JPH0742572B2 - Transparent conductive film - Google Patents

Description

TECHNICAL FIELD The present invention relates to a transparent conductive film having corrosion resistance, which is optimal as a substrate for solar cells.

[Prior Art] Generally, a transparent conductive film that is transparent and conductive in the visible light region is used as a transparent electrode in a new display system such as a liquid crystal display or an EL display, or as a transparent electrode in an amorphous silicon solar cell. It is used by forming a film on a transparent glass substrate for preventing photomask electrification. As a material for these transparent conductive films, indium oxide containing tin as a dopant, tin oxide containing antimony or fluorine as a dopant is mainly used. Hereinafter, these conductive films are assumed to be doped unless otherwise specified. Indium oxide, in particular, can have a lower resistance, and the selection of manufacturing conditions,
At present, about 10 ^-4 Ω.cm is obtained by controlling or adjusting the addition ratio of tin oxide.

[Problems to be Solved by the Invention] As described above, a transparent conductive film, particularly an indium oxide film, has excellent properties in terms of conductivity, but is extremely weak when examined for acid resistance or reduction resistance. Is. For example 100%
When a film formed by vapor-depositing indium oxide having a film thickness of 500 to 1000Å on a glass plate is immersed in hydrochloric acid having a concentration, it may be completely useless because the film is dissolved in 1 to 3 seconds. The indium oxide film is an oxygen-deficient semiconductor and has donor-type conductivity. In this film, the bond between indium and oxygen is weak, so that when ion bombardment is performed in a high-temperature atmosphere containing hydrogen or plasma, oxygen is liberated and metallic indium precipitates, resulting in devitrification. . This poses a serious problem when this indium oxide film is used as a semiconductor film for solar cells.
This is because the amorphous silicon film currently used as a semiconductor film for solar cells is mainly formed by a plasma CVD method using hydrogen plasma. As a method of improving such a problem, it is considered to form an oxide layer such as TiO ₂ or SiO ₂ as a blocking layer on the indium oxide film, but it may cause a problem in conductivity. There is a drawback that the plasma resistance is not sufficient. Further, the transparent conductive film mainly composed of tin oxide is also insufficient in terms of plasma reactivity.

[Means for Solving Problems] The present invention has been made to solve the above-mentioned problems, and the surface of a transparent conductive film containing indium oxide or tin oxide as a main component is coated with conductive zinc oxide. The present invention provides a transparent conductive film having improved corrosion resistance, which is characterized by forming a protective film comprising: a transparent conductive film mainly composed of indium oxide or tin oxide; It is intended to provide a transparent conductive film in which zinc oxide doped with either one is laminated.

Hereinafter, the present invention will be described in more detail.

FIG. 1 is a view showing an embodiment of a transparent conductive film according to the present invention, in which 1 is a substrate, 2 is a transparent conductive film containing indium oxide or tin oxide as a main component, 3 is a protective film, and 4 is a protective film. 3 shows an alkali barrier film.

As the transparent conductive film containing indium oxide as a main component in the present invention, tin is contained in an amount of 0.5 to 30% by weight, preferably about 2 to 10% by weight based on indium oxide, and a tin-doped indium oxide conductive film having conductivity is provided. The transparent conductive film mainly made of tin oxide is a fluorine-doped oxidation film containing fluorine in an amount of 0.1 to 5% by weight, preferably 0.3 to 2% by weight, based on tin oxide, and having conductivity. A tin conductive film or an antimony-doped tin oxide conductive film containing 0.1 to 30% by weight, preferably 0.3 to 5% by weight, of antimony with respect to tin oxide and imparting conductivity.

Such a tin-doped indium oxide conductive film can be manufactured by a sputtering method, a vacuum deposition method, or the like, and a fluorine-doped tin oxide conductive film is formed by a CVD method (Chemical Vapor Deposition).
deposition), sputtering method, vacuum deposition method, solution spray method, etc., and antimony-doped tin oxide conductive film can be manufactured by CVD method, sputtering method, vacuum deposition method, solution spray method, etc. it can. The film thickness of such a transparent conductive film is determined by the resistance value, optical characteristics, etc. to be obtained, but it is usually in the range of about 500 to 2 μ.

