JP2012252872A - Dye-sensitized solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dye-sensitized solar cell offering high power generation efficiency and capable of storing electricity.SOLUTION: The dye-sensitized solar cell comprises a semiconductor layer containing a tungsten oxide powder having an average particle size of 548 nm or smaller and a dye, and the potential difference (B-A) is equal to or greater than 0.3 (eV), where A (eV) represents the energy potential of the conduction band of the tungsten oxide powder, and B (eV) represents the energy potential of the LUMO of the dye. The tungsten oxide powder is preferably composed of WO.

Description

  The present invention relates to a dye-sensitized solar cell.

Solar cells using sunlight are attracting attention as clean electric energy. As solar cells, those using a single crystal silicon substrate or a polycrystalline silicon substrate are mainly used because of their excellent power generation efficiency. In addition, the use of thin-film amorphous silicon obtained by reducing the thickness of a silicon substrate has been studied for cost reduction. In addition, compound semiconductor solar cells using gallium, arsenic, phosphorus, germanium, indium or the like are also known as non-silicon solar cells.
Conventional solar cells have a problem that the cost is high due to the increase in the size of the silicon substrate that receives sunlight and the complexity of the compound synthesis process, and the diffusion of the solar cell does not proceed as expected. On the other hand, for the purpose of reducing production costs, development of organic solar cells using an organic compound in a light absorption layer (photoelectric conversion layer) has been promoted. Among organic solar cells, development of dye-sensitized solar cells using dyes is expected. In Japanese Patent Laid-Open No. 2001-76772 (Patent Document 1), an oxide semiconductor layer such as TiO ₂ , SnO ₂ , ZnO, ZrO ₂ , Nb ₂ O ₅ , CeO ₂ , WO ₃ , SiO ₂ , and Al ₂ O ₃ is used. It has been proposed.

JP 2001-76772 A

In Patent Document 1, various oxide semiconductors are listed, but in the examples, only the case of using TiO ₂ powder has been studied. It has been shown that a dye-sensitized solar cell using only TiO ₂ powder is excellent in power generation efficiency (photoelectric conversion efficiency).
By the way, conventional solar cells include silicon-based solar cells, compound-based solar cells, and dye-sensitized solar cells using only TiO 2 semiconductors as described above. These solar cells literally use sunlight as an energy source. Therefore, when there is no sunlight (the amount of light decreases), the generated power becomes zero. In particular, since the amount of power generation decreases vertically, if it is used as the starting power for power load devices such as electronic devices that run on solar cells, the standby power supply can be turned on and off depending on the weather (light quantity of sunlight). It must be made. On the other hand, even with current weather forecasts that can be forecast in units of several hours, the actual amount of light on the spot cannot be predicted. Therefore, if the power load device is activated with the power of only the solar cell, the power generation amount of the solar cell will be zero vertically when the amount of light is reduced, so there is a state where electricity cannot be passed to the power load device. It will occur. For this reason, it has been necessary to always turn on a standby power source such as a commercial power source supplied from an electric power company.
The present invention is for solving such problems, and provides a dye-sensitized solar cell having a power storage function capable of maintaining power generation capacity for a while even when the amount of sunlight falls below a certain level. Is for.

The dye-sensitized solar cell of the present invention is a dye-sensitized solar cell having a tungsten oxide powder having an average particle diameter of 548 nm or less and a semiconductor layer having a dye, and the energy potential of the tungsten oxide powder conduction band is A (eV). When the LUMO energy potential of the dye is B (eV), the potential difference (B−A) is 0.3 (eV) or more.
The tungsten oxide powder is preferably within the range of WO _{2.0 to 3.3} . The tungsten oxide powder preferably has an average particle size of 1 to 100 nm. Moreover, it is preferable that the titanium oxide powder with an average particle diameter of 500 nm or less is contained in the semiconductor layer. Further, the semiconductor layer preferably has a two-layer structure with a titanium oxide powder layer having an average particle size of 500 nm or less.

  In the dye-sensitized solar cell of the present invention, the energy potential of the conduction band is A (eV) and the LUMO energy potential of the dye is B (eV) and the potential difference (BA) is 0.3 (eV) or more. Therefore, it has excellent power generation efficiency and has a power storage function. Therefore, even if the light quantity of sunlight falls, power generation capability can be functioned for a while.

