JPH0518272B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an amorphous silicon solar cell that has flexibility and can significantly improve conductivity. In general, solar cells in which an amorphous thin film is formed on a non-flexible substrate such as a stainless steel plate or glass plate, or a solar cell in which an amorphous thin film is formed on a relatively flexible substrate such as a resin thin film such as polyimide, are well known. It is being When manufacturing amorphous solar cells, the characteristics achieved by using a flexible substrate are that the necessary amorphous silicon can be continuously provided on the substrate, and the manufacturing cost and ease of manufacturing are reduced. This is an extremely advantageous feature compared to non-flexible substrates. Furthermore, unlike solar cells formed on conventional non-flexible substrates, amorphous solar cells formed on flexible substrates are sheet-like, so they can be shaped arbitrarily. , It is expected that its applications will expand with future application development. However, when forming such an amorphous solar cell on a flexible substrate, according to the glow discharge method commonly used at present, when trying to obtain a high-quality amorphous silicon thin film, the temperature is 250°C ~ It requires a high temperature of 350°C, and when using a polymer film, only polyimide film, which has 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 thermal stress during the production of amorphous silicon. In other words, in the case of a substrate that does not have sufficient film strength, when an amorphous silicon 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 may be generated. The mechanical strength of the substrate will be exceeded and the substrate 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. Conventionally, in order to realize amorphous silicon solar cells using flexible substrates, in addition to heat resistance of at least 250°C, it is necessary to be able to withstand thermal stress during film formation at such high temperatures. A sturdy substrate must be provided. Furthermore, providing appropriate surface roughness or unevenness to the substrate surface is also effective in improving photoelectric conversion efficiency, but regarding surface roughness or unevenness, conventional polyimide films have too smooth surfaces, so The reflected light is emitted outside the solar cell without being used again, making it difficult to obtain high photoelectric conversion efficiency. Therefore, the main object of the present invention is to use a fiber cloth as a flexible substrate and form a conductive paste layer thereon, thereby preventing the substrate from curling and having an appropriate surface roughness. Another object of the present invention is to provide a solar cell that realizes unevenness and enables multiple reflections of incident light on the surface, thereby improving light absorption and increasing the current-carrying area. In other words, although solar cells using fabric as a substrate have the above-mentioned advantages,
When forming a lower electrode (also called a back electrode), in the method of simply depositing metal on the fabric, there are gaps between the fibers, so the metal is deposited only on the surface of the fabric, which limits the current carrying area. There is also an inherent disadvantage that the current carrying efficiency is reduced due to cases where metals are not connected. Unless such electrical conductivity is improved, it will ultimately be impossible to create a solar cell with high conversion efficiency. The inventors lowered the substrate temperature required for forming an amorphous silicon thin film, expanded the selection range of flexible substrates, prevented curling during film formation, and created an appropriate surface roughness or unevenness. As a result of intensive research into producing amorphous silicon solar cells on a substrate, we decided to use a fabric as the substrate for an amorphous silicon solar cell, apply a conductive paste to the surface of the fabric, and fill in the gaps between the fibers. , Furthermore, by forming an amorphous silicon thin film on this conductive paste, preferably by a cluster ion beam method,
The object of the invention has been advantageously achieved. This invention uses a fiber cloth as a flexible substrate, coats this cloth with a conductive paste, and further forms an amorphous silicon thin film as a photoelectric conversion element thereon, preferably by a cluster ion beam method. It is characterized by causing
Fabric here includes fiber fabrics such as woven fabrics, knitted fabrics, and non-woven fabrics, and has a basis weight of 10 to 10.
