METHOD OF MANUFACTURING Cu2O CRYSTAL-CONTAINING LAYER, AND STRUCTURE COMPRISING SEMICONDUCTOR HAVING Cu2O CRYSTAL-CONTAINING LAYER AND USE THEREOF

Abstract

To manufacture a thin layer made of a highly pure Cu2O crystal by a new method more excellent than a conventional method, from the standpoint of operability, a manufacturing cost and the like.SOLUTION: In a method according to the present invention, a layer containing a Cu2O crystal exhibiting higher light emission characteristics than a Cu2O crystal of a precursor is manufactured on a conductive substrate by forming a layer made of a cuprous halide on a layer made of the Cu2O crystal of the precursor formed on the conductive substrate.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing a layer containing CuO crystals on a conductive substrate that exhibits higher luminescence properties than precursor CuO crystals, as well as a structure including a conductive substrate and a semiconductor having a layer containing CuO crystals with predetermined luminescence properties on the substrate, and uses thereof (particularly, solar cells). [Background technology]

[0002] Addressing environmental issues has become a global challenge, and achieving carbon neutrality is a pressing need. To achieve this, renewable energy plays a crucial role. Solar cells have therefore attracted attention, and research is underway to develop better solar cells in terms of photoelectric conversion efficiency and manufacturing costs. In particular, the development of tandem (multi-junction) solar cells has attracted attention as a next-generation solar cell. Tandem solar cells have a structure in which different types of solar cells are stacked to improve power generation efficiency. For example, in a two-junction tandem solar cell, two solar cells (bottom and top cells) are stacked to form a single solar cell, and power is generated by both cells, improving overall power generation efficiency. Tandem solar cells, which have already been put into practical use, include two-junction and three-junction tandem solar cells, in which a germanium-based solar cell is used as the bottom cell and a III-V semiconductor, such as gallium arsenide (GaAs), is used as the top or middle cell. However, solar cells using III-V semiconductors have challenges in terms of manufacturing costs. In addition, tandem solar cells, which are two-junction solar cells that use a solar cell that uses crystalline silicon as a semiconductor for the bottom cell and a solar cell that uses a perovskite semiconductor for the top cell, are being actively researched and developed. However, solar cells that use perovskite semiconductors have issues with durability (specifically, deterioration over time due to external environments such as air and moisture).

[0003] Therefore, currently, attention is being paid to the development of tandem solar cells, which are two-junction solar cells that use a solar cell that uses crystalline silicon as a semiconductor for the bottom cell and a solar cell that uses CuO crystal as a semiconductor for the top cell, as a low-cost, highly reliable solar cell (for example, Patent Document 1). It has been reported that the power generation efficiency of such tandem solar cells exceeds 30% (for example, Non-Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-57536 [Non-patent literature]

[0005] [Non-Patent Document 1] Highly transparent Cu2O absorbing layer for thin film solar cells, Appl. Phys. Lett. 119, 242102 (2021) Summary of the Invention [Problem to be solved by the invention]

[0006] Solar cells using Cu2O crystals as a semiconductor are primarily made of Cu2O, a compound of copper and oxygen that is abundant on Earth. Compared to III-V semiconductors such as gallium arsenide (GaAs), the material itself is significantly less resource-constrained and less expensive. Furthermore, because they are thin-film solar cells that do not require expensive single-crystal substrates, significant cost reductions are expected. Furthermore, Cu2O is non-toxic and environmentally friendly. Cu2O is a semiconductor with a bandgap of approximately 2 eV and is readily available as p-type Cu2O crystals that are conductive at room temperature. Furthermore, solar cells using Cu2O crystals as a semiconductor are not subject to the aging degradation factors seen in conventional perovskite semiconductors, resulting in excellent durability. Because silicon solar cells are extremely durable, high durability (i.e., excellent durability) is an important characteristic for multijunction solar cells that combine silicon solar cells. In a tandem solar cell, a two-junction solar cell, in which a solar cell using a CuO crystal semiconductor is used as the top cell and a solar cell using a crystalline silicon semiconductor is used as the bottom cell, the CuO crystal absorbs light in a different wavelength range from that of the crystalline silicon to generate electricity, so the solar cell using a CuO crystal semiconductor (top cell) hardly interferes with the power generation of the solar cell using a crystalline silicon semiconductor (bottom cell). As a result, the power generation efficiency of this tandem solar cell has been reported to exceed 30%, as mentioned above (for example, Non-Patent Document 1).

[0007] For these reasons, as mentioned above, attention is currently being paid to the development of solar cells that use CuO crystals as semiconductors, in particular, two-junction solar cells (tandem solar cells) that use a solar cell that uses crystalline silicon as a semiconductor for the bottom cell and a solar cell that uses CuO crystal as a semiconductor for the top cell (see, for example, Patent Document 1 and Non-Patent Document 1).

[0008] When CuO crystals are used as semiconductors, increasing the purity of the CuO crystals (specifically, reducing the amount of impurities such as metallic copper (Cu) and divalent copper oxide (CuO) in the crystals) improves the semiconductor properties and therefore the power generation efficiency. Therefore, it is desirable to form a layer (particularly a thin film) made of high-purity CuO crystals (i.e., CuO crystals with little impurity inclusion) on a conductive substrate such as ITO glass. In this application, "thin film" refers to a layer with a thickness of 10 μm or less.

[0009] Furthermore, from the viewpoint of power generation efficiency, it is desirable for CuO crystals to have high crystallinity. In this application, "high crystallinity" means a state in which there are few defects within the crystal and single crystals are tightly adhered to each other as single crystals. Therefore, it is desirable for the CuO crystal to be single crystal. In this application, "single crystal" does not necessarily have to be a perfect single crystal; it is sufficient if those skilled in the art can recognize it as substantially single crystal in terms of device operation. However, it is more desirable for it to be close to a perfect single crystal, and it is most desirable for it to be a perfect single crystal. Figure 4(A) is a specific example of "high crystallinity," and Figure 4(B) is a specific example that does not satisfy the "high crystallinity" requirement.

[0010] When CuO crystals are used as semiconductors, if the purity and crystallinity of the CuO crystals are high, the semiconductors exhibit the desired semiconductor properties. Specifically, when the photoluminescence spectrum is measured at 25°C, the semiconductors exhibit an emission spectrum with a peak at approximately 2 eV (specifically, in the range of 1.9 eV to 2.2 eV).

[0011] However, conventional methods have the problem that it is extremely difficult to form a layer (especially a thin film) of CuO crystals with high purity and high crystallinity on a conductive substrate. The reason for this is that when forming a CuO crystal layer using conventional methods, impurities such as metallic copper (Cu) and divalent copper oxide (CuO) are likely to be mixed in, making it extremely difficult to form CuO crystals with high purity and high crystallinity.

[0012] As a result, there is a problem that it is extremely difficult to obtain a semiconductor having the desired semiconductor characteristics described above (specifically, the characteristic of exhibiting an emission spectrum with a peak in the range of 1.9 eV to 2.2 eV when the photoluminescence spectrum is measured at 25°C).

[0013] Furthermore, when attempting to form a layer (especially a thin film) of high-purity CuO crystals (especially CuO single crystals) on a conductive substrate by conventional methods, it is necessary to form the layer from low-purity CuO crystals (i.e., coarsely crystalline CuO crystals) without introducing impurities such as metallic copper (Cu) or divalent copper oxide (CuO), or to remove the impurities. This requires the establishment of conditions to prevent the introduction of such impurities, which complicates the process and increases production costs. Therefore, from the viewpoints of operability and production costs, there is a need for a novel method for producing a layer of high-purity CuO crystals that is superior to conventional methods. Furthermore, there is a need for a method for producing a layer of high-purity CuO crystals that also has high crystallinity. [Means for solving the problem]

[0014] As a result of extensive research, the present inventors have found for the first time that CuO crystals having high purity and high crystallinity can be formed on a conductive substrate by forming a layer of CuO crystals on a conductive substrate, forming a layer of cuprous halide (e.g., CuCl) on the layer, heating both layers to form a molten layer of CuO and CuCl at the contact surface between the layer of CuO crystals and the layer of cuprous halide, and cooling the molten layer to recrystallize it, thereby completing the present invention.

