JP7799650B2 - Solar cells, multi-junction solar cells, solar cell modules and solar power generation systems - Google Patents

Description

The present invention relates to solar cells, multi-junction solar cells, solar cell modules, and solar power generation systems.

One of the new solar cells is one that uses cuprous oxide (Cu ₂ O) as the light absorption layer. _{Cu 2} O is a wide-gap semiconductor. Cu ₂ O is a safe and inexpensive material made from copper and oxygen, which are abundant on Earth, and is therefore expected to enable the realization of highly efficient, low-cost solar cells.

JP 2016-154171 A

The problem that this invention aims to solve is to provide solar cells, multi-junction solar cells, solar cell modules, and solar power generation systems with excellent conversion efficiency.

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.

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

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). Furthermore, the average indicates an arithmetic mean value. Each concentration is the average concentration of the target region or layer. In each layer, the presence of a specific element refers to an element whose presence is confirmed by, for example, SIMS (Secondary Ion Mass Spectrometry), and the absence of a specific element refers to 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 (4S/π) ^1/2 .

In the specification, "/" represents a division symbol. However, the "/" in "or/and" means "or." In the specification, "・" represents a multiplication symbol. In numerical values in the specification, "." represents a decimal point.

(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 opposite the p-electrode 2 from the p-type light absorbing layer 3. However, the substrate 1 may also be provided on the side opposite the n-electrode 5 from the n-type layer 4. The following describes the configuration shown in FIG. 1. However, a configuration in which the substrate 1 is provided on the n-electrode 5 side is similar, except for the position of the substrate 1. In the solar cell 100 of the embodiment, light is incident from the n-electrode 5 side toward the p-electrode 2 side.

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 (polytetrafluoroethylene (PTFE), perfluoroethylene propene copolymer (FEP), ethylene tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxyalkane (PFA)), polyarylate, polysulfone, polyethersulfone, or polyetherimide; or inorganic materials such as soda-lime glass, white glass, chemically strengthened glass, or quartz. Substrate 1 may also be made by laminating the above-listed substrates.

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 contact 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 Figure 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), but are not particularly limited thereto. The oxide transparent conductive film may also be a laminated film having a plurality of 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. Furthermore, 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, its thickness is preferably approximately 5 nm or less from the perspective 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.

It is preferable that a doped tin oxide film that forms an ohmic junction with the p-type light absorbing layer 3 is 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 portion of the doped tin oxide film that is 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.

The p-type light absorbing layer 3 is a p-type semiconductor layer. The p-type light absorbing layer 3 is disposed 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 is 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. When the p-type light absorbing layer 3 is undoped, it is preferably a p-type compound semiconductor. When undoped, the p-type compound semiconductor layer is preferably a semiconductor layer containing only 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. When a semiconductor layer containing one of a compound having a chalcopyrite structure, a compound having a kesterite structure, and a compound having a stannite structure is used as the p-type light absorbing layer 3, or when cuprous oxide is used as the p-type light absorbing layer 3, the optical path length is increased and the quantum efficiency of light with wavelengths of 500 nm or longer is improved.

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.

The number of oxygen atoms contained in the cuprous oxide compound is preferably 0.48 or more and 0.56 or less, assuming the number of copper atoms is 1. If the oxygen content is too high relative to the copper, the ratio of copper oxide contained in the cuprous oxide compound will be high, narrowing the band gap and reducing the translucency of the p-type light absorbing layer 3, which is undesirable. If the oxygen content is too low relative to the copper content, the copper content in the cuprous oxide compound will be too high, reducing the translucency, which is undesirable.

Cuprous oxide compounds are oxides primarily 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 amount of copper contained 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%.

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.

The p-type light absorbing layer 3 is preferably a polycrystal of cuprous oxide or/and a composite oxide of cuprous oxide. The composite oxide of cuprous oxide is an oxide represented by Cu _a M1 _b O _c . 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. It is preferable that a, b, and c satisfy the following relationships: 1.80≦a≦2.01, 0.00≦b≦0.20, and 0.98≦c≦1.02. It is preferable that the p-type light absorbing layer 3 contains almost no Cu and/or CuO, which are heterophases. The p-type light absorbing layer 3 preferably contains few heterophases and has good crystallinity, which 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 using Si as a light absorbing 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.

