JP5614807B2 - Electrolyte composition for photoelectric conversion element and photoelectric conversion element - Google Patents

Description

  The present invention relates to an electrolyte composition for a photoelectric conversion element including a redox pair composed of a quaternary phosphonium salt ionic liquid and a quaternary phosphonium iodide, and a photoelectric conversion element using the same.

  In recent years, dye-sensitized solar cells proposed by Gretzell et al. Have attracted attention. This solar cell has a structure in which an electrolytic solution is interposed between a titanium oxide porous electrode carrying a sensitizing dye and a counter electrode. This solar cell can be significantly reduced in cost compared to silicon-based solar cells in terms of materials and manufacturing method.

  In dye-sensitized solar cells, for example, an ionic liquid that is a non-volatile and non-flammable substance, that is, a room temperature molten salt, has been proposed as an electrolyte. For example, the present inventors previously proposed an ionic liquid containing a quaternary phosphonium cation as an electrolyte used in a dye-sensitized solar cell (see Patent Document 1). This ionic liquid contains a trialkyl (alkoxyalkyl) phosphonium cation. This ionic liquid has the advantage that it is chemically and thermally stable and further has flame retardancy (self-extinguishing) due to the inclusion of phosphorus. Furthermore, this quaternary phosphonium salt ionic liquid has the advantages of low viscosity and high ionic conductivity.

  Apart from this technique, ionic liquids using tricyanomethide ions as anionic components are also known. For example, Patent Document 2 proposes an ionic liquid composed of an imidazolinium derivative and tricyanomethide. However, this ionic liquid cannot have a sufficiently low viscosity due to the structure of the imidazolinium derivative that is a cation component.

JP 2009-70811 A JP 2010-171087 A

  An object of the present invention is to provide an electrolyte composition in which various performances are further improved as compared with conventionally proposed electrolyte compositions for dye-sensitized solar cells, and a photoelectric conversion element using the same.

  In such a situation, as a result of intensive studies, the present inventors have found that a quaternary phosphonium salt ionic liquid composed of a combination of a specific quaternary phosphonium cation and a specific anion has extremely low viscosity and high ionic conductivity, And since it is rich in heat resistance and flame retardancy, it was found useful for an electrolyte composition of a dye-sensitized solar cell, and the present invention was completed.

That is, the present invention contains a quaternary phosphonium salt ionic liquid represented by the following general formula (1), and a redox pair consisting of iodide ions and polyiodide ions,
What solved the said subject by providing the electrolyte composition for photoelectric conversion elements characterized by using the quaternary phosphonium iodide represented by following General formula (2) as a supply source of the said iodide ion. It is.

  Moreover, this invention provides the photoelectric conversion element using the said electrolyte composition.

  According to the electrolyte composition for a photoelectric conversion element of the present invention, a photoelectric conversion element that can achieve a high short-circuit photocurrent density and a high photoelectric conversion efficiency can be obtained by using this for the photoelectric conversion element.

FIG. 1 is a schematic view showing the structure of one embodiment of the photoelectric conversion element of the present invention.

Hereinafter, the present invention will be described based on preferred embodiments thereof. The electrolyte composition for photoelectric conversion elements of the present invention (hereinafter also simply referred to as “electrolyte composition”) contains a quaternary phosphonium salt ionic liquid represented by the general formula (1) as one component thereof. . Three of the four groups in the quaternary phosphonium salt represented by the general formula (1) are the same alkyl group represented by R ₁ , and the remaining one is — (CH ₂ ) _n O—. An alkoxyalkyl group represented by R ₂ ; The viscosity of the quaternary phosphonium salt having such a structure is much lower than that of a quaternary phosphonium salt in which all the groups bonded to phosphorus are alkyl groups. The reason for this is not completely elucidated at the present time, but is considered to be caused by weakening the cationic charge due to the electron donating property of the alkoxy group. Moreover, when all three alkyl groups are the same group, electrochemical stability and heat resistance are improved.

Specific alkyl groups represented by R _{1 in} the general formula (1) include methyl group, ethyl group, n-propyl group, n-butyl group, n-hexyl group, i-propyl group, i-butyl group, n -Pentyl group, n-hexyl group, cyclopentyl group, cyclohexyl group and the like. Of these groups, a methyl group, an ethyl group, or an n-butyl group is particularly preferable from the viewpoint of particularly decreasing the viscosity of the quaternary phosphonium salt ionic liquid.

It is preferable to use a methyl group or an ethyl group as R ₂ in the general formula (1) from the viewpoint of particularly lowering the viscosity of the quaternary phosnium salt ionic liquid. Moreover, n in General formula (1) is 1-6 as mentioned above, Preferably it is 1-3, More preferably, it is 1 or 2, Most preferably, it is 1. When the value of n is within this range, the viscosity of the quaternary phosphonium salt ionic liquid can be particularly lowered, and the solubility of the organic compound contained in the electrolyte composition can be further enhanced.

Specific alkoxyalkyl groups represented by — (CH ₂ ) _n O—R ₂ in the general formula (1) include a methoxymethyl group, a 2-methoxyethyl group, a 3-methoxy-n-propyl group, 4 -Methoxy-n-butyl group, 5-methoxy-n-pentyl group, 6-methoxy-n-hexyl group, ethoxymethyl group, 2-ethoxyethyl group, 3-ethoxy-n-propyl group, 4-ethoxy-n -Butyl group, 5-ethoxy-n-pentyl group, 6-ethoxy-n-hexyl group and the like.

As the anion component in the general formula (1), tricyanomethide ion, that is, C (CN) ₃ ⁻ is used. The present inventors have found that the viscosity of the ionic liquid is unexpectedly reduced by combining this anionic component with the above-described quaternary phosphonium cation component.

