Method for producing a glass substrate for a solar cell

Abstract: A glass substrate for a solar cell, comprising a glass sheet (21) having a curved surface shape and a transparent electroconductive film (22) formed on the concave surface side of said glass sheet.

Description: This invention relates to a method for producing a glass substrate for a solar cell, particularly a solar cell-installed window glass panel for an automobile, and more particularly a glass panel suitable as a sunroof, opera window or the like for an automobile. A solar cell is regarded as a prospective photoelectric conversion apparatus which is capable of cleanly producing electric power without causing environmental pollution by using sunlight as an inexhaustible energy source unlike other energy sources such as petroleum, coal or nautral gas. Among such solar cells, a solar cell using amorphous silicon (a-Si) as a semi-conductor layer for effecting photoelectric conversion is widely used since it can be produced at a relatively low cost and it assures a relatively satisfactory photoelectric conversion efficiency. As a typical example of such an amorphous silicon (a-Si) solar cell, Figure 4 illustrates an a-Si solar cell 48 having an a-Si semiconductor layer 46 composed of p-type a-Si layer 43, i-type a-Si layer 44 and n-type a-Si layer 45 and an aluminum electrode 47 laminated sequentially on a transparent electroconductive film 42 formed on a transparent insulating sheet 41. In the a-Si solar cell 48, light 49 incident on the transparent insulating sheet 41 is absorbed in the a-Si layer 46 thereby generating an electromotive force between the transparent electroconductive film 42 and the aluminum electrode 47, and the electric power thus generated is taken out through lead wires 50. Small size types of such a-Si solar cells are already used as cells for table computers, watches and the like. In future, such cells are expected to be used for power generation, automobiles, ships, and the like by improving their photoelectric conversion, enlarging their sizes, and lowering their manufacturing costs. particularly, a solar cell for an automobile is expected to be loaded on an automobile for use as an electric power supply to drive a ventilator during parking or other electronic equipments in the inside of the automobile. It is expected that a solar cell is used in the above-mentioned fields. For a practical application, however, a solar cell having a curved surface is desired depending on its particular purpose. For exampe, an automobile body is desired to have a curved shape in view of aerodynamic aptitude and design, and acocrdingly a solar cell loaded thereon is also required to have a curved surface fitted thereto. Heretofore, it is known to use a bilayer glass having a solar cell of a flat plate-like small silicon wafer embedded between two glass plates, as a sunroof and the like. However, there has been a restriction such that the silicon wafer itself can not be subjected to a bending treatment, and it must be used in the state of a flat plate. It is also inferior from the viewpoint of manufacturing costs and productivity. From the viewpoint of the design, productivity and the like, it is desired that a solar cell itself has a curved surface shape. It is conceivable to use a flexible plastic film as the transparent insulating sheet in order to make a curve-shaped solar cell, but the plastic film is generally inferior in weather-resistance. Furthermore, when it is used for an automobile in the state of being exposed to atmosphere, it is weak in impact-resistance, scratch-resistance and the like. It is also undurable to high temperature at which an a-Si semiconductor film of a solar cel is formed. Thus, a plastic film has been generally unsuitable as the transparent insulating sheet for a solar cell. As mentioned above, there have been attempts to use a solar cell as an electric power source for driving an air ventilator during parking or other electronic equipments of an automobile. For example, Japanese Unexamined Patent Publication No. 115719/1986 discloses a sunroof for an automobile having a solar cell mounted thereon. However, the solar cell used therein is not visible light-transmissive, but rather it is intended to impart a light-shielding property to the sunroof by using a light-shielding solar cell. Japanese Unexamined Patent Publication no. 199466/1988 discloses a light-transmissive solar cell, but there is no provision for a protective layer. On the other hand, an automobile having a glass plate panelled on a sunroof window is popular as the glass-panelled sunroof gives open-minded feeling to drivers and passengers. Accordingly, a solar cell loaded on the automobile and a protective layer therefor are required to be visible light-transmissive for this purpose. However, if the protective layer is light-transmissive, a grid electrode, a collective busbar and the like of the solar cell are visible from the inside of the automobile, thus spoiling the appearance. Moreover, there has been a problem that when a highly reflective metal such as Ag, Al, Cr and the like is used as a back electrode of the solar cell, drivers and passengers are dazzled by the glare from the highly reflective metal goods and their eyes are sometimes fatigued. The present invention provides a method for producing a glass substrate for a solar cell having a curved surface shape, which comprises heating a glass sheet, forming a transparent electroconductive film on said glass sheet and then subjecting the resultant glass substrate to a bending treatment in such a manner as to make the transparent electroconductive film surface of the glass substrate in the shape of a concave surface. FR-A- 2 550 007 discloses non planar glass substrates with subsequently deposited transparent electroconductive films. Now, the present invention will be described in detail with reference to the preferred embodiments. In the accompanying drawings: Figures 1a to 1c are cross-sectional views illustrating different embodiemnts of the glass substrate for a solar cell having a curved surface shape in accordance with the present invention. Figure 2 is a cross-sectional view illustrating an outline of a solar cell for a sunroof having a curved surface shape, wherein the glass substrate of the present invention is employed. Figure 3 shows a graph illustrating the rising rate of resistance in relation to the heating temperature and time with regard to a glass substrate having a transparent electroconductive film. Figure 4 is a cross-sectional view illustrating a conventional amorphous silicon solar cell. Figure 5 is a cross-sectional view illustrating an embodiment of the solar cell-installed window glass panel for