Solar cell and manufacturing method thereof

Abstract

This solar cell (1) is provided with, in order from a substrate (2), an electron transport layer (4), a spacer layer (6), and a hole transport layer (7). The spacer layer (6) is a porous layer having a gap. The electron transport layer (4) is a denser layer than the spacer layer (6).

Description

Solar cell and its manufacturing method

[0001] The present disclosure relates to a solar cell using a perovskite compound and a method for producing the same.

[0002] In recent years, solar cells have become increasingly popular as a method for utilizing renewable energy. While solar cells using inorganic photoelectric conversion elements (e.g., silicon-based solar cells, CIGS-based solar cells, and CdTe-based solar cells) are widely used, solar cells using organic photoelectric conversion elements (e.g., organic thin-film solar cells, dye-sensitized solar cells, and perovskite solar cells) are also being considered. Perovskite solar cells, in particular, have been actively researched because they can be fabricated at lower cost than conventional solar cells (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2016-178167

[0004] A conventional solar cell has a substrate, an underlayer laminated on the substrate, and a light-absorbing layer containing an organic-inorganic perovskite compound formed inside the underlayer. The underlayer is formed by spraying at least one of N-type semiconductor particles and insulating particles to which the organic-inorganic perovskite compound is attached onto the substrate.

[0005] In the solar cell described above, when forming the underlayer, a perovskite compound is sprayed onto the substrate, but there is a problem that a dense electron transport layer that does not contain a perovskite compound cannot be formed. Furthermore, the connectivity between the fine particles has not been considered, and when a porous film is formed using only the fine particles to increase the connectivity between the fine particles and then the perovskite compound is applied, the penetration of the perovskite compound into the porous film has not been considered, so there is a risk that the perovskite compound will not penetrate the entire underlayer.

[0006] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a solar cell with improved permeability of a perovskite solution and a method for manufacturing the same.

[0007] The solar cell according to the present disclosure is a solar cell including, in order from a substrate, an electron transport layer, a spacer layer, and a hole transport layer, wherein the spacer layer is a porous layer having voids, and the electron transport layer is a layer denser than the spacer layer.

[0008] In the solar cell according to the present disclosure, the electron transport layer may be a dense layer.

[0009] In the solar cell according to the present disclosure, the electron transport layer may be configured so that the maximum width of the voids is less than 5 nm.

[0010] In the solar cell according to the present disclosure, the electron transport layer may be configured such that no perovskite compound is present on one side in the thickness direction of the electron transport layer.

[0011] In the solar cell according to the present disclosure, the electron transport layer may have a configuration in which there is no portion in which the perovskite compound exists throughout the thickness of the layer.

[0012] In the solar cell according to the present disclosure, the spacer layer may have insulating properties and be arranged adjacent to and in contact with the electron transport layer.

[0013] The solar cell according to the present disclosure may be configured such that there is no porous electron transport layer between the electron transport layer that is a dense layer and the spacer layer.

[0014] In the solar cell according to the present disclosure, the electron transport layer may be formed of a metal oxide selected from titanium oxide, tin oxide, zirconium dioxide, aluminum oxide, and silicon dioxide, either alone or in combination, and having a particle size of 1 to 80 nm.

[0015] In the solar cell according to the present disclosure, the electron transport layer may be formed of tin oxide.

[0016] In the solar cell according to the present disclosure, the substrate may be a flexible film.

[0017] The method for manufacturing a solar cell according to the present disclosure includes the steps of providing, in order from a substrate, an electron transport layer, a spacer layer, and a hole transport layer, wherein the spacer layer is a porous layer having voids, and the electron transport layer is a layer that is denser than the spacer layer.

[0018] According to the present disclosure, by laminating a spacer layer on the electron transport layer, the porous layer through which the perovskite solution penetrates can be made thinner, improving the permeability, thereby providing a solar cell with high conversion efficiency and improved variability.

