20 June 2011

Alta raises single-junction solar cell efficiency record again to 28.2%

At the 37th IEEE Photovoltaic Specialist Conference (PVSC37) in Seattle, WA, USA this week (19–24 June), Dr Brendan Kayes of Alta Devices in Santa Clara, CA, USA is presenting details of how it raised the record for single-junction solar cell efficiency (under non-concentrated 1-sun illumination) from 26.4% to 27.6% late last year, and most recently to 28.2% — as verified by the US National Renewable Energy Laboratory (NREL). The theoretical Shockley–Queisser limit for efficiency has been shown to be 33.5%, but efforts to attain this have been slow in coming.

“Up until now it was understood that to increase the current from our best solar materials, we had to find ways to get the material to absorb more light,” says co-founder Eli Yablonovitch, director of the US National Science Foundation (NSF) Center for Energy Efficient Electronics Science and professor at the University of California at Berkeley. “But the voltage is a different story. It was not recognized that to maximize the voltage, we needed the material to generate more photons inside the solar cell. Counter-intuitively, efficient light emission is the key for these high efficiencies,” he reports in the paper ‘The Physics Required to Approach the Shockley–Queisser Limit’.

Founded in 2007, Alta Devices is a development-stage company focused on improving the production economics of high-efficiency solar photovoltaic applications, as well as on making breakthroughs in both manufacturing and form factor. So far the firm has received $72m in venture capital funding from investors including August Capital, Kleiner Perkins Caufield & Byers, Crosslink Capital, DAG Ventures, New Enterprise Associates (NEA), Presidio Ventures, Technology Partners, Dow Chemical, and Alberta Investment Management Corporation (AIMCo).

Alta fabricates single-junction gallium arsenide (GaAs) photovoltaic cells in a micron-thick thin-film that it then lifts off the growth GaAs substrate (which can then be reused multiple times to amortize its high cost). The thin-film cell can then be placed on a flexible substrate.

“In 2009, our team came to me with an aggressive timeline for solar cell efficiency advances, but with a few caveats: they were butting up against what appeared to be entrenched, practical limits,” says president & CEO Christopher Norris. “Nevertheless, over the past two years, the team has succeeded in meeting each of its milestones.” According to Norris, Alta has achieved new cell efficiency improvements about once every two months.

“We are committed to using new scientific understanding, such as internal light generation and extraction, to push the limits of solar cell and module efficiencies while simultaneously driving production costs down through other important developments,” Norris says. “The goal of achieving the $1 per installed watt target set by the Department of Energy has energized our entire company.”

Also at PVSC37, a plenary talk ‘Paths to High Efficiency Low Cost Photovoltaics’ is being given by professor Harry Atwater, Alta co-founder and director of the Energy Frontier Research Center on Light-Matter Interactions as well as director of the Resnick Institute for Science, Energy and Sustainability at California Institute of Technology (CalTech) in Pasadena.

“The energy conversion efficiency results being achieved by Alta, in combination with other manufacturing and form-factor advances, will enable new ways to deploy solar without the economic compromises of other technologies,” reckons Atwater.

See related items:

Alta raises $72m to boost economics of high-efficiency PVs

DOE provides $12m for four early-stage PV firms

Tags: Alta Devices GaAs PV

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