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Monday, November 7, 2011

The Ins and Outs of Solar Photovoltaics

Berkeley Lab Research Sparks Record-Breaking Solar Cell Performances

http://newscenter.lbl.gov/feature-stories/2011/11/07/record-breaking-solar-cell-performances/

November 07, 2011

Theoretical research by scientists with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has led to record-breaking sunlight-to-electricity conversion efficiencies in solar cells. The researchers showed that, contrary to conventional scientific wisdom, the key to boosting solar cell efficiency is not absorbing more photons but emitting more photons.

“A great solar cell also needs to be a great Light Emitting Diode,” says Eli Yablonovitch, the Berkeley Lab electrical engineer who led this research. “This is counter-intuitive. Why should a solar cell be emitting photons? What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce.”

Yablonovitch holds joint appointments with Berkeley Lab’s Materials Sciences Division and the University of California (UC) Berkeley, where he is the James and Katherine Lau Chair in Engineering, and also directs the NSF Center for Energy Efficient Electronics Science. He is the corresponding author of a paper describing this work titled “Intense Internal and External Fluorescence as Solar Cells Approach the Shockley-Queisser Efficiency Limit.” Co-authoring this paper with Yablonovitch were Owen Miller of Berkeley Lab, and Sarah Kurtz, at the National Renewable Energy Laboratory.

In their paper, Yablonovitch, Miller and Kurtz describe how external fluorescence is the key to approaching the theoretical maximum efficiency at which a solar cell can convert sunlight into electricity. This theoretical efficiency, called the Shockley-Queisser efficiency limit (SQ Limit), measures approximately 33.5-percent for a single p-n junction solar cell. This means that if a solar cell collects 1,000 Watts per square meter of solar energy, the most electricity it could produce would be about 335 Watts per square meter.

Calculations by Miller, who is a member of Yablonovitch’s research group, showed that the semiconductor gallium arsenide is capable of reaching the SQ Limit. Based on this work, a private company co-founded by Yablonovitch, Alta Devices Inc., has been able to fabricate solar cells from gallium arsenide that have achieved a record conversion efficiency of 28.4 percent.

“Owen Miller provided an accurate theory on how to reach the SQ Limit that for the first time included external fluorescence efficiency,” Yablonovitch says. “His calculations for gallium arsenide showed that external fluorescence provides the voltage boost that Alta researchers subsequently observed.”

Berkeley Lab’s Eli Yablonovitch (left) and Owen Miller showed that counter-intuitively, a great solar cell also needs to be a great Light Emitting Diode. (Photo by Roy Kaltschmidt, Berkeley Lab)

Berkeley Lab’s Eli Yablonovitch (left) and Owen Miller showed that counter-intuitively, a great solar cell also needs to be a great Light Emitting Diode. (Photo by Roy Kaltschmidt, Berkeley Lab)

Solar or photovoltaic cells represent one of the best possible technologies for providing an absolutely clean and virtually inexhaustible source of electricity. However, for this dream to be realized, solar cells must be able to efficiently and cost-competitively convert sunlight into electricity. They must also be far less expensive to make.

The most efficient solar cells in commercial use today are made from monocrystalline silicon wafers and typically reach a conversion efficiency of about 23-percent. High grade silicon is an expensive semiconductor but is a weak collector of photons. Gallium arsenide, although even more expensive than silicon, is more proficient at absorbing photons, which means much less material is needed to make a solar cell.

“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.”

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. Miller’s calculations showed 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.

Thin film solar cells fabricated from gallium arsenide have achieved a record sunlight-to-electricity conversion efficiency of 28.4 percent. (Image courtesy of Alta Devices, Inc.)

Thin film solar cells fabricated from gallium arsenide have achieved a record sunlight-to-electricity conversion efficiency of 28.4 percent. (Image courtesy of Alta Devices, Inc.)

As Miller explains, “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 SQ Limit.”

Using a single-crystal thin film technology developed earlier by Yablonovitch, called “epitaxial liftoff,” Alta Devices was able to fabricate solar cells based on gallium arsenide that not only smashed previous solar conversion efficiency records, but can be produced at well below the cost of any other solar cell technology. Alta Devices expects to have gallium arsenide solar panels on the market within a year.

“The SQ Limit is still the foundation of solar cell technology,” says Yablonovitch. “However, the physics of light extraction and external fluorescence are clearly relevant for high performance solar cells.”

Yablonovitch believes that the theoretical work by he and his co-authors, in combination with the performance demonstrations at Alta Devices, could dramatically change the future of solar cells.

“We’re going to be living in a world where solar panels are very cheap and very efficient,” Yablonovitch says.

This research was funded by a grant from DOE’s Light-Material Interactions in Energy Conversion Energy Frontier Research Center (LMI-EFRC).

