JP2000044238A - Production of tin dioxide film and solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for inexpensively producing a tin dioxide film having uniform film quality, excellent in transparency, electrical conductivity and weather resistance and capable of enlarging the surface area. SOLUTION: The tin dioxide film is formed on a substrate by atomizing a mixed solution of a tin compound with a fluorine compound or an antimony compound with ultrasonic vibration or spraying and pyrolyzing the atomized particulate in the vicinity of the previously heated substrate 1 for film forming. Further the film obtained by this way is used as a transparent electrically conductive film 2 to constitute various solar cells such as a CdS/CdTe solar cell.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a tin dioxide film and a solar cell using the tin dioxide film as a transparent conductive film.

[0002]

_2. Description of the Related Art In recent years, tin dioxide (hereinafter referred to as SnO ₂ ) films have become transparent and conductive films, and have recently been used as a current collecting film for various solar cells, various electronic devices such as liquid crystal clocks and portable electronic calculators. , Such as liquid crystal panels and plasma displays. As for the method of producing the SnO ₂ film, the following methods have been conventionally proposed or implemented, but each has advantages and disadvantages.
When the SnO ₂ film is formed by using a vacuum deposition method or a sputtering method, the cost of the manufacturing apparatus is high because the film is formed by vacuum, and the apparatus is particularly large in order to manufacture a large film having a manufacturing area of 1000 cm ² or more. High equipment costs are required. Further, in order to form a high quality film, it is necessary to reduce the film forming speed. For example, in order to form a film having a thickness of 1000 Å, a film forming time of 10 to 30 minutes is required.
Further, the uniformity of the film and the uniformity of the film thickness are poor, and 30 × 30
The variation in the film thickness on the cm substrate is as large as 30 to 40%.

In addition, a metal compound such as an organometallic compound is placed in a heating vessel, and a glass substrate for forming a film is placed in an atmosphere of vapor generated by vaporizing the compound, whereby SnO is placed on the glass substrate. A method of forming a transparent metal oxide film such as ₂ has also been proposed (for example,
JP-B-55-18785). In this case, SnO ₂
Since a transparent metal oxide film such as that described above originally has a high resistance, it is necessary to dope the film with fluorine, antimony, or the like at the time of film formation in order to increase conductivity. For this reason, Sn
The O ₂ film is formed by evaporating a mixture of a dope material such as ammonium fluoride and a tin compound as a raw material. However, in the above method, even when the molar mixture of the raw material and the dope material is kept constant, the saturated vapor pressure of both in the source gas changes during film formation. It was difficult. To control these,
High-precision control equipment is required, and the equipment cost is high.

As another film forming method, there is a method of applying a paste containing an organotin compound on a film forming substrate and thermally decomposing the paste to form a SnO ₂ film. Can be formed. However, since the film produced by this method has poor crystallinity and is easily decomposed, when left in a high-temperature and high-humidity atmosphere for a long time, the resistance value becomes 20 to 30.
It is currently rarely used as it increases by a percentage.

In general, a transparent conductive film of a solar cell is formed by forming a SnO ₂ film on a light-transmitting insulating substrate such as a glass substrate by the above-described vacuum deposition method or sputtering method. For example, a cadmium sulfide (hereinafter, referred to as CdS) film and a cadmium telluride (hereinafter, referred to as CdTe) film are sequentially laminated on a transparent conductive film formed of an SnO ₂ film formed on the substrate by these methods. Cd
In the case of an S / CdTe solar cell, the above-mentioned transparent conductive film has low resistance and high light transmittance, but it has been difficult to form a low-cost solar cell because of its high manufacturing cost. In addition, when the area is large, the film quality becomes non-uniform, and the film resistance and the light transmittance vary, so that the characteristics of the solar cell are deteriorated, and it is difficult to obtain a high conversion efficiency of 10% or more. Was.

Further, as a transparent conductive film of another solar cell such as a copper indium selenium solar cell (hereinafter referred to as a CIS solar cell), S is formed by the above-mentioned vacuum deposition method or sputtering method.
Also in the case of using the nO ₂ film, there are problems in the manufacturing cost and the characteristics as in the case of the CdS / CdTe solar cell. In addition, SnO formed by thermal decomposition of an organotin compound is used.
_{2 When} any of the above solar cells is manufactured using the film as a transparent conductive film, the film formation cost is low, but when used outdoors for a long time, the resistance value of the film increases and the conversion efficiency decreases. There was a problem of doing.

