technology photovoltaics
Left: MicroLink’s solar cell arrays,which are made with its proprietary epitaxial lift-off technology,are an attractive option for powering electric unmanned aerial vehicles,thanks to their high efficiency,low weight and excellent flexibility an aerial mass density of less than 350 g/m2 power density exceeding 250 W/m2
,these arrays
have doubled the endurance of the
Raven.This small,hand-launched,remote-controlled aerial vehicle that is powered by a lithium ion battery can fly at altitudes of 10,000 feet to 15,000 feet and reach speeds of 60 miles per hour
.With and a
Technology published a paper on the fabrication of high-efficiency GaAs solar cells with a ‘peeled film’ technology. Progress in the 1980s and 1990s included efforts by Eli Yablonovitch from Bell Communications Research, who revealed the extreme etching selectivity of AlAs compared to GaAs. And further strides on epitaxial lift-off technology have been made during the last 15 years, with John Schermer’s group from Radboud University reaching a new level of understanding of the process, and devising new ways to increase the etch rate.
The performance of these devices, which are mounted on a flexible metal backing, is very promising. Measurements replicating the sun’s spectrum yield efficiencies for MicroLink’s best 1 cm2
cells of
31 percent in space (AM0) and 33.9 percent on the ground (AM1.5). Scale the cell size to 20 cm2
, and
efficiencies fall by just one percentage point. These high-efficiencies, combined with an incredibly low cell weight, make these photovoltaics attractive candidates for powering satellites. These cells are also flexible, so they can wrap around the topside of the wings of battery-powered, unmanned aerial vehicles (UAVs), increasing their endurance.
One organization starting to look at doing just this is the US Air Force Research Laboratory, and it is funding a project involving MicroLink. If this project has widespread success, it could do far more than just aiding the military – those working in the real-estate sector, for example, view UAVs as an attractive approach to surveying vast areas of land.
MicroLink’s cells could also be a competitive product for the terrestrial concentrating market, thanks to relatively low production costs that stem from multiple re-use of GaAs substrates, which just require a polish before they re-enter the MOCVD chamber. Using re-polished substrates makes no impact on device performance. “The substrate is a considerable fraction of the bill of materials for the device,” explains Chris Youtsey, Fab Director at MicroLink. “The limitations on how many times you can re-use the substrate come down to how much material you remove from the polishing and [the frequency of] breakages.”
Youtsey claims that a reasonable goal is to re-polish the wafer ten times. He points out that polishing removes as little as 10 µm, and says that thinning a GaAs substrate by 100 µm or so should have no impact on the deposition of a high-quality epitaxial structure. Even higher rates of substrate re-use are theoretically possible, but they produce ever diminishing returns. Enter this regime and the rewards do not justify the efforts.
Peeling it off The epitaxial lift-off technology that MicroLink uses is certainly not new. Reports of such efforts date back to 1978, when a Japanese team from Tokyo Institute of
However, epitaxial lift-off is still an intrinsically slow process, according to Youtsey: “It’s not seconds or minutes – it’s hours. So if you want to get high throughput, you have do it in batches.”
Switching to batch processing is one of the two big breakthroughs in epitaxial lift-off technology made by MicroLink Devices, whose team has a background in high-volume GaAs fabs, including TriQuint. Running in pilot product, MicroLink processed 1400 4-inch wafers in 2011, and it could increase this throughput substantially – each etching bath that it uses is capable of processing 1500 wafers per month. Youtsey says that another significant breakthrough made by his team is the simplification of the process – no longer are weights used to separate substrates and epilayers: “Our approach uses a proprietary support layer, which allows us to efficiently batch process.”
At MicroLink, photovoltaic production begins with MOCVD growth of an inverted metamorphic structure featuring InGaP, GaAs and InGaAs cells. This epitaxial stack contains a 5 nm-thick AlAs layer sandwiched between the substrate and triple-junction cell.
After attaching a thin, flexible metal carrier layer to the uppermost epitaxial layer, the resulting composite is immersed in a bath of concentrated hydrofluoric acid, which selectively dissolves the release layer – the etch
MicroLink forms its metamorphic cells on a GaAs substrate. Etching a sacrificial,AlAs layer about 5 nm-thick with hydrofluoric acid allows separation of substrate and epilayers
July 2012
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