JPH07131044A - Transparent conductive substrate - Google Patents

Abstract

(57) [Summary] [Structure] The first transparent conductive thin film 2 is formed on the transparent insulating substrate 1.
And a second transparent conductive thin film 3 different from the transparent conductive thin film 3 formed on the transparent conductive substrate, wherein the second transparent conductive thin film 3 is a crystalline thin film. . [Effect] It is possible to provide a transparent conductive substrate for a solar cell, which has excellent plasma resistance even under highly reducing plasma conditions such as high substrate temperature and high power density.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transparent conductive substrate most suitable as a transparent electrode for amorphous silicon solar cells, display elements and the like.

[0002]

2. Description of the Related Art A transparent conductive substrate for a solar cell has a concavo-convex structure having a high light confining effect and is required to have high transparency and conductivity. A tin oxide film has been used as a transparent conductive thin film for solar cells that satisfies the above characteristics. However, the tin oxide film has a drawback that its transmittance decreases when it is exposed to reducing plasma. Currently, a plasma CVD method is mainly used as a method for forming an amorphous silicon film, which is a photoelectric conversion layer of a solar cell, on a transparent conductive substrate.

Therefore, the transparent conductive substrate is exposed to reducing hydrogen plasma in the amorphous silicon film. At this time, due to ion bombardment from hydrogen plasma, weak oxygen and tin are broken, oxygen is released and metal tin is deposited, and thus the transmittance is reduced. Such a decrease in the substrate transmittance is a problem because it causes a decrease in the conversion efficiency of the battery. Another problem is that tin oxide or oxygen liberated during film formation diffuses into the amorphous silicon layer, resulting in a decrease in the film quality of the amorphous silicon that is the photoelectric conversion layer and a decrease in conversion efficiency of the solar cell. Thus, since the tin oxide film has low resistance to reducing plasma, the conditions for forming the amorphous silicon layer have been limited to a range that does not cause a decrease in the substrate transmittance.

Therefore, in order to improve such a problem, the group of the present inventors formed a protective film made of conductive zinc oxide on the surface of a transparent conductive thin film containing tin oxide as a main component, and thus the transmittance was improved. Has been proposed (Japanese Patent Laid-Open No. 63-89).
No. 657), the plasma resistance is significantly improved by this method as compared with the case where the protective film is not formed, but the plasma resistance is not always sufficient under the hydrogen plasma exposure condition having a high reducing property. Therefore, there is a drawback that it is not sufficient as a substrate for producing an extremely high-performance battery.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, and provides high resistance to high reducing plasma exposure conditions such as high substrate temperature and high power density. An object of the present invention is to provide a transparent conductive substrate for a solar cell that exhibits plasma properties.

[0006]

SUMMARY OF THE INVENTION The present invention is a transparent conductive substrate in which a first transparent conductive thin film is formed on a transparent insulating substrate, and a different second transparent conductive thin film is formed on the first transparent conductive thin film. In the second aspect, there is provided a transparent conductive substrate, wherein the second transparent conductive thin film is a crystalline thin film.

The present invention will be described in more detail below. Among the transparent conductive thin films in the present invention, tin oxide and fluorine are 0.1 to 5 relative to tin oxide as the first transparent conductive thin film.
Electrical characteristics such as tin oxide doped by weight%, tin oxide doped with antimony by 0.1 to 30 weight% with respect to tin oxide, and indium oxide doped with tin by 0.5 to 30 weight% with respect to indium oxide. Of a transparent metal oxide having a good transparency is suitable. Above all, the transparent conductive thin film made of fluorine-doped tin oxide has a sheet resistance of 30 Ω /
□ The following low resistance is easily obtained, and a predetermined surface irregularity shape exhibiting a high light confining effect is easily obtained, which is the most suitable as a solar cell substrate.

In the present invention, the second transparent conductive thin film formed on the first transparent conductive thin film is preferably a crystalline thin film, particularly a transparent conductive thin film containing zinc oxide as a main component. Is preferred. For example, in order to improve the plasma resistance of the transparent conductive thin film containing tin oxide as a main component, a protective film made of zinc oxide is formed in the uppermost layer in an amount of 20 to 2000 Å.

In the transparent conductive substrate according to the present invention,
Reflection high-energy electron diffraction (RHEED) when the thickness of zinc oxide, which is a protective film, is as thin as 20 to 2000Å
A diffraction pattern of a zinc oxide crystal is observed by the method, and in such a zinc oxide protective film, it is preferable that the (002) plane of zinc oxide is mainly parallel to the surface of the first transparent conductive thin film. For example, as described above, when a tin oxide film having a surface irregularity is used as the first transparent conductive thin film, the tin oxide film has a facet surface (mainly composed of a low index surface of tin oxide) on the tin oxide surface. Then, the zinc oxide transparent conductive thin film crystal is mainly formed so that the (002) planes are parallel to each other. This makes it possible to provide a transparent conductive substrate that solves the above problems.

