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High-efficiency multijunction solar cells
The efficiency of a solar cell can be increased by stacking multiple solar cells with a range of bandgap energies, resulting in a multijunction solar cell with a maximum theoretical efficiency limit of 86.8%. III-V compound semiconductors are good candidates for fabricating such multijunction solar cells for 2 reasons: they can be grown with excellent material quality; and their bandgaps span a wide spectral range, mostly with direct bandgaps, implying a high absorption coefficient. These factors are the reason for the success of this technology, which has achieved 39% efficiency, the highest solar-to-electric conversion efficiency of any photovoltaic device to date. This article explores the materials science of today's high-efficiency multijunction cells and describes challenges associated with new materials developments and how they may lead to next-generation, multijunction solar cell concepts. In these structures, the material quality is more important than the exactbandgap combination in achieving very high efficiencies. GalnP/GalnAs/Ge device efficiencies are nearing 40% and have attracted interest concentrator systems. A number of advanced ap-proaches include solar cells that integrate new materials (such as the dilute nitrides), accommodate lattice-mismatched growth, and make use of today's most advanced technologies for wafer manipulation. The challenge is to achieve the necessary quality in a config-uration that makes optimal use of each material. The solar electric conversion efficiency is expected to pass 40% soon and to move toward 50% in years to come. Concentrators will be the platform for making these high-efficiency technologies cost-competitive and enabling continuous cost reductions in the future.