Method for manufacturing tin iodide perovskite thin films, method for manufacturing perovskite solar cells, perovskite solar cells

Abstract

This disclosure aims to provide a method for producing a tin iodide perovskite thin film that uses tin chloride as the tin source and water as the solvent while being processed in air, a method for producing a perovskite solar cell, and a perovskite solar cell. [Solution] The method comprises preparing a precursor aqueous solution containing SnCl2, ascorbic acid, and a cation, and a substrate; supplying the precursor aqueous solution onto the substrate; removing the water from the precursor aqueous solution; and forming a tin iodide perovskite thin film on the substrate.

Description

[Technical Field]

[0001] This disclosure relates to a method for manufacturing a tin iodide perovskite thin film, a method for manufacturing a perovskite solar cell, and a perovskite solar cell. [Background technology]

[0002] Perovskite solar cells have attracted attention in recent years due to their lightweight, flexibility, and high efficiency. Furthermore, the perovskite layer, which is the photovoltaic layer, can be coated and fabricated, offering high convenience. However, lead-based perovskites, which are the mainstream photovoltaic layer, contain lead, raising concerns about toxicity and are subject to the RoHS directive. Additionally, the use of harmful anhydrous organic solvents in an inert atmosphere during the manufacturing process of the photovoltaic layer is undesirable from an environmental perspective.

[0003] Patent Document 1 discloses a method for manufacturing a Sn-based perovskite layer and a solar cell. In the section describing a solution containing a Sn-based perovskite compound, this document discloses that examples of solvents include organic solvents such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO).

[0004] Patent Document 2 discloses a method for forming a tin(II) halide-based perovskite thin film by applying an aqueous solution of a tin(II) halide-based perovskite compound to a substrate surface and removing the solvent. The precursors used are SnBr2 and CsBr, both of which are water-soluble. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2019 / 182058 [Patent Document 2] International Publication No. 2016 / 121700 [Overview of the project] [Problems that the invention aims to solve]

[0006] Considering the costs and environmental impact associated with processing and management, the use of harmful anhydrous organic solvents should be avoided when forming tin-based perovskite thin films. For large-scale industrial production, it is preferable to be able to form tin-based perovskite thin films in the atmosphere using water instead of organic solvents. However, tin chloride, like other tin sources, is poorly soluble in water, making it difficult to dissolve the necessary concentration of tin in water for thin film formation. Furthermore, tin readily oxidizes in the atmosphere or water. Therefore, when using tin chloride as the tin source, forming tin-based perovskite thin films in the atmosphere using water instead of organic solvents has been difficult.

[0007] This disclosure was made to solve the problems described above, and aims to provide a method for manufacturing a tin iodide perovskite thin film that uses tin chloride as the tin source and water as the solvent, and can be processed in air, as well as a method for manufacturing a perovskite solar cell and a perovskite solar cell. [Means for solving the problem]

[0008] The method for producing a tin iodide perovskite thin film according to this disclosure comprises preparing a precursor aqueous solution containing SnCl2, ascorbic acid, and a cation, and a substrate, supplying the precursor aqueous solution onto the substrate, removing the water from the precursor aqueous solution, and forming a tin iodide perovskite thin film on the substrate.

[0009] Other features of this disclosure are outlined below. [Effects of the Invention]

[0010] Thin films of tin iodide can be manufactured using a method suitable for large-scale industrial production. [Brief explanation of the drawing]

[0011] [Figure 1]It is a flowchart of a method for manufacturing a tin iodide perovskite thin film. [Figure 2] It is a photograph and a cross-sectional SEM photograph of a tin iodide perovskite thin film. [Figure 3] It is a photograph of a plurality of precursor aqueous solutions with different molar concentration ratios of SnCl2 and AA. [Figure 4] It is a photograph of a plurality of precursor aqueous solutions with different molar concentration ratios of SnCl2 and AA. [Figure 5] It is a photograph taken from the upper surface side of the tin iodide perovskite thin film obtained from each sample in FIG. 4. [Figure 6] It is a diagram showing the XRD profile of the tin iodide perovskite thin film in FIG. 5. [Figure 7] It is a diagram showing the results of elemental analysis by EDS. [Figure 8] It is an appearance photograph of a plurality of tin iodide perovskite thin films formed with different MAI / SnCl2 molar concentration ratios. [Figure 9] It is a diagram showing the profile obtained by performing X-ray diffraction method (XRD). [Figure 10] It is an appearance photograph of a plurality of tin iodide perovskite thin films formed at different preheating temperatures. [Figure 11] It is an appearance photograph of a plurality of tin iodide perovskite thin films formed at different substrate rotation speeds. [Figure 12] It is the XRD result for a plurality of tin iodide perovskite thin films formed at different post-annealing temperatures. [Figure 13] It is an appearance photograph of the five samples in FIG. 12. [Figure 14] It is a diagram showing the evaluation results of stability in an atmospheric environment. [Figure 15] It is a diffraction pattern showing the XRD results. [Figure 16] It is a cross-sectional SEM photograph of a prototype solar cell. [Figure 17] It is a photograph taken from above of the prototype solar cell. [Figure 18]This is an SEM image of a tin iodide perovskite thin film made from MAI. [Figure 19] This is an SEM image of a tin iodide perovskite thin film made from PEAI. [Figure 20] This is an SEM image of a tin iodide perovskite thin film made from both PEAI and MAI as raw materials. [Modes for carrying out the invention]

[0012] The following describes embodiments. Figure 1 is a flowchart showing a method for producing a tin iodide perovskite thin film according to an embodiment. This method for producing a tin iodide perovskite thin film includes preparing a precursor aqueous solution and a substrate, supplying the precursor aqueous solution to a preheated substrate, removing water from the precursor aqueous solution, and annealing the substrate. This process allows for the supersaturation deposition of a tin iodide perovskite thin film on the substrate. For example, this tin iodide perovskite thin film can be represented by the chemical formula ASnI3 (where the A site is at least one monovalent cation).

