A back passivation process to improve the photoelectric conversion efficiency of PERC solar cells

Abstract

The present invention provides a back passivation process for improving the photoelectric conversion efficiency of PERC cells. No need to improve the existing equipment, only the process parameters of the alumina cavity and the silicon nitride cavity of the MAIA back passivation machine need to be adjusted to improve the back passivation effect of Al2O3 and enhance the SiN x The internal back reflection function can improve the photoelectric conversion efficiency of the cell, while reducing TMA consumption, increasing machine capacity and reducing production costs.

Description

Technical Field

[0001] The present invention belongs to the technical field of silicon solar cell manufacturing technology, and in particular relates to a back passivation process for improving the photoelectric conversion efficiency of a PERC cell. Background Art

[0002] PERC technology (passivated emitter and rear cell) is a passivated emitter rear contact. By forming a passivation layer on the back of a silicon solar cell and using metal local contact, it can significantly reduce the electrical recombination rate on the back surface, form a good internal optical back reflection mechanism, and increase the open circuit voltage and short circuit current of the battery, thereby improving the conversion efficiency of the battery.

[0003] The core technology of PERC solar cells is back passivation technology, which is to coat a layer of aluminum oxide film (Al 2 O 3 ) and covered with a layer of silicon nitride film (SiN x ) implements passivation technology to enhance the internal back reflection of light in the silicon substrate and reduce back recombination, thereby effectively improving the efficiency of PERC cells. The two aluminum oxide deposition technologies widely used by battery manufacturers for large-scale production are plasma enhanced chemical vapor deposition (PECVD) and atomic layer deposition (ALD). Currently, PECVD equipment mainly uses the German MAIA (Multiple Application Inline Apperatus) back passivation machine, which has a high degree of integration and completes Al in the aluminum oxide chamber and silicon nitride chamber. 2 O 3 With SiN x The deposition of thin films is slightly worse than that of atomic layer deposition, and the TMA consumption is high and the production capacity is low. Therefore, the process of depositing aluminum oxide and silicon nitride films by MAIA back passivation machine still needs to be continuously improved to improve the photoelectric conversion efficiency and reduce production costs. Summary of the invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a back passivation process which is simple in process and easy to implement to improve the photoelectric conversion efficiency of PERC cells.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A back passivation process for improving the photoelectric conversion efficiency of a PERC cell, using a MAIA back passivation machine, the MAIA back passivation machine comprising an aluminum oxide chamber and a silicon nitride chamber, the aluminum oxide chamber being used to coat an aluminum oxide film on a silicon wafer, the silicon nitride chamber being used to coat a silicon nitride film on a silicon wafer, the silicon nitride film covering the surface of the aluminum oxide film, comprising the following steps:

[0007] (a) After conventional texturing, diffusion, SE laser, etching and annealing processes, the silicon wafer is waiting to enter the MAIA back passivation machine;

[0008] (b) setting the process belt speed in the alumina chamber to 220-250 cm / min, the chamber temperature to 350-450° C., and the process pressure to 0.11-0.15 mbar;

[0009] (c) The nitrous oxide flow rate of the first gas path in the alumina cavity is set to 300-700 sccm, the argon gas flow rate is set to 0 sccm, the TMA flow rate is set to 0 mg / min, the microwave peak power is set to 1500-2500 W, and the duty cycle is set to 4 / 18; the nitrous oxide flow rate of the second gas path in the alumina cavity is set to 450-850 sccm, the argon gas flow rate is set to 600-1000 sccm, the TMA flow rate is set to 200-400 mg / min, the microwave peak power is set to 1800-3200 W, and the duty cycle is set to 4 / 18; the reaction equation in the alumina cavity is: 2Al(CH 3 ) 3 +5Ar+20N 2 O→Al 2 O 3 +2CO 2 +4CO+9H 2 O+20N 2 +5Ar;

[0010] (d) setting the process belt speed in the silicon nitride chamber to 250-290 cm / min, the chamber temperature to 350-450° C., and the process pressure to 0.23-0.27 mbar;

[0011] (e) The ammonia flow rate of the first to third gas paths in the silicon nitride chamber is set to 400-600 sccm, and the silane flow rate is set to 250-350 sccm. The ammonia flow rate of the fourth and fifth gas paths in the silicon nitride chamber is set to 800-1000 sccm, and the silane flow rate is set to 220-320 sccm; the ammonia flow rate of the left, middle and right sections of the sixth gas path is set to 0-250 sccm, 0-700 sccm, and 0-250 sccm respectively; the silane flow rate of the left, middle and right sections is set to 0-100 sccm, 0-250 sccm, and 0-100 sccm respectively; the reaction equation in the silicon nitride chamber is: SiH 4 +NH 3 →Si x N y H z +H 2 ;

[0012] (f) setting the microwave peak power of the six gas paths in the silicon nitride cavity to 3800 W and the duty cycle to 8 / 8;

[0013] (g) The silicon wafer first enters the alumina chamber with the coated rectangular graphite carrier to be coated with an alumina film, and then enters the silicon nitride chamber to be coated with a silicon nitride film, completing the back passivation process of the silicon wafer.

