Perovskite film, thin film solar cell, method for manufacturing perovskite film, and method for manufacturing thin-film solar cell

Abstract

To provide a highly durable perovskite film, a thin-film solar cell, and a method for manufacturing the film and the cell.SOLUTION: The perovskite film according to the present embodiment has a plurality of grains when viewed in plan along the stacking direction, and the mode grain size of the plurality of grains is 1 μm or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a perovskite film, a thin-film solar cell, a method for manufacturing a perovskite film, and a method for manufacturing a thin-film solar cell.

Background Art

[0002] In recent years, development of thin-film solar cells using a thin film having a perovskite structure has been carried out. The perovskite structure is a cubic crystal structure represented by the general formula ABX3. Different atoms or molecules are arranged at the vertices of the cube (A site), the centers of each face of the cube (B site), and the center of the cube (X site), respectively. For example, organic materials such as CH3NH3 + , CH5N2 + etc. are used for the A site.

[0003] Perovskite films are vulnerable to water. Therefore, production in an inert gas atmosphere or a dry atmosphere with a relative humidity of 20% or less is required, which increases the production cost. Also, their durability in an atmosphere containing moisture is low, and their performance significantly deteriorates in about one month.

[0004] Patent Document 1 discloses that an ionic liquid is added to a precursor solution of a perovskite film, and film formation is performed by spin coating to produce a perovskite film. The perovskite film of Patent Document 1 has a film structure in which nano-sized fine particles are aggregated.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Perovskite films are expected to be applied to various devices such as photoelectric conversion devices, piezoelectric conversion devices, and thermoelectric conversion devices, and further improvement in performance and reduction in manufacturing cost are required. In addition, a method for manufacturing perovskite films under normal atmosphere rather than in a limited environment such as in an inert gas is required. For example, as described in Patent Document 1, by adding an ionic liquid to a precursor solution, it has become possible to manufacture perovskite films under the atmosphere. However, the manufacturing method disclosed in Patent Document 1 is not suitable for mass production, and further improvement in the durability of the manufactured perovskite films is required.

[0007] The present invention has been made in view of the above problems, and an object thereof is to provide a perovskite film with high durability, a thin-film solar cell, and methods for manufacturing them.

Means for Solving the Problems

[0008] In order to solve the above problems, the present invention provides the following means.

[0009] The perovskite film according to the first aspect has a plurality of grains when viewed in a plan view from the stacking direction, and the most frequent grain size of the plurality of grains is 1 μm or more.

Effects of the Invention

[0010] The perovskite film and the thin-film solar cell according to the present embodiment have high durability. The method for manufacturing a perovskite film and the method for manufacturing a thin-film solar cell according to the present embodiment can manufacture a perovskite film or a thin-film solar cell at low cost.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0012] Hereinafter, this embodiment will be described in detail. The following description is an example of the present invention, and the present invention is not limited thereto, and can be appropriately modified and implemented without changing the gist thereof.

[0013] FIG. 1 is a scanning electron microscope (SEM) image of the perovskite film 10 according to this embodiment. The perovskite film 10 according to this embodiment has a plurality of grains when viewed in a plan view from the stacking direction. The most frequent grain size of the grains is 1 μm or more, 2 preferably μm or more, more preferably 10 μm or more, still more preferably 30 μm or more, and particularly preferably 50 μm or more. The most frequent grain size of the perovskite film shown in FIG. 1 is 70 μm.

[0014] The most frequent grain size is obtained by the following procedure. First, using a scanning electron microscope (SEM), images at a magnification of 800 times are taken at three locations. Then, 10 grains are arbitrarily extracted from each of the taken images. The major axis diameters of the extracted grains are measured respectively. The major axis lengths of the measured grains are made into a histogram, and the value with the highest frequency is taken as the most frequent grain size.

[0015] FIG. 2 is a diagram schematically showing a characteristic portion of the perovskite film 10 according to this embodiment. The perovskite film 10 may contain a hydrophobic functional group H. The hydrophobic functional group H is, for example, at least at the grain boundaries of a plurality of grains G. Further, the hydrophobic functional group H may be attached to the entire surface of the perovskite film 10.

[0016] The presence of the hydrophobic functional group H can be confirmed by measuring the water contact angle. In the water contact angle measurement, a water droplet is dropped one by one onto the membrane surface, and the contact angle between the water droplet and the membrane is measured. When the hydrophobic functional group H is present on the surface of the perovskite membrane 10, the water contact angle is 70° or more. The water contact angle of the perovskite membrane 10 is more preferably 75° or more.

