Solar cell and method of manufacturing the same

Abstract

The application provides a solar cell and a preparation method thereof. The solar cell comprises a silicon base, an intrinsic amorphous silicon layer, a doped amorphous silicon layer, an ITO thin film and a metal electrode which are sequentially arranged on one side of the silicon base. The ITO thin film comprises at least two ITO films, and the grain sizes of the different ITO films are different. Compared with the prior art, the ITO thin film of the application comprises at least two film layers with different grain sizes, so that the IV performance of the solar cell is better, and the product reliability is further guaranteed.

Description

Technical Field

[0001] This invention relates to the field of photovoltaics, and more particularly to a solar cell and its preparation method. Background Technology

[0002] Indium tin oxide (ITO) film is a commonly used transparent conductive film with advantages such as high conductivity, good light transmittance, high mechanical hardness and good stability. It is a preferred electrode material for solar cells, OLED displays, liquid crystal displays and other applications.

[0003] While ITO coatings currently possess excellent electrical and optical properties and a moderate degree of compatibility with adjacent layers, they are not optimal enough, which can negatively impact device performance. For example, using ITO thin films as transparent conductive films in solar cells cannot simultaneously achieve optimal light transmittance, carrier mobility, and contact resistance with adjacent layers, thus affecting the cell's conversion efficiency.

[0004] In view of this, it is necessary to provide a solar cell and a method for its fabrication to solve the above-mentioned technical problems. Summary of the Invention

[0005] The purpose of this invention is to provide a solar cell and a method for its preparation.

[0006] To solve one of the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A solar cell includes a silicon substrate, an intrinsic amorphous silicon layer, a doped amorphous silicon layer, an ITO thin film, and a metal electrode, which are sequentially located on one side of the silicon substrate. The ITO thin film includes at least two ITO films with different grain sizes.

[0008] Furthermore, in two adjacent ITO films, the grains of the film layer closer to the metal electrode are larger.

[0009] Furthermore, the grain size of the ITO film in contact with the doped amorphous silicon layer is 1 nm to 20 nm.

[0010] Furthermore, the grain size of the ITO film in contact with the metal electrode is 20 nm to 500 nm.

[0011] Furthermore, the ITO film in contact with the doped amorphous silicon layer has a columnar crystal structure, while the ITO film in contact with the metal electrode has an equiaxed crystal structure.

[0012] Furthermore, the ITO thin film includes a small-grain ITO film in contact with the doped amorphous silicon layer and a large-grain ITO film in contact with the metal electrode, wherein the thickness of the large-grain ITO film is greater than the thickness of the small-grain ITO film.

[0013] Furthermore, the thickness of the ITO film is 50nm to 120nm, and the thickness of the large-grain ITO film is 40nm to 90nm.

[0014] A method for fabricating a solar cell includes: sequentially forming an intrinsic amorphous silicon layer, a doped amorphous silicon layer, an ITO thin film, and a metal electrode on one side of a silicon substrate; forming the ITO thin film includes: forming at least two ITO films, wherein the different ITO films have different grain sizes.

[0015] Furthermore, the process for forming a small-grain ITO film in contact with the doped amorphous silicon layer is as follows: the coating pressure is 0.7 to 1.5 Pa, and the power density is 2 to 8 kW / m.

[0016] Furthermore, the process for forming a large-grain ITO film in contact with the metal electrode is as follows: the coating pressure is 0.3 Pa to 0.7 Pa, and the power density is 5 to 15 kW / m.

[0017] Furthermore, forming the ITO thin film includes: forming a small-grain ITO film in contact with the doped amorphous silicon layer and forming a large-grain ITO film in contact with the metal electrode. The grain size of the small-grain ITO film is 1 nm to 20 nm, and the grain size of the large-grain ITO film is 20 nm to 500 nm.

[0018] Furthermore, the grain size of the small-grain ITO film is 1-20 nm, and the thickness of the small-grain ITO film is 10 nm-30 nm.

