Solar cell and solar cell manufacturing method

Abstract

To provide a solar cell having high photoelectric conversion efficiency and a method for manufacturing the same.SOLUTION: A solar cell 1 includes: an anode layer 21; a hole transportation layer 22; a photoelectric conversion layer 23 that contains a perovskite compound and is in contact with the hole transportation layer 22; an electron transportation layer 24; and a cathode layer 26 in order. The hole transportation layer 22 is a monomolecular film formed by two kinds of carbazole compound molecules. With the two kinds of carbazole compound molecules, the same carbon chain is bonded to a nitrogen atom of carbazole, and only a substituent of a benzene ring is different.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] The use of solar cells is expanding as an energy source with a low environmental impact. When installing solar cells in various devices, vehicles, buildings, etc., the available installation area is limited, making the photoelectric conversion efficiency of the solar cell important. Perovskite solar cells, which use organic materials, are being researched as solar cells with high photoelectric conversion efficiency. A basic perovskite solar cell is composed of a substrate and a first electrode (anode or cathode), a hole transport layer (hole transport layer or electron transport layer), a photoelectric conversion layer (perovskite layer), an electron transport layer (electron transport layer or hole transport layer), and a first electrode (cathode or anode) stacked in this order. Furthermore, it is known that the photoelectric conversion efficiency can be improved by providing a first buffer layer between the first electrode and the hole transport layer or a second buffer layer between the electron transport layer and the second electrode.

[0003] Patent Document 1 also describes a solar cell in which a monomolecular film is formed on the surface of a first electrode laminated on a substrate, a photoelectric conversion layer is directly laminated on the monomolecular film, and an electron transport layer and a transparent electrode are further laminated. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-141165 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 recommends adding a hole transport layer between the monolayer and the photoelectric conversion layer to enable more efficient charge extraction. The hole transport layer improves photoelectric conversion efficiency by blocking electrons and selectively allowing holes to reach the electrode, but it also limits photoelectric conversion efficiency by increasing the hole migration distance. In other words, the solar cell described in Patent Document 1 lacks sufficient hole selection (electron blocking) function of the monolayer, so the benefits of improving the electron blocking effect obtained by adding a hole transport layer outweigh the disadvantages of increasing the hole migration distance. Conversely, if a monolayer with high hole selectivity can be formed, it will function sufficiently as a hole transport layer, allowing the production of solar cells with higher photoelectric conversion efficiency.

[0006] An object of the present invention is to provide a solar cell with high photoelectric conversion efficiency and a method for manufacturing the same. [Means for solving the problem]

[0007] A solar cell according to one aspect of the present invention includes an anode layer, a hole transport layer, a photoelectric conversion layer containing a perovskite compound and in contact with the hole transport layer, an electron transport layer, and a cathode layer, in this order. The hole transport layer is a monomolecular film formed of two types of carbazole compound molecules, and the two types of carbazole compound molecules have the same carbon chain bonded to a nitrogen atom of carbazole and differ only in the substituents on their benzene rings.

[0008] In the above solar cell, the two types of carbazole compound molecules may be 2PACz and MeO-2PACz.

[0009] In the solar cell described above, the mass ratio of the 2PACz to the MeO-2PACz in the hole transport layer may be 1:99 or more and 60:40 or less.

[0010] In the solar cell described above, the area ratio of the 2PACz to the MeO-2PACz in the hole transport layer may be 1:99 or more and 50:50 or less.

[0011] A solar cell manufacturing method according to another embodiment of the present invention includes the steps of: laminating a conductive material on a substrate; applying a monolayer-forming material solution containing two types of self-assembled monolayer-forming molecules to the conductive material layer; drying the coating of the monolayer-forming material solution; and applying a photoelectric conversion layer-forming material solution to the dried coating of the monolayer-forming material solution, wherein the two types of carbazole compound molecules have the same carbon chain bonded to a nitrogen atom of carbazole and differ only in the substituents on the benzene ring.

[0012] In the above-described solar cell manufacturing method, the two types of carbazole compound molecules may be 2PACz and MeO-2PACz, and the mass ratio of the 2PACz to the MeO-2PACz in the photoelectric conversion layer-forming material solution may be 1:99 or more and 70:30 or less.

