WO2012115094A1 - Photoelectric conversion element - Google Patents

Abstract

Thie photoelectric conversion element is provided with a semiconductor electrode containing a semiconductor layer and a photosensitizer (pigment), a counter electrode, and an electrolyte layer provided between the semiconductor electrode and the counter electrode. The electrolyte layer contains water, and at least two types of cyclic nitroxyl radical compounds with different affinities to water.

Description

Photoelectric conversion element

The present invention relates to a photoelectric conversion element that can be used for a dye-sensitized solar cell, an optical sensor, and the like.

Conventionally, various solar cells have been proposed as photoelectric conversion elements that convert light energy into electrical energy. Among such solar cells, a dye-sensitized solar cell was developed by Gretzel et al. Of Lausanne University of Technology in Switzerland in 1991, and is generally a photoelectric conversion layer made of a semiconductor in which a dye is adsorbed on a conductive substrate. And a counter electrode made of a conductive base material provided opposite to the semiconductor electrode, and an electrolyte layer (charge transport layer) held between the semiconductor electrode and the counter electrode. .

Photoelectric conversion elements formed from a semiconductor electrode having a semiconductor layer adsorbed with a dye, an electrolyte layer, a counter electrode, and the like are expected to be applied to energy devices such as dye-sensitized solar cells, optical sensors, and the like. Among them, dye-sensitized solar cells are attracting wide attention because they exhibit high conversion efficiency among organic solar cells. As the semiconductor layer made of a photoelectric conversion material used in the dye-sensitized solar cell, a semiconductor layer in which a spectral sensitizing dye having absorption in the visible light region is adsorbed on the semiconductor surface is used.

Conventionally, the electrolyte layer has been formed by injecting an electrolytic solution in which iodine / iodide ions are dissolved in an organic solvent between a semiconductor electrode and a counter electrode. For this reason, the composition of the electrolytic solution may change due to volatilization of the solvent, which may cause a problem in long-term stability. In addition, in an electrolyte containing an organic solvent, the manufacturing process must be performed in a non-aqueous environment, preferably in a dehydrated state. Therefore, the manufacturing process becomes complicated and the manufacturing cost required for environmental maintenance increases. There was a problem. In addition, when the organic solvent leaks from the dye-sensitized solar cell, there is a problem that may cause environmental pollution due to volatilization of the organic solvent, and there is a possibility of further ignition or explosion.

Considering these dangers, development of an aqueous electrolyte in which the main solvent constituting the electrolytic solution is not an organic solvent is eagerly desired. Thus, many inventions in which the organic solvent is replaced with water have been disclosed so far. Patent Documents 1 and 2 propose an electrolyte containing no organic solvent, specifically, an electrolytic solution containing lithium iodide, iodine and water. By using this, an electromotive force of about 0.7 V is proposed. Is obtained. However, when the solvent is changed from organic to aqueous, the output tends to be low.

In recent years, Non-Patent Document 1 proposes an electrolytic solution in which 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical is substituted for iodine / iodide ions. Gretzell et al. Proposed the use of the redox reaction of the TEMPO radical, but at the same time the nitrosonium tetrafluoroborate (NOBF ₄ ), which is a deleterious reagent, does not work and cannot be handled safely. there were. Patent Documents 3 to 6 disclose dye-sensitized solar cells using a cyclic nitroxyl radical compound as an electrolyte.

JP 2003-308889 A JP 2003-308890 A JP 2009-021212 A JP 2009-076369 A JP 2009-098225 A JP 2010-205704 A

The present invention has been made in view of the above points. Instead of using an electrolyte in which iodide ions are dissolved in an organic solvent, high output can be obtained by using an electrolyte containing water and a cyclic nitroxyl radical compound at the same time. An object of the present invention is to provide a photoelectric conversion element having a possible aqueous electrolyte.

The photoelectric conversion element according to the present invention for solving the above problems is
A semiconductor electrode comprising a photosensitizer and a semiconductor layer;
A counter electrode;
An electrolyte layer provided between the semiconductor electrode and the counter electrode;
The electrolyte layer contains water and at least two kinds of cyclic nitroxyl radical compounds having different affinity for water at the same time.

According to the present invention, in a photoelectric conversion element having a semiconductor layer electrode having a photosensitizer and a semiconductor layer, a counter electrode, and an electrolyte layer that performs charge transport between the two, as an electrolyte, water is used as a solvent. By containing at least two types of cyclic nitroxyl radicals having different affinities, the problems of organic solvent electrolytes can be solved, and a highly efficient photoelectric conversion element can be provided as compared with conventional aqueous electrolytes.

1 is a schematic cross-sectional view showing a basic structure of a photoelectric conversion element according to an embodiment of the present invention. 7 is an IV curve of the photoelectric conversion element obtained in Example 6.

The photoelectric conversion element of the present invention comprises a semiconductor electrode, a counter electrode, and an electrolyte layer held between both electrodes. Preferably, a semiconductor electrode having a photoelectric conversion layer made of a semiconductor (for example, an n-type semiconductor) in which a photosensitizer (particularly a dye) is adsorbed on a conductive substrate, and a conductive electrode provided opposite to the semiconductor electrode It is a dye-sensitized photoelectric conversion element provided with the counter electrode which consists of a base material, and the electrolyte layer hold | maintained between these semiconductor electrodes and counter electrodes.

Hereinafter, each component of the photoelectric conversion element will be described in detail.

1. Semiconductor electrode A semiconductor electrode consists of a conductive substrate and the semiconductor layer formed on it, for example. The semiconductor layer has a structure in which a dye functioning as a photosensitizer is adsorbed.

1-1. Conductive substrate The conductive substrate may be a substrate that is conductive, or may be a substrate that is made conductive by forming a conductive layer on the insulating substrate. . Examples of the substrate include a glass substrate, a plastic substrate, and a metal plate. Among them, a substrate with high transparency (transparent substrate) is particularly preferable. Type conductive layer formed on the substrate is not particularly limited, for example ITO, FTO, a transparent conductive layer such as SnO ₂ is preferred. A method for manufacturing the conductive layer, a film thickness, and the like can be selected as appropriate, but those having a thickness of about 1 nm to 5 μm can be used.

