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Thin-film photoelectric conversion device and a method of manufacturing the sameRelated Patent Categories: Batteries: Thermoelectric And Photoelectric, Photoelectric, CellsThin-film photoelectric conversion device and a method of manufacturing the same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060213550, Thin-film photoelectric conversion device and a method of manufacturing the same. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a thin-film photoelectric conversion device, especially a solar cell which is formed on a substrate, and more particularly to a thin-film solar cell having a photoelectric conversion layer formed of a crystalline silicon film. [0003] 2. Description of the Related Art [0004] A solar cell or a solar battery can be manufactured using a variety of semiconductor materials or organic compound materials. However, from an industrial viewpoint, silicon is mainly used for the solar cell. The solar cells using silicon can be classified into a bulk solar cell using a wafer of monocrystal silicon or polycrystal silicon and a thin-film solar cell having a silicon film formed on a substrate. Reduction of manufacturing costs is required, and the thin-film solar cell is expected to have the effect of reducing the costs because less raw materials are used for the thin-film solar cell than for the bulk solar cell. [0005] In the field of thin-film solar cells, an amorphous silicon solar cell has been placed into practical use. However, since the amorphous silicon solar cell is lower in conversion efficiency compared with the monocrystal silicon or polycrystal silicon solar cell and also suffers from problems such as deterioration due to light exposure and so on, the use thereof is limited. For that reason, as another means, a thin-film solar cell using a crystalline silicon film has been also developed. [0006] A melt recrystallization method and a solid-phase growth method are used for obtaining a crystalline silicon film in the thin-film solar cell. In both the methods an amorphous silicon layer is formed on a substrate and recrystallized, thereby obtaining a crystalline silicon film. In any event, the substrate is required to withstand the crystallization temperature, whereby usable materials are limited. In particular, in the melt recrystallization method, the substrate has been limited to a material that withstands 1,412.degree. C., which is the melting point of silicon. [0007] The solid-phase growth method is a method in which an amorphous silicon film is formed on the substrate and crystallized thereafter through a heat treatment. In such a solid-phase growth method, in general, as the temperature becomes high, the processing time may be shortened more. However, the amorphous silicon film is hardly crystallized at a temperature of 500.degree. C. or lower. For example, when the amorphous silicon film which has been grown through a gas-phase growth method is heated at 600.degree. C. so as to be crystallized, 10 hours are required. Also, when the heat treatment is conducted at the temperature of 550.degree. C., 100 hours or longer is required for the heat treatment. [0008] For the above reason, a high heat resistance has been required for the substrate of the thin-film solar cell. Therefore, glass, carbon, or ceramic was used for the substrate. However, from the viewpoint of reducing the costs of the solar cell, those substrates are not always proper, and it has been desired that the solar cell be fabricated on a substrate which is most generally used and inexpensive. However, for example, the #7059 glass substrate made by Corning, which is generally used, has a strain point of 593.degree. C., and the conventional crystallization technique allows the substrate to be strained and largely deformed. For that reason, such a substrate could not be used. Also, since a substrate made of a material essentially different from silicon is used, monocrystal cannot be obtained even through crystallization is conducted on the amorphous silicon film through the above means, and silicon having large crystal grains is hard to obtain. Consequently, this causes a limit to an improvement in the efficiency of the solar cell. [0009] In order to solve the above problems, a method of crystallizing an amorphous silicon film through a heat treatment is disclosed in U.S. Pat. No. 5,403,772. According to the method disclosed in this patent, in order to accelerate crystallization at a low temperature, a small amount of a metal element is added to the amorphous silicon film as a catalyst material. Further, it is therein disclosed that a lowering of the heat treatment temperature and a reduction of the treatment time are enabled. Also, it is disclosed therein that a simple elemental metal substance, e.g. nickel (Ni), iron (Fe), cobalt (Co), or platinum (Pt), or a compound of any one of those metals and silicon, or the like is suitable for the catalyst material. [0010] However, since the catalyst materials used for accelerating crystallization are naturally undesirable for crystalline silicon, it has been desired that the concentration of the catalyst material is as low as possible. The concentration of catalyst material necessary for accelerating crystallization was 1.times.10.sup.17/cm.sup.3 to 1.times.10.sup.20/cm.sup.3. However, even when the concentration is relatively low, since the above catalyst materials are heavy metal elements, the material contained in silicon