Silicon solar cells with up to 20% conversion efficiency have been demonstrated in the laboratory. These cells are not yet space qualified and not currently available in the market. Advantages of silicon cells are that large area cells are available (8 by 8 cm cells are being manufactured for Space Station Freedom), the array manufacturing technology is well developed, and the technology is well characterized for vibration, thermal-cycling, and other environmental loads of the space environment. GaAs cells have higher efficiency than silicon cells. GaAs cells currently available on the market have an average conversion efficiency of 18.5%. Efficiency of 21.5% has been achieved in the laboratory. Gallium arsenide cells are smaller and more brittle than silicon cells, but the technology is being rapidly developed. Gallium arsenide cells are currently heavier than silicon cells. Several technologies under development will make GaAs cells much lighter in weight [5]. The most well-developed of these technologies is CLEFT [6], where an extremely thin (5 micron) large-area cell is separated from a single-crystal substrate. An important measure of power system performance is the specific power (power output per unit mass). Note that this is typically specified for Earth orbit conditions. Operation at the Mars surface will decrease the specific power by the ratio of Mars sun intensity to Earth orbit intensity. Note that it is possible to measure specific power at the cell level, at the blanket level, at the array level, or at the power system level. Specific power at the cell level does not include array structure and is many times higher than array level specific power. At the blanket level, specific power includes the coverglass, interconnections, and the backing material, but not the array structure. This may be an appropriate figure of merit if a flexible or semi-flexible array is to be simply unrolled horizontally onto the Mars surface without support structure. Specific power at the photovoltaic array level (including array structure) for the best arrays developed to date are shown in Table 1. For currently designed space power systems, the photovoltaic blanket weight is only about a quarter of the total power generation system mass (excluding storage). This is shown in Table 2 for the space station Freedom solar array. The array structure and the power management and distribution (PMAD) system account for three-quarters of the power system mass. This provides a powerful incentive to develop new and more efficient PMAD systems and to design new array structures to take advantage of the ultralight blankets. The second approach to photovoltaic arrays is to use a thin layer of photovoltaic material deposited onto a flexible substrate [7]. In the 1980's considerable research was devoted to development and commercialization of thin-film photovoltaics for terrestrial power generation. Thin-film solar cells consist of thin (—1-5 pm) films of photovoltaic material deposited on a supporting substrate. This approach has lower conversion efficiencies, but due to the low amount of the active material used, has the potential for high specific power.
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