Bifacial solar cell

Abstract

Provided is a bifacial solar cell having a tandem structure that can generate electricity from light incident from both the front light-receiving surface side and the back light-receiving surface side. [Solution] A bifacial solar cell with a tandem structure has a top cell 10 and a bottom cell 20, which are different types of photoelectric conversion cells, arranged in this order from the top light receiving surface 6 side toward the bottom light receiving surface 7 side between a top light receiving surface and a bottom light receiving surface that can receive light, and a top cell 10 and a bottom cell 20, which are different types of photoelectric conversion cells, and which are arranged in this order from the top light receiving surface 6 side toward the bottom light receiving surface 7 side, and which is characterized in that the bottom cell 20 can receive light incident from the top light receiving surface 3 and light incident from the bottom light receiving surface, and under standard test conditions (STC), the magnitude of the photocurrent of the bottom cell 20 due to light 3 incident from the top light receiving surface is smaller than the magnitude of the photocurrent of the top cell 10 due to light 3 incident from the top light receiving surface.

Description

[Technical Field]

[0001] The present disclosure relates to bifacial solar cells. [Background technology]

[0002] In order to improve the power generation efficiency of solar cells, solar cells with a tandem structure in which different types of light absorption layers (or photoelectric conversion layers) are stacked have been proposed. One example of such solar cells is a perovskite / silicon solar cell (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-910 Summary of the Invention [Problem to be solved by the invention]

[0004] In a solar cell with a tandem structure, different types of photoabsorption layers are connected in series, but because the current-voltage characteristics of each photoabsorption layer are different, the current that can be extracted from the solar cell is limited by the smallest current generated in each photoabsorption layer. Therefore, the solar cell is designed so that the current generated in each photoabsorption layer is equal, i.e., so that current matching can be achieved.

[0005] In this solar cell, light incident on the front light-receiving surface is received by the top cell and bottom cell to generate electricity. By changing the composition of the perovskite compound contained in the top cell light-absorbing layer and thereby changing its band gap, the current generated in the top cell and the current generated in the bottom cell are matched.

[0006] However, conventional solar cells are designed to generate electricity only using light incident on the front light-receiving surface, so there was a need for a configuration that could increase photoelectric conversion efficiency by using light incident on the back side.

[0007] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide a bifacial solar cell having a tandem structure that can generate electricity from light incident on both the front light-receiving surface side and the back light-receiving surface side. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the bifacial solar cell according to the present disclosure is a bifacial solar cell with a tandem structure, which has a top light-receiving surface and a bottom light-receiving surface that can receive light, and a top cell and a bottom cell that are different types of photoelectric conversion cells and are arranged in this order from the top light-receiving surface side toward the bottom light-receiving surface side, between the top light-receiving surface and the bottom light-receiving surface side, wherein the bottom cell is capable of receiving light incident from the top light-receiving surface and light incident from the bottom light-receiving surface, and under standard test conditions (STC), the magnitude of the photocurrent of the bottom cell caused by light incident from the top light-receiving surface is smaller than the magnitude of the photocurrent of the top cell caused by light incident from the top light-receiving surface.

[0009] The configuration of conventional solar cells with a tandem structure is not designed to allow light to enter the bottom cell from the back side of the solar cell, so simply allowing light to enter the bottom cell from the back side of the solar cell did not result in a significant improvement in photoelectric conversion efficiency.

[0010] With the above-described configuration, the bottom cell can generate electricity from light incident on the front light-receiving surface and light incident on the back light-receiving surface. Since the magnitude of the photocurrent of the bottom cell caused by light incident on the front light-receiving surface is smaller than the magnitude of the photocurrent of the top cell caused by light incident on the front light-receiving surface, the amount of power generated by the bottom cell increases when light is incident on the back light-receiving surface, and the currents of the top cell and bottom cell match in this state, resulting in an increase in the total amount of power generated.

[0011] In addition, under standard test conditions (STC), the solar radiation intensity is 1000W / m 2 The magnitude of the photocurrent is measured under test conditions of an air mass of 1.5 and an array representative temperature of 25±2°C.

[0012] In the present disclosure, the top cell and the bottom cell may be configured such that, under standard test conditions (STC), the ratio of the magnitude of the photocurrent of the top cell to the magnitude of the photocurrent of the bottom cell is in the range of greater than 1 and less than 1.5.

[0013] In the present disclosure, the top cell and the bottom cell may be electrically connected in series.

[0014] In the present disclosure, the top cells may be arranged in multiple numbers between the front light receiving surface and the back light receiving surface, with adjacent top cells being electrically connected in series; the bottom cells may be arranged in multiple numbers between the front light receiving surface and the back light receiving surface, with adjacent bottom cells being electrically connected in series; and the top cell group and the bottom cell group may be electrically connected in series.

