typically manufactured by vacuum vapor deposition onto thin plastic films, but characteristically these obtain only low efficiency (up to perhaps 10 percent). Further study of various thin film systems may indicate advantages for their use on SPS due to the potential low cost, low weight, and simple construction. Figure 7-5(c) depicts the solar cell selected for this study — a thin silicon cell with wraparound contacts, low area surface contacts, and some form of textured surface to improve optical performance. Cells typical of this design (constructed in COMSAT Corp, laboratories and called ''nonreflective" or ''black" cells), have recently shown conversion efficiencies in the 15 percent range. Further advances are expected in the future. Solar cell performance is adversely affected by increased cell temperature. It is desirable to prevent that part of the solar spectrum unusable by solar cells from being absorbed in the cell and heating it. Figure 7-5(d) is a plot of the characteristics of an ''optical bandpass" filter that performs this function. Only the spectral region capable of exciting the photovoltaic process is passed; ultraviolet and infrared radiation are absorbed or reflected by the filter. The filter may be integrated into the reflector surface or the solar cell and cover, or may be on all surfaces. Such filters are composed of fractional wavelength thick layers of various material applied by vacuum vapor deposition. An optical filter is sensitive to the direction of impinging light, but the SPS array pointing is adequate to prevent detuning of the filters. The manufacturing technology for optical filters is well known, being applied to virtually all optical surfaces on a commercial basis, but application to the large SPS surfaces will require study and development of automated (possibly in space) manufacturing techniques. 7. 1. 2. 2 SOLAR ARRAY FLEXIBLE SUBSTRATES The plastic composite substrate selected for the baseline SPS solar array is composed of Kapton, a polyimide film, and FEP fluoro ethylene, such as Teflon. These materials have received extensive study for space applications, and studies of space station and electric propulsion arrays have selected them also. Present technology enables fabrication of printed circuitry on such substrates, much like the circuits needed for solar cell interconnection. Small arrays using this construction have been operated. For the SPS, a light low cost interconnection method is desirable. This indicates that welding methods may be preferable to the soldering methods most commonly used in the past. The HELIOS solar probe used an array with automatically welded interconnections, and other projects have used semiautomated or fully automated interconnections on a small scale. These techniques form a basis for the study and development of an SPS oriented assembly process.
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