Solar cells, multi-junction solar cells, solar cell modules and solar power generation systems

Abstract

To provide a solar cell, a multi-junction solar cell, a solar cell module, and a photovoltaic power generation system having high conversion efficiency.SOLUTION: A solar cell according to one embodiment includes: a p-electrode; an n-electrode; a p-type light-absorbing layer provided on the p-electrode and including a first region and a second region; and an n-type layer provided between the p-type light-absorbing layer and the n-electrode. The first region exists in the p-type light-absorbing layer of the n-type layer side. The second region exists in the p-type light-absorbing layer of the p-electrode side. A first average crystal grain diameter that is an average crystal grain diameter of the first region is larger than a second average crystal grain diameter that is an average crystal grain diameter of the second region.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a solar cell, a multi-junction solar cell, a solar cell module, and a photovoltaic power generation system. [Background technology]

[0002] One new type of solar cell is one that uses cuprous oxide (Cu2O) as the light absorption layer. Cu2O is a wide-gap semiconductor. Because Cu2O is a safe and inexpensive material made from copper and oxygen, which are abundant on Earth, it is expected to realize highly efficient, low-cost solar cells. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-154171 Summary of the Invention [Problem to be solved by the invention]

[0004] The problem to be solved by the present invention is to provide a solar cell, a multi-junction solar cell, a solar cell module, and a solar power generation system that are excellent in conversion efficiency. [Means for solving the problem]

[0005] The solar cell of the embodiment includes a p-electrode, an n-electrode, a p-type light absorbing layer provided on the p-electrode and including a first region and a second region, and an n-type layer provided between the p-type light absorbing layer and the n-electrode. The first region is present on the n-type layer side of the p-type light absorbing layer. The second region is present on the p-electrode side of the p-type light absorbing layer. The first average crystal grain size, which is the average crystal grain size of the first region, is larger than the second average crystal grain size, which is the average crystal grain size of the second region. The p-type light absorbing layer is mainly composed of cuprous oxide or a composite oxide of cuprous oxide. The thickness of the p-type light absorbing layer is 2000 nm or more and 15000 nm or less. The thickness of the second region is 70 nm or more and 1000 nm or less. [Brief explanation of the drawings]

[0006] [Figure 1]FIG. 1 is a schematic cross-sectional view of a solar cell according to an embodiment. [Figure 2] FIG. 2 is a diagram illustrating an analysis spot of a solar cell according to an embodiment. [Figure 3] FIG. 3 is a schematic cross-sectional view of a second region 3b according to the embodiment. [Figure 4] FIG. 4 is a flowchart of a method for manufacturing a p-type absorber layer according to an embodiment. [Figure 5] FIG. 5 is a cross-sectional view of a multi-junction solar cell according to an embodiment. [Figure 6] FIG. 6 is a perspective view of a solar cell module according to an embodiment. [Figure 7] FIG. 7 is a cross-sectional view of a solar cell module according to an embodiment. [Figure 8] FIG. 8 is a configuration diagram of a solar power generation system according to an embodiment. [Figure 9] FIG. 9 is a schematic diagram of a vehicle according to an embodiment. [Figure 10] FIG. 10 is a schematic diagram of a flying object according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0007] A preferred embodiment of the present invention will be described in detail below with reference to the drawings. Unless otherwise specified, the physical property values ​​are those at 25°C and 1 atmosphere (atmosphere). Averages indicate arithmetic mean values. Each concentration is the average concentration of the target region or layer. In each layer, the presence of a specific element means, for example, an element whose presence is confirmed by SIMS (Secondary Ion Mass Spectrometry), and the absence of a specific element means, for example, an element whose presence cannot be confirmed by SIMS. When the cross-sectional area of ​​a particle captured in a cross-sectional image is S, the particle size (diameter) of each particle is calculated as (4S / π) 1 / 2 is.

[0008] In the specification, " / " represents a division symbol. However, the " / " in "or / and" means "or." In the specification, "·" represents a multiplication symbol. In the numerical values ​​in the specification, "." represents a decimal point.

[0009] (First embodiment) The first embodiment relates to a solar cell. FIG. 1 shows a schematic cross-sectional view of a solar cell 100 according to the first embodiment. As shown in FIG. 1, the solar cell 100 according to this embodiment includes a substrate 1, a p-electrode 2 (first electrode), a p-type light absorbing layer 3, an n-type layer 4, and an n-electrode 5 (second electrode). An intermediate layer (not shown) may be included between the n-type layer 4 and the n-electrode 5. Sunlight may be incident on either the n-electrode 5 side or the p-electrode 2 side, but it is more preferable for it to be incident on the n-electrode 5 side. Because the solar cell 100 according to this embodiment is a transparent solar cell, it is preferably used on the top cell side (light incident side) of a multi-junction solar cell. In FIG. 1, the substrate 1 is provided on the side of the p-electrode 2 opposite the p-type light absorbing layer 3. However, the substrate 1 may also be provided on the side of the n-electrode 5 opposite the n-type layer 4. The following describes the embodiment shown in FIG. 1. However, a similar embodiment is possible in which the substrate 1 is provided on the n-electrode 5 side, except for the position of the substrate 1. In the solar cell 100 of this embodiment, light is incident from the n-electrode 5 side toward the p-electrode 2 side.

[0010] Substrate 1 is a transparent substrate. Substrate 1 can be made of organic materials such as light-transmitting acrylic, polyimide, polycarbonate, polyethylene terephthalate (PET), polypropylene (PP), fluorine-based resins (such as polytetrafluoroethylene (PTFE), perfluoroethylene propene copolymer (FEP), ethylene tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), and perfluoroalkoxyalkane (PFA)), polyarylate, polysulfone, polyethersulfone, and polyetherimide, or inorganic materials such as soda-lime glass, white glass, chemically strengthened glass, and quartz. Substrate 1 can also be made by laminating the above-listed substrates.

[0011] The p-electrode 2 is provided on the substrate 1 and is disposed between the substrate 1 and the p-type light absorbing layer 3. The p-electrode 2 preferably forms an ohmic junction with the p-type light absorbing layer 3. The p-electrode 2 is a light-transmitting conductive layer provided on the p-type light absorbing layer 3 side. The thickness of the p-electrode 2 is typically 100 nm or more and 2000 nm or less. In FIG. 1, the p-electrode 2 is in direct contact with the p-type light absorbing layer 3. The p-electrode 2 preferably includes one or more transparent conductive oxide films. Examples of the oxide transparent conductive film include semiconductor conductive films such as indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), boron-doped zinc oxide (BZO), gallium-doped zinc oxide (GZO), doped tin oxide, titanium-doped indium oxide (ITiO), indium zinc oxide (IZO), indium gallium zinc oxide (IGZO), and hydrogen-doped indium oxide (IOH). The oxide transparent conductive film may be a laminated film having multiple films. The dopant for the tin oxide film or the like is not particularly limited as long as it is one or more elements selected from the group consisting of In, Si, Ge, Ti, Cu, Sb, Nb, Ta, W, Mo, F, Cl, etc. The p-electrode 2 preferably includes a tin oxide film doped with one or more elements selected from the group consisting of In, Si, Ge, Ti, Cu, Sb, Nb, Ta, W, Mo, F, Cl, etc. In the doped tin oxide film, the one or more elements selected from the group consisting of In, Si, Ge, Ti, Cu, Sb, Nb, Ta, W, Mo, F, Cl, etc. are preferably contained in an amount of 10 atomic % or less relative to the tin contained in the tin oxide film. The p-electrode 2 can be a laminated film formed by stacking a transparent conductive oxide film and a metal film.The metal film preferably has a thickness of 1 nm to 2 μm, and the metal (including alloy) contained in the metal film is not particularly limited, and may be Mo, Au, Cu, Ag, Al, Ta, or W. The p-electrode 2 preferably includes a dot-, line-, or mesh-shaped electrode (one or more selected from the group consisting of metals, alloys, graphene, conductive nitrides, and conductive oxides) between the oxide transparent conductive film and the substrate 1, or between the oxide transparent conductive film and the p-type light absorbing layer 3. The dot-, line-, or mesh-shaped metal preferably has an aperture ratio of 50% or more relative to the oxide transparent conductive film. The dot-, line-, or mesh-shaped metal may be Mo, Au, Cu, Ag, Al, Ta, or W, and is not particularly limited. When a metal film is used for the p-electrode 2, the thickness is preferably approximately 5 nm or less from the viewpoint of transparency. When a line- or mesh-shaped metal film is used, transparency is ensured by the apertures, so the thickness of the metal film is not limited to this.

[0012] A doped tin oxide film that forms an ohmic junction with the p-type light absorbing layer 3 is preferably provided on the outermost surface of the oxide transparent conductive film on the p-type light absorbing layer 3 side. It is preferable that at least a part of the doped tin oxide film provided on the outermost surface of the oxide transparent conductive film on the p-type light absorbing layer 3 side is in direct contact with the p-type light absorbing layer 3.

