JP2012195501A - Thin film photoelectric conversion element and solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin film photoelectric conversion element including an oxide transparent electrode film exhibiting excellent transmissivity not only in the visible light region but also in the infrared region, having a low resistance and excellent chemical durability, and to provide a solar cell using the thin film photoelectric conversion element.SOLUTION: In the thin film photoelectric conversion element, a first electrode layer, a photoelectric conversion layer and a second electrode layer on the light-receiving side are arranged in this order. At least the second electrode layer is primarily composed of zinc oxide, and is deposited by sputtering, ion plating, pulse laser deposition (PLD method) or electron beam (EB) vapor deposition by using a target or a tablet doped with a low valence metal oxide. The low valence metal oxide is doped so that the atomic ratio of the low valence metal and zinc will be 0.02-0.1, and the specific resistance of an oxide transparent conductive film is 2.0×10Ω cm or lower.

Description

  The present invention relates to a thin film photoelectric conversion element having an oxide transparent conductive film having excellent permeability and chemical durability and having a low resistance value, and a solar cell using the same.

In recent years, in order to achieve both cost reduction and high efficiency of a photoelectric conversion element, a thin film photoelectric conversion element having almost no problem in terms of resources has been attracting attention and has been vigorously developed.
A thin-film silicon solar cell using one of the thin-film photoelectric conversion elements can be formed on a large-area glass substrate or stainless steel substrate at a low temperature, so that cost reduction can be expected.

In order to improve the conversion efficiency of this thin-film silicon solar cell, conventionally, as a method for increasing the amount of absorption of sunlight, contrivance has been made to increase the optical path length of light incident on the photoelectric conversion layer in the photoelectric conversion layer. It was. For example, in the case of a thin film silicon solar cell using a glass substrate, a tin oxide (SnO ₂ ) film, which is a transparent electrode, is formed by a thermal chemical vapor deposition (TCVD) method in various shapes on the surface of the transparent electrode. A method of forming a texture (microstructure), a method of forming a texture by etching the surface of the transparent electrode, a method of mechanically polishing the surface of the transparent electrode, or a layer having an uneven structure between the transparent electrode and the glass substrate Increasing the scattering of the incident light of the photoelectric conversion layer is performed by the method of forming.

In addition, from the viewpoint of achieving both low cost and high efficiency of the thin film photoelectric conversion element, in order to effectively use the main wavelength region (400-1500 nm) of sunlight, two or more different wavelength regions that can be absorbed are used. Multi-junction (tandem) photoelectric conversion elements in which photoelectric conversion units are stacked have been developed.
For example, an amorphous silicon photoelectric conversion unit that uses light having a wavelength of about 350 to 800 nm and a crystalline silicon photoelectric conversion unit that uses light having a longer wavelength than the amorphous silicon photoelectric conversion unit are stacked. Junction-type thin film silicon solar cells have been put into practical use.
Thin film crystalline silicon solar cells such as thin film polycrystalline silicon solar cells and thin film microcrystalline silicon solar cells that have been actively studied in recent years have higher cell sensitivity in the long wavelength region than amorphous silicon solar cells. Therefore, the oxide transparent conductive film in the thin-film crystalline silicon solar cell is required to have higher light transmittance and light scattering property in a longer wavelength region than the amorphous silicon solar cell.

  In order to enhance light scattering at a long wavelength, it is effective to make the surface uneven structure of the oxide transparent conductive film larger. For example, if the film thickness is increased, the crystal grain size can be increased and the surface unevenness can be increased. However, as such a substrate with an oxide transparent conductive film for a multi-junction photoelectric conversion element, the size of the unevenness By scattering the light on the long wavelength side with a texture with a relatively large texture, and scattering the light on the short wavelength side with a texture with a relatively small size of irregularities, the wavelength range of the light scattered is widened, It has been proposed to improve the conversion characteristics.

  As an oxide transparent conductive film in a photoelectric conversion element that meets such requirements, low resistance and excellent transparency in the main wavelength region (400 to 1200 nm) of sunlight in order to increase photoelectric conversion efficiency, and easily texture It can be formed and has high light scattering performance.

Oxide transparent conductive films typified by fluorine-doped SnO ₂ film (FTO film) and indium / tin oxide film (ITO film) are used for ordinary electron beam vapor deposition, vacuum deposition, sputtering, chemical vapor It is formed by a phase growth (CVD) method or a spray method.
However, since indium / tin oxide film uses indium, which is a rare metal, there are significant problems of resource and high cost.
Further, the SnO ₂ film doped with fluorine has a high specific resistance, and in order to make it 10Ω / □ or less which is a sheet resistance suitable as an oxide transparent conductive film in a photoelectric conversion element, it is necessary to make the film thickness 1 μm or more. As a result, the transmittance of the main wavelength region (400 to 1200 nm) of sunlight is greatly reduced.

In order to solve these problems, aluminum-doped zinc oxide (AZO) films and gallium-doped zinc oxide (GZO) films are replacing FTO films and ITO films (see Patent Document 1). ).
The AZO film and the GZO film have (a) no rare metal, so there are no resource problems and are inexpensive, and (b) low resistance, so there is no need to increase the film thickness, and the transmittance in the visible light region. And (c) has a feature that a texture can be easily formed.

International Publication No. 2010/050338

  However, the conventional zinc oxide (ZnO) transparent conductive film using aluminum or gallium as a dopant as described above (i) has low resistance and high transmittance in the visible light region, but transmittance in the near infrared region. However, (ii) it is inferior to chemical durability, and the thin film photoelectric conversion element using these has two big problems that durability is large inferior.

Due to the problem (i) above, solar cells using these films on the light incident side could not sufficiently utilize solar energy in the infrared region. This is because the reflection and absorption of infrared rays by the oxide transparent conductive film increases as the carrier electron concentration increases, and the transmittance in the infrared region of a zinc oxide (ZnO) film using aluminum or gallium as a dopant is low. This is probably because the carrier electron concentration is high as a flip of low resistance.
The problem (ii) is a problem related to the reliability of the thin film photoelectric conversion element, and directly affects the product life. Usually, long-term outdoor use tolerance of 20 years is required, but it is still unclear whether the current thin film photoelectric conversion element can be guaranteed for 20 years at a practical level.

  The present invention is a material proposed for the purpose of overcoming the above-mentioned problems, and has excellent transparency not only in the visible light region but also in the infrared region, and has a low resistance value and excellent chemical durability. It aims at providing the thin film photoelectric conversion element provided with the oxide transparent conductive film, and the solar cell using the same.

As a result of various studies to solve the above problems, the present inventor has made use of a target or tablet containing zinc oxide as a main component and containing a low-valent metal oxide, sputtering method, ion plating method, pulse laser deposition. An oxide transparent conductive film formed by a (PLD) method or an electron beam (EB) vapor deposition method, wherein a low-valent metal is added to zinc oxide in a low-valence metal / zinc atomic ratio of 0.02. The oxide transparent conductive film that is substituted at a ratio of ˜0.1 and the specific resistance of the oxide transparent conductive film is 2.0 × 10 ⁻³ Ω · cm or less has excellent transparency in the near infrared region, When the oxide transparent conductive film is applied to the oxide transparent conductive film in a thin film photoelectric conversion element, it has been found that conversion efficiency and durability can be improved and long-term reliability can be improved, and the present invention has been completed. .

