JP4537693B2 - Dye-sensitized solar cell - Google Patents

Description

  The present invention relates to a dye-sensitized solar cell. More specifically, the present invention relates to a dye-sensitized solar cell having a high conversion efficiency and having a carrier transport layer containing an electrolytic solution.

  Conventionally, a silicon crystal solar cell is well known as a device for directly converting light energy into electric energy, and has already been used as a field of weak power consumption, as an independent power source, and as a power source for space. A silicon crystal solar cell is mainly composed of a silicon single crystal or amorphous silicon. However, a large amount of energy is required to produce amorphous silicon as well as a silicon single crystal. Therefore, in order to recover the energy spent for making the battery, it is necessary to continue power generation for a long period of nearly ten years.

  Under such circumstances, a dye-sensitized solar cell using a sensitizing dye has attracted widespread attention. This dye-sensitized solar cell is, for example, a porous semiconductor electrode (hereinafter also referred to as a semiconductor electrode) carrying a sensitizing dye formed on a transparent conductive film on a transparent substrate, a counter electrode, and sandwiched between these electrodes And is expected as a next-generation solar cell because of its simplicity of manufacturing method and low material cost.

  J. et al. Am. Ceram. Soc. , 80 (12) 3157-3171 (1997) (Non-Patent Document 1), a method for producing a dye-sensitized solar cell in which a sensitizing dye such as a transition metal complex is adsorbed on the surface of a titanium oxide electrode which is a porous semiconductor Is described. In this method, a semiconductor electrode made of titanium oxide formed on a transparent conductive film on a transparent substrate is immersed in a solvent in which a sensitizing dye is dissolved, whereby the sensitizing dye is supported on the semiconductor electrode. Thereafter, an electrolytic solution containing a redox system is dropped, and a counter electrode is stacked on the semiconductor electrode to produce a solar cell.

  In the solar cell, when visible light is irradiated to a semiconductor electrode, the sensitizing dye on the surface absorbs light, thereby exciting electrons in the dye molecule and injecting excited electrons into the semiconductor electrode. The excited electrons move to the transparent electrode, and further, the electrons move through the electric circuit to the counter electrode. The electrons that have moved to the counter electrode are carried by holes or ions in the carrier transport layer and return to the semiconductor electrode. Such a process is repeated to extract electric energy.

  However, in order to put it into practical use as a solar cell, further improvement in conversion efficiency is desired. For this purpose, an increase in the open circuit voltage as well as an increase in the generated current (short circuit current) and further improvement in durability are desired. It is rare.

In order to increase the open circuit voltage, it is necessary to suppress the reverse current from the semiconductor electrode to the sensitizing dye and / or the carrier transport layer. In an equivalent circuit in a silicon solar cell, the relationship between the reverse current I ₀ and the open circuit voltage V _oc is expressed by the following formula (1).

(In formula (1), I _ph is a photocurrent, n is a diode factor, k is a Boltzmann constant, T is an absolute temperature, and q is the number of charges of carriers.)
In the dye-sensitized solar cell, although it cannot be said that the formula (1) is strictly established, it is considered that the V _oc is lowered due to the increase of the reverse current as in the case of the silicon solar cell. In order to suppress reverse current in a dye-sensitized solar cell, for example, J. Org. Am. Ceram. Soc. , 80 (12) 3157-3171 (1997) (Non-Patent Document 1), t-butylpyridine (TBP) is mixed with an electrolytic solution, or a dye adsorption electrode is formed with a solution containing TBP. It is known that processing is effective.

  However, when TBP is used, the short circuit current is remarkably reduced, and the actually obtained open circuit voltage is lower than the theoretically expected open circuit voltage.

  On the other hand, Japanese Patent Application Laid-Open No. 2003-168494 (Patent Document 1) describes a technique in which a metal complex dye is used as a sensitizing dye and a compound of the same type as the ligand of the metal complex dye is added to an electrolytic solution. ing. According to this technology, the durability of the metal complex dye is improved by preventing the ligand of the metal complex dye from eluting into the electrolyte solution or being exchanged with the electrolyte component in the electrolyte solution. .

  Specifically, ketones, nitriles, alcohols, pyridines, carboxylic acids, dimethylformamide, dimethyl sulfoxide, hexamethylphosphoamide, tetrahydrofuran, and water are mentioned as the above compounds. The concentration of the compound relative to the entire electrolyte is 50 to 99% by weight.

