JP4391486B2 - Photovoltaic device manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a photovoltaic device such as a solar cell, and more particularly to a forming method for forming irregularities for improving light utilization efficiency in a solar cell using a crystalline silicon substrate.

  In a crystalline photovoltaic device, a texture structure is formed by forming irregularities on the surface of a substrate, and incident light is effectively used due to a light confinement effect to improve conversion efficiency for converting light energy into electric energy. As a method for forming irregularities on the surface of a crystalline silicon substrate, a wet etching method using an alkaline solution has been widely used so far. A single crystal silicon substrate oriented in the (100) plane has a pyramid. A good texture structure of the mold can be provided. However, in a polycrystalline silicon substrate, the concavo-convex shape formed differs depending on the crystal orientation, and in particular, a texture structure having a reflectance reduction effect is provided on the substrate surface oriented in the (111) plane horizontally to the substrate surface. Can hardly be established.

  Therefore, as a method of forming irregularities on a polycrystalline silicon substrate regardless of crystal orientation, there is a method of forming a V-shaped groove by machining or laser processing, but mechanical groove formation leaves damage during processing on the substrate. Therefore, it is not generally used. Furthermore, in the crystalline silicon-based solar cell, the substrate tends to be thin for effective use of the substrate raw material and cost reduction, and mechanical processing and laser processing of the thinned substrate become more difficult.

Therefore, there is a dry etching process as a method for forming irregularities on a thin silicon substrate having a thickness of 200 μm or less. As a dry etching process, a reactive ion etching method (reactive ion etching) for forming fine irregularities has been proposed. According to this method, the product formed when the substrate is etched with a fluorine-based etching gas adheres to the surface of the substrate in a striped manner, and this serves as an etching mask to provide an isotropic texture structure ( For example, see Patent Document 1).
However, because reactive ion etching uses plasma, etching damage is likely to occur on the substrate, the product that serves as an etching mask adversely affects the characteristics of the solar cell, and adverse effects on the environment such as fluorine gas. There are problems such as using a large amount of gas which affects
As a dry etching process that does not use plasma, there is a hot wire method (Hot Wire). In the hot wire method, hydrogen gas is brought into contact with a refractory metal filament to generate atomic hydrogen, the semiconductor substrate is etched with the atomic hydrogen, and irregularities are formed (see, for example, Patent Document 2).

In this hot wire method, it is possible to form a texture having an effect of reducing the reflectance even if the substrate surface is etched using the difference in etching rate with respect to the crystal orientation of the substrate without using an etching mask. However, the etching without using an etching mask does not sufficiently reduce the reflectance, and it is necessary to further reduce the reflectance in order to improve the characteristics of the photovoltaic device.
In order to further reduce the reflectivity, it is effective to form an etching mask on the substrate surface and perform deep etching to form deep irregularities.

As a method for forming an etching mask, photoengraving for patterning a resist in a semiconductor formation process is generally used, but it is not preferable for a solar cell in which cost reduction is desired because the process becomes complicated.
Therefore, colloidal silicon oxide particles dispersed in a resist-like material are applied to the substrate, etched back to leave the silicon oxide particles, which are used as an etching mask, and the etching rate is anisotropic in the direction perpendicular to the substrate. There has been proposed a method for forming deep sword-like irregularities by a simple process by performing etching (see, for example, Patent Document 3).

JP-A-9-102625 JP 2003-332605 A Japanese Patent Laid-Open No. 10-50954

  However, when silicon oxide particles adhering to the substrate surface are used as etching masks to form irregularities on the substrate surface with atomic hydrogen generated by the hot wire method, the atomic hydrogen generated by the hot wire method is separated from the substrate surface. Since there is no etching direction in the vertical direction, isotropic etching is performed. In the state where each silicon oxide particle is uniformly dispersed, the substrate is etched in the vertical direction and etched in the horizontal direction. There is a problem that deep concaves and convexes cannot be formed because the concaves formed by the above are connected.