1-2. Semiconductor layer 1-2-1. Semiconductor Layer Material / Structure Semiconductor materials constituting the semiconductor layer include known semiconductors such as titanium oxide, zinc oxide, tungsten oxide, barium titanate, strontium titanate, and cadmium sulfide. Two or more kinds of these semiconductor materials can be mixed and used. Among these, titanium oxide is particularly preferable in terms of conversion efficiency, stability, and safety. Examples of such titanium oxide include anatase-type titanium oxide, rutile-type titanium oxide, amorphous titanium oxide, various titanium oxides such as metatitanic acid, orthotitanic acid, and titanium oxide-containing composites. Among these, anatase type titanium oxide is preferable.

Examples of the shape of the semiconductor layer include a porous semiconductor layer obtained by sintering semiconductor fine particles, a thin-film semiconductor layer obtained by a sol-gel method, a sputtering method, a spray pyrolysis method, and the like, and other fibers. It can be appropriately selected according to the purpose of use of the photoelectric conversion element such as a semiconductor layer or a semiconductor layer made of needle crystals.

The semiconductor layer of the present invention is preferably a semiconductor layer having a large specific surface area, such as a porous semiconductor layer or a needle-like semiconductor layer, from the viewpoint of the amount of dye adsorbed. A porous semiconductor layer formed from semiconductor fine particles is preferable from the viewpoint that the utilization factor of incident light can be adjusted by the particle size of the semiconductor fine particles.

Further, the semiconductor layer may be a single layer or a multilayer. By using a multilayer structure, a semiconductor layer having a sufficient thickness can be easily formed. The porous multilayer semiconductor layer may be composed of semiconductor fine particle layers having different average particle diameters. For example, by making the average particle size of the semiconductor fine particles constituting the semiconductor layer closer to the light incident side (first semiconductor layer) smaller than that of the farther semiconductor layer (second semiconductor layer), the first semiconductor layer A large amount of light is absorbed, and the light that has passed through the first semiconductor layer is scattered by the second semiconductor layer, returned to the first semiconductor layer, and absorbed by the first semiconductor layer, thereby improving the overall light absorption rate. be able to. The film thickness of the semiconductor layer is not particularly limited, but is preferably about 0.5 to 45 μm from the viewpoints of permeability and conversion efficiency.

The specific surface area of the semiconductor layer is preferably 10 to 200 m ² / g in order to adsorb a large amount of dye. Further, the porosity is preferably 40 to 80% for adsorbing the dye and for carrying out charge transport by sufficiently diffusing ions in the electrolyte. Note that the porosity means the ratio of the volume occupied by the pores in the semiconductor layer in% in the volume of the semiconductor layer.

1-2-2. Next, a method for forming the semiconductor layer will be described by taking a porous semiconductor layer as an example. The porous semiconductor layer is prepared, for example, by adding a semiconductor fine particle together with an organic compound such as a polymer and a dispersing agent to a dispersion medium such as an organic solvent or water, and applying the suspension onto a conductive substrate. And it can form by drying and baking this.

When an organic compound is added to the dispersion medium together with the semiconductor fine particles, the organic compound burns during firing, and a gap can be secured in the porous semiconductor layer. Moreover, the porosity can be changed by controlling the molecular weight and the addition amount of the organic compound combusted during firing. The type and amount of the organic compound can be appropriately selected and adjusted depending on the state of the fine particles used, the total weight of the entire suspension, and the like. However, when the proportion of the semiconductor fine particles is 10 wt% or more with respect to the total weight of the whole suspension, the strength of the produced film can be sufficiently increased, and the proportion of the semiconductor fine particles is the total weight of the whole suspension. If the content is 40 wt% or less, a porous semiconductor layer having a large porosity can be obtained. Therefore, the ratio of the semiconductor fine particles is preferably 10 to 40 wt% with respect to the total weight of the entire suspension.

Examples of the semiconductor fine particles include single or compound semiconductor particles having an appropriate average particle size, for example, an average particle size of about 1 nm to 500 nm. Among them, those having an average particle diameter of about 1 to 50 nm are preferable from the viewpoint of increasing the specific surface area. Further, from the viewpoint of increasing the utilization factor of incident light, semiconductor particles having a large average particle diameter of about 200 to 400 nm may be added.

Examples of the method for producing the semiconductor fine particles include a sol-gel method such as a hydrothermal synthesis method, a sulfuric acid method, and a chlorine method. Any method can be used as long as it can produce the desired fine particles. From the viewpoint of crystallinity, it is preferably produced by a hydrothermal synthesis method.

Any organic compound can be used as long as it dissolves in the suspension and can be removed by burning when baked. Examples thereof include polymers such as polyethylene glycol and ethyl cellulose. Examples of the dispersion medium for the suspension include glyme solvents such as ethylene glycol monomethyl ether, alcohols such as isopropyl alcohol, mixed solvents such as isopropyl alcohol / toluene, and water.

Examples of the method for applying the suspension include known methods such as a doctor blade method, a squeegee method, a spin coating method, and a screen printing method. Thereafter, the coating film is dried and fired. The drying and firing conditions may be about 10 seconds to 12 hours in the range of about 50 to 800 ° C. in the air or in an inert gas atmosphere. This drying and baking can be performed once at a single temperature or twice or more by changing the temperature.

In addition, although the formation method of the porous semiconductor layer was explained in full detail here, another kind of semiconductor layer can also be formed using various well-known methods.

1-3. Dye that functions as a photosensitizer In the present invention, a dye can be used as a photosensitizer. A dye functioning as a photosensitizer (hereinafter simply referred to as “dye”) has absorption in various visible light regions and infrared light regions, and is strongly adsorbed to the semiconductor layer. Those having an interlock group such as a COOH group, an alkoxy group, a hydroxyl group, a hydroxyalkyl group, a sulfonic acid group, an ester group, a mercapto group, and a phosphonyl group in the dye molecule are preferable. Of these, those having a COOH group are particularly preferred.

