JP2003158275A - Photoelectric conversion element and method for manufacturing the same - Google Patents

Abstract

(57) [Problem] To compensate for an increase in series resistance when the junction depth is reduced without reducing the light receiving area. SOLUTION: An insulating film 3 is formed on an n-type semiconductor layer 2 from which a high-concentration impurity-doped layer on the surface has been removed by etching. Thus, the surface recombination of minority carriers due to the presence of dangling bonds and the presence of impurity levels on the surface of the n-type semiconductor layer 2 is significantly reduced as compared with the conventional violet cell. In addition, a contact opening 3a is formed in the insulating film 3, and a transparent conductive film 4 is formed over the insulating film 3 and the entire surface of the contact opening 3a. In this way, the series resistance that increases with the thinning of the n-type semiconductor layer 2 is compensated for without reducing the light receiving area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion element having a high conversion efficiency, which has a large effect when applied to a silicon polycrystalline solar cell, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, when a silicon polycrystalline solar cell is manufactured, an n-type semiconductor layer is formed by keeping a p-type semiconductor substrate at a high temperature of about 900 ° C. and performing thermal diffusion of POCl ₃ or the like. A pn junction is formed at the interface of this semiconductor layer. Here, the impurity concentration of the n-type semiconductor layer is on the order of 10 ¹⁹ cm ⁻³ , and particularly, a very thin layer of several 10 Å to 100 Å on the surface of the n-type semiconductor layer has a high concentration of 10 ²¹ cm ^−3. It is a doped layer.

In such a high-concentration impurity-doped layer, minority carriers (holes) generated by light irradiation.
However, there is a problem that the conversion efficiency is lowered by recombining via the recombination center before reaching the pn junction.

Therefore, by removing the high-concentration impurity-doped layer on the surface of the n-type semiconductor layer and thinning the n-type semiconductor layer as much as possible to make the junction depth shallow, minority carriers generated by light irradiation. A violet cell has been devised in which the photocurrent can be easily reached to the pn junction without recombination, and in particular, the photocurrent for short wavelength light of 500 nm or less is increased (j.Lindmayer and j.Allison, Conf.
Rec.IEEE Photo.Spec.Conf., 9th, p.83 (1972)).

The film thickness of the n-type semiconductor layer in the solar cell is usually 0.3 μm to 0.5 μm. However, in the case of the violet cell, by removing the high-concentration impurity-doped layer, an extremely thin n-type semiconductor layer having an appropriate impurity concentration of 0.1 μm to 0.2 μm is formed, and the n-type semiconductor layer surface is formed. The carrier recombination in the can be reduced.

On the other hand, however, the thinning of the n-type semiconductor layer increases the resistance in the lateral direction (direction parallel to the film surface), and the series resistance of the solar cell accordingly increases. Therefore, the series resistance is reduced by increasing the number of linear electrodes formed on the surface of the solar cell (j. Lindmayer and j. Allison,
H.fisher and W.Pschunder, International Conference
Record of Photovoltaic Power Generation, p.69, Sep.
(1974)). Hereinafter, the structure having the large number of surface electrodes will be referred to as a conventional structure 1.

However, in the case of forming the conventional structure 1, a large number of surface electrodes must be formed in order to compensate for the series resistance increased as the n-type semiconductor layer is made thinner. As a result, another problem occurs that the light receiving area is reduced.

Therefore, in order to reduce the series resistance without increasing the number of electrodes on the surface of the n-type semiconductor layer, an antireflection film is also formed on the n-type semiconductor layer formed on the p-type single crystal silicon substrate. A method of forming a low-resistance transparent conductive film has been proposed (N. Mardesich, Proc. IEEE Photovoltaic S
pec.Conf., 15th, p.446 (1981)). Hereinafter, the structure having this low resistance transparent conductive film will be referred to as a conventional structure 2.

[0009]

However, in the above-mentioned conventional structure 2, according to the additional test conducted by the inventors of the present invention using the polycrystalline substrate, there is a problem that the cell characteristics are deteriorated. It is considered that this is because the polycrystalline substrate has more defects such as crystal grain boundaries than the single crystal substrate, and carrier recombination is likely to occur at the interface with the transparent conductive film.

