Crystalline silicon solar cell, preparation method thereof and groove body device
By performing multi-stage temperature regulation on silicon wafers under a temperature variable environment, the problem of efficiency reduction caused by constant temperature velvet making is solved, forming a more uniform suede structure, and improving the photoelectric conversion efficiency of solar cells.
Patent Information
- Application Number
- CN202411232198.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-03
- Publication Date
- 2025-01-14
AI Technical Summary
The pyramid suede structure produced by the constant temperature velvet making method is not good for subsequent passivation and metallization processes, resulting in a decrease in solar cell efficiency, and the alkali and silicon wafer reaction temperature sensitivity are strong, making it difficult to regulate the suede structure.
Silicon wafers are fleece-making under a variable temperature environment, and through different temperature controls over multiple time periods, a more uniform pyramid suede structure is formed, including polishing, pre-cleaning, fleece making, cleaning and drying, using alkali and hydrogen peroxide. The mixed solution is corroded and cleaned, and a mixed acid solution of hydrofluoric acid and hydrochloric acid is used to remove metal ions.
It improves the photoelectric conversion efficiency of solar cell cells, forms a more uniform suede structure, enhances suede control ability, improves light absorption efficiency, and is suitable for a variety of solar cell process technologies.
Smart Images
Figure CN119317217A_ABST
Abstract
Description
Technical Field
[0001] The present disclosure relates to the field of solar cells, and more specifically, to a crystalline silicon solar cell and a preparation method and a trough device thereof. Background Art
[0002] In the field of solar cells, a constant temperature texturing and cleaning method is currently widely used, which uses a mixed solution of alkali, texturing additives and pure water to corrode the surface of the silicon wafer at a constant temperature to form a random pyramid light-trapping structure.
[0003] However, the pyramid velvet structure produced by the constant temperature velvet cleaning method will have an adverse effect on subsequent passivation, metallization and other processes, thereby reducing efficiency. Summary of the invention
[0004] In order to solve the above technical problems, the present disclosure provides a crystalline silicon solar cell and a preparation method and a trough device, which can better control the preparation process of the velvet structure, improve the uniformity of the velvet structure, and further improve the photoelectric conversion efficiency of the solar cell.
[0005] The present disclosure provides a method for preparing a velvet structure of a crystalline silicon solar cell, comprising: texturing the silicon wafer in a variable temperature environment to obtain the velvet structure of the crystalline silicon solar cell, wherein the variable temperature environment corresponds to multiple time periods, and the temperatures in different time periods are not all the same.
[0006] In some embodiments, the method for preparing a crystalline silicon solar cell further includes: in a plurality of time periods corresponding to the variable temperature environment, the temperature in a preceding time period is greater than the temperature in a succeeding time period.
[0007] In some embodiments, texturing the silicon wafer in a variable temperature environment also includes: texturing the silicon wafer at a first temperature in a first time period; and texturing the silicon wafer at a second temperature in a second time period; wherein the first temperature is greater than the second temperature.
[0008] In some embodiments, the variable temperature environment also includes a third time period; the third time period is located between the first time period and the second time period; texturing the silicon wafer in the variable temperature environment also includes: in the third time period, texturing the silicon wafer in the process of the temperature decreasing from the first temperature to the second temperature.
[0009] In some embodiments, before texturing the silicon wafer in a variable temperature environment, the process further includes: polishing the silicon wafer, including: polishing the silicon wafer at 65 to 85° C. for 20 to 100 seconds using an alkaline solution, wherein the concentration of the alkaline solution is 3.0 to 8.0 wt %.
[0010] In some embodiments, after polishing the silicon wafer and before texturing the silicon wafer in a variable temperature environment, it also includes: pre-cleaning the silicon wafer, including: pre-cleaning the silicon wafer at 60 to 70°C using a first mixed solution of alkali and hydrogen peroxide for 100 to 240 seconds, wherein in the first mixed solution, the concentration of the alkali is 0.5 to 3.0wt%, and the concentration of the hydrogen peroxide is 2.0 to 5.0wt%.
[0011] In some embodiments, texturing the silicon wafer in a variable temperature environment includes: using a second mixed solution of alkali and a texturing additive to react with the silicon wafer to perform texturing in a variable temperature environment.
[0012] In some embodiments, in the second mixed solution, the concentration of the alkali is 0.4 to 2.0 wt %, and the concentration of the texturing additive is 0.3 to 1.0 wt %.
[0013] In some embodiments, the variable temperature environment includes a first time period, a second time period, and a third time period; the first temperature of the first time period is 80 to 85°C, and the first time period is 120 to 240 seconds; the second time period is 200 to 300 seconds, and the second temperature of the second time period is 70 to 75°C; the third time period is 120 to 360 seconds.
[0014] In some embodiments, after the silicon wafer is textured in a variable temperature environment, the method further includes: cleaning the silicon wafer with a third mixed solution of alkali and hydrogen peroxide; soaking the silicon wafer with a mixed acid solution including hydrofluoric acid and hydrochloric acid; and pre-dehydrating and drying the silicon wafer.
[0015] In some embodiments, in the third mixed solution, the concentration of alkali is 0.4 to 2.0 wt %, the concentration of hydrogen peroxide is 2.0 to 5.0 wt %, and cleaning the silicon wafer using the third mixed solution of alkali and hydrogen peroxide includes: cleaning the silicon wafer at 60 to 70° C. for 150 to 250 seconds.
[0016] In some embodiments, in the mixed acid solution, the concentration of hydrofluoric acid is 3.0 to 6.0 wt %, and the concentration of hydrochloric acid is 2.0 to 5.0 wt %, and soaking the silicon wafer in the mixed acid solution including hydrofluoric acid and hydrochloric acid includes: soaking the silicon wafer at 20 to 25° C. for 120 to 250 seconds.
[0017] In some embodiments, the pre-dehydration includes using 25 to 60°C water to pre-dehydrate the silicon wafer by slow pulling for 60 to 120 seconds; the drying includes using 80 to 100°C circulating hot air flow to dry the silicon wafer for 500 to 1000 seconds.
[0018] In some embodiments, the alkali in the alkaline solution used when polishing the silicon wafer, the first mixed solution used when pre-cleaning the silicon wafer, the second mixed solution used when texturing the silicon wafer, and the third mixed solution used when cleaning the silicon wafer each independently includes potassium hydroxide and / or sodium hydroxide.