The interlock group provides an electrical bond that facilitates electron transfer between the excited dye and the semiconductor conduction band. Examples of these dyes containing an interlock group include ruthenium metal complex dyes (ruthenium bipyridine metal complex dyes, ruthenium terpyridine metal complex dyes, ruthenium quarterpyridine metal complex dyes, etc.), azo dyes, and quinone dyes. Quinone imine dyes, quinacridone dyes, squarylium dyes, cyanine dyes, merocyanine dyes, triphenylmethane dyes, xanthene dyes, porphyrin dyes, phthalocyanine dyes, perylene dyes, indigo dyes, naphthalocyanine dyes Examples thereof include dyes and coumarin dyes. Of these, organic dyes are preferred.

Examples of the method for adsorbing the dye on the semiconductor layer include a method of immersing the semiconductor layer formed on the substrate in a solution in which the dye is dissolved. Solvents used to dissolve the dye include alcohols such as ethanol, ketones such as acetone, ethers such as diethyl ether and tetrahydrofuran, nitrogen compounds such as acetonitrile, halogenated aliphatic hydrocarbons such as chloroform, and hexane. Examples thereof include aliphatic hydrocarbons, aromatic hydrocarbons such as benzene, and esters such as ethyl acetate. These solvents may be used as a mixture of two or more.

The dye concentration in the solution can be adjusted as appropriate depending on the type of the dye and the solvent to be used. In order to improve the adsorption function, it is preferable that the concentration is somewhat high. For example, the concentration can be m5 × 10 ⁻⁵ mol / L or more.

When the semiconductor is immersed in the solution in which the dye is dissolved, the temperature and pressure of the solution and the atmosphere are not particularly limited, and examples include room temperature and atmospheric pressure, and the immersion time is the dye and solvent to be used. It can be appropriately adjusted depending on the kind of the solution, the concentration of the solution, and the like. In order to effectively perform the immersion, the immersion may be performed under heating. Thereby, a pigment | dye can be made to adsorb | suck to a semiconductor layer.

When adsorbing the dye, deoxycholic acid or guanidine is added to the solution in which the dye is dissolved in order to control the dye and its adsorption state, the surface of fine particles such as TiO ₂ constituting the porous semiconductor layer, and the like. Organic compounds such as thiocyanate, tert-butylpyridine, ethanol, and the like may be added.
As a photosensitizer, semiconductor fine particles, so-called quantum dots, can be used instead of a dye.

2. Counter electrode Examples of the counter electrode include those in which a metal catalyst such as platinum or a carbon film is present on a support substrate. In particular, platinum is preferable. These film thicknesses may be any thickness that can exhibit a catalytic function, and are preferably about 1 to 2000 nm. Examples of the support substrate include glass, a polymer film, and a metal plate (foil). In particular, in order to reduce the resistance value, the support substrate is preferably a conductive substrate. The counter electrode may be formed on the same substrate as the semiconductor electrode. In this case, two or more electrodes can be formed on the same substrate by forming a conductive layer on the insulating substrate and cutting the conductive layer between the counter electrode and the semiconductor electrode by laser scribing or the like.

3. Electrolyte Layer 3-1 Cyclic Nitroxyl Radical Compound In the present invention, the electrolyte layer contains water and at least two types of cyclic nitroxyl radical compounds having different affinity for water. The “cyclic nitroxyl radical compound” in the present invention is a compound containing a nitrogen atom constituting a nitroxyl radical (═N—O.) As a ring constituent atom. As the ring structure, a 5- to 7-membered ring is preferable, and a nitroxyl radical compound having a piperidinoxyl radical ring structure is particularly preferable. Inclusion of at least two types of cyclic nitroxyl radical compounds is not simply an effect of increasing types. That is, it means that at least two kinds of nitroxyl radicals having different nitrogen-containing heterocycles act as redox species at different concentrations. Therefore, when there are differences in the properties and characteristics of at least two types of nitroxyl radicals with respect to the solvent (in the case of the present invention, water), it is possible to separate the roles in the solvent, which have been disclosed so far. An effect that is fundamentally different from that of the present invention can be expected.

The present invention uses at least two types of nitroxyl radicals having different affinity for water. Due to the difference in affinity for water, the positional relationship of each radical with water is different. That is, radicals with low affinity for water (hereinafter also referred to as hydrophobic radicals) tend to gather near the surface of the water as the electrolyte solvent, and radicals with excellent affinity for water (hereinafter referred to as hydrophilic radicals). (Also called) concentrates relatively inside the electrolyte. Such a difference in location in the electrolyte makes it possible to divide the function of each radical.

The electrolyte is present in the pores of the semiconductor layer of the semiconductor electrode, and the surface of the electrolyte is ideally in contact with the dye adsorbed on the semiconductor layer. Therefore, the hydrophobic radical mainly functions to exchange charges from the dye, that is, to deliver electrons to the dye. On the other hand, hydrophilic radicals exist mainly in the electrolyte solution, and carry electrons from the counter electrode to the hydrophobic radical in the electrolyte solution. In this way, by dividing the role of radicals and selecting a suitable radical, the efficiency of the photoelectric conversion element can be improved, and when the solvent becomes water, the problems shown in the background art can be improved and durability is improved. It becomes possible to improve the performance.

Hydrophobic radicals are low solubility in water for hydrophilic radicals. Therefore, generally, it is not possible to dissolve more than the limit amount in water. However, when mixed with a hydrophilic radical and dissolved in water, the hydrophobic radical may be dissolved in a larger amount than the limit amount. If this property is utilized, it will result in the increase of the radical seed | species which is a redox seed | species which exists near the surface, and an electron can be efficiently passed to a pigment | dye. Such an effect is also considered to contribute to efficiency improvement.

As the above consideration shows, the present invention uses a difference in affinity for water (a difference between hydrophilicity and hydrophobicity) in an electrolyte using water as a solvent. This basic principle can be extended to utilizing the difference in affinity for the main solvent constituting the electrolyte. Therefore, even if a solvent is an organic solvent, it is guessed that the same effect is exhibited by utilizing the difference in affinity with respect to the organic solvent.

In the present invention, the electrolyte is preferably liquid, but may be solid or gel. Here, the solid electrolyte is a solid electrolyte formed of the nitroxyl radical compound. The gel electrolyte is an electrolyte including a network structure formed by a polymer of a nitroxyl radical. There is a possibility that a polymer having a nitroxyl radical can be used even in a crosslinked structure insoluble in an organic solvent or water.