[0015] The specific aspects of the present invention are as follows [1] to

[24] .

[0016] [1] A method for producing a layer containing Cu2O crystals on a conductive substrate, the layer exhibiting higher luminescence properties than precursor Cu2O crystals, comprising the following steps 1 to 5: Step 1: forming a layer of precursor CuO crystals on a conductive substrate; Step 2: forming a layer of cuprous halide on the layer of CuO crystals of the precursor; Step 3: forming a melt layer composed of cuprous halide and CuO by heating at a contact surface between the layer composed of cuprous halide and the layer composed of CuO crystals of the precursor, wherein the cuprous halide constituting the melt layer is formed by melting the layer composed of cuprous halide, and the CuO constituting the melt layer is formed by melting the surface of the layer composed of CuO crystals of the precursor which is in contact with the layer composed of cuprous halide; Step 4: cooling the melt layer to recrystallize the cuprous halide and CuO in the melt layer, thereby forming a layer containing CuO crystals on the conductive substrate, the layer exhibiting higher luminescence properties than the precursor CuO crystals, and further forming a layer made of cuprous halide on the layer containing CuO crystals; and Step 5: A step of removing the layer made of cuprous halide formed on the layer containing Cu2O crystals. [2] The method according to [1], wherein the heating in step 3 is carried out at a temperature of 400°C or higher and 600°C or lower. [3] The method according to [2], wherein the heating in step 3 is carried out at a temperature of 450°C or higher and 550°C or lower. [4] The method according to any one of [1] to [3], wherein the step 1 is a step of forming a layer made of CuO on a conductive substrate by any one of physical vapor deposition, chemical vapor deposition, chemical liquid deposition, thermal oxidation, aerosol deposition using aerosol, and mist deposition. [5] The method according to any one of [1] to [4], wherein step 1 is a step of forming a layer made of precursor Cu2O crystals on a conductive substrate to a thickness of 0.5 μm or more and 5 μm or less. [6] The method according to any one of [1] to [5], wherein the step 2 is a step of forming a layer of cuprous halide on the layer of CuO crystals of the precursor by sputtering or vacuum deposition. [7] The method according to any one of [1] to [6], wherein the step 2 is a step of forming a layer of cuprous halide having a thickness of 10 nm or more and 2000 nm or less on the layer of CuO crystals of the precursor. [8] The method according to [7], wherein the step 2 is a step of forming a layer of cuprous halide having a thickness of 20 nm or more and 800 nm or less on the layer of CuO crystals of the precursor. [9] The method according to any one of [1] to [8], wherein the step 4 is a step of cooling the melt layer to room temperature at a temperature decrease rate of 25°C / min to 150°C / min to recrystallize the cuprous halide and CuO in the melt layer, thereby forming, on the conductive substrate, a layer containing CuO crystals that exhibits higher luminescence properties than the precursor CuO crystals, and a layer made of cuprous halide on the layer containing CuO crystals.

[10] The method according to any one of [1] to [9], wherein the step 5 is a step of removing the layer of cuprous halide formed on the layer containing CuO crystals by washing with water.

[11] The method according to any one of [1] to

[10] , wherein the cuprous halide is cuprous chloride.

[12] The method according to any one of [1] to

[11] , wherein the layer containing the Cu2O crystals exhibits higher luminescence properties than the precursor Cu2O crystals, and has a thickness of 0.5 μm or more and 5 μm or less.

[13] The method according to any one of [1] to

[12] , wherein the layer containing Cu2O crystals exhibiting higher luminescence properties than the precursor Cu2O crystals is made of crystal grains having an average grain size of 0.05 μm or more.

[14] The method according to

[13] , wherein the layer containing Cu2O crystals exhibiting higher luminescence properties than the precursor Cu2O crystals is made of crystal grains having an average grain size of 0.08 μm or more.

[15] The method according to any one of [1] to

[14] , wherein the conductive substrate is a substrate provided with a film selected from the group consisting of a transparent conductive film, a metal film, and a laminate film of these films.

[16] The method according to

[15] , wherein the conductive substrate is a transparent substrate provided with a transparent conductive film.

[17] The method according to

[16] , wherein the layer containing the Cu2O crystals exhibiting higher luminescence properties than the precursor Cu2O crystals is a semiconductor layer for a solar cell.

[18] a conductive substrate; a semiconductor having a layer containing CuO crystals on the conductive substrate; A structure comprising: the semiconductor exhibits an emission spectrum having a peak in the range of 1.9 eV or more and 2.2 eV or less when the photoluminescence spectrum is measured at 25°C; structure.

[19] The structure according to

[18] , wherein the layer containing Cu2O crystals on the conductive substrate contains halogen atoms.

[20] The structure according to

[18] or

[19] , wherein the layer containing Cu2O crystals on the conductive substrate has a thickness of 0.5 μm or more and 5 μm or less. [twenty one] The structure according to any one of

[18] to

[20] , wherein the layer containing Cu2O crystals on the conductive substrate is made of crystal grains having an average grain size of 0.05 μm or more. [twenty two] The structure according to

[21] , wherein the layer containing Cu2O crystals on the conductive substrate is made of crystal grains having an average grain size of 0.08 μm or more. [twenty three] A solar cell comprising the structure according to any one of

[18] to

[22] . [twenty four]

[18] to

[22] , comprising a structure according to any one of

[18] to

[22] as a top cell; A silicon solar cell is provided as the bottom cell. Solar cell. [Effects of the Invention]

[0017] According to the present invention, a layer (particularly a thin film) made of CuO crystals having high purity and high crystallinity can be formed on a conductive substrate such as ITO-coated glass (so-called ITO glass) by a novel method different from conventional methods.

[0018] According to the present invention, it is possible to provide a semiconductor that exhibits an emission spectrum having a peak at about 2 eV (specifically, in the range of 1.9 eV to 2.2 eV) when the photoluminescence spectrum is measured at 25°C.

[0019] Furthermore, according to the present invention, when forming a layer made of high-purity CuO crystals on a conductive substrate such as glass, it is not necessary to set complicated conditions to prevent the incorporation of impurities (e.g., metallic copper (Cu) or divalent copper oxide (CuO)), which is required in conventional methods. Therefore, it is possible to provide a semiconductor having the desired semiconducting properties as described above in a simple manner. Therefore, a layer (particularly a thin film) made of high-purity CuO crystals is produced by a novel method that is superior to conventional methods in terms of production cost and operability. Furthermore, the layer made of high-purity CuO crystals also has high crystallinity.