[0019] Furthermore, the grain size of the large-grain ITO film is 15–500 nm, and the thickness of the large-grain ITO film is 40 nm–90 nm.

[0020] The beneficial effects of the present invention are: compared with the prior art, the ITO thin film of the present invention includes at least two film layers with different grain sizes, which makes the IV performance of the solar cell better, and at the same time the product reliability is further guaranteed. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a solar cell according to an embodiment of the present invention;

[0022] Figure 2 This is a schematic diagram of the structure of a solar cell according to another embodiment of the present invention.

[0023] Among them, 1 is a solar cell, 1-silicon-based, 2-first intrinsic amorphous silicon layer, 3-first doped amorphous silicon layer, 4-first ITO thin film, 41-small-grain ITO film, 42-large-grain ITO film, 5-first metal electrode, 6-second intrinsic amorphous silicon layer, 7-second doped amorphous silicon layer, 8-second ITO thin film, and 9-second metal electrode. Detailed Implementation

[0024] The present invention will now be described in detail with reference to the specific embodiments shown in the accompanying drawings. However, these embodiments do not limit the present invention, and any structural, methodological, or functional modifications made by those skilled in the art based on these embodiments are included within the scope of protection of the present invention.

[0025] In the various illustrations of this invention, for ease of illustration, certain dimensions of structures or parts may be exaggerated relative to other structures or parts; therefore, only the basic structure of the subject matter of this invention is used to illustrate the invention.

[0026] Please refer to Figure 2 As shown, a solar cell 100 of a preferred embodiment of the present invention includes a silicon substrate 1, and intrinsic amorphous silicon layers 2 and 6, doped amorphous silicon layers 3 and 7, ITO thin films 4 and 8, and metal electrodes 5 and 9, which are sequentially located on one side of the silicon substrate.

[0027] When the solar cell 100 is a bifacial heterojunction cell, it includes a silicon substrate 1, a first intrinsic amorphous silicon layer 2, a first doped amorphous silicon layer 3, a first ITO thin film 4, and a first metal electrode 5 located sequentially on the first side of the silicon substrate 1, and a second intrinsic amorphous silicon layer 6, a second doped amorphous silicon layer 7, a second ITO thin film 8, and a second metal electrode 9 located sequentially on the second side of the silicon substrate 1.

[0028] The silicon substrate 1 is formed by texturing a silicon wafer. In one embodiment, an N-type silicon wafer is selected, with a resistivity of 0.5 Ω·cm to 3 Ω·cm, a thickness of 150 μm to 200 μm, and a size of 156.75 cm. The silicon wafer is texturized using an alkaline solution to form a pyramid structure, which improves the capture of sunlight.

[0029] The first intrinsic crystalline silicon layer 2 and the second intrinsic crystalline silicon layer 6 are collectively referred to as the intrinsic amorphous silicon layers 2 and 6, which passivate the silicon substrate 1.

[0030] The first doped amorphous silicon layer 3 and the second doped amorphous silicon layer 7 are collectively referred to as doped amorphous silicon layers 3 and 7, respectively, and their doping types are opposite. In one embodiment, the first doped amorphous silicon layer 3 is an N-type doped amorphous silicon layer, such as a phosphorus-doped amorphous silicon layer; and the second doped amorphous silicon layer 5 is a P-type doped amorphous silicon layer, such as a boron-doped amorphous silicon layer. Of course, the doping types of the first doped amorphous silicon layer 3 and the second doped amorphous silicon layer 5 can also be interchanged.

[0031] The first ITO film 4 and the second ITO film 8 are collectively referred to as ITO films 4 and 8. As transparent conductive films, their conductivity, light transmittance, and stability have a great influence on the performance of the solar cell 100.

[0032] The first metal electrode 5 and the second metal electrode 9 are collectively referred to as metal electrodes 5 and 9, which include a main grid and a fine grid, and are used to collect and output electrical energy.

