Solar cell back passivation film, preparation method of solar cell back passivation film, solar cell and preparation method of solar cell

Abstract

The invention relates to a solar cell back passivation film and a preparation method thereof, and a solar cell and a preparation method thereof, and the preparation method of the back passivation film comprises the steps: carrying out the first coating on the back surface of a silicon wafer through employing a PECVD method, and sequentially depositing an aluminum oxide film and a first silicon nitride film; performing secondary coating on the surface of the first silicon nitride film by adopting a PECVD (Plasma Enhanced Chemical Vapor Deposition) method, and depositing a second silicon nitride film to obtain a back passivation film; wherein the step of coating for the first time mainly comprises the steps of adopting protective gas and trimethylaluminum gas for pre-ventilation, adopting an oxygen-containing precursor and trimethylaluminum gas for depositing an aluminum oxide film, and depositing a first silicon nitride film; wherein the trimethylaluminum gas is obtained by converting liquid trimethylaluminum by adopting an evaporator, the temperature of the evaporator is 60-120 DEG C, and the flow rate of the liquid trimethylaluminum when the liquid trimethylaluminum is introduced into the evaporator is 40-70 mg / min. The solar cell obtained by the preparation method can effectively reduce EL local pollution, and the conversion efficiency is high.

Description

Technical Field

[0001] The present invention relates to the technical field of solar cells, and particularly to a back passivation film for a solar cell, a preparation method thereof, a solar cell and a preparation method thereof. Background Art

[0002] In solar cells such as PERC cells and TOPCon cells, the back passivation film generally includes an aluminum oxide film and a silicon nitride film, which are obtained by two times of film coating. Among them, the mainstream deposition methods of the aluminum oxide film are atomic layer deposition (ALD) and plasma enhanced chemical vapor deposition (PECVD), and each method has its own advantages and disadvantages.

[0003] For example, compared with ALD, the aluminum oxide film deposited by the PECVD method has poor compactness. In order to ensure the passivation effect, the thickness of the aluminum oxide film deposited by the PECVD method is 3 to 6 times that of ALD. However, with the increase of the thickness, the uniformity of the aluminum oxide film will become worse. At the same time, the aluminum oxide film is easily partially decomposed during the subsequent high-temperature deposition of the silicon nitride film, resulting in local dark pollution in the electroluminescence detection (EL) of the manufactured solar cell and reducing the conversion efficiency of the cell. Summary of the Invention

[0004] Based on this, in view of the above problems, it is necessary to provide a back passivation film for a solar cell, a preparation method thereof, a solar cell and a preparation method thereof. The solar cell obtained by using the preparation method of the back passivation film for a solar cell can effectively reduce the situation of EL local pollution and has a high conversion efficiency.

[0005] A preparation method of a back passivation film for a solar cell includes the following steps:

[0006] Perform the first film coating on the back of the silicon wafer by using the PECVD method, and sequentially deposit an aluminum oxide film and a first silicon nitride film;

[0007] Perform the second film coating on the surface of the first silicon nitride film by using the PECVD method, and deposit a second silicon nitride film to obtain a back passivation film;

[0008] Among them, the step of the first film coating mainly includes: performing pre-ventilation by using a protective gas and trimethylaluminum gas, depositing the aluminum oxide film by using an oxygen-containing precursor and the trimethylaluminum gas, and depositing the first silicon nitride film; wherein, the trimethylaluminum gas is all obtained by converting liquid trimethylaluminum by using an evaporator, the temperature of the evaporator is 60°C - 120°C, and the flow rate of the liquid trimethylaluminum when passing through the evaporator is 40 mg / min - 70 mg / min.

[0009] In one embodiment, the step of performing pre-ventilation by using nitrogen gas and trimethylaluminum gas further satisfies at least one of the following conditions:

[0010] (1) The flow rate of the protective gas is 5000 sccm - 10000 sccm;

[0011] (2) The pressure is ≤ 100 mTorr;

[0012] (3) The time is 10 s - 60 s;

[0013] (4) The temperature of the gas delivery pipe is 30 °C - 50 °C.

[0014] In one embodiment, the step of depositing the alumina film further satisfies at least one of the following conditions:

[0015] (1) The oxygen-containing precursor is selected from nitrous oxide;

[0016] (2) The flow rate of the oxygen-containing precursor is 3000 sccm - 8000 sccm;

[0017] (3) The deposition time is 80 s - 200 s;

[0018] (4) The deposition pressure is 1300 mTorr - 1700 mTorr;

[0019] (5) The deposition temperature is 280 °C - 350 °C;

[0020] (6) The thickness of the alumina film is 5 nm - 15 nm.

[0021] In one embodiment, a nitrogen-containing precursor and a silicon-containing precursor are used to deposit the first silicon nitride film.