an automobile in accordance with the present invention. Figure 6 is a cross-sectional view illustrating another embodiment of the solar cell-installed window glass panel for an automobile in accordance with the present invention. Figure 7 is a plain outline view of an embodiment of the solar cell-installed window glass panel for an automobile in accordance with the present invention, as viewed from the outside or the inside of the automobile. Figure 8 is a cross-sectional view illustrating an embodiment of the solar cell-installed window glass panel for an automobile, sealed along the peripheral margin, in accordance with the present invention. Figures 9a to 9d are cross-sectional views illustrating different embodiments of the solar cell-installed window glass panel having a gasket integrally molded along the peripheral margin, thereby being fixed to an automobile body, in accordance with the present invention. Figure 10 shows a graph illustrating the deterioration tendency with the passage of time of the power output of the solar cells embedded in the sunroofs of Example 4 and Comparative Example 1 after being naturally exposed to atmosphere. Figure 11 is a diagram illustrating a temperature-humidity cycle test applied to the sunroofs prepared in accordance with Example 5, Comparative Examples 2 and 3. Figure 12 shows a graph illustrating the deterioration in the power output of the solar cells embedded in the sunroofs of Example 5, Comparative Exmples 2 and 3 after being subjected to the temperature-humidity cycle test as illustrated in Figure 11. Figure 13 shows a graph illustrating the deterioration in the power output of the solar cells embedded in the sunroofs of Example 5, Comparative Examples 2 and 3 after being naturally exposed to atmosphere. Figures 1a to 1c are cross-sectional views illustrating various embodiments of the glass substrate for a solar cell having the curved surface shape in accordance with the present invention. The glass sheet 1 used in the present invention is an insulating transparent sheet having a high light-transmittance, for example a transmittance of at least 85%, in the wavelength range of 350 - 800 nm, and having a high chemical and physical durability and good optical properties. For example, soda-lime silicate glass, aluminosilicate glass, borosilicate glass, lithium aluminosilicate glass, or various other types of glass may be employed. The thickness of the the glass sheet 1 is not particularly limited, but is suitably from 1.0 to 5.0 mm when various factors such as the easiness of bending treatment, light transmittance, weight, strength, the convenience of handling, and the like are taken into consideration. The curvature of a glass substrate for a solar cell having a curved surface shape of the present invention is not particularly limited, but if the glass substrate having a transaparent electroconductive film is bended into a too small radius of curvature, it sometimes causes such strain as to deteriorate the transparent electrocondctive film, and it is difficult to uniformly form an a-Si film on the curved surface. Thus, a suitable radius of curvature (R) is not less than 80 mm, preferably not less than 500 mm. Examples of a transparent electroconductive film 2 formed on a glass sheet 1 include highly electroconductive transparent metallic oxides such as tin oxide doped with fluorine in an amount of from 0.1 to 5% by weight, tin oxide doped with antimony in an amount of from 0.1 to 30% by weight, indium oxide doped with tin in an amount of from 0.5 to 30% by weight, and the like. Among them, a transparent electroconductive film comprising tin oxide doped with fluorine is most suitable as the transparent elctroconductive film for a solar cell since it easily provides a low sheet resistance of not higher than 10 Ω/□, and it has a high durability to a highly reductive hydrogen plasma used for the formation of a-Si by plasma CVD method. The thickness of the transparent electroconductive film is suitably from 300 to 10,000 Å (10 Å = 1 nm). The transparent electroconductive film used in the present invention can be formed by various conventional methods such as CVD method, spraying method, sputtering method, vacuum depositing method, ion plating method, dipping method and the like. Among them, CVD method and sputtering method are most suitable since they produce a satisfactory uniform electroconductive film of low resistance. As seen from Figures 1a to 1c, the glass substrate for a solar cell having a curved surface of the present invention comprises a glass sheet 1 having a curved surface shaped and a transparent electroconductive film 2 formed on the concave surface side of the glass sheet 1. If a flat plate-like glass substrate having a transparent electroconductive film formed thereon is heated to a bending treatment temperture and the heated substrate is bended in such a manner as to make the transparent electrocondctive film surface concave, the transparent electroconductive film is liable to remarkable deterioration as it cracks and/or peels off from the glass sheet. Furthermore, in many practical cases, a solar cell is prepared by forming a photoelectric conversion layer on the concave surface of a glass substrate. Thus, it is preferable also for practical use to form a transparent electroconductive film on the concave surface side of a glass sheet. For example, as seen from Figure 2, the desired solar cell for a sunroof of an automobile comprises a glass sheet 21, the convex side of which is exposed to atmosphere, a transperent electroconductive film 22 formed on the concave side of the glass sheet 21, i.e. on the inner side of the automobile, an a-Si semiconductor layer 25 formed on the transparent electroconductive film 22, and a back electrode 26 formed on the a-Si semiconductor layer 25. When the glass sheet 1 comprises an alkali metal-containing glass such as soda lime silicate glass or the like, as shwon in Figures 1b and 1c, an alkali barrier coating 11 may be optionally applied on the glass sheet 1 in order to prevent the transparent electroconductive film 6 from being adversely affected by alkali eluted from the surface of the glass sheet 1. Examples of the alkali barrier coating 11 include silicon oxide film, aluminum oxide film and zirconium oxide film, and the thickness of the alkali barrier coating is suitably from 500 to 800 Å. As seen from Figure 1c, a light-shielding colored layer 13 may be formed beneath the transparent electroconductive film 6 and the alkali barrier coating 11, along the peripheral margin of the glass substrate for a solar cell having a curved surface shape of the present invention in order to shield a collective busbar usually formed along the peripheral margin