Materials scientists make solar energy chip 100 times more efficient

March 20, 2013 by Mike Ross
Part of a 2-inch-diameter gallium-arsenide wafer used as a base for photon-enhanced thermionic emission chips. Credit: Brad Plummer / SLAC

( —Scientists working at the Stanford Institute for Materials and Energy Sciences (SIMES) have improved an innovative solar-energy device to be about 100 times more efficient than its previous design in converting the sun's light and heat into electricity.

"This is a major step toward making practical devices based on our technique for harnessing both the light and heat energy provided by the sun," said Nicholas Melosh, associate professor of materials science and engineering at Stanford and a researcher with SIMES, a joint SLAC/Stanford institute.

The new device is based on the photon-enhanced thermionic emission (PETE) process first demonstrated in 2010 by a group led by Melosh and SIMES colleague Zhi-Xun Shen, who is SLAC's advisor for science and technology. In a report last week in Nature Communications, the group described how they improved the device's efficiency from a few hundredths of a percent to nearly 2 percent, and said they expect to achieve at least another 10-fold gain in the future.

Concentrated sunlight (red arrows at the top) heats up the device's semiconductor cathode (beige and grey upper plate) to more than 400 degrees Centigrade. Photoexcited hot electrons (blue dots) stream out of the cathode's nanotextured underside down to the anode (white/gray surface), where they are collected as direct electrical current. Additional solar and device heat is collected below the anode (arrow shows the cool-to-hot, blue-to-red flow) to run electricity-generating steam turbines or Stirling engines. Credit: Nick Melosh

Conventional use a portion of the sun's spectrum of wavelengths to generate electricity. But PETE uses a special to make electricity by using the entire spectrum of sunlight, including wavelengths that generate heat. In fact, the efficiency of thermionic emission improves dramatically at high temperatures, so adding PETE to utility-scale plants, such as multi-megawatt power tower and parabolic trough projects in California's , may increase their by 50 percent. Those systems use mirrors to focus sunlight into superbright, blazingly hot regions that boil water into steam, which then spins an .

"When placed where the sunlight is focused, our PETE chips produce electricity directly; and the hotter it is, the more electricity it will make," Melosh said.

The heart of the improved PETE chip is a sandwich of two semiconductor layers: One is optimized to absorb sunlight and create long-lived free electrons, while the other is designed to emit those electrons from the device so they can be collected as an electrical current. A cesium oxide coating on the second layer eases the electrons' passage from the chip. Future research is aimed at making the device up to an additional 10 times more efficient by developing new coatings or surface treatments that will preserve the atomic arrangement of the second layer's outer surface at the it will encounter in the concentrating solar power plant.

"We expect that other materials, such as those incorporating barium or strontium, will make the surface much more stable up to at least 500 degrees Celsius," said Jared Schwede, a Stanford graduate student who performed many of the PETE experiments. (play video to see Schwede explain the PETE technology)

An additional challenge will be to engineer the device to withstand the dramatic 500-degree daily temperature swings at , as their systems heat up during the day and then cool down at night.

Explore further: New solar energy conversion process could revamp solar power production

More information: Schwede, J. et al., Nature Communications, 12 Mar 2013. DOI: 10.1038/ncomms2577

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3.9 / 5 (7) Mar 20, 2013
I thought solar cells are already up to 20% efficiency with a theoretical limit between 80% and 90%

so 4 times OK, 100?
3 / 5 (2) Mar 20, 2013
IIRC The theoretical max for a 'conventional' silicon solar cell was 22% but, there are many variations to that old design which can sport more efficiency - the 22% may reflect the silicon only...

What we are talking about above is in reference to the para re 'few hundredths of a %' etc...
5 / 5 (2) Mar 20, 2013
Single-junction silicon cells have a limit of 33.7%, but for mufti-junction cells it is 86%. Since this specific technology can use the entire spectrum, it would also have a limit around 86%.
Cells for concentrators that can run at high temperatures without extensive heatsinks will be a major step up.
not rated yet Mar 20, 2013
Yup, this particular type of chip/cell is significantly less efficient (article says 2%) than a conventional photovoltaic cell (what most people think of when they think about solar panels), but uses more of the spectrum than conventional cells, including portions that produce heat. Conventional cells become less efficient the hotter they get, which is why cooling ideas like what V3 Solar ( came up with hold interest for solar cell users.
not rated yet Mar 20, 2013
How about 70% efficiency instead, with IR "rectennas":
4.5 / 5 (8) Mar 20, 2013
in the world of solar energy articles, I always have trouble figuring out whether this is just "we made this very specific technology a lot better but it's still average overall and problematic" or "we just advanced the field and this will be in production in a year or two." Most of course fall into the former.
2 / 5 (4) Mar 20, 2013
the biggest problems with concentrated intense radiation is that it tends to destroy whatever it touches over time. creating massive maintanance costs.
5 / 5 (1) Mar 20, 2013
Wealthy, depending how well the material holds up (referring to what Jeddy is saying) this really just moved the material from impractical' to 'supplemental', according to information in the article and a colleague that works with solar energy quite a bit. I realize that it's a new material, so they can't say for sure that 'it will last this long before it's at 80% efficiency', but it would be nice if some kind of estimate was provided.
1 / 5 (1) Mar 21, 2013
I don't get it whats the point in a 2 or 3 percent efficient solar cell....what am I missing?
4 / 5 (5) Mar 21, 2013
I don't get it whats the point in a 2 or 3 percent efficient solar cell....what am I missing?

You're missing that very solar cell technology started out with very low efficiency.
The only other type of solar cell that can harness many types of frequencies is a multijunction cell. And those are freakishly expensive to make by comparison.
How about 70% efficiency instead, with IR "rectennas":

They are very comlicated (and expensive) to make.

The performance of other solar cell designs drops quickly at high temperatures. This one seems to thrive on it.
1 / 5 (1) Mar 21, 2013
I don't get it whats the point in a 2 or 3 percent efficient solar cell....
100% efficient is less than 1350 Watts per square meter.
5 / 5 (1) Mar 24, 2013
It would be nice to be able to utilize the heat to generate electricity as well as using the light absorbed by a solar cell.
1 / 5 (2) Mar 25, 2013
Jeddy Mctedder I agree with you "radiation tend to destroy whatever it touch" and on top of that solar panels are already too expensive!

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