JP2009147272A - Method of manufacturing solar battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a solar battery of excellent characteristics in a simple process by using a phosphor based dopant coating liquid in a form suitable for application by an inkjet method. <P>SOLUTION: The method of manufacturing the solar battery includes: a step of applying the alcoholic solution of phosphorous acid to a p-type semiconductor substrate by the inkjet method so that the dots of the alcoholic solution of the phosphorous acid partially overlap; and a step of forming the n-type diffusion layer of a different phosphor concentration on the p-type semiconductor substrate by thermally diffusing phosphor. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a solar cell.

  In a general solar cell, a pn junction is required to separate carriers generated by irradiation with sunlight. For example, when a p-type semiconductor substrate is used as the substrate, this pn junction can be formed by thermally diffusing a group 5 element such as phosphorus as an n-type dopant on the light-receiving surface side, In the case of using a semiconductor substrate, it can be formed by thermally diffusing a group 3 element such as boron on the light receiving surface side as a p-type dopant.

  More specifically, taking a crystalline silicon solar cell as an example, the crystalline silicon solar cell is generally formed by thermally diffusing a phosphorus-based dopant at a temperature of about 700 ° C. to 900 ° C. on both surfaces of the p-type silicon substrate. A diffusion layer is formed, an unnecessary n-type diffusion layer on the back surface side is removed, and then an antireflection film made of a silicon nitride film or the like is formed on the n-type diffusion layer on the light receiving surface side. It is manufactured by printing the silver paste in a grid shape, solid-printing the aluminum paste on the back surface opposite to the surface on which the antireflection film is formed, and then baking the substrate. In addition, without removing the n-type diffusion layer on the back side, aluminum may be diffused from the aluminum paste to the substrate, and the n-type diffusion layer may be forcibly inverted to a p-type layer or a high-concentration p + type layer. .

  By the way, in order to increase the photoelectric conversion efficiency of the solar cell, it is necessary to reduce the thickness of the diffusion layer. However, if the thickness of the diffusion layer is too thin, current collection at the electrode portion cannot be performed well due to high resistance, and the diffusion layer portion is easily broken by the electrode, which is said to penetrate. Therefore, the portion corresponding to the light receiving surface is a high resistance layer (low concentration dopant diffusion layer) with a thin diffusion layer, and the portion corresponding to the electrode is a low resistance layer (high concentration with a high diffusion layer thickness). A structure called a selective emitter as a dopant diffusion layer) is preferable. Conventionally, when such a selective emitter is formed, a complicated process combining a plurality of times of diffusion and partial etching by masking is required.

  Therefore, using an inkjet apparatus, a coating liquid having a high phosphorus-based dopant concentration is applied to the surface on which the surface electrode of the semiconductor substrate is to be formed, and a coating liquid having a low phosphorus-based dopant concentration is used as the light receiving region of the semiconductor substrate. There has been proposed a method of forming a selective emitter on a semiconductor substrate by applying a heat treatment and diffusing a phosphorus-based dopant after coating (see, for example, Patent Document 1).

  In addition, a phosphoric acid-containing coating medium that acts as both an etching component and a doping component is locally applied to an antireflection film formed on a silicon substrate in which phosphorus is diffused, by screen printing, an inkjet method, or the like, and then the substrate is There has been proposed a method in which a selective emitter is formed on a silicon substrate by etching the antireflection film at a portion where the coating medium is applied by heating, and then further heating the substrate to locally diffuse phosphorus. (For example, see Patent Document 2).

JP 2003-224285 A JP 2005-506705 gazette

However, since the coating liquid used in the ink jet method is required to have low viscosity and surface tension, there is a great limitation on the composition. That is, in both Patent Documents 1 and 2, it is necessary to prepare a coating solution under the condition that the viscosity and the surface tension are small enough to be applied by an ink jet method. No consideration is given to the relationship between the components of the liquid and the viscosity and surface tension. As a general method for reducing the viscosity and surface tension, for example, the method of simply diluting with water can reduce the viscosity of the coating solution, but cannot reduce the surface tension, and the concentration of the phosphorus dopant is low. There is a possibility that the diffusion of phosphorus to the substrate becomes insufficient because the film becomes too thin. For example, in the method of diluting with an aqueous solution in which a surfactant is added to water, the viscosity and the surface tension can be lowered. There is a problem that impurities derived from the activator are mixed into the n-type diffusion layer to deteriorate the characteristics of the solar cell.
As described above, in the above-described conventional technology, no consideration is given to the viscosity and surface tension of the phosphorus-based dopant coating solution for use in the ink jet method, so that the phosphorus-based dopant coating solution is uniformly and stably applied. However, it is difficult to manufacture a solar cell with good characteristics by a simple process.