The above composition ratio of the p-type light absorbing layer 3 is the overall composition ratio of the p-type light absorbing layer 3.

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 minimal unevenness on its 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.

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. It is preferable that the first region 3a and the second region 3b are directly connected.

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.

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 from the n-type layer 4 may diffuse into the p-type light absorbing layer 3 and/or elements from 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. Less than 90 atom % of the metal elements in the mixed region are metals contained in the p-type light absorbing layer 3, and 10 atom % or more of the metal elements in the mixed region are 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.

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 minimal unevenness on its 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.

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 undesirable 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 preferably more than 70 nm and 1000 nm or less, and more preferably 100 nm or more and 1000 nm or less.

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.

The method for measuring the thickness of the first region 3a and the second region 3b will be explained below. Observe the cross sections of analysis spots A1 to A9 in Figure 2, and identify the large grain size region on the n-type layer 4 side as the first region 3a. Also, observe the cross sections of analysis spots A1 to A9 in Figure 2, and identify the small grain size region on the p-electrode 2 side as the second region 3b.

The thickness of the second region 3b is preferably 70 nm or more and 1000 nm or less, and 1% or more but less than 20% of the thickness of the p-type light absorbing layer 3. A relatively thin second region 3b compared 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 compared to the thickness of the p-type light absorbing layer 3 is undesirable because it increases the number of 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 3% or more but 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 greater than 70 nm and less than 1000 nm, and is 3% to less than 15% of the thickness of the p-type light absorbing layer 3, and more preferably is 100 nm to 1000 nm, and is 3% to less than 15% of the thickness of the p-type light absorbing layer 3.

The thickness of the second region 3b is preferably 150 nm to 600 nm, and is preferably 1% to less than 20% of the thickness of the p-type light absorbing layer 3. A relatively thin second region 3b relative to the thickness of the p-type light absorbing layer 3 is undesirable because it prevents the optical path length from increasing. A relatively thick second region 3b relative 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 to 600 nm, and is preferably 3% to 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 to 500 nm, and is preferably 3% to less than 15% of the thickness of the p-type light absorbing layer 3.

When the thickness of the second region 3b is 1% or more and less than 20% of the thickness of the p-type light absorbing layer 3, the thickness of the first region 3a is preferably 80% or more and 99% or less of the thickness of the p-type light absorbing layer 3. Having the first region 3a occupy most of the p-type light absorbing layer 3 results in a solar cell 100 with high conversion efficiency.

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. By having the first region 3a occupy the majority of the p-type light absorbing layer 3, a solar cell 100 with high conversion efficiency can be obtained.

The first average crystal grain size d1 of the first region 3a is preferably 1.5 μm or greater. The first average crystal grain size d1 is preferably 80% or greater but less than 100% of the thickness of the p-type photoabsorption layer 3. The first region 3a is the primary region in the p-type photoabsorption layer 3 that absorbs light and generates electricity. When the first average crystal grain size d1 of the first region 3a is large, the first region can absorb light of 350 nm or greater but less than 650 nm, generating electricity with high efficiency. The first average crystal grain size d1 is preferably 90% or greater but 100% of the thickness of the p-type photoabsorption layer 3 minus the thickness of the second region 3b, i.e., the thickness of the first region 3a, and more preferably 95% or greater but 100%. 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.

The first average crystal grain size d1 is determined by observing the p-type light absorbing layer 3 in each cross-sectional image of analysis spots A1 to A9 in Figure 2. For example, a scanning electron microscope (SEM) is used to obtain the cross-sectional image. The magnification is approximately 10,000x to 200,000x, and the image capture range 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 crystal stacking direction are selected. The average diameters in the stacking direction of the five selected crystals are then calculated. The average values calculated from the nine analysis spots A1 to A9 are further averaged, and this average value is defined as the average crystal grain size d1.

