Photovoltaic device and method of manufacturing the same

Abstract: A photovoltaic device of this invention has a semiconductor layer for generating a photovoltaic power and a collector electrode formed on the semiconductor layer to collect the power generated by the semiconductor layer. A conductive layer containing a conductive powder is formed on at least one side of the collector electrode closer to the semiconductor layer, and a metal layer is formed on the side away from the semiconductor layer. The metal layer covers the surface of the conductive layer away from the semiconductor layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an electrode of a semiconductor device and, more particularly, to a photoelectric converting device having a collector electrode which causes lesser deterioration with time, has a low resistance, and can be made thick. Related Background Art A photoelectric converting device is a semiconductor device known as a photosensor, a photoelectric cell, or a solar cell, which is an energy generating device using the photovoltaic power of a semiconductor. To alleviate an energy crisis caused by an insufficient energy supply power due to a recent abruptly increasing demand for energy or to mitigate a global environmental disruption by a warming phenomenon resulting from a greenhouse effect caused by an increase in carbon dioxide quantity, an energy supply source which is less harmful to environments has been desired. In particular, a photoelectric converting device (which will include a photovoltaic device and a solar cell hereinafter) which is clean, has a high safety, and can supply energy permanently for extended periods of time is considered most prominent as a novel energy supply source that can meet the above requirement. Well-known examples of the photovoltaic device are as follows. (1) Crystal-based solar cell A crystal-based solar cell is manufactured by forming a p-n junction by doping an n-type or p-type impurity into a p-type or n-type single-crystal or polycrystalline wafer. (2) Amorphous silicon-based solar cell An amorphous silicon-based solar cell is manufactured by decomposing monosilane gas or disilane gas by using, e.g., heat, an RF electric field, or light, thereby producing and depositing a-Si or the like. In this case, an amorphous silicon-based solar cell with a pin structure is formed by forming a p-type semiconductor and an n-type semiconductor by doping a gas, such as B 2 H 6 or BF 3 , as a p-type dopant and PH 3 as an n-type dopant simultaneously with monosilane or disilane. (3) Compound semiconductor solar cell A compound semiconductor solar cell is a GaAs solar cell manufactured by forming a p-n structure by growing p-type GaAs on n-type GeAs by liquid-phase epitaxial growth. A solar cell in which a p-n structure is formed by stacking and calcining n-type CdS and p-type CdTe is also a compound semiconductor solar cell. In addition to the above solar cells, a CuInSe 2 solar cell and an n-type CdS/p-type CuInS 2 solar cell are usable as photovoltaic devices. These photovoltaic devices aim at efficiently extracting an incident optical energy as an electrical energy through a collector electrode. To obtain a photovoltaic device with a high conversion efficiency, therefore, the following two points are important. (1) To increase the efficiency of conversion from an optical energy to an electrical energy. (2) To form a collector electrode with a lower electrical resistance for the purposes of decreasing a Joule loss by decreasing the electrical resistance at the collector electrode in collecting an electrical energy through the collector electrode and of reducing blocking of light by the collector electrode. The following two methods are known as a method of forming a collector electrode. One is a method of forming a collector electrode by vapor-depositing a conductive metal, as disclosed in Japanese Patent Publication No. 57-5066. The other is a method of forming a collector electrode by printing and calcining a conductive paste prepared by binding a conductive powder using glass or a polymer. In particular, the method using a conductive paste as a collector electrode is one of techniques which can easily form large-area devices through a simple manufacturing process and are therefore expected to have a surprising effect of reducing the manufacturing cost. At the present, however, collector electrodes manufactured by these conventional methods have a problem of a decrease in conversion efficiency or an inability to sufficiently achieve a stable operation over long periods of time resulting from the following respects. (1) The electrical resistance of a collector electrode itself is relatively high, leading to an increase in Joule loss, and this increases the loss of a conversion efficiency. (2) Since a collector electrode oxidizes with a long-term use, its electrical resistance further rises from the initial electrical resistance, increasing the loss of a conversion efficiency. (3) A collector electrode peels from a semiconductor or a transparent electrode under the influence of humidity to gradually degrade the function as a collector electrode. (4) Some material, such as a polymer binder, optically degrades upon irradiation with light to produce fine cracks or cause breaking of a collector electrode, causing the collector electrode to lose its function. In these respects, some improvements have been made to decrease the electrical resistance of a collector electrode or to stabilize the operation of a collector electrode over a long time period. One example is a method of binding conductive powders of gold and silver with low-melting glass, as disclosed in Japanese Patent Publication No. 