  However, in these weight concentration ranges, the solubility of the imidazole salt generally used as an additive to the electrolytic solution becomes poor. In addition, the short circuit current and the fill factor are significantly reduced due to the increase in the viscosity of the solvent in the electrolytic solution, and as a result, the conversion efficiency is greatly reduced. In particular, tetrahydrofuran has high volatility, and if the content ratio with respect to the electrolytic solution increases, the durability of the solar cell may be significantly reduced.

J. et al. Am. Ceram. Soc. , 80 (12) 3157-3171 (1997) Japanese Patent Laid-Open No. 2003-168494

  The inventors of the present invention have intensively studied in view of the above problems, and as a result, by containing a specific heterocyclic compound in the electrolytic solution, the reverse current is suppressed, the open circuit voltage is high, and the photoelectric conversion efficiency is high. The inventors have found that a dye-sensitized solar cell can be provided, and have reached the present invention.

Thus, according to the present invention, a porous semiconductor layer having a sensitizing dye adsorbed between a transparent electrode and a counter electrode, and a carrier transport layer containing an electrolytic solution, the electrolytic solution comprising furan, tetrahydrofuran , Pyran, tetrahydropyran, and a heterocyclic compound selected from the group consisting of those substituted with lower alkyl groups, and the concentration of the heterocyclic compound with respect to the electrolyte is 5 to 40% by volume. A featured dye-sensitized solar cell is provided.

  According to the present invention, a dye-sensitized solar cell having a high open-circuit voltage can be obtained by adding a heterocyclic compound having one oxygen atom in the ring structure to the electrolytic solution. The reason why the open-circuit voltage can be increased is that the oxygen atoms in the compound are adsorbed at the site where electrons leak from the semiconductor electrode and / or sensitizing dye, and the electrons are prevented from flowing out (reverse current is suppressed). it is conceivable that.

  The dye-sensitized solar cell of the present invention includes at least a porous semiconductor layer on which a sensitizing dye is adsorbed and a carrier transport layer containing an electrolytic solution between a transparent electrode and a counter electrode. Each of the transparent electrode and the counter electrode is usually formed on a support substrate. A transparent substrate is used as the support substrate on the transparent electrode side.

A schematic cross-sectional view of an example of the dye-sensitized solar cell of the present invention is shown in FIG.
Examples of the support substrates 1 and 8 include glass substrates and plastic substrates. The plate thickness is not particularly limited as long as it can give the solar cell appropriate strength. Further, at least one of the support substrates 1 and 8 is transparent.

Transparent conductive films (transparent electrode and counter electrode) 2 and 7 are formed on the support substrates 1 and 8. As the transparent conductive film, e.g., ITO, SnO _2, CuI, include film made of a transparent conductive material such as ZnO. The transparent conductive film is formed by a conventional method (for example, a sol-gel method, a sputtering method, etc.), and the film thickness is suitably about 0.1 to 5 μm. In FIG. 1, the transparent conductive film 2 is formed under the counter electrode 3, but the transparent conductive film 2 may not be formed.

The porous semiconductor layer 6 is formed on the transparent conductive film 7 and is composed of semiconductor fine particles. As the semiconductor fine particles, any material generally used for photoelectric conversion materials can be used. For example, titanium oxide, zinc oxide, tin oxide, niobium oxide, zirconium oxide, cerium oxide, tungsten oxide. , Silicon oxide, aluminum oxide, nickel oxide, barium titanate, strontium titanate, cadmium sulfide, CuAlO ₂ , SrCu ₂ O _{2 and the} like.

  Of the semiconductor fine particles, titanium oxide particles are preferable from the viewpoint of stability and safety. This titanium oxide includes various narrowly defined titanium oxides such as anatase type titanium oxide, rutile type titanium oxide, amorphous titanium oxide, metatitanic acid, orthotitanic acid, titanium hydroxide, hydrous titanium oxide and the like. The semiconductor layer may be in the form of particles or film, but is preferably in the form of a porous film.

The porous semiconductor layer can be formed on the transparent conductive film by various known methods. In particular,
(1) A method of forming a porous semiconductor layer by applying a suspension containing semiconductor particles on a transparent conductive film and drying and / or firing.
(2) A method of forming a porous semiconductor layer on a transparent conductive film by CVD or MOCVD using a necessary source gas,
(3) A single or combined method such as a method of forming a porous semiconductor layer by a PVD method using a solid raw material, a vapor deposition method, a sputtering method, a sol-gel method, or the like can be given.

  The semiconductor particles used to produce the porous semiconductor layer preferably have an average particle size in the range of 1 to 2000 nm, for example. Here, the average particle diameter was determined by analyzing dynamic scattering of laser light using a light scattering photometer (manufactured by Otsuka Electronics Co., Ltd.). In addition, what is marketed can be used for a semiconductor particle.

