Tandem solar cell and method for manufacturing same

Abstract

The present invention addresses the problem of providing a tandem solar battery (1) having a simple configuration and exceptional battery performance. A tandem solar battery (1) that solves the aforementioned problem includes a first solar battery cell (CB1) that includes a first light absorption layer (10) containing n-type or p-type crystal silicon, a titanium oxide layer (31) that is disposed on the first light absorption layer (10) and contains amorphous titanium oxide, an intermediate layer (32) that is disposed adjacent to the titanium oxide layer (31) and contains tin oxide or titanium nitride having a higher nitrogen composition than the titanium oxide layer (31), and a second solar battery cell (CT1) disposed on the intermediate layer (32).

Description

Tandem solar cell and its manufacturing method

[0001] The present invention relates to a tandem solar cell and a method for manufacturing the same.

[0002] Tandem solar cells have a structure in which two or more solar cells with different bandgaps are stacked. Because the solar spectrum is made up of photons with a wide range of energy, from ultraviolet to near-infrared, solar cells with multiple bandgaps can generate electricity with higher conversion efficiency than solar cells with a single bandgap.

[0003] Tandem solar cells are known in which a perovskite solar cell is arranged on the top cell side and a crystalline silicon solar cell is arranged on the bottom cell side. Between the light absorption layer (perovskite crystalline material layer) of the perovskite solar cell and the light absorption layer (crystalline silicon layer) of the crystalline silicon solar cell, a top cell-side charge selection layer, a recombination layer, a bottom cell-side charge selection layer, and a bottom cell-side passivation layer are typically arranged in this order (see, for example, Non-Patent Documents 1 and 2). The charge selection layer is also referred to as a charge transport layer.

[0004] Here, in the above configuration, the bottom cell side passivation layer is made of amorphous silicon or ultra-thin silicon oxide (SiO ２ The bottom cell side charge selection layer uses amorphous silicon doped with phosphorus, boron, or the like, nanocrystalline silicon or its alloy, or polycrystalline silicon. The recombination layer is a layer for exchanging charges between the perovskite solar cell and the crystalline silicon solar cell, and the recombination layer uses a transparent conductive layer with a thickness of about 20 nm made of indium tin oxide (ITO), indium zinc oxide (IZO), or the like, or doped nanocrystalline silicon.

[0005] A. Al-Ashouri et al., Science, vol.370, issue 6522, pp.1300-1309, 2020C. McDonald et al., ACS Appl. Mater. Interfaces 14, pp.33505-33514, 2022

[0006] As mentioned above, perovskite / crystalline silicon tandem solar cells require the formation of multiple layers at the interface between the top and bottom cells, which makes the manufacturing process complicated and further improvements are needed.

[0007] To address these issues, the inventors proposed replacing the three-layer structure of the recombination layer, bottom cell-side charge selection layer, and bottom cell-side passivation layer with a single amorphous titanium oxide layer approximately 5 nm thick (unpublished at the time of filing). In this tandem solar cell, an amorphous titanium oxide layer is disposed between the top and bottom cells. This amorphous titanium oxide layer simultaneously functions as a surface passivation layer for the bottom cell's light absorption layer (crystalline silicon), a hole selection layer for extracting holes from the bottom cell's light absorption layer, and a recombination layer for exchanging electrons from the top cell with holes from the bottom cell. Tandem solar cells with this structure can be efficiently manufactured without the need for multiple layers between the top and bottom cells. Furthermore, they do not require the formation of a layer containing indium, a rare metal (a conventional recombination layer). Energy loss due to interfacial reflection between the indium-containing layer (a conventional recombination layer) and other layers can also be suppressed.

[0008] However, further investigation by the inventors revealed that in tandem solar cells using the amorphous titanium oxide layer, contact resistance at the interface between each layer is likely to be high, and that there is room for further improvement in the passivation performance at the interface between the titanium oxide layer and the light absorption layer (crystalline silicon layer).

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a tandem solar cell with a simple structure and excellent cell performance, and a method for manufacturing the tandem solar cell.

[0010] The present invention provides the following tandem solar cells. [1] A tandem solar cell including: a first solar cell including a first light absorbing layer containing n-type or p-type crystalline silicon; a titanium oxide layer containing mainly amorphous titanium oxide arranged on the first light absorbing layer; an intermediate layer arranged adjacent to the titanium oxide layer and containing titanium nitride or tin oxide having a higher nitrogen content than the titanium oxide layer; and a second solar cell arranged on the intermediate layer. [2] The tandem solar cell according to [1], wherein the ratio of the thickness of the intermediate layer to the thickness of the titanium oxide layer is 0.5 to 1.5. [3] The tandem solar cell according to [1] or [2], wherein the second solar cell includes a second electron selection layer and a second light absorbing layer, in this order from the first solar cell side, and the intermediate layer and the electron selection layer are arranged adjacent to each other. [4] The tandem solar cell according to [3], wherein the second light absorbing layer contains a perovskite crystalline material. [5] The tandem solar cell according to [3] or [4], wherein the second electron selection layer contains tin oxide. [6] The tandem solar cell according to [1] or [2], wherein the second solar cell includes a second light absorbing layer made of an n-type semiconductor, and the intermediate layer and the second light absorbing layer are disposed adjacent to each other.

[0011] The present invention provides the following methods for manufacturing a tandem solar cell. [7] A method for manufacturing a tandem solar cell, comprising the steps of preparing a first light-absorbing layer containing n-type or p-type crystalline silicon, forming a titanium oxide layer mainly containing amorphous titanium oxide on the first light-absorbing layer, forming an intermediate layer containing titanium nitride or tin oxide having a higher nitrogen content than the titanium oxide layer so as to be in contact with the titanium oxide layer, and forming a second solar cell on the intermediate layer. [8] The method for manufacturing a tandem solar cell according to [7], wherein the ratio of the thickness of the intermediate layer to the thickness of the titanium oxide layer is 0.5 to 1.5. [9] The method for manufacturing a tandem solar cell according to [7] or [8], wherein the step of forming the second solar cell comprises the steps of forming a second electron selection layer so as to be in contact with the intermediate layer, and forming a second light-absorbing layer on the second electron selection layer.

[10] The method for manufacturing a tandem solar cell according to [9], wherein the second electron selection layer contains tin oxide and the second light-absorbing layer contains a perovskite crystalline material.

[11] The method for manufacturing a tandem solar cell according to [7] or [8], wherein the step of forming the second solar cell is a step of forming a second light absorbing layer made of an n-type semiconductor so as to be in contact with the intermediate layer.

[0012] According to the present invention, a tandem solar cell having a simple configuration and excellent cell performance, and a method for manufacturing the same are provided.

[0013] 1A and 1B are diagrams showing a schematic device structure of a tandem solar cell of the first embodiment. FIG. 2 is a flowchart illustrating a method for manufacturing a tandem solar cell of the first embodiment. FIGS. 3A and 3B are diagrams showing a schematic device structure of a tandem solar cell of the second embodiment. FIG. 4 is a flowchart illustrating a method for manufacturing a tandem solar cell of the second embodiment. FIG. 5 is a diagram showing a schematic device structure of a tandem solar cell of the third embodiment. FIG. 6 is a flowchart illustrating a method for manufacturing a tandem solar cell of the third embodiment. FIGS. 7A and 7B are diagrams showing a schematic device structure of a tandem solar cell of the fourth embodiment. FIG. 8 is a flowchart illustrating a method for manufacturing a tandem solar cell of the fourth embodiment. FIG. 9 is a flowchart illustrating a second electron selection layer (SnO ２ Fig. 10 is a cross-sectional transmission electron microscope image of the titanium nitride layer / titanium oxide layer / first light absorbing layer (Si) interface. Fig. 10 is a diagram showing the results of EELS analysis of each layer of the tandem solar cell fabricated in the example. Fig. 11 is a diagram showing the cell performance of the tandem solar cells fabricated in the example and comparative example. Fig. 12 is a diagram showing photoluminescence images of the tandem solar cells fabricated in the example and comparative example. Fig. 13 is a diagram showing the cell performance of a tandem solar cell having a titanium nitride layer as an intermediate layer and a tandem solar cell having a tin oxide layer as an intermediate layer.

[0014] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same reference numerals indicate the same or corresponding parts. Furthermore, in this specification, when a numerical range is indicated, the upper and lower limits are included.

[0015] The tandem solar cell of the present invention has a structure in which a first solar cell (bottom cell) including a first light absorption layer containing crystalline silicon, a titanium oxide layer containing mainly amorphous titanium oxide, an intermediate layer adjacent to the titanium oxide layer and containing titanium nitride or tin oxide with a higher nitrogen content than the titanium oxide layer, and a second solar cell (top cell) are stacked in this order. Note that if the intermediate layer contains titanium nitride, the layer will contain more oxygen than nitrogen during the formation of the intermediate layer or during subsequent processes. This layer is also referred to simply as an intermediate layer containing titanium nitride or a titanium nitride layer. While other layers may be included between the intermediate layer and the second solar cell, it is preferable to not include other layers between them from the perspective of manufacturing efficiency of the tandem solar cell. Furthermore, other layers may be included between the first light absorption layer and the titanium oxide layer as long as they do not impair the objectives and effects of the present invention, but it is preferable to not include other layers between them from the perspective of manufacturing efficiency of the tandem solar cell.

[0016] In the tandem solar cell of the present invention, the amorphous titanium oxide layer disposed adjacent to the first light absorption layer of the first solar cell functions as a hole selection layer for the first solar cell, a passivation layer for the first solar cell, and a recombination layer for the first solar cell and the second solar cell. The intermediate layer functions to enhance the passivation performance of the interface between the first light absorption layer and the titanium oxide layer and to reduce the contact resistance between the titanium oxide layer and the second solar cell. Because the tandem solar cell of the present invention includes this intermediate layer, its cell performance (e.g., fill factor and open-circuit voltage) is significantly better than that of tandem solar cells of conventional configurations, as shown in the examples below.

