JP2014175245A - Semiconductor electrode, and photoelectric conversion element and photochemical reaction element using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor electrode that allows preventing reverse electron reaction in an electrolytic solution.SOLUTION: A semiconductor electrode includes a metal oxide n-type semiconductor with porous properties. The semiconductor electrode (10) has a basal portion (101) including a first n-type semiconductor; and a surface portion (102) provided at least a part of a surface of the basal portion and including a second n-type semiconductor having oxygen deficiency.

Description

  Embodiments described herein relate generally to a semiconductor electrode, a photoelectric conversion element using the semiconductor electrode, and a photochemical reaction element.

A porous semiconductor electrode made of an aggregate of n-type semiconductor fine particles is used in a photoelectric conversion element such as a dye-sensitized solar cell or a photovoltaic battery. In recent years, such porous semiconductor electrodes are also used in photochemical energy conversion elements that generate hydrogen by sunlight or reduce CO ₂ . In an element using a porous semiconductor electrode, the porous semiconductor electrode is basically disposed in an electrolytic solution containing a redox pair (for example, I ⁻ and I ₃ ⁻ ). Charges such as holes are generated in the semiconductor electrode.

Of the charges generated in the n-type semiconductor electrode, it is desired that only the holes move into the electrolytic solution and the reaction proceeds, but the electrons that should move to the counter electrode also move into the electrolytic solution and become carriers. May cause reduction of (I ₃ ⁻ ). This reaction, called the reverse electron reaction, becomes an internal short circuit and causes a decrease in power generation efficiency. In order to increase the power generation efficiency while suppressing the reverse electron reaction, it is necessary to suppress the movement of electrons into the electrolyte.

  For example, it has been proposed to suppress a reverse electron reaction by providing a thin film of an insulator or a semiconductor on the surface of a porous semiconductor electrode. Since the surface of the electrode will be coated with a material with low electron conductivity, the coating is required to be sufficiently thin (<1 nm). However, it is difficult to uniformly form a film having a thickness of less than 1 nm on the surface of the porous semiconductor electrode. When it was formed, there was actually a portion where the electrode surface was exposed, and therefore the reverse electron reaction could not be sufficiently suppressed.

JP 2003-298032 A JP 2003-298082 A

Chem. Mater. 14 (2002) 2930-2935.

  The problem to be solved by the present invention is to provide a semiconductor electrode capable of suppressing a reverse electron reaction in an electrolytic solution.

  The semiconductor electrode of the embodiment includes a porous metal oxide n-type semiconductor. The semiconductor electrode includes a base including a first n-type semiconductor and a surface including a second n-type semiconductor having an oxygen vacancy provided on at least a part of the surface of the base. .

Schematic showing the structure of the semiconductor electrode concerning one Embodiment. The schematic diagram which shows the state of the electron in an n-type semiconductor. The schematic diagram explaining the barrier in the interface of a surface part and electrolyte solution. Schematic showing the structure of the photoelectric conversion element concerning one Embodiment. Schematic showing the structure of the dye-sensitized solar cell concerning one Embodiment. Schematic showing the manufacturing process of the dye-sensitized solar cell concerning one Embodiment. The schematic diagram explaining the movement of the surface of a semiconductor electrode and electron concerning one Embodiment. The figure explaining the electric power generation area | region and electrical storage area | region in the semiconductor electrode concerning one Embodiment. The schematic diagram explaining the surface of the conventional semiconductor electrode, and the movement of an electron. The figure explaining the electric power generation area | region and electrical storage area | region in the conventional semiconductor electrode. Schematic showing the structure of the photochemical reaction element concerning one Embodiment. The graph which shows the electric power generation characteristic of a dye-sensitized solar cell. The graph which shows the dark current characteristic of a dye-sensitized solar cell.

  The embodiment will be specifically described below.

  FIG. 1 is a schematic diagram illustrating a configuration of a semiconductor electrode according to an embodiment. The illustrated semiconductor electrode 10 is made of a porous metal oxide n-type semiconductor, and includes a base portion 101 and a surface portion 102 provided on at least a part of the surface of the base portion. Base 101 includes an aggregate of first n-type semiconductor particles, and pores 13 are formed between adjacent particles. If the pore 13 is maintained, the surface portion 102 may also exist on the inner surface of the pore 13. The surface portion 102 includes a second n-type semiconductor having oxygen vacancies, and has a higher carrier density than the base portion 101 made of the first n-type semiconductor. The semiconductor electrode 10 may be supported by the substrate 11 having the transparent conductive layer 12 to constitute the semiconductor electrode member 14. The semiconductor electrode of this embodiment can be referred to as an “optical semiconductor electrode” in that charges are generated by light irradiation.

  The semiconductor electrode according to one embodiment can be used as one electrode in an element such as a photoelectric conversion element such as a dye-sensitized solar cell or a photochemical reaction element. When used in such an element, the semiconductor electrode 10 is in contact with the electrolytic solution.

  The light applied to the semiconductor electrode 10 in contact with the electrolytic solution is absorbed by the semiconductor electrode, and electron-hole pairs are generated in the semiconductor electrode. The generated electrons move to the substrate 11 side, and one hole moves to the electrolytic solution. In addition, when the board | substrate 11 which supports the semiconductor electrode 10 is comprised from a translucent material, light can be irradiated from the board | substrate 11 side.

FIG. 2 schematically shows the state of electrons generated at the semiconductor electrode in the electrolytic solution in which carriers (I ⁻ and I ₃ ⁻ ) exist. In the illustrated example, the base 101 including the first n-type semiconductor is shown as a single particle. The surface of the base 101 is entirely covered with the surface 102 made of the second n-type semiconductor. Since the surface portion 102 includes a second n-type semiconductor having an oxygen vacancy, it can be referred to as an oxygen vacancy surface portion. The presence of such a surface portion 102 creates a high potential barrier for electrons at the semiconductor electrode / electrolyte interface. Electrons e ⁻ generated in the semiconductor electrode cannot move into the electrolytic solution, and only holes can move preferentially from the semiconductor electrode to the electrolytic solution. As a result, charge (electron, hole) separation was improved.