In the photoelectric conversion element of the present invention, the electrolyte layer has water selected from a cyclic nitroxyl radical compound represented by the following general formula (1) and a cyclic nitroxyl radical compound represented by the following general formula (2). It is preferable to have at least two cyclic nitroxyl radicals having different affinity at the same time.

(In the above general formula (1), A is a divalent group constituting a 5- to 7-membered heterocyclic ring containing nitrogen and may have a hydrophobic substituent. R ¹ , R ² , R ³ and R ⁴ each independently represent a hydrogen atom or a methyl group.)

(In the general formula (2), B is a divalent group constituting a 5- to 7-membered heterocyclic ring containing nitrogen and has a hydrophilic substituent. R ¹ , R ² , R ³ , R ⁴ may be different from each other, and represents a hydrogen atom or a methyl group.

In the general formulas (1) and (2), the divalent group represented by A or B is a group constituting a 5- to 7-membered heterocyclic ring containing nitrogen together with the nitrogen atom of the nitroxyl radical, Examples thereof include an alkylene group having 2 to 4 carbon atoms or an alkenylene group. In addition, some of the carbon atoms of the alkylene group may be replaced with oxygen atoms, nitrogen atoms, sulfur atoms, silicon atoms, phosphorus atoms, or boron atoms. Each carbon atom of the alkylene group may have a substituent, and in the general formula (1), it may have a hydrophobic substituent such as an aliphatic group or an aromatic group. On the other hand, the general formula (2) has a hydrophilic substituent. Examples of the hydrophilic substituent include a hydroxyl group, an alkoxy group, an aldehyde group, a carboxyl group, an alkoxycarbonyl group, a cyano group, an amino group, a nitro group, and a nitroso group. Also, there is a difference in affinity for water between hydrophilic substituents. That is, at least two types of cyclic nitroxyl radicals having different affinity for water may be selected from general formula (2), and one or more types of cyclic nitroxyl radicals may be selected from general formula (1) and general formula (2), respectively. Nitroxyl radicals may be selected. Also, the affinity for water varies depending on whether the group represented by R ¹ to R ⁴ is a hydrogen atom or a methyl group, and the number thereof. Preferably, at least one cyclic nitroxyl radical represented by the general formula (1) and at least one cyclic nitroxyl radical represented by the general formula (2) are selected and have an affinity for water. The larger the difference in sex, the better. In particular, at least one kind of cyclic nitroxyl radical of 0.1 mol / l or more and at least one kind of less than 0.1 mol / l, each having a solubility in water at 25 ° C. of 0.1 mol / l alone. It is preferable that it is the combination of these.

In the electrolyte layer according to the present invention, an oxoammonium salt represented by the following general formula (3) corresponding to the above general formulas (1) and (2) and an oxoammonium represented by the following general formula (4) It is desirable to include one or both of the salts.

(In the general formula ^{(3), A, R 1} , R 2, R 3, R 4 is ^A in the formula ^{(1), R 1, R} 2, R 3, R 4 represent the same meanings as, X is , AlCl ₄ ⁻ , Al ₂ Cl ₇ ^— and other metal chlorides, fluorine, chlorine, bromine, iodine, BF ₄ , PF ₆ , CF ₃ SO _3, N (SO ₂ CF ₃ ) ₂ , N (SO ₂ F) ₂ , CF ₃ COO, N (C ₂ F ₅ SO ₂ ), or ClO _4. )

In (formula ^{(4), B, R 1} , R 2, R 3, R 4 is ^B in the general formula ^{(2), R 1, R} 2, R 3, R 4 represent the same meanings as, X is , AlCl ₄ ⁻ , Al ₂ Cl ₇ ^— and other metal chlorides, fluorine, chlorine, bromine, iodine, BF ₄ , PF ₆ , CF ₃ SO _3, N (SO ₂ CF ₃ ) ₂ , N (SO ₂ F) ₂ , CF ₃ COO, N (C ₂ F ₅ SO ₂ ), or ClO _4. )

The cyclic nitroxyl radical in the electrolyte is oxidized and reduced in a radical state and a cation state. For example, a 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical useful as the general formula (1) has an equilibrium state between a radical state and a cation state as shown in the following formula (5). Show.

Therefore, when a cyclic nitroxyl radical is used as the redox couple of the electrolyte, it is necessary that the corresponding oxoammonium salt is present in the electrolyte.

The present invention contains at least two kinds of cyclic nitroxyl radicals having different affinity for water in the electrolyte. For example, when one kind of hydrophobic radical and one kind of hydrophilic radical are combined, in order for both of them to act as a redox pair, it is necessary to contain a corresponding oxoammonium salt. Further, if only one nitroxyl radical is allowed to act as a redox pair, one corresponding oxoammonium salt may be included.

Specific examples of the compounds represented by the general formulas (1) and (2) as the cyclic nitroxyl radical contained in the electrolyte include compounds represented by the following formulas (6) to (12). Examples of the oxoammonium salts represented by the general formulas (3) and (4) include oxoammonium salts corresponding to nitroxyl radicals represented by the following formulas (6) to (12).

3-2. Liquid Electrolyte As the liquid electrolyte, any liquid electrolyte may be used as long as it contains at least two types of cyclic nitroxyl radicals having different affinity for water. The electrolyte contains water as a solvent.
In addition, an organic compound can be added to the electrolyte as appropriate. For example, as an organic solvent to be added, nitrogen-containing compounds such as N-methylpyrrolidone and N, N-dimethylformamide, nitrile compounds such as methoxypropionitrile and acetonitrile, lactone compounds such as γ-butyrolactone and valerolactone, ethylene carbonate , Carbonate compounds such as diethyl carbonate, dimethyl carbonate, propylene carbonate, ethers such as tetrahydrofuran, dioxane, diethyl ether, ethylene glycol dialkyl ether, alcohols such as methanol, ethanol, isopropyl alcohol, and further imidazoles, etc. These can be used alone or in admixture of two or more. In particular, the addition of ethanol improves the penetration of the electrolyte into the pores of the porous semiconductor electrode.