[0017] The tandem solar cell of the present invention requires more manufacturing steps than a tandem solar cell that does not include an intermediate layer. However, the titanium oxide layer and the intermediate layer can usually be formed successively. Therefore, the tandem solar cell can be manufactured more efficiently than a conventional tandem solar cell that uses an ITO layer as a recombination layer.

[0018] The tandem solar cell of the present invention and the method for manufacturing the same will be described below using the following four embodiments as examples, although the tandem solar cell of the present invention and the method for manufacturing the same are not limited to these.

[0019] (1) First Embodiment The schematic structure of a tandem solar cell 1 of the first embodiment is shown in FIGS. 1A and 1B. The tandem solar cell 1 (1A and 1B) of this embodiment has a structure in which a crystalline silicon first solar cell CB1, a titanium oxide layer 31, an intermediate layer containing titanium nitride (hereinafter also referred to as the "titanium nitride layer") 32, and a perovskite second solar cell CT1 are stacked in this order. In this tandem solar cell 1, light is incident from the second solar cell CT1 side. The tandem solar cell 1A shown in FIG. 1A and the tandem solar cell 1B shown in FIG. 1B differ only in whether the first light absorption layer 10 contains n-type crystalline silicon (10n) or p-type crystalline silicon (10p), and are otherwise identical in configuration. Each component of the tandem solar cell 1 (1A and 1B) of this embodiment will be described in detail below. As will be described later, the titanium nitride layer contains a certain amount of oxygen, but the titanium nitride layer itself is formed using raw materials that do not contain oxygen.

[0020] (First solar cell CB1) The first solar cell CB1 has, from the titanium oxide layer 31 side, a first light absorbing layer 10 (n-type crystalline silicon 10n or p-type crystalline silicon 10p), a first electron selection layer 12, a first translucent electrode 13, and a first metal electrode 14, in this order.

[0021] The first light absorbing layer 10 may be any layer primarily containing crystalline silicon, preferably a layer made of single-crystalline silicon whose surface is a (100) plane and whose resistivity is approximately 0.1 to 10 Ω. The n-type crystalline silicon 10n is preferably doped with n-type impurities such as phosphorus (P), while the p-type crystalline silicon 10p is preferably doped with p-type impurities such as boron (B) or gallium (Ga). The thickness of the first light absorbing layer 10 is not particularly limited, but is typically preferably approximately 50 μm to 300 μm. Furthermore, the first light absorbing layer 10 may have a p+ diffusion layer in which boron or gallium is diffused at the interface with the titanium oxide layer 31 to improve passivation and hole selectivity at the interface between the first light absorbing layer 10 and the titanium oxide layer 31. Furthermore, as long as the object and effect of this embodiment are not impaired, the first light absorbing layer 10 may include an i-type amorphous silicon layer and a p-type amorphous silicon layer (not shown) on the interface side with the titanium oxide layer 31 in order to improve passivation and hole selectivity at the interface between the first light absorbing layer 10 and the titanium oxide layer 31.

[0022] 1A and 1B, the surface of the first light absorbing layer 10 facing the titanium oxide layer 31 is flat, but may have a random texture structure having a (111) facet. Furthermore, the surface of the first light absorbing layer 10 facing the first electron selection layer 12 has a random texture structure in FIGS. 1A and 1B, but may also be flat. When the surface of the first light absorbing layer 10 has a random texture structure, the reflection-reducing effect and light-trapping effect resulting from this structure improve the conversion efficiency of the first solar cell CB1.

[0023] The first electron selection layer 12 is a layer disposed adjacent to the first light absorbing layer 10, and may be any layer that can selectively extract electrons from the first light absorbing layer 10. There are no particular limitations on the material that constitutes the first electron selection layer 12.

[0024] The type of the first electron selection layer 12 is not particularly limited. The first electron selection layer 12 may be, for example, a stack (a-Si:H i-n layer) of a hydrogen-doped intrinsic amorphous silicon layer 12a and a hydrogen-doped n-type amorphous silicon layer 12b. In the first electron selection layer 12, the hydrogen-doped intrinsic amorphous silicon layer 12a functions as a passivation layer, and the hydrogen-doped n-type amorphous silicon layer 12b functions as a layer for electron selection. This results in improved passivation characteristics and electron selectivity. However, the first electron selection layer 12 may also be a stack of a silicon oxide film with the same function as the a-Si:H i-n layer and n-type doped polycrystalline silicon. Furthermore, the first electron selection layer 12 may be composed of only one layer, such as a titanium oxide layer with electron selectivity. The thickness of the first electron selection layer 12 is not particularly limited, but is preferably 5 nm to 20 nm.

[0025] The first translucent electrode 13 may be any layer that is transparent to visible light and near-infrared light and conductive. Examples of materials for the first translucent electrode 13 include indium-tin oxide (ITO), zinc oxide (ZnO), indium-zinc oxide (IZO), and the like. The thickness of the first translucent electrode 13 is not particularly limited and may be, for example, approximately 70 nm to 150 nm. However, if the first electron selection layer 12 is a stack of an electron-selective silicon oxide layer and an n-type doped polycrystalline silicon layer instead of an amorphous silicon layer, the first translucent electrode 13 may be omitted or replaced with a thin insulator such as silicon nitride or aluminum oxide. Furthermore, if an electron-selective titanium oxide layer is selected instead of an amorphous silicon layer for the first electron selection layer 12, in addition to the above materials, a lithium fluoride layer several nanometers thick may also be used.

[0026] The first metal electrode 14 may be an electrode formed in a planar shape so as to be in contact with the entire first translucent electrode 13, or may be an electrode formed in a grid shape. When the first metal electrode 14 is formed in a planar shape, the first metal electrode 14 reflects light incident on the tandem solar cell 1. This tends to increase the short-circuit current density and conversion efficiency of the tandem solar cell 1. On the other hand, when the first metal electrode 14 is formed in a grid shape, light can be collected from the back side of the tandem solar cell 1, allowing the tandem solar cell 1 to be a bifacial solar cell.

[0027] Examples of metal materials constituting the first metal electrode 14 include metals such as aluminum (Al), titanium (Ti), silver (Ag), copper (Cu), nickel (Ni), gold (Au), and platinum (Pt), as well as alloys of these. When the first metal electrode 14 is formed in a planar shape, a metal with a relatively high reflectance in the near-infrared region is preferred, and examples thereof include aluminum, gold, silver, and copper. The thickness of the first metal electrode 14 is selected appropriately depending on the type of material constituting the first metal electrode 14 and the shape of the first metal electrode 14, but is typically preferably 100 nm to 1000 nm.

[0028] (Second solar cell CT1) The second solar cell CT1 has, from the titanium nitride layer 32 side, a second electron selection layer 25, a second light absorption layer 26, a second hole selection layer 27, a second translucent electrode 28, and a second metal electrode (grid electrode) 29, in this order.

[0029] The second electron selection layer 25 may be any layer that can selectively extract electrons from the second light absorption layer 26 described below. Examples of materials that make up the second electron selection layer 25 include tin oxide (SnO ２ ), titanium oxide (TiO ｘ ), niobium oxide (NbO ｘ ), fullerene (C ６０ The second electron selection layer 25 may contain only one of these, or may contain two or more of them. Among these, tin oxide (SnO ２The thickness of the second electron selection layer 25 is not particularly limited, but is preferably in the range of 10 nm to 50 nm.

[0030] The second light absorbing layer 26 may be any layer that contains a perovskite crystal material and is capable of generating electrons and holes when irradiated with sunlight. The perovskite crystal material is represented by the general formula ABX ３ In the general formula, A is an organic group MA (methylammonium (CH ３ NH ３ ＋ )), organic group FA (formamidinium (HC(NH ２ ) ２ ＋ )), cesium cation (Cs ＋ ), potassium cation (K ＋ ), rubidium cation (Rb ＋ ) and may be a mixture thereof. In addition, B in the general formula represents a metal ion Pb ２＋ or Sn ２＋ Furthermore, X in the general formula represents a halogen ion F － , Cl － , I － , or Br － The three X's may all be the same halogen atom or may be different atoms.

[0031] Specific examples of perovskite crystalline materials include those of the formula (Rb,MA,FA): １ Pb １ (I １－ｙ Br ｙ ) ３ (where 0≦y≦1). The band gap of the perovskite crystal material can be changed by changing the type and ratio of halogens. The second light absorbing layer 26 may contain only one type of perovskite crystal, or may contain two or more types. There are no particular restrictions on the thickness of the second light absorbing layer 26, but it is usually preferably 300 nm or more and 1000 nm or less.

[0032] The second hole selection layer 27 is a layer disposed on the opposite side of the second electron selection layer 25, with the second light absorption layer 26 sandwiched therebetween. The second hole selection layer 27 may be any layer that can selectively extract holes from the second light absorption layer 26. Examples of materials for the second hole selection layer 27 include 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (abbreviation: Spiro-OMeTAD), poly(3-hexylthiophene-2,5-diyl) (abbreviation: P3HT), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (abbreviation: PTAA), N,N-bis(3-methylphenyl) )-N,N-diphenylbenzidine (abbreviation: TPD), N,N-di[(1-naphthyl)-N,N-diphenyl]-1,1-biphenyl)-4,4-diamine (abbreviation: NPD), tris(4-carbazoyl-9-ylphenyl)amine (abbreviation: TCTA), poly(9-vinylcarbazole) (abbreviation: PVK), 4,4-bis(N-carbazolyl)-1,1-biphenyl (abbreviation: CBP), etc. The second hole selection layer 27 may contain only one of these, or may contain two or more of them. Among the above materials, Spiro-OMeTAD is preferred from the viewpoints of ease of availability and ease of application. The second hole selection layer 27 may further contain a dopant such as lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) or TBA-TFSI, whose cation is tetrabutylammonium cation (TBA). It may also contain an additive such as 4-tert-butylpyridine. The hole selection layer materials described above are typically formed by solution coating, but a thinner second hole selection layer 27 can be formed by depositing other materials by vacuum deposition. For example, 2,2',7,7'-tetrakis(di-p-tolylamino)-9,9'-spirobifluorene (abbreviated as Spiro-TTB) or 2-(3,6-dimethoxy-9H-carbazol-9-yl)ethylphosphonic acid (abbreviated as MeO-2PACz) may be selected. The thickness of the second hole selection layer 27 is not particularly limited, but is typically preferably approximately 5 nm to 300 nm.

