JP5709502B2 - Electrolytic solution and photoelectric conversion element - Google Patents

Description

  The present invention relates to an electrolytic solution and a photoelectric conversion element.

  In order to improve the conductivity of the electrolyte solution of a dye-sensitized solar cell, studies using imidazolium salts have been made. When the counter anion is iodine, liquid 1-methyl-3-propylimidazolium iodide, 1-butyl-3-methylimidazolium iodide, 1-hexyl-3-methylimidazolium iodide, etc. are used. Has been. For example, Patent Document 1 describes that a quaternary salt electrolyte having a proton dissociator such as a carboxylic acid group, a sulfonic acid group, a phosphonic acid group, or a phosphoric acid group is contained. Here, the reason why only the liquid electrolyte is used is to suppress an increase in viscosity.

  On the other hand, among the imidazolium salts, 1-ethyl-3-methylimidazolium iodide is a solid and is considered to increase in viscosity when added to an electrolyte, so that the diffusion rate of iodine ions is not lowered. The use of solid electrolyte salts such as 1-ethyl-3-methylimidazolium iodide has been avoided.

JP 2003-17148 A

  However, when the above liquid 1-methyl-3-propylimidazolium iodide, 1-butyl-3-methylimidazolium iodide, 1-hexyl-3-methylimidazolium iodide or the like is used, The effect of improving the conversion efficiency is not sufficient.

  Furthermore, in order to lower the viscosity of the electrolyte, it may be possible to use an imidazolium salt whose counter anion is dicyanamide, bis (fluorosulfonyl) imide, bis (trifluoromethanesulfonyl) imide, trifluoroacetate, trifluoromethanesulfonate, or the like. However, since an iodine anion and an anion other than iodine coexist in the electrolytic solution, there are problems in terms of stability and durability of the dye-sensitized solar cell.

  For these reasons, there is no electrolyte solution having sufficient photoelectric conversion efficiency when the dye-sensitized solar cell is assembled, and it is an object of the present invention to provide such an electrolyte solution.

  As a result of extensive research, the present inventor has solved the problem by containing iodine, lithium iodide, and 1-ethyl-3-methylimidazolium iodide in the electrolytic solution. That is, the short circuit current density (Jsc) is increased by the addition effect of lithium iodide, and the conductivity of the electrolyte is improved by the addition effect of 1-ethyl-3-methylimidazolium iodide, which has been avoided. The inventors have succeeded in developing an electrolyte solution for a dye-sensitized solar cell that exhibits high photoelectric conversion efficiency by reducing the resistance component in the dye-sensitized solar cell and increasing the fill factor (FF). In particular, by combining lithium iodide and 1-ethyl-3-methylimidazolium iodide, the short-circuit current density is unexpectedly higher than when a liquid imidazolium salt that has been conventionally used is used. As a result, an effect more than the effect of improving the short-circuit current density by the addition of lithium iodide was obtained, and the photoelectric conversion efficiency was greatly improved. The present invention has been completed based on such findings. That is, the present invention includes the following configurations.

  Item 1. An electrolytic solution containing iodine, lithium iodide, and 1-ethyl-3-methylimidazolium iodide.

  Item 2. Item 2. The electrolyte according to Item 1, comprising 1 to 5 mol of lithium iodide and 5 to 15 mol of 1-ethyl-3-methylimidazolium iodide with respect to 1 mol of iodine.

  Item 3. Item 3. The electrolyte according to Item 1 or 2, further comprising at least one basic substance selected from the group consisting of 4-tertiarybutylpyridine and N-methylbenzimidazole.

  Item 4. Item 4. The electrolytic solution according to Item 3, containing 5 to 15 mol of the basic substance with respect to 1 mol of iodine.

  Item 5. Item 5. The electrolytic solution according to any one of Items 1 to 4, wherein the counter anion present is only iodine ions.

  Item 6. Item 6. The electrolytic solution according to any one of Items 1 to 5, further comprising at least one solvent selected from the group consisting of nitrile compounds, lactone compounds, carbonate compounds, ethers, alcohols, and sulfones.

  Item 7. Item 7. A photoelectric conversion element obtained using the electrolyte solution according to any one of items 1 to 6.

  Item 8. Item 8. A dye-sensitized solar cell obtained using the photoelectric conversion device according to Item 7.

  When the electrolytic solution of the present invention is used for a photoelectric conversion element, the photoelectric conversion efficiency can be remarkably improved as compared with the case where a conventional liquid imidazolium salt is used.