[0007]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a high quality SnO ₂ having excellent conductivity and transparency.
An object of the present invention is to provide a manufacturing method capable of forming a film uniformly at a low cost and in a large area. Another object of the present invention is to provide an inexpensive and highly reproducible solar cell with high conversion efficiency by using the SnO ₂ film obtained by the above method as a transparent conductive film.

[0008]

According to the present invention, there is provided a method for producing a SnO ₂ film, comprising the steps of atomizing a solution in which a tin compound and a fluorine compound or an antimony compound are dissolved, to form fine particles, and heating the fine particles. Forming a SnO ₂ film on the substrate by contacting the substrate with the substrate. Thus, using a simple apparatus, a uniform SnO in which the concentration of the atoms doped in the film is controlled to be constant.
_Two films can be formed. In addition, even when a large-area film is formed, a SnO ₂ film with little variation in film resistance, excellent transparency and conductivity, and high long-term reliability can be obtained at low cost and in a short time. . In addition, by using the SnO ₂ film formed according to the present invention as a transparent conductive film, various types of solar cells such as CdS / CdTe solar cells and CIS solar cells having low cost and high conversion efficiency can be formed.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is directed to a method in which a source solution in which a tin compound and a fluorine compound or an antimony compound as a dope material are dissolved is atomized into fine particles, and the fine particles are sprayed on the surface of a previously heated film-forming substrate. Upon contact, the finely divided compound in the solution is thermally decomposed on or near the substrate to form a SnO ₂ film doped with fluorine or antimony on the substrate surface.
In the present invention, the concentration in the fine particles can be controlled to be constant by keeping the concentration of the tin compound and the dope material in the source solution constant, so that fluorine or antimony is added to the SnO ₂ film formed on the substrate. Can be controlled to be constant. Thereby, a SnO ₂ film having uniform transparency and conductivity can be formed.

As a method of forming the source solution into fine particles, a method using ultrasonic vibration is effective. The particle size of the fine particles can be freely controlled by adjusting the energy of the ultrasonic vibration. Thus, when the fine particles reach the surface of the substrate heated to a predetermined temperature, the particle diameter can be controlled such that the solvent in the fine particles is vaporized, and further, the tin compound and the doped material in the fine particles are also vaporized. As a result, a thermal decomposition reaction between the tin compound and the dope material can be uniformly generated, and uniform and high-quality SnO ₂
The film can be formed over a large area. Moreover, since a simple method called ultrasonic vibration is used, such a high-quality SnO ₂ film can be formed at low cost. In order to obtain a more uniform and high-quality SnO ₂ film at low cost, it is preferable to use an ultrasonic oscillator having an ultrasonic vibration frequency of 10 kHz to 3 MHz. If the frequency is within this range, SnO
_{(2) The} film formation speed can be increased and the film quality can be stabilized. If the frequency of the ultrasonic vibration exceeds 3 MHz, the particle size of the fine particles becomes small, but since there is no ultrasonic vibrator having a high output, the amount of the source solution that can be made fine is insufficient, and the SnO ₂ film is formed. Speed slows down. Further, when the frequency of the ultrasonic vibration is less than 10 kHz, the particle size of the fine particles becomes excessively large, so that the decomposition reaction of the tin compound on the substrate surface becomes non-uniform, and the film quality uniformity of the formed SnO ₂ tin is reduced. Tends to decrease.

[0011] Further, when a spray injection method is used instead of the above-described atomization by ultrasonic vibration, the density of fine particles in a carrier gas such as air tends to decrease, and the film forming speed is reduced by the ultrasonic wave. Although it is slightly slower than the atomization method by vibration, an excellent SnO ₂ film can be formed with a lower-cost apparatus as in the method by ultrasonic vibration. In any case where the source solution is made into fine particles by any of the above methods, the center particle diameter of the fine particles (the center value of the particle size distribution range) is preferably 1 to 20 μm.
It is possible to form _two films. If the center particle diameter is smaller than 1 μm, a thermal decomposition reaction is likely to occur before the tin compound reaches the substrate surface, and it is difficult to form a SnO ₂ film with a high yield. When the center particle diameter is larger than 20 μm, the decomposition reaction of the tin compound on the substrate surface becomes non-uniform,
The uniformity of the quality of the formed SnO ₂ film tends to decrease.

