JPH059946B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a solar cell in which an amorphous silicon thin film is provided as a photovoltaic force generating element on a flexible film substrate. More specifically, the present invention relates to a solar cell using a ceramic film as the substrate for the solar cell. Solar cells are known in which an amorphous thin film is provided on a non-flexible substrate such as a stainless steel or glass plate, and a solar cell in which a resin thin film such as polyimide is used as a flexible substrate. The feature of using a flexible substrate when manufacturing an amorphous solar cell is that the necessary amorphous silicon layer can be continuously provided on the substrate, making it non-flexible in terms of manufacturing cost and ease of manufacturing. It is extremely advantageous compared to sexual substrates. Furthermore, unlike solar cells formed on conventional non-flexible substrates, amorphous solar cells formed on flexible substrates are film-shaped, so the product shape can be arbitrarily shaped, such as curved surfaces. It can be used in any state, and its applications are expected to expand. However, when forming such an amorphous solar cell on a flexible substrate, a high temperature of at least 250 to 350°C is desirable as the amorphous silicon formation temperature.
When using a polymer film, only a polyimide film with excellent heat resistance can be used. However, the polyimide film has a problem in that the initial Young's modulus at such high temperatures is not very large and the film does not have sufficient film strength to withstand the thermal stress during the formation of an amorphous silicon film. In other words, in the case of a substrate that does not have sufficient film strength, when an amorphous thin film is provided on the substrate, thermal stress due to the difference in thermal expansion coefficient between the amorphous silicon thin film and the substrate will be applied to the substrate. The mechanical strength will be exceeded and the board will curl. It has been confirmed that when the degree of curl increases, a serious defect occurs in that the efficiency as a solar cell is significantly reduced. Furthermore, the surface of conventional polyimide films is too smooth, making it difficult to obtain high photoelectric conversion efficiency. Therefore, in order to realize an amorphous silicon solar cell using a flexible substrate, in addition to heat resistance of at least 250°C, it must also have sufficient strength to withstand the thermal stress during film formation at such high temperatures. and a substrate with appropriate surface roughness must be provided. One of the purposes of the present invention is to prevent such curling during film formation, but another purpose is to prevent the surface roughness of the substrate, which has a large effect on the photoelectric conversion efficiency. An object of the present invention is to provide a substrate having an appropriately rough surface that makes it possible to obtain high photoelectric conversion efficiency in a solar cell formed with a thin silicon film. Regarding the relationship between the surface roughness of the substrate and the conversion efficiency of the solar cell, in order to improve the conversion efficiency, it is important to prevent the reflection of sunlight on the surface of the solar cell, that is, to reduce the reflectance of sunlight. However, if the surface is too roughened, pores are formed in the amorphous silicon thin film and many short circuits occur in the electromotive force elements, and the characteristics of the solar cell itself deteriorate.
This deviates from the original purpose of solar cells. Therefore, the substrate needs to have an appropriate surface roughness in order to prevent reflection and maintain battery characteristics. The present inventor has developed a thin film solar cell using an amorphous silicon thin film as a photovoltaic element, which can sufficiently withstand thermal stress when forming an amorphous silicon thin film on a substrate, thereby preventing curling. As a result of our earnest efforts to achieve the objective of improving battery characteristics by providing amorphous silicon with a suitable surface roughness, we have succeeded in achieving this goal by using a flexible ceramic film as a substrate for amorphous silicon thin film solar cells. The object of the invention has been achieved and the present invention has been achieved. As mentioned above, the present invention is characterized in that a flexible ceramic film having a moderately rough surface is used as the substrate in a solar cell in which an amorphous silicon thin film is provided as a photovoltaic element on a flexible film substrate. However, the ceramic film used in the present invention will be mentioned below. The ceramic film according to the present invention is not particularly limited as long as it can be formed into a film shape. A flexible mica film will be mentioned as a preferred example of the ceramic film. Mica such as _M.7 (MG _{2.3 Li.7} )
It has the composition formula Si ₄ O ₁₀ F ₂ .nH ₂ O, and M is Li or K.
can be easily molded into a film. When molding into a film, even the composition having the above composition formula alone can be molded into a film. The surface of the mica film formed in this manner has a moderate roughness. Even if glass fiber is mixed in, it can be sufficiently molded, so it is extremely effective for improving strength. The film can be formed to a thickness of 10 to 40 μm, and the weight per unit area is 15 to 40 μm.
