JPH11145497A - Manufacture of solar battery and semiconductor thin film used therein - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method with which a homogeneous and good quality solar battery semiconductor film of large area can be formed in a short time. SOLUTION: In a thin film forming method, wherein a semiconductor material formed by compounding a substrate and a plate are superposed after a substrate 4 to be used for formation of a semiconductor thin film 3 and a heat resistant plate 5 are superposed with the semiconductor material interposed therebetween the substrate 4 is heated to a temperature higher than the plate 5. The plate 5 and the substrate 4 are directly heated by different heating means respectively, or only the plate 4 is heated directly.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a thin-film solar cell,
In particular, the present invention relates to a CdS / CdTe solar cell, and more particularly, to a method for manufacturing a semiconductor thin film used therein.

[0002]

2. Description of the Related Art Cadmium telluride (CdTe) has attracted attention as a light absorbing layer for thin-film solar cells. CdT
e has a high theoretical conversion efficiency because it has a forbidden bandwidth matching the peak of the solar spectrum. Also, CdTe
Is a semiconductor showing a direct transition, so that it has a large light absorption coefficient and can be made thin. Therefore, it is expected that a solar cell with high conversion efficiency can be obtained by using CdTe for the light absorbing layer. As a method for producing a semiconductor thin film for a solar cell, particularly a CdTe film, a proximity sublimation method is widely used. According to the proximity sublimation method, a semiconductor thin film having good crystallinity can be obtained. For example, among CdTe-based solar cells, a solar cell exhibiting the world's highest conversion efficiency of 15.8% has been obtained by the same method. In the proximity sublimation method, a semiconductor material (hereinafter, referred to as a source) and a thin film forming substrate on which a semiconductor thin film is to be formed have a thickness of 0.5 to 5 mm.
After the source is vaporized by heating the source mainly under a reduced pressure inert gas atmosphere, the semiconductor to be obtained on a thin film formation substrate at a lower temperature than the source is placed opposite to each other with a slight gap therebetween. Precipitate.

A method of forming a CdTe film by the proximity sublimation method is described, for example, in TL Chu et al., “The Conference Rec.
ord of the 22nd IEEE Photovoltaic SpecialistsConfe
rence (1991) ", vol. 2, pp. 952-956. In this report, commercially available 5N pure CdTe polycrystals or CdTe polycrystals to which dopants are directly added are set in a suitable holder. However, in the conventional proximity sublimation method including the above-mentioned report, since the thin film forming substrate is supported by the peripheral portion at the time of heating, the large area thin film forming substrate is used. When the substrate is used, it is difficult to keep the distance between the thin film formation substrate and the source constant over the entire surface of the substrate because the thin film formation substrate is bent by its own weight. It was difficult to form CdTe in FIG.
The relationship between the distance between the source and the thin film forming substrate when forming a film and the conversion efficiency of a CdTe / CdS solar cell using the obtained CdTe film is shown. As shown in the figure, the characteristics of the CdTe film obtained by a slight change in the distance between the two greatly vary.

On the other hand, JP-A-7-94769 discloses that
CdTe is formed on the surface of the thin film forming substrate by heat-treating the thin film forming substrate and the source in close contact with each other in a high temperature atmosphere.
A method for forming a film has been proposed. According to this, C
A dry film formed by kneading a mixed powder of d, Te and CdTe with propylene glycol to prepare a paste, applying the obtained paste on a glass substrate and drying the paste is used as a source. The source is disposed in close contact with a CdS film formed in advance on a thin film forming substrate, and then heat-treated in a high-temperature atmosphere. This allows
The source is vaporized in the same manner as in the above proximity sublimation method, and then CdS
The CdTe film is formed by depositing on the film. According to this method, a CdTe film can be easily formed with simple equipment. However, according to this method, a long heat treatment is required for forming the CdTe film. Also, a high quality CdTe film cannot be obtained. In fact, in this publication, a thin film forming substrate and a source that are in close contact with each other are placed under atmospheric pressure at 66 ° C.
The heat treatment was performed in a nitrogen atmosphere at 0 ° C. for 60 minutes, and the conversion efficiency of the obtained solar cell was not so high at 11.1%.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a method for efficiently producing a high-quality and uniform semiconductor thin film for a solar cell in a large area.