  Specific quaternary phosphonium salts represented by the general formula (1) include, for example, triethyl (methoxymethyl) phosphonium tricyanomethide, triethyl (2-methoxyethyl) phosphonium tricyanomethide, tri-n-propyl (methoxy). Methyl) phosphonium tricyanomethide, tri-n-propyl (2-methoxyethyl) phosphonium tricyanomethide, tri-n-butyl (methoxymethyl) phosphonium tricyanomethide, tri-n-butyl (2-methoxyethyl) phosphonium tricia Nomethide, tri-n-pentyl (methoxymethyl) phosphonium tricyanomethide, tri-n-pentyl (2-methoxyethyl) phosphonium tricyanomethide, tri-n-hexyl (methoxymethyl) phosphonium tricyanomethide, tri-n − Such as hexyl (2-methoxyethyl) phosphonium tricyanomethide the like. Of these, triethyl (methoxymethyl) phosphonium tricyanomethide is preferable from the viewpoint of particularly low viscosity.

  The quaternary phosphonium salt represented by the general formula (1) is a liquid having ionic conductivity at room temperature (25 ° C.), that is, an ionic liquid. This quaternary phosphonium salt ionic liquid has a viscosity at 25 ° C. of preferably 200 mPa · sec or less, more preferably 100 mPa · sec or less, and even more preferably 50 mPa · sec or less. It is preferable for the viscosity to be 200 mPa · sec or less because the dehydration efficiency in the purification of the ionic liquid is increased. It is preferable for the viscosity to be 100 mPa · sec or less because the diffusion of iodine redox couples is highly efficient. Furthermore, when the viscosity is 50 mPa · sec or less, the ionic conductivity is remarkably increased and the photoelectric conversion efficiency is increased, which is preferable. The lower limit of the viscosity of the ionic liquid comprising a quaternary phosphonium salt is not particularly limited, and it is preferably as low as possible. However, when the viscosity at 25 ° C. is as low as 20 mPa · sec, the ionic conductivity is sufficiently high, and photoelectric conversion is achieved. This is preferable because the efficiency is sufficiently increased. The measuring method of a viscosity is demonstrated in the Example mentioned later.

  The quaternary phosphonium salt represented by the general formula (1) can be obtained by reacting a quaternary phosphonium halide with a metal salt of an anion component to perform anion exchange. The quaternary phosphonium halide is a general term for those in which the anion moiety in the quaternary phosphonium salt represented by the general formula (1) is halogen.

When the quaternary phosphonium halide is a trialkyl (alkoxyalkyl) phosphonium halide, this compound can be obtained, for example, by reacting a trialkylphosphine with a halogenated alkoxyalkyl. In particular, a trialkylphosphine (general formula: (R _a ) ₃ P) in which _three alkyl groups bonded to a phosphorus atom are the same, and an alkoxyalkyl halide (general formula: X— (CH ₂ ) _n O— Adopting the method of reacting R _b ) is preferable because an object having few impurities can be obtained. Further, it is preferable that the halogen of the quaternary phosphonium halide is bromine or iodine because the quaternary phosphonium halide can be purified by recrystallization. From this viewpoint, it is preferable to use an alkoxyalkyl bromide or an alkoxyalkyl iodide as the halogenated alkoxyalkyl. When halogen in the quaternary phosphonium halide is an element other than bromine and iodine, chlorine can be replaced with iodine or bromine by using sodium iodide or the like, for example, chloride.

  When the quaternary phosphonium halide is a trialkyl (alkoxyalkyl) phosphonium halide, in order to form this compound, the alkoxyalkyl halide is preferably 0.5 to 2 moles, more preferably, to the trialkylphosphine. Add 0.9 to 1.2 moles. Then, in an inert solvent not containing chlorine, for example, in toluene, the reaction is preferably performed at 20 to 150 ° C., more preferably 30 to 100 ° C., preferably 3 hours or more, and more preferably 5 to 12 hours. The reaction atmosphere is preferably an atmosphere without oxygen. For example, a nitrogen atmosphere or an argon atmosphere is preferable. When a trialkylphosphine and a halogenated alkoxyalkyl are reacted in an atmosphere in which oxygen is present, a trialkylphosphine oxide in which oxygen is bonded to the trialkylphosphine is generated, and the yield tends to decrease. Trialkylphosphine oxide can be removed by washing with an organic solvent as appropriate. However, when the total number of carbon atoms of the quaternary phosphonium halide is increased, the quaternary phosphonium halide tends to be dissolved in the organic solvent, making it difficult to remove. Therefore, it is preferable to carry out the reaction under an inert atmosphere so as not to generate trialkylphosphine oxide.

  As the metal salt of the anion component used for introducing another anion into the quaternary phosphonium halide by anion exchange, for example, an alkali metal salt such as the above-described Li salt of tricyanomethide which is an anion component can be used. The use of an alkali metal salt is preferable because the alkali halide generated by the reaction between the salt and the quaternary phosphonium halide can be easily removed by washing with water or an adsorbent. The alkali metal salt of tricyanomethide is a commercially available compound.

  As the water used for washing, ultrapure water or deionized water can be used. Washing with water is preferably repeated as appropriate until the impurity content decreases. Examples of impurities to be removed by washing with water include unreacted raw materials and alkali metal halides. By using the adsorbent, the alkali metal halide can be efficiently removed. Moreover, in order to remove unreacted raw materials, by-products and the like, washing with an organic solvent can be appropriately performed. As an organic solvent that can be used for washing, it is preferable to use a nonpolar solvent that does not contain chlorine, such as pentane, hexane, or heptane. By using these nonpolar solvents, nonpolar organic compounds such as impurities can be efficiently removed without dissolving the quaternary phosphonium salt.