The potential barrier at the interface between the surface portion and the electrolytic solution is expressed as shown in FIG. Here, q is the charge of electrons, and V _s is expressed by the following mathematical formula (1).

In the above formula, N _D is the carrier density of the semiconductor surface portion, W is the thickness of the depletion layer, and ε is the dielectric constant. Incidentally, E _F in FIG. 3 represents the Fermi level of the semiconductor.

Since the surface portion 102 has a higher carrier density than the base portion 101, as shown in FIG. 3, the barrier at the interface between the surface portion and the electrolyte is increased. The electron e ⁻ cannot cross this barrier. The electron e ⁻ could not move into the electrolytic solution, and the reverse electron reaction was suppressed.

  As will be described later, since the second n-type semiconductor constituting the surface portion has high electrical conductivity, the function as an electrode is not impaired even if it is formed with a thickness of 1 nm or more. When the surface portion 102 is formed as a coating covering the entire surface of the base 101, the uniformity of the thickness of the coating is improved. As a result, it is possible to reduce the exposed portion of the base portion 101 including the first n-type semiconductor.

The first n-type semiconductor constituting the base 101 is not particularly limited as long as it is an n-type semiconductor capable of absorbing at least a part of irradiated light, and an arbitrary metal oxide n-type semiconductor is used. it can. Examples include transition metal oxides such as titanium, zinc, tungsten, and tantalum; perovskites such as SrTiO ₃ , CaTiO ₃ , BaTiO ₃ , MgTiO ₃ , and SrNb ₂ O ₆ ; and complex oxides or oxide mixtures thereof. It is done. In addition, sulfides such as CdS and ZnS, and nitrides such as GaN are preferably used because they have a narrower band gap and higher light absorption capability than oxides.

  The base 101 including the first n-type semiconductor can be composed of an aggregate of particles. The average particle size of the first n-type semiconductor particles is not particularly limited, but is preferably in the range of 5 to 1000 nm. When particles in such a range are used, the effect of light scattering becomes remarkable and there is no possibility that the energy conversion efficiency is lowered, and a desired effect can be obtained. The average particle diameter of the first n-type semiconductor particles can be obtained by a method of estimating by the BET method or by directly observing and measuring with an electron microscope.

  When semiconductor particles having an average particle diameter in the range of 5 to 100 nm are used, the energy conversion efficiency can be further improved.

  Further, n-type semiconductor particles having different average particle diameters may be mixed and used. In this case, the packing density increases and the effective surface area increases. In addition to this, light incident on the surface of a large particle within the n-type semiconductor electrode is irregularly reflected, and the amount of light absorption is improved. As a result, the efficiency can be further increased.

  The particles containing the first n-type semiconductor can be produced by, for example, a sputtering method, a spray pyrolysis method (SPD), a screen printing method, a doctor blade method, a dip coating method, or a spin coating method.

  The first n-type semiconductor particles can be used for forming the base 101 as a paste. The paste can be prepared by mixing the first n-type semiconductor particles with a binder by any method. As the binder, for example, polyethylene glycol, cellulose or the like can be used.

  An additive selected from acetylacetone, a surfactant, a chelating agent, and the like may be added to the paste in order to prevent the n-type semiconductor particles from which secondary aggregation has been eliminated from aggregating again. If necessary, the viscosity of the paste can be increased by adding a polymer such as polyethylene oxide or polyvinyl alcohol, or various thickeners such as a cellulose-based thickener.

  The prepared paste is coated on a support using an arbitrary method to form a coating film. Examples of the coating method include a dip method, a spray method, a wire bar method, a spin coating method, a roller coating method, a blade coating method, and a gravure coating method.

  Next, drying and baking are performed to obtain the base 101 including the first n-type semiconductor. Firing is performed at an appropriate temperature according to the type of the n-type semiconductor so that the first n-type semiconductor particles are sufficiently sintered. When the firing temperature is too low, the contact between the particles is insufficient and the electron transfer resistance is increased. On the other hand, when the firing temperature is too high, grain growth proceeds and the specific surface area decreases. As will be described later, when a semiconductor electrode is formed on a translucent substrate, the baking temperature of the coating film of the paste containing n-type semiconductor particles can be determined in consideration of the heat resistance of the translucent substrate. desired.

  The coating film of the paste containing n-type semiconductor particles is preferably baked at 100 to 800 ° C. The firing time varies depending on the firing temperature and is about 3 minutes to 12 hours. If the firing temperature is high, the firing is performed in a short time, and if the firing temperature is low, the firing is required for a long time. The firing temperature is more preferably 200 to 600 ° C, particularly preferably 350 to 550 ° C.

  By firing, a base 101 composed of an aggregate of first n-type semiconductor particles is formed on the support. The thickness of the base 101 is preferably 0.01 to 500 μm, and more preferably 0.1 to 100 μm.

  At least a part of the surface of the base 101 is provided with a surface portion 102 containing a second n-type semiconductor. The surface portion 102 does not necessarily completely cover the entire surface of the base portion 101, but when the entire surface is covered, the movement of electrons into the electrolytic solution can be almost certainly prevented.

  The second n-type semiconductor constituting the surface portion 102 is an n-type semiconductor having oxygen vacancies. Specifically, the second n-type semiconductor can be at least one selected from zinc oxide, indium oxide, and tin oxide. In these semiconductors, since oxygen vacancies form donor levels, a surface portion having a higher carrier density than a base including the first n-type semiconductor can be formed.

  Since the generated electrons are moved to the conductive film, the conduction band level of the second n-type semiconductor is preferably equal to or higher than the conduction band level of the first n-type semiconductor. The conduction band level can be determined by, for example, directly measuring with a forward / reverse photoelectron spectrometer (IPES / PES) or using a spectrophotometer and ultraviolet photoelectron spectroscopy (UPS) in combination.

  Further, when the isoelectric point of the second n-type semiconductor is 7 or more, the surface characteristics of the semiconductor electrode can be improved. For example, when applied to a dye-sensitized solar cell, the amount of dye adsorbed is increased and the number of electrons generated by light irradiation increases. A typical dye used in a dye solar cell has a negative carboxylic chain at the terminal. Therefore, by increasing the isoelectric point of the second n-type semiconductor layer to 7 or more, the dye adsorptivity is improved, and the characteristics of the dye-sensitized solar cell are further enhanced. The isoelectric point can be determined by, for example, zeta potential measurement.