The second average crystal grain size d2, which is the average crystal grain size 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 smaller grain size on the p-electrode 2 side allows the second region 3b to absorb light with longer 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.

The maximum crystal grain size of 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.

The second average crystal grain size d2 was determined by measuring 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 light-absorbing layer 3 was observed. A region where the thickness of the second region 2b could be confirmed was selected during 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. This backscattered electron diffraction was analyzed to determine the crystal plane orientation in the image. Boundaries with a crystal plane orientation difference of 5° or more were treated as crystal 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 diameter of all particles included in the measurement area is 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 included in the measurement areas of analysis spots A1 to A9 is taken as the second average crystal grain size d2.

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 or more and 0.300 or less, more preferably 0.010 or more and 0.300 or less, and even more preferably 0.020 or more and 0.100 or less. Having d2/d1 within the preferred range is preferable because it reduces electron recombination and increases the optical path length on the back side.

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. Keeping the voids within this range may prevent an increase in electron recombination.

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

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

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.

When examining the grain boundaries of analysis spots A1 to A9 in Figure 2 using SEM-EBSD, areas determined to be other than crystals are considered to be voids G. The diameter of the voids G is calculated in the same way as the second average crystal grain size d2. The average value of the diameters of all voids included in the measurement areas 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 second region 3b is calculated by adding up the diameters of each void G in the first region 3a or second region 3b.

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 light absorbing layer 3 produced 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.

As shown in the schematic cross-sectional view of FIG. 3, the second region 3b of the embodiment preferably contains large particles A, small particles B, small voids G 1 _S , and large voids G 1 _L.

The particle size distribution in the second region 3b, in the range of 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 of 0.075 μm to 0.125 μm.

When the proportion of particles in the second region 3b with a crystal grain size of 0.075 μm or more and 2.00 μm or less is taken as 100%, 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 with 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 with a crystal grain size of 0.075 μm or more and 2.00 μm or less]))) are each preferably 10% or more and 80% or less.

If the proportion of particles in the second region 3b with 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 be highest in the range of crystal grain size of 0.075 μm or more and 2.00 μm.

It is preferable that the ratio of particles contained in the first peak P1 or the ratio of particles contained in the second peak P2 is 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.

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.

The grain size of each analysis spot A1 to A9 is determined in the same way as the method for determining the second average grain size d2 using 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 second peak P2 can be identified from the created grain size distribution.

The crystal orientation of the particles in the second region 3b is preferably random. The p-type light absorbing layer 3 produced by the manufacturing method of the embodiment has a 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.

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

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

The second region 3b preferably contains 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.

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, a 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 ¹⁰⁰ to ¹⁰⁶ times the average carrier concentration of the first region 3a.

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 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.

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

When the average carrier concentration of the second region 3b is ¹⁰⁰ to ^{105 times the average carrier concentration of the first region 3a, the thickness of the second region 3b is preferably 0.07 μm to 0.1 μm. When the average carrier concentration of the second region 3b is 100 to 105 times the} average carrier concentration of the first region 3a, the thickness of the second region 3b is preferably 0.01 to 0.05 times the thickness of the first region 3a. When the average carrier concentration of the second region 3b is ¹⁰¹ to ¹⁰⁶ times the average carrier concentration of the first region 3a, the thickness of the second region 3b is preferably 0.1 to 0.6 μm, and the thickness of the second region 3b is preferably 0.05 to 0.3 times the thickness of the first region 3a.

The hole concentration of the p-type light absorbing layer 3 is measured. For example, suppose 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 facing the n-type layer 4 toward the p-electrode 2 to a depth of 7 μm, and the second region 3b is the region of the p-type light absorbing layer 3 excluding the first region 3a and extending 1 μm toward the p-electrode 2. For example, first, remove material from the n-type layer 4 toward the p-type light absorbing layer 3 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, measure the hole concentration while removing the p-type light absorbing layer 3 from the side where the p-electrode 2 was located. For example, measure the hole concentration every 200 nm of thickness removed. Setting the 200 nm interval, which is the interval for measuring the hole concentration, to a constant interval of 10 nm or more and 100 nm or less, can improve measurement accuracy.

