also been demonstrated. Energy Conversion Devices (ECD), has reported an efficiency of 10% measured at AMO for a three junction, two bandgap cell [47]. Future Bottom Cell Materials. CuInSe2 is nearly ideal for the bottom cell for a cascade. The bandgap of CuInSe2 can be modified by alloying with related I-III-VI2 materials; for example, CuInTexSe(2_x), will have a lower bandgap, with x selected to form the bandgap required; a higher bandgap material can be formed in CuGaxIn(1_x)Se2. This may be important for monolithic cascades requiring current-matched cells. Mercury-Cadmium Telluride, with a bandgap continuously variable from 0 to 1.5 eV, is also a good candidate for a bottom cell. Other materials for bottom cells are polycrystalline silicon and crystalline silicon. Top Cell Materials. The optimum material for the top cell of a two-element cascade would have a bandgap near 1.75 eV. Of the wide-bandgap thin-film solar cell materials, CdTe is the most well developed. The bandgap of CdTe, 1.5 eV, is slightly low for an optimum cascade. For a current-matched cascade this could be remedied by use of a ‘perforated' cell; alternatively, a bottom cell (for example, HgCdTe) with correspondingly lower bandgap could be used and the small penalty for off-optimum performance accepted. The related ternary alloys with Mn, CdxMnu_x)Te; Zn, CdxZn(1_x)Te; or Se, CdTexSe(1_x), could be used to increase the bandgap to closer to optimum [33]. A related wide bandgap material is cadmium selenide, CdSe [48]. Electronic properties
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