  The object of the present invention is to use an atomic hydrogen generated by the hot wire method to form an etching mask effective for non-directional etching such as the hot wire method by a simple process other than photolithography and the like. It is another object of the present invention to provide a highly productive process for etching a substrate to form deep irregularities having a reflectance reduction effect.

  In the method of manufacturing a photovoltaic device according to the present invention, a hydrogen gas is brought into contact with a heated refractory metal filament with a texture that reduces the reflectance of the surface on which light of a semiconductor substrate constituting the photovoltaic device is incident. In the method of manufacturing a photovoltaic device formed by etching with atomic hydrogen generated in this way, prior to the etching, silicon oxide particles having a primary particle diameter in the range of 80 nm or more and 400 nm or less are aggregated in a mesh shape on the surface Adhering to form an etching mask.

  The effect of the method for producing a photovoltaic device according to the present invention is that, in the portion where the silicon oxide particles are aggregated and adhered, the silicon oxide particles obstruct the path of atomic hydrogen in the direction perpendicular to the substrate surface. Since a ridgeline is formed and a large difference in height is obtained between this ridgeline and the etched part where silicon oxide particles are not attached, the reflectivity is reduced by deep irregularities and the conversion efficiency of the solar cell is improved. Can do.

Embodiment 1 FIG.
FIG. 1 is a sectional view of a hot wire etching apparatus used for hot wire etching according to the present invention. FIG. 2 is a diagram illustrating a state in which the cooling block is in contact with the substrate holder by the hot wire etching apparatus.
As a photovoltaic device according to the present invention, a manufacturing method will be described taking a solar cell using a silicon single crystal substrate or a polycrystalline substrate as an example.
In addition, the hot wire etching according to the present invention uses a hot wire CVD method, also known as a catalytic chemical vapor deposition method, to etch a semiconductor substrate with atomic hydrogen instead of depositing a semiconductor film. That is.

The hot wire etching apparatus 1 is housed in a chamber 2, a hydrogen inlet 3 for introducing hydrogen gas into the chamber 2 is disposed at the top of the chamber 2, and the gas in the chamber 2 is disposed on the side of the chamber 2. An exhaust port 4 for exhausting the components to the outside is provided and drawn by a vacuum pump (not shown).
The hot wire etching apparatus 1 includes a substrate holder 6 that holds a semiconductor substrate 5 in a chamber 2, a filament 7 that is disposed above the substrate holder 6 and that is heated by a current supplied from a current source device (not shown), filament The heating lamp 8 as a lamp heating device that heats the semiconductor substrate 5 held by the substrate holder 6 is disposed below the substrate holder 6, and is moved up and down by a vertical drive device (not shown) so that the coolant flows. A cooling block 10 is disposed as a cooling device in which the refrigerant pipe 9 is built.

The semiconductor substrate 5 is fixed on the substrate holder 6 by a chuck (not shown).
The cooling block 10 is elastically supported by the bellows 11 while maintaining a vacuum with respect to the chamber 2, and the cooling block 10 is brought into contact with the substrate holder 6 as shown in FIG. Moreover, as shown in FIG. 1, it is made so that it can be separated from the substrate holder 6 during heating.
The cooling block 10 incorporates a refrigerant pipe 9 therein, and when a refrigerant from a cooling chiller (not shown) flows through the refrigerant pipe 9, the temperature of the cooling block 10 decreases. When the cooling block 10 is moved to the cooling position and brought into contact with the substrate holder 6, the heat transmitted from the substrate holder 6 is carried away to the cooling chiller by the refrigerant.
The temperature of the semiconductor substrate can be changed by adjusting the irradiation condition of the heating lamp 8.

  The filament temperature of the filament 7 is adjusted by the current flowing from the current source device. Then, the hydrogen molecules that collide with the heated filament 7 are dissociated into atomic hydrogen (hydrogen radicals) and collide with the surface of the semiconductor substrate 5. Tungsten is used as the refractory metal used for the filament, but any one or a plurality of alloys of tantalum, molybdenum, rhenium, nickel, chromium and platinum may be used.