[0017] FIG. 3 shows the water contact angles of the perovskite membrane according to this embodiment and the perovskite membrane having no hydrophobic functional group. The left figure in FIG. 3 is the measurement result of the water contact angle of the perovskite membrane according to this embodiment in which an ionic liquid is used when producing the perovskite membrane. The right figure in FIG. 3 is the measurement result of the water contact angle of the perovskite membrane according to the comparative example in which no ionic liquid is used when producing the perovskite membrane. As shown in FIG. 3, the water contact angle of the perovskite membrane 10 according to this embodiment having the hydrophobic functional group H exceeds 70°, whereas the water contact angle of the perovskite membrane 10 according to the comparative example having no hydrophobic functional group H is about 50°.

[0018] The hydrophobic functional group H may be a hydrophobic group derived from a surfactant or a hydrophobic group derived from an ionic liquid. For example, the ionic liquid binds to the perovskite through an N-H bond. When the ionic liquid binds to the perovskite membrane, the hydrophobic group of the ionic liquid is exposed on the surface of the perovskite membrane 10.

[0019] Further, the perovskite membrane 10 according to this embodiment has few holes confirmed when viewed in plan from the stacking direction. The holes are confirmed as black dots in a scanning electron microscope (SEM) image. The average density of the holes in the perovskite membrane 10 according to this embodiment is, for example, 0.2 holes / μm 2 or less, and preferably 1.0 hole / μm 2 or less, and more preferably 0.5 hole / μm 2 or less.

[0020] The average density of the holes is obtained by the following procedure. First, using a scanning electron microscope (SEM), take three images at a magnification of 10,000 times. Then, in each image, check the number of holes, and divide the number of holes by the area of the image to obtain the hole density in each image. By obtaining the average value of the hole densities obtained in each image, the average density of the holes is obtained.

[0021] Next, a method for manufacturing the perovskite film 10 according to the present embodiment will be described. FIG. 4 is a flowchart of the method for manufacturing the perovskite film according to the present embodiment. The method for manufacturing the perovskite film according to the present embodiment includes a mixed solution preparation step S1, a base layer preparation step S2, a coating step S3, a homogenization step S4, and an annealing step S5.

[0022] FIG. 5 is a diagram for explaining the mixed solution preparation step S1 of the method for manufacturing the perovskite film according to the present embodiment. In the mixed solution preparation step S1, an ionic liquid 2 is added to the perovskite precursor solution 1.

[0023] The perovskite precursor solution 1 contains a precursor substance that forms a perovskite crystal. The perovskite precursor solution 1 contains, for example, an alkylammonium halide and a metal halide as precursor substances. The alkylammonium halide is, for example, CH3NH3I, CH3NH3Br, CH4N2HI. The metal halide is, for example, PbI2, PbBr2.

[0024] The solvent constituting the perovskite precursor solution 1 is, for example, an amide-based solvent, butyrolactone, dimethyl sulfoxide (DMSO). The amide-based solvent is, for example, dimethylformamide (DMF), diethylformamide (DEF), diethylacetamide (DMAC), N-methylpyrrolidone (MPD), tetramethylurea (TMU), hexamethylphosphoric triamide (HMPA).

[0025] Ionic liquid 2 is a salt that remains liquid even below 100 °C, obtained by combining a cation and an anion. Since an ionic liquid consists only of ions, it has strong electrostatic interactions and is characterized by non-volatility and non-flammability. Ionic liquid 2 is, for example, an imidazolium salt, pyridinium salt, ammonium salt, pyrrolidinium salt, phosphonium salt, or sulfonium salt. The imidazolium cation that forms an imidazolium salt is, for example, 1-hexyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-ethyl-2,3-dimethylimidazolium, or 1-dodecyl-3-methylimidazolium. The salt is, for example, a halide salt, tetrafluoroborate, hexafluorophosphate, acetate, hydrogensulfate, alkyl sulfate, tosylate, methanesulfonate, or the like. The ionic liquid is, for example, 1-hexyl-3-methylimidazolium chloride (HMImCl) or 1-hexyl-3-methylimidazolium iodide (HMImI).

[0026] By adding ionic liquid 2 to perovskite precursor solution 1, a mixed solution is obtained. The mass ratio of ionic liquid 2 in the mixed solution is preferably 0.17 wt% or more and 0.83 wt% or less, more preferably 0.33 wt% or more and 0.66 wt% or less, and even more preferably 0.42 wt% or more and 0.58 wt% or less. When the mass ratio of ionic liquid 2 in the mixed solution is within the above range, when the perovskite structure crystallizes, the crystallization of the crystals is suppressed from becoming finer, and a perovskite film with a large grain size can be obtained. For example, if the concentration of ionic liquid 2 contained in the mixed solution is high, the crystal nuclei may be micellized by the ionic liquid. On the other hand, if the concentration of ionic liquid 2 contained in the mixed solution is sufficiently low, the risk that the crystal growth of fine particles is inhibited by micellization decreases.

