Tandem solar battery and method for manufacturing tandem solar battery

Abstract

The present invention addresses the problem of providing a tandem solar battery and a method for manufacturing the tandem solar battery which make it possible to achieve cost reduction with a simple configuration. Provided is a tandem solar battery in which a first solar battery SB1 including a light absorption layer of an n-type or p-type crystal silicon layer 10 is laminated with a second solar battery SB2 including an electron selection layer 12 or an n-type light absorption layer. An amorphous titanium oxide layer 11 that does not contain a crystal phase is provided in contact with the crystal silicon layer 10 and the electron selection layer 12, or with the crystal silicon layer 10 and the n-type light absorption layer. Also provided is a method for manufacturing a tandem solar battery, the method comprising: a step for directly forming, on the surface of an n-type or p-type crystal silicon layer 10, an amorphous titanium oxide layer 11 that does not contain a crystal phase; and a step for directly forming, on a surface of the titanium oxide layer 11 opposite to the crystal silicon layer 10, the electron selection layer 12 or the n-type light absorption layer.

Description

Tandem solar cell and method for manufacturing tandem solar cell

[0001] The present invention relates to a multi-junction tandem solar cell, such as a top cell and a bottom cell, and a manufacturing technique for the same.

[0002] Tandem solar cells, also known as multijunction solar cells, are solar cells that stack two or more solar cells (cells) with different bandgaps and generate electricity using these cells to increase conversion efficiency and overall power generation efficiency. Each solar cell has a light-absorbing layer that absorbs light of different wavelengths. For example, a perovskite / crystalline silicon (Si) tandem solar cell is a solar cell in which two solar cells are stacked as a bottom cell and a top cell. The basic structure of the junction interface between the perovskite-top cell and the crystalline Si-bottom cell is stacked in the following order from the light-receiving surface side: top cell charge selection film / recombination layer / bottom cell charge selection film / bottom cell passivation film / crystalline Si (Non-Patent Documents 1 and 2). The charge selection film is also called a charge transport film. The passivation film of the bottom cell is made of an amorphous silicon film or an ultrathin silicon oxide (SiO ２ The charge selection film of the bottom cell is made of an amorphous silicon film, nanocrystalline silicon film, or poly-Si film (polycrystalline silicon film) doped with phosphorus or boron, etc. The recombination layer that forms the interface between the top cell and bottom cell and exchanges charges is made of a highly conductive transparent conductive film (ITO, IZO, etc.) with a thickness of about 20 nm or a doped Si layer film (nanocrystalline silicon, etc.).

[0003] Furthermore, Non-Patent Document 3 uses a crystalline TiOx film (thickness: 54 nm) formed by atomic layer deposition (ALD) as an attempt to use a material to replace recombination layers such as ITO films and nanocrystalline silicon films. This crystalline TiOx film functions only as a recombination layer that recombines electrons in the perovskite-top cell and holes in the crystalline Si-bottom cell, and does not have the function of passivating the surface of the crystalline silicon. Instead, SiO ２ / By forming a p-type poly-Si film, a passivation function is provided.

[0004] A. Al-Ashouri et al., Science, vol 370, issue 6522, pp.1300-1309 (2020)C. McDonald et al., ACS Appl. Mater. Interfaces 14, pp.33505-33514 (2022)H. Shen et al., Science Advances vol 4, issue 12, eaau9711 (2018)

[0005] As mentioned above, the interface between the top and bottom cells of a perovskite / crystalline Si tandem solar cell is composed of a multilayer structure. Specifically, in addition to the two layers required for the bottom cell of a crystalline Si solar cell, a passivation film and a charge-selection film, a recombination layer is used at the top and bottom interfaces. Therefore, tandem solar cells have a higher number of layers than single solar cells, which tends to increase costs in exchange for increased conversion efficiency. Furthermore, there is a problem in industrial applications in that the cost-effectiveness is not realized unless the reduction in power generation costs due to improved conversion efficiency significantly exceeds the cumulative increase in manufacturing costs.

[0006] In particular, when an ITO film is used for the recombination layer, a process for depositing the ITO film is required between the manufacture of the bottom cell and the top cell, which raises the issue of high equipment and process costs. There is also the issue of using In (indium), a rare metal. Furthermore, the presence of the ITO film between the top and bottom cells causes optical reflection and absorption, resulting in optical loss. Regarding the optical loss resulting from the ITO film, a method has been used in which a refractive index adjustment layer (nanocrystalline SiOx) approximately 100 nm thick is deposited between the ITO film and the crystalline Si solar cell to reduce reflection (Non-Patent Document 1), but this further increases the number of layers and the film thickness, which raises the issue of increased manufacturing costs.

[0007] On the other hand, solar cells that use nanocrystalline silicon films for the recombination layer solve the above-mentioned problems inherent to ITO films, but have the problem of large variations in contact resistance with the electron selection layer of the top cell, resulting in large variations in cell performance (see Supporting Information, Fig. S3 of Non-Patent Document 2). In addition, the film deposition speed of nanocrystalline materials is generally slow, and it is difficult to deposit films uniformly and quickly over large areas, which has been a problem for industrial application.

[0008] Furthermore, in a method using a crystalline TiOx film as a recombination layer (Non-Patent Document 3), in addition to forming a crystalline TiOx film, SiO ２ Since it is necessary to form a film having a passivation function such as a p-type poly-Si film, the number of layers and film thickness increases, which leads to the same problems as above, such as an increase in manufacturing costs and the number of steps.

[0009] Therefore, an object of the present invention is to provide a tandem solar cell and a method for manufacturing a tandem solar cell that has a simple configuration and can be manufactured at low cost.

[0010] According to the present invention, by providing a titanium oxide layer between the bottom cell and the top cell, the titanium oxide layer can serve as both a passivation film and a charge (hole) selective film for a crystalline Si solar cell. Furthermore, the interface between the titanium oxide layer and the charge (electron) selective layer of the top cell, which is in contact with the titanium oxide layer, forms the interface connecting the top cell and the bottom cell, and the titanium oxide also functions as a recombination layer that recombines holes collected from the n-type crystalline Si layer or the p-type crystalline Si layer with electrons in the electron selective layer. This allows the structure of the interface between the top cell and the bottom cell in a tandem solar cell to be, from the light-receiving surface side, a top cell electron selective layer / titanium oxide layer / crystalline Si layer. This reduces the number of layers required by the conventional three-layer structure of recombination layer / bottom cell charge selective film / bottom cell passivation film to a single-layer structure of titanium oxide layer.

