1. Field of the Invention
The present invention concerns a back reflector layer, a method for forming it, a photovoltaic element using it, and a process for fabricating the photovoltaic element. Particularly, the invention relates to a back reflector layer having a high reflectivity and an asperity structure (hereinafter referred to as a texture structure), and a photovoltaic element high in performance, high in reliability, and capable of being mass-produced.
2. Related Background Art
At present the energy supply is greatly dependent upon thermal power generation using fossil fuels such as petroleum and coal, and nuclear power generation. However, there is a serious question whether we may continue our energy dependence upon the fossil fuels, which cause global warming because of carbon dioxide etc. produced upon use of the thermal power generation, or upon the nuclear power generation, which involves some danger of radiation not only due to an accident, but also in the course of normal operation. Thus, attention has been drawn to solar-electric power generation using solar cells, which has very little effect on the global environment, and wider use thereof is expected.
There are, however, some problems which hinder widespread use of solar-electric power at present.
Single-crystal or polycrystal silicon have been commonly used heretofore for the solar cells for solar-electric power generation. For such solar cells using single-crystal or polycrystal silicon, the growth of crystals necessitates a lot of energy and time, and complex steps are necessary after the crystal growth. Thus, they are not suitable for mass production, and it is difficult to provide them at a low price. On the other hand, active research and development has been made on the so-called thin-film semiconductor solar cells using amorphous silicon (hereinafter referred to as a-Si) and compound semiconductor such as CdS and CuInSe.sub.2. These solar cells can be fabricated simply by forming a necessary thickness of a semiconductor layer on a cheap substrate such as glass or stainless steel, the fabrication steps thereof are relatively simple, and they include a possibility of cost reduction. The thin-film semiconductor solar cells, however, have lower efficiencies of photoelectric conversion than the single-crystal or polycrystal silicon solar cells and are not of sufficient reliability in long-term use. Thus, they are not widely used presently. A variety of ideas as discussed below have been presented in order to solve such problems and to improve the performance of the thin-film semiconductor solar cells. enhancing the reflectivity of light on the surface of substrate in order to cause the light energy (such as sunlight) not absorbed by the thin-film semiconductor for photoelectric conversion to travel back again into the thin-film semiconductor, i.e., in order to more effectively utilize incident light. For this purpose, when the substrate is optically transparent and the sunlight is incident from the substrate side, after formation of the thin-film semiconductor on the substrate an electrode is formed on the surface thereof, using a metal with high reflectivity, such as silver (Ag), aluminum (Al), or copper (Cu). In contrast, when the sunlight is incident from the exposed surface of the thin-film semiconductor, a layer of the same metal as listed above is first formed on the substrate and then the semiconductor layer is formed thereon.
Further, the reflectivity can be further enhanced by the multiple interference effect when a transparent layer having appropriate optical properties is interposed between the metal layer and the thin-film semiconductor layer, for example as shown in FIGS. 6A and 6B. FIGS. 6A and 6B show simulation results to verify that the reflectivity is improved when zinc oxide (ZnO) is interposed as a transparent layer between silicon and a metal selected from the above listed metals: FIG. 6A shows reflectivities when a-Si is formed on the metals; FIG. 6B shows reflectivities when ZnO is formed on the metals and then a-Si is formed on the ZnO.
The use of such a transparent layer is also effective for enhancing the reliability of thin-film solar cells. For example, Japanese Patent Publication No. 60-41878 discloses that use of a transparent layer can prevent the semiconductor and the metal layer from forming an alloy. Also, U.S. Pat. Nos. 4,532,372 and 4,598,306 disclose that use of a transparent layer with an appropriate resistance can prevent an excessive electric current from flowing between the electrodes even if the semiconductor layer has a short-circuited portion.
Another proposal for enhancing the conversion efficiency of solar cells is a method for making fine asperities (texture structure) on the surface of the solar cell and/or an interface between the back reflector layer and the semiconductor layer. With such a structure, the sunlight is scattered by the surface of solar cell and/or the interface between the back reflector layer and the semiconductor layer and further is confined inside the semiconductor (light trapping effect), whereby the sunlight can be effectively absorbed by the semiconductor. For example, when a transparent substrate is used and the sunlight is incident from the transparent substrate side, a good result can be obtained by texturing the surface of the transparent electrode such as tin oxide (SnO.sub.2) on the substrate. When the sunlight is made incident from the surface of the thin-film semiconductor, a good result can be obtained by texturing the surface of the metal layer used as the back reflector layer.
M. Hirasaka, K. Suzuki, K. Nakatani, M. Asano, M. Yano, and H. Okaniwa disclose that the texture structure for back reflector layer can be obtained by depositing Al while adjusting the temperature of the substrate and the deposition rate (Solar Cell Materials, Vol. 20 (1990) pp. 99-110). FIG. 7 shows an example in which absorption of incident light increases by use of the back reflector layer in such a texture structure. In FIG. 7, curve (a) represents the spectral sensitivity of an a-Si solar cell using a flat Ag layer as the metal layer, whereas curve (b) represents the spectral sensitivity of an a-Si solar cell fabricated in the same manner except that the Ag layer is formed in the texture structure.
One of the methods for obtaining the texture structure of the transparent layer involves introducing water vapor into the discharge gas used during layer formation ("Effect of Water Vapor on the Textured ZnO-Based Films for Solar Cells by DC-Magnetron Sputtering", by Tokio Nakada, Yukinobu Ohkubo and Akio Kunioka, Japanese Journal of Applied Physics, Vol. 30 No. 12A, December, 1991, pp. 3344-3348). Observation by SEM confirmed that the texture structure was more developed by depositing a metal oxide on a glass plate while introducing water vapor than by using Ar gas as the discharge gas. Although the conventional technology required a thick film in order to obtain the necessary texture structure for improving the characteristics of the solar cell, this method made it possible to decrease the film thickness.
It was, however, found that the solar cells using the back reflector layer obtained by forming a transparent layer on a metal layer by introducing water vapor failed to show the expected improvement of characteristics, or on the contrary, the characteristics, for example, the conversion efficiency, could be lower.
As described above, although at present there is a possibility of producing high-performance solar cells at low cost, they are not available for practical use as yet.