Silicon nitride thin film preparation method and silicon heterojunction solar cell with silicon nitride thin film as passivation layer and anti-reflection layer

Abstract

The present application relates to a kind of silicon nitride film preparation method and the silicon nitride film prepared by the method as passivation layer and the silicon heterojunction solar cell of antireflection layer, belong to semiconductor thin film preparation and photovoltaic technical field.The silicon nitride film preparation method described in the present application is reaction plasma deposition method, the main advantage of this process relative to the current mainstream silicon nitride film preparation method (PECVD) is that deposition temperature is low (<200 ℃), the bombardment damage of substrate is low, while having raw material selection simple, process parameter controllability is high, thin film purity and so on advantage;The silicon nitride film material prepared by the method described in the present application has the characteristics of material distribution uniform, surface smooth, high light transmittance.The photoelectric conversion efficiency of new type silicon heterojunction solar cell with the silicon nitride film prepared by the reaction plasma deposition method provided in the present application as passivation layer and antireflection layer is >22%.

Description

Technical Field

[0001] This invention relates to a method for preparing silicon nitride thin films and a silicon heterojunction solar cell using silicon nitride thin films prepared by this method as antireflection layers and passivation layers, belonging to the field of semiconductor thin film material preparation and photovoltaic application technology. Background Technology

[0002] Since its inception, silicon heterojunction (SHJ) solar cells have received great attention from the photovoltaic industry. They combine the advantages of crystalline silicon cells and thin-film cells, and have advantages such as high conversion efficiency (>26%), low processing temperature (<200 ℃), short process flow, use of ultra-thin silicon (for fabricating flexible cells), and easy achievement of bi-lateral light input (10% power generation gain).

[0003] Currently, most commercially available high-efficiency SHJ batteries use n-type crystalline silicon as the substrate and employ a front-contact, rear-emitter structure. The layers from top to bottom are: silver grid lines, indium-based transparent conductive oxide (TCO), a wide-bandgap n-type hydrogenated nanocrystalline silicon oxide (nc-SiOx:H(n+)) front contact layer (also called the front field layer), a front intrinsic amorphous silicon passivation layer, an n-type monocrystalline silicon substrate, a back intrinsic amorphous silicon passivation layer, a p-type hydrogenated nanocrystalline silicon (nc-Si:H(p+)) rear emitter, indium-based transparent conductive oxide (TCO), and silver grid lines. The use of indium-based transparent conductive oxide (TCO) leads to higher battery costs; currently, mass-produced SHJ batteries cost approximately 0.1 yuan / W more than TOPCon batteries.

[0004] Reducing sunlight reflection at the solar cell's light-receiving surface and improving sunlight utilization are effective ways to increase the photoelectric conversion efficiency of solar cells and break through the theoretical limits of solar cells. Silicon nitride (SiNx) thin films are transparent dielectric films with high dielectric constant, good insulation, and excellent optical properties, and have broad application prospects in the field of solar cells. x The refractive index of the thin film can vary between 1.9 and 2.3 depending on the x value. Compared with other materials, it is closer to the optimal refractive index of 2.35 required for antireflection of solar cells, making it the best antireflection material known to date.

[0005] Applying high-performance silicon nitride thin films as antireflection layers in silicon heterojunction solar cells is beneficial for improving cell efficiency. However, the current mainstream method for depositing silicon nitride thin films, plasma-enhanced chemical vapor deposition (PECVD), uses SiH4 and NH3 as raw materials and has a deposition temperature above 300℃, which can damage the passivation layer (amorphous silicon or amorphous silicon oxide) of silicon heterojunction solar cells, leading to a decrease in cell performance.

[0006] For the reasons mentioned above, silicon nitride thin films are prepared by reactive plasma deposition using low-temperature deposition technology and used as antireflection and passivation layers in SHJ batteries, replacing part of the TCO. This is of great significance for improving battery efficiency and reducing battery cost. Summary of the Invention

[0007] The purpose of this invention is to address the problem that existing fabrication methods damage the intrinsic passivation layer of silicon heterojunction solar cells, leading to a decrease in open-circuit voltage and performance degradation. This invention proposes a method for preparing silicon nitride thin films based on reactive plasma deposition (RPD). This method efficiently utilizes non-toxic raw materials crystalline silicon and nitrogen gas to prepare silicon nitride thin films under low-temperature, low-substrate bombardment conditions, achieving efficient control of various parameters during material preparation and obtaining higher-quality silicon nitride thin films. This invention also proposes a silicon heterojunction solar cell using reactive plasma-deposited silicon oxynitride thin films as both a passivation layer and an antireflection layer, demonstrating potential for high efficiency and low cost.

