JP2014212219A - Solar cell and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a solar cell with excellent surface passivation, and a solar cell with high conversion efficiency manufactured by the method.SOLUTION: A method for manufacturing a solar cell comprises the steps of: forming a light-receiving surface side diffusion layer having a conductivity type opposite to a first conductivity type on a light-receiving surface side of a silicon substrate of a first conductivity type; processing a surface of the silicon substrate by applying a solution obtained by dissolving a halogen element in a solvent to the silicon substrate after the step of forming the light-receiving surface side diffusion layer; forming a dielectric film serving as an antireflection film on the light-receiving surface side diffusion layer after the step of processing the surface of the silicon substrate; and forming a light-receiving surface electrode electrically connected to the light-receiving surface side diffusion layer after the step of forming the dielectric film.

Description

  The present invention relates to a solar cell that directly converts light energy into electric energy and a method for manufacturing the solar cell.

A solar cell is a semiconductor element that converts light energy into electric power, and includes a pn junction type, a pin type, a Schottky type, and the pn junction type is widely used. Further, when solar cells are classified based on their substrate materials, they are broadly classified into three types: silicon crystal solar cells, amorphous (amorphous) silicon solar cells, and compound semiconductor solar cells. Silicon crystal solar cells are further classified into single crystal solar cells and polycrystalline solar cells. Since a silicon crystal substrate for a solar cell can be manufactured relatively easily, its production scale is currently the largest and is expected to become more widespread in the future (for example, Patent Document 1: JP-A-8-073297). Publication).
The output characteristics of a solar cell are generally evaluated by measuring an output current voltage curve using a solar simulator. The product of the output current I _max and the output voltage V _max on this curve, the point where I _max × V _max is maximum is called the maximum output P _max , and this P _max is the total light energy (S × (I: S is the element area, I is the intensity of the irradiated light))
η = {P _max / (S × I)} × 100 (%)
Is defined as the conversion efficiency η of the solar cell.

In order to increase the conversion efficiency η, the short-circuit current Isc (output current value when V = 0 in the current-voltage curve) or Voc (output voltage value when I = 0 in the current-voltage curve) is increased. It is important to make the output current voltage curve as close to a square as possible. The squareness of the output current voltage curve is generally
FF = P _max / (Isc × Voc)
It can be evaluated by the fill factor (curve factor) defined by the equation (1). The closer the FF value is to 1, the closer the output current-voltage curve becomes to an ideal square, and the higher the conversion efficiency η.
In order to improve the conversion efficiency η, it is important to reduce the surface recombination of carriers. In a silicon crystal solar cell, minority carriers generated by incident light of sunlight reach the pn junction surface mainly by diffusion, and then are transferred to the outside as majority carriers from the electrodes attached to the light receiving surface and the back surface. It is taken out and becomes electric energy. At that time, carriers that could be taken out as current through interface states existing on the substrate surface other than the electrode surface may be recombined and lost, leading to a decrease in conversion efficiency η.

Therefore, in a high-efficiency solar cell, a passivation film made of SiO ₂ is formed on the surface of the silicon substrate except for the contact portion with the electrode to suppress carrier recombination at the interface between the silicon substrate and the passivation film, thereby converting the conversion efficiency η Improvements are being made.
As a method for forming the passivation film, a thermal oxidation method is mainly used, and the silicon substrate is heated to 900 ° C. or more in a thermal oxidation furnace in an oxygen atmosphere or an air atmosphere to form an oxide film on the surface of the silicon substrate. . However, when the passivation film is formed by a thermal oxidation method, it is necessary to process the silicon substrate at a high temperature of 900 ° C. or higher, and the lifetime of the silicon substrate is reduced due to re-diffusion of impurities by such high temperature processing. A problem occurs.

  Further, in Patent Document 2 (Japanese Patent Laid-Open No. 58-23486), a passivation film is formed by applying a tantalum oxide film and a niobium oxide film to a substrate surface by spin coating, spraying, or dipping as a passivation film, followed by baking. A method for forming the above has been reported. According to this method, it is possible to form a passivation film without reducing the lifetime of the silicon substrate because it does not require high-temperature processing, but tantalum oxide and niobium oxide films themselves have low passivation ability, so it is sufficient There is a problem that a sufficient passivation effect cannot be obtained and carrier recombination increases on the surface of the silicon substrate, so that the conversion efficiency of the solar cell is lowered.