It is in the range of 400g/ ^m2 . The major factors that influence the composition, organization, and appearance of fabrics are:
There are thread selections, woven fabrics, and knitting methods to choose from, but the thread thickness, cross-sectional shape, monofilament, multifilament, etc. should be selected based on the desired fabric structure, and the appropriate woven fabric and knitting method should be selected. ,
A substrate suitable for an amorphous solar cell is formed. Although there are no particular restrictions on the fiber material constituting the fabric, for example, nonwoven fabrics or woven fabrics made of heat-resistant aromatic polyamide (Nomex) may also be used. One of the features of using such fiber fabrics as substrates for solar cells is their good flexibility. Conventionally, polyimide film has been used as a flexible substrate, but the flexibility of the film is, so to speak, unidirectional, and if you try to make it conform to a quadratic curved surface, such as a spherical surface, hard creases will occur. It occurs and is not desirable. That is,
It may cause electrical disconnection or cause discomfort to the human body. As described above, solar cells using polyimide film as a substrate, for example, are significantly lacking in flexibility. On the other hand, solar cells fabricated using fabric as a substrate have sufficient flexibility. Another advantage of using cloth is that it protects the silicon surface when the silicon thin film solar cell is rolled up. Furthermore, the solar cell having an amorphous silicon thin film according to the present invention is a solar cell in which a shot type, pin type, or tandem type element structure is formed using a silicon-based amorphous thin film. Note that silicon-based amorphous thin films include Si, Si-Ge, Si-C,
A hydrogenated amorphous amorphous film or a fluorinated amorphous amorphous film made of a simple substance or a compound such as Si-N is included. Next, a cluster ion beam method will be described which is a preferred method of forming the above-mentioned amorphous silicon thin film thereon using a fabric as a substrate according to the present invention. In the cluster ion beam method, the component elements of the substance to be formed are stored in a closed crucible with at least one injection nozzle, heated and vaporized, and the vapor is heated to a pressure sufficiently lower than that of the vapor. It is injected from an injection nozzle into a high vacuum with a pressure of 1/100 or less than the vapor pressure in the crucible.
Approximately 2,000 atoms form a so-called cluster, a mass of atoms loosely bound together by Van der Waals forces, and further ionize at least a portion of this cluster, and ionize it by the kinetic energy imparted during nozzle jetting or as needed. The electric field applied thereto accelerates the element to reach the surface of the substrate, thereby forming a thin film of the component element on the substrate. In this case, by setting various settings such as the substrate temperature, the degree of vacuum in the space surrounding the crucible, the ionization rate of clusters, or the acceleration voltage when accelerating cluster ions, the material to be grown on the substrate can be changed to an amorphous state, a polycrystalline state, or It has the characteristics of a single crystal state and can be controlled arbitrarily, and by introducing a reactive gas into the cluster formation space,
Thin films of compounds can also be easily formed. In a preferred embodiment of the present invention, this cluster ion beam evaporation method is used to form an amorphous silicon thin film as a photoelectric conversion element. The inventors have conducted various studies on a method for manufacturing amorphous silicon thin films as photoelectric conversion elements using this cluster ion beam evaporation method, and found that if conditions are appropriately selected, temperatures below 200°C It was confirmed that extremely high-quality amorphous silicon thin films could be obtained even at substrate temperatures of . In the conventional glow discharge method, in order to obtain a high-quality amorphous silicon thin film, a substrate temperature of at least 250° C. or higher is required, so the range of selection of flexible substrate materials is extremely narrow. On the other hand, when cluster ion beam evaporation is used, the substrate temperature only needs to be 200°C or less.
Specifically, we confirmed that high-quality amorphous silicon thin films can be obtained even at temperatures of around 180°C. It has therefore been found that fabrics made from very common fibrous materials such as polyester, rayon, and polynosic fibers can also serve as substrates for amorphous silicon solar cells. The fiber materials that can be used to form the fabric of the present invention are not limited to the organic materials mentioned above, and may include the aromatic polyamide fibers mentioned above, as well as inorganic materials such as glass fibers and steel fibers. In short, any fiber material that can be used to form a fabric may be used. In addition, the cluster ion beam method has excellent step covering properties, and can form a thin film with excellent uniformity even on a substrate with steps, such as the fabric substrate used in this invention. Further, the step covering property can be improved by appropriately arranging a plurality of crucibles and simultaneously using them to form a film. Regarding the heat resistance of the substrate, as mentioned above, we confirmed that it is possible to obtain a high-quality amorphous silicon thin film even at around 180℃ using the cluster ion beam method, but there are still issues with the strength, rigidity, and surface roughness of the substrate. As for the unevenness, it can be set arbitrarily by appropriately selecting the structure of the fabric, the structure of the thread, etc., so it is possible to prevent curling during the formation of an amorphous silicon thin film, and it can be used as a substrate for solar cells without roughening the surface. It is also possible to provide unevenness. In this invention, since a fabric is used as a substrate of a solar cell, it is necessary to form an electrode on the surface of the fabric substrate. In this invention, the formation of such electrodes is important. That is, a lower electrode having low electrical resistance is formed by applying a conductive paste to a fiber fabric. Such a conductive paste is made by mixing metal powder into a resin that can withstand the temperatures required for producing an amorphous silicon thin film, and then plasticizing the resin. As the resin, polyesterimide, polyimide amide, etc. can be used, and as the metal powder, Ag, Cu, Al, Sn, etc. can be used.
