Perovskite solar cell using graphene transparent conductive electrode

Abstract

[Problem] To provide a perovskite solar cell that has excellent flexibility and electrical conductivity and that is provided with a transparent conductive electrode. [SOLUTION] A solar cell in which one or more laminates of a perovskite layer (5) and a transparent conductive electrode are stacked, and a metal electrode (7) is provided on the uppermost perovskite layer (5), wherein the transparent conductive electrode is constituted from a graphene sheet (2) synthesized, after performing polishing and ion milling of a metal substrate for graphene growth, by thermal CVD with a gas containing hydrogen, nitrogen and carbon.

Description

Perovskite solar cells using graphene transparent conductive electrodes

[0001] The present invention relates to perovskite solar cells using graphene transparent conductive electrodes.

[0002] Perovskite solar cells are a new type of solar cell that uses perovskite crystalline structure materials. They are attracting attention because they are inexpensive to manufacture, do not require rare metals, are "light," "thin," and "soft," are flexible and can be made into various shapes, and achieve high conversion efficiency comparable to "silicon-based solar cells" and "compound-based solar cells."

[0003] The rare metal indium tin oxide (ITO) is used as the transparent conductive electrode for perovskite solar cells, but it is expensive, its supply is unstable, and there are problems with the flexibility of ITO conductive glass. ITO conductive glass, which contains the rare metal indium, has a resistance value of 2 Ω / sq to 1000 Ω / sq and a light transmittance of only 50% to 90%, making it difficult to achieve low resistance and high light transmittance, and therefore alternative materials are being sought.

[0004] Japanese Patent Application Laid-Open No. 2022-181466

[0005] Amol Nande, Ashish Tiwari, Swati Raut, SJDhoble, “Graphene and graphene oxide: Application in luminescence and solar cell”, Woodhead Publishing Series in Electronic and Optical Materials, 2023, Pages 107-133.

[0006] The objective of the present invention is to develop a transparent conductive electrode that can replace ITO and a perovskite solar cell equipped with the electrode.

[0007] As a result of the research conducted by the present inventors, structures with specific microstructures composed mainly of carbon (carbon nanostructures) are known. Such carbon nanostructures include fullerenes, carbon nanotubes, carbon nanowalls, and graphene. Among these, it has become clear that the application of graphene materials is expected to be used as transparent conductive materials in perovskite solar cells, which require high conversion efficiency, high conductivity, and high transparency.

[0008] In addition, graphene has a single-layer structure in which SP2 carbon electrons are bonded in a honeycomb pattern, and the carbon atoms form a single-atom thick layer, making it transparent.The electron mobility of graphene is 200,000 cm2V-1s-1 at room temperature, making it highly valuable as a transparent conductive material.

[0009] The present invention proposes a perovskite solar cell comprising one or more laminates of a perovskite layer and a transparent conductive electrode, with a metal electrode provided on the uppermost perovskite layer, characterized in that the transparent conductive electrode is made of graphene or carbon nanowalls.

[0010] The graphene used in the present invention is optimally synthesized by polishing and ion milling a metal substrate for graphene growth, and then growing graphene on the metal substrate by thermal CVD using a gas containing hydrogen, nitrogen, and carbon.

[0011] In conventional synthesis methods, the substrate surface is barely planarized, resulting in numerous defects in the synthesized graphene, which in turn leads to high resistance. To achieve the above objectives, the Cu substrate (10) is mirror-polished and ion-milled to form an extremely flat Cu(10) surface. The key to achieving this goal is the synthesis of high-quality graphene sheets on the extremely flat Cu(10) surface.

[0012] This surface treatment method results in an extremely flat structure with few scratches or sagging on the CU substrate (10). The synthesized graphene has few defects and is close to a perfect graphene structure. Therefore, it is possible to reduce the resistance.

[0013] Furthermore, by setting the number of graphene layers to 1 to 3, it is possible to obtain an electrode that is flexible, has low resistance, and has high light transmittance.