61-59549. Other examples are a method of binding gold, silver, and aluminum with low-melting glass, as disclosed in Japanese Patent Publication No. 61-59551 (U.S. Patent No. 4,256,513) and a method of binding a conductive powder consisting of a nickel-antimony alloy by using a binder, as disclosed in Japanese Patent Publication No. 62-8958 (U.S. Patent No. 4,342,796). Japanese Patent Publication Nos. 63-11791, 2-3553, 2-3554 (U.S. Patent No. 4,486,232), or 2-3555 also discloses a method of adding various metals, such as bismuth and a rare earth element, to silver and binding the resultant material by using a binder. In any of the above methods, however, the volume resistivity of a collector electrode is approximately 30 to 50 µΩ·cm, i.e., is not sufficiently lowered as that of a collector electrode of a photovoltaic device, and this increases the loss of a conversion efficiency. In addition, no satisfactory solutions have been obtained yet in long-term stability, humidity resistance, and light resistance. As shown in Fig. 2, a conventional collector electrode of a photovoltaic device is often fabricated by printing a conductive paste material obtained by monodispersing fine silver particles about 1 to 10 µm in diameter in a polyester-based, polyimide-based, epoxy-based, or phenol-based thermosetting resin or in a glass frit, and mixing the dispersant with an organic solvent, such as Cellosolve, for controlling the viscosity on a transparent electrode layer 71 as an electrode formation surface of a photovoltaic device by using a stencil screen or the like, and thermally hardening the resultant structure. This structure makes it possible to form an electrode at a high .productivity and a high material yield by using a material with a very large area, largely contributing to a reduction in the manufacturing cost that is currently one major problem of photovoltaic devices. In this case, as a collector electrode 74 made from the conductive paste, one having a line width d of 100 to 150 µm and a thickness h of 10 to 20 µm is common for the mass-production purpose. Various substances can be used as the material of the conductive paste. In an amorphous silicon-based solar cell which is apt to be easily damaged by high-temperature treatments, however, it is common to use a polymer paste prepared by monodispersing fine copper or silver particles about 1 to 5 µm in diameter in a polyester-based, polyimide-based, epoxy-based, or phenol-based thermosetting resin. Recently, however, with improving photoelectric conversion efficiencies or increasing areas of photovoltaic devices, a demand has arisen for a collector electrode with a smaller loss than those of the conventional collector electrodes, and so collector electrodes formed using only a conductive paste have become unable to meet these requirements. As an example, the collector electrode 74 formed from the conductive paste consisting of the thermosetting resin has a specific resistance of 3 x 10 -5 to 5 x 10 -5 Ω·cm. In addition, when fine printing with a smaller line width is performed in order to reduce a loss produced by blocking of light by the collector electrode, a printing variation increases or the film thickness decreases accordingly, and a generation loss derived from the resistance of the collector electrode largely increases. Furthermore, heating for a long time period is required to harden such a conductive paste, so it is practically difficult to obtain a thick electrode by coating layers of the paste one atop the next. As shown in Fig. 3, therefore, a collector electrode according to the present invention has a structure in which a material 87 with a low specific resistance, such as solder, is placed on a conductive paste 84 as described above. This collector electrode causes lesser deterioration with time and has a low specific resistance. In addition, it is desirable to increase the film thickness in order to improve the efficiency of a collecting function. If, however, a material with a high wettability with respect to solder is simply used as a conductive paste to increase bonding properties with respect to the solder, the curvature of the solder material decreases and no sufficient film thickness can be obtained. Methods using vapor deposition or sputtering are also known as the electrode formation method. These methods, however, are almost never used in the manufacture of large-area devices because the manufacturing cost increases. Note that losses produced by a collector electrode in a photovoltaic device are roughly classified into a loss caused by blocking of light by an electrode material and a resistance loss derived from a resistance that the electrode material has. To reduce the loss resulting from blocking of light, the line width of an electrode must be decreased. When, however, the line width is decreased in order to reduce the loss caused by blocking of light, it becomes difficult to obtain a large thickness in screen printing, and the resistance increases compared to the decrease in the line width under the influence of printing variations. For example, the collector electrode formed by the conductive paste consisting of a thermosetting resin mentioned earlier in the conventional example has a volume resistivity of 3 x 10 -5 to 5 x 10 -5 Ω·cm. In addition, when fine printing is performed to reduce the loss derived from blocking of light by the electrode, a printing variation increases or the film thickness decreases accordingly, resulting in a large increase in the resistance loss of the collector electrode. Furthermore, since a conductive paste of the above sort must be heated for a long period of time to be hardened, it is difficult to obtain a large thickness by coating layers