Perovskite solar cells using graphene transparent conductive electrode

Abstract

To provide a perovskite solar cell that is excellent in flexibility and conductivity and is provided with a transparent conductive electrode that is highly transparent.SOLUTION: In a solar cell, one or more laminates of a perovskite layer and a transparent conductive electrode are stacked, with a metal electrode provided on the topmost perovskite layer. After polishing and ion milling a metal substrate for graphene growth, the solar cell has a transparent conductive electrode made of a graphene sheet (2) synthesized by thermal CVD using a gas containing hydrogen, nitrogen, and carbon.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Perovskite solar cells are a new type of solar cell that uses perovskite crystal structure materials. They are attracting attention because they are inexpensive to manufacture, do not require rare metals, are "light," "thin," and "soft," and are flexible and can be adapted to a variety of shapes. Moreover, they achieve high conversion efficiency comparable to "silicon-based solar cells" and "compound 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. The resistance value of ITO conductive glass containing the rare metal indium is 2Ω / sq to 1000Ω / sq, and its light transmittance is only 50% to 90%, making it difficult to achieve low resistance and high light transmittance. As it is difficult to achieve low resistance and high light transmittance, there is an urgent need to consider alternative materials. [Prior art documents] [Patent documents]

[0004] Patent No. 2022-181466 [Non-patent literature]

[0005] Amol Nande Ashish Twari Swati Raut?SJDhobe “Graphene and graphene oxide: Application in luminescence and solar cell”, Woodhead Publishing Series in Electronic and Optical Materials, 2023, Pages 107-133. Summary of the Invention [Problem to be solved by the invention]

[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 inventors of the present application, structures (carbon nanostructures) having a specific microstructure mainly composed of carbon are known. Such carbon nanostructures include fullerene, carbon nanotube, carbon nanowall, and graphene. Among them, it has become clear that the conversion efficiency of graphene material is expected to be suitable for use as a transparent conductive material in perovskite solar cells, which require high conductivity and high transmittance.

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

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

[0010] The graphene used in the present invention is optimally graphene that is synthesized by polishing and ion milling a metal substrate for graphene growth, and then growing the 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 hardly planarized, and the synthesized graphene has many defects, which results in a high resistance. In order to achieve the above object of the present invention, the Cu substrate (10) is mirror-polished and ion-milled to form an extremely flat Cu (10) surface. The key to realizing the present invention is to synthesize high-quality graphene sheets on the extremely flat Cu (10) surface.

[0012] This method of flattening the surface of the Cu substrate (10) produces an extremely flat structure with few scratches or sagging. The graphene synthesized has few defects and is very close to a perfect graphene structure. This makes it possible to reduce the resistance.

[0013] Furthermore, by limiting 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, to improve the quality of graphene, graphene can be synthesized using thermal CVD, which is more suitable for precise manufacturing than plasma CVD, using gases containing hydrogen, nitrogen and carbon, making it optically transparent and a good electrical conductor, and this synthesis can realize a new transparent conductive film.

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

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

[0017] Graphene or carbon nanowalls can also be used in multiple layers. Effect of the Invention

[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 excellent flexibility, conductivity, and light transmittance compared with commonly used ITO glass, and the thickness is significantly thinner and has excellent bending and deformation properties. Therefore, not only does it have the advantages of low resistance and high light transmittance, but it also reduces the weight of the entire solar cell. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a perspective view showing an embodiment of a perovskite solar cell according to the present invention. [Diagram 2] Mirror polishing of metal (Cu) surface using a polishing machine [Diagram 3] A diagram of removing scratches and sagging from mirror polishing using the Alcon ion beam device [Figure 4] Graphene synthesis using a thermal CVD device [Diagram 5] Schematic diagram showing the process of creating a graphene sheet [Figure 6] Schematic diagram showing the process of creating a perovskite solar cell [Figure 7] FIG. 1 is a perspective view showing another embodiment of a perovskite solar cell according to the present invention. [Figure 8] FIG. 1 is a perspective view showing yet another embodiment of a perovskite solar cell according to the present invention. [Figure 9] A diagram showing a plasma CVD device DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] In a solar cell in which one or more laminates of a perovskite layer and a transparent conductive electrode are stacked and a metal electrode is provided on the topmost 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 a gas containing hydrogen, nitrogen and carbon.

[0021] (First embodiment) A first embodiment will be described. In this embodiment, a case where graphene is applied to a perovskite solar cell structure will be described.

[0022] 1 shows a perovskite solar cell according to one embodiment of the present invention, in which 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 laminated in this order, and finally the graphene 2 and the metal electrode 7 are connected using wiring 8 to form the structure of the perovskite solar cell.

[0023] When exposed to sunlight, electrons-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-hole pairs 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 2 is a schematic diagram of a grinding jig 9. Unevenness is observed on the surface 10 of an unpolished Cu substrate. In order to synthesize high-quality graphene 2, it is necessary to mirror-polish the Cu substrate 10 on a polishing plate 11.