Also, an ionic liquid, that is, a molten salt can be added to the electrolyte as appropriate. Examples of the ionic liquid are disclosed in “Inorg. Chem.” 1996, 35, p1168-1178, “Electrochemistry” 2002.2, p130-136, JP-T-9-507334, JP-A-8-259543, and the like. Although it will not specifically limit if it can generally be used in the well-known battery, solar cell, etc. which have a melting point lower than room temperature (25 degreeC) or higher than room temperature However, a salt that liquefies at room temperature by dissolving other molten salt or additives other than the molten salt is preferably used.

Specifically, as the cation of the molten salt, ammonium, imidazolium, oxazolium, thiazolium, piperidinium, pyrazolium, isoxazolium, thiadiazolium, oxadiazolium, triazolium, pyrrolidinium, pyridinium, pyrimidinium, pyridazinium, pyrazinium, Triazinium, phosphonium, sulfonium, carbazolium, indolium, and derivatives thereof are preferable, and ammonium, imidazolium, pyridinium, piperidinium, pyrazolium, and sulfonium are particularly preferable. The anions of the molten salt include metal chlorides such as AlCl ₄ ⁻ and Al ₂ Cl ₇ ^— , PF ₆ ⁻ , BF ₄ ⁻ , CF ₃ SO ₃ ⁻ , N (CF ₃ SO ₂ ) ₂ ⁻ , CF _3. Fluorine-containing materials such as COO ⁻ , NO ₃ ⁻ , CH ₃ COO ⁻ , C ₆ H ₁₁ COO ⁻ , CH ₃ OSO ₃ ⁻ , CH ₃ OSO ₂ ⁻ , CH ₃ SO ₃ ⁻ , CH ₃ SO ₂ ⁻ , (CH Non-fluorine compounds such as ₃ O) ₂ PO ₂ ^— , chlorine, bromine. And halides such as iodine.

Molten salt can be synthesized by methods known in various literatures and publications. Taking a quaternary ammonium salt as an example, a quaternization of an amine is performed using a tertiary amine as an alkylating agent as an alkylating agent as a first step, and an ion exchange from a halide anion to a target anion is performed as a second step. Can be used. Alternatively, there is a method in which a tertiary amine is reacted with an acid having a target anion to obtain the target compound in one step.

3-3. Solid Electrolyte Examples of the solid electrolyte include a polymer electrolyte solidified by infiltrating an organic solvent into a polymer compound, and an electrolyte obtained by solidifying a liquid electrolyte containing a molten salt with fine particles. As a polymer compound to be used, there are a case where a polymer itself having a nitroxyl radical is used and a case where a polymer other than a nitroxyl radical polymer is used.

Other than the nitroxyl radical polymer, the polymer compound for solidifying the liquid electrolyte may be any polymer compound that can hold the liquid electrolyte, including a compound having a polyoxyalkylene chain, and gelling or solidifying the electrolyte. The polymer precursor (polymer gelling agent) having a polyoxyalkylene chain is usually used. For example, (i) alkylenes such as trifunctional terminal acryloyl-modified alkylene oxide polymers and tetrafunctional terminal acryloyl-modified alkylene oxide polymers disclosed in JP-A-5-109311 and JP-A-11-176442 Examples include acryloyl-modified polymer compounds having an oxide polymer chain. (B) In addition, it contains compound A having at least one type of isocyanate group and compound B that is reactive with at least one type of isocyanate group, and at least one of compound A and compound B has a polyoxyalkylene chain. (See Patent Documents 2 and 3 above). As the compound having a polyoxyalkylene chain, a compound having a polymer structure having a molecular weight of 500 to 50,000 is preferably used.

The trifunctional or tetrafunctional terminal acryloyl-modified alkylene oxide polymer (i) is, for example, glycerol or trimethylolpropane in the case of trifunctional, diglycerin or pentaerythritol in the case of tetrafunctional. Etc. as starting materials, alkylene oxides such as ethylene oxide and propylene oxide are added thereto, and unsaturated organic acids such as acrylic acid and methacrylic acid are further esterified, or acrylic acid chloride and methacrylic acid chloride It is a compound obtained by carrying out dehydrochlorination reaction of acid chlorides, such as.

Examples of (B) compound A include aromatic isocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate and xylylene diisocyanate; aliphatic isocyanates such as hexamethylene diisocyanate and trimethylhexamethylene diisocyanate; fats such as isophorone diisocyanate and cyclohexyl diisocyanate. Examples thereof include cyclic isocyanates, which may be multimers such as dimers and trimers, or modified products. Further, low molecular alcohols and adducts of these isocyanates, and compounds obtained by addition reaction of polyoxyalkylene and these isocyanates in advance are also included. Examples of the compound B include compounds having an active hydrogen group such as a carboxyl group, a hydroxyl group, and an amino group. More specifically, examples of the compound having a carboxyl group include hexanoic acid, adipic acid, phthalic acid, and azelaic acid. Carboxylic acids such as: hydroxyl group-containing compounds such as ethylene glycol, diethylene glycol, glycerin, pentaerythritol, sorbitol, sucrose polyols; amino group-containing compounds such as ethylenediamine, tolylenediamine, diphenylmethanediamine, diethylenetriamine, etc. These organic amines are listed. Compound B also includes compounds having at least one active hydrogen group as described above in a molecule and having a polyoxyalkylene chain.

3-4. Stabilizing additive The cyclic nitroxyl radical of the electrolyte is oxidized and reduced in a radical state and a cation state as shown in the above formula (5). For the purpose of stabilizing the generated cationic state, it is also possible to add a salt to the electrolyte. As a salt to be used, as a cation, lithium, sodium, potassium, ammonium, imidazolium, oxazolium, thiazolium, piperidinium, pyrazolium, isoxazolium, thiadiazolium, oxadiazolium, triazolium, pyrrolidinium, pyridinium, pyrimidinium, pyridazinium, Pyrazinium, triazinium, phosphonium, sulfonium, carbazolium, indolium, and derivatives thereof are preferable, and ammonium, imidazolium, pyridinium, piperidinium, pyrazolium, and sulfonium are particularly preferable. As anions, fluorine-containing substances such as PF ₆ ⁻ , BF ₄ ⁻ , CF ₃ SO ₃ ⁻ , N (CF ₃ SO ₂ ) ₂ ⁻ , F (HF) _n ⁻ , CF ₃ COO ⁻ , NO ₃ ⁻ CH ₃ COO ⁻ , C ₆ H ₁₁ COO ⁻ , CH ₃ OSO ₃ ⁻ , CH ₃ OSO ₂ ⁻ , CH ₃ SO ₃ ⁻ , CH ₃ SO ₂ ⁻ , (CH ₃ O) ₂ PO ₂ ⁻ , SbCl ₆ ⁻ And non-fluorine compounds such as iodine, bromine and the like. The aforementioned molten salt may function as a stabilizing additive.