[0013] The p-type light absorbing layer 3 is a p-type semiconductor layer. The p-type light absorbing layer 3 is provided on the p-electrode 2. The p-type light absorbing layer 3 may be in direct contact with the p-electrode 2, or other layers may be present as long as electrical contact with the p-electrode 2 can be ensured. The p-type light absorbing layer 3 is disposed between the p-electrode 2 and the n-type layer 4. The p-type light absorbing layer 3 is in direct contact with the n-type layer 4. The p-type light absorbing layer 3 includes a first region 3a. The p-type light absorbing layer 3 is preferably a compound semiconductor. The p-type light absorbing layer 3 is preferably a p-type compound semiconductor when undoped. As a p-type compound semiconductor layer when undoped, a semiconductor layer containing any one of cuprous oxide (cuprous oxide compound), a compound having a chalcopyrite structure, a compound having a kesterite structure, and a compound having a stannite structure is preferred. When a semiconductor layer containing any one of a compound having a chalcopyrite structure, a compound having a kesterite structure, and a compound having a stannite structure is used for the p-type light absorbing layer 3, and when cuprous oxide is used for the p-type light absorbing layer 3, the optical path length is increased and the quantum efficiency of light with a wavelength of 500 nm or more is improved.

[0014] When the p-type light absorbing layer 3 contains cuprous oxide, it is preferably a cuprous oxide compound mainly composed of cuprous oxide or a composite oxide of cuprous oxide. In other words, the p-type light absorbing layer 3 is preferably a semiconductor layer containing a cuprous oxide compound. The p-type light absorbing layer 3 is preferably a polycrystalline cuprous oxide compound. The cuprous oxide compound may contain a trace amount of one or more cuprous oxide impurities selected from the group consisting of copper (Cu), copper oxide (CuO), and copper hydroxide (Cu(OH)2) as a partial impurity.

[0015] The number of oxygen atoms contained in the cuprous oxide compound is preferably 0.48 or more and 0.56 or less, assuming that the number of copper atoms is 1. If the oxygen content is too high relative to the copper content, the ratio of copper oxide contained in the cuprous oxide compound increases, which is undesirable because it narrows the band gap and reduces the light transmittance of the p-type light absorbing layer 3. If the oxygen content is too low relative to the copper content, the copper content in the cuprous oxide compound increases, which is undesirable because it reduces the light transmittance.

[0016] The cuprous oxide compound is an oxide mainly composed of cuprous oxide. When the total amount of all metal elements contained in the p-type light absorbing layer 3 is taken as 100%, the copper content in the p-type light absorbing layer 3 is preferably 95% to 100%, more preferably 98% to 100%, and even more preferably 99% to 100%.

[0017] Preferably, 95 wt% to 100 wt% of the p-type light absorbing layer 3 is a cuprous oxide compound, more preferably 98 wt% to 100 wt% of the p-type light absorbing layer 3 is a cuprous oxide compound, and even more preferably 99 wt% to 100 wt% of the p-type light absorbing layer 3 is a cuprous oxide compound. 100 wt% of the p-type light absorbing layer 3 can be composed of a cuprous oxide compound.

[0018] The p-type light absorbing layer 3 is preferably a polycrystal of cuprous oxide or / and cuprous oxide complex oxide. a M1 b O c The oxide represented by the formula (1) is preferably an oxide represented by the formula (1). M1 is preferably one or more elements selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Pd, Ag, Zn, Cd, Al, Ga, In, C, Si, Ge, Sn, N, P, Sb, Ge, Bi, Cl, F, Br, I, Mn, Tc, and Re. Preferably, a, b, and c satisfy the following conditions: 1.80≦a≦2.01, 0.00≦b≦0.20, and 0.98≦c≦1.02. The p-type light absorbing layer 3 preferably contains almost no heterophase Cu and / or CuO. The p-type light absorbing layer 3 preferably contains fewer heterophases and has good crystallinity, as this increases the light transmittance of the p-type light absorbing layer 3. The band gap of the p-type light absorbing layer 3 is preferably 2.0 eV or more and 2.2 eV or less. With a band gap in this range, in a multi-junction solar cell using a solar cell with Si as the light absorption layer as the bottom cell and the solar cell of the embodiment as the top cell, sunlight can be efficiently utilized in both the top cell and the bottom cell.

[0019] The composition ratio of the p-type light absorbing layer 3 is the composition ratio of the entire p-type light absorbing layer 3.

[0020] The thickness of the p-type light absorbing layer 3 is determined by cross-sectional observation using an electron microscope or a step gauge, and is preferably 2000 nm to 15000 nm (2 μm to 15 μm), more preferably 2500 nm to 10000 nm, even more preferably 4000 nm to 10000 nm, and preferably 4000 nm to 8000 nm. The p-type light absorbing layer 3 has a small uneven surface, and the difference between the minimum and maximum thicknesses of the p-type light absorbing layer 3 is preferably 0 nm to 100 nm, more preferably 0 nm to 50 nm. The p-type light absorbing layer 3 has a rectangular parallelepiped shape.

[0021] The p-type light absorbing layer 3 includes a first region 3a with a large grain size and a second region 3b with a small grain size. The first region 3a is located on the n-type layer 4 side. The second region 3b is located on the p-electrode 2 side. The first region 3a and the second region 3b are preferably directly connected to each other.

[0022] The first average crystal grain size d1, which is the average crystal grain size of the first region 3a, is preferably larger than the second average crystal grain size d2, which is the average crystal grain size of the second region 3b.

[0023] When the p-type light absorbing layer 3 and the n-type layer 4 are made of different compounds and form a pn heterojunction, elements in the n-type layer 4 may diffuse into the p-type light absorbing layer 3 and / or elements in the p-type light absorbing layer 3 may diffuse into the n-type layer 4, resulting in a mixed region with a thickness of 20 nm or less between the p-type light absorbing layer 3 and the n-type layer 4 where the elements have diffused. 90 atom % or less of the metal elements in the mixed region are the metals contained in the p-type light absorbing layer 3, and 10 atom % or more of the metal elements in the mixed region are the metal elements contained in the n-type layer 4. When a mixed region exists, the surface of the p-type light absorbing layer 3 facing the n-type layer 4 is the surface of the p-type light absorbing layer 3 facing the n-type layer 4 excluding the mixed region, and the surface of the n-type layer 4 facing the p-type light absorbing layer 3 is the surface of the n-type layer 4 facing the p-type light absorbing layer 3 excluding the mixed region.

[0024] The thickness of the p-type light absorbing layer 3 is determined by cross-sectional observation using an electron microscope or a step gauge, and is preferably 2000 nm to 15000 nm (1 μm to 15 μm), more preferably 2500 nm to 10000 nm, even more preferably 4000 nm to 10000 nm, and preferably 4000 nm to 8000 nm. The p-type light absorbing layer 3 has a small uneven surface, and the difference between the minimum and maximum thicknesses of the p-type light absorbing layer 3 is preferably 0 nm to 100 nm, more preferably 0 nm to 50 nm. The p-type light absorbing layer 3 has a rectangular parallelepiped shape.

[0025] The thickness of the second region 3b is preferably 70 nm or more and 1000 nm or less. The presence of the second region 3b improves the quantum efficiency of light with a wavelength of 500 nm or more, contributing to improved conversion efficiency. If the thickness of the second region 3b is thin, the effect of improving the quantum efficiency of light with a wavelength of 500 nm or more is small. If the thickness of the second region 3b is thick, it is not preferable because it increases the number of defects in the light absorption layer and makes electron recombination more likely. From the above perspectives, the thickness of the second region 3b is preferably more than 70 nm and 1000 nm or less, and more preferably 100 nm or more and 1000 nm or less.

[0026] The thickness of the second region 3b is preferably 150 nm or more and 600 nm or less. The presence of the second region 3b improves the quantum efficiency of light with a wavelength of 500 nm or more, contributing to improved conversion efficiency. If the thickness of the second region 3b is thin, the effect of improving the quantum efficiency of light with a wavelength of 500 nm or more is small. If the thickness of the second region 3b is thick, it is not preferable because it increases the number of defects in the light absorption layer and makes electron recombination more likely. From the above perspective, the thickness of the second region 3b is more preferably 150 nm or more and 500 nm or less.

[0027] The method for measuring the thickness of the first region 3a and the second region 3b will now be described. Observing the cross sections of analysis spots A1 to A9 in Figure 2, the region with large grain size on the n-type layer 4 side is designated as the first region 3a. Observing the cross sections of analysis spots A1 to A9 in Figure 2, the region with small grain size on the p-electrode 2 side is designated as the second region 3b.

[0028] The thickness of the second region 3b is preferably 70 nm or more and 1000 nm or less, and is preferably 1% or more and less than 20% of the thickness of the p-type light absorbing layer 3. A relatively thin second region 3b with respect to the thickness of the p-type light absorbing layer 3 is undesirable because the effect of increasing the optical path length is not obtained. A relatively thick second region 3b with respect to the thickness of the p-type light absorbing layer 3 is undesirable because it increases defects in the light absorbing layer and makes electron recombination more likely. Therefore, the thickness of the second region 3b is preferably 70 nm or more and 1000 nm or less, and is preferably 3% or more and less than 15% of the thickness of the p-type light absorbing layer 3. From the above viewpoint, the thickness of the second region 3b is preferably greater than 70 [nm] and less than 1000 [nm], and is 3% or more and less than 15% of the thickness of the p-type light absorbing layer 3, and more preferably greater than 100 [nm] and less than 1000 [nm], and is 3% or more and less than 15% of the thickness of the p-type light absorbing layer 3.

