JP3294005B2 - Solar cell - Google Patents

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") 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, solar cells using Si single crystal wafers as substrates can be expected to have higher conversion efficiency than other materials and are more economical, so they can be made more efficient by various approaches regardless of whether they are for ground or space use. Research and development has been done for

FIG. 5 shows a schematic cross-sectional structure of a typical conventional solar cell (hereinafter simply abbreviated as a cell), and a conventional high-efficiency technique will be described below. FIG. 5A is a diagram showing the most basic cell structure. An n ⁺ layer 2 in which impurities such as phosphorus (P) are thermally diffused on one surface (light receiving surface side) of a p-type Si substrate 1 and boron (B) on the other surface (back surface side).
Impurities etc. have p ⁺ layer 3 is thermally diffused, n ⁺ layer on the surface comb of the n-side electrode 4, the back surface p-side electrode 5 is formed on the almost entire surface, reflecting the light receiving surface The prevention film (not shown) was coated on the entire surface.

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

The cell shown in FIG. 5 (b) has solved 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 a main junction by diffusion, and has a width of about several tens μm. FIG.
As shown in (b), the oxide film 6 may be finally left 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 a 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 but are caught and recombined at the surface,
There was a problem that it could not be taken out as a 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 speed S _F on the light receiving surface is reduced to suppress this effect.

FIG. 6A is characterized in that the back surface is further passivated. In the cell of FIG.
Some of the carriers (electrons) that have absorbed light of a relatively long wavelength and have been photoexcited in the p-type substrate do not flow to the main junction side, are caught and recombined on the back surface of the cell, and cannot be extracted as photocurrent. Had the problem that 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 provided partially on this oxide film (partially). P ⁺ region 3a). As a result, the surface recombination speed S _R on the back surface was reduced, and the above effect was suppressed.

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

[0009]

However, despite the various improvements described above, the conversion efficiency of the current cell is not sufficiently high at about 22% for ground use and about 17% for space use. Had the problem that The inventors have conducted various studies and discussions on the effects of surface recombination among the above techniques. For the relationship between surface recombination rate and conversion efficiency, see, for example,
II "(Ryoichi Yamamoto, Kaibundo Shuppan Co., Ltd.) 161 shows the result of a computer simulation, which shows that if the surface recombination speed is reduced to 10 ² to 10 ³ cm / sec, a conversion efficiency of 24% or more can be expected under terrestrial light. (P.161, FIG. 7.11), the current cell has not reached that level.

The present inventors have measured the CV characteristics of a Si / SiO ₂ MOS device formed under the same conditions as those of 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 the interface between the (diffusion layer) and the oxide film. Therefore, when the solar cell is irradiated with light, photo-excited holes are accumulated at the interface between the n-Si and the oxide film (shown in FIG. 4A), and the photo-excitation occurs at the interface between the p-Si and the oxide film. The accumulated electrons are accumulated (FIG. 4B).
Shown). In the conventional structure shown in FIGS. 5B to 6B, since the n-type region and the p-type region are immediately adjacent to the surface immediately below the oxide film, the cells are formed through the interface state existing at the interface. As a result, it was revealed that the photoexcited holes and electrons easily recombine.

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

The present invention has been made based on the above problems and considerations, and has as its object to eliminate the injection of electrons into the interface state below the surface oxide film and to improve the conversion efficiency of the cell. The purpose is to provide an improved solar cell.

[0013]

In order to solve the above problems, the present invention has the following arrangement. That is, the present invention provides
N ⁺ -type region and the n-type surface as a light receiving surface of the mold silicon down board
Region, and an acid is formed on the surface of the n ⁺ -type region and the n-type region.
Solar cell having a cell structure with an oxide film
On the surface of the light receiving surface immediately below the film, an n-type region is
Is formed only around the surface, and the n ⁺ -type region is formed outside the n-type region.
The solar cell is characterized by being formed so as to form a main junction inside the cell, wherein the carrier concentration in the n- type region is lower than the carrier concentration in the n ⁺ -type region.

In the present invention, the depth from the surface of the n- type region can be smaller than the depth of the n ⁺ -type region.

[0015] In the present invention, the depth from the surface of the n-type region is deeper than the depth of the n ⁺ -type regions, sequentially n from the surface at least a part of a cross-section including a main junction of the solar cell becomes the light receiving surface ⁺ -n-p-p ⁺ Ru structure der.

[0016]

Next, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic sectional view showing the structure of the 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 and formed, and n ⁺ forming a main junction of the present cell is formed ^.
Region 2 is formed, and n region 2a having a lower impurity concentration than n ⁺ region 2 is formed in a range of about several tens of μm from each end face of the cell.

The carrier concentration in 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 n region 2a is lower than 1 × 10 ¹⁶ to
It is about 5 × 10 ¹⁸ / cm ³ . The width (distance from the cell end face) of the n region 2a 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 of the p-type Si substrate 1, there is provided a p ⁺ layer 3 in which an impurity such as boron (B) is diffused. A p-side electrode 5 is formed on almost the entire surface, and an antireflection film (not shown) is coated on the entire 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 having.