[0011] In one embodiment, the tandem solar cell is a tandem solar cell in which a first solar cell including an n-type or p-type crystalline silicon light absorption layer is stacked with a second solar cell including an electron selection layer or an n-type light absorption layer, and an amorphous titanium oxide layer not including a crystalline phase is provided between the first solar cell and the second solar cell, in contact with the crystalline silicon layer and the electron selection layer, or the crystalline silicon layer and the n-type light absorption layer.

[0012] Furthermore, in one embodiment, a method for manufacturing a tandem solar cell is a method for manufacturing a tandem solar cell in which a first solar cell including an n-type or p-type crystalline silicon layer light absorption layer is stacked with a second solar cell including an electron selection layer or an n-type light absorption layer, the method comprising the steps of: directly forming an amorphous titanium oxide layer containing no crystalline phase on the surface of the crystalline silicon layer; and directly forming the electron selection layer or the n-type light absorption layer on the surface of the titanium oxide layer opposite the crystalline silicon layer.

[0013] According to one embodiment, it is possible to provide a tandem solar cell with a simple configuration and low cost, and a method for manufacturing the tandem solar cell.

[0014] FIG. 1 is a diagram showing a schematic device structure of a tandem solar cell of embodiment 1. FIG. 2 is a flowchart illustrating a manufacturing process for a tandem solar cell of embodiment 1. FIG. 3 is a diagram showing a schematic device structure of a tandem solar cell of embodiment 2. FIG. 4 is a flowchart illustrating a manufacturing process for a tandem solar cell of embodiment 2. FIG. 5 is a diagram showing a schematic device structure of a tandem solar cell of embodiment 3. FIG. 6 is a flowchart illustrating a manufacturing process for a tandem solar cell of embodiment 3. FIG. 7 is a cross-sectional transmission electron microscope image of a solar cell. FIG. 8 is a diagram showing a schematic device structure of a tandem solar cell of comparative example 1. FIG. 9 is a flowchart illustrating a manufacturing process for a tandem solar cell of comparative example 1. FIG. 10 is a diagram showing a schematic device structure of a tandem solar cell of comparative example 2. FIG. 11 is a graph showing the current-voltage characteristics of a tandem solar cell.

[0015] 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.

[0016] (Embodiment 1) Figure 1 shows a schematic device structure of a tandem solar cell according to this embodiment. In Figure 1, solar cells 1A and 1B each have a first solar cell SB1 as a bottom cell and a second solar cell SB2 as a top cell. In this embodiment, a tandem solar cell is described as a two-junction solar cell consisting of a bottom cell and a top cell, but a multi-junction solar cell with three or more junctions may also be used. A tandem solar cell is a solar cell that combines cells with different properties. It is composed of an upper "top cell" that receives direct sunlight and a lower "bottom cell." By combining different materials, it is possible to utilize a wide range of wavelengths of light, thereby improving conversion efficiency. The top cell absorbs short-wavelength light, while the bottom cell absorbs long-wavelength light that cannot be absorbed by the top cell. Tandem solar cells are sometimes simply referred to as solar cells.

[0017] Here, the first solar cell SB1 is preferably made of inexpensive silicon (silicon cell) that absorbs light in the long wavelength region as a light absorption layer. Examples include single crystal silicon, polycrystalline silicon, and microcrystalline silicon. The second solar cell SB2 is preferably made of GaAs, AlGaAs, InGaP, InGaAsP, AlGaInP, chalcogenide (Cu(In,Ga)Se), as well as highly efficient perovskite that absorbs light in the short wavelength region as a light absorption layer. ２ (CIGS), CdTe, organic dyes, amorphous silicon, etc. While Figure 1 shows an example of a perovskite-top cell (second solar cell SB2) and a crystalline Si-bottom cell (first solar cell SB1), this combination is not limiting, and the first solar cell SB1 may be made of polycrystalline silicon or microcrystalline silicon, and the second solar cell SB2 may be made of GaAs, CdTe, amorphous silicon, etc.

[0018] Specifically, the first solar cell SB1 is composed of a crystalline silicon layer 10, an electron selection layer 12, a transparent electrode 13, and a metal electrode 14. The second solar cell SB2 is composed of an electron selection layer 15, a perovskite layer 16, a hole selection layer 17, a transparent electrode 18, and a metal electrode 19. FIG. 1(A) shows a case where n-type crystalline silicon is used as the crystalline silicon layer 10, and FIG. 1(B) shows a case where p-type crystalline silicon is used. In these solar cells 1A and 1B, an amorphous titanium oxide layer 11 (hereinafter, titanium oxide may be referred to as TiOx. TiOx is mainly composed of TiO ２ The solar cell SB1 is characterized by the use of a TiOx film, which serves three functions: a passivation film for the first solar cell SB1, a hole selection film for the first solar cell SB1, and a carrier recombination layer between the first solar cell SB1 and the second solar cell SB2. The recombination layer is a layer that separates two or more active layers (light absorption layers) and at the same time, where holes and electrons generated in each active layer recombine.

[0019] (First Solar Cell SB1) In FIG. 1, the first solar cell SB1 of solar cells 1A and 1B has a crystalline silicon layer 10. This crystalline silicon layer 10 functions as a light absorption layer and is composed of, for example, an n-type silicon (Si) substrate (solar cell 1A) doped with n-type impurities such as phosphorus (P) or a p-type silicon (Si) substrate (solar cell 1B) doped with p-type impurities such as boron (B) or gallium (Ga). The front surface (surface 1, upper side in the drawing) of the crystalline silicon layer 10 is flat. Meanwhile, the back surface (surface 2, lower side in the drawing) of the crystalline silicon layer 10 has a random texture structure. While the front surface of the crystalline silicon layer 10 may be flat, the random texture structure can improve conversion efficiency by reducing reflection and trapping light. A random texture structure may also be formed on both the front and back surfaces of the crystalline silicon layer 10.

[0020] An electron selection layer 12 (e.g., an amorphous silicon film) is formed on the back surface of the crystalline silicon layer 10. The electron selection layer 12 is made of, for example, an amorphous silicon film containing hydrogen or a titanium oxide film having electron selectivity, but is not particularly limited to this in the present embodiment. In particular, an electron selection layer 12 with good passivation properties can be made of, for example, a stacked film (a-Si:H i-n layer film) of a hydrogen-doped intrinsic amorphous silicon film and a hydrogen-doped n-type amorphous silicon film.

