A solar cell passivation film and preparation method thereof

Abstract

The present invention discloses a solar cell passivation film and a preparation method thereof. The preparation method comprises the following steps: performing standard RCA cleaning on a crystalline silicon substrate after texturing; depositing a pure aluminum oxide film on the cleaned substrate by atomic layer deposition; inserting a hydrogen-containing gas plasma treatment step after depositing a fixed number of pure aluminum oxide films; inserting a hydrogenation step after a fixed number of pure aluminum oxide cycles is collectively referred to as a large cycle, and repeating this large cycle to achieve the deposition of a passivation film of hydrogenated aluminum oxide; annealing the substrate on which the hydrogenated aluminum oxide film is deposited to obtain the solar cell passivation film. In the present invention, the in-situ hydrogen-containing gas plasma treatment achieves effective hydrogen doping, and the in-situ hydrogen-doped aluminum oxide film improves the film field effect passivation effect, which is significantly improved compared to the passivation effect of the pure aluminum oxide film.

Description

Technical Field

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

[0002] In silicon-based solar cells, surface recombination losses greatly limit the improvement of cell efficiency. The reason for this is that the crystal structure of crystalline silicon is discontinuous, and a large number of dangling bonds appear, which in turn leads to a high surface defect state density, becoming additional recombination centers for electrons and holes, greatly reducing the electrical performance of solar cells. Therefore, passivation treatment of the silicon surface is one of the important processes for silicon solar cells. Passivation methods can generally be divided into chemical passivation and field effect passivation. The former achieves the passivation effect by directly passivating the dangling bonds on the silicon surface and reducing the defect state density; the latter achieves the passivation effect by forming a suitable electric field on the surface, reducing the number of one type of carrier and reducing recombination.

[0003] Currently, solar cells based on passivation layers such as intrinsic hydrogenated amorphous silicon, intrinsic hydrogenated microcrystalline silicon, and intrinsic hydrogenated microcrystalline silicon oxide have achieved highly efficient surface passivation, with open-circuit voltages typically reaching 750mV. The core of this technology is the hydrogen-containing thin film passivation of the silicon surface dangling bonds, achieving high-quality chemical passivation. However, in these solar cells, materials such as amorphous silicon, microcrystalline silicon, and microcrystalline silicon oxide exhibit strong parasitic absorption, which reduces the short-circuit current of the cell and affects cell efficiency. In tunnel oxide passivated solar cells, high-quality passivation is typically provided by ultrathin tunnel oxide that provides chemical passivation, but this material often lacks UV and thermal stability. Therefore, the development of a transparent and stable passivation layer material is urgent.

[0004] Aluminum oxide (AlO) has an optical band gap of approximately 8 eV, resulting in high light transmittance. This reduces parasitic optical absorption, increases the short-circuit current of solar cells, and thus improves cell efficiency. Due to the film's inherently high negative fixed charge density, AlO provides excellent field-effect passivation for silicon. Pure AlO's passivation performance is generally not particularly good and cannot fully meet the passivation requirements of high-efficiency silicon solar cells. It requires layering with other films, such as AlO and silicon nitride. Hydrogen doping of AlO films is one method for improving their passivation. Incorporating additional hydrogen into the AlO film facilitates electron transfer from silicon to AlO, enhancing the film's field-effect passivation and thus improving AlO's passivation performance. Current methods for hydrogenating AlO films include: 1. annealing with a hydrogen-containing cap layer; 2. Deposition in a hydrogen-containing atmosphere; and 3. annealing in a hydrogen-containing atmosphere. Annealing the hydrogen-containing capping layer increases the number of process steps for deposition and removal of the capping layer, thereby increasing process costs. Deposition and annealing in a hydrogen-containing atmosphere generally cannot efficiently incorporate hydrogen into the aluminum oxide film, resulting in limited improvement in the passivation effect of the aluminum oxide film. Summary of the Invention