[0008] The reactive plasma deposition method proposed in this invention (using the Jiezao Technology HCD450 hollow cathode reactive ion plating equipment) operates on the following principle: An arc discharge is first generated within the tantalum tube of the cathode plasma gun. After argon ionization, Ar+ ions bombard the tantalum tube, emitting a high-density plasma dominated by thermionic electrons. A permanent magnet and electromagnetic coil near the crucible bend and focus the plasma downwards, directing it onto the target material. The kinetic energy of the plasma is converted into thermal energy, heating and evaporating the target material. During its movement towards the substrate, the evaporated material interacts with the plasma above the crucible, resulting in partial ionization and significantly enhanced chemical reactivity. The reactive gas (N2) is typically introduced into the high-density plasma region above the crucible, allowing it to also be partially ionized. These ions diffuse towards the substrate and react there, forming a thin film material.

[0009] Technical solution of the present invention

[0010] A method for preparing silicon nitride thin films, using reactive plasma deposition, includes the following steps:

[0011] (1) Clean the substrate (including commonly used substrates such as silicon wafers, glass, and ceramics) to which silicon nitride films will be deposited, using the conventional RCA standard cleaning method for semiconductor processes.

[0012] (2) Place the cleaned substrate obtained in step (1) on the top substrate of the cavity of the reactive plasma deposition equipment, cover the sample with a baffle, and place high-purity silicon particles (diameter 1~5 mm, purity 99.9999%) as the target material in a graphite crucible. The crucible is placed at the bottom of the reactive plasma deposition cavity.

[0013] (3) Evacuate the cavity to make the vacuum level < 3 × 10 -3 Pa;

[0014] (4) Introduce argon gas with a flow rate of 180 sccm into the vacuum chamber, set the starting power supply voltage to 800V and the starting power supply current to 1.2A, and ignite to generate argon plasma;

[0015] (5) After successful ignition, adjust the flow rate of argon gas into the cavity to 25 sccm, introduce nitrogen gas with a flow rate of 20-30 sccm and 30% argon-hydrogen mixture gas with a flow rate of 1-5 sccm (hydrogen gas integral of 30%), set the working current to 50-60A, so that silicon reacts with nitrogen gas and silicon nitride film is deposited on the substrate.

[0016] (6) After the working current, voltage and air pressure and other parameters have stabilized, open the sample baffle, control the film thickness according to the film thickness gauge, and close the baffle when the film reaches the required thickness. The film deposition is then complete.

[0017] (7) Turn off the argon-hydrogen mixture with a nitrogen and hydrogen gas integral of 30%, turn off the working power supply, turn off the argon gas, and after the sample cools down, take out the sample to obtain the required silicon nitride film.

[0018] This invention also provides a silicon heterojunction solar cell using a silicon nitride thin film prepared by the above method as an antireflection layer and a passivation layer, wherein the silicon heterojunction cell (see appendix) Figure 1 and attached Figure 2 Using n-type crystalline silicon as a substrate, the structure adopts a front contact and rear emitter structure. The specific structure from top to bottom is as follows: silver grid line of the light-receiving electrode, silicon nitride thin film antireflection layer prepared by the method of the present invention, indium-based transparent conductive oxide film TCO layer complementary to silicon nitride thin film, N-type nanocrystalline silicon oxygen emitter, front intrinsic amorphous silicon passivation layer, N-type single crystal silicon substrate, back intrinsic amorphous silicon passivation layer, TCO layer and back light-receiving silver electrode.