That is, this invention consists of the following structures.
(1) In the thin film photoelectric conversion element in which the first electrode layer, the photoelectric conversion layer, and the second electrode layer are arranged in this order, and a photovoltaic power is generated by light incident from the second electrode layer side, The second electrode layer is an oxide transparent conductive film in which zinc oxide is doped with a low-valent metal oxide as a dopant, and the thin-film photoelectric conversion element.
(2) The photoelectric conversion layer is a photoelectric conversion layer used for an amorphous silicon (a-Si) solar cell, a polycrystalline silicon solar cell, or a microcrystalline silicon solar cell. Thin film photoelectric conversion element.
(3) The oxide transparent conductive film is a sputtering method, an ion plating method, a pulsed laser deposition (PLD) method, or a target or tablet containing zinc oxide as a main component and doped with a low-valent metal oxide. A transparent oxide conductive film formed by an electron beam (EB) vapor deposition method, wherein the low-valence metal oxide has a ratio of the atomic ratio of the low-valence metal to zinc of 0.02 to 0.1 The thin-film photoelectric conversion element according to (1) or (2) above, wherein the oxide transparent electrode layer is doped to have a specific resistance of 2.0 × 10 ⁻³ Ω · cm or less. .
(4) The above-mentioned (1) to (3), wherein the low-valent metal oxide is selected from low-valent titanium oxide, low-valent niobium oxide, low-valent tantalum oxide, and low-valent molybdenum oxide. ).
(5) The thin film photoelectric conversion element as described in (4) above, wherein the low-valent titanium oxide is TiO _{2 -X} (X = 0.1-1).
(6) The low-valent titanium oxide is selected from TiO (II), Ti ₂ O ₃ (III), Ti ₃ O ₅ , Ti ₄ O ₇ , Ti ₆ O ₁₁ , Ti ₅ O ₉ and Ti ₈ O _15. The thin-film photoelectric conversion element according to (4) or (5), wherein
(7) The thin film according to any one of (4) to (6), wherein the low-valence niobium oxide is selected from NbO (II), Nb ₂ O ₃ (III), and NbO ₂ (IV) Photoelectric conversion element.
(8) The thin film photoelectric conversion device according to any one of (4) to (7), wherein the low-valent tantalum oxide is selected from TaO ₂ (IV) and Ta ₂ O ₃ (III).
(9) The thin film photoelectric conversion element as described in any one of (4) to (8) above, wherein the low-valent molybdenum oxide is selected from MoO ₂ (IV) and Mo ₂ O ₃ (III).
(10) The thin film photoelectric conversion device according to any one of (1) to (9), wherein an average light transmittance at a wavelength of 780 to 1500 nm is 85% or more.
(11) The thin film photoelectric conversion according to any one of (1) to (9), wherein an average light transmittance at a wavelength of 780 to 1500 nm is 80% or more and a surface resistance is 20 Ω / □ or less. element.
(12) A solar cell having at least a substrate and the thin-film photoelectric conversion element according to any one of (1) to (11), wherein the first electrode layer is stacked on the substrate.
(13) A solar cell having at least a transparent substrate and the thin film photoelectric conversion element according to any one of (1) to (11), wherein the second electrode layer is laminated on the transparent substrate.

  According to the present invention, not only the visible light region but also the near infrared region has excellent transparency, low resistance, and light of a thin film photoelectric conversion element provided with an oxide transparent conductive film with good chemical durability. By using it as a transparent electrode on the incident side, solar energy in the infrared region, which was impossible in the past, can be used with high efficiency, the long-term reliability of the photoelectric conversion element can be improved, and this is extremely useful industrially. It can be said that it is an invention.

It is a graph which shows the measurement result of the transmittance | permeability of the oxide transparent conductive film in the wavelength range of 190 nm-2500 nm. It is a schematic explanatory drawing which shows an example of the layer structure of a silicon-type thin film solar cell.

  In the photoelectric conversion element of the present invention, the first electrode layer, the photoelectric conversion layer, and the second electrode layer are arranged in this order, and a photovoltaic force is generated by light incident from the second electrode layer side. It is a thin film photoelectric conversion element.

  Such a thin-film photoelectric conversion element is not particularly limited, except that at least the second electrode layer is an oxide transparent conductive film obtained by doping zinc oxide with a low-valent metal oxide as a dopant. By changing the conversion layer in various ways, for example, silicon-based thin film solar cells such as amorphous silicon (a-Si) solar cells, polycrystalline silicon solar cells, and microcrystalline silicon solar cells; Heterojunction solar cells; chalcopyrite thin film solar cells; solar cells using compound semiconductors such as GaAs and CdTe; organic solar cells using organic materials, among others, amorphous silicon (a-Si) It is preferably used for a solar cell, a polycrystalline silicon solar cell, or a microcrystalline silicon solar cell. The thickness of the thin film photoelectric conversion element is not particularly limited, and is preferably about 0.5 μm to 100 μm.

Hereinafter, as an embodiment of the solar cell of the present invention in which the photoelectric conversion element is used for a silicon-based thin film solar cell, the photoelectric conversion layer is an n-type semiconductor layer and an i-type semiconductor layer in order from the first electrode layer side. A silicon-based thin film solar cell including a laminate of p-type semiconductor layers will be described (this embodiment is referred to as a first embodiment).
In the first embodiment, a first electrode layer is formed on a substrate, and an n-type semiconductor layer, an i-type semiconductor layer, and a p-type semiconductor layer are sequentially formed as a photoelectric conversion layer on the first electrode layer. This is a silicon thin-film solar cell having a so-called substrate type structure in which a specific oxide transparent conductive film described later is formed as a second electrode layer on a p-type semiconductor layer. Further, in some cases, a metal comb electrode made of, for example, silver, aluminum, copper, molybdenum, or the like can be provided on the second electrode layer.

  The substrate does not need to be translucent, for example, a resin substrate such as polyimide, a flexible substrate such as a metal substrate such as stainless steel or aluminum; a glass substrate such as non-alkali glass; a resin substrate such as polyethylene terephthalate (PET) A translucent substrate such as is used.

  As the first electrode layer, for example, a metal film electrode made of silver, aluminum, copper, molybdenum or the like; a zinc oxide thin film to which gallium or the like is added; a conductive oxide layer such as a tin oxide thin film; A conductive film or the like is used. A zinc oxide thin film or a conductive oxide layer can also be used in combination with a metal film electrode.

The photoelectric conversion layer can be used as long as it is a photoelectric conversion layer that can be used for a general solar cell, and the structure thereof is formed of a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer in this order. Alternatively, a single structure composed of three layers (hereinafter sometimes referred to as a pin layer structure) may be used.
Examples of the material of the p-type semiconductor layer include hydrogenated amorphous silicon carbide (a-SiC: H).
Examples of the material of the i-type semiconductor layer include hydrogenated amorphous silicon (a-Si: H), crystalline silicon (c-Si), microcrystalline silicon (μc-Si), and hydrogenated amorphous silicon germanium (a-SiGe: H).
Examples of the material of the n-type semiconductor layer include hydrogenated amorphous silicon (a-Si: H), microcrystalline silicon (μc-Si), and the like.
Among these, three layers (hereinafter referred to as the a-SiC: H layer as the p-type semiconductor layer, the a-Si: H layer as the i-type semiconductor layer, and the a-Si: H layer as the n-type semiconductor layer in this order). A single structure made of a-Si p-i-n layer) is preferred.

In particular, the photoelectric conversion layer used in the amorphous silicon (a-Si) solar cell has a p-i-n layer structure, the i-type semiconductor layer is made of hydrogenated amorphous silicon (a-Si: H), and is p-type. The semiconductor layer is made of B having a valence lower than Si as a dopant, and the n-type semiconductor layer is made of P having a valence higher than Si as a dopant.
The photoelectric conversion layer used for the polycrystalline silicon solar cell has the same pin layer structure as the photoelectric conversion layer used for the amorphous silicon (a-Si) solar cell, and the composition of the layers constituting each layer is also the same. Basically, there is no change, but polycrystalline silicon is used instead of amorphous. Specifically, it is a photoelectric conversion layer using polycrystalline silicon having a crystal grain size of several millimeters. This photoelectric conversion layer can be manufactured using scraps and off-grade products generated in the manufacturing process of other silicon semiconductor elements such as single crystal silicon solar cells as silicon raw materials. Polycrystalline silicon solar cells have a lower output per area (photoelectric conversion efficiency) than single crystal silicon solar cells, but require less energy for production, energy balance, energy payback time (EPT), and greenhouse effect. It is superior to single crystal silicon solar cells in terms of gas (GEG) emissions. Thus, polycrystalline silicon solar cells are the mainstream of current solar cells because of the good balance between cost and performance. It is also possible to use a very thin polycrystalline silicon solar cell formed on glass.
The photoelectric conversion layer used for the microcrystalline silicon solar cell has the same p-i-n layer structure as the photoelectric conversion layer used for the amorphous silicon (a-Si) solar cell, and the composition of each layer is also the same. Basically, there is no change, but microcrystalline silicon is used instead of amorphous. This photoelectric conversion layer is obtained by forming a thin film composed of fine crystals by a CVD method or the like. Microcrystalline silicon can be regarded as one type of polycrystalline type, but it also has an amorphous property depending on the film forming conditions. Specific examples thereof include gaps between silicon crystals having a grain size of about 10 to 100 nm (grain boundaries). ) Is filled with amorphous silicon, and dense silicon in which crystal and amorphous are mixed.