The conductive paste layer at this time has a thickness of about 0.1 to 10 μm. By applying a conductive paste to the fiber fabric in this way, the gaps between the fibers are filled, and the surface undulations of the woven fabric appear as they are on the conductive paste side of the back electrode, so that electricity can be applied. The area can be significantly increased, and a solar cell with high conversion efficiency can be produced. As already mentioned, the cluster ion beam method is preferably used to form an amorphous silicon thin film as a photoelectric conversion element on a fabric substrate on which electrodes are formed. The drawing shows an example of an apparatus used for forming an amorphous silicon thin film according to the present invention. Referring to the drawings, a crucible 4 having a nozzle 5 with a hole diameter of approximately 0.5 to 2.0 mm is provided.
This crucible 4 is filled with silicon that has been crushed to an appropriate size. A fabric (coated with a conductive paste material) as a substrate 9 is placed in front of the nozzle 5 , an accelerating electrode 8 is placed in the middle of the fabric, and an ionizing electrode 7 is placed between the nozzle 5 and the accelerating electrode 8 .
is placed. A gas introduction pipe 6 is provided on the side of the ionization electrode toward the ionization electrode 7. Each of these members in the drawing is placed in a vacuum container (not shown).
The vacuum container is supported by a suitable support member and is evacuated to a high vacuum atmosphere of at least 10 ⁻⁵ Torr or less. Next, hydrogen gas (H ₂ ), phosphine gas (PH ₈ ), and diborane gas (B ₂ H ₆ ) are supplied into the vacuum container through the gas introduction pipe 6, and the pressure inside the vacuum container is increased to 10 ^-6 to 10 ^-3. Maintain torr. The substrate 9 is also heated to an appropriate temperature. Next, the crucible 4 is heated to melt the silicon filled in the crucible 4 and generate silicon vapor. In this case, the heating temperature of silicon is set according to the pressure in the surrounding space of the crucible 4, that is, the vacuum container.If the pressure in the crucible 4 is P and the pressure in the vacuum container is P0, It is set so that P/P0≧10 ² , preferably P/P0≧10 ⁴ . Actually, the heating temperature is set in the range of about 1400 to 2300°C. However, due to this pressure difference, silicon vapor is ejected from the nozzle 5 to the outside of the crucible 4, and at this time, due to the supercooling phenomenon based on adiabatic expansion, clusters of vaporized silicon atoms and molecules are loosely bonded by Van der Waals force. It is formed. By the way, the vapor flow that has gained kinetic energy by being ejected out of the crucible 4 enters the ionization chamber, where at least a portion of it is ionized to form so-called cluster ions. A suitable accelerating voltage is applied to the accelerating electrode 8 to accelerate the cluster ions and cause them to collide with the substrate 9. At this time, the cluster is separated into individual atoms due to the impact during the collision in the vacuum layer, and the clusters are moved on the surface of the substrate 9 in the atomic state, which is the so-called surface migration effect, and the partially ionized silicon ions are The ionization effect created promotes bonding between silicones. On the other hand, part of the hydrogen gas (H ₂ ) introduced into the vacuum container is ionized by electron bombardment, or becomes a single atom and hits the substrate 9 together with the silicon vapor flow, causing the back surface to be exposed. Moving. H is bonded to the unbonded portion of the silicon that is bonded while moving on the substrate 9, and an amorphous silicon thin film having a structure in which dangling bonds are closed by H is formed. More specifically, to make an i-type amorphous silicon thin film, only hydrogen gas is used as the introduced gas, and to make a p-type silicon thin film, hydrogen gas and diborane gas (B ₂ H ₆ ) are used as the introduced gas. In addition, hydrogen gas and phosphine gas (PH ₈ ) are used as introduced gases to form an n-type silicon thin film. Therefore, as an amorphous silicon layer as a photoelectric conversion element, first an n layer, then an i layer, and finally a p layer are sequentially formed on the fabric substrate 9 on which a back electrode made of a conductive paste is formed. I will let you do it. Next, in order to use this amorphous silicon thin film as a solar cell device, n
A fabric substrate with laminated layers, i-layers, and p-layers is mounted in a vacuum layer, and in the case of a shottock junction cell, for example, the shottock barrier metal is platinum,
Gold, palladium, etc. are sputtered, vacuum evaporated,
Deposit with a film thickness of about 100 Å using ion plating method (including cluster ion beam method).
