JP4097786B2 - Solar cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a solar cell comprising a metal oxide semiconductor electrode, a dye adsorbed on the surface of the electrode, an electrolyte having a redox couple, and a counter electrode, and particularly has high reliability without deterioration even when exposed to sunlight for a long time. It relates to solar cells.
[0002]
[Prior art]
There are several types of solar cells, but most of them are diode type using silicon semiconductor junctions. These solar cells are currently expensive to manufacture, which is a factor that hinders their spread. Dye-sensitized wet solar cells have been studied for a long time because of the possibility of cost reduction, but recently Graetzel et al. Announced that they have performance comparable to silicon solar cells (J. Am. Chem. Soc. 115 (1993) 6382), the expectation for practical use is increasing.
[0003]
The basic structure of a dye-sensitized wet solar cell includes a metal oxide semiconductor electrode, a dye adsorbed on the surface thereof, an electrolyte having a redox pair, and a counter electrode. Graetzel et al. Significantly improved the photoelectric conversion efficiency by making a metal oxide semiconductor electrode such as titanium oxide (TiO ₂ ) porous to increase the surface area and adsorbing a ruthenium complex as a dye on a single molecule basis. Since then, several proposals have been made to further improve the characteristics. For example, Japanese Patent Laid-Open No. 9-237641 discloses that the open circuit voltage is increased by using niobium oxide (Nb ₂ O ₅ ) as a metal oxide semiconductor. Japanese Patent Laid-Open No. 8-81222 discloses that lattice defects and impurities are removed by etching the surface of the TiO ₂ electrode film, thereby improving the conversion efficiency.
[0004]
[Problems to be solved by the invention]
However, a fundamental problem inherent in such a dye-sensitized wet solar cell is that the characteristics are greatly deteriorated by ultraviolet rays contained in sunlight or the like. The present invention solves such problems and provides a highly reliable solar cell with little characteristic deterioration. Further, the present invention provides a low-cost solar cell with little characteristic deterioration. An object of the present invention is to provide a solar cell having high initial photoelectric conversion efficiency and little characteristic deterioration.
[0005]
[Means for Solving the Problems]
According to the present invention, first, in a solar cell comprising a metal oxide semiconductor electrode, a dye adsorbed on the surface thereof, an electrolyte having a redox couple, and a counter electrode, a member that absorbs ultraviolet rays is provided at least on the light incident surface. A solar cell is provided.
[0006]
Secondly, in the solar cell described in the first aspect, the member that absorbs ultraviolet rays can absorb at least light having a wavelength corresponding to the band gap energy of the metal oxide semiconductor. A solar cell is provided.
[0007]
Third, in the solar cell described in the first or second aspect, the member that absorbs ultraviolet rays contains an inorganic or organic substance that absorbs ultraviolet rays in a transparent insulator. Is provided.
[0008]
Fourth, in the solar cell described in the third, the absorption edge energy of the transparent insulator containing an inorganic or organic substance that absorbs ultraviolet rays is larger than the energy difference between the excited state and the ground state of the dye. And the solar cell characterized by being smaller than the band gap energy of a metal oxide semiconductor is provided.
[0009]
Fifth, in the solar cell described in the first or second aspect, a solar cell is characterized in that the ultraviolet absorbing member has a semiconductor pn junction formed on a transparent insulating substrate. Provided.
[0010]
Sixth, in the solar cell described in the fifth aspect, the band gap energy of at least one of the p-type and n-type semiconductors forming the pn junction is greater than the energy difference between the excited state and the ground state of the dye, and A solar cell is provided that is smaller than the band gap energy of the metal oxide semiconductor.
[0011]
Seventhly, in any one of the first to sixth solar cells, the member that absorbs ultraviolet rays also serves as a base supporting a metal oxide semiconductor electrode or a counter electrode. Is provided.
[0012]
The present invention is described in detail below.
