elements optically coupled with a transparent encapsulant. This module configuration is shown in Fig. 6 (courtesy ARCO Solar). For higher specific power, it would be desirable to eliminate the intermediate layer by depositing the a-Si cell directly on the CuInSe2. An alternative technology, CdTe on CuInSe2, has shown about 8% AMO efficiency for a mechanically stacked prototype [46], A problem with existing CuInSe2 technology is that the current solar-cell structures use a heterojunction window layer which may not withstand the temperatures needed to directly deposit a second cell on top. Thus, either a technology must be developed to deposit the CuInSe2 after the top cell deposition, a low temperature top cell must be used, or a more robust window layer found. Cascade cells with amorphous silicon alloys for both top and bottom elements have
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