Solar cell module with improved weathering characteristics

Abstract: An improved solar cell characterized in that the grid electrode has a coat comprising an epoxy resin of 20 g/m².day.0.1 mm/40 °C 90%RH or less in moisture permeability which is disposed so as to cover the entire exposed exterior of said grid electrode. The solar cell module is free of occurrence of shortcircuit between the grid electrode and the lower electrode even upon repeated use under severe environmental conditions of high temperature and high humidity, and continuously exhibits a desirable photoelectric conversion efficiency over a long period of time.

Description: FIELD OF THE INVENTION The present invention relates to an improved solar cell module which excels in weatherability and exhibits a stable, desirable photoelectric conversion efficiency over a long period of time. More particularly, it relates to a solar cell module having an a-Si solar cell element provided a grid electrode having a coating of epoxy resin disposed so as to cover the entire exposed exterior of the grid electrode. RELATED BACKGROUND ART In recent years, heating of the earth because of the so-called greenhouse effect due to an increase of atmospheric CO₂ has been predicted. Thus, there is an increased demand for a means of power generation capable of providing clean energy without causing CO₂ buildup. In this regard, nuclear power generation has been considered to be advantageous in view of not causing CO₂ buildup. However, as for nuclear power generation, there are problems that it unavoidably causes radioactive wastes which are harmful for living things and there is a probability that leakage of injurious radioactive materials from the nuclear power generation system will happen when the system is damaged. In this respect, early realization of a power generation system capable of providing clean energy without causing CO₂ buildup as in the case of thermal power generation and without causing radioactive wastes and radioactive materials as in the case of nuclear power generation is an increased societal demand. There have been various proposals which are expected to meet such societal demand. Among those proposals, solar cells are expected to be a future power generation source since they supply electric power without causing those problems as above mentioned. There have been proposed a variety of solar cells for commercial and home appliances. Those solar cells include sigle crystal silicon solar cells, polycrystal silicon solar cells and amorphous silicon solar cells. As for single crystal silicon solar cells, there is a disadvantage that they are still costly because of using an expensive specific single crystal substrate. Hence, they have not yet come into general use as solar cells in commercial and home appliances used by the general public. In view of this, the public attention has been focused on polycrystal silicon solar cells and amorphous silicon solar cells since they can be relatively easily produced and are of low production cost although they do not provide a photoelectric conversion efficiency as high as that provided by the single crystal silicon solar cell. However, as for polycrystal silicon solar cells, there is a disadvantage that it is difficult to form their semiconductor layer comprised of polycrystal silicon (hereinafter referred to as poly-Si) in a large area with a relatively low production cost. On the other hand, amorphous silicon solar cells have been evaluated as being the most advantageous among the conventional solar cells because their semiconductor layer comprised of amorphous silicon (hereinafter referred to as a-Si) can be easily formed in a large area and in a desired form on a relatively inexpensive substrate of glass, metal or synthetic resin with a relatively low production cost. As for amorphous silicon solar cell (hereinafter referred to as a-Si solar cell), the production cost has been estimated to be markedly lower than that of single crystal silicon solar cells when the production reaches some hundreds megawatts. In view of this, various studies have been made on a-Si solar cells from various viewpoints such as reproducibility, productivity, production cost, etc. Now, the a-Si solar cell module having a capacity capable of outputting an electric power of some wattages or above is usually used outdoors and because of this, it is required to have relevant weathering resistances in terms of resistance to rain (water), resistance to dusts, resistance to ultraviolet rays, heat resistance, humidity resistance, and the like. In the production of an a-Si solar cell module, an a-Si solar cell is housed using a glass plate or a synthetic resin so that the requirements are fulfilled. In the case where the a-Si solar cell is housed using a glass plate, it is necessary for the glass plate used to be of a thickness of some millimeters in order to prevent the glass plate from being broken. In this case, the a-Si solar cell module product obtained unavoidably becomes relatively heavy and costly. In addition, the a-Si solar cell module product is poor in flexibility. In the case of producing a solar cell module using an a-Si solar cell element with a bendable substrate such as stainless steel foil, synthetic resin film, or the like, the a-Si solar cell is housed using a synthetic resin member such as film of fluorine-contained resin, film of ethylene-vinyl acetate copolymer (EVA), or the like so that the resulting a-Si solar cell module becomes bendable as desired. In the following, explanation will be made of the conventional a-Si solar cell module with respect to FIG. 2. Shown in FIG. 2 is a schematic view illustrating the configuration of a typical example of the conventional a-Si solar cell module. In FIG. 2, numeral reference 200 stands for an a-Si solar cell element which comprises a metal electrode layer 202, an amorphous silicon semiconductor layer (hereinafter referred to as a-Si semiconductor layer) 203 having a pin junction and a transparent and conductive layer 204 being disposed in this order on a conductive substrate 201. Numeral reference 205 stands for a grid electrode. Numeral reference 206 stands for a filler provided so as to enclose the entire exterior of the a-Si solar cell element 200. Numeral reference 207 stands for a weather-resistant protective member (or a surface protective member) made of a synthetic resin which is disposed on the surface of the filler 206. Numeral reference 208 stands for a base member (or a reverse face protective member in other words). The grid electrode 205 is usually formed using a paste composed of powdery Ag and binder resin in order to provide it with a desirable flexibility and a desirable resistance to changes in temperature. As the binder resin used in this case, there are known polyester resin exhibiting good flexibility, epoxy resin exhibiting good weather-resistance, and other than these, phenol resin. In order to obtain an a-Si solar cell module of the configuration shown in FIG. 2 which excels in flexibility, the grid electrode 205 is formed using a Ag paste comprising polyester resin