As described above, the present invention provides a solar cell comprising a metal oxide semiconductor electrode, a dye adsorbed on the surface thereof, an electrolyte having a redox couple, and a counter electrode, and a member that absorbs ultraviolet rays on at least a light incident surface of the solar cell. It is characterized by providing. That is, according to such a configuration, ultraviolet rays contained in sunlight or the like are absorbed by the ultraviolet ray absorbing member and do not reach the photoelectric conversion element (hereinafter referred to as a cell), and therefore, the factor causing the characteristic deterioration is removed. Is done. Further, since a sensitizing dye is selected which has little sensitivity to ultraviolet light, the original conversion efficiency is hardly lowered.
[0013]
In the solar cell, the present invention is characterized in that the member that absorbs ultraviolet rays absorbs light having a wavelength corresponding to at least the band gap energy of the metal oxide semiconductor. The mechanism of characteristic deterioration due to ultraviolet light is that a metal oxide semiconductor such as TiO ₂ is excited by absorbing ultraviolet light.
(1) It becomes a photocatalyst and decomposes the pigment.
(2) The injected electrons are recombined to generate holes.
And so on. Such an effect is caused only by light having an energy larger than the band gap energy of the metal oxide semiconductor (for example, 3.2 eV in the case of TiO ₂ ), in other words, by light having a short wavelength (for example, 380 nm or less in the case of TiO ₂ ). If light having such a wavelength is absorbed by a member that absorbs ultraviolet rays and does not reach the inside of the cell, deterioration of characteristics can be prevented.
[0014]
In the solar cell, the present invention is characterized in that the member that absorbs ultraviolet rays contains an inorganic or organic substance that absorbs ultraviolet rays in the transparent insulator. As the above, for example, lead glass containing CeO ₂ or the like, a resin containing an ultraviolet absorber such as hydroxybenzophenone, and the like can be used. Since these can be easily produced, the cost does not increase so much.
[0015]
Further, the present invention provides the above solar cell, wherein an absorption edge energy of an inorganic or organic substance that absorbs ultraviolet rays in the transparent insulator is larger than the energy difference between the excited state and the ground state of the dye, and the metal It is characterized by being smaller than the band gap energy of an oxide semiconductor. The operation of the dye-sensitized solar cell is
(1) Electrons in the dye excited by light absorption are injected into the conduction band of the metal oxide semiconductor, and the dye enters an oxidized state.
(2) The oxidized dye receives electrons from the electrolyte in the reduced state and returns to the ground state.
(3) The oxidized electrolyte receives electrons from the counter electrode and returns to the reduced electrolyte.
This is done in a loop. The light that can excite the dye needs to have an energy larger than the energy difference between the excited state and the ground state of the dye (usually about 1 to 2 eV). In order not to excite the metal oxide semiconductor, the energy must be lower than the band gap energy (usually 3 to 4 eV) of the metal oxide semiconductor. When the absorption edge energy of an inorganic or organic substance that absorbs ultraviolet rays in the transparent insulator is in the above range, such light can be selectively introduced into the cell, so that photoelectric conversion is effective. Done. A member that absorbs ultraviolet rays, such as lead glass containing CeO ₂ or the like or a resin containing an ultraviolet absorber such as hydroxybenzophenone, satisfies the above requirements.
[0016]
Further, the present invention is characterized in that in the above solar cell, the member that absorbs ultraviolet rays has a semiconductor pn junction formed on a transparent insulating substrate. When light having energy larger than the band gap energy is incident on a semiconductor having a pn junction, a photovoltaic force is generated. Therefore, if the member that absorbs ultraviolet rays is as described above, characteristic deterioration is prevented. In addition, the electromotive force can be increased.
[0017]
In the solar cell, at least one of the p-type and n-type semiconductors forming the pn junction has a band gap energy larger than the energy difference between the excited state and the ground state of the dye, and the metal oxidation. It is characterized by being smaller than the band gap energy of a physical semiconductor. In this way, at least one of the p-type and n-type semiconductors that form the pn junction absorbs light that excites the metal oxide semiconductor to generate an electromotive force, and the dye is excited in the cell. The original electromotive force is generated. That is, the electromotive force can be increased without impairing other characteristics. Examples of semiconductor materials that satisfy the above requirements include delafossite-based oxides (p-type) such as CdS, ZnO (n-type), CuAlO ₂ , and CuGaO ₂ .
