Manufacturing method for tandem solar cells

Abstract

This invention provides a method for manufacturing tandem solar cells that reduces variations in the balance of power generation current between the top cell and the bottom cell. [Solution] The method for manufacturing a tandem solar cell is a method for manufacturing a two-terminal tandem solar cell 2, and includes the steps of: forming a transparent electrode layer 35 on the junction side of a crystalline silicon bottom cell B; measuring the reflection spectrum of the transparent electrode layer 35; and forming a thin-film top cell T on the junction side of the bottom cell B via the transparent electrode layer 35, including a photoelectric conversion layer 21 with a band gap corresponding to the measured reflection spectrum.

Description

Technical Field

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

Background Art

[0002] As solar cells, crystalline silicon solar cells using a crystalline silicon substrate as a photoelectric conversion layer, and thin-film solar cells using an inorganic thin film such as an amorphous silicon thin film as a photoelectric conversion layer are known. Further, as a thin-film solar cell, a perovskite thin-film solar cell using a perovskite thin film which is an organic thin film (specifically, an organic / inorganic hybrid thin film) as a photoelectric conversion layer is known.

[0003] In recent years, multi-junction (tandem) solar cells in which two photoelectric conversion units including photoelectric conversion layers with different band gaps are stacked have been known for the purpose of effectively utilizing light in a wide wavelength range to improve the conversion efficiency of solar cells. For example, Patent Document 1 discloses a tandem solar cell including a bottom cell (first photoelectric conversion unit) containing a crystalline silicon substrate as a photoelectric conversion layer, and a top cell (second photoelectric conversion unit) containing a perovskite thin film as a photoelectric conversion layer.

[0004] As such tandem solar cells, there are a 2-terminal type in which the top cell and the bottom cell are connected in series, and a 4-terminal type in which electricity is extracted separately from the top cell and the bottom cell. Further, a 3-terminal tandem solar cell that can utilize the advantages of these 2-terminal type and 4-terminal type and has the potential to further improve the photoelectric conversion efficiency has been devised (for example, see Patent Document 2).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] In a two-terminal tandem solar cell, the thickness of the transparent electrode layer (TCO) on the junction side of the crystalline silicon bottom cell varies. For example, when forming the transparent electrode layer, the thickness of the transparent electrode layer varies depending on its position (center / edge) in the CVD chamber. As a result, the reflection spectrum (in other words, the reflection wavelength band) of the transparent electrode layer varies, the incident wavelength band to the bottom cell varies, the amount of incident light to the bottom cell varies, and the amount of current generated by the bottom cell varies. Therefore, the balance of current generated between the top cell and the bottom cell varies.

[0007] The present invention aims to provide a method for manufacturing tandem solar cells that reduces variations in the balance of power generation current between the top cell and the bottom cell. [Means for solving the problem]

[0008] The present invention relates to a method for manufacturing a tandem solar cell, comprising the steps of: forming a transparent electrode layer on the junction side of a crystalline silicon bottom cell; measuring the reflection spectrum of the transparent electrode layer; and forming a thin-film top cell on the junction side of the bottom cell via the transparent electrode layer, including a photoelectric conversion layer with a bandgap corresponding to the measured reflection spectrum. [Effects of the Invention]

[0009] According to the present invention, variations in the balance of power generation current between the top cell and the bottom cell in a tandem solar cell can be reduced. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic cross-sectional view showing a solar cell according to this embodiment. [Modes for carrying out the invention]

[0011] An example of an embodiment of the present invention will be described below with reference to the attached drawings. In each drawing, the same or corresponding parts will be denoted by the same reference numerals. Also, for convenience, hatching and component reference numerals may be omitted, in which case refer to other drawings.

[0012] (Solar cells) Figure 1 is a schematic cross-sectional view showing a solar cell according to this embodiment. The solar cell 2 shown in Figure 1 is a tandem (multi-junction) and two-terminal solar cell including a first photoelectric conversion unit 10 (also called a bottom cell B) and a second photoelectric conversion unit 20 (also called a top cell T) stacked on the light-receiving surface side of the first photoelectric conversion unit 10.