It is a figure which shows an example of the dye-sensitized solar cell of this invention. It is a figure which shows an example of the electrical storage effect of the dye-sensitized solar cell concerning a present Example.

The dye-sensitized solar cell of the present invention is a dye-sensitized solar cell having a tungsten oxide powder having an average particle diameter of 548 nm or less and a semiconductor layer having a dye, and the energy potential of the tungsten oxide powder conduction band is A (eV). When the LUMO energy potential of the dye is B (eV), the potential difference (B−A) is 0.3 (eV) or more.
The tungsten oxide powder used for the semiconductor electrode layer has an average particle size of 548 nm or less. When the average particle size exceeds 548 nm, the power generation efficiency is lowered and the power storage effect is also lowered. A preferable average particle diameter is 1 to 200 nm, and further 1 to 50 nm. In a dye-sensitized solar cell, a dye is supported on an oxide semiconductor powder that forms a semiconductor electrode layer. In order to increase the power generation efficiency, it is necessary to increase the amount of dye supported. In order to increase the amount of the dye supported, it is effective to reduce the particle size of the oxide semiconductor powder and increase the specific surface area. On the other hand, if the average particle size is less than 1 nm, there is a concern about an increase in production cost. Therefore, the average particle diameter of the tungsten oxide powder is 548 nm or less, more preferably 1 to 200 nm, and more preferably 1 to 50 nm.
The tungsten oxide powder is preferably within the range of WO _{2.0 to 3.3} . Tungsten oxide forms oxides with various atomic ratios of tungsten and oxygen. In order to achieve both the power generation effect and the power storage effect, the atomic ratio of tungsten to oxygen is preferably within the range of WO _{2.0 to 3.3} . Outside this range, the energy excited by light may not be stable.
Further, the semiconductor layer may be provided with titanium oxide powder having an average particle size of 500 nm or less. The titanium oxide powder may be mixed with the tungsten oxide powder or may be formed into two layers with the tungsten oxide powder layer.
Moreover, it is preferable to comprise the 1st titanium oxide powder with an average particle diameter of 100 nm or less and the 2nd titanium oxide powder with an average particle diameter of 200-500 nm. Moreover, when the particle size distribution of the titanium oxide powder is measured, it is preferable that peaks of the particle size distribution exist in the ranges of 1 to 50 nm and 200 to 500 nm, respectively.
The dye-sensitized solar cell can improve power generation efficiency by supporting a dye on a semiconductor. For this reason, the titanium oxide powder should preferably have an average particle size as small as 500 nm or less, more preferably 50 nm or less. However, for example, in the case of only titanium oxide powder of 50 nm or less, although the amount of the dye supported increases, the light intake efficiency decreases, and as a result, the improvement of the photoelectric conversion efficiency is limited. By putting a relatively large titanium oxide powder having an average particle size of 200 to 500 nm, a light scattering effect in the semiconductor layer is obtained, and an improvement in photoelectric conversion efficiency is obtained. Moreover, when the average particle diameter of the titanium oxide powder is less than 1 nm, there is a risk that the burden of manufacturing cost increases.
The mixing ratio of the titanium oxide powder having an average particle diameter of 1 to 50 nm and the titanium oxide powder having an average particle diameter of 200 to 500 nm is a titanium oxide having an average particle diameter of 1 to 50 nm when the total value is 100 parts by mass. It is preferable that the content of the powder is within a range of 10 to 90 parts by mass and the balance is a titanium oxide powder having an average particle size of 200 to 500 nm. If the first titanium oxide powder or the second titanium oxide powder is less than 10 parts by mass, the effect of adding cannot be sufficiently obtained.
Further, when the total of the tungsten oxide powder and the titanium oxide powder is 100 parts by mass, the tungsten oxide powder is preferably 10 to 90 parts by mass and the remaining titanium oxide powder. Further, when either the tungsten oxide powder or the titanium oxide powder is less than 10 parts by mass, the effect of adding cannot be sufficiently obtained.
The titanium oxide powder is preferably anatase type. Titanium oxide has a crystal structure such as anatase type or rutile type, but anatase type improves power generation efficiency.
In the present invention, when the energy potential of the conduction band (CB) of the tungsten oxide powder is A (eV) and the energy potential of the LUMO (lowest orbit) of the dye is B (eV), the potential difference (BA) is 0.3 (eV) or more.
In a dye-sensitized solar cell, electrons are injected from a dye into a tungsten oxide conduction band by light irradiation. At this time, intercalation of Li ions contained in the electrolyte composition occurs, and a power storage reaction occurs. When light is blocked, Li and electrons that react with tungsten oxide are released, and a discharge reaction occurs. By setting the potential difference (B−A) to 0.3 eV or more, the discharge reaction can be performed smoothly. For this reason, even if the light irradiation state suddenly changes to the light blocking state, a certain amount of discharge can be maintained for a certain period of time. Even if the light irradiation state is changed to the light blocking state, the fact that the discharge can be maintained for a certain time is referred to as a storage effect.
Next, the structure of the dye-sensitized solar cell will be described.
The dye-sensitized solar cell of the present invention has, as a cell structure, a transparent electrode layer on a transparent substrate, a semiconductor layer in which tungsten oxide having an average particle size of 548 nm or less and a dye are supported, a transparent electrode layer on a transparent substrate, and a conductive layer. It is preferable that the counter electrode provided with a conductive catalyst layer and the electrolyte composition be provided between the semiconductor layer and the counter electrode.
FIG. 1 shows an example of the dye-sensitized solar cell of the present invention. In the figure, 1 is a dye-sensitized solar cell, 2 is a transparent substrate, 3 is a transparent electrode layer, 4 is a semiconductor layer, 5 is an electrolyte composition, 6 is a conductive catalyst layer, 7 is a transparent electrode layer (counter electrode), 8 is a transparent substrate, and 9 is a spacer.