  Patent Document 3 (Japanese Patent Laid-Open No. 58-220477) reports that a passivation effect can be obtained by forming a silicon nitride film on the surface of a silicon substrate by a plasma CVD method. Since the silicon nitride film has a function as an antireflection film for a crystalline silicon solar cell and also has an excellent passivation effect on the surface and inside of the silicon substrate, the silicon nitride film is used as a useful film as a passivation film. However, since the plasma CVD method has a high film formation rate even at a relatively low process temperature of about 400 ° C., it is frequently used in the dielectric film formation process of solar cells, but is generated in plasma. Since high energy charged particles are likely to damage the deposited film and the surface of the silicon substrate, the resulting silicon nitride film has a high interface state density, and a sufficient passivation effect cannot be obtained with the silicon nitride film alone. was there.

  In recent years, research that focuses on the relationship between the fixed charge of the diffusion layer and the dielectric film has been actively conducted. In general, a silicon nitride film having a positive fixed charge has an effective passivation performance as a passivation film in an n-type region, but is considered inappropriate as a passivation film for a p-type region. There is a problem that a sufficient passivation effect cannot be obtained in the p-type region.

JP-A-8-073297 JP 58-23486 A JP 58-220477 A

  Therefore, the present invention has been made in view of the above circumstances, and by forming a suitable passivation film on the diffusion layer on the light-receiving surface side of the silicon substrate, particularly on the p-type diffusion layer, the sun having excellent surface passivation. It aims at providing the manufacturing method of a battery, and the high conversion efficiency solar cell manufactured by the method.

  As a result of diligent studies to achieve the above object, the silicon substrate surface is treated with a solution in which a halogen element is dissolved in a solvent before the dielectric film made of silicon nitride is formed by the plasma CVD method. The halogen element formed thereon is expected to have a negative charge, and it has been found that the passivation effect in the diffusion layer, particularly the p-type region, is improved, and the present invention has been made. Therefore, this invention provides the following solar cell and its manufacturing method.

  In order to achieve the above object, a step of forming a light receiving surface side diffusion layer having a conductivity type opposite to the first conductivity type on the light receiving surface side of the first conductivity type silicon substrate, and a step of forming the light receiving surface side diffusion layer After the step of coating the silicon substrate with a solution in which a halogen element is dissolved in a solvent and treating the surface of the silicon substrate, and the step of treating the surface of the silicon substrate, the light-receiving surface side diffusion layer is formed. Manufacturing a solar cell including a step of forming a dielectric film to be an antireflection film, and a step of forming a light receiving surface electrode electrically connected to the light receiving surface side diffusion layer after the step of forming the dielectric film Provide a method.

  The method for manufacturing a solar cell further includes a step of forming a back side diffusion layer of the same conductivity type as the first conductivity type on the back side opposite to the light receiving surface of the silicon substrate, and a step of forming a dielectric film In this case, the step of forming the dielectric film on the back surface side diffusion layer in addition to the light receiving surface side diffusion layer and treating the surface of the silicon substrate may be performed after the step of forming the back surface side diffusion layer.

  The concentration of the halogen element in the solution in which the halogen element is dissolved in a solvent is preferably 0.001 to 0.1 mol / l. A solution in which a halogen element is dissolved in a solvent may contain an organic solvent or water. The halogen element is preferably iodine.

  In the above solar cell manufacturing method, the light-receiving surface side diffusion layer may be a p-type diffusion layer in a silicon substrate. In this case, in the step of processing the surface of the silicon substrate, a solution in which a halogen element is dissolved in a solvent may be applied only to the light receiving surface of the silicon substrate.

  Moreover, in order to achieve the said objective, the solar cell manufactured with the manufacturing method of said solar cell is provided.

  According to the present invention, a solution in which a halogen element is dissolved in a solvent is applied on the light-receiving surface side diffusion layer of the silicon substrate, particularly on the p-type region, and is subjected to surface treatment. A light-receiving surface side diffusion layer, particularly a passivation film having an excellent passivation effect in the p-type region can be formed, and as a result, a solar cell with high conversion efficiency can be provided.

It is a schematic sectional drawing which shows an example of the manufacturing process of the solar cell of this invention, (A) is a board | substrate, (B) is the state which formed the n-type diffused layer in the board | substrate back surface, (C) is p-type diffusion on the substrate surface. (D) is a step of applying a solution in which a halogen element is dissolved in a solvent to the substrate surface, (E) is a state in which an antireflection film (silicon nitride film) is formed on the front and back surfaces of the substrate, and (F) ) Shows a state where a light receiving surface and a non-light receiving surface electrode are formed.