[0019] The present disclosure also provides a solar cell, which is a crystalline silicon solar cell prepared by the above preparation method.
[0020] The present disclosure also provides a tank device for preparing solar cells, comprising: a main tank, in which a temperature sensor is arranged; a sub-tank, wherein a cooling cycle is performed between the main tank and the sub-tank through a cooling cycle valve, a cooling cycle pump and a heat exchanger connected in sequence, and a daily cycle and a heating cycle are performed through a heating cycle valve, a heating cycle pump and a heater connected in sequence, so as to perform texturing on silicon wafers in a variable temperature environment; and a drainage system, which is used to discharge the liquid in the main tank and the sub-tank.
[0021] The present invention provides a method for preparing a crystalline silicon solar cell, which forms a more uniform pyramid velvet surface structure by texturing a silicon wafer in a variable temperature environment, wherein the variable temperature environment corresponds to multiple time periods, and the temperatures in different time periods are not all the same, thereby improving the photoelectric conversion efficiency of the solar cell.
[0022] In addition, the present disclosure also provides a tank device that can achieve rapid adjustment of the tank process temperature. The preparation method and tank device of the crystalline silicon solar cell provided by the present disclosure have high tolerance and can be adapted to a variety of solar cell silicon wafers, for example, a variety of single crystal silicon wafers, and can be applied to various solar cell process technologies such as emitter and back passivated cells (PERC), oxide layer passivated contacts (TOPCON), and heterojunction (HJT), and have broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Figure 1 A schematic flow chart of a method for preparing a crystalline silicon solar cell according to some embodiments of the present disclosure is shown.
[0024] Figure 2 A schematic flow chart of a method for preparing a crystalline silicon solar cell according to some embodiments of the present disclosure is shown.
[0025] Figure 3 A schematic diagram of a tank device according to some embodiments of the present disclosure is shown.
[0026] Figure 4 A graph showing the reflectivity of the textured structure of the crystalline silicon solar cells according to the embodiment of the present disclosure and the comparative example.
[0027] Among them: 1. Main tank, 2. Auxiliary tank, 3. Temperature sensor, 4. Cooling circulation valve, 5. Chiller, 6. Cooling circulation pump, 7. Heat exchanger, 8. Cooling circulation drain valve, 9. Heating circulation valve, 10. Heating circulation pump, 11. Heater, 12. Heating circulation drain valve, 13. Drainage system. DETAILED DESCRIPTION
[0028] The following examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.
[0029] The inventors of the present disclosure have found that the pyramid velvet structure produced by constant temperature velveting will have an adverse effect on subsequent passivation, metallization and other processes. This is mainly because the anisotropic corrosion of the silicon wafer surface at a constant temperature produces a poor uniformity of the pyramid velvet structure, and there are a large number of random pyramids of different sizes and unevenness, which are not conducive to passivation, metallization and other processes, and thus affect the battery efficiency. In addition, constant temperature velveting generally uses alkali to anisotropically corrode silicon wafers, but the reaction process between alkali and silicon wafers is highly sensitive to temperature, and the velvet structures under different temperature processes are quite different, so the constant temperature velveting process is difficult to control the velvet structure, and the process window is small.
[0030] The present disclosure provides a method for preparing a crystalline silicon solar cell, such as Figure 1 As shown, including:
[0031] S101, texturing a silicon wafer in a variable temperature environment to obtain a textured surface structure of a crystalline silicon solar cell, wherein the variable temperature environment corresponds to multiple time periods, and the temperatures in different time periods are not all the same.
[0032] The present invention regulates the texturing temperature and texturing the silicon wafer in a variable temperature environment, wherein the variable temperature environment corresponds to multiple time periods, and the temperatures in different time periods are not all the same, so that a more uniform pyramid texturing structure can be formed, thereby improving the photoelectric conversion efficiency of solar cells.
[0033] In some embodiments, in the multiple time periods corresponding to the variable temperature environment, the temperature in the previous time period is greater than the temperature in the later time period. In this way, the pyramid growth period is first quickly passed at a higher temperature, and then the temperature is gradually lowered to reduce the pyramid nucleation rate, and then the velvet structure is further modified at a lower temperature, thereby forming a more uniform pyramid structure, thereby improving the light absorption efficiency of the solar cell.
[0034] In some embodiments, texturing the silicon wafer in a variable temperature environment includes: texturing the silicon wafer at a first temperature in a first time period; texturing the silicon wafer at a second temperature in a second time period; wherein the first temperature is greater than the second temperature. The reaction between alkali and silicon wafer is more intense at high temperatures, and it is more difficult to control the texture surface. The reaction is relatively mild or non-reactive at low temperatures, and the pyramid texturing cycle is prolonged. Specifically, for every 10°C decrease in reaction temperature, the etching rate of the alkali decreases by 1 to 2 times, and the lower second temperature is conducive to the control of the texturing reaction process. In this way, the pyramid growth period is first quickly passed at a higher first temperature, and then the texture surface structure is further modified at a lower second temperature, thereby forming a more uniform pyramid structure, thereby improving the light absorption efficiency of the solar cell.
[0035] In some embodiments, the variable temperature environment also includes a third time period, wherein the third time period is located between the first time period and the second time period. In some embodiments, texturing the silicon wafer in a variable temperature environment also includes: in the third time period, texturing the silicon wafer during the process of reducing the temperature from the first temperature to the second temperature. In this way, the pyramid growth period is quickly passed at a higher first temperature in the first time period, and then the temperature is gradually lowered in the third time period to reduce the pyramid nucleation rate, and then the velvet structure is further modified at a lower second temperature in the second time period, thereby forming a more uniform pyramid structure, thereby improving the light absorption efficiency of the solar cell.