Next, materials used for forming an etching mask on the surface of the semiconductor substrate 5 will be described.
The etching mask according to the present invention is composed of silicon oxide particles attached to the surface of the semiconductor substrate 5. And this etching mask applies colloidal silica by apply | coating the dispersion liquid by which colloidal silica generally synthesize | combined by the sol-gel method was disperse | distributed on the semiconductor substrate, and removing isopropyl alcohol which is a solvent. Attached to the surface of the semiconductor substrate. This colloidal silica is silicon oxide particles.

  The dispersion used in the present invention is colloidal silica having a primary particle diameter of 15 nm (for example, grade PL-1 manufactured by Fuso Chemical Co., Ltd., and referred to as “small-diameter colloidal silica” in the following description) and a primary particle diameter of 300 nm. IPA sol in which colloidal silica (for example, grade PL-30 manufactured by Fuso Chemical Industry Co., Ltd., hereinafter referred to as “large diameter colloidal silica”) is dispersed at a weight ratio of 20% is used as a starting material. An IPA sol in which large-diameter colloidal silica is dispersed (hereinafter referred to as “large-diameter IPA sol”), an IPA sol in which small-diameter colloidal silica is dispersed (hereinafter referred to as “small-diameter IPA sol”), and ethanol. A dispersion is prepared by mixing at a volume ratio of 8: 1: 56.

  Although colloidal silica having a primary particle diameter of 15 nm is used as the small diameter colloidal silica, the primary particle diameter of the small diameter colloidal silica is preferably 10 nm or more and 80 nm or less. If it is less than 10 nm, it is difficult to stably produce colloidal silica. On the other hand, when the thickness exceeds 80 nm, large-diameter colloidal silica cannot be connected in a network.

  Further, colloidal silica having a primary particle diameter of 300 nm is used as the large diameter colloidal silica, but the primary particle diameter of the large diameter colloidal silica is preferably 80 nm or more and 400 nm or less. If it is less than 80 nm, it will be evenly dispersed as in the case of small-diameter colloidal silica. Moreover, when it exceeds 400 nm, it will disperse | distribute uniformly. Moreover, it is difficult to produce colloidal silica having a thickness of 400 nm or more.

Further, although an example in which an IPA sol in which colloidal silica is dispersed in isopropyl alcohol will be described, a sol in which an organic solvent such as other alcohols, ketones, or ethers is dispersed may be used.
Also, a sol in which colloidal silica is dispersed in water may be used, but immediately after removing the natural oxide film on the surface of the semiconductor substrate, or when the natural oxide film is very thin, the surface becomes hydrophobic, Colloidal silica dispersed in water aggregates, making it difficult to spread and adhere on the substrate. In that case, it can be attached by forming a thin silicon oxide film on the surface by a method such as heating in an atmosphere containing oxygen or moisture.

  By using colloidal silica dispersed in an organic solvent or water in this way, colloidal silica is not exposed to the atmosphere without preparing a special chamber, and stable without agglomerating colloidal silica. Can be saved.

Next, a method for applying the dispersion and forming an etching mask on the surface of the semiconductor substrate will be described.
The dispersion is dropped onto the semiconductor substrate to wet the substrate uniformly. The dispersion liquid is dropped, and the solvent is volatilized while blowing warm air to adhere the colloidal silica to the surface of the substrate.

As a means for attaching colloidal silica to the substrate, the means for dripping the dispersion and wetting the substrate has been described. However, the dispersion may be sprayed to form a coating film, or the dispersion may be formed using water vapor as a carrier. The mist may be sprayed on the substrate surface by two-phase spraying.
Further, the means for evaporating the solvent of the dispersion coating film by blowing with warm air to adhere the colloidal silica to the substrate surface has been described. However, the dispersion coating film may be formed on a heated semiconductor substrate.
Further, even if powdered silicon oxide particles are used and adhered to the substrate by a powder spray method, the silicon oxide particles can be similarly adhered to the substrate surface.