so that the busbar can be concealed from one's eyes, thus improving external appearance. The light-shielding colored layer 13 is not particularly limited, but it is generally a ceramic color ink comprising a pigment to generate a desired color, a low melting glass frit to form a paint film intimately adhered with the glass sheet, various refractory fillers, oil for screen-printing, and the like. It is preferable that the ceramic color ink does not deteriorate during forming the alkali barrier coating 11 or the transparent electoconductive film 6 thereon, and that it does not adversely affect the upper layer formed in contact therewith. A suitable method for preparing a glass substrate for a solar cell having a curved surface shape as shown in Figure 1c, comprises screen-printing a ceramic color ink 13 on a glass sheet 1 along the peripheral margin, drying the printed glass sheet 1, forming an alkali barrier coating 11 on the dried glass sheet 1 in the alkali barrier coating zone of a CVD furnace, forming a transparent electroconductive film 6 on the alkali barrier coating in the transparent electroconductive film forming zone of the CVD furnace, and finally subjecting the glass substrate thus prepared to a bending treatment. In the present invention, the glass substrate for a solar cell having a transprent electroconductive film is subjected to a bending treatment. Example of a bending method used for this treatment generally include a press-bending method which comprises press-bending a glass substrate heated to a bending temperature by a press mold (e.g. the bending temperature of soda lime silicate glass is from 570 to 700°C); a gravity-bending method using the self-weight of a glass substrate in bending; an air forming method which comprises having a glass substrate adsorbed and bended by an adsorbing mold; and a blow-bending method by a gas blow. The glass substrate thus bended may be subjected to a strengthening treatment. The strengthening treatment can be effected by blowing a quenching air to the bended glass substrate in the quenching step after the bending treatment, thereby quench-strengthening the bended glass substrate. Another way of strengthening the bended glass sheet is a chemical strengthening method by ion exchange. In the preparation of the glass substrate for a solar cell in accordance with the present invention, the bending treatment may be carried out any time before or after forming the transparent electroconductive film on a glass sheet. However, when it is difficult to uniformly form the transparent electroconductive film on the bended glass sheet (for example when the film formation is effected by CVD method), it is preferable to carry out the bending treatment after forming the transparent electroconductive film on a flat plate-like glass sheet. Also, when it is necessary for the formation of a transparent electroconductive film to heat the glass sheet to such a high temperature as to substantially deform the glass sheet (for example when the film is formed by CVD method), it is preferable to effect the bending treatment after the formation of the film. Because the accuracy of the bending deteriorates and the desired curved surface shape can not be obtained if the bended glass sheet is heated to a high temperature for the formation of the film. A great care must be taken to heat the glass substrate for bending after the formation of the transparent electroconductive film. If the glass substrate is heated to an excessively high temperature, the transparent electroconductive film is likely to be impaired by the thermal expansion difference with glass, oxidation, or other factors, thereby resulting in the rise of resistance value and the degradation of light-transmittance. We have carried out an experiment to determine preferable conditions for bending the glass substrate having a transparent electroconductive film, and obtained the results as shown in Figure 3. That is, Figue 3 is a graph showing a relation between the rising rate of resistance value Rs/Ro (Ro shows the resistance values of the transparent electroconductive film before heating, and Rs shows those after heating) and the heating temperature and heating time when a glass substrate having a fluorine-doped tin oxide film is heated in a heating furnace in the atmosphere of air. As can be seen from these results, if the heating temperature is set not higher than 650°C and the heating time is set not longer than 2 minutes, the rise of resistance value of the transparent electroconductive film can be controlled to a minimum. Thus, these heating conditions are most preferable for bending the glass substrate having the transparent electroconductive film of the present invention. That is, the most suitable method for producing a bending-treated glass substrate for a solar cell comprises heating a glass sheet at from 500 to 600°C, coating a transparent electroconductive film on one side of the glass sheet by CVD method, and bending the formed glass substrate at a bending temperature of from 580 to 650°C. According to this method, a glass substrate having stable performance can be produced at a high mass-productivity and at a low cost. Furthermore, when a strengthening treatment step is added to this method, a glass substrate produced thereby is durable to external impacts when exposed to the outside atmosphere, and is not damaged by hails, small stones, and the like. This strengthening treatment can be effected by a quench-strengthening method which comprises blowing cooling air at the cooling step after the bending treatment, or other methods including a chemical strengthening method using ion-exchange. The solar cell having a curved surface shape using the glass substrate of the present invention is suitable for use where a good design is required, and is particularly suitable for bending mounted in a sunroof of an automobile. Figure 2 shows an embodiment of the solar cell using the glass substrate of the present invention to be mounted in a sunroof of an automobile. That is, the solar cell shown in Figure 2 comprises a curve-shaped glass sheet 21 having a ceramic color ink layer 24 on the concave surface along the peripheral margin, an alkali-barrier coating 23 formed thereon, a transparent electroconductive film 22, an amorphous silicon photoelectric conversion layer 25, and a back electrode 26 formed thereon. The solar cell is then laminated with a glass plate 28 having substantially the same curved surfce as that of the glass sheet 21 with an interlayer 27 interposed therebetween. Light 29 incident on the glass sheet 21 of the solar cell thus prepared is absorbed in the a-Si layer 25, thereby generating an electromotive force between the transparent