Therefore, in the conventional silicon polycrystalline solar cell, the number of surface electrodes must be increased in order to compensate for the increase in series resistance when the n-type semiconductor layer is thinned. As a result, the problem of the conventional structure 1 that the light receiving area is reduced cannot be solved.

Therefore, an object of the present invention is to provide a highly efficient photoelectric conversion element capable of compensating for an increase in series resistance when the junction depth is made shallow without reducing the light receiving area, and a method for manufacturing the photoelectric conversion element. To provide.

[0012]

To achieve the above object, a first invention is a photoelectric conversion element having a semiconductor layer surface as a light receiving surface, which is formed on the semiconductor layer surface and has an opening. It is characterized in that it has an insulating film which it has, and a transparent conductive film which is formed on the entire surface including the inside of the opening and on the insulating film and is electrically connected to the semiconductor layer through the opening.

According to the above structure, the insulating film is formed on the surface of the semiconductor layer. Therefore, surface recombination of minority carriers due to the presence of dangling bonds and the presence of impurity levels on the surface of the semiconductor layer is reduced. Furthermore, an opening is provided in the insulating film, and a transparent conductive film that is electrically connected to the semiconductor layer through the opening is formed on the entire surface. Therefore, the light receiving area is not reduced by the pitch of the openings or the presence of the insulating film, and the series resistance of each cell when the semiconductor layer is thinned to reduce the recombination of minority carriers. It is compensated without reducing the light receiving area.

Further, in one embodiment, in the photoelectric conversion element of the first invention, the shape of the opening is linear.
The width is 50 μm and the number is 45 / cell.

According to this embodiment, it is possible to reduce the series resistance of the 2 cm square cell to about 0.1 Ω or less.

In one embodiment, in the photoelectric conversion element of the first invention, the transparent conductive film formed in the opening and the transparent conductive film formed on the insulating film are made of different materials. It is configured.

A second aspect of the present invention is a method for manufacturing a photoelectric conversion element having a semiconductor layer surface as a light-receiving surface, the method including the step of forming an insulating film having an opening on the semiconductor layer surface, the inside of the opening, and The method is characterized by including a step of forming a transparent conductive film over the entire surface including the insulating film so as to be electrically connected to the semiconductor layer through the opening.

According to the above structure, the insulating film is formed on the surface of the semiconductor layer. Therefore, surface recombination of minority carriers due to the presence of dangling bonds and the presence of impurity levels on the surface of the semiconductor layer is reduced. Further, an opening is provided in the insulating film, and a transparent conductive film electrically connected to the semiconductor layer is formed on the entire surface through the opening. Therefore, the light receiving area does not decrease due to the pitch of the openings and the presence of the insulating film, and the series resistance of each cell when the semiconductor layer is thinned to reduce recombination of minority carriers It is compensated without reducing the area.

Further, in one embodiment, in the method for manufacturing a photoelectric conversion element of the second invention, the high concentration impurity layer on the surface of the semiconductor layer is removed prior to forming the insulating film on the surface of the semiconductor layer. Including the process.

According to this embodiment, the high concentration impurity layer on the surface of the semiconductor layer is removed. Therefore, the recombination of the minority carriers formed by the light of short wavelength is reduced. As a result, in combination with the effect of reducing the surface recombination of the minority carriers due to the formation of the insulating film, the recombination of the minority carriers is significantly reduced, and the open circuit voltage (Voc) and fill factor (FF) are reduced. Will be increased.

In one embodiment, in the method of manufacturing a photoelectric conversion element of the second invention, prior to forming the transparent conductive film, the semiconductor layer in the opening of the insulating film is heavily doped. Including the process.

According to this embodiment, the semiconductor layer is heavily doped at the position within the opening. Therefore, the contact characteristics between the transparent conductive film formed later and the semiconductor layer are improved.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to the embodiments shown in the drawings. FIG. 1 is a vertical sectional view showing a structure of a silicon polycrystalline solar cell as a photoelectric conversion element of the present embodiment.

