and performance of solar cells made from these materials are still comparatively uninvestigated. CuGaSe2, with a bandgap of 1.7 eV, and CuInS2, with bandgap 1.5 eV, are also promising choices, as discussed in the previous section. Amorphous silicon, with an effective bandgap of around 1.6 to 1.7 eV, may also make a good choice. Alloys with Ge, Sn, SiC and SiN can tailor the bandgap as necessary. Amorphous materials have the advantage that tunnel junctions are relatively easily formed. The efficiency and lifetime of these materials require improvements to allow them to be used for efficient elements in cascades, however, it should be noted that intensive research into amorphous silicon alloys is in progress. While mechanically stacked modules are easier to build, for high specific power, arrays will probably require monolithic construction. 6. Applications Future thin-film solar cells are likely to have greatly increased specific power at the solar cell level compared to conventional technology solar cells. Table II compares existing and projected efficiency for the best single crystal and thin-film cells (where ‘current' means for the best cells achieved in the lab, not for cells currently manufactured into space arrays). Table III shows these figures converted into specific power at the cell level. These specific powers are for the cell only, not including the radiation shielding, interconnections, support layers, array structure, etc., all of which are major contributors to the actual mass. It must be noted that cell mass is only a small component of the array mass, and thus of array specific power.
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