as the binder dispersed in powdery Ag. The weather-resistant protective member 207 is usually formed of a fluorine-contained resin so that it exhibits water-resistance, resistance to dusts, weather-resistance, and the like required therefor. As for the filler 206 which encloses the solar cell element 200, it is required to be transparent and highly insulative and to have weather-resistance, water-resistance, and high-impact properties. Other than these requirements, it is also required to exhibit a good adhesion property to the solar cell element 200 and not to give any negative influence to other constituent members. In view of this, as the filler 206, there is used EVA, silicone resin or polyvinyl butyral (PVB). Among these resins, EVA is most generally used since it has various advantages such that it can be handled in a sheet-like state, it can be easily processed, and it is relatively inexpensive. Now, there is a problem for the above conventional a-Si solar cell module such that not only the weather-resistant protective member 207 formed of a fluorine-contained resin but also the filler 206 comprised of EVA has a tendency of allowing moisture to pass through them although they have relevant weather-resistance and water-resistance. Other than this problem, there is other problem for the above conventional solar cell module such that the grid electrode 205 formed of the foregoing Ag paste comprising powdery Ag and polyester resin binder is apt to allow moisture to pass through the grid electrode and the moisture eventually arrives at the portion situated under the grid electrode because the Ag paste contains gaps although it meets the requirement relative to flexibility required therefor. This situation often occurs when the above conventional solar cell module is irradiated with light (sunlight) under conditions of high temperature and high humidity wherein moisture penetrates through the filler 206 and the grid electrode 205, followed by arriving at the a-Si semiconductor layer 203 through the transparent and conductive layer 204. When the a-Si semiconductor layer 203 is accompanied by defective portions such as pinholes in this case, the moisture passes through those defective portions of the a-Si semiconductor layer and arrives at the metal electrode layer 202 (that is, the lower electrode), resulting in making the grid electrode 205 short-circuited with the metal electrode 201 and in reducing the solar cell characteristics. In order to confirm the above problematic situation with respect to the conventional a-Si solar cell module in which the grid electrode is formed of the foregoing Ag paste comprising powdery Ag and polyester resin binder, a voltage was externally applied between the grid electrode and the metal electrode layer (that is, the lower electrode) so that a positive bias voltage was applied to the grid electrode. As a result, the above problems relative to the occurrence of shortcircuit between the grid electrode and the lower electrode and to the reduction in photoelectric conversion efficiency entailed. Occurrence of the above problems in the conventional a-Si solar cell module are considered to be due to the following reasons. That is, in the case where moisture has penetrated into the a-Si solar cell element 200 as above described upon subjecting the a-Si solar cell element to irradiation of light (sunlight) under conditions of high temperature and high humidity, a positive voltage is applied to the grid electrode 205 because of the moisture penetrated and a self-photoelectromotive force of the a-Si solar cell element to cause electrochemical reaction of the Ag contained in the grid electrode, whereby the Ag is oxidized in the moisture media to provide Ag⁺ ions. When the Ag⁺ ions thus thus provided are diffused into the a-Si semiconductor layer, a remarkable reduction is caused in the photoelectric conversion efficiency of the a-Si solar cell element. And in the case where the a-Si semiconductor layer of the a-Si solar cell element has defective portions such as pinholes, those Ag⁺ ions arrive at the lower electrode (the metal electrode) through these defective portions, and they are reduced to metallic Ag. This reaction should be continuously caused, the metallic Ag eventually becomes grown to produce dendrite crystal, resulting in electrically connecting the grid electrode to the lower electrode whereby entailing shortcircuit in the a-Si solar cell element. As a result, a marked reduction is caused in the photoelectric conversion efficiency of the a-Si solar cell element. The detailed reaction mechanism in this case is not clear enough but it is considered that the following reactions would be caused in the a-Si solar cell element: at the grid electrode (which serves as an anode): Ag₂O + H₂O --- 2Ag⁺ + 2OH⁻, and at the lower electrode (which serves as a cathode): Ag⁺ + e⁻ --- Ag (precipitation of dendrite crystal). In order to prevent occurrence of the above shor-circuit between the grid electrode and the lower electrode, there is a proposal to use epoxy resin instead of the polyester resin as the binder resin of the Ag paste for the formation of the grid electrode. However, as for an a-Si solar cell of the configuration shown in FIG. 2 in which the grid electrode 205 is formed of a paste comprising powdery Ag and epoxy resin, because the epoxy resin is hard and is insufficient in flexibility although its moisture permeability is low, the grid electrode is likely to separate from the a-Si solar cell element body upon exposing to changes in temperature in repeated use, wherein the contact resistance between the grid electrode and the a-Si solar cell element body is raised to cause a marked reduction in the photoelectric conversion efficiency. Thus, the use of epoxy resin instead of the polyester resin as the binder resin of the Ag paste is not effective to solve the above problem. Other than the above, in order to prevent occurrence of the above shor-circuit between the grid electrode and the lower electrode, it is considered to be effective that EVA as the filler 206 is replaced by other relevant resin having a moisture permeability which is lower than that of EVA. In the technical field to which the present invention pertains, the degree of moisture to be permeated through a given material is evaluated by observing the so-called moisture permeability in accordance with the method stipulated in JIS-Z0208 in which how much moisture said given material with a definite thickness and a definite square permeates within a prescribed period of time under the conditions of a definite temperature and a definite moisture is measured. The moisture permeability of EVA (ethylene-vinyl acetate copolymer) is 20 to 200 g/m².day.0.1 mm/40°C.90%RH. In view of this, aforesaid other relevant resin having a moisture permeability which is lower than that of EVA is a resin having a moisture permeability of less than the above-mentioned moisture permeability value of EVA, that is, 20g/m².day.0.1 mm/40°C.90%RH. Specific examples of such resin are polyethylene, polypropylene, polyvinyl alcohol, polymethyl methacrylate, polyethylene