[0027] The mixed solution may also be prepared in several steps. For example, a solution with an ionic liquid added as the first solution may be prepared, a solution without an ionic liquid added as the second solution may be prepared, and these may be mixed to prepare the mixed solution. The first solution is, for example, a solution obtained by adding 1-hexyl-3-methylimidazolium chloride (HMImCl) as an ionic liquid to CH3NH3I and PbI2. The solvent of the first solution is, for example, dimethylformamide. The second solution is, for example, a solution obtained by adding CH4N2HI and PbI 2、 PbBr2 to a mixed solvent of dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The mass ratio of the ionic liquid in the first solution is preferably, for example, 1 wt% or more and 5 wt% or less, more preferably 2 wt% or more and 4 wt% or less, and even more preferably 2.5 wt% or more and 3.5 wt% or less.

[0028] Alternatively, instead of the ionic liquid, any additive having a hydrophobic functional group may be used. Examples of the additive include phenethylammonium chloride (PEACl), potassium thiocyanate (KSCN), guanabenz acetate, L-α-phosphatidylcholine, tetradecyldimethyl(3-sulfopropyl)ammonium hydroxide inner salt (TAH), etc.

[0029] FIG. 6 is a schematic diagram for explaining the underlayer preparation step S2 of the method for manufacturing a perovskite film according to this embodiment. In the underlayer preparation step S2, a substrate 3 having an underlayer on which a perovskite film is to be formed is prepared. The underlayer preparation step S2 is not an essential step and may not be performed. For example, a commercially available substrate may be used as it is.

[0030] The substrate 3 having an underlayer is, for example, glass on which a transparent conductive film is formed. The transparent conductive film is, for example, indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide, fluorine-doped tin oxide (FTO), titanium oxide, etc.

[0031] First, it is preferable to ultrasonically clean the substrate 3 having the underlayer. In ultrasonic cleaning, for example, ultrapure water cleaning, acetone cleaning, and ethanol cleaning are performed.

[0032] Next, the substrate 3 with the underlayer is subjected to plasma treatment. In the plasma treatment, oxygen plasma is irradiated onto the surface of the underlayer of the substrate 3. When the underlayer is surface-treated, impurities adhering to the surface of the underlayer are removed, and the activity of the surface of the underlayer is enhanced. When the surface of the underlayer is plasma-treated, the wettability of the mixed solution with respect to the underlayer is improved. The plasma treatment is performed, for example, at room temperature for 20 minutes under the conditions of 115 V, 18 W, and a vacuum degree of 0.1 MPa.

[0033] FIG. 7 is a schematic diagram for explaining an example of the coating step S3 of the method for manufacturing a perovskite film according to the present embodiment. In the coating step S3, the mixed solution 4 is applied onto the underlayer by a predetermined coating method to form a coating film 8.

[0034] Here, the predetermined coating method refers to a bar coating method, a blade coating, a slot die coating, a spray coating, a gravure coating, a reverse coating, and a comma coating. When the coating film 8 is formed by these predetermined coating methods, the most frequent grain size of the grains G of the perovskite film 10 becomes larger. The reason for this is not clear, but it is considered that gentle removal of the solvent promotes grain growth of the grains G and increases the size of the grains G. When the spin coating method that forcibly removes the solvent by rotation is used, the size of the grains G becomes smaller.

[0035] FIG. 7 is a specific example when a predetermined coating method is bar coating. First, the mixed solution 4 is dropped onto the dummy substrate 5. The dummy substrate 5 is not particularly limited. Next, the bar 6 is brought into contact with the mixed solution 4 dropped on the dummy substrate 5, and the mixed solution 4 is adhered to the surface of the bar 6. Next, the bar 6 is installed with respect to the substrate 3 with an underlayer, which is the object to be coated. The support 7 may be disposed on the side surface of the substrate 3 with an underlayer so that the bar 6 does not tilt during bar coating. Next, the bar 6 is translated with respect to the substrate 3 with an underlayer. For example, when the apparatus is tilted, the bar 6 rolls. Here, the bar 6 is moved by tilting the apparatus, but the bar 6 may be simply translated. The mixed solution 4 adhered to the bar 6 adheres to the surface of the underlayer, and the coating film 8 is formed.

[0036] FIG. 8 is a schematic diagram for explaining the homogenization step S4 of the method for manufacturing a perovskite film according to the present embodiment. The homogenization step S4 is not an essential step, but it is preferably performed. In the homogenization step S4, before annealing the coating film 8, it is held for a certain period of time. Holding for a certain period of time means that heating or the like is not performed while maintaining the relative positional relationship of the coating film 8 with respect to the substrate 3 with an underlayer, and the coating film 8 may move integrally with the substrate 3 with an underlayer. The homogenization step is preferably performed for 5 minutes or more, more preferably 5 minutes or more and 10 minutes or less.