[0015] In the present disclosure, the top cell may be made of a perovskite solar cell, and the bottom cell may be made of a crystalline silicon solar cell. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic cross-sectional view of a solar cell according to the present disclosure. [Figure 2] FIG. 2 is a schematic diagram of the band gap of the solar cell shown in FIG. [Figure 3] Figure 3 is a graph showing the photoelectric conversion efficiency of six types of solar cells with different top cells. [Figure 4] Figure 4 is a graph of the rear-illuminated characteristics of solar cells with a Top / Bottom ratio of 1.0 or more. [Figure 5] FIG. 5 is a graph of the rear-illuminated characteristics of solar cells with a Top / Bottom ratio of 1.0 or less. [Figure 6] FIG. 6 is a schematic cross-sectional view of a solar cell according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] Preferred embodiments of the present disclosure will be described in detail below with reference to the drawings. The embodiments described below do not unnecessarily limit the content of the present disclosure as defined in the claims, and not all of the configurations described in the following embodiments are necessarily essential to the solutions of the present disclosure. In addition, in this disclosure, solar cells are sometimes referred to as having a front light-receiving surface side as the upper side and a back light-receiving surface side opposite the front light-receiving surface side as the lower side. However, this is for convenience and does not affect the installation orientation or recommended installation orientation. It is sufficient to consider either surface as the front light-receiving surface, and when either surface is considered as the front light-receiving surface, the opposite surface can be considered as the back light-receiving surface. In other words, as long as at least one surface has the configuration of the present disclosure when considered as the front light-receiving surface, it can be considered to fall within the technical scope of the present disclosure. In other words, even if one surface does not have the configuration of the present disclosure when considered as the front light-receiving surface, it can be considered to fall within the technical scope of the present disclosure as long as the other surface has the configuration of the present disclosure when considered as the front light-receiving surface. In the following description, unless otherwise specified, the same components are generally designated by the same reference numerals, and their names and functions are also the same, so that detailed description thereof will not be repeated in such cases.

[0018] First Embodiment Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Figure 1 shows one embodiment of the present disclosure and is a cross-sectional view showing a schematic configuration of a tandem-structure photoelectric conversion element 2 included in a bifacial solar cell (hereinafter simply referred to as a solar cell) 1 having a tandem structure in which a perovskite compound is used in the photoelectric conversion layer.

[0019] In the present disclosure, a solar cell having a tandem structure means a solar cell configured so that some or all of the light (more specifically, light having a certain wavelength band) incident from the light-receiving surface side of the photoelectric conversion element can be absorbed in sequence by two or more light-absorbing layers.

[0020] In terms of configuration, in the present disclosure, a solar cell having a tandem structure may refer to a solar cell in which two light-absorbing layers are provided in order from the light-receiving surface side to the back side of a photoelectric conversion element. It is not necessary for the two light-absorbing layers to completely overlap when viewed from the light-receiving surface side; at least a partial overlap is sufficient. It is also desirable for at least one light-absorbing layer to completely overlap the other light-absorbing layer. That is, in the present disclosure, a solar cell having a tandem structure is one in which some or all of the light incident from the light-receiving surface side of the photoelectric conversion element (specifically, light having a certain wavelength band) passes through one light-absorbing layer and enters the other light-absorbing layer. The light-absorbing layer referred to here does not need to be a single layer, but may be multiple layers. The multiple layers referred to here may refer to, for example, a stacked structure consisting of a PN junction.

[0021] The light absorbing layer is a layer that can absorb light incident on a photoelectric conversion element and generate electrons and holes. It is self-evident that the light absorbing layer absorbs light and generates electrons and holes as long as the solar cell functions as a solar cell, and so as long as the light absorbing layer is made of an appropriate material, there is no need to confirm that it absorbs light and generates electron-hole pairs, which is very difficult to confirm. Note that when referring to "different types of photoelectric conversion cells," the term "different types" refers to, for example, a top cell and a bottom cell, and if even one of the constituent layers is different, they naturally qualify as different types. However, even if the constituent layers are the same type, they also qualify as "different types" if their configurations, such as film thickness or composition, are slightly different. However, when comparing the top cell and the bottom cell, it is preferable that the constituent light-absorbing layers are different. Also, when comparing the top cell and the bottom cell, it is preferable that all of the constituent layers are different. Furthermore, it is most preferable that the top cell is composed of a perovskite solar cell and the bottom cell is composed of a crystalline silicon solar cell.

[0022] In FIG. 1, sunlight (an example of light) 3 enters the photoelectric conversion element 2 on the upper side, and reflected light (an example of light) 4 of sunlight 3 that corresponds to the environment in which the solar cell 1 is installed enters the photoelectric conversion element 2 on the lower side.

[0023] As shown in FIG. 1, the photoelectric conversion element 2 has a structure in which a top cell 10 and a bottom cell 20 are stacked, with the top cell 10 being disposed on the front light-receiving surface side and the bottom cell 20 being disposed on the back light-receiving surface side.

[0024] The top cell 10 has, in order from the front light-receiving surface side, a surface grid electrode 11, a surface transparent electrode 12, a top cell electron transport layer 13, a top cell light absorption layer 14, and a top cell hole transport layer 15.

