JP2003101052A - Conductive light-reflecting film, forming method therefor and solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a conductive light-reflecting film on which proper texture structure is installed and light confinement effect is large, and to provide its forming method. SOLUTION: An Ag alloy layer containing Al in a range of 0.1-1.0 atm.%, or an Ag layer and an Ag alloy layer containing Al in a range of 0.1-1.0 atm.% are laminated on a substrate, and a reflection film is formed. As a result, a conductive light-reflecting film having scattering reflectivity in the long wavelength region is high is obtained. When the film is applied to a thin film solar battery such as and a-Si solar battery, short-circuiting current is increased due to light confinement effect, so that conversion efficiency is increased. Moreover, the Ag layer, the Ag alloy layer containing Al, and an oxide transparent electrode film can be laminated under the same temperature conditions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a conductive light reflecting film used for a back electrode of a solar cell and a method for forming the same.

[0002]

2. Description of the Related Art In a thin film solar cell, the photoelectric conversion amount is limited by the film thickness when the absorption coefficient of the photoelectric conversion layer is not sufficiently large. Therefore, in order to effectively use the light incident on the photoelectric conversion layer, a conductive light reflection film made of a metal thin film having a high light reflectance is formed on the back side of the photoelectric conversion layer, and may also serve as a back electrode. Many.

Further, this conductive light reflecting film has irregularities (texture structure) formed on the surface thereof to scatter and reflect light to the photoelectric conversion layer. This conductive light reflection film is
A method using a metal or alloy such as Al or Ag, or an alloy of these and Si is known [Japanese Patent Application Laid-Open No. 4-334069].
Issue bulletin]. Further, as a method of forming a conductive light-reflecting film having such a concavo-convex shape, a method of using a metal two-layer structure of a semi-continuous film and a continuous film is disclosed in JP-A-9-162430. JP-A-8-288529 discloses a metal two-layer structure of an Ag film and an Al or Al alloy film, and a method of using a thin metal film having an appropriate uneven shape as a lower electrode. . The semi-continuous film is a film having a large nonuniformity in thickness and has a discontinuous portion in a part of width or length, and the continuous film is a film without such a discontinuous portion.

[0004]

Conventionally, the means for improving the texture structure of the conductive light reflecting film has been to adjust the substrate temperature and the deposition rate at the time of forming the conductive light reflecting film. However, only by adjusting these conditions, there was a limitation due to the structure of the thin film forming apparatus, and it was not possible to obtain a textured structure having satisfactory characteristics.

An object of the present invention is to obtain a conductive light-reflecting film having a textured structure with satisfactory characteristics and a method for forming the same by optimizing the amount of additives in the material for forming the conductive light-reflecting film. .

[0006]

In order to solve the above-mentioned problems, the conductive light-reflecting film of the present invention is provided on a substrate with at least 0.1.
An Ag alloy layer containing Al in the range of 1.0 at% is laminated. When a Ag alloy layer containing Al in the range of 0.1 to 1.0 at% is laminated, the scattering reflectance particularly in the long wavelength region increases and the light confinement effect improves, as described later. According to the scanning electron microscope observation, the grain shape becomes clear and the unevenness becomes clear. The reason why this is the case is not clear, but the inventor believes that the inclusion of Al lowers the melting point, which promotes the diffusion of atoms and facilitates the formation of grains.

If the Al content is less than 0.1 at%, the increase in scattering reflectance is small and the effect is small. Further, when the Al content exceeds 1.0 at%, when the transparent conductive film is deposited on the Al content, adverse effects such as coloring of the transparent conductive film occur. Therefore, the Al content is appropriately within the above range. Kohei
It is considered that the Ag alloy containing Al in the range of 0.1 to 1.0 at% of the present invention is not applicable, because it is Al or Al alloy disclosed in the 9-162430 publication.

Particularly, it is preferable that the film of the Ag alloy layer containing Al is a continuous film and the film thickness thereof is within the range of 20 to 200 nm. If it is continuous, it can be used even if the Ag alloy layer containing Al is a single layer. If the film thickness is less than 20 nm, it will not be a continuous film. On the contrary, if the thickness exceeds 200 nm, the grain shape is flattened again. It is useless to thicken it too much.
In the above-mentioned Japanese Patent Laid-Open No. 4-218977, the base film is semi-continuous, which is different from the present invention in which it is a continuous film. In addition, Al disclosed in JP-A-9-162430
Alternatively, the Al alloy has a thickness of 1 to 20 nm, and the present invention is also a different invention in this respect.

