JP4657630B2 - SOLAR CELL, MANUFACTURING METHOD THEREOF, AND ANTI-REFLECTION FILM DEPOSITION DEVICE - Google Patents

Description

  The present invention relates to a solar cell, a manufacturing method, and an antireflection film forming apparatus.

  Solar cells are electronic devices that convert solar energy into electrical energy, and have been put to practical use as power sources for artificial satellites, wristwatches, and calculators. A solar cell is basically composed of a stacked semiconductor composed of n-type silicon and p-type silicon, and electricity is generated by the photoelectric effect when light is applied to the semiconductor. In solar cells, in order to efficiently absorb sunlight, the light receiving surface of the solar cell is usually covered with an antireflection film. Conventionally, as this kind of antireflection film, a technique of forming a silicon nitride film containing hydrogen on a light receiving surface of a solar cell by plasma CVD is known (Patent Document 1). In addition, a passivation effect is known in which hydrogen in a silicon nitride film, which is an antireflection film of a solar cell, is diffused into the silicon substrate of the solar cell and the efficiency of the solar cell is increased (Patent Document 2).

JP 2000-299482 A JP 2003-273382 A

In the silicon nitride films of Patent Document 1 and Patent Document 2, the concentration of hydrogen in silicon nitride is adjusted according to the film formation conditions during film formation by plasma CVD. For this reason, if an attempt is made to increase the content of hydrogen in silicon nitride in order to enhance the H ₂ passivation effect, the amount of reaction gas used during film formation increases and control of film formation conditions becomes difficult. There was a problem of causing a decline in sex. Further, in Patent Document 2, the film must be formed at a temperature of 350 ° C. or higher in order to enhance the H ₂ passivation effect. For this reason, there is a problem that the amount of products adhering to the inside of the plasma CVD apparatus increases and the frequency of cleaning in the plasma CVD apparatus increases.

According to a first aspect of the present invention, there is provided a method for manufacturing a solar cell comprising: forming a silicon nitride antireflection film on a light-receiving surface side of a crystalline silicon substrate; And a heat treatment step in which the silicon nitride film formed in the film formation step is heat-treated in an atmosphere containing hydrogen.
The anti-reflection film forming apparatus for the photovoltaic devices according to the invention of claim 4, in the antireflection film forming apparatus for the photovoltaic devices obtained by forming an antireflection film of the crystalline silicon substrate a silicon nitride film on the light-receiving surface side of the A film forming unit that forms a silicon nitride film by a high-density plasma CVD method and a heat treatment unit that heat-treats the silicon nitride film formed by the film forming unit in an atmosphere containing hydrogen.

According to the present invention, it is possible to form a silicon nitride film having a high H ₂ passivation effect as an antireflection film of a solar cell without reducing productivity and without increasing the temperature of the plasma CVD apparatus.

Hereinafter, a solar cell according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a cross-sectional view of a solar cell according to an embodiment of the present invention. The solar cell 1 includes a p-type silicon substrate layer 11, an n-type diffusion layer 12, a p ⁺ backside impurity diffusion layer 13, an antireflection film 14, a surface electrode 15, a backside electrode 16, and solder layers 17 and 18.

The p-type silicon substrate layer 11 is formed of a silicon substrate manufactured by adding a small amount of a trivalent element such as boron. The p-type silicon substrate layer may be monocrystalline or polycrystalline. The n-type diffusion layer 12 is formed by diffusing phosphorus as an n-type dopant on the surface of the p-type silicon substrate, and a pn bond is formed by the p-type silicon substrate layer 11 and the n-type diffusion layer 12. The p ⁺ back side impurity diffusion layer 13 is formed by diffusing aluminum as a p-type dopant on the surface of the p-type silicon substrate. Silicon nitride is used for the antireflection film 14, and Ag electrodes are used for the front electrode 15 and the back electrode 16.

Hereinafter, the manufacturing method of the solar cell of this Embodiment is demonstrated, referring FIG.
In step S201, surface treatment is performed to form a fine relief structure on the surface of the p-type silicon substrate. Surface treatment is performed by an etching method using an alkaline aqueous solution or a reactive ion etching method. By forming a fine concavo-convex structure on the surface of the silicon substrate, reflection of light on the surface of the silicon substrate can be suppressed. In step S202, the n-type dopant is diffused from the surface of the p-type silicon substrate to form the n-type diffusion layer 12. Phosphorus (P) is used as the n-type dopant. Phosphorus is diffused on the surface of the p-type silicon substrate by a vapor phase diffusion method using POCl ₃ , a coating diffusion method using P ₂ O ₅ , an ion implantation method for directly diffusing P ions, or the like. In step S203, an etching process is performed to remove the n-type diffusion layer on one surface of the p-type silicon substrate into which the n-type dopant has been diffused. In step S204, a p-type dopant is diffused from the surface of the etched p-type silicon substrate to form a p + back-side impurity diffusion layer 13. Aluminum (Al) is used as a p-type dopant. Al paste is applied to the surface of the etched p-type silicon substrate and heat-treated to diffuse aluminum into the surface of the p-type silicon substrate.