electroconductive film 23 and the back electrode 26, and the electromotive force thus generated is taken out by lead wire or other means (not shown in Figure 2). As mentioned above, the present invention provides a glass substrate having a transparent electroconductive film suitable for a solar cell having a curved surface shape, which is superior in weather-resistance and crack-resistance to a solar cell using a plastic film as a substrate, and it is thereby possible to produce solar cells of various curved surface shapes. According to the method for producing a glass substrate of the present inveniton, a curve-shape glass substrate having a transparent electroconductive film of good quality can be produced at a low cost by forming the transparent electroconductive film on a glass sheet and then subjecting the resultant glass substrate to a bending treatment without substantially deteriorating the transparent electroconductive film. The embodiments below are not part of the present invention, they are only mentioned as examples of the different uses of the claimed glass substrate, and are explained hereinafter in accordance with the attached drawings. A solar cell-installed window glass panel of the present invention comprises an outer glass plate 1, a light-transmissive solar cell 2 formed on the outer glass plate 1 on the inner side of an automobile, and a transparent protective layer 3 formed on the solar cell 2 on the inner side of an automobile. The protective layer 3 is made of a material having a penetration resistance, which controls the transmission of visible light. Figure 5 shows a typical embodiment. Said protective layer 3 is composed of a transparent interlayer 4 and a transparent inner glass plate 5, and the inner plate 5 is laminated with the light-transmissive solar cell 2 with the interlayer 4 interposed therebetween. At least one of the interlayer 4 and the inner glass plate 5 used as a protective layer is colored. It is preferable that the interlayer 4 is colored and that the visible light-transmittance is from 5 to 60%. If the visible light-transmittance of the colored interlayer is less than 5%, the total visible light-transmittance of the window glass panel is greatly reduced and drivers or passengers can not fully enjoy open-minded feeling. On the other hand, if the visible light-transmittance of the colored interlayer is higher than 60%, it is difficult to reduce the glare from the back electrode and/or grid electrode of a solar cell. The color of the colored transparent interlayer 4 can be optionally selected. For example, bronze, blue, grey or green may be mentioned. The material of the interlayer 4 is preferably polyvinyl butyral, ethylene vinyl acetate or urethane, which is commonly used for a laminted glass of an automobile. The thickness of the interlayer 4 is preferably from 0.3 to 0.8 mm. Among them, polyvinyl butyral is preferable, since it has satisfactory penetration-resistance and good adhesive property with glass. However, polyvinyl butyral has a water content of about 0.5% by weight. Therefore the performance of a solar cell is liable to be deteriorated by water freed from polyvinyl butyral when polyvinyl butyral is placed in direct contact with an amorphous silicon photoelectric conversion area. On the other hand, the water content of ethylene-vinyl acetate is adequately low, but the penetration-resistance is a little poor as compared with that of polyvinyl butyral. Accordingly, a double-layered interlayer prepared by laminating the two materials has both advantages of the two. Figure 6 is a partially sectional view illustrating an embodiment of the solar cell using a double-layered interlayer composed of two layers 4a and 4b. In this embodiment, at least one of the first interlayer 4a, the second interlayer 4b and an inner glass plate 5, which make a protective layer 3, is colored. Preferably, either interlayer 4a or 4b is colored, and the visible light-transmittance of the colored interlayer is prefereably from 5 to 60%. If the visible light-transmittance of the colored interlayer is less than 5%, drivers and passengers can not fully enjoy open-minded feeling and therefore an object of the present invention can not be achieved. On the other hand, if the visible light-transmittance of the colored interlayer is higher than 60%, it is difficult to reduce the glare from the back electrode and/or grid electrode of the solar cell. The color of the colored transparent interlayer 4a and/or 4b can be optionally selected, including bronze, blue, grey and green. The material of the second interlayer 4b is preferably polyvinyl butyral or urethane, which is commonly used for a laminated glass of an automobile, and the thickness of the interlayer 4b is preferably from 0.2 to 0.8 mm. On the other hand, the material of the first interlayer 4a in direct contact with a solar cell should preferably have a moisture-permeability coefficient of not higher than 20 (g/hr/cm²/cm). A preferable example is ethylene-vinyl acetate (EVA). Ethylene-vinyl acetate is pereferable in view of transparency and adhesive property with the back electrode 8 of the solar cell 2. Ethylene-vinyl acetate having a thickness of larger than 0.05 mm satisfactorily shields a solar cell from moisture. From the practical view point, the thickness of ethylene-vinyl acetate is preferably from about 0.2 to 0.4 mm, which is easy to handle. In the embodiments as shown in Figures 5 and 6, when a colored transparent inner glass plate 5 is used, it is favorable to use a thermal ray-absorbing glass prepared by adding a minor amount of a metal oxide or metal such as iron, nickel, cobalt or selenium. It is also favorable to use a thermal ray-refective glass with its surface in contact with the second interlayer being treated. The color of the inner glass plate 5 is influenced by the type of metal or metal oxide to be added and the type of surface treatment to be applied, and can be optionally selected, for example, from bronze, blue, grey and green. The visible light-transmittance of the transparent inner glass plate 5 should also be preferably from 5 to 60%. The transparent protective layer 3 may be composed of one or more plastic layer. For example, polyethylene terephthalate (PET) having a sufficient durability is preferably used. Furthermore, a urethane type plastic film having a self-healing properting can also be used. When the protective layer is composed of a plastic type film only, a window glass panel for an automobile having a light weight and an anti-lacerative property to human body can be provided. The light-transmissive solar cell 2 formed on an outer glass plate 1 on the inner side of an automobile is not particularly limited. As shown in Figures 5 and 