Front cleaning method for high-efficiency gettering of crystalline silicon heterojunction solar cells

Abstract

This application relates to the technical field of silicon wafer processing, specifically disclosing a pre-cleaning method for high-efficiency gettering in crystalline silicon heterojunction solar cells. This method includes steps such as pre-cleaning with a mixed acid solution, texturing, post-cleaning with a mixed acid solution and hydrofluoric acid, followed by phosphorus diffusion gettering treatment of the resulting silicon wafer. The mixed acid solution can micro-etch the silicon wafer surface to remove metallic impurities. During the micro-etching texturing process, the silicon wafer surface develops a pyramidal morphology, increasing the specific surface area and thus expanding the gettering area. Simultaneously, because the texturing process uses micro-etching to preserve the mechanically damaged areas on the silicon wafer surface, the combination of texturing and the mechanically damaged areas creates dual gettering centers in the subsequent gettering process. Even if the back side of the silicon wafer is not subjected to phosphorus diffusion gettering, the presence of the mechanically damaged areas on the back side can still achieve a good gettering effect, thereby enabling gettering on both the diffused surface and the non-diffused back side of the silicon wafer, improving the gettering efficiency.

Description

Technical Field

[0001] This application relates to the technical field of silicon wafer processing, and more specifically, to a pre-cleaning method for efficient gettering of crystalline silicon heterojunction solar cells. Background Technology

[0002] A solar cell is a device that converts light energy into electrical energy using the photovoltaic effect. The core material of a solar cell is the silicon wafer; therefore, the quality of the silicon wafer is crucial in the manufacturing process of high-efficiency crystalline silicon solar cells.

[0003] During the manufacturing process of silicon wafers, various metallic impurities, such as iron, copper, cobalt, and nickel, are inevitably introduced. These impurities, when doped into the silicon wafer, form deep energy levels, becoming recombination centers for minority carriers and reducing the minority carrier lifetime. A shorter minority carrier lifetime means that excess carriers generated by illumination are more likely to reach the PN junction of the cell and are less likely to be separated by the PN junction's electric field to generate photocurrent. This directly leads to a decrease in the short-circuit current of the silicon wafer, resulting in a decline in the photoelectric conversion efficiency and the overall performance of the solar cell.

[0004] To improve silicon wafer quality, gettering is necessary. Various methods exist for silicon wafer gettering, with phosphorus diffusion gettering and mechanical damage gettering being two important approaches. Currently, gettering typically involves polishing the silicon wafer to remove the damaged layer on its surface, followed by phosphorus diffusion gettering. This method has the following drawbacks: During gettering, two silicon wafers are overlapped back-to-back for phosphorus diffusion gettering, meaning it's a single-sided process. During diffusion, phosphorus only diffuses onto one side of the wafer, forming a heavy phosphorus doped layer and a phosphorus-silicon glass layer. Removing these layers removes the impurities. However, this process only removes impurities from one side of the wafer in a single phosphorus diffusion process, limiting gettering efficiency. Summary of the Invention

[0005] In order to improve the gettering efficiency of silicon wafers, this application provides a pre-cleaning method for efficient gettering of crystalline silicon heterojunction solar cells.

[0006] This application provides a pre-cleaning method for high-efficiency gettering of crystalline silicon heterojunction solar cells, employing the following technical solution:

[0007] A pre-cleaning method for high-efficiency gettering in crystalline silicon heterojunction solar cells includes the following steps:

[0008] Pre-cleaning: The cut raw silicon wafers are first cleaned in a mixed acid solution, and then washed in deionized water. The mixed acid solution contains oxidant, hydrochloric acid and hydrofluoric acid.

[0009] Texturing: The silicon wafers obtained from the previous cleaning are placed in an alkaline solution for texturing and cleaning, and then washed in deionized water. The alkaline solution contains a strong alkali with a concentration of 0.5wt% to 1wt% and a texturing additive with a concentration of 0.2wt% to 0.5wt%. The strong alkali is KOH and / or NaOH. The texturing temperature is 60 to 80℃ and the texturing time is 80 to 120 seconds.

[0010] Post-cleaning: The silicon wafers obtained after texturing are placed in a mixed acid solution for cleaning, then washed in deionized water, then washed in hydrofluoric acid solution, and finally washed in deionized water. After washing, they are dried.