3-5 Electrolyte Preparation Method In order to simultaneously cause two kinds of cyclic nitroxyl radicals having different affinity for water according to the present invention in the electrolyte, there is a method of dissolving each cyclic nitroxyl radical in water. For example, there is a method of dissolving the cyclic nitroxyl radicals represented by the above general formulas (1) and (2) in water, respectively.

In addition to the above method, there can be mentioned a method in which one kind of cyclic nitroxyl radical and an oxoammonium salt corresponding to a cyclic nitroxyl radical having different affinity for water are dissolved in water. For example, a method of dissolving the cyclic nitroxyl radical represented by the general formula (1) and the oxoammonium salt represented by the general formula (4) in water, or the cyclic nitroxyl radical represented by the general formula (2) Any of the methods for dissolving the oxoammonium salt represented by the general formula (3) in water can be used.

When the cyclic nitroxyl radical represented by the general formula (1) and the oxoammonium salt represented by the general formula (4) are dissolved in water, as shown in the above formula (5), the general formula (1 A part of the cyclic nitroxyl radical of) becomes an oxoammonium salt of the general formula (3), and at the same time, a part of the oxoammonium salt represented by the general formula (4) Becomes a nitroxyl radical.

Further, when the cyclic nitroxyl radical represented by the general formula (2) and the oxoammonium salt represented by the general formula (3) are dissolved in water, one of the cyclic nitroxyl radicals represented by the general formula (2) is obtained. Part is an oxoammonium salt of the general formula (4), and at the same time, a part of the oxoammonium salt represented by the general formula (3) is represented by the general formula (1) as shown in the above formula (5). It becomes a cyclic nitroxyl radical.

For this reason, in order for the cyclic nitroxyl radical of the general formula (1) and the cyclic nitroxyl radical compound of the general formula (2) to be simultaneously present in the electrolyte, the cyclic nitroxyl radical represented by the general formula (1) is used. And a method of dissolving the oxoammonium salt represented by the general formula (4) in water, or the cyclic nitroxyl radical represented by the general formula (2) and the oxoammonium salt represented by the general formula (3) in water. This can be realized by using any one of the dissolving methods.

4). Photoelectric Conversion Element (Solar Cell) Structure FIG. 1 is a schematic cross-sectional view showing the basic structure of a photoelectric conversion element according to an embodiment of the present invention. Here, although the solar cell used for a dye-sensitized solar cell is shown as an example, it is not limited to this. As shown in the figure, the solar battery cell includes a transparent substrate (1) having optical transparency, a transparent conductive film (2) having optical transparency formed on the surface of the transparent substrate (1), and transparent conductive material. A semiconductor electrode (8) comprising a photoelectric conversion layer (3), which is a porous semiconductor layer adsorbing a dye formed on the film (2), and a counter electrode provided at a position facing the semiconductor electrode (8) (9) and an electrolyte layer (4) held between the semiconductor electrode (8) and the counter electrode (9). The counter electrode (9) is composed of a transparent substrate or a support substrate (7) on which a conductive film (6) is formed, and a platinum catalyst layer (5) is formed on the surface of the conductive film (6). In this embodiment, the electrolyte layer (4) is a liquid containing water and at least two nitroxyl radicals having different affinity for water, and at least two types of nitroxyl having different affinity for water. It consists of a gel or solid electrolyte impregnated with an electrolyte containing radicals.

In the photoelectric conversion element having this structure, when light such as sunlight is irradiated from the transparent substrate (1) side, the light is transmitted through the transparent substrate (1) and the transparent conductive film (2) to form a photoelectric conversion layer. The dye adsorbed in (3) is irradiated, and the dye absorbs light and is excited. Electrons generated by this excitation move from the photoelectric conversion layer (3) to the transparent conductive film (2). The electrons moved to the transparent conductive film (2) move to the counter electrode (9) through the external circuit, and return to the dye from the counter electrode (9) through the charge transport layer, that is, the electrolyte layer (4). In this way, a current flows and a photoelectric conversion element (solar cell) can be configured.

Hereinafter, the present invention will be specifically described by way of examples, but the scope of the present invention is not limited thereto.

Example 1
A photoelectric conversion element having the structure shown in FIG. 1 was produced as follows.
1. Formation of porous semiconductor layer The semiconductor layer was produced in the following procedures. First, 20 mL of an acetic acid aqueous solution having a concentration of 15 vol% was used as a solvent, and 5 g of a commercially available porous titanium oxide powder (P25, Nippon Aerosil Co., Ltd.) and a surfactant 0.1 mL (polyoxyethylene-p-isooctylphenol (product) Name: “Triton X-100” (manufactured by Sigma Aldrich) and polyethylene glycol 0.3 g (molecular weight 20000) were added, and the mixture was stirred for about 1 hour (once for 10 minutes) to prepare a titanium oxide paste. Next, an appropriate amount of this titanium oxide paste is applied to a glass substrate (6 cm × 4 cm, sheet resistance: 20Ω / □) with ITO as a transparent conductive film (2) so that the film thickness is about 20 μm by the doctor blade method ( Application area: 3 cm × 3 cm) This electrode was inserted into an electric furnace and at 450 ° C. in an air atmosphere. To obtain a semiconductor electrode was baked for 30 minutes.

2. Adsorption of dye Next, ruthenium dye (Ru (2,2′-bipyridine-4,4′-dicboxylic acid) ₂ (NSC) ₂ , manufactured by Kojima Chemical Co., Ltd.) in anhydrous ethanol at a concentration of 4 × 10 ⁻⁴ mol / L The dye solution for adsorption was prepared. This adsorption dye solution and the semiconductor electrode obtained above were put in a container and allowed to stand for 12 hours to adsorb the dye. Thereafter, it was washed several times with absolute ethanol and naturally dried at 50 ° C. for about 30 minutes.

