is typically 0.25 in thick glass; cheap and rugged but not light. There is little or no research on alternative, lightweight substrates. Advantages of thin-film solar cells are: • high radiation tolerance; • high specific power; potentially in the kilowatt/kilogram range; • large area solar cells with integral series interconnections; • flexible blankets; • large (by spacecraft standards) body of array manufacturing experience. The disadvantages of thin-film solar cells are: • lower efficiency; • lack of spacecraft experience; • not currently produced on lightweight substrates. Experimental measurements on thin-film solar cells are almost always quoted for a solar spectrum filtered by passage through the atmosphere (Air Mass 1.5, or AMI.5 spectrum). Almost no measurements have been made of cells under the space (Air Mass Zero, or AMO) spectrum. Efficiency measured under space sunlight is lower than that under terrestrial sunlight because most of the added energy available in space is in the infrared and ultraviolet regions, to which solar cells are generally not very responsive. The conversion factor from AMI.5 to AMO efficiency is typically a reduction of 15-20% for cells with bandgaps in the range of 1 to 1.5 eV, varying slightly with the spectral response of the solar cell in question. For example, for one amorphous silicon cell discussed in the literature [21], conversion of AMI.5 efficiency to AMO is by a multiplicative factor of 0.80. In this paper we have converted all efficiency figures quoted at AM 1.5 to AMO using an assumed conversion factor of 0.80. While thin-film technologies have not yet been demonstrated in space, there is a very large (by space standards) manufacturing base on the Earth: tens of megawatts per year for a-Si, a rapidly increasing capability of perhaps one megawatt per year for CuInSe, and around a hundred kilowatts per year for CdTe. Very little actual flight experience is available on thin-film cells. CuInSe and a-Si cells are both now flying on the LIPS-III satellite [22], Table I summarizes the historical and projected efficiency of some of the most important solar cell types.
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