Examples of the transparent substrate 2 and the transparent substrate 8 include substrates having optical transparency and insulating properties such as a glass substrate and a plastic substrate. Examples of the plastic substrate include plastic films such as polymethyl methacrylate, polycarbonate, polystyrene, polyethylene sulfide, polyethersulfone, polyolefin, polyethylene terephthalate, polyethylene naphthalate, and triacetyl cellulose. In view of direct contact with sunlight, a material having light resistance and heat resistance is preferable from the viewpoint of environment and life. In order to efficiently use incident light, an antireflection film or the like may be formed on the side where the semiconductor layer is not stacked. If a plastic substrate is used, the dye-sensitized solar cell can be made flexible. The surface on which the semiconductor electrode layer is formed (on the transparent substrate 2 side) is the sunlight receiving side.
A transparent electrode layer 3 is provided on the transparent substrate 2, and a transparent electrode layer 7 is provided on the transparent substrate 8. The transparent electrode layer is not particularly limited as long as it is a material having transparency and conductivity. However, ITO (complex oxide of indium and tin), tin oxide film doped with fluorine or indium, boron, etc. Alternatively, a zinc oxide film doped with potassium or the like is preferable. The transparent electrode layer 7 (the side opposite to the light receiving side) may be a metal film such as gold, silver, aluminum, titanium, or indium. The transparent electrode layer 7 provided on the transparent substrate 8 is called a counter electrode. Further, the transparent substrate 2 side on which the semiconductor layer is provided becomes a sunlight receiving surface.

  Moreover, the semiconductor layer is provided on the transparent electrode layer 3 by screen printing after slurrying by mixing a predetermined tungsten oxide powder (adding a titanium oxide powder as necessary) with a resin binder and, if necessary, a solvent, Then, it forms by heating at 350-500 degreeC and making it dry.