6, it is generally composed of a light-incident side transparent electrode 6 made of a transparent electroconductive film comprising fluorine-containing tin oxide, tin-containing indium oxide or the like as the main component; a p-, i-, n-type amorphous silicon layer 7 as a photoelectric conversion area; and a transparent back electrode 8 made of a metal oxide, a metal such as Ag, Al and Cr, or a laminted layer of the two. The thickness of the p-, i-, n-type amorphous silicon layer used for the transparent solar cell-installed window glass panel may be from 2,000 to 5,000 Å. Reference numeral 9 in the Figures represents a grid electrode made of e.g. Ag which connects a light-incident side electrode 6 with the corresponding back electrode 8 of the respective solar cell. Numeral 12 represents a pair of collective electrodes to take out electromotive force generated in the solar cell. When an alkali metal-containing glass is used as the outer glass plate 1, an alkali barrier film 11 such as a film comprising silicon dioxide or aluminum oxide as the main component, may be formed between the outer glass plate 11 and the light-incident side electrode 6 in order to protect the light-incident side electrode 6 from being damaged by the alkali component, thereby preventing the electroconductivity of the electrode 6 from lowering. The window glass panel comprises a light-transmissive solar cell 2 formed on a transparent outer glass plate 1 on the inner side of an automobile and a transparent protective layer 3 formed on the solar cell 2 on the inner side of the automobile, the protective layer 3 being composed of an interlayer 4 and an inner glass plate 5, at least one of which is colored, whereby the glare from the back electrode of the solar cell 2 in the inside of an automobile can be reduced by the colored protective layer. Furthermore, as shown in Figue 6, when the interlayer 4 is composed of two layers, and the interlayer 4a in direct contact with the solar cell 2 is made of ethylene-vinyl acetate having less water content and less moisture permeability, the electric properties of the solar cell 2 is not deteriorated by water, thereby providing a solar cell having a good penetration resistance, the electric properties of which are stable for a long period of time. The total visible light-transmittance of the solar cell-installed window glass panel for an automobile using the above-mentioned colored transparent protective layer should be controlled preferably 20% or less, thereby removing the glare as viewed from the inside of the automobile. The open-minded feeling in the inside of the automobile can be assured if the visible light-transmittance is at least a few percents, e.g. 5% or more. As shown in Figures 5 and 6, in a window glass panel comprising a solar cell 2 having a collective electrode 12 along the peripheral margin formed on a transparent outer glass plate 1 and a transparent protective layer 3 formed on the solar cell 2, the collective electrode 12 is visible from both the inside and the outside of the automobile. It is therefore preferable for beautiful external appearance to conceal the colllective electrode. For example, as shown in Figure 7, it is preferable to form a light-shielding colored layer 13 along the entire peripheral margin of the window glass panel in such a manner as to conceal the collective electrode 12. Figures 5 and 6 illustrate the cross sections taken along the line A-A in Figure 7. The light-shielding colored layer 13 is not particularly limited, but it is generally a ceramic color ink, as shown in Figure 1c and Figure 2, which comprises a pigment to give a desired color, a low melting glass frit to form a paint film intimately adhered with the glass sheet, various refractor fillers and oil for screen-printing. It is preferable that the ceramic color ink does not deteriorate during forming an alkali barrier coating 11 or a light-incident side electrode 6 thereon. Further, it should not adversely affect the upper layer to be formed in contact therewith. Figrues 5 and 6 illustrate embodiments wherein light-shielding colored layers 13a and 13b are formed on an outer glass plate 1 on the inside of an automobile and on an inner glass plate 5 on the inside of an automobile, respectively, but they may be formed on the outer glass plate 1 and on the inner glass plate 5 both on the outside of the automobile. However, it is generally preferred to form the light-shielding colored layer 13a on the outer glass plate 1 along the peripheral margin not on the outside of an automobile but on the inside of an automobile as shown in Figures 5 and 6 in view of weather-resistance and external appearance. With regard to the inner glass plate 5, a light-shielding colored layer may be formed on either the inside or the outside of an automobile. Figures 5 and 6 illustrate the cases wherein the light-shielding colored layer 13b is formed on the inner glass plate 5 along the peripheral margin on the inside of an automobile. In the preparation of a solar cell having a curved surface shape, the glass substrate having a curved surface shape for the solar cell is prepared by firstly forming a light-shielding colored layer 13a on a flat outer glass plate along the peripheral margin, forming thereon a light-incident side electrode 6, a collective electrode 12 and a grid electrode 9, placing the resultant outer glass plate in such as manner as to make the light-incident side electrode 6, etc. upward, placing thereon a flat inner glass plate having a light-shielding colored layer 13b along the peripheral margin, heating the outer glass plate and the inner glass plate thus placed to overlap each other, and finally bending the resultant sandwiched stracture by a self-weight bending method in such a manner as to make the surface of the light-incident electrode side of the outer glass plate into the shape of a concave surface. In such a preparation step as mentioned above, when a light-shielding colored layer is formed on the inner glass plate on the outside of an automobile, the light-shielding colored layer formed on the inner glass plate along the peripheral margin is heated in the state of being in contact with the outer glass plate in the bending step. Therefore, there is a fear that the light-shielding colored layer is melted and fused to deposit on the outer glass plate located thereunder. It is therefore preferable to form the light-shielding colored layer 13b on the inner glass plate on the inside of an automobile when the bending step is carried out by the above-mentioned method. When an outer glass plate and an inner glass plate are individually bended, a light-shielding colored layer may be formed on the inner glass plate on either the