the ground, and against great stress during launch and mission life in space. Terrestrialuse solar cells have the contacts formed by mass production low-cost screen patterning, because it is packaged in a strong humidity-proof structure, although it is exposed to severe outdoor weather. Although the SPS cell which aims for economic advantage should basically apply the process used for terrestrial cells, new developments in materials and process technology are likely to be necessary, because the usual process for terrestrial-use cells brings disadvantages in electrical output, reliability and mechanical characteristics. Radiation Shield The present space-use solar cell uses the expensive cover glass for a radiation shield. For the large area of an SPS array, it is necessary to develop new materials for the radiation shield which are cheaper and processed more quickly. As one idea, the solar array may be covered with a transparent resin film which has superior radiation resistance. Application of Amorphous Si Cells for SPS Amorphous Si cells deposited on glass substrates are effective, because the substrates act as a shield from irradiation. However, these cannot be used if the following problems are not solved. The problems of amorphous Si cells are the low efficiency mentioned above and degradation by sunlight. The effective thickness of amorphous Si cells is very thin, less than 1 /zm, so it is thought that amorphous Si cells have good radiation resistance because of their small absorption. But we have not yet achieved good results in our preliminary investigation by comparison to normal Si cells. It is thought that a-Si has smaller activation energy than crystalline Si, sufficient to confuse the atomic structure, like photon degradation. It is also thought that a lightweight array for space will be realized only when a-Si cells with lower degradation against irradiation are developed. There is a report that 1,620 watt per kg in the power-weight ratio of a-Si cells deposited on a flexible blanket (for example, Kapton) has been realized. This is five times more than the 300 watts per kg of one made of crystalline Si [2], When a-Si is suitable for space use and a large-area sheet array is required, it will become unnecessary to carry solar cells fabricated on the ground into space, because they will be able to be made in space where a high vacuum is readily available [3]. 4. Cost Forecast of Solar Cells for SPS From the above, the calculation of the expected costs of SPS solar cells at production capacity of 10 GW is shown in Fig. 5, compared with that for terrestrial-use solar cells. In this Figure the cost is shown according to each process for material, substrate and cell fabrication. As for a-Si, the cost of the cell substrate is not contained, because array substrate is used as the cell substrate at the same time. Crystalline solar cells for SPS are considered to increase costs because it is necessary to improve the conversion efficiency in comparison to terrestrial-use cells. A crystalline solar requires, for example, high purification of source material, thinning of Si substrate, a shallower junction and fine pattern contact formation, and development of the contact material to improve its reliability and resistance against the space environment, and of radiation shielding materials. A-Si cannot be free from the cost-raising factors of a new venture,
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