Solar cell and fabrication method thereof

Abstract: A thin film solar cell having a semiconductor layer (103,104,105) deposited on a substrate (101) is composed of a passivation layer made of a polymer resin (106) coated on the upper portion of the semiconductor layer (105), and an upper electrode (107) made of a conductive paste laminated on the passivation layer (106). Also, a collector electrode may be laminated on the upper electrode by electroplating. A method for fabricating a solar cell by depositing a semiconductor layer on the substrate includes coating a passivation layer made of a polymer resin on the upper portion of the semiconductor layer, and laminating an upper electrode made of a conductive paste containing a component capable of dissolving the polymer resin on the passivation layer. Also, the method may include laminating a collector electrode on the upper electrode by electroplating.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a photoelectric conversion element, or a photovoltaic element which converts the light to the electric energy, and more particularly to a solar cell having an excellent mass productivity and a high reliability and a fabrication method of the solar cell. Related Background Art Solar cell which is a photoelectric conversion element from the solar light to the electric energy has been noted as the oil substitute energy source. Examples of solar cell include a single crystal silicon solar cell, a polycrystalline silicon solar cell, an amorphous silicon-type solar cell or the like. The fabrication method of the single crystal silicon relies on the semiconductor process with a high production cost. Since the single crystal silicon has a small light absorption coefficient due to indirect transition, the solar cell of single crystal must be at least 50 microns thick to absorb the incident solar light, resulting in a high cost of material, and with the band gap being about 1.1eV, the short wavelength component having a wavelength of not more than about 500nm within the solar light spectra can not be effectively used because of the problem of the surface reconnection or wasteful energy at or below the band gap. The polycrystalline silicon has still an indirect transition problem even though it can have a reduced production cost, as compared with the single crystal silicon, so that the thickness of a solar cell can not be reduced. Moreover, the polycrystalline silicon has also other problems such as sub-boundary. Meanwhile, as one example of actually substituting the solar cell for the electric power, the necessary electric power used in an individual house is about 3KW. On the other hand, since the solar energy is 1Kw/m 2 at peak, the solar cells as large as 30m 2 are necessary if the conversion efficiency is 10%. Accordingly, it is necessary to fabricate a solar cell module as large as possible to use the solar cell for the electric power. However, in the single crystal silicon, a large wafer can not be fabricated, so that the realization of large area is difficult, whereby it is requisite to make the wiring to connect unit elements in series or parallel in order to obtain a large power. Further, since the single crystal silicon is mechanically weak, an expensive encapsulation made of glass, polymer resin, or aluminum frame is necessary in order to protect the solar cell from a mechanical damage caused by various weather conditions when it is used outdoors. From the above reason, there is a problem that the production cost per unit amount of power generation is higher than the existing power generation method. From these matters, studies have been made for the amorphous silicon-type solar cell which can be produced at a low cost and with a large area. Figs. 7A to 7C show typical examples of the conventional solar cell. In the figures, 700 is a solar cell main body, 702 is a lower electrode, 703 is an n layer, 704 is an i layer, 705 is a p layer, 706 is a transparent electrode, 707 is a passivation layer, 708 is a collector electrode, and 709 is a bus bar. The structure of an amorphous silicon-type solar cell is generally obtained by laminating the transparent electrode 706 for the reduction of surface resistance on one or more pairs of semiconductor junctions composed of the thin film p layer 705, the i layer 704 and the n layer 703 provided on the substrate 701, thereafter depositing the collector electrode 708 made of a relatively slender metal for collecting the electric current, and further depositing an electrode made of a relatively thick metal called the bus bar 709 for the collection of the current collected by the collector electrode 708. The problem as encountered when such an amorphous silicon-type solar cell is deposited on a large substrate e.g. about 30 cm square, or continuously deposited on a long substrate with a roll-to-roll method is that pin holes or other defects occur in the fabrication process, causing a shunt or short circuit, so that the conversion efficiency of solar cell may decrease. This is due to the fact that since the substrate surface is not a complete smooth plane, which may contain some flaws, dimples or spike-like projections, or which may be provided with an irregular back reflector for the purpose of reflecting irregularly the light against the substrate, a thin film semiconductor layer as thick as several hundreds Å (10Å = 1 nm) such as the p or n layer can not cover such a surface, or as another cause, pin holes occur due to the dust at the film formation. There is no problem if intrinsic normal semiconductor junction exists between the lower and upper electrodes, but if the semiconductor junction is lost due to a defect as mentioned above so that the upper and lower electrodes may be directly contacted with each other, or the spike of substrate may be placed into contact with the upper electrode, or if the semiconductor layer becomes a low resistance even though being not completely lost, the electric current produced by the light will flow into such defective portion, whereby the produced current can not be effectively collected. When the defective position is left away from the collector electrode or the bus bar, the resistance of the current flowing into the defective portion is great, with a relatively small current loss, but conversely, if it is under the collector electrode or the bus bar, the lost current is greater due to such defect. Owing to the opacity of the collector electrode and the bus bar, the effective area is reduced. In order to reduce the area of the collector electrode and the bus bar, and take out the electric current effectively, effective means is to reduce the specific resistances of the collector electrode and the bus bar, and to increase the cross-section of the electrode. Accordingly, a material having a small resistivity such as silver or copper is suitable at the electrode material. For example, the resistivity of silver is 1.62 x 10 -6 Ωcm, and the resistivity of copper is 1.72 x 10 -6 Ωcm, while aluminum has a resistivity of 2.75 x 10 -6 Ωcm, and zinc has a resistivity of 5.9 x 10 -6 Ωcm. The method for forming such an electrode is conventionally any one of the vapor deposition method, the plating method or the printing method if the solar cell is of crystal type. With the vapor deposition method, a good quality metal can be deposited in excellent ohmic contact with the semiconductor, but there is a disadvantage that the deposition rate is slow, the throughput is low due to the vacuum process, and the mask is required to form a specific pattern. With the plating method, the electroless plating of Ni is generally conducted, but has a problem that exfoliation easily occur and the mask is required. The printing method has a feature that automation is realized most easily and the mass productivity is high, one method being adopted in which Ag paste is screen printed, sintered at high temperature, and contacted. To decrease the resistance, it is contemplated that a plating or soldering coat is applied on the printed electrode. In the amorphous silicon-type solar cell, all the above methods are investigated, but practically, the printing method has been put into use because of its superior mass productivity. However, the amorphous silicon can not be sintered unlike the above-mentioned crystal type, so that it becomes a high resistive electrode. That is, since conductive paste of silver contains a polymer resin as the binder, the resistivity is about 4 x 10 -5 Ωcm, which is one digit higher than the pure silver. Accordingly, in order to reduce the resistance without changing the area of collector electrode, it is desirable to increase the thickness of the electrode. However, with the increased thickness, it is necessary to raise the viscosity of the conductive paste, whereby there is a limit as the screen may be clogged. Therefore, the thickness of the electrode fabricated practically by the screen printing is in a range of 10 µm to 20 µm. Accordingly, the collector electrode fabricated by the screen printing tended to be wider to decrease the resistance. From such matters, in the current situation, when the screen printing is used, a minute electrode can not be formed, resulting in a great loss of effective area. Meanwhile, as a countermeasure against the shunt or short circuit as described above, it is an effective means for preventing the decrease in the conversion efficiency that only the defective part is selectively covered with an insulating material