JP2006344883A - Manufacturing method of solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a solar cell with high conversion efficiency at low cost.
A method of manufacturing a solar cell according to the present invention includes a pn junction forming step of diffusing a dopant having a second conductivity into a Si substrate having a first conductivity, and titanium oxidation on a light-receiving surface of the Si substrate. An antireflection film forming step of forming an antireflection film having a film, and an electrode formation step of forming a light receiving surface electrode on the light receiving surface of the Si substrate and forming a back electrode on the back surface opposite to the light receiving surface. A hydrogen treatment step is provided after the prevention film forming step.
[Selection] Figure 1

Description

  The present invention relates to a solar cell manufacturing method that can provide a solar cell with high conversion efficiency at low cost.

In recent years, solar cells have attracted attention as a means for solving global environmental problems and energy problems, and their production volume has been dramatically increased. FIG. 4 is a schematic cross-sectional view showing the structure of a typical conventional solar cell. As shown in FIG. 4, in the conventional solar cell, an n ⁺ region 41 is formed on a p-type Si substrate 40, and an antireflection film 43, a light receiving surface electrode 44, and a back surface electrode 45 are formed. Moreover, in order to reduce surface reflection in a light-receiving surface, it has the texture structure 40a.

FIG. 5 is a process diagram showing a conventional method for manufacturing a solar cell. As shown in FIG. 5, first, a p-type Si substrate sliced to a predetermined thickness with a wire saw or the like is subjected to chemical treatment to form a texture structure to remove surface distortion and reduce surface reflection. (Step 51) (hereinafter, "Step" is abbreviated as "S"). Next, a group V dopant such as P (phosphorus) is diffused to form an n ⁺ region and a pn junction is formed (S52). P is diffused by a gas diffusion method using POCl ₃ or the like, or a solid phase diffusion method in which a diffusing agent containing P is applied to the surface and treated at 850 ° C. to 950 ° C. Thereafter, an antireflection film is formed in order to further reduce surface reflection (S53). As the antireflection film, a SiN film is formed by a p-CVD method. Thereafter, an Ag paste is formed as a light-receiving surface electrode, and an Al paste is printed as a back electrode, dried and fired to form an electrode (S54). Furthermore, inspections such as characteristics and appearance are performed (S55), and the process proceeds to the module manufacturing process.

  Thus, in the production of solar cells, chemical treatment tanks (etching tanks), diffusion furnaces (heat treatment furnaces), screen printing machines, baking furnaces, etc. are used, and simple and inexpensive manufacturing equipment is used. Only the antireflection film made of SiN is manufactured by an expensive p-CVD apparatus. The p-CVD apparatus used for forming the antireflection film will be described in more detail. The p-CVD apparatus requires equipment such as a vacuum reaction tank, a plasma generation power source, gas supply / exhaust equipment, and gas exclusion equipment, and the price of the entire apparatus is high. Moreover, since plasma is generated, it is difficult to form a film uniformly and at high speed with a large-area electrode serving as a substrate stage. Therefore, the number of substrates that can be processed simultaneously is small, and the throughput is low. Therefore, the ratio of the antireflection film in the manufacturing cost of the solar cell is large, and as a result, the manufacturing cost of the solar cell is increased. However, the SiN film formation by the p-CVD method has an advantage that the formation of the antireflection film and the passivation effect can be obtained at the same time. This is considered to terminate defects such as dangling bonds (DB) of the Si substrate by active hydrogen generated simultaneously with the formation of the SiN film, which is useful for improving the characteristics of the solar cell, that is, improving the conversion efficiency. Yes.

In addition to the SiN film formed by the p-CVD method, the antireflection film in the original sense for reducing the light reflection and improving the power generation efficiency is, for example, a SiO film, a SnO ₂ film or a TiO near a refractive index of 2. There are _two films, which can be formed by vapor deposition, sputtering, thermal CVD, spin-on, spraying, or the like (see Non-Patent Document 1). On the other hand, as a passivation method using activated hydrogen, a hydrogen plasma method, a hydrogen ion implantation method, a forming gas annealing (FGA) method, a hot wire method, or the like is known. For example, the diffusion coefficient in the polycrystalline Si film is improved by the active hydrogen treatment by the hot wire method, and the efficiency of the solar cell is improved (see Non-Patent Document 2). However, this information is general, and as a specific solar cell manufacturing technology, what kind of activated hydrogen treatment process should be introduced, and the effectiveness of hydrogen treatment after formation of the antireflection film Etc. are not known and could not be applied to the actual manufacturing process.

