JP2000332279A - Manufacture of solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode structure exhibiting sufficient adhesive strength, even when the front side electrode of the solar battery has a small width. SOLUTION: In a method of manufacturing a solar battery, wherein electrodes 1e and 1f are adhered to both main surfaces by diffusing, on one main surface of one conductivity type polycrystalline silicon substrate 1, a reverse conductivity type impurity 1b, parts of such one or both main surfaces of the substrate 1 for adhesion to the electrodes 1e and 1f are provided with a multitude of fine projections la formed by reactive ion etching, to have the electrodes 1e adhered and formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a solar cell, and more particularly to a method for manufacturing a solar cell using a polycrystalline silicon substrate.

[0002]

2. Description of the Related Art In recent years, as energy consumption has increased, securing an energy source has been regarded as important. At present, most of the energy depends on thermal power and nuclear power. However, thermal power generation has a problem of global warming due to carbon dioxide, which is a major problem. In nuclear power generation, the possibility of radioactive contamination at the time of an accident and the problem of radioactive waste disposal methods have been pointed out, and from a long-term perspective, there are global environmental issues. Under such circumstances, solar cells that directly convert solar energy into electric energy have been highlighted in recent years because of their non-polluting nature, and the importance of mass production technology and cost reduction and high efficiency technology is increasing. Is growing.

[0003] Main solar cells are classified into crystalline, amorphous, and compound solar cells according to the type of materials used. Among them, most of them currently available in the market are crystalline silicon solar cells. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. The single crystal silicon solar cell has the advantage that the efficiency of the substrate is easy because of the good quality of the substrate, but has the disadvantage that the manufacturing cost of the substrate is large. On the other hand, the polycrystalline silicon solar cell has a disadvantage that it is difficult to increase the efficiency due to the poor quality of the substrate, but has an advantage that it can be manufactured at low cost. In recent years, a conversion efficiency of about 18% has been achieved at the research level due to the improvement in quality of polycrystalline silicon substrates and advances in cell technology.

On the other hand, mass-produced polycrystalline silicon solar cells have been distributed to the market because of their lower cost than before. High conversion efficiency has been required.

[0005] One elemental technology for increasing the efficiency of polycrystalline silicon solar cells is to increase the light receiving area. As a result, since a large amount of sunlight can be taken in, an improvement in output current can be expected. For this purpose, it is necessary to reduce the area occupied by the electrodes on the light receiving surface side, and a technique for forming fine electrodes is required. As a method of forming fine electrodes, a method using photolithography is widely known, but this method is very expensive to use for a solar cell and is difficult in practice. Conventionally, mass-produced solar cells have been formed by screen-printing and firing a conductive paste whose main component is silver powder. Making the electrodes thinner increases the electrical resistance and leads to a decrease in the current value. In order to avoid this, an electrode having a small area and a high height, that is, an electrode having a large aspect ratio is required. Conventionally, the width of the electrode was about 200 μm. However, in order to achieve higher efficiency, it is necessary to further reduce the size, and the contact area with the substrate is accordingly reduced, so that the bonding strength of the electrode is reduced. There is a problem.

As a means for improving the bonding strength of the electrodes, an increase in the contact area can be considered. Further, the tensile strength of the electrode changes not only according to the surface area per unit of the substrate to be adhered but also according to the angle of close contact. For example, the electrodes are less likely to peel off when pulled obliquely than when pulled vertically. This means that it is difficult to peel off when the angle of the portion where the electrode and the substrate are in close contact is closer to the direction parallel to the pulling direction.

In the case of a single crystal substrate, a texture structure is generally formed with an aqueous alkali solution or the like in order to reduce the reflection of incident light. However, if this structure is used as it is, the surface area of the substrate to be adhered is increased. As a result, the contact area with the electrode also increases, and the bonding strength of the electrode improves.

In the case of single crystal silicon, it is usually (100)
Oriented substrates are used. When this is etched with an alkaline aqueous solution, the (111) plane is exposed, and a pyramid having a projection apex angle of 70.5 ° is formed. If the aspect ratio of this pyramid is defined by (the height of the pyramid / the bottom), it is 0.71. However, this unevenness may not be able to obtain sufficient electrode bonding strength.

Therefore, there has been a demand for an electrode underlayer structure having a larger aspect ratio. That is, there has been a demand for a base structure of an electrode having a large surface area per unit, a large contact area, and a contact angle closer to the vertical to the solar cell surface.

