The single crystal silicon cells are the photovoltaic devices with which the most experience has been made during the spacemissions of the recent two decades. The development state is rather high and 19.7 % efficiency has been achieved with special small-area experimental cells under laboratory conditions. Since the solar collector of a SPS is the heaviest part of the satellite, the cell weight is a very important factor. In the case of silicon, the minimum weight has been reached with the minimum thickness of 0.05 mm for optimum efficiency. Gallium Arsenide has the potential of weight savings, but this is not as much as one might expect because of its relatively high specific gravity of 5.32 compared with silicon which is 2.34. But another advantage of GaAs over Silicon makes it the most serious contender to displace Silicon as the prim photovoltaic material in the future: it has the potential of higher efficiencies - more than 20 % have been achieved under laboratory conditions - it has a lower temperature sensitivity, and its radiation resistance is much better. Disadvantages of GaAs are the facts, that this semiconductor material is binary compound and therefore more difficult and expensive to produce, and that the availability of Gallium is very scarce. Recent approaches in replacing the single crystal Silicon cells by polycrystalline or amorphous ones have shown encouraging results. Efficiencies of more than 10 % have been achieved. Two additional novel devices for photovoltaic power converters are the photoemissive cells and the beam splitter generators. It is expected to achieve efficiencies of 30 % and higher by these more complex devices with considerably higher masses. 1.2.1.4 Summary Fig-2.1.1: SILICON SOLAR CELL CROSS SECTION [ Source: Boeing ]
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