As described above, in the present invention, the surface of the first transparent conductive thin film has unevenness, and the crystal orientation of the second transparent conductive thin film is controlled with respect to each uneven surface. It is intended to provide a transparent conductive substrate which is a crystalline thin film.

[0011]

The present invention utilizes the high plasma resistance and high transmittance of a crystalline zinc oxide film with controlled orientation to improve the plasma resistance of a tin oxide transparent conductive substrate. The zinc oxide film has a strong resistance to the reducing power of hydrogen plasma. Among them, in particular, when the orientation is controlled so that the crystalline zinc oxide film, preferably the (002) plane of the zinc oxide crystal is parallel to the surface of the first transparent conductive thin film, Higher plasma resistance than high quality zinc oxide film. In particular, when exposed to hydrogen plasma at a substrate temperature of 300 ° C. or higher, when the surface of the tin oxide transparent conductive substrate is coated with an amorphous zinc oxide film, reductive deterioration occurs and the transmittance of the substrate decreases. On the other hand, when the crystalline zinc oxide film is coated, reduction deterioration is significantly suppressed and the transmittance is maintained.

It is considered that a large number of crystal grain boundaries exist in the zinc oxide film having poor orientation. Since hydrogen easily moves at the crystal grain boundaries, when exposed to hydrogen plasma, hydrogen penetrates through the crystal grain boundaries of the zinc oxide film to reduce tin oxide which is the base. Therefore, it is considered that the plasma resistance is improved when the surface of tin oxide is uniformly covered with a zinc oxide film having a uniform orientation and a small number of crystal grain boundaries. It is considered that this effect becomes remarkable especially when the film thickness of the zinc oxide layer is as thin as about 100Å as in the present embodiment.

[0013]

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows a sectional structure of a main part of a transparent conductive substrate for a solar cell according to the present invention. In FIG.
Reference numeral 1 is a soda lime silicate glass substrate with an alkali barrier (silica) film, 2 is a tin oxide film, and 3 is a crystalline zinc oxide film. This transparent conductive substrate was produced as follows.

A SiO ₂ film (thickness 800
Å) Alkaline barrier (silica) film-formed soda lime silicate glass substrate 1 (1.2 m thick)
m) from one end of the CVD furnace to a belt conveying speed of 0.20 m /
Introduced into the furnace in minutes, after heating the substrate temperature to 600 ℃,
A reaction gas was blown onto the substrate from a gas introduction nozzle to form a fluorine-doped tin oxide film 2 (thickness 8000Å) on the SiO ₂ film on the substrate. The tin oxide film 2 has an uneven surface.

A method for producing tin oxide having an uneven surface is described in detail in JP-A-63-313874.
SnCl ₄ , H ₂ O, CH ₃ O entrained in N ₂ gas
A belt having H, HF and N ₂ gas for dilution as a reaction gas, a heating part and a cooling part, and a heating part having a nozzle for introducing the reaction gas and an exhaust hole for exhausting the gas after the reaction. A tin oxide film was formed using a transfer type CVD furnace.

FIG. 2 shows the result of observing the tin oxide film 2 using the reflection high-energy electron diffraction (RHEED) method to examine the crystallinity. It can be seen that the tin oxide film has crystallinity and orientation with respect to the substrate because the diffraction pattern is a point shape with a spread. Assuming that the tin oxide crystal having a rutile structure has a (200) orientation with respect to the substrate and the angle between the [200] direction of the tin oxide crystal and the substrate normal direction is distributed within 20 degrees in FIG. Shows the expected theoretical diffraction pattern. When FIG. 2 and FIG. 3 are compared, since the positions of the diffraction points are in good agreement, the tin oxide crystals are mainly (200) oriented with respect to the substrate, and the [200]
It can be seen that the distribution of the angle formed by the direction and the normal to the substrate is within 20 degrees.

Next, the surface of the tin oxide transparent conductive substrate described above was coated with a crystalline zinc oxide film 3 having a film thickness of 100 liters. The crystalline zinc oxide film was formed by RF sputtering using Ar gas at a substrate temperature of 250 ° C. and a pressure of 1.0 × 10 ⁻³ T.
The film was formed under the conditions of orr and RF power of 1.5 kW. RHEED of transparent conductive thin film coated with such zinc oxide film 3
The observation result is shown in FIG. Compared with the diffraction pattern of tin oxide in FIG. 2, since the pattern clearly changes,
FIG. 4 is considered to be a diffraction pattern by the zinc oxide crystal with which the surface of the substrate is coated.