[0013] In the above-described method for manufacturing a solar cell, the photoelectric conversion layer forming material solution may be applied by spin coating. [Effects of the Invention]

[0014] According to the present invention, a solar cell with high photoelectric conversion efficiency can be provided. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic cross-sectional view showing a layer structure of a solar cell according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view showing the configuration of a hole transport layer in the solar cell of FIG. [Figure 3] 2 is a flowchart showing the steps of a method for manufacturing the solar cell of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. Fig. 1 is a schematic cross-sectional view showing the layer structure of a solar cell 1 according to one embodiment of the present invention.

[0017] Solar cell 1 comprises substrate 10, perovskite solar cell section 20 laminated on the surface of substrate 10, back surface collecting electrode 31 laminated on the back surface of substrate 10, front surface collecting electrode 32 partially laminated on the surface of perovskite solar cell section 20, and antireflection layer 33 covering the surfaces of perovskite solar cell section 20 and front surface collecting electrode 32. Light enters solar cell 1 through antireflection layer 33, and photoelectric conversion occurs in perovskite solar cell section 20.

[0018] The substrate 10 is a structure that supports the perovskite solar cell unit 20 and may have a photoelectric conversion function. Specifically, the substrate 10 may be, for example, a glass substrate or a resin sheet, but in the solar cell 1 of this embodiment, the substrate 10 is a crystalline silicon solar cell. In the solar cell 1, light incident through the antireflection layer 33 is first photoelectrically converted by the perovskite solar cell unit 20, and light of a wavelength that transmits through the perovskite solar cell unit 20 is photoelectrically converted by the crystalline silicon solar cell 10. Because the crystalline silicon solar cell has sufficient strength, it can function as the substrate 10 that supports the perovskite solar cell unit 20 and can improve photoelectric conversion efficiency by converting light that transmits through the perovskite solar cell unit 20 into electricity. In the solar cell 1, the perovskite solar cell unit 20 is stacked on the crystalline silicon solar cell that serves as the substrate 10, so that the crystalline silicon solar cell and the perovskite solar cell unit 20 are connected in series.

[0019] The base material 10 of this embodiment includes a semiconductor substrate 11, a first semiconductor layer 12 laminated on the front side of the semiconductor substrate 11, and a second semiconductor layer 13 laminated on the back side of the semiconductor substrate 11.

[0020] The perovskite solar cell section 20 includes, from the substrate 10 side, an anode layer 21, a hole transport layer 22, a photoelectric conversion layer 23, an electron transport layer 24, a buffer layer 25, and a cathode layer 26 in this order.

[0021] The anode layer 21 is one of the electrodes of the perovskite solar cell unit 20. The anode layer 21 can be formed of a transparent conductive oxide (TCO) that is conductive and optically transparent. Examples of the transparent conductive oxide that can be used to form the anode layer 21 include indium oxide, tin oxide, zinc oxide, titanium oxide, and composite oxides thereof. Among these, indium-based composite oxides containing indium oxide as the main component are preferred. Indium oxide is particularly preferred from the viewpoints of high conductivity and transparency. Furthermore, it is preferable to add a dopant to indium oxide to ensure reliability or higher conductivity. Examples of dopants include Sn, W, Zn, Ti, Ce, Zr, Mo, Al, Ga, Ge, As, Si, and S. For example, ITO (indium tin oxide), in which tin is added to indium oxide, is widely known.

[0022] The anode layer 21 can be laminated on the substrate 10 by, for example, sputtering or vacuum deposition. The thickness of the anode layer can be, for example, 5 nm or more and 100 nm or less when power is output from the perovskite solar cell unit 20 to the substrate 10 on its surface (when current flows in the thickness direction), and can be, for example, 500 nm or more and 1000 nm or less when power is output to wiring connected to the anode layer (when current flows in the surface direction).