[0037] FIG. 9 is a comparison image of perovskite films of an example in which the homogenization step S4 is performed and an example in which it is not performed. In FIG. 9, the upper two samples are perovskite films in which the homogenization step S4 is not performed, and the lower two samples are perovskite films in which the homogenization step S4 is performed. The upper two samples in which the homogenization step S4 is not performed are lighter in color and have more unevenness than the lower two samples in which the homogenization step S4 is performed. That is, the quality of the upper two samples in which the homogenization step S4 is not performed is worse than that of the lower two samples in which the homogenization step S4 is performed.

[0038] In the homogenization step S4, as shown in FIG. 8, before holding the substrate 3 with the coating film 8 for a certain period of time, the liquid pool 8a formed in the coating film 8 may be removed. By removing the liquid pool 8a, the unevenness of the coating film 8 can be further suppressed.

[0039] FIG. 10 is a schematic diagram for explaining the annealing step S5 of the method for manufacturing a perovskite film according to the present embodiment. In the annealing step, the coating film 8 is annealed. For example, the coating film 8 is annealed by heating the substrate 3 with the coating film 8 using a heater 9. The annealing temperature is, for example, 100° C. or higher, and the annealing time is, for example, 1 hour or longer. When the coating film 8 is annealed, the solvent is removed from the coating film 8, and at the same time, the perovskite crystallizes to obtain the perovskite film 10.

[0040] The perovskite film 10 has a cubic crystal structure represented by the general formula ABX3. Different atoms or molecules are arranged at the vertices (A sites) of the cube, the centers of each face (X sites) of the cube, and the center (B site) of the cube, respectively. For example, materials such as CH3NH3 and CH5N2 are used for the A site. For example, materials such as Pb, Sn, and Ge are used for the B site. For example, materials such as I, Cl, and Br are used for the X site. The perovskite film 10 is, for example, CsFAMA (general formula: Cs 0.05 (FA 0.83 MA 0.17 ) 0.95 Pb(I 0.83 Br 0.17 )3) perovskite film.

[0041] The preparation process S1 of the mixed solution, the underlying layer preparation process S2, the coating process S3, the homogenization process S4, and the annealing process S5 can be carried out even in an atmosphere with a humidity of 40% or more. The perovskite film 10 is vulnerable to moisture. Therefore, generally, the perovskite film 10 is produced in an inert gas atmosphere such as nitrogen or argon. On the contrary, in the method for producing a perovskite film according to the present embodiment, adding the ionic liquid 2 into the mixed solution improves the hydrophobicity of the perovskite film. Further, in the perovskite film by the bar coating method, the rapid volatilization of the solvent is suppressed, and the film quality of the produced perovskite film 10 can be enhanced. As a result, even under the film formation conditions of an atmosphere with a humidity of 40% or more, the perovskite film 10 can be appropriately produced.

[0042] The method for manufacturing a perovskite film according to the present embodiment is carried out using a coating method that is easy to apply to roll-to-roll methods such as the bar coating method. The spin coating method can only form a perovskite film in a single-wafer manner. The roll-to-roll method is easier to increase the area and has a lower process cost compared to the single-wafer spin coating method.

[0043] The perovskite film 10 according to the present embodiment has high durability. This is presumably because it is difficult for moisture to penetrate into the perovskite film 10.

[0044] The perovskite film 10 according to the present embodiment has a large size of the grains G and few grain boundaries between the grains G.

[0045] FIG. 11 is a scanning electron microscope image of a perovskite film according to the first comparative example. The first comparative example is different from the method for manufacturing a perovskite film according to the present embodiment in that the coating film 8 is formed by the spin coating method. The perovskite film according to the first comparative example has a most frequent grain size of the grains of 1 μm or less. The perovskite film according to the first comparative example has a smaller grain size and more grain boundaries of the grains than the perovskite film 10 according to the present embodiment.

[0046] Water (H2O) penetrates from the grain boundaries of the grains G that make up the perovskite film 10. Since the perovskite film 10 according to this embodiment has large-sized grains G, there are few grain boundaries between the grains G, and there are few paths for water to penetrate.

[0047] In addition, the use of an ionic liquid when manufacturing the perovskite film 10 according to this embodiment is one of the factors that enhance the durability of the perovskite film 10. FIG. 12 is a schematic diagram for explaining one of the reasons why the perovskite film 10 according to this embodiment has high durability.

[0048] As shown in FIG. 12, when at least the grain boundaries of a plurality of grains G contain a hydrophobic functional group H, the hydrophobic functional group H repels water and prevents water from entering the grain boundaries of the grains G. A part of the ionic liquid forms an N-H bond with the perovskite and functions as the hydrophobic functional group H. At least at the grain boundaries of the grains G, the hydrophobic functional group H of the ionic liquid 2 is exposed. Since the hydrophobic functional group H has low affinity for water, it prevents water from entering the grain boundaries of the grains G.