The carrier density of the surface portion 102 is desirably 1 × 10 ¹⁷ or more and 1 × 10 ²² cm ⁻³ or less. By having the carrier density within such a range, the potential barrier formed at the electrolyte / surface interface can be increased as described above. As a result, the movement of electrons to the electrolytic solution is suppressed, and only holes can preferentially move to the electrolytic solution. If the carrier density is too small, the resistance loss in the second semiconductor layer may increase. On the other hand, if the carrier density is too high, absorption loss may occur in the visible light region due to free carrier absorption. It has been confirmed by the present inventors.

  When forming the surface portion, the carrier density of the surface portion can be increased by performing heat treatment at a predetermined temperature in a predetermined atmosphere. The atmosphere can be, for example, a low oxygen concentration (for example, 20% or less by volume) or a reducing atmosphere using hydrogen gas. If the concentration of hydrogen gas in the atmosphere is 1% by volume or more, a desired effect can be obtained. In such an atmosphere, heat treatment is preferably performed at a temperature of 300 ° C. or higher and 700 ° C. or lower, for example. By performing heat treatment under a low oxygen concentration or in a reducing atmosphere, oxygen vacancies can be maintained, and in some cases, the number of oxygen vacancies increases. As a result, it is possible to achieve a high carrier density.

  Although it is difficult to directly measure the carrier density of the surface portion provided on the surface of the base portion, the carrier density can be obtained using the second n-type semiconductor layer. Specifically, a layer containing only the second n-type semiconductor is formed on the substrate, and the second n-type semiconductor is measured by measuring Hall effect and capacitance-voltage (CV) characteristics of this layer. It is possible to determine the carrier density of the surface portion including

  As described above, since it has a high carrier density, the resistivity of the surface portion is low, and it can be formed with a thickness that was difficult with a conventional coating. If the surface portion including the second n-type semiconductor has a thickness of at least 1 nm or more, a desired effect can be obtained, and it is more preferably 5 nm or more. When the thickness of the surface portion increases, the pores of the porous semiconductor may be filled with the second semiconductor. In this case, the specific surface area decreases, leading to deterioration of device characteristics. For example, in the case of the immersion method, the thickness of the surface portion can be controlled by repeated treatment. In the case of chemical vapor deposition (CVD) or atomic layer deposition (ALD), the thickness of the surface portion can be controlled by the film formation time.

  In order to obtain the effect of the surface portion without deteriorating the element characteristics, there is an appropriate range for the thickness of the surface portion. Specifically, the upper limit of the thickness of the surface portion is preferably ½ or less of the average pore diameter in the base including the first n-type semiconductor. The average pore diameter can be measured by a mercury intrusion method (mercury intrusion porosimeter) or a gas adsorption method.

  The surface portion including the second n-type semiconductor can be formed on at least a part of the surface of the base portion by, for example, chemical vapor deposition (CVD) or atomic layer deposition (ALD). Under the present circumstances, the carrier density of the surface part obtained can be raised by performing heat processing by the above-mentioned predetermined atmosphere and predetermined temperature. Alternatively, a dipping method in which heat treatment is performed after the base portion containing the fine particles of the first n-type semiconductor is immersed in a solution containing metal ions contained in the second n-type semiconductor is also effective. Even in this case, the carrier density of the surface portion can be increased by performing heat treatment at a predetermined temperature in a predetermined atmosphere in the above-described atmosphere.

  The semiconductor electrode 10 composed of the base 101 including the first n-type semiconductor and the surface portion 102 including the second n-type semiconductor can be used in a state of a particle layer. Alternatively, as shown in FIG. 1, such a particle layer can be supported by a substrate and used as the semiconductor electrode member 14.

  The material of the board | substrate 11 can be selected from a metal, a semiconductor, glass, a plastics etc., for example. When light enters from the substrate side, a translucent material such as glass and plastic is used. In particular, when a transparent plastic plate is used, the weight of the element can be reduced. Examples of the transparent plastic include polycarbonate, polyethylene terephthalate, acrylic substrate, polyimide, and fluorinated polyimide. Polycarbonate is preferred because it is excellent in heat resistance, transparency and impact resistance and is relatively inexpensive.

As the transparent conductive layer 12 formed on a light-transmitting substrate such as glass or plastic, a material having little visible light absorption and having conductivity is preferable. The material of the transparent conductive layer 12 can be selected from, for example, ITO (indium tin oxide), SnO ₂ , fluorine-doped SnO ₂ , and zinc oxide doped with fluorine or indium. When a plastic plate is used as the light-transmitting substrate, it is desirable to use ITO capable of forming a film at a low temperature.

  From the viewpoint of improving conductivity and preventing an increase in resistance, it is desirable to wire a low-resistance metal or carbon matrix in combination with the transparent conductive layer. The low resistance metal can be selected from, for example, Ag, Cu, Ti, and W.

  The semiconductor electrode according to one embodiment can be applied to a photoelectric conversion element as shown in FIG.

  In the illustrated photoelectric conversion element 16, the semiconductor electrode 10 is supported on a substrate 11 provided with a transparent conductive layer 12. Opposite the semiconductor electrode 10, a counter electrode 23 including a counter substrate 21 and a catalyst layer 22 provided thereon is disposed via a spacer 24. An electrolyte layer 25 is interposed between the substrate 11 provided with the semiconductor electrode 10 via the transparent conductive film 12 and the counter substrate 21. In the case of the photoelectric conversion element 16 in which a translucent substrate is used as the substrate 11, light irradiation is performed from the substrate 11 side.

  As shown in FIG. 5, when the dye 15 is adsorbed on the surface of the semiconductor electrode 10 to form the dye-carrying semiconductor electrode 17, the dye-sensitized solar cell 20 can be configured. Except for this point, the illustrated dye-sensitized solar cell 20 has the same configuration as that of the photoelectric conversion element 16 described above. Similarly to the above, in the case of the dye-sensitized solar cell 20 in which a light-transmitting substrate is used as the substrate 11, light irradiation is performed from the substrate 11 side.