[0011] Gettering: The silicon wafers obtained after cleaning are subjected to phosphorus diffusion gettering.

[0012] Furthermore, the oxidant in the mixed acid solution is ozone water and / or hydrogen peroxide.

[0013] Furthermore, the oxidant in the mixed acid solution is ozone water with a concentration of 40-60 ppm.

[0014] Furthermore, the concentration of hydrochloric acid in the mixed acid solution is 0.01 wt% to 0.05 wt%.

[0015] Furthermore, the concentration of hydrofluoric acid in the mixed acid solution is 0.2 wt% to 0.5 wt%.

[0016] Furthermore, the soaking time of the mixed acid solution in the pre-cleaning step is 100-120 seconds.

[0017] Furthermore, in the flocking step, the concentration of the strong alkali is 0.8 wt%, the concentration of the flocking additive is 0.3 wt%, the flocking temperature is 80°C, and the flocking time is 100 s.

[0018] Furthermore, the texturing additive is a mature chemical used in the photovoltaic industry, and its main components are 3-4 wt% nucleating agent, 6-8 wt% defoaming agent, 6-8 wt% nonionic surfactant, and 2-3 wt% other additives, with the remainder made up with water.

[0019] Furthermore, the soaking time of the mixed acid solution in the subsequent cleaning step is 100-120 seconds.

[0020] Furthermore, in the post-cleaning step, the concentration of hydrofluoric acid is 3wt% to 10wt%.

[0021] Furthermore, in the subsequent cleaning step, the immersion time in hydrofluoric acid is 100–120 seconds.

[0022] Furthermore, the resistivity of the deionized water is 18.2 MΩ·cm.

[0023] In summary, this application has at least the following advantages:

[0024] First, the polishing step of the silicon wafer is omitted in this application. After cleaning the original silicon wafer after cutting, texturing is performed while retaining the mechanical damage of the silicon wafer. Double getter centers are formed on the silicon wafer. Getter centers exist on both the diffusion surface and the back side of the silicon wafer. In particular, due to the mechanical damage area and pyramid shape on the back side of the silicon wafer, there are a large number of lattice defects in the mechanical damage area, which allows getter centers to be formed on the back side of the silicon wafer without diffusion, thus having a certain gettering ability.

[0025] Secondly, the pyramidal morphology of the silicon wafer surface increases its specific surface area. Furthermore, due to the inherent mechanical damage on the silicon wafer surface, the specific surface area after texturing is even larger than that of a polished wafer. This maximizes the getter area on both the exposed and back surfaces of the silicon wafer, facilitating thorough gettering and efficient removal of metallic impurities from the wafer surface.

[0026] Secondly, this application controls the concentration of strong alkali in the texturing step to make the concentration of strong alkali moderate, so that the pyramid morphology on the silicon wafer surface is more complete and the specific surface area of ​​the silicon wafer surface is maximized, thereby effectively forming efficient gettering centers. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the original silicon wafer used in the pre-cleaning step of Embodiment 1 of this application.

[0028] Figure 2 This is a schematic diagram of the silicon wafer structure obtained by the texturing step in Embodiment 1 of this application.

[0029] Figure 3 This is a schematic diagram of the silicon wafer structure obtained by the gettering step in Embodiment 1 of this application. Detailed Implementation

[0030] The applicant has modified the pre-cleaning process before the silicon wafer getter step. While retaining the mechanically damaged areas on the silicon wafer surface, texturing is performed directly, increasing the specific surface area of ​​the silicon wafer. Both the diffusion surface and the back side of the silicon wafer exhibit a pyramidal morphology, and both contain mechanically damaged areas. The mechanically damaged areas retained on the non-diffusion surface allow getter centers to form on the back side of the silicon wafer without phosphorus diffusion. In a single getter step, gettering can be performed on the back sides of back-to-back overlapping silicon wafers, rather than on one side, significantly improving the gettering efficiency. Actual testing shows that the silicon wafers produced using this application have a significantly improved average minority carrier lifetime compared to traditional cleaning and gettering methods, and the photoelectric conversion efficiency of the resulting solar cells can be improved by more than 0.1%.

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this application will be further described below in conjunction with embodiments and comparative examples.