However, in the above-described wet etching using an alkaline aqueous solution, since irregularities (pyramids) are formed using the plane orientation of the crystal of the substrate to be etched, the irregularities cannot be controlled.

In the case of a polycrystalline substrate, a texture structure as in a single crystal can hardly be formed due to its various plane orientations.

The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a method for forming an electrode having a sufficient bonding strength even when the width of a surface electrode of a solar cell is reduced. I do.

[0013]

In order to achieve the above object, a method for manufacturing a solar cell according to the present invention comprises diffusing impurities of another conductivity type into one principal surface of a polycrystalline silicon substrate of one conductivity type. In a method for manufacturing a solar cell in which electrodes are formed on both principal surfaces, fine projections are formed on one principal surface or on both principal surfaces of the polycrystalline silicon substrate by reactive ion etching. Are formed, and the electrodes are formed by deposition.

In the method for manufacturing a solar cell, it is desirable that the width and height of the fine projections be 2 μm or less.

In the method for manufacturing a solar cell, it is preferable that the aspect ratio of the fine projection is 0.75 to 2 or less.

[0016]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a sectional view showing the structure of a solar cell manufactured by the method for manufacturing a solar cell according to the present invention. In FIG. 1, reference numeral 1 denotes a polycrystalline silicon substrate;
a is a fine protrusion, 1b is a light-receiving surface side impurity diffusion layer, 1c is a back surface side impurity diffusion layer (BSF), 1d is a surface antireflection film, 1e is a front surface electrode, and 1f is a back surface electrode.

The polycrystalline silicon substrate 1 is of p-type or n-type.
Any of the types may be used. This polycrystalline silicon substrate 1 is formed by a casting method or the like. Polycrystalline silicon can be mass-produced and is extremely advantageous over single crystal silicon in terms of manufacturing cost. An ingot formed by casting or the like is sliced to a thickness of about 300 μm,
It is cut into a size of about 10 cm or 15 cm × 15 cm to form a silicon substrate.

Fine projections 1a are formed at least on one principal surface side of the polycrystalline silicon substrate 1 below the electrodes 1e.
The fine projections 1a have a conical shape or a continuous shape, and are formed by reactive ion etching (RI).
The size can be changed by controlling the gas concentration or the etching time by the method E).
Each of the fine projections 1a is formed to have a width and a height of 2 μm or less. In order to form the fine projections 1a uniformly and accurately with controllability over the entire required portion of the silicon substrate 1, the thickness is preferably 1 μm or less. The aspect ratio (width / height of the projection 1a) of the fine projection 1a is
It is desirably 2 or less. If the aspect ratio is 2 or more, the fine projections 1a are damaged during the manufacturing process, and when a solar cell is formed, the leakage current becomes large and good output characteristics cannot be obtained. When the aspect ratio is less than 0.75, only the same bonding strength as that of the conventional product can be obtained.

In the reactive ion etching method, for example, methane trifluoride (CHF ₃ ) is added at 12.00 sccm.
About 72 sccm of chlorine (Cl ₂ ), about 9 sccm of oxygen (O ₂ ), and sulfur hexafluoride (SF).
₂ ) While flowing about 65 sccm, the reaction pressure was 50 mT
This is performed for about 10 seconds to 15 minutes at about orr and RF power of about 500 W for applying plasma.

On the surface side of the polycrystalline silicon substrate 1, a layer 1b in which a semiconductor impurity of the opposite conductivity type is diffused is formed. The layer 1b in which the opposite conductivity type semiconductor impurity is diffused is
This is provided to form a semiconductor junction in the polycrystalline silicon substrate 1. For example, when an n-type impurity is diffused, a gas phase diffusion method using POCl _{3 or} a coating diffusion method using P ₂ O ₅ , And an ion implantation method for directly diffusing P ⁺ ions. The layer 1 containing the opposite conductivity type semiconductor impurity is formed at a depth of about 0.3 to 0.5 μm.

On one main surface side of the polycrystalline silicon substrate 1, an antireflection film 1d is formed. This antireflection film 1d is provided to prevent light from being reflected on the surface of the polycrystalline silicon substrate 1 and to effectively take light into the polycrystalline silicon substrate 1. This antireflection film is made of a material having a refractive index of about 2 in consideration of a refractive index difference from the polycrystalline silicon substrate 1 and the like, and has a silicon nitride film or a silicon oxide (SiO ₂ ) film having a thickness of about 500 to 2000 °. Etc.

