Conversion efficiencies of the best research solar cells worldwide from 1976 through 2011 for various photovoltaic technologies; efficiencies determined by certified agencies/laboratories. Credit: NREL.
The steady march to grid parity for solar energy devices continues: A Santa Clara, Calif., maker of gallium arsenide photovoltaic panels, Alta Devices, announced Tuesday that the NREL verified that its top-line panels operate at 23.5% efficiency. The ability to deliver an entire high-efficiency panel is a big step forward for the company’s business, which last year achieved verified record-setting efficiencies as high as 28.2% with a single, single-junction PV cell.
This looks to be a record efficiency-level for PV panels. Although, as the chart above indicates, NREL has verified higher efficiencies in other PV arrangements, these have been for a single or small sets of PV cells, not full panels. (It should probably be noted that Sanyo asserts that its in-production silicon heterostructure (HIT) panels come near to the numbers achieved by Alta, but this has not been verified by NREL.)
In a news release, Alta Devices explains a little bit about its interest in GaAs-based devices. The company says, “Alta chose to focus on GaAs because of its intrinsic efficiency advantages as well as its ability to generate electricity at high temperatures and in low light. This means that Alta’s panels have substantially higher energy density than other technologies, generating more kilowatt–hours of energy over the course of a year in real life conditions.”
Some investors have been cautious about GaAs-based solar technologies because they generally have appeared to require higher-priced materials than, for example, silicon. But the company says, “though GaAs is known for being expensive to produce, Alta has invented a manufacturing technique that enables extremely thin layers of GaAs that are a fraction of the thickness of earlier GaAs solar cells. Alta’s cells are about one micron thick… In utilizing very thin devices that have the highest energy density possible, the cost of the material needed in Alta panels remains low and the potential costs of an entire solar energy system based on Alta’s technology could be dramatically reduced.”
Alta deposits the GaAs on a thin, flexible film substrate. By focusing on this form factor, Alta says its film “has the potential to be integrated in wholly unique ways and into a variety of applications, including roof and building materials, and numerous military, consumer and transportation products.”
The company was cofounded by two well known California scientists engaged in academic-based energy research, CalTech’s Harry Atwater and University of California at Berkeley’s Eli Yablonovitch. Atwater is director of the Energy Frontier Research Center on Light-Matter Interactions and director of the Resnick Institute for Science, Energy and Sustainability, and Yablonovitch is director of the NSF Center for Energy Efficient Electronics Science, at their respective schools. Alta has received venture capital funding from high profile groups, such as August Capital and Kleiner Perkins Caufield & Byers.
In a recent story on the Lawrence Berkeley National Lab website, Yablonovitch offered a fundamental defense of GaAs. He said, “Gallium arsenide absorbs photons 10,000 times more strongly than silicon for a given thickness but is not 10,000 times more expensive,” says Yablonovitch. “Based on performance, it is the ideal material for making solar cells.”
But the trick is to extract the high efficiency from GaAs. In a June 2011 release, Yablonovitch explained, “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. 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. Counterintuitively, efficient light emission is the key for these high efficiencies.”
How are these efficiencies and energy density being achieved? For one thing, it required some open-mindedness. The LBL story describes that a leap in logic had to occur: “Past efforts to boost the conversion efficiency of solar cells focused on increasing the number of photons that a cell absorbs. Absorbed sunlight in a solar cell produces electrons that must be extracted from the cell as electricity. Those electrons that are not extracted fast enough, decay and release their energy. If that energy is released as heat, it reduces the solar cell’s power output. [LBL’s Owen Miller] calculated that if this released energy exits the cell as external fluorescence, it would boost the cell’s output voltage. ‘This is the central counter-intuitive result that permitted efficiency records to be broken,’ Yablonovitch says.”
“In the open-circuit condition of a solar cell, electrons have no place to go so they build up in density and, ideally, emit external fluorescence that exactly balances the incoming sunlight. As an indicator of low internal optical losses, efficient external fluorescence is a necessity for approaching the [theoretical efficiency] limit,” Miller said.
In other words, the Alta Devices PV panels achieve high efficiency by emitting certain light while converting solar energy, instead of allowing excess electron energy to build up internal heat.
On the processing side of things, the company says it “is making substantial progress on the build-out of its pilot manufacturing line, which uses mostly off–the–shelf equipment with some proprietary optimizations unique to Alta’s process. Moreover, Alta is starting to plan for full–scale production, with activities such as building strategic manufacturing partnerships and selecting its first large, commercial manufacturing site.”