  The quaternary phosphonium salt washed with water or an organic solvent is preferably purified in order to remove moisture or the organic solvent. Examples of the purification method include methods such as dehydration by molecular sieve and desolvation by vacuum drying. Purification by vacuum drying is preferable because it prevents contamination of impurities and removes water and organic solvent at a time. In purification by vacuum drying, the drying temperature is preferably 70 to 120 ° C, more preferably 80 to 100 ° C, and the degree of vacuum is preferably 0.1 to 1.0 kPa, more preferably 0.1 to 0.5 kPa. is there. The time is preferably about 2 to 8 hours, more preferably about 5 to 12 hours.

  The ionic liquid composed of the quaternary phosphonium salt represented by the general formula (1) thus obtained has a low viscosity, a high ionic conductivity, an appropriate amount derived from the structure of the quaternary phosphonium cation and the structure of the anion component. Since it has properties of solubility, chemical stability and thermal stability, it is suitably used as one component of the electrolyte composition of the photoelectric conversion element. When the viscosity is low, diffusion and convection are promoted and the ionic conductivity is remarkably improved, and the increase in viscosity due to cooling is low, which is advantageous from the viewpoint that the ionic liquid can be used at a low temperature. . Moreover, since the alkoxyalkyl group is introduced, the solubility of the organic compound tends to be improved. In other words, by shortening the alkyl group, the molecular weight can be reduced to obtain a low-viscosity ionic liquid, while the problem that the solubility of the organic compound-based additive decreases due to the shortened alkyl group, This can be solved by introducing an alkoxyalkyl group.

  Furthermore, the organophosphorus compounds including the quaternary phosphonium salt represented by the general formula (1) exhibit flame retardancy and self-extinguishing properties. And the ionic liquid consisting of a quaternary phosphonium salt represented by the general formula (1) has a short alkyl group (1 to 6 carbon atoms) and a small molecular weight. And self-extinguishing. Therefore, the ionic liquid can be used as a flame retardant electrolyte of a photoelectric conversion element.

  From the above description, the ionic liquid composed of the quaternary phosphonium salt represented by the general formula (1) has high ionic conductivity because of low viscosity, and as a result, high short-circuit photocurrent density and photoelectric conversion efficiency are obtained, and It is understood that the heat resistance and flame retardancy are high. Therefore, it is clear that the ionic liquid can be advantageously used as an electrolyte composition of a photoelectric conversion element.

The electrolyte composition of the present invention contains a redox couple in addition to the ionic liquid composed of the quaternary phosphonium salt represented by the general formula (1). The redox couple is not particularly limited as long as the redox potential of the redox couple is between the reduction potential of the excitation dye and the oxidation potential of the counter electrode, but in the present invention, iodide ion (I ⁻ ) and I ₃ ^A halogen-containing redox pair consisting of polyiodide ions such as ⁻ , I ₅ ⁻ , I ₇ ⁻ , Cl ₂ I ⁻ , ClI ₂ ⁻ , Br ₂ I ⁻ and BrI ₂ ⁻ is used. The concentration of the redox couple in the electrolyte composition (that is, the total concentration of iodide ions and polyiodide ions) is preferably 0.05 to 4.0 M in terms of molar concentration (“M” is “mol / l”). The same shall apply hereinafter.), More preferably 0.1 to 3.0M, and even more preferably 0.5 to 2.0M. This iodine redox couple can be obtained by reacting iodine molecules with iodide ions. The ratio of iodine molecules to iodide ions is not particularly limited, but is preferably 1 to 100% in molar ratio, more preferably 2 to 50%, and still more preferably 3 to 30%. A quaternary phosphonium ion iodide is used as a source of iodide ions.

  In the present invention, the quaternary phosphonium iodide used as a source of iodide ions is a quaternary phosphonium iodide represented by the general formula (2).

In the quaternary phosphonium iodide represented by the general formula (2), R _{3 to} R ₆ are either (i) all of which are alkyl groups, or (b) R _{3 to} R ₅ are alkyl groups, R ₆ is preferably any one of alkoxyalkyl groups.

In the case of (a), the four alkyl groups may be the same or different. In some cases, among the four alkyl groups, R ₃ and R ₄ may be bonded to form a ring. Of the four alkyl groups, three alkyl groups from R _{3 to} R ₅ are the same alkyl group, and R ₆ is an alkyl group different from R _{3 to} R _5, which further improves the photoelectric conversion efficiency. From the point of view, it is preferable. In the general formula (2), towards the case the number of carbon atoms of R ₆ is smaller than the number of carbon atoms in R ₃ to R ₅ is, than the number of carbon atoms in R ₆ is greater than the number of carbon atoms in R ₃ to R _5, It has been found that the photoelectric conversion efficiency is further increased. If the number of carbon atoms of R ₆ is smaller than the number of carbon atoms in R ₃ to R _5, the difference between the number of carbon atoms in the carbon number of the R ₃ to R ₅ in R ₆ is preferably 1 to 5, in particular 2 to 4 is preferred. The reason why the photoelectric conversion efficiency is improved when the number of carbon atoms in R ₆ is smaller than the number of carbon atoms in R _{3 to} R ₅ has not been completely elucidated at present. The reason for this is not because the redox pair redox efficiency is increased by the presence of the quaternary phosphonium iodide represented by the general formula (2), and the efficiency of charge transfer in the electrolyte composition is increased. I think.

