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【正文】 duction and lower materials cost. Thinfilm is also suitable for buildingintegrated systems because the semiconductor films may be applied to building materials such as glass, roofing, and siding. Figure 2 Typical thinfilm amorphous silicon constructions Using thin films instead of silicon wafers greatly reduces the amount of semiconductor material required for each cell and therefore lowers the cost of producing photovoltaic cells. Gallium arsenide (GaAs), copper indium diselenide (CuInSe2), cadmium telluride (CdTe) and titanium dioxide (TiO2) are materials that have been used for thin film PV cells. Titanium dioxide thin films have been recently developed and are interesting because the material is transparent and can be used for windows. Tin Oxide Tin oxide is a conductive material that is transparent when in a thin layer. Tin oxide is used in place of a metallic grid for the top layer of thin film photovoltaic sheets. Amorphous Silicon (aSi) Amorphous (uncrystallized) silicon is the most popular thinfilm technology. It is prone to degradation and produces cell efficiencies of 57%. Double and triplejunction designs raise efficiency to 810%. The extra layers capture different wavelengths of light. The top cell captures blue light, the middle cell captures green light, and the bottom cell captures red light. Variations include amorphous silicon carbide (aSiC), amorphous silicon germanium (a Si), and amorphous siliconnitride (aSiN). Cadmium Telluride (CdTe) and Cadmium Sulphide (CdS) Photovoltaic cells using these materials are under development by BP Solar and Solar Cells Inc . Polycrystalline Silicon Polycrystalline silicon offers an efficiency improvement over amorphous silicon while still using only a small amount of material. Copper Indium Diselenide and Copper Indium Gallium Diselenide These materials are currently being investigated, and have not been used mercially for photovoltaics. Concentrating Collectors By using a lens or mirror to concentrate the sun’s rays on a small area, it is possible to reduce the amount of photovoltaic material required. A second advantage is that greater cell efficiency can be achieved at higher light concentrations. To acmodate the higher currents in the photocells, a larger metallic grid is used. For example, in a system with a 22X concentration ratio, the grid covers about 20% of the surface of the solar cell. To prevent this from blocking 20% of the sunlight, a prism is used to redirect sunlight onto the photovoltaic material, as shown in Figure 3. A second problem is the higher temperatures of a concentrating system. The cells may be cooled with a heat sink or the heat can be used to heat water. Only direct sunlight, not scattered by clouds or haze, can be concentrated. Therefore, the concentrating collectors are less effective in locations
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