[0005] In order to overcome the problem of poor passivation effect of pure aluminum oxide in the prior art, the present invention improves the passivation effect of aluminum oxide by efficiently doping hydrogen into the aluminum oxide film. By hydrogen plasma treatment, the atomic layer deposited aluminum oxide film can be in situ hydrogenated to achieve in situ hydrogen doping of hydrogenated aluminum oxide. From the perspective of material passivation, the passivation effect of plasma in situ hydrogenated aluminum oxide with the same thickness is significantly improved compared with pure aluminum oxide. From the perspective of preparation method, compared with plasma chemical vapor deposition, plasma atomic layer deposition in situ hydrogenation has the following advantages: (1) Based on the atomic layer deposition technology of self-limiting reaction, the thickness and doping level of aluminum oxide can be controllably adjusted through super-cycle process and plasma process; (2) By regulating the growth state of the plasma treatment film, especially the regulation of the initial growth state of the film, the performance of the film after stable growth can be affected; (3) In one deposition process, multiple levels of different doping states can be continuously changed in the same layer of film to adapt to the design and application of the film in the device. Finally, the passivation effect of the film obtained by plasma atomic layer deposition in situ plasma hydrogen treatment is better than that of pure aluminum oxide film.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A first aspect of the present invention provides a method for preparing a solar cell passivation film, comprising the following steps:

[0008] (1) Perform standard RCA cleaning on the crystalline silicon substrate after texturing;

[0009] (2) depositing a pure aluminum oxide film on the cleaned substrate by atomic layer deposition; inserting a hydrogen-containing gas plasma treatment step after depositing a fixed number of pure aluminum oxide cycles (n value); inserting a hydrogenation step after a fixed number of pure aluminum oxide cycles is collectively referred to as a large cycle, and repeating this large cycle to achieve the deposition of a hydrogenated aluminum oxide passivation film;

[0010] (3) The substrate sheet on which the hydrogenated aluminum oxide film is deposited is annealed to obtain the solar cell passivation film.

[0011] Preferably, in step (1), the specific steps of the standard RCA cleaning include: immersing the texturized crystalline silicon substrate in a 70-80°C RCA1 solution for cleaning, taking it out, immersing it in a 2% hydrofluoric acid solution, taking it out, immersing it in a 70-80°C RCA2 solution for cleaning, taking it out, immersing it in a 2% hydrofluoric acid solution, taking it out, cleaning it with ultrapure water, and setting it aside.

[0012] Preferably, in step (2), the specific steps of the atomic layer deposition method include: placing the cleaned substrate sheet in the atomic layer deposition equipment chamber heated to a fixed temperature, evacuating the chamber, and performing an aluminum precursor pulse followed by an oxygen precursor pulse to achieve atomic layer deposition of a layer of aluminum oxide film, and depositing a fixed number of pure aluminum oxide films in this cycle; further preferably, the temperature of the atomic layer deposition equipment chamber is 100-250°C.

[0013] More preferably, the aluminum precursor is trimethylaluminum, and the oxygen precursor is ultrapure water or ozone.

[0014] Preferably, the number of cycles (n value) of depositing the pure aluminum oxide film with a fixed number of cycles is 3 to 100; further preferably, the number of cycles (n value) of depositing the pure aluminum oxide film with a fixed number of cycles is 10 to 50.

[0015] Preferably, in step (2), the hydrogen-containing gas is selected from nitrogen-hydrogen mixed gas, argon-hydrogen mixed gas, and hydrogen.

[0016] Preferably, in step (2), the plasma power is 10 to 300 W, the chamber pressure is 0.5 Pa to 100 Pa, and the plasma time is 1 s to 600 s.

[0017] Preferably, in step (2), the thickness of the passivation film of hydrogenated aluminum oxide is 5 nm to 200 nm.

[0018] Preferably, in step (3), the annealing atmosphere is selected from nitrogen, argon, nitrogen-hydrogen mixture or argon-hydrogen mixture, the annealing temperature is 300°C to 500°C, and the annealing time is 0.5min to 60min; further preferably, the annealing temperature is 400°C to 450°C, and the annealing time is 5min to 20min.

