JPH0823111A - Solar cell - Google Patents

Abstract

PURPOSE:To provide a solar cell having an improved cell conversion efficiency by eliminating the injection of electrons to the phase boundary level below an oxide film on the surface. CONSTITUTION:In a solar cell having an n-type regions 2 and 2a and an oxide film 6 formed on a surface as a light receiving surface for a p-type Si substrate 1, the carrier concentration of n-type region 2a in the vicinities of cell end face is characteristically lower than the carrier concentration of n<+> type region 2 forming the main junction inside the cell in this solar cell.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon (hereinafter referred to as "Si" symbol) solar cell, and more particularly to a solar cell structure having improved photoelectric conversion efficiency.

[0002]

2. Description of the Related Art In recent years, solar cells are regarded as important candidates for clean energy sources in the near future, and their development and structure are being spurred. In particular, a solar cell using a Si single crystal wafer as a substrate is expected to have higher conversion efficiency than other materials and is excellent in economic efficiency. Therefore, it is possible to improve efficiency by various approaches regardless of whether it is for the ground or space. Research and development for.

FIG. 5 shows a schematic cross-sectional structure of a typical conventional solar cell (hereinafter simply referred to as a cell), and a conventional high efficiency technology will be described below. FIG. 5A shows the most basic cell structure. An n ⁺ layer 2 formed by thermally diffusing impurities such as phosphorus (P) on one surface (light-receiving surface side) of the p-type Si substrate 1 and boron (B) on the other surface (back surface side) that faces the n ⁺ layer 2.
Has a p ⁺ layer 3 in which impurities such as the like are thermally diffused, a comb-shaped n-side electrode 4 is formed on the surface of the n ⁺ layer, and a p-side electrode 5 is formed on almost the entire back surface thereof, and is reflected on the light receiving surface. The prevention film (not shown) was coated on the entire surface.

5 (b), 5 (c), 6 (a), 6 (b)
5 shows variously improved cell structures for increasing the efficiency of the cell having the structure of FIG. 5A (hereinafter abbreviated as basic cell). FIG. 5B is characterized in that the n ⁺ layer 2 has a so-called planar structure formed inside the end face of the substrate 1. Since the pn junction (main junction) that largely controls the output characteristics of the basic cell appears on the end surface of the cell, there is a problem that the main junction is damaged when the cell is cut and separated from the wafer into a predetermined shape. Was there. Further, there is a problem in reliability such as adhesion of water vapor and the like on the end face, leak occurrence due to formation of products and the like, and leak occurrence due to end face damage during cell handling.

The cell shown in FIG. 5 (b) solves these problems by forming a main junction from the end face of the cell to the inside. The oxide film 6 is a mask for forming the main junction by diffusion and has a width of about several tens of μm. Figure 5
As shown in (b), the oxide film 6 may be left finally or may be removed by etching.

FIG. 5C is characterized in that the surface is passivated. In the basic cell and the cell of FIG. 5B, the Si surface is exposed as the light receiving surface. However, since many dangling bonds exist on the Si surface, some of the carriers (holes) photoexcited in the n ⁺ layer do not flow to the main junction side and are trapped on the surface and recombined.
There was a problem that it could not be taken out as photocurrent. In the cell of FIG. 5C, a thin oxide film 6 is formed on the entire light receiving surface to inactivate the surface, and the surface recombination rate S _F on the light receiving surface is reduced to suppress this effect.

FIG. 6 (a) is characterized in that back surface passivation is further applied. In the cell of FIG. 5 (c),
Some of the carriers (electrons) that absorb light of a relatively long wavelength and are photoexcited in the p-type substrate do not flow to the main junction side, are captured on the back surface of the cell and recombine, and cannot be taken out as a photocurrent. Had a problem. In the cell of FIG. 6A, a thin oxide film 7 is once formed on the entire back surface, and the p ⁺ layer 3a is formed by diffusing impurities from a window partially provided in this oxide film (partial). P ⁺ region 3a). As a result, the surface recombination rate S _R on the back surface was reduced, and the above effect was suppressed.

