JPS61267373A - Backplane electrode of photoelectric conversion element - Google Patents

Abstract

PURPOSE:To obtain an Al backplane electrode with excellent corrosion resistance by composing the backplane electrode of an Al film under compression stress. CONSTITUTION:On a transparent glass substrate 10, an ITO film 11 is formed as a transparent electrode and an amorphous silicon film 12 is formed as a semiconductor film so as to expose a part of the electrode 11. An Al film 13 is formed as a backplane electrode by an ion plating method and a Schottky junction between the Al film 13 and the amorphous silicon film 12 is formed. Lead wires 14 are bonded to bonding parts 15 with aluminum solder and the hole body is sealed with silicon resin 16 to obtain a solar battery. As a means to form the Al film, an ion plating method utilizing ion beam or cluster ion beam or a sputtering method is employed to leave compression stress in the Al film.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a back electrode of a photoelectric conversion element, and more specifically, to an Al back electrode of a photoelectric conversion element that has extremely excellent corrosion resistance. .

2. Description of the Related Art Currently, photoelectric conversion elements are widely used in solar cells, light receiving elements for optical communications, and the like. These photoelectric conversion elements have become longer-lasting due to improvements in the semiconductor that constitutes the central part of the element, and can now be used for long periods of time without the need for replacement. However, photoelectric conversion elements have not yet achieved a lifespan comparable to that of semiconductors. One of the reasons for this is the deterioration of electrodes formed on semiconductors.

This will be explained using the following specific example. FIG. 3 is a cross-sectional view illustrating the structure of a solar cell as an example of a conventional photoelectric conversion element. The illustrated solar cell has a transparent substrate 1, on which a transparent electrode 2 is formed, and further on which a semiconductor film 3 is formed so as to expose a part of the transparent electrode 2.

A back electrode 4 is formed on the semiconductor film 3 and is in a Schottky junction with the semiconductor film 3. Furthermore, a lead wire 5 is connected to the transparent electrode 2 and the back electrode 4 through a bonding portion 6 . Further, the entire area closer to the semiconductor film 3 than the transparent electrode 2 is sealed with a resin 7. In addition,
The relationship between the thicknesses of the respective layers of the solar cell is not actually the dimensional relationship shown in the drawings, and the drawings appropriately exaggerate the thinner layers to make them thicker.

In such a solar cell, light 8 passes through a substrate 1 and a transparent electrode 2, enters a semiconductor film 3, and is converted into electricity by the semiconductor film 3.
It is taken out of the device through two lead wires 5 connected to the transparent electrode 2 and the back electrode 4 through bonding portions 5, respectively.

The efficiency of the solar cell described above can be evaluated based on the surface reflectance, the absorption coefficient of the semiconductor, the electrical resistance and light reflectance of the back electrode □, etc. Among these, the stability of the characteristics of the back electrode is now relatively important. This is because the surface reflectance is
This is because it can be almost permanently improved by providing an antireflection film, and the absorption coefficient of a semiconductor is determined by the properties of the semiconductor material.

If the electrical resistance of the back electrode is large, the amount of power extracted will be lost, so it is necessary that the electrical resistance be low. On the other hand, in order to efficiently convert the light that has reached the back electrode into electricity, it is necessary to return the light to the semiconductor film.
On the other hand, as the back electrode absorbs light, the temperature increases accordingly, so the light reflectance of the back electrode needs to be high.

Furthermore, aluminum wires are now being widely used as lead wires, and it is preferable not to form insulating intermetallic compounds between the lead wires and the aluminum wires. Therefore, an aluminum (81) film is currently used for the back electrode.

Problems to be Solved by the Invention As described above, in solar cells using conventional Al film back electrodes, as the usage time becomes longer, a pair of lead wires becomes weaker even though the life of the semiconductor film itself has not been reached. There was a problem in that the electrical resistance between the electrodes increased and the photoelectric conversion rate decreased.

The reason for this was considered to be that the electrical resistance of the back electrode increased. In order to confirm this, a photoelectric conversion device having the structure shown in FIG. 4 was fabricated as a prototype to measure the electrical resistance of the back electrode on the semiconductor film, and the electrical resistance of the back electrode on the semiconductor film was measured.