[0019] A second aspect of the present invention provides a solar cell passivation film, which is prepared by the above-mentioned method for preparing a solar cell passivation film.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] In-situ hydrogen plasma treatment of pure aluminum oxide films during atomic layer deposition achieves effective hydrogen incorporation. The additional hydrogen facilitates the transfer of electrons from silicon to aluminum oxide, which enhances the field-effect passivation of the film (significantly increases the fixed charge density), thereby improving the passivation effect of aluminum oxide (significantly increases the minority carrier lifetime and pseudo open-circuit voltage). The significant enhancement of material passivation makes it possible to subsequently reduce the thickness of aluminum hydroxide as a tunneling passivation layer. From the perspective of material improvement, in-situ hydrogen gas plasma atomic layer deposition provides a larger design space. On the one hand, it can precisely control the doping level and thickness of the material. On the other hand, compared with other doping methods, the plasma doping efficiency is higher. Finally, the film can be processed to adapt to the multi-level changes of the device in the same deposition process.

[0022] The in-situ hydrogen-containing gas plasma treatment in this invention achieves effective hydrogen doping. In-situ hydrogen-doped aluminum oxide films enhance the field-effect passivation performance of the films, significantly improving the passivation performance compared to pure aluminum oxide films. Furthermore, this superior aluminum oxide passivation performance opens the door to the development of ultra-thin aluminum oxide tunneling passivation contacts. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 CV test curve of pure aluminum oxide / hydrogenated aluminum oxide film in Example 1;

[0024] Figure 2 Graph showing minority carrier lifetime and pseudo open circuit voltage of pure aluminum oxide / hydrogenated aluminum oxide film in Example 2. DETAILED DESCRIPTION

[0025] The following is a further description of specific embodiments of the present invention. It should be noted that the description of these embodiments is intended to facilitate understanding of the present invention and does not constitute a limitation of the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0026] The experimental methods in the following examples are conventional methods unless otherwise specified, and the experimental materials used in the following examples are commercially available unless otherwise specified.

[0027] Example 1

[0028] This embodiment provides a method for preparing a solar cell passivation film, which specifically includes the following steps:

[0029] (1) Standard RCA cleaning: Cut the textured crystalline silicon substrate (n-type silicon wafer, resistivity 1.5Ω·cm) into 4×4cm small pieces, immerse the silicon wafer in RCA1 solution (50 ml ammonia + 50 ml hydrogen peroxide + 300 ml ultrapure water) heated and kept at 75 degrees Celsius in a water bath for 10 minutes; remove the silicon wafer from the solution, rinse it with ultrapure water three times, and immerse it in 2% hydrofluoric acid solution for 1 minute. The silicon wafer was removed from the solution, rinsed three times with ultrapure water, and immersed in an RCA2 solution (50 ml hydrochloric acid + 50 ml hydrogen peroxide + 300 ml ultrapure water) heated in a water bath and kept warm to 75 degrees Celsius for 10 minutes; the silicon wafer was removed from the solution, rinsed three times with ultrapure water, and immersed in a 2% hydrofluoric acid solution for 1 minute; the silicon wafer was removed from the solution, rinsed three times with ultrapure water, and then taken out and blown dry with nitrogen for later use.

[0030] (2) Deposition of aluminum oxide film: Atomic layer deposition is used to deposit aluminum oxide film. A clean silicon wafer is placed in the chamber of an atomic layer deposition device heated to 200 degrees Celsius, and vacuum is applied. A pulse of aluminum precursor (trimethylaluminum) is applied followed by a pulse of oxygen precursor (ultrapure water) to achieve atomic layer deposition of a layer of aluminum oxide film. This is a cycle, and a fixed number of cycles (25) of pure aluminum oxide film is deposited.

[0031] (3) Hydrogenation step: After depositing a fixed number of pure aluminum oxide thin films, a plasma treatment step using an argon-hydrogen mixture (Ar:H2 = 90%:10%) is inserted to achieve hydrogenation of the film. The mixture flow rate is 50 sccm, the plasma power is 80 W, and the plasma treatment time is 4 s. The fixed number of pure aluminum oxide cycles (25) plus the hydrogenation step can be collectively referred to as a large cycle. This large cycle is repeated four times, resulting in a total of 100 pure aluminum oxide cycles to deposit a hydrogenated aluminum oxide passivation film (thickness approximately 11 nm).

[0032] (4) Annealing: The silicon wafer sample with the atomic layer deposition film was annealed in an annealing atmosphere of nitrogen and hydrogen mixed gas (N2:H2=95%:5%), an annealing temperature of 425 degrees Celsius, and an annealing time of 10 minutes.