or a material having an adequate high resistance to increase the contact resistance with the transparent electrode, the collector electrode or the bus bar. The selective treatment for only the defective part is the best method, but requires a complex process. Other than such a selective insulation, there are known methods in which the portion under the bus bar having the greatest influence is insulated with polymer or oxide as disclosed in United States Patent No. 4,590,327, and shown in Fig. 7C, and in which the portion corresponding to the pattern of bus bar and collector electrode between the semiconductor layer and the transparent electrode is provided with a material having a low electrical conductivity of 3000 Ω/□ or greater or an insulating material as disclosed in United States Patent No. 4,633,033 and United States patent No. 4,633,034. However, since in the conventional method the insulating material or high resistive material was disposed in accordance with the pattern of the bus bar and collector electrode, it was necessary that the insulating material having substantially the same shape be patterned at the same position as the bus bar, thus resulting in an impractical process due to the necessity of the patterning process. Also, the bus bar was laminated on the insulating material or high resistive material, the adhesion strength was weak, thus requiring an adhesive for the bus bar. Also, since in the conventional method the collector electrode itself was provided with the resistance, instead of providing the insulating material under the collector electrode, the series resistance of the solar cell became great, resulting in a low conversion efficiency. If only the collector electrode was fabricated by the normal method to avoid such a thing, there was a great possibility of causing a short circuit when the under portion of the collector electrode contained some defects. Also, the solar cell was unstable to the stimulus such as heat, light or humidity, even though the initial characteristic of solar cell module was sufficient. For this reason, there was a possibility that the short circuit might occur in the actual use. The plating method can be used as with the crystal system, and for example, in Japanese Patent Application Laid-Open No. 60-66426, there is disclosed a method of forming an electrode on the transparent electrode of the solar cell by the plating method. However, with this method, it is necessary to make the patterning with a resist to protect a portion other than the collector electrode from being plated. The plating method has associated therewith several problems that due to the necessity of the patterning, the fabrication process becomes complex, or if a thick plating membrance is made on the transparent electrode, the solar cell may be adversely affected as the solar cell is immersed in the acid or alkali plating bath for a long time, or the formed electrode is easily exfoliated due to its thick membrane. Moreover, in plating, a problem may arise that the transparent electrode is easily affected by hydrogen produced in the competitive reaction. SUMMARY OF THE INVENTION An object of the present invention is to solve the aforementioned problems associated with a thin film solar cell, and to provide a solar cell having an excellent characteristic. Another object of the present invention is to provide a solar cell having an excellent durability in the actual use. Also, another object of the present invention is to provide a fabrication method for a solar cell having an excellent mass productivity and a high reliability. A further object of the present invention is to provide a fabrication method for a solar cell having an excellent characteristic and a high yield by passivating defects of the solar cell. In order to accomplish the above-mentioned objects, the present invention provides a thin film solar cell having a semiconductor layer deposited on a substrate, comprising a substantially insulating and light transmissive passivation layer made of a polymer resin and coated on the upper portion of said semiconductor layer; and an upper electrode made of a solidified conductive paste containing a conductive filler, the upper electrode being located on, and etablishing an electrical contact through, said passivation layer. Also, it is characterized in that a collector electrode is laminated on the upper electrode by electroplating. The polymer resin is preferably a component selected from polyester, a copolymer of ethylene and vinyl acetate, acrylic, epoxy, and urethan, and desirably a polymer resin containing at least one of them. Also, the polymer resin has desirably a transmittance of 90% or greater with respect to the solar light, and a substantially electrical insulating property. The metallic component of the conductive paste preferably contains at least one of silver, palladium, copper, carbon and their alloy. The present invention provides a method for fabricating a thin film solar cell by depositing a semiconductor layer on a substrate, comprising the steps of: coating a substantially insulating and light transmissive passivation layer made of a polymer resin on the upper portion of said semiconductor layer; setting a conductive paste containing a conductive filler and a component capable of dissolving said polymer resin on said passivation layer; and forming an upper electrode by solidifying the conductive paste. Also, it is characterized in that a collector electrode is laminated on the upper electrode by electroplating. The fabrication method for the polymer resin is desirably any one selected from electrocoating method, electrolytic polymerization method, plasma polymerization method, spin coating method, roll coating method, and dipping method. The component of conductive paste for dissolving the polymer resin desirably contains at least one of ethyl acetate, methyl ethyl ketone and toluene, and in some cases, may be an unreacted component of polymer resin binder. The metal for electroplating desirable contains at least one of copper, aluminum, and chromium. As described above, by forming the passivation layer of polymer resin and the upper electrode of conductive paste on the semiconductor layer of the solar cell, the upper electrode is prevented to make contact with a defective portion of the semiconductor layer which may cause a shunt or a short circuit. Also, by forming the collector electrode by electroplating, it is possible to form a low resistive collector electrode. As a result, the loss of photoelectric current owing to the short circuit is avoided, and the effective area will increase with the slender width of collector electrode, whereby a high efficient solar cell can be obtained. That is, when the conductive paste is applied, a solvent contained in the paste or a monomer of polymer resin binder partially will dissolve the passivation layer, causing a conductive filler to place the transparent electrode and the collector electrode in electrical contact, while at the same time the passivation and the polymer resin binder cover the defective portion, whereby the short circuit can be suppressed. Moreover, the collector electrode having an excellent adhesion can be selectively formed by electroplating on the upper electrode formed of conductive paste, and the wiring resistance is made smaller, whereby a solar cell having a high conversion efficiency and a high durability can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A to 1C are perspective views schematically illustrating the constitution of a solar cell according to the present invention. Figs. 2A to 2D are perspective views schematically illustrating the constitution of another solar cell according to the present invention. Figs. 3A and 3B are schematic views illustrating an electrocoating apparatus. Fig. 4 is a schematic view illustrating a solar cell module according to the present invention. Figs. 5A and 5B are schematic views illustrating a plating apparatus according to the present invention. Fig. 6 is a schematic view illustrating another solar cell module according to the present invention. Figs. 7A to 7C are views schematically illustrating the constitution of a conventional solar cell. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1A shows schematically a constitutional example of a pin-type amorphous solar cell. Fig. 1B is a view representing the cross section of Fig. 1A. Fig. 1C is a view from above of Fig. 1A. Fig. 1A shows a solar cell upon which the light 100 is incident from above of the figure. In the figure, 100 is a solar cell main body, 101 is a substrate, 102 is a lower electrode, 103 is an n-type semiconductor layer, 104 is an i layer, 105 is a p layer, 106 is a transparent electrode, 107 is a passivation layer, 108 is an upper electrode, and 109 is a bus bar. The substrate 101 may be either of electrical conductive or dielectric. Specifically, a metal such as Fe, Ni, Cr, Al, Mo, Au, Nb, Ta, V, Ti, Pt, Pb or the like band their alloy, a thin plate made of brass, stainless or the like and its composite, and a heat resistant synthetic resin such as polyester, polyethylene, polycarbonate, cellulose acetate, polypropylene, polyvinylchloride, polyvinylidene chloride, polystyrene, polyamide, polyimide, epoxy, or the sheet thereof, or their composite with glass fiber, carbon fiber, boron fiber, and metallic fiber, a material treated with the surface coating in which a metallic thin film of dissimilar material and/or an insulating thin film of e.g. SiO 2 , Si 3 N 4 , Al 2 O 3 , AIN or the like is coated on the surface of a thin metal plate or a resin sheet, for example, by sputtering, vapor deposition, plating or the like, and glass or ceramics may be used. When the substrate 101 is electrical insulating, a lower electrode 102 is formed by depositing a so-called metal simple substance such Al, Ag, Pt, Au, Ni, Ti, Mo, W, Fe, V, Cr, Cu, stainless, brass, nichrome, SnO 2 , In 2 O 3 , ZnO, ITO, or their alloy, and a transparent electrical conductive oxide (TCO) on the surface where a deposited film is formed, for example, by plating, vapor deposition and sputtering. Also, even if the substrate 101 is electrical conductive, the lower electrode 102 can be formed by using the same material and means in order to make the surface smoother or textural. The n layer 103, the i layer 104 and the p layer 105 are fabricated by the normal thin film fabrication process, for example, any one of well known methods such as vapor deposition, sputtering, HF plasma CVD, method, microwave plasma CVD, ECR plasma CVD method, heating CVD, or LPCVD, as desired. Industrially, a plasma CVD method is preferably adopted in which a source gas is decomposed by the plasma and deposited on the substrate. Also, for the reactor, a batch apparatus or a continuous film formation apparatus can be used as needed. The solar cell which is a photovoltaic element according to the present invention can be used for a so-called tandem cell in which two or more pairs of pin junctions are laminated for the purpose of improving the spectral sensitivity or the voltage. Examples of the semiconductor material making up the i layer 104 suitably used for the solar cell of the present invention include IV-group or IV-group alloy-type amorphous semiconductors such as a-Si:H, a-Si:F, a-Si:H:F, a-SiGe:H, a-SiGe:F, a-SiGe:H:F, a-SiC:H, a-SiC:F, a-SiC:H:F or the like. The semiconductor material making up the p layer 105 or the n layer 104 suitably used for the solar cell of the present invention can be obtained by doping a valence electron control agent into the semiconductor material constituting the i layer 104. The same fabrication method as that for the i layer 104 can be suitably used. When the deposited film for the IV-group in the periodic table is obtained, a compound containing an element of the III-group in the periodic table is used as the valence electron control agent to obtain the p-type semiconductor. Examples of the III-group element include B, Al, Ga In or the like, for example. A compound containing an element of the V-group in the periodic table can be used for the valence electron control agent to obtain an n-type semiconductor. Examples of the V-group include p, N, As, Sb or the like, for example. The transparent electrode 106 for use in the present invention has desirably a light transmittance of 85% or greater to efficiently absorb the light from the sun, a white fluorescent lamp or the like into the semiconductor layer, and further has desirably a sheet resistance of 100 Ω/□ or less to cause the electric current produced by the light to electrically flow transversally across the semiconductor layer. Examples of the material having such characteristics include metallic oxides such as SnO 2 , In 2 O 3 , ZnO, CdO, CdSnO 4 and ITO (In 2 O 3 + SnO 2 ). The fabrication method may be any one of the resistance heating vapor deposition method, the electron beam heating vapor deposition method, the sputtering method, the spray method or the like, which is appropriately selected as desired. The passivation layer 107 for use in this invention is an insulating material for preventing defective portions having low resistance causing a shunt or short circuit from contacting with the upper electrode 108 and the bus bar 109. Since the passivation layer 107 is laminated over the entire surface of semiconductor layer, it is requisite to the made of a light transmissive material not to prevent the solar light from entering therein. Also, then the solar cell is used outdoors, it is required to have an excellent weatherproof and the stability against the heat, humidity and light in consideration of the environment. Moreover, since the solar cell may be bent or impacted in some cases, a mechanical strength is also required. For such a material, a polymer resin is preferable, and specific examples thereof include polyester, a copolymer of ethylene and vinyl acetate, acrylic resin, epoxy resin, urethan, etc. The preferable molecular weight for the polymer resin is about ten thousands to twenty thousands in average molecular weight. The film thickness, which should be sufficient to retain the insulation and not to lose the light transmittance, is appropriately selected from 1000Å to 100 µm depending on the material of polymer resin, preferably from 1 µm to 50 µm, and more preferably from 5 µm to 20 µm. The method for laminating the polymer resin may be an ordinary method. For example, a method of spin coating or dipping by dissolving the polymer resin in the solvent, a method of coating with a roller by dissolving it with heat, a method of depositing by electrolytic polymerization, a method of depositing by electrocoating, and a method of depositing by plasma polymerization may be used, and can be appropriately selected, depending on various conditions such as the material of polymer resin and a desired film thickness, but from the standpoint of mass productivity, the dipping method, the roller coat method and the electrocoating method are preferable. In particular, the electrocoating method is preferable because owing to the ability of using a water soluble solvent, the treatment of waste liquid is simple and the film thickness can be easily controlled by controlling the electric current. Such an electrocoating apparatus is as shown in Fig. 3A or 3B, for example. In Fig. 3, 301 is an electrocoating bath, 302 is an electrocoating liquid, 303 is an opposed electrode, 304 is a substrate, 305 is a transparent electrode, 306 is a power source, and 307 is a conductive wire. In the electrocoating, a voltage can be applied to the substrate 304 of solar cell. In some cases, a voltage may be applied by providing an appropriate terminal on the transparent electrode 106 (Figs. 1A to 1C). In the later case, since the voltage is not applied to the inside of the solar cell, unnecessary electric current will not flow through the solar cell, so that the solar cell is not destroyed during the electrocoating process. Also, the electrocoating rate can be higher, and the electrocoating liquid can be saved because the electrocoating is not applied on the back side of the substrate 304. A commercially available electrocoating liquid 302 can be used, including anion and cation type which is selected as necessary because the polarity of the voltage applied to the substrate 304 is different. The upper electrode 108 for use in this invention is a low resistance electrode to collect the electric current flowing through the transparent electrode 106. An example of the electrode material is a conductive paste for which a binder of polymer resin, such as epoxy, polyester and urethan, and a solvent of the binder are mixed at an appropriate ratio into a powder of a conductive filler such as Ag, Pt, Cu, C or the like, or their alloy to produce a paste. The resistivity is desirably 10 -2 Ωcm to 10 -5 Ωcm because a sufficient amount of current must flow during the plating, and for such a conductive paste, a commercially available paste can be appropriately used. Examples of the commercially available paste with silver filler and polyester binder include No. 5007 made by Dupon and No. FC-301CA made by Fujikura Kasei Co., Ltd. Examples of the paste with silver filler and epoxy binder include No. 84-1LM-1 and No. 959-1 made by Able Bond. Examples of the paste with silver filler and polyurethan binder include No. 3322 and No. 3320D made by Three Bond. An appropriate solvent may be added to such a conductive paste for the adjustment of viscosity. The electrode of such conductive paste can be fabricated with excellent productivity by using the conventional screen printing method. The screen printing method is to use a conductive paste as the print ink by using a screen in which a desired pattern is applied to a mesh made of nylon or stainless, the electrode width being about 50 µm at minimum. A preferable example of the printing machine is a commercially available printing machine such as Hk-4060 made by Tokai Seiki K.K.. The conductive paste applied by the screen printing is heated in a drying furnace to dry the binder and volatilize the solvent. In the present invention, it is necessary to select a material for the conductive paste in accordance with the material of the passivation layer 107. That is, since the passivation layer 107 is insulating or highly resistive, the ohmic contact between the upper electrode 108 and the transparent electrode 106 are not sufficient even if the upper electrode 108 is set (laminated) on the passivation layer 107, and the solvent or monomer contained in the conductive paste will dissolve the passivation layer 107, causing the conductive filler to make contact with the transparent electrode 106, whereby the upper electrode 108 and the transparent electrode 106 are sufficiently contacted electrically. In this case, there is a possibility that the defective portion and the conductive filler may be contacted at the same time, but the degree of short circuit