In addition, the photoelectric conversion layer is formed on, for example, the above-described a-Si p-i-n layer (hereinafter, sometimes referred to as a bottom cell), and another p-i-n layer having a different absorbable wavelength region ( Hereinafter, the tandem structure in which the top cell may be electrically connected in series may be used. More preferably, the top cell formed on the bottom cell has an a-Si: H layer as a p layer, Three layers in which a microcrystalline Si layer as an i layer and an a-Si: H layer as an n layer are formed in this order, or an a-Si: H layer as a p layer, an a-SiGe: H layer as an i layer, and n It is preferable that the layer has a tandem structure in which three layers of a-Si: H layers are formed in this order.
By making the photoelectric conversion layer into a tandem structure, it becomes possible to photoelectrically convert light not only at a short wavelength but also at a long wavelength side. For example, a substrate with an oxide transparent conductive film having a double texture structure has the above tandem structure. If a photoelectric conversion layer is used, the improvement effect of photoelectric conversion efficiency becomes clearer.

The first embodiment is a silicon thin-film solar cell having a substrate-type structure in which a back electrode is provided on the substrate side, but is another embodiment of the solar cell of the present invention. Light is applied from the substrate side to the photoelectric conversion layer. A solar cell having a super straight type structure that takes in the carbon will be described (this embodiment is referred to as a second embodiment).
In the second embodiment, a specific oxide transparent conductive film, which will be described later, is formed on a substrate as a second electrode layer, and a p-type semiconductor layer, i is formed on the second electrode layer as a photoelectric conversion layer. A silicon thin film solar cell in which a n-type semiconductor layer and an n-type semiconductor layer are sequentially formed, and a first electrode layer is formed on the n-type semiconductor layer.
In this case, it is preferable to use a transparent substrate such as a glass substrate as the substrate, and to use the oxide transparent conductive film of the present invention as the first electrode layer, and the other can be the same as in the first embodiment. .

In the photoelectric conversion element of the present invention, the photoelectric conversion layer has not only the above-described p-i-n layer structure but also an n-type semiconductor layer, an i-type semiconductor layer, and a single crystal in order from the first electrode layer side. Or it is good also as a photoelectric converting layer used for a heterojunction solar cell which laminates | stacks a polycrystalline silicon, an i-type semiconductor layer, and a p-type semiconductor layer.
In the photoelectric conversion element, an i-type semiconductor layer and an n-type semiconductor layer are sequentially formed on one surface (a side that is not a light receiving portion) of single crystal or polycrystalline silicon, and the first is formed on the n-type semiconductor layer. In addition, an i-type semiconductor layer and a p-type semiconductor layer are sequentially formed on the other surface (light-receiving portion side) of single crystal or polycrystalline silicon, and the p-type semiconductor layer is formed on the p-type semiconductor layer. An oxide transparent conductive film according to the present invention, which will be described later, is formed as the second electrode layer.
Further, in some cases, as in the first embodiment, a metal comb-shaped electrode can be provided on the second electrode layer, or on the surface opposite to the light receiving side of the first electrode layer. A back electrode layer can also be provided.

As the electrode material for the back electrode layer, for example, a layer mainly composed of Ag, Ag alloy, Al, Al alloy or the like can be used, and preferably 95 mol% or more of crystalline Ag is contained in the film. It is preferable to use a metal film. By using crystalline Ag for the metal film of the back electrode, it is possible to reflect the light transmitted through the photoelectric conversion layer and return the reflected light to the photoelectric conversion layer again, thereby improving the photoelectric conversion efficiency. Can be effective.
As the single crystal or polycrystalline silicon, for example, an n-type silicon wafer, a p-type silicon wafer, or the like is used.
In addition, about the 1st electrode layer, n-type semiconductor layer, i-type semiconductor layer, and p-type semiconductor layer in the photoelectric converting layer used for this heterojunction solar cell, the thing similar to 1st embodiment mentioned above is used. be able to. Moreover, you may use the electrode material of the said back surface electrode layer for a 1st electrode layer.

  The photoelectric conversion layer in the photoelectric conversion element of the present invention may have an nip type structure in which the light incident side is an n type semiconductor layer, and the same material as the pin type structure can be used. Furthermore, various photoelectric conversion layers used in conventionally known silicon-based thin-film solar cells, such as a pn-type structure in which a p-type semiconductor layer and an n-type semiconductor layer are stacked, may be used in order from the first electrode layer side.

In the photoelectric conversion element of the present invention, the average light transmittance at a wavelength of 780 to 1500 nm is preferably 85% or more, and more preferably 88% or more.
The photoelectric conversion element of the present invention has an average light transmittance of 80% or more at a wavelength of 780 to 1500 nm, preferably a surface resistance of 20 Ω / □ or less, and an average light transmittance of 85 to 850 to 1500 nm. It is more preferable that the surface resistance is 10Ω / □ or less. If the average light transmittance and the surface resistance are within the above ranges, it can contribute to an improvement in photoelectric conversion efficiency.
The average light transmittance is measured by a visible ultraviolet spectrophotometer, and the surface resistance is measured by a four-terminal four-probe method.

  The formation method of each layer other than the oxide transparent conductive film in each of the above embodiments, the thickness of each layer, and the like are not particularly limited, and a conventionally known method may be appropriately performed.

(Oxide transparent conductive film)
The transparent oxide conductive film in the present invention is composed of a transparent oxide conductive film obtained by doping zinc oxide with a low-valent metal oxide as a dopant. The film is preferably formed by sputtering, ion plating, PLD, or EB vapor deposition.

  For example, in the sputtering method, a zinc oxide sintered body target using a low-valent metal oxide as a dopant raw material is used as a sputtering target as a raw material, and the base and the target are arranged in a sputtering apparatus, and oxygen gas is contained. A part of zinc oxide zinc is formed by heating the substrate to a predetermined temperature in an argon inert gas atmosphere and applying an electric field between the substrate and the target to generate plasma between the target substrates. An oxide transparent conductive film in which is substituted with a low-valent metal can be formed on a substrate.

  On the other hand, in the ion plating method, a zinc oxide sintered body tablet containing a low-valence metal oxide is used as a raw material tablet for ion plating, and a substrate and the tablet are placed in a copper hearth in an ion plating apparatus. In an argon inert gas atmosphere containing oxygen gas, the substrate is heated at a predetermined temperature, the tablet is evaporated from the copper hearth using an electron gun, and plasma is generated near the substrate, An oxide transparent conductive film in which tablet vapor is ionized and a part of zinc oxide zinc is substituted with a low-valent metal can be formed on a substrate.

  The substrate is, for example, one having a photoelectric conversion layer on the surface; when producing a super straight type solar cell, the transparent substrate described above; when producing a substrate type solar cell, on the substrate And those obtained by laminating a photoelectric conversion layer.

  Note that the content of the low-valent metal in the film can be changed by changing the content of the low-valent metal in the target or tablet. At this time, the structure and crystallinity of the oxide transparent conductive film to be produced are determined according to the film formation such as the content of the low-valent metal in the film, the substrate heating temperature, the oxygen partial pressure in the inert gas atmosphere, and the film formation rate. Depends on conditions.

  Such a method is an example, but an oxide transparent conductive film containing zinc oxide as a main component and containing a low-valent metal, that is, the second electrode layer in the present invention can be obtained.

A target or tablet using a low-valent metal oxide as a raw material for a dopant is an oxide sintered body substantially composed of zinc and a low-valent metal.
Here, “substantially” means that 99% or more of all atoms constituting the oxide sintered body are composed of zinc, a low-valent metal, and oxygen.