  Examples of the dye 15 supported on the surface of the semiconductor electrode 10 include a ruthenium-tris transition metal complex, a ruthenium-bis transition metal complex, an osmium-tris transition metal complex, and an osmium-bis transition metal. Examples include complexes, ruthenium-cis-diaqua-bipyridyl complexes, phthalocyanines, and porphyrins. Such a dye is desirably adsorbed on the surface of the semiconductor electrode as a single molecule.

  For example, a glass substrate can be used as the counter substrate 21.

  The catalyst layer 22 provided on the counter substrate 21 can be selected from, for example, a metal film such as platinum, gold, and silver, or a carbon film. If necessary, a conductive film such as a tin oxide film, a tin oxide film doped with fluorine, or a zinc oxide film may be used between the counter substrate and the catalyst layer. In view of durability against the electrolytic solution, platinum is particularly preferable. The catalyst layer 22 containing platinum can be formed with a thickness of about 3 to 100 nm by sputtering, for example.

The electrolytic solution in the electrolytic solution layer 25 can be composed of a charge transporter or a diluted solution thereof. The charge transporter may be either liquid or gel, but preferably includes a reversible redox pair. 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 I ₃ ⁻ 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 lower than the oxidation potential of the dye. In a 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 containing such a redox couple in the electrolytic solution, the open-circuit voltage can be increased.

  The charge transporter preferably contains iodide. Examples of iodides include alkali metal iodides and organic compound iodides. In addition to these, a molten salt of iodide can also be used.

  As the molten salt of iodide, a molten salt of an iodide of a heterocyclic nitrogen-containing compound can be used. Examples thereof 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.

  Examples of the molten salt of iodide include 1,3-dimethylimidazolium iodide, 1-ethyl-3-methylimidazolium iodide, 1-methyl-3-propylimidazolium iodide, and 1-methyl-3-pentylimidazole. Rium iodide, 1-methyl-3-isopentylimidazolium iodide, 1-methyl-3-hexylimidazolium iodide, 1-methyl-3-isohexyl (branched) imidazolium iodide, 1-methyl-3- Ethyl imidazolium iodide, 1,2-dimethyl-3-propylimidazole iodide, 1-ethyl-3-isopropyl imidazolium iodide, 1-propyl-3-propyl imidazolium iodide, pyrrolidinium iodide, etc. You can choose from That.

  The molten salt of iodide can be used alone or in combination of two or more. The content of the molten salt of iodide is preferably about 0.005 mol / L or more and 7 mol / L or less in the electrolytic solution. If iodide is contained in the electrolytic solution in such an amount, a desired effect can be obtained without reducing the ionic conductivity with an appropriate viscosity.

  Iodine is mixed with iodide in the electrolyte and acts as a reversible redox pair. Therefore, if the content of iodine is too small, the oxidized form of the redox couple may be insufficient and it may be difficult to transport charges. On the other hand, when there is too much content of iodine, there exists a possibility that the light absorption of a solution may increase and light may not be efficiently given to a semiconductor like a titania. The iodine content in the electrolytic solution is preferably in the range of 0.01 mol / L to 3 mol / L, and more preferably in the range of 0.03 mol / L to 1 mol / L.

  The electrolyte solution can be prepared by diluting the charge transporter with an organic solvent. By containing the organic solvent, the viscosity of the electrolytic solution can be further reduced, so that the organic solvent 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, propionic acid Examples include methyl and ethyl propionate.

  Furthermore, cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; chain ethers such as dimethoxyethane and diethoxyethane; nitrile solvents such as acetonitrile, propionitrile, glutaronitrile, and methoxypropionitrile It is done. 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 mass or less in the electrolytic solution. When the content of the organic solvent is 30% by mass or less in the electrolytic solution, it is possible to avoid the possibility that the organic solvent volatilizes and the performance deteriorates.

  The dye-sensitized solar cell according to this embodiment can be manufactured, for example, by the method described below.

First, the transparent conductive layer 12 and the semiconductor electrode 10 are sequentially formed on the surface of the substrate 11 to produce a laminated substrate (semiconductor electrode member) 14 as shown in FIG. As the substrate 11, for example, a translucent substrate made of a glass substrate can be used. By immersing the semiconductor electrode member 14 in a dye solution dissolved in a medium such as ethanol, the dye 15 is adsorbed on the surface of the semiconductor electrode 10 as shown in FIG. Is obtained. A structure in which the substrate 11 having the transparent conductive layer 12 and the dye-carrying semiconductor electrode 17 supported by the substrate 11 are combined can be referred to as a dye-carrying semiconductor electrode member 14 ′. As shown in FIG. 7, the dye 15 is adsorbed on the surface of the semiconductor electrode 10 by an ester bond (—OOC—). I ⁻ of the oxidation-reduction pair in the electrolytic solution is oxidized by giving an electron e ⁻ to the dye 15 to generate I ₃ ⁻ to generate a power generation reaction.

  A catalyst layer 22 is formed on the surface of the counter substrate 21, and the counter electrode 21 and the dye-carrying semiconductor electrode member 14 ′ are opposed to each other via a spacer 24 to assemble a battery unit. The spacer is not particularly limited, and for example, a thermoplastic ionomer resin or the like can be used. As the counter substrate 21, for example, a glass substrate can be used, and as the catalyst layer 22, for example, a platinum film formed by a sputtering method can be used.

  Next, an electrolytic solution 25 containing a charge transporter is injected into the gap between the dye-carrying semiconductor electrode member 14 ′ and the counter substrate 21, and the battery unit is sealed using a spacer 24. When the electrolytic solution is a gel electrolytic solution precursor composition, the dye-sensitized solar cell according to this embodiment is obtained by gelling the gel-like charge transporter precursor composition.

Since the movement of electrons e ⁻ from the base 101 to the electrolytic solution is hindered by the presence of the surface portion 102, the reverse electron reaction in the electrolytic solution is suppressed. When tungsten oxide is used as the first semiconductor layer, I ⁻ in the electrolytic solution is oxidized to I ₃ ⁻ to cause a power generation reaction, while positively charged lithium ions (Li ⁺ ) The second semiconductor layer is attracted to the exposed surface of the semiconductor electrode 10 to cause a storage reaction. Since it is suppressed that the carrier I ₃ ^{− in the} oxidized state receives the electron e ⁻ from the semiconductor electrode 10 and is reduced, a reduction in power generation efficiency can be avoided.