Thus, according to the spray method or the spray method, the colloidal silica dispersed in the solution can be efficiently attached to the substrate.
Further, by using the powder spray method, the powdered silicon oxide particles can be attached to the substrate surface by a simple process without requiring a process using a solution or the like.

Next, a procedure for forming irregularities on the surface of the semiconductor substrate 5 using the hot wire etching apparatus 1 described above will be described with reference to FIG. FIG. 3 is a flowchart showing a procedure for forming irregularities on the surface of the semiconductor substrate.
In step S1, the natural oxide film is removed from the semiconductor substrate 5 with buffered hydrofluoric acid immediately before forming the etching mask.
In step S2, after removing the natural oxide film, the dispersion is dropped onto the semiconductor substrate to wet the substrate uniformly.
In step S3, the solvent is volatilized while blowing hot air, and colloidal silica is attached to the surface of the substrate to form an etching mask.

  In step S4, the semiconductor substrate 5 is set on the substrate holder 6, and the gap between the filament 7 and the semiconductor substrate 5 is set to 4 cm and inserted into the chamber 2. Next, 50 sccm (1,013 hPa, 0 ° C.) of hydrogen gas is supplied from the hydrogen inlet 3 to set the degree of vacuum in the chamber 2 to 10 Pa, the filament temperature to 2150 ° C., and the semiconductor substrate 5 is etched for 15 minutes. Thereafter, the colloidal silica adhering to the substrate is removed with hydrofluoric acid to finish the formation of the texture.

  Next, the state of the etching mask formed on the surface of the semiconductor substrate will be described. FIG. 4A is a diagram showing a distribution state of colloidal silica when a dispersion liquid in which 300 nm IPA sol, 15 nm IPA sol and ethanol are mixed at a volume ratio of 8: 1: 56 is used. FIG. 4A shows the distribution of colloidal silica observed with an electron microscope from a direction in which the substrate is tilted 40 degrees from the vertical. For comparison, FIG. 4B is a diagram showing a distribution state of colloidal silica when a dispersion liquid in which 300 nm IPA sol, 15 nm IPA sol, and ethanol are mixed at a volume ratio of 1: 1: 14 is used. In FIGS. 4A and 4B, a circle indicates colloidal silica having a primary particle diameter of 300 nm. Although colloidal silica with a primary particle diameter of 15 nm is uniformly distributed, it is omitted because it cannot be shown on this scale.

By applying a dispersion containing a colloidal silica having a primary particle size of 300 nm in a volume ratio of 8: 1 to colloidal silica having a primary particle size of 15 nm, the colloidal silica having a primary particle size of 300 nm is linearly aggregated. Are distributed in a mesh pattern. Further, colloidal silica having a primary particle diameter of 15 nm is evenly distributed.
On the other hand, when the number of particles of colloidal silica having a primary particle diameter of 15 nm is large such that the volume ratio is 1: 1, the aggregation of colloidal silica having a primary particle diameter of 300 nm is reduced.

Thus, by applying a dispersion containing a large amount of colloidal silica having a large primary particle size as a volume ratio with respect to colloidal silica having a small primary particle size, the colloidal silica having a large primary particle size is aggregated in a network shape to form primary particles. It is possible to form an etching mask having a region in which colloidal silica having a large diameter is densely distributed and a region in which the colloidal silica is sparsely distributed in a predetermined cycle.
Thus, the distribution of colloidal silica can be controlled by changing the ratio of colloidal silica having a large primary particle size and the dilution ratio of ethanol.

And since the area where the large-diameter colloidal silica aggregates and adheres and the area where the large-diameter colloidal silica does not adhere are arranged in a predetermined cycle, an area that is easily etched and an area that is difficult to be etched are formed on the surface of the substrate. Can be formed.
Note that the mesh pitch is preferably in the range of 500 nm or more and 5 μm or less. If the pitch is less than 500 nm, the large horizontal protrusions under the mask are connected before the deep unevenness is formed, making it impossible to form deep unevenness. When it exceeds 5 μm, the reduction in reflectance is reduced.