[0013] According to the example in Figure 1, first, in step S10, the precursor aqueous solution and the substrate are prepared. In one example, the precursor aqueous solution contains SnCl2, ascorbic acid (hereinafter sometimes referred to as AA), and a cation. The solvent of the precursor aqueous solution is water. In one example, the precursor aqueous solution can be kept at room temperature. In another example, the precursor aqueous solution can be heated to control the supersaturation deposition rate, for example, by setting the solubility of SnCl2 to 0.36 mol / L. Heating is performed, for example, by heating to 70°C in an aluminum bath. Below, SnCl2, ascorbic acid, and the cation will be explained one by one.

[0014] 1. About SnCl2 (Tin Chloride) The SnCl2 in the precursor aqueous solution supplies the tin source for the tin iodide perovskite thin film. If the molar concentration of SnCl2 in the precursor aqueous solution is too high, the deposition rate of the tin iodide perovskite thin film becomes too fast, resulting in large perovskite particles being sparsely distributed on the substrate. In other words, uniform film formation is not possible, the coverage rate deteriorates, or the thin film becomes too thick. In this case, deposition begins, for example, before or at the same time as the substrate starts rotating to deposit the thin film. On the other hand, if the molar concentration of SnCl2 in the precursor aqueous solution is too low, the deposition of the tin iodide perovskite thin film slows down, resulting in a uniform film but failing to obtain the required film thickness. For example, the film thickness becomes less than 300 nm. Therefore, the molar concentration of SnCl2 in the precursor aqueous solution should be adjusted to achieve a film thickness that provides good film thickness uniformity and good conversion efficiency. For example, the molar concentration of SnCl2 in the precursor aqueous solution can be between 0.30 mol / L and 0.42 mol / L. For another example, the molar concentration of SnCl2 in the precursor aqueous solution can be between 0.30 mol / L and 0.40 mol / L, between 0.30 mol / L and 0.38 mol / L, or between 0.30 mol / L and 0.36 mol / L.

[0015] 2. About Ascorbic Acid (Vitamin C) Ascorbic acid is a material that is safe for humans and the environment. In aqueous precursor solutions, ascorbic acid acts as a chelating agent, dissolving the poorly soluble SnCl2 in water. Specifically, ascorbic acid forms a high-coordination complex with SnCl2 as shown in the following reaction equation. SnCl2+ nC6H8O6→ [Sn(C6H6O6)nCl2]2n- (aq.) + 2nH + In this way, SnCl2 can be dissolved in water. Furthermore, ascorbic acid acts as a reducing agent, and the tin ion becomes Sn 2+ From Sn 4+This suppresses oxidation. If there is too little ascorbic acid in the precursor aqueous solution, the dissolution of Sn will be insufficient. On the other hand, if there is too much ascorbic acid in the precursor aqueous solution, the ascorbic acid itself will become an impurity in the tin iodide perovskite thin film. Taking these into consideration, the molar concentration of ascorbic acid in the precursor aqueous solution can be set to, for example, 0.2 times or more the molar concentration of SnCl2. In other words, the molar concentration ratio can be set to 0.2 or more. According to another example, this molar concentration ratio can be set to 0.4 or more. However, if this molar concentration ratio is increased to 1.0, a large amount of ascorbic acid will remain as an impurity in the precursor aqueous solution. On the other hand, if this molar concentration ratio is set to, for example, 0.1 or less, Sn cannot be sufficiently dissolved in water, and a considerable amount of turbidity remains in the aqueous solution. Therefore, if the value obtained by dividing the molar concentration of ascorbic acid by the molar concentration of SnCl2 is, for example, between 0.2 and 0.9, it is possible to dissolve SnCl2 in water while suppressing the impurity of ascorbic acid. Another example suggests that setting the molar concentration of ascorbic acid / molar concentration of SnCl2 between 0.2 and 0.5 can further suppress ascorbic acid impurities. Yet another example suggests that a different range of molar concentration ratios can be adopted. The amount of ascorbic acid can be determined to be appropriate for dissolving the Sn raw material in water while suppressing oxidation, and also for suppressing impurities. Citric acid can be used as both a chelating and reducing agent, but its reducing power is weaker than that of ascorbic acid, and it cannot suppress the oxidation of tin(II) ions.

[0016] 3. About cations The cations contained in the precursor aqueous solution replace chloride ions with iodine, serving as raw materials for tin iodide perovskite thin films. Various cations can be used in the precursor aqueous solution. Examples of cations include methylammonium iodide (MAI), formamidinium iodide (FAI), cesium cations, and a combination of these. For example, both MAI and FAI can be provided as cations in the precursor aqueous solution. In another example, the cations are phenylethylammonium iodide (PEAI) or both PEAI and methylammonium iodide (MAI).

