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Uncategorized An operator inspects a photolithography tool used to manufacture high-efficiency Solar Junction concentrator solar cells. NREL's pioneering multijunction work led to the Solar Junction SJ3 solar cell with tunable bandgaps, lattice-matched architecture, and ultra-concentrated tunnel junctions. Credit: Daniel Derkacs/Solar Junction

Published on December 30th, 2012 | by Zachary Shahan

11

Solar PV’s 44% Efficiency Record, Thanks To NREL & Solar Junction

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December 30th, 2012 by Zachary Shahan
 
 
Here’s a great solar PV story from the good folks over at NREL (note that we’ve already covered Solar Junction’s 44% solar cell efficiency record, but this post below goes above & beyond that first one):

An operator inspects a photolithography tool used to manufacture high-efficiency Solar Junction concentrator solar cells. NREL’s pioneering multijunction work led to the Solar Junction SJ3 solar cell with tunable bandgaps, lattice-matched architecture, and ultra-concentrated tunnel junctions. Credit: Daniel Derkacs/Solar Junction

It takes outside-the-box thinking to outsmart the solar spectrum and set a world record for solar cell efficiency. The solar spectrum has boundaries and immutable rules. No matter how much solar cell manufacturers want to bend those rules, they can’t.

So how can we make a solar cell that has a higher efficiency than the rules allow?

That’s the question scientists in the III-V Multijunction Photovoltaics Group at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) faced 15 years ago as they searched for materials they could grow easily that also have the ideal combinations of band gaps for converting photons from the sun into electricity with unprecedented efficiency.

A band gap is an energy that characterizes how a semiconductor material absorbs photons, and how efficiently a solar cell made from that material can extract the useful energy from those photons.

“The ideal band gaps for a solar cell are determined by the solar spectrum,” said Daniel Friedman, manager of the NREL III-V Multijunction Photovoltaics Group. “There’s no way around that.”

But this year, Friedman’s team succeeded so spectacularly in bending the rules of the solar spectrum that NREL and its industry partner, Solar Junction, won a coveted R&D 100 award from R&D Magazine for a world-record multijunction solar cell. The three-layered cell, SJ3, converted 43.5% of the energy in sunlight into electrical energy — a rate that has stimulated demand for the cell to be used in concentrator photovoltaic (CPV) arrays for utility-scale energy production.

Last month, that record of 43.5% efficiency at 415 suns was eclipsed with a 44% efficiency at 947 suns. Both records were verified by NREL. This is NREL’s third R&D 100 award for advances in ultra-high-efficiency multijunction cells. CPV technology gains efficiency by using low-cost lenses to multiply the sun’s intensity, which scientists refer to as numbers of suns.

Friedman says earlier success with multijunction cells — layered semiconductors each optimized to capture different wavelengths of light at their junctions — gave NREL a head start.

The SJ3 cells fit into the market for utility-scale CPV projects. They’re designed for application under sunlight concentrated to 1,000 times its normal intensity by low-cost lenses that gather the light and direct it at each cell. In regions of clear atmosphere and intense sunlight, such as the U.S. desert Southwest, CPV has outstanding potential for lowest-cost solar electricity. There is enough available sunlight in these areas to supply the electrical energy needs of the entire United States many times over.

Bending Material to the Band Gaps on the Solar Spectrum

NREL Principal Scientist Jerry Olson holds examples of the first multijunction cells that were developed in the 1980s based on his scientific breakthrough.

Sunlight is made up of photons of a wide range of energies from roughly zero to four electron volts (eV). This broad range of energies presents a fundamental challenge to conventional solar cells, which have a single photovoltaic junction with a single characteristic band gap energy.

Conventional cells most efficiently convert those photons that very nearly match the band gap of the semiconductors in the cell. Higher-energy photons give up their excess energy to the solar cell as waste heat, while lower-energy photons are not collected by the solar cell, and their energy is completely lost.