As the basic 1 for forming the transparent conductive film 2 described above, soda lime silicate glass plate, aluminosilicate glass plate, borosilicate glass plate, lithium are used in terms of transparency, optical characteristics, durability, electric characteristics and the like. Alkali-containing glass plates such as aluminosilicate glass plates, low alkali-containing glass plates, non-alkali glass plates, quartz glass plates, etc. are preferable, but in some cases, transparent plastic plates or transparent plastic films can also be used. . In the case of an alkali-containing glass plate such as a soda lime silicate glass plate, or a low alkali-containing glass plate, when a halide is used as a starting material, the alkali component on the surface of the glass plate is eluted and the transparent conductive film formed on it Alkaline barrier mainly composed of oxides such as SiO ₂ , Al ₂ O ₃ and ZrO _{2 is provided} on the transparent conductive film forming surface side of the above glass plate so that haze (cloudiness) due to precipitation of microcrystals of salt in the film does not occur. It is preferable to form the film 4.

In the present invention, a protective film made of conductive zinc oxide is formed on the surface of the transparent conductive film in order to improve the corrosion resistance, particularly plasma resistance, of the transparent conductive film containing indium oxide or tin oxide as a main component. Particularly preferably, a protective film made of zinc oxide (ZnO) doped with at least one of aluminum, indium, or tin is used in order to endow higher conductivity.

As the protective film made of zinc oxide containing at least one of aluminum, indium or tin, 0.1 to 10 at least one of aluminum, indium or tin is used as the protective film.
% Is optimal for lowering the specific resistance without lowering the transmittance. Zinc oxide is a white powdery compound that becomes a transparent film when made into a thin film. Even a conductive zinc oxide film containing no dopant can be used, but in a solar cell, it is desirable that the film has high conductivity because a current flows through this film. Its value is 10 ⁴
It should be Ω · cm or less.

Conventionally, zinc oxide is considered to be poor in electrical conductivity despite being an N-type semiconductor, but in recent years, a film having a specific resistance ρ of 10 ^-2 Ω · cm or less, which can be said to be conductive, has been produced. Depending on the case, low resistance films with ρ of about 10 ^-4 Ω · cm have also been created.
Further, since the raw material is relatively inexpensive, it is a material applicable as a transparent conductive film if the specific resistance further decreases. In addition, it has strong resistance to the reducing power of hydrogen plasma, and raises the glass substrate with a zinc oxide film to a substrate temperature of 300 ° C.
When exposed to a hydrogen plasma of about 00 mW / cm ² for 5 minutes, there was no change in the electroconductivity and the zinc oxide film thickness was not reduced at all. On the other hand, when the glass substrate of the tin oxide film is exposed to hydrogen plasma under the same conditions, the specific resistance increases and the transmittance greatly decreases. The present invention utilizes the strong hydrogen plasma resistance and high transmittance of a zinc oxide film to improve the low resistance, high transmittance, and inexpensive hydrogen oxide resistance of tin oxide.

As described above, zinc oxide can be a conductive thin film having a specific resistance of about 10 ^-3 Ωcm, but it is possible to lower the resistance by doping aluminum, tin or indium into the film. Yes, a product with a specific resistance of the order of 10 ^-4 Ω · cm is also available.

This fact indicates that even if the zinc oxide film is overcoated with zinc oxide on the transparent conductive film, the value of the sheet resistance of the solar cell substrate will not be affected. This fact is advantageous as compared with the case of overcoating another oxide having no electric conductivity.

In other words, the zinc oxide overcoat has excellent characteristics in terms of hydrogen plasma reactivity, transmittance, and resistance.

The thickness of the film does not have to be so thick as long as a film having characteristics close to the bulk can be formed, considering only that it improves the plasma reaction resistance of the underlying transparent conductive film, but the film thickness is small. As a result, the distribution is likely to be non-uniform, and the characteristics tend to be poor in terms of hydrogen plasma resistance. A film thickness of 50 Å or more is preferred to prevent distribution.
It is desirable to be 0 to 2000Å.

The method of forming the protective film in the present invention is not limited to a specific means such as a vapor deposition method or a sputtering method, but in order to obtain effective plasma reactivity resistance with the film thickness as shown above. It is necessary to form a film having a bulk property as close as possible, and for that purpose, it is desirable to use a plasma assisted film forming method such as a sputtering method, an ion plating method, or a plasma CVD method.