[0017] If the precursor aqueous solution contains too few cations, SnCl2 will remain. Furthermore, if MAI is used as the cation and the amount of MAI is too small, a large amount of chloride-based perovskite called MASnCl3 will precipitate. In other words, a large layer other than the tin iodide-based perovskite layer will precipitate. On the other hand, if the precursor aqueous solution contains too many cations, the cations themselves will remain unreacted as impurities. For example, if the concentration of MAI is too high, the MAI itself will remain as an unreacted impurity. The amount of cations can be controlled to suppress these problems.

[0018] The theoretical reaction equation for synthesizing MASNI3 using MAI as the cation in the precursor aqueous solution is as follows: SnCl2 + 3MAI → MASNI3 + 2MACI ↑ However, after diligent research by the inventors, it was found that if the molar concentration of MAI was set to three times the molar concentration of SnCl2 based on this reaction equation, there would be an excess of MAI, and unreacted MAI would be mixed into the tin iodide perovskite thin film. Furthermore, even when the molar concentration of MAI was set to twice the molar concentration of SnCl2, there was still a problem of unreacted MAI remaining in the tin iodide perovskite thin film. According to experiments conducted by the inventors, in order to resolve this problem, the molar concentration of MAI in the precursor aqueous solution can be set to, for example, 1.6 to 1.8 times the molar concentration of SnCl2. In another example, the molar concentration of MAI can be set to 1.65 to 1.75 times the molar concentration of SnCl2. In yet another example, the molar concentration of MAI can be set to 1.68 to 1.72 times the molar concentration of SnCl2.

[0019] The theoretical reaction equation for synthesizing (PEA)2SnI4 using PEAI as the cation in the precursor aqueous solution is as follows: 4PEAI+SnCl2→(PEA)2SnI4+2PEACl However, after diligent research by the inventors, it was found that if the molar concentration of PEAI was set to four times the molar concentration of SnCl2 based on this reaction equation, PEAI would be in excess, and unreacted PEAI would be mixed into the tin iodide perovskite thin film. Experiments conducted by the inventors showed that no significant amount of PEAI was detected in the tin iodide perovskite thin film even when the molar concentration of PEAI was 3.0 times or 3.5 times the molar concentration of SnCl2. Therefore, the molar concentration of PEAI can be set to between 3.0 times and 3.5 times the molar concentration of SnCl2.

[0020] The substrate prepared in step S10 of Figure 1 is any substrate. The substrate is a component on which the tin iodide perovskite thin film is deposited, and can be any material. For example, the tin iodide perovskite thin film can be formed on a known substrate for solar cells or organic EL displays.

[0021] Next, the process proceeds to step S12 in Figure 1. Step S12 relates to preheating the substrate. In this step, the substrate is preheated to a temperature of, for example, 125°C or higher. Preheating speeds up the reaction when removing water from the precursor aqueous solution. However, if the reaction is too fast, a uniform thin film cannot be formed. Therefore, it is necessary to set an optimal preheating temperature according to the solute concentration of the precursor aqueous solution, especially the molar concentration of SnCl2.

[0022] Next, the process proceeds to step S14 in Figure 1. In step S14, the precursor aqueous solution is supplied onto the substrate. For example, the substrate preheated in step S12 is moved to the turntable of a spin coater without a heating mechanism, and the precursor aqueous solution is supplied to the upper surface of the substrate. At this stage, SnCl2 and cations react in the precursor aqueous solution to form perovskite seed crystals. Ascorbic acid is bonded around these seed crystals, and the ascorbic acid functions as a protective film.

[0023] Next, the process proceeds to step S16 in Figure 1. In step S16, the water in the precursor aqueous solution is removed. For example, the substrate is rotated on the spin coater's turntable to give it movement. As a result, the water in the precursor aqueous solution is blown off the substrate by centrifugal force, but the aforementioned seed crystal and thin film material remain on the substrate because they have a large atomic weight. As the water in the precursor aqueous solution decreases, the amount of substance that can dissolve in the precursor aqueous solution decreases, and tin iodide perovskite supersaturates and precipitates. If the rotational motion of the substrate continues, the water in the precursor aqueous solution gradually decreases, and the layer of tin iodide perovskite increases accordingly. In other words, the aforementioned seed crystal grows.

[0024] The purpose of applying movement to the substrate, such as rotation, is to evaporate water from the precursor aqueous solution while leaving behind seed crystals and thin-film materials. If the substrate is moved too much, not only water but the entire precursor aqueous solution will be scattered from the substrate. On the other hand, if the substrate is not moved enough, water cannot be removed from the substrate. Therefore, the movement of the substrate must be appropriate so that water is removed while tin iodide perovskite is supersaturated and precipitated. As an example of such appropriate substrate movement, the rotation speed of the substrate can be set to between 500 rpm and 700 rpm. A rotation speed of several hundred rpm is quite slow for spin coating. For example, if the rotation speed is as slow as 300 rpm, the water will not be ejected by centrifugal force effectively, and a uniform film cannot be formed, resulting in a thick thin film with large irregularities. Also, if the rotation speed is slow, the precursor aqueous solution is likely to flow around to the back of the substrate, and the problem of perovskite precipitation on the back of the substrate is likely to occur. On the other hand, if the rotation speed is as high as, for example, 2000 rpm, there is a risk that not only water but also perovskite crystals and the materials used to form them will be scattered. The rotation speed of the circuit board can be set to an appropriate range, taking these effects into consideration.

