Solar cell manufacturing method

Abstract

A highly productive method for manufacturing solar cells is provided, which improves the performance of the solar cells. [Solution] A method for manufacturing a solar cell, comprising a spraying step of spraying an aqueous solution containing at least one element selected from alkali metal elements onto a surface of a light absorbing layer, wherein in the spraying step, the droplets of the aqueous solution have a Sauter mean particle size of 100 μm or less.

Description

[Technical Field]

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

[0002] Solar cells include silicon solar cells manufactured using materials such as monocrystalline or polycrystalline silicon. While these solar cells have excellent durability, they are expensive to manufacture and thick, making them suitable for use in large-scale power generation facilities. Thin-film solar cells are also available, in which a thin, film-like light-absorbing layer is formed on a substrate such as glass or metal. Thin-film solar cells are inexpensive to manufacture and are very thin, making them suitable for flexible use in electronic devices. Due to these characteristics, various studies have been conducted in recent years to improve conversion efficiency and reduce manufacturing costs in order to apply thin-film solar cells.

[0003] For example, Patent Document 1 discloses that, for the purpose of suppressing a decrease in photoelectric conversion efficiency due to an insufficient concentration of alkali metal elements in a photoelectric conversion layer, the photoelectric conversion layer is immersed in a solution in which alkali elements are dissolved to deposit the alkali elements, and then annealed in a selenium or sulfur atmosphere, thereby achieving the same effect as when the alkali elements are deposited by vacuum deposition. Furthermore, Non-Patent Document 1 reports that the photoelectric conversion efficiency of alkali elements is improved by using, in combination, an element with a relatively large atomic weight, such as Rb or Cs, with an element with a relatively small atomic weight, such as Na or K. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-181922 [Non-patent literature]

[0005] [Non-Patent Document 1] Adv.Energy Mater.2019, 9, 1900408. Summary of the Invention [Problem to be solved by the invention]

[0006] It has been known that in the manufacture of thin-film solar cells, the photoelectric conversion efficiency of solar cells can be improved by depositing an alkali element after the formation of a light absorbing layer and then performing a process (Post Deposition Treatment, PDT) in which annealing is performed in a selenium or sulfur atmosphere. However, even when the PDT treatment methods described in Patent Document 1 and Non-Patent Document 1 are performed, the results are still insufficient in terms of reducing the manufacturing cost of solar cells and improving the conversion efficiency.

[0007] The present invention has been made in view of the above problems, and has an object to provide a method for manufacturing a solar cell that achieves both high performance and high productivity. [Means for solving the problem]

[0008] A method for manufacturing a solar cell according to one embodiment of the present invention includes a spraying step of spraying an aqueous solution containing at least one element selected from alkali metal elements onto a surface of a light absorbing layer, wherein the droplets of the aqueous solution in the spraying step have a Sauter mean particle size of 100 μm or less. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a method for manufacturing a solar cell that achieves both high performance and high productivity. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic cross-sectional view of a surface-modified light-absorbing layer according to one embodiment of the present invention. [Figure 2] 1 is a schematic cross-sectional view of a solar cell according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail with reference to the drawings as necessary, but the present invention is not limited to this, and various modifications are possible without departing from the spirit of the present invention. In the drawings, the same elements are given the same reference numerals, and redundant explanations will be omitted. Furthermore, positional relationships such as up, down, left, and right will be based on the positional relationships shown in the drawings unless otherwise specified. Furthermore, the dimensional ratios of the drawings are not limited to those shown in the drawings.

[0012] In the past, in the manufacture of thin-film solar cells, it has been known that forming a layer containing an alkali element after the formation of a light-absorbing layer and then annealing it in a selenium or sulfur atmosphere (PDT treatment) can improve the photoelectric conversion efficiency of the solar cell. However, because PDT treatment is generally performed in a vacuum deposition apparatus, there have been issues with manufacturing costs. Therefore, methods have been investigated in which the light-absorbing layer is immersed in a solution containing an alkali element and annealed without using a vacuum deposition apparatus, but these methods have been limited to elements with relatively small atomic weights, such as Na and K.

[0013] Incidentally, in recent years, it has been reported that the effectiveness of PDT treatment can be improved by combining alkali elements with relatively large atomic weights, such as Rb or Cs, with elements with relatively small atomic weights, such as Na or K. Therefore, the deposition of elements such as Rb or Cs using the immersion method was also investigated, but it was found that the Na and K contained in the light absorption layer were eluted into the solution during immersion, resulting in solar cells with lower performance than those treated with PDT by vacuum deposition.

[0014] Therefore, the inventors have found that when a spraying step is provided in which an aqueous solution containing at least one element selected from alkali metal elements is sprayed onto the surface of a light absorbing layer and the Sauter mean particle size of the droplets of the sprayed aqueous solution is controlled to 100 μm or less, the resulting solar cell can have high performance, and since a vacuum device or immersion method is not used, production costs are low and conversion efficiency is improved, thereby achieving both high performance and high productivity. The reasons why the above method is superior are not entirely clear, but it is thought that spraying droplets with a Sauter mean particle size of 100 μm or less contributes to the evaporation of the solvent components of the aqueous solution before the Na and K dissolve from the light-absorbing layer into the aqueous solution, preventing the Na and K contained in the light-absorbing layer from dissolving into the solution and improving the effect of the PDT treatment, resulting in high-performance solar cells. However, the reasons are not limited to those mentioned above.