[0026] As shown in Fig. 3, it is a schematic diagram of an ion milling apparatus. Although the unevenness of the Cu substrate 10 can be improved by mirror polishing, ion milling is necessary to remove surface processing scratches and sagging layers of the Cu substrate 10 to 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 started to process the Cu substrate 10.

[0027] Clean the milled Cu substrate with ethanol (purity: 99.5%) in an ultrasonic device for, e.g., 15 min.

[0028] As shown in FIG. 4, it is a schematic diagram of a thermal CVD apparatus. A Cu substrate 10 washed with ultrapure water is placed in the thermal CVD apparatus, and after starting the vacuum pump 16 to draw a vacuum, nitrogen gas is turned on and the air in the thermal CVD chamber is removed for 10 minutes. The temperature is raised to the synthesis temperature (600 to 1000°C), and the temperature is maintained for 30 minutes while flowing a nitrogen-hydrogen mixed gas (hydrogen and nitrogen (2:1)) to remove the oxide layer on the Cu substrate surface, and finally the source gas is turned on to introduce the source gas at a flow rate of 10 to 50 sccm to synthesize graphene.

[0029] The processing steps after polishing and processing of the CU substrate (10) will be described with reference to FIG.

[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 the adhesion to the graphene 2.

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

[0032] As shown in FIG. 5C, the synthesized graphene 2 is mechanically peeled off from the Cu substrate 10 by a thermal peeling tape 18.

[0033] As shown in FIG. 5D, the graphene 2 peeled off with a thermal peeling tape 18 is transferred to a protective glass 1.

[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 FIG. 5F, the heated thermal peeling 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 FIG. 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 dirt and dust.

[0038] [Step shown in FIG. 6B] Graphene 2 is attached onto a glass substrate to form a transparent and conductive electrode. For example, the electron mobility is 200,000 cm2V-1s-1 at room temperature, and the light transmittance reaches 97 to 99.99%.

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

[0040] [Step shown in FIG. 6D] Poly TPD4, as a hole conducting layer, is coated onto a transparent conductive polymer (PEDOT:PSS)3. It allows 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 FIG. 6F] PCBM 6 is applied to the perovskite layer 5. It allows electrons to flow freely while blocking the passage of holes.

[0043] [Step shown in FIG. 6G] PCBM6 is coated with metal (Au)7 and used as a metal electrode.

[0044] [Step shown in FIG. 6H] The metal conductor is wired 8 between the transparent conductive electrode graphene 2 and the Au metal electrode 7. This forms the perovskite solar cell structure shown in Figure 1.

[0045] (Second embodiment) 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 graphene 2b, a second perovskite layer 5b, and a metal (Au) electrode 7 are laminated in this order, and finally, the graphenes 2a and 2b and the metal (Au) electrode 7 are connected using wiring 5. The manufacturing process is as follows.

[0046] The perovskite layer 5a on the graphene 2a is applied to form a homogeneous layer on the graphene thin film.

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

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

[0049] A metal (Au) 7 is coated onto the perovskite layer 5b.

[0050] The metal conductor is wired 8 between the transparent conductive graphene electrodes 2a and 2b and the Au metal electrode 7. The multi-layer perovskite solar cell constructed as above is operated by light irradiation.

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

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

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

[0054] First, a carbon nanowall layer 20a is synthesized on a protective glass 1 in a plasma CVD apparatus to form a transparent and conductive electrode.

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

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

[0057] A perovskite layer 5b is applied onto the carbon nanowall layer 20b.

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

[0059] Metal conductors are wired 8 between the transparent conductive electrode carbon nanowall layers 20a, 20b and the Au metal electrode 7. The multi-layer perovskite solar cell constructed as above is operated by light irradiation.

[0060] For example, in the above-mentioned implementation process, the perovskite solar cell structure may be a multi-layer structure as shown in Figures 7 and 8, but other structures are also possible. That is, since the graphene 2a, 2b and the carbon nanowall layers 20a, 20b are very thin and have excellent light transmission effect, in principle, a multi-layer stack structure can be formed, and the photoelectric conversion rate of the perovskite solar cell can be significantly improved. [Industrial Applicability]

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

[0062] 1 Protective glass 2, 2a,2b Graphene 3. Transparent conductive polymer (PEDOT:PSS) 4 Poly TPD 5,5a,5b Perovskite layers 6 PCBM 7 Metal (Au) 8 Wiring 9. Jig 10 Metal (Cu) 11 Polishing machine 12 Alcon Ion Beam 13 Workbench 14 Electric furnace 15 Raw gas (methane gas) 16 Vacuum Pump 17 Ultrapure water 18 Heat Release Tape 19 Dryer heating 20a, 20b Carbon nanowall layer 21 Heating plate 22 Plasma 23 Plasma Source

Claims

1. A perovskite solar cell comprising one or more laminates of a perovskite layer and a transparent conductive electrode, and a metal electrode provided on the uppermost 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. 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. 2. 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 formed into a multi-layer structure.

6. 2. The solar cell according to claim 1, wherein the perovskite layer and the carbon nanowalls are formed into a multi-layer structure.

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