Photoelectric conversion element and method for manufacturing photoelectric conversion element

Abstract

A photoelectric conversion element (10) includes a photoelectric conversion section (12) and a proton beam shielding layer (13) formed on the photoelectric conversion section (12) to shield the photoelectric conversion section (12) from a proton beam. The product of the electron density and the film thickness of the proton beam shielding layer (13) is 5 x 10 20 (cm ‑2 ) or more.

Description

Technical Field

[0001] This invention relates to photoelectric conversion elements and methods for manufacturing photoelectric conversion elements. Background Technology

[0002] Solar cells, as one of the photoelectric conversion elements, have long been used as power supply units for satellites and other devices operating in space. It is known that in solar cells used in space, lattice defects are generated in the semiconductor due to irradiation by cosmic rays containing proton beams, resulting in a decrease in output power. Therefore, various sealing structures for blocking proton beams have been proposed for solar cells used in space.

[0003] For example, Non-Patent Document 1 discloses a sealing structure for a space-use solar cell using a special cover glass and resin adhesive.

[0004] In addition, for example, Patent Document 1 discloses that, in order to improve the heat dissipation to outer space based on infrared radiation, an optical thin film is formed on the surface of a solar cell to increase the infrared emissivity.

[0005] Existing technical documents

[0006] Non-patent literature

[0007] Non-Patent Document 1: Juno Matsuda, "Cover Glass for Protecting Solar Cells from Cosmic Rays," NewGLASS Vol. 14 No. 4, 1999, pp. 27-30, New Glass Forum Company.

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent No. 4565105 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] As mentioned above, the sealing structure of space-grade solar cells using special cover glass is costly due to the high cost of the cover glass itself and the complexity of its application. Furthermore, the increased weight of the solar cells leaves room for improvement in terms of controlling the load weight during emission.

[0012] On the other hand, the optical thin film in Patent Document 1 cannot shield low-energy proton beams, making it difficult to prevent the degradation of solar cells caused by proton beams.

[0013] The present invention was made in view of the above-mentioned situation, and provides a photoelectric conversion element that can suppress the performance degradation of the element caused by proton beams, and is lightweight and cost-effective.

[0014] Methods for solving problems

[0015] One embodiment of the photoelectric conversion element of the present invention includes: a photoelectric conversion section; and a proton beam shielding layer formed on the photoelectric conversion section to shield the photoelectric conversion section and block the proton beam. The product of the electron density and the film thickness of the proton beam shielding layer is 5 × 10⁻⁶. 20 (cm -2 )above.

[0016] Invention Effects

[0017] According to one aspect of the present invention, it is possible to provide a photoelectric conversion element that can suppress the performance degradation of the element caused by proton beams, and is lightweight and cost-effective. Attached Figure Description

[0018] Figure 1 This is a cross-sectional view in the thickness direction showing a configuration example of the solar cell of this embodiment.

[0019] Figure 2 This is a cross-sectional view in the thickness direction showing another configuration example of the solar cell of this embodiment.

[0020] Figure 3 This is a schematic diagram illustrating the effect of a proton beam in a solar cell.

[0021] Figure 4 It is a graph showing the relationship between the energy of the proton beam and the corresponding flux (Japanese: フルエンス) during the operation of satellites, etc.

[0022] Figure 5 It is a graph showing the electron density and the shielding effect of proton beams for various materials.

[0023] Figure 6 This is a graph showing the simulation results of Example 7.

[0024] Figure 7 This is a graph showing the simulation results of Example 7.

[0025] Figure 8 This is a graph showing the simulation results of Example 7.

[0026] Figure 9 This is a graph showing the simulation results of Example 7.

[0027] Figure 10 This is a graph showing the simulation results of Example 7. Detailed Implementation

[0028] The embodiments will now be described with reference to the accompanying drawings.

[0029] In the embodiments, to facilitate understanding, structures or elements other than the main parts of the invention are described in a simplified or omitted manner. Furthermore, in the accompanying drawings, the same elements are labeled with the same symbols. It should be noted that the shapes, dimensions, etc., of the elements in the accompanying drawings are schematic and do not represent actual shapes, dimensions, etc.

[0030] The photoelectric conversion element of this embodiment is installed in satellites or similar devices used in space, and is used in environments exposed to cosmic rays containing proton beams. In this embodiment, a solar cell for space use will be described as an example of a photoelectric conversion element.

[0031] First, an overview of proton beam degradation and heat dissipation in solar cells used in space is provided.

[0032] <Proton Beam Degradation in Solar Cells>

[0033] It is known that in solar cells used in space, if a proton beam stops near the photoelectric conversion layer of the solar cell unit, especially near the pn junction, it will cause a significant decrease in the cell's output power. To suppress the degradation of solar cells caused by proton beams, proton beam-resistant solar cell units can be used, or the shielding area for the proton beam in the thickness direction of the element can be increased to a level where the total amount of protons stopping near the pn junction of the solar cell unit does not affect the cell's maintenance rate. For example, the shielding area for the proton beam can be increased by thickening the cover glass.

[0034] Figure 3 (a) is a schematic diagram illustrating the effect of proton beams on the shielded area of ​​a solar cell without a cover glass. Figure 3 (b) is a schematic diagram illustrating the effect of the proton beam in a solar cell with a cover glass shield.

[0035] like Figure 3 As shown in (a), if the solar cell without the shielding of the cover glass is exposed to cosmic rays, low-energy proton beams (e.g., around 0.2 MeV) will stop near, for example, the pn junction portion of the solar cell unit. In contrast, as... Figure 3 As shown in (b), when a solar cell with a shielded area having a cover glass (100 μm thick) is exposed to cosmic rays, low-energy proton beams stop in the cover glass, but higher-energy proton beams (e.g., around 3 MeV) pass through the cover glass and stop near, for example, the pn junction portion of the solar cell unit.

[0036] exist Figure 3In (a) and (b), the damage caused by a single proton to the pn junction of the battery cell is roughly the same. However, the total amount of high-energy protons is less than that of low-energy protons; therefore, as... Figure 3 (b) When there is a shielded area, with Figure 3 (a) In comparison, the degradation caused by the proton beam is less. However, if the cover glass is thickened to increase the shielding area, it will lead to an increase in the load weight.

