solar cells is aiming at larger cell area, higher efficiency, higher production speed and substrate technology in the field of amorphous solar cells. The results of the current development will not necessarily meet the SPS requirement, but fundamental technology will be applied for the improvement of the performance of solar cells for SPS. Thus the next goal of research and development of terrestrial-use solar cells should be the high-performance requirement of SPS solar cells. In this paper, the main properties of terrestrial-use solar cells will be examined from the standpoint of their applicability to the space environment, and the approach to higher-performance solar cells towards an extension of terrestrial-use solar cell development will be discussed. 1. Forecast of Development of the Terrestrial-Use Solar Cell In the 1970s, the main uses of solar cells were for satellites in the USA, and for independent power sources in remote areas in Japan. And the solar cell was thought of as a special electrical part which was used by particularly limited specialists. For example, terrestrial-use cells were installed in about 2,000 places in Japan in those days as telemeter power sources; less than 10 W per system was a typical example, and even the biggest solar system for a lighthouse had only 1 KW output. The cost was about ¥30,000 per watt and it was still very expensive. Nevertheless the solar system had enough merits by comparison to other power sources, such as the commercial power source which had to erect utility poles and run wires from a commercial power station, or the heavy batteries which had to be changed at intervals, or the diesel dynamo which required maintenance. Single Crystal Solar Cells With the SEG (semiconductor grade Si) single crystal wafer process used for large- scale integration (LSI), integrated circuit (IC) technology has been developed as a mass production process as the highest refined technology. The wafers used for solar cells do not require such high quality as those for LSI use, in the respect of physical characteristics and configuration accuracy, and so low-cost wafers can be supplied in accordance with the demand for LSI use if we use wafers out of specification or standard, or we miss out some processes. In Japan, 16.5% (AMI.5) efficiency solar cells fabricated from single crystal wafers have recently been sold in the market at a price of over ¥1,000 per watt, and the market size is about 4 MW per year. The demand for wafers for solar cells will increase rapidly, but cost reduction cannot be expected only from the additional mass production effect if we persist in the present high quality for single crystal wafers. The final cost of single crystal solar cells will be about ¥800 per watt, although the figure depends on the production quality. Solar Grade Si Materials and Substrate Manufacturing Technology The final limit of the cost reduction of a single crystal solar cell depends on the Si raw material and the substrate manufacturing technology. Generally there are two methods of material purification. One is to get polycrystalline Si by reduction through halogenation of Si, and the other is direct synthesis of SOG polycrystal by means of some purification process from metallurgical Si or SiO2 not through halogenation. In Japan,
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