Dr. Frank Dimroth and his team made a metamorphic triple-junction solar cell that reached a record efficiency of 41.1%. To accomplish this, researchers created a multi-junction solar cell made of III-V compound semiconductors by stacking three single-junction cells on top of each other, each one particularly efficient in converting a limited spectral bandwidth of sunlight to electricity. The result: efficiency levels that nearly double the efficiency of conventional silicon-based solar cells.
A multi-junction solar cell is created with the aid of processes similar to those used in the semiconductor industry. "Our work involves a modern epitaxial process known as metal organic vapour phase epitaxy," Dr. Dimroth explained. The process involves successively depositing solar sub-cells on top of each other on a substrate of germanium. The result is a wafer-thin solar cell structure just a few µm thick, with a well-hidden complex inner structure of up to 50 monocrystalline layers. With the development of metamorphic crystal growth, Frank Dimroth and his colleagues have made it possible to use a larger range of III-V compound semiconductors to grow multi-junction solar cells. This makes the solar cells better adapted to the spectrum of wavelengths found in sunlight.
multi-junction solar cells have been used in space technology to supply satellites with energy. To tap into the high potential efficiencies for regenerative power generation here on earth, Frank Dimroth and his colleagues came up with a special design: They developed a photovoltaic concentrator module that uses Fresnel lenses to concentrate solar radiation by a factor of 500 onto triple-junction solar cells only 3 mm2 in area. This reduces the costly semiconductor surface area required and makes III-V multi-junction solar cells for electricity generation an attractive alternative in regions rich in direct sunlight. Prof. Eicke R. Weber, Director of Fraunhofer ISE, is convinced: "We expect that high-efficiency concentrator technology - in addition to photovoltaics using crystalline silicon and the classic thin-layer technology - will become established as a third technology for cost-efficient generation of solar electricity in the sunny regions of the world."
The combination of highly efficient cells with a lens amplifier ensures that - in comparison to conventional solar modules - only one five-hundredth of the semiconductor surface will be needed. The cells within their own specially designed concentrator modules measure only three square millimeters in size. A Fresnel lens is situated over these mini solar cells, at a distance of approximately ten centimeters. This configuration concentrates the sunlight by a factor of 400 to 500. To prevent the cells from overheating, they are attached to a copper support that distributes the heat sufficiently well. Thus, passive cooling of the solar cells suffices. „Thanks to this construction, we were able to produce modules with a degree of efficiency in excess of 29 percent," says Dr. Andreas Bett.
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