inside or the outside of an automobile. When a light-shielding colored layer is required to be formed in a large thickness, it is preferable to form the light-shielding colored layer on the inner glass plate on the inside of an automobile. Otherwise, the outer glass plate having the thick light-shielding colored layer formed on the inside of an automobile and the inner glass plate having the thick light-shielding colored layer formed on the outside of the automobile are not satisfactorily laminated with an interlayer 4 located therebetween. The light-shielding colored layer 13 has another merit of concealing a dirty part produced when an interlayer constituting a window glass panel is deteriorated along the peripheral margin by reaction with water, thereby spoiling external appearance. A method for producing a glass substrate having such a curved surface suitable for a solar cell-installed window glass panel, comrises firstly washing a flat outer glass plate 1; forming a light-shielding colored layer 13, for example ceramic color ink, on the washed outer glass plate along the peripheral margin by printing and baking; forming an alkali barrier coating 11 and a light-incident side electrode 6 thereon; forming a grid electrode 9 and a collective electrode 12 made of e.g. Ag paste by screen-printing to have appropriate patterns at appropriate parts and baking them; placing a flat inner glass plate 5 having a light-shielding colored layer 13 printed and baked along the peripheral margin on the above treated outer glass plate to overlap each other;and finally bending the sandwiched structure in such a manner as to make the surface of the light-incident electrode side of the outer glass plate into the shape of a concave surface. The final bending treatment is effected preferably by a self-weight bending method so as not to deteriorate the light-incident side electrode 6, etc, after placing the inner glass plate on the light-incident side electrode 6, etc. of the outer glass plate. Thereafter, the inner glass plate is separated from the outer glass plate, and a photoelectric conversion layer 7 and a back electrode 8 are formed on the light-incident side electrode on the outer glass plate. The outer glass plate thus treated is then laminated with the inner glass plate preferably by interposing an interlayer 4 therebetween. A solar cell-installed window glass panel laminated by interposing an interlayer therebetween as shown in Figures 2, 5 and 6 is preferably sealed along its peripheral margin in order to protect the peripheral margin from being damaged by water, therevy preventing the lowering of the performance of the solar cell and also preventing Ag in the grid electrode 9 and the collective electrode 12 from whitening. Figure 8 is a cross sectional view illustrating an embodiment of sealing the peripheral margin of the solar cell-installed window glass panel as shown in Figure 5. A sealing material 14 to seal the peripheral margin should preferably have a moisture-permeability coefficient of less than 20 (g/hr/cm²/cm) in order to achieve satisfactory sealing effect. Preferable examples of the sealing material include butyl rubber (moisture-permeability coefficient = 0.1 to 0.5), fluorine type resin (moisture-permeability coefficient = 0.1 to 0.5), EVA (moisture-permeability coefficient = 2 to 3), epoxy resin (moisture-permeability coefficient = 4 to 6) and vinyl chloride type resin (moisture-permeability coefficient = 15). The shape of the sealing material is not particularly limited, but the sealing material may have a shape suitable for fixing a solar cell-installed window glass panel into an automobile body and it may work not only for sealing but also as a mole. As mentioned above, the damage by water at the peripheral margin of the glass panel can be prevented by sealing the peripheral margin with a sealing material having a less moisture-permeability coefficient, thereby preventing the lowering of the performance of a solar cell caused by reaction of water with the a-Si layer 7 constituting the photoelectric convertion area of a solar cell, with the grid electrode 9, or with the collective electrode 12, and also preventing the whitening of Ag which is the main component of the grid electrode 9 or collective electrode 12. As shown in Figure 9a, a synthetic resin can be molded into a desired shape along the peripheral margin of a solar cell-installed window glass panel of the present invention by an extrusion molding, injection molding, transfer molding, RIM (reaction injection molding) or LIM (liquid injection molding) method, so that the window glass panel can be easily fixed into an automobile body 16, and in such a case a decorative mole and a gasket 15 can be integrally molded. The type of the synthetic resin used for this purpose is not particularly limited if it is molded and solidified by the above-mentioned various molding methods. However, it is preferable for productivity and manufacturing cost to mold a mole or gasket made of polyurethane by RIM method from a polyol and an isocyanate compound or to mold a mole or gasket from a vinyl chloride type resin by injection molding method. In addition to the above-mentioned synthetic resins, thermoplastic resins or thermoplastic rubbers such as an ethylene-vinyl acetate copolymer (EVA), butyl rubber or polyvinyl chloride, thermosetting resins such as epoxy resin, and fluorine resin, may also be used. As shown in Figure 9a, the mole or gasket 15 is molded preferably so as to cover the sealing material 14 sealing the peripheral margin. When a synthetic resin having a sufficiently small moisture-permeability coefficient such as EVA, butyl rubber, fluorine resin or epoxy resin is used for molding a mole or gasket, the mole or gasket may be molded directly along the peripheral margin of a solar cell-installed window glass panel so as to serve also as the sealing material 14. Generally, a gasket is regarded as intended for sealing a window glass panel fixed to an automobile body, and a mole is regarded as intended for decoration. However, for the purpose of the present invention, the gasket and mole are not particulrly discriminated and used in substantially the same meaning. A gasket 15 made of a resin should preferably have such a shape as to be easily mounted on an automobile body 16. The gasket 15 may be integrally molded, together with a mounting element to the automobile body 16, for example, with a part of a retainer embedded in the gasket. As shown in Figure 9a, a lead wire 17 to take out electric power from a pair of collective electrodes 12 may also be integrally embedded, thereby providing a good external appearance and protecting the contact point between the collective electrode 12 and