[0020] The present invention also provides a semiconductor that exhibits an emission spectrum with a peak in the range of 1.9 eV to 2.2 eV when measured at 25°C. Therefore, the semiconductor can be used in a variety of applications (e.g., solar cells, electronic devices, etc.) that utilize the semiconductor's properties. When used in a solar cell, the semiconductor can be used as a single-cell solar cell or a tandem-cell solar cell (i.e., a tandem (multi-junction) solar cell). For example, the tandem solar cell can be a two-junction solar cell, using a solar cell that uses crystalline silicon as a semiconductor for the bottom cell and a solar cell that uses CuO crystal as a semiconductor for the top cell. Given that the power generation efficiency of the tandem solar cell can exceed 30%, for example, installing the two-junction solar cell on the roof of a vehicle can be expected to enable the vehicle to travel approximately 30 km / day without refueling. Thus, the present invention is expected to significantly improve the power generation efficiency of solar cells compared to conventional solar cells. [Brief explanation of the drawings]

[0021] [Figure 1] 1A and 1B are schematic diagrams showing an example of a solar cell of the present invention, in which Fig. 1A is a schematic diagram showing an example of a single-cell solar cell including the structure of the present invention, and Fig. 1B is a schematic diagram showing an example of a tandem-cell solar cell including the structure of the present invention. [Figure 2] 1 is a schematic diagram of the manufacturing process of each step from step 2 to step 5 constituting the method of the present invention. Here, the schematic diagrams of steps 3 and 4 include a schematic diagram showing the relationship between heating temperature and heating time. [Figure 3] FIG. 1 shows the results of XRD measurement of the surface of the FTO glass substrate used as the conductive substrate in this example and this comparative example, and the results of XRD measurement of the thin film formed on the FTO glass substrate that constitutes the structure produced in "Example 3." [Figure 4]4A and 4B are diagrams showing SEM images of the surface of a thin film formed on an FTO glass substrate constituting each of the structures fabricated in "Example 3" of this example and "Comparative Example 1" of this comparative example. Here, Fig. 4A is an SEM image of the surface of a thin film formed on an FTO glass substrate constituting the structure fabricated in "Example 3," and Fig. 4B is an SEM image of the surface of a thin film formed on an FTO glass substrate constituting the structure fabricated in "Comparative Example 1." [Figure 5] 1 shows the results of emission spectra measured by a photoluminescence measurement system for each structure fabricated in "Example 3" of this example and "Comparative Example 1" of this comparative example, where the vertical axis represents intensity and the horizontal axis represents photon energy. [Figure 6] FIG. 1 is a diagram showing an SEM image of the CuO single crystal powder produced in "Reference Example 1." [Figure 7] FIG. 1 is a diagram showing the results of XRD measurement of the CuO single crystal powder produced in "Reference Example 1." [Figure 8] 1 shows the results of the emission spectrum measured by a photoluminescence measurement system for the CuO single crystal powder produced in Reference Example 1. The vertical axis represents intensity, and the horizontal axis represents photon energy. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be described in detail below with reference to the accompanying drawings. However, the present invention is not limited to the matters shown in the following description, and can be implemented with appropriate modifications within the scope of the present invention.

[0023] <First embodiment of the present invention> The "method for producing a layer containing CuO crystals on a conductive substrate that exhibits higher luminescence properties than precursor CuO crystals" (also referred to as the "method of the present invention" in this application) of the present invention comprises the following steps 1 to 5. Step 1 is a step of forming a layer of precursor CuO crystals on a conductive substrate; Step 2 is a step of forming a layer of cuprous halide on the layer of CuO crystals of the precursor; Step 3 is a step of forming a melt layer composed of cuprous halide and CuO by heating at a contact surface between the layer composed of cuprous halide and the layer composed of CuO crystals of the precursor, wherein the cuprous halide constituting the melt layer is formed by melting the layer composed of cuprous halide, and the CuO constituting the melt layer is formed by melting the surface of the layer composed of CuO crystals of the precursor which is in contact with the layer composed of cuprous halide; Step 4 is a step of forming a layer containing CuO crystals on the conductive substrate, the layer exhibiting higher luminescence properties than the precursor CuO crystals, by cooling the melt layer to recrystallize the cuprous halide and CuO in the melt layer, and further forming a layer made of cuprous halide on the layer containing CuO crystals; and Step 5 is a step of removing the layer made of cuprous halide formed on the layer containing Cu2O crystals.

[0024] The "step 1" is a step of forming a layer made of precursor Cu2O crystals on a conductive substrate.

[0025] The "conductive substrate" may be any substrate that is conductive. Specific examples of conductive substrates include substrates made of conductive carbon materials such as carbon nanotubes and graphite; substrates made of metals such as titanium, stainless steel, and copper; substrates made of alloys such as aluminum; substrates made of conductive resins such as polyethylene naphthalate; and glass substrates having a conductive film formed on glass (e.g., alkali glass). The conductive film is preferably a substrate having a film selected from the group consisting of a transparent conductive film, a metal film, and a laminated film of these films, and more preferably a substrate having a transparent conductive film. Furthermore, the conductive film is preferably a thin film. The term "having a film" does not particularly limit the form of the film, as long as it has a film. For example, the substrate may be a substrate in which a thin film of a material having the same conductivity as the substrate is grown, or a substrate (e.g., glass) in which a thin conductive film is formed. Generally, a substrate (e.g., glass) in which a thin conductive film is formed is preferably a thin conductive film. Examples of transparent conductive films include thin films made of indium tin oxide (ITO), fluorine-doped tin oxide (FTO), antimony-doped tin oxide (ATO), aluminum-doped zinc oxide (AZO), zinc oxide doped with at least one element selected from the group consisting of boron, gallium, and indium (e.g., boron-doped zinc oxide (BZO), gallium-doped zinc oxide (GZO), indium zinc oxide (IZO), indium gallium zinc oxide (IGZO)), titanium-doped indium oxide (ITiO), and hydrogen-doped indium oxide (IOH). Among these, a thin film made of fluorine-doped tin oxide (FTO) is preferred.

[0026] The "precursor CuO crystal" refers to CuO crystal used to produce CuO crystal exhibiting high luminescence properties produced by the method of the present invention. Specifically, the "precursor CuO crystal" refers to CuO crystal (i.e., crude CuO crystal before recrystallization) in the stage preceding the CuO crystal exhibiting high luminescence properties produced by recrystallization in the method of the present invention (specifically, "Step 4" constituting the method of the present invention). Therefore, the purity of the "precursor CuO crystal" is lower than the purity of the CuO crystal exhibiting high luminescence properties produced by the method of the present invention.

[0027] The method for "forming a layer of precursor CuO crystals on a conductive substrate" is not particularly limited as long as it can achieve the object of the present invention, but any of physical vapor deposition, chemical vapor deposition, chemical liquid deposition, thermal oxidation, aerosol film formation using aerosol, and mist film formation is preferred. Examples of physical vapor deposition include vacuum deposition, ion plating, and sputtering, while examples of chemical vapor deposition include thermal CVD, plasma CVD, photo-CVD, and laser CVD.

[0028] The thickness of the layer made of precursor CuO crystals formed on the conductive substrate is not particularly limited as long as the object of the present invention can be achieved, but is preferably 0.5 μm or more and 5 μm or less, more preferably 1 μm or more and 4 μm or less, and even more preferably 1.5 μm or more and 3 μm or less.

[0029] The "step 2" is a step of forming a layer of cuprous halide on the layer of CuO crystals of the precursor. Here, "cuprous halide" means CuF (cuprous fluoride), CuCl (cuprous chloride), CuBr (cuprous bromide), or CuI (cuprous iodide). Therefore, the valence of copper is monovalent. As the "cuprous halide," CuCl is preferred.

[0030] The method for "forming a layer of cuprous halide on the layer of CuO crystals of the precursor" is not particularly limited as long as it can achieve the object of the present invention. Examples of the method include physical vapor deposition methods such as vacuum deposition, ion plating, and sputtering, and chemical vapor deposition methods such as thermal CVD, plasma CVD, photo-CVD, and laser CVD. However, in the present invention, which uses a monovalent "cuprous halide" (particularly CuCl) that is the same as the Cu in CuO crystals, sputtering or vacuum deposition is preferred, and sputtering is more preferred.

[0031] The thickness of the cuprous halide layer formed on the layer of CuO crystals of the precursor is not particularly limited as long as the object of the present invention can be achieved, but is preferably 10 nm or more and 2000 nm or less, more preferably 20 nm or more and 800 nm or less, and even more preferably 50 nm or more and 500 nm or less.

