solar cells

Abstract

A solar cell according to one embodiment of the present invention has high photoelectric conversion efficiency and comprises a semiconductor substrate 11, and a first semiconductor layer 14 having first conductivity and a second semiconductor layer 15 having second conductivity that are each layered on the semiconductor substrate 11. The material volume Vmp at 10% of the load area of at least a first main surface of the semiconductor substrate 11 is 0.003 µm3 / µm2 to 0.010 µm3 / µm2.

Description

[Technical Field] 【0001】 This invention relates to a solar cell. [Background technology] 【0002】 The use of solar cells is expanding as an energy source with a low environmental impact. When installing solar cells in various devices, vehicles, buildings, etc., the available installation area is limited, making the photoelectric conversion efficiency of the solar cells important. As a method to improve the photoelectric conversion efficiency of solar cells, a technique is known in which a textured structure with numerous pyramidal irregularities is formed on the main surface of the semiconductor substrate (see, for example, Patent Document 1). 【0003】 By forming a textured structure on the surface of a semiconductor substrate, specifically on the light-receiving surface, the reflectivity of light can be reduced, allowing more light to enter the substrate, thereby improving photoelectric conversion efficiency. Transparent electrodes, for example, are laminated on the light-receiving side of the semiconductor substrate. By using film deposition techniques such as vacuum deposition or sputtering, the material can be uniformly laminated on the semiconductor substrate, maintaining the textured structure. 【0004】 Another method for improving the photoelectric conversion efficiency of solar cells is to stack a perovskite solar cell, such as an organic photoelectric conversion layer containing a perovskite compound, on the light-receiving surface of a crystalline silicon solar cell formed using a semiconductor substrate (see, for example, Patent Document 2). [Prior art documents] [Patent Documents] 【0005】 [Patent Document 1] Japanese Patent Publication No. 2021-57435 [Patent Document 2] Japanese Patent Publication No. 2018-163959 [Overview of the project] [Problems that the invention aims to solve] 【0006】 Generally, the organic photoelectric conversion layer of a perovskite solar cell is formed by coating it with a material that forms the organic photoelectric conversion layer. As described in Patent Document 1, when attempting to form an organic photoelectric conversion layer containing a perovskite compound by coating the surface of a solar cell having a textured structure, it is not possible to form a uniform coating film, and in particular, pinholes in the coating film may be formed at the vertices of the textured structure, which can reduce the photoelectric conversion efficiency of the perovskite solar cell. Furthermore, even when not stacking perovskite solar cells, but when coating a resist for insulation on a solar cell having a textured structure, coating defects may similarly reduce the performance of the solar cell. 【0007】 Therefore, the object of the present invention is to provide a solar cell with high photoelectric conversion efficiency. [Means for solving the problem] 【0008】 A solar cell according to one aspect of the present invention comprises a semiconductor substrate and a first conductive semiconductor layer and a second conductive semiconductor layer laminated on the semiconductor substrate, wherein the actual volume Vmp at a load area ratio of 10% of at least the first main surface of the semiconductor substrate is 0.003 μm². 3 / μm 2 Above 0.010 μm 3 / μm 2 The following applies: 【0009】 In the solar cell described above, the first semiconductor layer and the second semiconductor layer may be stacked on different main surfaces. 【0010】 The solar cell described above may further include an organic photoelectric conversion layer containing a perovskite compound, which is laminated on the first main surface side. 【0011】 In the solar cell described above, the second main surface of the semiconductor substrate may have a textured structure having a plurality of pyramidal protrusions. 【0012】 The solar cell described above may receive light from the first main surface side. 【Advantages of the Invention】 【0013】 According to the present invention, a solar cell with high photoelectric conversion efficiency can be provided. 【Brief Description of the Drawings】 【0014】 [Figure 1] It is a schematic cross-sectional view showing a solar cell according to an embodiment of the present invention. [Figure 2] It is a graph showing the reflectance reduction rate and the number of non-coated steps of a processing example of a semiconductor substrate. 【Embodiments for Carrying Out the Invention】 【0015】 Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing a solar cell 1 according to an embodiment of the present invention. 