of the paste one on top of another. Fig. 9 shows the structure of a collector electrode in which a material with a low volume resistivity is placed on a bound conductive member consisting of the conductive paste described above. Figs. 8A to 8F illustrate a practical example of the steps of fabricating this structure. First, a conductive paste 20202 of the above sort is printed on a transparent electrode (SnO 2 , InO 3 , or ITO) 20201 of a photovoltaic device (Fig. 8A), and the resultant structure is thermally hardened in a thermodrying oven (Fig. 8B). It was found, however, that when a solder layer 20203 was formed on the conductive paste 20202 by using molten solder, this solder layer 20203 was very thin and hence unsatisfactory to improve the resistance loss although the layer had an effect of increasing an environmental resistance. Fig. 7 is a graph showing the thickness of a collector electrode with solder formed when molten solder was dip-brazed onto a bound conductive member consisting of a polymer type conductive paste. The present inventor, therefore, printed a solder paste, formed by mixing fine solder particles 30 to 50 µm in diameter with a cream flux, by using a metal plate, and thermally melting the resultant structure in a hot-air drying oven. As a result, a thick electrode with an average film thickness of 30 µm for a width of 300 µm was realized, indicating a considerable improvement compared to the electrode formed by the dip-brazing. If, however, the amount of a solder paste to be placed is simply increased by increasing the film thickness of a metal plate in order to realize a larger thickness than that obtained by the above process, the solder on the bound conductive member sometimes reaches a saturated critical amount. In such a case, the solder concentrates on a particular portion of the bound conductive member, forming a giant solder ball. SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor device which includes a collector electrode having excellent characteristics described below, which has a high conversion efficiency, and which can stably operate over long periods of time. More specifically, the semiconductor devices of the present invention are as follows. (1) A photovoltaic device in which the electrical resistance of a collector electrode is low enough to decrease a Joule loss, thereby decreasing the loss of a conversion efficiency. (2) A photovoltaic device which maintains an initial conversion efficiency of the device at a high level and can therefore operate stably over a long-term use. (3) A photovoltaic device in which no peeling of a collector electrode from a semiconductor or a transparent electrode occurs against various environmental factors, such as a high-humidity state, a high-temperature state, a low-temperature state, and repetition of these states, and which can therefore keep the initial function of the collector electrode at a high level. (4) A photovoltaic device in which a collector electrode hardly causes optical degradation, such as fine cracks or breaking, upon irradiation with light, and which can therefore keep the initial function of the collector electrode at a high level. (5) A photovoltaic device having an electrode structure which can decrease a loss caused by blocking of light while maintaining a low resistance loss and a high conversion efficiency, and can be fabricated at a low cost. A method and an apparatus for fabricating this photovoltaic device are also provided. According to the present invention, there is provided a photoelectric converting device as defined in claim 1. In the above photoelectric converting device of the present invention, the conductive particles may consist of two or more types of metal materials. In the above photoelectric converting device of the present invention, the conductive member consists of a low-melting metal which is a metal, an alloy, or a mixture thereof, and a conductive powder contained in the low-melting metal and having a higher melting point than that of the low-melting metal. In the above photoelectric converting device of the present invention, the low-melting metal may be a eutectic alloy. In the above photoelectric converting device of the present invention, the conductive particles have wettability with respect to the eutectic alloy. In the above photoelectric converting device of the present invention, the metal having wettability may be a ferromagnetic metal. In the above photoelectric converting device of the present invention, the metal member may be solder consisting primarily of Sn, Pb, or In. In the above photoelectric converting device of the present invention, the metal member may consist primarily of Cu. The above photoelectric converting device of the present invention further comprises a transparent conductive film between the semiconductor and the collector electrode. In the above photoelectric converting device of the present invention, the semiconductor may be amorphous silicon. In the above photoelectric converting device of the present invention, the conductive powder has wettability with respect to the eutectic alloy. In the above photoelectric converting device of the present invention, the conductive powder may consist of silver, copper, gold, palladium, or tin. In the above photoelectric converting device of the present invention, the ferromagnetic metal may consists of at least one of nickel and iron. In the above photoelectric converting device of the present invention, the conductive powder consists of a high conductive metal and a ferromagnetic metal. In the above photoelectric converting device of the present invention, the conductive powder may be present on the surface or in the vicinity of the surface of