[0025] In the example above, the substrate was rotated, but moisture can also be removed by moving the substrate in other ways. For example, arbitrary linear reciprocating motion or circumferential motion can be applied to the substrate. The method of moving the substrate is arbitrary, but regardless of the method, conditions can be adopted to remove water from the precursor aqueous solution while leaving the seed crystals and thin film material of the precursor aqueous solution intact. In yet another example, water can be removed from the precursor aqueous solution on the substrate by heating and evaporating the water solvent using the drop-casting method. In this case, it is not necessary to move the substrate. Note that perovskite can be supersaturated and precipitated by heating the precursor aqueous solution in a container without dropping it onto the substrate, but in that case, powdered perovskite will be obtained instead of a thin film.

[0026] Preheating the substrate before providing the precursor aqueous solution raises the temperature of the precursor aqueous solution above room temperature when removing water from it. Therefore, preheating can be replaced by another method of raising the temperature of the precursor aqueous solution above room temperature when the substrate is subjected to movement. For example, a heating device can be attached to the stage that holds the substrate when it is subjected to movement, and the substrate can be heated by the heating device while the substrate is subjected to movement.

[0027] Next, the process proceeds to step S18 in Figure 1. In step S18, the substrate is annealed. That is, after forming the tin iodide perovskite thin film, the movement of the substrate is stopped and the substrate is post-annealed. For example, the post-annealing temperature is between 75°C and 100°C. This post-annealing is performed to sufficiently remove moisture from the tin iodide perovskite thin film. If the post-annealing temperature is too high, the quality of the tin iodide perovskite thin film may deteriorate or irregularities may form on the thin film. Therefore, the post-annealing temperature should be, for example, 100°C or lower. In this way, a tin iodide perovskite thin film is formed on the substrate. This thin film formation method can be adopted as part of the manufacturing process for perovskite solar cells.

[0028] All of the above steps, including the preparation of the precursor aqueous solution and the formation of the tin iodide perovskite thin film, can be carried out in the atmosphere. The oxidation of tin in the precursor aqueous solution is suppressed by the function of ascorbic acid as a reducing agent. Furthermore, using water as a solvent avoids adverse effects on the human body and the environment compared to using organic solvents, and is also easy to handle. Therefore, the method for producing a tin iodide perovskite thin film of this embodiment allows for the formation of a perovskite thin film at low cost and safely. Moreover, according to one example, the tin iodide perovskite thin film produced by this method contains ascorbic acid. The ascorbic acid remaining in the tin iodide perovskite thin film functions as a sacrificial reducing agent that oxidizes itself to suppress the oxidation of tin, thereby increasing the durability of the thin film against atmospheric exposure. According to one example, the tin iodide perovskite thin film produced by this method, due to the use of the precursor aqueous solution, does not contain impurities originating from organic solvents, but may contain water as an impurity.

[0029] The fabricated tin iodide perovskite thin film can be represented by the chemical formula ASnI3 (where the A site is at least one monovalent cation). By controlling the composition of the A site, it is possible to optimize the band gap and impart stability. Using methylammonium as the A site allows for the production of MASnI3 thin films. In another example, the A site can be partially replaced by using a slightly larger molecular weight substance such as formamidinium (FA) or by using cesium ions (Cs) as the A site. As a result, MASnI3 thin films or CsFASnI3 thin films can be produced as tin iodide perovskite thin films. The A site can be adjusted so that the Cs / MA / FA ratio is arbitrary. Such control of the A site is possible by arbitrarily selecting one or more methylammonium iodide (MAI), formamidinium iodide (FAI), and cesium cations as cations in the precursor aqueous solution and adjusting their ratios.

[0030] For example, from the perspective of improving the performance of photovoltaic power generation equipment, the composition of site A can be controlled to achieve a desired band gap. To evaluate the band gap of a manufactured tin iodide perovskite thin film, the amount of light absorbed by the thin film at each wavelength is measured using UV-Vis-NIR (ultraviolet-visible near-infrared spectroscopy). The band gap is an indicator of how effectively sunlight can be used. For MASnI3, the band gap is 1.23 eV, and it can absorb all near-infrared light from wavelengths smaller than 1000 nm to visible light. For CsFASnI3, which is called a mixed-cation type, the band gap is 1.30-1.32 eV. According to the Shockley-Queisser limit theory, the band gap that achieves the theoretical maximum conversion efficiency of a single-junction solar cell is 1.34 eV, so the aforementioned 1.30-1.32 eV can be said to be an excellent light absorption characteristic. It is well known that mixed-cation perovskite crystals exhibit excellent light absorption properties, particularly in lead-based perovskites, but it has now been found that this can be extended to tin-based perovskites as well.

[0031] When using a perovskite thin film as a photovoltaic layer, impurities in the thin film must be reduced in order to increase its carrier mobility and extend its lifespan. By adopting optimal conditions for each of the above-mentioned conditions, impurities can be reduced to a minimum. However, the conditions described in each of the above-mentioned steps, such as the molar concentration of SnCl2, the molar ratio of MAI to SnCl2, and the molar ratio of ascorbic acid to SnCl2, are examples and can be appropriately changed depending on the purpose and the range of acceptable film quality. One of the features of this disclosure is the discovery that a tin iodide perovskite thin film can be formed in an atmospheric environment from a precursor aqueous solution containing SnCl2, ascorbic acid, and cations.