[0049] FIG. 13 is a scanning electron microscope image of the perovskite film according to the second comparative example. FIG. 14 is a scanning electron microscope image of the perovskite film according to the third comparative example. The second comparative example is different from the perovskite film according to this embodiment in that no ionic liquid is used. The third comparative example is different from the perovskite film according to the first comparative example in that no ionic liquid is used.

[0050] As shown in FIGS. 13 and 14, when no ionic liquid is used, pinholes occur in the perovskite film. The pinholes serve as paths for water to penetrate and reduce the durability of the perovskite film. That is, the fact that an ionic liquid is used in the manufacturing method of the perovskite film 10 according to this embodiment is also one of the factors that enhance the durability of the perovskite film 10.

[0051] Note that, as shown in FIGS. 13 and 14, even when no ionic liquid is used, a difference in grain size due to a difference in the coating method was confirmed.

[0052] The perovskite film according to this embodiment can be applied to various elements such as a photoelectric conversion element, a piezoelectric conversion element, and a thermoelectric conversion element. Hereinafter, as an example of an application example of the perovskite film, an example in which the perovskite film is used for a thin-film solar cell is shown.

[0053] FIG. 15 is a side view of a thin-film solar cell 100 according to this embodiment. The thin-film solar cell 100 includes a substrate 101, an electron transport layer 102, a light-emitting layer 103, a hole transport layer 104, an electrode layer 105, an electrode layer 106, and an electrode layer 107. The electrode layer 105 is a positive electrode layer, and the electrode layers 106 and 107 are negative electrode layers. In FIG. 15, the electron transport layer 102, the light-emitting layer 103, and the hole transport layer 104 are sequentially stacked on the substrate 101. The positional relationship between the electron transport layer 102 and the hole transport layer 104 may be reversed.

[0054] The substrate 101 is not particularly limited as long as it can support the structure composed of the above-mentioned layers. The substrate 101 is preferably a transparent conductive substrate having excellent light transmittance. For example, it is more preferable if the transmittance of visible light is 90% or more. Further, the substrate 101 is preferably flexible. Examples of the constituent material of the substrate 101 include glass, plastic film, and the like. As the resin constituting the plastic film, for example, polyethylene terephthalate, polyethylene naphthalate, or the like can be used.

[0055] The electrode layer 106 is, for example, a transparent electrode. The electrode layer 106 is on the substrate 101. A known electrode layer 106 can be applied. The electrode layer 106 is, for example, a conductive transparent oxide film, graphene, carbon nanotube, or conductive polymer film. The conductive transparent oxide is, for example, fluorine-doped tin oxide (FTO), indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide, tin oxide, titanium oxide, or the like.

[0056] The electron transport layer 102 is a layer that collects the electrons generated in the power generation layer 103 and efficiently transports them toward the electrode layer 106. As the constituent material of the electron transport layer 102, for example, inorganic materials such as zinc oxide, titanium oxide, tin oxide, niobium oxide, aluminum oxide, or organic materials such as PCBM, fullerene C 60 and other n-type semiconductors such as organic materials can be mentioned.

[0057] The power generation layer 103 is a layer (photoelectric conversion layer) that generates electricity upon receiving light. The power generation layer 103 is composed of a material having a perovskite structure. The above-described perovskite film 10 can be applied to the power generation layer 103.

[0058] The hole transport layer 104 is a layer that collects the holes generated in the power generation layer 103 and efficiently transports them toward the electrode layer 105. As the constituent material of the hole transport layer 104, for example, inorganic materials such as nickel oxide, copper iodide, copper oxide, copper sulfide, or organic materials such as Spiro-OMeTAD, PTAA, PEDOT:PSS (a composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonic acid (PSS)) and other p-type semiconductors can be mentioned.

[0059] The electrode layers 105 and 107 are each composed of, for example, a metal such as gold, silver, aluminum, or a conductive material such as an organic conductive ink such as PEDOT:PSS.

[0060] The thin-film solar cell 100 according to the present embodiment is obtained by sequentially laminating the layers constituting the thin-film solar cell 100.

[0061] For example, first, a substrate 101 on which the electrode layer 106 is formed is prepared. For example, the electrode layer 106 is FTO and the substrate 101 is glass. The substrate 101 with the electrode layer 106 may be purchased as a commercial product.

[0062] Next, after subjecting the substrate 101 with the electrode layer 106 to ultrasonic cleaning and plasma treatment, an electron transport layer 102 is formed on one surface of the electrode layer 106. The electron transport layer 102 can be obtained, for example, by sequentially forming a dense TiO2 layer and a TiO2 nanoparticle layer by spin coating. Each layer is obtained by repeating spin coating and annealing a plurality of times. This electron transport layer 102 corresponds to the underlayer in the manufacturing method of the perovskite film described above.

[0063] Next, the surface of the electron transport layer 102 is subjected to plasma treatment. This plasma treatment corresponds to the plasma treatment for the underlayer in the manufacturing method of the perovskite film described above.