[0037] Here, the cumulative proton beam irradiation dose is referred to as the fluence (cm). -2 ).

[0038] Because defects are generated in the pn junction, the energy that stops at the photoelectric conversion layer of the solar cell, such as the pn junction, depends on a certain flux of the solar cell type, the minority carrier lifetime is reduced, which in turn leads to a decrease in the electrical characteristics of the solar cell.

[0039] Regarding GaAs and CIGS solar cells, various publications report the flux and maintenance rate of electrical properties for a given energy level. According to these reports, if the flux is less than 1 × 10⁻⁶... 10 (cm -2 If so, it can be said that the electrical characteristics of the solar cell are maintained at a sufficiently high level and the solar cell deteriorates little.

[0040] Furthermore, the relationship between the proton beam energy calculated based on a representative cosmic radiation environment model (NASA's AP8 model) and the corresponding flux during satellite operations is presented. Figure 4 .

[0041] As mentioned above, in order to suppress the degradation of solar cells caused by proton beams, it is required that the flux corresponding to the operation period of satellites, etc., be set to less than 1 × 10⁻⁶. 10 (cm -2 ).according to Figure 4 It is known that if the energy of the proton beam is greater than 0.6 MeV, the corresponding flux during the operation of satellites, etc., can be reduced to less than 1 × 10⁻⁶. 10 (cm -2 Therefore, the shielding region for a proton beam must be able to prevent proton beams below 0.6 MeV.

[0042] Next, the calculation method for shielding the proton beam will be explained.

[0043] When a proton beam irradiates a substance, the protons lose energy as they pass through the substance. The amount of energy lost by the protons during this process is called the stopping power. The stopping power S(E) can be calculated, for example, using the simulation software Stopping Range of Irons in Solid (http: / / www.srim.org / ). Furthermore, the distance from which the protons stop losing energy relative to the stopping power S(E) is called the range X. As shown in equation (1), the range X is equal to the value obtained by integrating the energy E until the protons stop with respect to the reciprocal of the stopping power S(E).

[0044]

Mathematical Formula 1

[0045]

[0046] Based on the range X calculated by equation (1), the thickness of the material that can stop a proton beam of a certain energy within a film of any material can be calculated (i.e., the thickness of the film that can shield the proton beam).

[0047] Furthermore, the blocking ability is related to the electron density n of the material as shown in equation (2). Here, N A Let ρ be Avogadro's constant, Z be the average atomic number of the material, ρ be the density of the material, and A be the average mass number of the material.

[0048]

Mathematical Formula 2

[0049]

[0050] Based on the above, the energy of the protons that are to be shielded can be represented by the product of the electron density of the material and the film thickness.

[0051] <Heat dissipation of solar cells in outer space>

[0052] Typically, semiconductors such as solar cells experience performance degradation and reduced lifespan when subjected to high temperatures, thus requiring cooling. In space, unlike on Earth, there is no atmosphere; solar cells exist in a vacuum, making atmospheric heat exchange impossible. Therefore, they are primarily cooled through infrared radiation.

[0053] The cooling efficiency based on infrared radiation is defined by the emissivity ε shown in equation (3). The higher the emissivity, the higher the cooling efficiency of the solar cell.

[0054]

Mathematical Expression 3

[0055]

[0056] Here, R is the reflectivity, U is the radiation intensity of the blackbody, T is the temperature (K), and λ is the wavelength (μm). It should be noted that since substrates containing metal films, such as those in solar cells, do not transmit light, the transmittance is set to 0. The emissivity at any temperature T can be calculated using equation (3). In the embodiments and examples, as an example, the emissivity is calculated from room temperature.

[0057] The denominator of equation (1) represents the radiation intensity of the blackbody. The numerator of equation (3) is obtained by multiplying the radiation intensity of the blackbody by (1-R). Furthermore, the size of the numerator of equation (3) relative to the denominator corresponds to the emissivity ε. Therefore, it can be seen that in order to increase the emissivity ε, the reflectivity R can be decreased by increasing the value of the numerator of equation (3).

[0058] The wavelength range for integration is from 2.5 μm to Y μm, where Y is arbitrarily chosen depending on the environment in which it is used. In the implementation and examples, as one example, the emissivity ε is calculated within a wavelength range of 2.5 μm to 25 μm. It should be noted that the emissivity can also be calculated from a wider wavelength range, i.e., a wavelength range of 2.5 μm to 35 μm.

[0059] For example, the reflectivity R of a substrate with a single-layer thin film is calculated according to Fresnel's thin-film interference formula as follows.

[0060]

Mathematical Expression 4

[0061]

[0062]

[0063] In the case of perpendicular incidence, according to Snell's formula

[0064]

[0065]

[0066] Here, N a N is the complex refractive index of the outer side (vacuum) of the light-receiving surface. α Let N be the complex refractive index of the thin film α. s d represents the complex refractive index of the substrate. α Let φ be the film thickness of thin film α. α Let α be the optical path difference of the thin film. Additionally, n is the refractive index (the real part of the complex refractive index), and k is the extinction coefficient (the imaginary part of the complex refractive index). It should be noted that when the thin film is a single layer, the variable α is 1.

[0067] Additionally, for example, the reflectivity R of a substrate with three thin films is calculated using the following formula. Here, the thin film on the substrate side is designated as thin film 1 (α = 1), the thin film facing thin film 1 is designated as thin film 2 (α = 2), and the thin film on the light-receiving surface side facing thin film 2 is designated as thin film 3 (α = 3).

[0068]

Mathematical Expression 5

[0069]

[0070] According to the above formulas, the reflectivity R is calculated based on the film thickness d, the refractive index n of the film material, and the extinction coefficient k. In other words, by combining the parameters n and k of the film material with the film thickness d, a film with the desired heat dissipation characteristics suitable for space environments can be obtained.