[0019] 1 is a schematic cross-sectional view showing a solar cell according to an embodiment of the present disclosure; 2 is a schematic explanatory diagram showing a structure of a perovskite compound; 3 is a characteristic diagram showing the results of comparing the characteristics of a solar cell according to an embodiment of the present disclosure and a comparative example; 4 is a cross-sectional SEM photograph of a spacer layer;

[0020] Solar cells according to embodiments of the present disclosure will be described below with reference to the drawings.

[0021] FIG. 1 is a schematic cross-sectional view showing a solar cell according to an embodiment of the present disclosure.

[0022] A solar cell 1 according to an embodiment of the present disclosure includes a substrate 2 having a transparent electrode layer 3 formed on its surface, and an electron transport layer 4, a spacer layer 6, and a hole transport layer 7 stacked on top of the substrate 2.

[0023] The base 2 is the base of the solar cell 1 and may be the same as or include the substrate or base material. Examples of the shape of the base 2 include a flat plate and a film. When light is irradiated onto the surface of the solar cell 1 facing the base 2 (the underside of the base 2 in FIG. 1 ), the base 2 is preferably transparent. In this case, examples of materials for the base 2 include transparent glass and heat-resistant transparent resins. When light is irradiated from the opposite side, the base 2 may be opaque. Note that "transparent" means that light is transmitted through the base 2, but this does not exclude materials that reflect or absorb light even slightly. It is sufficient for the base 2 to transmit light appropriately, and can be considered synonymous with being provided on the light-receiving surface side of the solar cell (including the portion where light is incident, as in the present disclosure). Therefore, being provided at least on the light-receiving surface side of the solar cell can be considered transparent.

[0024] The transparent electrode layer 3 is formed on the substrate 2 or on the surface of the substrate 2 and functions as an electrode for extracting photovoltaic power from the solar cell 1. The transparent electrode layer 3 is formed of a transparent conductive material, such as FTO (fluorine-doped tin oxide), CuI (copper iodide), ITO (indium tin oxide), SnO2 (tin oxide), AZO (aluminum zinc oxide), IZO (indium zinc oxide), GZO (gallium zinc oxide), ATO (antimony-doped tin oxide), or a conductive transparent polymer. Note that the chemical formula is a representative example and may be any compound name (as in the present disclosure). Furthermore, the composition ratio of the chemical formula is preferably stoichiometric, but does not necessarily have to be stoichiometric (as in the present disclosure).

[0025] In this embodiment, the transparent electrode layer 3 is divided into two parts. For the sake of explanation, one of the transparent electrode layers 3 may be referred to as the first electrode portion 3a (the right side in FIG. 1 ) and the other as the second electrode portion 3b (the left side in FIG. 1 ). The term "film" or "layer" does not specify thickness or width, but includes patterned or island-shaped structures and structures with portions of different thicknesses. Preferably, the term "film" or "layer" has a substantially constant thickness. Unless otherwise specified, "substantially" means that variations within the range of manufacturing error are permitted, and preferably, variations of plus or minus 15% of the numerical value are permitted. In this disclosure, thickness, width, etc. are confirmed by cross-sectional observation unless otherwise specified. For example, SEM observation is performed by observing a cross-sectional SEM image of a 400 nm width. It is sufficient to confirm thickness, width, etc. within that range; it is not necessary to confirm all cross sections. That is, for example, "substantially constant thickness" can be said to be "substantially constant thickness" if the thickness is within a range of plus or minus 15% of the average thickness in a cross-sectional SEM image of a width of 400 nm.

[0026] The electron transport layer 4 is a layer capable of transporting electrons generated in the perovskite compound, and naturally has this function as long as it functions as a solar cell, and can be considered synonymous with being disposed on the electron transport side of the perovskite compound of the solar cell. Therefore, as long as it is disposed at least on the electron transport side of the perovskite compound, it can be said to have an electron transport function. The electron transport layer 4 preferably has a function of blocking the transport of holes. The electron transport layer 4 is preferably dense. In this embodiment, the electron transport layer 4 is formed of tin oxide and is provided only on the first electrode portion 3a, and is not present on the second electrode portion 3b.

