JP2002299660A - Thin-film crystalline Si solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost and high-efficiency thin-film crystalline Si with high light confinement efficiency and sufficiently suppressed generation of leakage current
It is intended to provide a solar cell. SOLUTION: On a substrate 1, a back electrode layer 2, a semiconductor layer 3 having a semiconductor junction in which a photoactive layer portion is formed of crystalline Si,
This is a thin-film crystalline Si solar cell in which a transparent conductive film 4 and a surface electrode 5 are sequentially laminated, and a large number of triangular pyramids or more pyramids are formed on the surface of the substrate 1 on the side in contact with the back electrode layer 2. It has a fine concavo-convex structure, the angle between each surface of the pyramid with respect to the direction horizontal to the substrate is 8 ° or more and 40 ° or less, and the distance between the vertices of adjacent pyramids is 5 μm or less. The thickness of the back electrode layer 2 is set to 0.05 to 2 μm.
The surface of the back electrode layer 2 has a fine uneven structure reflecting the fine uneven structure on the surface of the substrate 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a photoelectric conversion element represented by a thin-film crystalline Si solar cell and the like.

[0002]

2. Description of the Related Art Thin-film crystalline Si solar cells typified by thin-film polycrystalline Si solar cells have attracted attention as next-generation solar cells.
Since the light absorption coefficient is not sufficiently large with respect to the thickness of the thin film, it is particularly important to improve the light use efficiency by introducing a light confinement structure to obtain a sufficient photocurrent value.

As a light confinement technique, it has been known to form an anti-reflection film on the light incident surface side or to form a concavo-convex structure, and has long been applied to solar cells and put to practical use.

However, in a thin-film crystalline Si solar cell, since the light absorption coefficient of crystalline Si is small on the long wavelength side, light absorption is sufficiently generated at a film thickness of about several μm or less to more efficiently perform photoelectric conversion. To do so, the incident light must be crystalline S
It is particularly important to form a structure in which light can be more effectively confined by reflecting and reciprocating a number of times in the i-film. For this reason, in the thin-film crystalline Si solar cell, in addition to forming the concavo-convex structure on the light incident surface side surface of the conventional semiconductor layer, the concavo-convex structure is also formed on the opposite side of the semiconductor layer from the light incident surface. Studies are underway to more effectively confine light.

In a super-straight solar cell in which light is incident from the substrate side using a light-transmitting substrate, a transparent conductive film formed on the light-transmitting substrate can have a spontaneous uneven structure. An uneven structure reflecting the above uneven structure can be naturally formed also at the interface between the semiconductor and the back electrode, but a substrate type thin film crystalline Si solar cell having a light incident surface opposite to the substrate side. In this method, a flat glass plate or a stainless steel plate not particularly subjected to unevenness processing is usually used as a substrate, so that it is necessary to devise a method for making the back electrode layer uneven, and several prior arts are known.

For example, a conventional example shown in FIG.
In Japanese Patent Application Laid-Open No. 3-12014), a back electrode layer 22 having a concavo-convex structure, a semiconductor layer 23, a transparent conductive film 24, and a surface collecting electrode 25 are sequentially laminated on a substrate 21 to specify a material, its forming conditions, and the like. Back electrode layer 22 having a desired concavo-convex structure
It is stated that you get However, in this conventional example, there is no mention of the inclination angle of the surface on which the concavo-convex structure is formed with respect to the horizontal direction of the substrate 21, and if there is a portion where the inclination angle is steep, it becomes a leak current generating source and becomes a characteristic. The risk of lowering cannot be eliminated. Also, the substrate 21
There is no mention of a structure or a process for making the surface itself uneven, and in the method described above, formation of an uneven structure consisting of a surface having a desired inclination angle is not actually easy, and it is difficult to reduce the cost. is there.

Further, a conventional example shown in FIG.
50209), a back electrode layer 32a,
A back transparent conductive film 32b having an uneven structure, a semiconductor layer 33,
A table transparent conductive film 34 and a table electrode 35 are sequentially laminated and provided,
It is described that the inclination angle of the uneven surface of the back transparent conductive film 32b forming the uneven structure is preferably set to a value of 15 ° to 45 °. However, as for the method of forming the concave-convex structure, it is only described that the conditions for forming the back metal electrode 22a are controlled and that the transparent conductive layer 22b on which the spontaneous concave-convex structure is easily formed is laminated. No description is given of a structure or a process for making the surface itself uneven, and in the above-described method, formation of an uneven structure including a surface having a desired inclination angle is not actually easy,
There is still difficulty in reducing costs.