[0033] The focus of this invention is to improve the ITO thin film 4,8 and its preparation process to enhance the performance of solar cells. The inventors discovered that small-grain ITO films have relatively high carrier concentrations, but due to numerous grain boundaries and increased grain boundary scattering, carrier mobility is low. Therefore, small-grain ITO films suffer from severe optical absorption, leading to a decrease in the cell's optical flux density (Isc). Furthermore, some metal ions migrate more easily in this film layer, posing a certain risk to device reliability (such as PID and DH). Conversely, large-grain ITO films have relatively low carrier concentrations and high mobility, resulting in greater contact losses with the amorphous silicon layer and a decrease in the cell's final flux (FF).

[0034] In view of this, the ITO thin film 4,8 of the present invention includes at least two ITO films with different grain sizes, which can balance and take into account the conductivity, light transmittance and stability of the entire ITO thin film 4,8, improve its matching degree with other adjacent film layers in terms of optics, electricity, etc., and improve the performance of solar cells.

[0035] Preferably, in two adjacent ITO films, the ITO film closer to the metal electrodes 5,9 has larger grains. The small-grained ITO film 41 with high carrier concentration and low mobility is located on the inner side, and has low contact resistance with the doped amorphous silicon layers 3,7, which can improve the FF of the cell; the large-grained ITO film 42 with fewer grain boundaries, high stability, and high mobility is located on the outer side, which improves the stability of the entire ITO film 4,8, reduces the probability of short-circuit diffusion, and is beneficial to improving the stability of the solar cell 100.

[0036] Furthermore, when the ITO thin film 4,8 includes at least three ITO films, the grain size of the at least three ITO films gradually increases along the direction from the doped amorphous silicon layer 3,7 to the metal electrode 5,9, which can further improve the performance of the solar cell 100.

[0037] Specifically, the ITO film in contact with the doped amorphous silicon layers 3,7 is a small-grain ITO film 41 with a grain size of 1nm to 20nm, which is beneficial for the contact between the ITO film 4,8 and the doped amorphous silicon layers 3,7, resulting in low contact resistance and improved battery FF.

[0038] The ITO film in contact with the metal electrodes 5 and 9 is a large-grain ITO film 42 with grains ranging from 20 nm to 500 nm; it has fewer grain boundaries and higher stability, which is beneficial to improving optical performance and reliability.

[0039] Preferably, the ITO films of different layers have different crystal forms.

[0040] Preferably, the ITO film in contact with the doped amorphous silicon layer 3,7 has a columnar crystal structure. The columnar crystal grows longitudinally, with a small lateral dimension but a long longitudinal length, which is beneficial for the carriers of the inner doped amorphous silicon layer 3,7 to be transported longitudinally to the outer metal electrode 5,9.

[0041] The ITO film in contact with the metal electrodes 5,9 has an equiaxed crystal structure and excellent carrier transfer performance in both the lateral and longitudinal directions. This is beneficial for the longitudinal transfer of carriers from the doped amorphous silicon layer 3,7 and the inner ITO film to the metal electrode layer 5,9, and also for the carriers to migrate laterally along the ITO film and be collected by the fine gate.

[0042] In addition, the ITO thin films 4 and 8 include a small-grain ITO film 41 in contact with the intrinsic amorphous silicon layer and a large-grain ITO film 42 in contact with the metal electrodes 5 and 9. The thickness of the large-grain ITO film 42 is greater than that of the small-grain ITO film 41. By having a thin layer of small-grain ITO film 41 in contact with the doped amorphous silicon layer 3 and 7, the contact resistance is reduced. Then, by increasing the thickness of the large-grain ITO film 42 as much as possible, the performance of the entire ITO thin film 4 and 8 can be made to approach that of the large-grain ITO film 42, thereby improving its stability and optical performance.

[0043] For example, the thickness of the ITO thin film 4,8 is 50nm to 120nm, the thickness of the large-grain ITO film 42 is 40nm to 90nm, and the remainder is the thickness of the small-grain ITO film 41.