Since the diffraction pattern is in the form of dots having a spread, it can be seen that the zinc oxide film has crystallinity and orientation with respect to the substrate. As a result of analyzing the diffraction pattern, zinc oxide [10
It became clear that the [1] direction was almost perpendicular to the glass substrate. On the other hand, from the observation of the surface shape by SEM which was separately performed, the uneven surface of the tin oxide film in which the (200) plane was oriented parallel to the glass substrate was (111) tin oxide.
It turned out to be composed of faces. FIG. 6 is a schematic diagram showing the orientation relationship between the tin oxide film and the zinc oxide film.

The (200) plane of tin oxide is parallel to the glass substrate. That is, the [200] direction of tin oxide is perpendicular to the glass substrate. The angle between the [200] direction of tin oxide and the [111] direction of tin oxide ((20
The angle between the (0) plane and the (111) plane of tin oxide is about 61.
[002] direction of zinc oxide and [1] of zinc oxide.
The angle formed by the [01] direction (the angle formed by the (002) plane of zinc oxide and the (101) plane of zinc oxide) is about 62 degrees. From the above, it is considered that the (002) plane of zinc oxide grows in parallel on the (111) surface of tin oxide (C-axis oriented). The theoretical pattern obtained in such a case is shown in FIG.

In FIG. 5, it was assumed that the (002) plane of zinc oxide was parallel to the (111) plane of tin oxide, that is, the orientation distribution of zinc oxide with respect to tin oxide was 0 degree. Since the orientation distribution with respect to the substrate is 20 degrees, the orientation distribution of zinc oxide with respect to the glass substrate is 20 degrees. Since the pattern in FIG. 5 is in good agreement with the pattern actually obtained (FIG. 4), the (002) plane of the zinc oxide crystal is substantially parallel to the (111) facet plane of the underlying tin oxide film. It is understood (Example). In this case, if they are substantially parallel, there is no practical problem, and specifically, it is desirable that the angle formed by the [002] direction of zinc oxide and the [111] direction of tin oxide be within about 10 degrees.

The transparent conductive thin film coated with 100 Å of the zinc oxide film 3 has a specific resistance of 5.0 × 10 ⁻⁴ Ω · cm and a transmittance of 85%, which is higher than that before coating (Comparative Example 1 described later). As a result, there was no change in resistivity or transmittance. The transmittance here is an average value of the spectral transmittances at 400 nm to 800 nm, and will be simply referred to as transmittance hereinafter.

The conditions for forming such a crystalline zinc oxide film are not particularly limited as long as the zinc oxide film covering the surface has the above-mentioned crystallinity and orientation, but the substrate temperature is 20.
0 degreeC-600 degreeC is preferable. Further, the method for forming the zinc oxide film is not limited to the RF sputtering method,
It can also be prepared by a DC sputtering method, an ion plating method, a plasma CVD method, an atmospheric pressure CVD method, or the like.

In application to a solar cell, since light passes through such a zinc oxide film and electricity flows, it is necessary to reduce the series resistance component of the cell and reduce light absorption in order to improve conversion efficiency. Is. Therefore, it is desirable that the film thickness of the zinc oxide film is thin as long as the crystalline film can completely cover the surface of the underlying transparent conductive substrate having the uneven shape. However, as the film thickness becomes thinner, the distribution tends to become nonuniform, and the characteristics tend to deteriorate in terms of hydrogen plasma resistance. In order to prevent distribution, it is desirable that the film thickness is 20 Å or more, preferably 50 to 2000 Å.

A second component containing such crystalline zinc oxide as a main component
The transparent conductive thin film of aluminum, indium,
Containing 0.1 to 12% of at least one dopant selected from the group consisting of boron, silicon and gallium is optimal for reducing the specific resistance without reducing the transmittance. Even a conductive zinc oxide film containing no dopant can be used, but when applied to a solar cell or the like, it is desirable that the film has high conductivity because a current flows through this film.

[0025]

Comparative Example On the other hand, as a comparative example, a substrate in which a zinc oxide film which is a second transparent conductive thin film is not formed on the surface in the structure shown in FIG. 1 (Comparative Example 1) and a transparent conductive substrate having the same structure as FIG. A transparent conductive substrate (Comparative Example 2) in which the zinc oxide film, which is the second transparent conductive thin film, was amorphous was prepared. The manufacturing method of the zinc oxide film in Comparative Example 2 is the same as that of the Example except that the substrate temperature is set to room temperature. As a result of RHEED observation of the zinc oxide film 3 in Comparative Example 2, any reflection other than a halo pattern (a state in which a regular pattern is not obtained because the electron beam is scattered irregularly and the background becomes bright as a whole) Was also not observed,
It was confirmed that the zinc oxide film 3 was amorphous.