This behavior sets a fundamental limit on the efficiency of a conventional solar cell. Scientists overcome this limitation by using multijunction solar cells. Using multiple layers of materials in the cells, they create multiple junctions, each with different band gap energies. Each converts a different energy range of the solar spectrum. An invention in the mid-1980s by NREL’s Jerry Olson and Sarah Kurtz led to the first practical, commercial multijunction solar cell, a GaInP/GaAs two-junction cell with 1.85-eV and 1.4-eV bandgaps that was recognized with an R&D 100 award in 1990, and later to the three-junction commercial cell based on GaInP/GaAs/Ge that won an R&D 100 award in 2001.

The researchers at NREL knew that if they could replace the 0.67-eV third junction with one better tuned to the solar spectrum, the resulting cell would capture more of the sun’s light throughout the day. But they needed a material that had an atomic structure that matched the lattice of the layer above it — and that also had the ideal band gap.

“We knew from the shape of the solar spectrum and modeling solar cells that what we wanted was a third junction that has a band gap of about 1.0 electron volt, lattice-matched to gallium arsenide,” Friedman said. “The lattice match makes materials easier to grow.”

They concentrated on materials from the third and fifth columns of the periodic table because these so-called III-V semiconductors have similar crystal structures and ideal diffusion, absorption, and mobility properties for solar cells.

But there was seemingly no way to capture the benefits of the gallium arsenide material while matching the lattice of the layer below, because no known III-V material compatible with gallium arsenide growth had both the desired 1-eV band gap and the lattice-constant match to gallium arsenide.

That changed in the early 1990s, when a research group at NTT Laboratories in Tokyo working on an unrelated problem made an unexpected discovery. Even though gallium nitride has a higher band gap than gallium arsenide, when you add a bit of nitrogen to gallium arsenide, the band gap shrinks — exactly the opposite of what was expected to happen.

“That was very surprising, and it stimulated a great deal of work all over the world, including here at NREL,” Friedman said. “It helped push us to start making solar cells with this new dilute nitride material.”

Good Band Gaps, but Not So Good Solar Material

The NREL team that shared the 2012 R&D 100 award for the world-record SJ3 multijunction solar cell include, from left, Aaron Ptak, John Geisz, Sarah Kurtz, Brian Keyes, Bob Reedy, and Daniel Friedman; unpictured team members are Jerry Olson and Steve Johnston. Credit: Dennis Schroeder / NREL

The new solar cells NREL developed had two things going for them — and one big issue.

“The good things were that we could make the material very easily, and we did get the band gap and the lattice match that we wanted,” Friedman said. “The bad thing was that it wasn’t a good solar cell material. It wasn’t very good at converting absorbed photons into electrical energy. Materials quality is critical for high-performance solar cells, so this was a big problem.”

Still, NREL continued to search for a solution.

“We worked on it for quite a while, and we got to a point where we realized we had to choose between two ways of collecting current from a solar cell,” Friedman said. “One way is to let the electrical carriers just diffuse along without the aid of an electric field. That’s what you do if you have good material.”

If the material isn’t good, though, “you have to introduce an electric field to sweep the carriers out before they recombine and are lost,” Friedman said.

But to do that, virtually all impurities would have to be removed. And the only way to remove the impurities would be to use a different growth technique.

Using Molecular Beam Epitaxy to Virtually Eliminate Impurities

Solar cells are typically grown using metalorganic vapor-phase epitaxy, or MOVPE.

“It works great, except you always get a certain level of impurities in the material. That’s usually not a problem, but it would be an issue for this novel material, with the gallium arsenide diluted with nitrogen,” Friedman said.

A different growth technique, molecular beam epitaxy (MBE), is done in such an ultra-high vacuum — 10 to the minus 13 atmospheres — that it can lower the impurities to the point where an electric field can be created in the resulting photovoltaic junction. And that would make the otherwise promising gallium-arsenide-dilute-nitride material work as a solar cell.

“The only problem was that there was no one in the entire world manufacturing solar cells by MBE,” Friedman said.

But that was soon to change.