[0015] 1. Solar cell manufacturing method The method for manufacturing a solar cell of this embodiment (hereinafter also referred to as "this manufacturing method") includes a spraying step of spraying an aqueous solution containing at least one element selected from alkali metal elements onto the surface of the light absorbing layer, and in the spraying step, the droplets of the aqueous solution have a Sauter mean particle size of 100 μm or less.

[0016] In this specification, "a solar cell has high performance" means that the cell characteristics related to the power generation performance of the solar cell are excellent in at least one parameter. Furthermore, "a solar cell has high productivity" means that the production cost of the solar cell is low and furthermore, the manufacturing method is highly reliable. This manufacturing method will be described in detail below.

[0017] 1.1. Spraying process In the spraying step of this embodiment, an aqueous solution containing at least one element selected from alkali metal elements is sprayed onto the surface of the light absorbing layer.

[0018] Fig. 1 shows a schematic cross-sectional view of one example of the surface-modified light-absorbing layer of this embodiment. As shown in Fig. 1, an aqueous solution containing at least one alkali metal element is sprayed onto the surface of the light-absorbing layer 101 of this embodiment to form an alkali metal element layer 102. As a result, a surface-modified light-absorbing layer 10 is obtained in which the light-absorbing layer 101 and the alkali metal element layer 102 are combined.

[0019] The alkali metal elements include Li, Na, K, Rb, Cs, and Fr. From the viewpoint of more effectively and reliably achieving the effect of improving the battery performance by the spraying step of this embodiment, it is preferable to include one or more selected from the group consisting of Na, K, Rb, and Cs, more preferably to include one or more of Rb and Cs, and even more preferably to include Cs.

[0020] In the aqueous solution used in the spraying step, the total concentration of Rb and Cs elements is preferably 50 mMol / L or more and 500 mMol / L or less. When the total concentration of Rb and Cs elements in the aqueous solution is within the above range, the effect of improving battery performance by the spraying step tends to be more effective and reliable. From the same viewpoint, the total concentration of Rb and Cs is more preferably 100 mMol / L or more and 400 mMol / L or less, and even more preferably 150 mMol / L or more and 300 mMol / L or less.

[0021] When the aqueous solution used in the spraying step contains Rb element, the concentration thereof is preferably 50 mMol / L or more and 500 mMol / L or less, 100 mMol / L or more and 400 mMol / L or less, or 150 mMol / L or more and 350 mMol / L or less.

[0022] In the aqueous solution used in the spraying step, the concentration of Cs element is preferably 50 mMol / L or more and 500 mMol / L or less. When the concentration of Cs element in the aqueous solution is within the above range, the effect of improving battery performance by the spraying step tends to be more effective and reliable. From the same viewpoint, the concentration of Cs is more preferably 100 mMol / L or more and 400 mMol / L or less, even more preferably 120 mMol / L or more and 350 mMol / L or less, and even more preferably 150 mMol / L or more and 300 mMol / L or less.

[0023] When the aqueous solution used in the spraying step contains Na or K, the respective concentrations are preferably 50 mMol / L to 500 mMol / L. By setting the Na and K concentrations in the aqueous solution within the above ranges, the effect of improving battery performance by the spraying step tends to be more effective and reliable. From the same viewpoint, the Na or K concentrations are more preferably 100 mMol / L to 400 mMol / L, and even more preferably 150 mMol / L to 300 mMol / L, respectively.

[0024] In the alkali metal element layer 102, the total amount of the Rb element and the Cs element is preferably 0.1 nmol / cm 2 per unit area of ​​the light absorbing layer 101. 2 More than 100nmol / cm 2 By setting the total layer amount of Rb element and Cs element in the alkali metal element layer 102 within the above range, the effect of improving the battery performance according to the present invention tends to be more effective and reliable. From the same viewpoint, the total layer amount of Rb element and Cs element is set to 0.5 nmol / cm 2 More than 45nmol / cm 2 More preferably, it is 1 nmol / cm or less. 2 More than 30nmol / cm 2 It is more preferable that the ratio is less than or equal to the above.

[0025] When the alkali metal element layer 102 contains Rb element, the layer amount thereof is preferably 0.1 nmol / cm 2 per unit area of ​​the light absorbing layer 101. 2 More than 50nmol / cm 2 less than 0.3 nmol / cm 2 More than 40nmol / cm 2 less than or equal to 0.5 nmol / cm 2 More than 30nmol / cm 2 The following is the result.