  Since electrons are exchanged as described above, on the surface of the semiconductor electrode 10, a region where the dye 15 is adsorbed becomes a power generation region as shown in FIG. 8. The region where the dye is not adsorbed on the surface of the semiconductor electrode 10 is a power storage region where a reduction reaction of lithium ions occurs. Since the surface part 102 containing the 2nd n-type semiconductor which has an oxygen deficiency exists in the surface of the semiconductor electrode 10, a reverse electron reaction is suppressed by this.

In the conventional semiconductor electrode, as shown in FIG. 9, an electron e ⁻ moves from the semiconductor electrode 18 into the electrolytic solution, and this electron causes a reverse electron reaction in which I ₃ ⁻ is reduced to I ^−. It was. As shown in FIG. 10, also on the surface of the conventional semiconductor electrode 18, the region where the dye 15 is adsorbed is a power generation region, and the region where the dye is not adsorbed is a power storage region. Since the electron e ⁻ can move from the semiconductor electrode 18 into the electrolytic solution, a reverse electron reaction has progressed, and this reverse electron reaction has caused a decrease in efficiency.

  In this embodiment, since the reverse electron reaction is suppressed by the presence of the surface portion including the second n-type semiconductor having an oxygen vacancy on the surface of the semiconductor electrode, it is possible to avoid a decrease in efficiency. It became.

  The semiconductor electrode according to one embodiment can also be applied to a photochemical reaction element as shown in FIG.

  The illustrated photochemical reaction element 30 includes an anode electrode 31 and a cathode electrode 33 arranged in a reaction tank 37 containing an electrolytic solution 38, and these electrodes are electrically connected by a wiring 36. The anode electrode 31 includes a substrate 11 provided with the transparent conductive layer 12 and a semiconductor electrode 10 disposed on the transparent conductive layer 12. A cocatalyst 32 can be supported on the semiconductor electrode 10 as necessary. As the promoter 32, for example, ruthenium oxide, iridium oxide, nickel oxide or the like can be used. The co-catalyst 32 can be supported on the surface of the semiconductor electrode 10 by a technique such as immersion or electrodeposition.

  The cathode electrode 33 includes a substrate 34 and a reduction catalyst layer 35 formed thereon. The reduction catalyst layer 35 can be formed by depositing a catalyst such as platinum, nickel, silver, gold, or an alloy containing them by sputtering. The cathode electrode is not limited to the form of the film formed on the substrate, but may be a wire, mesh, or plate as long as the reduction catalyst has conductivity.

  The co-catalyst 32 in the anode electrode 31 promotes the oxidation reaction, and the reduction catalyst layer 35 in the cathode electrode 35 promotes the reduction reaction. That is, the co-catalyst 32 and the reduction catalyst layer 35 have an action of reducing the activation energy of the oxidation-reduction reaction.

  In the illustrated photochemical reaction element 30, an electrolyte layer exists between the anode electrode having the semiconductor electrode 10 and the cathode electrode 33 as a counter electrode. In this photochemical reaction element 30, light is irradiated to the surface of the semiconductor electrode 10 provided on the anode electrode 31. The semiconductor electrode 10 absorbs light, and charges such as electrons and holes are generated. The generated holes move to the promoter 32 supported on the semiconductor electrode 10 to cause an oxidation reaction. On the other hand, the generated electrons move through the wiring 36 to the surface of the reduction catalyst layer 35 of the counter electrode 33, and a reduction reaction occurs.

  Usually, whether or not an oxidation reaction and a reduction reaction occur by light irradiation is determined according to the positional relationship between the valence band level and the conduction band level of the n-type semiconductor constituting the semiconductor electrode. For example, in order for the oxidation reaction to occur, the valence band level of the n-type semiconductor needs to be at a positive potential position relative to the standard redox potential of the redox pair in the electrolytic solution. On the other hand, in order for the reduction reaction to occur, the position of the conduction band level of the n-type semiconductor needs to be at a position that is more negative than the standard redox potential of the redox pair in the electrolytic solution.

  In general, an n-type semiconductor made of an oxide has a low conduction band level. For this reason, although an oxidation reaction occurs by irradiating light, it is difficult to cause a reduction reaction. In that case, as shown in FIG. 11, it is possible to cause a reduction reaction by performing voltage assist using an external power source 39.

  For example, when water is used as the electrolytic solution 38, oxygen generation occurs at the anode electrode 31, and hydrogen generation occurs at the cathode electrode 33, thereby causing water decomposition reaction. Oxygen generation at the anode electrode 31 and hydrogen generation at the cathode electrode 33 are expressed as follows, respectively.

Oxygen generation: 2H ₂ O → O ₂ + 4H ⁺ + 4e ⁻
Hydrogen production: 4H + 4e ⁻ → 2H ₂
For example, when a saturated CO ₂ aqueous solution is used as the electrolytic solution 38, oxygen generation (2H ₂ O → O ₂ + 4H ⁺ + 4e ⁻ ) occurs at the anode electrode 33, and CO ₂ reduction occurs at the cathode electrode. The reducing reaction of CO _2, for example, carbon monoxide from the CO _2, formic acid, methane, formaldehyde, ethylene, ethanol, and propanol or reduction of methanol, is.

  The photochemical reaction element according to one embodiment can efficiently perform the oxidation-reduction reaction of the electrolytic solution by light irradiation by including the semiconductor electrode according to the present embodiment in the anode electrode.

<Example I>
In Example I, a surface portion made of a second n-type semiconductor is formed on the surface of a base portion made of tungsten oxide (WO ₃ ) as a first n-type semiconductor to obtain a semiconductor electrode. The dye-sensitized solar cell shown in FIG. 5 was produced.