FIG. 6B is further characterized in that a non-reflection surface shape is formed on the light receiving surface. That is, the above cell has a problem that a considerable part of incident light is emitted to the outside of the cell due to surface reflection and is not photoelectrically converted. The cell of FIG. 6 (b) has an uneven shape formed on the surface by anisotropic etching, and the reflection loss is reduced by multiple reflection between the uneven shapes.

[0009]

However, in spite of various improvements as described above, the current cell conversion efficiency is not sufficiently high at about 22% for ground use and about 17% for space use. Had a problem. The inventors conducted various studies and considerations regarding the effect of surface recombination among the above techniques. For the relationship between the surface recombination rate and the conversion efficiency, see, for example, “Computerized Silicon Technology
II ”(edited by Ryoichi Yamamoto, Kaibundou Publishing Co., Ltd.) 161 shows the result of the computer simulation, and it is shown that the conversion efficiency of 24% or more can be expected under the ground light when the surface recombination velocity is reduced to 10 ² to 10 ³ cm / sec. (P. 161, FIG. 7.11), the current cell has not reached that level.

Further, the inventors of the present invention measured the CV characteristics of the Si / SiO ₂ MOS element formed under the same conditions as the cell manufacturing process, and found that the interface between the p-Si (substrate) and the oxide film and the n-Si.
It was found that a depletion layer was formed at each interface between the (diffusion layer) and the oxide film. Therefore, when the solar cell is irradiated with light, photoexcited holes are accumulated at the interface between the n-Si and the oxide film (shown in FIG. 4A), and photoexcited at the interface between the p-Si and the oxide film. Accumulation of generated electrons occurs (Fig. 4 (b)).
Shown in). In the cells of FIGS. 5 (b) to 6 (b) having the conventional structure, the n-type region and the p-type region are adjacent to each other immediately below the oxide film on the surface, so that the interfacial level existing at the interface Have revealed that photoexcited holes and electrons are easily recombined.

Since this recombination occurs at the interface between Si and the oxide film, even if the oxide film is formed on the Si surface for the purpose of surface passivation, the surface recombination cannot be substantially suppressed.
There is a problem that the number of carriers flowing to the main junction is reduced and the output characteristics of the cell are degraded.

The present invention has been made on the basis of the above problems and consideration. The object of the present invention is to eliminate the injection of electrons into the interface state below the surface oxide film and improve the conversion efficiency of the cell. To provide a solar cell that does.

[0013]

In order to solve the above problems, the present invention has the following constitution. That is, the present invention is p
In a solar cell in which an n-type region and an oxide film are formed on the surface serving as a light-receiving surface of a type silicon (Si) substrate, the carrier concentration in the n-type region around the cell end face is an n ⁺ -type that forms a main junction inside the cell. The solar cell is characterized by having a carrier concentration lower than that of the region.

Further, in the present invention, n around the cell end face is
The depth from the surface of the type region can be made shallower than the depth of the n ⁺ type region forming the main junction. Further, in the present invention, the depth from the surface of the n-type region around the cell end face is deeper than the depth of the n ⁺ -type region forming the main junction, and at least a part of the cross section including the main junction of the solar cell serves as the light receiving surface. Can be an n ⁺ -n-p-p ⁺ structure sequentially from the surface side.

[0015]

In the present invention having the above-described structure, the region other than the n ⁺ region immediately below the surface oxide film is made an n-type region,
Since the injection of electrons into the interface state under the surface oxide film can be eliminated, carrier recombination under the interface can be suppressed, and the output characteristics of the cell can be improved.

[0016]

Next, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing the structure of an embodiment of the solar cell according to the present invention. FIG. 1A shows the first
It is sectional drawing which shows the cell structure of an Example. In the figure,
On the light-receiving surface side of the p-type Si substrate 1, for example, an impurity such as phosphorus (P) is diffused to form n ⁺ which forms the main junction of this cell.
Although the region 2 is formed, an n region 2a having an impurity concentration lower than that of the n ⁺ region 2 is formed in a range of several tens of μm from each end face of the cell.