Moreover, it is preferable that the thickness of a semiconductor is 1-40 micrometers. If the thickness of the semiconductor layer is less than 1 μm, the effect of providing the semiconductor layer cannot be sufficiently obtained. If the thickness exceeds 40 μm, the effect is not obtained because it is too thick, and the light is not sufficiently transmitted. Efficiency may be reduced. Preferably it is 5-15 micrometers.
Moreover, since it is necessary to carry | support a pigment | dye to each oxide powder and to transmit light between each powder, the semiconductor layer has a porosity of 20 to 60 vol% as a semiconductor layer. These semiconductor layers function as n-type semiconductor layers.
In addition, a dye is supported on each semiconductor layer. As described above, a dye having a potential difference (B-A) of 0.3 eV or more is used. Examples of the dye having such characteristics include a ruthenium-tris transition metal complex, a ruthenium-bis transition metal complex, an osmium-tris transition metal complex, an osmium-bis transition metal complex, and a ruthenium- Examples thereof include cis-diaqua-bipyridyl complexes, phthalocyanines, and porphyrins.
Next, an electrolyte composition is provided. An electrolyte composition is sandwiched between a counter electrode having a semiconductor layer and a conductive catalyst layer. The injected electrolyte composition is held in the pores (pores) in the semiconductor layer and interposed between the semiconductor layer and the counter electrode. In a dye-sensitized solar cell, when light is incident from the transparent substrate side, the dye supported on the surface of the semiconductor layer is first excited by absorbing incident light, and the excited dye passes electrons to the semiconductor layer electrode. At the same time, photoelectric conversion is performed by passing holes to the electrolyte composition.

The electrolyte composition preferably includes a reversible redox couple. The reversible redox _couple can be supplied from, for example, a mixture of iodine (I ₂ ) and iodide, iodide, bromide, hydroquinone, TCNQ complex, and the like. In particular, a redox pair consisting of I- and I3- supplied from a mixture of iodine and iodide is preferable.
The redox couple as described above desirably exhibits a redox potential that is 0.1 to 0.6 V smaller than the oxidation potential of the dye described later. In the redox pair showing a redox potential 0.1 to 0.6 V lower than the oxidation potential of the dye, for example, a reducing species such as I ⁻ can receive holes from the oxidized dye. By including such a redox pair in the electrolyte, the speed of charge transport between the semiconductor layer and the conductive catalyst film can be increased, and the open-circuit voltage can be increased.
Moreover, in order to improve the electrical storage effect of tungsten oxide powder, an electrolyte composition containing Li is preferable.
The electrolyte composition further contains iodide. Examples of iodides include alkali metal iodides, iodides of organic compounds, and molten salts of iodides. Examples of the molten salt of iodide include iodides of heterocyclic nitrogen-containing compounds such as imidazolium salts, pyridinium salts, quaternary ammonium salts, pyrrolidinium salts, pyrazolidium salts, isothiazolidinium salts, and isoxazolidinium salts. Can be used.
Examples of the molten salt of iodide include 1,1-dimethylimidazolium iodide, 1-methyl-3-ethylimidazolium iodide, 1-methyl-3-pentylimidazolium iodide, and 1-methyl-3. -Isopentylimidazolium iodide, 1-methyl-3-hexylimidazolium iodide, 1-methyl-3-isohexyl (branched) imidazolium iodide, 1-methyl-3-ethylimidazolium iodide, 1, Examples include 2-dimethyl-3-propylimidazole iodide, 1-ethyl-3-isopropylimidazolium iodide, 1-propyl-3-propylimidazolium iodide, and pyrrolidinium iodide.

Such a molten salt of iodide can be used alone or in combination of two or more. Moreover, it is preferable that the content is about 0.005 mol / L or more and 7 mol / L or less in electrolyte solution. In the case of less than 0.005 mol / L, it is difficult to obtain sufficient effects. On the other hand, if it exceeds 7 mol / L, the viscosity is high and the ionic conductivity may be significantly reduced.
Furthermore, the electrolyte composition in the present invention preferably contains iodine in a range of 0.01 mol / L to 3 mol / L. Iodine is mixed with iodide in the electrolyte composition of the present invention and acts as a reversible redox pair. Therefore, when the content of iodine is less than 0.01 mol / L, the oxidized form of the redox couple is insufficient and it becomes difficult to sufficiently transport charges. On the other hand, if it exceeds 3 mol / L, the light absorption of the solution increases, and there is a possibility that light cannot be efficiently applied to the semiconductor layer. The iodine content is more preferably 0.03 mol / L or more and 1.0 mol / L or less.