In FIG. 1, reference numeral 1 denotes a film thickness of 0.3 mm to 0.4.
It is a p-type silicon substrate of about mm. 2 has a film thickness of 0.1
μm-0.2μm, sheet resistance 60Ω / □ -1
It is an n-type semiconductor layer of about 00Ω / □. Reference numeral 3 is an insulating film such as SiO ₂ or SiNx having a film thickness of several tens of Å to 100 Å, and the contact opening 3a is formed therein. 4 is ITO having a film thickness of about 800 Å to 1000 Å
A transparent conductive film such as (indium tin oxide), which is formed on the entire surface of the contact opening 3a and on the insulating film 3 and also through the contact opening 3a.
Is electrically connected to. Reference numeral 5 is a back surface electrode, and 6 is a front surface electrode.

The method of manufacturing the polycrystalline silicon solar cell 7 shown in FIG. 1 will be described below. (1) On the light-receiving surface side of the p-type silicon substrate 1, thermal diffusion is performed in a gas phase containing a group V dopant to form an n-type semiconductor layer 2 with a thickness of about 0.3 μm to 0.4 μm. In this embodiment, as the p-type silicon substrate 1, a p-type polycrystalline silicon substrate having a thickness of 0.35 mm and a specific resistance of 1 Ωcm and 2 cm square is used, and thermal diffusion is performed on the light receiving surface side thereof in PoCl _3. Then, the n-type semiconductor layer 2 was formed. The sheet resistance of the n-type semiconductor layer 2 formed in this case is about 50Ω / □, and a high-concentration impurity-doped layer of about 10 ¹⁹ cm ⁻³ is formed on the surface thereof.

(2) The surface of the n-type semiconductor layer 2 is subjected to reactive ion etching (RIE) in a mixed gas atmosphere of CF ₄ and O ₂ to make the n-type semiconductor layer 2 0.1.
The thickness is reduced to about μm to 0.2 μm. By this, n
The sheet resistance of the type semiconductor layer 2 is about 60Ω / □ to 100Ω / □.

(3) An insulating film 3 is formed on the surface of the n-type semiconductor layer 2 with, for example, SiO ₂ by a thermal oxidation method or the like. The film thickness of the insulating film 3 is about several 10Å to 100Å. Then, a contact opening 3a is formed in the insulating film 3 by photolithography. The contact opening 3a is formed, for example, in a linear shape as shown in FIG. Here, the number and position of the contact openings 3a are set to 0 in a cell with a series resistance of the silicon polycrystalline solar cell 7 of 2 cm square.
It is set appropriately so that it is about 1 Ω or less. In the case of this embodiment, the width of the linear contact opening 3a is set to 5
The number was 0 μm and the number was 45 / cell.

(4) The transparent conductive film 4 (eg ITO) is formed by sputtering. In that case, the film thickness is 800
It is about Å to 1000Å. In the present embodiment,
The sheet resistance of the ITO film 4 on the insulating film 3 is 40Ω /
□, and the refractive index at a wavelength of 600 nm was 1.73.

(5) The diffusion layer on the back surface of the p-type silicon substrate 1 is removed by etching, and the back electrode 5 is formed by screen printing an Al paste.

(6) The surface electrode 6 is formed on the transparent conductive film 4 by vacuum vapor deposition or screen printing of Al, Ag or the like. In the present embodiment, the surface electrode 6 is formed on the contact opening 3a by vacuum vapor deposition. The grid width was set to 50 μm, and the number was 15 / cell, which was smaller than the number of the contact openings 3a (1/3 of the contact openings 3a).

When the characteristics of the silicon polycrystalline solar cell 7 produced as described above were evaluated, a high conversion efficiency of 17% was obtained. This is because the increase in series resistance when the junction depth is made shallow is compensated for without decreasing the light receiving area, and the formation of the insulating film 3 serves as a dangling bond that becomes a recombination center on the surface of the n-type semiconductor layer 2. It is thought that this is because the (dangling bond) was inactivated.

When the etching amount of the n-type semiconductor layer 2 was reduced so that the sheet resistance of the n-type semiconductor layer 2 was smaller than 60 Ω / □, the photocurrent in the short wavelength region decreased. It is considered that this is because the high-concentration impurity-doped layer on the surface of the n-type semiconductor layer 2 was not sufficiently removed.