[0023] The hole transport layer (HTL) 22 selectively transports holes generated in the photoelectric conversion layer 23 to the anode layer 21. The hole transport layer 22 is a self-assembled monolayer (SAM) formed from two types of carbazole compound molecules. The two types of carbazole compound molecules forming the hole transport layer 22 have the same carbon chain bonded to the nitrogen atom of the carbazole, and differ only in the substituents on the benzene ring (one of which may be hydrogen). The two types of carbazole compound molecules are preferably 2PACz ([2-(9H-Carbazol-9-yl)ethyl]phosphonic Acid) and MeO-2PACz ([2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic Acid). The self-assembled monolayers formed by two types of carbazole compound molecules, particularly 2PACz and MeO-2PACz, which have the same carbon chain bonded to the nitrogen atom of carbazole and differ only in the substituents on the benzene ring, have sufficient hole-selective function, so that even when the photoelectric conversion layer 23 is directly stacked on top of them, holes can be efficiently extracted while electrons are blocked.

[0024] The hole transport layer 22 can be formed by coating and drying a monolayer-forming material solution prepared by dissolving 2PACz and MeO-2PACz in an organic solvent such as ethanol or isopropanol. The total concentration of 2PACz and MeO-2PACz in the monolayer-forming material solution is about 0.3 mg / mL.

[0025] Considering only the wettability of the photoelectric conversion layer-forming material that forms the photoelectric conversion layer 23 and the hole selectivity with the perovskite layer due to the methoxy group, MeO-2PACz is considered to be advantageous. However, a monolayer-forming material solution containing only MeO-2PACz generates pinholes between the individual domains of the MeO-2PACz monolayer, which grow separately like crystal grains, and therefore fails to completely cover the surface of the anode layer 21. Therefore, by using a monolayer-forming material solution containing 2PACz in addition to MeO-2PACz, individual domains of 2PACz monolayer, which have a high degree of freedom in the shape of the individual domains, can be formed between the individual domains of the MeO-2PACz monolayer, as shown schematically in Figure 2, and pinholes in the hole-transport layer 22 can be reduced. In a monolayer-forming material solution containing 2PACz and MeO-2PACz, the growth of the monolayer of MeO-2PACz is dominant, and therefore the ratio of the area fraction of 2PACz to the area fraction of MeO-2PACz in the formed hole transport layer 22 is smaller than the ratio of the content of 2PACz to the content of MeO-2PACz in the monolayer-forming material solution.

[0026] The lower limit of the mass ratio of 2PACz to MeO-2PACz (2PACz proportion) in hole transport layer 22 is preferably 1:99, more preferably 10:90. Meanwhile, the upper limit of the mass ratio of 2PACz to MeO-2PACz is preferably 60:40, more preferably 55:45, even more preferably 50:50, and particularly preferably 30:70. By setting the mass ratio of 2PACz to MeO-2PACz within the above range, pinholes in hole transport layer 22 are reduced while wettability with a photoelectric conversion layer-forming material solution for forming photoelectric conversion layer 23 is improved, thereby enabling the photoelectric conversion layer 23 to be properly formed. This optimizes the generation of carriers (holes and electrons) by photoelectric conversion layer 23 and the transfer of holes from photoelectric conversion layer 23 to hole transport layer 22, thereby improving the photoelectric conversion efficiency of solar cell 1.

[0027] The lower limit of the mass ratio of 2PACz to MeO-2PACz in hole transport layer 22 is preferably 1:99, more preferably 10:90. On the other hand, the upper limit of the area ratio of 2PACz to MeO-2PACz is preferably 50:50, more preferably 30:70. By setting the area ratio of 2PACz to MeO-2PACz within the above range, pinholes in hole transport layer 22 are reduced while wettability with a photoelectric conversion layer-forming material solution for forming photoelectric conversion layer 23 is appropriate, thereby enabling photoelectric conversion layer 23 to be appropriately formed.

[0028] The lower limit of the mass ratio of 2PACz to MeO-2PACz in the monolayer-forming material solution is preferably 1:99, more preferably 30:70. On the other hand, the upper limit of the mass ratio of 2PACz to MeO-2PACz is preferably 70:30, more preferably 60:40. By setting the mass ratio of 2PACz to MeO-2PACz in the monolayer-forming material solution at or above the lower limit, a 2PACz monolayer can be formed between MeO-2PACz monolayers, thereby reducing pinholes in the hole transport layer 22. Furthermore, by setting the mass ratio of 2PACz to MeO-2PACz in the monolayer-forming material solution at or below the upper limit, 2PACz does not inhibit the growth of the MeO-2PACz monolayer, ensuring the formation of the hole transport layer 22.