Examples of the tin compound as a source material include tin halide alkyl compounds such as dimethyltin dichloride and trimethyltin chloride, tin carboxylate, tin β-diketone complex, and tin alkoxide alkyl halide compound. Although tin halides such as tin tetrachloride can be used, it is particularly preferable to use dimethyltin dichloride. Dimethyltin dichloride has high chemical stability to air and moisture, does not deteriorate during storage or work, has little adverse effect on the human body, is easily available, and is a solvent that is inexpensive and highly safe. By using this, there is an advantage that a high-quality SnO ₂ film can be formed at high speed, safely and at low cost because of its high solubility in water.

Since all of the above-mentioned tin compounds are thermally decomposed at a low temperature of 600 ° C. or less, the surface temperature of the film-forming substrate is set to be higher than the thermal decomposition temperature of the tin compound and 600 ° C. or less, thereby achieving a dense and light transmittance. , A low resistance SnO ₂ film can be formed. When diethyltin dichloride is used as the tin compound, a high-quality SnO ₂ film can be formed in a temperature range of 400 to 600 ° C., and particularly 480 to 58.
In a temperature range of 0 ° C., a SnO ₂ film having a higher light transmittance can be formed with a high yield. As a solvent for dissolving these tin compounds, water or an organic solvent diluted with water,
Further, an alcoholic solvent having an OH group can be used alone. In particular, when dimethyltin dichloride is used as the tin compound, it is preferable to use water as the solvent for the above-described reason. Examples of the doping material for the SnO ₂ film include fluorine compounds such as ammonium fluoride and sodium fluoride, and antimony chloride (SbCl ₃ , SbCl ₂₎ .
An antimony compound such as bCl ₅ ) can be used. Of these, the use of a fluorine compound is more effective for reducing the film resistance and improving the light transmittance.

The transparent conductive film made of the SnO ₂ film formed on the light-transmitting insulating substrate by the above-described method of the present invention has high conductivity and light transmittance and is uniform and can have a large area. By forming a solar cell by forming a semiconductor layer on the transparent conductive film, a solar cell having high conversion efficiency and stable characteristics can be obtained. Also, besides this,
By applying the present invention to the manufacture of the display surfaces of the various electronic devices described above, these can be improved in performance and reduced in cost. In the case of a solar cell, for example, an SnO ₂ film according to the present invention is formed as a transparent conductive film on a light-transmitting insulating substrate such as a glass substrate, and CdS is formed on this film by thermal decomposition of an organic cadmium compound containing sulfur. A film is formed as an n-type semiconductor window layer, and a CdTe film is formed on this film as a p-type semiconductor layer by proximity sublimation or the like, and a carbon film is formed on the p-type semiconductor film to form CdS / CdTe. A solar cell element can be configured. Further, by connecting these elements with silver electrodes or silver indium electrodes, it is possible to configure a solar cell module in which high efficiency single cells or single cell groups are connected in series.

Further, + on a substrate having conductivity and heat resistance
A p-type semiconductor film made of a CdTe film or a copper-indium-selenium compound film formed on the side electrode, a CdS film as a window layer of an n-type semiconductor formed on this film, and a p-type semiconductor film formed on this film In a CIS solar cell having a negative electrode made of a transparent conductive film, an SnO ₂ film may be formed as the transparent conductive film by the method according to the present invention.