[0026] The amount of Cs element in the alkali metal element layer 102 is preferably 0.1 nmol / cm 2 per unit area of ​​the light absorption layer 101. 2 More than 50nmol / cm 2By setting the amount of Cs element in the alkali metal element layer 102 within the above range, the effect of improving the battery performance according to the present invention tends to be more effective and reliable. From the same viewpoint, the amount of Cs element is set to 0.5 nmol / cm 2 More than 45nmol / cm 2 More preferably, it is 1 nmol / cm or less. 2 More than 30nmol / cm 2 More preferably, it is 5 nmol / cm or less. 2 More than 25nmol / cm 2 It is even more preferable that the ratio is less than or equal to:

[0027] When the alkali metal element layer 102 contains Na or K, the amount of Na or K contained therein is preferably 0.1 nmol / cm 2 per unit area of ​​the light absorbing layer 101. 2 More than 40nmol / cm 2 By setting the amount of Na or K in the alkali metal element layer 102 within the above range, the effect of improving the battery performance according to the present invention tends to be more effective and reliable. From the same viewpoint, the amount of Na is set to 1 nmol / cm 2 More than 30nmol / cm 2 More preferably, it is 5 nmol / cm or less. 2 More than 20nmol / cm 2 From the same viewpoint, the amount of K stacked is more preferably 5 nmol / cm or less. 2 More than 35nmol / cm 2 It is more preferable that it is 10 nmol / cm or less. 2 More than 30nmol / cm 2 It is more preferable that the ratio is less than or equal to the above.

[0028] The method for measuring the amount of each alkali metal element in the alkali metal element layer 102 is not particularly limited, and examples thereof include inductively coupled plasma atomic emission spectroscopy (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS), and secondary ion mass spectrometry (SIMS). The amount of the layer can be measured immediately after obtaining the surface-modified light absorbing layer 10, and can also be measured in the state of the solar cell.

[0029] In the spraying step, the alkali metal compound contained in the aqueous solution is not particularly limited as long as it contains an alkali metal element, but is preferably an alkali metal halide. By preparing an aqueous solution containing an alkali metal halide, the effect of improving battery performance according to the present invention tends to be more effective and reliable. From the same viewpoint, it is more preferable that the aqueous solution contains a chloride of one element selected from the group consisting of Li, Na, K, Rb, and Cs, even more preferable that the aqueous solution contains a chloride of one element selected from the group consisting of Na, K, Rb, and Cs, and even more preferable that the aqueous solution contains one or more of RbCl and CsCl.

[0030] The aqueous solution may contain other components as needed in addition to the alkali metal compound and water, and in this case, the content of the other components is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less, relative to 100% by mass of the entire aqueous solution.

[0031] In the spraying step, the aqueous solution to be sprayed has a Sauter mean particle size of 100 μm or less. By setting the Sauter mean particle size to 100 μm or less, the liquid components of the aqueous solution evaporate before Na or K elutes from the light absorbing layer 101 side into the aqueous solution, resulting in high performance of the solar cell. From the same viewpoint, the Sauter mean particle size of the aqueous solution to be sprayed is preferably 1 μm or more and 100 μm or less, more preferably 10 μm or more and 90 μm or less, and even more preferably 30 μm or more and 80 μm or less.

[0032] The Sauter mean particle size is a value obtained by calculating the diameter of a sphere having the same volume / surface area ratio as the target particle, and is generally suitable as a method for calculating the mean particle size of non-spherical spray particles. The method for measuring the Sauter mean particle size of aqueous solution particles is not particularly limited, and may be performed by a conventionally known method. For example, it can be measured by the immersion method or the phase Doppler method. Alternatively, a measuring device that performs measurements based on a known method may be used.

[0033] The device used for spraying is not particularly limited as long as it can reduce the Sauter mean particle size to 100 μm or less, and examples thereof include ultrasonic spraying, single-fluid spraying, two-fluid spraying, electrostatic spraying, and high-pressure spraying. Among these, ultrasonic spraying is more preferable from the viewpoint of more effectively and reliably achieving the effect of improving battery performance according to the present invention.

[0034] The gas atmosphere used in the spraying step is not particularly limited as long as it does not adversely affect the properties of the aqueous solution containing an alkali metal element. From the viewpoint of more effectively and reliably achieving the effect of improving battery performance according to the present invention, an inert gas atmosphere or an atmosphere of a gas containing Se or S element is preferred, and an inert gas atmosphere is more preferred. Furthermore, the pressure used in the spraying step is preferably atmospheric pressure. By performing the spraying step at atmospheric pressure, it is possible to improve battery performance and reduce production costs at the same time.

[0035] In the spraying step, the surface temperature of the light-absorbing layer 101 onto which the aqueous solution is sprayed is, for example, 20°C or higher and 500°C or lower. Because the surface temperature of the light-absorbing layer 101 can affect the reaction with the aqueous solution and the gas atmosphere, it is preferable to set the temperature taking into consideration the properties and purpose of each. In this embodiment, for example, performing the spraying step at room temperature around 25°C is preferable because the effects of the spraying step can be fully obtained while reducing production costs. Furthermore, for example, controlling the surface temperature of the light-absorbing layer 101 to a high temperature of 200°C or higher and 500°C or higher and 250°C or higher and 400°C or lower during the spraying step tends to achieve the effect of improving battery performance without requiring a separate annealing treatment step after the spraying step and before forming other layers, which is preferable. Note that temperatures higher than 500°C may cause abnormal diffusion of elements within the light-absorbing layer.

