A large on orbit demonstration of gallium arsenide cells has already flown on Uo-sat as a part of the on orbit technology demonstration programme. These first generation cells are not particularly useful in most applications, since they are about five times as heavy as equivalent silicon cells. Interest in these first generation cells is limited to military satellites, and spinning satellites with body mounted cells, which require a high areal conversion efficiency. These two types of missions do not appear at present in the European mission scenario. This scenario emphasizes the need for low mass deployable solar arrays. In order to be mass competitive at the subsystem level with advanced silicon cells, gallium arsenide cells must be thinner than 100 microns. The OMCVD production process has the potential of producing appropriate advanced gallium arsenide cells, while the more established LPE process appears to have insurmountable limitations. R&D efforts in all major European laboratories are now focussed on second generation gallium arsenide cells. Gallium arsenide on germanium is seen as an intermediate step towards a lightweight cell with over 20% efficiency. Most current effort is spent on putting III-V compound cells on silicon. The serious thermal expansion and lattice spacing mismatches between silicon and the usual III-V compounds have posed serious obstacles to progress in the past, but many European, Japanese and American workers are developing their own "secret recipes" to overcome these difficulties. Longer term development activities are focussed on tandem cells, cells for concentrated light, and alternative III-V compounds such as indium phosphide. Tandem cells (which have a high band gap junction lain over a lower band gap junction) show better efficiency at room temperature, but many specific designs lose the low band gap junction contribution at the temperatures characteristic of operation in orbit. Bifacial tandem cells where the rear side junction is matched to the spectrum of light reflected from the Earth are of special interest for LEO applications. Relatively heavy cells, optimized for high conversion efficiency, which cannot be used in a planar configuration due to mass and costs constraints can be used with high intensity light concentrators. Twenty five to thirty percent efficiency is the development target for such cells for the late nineties. Successful application of these devices requires that major problems relating to concentrator configuration be solved at the solar array system level. The European gallium arsenide technology program is directed towards developing lightweight second generation cells in labs in Italy, Germany and Great Britain, in line with ESA policy as described above. Lightweight gallium arsenide cells have the potential to replace silicon cells on most future European missions. Significant development efforts will be required to reduce production costs. Several cell design requirements are controlled by later integration steps (e. g. bonding to the substrate, interconnection, glazing). There is no European experience in this area with gallium arsenide. Since these requirements strongly influence cell design, a gallium arsenide module technology program was begun in
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