1. Field of the Invention
The present invention relates to photovoltaic element useful as a solar cell or the like, the structure of an electrode thereof, and a process for producing the photovoltaic device.
2. Related Background Art
In recent years, the global temperature rise caused by increase of carbon dioxide in the atmosphere, namely the greenhouse effect, has become a great problem. Therefore, clean energy sources are increasingly demanded. Nuclear power generation which does not cause the greenhouse effect involves the problem of disposal of radioactive waste. A safer and cleaner energy source is thus required.
Among promising clean energy sources, solar cells are attracting particular attention because of the safety, cleanness, and ease of handling thereof.
Of the solar cells, amorphous silicon type solar cells which employ an amorphous silicon semiconductor are promising because they have advantages of ease of large area cell production, high light absorbance enabling thin film operation, and so forth, although the photovoltaic conversion efficiency is lower than that of crystalline silicon type solar cells.
FIGS. 24A to 24D illustrate schematically a conventional photovoltaic element for comparison with the one of the present invention. FIGS. 24A to 24C illustrate the steps of forming a collecting electrode, and FIG. 24D is a cross-sectional view along the line 24D--24D in FIG. 24C. The solar cell element 500 of FIGS. 24A to 24D is prepared by successively laminating a lower electrode layer 502 on the surface of a substrate 501, a semiconductor layer 503 thereon, and an upper electrode layer 504 further thereon.
In such a solar cell element, in order to completely electrically separate the upper electrode layer from the lower electrode layer, a part 505 of the upper electrode layer is removed, and collecting electrodes 506 for the upper electrode layer 504 are provided on the surface of the upper electrode layer (FIG. 24A). In one method, for example, the collecting electrodes 506 are prepared by applying an electroconductive paste on the face of the photovoltaic laminate by screen printing and heat curing it. By this method, electrodes having a line width of 100 to 150 .mu.m, and thickness of 10 to 20 .mu.m are obtainable industrially. The electroconductive paste includes various materials. For amorphous silicon type solar cells, for which high temperature treatment is not suitable, polymer pastes comprising a thermosetting resin such as of polyester, epoxy and phenolic type, and a fine particulate material such as silver and copper dispersed therein are frequently used.
On the collecting electrodes 506 a bus bar electrode 507 is provided which further collects the generated power from the above collecting electrode 506 (FIG. 24B). Then crossing points of the bus bar electrode 507 over the collecting electrodes 506 are connected by applying an electroconductive adhesive 508 in spots and curing it in an air drier (FIG. 24C). Thus a leadout electrode which outputs the power from the upper electrode 504 is prepared by electrically connecting the collecting electrodes 506 with the bus bar electrode 507. Insulation tapes 509 are applied at the ends of the solar cell element 500 to ensure electrical separation of the bus bar electrode 507 from the substrate.
In producing this type of element, three steps are necessary: (1) registration of bus bar electrode to a prescribed position, (2) application of an electroconductive adhesive in spots at prescribed positions with the bus bar electrode fixed, and (3) curing the electroconductive adhesive by heating in an air drier or an IR oven. The steps involve many working operations and take a long time, and thus are disadvantageously not suitable for mass production. In the example shown in FIGS. 24A and 24B, the problems are not so serious since the area of the photovoltaic element is small and the number of the collecting electrodes is small. However, in the production of a photovoltaic element of a larger area, the number of the collecting electrodes is larger and a plurality of the bus bar electrodes are necessarily employed. In such a case, the electrically connected points increase in number and the working time becomes correspondingly longer, which is undesirable in respect of productivity.
Usual family consumption of electric power is about 3 KW per family. To supply 3 KW of power by means of solar cells, the solar cell needs to have a light-receiving area of as large as 30 m.sup.2, by assuming the photovoltaic conversion efficiency of the cell to be 10%. Such a large solar cell is required to have a bus bar to collect the generated power, which increases the cell element production steps. The larger the number of the production steps, and the larger the area of the cell, the more the defects of the element are developed. The defects cause shunting and short circuits which lower the photovoltaic conversion efficiency. If the defect is distant from the electrodes or the bus bar, loss of current is relatively small because of high resistance to the current flowing into the defective portion. On the other hand, if the defect is beneath the electrode or the bus bar, the loss of current is large.