  Accordingly, an object of the present invention is to provide a method for producing a solar cell with good characteristics by a simple process using a phosphorous dopant coating solution in a form suitable for coating by an ink jet method.

  Accordingly, as a result of intensive studies to solve the above problems, the present inventors have diluted phosphoric acid with alcohol to obtain an alcohol solution of phosphoric acid, and the p-type semiconductor substrate is formed so that the dots of this alcohol solution partially overlap. It has been found that a method of forming n-type diffusion layers having different phosphorus concentrations on a p-type semiconductor substrate by applying ink jet and thermally diffusing phosphorus can solve the above-mentioned problems, and has completed the present invention.

  That is, the method for manufacturing a solar cell according to the present invention includes a step of applying an alcohol solution of phosphoric acid to a p-type semiconductor substrate by an inkjet method so that the dots of the alcohol solution of phosphoric acid partially overlap, and thermally diffusing phosphorus. And a step of forming n-type diffusion layers having different phosphorus concentrations on the p-type semiconductor substrate.

  ADVANTAGE OF THE INVENTION According to this invention, the method of manufacturing a solar cell with a favorable characteristic can be provided by a simple process using the phosphorus type dopant coating liquid of the form suitable for the application | coating by the inkjet method.

Hereinafter, embodiments of the present invention will be described.
Embodiment 1 FIG.
The method for manufacturing a solar cell according to Embodiment 1 includes a step of inkjet-coating a phosphoric alcohol solution as a phosphorus-based dopant coating solution onto a p-type semiconductor substrate, and thermally dispersing the phosphorus on the p-type semiconductor substrate. Forming n-type diffusion layers having different phosphorus concentrations.

First, the process of apply | coating a phosphorus dopant coating liquid to a p-type semiconductor substrate by the inkjet method is demonstrated.
The phosphorus-based dopant coating solution used in the present embodiment is one in which viscosity and surface tension are adjusted by diluting phosphoric acid with alcohol. The viscosity of the phosphoric alcohol solution is preferably 1 mPa · s to 20 mPa · s at 25 ° C. from the viewpoint of more uniformly and stably coating, particularly about 5 mPa · s to 10 mPa · s. preferable. The surface tension of the phosphate alcohol solution is preferably 20 mN / m to 50 mN / m at 25 ° C. from the viewpoint of more uniformly and stably coating, and 20 mN / m to 30 mN / m. Is particularly preferred. In order to prepare a phosphate alcohol solution having the above-described viscosity and surface tension, phosphoric acid is preferably 5% by mass to 50% by mass, particularly preferably 10% by mass, depending on the type of alcohol used for dilution. What is necessary is just to dilute with alcohol so that it may be contained in the range of% -40 mass%. However, if the phosphoric acid concentration is too low, the diffusion of phosphorus into the p-type semiconductor substrate in the post-process may be insufficient. On the other hand, if the phosphoric acid concentration is too high, the viscosity and the surface tension increase, and the inkjet Application may be difficult.

  The alcohol used for dilution is not particularly limited, and examples thereof include ethanol, isopropanol, butanol, hexanol, heptanol, octanol, nonanol, and decanol. From the viewpoint of affinity for silicon substrate and appropriate evaporation Therefore, hexanol is particularly preferable.