1. Electrolyte <electrolyte>
The electrolytic solution of the present invention contains iodine, lithium iodide, and 1-ethyl-3-methylimidazolium iodide as the electrolyte.

Iodine and lithium iodide Iodine and lithium iodide form I ⁻ / I ₃ ⁻ which is a redox couple in the electrolytic solution of the present invention. Lithium ions generated by the addition of lithium iodide are adsorbed on, for example, porous titania used for a titania negative electrode of a dye-sensitized solar cell. As a result, there is an effect of lowering the titania conduction band and improving the electron injection rate from the dye to titania. It also has the effect of promoting the transport of electrons injected into titania. Thereby, a short circuit current density can be improved and a photoelectric conversion efficiency can be improved as a result.

  In addition, in the conventional electrolytic solution, the addition amount of lithium iodide is about 10 moles with respect to 1 mole of iodine in anticipation of an effect of improving the short-circuit current density. In order to prevent the open circuit voltage of the sensitized solar cell from being lowered, the amount of lithium iodide added is preferably about 1 to 5 mol, particularly about 2 to 4 mol, per 1 mol of iodine.

Addition of 1-ethyl-3-methylimidazolium iodide 1-ethyl-3-methylimidazolium iodide improved the conductivity in the electrolytic solution of the present invention and produced a dye-sensitized solar cell. In some cases, the resistance component of the cell can be lowered, the fill factor can be increased, and the photoelectric conversion efficiency can be improved. Further, as described above, by using 1-ethyl-3-methylimidazolium iodide in combination with lithium iodide, an effect more than the effect of improving the short-circuit current density by adding lithium iodide can be obtained. The photoelectric conversion efficiency can be greatly improved.

  In addition, as also shown in the examples described later, conventionally used liquid 1-methyl-3-propylimidazolium iodide, 1-butyl-3-methylimidazolium iodide, and the like were used. In this case, although there is an effect of improving the fill factor, the short-circuit current density is lowered, and as a result, sufficient photoelectric conversion efficiency cannot be obtained.

  In addition, in order to prevent the addition amount of 1-ethyl-3-methylimidazolium iodide from being excessively increased to increase the viscosity of the electrolytic solution of the present invention and decrease the diffusion rate of iodine ions, The added amount of ethyl-3-methylimidazolium is preferably about 5 to 15 mol, particularly about 8 to 12 mol, per 1 mol of iodine.

Other Components In addition to the above-described components, the electrolytic solution of the present invention may contain a basic substance such as 4-tertiarybutylpyridine, N-methylbenzimidazole and the like. When these basic substances are contained, when the photoelectric conversion element is produced, it is adsorbed on the titania surface of the titania electrode, can prevent reverse electron transfer from the titania electrode, and further improve the open-circuit voltage, Photoelectric conversion efficiency can be further improved.

  Further, in order not to desorb the sensitizing dye adsorbed on titania, the addition amount of the basic substance is about 5 to 15 mol, particularly 8 to 12 mol, with respect to 1 mol of iodine when the metal complex dye is used. The degree is preferred. Moreover, when using an organic pigment | dye, about 0.5-1.5 mol with respect to 1 mol of iodine, Especially about 0.8-1.2 mol are preferable.

  In addition, guanidine thiocyanate, which has the effect of lowering the conduction band of titania and improving the electron injection rate from the dye to titania, may be added to the electrolytic solution of the present invention, as in the case of lithium iodide described above. it can. In this case, the amount of these added may be about 1 to 5 mol, particularly about 2 to 4 mol, relative to 1 mol of iodine.

<Solvent>
You may use a solvent for the electrolyte solution of this invention. The solvent used in this case is not particularly limited, and nitrile compounds (acetonitrile, valeronitrile, 3-methoxypropionitrile, glutarodinitrile, methoxyacetonitrile, propionitrile, benzonitrile, biscyanoethyl ether, etc.), Lactone compounds (γ-butyrolactone, γ-valerolactone, β-butyrolactone, β-propiolactone, δ-valerolactone, ε-caprolactone, etc.), carbonate compounds (ethylene carbonate, propylene carbonate, etc.), ethers (dioxane, diethyl) Ether, ethylene glycol dialkyl ether, propylene glycol dialkyl ether, polyethylene glycol dialkyl ether, polypropylene glycol dialkyl ether, etc.), alcohols ( Tanol, ethanol, ethylene glycol monoalkyl ether, propylene glycol monoalkyl ether, polyethylene glycol monoalkyl ether, polypropylene glycol monoalkyl ether, ethylene glycol, propylene glycol, polyethylene glycol, polypropylene glycol, glycerin, etc.), sulfones (ethyl isopropyl sulfone) , Ethyl isobutyl sulfone, isopropyl isobutyl sulfone, etc.), and these solvents may be used alone or in combination of two or more. Among these solvents, nitrile compounds, particularly acetonitrile, valeronitrile, and 3-methoxypropionitrile are preferable because the diffusion rate of iodine ions can be increased.