[0036] In some embodiments, before the silicon wafer is textured under a variable temperature environment, the preparation method further includes: polishing the silicon wafer. In some embodiments, by polishing the silicon wafer, the cutting line marks and other defects or damage on the surface of the silicon wafer can be removed, making the surface of the silicon wafer smoother, thereby facilitating the uniform formation of the subsequent velvet structure. Then, the residual liquid on the surface of the silicon wafer can be removed with pure water. In some embodiments, polishing the silicon wafer includes: polishing the silicon wafer at 65 to 85°C for 20 to 100 seconds with an alkaline solution to remove the cutting line marks and other defects or damage on the surface of the silicon wafer, wherein the concentration of the alkaline solution is 3.0 to 8.0wt%. In some embodiments, the higher the concentration of the alkaline solution, the faster the corrosion rate; conversely, the slower. If the concentration of the alkaline solution is too high, the corrosion rate will be too high, and the excessively high corrosion rate may cause defects on the surface of the silicon wafer; if the concentration of the alkaline solution is too low, the corrosion rate will be too low, and the excessively low corrosion rate will affect the polishing efficiency. In some embodiments, the concentration of the alkaline solution may be 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, or any suitable value therebetween.
[0037] In some embodiments, if the polishing temperature is too high, the reaction between the alkali and the silicon wafer is too violent, which may easily cause surface defects of the silicon wafer; if the polishing temperature is too low, the corrosion rate of the alkali is slow, which may affect the polishing efficiency. In some embodiments, the polishing temperature may be 65°C, 70°C, 75°C, 80°C, 85°C or any suitable temperature therebetween. Similarly, if the polishing time is too long, it may easily cause excessive etching of the silicon wafer; if the polishing time is too short, it may result in insufficient polishing, and the damage on the surface of the silicon wafer may not be fully removed. In some embodiments, the polishing time may be 20 seconds, 30 seconds, 40 seconds, 50 seconds, 60 seconds, 70 seconds, 80 seconds, 90 seconds, 100 seconds or any suitable value therebetween.
[0038] In some embodiments, after polishing the silicon wafer and before texturing the silicon wafer in a variable temperature environment, the preparation method further includes: pre-cleaning the silicon wafer to remove impurities on the surface of the silicon wafer, thereby eliminating the influence of the impurities on the subsequent texturing process. In some embodiments, pre-cleaning the silicon wafer includes: pre-cleaning the silicon wafer at 60 to 70° C. using a first mixed solution of alkali and hydrogen peroxide for 100 to 240 seconds. In some embodiments, the presence of hydrogen peroxide in the first mixed solution is conducive to enhancing the cleaning effect. Then, pure water can be used to remove the first mixed solution remaining on the surface of the silicon wafer.
[0039] In some embodiments, in the first mixed solution, the concentration of the alkali is 0.5 to 3.0 wt%, and the concentration of the hydrogen peroxide is 2.0 to 5.0 wt%. In some embodiments, the first mixed solution can remove pollutants and oxide layers on the surface of the silicon wafer, providing a clean foundation for the subsequent texturing step. In some embodiments, the alkali in the first mixed solution is mainly used to remove impurities on the surface of the silicon wafer, such as oil and wax. When the concentration of the alkali is too low, it may not be able to effectively remove all impurities; when the concentration of the alkali is too high, it may cause damage to the surface of the silicon wafer or even change the surface morphology. In some embodiments, the concentration of the alkali in the first mixed solution can be 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt% or any suitable value therebetween. Hydrogen peroxide is a strong oxidant that can accelerate the oxidation process and help remove organic matter and some inorganic impurities on the surface. When the concentration of hydrogen peroxide is too low, the oxidation effect is weak and the pollutants on the surface may not be fully oxidized; when the concentration of hydrogen peroxide is too high, it may cause an additional oxide layer on the surface of the silicon wafer, affecting the subsequent texturing effect. When hydrogen peroxide and alkali are used together, a synergistic effect can be produced to enhance the cleaning effect; the oxidation effect of hydrogen peroxide combined with the dissolution effect of alkali can more effectively remove impurities on the surface of the silicon wafer. In some embodiments, the concentration of hydrogen peroxide in the first mixed solution is 2wt%, 3wt%, 4wt%, 5wt% or any suitable value therebetween.
[0040] In some embodiments, the increase in the pre-cleaning temperature can speed up the chemical reaction, thereby improving the cleaning efficiency and removing impurities faster; however, too high a temperature may affect the stability of hydrogen peroxide, which is easy to decompose at high temperatures, thereby affecting the cleaning effect; in addition, too high a temperature may cause changes on the surface of the silicon wafer, such as microcracks or changes in the surface structure. In some embodiments, the pre-cleaning temperature may be 60°C, 62°C, 65°C, 68°C, 70°C, or any suitable temperature therebetween. In some embodiments, too long a pre-cleaning time may affect the efficiency of the pre-cleaning, and too long a pre-cleaning time may have limited effect on improving the pre-cleaning effect; while too short a pre-cleaning time may result in the inability to completely remove impurities on the surface of the silicon wafer. In some embodiments, the pre-cleaning time may be 100 seconds, 120 seconds, 140 seconds, 160 seconds, 180 seconds, 200 seconds, 220 seconds, 240 seconds, or any suitable value therebetween.
[0041] In some embodiments, texturing a silicon wafer under a variable temperature environment includes: using a second mixed solution of alkali and a texturing additive to react with the silicon wafer to texturize under a variable temperature environment. In some embodiments, after the second mixed solution is used to react with the silicon wafer, the second mixed solution remaining on the surface of the silicon wafer can be removed with pure water. In some embodiments, the use of the texturing additive in the second mixed solution can promote the formation of a velvet structure and reduce the weight loss of the silicon wafer. The texturing additive disclosed in the present invention can adopt any suitable texturing additive for texturing, for example, the texturing additive HJ21 (V01).
[0042] In some embodiments, in the second mixed solution, the concentration of the alkali is 0.4 to 2.0wt%, and the concentration of the texturing additive is 0.3 to 1.0wt%. In some embodiments, the concentration of the alkali in the second mixed solution affects the corrosion rate or etching rate of the silicon wafer. The higher the concentration of the alkali, the faster the corrosion rate, but too high an corrosion rate may cause defects on the surface of the silicon wafer, such as holes; too low a concentration of the alkali may cause the corrosion rate to be too slow, prolonging the texturing time and affecting production efficiency. In some embodiments, the concentration of the alkali in the second mixed solution may be 0.4wt%, 0.8wt%, 1.2wt%, 1.6wt%, 2wt% or any suitable value therebetween. In some embodiments, if the concentration of the texturing additive is too low, its effect of promoting the formation of the velvet structure is relatively limited; if the concentration of the texturing additive is too high, its effect of promoting the formation of the velvet structure is no longer significantly increased, and excessive organic matter and metal ion residues may be introduced. In some embodiments, the concentration of the texturing additive in the second mixed solution may be 0.3wt%, 0.5wt%, 0.8wt%, 1wt% or any suitable value therebetween.