R ₃ to R ₅ have the same alkyl group, and when the number of carbon atoms of the R ₆ is smaller than the number of carbon atoms in R ₃ to R ₅ are as R ₃ to R ₅ is, for example, ethyl group, n- propyl group, Examples include n-butyl group, n-hexyl group, i-propyl group, i-butyl group, n-pentyl group, i-hexyl group, cyclopentyl group, cyclohexyl group and the like. These alkyl groups may be substituted with a monovalent substituent. Examples of such substituents include alkenyl, alkynyl, alkoxy, carboxyl, alkylcarbonyloxy, alkoxycarbonyl, sulfoxy, alkylsulfonyl, hydroxy, hydroxysulfonyl, aryl, aryloxy Group, acyloxy group, galvamoyl group, sulfonic acid group, sulfamoyl group, cycloalkyl group, cyano group, nitro group, amino group, alkylamino group, heterocyclic group, aminosulfonyl group, halogen atom, 2-butoxyethyl group, 6 -Bromohexyl group, 2-carboxyethyl group, 3-sulfoxypropyl group, 4-sulfoxydibutyl group, 2-hydroxyethyl group, phenylmethyl group, 4-butoxyphenylmethyl group and the like can be mentioned. On the other hand, R ₆ is a methyl group, an ethyl group, an n-propyl group, an n-butyl group, an i-propyl group, an i-butyl group, n, provided that the number of carbon atoms is less than R _{3 to} R _5. -A pentyl group, a cyclopentyl group, etc. are mentioned. R ₆ may also be substituted with the same substituent as R _{3 to} R ₅ .

In the case of (b), R _{3 to} R ₅ which are alkyl groups may be the same or different. In particular, R _{3 to} R ₅ are preferably the same. Examples of R _{3 to} R ₅ include an ethyl group, n-propyl group, n-butyl group, n-hexyl group, i-propyl group, i-butyl group, n-pentyl group, i-hexyl group, cyclopentyl group, Examples include a cyclohexyl group. These alkyl groups may be substituted with the monovalent substituent described above. On the other hand, R ₆ is preferably a methoxymethyl group, a methoxyethyl group, or the like, provided that the total number of carbon atoms is 2-6.

In any of the cases (a) and (b), when R ₃ and R ₄ form a ring, examples of such a ring include a tetrahydrophosphole ring and a pentahydrophospholine. Ring, 9-H-9-phosphabicyclononane ring, and the like.

  In both cases (a) and (b), the quaternary phosphonium iodide has a structure in which a relatively short chain alkyl group is directly bonded to a phosphorus atom. The present inventors have studied that the photoelectric conversion efficiency is much higher than before by using the quaternary phosphonium salt ionic liquid represented by the general formula (1) as an electrolyte composition for a photoelectric conversion element. The result turned out. It has also been found that the use of the quaternary phosphonium iodide represented by the general formula (2) improves the chemical stability and heat resistance of an electrolyte composition containing the quaternary phosphonium iodide. The inventors believe that this is because the quaternary phosphonium cation does not have an active hydrogen site like the imidazolium cation. Furthermore, the quaternary phosphonium iodide represented by the general formula (2) has higher solubility in a solvent than the corresponding nitrogen-based cation salt, and it is easy to increase the concentration of anions. It is advantageous.

As particularly preferred combinations of R _{3 to} R ₆ , R _{3 to} R ₅ are ethyl groups, R ₆ is a methyl group; R _{3 to} R ₅ are n-propyl groups, and R ₆ is a methyl group; R _{3 to} R ₅ is an i-propyl group, R ₆ is a methyl group; R _{3 to} R ₅ are n-butyl groups, R ₆ is a methyl group; R _{3 to} R ₅ are i-butyl groups, and R ₆ is a methyl group; R _{3 to} R ₅ are n-pentyl groups, R ₆ is a methyl group; R _{3 to} R ₅ are cyclopentyl groups, R ₆ is a methyl group; R _{3 to} R ₅ are n-hexyl groups, and R ₆ is a methyl group And the combination of R _{3 to} R ₅ is a cyclohexyl group, R ₆ is a methyl group; R _{3 to} R ₅ are ethyl groups, and R ₆ is a methoxymethyl group. Particularly preferred combinations are those in which R _{3 to} R ₅ are n-butyl groups, R ₆ is a methyl group, and R _{3 to} R ₅ are ethyl groups and R ₆ is a methoxymethyl group.

  Preferable specific examples of the quaternary phosphonium iodide represented by the general formula (2) include tetramethylphosphonium iodide, tetraethylphosphonium iodide, triethylmethylphosphonium iodide, tetra-n-propylphosphonium iodide, tri-n. -Propylmethylphosphonium iodide, tri-n-propylethylphosphonium iodide, tetra-n-butylphosphonium iodide, tri-n-butylmethylphosphonium iodide, tri-n-butylethylphosphonium iodide, tri-i- Butylmethylphosphonium iodide, tetra-n-pentylphosphonium iodide, tri-n-pentylmethylphosphonium iodide, tri-n-pentylethylphosphonium iodide, tri-n-pentyl-n-propylphosphonium iodide , Tricyclopentylmethylphosphonium iodide, tetra-n-hexylphosphonium iodide, tri-n-hexylmethylphosphonium iodide, tri-n-hexylethylphosphonium iodide, tri-n-hexyl-n-propylphosphonium iodide , Tri-n-hexyl-n-butylphosphonium iodide, tri-n-hexyl-i-butylphosphonium iodide, tricyclohexylmethylphosphonium iodide, triethyl (methoxymethyl) phosphonium iodide, and the like. Among these compounds, triethylmethylphosphonium iodide, tri-n-propylmethylphosphonium iodide, tri-i-propylmethylphosphonium iodide, tri-n-butylmethylphosphonium iodide, tri-i-butylmethylphosphonium iodide , Tri-n-pentylmethylphosphonium iodide, tricyclopentylmethylphosphonium iodide, tri-n-hexylmethylphosphonium iodide, tricyclohexylmethylphosphonium iodide, triethyl (methoxymethyl) phosphonium iodide, etc. It is preferable from the viewpoint of developing stability and heat resistance. Further, among these, tri-n-butylmethylphosphonium iodide and triethyl (methoxymethyl) phosphonium iodide are preferable because they exhibit particularly excellent photoelectric conversion efficiency.