3. Injection of electrolyte Next, an electrolyte solution was prepared. A compound represented by the above formula (7) (referred to as TEMPO-OH) is used as a hydrophilic cyclic nitroxyl radical, and a compound represented by the above formula (6) (TEMPO as a hydrophobic cyclic nitroxyl radical). TEMPO-BF ₄ which is a cyclic nitroxyl radical having a concentration of 0.05M (M = mol / l) and an oxoammonium salt having a concentration of 0.05M. An aqueous solution in which both were dissolved was used.

The oxoammonium salt is an ionic substance composed of a cation TEMPO ⁺ and an anion BF ₄ ⁻ . Therefore, in an aqueous solution, since the cation can form an equilibrium state with the radical state, a part of the cation transitions to the radical state. At this time, it is considered that a part of the TEMPO-OH which is a radical simultaneously transitions to a cation.

Thus, by replacing the TEMPO cation of TEMPO · BF ₄ with TEMPO, which is a cyclic nitroxyl radical, a state where TEMPO-OH and TEMPO coexist can be created in the solution. In this case, it is confirmed from the evaluation of solubility that TEMPO-OH is more hydrophilic than TEMPO.

The electrolyte solution was dropped onto the semiconductor layer (photoelectric conversion layer) adsorbing the dye of the semiconductor electrode formed earlier, and further evacuated with a rotary pump for about 10 minutes to sufficiently soak the solution into the semiconductor layer. Then, the counter electrode (9) which equipped the platinum catalyst layer (5) was installed, and it fixed with the jig | tool. Thereafter, the device was left to stand at 50 ° C. for 60 minutes to produce a device having a TEMPO-OH and a TEMPO · BF ₄ electrolyte layer (7). Then, sealing which avoids a contact with the external world with an epoxy resin was implemented, and the photoelectric conversion element was produced.

(Example 2)
Example 2 is the same as Example 1 except that an aqueous solution in which both TEMPO-OH having a concentration of 0.09M (M = mol / l) and TEMPO · BF ₄ having a concentration of 0.01M was used as an electrolytic solution. The photoelectric conversion element was produced similarly.

(Example 3)
In Example 3, an aqueous solution in which both TEMPO-OH having a concentration of 0.05 M (M = mol / l) and TEMPO · BF ₄ having a concentration of 0.05 M was used as an electrolytic solution, and 10% of water was used. A photoelectric conversion element was produced according to Example 1 except that ethanol was added.

Example 4
In Example 4, as an electrolytic solution, an aqueous solution in which both TEMPO-OH having a concentration of 0.05 M (M = mol / l) and TEMPO · BF ₄ having a concentration of 0.05 M was used, and 30% of water was used. A photoelectric conversion element was produced according to Example 1 except that ethanol was added.

(Example 5)
In Example 5, an aqueous solution in which both TEMPO-OH having a concentration of 0.05 M (M = mol / l) and TEMPO · BF ₄ having a concentration of 0.05 M was used as the electrolytic solution was 50% of water. A photoelectric conversion element was produced according to Example 1 except that ethanol was added.

(Example 6)
In Example 6, an aqueous solution in which both TEMPO-OH having a concentration of 0.25 M (M = mol / l) and TEMPO · BF ₄ having a concentration of 0.25 M was used as an electrolytic solution, and 30% of water was used. A photoelectric conversion element was produced according to Example 1 except that ethanol was added.

(Comparative Example 1)
In Comparative Example 1, a photoelectric conversion element was prepared according to Example 1 except that only 0.1 M TEMPO was used instead of the liquid electrolyte in which both TEMPO-OH and TEMPO · BF ₄ were dissolved in Example 1. Produced.

(Comparative Example 2)
In Comparative Example 2, photoelectric conversion according to Example 1 except that only 0.1 M TEMPO-OH was used instead of the liquid electrolyte in which both TEMPO-OH and TEMPO · BF ₄ were dissolved in Example 1. An element was produced.

(Comparative Example 3)
In Comparative Example 3, Example 1 was used except that a liquid electrolyte in which both TEMPO and TEMPO · BF ₄ were dissolved was used instead of the liquid electrolyte in which both TEMPO-OH and TEMPO · BF ₄ were dissolved in Example 1. A photoelectric conversion element was produced according to the above.

(Evaluation of photoelectric conversion element characteristics)
Evaluation of the characteristics of the produced photoelectric conversion element was carried out by IV measurement under irradiation conditions of AM 1.5 and 100 mW / cm ² using a solar simulator. Here, both ends of the photoelectric conversion element were connected to an electronic load device, and potential scanning at a step of 5 mV / sec was repeated until the voltage was extracted from the open circuit voltage and became zero. FIG. 2 shows an IV curve of the photoelectric conversion element obtained in Example 6. Table 1 shows the results of the characteristic evaluation of the photoelectric conversion elements obtained in Examples 1 to 6 and Comparative Examples 1 to 3. It can be seen that Examples 1 to 6 have higher efficiency than Comparative Examples 1 to 3.

When only one type of cyclic nitroxyl radical shown in Comparative Example 1 or 2 is used, power generation is not performed or power generation is slight, which is not practical. In the combination of TEMPO and TEMPO · BF ₄ shown in Comparative Example 3, the power generation efficiency is higher than those in Comparative Examples 1 and 2. Further, as shown in Examples 1 to 6, the use of two types of cyclic nitroxyl radicals having different affinity for water is more efficient than Comparative Example 3. This is considered to be an effect caused by the difference in the location in the solvent or near the surface due to the difference in affinity for water.

(Evaluation of solubility)
Is a radical species used in this example, TEMPO-OH, TEMPO, to evaluate the solubility to water of TEMPO · BF _4. TEMPO and TEMPO · BF ₄ could not dissolve 0.1 M (M = mol / l) or more in water at room temperature of 25 ° C. On the other hand, it was confirmed that TEMPO-OH dissolves 0.25 M or more. Therefore, TEMPO-OH has a higher affinity for water (hydrophilicity) than TEMPO.