[0029] The photoelectric conversion layer 23 absorbs light and generates carriers. The photoelectric conversion layer 23 includes a perovskite compound and is formed in contact with the hole transport layer 22. As the perovskite compound, a compound represented by ABX3 can be used, which includes an organic atom A including at least one of a monovalent organic ammonium ion and an amidinium ion, a metal atom B that generates a divalent metal ion, and a halogen atom X including at least one of an iodide ion I, a bromide ion Br, a chloride ion Cl, and a fluoride ion F. In particular, when the photoelectric conversion layer 23 is formed by a vapor deposition method (dry process), the organic atom A is preferably methylammonium MA (CH3NH3), the metal atom B is preferably lead Pb, and the halogen atom X is preferably at least one of an iodide I, a bromide ion Br, and a chloride ion Cl.

[0030] Specifically, preferred perovskite compounds include methylammonium lead halide MAPbX3 (CH3NH3PbX3), MAPbI3, MAPbBr3, MAPbCl3, etc. Note that the halogen atom X may contain multiple types. Examples of perovskite compounds containing iodide I and other halogen atoms X include methylammonium lead iodide MAPbI y X (3-y) (CH3NH3PbI y X (3-y) ), MAPbI y Br (3-y) , MAPbI y Cl (3-y) etc. (y is any positive integer).

[0031] When the perovskite compound is methylammonium lead halide (MAPbX3(CH3NH3PbX3)), the photoelectric conversion layer 23 can be formed by sequentially depositing a lead halide (PbX2) material and a methylammonium halide (MAX) material, and reacting the thin films of these materials at a reaction temperature. For example, when the perovskite compound is methylammonium lead iodide (MAPbI y X (3-y) (CH3NH3PbI y X(3-y) In the case of the above, the photoelectric conversion layer 23 is formed by sequentially depositing a lead halide (PbX2) material and a methylammonium iodide (MAI) material, and reacting the resulting thin films at a reaction temperature. Alternatively, the photoelectric conversion layer 23 may be formed by mixing a lead halide (PbX2) material and a methylammonium iodide (MAI) material, dissolving the mixture in a solvent, depositing the resulting solution, and reacting the resulting solution at a reaction temperature.

[0032] The material for forming the photoelectric conversion layer 23 is preferably deposited by spin coating of a solution of the photoelectric conversion layer material dissolved in a polar solvent such as DMF (N,N-dimethylformamide) or DMSO (dimethyl sulfoxide), in order to form a uniform thin film. Furthermore, when the material is deposited by spin coating, the effect of optimizing the wettability of the hole transport layer 22 is significant. Therefore, by forming the photoelectric conversion layer 23 by stacking a material on the hole transport layer 22 by spin coating, the photoelectric conversion efficiency of the solar cell 1 can be particularly improved.

[0033] Specifically, the photoelectric conversion layer-forming material solution can be prepared by weighing lead(II) iodide, formamidine hydroiodide, lead(II) bromide, and methylamine hydrobromide, dissolving them in DMF and DMSO, and mixing them with a separately prepared DMSO solution of cesium iodide. In this case, the molar concentration of lead(II) iodide relative to the molar concentration of formamidine hydroiodide is preferably 1 to 1.1 times, and the molar concentration of lead(II) bromide relative to the molar concentration of methylamine hydrobromide is preferably 1 to 1.1 times, and more preferably 1.06 to 1.09 times, respectively. The molar concentration ratio of lead(II) iodide to lead(II) bromide is preferably 72:28 to 79:21, and even more preferably 74:26 to 77:23. The total molar concentration of formamidine hydroiodide and methylamine hydrobromide is preferably 1.5 to 2.0 M, more preferably 1.6 to 1.8 M. The ratio of DMF to DMSO as solvents for dissolving lead(II) iodide, formamidine hydroiodide, lead(II) bromide, and methylamine hydrobromide is preferably 3:1 to 5:1. When mixing a DMSO solution of cesium iodide to form a mixed solution, the molar concentration of cesium iodide relative to the molar concentration of formamidine hydroiodide is preferably 4 to 6%. Such a photoelectric conversion layer-forming material solution can be appropriately formed into a film by spin coating on the hole transport layer 22 composed of the above-mentioned monolayer.