[0014] Furthermore, in order to improve the quality of graphene, graphene can be synthesized by thermal CVD using gases containing hydrogen, nitrogen, and carbon, which is more suitable for precise manufacturing than plasma CVD, making it optically transparent and an excellent electrical conductor, and this synthesis can realize a new transparent conductive film.

[0015] Graphene has a resistance of only 10-10Ω / sq to 1000Ω / sq and a light transmittance of 97% to 99.99%. Theoretically, graphene is the most ideal transparent conductive material to date.

[0016] On the other hand, carbon nanowalls synthesized in a plasma CVD apparatus are used.

[0017] Graphene or carbon nanowalls can also be used in multiple layers.

[0018] As described above, the transparent conductive electrode of the perovskite solar cell according to the present invention can be replaced with graphene or carbon nanowalls, which have superior flexibility, conductivity, and light transmittance compared to commonly used ITO glass, and is significantly thinner and more resistant to bending and deformation, thereby not only offering the advantages of low resistance and high light transmittance, but also reducing the overall weight of the solar cell.

[0019] A perspective view showing an embodiment of a perovskite solar cell according to the present invention. A diagram of mirror polishing a metal (Cu) surface with a polishing machine. A diagram of removing scratches and sagging caused by mirror polishing with an argon ion beam device. A diagram schematically showing the process of synthesizing graphene in a thermal CVD device up to the creation of a graphene sheet. A diagram schematically showing the process of creating a perovskite solar cell. A perspective view showing another embodiment of a perovskite solar cell according to the present invention. A perspective view showing yet another embodiment of a perovskite solar cell according to the present invention. A diagram showing a plasma CVD device. Other Modes for Carrying Out the Invention

[0020] In a solar cell in which one or more laminates of perovskite layers and transparent conductive electrodes are stacked and a metal electrode is provided on the top perovskite layer, the metal substrate for graphene growth is polished and ion milled, and then the transparent conductive electrode of the solar cell is made of graphene synthesized by thermal CVD using gas containing hydrogen, nitrogen, and carbon.

[0021] First Embodiment A first example will be described. In this example, a case where the present invention is applied to a perovskite solar cell mechanism will be described.

[0022] Figure 1 shows a perovskite solar cell according to one embodiment of the present invention. Synthesized graphene 2 is attached to a protective glass 1 to form a transparent conductive electrode. Next, a hole transition layer (conductive polymer: PEDOT; PSS) 3, a conductive layer (Poly TPD) 4 that allows holes to flow and blocks electrons, a perovskite layer 5, a conductive layer (PCBM) 6 that allows electrons to flow and blocks holes, and a metal electrode 7 are layered in this order. Finally, graphene 2 and metal electrode 7 are connected using wiring 8 to complete the structure of the perovskite solar cell.

[0023] When exposed to sunlight, electrons and holes are excited in the perovskite layer 5, and the hole transition layer 3 has a low carrier transition rate, which increases the diffusion distance and lifetime of the carriers.

[0024] The electron-holes that do not recombine are collected in the electron conducting layer (PCBM) 6 and the hole conducting layer (Poly TPD) 4, and finally flow into the conductive electrode, generating a current.

[0025] Graphene Production Example As shown in Figure 2, a schematic diagram of a grinding jig (9) is shown. The surface of an unpolished Cu substrate (10) is uneven. To synthesize high-quality graphene (2), the Cu substrate (10) must be mirror-polished on a polishing plate (11).

[0026] Figure 3 shows a schematic diagram of an ion milling device. While mirror polishing can improve the irregularities of a Cu substrate 10, ion milling is necessary to remove surface processing scratches and sagging layers from the Cu substrate 10 that will be subjected to flat milling. The mirror-polished Cu substrate 10 is left on the work table 13, and the arc ion beam 12 is initiated to process the Cu substrate 10.

[0027] The milled Cu substrate is cleaned using ethanol (purity: 99.5%) in an ultrasonic device for, for example, 15 minutes.

