NEWS ANALYSIS
this ‘sub-cell’ to an underlying GaAs multi-junction solar cell. This move will side-step the lattice mismatch issues that prohibit InGaN growth on GaAs structures, and could catapult overall device efficiency to a breath-taking 50 percent or more.
As Nathan Young from the Materials Department at UCSB explains, the InGaN/GaN solar sub-cell produces a relatively small current, and efficient current matching with the underlying junctions simply isn’t possible. Consequently the bonding layer – a transparent and non-conducting polymer – is critical to isolate the this cell from the underlying multi-junction cell.
“There’s no electrical connection [between the junctions], we’re just keeping the device optically coupled,” he says. “We basically place the InGaN/GaN sub-cell onto the multi-junction cell and bond it with a transparent polymer. We then take out an extra contact so we can link the structures electrically.”
Proposed cross-section of the device structure including contacts and optical coatings
According to Young, while the additional contact introduces an extra step to device manufacture, depositing one more epitaxial, current-matched junction simply isn’t an option. As he explains: “This inability to current match an InGaN cell with a 2.65 eV bandgap is unavoidable... InGaN is the only material that can perform efficiently at this wide bandgap.”
Broadband coatings A crucial part of the research has been to add optical coatings – as is standard practice in today’s multi-junction cells – to the InGaN/GaN cells. As Young highlights: “Before [coatings] were added to the cell, it just wasn’t performing that well as it wasn’t absorbing enough light.”
In this case, the challenge was to develop coatings that exhibited anti- reflection properties across sufficiently wide wavelength ranges, and, for the first time, apply these to InGaN/GaN cells.
“We’re trying to get as much light as possible into our device, without sacrificing any light that would not be absorbed by the InGaN multi-quantum well layer,” says Young’s colleague, Emmett Perl, from the Department of
Left: Concentrated photovoltaic systems: Will next generation multi-junction cells take efficiencies over 50%?
Electrical and Computer Engineering at UCSB. To this end, the team designed a system of high performance broadband optical coatings – a front- side anti-reflective coating and back-side dichromic mirror – for the InGaN/GaN solar cell. These actually minimise front surface reflections across the broad spectral range while maximising rear surface reflection only in the spectral range absorbed by the InGaN. The dichromic mirror also allows for the InGaN/GaN cell to absorb additional light on a second pass.
So far, the results are good. Application of the coatings increased the peak external quantum efficiency of the InGaN/ GaN cell by 56 percent, and conversion efficiency by 37.5 percent, relative to an uncoated structure.
The team is now looking to boost InGaN device efficiency. Young reckons the sub-cell will need to be at least 50 percent more efficient than current demonstrations, if integration to a five junction stack is to make sense.
Work is also underway to develop a final bonding process for integration. In other multi-junction architectures, the bonding layer has to allow electrical conduction and optical transmission, but as Perl
highlights: “Our optical coatings will provide excellent optical transmission into the underlying four junction structure.”
“Our electrically isolated terminal configuration also means electrical conduction across this bonding interface isn’t necessary,” he adds.
Perl is now optimising thermal conduction across the layer as well as its fabrication processes, while at the same time improving the quality of the broadband coatings. Currently investigating the use of anti-reflective nanostructures in the coatings, he believes this additional work will reduce reflections to near zero, for GaN, and is confident any increase in manufacturing costs will be offset by the gains in cell efficiency.
“With the right resources in place, I think an integrated device could be demonstrated in two-to-three years, with a 50 percent [efficient] device in the next five to seven years,” says Young. “With time and effort, we believe the inherent scalability of this technology provides good commercial opportunities for CPV.”
© 2014 Angel Business Communications. Permission required.
June 2014
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