can be reduced because the binder left undissolved by the solvent and the passivation layer 107 cover the defective portion, thereby producing an appropriate resistance. The shape and area of the upper electrode 108 are appropriately designed so as not to prevent the light from entering the semiconductor layer and to efficiently collect the electric current. That is, the area is made minimum not to have a shade for the incident light, and the electrical conductivity is made as high as possible not to be resistive for the current. The bus bar 109 for use in this invention is an electrode for further collecting the electric current flowing through the upper electrode 108. Examples of the electrode material include a metal such as Ag, Pt, Cu or the like, C, and their alloy, in which the wire- or foil-like electrode is bonded, or the conductive paste like the upper electrode 108 may be used. As the foil-like electrode, for example, a copper foil or a tin plated copper foil is used, and in some cases an adhesive may be applied. An example thereof is No. 1345 by 3M which is commercially available. Since a great amount of electric current will flow through the bus bar, it needs have a larger cross section and thus a lower resistivity than the collector electrode. The solar cell thus fabricated is modularized with the encapsulation by a well known conventional method to provide a better weatherproof for the outdoor use. The material for the encapsulation is preferably EVA (ethylene vinylacetate) or the like of ethylene vinyl acetate type by virtue of adhesiveness with the solar cell as an adhesive, the weatherproof and the buffer effect. To improve the moisture proof or the damage proof, a fluorine-type resin is used for the surface protective layer. Examples of the fluorine-type resin include TFE (Teflon made by Dupon) which is a polymer of 4-fluoroethylene, ETFE (Tefzel made by Dupon) which is a copolymer of 4-fluoroethylene and ethylene, polyvinyl fluoride (Tedler made by Dupon), and polychlorofluoroethylene CTFE (Neoflon made by Daikin Industries). The addition of a ultraviolet light absorbent to such a resin may improve the weatherproof. The method for laminating such a resin on the solar cell substrate is accomplished by heating and pressure bonding it by using a commercially available apparatus such as a vacuum laminater (VTL-100 type made by Nippon Physitec Equipment). The structure of a solar cell module of this invention thus fabricated is as shown in Fig. 4. In the figure, 400 is a solar cell module main body, 401 is a substrate, 402 is a lower electrode, 403 is an n-type semiconductor layer, 404 is an i layer, 405 is a p-type layer, 406 is a transparent electrode, 407 is a passivation layer, 408 is an upper electrode, 409 is a bus bar, 410 is an adhesive layer, and 411 is a surface protective layer. As described above, by providing the passivation layer made of a polymer resin between the semiconductor layer of solar cell and the upper electrode provided on the semiconductor layer, and forming the upper electrode using a conductive paste, the defective portions of the semiconductor layer causing the shunt or short circuit can be prevented from making contact with the upper electrode. As a result, the loss of photoelectric current owing to the short circuit is prevented, whereby a solar cell having a high conversion efficiency can be obtained. That is, when the conductive paste is applied, a solvent contained in the paste or a monomer of polymer resin binder will partially dissolve the passivation layer, causing a conductive filler to place the transparent electrode and the upper electrode into contact with each other and make an electrical contact, while at the same time the passivation layer and the polymer resin binder cover the defective portion, so that the short circuit can be suppressed. The adhesion of the passivation layer as formed above to the upper electrode, as well as the adhesion to the material for encapsulation are high, whereby the durability of the solar cell is greatly improved. Further, in the configuration of the present invention as described above, the pattering process for the passivation is unnecessary, resulting in a simplified fabrication process, whereby the solar cell can be fabricated at higher yield. Fig. 2A shows schematically another constitutional example of a pin-type amorphous solar cell according to the present invention. Fig. 2B is a view representing the cross section of Fig. 2A. Fig. 2C is a view from above of Fig. 2A. Fig. 2A shows a solar cell in which light 220 is incident from above of the figure. In the figure, 200 is a solar cell main body, 201 is a substrate, 202 is a lower electrode, 203 is an n-type semiconductor layer, 204 is an i layer, 205 is a p-type layer, 206 is a transparent electrode, 207 is a passivation layer, 208 is an upper electrode, 209 is a collector electrode, and 210 is a bus bar. The components of Fig. 2 up to the upper electrode can be formed of the same material as the corresponding components of Fig. 1 up to the upper electrode. The passivation layer 207 for use in this invention serves as a protective layer to protect the solar cell from being exposed to the plating liquid in forming the collector electrode 209 by electroplating, and as a mask to prevent the electrode from being formed on this portion. Also, the upper electrode 208 for use in this invention serves as a pattern in forming the collector electrode 209, and in the process, is fabricated prior to the formation of the collector electrode 209, but fulfills the same function as that of the collector electrode 208. The shape and area of the collector electrode 209 of this invention are optimally designed based on the resistivity or the thickness of the electrode material, the amount of electric current produced by the solar cell, and the sheet resistance of the transparent electrode. Also, it is designed to have a minimum area so as not to be shady for the incident light and not to be resistive for the current. The collector electrode 209 of this invention is fabricated preferably by electroplating method, with a metal depositable by the electroplating. Examples of such metal include copper, nickel, silver, gold, aluminum, chromium or the like, and their mixture. When electroplated with such a metal, the plating bath may be adjusted in accordance with deposited metal, and is formulated with a conventionally known method, for which a commercially available plating liquid can be used. The plating apparatus is exemplified in Figs. 5A and 5B. In these figures, 501 is a plating bath, 502 is a plating liquid, 503 is an opposed electrode, 504 is a substrate, 505 is all layers up to the passivation layer 207 (Fig. 2B), 506 is an upper electrode, 507 is a power source, and 508 is a conductive wire. In plating, a minus voltage can be applied to the substrate 504 of the solar cell, as shown in Fig. 5A, or in some cases, a minus voltage may be applied to the upper electrode 506, as shown in Fig. 5B. In the latter case, since the voltage is not applied to the inside of the solar cell, unnecessary electric current will not flow through the solar cell, so that the solar cell is not destroyed during the electrocoating process. Also, the electrocoating rate can be made faster, and the electrocoating liquid can be saved because the electrocoating is not applied on the back side of the substrate 504. The bus bar 210 for use in this invention is an electrode for further collecting the electric current flowing through the collector electrode 209. Examples of the electrode material include a metal such as Ag, Pt and Cu, C, and their alloy. The bus bar may be fabricated by bonding the wire-or foil-like electrode, or by the use of a conductive paste like the upper electrode 208, or by electroplating like the collector electrode 209. As the foil-like electrode, for example, a copper foil or a tin plated copper foil may be used, and in some cases an adhesive is applied. No. 1345 by 3M, which is commercially available is used for such an electrode, in which it needs have a larger cross section to show a lower resistivity than the collector electrode 209 because a great amount of electric current will flow through the bus bar 210. As in Fig. 4, the module structure of a solar cell formed with the collector electrode thus fabricated according to the present invention is as shown in Fig. 6. In the figure, 600 is a solar cell module main body, 601 is a substrate, 602 is a lower electrode, 603 is an n-type semiconductor layer, 604 is an i layer, 605 is a p-type layer, 606 is a transparent layer, 607 is a passivation layer, 608 is an upper electrode, 609 is a collector electrode, 610 is a bus bar, 611 is an adhesive layer, and 612 is a surface protective layer. In the following, the present invention will be described in detail by way of example, but the present invention is not limited to such examples. [Example 1] A solar cell having a layer configuration as shown in Fig. 1A was fabricated in the following way. First, a SUS430BA substrate 101 (30cm square, 0.1 mm thick) sufficiently degreased and cleaned was placed in a DC sputter apparatus (not shown) to form a lower electrode 102 by depositing Cr 2000 Å thick. The substrate 101 was taken out and placed in RF plasma CVD film formation apparatus (not shown), after which the semiconductor layer was deposited on the lower electrode 102 in the order of n layer 103, i layer 104 and p layer 105. Thereafter, it was