  The solar cell manufacturing method of the present embodiment includes a first conductivity type silicon substrate, a light receiving surface side diffusion layer of a conductivity type opposite to the first conductivity type formed on the light receiving surface side of the silicon substrate, and a light receiving surface side. A dielectric film made of silicon nitride is formed on the diffusion layer except for a contact portion with the electrode, and includes a light receiving surface electrode electrically connected to the light receiving surface side diffusion layer, and a back surface side of the silicon substrate. A backside diffusion layer of the same conductivity type as the first conductivity type formed at least in part, and a dielectric film made of silicon nitride is formed on the backside diffusion layer, excluding the contact portion with the electrode, and the backside diffusion A method of manufacturing a solar cell including a back electrode electrically connected to a layer, and before forming a dielectric film made of a silicon nitride film, a solution in which a halogen element is dissolved in a solvent is applied to the surface of the silicon substrate. Characterized by treating the surface .

  Hereinafter, although one Embodiment of the manufacturing method of the solar cell of this invention is described using drawing, this invention is not a thing limited by this description.

  1 (A) to 1 (F) are schematic cross-sectional views showing a manufacturing process of one embodiment in the method for manufacturing a solar cell of the present invention. Hereinafter, each step will be described in detail.

(1) An n-type substrate is used as the silicon substrate 1 [FIG. 1 (A)]. This silicon single crystal substrate may be produced by any of the Czochralski (CZ) method and the float zone (FZ) method. The specific resistance of the silicon substrate 1 is preferably 0.1 to 20 Ω · cm, and more preferably 0.5 to 2.0 Ω · cm from the viewpoint of producing a high-performance solar cell. As the silicon substrate 1, a phosphorus-doped n-type single crystal silicon substrate is preferable. The dopant concentration of the phosphorus dope is preferably 1 × 10 ¹⁵ cm ^{−3 to} 5 × 10 ¹⁶ cm ⁻³ .

(2) Damage etching / texture formation For example, the silicon substrate 1 is immersed in an aqueous sodium hydroxide solution, and the damaged layer is removed by etching. For removing damage from the substrate, a strong alkaline aqueous solution such as potassium hydroxide may be used, and a similar purpose can be achieved with an acid aqueous solution such as hydrofluoric acid. A random texture is formed on the silicon substrate 1 subjected to damage etching. In general, a solar cell preferably has an uneven shape on the surface. This is because it is necessary to cause the light receiving surface to perform reflection at least twice as much as possible in order to reduce the reflectance in the visible light region. The size of each concavo-convex shape is preferably about 1 to 20 μm. Typical surface uneven structures include V-grooves and U-grooves. These can be formed using a grinding machine. Moreover, a random uneven structure can be made by dipping in an aqueous solution obtained by adding isopropyl alcohol to sodium hydroxide and performing wet etching. In addition, a random concavo-convex structure can be formed by using acid etching, reactive ion etching, or the like. In the drawing, the texture structure formed on both sides is omitted because it is fine.

(3) Formation of n-type diffusion layer When the silicon substrate 1 is n-type, the n-type diffusion layer 3 is formed on the back surface by performing a heat treatment after applying a coating agent containing a dopant on the back surface [FIG. ]. After the heat treatment, the glass component attached to the silicon substrate 1 is washed by glass etching or the like. The dopant is preferably phosphorus. The surface dopant concentration of the n-type diffusion layer 3 is preferably 1 × 10 ¹⁸ cm ^{−3 to} 5 × 10 ²⁰ cm ^{−3 and} more preferably 5 × 10 ¹⁸ cm ⁻³ to 1 × 10 ²⁰ cm ⁻³ .

(4) Formation of p-type diffusion layer A similar process is performed on the light-receiving surface to form the p-type diffusion layer 2 over the entire light-receiving surface [FIG. 1 (C)]. The p-type diffusion layer 2 is formed by vapor phase diffusion with BBr ₃ by combining the back surfaces on which the n-type diffusion layer 3 is formed. The dopant is preferably boron, and the surface dopant concentration of the p-type diffusion layer 2 is preferably 1 × 10 ¹⁸ cm ^{−3 to} 5 × 10 ²⁰ cm ^{−3 and} more preferably 5 × 10 ¹⁸ cm ⁻³ to 1 × 10. ²⁰ cm ⁻³ is more preferable.

(5) Pn junction isolation Pn junction isolation is performed using a plasma etcher. In this process, the sample is stacked so that plasma and radicals do not enter the light-receiving surface and the back surface, and the end surface is cut by several microns in that state. After bonding and separation, glass components, silicon powder, and the like attached to the silicon substrate 1 are washed by glass etching or the like.