[0071] <Example of a solar cell configuration>

[0072] Next, the configuration of the solar cell for space use in this embodiment will be described with reference to the accompanying drawings. Figure 1 This is a cross-sectional view along the thickness direction showing an example of the configuration of the solar cell in this embodiment. Figure 1 In the example, a configuration example of a CIS-based solar cell will be described.

[0073] The solar cell 10 has a stacked structure in which a photoelectric conversion unit 12, a proton beam shielding layer 13, and a thermal radiation layer 14 are sequentially stacked on a conductive substrate 11 from bottom to top (substrate side). Sunlight and other light originate from the side opposite to the conductive substrate 11. Figure 1 The light is incident on the upper side of the photoelectric conversion unit 12.

[0074] (Conductive substrate 11)

[0075] The conductive substrate 11 is formed, for example, from titanium (Ti), stainless steel (SUS), copper, aluminum, or alloys thereof. The conductive substrate 11 can be a rigid metal substrate or a flexible metal substrate. The conductive substrate 11 can also be a laminated structure of multiple metal substrates, for example, stainless steel foil, titanium foil, or molybdenum foil can be formed on the surface of the substrate.

[0076] The shape and size of the conductive substrate 11 are appropriately determined according to the size of the solar cell 10, etc. In this embodiment, the overall shape of the conductive substrate 11 is, for example, a rectangular flat plate, but it is not limited thereto.

[0077] When a flexible metal substrate is used as the conductive substrate 11, the solar cell 10 can be bent, and the substrate cracking caused by bending can be suppressed. Furthermore, in the above-described case, compared with glass substrates and resin substrates, it is easier to achieve a lighter and thinner solar cell 10.

[0078] It should be noted that, in solar cells for space applications, from the viewpoint of suppressing the load weight during emission and achieving high strength of the solar cell, it is preferable to form the conductive substrate 11 from titanium or an alloy containing titanium.

[0079] (Photoelectric conversion unit 12)

[0080] The photoelectric conversion unit 12 has a stacked structure in which a first electrode layer 21, a photoelectric conversion layer 22, a buffer layer 23, and a second electrode layer 24 are stacked sequentially from the bottom (substrate side).

[0081] (First electrode layer 21)

[0082] The first electrode layer 21, for example, is a metal electrode layer such as molybdenum (Mo), and is formed on the conductive substrate 11. The first electrode layer 21 is not facing the light-receiving side of the photoelectric conversion layer 22, but rather facing the back side (the conductive substrate 11 side), and is therefore also referred to as the back electrode. Although not particularly limited, the thickness of the first electrode layer 21 is, for example, 50 nm to 1000 nm.

[0083] (Photoelectric conversion layer 22)

[0084] A photoelectric conversion layer 22 is formed on the first electrode layer 21. The photoelectric conversion layer 22 may also have a portion on the light-receiving side (…). Figure 1 (upper side) and back side ( Figure 1 The structure is a double-gradient structure with a large band gap on the lower side and a small band gap on the inner side of the photoelectric conversion layer 22 in the thickness direction. Although not particularly limited, the thickness of the photoelectric conversion layer 22 is, for example, 1.0 μm to 3.0 μm.

[0085] The photoelectric conversion layer 22 functions as a polycrystalline or microcrystalline p-type compound semiconductor layer. For example, the photoelectric conversion layer 22 is a CIS-based photoelectric conversion element using a chalcopyrite-structured I-III-VI2 compound semiconductor containing Group I, III, and VI elements (chalcogenides). Group I elements can be selected from copper (Cu), silver (Ag), gold (Au), etc. Group III elements can be selected from indium (In), gallium (Ga), aluminum (Al), etc. In addition to selenium (Se) and sulfur (S), the photoelectric conversion layer 22 can also contain tellurium (Te) as a Group VI element. Furthermore, the photoelectric conversion layer 22 can also contain alkali metals such as Li, Na, K, Rb, and Cs.

[0086] (Buffer layer 23)

[0087] A buffer layer 23 is formed on the photoelectric conversion layer 22. Although not particularly limited, the thickness of the buffer layer 23 is, for example, 10 nm to 100 nm.

[0088] The buffer layer 23 is, for example, an n-type or i-type (intrinsic) high-resistivity conductive layer. Here, "high resistance" means having a resistance value higher than that of the second electrode layer 24, which will be described later.

[0089] The buffer layer 23 can be selected from compounds containing zinc (Zn), cadmium (Cd), and indium (In). Examples of zinc-containing compounds include ZnO, ZnS, Zn(OH)₂, or mixed crystals thereof such as Zn(O,S), Zn(O,S,OH), ZnMgO, and ZnSnO. Examples of cadmium-containing compounds include CdS, CdO, or mixed crystals thereof such as Cd(O,S) and Cd(O,S,OH). Examples of indium-containing compounds include InS, InO, or mixed crystals thereof such as In(O,S) and In(O,S,OH), and In₂O₃, In₂S₃, and In(OH) can be used. x Additionally, buffer layer 23 may also have a stacked structure of these compounds.

[0090] It should be noted that the buffer layer 23 has the effect of improving photoelectric conversion efficiency, but the buffer layer 23 can be omitted. When the buffer layer 23 is omitted, the second electrode layer 24 is formed on the photoelectric conversion layer 22.

[0091] (Second electrode layer 24)

[0092] The second electrode layer 24 is formed on the buffer layer 23. The second electrode layer 24 is, for example, an n-type conductive layer. Although not particularly limited, the thickness of the second electrode layer 24 is, for example, 0.5 μm to 2.5 μm.

[0093] The second electrode layer 24 is preferably made of a material with a wide bandgap and sufficiently low resistance. Furthermore, since the second electrode layer 24 serves as a channel for light such as sunlight, it is preferably designed to transmit light of wavelengths that can be absorbed by the photoelectric conversion layer 22. In this sense, the second electrode layer 24 is also referred to as a transparent electrode layer or a window layer.