the lead wire 17 from being attacked by water, chemicals, etc. In this manner, the collection of electric power can be effected without being adversely affected by external physical factors including tensile force, impact force or the like. Further, as shown in Figue 9b, a terminal 18 and a lead wire 17 for taking out electric power from a solar cell may also be integrally embedded by molding. Still further, as shown in Figure 9c, external circuits 19 to support a solar cell, including a diode for preventing reverse current, a circuit for stabilizing electric voltage, a circuit for stabilizing electric current and other circuits necessary for an automobile, may also be embedded by molding. In this manner, these external circuits can be placed without spoiling the external appearance, and can be stabilized for a long period of time, thus being protected from external impact force, water chemicals and the like. Figure 9d illustrates still another embodiment. When a gasket material is integrally molded into a cavity space along the peripheral margin of a window glass panel by RIM method, injection molding method or the like, it is sometimes difficult to fix flexible elements such as a lead wire or the like into a desired place. In such a case, it is convenient to use, instead of the flexible element, a metallic element having a fixed shape to take out electric power from a solar cell. Figure 9d illustrates an embodiment wherein a fixed terminal 18 electrically connected with a light-incident side electrode 6 is provided in place of the flexible lead wire, so that it protrudes out from a gasket 15 and a lead wire may be electrically connected to the protruded portion of the fixed terminal 18. When various circuits including a circuit for preventing reverse current are embedded as shown in Figure 9c, the circuits may be preferably connected with the fixed terminal to take out electric power. The present invention will be described in further detail with reference to the following Examples. EXAMPLE 1 A soda lime silicate glass sheet of 870 x 285 x 2 (mm) was thoroughly washed, dried and thereafter placed in a CVD furnace of belt conveyor conveying type. SiH₄ gas and O₂ gas were then introduced into the CVD furnace, in an SiH₄:0₂ ratio of about 1:10, and a silicon oxide film (700 Å) was formed by CVD method at a glass sheet surface temperature of about 450°C as an alkali barrier coating. After heating the glass sheet at 580°C, a gaseous mixture of tin tetrachloride in an amount of 1 x 10⁻² ℓ/min., water vapor (10 times the amount of the tin tetrachloride), methyl alcohol (0.5 time the amount of the tin tetrachloride), hydrofluoric acid (0.5 time the amount of the tin tetrachloride) and nitrogen (250 times the amount of the tin tetrachloride) was blown onto the silicon oxide film on the glass sheet, thereby forming a transparent electroconductive film (6,000 Å) comprising tin oxide containing 1.0% by weight of fluorine by CVD method. The conveying speed of the glass sheet in the alkali barrier coating and tin oxide film coating zone was 0.60 m/min. The resultant transparent electroconductive film-coated glass sheet (hereinafter referred to as "TCO substrate") had a surface resistance of 8.0 Ω/□. Thereafter, the TCO substrate thus obtained was placed on a frame having a desired curved surface shape for self-weight bending by gravity bending method, so that the tin oxide film is located above, and the substrate was heated to 590°C. The self-weight bending was then effected to make the tin oxide side surface concave. Various properties of the TCO substrate having the curved surface shape thus obtained are shown in Table 1. In Table 1, the transmittance *1 was measured by an integrating-sphere photometer using C Light Source (JIS Z8720) as a light source. Bending A *2 shows the maximum displacement of the longer edge, and Bending B *2 shows the maximum displacement of the shorter edge. Thus, the TCO substrate having the desired curved surface was obtained, with its physical properties substantially unchanged before and after bending. EXAMPLE 2 A soda lime silicate glass sheet of 1013 x 445 x 3 (mm) was thoroughly washed, and placed in the same CVD furnace as used in Example 1. A silicon oxide film (700 Å) and a transparent electroconctive film (7,000 Å) of tin oxide containing 1.0% by weight of fluorine were then formed on the glass sheet by the CVD method in the same manner as in Example 1, except that the conveying speed of the glass sheet was changed to 0.45 m/min. The TCO substrate thus obtained had a surface resistance of 6.0 Ω/□. The TCO substrate was then suspended by a suspender, and heated to 630°C. The heated TCO substrate was then press-bended to make the tin oxide side surface concave, and the press-bended TCO substrate was subjected to a strengthening treatment by air cooling. Various properties of the TCO substrate having the curved surface shape thus obtained are shown in Table 1. Thus, the TCO substrate having the desired curved surface was obtained, with its physical properties substantially unchanged before and after bending. Also, the TCO substrate had a surface compressive stress of 400 kgf/cm². Example 1 Example 2 Before bending Surface resistance (Ω/□) 8.0 6.0 Transmittance (%) 80 79 After bending Surface resistance (Ω/□) 8.1 6.3 Transmittance (%) 80 79 Bending A (mm) 19 31 Bending B (mm) 5 8.5 External appearance No change No change The following examples do not form part of the present invention. EXAMPLE 3 A window glass panel for a sunroof of an automobile was prepared by laminating an outer glass plate 1 of transparent float glass (thickness = 2.0 mm, visible light transmitance, T v = 7.5%) having a transparent solar cell 2 comprising a fluorine-containing tin oxide film (6,000 Å), p-type amorphous SiC layer (100 Å), i-type amorphous silicon layer (3,000 Å), n-type amorphous silicon layer (300 Å) and an Ag back electrode (400 Å) formed thereon on the inside of an autombile, and an inner glass plate 5 of transparent float glass (thickness = 2.0 mm) with an interlayer 4 (thickness = 0.75 mm, T v = 55%) of bronze-colored polyvinyl butyral interposed between the outer and the inner glass plates. The optical properties of the window glass panel are shown in Table 2 together with those of Comaravtive Example 1. EXAMPLE 4 A window glass panel for a sunroof of an automobile was prepared by laminating an outer glass plate of transparent float glass (thickness = 2.0 mm, T v = 7.5% ) having a transparent solar cell 2 comprising a fluorine-contianing tin oxide film (6,000 Å), p-type amorphous SiC layer (100 Å), i-type amorphous silicon layer (3,000 Å), n-type amorphous silicon layer (300 Å) and an Ag back electrode (400 Å) formed thereon on