[0029] The thickness of the second region 3b is preferably 150 nm or more and 600 nm or less, and is preferably 1% or more and less than 20% of the thickness of the p-type light absorbing layer 3. A relatively thin second region 3b with respect to the thickness of the p-type light absorbing layer 3 is undesirable because the effect of increasing the optical path length is not obtained. A relatively thick second region 3b with respect to the thickness of the p-type light absorbing layer 3 is undesirable because it increases defects in the light absorbing layer and makes electron recombination more likely. Therefore, the thickness of the second region 3b is preferably 150 nm or more and 600 nm or less, and is preferably 3% or more and less than 15% of the thickness of the p-type light absorbing layer 3. From the above perspective, the thickness of the second region 3b is preferably 150 nm or more and 500 nm or less, and is preferably 3% or more and less than 15% of the thickness of the p-type light absorbing layer 3.

[0030] When the thickness of the second region 3b is equal to or greater than 1% and less than 20% of the thickness of the p-type light absorbing layer 3, the thickness of the first region 3a is preferably equal to or greater than 80% and less than 99% of the thickness of the p-type light absorbing layer 3. When the first region 3a occupies most of the p-type light absorbing layer 3, a solar cell 100 with high conversion efficiency is obtained.

[0031] When the thickness of the second region 3b is 3% or more and less than 15% of the thickness of the p-type light absorbing layer 3, the thickness of the first region 3a is preferably 85% or more and 97% or less of the thickness of the p-type light absorbing layer 3. When the first region 3a occupies most of the p-type light absorbing layer 3, a solar cell 100 with high conversion efficiency is obtained.

[0032] The first average crystal grain size d1, which is the average crystal grain size of the first region 3a, is preferably 1.5 μm or more. The first average crystal grain size d1 is preferably 80% or more and less than 100% of the thickness of the p-type light absorbing layer 3. The first region 3a is the main region in the p-type light absorbing layer 3 that absorbs light and generates electricity. If the first average crystal grain size d1 of the first region 3a is large, the first region can absorb light of 350 nm or more and less than 650 nm, thereby generating electricity with high efficiency. The first average crystal grain size d1 is preferably 90% or more and 100% or less of the thickness of the p-type light absorbing layer 3 minus the thickness of the second region 3b, i.e., the thickness of the first region 3a, and more preferably 95% or more and 100% or less. Furthermore, it is preferable that particles contained in the p-type light absorbing layer 3, whose particle size is 50% or more and less than 100% of the thickness of the p-type light absorbing layer 3, are not in direct contact with the p-electrode 2.

[0033] The first average crystal grain size d1 is determined by observing the p-type light absorbing layer 3 in each cross-sectional image of the analysis spots A1 to A9 in FIG. 2. For example, a scanning electron microscope (SEM) is used to obtain the cross-sectional image. The magnification is approximately 10,000 times to 200,000 times, and the photographed area is equal to or greater than the thickness of the p-type light absorbing layer 3. The cross-sectional image includes the p-type light absorbing layer 3, the n-type layer 4, and the p-electrode 2. In the p-type light absorbing layer 3 on the n-type layer 4 side of the cross-sectional image, five crystals with the longest diameters in the stacking direction are selected. The average diameters in the stacking direction of the five selected crystals are calculated. The average values ​​calculated from the nine analysis spots A1 to A9 are further averaged, and the average value obtained is defined as the average crystal grain size d1.

[0034] The second average crystal grain size d2 of the second region 3b is preferably 0.005 μm or more and 0.60 μm or less, and more preferably 0.15 μm or more and 0.45 μm or less. The presence of the second region 3b with a small grain size on the p-electrode 2 side allows the second region 3b to absorb light with long wavelengths that is not absorbed by the first region 3a. This improves the quantum efficiency of the solar cell 100 for light with wavelengths of 500 nm or more.

[0035] The maximum value of the crystal grain size in the second region 3b is preferably 0.05 μm or more and 2.00 μm or less, and more preferably 0.30 μm or more and 1.0 μm or less.

[0036] The second average crystal grain size d2 was determined by measuring the analysis spots A1 to A9 in Figure 2 using SEM-EBSD (scanning electron microscope-backscattered electron diffraction). Specifically, the boundary between the p-electrode 2 and the p-type absorber layer 3 was observed. An area where the thickness of the second region 2b could be confirmed was selected for the observation. The accelerating voltage was 1 kV, the sample tilt was 70°, the measurement area was 0.8 μm × 11 μm, and the step was 0.01 μm. By irradiating the sample tilted by 70° with an electron beam, diffracted electron beams were generated from each crystal plane. These backscattered electron diffraction patterns were analyzed to determine the crystal plane orientation in the image. Boundaries with a crystal plane orientation difference of 5° or more were treated as grain boundaries, and each particle was color-coded based on its crystal plane orientation. This color coding allows for easy visual identification of grain boundaries. The diameters of all particles contained in the measurement area are calculated by multiplying the number of pixels by the square of the step size (0.01 μm), and then calculating the particle area by multiplying the particle area by 4 / π^(1 / 2). When calculating the particle diameter, the area including the grain boundary is considered to be a particle, and the particle diameter is calculated. The average value of the diameters of all particles contained in the measurement areas of analysis spots A1 to A9 is taken as the second average crystal grain size d2.

[0037] The ratio of the second average crystal grain size d2 of the second region 3b to the first average crystal grain size d1 of the first region 3a (d2 / d1) is preferably 0.005 to 0.300, more preferably 0.010 to 0.300, and even more preferably 0.020 to 0.100. When d2 / d1 is within the preferred range, electron recombination is reduced, which is preferable because it increases the optical path length on the back side.

[0038] The second region 3b preferably includes voids G. The average cross-sectional diameter of the voids G is preferably 0.10 μm or more and 0.60 μm or less, and more preferably 0.15 μm or more and 0.45 μm or less. By keeping the voids within the above range, it is possible to prevent an increase in electron recombination.

[0039] It is preferable that the first region 3a contains almost no voids G. The area ratio R1 of the voids G to the cross section of the first region 3a ([total cross-sectional area of ​​the voids G in the first region 3a] / [cross-sectional area of ​​the first region 3a]) is preferably 0 or more and 0.1 or less, more preferably 0 or more and 0.01 or less.

[0040] The area ratio R2 of the voids G to the cross section of the second region 3b ([total cross-sectional area of ​​the voids G in the second region 3b] / [cross-sectional area of ​​the second region 3b]) is preferably 0.01 or more and 0.3 or less, more preferably 0.05 or more and 0.1 or less.

[0041] When the first region 3a includes voids G, the area ratio R1 (R1 / R2) of the voids G in the cross section of the first region 3a to the area ratio R2 of the voids G in the cross section of the second region 3b is preferably 0.001 or more and 0.1 or less.

[0042] When examining the grain boundaries of analysis spots A1 to A9 in Figure 2 using SEM-EBSD, regions determined to be other than crystals are regarded as voids G. The diameter of the voids G is calculated in the same manner as the second average crystal grain size d2. The average value of the diameters of all voids included in the measurement regions of analysis spots A1 to A9 is taken as the average cross-sectional diameter of the voids G. In addition, the total cross-sectional area of ​​the voids G in the first region 3a or the second region 3b is calculated by adding up the diameters of the voids G in the first region 3a or the second region 3b.

[0043] The particle size distribution of the second region 3b preferably includes two to three peaks in the range of 0.005 μm to 0.70 μm, and more preferably includes two peaks (only two peaks) in the range of 0.005 μm to 0.70 μm. The particle size distribution of the second region 3b preferably includes a first peak P1 in the range of 0.005 μm to less than 0.30 μm (first range) and a second peak P2 in the range of 0.30 μm to 0.70 μm (second range). The p-type absorber layer 3 fabricated by the manufacturing method of the embodiment has a second region 3b whose particle size distribution includes peaks in both the first and second ranges.

[0044] As shown in the schematic cross-sectional view of FIG. 3, the second region 3b of the embodiment contains particles A with large particle diameters, particles B with small particle diameters, and small voids G. S and large gap G L It is preferred that the following be included:

[0045] The particle size distribution in the second region 3b, ranging from 0.10 μm to 0.70 μm, is described below. The class width is 0.05 μm. For example, the 0.10 μm class includes particles with particle sizes ranging from 0.075 μm to 0.125 μm.

[0046] If the proportion of particles in the second region 3b having a crystal grain size of 0.075 μm or more and 2.00 μm or less is taken as 100%, it is preferable that the abundance ratio of the first peak (the proportion of particles in the first peak P1 ([number of particles in the first peak] / [number of particles having a crystal grain size of 0.075 μm or more and 2.00 μm or less])) and the abundance ratio of the second peak (the proportion of particles in the second peak P2 ([number of particles in the second peak] / [number of particles having a crystal grain size of 0.075 μm or more and 2.00 μm or less]))) are each 10% or more and 80% or less.

[0047] If the proportion of particles in the second region 3b having a crystal grain size of 0.075 μm or more and 2.00 μm is taken as 100%, it is preferable that the proportion of particles in the first peak P1 or the proportion of particles in the second peak P2 is highest in the range of crystal grain size of 0.075 μm or more and 2.00 μm.

[0048] The ratio of particles contained in the first peak P1 or the ratio of particles contained in the second peak P2 is preferably 20% or more and 70% or less. It is preferable that the first peak P1 and the second peak P2 are neither very sharp peaks nor nearly flat peaks.

[0049] In the particle size distribution of the second region 3b, it is preferable that a valley exists between the first peak P1 and the second peak P2. More specifically, it is preferable that the particle size distribution of the second region 3b includes a class with a ratio of 0% or more and less than 4% between the peak top of the first peak P1 and the peak top of the second peak P2.