In the oxide sintered body, the number of low-valent metal atoms is preferably 2% or more and 10% or less with respect to the total number of metal atoms, and more preferably the number of low-valent metal atoms. Is contained in a ratio of 3% or more and 9% or less, more preferably 3% or more and 6% or less of the total number of metal atoms. When the ratio of the number of atoms of the low-valent metal is less than 2%, chemical durability such as chemical resistance of a film formed using the oxide sintered body as a target may be insufficient. On the other hand, when the ratio of the number of atoms of the low-valent metal exceeds 10%, the low-valent metal cannot be sufficiently substituted and dissolved in the zinc site, and the conductivity of the film formed using this oxide sintered body as a target can be reduced. There is a risk of insufficient transparency.
Here, the total number of metal atoms is the total number of metal atoms contained in the raw material powder used for producing the oxide sintered body, and zinc occupies about 90 to 98% of the total number of metal atoms. Therefore, zinc oxide is the main component in the target or tablet. The low valent metal need not be a single component, and may be a plurality of low valent metal components.

The oxide sintered body has an atomic ratio of X / (Zn + X) of 0.02 or more and 0.1 or less (wherein X is at least one selected from the group consisting of titanium, niobium, tantalum and molybdenum). That is, substantially free of low-valent metal oxide crystal phases. In an oxide sintered body in which the value of X / (Zn + X) is less than 0.02 in terms of atomic ratio, since a low-valent metal usually reacts completely with zinc oxide, a low atom is present in the oxide sintered body. In general, in the oxide sintered body in which the value of X / (Zn + X) exceeds 0.1 in terms of the number ratio, the low-valent metal cannot fully react with zinc oxide. For this reason, low-valent metal oxides are easily generated in the oxide sintered body. However, if the oxide sintered body contains a low-valence metal oxide crystal phase, the resulting film may be uneven in physical properties such as specific resistance and may lack uniformity. In the oxide sintered body, the value of X / (Zn + X) is preferably within the above range, and substantially does not contain a crystal phase of a low-valent metal oxide.
Here, examples of the crystal phase of the low-valent metal oxide include a titanium oxide crystal phase, a niobium oxide crystal phase, a tantalum oxide crystal phase, and a molybdenum oxide crystal phase.

Specifically, the crystal phase of titanium oxide includes not only Ti ₂ O ₃ and TiO but also a substance in which other elements such as Zn are dissolved in these crystals.
Specifically, the niobium oxide crystal phase includes Nb ₂ O ₅ , NbO ₂ , Nb ₂ O ₃ , and NbO, as well as substances obtained by dissolving other elements such as Zn in these crystals.
Specifically, the crystalline phase of tantalum oxide includes not only Ta ₂ O ₅ , TaO ₂ , Ta ₂ O ₃ , and TaO but also substances in which other elements such as Zn are dissolved in these crystals.
Specifically, the crystal phase of molybdenum oxide includes, in addition to MoO ₃ , MoO ₂ , and Mo ₂ O ₃ , a substance in which other elements such as Zn are dissolved in these crystals.

The oxide sintered body uses a divalent, trivalent or tetravalent low valence niobium oxide as the niobium source, and the tantalum source has a divalent, trivalent or tetravalent valence. The low-valent tantalum element oxide is used, and the titanium source may be various valences. Preferably, the low-valent titanium element oxide described later is used, and the molybdenum source is trivalent or trivalent. A tetravalent low-valent molybdenum element oxide is preferably used, for example, zinc oxide powder and low-valent metal oxide powder are mixed and press-molded.
The low-valent metal oxide is preferably at least one selected from low-valent titanium oxide, low-valent niobium oxide, low-valent tantalum oxide, and low-valent molybdenum oxide.

Specifically, the low-valence titanium oxide is not only an oxide of a titanium element whose valence is a divalent or trivalent integer such as TiO (II) and Ti ₂ O ₃ (III), but also Ti _{3 O 5, Ti 4 O 7} , Ti 6 O 11, Ti 5 O 9, the general formula including a Ti ₈ O ₁₅ and the like: novel low represented by _{TiO 2-X (X = 0.1~1} ) It refers to valence titanium oxide. As the low-valent titanium oxide powder, one kind of titanium oxide represented by the above general formula may be used alone, or a mixture of two or more kinds may be used. Among these, it is particularly preferable to use Ti ₂ O ₃ (III) powder. This is because Ti ₂ O _{3 has} an ionic radius of 0.67 、 and an ionic radius of zinc of 0.74 、, so that it is very close to the ionic radius of zinc and easily dissolves in substitution during solid phase sintering. is there.

The general formula: TiO _2-X (X = 0.1-1) is difficult to produce a single component and is obtained as a mixture. Usually, it can be produced by heating titanium oxide (TiO ₂ ) in a reducing atmosphere such as a hydrogen atmosphere using carbon or the like as a reducing agent. By adjusting the hydrogen concentration, the amount of carbon as a reducing agent, and the heating temperature, the ratio of the low-valent titanium oxide mixture can be controlled.

Examples of the low-valence niobium oxide include niobium oxide (II), niobium oxide (III), niobium oxide (IV), and the like. These may be used alone or in combination of two or more.
Examples of the low-valent tantalum oxide include tantalum oxide (II), tantalum oxide (III), and tantalum oxide (IV). These may be used alone or in combination of two or more.
Examples of the low-valent molybdenum oxide include molybdenum (III) oxide and molybdenum (IV) oxide, and each may be used alone or in combination of two or more.
The structure of the low-valence metal oxide can be confirmed by instrumental analysis such as an X-ray diffractometer (XRD) or an X-ray photoelectron spectrometer (XPS).

  The oxide sintered body as described above is preferably obtained by a method for producing a zinc oxide-based oxide sintered body described later, but is not limited to those obtained by the production method. Normally, when the oxide sintered body is sintered in a reducing atmosphere, the specific resistance of the obtained oxide sintered body is reduced by introducing oxygen deficiency, and when sintered in an oxidizing atmosphere, the specific resistance is reduced. Becomes higher.

  The oxide sintered body may have a relative density of 93% or more, preferably 95 to 100%. Here, the relative density is defined as the density of the oxide sintered body divided by the theoretical density and multiplied by 100. If the relative density is less than 93%, the characteristics of the sintered body, such as the high film formation speed and the possibility of stable film formation, may be impaired.

(Method for producing zinc oxide-based oxide sintered body)
The method for producing a zinc oxide-based oxide sintered body is obtained by molding a raw material powder containing at least one of the following (A) and (B), and then sintering the obtained molded body to obtain the above-described oxidation. This is a method for obtaining a sintered product.
(A) A low-valent metal oxide powder and zinc oxide powder or zinc hydroxide powder (hereinafter, zinc oxide powder or zinc hydroxide powder may be referred to as “zinc oxide powder (zinc hydroxide powder)”). Mixed powder (B) Zinc and low-valent metal oxide compound powder (complex oxide powder in which low-valent metal oxide and zinc oxide are generated by solid-phase reaction)

  Specific examples of the raw material powder may be a mixed powder of low-valent metal oxide powder and zinc oxide powder (zinc hydroxide powder) or a powder containing zinc and low-valent metal oxide compound powder. .

As the zinc oxide powder, a powder of ZnO or the like having a wurtzite structure is usually used, and a powder obtained by firing this ZnO in advance in a reducing atmosphere and containing oxygen deficiency may be used.
The zinc hydroxide powder may be either amorphous or crystalline.

  Examples of the low-valent metal oxide powder include low-valent titanium oxide powder, low-valent niobium oxide powder, low-valent tantalum oxide powder, low-valent molybdenum oxide powder, and low-valent metal oxide compound powder. Among them, it is preferable to select from low-valent titanium oxide powder, low-valent niobium oxide powder, low-valent tantalum oxide powder, and low-valent molybdenum oxide powder.

As the low-valent molybdenum oxide powder, for example, a powder such as MoO ₂ or Mo ₂ O ₃ can be used, and it is particularly preferable to use a Mo ₂ O ₃ powder. This is because the ionic radius of Mo ₂ O ₃ is 0.69 、 and the ionic radius of zinc is 0.74 Å. This is because it is very close to the ionic radius of zinc and is liable to be dissolved by substitution during solid phase sintering. These powders may be used alone or in combination of two or more.