  In addition, in the electrolytic solution of the present invention, in addition to the above components, a viscosity modifier (polyethylene glycol, etc.), a dehydrating agent (zeolite, silica gel, etc.) and the like may be included within a range not impairing the effects of the present invention. it can.

  However, in the electrolytic solution of the present invention, it is preferable that the counter anion present is only iodine ions. Thereby, movements other than iodine ions can be made smooth.

2. Photoelectric Conversion Element and Dye-Sensitized Solar Cell The photoelectric conversion element of the present invention is obtained by forming a counter electrode (counter electrode) on a porous titania film of a titania electrode and filling between these electrodes with the electrolytic solution of the present invention. It is done.

  The titania electrode is formed, for example, by forming a porous titania film on a resin substrate or a glass substrate.

Examples of titania used for the porous titania membrane include, for example, known or commercially available titania nanoparticles; known or commercially available titania nanotubes; titania nanorods; titania nanofibers; Etc.) may be used alone or in combination of two or more. In addition, “titania” does not refer to only titanium dioxide, but is represented by titanium dioxide (Ti ₂ O ₃ ); titanium monoxide (TiO); Ti ₄ O ₇ , Ti ₅ O _{9 and the} like. This includes a composition having oxygen deficiency from titanium. Further, as represented by the terminal OH group, a group other than Ti—O—Ti resulting from the synthesis of titanium oxide may be included.

  The resin substrate is not particularly limited as long as it is a conductive resin substrate. For example, polyester such as polyethylene naphthalate resin substrate (PEN resin substrate) and polyethylene terephthalate resin substrate (PET resin substrate); polyamide; polysulfone; polyether Examples include sulfone, polyether ether ketone, polyphenylene sulfide, polycarbonate, polyimide, polymethyl methacrylate, polystyrene, cellulose triacetate, and polymethylpentene.

  There is no restriction | limiting in particular also as a glass substrate, What is necessary is just to use a well-known or commercially available thing, and any of colorless or colored glass, meshed glass, a glass block etc. may be sufficient.

  As this resin substrate or glass substrate, one having a thickness of about 0.05 to 10 mm may be used.

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

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

  The method for forming the porous titania film on the resin substrate or the glass substrate is not particularly limited. For example, the above-described film-forming composition containing titania is prepared, and the method is performed on the resin substrate or the glass substrate. What is necessary is just to apply | coat and dry the said composition for film formation. Further, after drying, the obtained film may be subjected to a heat treatment as necessary to be baked.

  There are no particular restrictions on the application method, and conventional methods such as screen printing, dip coating, spray coating, spin coating, and squeegee method may be employed.

  Moreover, there is no restriction | limiting in particular in drying conditions and baking conditions, What is necessary is just to make drying temperature into about 60-250 degreeC and baking temperature to about 250-800 degreeC.

  What is necessary is just to apply | coat so that the film thickness of the film | membrane obtained may be set to about 0.5-50 micrometers in preparation of a porous titania film | membrane.

  The counter electrode may have a single layer structure made of a conductive material, or may be composed of a conductive layer and a substrate. The substrate is not particularly limited, and the material, thickness, dimensions, shape, and the like can be appropriately selected according to the purpose. For example, metal, colorless or colored glass, meshed glass, glass block, etc. are used. Resin may be used. Examples of such resins include polyesters such as polyethylene terephthalate, polyamide, polysulfone, polyether sulfone, polyether ether ketone, polyphenylene sulfide, polycarbonate, polyimide, polymethyl methacrylate, polystyrene, cellulose triacetate, and polymethylpentene. Alternatively, the counter electrode may be formed by applying, plating, or vapor-depositing (PVD, CVD) a conductive material directly on the charge transport layer.

  As the conductive material, metals such as platinum, gold, nickel, titanium, aluminum, copper, silver, and tungsten, and materials having a small specific resistance such as carbon materials and conductive organic substances are used.