[0028] Figure 4 shows a schematic diagram of a thermal CVD apparatus. A Cu substrate (10) washed with ultrapure water is placed in the thermal CVD apparatus, and the vacuum pump (16) is started to evacuate the chamber. Then, nitrogen gas is turned on to remove the air from the thermal CVD chamber. The temperature is raised to the synthesis temperature, and the oxide layer on the Cu substrate surface is removed with a nitrogen / hydrogen gas mixture. Finally, the source gas is turned on to synthesize graphene.

[0029] The following describes the processing steps after polishing and processing the Cu substrate (10).

[0030] As shown in FIG. 5A, the polished Cu substrate (10) is immersed in ultrapure water (17) to oxidize the surface (10) of the Cu substrate, thereby reducing adhesion to the graphene (2).

[0031] As shown in Figure 5B, a thermal release tape (18) is pressed onto the synthesized graphene (2) on the Cu substrate (10).

[0032] As shown in Figure 5C, the synthesized graphene (2) is mechanically peeled off from the Cu substrate (10) using thermal peel tape (18).

[0033] As shown in Figure 5D, the peeled graphene (2) is transferred to a protective glass (1) using a thermal peel tape (18).

[0034] As shown in FIG. 5E, heat is applied with a dryer (19) to weaken the adhesive strength of the thermal peeling tape (18).

[0035] As shown in Figure 5F, the heated thermal peel tape (18) is removed by the protective glass (1).

[0036] Next, the perovskite solar cell manufacturing process will be described with reference to FIG.

[0037] [Step shown in Figure 6A] First, prepare a sheet of protective glass (1). The glass substrate (1) is used in the structure of this perovskite solar cell to prevent impurities such as dust and dirt from entering.

[0038] [Step shown in Figure 6B] Graphene material (2) is attached to a glass substrate to form a transparent and conductive electrode. For example, the electron mobility is 200,000 cm at room temperature. 2 V -1 s -1 It has a light transmittance of 97-99.99%.

[0039] [Step shown in Figure 6C] The transparent conductive polymer (PEDOT:PSS) (3) has extremely high conductivity and transparency compared to other conductive polymers. It can be applied to graphene (2) on a glass substrate (1), and exhibits excellent transparency. It forms a homogeneous layer on the graphene thin film, and is used as a material for the hole transition layer.

[0040] [Step shown in Figure 6D] Poly TPD (4) as a hole transport layer is applied to a transparent conductive polymer (PEDOT:PSS) (3), allowing holes to flow freely while blocking the passage of electrons.

[0041] [Step shown in FIG. 6E] A perovskite layer (5) is applied to the hole transport layer Poly TPD (4).

[0042] [Step shown in Figure 6F] PCBM (6) is applied to the perovskite layer (5), allowing electrons to flow freely while blocking the passage of holes.

[0043] [Step shown in Figure 6G] PCBM (6) is coated with metal (Au) (7) and used as a metal electrode.

[0044] [Step shown in Figure 6H] Metal wires are wired (8) between the transparent conductive graphene electrode (2) and the Au (7) metal electrode, forming the perovskite solar cell structure shown in Figure 1.

[0045] (Second Fourth Embodiment) Figure 7 shows another embodiment of a perovskite solar cell according to the present invention. Graphene 2a synthesized in a thermal CVD apparatus is attached to a protective glass (1) to form a transparent conductive electrode. Next, a perovskite layer 5a, a second perovskite layer 5b, and a metal (Au) electrode 7 are laminated in this order, and finally, the graphene 2a / 2b and the metal (Au) electrode 7 are connected using wiring 5. The manufacturing process is as follows:

[0046] A uniform layer is formed on the graphene thin film via the perovskite layer 5b on the graphene 2a.

[0047] Next, a second graphene 2b is transferred to the perovskite layer 5b.

[0048] A second perovskite layer 5b is applied to the graphene 2b.