In this step, the phosphate alcohol solution is applied to the p-type semiconductor substrate while adjusting the discharge amount and pitch with an ink jet apparatus so that the above-described dots of the phosphate alcohol solution partially overlap. The amount of the phosphoric alcohol solution applied to the p-type semiconductor substrate is usually in the range of 0.1 μl / cm ^{2 to} 50 μl / cm ^2. From the viewpoint of easy ejection, 0.2 μl / cm ² It is preferably in the range of ˜5 μl / cm ² . The phrase “so that the dots of the phosphate alcohol solution partially overlap” in the present embodiment means that at least the vicinity of the ends of the dots 1 of the phosphate alcohol solution partially overlap as shown in FIG. To do. In FIG. 1, the portion 3 where the dots overlap has a higher phosphoric acid concentration than the portion 2 where the dots do not overlap. Form a layer. Therefore, from the viewpoint of obtaining a solar cell with better characteristics, it is desirable to apply an alcoholic solution of phosphoric acid so that the dots partially overlap on the p-type semiconductor substrate corresponding to the portion where the electrode is formed. It is also desirable to apply so that there is no gap (unapplied portion) between the dots and there are continuous portions where the dots overlap. Moreover, it is also preferable that the overlapping portion of the dots is in a mesh shape from the viewpoint that the flow path through which the current easily flows is connected to the entire surface. In order to accurately apply the alcoholic solution of phosphoric acid in the form of dots so that these dots partially overlap, a method of applying and drying simultaneously by heating the substrate, or the original half of the dots so that the dots do not overlap A method of applying the remaining dots so as to connect the dots after applying at a density and drying may be used.

  Examples of the p-type semiconductor substrate used in this embodiment include a single crystal silicon substrate and a polycrystalline silicon substrate. Moreover, as a p-type semiconductor substrate, the thing of the thickness of 150 micrometers-300 micrometers is normally used. In order to further increase the photoelectric conversion efficiency of the solar cell, the damaged layer on the surface of the p-type semiconductor substrate may be removed by etching using an acid or alkali. Prior to the application of the phosphate alcohol solution, an appropriate surface treatment may be applied to the p-type semiconductor substrate. For example, the wettability of the phosphate alcohol solution by slightly oxidizing the surface of the p-type semiconductor substrate by plasma oxidation or the like. Alternatively, the wettability of the phosphate alcohol solution may be deteriorated (the phosphate alcohol solution is easily repelled) by removing the oxide film on the surface of the p-type semiconductor substrate with hydrofluoric acid.

As the ink jet device used in the present embodiment, an ink jet head, a p-type semiconductor substrate to be coated, or a known device having a moving mechanism on both can be used. In particular, the liquid can be obtained by compression using a piezoelectric element. A method of discharging is preferable. There may be a single inkjet nozzle or a plurality of inkjet nozzles arranged in the inkjet head, but the arrangement of a plurality of nozzles is preferable because the time required for coating can be shortened.
In addition, since phosphoric acid is a strong acid, if the material of the liquid contact part (part where the coating liquid contacts) such as the ink supply tube to the ink jet nozzle, ink tank, or ink jet nozzle is made of metal, dissolve it. In other words, the dissolved component may be mixed into the n-type diffusion layer to deteriorate the performance of the solar cell. For this reason, it is desirable that the liquid contact portion in the ink jet apparatus is covered with an acid resistant material or made of an acid resistant material. Examples of acid-resistant materials suitable for such applications include resins such as Teflon (registered trademark) and polypropylene, glass, and the like.

  As described above, in the present embodiment, since the phosphorus-based dopant coating solution in a form suitable for coating by the ink jet method is used, coating can be performed uniformly and stably on the p-type semiconductor substrate. Furthermore, in this embodiment, since the portions having different phosphorus concentrations are formed by overlapping the dots of the phosphoric alcohol solution, a plurality of types of coating solutions having different concentrations of phosphorus dopants are prepared as in the prior art. There is no need to change the inkjet head for each density, and there is an advantage that it is not necessary to prepare a plurality of inkjet heads.