【0016】 The solar cell 1 includes a crystalline silicon solar cell part 10 and a perovskite solar cell part 20 laminated on the light-receiving surface side (front surface side: the upper side in FIG. 1) of the crystalline silicon solar cell part 10. The solar cell 1 is configured such that the crystalline silicon solar cell part 10 and the perovskite solar cell part 20 are connected in series. First, the solar cell 1 photoelectrically converts the incident light in the perovskite solar cell part 20, and converts the light that has passed through the perovskite solar cell part 20 without being converted into electric power in the perovskite solar cell part 20 into electric power in the crystalline silicon solar cell part 10, thereby realizing a relatively high photoelectric conversion efficiency as a whole. 【0017】 The crystalline silicon solar cell unit 10 may have a structure including a semiconductor substrate 11, a first passivation layer 12 laminated on the first main surface on the light-receiving surface side of the semiconductor substrate 11, a second passivation layer 13 laminated on the second main surface on the side opposite to the first main surface (back surface side: the lower side in FIG. 1) of the semiconductor substrate 11, a first semiconductor layer 14 laminated on the first passivation layer 12 and having a first conductivity, a second semiconductor layer 15 laminated on the second passivation layer 13 and having a second conductivity, an intermediate electrode layer 16 laminated on the first semiconductor layer 14, and a back surface electrode layer 17 laminated on the second semiconductor layer 15. 【0018】 The perovskite solar cell unit 20 may have a structure including, in this order from the crystalline silicon solar cell unit 10 side, a first charge transport layer 21, an organic photoelectric conversion layer 22 laminated on the first charge transport layer 21, a second charge transport layer 23, and a surface electrode layer 24. 【0019】 The semiconductor substrate 11 can be formed of a crystalline silicon material such as single crystal silicon or polycrystalline silicon. Further, the semiconductor substrate 11 may be formed of another semiconductor material such as gallium arsenide (GaAs). The semiconductor substrate 11 may be, for example, an n-type semiconductor substrate doped with an n-type dopant in a crystalline silicon material. Examples of the n-type dopant include phosphorus (P). The semiconductor substrate 11 functions as a photoelectric conversion substrate that absorbs incident light from the light-receiving surface side and generates optical carriers (electrons and holes). By using crystalline silicon as the material of the semiconductor substrate 11, the dark current is relatively small, and a relatively high output (stable output regardless of illuminance) can be obtained even when the intensity of incident light is low. The thickness of the semiconductor substrate 11 may be, for example, 50 μm or more and 300 μm or less. 【0020】 The semiconductor substrate 11 has an uneven structure having a plurality of protrusions with rounded tops on the first main surface. Further, the semiconductor substrate 11 of the present embodiment has a texture structure having a plurality of pyramid-shaped protrusions on the second main surface. 【0021】 The uneven structure on the first main surface of the semiconductor substrate 11 reduces the reflectivity of light and increases the amount of light incident on the inside of the semiconductor substrate 11, thereby improving the photoelectric conversion efficiency of the crystalline silicon solar cell unit 10 and thus the solar cell 1. Further, since the top of the protrusion forming the uneven structure is rounded, when forming a coating film on the surface of the intermediate electrode layer 16, which is a layer having a surface shape following the first main surface and laminated with a uniform thickness on the first main surface or the first main surface in the present embodiment, pinholes are less likely to be formed in the coating film. As a result, even when the perovskite solar cell unit 20 is formed by coating, defects are less likely to occur in the perovskite solar cell unit 20, so that a decrease in the photoelectric conversion efficiency of the solar cell 1 can be suppressed. Note that the trough of the uneven structure on the first main surface of the semiconductor substrate 11 may have a V-shaped cross section forming a clear bottom line as shown in FIG. 1, or may be rounded like the top. 【0022】 The lower limit of the solid volume Vmp at a load area ratio of 10% of the first main surface is 0.003 μm 3 / μm 2 is preferable, and 0.005 μm 3 / μm 2 isisis more preferably. On the other hand, the upper limit of the solid volume Vmp at a load area ratio of 10% of the first main surface is 0.010 μm 3 / μm 2 is preferable, and 0.008 μm 3 / μm 2 is more preferably. Note that the "solid volume Vmp" is the volume of the peak measured in accordance with ISO25178, and the "load area ratio" is the ratio of the load area (the area of the region where the height is c or more) at a certain height c. By setting the solid volume Vmp at a load area ratio of 10% of the first main surface to be not less than the lower limit, the reflectivity of light can be reduced as compared with the case where the first main surface is smooth, and the photoelectric conversion efficiency of the crystalline silicon solar cell unit 10 can be improved. Further, by setting the solid volume Vmp at a load area ratio of 10% of the first main surface to be not more than the upper limit, it is possible to effectively suppress the formation of pinholes when a coating film is formed. 