When the Ag layer and the Ag alloy layer containing Al are laminated on the substrate, even if the Ag alloy layer containing Al is laminated on the Ag layer, the Ag layer is formed on the Ag alloy layer containing Al. You may laminate. The Ag layer and the Ag alloy layer containing Al may be combined and laminated a plurality of times on the substrate. Since the Ag layer has a substantially uniform thickness, the unevenness of the surface of the Ag alloy layer containing Al is maintained regardless of whether the Ag layer is on the upper side or the lower side.

The grain size of the Ag alloy layer containing Al is 0.2 to 1 μm.
It should be in the range of m. Further, it is preferable that the height difference of the uneven shape of the Ag alloy layer containing Al is 0.06 μm or more. Since the grain shape of the Ag alloy layer containing Al is similar to each other, if the grain size becomes smaller than 0.2 μm, the height difference of the uneven shape becomes small and the scattering reflection also becomes small. On the other hand, if the particle size is too large, the difference in height becomes large and it becomes difficult to maintain the continuity of the alloy layer. As in Examples described later, the above range is an appropriate particle size and height difference.

Even if there is a conductive transparent electrode film above the laminated layer with the Ag alloy layer containing Al, if it is transparent, the scattering reflection is largely maintained. As the substrate material, various substrates such as an insulating substrate, a flexible substrate, and a plastic film substrate can be used. As a method of forming the conductive light reflection film, the substrate is set to 250.
The Ag layer, the Ag alloy layer containing Al, and the conductive transparent electrode film are formed while being heated to ˜360 ° C.

Since the films can be formed at the same temperature, there is an advantage that the process can be simplified. The present invention is also different from the above-mentioned JP-A-4-218977 in that the film forming temperature is not changed.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment] FIG. 1 is a sectional view of a conductive light reflecting film of a first embodiment according to the present invention. An aramid-based polymer resin material was used as the transparent substrate 1, and a conductive light reflecting film was formed thereon.

The conductive light-reflecting film has a thickness of 99.
99% (4N) pure silver layer (hereinafter referred to as Ag layer) 2 and thickness 100
Ag-Al containing 0.3 atomic% of nm (hereinafter referred to as at%) Al
And alloy layer 3. A 60 nm thick zinc oxide film (hereinafter referred to as a ZnO film) is formed thereon as a transparent electrode film 4. Each layer has a forming temperature of 350 ° C and an Ar atmosphere of 0.5 Pa.
Then, using the roll-to-roll method in the same vacuum tank,
The film was continuously formed by the sputtering method. The deposition rate of Ag layer 2 and Ag-Al alloy layer 3 is 6.4 nm / s, and the deposition rate of ZnO film is 1.
It is 9 nm / s.

The evaluation was carried out by comparing the reflectances of the electrode surfaces. From the ratio of the total reflected light to the incident light (represented by the total reflectance is Rt) and the ratio of the reflected light in the −5 ° direction to the incident light inclined 5 ° from the vertical direction (represented by the vertical reflectance Rv) , Scattering reflectance = (Rt−Rv) / Rt was determined. It can be judged that the higher the scattering reflectance, the greater the light confinement effect.

FIG. 2 is a characteristic diagram for examining the wavelength dependence of the conductive light reflecting film according to the present invention. The vertical axis represents the scattering reflectance and the horizontal axis represents the wavelength. As a comparative example, the thickness is 100 under the same conditions.
The wavelength dependence of the conductive light-reflecting film in which a pure silver layer of 99.99% (4N) of nm was deposited twice and a ZnO film having a thickness of 60 nm was formed thereon as a transparent electrode layer was also described.

Compared with the conventional film having two pure Ag films (A in FIG. 2), the conductive light-reflecting film of this embodiment in which an Ag.Al alloy layer is formed on the Ag layer (B in FIG. 2). Then, the scattering reflectance is 60
It can be seen that the maximum improvement is 40% in the wavelength range of 0 to 800 nm. 3 and 4 are scanning electron microscope (SEM) photographs of the surfaces of the conventional film and the conductive light-reflecting film of this example, respectively.

From this observation result, the light-reflecting film of this embodiment shown in FIG. 4 is more granular than the conventional light-reflecting film shown in FIG. Next, in order to confirm the light confining effect of the light reflecting film according to the present embodiment, amorphous silicon (a-Si) and amorphous silicon germanium (a
-SiGe) was used for the i-layer, and a triple junction solar cell with an a-Si / a-SiGe / a-SiGe structure was prototyped. FIG. 6 is a sectional view showing a laminated state of the solar cell. For comparison, a-Si / a-SiGe / a-S having the light-reflecting film of the above comparative example is used.
An iGe structure triple junction solar cell was also prototyped at the same time.