[0019] The fabrication of a silicon heterojunction solar cell using silicon nitride thin film as a passivation layer and antireflection layer includes the following steps:

[0020] Step 1: Using a semi-finished solar cell, its structure from top to bottom is as follows: N-type nanocrystalline silicon-oxygen emitter, front intrinsic amorphous silicon passivation layer, N-type monocrystalline silicon substrate, and back intrinsic amorphous silicon passivation layer. Using mask A, a silicon nitride thin film is deposited on the front side as an antireflection layer and passivation layer using reactive plasma deposition technology;

[0021] Step 2: Using a matching mask B that is complementary to mask A, deposit 2% ICO on the front side of the semi-finished solar cell with silicon nitride film obtained in Step 1 using RPD technology. The silicon nitride film and the indium-based transparent conductive oxide film (TCO) are in a complementary state.

[0022] Step 3: On the back side of the semi-finished battery cell obtained in Step 2, deposit 2% ICO as a TCO layer using RPD technology.

[0023] Step 4: On the back of the semi-finished battery cell obtained in Step 3, cubic silver is deposited as the back electrode by thermal evaporation.

[0024] Step 5: On the front side of the semi-finished solar cell obtained in Step 4, silver grid lines are screen-printed onto the TCO on the front side, and then annealed to obtain a partially contacted silicon heterojunction solar cell.

[0025] Advantages and beneficial effects of the present invention:

[0026] (1) The equipment used in the method of the present invention for preparing silicon nitride thin film is a reactive plasma deposition equipment with low substrate deposition temperature (<200℃) and low ion bombardment of substrate, which will not damage the passivation layer in silicon heterojunction cell.

[0027] (2) The raw materials used in the method of the present invention for preparing silicon nitride thin films are high-purity silicon particles and nitrogen gas, which are simple and readily available, can significantly reduce the cost of thin film preparation, and are non-toxic and harmless;

[0028] (3) The silicon nitride thin film material prepared by the method of the present invention has the characteristics of smooth surface, high light transmittance, uniform deposition on textured substrate, and effective improvement of minority carrier lifetime of silicon wafer substrate.

[0029] (4) The silicon heterojunction solar cell prepared by the method of the present invention, which uses silicon nitride thin film as passivation layer and antireflection layer, has a relatively simple preparation process and can achieve a cell efficiency of >22%. The silicon nitride thin film replaces part of the TCO, which has the effect of reducing the cost of the cell. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the front structure of a silicon heterojunction solar cell with silicon nitride thin film as antireflection layer and passivation layer. 1 represents silicon nitride, 2 represents the front TCO, and 3 represents the front silver grid line.

[0031] Figure 2 This is a schematic diagram of the cross-sectional structure of a silicon heterojunction solar cell with silicon nitride thin film as antireflection layer and passivation layer. 4 represents the front silver grid line, 5 represents silicon nitride, 6 represents the front TCO, 7 represents the front intrinsic amorphous silicon, 8 represents n-type silicon, 9 represents the back intrinsic amorphous silicon, 10 represents the back emitter, 11 represents the back TCO, and 12 represents the back square silver.

[0032] Figure 3 The transmittance curve is that of the silicon nitride thin film prepared in Example 1;

[0033] Figure 4This is an SEM image of the silicon nitride film prepared on a polished silicon wafer in Example 2;

[0034] Figure 5 This is a SEM cross-sectional morphology image of the silicon nitride thin film prepared on a polished silicon wafer in Example 2;

[0035] Figure 6 This is the light IV curve of the silicon heterojunction solar cell in Example 3, which uses silicon nitride as the antireflection layer and passivation layer. Specific implementation methods

[0036] This invention proposes a method for preparing silicon nitride thin film materials by reactive plasma deposition, the specific steps of which are as follows:

[0037] (1) Clean the substrate (including commonly used substrates such as silicon wafers, glass, and ceramics) to which silicon nitride films will be deposited, using the conventional RCA standard cleaning method for semiconductor processes.

[0038] (2) Place the cleaned substrate obtained in step (1) on the top substrate in the cavity of the reactive plasma deposition equipment, cover the sample with a baffle, and place high-purity silicon particles (diameter 1~5 mm, purity 99.9999%) as the target material in the graphite crucible.