[0032] For example, the thin film formation method described above can be adopted as part of the manufacturing process for a perovskite solar cell. For instance, a glass substrate (FTO) coated with fluorine-doped tin oxide containing tin, halogen, and oxygen is prepared as a substrate, and an oxide layer containing group IV elements is formed on this substrate. The oxide layer is, for example, TiO2. A tin iodide perovskite thin film that functions as a photovoltaic layer is formed on the substrate thus created using the method of this disclosure. The thickness of the perovskite thin film that maximizes conversion efficiency is said to be between 300 nm and 800 nm. Within this thickness range, light is strongly absorbed and charge transfer loss is small. According to the method of this disclosure, a tin iodide perovskite thin film having a thickness of 300 nm to 800 nm can be manufactured. Furthermore, a hole transport layer can be formed on this perovskite thin film, and a back electrode can be formed on the hole transport layer. For example, the hole transport layer can be formed with Spiro-OMeTad. The chemical name of Spiro-OmeTad is 2,2',7,7'-tetrakis[N,N-di-p-methoxyphenylamino]-9,9'-spirobifluorene. In another example, the hole transport layer can be one of the following: PEDOT:PSS = Poly(3,4-ethylenedioxythiophene):Polystyrene sulfonic acid PTAA = Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] ·TOP-HTM-α2=(E,E,E,E)-4,4',4'',4'''-[benzene-1,2,4,5-tetrayltetrakis(ethene-2,1-diyl)]tetrakis[N,N-bis(4-methoxyphenyl)aniline] The back electrode can be formed, for example, by gold deposition. The deposition rate is, for example, 0.1 nm / s. The thickness of the gold electrode is, for example, 80 nm.

[0033] Next, we will describe the examples. In these examples, we will assume the adoption of the optimal conditions described above, and will mainly discuss the points that differ from those assumptions.

[0034] Example 1 A tin iodide perovskite thin film was formed using a precursor aqueous solution with a molar concentration of SnCl2 of 0.36 mol / L. Figure 2A is a photograph of the top surface of the obtained tin iodide perovskite thin film. From Figure 2A, it was observed that a uniformly formed, completely black tin iodide perovskite thin film was observed. Figure 2B is a cross-sectional SEM image of the obtained tin iodide perovskite thin film. The area indicated by the double-headed arrow is the tin iodide perovskite thin film. The inventors measured 100 locations on multiple SEM images, including Figure 2B, and observed that the film thickness of the tin iodide perovskite thin film was in the range of 400 nm to 520 nm, with an average value of 494 nm (±38 nm), indicating a good result. The values ​​in parentheses are the standard deviations. Furthermore, the inventors confirmed that when forming a tin iodide perovskite thin film with a molar concentration of SnCl2 in the precursor aqueous solution of 0.42 mol / L, uniform film formation was not possible.

[0035] Example 2 The degree of turbidity of the aqueous solutions was observed by varying the molar concentration ratio of SnCl2 to ascorbic acid in the precursor aqueous solutions. Furthermore, elemental analysis was performed by detecting characteristic X-rays using energy-dispersive X-ray spectroscopy (EDS). Figure 3 shows photographs of several precursor aqueous solutions with varying molar concentration ratios of SnCl2 to ascorbic acid. In this example, five aqueous solutions were prepared with molar concentration ratios of 0, 0.25, 0.5, 0.75, and 1.0, obtained by dividing the molar concentration of ascorbic acid (AA) in the precursor aqueous solution by the molar concentration of SnCl2. In this example, the molar concentration of SnCl2 was fixed at 0.36 mol / L, and the molar concentrations of ascorbic acid were set to 0, 0.09 mol / L, 0.18 mol / L, 0.27 mol / L, and 0.36 mol / L, as described above. The leftmost aqueous solution in Figure 3 has a molar concentration ratio of 0, meaning it does not contain ascorbic acid, so SnCl2 does not dissolve in water and is very turbid. In Figure 3, the second aqueous solution from the left has a molar concentration ratio of 0.25, and it was found that the white turbidity of the solution was reduced due to the chelating effect, which caused SnCl2 to dissolve in the water. Even when this molar concentration ratio was increased to 0.5, 0.75, and 1.0, the degree of turbidity did not change significantly from the case with a molar concentration ratio of 0.25.

[0036] Figure 4 shows photographs of several precursor aqueous solutions with varying molar ratios of SnCl2 and ascorbic acid. In this example, six aqueous solutions were prepared with molar ratios of 0, 0.1, 0.2, 0.3, 0.4, and 0.5, obtained by dividing the molar concentration of ascorbic acid (AA) in the precursor aqueous solution by the molar concentration of SnCl2. When the molar concentration ratio was 0.1, the amount of ascorbic acid was insufficient, and strong turbidity was observed. Figure 5 shows photographs of the top surface of the tin iodide perovskite thin films obtained from each sample in Figure 4. Figure 6 shows the XRD profile of the tin iodide perovskite thin film in Figure 5. From Figure 6, it can be seen that with the addition of ascorbic acid, Sn4 + It was found that the peak intensity of MA2SnCl6, the oxidized phase derived from the ascorbic acid, weakened. Furthermore, it was found that when the AA / SnCl2 ratio was 0.4 and 0.5, the peak of SnCl2 derived from the undissolved raw material disappeared. Therefore, it was found that in order for ascorbic acid to function as a chelating agent to dissolve almost all of the tin chloride in water, while simultaneously functioning as a reducing agent to suppress the oxidation of tin, this molar concentration ratio should be 0.4 or higher. Figure 7 shows the results of elemental analysis by EDS. Figure 7A shows the results of elemental analysis of a precursor aqueous solution with an AA / SnCl2 ratio of 0.25. Figure 7B shows the results of elemental analysis of a precursor aqueous solution with an AA / SnCl2 ratio of 1.0. When the molar concentration ratio was 1.0, the elemental ratios of carbon (C) and oxygen (O) increased compared to when the molar concentration ratio was 0.25, reaching 28.01% and 51.42%, respectively. The increase in the elemental ratios of carbon (C) and oxygen (O) indicates the presence of unreacted AA impurities. It was found that when the molar concentration ratio is 1.0, there is a large amount of unreacted AA, which becomes an impurity.