[0050] The grain size of each of the analysis spots A1 to A9 is determined in the same manner as in determining the second average grain size d2 by SEM-EBSD. A grain size distribution is created using the determined grain sizes with a class width of 50 [nm]. The vertical axis of the grain size distribution shows the abundance ratio (the ratio of the number of particles belonging to a class to the number of particles contained in the second region 3b). The first peak P1 and the second peak P2 can be identified from the created grain size distribution.

[0051] The crystal orientation of the particles in the second region 3b is preferably random. The p-type absorber layer 3 produced by the manufacturing method of the embodiment has the second region 3b with random crystal orientation. Whether the crystal orientation of the second region 3b in the embodiment is random or not is determined as follows: Observe each analysis spot to obtain an IPF (Inverse Pole Figure) map. Select the highest particle (selected central particle) at the center of the IPF map. Select 20 particles (selected peripheral particles) that are closest to the particle closest to the selected central particle. Calculate the misorientation between each of the selected 20 peripheral particles and the central particle. If there are 10 or more particles with a misorientation of 15° or more, the crystal orientation of the particles in the second region 3b is considered to be random.

[0052] The cross section of the second region 3b preferably includes the (001) plane ((200) plane), the (101) plane, and the (111) plane. Even if the crystal orientation is random according to the above criteria, it is preferable that the cross section of the second region 3b includes a plurality of grains with different plane orientations, rather than being biased toward a specific plane orientation.

[0053] The crystal plane orientation in the cross section of the second region 3b is determined as follows: In the same way as the method for determining the second average crystal grain size d2 by SEM-EBSD, the misorientation between the crystal plane orientation of each crystal and the central grain is determined from the IPF maps of the analysis spots A1 to A9.

[0054] The second region 3b preferably contains small voids with a cross-sectional diameter of 0.01 μm or more and less than 0.1 μm, and large voids with a cross-sectional diameter of 0.1 μm or more and 0.6 μm or less.

[0055] The carrier concentration of the second region 3b is preferably higher than that of the first region 3a. When the carrier concentration of the second region 3b is higher than that of the first region 3a, the back surface field (BSF) suppresses recombination, improving the short-circuit current density and contributing to improved conversion efficiency. The average carrier concentration of the second region 3b is preferably 10 times higher than that of the first region 3a. 0 more than 10 times 6 The following is preferred:

[0056] When the carrier concentration of the second region 3b is higher than that of the first region 3a, the second region 3b is preferably one or more elements selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Pd, Ag, Zn, Cd, Al, Ga, In, C, Si, Ge, Sn, N, P, Sb, and Bi. When the second region 3b contains one or more elements selected from the group consisting of Cl, F, Br, I, Mn, Tc, and Re, the total concentration of the one or more elements selected from the group consisting of Cl, F, Br, I, Mn, Tc, and Re in the second region 3b is preferably less than 100% of the total concentration of the one or more elements selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Pd, Ag, Zn, Cd, Al, Ga, In, C, Si, Ge, Sn, N, P, Sb, and Bi in the second region 3b, and more preferably 10% or less.

[0057] From the viewpoint of preventing recombination, it is preferable that the carrier concentration in second region 3b is lower on the first region 3a side and higher on the p electrode 2 side. From the viewpoint of preventing recombination, it is preferable that the carrier concentration in second region 3b is lower on the first region 3a side and higher on the p electrode 2 side, with the carrier concentration gradually changing from the first region 3a side toward the p electrode 2 side.

[0058] The average carrier concentration in the second region 3b is 10 times the average carrier concentration in the first region 3a. 0 more than 10 times 5 When the average carrier concentration of the second region 3b is 10 times or less than the average carrier concentration of the first region 3a, the thickness of the second region 3b is preferably 0.07 μm or more and 0.1 μm or less. 0 more than 10 times 5 When the average carrier concentration of the second region 3b is 10 times or less than the average carrier concentration of the first region 3a, the thickness of the second region 3b is preferably 0.01 times or more and 0.05 times or less than the thickness of the first region 3a. 1 more than 10 times 6 If the thickness of the second region 3b is equal to or greater than 0.1 μm and equal to or less than 0.6 μm, it is preferable that the thickness of the second region 3b is equal to or greater than 0.05 and equal to or less than 0.3 times the thickness of the first region 3a.

[0059] The hole concentration of the p-type light absorbing layer 3 is measured. For example, if the p-type light absorbing layer 3 is 8 μm thick, the first region 3a extends from the surface of the p-type light absorbing layer 3 on the n-type layer 4 side to a depth of 7 μm toward the p-electrode 2 side, and the second region 3b is a 1 μm-thick region of the p-type light absorbing layer 3 on the p-electrode 2 side excluding the first region 3a, for example, first, remove the material from the n-type layer 4 side toward the p-type light absorbing layer 3 side by polishing or etching, then remove the substrate 1 and p-electrode 2 from the p-electrode 2 side, and measure the overall hole concentration of the p-type light absorbing layer 3 alone. Next, the hole concentration is measured while removing the p-type light absorbing layer 3 from the side where the p-electrode 2 was located. For example, the hole concentration is measured every 200 nm of thickness removed. Setting the 200 nm interval, which is the interval between removal thicknesses used to measure the hole concentration, to a fixed interval of 10 nm or more and 100 nm or less can improve measurement accuracy.

[0060] The first region 3a is preferably in direct contact with the second region 3b. The surface of the first region 3a facing the second region 3b is preferably in direct contact with the surface of the second region 3b facing the first region 3a. The surface of the first region 3a facing the n-type layer 4 is preferably in direct contact with the surface of the n-type layer 4 facing the p-type light absorbing layer 3. The surface of the second region 3b facing the p-electrode 2 is preferably in direct contact with the surface of the p-electrode 2 facing the p-type light absorbing layer 3.

[0061] The p-type light absorbing layer 3 is preferably formed by, for example, sputtering. Fig. 4 shows a flowchart of a method for manufacturing the p-type light absorbing layer 3. The method for manufacturing the p-type light absorbing layer 3 shown in the flowchart of Fig. 4 includes a low-temperature film formation step (S01) and a high-temperature film formation step (S02). By performing two sputtering steps at different temperatures, it is possible to form a p-type light absorbing layer 3 including both the first region 3a and the second region 3b. An oxidation step (S03) may also be performed.

[0062] When the substrate 1 is provided on the p-electrode 2 side, it is preferable to carry out the low-temperature film-forming step (S01), the high-temperature film-forming step (S02), and the oxidation step (S03) in this order.

[0063] When the substrate 1 is provided on the n-electrode 5 side, it is preferable to partially reverse the order of the steps in the flowchart of FIG. 4 and perform the high-temperature film formation step (S02), the low-temperature film formation step (S01), and the oxidation step (S03) in that order.

[0064] During sputtering, atoms other than Cu can be introduced into the chamber to form a doped Cu2O film or a composite oxide film of cuprous oxide.

[0065] In sputtering, a component on which the p-type light absorbing layer 3 is to be formed (for example, a component having a p-electrode 2 formed on a substrate 1 or a component having an n-electrode 5 and an n-type layer 4 formed on a substrate 1) is placed on a stage in a chamber, the component is heated by heating the stage, and a target containing copper as its main component is used to oxidize the copper in an oxidizing atmosphere and deposit it on the component to form a film.

[0066] The sputtering temperature is the temperature of the underlying material on which the p-type light absorbing layer 3 is formed. When the substrate 1 is provided on the p-electrode 2 side, the sputtering temperature is the temperature of the substrate 1 and / or the temperature of the p-electrode 2. When the substrate 1 is provided on the n-electrode 5 side, the sputtering temperature is the temperature of the substrate 1 and / or the temperature of the n-electrode 5. The stage temperature can be considered to be the temperature of the underlying material.

[0067] The sputtering temperature in the low-temperature film formation step (S01) is preferably 200°C or higher and lower than 350°C, and more preferably 250°C or higher and 350°C or lower. The low temperature of the underlying layer makes it difficult for crystals of the p-type light absorbing layer 3 to grow, and the second region 3b with small grain size is formed.

[0068] The oxygen partial pressure in the chamber in the low-temperature film-forming step (S01) is preferably 0.01 [Pa] or more and 4.8 [Pa] or less.

[0069] When the deposition rate of the p-type absorber layer 3 in the low-temperature film formation step (S01) is d [μm / min], the oxygen partial pressure is preferably 0.5 to 1.5 times d (the dimension of the deposition rate is ignored in this calculation).

[0070] In the low-temperature film formation step (S01), the carrier concentration can be increased by introducing one or more atoms selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Pd, Ag, Zn, Cd, Al, Ga, In, C, Si, Ge, Sn, N, P, Sb, Ge, and Bi into the chamber together with Cu atoms.

[0071] The sputtering temperature in the high-temperature film formation step (S02) is preferably 350°C or higher and 600°C or lower, and more preferably 400°C or higher and 550°C or lower. The high temperature of the underlying layer allows crystals of the p-type light absorbing layer 3 to grow, and the first regions 3a with large grain sizes are formed.

[0072] The oxygen partial pressure in the chamber in the high-temperature film-forming step (S02) is preferably 0.01 [Pa] or more and 4.8 [Pa] or less.

[0073] When the deposition rate of the p-type absorber layer 3 in the high-temperature film formation step (S02) is d [μm / min], the oxygen partial pressure is preferably 0.5 to 1.5 times d (the dimension of the deposition rate is ignored in this calculation).

[0074] The sputtering temperature in the high-temperature film-forming step (S02) is preferably 100° C. or more and 400° C. or less higher than the sputtering temperature in the low-temperature film-forming step (S01).