(6) Application of Halogen Element-Containing Solution Subsequently, a solution in which a halogen element is dissolved in a solvent is applied on the p-type diffusion layer 2, that is, the light receiving surface of the silicon substrate 1 to inactivate the surface of the silicon substrate 1 [ FIG. 1 (D)]. That is, the substrate surface 4 on the p-type diffusion layer 2 shown in the figure is treated with a solution in which a halogen element is dissolved in a solvent.

  Examples of the solvent used here include alcohols such as methanol, ethanol, n-propanol, isopropanol and n-butanol, ethers such as diethyl ether, tetrahydrofuran and dioxane, aromatic hydrocarbons such as benzene and toluene, dimethyl sulfoxide, Examples thereof include polar solvents such as N, N-dimethylformamide, N, N-dimethylacetamide, formamide, N-methylformamide, N-methylpyrrolidone, ethylene carbonate, propylene carbonate, and potassium iodide solution. Among these, alcohols are more preferable.

As the concentration to be diluted, the halogen element concentration is preferably in the range of 0.001 mol / l to 0.1 mol / l. If the halogen element concentration is too low, the surface of the silicon substrate 1 cannot be sufficiently inactivated. Further, if the halogen element concentration is too high, the productivity may be deteriorated.
The halogen element used here is preferably iodine from the viewpoint of easy handling.
Further, the method for applying the halogen element-containing solution to the surface of the silicon substrate 1 is not particularly limited, such as a spin coating method, a spray coating method, a dip coating method, and a roll coating method, but the spin coating method is simple and preferable. is there. The halogen element contained in the solution applied in this step terminates dangling bonds on the treated substrate surface 4 and also acts as a negative charge to improve the passivation effect on the p-type diffusion layer 2.

(6) Dielectric Film Formation Subsequently, as shown in FIG. 1E, a silicon nitride film as the dielectric film 5 is deposited on the n-type diffusion layer 3 and the p-type diffusion layer 2 using a CVD apparatus. This film thickness is preferably 70 to 100 nm. Other antireflection films include a titanium dioxide film, a zinc oxide film, a tin oxide film, a tantalum oxide film, a niobium oxide film, a magnesium fluoride film, and an aluminum oxide film, which can be substituted. In addition to the above, the formation method includes a remote plasma CVD method, a coating method, a vacuum deposition method, and the like. From the economical viewpoint, it is preferable to form the silicon nitride film by the plasma CVD method.

(7) Electrode formation Using a screen printing device or the like, a paste made of, for example, silver is printed on the p-type diffusion layer 2 and the n-type diffusion layer 3 using a screen printing device on the light-receiving surface side and the back surface side, and comb-shaped Apply to electrode pattern and dry. When using a p-type for a silicon substrate, the paste which mixed Al powder with the organic binder on the back side is screen-printed and dried. Finally, firing is performed at 500 to 900 ° C. for 1 to 30 minutes in a firing furnace, and the finger electrode 6, the back electrode 7, and the bus bar electrode 8 that are electrically connected to the p-type diffusion layer 2 and the n-type diffusion layer 3 are provided. Form [FIG. 1 (F)].

  In FIG. 1 (F), the finger electrode 6 and the back electrode 7 are shown not connected to the p-type diffusion layer 2 and the n-type diffusion layer 3, respectively. Connected.

  As described above, before the dielectric film 5 is formed, a solution in which a halogen element is dissolved in a solvent is applied on the p-type diffusion layer 2 and the substrate surface 4 is treated, so that the halogen element is dangling bonded. At the same time, carrier recombination on the surface of the silicon substrate 1 is reduced by the effect of negative charges, and as a result, a solar cell with high conversion efficiency can be manufactured.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated concretely, this invention is not restrict | limited to the following Example.

[Example 1]
Crystal plane orientation (100), 15.65 cm square 200 μm thickness, as slice specific resistance 2 Ω · cm (dopant concentration 7.2 × 10 ¹⁵ cm ⁻³ ) Phosphorus-doped n-type single crystal silicon substrate is immersed in an aqueous sodium hydroxide solution The damaged layer was removed by etching, and texture formation was performed by soaking in an aqueous solution obtained by adding isopropyl alcohol to an aqueous potassium hydroxide solution and performing alkali etching. After applying the coating agent containing a phosphorus dopant to the back surface of the obtained silicon substrate 1, heat treatment was performed at 900 ° C. for 1 hour to form the n-type diffusion layer 3 on the back surface. After the heat treatment, the glass component attached to the substrate was removed by high concentration hydrofluoric acid solution and then washed.

