1. Field of the Invention
The present invention relates to a photovoltaic device with a region containing germanium therein, with a high photoelectric conversion efficiency and a high reliability, adapted for use as a solar cell or the like.
2. Related Background Art
The solar cell, a photoelectric converting device for converting sunlight into electric energy, is already widely used as a small power source in various consumer products such as desk-top calculators and wrist watches, and is considered as a promising technology as a substitute for the so-called fossil fuels such as petroleum and coal. The solar cell relies on the photovoltaic force generated by a semiconductor p-n junction, and is based on the principle of generating photocarriers, namely electrons and positive holes by the absorption of sunlight by a semiconductor such as silicon, causing said photocarriers to drift by the internal electric field of the p-n junction and outputting said photocarriers to an external load Such solar cells can be prepared substantially through a semiconductor manufacturing process. More specifically, the p-n junction is obtained by preparing a monocrystalline silicon ingot doped to p- or n-conductivity type by a crystal growing method such as the CZ method, slicing the obtained single crystal ingot to prepare a silicon wafer of a thickness of about 300 .mu.m, and forming a layer of a conductivity type different from that of said wafer, for example, by diffusion of a valence electron controlling material.
The currently commercialized solar cells are principally based on monocrystalline silicon in consideration of their reliability and conversion efficiency, but the production cost is high because of the semiconductor process employed in the preparation. Such monocrystalline silicon solar cells are also associated with other drawbacks; for example, they require a thickness of at least 50 .mu.m because monocrystalline silicon has a low light absorption coefficient because of an indirect band gap transition, and they are unable to effectively utilize short wavelength components because the band gap is about 1.1 eV, which is narrower than 1.5 eV, which is desirable for solar cells. The use of polycrystalline silicon may reduce the production cost, but the problem of indirect band gap transition still remains, so that the thickness of the solar cell cannot be reduced. Besides, polycrystalline silicon is associated with other problems, such as those resulting from crystal grain boundaries.
Furthermore, the cost per unit amount of generated electric power becomes higher than in the conventional power generating methods, because wiring for serial or parallel connection of unit devices is required for obtaining a large electric power, as large-sized wafers are difficult to prepare with the crystalline material, and because expensive instrumentation is required for protecting the solar cells from mechanical damages resulting from various climates they are subjected to in outdoor use. In development of solar cells for large power generation, therefore, cost reduction and device area extension are important issues, and investigations are being made for materials of lower cost or higher conversion efficiency. Examples of such material for solar cells include tetrahedrally bonded amorphous semiconductors such as amorphous silicon or amorphous silicon-germanium, II-VI compound semiconductors such as CdS, and Group III-V compound semiconductors such as GaAs or GaAIAs. Particularly, a thin film solar cell employing an amorphous semiconductor in the photovoltaic layer is considered promising because it can be prepared in a larger area, with a smaller thickness, and on an arbitrary substrate, in comparison with the monocrystalline solar cell.
However, such solar cells employing amorphous semiconductors still require improvement in the photoelectric conversion efficiency and reliability. A method for improving the photoelectric conversion efficiency is to reduce the band gap, thereby increasing the sensitivity for the light of the longer wavelength region. Since amorphous silicon, with a band gap of ca. 1 7 eV, cannot absorb and utilize the light of a wavelength of 700 nm or longer, there have been investigated materials of a narrower band gap, sensitive to the light of such longer wavelength region. An example of such materials is amorphous silicon-germanium, of which the band gap is arbitrarily variable from 1.3 to 1.7 eV by a change in the ratio of silicon-containing gas and germanium-containing gas employed during film formation.
Another method for improving the photoelectric conversion efficiency of the solar cell is based on the use of a tandem cell in which plural unit solar cells are laminated, as disclosed in U.S. Pat. No. 2,949,498. Said tandem cell, originally applied to crystalline semiconductors including a p-n junction, achieves improvement in the power generating efficiency by efficiently absorbing the solar spectrum with photovoltaic devices of different band gaps, thereby increasing V.sub.oc., and this principle is likewise applicable to an amorphous or crystalline material. In said tandem solar cell, the devices of different band gaps are stacked to efficiently absorb different regions of the solar spectrum in order to increase the energy conversion efficiency. The so-called top cell, positioned at the light entrance side of the stacked devices, is designed to have a wider band gap than the so-called bottom cell, positioned under said top cell. In the case of the two-layer tandem cell, the top and bottom cells are respectively composed of amorphous silicon and amorphous silicon-germanium, as a preferred combination of materials.
Another drawback of solar cells based on amorphous semiconductors is deterioration of conversion efficiency, caused by light irradiation. This phenomenon is caused by lowered carrier drift-mobility resulting from deterioration of the film quality of amorphous silicon or amorphous silicon alloys by light irradiation, and is known as the Staebler-Wronski effect, specific to amorphous semiconductors. This phenomenon leads to a lack of reliability in large power applications, and is a major obstacle to the commercial use of such solar cells. Therefore, intensive efforts are being conducted for avoiding such photodeterioration of amorphous semiconductors, by improvements in the film quality and in the cell structure. The use of the above-mentioned tandem cell structure is an effective method for avoiding such drawbacks, and it has been confirmed that the deterioration of cell characteristics by light irradiation is reduced by a reduction in the free path of carriers through the use of a thinner intrinsic layer.
However, further improvement in the reliability of amorphous silicon solar cells is still required, as the drawback of photodeterioration has not completely been resolved, despite continued efforts for clarifying the mechanism thereof and avoiding such photodeterioration. The maximum efficiency in such tandem cells can be obtained by so-called current matching, in which the currents obtained by the different cells are made equal. A bottom cell composed of amorphous silicon-germanium can sufficiently absorb the light even with a small thickness because the band gap thereof is smaller than that of the top cell composed of amorphous silicon, but efficient power generation becomes impossible because of an imbalance between the top cell and the bottom cell after prolonged light irradiation, because the deterioration rate of the amorphous silicon-germanium constituting the bottom cell is larger than that of the amorphous silicon constituting the top cell. While in such stacked cell, the use of a thinner intrinsic layer is effective for preventing the photodeterioration because the light absorption of the entire device is reduced, it, however, leads to a decreased initial photovoltaic force. Thus, a photovoltaic device meeting the requirements of high reliability and high efficiency at the same time has not been realized.
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