  In step S <b> 205, the antireflection film 14 is formed on the surface of the n-type diffusion layer 12. A silicon nitride film is used as the antireflection film and is formed using a high density plasma CVD apparatus. In step S206, the front electrode 15 and the back electrode 16 are patterned. The patterning is performed by screen printing an Ag paste made of Ag powder, a binder, and a frit. In order to increase the efficiency of the solar cell, the electrodes are formed in a comb pattern. In step S207, the printed Ag paste is baked to form an electrode. In step S208, solder layers 17 and 18 are formed by a solder dipping method.

  The solar cell that has been completed up to the manufacturing process of step S204 is conveyed to the antireflection film forming apparatus 301 shown in FIG. 3, and the antireflection film 14 is formed. For the sake of convenience, the solar cell that has been completed up to the manufacturing process of step S204 is referred to as an antireflection film pre-formed substrate 311. The antireflection film forming apparatus 301 includes a chamber 302, a surface wave excitation plasma CVD apparatus 303, a heating chamber 304, and a transfer chamber 305. The chamber 302 is an apparatus that replaces the atmosphere from the outside air with the atmosphere inside the antireflection film forming apparatus 301 when the substrate 311 before forming the antireflection film is transferred to the antireflection film forming apparatus 301. The surface wave excitation plasma CVD apparatus 303 is an apparatus that forms a silicon nitride film on the surface of the substrate 311 before forming the antireflection film. The heating chamber 304 is a device that performs heat treatment on the substrate 311 before forming the antireflection film on which the silicon nitride film is formed in the surface wave excitation plasma apparatus 303. In the transfer chamber 305, the substrate 311 before forming an antireflection film is transferred from the chamber 302 to the surface wave plasma CVD apparatus 303, and the substrate 311 before forming an antireflection film in which a silicon nitride film is formed by the surface wave excitation plasma CVD apparatus 303. A transport device for transporting the substrate 311 before forming an antireflection film, which has been transported to the heating chamber 304 and completed the heat treatment in the heating chamber 304, to the chamber 302 is stored. Arrows A to C in FIG. 3 indicate the movement of the loaded substrate 311 before forming the antireflection film in order.

  Details of the surface wave excitation plasma CVD apparatus 303 shown in FIG. 3 will be described with reference to FIG. FIG. 4 is a cross-sectional view showing a schematic configuration of the surface wave excitation plasma CVD apparatus. A surface wave excitation plasma CVD apparatus (hereinafter referred to as SWP-CVD apparatus) can easily generate a high-density plasma with a large area using surface waves, and this plasma is a surface wave excitation plasma (SWP: It is called “Surface Wave Plasma”.

  In FIG. 4, the SWP-CVD apparatus 303 includes a chamber 401, a microwave waveguide 402, a termination matching unit 403, a slot antenna 404, a dielectric plate 405, a side reflector 406, a process gas introduction tube 407, and a material gas introduction tube 408. The vacuum exhaust pipe 409 and the substrate holder 410 are provided. The chamber 401 is a sealed container for forming a film on the surface of the substrate held by the substrate holder 410 using the surface wave excitation plasma P generated in the internal space. The substrate holder 410 can move and rotate in the vertical direction, and is configured to be able to heat, cool, apply an electric field, etc., as necessary, to the substrate that is the film formation target.

  A dielectric plate 405 made of quartz, alumina, zirconia or the like is provided on the upper portion of the chamber 401. A microwave waveguide 402 is placed in contact with the upper surface of the dielectric plate 305. A plurality of slot antennas 404 having long rectangular openings are provided on the bottom plate of the microwave waveguide 402 in contact with the dielectric plate 405.

Among the film forming gases, the process gas introduced from the process gas introduction pipe 407 into the chamber 401 is a reactive active species such as N ₂ gas, O ₂ gas, H ₂ gas, NO ₂ gas, NO gas, NH ₃ gas, etc. And a rare gas such as Ar gas, He gas, Ne gas, Kr gas, and Xe gas. Of the deposition gas, the material gas introduced into the chamber 401 from the material gas introduction pipe 408 is a gas containing Si element, which is a component of a silicon thin film or a silicon compound thin film, such as SiH ₄ gas or Si ₂ H ₆ gas.