[0021] A translucent electrode 13 is disposed on the electron selection layer 12. This translucent electrode 13 is composed of a translucent film that is transparent to at least the visible light contained in sunlight and is electrically conductive. Examples include indium-tin oxide (ITO), zinc oxide (ZnO), and indium-zinc oxide (IZO). A metal electrode 14 is disposed on the translucent electrode 13. This metal electrode 14 is formed from a metal material such as aluminum (Al), titanium (Ti), silver (Ag), copper (Cu), nickel (Ni), gold (Au), or platinum (Pt). For example, the use of a film of aluminum, gold, silver, or copper, which has a relatively high reflectivity in the near-infrared region, is preferable because it increases the short-circuit current density and conversion efficiency of the solar cell. The metal material may also be an alloy or a laminated film made of two or more metals. For example, the costly Ag may be used to a minimum necessary thickness, and the remaining film may be made of a less expensive metal such as Al.

[0022] (Second solar cell SB2) The second solar cell SB2 of the solar cells 1A and 1B has a perovskite layer 16. The perovskite layer 16 functions as a light absorbing layer. The compound constituting the perovskite type crystalline material of the perovskite layer 16 is represented by the general formula A １ B １ X ３ In the formula, A is an alkali metal (K, Rb, Cs) or CH ３ NH ３ , HC(NH ２ ) ２ etc., and a mixture of these is used. ３ NH３ Methylammonium (MA), HC (NH ２ ) ２ is called Formamidinium (FA). B is a divalent metal ion, preferably Pb or Sn. X is a halogen, such as Cl, Br, or I. All three Xs may be the same halogen element, or multiple halogens may be present.

[0023] For example, a preferred example of a compound constituting a perovskite crystal material is a compound represented by the formula (RbMAFA): １ Pb １ (I １－Ｘ Br Ｘ ) ３ (where 0≦x≦1). The spectral sensitivity characteristics of the perovskite crystal material can be changed by changing the type and ratio of halogens. The perovskite layer 16 can be formed by various dry processes or solution film formation such as spin coating. For example, the perovskite layer 16 can be produced by dissolving the perovskite crystal material in a solvent, applying it to the electron selection layer 15 described below, and drying it. Alternatively, the perovskite layer 16 may be produced by applying a precursor of the perovskite layer 16 and then applying a poor solvent such as chlorobenzene, toluene, or anisole.

[0024] An electron selection layer 15 is disposed on the back surface of the perovskite layer 16, and a hole selection layer 17 is disposed on the front surface. Examples of compounds constituting the electron selection layer 15 include tin oxide (SnO ２ ) and TiOx, NbOx, C ６０ These compounds may be used in combination of two or more kinds. ２ has the characteristic that it can be formed into a film at room temperature by solution coating, ２Examples of the hole selection layer 17 include 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD), poly(3-hexylthiophene-2,5-diyl) (P3HT), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), N,N-bis(3-methylphenyl)-N, Examples of hole transport materials include N-diphenylbenzidine (TPD), N,N-di[(1-naphthyl)-N,N-diphenyl]-1,1-biphenyl-4,4-diamine (NPD), tris(4-carbazoyl-9-ylphenyl)amine (TCTA), poly(9-vinylcarbazole) (PVK), and 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP). Two or more of these hole transport materials may be used. In particular, Spiro-OMeTAD is preferred because it is the most widely used hole transport material, and Spiro-OMeTAD is particularly preferred because it can be easily applied to the perovskite layer 16. Furthermore, these hole transport materials may be doped.

[0025] A translucent electrode 18 is disposed on the hole selection layer 17, and a metal electrode 19 such as a silver film is formed on the translucent electrode 18 and processed into a grid shape to ensure a region through which light passes. The grid-shaped metal electrode 19 may penetrate the translucent electrode 18 and be in contact with the hole selection layer 17. The translucent electrode 18 may be made of any material as long as it is translucent to at least visible light to near-infrared light, and examples of the material include ITO, ZnO, and IZO.

[0026] (Recombination Layer) A recombination layer is disposed at the junction interface between the first solar cell SB1 and the second solar cell SB2, and is formed of an amorphous titanium oxide layer 11 that does not contain a crystalline phase. This titanium oxide layer 11 is clearly distinguishable from a crystalline TiOx film that contains a crystalline phase. This titanium oxide layer 11 is disposed so as to be in direct contact with the crystalline silicon layer 10 of the first solar cell SB1 and the electron selection layer 15 of the second solar cell SB2. In this case, the titanium oxide layer 11 not only functions as a carrier recombination layer, but also as a passivation film and a hole selection film for the crystalline silicon layer 10 of the first solar cell SB1. The titanium oxide layer 11 is formed of TiO ２ In addition, the titanium oxide layer 11 may contain hydrogen, silicon, etc. The titanium oxide layer 11 serves to bond the first solar cell SB1 and the second solar cell SB2. As a result, the second solar cell SB2 is electrically connected to the first solar cell SB1 via the recombination layer. Here, the "passivation characteristics" focusing on the titanium oxide layer 11 refers to the function of suppressing the recombination of electrons and holes caused by dangling bonds present on the surface of the crystalline silicon layer 10 with which the titanium oxide layer 11 is in contact. In addition, the titanium oxide layer 11 functions as a hole-selective film to selectively extract holes from the crystalline silicon layer 10 among electron-hole pairs photogenerated in the crystalline silicon layer 10. On the other hand, the titanium oxide layer 11 also serves as a recombination layer, which promotes recombination between electrons contained in the electron selection layer 15 of the second solar cell SB2, with which the titanium oxide layer 11 is in contact, and holes supplied from the crystalline silicon layer 10 through the titanium oxide layer 11, thereby allowing charges to be exchanged between the first solar cell SB1 and the second solar cell SB2, and allowing current to flow in an external circuit.

[0027] In the solar cells 1A and 1B configured as described above, for example, when sunlight containing visible light and infrared light is irradiated from above the translucent electrode 18, the short-wavelength light (blue-green) of the incident sunlight is absorbed by the perovskite layer 16 of the second solar cell SB2, which has a large energy band gap, and converted into electricity. The long-wavelength light (red-infrared light) not absorbed by the second solar cell SB2 is absorbed by the crystalline silicon layer 10 of the first solar cell SB1, which has a smaller energy band gap, and converted into electricity. That is, the short-wavelength light is absorbed by the upper perovskite layer 16 to generate electric charges, and the long-wavelength light that passes through the perovskite layer 16 is absorbed by the lower crystalline silicon layer 10 to generate electric charges.

[0028] In this way, the upper second solar cell SB2 absorbs light in the short wavelength range and generates electricity, and the lower first solar cell SB1 absorbs light in the long wavelength range and generates electricity, so that when a load is connected between the negative metal electrode 14 and the positive grid-shaped metal electrode 19, the load can be driven by electromotive force. This is how solar cells 1A and 1B operate.