[0018]
Further, the present invention is characterized in that, in the solar cell, the member that absorbs ultraviolet rays also serves as a substrate supporting the metal oxide semiconductor electrode or the counter electrode. With such a structure, the base material and the process can be reduced, so that the cost can be reduced. In addition, it does not impair the function of the solar cell of the present invention described above.
[0019]
Furthermore, the present invention is characterized in that, in the solar cell, the metal oxide semiconductor electrode is an aggregate of fine particles having a large number of voids, and the dye is adsorbed on the peripheral surface of the fine particles. By making the metal oxide semiconductor electrode into an aggregate of fine particles having a large number of voids, the surface area is increased, so that the adsorption site of the dye increases, thereby increasing the amount of electrons injected from the dye, and the photoelectric conversion efficiency. Is significantly improved.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
First, a first embodiment will be described with reference to FIG. In the figure, reference numeral 1 denotes a member that absorbs ultraviolet light, and contains a transparent insulator containing an inorganic or organic material that absorbs ultraviolet light. Specifically, lead glass containing CeO ₂ or the like, or ultraviolet absorption of hydroxybenzophenone or the like. Resin containing an agent. 2 is a substrate made of glass, plastic or the like, 3 is a transparent conductive film made of ITO, SnO ₂ : F, ZnO: Al, etc. 4 is made of TiO ₂ , SrTiO ₂ , SnO ₂ , ZnO, WO ₃ , Nb ₂ O _5, etc. 5 is a dye such as ruthenium bipyridyl, zinc porphyrin, copper phthalocyanine, chlorophyll, rose bengal, eosin, and 6 is a redox pair such as I ⁻ / I ₃ ⁻ or Br ⁻ / Br ₃ ^−. An electrolyte 7 is a counter electrode made of Pt or the like. This solar cell is formed by attaching the member 1 that absorbs ultraviolet rays to the cells composed of 2 to 7 described above. Light is incident from above.
[0021]
Next, an example of a method for manufacturing the solar cell will be described.
First, two sheets are prepared by forming, for example, a SnO ₂ : F film 3 on a glass substrate 2 by a sputtering method, a CVD method, a sol-gel method or the like. Since the SnO ₂ : F film functions as a current collector, the sheet resistance is 50 Ω / □ or less, preferably 10 Ω / □ or less. One of them forms, for example, a TiO ₂ electrode 4 on the SnO ₂ : F film, and then adsorbs a sensitizing dye such as ruthenium bipyridyl 5.
[0022]
The TiO ₂ electrode can be formed by (1) spraying or applying an organic titanium compound to the substrate and thermally decomposing it, and (2) evaporating the titanium halide or the organic titanium compound having a high vapor pressure, A method of depositing, (3) a method of gelling a titanium oxide sol on a substrate, and (4) a titanium oxide particle obtained by hydrolyzing a titanium compound is grown by hydrothermal treatment or the like, and then sprayed or applied to the substrate. There are methods for firing. Among these, the method (4) is desirable because the structure of the TiO ₂ electrode can be an aggregate of fine particles having a large number of voids and can be firmly attached to the substrate (SnO ₂ : F film). It is. The film thickness of the TiO ₂ electrode is preferably about 1 to 50 μm.
[0023]
The TiO ₂ electrodes to adsorb the dye to TiO ₂ electrode, water, alcohols, may be immersed in a solution of dye in a solvent such as toluene. It is preferable to have a functional group such as a carboxyl group, a hydroxyl group, or a sulfone group in the dye molecule because the dye is chemically fixed to the TiO ₂ surface. A typical example is a ruthenium complex represented by [ruthenium (4,4′-dicarboxy-2,2′-bipyridine) ₂ (isothiocyanato) ₂ ].
[0024]
On the other SnO ₂ : F film, for example, a Pt (fine particle) film 7 is formed by sputtering, vapor deposition, coating, or the like. The film thickness is preferably about 1 to 50 nm.