  A vacuum exhaust pipe 409 connected to a vacuum exhaust pump (not shown) is disposed on the bottom plate of the chamber 401. By exhausting while introducing a predetermined gas into the chamber 401 at a predetermined flow rate through the process gas introduction pipe 407 and the material gas introduction pipe 408, the inside of the chamber 401 can be maintained at a predetermined pressure.

  In the SWP-CVD apparatus 303 configured as described above, a microwave having a frequency of 2.45 GHz is propagated from the microwave generation source into the microwave waveguide 402, and the TE10 mode electromagnetic wave standing wave is transmitted by the termination matching unit 403. T is generated. The standing wave T is radiated to the dielectric plate 405 from the slot antenna 404 disposed at the wavelength interval of the standing wave T. The electromagnetic wave radiated from the slot antenna 404 propagates inside the dielectric plate 405 and generates a unique standing wave in a range surrounded by the side reflector 406. Then, a surface wave SW is generated on the surface of the dielectric plate 405. The surface wave SW ionizes and dissociates the deposition gas immediately below the dielectric plate 405 to generate the surface wave excited plasma P.

  The surface-wave-excited plasma P has an electron density equal to or higher than the cut-off density of the microwave, totally reflects the electromagnetic wave at the plasma interface, and does not absorb the electromagnetic wave into the plasma. The ion energy maintains a low temperature of 10 eV or less. In the surface wave excitation plasma P, energy is transferred between the surface of the dielectric plate 405 and the plasma boundary surface, and high-energy plasma is distributed only in the vicinity of the surface of the dielectric plate 405, and the index increases as the distance from the dielectric surface increases. The energy level decreases functionally. At a distance of about 200 mm from the dielectric plate 405, the ion energy is 20 ev or less. As described above, the surface wave excitation plasma P has a high energy region and a low energy region. Therefore, radical generation is performed in the high energy region, and material gas is introduced into the low energy region. Damage high-speed film formation is possible.

  Since the surface wave SW spreads over the entire inner surface of the dielectric plate 405, the surface wave-excited plasma P also spreads in the corresponding region in the chamber 401. Therefore, it is possible to cope with a large area by expanding the dielectric plate 405. Using this surface wave excitation plasma P, the antireflection film 14 is formed on the substrate 311 before forming the antireflection film.

Details of the heating chamber 304 shown in FIG. 3 will be described with reference to FIG. FIG. 5 is a cross-sectional view illustrating a schematic configuration of the heating chamber 304. The heating chamber 304 includes a lamp heater 501, a substrate holder 502, a process gas introduction pipe 503, and a vacuum exhaust pipe 504. The lamp heater 501 heats the substrate 311 before forming an antireflection film mounted on the sample holder 502 with infrared rays emitted from the lamp. The process gas introduced from the process gas introduction pipe 503 into the heating chamber 304 is H ₂ gas. A vacuum exhaust pipe 504 connected to a vacuum exhaust pump (not shown) is disposed on the bottom plate of the heating chamber 304. By exhausting while introducing a predetermined gas into the heating chamber 304 through the process gas introduction pipe 407 at a predetermined flow rate, the inside of the heating chamber 304 can be maintained at a predetermined pressure.

Hereinafter, the formation process of the antireflection film in step S205 will be described in detail.
(1) Formation of silicon nitride layer The substrate 311 before the antireflection film is formed is set on the substrate holder 410 of the SWP-CVD apparatus 303 by the transfer device stored in the transfer chamber 305, and the inside of the chamber 401 is about 0.01 Pa. Exhaust to high vacuum. NH ₃ gas and Ar gas are respectively introduced into the chamber 401 through the process gas introduction pipe 407, SiH ₄ gas is introduced into the chamber 401 through the material gas introduction pipe 408, and the pressure in the chamber 401 is maintained at 4.0 Pa. Film formation is performed for 20 seconds by surface wave plasma P generated with a microwave power of 2.0 kW to form a silicon nitride layer 14 having a thickness of 800 mm. The gas flow rates in this case were SiH ₄ gas, 50 sccm, NH ₃ gas, 50 sccm, and Ar gas, 100 sccm. The film formation temperature was 70 to 100 ° C.

(2) Heat treatment of silicon nitride layer The substrate 311 before formation of the antireflection film on which the silicon nitride layer is formed by the SWP-CVD apparatus 303 is transferred from the SWP-CVD apparatus 303 to the heating chamber 304 by the transfer apparatus stored in the transfer chamber 305. To do. Then, the substrate 311 before forming the antireflection film is set on the substrate holder 502. H ₂ gas is introduced into the heating chamber 304 through the process gas introduction pipe 503. And it heats at 300 to 350 degreeC with the lamp heater 501 for 30 minutes. As described above, the formation of the antireflection film 14 on the substrate 311 before formation of the antireflection film is completed.