terephthalate, vinylidene chloride-vinyl chloride copolymer, and epoxy resin. However, of these resins, polyethylene, polypropylene, polyvinyl alcohol, polymethyl methacrylate, polyethylene terephthalate and vinylidene chloride-vinyl chloride copolymer are not usable instead of EVA as the filler 206 because any of them is poor in adhesion with the exterior of the solar cell element 200. Particularly, when an a-Si solar cell module of the configuration shown in FIG. 2 in which the filler 206 is comprised of a member selected from polyethylene, polypropylene, polyvinyl alcohol, polymethyl methacrylate, polyethylene terephthalate and vinylidene chloride-vinyl chloride copolymer is positioned while being bent, the filler 206 is likely to separate from the solar cell element. In addition, when said solar cell module is repeatedly exposed to high temperature, the filler 206 situated in the vicinity of the interface with the exterior of the solar cell element 200 is colored in white and eventually separated from the exterior of the solar cell element. Further in addition, when said solar cell is placed outdoors so as to expose to sunlight over a long period of time, the filler 206 is yellowed. In the case of an a-Si solar cell module of the configuration shown in FIG. 2 in which the filler 206 is comprised of epoxy resin, there is a problem that the filler comprised of epoxy resin is yellowed when the solar cell module is placed outdoors so as to expose to sunlight over a long period of time, resulting in causing remarkable reduction in the photoelectric conversion efficiency of the solar cell element. This situation is apparent from what shown in FIG. 3. FIG. 3 contains a graph with respect to changes in light transmittance as for EVA observed when it was exposed to sunlight and other graph with respect to changes in light transmittance as for epoxy resin observed when it was exposed to sunlight. Thus, it is apparent that epoxy resin is also not usable instead of EVA as the filler 206. SUMMARY OF THE INVENTION The present invention is aimed at eliminating the foregoing problems in the known solar cell module. Another object of the present invention is to provide an improved solar cell module which is free of occurrence of shortcircuit between the grid electrode and lower electrode even upon repeated use under severe environmental conditions of high temperature and high humidity, and continuously exhibits a desirable photoelectric conversion efficiency over a long period of time. A further object of the present invention is to provide an improved, reliable solar cell module which excels in weatherability and continuously exhibits a desirable photoelectric conversion efficiency without being deteriorated even upon repeated use over a long periods of time under severe environmental conditions of high temperature and high humidity. A further object of the present invention is to provide an improved, reliable amorphous silicon solar cell module which excels in permeability to light and exhibits a desirable photoelectric conversion efficiency over a long period of time without the permeability to light being deteriorated. A further object of the present invention is to provide a reliable amorphous silicon solar cell module provided with an improved grid electrode, which can be mass-produced at a reduced production cost. The above described and other objects, advantages, and features of the present invention will become more apparent upon making reference to the following specification, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view illustrating the configuration of an embodiment of an a-Si solar cell module according to the present invention. FIG. 2 is a schematic view of the configuration of a conventional a-Si solar cell module. FIG. 3 shows graphs of the results obtained in climatic tests for EVA and epoxy resin. FIG. 4 shows a graph of the results obtained as a result of performing evaluation of the photoelectric conversion efficiency for the various a-Si solar cell elements respectively provided with an epoxy resin coat of a different thickness at the grid electrode. FIG. 5 is a schematic top view illustrating the configuration of an embodiment of an a-Si solar cell module comprising a plurality of a-Si solar cell elements being integrated in series connection. FIG. 6 is a schematic cross section view taken along line A-A' in FIG. 5. FIG. 7 shows graphs of the evaluated results of the photoelectric conversion efficiency with respect each of the a-Si solar cell modules obtained in Examples 1 and 2 and Comparative Examples 1 and 2. DESCRIPTION OF THE INVENTION The present invention is to provide an improved a-Si solar cell module which is free of the foregoing problems in the conventional a-Si solar cell module and which attains the above objects. The feature of the a-Si solar cell module according to the present invention lies in an improvement in the solar cell module comprising an amorphous silicon solar cell element enclosed by a filler, said amorphous solar cell element comprising a metal electrode layer, an amorphous silicon semiconductor layer (hereinafter referred to as a-Si semiconductor layer) having a pin junction, a transparent and conductive electrode layer and a grid electrode being disposed in this order on a conductive substrate, said solar cell element being disposed on a base member serving a reverse face protective member, said improvement is characterized in that said grid electrode has a coating of epoxy resin of 20 g/m².day.0.1 mm/40 °C 90 %RH or less in moisture permeability which is disposed so as to cover the entire exposed exterior of said grid electrode. The a-Si solar cell module according to the present invention usually comprises a plurality of a-Si solar cell element bodies respectively of the above configuration, said plurality of a-Si solar cell element bodies being spacedly arranged on a base member serving as a reverse face protective member, and the respective a-Si solar cell elements of said plurality of a-Si solar cell element bodies being electrically connected with each other in series by wiring the grid electrode of one a-Si solar cell element with the conductive substrate of the other a-Si solar cell element. The a-Si solar cell module thus configured according to the present invention is free of the foregoing problems relative to occurrence of shortcircuit in the prior art even upon repeated use under severe environmental conditions of high temperature and high humidity and exhibits a stable, desirable photoelectric conversion efficiency over a long period of time without being deteriorated. Particularly, in the a-Si solar cell module of the present invention, there is no occasion for moisture to be permeated through the grid electrode upon continuously exposing the a-Si solar cell element of the a-Si solar cell module to sunlight even under severe environmental conditions of high temperature and high humidity because the grid electrode is covered by the specific epoxy resin as above described. The a-Si solar cell element of the a-Si solar cell module is always maintained in