[0029] Next, a method for manufacturing the solar cells 1A and 1B shown in Fig. 1 will be described. Fig. 2 shows a flowchart illustrating the manufacturing process. In this flowchart, the crystalline silicon layer 10 is an n-type crystalline silicon substrate or a p-type crystalline silicon substrate, the electron selection layer 12 is an a-Si:H i-n layer film, the transparent electrode 13 of the first solar cell SB1 is ITO, the metal electrode 14 is a silver film, and the electron selection layer 15 is SnO ２ The perovskite crystal material of the perovskite layer 16 is Rb ０．０５ (FA ０．８３ MA ０．１７ ) ０．９５ Pb(I ０．８３ Br ０．１７ ) - ３ 2 shows a case where Spiro-OMeTAD is used as the hole selection layer 17, ITO is used as the translucent electrode 18 of the second solar cell SB2, and a silver film is used as the metal electrode 19.

[0030] First, an n-type or p-type crystalline silicon substrate is prepared to form the crystalline silicon layer 10. The surface of these silicon substrates has a (100) plane, a predetermined resistivity (2-5 Ωcm), and a predetermined thickness (approximately 280 μm). Next, a silicon nitride film (SiNx film) is deposited on one side (side 1) of the silicon substrate using plasma-assisted chemical vapor deposition (plasma CVD) to a thickness of approximately 140 nm (S1). Subsequently, the other side (side 2) of the silicon substrate is anisotropically etched to form a random texture structure in the silicon substrate (S2). At this time, the SiNx film formed on side 1 acts as a protective film for the etching solution, allowing the random texture to be formed only on side 2. The SiNx layer is then removed using dilute hydrofluoric acid (S3). Although these processes can form a random texture on only one side of the crystalline silicon layer 10, a more industrially viable method can also be used to fabricate an equivalent substrate by first forming a random texture on both sides of the crystalline silicon layer 10 and then etching one side. In the embodiments, for ease of understanding, surface 1 refers to the light-receiving side surface, and surface 2 refers to the non-light-receiving side surface.

[0031] Next, the silicon substrate is cleaned, and then the native oxide film formed on surfaces 1 and 2 of the silicon substrate is removed using dilute hydrofluoric acid (S4). Then, an electron selection layer 12 with excellent passivation properties is formed on surface 2 of the silicon substrate (S5). Here, the electron selection layer 12 with excellent passivation properties can be composed of a hydrogen-doped intrinsic amorphous silicon film 12a and an n-type amorphous silicon film 12b (a-Si:H i-n layer) doped with hydrogen and phosphorus. The a-Si:H i-n layer can be formed using a plasma CVD method. Here, the hydrogen-doped intrinsic amorphous silicon film 12a functions as a passivation film, while the hydrogen-doped n-type amorphous silicon film 12b functions as an electron selection layer. As a result, the stacked film of the hydrogen-doped intrinsic amorphous silicon film and the hydrogen-doped n-type amorphous silicon film forms the electron selection layer 12 with excellent passivation properties. When a p-type crystalline silicon substrate is used as the crystalline silicon layer 10, a p-n junction is formed between the crystalline silicon layer 10 and the electron selection layer 12. In the case of a p-type crystalline silicon substrate, it is necessary to form an electron selection layer and an n-type diffusion layer made of amorphous silicon on a surface different from that of the titanium oxide layer 11 to form a p-n junction.

[0032] In this embodiment, an amorphous silicon film, which is commonly used in heterojunction solar cells, is used as the electron selection layer 12 with good passivation properties. However, the electron selection layer with good passivation properties in this embodiment is not limited to this, and can also be composed of a film other than an amorphous silicon film. For example, an electron selection layer known in PERC solar cells may be used as the electron selection layer with good passivation properties. That is, as in Embodiment 3 ( FIGS. 5 and 6 ) described below, a phosphorus (P)-diffused n-type Si layer, a passivation film such as SiNx, and a point contact of a metal electrode can be combined on the non-light-receiving surface of surface 2 to provide functions similar to those of an electron selection layer.

[0033] Next, the native oxide film on the surface 1 is removed again by using dilute hydrofluoric acid on the silicon substrate (crystalline silicon layer 10) (S6). After that, a titanium oxide layer 11 is formed on the surface 1 as a passivation film, a hole selection film, and a recombination layer (S7). The titanium oxide layer 11 is formed by thermal atomic layer deposition. In this embodiment, the atomic layer deposition apparatus used in this method is a FLexAL model manufactured by Oxford Instruments. At this time, TTIP (Titanium isopropoxide) is used as the titanium precursor, and water vapor (H ２ O) is used. The TTIP dose time per cycle is 1.2 seconds, and the water vapor dose is repeated three times at 1.2-second intervals. The titanium oxide film formation temperature is preferably in the range of 120-350°C, and preferably 200-300°C. Higher temperatures promote crystallization of titanium oxide, so the temperature should be 350°C or less, and preferably 300°C or less. By repeating this ALD cycle 128 times, a titanium oxide layer 11 having a thickness of approximately 5 nm is formed on the surface of the silicon substrate. The film thickness of the titanium oxide layer 11 can be changed by adjusting the source material dose time, the ALD cycle, etc., and is preferably 15 nm or less, and particularly 10 nm or less. If the thickness of the titanium oxide layer 11 is increased, the titanium oxide tends to crystallize, resulting in a decrease in passivation performance (reference: Zu-Po Yang et al., Appl. Sci. Abstract, 2016, 6, 233). However, according to this embodiment, by reducing the thickness of the titanium oxide layer 11, it is possible to prevent the titanium oxide from crystallizing and improve the passivation properties of the amorphous titanium oxide layer 11. Furthermore, by reducing the thickness of the titanium oxide layer 11, it is possible to reduce the consumption of raw materials and the film formation time.