[0044] In one specific embodiment, the ITO thin film 4,8 comprises two layers: a small-grain ITO film 41 in contact with the doped amorphous silicon layer 3,7, and a large-grain ITO film 42 in contact with the metal electrodes 5,9. The small-grain ITO film 41 has a grain size of 1nm to 20nm, a columnar crystal structure, and a thickness of 10nm to 30nm; the large-grain ITO film 42 has a grain size of 20nm to 500nm, an equiaxed crystal structure, and a thickness of 40nm to 90nm.

[0045] Preferably, the ITO films 4 and 8 located on both sides of the silicon substrate 1 are symmetrically arranged, that is, the inner film layer and the outer film layer on both sides are the same.

[0046] The present invention also provides a method for fabricating a solar cell, comprising: sequentially forming the intrinsic amorphous silicon layer 2, 6, the doped amorphous silicon layer 3, 7, the ITO thin film 4, 8, and the metal electrode 5, 9 on one side of the silicon substrate 1.

[0047] This invention improves the performance of solar cells by modifying the preparation process of the ITO thin films 4 and 8. The processes for other film layers are based on existing technologies and will not be described in detail here.

[0048] Specifically, forming the ITO thin film 4,8 includes forming at least two ITO films with different grain sizes, which can balance and take into account the conductivity, light transmittance and stability of the entire ITO thin film 4,8, improve its matching degree with other adjacent film layers in terms of optics, electricity, etc., and improve the performance of the solar cell.

[0049] The process for forming the small-grain ITO film 41 in contact with the doped amorphous silicon layer is as follows: the deposition pressure is 0.7–1.5 Pa, and the power density is 2–8 kW / m. The small-grain ITO film has low contact resistance in contact with the doped amorphous silicon layer.

[0050] The process for forming the large-grain ITO film 42 in contact with the metal electrode is as follows: the coating pressure is 0.3 Pa to 0.7 Pa, and the power density is 5 to 15 kW / m. The large-grain ITO film 42, which has fewer grain boundaries, higher stability, and higher mobility, is located on the outer side, which improves the stability of the entire ITO film 4,8 and is beneficial to improving the stability of the solar cell 100.

[0051] Typically, the ITO thin film can consist of two ITO films. When it consists of three or more ITO films, the grain size of the at least three ITO films gradually increases along the direction from the doped amorphous silicon layer 3,7 to the metal electrode 5,9, which can further improve the performance of the solar cell 100.

[0052] In one specific embodiment, forming the ITO thin film 4,8 includes: forming a small-grain ITO film 41 in contact with the intrinsic amorphous silicon layer and forming a large-grain ITO film 42 in contact with the metal electrode.

[0053] The grain size and thickness of the small-grain ITO film 41 and the large-grain ITO film 42 are as described above and will not be repeated here.

[0054] The following provides a method for fabricating a solar cell, including the following steps:

[0055] S1 silicon wafer: N-type silicon wafer with resistivity of 0.5-3 Ω·cm, thickness of 150-200 μm, and dimensions of 156.75 cm.

[0056] S2 cleaning and texturing: The surface oxide layer is removed using a 5% HF solution. A shallow pyramid structure is formed on the silicon wafer surface by using KOH, NaOH, or tetramethylammonium hydroxide (TMAH) with alcohol and anisotropic etching of single-crystal silicon.

[0057] S3 forms the intrinsic amorphous silicon layers 2 and 6, and the doped amorphous silicon layers 3 and 7:

[0058] Silane (SiH4) gas is introduced into a vacuum chamber, and the first intrinsic amorphous silicon layer 2 is formed over the entire area of ​​the first surface of the silicon substrate 1 by plasma CVD. Then, SiH4 gas, H2 gas, and PH3 gas are introduced into the vacuum chamber, and an N-type amorphous silicon layer 3 is formed on the first intrinsic amorphous silicon layer 2 by plasma CVD.

[0059] Next, the silicon substrate is flipped over, the tray is changed, and SiH4 gas is introduced into the vacuum chamber. The second intrinsic amorphous silicon layer 6 is then formed over the entire area of ​​the second surface of the silicon substrate 1 by plasma CVD. Then, SiH4 gas, H2 gas, and B2H6 (diborane) gas are introduced into the vacuum chamber, and a p-type amorphous silicon layer 7 is formed on the second intrinsic amorphous silicon layer 6 by plasma CVD.