[0039] (3) Evacuate the cavity to make the vacuum level < 3 × 10 -3 Pa;

[0040] (4) Introduce argon gas with a flow rate of 180 sccm into the vacuum chamber, set the starting power supply voltage to 800V and the starting power supply current to 1.2A, and ignite to generate argon plasma;

[0041] (5) After the ignition is successful, adjust the flow rate of argon gas into the cavity to 25 sccm, introduce nitrogen gas with a flow rate of 20-30 sccm and 30% argon-hydrogen mixture with a flow rate of 1-5 sccm (hydrogen gas integral of 30%), set the working current to 50-60A, so that silicon reacts with nitrogen gas and silicon nitride film is deposited on the substrate.

[0042] (6) After the working current, voltage and air pressure and other parameters have stabilized, open the sample baffle, control the film thickness according to the film thickness gauge, and close the baffle when the film reaches the required thickness. The film deposition is then complete.

[0043] (7) Turn off the argon-hydrogen mixture with a nitrogen and hydrogen gas integral of 30%, turn off the working power supply, turn off the argon gas, and after the sample cools down, take out the sample to obtain the required silicon nitride film.

[0044] The specific structure of the silicon heterojunction solar cell proposed in this invention, using a silicon nitride thin film prepared by reactive plasma deposition as the antireflection layer and passivation layer, consists of the following layers from top to bottom: silver grid lines for the light-receiving electrode, silicon nitride thin film antireflection layer, indium-based transparent conductive oxide (TCO) layer complementary to the silicon nitride thin film, N-type nanocrystalline silicon-oxygen emitter, front intrinsic amorphous silicon passivation layer, N-type single-crystal silicon substrate, back intrinsic amorphous silicon passivation layer, TCO layer, and back silver electrode. A schematic diagram of the structure is attached. Figure 1 and 2 The specific preparation steps are as follows:

[0045] Step 1: Using a semi-finished solar cell, its structure from top to bottom is as follows: N-type nanocrystalline silicon-oxygen emitter, front intrinsic amorphous silicon passivation layer, N-type monocrystalline silicon substrate, and back intrinsic amorphous silicon passivation layer. Using mask A, a silicon nitride thin film is deposited on the front side as an antireflection layer and passivation layer using reactive plasma deposition technology;

[0046] Step 2: Using a matching mask B that is complementary to mask A, deposit 2% ICO on the front side of the semi-finished solar cell with silicon nitride film obtained in Step 1 using RPD technology. The silicon nitride film and the indium-based transparent conductive oxide film (TCO) are in a complementary state.

[0047] Step 3: On the back side of the semi-finished battery cell obtained in Step 2, deposit 2% ICO as a TCO layer using RPD technology.

[0048] Step 4: On the back of the semi-finished battery cell obtained in Step 3, cubic silver is deposited as the back electrode by thermal evaporation.

[0049] Step 5: On the front side of the semi-finished solar cell obtained in Step 4, silver grid lines are screen-printed onto the TCO on the front side, and then annealed to obtain a partially contacted silicon heterojunction solar cell.

[0050] The following specific embodiments are provided to illustrate the present invention: These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Any modifications or substitutions made to the methods, steps, or conditions of the present invention without departing from the spirit and essence of the invention are within the scope of protection of the present invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0051] Example 1

[0052] According to the method for preparing silicon nitride thin films of the present invention, a glass sheet is selected as a substrate, and a silicon nitride thin film is prepared on it.

[0053] (1) Clean the glass substrate on which silicon nitride film will be deposited. The specific process is to refer to the conventional glassware cleaning method, and clean the substrate in sequence with detergent, concentrated sulfuric acid solution, acetone and ethanol. During the cleaning process, the glass substrate is repeatedly rinsed with deionized water. The cleaned glass substrate is then blown dry with nitrogen gas.

[0054] (2) Place the cleaned substrate obtained in step (1) into the cavity of the reactive plasma deposition equipment, cover the sample with a baffle, and place high-purity silicon particles (diameter 1~5 mm, purity 99.9999%) as the target material in a graphite crucible;

[0055] (3) Evacuate the cavity to make the vacuum level <3×10-3 Pa;

[0056] (4) Introduce argon gas with a flow rate of 180 sccm into the vacuum chamber, set the starting power supply voltage to 800V and the starting power supply current to 1.2A, and ignite to generate argon plasma;

[0057] (5) The flow rate of argon gas introduced into the cavity is adjusted to 25 sccm, and nitrogen gas with a flow rate of 20 sccm and 30% argon-hydrogen mixture with a flow rate of 2 sccm are introduced. The working current is set to 54A so that silicon reacts with nitrogen gas to generate silicon nitride.