[0037] Example 3 The appearance of tin iodide perovskite thin films precipitated using the aforementioned method with varying molar ratios of MAI and SnCl2 was observed. Figure 8 shows photographs of the appearance of tin iodide perovskite thin films formed with molar ratios obtained by dividing the molar concentration of MAI by the molar concentration of SnCl2 in the precursor aqueous solution of 1.5, 1.6, 1.7, 1.8, 1.9, and 2.0, respectively. The black, uniform thin films are high-quality tin iodide perovskite thin films with suppressed impurities. Observation of Figure 8 revealed that a black, uniform thin film was obtained when the MAI / SnCl2 molar ratio was 1.7.

[0038] Figure 9 shows the profiles obtained by X-ray diffraction (XRD) for these six samples. Figure 9 illustrates the XRD profiles for six thin films formed from precursor aqueous solutions with MAI / SnCl2 molar concentration ratios of 1.5, 1.6, 1.7, 1.8, 1.9, and 2.0, respectively. If the MASnI3 crystals intended for this manufacturing method are present, strong diffraction patterns appear at 14° and 28°. It is known that if the impurity layer, MASnCl3, is present, a diffraction pattern is observed at around 15°. In the figure, the two positions indicated by triangles correspond to the MASnI3 layer, the two positions indicated by inverted triangles correspond to MAI, and the position indicated by a square corresponds to MASnCl3. It was found that MASnI3 was synthesized as the main phase in each case of MAI / SnCl2 molar concentration ratios of 1.5, 1.6, 1.7, 1.8, and 1.9. In particular, good thin films with few MASnCl3 layers and MAI were formed when the molar concentration ratios were 1.6, 1.7, and 1.8. Among these, a particularly good thin film with almost no MASnCl3 layers or MAI was formed when the molar concentration ratio was 1.7. In this case, it is presumed that almost 100% of the MAI reacted with the tin source.

[0039] Figure 9 shows that when the molar concentration ratio of MAI / SnCl2 is high, i.e., when there is a large amount of MAI, the tendency for MAI to remain in the thin film increases. For example, it was found that when the molar concentration ratio is 1.9 or higher, MAI remains in the thin film. On the other hand, when the molar concentration ratio of MAI / SnCl2 is low, i.e., when there is a small amount of MAI, the chloride-based perovskite tends to be more prominent. For example, it was found that when the molar concentration ratio is 1.5, a MASnCl3 layer is present in the thin film.

[0040] Example 4 The effect of preheating temperature on tin iodide perovskite thin films was investigated. Specifically, the appearance of tin iodide perovskite thin films deposited using the method described above was observed for multiple samples with varying preheating temperatures before spin coating. Figure 10 shows photographs of the appearance of tin iodide perovskite thin films formed at room temperature, 50°C, 75°C, 100°C, and 125°C, respectively. The closer to black the color, the more sufficient the thickness of the perovskite layer formed. Furthermore, uniform color indicates a uniform film. Comparing the five photographs, it was found that the preheating temperature has a significant effect on the deposition rate of the thin film. It was found that a uniform black film was formed at 125°C.

[0041] Example 5 The effect of substrate rotation speed on tin iodide perovskite thin films was investigated. Specifically, the appearance of multiple tin iodide perovskite thin films fabricated at varying substrate rotation speeds was observed. Figure 11 shows photographs of the appearance of tin iodide perovskite thin films formed at substrate rotation speeds of 300 rpm, 600 rpm, 1000 rpm, and 2000 rpm, respectively. For all samples, 400 μL of precursor aqueous solution was dropped onto the substrate, and the rotation time was set to 10 seconds. Referring to Figure 11, it can be seen that a good, uniform, black thin film was formed at 600 rpm. When the spin coating speed was increased to around 2000 rpm, radial unevenness occurred. In such high-speed spin coating, not only water but also perovskite was scattered, and a uniform, black thin film could not be obtained. In addition, experiments were conducted by changing the amount of aqueous solution provided during spinning from 400 μL, but the amount of aqueous solution did not have much effect on the film thickness.

[0042] Example 6 The effect of post-annealing temperature on tin iodide perovskite thin films was investigated. Specifically, several samples were prepared by varying the annealing temperature after the deposition of black tin iodide perovskite by spin coating. XRD profiles were examined, and their appearance photographs were observed. Figure 12 shows the profiles obtained by X-ray diffraction (XRD) for five tin iodide perovskite thin films formed at post-annealing temperatures of 50°C, 75°C, 100°C, 125°C, and 150°C, respectively. For post-annealing temperatures of 50°C, 75°C, and 100°C, strong diffraction patterns were observed at the 14° and 28° positions corresponding to the MASnI3 crystal. The intensity of the diffraction pattern at 14° and 28° in the 50°C sample was smaller than that of the 75°C and 100°C samples. No large peaks attributable to impurities were observed in any of the 50°C, 75°C, and 100°C cases. In contrast, when the post-annealing temperature was set to 125°C and 150°C, in addition to the peaks at 14° and 28°, diffraction patterns originating from impurities appeared at the positions indicated by the squares. These impurities are thought to be MASnCl3. It was found that when the post-annealing temperature is 125°C or higher, the evaporation rate of water is excessively high, and impurities remain in the thin film.