[0075] When shifting from the low-temperature film-forming process (S01) to the high-temperature film-forming process (S02), the temperature of the component may be changed after stopping sputtering, or the temperature may be changed while sputtering is being performed. When the temperature of the component is changed while sputtering is being performed, it is preferable to change the component temperature at a rate of 10°C / min to 100°C / min.

[0076] When transitioning between the high-temperature film formation process (S02) and the low-to-high-temperature film formation process (S01), the temperature of the component may be changed after stopping sputtering, or the temperature may be changed while sputtering is being performed. When changing the temperature of the component while sputtering is being performed, it is preferable to change the component temperature at a rate of -100°C / min or more and -10°C / min or less.

[0077] In the high-temperature film formation step (S01), one or more atoms selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Pd, Ag, Zn, Cd, Al, Ga, In, C, Si, Ge, Sn, N, P, Sb, Ge, Bi, Cl, F, Br, I, Mn, Tc, and Re can be introduced into the chamber together with Cu atoms.

[0078] The oxidation step (S03) is performed by placing the member on which the p-type absorber layer 3 has been formed in an atmosphere having a low oxygen concentration (oxygen partial pressure of 100 [Pa]) to a high oxygen concentration (oxygen partial pressure of 200,000 [Pa]) or less, or in an atmosphere containing ozone. The oxidation treatment (S03) may be omitted. The oxidation treatment (S03) is preferably performed after the low-temperature film-forming step (S01) and the high-temperature film-forming step (S02) have been performed, and before the formation of the n-type layer 4 (when the substrate 1 is provided on the p-electrode 2 side).

[0079] When the p-type light absorbing layer 3 contains a compound having a chalcopyrite structure, examples of the compound having a chalcopyrite structure include compounds containing Group 11 elements (Cu, Ag), Group 13 elements (Al, Ga, In), and Group 16 elements (S, Se, Te). As the n-type dopant for the compound having a chalcopyrite structure, Zn and / or Cd are preferred.

[0080] When the p-type light absorbing layer 3 contains a compound having a kesterite structure, an example of the compound having a kesterite structure is Cu2ZnSn(S,Se)4. As an n-type dopant for the compound having a kesterite structure, Cd is preferred.

[0081] When the p-type light absorbing layer 3 contains a compound having a stannite structure, for example, Cu2(Fe,Zn)Sn(S,Se)4 can be given as the compound having a stannite structure. Cd is preferred as the n-type dopant for the compound having a stannite structure.

[0082] The n-type layer 4 is an n-type semiconductor layer. The n-type layer 4 is disposed between the p-type light absorbing layer 3 and the n-electrode 5. The n-type layer 4 is preferably provided on the p-type light absorbing layer 3. The n-type layer 4 is in direct contact with the surface of the p-type light absorbing layer 3 opposite to the surface in contact with the p-electrode 2. The n-type layer 4 preferably includes a semiconductor layer including an oxide containing Ga or a semiconductor layer including an n-type cuprous oxide compound. The semiconductor layer including an oxide containing Ga or the semiconductor layer including an n-type cuprous oxide compound forms a pn junction (a pin junction if the mixed region is i-type) with the p-type light absorbing layer 3. When a semiconductor layer including an oxide containing Ga is used for the n-type layer 4, the solar cell 100 is a heterojunction solar cell. When a semiconductor layer including a cuprous oxide compound is used for the n-type layer 4, the solar cell 100 is a homojunction solar cell. The n-type layer 4 may be a single layer or multiple layers of a semiconductor layer including an oxide containing Ga or a semiconductor layer including an n-type cuprous oxide compound. The multilayer n-type layer 4 may include a plurality of semiconductor layers containing oxides containing Ga with different compositions, or a plurality of semiconductor layers containing an n-type cuprous oxide compound and a plurality of semiconductor layers containing oxides containing Ga with different compositions. In both heterojunction solar cells and homojunction solar cells, the short-circuit current density is theoretically improved by increasing the depletion layer width, and therefore, in heterojunction solar cells and homojunction solar cells, the short-circuit current density and conversion efficiency are improved by providing the first region 3a.

[0083] It preferably contains a compound (oxide) mainly composed of Ga. The n-type layer 4 may be a mixture of an oxide mainly composed of Ga with other oxides, an oxide mainly composed of Ga doped with other elements, or a mixture of an oxide mainly composed of Ga doped with other elements and other oxides. The n-type layer 4 is a single layer or a multilayer. Of the metal elements contained in the n-type layer 4, Ga is preferably 40 atomic % or more, and more preferably 50 atomic % or more. The metal elements contained in the n-type layer 4 may be arranged in a gradient from the p-type light absorbing layer 3 side to the n-electrode 5 side.

[0084] The n-type layer 4 preferably contains an oxide containing Ga and an element represented by M3. The oxide containing Ga as a main component is, for example, an oxide containing Ga and an element represented by M3. The n-type layer 4 preferably contains an oxide containing Ga and one or more elements M3 selected from the group consisting of H, Sn, Sb, Cu, Ag, Li, Na, K, Cs, Rb, Al, In, Zn, Mg, Si, Ge, N, B, Ti, Hf, Zr, and Ca. The n-type layer 4 preferably contains 90 wt% to 100 wt% of an oxide containing Ga and one or more elements M3 selected from the group consisting of H, Sn, Sb, Cu, Ag, Li, Na, K, Cs, Rb, Al, In, Zn, Mg, Si, Ge, N, B, Ti, Hf, Zr, and Ca. The compound containing Ga as a main component of the n-type layer 4 has an average composition of Ga. h M3 i O j It is preferable that the oxide contains M3 and Ga, and is represented by the following formula: h, i, and j preferably satisfy 1.8≦h≦2.1, 0≦i≦0.2, and 2.9≦j≦3.1.

[0085] Preferably, 90 wt% to 100 wt% of the n-type layer 4 is an oxide containing M3 and Ga. More preferably, 95 wt% to 100 wt% of the n-type layer 4 is an oxide containing M3 and Ga. Even more preferably, 98 wt% to 100 wt% of the n-type layer 4 is a compound represented by an oxide containing M3 and Ga. The Cu contained in the n-type layer 4 is not contained in the raw materials used to form the n-type layer 4, but is Cu contained in the p-type light absorbing layer 3 that has diffused into the n-type layer 4. If other elements are used when forming the p-type light absorbing layer 3, these elements may also diffuse into the n-type layer 4.

[0086] The thickness of the n-type layer 4 is typically 3 nm or more and 100 nm or less. If the thickness of the n-type layer 4 is less than 3 nm, poor coverage of the n-type layer 4 may result in leakage current, degrading the characteristics. If the coverage is good, the thickness is not limited to the above. If the thickness of the n-type layer 4 exceeds 50 nm, excessively high resistance of the n-type layer 4 may degrade the characteristics, or a short-circuit current may decrease due to a decrease in transmittance. Therefore, the thickness of the n-type layer 4 is more preferably 3 nm or more and 20 nm or less, and even more preferably 5 nm or more and 20 nm or less.

[0087] Unless otherwise specified, the composition of the compound in the n-type layer 4 is the average composition of the entire n-type layer 4. The composition of the n-type layer 4 is the average value of the composition at depths of 0.2d4, 0.5d4, and 0.8d4 from the surface of the n-type layer 4 on the p-type light absorbing layer 3 side, where d4 is the thickness of the n-type layer 4. Except for conditions such as a gradient in the elemental composition ratio of the compound in the n-type layer 4, it is preferable that the n-type layer 4 satisfy the above and following preferred compositions at each depth. If the n-type layer 4 is very thin (e.g., 5 nm or less), the composition at a depth of 0.5d from the surface of the n-type layer 4 on the p-type light absorbing layer 3 side can be considered the overall composition of the n-type layer 4. The composition analysis of the n-type layer 4 is the same as that of the p-type light absorbing layer 3.

[0088] In a configuration in which the p-type light absorbing layer 3 does not contain cuprous oxide, a suitable n-type semiconductor layer can be used as the n-type layer 4. In a configuration in which the p-type light absorbing layer 3 does not contain cuprous oxide, an n-type oxide can be used as the n-type layer 4.

[0089] The n-electrode 5 is an electrode on the n-type layer 4 side that is optically transparent to visible light. The n-electrode 5 is preferably provided on the n-type layer 4. The n-type layer 4 is sandwiched between the n-electrode 5 and the p-type light absorbing layer 3. An intermediate layer (not shown) can be provided between the n-type layer 4 and the n-electrode 5. A transparent conductive oxide film is preferably used for the n-electrode 5. The transparent conductive oxide film used for the n-electrode 5 is preferably one or more semiconductor conductive films selected from the group consisting of indium tin oxide, aluminum-doped zinc oxide, boron-doped zinc oxide, gallium-doped zinc oxide, indium-doped zinc oxide, titanium-doped indium oxide, indium gallium zinc oxide, and hydrogen-doped indium oxide. Dopants for films such as tin oxide are not particularly limited as long as they are one or more selected from the group consisting of In, Si, Ge, Ti, Cu, Sb, Nb, Ta, W, Mo, F, Cl, etc. The n-electrode 5 may include a mesh- or line-shaped electrode to reduce the resistance of the transparent conductive oxide film. The mesh or line-shaped electrode may be made of any material, including, but not limited to, Mo, Au, Cu, Ag, Al, Ta, and W. Graphene may also be used for the n-electrode 5. Graphene is preferably laminated with silver nanowires.