[0022] In one embodiment, the step of depositing the first silicon nitride film using a nitrogen-containing precursor and a silicon-containing precursor further satisfies at least one of the following conditions:

[0023] (1) The flow rate of the nitrogen-containing precursor is 3000 sL / min - 8000 sL / min, and the flow rate of the silicon-containing precursor is 500 sL / min - 1300 sL / min;

[0024] (2) The nitrogen-containing precursor is selected from ammonia, and the silicon-containing precursor is selected from silane;

[0025] (3) The deposition time is 15 s - 40 s;

[0026] (4) The deposition pressure ≤ 1500 mTorr;

[0027] (5) The deposition temperature is 280 °C - 350 °C;

[0028] (6) The thickness of the first silicon nitride film is 2 nm - 5 nm;

[0029] (7) The refractive index of the first silicon nitride film is 1.9 - 2.4.

[0030] In one embodiment, before pre - purging, it also includes evacuating and purging to keep the temperature at 280°C - 350°C.

[0031] In one embodiment, before depositing the first silicon nitride film, it also includes introducing nitrous oxide and ammonia for 100s - 400s at a pressure of 1000mTorr - 1500mTorr, where the flow rate of nitrous oxide is 500sccm - 5000sccm and the flow rate of ammonia is 500sccm - 5000sccm.

[0032] A solar cell back - passivation film made by the preparation method as described above.

[0033] A preparation method of a solar cell, wherein the back - passivation film adopts the preparation method as described above.

[0034] A solar cell obtained by the preparation method of a solar cell as described above.

[0035] In the present invention, before depositing the alumina film, a protective gas and trimethylaluminum (TMA) gas are introduced for pre - purging treatment, and the temperature of the evaporator for evaporating liquid TMA and the flow rate of liquid TMA introduced into the evaporator are controlled, so as to control the flow rate of TMA gas, so that a sufficient amount of TMA atmosphere is formed before depositing the alumina film, which helps to improve the thickness uniformity of the deposited alumina film. And during the deposition of the alumina film, the temperature of the evaporator for evaporating liquid TMA and the flow rate of liquid TMA introduced into the evaporator are continuously controlled, so as to control the flow rate of TMA gas, and further effectively improve the deposition effect of the alumina film during the deposition of the alumina film.

[0036] In addition, under the temperature conditions for depositing the alumina film in the present invention, a first silicon nitride film is also deposited, so that the first silicon nitride film plays a protective role for the alumina film, and the uniformity of the alumina film will not be damaged in the high - temperature environment when depositing the second silicon nitride film for the second coating, thus avoiding the problem that the alumina film will be partially decomposed when depositing the second silicon nitride film.

[0037] Therefore, for the solar cell prepared by the preparation method of the present invention, the proportion of EL dark pollution is significantly reduced, and it has excellent conversion efficiency. Description of the Drawings

[0038] Figure 1 It is the EL diagram of Example 3;

[0039] Figure 2 It is the EL diagram of Comparative Example 2;

[0040] Figure 3 It is the EL diagram of Comparative Example 6. Detailed implementation manners

[0041] For the convenience of understanding the present invention, the present invention will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. On the contrary, these embodiments or examples are provided to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments or examples and are not intended to limit the present invention. The optional scope of the term "and / or" used herein includes any one of two or more related listed items, and also includes any and all combinations of the related listed items. The any and all combinations include any two related listed items, any more related listed items, or the combination of all related listed items.

[0043] The present invention provides a method for preparing a back passivation film for a solar cell, including: performing a first coating on the back surface of a silicon wafer by using a PECVD method, sequentially depositing an aluminum oxide film and a first silicon nitride film; and performing a second coating on the surface of the first silicon nitride film by using a PECVD method to deposit a second silicon nitride film, thereby obtaining the back passivation film.

[0044] Among them, the steps of the first coating mainly include: performing pre-gassing by using a protective gas and trimethylaluminum gas, depositing the aluminum oxide film by using an oxygen-containing precursor and the trimethylaluminum gas, and depositing the first silicon nitride film; wherein, the trimethylaluminum gas is all obtained by converting liquid trimethylaluminum by using an evaporator, the temperature of the evaporator is 60°C - 120°C, and the flow rate of the liquid trimethylaluminum when passing through the evaporator is 40 mg / min - 70 mg / min.

[0045] Specifically, before depositing the aluminum oxide film, the annealed silicon wafer is first inserted into a graphite boat, sent into a furnace tube, the air in the furnace tube is evacuated, and a protective gas is introduced for purging, so that the temperature in the furnace tube is maintained at 280°C - 350°C.