a desirable state such that it exhibits a stable, desirable photoelectric conversion efficiency. The a-Si solar cell module according to the present invention is greatly advantageous since it functions as desired without having any surface protective member. This results in remarkably reducing the production cost. It is a matter of course that the a-Si solar cell module may be provided with a surface protective member. In this case, the stability of the a-Si solar cell element is further secured. In the following, the a-Si solar cell module will be explained in more detail while referring to the drawing. Shown in FIG. 1 is a schematic view illustrating the configuration of an embodiment of the a-Si solar cell module according to the present invention. In FIG. 1, numeral reference 100 stands for an a-Si solar cell element which comprises a metal electrode layer 102, an a-Si semiconductor layer 103 having a pin junction and a transparent and conductive layer 104 being disposed in this order on a conductive substrate 101. Numeral reference 105 stands for a grid electrode. Numeral reference 109 stands for a coating of epoxy resin of 20 g/m².day.0.1 mm/40 °C 90 %RH or less in moisture permeability which is disposed so as to cover the entire exposed exterior of the grid electrode 105. Numeral reference 106 stands for a filler provided so as to enclose the entire exterior of the a-Si solar cell element 100. Numeral reference 107 stands for a weather-resistant protective member (or a surface protective member), which is disposed on the surface of the filler 106 in case where necessary. Numeral reference 108 stands for a base member (or a reverse face protective member in other words). As for the substrate 101 of the a-Si solar cell element 100, there is not any particular restriction as long as it is conductive. Specifically, it may be a conductive member composed of a metal such as Al, Cu, etc., a conductive member composed of an alloy such as stainless steel, etc., or a conductive member composed of other conductive material such carbon, etc. In a preferred embodiment, the substrate 101 comprises a stainless steel member. The metal electrode layer 102 (the lower electrode in other words) of the a-Si solar cell element is desired to be formed of a metal such as Ti, Cr, Mo, Fe, Cu, Nb, Zn, W, Al, Ag, Ni, etc. The metal electrode layer 102 may be properly formed by means of a conventional resistance heating evaporation technique, electron beam evaporation technique, or sputtering technique. In the case where the substrate 101 is desirably conductive, the metal electrode layer 102 is not always necessary to be disposed. The a-Si semiconductor layer 103 of the a-Si solar cell element 100 may be a single p/i/n amorphous silicon semiconductor cell, a tandem type semiconductor cell comprising two p/i/n amorphous silicon semiconductor cells being stacked, or a triple type semiconductor cell comprising three p/i/n amorphous silicon semiconductor cells being stacked. The a-Si semiconductor layer 103 may be properly formed by means of a conventional plasma CVD technique or light-induced CVD technique selectively using silane gas, hydrogen gas, and gas capable of imparting an impurity such as phosphine, diborane or the like. Alternatively, the a-Si semiconductor layer 103 may be comprised of a p/i/n amorphous silicon semiconductor cell being stacked on a pn junction semiconductor layer formed by a conventional epitaxially growing technique. The transparent and conductive layer 104 of the a-Si solar cell 100 is desired to be formed of In₂O₃, SnO₂, In₂O₃-SnO₂ (ITO), ZnO, TiO₂, or CdSnO₄. The transparent and conductive layer 104 may be properly formed by means of a conventional resistance heating evaporation technique, electron beam evaporation technique, sputtering technique, spraying technique or CVD technique. The grid electrode 105 of the a-Si solar cell element 100 is formed a material comprising powdery Ag dispersed in a binder resin such as phenol resin, urethane resin, rubber, polyester or acrylic resin. The grid electrode 105 may be properly formed by providing a Ag paste comprising powdery Ag dispersed in one of the above-mentioned binder resin and subjecting the Ag paste to screen printing. The Ag paste may contain powder of a metal, other than Ag, such as Ti, Au, Zn, Cr, Mo, W, Al, Ni, Cu or Sn in an appropriate amount. A pronounced effect is provided in the case where the grid electrode 105 is formed of the Ag paste. As above described, the principal feature of the present invention lies in the coating 109 of epoxy resin of 20 g/m².day.0.1 mm/40 °C 90 %RH or less in moisture permeability which is disposed so as to cover the entire exposed exterior of the grid electrode 105. The above epoxy resin to constitute the coat 109 is desired to have a light-transmitting property for the reason that if the epoxy resin is not light transmissive, it becomes a shadow loss factor leading to a reduction in the photoelectric conversion efficiency of the a-Si solar cell element. In addition, the epoxy resin to constitute the coating 109 is desired to have a refractive index which is greater than that of the filler 106. In the case where the epoxy resin to constitute the coat 109 is of a greater refractive index than that of the filler 109, light impinged through the surface protective member 107 and the filler 106 effectively reflects at the interface between the epoxy resin and the filler 106 before arriving at the grid electrode, resulting in providing an improvement in the photoelectric conversion efficiency. Examples of such epoxy resin are bisphenol A epoxy resin, bisphenol F epoxy resin, cycloaliphatic epoxy resin and phenol novolak epoxy resin. Among these epoxy resins, bisphenol A epoxy resin is the most desirable in view of humidity resistance and adhesion. As the curing agent used together with the epoxy resin, there can be mentioned aliphatic polyamine, modified aliphatic polyamine, aromatic polyamine, modified aromatic polyamine, cycloaliphatic polyamine, modified cycloaliphatic polyamine, polyamideamine, modified polyamideamine, imidazole and derivatives thereof. These epoxy resins are used in combination with appropriate curing agents. There are various commercially available epoxy resins as the above epoxy resins in combination with appropriate curing agents. Commercially available examples as the foregoing epoxy resins are those produced by Three Bond Co., Ltd. under the trading identifications 2001, 2002H, 2003, 2016B, and 2022; and commercially available curing agents used in combination with these epoxy resins are those produced by the same company under the trading identifications 2102B, 2103, 2104, 2105C, 2105F, 2106, 2131B, 2131D, 2131F and 2163. These epoxy resins are used in combination with these curing agents with an appropriate mixing ratio. Commercially available examples of the combination of the epoxy resin with an appropriate curing agent are those produced by Sumitomo 3M Ltd. under the trademark names of EW-2 (one-part type), S/W-2214 (one-part type), XA7416 (one-part type), JA7437 (one-part type), 1838B/A (two-part