the low-melting metal. In the above photoelectric converting device of the present invention, the melting point of the conductive powder may be higher by 100°C or more than that of the low-melting metal. According to still another aspect of the present invention, there is also provided a method of manufacturing a photoelectric converting device, as defined in claim 6. The conventional problems mentioned earlier can be solved by the above means of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic sectional view showing a photovoltaic device of the present invention; Fig. 2 is a schematic perspective view showing an example of a conventional photovoltaic device; Fig. 3 is a schematic perspective view showing a practical example of a photovoltaic device; Figs. 4A to 4F are sectional views showing the manufacturing steps of Example 1; Figs. 5A, 5B, and 5C are schematic sectional views showing solder pastes according to Examples 1, 2, and 3, respectively; Figs. 6A to 6G are sectional views showing the manufacturing steps of Example 4; Fig. 7 is a graph showing the relationship between the line width and the thickness of a collector electrode; Figs. 8A to 8F are schematic perspective views for explaining steps of manufacturing a photovoltaic device; and Fig. 9 is a perspective view showing the main part of the photovoltaic device shown in Figs. 8A to 8F. DESCRIPTION OF THE PREFERRED EMBODIMENTS The first effect of the present invention is the ability to significantly decrease the electrical resistance of a collector electrode. A bound conductive member formed by, e.g., calcining a pasty substance (which will be sometimes referred to as a conductive paste hereinafter) consisting of a conductive powder and glass or a polymer (binder) has a volume resistivity of 30 to 50 µΩ·cm or more. When a collector electrode with a line width of 200 µm and an average film thickness of 10 µm is formed by using only the conductive paste of this type, a linear resistance becomes 1.5 Ω/cm or more, resulting in a collector electrode with a high electrical resistance. Consequently, the collecting loss extremely increases to decrease the conversion efficiency significantly. Note that the above effect is not limited to bound conductive members with a volume resistivity of 30 to 50 µΩ·cm or more but can be similarly obtained in bound conductive members with lower volume resistivities. Since light can be blocked by forming the metal member, an effect of significantly decreasing all deteriorations caused by light can be expected as well as the effect of preventing the optical decomposition described above. (Conductive particles) Preferable examples of conductive particles are particles of metals, such as copper, nickel, tin, lead, zinc, aluminum, iron, chromium, and titanium in addition to gold, silver, platinum, and palladium; particles of alloys of these metals and other metals: particles of doped metals; particles of these metals or other metals, which are imparted with a special distribution; surface-coated particles of these metals or other metals; and particles of oxides, such as ITO, ZnO, and SnO. The conductive particles may also be mixtures of these particles. The characteristic feature of the present invention is that even structures mainly consisting of metals, such as copper, nickel, tin, zinc, aluminum, and iron, which have been conventionally considered difficult to apply because of a problem of oxidation and the like, very hardly oxidize and therefore can be used stably. In addition, according to the present invention, even structures mainly consisting of metals, such as indium, palladium, chromium, titanium, tin, lead, iron, cobalt, and nickel, or of fine particles of metal oxides, such as ITO, ZnO, and SnO, which have been conventionally considered difficult to apply because of their high electrical resistances, can be used with electrical resistances lowered. Examples of the structures of conductive particles that can provide the above effects are as follows. (1) A so-called graded alloy particle which consists of a plurality of metals and in which the distribution of one of the metals gradually decreases from the surface to the interior of the particle. Examples are a copper alloy in which the distribution of silver has a gradient, a copper alloy in which the distribution of tin has a gradient, a copper alloy in which the distribution of lead has a gradient, a silver alloy in which the distribution of copper has a gradient, a silver alloy in which the distribution of tin has a gradient, a silver alloy in which the distribution of palladium has a gradient, and an aluminum alloy in which the distribution of copper has a gradient. The same surprising effects can be obtained by a tertiary metal system and a quaternary metal system as well as a binary metal system. Examples of the method of forming these particles are a method of obtaining a gradient by controlling metal supply quantities from a plurality of metal sources through vacuum heating or sputtering; a method of obtaining a gradient by making use of the difference between ion precipitation tendencies in a process of reduction precipitation in a solution; and a method of obtaining a gradient by using the difference in melting point when a molten alloy is atomized. The graded distribution can also be controlled by further performing a heat treatment for these particles. (2) A particle which consists of a certain kind of a metal and is surface-coated with a different metal. Examples are a copper particle surface-coated with silver, a copper particle surface-coated with tin, a copper particle surface-coated with lead, a silver particle surface-coated with copper, a silver particle surface-coated with tin, a