[0032] The "step 3" is a step of forming a melt layer composed of cuprous halide and CuO by heating at the contact surface between the layer composed of cuprous halide and the layer composed of CuO crystals of the precursor, wherein the cuprous halide constituting the melt layer is formed by melting the layer composed of cuprous halide, and the CuO constituting the melt layer is formed by melting the surface of the layer composed of CuO crystals of the precursor that is in contact with the layer composed of cuprous halide.

[0033] The "melt layer" is formed by the melting (i.e., dissolving) of the "layer made of precursor CuO crystals" in step 1 and the "layer made of cuprous halide" in step 2. Therefore, the "melt layer" is a mixed melt layer made of CuO constituting the "layer made of precursor CuO crystals" in step 1 and the cuprous halide constituting the "layer made of cuprous halide" in step 2. However, in the process of forming the "melt layer," the portion of the "layer made of precursor CuO crystals" in step 1 that melts is only the surface that comes into contact with the "layer made of cuprous halide" in step 2 (i.e., the surface of the "layer made of precursor CuO crystals" in step 1 that comes into contact with the "layer made of cuprous halide" in step 2). In other words, in the process of forming the "melt layer," the portion of the "layer made of precursor CuO crystals" in step 1 that melts is limited. For example, the contact surface of the "layer made of precursor CuO crystals" in contact with the "conductive substrate" in step 1 does not melt during the formation of the "melt layer," but remains in its original state (i.e., the state when the layer made of precursor CuO crystals is formed on the conductive substrate in step 1). Thus, the CuO constituting the "melt layer" is formed by melting a portion of the "layer made of precursor CuO crystals" in step 1 (specifically, the surface of the "layer made of precursor CuO crystals" in step 1 that is in contact with the "layer made of cuprous halide" in step 2). On the other hand, the cuprous halide constituting the "melt layer" is formed by melting the entire "layer made of cuprous halide" in step 2. The cuprous halide constituting the "layer made of cuprous halide" in step 2, which is one component of the "melt layer," functions as a flux. In the process of forming the "melt layer," the thickness of the melted portion of the "layer made of precursor CuO crystals" in step 1 is not particularly limited as long as the object of the present invention can be achieved. However, from the viewpoint of preventing all of the precursor CuO crystals from dissolving, the thickness is preferably in the range of 1 / 500 to 9 / 10, and more preferably 1 / 250 to 1 / 3, of the thickness of the "layer made of precursor CuO crystals" in step 1 from the contact surface with the "layer made of cuprous halide" in step 2.

[0034] As described above, the "melt layer" is formed by melting (i.e., dissolving) the "layer made of precursor CuO crystals" in step 1 and the "layer made of cuprous halide" in step 2. In this case, both the melt obtained by melting (i.e., dissolving) the "layer made of precursor CuO crystals" in step 1 and the melt obtained by melting (i.e., dissolving) the "layer made of cuprous halide" in step 2 have a high enough viscosity to form a layer. Therefore, a layer (specifically, a (mixed) melt layer made of cuprous halide and CuO) is formed directly by the melting.

[0035] The "melt layer" is formed by heating, and the heating temperature is preferably 400°C or higher and 600°C or lower, and more preferably 450°C or higher and 550°C or lower.

[0036] The "step 4" is a step of forming a layer containing CuO crystals on the conductive substrate, the layer exhibiting higher luminescence properties than the precursor CuO crystals, by cooling the melt layer to recrystallize the cuprous halide and CuO in the melt layer, and further forming a layer made of cuprous halide on the layer containing CuO crystals.

[0037] The "cooling" is preferably performed slowly. Specifically, the "melt layer" in step 3 is preferably cooled to room temperature at a rate of 150°C / min or less, more preferably 100°C / min or less, and even more preferably 50°C / min or less.

[0038] By cooling the melt layer to recrystallize the cuprous halide and CuO in the melt layer, the "layer made of precursor CuO crystals" formed on the "conductive substrate" in step 1 becomes a layer containing CuO crystals that exhibits higher luminescence characteristics than the "layer made of precursor CuO crystals." In other words, by cooling the melt layer to recrystallize the cuprous halide and CuO in the melt layer, the "layer made of precursor CuO crystals" formed on the "conductive substrate" in step 1 becomes a layer containing CuO crystals with high purity and crystallinity, rather than a layer made of coarsely crystalline CuO crystals. In this application, CuO crystals with high purity and high crystallinity are also referred to as "high-quality CuO crystals."

[0039] Furthermore, by cooling the melt layer to recrystallize the cuprous halide and CuO in the melt layer, the "layer made of precursor CuO crystals" formed on the "conductive substrate" in step 1 becomes a layer containing CuO crystals that exhibit higher luminescence properties than the "layer made of precursor CuO crystals," and a layer made of cuprous halide is formed again on the layer containing CuO crystals that exhibits higher luminescence properties.

[0040] The "step 5" is a step of removing the layer made of cuprous halide formed on the layer containing CuO crystals. That is, the layer made of cuprous halide formed again in the step 4 is removed in the step 5.

[0041] The method of "removal" is not particularly limited as long as the object of the present invention can be achieved. Usually, removal by washing with water is preferred.

[0042] The thickness of the layer containing CuO crystals exhibiting high luminescence properties produced by the method of the present invention (i.e., the layer containing CuO crystals produced through steps 1 to 5) is preferably 0.5 μm or more and 5 μm or less, more preferably 1 μm or more and 4 μm or less, and even more preferably 1.5 μm or more and 3 μm or less.

[0043] The average particle size of the crystal grains constituting the layer containing CuO crystals exhibiting high luminescence properties, produced by the method of the present invention, is preferably as large as possible, specifically, preferably 0.05 μm or more, more preferably 0.08 μm or more, and even more preferably 0.4 μm or more.

[0044] As described above, the method of the present invention is a method for forming a layer (particularly a thin film) made of high-purity CuO crystals simply by going through steps 1 to 5. Therefore, it is not necessary to set complicated conditions for removing impurities (e.g., metallic copper (Cu) and divalent copper oxide (CuO)) from a layer made of low-purity CuO crystals (i.e., coarsely crystalline CuO crystals), which is required in conventional methods. Therefore, the method of the present invention is a method that is excellent in terms of production cost and operability when producing a layer made of high-purity CuO crystals. The method of the present invention is a method that provides a layer made of CuO crystals that has high crystallinity in addition to high purity.

[0045] <Second embodiment of the present invention> A "structure" according to a second embodiment of the present invention (the structure is also referred to herein as the "structure of the present invention") comprises a conductive substrate and a semiconductor having a layer containing CuO crystals on the conductive substrate, wherein the semiconductor exhibits an emission spectrum having a peak in the range of 1.9 eV to 2.2 eV when the photoluminescence spectrum is measured at 25° C. With regard to the terms used in <Second embodiment of the present invention> in this section, the explanations in the <First embodiment of the present invention> section apply as they are to terms already explained in the <First embodiment of the present invention> section, unless otherwise specified.

[0046] The "layer comprising CuO crystals on the conductive substrate" constituting the structure of the present invention may contain halogen atoms as long as the object of the present invention can be achieved. Examples of such halogen atoms include halogens constituting the cuprous halide used in the method of the present invention. As described above, when CuCl, which is preferred as the cuprous halide, is used, the halogen atoms are Cl atoms.

[0047] The thickness of the "layer containing CuO crystals on the conductive substrate" constituting the structure of the present invention is preferably 0.5 μm or more and 5 μm or less, more preferably 1 μm or more and 4 μm or less, and even more preferably 1.5 μm or more and 3 μm or less.