[0094] The second electrode layer 24 may be a metal oxide with a group III element (B, Al, Ga, or In) added as a dopant. Examples of metal oxides include ZnO or SnO2. The second electrode layer 24 may be selected from, for example, In2O3 (indium oxide), ITO (indium tin oxide), ITiO (indium titanium oxide), IZO (indium zinc oxide), ZTO (zinc tin oxide), FTO (fluorine-doped tin oxide), GZO (gallium-doped zinc oxide), BZO (boron-doped zinc oxide), AZO (aluminum-doped zinc oxide), etc.

[0095] (Proton beam shielding layer 13)

[0096] The proton beam shielding layer 13 is an optical thin film formed on the second electrode layer 24 of the photoelectric conversion unit 12, and functions as a shielding region for the proton beam incident from the light-receiving surface side.

[0097] The proton beam shielding layer 13 is made of a material with high transmittance and a refractive index between that of the photoelectric conversion unit 12 and vacuum. As an example, the proton beam shielding layer 13 is formed from one or more materials selected from Al2O3, Y2O3, ZrO2, MgO, HfO2, Bi2O3, TiO2, ZnO, In2O3, SnO2, Nb2O5, and Ta2O5. These materials satisfy the requirements for transmittance and refractive index, and are also cost-effective.

[0098] It should be noted that the shielding effect of the proton beam depends on the electron density of the material, not on the bonding state. Therefore, the material of the proton beam shielding layer 13 can be a mixture of any one or more of the materials mentioned above.

[0099] In this embodiment, the proton beam shielding layer 13 is configured to shield proton beams with energies below 0.6 MeV, with the product of electron density and film thickness set to 5 × 10⁻⁶. 20 (cm -2 The above is true. Preferably, the proton beam shielding layer 13 has an electron density-to-film thickness product of 1.06 × 10⁻⁶ Ω·cm to shield proton beams with energies below 1 MeV. 21 (cm -2 )above.

[0100] Figure 5 This is a graph showing the electron density and proton beam shielding effect of various materials. In Figure 5 In the diagram, for various materials, the electron density, film thickness for shielding 0.6 MeV and 1 MeV proton beams, and film thickness × electron density for shielding 0.6 MeV and 1 MeV proton beams are shown respectively. It should be noted that this is from the perspective of film formation on solar cells. Figure 5 The materials selected are those with a refractive index between the surface of the solar cell and the vacuum, high light transmittance, and excellent cost-effectiveness.

[0101] The energy used to stop a proton beam within various materials is calculated based on the range. For example... Figure 5 As shown, the minimum film thickness × electron density for shielding a 0.6 MeV proton beam is 5 × 10⁻⁶ times that of MgO. 20 (cm -2 Therefore, it can be concluded that when shielding proton beams below 0.6 MeV, the film thickness × electron density is 5 × 10⁻⁶. 20 (cm -2 That's all.

[0102] Furthermore, the proton beam shielding layer 13 is more preferably capable of shielding proton beams below 1 MeV. For example... Figure 5 As shown, the minimum film thickness × electron density for shielding a 1 MeV proton beam is 1.06 × 10⁻⁶ for MgO. 21 (cm -2 Therefore, it can be concluded that when shielding proton beams below 1 MeV, the film thickness × electron density is 1.06 × 10⁻⁶. 21 (cm -2 That's all.

[0103] Furthermore, the proton beam shielding layer 13 also increases in weight with increasing thickness. Here, the electron density × film thickness of a typical space-grade cover glass (100 μm thick) is 6.68 × 10⁻⁶. 21 (cm -2 Therefore, from the viewpoint that the proton beam shielding layer 13 is lighter than the cover glass, it is preferable that the electron density × film thickness is less than 6.68 × 10⁻⁶. 21 (cm -2 ).

[0104] Furthermore, the proton beam shielding layer 13 can be a multilayer film formed by stacking different materials selected from the above-mentioned materials. Even in the case of a multilayer film, the film thickness required to shield the proton beam is calculated based on the range, so even if the proton beam shielding layer 13 has multiple layers, the shielding effect of the proton beam will not be compromised. In addition, from the viewpoint of improving the infrared emissivity of the solar cell 10, it is preferable to also stack materials with different refractive indices and extinction coefficients in the proton beam shielding layer 13.

[0105] Furthermore, ideally, the thin film stacked on the photoelectric conversion unit 12 should have a structure in which the refractive index monotonically increases in the thickness direction from the light-receiving surface to the transparent electrode layer across a broad wavelength region from visible to infrared light. However, in most cases, the refractive index does not monotonically increase in the thickness direction in any wavelength region, and an optical design is performed as a whole to approximate the desired optical performance. In this case, by making the proton beam shielding layer 13 into a multilayer film to adjust the overall optical performance of the thin film, the optical design of the thin film stacked on the photoelectric conversion unit 12 can be easily and flexibly performed.

[0106] (Thermal radiation layer 14)

[0107] The thermal radiation layer 14 is an optical thin film formed on the proton beam shielding layer 13, which serves to promote cooling caused by the radiation of the solar cell 10. Although not particularly limited, in order to achieve efficient cooling of the infrared radiation-based solar cell 10, the emissivity of the thermal radiation layer 14 is preferably 0.80 or higher.

[0108] If only a proton beam shielding layer 13 is formed in the photoelectric conversion unit 12, it is difficult to meet the desired emissivity. In addition, if the desired emissivity is met, the film thickness may become very thick. Therefore, by stacking a thermal radiation layer 14 on the proton beam shielding layer 13, the shielding effect of the proton beam can be ensured, and the desired emissivity can be ensured.

[0109] As an example, SiO2 or Al2O3, which have high emissivity, can be used as materials in the thermal radiation layer 14. The thermal radiation layer 14 can be a single-layer film of any material, either SiO2 or Al2O3, or it can be a laminated film formed by stacking thin films of SiO2 and Al2O3. From the viewpoint of improving the infrared emissivity of the solar cell 10, the thermal radiation layer 14 is preferably made of a laminate of materials with different refractive indices and extinction coefficients.

[0110] Furthermore, in order to achieve an emissivity of 0.8 or higher, the thermal radiation layer 14 is preferably 210 nm or thicker. Additionally, when the thermal radiation layer 14 is a laminated film, in order to achieve an emissivity of 0.8 or higher, it is preferable that the thickness of any film contained in the laminated film is 110 nm or thicker.