As a method of manufacturing a solar cell that performs hydrogen treatment without using a SiN antireflection film by the p-CVD method, for example, after electrode formation, in a hydrogen atmosphere or a mixed gas atmosphere of nitrogen and hydrogen, a temperature of 350 ° C. to 500 ° C. A manufacturing method is known in which an antireflection film such as TiO ₂ or Ta ₂ O ₅ is formed by performing heat treatment (FGA) at a temperature of 5 ° C. for about 5 minutes (see Patent Document 1). However, this document only describes the FGA treatment effect before forming the antireflection film, and does not disclose the effect of the hot wire method and the effect after forming the antireflection film. Although there is no specific description in the same document, since the antireflection film is formed after the electrode is formed, the antireflection film in the contact portion for connecting the solar cells is removed after the formation of the antireflection film. It is necessary to take measures such as forming a mask so that the antireflection film does not adhere to the contact portion at the time of forming the antireflection film, which complicates the process and makes the apparatus configuration complicated.

  Further, a method is known in which dopant diffusion for forming a pn junction and formation of an antireflection film are performed in the same process, and hydrogen plasma treatment is performed after electrode formation (see Patent Document 2). However, in this method, an organic solution containing Ti and P is applied as a dopant diffusing agent solution by a spin method or the like and treated at 900 ° C. to 1000 ° C. to simultaneously diffuse the dopant and form an antireflection film. Therefore, a special film different from the Ti oxide antireflection film formed by a normal method is formed. In the same document, the effect of the hydrogen plasma method is described for active hydrogen, but the effect of the hot wire method is not described.

  On the other hand, a method is known in which hydrogen is contained in a titanium oxide film, and the titanium oxide film containing hydrogen is applied to a solar cell (see Patent Document 3). When this method is applied to a solar cell by adding hydrogen to the titanium oxide film by the hydrogen plasma method and improving the light transmittance, the amount of light incident on the solar cell increases and the short-circuit photocurrent increases. The conversion efficiency is improved. However, there is no description about the improvement of the open circuit voltage, which is one of the hydrogen passivation effects, and the effect of active hydrogen by the hot wire method is not disclosed.

Furthermore, a method of forming an antireflection film made of TiO ₂ by spray pyrolysis and a hydrogen treatment method before forming the antireflection film are known (see Patent Document 4). However, this document only discloses the effect of the active hydrogen treatment before the formation of the antireflection film, and does not describe the effect after the formation of the antireflection film. Further, the passivation effect by the active hydrogen treatment is reduced after the hydrogen treatment step, after passing through a high-temperature treatment step such as electrode firing, once the hydrogen that has terminated the defect is detached again.
JP 59-117276 A Japanese Patent Laid-Open No. 61-160980 Japanese Patent Laid-Open No. 10-226598 JP 2004-266194 A Hamakawa Yasuhiro and Kuwano Kotonori, "Solar Energy Engineering", Bafukan, pp.76-77 R. Plieninger et al., Efficient defect passivation by hot-wire hidrogenation, Appl. Phys. Lett., American Institute of Physics, 13 October 1997, vol. 71, no. 15, pp. 2169-2171

  An object of the present invention is to provide a method for forming a solar cell with high conversion efficiency at low cost by forming an antireflection film without using a p-CVD apparatus, effectively performing hydrogen treatment of an Si substrate with active hydrogen. There is.

  The method for manufacturing a solar cell according to the present invention includes a pn junction forming step of diffusing a dopant having a second conductivity in a Si substrate having a first conductivity, and a reflection having a titanium oxide film on a light receiving surface of the Si substrate. An antireflection film forming step of forming an antireflection film, and an electrode formation step of forming a light receiving surface electrode on the light receiving surface of the Si substrate and forming a back electrode on the back surface opposite to the light receiving surface. And a hydrogen treatment step.

  The aspect which implements the hydrogen treatment process after the electrode formation process heated to the temperature of 300 degreeC or more is preferable, and the aspect implemented by a hot wire method is preferable for a hydrogen treatment process. The titanium oxide film can be formed by CVD, spray pyrolysis, or vapor deposition.

  According to the present invention, an antireflection film can be produced at low cost, and a highly efficient solar cell can be obtained by effective hydrogen treatment. The antireflection film to be formed has a low reflectance, and the light transmittance is not deteriorated even by a hydrogen treatment performed in a later step, and the hydrogen treatment can be effectively performed on the Si substrate.

