Numerical Analysis of a Solar Cell with Tensile-Strained Ge as a Novel Narrow-Band-Gap Absorber

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A narrow-band-gap (0.6--0.7 eV) bottom cell absorber material is one of the most important and relatively undeveloped components for future five- or six-junction solar cells. Tensile-strained Ge is a promising material for a novel bottom cell absorber since it has a high absorption coefficient and an ``adjustable'' lattice constant. In this study, we numerically demonstrate the possibility of tensile-strained Ge as a bottom cell material for multijunction solar cells. The design examples of lattice-matched five-junction cells using tensile-strained Ge as bottom cells are also presented. It is shown that sub-μm tensile-strained Ge can produce same efficiency as that of a 100--300 μm bulk unstrained Ge cell. Using tensile-strained Ge as a bottom cell gives a higher efficiency and solves the lattice mismatch problem compared with traditional bottom cell materials such as a Ge substrate and a high In-composition InxGa1-xAs.
2011-04-25