[0048] The "layer containing CuO crystals on the conductive substrate" constituting the structure of the present invention preferably has a larger average particle size of the crystal particles constituting the layer. Specifically, the average particle size is preferably 0.05 μm or more, more preferably 0.08 μm or more, and even more preferably 0.2 μm or more.

[0049] <Third embodiment of the present invention> A "solar cell" according to a third embodiment of the present invention (the solar cell is also referred to herein as a "solar cell of the present invention") comprises a "structure" according to the second embodiment of the present invention. With regard to the terms used in <Second embodiment of the present invention> in this section, the explanations in <First embodiment of the present invention> and <Second embodiment of the present invention> apply to terms already explained in the above sections <First embodiment of the present invention> and <Second embodiment of the present invention>, unless otherwise specified.

[0050] The solar cell of the present invention may be, for example, a single-cell solar cell having the structure of the present invention as shown in Fig. 1(A), or a tandem-cell solar cell (i.e., a tandem (multi-junction) solar cell). For example, an example of a tandem solar cell of a two-junction solar cell as shown in Fig. 1(B) is a two-junction solar cell having a structure in which the "structure" according to the second embodiment of the present invention is the top cell and a silicon solar cell is the bottom cell.

[0051] In a single-cell solar cell equipped with the structure of the present invention as shown in Figure 1(A), when irradiated with light, electrons and holes generated at the junction between the n-type semiconductor and the p-type semiconductor (specifically, a layer made of Cu2O crystals) flow to the transparent conductive film of the counter electrode, generating a voltage, and the cell operates. Therefore, improving the crystallinity of the surface of the layer made of Cu2O crystals in contact with the n-type semiconductor is an important factor that determines the performance of the solar cell.

[0052] In a tandem solar cell having the structure of the present invention as shown in Figure 1(B), a solar cell using a CuO crystalline semiconductor is used as the top cell, and a solar cell using a semiconductor with a smaller band gap than CuO (e.g., a crystalline silicon semiconductor) is used as the bottom cell. Utilizing this structure, when power is generated in the top cell, the short-wavelength light transmitted through the top cell is used to generate power in the bottom cell as well. As a result, the total power generation amount of the top and bottom cells can be secured with the same area.

[0053] There are no particular limitations on the conditions not specified in this application as long as the object of the present invention can be achieved. [Example]

[0054] Next, embodiments of the present invention will be described in more detail, but the embodiments of the present invention are not limited to the following examples as long as they do not depart from the gist of the invention.

[0055] <Preparation of a Layer Containing CuO Crystals Exhibiting Higher Luminescence Properties Than Precursor CuO Crystals on a Conductive Substrate and a Structure Including the Layer Containing CuO Crystals> Example 1: In this example, a layer containing CuO crystals that exhibits higher luminescence properties than precursor CuO crystals was formed on a conductive substrate, and a structure including the layer containing CuO crystals was fabricated according to the following steps 1 to 5. In "Example 1" of the present application, a schematic diagram of the manufacturing process for each step from step 2 to step 5 is shown in Figure 2.

[0056] Step 1: forming a layer of precursor CuO crystals on a conductive substrate (not shown) A conductive substrate (simply referred to as "substrate" in Table 1) was an FTO glass substrate (Huanyu FTO glass (sheet resistance: 15 ohm / sq)) with a thin FTO (fluorine-doped tin oxide) transparent conductive film deposited on a glass plate (hereinafter, this glass substrate will be simply referred to as "FTO glass substrate" and simply referred to as "FTO" in Table 1). A 0.7 μm-thick thin film composed of CuO crystals was formed (i.e., deposited) on the FTO glass substrate (specifically, on the FTO transparent conductive film) using a 1-inch (=2.54 cm) Cu target in an RF (radio frequency) magnetron sputtering system (Pascal) under the following conditions: a distance of 30 mm between the Cu target and the FTO glass substrate, a total pressure of 1.0 Pa, an input power (RF output) of 27 W, an argon gas flow with an oxygen concentration of 0.8%, and room temperature (25°C) (see Example 1 in Table 1). For convenience, this layer (thin film) made of CuO crystals is also referred to simply as the "CuO layer" or "layer made of precursor CuO crystals" in this application. The thickness of the thin film made of CuO crystals was determined from the weight change before and after film formation using a microbalance (ME5 manufactured by Sartrius). The conditions used in step 1 and the thickness of the thin film formed are summarized in "Example 1" in Table 1.

[0057] Step 2: forming a layer of cuprous halide on the layer of CuO crystals of the precursor (see step 2 in FIG. 2). After performing step 1, a thin CuCl layer with a thickness of 800 nm was formed (i.e., deposited) on the thin film of CuO crystals (CuO layer) formed in step 1 using a homemade 1-inch (=2.54 cm) CuCl target in the RF magnetron sputtering apparatus under the following conditions: a distance between the CuCl target and the FTO glass substrate of 30 mm, an argon pressure of 1.8 Pa, an input power (RF output) of 11 W, and room temperature (25°C). For convenience, this CuCl layer (thin film) is also referred to simply as the "CuCl layer" in this application. The thickness of the formed CuCl thin film was determined from the weight change before and after deposition using a microbalance (ME5 manufactured by Sartrius). The operating conditions and the thickness of the deposited thin film in step 2 are summarized in "Example 1" in Table 1.

[0058] Step 3: A step of forming a melt layer composed of cuprous halide and CuO by heating at the contact surface between the layer composed of cuprous halide and the layer composed of CuO crystals of the precursor, wherein the cuprous halide constituting the melt layer is formed by melting the layer composed of cuprous halide, and the CuO constituting the melt layer is formed by melting the surface of the layer composed of CuO crystals of the precursor that is in contact with the layer composed of cuprous halide (see Step 3 in FIG. 2). After step 2, the FTO glass substrate prepared in step 2 (specifically, the FTO glass substrate having the CuCl thin film (CuCl layer) formed in step 2 on the thin film (CuO layer) made of CuO crystals formed in step 1) was placed in an RTA (Rapid Thermal Annealing) device (MILA-5050, manufactured by Advance Riko Co., Ltd.), and the sample chamber in the RTA was heated to 2 × 10°C using a turbomolecular pump. -2After the pressure was reduced to 1 Pa, argon gas was introduced up to 1 atmosphere. Then, the FTO glass substrate prepared in step 2 placed in the RTA apparatus was heated to 470°C at a heating rate of 15.7°C / sec (15.7°C / sec). After that, the temperature of 470°C was maintained for 3 minutes, melting the thin film (CuO layer) made of CuO crystals formed in step 1 and the CuCl thin film (CuCl layer) formed in step 2 on the thin film made of CuO crystals. During this process, only the surface of the thin film (CuO layer) made of CuO crystals formed in step 1 that was in contact with the CuCl thin film (CuCl layer) formed in step 2 on the thin film made of CuO crystals was melted, and the CuCl thin film (CuCl layer) formed in step 2 was completely melted. As a result, a melt layer consisting of CuO and CuCl was formed at the contact surface between the thin film (CuO layer) made of CuO crystals formed in step 1 and the CuCl thin film (CuCl layer) formed on the thin film (CuO layer) made of CuO crystals in step 2. For convenience, this melt layer is also referred to as a "CuO-CuCl mixed melt layer" in this application. The heat treatment conditions in step 3 are summarized in "Example 1" in Table 1.