On the other main surface side of the polycrystalline silicon substrate 1,
It is desirable to form a layer 1c in which one-conductivity-type semiconductor impurity is diffused at a high concentration. The layer 1c in which the one-conductivity-type semiconductor impurity is diffused at a high concentration forms an internal electric field on the back surface side of the polycrystalline silicon substrate 1 in order to prevent a decrease in efficiency due to carrier recombination near the back surface of the polycrystalline silicon substrate 1. To form. In other words, carriers generated near the back surface of the silicon substrate 1 are accelerated by this electric field, so that power is effectively extracted, and in particular, photosensitivity at long wavelengths increases and deterioration of solar cell characteristics at high temperatures is reduced. it can. As described above, the sheet resistance on the back surface side of the polycrystalline silicon substrate 1 on which the layer 1c in which the one conductivity type semiconductor impurity is diffused at a high concentration is formed is about 15Ω / □.

On one main surface of the polycrystalline silicon substrate 1, a surface electrode 1e is formed. The surface electrode 1e is made of, for example, A mainly composed of Ag powder, binder, frit, or the like.
g paste at 600-800 ° C by screen printing.
After baking for about 30 minutes, a solder layer is formed thereon to a thickness of about 10 to 40 μm. The surface electrode 1e has, for example, a width 2
It is composed of a large number of finger electrodes formed with a thickness of about 00 μm and a pitch of about 3 mm, and two bus bar electrodes interconnecting the large number of finger electrodes. This bus bar electrode is formed to have a width of about 2 mm and a thickness of about 10 μm.

As shown in FIG. 2, a copper foil coated with solder was applied to the solar cell electrode formed as described above by 2 mm.times.
A tensile test was performed in which the electrode was attached in an area of about 2 mm and pulled in the vertical direction, and the tensile strength, which is the strength when the electrode was peeled off, was measured. Projections having three different aspect ratios were formed by RIE. For comparison, a substrate in which a texture structure was formed on the surface side of the single crystal silicon substrate 1 with an alkaline aqueous solution was also prepared. Table 1 shows the results.

[0025]

[Table 1]

As can be seen from Table 1, it was confirmed that the tensile strength of the electrode was improved when the aspect ratio was large in a solar cell having fine projections on the surface side of the substrate 1 formed by RIE.

[0027]

As described above, according to the method for manufacturing a solar cell according to the present invention, the reactive ion etching method is applied to the portion of the polycrystalline silicon substrate to which the electrode is to be attached on one main surface or both main surfaces. Since a large number of fine projections are formed and the electrodes are formed, the surface area per unit of the base increases, and
Because the contact angle is closer to the perpendicular to the board surface,
The adhesion strength between the electrode and the substrate is increased, which contributes to the improvement in the yield of solar cell production and the reliability of the product.

[Brief description of the drawings]

FIG. 1 is a diagram showing a general structure of a solar cell according to the present invention.

FIG. 2 is a diagram showing a method for measuring the electrode strength of a solar battery cell according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate, 1a ... Surface uneven structure, 1b ... Impurity diffusion layer, 1c ... Backside diffusion layer, 1d ... Antireflection film, 1e
... front electrode, 1f ... back electrode, 2a ... front electrode,
2b: Copper foil with solder coating

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5F004 AA16 DA00 DA04 DA16 DA26 DB02 EA34 5F051 AA03 CB22 DA03 FA14 FA19 GA04 GA14

Claims (3)

[Claims] 1. A method for manufacturing a solar cell, comprising: diffusing impurities of another conductivity type on one main surface of a polycrystalline silicon substrate of one conductivity type to form electrodes on both main surfaces. A method for manufacturing a solar cell, comprising: forming a large number of fine projections by reactive ion etching on a portion where one of said main surfaces or both main surfaces is to be attached to said electrode; 2. The method for manufacturing a solar cell according to claim 1, wherein the width and height of the fine projections are 2 μm or less. 3. The fine projections have an aspect ratio of 0.7.
3. The method according to claim 1, wherein the number is from 5 to 2.
3. The method for manufacturing a solar cell according to 1.