Further, a conventional example shown in FIG.
00-49372) describes formation of an insulating layer 42 having a surface uneven structure on a metal substrate 41. However, since the surface on which the uneven structure is formed includes a steeply inclined surface whose inclination angle ranges from 30 ° to 80 °, deterioration of characteristics due to generation of a leak current is unavoidable as it is.

[0009] The present invention has been made in view of the above,
An object of the present invention is to provide a low-cost and high-efficiency thin-film crystalline Si solar cell having high light confinement efficiency and sufficiently suppressing generation of leak current.

[0010]

According to a first aspect of the present invention, there is provided a thin-film crystalline Si solar cell having a back electrode layer and a photoactive layer formed of crystalline Si on a substrate. In a thin-film crystalline Si solar cell in which a semiconductor layer having a junction, a transparent conductive film, and a surface electrode are sequentially laminated, a large number of triangular pyramids or more are formed on the substrate surface in contact with the back electrode layer. It has a fine concavo-convex structure, the angle (tilt angle) between each surface of the pyramid with respect to the direction horizontal to the substrate is 8 ° or more and 40 ° or less, and the distance between the vertices of adjacent pyramids is Is not more than 5 μm, and the thickness of the back electrode layer is 0.05 to 2 μm, and the back electrode layer surface has a fine uneven structure reflecting the fine uneven structure of the substrate surface. I do.

Further, in the thin film crystalline Si solar cell according to the second aspect, the back electrode layer and the photoactive layer are formed on the substrate by the crystalline S
a thin film crystalline S formed by sequentially laminating a semiconductor layer having a semiconductor junction, a transparent conductive film, and a collecting electrode formed by i.
In the i solar cell, the substrate surface on the side in contact with the back electrode layer has a fine concavo-convex structure, a concave portion of this fine concavo-convex structure is formed with a curved surface, and the tangent of the curved surface of the concave portion is horizontal to the substrate. The maximum value of the angle (inclination angle) with respect to the direction is 8 ° or more and 40 ° or less, the distance between the lowest points of adjacent concave portions is 5 μm or less, and the thickness of the back electrode layer is 0.05 or less. The surface of the back electrode layer has a fine uneven structure reflecting the fine uneven structure of the substrate surface.

In the above-mentioned thin-film crystalline Si solar cell, it is preferable that the uneven structure on the substrate surface is formed by a dry etching method or a wet etching method.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A thin-film crystalline Si solar cell according to each claim will be described below with reference to FIGS. FIG. 1 is a view showing an embodiment of a thin-film crystalline Si solar cell according to claim 1, wherein 1 is a substrate, 2 is a back electrode layer, 3 is a semiconductor layer, 4 is a transparent conductive film, and 5 is a collecting electrode. It is.

The substrate 1 is made of a plate or film made of glass, metal, plastic or the like.
The surface of the substrate 1 has a fine concavo-convex structure composed of a large number of polygonal pyramids of three or more pyramids. The angle between each surface of the polygonal pyramid of the concavo-convex structure and the direction horizontal to the substrate 1 is set to 8 ° or more and 40 ° or less, and the distance between vertices of adjacent polygonal pyramids is set to 5 μm or less.

When it is desired to form a fine uneven structure composed of a large number of polygonal pyramids having three or more pyramids as the uneven structure on the surface of the substrate 1, a negative replica such as a mold having the negative structure is prepared in advance. Thus, if the surface of the substrate 1 is pressed under an appropriate temperature condition, it can be realized at a relatively low cost.

The substrate 1 having an arbitrary inclined surface of this fine uneven structure.
The inclination angle with respect to the horizontal direction is in the range of 8 ° to 40 °, more preferably in the range of 15 ° to 40 °,
By setting the distance between the vertices of adjacent polygonal pyramids to 5 μm or less, a sufficient light confinement effect can be obtained, and a decrease in characteristics due to generation of leak current can be prevented.