[0046] Then, a protective gas carrying TMA gas is introduced for pre-gassing treatment, which can ensure that there is enough TMA gas in the furnace tube before depositing the aluminum oxide film, so that when depositing the aluminum oxide film, a sufficient amount of TMA gas can be provided to obtain a uniformly deposited aluminum oxide film, thereby significantly reducing the proportion of local darkening pollution of the solar cell EL.

[0047] Optionally, the protective gas is preferably at least one of nitrogen or argon.

[0048] Specifically, since the liquid TMA flows out of the source bottle and is converted into TMA gas through the evaporator, and then the TMA gas is mixed with the protective gas and enters the furnace tube for depositing the alumina film, the temperature of the evaporator for evaporating the liquid TMA and the flow rate of the liquid TMA introduced into the evaporator are crucial for the uniformity of the alumina film. For example, if the flow rate of the liquid TMA is too small, the amount of TMA gas required for depositing the alumina film is insufficient, resulting in a thin and non-uniform alumina film. If the flow rate is too large, the deposited alumina film will be thick in the middle and thin around, causing the overall EL to be dark. If the temperature of the evaporator is too low, the gasification amount of the liquid TMA is insufficient, leading to a thin alumina film. If the temperature of the evaporator is too high, the evaporation and gasification rate of the liquid TMA fluctuates, and the gasification amount is unstable, resulting in a non-uniform thickness of the deposited alumina film. Therefore, the temperature of the evaporator is 60°C - 120°C, and the flow rate of the liquid TMA introduced into the evaporator is 40 mg / min - 70 mg / min.

[0049] Optionally, the pre-gassing time is preferably 10 s - 60 s, which is beneficial to having a sufficient amount of TMA gas in the furnace tube before depositing the alumina film, further improving the uniformity of the deposited alumina film.

[0050] Optionally, the flow rate of the protective gas is preferably 5000 sccm - 10000 sccm. The protective gas carries the TMA gas into the furnace tube through the gas transmission pipe. The temperature of the gas transmission pipe is preferably 30°C - 50°C, and the pressure is preferably ≤100 mTorr, which is beneficial to ensuring the flow rate of the TMA gas entering the furnace tube during the pre-gassing process.

[0051] After the pre-gassing is completed, an oxygen-containing precursor and TMA gas are introduced to deposit the alumina film. Among them, the flow rate of the liquid TMA introduced into the evaporator is the same as that during the pre-gassing process, and the temperature of the evaporator is the same as that during the pre-gassing process. Thus, by controlling the flow rate of the liquid TMA and the temperature of the evaporator during the pre-gassing process to be the same as those during the alumina film deposition process, the stability of the TMA gas flow rate during the alumina film deposition process is further ensured, and then a uniformly deposited alumina film is obtained, further reducing the proportion of local EL darkening pollution of the solar cell and improving the conversion efficiency.

[0052] Optionally, the oxygen-containing precursor is preferably nitrous oxide.

[0053] Optionally, the flow rate of the oxygen-containing precursor is preferably 3000 sccm - 8000 sccm, the deposition time is preferably 80 s - 200 s, the deposition pressure is preferably 1300 mTorr - 1700 mTorr, and the deposition temperature is preferably 280 °C - 350 °C.

[0054] Optionally, the thickness of the alumina film is preferably 5 nm - 15 nm, and the thickness difference is ≤ 2 nm. The alumina film has good thickness uniformity and is relatively thin.

[0055] After depositing the alumina film by PECVD, if directly heating up to deposit the silicon nitride film by PECVD, the high-temperature environment will cause partial decomposition of the alumina film. Therefore, at a temperature of 280 °C - 350 °C, a nitrogen-containing precursor and a silicon-containing precursor are first introduced to deposit a first silicon nitride film, and the thickness of the first silicon nitride film is preferably 2 nm - 5 nm.

[0056] Thus, the first silicon nitride film plays a role in protecting the alumina film. When subsequently heating up to a second temperature to deposit the second silicon nitride film, the higher temperature will not damage the uniformity of the alumina film, thereby avoiding the problem of an increase in the proportion of EL dark pollution of the battery caused by partial decomposition of the alumina film during the deposition of the second silicon nitride film.

[0057] Optionally, the flow rate of the nitrogen-containing precursor is preferably 3000 sL / min - 8000 sL / min, the flow rate of the silicon-containing precursor is preferably 500 sL / min - 1300 sL / min, the deposition time is preferably 15 s - 40 s, and the deposition pressure is preferably ≤ 1500 mTorr.

[0058] Optionally, the nitrogen-containing precursor is preferably ammonia, and the silicon-containing precursor is preferably silane.

[0059] Optionally, the refractive index of the first silicon nitride film is preferably 1.9 - 2.4.