In addition, in the case of a heteroface junction cell, a film of indium oxide, tin oxide, tin oxide-indium oxide, etc. is deposited to a thickness of about 200 to 5000 Å using sputtering, vacuum evaporation, or ion plating (cluster ion). (including a beam method) to form a surface electrode. A collection electrode is then provided on the surface of the shot barrier metal, heteroface electrode to complete the amorphous silicon solar cell device. According to this invention, an amorphous silicon solar cell is produced by forming a back electrode made of a conductive paste on a fabric substrate, and applying amorphous layers of n-layer, i-layer, and p-layer onto this electrode by cluster ion beam method. It has a basic structure in which a silicon film is sequentially provided, a shot barrier metal or a hetero (face) electrode is provided thereon, and a collector electrode is further provided thereon. As described above, the solar cell of the present invention is characterized in that a fabric is used as a substrate, a conductive paste is applied to the fabric, and an amorphous silicon thin film is formed on the fabric. As a result, we were able to fabricate a solar cell with a large current-carrying area, improved current-carrying performance, and high conversion efficiency. Furthermore, since appropriate surface roughness or unevenness is achieved on the paste-like material side, the incident light that enters the solar cell is multi-reflected, and the light absorption rate is improved, resulting in a solar cell with higher conversion efficiency. is obtained. In addition, good flexibility is achieved by using fabric as a substrate, and as a result, the uses of the solar cell of the present invention are expanded.For example, it is easy to think of product development for tents, yacht sails, etc. When the solar cell of this invention is used in clothing, etc., the discomfort caused to the human body is reduced. In a preferred embodiment of the present invention, a cluster ion beam method is used as a method for forming the amorphous silicon thin film. By using this cluster ion beam method, the substrate temperature is below 200℃,
Specifically, extremely high quality amorphous silicon thin films can be obtained even at 180°C. In this way, the required substrate temperature is significantly lowered, which widens the range of substrate selection.Of course, aromatic polyamide fibers (Nomex) and glass fibers can be used, but common materials such as polyester, rayon, and polynosic can be used. Textile materials can also be used. Also, the thickness of the thread,
By appropriately selecting the material, cross-sectional shape, multifilament, monofilament, etc., as well as the fabric structure, basis weight, etc., it is possible to set the desired surface roughness or unevenness, strength, and rigidity. As a result, it is possible to prevent curling during film formation, and as described above, it is possible to achieve appropriate surface roughness or unevenness, achieving excellent photoelectric conversion efficiency as shown in the examples below. can do. In addition, by adopting the cluster ion beam method for forming an amorphous silicon thin film and using fabric as a substrate, it becomes possible to realize a solar cell with rigidity that can withstand thermal stress during film formation. Examples of the present invention will be described below. Example 1 Warp 75 denier, weft 75 denier, basis weight 60 g/
A conductive paste made of polyimide amide resin mixed with silver powder was applied to a polyester plain-woven fabric measuring 2 m ² and dried at 100° C. for 5 minutes to form a back electrode. On the paste side of this back electrode, the surface undulations of the woven fabric used appeared as they were. The woven fabric substrate with this back electrode formed was placed under a vacuum of 1×10 ² Torr.