silver particle surface-coated with palladium, an aluminum particle surface-coated with copper, and an iron particle which is carbon-coated. Particles of this type can be formed by a method of implanting metal ions into various metal particles, a method of performing wet plating, or a method of treating a plurality of kinds of metal particles under forced stirring. (3) A so-called alloy particle which consists of a plurality of metals and in which the distribution is relatively even. Examples are a brass particle, a bronze particle, a gold-copper alloy particle, a phosphor bronze particle, a purified silver particle, and a tin-lead alloy particle. (4) A particle formed by mixing different metal particles. Examples are a copper-silver mixture particle, a copper-tin mixture particle, and a palladium-silver mixture particle. Although various particle sizes are usable as the particle size of the conductive particles, the number-average particle size is preferably 0.01 to 100 µm in respect of electrical resistance or long-term operating stability (the number-average particle size is an average value of the diameters of particles measured by a sedimentation method or an average value of the diameters of true circles equivalent to the sectional areas of individual particles). As the binder for binding the conductive particles, both inorganic and organic materials generally known as binders can be applied. For example, various glass materials are usable as the inorganic binder, and a thermoplastic polymer and a curable polymer are usable as the organic binder. Usable examples of the thermoplastic polymer binder are a saturated polyester resin, a phenolic resin, an acrylic resin, a styrene resin, an epoxy resin, an urethane resin, a vinyl acetate resin, a vinyl chloride resin, a vinyl alcohol resin, an acetal resin, an amide resin, and modified resins and copolymer resins of these resins. Usable examples of the curable polymer binder are an imide resin, an unsaturated polyester resin, a phenolic resin, an alkyd resin, an unsaturated acrylic resin, an epoxy resin, a polyurethane resin, a melamine resin, a diallylphthalate resin, and oligomers and modified resins of these resins. In particular, the curable polymer binders can provide a superior effect in terms of long-term operating stability. Especially when a number-average molecular weight was 5,000 or less, the adhesive power of a metal to be stacked was further enhanced to achieve more surprising effects. In some cases, the effects of the present invention can be further improved by adding a small amount of a thermoplastic polymer to a curable polymer or by adding various additives, such as coupling agents and dispersants, for the purpose of controlling the dispersibility of the conductive particles or the wettability with respect to the binder. The mixing ratio of the conductive particles to the binder is represented by: ( weight of conductive particles ) ( weight of conductive particles ) + ( weight of binder ) x 100 The mixing ratio is preferably 3% to 50% when electrical resistance and operating stability over a long period of time are taken into account. As mixing means, stirring or milling is used while adding a proper amount of a diluting solvent as needed. As a milling apparatus, it is possible to use various milling apparatuses commonly used, such as a roll mill, a bead mill, a sand mill, an ultrasonic mill, an injection collision mill, and a ball mill. The bound conductive member having the conductive particles can be formed by printing means, such as screen printing, planographic printing, intaglio printing, or relief printing, when the member is in the form of an ink. The bound conductive member can also be formed by supplying using supplying means, such as a dispenser, and by hardening by heat drying or thermal hardening, or by using light or electron beams. The film thickness of the bound conductive member preferably ranges between 0.1 and 500 µm in respect of electrical resistance and long-term operating stability. The effects of the present invention were not reduced at all even when a transparent conductive film, an antireflection film, a protective film, and the like were formed between the semiconductor layer and the bound conductive member having the conductive particles. (Metal member) Examples of the metal are gold, silver, platinum, palladium, copper, nickel, tin, lead, zinc, aluminum, indium, iron, chromium, titanium, metals mainly consisting of these metals, and alloys of these metals. A metal with a volume resistivity of 30 µΩ·cm or less is most preferred. Examples are gold, silver, platinum, palladium, copper, nickel, tin, lead, zinc, aluminum, indium, iron, chromium, metals mainly consisting of these metals, and alloys of these metals. Examples of a metal which is very excellent in particularly adhesion properties and operating stability are various low-melting solders mainly consisting of tin and lead, a low-melting alloy containing indium, a metal with a relatively high stretchability, such as copper, tin, lead, zinc, aluminum, indium, gold, silver, and palladium, and alloys containing these metals. Examples of means of forming the metal member are means using vacuum vapor deposition or sputtering, wet forming means using plating, means of dip-coating in a molten metal, and powdery metal melting means. It is also possible to use other general metal member forming means. The film thickness is preferably 0.01 to 500 µm in terms of electrical resistance and operating stability over a long time period. The conductive powder of the present invention has an effect of adsorbing a low-melting metal, such as a solder material, an alloy, or a mixture (to be referred to as a low-melting