[0090] The thickness of the n-electrode 5 is determined by cross-sectional observation using an electron microscope or a step gauge, and is not particularly limited, but is typically 50 nm or more and 2 μm or less.

[0091] The n-electrode 5 is preferably formed by, for example, sputtering.

[0092] (Second embodiment) The second embodiment relates to a multi-junction solar cell. Fig. 5 shows a cross-sectional conceptual diagram of a multi-junction solar cell of the second embodiment. The multi-junction solar cell 200 of Fig. 5 has a solar cell (first solar cell) 100 of the first embodiment and a second solar cell 201 on the light incident side. The light absorption layer of the second solar cell 201 has a band gap smaller than that of the p-type light absorption layer 3 of the solar cell 100 of the first embodiment. Note that the multi-junction solar cell 200 of this embodiment also includes a solar cell in which three or more solar cells are joined together.

[0093] Since the band gap of the p-type light absorbing layer (cuprous oxide) 3 of the first solar cell 100 of the first embodiment is approximately 2.0 eV or more and 2.2 eV or less, the band gap of the light absorbing layer of the second solar cell 201 is preferably 1.0 eV or more and 1.6 eV or less. The light absorbing layer of the second solar cell 201 is preferably one or more compound semiconductor layers selected from the group consisting of CIGS-based and CdTe-based compounds with a high In content, crystalline silicon, and one selected from the group consisting of perovskite-type compounds.

[0094] (Third embodiment) The third embodiment relates to a solar cell module. Fig. 6 shows a perspective view of a solar cell module 300 of the third embodiment. The solar cell module 300 of Fig. 6 is a solar cell module in which a first solar cell module 301 and a second solar cell module 302 are stacked. The first solar cell module 301 is on the light incident side and uses the solar cell 100 of the first embodiment. It is preferable to use the second solar cell 201 for the second solar cell module 302.

[0095] FIG. 7 shows a cross-sectional view of a solar cell module 300. FIG. 7 shows the structure of the first solar cell module 301 in detail, but does not show the structure of the second solar cell module 302. For the second solar cell module 302, the structure of the solar cell module is selected appropriately depending on the light absorption layer of the solar cell to be used, etc. The solar cell module 300 in FIG. 6 includes a plurality of sub-modules 303 surrounded by dashed lines, in which a plurality of solar cells 100 (solar cells) are arranged horizontally and electrically connected in series by wiring 304, and the plurality of sub-modules 303 are electrically connected in parallel or in series. Adjacent sub-modules 303 are electrically connected by bus bars 305.

[0096] Adjacent solar cells 100 are connected by wiring 304, with the n-electrode 5 on the upper side and the p-electrode 2 on the lower side. Similar to the solar cell 100 of the first embodiment, the solar cell 100 of the third embodiment also has a substrate 1, a p-electrode 2, a p-type light absorbing layer 3, an n-type layer 4, and an n-electrode 5. Both ends of the solar cells 100 in a submodule 303 are connected to a bus bar 305, and the bus bar 305 is preferably configured to electrically connect multiple submodules 303 in parallel or series and adjust the output voltage with the second solar cell module 302. Note that the connection configuration of the solar cells 100 shown in the third embodiment is one example, and a solar cell module can be configured using other connection configurations.

[0097] (Fourth embodiment) The fourth embodiment relates to a solar power generation system. The solar cell module of the fourth embodiment can be used as a generator that generates power in the solar power generation system of the fourth embodiment. The solar power generation system of the embodiment generates power using a solar cell module, and specifically includes a solar cell module that generates power, a means for converting the generated electricity, and a storage means for storing the generated electricity or a load that consumes the generated electricity. FIG. 8 shows a configuration diagram of a solar power generation system 400 of the embodiment. The solar power generation system of FIG. 8 includes a solar cell module 401 (300), a converter 402, a storage battery 403, and a load 404. Either the storage battery 403 or the load 404 may be omitted. The load 404 may be configured to utilize the electrical energy stored in the storage battery 403. The converter 402 is a device including a circuit or element that performs power conversion such as voltage transformation or DC-AC conversion, such as a DC-DC converter, a DC-AC converter, or an AC-AC converter. The converter 402 may have a suitable configuration depending on the power generation voltage and the configurations of the storage battery 403 and the load 404 .

[0098] The solar cell included in the submodule 303 included in the solar cell module 300 receives light and generates electricity, and the electrical energy is converted by the converter 402 and stored in the storage battery 403 or consumed by the load 404. It is preferable to provide the solar cell module 401 with a solar tracking drive device for always directing the solar cell module 401 toward the sun, a concentrator for concentrating sunlight, or other devices for improving power generation efficiency.

[0099] The solar power generation system 400 is preferably used in real estate such as residences, commercial facilities, factories, etc., or in movable property such as vehicles, aircraft, electronic devices, etc. By using the solar cell with excellent conversion efficiency of the embodiment in a solar cell module, an increase in the amount of power generation is expected.

[0100] A vehicle is shown as an example of use of the solar power generation system 400. FIG. 9 shows a conceptual diagram of the configuration of the vehicle 500. The vehicle 500 in FIG. 9 includes a vehicle body 501, a solar cell module 502, a power converter 503, a storage battery 504, a motor 505, and tires (wheels) 506. The power generated by the solar cell module 502 provided on the top of the vehicle body 501 is converted by the power converter 503 and charged in the storage battery 504, or consumed by loads such as the motor 505. The vehicle 500 can be moved by rotating the tires (wheels) 506 using power supplied from the solar cell module 502 or the storage battery 504 via the motor 505. The solar cell module 502 need not be a multi-junction type, and may instead be composed solely of a first solar cell module including the solar cell 100 of the first embodiment. When a transparent solar cell module 502 is used, it is also preferable to use the solar cell module 502 as a power generation window on the side of the vehicle body 501 in addition to the top of the vehicle body 501.

[0101] An air vehicle (drone) is shown as an example of use of the solar power generation system 400. The air vehicle uses a solar cell module 401. The configuration of the air vehicle according to this embodiment will be briefly described using the schematic diagram of an air vehicle 600 in FIG. 10. The air vehicle 600 has a solar cell module 401, a body frame 601, a motor 602, rotors 603, and a control unit 604. The solar cell module 401, the motor 602, the rotors 603, and the control unit 604 are arranged on the body frame 601. The control unit 604 converts the power output from the solar cell module 401 and adjusts the output. The motor 602 uses the power output from the solar cell module 401 to rotate the rotors 603. By using the air vehicle 600 with this configuration that includes the solar cell module 401 of the embodiment, an air vehicle that can fly using more power is provided.

[0102] The present invention will be described in more detail below based on examples, but the present invention is not limited to the following examples.

[0103] (Example A: Examples A-1 to A-12, Comparative Examples A-1 to A-6) On the glass substrate 1, ITO (In:Sn=80:20, 20 nm thick) and ATO (Sn:Sb=98:2, 150 nm thick) are deposited on the top surface to form the backside p-electrode 2, which is in contact with the glass. A CuO layer is formed on the ATO as the p-type light-absorbing layer 3 by sputtering in an oxygen and argon gas atmosphere. The CuO layer is formed in two stages. First, a 0.5 μm thick CuO film (second region 3b) is formed on the substrate 1 at a temperature of 250°C. Next, a 5.0 μm thick CuO film (first region 3a) is formed on the substrate 1 at a temperature of 450°C. After the formation of the p-type light-absorbing layer 3, a 10 nm thick GaO film is formed as the n-type layer 4. Then, a 0.1 μm thick film of AZO (ZnO:Al) is formed as the n-electrode 5, and the solar cell 100 is obtained.

[0104] Examples A-2 to A-12 and Comparative Examples A-1 to A-6 are similar to Example A-1 except that the thicknesses of the first region 3a and the second region 3b are different. Table 1 shows the thicknesses, grain sizes, and d1>d2 of the first region 3a and the second region 3b of Examples A-1 to A-12 and Comparative Examples A-1 to A-6. In Example A, the regions on the n-type layer 4 side are the first region 3a and the second region 3b. Table 1 shows comparative examples for each Example and Comparative Example. Here, the grain sizes listed in Table 1 refer to the average crystal grain size of the first region 3a and the average crystal grain size of the second region 3b. If d1>d2 is satisfied, the sample is evaluated as A. If d1>d2 is not satisfied, the sample is evaluated as B. The evaluation of d1>d2 is common to Examples A to C.

[0105] [Table 1]

[0106] A solar simulator simulating an AM1.5G light source is used, and a reference Si cell is used under that light source, with the light intensity adjusted to achieve 1 sun. Measurements are taken at atmospheric pressure with the temperature of the measurement room at 25°C. The voltage is swept and the current density (current divided by the cell area) is measured. With the horizontal axis representing voltage and the vertical axis representing current density, the point where they intersect is the open-circuit voltage Voc. On the measurement curve, multiply the voltage and current density and define the maximum points as Vmpp and Jmpp (maximum power point), respectively. The fill factor can be calculated from FF = (Vmpp * Jmpp) / (Voc * Jsc). Conversion efficiency can also be calculated from Eff. = Voc * Jsc * FF.

[0107] The results of Example A are shown in Table 2. Table 2 shows the open circuit voltage (Voc), current density (Jsc), fill factor (FF), conversion efficiency (Eff.), and transmittance of light with a wavelength of 700 nm or more and 1000 nm or less for the example.

[0108] A light transmittance of 60% or more and 100% or less was evaluated as A, a light transmittance of 50% or more and less than 60% was evaluated as B, and a light transmittance of less than 50% was evaluated as C. The evaluation criteria are common to Examples A to C.