[0111] Figure 2 This is a cross-sectional view along the thickness direction showing another configuration example of the solar cell according to this embodiment. Figure 2 In the examples, we will illustrate the configuration of solar cells using crystalline Si-based, GaAs-based, and other semiconductor substrates. It should be noted that... Figure 2 In the example, for and Figure 1 Common elements are marked with the same symbol, and repeated descriptions are omitted.

[0112] Figure 2 The solar cell 10a shown has a stacked structure in which a photoelectric conversion unit 12, a proton beam shielding layer 13, and a thermal radiation layer 14 are sequentially stacked. Sunlight and other light enter from the thermal radiation layer 14 side ( Figure 2 (The light) is incident on the upper side of the photoelectric conversion unit 12.

[0113] The photoelectric conversion unit 12 has a semiconductor substrate 11a of a first conductivity type, a first electrode layer 31, a semiconductor layer 32 of a second conductivity type, and a second electrode layer 33. Here, the first conductivity type is either p-type or n-type, and the second conductivity type is either p-type or n-type.

[0114] The semiconductor substrate 11a is, for example, a silicon substrate formed of a single crystal or polycrystalline material of the first conductivity type. In the case of a p-type first conductivity type, the semiconductor substrate 11a is formed by doping with trace amounts of group III elements (e.g., boron, aluminum, etc.). In the case of an n-type first conductivity type, the semiconductor substrate 11a is formed by doping with trace amounts of group V elements (e.g., phosphorus, arsenic, etc.).

[0115] The first electrode layer 31 is formed on one side of the semiconductor substrate 11a. Figure 2 The back electrode (on the lower side). The first electrode layer 31 is composed of, for example, a thin film of a conductive metal such as silver (Ag), aluminum (Al), or titanium (Ti).

[0116] The second conductivity type semiconductor layer 32 is, for example, a layer formed of crystalline silicon or polycrystalline silicon of the second conductivity type, forming a pn junction with the first conductivity type semiconductor substrate 11a. The semiconductor layer 32 can be disposed entirely on the other side of the semiconductor substrate 11a, or it can be disposed on a portion of the other side of the semiconductor substrate 11a. Furthermore, when the second conductivity type is n-type, for example, the semiconductor layer 32 is formed by doping with trace amounts of group V elements (e.g., phosphorus, arsenic, etc.). When the second conductivity type is p-type, for example, the semiconductor layer 32 is formed by doping with trace amounts of group III elements (e.g., boron, aluminum, etc.).

[0117] The second electrode layer 33 is formed on the semiconductor layer 32. The second electrode layer 33 may be configured as, for example, a transparent electrode layer that is transparent to light or a comb-shaped busbar electrode.

[0118] Figure 2 The proton beam shielding layer 13 shown is formed on the second electrode layer 33, and the thermal radiation layer 14 is formed on the proton beam shielding layer 13. The proton beam shielding layer 13 and the thermal radiation layer 14 are... Figure 1 They have the same composition.

[0119] <Methods for Manufacturing Solar Cells>

[0120] Next, the manufacturing method of the solar cell according to the above embodiment will be described.

[0121] exist Figure 1 The process of forming the photoelectric conversion section 12 on the conductive substrate 11 of the solar cell 10 shown is the same as the manufacturing process of a typical CIS solar cell. That is, a precursor layer is formed on the surface of the conductive substrate 11 on which the first electrode layer 21 is formed, and the precursor layer is sulfided to form the photoelectric conversion layer 22. Then, the photoelectric conversion section 12 is formed by sequentially stacking a buffer layer 23 and a second electrode layer 24 on the photoelectric conversion layer 22.

[0122] On the other hand, Figure 2 In the case of the solar cell 10a shown, in the usual semiconductor layer and electrode layer formation process, a first electrode layer 31 is formed on one side of the semiconductor substrate 11a, and a semiconductor layer 32 and a second electrode layer 33 are formed on the other side of the semiconductor substrate 11a, thereby forming the photoelectric conversion unit 12.

[0123] Next, a proton beam shielding layer 13 and a thermal radiation layer 14 are sequentially formed on the second electrode layer 24 (or the second electrode layer 33) of the photoelectric conversion unit 12. The proton beam shielding layer 13 and the thermal radiation layer 14 are formed using semiconductor thin film formation processes such as sputtering, CVD (Chemical vapor deposition), evaporation, photo-mod (Metal Organic Deposition), and AD (Aerosol Deposition). It should be noted that the combined thickness of the proton beam shielding layer 13 and the thermal radiation layer 14 is, for example, in the range of 1 μm to 65 μm.

[0124] As an example, examples of film formation conditions for forming Y2O3 thin films, Al2O3 thin films as proton beam shielding layers 13, and SiO2 as thermal radiation layers 14 are shown in Table 1 below. In the following examples, thin films for proton beam shielding layers 13 and thermal radiation layers 14 are formed by electron beam evaporation.

[0125] Table 1

[0126]

[0127] As described above, the solar cells 10 and 10a of this embodiment have a proton beam shielding layer 13 on the second electrode layer 24 (or second electrode layer 33) of the photoelectric conversion unit 12 to shield the photoelectric conversion unit 12 from the proton beam. The product of the electron density and the film thickness of the proton beam shielding layer 13 is 5 × 10⁻⁶. 20 (cm -2 )above.

[0128] Therefore, the solar cells 10 and 10a of this embodiment can shield proton beams below 0.6 MeV using the proton beam shielding layer 13, enabling the flux corresponding to operation with satellites, etc., to be less than 1 × 10⁻⁶. 10 (cm -2 Therefore, according to this embodiment, during the use of solar cells for space applications, it is possible to suppress the performance degradation of the components caused by proton beams.