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.

The p-type light absorbing layer 3 is preferably formed by, for example, sputtering. Figure 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 Figure 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.

When the substrate 1 is provided on the p-electrode 2 side, it is preferable to perform the processes in the order of the low-temperature film formation process (S01), the high-temperature film formation process (S02), and the oxidation process (S03).

When the substrate 1 is provided on the n-electrode 5 side, it is preferable to reverse the order of the steps in the flowchart of Figure 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.

During sputtering, atoms other than Cu can be introduced into the chamber to form a doped Cu ₂ O film or a composite oxide film of cuprous oxide.

In sputtering, the component on which the p-type light absorbing layer 3 is to be formed (for example, a component with a p-electrode 2 formed on a substrate 1, or a component with an n-electrode 5 and n-type layer 4 formed on a substrate 1) is placed on a stage inside a chamber, the component is heated by heating the stage, and a target primarily composed of copper is used to oxidize the copper in an oxidizing atmosphere, causing it to adhere to the component and form a film.

The sputtering temperature is the temperature of the underlying material on which the p-type light absorbing layer 3 is formed. If 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. If 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.

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 in the p-type light absorbing layer 3 to grow, resulting in the formation of small-grain second regions 3b.

The oxygen partial pressure in the chamber during the low-temperature film formation process (S01) is preferably 0.01 Pa or higher and 4.8 Pa or lower.

When the deposition rate of the p-type light absorbing 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).

In the low-temperature film formation process (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.

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 in the p-type light absorbing layer 3 to grow, forming large-grain first regions 3a.

The oxygen partial pressure in the chamber during the high-temperature film formation process (S02) is preferably 0.01 Pa or higher and 4.8 Pa or lower.

When the deposition rate of the p-type light absorbing 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).

The sputtering temperature in the high-temperature film formation process (S02) is preferably 100°C or more and 400°C or less higher than the sputtering temperature in the low-temperature film formation process (S01).

When transitioning 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 sputtering is stopped, or the temperature may be changed while sputtering is continuing. If the temperature of the component is changed while sputtering is continuing, it is preferable to change the component temperature at a rate of 10°C/min or more and 100°C/min or more.

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 sputtering has stopped, or the temperature may be changed while sputtering is continuing. If the temperature of the component is changed while sputtering is continuing, it is preferable to change the component temperature at a rate of -100°C/min or higher and -10°C/min or lower.

In the high-temperature film formation process (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.

The oxidation step (S03) involves placing the p-type light absorbing layer 3 on a substrate in an atmosphere with 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 formation step (S01) and the high-temperature film formation step (S02) and before the formation of the n-type layer 4 (when the substrate 1 is provided on the p-electrode 2 side).

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). Zn and/or Cd are preferred as n-type dopants for compounds having a chalcopyrite structure.

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 Cu ₂ ZnSn(S,Se) _4. As an n-type dopant for the compound having a kesterite structure, Cd is preferred.

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.

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 disposed 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 the surface in contact with the p-electrode 2. The n-type layer 4 preferably includes a semiconductor layer containing a Ga-containing oxide or a semiconductor layer containing an n-type cuprous oxide compound. The semiconductor layer containing a Ga-containing oxide or the semiconductor layer containing an n-type cuprous oxide compound forms a pn junction (a pn junction if the mixed region is i-type) with the p-type light absorbing layer 3. When a semiconductor layer containing a Ga-containing oxide is used for the n-type layer 4, the solar cell 100 is a heterojunction solar cell. When a semiconductor layer containing a cuprous oxide 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 containing an oxide containing Ga or a semiconductor layer containing an n-type cuprous oxide compound. A multilayer n-type layer 4 may include multiple semiconductor layers containing oxides containing Ga with different compositions, or multiple semiconductor layers containing an n-type cuprous oxide compound and multiple 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. Therefore, the short-circuit current density and conversion efficiency are improved by providing the first region 3a in heterojunction solar cells and homojunction solar cells.