[0043] Figure 13 shows photographs of the appearance of the five samples in Figure 12. The sample post-annealed at 50°C has fewer black areas and a mottled pattern. This is thought to be because the water did not dry sufficiently at 50°C, and water remained in the perovskite layer. As mentioned above, the intensity of the diffraction pattern corresponding to the MASnI3 crystal was weaker in the 50°C XRD pattern compared to the 75°C and 100°C XRD patterns, but this result is consistent with the results of the visual inspection. The samples post-annealed at 125°C and 150°C contain many impurities, as mentioned above, so the conditions are not very good, even without visual inspection.

[0044] Example 7 The stability of tin iodide perovskite thin films produced by the method of this embodiment in an atmospheric environment was evaluated. Figure 14A shows the evaluation results of the stability of MASnI3 thin films produced by the method of the present invention in an atmospheric environment. The lower part of Figure 14A shows the XRD pattern of the MASnI3 thin film immediately after production, and the upper part shows the XRD pattern 12 hours after production. Both diffraction patterns have strong diffraction peaks at 14° and 28°, which correspond to the crystal of MASnI3. No significant degradation of the crystalline phase was observed in the XRD after 12 hours. Figure 14B is a quote from the evaluation results of the atmospheric stability of tin iodide perovskites published in the scientific journal Advanced Functional Materials. The XRD pattern labeled "fresh" in the lower part was obtained immediately after thin film production, and the XRD pattern labeled "aged (12h)" in the upper part was obtained 12 hours after thin film production. After 12 hours, an oxide layer called MA2SnI6, which was not present immediately after production, was detected. The appearance of an oxide layer indicates degradation of the perovskite thin film. Therefore, the stability of the MASnI3 thin film produced by the manufacturing method of this disclosure shown in Figure 14A in an atmospheric environment is superior to that of the conventional example shown in Figure 14B.

[0045] Example 8 The orientation of a tin iodide perovskite thin film manufactured by the method of this embodiment was evaluated. When the perovskite crystals are aligned parallel to the substrate, charge transfer improves, and the evaluation of orientation involves assessing this parallelism. Specifically, it is evaluated by XRD. Figure 15 shows the diffraction pattern of the XRD results. The lower part of Figure 15 shows the XRD results when MASnI3 powder was used as a sample. The upper part of Figure 15 shows the XRD results when a MASnI3 thin film manufactured by the method of this disclosure was used as a sample. From the diffraction pattern in the upper part of Figure 15, it can be seen that this thin film is oriented to the (00n) plane, where n is a positive integer. In other words, it can be seen that it is a highly oriented thin film in which only the (00n) plane is almost dominant. On the other hand, from the diffraction pattern in the lower part of Figure 15, it can be seen that in the case of powder, not only the (00n) plane but also the (011) plane and the (111) plane are present.

[0046] Example 9 A solar cell was fabricated using the tin iodide perovskite thin film of this embodiment. Figure 16 is a cross-sectional SEM image of the fabricated solar cell. This solar cell has, from the bottom, an FTO substrate that receives sunlight, a TiO2 electron transport layer, a perovskite layer manufactured by the method of this disclosure, a hole transport layer, and a gold deposition layer (electrode). When the thickness of the thin film portion of the perovskite crystalline phase was evaluated by cross-sectional SEM, it was found to be approximately 500 nm, which is the optimal film thickness. It was found that when the concentration of SnCl2 in the precursor aqueous solution was increased to 0.6 mol / L, the flatness was poor and the film thickness exceeded 1 μm. However, even with such a high concentration of SnCl2, it was possible to dissolve it in water using ascorbic acid. The inventors investigated what the optimal molar concentration of SnCl2 should be when it was lowered from 0.6, and as described above, they found that a molar concentration of approximately 0.36 was optimal. Figure 17 is a photograph of the fabricated solar cell taken from above. Figure 17 shows a 25mm x 25mm substrate with 5mm x 5mm solar cells formed in four locations in the center of the substrate. These four 5mm x 5mm regions contain solar cells with a layered structure of FTO / Compact-TiO2 / mesoporous-TiO2 / Perovskite / Spiro-MeOTAD / Au. The photoelectric efficiency of these solar cells was measured. For the photoelectric conversion efficiency measurement, a 3mm x 3mm photomask was added to the aforementioned 5mm x 5mm region, reducing the effective light irradiation area to 0.09cm². 2 This was defined as follows: The area containing this photomask is illuminated from above with simulated sunlight (100mW / cm²) from the solar simulator. 2The solar cell was irradiated with simulated sunlight. The OTENTO-SUN3 spectrometer from Spectrometer Co., Ltd. was used as the solar simulator. With the cell irradiated with simulated sunlight, a voltage of -0.1 to 1.0V was applied using a Keithley Instruments source meter (Keithley 2400), and the current was measured. The conversion efficiency was calculated from the obtained IV curve, and it was found to be a photoelectric conversion efficiency of 0.6%. In this way, the photovoltaic power of a solar cell having a tin iodide perovskite thin film manufactured from a precursor aqueous solution containing ascorbic acid was confirmed. It should be noted that the efficiency obtained with a tin-based perovskite thin film manufactured using 2-propanol, which is a different manufacturing method from that of this embodiment, is 0.004%, so the value of 0.6% obtained in this example can be said to be a high photoelectric conversion efficiency.