[0043] In some embodiments, the variable temperature environment includes a first time period, a second time period, and a third time period. The first temperature of the first time period is 80 to 85°C, and the first time period is 120 to 240 seconds; the second time period is 200 to 300 seconds, and the second temperature of the second time period is 70 to 75°C; the third time period is 120 to 360 seconds. In some embodiments, the selection of the first temperature and the first time period mainly considers the factors to prevent excessive etching of the silicon wafer and the efficiency of the velvet making process. In some embodiments, the second time period is the time required to reduce the first temperature to the second temperature. In some embodiments, the selection of the second temperature and the third time period mainly considers the uniformity modification of the velvet structure, which can not only etch the silicon wafer to modify the velvet structure, but also avoid excessive etching to cause uneven velvet structure.
[0044] In some embodiments, if the first temperature is too high, the reaction between the alkali and the silicon wafer is more intense, the velvet control is more difficult, and the silicon wafer is prone to over-etching and uneven formation of the velvet structure; if the first temperature is too low, the pyramid velveting cycle is extended, affecting the velvet efficiency of the silicon wafer. Specifically, the etching rate of the alkali decreases by 1 to 2 times for every 10°C decrease in temperature. In some embodiments, the first temperature may be 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, or any suitable value therebetween. In some embodiments, if the first time period is too long, it is easy to cause over-etching of the silicon wafer and uneven formation of the velvet structure; if the first time period is too short, it is easy to cause insufficient formation of the velvet structure. In some embodiments, the first time period may be 120 seconds, 140 seconds, 160 seconds, 180 seconds, 200 seconds, 220 seconds, 240 seconds, or any suitable value therebetween. In some embodiments, the second time period is the time allowed to decrease from the first temperature to the second temperature, and the second time period may be 200 seconds, 220 seconds, 240 seconds, 260 seconds, 280 seconds, 300 seconds, or any suitable value therebetween. In some embodiments, if the second temperature is too high, the velvet control is more difficult, affecting the modification effect of the velvet structure; if the second temperature is too low, the velvet modification time needs to be extended, affecting the velvet efficiency of the silicon wafer. In some embodiments, the second temperature may be 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, or any suitable value therebetween. In some embodiments, if the third time period is too long, the modification effect of the velvet structure is no longer significantly increased, and the velvet efficiency is affected; if the third time period is too short, insufficient modification of the velvet structure may result. In some embodiments, the third time period may be 120 seconds, 160 seconds, 200 seconds, 240 seconds, 280 seconds, 320 seconds, 360 seconds, or any suitable value therebetween.
[0045] like Figure 2As shown, in some embodiments, the preparation method of the present disclosure further includes: using a third mixed solution of alkali and hydrogen peroxide to clean the silicon wafer to remove organic residues on the surface of the silicon wafer. Generally, the organic residues mainly come from the texturing additives. In some embodiments, the presence of hydrogen peroxide in the third mixed solution is conducive to enhancing the cleaning effect. Then, the third mixed solution remaining on the surface of the silicon wafer can be removed with pure water.
[0046] like Figure 2 As shown, in some embodiments, the preparation method of the present disclosure further includes: soaking the silicon wafer in a mixed acid solution including hydrofluoric acid and hydrochloric acid to remove metal ions on the surface of the silicon wafer. In some embodiments, the metal ions on the surface of the silicon wafer mainly come from alkali and texturing additives. Then, pure water can be used to remove the residual mixed acid solution on the surface of the silicon wafer. Figure 2 As shown, in some embodiments, the preparation method of the present disclosure further includes: pre-dehydrating and drying the silicon wafer for further processing in the preparation of crystalline silicon solar cells.
[0047] In some embodiments, in the third mixed solution, the concentration of the alkali is 0.4 to 2.0 wt%, and the concentration of the hydrogen peroxide is 2.0 to 5.0 wt%. In some embodiments, the third mixed solution can remove organic matter on the surface of the silicon wafer. The higher the concentration of the alkali in the third mixed solution, the stronger the ability to remove organic matter. Therefore, when the concentration of the alkali is too low, it may not be able to effectively remove all organic matter; when the concentration of the alkali is too high, it may cause excessive corrosion of the surface of the silicon wafer and produce microscopic defects. In some embodiments, the concentration of the alkali in the third mixed solution can be 0.4 wt%, 0.8 wt%, 1.2 wt%, 1.6 wt%, 2 wt% or any suitable value therebetween. Hydrogen peroxide is a strong oxidant that can oxidize organic matter on the surface of the silicon wafer in an alkaline environment. The presence of hydrogen peroxide can accelerate the decomposition of organic matter and improve the cleaning efficiency. When the concentration of hydrogen peroxide is too low, the oxidation effect is weak and the organic matter on the surface may not be fully oxidized; when the concentration of hydrogen peroxide is too high, it can cause excessive bubbles to form on the surface of the silicon wafer, affecting the cleaning effect. When hydrogen peroxide and alkali are used together, a synergistic effect can be produced to enhance the cleaning effect; the oxidizing effect of hydrogen peroxide combined with the dissolving effect of alkali can more effectively remove organic matter on the surface of the silicon wafer. In some embodiments, the concentration of hydrogen peroxide in the third mixed solution is 2wt%, 3wt%, 4wt%, 5wt% or any suitable value therebetween.
[0048] In some embodiments, cleaning the silicon wafer using a third mixed solution of alkali and hydrogen peroxide includes: cleaning the silicon wafer at 60 to 70°C for 150 to 250 seconds to remove organic residues on the surface of the silicon wafer. In some embodiments, increasing the cleaning temperature can accelerate the speed of the chemical reaction, thereby improving the cleaning efficiency and removing organic matter faster; however, too high a temperature may affect the stability of hydrogen peroxide, which is easily decomposed at high temperatures, thereby affecting the cleaning effect; in addition, too high a temperature may cause changes on the surface of the silicon wafer, such as microcracks or changes in the surface structure. In some embodiments, the cleaning temperature may be 60°C, 62°C, 65°C, 68°C, 70°C, or any suitable temperature therebetween. In some embodiments, a long cleaning time may affect the cleaning efficiency, and an excessively extended cleaning time has limited effect on improving the cleaning effect; while a short cleaning time may result in the inability to completely remove organic matter on the surface of the silicon wafer. In some embodiments, the cleaning time may be 150 seconds, 170 seconds, 190 seconds, 210 seconds, 230 seconds, 250 seconds, or any suitable value therebetween.