[0129] Furthermore, in the solar cells 10 and 10a of this embodiment, a proton beam shielding layer 13 is stacked on the photoelectric conversion unit 12 using a semiconductor thin-film forming process. In this embodiment, the proton beam shielding layer 13, which has excellent proton beam shielding capabilities, can be formed to be lighter than the cover glass, and complex assembly processes such as the bonding of the cover glass can be omitted. Therefore, the solar cells 10 and 10a of this embodiment are lighter and more cost-effective than solar cells with cover glass.

[0130] <Description of the Implementation>

[0131] Hereinafter, an embodiment of the solar cell of this embodiment will be described.

[0132] (Example 1)

[0133] In Example 1, four solar cells with different thin films formed on the transparent electrode layer were compared, and the differences in the product of the electron density and film thickness, as well as the emissivity, were calculated through simulation. The solar cells of Example 1 are all CIS solar cells, from the conductive substrate to the transparent electrode layer. The structure of the solar cells of Example 1 is shown below.

[0134] • Comparative Example 1: Solar cells without a thin film on the transparent electrode layer

[0135] • Example 1A: A solar cell with a 4.59 μm ZrO2 thin film formed on a transparent electrode layer

[0136] • Example 1B: A solar cell in which a 4.59 μm ZrO2 thin film and a 1.3 μm SiO2 thin film are sequentially stacked on a transparent electrode layer from bottom to top.

[0137] Example 1C: A solar cell in which a 4.59 μm ZrO2 thin film, a 0.65 μm Al2O3 thin film, and a 0.65 μm SiO2 thin film are sequentially stacked on a transparent electrode layer from bottom to top.

[0138] Regarding the product of the film thickness and electron density for shielding a 0.6 MeV proton beam, in Comparative Example 1, it is less than 5 × 10⁻⁶. 20 (cm -2 In Examples 1A, 1B, and 1C, the value is 5 × 10. 20 (cm -2 )above.

[0139] Furthermore, the emissivity in Comparative Example 1 was 31.3%, and the emissivity in Example 1A was 75.7%. In contrast, the emissivity in Example 1B was 80.6%, and the emissivity in Example 1C was 83.7%. Therefore, as in Examples 1B and 1C, in configurations where the proton beam shielding layer is stacked in two or more layers, the emissivity is increased compared to Comparative Example 1 and Example 1A. Additionally, compared to Example 1B, as in Example 1C, even if the total film thickness of the proton beam shielding layers after the second layer is the same, in configurations where proton beam shielding layers of different materials are stacked, the emissivity is increased compared to Example 1B.

[0140] Furthermore, Al2O3, the thin film material used in Example 1C, is also the material for the thermal radiation layer. Therefore, Example 1C can also be considered as having a thermal radiation layer composed of Al2O3 and SiO2 thin films stacked together. That is, it is known that compared to the case where a single layer of SiO2 thin film is formed on the proton beam shielding layer as the thermal radiation layer, forming a thermal radiation layer with a stacked structure of Al2O3 and SiO2 helps to improve the emissivity.

[0141] (Example 2)

[0142] The energy of the shielded proton beam is calculated based on the range. Therefore, in Example 2, the thickness of each film is calculated by simulation in the case of a thin film composed of multiple materials.

[0143] In Example 2, the film thickness for shielding 0.6 MeV and 1 MeV of energy was calculated for material Z (mixture composition) consisting of Al2O3, Y2O3, and In2O3 in a 1:1:1 ratio. The calculation results from Example 2 are shown in Table 2.

[0144] Table 2

[0145] Material Z 4.8 10.01

[0146] Thus, by combining the materials and film thickness shown in Example 2, a solar cell that can theoretically shield the energy of a proton beam of 0.6 MeV can be provided.

[0147] (Example 3)

[0148] In Example 3, the thickness of the laminated film, constructed by stacking Al2O3, Y2O3, and In2O3 films with the same thickness, was calculated to shield 0.6 MeV and 1 MeV of energy. The calculation results from Example 3 are shown in Table 3. It should be noted that the thickness of each Al2O3, Y2O3, and In2O3 film is 1 / 3 of the thickness of the laminated film.

[0149] Table 3

[0150]

[0151] Thus, the laminated film shown in Example 3 can provide a solar cell that can theoretically shield the energy of a proton beam of 0.6 MeV.

[0152] (Example 4)

[0153] In Example 4, the emissivity was calculated by simulation for the case where a SiO2 or Al2O3 thin film was formed on the proton beam shielding layer stacked on a CIS solar cell as a thermal radiation layer, and for the case where an In2O3 thin film was formed as a comparative example. Al2O3, HfO2, In2O3, ZrO2, and Y2O3 were selected as the materials for the proton beam shielding layer in Example 4. Furthermore, the film thickness of the proton beam shielding layer was set to the minimum thickness required to shield a 0.6 MeV proton beam for each material, and the film thickness of the thin film on the proton beam shielding layer was set to 1.0 μm. The calculation results for Example 4 are shown in Table 4.

[0154] Table 4

[0155]

[0156] Table 4 shows the ratio of emissivity when the emissivity is set to 1 for a proton beam shielding layer alone, and the ratio of emissivity when a thin film is stacked on the proton beam shielding layer. As shown in Table 4, it can be seen that when a SiO2 or Al2O3 thin film is formed on the proton beam shielding layer, the emissivity ratio is greater than 1, and the emissivity is improved through the thermal radiation layer. Specifically, it is believed that by using a SiO2 or Al2O3 thin film to suppress the reflection of light in the wavelength region of 5 μm to 15 μm, the emissivity is improved as described above. On the other hand, when an In2O3 thin film is formed on the proton beam shielding layer in the comparative example, the emissivity shows a decreasing trend due to the increased light reflectivity.

[0157] (Example 5)

[0158] In Example 5, the emissivity was calculated by simulation for the case where a SiO2 or Al2O3 thin film was formed on a proton beam shielding layer stacked on a solar cell using a crystalline Si-based semiconductor substrate as a thermal radiation layer, and for the case where an In2O3 thin film was formed as a comparative example. ZrO2 was selected as the material for the proton beam shielding layer in Example 5. Furthermore, the thickness of the proton beam shielding layer was set to the minimum thickness capable of shielding a 0.6 MeV proton beam, and the thickness of the thin film on the proton beam shielding layer was set to 1.0 μm. The calculation results from Example 5 are shown in Table 5.

