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Rapid progress is being made in increasing the efficiencies of multijunction solar cells based on Ⅲ-Ⅴ semiconductors.In particular, Solar Junction has fabricated a three-junction solar cell with 43.5% efficiency at 834 suns, and Sharp Corporation has duplicated that feat.The bandgaps of the three junctions, 1/1.43/1.9 eV, are ideal for current-matching under the direct solar spectrum, as well as the entire stack being lattice matched.A commercial cell, operating at 400 suns, had efficiency > 41%, which decreased only slightly even at 1000 suns.However, the high cost of all existing multijunction solar cells requires the use of very-high optical concentration and thus expensive, extremely precise tracking systems and optical concentrators.Thus, the primary thrust has been to increase cell efficiencies so as to reduce those costs, but that has led to increasingly complex cell processing, such as the removal of the growth substrate.To achieve grid parity, the cost of the cells, the tracking and the optical systems must be dramatically reduced.Here, we propose an innovative way to achieving grid parity: the use of CdZnTe/CdTe/Si three-junction solar cells, manufactured using high-throughput MBE or a less expensive vacuum epitaxial technology and costing an order of magnitude less than the presently used three-junction Ⅲ-Ⅴ cells.These cells would have efficiencies similar to those of the highest-efficiency Ⅲ-Ⅴ multijunction cells but would cost far less because of using less expensive growth, a rugged, inexpensive active Si substrate and simplified cell processing.This would yield lower cell costs per kW of electric power produced, even with much lower optical concentrations, X-3d 100.These much lower optical concentrations would allow the tracking, optics and installation costs to be greatly reduced and simplify cell cooling, and would reduce light loss in the concentrator optics and allow the collection of diffuse light as well as direct sunlight.