As described above, it is difficult to dissolve 0.1 M or more with TEMPO alone, but as shown in Example 6, the amount of dissolution is increased by the coexistence of TEMPO-OH. I understand that. As a result, it can be seen that the closed circuit current, the fill factor, and the conversion efficiency are the highest.

From the results of this solubility evaluation, the effect of the cyclic nitroxyl radical compound on the electrolyte used in the photoelectric conversion element clearly appears because the solubility in water at room temperature of 25 ° C is 0.1M. It is thought that it happens.

The above results show that high conversion efficiency can be realized by using an electrolyte containing water and at least two types of cyclic nitroxyl radicals different in affinity for water disclosed in the present invention. The effect of the presence of salt and the difference in the affinity of water for at least two nitroxyl radicals as a solvent (difference in the location of the solvent due to the difference in radical species and increase in the amount of dissolution )It suggests.

The photoelectric conversion element according to the present invention is suitably used as a dye-sensitized solar cell, and can be used not only as a solar cell but also as an optical sensor.
This application claims the priority on the basis of Japanese application Japanese Patent Application No. 2011-35855 for which it applied on February 22, 2011, and takes in those the indications of all here.

DESCRIPTION OF SYMBOLS 1 ... Transparent substrate 2 ... Transparent conductive film 3 ... Photoelectric conversion layer 4 ... Electrolyte layer 5 ... Platinum catalyst layer 6 ... Conductive film 7 ... Transparent substrate or support substrate 8 ... Semiconductor electrode 9 ... Counter electrode

Claims (10)

1. A semiconductor electrode comprising a photosensitizer and a semiconductor layer;
   A counter electrode;
   An electrolyte layer provided between the semiconductor electrode and the counter electrode;
   Including
   The photoelectric conversion element which contains simultaneously the water and the at least 2 sort (s) of cyclic nitroxyl radical compound from which affinity with water differs in the said electrolyte layer.
2. The at least two kinds of cyclic nitroxyl radical compounds having different affinity for water are a cyclic nitroxyl radical compound represented by the following general formula (1) and a cyclic nitroxyl radical compound represented by the following general formula (2). The photoelectric conversion element of Claim 1 selected.
   

   

   (In the above general formula (1), A is a divalent group constituting a 5- to 7-membered heterocyclic ring containing nitrogen and may have a hydrophobic substituent. R ¹ , R ² , R ³ and R ⁴ each independently represent a hydrogen atom or a methyl group.)
   

   

   (In the general formula (2), B is a divalent group constituting a 5- to 7-membered heterocyclic ring containing nitrogen and has a hydrophilic substituent. R ¹ , R ² , R ³ , R ⁴ may be different from each other, and represents a hydrogen atom or a methyl group.
3. The photoelectric conversion element according to claim 2, wherein the electrolyte layer further contains one or more selected from oxoammonium salts represented by the following general formula (3) and the following general formula (4).
   

   

   (In the general formula ^{(3), A, R 1} , R 2, R 3, R 4 is ^A in the formula ^{(1), R 1, R} 2, R 3, R 4 represent the same meanings as, X is , Metal chloride, fluorine, chlorine, bromine, iodine, BF ₄ , PF ₆ , CF ₃ SO _3, N (SO ₂ CF ₃ ) ₂ , N (SO ₂ F) ₂ , CF ₃ COO, N (C ₂ F ₅ SO ₂ ), or ClO _4. )
   

   

   In (formula ^{(4), B, R 1} , R 2, R 3, R 4 is ^B in the general formula ^{(2), R 1, R} 2, R 3, R 4 represent the same meanings as, X is , Metal chloride, fluorine, chlorine, bromine, iodine, BF ₄ , PF ₆ , CF ₃ SO _3, N (SO ₂ CF ₃ ) ₂ , N (SO ₂ F) ₂ , CF ₃ COO, N (C ₂ F ₅ SO ₂ ), or ClO _4. )
4. The at least two kinds of cyclic nitroxyl radical compounds having different affinity for water are each at least one kind having a solubility in water at 25 ° C. of not less than 0.1 mol / l and at least one kind having less than 0.1 mol / l. The photoelectric conversion element according to any one of claims 1 to 3, wherein the photoelectric conversion element is a combination thereof.
5. The at least two kinds of cyclic nitroxyl radical compounds having different affinity for water are represented by the following formula (6):
   

   

   A cyclic nitroxyl radical compound represented by formula (7):
   

   

   The photoelectric conversion element according to claim 4, which is a cyclic nitroxyl radical compound represented by the formula:
6. The photoelectric conversion element according to claim 1, further comprising ethanol in the electrolyte layer.
7. The photoelectric conversion element according to any one of claims 1 to 6, wherein the semiconductor substrate is obtained by adsorbing a dye as the photosensitizer on the semiconductor layer.
8. The photoelectric conversion element according to claim 7, wherein the semiconductor layer is porous titanium oxide.
9. The photoelectric conversion element according to any one of claims 1 to 8, wherein the photosensitizer is an organic dye.
10. A method for preparing the electrolyte in the photoelectric conversion device according to any one of claims 1 to 9, wherein at least one cyclic nitroxyl radical represented by the following general formula (1) and the following general formula: A method in which at least one oxoammonium salt represented by the formula (4) is dissolved in water, or at least one cyclic nitroxyl radical represented by the following general formula (2) and the following general formula (3) Any one of the methods of dissolving at least 1 sort of the oxoammonium salt to be dissolved in water is used.
    

    

    (In the formula, A is a divalent group constituting a 5- to 7-membered heterocyclic ring containing nitrogen and may have a hydrophobic substituent. B is a 5- to 7-membered containing nitrogen. A divalent group constituting a heterocyclic ring having a hydrophilic substituent, R ¹ , R ² , R ³ and R ⁴ may be different from each other and each represents a hydrogen atom or a methyl group. Is metal chloride, fluorine, chlorine, bromine, iodine, BF ₄ , PF ₆ , CF ₃ SO _3, N (SO ₂ CF ₃ ) ₂ , N (SO ₂ F) ₂ , CF ₃ COO, N (C ₂ F ₅ SO _2), or an ClO _4.)

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