[0036] 1.2.Annealing process The solar cell manufacturing method of this embodiment may include an annealing step of annealing in an atmosphere of a gas containing Se or S element or an inert gas after the spraying step and before the formation of other layers. The annealing step tends to improve cell performance and production efficiency. In this specification, the "other layer" in "after the spraying step and before the formation of other layers" refers to, for example, an electron transport layer, a transparent electrode layer, a grid electrode, or the like, which is formed in a step subsequent to the surface-modified light-absorbing layer 10 in the manufacturing process.

[0037] When the surface temperature of the light absorbing layer 101 is 300°C or higher in the spraying step, it is preferable not to include an annealing step of annealing in an atmosphere of a gas containing Se or S element or an inert gas after the spraying step and before forming other layers. By setting the surface temperature to 300°C or higher, the same effect as that of the annealing step tends to be obtained, and higher performance and productivity are achieved.

[0038] When the surface temperature of the light absorbing layer 101 is 300° C. or less in the spraying step, it is preferable to provide an annealing step after the spraying step in which annealing is performed in an atmosphere of a gas containing Se element or S element, or an inert gas. By including the annealing step, the yield is improved, and the effect of high productivity of the present manufacturing method tends to be more effectively and reliably achieved.

[0039] In the annealing step, the gas atmosphere preferably contains one or more selected from the group consisting of H2S gas, N2 gas, Ar gas, He gas, H2Se gas, and Se vapor. By using the above gas atmosphere in the annealing step, the effect of improving battery performance according to the present invention tends to be more effective and reliable. From the same viewpoint, the gas atmosphere in the annealing step more preferably contains one or more selected from the group consisting of H2S gas, N2 gas, Ar gas, and H2Se gas, even more preferably contains H2S gas or N2 gas, and even more preferably consists of only H2S and N2 gas or only N2 gas.

[0040] The surface temperature of the surface-modified light-absorbing layer 10 in the annealing treatment step is preferably 200°C or higher and 600°C or lower. By keeping the surface temperature of the surface-modified light-absorbing layer 10 in the annealing treatment step within the above range, the effect of improving battery performance according to the present invention tends to be more effective and reliable. From the same viewpoint, the surface temperature of the surface-modified light-absorbing layer 10 is more preferably 300°C or higher and 550°C or lower, and even more preferably 350°C or higher and 500°C or lower.

[0041] In the annealing step, the annealing time is preferably 1 minute or more and 30 minutes or less. By setting the annealing time within the above range, the effect of improving battery performance according to the present invention tends to be more effectively and reliably achieved. From the same viewpoint, the annealing time is more preferably 3 minutes or more and 20 minutes or less, even more preferably 5 minutes or more and 15 minutes or less, and even more preferably 7 minutes or more and 10 minutes or less.

[0042] An example of the cross-sectional structure of a solar cell obtained by the manufacturing method of this embodiment is shown in Fig. 2. For example, as shown in Fig. 2, a solar cell 20 includes a substrate 201, a back electrode layer 202 provided on the substrate 201, a hole transport layer 203 provided on the back electrode layer 202, a surface-modified light-absorbing layer 204 provided on the hole transport layer 203, an electron transport layer 205 provided on the surface-modified light-absorbing layer 204, a transparent electrode layer 206 provided on the electron transport layer 205, and a grid electrode 207 provided on the transparent electrode layer 206.

[0043] The steps for forming the solar cell 20, substrate 201, back electrode layer 202, hole transport layer 203, surface-modified light absorbing layer 204, electron transport layer 205, transparent electrode layer 206, and grid electrode 207 shown in FIG. 2 will be described below in order.

[0044] 1.3. Back electrode layer formation process In the process of forming the power generating portion of the solar cell, first, for example, a back electrode layer 202 is formed on a substrate 201. Methods for forming the back electrode layer 202 include a dry process and a wet process, with a dry process being preferred. The dry process is not particularly limited, and an example is a method for forming the back electrode layer 202, which is a metal conductive layer, by a sputtering method. The film formation conditions for the sputtering method are not particularly limited, and for example, applied power: 1.0 to 3.0 W / cm 2 The film formation atmosphere may be an argon atmosphere, and the film formation pressure may be 0.5 to 3.0 Pa. In the back electrode layer forming step, for example, the substrate 201 may be the substrate to be sputtered.

[0045] 1.4. Hole transport layer formation process Next, for example, a hole transport layer 203 is formed on the back electrode layer 202. Methods for forming the hole transport layer 203 include a dry process and a wet process, with a dry process being preferred. The dry process is not particularly limited, and an example thereof is a method for forming the hole transport layer 203, which is a p-type semiconductor containing an organic compound or an inorganic compound, by a sputtering method. The film formation conditions for the sputtering method are not particularly limited, and an example thereof is an applied power of 0.5 to 3.0 W / cm. 2 The film formation atmosphere may be an argon atmosphere or a mixed atmosphere of argon and oxygen, and the film formation pressure may be 0.5 to 3.0 Pa. When the surface-modified light-absorbing layer 204 is formed, a compound may be formed between the back electrode layer 202 and the surface-modified light-absorbing layer 204, thereby forming a hole-transporting layer 203 between the back electrode layer 202 and the surface-modified light-absorbing layer 204.