[0013] The first photoelectric conversion unit 10 (bottom cell B) includes a first semiconductor layer as a photoelectric conversion layer 11. The first semiconductor layer absorbs light and generates photocarriers. The first semiconductor layer as the photoelectric conversion layer 11 is a crystalline silicon substrate such as single-crystal silicon or polycrystalline silicon.

[0014] When the first semiconductor layer as the photoelectric conversion layer 11 is a single-crystal silicon substrate, the first photoelectric conversion unit 10 can be a diffusion cell in which a second conductivity type diffusion layer is provided on the light-receiving surface side of the first conductivity type single-crystal silicon substrate, or a heterojunction cell in which silicon-based thin films are provided on both sides of the first conductivity type single-crystal silicon substrate.

[0015] In the case of a heterojunction cell in which the first semiconductor layer as the photoelectric conversion layer 11 is a single-crystal silicon substrate and silicon-based thin films are provided on both the front and back surfaces of the single-crystal silicon substrate, the first photoelectric conversion unit 10 has a conductive silicon-based thin film 14 formed on the light-receiving surface side of the photoelectric conversion layer 11 and a conductive silicon-based thin film 15 formed on the back surface side of the photoelectric conversion layer 11.

[0016] The single-crystal silicon substrate can be either p-type or n-type. When comparing holes and electrons, electrons have higher mobility, so using an n-type single-crystal silicon substrate results in particularly excellent conversion characteristics. Conductive silicon-based thin films 14 and 15 are either p-type or n-type silicon-based thin films.

[0017] Intrinsic silicon thin films may be provided between the single-crystal silicon substrate, which serves as the photoelectric conversion layer 11, and the conductive silicon-based thin films 14 and 15. By providing an intrinsic silicon thin film on the surface of the single-crystal silicon substrate, surface passivation can be effectively performed while suppressing the diffusion of impurities into the single-crystal silicon substrate. By providing an intrinsic amorphous silicon thin film as an intrinsic silicon thin film on the surface of the single-crystal silicon substrate, a high passivation effect on the surface of the single-crystal silicon substrate can be obtained.

[0018] The second photoelectric conversion unit 20 (top cell T) includes a thin second semiconductor layer as the photoelectric conversion layer 21. The second semiconductor layer absorbs light and generates photocarriers. The second semiconductor layer has a different band gap than the first semiconductor layer described above. Therefore, the first and second semiconductor layers have spectral sensitivity characteristics in different wavelength ranges. Consequently, in a stacked photoelectric conversion unit in which the first photoelectric conversion unit 10, which includes the first semiconductor layer as the photoelectric conversion layer 11, and the second photoelectric conversion unit 20, which includes the second semiconductor layer as the photoelectric conversion layer 21, are stacked, light of a wider wavelength can contribute to photoelectric conversion.

[0019] Specifically, examples of the thin film constituting the second semiconductor layer include an inorganic semiconductor thin film, or an organic semiconductor thin film (specifically, an organic-inorganic hybrid semiconductor thin film). Examples of the inorganic semiconductor thin film include an amorphous silicon thin film and the like. Examples of the organic-inorganic hybrid semiconductor thin film include a perovskite thin film containing a photosensitive material having a perovskite crystal structure. In the following, the second photoelectric conversion unit 20 having a perovskite thin film as the second semiconductor layer serving as the photoelectric conversion layer 21 will be illustrated. The present embodiment is not limited thereto, and for example, the second photoelectric conversion unit 20 may be in the form of various thin-film cells having various semiconductor thin films such as an amorphous silicon thin film as the second semiconductor layer serving as the photoelectric conversion layer 21.

[0020] The perovskite thin film contains a perovskite compound. The perovskite compound is not particularly limited, and for example, a compound represented by ABX3 can be used, which includes: an organic atomic group A containing at least one of a monovalent organic ammonium ion and an amidinium-based ion; a metal atom B that generates a divalent metal ion; and a halogen atom X containing at least one of iodide ion I, bromide ion Br, chloride ion Cl, and fluoride ion F. It has also been studied to substitute part or all of the organic atomic group A with an alkali metal Am, and such a perovskite compound is not excluded from the present invention.

[0021] The band gap of the perovskite thin film can be continuously changed by adjusting the composition. As an example, in a perovskite thin film composed of iodide ion I and bromide ion Br as the halogen atom X, the band gap can be increased by increasing the composition ratio of bromide ion Br, and conversely, the band gap can be decreased by increasing the composition ratio of iodide ion I.