[0034] After forming the titanium oxide layer 11 on the surface of the silicon substrate, a hydrogen plasma treatment is performed for 60 minutes or less, in which the surface of the titanium oxide layer 11 is irradiated with hydrogen plasma (S8). The hydrogen plasma is generated 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 300-600 W. This hydrogen plasma treatment improves the passivation performance and hole selectivity of the interface between the titanium oxide layer 11 and the crystalline silicon layer 10. A laminated film of a translucent electrode 13 and a metal electrode 14 is then formed by sputtering on the surface 2 of the electron selection layer 12 of the first solar cell SB1 (S9). These films may be fabricated by vapor deposition, or other film formation methods are also applicable. The translucent electrode 13 is transparent to at least visible light and near-infrared light and is composed of, for example, an ITO film with a thickness of 70 nm-150 nm. The metal electrode 14 has high reflectivity at least in the near-infrared region and is, for example, a 200 nm thick silver film. The metal electrode 14 can be formed into a grid to produce a bifacial solar cell that can capture light from the back side as well. The resulting solar cell is then annealed in an oven at 180°C for a predetermined time (approximately 2 hours) (S10). The annealing is performed in a vacuum, a nitrogen atmosphere, or the air. The annealing temperature is preferably 200°C or less to achieve both passivation performance and hole selectivity of the titanium oxide layer 11. This process forms the first solar cell SB1 (bottom cell).

[0035] Next, a second solar cell SB2 (top cell) is formed on the titanium oxide layer 11 deposited on the surface 1 of the silicon substrate using the following procedure. The second solar cell SB2 is formed by stacking an electron selection layer 15, a perovskite layer 16 (light absorption layer), a hole selection layer 17, and a translucent electrode 18 in this order. First, a surface modification treatment is performed (S11) to decompose and remove organic matter adhering to the surface of the titanium oxide layer 11 on the surface 1, thereby increasing the hydrophilicity of the surface. Surface modification treatments typically involve UV ozone treatment or oxygen plasma treatment. Considering the impact of UV light irradiation and excessive oxygen supply on the passivation performance of the titanium oxide layer / crystalline silicon layer interface and the hole selectivity of the titanium oxide layer 11 (reference: T. Matsui et al., ACS Appl. Mater. Interfaces, 12, pp. 49777-49785, 2020), alternative methods to these surface modification treatments may be employed. For example, ozone treatment or hydrogen plasma treatment, which does not irradiate the titanium oxide layer with UV light, can be used. In this embodiment, however, hydrogen plasma treatment (hydrogen flow rate 50 sccm, pressure 10 Pa, discharge power 300 W-600 W, 2 minutes) is performed using the apparatus used in S8 above. Note that this hydrogen plasma treatment can also be performed in S8 above. In this case, it is preferable to omit S11 and to replace steps S9 and S10 with steps S12. This makes it possible to prevent organic matter from adhering to the surface-modified titanium oxide layer 11 again during the processes of S9 and S10, which would impair its hydrophilicity.

[0036] Next, nanocrystalline SnO is deposited on the titanium oxide layer 11 of the surface 1 that has been subjected to the surface modification treatment. ２ A dispersion solution containing SnO was applied by spin coating to form a film. ２ A film is formed (S12). The coating method may be a dip coating method or a spray method, which is also common to other film forming methods. ２ The raw material for the film is nanocrystalline SnO ２ For example, a reagent (number 44592, 15% aqueous solution) from Alfa Aesar is used as the aqueous dispersion containing SnO. ２The aqueous dispersion containing SnO was diluted with deionized water to a concentration of about 7.5%. ２ The titanium oxide layer 11 on the surface 1 is spin-coated (at 2000 rpm for 30 seconds) with a dispersion solution containing SnO, and then the titanium oxide layer 11 is heat-treated on a hot plate (at 100° C. for 1 hour) (S13). ２ The heat treatment of the film may be performed either outside the glove box or outside the glove box. The heat treatment of S13 may be performed under the same conditions as the annealing treatment of S10 (for example, at a temperature of 180° C. for about 2 hours). In this case, the step of S10 is replaced with the step of SnO ２ By carrying out the heat treatment after the film formation, the heat treatment process can be shortened.

[0037] Next, the perovskite precursor solution is applied by spin coating to form a film of a perovskite layer 16 having a predetermined thickness (about 500 nm) (S14). ２ ,0.25M PbBr ２ , 0.09M RbI, 0.25M methylammonium bromide (MABr), 1.26M formamidinium iodide (FAI)) dissolved in a mixed solution of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) (DMF:DMSO = 4:1) is recommended. This perovskite precursor solution is used as a SnO ２The solution is applied to the film by spin coating. It is recommended that the spin coating be performed at a low rotation speed initially, gradually increasing the rotation speed, and maintaining a high rotation speed for a certain period of time. In this embodiment, the rotation speed is increased from 200 rpm / s to 1300 rpm, which is maintained for 5 seconds. Then, the rotation speed is further increased to 2000 rpm / s, which is maintained for 30 seconds at 5000 rpm. A few seconds before the end of the spin coating, an anisole solution is supplied to the substrate, and a poor solvent is added to remove the solvent contained in the perovskite precursor. Other poor solvents that can be used include chlorobenzene and toluene. After the spin coating is completed, the silicon substrate is heated to a predetermined temperature (110°C) on a hot plate for a predetermined time (15 minutes) (S15), crystallizing the perovskite layer 16.

[0038] Next, a precursor solution for the hole selection layer 17 is spin-coated to form a hole selection layer 17 (Spiro-OMeTAD) with a thickness of approximately 200 nm (S16). The precursor solution for the hole selection layer is a solution prepared by dissolving raw material powder in a solvent such as chlorobenzene. To increase the hole mobility of the hole selection layer 17, a solution prepared by dissolving a dopant such as lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) in a solvent such as acetonitrile and an appropriate amount of an additive such as 4-tert-butylpyridine are added to this precursor solution. While the addition of a dopant is not essential, adding a dopant facilitates hole mobility and can achieve high photoelectric conversion efficiency. The dopant is not limited to Li-TFSI; TBA-TFSI, in which the cation is tetrabutylammonium cation (TBA), may also be used. The formed hole selection layer 17 is then stored in dry air for a predetermined time (approximately 16 hours), and a step of doping an appropriate amount of oxygen into the hole selection layer 17 is carried out (S17).

[0039] The above steps S13 to S16 are carried out in a dry air atmosphere using, for example, a spin coater and hot plate installed in a glove box. Next, a translucent electrode 18 is formed on the hole selection layer 17 of the second solar cell SB2 by, for example, sputtering (S18). This translucent electrode 18 is only required to be translucent to at least visible light. In this example, it is composed of an ITO film of a predetermined thickness (100 nm-150 nm). In this embodiment, to minimize damage to the hole selection layer 17 during sputtering, it is recommended that the substrate and the ITO target be separated (250 mm) from each other in the vacuum chamber where the sputtering is performed. This allows for the formation of a satisfactory ITO film / hole selection layer interface without the need for a buffer layer such as MoOx between the translucent electrode 18 (ITO film) and the hole selection layer 17. Furthermore, a trace amount of oxygen may be added to the Ar gas used for sputtering to improve the translucency of the ITO film.