placed in a resistance heating vapor deposition apparatus (not shown) to deposit In and Sn by resistance heating, whereby a transparent conductive film 106 also serving for the anti-reflection effect was deposited 700 Å thick. Then, the passivation layer 107 was deposited using an electrocoating apparatus of Figs. 3A and 3B in the following procedure. The deionized water was poured 10 liters within an electrocoating bath 301, and acrylic anion electrocoating paint (Acrylclear A-7X made by Uemura Industries) was put 2 liters therein while agitating at a rate of 500 rpm by means of an agitator (not shown). After the continuous agitation for one hour, it was left stationary for two days to make the aging, whereby an electrocoating liquid 302 was fabricated. Then, the substrate 304 was put into the electrocoating bath 301, and a voltage of 30V was applied to the opposed electrode 303 so that the substrate 304 was placed at minus and held for two minutes. After the passivation layer 107 was deposited on the transparent electrode 106, the substrate 304 was taken out from the electrocoating bath to wash it with the deionized water, and then put into a thermostat and held at 150°C for five minutes to dry out the passivation layer 107. The measurement of this coating thickness by profilometer indicated 5 µm. This membrane was an insulating film, the resistivity of which is too great to be measured by micro ammeter 4140B made by HP. Moreover, the measurement for the transmittance of this membrane deposited on the specular surface of a silver plate by reflection measurement indicated 90% or greater. Then, the substrate 304 was installed on a screen printing machine (HK-4060 made by Tokai Seiki K.K.) (not shown), and the upper electrodes 108, 300 µm wide and 8 cm long, were printed at an interval of 1 cm. In this case, used conductive paste had a composition containing 70 parts of Ag filler, 30 parts of polyester binder (volume ratio), and 5 parts of ethyl acetate as the solvent. After printing, the substrate 304 was put into the thermostat and held at 150°C for five minutes to cure the conductive paste. Moreover, the bus bar 109 of a copper foil (No. 1345 made by 3M) having a width of 5 mm with an adhesive applied thereon was bonded as shown in Fig. 1C to become electrical conductive, whereby a single cell 30 cm square was fabricated. Then, the encapsulation of this sample was made in the following way. EVA (ethylene vinyl acetate) was laminated over and under the substrate 304, and further PTFE (polytetrafluoroethylene) resin (Tefzel made by Dupon) was laminated over and under the substrate 304 as once laminated, and then the substrate 304 was placed into a vacuum laminator (VTL-100 type made by Nippon Physitec Equipment K.K. and held at 150°C for sixty minutes to make the vacuum lamination. The obtained sample was made No. 1-1. The initial characteristics of obtained sample were measured in the following way. First, the current/voltage characteristics of the obtained sample in the dark state were measured, in which the shunt resistance was measured from an inclination near the origin. Next, each sample was irradiated by the solar light spectrum of AM 1.5 at an intensity of 100 mW/cm 2 , using a pseudo solar light source (thereinafter called a simulator) with a xenon lamp, whereby the solar cell characteristics were measured to obtain the conversion efficiency. The series resistance was obtained from a current/voltage curve thus obtained. In order to evaluate the difference based on the composition of a conductive paste used in the upper electrode 108, the films up to the passivation layer 107 were formed on another substrate 304 with the above-mentioned method, and the composition of the conductive paste was changed in various ways to form the upper electrode 108. Subsequent processes were performed as in the above method, and the encapsulation was made, whereby the evaluation was performed as above. Samples ranging from sample No. 1-2 to No. 1-4 have the same binder as the sample No. 1-1 with the quantity of solvent varied, and the composition and evaluation results are shown in Table 1. Samples ranging from sample No. 1-5 to No. 1-8 have the polyurethane as the binder and methylethylketon (MEK) as the solvent with the quantity of solvent varied, and the results are shown in Table 2. Samples ranging from sample No. 1-9 to No. 1-12 have epoxy as the binder and MEK as the solvent with the quantity of solvent varied, and the results are shown in Table 3. As shown in Tables 1 and 2, the solar cell characteristics greatly vary depending on the quantity of solvent, that is, the more the solvent, the smaller the shunt resistance and the series resistance, and vice versa. This is believed due to the fact that when the amount of solvent is small, it is difficult to make Ag contact with the transparent electrode 106 by dissolving the passivation layer 107, and the series resistance increases, while the leak current can be prevented, whereby the shunt resistance becomes large. On the other hand, when the amount of solvent is large, the passivation layer 107 is too dissolved, resulting in a large contact area between Ag and the transparent electrode 106, whereby the passivation effects are almost lost. This is believed due to the fact that the shunt resistance decreases with the increase in the leak current, with the contact resistance between the upper electrode 108 and the transparent electrode 106 being small, so that the series resistance decreases. On the other hand, in connection with Table 3, even with no solvent, an excellent conversion efficiency can be obtained, and the shunt resistance and the series resistance decrease with increasing amount of solvent, which is believed due to the fact that the monomer within the binder has a dissolving effect for the passivation layer. For the samples No. 1-3, No. 1-7 and No. 1-10 which had excellent initial characteristics among these samples, the dispersion in the initial characteristics and the durability characteristic were evaluated in the following way. First, with the above method, the samples corresponding to No. 1-3, No. 1-7 and No. 1-10 were fabricated each ten sheets, and designated No. 1-20 to No. 29, No. 1-30 to No. 1-39, and No. 1-40 to No. 1-49, respectively The initial characteristics were measured and the results are shown in Table 4. From Table 4, it can be found that the dispersion in the characteristics is small and the yield is excellent. Next, the durability characteristic was tested based on a temperature/humidity cycle test A-2 determined in the environmental test method and the durability test method for the crystal system solar cell module according to the Japanese Industrial Standard C8917. First, the sample was placed into a thermohygrostat which can control the temperature and the humidity, and the cycle test of changing the temperature from - 40°C to +85°C (with a relative humidity of 85%) was repeated ten times. For the sample after termination of the test, the solar cell characteristics were measured, using the shunt resistance and the simulator in the dark state in the same way as the measurement of initial characteristics. The results are shown in Table 4. As clearly seen from Table 4, no degradation after the durability test was seen, and it could be found that the excellent passivation effect was provided. No exfoliation of the encapsulation resin from the substrate was seen, and it could be found that the passivation layer 107 had the excellent adhesion effect. From the above results, it can be found that the solar cell of the present invention with the solar cell fabrication method of the present invention has an excellent yield, a superior durability and a high reliability. Sample No. Composition conductive paste Initial characteristics of solar cell Conversion efficiency Shunt resistance Series resistance No. 1-1 Ag 70 parts Polyester 30 parts 3.5% 1000KΩ cm 2 50Ω cm 2 Ethyl acetate 5 parts No. 1-2 Ag 70 parts Polyester 30 parts 4.8% 250KΩ cm 2 20Ω cm 2 Ethyl acetate 10 parts No. 1-3 Ag 70 parts Polyester 30 parts 6.5% 50KΩ cm 2 10Ω cm 2 Ethyl acetate 20 parts No. 1-4 Ag 70 parts Polyester 30 parts 4.5% 10KΩ cm 2 5Ω cm 2 Ethyl acetate 40 parts Sample No. Composition conductive paste Initial characteristics of solar cell Conversion efficiency Shunt resistance Series resistance No. 1-5 Ag 70 parts Polyurethan 30 parts 4.5% 800KΩ cm 2 30Ω cm 2 MEK 5 parts No. 1-6 Ag 70 parts Polyurethan 30 parts 5.1% 500KΩ cm 2 15Ω cm 2 MEK 10 parts No. 1-7 Ag 70 parts Polyurethan 30 parts 6.1% 35KΩ cm 2 10Ω cm 2 MEK 20 parts No. 1-8 Ag 70 parts Polyurethan 30 parts 5.1% 10KΩ cm 2 5Ω cm 2 MEK 40 parts Sample No. Composition conductive paste Initial characteristics of solar cell Conversion efficiency Shunt resistance Series resistance No. 1-9 Ag 70 parts Epoxy 30 parts 5.7% 120KΩ cm 2 10Ω cm 2 MEK 0 parts No.1-10 Ag 70 parts Epoxy 30 parts 6.2% 50KΩ cm 2 8Ω cm 2 MEK 5 parts No.1-11 Ag 70 parts Epoxy 30 parts 5.5% 35KΩ cm 2 8Ω cm 2 MEK 10 parts No.1-12 Ag 70 parts Epoxy 30 parts 3.5% 10KΩ cm 2 5Ω cm 2 MEK 20 parts [Comparative example 1] A conventional solar cell having a layer configuration as shown in Fig. 7A was fabricated in the following way. In the same way as example 1, a transparent electrode 706 was formed on a substrate 701. Then, a passivation layer 707 was fabricated by electrocoating in the same way as the example 1. The difference from the example 1 was that the passivation layer 707 was patterned. Further, in