[0049] In some embodiments, in the mixed acid solution, the concentration of hydrofluoric acid is 3.0 to 6.0wt%, and the concentration of hydrochloric acid is 2.0 to 5.0wt%. In some embodiments, the role of hydrofluoric acid is mainly to remove the oxide layer on the surface of the silicon wafer. The concentration of hydrofluoric acid in the mixed acid solution affects the corrosion rate. A lower hydrofluoric acid concentration will result in a slower corrosion rate, which may extend the processing time; while a higher hydrofluoric acid concentration may cause a faster corrosion rate, but it also increases the potential risk of damage to the surface structure of the silicon wafer. In some embodiments, the concentration of hydrofluoric acid in the mixed acid solution can be 3wt%, 4wt%, 5wt%, 6wt% or any suitable value therebetween. In some embodiments, the hydrochloric acid in the mixed acid solution has the dual effects of acidity and complexation. It can form water-soluble complexes with a variety of metal ions, thereby effectively removing metal ions on the surface of the silicon wafer. In some embodiments, a too low concentration of hydrochloric acid in the mixed acid solution may not be able to fully remove all metal ions, while a too high concentration of hydrochloric acid may cause unnecessary corrosion or increase the difficulty of subsequent cleaning. In addition, the concentration of hydrochloric acid also affects the corrosion rate. A lower concentration may mean a slower processing speed, while a higher concentration may bring a faster corrosion rate, but care should also be taken to avoid excessive corrosion of the silicon wafer. In some embodiments, the concentration of hydrochloric acid in the mixed acid solution may be 2wt%, 3wt%, 4wt%, 5wt% or any suitable value therebetween.
[0050] In some embodiments, soaking the silicon wafer in a mixed acid solution including hydrofluoric acid and hydrochloric acid includes: soaking the silicon wafer for 120 to 250 seconds at 20 to 25°C to remove metal ions on the surface of the silicon wafer. In some embodiments, increasing the temperature of the acid soaking can accelerate the chemical reaction rate, thereby improving the removal efficiency of metal ions, but too high a temperature may cause excessive etching of the surface of the silicon wafer or other adverse effects. In addition, too high a temperature of the acid soaking will also affect the stability of the mixed acid solution, especially for volatile hydrochloric acid and hydrofluoric acid. In some embodiments, the temperature of the acid soaking can be 20°C, 21°C, 22°C, 23°C, 24°C, 25°C or any suitable temperature therebetween. In some embodiments, if the acid soaking time is too long, it may cause excessive etching of the surface of the silicon wafer and affect the pickling efficiency; if the acid soaking time is too short, all metal ions may not be completely removed. In some embodiments, the acid soaking time can be 120 seconds, 150 seconds, 170 seconds, 190 seconds, 210 seconds, 230 seconds, 250 seconds or any suitable value therebetween.
[0051] In some embodiments, pre-dehydration includes using water at 25 to 60°C to pre-dehydrate the silicon wafer by slow pulling for 60 to 120 seconds. Slow pulling is a pre-treatment process for drying. Its working principle is to use the surface tension of water to allow the cleaned workpiece to slowly remove water from the surface of the workpiece during the process of slowly pulling it out of the water surface, thereby obtaining a relatively dry workpiece and preparing for the subsequent drying step. In some embodiments, the higher the pre-dehydration temperature, the faster the water evaporates; the lower the temperature, the longer it takes to complete the pre-dehydration, thereby affecting the pre-dehydration efficiency; however, if the temperature is too high, it may cause the water on the surface of the silicon wafer to evaporate too quickly, leaving water marks or affecting the uniformity of the surface. In some embodiments, the pre-dehydration temperature can be 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C or any suitable temperature therebetween. In some embodiments, too long a pre-dehydration time will affect the efficiency of the pre-dehydration, while too short a pre-dehydration time may result in incomplete water removal, affecting the execution of subsequent processes. In some embodiments, the pre-dehydration time can be 60 seconds, 70 seconds, 80 seconds, 90 seconds, 100 seconds, 110 seconds, 120 seconds, or any suitable value therebetween.
[0052] In some embodiments, drying includes drying the silicon wafer for 500 to 1000 seconds using a circulating hot air flow at 80 to 100°C. In some embodiments, the higher the drying temperature, the faster the water evaporates, shortening the drying time; the lower the temperature, the longer it takes to complete the drying, thereby affecting the drying efficiency; however, if the temperature is too high, it may cause thermal damage to the surface of the silicon wafer, such as changes in the surface structure, the generation of microcracks, etc. In addition, if the drying temperature is too low, the water may not evaporate completely, thereby affecting the execution of subsequent processes. In some embodiments, the drying temperature may be 80°C, 85°C, 90°C, 95°C, 100°C, or any suitable temperature therebetween. In some embodiments, a drying time that is too long may affect the drying efficiency, while a drying time that is too short may result in incomplete water removal, affecting the execution of subsequent processes. In some embodiments, the drying time may be 500 seconds, 600 seconds, 700 seconds, 800 seconds, 900 seconds, 1000 seconds, or any suitable value therebetween.
[0053] In some embodiments, the alkali in the alkaline solution used when polishing the silicon wafer, the first mixed solution used when pre-cleaning the silicon wafer, the second mixed solution used when texturing the silicon wafer, and the third mixed solution used when cleaning the silicon wafer each independently includes potassium hydroxide and / or sodium hydroxide. The atomic radius of other elements in Group I elements is larger, and the barrier effect of the formed alkali on the reaction is also larger, the corresponding reaction etching rate will decrease, and the cost will increase. Therefore, in the present disclosure, by using potassium hydroxide and / or sodium hydroxide, both corrosion efficiency and cost can be benefited. In some embodiments, potassium hydroxide and sodium hydroxide can be replaced with each other in a mass ratio of 1:1.6, which has little effect on various reactions in the process.