In the above embodiment, the contact opening 3a has a linear shape, but it may have a circular shape as shown in FIG. In addition, the n-type semiconductor layer 2 is subjected to high-concentration doping at a position within the linear or circular contact opening 3a to form an ITO film.
The contact characteristics at the interface of the film 4 / n-type semiconductor layer 2 may be improved.

Further, although the transparent conductive film 4 is formed by the sputtering method in the above-mentioned embodiment, it is formed by the coating method or the spraying method so that the cell is not damaged by plasma particles. can do. In that case, the cell characteristics can be further improved, and the film forming cost can be reduced because the vacuum exhaust device is not required.

In the above embodiment, the n
Although the type semiconductor layer 2 is thinned by RIE dry etching, it may be thinned by wet etching with hydrofluoric nitric acid or the like. In that case, since the etching can be performed without damaging the plasma particles, the cell characteristics can be further improved.

Further, in the above embodiment, ITO is used as the transparent conductive film 4, but a low resistance transparent conductive film such as an indium oxide film or a tin oxide film may be used. Further, the transparent conductive film 4 formed in the contact opening 3a and the transparent conductive film 4 formed on the insulating film 3 may be made of different materials.

Although SiO ₂ is used as the insulating film 3 in the above-mentioned embodiment, other materials such as SiNx can be used as long as they can inactivate the dangling bonds on the surface of the n-type semiconductor layer 2. May be Although the thermal oxidation method was used for forming the insulating film 3, the plasma CV was used.
A D (chemical vapor deposition) method, a thermal CVD method, a coating method or the like may be used.

Although the p-type silicon substrate 1 is used as the substrate in the above embodiment, glass may be used. In that case, plasma CV on the glass substrate
PIN type or PN in which semiconductor layers are sequentially stacked by the D method or the like
Type thin film solar cells can also be formed.

As described above, in the present embodiment, (A) the high-concentration impurity-doped layer on the surface is removed by thinning the n-type semiconductor layer 2 by etching. Therefore, as in the case of the above-described conventional violet cell, recombination of minority carriers (holes) formed by light with a short wavelength can be particularly reduced.

(B) An insulating film 3 is formed on the n-type semiconductor layer 2 except for the contact openings 3a. Therefore, surface recombination of minority carriers due to the presence of dangling bonds and the presence of impurity levels on the surface of the n-type semiconductor layer 2 can be further reduced.

(C) The transparent conductive film 4 has a light-transmitting property. Therefore, even if it is formed on the contact openings 3a formed at a narrow pitch, the light receiving area is not reduced. Furthermore, 800Å to 1000Å on the insulating film 3
The light receiving area is not reduced even if the film is formed with a film thickness (corresponding to a sheet resistance of about 40Ω / □). Therefore,
As described in (A), in order to reduce the recombination of minority carriers (holes) formed by light with a short wavelength, the series resistance of the cell increased by thinning the n-type semiconductor layer 2 is It is possible to compensate without reducing the light receiving area.

(D) The transparent conductive film 4 has a light-transmitting property. Therefore, the antireflection effect can be obtained by adjusting the film thickness. In particular, since the ITO film has a refractive index of 1.6 to 1.8, which is smaller than that of the n-type semiconductor layer 2, a thickness of about 800Å to 1000Å is excellent as an antireflection film. It can be effective.

The effect of increasing the light receiving area by forming the transparent conductive film 4 and reducing the carrier recombination on the surface of the n-type semiconductor layer 2 by forming the insulating film 3 will be described below.
A comparative example will be described.

In FIG. 4, the surface electrode 1 is provided at the contact opening 3a in FIG. 1 without providing the transparent conductive film 4.
5 is a comparative example 1 in which No. 5 is formed, and corresponds to the one in which the insulating film 13 is provided on the silicon polycrystalline solar cell having the conventional structure 1. Reference numeral 11 is a p-type silicon substrate, 12 is an n-type semiconductor layer, and 14 is a back surface electrode.