In FIG. 4, parts similar to those in FIG. 3 are given the same reference numerals and their explanations will be omitted. The solar cell for measurement shown in FIG. 4 differs from the solar cell shown in FIG. 3 in that a pair of lead wires 5 are wire-bonded with a distance between them on the back electrode. When such a semiconductor device was left in a high humidity atmosphere for a predetermined period of time, the resistance between the pair of lead wires 5 increased.

This is because the corrosion resistance of the AI electrode is poor, and moisture enters from the leak path 9 between the transparent electrode 3 and the sealing resin 7, causing corrosion and deterioration of the aluminum of the back electrode, increasing the resistance value. It is judged that.

Corrosion of the aluminum wire is also considered in this connection.

However, ■The aluminum wire is far from the leak path, and ■The AI back electrode has a thickness of several hundred to one thousand people, and its corrosion has a large effect on the electrical resistance, whereas the aluminum wire has a thickness of several tens of μm, which is somewhat It is judged that corrosion of the aluminum wire is not the cause for two reasons: the corrosion has almost no effect on the electrical resistance.

For this reason, it can be determined that the increase in resistance between the pair of lead wires 5 of the solar cell for measurement shown in FIG. 4 is due to deterioration of the back electrode.

The deterioration of the back electrode as described above is caused by the intrusion of moisture through the leak path 9, etc., and deterioration of the back electrode can be prevented by ensuring complete sealing between the sealing resin and the semiconductor film. However, in reality, since the sealing resin and the semiconductor are different materials, it is impossible to achieve complete sealing over a long period of time under various temperature conditions.

The above problems are common not only to solar cells but also to other photoelectric conversion elements such as light receiving elements for optical communication.

Therefore, the present invention aims to provide an Al back electrode with extremely excellent corrosion resistance for a photoelectric conversion element.

Means for Solving the Problems In view of the above-mentioned current situation, the inventor of the present invention has conducted various studies to improve the corrosion resistance of the back electrode of a photoelectric conversion element, and has developed the following.
It has been found that the corrosion of the l electrode occurs from its grain boundaries, the degree of corrosion depends on the stress state of the Al film, and that the corrosion can be solved by placing the Al film in a compressive stress state. The present invention is based on this knowledge.

That is, the present invention provides a photoelectric conversion element having a semiconductor film, an incident side electrode provided on the light incident side of the semiconductor film, and a back electrode provided on the side opposite to the incident side electrode of the semiconductor film. , the back electrode is made of an Al film under compressive stress.

Further, the semiconductor film described above can be made of any conventionally known material, for example, it is composed of a single layer of a single semiconductor such as amorphous silicon, amorphous germanium, crystalline silicon, or crystalline germanium. Alternatively, it can be made of an elemental semiconductor such as silicon or germanium having a PN junction or a PIN junction, or a compound semiconductor such as a ■-■ group semiconductor. In particular, amorphous silicon-based and amorphous φ germanium-based
It is a particularly suitable material for photoelectric conversion elements used in harsh corrosive environments such as outdoors.

Although the compressive stress state of the Al film described above can be provided by a sealing material, it is certainly and preferably achieved by the residual stress remaining after forming the Al film on the semiconductor film.

As an Al film forming means, in the sense of protecting the semiconductor film,
Physical vapor deposition (PVD) using vacuum means and chemical vapor deposition (C
VD) method must be used. Among these methods, A
Regarding the I film, various methods such as vacuum evaporation are being considered from the viewpoint of cost, but the Al film formed by these methods is generally in a state of tensile stress. Therefore, as mentioned above, in order to leave compressive stress in the Al film, the ion plating method using an ion beam or cluster ion beam,
It is advantageous to use sputtering methods.

Further, the residual stress of the Al film is at least 6 kg/m
m2, and further preferably, the crystal grain size of the Al film is 1 μm or less.

As explained above, by maintaining the AI back electrode in a compressive stress state, it is possible to prevent corrosion of the Al back electrode, which has been a problem in the past, especially corrosion caused by moisture entering through the 9 shortest leak paths. .