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

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

  The dye is not particularly limited as long as it has absorption characteristics in the visible region and the near infrared region and improves (sensitizes) the light absorption efficiency of titania, but metal complex dyes, organic dyes, natural dyes, semiconductors, etc. preferable. In addition, in order to impart adsorptivity to the porous titania film, a carboxyl group, hydroxyl group, sulfonyl group, phosphonyl group, carboxylalkyl group, hydroxyalkyl group, sulfonylalkyl group, phosphonylalkyl group, etc. in the dye molecule Those having the functional group are preferably used.

  As the metal complex dye, for example, a ruthenium, osmium, iron, cobalt, zinc, mercury complex (for example, mellicle chromium), metal porphyrin, metal phthalocyanine, chlorophyll, or the like can be used. Examples of organic dyes include cyanine dyes, hemicyanine dyes, merocyanine dyes, xanthene dyes, triphenylmethane dyes, metal-free phthalocyanine dyes, perylene dyes, coumarin dyes, polyene dyes, indolines. Examples thereof include, but are not limited to, system dyes and caribasol dyes. As a semiconductor that can be used as a dye, an amorphous semiconductor having a large i-type light absorption coefficient, a direct transition semiconductor, or a fine particle semiconductor that exhibits a quantum size effect and efficiently absorbs visible light is preferable. Usually, one of various semiconductors, metal complex dyes and organic dyes, or two or more kinds of dyes can be mixed in order to make the wavelength range of photoelectric conversion as wide as possible and increase the conversion efficiency. Moreover, the pigment | dye to mix and its ratio can be selected so that it may match with the wavelength range and intensity distribution of the target light source.

  As a method for adsorbing the dye to the porous titania film, for example, a solution in which the dye is dissolved in a solvent is applied by spray coating or spin coating on the porous titania film and then dried. it can. In this case, the substrate may be heated to an appropriate temperature. Further, a method in which a porous titania film is immersed in a solution and adsorbed can also be used. The immersion time is not particularly limited as long as the dye is sufficiently adsorbed, but is preferably 10 minutes to 30 hours, more preferably 1 to 20 hours. Moreover, you may heat a solvent and a board | substrate when immersing as needed. The concentration of the dye in the case of a solution is about 0.01 to 100 mmol / L, preferably about 0.1 to 10 mmol / L.

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

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

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

  After adsorbing the dye, porous using amines, quaternary ammonium salts, ureido compounds having at least one ureido group, silyl compounds having at least one silyl group, alkali metal salts, alkaline earth metal salts, etc. The surface of the quality titanium oxide film may be treated. Examples of preferred amines include pyridine, 4-t-butylpyridine, polyvinylpyridine and the like. Examples of preferred quaternary ammonium salts include tetrabutylammonium iodide, tetrahexylammonium iodide and the like. These may be used by dissolving in an organic solvent, or may be used as they are in the case of a liquid.

  The dye-sensitized solar cell of the present invention can be manufactured by modularizing the photoelectric conversion element of the present invention and providing predetermined electrical wiring.

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

[Preparation of a film-forming composition containing titania]
Acetic acid 0.05 mol was added to titanium isopropoxide 0.05 mol and stirred for 15 minutes. Distilled water 73 ml was added and stirred for 1 hour. Further, 1 ml of concentrated nitric acid was added, and the mixture was heated and stirred at 80 ° C. for 75 minutes. Distilled water was added thereto to make a total volume of 93 ml to obtain an aqueous titania sol solution. 40 ml of this titania sol aqueous solution was placed in a pressure reaction vessel having an internal volume of 125 ml and heated at 250 ° C. for 12 hours. The obtained white precipitate (titania) was subjected to solvent substitution with ethanol, and then made into a 100 ml ethanol dispersion. To this, 7 g of α-terpineol and 8.65 g of a 10 wt% ethanol solution of ethyl cellulose were added and stirred. After sufficiently stirring, ethanol was distilled off using an evaporator to obtain 10 g of a film forming composition containing titania.

[Production of titania negative electrode]
A film-forming composition containing titania prepared as described above was prepared using a polyester screen printing plate (225 mesh) on a glass with fluorine-doped tin oxide (FTO) film (manufactured by Nippon Sheet Glass Co., Ltd .; 4 mm thickness). Screen printing was repeatedly performed until the film thickness was 14 μm in the size of millimeter square. Furthermore, it put into the electric furnace and baked at 500 degreeC for 1 hour.