[0025] The bottom cell 20 has, in order from the top, a bottom cell n-type doped layer 21, a bottom cell light absorbing layer 22, a bottom cell p-type doped layer 23, a back surface transparent electrode 24, and a back surface grid electrode 25. In the photoelectric conversion element 2, an intermediate electrode 5 is provided as an intermediate layer between the top cell 10 and the bottom cell 20, and the top cell 10 and the bottom cell 20 are connected in series by the intermediate electrode 5.

[0026] FIG. 2 shows a schematic diagram of the band gap of the photoelectric conversion element 2 shown in FIG.

[0027] Such a photoelectric conversion element 2 is sealed between a front glass 6 and a rear glass 7 to form a solar cell 1. The area between the front glass 6 and the rear glass 7 around the photoelectric conversion element 2 is filled with sealing resin 8. The front glass 6 is the front light-receiving surface, and the rear glass 7 is the rear light-receiving surface.

[0028] In this embodiment, the top cell 10 is a perovskite solar cell, and the bottom cell 20 is a silicon-based solar cell that is a crystalline silicon solar cell. That is, the top cell light-absorbing layer 14 is a layer containing a perovskite compound, which is a photoelectric conversion material, and may be composed of a perovskite compound alone, or may contain a substance other than a perovskite compound.

[0029] Furthermore, in a solar cell (or photoelectric conversion module) using multiple photoelectric conversion elements 2, it is preferable that the upper surface of a photoelectric conversion element 2 (i.e., the upper surface of the top cell 10) and the back surface of an adjacent photoelectric conversion element 2 (i.e., the lower surface of the bottom cell 20) are connected in series by an interconnector (not shown), and it is even more preferable that this is repeated in multiple stages and connected in series.

[0030] The photoelectric conversion element 2 shown in FIG. 1 can be fabricated by depositing each layer required for the top cell 10 on the bottom cell 20. However, the fabrication procedure for the photoelectric conversion element 2 of the present disclosure is not particularly limited. The photoelectric conversion element 2 may also be fabricated by other methods, such as fabricating the top cell 10 and the bottom cell 20 separately and then bonding these cells together. Note that known techniques can be applied to the materials and deposition methods of each layer in the photoelectric conversion element 2, and therefore detailed explanations thereof will be omitted here.

[0031] Although the photoelectric conversion element 2 shown in FIG. 1 is configured such that current flows from top to bottom, the direction of current flow is not limited thereto and may be configured such that current flows from bottom to top. In a configuration in which current flows from bottom to top, the positions of the top cell electron transport layer 13 and the top cell hole transport layer 15 are interchanged in the top cell 10, and the positions of the bottom cell n-type doped layer 21 and the bottom cell p-type doped layer 23 are interchanged in the bottom cell 20. The type of doped layer (bottom cell n-type doped layer 21 or bottom cell p-type doped layer 23) in the bottom cell 20 is not particularly limited and may be any of a PERC (Passivated Emitter and Rear Cell) type, a TOPCon (Tunnel Oxide Passivated Contact) type, or a heterojunction type. In addition to the doped layer, a PERC portion, a TOPCon portion, or a heterojunction portion may be provided.

[0032] As shown in FIG. 1, in a photoelectric conversion element 2, for example, sunlight 3 incident from the front light-receiving surface side (upper side in FIG. 1), typically light in the visible light region, is photoelectrically converted in a top cell 10, while sunlight 3, typically light in the infrared light region, passes through the top cell 10 and enters a bottom cell 20. Reflected light 4 also enters the bottom cell 20 from the back light-receiving surface side (lower side in FIG. 1). This reflected light 4 includes light in the visible light region and light in the infrared light region. A current is generated in the bottom cell 20 by the sunlight 3 and reflected light 4. Note that the term "reflected light" used here is merely a typical example of sunlight "reflected" by some object, when describing light incident from one side, such as sunlight, and is not limited thereto. In other words, it goes without saying that any light source can be used as long as the light is incident from the back side.

[0033] In this embodiment, since the intermediate electrode 5 is provided on the bottom cell 20, strictly speaking, the sunlight 3 incident on the front light-receiving surface passes through the intermediate electrode 5 before reaching the bottom cell 20. A variation of this embodiment can include a configuration in which the intermediate electrode 5 is not provided between the top cell 10 and the bottom cell 20 (a configuration in which the top cell 10 and the bottom cell 20 are directly bonded).

[0034] The following describes an example of the configuration of each layer in the photoelectric conversion element 2, but known techniques can be applied to the materials and film-forming methods of each layer in the photoelectric conversion element 2, and therefore the configuration of each layer that can be applied to this embodiment is not limited to this example. In other words, as long as it functions as a photoelectric conversion element in a solar cell having a tandem structure, optional layers may be omitted, layers other than those described below may be included, and one layer may also serve the function of another layer.

[0035] (Front grid electrode, rear grid electrode) The front grid electrode 11 and the back grid electrode 25 are made of conductive members that are parallel to each other. The material of the conductive members is not particularly limited, but examples include metals such as silver, copper, and aluminum. The conductive members of the front grid electrode 11 and the back grid electrode 25 are configured so that sunlight 3 and reflected light 4 can enter the inside of the photoelectric conversion element 2 through gaps between adjacent conductive members.