For example, when forming a film by a sputtering method,
A dense film can be easily obtained by setting a zinc target in a vacuum chamber of a sputtering apparatus, introducing argon gas or argon gas and oxygen gas, and applying an RF voltage to the target to perform sputtering. High conductivity,
In order to obtain a zinc oxide film doped with aluminum or indium or both having high transmittance and plasma reactivity, set a zinc oxide or zinc target and aluminum or indium or aluminum and indium target in a vacuum chamber. Then, an optimum film can be obtained by introducing an argon gas or an argon gas mixed with an oxygen gas in the range of 1 to 20 vol%, vacuuming and sputtering. Under these conditions, the in-plane specific resistance and transmittance of the transparent conductive film on which about 50 to 2000 Å of zinc oxide overcoated with at least one of aluminum, indium, and tin is applied, are the same as those before and after overcoating. There is no change. It is also considered that a dense film close to a bulk also serves as an effective diffusion barrier. Further, if the underlying transparent conductive film is also formed by the CVD method and the film of the present invention is also formed by the plasma CVD method, online production up to amorphous silicon is possible.

Since the transparent conductive film of the present invention has high plasma reactivity resistance,
Various films can be formed on the transparent conductive film by the plasma CVD method. Therefore, such a transparent conductive film is most suitable as a transparent electrode for an amorphous solar cell.

In manufacturing an amorphous solar cell, for example, a p-type amorphous Si film, an i-type amorphous Si film, and an n-type amorphous Si film are formed on a transparent conductive film of the present invention formed on a glass substrate by a plasma CVD method. It is sequentially formed and manufactured.

[Examples] Examples of the present invention will be described below.

Example 1 A metallic indium target containing 10 at% (atomic ratio) of tin and a pure metallic zinc sputtering target are set on a cathode in a vacuum chamber of a sputter apparatus. 1.1mm whose surface was cleaned by ceria polishing and washing with water
Place a thick soda lime silicate glass substrate in a vacuum chamber and evacuate to 5.0 × 10 ^-5 Torr or less with an oil diffusion pump.
The substrate temperature is raised to about 370 ° C. Next, in the vacuum chamber A
Filled with r: O ₂ = 62: 38 mixed gas, vacuum degree 2.2 × 10 ^-3 Torr
Set to 500 V DC on a tin-indium alloy target.
A voltage is applied and pre-sputtering is performed for 10 minutes. After pre-sputtering, the shutter was opened and sputtering was performed for 5 minutes. As a result, an In ₂ O ₃ conductive film having a film thickness of 4200 Å and containing 10 at% of transparent tin was obtained. Next, the atmosphere in the vacuum chamber was completely replaced with O ₂ without breaking the vacuum, the vacuum degree was adjusted to 1.2 × 10 ^-3 Torr, and the RF voltage of 2 KV was applied to the zinc target to perform pre-sputtering for 10 minutes. After that, sputtering was performed for 5 minutes. The thickness of the overcoated zinc oxide protective film was about 500Å. The transparent conductive film thus obtained has a specific resistance of 2.5 × 10 ⁻⁴ Ω · c.
m, the transmittance was 82%, and there was almost no change from that which was not overcoated. An ordinary plasma CVD apparatus for producing amorphous silicon is used on these transparent conductive film substrates, and the inside of the chamber of the apparatus is set to 1 × 10 ^-5 by an oil diffusion pump.
After exhausting to about Torr, SiH ₄ gas and B ₂ H ₆ gas diluted with hydrogen to 100 ppm were introduced into the chamber at a volume ratio of 1:10.
After forming a P-type amorphous silicon film at an RF output of 100 mW / cm ² and a substrate temperature of 300 ° C., the amorphous silicon film was etched using hydrazine hydrate to measure the specific resistance and transmittance of the transparent conductive film. . As a result, in the film without any overcoating, the specific resistance was increased by 1.6 times, and the transmittance was changed by 0.9 times, whereas the protective film of zinc oxide of this example was overcoated, and the specific resistance was changed in the film. The transmittance was not changed at all.