[0027] The term "layer" does not specify thickness or width, and includes a pattern or island shape, or a layer having portions of different thickness. A layer is preferably a member having a substantially constant thickness.

[0028] Dense is also called dense, compact, compact quality, etc., and is the same as or includes these.

[0029] Porous is also called porous, porous, or mesoporous, and includes or refers to the same thing. Furthermore, porous refers to something that can contain a perovskite compound in the voids. The term "denser than porous" refers to something that is denser than another material, specifically, something that has less porosity than the voids of a porous material and can be confirmed to be denser. It should be noted that the evaluation criteria for "density" differ between absolute "density" (i.e., not relative) and relative "density."

[0030] Here, "dense," i.e., "dense" used without comparison with other terms, means that the voids are extremely small. For example, dense specifically refers to a material in which the maximum void width is less than 5 nm when observed through a cross section by SEM. Here, the maximum void width refers to the maximum portion of the void width when observed through a cross section by SEM. For example, it is sufficient to observe a cross-sectional SEM image of a 400 nm width and confirm that the maximum void width within that range is less than 5 nm. Preferably, dense refers to a material that suppresses the penetration of perovskite compounds and is free of perovskite compounds on one side of the thickness direction of the dense layer. More preferably, dense refers to a material that cannot contain perovskite compounds in the voids or does not have a portion where the perovskite compound is continuously present throughout the thickness of the dense portion. That is, when the dense layer cannot be confirmed at the maximum void width, it is sufficient to confirm by SEM and EDX observation that the perovskite compound is not present on one side of the dense layer in the thickness direction. The absence of the perovskite compound can be determined, for example, by observing a cross-sectional SEM (or EDX) image with a width of 400 nm using SEM and EDX, and if the perovskite compound is not present within that range, it can be determined that the perovskite compound is not present. Alternatively, when the absence of the perovskite compound cannot be confirmed, it is sufficient to confirm by SEM and EDX observation that there is no portion where the perovskite compound is present throughout the layer thickness. In the present disclosure, unless otherwise specified, it is sufficient to confirm by SEM observation by observing a cross-sectional SEM (or EDX) image with a width of 400 nm. For example, when a 400 nm wide cross-section is observed by SEM or EDX, and there is no portion in which the perovskite compound is present throughout the thickness of the layer, the layer can be said to be a dense layer. Note that the definition of dense as being one in which the voids are extremely small takes precedence, and when this cannot be confirmed, the above preferred definition (e.g., a state in which the perovskite compound is not present on one side in the thickness direction) can be applied, and when this still cannot be confirmed, the above more preferred definition (e.g., there is no portion in which the perovskite compound is present continuously throughout the thickness of the dense portion) can be applied.

[0031] The spacer layer 6 is made of a porous material and is preferably an insulator. That is, it is preferably an insulating porous spacer layer. Examples of materials for the spacer layer 6 include metal oxides such as titanium oxide, zirconium dioxide, aluminum oxide, and silicon dioxide. Furthermore, examples of materials for the insulating spacer layer 6 include insulating metal oxides such as insulating titanium oxide, insulating zirconium dioxide, insulating aluminum oxide, and insulating silicon dioxide. The spacer layer 6 is a porous layer including voids. The spacer layer 6 preferably has a large number of voids with a size of 20 nm or more. That is, the spacer layer 6 is occupied by, for example, the metal oxide 6c that constitutes the spacer layer itself and the voids between the metal oxides 6c. A porous layer has a large number of pores within the layer, and these pores are connected. Porosity means that when a liquid (with good wettability) is dropped onto a porous layer (a layer with hollow pores), the liquid penetrates into the porous layer.

