technology photovoltaics
selectivity of AlAs relative to the GaAs epitaxial structure exceeds 105
. After etching for 12 hours or
so, the metal carrier and solar cell epilayers are completely separated from the GaAs substrate. Engineers at MicroLink mount these epitaxial lift-off foils to a temporary, rigid carrier, so that they can process the wafer into devices. This involves evaporation and lift- off of a metal ohmic contact, wet etch isolation, evaporation of an anti-reflection coating made from a bilayer dielectric stack, and dicing of the processed wafer into individual devices. Following that, the solar cells are removed from the temporary carrier.
Epitaxial lift-off does not degrade the material quality of the cells. Transmission electron microscopy images from the National Renewable Energy Laboratory fail to uncover any delamination, cracking, threading dislocations or voids in the cells. An absence of cracks and defects holds the key to the fabrication of cells with very large areas.
A better base
MicroLink’s metamorphic structure features: An InGaAs bottom cell with a bandgap that can be tuned from 0.9 eV to 1.1 eV; a 1.42 eV GaAs middle cell; and an (Al)InGaP top cell with a bandgap that is adjustable from 1.88 eV to 2.00 eV. “I think that’s pretty close to optimum,” claims Youtsey. To evaluate the impact of re-polishing on device quality, engineers at Micolink have compared the efficiency of 1 cm2
cells formed on 25 substrates that were initially pristine, and then
After a flexible metal carrier is attached to the solar cell epi-structure,the GaAs substrate is removed by etching in hydrofluoric acid
re-polished once, twice, three and then four times. Their conclusion: The average cell efficiency on the three re-polished device populations is comparable to that of cells grown on original, prime GaAs substrates.
If these cells are to be used in space applications, they need to be able to withstand bombardment from various forms of radiation. Youtsey believes that the cells can meet this demand: “The radiation hardness of individual epitaxial lift-off junctions, such as InGaP, GaAs and InGaAs, follows the expected trends for these materials.”
24
www.compoundsemiconductor.net July 2012
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