It was in the range of 35g/ ^m2 . The electrical properties of the produced film, such as dielectric breakdown resistance, dielectric constant, and specific resistance, were extremely excellent. Furthermore, in terms of strength, rigidity, and heat resistance, ceramics have extremely superior properties compared to polymer films, which are commonly used as flexible substrates for solar cells. shows. Heat resistance is 400℃
It is extremely advantageous for producing high-quality amorphous silicon thin films without any problems even when heated to a certain degree. As for rigidity, as well as heat resistance, since it is made of ceramics, it is stiff when molded into a film, and can sufficiently withstand thermal stress during the formation of an amorphous silicon film. In order to use a flexible ceramic film as a substrate for a solar cell, electrodes are created on the surface of the substrate.
The electrode is not particularly limited, and a thin film of aluminum, iron, stainless steel, nickel, tungsten, or the like is formed on a substrate by vapor deposition, sputtering, ion plating, or the like. In order to form an amorphous silicon thin film on a flexible substrate, a known method such as a glow discharge method, a sputtering method, an ion plating method, or a thermal decomposition method is used. For example, in the case of the glow discharge method, a flexible substrate is rolled up in a vacuum chamber maintained at 0.1 to 10 torr, and then the substrate is pulled out and brought into close contact with a substrate holder heated to 200 to 350°C. This substrate holder is used as one electrode, and between this and the opposing electrode, for example,
Provides 13.56MHz high frequency power. Inside the vacuum chamber, silane gas (SiH ₄ ), diborane gas (B ₂ H ₆ ),
Phosphine gas (PH ₃ ) and hydrogen gas (H ₂ ) are introduced to cause glow discharge, and source gas is supplied until a predetermined film thickness is reached, forming an amorphous silicon thin film that is a photovoltaic element. More specifically, to create an i-type silicon thin film, film formation is performed by supplying silane gas and H ₂ gas, and to create a p-type silicon thin film, film formation is performed by supplying silane gas, hydrogen gas, and diborane gas. Let's do it. Further, an n-type silicon thin film is formed by supplying silane gas, hydrogen gas, and phosphine gas. Next, in order to use the amorphous silicon thin film as a solar cell device, a back electrode is formed, and then a flexible ceramic film substrate on which a P layer, an i layer, and an n layer are laminated is mounted in a vacuum chamber. For Schottky junction cells,
As a Schottky barrier metal, platinum, gold, palladium, or the like is deposited to a thickness of about 100 Å by sputtering, vacuum evaporation, ion plating, or the like.
In addition, in the case of a hetero (face) junction cell, indium oxide, tin oxide, or tin oxide-indium oxide film is formed to a film thickness of about 200 to 5000A by sputtering, vacuum evaporation, ion plating, etc. to form a surface electrode. A collection electrode is then provided on the Schottky barrier metal, heteroface electrode surface to form an amorphous silicon solar cell device. In the amorphous silicon solar cell of the present invention, a back electrode is formed on a flexible ceramic film substrate, a multilayer amorphous silicon film is provided on the electrode, and a Schottky barrier metal or heteroface electrode is provided on the back electrode. It has a basic structure in which a collecting electrode is further provided on top of the collecting electrode. The amorphous silicon solar cell of the present invention is characterized by the use of a ceramic film as a flexible substrate, and the following characteristics are attributable to the fact that this flexible ceramic film is made of ceramics. has. It has high rigidity and can withstand thermal stress during film formation. It has excellent heat resistance, that is, there is no problem even when heated to 400°C. Although it has excellent strength, it can be made even stronger by incorporating glass fiber. Since it has an appropriate surface roughness, it is possible to obtain excellent photoelectric conversion efficiency as shown in Examples described later. By using a ceramic film as a flexible substrate in this way, it is possible to manufacture continuous solar cells in roll form. In addition, it has become possible for the first time to realize a solar cell with excellent photoelectric conversion efficiency, which is due to the fact that it has the rigidity to withstand thermal stress during film formation and has an appropriate surface roughness. Summer. The present invention will be explained below with reference to Examples. Example 1 Mica having the composition formula Li _.7 (MG _2.3 Li _.7 ) Si ₄ O ₁₀ F ₂ ·nH ₂ O was molded into a film to obtain a flexible mica film with a thickness of 30 μm. This mica film was heated at 10 ^-2 torr under vacuum.
