A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell is shown to provide current-matching of all three subcells and thus composes a device structure with virtually ideal band gap combination. We demonstrate that the key for the realization of this device is the improvement of material quality of the lattice-mismatched layers as well as the development of a highly relaxed Ga1-yInyAs buffer structure between the Ge substrate and the middle cell. This allows the metamorphic growth with low dislocation densities below 10(6) cm(-2). The performance of the approach has been demonstrated by a conversion efficiency of 41.1% at 454 suns (454 kW/m(2), AM1.5d ASTM G173-03)
GaxIn1−xAs and GayIn1−yP layers were grown lattice mismatched to GaAs and Ge by low-pressure metal organic vapor phase epitaxy (LP-MOPVE). These materials are very promising for further increasing the efficiency of monolithic triple-junction solar cells. Different buffer layer structures were realized. Transmission electron microscopy and x-ray diffraction analysis were used to characterize the quality of the crystal. Both linear and step-graded buffers in GaxIn1−xAs were successfully used under an active solar cell structure. GayIn1−yP as buffer material showed a worse performance. Excellent solar cell performance was achieved for lattice mismatched single-, dual- and triple-junction solar cells.
Defect formation and strain relaxation in step-graded GaAs 1−x N x and GaAs 1−y P y buffer structures grown by metal-organic vapor phase epitaxy on GaAs͑001͒ substrates have been investigated by transmission electron microscopy and high-resolution x-ray diffractometry. From the comparison of different buffer concepts, it is shown that, by introducing intermediate GaAs 1−x N x layers with N concentrations x ജ 2% into a GaAs 1−x P x buffer structure, dislocation formation and strain relaxation are effectively suppressed during subsequent growth of layers with tensile strains. It is argued that a similar concept, however, modified by using layers of differing alloy composition, can be used for layer systems with compressive strains. Appropriately alloyed intermediate dilute nitride layers appear to offer a powerful concept for engineering defect distributions and layer strain in semiconductor technology.
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