metal hereinafter). In the formation of an electrode layer consisting of a low-melting metal, the conductive powder suppresses the flowability of a molten metal to fix a larger amount of the low-melting metal on a semiconductor layer (or a transparent conductive layer). This makes formation of a low-melting metal layer with a larger thickness possible. The resistance of the collector electrode can be further decreased by forming aggregates of the conductive powder relatively close to one another in the vicinity of the surface of the low-melting metal. This can consequently decrease the resistance of the electrode and thereby increase the efficiency of the photovoltaic device. In addition, by decreasing the line width in accordance with the decrease in resistance achieved by the large thickness, the loss caused by blocking of light by the collector electrode can be reduced to further increase the generation efficiency of the photovoltaic device. The arrangements of the photovoltaic devices according to the present invention will be described in detail below with reference to Fig. 1. Referring to Fig. 1, this photovoltaic device comprises a transparent conductive film 11, a semiconductor layer 12, a substrate 13, a conductive member 15 for fixing a conductive powder 14, and a low-melting metal layer 17. (Low-melting metal) Any metal or alloy can be used as the low-melting metal of the present invention as long as the melting point of the metal or alloy is lower than that of the conductive powder over a temperature range within which a semiconductor is not damaged. However, the low-melting metal preferably has a melting point different by 100°C or more from that of the conductive powder because an electrode layer with a larger thickness can then be formed. A practical example of the low-melting metal is solder, and examples of the solder material are Sn-Pb-based solder, Sn-Ag-based solder, Pb-group solder, and In-group solder. It is also possible to use cream solder, and the use of the cream solder is more preferable because the thickness of the solder can be controlled. (Conductive powder) As the conductive powder 14 fixed to the semiconductor layer side in the present invention, a powder consisting of palladium, gold, silver, copper, a copper alloy, nickel, or zinc, or a powder containing these metals can be preferably used. The use of these metals suppresses the flowability of solder because of the adsorption function that the conductive powder has in heat melting, increasing the solder amount per unit area, and further decreasing the electrical resistance of a collector electrode. The conductive powder particles of the present invention are not limited to spherical particles but may be scaly particles or needle-like particles. Although the size of the particles depends on the width of a collector electrode to be formed, it is preferably about 0.1 to 100 µm. Any material can be used as the material of the conductive members 15 of the present invention provided that the material has a high bonding power (high wettability) with respect to the semiconductor layer (or the transparent conductive film) and the low-melting metal. For example, it is preferable to use a bound conductive member formed by dispersing particles of a metal, such as gold, silver, copper, nickel, or iron, into a binder consisting of an organic resin, such as an epoxy resin, a phenolic resin, a polyester resin, or a polyimide resin. In situations where a treatment at a relatively high temperature is possible, a calcined paste formed by mixing the above metal material with a glass frit is also usable. By co-dispersing fine particles of at least one type of a metal with a solder wettability or fine particles of two or more different types of metals with a solder wettability into solder of a eutectic alloy as the low-melting metal on the bound conductive member, it is possible to suppress the flowability of the solder in heat melting to thereby increase the saturated critical amount of the solder on the bound conductive member to twice or more those of comparative examples. In this case, any metal particles can be dispersed in the low-melting metal as long as the particles have wettability with respect to solder. Examples of the metal are silver, tin, gold, copper, brass, bronze, lead, nickel, a monel metal, zinc, iron, stainless steel, nichrome, aluminum, and palladium. In addition, the solder as the low-melting metal is not limited to a so-called Sn-Pb-based solder but may be any of Sn-group solder, Pb-group solder, and In-group solder. The present invention will be described in more detail below by way of its examples. (Example 1) Figs. 24A to 24F illustrate the steps of manufacturing a photovoltaic device according to Example 1 of the present invention. This example relates to the manufacture of an amorphous photovoltaic device. As shown in Fig. 4A, an amorphous semiconductor layer 14142 was formed on a stainless steel substrate 14143. An electrode formation surface was on the light incident side, and the amorphous semiconductor layer 14142 as a photovoltaic layer had a high electrical resistance. Therefore, the surface of the amorphous semiconductor layer 14142 was covered with a thin transparent electrode (ITO) 14141, and the volume resistivity of the layer 14142 was 1 x 10 -3 to 2 x 10 -4 Ω•cm. It is impossible to solder an electrode directly to the transparent conductive film 14141 because ITO is an oxide and hence has no wettability with respect to solder. Since, therefore, an undercoating electrode agent serving as a portion for soldering is required in order to form a thick collector electrode, a metal film was formed on the transparent conductive film 14141 by vapor