【0023】 The root mean square height Sq (ISO25178) of the uneven structure of the first main surface is set to, for example, 0.02 μm or more and 0.25 μm or less. Furthermore, the number of protrusions per 10 μm square is set to, for example, 5 or more and 40 or less. By satisfying these conditions, it becomes easy to set the actual volume Vmp of the first main surface at a load area ratio of 10% within the aforementioned range. 【0024】 The uneven structure of the semiconductor substrate 11 can be formed by first anisotropically etching a crystalline silicon substrate with smooth first and second main surfaces to form a pyramidal texture structure on at least the first main surface, and then polishing the first main surface to round off the tops of the pyramidal texture. Polishing of the semiconductor substrate 11 is preferably performed by chemical mechanical polishing using a chemical solution that erodes the semiconductor substrate 11. This makes it possible to smoothly round off the texture, and the actual volume Vmp at a load area ratio of 10% of the first main surface can be brought within the above range relatively easily. A mixture of nitric acid and hydrofluoric acid is preferably used as the chemical solution for chemical mechanical polishing of the semiconductor substrate 11 made of crystalline silicon. 【0025】 The textured structure of the second main surface of the semiconductor substrate 11 can be formed by anisotropic etching of the crystalline silicon substrate. In other words, the second main surface of the semiconductor substrate 11 may be an unpolished surface, unlike the first main surface. If coating is not performed on the second main surface side of the semiconductor substrate 11 during the manufacturing process of the solar cell 1, the textured structure of the second main surface can further reduce the light reflectivity compared to the first main surface. When a reflective layer is provided on the back side of the solar cell 1 to allow light transmitted to the back side to be incident on the solar cell 1, providing a textured structure on the second main surface of the semiconductor substrate 11 can improve the utilization rate of light and further improve the photoelectric conversion efficiency of the solar cell 1. Furthermore, anisotropic etching of only one side of the crystalline silicon substrate to form the above-mentioned uneven structure on the first main surface of the semiconductor substrate 11 is more costly than anisotropic etching of both sides. Therefore, by providing a textured structure on the second main surface of the semiconductor substrate 11, productivity can be improved and the solar cell 1 can be provided at a lower cost. 【0026】 The first passivation layer 12 and the second passivation layer 13 suppress carrier recombination at the interface between the semiconductor substrate 11 and the first semiconductor layer 14 or the second semiconductor layer 15. The first passivation layer 12 and the second passivation layer 13 may be intrinsic semiconductor thin layers formed from amorphous silicon. The first passivation layer 12 and the second passivation layer 13 can be stacked by methods such as sputtering. The thickness of the first passivation layer 12 and the second passivation layer 13 may be, for example, 2 nm to 20 nm. 【0027】 The first semiconductor layer 14 and the second semiconductor layer 15 collect charges of opposite polarities by inducing carriers of opposite polarities from within the semiconductor substrate 11. Specifically, the first semiconductor layer 14 may be formed from an n-type semiconductor, and the second semiconductor layer 15 may be formed from a p-type semiconductor. The first semiconductor layer 14 and the second semiconductor layer 15 can be formed from an amorphous silicon material containing a dopant that imparts a desired conductivity type, for example. An example of a p-type dopant is boron (B), and an example of an n-type dopant is phosphorus (P) as described above. 【0028】 The intermediate electrode layer 16 serves as both an electrode for the crystalline silicon solar cell section 10 and an electrode for the perovskite solar cell section 20. The intermediate electrode layer 16 can be formed from a transparent conductive oxide (TCO) that is conductive and light-transmitting, in order to allow light transmitted through the perovskite solar cell section 20 to be incident onto the semiconductor substrate 11. Examples of transparent conductive oxides that can form the intermediate electrode layer 16 include indium oxide, tin oxide, zinc oxide, titanium oxide, and composite oxides thereof. Among these, indium-based composite oxides with indium oxide as the main component are preferred. Indium oxide is particularly preferred from the viewpoint of high conductivity and transparency. Furthermore, it is preferable to add a dopant to the indium oxide to ensure reliability or higher conductivity. Examples of dopants include Sn, W, Zn, Ti, Ce, Zr, Mo, Al, Ga, Ge, As, Si, and S. For example, ITO (Indium Tin Oxide), which is indium oxide with added tin, is widely known. The intermediate electrode layer 16 can be formed by methods such as sputtering or vacuum deposition. The thickness of the anode layer can be, for example, 5 nm to 100 nm. 