[0159] Table 5

[0160]

[0161] Table 5, similar to Table 4, shows the ratio of emissivity when the emissivity of the proton beam shielding layer alone is set to 1, and the ratio when a thin film is stacked on the proton beam shielding layer. As shown in Table 5, it can be seen that when a SiO2 or Al2O3 thin film is formed on the proton beam shielding layer, the emissivity ratio is greater than 1, and the emissivity is improved through the thermal radiation layer. Specifically, it is believed that by using a SiO2 or Al2O3 thin film to suppress the reflection of light in the wavelength region of 5 μm to 15 μm, the emissivity is improved as described above. On the other hand, when an In2O3 thin film is formed on the proton beam shielding layer in the comparative example, the emissivity shows a decreasing trend due to the increased light reflectivity.

[0162] As can be seen from Examples 4 and 5 above, in either a CIS-based solar cell or a solar cell using a crystalline silicon (Si)-based semiconductor substrate, if a 1.0 μm thin film of SiO2 or Al2O3 is formed on a proton beam shielding layer with a thickness capable of shielding a proton beam of 0.6 MeV as a thermal radiation layer, the emissivity can be improved.

[0163] (Example 6)

[0164] In Example 6, when a thin film of SiO2 or Al2O3 was formed on the proton beam shielding layer as a thermal radiation layer, the film thickness used to achieve an emissivity of 80% or higher was calculated through simulation. It should be noted that the solar cell in Example 6, from the conductive substrate to the transparent electrode layer, is a CIS solar cell.

[0165] Al₂O₃, HfO₂, In₂O₃, ZrO₂, and Y₂O₃ were selected as the materials for the proton beam shielding layer in Example 6. Furthermore, the film thickness of the proton beam shielding layer was set to the minimum thickness required to shield a 0.6 MeV proton beam for each material. The calculation results from Example 6 are shown in Table 6.

[0166] Table 6

[0167]

[0168] As shown in Table 6, it can be seen that when a thin film of SiO2 or Al2O3 is formed on the proton beam shielding layer, the film thickness needs to be set to at least 210 nm in order to achieve an emissivity of more than 80%.

[0169] (Example 7)

[0170] In Example 7, when SiO2 and Al2O3 films were stacked on a proton beam shielding layer, the film thicknesses used to achieve an emissivity of 80% or higher were calculated through simulation. It should be noted that the solar cell in Example 7, from the conductive substrate to the transparent electrode layer, is a CIS solar cell.

[0171] In Example 7, an Al2O3 thin film and a SiO2 thin film are sequentially stacked on a proton beam shielding layer. Furthermore, Al2O3, HfO2, In2O3, ZrO2, and Y2O3 are selected as the materials for the proton beam shielding layer. The film thickness of the proton beam shielding layer is set to the minimum thickness required to shield a 0.6 MeV proton beam for each material.

[0172] Figures 6-10 This represents the simulation results in Example 7. Figures 6-10 The vertical axis of each figure represents the film thickness (nm) of the SiO2 thin film. Figures 6-10 The horizontal axis of each graph represents the film thickness (nm) of the Al2O3 thin film. Additionally, in Figures 6-10 Each figure shows the distribution of regions with emissivity less than 75%, regions with emissivity greater than 75% but less than 80%, and regions with emissivity greater than 80%.

[0173] Figure 6 This is a distribution diagram showing the relationship between the film thicknesses of Al2O3 and SiO2 films when the proton beam shielding layer is made of Al2O3. Figure 6 In the case where both the Al2O3 film thickness and the SiO2 film thickness are 0.21 μm, a region with an emissivity of over 80% was identified. Furthermore, Table 7 shows an example of a combination of Al2O3 and SiO2 film thicknesses for a thermal radiation layer with an emissivity of 80% when the proton beam shielding layer is Al2O3.

[0174] Table 7

[0175] 1 0 300 2 100 250 3 240 200 4 420 150 5 720 100 6 1020 70 7 1340 50 8 1820 30 9 2830 0

[0176] Figure 7 This is a distribution diagram showing the relationship between the film thicknesses of Al2O3 and SiO2 films when the proton beam shielding layer material is HfO2. Figure 7 In this case, when the thickness of the Al2O3 film was set to 1.83 μm and the thickness of the SiO2 film was set to 0.86 μm, a region with an emissivity of 80% or higher was identified. Furthermore, an example of a combination of Al2O3 and SiO2 film thicknesses for a thermal radiation layer with HfO2 as the proton beam shielding layer and an emissivity of 80% is shown in Table 8.

[0177] Table 8

[0178] 1 1130 2990 2 1590 2210 3 1700 2000 4 1760 1700 5 1780 1510 6 1810 990 7 1850 810 8 1900 660 9 1980 540 10 2110 410 11 2290 310 12 2490 230 13 2750 180 14 2930 160 15 2990 150

[0179] Figure 8 This is a distribution diagram showing the relationship between the film thicknesses of Al2O3 and SiO2 films when the proton beam shielding layer is made of In2O3. Figure 8In this case, when the thickness of the Al2O3 film was set to 1.37 μm and the thickness of the SiO2 film was set to 0.50 μm, a region with an emissivity of 80% or higher was identified. Furthermore, an example of a combination of Al2O3 and SiO2 film thicknesses for a thermal radiation layer with an emissivity of 80% and a proton beam shielding layer of In2O3 is shown in Table 9.