Example 2 A metallic indium target containing 10 at% tin, a pure zinc oxide sputtering target and an aluminum target were set on the cathode in the vacuum chamber of the sputter apparatus. A soda lime silicate glass substrate with a silica alkali barrier film (sheet thickness; 1.1 mm) whose surface has been washed by ceria polishing and water washing is placed in a vacuum chamber and evacuated to 5.0 × 10 ^-5 Torr or less with an oil diffusion pump. The substrate temperature is raised to about 370 ° C. Next, Ar: O ₂ = in the vacuum chamber
It is filled with a mixed gas of 62:38, the vacuum degree is set to 2.2 × 10 ⁻³ Torr, a DC voltage of 500 V is applied to the tin-indium alloy target, and pre-sputtering is performed for 10 minutes. After pre-sputtering, the shutter was opened and sputtering was performed for 5 minutes. As a result, an In ₂ O ₃ conductive film having a film thickness of 4200 Å and containing 10 at% of transparent tin was obtained. Next, the atmosphere in the vacuum chamber was changed to Ar: O ₂ = 4: 6 without breaking the vacuum.
After completely replacing it with a mixed gas of, and adjusting the degree of vacuum to 2.7 × 10 ^-3 Torr, 1.5K for a zinc oxide and aluminum target.
An RF voltage of V was applied to perform pre-sputtering for 10 minutes and then sputtering for 2 minutes. The thickness of the overcoated protective film made of zinc oxide was about 100Å. The transparent conductive film thus obtained had a specific resistance of 2.5 × 10 ⁻⁴ Ω · cm and a transmittance of 82%, which was almost the same as that which was not overcoated. A normal plasma CVD device for producing amorphous silicon was used on these transparent conductive film substrates, the chamber of the device was evacuated to about 1 × 10 ^-5 Torr by an oil diffusion pump, and then SiH ₄ gas and 1000 ppm were added. B ₂ H ₆ gas diluted with hydrogen was introduced into the chamber at volume ratio 1:10 and RF
After forming a P-type amorphous silicon film at an output of 100 mW / cm ² and a substrate temperature of 300 ° C., the amorphous silicon film was etched using hydrazine hydrate to measure the specific resistance and transmittance of the transparent conductive film. As a result, in the film without any overcoating, the specific resistance was changed to 1.6 times and the transmittance was changed to 0.9 times, whereas the film overcoated with the aluminum-doped zinc oxide protective film of this example was used. In, the specific resistance and the transmittance did not change at all.

Example 3 SiO ₂ formed by a CVD method as an alkali barrier film
Soda / lime / silicate glass substrate with silica / alkali barrier film (film thickness 2. 00Å) on the surface (plate thickness 2.
0 mm) was thoroughly washed, and then this glass substrate was put into a CVD device. After heating the glass substrate to 500 ° C, tin tetrachloride 1 x 10 ^-2 / min was set to 1 on this glass substrate surface and steam (1.1 x 10 ^-4 mol / min), water vapor (30 min), methyl alcohol (1 ) And nitrogen gas containing hydrofluoric acid,
A transparent conductive film (thickness 4000 Å) made of tin oxide doped with 1.0 wt% of fluorine at 5000 Å / min was formed. Then, the transparent conductive film-coated glass substrate was placed in a vacuum chamber of a sputtering apparatus in which a sputtering target of zinc oxide and a target of metal indium were set, and after evacuating the vacuum chamber to 1.0 × 10 ⁻⁵ Torr, Ar: A mixed gas of O ₂ = 4: 6 was introduced, the degree of vacuum was adjusted to 2.7 × 10 ⁻³ Torr, an RF voltage of 1.5 KV was applied to both targets, pre-sputtering was performed for 10 minutes, and then sputtering was performed for 2 minutes. The thickness of the overcoated indium-doped zinc oxide was 100Å. The transparent conductive film thus obtained has a specific resistance of 3.
The value was 0 × 10 ^-4 Ω · cm and the transmittance was 80%, which was almost the same as that without overcoating.