As the low-valence niobium oxide powder, for example, a powder of NbO ₂ , NbO, Nb ₂ O ₃ or the like can be used, and in particular, a powder of Nb ₂ O ₃ is preferably used. This is because the ionic radius of Nb ₂ O ₃ is 0.72 、 and the ionic radius of zinc is 0.74 、, so that it is very close to the ionic radius of zinc and easily dissolves in substitution during solid phase sintering. . These powders may be used alone or in combination of two or more.

As the low-valent tantalum oxide powder, for example, a powder of TaO ₂ , TaO, Ta ₂ O ₃ or the like can be used, and it is particularly preferable to use a Ta ₂ O ₃ powder. This is because the ionic radius of Ta ₂ O ₃ is 0.72 、 and the ionic radius of zinc is 0.74 、, which is very close to the ionic radius of zinc and is easily dissolved by substitution during solid phase sintering. These powders may be used alone or in combination of two or more.

The low valence titanium oxide is not only an oxide of a titanium element whose valence is a divalent or trivalent integer such as TiO (II) and Ti ₂ O ₃ (III), but also Ti ₃ O _5. , Ti ₄ O ₇ , Ti ₆ O ₁₁ , Ti ₅ O ₉ , Ti ₈ O _{15 and} other general formula: TiO _2-X (X = 0.1-1) Says titanium. As the low-valent titanium oxide powder, one kind of titanium oxide represented by the above general formula may be used alone, or a mixture of two or more kinds may be used. Among these, it is particularly preferable to use Ti ₂ O ₃ (III) powder. This is because Ti ₂ O _{3 has} an ionic radius of 0.67 、 and an ionic radius of zinc of 0.74 、, so that it is very close to the ionic radius of zinc and easily dissolves in substitution during solid phase sintering. is there.

The low-valence titanium oxide powder represented by the general formula: TiO _{2 -X} (X = 0.1-1) is difficult to produce a single component and is obtained as a mixture. Usually, titanium oxide (TiO ₂ ) can be produced by heating in a reducing atmosphere such as a hydrogen atmosphere using carbon or the like as a reducing agent. By adjusting the hydrogen concentration, the amount of carbon as a reducing agent, and the heating temperature, the ratio of the low-valent titanium oxide mixture can be controlled.

  The structure of the low-valent metal oxide can be confirmed by instrumental analysis such as an X-ray diffractometer (X-Ray Diffraction, XRD) and an X-ray photoelectron spectrometer (XPS).

  Specific examples of zinc and low valent metal oxide compound powder (composite oxide powder produced by solid phase reaction of low valent metal oxide and zinc oxide) include zinc niobate compound powder, zinc tantalate compound powder, Examples thereof include zinc molybdate powder.

As the zinc niobate compound powder, for example, powders such as Zn ₃ Nb ₂ O ₈ , ZnNb ₂ O ₆ , Zn ₄ Nb ₂ O ₉ can be used, and in particular, powders of Zn ₃ Nb ₂ O ₈ are used. Is preferred.
As the zinc tantalate compound powder, for example, a powder of ZnTa ₂ O ₆ , Zn ₃ Ta ₂ O ₈ , Zn ₄ Ta ₂ O ₉ or the like can be used, and in particular, a powder of Zn ₃ Ta ₂ O ₈ is used. Is preferred.
Examples of the zinc molybdate powder (composite oxide by solid phase reaction of zinc oxide and low-valent molybdenum oxide) include, for example, ZnMoO ₄ , Zn ₂ Mo ₃ O ₈ , Zn ₃ Mo ₂ O ₉ , ZnMo ₂ O. ₇ , ZnMoO ₃ , ZnMoO ₄ , Zn ₃ Mo ₃ O ₈ , ZnMo ₈ O ₁₀ , ZnMoO _{3 and the} like can be used, and it is particularly preferable to use ZnMoO ₃ .

In the raw material powder, the number of low-valent metal atoms is preferably 2% to 10% of the zinc oxide atoms, and more preferably the number of low-valent metal atoms is zinc oxide. It is contained in a proportion of 3% to 9%, more preferably 3% to 6%, based on the number of atoms. When the ratio of the number of atoms of the low-valent metal is less than 2%, the chemical resistance of the film formed using the obtained oxide sintered body, target or tablet (hereinafter sometimes simply referred to as target), etc. There is a risk that the chemical durability will be insufficient. On the other hand, when the ratio of the number of atoms of the low-valent metal exceeds 10%, the low-valent metal cannot be sufficiently substituted and dissolved in the zinc site, and the obtained oxide sintered body or the film formed using the target There is a risk that the conductivity and transparency may be insufficient.
Since zinc occupies about 90 to 98% of the total number of metal atoms contained in the raw material powder, zinc oxide is the main component in the raw material powder. The low valent metal need not be a single component, and may be a plurality of low valent metal components.

The raw material powder may contain at least one element selected from the group consisting of gallium, aluminum, tin, silicon, germanium, zirconium, and hafnium (hereinafter, these may be referred to as “additive elements”). . Thereby, in addition to the specific resistance of the film formed using the obtained oxide sintered body or target, the specific resistance of the obtained oxide sintered body or target itself can be reduced. The total content of additive elements is preferably 0.05% or less in terms of atomic ratio with respect to the total amount of all metal elements contained in the raw material powder. When there is more content of an additional element than the said range, there exists a possibility that the specific resistance of the film | membrane formed using the oxide sintered compact or target obtained may increase.
Furthermore, the raw material powder may contain other elements such as indium, iridium, ruthenium, and rhenium as impurities. The total content of elements contained as impurities is preferably 0.1% or less in terms of atomic ratio with respect to the total amount of all metal elements contained in the raw material powder.

  The average particle size of each compound (powder) used as the raw material powder is preferably 1 μm or less.

  When the oxide sintered body contains zinc oxide and molybdenum oxide, that is, when a mixed powder of low-valent molybdenum oxide powder and zinc oxide powder (zinc hydroxide powder) is used as the raw material powder, or low-valent oxidation The mixing ratio of each powder when using a mixed powder of molybdenum powder, zinc oxide powder (zinc hydroxide powder) and zinc molybdate compound powder is finally obtained according to the type of compound (powder) used. What is necessary is just to set suitably so that the value of said X / (Zn + X), ie, Mo / (Zn + Mo), may become the range mentioned above by atomic ratio in oxide sinter. In that case, considering that zinc has a higher vapor pressure than molybdenum and is likely to volatilize when sintered, the desired composition of the desired oxide sintered body (atomic ratio of Zn and Mo) It is preferable to set the mixing ratio in advance so that the amount of zinc increases.

  When the oxide sintered body contains zinc oxide, niobium oxide, and tantalum oxide, that is, the raw material powder is composed of low-valent niobium oxide powder, low-valent tantalum oxide powder, and zinc oxide powder (zinc hydroxide powder). Or low-valent niobium oxide powder and / or low-valent tantalum oxide powder, zinc oxide powder (zinc hydroxide powder), zinc niobate compound powder and / or zinc tantalate compound powder. A raw material powder comprising low-valence niobium oxide powder, zinc oxide powder (zinc hydroxide powder) and zinc niobate compound powder, and low-valent tantalum oxide powder and zinc oxide powder. The mixing ratio of each raw material powder (except when the raw material powder is composed of zinc hydroxide powder and zinc tantalate compound powder) is finally obtained according to the type of compound (powder) used. The value of the ratio of the number of atoms in the oxide sintered body X / (Zn + X), i.e. (Nb + Ta) / value of (Zn + Nb + Ta) may be set as appropriate so that the above-mentioned range. In that case, considering that zinc has a higher vapor pressure than niobium and is likely to volatilize when sintered, the desired composition of the desired oxide sintered body (atomic ratio of Zn, Nb and Ta) However, it is preferable to set the mixing ratio in advance so that the amount of zinc increases.