Next, a process of forming n-type diffusion layers having different phosphorus concentrations on the p-type semiconductor substrate by thermally diffusing phosphorus will be described.
In this step, the p-type semiconductor substrate coated with the phosphate alcohol solution is heated to thermally diffuse phosphorus. When phosphorus is thermally diffused in the p-type semiconductor substrate obtained in the previous step, the portion where the dots of the phosphoric alcohol solution overlap is a low-resistance n-type diffusion layer because the phosphoric acid concentration is high, and the dots do not overlap. The portion becomes a high resistance n-type diffusion layer because the phosphoric acid concentration is low. Usually, by increasing the phosphoric acid concentration, increasing the amount of application, increasing the heating temperature and increasing the heating time, the diffusion layer tends to be thicker and the sheet resistance tends to be lower. By reducing the coating amount, lowering the heating temperature and shortening the heating time, the diffusion layer tends to become thinner and the sheet resistance tends to increase. Therefore, the optimum sheet resistance as a selected emitter can be obtained by changing these conditions as appropriate. Each sheet resistance can be adjusted so as to obtain a ratio. In general, if the sheet resistance is 40Ω / □ to 60Ω / □, it can function as a diffusion layer for solar cells, but in this embodiment, the sheet resistance of the high resistance n-type diffusion layer is 60Ω / □. It is preferable that □ to 100Ω / □ and the low resistance n-type diffusion layer be 30Ω / □ to 50Ω / □.

  The method for heating the p-type semiconductor substrate is not particularly limited, and examples thereof include a method using an electric furnace and a laser device, and a method combining these. In the case where phosphorus is thermally diffused using an electric furnace, the p-type semiconductor substrate may be heat-treated at 700 ° C. to 1000 ° C. for 1 minute to 60 minutes in an air atmosphere. When phosphorus is thermally diffused using a laser device, the laser may be irradiated so that the surface temperature of the p-type semiconductor substrate instantaneously becomes 800 ° C. to 1200 ° C. The laser irradiation may be performed on the entire surface of the p-type semiconductor substrate coated with the phosphate alcohol solution, or may be performed only on a part of a portion where an electrode is to be formed. However, crystal distortion occurs due to a rapid temperature change in the portion irradiated with the laser, so that the entire p-type semiconductor substrate is heated by an electric furnace in the subsequent process, or the entire surface of the p-type semiconductor substrate is irradiated with a low energy laser ( It is preferable to relieve crystal distortion by so-called laser annealing.

Among the thermal diffusion processes described above, the process of heating the p-type semiconductor substrate in an electric furnace after irradiating only the portion where the electrode of the p-type semiconductor substrate coated with the phosphate alcohol solution is to be formed is performed by laser. Since only the selective irradiation portion can be reduced in resistance, it is easy to form a resistance difference between the electrode portion and other light receiving portions. However, it is necessary to perform alignment at the time of electrode formation.
In addition, the entire p-type semiconductor substrate can be processed without irradiating the entire p-type semiconductor substrate at a high temperature by irradiating the entire surface of the p-type semiconductor substrate coated with the phosphoric alcohol solution with a laser and then irradiating a low energy laser to relieve crystal distortion. An n-type diffusion layer can be formed. For this reason, firing in forming electrodes in the subsequent process is also performed by irradiating a laser, or by using an electrode that does not require firing, for example, a conductive paste, a solar cell can be apparently manufactured in a low-temperature process. it can. This method is effective when a point contact structure is used on the back surface, but it is necessary to peel off the antireflection film at the corresponding portion for the front side electrode in advance.

  Examples of the laser to be used include a YAG fundamental wave laser, a second harmonic, a third harmonic, and the like. In general, when a laser with a short wavelength is used, energy is concentrated on the surface, so that an n-type diffusion layer with a high phosphorus concentration is formed thin. Conversely, when a laser with a long wavelength is used, an n-type with a low phosphorus concentration is used. Since the diffusion layer tends to be formed thick, a laser may be appropriately selected in consideration of these.

  As described above, in the prior art, a selective emitter cannot be formed without preparing a coating solution having a high phosphorus dopant concentration and a coating solution having a low concentration of phosphorous dopant, and applying each of them to separate regions on the substrate. On the other hand, in this embodiment, the selective emitter can be easily formed without going through such a process.

  In addition to the above steps, the solar cell manufacturing method according to the present embodiment includes a step of forming an antireflection film on the n-type diffusion layer, a step of forming a surface electrode on the antireflection film, and an antireflection And a step of forming a back electrode on the surface opposite to the surface on which the film is formed.