[0058] (6) After the working current, voltage and air pressure and other parameters have stabilized, open the sample baffle, control the film thickness according to the film thickness gauge, and close the baffle when the film reaches the required thickness. The film deposition is then complete.

[0059] (7) Turn off the nitrogen and 30% argon-hydrogen mixture, turn off the power supply, turn off the argon gas, and after the sample cools down, take out the sample to obtain the required silicon nitride film.

[0060] (8) The thickness of the silicon nitride thin film prepared in this example was measured to be approximately 100 nm, and its transmission spectrum is shown in the attached figure. Figure 3 As shown, the absorption wavelength range of crystalline silicon solar cells is 300-1200 nm. Figure 3 The transmission spectrum shows that the silicon nitride thin film prepared by this invention has a transmittance of over 90% in the 300-400nm wavelength range and over 95% in the 400-1200nm wavelength range. Its high transmittance across the entire wavelength range perfectly matches the requirements of the core absorption band of crystalline silicon solar cells.

[0061] Example 2

[0062] According to the method for preparing silicon nitride thin films of the present invention, a polished silicon wafer is selected as a substrate, and a silicon nitride thin film is prepared on it.

[0063] (1) Clean the polished silicon wafer substrate to which silicon nitride film will be deposited. The silicon wafer substrate can be cleaned using the conventional RCA standard cleaning method for semiconductor processes, followed by nitrogen purging and drying.

[0064] (2) Immerse the polished silicon wafer substrate after RCA standard cleaning in a 2% (volume ratio: hydrofluoric acid / deionized water = 2 / 100) hydrofluoric acid solution for 1 min to remove the oxide layer on the polished silicon wafer substrate.

[0065] (3) Place the cleaned substrate obtained in step (2) into the cavity of the reactive plasma deposition equipment, cover the sample with a baffle, and place high-purity silicon particles (diameter 1~5 mm, purity 99.9999%) as the target material in the graphite crucible;

[0066] (4) Evacuate the cavity to a vacuum level of <3×10-3 Pa;

[0067] (5) Introduce argon gas with a flow rate of 180 sccm into the vacuum chamber, set the starting power supply voltage to 800V and the starting power supply current to 1.2A, and ignite to generate argon plasma;

[0068] (6) The flow rate of argon gas introduced into the cavity is adjusted to 25 sccm, and nitrogen gas with a flow rate of 20 sccm and 30% argon-hydrogen mixture with a flow rate of 2 sccm are introduced. The working current is set to 54A so that silicon reacts with nitrogen gas to generate silicon nitride.

[0069] (7) After the working current, voltage and air pressure and other parameters have stabilized, open the sample baffle, control the film thickness according to the film thickness gauge, and close the baffle when the film reaches the required thickness. The film deposition is then complete.

[0070] (8) Turn off the nitrogen and 30% argon-hydrogen mixture, turn off the power supply, turn off the argon, and after the sample cools down, take out the sample to obtain the required silicon nitride film.

[0071] (9) The thickness of the silicon nitride film prepared in this example was measured to be approximately 188 nm, and its SEM surface morphology image is attached. Figure 4 As shown in the image, the film surface is very smooth and dense, with no obvious large pores or cracks, and only a few isolated microparticles (such as the bright spots in the upper left corner), which may be impurities or process residues from the deposition process. At 12kx magnification, the surface exhibits a uniform grayscale, indicating that the film thickness and composition are very uniform on a macroscopic scale, consistent with the characteristics of a high-quality silicon nitride film. Its SEM cross-sectional morphology is attached. Figure 5As shown, the silicon nitride film is approximately 188 nm thick and exhibits very uniform thickness. The interface with the substrate is clear, with no obvious interdiffusion layer. The film as a whole displays an amorphous, dense structure, without columnar crystals or obvious layered structures, indicating excellent barrier properties and mechanical stability. The film is tightly bonded to the silicon substrate, without voids or peeling.

