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Plasmonic nanostructures could prove a boon to solar cell technology

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September 15, 2013

Do 'plasmonic nanostructures' hold the key to next-generation solar power?  (Photo: Shutte...

Do 'plasmonic nanostructures' hold the key to next-generation solar power? (Photo: Shutterstock/Pavelk)

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Researchers at the University of Pennsylvania have found a way to harvest energy from sunlight more efficiently, with the help of so-called plasmonic nanostructures. The new findings suggest that plasmonic components can enhance and direct optical scattering, creating a mechanism that is more efficient than the photoexcitation that drives solar cells. The development could therefore provide a real boost to solar cell efficiency and lead to faster optical communication.

When photons hit the surface of a solar cell, the energy they carry is absorbed by the atoms of a doped semiconductor. If the energy absorbed is higher than a set threshold, known as the energy gap, then electrons are set free and can be used to generate electricity.

Theoretically, the energy gap can be manipulated to maximize the number of electrons that will be set free by a photon; but setting this threshold isn't straightforward, because some photons carry more energy than others.

Photons in the infrared typically don't carry enough energy to knock electrons off a silicon atom. Red photons carry just enough energy to knock down a single electron, and photons in the blue spectrum and beyond carry enough to knock off one electron, but the rest of the energy is wasted as heat. This large amount of wasted energy compromises solar cell efficiency.

Plasmonic nanostructures could significantly enhance the efficiency of solar cells (Image:...

Plasmonic nanostructures could significantly enhance the efficiency of solar cells (Image: University of Pennsylvania)

Building on their previous work, Prof. Dawn Bonnell and colleagues have now demonstrated that there is another way to harvest energy from light – a method that has tested up to 10 times more efficient than conventional photoexcitation and that could greatly improve the efficiency of solar cells and optoelectronic devices that convert light signals into electricity.

The University of Pennsylvania researchers focused on plasmonic nanostructures, materials made from arrays of gold nanoparticles and light-sensitive molecules of porphyin arranged in specific patterns.

When a photon hits these structures, it generates an electrical current that moves in a direction controlled by the size and the layout of the gold particles. By controlling and enhancing the way light scatters across them, these nanostructures can transduce light into electricity more efficiently than was previously possible. The freed up electrons can then be extracted from the plasmons and used to power molecular electronic optoelectronic devices.

Since their first results in 2010, the researchers led by Prof. Bonnell had suspected that their method could lead to significant increases in performance, but they couldn't prove it. Now, in this new study, they managed to do just that.

"In our measurements, compared to conventional photoexcitation, we saw increases of three to 10 times in the efficiency of our process," Bonnell says. "And we didn’t even optimize the system. In principle you can envision huge increases in efficiency."

"Light impinges on an array of metal nanoparticles connected with optically active molecules, and the resulting current is detected across the array," Prof. Bonnell tells Gizmag. "This process can produce more electrons in principle. You can imagine building energy harvesting devices made of the nanoparticles and organic molecules, or you could envision putting the nanoparticles into a silicon solar cell."

The nanostructures can be optimized for specific applications by changing the size and spacing of the nanoparticles, which would alter the wavelength of light to which the plasmon responds, in the same way that multi-junction solar cells are built to absorb photons of different wavelengths more effectively.

Applications could include the more efficient transduction of optical signals (e.g. from fiber optics) into electrical signals and, of course, more efficient solar cell technology. "You could imagine having a paint on your laptop that acted like a solar cell to power it using only sunlight," Bonnell says.

Promising as they may sound, we must remember that these results are largely theoretical; through this study the researchers have shown that generating electricity using plasmonic nanostructures can be more efficient than by using standard photoexcitation, but there's no telling how soon devices exploiting this principle could reach mass-production, or even what kind of actual efficiency gains they could bring.

The results were recently published on the journal ACS Nano.

Source: University of Pennsylvania

About the Author
Dario Borghino Dario studied software engineering at the Polytechnic University of Turin. When he isn't writing for Gizmag he is usually traveling the world on a whim, working on an AI-guided automated trading system, or chasing his dream to become the next European thumbwrestling champion.   All articles by Dario Borghino
5 Comments

Now all we need is a good source of gold. Asteroid wranglers?

Seth Miesters
15th September, 2013 @ 06:42 am PDT

I guess i don't understand...i've heard of a lab created solar cell that managed 47% efficiency. 3 to 10 times that would make these researchers

solar cell producing more energy than the sunlight hitting it.

that should challenge the law of conservation of energy.

i can see that the process might make the solar cell more efficient, but producing more energy than the input sounds wrong to me.

can someone "enlighten" me?

notarichman
15th September, 2013 @ 06:56 am PDT

notarichman

My guess is this. The article says "In our measurements, compared to conventional photoexcitation, we saw increases of three to 10 times in the efficiency of our process". Photoexcitation is the process of using the light to excite electrons in the material and make them available for harvest. The process of harvesting these electrons for use in electrical power is the next step and depending on the material there are plenty of losses along the way and many of these photoexcited electrons will recombine and be lost. So 10 times photoexcitation does not necessarily translate as 10 times power generation.

Oldboot
15th September, 2013 @ 07:34 pm PDT

Seems to me that solar is the smart energy source that has very little effect on the climate, like wind or hydro generators do.

If they could get them sensitive enough to work off moonlight or starlight that would be a big plus, and the way it looks it might not be far away.

Solar is our world's most wasted energy.

Jay Finke
16th September, 2013 @ 10:22 am PDT

Gizmag has printed (I'm guessing) hundreds of articles on improvements to solar cells that "could" boost their spectral and other efficiency's. Yet I can't recall reading of any of these advancements that have been commercialized. Personally I'm waiting for lazer-like positive feedback in solar cells that boosts their efficiency's to over 95%. "Energetic electrons go crazy in new solar cells"

grtbluyonder
20th September, 2013 @ 09:49 am PDT
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