technology photovoltaics
Bigger and better MicroLink has also fabricated 61 cm2
cells from 6-inch
GaAs wafers. Part of the motivation behind producing such large cells is to demonstrate the quality of MicroLink’s process. “But there is interest, in terms of panels assembly, in working with large cells: Fewer part counts, fewer interconnects,” explains Youtsey.
Processing begins with the growth of an AlAs release layer and a solar cell structure on a GaAs
substrate.After etching in hydrofluoric acid the GaAs substrate is re-polished and used again,while the solar cell and back metal composite is temporarily bonded to a carrier wafer for device processing
Scaling cell size also appears to have no impact of device performance – typical conversion efficiencies of 29 percent were recorded for 1 cm2
cells and 20 cm2
equivalents, which were both measured at NASA Glenn using the AM0 spectrum.
Flexible solar sheets have been produced at MicroLink by interconnecting 30 large cells with silver-based foil ribbons and laminating the structure between transparent sheets to yield flexible solar sheets. The resulting composite is highly flexible, and can wrap around curved structures, such as the wings of solar- powered planes. One great attribute of this solar sheet is its incredibly high specific power, which exceeds 400 W/kg.
According to Youtsey, triple-junction germanium cells with a 150 µm thickness are at least four times as heavy: “From an application point of view, that is a big deal.” In addition, cells formed on germanium substrate are very brittle. “You have to build a wafer fab to accommodate them,” explains Youtsey.
Today, however, MicroLink's primary focus is not on making bigger and bigger cells. Instead, it is setting its sights on increasing efficiency and streamlining process flow. Success will simplify ramping to production volumes and it will also increase yield. To boost efficiency, efforts will focus on increasing the open- circuit voltage and fill factor. Improvements in material quality will underpin these programmes. Further efficiency gains could result from increasing the number of junctions to four or five, but Youtsey acknowledges that this step is not easy to execute in high-volume production.
MicroLink is currently in pilot production, and it plans to progress to full commercialisation within the next two years. When it hits that milestone, customers of multi- junction cells will have some head-scratching to do. Up until that point, selecting a device has been based on efficiency, reliability and price, but from then on factors such as weight, flexibility and robustness will come into the play.
© 2012 Angel Business Communications. Permission required.
Two 61 cm2 triple-junction cells formed by applying MicroLink’s epitaxial lift-off process to a 6-inch GaAs wafer
A 4-inch film produced by MicroLink’s epitaxial lift- off process features two,20 cm2 flexible metal backing
solar cells on a thin,
Thermophotovolatics MicroLink’s epitaxial lift-off process is an attractive approach for making any type of device that has a large proportion of its cost associated with that of the substrate. One such example is thermo-photovoltaics, devices built on InP substrates that convert heat into electricity. Reducing the number of InP substrates needed to make thermo-photovoltaics could lead to massive cost savings, because InP substrates are nearly an order of magnitude more expensive than GaAs equivalents.
July 2012
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