Method of manufacture of solar battery
BACKGROUND OF THE INVENTION 
     This invention relates to a film type solar battery comprising a plurality of series-connected photoelectric conversion elements formed on a translucent insulated substrate. 
     Non-crystalline or amorphous silicon (hereinafter referred to as &#34;a--Si&#34;) is formed by subjecting silane gas to glow discharge decomposition. Because a--Si grows in the gaseous phase, it theoretically can readily have a large area. It is hoped that non-crystalline silicon can be developed as a high output element in a solar battery. 
     In order to efficiently use the electric power generated by a solar battery, it is desirable that the structure of the solar battery be as shown in FIG. 2, for example, where unitary cells are connected in series to one another. In a solar battery of the type shown in FIG. 2, strip-shaped transparent electrodes 21, 22, 23, 24 and so on are formed on a translucent insulated substrate 1 such as a glass substrate. For example, an ITO (indium tin oxide) film, SnO 2  (tin oxide) film, or ITO/SnO 2  compound film is formed on the entire upper surface of the glass substrate 1 by electron beam vacuum deposition, sputtering, or thermal chemical vapor deposition (CVD), and the film thus formed is subjected to optical etching to form the strip-shaped transparent electrodes 21, 22, 23, 24 and so on. In the same manner, a--Si layers 31, 32, 33, 34 and so forth and metal electrodes 41, 42, 43, 44 and so forth are formed. In order to electrically connect the transparent electrode layers to the metal electrode layers, the metal electrode 41 is brought into contact with transparent electrode layer 22, the metal electrode 42 is brought into contact with the transparent electrode layer 23 and the metal electrode 43 is brought into contact with the transparent electrode layer 24, and so on until all the corresponding metal electrodes are brought into contact with their corresponding transparent electrode layer. Each of the a--Si layers 31, 32, 33, 34 and so on consists of a p-layer of about 100 Å in thickness, a non-doped (i) layer 0.5 um in thickness, and an n-layer of about 500 Å in thickness which are laid one on another in the stated order as viewed from the side of the transparent electrode. 
     However, the manufacture of the above-described series connection type solar battery of the prior art suffers from the following difficulties: 
     (1) Pinholes are liable to be formed because of defects in photo-resist layers, thus decreasing the output power of the solar battery; 
     (2) A chemical treatment is carried out for every film formation, which contaminates the interfaces of the films, thus lowering the output power of the solar battery; and 
     (3) The manufacturing processes are intricate, and thus the manufacturing cost is increased as the area of the solar battery increases. 
     In order to eliminate the above-described difficulties, a method has recently been proposed in the art in which the energy of a laser beam is utilized to cut the transparent electrode layer, the a--Si layer and the metal electrode layer into pieces. 
     Additionally, the present inventors have described a method in Japanese Patent Application No. 213736/1984 in which the a--Si layer is not cut, but instead electrically conductive polycrystalline regions are formed in the a--Si layer by utilizing the energy of a laser beam so that the transparent electrode layer of any one of the photoelectric conversion elements is connected to the metal electrode layer of the next photoelectric conversion element. However, as is described in the specification of the above mentioned patent application, there are several cases where the applied laser beam is either not absorbed or is excessively absorbed. For example, in both cases where the laser beam is applied to the a--Si layer through the metal electrode layer or applied directly to the a--Si layer before the metal electrode layer is formed, a non-uniform thickness in either the a--Si layer or the metal electrode layer causes reflection of the beam to fluctuate. This results in the laser beam being reflected by the layer or passed through it in an unpredictable manner. As a result, the energy necessary for forming polycrystalline regions in the a--Si layer is not absorbed therein, or, in the alternative, the energy of the laser beam is excessively absorbed therein, so that the a--Si layer may be cut and the transparent electrode layer may also be cut. The problems associated with over or under absorption of light are evident. When the a--Si layer is not made polycrystalline, it is so high in resistance that the metal electrode layer on the a--Si layer is not electrically connected to the transparent electrode layer. If the transparent electrode layer is also cut, the electrical connection cannot be obtained. Even if the metal electrode layer is formed later so that the gap formed by cutting of the transparent electrode layer is filled with metal, the metal layer in the gap is brought into contact only with the cross-sectional areas of the transparent electrode layer. The resulting contact area is extremely small such that it is impossible for the metal electrode layer to sufficiently electrically contact the transparent electrode layer. Accordingly, it is necessary for the prior art to monitor and adjust the laser output according to the conditions of the layers, which lowers the manufacturing efficiency in the mass production of solar batteries. 
