Hydroelectric power generation, wind-power generation, photovoltaic power generation and the like, which can be used to attempt to reduce carbon dioxide or improve other environmental problems by using inexhaustible natural energy, have received much attention. Among these, photovoltaic power generation has seen a remarkable improvement in performance such as the power generation efficiency of solar cell modules, and an ongoing decrease in the price, and national and local governments have worked on projects to promote the installation of residential photovoltaic power generation systems. Thus, in recent years, spread of photovoltaic power generation systems has advanced considerably.
Photovoltaic power generation directly converts solar energy to electric energy using a silicon cell semiconductor (solar cell element). The solar cell element as used herein undergoes a decrease in function when directly brought into contact with air from the outside. Therefore, a solar cell element is placed between sealing materials so as to provide a buffer as well as to prevent incorporation of foreign materials or infiltration of moisture. This sealing material is required to have various performance criteria, such as that the material is transparent and does not inhibit the power generation by light (transparency), that the material does not elute or disintegrate under the action of heat (heat resistance), that the material has good adherence to a protecting material such as glass or a back sheet (adhesiveness), and that the material does not undergo significant deterioration or yellowing under sunlight (durability). Thus, in order to satisfy these requirements, various formulations of blending in various components have been investigated. For example, there is known, as a representative formulation with consideration for transparency, heat resistance, adhesiveness, flexibility, moldability, durability and the like, a mixture of an ethylene-vinyl acetate copolymer combined with a peroxide and a silane coupling agent is used as a solar cell sealing material (see, for example, Patent Document 1).
Such a solar cell sealing material needs to employ a two-step process including a step of producing a sheet of an ethylene-vinyl acetate copolymer with various additives incorporated, and a step of sealing a solar cell element using the sheet thus obtained. In this step of producing a sheet, since it is necessary to perform molding at a low temperature at which the organic peroxide included for crosslinking does not undergo decomposition, the extrusion molding speed cannot be increased. Furthermore, in the step of sealing a solar cell element, it is necessary to perform an adhesion process which takes time and has two steps, including a step of performing temporary bonding in a laminator over several minutes to several tens of minutes, and a step of performing main bonding in an oven at a high temperature at which the organic peroxide is decomposed for several tens of minutes to one hour. Therefore, the production of a solar cell module requires effort and time, and also is one contributory factor for increasing the production cost.
In light of such circumstances, investigations are being undertaken on substitute materials which do not require the use of an organic peroxide such as described above, can significantly improve the production efficiency for solar cell modules, and have excellent characteristics as sealing materials for solar cells. Specifically, there has been suggested a solar cell element sealing material using an ethylene-unsaturated carboxylic acid copolymer having an unsaturated carboxylic acid content of 4 mass % or more and a melting point of 85° C. or higher, or an ionomer thereof (see, for example, Patent Document 2).
Furthermore, a press sheet has been disclosed, which is obtained by performing press molding using, as a sealing material that constitutes a solar cell module, an ethylene copolymer composition containing an ethylene-methacrylic acid copolymer and an amino group-containing trimethoxysilane at a predetermined ratio (see, for example, Patent Documents 3 and 4).
On the other hand, a solar cell uses various backing films on the side opposite to side of the solar cell element where sunlight is incident, and the amount of addition of a silane coupling agent in the sealing material may be increased in order to adjust the adhesiveness to the backing films
Patent Document 1: WO 2006/095762 A1
Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 2000-186114
Patent Document 3: JP-A No. 2004-31445
Patent Document 4: JP-A No. 2001-144313