Partnering with a Startup out of Stanford University: Solar Junction

A Stanford University research group with expertise in the use of MBE for other electronic devices saw an opportunity, and around 2007, they spun out a startup company they named Solar Junction.

Because Solar Junction was a mix of enthusiastic recent Ph.D.s and experienced hands from outside the established solar cell field, “they weren’t tied to the constraints of thinking this couldn’t be done, that the only economically viable way to make solar cells was with MOVPE,” Friedman said.

The federal lab and the startup got together. Solar Junction won a $3 million DOE/NREL Photovoltaic Technology Incubator contract to develop a commercial multijunction cell using dilute nitrides, and also received more than $30 million of venture-capital funding for this commercialization effort. To see more about NREL’s Incubator projects, see the NREL news release.

“So Solar Junction had this good idea. But now they had to prove that you could actually make a high-efficiency solar cell with this,” Friedman said. “Otherwise, who cares? People can make a lot of claims, but it’s very simple to know whether you have a good solar cell or not — you just measure it.”

It didn’t take that long, Friedman said. By 2011, NREL had certified a new efficiency record for Solar Junction’s SJ3 cell. The cell achieved an efficiency of 43.5% under concentrated sunlight, a significant step beyond the previous multijunction efficiency record of 41.6%, and far beyond the maximum theoretical efficiency of 34% for traditional one-sun single-junction cells.

Dilute-Nitride Junction Eliminates Need for Heavy Germanium Layer

With the new dilute-nitride junction, the germanium layer, which constitutes about 90% of the weight of the cell, is no longer needed. That may not be a big deal when it’s part of a huge fixed utility-scale array. But when solar cells are used to power satellites, reduction in weight means a smaller rocket is needed to launch into space, potentially reducing costs significantly. The lighter weight is also essential for the military, which is increasingly asking soldiers to carry backpacks that include solar devices to power electronics.

Serendipitously, if the germanium substrate is retained, it has essentially the ideal band gap of 0.7 eV for a fourth junction, perfect for capturing longer wavelengths of the solar spectrum. That paves the way for a 50%-efficient solar cell in the not-distant future.

The cost to manufacture the SJ3 cell is competitive with that of the industry-standard GaInP/GaAs/Ge cell, according to Solar Junction. Its greater efficiency translates to significant cost-of-energy savings.

According to a report released this fall from IMS Research, the CPV market is forecast to double in 2012 and reach almost 90 megawatts. The World Market for Concentrated PV (CPV) — 2012 predicts installations of CPV will grow rapidly over the next five years to reach 1.2 gigawatts by 2016.

Because of its design and size, SJ3 is an instant plug-in replacement for the standard cell now used by the space and CPV industries. So, for example, if a 40%-efficient cell were replaced with a 44%-efficient cell, this would instantly increase the entire system power output by close to 10%.

“This is really a classic example of NREL developing something and then industry picking it up and running with it and making it a great commercial success,” Friedman said. “We started with some very basic materials research. We took it to the point where it made sense for industry to take over and take it to the marketplace.”

“We conceived the cell, demonstrated the individual parts, and let the world know about it,” Friedman said. “But Solar Junction put all the parts together with record-breaking results, made it work with MBE, and commercialized it at a time when no one else seemed to be interested in or able to do it.”

And now, utilities are ordering the SJ3 cells so fast that Solar Junction has depleted its pilot-scale stock and gone into partnership with manufacturer IQE to ramp up to full manufacturing scale.

Learn more about NREL’s photovoltaic research.

— Bill Scanlon

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About the Author

is the director of CleanTechnica, the most popular cleantech-focused website in the world, and Planetsave, a world-leading green and science news site. He has been covering green news of various sorts since 2008, and he has been especially focused on solar energy, electric vehicles, and wind energy since 2009. Aside from his work on CleanTechnica and Planetsave, he's the founder and director of Solar Love, EV Obsession, and Bikocity. To connect with Zach on some of your favorite social networks, go to ZacharyShahan.com and click on the relevant buttons.



  • jburt56

    Looks like quantum dots will be the next big step up.