[0060] Optionally, before depositing the first silicon nitride film, nitrous oxide and ammonia can also be introduced to repair the alumina film. The flow rate of the nitrous oxide is preferably 500 sccm - 5000 sccm, the flow rate of the ammonia is preferably 500 sccm - 5000 sccm, the time is preferably 100 s - 400 s, and the pressure is preferably 1000 mTorr - 1500 mTorr.

[0061] After the first coating, the temperature is raised for the second coating, and a second silicon nitride film is deposited using a nitrogen-containing precursor and a silicon-containing precursor. The second silicon nitride film may include multiple sub-layers, preferably 3 sub-layers, namely the first silicon nitride sub-layer, the second silicon nitride sub-layer, and the third silicon nitride sub-layer. When depositing different sub-layers, the flow ratio of the silicon-containing precursor to the nitrogen-containing precursor can be selected to obtain silicon nitride films with different refractive indices.

[0062] Optionally, the temperature for depositing the second silicon nitride film is preferably 400°C - 460°C.

[0063] Optionally, the thickness of the second silicon nitride film is preferably 75 nm - 90 nm.

[0064] Optionally, the nitrogen-containing precursor is preferably ammonia, the silicon-containing precursor is preferably silane, the flow rate of the ammonia is preferably 4000 sL / min - 8000 sL / min, the flow rate of the silane is preferably 500 sL / min - 1000 sL / min, the deposition time is preferably 100 s - 300 s, and the deposition pressure is preferably 1500 mTorr - 2000 mTorr.

[0065] The present invention also provides a back passivation film for a solar cell prepared by the above preparation method. The back passivation film includes an alumina film, a first silicon nitride film, and a second silicon nitride film. The thickness variation range of the alumina in the present invention is less than or equal to 2 nm, and it has excellent uniformity.

[0066] The present invention also provides a preparation method for a solar cell. The back passivation film adopts the preparation method for the back passivation film of the solar cell described above. Among them, the solar cell can be a PERC cell, a TOPCon cell, etc.

[0067] Specifically, for example, when preparing a PERC cell, before depositing the back passivation film, the silicon wafer can be subjected to texturing, boron diffusion, SE laser, etching, and annealing treatments to form a cell substrate to be deposited with the back passivation film, and then the back passivation film is deposited on the cell substrate. Since the front passivation film also needs to be deposited on the cell substrate, when depositing the back passivation film, it can be selected to use a two-in-one line to perform the first coating and the second coating in sequence, and then deposit the front passivation film, or first perform the first coating described in the present invention, then deposit the front passivation layer, and then perform the second coating described in the present invention.

[0068] The present invention also provides a solar cell obtained by the above preparation method for a solar cell. Among them, the solar cell can be a PERC cell, a TOPCon cell, etc., and the proportion of defective EL pollution of the solar cell is significantly reduced.

[0069] Hereinafter, the solar cell back passivation film, its preparation method, the solar cell and its preparation method will be further described through the following specific embodiments.

[0070] Example 1

[0071] Place the graphite boat loaded with the annealed silicon wafers in the PECVD furnace tube, evacuate the air in the furnace tube, purge the silicon wafers with nitrogen, and stabilize the temperature in the furnace tube at 280°C.

[0072] Pre-introduce TMA gas and nitrogen into the furnace tube, and maintain the pressure at 90 mTorr. Among them, the TMA gas is obtained by evaporating liquid TMA through an evaporator. The flow rate of liquid TMA introduced into the evaporator is 40 mg / min, the evaporation temperature is 60°C, the nitrogen flow rate is 8000 sccm, the temperature of the gas transmission pipe is maintained at 30°C, and the pre-gas introduction time is 15 s.

[0073] Under the condition of 280°C, introduce nitrous oxide and TMA gas with a flow rate of 3000 sccm, maintain the pressure at 1300 mTorr, turn on the radio frequency, and deposit for 150 s to form an alumina film with an average thickness of 10.06 nm. Among them, the TMA gas is obtained by evaporating liquid TMA through an evaporator. The flow rate of liquid TMA introduced into the evaporator is 40 mg / min, and the evaporation temperature is 60°C.

[0074] Evacuate the gas in the furnace tube, maintain the pressure in the tube at 25 mTorr, and then introduce nitrous oxide and ammonia for 300 s to repair the alumina film. Among them, the nitrous oxide flow rate is 2500 sccm, the ammonia flow rate is 2500 sccm, and the pressure is 1300 mTorr.

[0075] Evacuate the gas in the furnace tube, maintain the pressure in the tube at 25 mTorr. Under the condition of 280°C, introduce ammonia and silane, and deposit for 15 s to form a first silicon nitride film with a thickness of 2 nm and a refractive index of 2.2. Among them, the ammonia flow rate is 3000 sL / min, the silane flow rate is 500 sL / min, and the deposition pressure is 900 mTorr.