  When a mixed powder of low-valent titanium oxide powder and zinc oxide powder (zinc hydroxide powder) is used as the raw material powder, or low-valent titanium oxide powder, zinc oxide powder (zinc hydroxide powder) and zinc titanate When using mixed powder with compound powder, the mixing ratio of each powder depends on the type of compound (powder) to be used, and the above-mentioned X / (Zn + X) in atomic ratio in the finally obtained oxide sintered body The value of Ti and the value of Ti / (Zn + Ti) may be appropriately set so as to be in the above-described range. At that time, considering that zinc has a higher vapor pressure than titanium and is likely to be volatilized when sintered, the desired composition of the desired oxide sintered body (atomic ratio of Zn and Ti), It is preferable to set the mixing ratio in advance so that the amount of zinc increases.

  When the low-valent metal oxide is annealed in the air, it is oxidized to the main valence (for example, niobium is pentavalent, tantalum is pentavalent, molybdenum is hexavalent, titanium is tetravalent). End up. Therefore, it is preferable to sinter in an inert atmosphere and a reducing atmosphere (atmospheric atmosphere sintering followed by reduction annealing). Specifically, the easiness of volatilization of zinc varies depending on the atmosphere during sintering.For example, when zinc oxide powder is used, only the volatilization of zinc oxide powder itself occurs in an air atmosphere or an oxidizing atmosphere. When sintered in an inert atmosphere or a reducing atmosphere, zinc oxide is reduced and becomes metal zinc that is more easily volatilized than zinc oxide, so the amount of zinc lost increases (however, as described later, When annealing is performed in a reducing atmosphere after sintering, the composite oxide is already formed at the time of annealing, so that zinc is not easily evaporated. Therefore, the amount of zinc to be increased with respect to the target composition may be set in consideration of the sintering atmosphere or the like. For example, it is desirable when sintering is performed in an air atmosphere or an oxidizing atmosphere. If the sintering is performed in an inert atmosphere or a reducing atmosphere, the amount is about 1.1 to 1.3 times the amount of the desired atomic ratio. That's fine. In addition, each compound (powder) used as the raw material powder may be only one kind or two or more kinds.

The raw material powder may be pulverized before being molded. By performing the pulverization treatment, the raw material powder is adjusted to have a narrow particle size distribution, and can be uniformly solid-phase sintered in the sintering described later to obtain a high-density oxide sintered body. it can.
The method for pulverization is not particularly limited. For example, when media is used, a method using a pulverizer equipped with a bead mill, a ball mill, a planetary mill, a sand grinder, a vibration mill, or an attritor, and no media are used. Examples thereof include a method using a wet ultrahigh pressure atomizer such as a jet mill, a nanomizer, and a starburst.

The method for forming the raw material powder is not particularly limited. For example, the raw material powder and an aqueous solvent are mixed, and the resulting slurry is sufficiently mixed by wet mixing. What is necessary is just to dry and granulate and shape | mold the obtained granulated material.
The aqueous solvent contains water as a main component and may be water alone, or may be a mixture of water and alcohol such as methanol or ethanol.
The wet mixing may be performed by, for example, a wet ball mill using a hard ZrO ₂ ball or a vibration mill, and the mixing time when using the wet ball mill or the vibration mill is preferably about 12 to 78 hours. In addition, although raw material powder may be dry-mixed as it is, wet mixing is more preferable.
For solid-liquid separation / drying / granulation, known methods may be employed.
When the obtained granulated product is molded, for example, the granulated product is put into a mold, and a pressure of 1 ton / cm ² or more is applied using a cold molding machine such as a cold press or a cold isostatic press. Can be molded. At this time, when hot forming is performed using a hot press or the like, it is disadvantageous in terms of manufacturing cost and it is difficult to obtain a large-sized sintered body.
In addition, when obtaining a granulated material as a molded object, what is necessary is just to granulate by a well-known method after drying, In that case, it is preferable to mix a binder with raw material powder. As the binder, for example, polyvinyl alcohol, vinyl acetate, ethyl cellulose and the like can be used.

  The obtained compact is sintered in an air atmosphere, an inert atmosphere, a reducing atmosphere (for example, nitrogen, argon, helium, carbon dioxide, vacuum, hydrogen, etc.) and an oxidizing atmosphere (an atmosphere having a higher oxygen concentration than the air). It is performed at 600 to 1500 ° C. in any atmosphere. Then, when sintering is performed in an air atmosphere or an oxidizing atmosphere, annealing is further performed in a reducing atmosphere. The annealing treatment in the reducing atmosphere that is performed after sintering in the air atmosphere or in the oxidizing atmosphere is performed to cause oxygen deficiency in the oxide sintered body and to reduce the specific resistance. Accordingly, it is needless to say that the annealing treatment is preferably performed after the sintering when it is desired to further reduce the specific resistance even when the sintering is performed in an air atmosphere or a reducing atmosphere.

  In sintering in any atmosphere, the sintering temperature is 600 to 1500 ° C., preferably 1000 to 1300 ° C. If the sintering temperature is lower than 600 ° C., the sintering does not proceed sufficiently, so that the target density is lowered. On the other hand, if it exceeds 1500 ° C., zinc oxide itself is decomposed and disappears. When the molded body is heated to the sintering temperature, the rate of temperature increase is 5 to 10 ° C./min up to 1000 ° C., and 1 to 4 ° C./min up to 1000 ° C. It is preferable to make the sintered density uniform.

  When sintering in any atmosphere, the sintering time (that is, the holding time at the sintering temperature) is preferably 3 to 15 hours. If the sintering time is less than 3 hours, the sintered density tends to be insufficient, and the strength of the resulting oxide sintered body tends to decrease. On the other hand, if it exceeds 15 hours, the crystal grains of the sintered body As the growth becomes remarkable, there is a tendency that the pores are coarsened and consequently the maximum pore diameter is increased, and as a result, the sintered density may be lowered.

The method for performing the sintering is not particularly limited, and is a normal pressure firing method, a hot press method, a hot isostatic pressing (HIP) method, a cold isostatic pressing (CIP) method, A microwave sintering method, a millimeter wave sintering method, or the like can be employed.
Examples of the reducing atmosphere when performing the annealing treatment include an atmosphere made of at least one selected from the group consisting of nitrogen, argon, helium, carbon dioxide, vacuum, and hydrogen.
As a method for the annealing treatment, for example, a method of heating at normal pressure while introducing a non-oxidizing gas such as nitrogen, argon, helium, carbon dioxide, or hydrogen, or heating under a vacuum (preferably 2 Pa or less). Although it can be performed by a method or the like, the former method at normal pressure is advantageous from the viewpoint of production cost.

  In performing the annealing treatment, the annealing temperature (heating temperature) is preferably 1000 to 1400 ° C., more preferably 1100 to 1300 ° C. The annealing time (heating time) is preferably 7 to 15 hours, and more preferably 8 to 12 hours. If the annealing temperature is less than 1000 ° C., introduction of oxygen deficiency due to the annealing treatment may be insufficient. On the other hand, if it exceeds 1400 ° C., zinc tends to be volatilized, and the composition of the obtained oxide sintered body There is a possibility that (atom ratio of Zn and metal) is different from a desired ratio.

Examples of the inert atmosphere and reducing atmosphere when performing the annealing treatment include an atmosphere made of at least one selected from the group consisting of nitrogen, argon, helium, carbon dioxide, vacuum, and hydrogen.
As a method for the annealing treatment, for example, a method of heating at normal pressure while introducing a non-oxidizing gas such as nitrogen, argon, helium, carbon dioxide, or hydrogen, or heating under a vacuum (preferably 2 Pa or less). Although it can be performed by a method or the like, the former method at normal pressure is advantageous from the viewpoint of production cost.

(target)
The target in the present invention is a target used for film formation by sputtering, ion plating, PLD, or EB vapor deposition. In addition, although the solid material used in the film formation may be referred to as “tablet”, in the present invention, these are referred to as “target”.