[0033] The second translucent electrode 28 may be any layer that is transparent to visible light and conductive. The material constituting the second translucent electrode 28 is the same as the material constituting the first translucent electrode 13 of the first solar cell CB1 described above. The thickness of the second translucent electrode 28 is not particularly limited, but may be, for example, about 100 nm to 150 nm.

[0034] The second metal electrode (grid electrode) 29 is a grid-shaped electrode to ensure light transmittance. The second metal electrode 29 may be disposed on the second translucent electrode 28, or may be disposed so as to penetrate the second translucent electrode 28 and be in direct contact with the second hole selection layer 27. The total area of ​​the second metal electrode 29 in a planar view is preferably approximately 1% to 5% of the cell area of ​​the second solar cell CT1. In this specification, the cell area refers to the area of ​​the second translucent electrode 28 in a planar view. The metal material constituting the second metal electrode 29 is the same as the material constituting the first metal electrode 14 of the first solar cell CB1. There are no particular limitations on its thickness, but it is generally preferably approximately 0.5 μm to 100 μm.

[0035] The second solar cell CT1 may include layers other than those described above, provided that the objects and effects of this embodiment are not impaired. For example, the second solar cell CT1 may include an anti-reflection layer (not shown) that covers the second metal electrode 29 and the second translucent electrode 28. The anti-reflection layer may be similar to known anti-reflection layers, and examples thereof include a layer made of magnesium fluoride. There are no particular limitations on the thickness of the anti-reflection layer, but it is generally preferably about 50 nm to 150 nm.

[0036] (Titanium Oxide Layer 31) The titanium oxide layer 31 is a layer disposed on (adjacent to, in this embodiment) the first light absorbing layer 10 of the first solar cell CB1 and is a layer primarily containing amorphous titanium oxide. As described below, when amorphous titanium oxide is deposited on the first light absorbing layer and hydrogen plasma treatment is performed to form the titanium oxide layer 31, a silicon oxide layer or a layer containing silicon and titanium is typically formed near the interface between the first light absorbing layer 10 and the titanium oxide layer 31. These layers are formed, for example, by a reaction between oxygen supplied during annealing and crystalline silicon, or by interdiffusion between the first light absorbing layer 10 containing crystalline silicon and the titanium oxide layer 31, and can be considered part of the first light absorbing layer 10. Therefore, in this specification, even in such a case, the first light absorbing layer 10 and the titanium oxide layer 31 are considered to be adjacent to each other.

[0037] In this embodiment, the titanium oxide layer 31 functions as a carrier recombination layer between the first solar cell CB1 and the second solar cell CT1. More specifically, the titanium oxide layer 31 functions as a layer that promotes recombination between electrons extracted from the second light absorbing layer 26 of the second solar cell CT1 via the second electron selection layer 25 and holes extracted from the first light absorbing layer 10 of the first solar cell CB1. The titanium oxide layer 31 allows charge exchange between the first solar cell CB1 and the second solar cell CT1, allowing current to flow to an external circuit. In other words, the first solar cell CB1 and the second solar cell CT1 are electrically connected via the titanium oxide layer 31.

[0038] Meanwhile, the titanium oxide layer 31 also functions as a passivation layer for the first light absorbing layer 10 of the first solar cell CB1, and further functions as a hole selection layer for the first solar cell CB1. The titanium oxide layer 31 functioning as a passivation layer for the first light absorbing layer 10 of the first solar cell CB1 means that it has the function of suppressing recombination of electrons and holes resulting from dangling bonds present on the surface of the first light absorbing layer 10. Furthermore, the titanium oxide layer 31 functioning as a hole selection layer for the first light absorbing layer 10 means that the titanium oxide layer 31 functions to selectively extract holes from the first light absorbing layer 10 out of electron-hole pairs photogenerated in the first light absorbing layer 10.

[0039] The titanium oxide layer 31 may partially contain a crystalline phase as long as it does not impair the objectives and effects of this embodiment; however, it is preferable that the titanium oxide layer 31 does not contain a crystalline phase in terms of hole selectivity, passivation performance, and the like. The presence or absence of a crystalline phase can be confirmed, for example, by observation with a transmission electron microscope. The titanium oxide layer 31 may be composed mainly of titanium oxide (for example, the total amount of oxygen and titanium in the composition of the titanium oxide layer 31 is 90 atomic % or more). However, in addition to titanium oxide, the titanium oxide layer 31 may partially contain hydrogen, silicon, etc., and may partially contain nitrogen due to diffusion from the adjacent titanium nitride layer 32, for example.

[0040] The thickness of the titanium oxide layer 31 is preferably 15 nm or less, and more preferably 10 nm or less. As the thickness of the titanium oxide layer 31 increases, it becomes more likely to crystallize, which may result in a decrease in passivation performance. In contrast, when the thickness of the titanium oxide layer 31 is 15 nm or less, the titanium oxide is less likely to crystallize, and the passivation properties are further improved. Furthermore, by reducing the thickness of the titanium oxide layer 31, the tandem solar cell 1 can be manufactured more efficiently. On the other hand, the thickness of the titanium oxide layer 31 is preferably 2 nm or more, and more preferably 3 nm or more. By making the thickness of the titanium oxide layer 31 2 nm or more, the effects of the titanium oxide layer 31 can be more easily obtained.

[0041] As will be described later in the manufacturing method, the titanium oxide layer 31 is preferably a layer formed by thermal atomic layer deposition, and more preferably a layer that has been subjected to hydrogen plasma treatment after being formed by the above method. If the titanium oxide layer 31 is a layer that has been subjected to hydrogen plasma treatment, the passivation performance and hole selectivity are likely to be further improved.

[0042] (Titanium Nitride Layer 32) The titanium nitride layer 32 is a layer disposed adjacent to the titanium oxide layer 31, and has a higher nitrogen content than the titanium oxide layer 31. The fact that the titanium nitride layer 32 has a higher nitrogen content than the titanium oxide layer 31 can be confirmed by analysis using, for example, an energy dispersive X-ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS) attached to a transmission electron microscope (TEM). As described above, in this embodiment, the titanium nitride layer 32 functions as a protective layer for the titanium oxide layer 31, and as a layer for reducing contact resistance between the titanium oxide layer and the second solar cell.

[0043] The titanium nitride layer 32 may be, for example, a layer that mainly contains titanium nitride (for example, the total amount of nitrogen and titanium in the composition of the titanium nitride layer 32 is 35 atomic % or more), and may further contain hydrogen, oxygen, etc. in addition to titanium nitride. As described above, the titanium nitride layer 32 may contain more oxygen than nitrogen. Furthermore, the crystalline structure of the titanium nitride layer is not particularly limited, but it is preferable that the titanium nitride layer contains a crystalline phase.

[0044] Here, the ratio of the thickness of the titanium nitride layer 32 to the thickness of the titanium oxide layer 31 (thickness of the titanium nitride layer 32 / thickness of the titanium oxide layer 31) is preferably 0.5 to 1.5, and more preferably 0.75 to 1.25. The specific thickness of the titanium nitride layer 32 is preferably 2 nm to 15 nm, and more preferably 3 nm to 10 nm. When the thickness of the titanium nitride layer 32 is within this range, the titanium nitride layer 32 is more likely to function as a protective layer for the titanium oxide layer 31, and the cell performance of the tandem solar cell is likely to be particularly good. Furthermore, passivation performance at the interface between the titanium oxide layer 31 and the first light absorbing layer 10 is also likely to be good.

[0045] The method for forming the titanium nitride layer 32 is not particularly limited, but it is preferable that the layer be formed by thermal atomic layer deposition, as will be explained later in the manufacturing method.

[0046] (Operation of Tandem Solar Cell of First Embodiment) In the tandem solar cell 1 of this embodiment, sunlight is incident from the second metal electrode (grid electrode) 29 side. Then, the short-wavelength light of the sunlight (blue to green light) is absorbed by the second light absorption layer (perovskite crystal material) 26 of the second solar cell CT1, which has a large energy band gap, and converted into electricity. Furthermore, the long-wavelength light (red to infrared light) that is not absorbed by the second light absorption layer 26 is absorbed by the first light absorption layer 10 of the first solar cell CB1, which has a smaller energy band gap, and converted into electricity.

[0047] (Modification of Tandem Solar Cell 1 of First Embodiment) In the above description, the tandem solar cell 1 (1A, 1B) has a structure including two solar cells (first solar cell CB1 and second solar cell CT1). However, the tandem solar cell may also be one in which three or more solar cells are stacked.

[0048] The first light absorbing layer 10 of the first solar cell CB1 may be any layer containing crystalline silicon, and may be a layer made of, for example, polycrystalline silicon or microcrystalline silicon.

[0049] (Method for manufacturing tandem solar cell of first embodiment) A method for manufacturing the tandem solar cell 1 of the first embodiment will be described. In the following description, the light-receiving surface side of each component will also be referred to as the "first surface" and the back surface side will also be referred to as the "second surface." Figure 2 shows the flow of this manufacturing method.

[0050] The method for manufacturing a tandem solar cell of this embodiment includes a first light absorbing layer preparation step S110 of preparing a first light absorbing layer 10 of a first solar cell CB1, a first electron selection layer formation step S113 of forming a first electron selection layer 12 on one surface (second surface) of the first light absorbing layer 10, a titanium oxide layer formation step S120 of forming a titanium oxide layer 31 on the other surface (first surface) of the first light absorbing layer 10, a titanium nitride layer formation step S130 of forming a titanium nitride layer 32 on the titanium oxide layer 31, an electrode formation step S115 of forming a first translucent electrode 13 and a first metal electrode 14 on the first electron selection layer 12, and a second solar cell formation step 140 of forming a second solar cell CT1 on the titanium nitride layer 32. However, other steps may also be included. Furthermore, the method for manufacturing the above-described tandem solar cell 1 is not limited to the following method, but can be modified as appropriate according to the materials of each layer and the desired performance.