A normal plasma CVD device for producing amorphous silicon was used on these transparent conductive film substrates, the chamber of the device was evacuated to about 1 × 10 ^-5 Torr by an oil diffusion pump, and then SiH ₄ gas and 1000 ppm were added. B ₂ H ₆ diluted with hydrogen
Introduce gas into chamber with volumetric 1:10 and RF output 100mW /
After forming a P-type amorphous silicon film at cm ² and a substrate temperature of 300 ° C., the amorphous silicon film was etched using hydrazine hydrate to measure the specific resistance and transmittance of the transparent conductive film. As a result, in the film without any overcoating, the specific resistance was changed to 1.6 times and the transmittance was changed to 0.9 times, whereas the film overcoated with the protective film of indium-doped zinc oxide of the present example. In, the specific resistance and the transmittance did not change at all.

Example 4 SiO ₂ formed by a CVD method as an alkali barrier film
Silica, alkali barrier film-equipped soda, lime, silicate glass substrate (thickness 800 Å)
(2.0 mm) was thoroughly washed, and then this glass substrate was placed in a CVD device. After heating the glass substrate to 600 ℃, tin tetrachloride (1 × 10 ^-2 / min) was set to 1 and steam (1.1 × 10 ^-4 mol / min), steam (30 min), methyl alcohol (1 ) And nitrogen gas containing hydrofluoric acid,
Textured transparent conductive film made of tin oxide with 1.0 wt% fluorine at 5000Å / min (film thickness 9000Å)
Was formed. Then, the glass substrate with the transparent conductive film is placed in a vacuum chamber of a sputtering apparatus in which a sputtering target of zinc oxide and a target of metal tin are set,
After evacuating the vacuum chamber to 1.0 × 10 ⁻⁵ Torr, Ar: O ₂ = 4:
A mixed gas of 6 was introduced, the degree of vacuum was adjusted to 2.7 × 10 ⁻³ Torr, and an RF voltage of 1.5 KV was applied to both targets to perform pre-sputtering for 10 minutes and then sputter for 2 minutes. The film thickness of overcoated tin-doped zinc oxide was 100Å. The transparent conductive film thus obtained has a specific resistance of 2.8.
It was × 10 ^-4 Ω · cm and the transmittance was 80%, which was almost the same as that of the film without overcoating.

A normal plasma CVD device for producing amorphous silicon was used on these transparent conductive film substrates, the chamber of the device was evacuated to about 1 × 10 ^-5 Torr by an oil diffusion pump, and then SiH ₄ gas and 1000 ppm were added. B ₂ H ₆ diluted with hydrogen
Introduce gas into chamber with volumetric 1:10 and RF output 100mW /
After forming a P-type amorphous silicon film at cm ² and a substrate temperature of 300 ° C., the amorphous silicon film was etched using hydrazine hydrate to measure the specific resistance and transmittance of the transparent conductive film. As a result, in the film without any overcoating, the specific resistance was changed to 1.6 times and the transmittance was changed to 0.9 times, whereas the film overcoated with the tin-doped zinc oxide protective film of the present example. In, the specific resistance and the transmittance did not change at all.

[Effect of the Invention] As described above, according to the present invention, it is possible to remarkably improve the durability against a reducing plasma without impairing the transparency and conductivity of the transparent conductive film, particularly the indium oxide film and the tin oxide. This is very advantageous in using this film structure as a solar cell substrate using amorphous silicon as a substrate.

Further, the zinc oxide film having the conductivity according to the present invention can be formed by using a method other than the sputtering method such as the vacuum deposition method, the CVD method, etc. without losing the hydrogen plasma resistance by the manufacturing method. .

[Brief description of drawings]

1 and 2 are cross-sectional views for explaining the transparent conductive film according to the present invention. 1: substrate, 2: transparent conductive film, 3: protective film, 4: alkali barrier film

Claims (3)

[Claims] 1. A transparent conductive film having improved corrosion resistance, characterized in that a protective film made of conductive zinc oxide is formed on the surface of a transparent conductive film containing indium oxide or tin oxide as a main component. 2. The transparent conductive film according to claim 1, wherein the protective film is made of conductive zinc oxide doped with at least one of aluminum, indium, and tin. 3. The protective film has an atomic ratio of zinc oxide of at least one of aluminum, indium, and tin of 0.1 to 10.
%, The transparent conductive film according to claim 1.