Here, the reason why the maximum value of the inclination angle is set to 40 ° or less is to suppress generation of a leak current. The distance between the vertices of adjacent polygonal pyramids forming the concavo-convex structure on the substrate surface is set to 5 μm or less when the thickness of the semiconductor layer is about 0.5 to several μm. This is to obtain a significant uneven structure that cannot be regarded as flat with respect to the thickness.

The original concavo-convex structure for producing a negative replica is, for example, a concavo-convex structure that reflects the plane orientation of a Si crystal formed by etching a crystalline Si substrate under predetermined wet or dry etching conditions. And SnO ₂
It is also possible to use a spontaneous surface uneven structure obtained by forming a transparent conductive film such as a film under predetermined conditions,
Various materials can be used depending on the uneven structure to be obtained.

Next, a metal film to be the back electrode layer 2 is formed. As the metal film material, Al, A having excellent light reflection characteristics
It is desirable to use g or the like. Known techniques such as a vapor deposition method, a sputtering method, and an ion plating method can be used as a film forming method. The back electrode layer 2 has a thickness of 0.05 to 2 μm, and the surface of the back electrode layer 2 has a fine uneven structure reflecting the fine uneven structure of the surface of the substrate 1. The thickness of the back electrode layer 2 is 0.05 to 2 μm
In this case, the unevenness on the surface of the back electrode layer 2 substantially reflects the uneven structure on the surface of the substrate. The angle of inclination of the surface of the layer 2 can be set to about 8 ° or more. At this time, the light is vertically incident from the element surface and the back electrode layer 2
The light reflected on the surface can obtain the condition of total reflection at the interface between the transparent conductive film 4 formed on the semiconductor layer 3 and the air layer, and can perform very efficient light confinement.

In actual commercialization, since the solar cell element is modularized, the medium in contact with the surface of the transparent conductive film 4 is not air but sealing resin or glass, which is a constituent material of the module. If the angle of inclination of the uneven surface is set to 15 ° or more, total reflection conditions can be obtained at the interface between the Si semiconductor layer and the transparent conductive film, and very efficient light confinement can also be performed. In addition, when the element surface can be regarded as flat, it is formed into the above-described uneven structure, but even if there is an uneven structure reflecting the autonomous uneven structure caused by the crystal structure of the semiconductor layer on the element surface, As long as the numerical value is an angle, the condition is still satisfied. If the thickness of the back electrode layer 2 is 0.05 μm or less, the deterioration of the characteristics due to the increase in the series resistance component of the element cannot be ignored, and if it is 2 μm or more, the uneven structure on the surface of the substrate will At the same time, the cost is not realistic. In addition, in order to increase the adhesive strength between the substrate 1 and the back electrode layer 2,
For example, a metal layer such as Ti may be inserted between the substrate 1 and the back electrode layer 2 with a thickness of 0.5 to 200 nm.

Further, when diffusion of a metal component from the back electrode layer 2 to the semiconductor layer 3 described later becomes a problem, a diffusion barrier layer may be inserted between the back electrode 2 and the semiconductor layer 3.
When a metal film such as is used for the diffusion barrier layer, the thickness is 10 n.
When a transparent conductive film such as ZnO, SnO ₂ , or ITO is used for the diffusion barrier layer, the thickness may be 100 nm or less. In the case where the latter transparent conductive film is used, a function of improving the effective reflectance of the back electrode layer 2 can be provided in addition to the function as the diffusion barrier layer.

Next, a semiconductor layer 3 serving as a photoelectric conversion layer having a semiconductor junction is formed. The photoelectric conversion layer is roughly composed of an underlayer, a photoactive layer, and a bonding layer.

First, as a base layer (not shown), a non-single crystal
Si film is formed by a method such as catalytic CVD or plasma CVD.
To form. The thickness is about 10 to 500 nm. Do
1E18-1E21 / cm
^ThreeP as degree⁺Type (or n ⁺Type).

Next, a crystalline Si film is formed as a photoactive layer (not shown) by a method such as catalytic CVD or plasma CVD. The thickness is about 0.5 to 10 μm. In addition,
The conductivity type is the same conductivity type with a lower doping concentration than the above-described underlayer, or substantially i-type.