[0006]

According to the present invention, a substrate for forming a thin film and a heat-resistant plate are overlapped with a source interposed therebetween, and then the substrate and the plate are heated so that the plate has a higher temperature than the substrate. And heat. The reason why a CdTe film having satisfactory characteristics cannot be obtained by the method of the above publication is as follows.
It is estimated as follows. In this publication, since the thin film forming substrate and the source are heat-treated in a high-temperature atmosphere,
It is considered that the source and the substrate for forming a thin film are heated at substantially the same temperature. Therefore, the rate at which the vaporized source is deposited on the substrate for forming a thin film is low, and as a result, long-time heating is required. Also, according to the above publication, general Cd
As in the manufacturing process for Te-based solar cells, C
A transparent heat-resistant substrate on which a dS film is formed is used as a substrate for forming a thin film, and a CdTe film is formed on the CdS film. As a result, the temperature of the CdS film rises excessively during the formation of the CdTe film and is further heated for a long time, so that the S atoms in the CdS film diffuse into the CdTe film and the CdS _x Te _1-x
It is believed that mixed crystals form.

In view of the above, the present invention has a heating means disposed on the back surface of a thin film forming substrate and a plate which are superimposed on each other with a source interposed therebetween, and the thin film forming substrate and the plate are heated by heating means located on the respective back surfaces. Forming a semiconductor thin film on the thin film forming substrate by heating the thin film forming substrate and the plate so that the plate has a higher temperature than the thin film forming substrate. As a result, the Cd previously formed on the surface of the thin film
When a CdTe film is formed on an n-type semiconductor thin film such as an S film or CdZnS, the constituent element of the n-type semiconductor thin film is C
Prevents diffusion into dTe film. Further, a heating means is arranged only on the back surface of the plate to control the temperature of the plate to prevent the same diffusion.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The method for manufacturing a semiconductor thin film for a solar cell according to the present invention comprises the steps of: stacking a heat-resistant plate and a substrate for forming the semiconductor thin film with a semiconductor material interposed therebetween; And heating the substrate, comprising evaporating a semiconductor material and then depositing the semiconductor material on the substrate to form a semiconductor thin film containing a semiconductor as a main component. Also heat to high temperature. Here, the paste containing the semiconductor material is applied to either the plate or the substrate and dried to form a coating film, and the plate and the substrate are overlapped so as to sandwich the coating film, so that both can be easily adhered to each other. it can. By directly heating the plate and the substrate by separate heating means, a temperature difference is provided between the two. Alternatively, only the plate may be directly heated. At this time, the substrate is heated to a lower temperature than the plate due to the propagation of heat from the plate in close contact.

It is preferable to use a lamp heater as a means for heating the plate and the substrate for forming the thin film. By using the lamp heater, only one of the plate and the substrate can be preferentially heated. In the case where both the plate and the substrate are directly heated, lamp heaters are respectively arranged on their back surfaces. By using the lamp heater, the temperature of the plate and the temperature of the substrate can be easily controlled. Further, the plate and the substrate can be effectively heated, and for example, the temperature rising rate can be increased. Furthermore, the plate and the substrate can be uniformly heated, respectively, and the temperature variation inside them can be reduced. As described above, even when a large-area semiconductor thin film is formed, the distance between the source and the substrate can be kept constant by heating the plate and the substrate for thin film formation in close contact with each other. Is obtained.

The present invention can be applied to the formation of various semiconductor thin films, and is particularly useful for forming a CdTe film for a solar cell. When forming a CdTe film,
It is preferable that a mixture of Cd and Te or a source made of CdTe is sandwiched between a plate and a substrate for forming a thin film, and the plate is heated to 600 to 750 ° C.
When the heating temperature of the plate is lower than 600 ° C., the evaporation rate of the source becomes slow, and it takes a long time to form a film.
Prolonged heating lowers the production efficiency and also deteriorates the obtained semiconductor thin film. When heating is performed at a temperature higher than 750 ° C., the temperature of the substrate for forming a thin film excessively increases due to heat transfer from the plate, and the semiconductor thin film formed on the substrate surface is deteriorated. Here, the heating time of the plate and the substrate is desirably 30 minutes or less. If the substrate is heated for a long time, the temperature of the substrate rises due to the heat from the plate, and the CdTe film formed on the substrate is exposed to excessive heat and deteriorates. In particular, CdS
When a film is formed, diffusion of S atoms into the CdTe film is promoted. If the heating time is 30 minutes or less, the temperature difference between the plate and the substrate can be maintained,
Such a decrease in the quality of the CdTe film can be suppressed.

The above-mentioned heat treatment is performed by using helium at a pressure of 2 atm or less,
It is preferable to carry out in an atmosphere of an inert gas such as argon or nitrogen or a hydrogen atmosphere. According to the present invention, the formation of the semiconductor thin film does not need to be performed in a reduced pressure atmosphere, but can be performed, for example, under atmospheric pressure. Therefore, a semiconductor thin film can be formed with an inexpensive manufacturing apparatus. Further, by using an inexpensive inert gas such as nitrogen or argon, a CdTe film can be formed at low cost. A heat-resistant material such as glass or carbon is used for the plate and the substrate.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, as an embodiment of the present invention, a method for manufacturing a CdTe film in a CdTe / CdS solar cell as shown in FIG. 1 will be described in detail. As the glass substrate 1, borosilicate glass, low alkali glass, white plate glass or soda lime glass is used. First, a thickness of 1000 was applied to the surface of the glass substrate 1 by chemical vapor deposition or sputtering.
A transparent conductive film 2 having a thickness of 10,000 to 10000 angstroms is formed. This transparent conductive film 2 is made of, for example, tin oxide or indium tin oxide. Here, a silicon oxide film may be formed between the glass substrate 1 and the transparent conductive film 2 in order to prevent the alkali component from diffusing. Next, a cadmium sulfide (CdS) film 3 having a thickness of 500 to 2,000 angstroms is formed by thermally decomposing an organic complex of cadmium such as cadmium isopropyl xanthate on the surface of the transparent conductive film 2 to form a thin film forming substrate 4.
Get. A CdTe film was formed on the surface of the obtained thin film forming substrate 4 as follows. In addition, CdTe / CdS solar cells were manufactured using these, and the characteristics thereof were evaluated.

Example 1 A paste was prepared by mixing CdTe powder as a source with propylene glycol. The obtained paste was printed on a square glass plate 5 having a side of 100 mm and dried to form a coating film 6,
A source substrate 7 was obtained. On the other hand, a CdS film having a thickness of 0.1 μm was formed on the surface of a square glass substrate 1 having a side of 100 mm to obtain a thin film forming substrate 4. The thin film forming substrate 4 and the source substrate 7 obtained as described above were overlaid so that the CdS film 3 and the coating film 6 adhered to each other as shown in FIG. Next, the thin film forming substrate 4 and the source substrate 7 which are superimposed on each other are arranged between a pair of lamp heaters 12 in a nitrogen atmosphere of 1 atm, and the output of each lamp heater 12 is controlled to control the thin film forming substrate. 4 to 500 ~
At 650 ° C., the source substrate 7 is 5
Hold at -100 ° C higher temperature for 5-30 minutes.

As described above, C is formed on the substrate 4 for forming a thin film.
After forming the dTe film 8, a saturated solution of cadmium chloride in methanol is applied on the CdTe film 8, and after evaporating the methanol, this is heat-treated at 400 to 430 ° C. for 30 minutes to grow the grains in the CdTe film 8. I let it. Note that a saturated aqueous solution of cadmium chloride may be used instead of the methanol saturated solution of cadmium chloride. Meanwhile, carbon powder,
A carbon paste was prepared by kneading a solution of polyvinyl butyral in diethylene ether monobutyl ether as a thickener. The obtained carbon paste was applied on the CdTe film 8 by a screen printing method, dried and baked to form a carbon electrode layer 9. Thereafter, a mixed powder of silver and indium was kneaded with an alcohol solution containing epoxy and terpineol as main components to prepare a paste. The obtained paste was applied on the transparent conductive film 2 and the carbon electrode layer 9 by screen printing, respectively, and dried and baked to form the + side electrode 10 and the − side electrode 11.

<< Comparative Example >> Further, as a comparative example,
The substrate 4 for thin film formation provided with a μm CdS film and the source substrate 7 were brought into close contact with each other, and were held at 660 ° C. in a nitrogen atmosphere at atmospheric pressure for 60 minutes to form a CdTe film. Obtained C
A solar cell was manufactured in the same manner using the dTe film.

Various characteristics of the solar cell obtained as described above were measured. Among them, the temperature of the thin film forming substrate 4 is set to 5
After fixing at 80 ° C., the source substrate 7 was heated at 8 ° C. at various temperatures.
Table 1 shows the characteristics of the solar cell using the CdTe film formed by heating for about one minute and the solar cell of the comparative example.

[0017]

[Table 1]

As shown in Table 1, when the heating temperature of the source substrate is set at 600 to 750 ° C., a CdTe film having better characteristics than that obtained in the comparative example can be obtained. That is, Cd of higher quality in a shorter time than the comparative example
A Te film can be obtained. However, when the heating temperature of the source substrate is set to 575 ° C. or less, the obtained CdTe
The conversion efficiency of a solar cell manufactured using the film is low. This is because the evaporation rate of the source is low, and the obtained CdTe film is thin, and it is considered that this can be improved by heating for a longer time. On the other hand, the heating temperature of the source
Even in the case described above, the conversion efficiency similarly decreases. It is considered that this is because the thin film forming substrate is heated together with the source substrate, the temperature becomes higher than the initially set temperature, and S atoms in the CdS film diffuse into the CdTe film. Therefore, the heating temperature of the source substrate is 600 to 750 ° C., particularly 6 ° C.
75-725 ° C is desirable.

Example 2 In the same manner as in Example 1, both the glass substrate 1 of the thin film forming substrate 4 and the plate 5 of the source substrate 7 were glass plates each having a side of 35 cm. Is heated to 580 ° C. and the source substrate 7 is heated to 700 ° C., respectively, so that C
A dTe film 8 was formed. The thin film-forming substrate 4 having the CdTe film 8 formed on its surface is divided into nine equal parts as shown by broken lines in FIG. 3, and solar cells having an area of 1 cm ² are formed at the center of each of them. The properties were evaluated. Table 2 shows these conversion efficiencies.

[0020]

[Table 2]

As is clear from Table 2, a solar cell with high conversion efficiency can be obtained by using any part of the obtained large-area CdTe film. In addition, variations in characteristics between them are small. That is, according to the present invention, a uniform CdTe film having a large area can be formed. The source disposed between the thin film forming substrate and the plate is considered to vaporize preferentially from the higher temperature plate side. However, by heating the thin film forming substrate and the source substrate in close contact with each other, the CdT
Although the distance between the thin film forming substrate and the plate of the source substrate decreases as the formation of the e film progresses, the variation in the distance between the two over the entire surface of the source is smaller than in the case of the proximity sublimation method. It is considered that a CdTe film having small variation in pod quality can be obtained.

<< Embodiment 3 >> In the same manner as in Embodiment 1, the source substrate was set at 650 ° C., and the thin film forming substrate was set at 580 ° C.
C. for 5 to 60 minutes to form a CdTe film. A solar cell was manufactured using the obtained CdTe film, and these characteristics were evaluated. Table 3 shows the results.

[0023]

[Table 3]

As is clear from Table 3, the heating time was 30 minutes.
If the time exceeds minutes, the conversion efficiency of a solar cell using the obtained CdTe film decreases. At this time, it was confirmed that the temperature of the thin film forming substrate gradually increased from the set temperature. Therefore, it is considered that the decrease in the conversion efficiency of the solar cell is caused by deterioration of the film due to heating at a high temperature for a long time. As is clear from Table 3, according to the present example, a good quality CdTe film can be obtained by heating for 30 minutes or less. Even when the heating temperature of the source substrate is set to 650 ° C. or less, the temperature of the substrate for thin film formation gradually increases after 30 minutes, and the obtained C
The conversion efficiency of the solar cell using the dTe film tended to decrease.

Example 4 A CdTe film was formed in the same manner as in Example 1 except that the substrate for thin film formation and the source substrate were overlapped, and only the source substrate was heated at 550 to 800 ° C. That is, the source substrate to be heated to a higher temperature was heated, and the thin film forming substrate was heated by heat transfer from the source substrate. Since the temperature around the thin film forming substrate is lower than the heating temperature of the source substrate, the film forming substrate is stable at a lower temperature than the source substrate. Using the obtained solar cell, various characteristics of a solar cell similarly manufactured were evaluated. Table 4 shows the results.

[0026]

[Table 4]

When the heating temperature of the source substrate is set to 575 ° C. or lower, the conversion efficiency of the solar cell using the obtained CdTe film decreases. This is because the heating temperature is low,
This is because the evaporation rate of the source is low and the obtained film is thin. By performing the heat treatment for a longer time, the film can be thickened. However, as described above, long-time heating is not preferable because the film is deteriorated. On the other hand, when the heating temperature of the source substrate is set to 775 ° C. or higher, the temperature of the thin film forming substrate excessively increases, and the characteristics of the obtained solar cell deteriorate. Therefore, the heating temperature of the source substrate is 600-75.
0 ° C, especially 675-725 ° C, is desirable.

Example 5 A CdTe paste was printed and dried on the same thin film forming substrate as in Example 1 to form a coating film. Next, the substrate for thin film formation and a glass plate were overlaid so that the coating film was in close contact with the plate. The substrate for forming the thin film was heated at 580 ° C.
CdS of thin film formation substrate by heating at ~ 800 ° C for 8 minutes
A CdTe film was formed on the film. Using the obtained CdTe film, a solar cell similar to that of Example 1 was produced, and its characteristics were evaluated. Table 5 shows the results.

[0029]

[Table 5]

As shown in Table 5, when the heating temperature of the plate is set at 575 ° C. or less, the obtained CdTe film becomes extremely thin, so that various characteristics of the solar cell, especially the conversion efficiency, become low. On the other hand, when the heating temperature of the glass substrate is 775 ° C. or higher, the temperature of the substrate for forming a thin film becomes higher than the set temperature due to the heat from the plate, so that various characteristics of the solar cell are lowered. Therefore, the heating temperature of the source substrate is 6
00-750 degreeC, Especially 675-725 degreeC is desirable.

Example 6 A CdTe paste was printed and dried on the same thin film forming substrate as in Example 1 to form a coating film. Then, a glass plate was closely attached onto the coating film of the thin film-forming substrate and was superposed. The plate was heated at 550 to 800 ° C. for 8 minutes to form a CdTe film on the CdS film of the thin film forming substrate. CdTe obtained
Using the film, a solar cell similar to that of Example 1 was manufactured, and these characteristics were evaluated. Table 6 shows the results.

[0032]

[Table 6]

As shown in Table 6, when the heating temperature of the plate is set to 575 ° C. or less, the obtained CdTe film becomes extremely thin, so that various characteristics of the solar cell, especially the conversion efficiency, become low. On the other hand, when the heating temperature of the plate is set to 775 ° C. or higher, the temperature of the thin film forming substrate is excessively increased by the heat from the plate, and thus various characteristics of the solar cell are reduced. Therefore, the heating temperature of the source substrate is 600 to 7
50 ° C, especially 675-725 ° C, is desirable.

In each of the above embodiments, the case where CdS is used as the n-type semiconductor has been described. However, similar effects can be obtained when other n-type semiconductors, for example, CdZnS are used.

[0035]

According to the present invention, a high-quality, uniform, large-area semiconductor thin film for a solar cell can be efficiently and inexpensively manufactured. Further, by using the obtained semiconductor thin film, a high-performance solar cell can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view schematically showing the structure of a solar cell according to one embodiment of the present invention.

FIG. 2 is a longitudinal sectional view showing an arrangement of a source substrate and a thin film forming substrate when a semiconductor thin film is formed by heating in the embodiment.

FIG. 3 shows CdTe formed in another embodiment of the present invention.
It is a figure showing arrangement of a film.

FIG. 4 is a characteristic diagram showing a relationship between a distance between a source and a substrate for forming a thin film and a conversion efficiency of a solar cell using the obtained CdTe film in a conventional proximity sublimation method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Transparent conductive film 3 Cadmium sulfide film 4 Thin film formation substrate 5 Plate 6 Coating film 7 Source substrate 8 Cadmium telluride film 9 Carbon electrode layer 10 + side electrode 11-side electrode 12 Lamp heater

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Mikio Murosono 1-1 Matsushita-cho, Moriguchi-shi, Osaka Matsushita Battery Industry Co., Ltd.

Claims (10)

[Claims] 1. A step of stacking a heat-resistant plate and a substrate for forming a semiconductor thin film with a semiconductor material interposed therebetween, and evaporating the semiconductor material by heating the plate and the substrate. Forming a semiconductor thin film containing the semiconductor material as a main component by depositing the semiconductor material on the substrate, and heating the plate and the substrate to a higher temperature than the substrate in the step of heating the plate and the substrate. A method for producing a semiconductor thin film for a solar cell. 2. The method for manufacturing a semiconductor thin film for a solar cell according to claim 1, wherein the paste containing the semiconductor material is applied to the plate or the substrate and dried, and then the plate and the substrate are brought into close contact with each other. 3. The method according to claim 1, wherein the plate and the substrate are directly heated by separate heating means. 4. The method according to claim 1, wherein only the plate is directly heated. 5. The method according to claim 1, wherein the plate and the substrate are heated by a lamp heater. 6. The method according to claim 1, wherein the semiconductor thin film to be formed is made of cadmium telluride. 7. The heating temperature of the plate is from 600 to 75.
The method according to claim 6, wherein the temperature is 0 ° C. 8. The method according to claim 6, wherein the heating time of the plate and the substrate is 30 minutes or less. 9. The method according to claim 1, wherein the plate and the substrate are heated in an inert gas atmosphere of 2 atm or less. 10. A transparent substrate, a transparent conductive film formed on the surface of the substrate, a cadmium sulfide semiconductor film formed on the surface of the transparent conductive film, and telluride formed on a surface of the cadmium sulfide semiconductor film. A cadmium semiconductor film, a positive electrode electrically connected to the cadmium sulfide semiconductor film, and a negative electrode electrically connected to the cadmium telluride semiconductor film, wherein the cadmium telluride semiconductor film is A solar cell formed by a method according to any one of claims 1 to 9 on a surface of a cadmium sulfide semiconductor film previously formed on a substrate with the transparent conductive film interposed therebetween.