the same way as the example 1, an upper electrode 708 which was a collector electrode was printed. In this case, the conductive paste was FC-301CA made by Fujikura Kasei Co., Ltd. A copper foil bus bar 709 with an adhesive applied thereon was laminated on the passivation layer 707, whereby a single cell 30cm square was fabricated. With the above fabrication method, ten sheets of sample were fabricated. Then, the encapsulation of these sample was performed in the same way as the example 1. Obtained samples were designated R-1 to R-10. The initial characteristics of obtained samples were such that the conversion efficiency was 5.2%±2% and the shunt resistance was in a range of 5KΩ cm 2 to 150KΩ cm 2 ; the dispersion was larger and the shunt was greater than in the example 1. Next, the durability characteristic of these samples was evaluated in the same way as in the example 1. The measurement of the solar cell characteristic of the samples after termination of the humidity/temperature cycle test indicated an average value of 90% relative to the initial value, and thus the occurrence of a significant degradation was indicated. Also, the measurement of shunt resistances R-1 to R-10 indicated a reduction of 50% in average. The shunt portion of the sample was confirmed in the following way. A reverse bias of 1.5 volts was applied to the sample. In the shunt portion, the electric current will flow and generate the heat, while in the normal portion, the electric current will not flow due to the reverse bias, and therefore will generate no heat. Observing the sample surface in this state with an infrared camera, a heat generating portion was detected, and it could be found that the sample was shunted under the collector electrode. Also, the investigation for the series resistance before and after the temperature/humidity cycle test indicated that exfoliation of the encapsulation resin from the substrate was partially made. [Comparative example 2] A conventional solar cell having a passivation layer as shown in Fig. 7B was fabricated in the following way. In the same way as in example 1, membranes up to a p layer 705 were formed on a substrate 701. Then, a positive pattern mask having the same shape as the bus bar 709 made of a polyester film was laminated on the p layer 705, and a passivation layer 707 was formed by electrocoating in the same way as in the example 1, whereby the passivation layer 707 was formed on the p layer 705 in the same pattern as that of the bus bar 709. Then, in the same way as the comparative example 1, an upper electrode 708 which was a collector electrode was printed by a screen printing machine not shown. Further, a copper foil bus bar 709 with an adhesive applied thereon was laminated, after which a transparent electrode 706 was deposited, and a single cell 30cm square was fabricated. Ten sheets of the sample were fabricated in the same way. Then, the encapsulation of these sample was performed in the same way as in the example 1. Obtained samples were designated R-11 to R-20. The initial characteristics of obtained samples were such that the conversion efficiency was 5.8%±1.5% and the shunt resistance was 15KΩ cm 2 to 200KΩ cm 2 ; the dispersion was larger and the shunt was greater than in the example 1. The series resistance was in a range of 10Ω cm 2 to 30Ω cm 2 . Next, the durability characteristic of these samples was evaluated in the same way as in the example 1. The measurement of the solar cell characteristics of the samples after termination of the humidity/temperature cycle test indicated an average value of 90% relative to the initial value, and thus the occurrence of a significant degradation was indicated. Also, the measurement of shunt resistances of the samples indicated a reduction of about 30% in average, whereby the shunt was found. Observing the shunt portion of this sample with an infrared camera, as in the comparative example 1, it could be found that the sample was shunted under the collector electrode. [Example 2] By depositing a passivation layer 107 with a cation electrocoating resin, a solar cell having a layer configuration as shown in Fig. 1A was fabricated in the substantial same way as in the example 1. First, membranes up to the transparent electrode 106 were formed on the substrate 101 in the same way as in the example 1. Then, the passivation layer 107 was deposited by cation electrocoating in the following procedure. The deionized water was poured 10 liters into an electrocoating bath 301 of Fig. 3A, and epoxy-type cation electrocoating paint (AUED800-FH made by Uemura Industries K.K.) was added 2 liters therein while agitating at a rate of 1000rpm with an agitator (not shown). After continuous agitation for one hour, it was left stationary for two days to make the aging. Then, the substrate 304 was put into the electrocoating bath 301, and a voltage of 100V was applied to the opposed electrode 302 so that the substrate 304 was placed at plus, and held for two minutes. After the passivation layer 107 was deposited on the transparent electrode 106, the substrate 304 was taken out from the electrocoating bath to wash it with the deionized water, and then put into a thermostat and held at 175°C for twenty minutes to dry out the passivation layer 107. The measurement of this coating thickness by profilometer indicated 10 µm. Then, using a conductive paste having a composition containing 60 parts of Pd filler, 40 parts of epoxy (volume ratio), and 15 parts of MEK as the solvent, the upper electrode was printed, and a copper foil with an adhesive applied thereon was laminated on the bus bar 109, whereby a single cell 30cm square was fabricated. Ten sheets of the above sample were fabricated. Further, the encapsulation of these samples was made in the same way as in the example 1. Obtained samples were designated No. 2-1 to No. 2-10. The initial characteristics of obtained samples showed 6.1%±0.5% and a shunt resistance of 300KΩ cm 2 to 500KΩ cm 2 , which was excellent characteristics with a small dispersion. Next, the durability characteristic of these samples was evaluated in the same way as in the example 1. The measurement of the solar cell characteristics of the samples after termination of the humidity/temperature cycle test indicated an average value of 97% relative to the initial conversion efficiency, and thus no occurrence of a significant degradation was indicated. Also, the measurement of the shunt resistances indicated a reduction of about 10% in average, with no significant degradation. No exfoliation of the encapsulation resin from the substrate was seen, and it could be found that the passivation layer 107 had the excellent adhesion effect. From the above results, the solar cell of the present invention using the solar cell fabrication method of the present invention had an excellent yield, a good characteristic and a superior durability. [Example 3] By using a dipping coat method as the fabrication method of a passivation layer 107, a solar cell having a layer configuration as shown in Fig. 1A was fabricated in the following way. First, membranes up to the transparent electrode 106 were formed on the substrate 101 in the same way as in the example 1. Then, polyester resin having an average molecular weight of ten thousands to twenty thousands was dissolved in a solvent MEK. The substrate 101 was put into this bath and then taken out to be held in a thermostat at 70°C for five minutes to dry out the solvent and form the passivation layer 107. The thickness of the passivation layer 107 was 5 µm. Then, in the same way as in the example 1, the upper electrode 108 was printed, and a copper foil with an adhesive applied thereon was laminated on the bus bar 109, whereby a single cell 30cm square was fabricated. Ten sheets of the sample were fabricated in the same way. Then, the encapsulation of these samples was made in the same way as in the example 1. Obtained samples were designated No. 3-1 to No. 3-10. The initial characteristic of obtained samples showed 6.3%±0.7% and a shunt resistance of 100KΩ cm 2 to 300KΩ cm 2 , which was excellent characteristics with a small dispersion. Next, the durability characteristic of these samples was evaluated in the same way as in the example 1. The measurement of the solar cell characteristics of the samples after termination of the humidity/temperature cycle test indicated an average value of 98% relative to the initial conversion efficiency, and thus no occurrence of a significant degradation was indicated. Also, the measurement of the shunt resistances indicated a reduction of about 10% in average, with no significant degradation. No exfoliation of the encapsulation resin from the substrate was seen, and it could be found that the passivation layer 107 had the excellent adhesion effect. From the above results, the solar cell of the present invention using the solar cell fabrication method of the present invention had an excellent yield, a good characteristic and a superior durability. [Example 4] A solar cell having a layer configuration as shown in Fig. 2A was fabricated in the following way. First, a SUS430BA substrate 201 (30cm square, 0.1mm thick) sufficiently degreased and cleaned was placed in a DC sputter apparatus (not shown) to form a lower electrode 202 by depositing Cr 2000 Å thick. The substrate 201 was taken out and placed in RF plasma CVD film formation apparatus (not shown) for the deposition in the order of n layer 203, i layer 204 and p