  Since a phosphorous glass layer is usually formed on the n-type diffusion layer (consisting of a high-resistance n-type diffusion layer and a low-resistance n-type diffusion layer) of the p-type semiconductor substrate obtained in the above process, If necessary, the phosphorus glass layer is removed by washing with hydrofluoric acid. Next, an antireflection film made of silicon nitride (SiN) or the like is formed on the n-type diffusion layer by a known method such as a plasma CVD method. This antireflection film made of silicon nitride or the like also functions as a passivation film for terminating and protecting the surface of the p-type semiconductor substrate.

Next, a solar cell can be obtained by forming a surface electrode on the antireflection film and forming a back electrode on the surface opposite to the surface on which the antireflection film is formed.
For example, the surface electrode is formed by screen-printing a silver paste in a grid shape on an antireflection film corresponding to a portion where the surface electrode of the p-type semiconductor substrate is to be formed, and then baking at 700 to 900 ° C. for 3 to 30 minutes. Can be formed. The back electrode can be formed, for example, by solid-printing an aluminum paste on the surface of the p-type semiconductor substrate opposite to the surface on which the antireflection film is formed, and then baking it simultaneously with the surface electrode.

  According to the present embodiment, since the phosphorus-based dopant coating liquid can be uniformly and stably applied onto the p-type semiconductor substrate by the inkjet method, the amount of use and waste of the coating liquid can be reduced, In addition, since the selective emitter can be formed without going through complicated steps, a solar cell with good characteristics can be manufactured at low cost.

EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.
<Example 1>
High concentration phosphoric acid (85% by mass or more) was diluted with hexanol to prepare a 30% by mass phosphoric acid hexanol solution. A Teflon (registered trademark) ink tank, a glass inkjet nozzle (piezo method), a Teflon (registered trademark) tube for connecting the ink tank and the inkjet nozzle, and an inkjet head having a moving mechanism were provided. By adjusting the discharge amount and pitch of the ink jet device, a polycrystalline silicon substrate (10 μm on both sides of a thickness of 200 μm, so that the dots of the hexanol phosphate solution prepared previously partially overlap as shown in FIG. The surface of the damaged layer was removed by alkali etching, and a hexanol phosphate solution was applied at a coating amount of 1 μl / cm ² . Next, the polycrystalline silicon substrate coated with the hexanol phosphate solution was put in an electric furnace and heated at 850 ° C. for 15 minutes in an air atmosphere to thermally diffuse phosphorus in the polycrystalline silicon substrate. Next, the polycrystalline silicon substrate was taken out from the electric furnace, washed with hydrofluoric acid to remove the phosphorus glass layer, and only the phosphorus diffusion layer was left on the polycrystalline silicon substrate. Subsequently, a SiN film was formed on the phosphorus diffusion layer by a plasma CVD method. Finally, silver paste is screen-printed on the portion (grid shape) where the surface electrode is to be formed on the SiN film of the polycrystalline silicon substrate, and the aluminum paste is solid on the surface opposite to the surface on which the SiN film is formed. After printing, the polycrystalline silicon substrate was baked at 900 ° C. for 5 minutes to form a front electrode and a back electrode to obtain a solar battery cell. When the characteristic of the obtained photovoltaic cell was measured, the photoelectric conversion efficiency of 16.2% was obtained.

<Example 2>
High concentration phosphoric acid (85% by mass or more) was diluted with hexanol to prepare a 30% by mass phosphoric acid hexanol solution. A Teflon (registered trademark) ink tank, a glass inkjet nozzle (piezo method), a Teflon (registered trademark) tube for connecting the ink tank and the inkjet nozzle, and an inkjet head having a moving mechanism were provided. By adjusting the discharge amount and pitch of the inkjet device, a polycrystalline silicon substrate (10 μm on both sides with a thickness of 200 μm is formed so that the dots of the hexanol phosphate solution prepared above partially overlap at the portion where the surface electrode should be formed. The hexanol phosphate solution was applied at a coating amount of 1 μl / cm ² on the surface of which the damaged layer on the surface was removed by alkali etching. Next, the YAG fundamental wave laser was irradiated to the portion where the surface electrode on the polycrystalline silicon substrate coated with the hexanol phosphate solution was to be thermally diffused. Thereafter, the polycrystalline silicon substrate coated with the hexanol phosphate solution is put into an electric furnace and heated at 800 ° C. for 10 minutes in an air atmosphere to thermally diffuse phosphorus in the polycrystalline silicon substrate and relieve crystal distortion. I let you. Next, the polycrystalline silicon substrate was taken out from the electric furnace, washed with hydrofluoric acid to remove the phosphorus glass layer, and only the phosphorous diffusion layer was left on the polycrystalline silicon substrate. Subsequently, a SiN film was formed on the phosphorus diffusion layer by a plasma CVD method. Finally, silver paste is screen-printed on the portion (grid shape) where the surface electrode is to be formed on the SiN film of the polycrystalline silicon substrate, and the aluminum paste is solid on the surface opposite to the surface on which the SiN film is formed. After printing, the polycrystalline silicon substrate was baked at 900 ° C. for 5 minutes to form a front electrode and a back electrode to obtain a solar battery cell. When the characteristic of the obtained photovoltaic cell was measured, the photoelectric conversion efficiency of 16.4% was obtained.

