Technology to manufacture amorphous silicon solar cells on lightweight thin substrates has been demonstrated, on thin polymer and metals by ECD [21] and on thin polyimide by 3M [36], and there is some interest in lightweight, high specific power amorphous Si arrays for space [21], Thin Poly crystalline Silicon A final thin-film technology which should be mentioned is thin polycrystalline silicon. Recently results of up to 12.6% AMO have been reported by a proprietary technique developed by Astropower [37]. Crystalline silicon is an indirect bandgap material and does not have the extremely high absorption constant typical of the other thin-film materials; consequently, a ‘thin' polycrystalline silicon cell is considerably thicker and heavier than other thin-film technologies. The silicon is deposited on a ceramic substrate; due to the high-temperatures typical of most silicon deposition processes it is not clear if it will ever be possible to produce the material on lightweight substrates. Nevertheless, future developments in this technology may make it of interest, especially for the bottom element of a cascade. 4. Radiation Tolerance of Thin-film Solar Cells Thin-film copper indium selenide solar cells have the highest radiation tolerance of any solar cell measured to date. Existing experimental data show no noticeable degradation in performance at 1-MeV electron fluences of up to 1E16 electrons/cm2, a dose equivalent to about 200 years of exposure at geosynchronous orbit if standard coverglass protection is assumed (in fact, the measured efficiencies actually improved slightly) [38], Under 1-MeV proton irradiation, the cells do show some loss of power; to about 90% after 1E12 protons/cm2, as shown in Fig. 3 (courtesy Boeing [39]). This represents about 50 times greater resistance to 1-MeV proton radiation than either Si or GaAs. The damage from the proton irradiation could be almost completely recovered by a low-temperature anneal. The cells exhibited 95% recovery of initial power after 6 min at 225°C. While it remains to be seen whether the high radiation tolerance will remain for future high-performance versions of the cell technology, this radiation tolerance is so extraordinary that the end of life (EOL) efficiency of even present-day CuInSe2 cells may out-perform conventional cell technologies in some high radiation orbits. Thin-film cadmium telluride cells have not, to date, been extensively tested for radiation tolerance. Preliminary results of 1-MeV electron irradiation, quoted by Zwiebel [40], show moderately high radiation tolerance with some loss of short circuit current but negligible loss of voltage or fill factor. All the degradation witnessed could be attributed to darkening of the glass superstrates used for the cells, which could be avoided by using radiation tolerant glass. Bernard et al. [41] also noticed little change in CdTe cell performance at 1-MeV electron fluence of up to 3E16/cm2. Amorphous silicon cells from Arco Solar exposed to 1-MeV electrons degraded from 8.57% AMO to 8% at 1E15 electrons/cm2 [38]. The efficiency dropped to 0.95% at 1E16 electrons/cm2. The damage could be almost completely removed with a low temperature anneal; the cells showed 97% recovery after a 15 minute treatment at 175°C.
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