the inside of an automobile, and an inner glass plate 5 of transparent float glass (thickness = 2.0 mm), with a double-layered interlayer comprising the first interlayer 4a of ethylene-vinyl acetate (thickness = 0.4 mm, T v = 90%) and the second interlayer 4b of bronze-colored polyvinyl butyral (thickness = 0.37 mm, T v = 55%) interposed between the outer and the inner glass plates. The optical properties of the window glass panel are shown in Table 2, together with those of Comparative Example 1. The initial value of the maximum output power (Pmax) of the solar cell embedded in this sunroof is defined as 1, and the lowering of the maximum output power with the passage of time when exposed to the atmosphere is expressed by Line A in Figure 10. COMPARATIVE EXAMPLE 1 A window glas panel for a sunroof of an automobile was prepared in the same manner as in Example 3, except that non-colord polyvinyl butyral (thickness = 0.75 mm, T v = 90%) was used as the interlayer 4. The optical properties of this comparavtive window glass panel are shown in Table 2. The initial value of the maximum output power (Pmax) of the solar cell embedded in this comparative sunroof is defined as 1, and the lowering of the maximum output power with the passage of time when exposed to the atmosphere is expressed by Line B in Figure 10. Example 3 Example 4 Comparative Example 1 Visible light-transmittance (%) 4.2 3.7 6.7 Reflectance in the inside of automobile (%) 5.7 5.9 23.4 Glare in the inside of automobile Nil Nil Much Concealment of grid electrode Good Good No good As can be seen from Figure 10, the output power of a solar cell is more stable for a long period of time when ethylene-vinyl acetate having a less water content and a smaller moisture-permeability as the first interlayer in contact with the solar cell 2 (Example 4, Line A) is used in comparison with when polyvinyl butyral (Comparative Example 1, Line B) is used. EXAMPLE 5 A window glass panel for a sunroof of an automobile was prepared by laminating an outer glass palte 1 of transparent float glass (thickness = 2.0 mm, T v = 7.5%) having a transparent solar cell 2 comprising a fluorine-containing tin oxide film (6,000 Å), p-type amorphous SiC layer (100 Å), i-type amorphous silicon layer (3,000 Å), n-type amorphous silicon layer (300 Å) and an Ag back electrode (400 Å) formed thereon on the inside of an automobile and an inner glass plate 5 of transparent float glass (thickness = 2.0 mm) with an interlayer 4 (thickness = 0.75 mm, T v = 55%) of bronze-colored polyvinyl butyral interposed between the outer and the inner glass plates. The window glass panel was sealed along the entire peripheral margin with butyl rubber having a moisture-permeability of from 0.1 to 0.5 (g/hr/cm²/cm). COMPARATIVE EXAMPLE 2 A window glass panel for a sunroof was prepard in the same manner as in Example 5, except that Thiokol having a moisture-permeability of 80 (g/hr/cm²/cm) was used as a sealing material. COMPARATIVE EXAMPLE 3 A window glass panel for a sunroof was prepared in the same manner as in Example 5, except that silicone having a moisture-permeability of 350 (g/hr/cm²/cm) was used as a sealing material. The window glass panels as prepared in Example 5, Comparative Examples 2 and 3 were subjected to a temperature-humidity cycle test under the conditions shown in Figure 11. The results are shown in Figure 12. Other window glass panels prepared in the same manner as in Example 5, Comparative Examples 2 and 3 were exposed to the atmosphere, and the results are shown in Figure 13. As can be seen from Figures 12 and 13, the window glass panel sealed with Thiokal having a moisture-permeability of 8.0 (g/hr/cm²/cm) provided substantially the same results in the temperature-moisture cycle test as Example 5, but provided poorer durability in the exposure test as compared with Example 5. EXAMPLE 6 A winow glass panel for a sunroof of an automobile was prepared by laminating an outer glass plate 1 of transparent float glass thickness = 2.0 mm, T v = 7.5%) having a transparent solar cell 2 comprising a fluorine-containing tin oxide film (6,000 Å), p-type amorphous SiC layer (100 Å), i-type amorphous silicon layer (3,000 Å), n-type amorphous silicon layer (300 Å) and an Ag back electrode (400 Å) formed thereon on the inside of an automobile and an inner glass plate 5 of transparent float glass (thickness = 2.0 mm) with an interlayer 4 (thickness = 0.75 mm, T v = 55%) of bronze-colored polyvinyl butyral interposed between the outer and the inner glass plates. Vinyl chloride resin (moisture-permeability = 15 g/hr/cm²/cm) was integrally molded by injection-molding method to prepare a gasket 15 along the peripheral margin of the above solar cell-installed window glass panel. EXAMPLE 7 A solar cell-installed window glass panel for a sunroof was prepared in the same manner as in Example 6, except that the window glass panel was sealed with epoxy resin (moisture-permeability = 5 g/hr/cm²/cm) as a sealing material 14 and integrally molded with urethane type resin (moisture-permeability = 170 g/hr/cm²/cm) to prepare a gasket 15 along the peripheral margin. COMPARATIVE EXAMPLE 4 A solar cell-installed window glass panel for a sunroof was prepared in the same manner as in Example 6, except that neither a sealing material 14 nor a gasket 15 was formed. COMPARATIVE EXAMPLE 5 A solar cell-installed window glass panel for a sunroof was prepared in the same manner as in Example 7, except that the sealing material 14 was omitted. The above solar cell-installed window glass panels were subjected to the atmospheric exposure test to evaluate the lowering of the solar cell performance, and the test results are shown in Table 3. Output power after 90 days/Initial output power Example 6 0.88 Example 7 0.90 Comparative Example 4 0.53 Comparative Example 5 0.61

Claims: A method for producing a glass substrate for a solar cell having a curved surface shape, which comprises heating a glass sheet, forming a transparent electroconductive film on said glass sheet and then subjecting the resultant glass substrate to a bending treatment in such a manner as to make the transparent electroconductive film surface of the glass substrate in the shape of a concave surface. The method according to Claim 1, which further comprises subjecting said glass substrate to a strengthening treatment after the bending treatment.

Inventors: KIKUGAWA SINYA, OMAE MASARU, OSADA KOICHI, SATO KATSUHITO, TAKIGAWA TOMOYA, YABA SUSUMU, KIKUGAWA, SINYA, OMAE, MASARU, OSADA, KOICHI, SATO, KATSUHITO, TAKIGAWA, TOMOYA, YABA, SUSUMU
Applicants: ASAHI GLASS CO LTD, ASAHI GLASS COMPANY LTD.
Application year: 1989
Main classification: H01L25
IPC codes: H01L31, B60J7, C03C17, B60J1, H01L27, B32B17