deposition. Fig. 4B shows this vapor deposition step. Referring to Fig. 4B, the transparent conductive film 14141 was patterned in advance through a metal mask 14144, and evaporated metal particles were adhered to the transparent conductive film 14141 by electron beam heating or resistance heating, thereby forming a metal film 14145. As the metal particles to be vapor-deposited, nickel having good adhesion properties with respect to ITO and a high solder wettability was used. Fig. 4C shows a step after the mask 14144 was removed, in which the metal collector electrode 14145 about 1 µm thick could be formed on the electrode formation surface. The manufacturing cost is extremely increased when vapor deposition is adopted because the film formation rate of vapor deposition is low. In this example, however, since a very thin film is used as a portion for soldering, no major problem arises in terms of manufacturing cost. As shown in Fig. 4D, a cream solder material 14147 was adhered by screen printing to the metal film 14145 formed as a portion for soldering. In this case, the printing precision of the cream solder is not as strict as that of the metal film 14145 consisting of nickel. As an example, when the line width of the nickel electrode 14145 is 100 µm, the cream solder can be printed with a margin extending by about 100 µm on each of the left and right sides. Subsequently, as shown in Fig. 4E, the cream solder 14147 adhered to the nickel electrode 14145 was heat-melted at 180°C to 240°C by an infrared heating reflow or in a vapor phase oven. In this heat melting, the nickel electrode 14145 with a high solder wettability aggregated by the surface tension, and at the same time the solder material and the copper particles present outside the nickel electrode 14145 were attracted onto the nickel electrode. The copper particles thus moved onto the nickel electrode 14145 rose on the nickel electrode to form a thick collector electrode 14146 while suppressing the flowability of the molten solder, preventing the solder from assembling into a ball on a portion of the electrode. A flux material and a thixotropic material 14148 contained in the cream solder remained on the surface of the solder or around the solder. If this material 14148 is left behind, it may contaminate the surface owing to its intrinsic properties to thereby interfere with incident light or may deteriorate the reliability of a product under high-temperature, high-humidity conditions. Therefore, the resultant structure was shower-washed with, e.g., an organic solvent, such as a modified alcohol or Cellosolve, and then dried (Fig. 4F). The cream solder 14147 had a composition and an arrangement as shown in Fig. 5A. Referring to Fig. 5A, copper particles 15151 with a particle size of about 50 to 100 µm were dispersed at a weight ratio of 5% to 25% with respect to Sn-Pb (Sn63% : Pb37%) solder particles 15152 about 5 to 30 um in particle size. A flux as a binder and a thixotropic material 15153 as a pasting agent for controlling viscosity were blended between the solder particles 15152 and the copper particles 15151. This makes it possible to print the cream solder easily by using the screen printing. In addition to the cream solder shown in Fig. 5A, those shown in Figs. 5B and 5C can be used. (Example 2) Fig. 5B shows Example 2 of the present invention, in which copper particles 15154 are covered with Sn63%Pb37% solder 15155. In this arrangement, the range of wt% of copper can be widened to about 3 to 40 wt% although the diameters of the individual particles are increased by the solder covering them to cause a disadvantage in printing properties. (Example 3) Fig. 5C shows Example 3 of the present invention, in which Sn-Pb solder 15158 is not only dispersed as particles in a paste but in part bound to copper particles 15157. In this arrangement, the copper particles adapt themselves to the solder particles rapidly in melting. The shape of these particles is not limited to a sphere, and any of Sn-Ag, Pb-group, Au-group, and In-group solder materials can also be used in addition to the Sn-Pb solder material. (Example 4) Figs. 6A to 6G illustrate the steps of manufacturing a photovoltaic device according to Example 4 of the present invention. This example relates to the manufacture of a polycrystalline solar cell. The electrode formation surface of this polycrystalline solar cell is a p-type silicon substrate 16161 (Fig. 6A) with a relatively high electrical conductivity, but solder as a collector electrode cannot be formed directly on the substrate 16161. Therefore, a calcined paste 16163 normally used in a thick-film hybrid IC or the like was used to form an undercoating electrode by screen printing to be described below (Fig. 6B). The calcined paste 16163 mainly consisted of silver particles about 3 to 10 µm in diameter and further added with a glass frit as a binder and a thixotropic material for controlling the viscosity of the paste. The pattern of the calcined paste 16163 can be formed easily by screen printing. In the screen printing, widths around 100 µm can be easily realized by using the state-of-the-art techniques. In this example, therefore, an undercoating electrode with a line width of 80 to 100 µm was patterned using a 325-mesh screen plate with an aperture ratio of 60% available from Tokyo Process Service. The grass frit contained in the paste melts at 500°C to 900°C in about 30 minutes, making formation of an electrode with a resistance of approximately 5 x 10 -6 to 8 x 10 -6 Ω•cm possible. However, it becomes difficult to form a thick electrode as the line width of the electrode decreases. For this reason, after calcining was performed by heating (Fig. 6C), a cream solder 16164 