[0036] (Surface transparent electrode, back transparent electrode) The front transparent electrode 12 and the back transparent electrode 24 are thin-film electrodes that are conductive and optically transparent. Examples of materials for these electrodes include conductive transparent materials such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), indium zinc oxide (IZO), and gallium-doped zinc oxide (GZO). These may be used alone or in combination of two or more.

[0037] (Top cell electron transport layer) The top cell electron transport layer 13 is a layer that transports electrons generated in the top cell light absorption layer 14 to the surface transparent electrode 12. The top cell electron transport layer 13 preferably also functions as a hole blocking layer that inhibits holes generated in the top cell light absorption layer 14 from migrating to the surface transparent electrode 12. Examples of materials for the top cell electron transport layer 13 include tin oxide, titanium oxide, and zinc oxide.

[0038] Note that, as long as the top cell 10 has a photoelectric conversion function, it is self-evident that a portion located on the electron transport side (or on the negative electrode side, similarly in the present disclosure) of the top cell light absorbing layer 14 or on the electron transport side within the top cell light absorbing layer 14 has an electron transport function, and there is no need to confirm the electron transport function, which is difficult to confirm in practice. In other words, as long as the top cell 10 has a photoelectric conversion function, a layer located on the electron transport side of the top cell light absorbing layer 14 or on the electron transport side within the top cell light absorbing layer 14 and made of an appropriate material can be considered to be the top cell electron transport layer 13.

[0039] Furthermore, the top cell electron transport layer 13 may also function as the surface transparent electrode 12, and vice versa. Therefore, the photoelectric conversion element 2 does not necessarily have to have both the surface transparent electrode 12 and the top cell electron transport layer 13, and may have only one of them, with one layer also functioning as the other.

[0040] (Top cell light absorption layer) The top cell light absorbing layer 14 is a layer containing a perovskite compound, which is a photoelectric conversion material, and is a layer that absorbs at least a portion of light incident on the photoelectric conversion element 2 and generates electrons and holes. Of these, the electrons move to the top cell electron transport layer 13, and the holes move to the top cell hole transport layer 15. The top cell light absorbing layer 14 may be composed of a perovskite compound alone, or may contain a substance other than a perovskite compound.

[0041] The perovskite compound is composed of compounds represented by the general formula: ABX3 (1). The composition ratio of each element is preferably 1:1:3, but it does not necessarily have to be 1:1:3, and the content of each element may be increased or decreased as appropriate.

[0042] In general formula (1), A is an organic molecule (including an organic group or an organic cation, the same applies in this disclosure), an inorganic atom or molecule (including an inorganic group or an inorganic cation, the same applies in this disclosure), or a combination thereof, B is a metal atom or molecule (including a metal cation, the same applies in this disclosure), and X is a halogen atom or molecule or a chalcogen atom or molecule (including a halogen anion or a chalcogen anion, the same applies in this disclosure). In general formula (1), the three Xs may be the same or different from one another.

[0043] In general formula (1), the organic molecule represented by A is preferably a molecule containing carbon, nitrogen, and hydrogen, and the inorganic atom represented by A is preferably cesium or rubidium.

[0044] It is possible to determine that a compound is a perovskite compound if it is known that the top cell light absorbing layer 14 has a photoelectric conversion function and contains A, B, and X, and it is not necessary to confirm that it has a crystalline structure. For example, it can be determined if it is known that A, B, and X contain organic molecules, metal atoms, and halogen atoms, or that A, B, and X contain inorganic atoms, metal atoms, and halogen atoms.

[0045] In the general formula (1), examples of the organic molecule represented by A include alkylamine, alkylammonium, and nitrogen-containing heterocyclic compounds. In the perovskite compound (1), the organic molecule represented by A may be only one type of organic molecule, or may be two or more types of organic molecules.

[0046] Examples of alkylamines include methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, trimethylamine, triethylamine, tripropylamine, tributylamine, tripentylamine, trihexylamine, ethylmethylamine, methylpropylamine, butylmethylamine, methylpentylamine, hexylmethylamine, ethylpropylamine, and ethylbutylamine.

[0047] The alkylammonium is an ionized product of the alkylamine. Examples of the alkylammonium include methylammonium, ethylammonium, propylammonium, butylammonium, pentylammonium, hexylammonium, dimethylammonium, diethylammonium, dipropylammonium, dibutylammonium, dipentylammonium, dihexylammonium, trimethylammonium, triethylammonium, tripropylammonium, tributylammonium, tripentylammonium, trihexylammonium, ethylmethylammonium, methylpropylammonium, butylmethylammonium, methylpentylammonium, hexylmethylammonium, ethylpropylammonium, and ethylbutylammonium.