[0064] On the surface of the plasma-treated electron transport layer 102, a power generation layer 103 is formed by a predetermined coating method. The predetermined coating method is the coating method used in the manufacturing method of the perovskite film described above. The predetermined coating method is, for example, the bar coating method. The power generation layer 103 is a perovskite film, and the power generation layer 103 is formed according to the manufacturing method of the perovskite film described above.

[0065] A hole transport layer 104 is formed on one surface of the power generation layer 103. For example, Spiro-OMeTAD is spin-coated as the hole transport layer 104.

[0066] Next, a part of the electron transport layer 102, the power generation layer 103, and the hole transport layer 104 is cut out, and electrode layers 105 and 107 are formed thereon. The electrode layers 105 and 107 are, for example, gold and can be formed by vapor deposition. By such a procedure, the thin-film solar cell according to the present embodiment can be manufactured.

[0067] The thin-film solar cell according to the present embodiment is excellent in output characteristics and durability. The reason why the thin-film solar cell according to the present embodiment is excellent in durability is that, as described above, the perovskite film constituting the power generation layer 103 has high durability.

[0068] Also, the reason why the output characteristics of the thin-film solar cell according to the present embodiment are excellent is considered to be that the grains of the power generation layer 103 are large.

[0069] FIG. 16 is a schematic diagram for explaining the characteristics of the thin-film solar cell according to the present embodiment. FIG. 16(a) shows the configuration of the thin-film solar cell of the first comparative example, which is a thin-film solar cell in which the power generation layer 103 is formed by a spin coating method. FIG. 16(b) shows the configuration of the thin-film solar cell according to the present embodiment, which is a thin-film solar cell in which the power generation layer 103 is formed by a bar coating method. The crystal grain size (grain size) of the power generation layer 103 (FIG. 16(b)) of the present embodiment is larger than the crystal grain size (grain size) of the power generation layer 103 (FIG. 16(a)) formed by the spin coating method.

[0070] Due to irradiation such as sunlight L, the electrons E and holes H generated in the power generation layer 103 flow toward the electrode layer 106 and the electrode layer 105, respectively, but a part of them is trapped at the grain boundaries 103C existing in the flow path. Since the trapped electrons E and holes H recombine, the number of electrons E reaching the electrode layer 106 and the number of holes H reaching the electrode layer 105 decrease, resulting in a loss of output current accordingly. In the power generation layer 103 of the present embodiment shown in FIG. 16(b), the crystal particle size is large and the distribution density (volume density) of the grain boundaries 103C is low. Therefore, in the power generation layer 103 of the present embodiment, there are few carriers trapped and recombined at the grain boundaries 103C. The more carriers that reach the electrodes without recombination, the larger the output current, and the output characteristics (PCE) of the thin-film solar cell are improved.

[0071] As described above, the preferred embodiments of the present invention have been described in detail. However, the present invention is not limited to specific embodiments, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

Example

[0072] (First Example) In the first example, each configuration shown in FIG. 15 was used to fabricate the following thin-film solar cell. Substrate 101: Glass Electrode layer 106: FTO Electron transport layer 102: TiO2 Power generation layer 103: Cs0.05 (FA 0.83 MA 0.17 ) 0.95 Pb(I 0.83 Br 0.17 )3 hole transport layer 104: Spiro-OMeTAD Electrode layers 105, 107: Gold

[0073] The power generation layer 103 of the first embodiment was prepared by coating a mixed solution, which was obtained by mixing a first solution containing an ionic liquid added at a mass ratio of 3 wt% and a second solution without the addition of the ionic liquid, using the bar coating method and then annealing it.

[0074] The ionic liquid was 1-hexyl-3-methylimidazolium chloride (HMImCl). The perovskite precursors contained in the first solution were CH3NH3I and PbI2, and the solvent was dimethylformamide (DMF). The perovskite precursors contained in the second solution were CH4N2HI, PbI 2、 PbBr2, and CsI, and the solvent was a mixed solvent of dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). 200 μl was taken out from this first solution and 373 μl was taken out from the second solution and mixed to obtain Cs 0.05 (FA 0.83 MA 0.17 ) 0.95 Pb(I 0.83 Br 0.17 )3-containing mixed solution. The mass ratio of the ionic liquid in the mixed solution was 0.5 wt%.

[0075] Before coating the mixed solution by the bar coating method, the surface of the electron transport layer 102 was treated with oxygen plasma. The plasma treatment was carried out at room temperature for 20 minutes under the conditions of 115 V, 18 W, and a vacuum degree of 0.1 MPa. Also, after coating the mixed solution by the bar coating method, a standing time of 5 minutes was ensured as a homogenization process. The coated film was annealed at 100 °C for 1 hour. The production of the power generation layer 103 was carried out in an atmosphere with a humidity of 40% or more.