# # #

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

Additional Information

For more information about the research of Eli Yablonovitch, visit the Website at http://optoelectronics.eecs.berkeley.edu/

For more information about the LMI-EFRC, visit the Website at http://www.lmi.caltech.edu/

For more information about Alta Devices, Inc., visit the Website at https://www.altadevices.com/

Friday, November 4, 2011

Solar To Get Better and Cheaper

http://scienceprogress.org/2011/10/new-solar-technology-you-never-heard-of/

PROCESS INNOVATION

The Coolest New Solar Manufacturing Technology You’ve Never Heard Of

Publicly Funded Research Leads to Breakthrough in Solar Cell Production


SOURCE: NREL/Dennis Schroeder
The cavity inside the Solar Optical Furnace glows white hot during a simulated firing of a solar cell.

Too often, when talking about research and innovation on clean energy technologies, policymakers, pundits, and the media tend to assume that the biggest breakthrough will come from a completely novel technology. The discovery of some new and sexy clean energy technology will suddenly change the game and make clean energy abundant and affordable overnight.

In practice that rarely happens. A more likely scenario is that humble, behind-the-scenes “process innovations” will continue to increase the efficiency and drive down the costs of manufacturing the technologies we already know work.

The Department of Energy has recently completed testing on just such a humble breakthrough. The Optical Cavity Furnace is a new piece of equipment for making solar cells that is about to rock the photovoltaic industry by slashing costs and increasing efficiency. The news should not just excite tech nerds—by reducing the cost of producing solar cells by nearly three-quarters, this new technology represents another big step on the path to making clean energy the cheap kind of energy.

Here’s how it works. By using optics to more efficiently focus visible and infrared light, the Optical Cavity Furnace can heat silicon wafers used in solar cell production much more precisely and uniformly than previous forms of solar cell manufacture. The resulting solar cells are stronger, more efficient, and have fewer impurities. The National Renewable Energy Lab, or NREL, the DOE office responsible for the research, and a corporate partner AOS Inc. are now working to bring this technology to scale. The partners plan to build an industrial-scale Optical Cavity Furnace capable of producing 1,200 highly efficient solar cells per hour. NREL has cooperative research agreements with many of the country’s biggest solar cell producers.

Even better, in addition to producing solar cells more reliably, quickly, and therefore cheaply, the Optical Cavity Furnace itself is cheaper than traditional equipment used to produce cells. As the cost of manufacturing solar cells goes down, elementary economics suggests the accessibility of solar cells will soar. Then it’s a matter of harnessing their power in a myriad of other industries in a clean energy domino effect.

The White House has challenged the solar industry to produce clean electricity at $1 per watt. It has also set a national goal to achieve 80 percent clean energy use by 2035. Though some tout the idea that radically new breakthroughs in energy technology are needed to achieve these goals, incremental process innovation in existing technologies is perhaps a more important part of the solution. Innovations like the Optical Cavity Furnace that make the technologies we already know about cheaper, easier to produce, and more abundant can have game-changing impacts on bringing clean energy to scale.

The concept of “grid parity”—the point at which generating electricity from alternate energy sources is equivalent in cost to generating electricity from grid power—underlies the feasibility of using solar cells as a resource. Due to the competing forces of supply and demand, consumers likely will not choose clean energy until it is cheap and convenient. The good news is that researchers are racing toward that goal at an impressive rate.

In fact, the cost of photovoltaic, or PV, cells had already fallen 50 percent in the past two years prior to the DOE announcement. A June 2011 projection predicted PV module prices would hit the goal of $1 per watt by 2013; now the finish line of the proverbial “race to the bottom” seems even more imminent.

For consumers weary of the daily media promises of a cure-all solution to climate change, consider this: Deflating prices of solar cell manufacturing mirror the downward price slope of other technologies we now take for granted, like cell phones and DVD players. One important driver of those price declines is process innovation. And the government, instead of being an obstacle to competition, is uniquely poised to foster it, as evidenced by the new DOE solar furnace. In addition to the work being done at the Department of Energy’s National Renewable Energy Laboratory, dozens of other federal labs across the country under the DOE Office of Science, the National Institute of Standards and Technology, and the Manufacturing Extension Partnerships are helping push the bounds of process innovation in clean energy manufacturing.

The notion that science or innovation alone can solve our energy and climate challenges may seem like the overoptimistic ramblings of an enthusiastic technocrat. Yet new technologies like the Optical Cavity Furnace are piling up, creating a stronger and stronger rationale for increased federal investment in innovation. Through process innovation, we increase efficiency and lower costs, virtually negating the common arguments against climate-conscious energy policy. Like it or not, most consumers still make energy choices based on the impact those choices have on their wallets rather than based on the impact they have on the environment. With a vibrant national research ecosystem that fosters process innovation, before we know it, more and more consumers will be choosing clean energy not because it is the socially conscious choice but because it’s the cost-effective choice.