From the film substrate 1 side, there are a bottom cell, a middle cell, and a top cell. The i layer of the bottom cell and the middle cell is a-SiGe, and the i layer of the top cell is a-Si. The i-layer thickness of the bottom cell, middle cell, and top cell is 60 to 120 nm, 60 to 160 nm, and 70 to
It is 100 nm. Controlling the film thickness and bandgap of each layer is important for current matching of each cell. The optical gap of each i layer of the bottom cell, middle cell, and top cell is 1.35 to 1.6 eV, 1.5 to
It is 1.6 eV and 1.75 to 1.8 eV.

FIG. 7 is a reflection spectrum diagram of both solar cells. In the figure, the solid line represents the reflection spectrum of the solar cell to which the light reflecting film of this example was applied, and the broken line represents the reflection spectrum of the solar cell to which the light reflecting film of the comparative example was applied. From this figure, it is apparent that the cell to which the substrate of this example is applied has a smaller reflectance in the long-wavelength range, and the optical confinement performance is improved.

FIG. 8 is a characteristic diagram showing the wavelength dependence of the collection efficiency of both solar cells. It can be seen that the solar cell using the light-reflecting film of this example has improved collection efficiency in the long wavelength region, which corresponds well to the reflection spectrum of FIG. 7. As a result, the total current of the top, middle, and bottom cells obtained by integrating the collection efficiency under AM1.5, 1 sun was improved by about 1 mA / cm ² .

Table 1 shows comparison results of cell characteristics of triple cells formed on various electrodes.

[0023]

[Table 1] Among them, the solar cell to which the conventional light reflecting film is applied is the cell A,
The solar cell to which the light reflecting film of this embodiment is applied is the cell B. Each parameter of the cell characteristics is standardized by the value of the cell A.

From this table, it can be seen that the short circuit current (Isc) of cell B is 4.5% larger than that of cell A. This result corresponds well with the above-mentioned result of collection efficiency. However, the open voltage (Voc) and the fill factor (FF) of the cell B were reduced by about 1 to 2%, respectively, and as a result, the improvement of the conversion efficiency was 2.2%.

It is considered that the cause of the decrease in Voc and FF is due to the occurrence of defects called white stripes due to the severe irregularities of the electrodes (Japanese Journal o.
f Applied Physics, Vol. 29, No.4, April, 1990, P
P. 630-635). In fact, when the cell B was observed with a transmission electron microscope (TEM), it was confirmed that white stripes were generated from the valleys of the irregularities.

In order to suppress the decrease of Voc and FF, it is necessary to make the concavo-convex shape of the electrode slightly gentle to suppress the occurrence of white stripes. [Example 2] In the same manner as in Example 1, the thickness of each electrode layer was changed to Ag layer: 140 nm, Ag alloy layer: 60 nm, ZnO layer: 60 n.
It was m.

The measurement result of the scattering reflectance of this light reflecting film is also shown in FIG. 2 (C in the figure). Compared with Example 1, 600-80
It can be seen that the scattering reflectance in the wavelength range of 0 nm is slightly lowered, but no significant difference is observed. FIG. 5 is a scanning electron microscope (SEM) photograph of the surface of the conductive light-reflecting film of Example 2.

It can be seen that the light-reflecting film of Example 2 of FIG. 5 is slightly less uneven than the light-reflecting film of Example 1 of FIG. A triple cell solar cell was manufactured by applying the light reflecting film of Example 2. The characteristics of the solar cell are shown in Table 1.
It is described in the column of cell C in the inside. The improvement of Isc with respect to the cell A is 3.9%, which is slightly lower than that of the cell B of Example 1. However, the decrease of Voc and FF was suppressed, and the efficiency was slightly higher than that of cell B.

[Embodiment 3] In the same manner as in Embodiment 1, Ag
The Al concentration in the alloy layer was set to 0.1 at%, and a light reflecting film was similarly formed. The measurement result of the scattering reflectance of this light reflecting film is also shown in FIG. 2 (D in the figure). It can be seen that although the scattering reflectance is slightly lower than in Example 2, it is more textured than in the past.

