14 November 2011

Berkeley Lab shows external fluorescence is key to reaching Shockley-Queisser PV limit

Theoretical research at the Lawrence Berkeley National Laboratory (LBNL), which is managed by the University of California for US Department of Energy (DOE)’s Office of Science, has led to record 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 (‘Intense Internal and External Fluorescence as Solar Cells Approach the Shockley-Queisser Efficiency Limit’, Journal of PhotoVoltaics).

“A great solar cell also needs to be a great light-emitting diode,” says professor Eli Yablonovitch, the Berkeley Lab electrical engineer who led the 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,” he adds.

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 (National Science Foundation) Center for Energy Efficient Electronics Science (E3S). The paper is co-authored by Owen Miller of Berkeley Lab, and Sarah Kurtz of the National Renewable Energy Laboratory (NREL).

They describe how external fluorescence is the key to approaching the theoretical maximum efficiency at which a solar cell can convert sunlight into electricity. This Shockley-Queisser (SQ) limit is about 33.5% for a single p-n junction solar cell.

Calculations by Miller (a member of Yablonovitch’s research group) showed that gallium arsenide is capable of reaching the SQ Limit. Based on this work, the private development stage firm Alta Devices Inc - co-founded by Yablonovitch with Harry Atwater (Caltech’s Howard Hughers professor of Applied Physics and Materials Science) with the aim of making high-efficiency solar PV economically viable - has fabricated solar cells from GaAs that have achieved a record efficiency of 28.4%.

“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 GaAs showed that external fluorescence provides the voltage boost that Alta researchers subsequently observed,” he adds.

The most efficient solar cells in commercial use currently are made from monocrystalline silicon wafers and typically reach an efficiency of about 23%. High-grade silicon is an expensive semiconductor but is a weak collector of photons. Although more expensive than silicon, gallium arsenide is more proficient at absorbing photons, so 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 efficiency focused on increasing the number of photons that a cell absorbs. If photo-generated electrons are not extracted quickly enough from the cell as electricity, then they 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, then it would boost the cell’s output voltage. “This is the central counter-intuitive result that permitted efficiency records to be broken,” says Yablonovitch.

“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,” Miller explains. “As an indicator of low internal optical losses, efficient external fluorescence is a necessity for approaching the SQ Limit.”

Using an epitaxial liftoff single-crystal thin-film technology developed earlier by Yablonovitch, Alta Devices was able to fabricate solar cells based on GaAs that not only exceeded previous solar conversion efficiency records but can be produced at well below the cost of any other solar cell technology. Alta expects to have GaAs 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,” he adds.

Yablonovitch believes that the theoretical work, in combination with the performance demonstrations at Alta, 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,” he adds.

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

See related items:

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

Tags: Shockley-Queisser PV GaAs PV

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