was printed following the same procedures as in Example 1 (Fig. 6D). The printing was performed by using a metal plate with a film thickness of 100 µm. Thereafter, when the resultant structure was heated at about 180°C to 250°C, the molten solder aggregated onto the calcined paste while containing the metal particles, and the metal particles suppressed the flowability of the molten metal, forming a thick collector electrode. Fig. 6E illustrates the state after the heat-melting described above was performed. Referring to Fig. 6E, a flux and a thixotropic material 16165 remained on the surface of and around the collector electrode 16164. Fig. 6F shows a step of washing away the flux and the thixotropic material 16165. In this step, shower washing was performed by using an organic solvent, such as a modified alcohol or Cellosolve, or hot water. After the collector electrode was formed as described above, an antireflection film 16166 was coated on the entire electrode formation surface to prevent reflection (Fig. 6G). Any example of the present invention was superior to the conventional examples. According to the present invention, a collector electrode having a large thickness regardless of its small line width can be formed. This consequently, largely decreases the resistance of the collector electrode, making it possible to provide a photovoltaic device with a high photoelectric conversion efficiency at a low cost. According to the present invention, aggregates of metal particles relatively close to one another can be formed in the vicinity of the top portion of a low-melting metal layer. As a result, the resistance of a collector electrode can be further decreased, so there can be provided a photovoltaic device with a high photoelectric conversion efficiency. In addition, since the resistance is stable over a long time period, it is possible to provide a photovoltaic device with a high conversion efficiency, in which a decrease in output with time is small. According to the present invention, a thick solder material can be formed on an electrode, and this decreases the resistance loss of the electrode material. In addition, the flowability of a magnetic material contained in a paste material increases when the paste material is heat-melted, so the thickness of a solder layer or variations in the thickness can be controlled by applying a high magnetic field. That is, the magnetic metal particles with a high solder adsorptivity tend to form a thick solder layer under the influence of the magnetic field, and the solder loses its flowability when cooled in this magnetic field, forming a photovoltaic device electrode having a solder layer with a considerable thickness. Consequently, a fine, low-resistance electrode can be formed on a photovoltaic device, and this increases the module efficiency of the device. Furthermore, according to the present invention, a sufficiently large thickness can be maintained even when the line width is decreased. Therefore, a loss caused by blocking of light and a resistance loss can be reduced, and thereby the module conversion efficiency of a photovoltaic device can be effectively improved.

Claims: A photoelectric converting device comprising: a semiconductor (14142) for performing photoelectric conversion; a transparent conductive layer (14141); and a collector electrode formed on said transparent conductive layer (14141), wherein said collector electrode has: (a) a first conductive member (14145); and (b) a second conductive member (14147) composed of a low melting metal which is a metal, an alloy or a mixture thereof, and a conductive powder contained in said low-melting metal and having a higher melting point than that of said low-melting metal, said second conductive member covering said first conductive member and being electrically connected to said first conductive member. A device according to claim 1, wherein said first conductive member is composed of a metal. A device according to claim 2, wherein said first conductive member is nickel. A device according to claim 1, wherein said conductive powder is composed of copper. A device according to claim 1, wherein said first conductive member is a calcined paste. A method of manufacturing a photoelectric converting device as defined in one of claims 1 to 5, comprising the steps of: forming a first conductive member on a transparent conductive film formed on a semiconductor layer; placing a second conductive member composed of a low-melting metal, a conductive powder contained in said low-melting metal and having a higher melting point than that of said low melting metal, and a thixotropic material, thus covering said first conductive member; heating said second conductive member; and removing said thixotropic material. A method according to claim 6, wherein said forming step of said first conductive member comprises vapor-depositing a metal using a mask pattern. A method according to claim 6, wherein said placing step of said second conductive member comprises performing screen printing. A method according to claim 6, wherein said conductive powder (15154) is covered with said low-melting metal (15155). A method according to claim 6, wherein a part of said low-melting metal is bound to said conductive powder. A method according to claim 6, wherein said forming step of said first conductive member comprises printing and heating a calcined paste composed of at least silver, a glass frit, and a thixotropic material.

Inventors: FUJISAKI TATSUO, HYOSU YOSHIHIKO, MIMURA TOSHIHIKO, SAWAYAMA IPPEI, TOMA HITOSHI, FUJISAKI, TATSUO, HYOSU, YOSHIHIKO, MIMURA, TOSHIHIKO, SAWAYAMA, IPPEI, TOMA, HITOSHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L31
IPC codes: H01L31