It preferably contains a compound (oxide) primarily composed of Ga. The n-type layer 4 may be a mixture of an oxide primarily composed of Ga with other oxides, an oxide primarily composed of Ga doped with other elements, or a mixture of an oxide primarily composed of Ga doped with other elements and other oxides. The n-type layer 4 may be a single layer or multiple layers. Of the metal elements contained in the n-type layer 4, Ga preferably accounts for 40 atomic % or more, and more preferably 50 atomic % or more. The metal elements contained in the n-type layer 4 may be distributed in a gradient from the p-type light absorbing layer 3 side to the n-electrode 5 side.

The n-type layer 4 preferably contains an oxide containing Ga and an element represented by M3. The oxide mainly composed of Ga 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 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 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 in the n-type layer 4 is preferably an oxide containing M3 and Ga, whose average composition is represented by _GahM3iOj , where h, _i , and _j preferably satisfy the following conditions: 1.8≦h≦2.1, 0≦i≦0.2, and 2.9≦j≦3.1.

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.

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 characteristics. If coverage is good, the above thickness is not required. If the thickness of the n-type layer 4 exceeds 50 nm, excessively high resistance of the n-type layer 4 may result in degraded characteristics, or a decrease in short-circuit current due to reduced 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.

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. When the thickness of the n-type layer 4 is _d4 , 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. Except when there is a condition that the elemental composition ratio of the compound in the n-type layer 4 is graded, it is preferable that the n-type layer 4 satisfy the above and following preferred compositions at each depth. When the n-type layer 4 is very thin (for example, 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 to be 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.

When 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. When 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.

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 disposed 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 disposed 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, and Cl. The n-electrode 5 may include a mesh or line-shaped electrode to reduce the resistance of the transparent conductive oxide film. Materials for the mesh or line-shaped electrode include, but are not limited to, Mo, Au, Cu, Ag, Al, Ta, and W. Graphene can also be used for the n-electrode 5. Graphene is preferably laminated with silver nanowires.

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 between 50 nm and 2 μm.

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

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.

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 and CdTe systems with a high In content, or one type selected from the group consisting of crystalline silicon and perovskite-type compounds.

(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.

Figure 7 shows a cross-sectional view of the solar cell module 300. Figure 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 used, etc. The solar cell module 300 in Figure 6 includes multiple sub-modules 303 surrounded by dashed lines, in which multiple solar cells 100 (solar cells) are arranged horizontally and electrically connected in series by wiring 304, and the multiple sub-modules 303 are electrically connected in parallel or in series. Adjacent sub-modules 303 are electrically connected by bus bars 305.

Adjacent solar cells 100 are connected by wiring 304, with the upper n-electrode 5 and the lower p-electrode 2. Like the solar cell 100 of the first embodiment, the solar cell 100 of the third embodiment has a substrate 1, p-electrode 2, p-type light absorbing layer 3, n-type layer 4, and n-electrode 5. Both ends of the solar cells 100 in the submodule 303 are connected to a bus bar 305, and the bus bar 305 preferably electrically connects multiple submodules 303 in parallel or series, adjusting 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 solar cell modules can be configured using other connection configurations.

(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. Specifically, the solar cell module generates power, a means for converting the generated electricity, and a storage means for storing the generated electricity or a load for consuming 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 .

The solar cell cells included in the submodule 303 of the solar cell module 300 receive light and generate electricity. This 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 add a solar tracking drive device to the solar cell module 401 to always point the solar cell module 401 toward the sun, a concentrator to concentrate sunlight, or a device to improve power generation efficiency.

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

A vehicle is shown as an example of a use of the solar power generation system 400. Figure 9 shows a conceptual diagram of the vehicle 500. The vehicle 500 in Figure 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 mounted 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 a load 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, but may instead be composed solely of a first solar cell module equipped with 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.