[0051] First Light Absorbing Layer Preparation Step S110 In the first light absorbing layer preparation step, an n-type crystalline silicon substrate or a p-type crystalline silicon substrate is prepared and processed to obtain the desired first light absorbing layer 10. The method for processing the crystalline silicon substrate is appropriately selected according to the desired shape of the first light absorbing layer 10. As shown in FIG. 1A , the first light absorbing layer 10, which is flat on the titanium oxide layer 31 side and has a random texture structure on the first electron selection layer 12 side, can be prepared as follows.

[0052] A silicon nitride layer (SiN) of a desired thickness is formed on one main surface (for example, the (100) surface) of the crystalline silicon substrate. ｘ The SiN layer is then formed by plasma-assisted chemical vapor deposition (plasma CVD) (S1). ｘ The surface opposite to the surface on which the layer was formed was immersed in an anisotropic etching solution to remove SiN ｘ A random texture structure is formed only on the surface opposite to the surface on which the SiN film is formed (S2). ｘ The layer is removed (S3), and the flat surface is made the light-receiving surface (first surface), and the surface on which the random texture structure is formed is made the back surface (second surface).

[0053] However, the method for forming the random texture structure on only one surface of the silicon substrate is not limited to the above method. For example, both surfaces of the silicon substrate may be anisotropically etched without forming a silicon nitride layer, and then one surface (the first surface) may be etched to be flat.

[0054] After both sides of the silicon substrate are processed into the desired shape, the silicon substrate is washed and the native oxide film formed on the silicon substrate is removed with diluted hydrofluoric acid (S4).

[0055] First electron selection layer deposition step S113: A desired first electron selection layer 12 is deposited on the surface (second surface) of the first light absorbing layer 10 obtained above on which the random texture structure has been formed (S5). The deposition method for the first electron selection layer 12 is selected appropriately depending on the type of the first electron selection layer 12. The deposition of the first electron selection layer 12 can be performed by a known method, with plasma CVD being a preferred method.

[0056] Titanium Oxide Layer Forming Step S120 In the titanium oxide layer forming step S120, titanium oxide (TiO ｘ ) layer 31 is formed. At this time, it is preferable to remove the native oxide film present on the surface (first surface) of the first light absorbing layer 10 with dilute hydrofluoric acid (S6). The method for forming the titanium oxide layer 31 is not particularly limited, but forming the titanium oxide layer 31 by thermal atomic layer deposition is preferable from the viewpoint that the amorphous titanium oxide layer 31 having the above-mentioned excellent properties can be easily obtained. In this case, TTIP (titanium isopropoxide) can be used as a titanium precursor, and water vapor (H ２ O) can be used. Although the film formation conditions are not particularly limited, the titanium oxide layer 31 is preferably formed at 120°C to 350°C, and more preferably at 200°C to 300°C. By forming the titanium oxide layer 31 at 350°C or less, its crystallization can be suppressed. The thickness of the titanium oxide layer 31 can be adjusted by the dose time of the raw material, the number of atomic layer deposition (ALD) cycles, etc.

[0057] After depositing titanium oxide by the above method, it is preferable to further perform hydrogen plasma treatment (S8). The hydrogen plasma treatment can be performed, for example, using an inductively coupled plasma source attached to the atomic layer deposition apparatus, under conditions of a hydrogen flow rate of 50 sccm, a pressure of 10 Pa, and a discharge power of 300 to 600 W. This hydrogen plasma treatment further improves the passivation performance and hole selectivity of the interface between the titanium oxide layer 31 and the first light absorbing layer 10.

[0058] Titanium Nitride Layer Forming Step S130 In the titanium nitride layer forming step S130, a titanium nitride layer 32 is formed on the titanium oxide layer 31. There are no particular limitations on the method for forming the titanium nitride layer 32, but it can be formed by the same thermal atomic layer deposition method as the titanium oxide layer 31. By forming the titanium oxide layer 31 and the titanium nitride layer 32 by the same method, the titanium oxide layer forming step S120 and the titanium nitride layer forming step S130 can be performed continuously.

[0059] When the titanium nitride layer 32 is formed by thermal atomic layer deposition, TDMAT (tetrakis(dimethylamido)titanium) can be used as a titanium precursor, and ammonia (NH ３ Although the film formation conditions are not particularly limited, the film formation temperature for the titanium nitride layer 32 is preferably 120°C to 350°C, the same as in the titanium oxide layer formation step S120. The thickness of the titanium nitride layer 32 can be adjusted by the dose time of the raw material and the number of atomic layer deposition (ALD) cycles.

[0060] In the electrode formation step S115, the first translucent electrode 13 and the first metal electrode 14 are formed on the first electron selection layer 12 by a known method (S10). The method for forming the first translucent electrode 13 and the first metal electrode 14 is not particularly limited, and may be, for example, a sputtering method or a vapor deposition method.

[0061] Furthermore, after the first translucent electrode 13 and the first metal electrode 14 are formed, they are preferably annealed in an oven (S11). The annealing must be performed in an atmosphere containing oxygen or water. It may be performed under a low vacuum, a nitrogen atmosphere containing an appropriate amount of oxygen or water, or air. The annealing temperature is preferably 80°C or higher and 200°C or lower. This temperature allows for an appropriate amount of oxygen to be supplied to the interface between the titanium oxide layer 31 and the first light absorbing layer 10, improving passivation performance. It also prevents crystallization of the titanium oxide layer 31 and desorption of hydrogen contained in the titanium oxide layer 31. While the annealing time is not particularly limited, a time of, for example, 2 hours or less is preferable from the viewpoint of manufacturing efficiency.

[0062] Second solar cell formation step S140 In the second solar cell formation step S140, the second solar cell CT1 is formed on the above-described titanium nitride layer 32. In this embodiment, the second electron selection layer 25, the second light absorption layer 26, the second hole selection layer 27, the second translucent electrode 28, and the second metal electrode 29 are formed in this order.

[0063] First, a surface modification treatment is performed to decompose or remove organic matter adhering to the surface (first surface) of the titanium nitride layer 32 to enhance the hydrophilicity of the surface (S12). Common surface modification treatment methods include UV ozone treatment and oxygen plasma treatment. However, UV light irradiation or excessive oxygen supply can affect the passivation performance of the interface between the titanium oxide layer 31 and the first light absorbing layer 10 and the hole selectivity of the titanium oxide layer 31 (see T. Matsui et al., ACS Appl. Mater. Interfaces, 12, pp. 49777-49785, 2020). Therefore, it is preferable to perform surface modification using a method other than these, such as ozone treatment or hydrogen plasma treatment, so as not to expose the titanium nitride layer 32 or titanium oxide layer 31 to UV light. In particular, it is preferable to perform hydrogen plasma treatment (for example, hydrogen flow rate 50 sccm, pressure 10 Pa, discharge power 300 W to 600 W, 2 to 5 minutes) using the same equipment as that used in the hydrogen plasma treatment (S8) in the titanium oxide layer forming step S120 described above.

[0064] The surface modification treatment (S12) of the titanium nitride layer 32 and the hydrogen plasma treatment (S8) in the titanium oxide layer formation step S120 may be performed simultaneously. In this case, the hydrogen plasma treatment (S8) is not performed in the titanium oxide layer formation step S120, and the titanium nitride layer formation step S130 is performed first. Thereafter, hydrogen plasma treatment is performed from the surface side of the titanium nitride layer 32, thereby simultaneously performing the hydrogen plasma treatment of the titanium oxide layer 31 and the surface modification of the titanium nitride layer 32. In this case, the electrode formation step S115 is preferably performed after the second electron selection layer is formed (S13). Performing the electrode formation step S115 in this order can prevent foreign matter from adhering to the surface of the titanium nitride layer 32 after the hydrogen plasma treatment.

[0065] Next, the second electron selection layer 25 is formed on the titanium nitride layer 32 that has been subjected to the surface modification treatment (S13). The method for forming the second electron selection layer 25 is appropriately selected depending on the type of the second electron selection layer 25. For example, the CVD method may be used, but if the second electron selection layer 25 is made of tin oxide (SnO ２ ), the second electron selection layer 25 can be formed by a coating method. ２ The second electron selection layer 25 containing SnO ２ A dispersion containing nanocrystals of the above can be applied to the titanium nitride layer 32 by spin coating, dip coating, spraying, or the like, followed by annealing to form a film (S14). The annealing method for the second electron selection layer 25 is not particularly limited, and can be performed, for example, inside a glove box or on a hot plate installed outside the glove box. The heating time and heating temperature are not particularly limited, and may be the same temperature and time as those used in the annealing treatment (S11) of the electrode formation step S115. When the electrode formation step S115 is performed after the deposition of the second electron selection layer 25 (S13), the annealing treatment (S11) in the electrode formation step S115 and the annealing treatment (S14) of the second electron selection layer 25 may be performed simultaneously.

[0066] Furthermore, a perovskite precursor solution is applied onto the second electron selection layer 25 by spin coating (S15). The perovskite precursor solution contains raw material (1.4M PbI ２ ,0.25M PbBr ２ A solution prepared by dissolving 0.09 M RbI, 0.25 M methylammonium bromide (MABr), and 1.26 M formamidinium iodide (FAI) in a mixed solution of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) (DMF:DMSO = 4:1) can be used, but is not limited to this. The perovskite precursor solution is preferably applied by spin coating. During spin coating, it is preferable to gradually increase the rotation speed from a low speed and maintain it at a high rotation speed for a certain period of time, from the viewpoint of obtaining a second light-absorbing layer 26 of the desired thickness. Then, a poor solvent is supplied to the substrate several seconds before the end of spin coating to remove the solvent contained in the perovskite precursor. Examples of the poor solvent include anisole solution, chlorobenzene, and toluene. After spin coating, the coating film is then heated for a certain period of time using a hot plate or the like (S16) to crystallize the perovskite crystal material.