<Example I-1>
A glass substrate with an FTO (Fluorine doped Tin Oxide, 9Ω / □) film was prepared as the counter substrate 21, and a Pt film was formed to a thickness of 80 nm on the surface thereof by a sputtering method to form a catalyst layer 22. The glass substrate provided with the catalyst layer was cut into a size of 3 cm × 1.5 to 2 cm. Next, ultrasonic cleaning with a neutral detergent, rinsing with pure water, and acetone ultrasonic cleaning were performed for 10 minutes. Finally, the counter electrode 23 was obtained by drying with N ₂ blow.

WO ₃ nanoparticles (average primary particle size 10 to 20 nm, specific surface area 36 m ² / g) were prepared as the first n-type semiconductor particles, and EC vehicle 100FTP (manufactured by Nisshin Kasei) was prepared as a binder. The first n-type semiconductor particles and the binder agent were mixed at a mass ratio of 36:64 and stirred to obtain a WO ₃ paste. A glass substrate with an FTO film (3 cm × 1.5 to 2 cm) is used as the substrate 11 provided with the transparent conductive layer 12, and a squeegee method using a metal mask (printing area 5 × 20 mm ² ) thereon is used as a WO _Three pastes were applied.

After that, it was placed on a ceramic plate and fired in a nitrogen atmosphere at 450 ° C. (temperature increase rate: 10 ° C./min) for 30 minutes to form a WO ₃ layer serving as a base. The thickness of the obtained WO ₃ layer (base) was 10 μm.

A surface portion containing a second n-type semiconductor was formed on the surface of the WO ₃ electrode as the base by an immersion method using a metal nitrate aqueous solution. First, an aqueous solution of indium (III) nitrate trihydrate (4 mmol / L, 200 ml) was prepared and heated on a hot plate to 70 ° C. The aforementioned WO ₃ electrode was immersed in the heated solution and held for 1 hour. Thereafter, the WO ₃ electrode was pulled out of the solution, ethanol rinse, N ₂ blow dried, and dried on a hot plate at 120 ° C. for 10 minutes.

Finally, heat treatment was performed in a nitrogen atmosphere at 450 ° C. (temperature increase rate: 10 ° C./min) for 30 minutes. As a result, a surface portion including the _second n-type semiconductor (In ₂ O ₃ ) was formed, and the semiconductor electrode 10 was obtained. It was confirmed by a transmission electron microscope that the thickness of the surface portion was about 5 nm.

  N719 (Ruthenizer 535-bisTBA, Solaronics) was used as the dye. t-Butyl alcohol (t-BuOH) and acetonitrile (AN) were mixed at a volume ratio of 1: 1 to prepare a solvent. N719 was dissolved in a t-BuOH / AN solvent to prepare a solution having a concentration of 0.3 mM to obtain a dye solution.

  The electrolytic solution was prepared by dissolving predetermined components in acetonitrile. Specifically, in acetonitrile, lithium iodide (LiI) 0.5 mol / L, 1-ethyl-3-methylimidazolium dicyanamide 0.6 mol / L, t-butylpyridine 1.0 mol / L, and An electrolytic solution was obtained by dissolving 0.05 mol / L of iodine.

The semiconductor electrode immersed in the dye solution was taken out, rinsed with a t-BuOH / AN mixed solution for 1 minute in order to remove the associated dye, and dried with N ₂ blow. Thus, the dye-carrying semiconductor electrode 17 was formed on the substrate 11 having the catalyst layer 12. The resulting structure can be referred to as a dye-carrying semiconductor electrode member 14 ′.

  (High Milan 1702, 40 μm thickness, Mitsui Dupont Polychemical) as a spacer is disposed on both sides of the long side portion of the dye-carrying semiconductor electrode member 14 ′, and the counter electrode 23 is further disposed thereon, and the dye-carrying semiconductor electrode member A sandwich structure of / spacer / counter electrode was formed and pressed on a hot plate at 130 ° C.

  The electrolyte was injected from the short side using a pipette using a capillary phenomenon, and the short side was sealed using an epoxy resin. Finally, a conductive paste (Dotite D-500) was applied to the conductive surfaces of the dye-carrying semiconductor electrode member and the counter electrode to complete the dye-sensitized solar cell of this example.

<Example I-2>
A dye-supported semiconductor electrode member was formed by the same method as in Example I-1, except that the surface portion was formed by changing the metal nitrate aqueous solution used in the dipping method to a zinc nitrate hexahydrate aqueous solution (40 mmol / L). did. In the present embodiment, the second semiconductor contained in the surface portion is ZnO. A dye-sensitized solar cell of this example was produced in the same manner as in Example I-1, except that the obtained dye-carrying semiconductor electrode member was used.

<Example I-3>
A dye-carrying semiconductor electrode member was formed by the same method as in Example I-1, except that the surface portion was formed by changing the metal nitrate aqueous solution used in the dipping method to a stannic chloride aqueous solution (40 mmol / L). In this embodiment, the second semiconductor contained in the surface layer is SnO ₂ . A dye-sensitized solar cell of this example was produced in the same manner as in Example I-1, except that the obtained dye-carrying semiconductor electrode member was used.

<Comparative Example I-1>
A dye-carrying semiconductor electrode member was formed by the same method as in Example I-1 except that the surface portion was not formed on the surface of the base portion. A dye-sensitized solar cell of this comparative example was produced in the same manner as in Example I-1 except that the obtained dye-carrying semiconductor electrode member was used.

<Comparative Example I-2>
A dye-carrying semiconductor electrode member was formed by the same method as in Example 1-1 except that the surface portion was formed by changing the aqueous metal nitrate solution used in the dipping method to an aqueous yttrium nitrate n-hydrate solution (40 mmol / L, 200 ml). Formed. In this comparative example, although the surface portion is provided on the surface of the base portion, Y ₂ O ₃ contained in the surface portion is not an n-type semiconductor having oxygen vacancies. A dye-sensitized solar cell of this comparative example was produced by the same method as described above except that the obtained dye-carrying semiconductor electrode member was used.

About the dye-sensitized solar cell of an Example and a comparative example, the conversion efficiency was measured using the solar simulator with a xenon lamp, and a source meter. Specifically, the current density-voltage characteristics during light irradiation were measured, and the conversion efficiency was calculated based on the maximum output value obtained. The fill factor was determined by dividing the maximum power value by the short-circuit current density and the open circuit voltage. The obtained results are summarized in Table 1 below together with the material of the surface portion.