[0060] S4 deposition of ITO thin films 4,8:

[0061] ITO coating is performed on the doped amorphous silicon layers 3 and 7 on the front and back sides using reactive plasma deposition (RPD) or magnetron sputtering. The back side is shielded by an edge design using a carrier disk, i.e., by a mask. The specific shielding area around the edges is 0.8 mm, which plays a role in preventing skewing during the deposition process.

[0062] During ITO coating, ITO films with different grain sizes are sequentially deposited on the first and second sides of the substrate after the deposition of the doped amorphous silicon layers 3 and 7.

[0063] The specific method is as follows: The PVD mass production equipment has at least four non-contaminating coating target sites, each equipped with a different target material; the substrate is placed on a carrier plate and sequentially passes through different target sites for coating to obtain the desired film design. O2 atmosphere is adjusted at different target sites to regulate the TCO film's work function and carrier concentration to the required range.

[0064] After CVD, the silicon wafer is mounted on a carrier. The carrier is a perforated flat plate design with raised edges of about 0.6-0.8 mm at the cutouts to support the wafer and expose the front and back areas for coating. The coating process uses physical vapor deposition, which uses energy to bombard the phases in the target material while simultaneously introducing a specific gas to create a suitable atmosphere. This atmosphere typically consists of 90%–99% Ar, 1%–6% O2, and 0%–4% H2. Different Ar, O2, and H2 flow rates are used for different work functions. Specific process selections are described in detail in different embodiments.

[0065] S5 Printed Electrode: A layer of low-temperature conductive silver paste is printed on the front and back ITO films 4,8 using screen printing, and then sintered at a low temperature of 150℃~300℃ to form a good ohmic contact.

[0066] The present invention will be described below through specific embodiments and comparative examples.

[0067] Comparative Example 1: Please refer to Figure 1 The solar cell structure shown.

[0068] The first ITO thin film 4 on the front side is a single-layer film with an indium oxide to tin oxide ratio of 97:3 and a film thickness of 70nm to 75nm. The preparation process is as follows: Ar, O2, and H2 gases are introduced in a certain flow rate ratio to obtain an ITO thin film with an indium oxide to tin oxide ratio of 97:3. The deposition process pressure in the process chamber is 0.7Pa to 1.0Pa. The grain size of the obtained first ITO thin film 4 is in the range of 1-10nm.

[0069] The second ITO film 8 on the back side is a single-layer film with an indium oxide to tin oxide ratio of 90:10 and a film thickness of 70nm to 75nm. The fabrication process involves introducing Ar, O2, and H2 gases at specific flow rates to obtain an ITO film with an indium oxide to tin oxide ratio of 90:10. The deposition process pressure within the process chamber is 0.7Pa to 1.0Pa. The grain size of the obtained second ITO film 8 is in the range of 1 to 10nm.

[0070] Example 1: Please refer to Figure 1 The solar cell structure shown.

[0071] The first ITO thin film 4 on the front side is a single-layer film with an indium oxide to tin oxide ratio of 97:3 and a film thickness of 70nm to 75nm. The preparation process is as follows: Ar, O2, and H2 gases are introduced in a certain flow rate ratio to obtain an ITO thin film with an indium oxide to tin oxide ratio of 97:3. The deposition process pressure in the process chamber is 0.5 to 0.7 Pa. The grain size of the obtained first ITO thin film 4 is in the range of 20nm to 30nm.

[0072] The second ITO thin film 8 on the back side is a single-layer structure with an indium oxide to tin oxide ratio of 90:10 and a film thickness of 70nm to 75nm. Ar, O2, and H2 gases of a certain flow rate ratio are introduced to obtain an ITO thin film with an indium oxide to tin oxide ratio of 90:10. The deposition process pressure within the process chamber is 0.5 to 0.7 Pa. The grain size of the obtained second ITO thin film 8 is in the range of 20 to 30nm.