[0048] Examples of the nitrogen-containing heterocyclic compound include imidazole, azole, pyrrole, aziridine, azirine, azetidine, azeto, azole, imidazoline, and carbazole. The nitrogen-containing heterocyclic compound may be an ionized compound. As the ionized nitrogen-containing heterocyclic compound, phenethylammonium is preferred.

[0049] In general formula (1), the organic molecule represented by A is preferably methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, methylammonium, ethylammonium, propylammonium, butylammonium, pentylammonium, hexylammonium, or phenethylammonium, more preferably methylamine, ethylamine, propylamine, methylammonium, ethylammonium, or propylammonium, and even more preferably methylammonium.

[0050] In general formula (1), examples of the metal atom represented by B include lead, tin, zinc, titanium, antimony, bismuth, nickel, iron, cobalt, silver, copper, gallium, germanium, magnesium, calcium, indium, aluminum, manganese, chromium, molybdenum, and europium. In the perovskite compound, the metal atom represented by B may be only one type of metal atom, or may be two or more types of metal atoms. From the viewpoint of improving the light absorption properties and charge generation properties of the perovskite compound, the metal atom represented by B is preferably a lead atom or a tin atom. From the viewpoint of reducing lead, a tin atom is preferred.

[0051] In general formula (1), examples of halogen atoms represented by X include fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms. Examples of chalcogen atoms include oxygen atoms, sulfur atoms, selenium atoms, and tellurium atoms. In the perovskite compound, the halogen atoms or chalcogen atoms represented by X may be one type or two or more types. The halogen atom represented by X is preferably an iodine atom, from the viewpoint of enabling the perovskite compound to utilize light in a wide wavelength range. Specifically, of the three Xs, it is preferable that at least one X represents an iodine atom, and it is more preferable that all three Xs represent iodine atoms.

[0052] As the perovskite compound, a compound represented by the general formula "CH3NH3PbX3 (wherein X represents a halogen atom)" is preferred, with CH3NH3PbI3 being more preferred. By using a compound represented by the general formula "CH3NH3PbX3" (particularly CH3NH3PbI3) as the perovskite compound, electrons and holes can be generated more efficiently in the perovskite compound, and as a result, the photoelectric conversion efficiency of the solar cell can be further improved.

[0053] An example of a method for forming the top cell light absorbing layer 14 containing a perovskite compound is a method in which a precursor solution prepared by dissolving a precursor compound of the perovskite compound in an organic solvent is applied by a known method such as spin coating or bar coating to form a film.

[0054] (Top cell hole transport layer) The top cell hole transport layer 15 is a layer that transports holes generated in the top cell light absorbing layer 14 to the intermediate electrode 5. The top cell hole transport layer 15 preferably also functions as an electron blocking layer that suppresses the movement of electrons generated in the top cell light absorbing layer 14 to the intermediate electrode 5.

[0055] The top cell hole transport layer 15 is mainly composed of a hole transport material. Specifically, the top cell hole transport layer 15 preferably contains 70% by mass or more of the hole transport material, and more preferably 85% by mass or more and 100% by mass or less. Examples of hole transport materials include P-type organic semiconductors, conductive polymers, metal oxides, and metal sulfides (e.g., CuO, NiO, and ZnS), and spiro-OMeTAD is preferred.

[0056] As long as the top cell 10 has a photoelectric conversion function, it is self-evident that a portion located on the hole transport side (or on the positive electrode side, similarly in the present disclosure) of the top cell light absorbing layer 14 or on the hole transport side within the top cell light absorbing layer 14 has a hole transport function, and there is no need to confirm the hole transport function, which is difficult to confirm in practice. In other words, as long as the top cell 10 has a photoelectric conversion function, a layer located on the hole transport side of the top cell light absorbing layer 14 or on the hole transport side within the top cell light absorbing layer 14 and made of an appropriate material can be considered to be the top cell hole transport layer 15.

[0057] (middle class) In the present disclosure, an intermediate electrode 5 is exemplified as an intermediate layer. The intermediate electrode 5 is an electrode that electrically connects the top cell 10 and the bottom cell 20 in series, allowing light not absorbed by the top cell 10 to reach the bottom cell 20. That is, the intermediate electrode 5 can be made of a material that is conductive and optically transparent, such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO). These materials may be used alone or in combination of two or more. Furthermore, the intermediate layer is not limited to the intermediate electrode 5, and other known structures disposed between the tandem top and bottom cells, such as a highly doped impurity-doped layer, a highly doped PN junction layer, or a tunnel junction layer, can also be used. The intermediate layer may be any structure disposed between the top cell and the bottom cell of a tandem solar cell, and as long as the solar cell functions as a solar cell, it is self-evident that it is an intermediate layer and functions as such, regardless of whether its electrical conductivity, transmittance, or other physical properties are confirmed. Furthermore, it is not necessary to use an intermediate layer; a layer on the bottom cell side of the top cell and a layer on the top cell side of the bottom cell, having a similar structure to the intermediate layer, can be used in place of an intermediate layer, and as long as the top cell and bottom cell of a tandem solar cell function as a tandem solar cell, it is self-evident that it functions in place of an intermediate layer, regardless of whether its electrical conductivity, transmittance, or other physical properties are confirmed.