[0049] The perovskite layer 5b is coated with a metal (Au) 7.

[0050] Metal conductors are wired between the transparent conductive electrode graphene 2a, 2b and the Au metal electrode 7. The multilayer perovskite solar cell constructed as described above is activated by light irradiation.

[0051] (Third Embodiment) Figure 8 shows yet another embodiment of a perovskite solar cell according to the present invention. First, a transparent conductive electrode of carbon nanowall layer 20a is synthesized on protective glass 1 in a plasma CVD apparatus. Next, perovskite layer 5a, second carbon nanowall layer 20b, second perovskite layer 5b, and metal (Au) are laminated in this order. Finally, carbon nanowall layers 20a and 20b and metal (Au) electrode 7 are connected with wiring 8 to complete the structure of the perovskite solar cell. The manufacturing process is as follows:

[0052] 9 is a structural diagram of a plasma CVD apparatus. The protective glass 1 having the perovskite layer 5a / 5b is placed on a heating plate 21, and the carbon nanowall layer 20a / 20b is synthesized by plasma 22 from a plasma source 23.

[0053] First, prepare a sheet of protective glass 1. Protective glass 1 is used in the structure of this perovskite solar cell to prevent impurities such as dust and dirt.

[0054] First, a carbon nanowall layer 20a is formed on a protective glass 1 using a plasma CVD apparatus, and a transparent and conductive electrode is formed on the protective glass 1 using a synthetic paper.

[0055] The perovskite layer 5a is applied onto the carbon nanowall layer 20a to form a uniform layer on the thin carbon nanowall layer (20a).

[0056] Next, a carbon nanowall layer 20b is synthesized on the perovskite layer 5a in a plasma CVD apparatus.

[0057] The perovskite layer 5b is applied to the carbon nanowall layer 20b.

[0058] A metal (Au) 7 is coated on the perovskite layer 5bn.

[0059] The metal conductor is wired 8 between the transparent conductive electrode carbon nanowall layers 20a and 20b and the Au metal electrode 7. The multilayer perovskite solar cell configured as described above is activated by light irradiation.

[0060] For example, in the above-described process, the perovskite solar cell structure may be a multilayer structure as shown in Figures 7 and 8, but other structures are also possible. That is, because graphene is very thin and has excellent light transmission properties, it can in principle be made into a multilayer stacked structure, which can significantly improve the photoelectric conversion efficiency of perovskite solar cells.

[0061] The present invention can provide a perovskite solar cell that has excellent flexibility and conductivity and is equipped with a transparent conductive electrode that is highly transparent. Explanation of symbols

[0062] 1 Protective glass 2 Graphene 3 Transparent conductive polymer (PEDOT:PSS) 4 Poly-TPD 5 Perovskite material 6 PCBM6 7 Metal (Au) 8 Wiring 9 Jig 10 Metal (Cu) 11 Polishing plate 12 Alcon ion beam 13 Work table 14 Electric furnace 15 Raw material gas (methane gas) 16 Vacuum pump 17 Ultrapure water 18 Thermal release tape 19 Heating with a dryer

Claims

1. A perovskite solar cell comprising one or more laminates of a perovskite layer and a transparent conductive electrode, with a metal electrode provided on the topmost perovskite layer, characterized in that the transparent conductive electrode is made of graphene or carbon nanowalls.

2. The solar cell according to claim 1, wherein the graphene is a graphene sheet.

3. The perovskite solar cell according to claim 2, wherein the graphene is a graphene sheet synthesized by thermal CVD using a gas containing hydrogen, nitrogen and carbon after polishing and ion milling a metal substrate for graphene growth.

4. The solar cell according to claim 1, wherein the carbon nanowalls are synthesized using a plasma CVD apparatus.

5. The solar cell according to claim 1, wherein the perovskite layer and the graphene are arranged in a multi-layer structure.

6. The solar cell according to claim 1, which has a multi-layer structure of perovskite layers and carbon nanowalls.

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