[0046] 1.5. Surface modification light absorption layer formation process Next, a surface-modified light-absorbing layer 204 is formed on the hole-transporting layer 203. The process of forming the surface-modified light-absorbing layer of this embodiment includes a step of forming the light-absorbing layer 101 and a step of obtaining the alkali metal element layer 102 by the above-mentioned spraying step, and also includes an annealing treatment step as needed. Methods for forming the light-absorbing layer 101 include, for example, a dry process and a wet process, with a dry process being preferred. The dry process is not particularly limited, but examples thereof include a method of forming the light-absorbing layer 101 containing a chalcopyrite compound or a kesterite compound by a sputtering method. The film formation conditions for the sputtering method are not particularly limited, but examples include applied power: 0.5 to 3.0 W / cm 2 The film formation atmosphere may be an argon atmosphere, and the film formation pressure may be 0.5 to 3.0 Pa. The temperature of the atmosphere and the temperature of the substrate to be sputtered may not be controlled during sputtering. After sputtering, annealing may be performed at 350°C or higher and 650°C or lower in a nitrogen or selenium and sulfur atmosphere.

[0047] In this manufacturing method, the light absorbing layer 101 preferably includes a chalcopyrite thin film or a kesterite thin film. When the light absorbing layer 101 includes a chalcopyrite thin film or a kesterite thin film, the effect of improving battery performance by the spraying step of the present invention tends to be more effective and reliable.

[0048] 1.6.Electron transport layer formation process Next, the electron transport layer 205 is formed on the surface-modified light absorbing layer 204. For example, the electron transport layer 205 may be formed on the sputtered substrate including the surface-modified light absorbing layer 204 by depositing an n-type oxide semiconductor by sputtering while supplying a gas containing an oxygen source and a hydrogen source, or the n-type oxide semiconductor may be deposited by sputtering while supplying a gas not containing a hydrogen source. The deposition conditions for the sputtering method are not particularly limited, and examples thereof include applied power of 0.5 to 3.0 W / cm. 2 The film formation atmosphere may be an argon atmosphere which may contain oxygen, and the film formation pressure may be 0.5 to 3.0 Pa. It is also preferable to heat the substrate to be sputtered during sputtering.

[0049] 1.7. Second electrode layer formation process Next, the transparent electrode layer 206 is formed on the electron transport layer 205. Methods for forming the transparent electrode layer 206 include a dry process and a wet process, with the dry process being preferred. The dry process is not particularly limited, and an example thereof is a method for forming the transparent electrode layer 206 by a sputtering method. The film formation conditions for the sputtering method are not particularly limited, and an example thereof is an applied power of 0.5 to 3.0 W / cm. 2 The film formation atmosphere may be an argon atmosphere or a mixed atmosphere of argon, oxygen, and hydrogen, and the film formation pressure may be 0.5 to 3.0 Pa.

[0050] 1.8.Grid electrode formation process This manufacturing method includes a step of forming a grid electrode 207, in which the grid electrode 207 is formed on the transparent electrode layer 206 on the light-receiving surface side of the solar cell. The method for forming the grid electrode 207 is not particularly limited, and examples thereof include a dry process and a wet process. Specific examples include a sputtering method, a vapor deposition method, a method of printing a paste-like conductive material on the transparent electrode layer, and a method of crimping a wire.

[0051] In addition to the steps described above, the present manufacturing method may include other steps as necessary. For example, a step of forming an oxide insulating film on the grid electrode may be performed, or a step of covering the entire solar cell 20 obtained with a sealing film may be performed.

[0052] 2. Solar cells The solar cell 20 of this embodiment is obtained by the above-described manufacturing method, and from the substrate 201 to the grid electrode 207, for example, has the following characteristics.

[0053] The substrate 201 is not particularly limited, and examples thereof include glass substrates such as soda lime glass and low-alkali glass, metal substrates such as stainless steel foil, aluminum foil, and titanium foil, and resin substrates such as polyimide resin film and epoxy resin film. The thickness of the substrate 201 is not particularly limited, and examples thereof are 10 μm to 500 μm, 20 μm to 250 μm, and 30 μm to 100 μm.

[0054] The back electrode layer 202 is generally provided to extract current due to holes generated in the surface-modified light absorbing layer 204 described below. The back electrode layer 202 is not particularly limited as long as it is conductive, and examples thereof include a metal conductive layer made of a metal such as Mo, Cr, or Ti; a conductive inorganic compound conductive layer made of a conductive inorganic compound other than a metal; and a conductive organic compound conductive layer made of a conductive organic compound. The thickness of the back electrode layer 202 is not particularly limited, and is, for example, 200 nm to 800 nm, or 300 nm to 700 nm.