[0016]

The present invention will be described below in more detail with reference to specific examples. Example 1 An SnO ₂ film 2 was formed on a film-forming substrate 1 by using a tin dioxide film forming apparatus shown in FIG. 1 using dimethyltin dichloride as a tin compound. 100 g of dimethyltin dichloride powder and 4 g of ammonium fluoride powder are applied for 360 c.
The source solution 8 prepared by dissolving in water c was placed in the source container 3, and the ultrasonic vibrator 4 having a frequency of 1 MHz was operated to atomize the source solution 8 into fine particles 7 having a center particle diameter of 10 μm. The atomized fine particles 7 are supplied to a carrier gas introduction pipe 6.
The particles were ejected from the fine particle outlet 5 together with air as a carrier gas introduced from the reactor, and were introduced into the muffle furnace 11 through the fine particle introduction tube 10. The atomized fine particles 7 introduced into the muffle furnace 11 are brought into contact with the surface of a glass film-forming substrate 1 placed on a metal conveyor belt 12 moving in the muffle furnace 11 to form an SnO ₂ film 2. I let it.

The surface temperature of the film forming substrate 1 was maintained at 550 ° C. by the heat transfer from the conveyor belt 12 heated by the heater 9 and the radiant heat from the inside of the muffle furnace 11. The atomized fine particles 7 and the vaporized tin compound, fluorine compound and water not used for film formation were discharged through the waste gas discharge pipe 13. The film formation time was 30 seconds, and the thickness of the SnO ₂ film 2 formed on the film forming substrate of 30 × 30 cm was 400 seconds.
It was 0 angstroms.

At this time, the temperature of the film-forming substrate is adjusted such that the atomized fine particles evaporate in the vicinity of the heated film-formed substrate, and the dimethyltin dichloride and ammonium fluoride in the atomized fine particles are vaporized. It is necessary to set an appropriate temperature for forming a SnO ₂ film by thermal decomposition. This temperature needs to be treated at a temperature suitable for the type of tin compound used as the source material, and the temperature conditions when diethyltin dichloride was used were studied in detail. As a result, it was confirmed that in a temperature range of 400 to 600 ° C., a film could be formed with a yield (a ratio of dimethyltin dichloride used as a component of the SnO ₂ film) of 10% or more. Among them, in the temperature range of 480 to 580 ° C., the light transmittance in the visible region of the light wavelength of 400 nm to 800 nm is 50 to 95%, and the volume resistance is 1 × 10 ^−6.
S that satisfies the electrical and optical characteristics required of a transparent conductive film of Ω · cm to 1 × 10 ^-2 Ω · cm and is free from cloudiness
An nO ₂ film was obtained with a yield of 40% or more. In this example, among these, this high-quality film was obtained with the highest yield.
The surface temperature of the film-forming substrate was set to 50 ° C.

The surface temperature of the film forming substrate is 400 ° C.
If it is less than 1, the rate of thermal decomposition of dimethyltin dichloride decreases rapidly, so that the rate of formation of the SnO ₂ film greatly decreases,
The film quality deteriorated and the film resistance increased. Also, when heated to a temperature exceeding 600 ° C., the glass film forming substrate is deformed,
Further, the formation of the SnO ₂ film rapidly occurred, the crystal particle diameter of SnO ₂ became large, and the film became cloudy due to scattering of incident light due to unevenness of the film surface. Further, the thickness of the SnO ₂ film 2 formed on the formation substrate was also uneven.

<< Comparative Example 1 >> SnO ₂ was formed by a conventional vacuum deposition method.
A film was prepared. That is, an SnO ₂ film was formed on a 30 cm × 30 cm glass substrate using tin oxide as a target under a reduced pressure of 1 torr. The film was formed at a rate of 200 Å / min for 20 minutes.

With respect to the thus formed SnO ₂ films of Example 1 and Comparative Example 1, the surface resistivity was measured at 30 places as shown in FIGS. 2 and 3, and the distribution was examined. As a result, as shown in FIG. 2, in Example 1, 5.1 Ω / □ to 5.4 Ω / □ (in terms of volume resistance: 2.04
× 10 ^{−4 to} 2.16 × 10 ⁻⁴ Ω · cm), and the variation was within 6%. On the other hand, as shown in FIG. 3, the distribution of the surface resistivity of Comparative Example 1 was 20Ω / □ to 28Ω / □.
(Volume resistance conversion: 8.0 × 10 ^{-4 to} 1.12 × 10 ^-3 Ω
Cm), and a large variation of 40% was observed. Also, a large area SnO ₂ of 100 cm × 30 cm is used.
When the film was formed in the same manner as in Example 1, the variation in the surface resistivity of the film was within 10%, and it was confirmed that the film could be formed in a large area extremely stably. In addition, it was confirmed that the light transmittance of the SnO ₂ film thus produced had a light transmittance of 80 to 95% at a wavelength of 450 nm.