[0073] Example 2: Please refer to Figure 2 The solar cell structure shown.

[0074] The first ITO film 4 on the front side consists of two ITO layers. The ITO film in contact with the first doped amorphous silicon layer 3 has an indium oxide to tin oxide ratio of 97:3 and a film thickness of 5 nm to 10 nm. The fabrication process involves introducing Ar, O2, and H2 gases at specific flow rates to achieve an indium oxide:tin oxide ratio of 97:3. The deposition pressure within the inner ITO film process chamber is 0.7 Pa to 1.0 Pa, and the film grain size is between 1 nm and 10 nm. The ITO film in contact with the first metal electrode 5 has an indium oxide to tin oxide ratio of 97:3 and a film thickness of 65 nm to 70 nm. The fabrication process involves introducing Ar, O2, and H2 gases at specific flow rates to achieve an indium oxide:tin oxide ratio of 97:3. The deposition pressure within the outer ITO film process chamber is 0.5 Pa to 0.7 Pa, and the film grain size is between 20 nm and 30 nm.

[0075] The second ITO film 8 on the back side consists of two ITO layers. The ITO film in contact with the second doped amorphous silicon layer 7 has an indium oxide to tin oxide ratio of 90:10, a work function of 4.6–4.8 eV, and a film thickness of 5 nm–10 nm. The fabrication process involves introducing Ar, O2, and H2 gases at specific flow rates to achieve an indium oxide:tin oxide ratio of 90:10. The deposition pressure within the inner ITO film process chamber is 0.7 Pa–1.0 Pa, and the film grain size is between 1 nm and 10 nm. The ITO film in contact with the second metal electrode 9 also has an indium oxide to tin oxide ratio of 90:10 and a film thickness of 65 nm–70 nm. The preparation process is as follows: Ar, O2 and H2 gases with a certain flow rate ratio are introduced so that the ratio of indium oxide to tin oxide is 90:10, the coating process pressure in the outer ITO film process chamber is 0.5 Pa to 0.7 Pa, and the film grain size is between 20 nm and 30 nm.

[0076] The performance of the solar cells 100 corresponding to the comparative examples, Example 1, and Example 2 is shown in Table 1, where the data for Example 1 and Example 2 are changes relative to the comparative examples.

[0077] Table 1

[0078] Group Eff(%) Voc(V) Isc(A) FF (%) Comparative Example 24.51 744.9 6.416 84.92 Example 1 +0.05 +0.1 +0.028 -0.2 Example 2 +0.10 +0.1 +0.025 +0.01

[0079] In the comparative example, both the front and back ITO films are small-grained films formed via a high-voltage route, exhibiting numerous grain boundaries, poor light transmittance, but superior electrical properties and a high fairing (FF). In Example 1, both the front and back ITO films are large-grained films formed via a low-voltage route, offering better light transmittance but slightly inferior electrical performance. In Example 2, both the front and back ITO films utilize a combination of ITO films with different grain sizes: the inner layer is a small-grained ITO film 41 formed via a high-voltage route, while the outer layer is a large-grained ITO film 42 formed via a low-voltage route.

[0080] Comparing the comparative example with Example 1, it can be seen that the optical performance of the front ITO film in Example 1 is improved, and the short-circuit current Isc increases significantly by 28mA; however, the contact deteriorates, the FF decreases by 0.2, the open-circuit voltage Voc increases slightly by 0.1mV, and the efficiency gain is 0.05%.

[0081] Comparison of the comparative example and Example 2 shows that the optical performance of the front ITO film is improved, the short-circuit current Isc increases by 25mA, the contact is comparable to the comparative example, the FF remains unchanged, the open-circuit voltage Voc increases slightly by 0.1mV, and the efficiency gain is 0.10%.