[0076] (Second Embodiment) The second embodiment is different from the first embodiment in that plasma treatment on the surface of the electron transport layer 102 and a standing step when fabricating the power generation layer were not performed. Other conditions were the same as those in the first embodiment.

[0077] The above-mentioned FIG. 9 shows the comparison results between the first embodiment and the second embodiment. As shown in FIG. 9, the power generation layer according to the first embodiment (the lower two samples in FIG. 9) had less unevenness and was more uniform than the power generation layer according to the second embodiment (the upper two samples in FIG. 9).

[0078] (First Comparative Example) The first comparative example is different from the first embodiment in that the power generation layer 103 was fabricated by a two-step spin coating method. Other conditions were the same as those in the first embodiment, and a thin film solar cell was fabricated.

[0079] First, in the first step, a solution obtained by adding an ionic liquid to a perovskite precursor solution was spin-coated. The mass ratio of the ionic liquid at this time was, for example, 1 wt%. Next, in the second step, a solution containing CsFAMA perovskite was spin-coated. Then, the coated layer was annealed at 100 °C for 1 hour.

[0080] (Second Comparative Example) The second comparative example is different from the first embodiment in that no ionic liquid was used when fabricating the power generation layer 103. Other conditions were the same as those in the first embodiment, and a thin film solar cell was fabricated.

[0081] (Third Comparative Example) The third comparative example is different from the first comparative example in that no ionic liquid was used when fabricating the power generation layer 103. Other conditions were the same as those in the first comparative example, and a thin film solar cell was fabricated.

[0082] SEM images of the surfaces of the power generation layers of the first embodiment, the first comparative example, the second comparative example, and the third comparative example were observed. The power generation layer of the first embodiment corresponds to FIG. 1. The power generation layer of the first comparative example corresponds to FIG. 11. The power generation layer of the second comparative example corresponds to FIG. 13. The power generation layer of the third comparative example corresponds to FIG. 14.

[0083] In the second and third comparative examples that did not use an ionic liquid, pinholes were formed in the power generation layer 103. Also, the grain size of the power generation layer 103 in the first example and the second comparative example fabricated by the bar coating method was larger than the grain size of the power generation layer 103 in the first and third comparative examples fabricated by the spin coating method.

[0084] Also, FIG. 17 shows the measurement results of X-ray diffraction (XRD) of the power generation layers of the first example and the second comparative example. In FIG. 17, "w / IL" indicates that an ionic liquid was used and corresponds to the result of the first example. In FIG. 17, "w / o IL" indicates that no ionic liquid was used and corresponds to the result of the second comparative example. As shown in FIG. 17, in the second comparative example that did not use an ionic liquid, a peak attributed to PbI2 was confirmed near 2θ = 13°. PbI2 is a decomposition product of perovskite. That is, it can be seen that the film quality of the power generation layer according to the first example is superior to that of the power generation layer according to the second comparative example.

[0085] Also, FIG. 18 shows the characteristics of the thin-film solar cells of the first example and the second comparative example. In FIG. 18, "w / IL" indicates that an ionic liquid was used and corresponds to the result of the first example. In FIG. 18, "w / o IL" indicates that no ionic liquid was used and corresponds to the result of the second comparative example. As shown in FIG. 18, the output characteristics (PCE) of the thin-film solar cell were superior in the first example compared to the second comparative example.

[0086] Figure 19 shows the results of measuring the durability of thin-film solar cells using perovskite films of the first embodiment, the first comparative example, the second comparative example, and the third comparative example. In Figure 19, "Bar coat w / IL" indicates that an ionic liquid was used and the power generation layer was formed by the bar coating method, corresponding to the results of the first embodiment. "Bar coat w / o IL" indicates that the power generation layer was formed by the bar coating method without using an ionic liquid, corresponding to the results of the second comparative example. "Spin coat w / IL" indicates that an ionic liquid was used and the power generation layer was formed by the spin coating method, corresponding to the results of the first comparative example. "Spin coat w / o IL" indicates that the power generation layer was formed by the spin coating method without using an ionic liquid, corresponding to the results of the third comparative example. The horizontal axis in Figure 19 represents the operating time of the thin-film solar cell, and the vertical axis represents the change in PCE when the PCE of the initial thin-film solar cell is normalized to 1. The PCE in this durability test was obtained by leaving the unsealed element in a dark atmosphere with a relative humidity (RH) of 40% or more and 70% or less and measuring the I-V measurement under sunlight irradiation approximately every week.

[0087] As shown in Figure 19, the thin-film solar cells according to the first embodiment and the second comparative example produced by the bar coating method were more durable than the thin-film solar cells according to the first comparative example and the third comparative example produced by the spin coating method. This is presumably because the grain size of the power generation layer increased and the grain boundaries through which water could penetrate decreased by using the bar coating method. In addition, the first embodiment containing an ionic liquid was more durable than the second comparative example not containing an ionic liquid. This is presumably because pinholes were less likely to be formed in the power generation layer by containing the ionic liquid, and the hydrophobic group of the ionic liquid prevented the intrusion of water.

