a. Efficiency Increase The maximum theoretical efficiency of a silicon solar cell is about 22%. The most widely used single-crystal silicon solar cells can routinely reach an efficiency of 11% and efficiencies of up to 16% have been reported (13). Development-programs to increase the efficiency of silicon solar cells up to 20% have been outlined (14). The silicon solar cell which is produced from single-crystal silicon is typically arranged with the P-N junctions positioned horizontally. More recently, vertically illuminated multijunction silicon solar cells have been investigated (15). These have the potential to reach higher efficiencies and to be more resistant to solar radiation damage. Solar cells made from single-crystal gallium arsenide exhibit an efficiency of about 14% with a theoretical limit of about 26%. Recently, a modified gallium arsenide solar cell was reported to have reached an efficiency of 18% (16). This substantial increase in efficiency is particularly significant, because these cells can operate at higher temperatures than silicon solar cells, are more radiationresistant and can be prepared in thicknesses about one-tenth that of a silicon solar cell. Several other materials may be suitable for photovoltaic solar energy conversion. Among these are various combinations of inorganic semiconductors which have only partially been investigated. Organic semiconductors which exhibit the photovoltaic effect and which do not have known boundaries to the theoretical efficiency also remain to be explored, so that their potential for photovoltaic solar-energy conversion can be established (17). b. Weight Single-crystal silicon solar cells are presently 500 to 1000 microns thick, although their thickness could be reduced to about 50 microns without compromising efficiency. However, gallium arsenide cells need be only a few microns thick. The individual solar cells have to be assembled to form the solar collector. The weight of a solar cell array can be reduced by assembling the solar cells in a blanket between thin plastic films, with electrical interconnections between individual cells obtained by vacuum-depositing metal alloy contact materials. The collector weight can be further reduced when solar energy concentrating minors arranged to form flatplate channels are used so that a smaller area of solar cells is required for the same electrical power output (see Figure 3). The weight and cost of a given area of a reflecting mirror used to concentrate solar energy are considerably less than those for the same area of solar cells. Suitable coatings on mirrors to reflect only the component of the solar spectrum most useful for photovoltaic conversion can reduce heating of the solar cells and thus increase efficiency. There is a balance between concentrating the solar radiation per unit area of the cell, which may lead to a rise in temperature and a consequent decrease in solar cell efficiency, and the desire to maximize the collection of solar energy. An array configuration that includes mirrors with a concentration factor of about 2 has been chosen for the solar collector.
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