type, the volumetric mixing ratio of the epoxy resin to the curing agent is 4 : 5), S/W-2216B/A, DP-100 (two-part type, the volumetric mixing ratio of the epoxy resin to the curing agent is 1 : 1), DP-100 (two-part type, the volumetric mixing ratio of the epoxy resin to the curing agent is 1 : 1), DP-110 (two-part type, the volumetric mixing ratio of the epoxy resin to the curing agent is 1 : 1), DP-190 (two-part type, the volumetric mixing ratio of the epoxy resin to the curing agent is 1 : 1), DP-PURE60 (two-part type, the volumetric mixing ratio of the epoxy resin to the curing agent is 1 : 1), and DP-270 (two-part type, the volumetric mixing ratio of the epoxy resin to the curing agent is 1 : 1). There are other commercially available examples as the foregoing epoxy resins; Epicote 812, 815, 827, 828 and 834 produced by Yuka Shell Epoxy Kabushiki Kaisha. These epoxy resins are used in combination with appropriate curing agents. Further commercially available examples of the combination of the epoxy resin with an appropriate curing agent are those produced by Asahi-Ciba Ltd.; epoxy resin AY-103/curing agent HY956 (the mixing ratio (part by weight): 100/17), epoxy resin AY-105/curing agent HY956 (the mixing ratio (part by weight): 100/17), epoxy resin AW-106/curing agent HY953U (the mixing ratio (part by weight): 100/60), epoxy resin AV-121/curing agent HY956 (the mixing ratio (part by weight): 100/8), epoxy resin AV-123B/curing agent HY956 (the mixing ratio (part by weight): 160/11), epoxy resin AV-129/curing agent HY956 (the mixing ratio (part by weight): 100/13), epoxy resin AW-136/curing agent HY994 (the mixing ratio (part by weight): 100/40), epoxy resin AV-138/curing agent HV998 (the mixing ratio (part by weight): 100/40), epoxy resin AV-144/curing agent HY997 (the mixing ratio (part by weight): 100/60), epoxy resin AW-2101/curing agent HW2951 (the mixing ratio (part by weight): 100/100), epoxy resin AW-2104/curing agent HW2934 (the mixing ratio (part by weight): 100/100), and epoxy resin AV-1580/curing agent HV1580 (the mixing ratio (part by weight): 100/100). Still further commercially available examples of the foregoing epoxy resins are those produced by Tohto Kasei Co., Ltd. under the trademarks of YD-115, YD-116, YD-119, YD-126, YD-127, YD-128, YD-134, YD-001, YD-011, YD-012, YD-013, YD-014, YD-017, YD-019 and YD-020. And examples of the curing agent used in combination with these epoxy resins are those produced by the same company under the trademark names of G-series curing agent, TH-series curing agent, KHX-series curing agent and ZX-series curing agent. The thickness of the coat 109 constituted by the above specific epoxy resin is be preferably in the range of 10 to 200 um or more preferably in the range of 50 to 150 um. This situation with respect to the thickness of the coat 109 is based on the findings shown in FIG. 4. Shown in FIG. 4 are the results obtained as a result of performing evaluation as follows. That is, there were provided a plurality of a-Si solar cell module samples of the configuration shown in FIG. 1 respectively having an epoxy resin coat 109 of a different thickness disposed so as to cover the entire exposed exterior of the grid electrode 105. As for each of the a-Si solar cell module samples, there was observed (i) an initial photoelectric conversion efficiency by subjecting the a-Si solar cell module sample to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted; then, after a forward bias voltage of 0.3 volt having been applied between the grid electrode and the lower electrode (wherein a positive bias voltage was applied to the grid electrode) for 24 hours under conditions of high temperature and high humidity, there was observed (ii) a photoelectric conversion efficiency by subjecting the a-Si solar cell module sample to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted. And a proportion of a change between the initial photoelectric conversion efficiency (i) and the photoelectric conversion efficiency (ii) as for each of the a-Si solar cell module samples. The evaluated results were graphically shown in the relationship between the thickness of the coat 109 and the change in photoelectric conversion efficiency. Each of the above plurality of a-Si solar cell module samples was prepared as follows. That is, on a stainless steel substrate of 0.1 mm in thickness as the substrate 101 were formed a 5000 Å thick Al-Si alloy layer as the lower electrode 102, a 4000 Å thick pin structured a-Si layer as the a-Si semiconductor layer 103, and a 800 Å thick layer composed of In₂O₃-SnO₂ as the transparent and conductive layer 104 respectively by means of a conventional film-forming technique. (10Å=1nm). On the transparent and conductive layer 104 was formed a grid electrode 105 by applying an Ag paste (Ag Paste 5007 : trademark name, produced by Du Pont Company) by means of a conventional screen printing technique. The entire exposed exterior of the grid electrode thus formed was covered by applying an epoxy resin of 2.5 g/cm².day.0.1 mm/40 °C90 %RH in moisture permeability (trademark name : DP-190GRAY, produced by 3M Company) at a given thickness by means of a conventional screen printing technique. Thus, there was obtained an a-Si solar cell element. The resultant a-Si solar cell element was sandwiched between the corresponding members to obtain a laminate of ethylene-tetrafluoroethylene copolymer film (trademark name : Tefzel film, produced by Du Pont Company)/EVA sheet/the a-Si solar cell element/EVA sheet/galvanized steel (produced by Daido Steel Sheet Mgf. Co., Ltd.) being laminated in this order from the top toward the bottom. The laminate was introduced into a vacuum laminator, wherein the EVA sheets were fused at a temperature of 150 °C to thereby seal the laminate. Thus, there were obtained a plurality of a-Si solar cell modules which were different with respect to the thickness of the epoxy resin coat 109. From the results shown in FIG. 4, the following facts were found. That is, in the case where the epoxy resin coat 109 disposed on the grid electrode 105 of the a-Si solar cell element 100 is of a thickness in the range of 10 to 200 µm, there is not a distinguishable change between the initial photoelectric conversion efficiency and the photoelectric conversion efficiency after a prescribed voltage had been applied between the grid electrode and the lower electrode under severe conditions of high temperature and high humidity so that a positive bias voltage was applied to the grid electrode. And, a marked effect is provided in the case where the thickness of the epoxy resin coat 109 is in the range of 50 to 150 µm. On the other hand, in the case where the thickness of the epoxy resin coat 109 is less than 10 µm, pinholes are apt to cause in the epoxy resin coat wherein moisture permeate through those pinholes to arrive at the portion under the grid electrode under conditions of high temperature and high humidity, resulting in causing occurrence of shortcircuit between the grid electrode and the lower electrode, and a a result, the photoelectric conversion efficiency is remarkably