[0072] Example 3

[0073] The silicon nitride thin film prepared by this invention can be used as an antireflection layer and passivation layer in novel silicon heterojunction solar cells. The steps are as follows:

[0074] Step 1: Using a semi-finished solar cell, its structure from top to bottom is as follows: N-type nanocrystalline silicon-oxygen emitter, front intrinsic amorphous silicon passivation layer, N-type monocrystalline silicon substrate, and back intrinsic amorphous silicon passivation layer. Using mask A, a silicon nitride thin film is deposited on the front side as an antireflection layer and passivation layer using reactive plasma deposition technology;

[0075] Step 2: Using a matching mask B that is complementary to mask A, deposit 2% ICO on the front side of the semi-finished solar cell with silicon nitride film obtained in Step 1 using RPD technology. The silicon nitride film and the indium-based transparent conductive oxide film (TCO) are in a complementary state.

[0076] Step 3: On the back side of the semi-finished battery cell obtained in Step 2, 2% ICO is deposited as a TCO layer using RPD technology. The battery structure designed in this invention deposits only a portion of TCO below the silver grid line using a masking method, accounting for approximately 20% of the battery area. Compared to conventional batteries that deposit a full layer of TCO, this reduces the amount of TCO used by 80%, thereby achieving the effect of reducing costs.

[0077] Step 4: On the back of the semi-finished battery cell obtained in Step 3, cubic silver is deposited as the back electrode by thermal evaporation.

[0078] Step 5: On the front side of the semi-finished solar cell obtained in Step 4, silver grid lines are screen-printed onto the TCO on the front side, and then annealed to obtain a partially contacted silicon heterojunction solar cell.

[0079] The photoluminescence IV curve of the novel heterocrystalline silicon solar cell obtained in this embodiment is attached. Figure 6 Among them, the open-circuit voltage (V) oc The voltage is 723.8 mV, and the short-circuit current (J) is... sc The value was 40.54 mA / cm. 2 The fill factor (FF) is 76.68%, and the conversion efficiency (E) is... ff The figure was 22.49%.

Claims

1. A method for preparing a silicon nitride thin film, the method comprising the following steps: Step 1: Place the cleaned substrate on the top substrate of the reactive plasma deposition chamber, cover the sample with a baffle, place high-purity silicon particles as the target material in the graphite crucible, and place the crucible at the bottom of the reactive plasma deposition chamber. Step 2: Evacuate the cavity to a vacuum level < 3 × 10⁻⁶. -3 Pa; Step 3: Introduce argon gas at a flow rate of 180 sccm into the vacuum chamber, set the power supply voltage to 800V and the power supply current to 1.2A, and ignite to generate argon plasma. Step 4: After successful ignition, adjust the argon gas flow rate into the cavity to 25 sccm, introduce nitrogen gas with a flow rate of 20-30 sccm and a 30% argon-hydrogen mixture with a flow rate of 1-5 sccm, and set the working current to 50-60A to allow silicon to react with nitrogen gas and deposit a silicon nitride thin film on the substrate. Step 5: After the working current, voltage and gas pressure parameters stabilize, open the sample baffle, control the film thickness according to the film thickness gauge, and close the baffle when the film reaches the required thickness. The film deposition is then complete, and the desired silicon nitride film is obtained.

2. The preparation method according to claim 1, characterized in that, The substrate is a commonly used substrate, including silicon wafers, glass, and ceramics.

3. The preparation method according to claim 1, characterized in that, The high-purity silicon particles have a diameter of 1-5 mm and a purity of 99.9999%.

4. A silicon nitride thin film prepared by the method according to any one of claims 1 to 3.

5. A silicon heterojunction solar cell using the silicon nitride thin film as described in claim 4 as an antireflection layer and a passivation layer.

6. The silicon heterojunction solar cell according to claim 5, characterized in that, The solar cell structure, from top to bottom, consists of: a silver grid line for the light-receiving electrode, a silicon nitride thin film antireflection layer as described in claim 4, an indium-based transparent conductive oxide (TCO) layer complementary to the silicon nitride thin film, an N-type nanocrystalline silicon oxide emitter, a front intrinsic amorphous silicon passivation layer, an N-type monocrystalline silicon substrate, a back intrinsic amorphous silicon passivation layer, a TCO layer, and a back light-receiving silver electrode.

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