【0029】 The back electrode layer 17 is one electrode for outputting power from the crystalline silicon solar cell portion 10 and, consequently, the solar cell 1. The back electrode layer 17 can be formed from a conductive material, such as a transparent conductive oxide such as ITO, a cured product of a conductive paste such as silver paste, or a metal such as Cu or Ni. The back electrode layer 17 may have a multilayer structure, for example, a layer formed from ITO which has excellent adhesion to the second semiconductor layer 15, and a layer formed from silver paste which can be easily and inexpensively increased in thickness to reduce electrical resistance. The method for forming the back electrode layer 17 is selected according to the material, and methods such as sputtering, coating, and plating can be used. The thickness of the back electrode layer 17 can be, for example, 100 nm to 300 nm. 【0030】 The first charge transport layer 21 is a hole transport layer (HTL) that selectively transfers holes, one of the carriers generated in the organic photoelectric conversion layer 22, to the intermediate electrode layer 16 in this embodiment. Examples of the main material of the first charge transport layer 21 include metal oxides such as nickel oxide (NiO) and copper oxide (Cu2O), and organic materials such as PTAA (Poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine)) and Spiro-MeOTAD. Furthermore, the first charge transport layer 21 may be a self-assembled monolayer (SAM) formed from, for example, 2PACz ([2-(9H-Carbazol-9-yl)ethyl]phosphonic Acid), MeO-2PACz ([2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic Acid), Me-4PACz ([4-(3,6-Dimethyl-9H-carbazol-9-yl)butyl]phosphonic Acid), etc. 【0031】 The first charge transport layer 21, made of a metal oxide, can be formed by methods such as sputtering or vacuum deposition. Alternatively, the first charge transport layer 21 containing organic matter can be formed by methods such as coating and drying an organic solution. In particular, when the first charge transport layer 21 is formed by coating and drying a solution, the physical volume Vmp of the semiconductor substrate 11 is reduced as described above, allowing for the formation of a pinhole-free coating. This prevents the formation of defects in the perovskite solar cell portion 20, thereby improving the photoelectric conversion efficiency of the perovskite solar cell portion 20 and, consequently, the solar cell 1. The thickness of the first charge transport layer 21 can vary significantly depending on its material, the composition of adjacent layers, etc., but can be, for example, between 1 nm and 200 nm, and in the case of a self-assembled monolayer, it can be the thickness of the material molecules. 【0032】 The organic photoelectric conversion layer 22 contains a perovskite compound that absorbs light and generates carriers. The perovskite compound contained in the organic photoelectric conversion layer 22 may be a compound represented by ABX3, which contains an organic atom A containing at least one of monovalent organic ammonium ions and amidinium-based ions, a metal atom B that generates a divalent metal ion, and a halogen atom X containing at least one of iodide ions I, bromide ions Br, chloride ions Cl, and fluoride ions F. 【0033】 The organic photoelectric conversion layer 22 can be formed by methods such as vapor deposition, but it is assumed that it will be formed by methods such as the sol-gel method (a method of synthesizing perovskite compounds in a coating film) or the coating method (a method of coating a solution containing a pre-synthesized perovskite compound). When the organic photoelectric conversion layer 22 is formed by coating a solution, by reducing the actual volume Vmp of the semiconductor substrate 11 as described above, an organic photoelectric conversion layer 22 without pinholes and uneven distribution of perovskite compounds can be formed, thereby improving the photoelectric conversion efficiency of the perovskite solar cell portion 20 and, consequently, the solar cell 1. The thickness of the organic photoelectric conversion layer 22 depends on the forming material, but it is preferable to set it to 100 nm or more and 1000 nm or less in order to increase the light absorption rate while reducing the distance the generated charge travels. 【0034】 The second charge transport layer 23 is an electron transport layer (ETL) that selectively transfers carriers generated in the organic photoelectric conversion layer 22, specifically electrons in this embodiment, to the surface electrode layer 24. Examples of main materials for the second charge transport layer 23 include PTAA (Poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine)), Spiro-MeOTAD, and fullerenes. Examples of fullerenes include C60, C70, their hydrides, oxides, metal complexes, and derivatives with added alkyl groups. By forming the second charge transport layer 23 from a material containing lithium Li-encapsulated fullerene, the electron transport efficiency can be improved. The second charge transport layer 23 can be formed by methods such as the sol-gel method or coating method. The thickness of the second charge transport layer 23 can be, for example, 3 nm to 30 nm. 【0035】 The surface electrode layer 24 is an electrode that pairs with the intermediate electrode layer 16 in the perovskite solar cell portion 20. The surface electrode layer 24 is a transparent electrode that transmits light incident through the anti-reflective layer 33, and can be formed using the same material as the intermediate electrode layer 16 and in the same manner as the intermediate electrode layer 16. The thickness of the surface electrode layer 24 can be, for example, 500 nm to 1000 nm. 