subject the cells to a sequential thermal annealing in orbit in order to minimize solar cell degradation. The annealing temperature will be in the order of 500°C. SOLAR CELLS — STATUS AND DEVELOPMENT TRENDS Actual solar arrays are mostly manufactured from approximately 10 cm2 solar cells. 2x4 cm2 and 2x6 cm2 solar cells with a thickness of about 200 gm have become the standard cell-types for most of the modern space arrays. Two different types of silicon base material are commonly used, depending on the particle environment of the specific space missions. The low ohmic cells (1-2 O cm) have a better beginning-of-life (BOL) efficiency than the high-ohmic cells (10-25 fl cm) but they are more sensitive to particle radiation. Large quantities of 1 (Fem cells are economically manufactured with a typical minimum-average efficiency of 13%, while the corresponding efficiency of 10 (1-cm cells is 12%. A 30-year mission in geosynchronous orbit would degrade the performance of 1 (1cm cells by 45%, and the 10 (1cm cells would suffer from a 35% degradation with a resulting end-of-life (EOL) efficiency of 7)! = 7.2% (EOL) 7)10 = 7.8% (EOL). This shows that the EOL efficiency has to be increased by 10% absolute in order to meet the reference system requirement of 17.3%, i.e., the EOL output must be more than doubled. The interconnection of individual solar cells to large solar cell modules is either done by soft soldering or by welding. Both technologies are used today. In order to be weldable the solar cells need a special contact system. This contact system must be passivated against corrosion and must be optimized for the different parameters of the welding process. Due to the excellent characteristics of welded solar cells this technology has been proposed for the SPS solar array. This technology has already proven its outstanding reliability in many space programs. The quality of the welds is extremely reproducible. Welded solar cell modules are extremely temperature resistant and compatible with a large cycle lifetime. The fact that the welding technology for solar cells can be automated for reliable mass production is of major importance for the SPS solar array. AEG-TELEFUNKEN have introduced this technology in 1970 for the solar array of the sun probe HELIOS which had to withstand temperature extremes up to 220°C. From that program all succeeding solar arrays were manufactured with this welding technology at AEG-TELEFUNKEN. These programs represent a total number of more than 500,000 solar cells. The large cycle-lifetime of welded solar arrays has recently been demonstrated for the Space Telescope solar array. Representative solar cell modules have been subjected to 30,000 thermal cycles with temperature extremes -90 to +100°C and passed this test successfully. For comparison, the SPS solar array must only survive 3000 cycles during 30 years operation. The compatibility of the welding technology with mass production, which is an essential factor for the SPS, is demonstrated by AEG-TELEFUNKEN's production line for terrestrial so'ar array. Solar cell modules from single-crystalline cells with dimensions up to 5x5 cm2 and from polycrystalline cells with dimensions up to 10 x 10
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