Drying was performed at 150°C for 2 hours. This dried substrate was set under tension on a substrate support stand for the cluster ion beam method, and was heated to ¹⁸⁰ m
The substrate was heated to . When a vacuum of 5×10 ^-7 Torr was reached, hydrogen gas and phosphine gas (PH ₈ ) diluted to 1% with hydrogen gas were introduced into the vacuum layer at a flow rate ratio of 5:1, and the vacuum layer was heated to 1× ^10-4 Torr
maintained. An n-type amorphous silicon thin film was formed to a thickness of 200 Å under conditions of a crucible heating temperature of 2000 to 2200°C, an ionization current of 200 mA (300 V), and a substrate temperature of 180°C. Next, an i-type amorphous silicon thin film with a thickness of 3000 Å was formed on the n-type amorphous silicon thin film using only hydrogen gas as the introduced gas in the same manner as described above. Next, hydrogen gas and diborane gas (B ₂ H ₆ ) diluted to 10% with hydrogen gas were introduced into the vacuum container.
A p-type amorphous silicon thin film was formed to a thickness of 100 Å, and a pin-type amorphous silicon thin film was provided on a fabric substrate. The pin-type amorphous silicon thin film obtained in this way was mounted on a sputtering device, and tin oxide
A 100 Å thin film of indium oxide was deposited to form a heteroface layer. Finally, palladium was deposited in a comb shape with a thickness of 1000 Å as a collection electrode on this heteroface layer, and a pin heteroface type solar cell device was realized on the fabric substrate. The performance of this solar cell device is shown in Table 1. Example 2 The fabric substrate obtained in Example 1 (after forming the back electrode) was mounted on a sputtering device, and a 1.5 μm thick tungsten thin film was further formed on the back electrode using tungsten as a target. The surface irregularities of the woven fabric still appeared in this tungsten thin film. A pin heteroface type solar cell device was produced under the same conditions as in Example 1. Its performance is the first priority.
Shown in the table. Comparative example: A polyimide film with a thickness of 125μ was selected as a substrate, and this film was exposed to a vacuum of ×10 ^-2 Torr.
Drying was performed at 150°C for 2 hours. This dried polyimide film was mounted on a sputtering device, and a 1.5 μm thick tungsten thin film was formed as a back electrode using tungsten as a target.
The amorphous silicon thin film was prepared using a capacitively coupled high frequency (13.56 MHz) glow discharge device, and the substrate on which the back electrode was formed was mounted under tension on the anode side electrode of the glow discharge device. ^-6 Torr
This substrate was heated to 250 °C while being placed at a temperature of 250 °C. Then, nitrogen gas (N ₂ ) was introduced at 500ml/min.
The substrate was cleaned by applying 200 W of high frequency power in a 1.0 Torr nitrogen gas atmosphere to ion bombard the substrate for 20 minutes. Next, silane gas diluted to 10% with hydrogen gas and phosphine gas diluted to 0.1% with hydrogen gas were introduced into the glow discharge device.
In this gas atmosphere of 6×10 ⁻¹ Torr, a high frequency power of 100 W was applied 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 was 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 with hydrogen gas to 0.1% were introduced into the glow discharge device, and 300% diborane gas diluted with hydrogen gas was applied to the i-type amorphous silicon thin film.
A p-type amorphous silicon thin film with a thickness of 1.5 Å was formed, and a pin-type amorphous silicon thin film was provided on a fabric substrate. 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 the form of a 1000 Å comb shape as a collector electrode on this heteroface layer, and a pin heteroface type solar cell device was obtained on a flexible polyimide film substrate. The performance of this device is also listed in Table 1. 【table】

[Brief explanation of drawings]

The drawing is an explanatory diagram of the basic configuration of a cluster ion beam device. In the figure, 4 is a crucible, 5 is a nozzle, 6 is a gas introduction tube, 7 is an ionization electrode, 8 is an acceleration electrode, and 9
is the substrate.

Claims (1)

[Scope of Claims] 1. A flexible substrate made of a fiber fabric, and at least on the surface of the fibers in the substrate.
It is characterized by comprising: an electrically continuous back electrode layer coated with a conductive paste to a thickness within the range of 0.1 to 10 μm; and an amorphous silicon thin film formed on the back electrode layer. Amorphous silicon solar cells. 2. The amorphous silicon solar cell according to claim 1, wherein the amorphous silicon thin film is formed by a cluster ion beam method.