     OBJECTS OF THE INVENTION 
     An object of this invention is to provide a film type solar battery suitable for mass production in which the above-described difficulties have been eliminated. 
     It is a further object of this invention to form polycrystalline regions in an a--Si layer by a laser beam to connect the photoelectric conversion elements, wherein adjustment of the laser output is unnecessary. 
     SUMMARY OF THE INVENTION 
     The foregoing objects of the invention have been achieved by the provision of a film type solar battery in which polycrystalline regions for connecting photoelectric conversion elements are formed in a non-crystalline semiconductor layer. These regions are covered by elongated portions of the metal electrode layers of adjacent photoelectric conversion elements. According to the invention, conductive layers are provided between a translucent insulated substrate and the transparent electrode layers under the polycrystalline regions, to sandwich the transparent electrode between itself and the semiconductor layer, whereby, even if the non-crystalline semiconductor layer and the transparent electrode layer are cut by the excessively large energy of a laser beam, the gaps formed in these layers are filled with the metal of the metal electrode layer to electrically connect the photoelectric conversion elements to one another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a-1d are sectional views showing the steps of manufacturing a film type solar battery in one embodiment of this invention; 
     FIG. 2 is a sectional view showing a conventional film type solar battery; and 
     FIGS. 3a-3b are sectional views showing an abnormal condition occurring in the steps of manufacturing the solar battery shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The parts (a) through (d) of FIG. 1 schematically illustrate the steps of manufacturing a film type solar battery in sequence, in one embodiment of the present invention. In FIG. 1, parts corresponding functionally to those already described with reference to FIG. 2 are therefore designated by the same reference numerals or characters. 
     FIG. 1a illustrates a pattern of a conductive layer according to one embodiment of the invention. In FIG. 1a, conductive films 51, 52, 53, 54 and so forth are formed by printing the conductive films on a glass substrate 1 with conductive paste containing silver by a screen printing technique, followed by sintering at about 500° C. for about ten minutes. 
     Thereafter, a transparent electrode layer is formed to a thickness of 0.4 to 1 μm by an electronic beam vacuum deposition method of hot CVD method in such a manner as to cover all the conductive films. The transparent electrode layer thus formed is scanned with a yttrium-aluminum-garnet (YAG) laser beam about 50 μm in diameter in such a manner that the laser beam is applied along a line about 20 μm to the left of each of the conductive films 51, 52, 53, 54 and so forth, thereby to pattern (form) transparent electrodes 21, 22, 23, 24 and so forth as shown in FIG. 1B. A suitable output power of the laser in this operation is 2×10 5  to 5×10 5  W/cm 2 . 
     Next, an a--Si layer 3 is formed consisting of a p-layer about 100 Å in thickness, an i-layer about 0.5 μm in thickness, and an n-layer of about 500 Å in thickness laid one over the other in the stated order as viewed from the side of the transparent electrodes. These layers are formed in the stated manner so as to cover all the transparent electrodes. The p-layer can be formed by decomposing a reaction gas which is formed by mixing about 1% B 2  H 6  with SiH 4  and subjecting the gas to glow discharge. The i-layer can be formed by decomposing SiH 4  by the same technique. The n-layer can be formed by decomposing a gas which is prepared by mixing about 1% PH 3  with SiH 4 . 