  A commercially available product can be used as the quaternary phosphonium iodide represented by the general formula (2). Examples of such commercially available products include Hishicolin (registered trademark) PX-4MI (tri-n-butylmethylphosphonium iodide), which is a compound available from Nippon Chemical Industry Co., Ltd. The quaternary phosphonium iodide represented by the general formula (2) can also be synthesized by a nucleophilic reaction between a trialkylphosphine and an alkyl iodide. For example, the alkyl iodide is preferably added in an amount of 0.5 to 2 times mol, more preferably 0.9 to 1.2 times mol with respect to the trialkylphosphine, and in an inert solvent not containing chlorine, for example, in toluene. The reaction is preferably carried out at 20 to 150 ° C., more preferably 30 to 100 ° C., preferably 3 hours or more, more preferably 5 to 12 hours. Since trialkylphosphine is easily oxidized, the reaction atmosphere is preferably an atmosphere free of oxygen. For example, a nitrogen atmosphere or an argon atmosphere is preferable. When a trialkylphosphine and an alkyl iodide are reacted in an atmosphere in which oxygen is present, a trialkylphosphine oxide in which oxygen is bonded to the trialkylphosphine may be generated and the yield may be lowered. Trialkylphosphine oxide can be removed by washing with an organic solvent as appropriate. However, when the total number of carbon atoms in the quaternary phosphonium iodide is increased, the quaternary phosphonium iodide tends to dissolve in the organic solvent, making it difficult to remove. . Therefore, it is preferable to carry out the reaction under an inert atmosphere so as not to generate trialkylphosphine oxide.

  The obtained quaternary phosphonium iodide can be purified by recrystallization. The recrystallization is preferably repeated as appropriate until the impurity content decreases. Examples of impurities to be removed by recrystallization include unreacted raw materials and trialkylphosphine oxide. As an organic solvent that can be used for recrystallization, for example, water, methanol, acetone, toluene, hexane, or the like is preferably used alone or in combination.

  The quaternary phosphonium iodide purified by recrystallization is preferably dried to remove moisture and organic solvent. As a drying method, a vacuum drying method is preferable because it prevents contamination of impurities and can remove moisture and an organic solvent at a time. In the vacuum drying method, the drying temperature is preferably 70 to 120 ° C, more preferably 80 to 100 ° C, and the degree of vacuum is preferably 0.1 to 1.0 kPa, more preferably 0.1 to 0.5 kPa. . The drying time is preferably about 2 to 8 hours, more preferably about 5 to 12 hours.

  When an electrolyte composition containing an ionic liquid composed of a quaternary phosphonium salt represented by the general formula (1) and an iodine redox couple is used as an electrolyte layer of a dye-sensitized solar cell, The electrolyte composition includes 4-tert-butylpyridine (hereinafter referred to as “TBP”), organic nitrogen compounds such as 2-vinylpyridine and N-vinyl-2-pyrrolidone, lithium salt, sodium salt, magnesium salt, iodide, thiocyanate. It is preferable to add various additives such as acid salts and water in order to increase the photoelectric conversion efficiency. As illustrated in the examples described later, it is preferable to use TBP as an additive or to use TBP in combination with water since the photoelectric conversion efficiency is further improved. The amount of these additives is not particularly limited, but the molar concentration of each additive in the electrolyte composition is preferably 0.01 to 4.0M, more preferably 0.05 to 3.0M, and still more preferably. 0.1 to 2.0M.

  As a photoelectric conversion element comprising an ionic liquid composed of a quaternary phosphonium salt represented by the general formula (1) and an electrolyte composition containing an iodine redox pair, an element that converts light into electric energy and conversely electric energy. Elements that convert to light are included. Typical examples of the former include power generation devices such as dye-sensitized solar cells and photodiodes. Typical examples of the latter include light emitting devices such as light emitting diodes and semiconductor lasers. In the case where the photoelectric conversion element is either a power generation device or a light emitting device, for example, as illustrated in FIG. 1, the photoelectric conversion element 10 includes a semiconductor layer 11, a dye layer 12 provided on one surface of the semiconductor layer 11, A transparent electrode layer 13 provided on the other surface of the semiconductor layer 11, a counter electrode 14 disposed to face the dye layer 12, and an electrolyte layer 15 disposed between the dye layer 12 and the counter electrode 14. The distance between the dye layer 12 and the counter electrode 14, that is, the thickness of the electrolyte layer 15 can generally be set to 50 to 500 μm. The electrolyte layer 15 is made of an electrolyte composition containing an ionic liquid composed of a quaternary phosphonium salt represented by the general formula (1) and an iodine redox couple. When the photoelectric conversion element 10 shown in FIG. 1 is used as a power generation device, an electromotive force is generated between the transparent electrode layer 13 and the counter electrode 14 by irradiating sunlight (visible light) from the transparent electrode layer 13 side. Arise. When the photoelectric conversion element 10 shown in the figure is used as a light emitting device, light is emitted between the semiconductor layer 11 and the dye layer 12 by applying a voltage between the transparent electrode layer 13 and the counter electrode 14. In FIG. 1, conductive wires are connected to the transparent electrode layer 13 and the counter electrode 14, but conductive wires may be connected to the semiconductor layer 11 instead of the transparent electrode layer 13. In this case, the transparent electrode layer 13 is not essential.

  The photoelectric conversion element 10 using an electrolyte composition containing an ionic liquid composed of a quaternary phosphonium salt represented by the general formula (1) and an iodine redox couple is particularly used as a dye-sensitized solar cell which is a kind of power generation device. It is useful. Since the quaternary phosphonium salt represented by the general formula (1) contains a tricyanomethide ion as an anion component, the viscosity is remarkably lower than that of a conventional quaternary phosphonium salt ionic liquid. The effect of improving ion conductivity can be expected, which is preferable. Moreover, when the electrolyte composition of this invention is used for a dye-sensitized solar cell, compared with the case where another ionic liquid is used, since a high short circuit photocurrent density and a high open circuit voltage are obtained, it is preferable.