Next, the plasma resistance of each substrate was evaluated by the following method. First, the substrate was placed in a plasma CVD apparatus and exposed to hydrogen plasma at a substrate temperature of 200 ° C and 300 ° C. The hydrogen plasma exposure conditions are shown in Table 1. Table 2 shows changes in the substrate transmittance before and after exposure to hydrogen plasma.

According to the example and the comparative example 1, when the surface of the tin oxide having the uneven shape is coated with the crystalline zinc oxide film (the example), compared with the case where it is not coated (the comparative example 1). , No decrease in transmittance due to hydrogen plasma exposure.
In addition, no increase in the specific resistance of the substrate occurred before and after the exposure.

According to Comparative Examples 1 and 2, when the tin oxide surface was coated with the amorphous zinc oxide film (Comparative Example 2), the hydrogen plasma exposure at the substrate temperature of 300 ° C. gave the transmittance before exposure. It drops from 85% to 70%. This result shows that the substrate whose surface is coated with the amorphous zinc oxide film suppresses the decrease in transmittance as compared with the case where the substrate is not coated (Comparative Example 1), but the substrate has a plasma resistance at 300 ° C. It shows that it is insufficient.

According to the example and the comparative example 2, when the amorphous zinc oxide film is coated on the tin oxide surface (comparative example 2), when the hydrogen plasma is exposed at the substrate temperature of 300 ° C., the transmittance is 85 before the exposure. % To 70%. On the other hand, when coated with a crystalline zinc oxide film (Example), it is 85%,
No decrease in transmittance was observed. From these results, under the strongly reducing plasma conditions as shown in Table 1, the zinc oxide coating the tin oxide surface should have the same crystallinity and orientation as described in Examples in order to exhibit sufficient plasma resistance. It is clear that they must have sex.

[0030]

[Table 1]

[0031]

[Table 2]

Needless to say, the present invention is not limited to the transparent conductive substrates for solar cells shown in the examples, but is also effective as transparent electrodes for liquid crystal displays, EL displays, plasma displays and the like. .

[0033]

According to the present invention, in a transparent conductive substrate, a first transparent conductive thin film is formed on a transparent insulating substrate, and a different second transparent conductive thin film is further formed thereon. By using at least the second transparent conductive thin film as a crystalline zinc oxide thin film with controlled orientation, high substrate temperature,
It is possible to provide a transparent conductive substrate for a solar cell that has excellent plasma resistance even under highly reducing plasma conditions such as high power density.

Further, since the crystalline zinc oxide film whose crystallinity is controlled is very stable in terms of crystal structure, by using it as a transparent conductive substrate for solar cells, element diffusion into the photoelectric conversion layer is suppressed. As a result, the film quality of the photoelectric conversion layer is improved,
It is expected that the conversion efficiency of solar cells will be improved.

[Brief description of drawings]

FIG. 1 is a sectional view of a transparent conductive substrate for a solar cell according to an embodiment of the present invention.

FIG. 2 is an electron diffraction photograph showing a RHEED pattern of a tin oxide film.

FIG. 3 is expected when it is assumed that the tin oxide crystal is [200] oriented with respect to the substrate and the angle formed by the [200] direction of the tin oxide crystal and the substrate normal is distributed within 20 degrees. Diagram showing theoretical diffraction pattern

FIG. 4 is an electron diffraction photograph showing a RHEED pattern when a tin oxide film surface is coated with a crystalline zinc oxide film.

FIG. 5 is a view showing a theoretical diffraction pattern obtained when it is assumed that the [101] direction of zinc oxide has a distribution within 20 degrees with respect to the substrate normal direction.

FIG. 6 is an explanatory diagram showing the orientation relationship between tin oxide and zinc oxide crystals.

[Explanation of symbols]

1: Soda lime / silicate glass substrate with alkali barrier (silica) film 2: First transparent conductive thin film 3: Second transparent conductive thin film

Claims (5)

[Claims] 1. A transparent conductive substrate in which a first transparent conductive thin film is formed on a transparent insulating substrate, and a different second transparent conductive thin film is further formed on the first transparent conductive thin film. A transparent conductive substrate, wherein the thin film is a crystalline thin film. 2. The second transparent conductive thin film has a thickness of 20.
The transparent conductive substrate according to claim 1, which is a transparent conductive thin film whose main component is zinc oxide of about 2000 Å. 3. The transparent conductive substrate according to claim 1, wherein the second transparent conductive thin film is a crystalline thin film whose orientation is controlled. 4. The (002) plane of the zinc oxide crystal of the second transparent conductive thin film is substantially parallel to the surface of the first transparent conductive thin film. Transparent conductive substrate. 5. The surface of the first transparent conductive thin film has unevenness, and the second transparent conductive thin film is a crystalline thin film in which crystal orientation is controlled with respect to each of the uneven surfaces. The transparent conductive substrate according to any one of claims 1 to 4, which is characterized in that.