A triple cell type solar cell was manufactured by applying the light reflecting film of this example. The characteristics of the solar cell are shown in the column of cell D in Table 1. The improvement in Isc with respect to the cell A is 2.6%, which is about half that of the cell B.
However, the decrease in Voc and FF was suppressed, so the efficiency was 1.6% higher than that of cell A.

[0031]

As described above, according to the present invention, when a conductive light reflecting film is formed on a transparent substrate, by stacking a small amount of Al-doped Ag alloy, the same film forming temperature can be obtained. Can form a film having an improved light confinement effect. When the conductive light-reflecting film of the present invention is applied to a thin-film solar cell such as an a-Si solar cell, the short-circuit current increases due to the light confinement effect. Therefore, it becomes possible to easily provide a solar cell having higher conversion efficiency than conventional ones.

[Brief description of drawings]

FIG. 1 is a sectional view of a conductive light-reflecting film of Example 1 of the present invention.

FIG. 2 is a characteristic diagram in which the wavelength dependence of the scattering reflectance of the conductive light-reflecting films of Examples 1, 2 and 3 of the present invention and a comparative example is examined.

FIG. 3 is an SEM observing the surface of a conventional conductive light reflecting film.
Photo

FIG. 4 is an SEM photograph of the surface of the conductive light-reflecting film of Example 1 of the present invention.

FIG. 5 is an SEM photograph of the surface of the conductive light-reflecting film of Example 2 of the present invention.

FIG. 6 is a cross-sectional view showing a stacked state of a prototype a-Si / a-SiGe / a-SiGe structure triple-junction solar cell.

FIG. 7 is a reflectance spectrum diagram of solar cells to which the light-reflecting films of Examples of the present invention and Comparative Examples are applied.

FIG. 8 is a characteristic diagram showing the wavelength dependence of the collection efficiency of the solar cells to which the light reflecting films of the example of the present invention and the comparative example are applied.

[Explanation of symbols]

1. Transparent substrate 2. Ag (4N) layer 3. Al-Ag alloy layer Four. Transparent electrode film Five. Bottom n layer 6. Bottom i layer 7. Bottom p layer 8. Middle n layer 9. Middle i layer Ten. Middle p layer 11. Top n tier 12. Top i tier 13. Top p layer

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Claims (15)

[Claims] 1. A conductive light-reflecting film comprising an Ag alloy layer containing Al in the range of 0.1 to 1.0 at% laminated on a substrate. 2. The conductive light-reflecting film according to claim 1, wherein the film of the Ag alloy layer containing Al is continuous and has a film thickness of 20 to 200 nm. 3. A substrate having at least an Ag layer, and 0.1-1.
A conductive light-reflecting film, comprising: an Ag alloy layer containing Al in an amount of 0 at%. 4. The conductive light reflecting film according to claim 3, wherein the film of the Ag alloy layer containing Al is continuous and has a film thickness of 20 to 200 nm. 5. The conductive light reflecting film according to claim 3, wherein an Ag layer and an Ag alloy layer containing Al are laminated in this order on the substrate. 6. The conductive light reflecting film according to claim 3, wherein an Ag alloy layer containing Al and an Ag layer are laminated in this order on the substrate. 7. The conductive light-reflecting film according to claim 3, wherein an Ag layer and an Ag alloy layer containing Al are laminated a plurality of times on a substrate and laminated. 8. The conductive light-reflecting film according to claim 1, wherein the grain size of the stacked Ag layer or the Ag alloy layer containing Al is 0.2 to 1 μm. . 9. The conductive light-reflecting film according to claim 1, wherein the unevenness of the laminated Ag alloy layer containing Al is 0.06 μm or more. 10. The conductive light-reflecting film according to claim 1, further comprising a conductive transparent electrode film above a stack of Ag alloy layers containing Al. 11. The conductive light-reflecting film according to claim 1, wherein an insulating substrate is used as the substrate material. 12. The conductive light reflection film according to claim 1, wherein a flexible substrate is used as the substrate material. 13. The conductive light-reflecting film according to claim 11, wherein a plastic film substrate is used as the substrate material. 14. A method of forming a conductive light-reflecting film comprising a substrate, an Ag layer, an Ag alloy layer containing Al, and a conductive transparent electrode film laminated on the substrate, wherein the substrate is heated to 250 to 360 ° C.
A method for forming a conductive light-reflecting film, which comprises forming a layer, an Ag alloy layer containing Al, and a conductive transparent electrode film. 15. A solar cell comprising the conductive light reflecting film according to claim 1 as a back electrode.