[0067] A second hole selection layer 27 is formed on the second light absorption layer 26. The method for forming the second hole selection layer 27 is not particularly limited and can be appropriately selected depending on the type of second hole selection layer 27. For example, when forming a second hole selection layer 27 containing Spiro-OMeTAD, the second hole selection layer 27 can be formed by spin coating (S17). Specifically, a solution is prepared by dissolving Spiro-OMeTAD powder in a solvent such as chlorobenzene. At this time, various dopants and additives may be added to the solution. While the addition of a dopant is not essential, adding a dopant facilitates hole mobility and facilitates high photoelectric conversion efficiency. Furthermore, when forming a second hole selection layer 27 containing Spiro-OMeTAD, it is preferable to store the resulting layer in dry air for a predetermined period of time (e.g., approximately 16 hours) after spin coating (S18). This allows the second hole selection layer 27 to be doped with an appropriate amount of oxygen.

[0068] Furthermore, a second translucent electrode 28 is formed on the second hole selection layer 27 (S19). The method for forming the second translucent electrode 28 is not particularly limited, and it can be performed by, for example, a sputtering method. When forming the second translucent electrode 28 by a sputtering method, it is preferable to increase the distance between the second hole selection layer 27 and the target in the vacuum chamber where sputtering is performed, more than usual, in order to suppress damage to the second hole selection layer 27. This prevents a MoO ｘ A good interface can be formed without sandwiching a buffer layer such as a SiO 2 layer or the like. In order to improve the transparency of the second translucent electrode 28, a small amount of oxygen may be added to the Ar gas used in sputtering.

[0069] A second metal electrode 29 (silver layer) is further formed on the surface (first surface) of the second translucent electrode 28 obtained above (S20). The method for forming the second metal electrode 29 is not particularly limited, and a sputtering method or a vacuum heating evaporation method can be used, as in the case of the second translucent electrode 28. At this time, the second metal electrode 29 is processed into a grid shape to ensure a region through which light passes. Thereafter, an annealing treatment (S21) is performed in a vacuum to reduce damage to the second translucent electrode 28 and the second metal electrode 29 caused by sputtering.

[0070] If necessary, magnesium fluoride may be deposited on the second metal electrode 29 by evaporation or the like (S22) to form an anti-reflection layer (not shown).

[0071] (2) Second Embodiment The schematic structure of a tandem solar cell 2 of the second embodiment is shown in Figures 3A and 3B. The tandem solar cell 2 (2A and 2B) of the second embodiment has a structure in which a crystalline silicon first solar cell CB1, a titanium oxide layer 31, a titanium nitride layer 32, and a second solar cell CT2 having a second light absorbing layer 261 made of an n-type semiconductor are stacked in this order. In the tandem solar cell 2, light is incident from the second solar cell CT2 side. The tandem solar cell 2A shown in Figure 3A and the tandem solar cell 2B shown in Figure 3B are identical in all other configurations, except for whether the first light absorbing layer 10 is made of n-type crystalline silicon (10n) or p-type crystalline silicon (10p). Furthermore, these tandem solar cells 2 (2A, 2B) are similar to the tandem solar cells 1 (1A, 1B) of the first embodiment, except for the configuration of the solar cell CT2. The same components are denoted by the same reference numerals, and detailed explanations thereof will be omitted here.

[0072] (Second solar cell CT2) In the second solar cell CT2 of this embodiment, a second light absorbing layer 261, a second hole selection layer 271, a second translucent electrode 28, and a second metal electrode 29 are arranged in this order from the first solar cell CB1 side.

[0073] In this embodiment, the second light absorbing layer 261 is made of an n-type semiconductor, and the second light absorbing layer 261 itself serves as an electron selection layer. Therefore, there is no need to provide an electron selection layer, and the second light absorbing layer 261 is disposed adjacent to the titanium nitride layer 32.

[0074] Here, the second light absorption layer 261 may be made of an n-type semiconductor. Examples of n-type semiconductors include GaAs, AlGaAs, InGaP, InGaAsP, AlGaInP, and chalcogenide-based (Cu(In,Ga)Se ２ ), CdTe, organic dyes, amorphous silicon, etc. Among these, GaAs is preferable from the viewpoint of availability and cell performance. There is no particular limitation on the thickness of the second light absorption layer 261.

[0075] The second hole selection layer 271 may be any layer that can selectively extract holes from the second light absorption layer 26. The second hole selection layer 271 is appropriately selected depending on the type of the second light absorption layer 261. When the second light absorption layer 261 is GaAs, for example, p-type GaAs or a stack of p-type GaAs and p-type InGaP is selected. There are no particular limitations on the thickness of the second hole selection layer 271.

[0076] The second translucent electrode 28 and the second metal electrode 29 are similar to the second translucent electrode 28 and the second metal electrode 29 in the first embodiment.

[0077] (Operation of Tandem Solar Cell of Second Embodiment) In the tandem solar cell 2 of this embodiment, sunlight is incident from the second metal electrode (grid electrode) 29 side. Then, the short-wavelength light of the sunlight (blue to green light) is absorbed by the second light absorbing layer (n-type semiconductor) 261 of the second solar cell CT2, which has a large energy band gap, and converted into electricity. Furthermore, the long-wavelength light (red to infrared light) that is not absorbed by the second light absorbing layer 261 is absorbed by the first light absorbing layer 10 of the first solar cell CB1, which has a smaller energy band gap, and converted into electricity.

[0078] (Method of Manufacturing Tandem Solar Cell of Second Embodiment) The flow of the method of manufacturing a tandem solar cell of the second embodiment is shown in Figure 4. The method of manufacturing a tandem solar cell of this embodiment includes a first light absorbing layer preparation step S110 of preparing a first light absorbing layer 10 of a first solar cell CB1, a first electron selection layer formation step S113 of forming a first electron selection layer 12 on one surface (second surface) of the first light absorbing layer 10, a titanium oxide layer formation step S120 of forming a titanium oxide layer 31 on the other surface (first surface) of the first light absorbing layer 10, a titanium nitride layer formation step S130 of forming a titanium nitride layer 32 on the titanium oxide layer 31, an electrode formation step S115 of forming a first translucent electrode 13 and a first metal electrode 14 on the first electron selection layer 12, and a second solar cell formation step 240 of forming a second solar cell CT1 on the titanium nitride layer 32. However, other steps may also be included. Furthermore, the method for manufacturing the above-described tandem solar cell 1 is not limited to the following method, but can be modified as appropriate according to the materials of each layer and the desired performance.

[0079] This manufacturing method differs from the manufacturing method of the first embodiment only in the second solar cell forming step S240, and therefore only the second solar cell forming step S240 will be described.

[0080] Second Solar Cell Forming Step S240 In the second solar cell forming step S240, the second light absorbing layer 261, the second hole selection layer 271, the second translucent electrode 28, and the second metal electrode 29 are formed in this order on the titanium nitride layer 32.

[0081] In the second solar cell formation step S240, first, a surface modification treatment is performed (S201) to decompose or remove organic matter adhering to the surface (first surface) of the titanium nitride layer 32 and increase the hydrophilicity of the surface. The surface modification treatment method is the same as the surface modification treatment (S12) in the first embodiment.

[0082] A second light absorption layer 261 made of an n-type semiconductor layer is formed on the surface-treated titanium nitride layer 32 (S202). The method for forming the second light absorption layer 261 is appropriately selected depending on the type of n-type semiconductor. For example, if the second light absorption layer 261 is made of GaAs, an n-type GaAs layer and a p-type GaAs layer as the second hole selection layer 271 described below may be epitaxially grown sequentially on another GaAs substrate, and the grown layers may be peeled off from the substrate and directly bonded to the substrate by a bonding method. Note that if the second hole selection layer 271 is made of p-type GaAs, a p-type InGaP layer or the like may be further laminated thereon.

[0083] A second hole selection layer 271 is formed (S203) on the second light absorption layer 261. The method for forming the second hole selection layer 271 is appropriately selected depending on the type of the second hole selection layer 27, and may be any method such as a wet film formation method, a dry film formation method, or a coating method.

[0084] Thereafter, a second translucent electrode and a second metal electrode are formed on the second hole selection layer 271 (S024, S205). ２ The anti-reflection layer is formed by vapor deposition of, for example, a material such as a silicon dioxide powder (S206). These methods are the same as the methods (S19 to S22) performed in the second solar cell formation step S140 in the first embodiment, and therefore descriptions thereof will be omitted.

[0085] (3) Third Embodiment A schematic structure of a tandem solar cell 3 of the third embodiment is shown in Figure 5. The tandem solar cell 3 of the third embodiment has a structure in which a crystalline silicon (p-type) first solar cell CB3, a titanium oxide layer 31, a titanium nitride layer 32, and a perovskite second solar cell CT1 are stacked in this order. In this tandem solar cell 3, light is incident from the second solar cell CT1 side. Furthermore, except for the configuration of the first solar cell CB3, this tandem solar cell 3 is similar to the tandem solar cell 1B of the first embodiment. Identical components are designated by the same reference numerals, and detailed description thereof will be omitted here.

[0086] (First solar cell CB3) The first solar cell CB3 of this embodiment includes, in order from the titanium oxide layer 31 side, a first light absorbing layer 10p made of p-type crystalline silicon 10p, ＋ A diffusion layer 312, a silicon nitride layer 313, and a first metal electrode 314 are arranged in this order. The first solar cell CB3 has the same structure as a general PERC (Passivated Emitter Rear Cell) solar cell.