[0022] As methods for forming the perovskite thin film, a wet process such as a printing method, a coating method or a solution method, and a dry process such as a vapor deposition method are known.

[0023] When the photoelectric conversion layer 21 includes a perovskite semiconductor thin film, the second photoelectric conversion unit 20 has charge transport layers 24 and 25. One of the charge transport layers 24 and 25 is a hole transport layer, and the other is an electron transport layer.

[0024] The material for the hole transport layer is not particularly limited, and examples thereof include polythiophene derivatives such as poly-3-hexylthiophene (P3HT) and poly(3,4-ethylenedioxythiophene) (PEDOT), fluorene derivatives such as 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD), carbazole derivatives such as polyvinylcarbazole, triphenylamine derivatives, diphenylamine derivatives, polysilane derivatives, and polyaniline derivatives.

[0025] The material for the electron transport layer is not particularly limited, and examples thereof include metal oxides such as titanium oxide, zinc oxide, niobium oxide, zirconium oxide, and aluminum oxide.

[0026] The first photoelectric conversion unit 10 and the second photoelectric conversion unit 20 described above are connected in series. Examples of the combination of the charge transport layer 24 / charge transport layer 25 / conductive semiconductor layer 14 / conductive semiconductor layer 15 of the second photoelectric conversion unit 20 (top cell T) and the first photoelectric conversion unit 10 (bottom cell B) include the following combinations. ·Hole transport layer (HTM) / Electron transport layer (ETM) / n-type amorphous silicon semiconductor layer (n-a-Si) / p-type amorphous silicon semiconductor layer (p-a-Si): pn-np junction type ·Electron transport layer (ETM) / Hole transport layer (HTM) / n-type amorphous silicon semiconductor layer (n-a-Si) / p-type amorphous silicon semiconductor layer (p-a-Si): np-np junction type ·Hole transport layer (HTM) / Electron transport layer (ETM) / p-type amorphous silicon semiconductor layer (p-a-Si) / n-type amorphous silicon semiconductor layer (n-a-Si): pn-pn junction type • Electron transport layer (ETM) / Hole transport layer (HTM) / p-type amorphous silicon semiconductor layer (pa-Si) / n-type amorphous silicon semiconductor layer (na-Si): np-pn junction type

[0027] The conductive semiconductor layer 14 of the first photoelectric conversion unit 10 is not necessarily required. Furthermore, an intermediate layer may be provided between the first photoelectric conversion unit 10 and the second photoelectric conversion unit 20. A transparent electrode layer (ITO) 35 can be used as the intermediate layer.

[0028] The intermediate layer is provided for purposes such as adjusting the band gap between two stacked photoelectric conversion units, selective carrier movement, tunnel junction formation, and wavelength-selective reflection. The configuration of the intermediate layer is selected according to the type and combination of photoelectric conversion units 10 and 20. The intermediate layer can also be omitted by giving the conductive semiconductor layer 14 and charge transport layer 25 provided at the interface between the first photoelectric conversion unit 10 and the second photoelectric conversion unit 20 the function of an intermediate layer.

[0029] A first electrode 31 for extracting photogenerated carriers is formed on the main surface of the second photoelectric conversion unit 20 opposite to the first photoelectric conversion unit 10, that is, on the light-receiving surface side of the solar cell 2. A second electrode 32 for extracting photogenerated carriers is formed on the main surface of the first photoelectric conversion unit 10 opposite to the second photoelectric conversion unit 20, that is, on the back surface side of the solar cell 2.

[0030] The first electrode 31 may include a transparent electrode 311 and a metal electrode 312. Similarly, the second electrode 32 may include a transparent electrode 321 and a metal electrode 322. The materials for the transparent electrodes 311 and 321, and the transparent electrode layer 35 described above are not particularly limited, but examples include metal oxides such as ITO, zinc oxide, and tin oxide. The materials for the metal electrodes 312 and 322 are not particularly limited, but examples include silver, copper, and aluminum.

[0031] The metal electrode 312 of the first electrode 31 on the light-receiving surface side may be a grid-shaped or slit-shaped electrode. Similarly, the metal electrode 322 of the second electrode 32 on the back side may also be a grid-shaped or slit-shaped electrode. This allows light from both the light-receiving surface and the back side to be incident, thereby improving the photoelectric conversion efficiency.