[0109] When Voc is less than that of the comparative example, it is evaluated as B, and when it is equal to or greater than that of the comparative example, it is evaluated as A. The evaluation criteria are the same for Examples A to C.

[0110] When Jsc is equal to or less than the Jsc of the comparative example, it is evaluated as B, and when it is greater than the Jsc of the comparative example, it is evaluated as A. The evaluation criteria are common to Examples A to C.

[0111] When the FF is less than the FF of the comparative example, it is evaluated as B, and when it is equal to or greater than the FF of the comparative example, it is evaluated as A. The evaluation criteria are common to Examples A to C.

[0112] When Eff is equal to or less than the Eff of the comparative example, it is evaluated as B, and when it is greater than the Eff of the comparative example, it is evaluated as A. The evaluation criteria are common to Examples A to C.

[0113] [Table 2]

[0114] In Examples A-1 to A-12, the first average crystal grain size d1 is larger than the second average crystal grain size d2. In Example A, when a film with a small crystal grain size is formed in the CuO layer of the second region 3b, the optical path length is increased, the amount of light absorption in the entire CuO layer including the second region 3b is increased, the current density (Jsc) is increased, and the conversion efficiency (Eff.) is increased.

[0115] (Example B: Examples B-1 to B-12, Comparative Examples B-1 to B-10) On the glass substrate 1, ITO (In:Sn=80:20, 20 nm thick) and ATO (Sn:Sb=98:2, 150 nm thick) are deposited on the top surface to form the backside p-electrode 2, which is in contact with the glass. A CuO layer is formed on the ATO as the p-type light-absorbing layer 3 by sputtering in an oxygen and argon gas atmosphere. The CuO layer is formed in two stages. First, a 0.5 μm thick CuO film (second region 3b) is formed on the substrate 1 at a temperature of 250°C. Next, a 5.0 μm thick CuO film (first region 3a) is formed on the substrate 1 at a temperature of 500°C. After the formation of the p-type light-absorbing layer 3, a 10 nm thick GaO film and a 15 nm thick ZTO film are sequentially formed as the n-type layer 4. Then, a 0.1 μm thick AZO film is formed as the n-electrode 5 to obtain the solar cell 100.

[0116] Solar cells 100 of Examples B-2 to B-12 and Comparative Examples B-1 to B-10 are fabricated by changing the thickness of the second region 3b and the thickness of the p-type absorber layer 3. Table 3 shows examples for comparison with each of the Examples and Comparative Examples.

[0117] Table 3 summarizes the thickness of the second region 3b, the thickness of the p-type light absorbing layer 3, [thickness of the second region 3b] / [thickness of the p-type light absorbing layer 3], and d1>d2 for the examples and comparative examples. The thickness values ​​in Table 3 are shown to one significant digit (rounded off) except for comparative example B-4. Table 3 also shows comparative examples for each example and comparative example. Since comparative example B-1 and comparative examples B-5 to B-10 do not have the second region 3b, d1>d2 is shown with a hyphen.

[0118] [Table 3]

[0119] The results of Example B are shown in Table 4. Table 4 shows the open circuit voltage (Voc), current density (Jsc), fill factor (FF), conversion efficiency (Eff.), and transmittance of light with a wavelength of 700 nm or more and 1000 nm or less for the example.

[0120] Also, it is confirmed whether the crystal orientation of the particles in the second region 3b is random. When the crystal orientation of the particles in the second region 3b is random, it is evaluated as A, and when it is not random, it is evaluated as B. The evaluation criteria are common to Examples B and C.

[0121] The presence or absence of voids in the second region 3b was also evaluated. Cases where voids were present in the second region 3b were evaluated as A, and cases where voids were not present were evaluated as B. The evaluation criteria were the same for Examples B to C. In Comparative Example B-1 and Comparative Examples B-5 to B-10, in which the second region 3b was not present, the crystal orientation and voids were evaluated in the region from the interface between the p-type light absorbing layer 3 and the p-electrode 2 to a position 200 nm toward the n-type layer 4.

[0122] [Table 4]

[0123] When fabricated at a low substrate temperature and voids are intentionally created in the Cu2O layer of the second region 3b, light reflection and refraction occur more easily, the optical path length of light increases, the amount of light absorbed by the Cu2O layer of the entire second region 3b increases, the current density (Jsc) increases, and the conversion efficiency (Eff.) increases.

[0124] Table 5 shows the first average crystal grain size d1, the second average crystal grain size d2, and the maximum value of the crystal grain size in the second region 3b for Examples B-2 to B-6 and Comparative Examples B-1 to B-4.

[0125] [Table 5]

[0126] The average crystal grain size can be changed by adjusting not only the sputtering temperature but also the film thickness of the Cu2O layer in the second region 3b. Adjusting the average grain size in the second region 3b ensures a long optical path length, increases the amount of light absorption in the entire Cu2O layer including the second region 3b, improves the current density (Jsc), and increases the conversion efficiency (Eff.). Note that because the second region 3b is not confirmed in Comparative Example B-1, the second average crystal grain size d2, the maximum crystal grain size of the second region 3b, and d1 / d2 for Comparative Example B-1 in Table 5 are all indicated with hyphens.

[0127] In addition, the area ratio R1 of the voids G to the cross section of the first region 3a in the example satisfies the above-mentioned preferred range. In addition, the area ratio R2 of the voids G to the cross section of the second region 3b in the example satisfies the above-mentioned preferred range. In addition, R1 / R2 in the example satisfies the above-mentioned preferred range.

[0128] Next, the particle size distribution of the second region 3b of Examples B-2 to B-6 and Comparative Examples B-1 to B-4 was examined, and the results are shown in Table 6. The number of peaks in the particle size distribution of 0.005 μm or more and 0.70 μm or less (number of peaks: 0.10-0.70), the number of peaks in the particle size distribution of 0.005 μm or more and less than 0.30 μm (number of peaks: 0.10-0.30), the number of peaks in the particle size distribution of 0.30 μm or more and 0.70 μm or less (number of peaks: 0.30-0.70), the abundance ratio of the first peak P1, and the abundance ratio of the second peak P2 are shown.

[0129] [Table 6]

[0130] The distribution of the average crystal grain size can also be adjusted by changing the film thickness of the Cu2O layer in the second region 3b. Adjusting the average grain size ensures a long optical path length, increases the amount of light absorption in the Cu2O layer throughout the second region 3b, improves the current density (Jsc), and increases the conversion efficiency (Eff.). No peaks are observed in the Cu2O layer of the comparative example. Therefore, peaks and valleys are not observed in the comparative example, but peaks are observed in the example. Furthermore, both small and large voids are observed in the second region 3b of the example. Example C also satisfies d1>d2.

[0131] (Example C: Examples C-1 to C-25) On the glass substrate 1, ITO (In:Sn=80:20, 20 nm thick) and ATO (Sn:Sb=98:2, 150 nm thick) were deposited on the top surface, which served as the backside p-electrode 2 (contacting the glass). A CuO layer was deposited on the ATO as the p-type light-absorbing layer 3 by sputtering in an oxygen and argon gas atmosphere. The CuO layer was formed in two stages. First, a 0.5 μm thick N-doped CuO film (second region 3b) was deposited at a substrate temperature of 250°C. Next, a 5.0 μm thick undoped CuO film (first region 3a) was deposited at a substrate temperature of 480°C. After the deposition of the p-type light-absorbing layer 3, a 10 nm thick GaO film and a 15 nm thick ZTO film were sequentially deposited as the n-type layer 4. Then, a 0.1 μm thick AZO film is formed as the n-electrode 5 to obtain the solar cell 100.

[0132] Solar cells 100 of Examples C-2 to C-25 were fabricated by changing the thickness of the second region 3b and the dopant element. Table 7 shows the thickness [nm] of the first region 3a, the thickness [nm] of the second region 3b, the dopant, d1>d2, and the comparative example for each example and comparative example.

[0133] [Table 7]

[0134] Table 8 shows the open circuit voltage (Voc), current density (Jsc), fill factor (FF), conversion efficiency (Eff.), transmittance for light with a wavelength of 700 nm or more and 1000 nm or less, whether the crystallinity of the second region 3b is random, and whether or not there are voids in the second region 3b for Examples C-1 to C-25.

[0135] [Table 8]

[0136] Sputter deposition at a low substrate temperature while adding impurities results in smaller crystal grains in the Cu2O layer of the second region 3b and makes voids more likely to form, which increases the optical path length, increases the amount of light absorbed by the Cu2O layer throughout the second region 3b, increases the current density (Jsc), and improves the conversion efficiency (Eff.).

[0137] Although the embodiments of the present invention have been described above, the present invention should not be construed as being limited to the above-described embodiments, and the components can be modified and embodied in practice without departing from the spirit of the invention. Furthermore, various inventions can be formed by appropriately combining multiple components disclosed in the above-described embodiments. For example, components from different embodiments may be appropriately combined, as in modified examples. In the specification, some elements are shown only by their element symbols.