  The manufacturing method of the solar cell of the present invention is characterized in that an antireflection film including a titanium oxide film is formed, and hydrogen treatment is performed after the formation of the antireflection film. Since the titanium oxide film can be formed by atmospheric pressure CVD or reduced pressure CVD, spray pyrolysis, or vapor deposition, it effectively suppresses reflection of sunlight without using an expensive p-CVD apparatus. An inexpensive solar cell can be provided. In addition, after the formation of the antireflection film, hydrogen treatment is performed, and defects such as dangling bonds existing in the Si substrate are terminated with active hydrogen, so that a solar cell with high conversion efficiency can be provided.

Embodiment 1
FIG. 1 is a process diagram showing a method for manufacturing a solar cell in the present embodiment. As shown in FIG. 1, first, as a first conductive Si substrate, for example, on the light-receiving surface of a p-type polycrystalline Si substrate, the purpose is to remove a damaged layer by cutting the substrate and to increase the antireflection effect. A texture structure is formed (S1). Here, as a method of forming the texture structure, a method of forming by dry etching such as reactive ion etching using fluorine-based gas, or an alkali such as KOH (potassium hydroxide) or NaOH (sodium hydroxide) is used. There are a method of forming by wet etching using a solution, a method of forming by wet etching using a mixed solution of hydrofluoric acid and nitric acid, and the like.

Next, an n-type impurity such as P (phosphorus) is diffused as a second conductive dopant on the light receiving surface of the p-type polycrystalline Si substrate to form a pn junction (S2). By diffusing a dopant such as P, an n-type region is formed in the vicinity of the light-receiving surface of the p-type Si substrate. The diffusion can be formed, for example, by applying a solution containing n-type impurities to the surface of the p-type polycrystalline Si substrate and then heat-treating the Si substrate. In the step of forming the pn junction, it is not always necessary to form the n ⁺ region only on the light receiving surface side, but this embodiment exemplifies a preferable mode in which the n ⁺ region is formed on the light receiving surface side.

Subsequently, an antireflection film including a titanium oxide film (TiO ₂ ) is formed on the light receiving surface of the Si substrate (S3). The TiO ₂ film can be formed by, for example, an atmospheric pressure CVD method. Specifically, a solution containing titanium alkoxide, an organic solvent such as isopropyl alcohol or ethanol and a stabilizer such as acetylacetone is supplied as a mist to the film formation chamber, and the Si substrate is heated to 350 ° C. to 600 ° C. An embodiment in which a TiO ₂ film is formed on the surface of the Si substrate is preferable. By setting the temperature to 350 ° C. or higher, the reactivity of the material is improved, the characteristics such as the refractive index and the light transmittance are improved, and the adhesiveness to the Si substrate is good, so that it is difficult to peel off. On the other hand, inconveniences such as decomposition of the material are unlikely to occur even at high temperatures, but by setting the temperature to 600 ° C. or less, the temperature rise time can be shortened and the production efficiency can be increased. From this viewpoint, the heating temperature of the Si substrate is more preferably 450 ° C. to 550 ° C.

In general, when a thin film having a refractive index of about 2.2 is formed on the surface of a Si substrate, a high antireflection effect can be obtained with respect to sunlight. However, a refractive index of 2 is applied to the surface of the Si substrate by atmospheric pressure CVD. A TiO ₂ film of about 0.0 to 2.4 can be formed. Further, since the material is uniformly supplied to the surface of the Si substrate in a mist state, a TiO ₂ film having a uniform thickness can be formed along the textured surface formed on the substrate surface. Therefore, reflection of sunlight can be suppressed and a highly efficient solar cell can be supplied. Further, the structure of the film forming apparatus is also a simple structure such as a reaction chamber, a substrate heating mechanism, a material solution tank, a solution supply mechanism, and a substrate transport mechanism, and the apparatus cost can be reduced. Furthermore, by using an in-line flat-flow apparatus, there is an advantage that productivity can be improved.

The TiO ₂ film can also be formed by a low pressure CVD method. Low-pressure CVD requires an evacuation mechanism for reducing the pressure, but the others can be used in the same apparatus configuration as the atmospheric pressure CVD method, and the film quality and film thickness uniformity are the same as those of the atmospheric pressure CVD method. Can be obtained. Also, the spray pyrolysis method can be effectively applied since the materials used are the same. FIG. 3 is a schematic view illustrating an apparatus configuration used for the spray pyrolysis method. An Si substrate 31 is placed on a substrate heating stage 32 such as a hot plate and heated to 350 ° C. to 600 ° C. Thereafter, the raw material solution 33 is sprayed from the spray nozzle 34 onto the surface of the Si substrate 31 to form TiO ₂ (not shown) on the surface of the Si substrate 31.