[0032] (Method of manufacturing solar cells) Next, the method for manufacturing the solar cell of this embodiment will be described.

[0033] First, the first photoelectric conversion section 10 is formed. For example, a conductive semiconductor layer 14 is formed on the light-receiving side of the photoelectric conversion layer 11, and a conductive semiconductor layer 15 is formed on the back side of the photoelectric conversion layer 11. Known techniques, such as CVD or PVD, are used for forming the conductive semiconductor layers 14 and 15.

[0034] Next, a transparent electrode layer 35 is formed as an intermediate layer. For example, the transparent electrode layer 35 is formed on the conductive semiconductor layer 14 on the light-receiving surface side of the first photoelectric conversion unit 10, that is, on the junction surface side of the first photoelectric conversion unit 10 (bottom cell B). Known techniques such as CVD or PVD are used as methods for forming the transparent electrode layer 35.

[0035] Next, the reflection spectrum of the transparent electrode layer 35 is measured (measurement step). In the measurement step, the reflection spectrum of the transparent electrode layer 35 may be actually measured using a reflection spectrum measuring device, or the reflection spectrum of the transparent electrode layer 35 may be estimated by alternative measurement. Alternative measurement and estimation include, for example, color measurement and estimation of the transparent electrode layer 35 using an imaging device such as a camera. For example, the surface color of the transparent electrode layer 35 is imaged, the surface color of the transparent electrode layer 35 is determined by image processing of the captured image, and the reflection spectrum is estimated from the surface color of the transparent electrode layer 35. In this case, for example, the estimation can be based on data that correlates the surface color of the transparent electrode layer 35 with the reflection spectrum measured in advance.

[0036] For example, if the thickness of the transparent electrode layer 35 is thin, the surface color of the transparent electrode layer 35 will be predominantly red. In this case, there will be more light reflection in the long-wavelength band, and the reflection spectrum will shift towards the long-wavelength side. On the other hand, if the thickness of the transparent electrode layer 35 is thick, the surface color of the transparent electrode layer 35 will be predominantly blue. In this case, there will be more light reflection in the short-wavelength band, and the reflection spectrum will shift towards the short-wavelength side.

[0037] Next, the second photoelectric conversion unit 20 is formed. For example, a photoelectric conversion layer 21 with a band gap corresponding to the measured reflection spectrum, and charge transport layers 25 and 24 are formed on the transparent electrode layer 35 on the light-receiving surface side of the first photoelectric conversion unit 10, that is, on the junction surface side of the first photoelectric conversion unit 10 (bottom cell B) via the transparent electrode layer 35 (tandem process). In the tandem process, the charge transport layer 25, photoelectric conversion layer 21 and charge transport layer 24 may be formed sequentially after the measurement process, or a pre-formed second photoelectric conversion unit 20 may be used. When using a pre-formed second photoelectric conversion unit 20, for example, multiple types of second photoelectric conversion units 20 having photoelectric conversion layers 21 with different band gaps may be prepared in advance, and the second photoelectric conversion unit 20 may be selected according to the measured reflection spectrum.

[0038] For example, if the wavelength band of the reflection spectrum of the transparent electrode layer 35 is broad and there is a lot of reflected light, the band gap of the photoelectric conversion layer 21 is adjusted to be larger, and the generated current of the first photoelectric conversion unit 10 is adjusted to be larger. As described above, the band gap of the photoelectric conversion layer 21 can be adjusted by adjusting the material and composition.

[0039] For forming the charge transport layer 25 and the charge transport layer 24, known techniques such as CVD, PVD, printing, coating, solution, or vapor deposition methods are used. For forming the photoelectric conversion layer 21, known techniques such as printing, coating, solution, or vapor deposition methods are used.

[0040] Next, transparent electrodes 311 and 321 are formed. Specifically, transparent electrode 311 is formed on the light-receiving surface side of the second photoelectric conversion unit 20, and transparent electrode 321 is formed on the back side of the first photoelectric conversion unit 10. Known techniques, such as CVD or PVD, are used for forming the transparent electrodes 311 and 321.