In this case, the area occupied by the surface electrode 15 with respect to the surface of the silicon polycrystalline solar cell is larger than in the case of the silicon polycrystalline solar cell 7 shown in FIG. 1 in the present embodiment (in the case of this comparative example 1). 3 times). FIG. 5 shows current-voltage characteristics of the polycrystalline silicon solar cell 7 of the present embodiment and the polycrystalline silicon solar cell 16 of Comparative Example 1. According to FIG. 5, the silicon polycrystalline solar cell 7 of the present embodiment is better than the silicon polycrystalline solar cell 1 of Comparative Example 1.
The short-circuit current (Isc) is higher than that of No. 6. This is because in the silicon polycrystalline solar cell 7 of the present embodiment, by covering the n-type semiconductor layer 2 with the transparent conductive film 4 having a light-transmitting property, the number of surface electrodes 6 can be increased without increasing the series resistance. This is because the light receiving area can be reduced.

FIG. 6 shows a transparent conductive film 23 formed directly on the n-type semiconductor layer 2 in FIG. 1 without providing the insulating film 3 and having the same film thickness as in the case of the silicon polycrystalline solar cell 7 shown in FIG.
Comparative Example 2 is formed by 0Å to 1000Å, and the same number of surface electrodes 25 are formed on the same position as in the case of the silicon polycrystalline solar cell 7 shown in FIG. 1. This comparative example 2
Corresponds to the above-described conventional polycrystalline silicon solar cell having structure 2 provided with the transparent conductive film 23. Reference numeral 21 is a p-type silicon substrate, 22 is an n-type semiconductor layer, and 24 is a back surface electrode.

FIG. 7 shows current-voltage characteristics of the polycrystalline silicon solar cell 7 of the present embodiment and the polycrystalline silicon solar cell 26 of Comparative Example 2. According to FIG. 7, the silicon polycrystalline solar cell 7 of the present embodiment has a higher fill factor (FF) and open circuit voltage (Voc) than the silicon polycrystalline solar cell 26 of Comparative Example 2. This is because the recombination of carriers on the surface of the n-type semiconductor layer 2 is reduced by forming the insulating film 3 on the n-type semiconductor layer 2 like the silicon polycrystalline solar cell 7.

As described above, in the present embodiment,
An insulating film 3 is formed on the n-type semiconductor layer 2 which has been thinned by etching to remove the high-concentration impurity-doped layer on the surface. Therefore, the surface recombination of minority carriers due to the presence of dangling bonds and the presence of impurity levels on the surface of the n-type semiconductor layer 2 can be significantly reduced as compared with the case of the conventional violet cell.

A linear or circular contact opening 3a is formed in the insulating film 3, and a transparent conductive film 4 is formed on the entire surface of the insulating film 3 and the contact opening 3a. Therefore, the series resistance of the cell increased by thinning the n-type semiconductor layer 2 can be compensated without reducing the light receiving area.

Thus, according to the present embodiment, it is possible to realize a highly efficient photoelectric conversion element capable of compensating for an increase in series resistance when the junction depth is shallow, without reducing the light receiving area.

[0051]

As apparent from the above, in the photoelectric conversion element of the first invention, since the insulating film having the opening is formed on the surface of the semiconductor layer which becomes the light receiving surface, the dangling on the surface of the semiconductor layer is performed. Surface recombination of minority carriers due to the presence of ring bonds and the presence of impurity levels can be reduced. Therefore, when used in combination with thinning the semiconductor layer to remove the high-concentration impurity layer on the surface, recombination of minority carriers formed especially by short-wavelength light can be significantly reduced. Even when the semiconductor layer is a polycrystalline semiconductor layer, the recombination of the minority carriers can be effectively reduced.

Further, since the insulating film is provided with an opening, and the transparent conductive film which is electrically connected to the semiconductor layer through the opening is formed on the entire surface, the pitch of the opening and the above The presence of the insulating film does not reduce the light receiving area. Therefore, the series resistance of each cell, which increases as the semiconductor layer becomes thinner, can be compensated without reducing the light receiving area.

Further, in the photoelectric conversion element of Example 1, since the shape of the opening is linear and the width thereof is 50 μm and the number is 45 / cell, the series resistance of the 2 cm square cell is 0. It becomes possible to make it about 1Ω or less.