As the raw material solution 33, a solution containing titanium alkoxide, an organic solvent such as isopropyl alcohol or ethanol, and a stabilizer such as acetylacetone can be used. Further, the temperature of the Si substrate 31 is monitored by the control means 35, the timing of spraying is controlled, and it is possible to prevent the substrate temperature from being lowered by the heat of vaporization of the solvent contained in the raw material solution 33. Therefore, the spraying of the raw material solution 33 and the heating of the Si substrate 31 are alternately repeated a plurality of times, and by this repetition, a plurality of extremely thin layers are stacked on the surface of the Si substrate 31 to obtain a TiO having a desired thickness. _Two films can be formed. Therefore, an antireflection film having a uniform film quality in the thickness direction and a uniform film thickness along the texture surface can be obtained.

The TiO ₂ or Ti, it is possible to form the TiO ₂ film by a vapor deposition method for depositing in a vacuum atmosphere or an oxygen atmosphere. Although a vacuum apparatus is required, equipment for generating plasma necessary for the p-CVD apparatus, gas exclusion equipment, and the like are unnecessary, so that the apparatus cost can be greatly reduced. In addition, a TiO ₂ film formed by sputtering can also be used as an antireflection film, but there is room for improvement in terms of apparatus cost and plasma damage. An antireflection film made of TiO ₂ can also be formed by a spin-on (spin coating) method or a dip method, but it is not easy to form a uniform film thickness on the surface of a Si substrate having a texture structure.

  After the formation of the antireflection film, a light receiving surface electrode is formed on the light receiving surface of the Si substrate, and a back electrode is formed on the back surface opposite to the light receiving surface (S4). First, a silver paste containing silver powder is printed on the surface of the antireflection film formed on the light receiving surface. Also, an aluminum paste containing aluminum powder is printed on the back surface of the Si substrate. Thereafter, the Si substrate is subjected to heat treatment at 600 ° C. to 800 ° C. to form a light receiving surface electrode made of silver and a back electrode made of aluminum.

  After the formation of the antireflection film, hydrogen treatment is performed (S5). It is preferable that the hydrogen treatment is performed by a hot wire method. In the hot wire method, it is considered that a metal wire such as W (tungsten) or Ta (tantalum) heated to a high temperature has a catalytic function for activating the raw material gas. This is a technique frequently used in the formation method. First, a Ta wire serving as a catalyst installed in the reaction vessel is heated to about 1700 ° C. to 2000 ° C. Next, when a gas containing hydrogen or hydrogen gas is introduced into the reaction vessel, the hydrogen gas passing in the vicinity of the Ta wire having a high temperature is activated and decomposed into hydrogen radicals and / or hydrogen ions.

  Thereafter, hydrogen radicals and / or hydrogen ions come into contact with the Si substrate heated to about 200 ° C. to 400 ° C., so that hydrogen radicals and / or hydrogen ions are present at the interface and / or grain boundary of the Si substrate. Terminate defects such as ring bonds. The hydrogen partial pressure to be introduced is preferably 0.001 Pa to 1 Pa, and is determined by the amount of Si wafer to be processed, the length and arrangement of Ta wires arranged in the apparatus, and the like. In addition to Ta, the material of the wire used as the catalyst can be W, and the shape is not a linear wire, but is effective even in a ribbon shape with a width of several millimeters. Thereafter, the light-receiving surface electrode is coated with solder as necessary, and the characteristics and appearance are inspected (S6), whereby a solar battery cell can be obtained.

Table 1 shows the characteristics related to the conversion efficiency of the solar cell subjected to hydrogen treatment by the hot wire method. Table 1 shows changes in the characteristic value (%) depending on the hydrogen treatment time (5 minutes, 30 minutes) when the characteristic value of a solar cell not subjected to hydrogen treatment is 100%. Even when the processing time is 5 minutes, the open circuit voltage (Voc) is improved, and the cell conversion efficiency is improved. Further, in the treatment for 30 minutes, the short circuit current (Isc), the open circuit voltage (Voc), the fill factor (FF), and the like are improved as compared with the untreated solar cell. This is an effect of the hydrogen treatment based on the hot wire method, and not only the short-circuit current is improved, but even if a TiO ₂ film is formed on the surface, the defects in the Si substrate are passivated, resulting in an improvement in open-circuit voltage, etc. This was considered for the first time by the inventors of the present invention.

