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Triple-junction solar cell design could break 50 percent conversion barrier

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January 16, 2013

Schematic diagram of a multi-junction solar cell formed from materials lattice-matched to ...

Schematic diagram of a multi-junction solar cell formed from materials lattice-matched to InP and achieving the bandgaps for maximum efficiency (Photo: U.S. Naval Research Laboratory)

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The current world record for triple-junction solar cell efficiency is 44 percent, but a collaboration between the U.S. Naval Research Laboratory (NRL), the Imperial College of London, and MicroLink Devices Inc. has led to a multi-junction photovoltaic cell design that could break the 50 percent conversion efficiency barrier under concentrated solar illumination.

Presently, the best examples of traditional silicon solar cells top out at around 25 percent efficiency, whereas multi-junction cells have achieved more than 40 percent. Multi-junction solar cells contain several layers of semiconducting material stacked one on top of another, each designed to absorb a different wavelength of light, thereby boosting their efficiency. On top, high band gap semiconductor material absorbs the short wavelength radiation, while longer wavelengths are absorbed by the layers below.

The key to the new multi-junction cell design was identifying what are known as "InAlAsSb quaternary alloys" as a high band gap material that can be grown lattice-matched to an indium phosphide (InP) substrate. Used in the team's multi-junction solar cell, the alloys could achieve a maximum direct band gap of 1.8 electron volts (eV), which is higher than commonly used materials that, when lattice-matched to InP, result in a maximum of 1.4 eV.

"This research has produced a novel, realistically achievable, lattice-matched, multi-junction solar cell design with the potential to break the 50 percent power conversion efficiency mark under concentrated illumination," said Robert Walters, Ph.D., NRL research physicist.

The Naval Research Lab, MicroLink, and the Rochester Institute of Technology will develop the technology over the next three years with funding from the U.S. Department of Energy (DoE) and the Advanced Research projects Agency-Energy (ARPA-E). ARPA-E projects develop technology that is promising but too early for private-sector investment.

Source: Naval Research Laboratory

About the Author
Jason Falconer Jason is a freelance writer based in central Canada with a background in computer graphics. He has written about hundreds of humanoid robots on his website Plastic Pals and is an avid gamer with an unsightly collection of retro consoles, cartridges, and controllers.   All articles by Jason Falconer
14 Comments

I always ask myself, is it efficiency that matters or total system $/kWh produced over the lifetime of the cell?

splatman
17th January, 2013 @ 03:32 am PST

50% using concentration...I wonder what the attenuation is of the concentrator. More importantly, what is the material cost per kwatt generated?

JBar
17th January, 2013 @ 04:56 am PST

Splatman: It depends.

In applications where space is limited or weight is crucial, such as satellites or high-flying electric relay platforms (24-7 planes) to replace communication satellites, efficiency is everything, because one needs to carry as little as possible and has to get out as much as possible.

In applications where space doesn't matter, such as powering a large home in a suburb, cost per kW rules. If the home owner only needs to cover half of his roof to make 80% of his power (and that depends on efficiency of house and appliances), then the only thing he cares about is how much cash to pay to get there.

BeWalt
17th January, 2013 @ 09:40 am PST

Just a thought, why is 50% a barrier? Milestone perhaps?

Henry Davis
17th January, 2013 @ 10:19 am PST

Solar cells have always "almost" become economically viable, have "almost" achieved good efficiency but never in actuality. I used to design solar arrays for satellites and the target successful market price touted in the 1970's was 25 cents for a 2X2cm silicon solar cell. The price never came even close and still isn't. This is likely just another lab toy. There are thousands of variants of almost, soon, perhaps, etc. etc. etc. They have a lot in common with fuel cells in terms of their seemingly "almost" commercialization and economic viability.

grtbluyonder
17th January, 2013 @ 01:24 pm PST

to: BeWalt please try to think outside of 'roof' as the geophysical orientation that u have to contend with in any installation situation.low installations on a proper surface R much easier to align & maintain.

slayerwulfe
17th January, 2013 @ 01:44 pm PST

Henry: There is no 50%-barrier as such. It's just a good round number that would be nice to pass. Just like the 100mph-barrier in a car. Or my personal 180lbs-barrier, uhm, different story.