[0040] Then, a silver film that will become the metal electrode 19 is formed on the surface of the translucent electrode 18 formed on face 1 (S19). At this time, the silver film formed on the light incident side is processed into a grid pattern to ensure an area through which light passes. For example, the area of ​​the silver film occupies approximately 4% of the cell area. The cell area is defined as the area of ​​the translucent electrode 18 on the light receiving side of face 1. Finally, annealing (S20) is performed in a vacuum at a predetermined temperature (e.g., 50°C) for a predetermined time (e.g., 2 hours) to reduce damage to the first solar cell SB1 caused by sputtering of the translucent electrode 18 and metal electrode 19.

[0041] In this manner, the tandem solar cell of this embodiment can be manufactured. In the tandem solar cells 1A and 1B manufactured in this manner, the titanium oxide layer 11 formed directly on the surface of the silicon substrate (crystalline silicon layer 10) serves as a hole-selective film with excellent hole selectivity and passivation properties. The titanium oxide layer 11 also functions as a recombination layer where holes and electrons supplied from the first solar cell SB1 and the second solar cell SB2 recombine. As a result, the separate passivation film, hole-selective film, and recombination layer used in conventional tandem solar cells can be integrated into a single film, reducing the number of layers, lowering manufacturing costs, and shortening the manufacturing process. Thus, this embodiment provides a tandem solar cell with a simple configuration that generates electricity by depositing the titanium oxide layer 11 directly on the silicon substrate. Similar effects are also achieved in the following embodiments. In addition, in this embodiment, an example has been shown in which two solar cells are stacked as a bottom cell and a top cell, but three or more solar cells may be stacked, and this is also common to the following embodiments.

[0042] (Embodiment 2) In the second solar cell SB2, if the light absorption layer is an n-type light absorption layer, the light absorption layer itself serves as the electron selection layer, and therefore the step of depositing the electron selection layer 15 (FIG. 1) (S12 in FIG. 2) can be omitted. FIG. 3 shows a schematic device structure of such solar cells 2A and 2B. Examples of n-type light absorption layers 26 include GaAs. For example, solar cells 2A and 2B can be manufactured by the method shown in FIG. 4. In FIG. 4, the steps up to S11 in FIG. 2 (S211 in FIG. 4) are the same. In FIG. 4, after S211, an n-type light absorption layer 26 is provided (S212), and a hole selection layer 17 is further provided on the n-type light absorption layer 26 (S213). However, the subsequent steps are the same as S18-S20 in FIG. 2, and therefore a description of the overlapping portions will be omitted. When GaAs is used as the n-type light absorption layer 26 in the second solar cell, for example, a p-type GaAs layer that serves as the hole selection layer 17 is formed on the n-type GaAs layer, and the n-type GaAs layer and the titanium oxide layer 11 are directly bonded together to produce a tandem solar cell.

[0043] 5 shows a schematic device structure of a solar cell 3 according to another embodiment. In the first embodiment, amorphous silicon is used for the electron selection layer 12 of the first solar cell SB1. ＋ Formation of diffusion layer and SiO ２ In this example, the n-type poly-Si film used in a general PERC (Passivated Emitter Rear Cell) type can be replaced with an n-type poly-Si film. ＋ An example will be described in which a crystalline Si solar cell having a diffusion layer as an electron selective layer is used as the bottom cell (second solar cell SB2). FIG. 6 shows a flowchart illustrating the manufacturing process of the solar cell 3.

[0044] First, a p-type silicon substrate is prepared as the crystalline silicon layer 10. Then, both surfaces (surface 1 and surface 2) of the silicon substrate are anisotropically etched to form a random texture structure on the silicon substrate (S31). Next, the silicon substrate is cleaned, and then the native oxide films formed on surfaces 1 and 2 of the silicon substrate are removed using dilute hydrofluoric acid (S32). Next, n-type silicon is applied to surface 2 of the silicon substrate. ＋ A layer 23 (n: n-type semiconductor. + indicates high concentration) is formed (S33). ＋ The layer 23 is formed by, for example, using POCl containing phosphorus as an n-type dopant. ３ It is obtained by thermally diffusing phosphorus into a silicon substrate using as a raw material.

[0045] Then, a silicon nitride film 24 (SiNx film) is formed on both sides of the silicon substrate to a thickness of about 80 nm by plasma CVD (S34). ＋ A silver film (grid-shaped) that will become the negative metal electrode 25 is formed on the surface of the layer 23 (S35). At this time, the silver film is formed by, for example, screen printing. Then, baking is performed in an oven at a temperature of 500°C or higher (S35). At this time, the silver film of the metal electrode 25 diffuses and penetrates (also called fire-through) the SiNx film 24, so that the silver film becomes n ＋ The negative electrode is formed by contacting the layer 23. Next, one side (side 1) of the silicon substrate is subjected to single-side etching to flatten the side 1 and to remove the n-type surface of the side 2.＋ This allows the phosphorus diffusion layer that has spread to surface 1 during layer formation to be removed. Surface 1 is then subjected to HF treatment to remove the oxide film (S36). Then, titanium oxide layer 11, which is a hole selective film, is formed on light-receiving surface 1 of the silicon substrate (S37). As described above, titanium oxide layer 11 is formed by thermal atomic layer deposition. Then, hydrogen plasma treatment is performed for 60 minutes or less, in which hydrogen plasma is irradiated onto the surface of titanium oxide layer 11 (S38), and SnO is immediately deposited. ２ A film is formed by spin coating (S39). After that, annealing is performed in an oven at a temperature of 180° C. for 2 hours (S310). Here, annealing is performed in an oxygen-containing atmosphere such as low vacuum or air. The subsequent process is the same as in the first embodiment.