     A laser beam of about 50 μm in diameter is then applied to the a--Si layer 3 formed over the conductive films 51, 52, 53, 54 and so on as described above. The power of the applied laser beam is 2×10 5  W/cm 2  or less, preferably 0.5×10 5  to 2×10 5  W/cm 2 . As a result, the a--Si layer becomes molten and evaporates at irradiated regions 61, 62, 63, 64 and so forth and the heat produced in this operation makes the irradiated regions polycrystalline in the direction of the thickness of the a--Si layer 3 as shown in FIG. 1c. Thereafter, a metal electrode layer, for example, aluminum, is formed on the a--Si layer 3 by an electronic beam vacuum deposition or sputtering method. Irradiated regions 61, 62, 63, 64 of the a--Si layer which have been made polycrystalline are low in resistance, having a conductivity 1,000 times that of the surrounding a--Si layer. Thus, the metal electrode layer of aluminum or the like is electrically connected to the transparent electrodes through the polycrystalline regions 61, 62, 63, 64 as shown in FIG. 1d. Thereafter, the metal electrode layer is scanned with a laser beam about 50 μm in diameter in such a manner that the laser beam is applied along a line on the righthand side of each of the polycrystalline regions, thereby to pattern or form individual metal electrodes 41, 42, 43, 44. For this operation, a suitable power of the laser beam was found to be 5×10 6  to 10×10 6  W/cm 2 , and the separation belts 81, 82, 83, 84, or material removed from between the electrodes 41, 42, 43, 44 were found to be about 40 μm in width. 
     Conductive films 51, 52, 53, 54 formed on the glass substrate 1 absorb the energy of the laser beam which otherwise may be lost, thus contributing to the formation of the polycrystalline regions in the a--Si layer. Even if the energy of the laser beam utilized is so large that, as shown in FIG. 3a, the a--Si layer 3 and the transparent electrodes 21, 22, 23, 24 are cut to form gaps 71, 72, 73, 74, FIG. 3a, these gaps are filled with metal in the subsequent step of forming the metal electrode layer. As shown in FIG. 3b, the metal electrodes 41, 42, 44 are connected to the transparent electrodes 21, 22, 23, through a metal, such as aluminum, which fills in the gaps 72, 73, 74. Thus, when the power of the laser beam is sufficiently large for the formation of the polycrystalline regions in the a--Si layer, but the absorption is excessive and cuts through the a--Si layer then the elements can be connected in series to one another no matter how non-uniform the surface and thickness of the a--Si layer 3 when employing the method of the present invention. 
     The characteristics of a solar battery consisting of ten series-connected photoelectric conversion elements formed on a glass substrate 1 by the process as described above were V oc  =8.4, I sc  =130 mA, FF=0.65, and output=710 mW. 
     In the above-described embodiment, the conductive films 51, 52, 53, 54 are formed by printing them on the substrate with a conductive paste; however, they may also be formed by vacuum-depositing metal through a mask. They may also be formed by depositing a metal film layer which is thereafter subjected to patterning. However, since the a--Si layer is formed over them, these conductive films should be resistant to temperatures as high as about 500° C. so as to withstand the temperature which is used in forming the a--Si layer. Moreover, the conductive film should be scarcely scattered when irradiated by the laser beam. 
     In the solar battery produced by the method of the invention, a conductive layer 51, 52, 53, 54 is provided between the translucent insulated substrate 1 and the transparent electrode layer 21, 22, 23, 24 with each conductive layer positioned below its respective polycrystalline region 61, 62, 63, 64 of a--Si layer 3. The conductive layer 51, 52, 53, 54 also prevents the transmission of the laser beam through the transparent substrate 1. Thus, even when the non-crystalline semiconductor layer and the transparent electrode layer are cut by an excessively large energy or absorption of the laser beam, the present battery will still contain an electrical connection between the metal electrodes 41, 42, 43, and 44 and their respective transparent electrodes 21, 22, 23 and 24. Accordingly, even if, for instance, the non-crystalline semiconductor layer is not uniform in thickness or its surface fluctuates in reflection factor, sites for polycrystalline regions can be formed in the non-crystalline semiconductor layer by using the same laser beam. Even if the site comprises a void, the subsequent metal layer forming step will provide the electrical connection with the transparent electrode. Thus, automation of the manufacture of the solar battery of the invention can be readily achieved. 
     Although the invention has been described in conjunction with specific embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all of the alternatives and variations that fall within the spirit and scope of the appended claims.