forms a defect level, thereby lowering the characteristics of a fabricated element. [0011] The principle of operation of a solar cell containing a p-n junction can be roughly described as follows. The solar cell absorbs light and generates electron/hole charge pairs due to absorbed light energy. The electrons move toward the n-layer side of the junction, and the holes move toward the p-layer side due to drift caused by the junction electric field and diffusion. However, when the defect levels are high in silicon, the charges are trapped by the defect levels while they are moving in the silicon, thereby disappearing. In other words, the photoelectric conversion characteristics are lowered. The period of time from when the electrons/holes are generated until they disappear is called the "life time". In the solar cell, it is desirable that the lifetime is long. Hence, it has been necessary to reduce as much as possible the heavy metal elements that generate the defect levels in silicon. SUMMARY OF THE INVENTION [0012] The present invention has been made in view of the above circumstances, and therefore an object of the present invention is to provide a method of manufacturing a thin-film solar cell, which retains the feature of crystallization due to the above catalyst material and removes the catalyst material after the crystallization has been completed. [0013] Another object of the present invention is to provide a solar cell which has an excellent photoelectric conversion characteristic, using the above method. [0014] In accordance with the primary feature of the present invention, a method of manufacturing a photoelectric conversion device includes a step of forming a gettering layer on a crystallized semiconductor layer obtained by using a catalyst metal such as nickel. The gettering layer may be either insulative or semiconductive and contains phosphorus to absorb the catalyst metal such as nickel from the semiconductor layer after it is crystallized, thereby reducing the concentration of the catalyst metal in the semiconductor layer. Specifically, the method includes the steps of: [0015] disposing a metal containing layer in contact with an upper or lower surface of a non-single crystalline silicon semiconductor layer; [0016] crystallizing the non-single crystalline silicon semiconductor layer by heating, wherein the metal functions to promote the crystallization; [0017] forming a gettering layer on or within said semiconductor layer after crystallized, the gettering layer containing phosphorus; and [0018] heating said semiconductor layer and the gettering layer in order to getter the metal contained in the semiconductor layer. [0019] As the metal element, it is possible to use one or more elements chosen from Ni, Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au. [0020] In accordance with a preferred embodiment of the invention, the gettering layer may be a silicon layer to which phosphorus is added during the deposition thereof onto the crystallized semiconductor layer. In an alternative, the gettering layer may be a phosphorus doped region formed within the crystallized semiconductor layer, namely, a method of the present invention includes a step of introducing phosphorus ions into a surface region of the crystallized semiconductor layer by ion doping after crystallizing the semiconductor layer by the use of the catalyst metal. In a further alternative, the gettering layer may be a phospho-silicate glass (PSG) layer deposited on the crystallized semiconductor layer. [0021] In accordance with another aspect of the invention, the catalyst metal is provided by disposing the metal containing layer in contact with an upper or lower surface of a non-single crystalline semiconductor layer to be crystallized. In the case of disposing the metal containing layer under the non-single crystalline semiconductor layer, the metal containing layer may be used also as a lower electrode of the photoelectric conversion device. [0022] In accordance with still another aspect of the invention, a solar cell comprises a substrate, a first crystalline silicon film having conductivity type formed on the substrate, and a second crystalline silicon film having another conductivity type adjacent to the first crystalline silicon film, wherein the first crystalline silicon film contains a catalyst element for promoting crystallization of silicon at a concentration not higher than 5.times.10.sup.18 atoms/cm.sup.3. The concentration value disclosed in the present invention is determined by secondary ion mass spectroscopy and corresponds to a maximum value of the measured values. [0023] In accordance with a further aspect of the invention, in the above mentioned solar cell, the concentration of the catalyst contained in the second crystalline silicon film is higher than the concentration of the catalyst contained in the first crystalline silicon film. [0024] In accordance with a still further aspect of the invention, the crystalline semiconductor film obtained by using the catalyst metal such as nickel has a plurality of crystal grains in the form of needles. [0025] According to the present invention, the lifetime of carriers in the crystalline silicon film is increased, and the excellent characteristics of the thin-film solar cell are obtained. BRIEF DESCRIPTION OF THE DRAWINGS Continue reading about Thin-film photoelectric conversion device and a method of manufacturing the same... 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