[0054] In some embodiments, the silicon wafer of the present disclosure includes single crystal silicon. Due to the anisotropy of single crystal silicon, that is, different crystal directions have different corrosion rates, corrosion in an alkaline solution will cause a pyramid-shaped structure to form on the surface, which can effectively reduce reflection and increase light absorption, helping to improve the photoelectric conversion efficiency of solar cells.
[0055] In some embodiments, the present disclosure also provides a crystalline silicon solar cell prepared by the above preparation method. By adopting the variable temperature texturing method of the present disclosure, the texture structure of the prepared crystalline silicon solar cell is made more uniform, thereby reducing the reflectivity of the texture structure and improving the photoelectric conversion efficiency of the solar cell.
[0056] refer to Figure 3The present disclosure also provides a tank device for preparing solar cells, including a main tank 1 and a sub-tank 2. In some embodiments, a temperature sensor 3 is provided in the main tank 1. In some embodiments, a cooling cycle valve 4, a cooling cycle pump 6 and a heat exchanger 7 connected in sequence are used for cooling cycle between the main tank 1 and the sub-tank 2, and a heating cycle valve 9, a heating cycle pump 10 and a heater 11 connected in sequence are used for daily cycle and heating cycle, so as to texturize the silicon wafer under a variable temperature environment. In some embodiments, the tank device also includes a drainage system 13, and the drainage system 13 is used to drain or empty the liquid in the main tank 1 and the sub-tank 2. In some embodiments, the liquid in the cooling cycle process can be drained through the cooling cycle drain valve 8, and the liquid in the daily cycle or heating cycle process can be drained through the heating cycle drain valve 12.
[0057] The working principle steps of the tank device provided by the present disclosure include: S1, obtaining the temperature of the liquid medicine in the tank through the temperature sensor 3; S2, obtaining the process time and temperature corresponding to the tank; S3, determining whether the liquid medicine temperature meets the corresponding temperature set by the process; S4, if the liquid medicine temperature is less than the process set temperature, then open the heating circulation valve 9, start the heating circulation pump 10 and the heater 11, and heat the liquid medicine in a rapid circulation; S5, if the liquid medicine temperature meets the process set temperature, then open the heating circulation valve 9, start the heating circulation pump 10, turn off the heater 11, and circulate the liquid medicine daily; S6, if the liquid medicine temperature is greater than the process set temperature, then open the cooling circulation valve 4, start the cooling circulation pump 6 and the heat exchanger 7, and cool the liquid medicine in a rapid circulation. In some embodiments, the heating speed can be changed by adjusting the flow rate of the heating circulation pump 10 and the power of the heater 11. In some embodiments, the cooling speed can be changed by adjusting the flow rate of the cooling circulation pump 6, the cold water temperature of the chiller 5, and the circulation flow rate.
[0058] By adopting the tank device disclosed in the present invention, the temperature of the tank liquid can be automatically identified, the temperature of the liquid in the tank can be quickly adjusted, and rapid and accurate temperature control can be achieved, thereby meeting various process requirements. Specifically, the variable temperature texturing process disclosed in the present invention can be implemented in the tank device.
[0059] The following is a description with reference to specific embodiments to provide a better understanding of the technical solution of the present disclosure.
[0060] Example 1
[0061] A method for preparing a crystalline silicon solar cell, the method comprising the following steps:
[0062] S1. Polishing: polish the silicon wafer for 80 seconds at 82°C using a 5.5wt% potassium hydroxide solution to remove cutting marks and other defects on the surface of the silicon wafer; then wash with water for 120 seconds to remove residual liquid on the surface of the silicon wafer;
[0063] S2, pre-cleaning: at 65° C., using a mixture of 1.25 wt % potassium hydroxide and 2.50 wt % hydrogen peroxide to clean for 220 seconds to remove impurities on the surface of the silicon wafer; and then washing with water for 120 seconds to remove the residual mixture on the surface of the silicon wafer;
[0064] S3, variable temperature texturing: Use 0.90wt% potassium hydroxide and 0.60wt% Shichuang HJ21 (V01) texturing additive mixed liquid to texturize the silicon wafer, and the temperature parameters used are: 120s in the early stage, temperature 85℃; 300s in the middle stage, gradually cooling to 70℃; 240s in the late stage, temperature 70℃; then wash with water for 120 seconds to remove the residual mixed liquid on the surface of the silicon wafer;
[0065] S4, alkali washing: at 62°C, use a mixture of 1.25wt% potassium hydroxide and 4.50wt% hydrogen peroxide to wash for 240s to remove organic residues on the surface of the silicon wafer; then wash with water for 120s to remove the residual mixture on the surface of the silicon wafer;
[0066] S5, pickling: at 20°C, use a mixture of 6.00% hydrofluoric acid and 5.00% hydrochloric acid to soak for 120 seconds to remove metal ions on the surface of the silicon wafer; then rinse with water for 120 seconds to remove the residual mixed solution on the surface of the silicon wafer;
[0067] S6. Slow pulling: pre-dehydrate the silicon wafer by slow pulling for 60 seconds at 55°C; drying: use 100°C circulating hot air to dry the silicon wafer for 500 seconds to complete the texturing process.
[0068] Comparative Example 1
[0069] The variable temperature texturing in step S3 of Example 1 is changed to: using a mixed solution of 0.90wt% potassium hydroxide and 0.60wt% Shichuang HJ21 (V01) texturing additive to texturize the silicon wafer at a constant temperature of 85°C for 660s, and the other steps remain the same.
