SOLAR BATTERY AND METHOD OF MANUFACTURE

Abstract: A solar battery having a plurality of photoelectric conversion elements connected in series. Each element consists of a transparent electrode layer, a non-crystalline semiconductor layer and a metal electrode layer laid one upon another in the stated order. A site for a polycrystalline region in the non-­ crystalline semiconductor is provided. A conductive element sandwiches the transparent electrode between it and said non-­ crystalline material directly beneath said site. The site may comprise a void which is filled with metal from said metal elec­ trode layer. Methods of manufacturing the solar battery are disclosed.

Description: Background of the Invention This invention relates to a method of forming 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 "a-Si") 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 such as a glass substrate. For example, an ITO (indium tin oxide) film, SnO₂ (tin oxide) film, or ITO/SnO₂ compound film is formed on the entire upper surface of the glass substrate 1 by electron beam vacuum deposition, sputtering or hot 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 layer 22, the metal electrode 42 is brought into contact with the transparent layer 23 and the metal electrode 43 is brought into contact with the transparent layer 24, and so on until all the corresponding metal electrodes are brought into contact with their corresponding transparent layer. Each of the a-Si layers 31, 32, 33, 34 and so on consists of a p-layer of about 10nm (100 Å) in thickness, a non-doped (i) layer 0.5 µm in thickness and an n-layer of about 50nm (500 Å) in thickness which are laid one on another in the stated order as viewed from the side of the transparent electrode. GB-A-2 108 755 discloses a solar battery having a structure similar to the above-mentioned device except instead of the metal layer making direct contact with the transparent electrodes, metal strips are formed to connect to one edge of the transparent electrode layer. During the formation of the a-Si layer by a glow discharge process, these metal strips diffuse to form diffused connection regions with the upper tip of this region penetrating the upper face of the a-Si layer so that when the metal layer is deposited upon the a-Si layer and the required pattern of this metal layer is formed then series-connection of the photoelectric devices is achieved. 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; (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 (JP-A-61091971) 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 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 thereby, or, in the alternative, the energy of the laser beam is excessively absorbed thereby, 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. A solar battery is disclosed in US-A-4 461 922 whereby conductive films or stitch bars may be applied to the substrate either before or after the transparent conductive layer. The stitch bars can give protection against the laser beam cutting through the transparent electrode, however, according to this prior art the method of manufacturing is to perform the light exposure after the forming of the transparent top electrode on the non-crystalline material. The problem of this prior art is that if the intensity of the laser beam is large enough to make a hole in the top electrode portion, then the a-Si layer may be subjected to damage thereby affecting the conductivity of the overlying site. 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 method of forming 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 method of forming a film type solar battery comprising a plurality of photoelectric conversion elements connected in series, said method including the steps of: (a) providing a transparent insulating substrate; (b) forming a plurality of separate conductive films on said substrate; (c) forming a plurality of separate transparent electrodes on said substrate and on said conductive films, each transparent electrode covering a portion of said substrate between said conductive films and one of said conductive films; (d) blanket depositing an amorphous semiconductor layer on said transparent electrodes; (e) exposing the amorphous semiconductor layer to light having an intensity at least sufficient to form polycrystalline regions in said amorphous semiconductor layer, said polycrystalline regions being located over said conductive films; and (f) forming a plurality of separate metal electrodes on said amorphous semiconductor layer, each metal electrode overlying one of said polycrystalline regions so that said series connections between adjacent photoelectric conversion elements are formed. According to the invention, conductive films are provided between a translucent insulated substrate and the transparent electrode layers under the polycrystalline regions to sandwich the transparent electrode between the conductive film and the semiconductor layer, whereby, even if the amorphous semiconductor layer and the transparent electrode layer are cut by an excessively large energy of the 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 section 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 of a glass substrate l 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 or 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 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 x 10⁵ to 5 x 10⁵ W/cm². Next, an a-Si layer 3 consisting of a p-layer about 10nm (100 Å) in thickness, an i-layer about 0.5 µm in thickness and a n-layer of about 50nm (500 Å) in thickness were laid one on another 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 B₂H₆ of about 1% with SiH₄ and subjecting the gas to glow discharge. The i-layer can be formed by decomposing SiH₄ by the same technique. The n-layer can be formed by decomposing a gas which is prepared by mixing PH₃ of about 1% with SiH₄. A laser beam of about 50 µm in diameter is 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 x 10⁵ W/cm² or less, preferably 0.5 x 10⁵ to 2 x 10⁵ W/cm². 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 as high as 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 x 10⁶ to 10 x 10⁶ W/cm², and the separation belts, or material removed from between the electrodes 81, 82, 83, 84 were found to be about 40 µm in width. Conductive films 51, 52, 53, 54 formed on the glass substrate l 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 71, 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 l 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 scattering 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 l 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 layers 51, 52, 53, 54 also prevent the transmission of the laser beam through the transparent substrate l. 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.

Claims: A method of forming a film type solar battery comprising a plurality of photoelectric conversion elements connected in series, said method including the steps of: (a) providing a transparent insulating sustrate (1); (b) forming a plurality of separate conductive films (51,52,53,54) on said substrate; (c) forming a plurality of separate transparent electrodes (21,22,23,24) on said substrate and on said conductive films, each transparent electrode covering a portion of said substrate between said conductive films and one of said conductive films; (d) blanket depositing an amorphous semiconductor layer (3) on said transparent electrodes; (e) exposing the amorphous semiconductor layer to light having an intensity at least sufficient to form polycrystalline regions (61,62,63,64) in said amorphous semiconductor layer, said polycrystalline regions being located over said conductive films; and (f) forming a plurality of separate metal electrodes (41,42,43,44) on said amorphous semiconductor layer, each metal electrode overlying one of said polycrystalline regions so that said series connections between adjacent photoelectric conversion elements are formed. The method of Claim 1, wherein said exposing of step (e) forms voids (71,72,73,74) at the sites for the polycrystalline regions (61,62,63,64) and further comprising the step of filling the voids with metal extending from said metal electrodes (41,42,43,44). The method Claim 2, where said voids (71,72,73,74) extend through said amorphous semiconductor layer (3) and said transparent electrodes (21,22,23,24) and said step of filling electrically connects the metal electrodes (41,42,43,44) to said conductive films (51,52,53,54). The method of Claim 1, wherein said depositing of step (b) comprises printing the conductive films (51,52,53,54) on the substrate (1) and further comprises the step of sintering the films for about 10 minutes at about 500°C. The method of Claim 4, wherein said forming of step (c) comprises covering all the conductive films (51,52,53,54) with a transparent electrode layer and further comprises the step of scanning the transparent electrode layer with light of intensity sufficient to pattern the transparent electrode layer into said plurality of separate transparent electrodes (21,22,23,24). The method of Claim 1, wherein the depositing of step (d) comprises decomposing a reaction gas comprising SiH₄ to form said amorphous semiconductor layer (3). The method of Claim 6, wherein the reaction gas also contains a member selected from the group consisting of B₂H₆ and PH₃. The method of Claim 1, wherein said exposing of step (e) comprises exposing the amorphous semiconductor layer (3) to a laser beam. The method of Claim 1, where said metal electrodes (41,42,43,44) comprise aluminum.

Inventors: KAMIYAMA, MICHINARI FUJI ELECTRIC CORP.
Applicants: FUJI ELECTRIC CORPORATE RESEARCH AND DEVELOPMENT LTD.
Application year: 1986
Main classification: H01L27
IPC codes: H01L27, H01L31