[0032] It is preferable that the spacer layer be formed directly on the dense electron transport layer. That is, the spacer layer is not formed on the porous electron transport layer after a porous electron transport layer has been formed on the dense electron transport layer, but is instead formed directly on the dense electron transport layer. The structure can be described as the spacer layer being disposed adjacent to and in contact with the dense electron transport layer. Alternatively, from another perspective, the spacer layer can be described as being disposed adjacent to and in contact with the dense electron transport layer, with no porous electron transport layer between the dense electron transport layer and the spacer layer. This structure can be confirmed by observing a 400 nm-wide cross-sectional SEM (or EDX) image using a SEM or the like. That is, it can be confirmed in a 400 nm-wide cross-sectional SEM image that the spacer layer is disposed adjacent to and in contact with the dense electron transport layer. Alternatively, it is sufficient to confirm from another cut that there is no porous electron transport layer between the dense electron transport layer and the spacer layer in a 400 nm wide cross-sectional SEM image.

[0033] Furthermore, a light absorbing layer that absorbs irradiated light is provided in the gaps of the spacer layer 6. In this embodiment, the spacer layer 6, like the electron transport layer 4, has an insulating layer laminated portion 6a formed on the first electrode portion 3a and an inter-electrode portion 6b that covers the side surfaces of the electron transport layer 4 and fills between the first electrode portion 3a and the second electrode portion 3b. In the spacer layer 6, it is preferable that the particles of the metal oxide 6c are formed so as to be continuously connected.

[0034] The hole transport layer 7 is a layer that collects holes photoexcited in the light absorption layer and is made of a carbon electrode material that forms a porous layer. The hole transport layer 7 has a collection layer laminated portion 7a that covers the insulating layer laminated portion 6a with respect to the spacer layer 6, and an electrode connecting portion 7b that covers the side surface of the inter-electrode portion 6b and is laminated on the second electrode portion 3b.

[0035] The light absorbing layer described above is a layer containing a perovskite compound. As long as a solar cell has a photoelectric conversion function, the light absorbing layer naturally generates electrons and holes upon photoexcitation. The electrons generated in the light absorbing layer move to the electron transport layer, and the holes generated in the light absorbing layer move to the hole transport layer 7, resulting in charge separation. As long as a solar cell has a photoelectric conversion function, it can be said that electrons and holes are generated upon photoexcitation. The hole transport layer 7 also has a layer filled with a perovskite compound. That is, the voids in the hole transport layer 7 also contain a perovskite compound. It is desirable that the voids in the hole transport layer 7 are filled with a perovskite compound. The thickness of the light absorbing layer is preferably 500 nm or more and 2 μm or less, and more preferably 700 nm or more and 800 nm or less.

