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25 February 2008


IMEC claims record 24.7% efficiency for single-junction GaAs-on-Ge solar cell

As part of the ESA-IMAGER project, independent research center IMEC of Leuven, Belgium says that it fabricated a single-junction GaAs solar cell on a Ge substrate with what it claims is a record conversion efficiency of 24.7%, with an open-circuit voltage (Voc) of 999mV, a short-circuit current (Jsc) of 29.7mA/cm^², and a fill factor of 83.2%, as measured and confirmed by the US National Renewable Energy Laboratory (NREL).

IMEC achieved the efficiency on a single-junction GaAs cell measuring 0.25cm^² grown epitaxially on a Ge substrate with an improved micro-defect distribution. Materials technology group Umicore produced the Ge substrate through an optimized manufacturing technology, aimed at improving the intrinsic germanium crystal quality.


Picture:IMEC's single-junction GaAs solar cell on Ge substrate.


IMEC says that improving the efficiency of this single-junction GaAs cell is a further step in the development of a hybrid monolithic/mechanically stacked triple-junction solar cell (consisting of stacks of solar cells of different semiconductor materials, chosen to absorb different wavelengths of the solar spectrum as efficiently as possible.

Among the many possible combinations, IMEC focuses on stacked cells consisting of top cells with III-V materials and bottom cells made from Ge. With this combination, it is targeting a conversion efficiency of 35% and above.

The resulting stacks can be used in both satellite-based photovoltaic cells and terrestrial concentrator photovoltaic cells (CPVs), where high-efficiency energy conversion is paramount.

Graph: Performance characterization, measured by NREL.

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