[0055] The hole transport layer 203 has the function of, for example, efficiently extracting holes generated in the surface-modified light absorbing layer 204 (described later) from the surface-modified light absorbing layer 204 and preventing the recombination of electrons and holes generated simultaneously with the holes in the surface-modified light absorbing layer 204 (described later). The hole transport layer 203 is preferably a p-type semiconductor. The substance contained in the p-type semiconductor is not particularly limited, and examples thereof include organic compounds such as polythiophene derivatives, such as poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS), poly(3-hexylthiophene) (P3HT), and poly(3-octylthiophene) (P3OT), as well as inorganic compounds, such as nickel oxide, molybdenum oxide, copper gallium oxide, copper aluminum oxide, molybdenum selenide, and molybdenum sulfide selenide. The p-type semiconductor in the hole transport layer 203 may be used alone or in combination of two or more types. In the solar cell, the formation of the hole transport layer 203 may be omitted.

[0056] The surface-modified light-absorbing layer 204 has the function of absorbing light such as near-infrared light, visible light, and ultraviolet light to generate electrons and holes, and an example of such light is sunlight. Examples of compounds constituting the light-absorbing layer 101 in the surface-modified light-absorbing layer include those containing perovskite compounds, chalcopyrite compounds, and kesterite compounds, with those containing chalcopyrite compounds and kesterite compounds being preferred. Each compound may be used alone, or two or more types may be used in combination.

[0057] The chalcopyrite compound is preferably a I-III-VI2 group chalcopyrite compound. The I-III-VI2 group chalcopyrite compound is not particularly limited, but examples thereof include CuAlS2, CuAlSe2, CuAlTe2, CuGaS2, CuGaSe2, CuGaTe2, CuInS2, CuInSe2, CuInTe2, AgAlS2, AgAlSe2, AgAlTe2, AgGaS2, AgGaSe2, AgGaTe2, AgInS2, AgInSe2, AgInTe2, and combinations thereof. Among these chalcopyrite compounds, CuGaS2, CuGaSe2, CuInS2, CuInSe2, Cu(In x Ga 1-x )(Se y S 1-y )2 (0≦x≦1, 0≦y≦1) is preferred, and Cu(In x Ga 1-x )(Se y S 1-y )2 (0≦x≦1, 0≦y≦1) is more preferable. In this embodiment, the term "CIS compound" refers to a chalcopyrite compound containing Cu, In, and Se, the term "CIGS compound" refers to a chalcopyrite compound containing Cu, In, Ga, and Se, and the term "CIGSS compound" refers to a chalcopyrite compound containing Cu, In, Ga, Se, and S.

[0058] The kesterite compound is preferably a Group I2-II-IV-VI4 kesterite compound, and examples of the Group I2-II-IV-VI4 kesterite compound include, but are not limited to, Cu2ZnSnS4, Cu2ZnSnSe4, Cu2ZnGeS4, Cu2ZnGeSe4, Cu2MnSnS4, Cu2MnSnSe4, Cu2MnGeS4, Cu2MnGeSe4, Ag2ZnSnS4, Ag2ZnSnSe4, Ag2ZnGeS4, Ag2ZnGeSe4, Ag2MnSnS4, Ag2MnSnSe4, Ag2MnGeS4, Ag2MnGeSe4, and combinations thereof.

[0059] Examples of perovskite compounds include organic-inorganic perovskite compounds, particularly halide-based organic-inorganic perovskite compounds. Specific examples include CH3NH3PbI3, CH3NH3PbBr3, CH3NH3PbCl3, CH3NH3SnI3, CH3NH3SnBr3, CH3NH3SnCl3, and CH3NH3PbI (3-x) Cl x and compounds in which CFH2NH3, CF2HNH3, CF3NH3, or NH2CH=NH2 is used in place of CH3NH3 in the above compounds. In the above formula, x is an arbitrary value of 0 or more and 3 or less, and y is an arbitrary value of 0 or more and 1 or less.

[0060] The band gap of the surface-modified light-absorbing layer 204 is preferably 2.0 eV or less, 1.8 eV or less, 1.5 eV or less, 1.2 eV or less, or 1.1 eV or less, based on the minimum value in the depth direction. The lower limit of the band gap may be, for example, 0.5 eV or 0.8 eV or more. When the band gap of the surface-modified light-absorbing layer 204 satisfies the above range, the solar cell exhibits high performance.

[0061] The thickness of each surface-modified light-absorbing layer 204 is preferably 0.5 μm to 5 μm, 0.8 μm to 4 μm, or 1 μm to 3 μm. By setting the thickness of each surface-modified light-absorbing layer 204 within the above range, productivity of the solar cell is further improved, and it tends to be easier to make the solar cell lighter and more flexible.

[0062] The electron transport layer 205 has a function of, for example, efficiently extracting electrons generated in the surface-modified light-absorbing layer 204 from the surface-modified light-absorbing layer 204 and preventing recombination of electrons and holes generated simultaneously in the surface-modified light-absorbing layer 204. The electron transport layer 205 is preferably an n-type semiconductor. The material contained in the n-type semiconductor is not particularly limited, and may be, for example, C 60and organic compounds such as phenanthroline derivatives such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), as well as n-type oxide semiconductors consisting essentially of zinc oxide, tin oxide, titanium oxide, zinc oxide sulfide, magnesium zinc oxide, zinc tin oxide, or titanium zinc oxide, and n-type semiconductors containing cadmium sulfide, indium sulfide, or indium sulfide doped with oxygen or an alkali metal element.