layer 205. Thereafter, it was placed in a resistance heating vapor deposition apparatus (not shown) to deposit an alloy of In and Sn by resistance heating, whereby a transparent electrode 206 also serving for the anti-reflection effect was deposited 700 Å thick. Then, after the passivation layer 207 was deposited using an acrylic anion electrocoating paint (Acrylclear A-7X made by Uemura Industries), the substrate 201 was installed in a screen printing machine (HK-4060 made by Tokai Seiki) (not shown) to print upper electrodes 208, 100 µm wide and 8 cm long at an interval of 1cm. In this case, the conductive paste had a composition containing 70 parts of Ag filler, 30 parts of polyester binder (volume ratio), and 20 parts of ethyl acetate as the solvent. After printing, the substrate 201 was put into a thermostat and held at 150°C for five minutes to cure the conductive paste. Next, in the following way, using a plating bath of Fig. 5A, a collector electrode 209 of nickel was formed by electroplating. First, the substrate 504 was immersed in a neutral sulfamic acid solution for five seconds, and then immersed in a plating bath 501 filled with a plating liquid 502 held at a temperature of 50°C and pH4, which liquid was a commercially available electroplating liquid N-100ES made by High Purity Chemicals K.K. Thereafter, using a power source 507, a voltage of about -1 volt was applied to the substrate 504, and the plating was made at a current density of 20mA/cm 2 . Under these conditions, nickel was deposited 2 µm thick only on the upper electrode 506, and was not deposited on other portions. Moreover, the bus bar 209 of a copper foil having a width of 5mm with an adhesive applied thereon was bonded as shown in Fig. 2C, whereby a single cell 30cm square was fabricated. Ten sheets of the sample were fabricated in the same way. Then, the encapsulation of these samples was made in the following way. EVA (ethylene vinyl acetate) was laminated over and under the substrate 201, and further a fluororesin film ETFE (ethylene tetrafluoroethylene) (Tefzel made by Dupon) was laminated thereon. Then the substrate 304 was placed into a vacuum laminator and held at 150°C for sixty minutes to make the vacuum lamination. The initial characteristics of obtained samples were measured in the following way. First, the current/voltage characteristics of the obtained samples in the dark state were measured. The shunt resistance was measured from the inclination near the origin, so that the shunt resistance was in a range of 300KΩ cm 2 to 500KΩ cm 2 , which was the excellent characteristic with a small dispersion. Next, each sample was irradiated by the solar light spectrum of AM 1.5 at an intensity of 100mW/cm 2 , using a pseudo solar light source (thereinafter called a simulator) with a xenon lamp, whereby the solar cell characteristics were measured to obtain the conversion efficiency, so that the conversion efficiency was in a range of 6.1%±0.5% which was the excellent characteristic with a small dispersion. The series resistance was obtained from a current/voltage curve thus obtained, so that the series resistance was 5Ω cm 2 in average, which was the excellent characteristic with a low resistance of the collector electrode 209. Next, the durability characteristic was tested based on a temperature/humidity cycle test A-2 determined in the environmental test method and the durability test method for the crystal system solar cell module according to the Japanese Industrial Standard C8917. First, the sample was placed into a thermohygrostat which can control the temperature and the humidity, and the cycle test for changing the temperature from -40°C to +85°C (with a relative humidity of 85%) was repeated ten times. For the sample after termination of the test, the solar cell characteristic was measured, using a simulator in the same way as the measurement of initial characteristics, so that it was 98% in average relative to the initial conversion efficiency with no significant degradation. Also, the measurement of shunt resistance indicated a reduction of about 10% in average, with no significant degradation. No exfoliation of the resin of encapsulation from the substrate was seen, and it could be found that the passivation layer 207 had the excellent adhesion effect. From the above results, the solar cell of the present invention using the solar cell fabrication method of the present invention had an excellent yield, a good characteristic and a superior durability. [Comparative example 3] Another conventional solar cell having a layer configuration as shown in Fig. 7A was fabricated in the following way. In the same way as in the example 4, membranes up to a transparent electrode 706 were formed on a substrate 701. Then, a collector electrode 708 was printed in the same way as in the example 4. A copper foil bus bar 709 with an adhesive applied thereon was laminated, whereby a single cell 30cm square was fabricated. Then, the encapsulation of this sample was performed in the same way as in the example 4. The initial characteristics of obtained sample was such that the conversion efficiency was 5.2%, the shunt resistance was 5KΩ cm 2 , and the series resistance was 15Ω cm 2 . It is believed that since the series resistance was higher than in the example 4, the conversion efficiency was lower. Next, the durability characteristic of this sample was evaluated in the same way as in the example 4. The measurement of the solar cell characteristic of the sample after termination of the humidity/temperature cycle test indicated an average value of 90% relative to the initial value, and thus the occurrence of a significant degradation was indicated. Also, the shunt resistance was about 10KΩ cm 2 and the shunt was observed. The shunt portion of this sample was confirmed in the following way. A reverse bias of 1.5 volts was applied to the sample. In the shunt portion, the electric current will flow and generate heat, while in the normal portion, the electric current will not flow due to the reverse bias, and therefore will generate no heat. Observing the sample surface in this state with an infrared camera, the heat generating portion was detected, and it could be found that the sample was shunted under the collector electrode. [Comparative example 4] A conventional solar cell as shown in Fig. 7A was fabricated in the following way by a conventional plating method. In the same way as in the example 4, membranes up to a transparent electrode 706 were formed on a substrate 701. Then, a positive pattern mask having the same shape as the bus bar 709 made of a polyester film was laminated on the transparent electrode 706, and a resist layer was laminated by spin coating. Then, nickel was plated to form a collector electrode 708 in the same way as in the example 4. Further, a copper foil bus bar 709 with an adhesive applied thereon was laminated, and a single cell 30cm square was fabricated. Ten sheets of the sample were fabricated in the above-described way. Then, the encapsulation of these samples were performed in the same way as in the example 4. The initial characteristic of obtained samples was 6.1%±3% with a large dispersion. This is believed due to the fact that the shunt resistance for the sample having poor characteristic is lower, and the collector electrode 708 formed of nickel was placed into contact with the defective portion, thereby causing a short-circuit. The shunt resistance was 200Ω cm 2 to 15KΩ cm 2 . Next, the durability characteristic of these samples was evaluated in the same way as in the example 4. The measurement of the solar cell characteristic of the samples after termination of the humidity/temperature cycle test indicated an average value of 92% relative to the initial value, and thus the occurrence of a significant degradation was indicated. Also, the measurement of shunt resistances indicated a reduction of about 50% in average. Observing the shunt portion of this sample with an infrared camera, as in the comparative example 3, it could be found that the sample was shunted under the collector electrode. [Example 5] A collector electrode 209 was fabricated as shown in Fig. 5B, and a solar cell having a layer configuration as shown in Fig. 2A was fabricated in the substantial same way as in the example 4. Membranes up to the passivation layer 207 were deposited on the substrate 201 with the same procedure as in the example 4. Then, in the same way as in the example 4, an upper electrode 208 was printed using a conductive paste having a composition containing 60 parts of Pd filler, 40 parts of epoxy (volume ratio), and 15 parts of MEK as the solvent. Next, in the following way, using a plating bath of Fig. 5B, a collector electrode 209 was formed. First, the substrate 201 was immersed in a neutral sulfamic acid solution for five seconds, and then immersed in a plating bath 501 filled with a plating liquid 502 held at a temperature of 55°C and pH8.5, which liquid was a commercially available electroplating liquid C-100EP made by High Purity Chemicals K.K.. Thereafter, using a power source 507, a voltage of about -0.5 volt was applied to the substrate 504, and the plating was made at a current density of 20mA/cm 2 . Under these conditions, copper was deposited 2 µm thick only on the upper electrode 208 to form a collector electrode 209, and was not deposited on other portions. Moreover, the bus bar 210 of a copper foil with an adhesive applied thereon was laminated, whereby