[0036] The perovskite compound contained in the light-absorbing layer is composed of a compound represented by the general formula: ABX (1). While the composition ratio of each element is preferably 1:1:3, it does not necessarily have to be 1:1:3. The content of each element may vary as appropriate, and each element does not necessarily have to be a single type. As long as the solar cell has a photoelectric conversion function, the perovskite compound contained in the light-absorbing layer exhibits the photoelectric conversion function. Therefore, even if there is a degree of freedom in the composition as described above regarding the composition ratio and the type of constituent elements, it is reasonable to consider that the function is exhibited. In general formula (1), A is an organic molecule (including an organic group or an organic cation, as defined in the present disclosure) or an inorganic atom or molecule (including an inorganic group or an inorganic cation, as defined in the present disclosure) or a combination thereof; B is a metal atom or molecule (including a metal cation, as defined in the present disclosure); and X is a halogen atom or molecule or a chalcogen atom or molecule (including a halogen anion or a chalcogen anion, as defined in the present disclosure). In general formula (1), the three Xs may be the same or different. As long as the solar cell has a photoelectric conversion function, the perovskite compound contained in the light absorption layer exhibits the photoelectric conversion function, and this should be taken into consideration. That is, if it is confirmed that a compound is a perovskite compound, it is reasonable to consider it a perovskite compound exhibiting a photoelectric conversion function. For example, it is sufficient to know that it contains organic molecules, metal atoms, and halogen atoms. Furthermore, as long as the solar cell has a photoelectric conversion function, it can be confirmed that the compound is a perovskite compound if elements corresponding to A, B, and X are detected. For example, molecules containing carbon, nitrogen, and hydrogen are suitable as organic molecules, and therefore it is sufficient to detect carbon, nitrogen, hydrogen, a metal element, and a halogen or chalcogen. Alternatively, it is sufficient to know that a compound is a perovskite compound if it contains A, B, and X. For example, it is sufficient to know that it contains inorganic atoms, metal atoms, and halogen atoms. Furthermore, the perovskite compound can be confirmed if elements corresponding to A, B, and X are detected, so long as the solar cell has a photoelectric conversion function.For example, cesium or rubidium is suitable as the inorganic atom, and therefore, it is sufficient to detect cesium or rubidium, a metal element, and a halogen or chalcogen. Furthermore, since it is a natural consequence that a solar cell has a crystalline structure as long as it has a photoelectric conversion function, it is not necessary to confirm that the compound is a perovskite compound. This does not exclude the inclusion of compounds other than perovskite compounds in the light absorption layer.

[0037] The light absorption layer may contain an organic-inorganic hybrid compound. An organic-inorganic hybrid compound refers to a compound containing an inorganic material and an organic material. Organic-inorganic hybrid compounds also include perovskite compounds, and solar cells using perovskite compounds are also called organic-inorganic hybrid solar cells. "Organic" typically refers to a material composed of multiple carbon atoms. Note that graphite, graphene, carbon nanowires, carbon nanofibers, carbon nanotubes, and carbon materials such as carbon and carbon black that function as electrodes are not considered to be organic materials. In other words, organic refers to a material that contains multiple carbon atoms as one of its constituent elements, excluding the above-mentioned carbon materials such as graphite. "Inorganic" refers to a material that is not organic.

[0038] In 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] In general formula (1), examples of halogen atoms represented by X include fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms, and 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. As the halogen atom represented by X, an iodine atom is preferred from the viewpoint of enabling the perovskite compound to utilize light in a wide wavelength range. Specifically, of the three Xs, it is preferred that at least one X represents an iodine atom, and it is more preferred that all three Xs represent iodine atoms.

[0045] 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 1 can be further improved.

[0046] When fabricating the solar cell 1, first, a substrate is fabricated using a general photolithography process, screen printing process, etc., with appropriate settings made for the portions where each layer of the solar cell 1 will be provided. The fabricated substrate has a porous layer as described above, and the solar cell 1 is fabricated by dripping a perovskite precursor solution containing a perovskite compound (described below) from the top of the substrate and baking it.

[0047] In the process of producing the solar cell 1, after each layer is formed on the substrate 2, a perovskite precursor solution containing an organic onium salt is dropped onto the substrate, followed by baking to form a light absorbing layer. In this embodiment, the organic onium salt-containing perovskite precursor solution contains 100% Pb and 5% 5-aminovaleric acid hydroiodide (5-AVAI) added.

[0048] Although FIG. 1 shows a configuration in which an electrode layer is provided only on the substrate 2, the present invention is not limited to this, and a configuration in which an additional electrode layer is provided on the hole transport layer 7 may also be used.

[0049] As described above, the electron transport layer 4 is a layer that is denser than the spacer layer 6. By stacking the spacer layer 6 on the electron transport layer 4 in this way, the porous layer through which the perovskite solution permeates becomes thinner, thereby improving permeability. This makes it possible to provide a solar cell with high conversion efficiency and improved variability.

[0050] The electron transport layer 4 is preferably formed of a metal oxide selected from titanium oxide, tin oxide, zirconium dioxide, aluminum oxide, and silicon dioxide, either alone or in combination, and has a particle size of 1 to 80 nm, more preferably 1 to 20 nm. By using a metal oxide with an appropriate particle size, a dense electron transport layer 4 can be obtained.