[0087] The first photoabsorption layer 10 may be a layer made of p-type crystalline silicon 10p doped with p-type impurities such as boron (B) or gallium (Ga). The p-type crystalline silicon preferably has a (100) surface and a resistivity of approximately 0.5 to 5 Ω. The surface of the first photoabsorption layer 10 facing the titanium oxide layer 31 may be flat or have a random texture structure. To improve passivation and hole selectivity at the interface between the first photoabsorption layer 10 and the titanium oxide layer 31, a p+ diffusion layer in which boron or gallium is diffused may be formed on the surface of the p-type crystalline silicon 10p. The other surface of the first photoabsorption layer 10 (hereinafter also referred to as the "back surface") may also be flat or have a random texture structure.

[0088] n ＋ The diffusion layer 312 is a layer made of an n-type semiconductor, and + means that it is highly doped. ＋ The diffusion layer 312 is a layer formed by thermally diffusing an n-type dopant such as phosphorus onto the back surface side of the first light absorbing layer 10. The amount of the n-type dopant is 10 １８ ~10 ２０ cm ３ It is preferable that:

[0089] The silicon nitride layer 313 is ＋ It is sufficient that the silicon nitride layer 313 is formed adjacent to the diffusion layer 312. The thickness of the silicon nitride layer 313 can be, for example, about 80 nm. Furthermore, the first metal electrode 314 is a grid-shaped electrode, and can be an electrode made of, for example, silver. The first metal electrode 314 penetrates the silicon nitride layer 313 and is connected to the n ＋It is preferable that the insulating layer 312 is in electrical contact with the diffusion layer 312 .

[0090] (Operation of Tandem Solar Cell of Third Embodiment) In the tandem solar cell 3 of this embodiment, sunlight is incident from the second metal electrode (grid electrode) 29 side. Then, the short-wavelength light of the sunlight (blue to green light) is absorbed by the second light absorption layer (perovskite crystalline material) 26 of the second solar cell CT1, which has a large energy band gap, and converted into electricity. Furthermore, the long-wavelength light (red to infrared light) that is not absorbed by the second light absorption layer 26 is absorbed by the first light absorption layer (p-type crystalline silicon) 10 of the first solar cell CB3, which has a smaller energy band gap, and converted into electricity.

[0091] (Method of Manufacturing Tandem Solar Cell of Third Embodiment) The flow of the method of manufacturing a tandem solar cell of the third embodiment is shown in Figure 6. The method of manufacturing a tandem solar cell of this embodiment includes a first solar cell preparation step S310 of preparing a first solar cell CB3, a titanium oxide layer formation step S120 of forming a titanium oxide layer 31 adjacent to one surface (first surface) of the first light absorbing layer 10p, a titanium nitride layer formation step S130 of forming a titanium nitride layer 32 on the titanium oxide layer 31, and a second solar cell formation step S140 of forming a second solar cell CT1 on the titanium nitride layer 32. The titanium oxide layer formation step S120, the titanium nitride layer formation step S130, and the second solar cell formation step S140 are the same as those in the first embodiment, and therefore will not be described in detail here.

[0092] First Solar Cell Preparation Step S310 In the first solar cell preparation step S310, a p-type silicon substrate is prepared. Then, both surfaces (first surface and second surface) of the p-type silicon substrate are anisotropically etched to form a random texture structure on both surfaces (S31). Thereafter, the p-type silicon substrate is cleaned, and the native oxide films on both surfaces (first surface and second surface) of the p-type silicon substrate are removed using dilute hydrofluoric acid (S32).

[0093] Next, the p-type silicon substrate is ３The p-type silicon substrate is heated in an atmosphere containing the compound, and phosphorus, which is an n-type dopant, is thermally diffused into the p-type silicon substrate, forming n-type dopants on the desired surface (second surface) of the p-type silicon substrate. ＋ A diffusion layer 312 is formed (S33).

[0094] n ＋ A silicon nitride layer 313 (SiNx layer) is formed by plasma CVD on both surfaces (first surface and second surface) of the p-type silicon substrate on which the diffusion layer 312 has been formed (S34). After that, silver is applied to the silicon nitride layer 313 (second surface) in the pattern of the first metal electrode 314 by screen printing or the like, and then baked at a temperature of 500°C or higher (S35). By baking, the silver diffuses into the silicon nitride layer 313 and penetrates it (fires through). This results in the formation of n ＋ A first metal electrode 314 may be formed in contact with the diffusion layer 312 .

[0095] Next, the silicon nitride layer and the p-type silicon substrate on the surface (first surface) opposite to the side on which the first metal electrode 314 is formed are etched to flatten the surface and to form an n-type silicon nitride layer. ＋ The phosphorus diffusion layer formed on the first surface due to the penetration of phosphorus during the formation of the diffusion layer 312 is removed. Then, the surface is washed with hydrofluoric acid to remove the oxide film (S36). This results in the first solar cell SC3.

[0096] (4) Fourth Embodiment Schematic structures of a tandem solar cell 4 of the fourth embodiment are shown in Figures 7A and 7B. The tandem solar cell 4A shown in Figure 7A and the tandem solar cell 4B shown in Figure 7B differ only in whether the first light absorbing layer 10 contains n-type crystalline silicon (10n) or p-type crystalline silicon (10p), and are otherwise identical in configuration. In the tandem solar cell 4 of this embodiment, an intermediate layer 42 containing tin oxide (hereinafter also referred to as a "tin oxide layer") is disposed between the titanium oxide layer 31 and the second electron selection layer 25, which is a constituent layer of the second solar cell CT1. The intermediate layer 42 has different properties and is formed by a different method than the second electron selection layer 25. Furthermore, these tandem solar cells 4 (4A, 4B) are similar to the tandem solar cells 1 (1A, 1B) of the first embodiment, except that the intermediate layer 42 is a tin oxide layer. The same components are denoted by the same reference numerals, and detailed description thereof will be omitted here. The tin oxide layer 42 will be described below.

[0097] (Tin oxide layer 42) The tin oxide layer 42 is a layer disposed adjacent to the titanium oxide layer 31. In this embodiment, the tin oxide layer 42 functions as a protective layer for the titanium oxide layer 31 and as a layer for reducing contact resistance between the titanium oxide layer and the second solar cell. Note that the second electron selection layer 25 described below can also be a layer made of tin oxide as described above, but differs from the tin oxide layer 42 in that it is an aggregate of nanocrystalline particles of 10 nm or less. Therefore, the tin oxide layer 42 has different performance from the second electron selection layer 25 and can exhibit the above-mentioned function. The difference in crystal grain shape and morphology between the tin oxide layer 42 and the second electron selection layer 25 can be confirmed, for example, by analysis using cross-sectional transmission electron microscope observation.

[0098] The tin oxide layer 42 may be, for example, a layer that mainly contains tin oxide (for example, the total amount of oxygen and tin in the composition of the tin oxide layer 42 is 90 atomic % or more), but may also contain hydrogen, nitrogen, etc. In addition to tin oxide, the crystal structure of the tin oxide layer is not particularly limited, but may include a crystalline phase.

[0099] Here, the ratio of the thickness of the tin oxide layer 42 to the thickness of the titanium oxide layer 31 (thickness of the tin oxide layer 42 / thickness of the titanium oxide layer 31) is preferably 0.5 to 1.5, and more preferably 0.75 to 1.25. The specific thickness of the tin oxide layer 42 is preferably 2 nm to 15 nm, and more preferably 3 nm to 10 nm. When the thickness of the tin oxide layer 42 is within this range, the tin oxide layer 42 can easily function as a protective layer for the titanium oxide layer 31, and the cell performance of the tandem solar cell is likely to be particularly good. Furthermore, when the tandem solar cell is formed, good passivation performance is maintained at the interface between the titanium oxide layer 31 and the first light absorbing layer 10.

[0100] The method for forming the tin oxide layer 42 is not particularly limited as long as it can exhibit the desired performance (except for a coating method using a nanocrystalline tin oxide dispersion solution), but it is preferable that the layer be formed by plasma atomic layer deposition, as will be explained in the manufacturing method described below.

[0101] (Operation of Tandem Solar Cell of Fourth Embodiment) In the tandem solar cell 1 of this embodiment, sunlight is incident from the second metal electrode (grid electrode) 29 side. Then, the short-wavelength light of the sunlight (blue to green light) is absorbed by the second light absorption layer (perovskite crystal material) 26 of the second solar cell CT1, which has a large energy band gap, and converted into electricity. Furthermore, the long-wavelength light (red to infrared light) that is not absorbed by the second light absorption layer 26 is absorbed by the first light absorption layer 10 of the first solar cell CB1, which has a smaller energy band gap, and converted into electricity.

[0102] (Method of Manufacturing Tandem Solar Cell of Fourth Embodiment) A flow of the method of manufacturing a tandem solar cell of the fourth embodiment is shown in Figure 8. The method of manufacturing a tandem solar cell of this embodiment includes a first light absorbing layer preparation step S110 of preparing a first solar cell CB1, a titanium oxide layer formation step S120 of forming a titanium oxide layer 31 adjacent to one surface (first surface) of the first light absorbing layer 10p, a tin oxide layer formation step S430 of forming a tin oxide layer 42 on the titanium oxide layer 31, and a second solar cell formation step S140 of forming a second solar cell CT1 on the tin oxide layer 42. The first light absorbing layer preparation step S110, the titanium oxide layer formation step S120, and the second solar cell formation step S140 are the same as in the first embodiment, and therefore detailed description thereof will be omitted here.

[0103] Tin Oxide Layer Forming Step S430 In the tin oxide layer forming step S430, a tin oxide layer 42 is formed on the titanium oxide layer 31. The method for forming the tin oxide layer 42 is not particularly limited (except for the coating method using a nanocrystalline tin oxide dispersion solution, as described above), but it can be formed, for example, by a plasma atomic layer deposition method different from the thermal atomic layer deposition method for the titanium oxide layer 31. In this case, although the titanium oxide layer 31 and the tin oxide layer 42 differ in whether a thermal reaction or a plasma reaction is used in the oxidation process, they can be formed using the same atomic layer deposition apparatus, and the titanium oxide layer forming step S120 (including hydrogen plasma treatment) and the tin oxide layer forming step S430 can be performed consecutively.