<Comparative Example 1>
Except for diluting the high-concentration phosphoric acid with water, an attempt was made to produce a solar cell in the same manner as in Example 1. However, it was not possible to apply ink jet with a 30% by mass phosphoric acid aqueous solution. Therefore, although the phosphoric acid aqueous solution was further diluted with water, ink jet coating could not be performed. Thus, a solar cell was manufactured in the same manner as in Example 1 except that 30% by mass of a hexanol solution of phosphoric acid was uniformly applied to the entire surface of the substrate by spin coating, and 15.4% photoelectric conversion was performed. Efficiency was obtained. Similarly, a 30% by mass phosphoric acid aqueous solution was uniformly applied to the entire surface of the substrate by spin coating, and a solar cell was manufactured in the same manner as in Example 1 except that 15.5% photoelectric conversion was performed. Efficiency was obtained. Both of these photoelectric conversion efficiencies were lower than in Example 1.

<Comparative example 2>
A solar cell was produced in the same manner as in Example 2 except that 30% by mass of a hexanol solution of phosphoric acid was uniformly applied to the entire surface of the substrate by spin coating, and a photoelectric conversion efficiency of 15.5% was obtained. Obtained. Similarly, a 30% by mass phosphoric acid aqueous solution was uniformly applied to the entire surface of the substrate by spin coating. Otherwise, a solar cell was produced in the same manner as in Example 1. As a result, 15.6% photoelectric conversion was performed. Efficiency was obtained. Both of these photoelectric conversion efficiencies were lower than in Example 2.

FIG. 3 is a schematic diagram showing a state where dots of the phosphate alcohol solution in Embodiment 1 partially overlap each other.

Explanation of symbols

  1 A dot of a phosphate alcohol solution, 2 A portion where dots of a phosphate alcohol solution do not overlap, 3 A portion where dots of a phosphate alcohol solution overlap.

Claims (6)

Applying a phosphoric acid alcohol solution to a p-type semiconductor substrate by an inkjet method so that the dots of the phosphoric acid alcohol solution partially overlap;
And a step of forming n-type diffusion layers having different phosphorus concentrations on a p-type semiconductor substrate by thermally diffusing phosphorus.   2. The method of manufacturing a solar cell according to claim 1, wherein the coating is performed so that the dots partially overlap at a portion where an electrode is to be formed.   3. The method for manufacturing a solar cell according to claim 1, wherein the thermal diffusion is performed by irradiating a laser on the entire surface of the p-type semiconductor substrate on which the alcoholic solution of phosphoric acid has been applied.   The thermal diffusion is performed by heating the entire p-type semiconductor substrate after irradiating only a portion where the electrode of the p-type semiconductor substrate to which the alcoholic solution of phosphoric acid has been applied is to be formed. The manufacturing method of the solar cell of Claim 1 or 2.   The method for producing a solar cell according to any one of claims 1 to 4, wherein an ink-jet device in which the liquid contact portion is coated with an acid-resistant material or is made of an acid-resistant material is used. Forming an antireflection film on the n-type diffusion layer;
Forming a surface electrode on the antireflection film;
The method for manufacturing a solar cell according to claim 1, further comprising a step of forming a back electrode on a surface opposite to the surface on which the antireflection film is formed.

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