From the comparison between Comparative Example I-2 and Comparative Example I-1, it can be seen that the conversion efficiency is slightly improved by being present on the surface of the base portion even in the surface portion made of Y ₂ O ₃ . However, in Comparative Example I-2, the fill factor is lower than in Comparative Example I-1.

When a surface portion including an n-type semiconductor having an oxygen vacancy such as In ₂ O ₃ , ZnO, and SnO ₂ is provided (Examples I-1, I-2, I-3), the conversion efficiency is compared. This is even higher than in Example I-2. In these examples, the fill factor is also larger than that of Comparative Example I-2. The fill factor is a factor that depends on the internal resistance of the cell. By applying an n-type semiconductor having oxygen vacancies such as In ₂ O ₃ , ZnO, and SnO ₂ , the effect of suppressing the reverse electron reaction is further enhanced. The fill factor is increased by reducing the internal resistance.

  The graph of FIG. 12 shows the results of measuring current density-voltage characteristics during light irradiation for the dye-sensitized solar cell of Example I-1 and the dye-sensitized solar cell of Comparative Example I-1. Here, curve a is the result for Example I-1, and curve b is the result for Comparative Example I-1. As shown in the graph of FIG. 12, the dye-sensitized solar cell of Example I-1 is improved in dye adsorptivity and increased short-circuit current density by using a second semiconductor having a high isoelectric point. Yes. Moreover, since the reverse electron reaction is suppressed and the open circuit voltage is improved, it can be seen that the power generation characteristics are superior to those of Comparative Example I-1.

When a dye-sensitized solar cell using WO ₃ as an electrode is irradiated with light, not only power generation but also power storage occurs simultaneously. This is due to the intercalation of Li ions in the electrolyte into WO ₃ . The storage capacity of each dye-sensitized solar cell was evaluated by the following method. First, each dye-sensitized solar cell was irradiated with light for a certain time in an open state. Thereafter, a load resistance (510Ω) was connected, and the observed cell voltage (until the measurement range reached 0.01 V) was measured with a data logger.

As a light source, a solar simulator (AM 1.5, 100 mW / cm ² ) was used to irradiate the translucent substrate side of each dye-sensitized solar cell for 300 s. The obtained voltage value is converted into a current value, and this is integrated to calculate the charge (Q = I · t), thereby estimating the capacity per unit area (C / m ² ) of the dye-sensitized solar cell. It was. The results obtained are summarized in Table 2 below.

Table 2 shows that the same tendency as in the case of the power generation characteristic is observed in the case of the storage characteristic. That is, it can be seen from the comparison between Comparative Example I-2 and Comparative Example I-1 that even when the surface portion is made of Y ₂ O ₃ , the storage capacity is increased by being present on the surface of the base portion. Further, when a surface portion including an n-type semiconductor having an oxygen vacancy such as In ₂ O ₃ , ZnO, and SnO ₂ is provided (Examples I-1, I-2, I-3), it is even larger. The storage capacity is obtained.

  The presence of an n-type semiconductor having an oxygen vacancy suppresses the reverse electron reaction and can be used for an electricity storage reaction without losing electrons generated by light irradiation. Therefore, it exhibits high electricity storage characteristics. Was confirmed.

  Each dye-sensitized solar cell was charged by applying voltage (0.73 V) instead of light irradiation, and each storage capacity was evaluated in the same manner as described above. As a result, the same tendency as in the case of light irradiation was confirmed.

  The graph of FIG. 13 shows the results of measuring dark current for the dye-sensitized solar cells of Example I-1, Comparative Example I-1, and Comparative Example I-2. The dark current was determined by measuring the current-voltage characteristics without light irradiation. In the figure, curve c is the result for Example I-1, curve d is the result for Comparative Example I-1, and curve e is the result for Comparative Example I-2.

As shown by the curve c, the case of Example I-1 having the surface portion containing In ₂ O ₃ on the surface of the semiconductor electrode has the highest effect of reducing the leakage current, so that the reverse electron reaction is suppressed. It is confirmed that

<Example II>
In Example II, a dye-carrying semiconductor electrode member was produced by changing the thickness of the surface portion including the second n-type semiconductor, and the dye-sensitized solar cell shown in FIG. 5 was produced using this.

First, in the same manner as in Example I, a WO ₃ layer was formed on a glass substrate with an FTO film to obtain a base.

_{On the} base made of WO _{3, an} In ₂ O ₃ layer was formed by an ALD method to provide a surface portion. The thickness of the surface portion was changed by adjusting the number of cycles. Specifically, three types of surface portions having thicknesses of 0.5 nm, 1 nm, and 5 nm were formed to obtain three types of semiconductor electrode members.

The In ₂ O ₃ layer was formed on the Si substrate by the same method, and the thickness was measured by spectroscopic ellipsometry to obtain the thickness of the surface portion on the base surface.

  In the same manner as in Example I, a dye was supported on the surface of the semiconductor electrode to prepare a dye-supported semiconductor electrode member. Except for using the obtained dye-carrying semiconductor electrode member, three types of dye-sensitized solar cells were prepared in the same manner as in Example I, and were designated as Examples II-1, II-2, and II-3, respectively.

<Comparative Example II>
Three types of semiconductor electrodes having different surface thicknesses were formed by the same method as in Example II described above, except that insulating Al ₂ O ₃ was formed by ALD as the surface portion. Three types of dye-sensitized solar cells were prepared in the same manner as described above except that the obtained semiconductor electrode was used, and were designated as Comparative Examples II-1, II-2, and II-3, respectively.

About the dye-sensitized solar cell of an Example and a comparative example, the conversion efficiency was measured using the solar simulator with a xenon lamp. The obtained results are shown in Table 3 below.

  As shown in Table 3 above, when the thickness of the surface portion is 0.5 nm, there is no significant difference in the characteristics of Example II-1 and Comparative Example II-1, and the conversion efficiency is comparable. It is. In any of the examples and comparative examples, as the thickness of the surface portion increases, the open circuit voltage increases and the short circuit current density tends to decrease.