[0046] (Performance Evaluation of Solar Cells) Solar cell 1A (FIG. 1A) and solar cell 1B (FIG. 1B) of embodiment 1 were manufactured using the method described above (FIG. 2). Solar cells 1A and 1B were manufactured using crystalline silicon layers 10 of n-type and p-type silicon substrates, respectively, and these were designated Sample A (solar cell 1A with n-type silicon substrate) and Sample B (solar cell 1B with p-type silicon substrate). Note that a Topsil float zone Si substrate (thickness approximately 280 μm, surface orientation (100), resistivity approximately 2-5 Ωcm) was used as the n-type silicon substrate, and a Topsil float zone Si substrate (thickness approximately 280 μm, surface orientation (100), resistivity approximately 2-5 Ωcm) was used as the p-type silicon substrate. Silver (manufactured by Mitsubishi Materials Corporation) was used for metal electrodes 14 and 19, with thicknesses of approximately 400 nm and approximately 200 nm, respectively. The film thickness of the electron selection layer 12 (amorphous silicon film) was about 20 nm. The titanium oxide layer 11 was formed using the above-mentioned atomic layer deposition apparatus at the above-mentioned dose of 280°C, and the film thickness was about 5 nm (4.7 nm). The film thickness of the translucent electrode 13 (ITO film) was about 100 nm, and the film thickness of the translucent electrode 18 was about 130 nm. The electron selection layer 15 (SnO ２The thickness of the perovskite layer 16 was approximately 50 nm. The perovskite layer 16 was fabricated by preparing a precursor solution using raw materials (manufactured by Tokyo Chemical Industry Co., Ltd. and Greatcell Solar Material, Inc.), DMF (manufactured by Acros), and DMSO (manufactured by Acros), followed by a poor solvent method using an anisole solution (manufactured by Acros), as described above. The thickness of the layer was approximately 500 nm. The hole selection layer 17 was also fabricated by using raw materials Spiro-OMeTAD (manufactured by Nippon Fine Chemical Co., Ltd.), Li-TFSI (manufactured by Sigma-Aldrich), and a solution of 4-tert-butylpyridine (manufactured by Sigma-Aldrich) in chlorobenzene (manufactured by Acros), as described above. The thickness of the layer was approximately 200 nm. In the following comparative examples, unless otherwise specified, the solar cells were fabricated in the same manner and under the same conditions as in the first embodiment. ２ 1 shows a cross-sectional transmission electron microscope image of the SnO / TiOx / Si interface. Measurement was performed using a transmission electron microscope (JEM-ARM200F, manufactured by JEOL Ltd.) under conditions of an acceleration voltage of 80 kV, a bright field image, and a magnification of 2,000,000 times. According to this figure, the SnO ２ The film has clear lattice fringes and nanocrystalline SnO ２ However, since no lattice fringes are observed in the titanium oxide layer 11, it can be said that the titanium oxide layer 11 is amorphous and does not contain a crystalline phase. An oxide film (SiOx) about 1 nm thick is naturally formed at the interface between the titanium oxide layer 11 and the crystalline silicon layer 10. This oxide film is derived from the natural oxidation of the surface of the crystalline silicon when the titanium oxide layer 11 is formed, and can be considered to be part of the crystalline silicon layer 10. The interface between the titanium oxide layer 11 and the SiOx film is relatively sharp, and no significant interdiffusion was observed.

[0047] Next, Samples C and D were fabricated as comparative examples. Figure 8 shows the schematic device structures of Sample C (solar cell 4) and Sample D (solar cell 5). These solar cells 4 and 5 use heterojunction Si solar cells as the bottom cell, in which an undoped i-type amorphous silicon film 21 as a passivation film and a doped p-type amorphous silicon film 22 as a hole selective film are deposited by plasma CVD on the surface 1 of an n-type silicon substrate, which is a crystalline silicon layer 10. These solar cells do not have a titanium oxide layer. Furthermore, Solar Cell 4 in Figure 8(A) is a solar cell further stacked with an ITO film 27 as a recombination layer. Other than that, the solar cell has the same structure as Solar Cell 1A of Embodiment 1.

[0048] FIG. 9 shows a flowchart illustrating the manufacturing process of the solar cell 4. FIG. 9 differs from the process of FIG. 2 in that S46 is added after S5 (S45 in FIG. 9 ) in the flowchart of FIG. 2 , and the TiOx film deposition process (S7-8 in the flowchart of FIG. 2 ) is omitted, and an ITO film 27 deposition process (S47) is added. Other than that, the process is the same as that of FIG. 2 , and therefore, a description of the overlapping portions will be omitted. In S46, the hydrogen-containing i-type amorphous silicon film 21 can be deposited using a plasma CVD method using monosilane gas and hydrogen gas as raw materials, similar to the hydrogen-containing i-type amorphous silicon film 12a. Furthermore, the hydrogen-containing p-type amorphous silicon film 22 can be deposited on the hydrogen-containing i-type amorphous silicon film 21 using a plasma CVD method using monosilane gas, hydrogen gas, and diborane gas as raw materials. The thicknesses were approximately 10 nm and 5 nm, respectively. In step S47, a 20 nm ITO film 27 was deposited by the same sputtering method as in step S9 described in embodiment 1. By using an undoped amorphous silicon film as the passivation film and a doped amorphous silicon film as the hole selective film, a hole selective film with excellent passivation properties was obtained. Solar cell 5 in FIG. 8(B) was fabricated using the same method and conditions as solar cell 4 described above, except that step S47 in FIG. 9 was omitted.

[0049] Furthermore, Samples E and F were fabricated as comparative examples. FIG. 10 shows the schematic device structures of Sample E (solar cell 6) and Sample F (solar cell 7). Solar cells 6 and 7 in FIGS. 10(A) and 10(B) have the same structure as the electron selection layer 15 (SnO ２ Solar cells 6 and 7 have a structure without the film (film) and are otherwise similar in structure to solar cells 1A and 1B of embodiment 1. Solar cells 6 and 7 were fabricated by the same method and under the same conditions as solar cells 1A and 1B, except that S12 and S13 in FIG. 2 were omitted.

[0050] Figure 11 and Table 1 show the current-voltage characteristics of each solar cell. In the graph, (A) shows the current-voltage characteristics of sample A, (B) shows the current-voltage characteristics of sample B, (C) shows the current-voltage characteristics of sample C, (D) shows the current-voltage characteristics of sample D, (E) shows the current-voltage characteristics of sample E, and (F) shows the current-voltage characteristics of sample F. A Keithley Source Meter 2400 was used to measure the current-voltage characteristics of the solar cells. A solar simulator (manufactured by WACOM Corporation) consisting of two lamps, one Xe and one halogen, was used as the light source, and the light was measured at an air mass of 1.5 global, 100 mW / cm. ２ The solar cell was irradiated with reference sunlight.