[0014] The silicon wafer is an N-type single crystal silicon wafer or a P-type single crystal silicon wafer, and the P-type single crystal silicon wafer is preferably used.

[0015] By adopting the above technical solution, the beneficial effects of the present invention are:

[0016] (1) No new equipment investment is required. By adjusting the process parameters of the aluminum oxide cavity and silicon nitride cavity on the original MAIA back passivation machine, the compactness of the aluminum oxide film and silicon nitride film can be comprehensively improved, and the passivation effect on the silicon wafer and the inner back surface reflection effect can be increased. The purpose of improving the photoelectric conversion efficiency of the battery produced by the back passivation machine can be achieved;

[0017] (2) This solution reduces the use of TMA, increases machine capacity, and has a simple process and is easy to implement;

[0018] (3) Compared with the existing process, the cell conversion efficiency of the present invention is improved by more than 0.06% compared with the original process, which can actively promote the technological innovation and large-scale production of PERC solar cells and greatly improve the economic benefits of PERC solar cells;

[0019] In summary, the present invention provides a back passivation process for improving the photoelectric conversion rate of PERC solar cells. It does not require improvement of existing equipment. It only needs to adjust the process parameters of the aluminum oxide cavity and the silicon nitride cavity of the MAIA back passivation machine to improve the Al 2 O 3 Back passivation effect, enhanced SiN x The internal back reflection function can improve the photoelectric conversion efficiency of the cell, while reducing TMA consumption, increasing machine capacity and reducing production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the structure of the back passivation film layer in the present invention. DETAILED DESCRIPTION

[0021] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0022] The MAIA back passivation machine can deposit aluminum oxide film alone, silicon nitride film alone, or deposit aluminum oxide film first and then silicon nitride film at the same time.

[0023] Under existing process conditions, the thickness of the aluminum oxide film on the back of a PERC solar cell is generally 10-20 nm, and the thickness of the silicon nitride film is generally 60-110 nm. The process of the present invention improves the photoelectric conversion efficiency of the MAIA back passivation machine by changing the process parameters while ensuring that the thickness of the aluminum oxide and silicon nitride films remain unchanged.

[0024] The following will be combined with the drawings in the embodiments of the invention to clearly and completely describe the technical solutions in the embodiments of the invention. Obviously, the described embodiments are only part of the embodiments of the invention, not all of the embodiments. Based on the embodiments of the invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the invention.

[0025] Embodiment: Using the new process provided by the present invention:

[0026] A 166mm×166mm P-type single-crystal silicon wafer with a resistivity of 0.4 to 1.1Ω•cm was used. After conventional texturing, diffusion, SE laser, etching, and annealing, an aluminum oxide film and a silicon nitride film were sequentially coated on a MAIA back passivation machine.

[0027] The process belt speed in the alumina chamber is set to 220-250 cm / min, the chamber temperature is set to 350-450°C, and the process pressure is set to 0.11-0.15 mbar;

[0028] The nitrous oxide flow rate of the first gas path in the alumina cavity is set to 300-700 sccm, the argon flow rate is set to 0 sccm, the TMA flow rate is set to 0 mg / min, the microwave peak power is set to 1500-2500W, and the duty cycle is set to 4 / 18; the nitrous oxide flow rate of the second gas path in the alumina cavity is set to 450-850 sccm, the argon flow rate is set to 600-1000 sccm, the TMA flow rate is set to 200-400 mg / min, the microwave peak power is set to 1800-3200W, and the duty cycle is set to 4 / 18; the reaction equation in the silicon nitride cavity is: SiH4+NH3 →SixNyHz+H2;

[0029] The process belt speed in the silicon nitride chamber is set to 250-290 cm / min, the chamber temperature is set to 350-450°C, and the process pressure is set to 0.23-0.27 mbar;

[0030] The ammonia flow rates of the first to third gas paths in the silicon nitride chamber are set to 400-600 sccm, and the silane flow rate is set to 250-350 sccm. The ammonia flow rates of the fourth and fifth gas paths in the silicon nitride chamber are set to 800-1000 sccm, and the silane flow rate is set to 220-320 sccm. The ammonia flow rates of the left, middle and right sections of the sixth gas path are set to 0-250 sccm, 0-700 sccm, and 0-250 sccm, respectively. The silane flow rates of the left, middle and right sections are set to 0-100 sccm, 0-250 sccm, and 0-100 sccm, respectively. The reaction equation in the silicon nitride chamber is: SiH4+NH3 →SixNyHz+H2.