An air vehicle (drone) is shown as an example of the 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 explained using the schematic diagram of the air vehicle 600 in Figure 10. The air vehicle 600 has a solar cell module 401, a frame 601, a motor 602, rotors 603, and a control unit 604. The solar cell module 401, motor 602, rotors 603, and control unit 604 are arranged on the 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 and the solar cell module 401 of this embodiment, an air vehicle that can fly using more power is provided.

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

(Example A: Examples A-1 to A-12, Comparative Examples A-1 to A-6)
On a glass substrate 1, ITO (In:Sn=80:20, film thickness 20 nm) and ATO (Sn:Sb=98:2, film thickness 150 nm) are deposited on the top surface, which contacts the glass, as a p-electrode 2 on the rear side. A Cu ₂ O layer is formed on the ATO as a p-type light absorbing layer 3 by sputtering in an oxygen and argon gas atmosphere. The Cu ₂ O layer is formed in two stages. First, a 0.5 μm thick Cu ₂ O film (second region 3 b) is formed on the substrate 1 at a temperature of 250° C., and then a 5.0 μm thick Cu ₂ O film (first region 3 a) 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 an n-type layer 4. Then, a 0.1 μm thick AZO (ZnO:Al) film is formed as the n-electrode 5 to obtain the solar cell 100 .

Examples A-2 to A-12 and Comparative Examples A-1 to A-6 are similar to Example A-1, except for the thicknesses of the first region 3a and second region 3b. Table 1 shows the thicknesses, grain sizes, and d1>d2 of the first region 3a and second region 3b for 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 referred to as the first region 3a and the second region 3b. Table 1 shows comparative examples for each Example and Comparative Example. 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 evaluation is A. If d1>d2 is not satisfied, the evaluation is B. The evaluation of d1>d2 is common to Examples A to C.

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 in the measurement room at 25°C. The voltage is swept and the current density (current divided by cell area) is measured. With voltage on the horizontal axis and current density on the vertical axis, the point where they intersect is the open-circuit voltage (Voc). By multiplying the voltage and current density on the measurement curve and taking the maximum points as Vmpp and Jmpp (maximum power point), respectively, the fill factor can be calculated as FF = (Vmpp * Jmpp) / (Voc * Jsc). Conversion efficiency can be calculated as Eff. = Voc * Jsc * FF.

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 for light with a wavelength of 700 nm or more and 1000 nm or less for the example.

When the light transmittance is between 60% and 100%, it is rated as A, when the light transmittance is between 50% and less than 60%, it is rated as B, and when the light transmittance is less than 50%, it is rated as C. The evaluation criteria are the same for Examples A to C.

If the Voc is less than the Voc of the comparative example, it is rated as B, and if it is equal to or greater than the Voc of the comparative example, it is rated as A. The evaluation criteria are the same for Examples A to C.

If the Jsc is equal to or less than the Jsc of the comparative example, it is rated as B, and if it is greater than the Jsc of the comparative example, it is rated as A. The evaluation criteria are the same for Examples A to C.

If the FF is less than the FF of the comparative example, it is rated as B, and if it is equal to or greater than the FF of the comparative example, it is rated as A. The evaluation criteria are the same for Examples A to C.

If the Eff is equal to or less than the Eff of the comparative example, it is rated as B, and if it is greater than the Eff of the comparative example, it is rated as A. The evaluation criteria are the same for Examples A to C.

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 Cu ₂ O layer of the second region 3 b, the optical path length is increased, the amount of light absorption in the entire Cu ₂ O layer including the second region 3 b is increased, the current density (Jsc) is increased, and the conversion efficiency (Eff.) is increased.

(Example B: Examples B-1 to B-12, Comparative Examples B-1 to B-10)
On a glass substrate 1, ITO (In:Sn=80:20, film thickness 20 nm) and ATO (Sn:Sb=98:2, film thickness 150 nm) are deposited on the top surface, which contacts the glass, as a p-electrode 2 on the rear side. A Cu ₂ O layer is formed on the ATO as a p-type light absorbing layer 3 by sputtering in an oxygen and argon gas atmosphere. The Cu ₂ O layer is formed in two stages. First, a 0.5 μm thick Cu ₂ O film (second region 3 b) is formed on the substrate 1 at a temperature of 250° C. Next, a 5.0 μm thick Cu ₂ O film (first region 3 a) 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 an 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 .