Further, in the method for manufacturing a photoelectric conversion element of the second invention, since the insulating film having the opening is formed on the surface of the semiconductor layer, the presence of dangling bonds and the presence of impurity levels on the surface of the semiconductor layer. Surface recombination of minority carriers due to the above can be reduced. Furthermore, since the transparent conductive film that is electrically connected to the semiconductor layer is formed on the entire surface through the opening of the insulating film, the light receiving area is not decreased by the pitch of the opening and the presence of the insulating film. Absent. Therefore, it is possible to compensate the series resistance of each cell, which is increased due to the thinning of the semiconductor layer for reducing the recombination of minority carriers, without reducing the light receiving area.

In the method for manufacturing the photoelectric conversion element of the first embodiment, prior to forming the insulating film on the surface of the semiconductor layer,
Since the high-concentration impurity layer on the surface of the semiconductor layer is removed, recombination of minority carriers formed by short-wavelength light can be particularly reduced. Therefore, in combination with the effect of reducing the surface recombination of the minority carriers due to the formation of the insulating film, the recombination of the minority carriers can be significantly reduced, and the semiconductor layer is a polycrystalline semiconductor layer. However, the recombination of the minority carriers can be effectively reduced.

That is, according to this embodiment, a high efficiency photoelectric conversion element can be formed.

In the method for manufacturing the photoelectric conversion element of the first embodiment, prior to forming the transparent conductive film, the semiconductor layer in the opening of the insulating film is heavily doped, so that it is formed later. Contact characteristics between the transparent conductive film and the semiconductor layer can be improved. Therefore, the contact resistance between the transparent conductive film and the semiconductor layer can be reduced, and the increase in the series resistance can be more effectively compensated.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing the structure of a silicon polycrystalline solar cell as a photoelectric conversion element of the present invention.

FIG. 2 is a diagram showing an example of the shape of a contact opening in FIG.

FIG. 3 is a view showing a shape of a contact opening different from that in FIG.

FIG. 4 is a cross-sectional view of Comparative Example 1 in which a transparent conductive film is not provided and only an insulating film is formed.

5 is a diagram showing current-voltage characteristics of the silicon polycrystalline solar cell shown in FIG. 1 and Comparative Example 1. FIG.

FIG. 6 is a cross-sectional view of Comparative Example 2 in which only a transparent conductive film is formed without providing an insulating film.

FIG. 7 is a diagram showing current-voltage characteristics of the silicon polycrystalline solar cell shown in FIG. 1 and Comparative Example 2.

[Explanation of symbols]

1 ... p-type silicon substrate, 2 ... n-type semiconductor layer, 3 ... Insulating film, 3a ... contact opening, 4 ... Transparent conductive film, 5 ... Back electrode, 6 ... Surface electrode, 7 ... Silicon polycrystalline solar cell.

Claims (6)

[Claims] 1. A photoelectric conversion element having a semiconductor layer surface as a light-receiving surface, the insulating film being formed on the semiconductor layer surface and having an opening, and the entire surface including the inside of the opening and the insulating film. A photoelectric conversion element comprising a transparent conductive film that is formed and that is electrically connected to the semiconductor layer through the opening. 2. The photoelectric conversion element according to claim 1, wherein the shape of the opening is linear, the width thereof is 50 μm, and the number thereof is 45 / cell. . 3. The photoelectric conversion element according to claim 1, wherein the transparent conductive film formed in the opening and the transparent conductive film formed on the insulating film are made of different materials. A photoelectric conversion element characterized by. 4. A method of manufacturing a photoelectric conversion element having a semiconductor layer surface as a light-receiving surface, the method comprising: forming an insulating film having an opening on the semiconductor layer surface; A method of manufacturing a photoelectric conversion element, comprising the step of forming a transparent conductive film on the entire surface including the film so as to be electrically connected to the semiconductor layer through the opening. 5. The method of manufacturing a photoelectric conversion element according to claim 4, further comprising a step of removing a high-concentration impurity layer on the surface of the semiconductor layer before forming an insulating film on the surface of the semiconductor layer. A method for manufacturing a photoelectric conversion element, comprising: 6. The method for manufacturing a photoelectric conversion element according to claim 4, including a step of performing high-concentration doping on the semiconductor layer in the opening of the insulating film before forming the transparent conductive film. A method for manufacturing a photoelectric conversion element, comprising: