JPH0567797A - Transparent conductive substrate for solar battery and solar battery using it - Google Patents

Abstract

PURPOSE:To form a best particle of a laminated tin oxide film by providing a transparent conductive film with a diffraction peak by a (200) surface and a (110) surface and by providing diffraction strength by the (110) surface with specific relation to diffraction strength by the (200) surface in its X-ray diffraction pattern. CONSTITUTION:In an X-ray diffraction pattern which is mainly composed of tin oxide and measured by using a proportional counter tube, etc., it is desirable that an X-ray scattering strength (height of peak) at a strongest diffraction angle has ten or more times X-ray scattering strength in a back ground thereabouts. When an X-ray diffraction pattern of a film is appreciated by a diffraction strength defined by a product of a height of diffraction peak and a peak width at half height, it is desirable that the diffraction strength of the (110) surface is 20 or larger and 120 or smaller when diffraction strength of the (200) surface is 100, and that 30 or larger and 100 or smaller is more desirable. Crystal of an upper layer film of (200) orientation on a lower layer film of (110) orientation is the best.

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 for solar cells in which a transparent conductive thin film containing tin oxide as a main component is formed on a transparent substrate and a thin film semiconductor on the transparent conductive film of the substrate. , Improvement of a thin film solar cell in which a back electrode is sequentially formed.

[0002]

2. Description of the Related Art A method of forming a transparent conductive film containing tin oxide as a main component (hereinafter referred to as a conductive film) on a transparent substrate as a light receiving side electrode of a thin film solar cell is widely used. Examples of optimizing the conductive film by studying the fine structure of the film include JP-A-60-240166 and JP-A-61-1.
No. 15354 is known.

[0003]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
According to Japanese Patent No. 66, an amorphous silicon solar cell having a conversion efficiency of 9 to 9.6% by forming a conductive film with columnar crystals standing upright on a light receiving surface (translucent substrate in the present invention). It is said that can be created. Further, according to JP-A-61-115354, the X-ray diffraction pattern is (2
By using an extremely strongly oriented film on the (00) plane,
It is said that the conversion efficiency of the amorphous silicon solar cell is improved to about 9.2%.

Compared with the conversion efficiency of an amorphous silicon solar cell formed on a conductive film according to the prior art of about 7 to 8%, these inventions clearly show technological progress. The present invention provides a transparent conductive substrate for solar cells, which can obtain higher conversion efficiency.

[0005]

The present invention is a transparent conductive substrate for a solar cell in which a transparent conductive film containing tin oxide as a main component is formed on a substrate, and the transparent conductive film has its X-ray diffraction pattern. In the pattern, there are diffraction peaks due to the (200) plane and the (110) plane, and the diffraction intensity due to the (200) plane is 10
When 0, the diffraction intensity by the (110) plane is 20 or more and 120 or less, and the X-ray scattering intensity at the strongest diffraction angle is 10 times or more the X-ray scattering intensity in the background in the vicinity thereof. And a transparent conductive substrate for a solar cell.

The conductive film according to the present invention contains tin oxide as a main component, and in the X-ray diffraction pattern measured using a proportional coefficient tube or the like, the X-ray scattering intensity (peak height) at the strongest diffraction angle is in the vicinity thereof. It is preferable to have an X-ray diffraction pattern that is 10 times or more of the X-ray scattering intensity in the background (1), and more preferably 30 times or more, as shown in the following examples.

When various conductive films were formed and the X-ray diffraction patterns thereof were measured, the strongest diffraction angle ((2
A solar cell was prepared on a conductive film having a scattering intensity ratio of (00) plane at a diffraction angle of about 10 times, and the characteristics of the solar cell were measured. The results of forming a solar cell on the above are shown in Examples 2 and 3, and conversely, the result of forming a solar cell on a conductive film having a scattering intensity ratio of about 3 times is shown in Comparative Example 4.

The conversion efficiency of the solar cell is shown in Comparative Example 10 ((20
The result is obtained by a solar cell formed on a substrate using a conductive film strongly oriented on the (0) plane, and the details will be described later) are shown as relative values with 100. Examples 1, 2, and 3 are comparative examples 1.
The conversion efficiency of the solar cell is about 10% compared to 0, and 2
It improves by 0% and 15%. On the other hand, it can be seen that Comparative Example 4 is reduced to about 80% compared to Comparative Example 10.

Further, when the X-ray diffraction pattern of the film is evaluated by the diffraction intensity defined by the product of the height of the diffraction peak and the half width, when the diffraction intensity by the (200) plane is 100, the (110) plane is obtained. The diffraction intensity by 20 is preferably 20 or more and 120 or less, and 30 or more and 100 or less.
The following is more preferable.

Examples 1, 2 and 3 are examples satisfying such conditions, and all are preferable embodiments as the solar cell substrate of the present invention. On the other hand, Comparative Example 5,
As shown in FIGS. 6 and 7, when the diffraction intensity from the (110) plane of the conductive film is 120 or more, the solar cell is formed on the transparent conductive film according to the present invention even if the solar cell is formed thereon. It is not possible to obtain good battery characteristics as compared with (Examples 1, 2, 3). Also, when a solar cell is formed on a conductive film having a diffraction intensity from the (110) plane of 20 or less, as shown in Comparative Example 3, it is preferable as compared with the result using the transparent conductive substrate according to the present invention. No results are obtained.

The transparent conductive film of the present invention comprises two layers, and the first transparent conductive film (hereinafter referred to as the lower layer film) closer to the substrate has the highest diffraction intensity in the (110) plane in its X-ray diffraction pattern. It is preferable that the second transparent conductive film (hereinafter referred to as the upper layer film) which is strong and far from the substrate has the strongest diffraction intensity by the (200) plane in its X-ray diffraction pattern.

The reason for this will be described in detail in the section "Operation". The thickness of the first and second transparent conductive films is such that the thickness of the lower layer film is 500 Å or more and 5000 Å or less, and the thickness of the upper layer film is It is preferably 3000 Å or more and 15000 Å or less, more preferably 1000 Å or more and 4000 Å or less and the upper layer film is 3000 Å or more and 6000 Å or less.

Glass, plastic or the like can be used for the substrate of the present invention. When the substrate contains an alkali metal as its component, such as soda lime glass, in order to prevent the diffusion of the alkali metal from the substrate to the conductive film, Si, Al, Zr or the like is added between the substrate and the conductive film. It is more preferable to form an underlayer containing a metal oxide as a main component.

The method of forming the conductive film of the present invention is not particularly limited, but when the substrate is a material such as glass that can withstand high temperatures, the chemical vapor deposition method such as the spray method or atmospheric pressure CVD method is more preferable. Better conductive film characteristics can be obtained than those obtained by physical vapor deposition methods such as the method. Further, in order to realize the present invention, it is necessary to clearly separate the conductive film into two layers to form a film. Therefore, among the chemical vapor deposition methods, the atmospheric pressure CVD method having a two-stage film formation chamber is preferable. Is particularly preferable.

[0015]

[Function] As a transparent conductive substrate used in a thin film solar cell in which a transparent conductive film containing tin oxide as a main component, a thin film semiconductor, and a back electrode are sequentially formed on a transparent substrate, the resistance of the transparent conductive film is Low and high light transmission are the first requirements. However, in addition to that, when the particles forming the transparent conductive film have a grain boundary or the like, a defect occurs in the solar cell layer formed on the grain boundary as a starting point, and as a result, the conversion efficiency of the solar cell decreases. Therefore, it is preferable that the transparent conductive film has few grain boundaries, in other words, a film in which particles are sufficiently grown.

On the other hand, when a film is grown on a substrate such as glass, the particles of the film in contact with the substrate do not grow large, and a film having poor electrical and optical performance easily grows. As a method for forming a film that meets such a purpose, measures such as inserting an appropriate underlayer at the interface between the substrate and the film to be grown are generally taken.

According to the present invention, in addition to such a known technique, the growth process of the conductive film is examined in detail to realize an appropriate grain structure while maintaining low resistance and high transmittance.
It is intended to provide a method for producing a more preferable transparent conductive substrate for a solar cell.

As a result of detailed experiments, in the case of the tin oxide film, the exact reason is not clear, but when the film is grown from the substrate surface such as glass, when the film of (110) orientation is deposited, the film is It was found that the grains of the crystal forming the crystal grow the largest and the specific resistance thereof is the lowest as compared with the film oriented on other planes. An example of such a film is Comparative Example 1
Shown in.

However, when the film has a single-layer structure, the growth of the crystal grains of the film is limited even if an appropriate underlayer is formed on the substrate, and the single-layer structure is preferably used for solar cells.
It has been found that it is difficult to obtain a film with sufficiently large crystal grains. The present invention intends to solve this problem by allowing the conductive film to have a two-layer structure to sufficiently grow the particles of the upper layer film in contact with the solar cell layer.

From a series of experimental results, when tin oxide is re-laminated on the tin oxide film (first transparent conductive film), the re-laminated upper layer film (second transparent conductive film) has a single-layer film. In addition, the crystal growth of the upper layer film has a close relationship with the crystal state of the lower layer film, and the better the crystal state of the lower layer film, the more the crystal of the upper layer film tends to grow. all right.

That is, also in the case of the present invention, when tin oxide oriented in (110) was used as the lower layer film, the particles of the tin oxide film laminated thereon grew best. Also,
The reason for this is also not clear, but when the upper layer film is deposited on the lower layer film of (110) orientation, the crystal of the upper layer film grows best when the upper layer film has the (200) orientation. I understood. Furthermore, it was also found that with such a film configuration, it is possible to reduce the specific resistance of the conductive film without changing the light transmittance of the entire conductive film.

To summarize the above-mentioned examination results, high transmittance and low resistance are realized, and there are few grain boundaries, in other words,
In order to create a conductive film in which particles are sufficiently grown,
It is preferable to form an upper layer film of (200) orientation on a lower layer film of (110) orientation, and by making the conductive film have such a structure, the solar cell formed on this has high conversion efficiency. It was decided that he would be able to.

According to the result of examining the film thickness of the lower layer film from the viewpoint of grain growth of the conductive film, when the film thickness of the lower layer film oriented in (110) is less than 500Å, the upper layer film does not grow sufficiently. Since the effect of promoting grain growth is saturated even when the film thickness of the lower layer film is more than 5000Å, it was found that the effective film thickness range as the lower layer film is 500Å or more and 5000Å or less.

On the other hand, when the lower layer film is examined from the specific resistance of the conductive film, the specific resistance of the entire conductive film is not sufficiently reduced when the thickness of the lower layer film is 1000 Å or less. It is estimated that the cause of this is that, in addition to the low degree of growth of crystal grains, the lower layer film of (110) orientation, which has a lower specific resistance, is also affected. FIG. 16 shows the relationship between the film thickness of the lower layer film and the specific resistance and haze ratio (estimated to have a correlation with the degree of grain growth) of the entire conductive film. Through these studies, it was found that the preferable thickness of the lower layer film is 1000 Å or more and 5000 Å or less.

Further, it has been found that the upper layer film itself needs to have a certain thickness in order for grain growth to occur. When the film thickness of the upper layer film is less than 2000 Å, almost no grain growth is observed in the upper layer film even if there is a lower layer film of 1000 Å or more. On the other hand, as with other thin films, particles grow in the tin oxide film as the film thickness increases, but the amount of light absorption also increases as the film thickness increases. It is desirable to set the upper limit of about 15000Å.
That is, the range of the film thickness of the upper layer film can be 2000 Å or more and 14000 Å or less.

According to the experimental results, the film thickness of the upper layer film is 300.
If it is 0 Å or more, the crystal grows to a size that can sufficiently function as a substrate for solar cells. Therefore, in order to minimize the light absorption of the conductive film by the upper layer film, the film thickness of the upper layer film is set to 3
It is a more preferable option to set it to about 000Å or more and 6000Å or less.

[0027]

[Example] Using SnCl ₄ , CH ₃ OH, HF, and H ₂ O as raw materials, the substrate temperature was set to 540 ° C. and the atmospheric pressure CVD method was performed to obtain about 50
On a soda lime glass substrate coated with 0Å SiO ₂ ,
Various conductive films were laminated. In Comparative Examples 2, 3 and 10, the conductive film was laminated on the soda lime glass substrate by the same method. The X-ray diffraction measurement of the conductive film was performed by using a copper Kα ray and a rate meter using a proportional counter. The results are shown in Tables 1 and 2.

The numbers entered in the columns of SnCl ₄ , CH ₃ OH, and HF of the raw material compositions in the table indicate the carrier gas amounts for feeding the respective raw materials into the apparatus. However, for H ₂ O, the amount of water actually supplied to the equipment was measured and the value was entered. The X-ray intensity is the diffraction intensity of the (200) plane of 1
The relative intensity ratio when 00 is shown.

Hereinafter, description will be made according to the numbers. An example of a preferable film as the lower layer film of the conductive film according to the present invention is shown in Comparative Example 1.
An example of an unfavorable film is shown in Comparative Example 2. The film shown in Comparative Example 1 has a strong (110) orientation, whereas the film shown in Comparative Example 2 has a strong (200) orientation. The stronger the (110) orientation of the lower layer film is, the higher the tendency is to give a preferable result as a solar cell substrate having the upper layer film laminated. Furthermore, the stronger the degree of orientation of the lower layer film to the (110) plane is, the electric resistance of the conductive film itself The decrease in is as described in the section "Action". Note that there was no direct correlation between the orientation of the conductive film and the light absorption amount of the film.

In order to evaluate the performance as a transparent conductive substrate for solar cells, p, p
i-type and a-type a-Si (total of about 4000Å) and a silver back electrode (about 3000Å) were sequentially laminated to form a solar cell, and the conversion efficiency was measured.

In Comparative Example 10 in the table, an upper layer film was laminated on the lower layer film formed in the same manner as in Comparative Example 2 in the ratio of raw materials in the table, and is shown in JP-A-61-115354. It is an example of a conductive film that is extremely strongly oriented on the (200) plane as described above. The cell conversion efficiencies in the examples described below are conversion efficiency of each example shown as a relative value with the conversion efficiency of the solar cell formed on the conductive film shown in this comparative example as 100.

Examples 1, 2, and 3 are the results of laminating the film formed in the same manner as in Comparative Example 1 as the lower layer film, and laminating the upper layer film according to the raw material ratio shown in the table thereon. In the lower layer film, (2
The diffraction intensity of the (110) plane with respect to the diffraction intensity of the (00) plane (hereinafter referred to as the intensity ratio) is about 350, whereas the intensity ratio after the upper layer film is laminated is about 40-80.

Comparative Example 3 is a result of laminating the upper layer film on the lower layer film formed in the same manner as in Comparative Example 2 in the raw material ratio shown in the table. In this case, the strength ratio is about 13, and a film having a strong (200) orientation can be obtained. As you can see immediately from the evaluation results,
When the lower layer film having a strong (200) orientation is used, the conversion efficiency of the battery is equal to or lower than that of Comparative Example 10. It should be noted that the intensity ratio of X-ray diffraction in Comparative Example 3 is smaller than 20.

In Comparative Example 4, an upper layer film was laminated on the lower layer film formed in the same manner as in Comparative Example 1 in the raw material ratio shown in the table, and is also described in "Means for Solving the Problems". As described above, this is an example of a conductive film in which the scattering intensity at the strongest diffraction angle is only about three times the intensity of the background. Even if the specific resistance of the conductive film is sufficiently low, when a solar cell is produced using such a conductive substrate, the cell performance is significantly reduced.

Comparative Examples 5, 6, and 7 are also those in which the upper layer film was laminated at the raw material ratio shown in the table on the lower layer film formed in the same manner as in Comparative Example 1, and the "means for solving the problem" was obtained. This is an example described in section. Even if a battery is formed on a substrate using a conductive film having a strength ratio exceeding 120 as in these examples, the conversion efficiency is not improved.

In Comparative Examples 8 and 9, the film formed in the same manner as in Comparative Example 1 was used as the lower layer film, and the upper layer film was laminated at the raw material ratio shown in the table to form a conductive film, and the film thickness distribution between the upper and lower layers was determined. This is an example of changing. It can be seen that even if the lower layer film is thick and the upper layer film is thin as in Comparative Example 8, and the lower layer film is thin and the upper layer film is thick as in Comparative Example 9, the battery conversion efficiency decreases.

In the conductive film according to the present invention, the orientation of the lower layer film does not change by stacking the upper layer film. The conductive film having a two-layer structure according to the present invention was prepared (this X-ray diffraction pattern is shown in FIG. 14), and then the surface thereof was polished with diamond paste to remove the upper layer film by X-ray diffraction. The result of measuring the pattern is shown in FIG. Since the X-ray diffraction pattern of the conductive film obtained by removing the upper layer film by polishing agrees well with the pattern of the lower layer film alone, it can be seen that even if the upper layer film is laminated, the lower layer film does not change at all.

Tables 1 and 2 also show the diffraction intensities of the (211) plane and the (101) plane, but if the intensity of these diffraction lines is about 20% or less of the intensity of the (200) plane. , There was no direct correlation between the intensity of these diffraction lines and the solar cell performance.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

The growth temperature of the conductive film by the spray method disclosed in JP-A-61-115354 is 400 to 45.
The temperature is 0 ° C., which is lower than the growth temperature of a conductive film by a general spray method, and growing a film at such a low temperature is obviously a disadvantageous method from the viewpoint of the growth rate of the film. . In fact, as disclosed in the patent,
The growth rate of these conductive films was 10 to 30 Å / sec, which was a fatal problem from the viewpoint of industrial application.

In the case of the conductive film of the present invention, the deposition temperature of the conductive film can be raised to 500 ° C. or higher by inserting, for example, a SiO ₂ undercoat (underlayer) at the interface between the lower layer film and the substrate. .. Using the known atmospheric pressure CVD method, 5
If the film is formed at a sufficiently high temperature of 00 ° C. or higher, it is possible to improve the growth rate of the conductive film to several hundred Å / sec or more. Therefore, the present invention can be practically combined with a suitable underlayer. It also has the effect that the film can be grown at various film forming rates.

In general, when a conductive film is formed at a high temperature, if minute defects are present on the growth surface of the substrate, the particles of the film may grow abnormally, so that a good-quality conductive film may not be obtained. However, such problems can be solved at the same time by using an appropriate underlayer (preferably oxides of Si, Al, Zr, etc.).

When a solar cell is laminated on a conductive film on which particles are abnormally grown, it can be easily imagined that huge particles become a defect of the battery. Therefore, in order to realize high conversion efficiency, the conductive film should be formed. Although it is presumed that it is extremely important to constitute by homogeneous particles, the solar cell using the transparent conductive substrate according to the present invention shows stable and high conversion efficiency. It is presumed that the film is composed of extremely homogeneous particles while having a film growth rate of 100 Å / sec or more.

The conductive film according to the present invention is characterized in that the grain size of the crystals constituting the surface of the film is aligned, as can be imagined from the X-ray diffraction pattern. When observing such a film from the cross-sectional direction, it can be pointed out that the crystal grains have a pyramid shape and the apexes thereof have a crossing angle of approximately 90 to 120 degrees. This intersection angle is
For example, the crossing angle is obviously gentle as compared with the apex angle of the film recommended in JP-A-60-240166.

On the other hand, the crystal grains of the conductive film of the present invention have a flat grain surface, and therefore, they are disclosed in JP-A-61-2168.
As shown in No. 9, it is also a different structure than the rounded top membrane. The reason why good characteristics are obtained when a solar cell is formed on the conductive film of the present invention is
Imagine that because the intersection angle of the apex angles is gentle, structural defects are unlikely to occur in the solar cell layer, and because the top of the conductive film is not rounded, a sufficient light confinement effect can be secured. To be done.

Further, it can be understood from the X-ray diffraction diagram that the conductive film according to the present invention is composed of particles that have grown sufficiently and have a uniform particle size. That is, the full width at half maximum of the diffraction pattern for the (200) plane (the width of the diffraction angle 2θ at half the value of the diffraction peak intensity) is the same as in Example 3.
In the case of Comparative Example 4, it is about 0.7 degree to 1 degree.