[0082] In summary, compared to existing technologies, the ITO thin film of this invention comprises at least two film layers with different grain sizes, resulting in superior IV performance of the solar cell and further ensuring product reliability. The inner film uses smaller grains and higher carrier concentration to ensure contact; the outer film uses larger grains and higher mobility to ensure light transmittance.

[0083] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0084] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A solar cell, comprising a silicon substrate, and sequentially disposed on one side of the silicon substrate an intrinsic amorphous silicon layer, a doped amorphous silicon layer, an ITO thin film, and a metal electrode, characterized in that, The ITO thin film comprises at least two ITO films, each with a different grain size. The ITO thin film includes a small-grain ITO film in contact with the doped amorphous silicon layer and a large-grain ITO film in contact with the metal electrode. The grain size of the small-grain ITO film is smaller than that of the large-grain ITO film, and the thickness of the large-grain ITO film is greater than that of the small-grain ITO film. The small-grain ITO film has a columnar crystal structure, and the large-grain ITO film has an equiaxed crystal structure.

2. The solar cell according to claim 1, characterized in that: The ITO thin film comprises at least three ITO film layers, and the grain size of the at least three ITO film layers gradually increases along the direction from the doped amorphous silicon layer to the metal electrode.

3. The solar cell according to claim 1, characterized in that: The grain size of the small-grain ITO film is 1nm~20nm.

4. The solar cell according to claim 1, characterized in that: The grain size of the large-grain ITO film is 20nm~500nm.

5. The solar cell according to claim 1, characterized in that: The solar cell includes a silicon substrate, a first intrinsic amorphous silicon layer, a first doped amorphous silicon layer, a first ITO thin film, and a first metal electrode located sequentially on a first side of the silicon substrate, and a second intrinsic amorphous silicon layer, a second doped amorphous silicon layer, a second ITO thin film, and a second metal electrode located sequentially on a second side of the silicon substrate; the first ITO thin film and the second ITO thin film located on both sides of the silicon substrate are symmetrically arranged.

6. The solar cell according to claim 1, characterized in that: The small-grain ITO film has a grain size of 1nm to 20nm and a thickness of 10nm to 30nm; the large-grain ITO film has a grain size of 20nm to 500nm and a thickness of 40nm to 90nm.

7. The solar cell according to claim 6, characterized in that: The thickness of the ITO thin film is 50 nm to 120 nm, and the thickness of the large-grain ITO film is 40 nm to 90 nm.

8. A method for preparing a solar cell, comprising: An intrinsic amorphous silicon layer, a doped amorphous silicon layer, an ITO thin film, and a metal electrode are sequentially formed on one side of a silicon substrate. The ITO thin film is characterized by forming a small-grain ITO film in contact with the doped amorphous silicon layer and a large-grain ITO film in contact with the metal electrode. The grain size of the small-grain ITO film is smaller than that of the large-grain ITO film, and the thickness of the large-grain ITO film is greater than that of the small-grain ITO film. The small-grain ITO film has a columnar crystal structure, and the large-grain ITO film has an equiaxed crystal structure.

9. The method for preparing a solar cell according to claim 8, characterized in that: The process for forming a small-grain ITO film in contact with the doped amorphous silicon layer is as follows: the coating pressure is 0.7~1.5 Pa and the power density is 2~8 kW / m.

10. The method for preparing a solar cell according to claim 8, characterized in that: The process for forming a large-grain ITO film in contact with the metal electrode is as follows: the coating pressure is 0.3 Pa to 0.7 Pa, and the power density is 5 to 15 kW / m.

11. The method for preparing a solar cell according to claim 8, characterized in that: The grain size of small-grain ITO films is 1nm~20nm, while the grain size of large-grain ITO films is 20nm~500nm.

12. The method for preparing a solar cell according to claim 9 or 11, characterized in that: The grain size of the small-grain ITO film is 1~20nm, and the thickness of the small-grain ITO film is 10nm~30nm.

13. The method for preparing a solar cell according to claim 10 or 11, characterized in that: The grain size of the large-grain ITO film is 20~500nm, and the thickness of the large-grain ITO film is 40 nm~90nm.

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