reduced. Similarly, in the case where the thickness of the epoxy resin coat 109 is exceeding 200 µm, cracks are apt to cause in the epoxy resin coat wherein moisture permeate through those cracks to arrive at the portion under the grid electrode under conditions of high temperature and high humidity, resulting in causing occurrence of shortcircuit between the grid electrode and the lower electrode, and a a result, the photoelectric conversion efficiency is remarkably reduced. As the filler 106 which is provided so as to enclose the entire exterior of the a-Si solar cell element 100, there is used a light transmissive, insulating resin such as ethylene-vinyl acetate copolymer (EVA), polyvinyl butyral, silicone resin, and the like. As above described, the a-Si solar cell module according to the present invention is not always necessary to be provided with the weather-resistant protective member (the surface protective member) 107. However, in the case where the surface protective member 107 is disposed, the stability of the a-Si solar cell element 100 is further ensured. The surface protective member 107 is desired to a member capable of effectively allowing light to pass through and which is resistant to ultraviolet rays and to ozone and excels in weatherability. Specific examples of such member are fluorine-contained resin film, silicone resin film, and the like. The a-Si solar cell module of the configuration shown in FIG. 1 and as above described may be properly prepared by a conventional technique, for example, in the following manner. That is, an a-Si solar cell element 100 is firstly prepared by forming, on a substrate 101, a metal electrode layer 102, a pin-structured a-Si semiconductor layer 103 having a pin junction, a transparent and conductive layer 104 and a grid electrode 105 in this order respectively by the foregoing corresponding layer-forming technique. The entire exposed exterior of the grid electrode 105 is covered by a coat comprising the foregoing specific epoxy resin. The resultant is enclosed using the foregoing filler 106, the foregoing weather-resistant member (surface protective member) 107 and a base member (reverse face protective member) 108. The formation of the coat comprising the foregoing specific epoxy resin at the grid electrode may be carried out, for example, by applying the foregoing specific epoxy resin to the exposed exterior of the grid electrode by means of a conventional screen printing technique so as to form a coat of a predetermined thickness to cover the entire exposed exterior of the grid electrode. The enclosure of the a-Si solar cell element may be performed by firstly interposing the a-Si solar cell element of the present invention between two sheets capable of serving the filler 106 to obtain a laminate comprising base member 108/filler 106/a-Si solar cell element 100/filler 106/surface protective member 107 and subjecting the laminate to heat treatment at a prescribed temperature while pressing the laminate so that the fillers 106 are fused while removing gas contained in the laminate. By this, a desirable a-Si solar cell module of the present invention is obtained. DESCRIPTION OF THE PREFERRED EMBODIMENTS The advantages of the present invention are now described in more detail with reference to the following examples which are provided here for illustrative purpose only, and are not intended to limit the scope of the pesent invention. EXAMPLE 1 There was prepared an a-Si solar cell module comprising three a-Si solar cell elements being integrated in series connection as shown in FIG. 5. FIG. 5 is a schematic top view illustrating the configuration of said a-Si solar cell module. Shown in FIG. 6 is a schematic cross section view taken along line A-A' in FIG. 5. In FIGs. 5 and 6, numeral reference 301 stands for a stainless steel substrate, numeral reference 302 stands for a lower electrode layer composed of a Al-Si alloy, numeral reference 303 stands for a pin-structured a-Si semiconductor layer, numeral reference 304 stands for a transparent and conductive layer composed of ITO, numeral reference 305 stands for an Ag grid electrode, numeral reference 306 stands for a filler comprising ethylene-vinyl acetate copolymer (EVA), numeral reference 307 stands for a surface protective member composed of ethylene-tetrafluoroethylene copolymer, and numeral reference 308 stands for a base member made of galvanized steel. Numeral reference 310 stands for a tin-plated copper foil to electrically connect the grid electrode 305 to a copper foil 316 or 317 extending from the substrate side of the adjacent a-Si solar cell element, wherein the copper foil serves to electrically connect a pair of adjacent a-Si solar cell elements in series. Numeral reference 311 stands for a lead wiring copper foil on the cathode side. Numeral reference 316 stands for a lead wiring copper foil on the anode side. Numeral reference 312 stands for an adhesive Ag ink to connect the grid electrode 305 to the tin-plated copper foil 310. Numeral reference 313 stands for an element dividing portion. Numeral reference 314 stands for an exposed portion of the stainless steel substrate 301 for spotwelding the copper foils 311 and 317. Numeral reference 315 stands for an insulative tape for preventing occurrence of shortcircuit. Numeral reference 320 stands for an epoxy resin member to cover the grid electrode 305 and the adhesive Ag ink 312. Firstly, there was provided a stainless steel web of 0.1 mm in thickness to be the substrate 301. On the stainless steel web as the substrate 301 was formed a 5000 A thick Al-Si alloy layer to be the lower electrode 302 by means of a conventional sputtering process wherein a target comprising a Al-Si alloy containing Si in an amount of 1 % by weight was used. Then, on the Al-Si alloy layer as the lower electrode was formed a 4400 Å thick pin-structured a-Si semiconductor layer 303 by means of a conventional plasma CVD process wherein a 300 Å thick n-type a-Si semiconductor layer was formed using SiH₄ gas, H₂ gas and PH₃ gas, a 4000 Å thick i-type a-Si semiconductor layer was formed using SiH₄ gas and H₂ gas, and a 100 Å thick p-type a-Si semiconductor layer was formed using SiH₄ gas, H₂ gas and B₂H₆ gas. Thereafter, there was formed a 800 Å thick ITO layer to be the transparent and conductive layer 304 on the pin-structured a-Si semiconductor layer 303 by means of a conventional resistance heating evaporation process. Thus, there was obtained an intermediate of a-Si solar cell element. The resultant was cut into three pieces (A, B and C, see FIG. 5) of an equal size (10 cm x 5 cm) by means of a press machine. At the respective sections, the constituent members were crushed to cause shortcircuit between the ITO layer and the stainless steel substrate. In order to repair the shortcircuit, as for each of the three a-Si solar cell element intermediates, the corresponding portion of the ITO layer situated where the shortcircuit occurred was removed in the following manner, to thereby establish an element dividing portion 313 (see, FIG. 6). That is, an etchant comprising FeCl₃ capable of