The electrolyte composition in the present invention may be either liquid or gel and can contain an organic solvent. By containing the organic solvent, the viscosity of the electrolyte composition can be further reduced, so that it can easily penetrate into the semiconductor electrode. Examples of the organic solvent that can be used include cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC); chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate; γ-butyrolactone, acetonitrile, Examples include methyl propionate and ethyl propionate. Furthermore, cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; chain ethers such as dimethoxyethane and diethoxyethane; 弐 tolyl solvents such as acetonitrile, propionitrile, glutaronitrile, and methoxypropionitrile Can be mentioned.
These organic solvents can be used alone or as a mixture of two or more. The content of the organic solvent is not particularly limited, but is preferably 80% by weight or less, and more preferably 0% by weight or more and 30% by weight or less in the electrolyte composition.

The transparent substrate 9 is provided with a transparent electrode layer 7 and a conductive catalyst layer 6. The material of the transparent electrode layer 7 is as described above. Examples of the conductive catalyst layer 6 include platinum (Pt), gold (Au), silver (Ag), and carbon. In view of durability against the electrolyte composition, platinum is preferable.
Further, the dye-sensitized solar cell 1 can maintain a constant distance between the transparent substrate 2 and the transparent substrate 8 by the spacer 9 so that the electrolyte composition portion does not leak. Examples of the spacer include an insulating inorganic material such as resin and silica. The resin spacer is preferably a resin that is durable to the electrolyte composition, and is preferably an epoxy resin, an acrylic resin, or the like. Further, in order to prevent liquid leakage (electrolyte composition leakage), it is preferable to seal the resin separately from the spacer.

The dye-sensitized solar cell as described above is excellent in power generation efficiency and has a storage effect. The power storage effect indicates a function in which power generation is continued for a while even when the solar power is sensed and solar power is generated, and then the light is cut off and darkened. For example, the amount of power generated when irradiating a light amount of 12000 Lx (lux) is set to 100, and then the power amount of 30 to 70 can be maintained for several seconds to several minutes even if the light is cut off to make it dark. it can. Even if the sunlight is cut off, the power generation amount does not become zero vertically, and a constant amount of power generation is maintained for a certain period. In addition, the normal sunlight illuminance is 100,000 Lx. The fact that the effect can be obtained at 12000 Lx means that the light source is not limited to sunlight, and even a general light bulb functions sufficiently. Therefore, the dye-sensitized solar cell of the present invention can be used both outdoors and indoors. FIG. 2 shows an example of the storage effect. As can be seen from the figure, in the conventional dye-sensitized solar cell, when the light irradiation was stopped, the power generation amount was zero vertically as indicated by the dotted arrow. Since the dye-sensitized solar cell of the present invention has a power storage effect, even if light irradiation is stopped, the power generation capability is maintained for a while, and the power generation amount does not become zero immediately.
Further, by using the power storage function, power generation can be maintained even if the illuminance (light quantity) of sunlight is momentarily reduced. Therefore, even if the weather changes for a short time that cannot be predicted by the weather forecast, the power generation can be maintained. As a result, it is possible to stably supply power to the power load device. In addition, if the cloudy time is longer than the time during which the power storage effect is exhibited, it is possible to switch to a standby power source such as a commercial power source supplied in advance by the electric power company while the power storage effect is exhibited. According to such a system, the power generation of the dye-sensitized solar cell can be utilized as much as possible, and the use of a standby power source such as a commercial power source can be reduced as much as possible. Therefore, the energy saving effect is improved as compared with the conventional solar cell.
In addition, the standby power source includes an electric power system other than solar power generation such as a commercial power system of an electric power company, power from a storage battery, and a private power generation device (including cogeneration). Electricity load equipment uses electricity from home and office appliances such as personal computers, televisions, recorders, refrigerators and washing machines, various medical equipment such as CT, and various industrial equipment such as electric furnaces and elevators. It shows the equipment that operates in the same way.