[0032] Example

[0033] Example 1

[0034] A pre-cleaning method for high-efficiency gettering in crystalline silicon heterojunction solar cells is performed according to the following steps:

[0035] Pre-cleaning: Immerse the cut raw silicon wafers in a mixed acid solution of 25°C, 50ppm ozone water, 0.02wt% HCl, and 0.3wt% HF for 100 seconds.

[0036] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at room temperature for 100 seconds.

[0037] Texturing: The silicon wafer obtained from the pre-cleaning process is placed in a solution with a temperature of 80°C, a KOH concentration of 0.5%, and a texturing additive concentration of 0.3% for micro-texturing treatment for 100 seconds. In this embodiment, the texturing additive is TS53.

[0038] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at 25°C for 100 seconds.

[0039] Post-cleaning: The silicon wafers obtained after texturing are immersed in a mixed acid solution of 25°C, 50ppm ozone water, 0.02wt% HCl, and 0.3wt% HF for 100 seconds.

[0040] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at 25°C for 100 seconds.

[0041] The treated silicon wafers were immersed in a 5% HF solution at 25°C for 110 seconds.

[0042] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at 25°C for 100 seconds.

[0043] The processed silicon wafers are then dehydrated and dried.

[0044] Gettering: Using a tubular high-temperature phosphorus source diffusion method, two cleaned silicon wafers are placed back-to-back into the same slot on a quartz boat. The quartz boat has several slots.

[0045] The quartz boat with the silicon wafers inserted is sent into the tube diffusion furnace, and the furnace door is closed.

[0046] The main diffusion step temperature in the furnace is 860°C, and nitrogen, oxygen, and phosphorus oxychloride (carried by nitrogen) are introduced.

[0047] Example 2-3

[0048] A pre-cleaning method for high-efficiency gettering of crystalline silicon heterojunction solar cells differs from Example 1 in that the concentration of strong alkali in the texturing step is different, as detailed below:

[0049] In Example 2, the concentration of the strong alkali used in the texturing step was 0.8 wt%.

[0050] In Example 3, the concentration of the strong alkali used in the texturing step was 1 wt%.

[0051] Example 4

[0052] A pre-cleaning method for high-efficiency gettering in crystalline silicon heterojunction solar cells is performed according to the following steps:

[0053] Pre-cleaning: Immerse the cut raw silicon wafers in a mixed acid solution of 25°C, 40ppm ozone water, 0.05wt% HCl, and 0.2wt% HF for 100 seconds.

[0054] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at room temperature for 100 seconds.

[0055] Texturing: The silicon wafer obtained from the pre-cleaning process is placed in a solution with a temperature of 60°C, a KOH concentration of 0.8%, and a texturing additive concentration of 0.2% for micro-texturing treatment for 100 seconds. In this embodiment, the texturing additive is TS53.

[0056] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at 25°C for 100 seconds.

[0057] Post-cleaning: The silicon wafers obtained after texturing are immersed in a mixed acid solution of 25°C, 40ppm ozone water, 0.05wt% HCl, and 0.2wt% HF for 100 seconds.

[0058] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at 25°C for 100 seconds.

[0059] The treated silicon wafers were immersed in a 3% HF solution at 25°C for 120 seconds.

[0060] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at 25°C for 100 seconds.

[0061] The processed silicon wafers are then dehydrated and dried.

[0062] Gettering: Using a tubular high-temperature phosphorus source diffusion method, two cleaned silicon wafers are placed back-to-back into the same slot on a quartz boat. The quartz boat has several slots.

[0063] The quartz boat with the silicon wafers inserted is sent into the tube diffusion furnace, and the furnace door is closed.

[0064] The main diffusion step temperature in the furnace is 860°C, and nitrogen, oxygen, and phosphorus oxychloride (carried by nitrogen) are introduced.

[0065] Example 5

[0066] A pre-cleaning method for high-efficiency gettering in crystalline silicon heterojunction solar cells is performed according to the following steps:

[0067] Pre-cleaning: Immerse the cut raw silicon wafers in a mixed acid solution of 25°C, 60ppm ozone water, 0.01wt% HCl, and 0.5wt% HF for 100 seconds.

[0068] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at room temperature for 100 seconds.

[0069] Texturing: The silicon wafer obtained from the pre-cleaning process is placed in a solution with a temperature of 80°C, a KOH concentration of 0.8%, and a texturing additive concentration of 0.5% for micro-texturing treatment for 100 seconds. In this embodiment, the texturing additive is TS53.

[0070] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at 25°C for 100 seconds.