[0051]

[0052] Sample C, which has the ITO film 27 of the recombination layer, showed relatively high performance, but sample D, which does not have the recombination layer, showed relatively high performance due to the SnO ２The fill factor and conversion efficiency were lowered due to the depletion of charges at the interface between the p-type amorphous silicon film 22 and the silicon dioxide film 21. On the other hand, it was confirmed that Samples A and B generally had higher short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and conversion efficiency (Eff.) than Samples C-F. While the current density of Sample B was slightly lower than that of Samples C-D, Sample B had higher values ​​in all other parameters, and its conversion efficiency, an important performance factor for solar cells, was particularly high. Overall, Sample B performed better. While Sample A showed results almost identical to those of Sample C in some parameters, it also achieved very good values ​​for current density and conversion efficiency. Furthermore, Samples E and F, which do not have an electron selection layer, showed significantly lower performance, confirming that the titanium oxide layer 11 does not function as an electron selection layer for the second solar cell SB2. In this way, in Samples A and B, the titanium oxide layer 11 serves the triple roles of passivation film, hole selection film, and recombination layer, making it possible to reduce the number of layers and the film thickness. Therefore, the solar cell of the embodiment not only has good performance, but also has a simple configuration, does not require a complicated manufacturing process, and can achieve low manufacturing costs.

[0053] As described above, a tandem solar cell according to one embodiment and a method for manufacturing the tandem solar cell according to one embodiment are as follows: (1) A tandem solar cell in which a first solar cell including an n-type or p-type crystalline silicon light absorption layer is stacked with a second solar cell including an electron selection layer or an n-type light absorption layer, wherein an amorphous titanium oxide layer not including a crystalline phase is provided between the first solar cell and the second solar cell and in contact with the crystalline silicon layer and the electron selection layer, or the crystalline silicon layer and the n-type light absorption layer. (2) The tandem solar cell according to (1), in which the second solar cell includes the electron selection layer and a light absorption layer is in contact with the surface of the electron selection layer opposite to the titanium oxide layer. (3) The tandem solar cell according to (2), in which the light absorption layer includes a perovskite material. (4) The tandem solar cell according to either (2) or (3), in which the electron selection layer includes tin oxide. (5) The tandem solar cell according to any one of (1) to (3), wherein the titanium oxide layer has a thickness of 10 nm or less. (6) The tandem solar cell according to (4), wherein the titanium oxide layer has a thickness of 10 nm or less. (7) A method for manufacturing a tandem solar cell in which a first solar cell including an n-type or p-type crystalline silicon light absorption layer is stacked with a second solar cell including an electron selection layer or an n-type light absorption layer, the method comprising the steps of: directly forming an amorphous titanium oxide layer not including a crystalline phase on the surface of the crystalline silicon layer; and directly forming the electron selection layer or the n-type light absorption layer on the surface of the titanium oxide layer opposite the crystalline silicon layer. (8) A method for manufacturing a tandem solar cell in which a second solar cell including an electron selection layer is stacked on a first solar cell including a light absorption layer of an n-type or p-type crystalline silicon layer, the method comprising the steps of: forming an amorphous titanium oxide layer containing no crystalline phase directly on the surface of the crystalline silicon layer; forming the electron selection layer directly on the surface of the titanium oxide layer opposite the crystalline silicon layer; and forming a light absorption layer directly on the surface of the electron selection layer opposite the titanium oxide layer.(9) The method for manufacturing a tandem solar cell according to (8) above, wherein the light absorption layer is a layer containing a perovskite material. (10) The method for manufacturing a tandem solar cell according to (8) or (9) above, wherein the electron selection layer is a layer containing tin oxide. (11) The method for manufacturing a tandem solar cell according to any one of (7) to (9) above, wherein the titanium oxide layer has a film thickness of 10 nm or less. (12) The method for manufacturing a tandem solar cell according to (10) above, wherein the titanium oxide layer has a film thickness of 10 nm or less.

[0054] This application claims priority based on Japanese Patent Application No. 2023-117756, filed July 19, 2023. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety.

[0055] The present invention has applicability to various devices that use solar cells.

[0056] 1A, 1B, 2A, 2B, 3, 4, 5, 6, 7 Solar cell 10 Crystalline silicon layer 11 Titanium oxide layer 12, 15 Electron selection layer 13, 18 Light-transmitting electrode 14 Metal electrode 16 Perovskite layer 17 Hole selection layer 19 Grid-shaped metal electrode 21 i-type amorphous silicon film 22 p-type amorphous silicon film 23 n ＋ Layer 24: Silicon nitride film 25: Metal electrode 26: N-type light absorbing layer 27: ITO film

Claims

1. A tandem solar cell in which a first solar cell including an n-type or p-type crystalline silicon layer light absorption layer is stacked with a second solar cell including an electron selection layer or an n-type light absorption layer, characterized in that an amorphous titanium oxide layer containing no crystalline phase is provided between the first solar cell and the second solar cell, in contact with the crystalline silicon layer and the electron selection layer, or the crystalline silicon layer and the n-type light absorption layer.

2. The tandem solar cell according to claim 1, characterized in that the second solar cell includes the electron selection layer, and a light absorption layer is in contact with the surface of the electron selection layer opposite the titanium oxide layer.

3. The tandem solar cell according to claim 2, wherein the light absorbing layer comprises a perovskite material.

4. The tandem solar cell according to claim 2 or 3, wherein the electron selection layer contains tin oxide.

5. A tandem solar cell according to any one of claims 1 to 3, characterized in that the thickness of the titanium oxide layer is 10 nm or less.

6. The tandem solar cell according to claim 4, wherein the thickness of the titanium oxide layer is 10 nm or less.

7. A method for manufacturing a tandem solar cell in which a first solar cell including an n-type or p-type crystalline silicon layer light absorption layer is stacked with a second solar cell including an electron selection layer or an n-type light absorption layer, the method comprising the steps of: forming an amorphous titanium oxide layer containing no crystalline phase directly on the surface of the crystalline silicon layer; and forming the electron selection layer or the n-type light absorption layer directly on the surface of the titanium oxide layer opposite the crystalline silicon layer.

8. A method for manufacturing a tandem solar cell in which a first solar cell including a light absorption layer of an n-type or p-type crystalline silicon layer is stacked with a second solar cell including an electron selection layer, comprising the steps of: forming an amorphous titanium oxide layer containing no crystalline phase directly on a surface of the crystalline silicon layer; forming the electron selection layer directly on the surface of the titanium oxide layer opposite the crystalline silicon layer; and forming a light absorption layer directly on the surface of the electron selection layer opposite the titanium oxide layer.

9. The method for producing a tandem solar cell according to claim 8, wherein the light absorbing layer is a layer containing a perovskite material.

10. The method for producing a tandem solar cell according to claim 8 or 9, characterized in that the electron selection layer is a layer containing tin oxide.

11. The method for producing a tandem solar cell according to any one of claims 7 to 9, characterized in that the thickness of the titanium oxide layer is 10 nm or less.

12. The method for producing a tandem solar cell according to claim 10, wherein the thickness of the titanium oxide layer is 10 nm or less.

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