FIG. 2 shows the relationship between the hydrogen treatment time by the hot wire method and the change in conversion efficiency. In FIG. 2, the conversion efficiency of a solar cell not subjected to hydrogen treatment is 100%. As a comparative example, the results of hydrogen treatment by the hydrogen plasma method (H-plasma) and the FGA method are also shown. In the hydrogen plasma method, a result of treatment for 10 minutes in hydrogen plasma generated under conditions of a substrate temperature of 400 ° C., a hydrogen gas pressure of 0.01 Pa, and an applied power density of 0.3 W / cm ² (frequency of 13.56 MHz) is shown. The FGA method shows a result of treating N ₂ gas having a furnace temperature of 400 ° C. and a hydrogen gas concentration of 3% by volume at a flow rate of 5 L / min for 10 minutes.

As is clear from FIG. 2, the hydrogen treatment by the hydrogen plasma method and the FGA method has almost no effect, or conversely the characteristics deteriorate, whereas the conversion efficiency is improved by the hot wire method. It is clear that the hot wire method is superior as a hydrogen treatment method after forming the TiO ₂ film. When the substrate temperature in the hydrogen treatment by the hot wire method is set to a high temperature of about 400 ° C., the hydrogen treatment is performed before the solder coating step, but in the hydrogen treatment by the hot wire method, the substrate temperature is set to a low temperature of 200 ° C. or less. In this case, the hydrogen treatment can be performed even after the soldering process. In addition, when heating to 300 ° C. or higher during electrode formation, it is preferable to carry out the hydrogen treatment step after the electrode formation step in order to avoid the separation of hydrogen after the hydrogen treatment.

Embodiment 2
After the texture structure is formed and the pn junction is formed as in the first embodiment, an SiO ₂ film having a thickness of 50 to 200 mm is formed in an oxidation furnace. Thereafter, as in Embodiment 1, an antireflection film made of TiO ₂ is formed, electrodes are formed, hydrogen treatment is performed by a hot wire method, and inspection is performed. In the oxidation furnace, an oxide film can be formed by processing at a processing temperature of 700 ° C. to 900 ° C. and an O ₂ gas flow rate of 5 L / min for about 0.5 minutes to 5 minutes. An antireflection film having a two-layer structure of TiO ₂ / SiO ₂ is formed on a Si substrate, but the hydrogen treatment effect by the hot wire method is obtained, and the cell characteristics are superior to a solar cell not subjected to hydrogen treatment. 1% to 5% improvement.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  According to the present invention, an antireflection film is formed with an inexpensive manufacturing facility, and effective hydrogen treatment is performed after the formation of the antireflection film, so that a solar cell with good characteristics can be manufactured at low cost.

It is process drawing which shows the manufacturing method of the solar cell of this invention. It is a figure which shows the relationship between the hydrogen treatment time by a hot wire method, and the change of conversion efficiency. It is a schematic diagram which illustrates the apparatus structure used for the spray pyrolysis method. It is a schematic sectional drawing of the conventional solar cell. It is process drawing which shows the manufacturing method of the conventional solar cell.

Explanation of symbols

31 Si substrate, 32 substrate heating stage, 33 raw material solution, 34 spray nozzle, 35 control means, 40 p-type Si substrate, 40a texture structure, 41 n ⁺ region, 43 antireflection film, 44 light receiving surface electrode, 45 back electrode.

Claims (4)

A pn junction forming step of diffusing a dopant having the second conductivity into the Si substrate having the first conductivity;
An antireflection film forming step of forming an antireflection film having a titanium oxide film on the light receiving surface of the Si substrate;
Forming a light receiving surface electrode on the light receiving surface of the Si substrate, and forming a back electrode on the back surface opposite to the light receiving surface,
A method for producing a solar cell, comprising a hydrogen treatment step after the antireflection film forming step.   The method for manufacturing a solar cell according to claim 1, wherein the hydrogen treatment step is performed after an electrode formation step of heating to a temperature of 300 ° C. or higher.   The method for manufacturing a solar cell according to claim 1, wherein the hydrogen treatment step is performed by a hot wire method.   The said titanium oxide film is formed by CVD method, spray pyrolysis method, or a vapor deposition method, The manufacturing method of the solar cell in any one of Claims 1-3 characterized by the above-mentioned.

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