[0059] Step 4: A step of forming a layer containing CuO crystals on the conductive substrate, which exhibits higher luminescence properties than the precursor CuO crystals, by cooling the melt layer to recrystallize the cuprous halide and CuO in the melt layer, and further forming a layer of cuprous halide on the layer containing CuO crystals (see step 4 in Figure 2). After performing step 3, the molten layer (CuO-CuCl mixed molten layer) formed in step 3 was naturally cooled to room temperature at a temperature drop rate of 100°C / min (100°C / min). This caused CuO crystals with high purity and high crystallinity (high-quality CuO crystals) to re-grow (specifically, recrystallize) on the melted surface of the thin film (CuO layer) made of CuO crystals in step 3, thereby forming a layer made of high-quality CuO crystals. For convenience, this layer made of high-quality CuO crystals is also referred to as the "high-quality CuO layer (regrowth layer)" in this application. In this way, a thin film containing CuO crystals (high-quality CuO crystals) with higher purity and higher crystallinity than the thin film made of CuO crystals formed on the FTO glass substrate in step 1 was formed (i.e., deposited). A thin film consisting of CuCl crystals is regrown on the thin film containing high-quality CuO crystals. For convenience, this layer (thin film) consisting of CuCl crystals is also referred to as the "CuCl layer (regrowth layer)" in this application. The heat treatment conditions in step 4 are summarized in "Example 1" in Table 1.

[0060] Step 5: removing the layer made of cuprous halide formed on the layer containing CuO crystals (see step 5 in FIG. 2). Finally, the FTO glass substrate placed in the RTA apparatus (specifically, the FTO glass substrate having a CuCl thin film (CuCl layer (regrowth layer)) on the high-quality CuO layer (regrowth layer) after step 4) was removed, and the CuCl thin film was dissolved and removed with distilled water. In this way, a structure was fabricated on the FTO glass substrate, comprising a thin film containing CuO crystals (high-quality CuO crystals) with higher purity and crystallinity than the CuO crystals in step 1. The film thickness of the fabricated thin film containing high-quality CuO crystals was confirmed to be approximately twice the thickness of the thin film consisting of CuCl formed in step 2. The fact that the fabricated thin film containing CuO crystals had higher purity and crystallinity than the CuO crystals in step 1 (i.e., was of higher quality) was confirmed by XRD, SEM photographs, and emission spectrum measurements, as described below.

[0061] Examples 2 to 7: In these examples, six more types of thin films containing CuO crystals that exhibit higher luminescence properties than the precursor CuO crystals were fabricated on a conductive substrate, and six more structures including the thin films were fabricated using a method similar to that of Example 1 above. These were designated Example 2 to Example 7, respectively. The use conditions and heat treatment conditions in each of steps 1 to 4 were as shown in Table 1 for Example 2 to Example 7. In Example 2 to Example 7, the film thickness of the thin film made of CuO crystals formed in step 1 and the film thickness of the thin film made of CuCl formed in step 2 were appropriately changed as shown in Table 1. Conditions not listed in Table 1 (for example, the conductive substrate and apparatus used) were the same as those in Example 1 of this example.

[0062] Comparative Examples 1 and 2: For these comparative examples, two types of FTO glass substrates on which a layer (i.e., a thin film) made of CuO crystals was formed were produced by performing only the step 1 used in the present examples (i.e., "Example 1" to "Example 7"). These were designated "Comparative Example 1" and "Comparative Example 2," respectively. The conditions for using step 1 performed in "Comparative Example 1" and "Comparative Example 2" were as shown in "Comparative Example 1" and "Comparative Example 2" in Table 1. Conditions not listed in Table 1 (e.g., the conductive substrate and equipment used) were the same as those used in the present examples. The difference between "Comparative Example 1" and "Comparative Example 2" is the film thickness of the thin film made of CuO crystals formed, as shown in Table 1.

[0063] [Table 1]

[0064] <Characteristics evaluation> The properties of each structure fabricated in this example (i.e., each structure of "Example 1" to "Example 7") and each structure fabricated in this comparative example (i.e., each structure of "Comparative Example 1" and "Comparative Example 2") were evaluated by evaluating the surface properties of the thin film deposited on the FTO glass substrate constituting the structure and evaluating the luminescence properties of each structure fabricated in this example and this comparative example. The surface properties of the thin film were evaluated based on both measurements using an X-ray diffraction (XRD) device (Miniflex 600, manufactured by RIGAKU) and SEM images obtained using a scanning electron microscope (SEM). The luminescence properties of the structures were evaluated based on the results of emission spectrum measurements using a photoluminescence measurement system (measurement unit, manufactured by Lucile Co., Ltd.) under conditions of excitation light from a CW laser (457 nm), a spot diameter of 0.5 mm, an irradiation intensity of 50 mW, and room temperature (25 °C). The crystal grain size of the thin film deposited on the FTO glass substrate constituting each structure fabricated in this example and this comparative example was also measured based on the results of SEM images obtained using a scanning electron microscope (SEM). Specifically, the average particle diameter was determined by drawing a diagonal line on the SEM image, determining the particle diameter of each particle on the line in the direction of the longest diameter, and then dividing the sum of the particle diameters by the total number of particles measured to obtain an average value.

[0065] Surface characterization of thin films (1) Examples ("Example 1" to "Example 7") As an example of the results of measurement using the X-ray diffraction (XRD) device, FIG. 3 shows the results of measurement of the surface of a thin film formed on an FTO glass substrate constituting the structure fabricated in "Example 3" of this example. The results in FIG. 3 confirmed that the structure was composed solely of a single phase of CuO crystals, with no detectable impurities such as metallic copper (Cu) or divalent copper oxide (CuO). Similarly, although not shown, it was confirmed that the structures of "Example 1" to "Example 2" and "Example 4" to "Example 7" also yielded XRD measurement results similar to those of the structure fabricated in "Example 3." Therefore, in all of the structures of "Example 1" to "Example 2" and "Example 4" to "Example 7," as in "Example 3," the surface of the thin film formed on the FTO glass substrate was composed solely of a single phase of CuO crystals, with no detectable impurities such as metallic copper (Cu) or divalent copper oxide (CuO), and was therefore found to be composed of high-purity CuO crystals. From these results, it was determined that the surface of the thin film formed on the FTO glass substrate of each of the structures fabricated in "Example 1" to "Example 7" of this example was composed of CuO crystals.

[0066] As an example of the results of SEM imaging, FIG. 4(A) shows an SEM image of the surface of a thin film formed on an FTO glass substrate constituting the structure fabricated in Example 3 of this example. From the SEM image in FIG. 4(A), it was confirmed that the surface of the thin film formed on the FTO glass substrate constituting the structure fabricated in Example 3 was in a mirror-like state, had few intracrystalline defects, and the single crystals were in close contact with each other as single crystals. Although not shown, it was also confirmed that the surfaces of the thin films formed on the FTO glass substrates constituting the structures of other Examples (i.e., Examples 1 to 2 and Examples 4 to 7) obtained SEM images similar to those of the structure fabricated in Example 3. Therefore, it was confirmed that the surfaces of the thin films formed on the FTO glass substrates constituting the structures of Examples 1 to 2 and Examples 4 to 7 were in a mirror-like state, had few intracrystalline defects, and the single crystals were in close contact with each other as single crystals, similar to the structure fabricated in Example 3. Therefore, it was found that the surface of the thin film formed on the FTO glass substrate constituting each of the structures of "Example 1" to "Example 7" of this example had high crystallinity.

[0067] (2) Comparative Examples (“Comparative Example 1” and “Comparative Example 2”) As a result of measurement using the X-ray diffraction (XRD) device, although not shown, the surface of the thin film formed on the FTO glass substrate was identified as Cu2O.