(Example)
Example 1
A slurry in which tungsten oxide powder (WO ₃ , average particle size 30 nm) is dispersed in water at 10 wt% is prepared, and this slurry is coated on a glass substrate (thickness 1.1 mm, surface) with a transparent electrode layer (ITO film) on one side. A conductive film having a resistance of 5Ω / □ is applied by a spray method and fired at 450 ° C. for 30 minutes (heating rate: 10 ° C./min) to form a porous semiconductor layer (powder layer thickness 5 μm) made of tungsten oxide powder. Formed. This operation was repeated to produce a tungsten oxide semiconductor layer having a thickness of 15 μm (electrode area: 14 cm ² ).
The produced semiconductor layer (electrode) was immersed in a dye solution at room temperature for 24 hours to produce a photoelectrode. The dye solution used was N719 (Sigma Aldrich) 0.3 mM dissolved in a mixed solvent of acetonitrile and t-butyl alcohol (volume ratio 1: 1).
Here, the energy potential A of the conduction band of the tungsten oxide powder used in Example 1 is 4.5 (eV), the energy potential B of the LUMO of the dye is 3.8 (eV), and the potential difference (B−A ) Is 0.7 eV.
The counter electrode was a glass substrate with a transparent conductive film obtained by sputtering platinum with a thickness of 80 nm. A spacer resin having a film thickness of 60 μm was arranged so as to surround the periphery of the photoelectrode so that four electrolyte composition injection holes were opened, and a counter electrode heated to 110 ° C. was bonded thereto. The electrolyte solution was injected from the electrolyte composition injection port with a syringe, and the injection port was sealed with a UV curable resin. The electrolyte composition was prepared by dissolving 0.5M lithium iodide, 0.05M iodine, 0.58M t-butylpyridine, and 0.6M EtMeIm (CN) ₂ (1-ethyl-3-methylimidazolium amide) 0.6M in acetonitrile solvent. Used. Thereby, the dye-sensitized solar cell concerning Example 1 was produced.
The produced dye-sensitized solar cell was connected to a resistor of 750Ω, and the change in current during light irradiation and light interruption was measured. As a light source, an LED bulb equivalent to daylight white (manufactured by Toshiba Lighting & Technology, LEL-AW8N, 8.7 W) was used. After leaving still in the dark for 23 seconds, light irradiation (illuminance 12000 Lx) was performed for 27 seconds to obtain an output of 0.04 mA / cm ² . Next, the light was blocked to make a dark place. After light interruption, the current value gradually decreased, and the discharge capacity until the current value reached 0 mA / cm ² was determined to be 7379 μC / cm ² .
(Example 2)
The same thing as Example 1 was used except having made the average particle diameter of tungsten oxide powder into 10 nm.
(Comparative Example 1)
A dye-sensitized solar cell according to Comparative Example 1 was used as the semiconductor layer except that the semiconductor layer had a single-layer structure of 20 μm thick using titanium oxide powder having an average particle diameter of 50 nm.
Regarding the dye-sensitized solar cells according to Example 2 and Comparative Example 1, the power storage effect was examined by the same method as in Example 1. The results are shown in Table 1.

  As can be seen from the table, the dye-sensitized solar cell according to this example was found to have an excellent power storage effect.

DESCRIPTION OF SYMBOLS 1 ... Dye-sensitized solar cell 2 ... Transparent substrate 3 ... Transparent electrode layer 4 ... Semiconductor layer 5 ... Electrolyte composition 6 ... Conductive catalyst layer 7 ... Transparent electrode layer (counter electrode)
8 ... Transparent substrate 9 ... Spacer

Claims (5)

In a dye-sensitized solar cell having a semiconductor layer having a tungsten oxide powder having an average particle size of 548 nm or less and a dye,
When the energy potential of the conduction band of tungsten oxide powder is A (eV) and the energy potential of LUMO of the dye is B (eV), the potential difference (BA) is 0.3 (eV) or more. A dye-sensitized solar cell. 2. The dye-sensitized solar cell electrode according to claim 1, wherein the tungsten oxide powder is within a range of WO _{2.0 to 3.3} . 3. The dye-sensitized solar cell according to any one of claims 1 to 2, wherein the tungsten oxide powder has an average particle size of 1 to 100 nm. The dye-sensitized solar cell according to any one of claims 1 to 3, wherein the semiconductor layer contains titanium oxide powder having an average particle diameter of 500 nm or less. The dye-sensitized solar cell according to any one of claims 1 to 4, wherein the semiconductor layer has a two-layer structure including a titanium oxide powder layer having an average particle size of 500 nm or less.

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