SiN / SiON / SiN stacked thin film for a solar cell and method for preparing same

Abstract

The present invention relates to a SiN / SiON / SiN stacked thin film for a solar cell and a preparation method thereof, comprising the following steps: (1) depositing a silicon nitride thin film on the back surface of a substrate by using PECVD; (2) then depositing a silicon oxynitride thin film on the surface of the silicon nitride thin film by using PECVD; (3) then depositing a silicon nitride thin film on the surface of the silicon oxynitride thin film by using PECVD; finally, obtaining a SiN / SiON / SiN stacked thin film on the back surface of the substrate, with a comprehensive film thickness of 70 nm to 90 nm and a refractive index of 2.08 to 2.10. The SiN / SiON / SiN stacked thin film of the present invention can reduce the electrical recombination on the back surface of the solar cell and improve the passivation effect.

Description

Technical Field

[0001] The present invention relates to the technical field of coating films on substrates, and particularly to a SiN / SiON / SiN laminated film for a solar cell and a preparation method thereof. Background Art

[0002] Crystalline silicon solar cells are typical p-n junction solar cells that have been studied earliest and most widely applied. Their main technological processes include: preparation of a textured surface, preparation of a p-n junction, deposition of an antireflection film, preparation of an aluminum back surface field, and preparation of front and back electrodes. Preparation of a textured surface can effectively reduce sunlight reflection and enhance light absorption, and an antireflection film can also achieve this function. The basic principle of an antireflection film: When light is reflected from the upper and lower surfaces of the antireflection film, the optical path difference generated will cause the two reflected light beams to interfere destructively, thereby reducing light reflection and increasing light transmission. For crystalline silicon solar cells encapsulated with glass, the refractive index of glass is 1.5, and the refractive index of crystalline silicon is 3.6. It is known from theoretical calculations that the optical refractive index of the most suitable antireflection film for a specific silicon solar cell is 2.3 (sqrt(3.6×1.5)), and the optimal thickness of the antireflection film is 70 nm. In addition to the reflection of sunlight from the surface of the solar cell, another important factor affecting the conversion efficiency of the solar cell is the carrier recombination caused by surface states. Due to the disruption of the lattice periodicity on the surface of crystalline silicon, there are a large number of unsaturated dangling silicon bonds on the surface, and the corresponding energy states are surface states. The existence of surface states will introduce additional energy levels in the forbidden band and become effective recombination centers for carriers, thus seriously affecting the performance of the solar cell. In order to control the surface recombination of crystalline silicon solar cells within an acceptable range, these surface states must be passivated.

[0003] Common passivation dielectric films include silicon dioxide, silicon nitride, aluminum oxide, etc. These dielectric films all have some defects that are difficult to overcome. And silicon nitride contains a large number of fixed positive charges inside. During the back passivation process of a P-type cell, an inversion layer will be formed on the surface, resulting in poor passivation quality. Summary of the Invention

[0004] In order to solve the technical problem of poor passivation quality of the existing antireflection film, a SiN / SiON / SiN laminated film for a solar cell and a preparation method thereof are provided. The SiN / SiON / SiN laminated film obtained by the method of the present invention can reduce the electrical recombination on the back surface of the solar cell and improve the passivation effect.

[0005] In order to achieve the above object, the present invention is realized through the following technical solutions:

[0006] A preparation method of a SiN / SiON / SiN laminated film for a solar cell includes the following steps:

[0007] (1) Deposit a silicon nitride thin film on the back surface of the substrate by PECVD method;

[0008] (2) Then deposit a silicon oxynitride thin film on the surface of the silicon nitride thin film by PECVD method;

[0009] (3) Then deposit a silicon nitride thin film on the surface of the silicon oxynitride thin film by PECVD method;

[0010] Finally, a SiN / SiON / SiN stacked thin film is obtained on the back surface of the substrate.

[0011] Further, the refractive index of the silicon nitride thin film (represented by SiN) in step (1) is 2.20 - 2.25, and the film thickness is 25 nm to 35 nm.

[0012] Still further, the refractive index of the silicon nitride thin film is controlled by controlling the flow rate of the reaction gas NH3 to be 4500 sccm, the flow rate of SiH4 to be 1100 sccm, and the volume ratio of the reaction gases NH3 and SiH4 to be 9:2.2 in the PECVD method; the film thickness is obtained by controlling the deposition time to be 420 s.

[0013] Further, the refractive index of the silicon oxynitride thin film (represented by SOiN) in step (2) is 2.10 - 2.15, and the film thickness is 15 nm to 25 nm.