[0068] On the other hand, as an example of the results of SEM imaging, an SEM image of the surface of a thin film deposited on an FTO glass substrate constituting the structure fabricated in "Comparative Example 1," which is this comparative example, is shown in Figure 4(B). From the SEM image in Figure 4(B), it was confirmed that the surface of the thin film deposited on the FTO glass substrate constituting the structure fabricated in "Comparative Example 1," while mirror-finished, had many intracrystalline defects, indicating that the individual crystals existed as polycrystalline aggregates, which are single crystals connected to each other. In other words, it was confirmed that the SEM image in Figure 4(A) (i.e., the SEM image of the surface of a thin film deposited on an FTO glass substrate constituting the structure fabricated in "Example 3"), in which there were few intracrystalline defects and the single crystals were closely attached to each other as single crystals, and did not exist as polycrystalline aggregates, which are single crystals connected to each other, was completely different. Regarding the surface of a thin film deposited on an FTO glass substrate constituting the structure in "Comparative Example 2," which is this comparative example, it was confirmed that an SEM image similar to that of the structure fabricated in "Comparative Example 1" was obtained, although not shown. Therefore, it was found that the surface of the thin film formed on the FTO glass substrate constituting the structure of "Comparative Example 2" was in a mirror-like state, like the structure produced in "Comparative Example 1," but had many defects within the crystals, and the individual crystals existed as polycrystals, which were aggregates of single crystals connected to each other. Therefore, it was confirmed that the surfaces of the thin films formed on the FTO glass substrates constituting the structures produced in "Comparative Example 1" and "Comparative Example 2" of this comparative example did not have high crystallinity, unlike the surfaces of the thin films formed on the FTO glass substrates constituting the structures produced in "Example 1" to "Example 7" of this example.

[0069] In the comparative examples (i.e., "Comparative Example 1" and "Comparative Example 2") in which only step 1 was performed, the surface of the thin film deposited on the FTO glass substrate constituting each structure consisted of visually rough CuO crystals (specifically, CuO polycrystals) as shown in Figure 4(B). In contrast, the surfaces of the thin films deposited on the FTO glass substrates constituting each structure fabricated in the present examples (i.e., "Example 1" to "Example 7") in which steps 2 through 5 were performed in addition to step 1 consisted of visually clean CuO crystals. Thus, this example demonstrated that the surface of the thin film deposited on the FTO glass substrate fabricated in the comparative examples in which only step 1 was performed can be modified to a thin film consisting of CuO crystals with high crystallinity. This modification facilitates the lamination of an n-type semiconductor on the thin film when the thin film consisting of CuO crystals is used as a p-type semiconductor layer, and is understood to further improve the semiconductor properties of semiconductor devices using CuO crystals.

[0070] -Emission characteristics evaluation (1) Examples ("Example 1" to "Example 7") As an example of the photoluminescence spectrum measured by the photoluminescence measurement system, the emission spectrum of the structure produced in "Example 3" of this example is shown in FIG. 5. As shown in FIG. 5, it was confirmed that the structure produced in "Example 3" exhibited an emission spectrum having a peak at 2.1 eV, which is in the range of 1.9 eV to 2.2 eV. Although not shown, it was also confirmed that the structures of the other examples (i.e., "Example 1" to "Example 2" and "Example 4" to "Example 7") exhibited emission spectra having a peak at 2.1 eV, which is in the range of 1.9 eV to 2.2 eV. Thus, when the photoluminescence spectrum of each structure of this example (i.e., "Example 1" to "Example 7") was measured at 25°C, it exhibited an emission spectrum in the range of 1.9 eV to 2.2 eV, and therefore it was found that it is effective as a semiconductor having CuO crystals (particularly a semiconductor for solar cells).

[0071] (2) Comparative Examples (“Comparative Example 1” and “Comparative Example 2”) As an example of the results of measuring the photoluminescence spectrum using the photoluminescence measurement system, FIG. 5 also shows the emission spectrum of the structure produced in "Comparative Example 1," which is this comparative example. As shown in FIG. 5, it was confirmed that the structure produced in "Comparative Example 1" did not exhibit an emission spectrum in the range of 1.9 eV to 2.2 eV. Although not shown, it was also confirmed that the structure produced in "Comparative Example 2," which is this comparative example, did not exhibit an emission spectrum in the range of 1.9 eV to 2.2 eV, similar to the structure produced in "Comparative Example 1." As such, when the photoluminescence spectra of the structures produced in "Comparative Example 1" and "Comparative Example 2" of this comparative example were measured at 25°C, they did not exhibit an emission spectrum in the range of 1.9 eV to 2.2 eV, and therefore were found to be ineffective as semiconductors containing CuO crystals (particularly semiconductors for solar cells).

[0072] Taking into account the results of the "Evaluation of Surface Properties of Thin Films" section above, it was found that when a layer made of Cu2O crystals with high purity and high crystallinity is formed on a conductive substrate such as an FTO glass substrate, as in the structures produced in "Example 1" to "Example 7" of this example, an emission spectrum having a peak at 2.1 eV (i.e., an emission peak) within the range of 1.9 eV to 2.2 eV was exhibited. Therefore, it was found that the structures produced in "Example 1" to "Example 7" of this example are effective as semiconductors containing Cu2O crystals (particularly semiconductors for solar cells).

[0073] The measurement results of the "surface property evaluation of thin film" and the "emission property evaluation" in the present examples (i.e., "Example 1" to "Example 7") and the present comparative examples (i.e., "Comparative Example 1" and "Comparative Example 2") are shown in Table 2. Table 2 also shows the results of the measurement, as described above, of the crystal grain size of the thin film formed on the FTO glass substrate constituting each structure produced in the present examples and the present comparative examples.

[0074] [Table 2]

[0075] <Preparation of CuO single crystal powder with high luminescence properties and evaluation of the properties of the powder> Cu2O single crystal powder exhibiting high luminescence properties was prepared by the following method. Various crucibles made of any one of MgO, Al2O3, and WC compounds were prepared, and the raw material Cu2O powder and CuCl powder were placed in each crucible and heated to 500°C. The raw material Cu2O powder was melted in the CuCl by heating, and then slowly cooled and recrystallized to produce Cu2O single crystal powder. Four types of Cu2O single crystal powder were produced, designated "Reference Example 1" to "Reference Example 4." The crystal shape of each of the produced Cu2O single crystal powders was evaluated by observing SEM images using a scanning electron microscope (SEM). Table 3 shows the amount of CuO powder, amount of CuCl powder, crucible, melting temperature (°C), and slow cooling rate (°C / hour) used in each of the CuO single crystal powders produced in "Reference Example 1" to "Reference Example 4," as well as an evaluation of the crystal shape of each of the produced CuO single crystal powders.

[0076] [Table 3]

[0077] The CuO single crystal powders prepared in each Reference Example were identified using an X-ray diffraction (XRD) device (Miniflex 600, manufactured by RIGAKU) and their emission spectra were measured using a photoluminescence measurement system (measurement unit manufactured by Lucile Corporation). The measurement conditions were the same as those used in the present Examples (i.e., Examples 1 to 7). As an example of the measurement results, the SEM image of the CuO single crystal powder prepared in Reference Example 1 is shown in Figure 6, the XRD measurement results are shown in Figure 7, and the emission spectrum measured using the photoluminescence measurement system is shown in Figure 8. The results in Figure 6 confirmed that the CuO single crystal powder prepared in Reference Example 1 had few intracrystalline defects and that the single crystals were in close contact with each other as single crystals. Furthermore, the results in Figure 7 confirmed that the powder consisted of only a single phase of CuO crystals, and that no impurities such as metallic copper (Cu) or divalent copper oxide (CuO) were observed. Furthermore, from the results in Figure 8, it was confirmed that the emission spectrum had a peak in the range of 1.9 eV to 2.2 eV. Although not shown, it was confirmed that the SEM images, XRD measurements, and XRD measurement results for each of the CuO single crystal powders in other reference examples (i.e., "Reference Example 2" to "Reference Example 4") were similar to those of the CuO single crystal powder produced in "Reference Example 1." Incidentally, the SEM image of "Reference Example 4" showed a slightly rounded crystal shape, but few defects within the crystal, and the single crystals were in close contact with each other as single crystals. It was also confirmed that CuO crystals were not produced when the crucible was made of a SiN compound different from those used in Reference Examples 1 to 4.