The target in the present invention is obtained by processing the above-described zinc oxide-based oxide sintered body.
A processing method in particular is not restrict | limited, What is necessary is just to employ | adopt a well-known method suitably. For example, the target in the present invention can be obtained by subjecting the zinc oxide-based oxide sintered body to surface grinding or the like, and then cutting the zinc oxide-based oxide sintered body to a predetermined size, and then attaching it to a support base. Moreover, it is good also as a large area target (composite target) by arranging several oxide sintered compacts in a division | segmentation shape as needed.

  As a method for forming a zinc oxide-based oxide transparent conductive film, it is preferable to form a film by sputtering, ion plating, PLD, or EB vapor deposition. Specific methods and conditions at that time are described above. There is no particular limitation other than using a target, and a known sputtering method, ion plating method, PLD method, or EB vapor deposition method and conditions may be appropriately employed.

As described above, the oxide transparent conductive film, that is, the second electrode layer in the present invention is formed.
The film thickness of the zinc oxide-based oxide transparent conductive film in the present invention may be appropriately set depending on the application, and is not particularly limited, but is preferably 50 to 600 nm, more preferably 100 to 500 nm. If the thickness is less than 50 nm, sufficient specific resistance may not be ensured. On the other hand, if the thickness exceeds 600 nm, the film may be colored.
A film having the above properties is useful as a transparent electrode of a solar cell because of its extremely high transmittance in the infrared region and low resistance.

  As mentioned above, although typical embodiment of the photoelectric conversion element was mentioned, the photoelectric conversion element concerning this invention is not limited to these, The specific transparent conductive film mentioned above as a 2nd electrode layer which is a light-receiving side As long as is used, the configuration of a conventionally known photoelectric conversion element can be appropriately employed.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by a following example.
In addition, evaluation of the obtained transparent conductive substrate was performed by the following method.

<Resistivity>
The specific resistance was measured by a four-terminal four-probe method using a resistivity meter (“LORESTA-GP, MCP-T610” manufactured by Mitsubishi Chemical Corporation). Specifically, four needle-shaped electrodes are placed on a straight line on the sample, a constant current is passed between the outer two probes, a constant current is passed between the inner two probes, and the inner two probes are The resulting potential difference was measured to determine the resistance.
<Surface resistance>
The surface resistance (Ω / □) was calculated by dividing the specific resistance (Ω · cm) by the film thickness (cm).
<Transmissivity>
The transmittance was measured using an ultraviolet visible near infrared spectrophotometer (“V-670” manufactured by JASCO Corporation).

(Reference Example 1)
Zinc oxide powder (ZnO; manufactured by Wako Pure Chemical Industries, Ltd., special grade) and titanium oxide powder (TiO (II); manufactured by Kojundo Chemical Laboratory Co., Ltd., purity 99.99%) are used as raw material powders. Mixing was performed at a Zn: Ti atomic ratio of 97: 3 to obtain a raw material powder mixture.
Next, the obtained mixture was put in a mold and molded by a uniaxial press at a molding pressure of 500 kg / cm ² to obtain a disk-shaped molded body having a diameter of 100 mm and a thickness of 5 mm. This compact was annealed at 1000 ° C. for 4 hours in an argon atmosphere at normal pressure (100 Pa) to obtain an oxide sintered body (1).
When the obtained oxide sintered body (1) was analyzed with an energy dispersive fluorescent X-ray apparatus (“EDX-700L” manufactured by Shimadzu Corporation), the atomic ratio of Zn and Ti was Zn: Ti = 97: 3 (Ti / (Zn + Ti) = 0.03). When the crystal structure of this oxide sintered body (1) was examined with an X-ray diffractometer (“RINT2000” manufactured by Rigaku Corporation), crystals of zinc oxide (ZnO) and zinc titanate (Zn ₂ TiO ₄ ) were obtained. It was a mixture of phases, and no crystalline phase of titanium oxide was present.

  Next, the obtained oxide sintered body (1) was processed into a disk shape of 50 mmφ to prepare a target, and an oxide transparent conductive film was formed by sputtering using the target, and a transparent conductive substrate was formed. Got. That is, the above-mentioned target and a transparent substrate (quartz glass substrate) are installed in a sputtering apparatus (“E-200” manufactured by Canon Anelva Engineering Co., Ltd.), respectively, and Ar gas (purity 99.9995% or more, Ar pure gas) = 5N) was introduced at 12 sccm, and sputtering was performed under the conditions of a pressure of 0.5 Pa, a power of 75 W, and a transparent substrate temperature of 250 ° C. to form an oxide transparent conductive film having a thickness of 500 nm on the transparent substrate.

About the composition (Zn: Ti) in the formed oxide transparent conductive film, a wavelength dispersion type fluorescent X-ray apparatus (“XRF-1700WS” manufactured by Shimadzu Corporation) is used, and a calibration curve is used by a fluorescent X-ray method. As a result of quantitative analysis, it was Zn: Ti (atomic ratio) = 97: 3 (Ti / (Zn + Ti) = 0.03). The transparent oxide conductive film is subjected to X-ray diffraction using an attachment for thin film measurement using an X-ray diffractometer (“RINT2000” manufactured by Rigaku Corporation) and an energy dispersive X-ray microanalyzer ( TEM-EDX) was used to investigate the doping state of titanium with zinc, and further the field structure electron microscope (FE-SEM) was used to examine the crystal structure. It was found that titanium was substituted and dissolved in zinc.
The specific resistance of the oxide transparent conductive film on the obtained transparent conductive substrate was 4.2 × 10 ⁻⁴ Ω · cm, and the surface resistance was 8.4 Ω / □. The specific resistance distribution on the transparent substrate was uniform in the plane.
The transmittance of the obtained transparent conductive substrate was 89% on the average in the visible region (380 nm to 780 nm) and 89% on the average in the infrared region (780 nm to 1500 nm). The transmittance in the visible region (380 nm to 780 nm) of the quartz glass substrate before film formation was 94% on average, and the transmittance in the infrared region (780 nm to 2700 nm) was 94% on average.
Moreover, the measurement result of the transmittance | permeability of the obtained transparent conductive substrate is shown in FIG. As is clear from FIG. 1, the light transmittance was not only in the visible light region but also in the near infrared region (780 nm to 1500 nm).

  Therefore, when such an oxide transparent conductive film is used for the surface transparent electrode film (transparent conductive film) on the light receiving part side of the solar cell and / or the transparent conductive film on the back side of the pn junction part, solar energy in the infrared region is used. It can be effectively converted into electrical energy.

(Reference Example 2)
97.7 parts by weight of zinc oxide powder having an average particle diameter of 1 μm and 2.3 parts by weight of aluminum oxide powder having an average particle diameter of 0.2 μm are placed in a polyethylene pot and mixed for 72 hours using a dry ball mill. A raw material powder mixture was obtained. The obtained mixture was put into a mold and pressed at a molding pressure of 300 kg / cm ² to obtain a molded body. The compact was subjected to densification treatment with CIP at a pressure of 3 ton / cm ² and then sintered under the following conditions to obtain a sintered body of aluminum-doped zinc oxide. When the obtained sintered body was analyzed by X-ray diffraction, it was a two-phase mixed structure of ZnO and ZnAl ₂ O ₄ .
Sintering temperature: 1500 ° C
Temperature rising rate: 50 ° C./hour Holding time: 5 hours Sintering atmosphere: in air

Next, the obtained oxide sintered body is processed into a 4 inch φ, 6 mmt shape to produce a target, bonded to an oxygen-free copper backing plate using indium solder, and sputtered under the following conditions. Film formation was performed to form an oxide transparent conductive film having a thickness of 500 nm on a transparent substrate (quartz glass substrate) to obtain a transparent conductive substrate. The Al content in the formed film was 2.3% by weight.
Equipment: dc magnetron sputtering equipment Magnetic field strength: 1000 Gauss (horizontal component directly above the target)
Substrate temperature: 200 ° C
Ultimate vacuum: 5 × 10 ⁻⁵ Pa
Sputtering gas: Ar
Sputtering gas pressure: 0.5 Pa
DC power: 300W

The specific resistance of the oxide transparent conductive film on the obtained transparent conductive substrate was 4.1 × 10 ⁻⁴ Ω · cm, and the surface resistance was 8.2 Ω / □.
The transmittance of the obtained transparent conductive substrate was an average of 89% in the visible region (380 nm to 780 nm) and an average of 70% in the infrared region (780 nm to 1500 nm).
Moreover, the measurement result of the transmittance | permeability of the obtained transparent conductive substrate is shown in FIG. As is apparent from FIG. 1, the transmittance in the visible light region is high, but the light transmittance in the near infrared region (780 nm to 1500 nm) is low.