[0063] The transparent electrode layer 206 is provided, for example, to extract a current due to electrons generated in the surface-modified light-absorbing layer 204. In a solar cell, the surface-modified light-absorbing layer 204 typically absorbs light that has passed through the transparent electrode layer 206, and therefore this layer is made into a transparent electrode layer in order to increase the amount of light absorbed by the surface-modified light-absorbing layer 204. Known materials can be used as the transparent electrode material, such as indium tin oxide (ITO), hydrogen-containing indium oxide (IOH), fluorine-containing tin oxide (FTO), boron-containing zinc oxide (ZnO:B), and aluminum-containing zinc oxide (ZnO:Al).

[0064] The grid electrode 207 is provided, for example, to extract electricity from the transparent electrode layer 206. The material of the grid electrode 207 is not particularly limited as long as it is conductive, and examples thereof include metals such as Mo, Cr, Ag, Cu, Ni, Al, and Ti; conductive inorganic compounds other than metals; and conductive organic compounds.

[0065] The solar cell 20 shown in FIG. 2 is an example for explaining the solar cell of the present invention, and is not intended to limit the present invention to only this embodiment. The solar cell of the present invention can be modified in various ways without departing from the gist of the invention.

[0066] The solar cell 20 may have other layers between the layers, on the grid electrode 207, or under the substrate 201, as needed. Specifically, the hole transport layer 203 may have two or more hole transport layers each containing a different material. Alternatively, instead of the grid electrode 207, another hole transport layer may be provided on the transparent electrode layer 206, with an additional light absorbing layer thereon. The electron transport layer 205 may have two or more electron transport layers 205 each containing a different material.

[0067] Although not shown, the solar cell 20 may have two or three sets of a hole transport layer 203, a surface-modified light absorbing layer 204 provided on the hole transport layer 203, an electron transport layer 205 provided on the light absorbing layer, and a transparent electrode layer 206 provided on the electron transport layer 205, stacked on the back electrode layer 202. A grid electrode may be provided on the uppermost transparent electrode layer of such a stack.

[0068] When there are a plurality of layers 201 to 206, the plurality of layers may be the same or different. For example, when there are a plurality of surface-modified light-absorbing layers 204, each surface-modified light-absorbing layer may contain a compound with a different absorption spectrum, and the electron transport layer and hole transport layer in contact with each surface-modified light-absorbing layer may be selected according to the properties of the light-absorbing layer in contact with it.

[0069] The solar cell of this embodiment has high conversion efficiency. More specifically, the conversion efficiency of the solar cell of this embodiment is preferably 14% or more, 15% or more, or 16% or more. A conversion efficiency in the above range indicates that a high-performance solar cell has been obtained.

[0070] The thickness of the solar cell 20 excluding the substrate 201 is not particularly limited and is, for example, 1 μm to 40.0 μm, 2 μm to 30 μm, or 3 μm to 20 μm. The solar cell of the present invention can be a thin-film solar cell by forming each layer to be sufficiently thin. [Example]

[0071] The present embodiment will be described in more detail below using examples and comparative examples, but the present embodiment is not limited to the following examples.

[0072] <How to make solar cells> A solar cell with a single electron transport layer, as shown in Figure 2, was fabricated. A 50 μm-thick titanium foil was prepared as the substrate. A 600 nm-thick back electrode layer containing metallic molybdenum was formed on this substrate using a sputtering method. Next, a precursor to the CIGSS layer was deposited on the back electrode layer using a sputtering method. The resulting layer was then annealed at 350°C to 650°C in a nitrogen, selenium, and sulfur atmosphere to form a 2 μm-thick CIGSS layer. The necessary alkali metal element-containing layer formation process and annealing process were then carried out according to the methods described in each example.

[0073] Next, an n-type electron transport layer containing titanium zinc oxide doped with hydrogen and sulfur elements was formed on the light absorption layer containing the CIGSS compound by sputtering to a thickness of 70 nm to 120 nm. A transparent electrode layer of hydrogen-containing indium oxide was formed on the n-type electron transport layer to a thickness of 300 nm, and a silver grid electrode with a thickness of 5 μm was formed on the surface of the layer.

[0074] In the following examples and comparative examples, solar cells were fabricated and evaluated in the same manner as described above, except for the step of forming the alkali metal element-containing layer described below.

[0075] 1. Formation of an alkali metal element-containing layer 1.1. Spraying process In Examples 1 to 6, an alkali metal element-containing layer was formed on a substrate immediately after the formation of a CIGSS layer by the spraying process of this embodiment. First, a mixed solution of KCl and NaCl dissolved in pure water, and optionally CsCl, was sprayed onto the surface of the CIGSS layer for 20 seconds using an ultrasonic spray method at atmospheric pressure. The concentration of each component in the solution, the amount of each component layered, and the spraying conditions are shown in Table 1, and the ultrasonic spraying conditions are shown below. <Conditions for ultrasonic spraying> Equipment used: Sonaer particle generator 241PGT Ultrasonic frequency: 2.4MHz Atmosphere during spraying: Nitrogen The Sauter mean particle size of the spray particles produced under the conditions of the ultrasonic spray method is in the range of 5 μm to 50 μm.