  For example, in the above method (1), first, the semiconductor particles are suspended in a suitable solvent. Examples of such a solvent include glyme solvents such as ethylene glycol monomethyl ether, alcohols such as isopropyl alcohol, alcohol mixed solvents such as isopropyl alcohol / toluene, water, and the like. Application of the suspension of the semiconductor particles to the substrate includes known methods such as a doctor blade method, a squeegee method, a spin coating method, and a screen printing method. Thereafter, the porous semiconductor layer can be formed by drying and baking the coating solution.

  In the above method, the temperature, time, atmosphere, and the like necessary for drying and firing can be appropriately adjusted according to the type of substrate and semiconductor particles used. For example, it 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. Drying and firing may be performed only once at a single temperature, or may be performed twice or more by changing the temperature. Moreover, application | coating, drying, and baking may be performed only once and may be performed twice or more.

  As the raw material gas used in the method (2) described above, a single gas containing two or more kinds of mixed gases containing the elements constituting the porous semiconductor layer can be used.

  In the method (3) described above, the solid material can be a single solid containing a semiconductor constituent element, a combination of a plurality of solids, or a solid compound.

The thickness of a porous semiconductor layer is not specifically limited, For example, about 0.1-100 micrometers is mentioned. From another point of view, a porous semiconductor layer having a large surface area is preferable, for example, about 10 to 200 m ² / g.

  The sensitizing dye 5 is adsorbed on the surface of the porous semiconductor layer 6. As the sensitizing dye, those having absorption in various visible light regions and / or infrared light regions can be used. Examples thereof include organic dyes and metal complex dyes.

  Examples of organic dyes include azo dyes, quinone dyes, quinone imine dyes, quinacridone dyes, squarylium dyes, cyanine dyes, merocyanine dyes, triphenylmethane dyes, xanthene dyes, porphyrin dyes, and perylenes. And dyes such as indigo dyes and naphthalocyanine dyes. As metal complex dyes, Cu, Ni, Fe, Co, V, Sn, Si, Ti, Ge, Cr, Zn, Ru, Mg, Al, Pb, Mn, In, Mo, Y, Zr, Nb, Sb, Metals such as La, W, Pt, Ta, Ir, Pd, Os, Ga, Tb, Eu, Rb, Bi, Se, As, Sc, Ag, Cd, Hf, Re, Au, Ac, Tc, Te, Rh Preferred are phthalocyanine dyes, ruthenium bipyridine dyes, etc., having a central metal.

  In the present invention, in order to strongly adsorb the sensitizing dye and the porous semiconductor layer, an interlock group such as a carboxyl group, an alkoxy group, a hydroxyl group, a sulfonic acid group, an ester group, a mercapto group, or a phosphonyl group is included in the molecule. It is preferable to use a sensitizing dye having

  Among the sensitizing dyes, ruthenium bipyridine dyes are preferable. Ruthenium 535 dye represented by the following general formula (1), Ruthenium 535-bisTBA dye represented by the following general formula (2), and the following general formula (3) The Ruthenium 620-1H3TBA dye represented is particularly preferred.

Before adsorbing the sensitizing dye 5 to the surface of the porous semiconductor layer 6, a treatment for activating the surface of the porous semiconductor layer may be performed as necessary. In the step of adsorbing the sensitizing dye to the porous semiconductor layer, the porous semiconductor layer is immersed in a liquid containing the sensitizing dye, and the sensitizing dye is adsorbed on the surface of the semiconductor layer. The liquid is not particularly limited as long as it dissolves the sensitizing dye to be used. Specifically, an organic solvent such as alcohol, toluene, acetonitrile, chloroform, dimethylformamide, or the like can be used. In general, it is preferable to use a purified solvent. The concentration of the sensitizing dye in the solvent can be adjusted according to the sensitizing dye to be used, the type of the solvent, the conditions for the sensitizing dye adsorption step, and the like. The concentration of the sensitizing dye is preferably 1 × 10 ⁻⁵ mol / liter or more.

  In the step of immersing the porous semiconductor layer in the liquid containing the sensitizing dye, the temperature, pressure, and immersion time can be changed as necessary. Immersion may be performed once or multiple times. Moreover, you may dry suitably after the process of immersion.

  The sensitizing dye adsorbed on the porous semiconductor layer by the above-described method functions as a photosensitizer that sends electrons to the porous semiconductor layer by light energy. Generally, when the sensitizing dye has an interlock group, it is fixed to the porous semiconductor layer through the group. The interlock group provides an electrical bond that facilitates the transfer of electrons between the excited state sensitizing dye and the conduction band of the porous semiconductor layer.