  Therefore, when such an oxide transparent conductive film is used for the surface transparent electrode film (transparent conductive film) on the light-receiving part side of the solar cell and / or the transparent electrode film on the back side of the pn junction part, solar energy in the infrared region is used. It cannot be converted into electrical energy effectively.

Example 1
<Production of solar cells>
An amorphous silicon solar cell element was produced by the following procedure so that the layer structure shown in FIG. 2 was formed on the transparent electrode substrate produced in Reference Example 1.

(Amorphous p-type silicon layer formation)
Sample 1 (transparent conductive substrate obtained in Reference Example 1) was transported to a p-type silicon film forming chamber, and then silane (SiH ₄ ), hydrogen (H ₂ ), diborane (B ₂ H ₆ ), methane (CH ₄ ) Or the like is introduced into the p-type silicon film forming chamber at a constant flow rate and maintained at a substrate temperature of 150 ° C. and a pressure of 0.5 Torr. A boron-doped a-Si alloy film was obtained. Thereafter, only the introduction of diborane (B ₂ H ₆ ) gas under the above conditions was stopped in the same chamber, and a film with a thickness of 5 nm was formed using a non-doped a-SiC alloy film as a solar cell buffer layer to obtain Sample 2. After film formation was completed, the vacuum was exhausted again to high vacuum.

(Non-doped amorphous i-type silicon photoelectric conversion layer formation)
Next, after transporting the obtained sample 2 to the i-type silicon film forming chamber, SiH ₄ and H ₂ are introduced into the i-type silicon film forming chamber at a constant flow rate, and the substrate temperature is set to 150 ° C. and the high pressure is set to 1.0 Torr. After maintaining, discharge was started, and a sample 3 in which non-doped a-Si having a thickness of 0.35 μm was formed by film formation for 25 minutes was obtained. After film formation was completed, the vacuum was exhausted again to high vacuum.

(Amorphous n-type silicon layer formation)
Next, the obtained sample 3 is transported to the n-type silicon film forming chamber, SiH ₄ , H ₂ and phosphine (PH ₃ ) are introduced into the n-type silicon film forming chamber at a constant flow rate, and the substrate temperature is 150 ° C., pressure It was kept at 0.2 Torr. Discharge was started, and a sample 4 in which phosphorus-doped a-Si having a thickness of 30 nm was formed by film formation for 6 minutes was obtained. After film formation was completed, the vacuum was exhausted again.

(First electrode layer and back surface reflective electrode layer formation)
After forming the above-mentioned p-type layer-i-type layer-n-type layer three-layer photoelectric conversion unit, the sample 4 was cooled to room temperature and taken out into the atmosphere, and then the sample 4 was placed in the sputtering vacuum device again. A sample 5 in which a first electrode layer and a back surface reflective electrode layer (hereinafter sometimes collectively referred to as a back surface electrode) were formed by the following procedure was obtained.
Sample 5 was laminated with a gallium-added zinc oxide layer 20 nm, a silver layer 200 nm, and a gallium-added zinc oxide layer 20 nm in this order at room temperature to obtain Sample 6. After the sample 6 was taken out from the vacuum device, a solar cell element having an area of 0.25 cm ² was obtained by patterning the back electrode. Thereafter, post-annealing at 150 ° C. was performed for 2 hours.
It was 8.9% when the photoelectric conversion efficiency of the amorphous silicon solar cell obtained by the above process was measured.
In order to evaluate the chemical durability of the fabricated solar cell element, even when an exposure test was conducted for 1000 hours in an atmosphere of 85 ° C. and 85% humidity, only a slight deterioration of the transparent oxide conductive film occurred. The previous photoelectric conversion efficiency was slightly decreased.

(Comparative Example 1)
A solar cell having the layer configuration shown in FIG. 2 was produced under the same conditions and procedures as in Example 1 except that the AZO film of Reference Example 2 was used as the oxide transparent conductive film, and the characteristics were examined under the same conditions. As compared with Example 1, the photoelectric conversion efficiency was low.
In order to evaluate the chemical durability of the produced solar battery cell, an exposure test was conducted in an atmosphere of 85 ° C. and humidity of 85% for 1000 hours. As a result, the oxide transparent conductive film deteriorated and photoelectric conversion before the test was performed. The efficiency was significantly reduced.

  From this experiment, it is obvious that the solar cell to which the transparent electrode film of the present invention is applied has excellent conversion efficiency, excellent chemical durability, and excellent long-term reliability.

DESCRIPTION OF SYMBOLS 1 Back surface reflective electrode layer 2 1st electrode layer 3 Amorphous n type silicon layer 4 Non-dope amorphous i type silicon photoelectric conversion layer 5 Amorphous p type silicon layer 6 Zinc oxide type transparent conductive film (2nd electrode layer)
7 Transparent substrate

Claims (13)

In the thin film photoelectric conversion element in which the first electrode layer, the photoelectric conversion layer, and the second electrode layer are arranged in this order, and a photovoltaic force is generated by light incident from the second electrode layer side,
At least the second electrode layer is an oxide transparent conductive film obtained by doping zinc oxide with a low-valent metal oxide as a dopant.   The thin film photoelectric according to claim 1, wherein the photoelectric conversion layer is a photoelectric conversion layer used for an amorphous silicon (a-Si) solar cell, a polycrystalline silicon solar cell, or a microcrystalline silicon solar cell. Conversion element. The transparent oxide conductive film is made of a sputtering method, an ion plating method, a pulsed laser deposition (PLD) method or an electron beam using a target or tablet mainly composed of zinc oxide and doped with a low-valent metal oxide. EB) an oxide transparent conductive film formed by vapor deposition,
The low valent metal oxide is doped so that the atomic ratio of the low valent metal to zinc is 0.02 to 0.1, and the specific resistance of the oxide transparent electrode layer is 2.0 × The thin film photoelectric conversion element according to claim 1, wherein the thin film photoelectric conversion element is 10 ⁻³ Ω · cm or less.   The low-valent metal oxide is selected from low-valent titanium oxide, low-valent niobium oxide, low-valent tantalum oxide, and low-valent molybdenum oxide. Thin film photoelectric conversion element. The thin film photoelectric conversion element according to claim 4, wherein the low-valent titanium oxide is TiO _2−X (X = 0.1-1). The low-valent titanium oxide is selected from TiO (II), Ti ₂ O ₃ (III), Ti ₃ O ₅ , Ti ₄ O ₇ , Ti ₆ O ₁₁ , Ti ₅ O ₉ and Ti ₈ O _15. The thin film photoelectric conversion element according to claim 4 or 5. The thin film photoelectric conversion element according to claim 4, wherein the low-valence niobium oxide is selected from NbO (II), Nb ₂ O ₃ (III), and NbO ₂ (IV). The thin film photoelectric conversion element according to any one of claims 4 to 7, wherein the low-valent tantalum oxide is selected from TaO ₂ (IV) and Ta ₂ O ₃ (III). The thin film photoelectric conversion element according to any one of claims 4 to 8, wherein the low-valent molybdenum oxide is selected from MoO ₂ (IV) and Mo ₂ O ₃ (III).   The thin film photoelectric conversion device according to any one of claims 1 to 9, wherein an average light transmittance at a wavelength of 780 to 1500 nm is 85% or more.   The thin film photoelectric conversion element according to claim 1, wherein an average light transmittance at a wavelength of 780 to 1500 nm is 80% or more, and a surface resistance is 20Ω / □ or less.   The solar cell which has a board | substrate and the thin film photoelectric conversion element in any one of Claims 1-11 in which said 1st electrode layer was laminated | stacked on this board | substrate.   The solar cell which has a transparent substrate and the thin film photoelectric conversion element in any one of Claims 1-11 in which said 2nd electrode layer was laminated | stacked on this transparent substrate.

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