  When the photoelectric conversion element using an ionic liquid composed of a quaternary phosphonium salt represented by the general formula (1) and an electrolyte composition containing an iodine redox couple is a dye-sensitized solar cell, the dye-sensitized solar cell An example of a specific configuration of the battery is as follows. That is, the dye-sensitized solar cell is disposed on the transparent electrode layer, the nanoporous oxide semiconductor layer coated thereon and supporting the sensitizing dye, the counter electrode, and at least a part between the transparent electrode layer and the counter electrode It is comprised from the electrolyte layer containing an iodine redox couple. When sunlight (for example, visible light) irradiated from the transparent electrode side excites the dye on the oxide semiconductor, the excited dye injects electrons into the conduction band of the oxide semiconductor. The resulting dye oxidant accepts electrons from the reductant in the electrolyte layer, returns to the ground state dye, and the reductant becomes an oxidant. The electrons injected into the oxide semiconductor layer donate electrons to the oxidant in the electrolyte layer through the external circuit and at the counter electrode. Through the above cycle, a steady photocurrent flows in the circuit.

The type of the transparent electrode layer is not particularly limited as long as it has good light transmittance and has a conductive layer formed on the surface thereof. For example, transparent oxide semiconductors such as tin-added indium oxide (ITO), tin oxide (SnO ₂ ), fluorine-added tin oxide (FTO), and zinc oxide (ZnO) alone or in combination, glass, polyethylene terephthalate (PET), It is preferably formed as a thin film on a non-conductive and transparent substrate such as polyethylene naphthalate (PEN) or polycarbonate (PC). The nanoporous oxide semiconductor layer includes titanium oxide (TiO ₂ ), tin oxide (SnO ₂ ), tungsten oxide (WO ₃ ), zinc oxide (ZnO), niobium oxide (Nb ₂ O ₅ ), etc. alone or in combination. It is a porous thin film mainly composed of the oxide semiconductor fine particles used. The average particle size of the oxide semiconductor fine particles used is preferably 1 to 200 nm, more preferably 3 to 100 nm, and still more preferably 5 to 50 nm. An oxide semiconductor is generally n-type, but is not limited to this and may be p-type. The sensitizing dye supported on the nanoporous oxide semiconductor is not particularly limited as long as it efficiently absorbs sunlight (for example, visible light). For example, a metal complex such as a ruthenium complex or iron complex having a ligand including a bipyridine structure or a terpyridine structure, a metal complex such as a porphyrin or phthalocyanine complex, an organic dye such as eosin, rhodamine, merocyanine, or coumarin. Is preferable from the viewpoint of photoexcitation under sunlight irradiation conditions. The counter electrode is not particularly limited as long as it is an electrode that generates an electromotive force with the transparent electrode. For example, a conductive film such as gold, platinum, or a carbon-based material is formed into a vacuum by a sputtering method or a vapor deposition method. It is preferable to use an electrode formed on a substrate by a method such as a wet film-forming method in which a heat treatment is applied after a platinum-containing solution such as a chloroplatinic acid solution is applied.

  The present invention will be specifically described below with reference to examples. Unless otherwise specified, “%” means “% by weight”.

[Synthesis Example 1]
Synthesis of triethyl (methoxymethyl) phosphonium tricyanomethide and measurement of physical properties (1) Synthesis To 236 g (0.5 mol) of triethylphosphine 25% toluene solution (product name: Hishicolin (registered trademark) P-2 manufactured by Nippon Chemical Industry Co., Ltd.) Then, 62 g (0.5 mol) of bromomethyl methyl ether (reagent manufactured by Tokyo Chemical Industry Co., Ltd.) was added dropwise and reacted at 70-80 ° C. for 6 hours. After completion of the reaction, hexane was added for crystallization to obtain 97 g of triethyl (methoxymethyl) phosphonium bromide crystals (yield 80%). 97 g (0.4 mol) of this triethyl (methoxymethyl) phosphonium bromide was dissolved in 100 g of water, and 45 g (0.4 mol) of sodium tricyanomethide (Lonza Ltd. reagent) was added and reacted. Next, the mixture was aged by stirring at room temperature for 3 hours. After completion of the stirring, the product was extracted with dichloromethane, purified by passing through a silica gel column, and then the solvent was distilled off, followed by vacuum drying at 100 ° C. and a vacuum degree of 0.5 kPa for 5 hours. The product thus obtained was confirmed by ¹ H-NMR, ¹³ C-NMR, and ³¹ P-NMR. The yield of the product (reddish brown transparent liquid) was 51 g (yield 40%), and it was confirmed by ³¹ P-NMR that the purity was 99% or more.

(2) Measurement of physical properties Viscosity at 25 ° C. was measured using a rotational viscometer (BROOKFIELD, DV-II + Pro).
It measured using. The measurement results are shown in Table 1. The viscosity has an error of about ± 5% depending on the measurement conditions.