FIG. 5 is a graph showing the reflectance of the surface of the (100) substrate on which irregularities are formed by the texture forming method according to Embodiment 1 of the present invention. FIG. 6 is a graph showing the reflectance of the surface of the (111) substrate on which irregularities are formed by the texture forming method according to Embodiment 1 of the present invention. 5 and 6 show the reflectance of the surface of the semiconductor substrate that has been etched without an etching mask.
When the semiconductor substrate on which the etching mask in which colloidal silica having a large primary particle diameter is distributed like a network is etched as in the first embodiment of the present invention, as shown in FIG. When the etching mask is not used, the reflectance of light having a wavelength of 1000 nm is 23%, whereas the reflectance of light having a wavelength of 1000 nm is reduced to 19% when etching is performed using the etching mask.
In addition, as shown in FIG. 6, in the case of the (111) substrate without an etching mask, the reflectance of light with a wavelength of 1000 nm is 27%, whereas the light with a wavelength of 1000 nm is etched using the etching mask. The reflectivity has decreased to 19%.

  In this way, the reflectance is improved in a region with a large wavelength because the path of atomic hydrogen in the direction perpendicular to the substrate surface is hindered in the region where colloidal silica with a large primary particle size is agglomerated, and the substrate surface and the horizontal direction. As a result of the etching only, a network-like center line remains, a ridge line is formed, and deep unevenness having a large height difference with respect to the etched portion where the colloidal silica having a large primary particle diameter is not attached. It is thought that it is formed.

  In such a method for producing a photovoltaic device, the surface of the portion where the colloidal silica having a large primary particle size is agglomerated and adhered is blocked by the colloidal silica from the flight of atomic hydrogen, and is located below the surface of this portion. Since the substrate is left in a ridgeline shape and a large height difference is obtained between this ridgeline and the etched portion where the colloidal silica having a large primary particle size is not attached, the reflectivity is reduced by the deep unevenness, and the solar cell The conversion efficiency can be improved.

Embodiment 2. FIG.
The texture forming method according to the second embodiment of the present invention includes the texture forming method according to the first embodiment, the pyramid-like irregularities formed on the surface of the semiconductor substrate on which the etching mask is formed, and the composition of the dispersion liquid. Are the same except that they are different, the same reference numerals are given to the same parts, and the description thereof is omitted.
The dispersion according to Embodiment 2 of the present invention is prepared by adding an appropriate amount of hexamethyldisilazane to a liquid in which 300 nm IPA sol, 15 nm IPA sol and ethanol are mixed at a volume ratio of 8: 1: 56. Other organic silane raw materials may be used instead of hexamethyldisilazane.

Next, a texture forming method according to the second embodiment will be described. FIG. 7 is a flowchart showing the procedure of the texture forming method according to the second embodiment of the present invention.
In S11, the damaged layer is removed from the polycrystalline silicon semiconductor substrate and pyramidal irregularities are formed by alkali etching. The height of the pyramid shape is 1 μm to 50 μm.
In step S12, after forming the pyramidal irregularities, the dispersion liquid according to the second embodiment is dropped onto the semiconductor substrate to wet the substrate uniformly.
In step S13, the solvent is volatilized while blowing hot air, and colloidal silica is adhered to the surface of the substrate to form an etching mask.

  In step S14, the semiconductor substrate 5 is set on the substrate holder 6, and the gap between the filament 7 and the semiconductor substrate 5 is set to 4 cm and inserted into the chamber 2. The semiconductor substrate 5 is etched for 15 minutes by flowing hydrogen gas from the hydrogen inlet 3 at 50 sccm (1,013 hPa, 0 ° C.) to set the degree of vacuum in the chamber 2 to 10 Pa and the filament temperature to 2150 ° C. Thereafter, the colloidal silica adhering to the substrate is removed with hydrofluoric acid to finish the formation of the texture.