[0025]
After the pair of substrates formed as described above superposed through a spacer, for example, I ^- / I ₃ ^- electrolyte solution 6 having a redox pair is injected, sealed with sealant. As the electrolyte solution, a solution obtained by adding iodine and tetrapropylammonium iodide to a mixed solvent of ethylene carbonate and acetonitrile can be suitably used. As a member 1 that absorbs ultraviolet rays, for example, lead glass containing CeO ₂ or the like (a commercially available sharp cut filter such as L-40 or L-42 may be used) is bonded to the cell thus formed. Complete.
[0026]
Next, a second embodiment of the present invention will be described with reference to FIG. This is because the one substrate in the first embodiment also serves as a member that absorbs ultraviolet rays, and the substrate material and the bonding process can be reduced. The material and manufacturing method are the same as in the first embodiment.
[0027]
Next, a third embodiment of the present invention will be described with reference to FIG. Reference numeral 1 denotes a member that absorbs ultraviolet rays and has a semiconductor pn junction formed on a transparent insulating substrate. Specifically, it consists of a substrate 11 made of glass, plastic or the like, a lower electrode 12 made of a transparent conductive film such as ITO, SnO ₂ or ZnO: Al, a p-type semiconductor such as CuAlO ₂ or CuGaO _2, or an n-type semiconductor such as CdS. A light absorption layer 13, a bonding layer 14 made of an n-type semiconductor such as ZnO or CdS, or a p-type semiconductor such as CdS: Cu, and an upper electrode 15 made of a transparent conductive film such as ITO, SnO ₂ : F, ZnO: Al are sequentially formed. It is formed by depositing. Each of these layers can be produced using a thin film production method such as sputtering, vapor deposition, laser ablation, CVD, sol-gel, or solution growth. The film thickness is preferably about 0.1 to 2 μm for the lower electrode 12, 0.5 to 5 μm for the light absorption layer 13, 10 to 200 nm for the bonding layer 14, and about 0.1 to 2 μm for the upper electrode 15.
[0028]
The materials 2 to 7 constituting the cell and the production method are the same as in the first embodiment. A member 1 that absorbs ultraviolet rays having a semiconductor pn junction formed on the transparent insulating substrate is bonded to the cell and connected by a conductor 16 to complete. In this solar cell, the member 1 that absorbs ultraviolet rays absorbs ultraviolet rays and simultaneously generates an electromotive force, so that the photoelectric conversion efficiency is improved.
[0029]
Next, a fourth embodiment of the present invention will be described with reference to FIG. This is because the one substrate in the third embodiment also serves as a member that absorbs ultraviolet rays, and the substrate material and the steps of bonding and connecting can be reduced. The material and manufacturing method are the same as in the third embodiment.
[0030]
【Example】
Hereinafter, the embodiment will be described in more detail.
Example 1
The solar cell shown in FIG. 2 was produced as follows.
An L-42 sharp cut filter was used as the member (cum substrate) 1 that absorbs ultraviolet rays. On this, a SnO ₂ : F film 3 was formed so as to have a sheet resistance of 10Ω / □ by a sol-gel method. Next, hydrolyze the titanium sulfate aqueous solution, adjust the pH with nitric acid, and then add polyethylene glycol to the titanium oxide sol obtained by hydrothermal treatment at 180 ° C for 13 hours. After coating and drying, baked in air at 450 ° C for 30 minutes. Thus, a TiO ₂ electrode 4 was formed. The TiO ₂ electrode was found by SEM observation to form a porous film having a thickness of about 10 μm with primary particles (particle size: about 15 nm) bonded to each other without secondary aggregation and having a large number of voids. The L-42 absorbs light of 400 nm or less, that is, the band gap energy is 3.1 eV or more, and the absorption edge energy is 3.0 eV.
[0031]
Next, this was immersed in an ethanol solution of a ruthenium complex represented by [ruthenium (4,4′-dicarboxy-2,2′-bipyridine) ₂ (isothiocyanato) ₃ ], refluxed for 10 minutes, and TiO _2. Ruthenium complex 5 was adsorbed on the electrode surface. On the other hand, after the SnO ₂ : F film 3 was formed on the glass substrate 2 as described above, the Pt film 7 was deposited to a thickness of 20 nm by vacuum deposition.