[Synthesis Example 2]
Synthesis and measurement of physical properties of tri-n-butyl (methoxymethyl) phosphonium tricyanomethide (1) Synthesis Tri-n-butylphosphine 25% toluene solution (product name: Hishicolin (registered trademark) P-4 manufactured by Nippon Chemical Industry Co., Ltd.) ) 62 g (0.5 mol) of bromomethyl methyl ether (Tokyo Chemical Industry Co., Ltd. reagent) was added dropwise to 405 g (0.5 mol) and reacted at 70-80 ° C. for 6 hours. After completion of the reaction, hexane was added for crystallization to obtain 131 g of tributyl (methoxymethyl) phosphonium bromide crystals (yield 80%). 131 g (0.4 mol) of this tributyl (methoxymethyl) phosphonium bromide was dissolved in 65 g of water, and 45 g (0.4 mol) of sodium tricyanomethide (a reagent manufactured by Lonza Ltd.) was added and reacted in an aqueous system. Next, the mixture was aged by stirring at room temperature for 3 hours. After completion of the stirring, the product was extracted with dichloromethane, purified by passing through a silica gel column, and then the solvent was distilled off, followed by vacuum drying at 100 ° C. and a vacuum degree of 0.5 kPa for 5 hours. The product thus obtained was confirmed by ¹ H-NMR, ¹³ C-NMR, and ³¹ P-NMR. The yield of the product (reddish brown liquid) was 67 g (yield 40%), and it was confirmed by ³¹ P-NMR that the purity was 99% or more.
(2) Measurement of physical properties The viscosity of the product at 25 ° C. was measured in the same manner as in Synthesis Example 1. The results are shown in Table 1.

[Synthesis Example 3]
Synthesis of tri-n-butyl (methoxyethyl) phosphonium tricyanomethide and measurement of physical properties (1) Synthesis Tri-n-butylphosphine 25% toluene solution (manufactured by Nippon Chemical Industry Co., Ltd., product name: Hishicolin (registered trademark) P-4 ) 69 g (0.5 mol) of 2-bromoethyl methyl ether (a reagent manufactured by Aldrich) was added dropwise to 405 g (0.5 mol) and reacted at 70 to 80 ° C. for 6 hours. After completion of the reaction, hexane was added for crystallization to obtain 136 g of tributyl (methoxyethyl) phosphonium bromide crystals (yield 80%). 136 g (0.4 mol) of this tributyl (methoxyethyl) phosphonium bromide was dissolved in 68 g of water, and 45 g (0.4 mol) of sodium tricyanomethide (Lonza Ltd. reagent) was added and reacted in an aqueous system. Next, the mixture was aged by stirring at room temperature for 3 hours. After completion of the stirring, the product was extracted with dichloromethane, purified by passing through a silica gel column, and then the solvent was distilled off, followed by vacuum drying at 100 ° C. and a vacuum degree of 0.5 kPa for 5 hours. The product thus obtained was confirmed by ¹ H-NMR, ¹³ C-NMR, and ³¹ P-NMR. The yield of the product (reddish brown liquid) was 70 g (yield 40%), and it was confirmed by ³¹ P-NMR that the purity was 99% or more.
(2) Measurement of physical properties The viscosity of the product at 25 ° C. was measured in the same manner as in Synthesis Example 1. The results are shown in Table 1.

[Synthesis Example 4 (Comparative)]
Synthesis and physical property measurement of triethyl (methoxymethyl) phosphonium bis (trifluoromethylsulfonyl) imide (1) Synthesis 236 g of triethylphosphine 25% toluene solution (Nippon Chemical Industry Co., Ltd. product name: Hishicolin (registered trademark) P-2) 0.5 mol), 62 g (0.5 mol) of bromomethyl methyl ether (Tokyo Chemical Industry Co., Ltd. reagent) was added dropwise and reacted at 70-80 ° C. for 6 hours. After completion of the reaction, hexane was added for crystallization to obtain 97 g of triethyl (methoxymethyl) phosphonium bromide crystals (yield 80%). 97 g (0.4 mol) of this triethyl (methoxymethyl) phosphonium bromide was dissolved in 50 g of water, and 75 g (0.4 mol) of lithium bis (fluoromethylsulfonyl) imide (Mitsubishi Materials Co., Ltd. reagent) was added and reacted in an aqueous system. . Next, the mixture was aged by stirring at room temperature for 3 hours. After stirring, the lower layer (product) was separated. The separated product was washed with pure water four times, followed by hexane washing four times. After completion of the cleaning, vacuum drying was performed at 100 ° C. and a vacuum degree of 0.5 kPa for 5 hours. The product thus obtained was confirmed by ¹ H-NMR, ¹³ C-NMR, ³¹ P-NMR, and ¹⁹ F-NMR. The yield of the product (colorless transparent liquid) was 130 g (yield 76%), and it was confirmed by ³¹ P-NMR that the purity was 99% or more.
(2) Measurement of physical properties The viscosity of the product at 25 ° C. was measured in the same manner as in Synthesis Example 1. The results are shown in Table 1.

[Synthesis Example 5 (Comparative)]
Synthesis and measurement of physical properties of triethyl (methoxymethyl) ammonium tricyanomethide (1) Synthesis To 51 g (0.5 mol) of triethylamine (Kanto Chemical Co., Ltd.), 62 g of bromomethyl methyl ether (Reagent of Tokyo Chemical Industry Co., Ltd.) 5 mol) was added dropwise and reacted at 70-80 ° C. for 6 hours. After completion of the reaction, hexane was added for crystallization to obtain 81 g of triethyl (methoxymethyl) ammonium bromide crystals (yield 76%). 81 g (0.4 mol) of this triethyl (methoxymethyl) ammonium bromide was dissolved in 40 g of water, and 45 g (0.4 mol) of sodium tricyanomethide (Lonza Ltd. reagent) was added and reacted in an aqueous system. Next, the mixture was aged by stirring at room temperature for 3 hours. After completion of the stirring, the product was extracted with dichloromethane, purified by passing through a silica gel column, and then the solvent was distilled off, followed by vacuum drying at 100 ° C. and a vacuum degree of 0.5 kPa for 5 hours. The product thus obtained was confirmed by ¹ H-NMR, ¹³ C-NMR, and ¹⁵ N-NMR. The yield of the product (pale yellow transparent liquid) was 42 g (yield 38%), and it was confirmed by ¹ H-NMR that the purity was 99% or more.
(2) Measurement of physical properties The viscosity of the product at 25 ° C. was measured in the same manner as in Synthesis Example 1. The results are shown in Table 1.