[0070] Example 2
[0071] A method for preparing a crystalline silicon solar cell, the method comprising the following steps:
[0072] S1. Polishing: polish the silicon wafer with 3.5wt% potassium hydroxide solution at 80°C for 100 seconds to remove cutting line marks and other defects on the surface of the silicon wafer; then wash with water for 120 seconds to remove residual liquid on the surface of the silicon wafer;
[0073] S2, pre-cleaning: at 63°C, use a mixture of 2.50wt% potassium hydroxide and 3.25wt% hydrogen peroxide to clean for 120s to remove impurities on the surface of the silicon wafer; then wash with water for 120s to remove the residual mixture on the surface of the silicon wafer;
[0074] S3, variable temperature texturing: Use 1.50wt% potassium hydroxide and 0.90wt% Shichuang HJ21 (V01) texturing additive mixed solution to texturize the silicon wafer, and the temperature parameters used are: 200s in the early stage, temperature 82℃; 200s in the middle stage, gradually cooling to 73℃; 300s in the late stage, temperature 73℃; then wash with water for 120 seconds to remove the residual mixed solution on the surface of the silicon wafer;
[0075] S4, alkali washing: at 68°C, use a mixture of 1.50wt% potassium hydroxide and 3.25wt% hydrogen peroxide to wash for 200s to remove organic residues on the surface of the silicon wafer; then wash with water for 120s to remove the residual mixture on the surface of the silicon wafer;
[0076] S5, pickling: at 25°C, use a mixture of 4.40% hydrofluoric acid and 3.00% hydrochloric acid to soak for 150 seconds to remove metal ions on the surface of the silicon wafer; then rinse with water for 120 seconds to remove the residual mixed solution on the surface of the silicon wafer;
[0077] S6. Slow pulling: At 25°C, use slow pulling to pre-dehydrate the silicon wafer for 120 seconds; Drying: Use 85°C circulating hot air to dry the silicon wafer for 800 seconds to complete the texturing process.
[0078] Comparative Example 2
[0079] The variable temperature texturing in step S3 of Example 2 is changed to: using a mixed solution of 0.80wt% potassium hydroxide and 0.50wt% Shichuang HJ21 (V01) texturing additive to texturize the silicon wafer at a constant temperature of 82°C for 700s, and the other steps remain the same.
[0080] Example 3
[0081] A method for preparing a crystalline silicon solar cell, the method comprising the following steps:
[0082] S1, polishing: polishing the silicon wafer for 40 seconds using 7.5wt% potassium hydroxide solution at 68°C to remove cutting line marks and other defects on the surface of the silicon wafer; and then washing with water for 120 seconds to remove residual liquid on the surface of the silicon wafer;
[0083] S2, pre-cleaning: at 68° C., using a mixture of 0.90 wt % potassium hydroxide and 4.00 wt % hydrogen peroxide to clean for 150 seconds to remove impurities on the surface of the silicon wafer; and then washing with water for 120 seconds to remove the residual mixture on the surface of the silicon wafer;
[0084] S3, variable temperature texturing: Use 0.63wt% potassium hydroxide and 0.40wt% Shichuang HJ21 (V01) texturing additive mixed liquid to texturize the silicon wafer, and the temperature parameters used are: 240s in the early stage, temperature 80℃; 200s in the middle stage, gradually cooling to 75℃; 150s in the late stage, temperature 75℃; then wash with water for 120 seconds to remove the residual mixed liquid on the surface of the silicon wafer;
[0085] S4, alkali washing: at 65°C, use a mixture of 0.60wt% potassium hydroxide and 2.50wt% hydrogen peroxide to wash for 160s to remove organic residues on the surface of the silicon wafer; then wash with water for 120s to remove the residual mixture on the surface of the silicon wafer;
[0086] S5, pickling: at 25°C, use a mixture of 3.00% hydrofluoric acid and 2.00% hydrochloric acid to soak for 200 seconds to remove metal ions on the surface of the silicon wafer; then rinse with water for 120 seconds to remove the residual mixed solution on the surface of the silicon wafer;
[0087] S6. Slow pulling: pre-dehydrate the silicon wafer for 100 seconds using a slow pulling method at 30°C; drying: use 80°C circulating hot air to dry the silicon wafer for 1000 seconds to complete the texturing process.
[0088] Comparative Example 3
[0089] The variable temperature texturing in step S3 of Example 3 is changed to: using a mixed solution of 0.63wt% potassium hydroxide and 0.40wt% Shichuang HJ21 (V01) texturing additive to texturize the silicon wafer at a constant temperature of 80°C for 590s, and the other steps remain the same.
[0090] In order to verify the advantages of the velvet surface compared with the traditional constant temperature velvet surface, the reflectivity of the silicon wafer velvet surface and the electrical performance of the solar cell obtained by the velvet surface making process of each embodiment and the comparative example are compared.
[0091] like Figure 4 As shown in the figure, it is a comparison chart of the velvet reflectivity of silicon wafers prepared by the preparation methods of various embodiments and comparative examples. The D8 velvet reflectometer of Zhidong Optoelectronics is used to test the light reflectivity intensity of the velvet structure of the silicon wafer in the wavelength range of 360 to 1050nm. 25 sample silicon wafers are taken for each group of embodiments and comparative examples for measurement, and 9 points are taken from the center and edge of each silicon wafer for multi-point automatic measurement and the average value is taken.
[0092] Depend on Figure 4 It can be seen that the reflectivity of each embodiment is lower than that of the corresponding comparison example, and the average decrease range is about 0.13% to 0.25%, which shows that the variable temperature texturing of the embodiment has a more uniform and ideal velvet morphology than the constant temperature texturing of the comparison example.
[0093] As shown in Table 1, it is a comparison table of the electrical properties of solar cells prepared by the preparation methods of various embodiments and comparative examples, and Table 1 is based on the electrical properties of each group of comparative examples as a reference. In this experiment, an n-type single crystal silicon wafer is used as a substrate, the silicon wafer size is 210.1 mm×105.05 mm, the silicon wafer thickness is 110-130 μm, and the silicon wafer resistivity is 0.3-2.1 Ω·cm. The overall solution for manufacturing a Maiwei HJT heterojunction high-efficiency battery is firstly carried out through the texturing and cleaning steps described in the embodiments / comparative examples of the present invention, and then an intrinsic amorphous silicon layer and an n-type doped amorphous silicon layer are sequentially deposited on the front side of an n-type silicon wafer substrate, and an intrinsic amorphous silicon layer and a p-type doped amorphous silicon layer are sequentially deposited on the back side of the n-type silicon wafer substrate; then an indium tin oxide (ITO) transparent conductive film is deposited on the amorphous silicon layer on both the front and back sides; finally, silver grid lines are printed on the surfaces of the double-sided ITO transparent conductive films by screen printing technology, and metal silver electrodes with good ohmic contact are formed after drying and curing, and finally an HJT solar cell is prepared, and a Maiwei electrical performance tester (in compliance with the IEC60904 series industry standards) is used to test the HJT solar cell under standard test conditions (STC) (i.e., the light intensity is 1000W / m 2 , spectrum is AM1.5, temperature is 25°C), the difference in photoelectric conversion efficiency (including Eta, Uoc, Isc, FF parameters) of the HJT solar cells of the embodiment and the comparative example is tested.