Next, in order to form a semiconductor junction (not shown), a non-single-crystal Si film (joining layer) is formed by a method such as a catalytic CVD method or a plasma CVD method. The film thickness is 5-500n
m. Doping element concentration is 1E18-1E2
It is about 1 / cm ^3, which is n ⁺ (or p ⁺ type), which is the opposite conductivity type to the above-described underlayer. In order to further improve the bonding characteristics, a substantially i-type non-single-crystal Si layer may be inserted between the photoactive layer and the bonding layer. At this time, the thickness of the insertion layer is about 10 to 500 nm in the case of a crystalline Si layer, and about 1 to 20 nm in the case of amorphous Si.

Next, a transparent conductive film 4 is formed. Known materials such as SnO ₂ , ITO, and ZnO can be used as the material of the transparent conductive film 40. As the film forming method,
Known techniques such as an evaporation method, a sputtering method, and an ion plating method can be used. This film thickness is preferably about 60 to 300 nm in consideration of the optical interference effect.

Finally, a metal film to be the collecting electrode 5 is formed. As the metal film material, it is desirable to use Al, Ag, or the like which has excellent conductivity. As a film forming method, a vapor deposition method,
Known techniques such as a sputtering method and a screen printing method can be used. The electrode pattern can be formed into a desired pattern by using a masking method, a lift-off method, or the like. In order to enhance the adhesive strength between the transparent conductive film 4 and the transparent conductive film 4,
It is effective to insert a metal material having excellent adhesive strength with an oxide material such as.

As described above, a high-efficiency and low-cost thin-film crystalline Si solar cell having a high light confinement efficiency and a light confinement structure capable of sufficiently suppressing generation of a leak current can be obtained.

Next, a thin film crystalline Si solar cell according to claim 2 will be described with reference to FIG. In FIG. 2, 11 is a substrate,
12 is a back electrode layer, 13 is a semiconductor layer, 14 is a transparent conductive film,
Reference numeral 15 denotes a table electrode.

Although this thin-film crystalline Si solar cell is almost the same as the thin-film crystalline Si solar cell shown in FIG. 1, this thin-film crystalline Si solar cell is formed on the surface of the substrate 1 in contact with the back electrode layer 12. The concave portion of the fine concave and convex structure is formed with a curved surface, the maximum value of the angle formed by the tangent to the curved surface of the concave portion with respect to the direction horizontal to the substrate is set to 8 ° or more and 40 ° or less, and The distance between the points is 5 μm or less, and the thickness of the back electrode layer 12 is 0.05 to 2 μm, and the surface of the back electrode layer 12 has a fine uneven structure reflecting the fine uneven structure of the surface of the substrate 11. have.

When it is desired to form a fine concavo-convex structure in which a concave portion is formed on the surface of the substrate 1 by a curved surface, it can be realized at relatively low cost by processing using a dry etching method or a wet etching method. It is described in Japanese Patent Application No. 2000-301419, for example, that a desired fine uneven shape can be obtained by using an RIE method, which is a kind of dry etching method, depending on etching conditions such as a gas type, a gas pressure, and plasma power. .

The maximum value of the angle formed by the tangent to the curved surface of the concave portion of the fine uneven structure with respect to the direction parallel to the substrate is 8 ° to 4 °.
0 °, more preferably 15 ° to 40 °, and the distance between the lowest points of adjacent concave portions is 5 μm.
By setting m or less, not only a sufficient light confinement effect can be obtained, but also deterioration in characteristics due to generation of leak current can be prevented. It is to be noted that the above-described press working method using a negative replica can be used for forming the uneven structure.

The maximum value of the inclination angle of the curved surface of the concave portion is set to 8 ° or more and 40 ° or less, and the thickness of the back electrode layer 12 is set to 0.05 to 2 μm.
The reason for setting m is as described above, and the maximum value of the inclination angle is more preferably 15 ° or more and 40 ° or less. Further, the distance between the lowest points of the adjacent concave portions is 5 μm.
The reason for this is to obtain a significant concavo-convex structure that cannot be regarded as flat with respect to the thickness of the semiconductor layer when the thickness of the semiconductor layer is about 0.5 to several μm.