[0058] (Bottom cell n-type doped layer, bottom cell light absorption layer, bottom cell p-type doped layer) The bottom cell n-type doped layer 21, bottom cell light absorbing layer 22, and bottom cell p-type doped layer 23 may have the same configuration as that used in known silicon-based solar cells. For example, by adding a doping impurity to the surface of a crystalline silicon substrate, the bottom cell n-type doped layer 21 may be formed on one side of the bottom cell light absorbing layer 22, which is a crystalline silicon substrate, and the bottom cell p-type doped layer 23 may be formed on the other side. The bottom cell n-type doped layer 21 may be formed by adding phosphorus, arsenic, or the like as a doping impurity, and the bottom cell p-type doped layer 23 may be formed by adding boron, gallium, or the like.

[0059] Furthermore, in a configuration using heterojunction silicon, for example, a configuration can be adopted in which a non-monocrystalline silicon-based thin film such as amorphous silicon or microcrystalline silicon is formed as the bottom cell n-type doped layer 21 and the bottom cell p-type doped layer 23 on a single-crystalline silicon substrate as the bottom cell light absorption layer 22. Examples of materials for the silicon-based thin film as the bottom cell n-type doped layer 21 and the bottom cell p-type doped layer 23 include amorphous silicon, microcrystalline silicon, amorphous silicon alloys, and microcrystalline silicon alloys. Examples of silicon alloys include silicon oxide, silicon carbide, silicon nitride, and silicon germanium. These may be used alone or in combination of two or more.

[0060] FIG. 3 is a graph showing the results of an experiment on the change in photoelectric conversion efficiency when the ratio of the materials constituting the top cell light absorption layer 14 of the top cell 10 is changed to change the photocurrent generated in the top cell 10, and the ratio of this to the photocurrent generated in the bottom cell 20 is changed in the solar cell 1 having the structure shown in FIG.

[0061] In this experiment, top cells 10 were prepared having top cell light-absorbing layers 14 with six different band gaps, for example, by appropriately combining MASnI3 (band gap: 1.1 eV), FAPbI3 (band gap: 1.4 eV), MAPbI3 (band gap: 1.5 eV), MAPBI2Br (band gap: 1.8 eV), and MAPbBr3 (band gap: 2.2 eV) as perovskite compounds that make up the top cell light-absorbing layer 14.

[0062] Figure 3 plots the relationship between the ratio of the short-circuit current (Isc) of the bottom cell 20 to the short-circuit current (Isc) of the six types of top cells 10 with different bandgaps prepared above (the ratio of the photocurrent of the top cell 10 to the photocurrent of the bottom cell 20, hereinafter referred to as the Top / Bottom ratio), and the photoelectric conversion efficiency (Pmax).

[0063] The top cell 10 with a Top / Bottom ratio of 1.43 has a top cell light absorbing layer 14 made of MAPbI3. The other examples are examples where a desired band gap is created by combining these materials.

[0064] The magnitude of the short circuit current (Isc) is the surface incidence only, solar radiation intensity 1000W / m 2 The measurements were performed under standard test conditions (STC) of air mass 1.5 and array representative temperature 25±2°C. However, the short-circuit current (Isc) may be calculated from the external quantum efficiency (EQE) of the six types of top cells10 and the IEC-specified AM1.5 spectrum.

[0065] Figure 4 plots the relationship between the rear incidence ratio (= amount of light incident on the rear light-receiving surface / amount of light incident on the front light-receiving surface) and the photoelectric conversion efficiency (Pmax) for each solar cell that uses four types of top cells 10 with a Top / Bottom ratio of 1.0 or more out of the six types of top cells 10 shown in Figure 3, specifically, for those with a Top / Bottom ratio of 1.00, a Top / Bottom ratio of 1.10, a Top / Bottom ratio of 1.43, and a Top / Bottom ratio of 1.92.

[0066] The band gap of the top cell light absorption layer 14 can be changed by changing the composition, and thus the amount of light absorption can be adjusted. However, since other properties also change at the same time, the combinations that can provide sufficient performance and reliability for the top cell light absorption layer 14 are limited.

[0067] As an example of a decrease in performance and reliability, mixing MAPbI3 and MASnI3 and replacing part of the Pb with Sn narrows the band gap, but the photoelectric conversion efficiency and reliability decrease due to the instability of the oxidation state of Sn. Also, changing the composition of the top cell light absorption layer 14 changes the optimal film formation conditions and device configuration, so the combination of compositions is also limited from this perspective.

[0068] As shown in FIG. 3, among the solar cells using six types of top cells 10, the one with a Top / Bottom ratio of 1.00 (which is equivalent to a conventional solar cell that is designed to generate power only from light incident on the front light-receiving surface) had a photoelectric conversion efficiency (Pmax) of 27 mW / cm 2 However, as shown in FIG. 4, when the Top / Bottom ratio is 1.00, even if the rear surface incidence ratio increases from 0.00 to 0.30, that is, even if the amount of reflected light 4 incident from the rear light receiving surface increases, the photoelectric conversion efficiency (Pmax) in the bottom cell 20 remains at 27 mW / cm 2 to 28mW / cm 2 It only increases slightly to a certain extent.