[0094] Table 1
[0095]
[0096] It can be seen from Table 1 that the efficiency of each embodiment is about 0.05 to 0.10% higher than that of the comparative example, which is mainly due to the improvement in open circuit voltage and short circuit current. It also reflects that the texture morphology of the battery cell produced by the texture making process of the embodiments of the present disclosure is better.
[0097] It should be understood by those skilled in the art that the above embodiments are merely exemplary embodiments and that various changes, substitutions and alterations may be made without departing from the spirit and scope of the present invention.
Claims
1. A method for preparing a crystalline silicon solar cell, characterized in that: include: The silicon wafer is textured in a variable temperature environment to obtain the textured surface structure of the crystalline silicon solar cell, wherein the variable temperature environment corresponds to multiple time periods, and the temperatures in different time periods are not all the same.
2. The method for preparing a crystalline silicon solar cell according to claim 1, characterized in that: Among the multiple time periods corresponding to the variable temperature environment, the temperature in the previous time period is greater than the temperature in the subsequent time period.
3. The method for preparing a crystalline silicon solar cell according to claim 2, characterized in that: Texturing of silicon wafers under variable temperature conditions, including: In a first time period, texturing the silicon wafer at a first temperature; In a second time period, texturing the silicon wafer at a second temperature; Wherein, the first temperature is greater than the second temperature.
4. The method for preparing a crystalline silicon solar cell according to claim 3, characterized in that: The variable temperature environment further includes a third time period; the third time period is located between the first time period and the second time period; Texturing of silicon wafers under variable temperature environment also includes: In a third time period, the silicon wafer is textured while the temperature is reduced from the first temperature to the second temperature.
5. The method for preparing a crystalline silicon solar cell according to any one of claims 1 to 4, characterized in that: Before texturing the silicon wafer in a variable temperature environment, the method further includes: polishing the silicon wafer, including: polishing the silicon wafer with an alkaline solution at 65 to 85° C. for 20 to 100 seconds, wherein the concentration of the alkaline solution is 3.0 to 8.0 wt %.
6. The method for preparing a crystalline silicon solar cell according to any one of claims 1 to 4, characterized in that: After polishing the silicon wafer and before texturing the silicon wafer in a variable temperature environment, the method also includes: pre-cleaning the silicon wafer, including: pre-cleaning the silicon wafer for 100 to 240 seconds at 60 to 70° C. using a first mixed solution of alkali and hydrogen peroxide, wherein the concentration of the alkali in the first mixed solution is 0.5 to 3.0wt%, and the concentration of the hydrogen peroxide is 2.0 to 5.0wt%.
7. The method for preparing a crystalline silicon solar cell according to any one of claims 1 to 4, characterized in that: The silicon wafer is textured in a variable temperature environment, comprising: Under a variable temperature environment, a second mixed solution of alkali and a texturing additive is used to react with the silicon wafer to perform texturing.
8. The method for preparing a crystalline silicon solar cell according to claim 7, characterized in that: In the second mixed solution, the concentration of the alkali is 0.4 to 2.0 wt %, and the concentration of the texturing additive is 0.3 to 1.0 wt %.
9. The method for preparing a crystalline silicon solar cell according to claim 8, characterized in that: The variable temperature environment includes a first time period, a second time period, and a third time period; the first temperature of the first time period is 80 to 85°C, and the first time period is 120 to 240 seconds; the second time period is 200 to 300 seconds, and the second temperature of the second time period is 70 to 75°C; the third time period is 120 to 360 seconds.
10. The method for preparing a crystalline silicon solar cell according to any one of claims 1 to 4, characterized in that: After the silicon wafer is textured under a variable temperature environment, the method further comprises: Cleaning the silicon wafer using a third mixed solution of alkali and hydrogen peroxide; soaking the silicon wafer in a mixed acid solution comprising hydrofluoric acid and hydrochloric acid; The silicon wafer is pre-dehydrated and dried.
11. The method for preparing a crystalline silicon solar cell according to claim 10, characterized in that: In the third mixed solution, the concentration of alkali is 0.4 to 2.0 wt %, and the concentration of hydrogen peroxide is 2.0 to 5.0 wt %. Cleaning the silicon wafer using the third mixed solution of alkali and hydrogen peroxide includes: cleaning the silicon wafer at 60 to 70° C. for 150 to 250 seconds.
12. The method for preparing a crystalline silicon solar cell according to claim 10, characterized in that: In the mixed acid solution, the concentration of hydrofluoric acid is 3.0 to 6.0 wt %, and the concentration of hydrochloric acid is 2.0 to 5.0 wt %. Soaking the silicon wafer in the mixed acid solution including hydrofluoric acid and hydrochloric acid includes: soaking the silicon wafer at 20 to 25° C. for 120 to 250 seconds.
13. The method for preparing a crystalline silicon solar cell according to claim 10, characterized in that: The pre-dehydration includes using water at 25 to 60° C. to pre-dehydrate the silicon wafer by slow pulling for 60 to 120 seconds; the drying includes using a circulating hot air flow at 80 to 100° C. to dry the silicon wafer for 500 to 1000 seconds.
14. The method for preparing a crystalline silicon solar cell according to claim 10, characterized in that: The alkali in the alkaline solution used for polishing the silicon wafer, the first mixed solution used for pre-cleaning the silicon wafer, the second mixed solution used for texturing the silicon wafer, and the third mixed solution used for cleaning the silicon wafer independently include potassium hydroxide and / or sodium hydroxide.
15. A solar cell, characterized in that: The solar cell is a crystalline silicon solar cell prepared according to the method for preparing a crystalline silicon solar cell according to any one of claims 1 to 14.
16. A trough device for preparing a solar cell, characterized in that: include: A main tank (1), wherein a temperature sensor (3) is arranged in the main tank (1); A secondary tank (2), wherein a cooling cycle is performed between the main tank (1) and the secondary tank (2) through a cooling cycle valve (4), a cooling cycle pump (6) and a heat exchanger (7) connected in sequence, and a daily cycle and a heating cycle are performed through a heating cycle valve (9), a heating cycle pump (10) and a heater (11) connected in sequence, so as to perform texturing on the silicon wafer in a variable temperature environment; A liquid drainage system (13) is used to drain the liquid in the main tank (1) and the auxiliary tank (2).