The solar cells 100 of Examples B-2 to B-12 and Comparative Examples B-1 to B-10 were fabricated by varying the thickness of the second region 3b and the thickness of the p-type light absorbing layer 3. Table 3 shows examples for comparison with each Example and Comparative Example.

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. Because comparative example B-1 and comparative examples B-5 to B-10 do not have the second region 3b, d1>d2 is indicated with a hyphen.

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 for light with a wavelength of 700 nm or more and 1000 nm or less for the example.

We also checked whether the crystalline orientation of the particles in the second region 3b was random. If the crystalline orientation of the particles in the second region 3b was random, we evaluated it as A, and if it was not random, we evaluated it as B. The evaluation criteria were the same for Examples B and C.

The presence or absence of voids in the second region 3b is also evaluated. Cases where voids are present in the second region 3b are evaluated as A, and cases where voids are not present are evaluated as B. The evaluation criteria are common to Examples B to C. In Comparative Example B-1 and Comparative Examples B-5 to B-10, which do not have the second region 3b, the crystal orientation and voids in the region extending 200 nm from the interface between the p-type light absorbing layer 3 and the p-electrode 2 toward the n-type layer 4 are evaluated.

When fabricated at a low substrate temperature and voids are intentionally created in the Cu ₂ O layer of the second region 3 b, reflection and refraction of light become more likely, the optical path length of light becomes longer, the amount of light absorption in the Cu ₂ O layer of the entire second region 3 b increases, the current density (Jsc) increases, and the conversion efficiency (Eff.) increases.

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

The average crystal grain size can be changed by adjusting not only the sputtering deposition temperature but also the film thickness of the Cu ₂ O layer in the second region 3b. By adjusting the average grain size of the second region 3b, a long optical path length can be ensured, the amount of light absorption in the entire Cu ₂ O layer including the second region 3b can be increased, the current density (Jsc) can be improved, and the conversion efficiency (Eff.) can be improved. Note that, since the second region 3b is not confirmed in Comparative Example B-1, the second average crystal grain size d2 of Comparative Example B-1 in Table 5, the maximum value of the crystal grain size in the second region 3b, and d1/d2 are all indicated with a hyphen.

Furthermore, the area ratio R1 of the voids G in the cross section of the first region 3a in the example satisfies the above-mentioned preferred range. Furthermore, the area ratio R2 of the voids G in the cross section of the second region 3b in the example satisfies the above-mentioned preferred range. Furthermore, R1/R2 in the example satisfies the above-mentioned preferred range.

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 below. Table 6 shows 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.

By changing the film thickness of the Cu ₂ O layer in the second region 3b, the distribution of the average crystal grain size can also be adjusted. Adjusting the average grain size ensures a long optical path length, increases the amount of light absorption in the Cu ₂ O layer throughout the second region 3b, improves the current density (Jsc), and increases the conversion efficiency (Eff.). No peaks are observed in the Cu ₂ O layer of the comparative example. Therefore, peaks and valleys are not observed in the comparative example, but peaks are observed in the examples. Furthermore, both small and large voids are observed in the second region 3b of the examples. Furthermore, example C satisfies d1>d2.

(Example C: Examples C-1 to C-25)
On the glass substrate 1, ITO (In:Sn=80:20, film thickness 20 nm) and ATO (Sn:Sb=98:2, film thickness 150 nm) are deposited on the top surface, which contacts the glass, as the rear side p-electrode 2. A Cu ₂ O layer is formed as the p-type light absorbing layer 3 on the ATO by sputtering in an oxygen and argon gas atmosphere. The Cu ₂ O layer is formed in two stages. First, a 0.5 μm thick N-doped Cu ₂ O film (second region 3 b) is formed on the substrate 1 at a temperature of 250° C. Next, a 5.0 μm thick non-doped Cu ₂ O film (first region 3 a) is formed on the substrate 1 at a temperature of 480° C. After the formation of the p-type light absorbing layer 3, a 10 nm GaO film and a 15 nm ZTO film were formed in this order as the n-type layer 4. Then, a 0.1 μm AZO film was formed as the n-electrode 5, thereby completing the solar cell 100.

