tin oxide particle layer and a zinc oxide particle layer. A film electrode was prepared.

(2) Preparation of dye-sensitized solar cell Tetrabutylammonium salt of bisisocyanate bisbipyridyl Ru complex (N719) as a Ru bipyridyl complex dye was mixed in a mixed solvent of acetonitrile: tert-butanol (1: 1) at a concentration of 3 × 10 ⁻ The porous semiconductor film electrode substrate is immersed in a dye solution dissolved in ⁴ mol / liter, and left at 40 ° C. for 60 minutes with stirring to complete dye adsorption, and dye-sensitized titanium oxide ITO-PET film electrode Was made.

  A sheet resistance of 0.8 Ω / □ is formed by coating a 100-nm-thick platinum film on the ITO surface of a polyethylene terephthalate (PET) film having a thickness of 400 μm and a surface resistance of 15 Ω / □ supporting ITO as a conductive film as a counter electrode. A conductive film was used.

The semiconductor layer of the dye-sensitized ITO-PET film electrode was scraped off from the film to form a light receiving layer having a light receiving area of 40 cm ² (5 cm × 8 cm). The platinum-deposited ITO-PET film of the above-mentioned counter electrode was superimposed on this electrode, and a nonaqueous organic electrolyte composed of propylene carbonate, tert-butylpyridine, lithium iodide and iodine was injected into the gap by the capillary effect. An epoxy thermal effect type sealing material was injected into the edge portion of the sandwich type film battery thus produced, and a curing treatment was performed at 110 ° C. for 20 minutes. The film-type photovoltaic cell of the business card size thus assembled has a thickness of about 600 μm and a weight of 3.6 g.

(3) Evaluation of photoelectric conversion characteristics of film-type solar cell AM1.5 simulated solar with an incident light intensity of 100 mW / cm ² for the above-mentioned film-type photovoltaic cell using a solar simulator equipped with a 500 W xenon lamp. Light was irradiated from the dye-sensitized semiconductor film electrode side. The battery was fixed in close contact with the stage of the thermostat, and the temperature of the element during irradiation was controlled at 40 ° C. The photocurrent-voltage characteristics were measured by scanning the DC voltage applied to the element at a constant speed of 10 mV / sec and measuring the photocurrent output from the element using a current-voltage measuring device. Table 3 shows the short-circuit photocurrent density (J _sc ) and energy conversion efficiency (η) of the above-described various elements, together with the composition of the paste used for coating the film electrode.
In Tables 1 to 3, the paste No. 3 to 7, 14, 15, and 17 to 19 satisfy the definition of the present invention.

From the results in Table 1, the following is clear regarding the practicality of the paste for forming a porous semiconductor layer.
1) In a paste composition that does not contain a binder, those whose water content is less than 8% by mass are low in viscosity and flow in application, so that they cannot be applied and precipitate due to storage, so that they are used as application pastes. I can't. In addition, the water content exceeding 47% by mass cannot be used as a coating paste because it does not adhere to the surface of the film substrate when applied, in addition to the same low viscosity. In the case where the water content was in the range of 8% by mass to 47% by mass, the paste having particularly good coating performance was a paste having a water content of 15 to 35% by mass.
2) Regarding the type of alcohol in the composition, butanol (carbon number 4) and propanol (carbon number 3) gave pastes with good viscosity and coating performance, but pentanol (carbon number 5) gave paste quality. Decreased slightly. As a result of phase separation from water with hexanol (6 carbon atoms), a paste that can withstand use cannot be prepared.
3) Regarding the content of the semiconductor nanoparticles in the composition, the paste having a content of less than 8% by mass decreases the viscosity and the uniformity of the coating film, and the paste having a content exceeding 40% by mass has a viscosity. With a substantial increase, particle agglomeration occurs and the uniformity and quality of the coating film decreases.
4) A paste containing brookite-type crystals in the semiconductor nanoparticles in the composition obtained a good paste by adjusting the water content to an optimum range, whereas a paste containing no brookite-type crystals was coated film The uniformity and adhesion are slightly reduced.
5) In place of titanium dioxide instead of titanium dioxide, metal oxide semiconductor tin oxide, zinc oxide, sulfide semiconductor cadmium sulfide nanoparticles can also be used to adjust the water content to the optimum range, thereby creating a viscous paste. It was shown to be obtained.

  Also, from the results in Table 3, it is clear that when the paste of the present invention is used for production of a photoelectrode material for a solar cell, it gives excellent practical performance of photoelectric conversion.

  As described above, a liquid paste containing semiconductor nanoparticles having the constitutional conditions disclosed in the present invention exhibits high viscosity and good storage stability even under composition conditions that do not contain a binder, and is obtained by coating on a film. The resulting porous membrane gives good film quality and high peeling durability. Moreover, the semiconductor film obtained by applying this paste to a film and drying it at a low temperature of 150 ° C. or less showed high conductivity, indicating that this paste is also useful as a paste for film electrode preparation. By using this film electrode, a dye-sensitized solar cell excellent in photoelectric conversion performance can be provided.

By using the paste of the present invention as a coating paste for the doctor blade method or the screen printing method, a semiconductor porous film excellent in film quality and adhesion can be obtained. By performing film formation at a low temperature using this paste as a film substrate, a film type dye-sensitized solar cell excellent in photoelectric conversion performance is provided.

Claims (12)

In a dispersion medium, a dispersion liquid in which crystalline semiconductor nanoparticles made of a rutile, anatase, or brookite metal oxide having an average particle diameter of 10 nm or more and less than 150 nm are dispersed, the dispersion medium comprising : include parent-aqueous organic solvent from 53 to 92 wt% and the aqueous 8-47 wt%, a possible free solvent mixture comprising organic binder 2 mass% or more, 70 volumes of hydrophilic organic solvents is tert- butanol %, And the crystalline semiconductor nanoparticles are dispersed in an amount of 15 to 38 % by mass with respect to the total dispersion.   The viscous dispersion according to claim 1, which is used for forming a porous semiconductor layer. The viscous dispersion liquid according to claim 1 or 2, wherein the crystalline semiconductor nanoparticles have an average particle diameter of 15 nm or more and 100 nm or less . The viscous dispersion according to any one of claims 1 to 3, wherein the metal oxide is titanium dioxide . The viscous dispersion liquid according to claim 4 , wherein the crystalline semiconductor nanoparticles include brookite-type crystals of titanium dioxide. Viscous dispersion according to ing請 Motomeko 5 from a mixture of brookite-type crystal of the crystalline semiconductor nanoparticles anatase titanium dioxide titanium dioxide.   The viscous dispersion according to any one of claims 1 to 6, wherein the viscosity is 800 mPa · s or more.   The viscous dispersion liquid according to claim 7, wherein the viscosity is 3000 to 15000 mPa · s. The viscous dispersion according to claim 1, wherein the content of the hydrophilic organic solvent containing 70% by volume or more of tert-butanol in the solvent mixture is 65 to 85% by mass and the content of water is 15 to 35% by mass. liquid. The viscous dispersion liquid according to claim 1 , further comprising crystalline titanium dioxide particles having an average particle diameter of 150 nm or more and 600 nm or less in a proportion of 5 to 80% by mass with respect to the crystalline semiconductor nanoparticles .   The viscous dispersion according to claim 1, wherein the solvent mixture does not contain 1% by mass or more of the organic binder.   The viscous dispersion according to claim 1, wherein the organic binder is not contained in an amount of 2% by mass or more based on the crystalline semiconductor nanoparticles.