[Immobilization of sensitizing dye]
A titania negative electrode calcined at 500 ° C. in a solution of N-719 dye manufactured by Solaronix, Switzerland, dissolved in a mixed solvent of tertiary butyl alcohol and acetonitrile at a volume ratio of 1: 1 at a concentration of 0.5 mmol / L was used at 25 ° C. For 20 hours to fix the dye.

[Assembly of small cells]
Epoxy adhesive glass with fluorine-doped tin oxide (FTO) film sputtered with platinum (made by Pilkington; 2.2 mm thickness) using Kapton tape (35 μm thickness) as a spacer on the titania negative electrode on which the dye is fixed Attach together. Then, the electrolyte solution of Examples 1-3 and Comparative Examples 1-4 mentioned later was inject | poured and sealed, and the photoelectric conversion element was produced.

[Evaluation of small cells]
The produced small cell was irradiated with light having an intensity of 100 mW / cm ² under the condition of AM1.5 (JISC8912A rank) with a solar simulator manufactured by Mitsunaga Electric Manufacturing Co., Ltd., and the photoelectric conversion characteristics of the photoelectric conversion element were evaluated. did.

Example 1
Acetonitrile was used as a solvent, and the composition of the electrolytic solution was evaluated as follows to evaluate a small cell.
Iodine: 0.05M
Lithium iodide: 0.05M
1-ethyl-3-methylimidazolium iodide: 0.55M
4-tert-butylpyridine: 0.5M
The results are shown in Table 1.

Example 2
Acetonitrile was used as a solvent, and the composition of the electrolytic solution was evaluated as follows to evaluate a small cell.
Iodine: 0.05M
Lithium iodide: 0.1M
1-ethyl-3-methylimidazolium iodide: 0.5M
4-tert-butylpyridine: 0.5M
The results are shown in Table 1.

Example 3
Acetonitrile was used as a solvent, and the composition of the electrolytic solution was evaluated as follows to evaluate a small cell.
Iodine: 0.05M
Lithium iodide: 0.2M
1-ethyl-3-methylimidazolium iodide: 0.4M
4-tert-butylpyridine: 0.5M
The results are shown in Table 1.

Comparative Example 1
Acetonitrile was used as a solvent, and the composition of the electrolytic solution was evaluated as follows to evaluate a small cell.
Iodine: 0.05M
Lithium iodide: 0.6M
4-tert-butylpyridine: 0.5M
The results are shown in Table 1.

Comparative Example 2
Acetonitrile was used as a solvent, and the composition of the electrolytic solution was evaluated as follows to evaluate a small cell.
Iodine: 0.05M
1-ethyl-3-methylimidazolium iodide: 0.6M
4-tert-butylpyridine: 0.5M
The results are shown in Table 1.

Comparative Example 3
Acetonitrile was used as a solvent, and the composition of the electrolytic solution was evaluated as follows to evaluate a small cell.
Iodine: 0.05M
Lithium iodide: 0.1M
1-methyl-3-propylimidazolium iodide: 0.5M
4-tert-butylpyridine: 0.5M
The results are shown in Table 1.

Comparative Example 4
Acetonitrile was used as a solvent, and the composition of the electrolytic solution was evaluated as follows to evaluate a small cell.
Iodine: 0.05M
Lithium iodide: 0.1M
1-butyl-3-methylimidazolium iodide: 0.5M
4-tert-butylpyridine: 0.5M
The results are shown in Table 1.

Claims (8)

Dye sensitization containing iodine, lithium iodide and 1-ethyl-3-methylimidazolium iodide, and having a content of 1-ethyl-3-methylimidazolium iodide of 5 to 11 mol per mol of iodine Electrolyte for solar cells . The electrolytic solution according to claim 1, comprising 1 to 5 moles of lithium iodide with respect to 1 mole of iodine. Furthermore, the electrolyte solution of Claim 1 or 2 containing the at least 1 sort (s) of basic substance chosen from the group which consists of 4-tert-butyl pyridine and N-methylbenzimidazole. The electrolytic solution according to claim 3, comprising 5 to 15 mol of the basic substance with respect to 1 mol of iodine. The electrolyte solution in any one of Claims 1-4 whose counter anion which exists is only an iodine ion. Furthermore, the electrolyte solution in any one of Claims 1-5 containing the at least 1 sort (s) of solvent chosen from the group which consists of a nitrile compound, a lactone compound, a carbonate compound, ethers, alcohols, and sulfones. The photoelectric conversion element for dye-sensitized solar cells obtained using the electrolyte solution in any one of Claims 1-6. The dye-sensitized solar cell obtained using the photoelectric conversion element of Claim 7.