The counter electrode 3 can be formed directly on the support substrate or on the transparent conductive film 2. This conductive film may be transparent or opaque. For example, an N-type or P-type elemental semiconductor (eg, silicon, germanium, etc.) or a compound semiconductor (eg, GaAs, InP, ZnSe, CsS, etc.); a metal such as gold, platinum, silver, copper, aluminum, etc .; titanium, tantalum And high melting point metals such as tungsten, and electrodes made of transparent conductive materials such as ITO, SnO ₂ , CuI, and ZnO. The counter electrode is formed by an ordinary method, and the film thickness is suitably about 0.1 to 5 μm.

  In addition, it is preferable that the counter electrode is formed on the support substrate or protective layer which can support a solar cell. As the support substrate and the protective layer, a transparent or opaque substrate that is usually used as a substrate for solar cells can be used. Specifically, a platinum film as a counter electrode may be formed on a support substrate coated with a conductive film by a method such as sputtering, thermal decomposition of chloroplatinic acid, or electrodeposition. In this case, the thickness of the platinum film is preferably about 1 to 100 nm.

Any carrier transporting layer 4 can be used as long as it contains an electrolytic solution as long as it can transport any of electrons, holes, or ions. As the electrolytic solution in the electrolytic solution, an ionic conductor such as a liquid electrolyte or a polymer electrolyte can be used. The ionic conductor is preferably redox. The electrolyte is generally not particularly limited as long as it is an electrolyte that can be used in batteries or solar cells, specifically LiI, NaI, KI, metal iodide and iodine, such as CaI ₂ combination, LiBr, NaBr, Examples thereof include a combination of a metal bromide such as KBr or CaBr ₂ and bromine. Of these, a combination of LiI and iodine is preferable.

  The electrolyte concentration is suitably 0.01 to 1.5 mol / liter, preferably 0.1 to 0.7 mol / liter.

  Examples of the solvent for the electrolyte include carbonate compounds such as propylene carbonate, nitrile compounds such as acetonitrile, alcohols such as ethanol, water, aprotic polar substances, and the like. Compounds are preferred.

  In the present invention, a heterocyclic compound having one oxygen atom in the ring structure is added as an additive to the electrolytic solution. The heterocyclic compound preferably has a 5-membered or 6-membered ring structure.

  Further, the heterocyclic compound is preferably a compound selected from any of a 5-membered ring having one oxygen atom and a 6-membered ring having one oxygen atom in the ring structure.

  More specific heterocyclic compounds include compounds selected from the group consisting of furan, tetrahydrofuran, pyran, tetrahydropyran, and substituents thereof with lower alkyl groups. Examples of the lower alkyl group include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl and the like.

The above heterocyclic compounds may be used alone or in combination. For example,
A combination of furan with tetrahydrofuran, pyran, tetrahydropyran,
A combination of tetrahydrofuran with pyran, tetrahydropyran,
A combination of pyran and tetrahydropyran,
2 types of combinations are mentioned.

  In the present invention, the heterocyclic compound improves the generated current (short-circuit current) and the open-circuit voltage, which is considered to be due to the following reason. That is, in the heterocyclic compound having one oxygen atom in the ring structure, the oxygen atom in the molecule is adsorbed on the electron leakage site present in the porous semiconductor layer and / or the sensitizing dye, and the electron flows out. This is considered to be due to suppressing (reverse current suppression).

  An oxygen atom has a larger number of unshared electron pairs than a nitrogen atom. For this reason, the heterocyclic compound has a stronger adsorption power to the porous semiconductor layer and / or sensitizing dye than conventional nitrogen-containing compounds such as TBP, and the reverse current deterrence is improved. Conceivable.

  In particular, the ring structure is similar to nitrogen-containing heterocyclic compounds and imidazole salts, and (1) the molecular size is reduced compared to linear molecules having the same number of atoms, and the degree of freedom in the molecule. Therefore, it is considered that it has a role of making it easier to move in the porous semiconductor layer and (2) improving the stability.

  The concentration of the heterocyclic compound in the entire electrolyte solution takes into account the low boiling point of the compound, the low solubility of the imidazole salt in the compound, and the problems of short circuit current and fill factor reduction due to increased viscosity of the compound. Then, it is preferable that it is 5-40 volume%, and it is more preferable that it is 10-30 volume%.