[0014] Still further, the refractive index of the silicon oxynitride thin film is controlled by controlling the flow rate of N2O in the reaction gas used in the PECVD method to be 200 sccm, the flow rate of NH3 to be 1500 sccm, and the flow rate of SiH4 to be 3500 sccm, and the volume ratio of the reaction gases N2O, NH3, and SiH4 to be 1:7.5:17.5; the film thickness is obtained by controlling the deposition time to be 600 s.

[0015] Further, the refractive index of the silicon nitride thin film in step (3) is 2.10 - 2.15, and the film thickness is 30 nm to 40 nm.

[0016] Still further, the refractive index of the silicon nitride thin film is controlled by controlling the flow rate of NH3 in the reaction gas used in the PECVD method to be 6000 sccm and the flow rate of SiH4 to be 850 sccm, and the volume ratio of the reaction gases NH3 and SiH4 to be 7.06:1; the film thickness is obtained by controlling the deposition time to be 240 s.

[0017] On the other hand, the present invention provides a SiN / SiON / SiN laminated film prepared by the above preparation method, which has a silicon nitride film layer, a silicon oxynitride film layer, and a silicon nitride film layer in sequence on the back surface of the substrate. The total film thickness of the laminated film is 70 nm to 90 nm, the refractive index is 2.08 to 2.10, and the back surface of the silicon wafer is light blue.

[0018] Further, the substrate is single-crystalline silicon or polycrystalline silicon.

[0019] Beneficial technical effects:

[0020] In the present invention, a three-layer laminated film structure of silicon nitride film - silicon oxynitride film - silicon nitride film (SiN / SiON / SiN) is sequentially deposited on the back surface of the substrate by PECVD method. The structure of the multi-layer laminated film of the present invention can increase the band gap, further enhance the second absorption in the long wavelength band, and has a lower absorption coefficient. At the same time, it reduces the back electrical recombination, increases the internal optical reflection, further improves the minority carrier lifetime and the efficiency of the solar cell, and improves the passivation quality; the multi-layer laminated film structure of the present invention also improves the stress distribution at the interface of different film layers of the silicon wafer substrate, improves the anti-damage ability of the overall film, and thus improves the passivation effect. Specific embodiments

[0021] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and in no way limits the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0022] Unless otherwise specifically stated, the numerical values set forth in these embodiments do not limit the scope of the present invention. Technologies and methods known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, such technologies and methods should be regarded as part of the specification. In all examples shown and discussed herein, any specific value should be construed as merely exemplary and not as a limitation. Therefore, other examples of the exemplary embodiments may have different values.

[0023] In the following embodiments, the experimental methods without specific conditions are usually determined according to national standards; if there is no corresponding national standard, they are carried out according to general international standards or the standard requirements proposed by relevant enterprises. Unless otherwise specified, all parts are by weight and all percentages are by weight percentage.

[0024] Example 1

[0025] A method for preparing a SiN / SiON / SiN stacked thin film of a solar cell, comprising the following steps:

[0026] (1) Depositing a silicon nitride thin film on the back surface of the substrate by PECVD method. The refractive index of the silicon nitride thin film (denoted as SiN) is 2.10 - 2.15, and the film thickness is 15 nm to 25 nm;

[0027] (2) Then depositing a silicon oxynitride thin film on the surface of the silicon nitride thin film by PECVD method. The refractive index of the silicon oxynitride thin film (denoted as SOiN) is 2.10 - 2.15, and the film thickness is 15 nm to 25 nm;

[0028] (3) Then depositing a silicon nitride thin film on the surface of the silicon oxynitride thin film by PECVD method. The refractive index of the silicon nitride thin film is 2.10 - 2.15, and the film thickness is 30 nm to 40 nm;

[0029] Finally, a SiN / SiON / SiN stacked thin film is obtained on the back surface of the substrate.

[0030] Among them, in step (1), the refractive index of the silicon nitride thin film is controlled by controlling the flow rate of the reaction gas NH3 to be 4500 sccm, the flow rate of SiH4 to be 1100 sccm, and the volume ratio of the reaction gases NH3 and SiH4 to be 9:2.2; the film thickness is obtained by controlling the deposition time to be 420 s.

[0031] Among them, in step (2), the refractive index of the silicon oxynitride thin film is controlled by controlling the flow rate of the reaction gas N2O to be 200 sccm, the flow rate of NH3 to be 1500 sccm, and the flow rate of SiH4 to be 3500 sccm. The volume ratio of the reaction gases N2O, NH3, and SiH4 is 1:7.5:17.5; the film thickness is obtained by controlling the deposition time to be 600 s.

[0032] Among them, in step (3), the refractive index of the silicon nitride thin film is controlled by controlling the flow rate of the reaction gas NH3 to be 6000 sccm and the flow rate of SiH4 to be 850 sccm. The volume ratio of the reaction gases NH3 and SiH4 is 7.06:1; the film thickness is obtained by controlling the deposition time to be 240 s.