[0076] Evacuate the gas in the furnace tube, maintain the pressure in the tube at 25 mTorr, raise the temperature in the furnace tube to 400°C, and introduce ammonia and silane to deposit three layers of second silicon nitride films with different refractive indexes. Among them, when depositing the first layer of silicon nitride film, the ammonia flow rate is 4800 sccm, the silane flow rate is 1000 sccm, the time is 250 s, and the pressure is 1700 mTorr. When depositing the second layer of silicon nitride film, the ammonia flow rate is 4800 sccm, the silane flow rate is 600 sccm, the time is 200 s, and the pressure is 1700 mTorr. When depositing the third layer of silicon nitride film, the ammonia flow rate is 8000 sccm, the silane flow rate is 1000 sccm, the time is 130 s, and the pressure is 1700 mTorr.

[0077] Finally, take out the silicon wafer on which the alumina film, the first silicon nitride film and the second silicon nitride film have been deposited, place it on the shelf to cool and then unload the wafer.

[0078] Example 2

[0079] Place the graphite boat loaded with the annealed silicon wafer into the PECVD furnace tube, evacuate the air in the furnace tube, purge the silicon wafer with nitrogen, and stabilize the temperature in the furnace tube at 350 °C.

[0080] Pre-introduce TMA gas and nitrogen into the furnace tube, and maintain the pressure at 90 mTorr. Among them, the TMA gas is obtained by evaporating liquid TMA through an evaporator. The flow rate of liquid TMA introduced into the evaporator is 70 mg / min, the evaporation temperature is 120 °C, the nitrogen flow rate is 5000 sccm, the temperature of the gas transmission pipe is maintained at 40 °C, and the pre-gas introduction time is 10 s.

[0081] Under the condition of 350 °C, introduce nitrous oxide and TMA gas with a flow rate of 8000 sccm, maintain the pressure at 1700 mTorr, turn on the radio frequency, and deposit for 130 s to form an alumina thin film with an average thickness of 12.24 nm. Among them, the TMA gas is obtained by evaporating liquid TMA through an evaporator. The flow rate of liquid TMA introduced into the evaporator is 70 mg / min, and the evaporation temperature is 120 °C.

[0082] Evacuate the gas in the furnace tube, maintain the pressure in the tube at 25 mTorr, and then introduce nitrous oxide and ammonia for 250 s to repair the alumina film. Among them, the nitrous oxide flow rate is 2000 sccm, the ammonia flow rate is 2000 sccm, and the pressure is 1300 mTorr.

[0083] Evacuate the gas in the furnace tube, maintain the pressure in the tube at 25 mTorr. Under the condition of 350 °C, introduce ammonia and silane, and deposit for 40 s to form a first silicon nitride film with a thickness of 5 nm and a refractive index of 2.15. Among them, the ammonia flow rate is 8000 sL / min, the silane flow rate is 1300 sL / min, and the deposition pressure is 900 mTorr.

[0084] Pump out the gas inside the furnace tube, maintain the pressure inside the tube at 25 mTorr, raise the temperature inside the furnace tube to 400 °C, introduce ammonia and silane, and deposit three layers of second silicon nitride films with different refractive indices. Among them, when depositing the first layer of silicon nitride film, the ammonia flow rate is 4500 sccm, the silane flow rate is 900 sccm, the time is 300 s, and the pressure is 1700 mTorr. When depositing the second layer of silicon nitride film, the ammonia flow rate is 4800 sccm, the silane flow rate is 800 sccm, the time is 220 s, and the pressure is 1700 mTorr. When depositing the third layer of silicon nitride film, the ammonia flow rate is 5000 sccm, the silane flow rate is 500 sccm, the time is 150 s, and the pressure is 1700 mTorr.

[0085] Finally, take out the silicon wafer deposited with the alumina film, the first silicon nitride film, and the second silicon nitride film, place it on the shelf to cool, and then unload the wafer.

[0086] Example 3

[0087] Place the graphite boat loaded with the annealed silicon wafer into the PECVD furnace tube, pump out the air inside the furnace tube, purge the silicon wafer with nitrogen, and stabilize the temperature inside the furnace tube at 310 °C.

[0088] Pre-introduce TMA gas and nitrogen into the furnace tube, maintain the pressure at 90 mTorr. Among them, the TMA gas is obtained by evaporating liquid TMA through an evaporator. The flow rate of liquid TMA introduced into the evaporator is 55 mg / min, the evaporation temperature is 90 °C, the nitrogen flow rate is 10000 sccm, the temperature of the gas transmission pipe is maintained at 35 °C, and the pre-gas introduction time is 40 s.