selectively dissolving only the ITO layer without dissolving the a-Si semiconductor layer situated thereunder was applied to the periphery of the ITO layer situated slightly inside the section by means of a conventional screen printing technique to thereby remove the corresponding portion, followed by washing with water, whereby establishing the element dividing portion 313 at the ITO layer. On the ITO layer 304 of each of the three a-Si solar cell element intermediates thus treated was formed a grid electrode 305 by applying a Ag paste containing polyester resin as the binder resin (Ag Paste 5007 : trademark name, produced by Du Pont Company) by means of a conventional screen printing technique. Thus, there were obtained three a-Si solar cell elements (A, B and C)(see, FIG. 5). As for each of the resultant three a-Si solar cell elements, a tin-plated copper tape to be a collecting electrode of the grid electrode 305 was arranged at right angle to the grid electrode (see, 310 in FIG. 5). Then, ban adhesive Ag ink (C-220 : trademark name, produced by Emarson & Cuming Inc.) was dropwise applied to the intersection between the grid electrode 305 and the tin-plated copper tape as the collecting electrode 310, followed by drying at 150 °C for 30 minutes, whereby the grid electrode was connected to the tin-plated copper tape (see, 312 in FIG. 5). Successively, as for each of the three a-Si solar cell modules A, B and C, a polyimide tape was fixed to the position under the tin-plated copper tape 310 in order to insure the insulation between the grid electrode and the stainless steel substrate. Thereafter, as for each of the three resultants, part of the solar cell element comprising the ITO layer/the a-Si semiconductor layer/the Al-Si alloy layer was removed using a grinder to expose part of the stainless steel substrate (see, 314 in FIG. 5), followed by welding copper foils thereto using a spotweldor (see, 311 and 317 in FIG. 5). Then, the three a-Si solar cell elements A, B and C were serially integrated by way of soldering the copper foils 317 with the tin-plated copper tapes 310. On the respective grid electrodes 305 and the respective adhesive Ag inks 312, there was applied a bisphenol A two-part epoxy resin produced by Three Bond Kabushiki Kaisha (epoxy resin : 2022/curing agent : 2106) of 2 g/cm².day.0.1mm/40 °C90 %RH in moisture permeability at a thickness of 100 µm using a dispenser, followed by drying at 150 °C for 2 hours to thereby cure the epoxy resins applied, wherein the exposed exterior of the grid electrode of each of the three a-Si solar cell elements A, B and C was covered by a coat comprising the epoxy resin (see, 320 in FIG. 5). The resultant integrated a-Si solar cell element body comprising the three a-Si solar cell elements being connected in series was sandwiched between the corresponding members to obtain a laminate of ethylene-tetrafluoroethylene copolymer film (trademark name : Tefzel film, produced by Du Pont Company)/EVA sheet/the integrated a-Si solar cell element body/EVA sheet/galvanized steel (produced by Daido Steel Sheet Mgf. Co., Ltd.) being laminated in this order from the top toward the bottom. The laminate was introduced into a vacuum laminator, wherein the EVA sheets were fused at a temperature of 150 °C while removing gas contained in the laminate by virtue of pressure to thereby seal the laminate. The ethylene-tetrafluoroethylene copolymer film used in the above had been subjected to plasma treatment in advance in order to improve the adhesion with the EVA. Thus, there was obtained an a-Si solar cell module comprising three a-Si solar cell elements being integrated in series connection as shown in FIG. 5. As for the a-Si solar cell module thus obtained, there was observed (i) an initial photoelectric conversion efficiency by subjecting the a-Si solar cell module to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted; then, after a forward bias voltage of 0.3 volt having been applied between the grid electrode and the lower electrode (wherein a positive bias voltage was applied to the grid electrode) for 24 hours under conditions of 85 °C for environmental temperature and 85 % for environmental humidity, there was observed (ii) a photoelectric conversion efficiency by subjecting the a-Si solar cell module to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted. As a result, it was found that there was not a substantial change between the initial photoelectric conversion efficiency (i) and the photoelectric conversion efficiency (ii). In view of the above, it was recognized that the a-Si solar cell module excels in solar cell characteristics and it is practically usable. EXAMPLE 2 There was prepared an a-Si solar cell module of the configuration shown in FIGs. 5 and 6 by repeating the procedures of Example 1, except that there was not used ethylene-tetrafluoroethylene copolymer film to be the surface protective member. As for the a-Si solar cell module obtained, there was observed (i) an initial photoelectric conversion efficiency by subjecting the a-Si solar cell module to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted; then, after a forward bias voltage of 0.3 volt having been applied between the grid electrode and the lower electrode (wherein a positive bias voltage was applied to the grid electrode) for 24 hours under conditions of 85 °C for environmental temperature and 85 % for environmental humidity, there was observed (ii) a photoelectric conversion efficiency by subjecting the a-Si solar cell module to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted. As a result, it was found that there was not a substantial change between the initial photoelectric conversion efficiency (i) and the photoelectric conversion efficiency (ii). In view of the above, it was recognized that the a-Si solar cell module excels in solar cell characteristics and it is practically usable. COMPARATIVE EXAMPLE 1 There was prepared a conventional a-Si solar cell module by repeating the procedures of Example 1, except that there was not formed any epoxy resin coat at the grid electrode of each of the three a-Si solar cell elements A, B and C. As for the a-Si solar cell module obtained, there was observed (i) an initial photoelectric conversion efficiency by subjecting the a-Si solar cell module to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted; then, after a forward bias voltage of 0.3 volt having been applied between the grid electrode and the lower electrode (wherein a positive bias voltage was applied to the grid electrode) for 24 hours under conditions of 85 °C for environmental temperature and 85 % for environmental humidity, there was observed (ii) a photoelectric conversion efficiency by subjecting the a-Si solar cell module to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted. As a result, it was found that there was a significant change between the initial photoelectric conversion efficiency (i) and the photoelectric conversion efficiency (ii), particularly the initial photoelectric conversion efficiency was reduced by about 50 % after the above endurance. In view of this, it