  • http://profiles.google.com/vandammes James Van Damme

    Us folks with flat surfaces and no tracking concentrators are not going to be impressed with HE cells when we see the price tag. But keep at it, folks.

  • photosymbiont

    What about the potential for incorporating high-efficiency solar cells in the roof / hood / trunk panels of electric vehicles? The limited surface area available on an EV would point towards the use of high-efficiency panels – but how long would it take to charge a typical EV battery using a 44% efficient panel, perhaps two meters square?

    • Bob_Wallace

      It wouldn’t be an optimal use for the cells.

      First, it’s best to point cells at the Sun, most of the surface area of a vehicle is flat.

      Second, one would always need to park in the sunshine. Not under a tree, in a garage, in the shadow of a tall building.

      Third, once the vehicle batteries were full then the rest of the potential electricity would be wasted.

      Better to install them where they will get the most possible sunlight, at the best angle, and feed all the power to the grid. A plugged in vehicle can grab that power with little loss.

    • dynamo.joe

      I have been wondering for a couple of years now why none of the university teams that enter solar car competitions have attempted a CPV design. Maybe they did try it and it was less efficient? Instead you only see teams trying to make the lightest, most aerodynamic car they can and slap solar cells on it.

      C’mon guys start thinking outside of that box.

    • Pete Stiles

      100% efficiency is 1Kw per M^2 (in perfect sun) a 2×2 meter panel is therefore 4 Kw. Average annual insolation (amount of sunshine) is about 4 hrs in the US (depending on location).
      So 16 Kwhr per day which at 44% efficiency is 7Kwhr. We would need to deduct about 20% from this to compensate for losses in the batteries,wiring and controllers etc so 5 Kwhr per day is a nice round figure. A Chevy volt type battery bank is about 25 Kwhr so your answer is 5 days charging for 1 day driving, Mind you 4^m might be an ambitious area to find on a typical car.

  • DJ

    what would be the efficiency in 1 SUN

    • dynamo.joe

      Efficiency doesn’t seem to scale much with number of suns (~400 gives 43.5%, ~900 gives 44%). What will be a strong function of number of suns is cost/kWhr.

      So at 1 sun efficiency will probably be ~42%, but the cost would be 100 times more expensive than going down to radio shack and buying whatever polycrystalline Si solar cell they have there and it will only supply 2-3 times as much electricity. So economically a bad plan to use these in a non-concentrating environment.

      But at 900 suns you get 1800-2700 times the electricity that you would get from your 1 sun radio shack cell. You also have to supply a reflector, a tracking system, and an active cooling system. But as long as all of that costs less than 2700 times your radio shack cell, you still have a less expensive system on a $/kWhr basis.

      • http://www.facebook.com/matthew.t.peffly Matthew Todd Peffly

        So at the large CPV plants using these cells. What are they doing with the “waste” heat? Sounds like you could set up some stirling cycle engines and extra more power from the system. Please don’t say that they are setting up cooling towers.

        • Bob_Wallace

          Everyone seems to love Stirling engines but I’ve never heard of one in actual use. So I googled. From the American Stirling Company page…
          “The modern uses of Stirling engines are invisible to almost everyone. There have been many research engines built in recent years but there are only three areas where Stirling engines have made a dramatic impact.

          There are Stirling engines in Submarines, stirling machines used as cryocoolers,
          and Stirling engines in classrooms. ”

          http://www.stirlingengine.com/faq/#5

          Wiki tells me that -
          “A *Cryocooler* is a standalone cooler, usually of table-top size. It is used to cool some particular application to cryogenic
          temperatures.”

          Further searching leads me to no current applications. Seems like Coleman made a small cooler for a while that sold for several hundred dollars.

        • dynamo.joe

          That’s a good question. I’m sure if you asked Solar Junction they would say something like “that is a design decision that is made by each of our customers”.

          Like you I would hope they are finding some use for all that energy but if the project is far out in the desert I’m not sure what use they would find.

          If the project was on the roof/parking lot of a highschool or something you would think it would be easy to use the waste heat to heat and cool the school.

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