Subsequently, the back surfaces of the silicon substrate on which the n-type diffusion layer 3 was formed were put together to perform gas phase diffusion with BBr ₃ to form the p-type diffusion layer 2 over the entire light receiving surface.

  Next, pn junction isolation was performed using a plasma etcher. In order to prevent plasma and radicals from entering the light-receiving surface and back surface, the end face was cut several microns with the target stacked. The glass component attached to the substrate was removed with a high-concentration hydrofluoric acid solution and then washed.

  Subsequently, an iodine methanol solution having an iodine concentration of 0.05 mol / l was applied onto the p-type diffusion layer 2 by spin coating, and surface treatment was performed.

  Next, using a parallel plate type CVD apparatus, a mixed gas of monosilane, ammonia and hydrogen is used as a film forming gas, and dielectric is formed on the p-type diffusion layer 2 on the light-receiving surface side and the n-type diffusion layer 3 on the back surface. A silicon nitride film as the body film 5 was laminated. This film thickness was 70 nm.

  Subsequently, a silver paste was electrode-printed on each of the light-receiving surface side and the back surface side, dried, and then fired at 800 ° C. for 3 minutes to form the finger electrode 6, the back electrode 7, and the bus bar electrode 8.

[Comparative Example 1]
Before the silicon nitride film was formed, an iodine methanol solution was applied onto the p-type diffusion layer 2 by a spin coating method, and the same process as in Example 1 was performed except that the surface treatment step was omitted.

The current-voltage characteristics of the solar cells obtained in Examples and Comparative Examples were measured in a 25 ° C. atmosphere under a solar simulator (light intensity: 1 kW / m ² , spectrum: AM1.5 global). The results are shown in Table 1. In addition, the number in a table | surface is an average value of ten cells made as an experiment in an Example and a comparative example.

  As described above, in the solar cell of Example 1, the iodine-containing solution was applied on the p-type diffusion layer 2 and surface-treated before the dielectric film 5 was formed, so that the dangling bonds on the substrate surface with iodine were terminated. Due to the negative charge of iodine, a solar cell with high conversion efficiency and excellent passivation effect was obtained.

1 silicon substrate 2 p-type diffusion layer 3 n-type diffusion layer 4 substrate surface treated with a solution in which a halogen element is dissolved in a solvent 5 dielectric film (antireflection film)
6 Light-receiving surface electrode (finger electrode)
7 Back electrode 8 Bus bar electrode

Claims (8)

Forming a light-receiving surface side diffusion layer that has a conductivity type opposite to the first conductivity type on the light-receiving surface side of the first conductivity type silicon substrate;
After the step of forming the light receiving surface side diffusion layer, a step of applying a solution in which a halogen element is dissolved in a solvent to the silicon substrate to treat the surface of the silicon substrate;
After the step of processing the surface of the silicon substrate, a step of forming a dielectric film serving as an antireflection film on the light-receiving surface side diffusion layer;
And a step of forming a light receiving surface electrode electrically connected to the light receiving surface side diffusion layer after the step of forming the dielectric film. Further comprising forming a back side diffusion layer of the same conductivity type as the first conductivity type on the back side opposite to the light receiving surface of the silicon substrate;
In the step of forming the dielectric film, the dielectric film is formed on the back surface side diffusion layer in addition to the light receiving surface side diffusion layer,
The method for manufacturing a solar cell according to claim 1, wherein the step of processing the surface of the silicon substrate is performed after the step of forming the back-side diffusion layer.   3. The method of manufacturing a solar cell according to claim 1, wherein a concentration of the halogen element in a solution in which the halogen element is dissolved in a solvent is 0.001 to 0.1 mol / l.   The method for manufacturing a solar cell according to any one of claims 1 to 3, wherein a solution in which the halogen element is dissolved in a solvent contains an organic solvent or water.   The method for manufacturing a solar cell according to any one of claims 1 to 4, wherein the halogen element is iodine.   The method for manufacturing a solar cell according to claim 1, wherein the light receiving surface side diffusion layer is a p-type diffusion layer in the silicon substrate.   The method for manufacturing a solar cell according to claim 6, wherein in the step of treating the surface of the silicon substrate, a solution in which a halogen element is dissolved in a solvent is applied only to the light receiving surface of the silicon substrate.   The solar cell manufactured by the manufacturing method of the solar cell as described in any one of Claim 1 to 7.