[0089] Under the condition of 310 °C, introduce nitrous oxide and TMA gas with a flow rate of 5500 sccm, maintain the pressure at 1500 mTorr, turn on the radio frequency, and deposit for 130 s to form an alumina thin film with an average thickness of 10.18 nm. Among them, the TMA gas is obtained by evaporating liquid TMA through an evaporator. The flow rate of liquid TMA introduced into the evaporator is 55 mg / min, and the evaporation temperature is 90 °C.

[0090] Pump out the gas inside the furnace tube, maintain the pressure inside the tube at 25 mTorr, and then introduce nitrous oxide and ammonia for 400 s to repair the alumina film. Among them, the nitrous oxide flow rate is 5000 sccm, the ammonia flow rate is 5000 sccm, and the pressure is 1500 mTorr.

[0091] Pump out the gas inside the furnace tube, maintain the pressure inside the tube at 25 mTorr. Under the condition of 310 °C, introduce ammonia and silane, and deposit for 28 s to form a first silicon nitride film with a thickness of 3.5 nm and a refractive index of 2.18. Among them, the ammonia flow rate is 5500 sL / min, the silane flow rate is 900 sL / min, and the deposition pressure is 1500 mTorr.

[0092] The gas in the furnace tube is evacuated, and the pressure inside the tube is maintained at 25 mTorr. The temperature inside the furnace tube is raised to 400 °C, and ammonia and silane are introduced to deposit three layers of second silicon nitride films with different refractive indices. Among them, when depositing the first layer of silicon nitride film, the ammonia flow rate is 7000 sccm, the silane flow rate is 700 sccm, the time is 120 s, and the pressure is 1700 mTorr. When depositing the second layer of silicon nitride film, the ammonia flow rate is 6000 sccm, the silane flow rate is 750 sccm, the time is 200 s, and the pressure is 1700 mTorr. When depositing the third layer of silicon nitride film, the ammonia flow rate is 4800 sccm, the silane flow rate is 800 sccm, the time is 300 s, and the pressure is 1700 mTorr.

[0093] Finally, the silicon wafer deposited with the alumina film, the first silicon nitride film, and the second silicon nitride film is taken out, placed on the shelf to cool, and then unloaded.

[0094] Example 4

[0095] Place the graphite boat loaded with the annealed silicon wafer into the PECVD furnace tube, evacuate the air in the furnace tube, and purge the silicon wafer with nitrogen to stabilize the temperature inside the furnace tube at 310 °C.

[0096] Pre-introduce TMA gas and nitrogen into the furnace tube, and maintain the pressure at 90 mTorr. Among them, the TMA gas is obtained by evaporating liquid TMA through an evaporator. The flow rate of liquid TMA introduced into the evaporator is 60 mg / min, the evaporation temperature is 100 °C, the nitrogen flow rate is 8000 sccm, the temperature of the gas transmission pipe is maintained at 35 °C, and the pre-gas introduction time is 15 s.

[0097] Under the condition of 310 °C, introduce nitrous oxide and TMA gas with a flow rate of 5000 sccm, maintain the pressure at 1300 mTorr, turn on the radio frequency, and deposit for 130 s to form an alumina thin film with an average thickness of 10.18 nm. Among them, the TMA gas is obtained by evaporating liquid TMA through an evaporator. The flow rate of liquid TMA introduced into the evaporator is 60 mg / min, and the evaporation temperature is 100 °C.

[0098] Evacuate the gas in the furnace tube, maintain the pressure inside the tube at 25 mTorr, and then introduce nitrous oxide and ammonia for 300 s to repair the alumina film. Among them, the nitrous oxide flow rate is 4000 sccm, the ammonia flow rate is 4000 sccm, and the pressure is 1300 mTorr.

[0099] The gas in the furnace tube was evacuated, and the pressure inside the tube was maintained at 25 mTorr. At 310 °C, ammonia and silane were introduced, and deposition was carried out for 28 s to form a first silicon nitride film with a thickness of 3.5 nm and a refractive index of 2.18. Among them, the ammonia flow rate was 8000 sL / min, the silane flow rate was 1300 sL / min, and the deposition pressure was 900 mTorr.

[0100] The gas in the furnace tube was evacuated, and the pressure inside the tube was maintained at 25 mTorr. The temperature inside the furnace tube was raised to 400 °C, and ammonia and silane were introduced to deposit a second silicon nitride film with three different refractive indices. Among them, when depositing the first layer of silicon nitride film, the ammonia flow rate was 4800 sccm, the silane flow rate was 1000 sccm, the time was 250 s, and the pressure was 1700 mTorr. When depositing the second layer of silicon nitride film, the ammonia flow rate was 4800 sccm, the silane flow rate was 600 sccm, the time was 200 s, and the pressure was 1700 mTorr. When depositing the third layer of silicon nitride film, the ammonia flow rate was 8000 sccm, the silane flow rate was 1000 sccm, the time was 130 s, and the pressure was 1700 mTorr.