[0032]
These two substrates are overlapped with a bead or rod-like insulating spacer while maintaining a gap of about 10 μm, and an oxidation-reduction electrolyte solution 6 in which iodine and tetrapropylammonium iodide are added to a mixed solvent of ethylene carbonate and acetonitrile 6 Was injected and then sealed with an epoxy adhesive.
[0033]
The photoelectric conversion efficiency of this solar cell is 7.6% (AM1.5, 100 mW / cm ² ), and the deterioration rate after 6000 hours of continuous irradiation with pseudo-sunlight (AM1.5, 100 mW / cm ² ) is 21%. Met.
[0034]
Example 2
The solar cell shown in FIG. 3 was produced as follows.
A ZnO: Al thin film having a thickness of 2 μm was formed on the glass substrate 11 by a sputtering method to form the lower electrode 12. Next, a p-type CuGaO ₂ thin film having a thickness of 1 μm was formed at a substrate temperature of 550 ° C. by a laser ablation method using a CuGaO ₂ sintered body as a target, and the light absorption layer 13 was obtained. Further, an n-type ZnO thin film having a thickness of 100 nm was formed thereon by a sputtering method to form a bonding layer 14. Finally, an ITO thin film having a film thickness of 500 nm was formed as the upper electrode 15 by sputtering to produce a member 1 that absorbs ultraviolet rays. The band gap of the p-type CuGaO ₂ thin film is 3.0 eV.
[0035]
On the other hand, a cell similar to that of Example 1 is manufactured using two glass substrates 2, and the above-described member 1 that absorbs ultraviolet rays is bonded thereto, and conduction is made with a conductor (copper wire) 16. To complete the solar cell.
[0036]
The photoelectric conversion efficiency of this solar cell is 10.1% (AM1.5, 100 mW / cm ² ), and the deterioration rate after 6000 hours of continuous irradiation of pseudo-sunlight (AM1.5, 100 mW / cm ² ) is 22%. Met.
[0037]
Example 3
The solar cell shown in FIG. 4 was produced as follows.
A SnO ₂ : F thin film having a thickness of 1 μm was formed on the glass substrate 11 by a sol-gel method to form the upper electrode 15. Next, a CdS: Cu thin film having a film thickness of 100 nm was formed by a binary vapor deposition method of Cu and CdS, and then heat treatment was performed at 350 ° C. for 15 minutes to form a p-type bonding layer 14. Further, an n-type CdS thin film having a thickness of 1 μm was formed as the light absorption layer 13 thereon by a vacuum vapor deposition method, thereby producing a member 1 that absorbs ultraviolet rays. The band gap of the n-type CdS thin film is 2.4 eV.
[0038]
A SnO ₂ : F film 3 (also serving as the lower electrode of the member that absorbs ultraviolet rays) was formed thereon by a sol-gel method so that the sheet resistance was 10 Ω / □. Subsequent cell fabrication was performed in the same manner as in Example 1.
[0039]
The photoelectric conversion efficiency of this solar cell is 9.0% (AM1.5, 100 mW / cm ² ), and the deterioration rate after 6000 hours of continuous irradiation with pseudo-sunlight (AM1.5, 100 mW / cm ² ) is 19%. Met.
[0040]
Comparative Example A solar cell was produced in the same manner as in Example 1 except that a glass substrate was used instead of the member that absorbs ultraviolet rays ( L-42 sharp cut filter).
The photoelectric conversion efficiency of this solar cell is 6.9% (AM1.5, 100 mW / cm ² ), and the deterioration rate after 6000 hours of continuous irradiation with pseudo-sunlight (AM1.5, 100 mW / cm ² ) is 45%. Met.
[0041]
【The invention's effect】
As described above, the solar cell according to claim 1 is a solar cell comprising a metal oxide semiconductor electrode, a dye adsorbed on the surface thereof, an electrolyte having a redox couple, and a counter electrode, and at least a member that absorbs ultraviolet rays on a light incident surface. According to this, ultraviolet rays contained in sunlight or the like are absorbed by the member and do not reach the inside of the photoelectric conversion element, so that characteristic deterioration can be reduced and reliability can be improved.