You have a number, you aim higher, and try harder, and are happy to get there once it happens. Or lower, whatever.

The thing with solar cells is that once upon a time, "experts" said solar cells could never be made with more than "x" % efficiency. Until they realized they had not known enough to make that statement. Between making the statement and the realization, decades can pass. True for pretty much all technologies.

The silicon solar cell efficiency barrier was once thought to be around 30%. Then they started cooking up multilayer-multicompound-cells and thought it would be 40% which at some point was passed. So let's have a look at the number 50 now. It's more exciting than watching baseball!

Sure, there's no way to get close to or make more than 100%. But for theoretical considerations as of now, based on hypothetical multi-multi-super-dooper cells made from Indium Gallium Nitride, people have taken the word "seventy" into their mouths, some 10 years ago, in Berkeley, California. And then, as usually, got bashed for it by "experts".

It's a great spectacle. Get the chips, sit back and enjoy.

BeWalt
17th January, 2013 @ 02:43 pm PST

How much to produce said solar cells for home use.

Radical if doable.

Stephen N Russell
17th January, 2013 @ 06:23 pm PST

I've been hearing about solar cells for decades now, but the only cells available seem to be early 80's surplus at best. When will some of the newer technology become available for the average person to purchase? nw9784@yahoo.com

Norman Welch
17th January, 2013 @ 09:19 pm PST

Seems like my whole adult life (and I'm 59) we've been a decade away from practical fusion power and three years away from economically feasible solar energy. For the last 5 years I've been reading gizmag I have read about 1000 "ground breaking" developments in solar energy. Sooooooo..... does anybody know when the hell I'll finally be able to put solar cells on my roof that will, in the long run, be cheaper than buying oil/coal produced electricity?

Mike Kling
18th January, 2013 @ 09:11 am PST

Mike, that kind of depends upon where you live, whether or not your power company is subscribed to the protestant church ethic of "always ask for more money" and whether or not you have functional access to solar cells from more than one source, so there is some legitimate competition. That being said, there has never been a cheaper time than right now to install the state of the art in commercially available solar cells.

John Strohl
18th January, 2013 @ 11:02 am PST

This is highly misleading as it measures the results with a "concentrated" beam of light. More meaningful would be the conversion efficiency with sunlight as that is the intended application. At the present time the efficiency of solar panels is barely 21% for the very best ones.

Calson
22nd January, 2013 @ 12:29 pm PST

@grtbluyonder and @Mike Kling, solar panels are economically viable TODAY, depending on your application. The old story that they are for the future is no longer the case. If they weren't there wouldn't be a lot being installed in California, Germany, Colorado, Arizona ... and even in sunny Pacific Northwest. Keep in mind the fossil fuel industry is heavily subsidized, both directly in terms money, but also in terms of not having to pay for the pollution, both direct toxic output (e.g. tar sands) and carbon thus climate impacts. But solar panels are subsidized at state and federal levels as well. Check out tax credits available that can help pay for large sums of a project. If residential doesn't work for you, consider a community solar project.

Derek Hoshiko
24th January, 2013 @ 08:16 am PST

The efficiency is not misleading it just means you must couple the solar cell with a concentrator collector system and mount it on a heliostat. The concentrator is a simple reflector or lens that focuses the light from say a 200 mm square on a 5mm square solar cell. The cell needs a heat sink to keep it cool. In some ways that is an advantage. Your high cost material solar cell covers only a small percent of the area light is collected from. Most of the area is covered by lower cost lens or reflector material. Also because the heliostat keeps the system pointed full on the sun from sun up to sun down. The power output during daytime is maximized and nearly uniform. With flat non movable collectors you must have more area not just to compensate for the lower efficiency but also the decreased performance due to the sun hitting at a lower angle. The output varies during the day almost zero at sunrise max at noon and back to zero at sunset. Systems with concentrators and heliostats are very cost competitive with flat plate systems and offer advantages where space is a premium.

n2liberty
24th January, 2013 @ 09:09 am PST
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