The carrier concentration of the n ⁺ region 2 of the first embodiment is, for example, about 1 × 10 ^{19 to} 5 × 10 ²⁰ / cm ³ ,
The carrier concentration of the n region 2a is lower than this and is 1 × 10 ¹⁶ to
It is about 5 × 10 ¹⁸ / cm ³ . The width of the n region 2a (distance from the cell end face) is about several μm to 100 μm. The depth x _j2 of the n region 2a may be shallower or deeper than the depth x _j1 of the n ⁺ region 2.

On the back surface side of the p-type Si substrate 1, there is a p ⁺ layer 3 in which impurities such as boron (B) are diffused, and a comb-shaped n-side electrode 4 is formed on the light receiving surface side surface. The p-side electrode 5 is formed on almost the entire surface of the device, and an antireflection film (not shown) is formed on the entire surface of the light receiving surface.

FIG. 1B is a sectional view showing the cell structure of the second embodiment. In the structure of the present embodiment, the depth x _j2 of the n region 2a is deeper than the depth x _j1 of the n ⁺ region 2, and the structure including the main junction becomes n ⁺ -n-p-p ^{+ in} order from the light receiving surface side. It is characterized by

In the cell of the first embodiment, the main junction is n ⁺ p.
The cell according to the second embodiment has a main junction of the np structure, but has a back surface on each of the structures. It is characterized in that the photogenerated carriers are intended to flow toward the main junction by forming an electric field (Buck surface field). The other parts are similar to those of the cell of the first embodiment.

FIG. 1C is a sectional view showing the cell structure of the third embodiment. The present embodiment is characterized in that the front surface side structure of the cell of the first embodiment is combined with the back surface passivation technique described with reference to FIG. FIG. 2A is a sectional view showing the cell structure of the fourth embodiment. The present embodiment is characterized in that the front surface side structure of the cell of the second embodiment is combined with the back surface passivation technique described with reference to FIG.

FIG. 2B is a sectional view showing the cell structure of the fifth embodiment. The present embodiment is characterized in that the antireflection structure described in FIG. 6B is formed on the light receiving surface side surface of the cell of the third embodiment. FIG. 2C is a sectional view showing the cell structure of the sixth embodiment. The cell structure of the present embodiment is characterized in that the antireflection structure described in FIG. 6B is formed on the light-receiving surface side surface of the cell of the fourth embodiment.

Next, as a typical cell of the present invention, FIG.
An example of a method of manufacturing the cell of the fifth embodiment shown in FIG.
This will be described with reference to the process order cross-sectional views. First, a desired non-reflective surface shape is formed on the (100) surface of the Si-p type wafer 1 (FIG. 3A). This can be easily formed by anisotropic etching using an alkaline solution such as potassium hydroxide. In this example, the surface electrode forming portion has a shape in which it is left flat, but the shape is not limited to this. At this time, the back surface is protected by, for example, coating with an alkali-resistant resin or applying a SiO ₂ film by a CVD method (FIG. 3B).

Next, the n region 2a is formed by thermal diffusion using, for example, phosphine (PH ₃ ) as a diffusion source. On this occasion,
The carrier concentration of the n region 2a is set to n of the main junction formed later.
⁺ Set lower than carrier concentration in region 2. For example, 1
It is approximately 10 ^{16 to} 5 × 10 ¹⁸ / cm ³ . The diffusion depth of the n region 2a is set to be shallower than the diffusion depth of the n ⁺ region 2 and is, for example, about 0.05 to 0.1 μm. Further, in the case of forming by thermal diffusion, the n region is also formed on the back surface 2b and side surfaces (not shown) of the wafer, so this is removed by etching. In order to save this trouble, the n region 2a may be formed by ion implantation.

Next, after SiO ₂ is entirely deposited on the front surface and the back surface of the wafer by the CVD method, a window is opened in the main junction forming portion on the front surface side. Well-known photolithography is used for this (FIG.3 (d)).

Next, for example, phosphorus oxychloride (POC)
The n ⁺ region 2 is formed by a thermal diffusion method using 13 ₃ ) as a diffusion source, and then SiO ₂ on the wafer is once removed (FIG. 3E). The carrier concentration of the n ⁺ region 2 is, for example, about 1 × 10 ^{19 to} 5 × 10 ²⁰ / cm ³ , and the diffusion depth is about 0.1 to 0.5 μm.