  In JP-A-2003-168494, tetrahydrofuran is added to the electrolytic solution, and the concentration thereof is 50 to 99% by weight. In the range of these concentrations, in addition to the above-described problems of decreasing the solubility of the imidazole salt and increasing the viscosity, a decrease in durability due to containing a large amount of highly volatile tetrahydrofuran occurs. On the other hand, in the present invention, both the improvement of the open circuit voltage and the suppression of the decrease in the short circuit current and the improvement of the durability can be realized by setting the concentration of the heterocyclic compound with respect to the electrolyte to 5 to 40% by volume.

In addition, conventionally used nitrogen-containing heterocyclic compounds such as t-butylpyridine (TBP), dimethylpropylimidazole iodide (DMPII), methylpropylimidazole iodide (MPII), ethylmethyl Oxygen containing two or more oxygen atoms such as imidazole salts such as imidazole iodide (EMII), ethylimidazole iodide (EII), hexylmethylimidazole iodide (HMII), dioxolane, dioxane, dioxol, dioxene, dioxin, trioxane A sulfur-containing heterocyclic compound such as a heterocyclic compound, thiophene, and tetrahydrothiophene may be contained.
The electrolyte solution may contain the same type of compound as the ligand of the metal complex dye.

  The following examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof.

Examples 1-8 and Comparative Examples 1-7
The heterocyclic compound of Table 1 was used as an electrolytic solution at the concentration shown in Table 1, and a dye-sensitized solar cell was produced as follows.

-Production of porous semiconductor layer A transparent substrate on which a commercially available titanium oxide paste (manufactured by Solaronix, trade name: Ti-Nanoxide D, average particle size: 13 nm) is deposited by a doctor blade method on a SnO ₂ film, which is a transparent conductive film This was applied to a glass plate (manufactured by Nippon Sheet Glass Co., Ltd.). Thereafter, the coating film was preliminarily dried at 300 ° C. for 30 minutes and then baked at 500 ° C. for 40 minutes to obtain a titanium oxide film having a thickness of 20 μm as a porous semiconductor layer.

Adsorption of sensitizing dye A Ruthenium 535-bisTBA dye manufactured by Solaronix was dissolved in ethanol (manufactured by Aldrich Chemical Company) to a concentration of 4 × 10 ⁻⁴ mol / liter to prepare a solution. Next, the glass plate on which the titanium oxide film was formed was held in this solution for 30 minutes to adsorb the sensitizing dye to the titanium oxide film. The adsorbed dye concentration obtained was 7 × 10 ⁻⁸ mol / cm ² with respect to the titanium oxide film. Thereafter, the electrode was washed and dried with ethanol (manufactured by Aldrich Chemical Company).

-Preparation of electrolyte solution A redox electrolyte solution used as a carrier transport layer was prepared by mixing acetonitrile (manufactured by Aldrich Chemical Company) and a compound mixed with the compounds shown in Table 1 (all manufactured by Aldrich Chemical Company) at a concentration of 0.6. Mol / liter DMPII (manufactured by Shikoku Kasei), 0.1 mol / liter lithium iodide (manufactured by Aldrich Chemical Company), 0.05 mol / liter iodine (manufactured by Aldrich Chemical Company), concentration 0.5 mol / Liter of TBP (manufactured by Aldrich Chemical Company) was dissolved.

-Production of solar cell On the same transparent conductive glass plate as that on which the porous semiconductor layer described above was formed, a counter electrode was formed by depositing a platinum film by 1 µm. This counter electrode and the photoelectric conversion layer obtained above were overlapped with a spacer for preventing a short circuit. A redox electrolyte was injected from the gap, and the side surfaces were sealed with resin. A lead wire was attached to each electrode to obtain a solar cell.

The obtained solar cell was irradiated with light of 1 kW / m ² intensity (AM1.5 solar simulator), and the photoelectric conversion efficiency was measured. The results are shown in Table 2.

  As is apparent from Table 2, it is understood that the photoelectric conversion efficiency can be improved by adding a heterocyclic compound having one oxygen atom in the ring structure to the electrolytic solution.

It is a schematic diagram which shows the dye-sensitized solar cell of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 8 Support substrate 2, 7 Transparent conductive film 3 Counter electrode 4 Carrier transport layer 5 Sensitizing dye 6 Porous semiconductor layer

Claims (1)

A porous semiconductor layer having a sensitizing dye adsorbed between a transparent electrode and a counter electrode, and a carrier transport layer containing an electrolytic solution, wherein the electrolytic solution is furan, tetrahydrofuran, pyran, tetrahydropyran, and A dye-sensitized solar comprising a heterocyclic compound selected from the group consisting of those substituted with lower alkyl groups, wherein the concentration of the heterocyclic compound relative to the electrolyte is 5 to 40% by volume battery.

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