[0078] Therefore, it was found that all of the CuO single crystal powders of "Reference Example 1" to "Reference Example 4" were CuO crystal powders with high purity and high crystallinity. This is understood to be because, when producing the CuO single crystal powders of "Reference Example 1" to "Reference Example 4," all of the Cu in the melt layer was monovalent. Therefore, all of the CuO single crystal powders of "Reference Example 1" to "Reference Example 4" were produced by crystal growth from a monovalent Cu melt layer. This makes it difficult for impurities such as metallic copper (Cu) and divalent copper oxide (CuO) to be produced, and makes it easier to produce CuO single crystals with high purity. The high-purity CuO single crystal powders of "Reference Example 1" to "Reference Example 4" are expected to be used, for example, as raw materials for forming thin films made of high-quality CuO crystals by electrophoresis (i.e., thin films made of CuO crystals with high purity and high crystallinity). [Industrial Applicability]

[0079] According to the present invention, a novel and simple method different from conventional methods can be used to form a layer (specifically, a thin film) of CuO crystals with high purity and high crystallinity on a conductive substrate such as ITO glass or FTO glass. This method provides a semiconductor that exhibits an emission spectrum with a peak in the range of 1.9 eV to 2.2 eV when measured by photoluminescence at 25°C. This provides a semiconductor with CuO crystals that is excellent in terms of manufacturing cost and operability, and is expected to be industrially applicable to various applications where the use of such semiconductors is desired (e.g., solar cells, electronic devices, etc.). In particular, given the characteristics of this semiconductor, it is expected to be industrially applicable as a solar cell, where further improvement in power generation efficiency is desired. In particular, it is expected to be industrially applicable as a tandem (multi-junction) solar cell, for example, a two-junction solar cell in which a solar cell using crystalline silicon as a semiconductor is used as the bottom cell and a solar cell using CuO crystal as a semiconductor is used as the top cell.

Claims

1. The method includes the following steps 1 to 5: ２ Cu exhibits higher luminescence properties than O crystals ２ A method for producing a layer containing O crystals. Step 1: Precursor Cu on a conductive substrate ２ forming a layer made of O crystals; Step 2: Cu of the precursor ２ forming a layer of cuprous halide on the layer of O crystals; Step 3: The layer of the cuprous halide and the Cu of the precursor ２ By heating, copper halide and Cu are formed on the contact surface with the layer made of O crystals. ２ a step of forming a melt layer comprising copper(I) halide and copper(I) O, wherein the copper(I) halide constituting the melt layer is formed by melting the copper(I) halide layer, and the copper(I) halide constituting the melt ２ O is Cu of the precursor in contact with the layer of cuprous halide. ２ forming a layer of O crystals by melting the surface of the layer; Step 4: Cooling the melt layer to remove the cuprous halide and Cu in the melt layer. ２ By recrystallizing O, the precursor Cu is deposited on the conductive substrate. ２ Cu exhibits higher luminescence properties than O crystals ２ A layer containing O crystals is formed, and the Cu ２ further forming a layer of cuprous halide on the layer containing O crystals; and Step 5: The Cu ２ a step of removing the layer made of cuprous halide formed on the layer containing O crystals.

2. The method according to claim 1, wherein the heating in step 3 is carried out at a temperature of 400°C or higher and 600°C or lower.

3. The method according to claim 2, wherein the heating in step 3 is carried out at a temperature of 450°C or higher and 550°C or lower.

4. The step 1 is to deposit Cu on a conductive substrate by any one of physical vapor deposition, chemical vapor deposition, chemical liquid phase deposition, thermal oxidation, aerosol film formation using aerosol, and mist film formation. ２ The method according to claim 1 , wherein the layer is made of O.

5. The step 1 is a step of depositing a precursor Cu on a conductive substrate to a thickness of 0.5 μm or more and 5 μm or less. ２ The method according to claim 1 , wherein the layer is made of O crystals.

6. The step 2 is to deposit Cu of the precursor by sputtering or vacuum deposition. ２ The method according to any one of claims 1 to 5, wherein a layer of cuprous halide is formed on a layer of O crystals.

7. The step 2 is to ２ 7. The method according to claim 1, further comprising forming a layer of cuprous halide having a thickness of 10 nm to 2000 nm on a layer of O crystals.

8. The step 2 is to ２ 8. The method according to claim 7, further comprising forming a layer of cuprous halide having a thickness of 20 nm to 800 nm on the layer of O crystals.

9. The step 4 comprises cooling the melt layer to room temperature at a temperature decreasing rate of 25° C. / min to 150° C. / min to separate the cuprous halide and Cu in the melt layer. ２ By recrystallizing O, the precursor Cu is deposited on the conductive substrate. ２ Cu exhibits higher luminescence properties than O crystals ２ The layer containing O crystals and the Cu ２ 9. The method according to claim 1, further comprising forming a layer of cuprous halide on a layer containing O crystals.

10. The step 5 is ２ The method according to any one of claims 1 to 9, further comprising the step of removing the layer of cuprous halide formed on the layer containing O crystals by washing with water.

11. 11. The method of any one of claims 1 to 10, wherein the cuprous halide is cuprous chloride.

12. The precursor Cu ２ The Cu crystal exhibits higher luminescence properties than O crystal. ２ The method according to claim 1 , wherein the layer containing O crystals has a thickness of 0.5 μm or more and 5 μm or less.

13. The precursor Cu ２ The Cu crystal exhibits higher luminescence properties than O crystal. ２ The method according to claim 1 , wherein the layer containing O crystals is made of crystal grains having an average grain size of 0.05 μm or more.

14. The precursor Cu ２ The Cu crystal exhibits higher luminescence properties than O crystal. ２ The method according to claim 13 , wherein the layer containing O crystals is made of crystal grains having an average grain size of 0.08 μm or more.

15. The method according to claim 1 , wherein the conductive substrate is a substrate comprising a film selected from the group consisting of a transparent conductive film, a metal film, and a laminate film of these films.

16. The method of claim 15 , wherein the conductive substrate is a transparent substrate provided with a transparent conductive film.

17. The precursor Cu ２ The Cu crystal exhibits higher luminescence properties than O crystal. ２ The method of claim 16 , wherein the layer containing O crystals is a semiconductor layer for a solar cell.

18. a conductive substrate; Cu is deposited on the conductive substrate. ２ a semiconductor having a layer containing O crystals; A structure comprising: the semiconductor exhibits an emission spectrum having a peak in the range of 1.9 eV to 2.2 eV when the photoluminescence spectrum is measured at 25°C; structure.

19. Cu is deposited on the conductive substrate. ２ 20. The structure of claim 18, wherein the layer containing O crystals contains halogen atoms.

20. Cu is deposited on the conductive substrate. ２ 20. The structure according to claim 18 or 19, wherein the layer containing O crystals has a thickness of 0.5 μm or more and 5 μm or less.

21. Cu is deposited on the conductive substrate. ２ 21. The structure according to claim 18, wherein the layer containing O crystals is made of crystal grains having an average grain size of 0.05 μm or more.

22. Cu is deposited on the conductive substrate. ２ 22. The structure according to claim 21, wherein the layer containing O crystals is made of crystal grains having an average grain size of 0.08 μm or more.

23. A solar cell comprising a structure according to any one of claims 18 to 22.

24. 23. A structure according to any one of claims 18 to 22 as a top cell, A silicon solar cell is provided as the bottom cell. Solar cell.

Images