was recognized that the conventional a-Si solar cell module is apparently poor in solar cell characteristics. COMPARATIVE EXAMPLE 2 There was prepared a comparative a-Si solar cell module by repeating the procedures of Example 2, except that there was not formed any epoxy resin coat at the grid electrode of each of the three a-Si solar cell elements A, B and C. As for the a-Si solar cell module obtained, there was observed (i) an initial photoelectric conversion efficiency by subjecting the a-Si solar cell module to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted; then, after a forward bias voltage of 0.3 volt having been applied between the grid electrode and the lower electrode (wherein a positive bias voltage was applied to the grid electrode) for 24 hours under conditions of 85 °C for environmental temperature and 85 % for environmental humidity, there was observed (ii) a photoelectric conversion efficiency by subjecting the a-Si solar cell module to irradiation of AM 1.5 light (100 mW/cm²) to measure a power (W) outputted. As a result, it was found that there was a significant change between the initial photoelectric conversion efficiency (i) and the photoelectric conversion efficiency (ii), particularly the initial photoelectric conversion efficiency was reduced by about 30 % after the above endurance. In view of this, it was recognized that the conventional a-Si solar cell module is apparently poor in solar cell characteristics. Shown in FIG. 7 are of the results obtained in Examples 1 and 2 and Comparative Examples 1 and 2. From what shown in FIG. 7, it is understood that any of the a-Si solar cell modules obtained in Examples 1 and 2 of the present invention in which the grid electrode is provided with a coat comprising the specific epoxy resin is apparently surpassing any of the comparative a-Si solar cell modules obtained in Comparative Examples 1 and 2 particularly in view of the photoelectric conversions efficiency provided after endurance over a long period of time. More particularly, it is understood that any of the a-Si solar cell modules obtained in Examples 1 and 2 of the present invention continuously exhibits a desirable photoelectric conversion efficiency without being deteriorated even upon repeated used under severe environmental conditions of high temperature and high humidity. It is also understood that the a-Si solar cell module according to the present invention is greatly advantageous since it functions as desired without having any surface protective member. An improved solar cell characterized in that the grid electrode has a coating of epoxy resin of 20 g/m².day.0.1 mm/40 °C 90%RH or less in moisture permeability which is disposed so as to cover the entire exposed exterior of said grid electrode. The solar cell module is free of occurrence of shortcircuit between the grid electrode and the lower electrode even upon repeated use under severe environmental conditions of high temperature and high humidity, and continuously exhibits a desirable photoelectric conversion efficiency over a long period of time.

Claims: Solar cell module comprising an amorphous silicon solar cell element (100) enclosed by a filler (106), said amorphous silicon solar cell element comprising a metal electrode layer (102), an amorphous silicon semiconductor layer (103) having a pin junction, a transparent and conductive layer (104) and a grid electrode (105) being disposed in this order on a conductive substrate (101), said solar cell element being disposed on a base member (108), characterized in that said grid electrode (105) has a coating (109) of epoxy resin covering the entire exposed exterior of said grid electrode and having a moisture permeability of 20 g/m².day.0.1 mm/40 °C 90%RH or less. A solar cell module according to claim 1, wherein the epoxy resin is a member selected from the group consisting of bisphenol A epoxy resin, bisphenol F epoxy resin, cycloaliphatic epoxy resin, and phenol novolak epoxy resin. A solar cell module according to claim 1, wherein the filler (106) comprises a light transmissive, insulating resin. A solar cell module according to claim 3, wherein the light transmissive, insulating resin is a member selected from the group consisting of ethylene-vinyl acetate copolymer, polyvinyl butyral and silicone resin. A solar cell module according to claim 1, wherein the coating is of a thickness in the range of 10 to 200 µm. A solar cell module according to claim 5, wherein the epoxy resin is a member selected from the group consisting of bisphenol A epoxy resin, bisphenol F epoxy resin, cycloaliphatic epoxy resin, and phenol novolak epoxy resin. A solar cell module according to claim 5, wherein the filler comprises a light transmissive, insulating resin. A solar cell module according to claim 7, wherein the light transmissive, insulating resin is a member selected from the group consisting of ethylene-vinyl acetate copolymer, polyvinyl butyral and silicone resin. Solar cell module comprising a plurality of amorphous silicon solar cell elements (A,B,C) being integrated in series connection and being enclosed by a filler (306), each of said plurality of amorphous silicon solar cell elements comprising a metal electrode layer (302), an amorphous silicon semiconductor layer (303) having a pin junction, a transparent and conductive layer (304) and a grid electrode (305) being disposed in this order on a conductive substrate (301), said solar cell elements being disposed on a base member (308), characterized in that said grid electrode (305) of each of said plurality of amorphous silicon solar cell elements has a coating (320) of epoxy resin covering the entire exposed exterior of said grid electrode and having a moisture permeabiliy of 20 g/m².day.0.1 mm/40 °C 90%RH or less. A solar cell module according to claim 9, wherein the epoxy resin is a member selected from the group consisting of bisphenol A epoxy resin, bisphenol F epoxy resin, cycloaliphatic epoxy resin, and phenol novolak epoxy resin. A solar cell module according to claim 9, wherein the filler (306) comprises a light transmissive, insulating resin. A solar cell module according to claim 11, wherein the light transmissive, insulating resin is a member selected from the group consisting of ethylene-vinyl acetate copolymer, polyvinyl butyral and silicone resin. A solar cell module according to claim 9, wherein the coating is of a thickness in the range of 10 to 200 µm. A solar cell module according to claim 13, wherein the epoxy resin is a member selected from the group consisting of bisphenol A epoxy resin, bisphenol F epoxy resin, cycloaliphatic epoxy resin, and phenol novolak epoxy resin. A solar cell module according to claim 13, wherein the filler comprises a light transmissive, insulating resin. A solar cell module according to claim 15, wherein the light transmissive, insulating resin is a member selected from the group consisting of ethylene-vinyl acetate copolymer, polyvinyl butyral and silicone resin.

Inventors: INOUE YUJI, INOUE, YUJI, Inoue, Yuji, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L31
IPC codes: H01L31