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)

Improving costs and scale reflect looming opportunities

This work presents efforts to simulate numerically the current voltage (IV) curve of a III–V based Esaki tunnel diode. Using a tunneling model, which takes into account the full nonlocality of the barrier, a good agreement between measured and simulated IV curves of a GaAs tunnel diode was achieved. The influence of a series resistance effect as well as the importance of trap assisted tunneling (TAT) could be shown. In addition, we present a two‐dimensional model of a III–V multi‐junction solar cell including the numerical model of the investigated Esaki tunnel diode. Copyright © 2008 John Wiley & Sons, Ltd.

GaInP/GaAs//Si solar cells with three active p-n junctions were fabricated by surface activated direct wafer bonding between GaAs and Si. The direct wafer bond is performed at room temperature and leads to a conductive and transparent interface. This allows the fabrication of high-efficiency monolithic tandem solar cells with active junctions in both Si and the III-V materials. This technology overcomes earlier challenges of III-V and Si integration caused by the large difference in lattice constant and thermal expansion. Transmission electron microscopy revealed a 5-nm thin amorphous interface layer formed by the argon fast atom beam treatment before bonding. No further defects or voids are detected in the photoactive layers. First triple-junction solar cell devices on Si reached an efficiency of 23.6% under concentrated illumination.

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