FIG. 8 is a graph showing the reflectance of the surface of the semiconductor substrate on which irregularities are formed by the texture forming method according to Embodiment 2 of the present invention. FIG. 8 shows the reflectance of the surface of the (111) substrate that has been etched without an etching mask.
Since the pyramidal unevenness and the submicron unevenness are formed on the surface of the unevenness, as shown in FIG. 8, in the (111) substrate, the reflectance of light with a wavelength of 1000 nm is 26% when etched without an etching mask. Was reduced to 19% when etched using an etching mask.

  Thus, even in the case of a substrate having pyramidal irregularities formed by alkali etching, hot wire etching can be performed by forming an etching mask made of colloidal silica, and the reflectance can be reduced in a wide wavelength region.

  In addition, by adding an organic silane raw material such as hexamethyldisilazane, the surface of the colloidal silica can be modified to improve dispersibility, and the surface of the hydrophobic silicon substrate can be modified. Dispersibility can also be improved with respect to the substrate.

It is sectional drawing of the hot wire etching apparatus used for manufacture of the photovoltaic apparatus of this invention. It is sectional drawing of the hot wire etching apparatus of the state which contact | abutted the cooling block to the board | substrate holder. It is a flowchart which shows the procedure of the texture formation method concerning Embodiment 1 of this invention. It is a distribution map of colloidal silica having a primary particle size of 300 nm distributed on the substrate surface. It is the reflectance of the (100) surface where unevenness | corrugation was formed with the texture formation method concerning Embodiment 1. FIG. It is the reflectance of the (111) plane on which irregularities are formed by the texture forming method according to the first embodiment. It is a flowchart which shows the procedure of the texture formation method concerning Embodiment 2 of this invention. It is a reflectance of the (100) surface in which the unevenness | corrugation was formed by the texture formation method concerning Embodiment 2. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Hot wire etching apparatus, 2 chamber, 3 hydrogen introduction port, 4 exhaust port, 5 semiconductor substrate, 6 substrate holder, 7 filament, 8 heating lamp, 9 refrigerant pipe, 10 cooling block, 11 bellows.

Claims (8)

A texture that reduces the reflectivity of the surface on which light of a semiconductor substrate constituting the photovoltaic device is incident is formed by etching with atomic hydrogen generated by bringing hydrogen gas into contact with a heated refractory metal filament. In the method of manufacturing a photovoltaic device,
Prior to the etching, there is a step of forming an etching mask by agglomerating and adhering silicon oxide particles having a primary particle diameter in the range of 80 nm or more and 400 nm or less to the surface to form an etching mask. Method.   The method for manufacturing a photovoltaic device according to claim 1, wherein the silicon oxide particles are colloidal silica.   The method for manufacturing a photovoltaic device according to claim 1 or 2, wherein the mesh pitch is in a range of 500 nm or more and 5 µm or less.   In the step of forming the etching mask, the colloidal silica and a small-diameter colloidal silica having a primary particle diameter of 10 nm or more and 80 nm or less are dispersed in an organic solvent or water, and the surface of the semiconductor substrate on which the light is incident 3. The method of manufacturing a photovoltaic device according to claim 2, wherein the organic solvent or water is removed from the applied film and adhered to the surface.   In the step of forming the etching mask, the colloidal silica and a small diameter colloidal silica having a primary particle diameter of 10 nm or more and 80 nm or less are dispersed in an organic solvent or water to which an organosilane material is added, and the dispersed liquid is used as a semiconductor. 3. The method of manufacturing a photovoltaic device according to claim 2, wherein the substrate is applied to a surface on which light is incident, and the organic solvent or water is removed from the applied film and adhered to the surface.   The method of applying the dispersed liquid is any one of a method of dropping the dispersed liquid, a method of spray-coating the dispersed liquid, or a method of applying mist by using water vapor as a gas phase carrier from the dispersed liquid. The method for manufacturing a photovoltaic device according to claim 4 or 5, wherein:   The method for manufacturing a photovoltaic device according to claim 1, wherein the silicon oxide particles are powder and adhere to the surface by a powder spray coating method.   The method for producing a photovoltaic device according to claim 1, wherein, prior to the formation of the etching mask, irregularities having a large period of 1 µm or more are formed on the surface by anisotropic etching with an alkaline solution.

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