[0088] (Third Embodiment to Sixth Embodiment) Examples 3 to 6 differ from Example 1 in that the concentration of the ionic liquid was changed when forming the power generation layer. Other conditions were the same as in Example 1, and thin-film solar cells were fabricated using the same procedures. In each of Examples 3 to 6, the mass ratio of the ionic liquid in the first solution and the mass ratio of the ionic liquid in the mixed solution are as follows. For comparison, the mass ratios of the ionic liquid in Comparative Example 2 and Example 1 are also shown below.

[0089] Second Comparative Example Mass ratio of ionic liquid in the first solution: 0 wt% Mass ratio of ionic liquid in the mixed solution: 0 wt%

[0090] Third Example Mass ratio of ionic liquid in the first solution: 1 wt% Mass ratio of ionic liquid in the mixed solution: 0.17 wt%

[0091] First Example Mass ratio of ionic liquid in the first solution: 3 wt% Mass ratio of ionic liquid in the mixed solution: 0.5 wt%

[0092] Fourth Example Mass ratio of ionic liquid in the first solution: 5 wt% Mass ratio of ionic liquid in the mixed solution: 0.83 wt%

[0093] Fifth Example Mass ratio of ionic liquid in the first solution: 7 wt% Mass ratio of ionic liquid in the mixed solution: 1.17 wt%

[0094] Sixth Example Mass ratio of ionic liquid in the first solution: 10 wt% Mass ratio of ionic liquid in the mixed solution: 1.66 wt%

[0095] Figure 20 shows the output characteristics (PCE) of the thin-film solar cells of the first embodiment, the third to sixth embodiments, and the second comparative example. The specific values of PCE in each of the embodiments and comparative examples were as follows. Second Comparative Example: 12.59% Third Embodiment: 12.67% First Embodiment: 15.28% Fourth Embodiment: 13.18% Fifth Embodiment: 11.27% Sixth Embodiment: 10.64%

[0096] As shown in Figure 19, it was confirmed that when the mass ratio of the ionic liquid in the first solution was 3 wt% (the mass ratio of the ionic liquid in the mixed solution was 0.5 wt%), the output characteristics of the thin-film solar cell were most improved.

Description of Reference Numerals

[0097] 1 Perovskite precursor solution 2 Ionic liquid 3 Substrate 4 Mixed solution 5 Dummy substrate 6 Bar 7 Support 8 Coating film 9 Heater 10 Perovskite film 100 Thin-film solar cell 101 Substrate 102 Electron transport layer 103 Light-absorbing layer 103C Grain boundary 104 Hole transport layer 105, 106, 107 Electrode layer G Grain H Hydrophobic functional group S1 Mixed solution preparation process S2 Underlayer preparation process S3 Coating process S4 Homogenization process S5 Annealing process

Claims

1. When viewed in plan view from the stacking direction, it has a plurality of grains, A perovskite film in which the most frequent grain size of the plurality of grains is 1 μm or more.

2. Having a hydrophobic functional group, and the hydrophobic functional group is at least at the grain boundaries of the plurality of grains, the perovskite film according to Claim 1.

3. The perovskite film according to Claim 2, wherein the hydrophobic functional group is a hydrophobic group derived from an ionic liquid.

4. Having holes confirmed when viewed in plan view from the stacking direction, The average density of the holes is 0.2 holes / μm ２ The perovskite film according to claim 1, which is as follows.

5. A thin-film solar cell including the perovskite film according to Claim 1 as a light-emitting layer.

6. A step of preparing a mixed solution by adding an ionic liquid to a perovskite precursor solution, A step of applying the mixed solution to a base layer by bar coating, blade coating, slot die coating, spray coating, gravure coating, reverse coating, comma coating to form a coating film, A step of annealing the coating film, a method for manufacturing a perovskite film.

7. The method for manufacturing a perovskite film according to Claim 6, wherein the coating film is annealed after 5 minutes or more have elapsed since the coating film was formed.

8. The method for manufacturing a perovskite film according to Claim 6, wherein the surface of the base layer is plasma-treated before forming the coating film.

9. The method for manufacturing a perovskite film according to Claim 6, wherein the film-forming atmosphere is air with a humidity of 40% or more.

10. The method for manufacturing a perovskite film according to Claim 6, wherein the mass ratio of the ionic liquid in the mixed solution is 0.17 wt% or more and 0.83 wt% or less.

11. A method for manufacturing a thin-film solar cell, in which a light-emitting layer having a perovskite structure is formed using the method for manufacturing a perovskite film according to any one of Claims 6.

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