[0071] Post-cleaning: The silicon wafers obtained after texturing are immersed in a mixed acid solution of 25°C, 60ppm ozone water, 0.01wt% HCl, and 0.5wt% HF for 100 seconds.

[0072] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at 25°C for 100 seconds.

[0073] The treated silicon wafers were immersed in a 10% HF solution at 25°C for 100 seconds.

[0074] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at 25°C for 100 seconds.

[0075] The processed silicon wafers are then dehydrated and dried.

[0076] Gettering: Using a tubular high-temperature phosphorus source diffusion method, two cleaned silicon wafers are placed back-to-back into the same slot on a quartz boat. The quartz boat has several slots.

[0077] The quartz boat with the silicon wafers inserted is sent into the tube diffusion furnace, and the furnace door is closed.

[0078] The main diffusion step temperature in the furnace is 860°C, and nitrogen, oxygen, and phosphorus oxychloride (carried by nitrogen) are introduced.

[0079] Comparative Example

[0080] Comparative Example 1

[0081] A gettering method for crystalline silicon heterojunction solar cells comprises the following steps:

[0082] The original silicon wafer was immersed in a mixture of room temperature, 50 ppm ozone water, 0.02% HCl, and 0.3% HF for 100 seconds.

[0083] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at room temperature for 100 seconds.

[0084] The treated silicon wafers were placed in a solution with a temperature of 80℃ and a KOH concentration of 5% for SDE treatment for 100 seconds.

[0085] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at room temperature for 100 seconds.

[0086] The original silicon wafer was immersed in a mixture of room temperature, 50 ppm ozone water, 0.02% HCl, and 0.3% HF for 100 seconds.

[0087] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at room temperature for 100 seconds.

[0088] The treated silicon wafers were immersed in a 5% HF solution at room temperature for 110 seconds.

[0089] The treated silicon wafer was immersed in deionized water with a resistivity of 18.2 MΩ·cm at room temperature for 100 seconds.

[0090] The processed silicon wafers are then dehydrated and dried.

[0091] Gettering: Using a tubular high-temperature phosphorus source diffusion method, two cleaned silicon wafers are placed back-to-back into the same slot on a quartz boat. The quartz boat has several slots.

[0092] The quartz boat with the silicon wafers inserted is sent into the tube diffusion furnace, and the furnace door is closed.

[0093] The main diffusion step temperature in the furnace is 860°C, and nitrogen, oxygen, and phosphorus oxychloride (carried by nitrogen) are introduced.

[0094] Performance testing

[0095] Sample preparation:

[0096] The silicon wafers obtained in Examples 1-5 and Comparative Example 1 were all precisely sliced, with each wafer measuring 166mm × 166mm and having an area of ​​274.15cm². 2 The specifications of the N-type monocrystalline silicon wafers minimize testing errors caused by the wafer source.

[0097] The silicon wafers obtained in Examples 1-5 and Comparative Example 1 were sequentially subjected to phosphorus diffusion gettering, texturing, PECVD, PVD, and screen printing to obtain solar cells. The parameters in each processing step, such as phosphorus diffusion gettering, texturing, PECVD, PVD, and screen printing, were kept the same to further reduce the test errors caused by subsequent processing steps.

[0098] 1. Minority carrier lifetime: Minority carrier lifetime was tested on silicon wafers after PECVD processing. Tests were performed at 9 points on the silicon wafer using a WCT-120 instrument. The average value of the 9 tests was taken, in µs.

[0099] 2. Cell performance testing: The cell after screen printing is subjected to electrical performance tests, including open circuit voltage (Uoc, mV), short circuit current (Isc, A), fill factor (FF, %), and solar cell photoelectric conversion efficiency (EFF, %).

[0100] The tested light intensity was 1000 W / m. 2 The power of light on the silicon wafer is P0 = 0.1 × 274.15 = 27.415 W;

[0101] Maximum power Pm = Uoc × Isc × FF;

[0102] The photoelectric conversion efficiency of a solar cell is EFF = Pm / P0 × 100%.