[0047] Example 10 The crystal structure was observed when MAI was used as the cation in the precursor aqueous solution. In this case, the reaction proceeds according to the following reaction equation. SnCl2 + 1.7MAI → MASnI3 Figure 18 shows an SEM image of the crystal of a tin iodide perovskite thin film when MAI was used as the cation in the precursor aqueous solution. From this SEM image, it was found that a three-dimensional crystal structure of MASnI3 was synthesized. The three-dimensional crystal structure is a characteristic structure of three-dimensional perovskites. From UV-Vis-NIR and PL analysis, the optical band gap of this three-dimensional perovskite was measured to be 1.2 eV.

[0048] Example 11 The crystal structure was observed when PEAI was used as the cation in the precursor aqueous solution. In this case, the reaction proceeds according to the following reaction equation. SnCl2 + 3.4PEAI → (PEA)2SnI4 Figure 19 is a SEM photograph of the crystals of a tin iodide perovskite thin film when PEAI is employed as the cation of the precursor aqueous solution. It was found from this SEM photograph that plate-like (PEA)2SnI4 could be synthesized. The plate-like structure is a characteristic structure of two-dimensional perovskite. From UV-Vis-NIR and PL analyses, the optical band gap of this two-dimensional perovskite was measured to be 1.9 eV.

[0049] Example 12 The crystal structure was observed when both PEAI and MAI were employed as the cations of the precursor aqueous solution. In this case, the reaction proceeds according to the following reaction formula. nSnCl2 + 3.4PEAI + 1.7(n - 1)MAI → (PEA)2(MA) n-1 Sn n I 3n+1 In this reaction formula, n is either 3, 4, or 5. Figure 20 is a SEM photograph of the crystals of a tin iodide perovskite thin film when both PEAI and MAI are employed as the cations of the precursor aqueous solution. It was found from this SEM photograph that accordion-like (PEA)2(MA) n-1 Sn n I 3n+1 could be synthesized. The accordion-like structure is a characteristic structure of pseudo-two-dimensional perovskite. From UV-Vis-NIR and PL analyses, the optical band gap of this pseudo-two-dimensional perovskite was determined to be from 1.3 eV to 1.5 eV. In all perovskite thin films of the dimensions shown in Examples 10 - 12, those prepared from the precursor aqueous solution containing AA showed higher stability against air exposure than those obtained from the organic solvent-based precursor aqueous solution. This suggests that AA not only stabilizes the valence of tin species in the precursor solution but also remains in the obtained thin film, protecting it from oxidative decomposition.

Claims

1. SnCl ２ The process involves preparing an aqueous precursor solution containing ascorbic acid and cations, and a substrate. The process involves supplying the aforementioned precursor aqueous solution onto the substrate, Removing water from the aforementioned precursor aqueous solution, A method for producing a tin iodide perovskite thin film, comprising forming a tin iodide perovskite thin film on the aforementioned substrate.

2. The aforementioned cation is methylammonium iodide (MAI), In the aforementioned aqueous precursor solution, the molar concentration of methylammonium iodide is the same as that of SnCl. ２ The manufacturing method according to claim 1, wherein the molar concentration is 1.6 times or more and 1.8 times or less.

3. The aforementioned cation is Phenylateammonium iodide (PEAI) or, The manufacturing method according to claim 1, wherein the product is both phenylethylammonium iodide (PEAI) and methylammonium iodide (MAI).

4. The molar concentration of the ascorbic acid is the same as the SnCl ２ The manufacturing method according to claim 1, wherein the molar concentration is 0.4 times or more.

5. The SnCl of the aforementioned aqueous precursor solution ２ The manufacturing method according to claim 1, wherein the molar concentration is 0.30 mol / L or more and 0.40 mol / L or less.

6. The substrate is preheated to a temperature of 125°C or higher before the precursor aqueous solution is provided onto the substrate. The manufacturing method according to claim 1, comprising post-annealing the substrate at a temperature of 75°C to 100°C after forming the tin iodide perovskite thin film.

7. The manufacturing method according to claim 1, wherein removing water from the precursor aqueous solution includes rotating the substrate, and the rotation speed of the substrate is 500 rpm or more and 700 rpm or less.

8. The aforementioned tin iodide perovskite thin film is MASnI ３ Or CsFASnI ３ The manufacturing method according to claim 1.

9. The manufacturing method according to claim 1, wherein the tin iodide perovskite thin film is oriented in the (00n) plane, where n is a positive integer.

10. The manufacturing method according to claim 1, wherein the thickness of the tin iodide perovskite thin film is 300 nm or more and 800 nm or less.

11. The manufacturing method according to claim 1, wherein the preparation of the precursor aqueous solution and the formation of the tin iodide perovskite thin film are carried out in air.

12. Prepare a substrate containing tin, halogen, and oxygen. Forming an oxide layer containing a Group 4 element on the aforementioned substrate, SnCl ２ A precursor aqueous solution containing ascorbic acid and cations is supplied onto the oxide layer, and the water in the precursor aqueous solution is removed. A method for manufacturing a perovskite solar cell, comprising forming a tin iodide perovskite thin film on the oxide layer.

13. A method for producing a perovskite solar cell according to claim 12, comprising forming a hole transport layer on the tin iodide perovskite thin film.

14. circuit board and electron transport layer, A tin iodide perovskite thin film containing ascorbic acid and water, A perovskite solar cell comprising a hole transport layer.

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