The silicon nitride film manufactured as described above has a high H ₂ passivation effect. The reason will be described below.
Since the film forming speed using a high-density plasma CVD apparatus such as a SWP-CVD apparatus is very high, the antireflection film 14 of the solar cell having a film thickness of about several hundreds of liters is formed in a short time. For this reason, the crystallinity of the silicon nitride film generated by the high-density plasma CVD apparatus becomes very poor, and the diffusion of hydrogen easily proceeds during the heat treatment. Accordingly, since the diffusion of hydrogen in the silicon nitride film is easy, the hydrogen in the atmosphere is easily diffused into the n-type diffusion layer 12 and the p-type silicon substrate layer 11, and the hydrogen content of the silicon nitride film is increased. Can do. For this reason, the H ₂ passivation effect of the antireflection film 14 produced as described above is high. The silicon nitride film generated by the high-density plasma CVD apparatus is heat-treated in the heating chamber 304, and finally becomes a dense film with improved crystallinity. In addition, since the hydrogen content in the film is increased after the silicon nitride film is formed, the amount of reaction gas used during film formation increases, and it becomes difficult to control film formation conditions. The problem of deteriorating does not occur.

  In this embodiment, since the reaction gas is completely dissociated by the high-density plasma CVD apparatus, the amount of products attached to the inside of the high-density plasma CVD apparatus is small. For this reason, the frequency of cleaning of the high-density plasma CVD apparatus is reduced. In addition, since the temperature inside the high-density plasma CVD apparatus is low, the time required for the temperature to drop to a temperature at which the cleaning operation can be performed is short. For this reason, the time for stopping the high-density plasma CVD apparatus for cleaning can be shortened.

In the present embodiment, the SWP-CVD apparatus has been described as the film formation apparatus for the antireflection film 14, but a CVD apparatus using electron cyclotron resonance (ECR) plasma or inductively coupled plasma (ICP) can also be used. The SWP-CVD apparatus, the ECR-CVD apparatus, and the ICP-CVD apparatus can obtain high-density plasma with an electron density of about 10 ¹² cm ^{−3 and} have a very high film formation rate. Accordingly, a silicon nitride film that is incompletely generated can be manufactured, and therefore, the H ₂ hashing effect can be enhanced as compared with other plasma CVD apparatuses.

Hereinafter, in the present embodiment, SiH ₄ , NH ₃ , and Ar gas are used when forming the silicon nitride film with the SWP-CVD apparatus 303, but SiH ₄ , NH ₃ , and N ₂ gas may be used. . Further, when the heat treatment is performed in the heating chamber 304, the lamp heater 501 is used to heat the substrate 311 before forming the antireflection film, but a hot plate or a sheath heater may be used instead of the lamp heater 501.

It is sectional drawing of the solar cell of this invention. It is a flowchart which shows the manufacturing process of the solar cell of this invention. It is a figure explaining the anti-reflective film forming apparatus which is one Embodiment of this invention. It is a figure explaining the SWP-CVD apparatus which is one Embodiment of this invention. It is a figure explaining the heating chamber which is one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Solar cell 11 p-type silicon substrate layer 12 n-type diffusion layer 13 p ⁺ back surface side impurity diffusion layer 14 Antireflection film 15 Front surface electrode 16 Back surface electrodes 17 and 18 Solder layer 301 Antireflection film formation apparatus 301
302 Chamber 303 Surface wave excitation plasma CVD apparatus 304 Heating chamber 305 Transfer chamber

Claims (4)

In a method for manufacturing a solar cell, in which an antireflection film of a silicon nitride film is formed on a light receiving surface side of a crystalline silicon substrate,
A film forming step of forming the silicon nitride film by a high density plasma CVD method;
And a heat treatment step of heat-treating the silicon nitride film formed in the film formation step in an atmosphere containing hydrogen. In the manufacturing method of the solar cell of Claim 1,
The method of manufacturing a solar cell, wherein the film forming step forms a film with a surface wave excitation plasma apparatus.   The solar cell manufactured by the method of Claim 1 or 2. In an antireflection film forming apparatus for a solar cell formed by forming an antireflection film of a silicon nitride film on the light receiving surface side of a crystalline silicon substrate,
Film forming means for forming the silicon nitride film by a high density plasma CVD method,
An antireflection film forming apparatus for a solar cell, comprising: a heat treatment means for heat-treating the silicon nitride film formed by the film formation means in an atmosphere containing hydrogen.

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