[0033] The SiN / SiON / SiN stacked thin film prepared in this embodiment has a structure with a silicon nitride thin film layer, a silicon oxynitride thin film layer, and a silicon nitride thin film layer in sequence on the back surface of the substrate. The comprehensive film thickness of the stacked thin film is 70 nm to 90 nm, the refractive index is 2.08 to 2.10, and the back surface of the silicon wafer is light blue.

[0034] Comparative Example 1

[0035] This comparative example is single-crystalline silicon without coating.

[0036] The minority carrier lifetimes of the multi-layer stacked films prepared on single-crystalline silicon in the above examples and comparative examples were measured. The wavelength of the light irradiation in the minority carrier lifetime measurement was 1064 nm. The results are shown in Table 1.

[0037] Table 1 Minority carrier lifetimes of the stacked antireflection films of the examples and comparative examples

[0038] Test 1 Test 2 Test 3 Test 4 Test 5 Average value Comparative Example 1 36 33 54 27 31 36.2 Example 1 85 92 98 100 93 93.6

[0039] As can be seen from Table 1, the minority carrier lifetime of the double-layer stacked antireflection film of the silicon oxynitride film and silicon nitride film in Example 1 was increased by about 55 μs on average compared with the comparative example.

[0040] After preparing solar cells from the stacked antireflection films prepared on crystalline silicon cells in the above examples and comparative examples, the electrical performance and efficiency were tested. The results are shown in Table 2.

[0041] Table 2 Electrical performance and efficiency of solar cells prepared from multi-layer stacked films of the examples and comparative examples

[0042] Eta(%) Uoc(V) Isc(A) FF(%) Rs(Ω) Rsh(Ω) Comparative Example 1 20.768 0.6501 9.664 81.22 0.0022 788.54 Example 1 22.529 0.6599 10.353 81.05 0.0023 744.71 Difference 1.761 0.0098 0.689 -0.17 0.0001 -95.97

[0043] Note: In the table, Eta represents the conversion efficiency, Uoc represents the open-circuit voltage, Isc represents the short-circuit current, FF represents the fill factor, Rs represents the series resistance, and Rsh represents the shunt resistance.

[0044] As can be seen from Table 2, compared with the uncoated single-crystalline silicon, after depositing the SiN / SiON / SiN stacked film on the single-crystalline silicon by the PECVD method in this example, the conversion efficiency of the solar cell was increased by 1.761%, and the electrical performance was optimized.

[0045] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A preparation method of a SiN / SiON / SiN laminated film for a solar cell, characterized in that, The structure of the SiN / SiON / SiN stacked thin film is successively provided with a silicon nitride thin film layer, a silicon oxynitride thin film layer, and a silicon nitride thin film layer on the back surface of the substrate. The comprehensive film thickness of the SiN / SiON / SiN stacked thin film is 70 nm to 90 nm, and the refractive index is 2.08 to 2.10; It includes the following steps: (1) Deposit a silicon nitride thin film on the back surface of the substrate by PECVD. The refractive index of the silicon nitride thin film is 2.10 - 2.15, and the film thickness is 15 nm to 25 nm; Control the flow rate of the reaction gas NH3 used in the PECVD method to be 4500 sccm, the flow rate of SiH4 to be 1100 sccm, the volume ratio of the reaction gases NH3 and SiH4 to be 9:2.2, and control the deposition time to be 420 s to obtain it; (2) Then deposit a silicon oxynitride thin film on the surface of the silicon nitride thin film by PECVD. The refractive index of the silicon oxynitride thin film is 2.10 - 2.15, and the film thickness is 15 nm to 25 nm; Control the flow rate of N2O in the reaction gas used in the PECVD method to be 200 sccm, the flow rate of NH3 to be 1500 sccm, the flow rate of SiH4 to be 3500 sccm, the volume ratio of the reaction gases N2O, NH3, and SiH4 to be 1:7.5:17.5, and control the deposition time to be 600 s to obtain it; (3) Then deposit a silicon nitride thin film on the surface of the silicon oxynitride thin film by PECVD. The refractive index of the silicon nitride thin film is 2.10 - 2.15, and the film thickness is 30 nm to 40 nm; Control the flow rate of NH3 in the reaction gas used in the PECVD method to be 6000 sccm, the flow rate of SiH4 to be 850 sccm, the volume ratio of the reaction gases NH3 and SiH4 to be 7.06:1, and control the deposition time to be 240 s to obtain it; Finally, obtain the SiN / SiON / SiN stacked thin film on the back surface of the substrate in sequence.