The inventors conducted a thorough investigation on the layer quality and corrosivity of an Al film on a semiconductor film. As a result, as mentioned above, corrosion of the Al film occurs from the grain boundaries, that is, the corrosion of the Al film occurs from the grain boundaries.
It was found that it depends on the stress state of the membrane.

Specifically, using a 42% Ni-Fe alloy as a substrate, an Al film was formed by various methods, and the relationship between Al film stress and corrosivity was investigated and evaluated in four stages. The results are shown in Table 1. Regarding the conditions for the ion plating method and the sputtering method in Table 1, the deposition rate, bias, etc. should be adjusted while taking into account other characteristics of the back electrode of the photoelectric conversion element, such as the adhesion between the Al film and the semiconductor film. It is formed with consideration to voltage and substrate temperature. From the results in Table 1, AI
When the membrane stress is in a tensile state, corrosion resistance deteriorates extremely;
On the contrary, when the material is in a compressive stress state, the corrosion resistance is good, and it will be understood that the higher the compressive stress is, the better the corrosion resistance is.

Table 1 Note 1: In the 81 membrane stress, 10 indicates tensile force and - indicates compressive force.

Note 2: For corrosion resistance, ◎ is the best, followed by ○, △, and
× indicates the worst condition.

The Al film stress was determined by X-ray diffraction, and the corrosion test was conducted using
This was carried out using a pressure test (121°C, 2 atmospheres).

The exact relationship between the above phenomenon, that is, the stress state of the Al coating and its corrosion resistance, is currently unclear, but if residual tensile stress exists in the Al film, the stress will be concentrated near the grain boundaries and It is thought that stress corrosion has occurred. That is, when the membrane stress is in a tensile state, stress corrosion is likely to occur, and it is considered to be disadvantageous from the viewpoint of corrosion potential and hydrogen embrittlement.

Furthermore, regarding the grain size, the finer the grain, the less influence it has on the surroundings of crystal grains that are preferentially corroded due to crystal orientation differences. Furthermore, when an ion blating method or the like is used, the adhesion between the Al electrode and the semiconductor layer is improved, making it difficult to cause blisters and reducing the contact potential. Therefore,
In addition to bringing the Al film into a state of residual compressive stress, by setting the crystal grain size to 1 μm or less, the back electrode is less likely to bulge and becomes stable.

Thus, in the present invention, sufficient corrosion resistance can be imparted to the Al film forming the back electrode due to the above-mentioned effects.

EXAMPLES Hereinafter, examples of the back electrode of a photoelectric conversion element according to the present invention will be described with reference to the accompanying drawings.

Example 1 A solar cell having the structure shown in FIG. 1 was manufactured. On a transparent glass substrate 10, an ITO film 100 is placed as a transparent electrode.
Further, an amorphous silicon film 12 was formed as a semiconductor film to a thickness of 5,000 mm so as to expose a part of the transparent electrode 11. Then,
A helium film 13 is placed on top of the amorphous silicon film 12 as a back electrode using an ion-blating method (vacuum level 2
IO'torr, substrate temperature 200℃, bias voltage 1k
The film was formed to a thickness of 500 mm at a deposition rate of 15 people/sec) and Schottky bonded to the amorphous silicon film 12. Then, a lead wire 14 was connected to the ITO film 1 and the 1I film 13 at a bonding portion 15 using aluminum solder. Thereafter, the entire surface of the ITO film 1 and the amorphous silicon film 12 was sealed with a silicone resin 16. The residual stress of the Al film of the solar cell thus prepared was 8 Kg/mm2 in the compression direction. This solar cell is also used by irradiating light through the glass substrate 10 in the direction of the arrow 17.

Further, in order to test the corrosion resistance of the back electrode of the above-described example, a test solar cell as shown in FIG. 4 was prepared under the same conditions. Then, the pressure kicker test (12
1°C, 2 atm, and 100% relative humidity for 26 hours). The electrical resistance between the two lead wires 14 before and after the test is shown by a solid line in FIG.

Comparative Example 1 A solar cell was produced in the same manner as in Example 1 except that the AI film serving as the back electrode was formed by vacuum evaporation, and a test solar cell shown in FIG. 4 was also produced.

In Comparative Example 1, the residual stress of the Al film serving as the back electrode was 3 Kg/mm2 in the tensile force direction.