<< Comparative Example 2 >> SnO _{2 by a} conventional coating printing method
A film was prepared. That is, a paste containing tin toluate is applied on a 30 cm × 30 cm glass substrate by screen printing, and this is heat-treated at 500 to 550 ° C. and thermally decomposed to form a SnO film having a thickness of about 4000 Å.
_Two films were formed. Tables 1 and 2 show the results of measuring the change in the surface resistivity of the SnO ₂ films of Example 1 and Comparative Example 2 in a high temperature and high humidity test at 60 ° C. and 95% RH, respectively. In the case of Example 1 in Table 1, the increase in surface resistivity after 2000 hours was stable within 5%. On the other hand, in the case of Comparative Example 2 in Table 2,
A large increase of 0% was observed.

[0023]

[Table 1]

[0024]

[Table 2]

Embodiment 2 FIG. 4 shows a sectional structural view of a solar cell manufactured in this embodiment. 1cm square glass substrate 2
1, a SnO ₂ film 22 is formed in the same manner as in Example 1, a CdS film 23 is formed on this film 22 by a thermal decomposition method of an organic cadmium compound, and further a proximity sublimation method is formed on the CdS film 23. A CdTe film 24 is formed, and a CdTe film 24 is formed.
A carbon film 25 was formed thereon. Further, the carbon film 2
An AgIn electrode 26 was formed on the sample No. 5 and the SnO ₂ film 22 to form a compound semiconductor solar cell device.

The CdS film 23 is formed by heating the cadmium dimethyldithiocarbamate complex to vaporize the cadmium complex.
The glass substrate 21 on which the nO ₂ film 22 is formed is heated and placed at 450 ° C., and the vapor of the cadmium dimethyldithiocarbamate complex is thermally decomposed on the surface of the SnO ₂ film 22 to form a Sn substrate.
An O ₂ film 22 was formed. The CdTe film 24 is made of Cd
The SnO ₂ film 22 and Cd are contained in a container containing Te source.
The glass substrate 21 on which the S film 23 is laminated is placed, the pressure in the container is reduced to 1 torr, the CdTe source is set to 640 ° C., and the glass substrate 21 is set to 600 ° C., and is held for 2 minutes. Formed on top. Further, the CdS film 23
The CdTe film 24 and the CdTe film 24 were simultaneously partially removed by irradiating a laser beam from the glass substrate 21 side, and the SnO ₂ film 22 in that portion was exposed to form a negative electrode. A carbon paste is applied on the CdTe film 24,
After drying, a carbon film 25 as a positive electrode was formed.

<< Comparative Example 3 >> On a 1 cm square glass substrate,
In the same manner as in Comparative Example 1, 4000 angstroms of S
A CdS / CdTe solar cell element was produced in the same manner as in Example 2 except that the element formed with the nO ₂ film was used.

The photovoltaic characteristics of these solar cell elements were measured using a solar simulator at 25.degree.
It was measured under the condition of mW / cm ² . As a result, Example 2
Then, Jsc = 24 mA / cm ² , Voc = 0.84
The conversion efficiency was 14.5% at V, FF = 72.1%. On the other hand, in Comparative Example 3, Jsc = 22 mA / cm ² ,
The conversion efficiency was 9.8% at Voc = 0.74V and FF = 60.2%, and Example 2 showed more excellent values for each characteristic.

Example 3 Next, a solar cell module was manufactured and its characteristics were evaluated. First, an SnO ₂ film was formed on a 30 × 30 cm glass substrate in the same manner as in Example 2, and then patterned by laser scribe.
A CdS film and Cd formed thereon in the same manner as in Example 2
The Te film was laser-scribed at the same time and divided into 28 films of a single cell unit. Forming a carbon film on each CdTe film,
The surface of each carbon film and the exposed surface of the SnO ₂ film of the adjacent cell were connected by a silver electrode, and 28 cells were connected in series to produce a solar cell module.