[0042]
Furthermore, in the solar cell according to claim 2, the member that absorbs ultraviolet rays absorbs light having a wavelength equal to or less than the wavelength corresponding to the band gap energy of the metal oxide semiconductor. According to this, light having energy larger than the band gap energy of the metal oxide semiconductor (for example, TiO ₂ is 3.2 eV), in other words, light having a short wavelength (380 nm or less in the case of TiO ₂ ) is absorbed by the member. Degradation such as dye decomposition due to excitation of the oxide semiconductor can be prevented.
[0043]
In the solar cell of claim 3, the member that absorbs ultraviolet rays contains a substance that absorbs ultraviolet rays in the transparent insulator, and according to this, the member can be produced by a simple method. Therefore, it is possible to reduce characteristic deterioration without increasing the cost.
[0044]
Furthermore, the solar cell according to claim 4 has an absorption edge energy larger than the energy difference between the excited state and the ground state of the dye, although the transparent insulator contains an inorganic or organic substance that absorbs ultraviolet rays. This is smaller than the band gap energy of a physical semiconductor. As described above, the light that can excite the dye needs to have an energy larger than the energy difference between the excited state and the ground state of the dye (usually about 1 to 2 eV). In order not to excite the metal oxide semiconductor, the energy must be lower than the band gap energy (usually 3 to 4 eV) of the metal oxide semiconductor. When the absorption edge energy of the inorganic or organic substance that absorbs the ultraviolet light is within the above range, such light can be selectively introduced into the cell, so that the photoelectric conversion effect can be effectively performed.
[0045]
According to a fifth aspect of the present invention, the member for absorbing ultraviolet rays has a semiconductor pn junction formed on a transparent insulating substrate. When light having energy larger than the band gap energy is incident on a semiconductor having a pn junction, an electromotive force is generated. Thus, by configuring the member that absorbs ultraviolet rays in this way, characteristic deterioration can be reduced and initial characteristics can be reduced. The photoelectric conversion efficiency of can be improved.
[0046]
Furthermore, in the solar cell of claim 6, at least one of the p-type and n-type semiconductors forming the pn junction has a band gap energy larger than the energy difference between the excited state and the ground state of the dye, and a metal oxide This is smaller than the band gap energy of the semiconductor. According to this, at least one of the p-type and n-type semiconductors forming the pn junction absorbs light that excites the metal oxide semiconductor to generate an electromotive force, and the dye is excited in the cell. An electromotive force is generated. That is, the electromotive force can be increased without impairing other characteristics.
[0047]
According to a seventh aspect of the present invention, the member that absorbs ultraviolet rays also serves as a base for supporting the metal oxide semiconductor electrode or the counter electrode. According to this, since the base material and the process can be reduced, the manufacturing cost can be reduced.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically showing an example of a solar cell according to the present invention.
FIG. 2 is a cross-sectional view schematically showing another example of a solar cell according to the present invention.
FIG. 3 is a cross-sectional view schematically showing another example of the solar cell according to the present invention.
FIG. 4 is a cross-sectional view schematically showing another example of the solar cell according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Member which absorbs ultraviolet-ray 2 Board | substrate 3 Transparent electrically conductive film 4 Metal oxide semiconductor electrode 5 Dye 6 Electrolyte 7 which has a redox pair Counter electrode
11 Board
12 Bottom electrode
13 Light absorption layer
14 Bonding layer
15 Upper electrode
16 Conductor

Claims (3)

A metal oxide semiconductor electrode, and a dye adsorbed on the surface thereof, a solar cell having a cell having an electrolyte and a counter electrode having a redox pair, provided with a member which absorbs ultraviolet light at least on the light incident surface, the ultraviolet A solar cell , wherein the absorbing member has a pn junction of a semiconductor formed on a transparent insulating substrate, and the semiconductor is electrically connected to the cell. 2. The solar cell according to claim 1, wherein the band gap energy of at least one of the p-type and n-type semiconductors forming the pn junction is larger than an energy difference between an excited state and a ground state of the dye, and the metal oxide semiconductor. A solar cell characterized by being smaller than the band gap energy. 3. The solar cell according to claim 1, wherein the member that absorbs ultraviolet rays also serves as a base for supporting a metal oxide semiconductor electrode or a counter electrode.

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