[0101] Finally, the silicon wafer deposited with the alumina film, the first silicon nitride film, and the second silicon nitride film was taken out, placed on the shelf to cool, and then unloaded.

[0102] Comparative Example 1

[0103] The difference between Comparative Example 1 and Example 3 is that in the steps of pre - ventilation and depositing the alumina film, the temperature at which the evaporator evaporates liquid TMA is 50 °C.

[0104] Comparative Example 2

[0105] The difference between Comparative Example 2 and Example 3 is that in the steps of pre - ventilation and depositing the alumina film, the temperature at which the evaporator evaporates liquid TMA is 130 °C.

[0106] Comparative Example 3

[0107] The difference between Comparative Example 3 and Example 3 is that the deposition of the first silicon nitride film is not carried out. After depositing the alumina film, the temperature is immediately raised to deposit the second silicon nitride film.

[0108] Comparative Example 4

[0109] The difference between Comparative Example 4 and Example 4 is that after purging the silicon wafer with nitrogen to stabilize the temperature inside the furnace tube at 310 °C, the deposition of the alumina film is directly carried out without pre - ventilation treatment.

[0110] Comparative Example 5

[0111] The difference between Comparative Example 5 and Example 3 lies in that: in the steps of pre-ventilation and deposition of the aluminum oxide film, the flow rate of liquid TMA introduced into the evaporator is 30 mg / min.

[0112] Comparative Example 6

[0113] The difference between Comparative Example 6 and Example 3 lies in that: in the steps of pre-ventilation and deposition of the aluminum oxide film, the flow rate of liquid TMA introduced into the evaporator is 80 mg / min.

[0114] The silicon wafers with back passivation films prepared in Examples 1-4 and Comparative Examples 1-6 were respectively processed into 1 furnace of wafers on the same machine with the same incoming materials. The uniformity of the thickness of the aluminum oxide film on the back of the silicon wafers was detected, and PERC cells were made, and the EL contamination situation of the cells was tracked. The results are shown in Tables 1 and 2.

[0115] Table 1

[0116]

[0117] Table 2

[0118]

[0119]

[0120] As can be seen from Table 1, in Examples 1-4, the average thickness of the aluminum oxide film is between 10 nm and 13 nm, and the total proportion of contaminated defects is below 0.05%, and is close to or equal to 0.

[0121] As can be seen from Table 2, when the temperature of the evaporator in Comparative Example 1 is 50°C, the flow rate of TMA gas will decrease, resulting in a decrease in the average thickness of the final alumina film, a low center thickness, and a thickness range of 4.4 nm, with the highest total contamination ratio. In Comparative Example 2, when the temperature of the evaporator is 130°C, the average thickness of the alumina film is 10.58 nm, but the thickness range is 2.7 nm, so the thickness uniformity of the alumina film is poor, and the total contamination ratio is 1.89%. In Comparative Example 3, when the first silicon nitride film is not deposited after depositing the alumina film, the average thickness of the alumina film is 9.6, the thickness range is 1.9, and the total contamination ratio is 1.97%. This is because after depositing the alumina film, the temperature during the deposition of the second silicon nitride film is relatively high, and the high temperature will cause partial decomposition and volatilization of the alumina, thus affecting the uniformity of the alumina film and increasing the EL contamination ratio of the PERC cell. In Comparative Example 4, before depositing the alumina film, no pre-purging treatment is carried out, so the flow rate of TMA gas during the deposition of the alumina film becomes less, resulting in a decrease in its thickness, and the total contamination ratio is 0.29%. In Comparative Examples 5 and 6, when pre-purging and depositing alumina, if the flow rate of liquid TMA introduced into the evaporator is too small or too large, it will affect the alumina film. If the flow rate of liquid TMA is too small, the reaction rate in the middle region will be slow, and the thickness of the alumina film will be low; if the flow rate of liquid TMA is too large, the deposition rate of alumina at the edge and in the middle will be too fast, thus deteriorating the thickness uniformity of the alumina film.

[0122] From the conversion efficiency of the PERC cells in Table 1 and Table 2, it can be seen that the alumina films in Examples 1-4 have excellent uniformity, making the conversion efficiency of the fabricated PERC cells increase by 0.02%-0.09% compared with that in Comparative Examples 1-6.

[0123] In addition, Figure 1 In Example 3, when the temperature of the evaporator is within a suitable range, an alumina film with uniform thickness is deposited, thus effectively reducing the EL darkening during EL detection.