Furthermore, a pressure test was conducted under the same conditions as in Example 1, and the two lead wires 14 were tested before and after the test.
The electrical resistance between them was measured. The results are shown in FIG. 2 by dotted lines.

As is clear from the comparison between the solid line and the dotted line in the graph of FIG. 2, the resistance change in Example 1 according to the present invention is slight. That is, it can be seen that the moisture resistance of the Al back electrode according to the present invention is far superior to that of the conventional one, and it is suitable as a back electrode of a photoelectric conversion element.

Therefore, a solar cell having such a back electrode can be used for a longer period of time than before without decreasing photoelectric conversion efficiency.

Note that in this example, the semiconductor film is amorphous.
Although silicone and silicone resin are used as the sealing material, the effects of the present invention can be similarly achieved using other materials.

Further, although the TO film 11 is used as the transparent electrode, films of other transparent conductive materials may be used. Furthermore,
A comb-shaped opaque electrode may be used on the transparent electrode, and in that case, the product of the present invention can be used for the opaque electrode.

Furthermore, although the glass substrate 10 is used in the above embodiment, the glass substrate can be omitted if the semiconductor film 12 is used as a support for an incident side electrode such as a transparent electrode and a back electrode. can.

In addition, although the above-mentioned example applies the back electrode according to the present invention to a solar cell, A of the photoelectric conversion element according to the present invention
The back electrode can be applied to any photoelectric conversion element packaged in a resin-sealed package.

Effects of the Invention As is clear from the above explanation, the back electrode of the present invention has the following effects:
Since it is composed of an Al film to which compressive stress is applied, it has better moisture resistance than an Al film to which no stress is applied or an Al film with residual tensile stress. Therefore, the change in resistance between the pair of lead wires of the photoelectric conversion element over time is small.

Furthermore, the degree of stress corrosion is large in the tensile stress field and small in the compressive stress field, and especially when the residual compressive stress is 5 Kg/mm.
When the number is 2 or more, the effect is significant.

Therefore, the residual compressive stress of the back electrode according to the present invention can be reduced to 6Kg.
/mrn2J2J or above, intergranular corrosion of the Al electrode 3 can be significantly slowed down, and aging change in resistance between lead wires can be further suppressed.

Due to the above effects, having the Al back electrode according to the present invention,
A photoelectric conversion element provided with a resin-sealed package can maintain stable photoelectric conversion efficiency over a long period of time. Therefore, replacement intervals can be extended, and maintenance of the device can be made inexpensive.

Particularly, when the back electrode of the present invention is used in a solar cell used in a harsh environment, it is effective.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a solar cell having a back electrode according to the present invention, FIG. 2 is a graph showing an example of the change over time in the electrical resistance value of the back electrode according to the present invention and a conventional example, and FIG. 4 is a schematic cross-sectional view of a solar cell having a conventional back electrode, and FIG. 4 is a cross-sectional view of a photoelectric conversion element structure for testing the resistance value of the back electrode. [Main reference numbers] ■...Transparent substrate, 2...Transparent electrode 3...Semiconductor film, 4...Back electrode 5...Stone wire, 6...Ponding part 7...Sealing resin, 9..Leak path 10..Transparent glass substrate, 11..ITO film 12..Amorphous silicon film, 13..AI film film type 4.Lead, 15..Ponding part 16..Silicon resin

Claims (1)

Scope of Claims: (1) A photovoltaic device having a semiconductor film, an incident side electrode provided on the light incident side of the semiconductor film, and a back electrode provided on the side opposite to the incident side electrode of the semiconductor film. A back electrode of a photoelectric conversion element, wherein the back electrode is made of an Al film under compressive stress. (2) The photoelectric conversion according to claim (1), wherein the compressive stress state of the Al film is realized by residual compressive stress remaining after forming the Al film on the semiconductor film. Back electrode of element. (3) The residual compressive stress of the Al film is at least 6 kg.
/mm^2
2) A back electrode of the photoelectric conversion element described in section 2). (5) The Al film is formed by depositing Al on the semiconductor film using either an ion plating method or a sputtering method. A back electrode of a photoelectric conversion element according to any one of items 2) to 4).