In Example II, an n-type semiconductor (In ₂ O ₃ ) having oxygen vacancies is used, so even if it is formed with a thickness of 1 nm or more, the characteristics are not greatly impaired. Even when the surface portion containing In ₂ O ₃ is formed with a thickness of 5 nm, the short-circuit current density is 4.8 mA / cm ² , and a conversion efficiency of 1.08% is obtained. This is shown in the result of 3. As described above, when the oxygen deficient surface portion is provided on the surface of the semiconductor electrode, the short-circuit current density gradually decreases, and as a result, high conversion efficiency is obtained.

On the other hand, in Comparative Example II, since insulating Al ₂ O ₃ is used for the surface portion, the resistance increases when the film thickness of the surface portion is 1 nm or more. As a result of Comparative Example II-2, even when the thickness of the surface portion containing Al ₂ O ₃ is 1 nm, the short-circuit current density decreases to 3.2 mA / cm ² and the conversion efficiency decreases to 0.70%. Is shown in

<Example III>
In Example III, a surface portion made of In ₂ O ₃ was formed on the surface of a base portion made of tungsten oxide (WO ₃ ) to obtain a semiconductor electrode, and the photochemical reaction element shown in FIG. 11 was produced using this semiconductor electrode. . Similar to Example I-1 and Example II described above, in this example, the first n-type semiconductor is WO ₃ and the second n-type semiconductor is In ₂ O ₃ .

First, by the same method as in Example I, a WO ₃ layer was formed on a glass substrate with an FTO film to obtain a base. At this time, an appropriate solvent was added to the paste to adjust the viscosity, thereby forming a WO ₃ layer having a thickness of 1 μm. On the WO ₃ layer serving as the base, an In ₂ O ₃ layer as a surface portion was formed in the same manner as in Example I, whereby an anode electrode 31 was obtained. The FTO film was electrically connected to the copper wire.

A platinum mesh was prepared as the cathode electrode 33. The platinum mesh electrically connected the copper wire.
Next, the anode electrode of the produced working electrode, the cathode electrode of the counter electrode, and the saturated Ag / AgCl electrode as a reference electrode are inserted into a sealed triode cell using 0.1 mol / L sulfuric acid aqueous solution as an electrolyte. Each was connected to a potentiostat to produce a photochemical reaction element. The characteristics of the obtained device were evaluated by water splitting experiments.

<Comparative Example III>
An anode electrode was prepared in the same manner as in Example III except that the surface portion containing In ₂ O ₃ was not formed, and connected to the same potentiostat as described above.

The evaluation method is as follows. First, a solar simulator (AM 1.5, 100 mW / cm ² ) was irradiated for 10 minutes while a 1.0 V voltage was applied to the potentiostat with respect to the reference electrode. Thereafter, the gas in the cell was collected, and the amount of oxygen produced was measured by gas chromatography.

  As a result, it was confirmed that the amount of oxygen produced in Example III was about 10% higher than that in Comparative Example III. The amount of oxygen produced depends on the amount of holes transferred from the anode electrode to the electrolyte. In Example III, since the semiconductor electrode having a surface portion having oxygen vacancies is included in the anode electrode, the reverse electron reaction in the electrolytic solution is suppressed. As a result, the transfer of electrons from the anode electrode to the electrolyte was suppressed, and holes could be transferred efficiently, thereby increasing the amount of oxygen produced.

  According to at least one embodiment described above, since the reverse electron reaction is suppressed by the presence of the oxygen deficient surface portion on the surface of the semiconductor electrode, it is possible to avoid a decrease in efficiency. .

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 10 ... Semiconductor electrode; 101 ... Base part; 102 ... Surface part; 11 ... Substrate 12 ... Transparent conductive layer; 13 ... Pore; 14 ... Semiconductor electrode member 14 '... Dye carrying | support semiconductor electrode member; Element 17 ... Dye-sensitized semiconductor electrode; 18 ... Semiconductor electrode; 20 ... Dye-sensitized solar cell 21 ... Counter substrate; 22 ... Catalyst layer; 23 ... Counter electrode: 24 ... Spacer 25 ... Electrolytic solution; 31 ... Anode electrode; 32 ... Cocatalyst 33 ... Cathode electrode; 34 ... Substrate; 35 ... Reduction catalyst layer; 36 ... Wiring 37 ... Reactor; 38 ... Electrolyte;

Claims (10)

A semiconductor electrode comprising a porous metal oxide n-type semiconductor,
A base including a first n-type semiconductor;
And a surface portion including a second n-type semiconductor having an oxygen vacancy provided on at least a part of the surface of the base portion.   2. The semiconductor electrode according to claim 1, wherein the second n-type semiconductor includes at least one selected from the group consisting of indium oxide, zinc oxide, and tin oxide.   The semiconductor electrode according to claim 2, wherein the first n-type semiconductor includes tungsten oxide. The semiconductor electrode according to claim 1, wherein the surface portion has a carrier density of 1 × 10 ¹⁷ or more and 1 × 10 ²² cm ⁻³ or less when converted to a bulk.   5. The semiconductor according to claim 1, wherein a conduction band level of the second n-type semiconductor is equal to or higher than a conduction band level of the first n-type semiconductor. electrode.   The semiconductor electrode according to claim 1, wherein an isoelectric point of the surface portion is 7 or more.   The semiconductor electrode according to any one of claims 1 to 6, wherein a heat treatment is performed in a reducing atmosphere. The semiconductor electrode according to any one of claims 1 to 6,
A photoelectric conversion element comprising: a counter electrode disposed to face the semiconductor electrode; and an electrolyte layer disposed between the semiconductor electrode and the counter electrode. The semiconductor electrode according to any one of claims 1 to 6,
A photochemical reaction device comprising: a counter electrode disposed to face the semiconductor electrode; and an electrolyte layer disposed between the semiconductor electrode and the counter electrode. A dye-carrying semiconductor electrode comprising the semiconductor electrode according to any one of claims 1 to 6 and a dye bonded to a surface of the semiconductor electrode,
A dye-sensitized solar cell comprising: a counter electrode disposed to face the dye-carrying semiconductor electrode; and an electrolyte layer disposed between the dye-carrying semiconductor electrode and the counter electrode.

Images