[0104] When the tin oxide layer 42 is formed by plasma atomic layer deposition, TDMASn (tetrakis(dimethylamido)tin) can be used as a tin precursor, and oxygen plasma can be used as an oxygen source. Although the film formation conditions are not particularly limited, the film formation temperature for the tin oxide layer 42 is preferably 100°C to 350°C, the same as in the titanium oxide layer formation step S120. The thickness of the tin oxide layer 42 can be adjusted by the dose time of the raw material and the number of atomic layer deposition (ALD) cycles.

[0105] (Others) In the above description of the fourth embodiment, a tandem solar cell 4 (4A and 4B) having a configuration similar to that of the first embodiment has been described, i.e., a solar cell having a structure in which a crystalline silicon first solar cell CB1, a titanium oxide layer 31, a tin oxide layer 42, and a perovskite second solar cell CT1 are stacked in this order. However, the tin oxide layer can be used instead of the titanium nitride layer of the second embodiment. Specifically, a solar cell can be configured having a structure in which a crystalline silicon first solar cell, a titanium oxide layer, a tin oxide layer, and a second solar cell CT2 having a second light absorption layer made of an n-type semiconductor are stacked in this order. Furthermore, the tin oxide layer can be used instead of the titanium nitride layer of the third embodiment. Specifically, a solar cell can be configured having a structure in which a crystalline silicon (p-type) first solar cell, a titanium oxide layer, a tin oxide layer, and a perovskite second solar cell CT1 are stacked in this order. In either case, the tin oxide layer may be the same as the tin oxide layer 42 of the fourth embodiment. The other configurations may be the same as those of the second or third embodiment.

[0106] Examples 1 to 3 Tandem solar cells (samples A to C) of Examples 1 to 3 were fabricated by the following method.

[0107] (First Light Absorption Layer Preparation Step) An n-type crystalline silicon substrate (manufactured by Topsil, float zone Si (thickness: approximately 280 μm, plane orientation: (100), resistivity: approximately 2-5 Ωcm) was prepared. One surface (first surface) of the n-type silicon substrate was coated with SiN ｘ A layer of about 140 nm was formed by plasma-assisted chemical vapor deposition (plasma CVD). Subsequently, the other surface (second surface) of the n-type silicon substrate was subjected to anisotropic etching to form a random texture structure. At this time, the entire substrate was immersed in an etching solution, but the SiN ｘ The SiN layer acts as a protective film, and a random texture is formed only on the second surface. ｘThe layer was removed, and the native oxide layers formed on both sides of the n-type silicon substrate were then removed to obtain the desired first light absorbing layer.

[0108] (First electron selection layer formation process) A first electron selection layer (a-Si:H i-n layer) consisting of a hydrogen-added intrinsic amorphous Si film and a hydrogen-added n-type amorphous Si film was formed on the randomly textured surface (second surface) of the crystalline silicon substrate by plasma CVD.

[0109] (Titanium Oxide Layer Formation Process) The surface (first surface) of the first light absorption layer opposite the first electron selection layer was washed with dilute hydrofluoric acid to remove the native oxide layer formed on that surface. A titanium oxide layer was then formed on that surface by thermal atomic layer deposition using an atomic layer deposition apparatus. TTIP was used as the titanium precursor, and water vapor was used as the oxygen source. The TTIP dose time per cycle was 1.2 seconds, and a 1.2-second dose of water was repeated three times. The titanium oxide film formation temperature was in the range of 120 to 350°C. An amorphous titanium oxide layer with a thickness of approximately 4 nm was then formed on the surface of the Si substrate by repeating ALD (atomic layer deposition) cycles.

[0110] The titanium oxide layer was subjected to a hydrogen plasma treatment for 60 minutes or less using an inductively coupled plasma source attached to the atomic layer deposition apparatus under the conditions of a hydrogen flow rate of 50 sccm, a pressure of 10 Pa, and a discharge power of 600 W.

[0111] (Titanium Nitride Layer Formation Process) A titanium nitride layer was then formed on the titanium oxide by thermal atomic layer deposition. TDMAT was used as the titanium precursor, and ammonia was used as the nitrogen source. The TDMAT dose time per cycle was 0.05 seconds, and the ammonia dose time was 5 seconds. The titanium nitride layer was formed at a temperature in the range of 120 to 350°C, the same temperature range as the titanium oxide layer and the substrate temperature during hydrogen plasma treatment. This ALD cycle was repeated 25 times in Example 1, 50 times in Example 2, and 75 times in Example 3. This resulted in titanium nitride layers with thicknesses of approximately 2 nm (Example 1), 4 nm (Example 2), and 6 nm (Example 3), respectively.

[0112] After the titanium nitride layer was formed, a first translucent electrode made of indium tin oxide (ITO) and having a thickness of 70 to 150 nm and a silver electrode having a thickness of 200 nm were formed on the first electron selection layer by sputtering, followed by annealing at 180° C. for 2 hours.

[0113] (Second solar cell forming step) The titanium nitride layer was subjected to hydrogen plasma treatment (hydrogen flow rate 50 sccm, pressure 10 Pa, discharge power 300 W, 5 minutes) to modify the surface. ２ A dispersion containing nanocrystals of SnO (a reagent (Ceramace S-8) manufactured by Taki Chemical Co., Ltd.) was diluted with an appropriate amount of deionized water. The solution was applied to the surface-modified titanium nitride layer by spin coating (2000 rpm, 30 seconds). The coating was then heated on a hot plate at 100°C for 1 hour to form a SnO nanocrystal with a thickness of approximately 40 nm. ２ The spin coating was carried out in the air, and the heating with the hot plate was carried out in a glove box in a dry air atmosphere.

[0114] Next, the raw material (1.4M PbI ２ ,0.25M PbBr ２ A perovskite precursor solution was prepared by dissolving a mixture of 0.09 M RbI, 0.25 M methylammonium bromide (MABr), and 1.26 M formamidinium iodide (FAI) in a mixture of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) (DMF:DMSO = 4:1). The perovskite precursor solution was then applied to the second electron selection layer by spin coating. The spin coating was performed by increasing the rotation speed to 200 rpm / s, holding the rotation speed at 1300 rpm for 5 seconds, then increasing the rotation speed to 2000 rpm / s, and holding the rotation speed at 5000 rpm for 30 seconds. Furthermore, anisole solution was supplied to the substrate a few seconds before the end of spin coating to remove the solvent contained in the perovskite precursor. After spin coating, the coating film (laminate) was heat-treated on a hot plate heated to 110° C. for 15 minutes to obtain a second light-absorbing layer made of a crystallized perovskite layer.

[0115] Next, a solution was prepared by dissolving Spiro-MeOTAD powder in chlorobenzene. To this solution, a dopant solution prepared by dissolving Li-TFSI in acetonitrile and an appropriate amount of 4-tert-tetrabutylpyridine were added. The resulting solution was applied by spin coating in a glove box. It was then stored in the glove box in dry air for approximately 16 hours, and an appropriate amount of oxygen was doped. This resulted in a second hole-selective layer with a thickness of 200 nm.

[0116] Furthermore, ITO was deposited on the second hole selection layer by sputtering to obtain a second translucent electrode having a thickness of 100 nm to 150 nm. To prevent damage to the second hole selection layer during sputtering, the distance between the substrate (second hole selection layer) and the target in the vacuum chamber where sputtering was performed was set to 250 mm. Furthermore, a 250 nm-thick silver layer serving as a second metal electrode was formed in a grid pattern on the second translucent electrode. The pattern of the second metal electrode was adjusted so that the total area of ​​the second metal electrode in plan view was approximately 1% of the area of ​​the second translucent metal electrode in plan view. The resulting structure was then annealed in vacuum at 50°C for 2 hours.

[0117] Furthermore, to reduce optical reflection loss on the device surface, a 100 nm thick magnesium fluoride (MgF ２ The SiO 2 layer was formed by vacuum deposition.

[0118] Comparative Example A tandem solar cell (sample R) was obtained in the same manner as in Example 1, except that the titanium nitride layer formation step was not performed.

[0119] (Results) Figure 9 shows a cross-sectional transmission electron microscope image of the second electron selection layer (tin oxide layer) / titanium nitride layer / titanium oxide layer / first light absorption layer (crystalline silicon layer) interface of the tandem solar cell (Sample B) fabricated in Examples 1 to 3. Measurement was performed using a transmission electron microscope (JEOL Ltd., JEM-ARM200F) under conditions of an accelerating voltage of 80 kV, bright field image, and 300,000x magnification. This image shows that the second electron selection layer (tin oxide layer) has clear lattice fringes, indicating that it is composed of nanocrystalline tin oxide. Furthermore, the titanium nitride layer in contact with the tin oxide layer also shows contrast due to diffraction by the crystalline phase, revealing that it contains a partial crystalline phase. Since no lattice fringes are observed in the titanium oxide layer in contact with the titanium nitride layer, it can be said that the titanium oxide layer is amorphous and does not contain a crystalline phase. Note that an oxide layer (SiO ) with a thickness of approximately 1.5 nm is present at the interface between the titanium oxide layer and the crystalline silicon layer. ｚ ) is naturally formed, but this oxide layer is derived from the reaction of oxygen supplied during the deposition of the titanium oxide layer or during annealing with the surface of the crystalline silicon, resulting in natural oxidation. Therefore, it can be considered as part of the crystalline silicon layer. This transmission electron microscope revealed that the tin oxide layer, titanium nitride layer, titanium oxide layer, and SiO ｚ The layer thicknesses were 41 nm, 4.3 nm, 3.6 nm, and 1.5 nm, respectively.