[0069] On the other hand, as shown in Figure 4, the photoelectric conversion efficiency (Pmax) of the device with a Top / Bottom ratio of 1.10 was 26 mW / cm 2 Although this is lower than the top cell 10 with the above-mentioned Top / Bottom ratio = 1.00, as the rear surface incidence ratio increases from 0.00 to 0.30, that is, as the amount of reflected light 4 incident from the rear light receiving surface increases, the photoelectric conversion efficiency (Pmax) of the bottom cell 20 increases to 26 mW / cm 2 to 30mW / cm 2 increases to.

[0070] Furthermore, as shown in Figure 4, the photoelectric conversion efficiency (Pmax) of the device with a Top / Bottom ratio of 1.43 was 24 mW / cm 2 Although this is lower than the top cell 10 with the above-mentioned Top / Bottom ratio = 1.00 or Top / Bottom ratio = 1.10, as the rear surface incidence ratio increases from 0.00 to 0.30, that is, as the amount of reflected light 4 incident from the rear light receiving surface increases, the photoelectric conversion efficiency (Pmax) of the bottom cell 20 increases to 24 mW / cm 2 to 34mW / cm 2 The photoelectric conversion efficiency (Pmax) increases significantly. MAPbI3, which constitutes the top cell light absorption layer 14, is rarely used in conventional tandem solar cells because its band gap is too narrow. However, the photoelectric conversion efficiency (Pmax) increases when the reflected light 4 from the back light-receiving surface is used for power generation.

[0071] The one with a Top / Bottom ratio of 1.43 uses MAPbI3 for the top cell 10 and Si(HJT) for the bottom cell 20. MAPbI3 has too narrow a bandgap to be used as a top cell in conventional solar cells that do not utilize light incident from the back light-receiving surface, but it is found to be useful in solar cells that make effective use of light incident from the back light-receiving surface.

[0072] As shown in FIG. 4, in the case where the Top / Bottom ratio is 1.92, as the rear surface incidence rate increases from 0.00 to 0.30, that is, as the amount of reflected light 4 incident from the rear light receiving surface increases, the photoelectric conversion efficiency in the bottom cell 20 decreases to 20 mW / cm. 2 to 35mW / cm 2 However, when the back-illumination ratio is low, from 0.00 to 0.0.5, the photoelectric conversion efficiency (Pmax) is 20 mW / cm 2 to 23mW / cm 2 Low degree.

[0073] Under normal installation conditions, the solar cell 1 is expected to have a rear surface incidence ratio based on reflected light 4 of approximately 0.10 to 0.20. Therefore, it is preferable that the Top / Bottom ratio be in this range so that the reflected light 4 incident from the rear light-receiving surface can be utilized, i.e., so that an increase in photoelectric conversion efficiency (Pmax) is observed. Based on the above results, it is preferable that the solar cell is configured so that the Top / Bottom ratio is in the range greater than 1 and less than 1.5.

[0074] Figure 5 plots the relationship between the rear incidence rate and the photoelectric conversion efficiency (Pmax) for each solar cell that uses three types of top cells 10 with a Top / Bottom ratio of 1.0 or less out of the six types of top cells 10 shown in Figure 3, specifically, those with a Top / Bottom ratio of 0.59, a Top / Bottom ratio of 0.81, and a Top / Bottom ratio of 1.0.

[0075] It was confirmed that these solar cells showed almost no increase in photoelectric conversion efficiency (Pmax) even when the backside incidence ratio was increased from 0.00 to 0.30, i.e., even when the amount of reflected light 4 incident from the backside light-receiving surface increased. In other words, it was confirmed that simply using rear glass 7 as the backside light-receiving surface and configuring solar cell 1 to also allow sunlight to enter from the backside does not increase the output of bottom cell 20 even when the amount of reflected light 4 incident from the backside light-receiving surface increases, and that the incident reflected light 4 is not being utilized.

[0076] As described above, the band gap of the light absorption layer can be changed relatively easily by changing the composition of the perovskite compound used in the top cell 10, thereby adjusting the ratio of the photocurrent of the top cell 10 to the photocurrent of the bottom cell 20. However, in addition to changing the band gap, the photocurrent of the top cell 10 and the bottom cell 20 can also be adjusted by using means such as light absorption in the electron transport layer, hole transport layer, or transparent electrode, or surface reflection of the cell or module.

[0077] Second Embodiment 6 is a schematic cross-sectional view of a solar cell 1' according to a second embodiment of the present disclosure. The solar cell 1' according to this embodiment can have the same configuration as the first embodiment, except as described below.