[0138] The technical solutions of the embodiments are described below. Technical proposal 1 a p-electrode; an n-electrode; a p-type light absorbing layer provided on the p-electrode and including a first region and a second region; an n-type layer provided between the p-type light absorbing layer and the n-electrode, the first region is present on the n-type layer side of the p-type light absorbing layer, the second region is present on the p-electrode side of the p-type light absorbing layer, A solar cell, wherein a first average crystal grain size, which is the average crystal grain size of the first region, is larger than a second average crystal grain size, which is the average crystal grain size of the second region. Technical proposal 2 The solar cell according to Technical Scheme 1, wherein the p-type light absorbing layer is mainly made of cuprous oxide or a composite oxide of cuprous oxide. Technical proposal 3 the thickness of the p-type light absorption layer is 2000 [nm] or more and 15000 [nm] or less, The solar cell according to Technical Scheme 1 or 2, wherein the thickness of the second region is 70 nm or more and 1000 nm or less. Technical proposal 4 The thickness of the second region is 70 nm or more and 1000 nm or less, The solar cell according to any one of Technical Schemes 1 to 3, wherein the thickness of the second region is 3% or more and less than 15% of the thickness of the p-type absorber layer 3. Technical proposal 5 The thickness of the second region is 150 nm or more and 600 nm or less, The solar cell according to any one of Technical Schemes 1 to 4, wherein the thickness of the second region is 3% or more and less than 15% of the thickness of the p-type absorber layer 3. Technical plan 6 The solar cell according to any one of Technical Schemes 1 to 5, wherein the first average crystal grain size is 80% or more and less than 100% of the thickness of the p-type absorber layer. Technical proposal 7 The solar cell according to any one of Technical Schemes 1 to 6, wherein the second average crystal grain size is 0.005 μm or more and 0.60 μm or less. Technical proposal 8 The solar cell according to any one of Technical Schemes 1 to 7, wherein the maximum value of the crystal grain size in the second region is 0.05 μm or more and 2.00 μm or less. Technical proposal 9 A solar cell described in any one of technical proposals 1 to 8, wherein the ratio of the second average crystal grain size to the first average crystal grain size ([average crystal grain size of the second region] / [average crystal grain size of the first region]) is 0.005 or more and 0.300 or less. Technical proposal 10 The second region includes a void; The solar cell according to any one of Technical Schemes 1 to 9, wherein the average cross-sectional diameter of the voids is 0.1 μm or more and 0.60 μm or less. Technical proposal 11 A solar cell described in any one of technical proposals 1 to 10, wherein the area ratio of the voids to the cross section of the second region ([total cross-sectional area of ​​the voids in the second region] / [cross-sectional area of ​​the second region]) is 0.01 or more and 0.3 or less. Technical proposal 12 A solar cell described in any one of technical proposals 1 to 11, wherein the area ratio of voids to the cross section of the first region ([total cross-sectional area of ​​voids in the first region] / [cross-sectional area of ​​the first region]) is greater than or equal to 0.1 and less than or equal to 0.1. Technical proposal 13 the first region includes a void; the second region includes a void; A solar cell described in any one of technical proposals 1 to 12, wherein the area ratio of the voids in the cross section of the first region to the area ratio of the voids in the cross section of the second region is 0.001 or more and 0.1 or less. Technical proposal 14 The solar cell according to any one of Technical Schemes 1 to 13, wherein the particle size distribution in the second region in the range of 0.005 μm to 0.70 μm includes two peaks. Technical proposal 15 The particle size distribution in the second region in the range of 0.005 [μm] or more and 0.70 [μm] or less includes two peaks, The particle size distribution includes a first peak in the range of 0.005 [μm] or more and less than 0.30 [μm], The solar cell according to any one of Technical Schemes 1 to 14, wherein the particle size distribution has a second peak in the range of 0.30 μm or more and 0.70 μm or less. Technical proposal 16 A solar cell according to Technical Scheme 15, wherein the ratio of particles contained in the first peak or the ratio of particles contained in the second peak is 10% or more and 80% or less. Technical proposal 17 A solar cell according to any one of technical proposals 1 to 16, wherein the crystal orientation of the particles in the second region is random. Technical proposal 18 A solar cell described in any one of technical proposals 1 to 17, wherein the two regions include small voids with a cross-sectional diameter of 0.01 μm or more but less than 0.1 μm and large voids with a cross-sectional diameter of 0.1 μm or more but less than 0.6 μm. Technical proposal 19 The solar cell according to any one of Technical Schemes 1 to 18, wherein the second region contains one or more elements selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Pd, Ag, Zn, Cd, Al, Ga, In, C, Si, Sn, N, P, Sb, and Bi. Technical proposal 20 A multi-junction solar cell using the solar cell according to any one of Technical Schemes 1 to 19. Technical Scheme 21 A solar cell module using a solar cell according to any one of technical proposals 1 to 19. Technical proposal 22 A solar power generation system that generates electricity using the solar cell module described in Technical Proposal 21. Technical proposal 23 a p-electrode; an n-electrode; a p-type light absorbing layer provided on the p-electrode and including a first region with a large grain size and a second region with a small grain size; an n-type layer provided between the p-type light absorbing layer and the n-electrode, the first region is present on the n-type layer side of the p-type light absorbing layer, The second region is a solar cell that exists on the p-electrode side of the p-type light absorbing layer. [Explanation of symbols]

[0139] 1: Circuit board 2 :p electrode 3: p-type light absorbing layer 3a: 1st area 3b:Second area 4:N-type layer 5 :n electrode 100: Solar cell 200: Multijunction solar cell 201:Second solar cell 300: Solar cell module 301: First solar cell module 302: Second solar cell module 303: Submodule 304: Wiring 305: Busbar 400: Solar power generation system 401: Solar cell module 402: Converter 403: Storage battery 404: Load 500: Vehicle 501: Body 502: Solar cell module 503: Power conversion device 504: Storage battery 505: Motor 506: Tires (wheels) 600: Flying object 601: Aircraft frame 602: Motor 603: Rotor 604: Control unit

Claims

1. a p-electrode; an n-electrode; a p-type light absorbing layer provided on the p-electrode and including a first region and a second region; an n-type layer provided between the p-type light absorbing layer and the n-electrode, the first region is present on the n-type layer side of the p-type light absorbing layer, the second region is present on the p-electrode side of the p-type light absorbing layer, a first average crystal grain size that is an average crystal grain size of the first region is larger than a second average crystal grain size that is an average crystal grain size of the second region; the p-type light absorbing layer is mainly composed of cuprous oxide or a composite oxide of cuprous oxide, the thickness of the p-type light absorbing layer is 2000 nm or more and 15000 nm or less, The thickness of the second region is 70 nm or more and 1000 nm or less.

2. A solar cell as described in claim 1, wherein the thickness of the second region is 3% or more and less than 15% of the thickness of the p-type light absorption layer.

3. the thickness of the second region is 150 nm or more and 600 nm or less; The solar cell according to claim 1 , wherein the thickness of the second region is equal to or greater than 3% and less than 15% of the thickness of the p-type absorber layer.

4. The solar cell according to claim 1 , wherein the first average crystal grain size is equal to or greater than 80% and less than 100% of the thickness of the p-type light absorbing layer.

5. The solar cell according to claim 1 , wherein the second average crystal grain size is 0.005 μm or more and 0.60 μm or less.

6. 2. The solar cell according to claim 1, wherein the maximum value of the crystal grain size in the second region is 0.05 [μm] or more and 2.00 [μm] or less.

7. 2. The solar cell according to claim 1, wherein the ratio of the second average crystal grain size to the first average crystal grain size ([the second average crystal grain size] / [the first average crystal grain size]) is 0.005 or more and 0.300 or less.

8. The second region includes a void; 2. The solar cell according to claim 1, wherein the average cross-sectional diameter of the voids is 0.1 [mu]m or more and 0.6 [mu]m or less.

9. The solar cell according to claim 1, wherein the area ratio of the voids to the cross section of the second region ([total cross-sectional area of ​​the voids in the second region] / [cross-sectional area of ​​the second region]) is 0.01 or more and 0.3 or less.

10. The solar cell according to claim 1, wherein the area ratio of the voids to the cross section of the first region ([total cross-sectional area of ​​the voids in the first region] / [cross-sectional area of ​​the first region]) is greater than or equal to 0 and less than 0.

1.

11. the first region includes a void; the second region includes a void; The solar cell according to claim 1 , wherein the area ratio of the voids in the cross section of the first region to the area ratio of the voids in the cross section of the second region is 0.001 or more and 0.1 or less.

12. The solar cell according to claim 1 , wherein the particle size distribution in the second region in the range of 0.005 μm to 0.70 μm includes two peaks.

13. The particle size distribution in the second region in the range of 0.005 μm or more and 0.70 μm or less includes two peaks, The particle size distribution includes a first peak in the range of 0.005 [μm] or more and less than 0.30 [μm], The solar cell according to claim 1 , wherein the particle size distribution includes a second peak in the range of 0.30 μm to 0.70 μm.

14. The solar cell according to claim 13 , wherein the ratio of particles included in the first peak or the ratio of particles included in the second peak is 10% or more and 80% or less.

15. The solar cell according to claim 1 , wherein the crystal orientation of the grains in the second region is random.

16. 2. The solar cell according to claim 1, wherein the second region includes small voids having a cross-sectional diameter of 0.01 μm or more but less than 0.1 μm and large voids having a cross-sectional diameter of 0.1 μm or more but less than 0.6 μm.

17. 2. The solar cell according to claim 1, wherein the second region contains one or more elements selected from the group consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ni, Pd, Ag, Zn, Cd, Al, Ga, In, C, Si, Ge, Sn, N, P, Sb, and Bi.

18. A multi-junction solar cell using the solar cell according to any one of claims 1 to 17.

19. A solar cell module using the solar cell according to any one of claims 1 to 17.

20. A solar power generation system that generates power using the solar cell module according to claim 19.

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