[0034]

As described above, according to the thin-film crystalline Si solar cell according to the first aspect, the fine concavo-convex structure composed of a large number of triangular pyramids or more is formed on the substrate surface in contact with the back electrode layer. The angle between each surface of the polygonal pyramid with respect to the direction parallel to the substrate is 8 ° to 40 °, the distance between the vertices of adjacent polygonal pyramids is 5 μm or less, and the back electrode Since the thickness of the layer is 0.05 to 2 μm and the surface of the back electrode layer has a fine uneven structure reflecting the fine uneven structure of the substrate surface, light confinement efficiency is high and leakage current is reduced. By forming a concavo-convex structure on a substrate, a light confinement structure that can be sufficiently suppressed can be realized at a relatively low cost, and a high-efficiency and low-cost thin film that has a high short-circuit current density and sufficiently suppresses the decrease in open-circuit voltage Manufacture crystalline Si solar cells It is possible.

According to the thin-film crystalline Si solar cell of the second aspect, the substrate surface on the side in contact with the back electrode layer has a fine uneven structure, and the concave portion of the fine uneven structure has a curved surface. The maximum value of the angle formed by the tangent to the curved surface of the concave portion with respect to the direction horizontal to the substrate is 8 ° or more and 40 ° or less, and the distance between the lowest points of adjacent concave portions is 5 μm or less. Since the thickness of the back electrode layer is 0.05 to 2 μm and the surface of the back electrode layer has a fine uneven structure reflecting the fine uneven structure of the substrate surface, the light confinement efficiency is high and the leakage current is high. By forming the concavo-convex structure on the substrate, a light confinement structure that can sufficiently suppress the generation can be realized at a relatively low cost, and has a high short-circuit current density and a high efficiency and low cost with the open-circuit voltage characteristic deterioration sufficiently suppressed. Thin crystalline Si sun It is possible to produce a pond.

[Brief description of the drawings]

FIG. 1 is a view showing one embodiment of a thin-film crystalline Si solar cell according to claim 1;

FIG. 2 is a view showing one embodiment of a thin-film crystalline Si solar cell according to claim 2;

FIG. 3 is a view showing a conventional thin-film crystalline Si solar cell.

FIG. 4 is a view showing another conventional thin film crystalline Si solar cell.

FIG. 5 is a view showing another conventional thin film crystalline Si solar cell.

[Explanation of symbols]

1, 11: substrate, 2, 12: back electrode layer, 3, 13: semiconductor layer, 4, 14: transparent conductive film, 5, 15: front electrode

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Claims (3)

[Claims] 1. A thin-film crystalline Si having a back electrode layer, a semiconductor layer having a semiconductor junction in which a photoactive layer is formed of crystalline Si, a transparent conductive film, and a collecting electrode are sequentially laminated on a substrate. In the solar cell, the substrate surface on the side in contact with the back electrode layer has a fine uneven structure composed of a large number of pyramids of three or more pyramids, and each surface of the pyramids is in a direction horizontal to the substrate. The angle between 8 ° and 40 ° is formed, the distance between the vertices of adjacent pyramids is 5 μm or less, and the thickness of the back electrode layer is 0.05-2 μm. Characterized by having a fine uneven structure reflecting the fine uneven structure of
i solar cell. 2. A thin-film crystalline Si having a back electrode layer, a semiconductor layer having a semiconductor junction in which a photoactive layer portion is formed of crystalline Si, a transparent conductive film, and a collecting electrode are sequentially laminated on a substrate. In the solar cell, the substrate surface on the side in contact with the back electrode layer has a fine concave-convex structure, a concave portion of the fine concave-convex structure is formed with a curved surface, and a tangent of the curved surface of the concave portion is in a direction parallel to the substrate. The maximum value of the angle made with respect to
0 ° or less, and the distance between the lowest points of adjacent recesses is 5
μm or less, and the thickness of the back electrode layer is 0.05 to
A thin-film crystalline Si solar cell, wherein the surface of the back electrode layer has a fine uneven structure reflecting the fine uneven structure of the substrate surface as 2 μm. 3. The thin-film crystalline Si solar cell according to claim 2, wherein the uneven structure on the substrate surface is formed by a dry etching method or a wet etching method.