[0124] Figure 2 In Comparative Example 2, the temperature of the evaporator for evaporating liquid TMA is too high, resulting in unstable amount of TMA gas obtained by evaporation. Therefore, during the deposition of the alumina film, the EL darkening occurs at the position in contact with the graphite boat due to uneven gas flow.

[0125] Figure 3 In Comparative Example 6, the flow rate of liquid TMA is too large, resulting in a situation where the deposited alumina film is thick in the middle and thin around, thus causing EL darkening.

[0126] Therefore, as can be seen from the above embodiments and comparative examples, by means of pre-ventilation treatment, regulating the temperature of the evaporator for evaporating liquid TMA and the flow rate of liquid TMA, the deposition quality of the alumina film can be effectively controlled, making the formed alumina film more uniform. Moreover, by depositing an extremely thin first silicon nitride film before the heating stage, the deterioration of the uniformity of the alumina film during the heating and deposition of the second silicon nitride film can be avoided, thereby significantly reducing the EL contamination ratio and improving the conversion efficiency.

[0127] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, 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, it should be considered as the scope recorded in this specification.

[0128] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A method for preparing a back passivation film of a solar cell, characterized in that, it includes the following steps: Performing a first coating on the back surface of the silicon wafer by PECVD method, sequentially depositing an aluminum oxide film and a first silicon nitride film; Performing a second coating on the surface of the first silicon nitride film by PECVD method, depositing a second silicon nitride film to obtain a back passivation film; Among them, the steps of the first coating mainly include: pre-purging with a protective gas and trimethylaluminum gas, depositing the aluminum oxide film with an oxygen-containing precursor and the trimethylaluminum gas, and depositing the first silicon nitride film; wherein, the trimethylaluminum gas is all obtained by transforming liquid trimethylaluminum with an evaporator, the temperature of the evaporator is 60°C - 120°C, and the flow rate of the liquid trimethylaluminum when passing through the evaporator is 40 mg / min - 70 mg / min.

2. The method for preparing a back passivation film of a solar cell according to claim 1, characterized in that, The step of pre-purging with nitrogen and trimethylaluminum gas also satisfies at least one of the following conditions: (1) The flow rate of the protective gas is 5000 sccm - 10000 sccm; (2) The pressure is ≤100 mTorr; (3) The time is 10 s - 60 s; (4) The temperature of the gas delivery pipe is 30°C - 50°C.

3. The method for preparing a back passivation film of a solar cell according to claim 1, characterized in that, The step of depositing the aluminum oxide film also satisfies at least one of the following conditions: (1) The oxygen-containing precursor is selected from nitrous oxide; (2) The flow rate of the oxygen-containing precursor is 3000 sccm - 8000 sccm; (3) The deposition time is 80 s - 200 s; (4) The deposition pressure is 1300 mTorr - 1700 mTorr; (5) The deposition temperature is 280°C - 350°C; (6) The thickness of the aluminum oxide film is 5 nm - 15 nm.

4. The method for preparing a back passivation film of a solar cell according to claim 1, characterized in that, Depositing the first silicon nitride film with a nitrogen-containing precursor and a silicon-containing precursor.

5. The method for preparing a back passivation film of a solar cell according to claim 4, characterized in that, The step of depositing the first silicon nitride film with a nitrogen-containing precursor and a silicon-containing precursor also satisfies at least one of the following conditions: (1) The flow rate of the nitrogen-containing precursor is 3000 sL / min - 8000 sL / min, and the flow rate of the silicon-containing precursor is 500 sL / min - 1300 sL / min; (2) The nitrogen-containing precursor is selected from ammonia, and the silicon-containing precursor is selected from silane; (3) The deposition time is 15 s - 40 s; (4) The deposition pressure ≤1500 mTorr; (5) The deposition temperature is 280°C - 350°C; (6) The thickness of the first silicon nitride film is 2 nm - 5 nm; (7) The refractive index of the first silicon nitride film is 1.9 - 2.

4.

6. The method for preparing a back passivation film of a solar cell according to claim 1, characterized in that, Before pre-purging, it also includes vacuum pumping and purging to keep the temperature at 280°C - 350°C.

7. The method for preparing the back passivation film of a solar cell according to claim 1, characterized in that, before depositing the first silicon nitride film, it further includes introducing nitrous oxide and ammonia for 100 s - 400 s at a pressure of 1000 mTorr - 1500 mTorr, wherein the flow rate of the nitrous oxide is 500 sccm - 5000 sccm, and the flow rate of the ammonia is 500 sccm - 5000 sccm.

8. A back passivation film of a solar cell prepared by the preparation method according to any one of claims 1 - 7.

9. A method for preparing a solar cell, characterized in that, the back passivation film adopts the preparation method according to any one of claims 1 - 7.

10. A solar cell obtained by the method for preparing a solar cell according to claim 9.

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