[0120] Furthermore, EDX analysis of the titanium nitride layer revealed that the composition of the main constituent elements was titanium: 27.1 atomic %, oxygen: 57.2 atomic %, and nitrogen: 15.7 atomic %. The reason for the large amount of oxygen mixed in, despite the titanium nitride layer being formed using oxygen-free raw materials, is due to interdiffusion during the formation or processing of each layer. Furthermore, EELS (energy loss spectroscopy) analysis of each layer shown in Figure 10 revealed that signals due to nitrogen were detected only in the titanium nitride layer, and not in the titanium oxide layer or SnO layer adjacent to the titanium nitride layer. ２ Since the detection limit of nitrogen in EELS analysis is several atomic percent, the nitrogen contained in the titanium nitride layer is at least several atomic percent, while the nitrogen contained in the titanium oxide layer and the tin oxide layer is several atomic percent or less.

[0121] FIG. 11 shows the short-circuit current density J of the tandem solar cells (samples A to C) fabricated in Examples 1 to 3 and the tandem solar cell (sample R) fabricated in the comparative example. ｓｃ (mA / cm ２ ), open circuit voltage V ＯＣ The graph shows the luminance (V), fill factor (FF), and conversion efficiency (%). For statistical analysis, the graph shows the results of samples that were fabricated and evaluated multiple times under the same conditions.

[0122] Comparative Example Sample R (TiN ｙ layer thickness: 0 nm), compared with sample A of Example 1 (TiN ｙ Layer thickness: 2 nm), Sample B of Example 2 (TiN ｙ Layer thickness: about 4 nm), and Sample C of Example 3 (TiN ｙ The layer thickness (approximately 6 nm) results in a higher open circuit voltage V ＯＣ The results are consistent with previous results. This is due to the improved passivation performance of the interface between the first light-absorbing layer (crystalline silicon) and the titanium oxide layer, resulting in an increased output voltage for the first solar cell (bottom cell). The passivation performance of the titanium oxide layer for crystalline silicon depends not only on the deposition conditions and film quality of the titanium oxide layer, but also on the material of the capping layer deposited on top of the titanium oxide layer. It is believed that inserting the titanium nitride layer improved the passivation performance of the interface between the titanium oxide layer and crystalline silicon compared to when a tin oxide layer was directly deposited on top of the titanium oxide. Furthermore, it is known that perovskite-based elements such as lead, iodine, and bromine diffuse into the low-density second electron transport layer (tin oxide layer) during the formation of the second solar cell (perovskite-type top cell). The titanium nitride layer deposited on top of the titanium oxide layer functions as a barrier layer to prevent the diffusion of these elements, which is thought to have improved the passivation performance of the interface between the titanium oxide layer and crystalline silicon.

[0123] Figure 12 shows photoluminescence images of Samples A to C and R fabricated in Examples 1 to 3 and Comparative Example. The photoluminescence intensity depends on the concentration of photogenerated carriers in the crystalline silicon. The higher the brightness (brighter) in the photoluminescence image, the longer the carrier lifetime, i.e., the better the surface passivation. As shown in Figure 12, there is no significant difference in the images before the formation of the second solar cell (perovskite-type top cell). In contrast, after the formation of the second solar cell (perovskite-type top cell), Sample R, which does not contain a titanium nitride layer, exhibits low photoluminescence brightness. On the other hand, Samples A to C, which contain a titanium nitride layer, show a tendency for the photoluminescence brightness to increase, corresponding to the photogenerated carrier concentration (carrier lifetime) of the crystalline silicon. This indicates that the insertion of a titanium nitride layer at the interface between the titanium oxide layer and the tin oxide layer (second electron selection layer) improves the passivation performance of the interface between the crystalline silicon and the titanium oxide layer.

[0124] In addition, the short-circuit current density J ｓｃ Regarding the short-circuit current density J, sample A with a titanium nitride layer of about 2 nm is comparable to sample R (comparison example) without a titanium nitride layer. However, as the thickness of the titanium nitride layer increases (samples B and C), ｓｃ This is because the thickness of the titanium nitride layer increases with the increase in the refractive index of silicon and the low refractive index of TiO. ｘ , TiN ｙ , and SnO ２ This is because the optical reflectivity at the interface between the two layers has increased. It has been confirmed that the difference in Jsc between Sample A and Sample B can be reduced by thinning the tin oxide layer (second electron selection layer) instead of the titanium nitride layer in order to reduce the film thickness of the low refractive index layer, and the problem of a decrease in Jsc can be solved by fine-tuning the film thickness. ｙ One reason is that significant light absorption occurs in

[0125] On the other hand, the fill factor FF data tended to show a smaller variance when a titanium nitride layer was inserted. This indicates that the contact resistance at the interface between the titanium oxide layer and the titanium nitride layer, or between the titanium nitride layer and the tin oxide layer (second electron selection layer), was smaller than that at the contact between the titanium oxide layer and the tin oxide layer (second electron selection layer). Furthermore, Sample B, which had a titanium nitride layer thickness of approximately 4 nm, and Sample C, which had a titanium nitride layer thickness of approximately 6 nm, had significantly higher fill factors FF than Sample R, which did not contain a titanium nitride layer, and Sample A, which had a titanium nitride layer thickness of approximately 2 nm. This is thought to be primarily due to the increase in interface reflectance described above, which resulted in the current flowing through the first solar cell (bottom cell) being smaller than that of the second solar cell (top cell), resulting in a higher fill factor FF due to the current-limiting effect. However, this also reflects the reduced contact resistance caused by the insertion of the titanium nitride layer.

[0126] Concerning the conversion efficiency, Samples A to C of Examples 1 to 3 exhibited statistically higher values ​​than Sample R, which is a comparative example. In a comparison of Samples A, B, and C, which have different thicknesses of the titanium nitride layer, Sample B exhibited the highest conversion efficiency.

[0127] Example 4 A tandem solar cell (Sample D) of Example 4 was fabricated using the following method. In Example 4, a tandem solar cell was fabricated using the same process as in Example 1, except that a tin oxide layer was used instead of the titanium nitride layer used in Example 1. The tin oxide layer had a thickness of 4 nm. FIG. 13 shows the solar cell current density-voltage characteristics of the tandem solar cell (Sample D) fabricated in Example 4 and that of a tandem solar cell (Sample b) fabricated separately using the same method as in Example 2. These results demonstrate that Sample D exhibits nearly identical current-voltage characteristics to Sample b. This demonstrates that tin oxide, in addition to titanium nitride, can provide equivalent functionality as an intermediate layer when deposited on a titanium oxide layer. The tandem solar cell (Sample R) fabricated in the comparative example described above has a configuration in which a tin oxide layer is directly formed on a titanium oxide layer as a (second) electron transport layer, and thus has a similar configuration to Sample D. However, Sample R, which does not have an intermediate layer, exhibits a clear difference in performance from Sample B, which has a titanium nitride layer deposited thereon (see FIG. 11). Therefore, it is clear that even though they are both tin oxide layers, a tin oxide layer formed as an intermediate layer by atomic layer deposition and a tin oxide layer formed as a second electron transport layer by a coating method have different properties and functions.

[0128] This application claims priority based on Japanese Patent Application No. 2024-085725, filed May 27, 2024. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety.

[0129] INDUSTRIAL APPLICABILITY The present invention can be used in various devices that use solar cells.

[0130] 1A, 1B, 2A, 2B, 3, 4 Tandem solar cell 10 First light absorbing layer (crystalline silicon) 10n n-type crystalline silicon 10p p-type crystalline silicon 12 First electron selection layer 13 First translucent electrode 14, 314 First metal electrode 25 Second electron selection layer 26, 261 Second light absorbing layer 27, 271 Second hole selection layer 28 Second translucent electrode 29 Second metal electrode 31 Titanium oxide layer 32 Titanium nitride layer (intermediate layer) 42 Tin oxide layer (intermediate layer) 312n ＋Diffusion layer 313 Silicon nitride layer CT1, CT2 Second solar cell (top cell) CB1, CB3 First solar cell (bottom cell)

Claims

1. A tandem solar cell comprising: a first solar cell including a first light absorbing layer containing n-type or p-type crystalline silicon; a titanium oxide layer containing mainly amorphous titanium oxide arranged on the first light absorbing layer; an intermediate layer arranged adjacent to the titanium oxide layer and containing titanium nitride or tin oxide having a higher nitrogen composition than the titanium oxide layer; and a second solar cell arranged on the intermediate layer.

2. The tandem solar cell according to claim 1, wherein the ratio of the thickness of said intermediate layer to the thickness of said titanium oxide layer is 0.5 to 1.

5.

3. The tandem solar cell according to claim 1, wherein the second solar cell includes a second electron selection layer and a second light absorption layer in this order from the first solar cell side, and the intermediate layer and the second electron selection layer are arranged adjacent to each other.

4. The tandem solar cell according to claim 3, wherein the second light absorbing layer comprises a perovskite crystalline material.

5. The tandem solar cell according to claim 3, wherein the second electron selective layer comprises tin oxide.

6. The tandem solar cell according to claim 1, wherein the second solar cell includes a second light absorbing layer made of an n-type semiconductor, and the intermediate layer and the second light absorbing layer are disposed adjacent to each other.

7. A method for manufacturing a tandem solar cell, comprising the steps of: preparing a first light absorbing layer containing n-type or p-type crystalline silicon; forming a titanium oxide layer containing mainly amorphous titanium oxide on the first light absorbing layer; forming an intermediate layer containing titanium nitride or tin oxide having a higher nitrogen composition than the titanium oxide layer so as to be in contact with the titanium oxide layer; and forming a second solar cell on the intermediate layer.

8. The method for producing a tandem solar cell according to claim 7, wherein the ratio of the thickness of said intermediate layer to the thickness of said titanium oxide layer is 0.5 to 1.

5.

9. The method for manufacturing a tandem solar cell according to claim 7, wherein the step of forming the second solar cell comprises the steps of: forming a second electron selection layer in contact with the intermediate layer; and forming a second light absorption layer on the second electron selection layer.

10. The method for producing a tandem solar cell according to claim 9, wherein the second electron selection layer comprises tin oxide, and the second light absorption layer comprises a perovskite crystalline material.

11. A method for manufacturing a tandem solar cell according to claim 7, wherein the step of forming the second solar cell is a step of forming a second light absorbing layer made of an n-type semiconductor so as to be in contact with the intermediate layer.

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