[0103] Test results

[0104] Table 1. Minority carrier lifetime test results of Examples 1-5 and Comparative Example 1

[0105]

[0106] Table 2. Electrical performance test results of the battery cells prepared in Examples 1-5 and Comparative Example 1

[0107]

[0108]

[0109] Combining Examples 1-3 and Comparative Example 1 with Tables 1-2, it can be seen that Comparative Example 1 uses a traditional getter pretreatment process, first polishing with alkaline solution and then gettingtering. According to actual test results, the silicon wafers produced by polishing first and then gettingtering have a shorter minority carrier lifetime, only 2494 μs. The minority carrier lifetime of the silicon wafers obtained in the examples ranges from 2835 to 3191 μs, with Example 2 showing the highest at 3191 μs. Furthermore, the final photoelectric conversion efficiency of the solar cells obtained in the examples is higher than that of the comparative example, with Example 2 showing a 0.11% higher photoelectric conversion efficiency than the comparative example, consistent with theoretical results.

[0110] In Example 1, the low alkali concentration resulted in a poor textured surface with a small specific surface area and slightly poor getter removal effect. In Example 3, the textured surface was acceptable, but the increased corrosion resulted in a smaller damaged layer, leading to a slightly poorer getter removal effect. In Example 2, a better damaged layer and a better specific surface area were obtained, resulting in a better getter removal effect.

[0111] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0112] Furthermore, the above-described embodiments merely illustrate several implementation methods of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A front cleaning method for efficient gettering of a crystalline silicon heterojunction solar cell, characterized in that, Includes the following steps: Pre-cleaning: The cut raw silicon wafers are first cleaned in a mixed acid solution, and then washed in deionized water. The mixed acid solution contains oxidant, hydrochloric acid and hydrofluoric acid. Texturing: The silicon wafers obtained from the previous cleaning are placed in an alkaline solution for texturing and cleaning, and then washed in deionized water. The alkaline solution contains a strong alkali with a concentration of 0.5wt% to 1wt% and a texturing additive with a concentration of 0.2wt% to 0.5wt%. The strong alkali is KOH and / or NaOH. The texturing temperature is 60 to 80℃ and the texturing time is 80 to 120 seconds. Post-cleaning: The silicon wafers obtained after texturing are placed in a mixed acid solution for cleaning, then washed in deionized water, then washed in hydrofluoric acid solution, and finally washed in deionized water. After washing, they are dried. Gettering: The silicon wafers obtained after cleaning are subjected to phosphorus diffusion gettering.

2. The method for high efficiency of pre-cleaning of gettering of crystalline silicon heterojunction solar cells as claimed in claim 1, wherein: The oxidant in the mixed acid solution is ozone water and / or hydrogen peroxide.

3. The method for high efficiency of pre-cleaning of gettering of crystalline silicon heterojunction solar cells as claimed in claim 2, wherein: The oxidant in the mixed acid solution is ozone water with a concentration of 40-60 ppm.

4. The method for high efficiency of pre-cleaning of gettering of crystalline silicon heterojunction solar cells as claimed in claim 2, wherein: The concentration of hydrochloric acid in the mixed acid solution is 0.01 wt% to 0.05 wt%.

5. The method for high efficiency of pre-cleaning of gettering of crystalline silicon heterojunction solar cells as claimed in claim 2, wherein: The concentration of hydrofluoric acid in the mixed acid solution is 0.2 wt% to 0.5 wt%.

6. The pre-cleaning method for high-efficiency gettering of crystalline silicon heterojunction solar cells as described in claim 2, characterized in that: The soaking time of the mixed acid solution in the pre-cleaning step is 100-120 seconds.

7. The pre-cleaning method for high-efficiency gettering of crystalline silicon heterojunction solar cells as described in claim 1, characterized in that: In the flocking step, the concentration of the strong alkali is 0.8 wt%, the concentration of the flocking additive is 0.3 wt%, the flocking temperature is 70°C, and the flocking time is 100 s.

8. The pre-cleaning method for high-efficiency gettering of crystalline silicon heterojunction solar cells as described in claim 1, characterized in that: The soaking time of the mixed acid solution in the post-cleaning step is 100-120 seconds.

9. The pre-cleaning method for high-efficiency gettering of crystalline silicon heterojunction solar cells as described in claim 1, characterized in that: In the post-cleaning step, the concentration of hydrofluoric acid is 3wt% to 10wt%.

10. The pre-cleaning method for high-efficiency gettering of crystalline silicon heterojunction solar cells as described in claim 9, characterized in that, In the subsequent cleaning step, the soaking time in hydrofluoric acid is 100-120 seconds.

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