Next, SiO ₂ films 6 and 7 having a desired thickness are newly formed on both surfaces of the wafer by a thermal oxidation method (FIG. 3).
(F)). The thickness may be adjusted by etching after forming the oxide film. Next, a window of an oxide film having a desired shape is formed in the p ⁺ region forming portion on the back surface, for example, boron tribromide (BBr ₃ ).
The p ⁺ region 3a is formed by using thermal diffusion or ion plantation method with the diffusion source as a diffusion source.

Next, for example, a comb-shaped electrode 4 is formed on the front surface side by the well-known photolithography and vacuum deposition method (FIG. 3).
(H)). The electrode 4 contains Ag as a main component and has a total thickness of about 5 μm. Next, the electrode 5 is formed on the entire back surface by vacuum evaporation (FIG. 3 (i)). The electrodes are, for example, Al,
It is a stack of layers containing Ag as the main component, and the total thickness is about 5μ.
m.

After that, through a desired heat treatment step, an antireflection film is applied to the surface and then cut and separated into desired dimensions (not shown above) to complete a cell.

[0030]

As described above, according to the present invention, the recombination of photoexcited carriers generated at the interface of the surface oxide film can be prevented, so that the original effect of surface passivation is exhibited and the output characteristics of the solar cell are improved. Contribute. As a result, the conversion efficiency of the solar cell can be expected to be improved by at least 2% or more, and for example, under the terrestrial light, the conversion efficiency of 24% or more can be achieved.

According to the present invention, the pn junction is exposed on the end face of the cell, but it is experimentally confirmed that the above-mentioned junction damage and reliability problems at the time of separation and cutting are significantly smaller than those of the conventional cell. Was done. This is because the junction exposed at the cell end face is a pn junction (n ⁺ region 2 and p that forms the main junction of the cell).
It is considered that it is not the (type substrate) but the junction of the added n region 2a and the p-type substrate. As described above, the structure of the solar cell according to the present invention remarkably contributes to the high efficiency of the solar cell and has a great effect on the related industries.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing a cell structure of an embodiment of a solar cell according to the present invention, and (a) to (c) respectively show the embodiment.

FIG. 2 is a schematic cross-sectional view showing a cell structure of an embodiment of a solar cell according to the present invention, and (a) to (c) respectively show the embodiment.

FIG. 3 is a step-by-step cross-sectional view for explaining the manufacturing method of the cell of the fifth embodiment (FIG. 2 (b)) which is an example of the solar cell of the present invention.

FIG. 4 is an energy band diagram for explaining the rationale for the present invention, where (a) is an interface between n-type Si and an oxide film, and (b) is a diagram.
Indicates the state of the interface between p-type Si and the oxide film.

FIG. 5 is a schematic cross-sectional view showing a cell structure of a conventional solar cell, and FIGS. 5 (a) to 5 (c) are views for explaining various conventional techniques for increasing the efficiency of the cell.

FIG. 6 is a schematic cross-sectional view showing a cell structure of a conventional solar cell, and FIGS. 6 (a) and 6 (b) are diagrams for explaining various conventional techniques for increasing the efficiency of the cell.

[Explanation of symbols]

1 p-type Si substrate 2 n ⁺ layer 2a n region 3 p ⁺ layer 3a partial p ⁺ region 4 n-side electrode 5 p-side electrode 6 oxide film 7 oxide film

Claims (3)

[Claims] 1. In a solar cell in which an n-type region and an oxide film are formed on a surface serving as a light-receiving surface of a p-type silicon (Si) substrate, the carrier concentration in the n-type region around the cell end face is the main junction inside the cell. A solar cell characterized by having a carrier concentration lower than that of an n ⁺ type region forming the. 2. The solar cell according to claim 1, wherein the depth from the surface of the n-type region around the cell end face is shallower than the depth of the n ⁺ -type region forming the main junction. 3. The depth from the surface of the n-type region around the cell end face is deeper than the depth of the n ⁺ -type region forming the main junction, and at least a part of the cross section including the main junction of the solar cell serves as the light-receiving surface. The solar cell according to claim 1, wherein the solar cell has an n ⁺ -n-p-p ⁺ structure sequentially from the front surface side.