[0041] Next, metal electrodes 312 and 322 are formed. Specifically, metal electrode 312 is formed on the light-receiving surface side of the second photoelectric conversion unit 20, and metal electrode 322 is formed on the back side of the first photoelectric conversion unit 10. Known techniques such as CVD, PVD, and printing methods are used to form the metal electrodes 312 and 322.

[0042] Based on the above, the solar cell 2 shown in Figure 1 is obtained.

[0043] Here, the thickness of the transparent electrode layer 35 varies. For example, when forming the transparent electrode layer 35, the thickness of the transparent electrode layer 35 varies depending on its position (center / edge) in the CVD chamber. As a result, the reflection spectrum (in other words, the reflection wavelength band) of the transparent electrode layer 35 varies, the incident wavelength band of the first photoelectric conversion unit 10 varies, the amount of incident light to the first photoelectric conversion unit 10 varies, and the amount of generated current of the first photoelectric conversion unit 10 varies. Therefore, the balance of generated current between the second photoelectric conversion unit 20 and the first photoelectric conversion unit 10 varies. If the balance of generated current between the second photoelectric conversion unit 20 and the first photoelectric conversion unit 10 is poor, the power generation efficiency of the solar cell 2 decreases.

[0044] In this regard, according to the solar cell manufacturing method of this embodiment, the reflection spectrum of the transparent electrode layer 35 is measured, and a second photoelectric conversion unit 20 is formed on the junction side of the first photoelectric conversion unit 10 via the transparent electrode layer 35, including a photoelectric conversion layer 21 with a band gap corresponding to the measured reflection spectrum. This makes it possible to change the band gap of the photoelectric conversion layer 21 of the second photoelectric conversion unit 20 for each solar cell 2 in response to variations in the film thickness of the transparent electrode layer 35 on the junction side of the first photoelectric conversion unit 10. Therefore, the generated current of the first photoelectric conversion unit 10 (top cell T) and the second photoelectric conversion unit 20 (bottom cell B) can be balanced. In other words, variations in the balance of generated current of the first photoelectric conversion unit 10 (top cell T) and the second photoelectric conversion unit 20 (bottom cell B) can be reduced.

[0045] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and various modifications and variations are possible. In the embodiments described above, a method for manufacturing a tandem solar cell was described in which the photoelectric conversion layer 21 of the second photoelectric conversion unit 20 (top cell T) is composed of a perovskite thin film which is an organic semiconductor thin film (more specifically, an organic-inorganic hybrid semiconductor thin film). However, the present invention is not limited thereto and is applicable to various methods for manufacturing tandem solar cells in which the photoelectric conversion layer 21 of the second photoelectric conversion unit 20 (top cell T) is composed of various semiconductor thin films such as amorphous silicon thin films which are inorganic semiconductor thin films. [Explanation of Symbols]

[0046] 2 solar cells 10,B First photoelectric conversion unit (bottom cell) 11. Photoelectric conversion layer (first semiconductor layer) 14,15 Conductive semiconductor layer (conductive silicon-based thin film) 20,T Second photoelectric conversion unit (top cell) 21 Photoelectric conversion layer (second semiconductor layer) 24,25 Charge transport layer 31 1st electrode 32 2nd electrode 35 Transparent electrode layer (intermediate layer) 311, 321 Transparent Electrodes 312, 322 Metal electrodes

Claims

1. A method for manufacturing a two-terminal tandem solar cell, A process of forming a transparent electrode layer on the junction surface side of a crystalline silicon bottom cell, A measurement step of measuring the reflection spectrum of the transparent electrode layer, A tandem step is to form a top cell of a thin film system on the bonding surface side of the bottom cell, via the transparent electrode layer, which includes a photoelectric conversion layer with a band gap corresponding to the measured reflection spectrum. A method for manufacturing tandem solar cells, including [the specified part of the method].

2. The method for manufacturing a tandem solar cell according to claim 1, wherein in the measurement step, an imaging device is used to image the surface of the TCO layer of the bottom cell, and the reflectance spectrum of the TCO layer of the bottom cell is estimated based on the color of the surface of the TCO layer of the bottom cell.

3. The method for manufacturing a tandem solar cell according to claim 1 or 2, wherein in the tandem step, a plurality of types of top cells having photoelectric conversion layers with different band gaps are prepared in advance, and the top cell is selected according to the measured reflection spectrum.

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