[0180] Table 9

[0181] 1 1250 800 2 1250 980 3 1280 1100 4 1290 640 5 1330 1230 6 1380 1300 7 1390 470 8 1460 1360 9 1540 1390 10 1550 330 11 1610 1400 12 1680 1380 13 1690 250 14 1850 200 15 1870 1300 16 2040 170 17 2040 1180 18 2180 1060 19 2360 170 20 2390 860 21 2580 660 22 2600 230 23 2640 250 24 2680 510 25 2690 300 26 2700 470 27 2710 340 28 2710 440

[0182] Figure 9 This is a distribution diagram showing the relationship between the film thicknesses of Al2O3 and SiO2 films when the proton beam shielding layer is made of ZrO2. Figure 9 In this case, when the thickness of the Al2O3 film was set to 0.11 μm and the thickness of the SiO2 film was set to 0.2 μm, a region with an emissivity of 80% or higher was identified. Furthermore, an example of a combination of Al2O3 and SiO2 film thicknesses for a thermal radiation layer with a proton beam shielding layer of ZrO2 and an emissivity of 80% is shown in Table 10.

[0183] Table 10

[0184] 1 0 360 2 110 200 3 320 90 4 740 20 5 1090 20 6 1600 50 7 2190 50 8 2850 20 9 2990 20

[0185] Figure 10 This is a distribution diagram showing the relationship between the film thicknesses of Al2O3 and SiO2 films when the proton beam shielding layer is made of Y2O3. Figure 10 In this case, when the thickness of the Al2O3 film was set to 0.13 μm and the thickness of the SiO2 film was set to 0.17 μm, a region with an emissivity of 80% or higher was identified. Furthermore, an example of a combination of Al2O3 and SiO2 film thicknesses for a thermal radiation layer with a proton beam shielding layer of Y2O3 and an emissivity of 80% is shown in Table 11.

[0186] Table 11

[0187] 1 0 210 2 80 180 3 230 160 4 330 160 5 510 180 6 850 240 7 1020 260 8 1320 240 9 1510 220 10 2100 150 11 2850 80 12 2990 80

[0188] like Figures 6-10 As shown, when Al2O3 thin films and SiO2 thin films are stacked sequentially on a proton beam shielding layer, in order to achieve an emissivity of over 80%, the thickness of at least one of the films needs to be set to over 110 nm.

[0189] <Supplementary Notes on Implementation Methods>

[0190] In the above embodiments, a solar cell with a proton beam shielding layer was described as an example of a photoelectric conversion element for space use. However, the configuration of the photoelectric conversion element of the present invention is not limited to solar cells, and can also be widely applied to other semiconductor elements (such as imaging elements) with photoelectric conversion layers that are used in space.

[0191] Furthermore, this invention is not limited to CIS-based solar cells or solar cells using crystalline Si-based semiconductor substrates; it can also be applied to compound-based solar cells other than CIS-based solar cells, and other conventional solar cells. As an example, the structure of this invention can also be applied to compound-based solar cells such as CZTS-based solar cells, CIGS-based solar cells, CdTe-based solar cells, and GaAs-based solar cells, as well as organic-based solar cells.

[0192] In addition, the proton beam shielding layer or thermal radiation layer in this invention can also be configured as a laminated film of three or more layers.

[0193] As described above, embodiments of the present invention have been illustrated, but these embodiments are provided as examples and are not intended to limit the scope of the invention. Embodiments can be implemented in various ways other than those described above, and various omissions, substitutions, modifications, etc., can be made without departing from the spirit of the invention. Embodiments and their variations are included within the scope and spirit of the present invention, as are the inventions and their equivalents described in the technical solutions.

[0194] This application claims priority based on Japanese Patent Application No. 2021-057856, filed on March 30, 2021, the entire contents of which are incorporated herein by reference.

[0195] Explanation of reference numerals in the attached figures

[0196] 10, 10a…Solar cell, 11…Conductive substrate, 11a…Semiconductor substrate, 12…Photovoltaic conversion unit, 13…Proton beam shielding layer, 14…Thermal radiation layer, 21…First electrode layer, 22…Photovoltaic conversion layer, 23…Buffer layer, 24…Second electrode layer, 31…First electrode layer, 32…Semiconductor layer, 33…Second electrode layer

Claims

1. A photoelectric conversion element, characterized in that, It possesses: Photoelectric conversion unit; A proton beam shielding layer is formed directly on the photoelectric conversion unit to shield the photoelectric conversion unit and block the proton beam. as well as A thermal radiation layer, which is directly grounded, is formed on the proton beam shielding layer. The product of the electron density and the film thickness of the proton beam shielding layer is 5 × 10⁻⁶. 20 cm -2 above.

2. The photoelectric conversion element according to claim 1, wherein, The photoelectric conversion unit is formed on the substrate.

3. The photoelectric conversion element according to claim 1, wherein, The photoelectric conversion unit has a semiconductor substrate.

4. The photoelectric conversion element according to any one of claims 1 to 3, wherein, The proton beam shielding layer is a layer containing the first material. The first material is selected from one or more of Al2O3, Y2O3, ZrO2, MgO, HfO2, Bi2O3, TiO2, ZnO, In2O3, SnO2, Nb2O5, and Ta2O5.

5. The photoelectric conversion element according to claim 4, wherein, The proton beam shielding layer is a laminated film consisting of two or more layers containing the first material.

6. The photoelectric conversion element according to claim 1, wherein, The thermal radiation layer is a layer containing a second material. The second material is selected from at least one of SiO2 and Al2O3.

7. The photoelectric conversion element according to claim 6, wherein, The thickness of the thermal radiation layer is 210 nm or more.

8. The photoelectric conversion element according to claim 7, wherein, The thermal radiation layer is a laminated film consisting of two layers containing the second material.

9. The photoelectric conversion element according to claim 8, wherein, The thickness of any one of the laminated films in the thermal radiation layer is 110 nm or more.

10. A method for manufacturing a photoelectric conversion element, wherein, The photoelectric conversion element comprises: Photoelectric conversion unit; A proton beam shielding layer is formed directly on the photoelectric conversion unit to shield the photoelectric conversion unit and block the proton beam. as well as A thermal radiation layer is formed directly on the proton beam shielding layer and grounded thereon. The method for manufacturing the photoelectric conversion element includes: The process of forming the photoelectric conversion unit; The product of electron density and film thickness is 5 × 10⁻⁶ when the photoelectric conversion unit is directly grounded. 20 cm -2 The above-described process for the proton beam shielding layer; and The process of forming the thermal radiation layer directly grounded on the proton beam shielding layer.

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