[0034] The thickness of the photoelectric conversion layer 23 depends on the type of material used to form the photoelectric conversion layer, but is preferably 100 nm or more and 1000 nm or less in order to increase the light absorption rate while reducing the distance that the generated charges travel.

[0035] The electron transport layer (ETL) 24 selectively transfers electrons generated in the photoelectric conversion layer 23 to the cathode layer 26 through the buffer layer 25. Examples of the main material of the electron transport layer 24 include PTAA (Poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine)), Spiro-MeOTAD, and fullerene. Examples of fullerene include C60, C70, and their hydrides, oxides, metal complexes, and derivatives with alkyl groups or the like added thereto. Forming the electron transport layer 24 from a material containing fullerenes encapsulating lithium (Li) can improve the electron transport efficiency.

[0036] The electron transport layer 24 can be formed by, for example, a sol-gel method, a coating method, etc. The thickness of the electron transport layer 24 can be, for example, 3 nm or more and 30 nm or less.

[0037] The buffer layer 25 prevents the metal of the cathode layer 26 from diffusing into the electron transport layer 24, thereby preventing the resulting recombination of charges. The buffer layer 25 also functions as a buffer layer to prevent damage to the electron transport layer 24 when the cathode layer 26 is formed by sputtering or the like. The buffer layer 25 is made of a stable metal oxide such as tin oxide (SnO2) or zinc oxide (ZnO), or an organic material such as bathocuproine (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline).

[0038] The buffer layer 25 can be formed by a method such as vacuum evaporation, sputtering, atomic layer deposition, etc. The thickness of the buffer layer 25 can be set to, for example, 3 nm or more and 30 nm or less.

[0039] The cathode layer 26 is an electrode that forms a pair with the anode layer 21 of the perovskite solar cell section 20. The cathode layer 26 is a transparent electrode that transmits light that enters through the anti-reflection layer 33, and is made of amorphous ITO. In addition, the cathode layer 26 is preferably doped with tin oxide (SnO2).

[0040] The cathode layer 26 can be formed by sputtering. The lower limit of the thickness of the cathode layer 26 can be set to, for example, 10 nm or more and 200 nm or less.

[0041] The back surface collecting electrode 31 and the front surface collecting electrode 32 are a pair of electrodes for extracting power from the laminate of the crystalline silicon solar cell and the perovskite solar cell and outputting it to the outside. The back surface collecting electrode 31 may be laminated over the entire surface, but the front surface collecting electrode 32 has a plurality of linear portions, so-called finger electrodes, arranged at intervals to allow light to enter the perovskite solar cell section 20.

[0042] Back collector electrode 31 and front collector electrode 32 are made of a conductive material. Back collector electrode 31 and front collector electrode 32 can be formed by known methods such as sputtering and plating, but front collector electrode 32 is preferably formed by, for example, silver vapor deposition, which is a method that can be used to form front collector electrode 32 without damaging perovskite solar cell portion 20. Back collector electrode 31 and front collector electrode 32 can have a thickness of, for example, 100 nm to 300 nm.

[0043] The anti-reflection layer 33 reduces light reflection on the light-receiving surface of the solar cell 1. The anti-reflection layer 33 can be formed from a low-refractive-index material such as magnesium fluoride (MgF), silicon oxide (SiO), aluminum oxide (AlO), or cerium fluoride (CeF). The anti-reflection layer 33 may also be a multilayer film in which low-refractive-index materials and high-refractive-index materials are alternately stacked. The thickness of the anti-reflection layer 33 can be, for example, 50 nm or more and 200 nm or less.