【0036】 The solar cell 1 having the above configuration has good coating properties when forming the first charge transport layer 21 and the organic photoelectric conversion layer 22 because the actual volume Vmp of the first main surface of the semiconductor substrate 11 is within the above range, resulting in fewer defects and excellent photoelectric conversion efficiency. 【0037】 Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and various modifications and variations are possible. For example, in the solar cell according to the present invention, the surface shape of the second main surface of the semiconductor substrate is not particularly limited and may have an uneven structure having the same actual volume Vmp as the first main surface, or it may be a smooth mirror surface. The solar cell according to the present invention may not have some layers such as a perovskite solar cell portion, and may have further layers such as an anti-reflective layer laminated on the light-receiving surface. Furthermore, the solar cell according to the present invention may be a so-called back-electrode type solar cell in which the first semiconductor layer and the second semiconductor layer are provided on the back side of the semiconductor substrate in a complementary shape. Furthermore, in the solar cell according to the present invention, the first main surface having the above-mentioned actual volume Vmp may be the surface opposite to the light-receiving surface. For example, in a back-electrode type solar cell, when a reflective layer is provided on the back side and light transmitted to the back side is re-incident to the semiconductor substrate, the present invention makes it possible to improve the absorption rate of light from the back side while facilitating the formation of resists and the like when patterning the first semiconductor layer and the second semiconductor layer. [Examples] 【0038】 The present invention will be described in detail below based on examples, but the present invention is not limited to the following examples. 【0039】 We fabricated prototype semiconductor substrates: a mirror-finished crystalline silicon substrate (substrate number 1), substrates with a textured structure formed by anisotropic etching on the same crystalline silicon substrate (substrate numbers 2-4), and semiconductor substrates with the textured surface polished by chemical mechanical polishing (substrate numbers 5-8). For each semiconductor substrate, we measured the actual volume Vmp at a load area ratio of 10% and the decrease in light reflectance at a wavelength of 800 nm from the mirror-finished substrate. Furthermore, after coating each semiconductor substrate with a solution containing a perovskite compound, we observed the surface under a microscope and counted the number of uncoated areas (pinholes) within a 50 μm square area. The following table shows these measurement results. 【0040】 [Table 1] 【0041】 Furthermore, Figure 2 shows the graph of the actual volume Vmp and reflectivity reduction rate in the range where the actual volume Vmp is small. As shown in the figure, by setting the actual volume Vmp to 0.003 or higher, it is thought that the reflectivity can be reduced and the photoelectric conversion efficiency of the solar cell can be improved compared to a mirror-finished crystalline silicon substrate. Also, by setting the actual volume Vmp to 0.010 or lower, defects do not occur in the coating film, so it is thought that a decrease in photoelectric conversion efficiency due to manufacturing defects of the solar cell can be prevented. [Explanation of symbols] 【0042】 1. Solar cell 10. Crystalline silicon solar cell section 11 Semiconductor substrates 12. First Passivation Layer 13. Second Passivation Layer 14. First Semiconductor Layer 15. Second Semiconductor Layer 16 Intermediate electrode layer 17 Back electrode layer 20 Perovskite solar cell section 21 First charge transport layer 22 Organic photoelectric conversion layer 23 Second charge transport layer 24 Surface electrode layer

Claims

[Claim 1] Semiconductor substrate and A first semiconductor layer having first conductivity and a second semiconductor layer having second conductivity are laminated on the semiconductor substrate. Equipped with, The actual volume Vmp in a load area ratio of 10% of at least the first main surface of the semiconductor substrate is 0.003 μm². ３ / μm ２ The above 0.010 μm ３ / μm ２ The following is a solar cell. [Claim 2] The solar cell according to claim 1, wherein the first semiconductor layer and the second semiconductor layer are stacked on different main surfaces. [Claim 3] The solar cell according to claim 2, further comprising an organic photoelectric conversion layer laminated on the first main surface side and containing a perovskite compound. [Claim 4] A solar cell according to any one of claims 1 to 3, which receives light from the first main surface side. [Claim 5] The solar cell according to any one of claims 1 to 4, wherein the second main surface of the semiconductor substrate has a textured structure having a plurality of pyramidal protrusions.

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