[0031] The microwave power of the six gas paths in the silicon nitride cavity was set to 3800 W, and the duty cycle was set to 8 / 8.

[0032] The silicon wafer is placed with the back side facing up on a coated rectangular graphite carrier, and then enters the alumina chamber and the silicon nitride chamber in turn to be coated with an aluminum oxide film and a silicon nitride film respectively. A rectangular graphite carrier with 6 pieces long and 4 pieces wide is used to load a total of 24 pieces.

[0033] Comparative example: using existing process

[0034] A 166mm×166mm P-type single-crystal silicon wafer with a resistivity of 0.4 to 1.1Ω•cm was used. After conventional texturing, diffusion, SE laser, etching, and annealing, an aluminum oxide film and a silicon nitride film were sequentially coated on a MAIA back passivation machine.

[0035] The process belt speed in the alumina chamber is set to 200-220 cm / min, the chamber temperature is set to 250-350°C, and the process pressure is set to 0.11-0.15 mbar;

[0036] The nitrous oxide flow rate of the first gas path in the alumina cavity was set to 500-900 sccm, the argon flow rate was set to 200-300 sccm, the TMA flow rate was set to 0 mg / min, the microwave peak power was set to 2400-3000 W, and the duty cycle was set to 6 / 18; the nitrous oxide flow rate of the second gas path in the alumina cavity was set to 700-1100 sccm, the argon flow rate was set to 250-350 sccm, the TMA flow rate was set to 260-460 mg / min, the microwave peak power was set to 2600-3200 W, and the duty cycle was set to 4 / 18;

[0037] The process belt speed in the silicon nitride chamber is set to 220-250 cm / min, the chamber temperature is set to 350-450°C, and the process pressure is set to 0.23-0.27 mbar;

[0038] The ammonia flow rate of the first and second gas paths in the silicon nitride chamber is set to 500-650 sccm, and the silane flow rate is set to 200-300 sccm. The ammonia flow rate of the third, fourth and fifth gas paths in the silicon nitride chamber is set to 600-800 sccm, and the silane flow rate is set to 150-250 sccm; the ammonia flow rate of the left, middle and right sections of the sixth gas path is set to 0-250 sccm, 0-700 sccm, and 0-250 sccm respectively; the silane flow rate of the left, middle and right sections is set to 0-100 sccm, 0-250 sccm, and 0-100 sccm respectively.

[0039] The microwave peak power of the six gas paths in the silicon nitride cavity was set to 3800 W, and the duty cycle was set to 8 / 9.

[0040] The silicon wafer is placed with the back side facing up on a coated rectangular graphite carrier, and then enters the alumina chamber and the silicon nitride chamber in turn to be coated with an aluminum oxide film and a silicon nitride film respectively. A rectangular graphite carrier with 6 pieces long and 4 pieces wide is used to load a total of 24 pieces.

[0041] Comparison of aluminum oxide film thickness: 12 P-type single crystal polished silicon wafers from the same batch of raw materials were used, 6 of each were processed by the above two processes, and after the aluminum oxide film was deposited in the aluminum oxide chamber of the MAIA back passivation machine, the aluminum oxide film thickness was taken out and tested by an ellipsometer. The obtained aluminum oxide film thickness is shown in Table 1:

[0042] Table 1

[0043]

[0044] It can be seen that the thickness of the aluminum oxide film in the existing process is 10.85 nm, and the thickness of the aluminum oxide film in the process of the present invention is 11.05 nm, which is within the normal process requirement range and the in-sheet uniformity of aluminum oxide in the embodiment is significantly improved compared with the comparative example.

[0045] Comparison of silicon nitride film thickness: 12 P-type single crystal annealed silicon wafers from the same batch of raw materials were used, 6 of each were processed using the above two processes, and after silicon nitride film was deposited in the silicon nitride chamber of the MAIA back passivation machine, the silicon nitride film thickness was tested using an ellipsometer. The obtained silicon nitride film thickness is shown in Table 2:

[0046] Table 2

[0047]

[0048] It can be seen that the thickness of the silicon nitride film in the existing process is 80.08 nm, and the refractive index is 2.137; the thickness of the silicon nitride film in the process of the present invention is 80.64 nm, and the refractive index is 2.148, which is within the normal process requirement range.