[0051] The particle diameter in the electron transport layer 4 can be determined by calculating the diameter of a circle having an area equivalent to the cross-sectional area of ​​10 adjacent particles in SEM cross-section observation of the electron transport layer 4, and averaging the diameters.

[0052] The electron transport layer 4 is formed by depositing particles of various sizes, and is a layer having uneven thickness. The particle diameter of the metal oxide may be larger than the average film thickness of the electron transport layer 4. In other words, particles may exist that occupy the entire electron transport layer 4 from the bottom to the top.

[0053] It is possible to form a porous electron transport layer on a dense electron transport layer 4. Then, a porous spacer layer can be formed on the porous electron transport layer. This eliminates electrical connectivity issues between the electron transport layer and the hole transport layer, enabling excellent photoelectric conversion performance. However, in this case, after the porous electron transport layer and the porous spacer layer are formed, a precursor of the perovskite compound must be applied or otherwise formed to penetrate both porous layers. This can lead to problems with even penetration into both layers. As a result, some perovskite does not penetrate, resulting in resistive areas without the perovskite compound, which can lead to variations and reduced performance.

[0054] However, in this embodiment, a porous electron transport layer is not formed on the dense electron transport layer 4, but rather a porous spacer layer is formed directly on the dense electron transport layer. In this way, after forming only the porous spacer layer on the dense electron transport layer without the porous electron transport layer, it is only necessary to form a precursor of the perovskite compound by coating or the like so that it penetrates into the porous spacer layer, which makes it easy to penetrate the porous spacer layer evenly and allows the liquid to penetrate evenly. As a result, the perovskite compound is formed evenly throughout the layer, leading to improved conversion efficiency and improved variation.

[0055] As described above, in this embodiment, a film-shaped substrate 1 or a flexible film-shaped substrate 1 may be used. However, a problem with the film-shaped substrate 1 is that it cannot withstand high temperatures. When forming a mesoporous nanocrystal layer as in conventional solar cells, firing is performed at 500°C, but there is a risk that the film-shaped substrate 1 will not be able to withstand this temperature.

[0056] In contrast, in the present embodiment, there is no need to form a mesoporous nanocrystal layer, so that firing can be carried out at a low temperature, for example, 150°C or less, and solar cell 1 can be formed without damaging film-like substrate 1.

[0057] Next, the results of a comparison of the characteristics of solar cell 1 according to an embodiment of the present disclosure and a comparative example fabricated under different conditions will be described with reference to FIGS. 2 and 3. FIG.

[0058] FIG. 2 is a characteristic diagram illustrating the mobility of electrons in the comparative example.

[0059] FIG. 2 shows the energy band relationship between the electron transport layer 4 and the perovskite compound (light absorption layer) in a comparative example, where "La1" corresponds to the electron transport layer 4 and "La2" corresponds to the perovskite compound. In the comparative example, the electron transport layer 4 is formed of titanium oxide. In FIG. 2, the energy level represented by EL1 or EL2 indicates the energy level at the bottom of the conduction band, and the energy level indicated by the dashed dotted line below indicates the top of the valence band. The top of the valence band is sometimes referred to as the VBM (valence band maximum), and the bottom of the conduction band is sometimes referred to as the CBM (conduction band minimum). Furthermore, the absolute value of the difference between the vacuum level and the CBM can be rephrased as the electron affinity (or its absolute value), and the absolute value of the difference between the vacuum level and the VBM can be rephrased as the ionization potential (or its absolute value). "Deep" or "low" for a CBM or VBM means that the corresponding electron affinity or ionization energy is large or far from the vacuum level, and "shallow" or "high" for a CBM or VBM means that the corresponding electron affinity or ionization energy is small or close to the vacuum level.