[0076] 1.2.Immersion method In Comparative Examples 3 to 10, after forming the CIGSS layer, an alkali metal element-containing layer was formed by a dipping method. The solar cell precursor substrate immediately after forming the CIGSS layer was immersed in a mixed solution of KCl and NaCl dissolved in pure water, and optionally CsCl, at room temperature (25°C) for 2 minutes so that the entire surface of the CIGSS layer was immersed. The substrate was then removed from the solution and dried with an air knife. The concentration of each component in the solution, the amount of each component layered, and the spraying conditions are shown in Table 1.

[0077] In Comparative Examples 1 and 2, the step of forming an alkali metal element-containing layer was not carried out.

[0078] 2. Annealing process The solar cell precursor substrates of Examples 1 to 3, 5, and 6 and Comparative Examples 2 to 10 obtained above were subjected to an annealing treatment at atmospheric pressure under the conditions shown in Table 1. The solar cell precursor substrates of Example 4 and Comparative Example 1 were not subjected to the annealing treatment step. Here, the solar cell precursor substrate refers to a substrate in a state where up to the light absorbing layer or surface-modified light absorbing layer has been formed.

[0079] 3. Evaluation of battery characteristics: conversion efficiency ratio The IV curve of the solar cell obtained by the above process was measured under standard test conditions (light with spectrum AM1.5 and irradiance of 1 kW / m 2The conversion efficiency was measured under test conditions in which light was incident at 1000 kJ / s and the solar cell temperature was 25°C. The conversion efficiency of each solar cell was calculated using the following formula. The conversion efficiency (%) is the value obtained by dividing the output (maximum output: Pmax) at the optimum operating point on the IV curve by the light energy E received by the solar cell. As a result of measuring the conversion efficiency of each solar cell, the solar cell of Comparative Example 4 had the lowest conversion efficiency, so the conversion efficiency ratio of each solar cell to this value (vs. Comparative Example 4) was calculated and is shown in Table 1. Conversion efficiency (%) = (Pmax / E) x 100

[0080] [Table 1]

[0081] A method for manufacturing a solar cell according to one embodiment of the present invention includes the following aspects. [1] a spraying step of spraying an aqueous solution containing at least one element selected from alkali metal elements onto a surface of the light absorbing layer, In the spraying step, the Sauter mean particle size of the droplets of the aqueous solution is 100 μm or less. How solar cells are manufactured. [2] The aqueous solution contains at least one of an Rb element and a Cs element among alkali metal elements. [1] A method for producing a solar cell according to the present invention. [3] The total amount of the Rb element and the Cs element stacked relative to the unit area of ​​the light absorbing layer is 1 nmol / cm 2 More than 30nmol / cm 2 Below is the [2] A method for producing a solar cell according to [2]. [4] In the spraying step, the surface temperature of the light absorbing layer is 300°C or higher, There is no annealing treatment step of annealing in an atmosphere of a gas containing Se element or S element, or an inert gas, after the spraying step and before the formation of another layer. A method for manufacturing a solar cell in any one of [1] to [3]. [5] In the spraying step, the surface temperature of the light absorbing layer is less than 300°C, After the spraying step, an annealing step is performed in an atmosphere of a gas containing Se element or S element, or an inert gas. The method for producing a solar cell according to any one of [1] to [4]. [6] The light absorbing layer includes a chalcopyrite thin film or a kesterite thin film. The method for producing a solar cell according to any one of [1] to [5]. [Explanation of symbols]

[0082] 10, 204...surface-modified light-absorbing layer, 101...light-absorbing layer, 102...alkali metal element layer, 20...solar cell, 201...substrate, 202...back electrode layer, 203...hole transport layer, 205...electron transport layer, 206...transparent electrode layer, 207...grid electrode.

Claims

1. a spraying step of spraying an aqueous solution containing at least one element selected from alkali metal elements onto a surface of the light absorbing layer, In the spraying step, the Sauter mean particle size of the droplets of the aqueous solution is 100 μm or less. How solar cells are manufactured.

2. The aqueous solution contains at least one of an Rb element and a Cs element among alkali metal elements. The method for manufacturing the solar cell according to claim 1 .

3. The total amount of the Rb element and the Cs element to be stacked relative to the unit area of ​​the light absorbing layer is 1 to 30 nmol / cm 2 That is, The method for producing a solar cell according to claim 2 .

4. In the spraying step, the surface temperature of the light absorbing layer is 300° C. or higher, There is no annealing step of annealing in an atmosphere of a gas containing Se or S element or an inert gas after the spraying step and before the formation of another layer. A method for manufacturing a solar cell according to claim 1.

5. In the spraying step, the surface temperature of the light absorbing layer is less than 300°C, After the spraying step, an annealing step is performed in an atmosphere of a gas containing Se or S element, or an inert gas. The method for manufacturing the solar cell according to claim 1 .

6. The light absorbing layer includes a chalcopyrite thin film or a kesterite thin film. The method for manufacturing the solar cell according to claim 1 .

Images