a single cell 30cm square was fabricated. Ten sheets of the sample were fabricated with the above-described method. Then, the encapsulation of these samples was made in the same way as in the example 4. The initial characteristics of obtained samples were such that the conversion efficiency was in a range of 6.0%±1%, the shunt resistance was in a range of 200KΩ cm 2 to 350KΩ cm 2 , and the series resistance was 5Ω cm 2 in average, which were excellent characteristics. Next, the durability characteristic was evaluated in the same way as in the example 4. After termination of the temperature/humidity cycle test, the solar cell characteristics of the samples were measured and showed an average value of 97% relative to the initial value with no significant degradation. Also, the measurement of shunt resistances indicated almost no variation. From the above results, the solar cell of the present invention using the solar cell fabrication method of the present invention had an excellent yield, a good characteristic and a superior durability. [Example 6] A solar cell having a layer configuration as shown in Fig. 2A was fabricated in the following way, with the collector electrode 209 being the three layer structure of nickel/copper/nickel. Membranes up to the passivation layer 207 were deposited on the substrate 201 in the same way as in the example 4. Then, a collector electrode 209 was formed by depositing nickel 1 µm thick in the same way as in the example 4, depositing copper 1 µm thick in the same way as in the example 5, and finally depositing nickel 1 µm thick. Moreover, the bus bar 210 of a copper foil with an adhesive applied thereon was laminated, whereby a single cell 30cm square was fabricated. Ten sheets of the sample were fabricated by the above-described method. Then, the encapsulation of these samples was made in the same way as in the example 4. The initial characteristic of obtained samples was 6.4%, with the shunt resistance being 4Ω cm 2 , which were excellent characteristics. Next, the durability characteristic of these samples was evaluated in the same way as in the example 4. After termination of the temperature/humidity cycle test, the solar cell characteristics of the samples were measured and showed an average value of 98% relative to the initial value with no significant degradation. Also, the measurement of shunt resistance indicated almost no variation. From the above results, the solar cell of the present invention using the solar cell fabrication method of the present invention had an excellent yield, a good characteristic and a superior durability. [Example 7] By shaping the upper electrode 208, the collector electrode 209 and the bus bar 210 as shown in Fig. 2D, a solar cell having a configuration as shown in Fig. 2A was fabricated in the same way as in the example 4. In this case, the upper electrode was made by forming patterns each having a width of 50 µm and a length of 25cm at an interval of 1cm. Thereafter, by depositing nickel 3 µm thick in the same way as in the example 4, the bus bar 209 of a copper foil with an adhesive applied thereon was bonded, whereby a single cell 30cm square was fabricated. Without encapsulation, the measurement of initial characteristics indicated that the efficiency (effective efficiency) in the area including a shade portion in the collector electrode 209 and the bus bar 210 was 6.8%, and the series resistance was 4Ω cm 2 , which were excellent characteristics. The present invention provides a thin film solar cell comprising at least one pair of semiconductor junctions deposited on the substrate with good characteristics and a superior durability, having a structure in which the passivation layer of a polymer resin is coated on the semiconductor and the upper electrode made of a conductive paste is laminated on the passivation layer, the polymer resin being any one component selected from polyester, copolymer of ethylene and vinyl acetate, acrylic epoxy, and urethan, and having a transmittance to the solar light of 90% or greater to be substantially insulating, and the metal component of the conductive paste containing at least one component from silver, palladium, copper and carbon. Further, by laminating the collector electrode on the upper electrode by electroplating, it is possible to provide a solar cell having excellent characteristics and a superior durability. The present invention provides a solar cell fabrication method to provide a thin film solar cell comprising at least one pair of semiconductor junctions deposited on the substrate, in which the passivation layer of a polymer resin is coated on the semiconductor layer, and the upper electrode made of a conductive paste containing a component capable of dissolving the polymer resin is laminated on the passivation layer. The fabrication method for the polymer resin may be any one selected from electrocoating method, electrolytic polymerization method, plasma polymerization method, spin coating method, roll coating method, and dipping method. The component capable of dissolving the polymer resin of the conductive paste desirably contains at least one of ethyl acetate, methyl ethyl ketone and toluene, whereby the solar cell can be produced at an excellent yield. Further, by laminating the collector electrode on the upper'electrode by electroplating, it is possible to provide a solar cell having excellent characteristics and a superior durability at a high yield. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims.

Claims: A solar cell having a semiconductor layer (103, 104, 105) formed on a substrate (101) comprising: a substantially insulating and light transmissive passivation layer (107) made of a polymer resin and coated on the upper portion of said semiconductor layer; and an upper electrode (108) made of a solidified conductive paste containing a conductive filler, the upper electrode being located on, and etablishing an electrical contact through said passivation layer. The solar cell according to claim 1, wherein said polymer resin is one component selected from polyester, a copolymer of ethylene and vinyl acetate, acrylic, epoxy, and urethan. The solar cell according to claim 1, wherein the transmittance of said polymer resin with respect to the solar light is 90 % or greater. The solar cell according to claim 2, wherein the transmittance of said polymer resin with respect to the solar light is 90 % or greater. The solar cell according to claim 1, wherein the conductive paste contains a conductive filler including at least one of silver, palladium, copper, carbon and their alloy. The solar cell according to claim 1, wherein said conductive paste contains a binder including at least one of polyester, epoxy and polyurethan. A solar cell according to any of the claims 1 to 6 further comprising: a collector electrode formed on said upper electrode. The solar cell according to claim 7, wherein said collector electrode is comprised of at least one of copper, aluminum, and chromium. A method for fabricating a solar cell by depositing a semiconductor layer (103, 104, 105) on a substrate (101), comprising the steps of: coating a substantially insulating and light transmissive passivation layer made of a polymer resin on the upper portion of said semiconductor layer; setting a conductive paste containing a conductive filler and a component capable of dissolving said polymer resin on said passivation layer; and forming an upper electrode by solidifying the conductive paste. The method for fabricating a solar cell according to claim 9, wherein said polymer resin is formed by any one method selected from electocoating method, electrolytic polymerization method, plasma polymerization method, spin coating method, roll coating method, and dipping method. The method for fabricating a solar cell according to claim 9, wherein said conductive paste contains a conductive filler including at least one of silver, palladium, copper, carbon and their alloy. The method for fabricating a solar cell according to claim 10, said conductive paste contains a conductive filler including at least one of silver, palladium, copper, carbon and their alloy. The method for fabricating a solar cell according to claim 9, wherein said conductive paste contains a binder consisting of at least one of polyester, epoxy and polyurethan. The method for fabricating a solar cell according to claim 9, wherein the component for dissolving said polymer resin contained in said conductive paste contains at least one of ethyl acetate, methyl ethyl ketone and toluene. The method for fabricating a solar cell according to claim 13, wherein said binder contains an unreacted component. A method for fabricating a solar cell according to any of the claims 9 to 15 further comprising the step of: forming a collector electrode on said upper electrode. The method for fabricating a solar cell according to claim 16, wherein said polymer resin is formed by any one method selected from electrocoating method, electrolytic polymerization method, plasma polymerization method, spin coating method, roll coating method, and dipping method. The method according to claim 16, wherein said collector electrode is comprised of at least one of copper, aluminum, and chromium. The method according to claim 16, wherein said collector electrode is formed by electroplating.

Inventors: MURAKAMI TSUTOMU, MURAKAMI, TSUTOMU, MURAKAMI, TSUTOMU, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L31
IPC codes: H01L31