[Synthesis Example 6 (Comparative)]
Synthesis and measurement of physical properties of triethylpentylphosphonium tricyanomethide (1) Synthesis To 236 g (0.5 mol) of triethylphosphine 25% toluene solution (product name: Hishicolin (registered trademark) P-2) in 1-bromo 77 g (0.5 mol) of pentane (Tokyo Chemical Industry Co., Ltd. reagent) was added dropwise and reacted at 70-80 ° C. for 6 hours. After completion of the reaction, hexane was added for crystallization to obtain 113 g of triethylpentylphosphonium bromide crystals (yield 84%). 81 g (0.3 mol) of this triethylpentylphosphonium bromide was dissolved in 40 g of water, and 34 g (0.3 mol) of sodium tricyanomethide (Lonza Ltd. reagent) was added and reacted in an aqueous system. Next, the mixture was aged by stirring at room temperature for 3 hours. After completion of the stirring, the product was extracted with dichloromethane, purified by passing through a silica gel column, and then the solvent was distilled off, followed by vacuum drying at 100 ° C. and a vacuum degree of 0.5 kPa for 5 hours. The product thus obtained was confirmed by ¹ H-NMR, ¹³ C-NMR, and ³¹ P-NMR. The yield of the product (reddish brown liquid) was 38 g (yield 45%), and it was confirmed by ³¹ P-NMR that the purity was 99% or more.
(2) Measurement of physical properties The viscosity of the product at 25 ° C. was measured in the same manner as in Synthesis Example 1. The results are shown in Table 1.

[Synthesis Example 7 (Comparative)]
Synthesis of 1-ethyl-3-methylimidazolium tricyanomethide and measurement of physical properties (1) Synthesis 25 g (0.13 mol) of 1-ethyl-3-methylimidazolium bromide (Tokyo Chemical Industry Co., Ltd. reagent) in 12 g of water After dissolution, 15 g (0.13 mol) of sodium tricyanomethide (a reagent manufactured by Lonza Ltd.) was added and reacted in an aqueous system. Next, the mixture was aged by stirring at room temperature for 3 hours. After completion of the stirring, the product was extracted with dichloromethane, purified by passing through a silica gel column, and then the solvent was distilled off, followed by vacuum drying at 100 ° C. and a vacuum degree of 0.5 kPa for 5 hours. The product thus obtained was confirmed by ¹ H-NMR, ¹³ C-NMR, and ¹⁵ N-NMR. The yield of the product (reddish brown transparent liquid) was 9.2 g (35% yield), and it was confirmed by ¹ H-NMR that the purity was 99% or more.
(2) Measurement of physical properties The viscosity of the product at 25 ° C. was measured in the same manner as in Synthesis Example 1. The results are shown in Table 1.

[Examples 1 to 3 and Comparative Examples 1 to 5]
As shown in Table 2, the iodine redox couple shown in the same table was added and dissolved in the ionic liquid composed of the compound obtained in each synthesis example so as to have the molar concentration shown in the same table. Furthermore, the additives shown in the same table were added and dissolved so as to have a molar concentration shown in the same table. In this way, an electrolyte composition was prepared.

  Using the obtained electrolyte composition, a dye-sensitized solar cell was prepared by the following procedure, and the evaluation was performed by the following method. The results are shown in Table 3.

As a photoanode, a fluorine-added tin oxide transparent electrode (FTO; manufactured by Asahi Glass Co., Ltd., 10.8 Ωcm ⁻² ) in which titanium oxide nanoparticles (Solaronix D) were applied by a doctor blade so as to have a film thickness of 8 mil was obtained at 450 ° C. And baked for 30 minutes. This photoanode was immersed in a 0.3 mM N719 dye ethanol solution at 40 ° C. for 5 hours to carry the dye. Cells were assembled with a photoanode carrying a dye and a platinum-carrying counter electrode (interval: 100 μm), and the electrolyte compositions obtained in Examples and Comparative Examples were filled between the cells. The active area of the photoanode was 0.25 cm ² and the other surface was masked. Otherwise, a dye-sensitized solar cell was produced according to a conventional method. For the dye-sensitized solar cell thus obtained, the photocurrent-electromotive voltage characteristics were measured using an AM1.5 solar simulator (Peccel PEC-L10N) equipped with a Keithley 2400 type high-voltage power supply and a 500 W xenon lamp. did. The light intensity was adjusted using an ND filter (100 mWcm ⁻² ). All measurements were performed under normal temperature and normal pressure conditions.

  As is clear from the results shown in Table 3, the solar cell using the electrolyte composition of each example has a higher short-circuit photocurrent density and higher photoelectric conversion than the solar cell using the electrolyte composition of each comparative example. It turns out that it shows efficiency. In particular, when the ionic liquid: triethyl (methoxymethyl) phosphonium tricyanomethide and the redox pair: tri-n-butylmethylphosphonium iodide are used in combination, the photoelectric conversion efficiency is very high.

DESCRIPTION OF SYMBOLS 10 Photoelectric conversion element 11 Semiconductor layer 12 Dye layer 13 Transparent electrode layer 14 Counter electrode 15 Electrolyte layer

Claims (4)

Containing a quaternary phosphonium salt ionic liquid represented by the following general formula (1), and a redox pair consisting of an iodide ion and a polyiodide ion;
An electrolyte composition for a photoelectric conversion element, wherein a quaternary phosphonium iodide represented by the following general formula (2) is used as a supply source of the iodide ion.

The electrolyte composition for photoelectric conversion elements according to claim 1, wherein in general formula (1), R ₁ is an ethyl group or an n-butyl group, and n is 1.   The electrolyte composition for photoelectric conversion elements according to claim 1 or 2, comprising 0.05 to 4.0 M of the redox pair.   The photoelectric conversion element using the electrolyte composition for photoelectric conversion elements as described in any one of Claims 1 thru | or 3.

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