[0033] Control group (pure aluminum oxide): The same steps as above, excluding the hydrogenation step. The total number of aluminum oxide cycles in step (2) is 100, which is consistent with the number of cycles of hydrogenated aluminum oxide film (thickness is about 11 nm).

[0034] The passivation effect test method includes the following steps:

[0035] Passivation effect test: The minority carrier lifetime (τ eff =1317.24μs) and pseudo open circuit voltage (iV OC =720.9mV).

[0036] CV sample preparation and CV characterization: According to the above steps, a pure aluminum oxide / aluminum hydroxide film was deposited on one side of a silicon wafer, and an aluminum electrode (~350nm) was deposited on it. A full-area ohmic contact (1.2nm lithium fluoride / 350nm aluminum) was deposited on the back of the silicon wafer. The CV test curve is shown in the attached figure. Figure 1 Based on the CV test of the sample, the fixed charge density (Q f =-2.2×10 12 cm -2 ) and defect state density (D it =2.92×10 12 cm -2 ).

[0037] Blunt gain: compared with the control group (τ eff =248.52μs, iV OC =674.8mV, Q f =-8.4×10 10 cm -2 , D it =1.4×10 12 cm -2 ), the minority carrier lifetime and pseudo open-circuit voltage of hydrogenated aluminum oxide significantly increased, indicating enhanced passivation. CV testing revealed a significant increase in the fixed charge density of hydrogenated aluminum oxide compared to the control group, indicating enhanced field-effect passivation due to the additional hydrogen incorporation. The defect state density also increased slightly, indicating an increase in defects at the film-silicon interface, likely related to plasma radiation damage. Overall, enhanced field-effect passivation plays a dominant role in the film, resulting in a stronger passivation effect.

[0038] Example 2

[0039] This embodiment provides a method for preparing a solar cell passivation film, which specifically includes the following steps:

[0040] (1) Standard RCA cleaning: Cut the textured crystalline silicon substrate (n-type silicon wafer, resistivity 1.5Ω·cm) into 4×4cm small pieces, immerse the silicon wafer in RCA1 solution (50 ml ammonia + 50 ml hydrogen peroxide + 300 ml ultrapure water) heated and kept at 75 degrees Celsius in a water bath for 10 minutes; remove the silicon wafer from the solution, rinse it with ultrapure water three times, and immerse it in 2% hydrofluoric acid solution for 1 minute. The silicon wafer was removed from the solution, rinsed three times with ultrapure water, and immersed in an RCA2 solution (50 ml hydrochloric acid + 50 ml hydrogen peroxide + 300 ml ultrapure water) heated in a water bath and kept warm to 75 degrees Celsius for 10 minutes; the silicon wafer was removed from the solution, rinsed three times with ultrapure water, and immersed in a 2% hydrofluoric acid solution for 1 minute; the silicon wafer was removed from the solution, rinsed three times with ultrapure water, and then taken out and blown dry with nitrogen for later use.

[0041] (2) Deposition of aluminum oxide film: Atomic layer deposition is used to deposit aluminum oxide film. A clean silicon wafer is placed in the chamber of an atomic layer deposition device heated to 200 degrees Celsius. The chamber is evacuated and a pulse of aluminum precursor (trimethylaluminum) is applied followed by a pulse of oxygen precursor (ultrapure water) to achieve atomic layer deposition of a layer of aluminum oxide film. This cycle is repeated for a fixed number of cycles (5 / 10 / 15 / 20 / 25 / 50) of pure aluminum oxide film.

[0042] (3) Hydrogenation step: After depositing a fixed number of pure aluminum oxide thin films, a plasma treatment step using an argon-hydrogen mixture (Ar:H2 = 90%:10%) is inserted to achieve hydrogenation of the film. The mixture flow rate is 50 sccm, the plasma power is 80 W, and the plasma treatment time is 4 s. A fixed number of pure aluminum oxide cycles (5 / 10 / 15 / 20 / 25 / 50) plus a hydrogenation step can be collectively referred to as a large cycle. Deposition is performed using this large cycle, achieving a total of 100 pure aluminum oxide cycles to deposit a hydrogenated aluminum oxide passivation film (thickness approximately 11 nm).