In the cell of the first embodiment, the main junction is n ⁺ p
In the cell of the second embodiment, the main junction has an np structure, and each of the cells has a back surface, which is characterized by forming a large built-in voltage. The formation of an electric field (Buck surface field) is characterized in that the photogenerated carriers are intended to flow toward the main junction. Other parts are the same as those of the cell of the first embodiment.

FIG. 1C is a sectional view showing the cell structure of the third embodiment. This embodiment is characterized in that the backside passivation technique described with reference to FIG. 6A is combined with the front side structure of the cell of the first embodiment. FIG. 2A is a sectional view showing the cell structure of the fourth embodiment. This embodiment is characterized in that the back surface passivation technique described with reference to FIG. 6A is combined with the front surface structure of the cell of the second embodiment.

FIG. 2B is a sectional view showing the cell structure of the fifth embodiment. This embodiment is characterized in that the antireflection structure described with reference to 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 this embodiment is characterized in that the non-reflection structure described with reference to 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 cross-sectional views in the order of the steps. First, a desired non-reflective surface shape is formed on the surface of the (100) plane of the Si-p type wafer 1 (FIG. 3A). This can be easily formed by anisotropic etching using an alkali solution such as potassium hydroxide. In this example, the surface electrode forming portion has a shape to be 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 process such as 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 n region 2a is adjusted to n of a main junction to be formed later.
⁺ Set to be lower than the carrier concentration of region 2. For example, 1
It is about × 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. When formed by thermal diffusion, the n region is also formed on the back surface 2b and the side surface (not shown) of the wafer, and this is removed by etching. In order to save this trouble, the formation of the n region 2a may be performed by ion implantation.

Next, after the entire surface of the wafer is coated with SiO ₂ 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. 3D).

Next, for example, phosphorus oxychloride (POC
An n ⁺ region 2 is formed by a thermal diffusion method using l ₃ ) as a diffusion source, and then SiO ₂ on the wafer is once removed (FIG. 3E). The carrier concentration of 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 formed again 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, and for example, boron tribromide (BBr ₃ )
The p ⁺ region 3a is formed by using thermal diffusion or ion plantation using as a diffusion source.

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

Then, after a desired heat treatment step, an antireflection film is applied to the surface, and cut and separated into desired dimensions (not shown) to complete the cell.

[0030]

As described above, according to the present invention, 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 can be improved. Contribute. Thereby, the conversion efficiency of the solar cell can be expected to be improved by at least 2% or more. For example, the conversion efficiency of 24% or more can be achieved under terrestrial light.

According to the present invention, a pn junction is exposed at the end face of the cell. However, it has been experimentally confirmed that the junction damage and reliability problems at the time of the above-described 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 forms a pn junction (n ⁺ region 2 and p
It is considered that the added n region 2a is joined to the p-type substrate instead of the n-type substrate). As described above, the structure of the solar cell according to the present invention significantly contributes to the improvement of the efficiency of the solar cell, and has a great effect on related industries.

[Brief description of the drawings]

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

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

FIG. 3 is a cross-sectional view in a process order for explaining a method for manufacturing a cell of a fifth embodiment (FIG. 2B) which is an example of the solar cell of the present invention.

FIGS. 4A and 4B are energy band diagrams for explaining the basis of the present invention, wherein FIG. 4A is an interface between n-type Si and an oxide film, and FIG.
Indicates the interface state between the 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 (a) to (c) are diagrams for explaining various conventional techniques for increasing the efficiency of a cell.

FIGS. 6A and 6B are schematic cross-sectional views showing a cell structure of a conventional solar cell, and FIGS. 6A and 6B are diagrams for explaining various conventional techniques for increasing the efficiency of a cell.

[Explanation of symbols]

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

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 31/04-31/078

Claims (3)

(57) [Claims] To 1. A light receiving surface and becomes the surface of the p-type silicon down board
n ⁺ -type region and the n-type region is formed, further n ⁺ -type region and the n-type territory
Cells with an oxide film formed on the surface of a solar cell
An n-type region is formed on the surface of the light receiving surface immediately below the oxide film.
Formed only around the end face of the transistor, and the n ⁺ type region is other than the n type region
Is formed without a main junction of the internal cells, the carrier concentration of the n-type region, a solar cell characterized by lower than the carrier concentration of the n ⁺ -type region. 2. The solar cell according to claim 1, wherein the depth from the surface of the n- type region is smaller than the depth of the n ⁺ -type region. 3. A depth from the surface of the n-type region is deeper than the depth of the n ⁺ -type regions, sequentially from the surface of at least a portion of a cross-section including a main junction of the solar cell is the light-receiving surface n ^+-n- p-
The solar cell according to claim 1, wherein the solar cell has ap ⁺ structure.