1. Field of the Invention
The present invention relates to a highly reliable solar cell module. More particularly, the present invention relates to a highly reliable solar cell module comprising a photovoltaic element (a solar cell) as a photoelectric conversion member and a specific surface side covering material disposed to seal a front side (a light receiving face) of said photovoltaic element, which excels particularly in weatherability and moisture resistance and has good exterior appearance, and can be used as a building construction material such as a roofing material.
The present invention includes a process for the production of said solar cell module.
2. Related Background Art
There have been proposed a number of solar cell modules comprising a photovoltaic element (a solar cell) sealed by a resin, a surface protective member which covers a front side (a light receiving side) of said photovoltaic element, and a back side protective member which covers a back side of said photovoltaic element.
FIG. 5 is a schematic cross-sectional view illustrating the constitution of an example of such solar cell module.
In FIG. 5, reference numeral 501 indicates a transparent surface protective member, reference numeral 502 a transparent thermoplastic resin as a filler, reference numeral 503 a photovoltaic element (or a solar cell), reference numeral 504 an insulating back face protective member. In the solar cell module shown in FIG. 5, the photovoltaic element 503 is enclosed by the transparent thermoplastic resin 502, the front side (the light receiving side) of the photovoltaic element 503 is covered by the transparent surface protective member 501, and the back side of the photovoltaic element 503 is covered by the insulating back face protective member 504.
The surface protective member 501 comprises a transparent resin film made of a fluororesin such as ETFE (ethylene-tetrafluoroethylene copolymer) or PVF (polyvinyl fluoride), or a glass plate. The thermoplastic resin 502 comprises EVA (ethylene-vinyl acetate copolymer) or butyral resin. The back face protective member 504 comprises an organic film such as a nylon film or a composite comprising an aluminum foil sandwiched with TEDLAR (trademark name).
The back face protective member 504 is used in order to reinforce the solar cell module while adding an appropriate rigidity thereto.
The thermoplastic resin 502 serves as a filler enclosing the photovoltaic element 503 so as to prevent the photovoltaic element from being externally damaged and from external shock. The thermoplastic resin 502 also serves not only as an adhesive between the photovoltaic element 503 and the surface protective member 501 (that is, the fluororesin film) but also as an adhesive between the photovoltaic element 503 and the back face protective member 504.
Now, the filler (comprising the thermoplastic resin) is used for covering irregularities of the photovoltaic element while ensuring the adhesion of the photovoltaic element with not only the surface protective member but also the back face protective member. Therefore, it is required for the filler to have sufficient weatherability, adhesion properties, and heat resistance. Besides, the filler is also required to excel in long-term durability, endure against thermal expansion and heat contraction, and have flexibility.
As the filler, any transparent thermoplastic resins can be optionally used as long as they satisfy the above requirements. However, it is advantageous to use ethylene series copolymers because they excel particularly in heat resistance and adhesion properties and are relatively inexpensive. Of these ethylene series copolymers, EVA (ethylene-vinyl acetate copolymer) is the most often used.
In the case where the solar cell module is used under an environment with high temperatures, in order to ensure the adhesion of the filler (comprising the thermoplastic resin) with the photovoltaic element, the surface protective member, and the back face protective member under such severe environment, the filler is usually crosslinked with a crosslinking agent. Particularly in the case of using EVA as the filler, the EVA is usually crosslinked with a crosslinking agent comprising an organic peroxide.
However, in this case, such problems as will be described are liable to occur. The organic peroxide used as the crosslinking agent is liable to decompose an antioxidant added to the thermoplastic resin as the filler, resulting in deteriorating the weatherability of the filler. This situation becomes a serious drawback in the case where the thermoplastic resin as the filler situated on the light incident side (the front side) of the photovoltaic element is crosslinked with the organic peroxide as the crosslinking agent.
Besides, in the case where a solar cell module having a filler comprising a given thermoplastic resin such as EVA containing an organic peroxide as a crosslinking agent is prepared by means of vacuum lamination, sometimes the organic peroxide is not sufficiently removed and remains in the form of a bubble in the filler, whereby markedly deteriorating the bonding property and moisture resistance of the filler. In addition, gas is generated when the filler is crosslinked with the organic peroxide, and this gas tends to form a bubble in the filler, resulting in deteriorating the bonding property and moisture resistance of the filler. The formation of this bubble is more likely to occur upon rapid temperature rise.
Further, a relatively long period of time and a relatively large quantity of energy are required in order to crosslink the thermoplastic resin as the filler. This situation is a factor of making the resulting solar cell module costly.
For a photovoltaic element comprising a semiconductor layer as a photoelectric conversion member formed on an electrically conductive metal substrate and a transparent electrically conductive layer formed on said semiconductor layer which is used in a solar cell module, it is liable to have a short-circuit defect (a shunt) in which the substrate is short-circuited with the transparent electrically conductive layer due to irregularities present at the surface of the substrate or/and unevenness in the film formation. This short-circuit defect can be repaired by means of the so-called passivation treatment. However, the photovoltaic element thus repaired is potentially in a state that it is likely to be again shunted. This situation is facilitated particularly when moisture is present therein.
In the case of a solar cell module in which such repaired photovoltaic element is used having a fluororesin film as the surface protective member, the fluororesin film as the surface protective member has difficultly functioning as a barrier to sufficiently prevent moisture from invading into the solar cell module. In this case, when the photovoltaic element is sealed by a filler comprising an EVA having a high hygroscopic property, the solar cell module is not always satisfactory in terms of assuring the stability of the solar cell module upon continuous use under an environment with high temperature and high humidity over a long period of time.
In the case where a glass fiber is provided in the filler in order to protect the surface of the photovoltaic element, the above moisture invasion phenomenon is facilitated by way of the capillary action of the glass fiber.
For the EVA as filler by which the photovoltaic element is sealed, there is a tendency that the acetic acid radical is hydrolyzed in the present of moisture to liberate acetic acid. The liberated acetic acid is liable to corrode the surface of the transparent and electrically conductive layer, resulting in facilitating to shunt the photovoltaic element.