To solve these problems, electrode constructions are disclosed which are suitable for a large-area solar cell without using the bus bar. For example, U.S. Pat. No. 4,260,429 discloses a process in which a copper wire is covered with a solid polymer containing electroconductive particles and is attached as the electrode to a solar cell. U.S. Pat. No. 5,084,107 discloses a process in which a metal wire is connected and fixed by an electroconductive adhesive to a surface of a photovoltaic element. In these methods, the electrode is formed by covering an electroconductive wire with an electroconductive particle-containing solid polymer (electroconductive adhesive) with a low ohmic loss even with an electrode length of 10 cm or more.
However, in a study of electrode constitution and reliability of solar cells conducted by the inventors of the present invention, it was found that the electrodes formed by the methods of the above U.S. Patents are insufficient in adhesion at the interface between the electroconductive wire or the metal member and the electroconductive particle-containing solid polymer or the electroconductive adhesive, and the electrodes are not uniform in the width or diameter. The insufficient adhesion at the interface between the electroconductive wire or the metal member and the electroconductive adhesive causes initial power loss, increase of series resistance by peeling at the interface in a long-term run, resulting in a drop in conversion efficiency, and other problems in reliability. Further, in the above elements, shunting and low yield may be caused, depending on the resistance of the electrode layer formed from the electroconductive adhesive, and disadvantageous migration of ionic substances may be caused by interaction with water, resulting in leakage due to humidity in practical use.
In the above U.S. Patents, the coated wires are bonded by heating or pressing, but neither the apparatus nor the method are shown specifically. However, it has been found that the usual method of heat-press bonding causes spreading of the coating polymer, which increases shadow loss, and particularly in a large-area photovoltaic element, the pressure applied to the coated wire becomes non-uniform to give non-bonded portions which disadvantageously increase the series resistance.
Electrical connection is now further considered for outputting the generated power of the photovoltaic element.
FIG. 17 is a schematic plan view of a photovoltaic element from the front side (light-receiving side) for comparison. The photovoltaic element of FIG. 17 is constituted by an electroconductive substrate for supporting the entire photovoltaic element, a non-crystalline semiconductor layer, an electrode layer, collecting electrodes, and lead-out terminals successively formed over the substrate. The electroconductive substrate is made of a metallic material such as stainless steel. The semiconductor layer is constituted by a back-face reflection layer, a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer successively arranged from the bottom layer. These semiconductor layers are formed and laminated by a film-forming method such as CVD (chemical vapor deposition) so that the light may be efficiently absorbed and converted to electric power. The aforementioned electrode layer is a light-transmissive electroconductive film made of indium oxide or the like, serving both as a reflection-preventing means and as a power-collecting means.
The light-transmissive electroconductive film is formed by application of an etching-paste containing FeCl.sub.3, and AlCl.sub.3, or the like by screen printing, and subsequent heating. An etched groove 17401 is formed by removing the light-transmissive electroconductive film by etching in a line shape. This partial removal of the light-transmissive electroconductive film is conducted for preventing short circuits between the substrate and the light-transmissive electroconductive film on the effective light-receiving area of the photovoltaic element. Such short-circuits may occur in cutting of the outer periphery of the photovoltaic element.
On the surface of the above photovoltaic element, collecting electrodes 17402 are formed for efficiently collecting the generated power. The collecting electrodes 17402 are formed by using a fine metal wire of low resistance such as copper as the core material, applying an electroconductive adhesive on the outer surface of the metal wire for adhesion, drying the adhesive, cutting the wire in a predetermined length, arranging the cut wires, and heat-bonding the wires on the surface of the effective light-receiving area by hot pressing.
The power collected by the collecting electrode 17402 is transmitted to lead-out terminals 17403 provided on both ends of the element. The lead-out terminals 17403 are foils made of a low-resistance metal such as copper, with an insulating member 17404 as the lowest layer to insulate the foils from the surface of the photovoltaic element.
The connection between the collecting electrodes 17402 and the lead-out terminals 17403 is conducted by spot-like application of an electroconductive adhesive 17405 to ensure reliable connection.
This process involves the problems of: necessity of spot-like application of an electroconductive adhesive, requiring a heat-curing step for curing the electroconductive adhesive, thereby increasing the working steps, taking a long time, and requiring a complicated apparatus for conducting these steps; the high cost of the electroconductive adhesive; the protrusion of the spot-wise applied electroconductive adhesive requiring a thick surface coating material, thus raising the production cost; deterioration of the surface of the terminal material such as copper by oxidation and other causes during the heating process of the element prior to the application of the electroconductive adhesive, whereby a sufficiently low connection resistance is not obtained by the application of the electroconductive adhesive.