[0043] (4) Annealing: The silicon wafer sample with the atomic layer deposition film was annealed in an annealing atmosphere of nitrogen and hydrogen mixed gas (N2:H2=95%:5%), an annealing temperature of 425 degrees Celsius, and an annealing time of 10 minutes.

[0044] Control group (pure aluminum oxide): The same steps as above, excluding the hydrogenation step. The total number of aluminum oxide cycles in step (2) is 100, which is consistent with the number of cycles of hydrogenated aluminum oxide film (thickness is about 11 nm).

[0045] The passivation effect test method includes the following steps:

[0046] Passivation effect test: The minority carrier lifetime and pseudo open circuit voltage of the double-sided deposited film sample were measured using a minority carrier lifetime tester (WCT-120). Figure 2 It can be observed that the in-situ plasma hydrogen doping method can enhance the passivation of the material to the silicon surface, and the passivation effect is the best when n=25.

[0047] The embodiments of the present invention are described in detail above, but the present invention is not limited to the described embodiments. It is apparent to those skilled in the art that various changes, modifications, substitutions, and variations of these embodiments may be made without departing from the principles and spirit of the present invention, and the changes still fall within the scope of protection of the present invention.

Claims

1. A method for preparing a solar cell passivation film, characterized in that: The following steps are involved: (1) Perform standard RCA cleaning on the crystalline silicon substrate after texturing; (2) depositing a pure aluminum oxide film on the cleaned substrate using an atomic layer deposition method; A fixed number of cycles of pure aluminum oxide film deposition is followed by a hydrogen-containing gas plasma treatment step. A fixed number of pure aluminum oxide cycles followed by a hydrogenation step is referred to as a large cycle, and this large cycle is repeated to achieve hydrogenated aluminum oxide passivation film deposition. (3) The substrate sheet on which the hydrogenated aluminum oxide film is deposited is annealed to obtain the solar cell passivation film.

2. The method for preparing a solar cell passivation film according to claim 1, wherein: In step (1), the specific steps of the standard RCA cleaning include: immersing the texturized crystalline silicon substrate in a 70-80°C RCA1 solution for cleaning, taking it out, immersing it in a 2% hydrofluoric acid solution, taking it out, immersing it in a 70-80°C RCA2 solution for cleaning, taking it out, immersing it in a 2% hydrofluoric acid solution, taking it out, cleaning it with ultrapure water, and setting it aside.

3. The method for preparing a solar cell passivation film according to claim 1, wherein: In step (2), the specific steps of the atomic layer deposition method include: placing the cleaned substrate sheet in the atomic layer deposition equipment chamber heated to a fixed temperature, evacuating the chamber, and performing an aluminum precursor pulse followed by an oxygen precursor pulse to achieve atomic layer deposition of a layer of aluminum oxide film, and performing this as a cycle to deposit a fixed number of pure aluminum oxide films.

4. The method for preparing a solar cell passivation film according to claim 1, wherein: The number of cycles for depositing the pure aluminum oxide film with a fixed number of cycles is 3 to 100.

5. The method for preparing a solar cell passivation film according to claim 4, wherein: The number of cycles for depositing the pure aluminum oxide film with a fixed number of cycles is 10 to 50.

6. The method for preparing a solar cell passivation film according to claim 1, wherein: In step (2), the hydrogen-containing gas is selected from nitrogen-hydrogen mixed gas, argon-hydrogen mixed gas, and hydrogen.

7. The method for preparing a solar cell passivation film according to claim 1, wherein: In step (2), the plasma treatment power is 10 to 300 W, the chamber pressure is 0.5 Pa to 100 Pa, and the plasma treatment time is 1 s to 600 s.

8. The method for preparing a solar cell passivation film according to claim 1, wherein: In step (2), the thickness of the passivation film of hydrogenated aluminum oxide is 5nm to 200nm.

9. The method for preparing a solar cell passivation film according to claim 1, wherein: In step (3), the atmosphere of the annealing treatment is selected from nitrogen, argon, nitrogen-hydrogen mixed gas or argon-hydrogen mixed gas, the annealing temperature is 300° C. to 500° C., and the annealing time is 0.5 min to 60 min.

10. A solar cell passivation film, characterized in that: The solar cell passivation film is prepared by the preparation method of any one of claims 1 to 9.

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