[0060] In the comparative example, the energy level (first energy level EL1) of the CBM of the electron transport layer 4 is higher than the energy level (second energy level EL2) of the CBM of the perovskite compound, making it difficult to inject electrons from the perovskite compound. Therefore, unless a porous electron transport layer is provided between the electron transport layer 4 and the perovskite compound to increase the contact area, a suitable current cannot be obtained.

[0061] FIG. 3 is a characteristic diagram illustrating electron mobility according to an embodiment of the present disclosure.

[0062] 3 shows the relationship between the energy bands of the electron transport layer 4 and the perovskite compound (light absorbing layer) in this embodiment of the disclosure, where "La3" corresponds to the electron transport layer 4 and "La4" corresponds to the perovskite compound. In this embodiment, the electron transport layer 4 is made of tin oxide.

[0063] In the present embodiment, the energy level (third energy level EL3) of the CBM of the electron transport layer 4 is lower than the energy level (third energy level EL3) of the CBM of the perovskite compound, and electron injection from the perovskite compound is advantageous, so that a suitable current can be obtained without providing a porous electron transport layer.

[0064] FIG. 4 is a cross-sectional SEM photograph of the solar cell according to this embodiment.

[0065] 4 is an SEM photograph of a cross section of the solar cell 1 according to this embodiment. The lower region of the SEM photograph (first region AR1) corresponds to the electron transport layer 4 and appears dark overall. If the electron transport layer 4 is not porous, perovskite is not encapsulated, and therefore lead cannot be detected by EDX analysis or the like.

[0066] The central region (second region AR2) of the SEM photograph corresponds to the spacer layer 6, and in cross-sectional SEM observation, white areas are visible in places because the insulating properties make it difficult for electrons to escape.

[0067] It should be noted that 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.

[0068] This application claims priority based on Japanese Patent Application No. 2023-140871 filed on August 31, 2023, the contents of which are incorporated herein by reference. Furthermore, all references cited in this specification are specifically incorporated herein by reference in their entirety.

[0069] REFERENCE SIGNS LIST 1 Solar cell 2 Substrate 3 Transparent electrode layer 4 Electron transport layer 6 Spacer layer 7 Hole transport layer

Claims

1. A solar cell comprising, in order from a substrate, an electron transport layer, a spacer layer, and a hole transport layer, wherein the spacer layer is a porous layer having voids, and the electron transport layer is a layer denser than the spacer layer.

2. The solar cell according to claim 1, characterized in that the electron transport layer is a dense layer.

3. The solar cell according to claim 2, characterized in that the electron transport layer has a maximum gap width of less than 5 nm.

4. A solar cell according to claim 2 or 3, characterized in that the electron transport layer does not contain a perovskite compound on one side in the thickness direction of the layer.

5. A solar cell according to any one of claims 2 to 4, characterized in that the electron transport layer has no portion in which the perovskite compound exists throughout the thickness of the layer.

6. A solar cell according to any one of claims 2 to 5, characterized in that the spacer layer has insulating properties and is disposed adjacent to and in contact with the electron transport layer.

7. A solar cell according to claim 6, characterized in that there is no porous electron transport layer between the dense electron transport layer and the spacer layer.

8. A solar cell according to any one of claims 1 to 7, characterized in that the electron transport layer is formed of a metal oxide selected from the group consisting of titanium oxide, tin oxide, zirconium dioxide, aluminum oxide, and silicon dioxide, either alone or in combination, and has a particle size of 1 to 80 nm.

9. The solar cell according to claim 8, wherein the electron transport layer is made of tin oxide.

10. A solar cell according to any one of claims 1 to 9, characterized in that the substrate is a flexible film.

11. A method for manufacturing a solar cell, comprising the steps of providing, in order from a substrate, an electron transport layer, a spacer layer, and a hole transport layer, wherein the spacer layer is a porous layer having voids, and the electron transport layer is a layer denser than the spacer layer.

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