[0049] Electrical performance comparison: 2000 P-type single crystal silicon wafers from the same batch of raw materials were used, 1000 of each were processed by the above two processes, and after conventional front-side coating, laser grooving, and screen printing, the electrical parameters were tested by a Halm tester. The obtained electrical parameters (average value) are shown in Table 3 (three precise comparisons were performed):

[0050] Table 3

[0051]

[0052] By comparing the thickness of the aluminum oxide film and the thickness and refractive index of the silicon nitride film, it can be seen that the thickness of the aluminum oxide, the thickness of the silicon nitride film and the refractive index of the process of the present invention are not much different from those of the existing process, but the battery conversion efficiency is higher than that of the existing process, which is increased by more than 0.06%. Therefore, the aluminum oxide and silicon nitride films of the process of the present invention all meet the original requirements. On the one hand, the present invention improves the density and uniformity of the aluminum oxide film and its field effect passivation effect by improving the process temperature and microwave power of the aluminum oxide chamber of the MAIA back passivation machine; on the other hand, the film layer structure of the silicon nitride chamber of the MAIA back passivation machine is improved to enhance its protection of the aluminum oxide film and internal back emission. Without adding new equipment, the recombination of carriers can be reduced, the minority carrier lifetime can be increased, and the photoelectric conversion efficiency of the battery can be improved. At the same time, the present invention improves the utilization rate of TMA, reduces the consumption of TMA, and the belt speed of the MAIA back passivation process chamber is improved, which further releases the machine production capacity and reduces production costs.

[0053] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A back passivation process for improving the photoelectric conversion efficiency of PERC cells, using a MAIA back passivation machine, the MAIA back passivation machine includes an aluminum oxide chamber and a silicon nitride chamber, the aluminum oxide chamber is used to coat an aluminum oxide film on a silicon wafer, the silicon nitride chamber is used to coat a silicon nitride film on a silicon wafer, and the silicon nitride film covers the surface of the aluminum oxide film, characterized in that: The following steps are involved: (a) After conventional texturing, diffusion, SE laser, etching and annealing processes, the silicon wafer is waiting to enter the MAIA back passivation machine; (b) setting the process belt speed in the alumina chamber to 220-250 cm / min, the chamber temperature to 350-450° C., and the process pressure to 0.11-0.15 mbar; (c) The nitrous oxide flow rate of the first gas path in the alumina cavity is set to 300-700 sccm, the argon gas flow rate is set to 0 sccm, the TMA flow rate is set to 0 mg / min, the microwave peak power is set to 1500-2500 W, and the duty cycle is set to 4 / 18; the nitrous oxide flow rate of the second gas path in the alumina cavity is set to 450-850 sccm, the argon gas flow rate is set to 600-1000 sccm, the TMA flow rate is set to 200-400 mg / min, the microwave peak power is set to 1800-3200 W, and the duty cycle is set to 4 / 18; the reaction equation in the alumina cavity is: ； (d) setting the process belt speed in the silicon nitride chamber to 250-290 cm / min, the chamber temperature to 350-450° C., and the process pressure to 0.23-0.27 mbar; (e) The ammonia flow rate of the first to third gas paths in the silicon nitride chamber is set to 400-600 sccm, and the silane flow rate is set to 250-350 sccm. The ammonia flow rate of the fourth and fifth gas paths in the silicon nitride chamber is set to 800-1000 sccm, and the silane flow rate is set to 220-320 sccm; the ammonia flow rate of the left, middle and right sections of the sixth gas path is set to 0-250 sccm, 0-700 sccm, and 0-250 sccm respectively; the silane flow rate of the left, middle and right sections is set to 0-100 sccm, 0-250 sccm, and 0-100 sccm respectively; the reaction equation in the silicon nitride chamber is: ; (f) The microwave peak power of the six gas paths in the silicon nitride cavity is set to 3800 W, and the duty cycle is set to 8 / 8; (g) The silicon wafer along with the coated rectangular graphite carrier first enters the alumina chamber to be coated with an alumina film, and then enters the silicon nitride chamber to be coated with a silicon nitride film, completing the back passivation process of the silicon wafer.

2. The back passivation process for improving the photoelectric conversion efficiency of PERC cells according to claim 1, characterized in that: The silicon wafer is an N-type single crystal silicon wafer or a P-type single crystal silicon wafer.

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