Drying was performed at ℃ for 2 hours. This dried MyFilm sputtering device was installed, and a 1.5μ thick tungsten thin film was formed as a back electrode using tungsten as a target. Using a capacitively coupled high frequency (13.56 MHz) glow discharge device, the amorphous silicon thin film was prepared by mounting the substrate on which the back electrode was formed on the anode electrode of the glow discharge device under tension. 300℃ while exhausting to ^-6 torr
The substrate is heated. Then add _500c.c./N2 gas
ion bombardment of the substrate for 20 minutes by applying 200 W of high frequency power in a 1.0 torr N ₂ gas atmosphere to clean the substrate. Next, 10% silane gas diluted with hydrogen gas and 0.1% hydrogen gas.
A phosphine gas diluted to 100% is introduced into a glow discharge device, and a high frequency power of 100 W is applied in the gas atmosphere of 0.6 torr to form an n-type amorphous silicon thin film of 200 Å. Next, an i-type amorphous thin film with a thickness of 3000 Å is formed on the n-type amorphous silicon thin film using hydrogen gas and silane gas in the same manner as described above. Next, silane gas diluted to 10% with hydrogen gas and diborane gas diluted to 0.1% with hydrogen gas were introduced into the glow discharge device and were applied onto the i-type amorphous silicon thin film.
A P-type amorphous silicon thin film of 300 Å is formed, and a pin-type amorphous silicon thin film is provided on the flexible mica film. The pin-type amorphous silicon thin film thus obtained was mounted on a sputtering device, and a tin oxide-indium oxide thin film was deposited to a thickness of 1000 Å to form a heteroface layer. Finally, palladium was deposited in a 1000 Å comb shape as a collecting electrode on this heteroface layer, and then deposited on a flexible mica film substrate.
A pin heteroface type solar cell device was obtained. Example 2 Mica having the composition formula K _.7 (MG _2.3 Li _.7 ) Si ₄ O ₁₀ F ₂ ·nH ₂ O was molded into a film to obtain a flexible mica film with a thickness of 30 μm. A pin heteroface type solar cell device was produced under the same conditions as in Example 1. Example 3 Li _.7 (MG _2.3 Li _.7 ) Si ₄ O ₁₀ F ₂ ·nH ₂ O mica was mixed with 20 wt% of glass fiber based on the mica, formed into a film, and made into a glass-reinforced film with a thickness of 30μ. Obtained mica film. A pin heteroface type solar cell was produced under the same conditions as in Example 1. Example 4 Initial characteristics of the solar cell devices of Examples 1 to 3
Measurement was performed using a solar simulator manufactured by Oriel Co., Ltd. adjusted to AM=1. A polyimide film was selected as a comparative example, and a pin-type heteroface solar cell device was formed on this film in the same manner as in Example 1. In this measurement, the sample was used as a measurement sample without once releasing the tension throughout the solar cell device formation process. The results are shown in Table 1.

[Table] Example 5 Example 4 showed the results of measuring the initial characteristics of a solar cell device under conditions where the tension state was never resolved, but in this example, the tension state of each sample was resolved once. The results are shown below. For the samples of Examples 1 to 3, there was almost no curling even after the tension was released, and the battery properties were almost the same as before the tension was released, but the polyimide film had significant curling, and the battery properties also showed Before the tension was released, the conversion efficiency was 3.2%, but it had decreased to 2.5%.

Claims (1)

[Claims] 1. In a solar cell having an amorphous silicon thin film on a flexible substrate, a ceramic film having a thickness of 10 to 40 μm and a weight per unit area of 15 to 35 g/m ² is used as a substrate. Characteristics of amorphous silicon thin film solar cells. 2. The solar cell according to claim 1, wherein the ceramic film is a mica film. 3. The solar cell according to claim 1, wherein the ceramic film is a ceramic film reinforced with glass fibers.