[0078] Solar cell 1' has a plurality of top cells arranged between the front light-receiving surface and the back light-receiving surface, and adjacent top cells are electrically connected in series by interconnectors 30 to form a top cell group 10'. Also, a plurality of bottom cells are arranged between the front light-receiving surface and the back light-receiving surface, and adjacent bottom cells are electrically connected in series by interconnectors 31 to form a bottom cell group 20'. The top cell group 10' and bottom cell group 20' are electrically connected in series.

[0079] Even with such a solar cell 1', similar effects can be achieved by configuring it to have the same relationship between the top cell 10 and the bottom cell 20 as in the first embodiment. In particular, by configuring it so that the Top / Bottom ratio is greater than 1 and less than 1.5, it is possible to utilize reflected light 4 incident from the back light receiving surface when the rear surface incidence rate based on reflected light 4 is in the range of approximately 0.10 to 0.20.

[0080] (Addendum) (Aspect 1) A bifacial solar cell having a tandem structure, comprising a front light-receiving surface and a back light-receiving surface onto which light can be incident, and a top cell and a bottom cell, each made of a different type of photoelectric conversion cell, disposed between the front light-receiving surface and the back light-receiving surface in this order from the front light-receiving surface side toward the back light-receiving surface side, a bifacial solar cell, characterized in that the bottom cell is capable of receiving light incident from the front light-receiving surface and light incident from the back light-receiving surface, and under standard test conditions (STC), the magnitude of the photocurrent of the bottom cell caused by light incident from the front light-receiving surface is smaller than the magnitude of the photocurrent of the top cell caused by light incident from the front light-receiving surface. (Aspect 2) The bifacial solar cell of aspect 1, wherein the top cell and the bottom cell are configured such that, under standard test conditions (STC), the ratio of the photocurrent magnitude of the top cell to the photocurrent magnitude of the bottom cell is in the range of greater than 1 and less than 1.5. (Aspect 3) 3. The bifacial solar cell according to aspect 1 or 2, wherein the top cell and the bottom cell are electrically connected in series. (Aspect 4) a plurality of the top cells are arranged between the front light receiving surface and the rear light receiving surface, and adjacent top cells are electrically connected in series; a plurality of the bottom cells are arranged between the front light receiving surface and the rear light receiving surface, and adjacent bottom cells are electrically connected in series; 2. The bifacial solar cell according to embodiment 1, wherein the top cell group and the bottom cell group are electrically connected in series. (Aspect 5) 5. The bifacial solar cell according to any one of aspects 1 to 4, wherein the top cell is a perovskite solar cell, and the bottom cell is a crystalline silicon solar cell.

[0081] The embodiments disclosed herein are illustrative in all respects and are not intended to be limiting. Therefore, the technical scope of the present disclosure should not be interpreted solely by the above-described embodiments, but should be defined based on the claims. Furthermore, all modifications within the scope and meaning equivalent to the claims are included. [Explanation of symbols]

[0082] 1: Solar cell 1': Solar cell 2: Photoelectric conversion element 3: Sunlight 4:Reflected light 5: Intermediate electrode 6: Front glass 7: Rear glass 8: Sealing resin 10: Top cell 10': Top cell group 11: Surface grid electrode 12: Surface transparent electrode 13: Top cell electron transport layer 14: Top cell light absorbing layer 15: Top cell hole transport layer 20: Bottom cell 20': Bottom cell group 21: Bottom cell n-type doped layer 22: Bottom cell light absorption layer 23: Bottom cell p-type doped layer 24: Back side transparent electrode 25: Rear grid electrode 30: Interconnector 31: Interconnector

Claims

1. A bifacial solar cell having a tandem structure, comprising a front light-receiving surface and a back light-receiving surface onto which light can be incident, and a top cell and a bottom cell, each made of a different type of photoelectric conversion cell, disposed between the front light-receiving surface and the back light-receiving surface in this order from the front light-receiving surface side toward the back light-receiving surface side, a bifacial solar cell, characterized in that the bottom cell is capable of receiving light incident from the front light-receiving surface and light incident from the back light-receiving surface, and under standard test conditions (STC), the magnitude of the photocurrent of the bottom cell caused by light incident from the front light-receiving surface is smaller than the magnitude of the photocurrent of the top cell caused by light incident from the front light-receiving surface.

2. 2. The bifacial solar cell of claim 1, wherein the top cell and the bottom cell are configured such that, under standard test conditions (STC), the ratio of the photocurrent magnitude of the top cell to the photocurrent magnitude of the bottom cell is in the range of greater than 1 and less than 1.

5.

3. 2. The bifacial solar cell according to claim 1, wherein the top cell and the bottom cell are electrically connected in series.

4. a plurality of the top cells are arranged between the front light receiving surface and the rear light receiving surface, and adjacent top cells are electrically connected in series; a plurality of the bottom cells are arranged between the front light receiving surface and the rear light receiving surface, and adjacent bottom cells are electrically connected in series; 2. The bifacial solar cell according to claim 1, wherein the top cell group and the bottom cell group are electrically connected in series.

5. 5. The bifacial solar cell according to claim 1, wherein the top cell is a perovskite solar cell and the bottom cell is a crystalline silicon solar cell.

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