According to the general theory regarding the spread of the X-ray diffraction pattern, it is considered that the film of Example 3 has a particle size several times to several tens of times that of the film of Comparative Example 4. That is,
It is presumed that it is most effective for improving the conversion efficiency of the thin-film solar cell that the conductive film has an apex angle of about 90 to 120 degrees and is composed of sufficiently grown uniform particles with few defects. To be done.

Such a conductive substrate is not only effective as an electrode on the light-receiving side of a thin film solar cell, but a solar cell is deposited by using such a substrate as a back electrode, and the structure opposite to that of the present invention is obtained. A favorable effect can be expected also when producing a solar cell (in this case, a light-transmissive solar cell).

Further, taking advantage of the fact that the surface of the conductive film of the present invention has a vertical angle (90 to 120 degrees) preferable for solar cells, a metal thin film such as silver or stainless steel is formed on the surface of the base. It is also possible to deposit and use this as a back electrode to make a solar cell. That is, if the conductive substrate is processed, it can be used as a metal electrode for a solar cell that has a good light confinement effect and does not require anticorrosion sealing on the back surface.

It goes without saying that the transparent conductive substrate of the present invention can be used not only for solar cells but also for other conductive substrate applications (for example, display elements and heating elements).

[Brief description of drawings]

FIG. 1 is a diagram showing an X-ray diffraction pattern of Example 1.

FIG. 2 is a diagram showing an X-ray diffraction pattern of Example 2.

FIG. 3 is a diagram showing an X-ray diffraction pattern of Example 3.

FIG. 4 is a diagram showing an X-ray diffraction pattern of Comparative Example 1.

FIG. 5 is a diagram showing an X-ray diffraction pattern of Comparative Example 2.

FIG. 6 is a diagram showing an X-ray diffraction pattern of Comparative Example 3.

FIG. 7 is a diagram showing an X-ray diffraction pattern of Comparative Example 4.

FIG. 8 is a diagram showing an X-ray diffraction pattern of Comparative Example 5.

FIG. 9 is a diagram showing an X-ray diffraction pattern of Comparative Example 6.

FIG. 10 is a diagram showing an X-ray diffraction pattern of Comparative Example 7.

FIG. 11 is a diagram showing an X-ray diffraction pattern of Comparative Example 8.

FIG. 12 is a diagram showing an X-ray diffraction pattern of Comparative Example 9.

FIG. 13 is a diagram showing an X-ray diffraction pattern of Comparative Example 10.

FIG. 14 is a diagram showing an X-ray diffraction pattern of a conductive film having a two-layer structure of the present invention.

15 is a diagram showing an X-ray diffraction pattern of the conductive film obtained by removing the upper layer film of the conductive film of FIG.

FIG. 16 is a graph showing the relationship between the film thickness of the lower layer film and the specific resistance (ρ) and haze ratio of the conductive film after lamination.

Claims (3)

[Claims] 1. A transparent conductive substrate for a solar cell, wherein a transparent conductive film containing tin oxide as a main component is formed on a substrate, the transparent conductive film having a (200) plane and an X-ray diffraction pattern in the X-ray diffraction pattern. It has a diffraction peak due to the (110) plane,
When the diffraction intensity by the (0) plane is 100, (11
Diffraction intensity by the 0) plane is 20 or more and 120 or less, and the X-ray scattering intensity at the strongest diffraction angle is 10 times or more the X-ray scattering intensity in the background in the vicinity thereof. Conductive substrate. 2. The transparent conductive film is composed of two layers, and the first transparent conductive film closer to the substrate has the strongest diffraction intensity by the (110) plane in the X-ray diffraction pattern, and the first transparent conductive film is closer to the substrate. The transparent conductive substrate for a solar cell according to claim 1, wherein the second transparent conductive film has the strongest diffraction intensity by the (200) plane in its X-ray diffraction pattern. 3. A solar cell in which a photoelectric conversion layer and a back electrode are sequentially formed on the transparent conductive film of the transparent conductive substrate for a solar cell according to claim 1.