<< Comparative Example 4 >> A 4000 Å S film prepared in Comparative Example 1 using a 30 cm square glass substrate.
A solar cell module was manufactured in the same manner as in Example 3 except that the nO ₂ film was used.

The characteristics of all the cells (28 cells) in these solar cell modules were measured in the same manner as in the case of the solar cell element of Example 2. As a result, for each characteristic, Comparative Example 4 showed a large variation of 22%, whereas
In Example 3, the variation was 10% or less, and stable and good characteristics were exhibited.

[0032]

According to the present invention, it is possible to form a SnO ₂ film having excellent conductivity, light transmittance and weather resistance and uniform film quality even in a large area by using a simple manufacturing apparatus. Further, by using the SnO ₂ film as a transparent conductive film, a CdS / CdTe solar cell having good conversion efficiency, CI
Various solar cells such as S solar cells can be provided at low cost. In addition, by using the SnO ₂ film obtained by the present invention for a display surface of various electronic devices and the like, the cost and performance thereof can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view showing a film forming apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a distribution of surface resistivity of a tin dioxide film obtained according to an example of the present invention.

FIG. 3 is a view showing a distribution of surface resistivity of a tin dioxide film obtained by a conventional manufacturing method.

FIG. 4 is a schematic vertical sectional view of a CdS / CdTe solar cell element according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Film-forming substrate 2 Transparent conductive film 3 Source container 4 Ultrasonic oscillator 5 Fine particle ejection port 6 Carrier gas introduction pipe 7 Atomized fine particles 8 Source solution 9 Heater 10 Fine particle introduction pipe 11 Muffle furnace 12 Transport belt 13 Gas discharge Tube 21 glass substrate 22 tin dioxide film 23 CdS film 24 CdTe film 25 carbon film 26 AgIn electrode

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Miwa Tsuji 1-1, Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery Industry Co., Ltd. (72) Inventor Go Nishio 1-1, Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery (72) Inventor Takeshi Hibino 1-1, Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery Industrial Co., Ltd. (72) Mikio Murono 1-1, Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery F term (reference) in the company 5F051 AA09 AA10 BA04 BA05 CB11 CB27 FA03 FA08 FA24 GA03 5F103 AA01 BB01 BB27 DD21 DD30 HH04 KK10 LL20 NN01 PP11 RR03 RR08

Claims (10)

[Claims] 1. A step of atomizing a solution in which a tin compound and a fluorine compound or an antimony compound are dissolved to form fine particles, and a step of contacting the fine particles with a heated substrate to form a tin dioxide film on the substrate. A method for producing a tin dioxide film, comprising: 2. The method for producing a tin dioxide film according to claim 1, wherein the step of atomizing the solution to form fine particles is a step of atomizing the solution by ultrasonic vibration to form fine particles. 3. The ultrasonic vibration frequency is 10 kHz.
The method for producing a tin dioxide film according to claim 2, wherein the frequency is 3 MHz or less. 4. The method for producing a tin dioxide film according to claim 1, wherein the step of atomizing the solution is a step of atomizing the solution by spraying to atomize the solution. 5. The method according to claim 1, wherein the center value of the particle size distribution range of the fine particles is 1
The method for producing a tin dioxide film according to any one of claims 1 to 4, wherein the thickness is not less than 20 µm and not more than 20 µm. 6. The method for producing a tin dioxide film according to claim 1, wherein said tin compound is dimethyltin dichloride. 7. The tin dioxide film according to claim 1, wherein the tin compound is dimethyltin dichloride, and a temperature range of the surface of the heated substrate is 400 ° C. or more and 600 ° C. or less. Manufacturing method. 8. A transparent conductive film made of a tin dioxide film formed on a light-transmitting insulating substrate, a cadmium sulfide film formed on the transparent conductive film, and cadmium telluride formed on the cadmium sulfide film. A solar cell comprising a film, wherein the transparent conductive film is a tin dioxide film formed by the method according to claim 1. 9. The solar cell according to claim 8, wherein the cadmium sulfide film is a cadmium sulfide film formed by thermal decomposition of an organic cadmium complex. 10. The solar cell according to claim 8, wherein the cadmium telluride film is a cadmium telluride film formed by a proximity sublimation method.