The solar cells 100 of Examples C-2 to C-25 were fabricated by varying 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.

Table 8 shows the open circuit voltage (Voc), current density (Jsc), fill factor (FF), conversion efficiency (Eff.), light 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.

Sputtering deposition at a low substrate temperature while adding impurities reduces the crystal grain size in the Cu ₂ O layer of the second region 3 b and makes voids more likely to form, thereby lengthening the optical path length, increasing the amount of light absorption in the Cu ₂ O layer of the entire second region 3 b, and increasing the current density (Jsc) and conversion efficiency (Eff.).

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.

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 cosmetic grain size, which is the average crystal grain size of the second region.
Technical plan 2
The solar cell according to Technical Scheme 1, wherein the p-type light absorbing layer is mainly composed of cuprous oxide or a composite oxide of cuprous oxide.
Technical plan 3
the thickness of the p-type light absorbing 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 plan 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 proposals 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 light absorbing layer 3.
Technical plan 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 proposals 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 light absorbing 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 light absorbing layer.
Technical plan 7
The solar cell according to any one of technical proposals 1 to 6, wherein the second average crystal grain size is 0.005 μm or more and 0.60 μm or less.
Technical plan 8
A solar cell according to any one of technical proposals 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 plan 9
A solar cell described in any one of technical proposals 1 to 8, wherein the second average crystal grain size relative 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;
A solar cell according to any one of technical proposals 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 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 0 or more and 0.1 or less.
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 proposals 1 to 13, wherein 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.
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],
15. A solar cell according to any one of technical proposals 1 to 14, wherein the particle size distribution includes 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 Proposal 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 described in 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 having a cross-sectional diameter of 0.01 μm or more and less than 0.1 μm and large voids having a cross-sectional diameter of 0.1 μm or more and 0.6 μm or less.
Technical proposal 19
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 the 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 present on the p-electrode side of the p-type light absorbing layer.

1: Substrate 2: P-electrode 3: P-type light absorption layer 3a: First region 3b: Second region 4: N-type layer 5: N-electrode 100: Solar cell 200: Multi-junction solar cell 201: Second solar cell 300: Solar cell module 301: First solar cell module 302: Second solar cell module 303: Sub-module 304: Wiring 305: Bus bar 400: Solar power generation system 401: Solar cell module 402: Converter 403: Storage battery 404: Load 500: Vehicle 501: Vehicle body 502: Solar cell module 503: Power conversion device 504: Storage battery 505: Motor 506: Tire (wheel)
600: Flying object 601: Airframe frame 602: Motor 603: Rotor 604: Control unit

Claims (20)

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 . 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. 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. The solar cell described in 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. The solar cell described in claim 1, wherein the second average crystal grain size is 0.005 μm or more and 0.60 μm or less. The solar cell described in claim 1, wherein the maximum crystal grain size of the second region is 0.05 μm or more and 2.00 μm or less. The solar cell described in 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. 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. The solar cell described in 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. The solar cell described in 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 between 0 and 0.1. 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. The solar cell described in claim 1, wherein the particle size distribution of the second region in the range of 0.005 μm to 0.70 μm includes two peaks. 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. The solar cell described in claim 13, 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. The solar cell described in claim 1, wherein the crystal orientation of the particles in the second region is random. The solar cell described in claim 1, wherein the second region includes 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. The solar cell of 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. A multi-junction solar cell using the solar cell described in any one of claims 1 to 17. A solar cell module using the solar cell described in any one of claims 1 to 17. A solar power generation system that generates power using the solar cell module described in claim 19.