[0044] As is clear from the above description, one embodiment of the solar cell manufacturing method according to the present invention for manufacturing solar cell 1 includes, as shown in FIG. 3 , a step of laminating a conductive material that forms anode layer 21 on substrate 10 (step S1: anode conductive material laminating step), a step of applying a monolayer-forming material solution containing two types of carbazole compound molecules to the conductive material layer, i.e., anode layer 21 (step S2: monolayer-forming material solution applying step), a step of drying the coating of the monolayer-forming material solution (step S3: coating drying step), and a step of forming photoelectric conversion layer 23 on the coating of the monolayer-forming material solution after drying, i.e., hole transport layer 22. The method includes a step of applying a semi-dry photoelectric conversion layer forming material solution containing a material (step S4: photoelectric conversion layer forming material solution application step), a step of forming a photoelectric conversion layer 23 by reacting the photoelectric conversion layer forming material by heating (step S5: reaction step), a step of stacking a material that will form an electron transport layer 24 on the photoelectric conversion layer 23 (step S6: electron transport layer forming material stacking step), a step of stacking a material that will form a buffer layer 25 on the electron transport layer 24 (step S7: buffer layer forming material stacking step), and a step of stacking a conductive material that will form a cathode layer 26 on the buffer layer 25 (step S8: cathode conductive material stacking step).

[0045] The solar cell 1 manufactured by such a solar cell manufacturing method has a hole transport layer 22 that is a monomolecular film formed from two types of carbazole compound molecules, which gives the hole transport layer 22 sufficient hole selectivity and allows the photoelectric conversion layer 23 to be appropriately formed, resulting in a relatively high photoelectric conversion efficiency.

[0046] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and various modifications and variations are possible. For example, a solar cell according to the present invention may use a transparent substrate and convert light incident from the substrate side into electricity. Furthermore, the solar cell according to the present invention may further include an intermediate layer interposed to improve adhesion between layers, a blocking layer to prevent undesired interlayer movement of charge or material, a doped layer to provide a dopant to a specific layer (e.g., a lithium fluoride thin layer that may be provided to provide lithium fluoride to an electron transport layer containing fullerene), etc. For example, some layers, such as a buffer layer or antireflection layer, may be omitted, or multiple layers may be replaced with a single layer that combines these functions. Furthermore, processes for the solar cell according to the present invention may be omitted, added, or modified depending on the configuration of the solar cell to be manufactured. [Explanation of symbols]

[0047] 1. Solar cells 10 Base material 11 Semiconductor substrate 12 First semiconductor layer 13 Second semiconductor layer 20 Perovskite Solar Cells 21 Anode layer 22 Hole transport layer 23 Photoelectric conversion layer 24 Electron transport layer 25 buffer layer 26 Cathode layer 31 Rear collecting electrode 32 Surface collecting electrode 33 Anti-reflection layer

Claims

1. a cathode layer, an anode layer, a hole transport layer, a photoelectric conversion layer containing a perovskite compound and in contact with the hole transport layer, an electron transport layer, and a cathode layer, in this order; the hole transport layer is a monomolecular film formed of two types of carbazole compound molecules, The two types of carbazole compound molecules are 2PACz and MeO-2PACz.

2. 2. The solar cell according to claim 1, wherein a mass ratio of the 2PACz to the MeO-2PACz in the hole transport layer is 1:99 or more and 60:40 or less.

3. 2. The solar cell according to claim 1, wherein the area ratio of the 2PACz to the MeO-2PACz in the hole transport layer is 1:99 or more and 50:50 or less.

4. Laminating a conductive material to a substrate; applying a monomolecular film-forming material solution containing two types of carbazole compound molecules onto the conductive material layer; a step of drying the coating film of the monolayer-forming material solution; applying a photoelectric conversion layer forming material solution to the dried coating film of the monomolecular film forming material solution; Equipped with The two types of carbazole compound molecules are 2PACz and MeO-2PACz.

5. The solar cell of claim 4, wherein the mass ratio of 2PACz to MeO-2PACz in the monolayer-forming material solution is 1:99 or more and 70:30 or less.

6. 6. The method for manufacturing a solar cell according to claim 4, wherein the photoelectric conversion layer forming material solution is applied by spin coating.

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