Blunt nanostructures could make high-efficiency solar cells easier to fabricate

Jan 24, 2012 by Lisa Zyga feature
Showing data from four different crescent-shaped nanostructures, this figure demonstrates the strong dependence of SP excitations on crescent shape. Most significantly, the study shows that nanostructures can have a continuous light absorption spectrum even when having blunt edges, greatly simplifying fabrication requirements. Image credit: Yu Luo, et al. ©2012 American Physical Society

(PhysOrg.com) -- One of the most promising methods for increasing the efficiency of solar cells consists of coating the cells’ surfaces with a thin layer of metal nanoparticles. The nanoparticles scatter incoming light in different directions, which allows the solar cells to absorb more light than they otherwise would. The scattering occurs when the incoming light stimulates the nanoparticles’ surface plasmons (SPs), which are coherent electron oscillations in the metal atoms that can reach a resonance mode when the electrons’ frequency matches the photons’ frequency. Under these conditions, the resulting “surface plasmon resonance” induces light scattering and enhances the light absorption of the surface.

Until recently, scientists thought that metallic nanoparticles usually have SP resonances only at quantized, rather than continuous, frequencies. But in 2010, Professor Sir John Pendry of Imperial College London, along with Alexandre Aubry, Yu Luo, and others, found that this no longer holds true for nanostructures with sharp edges or corners. Such geometrical features act as singularities for the SP frequencies, causing them to propagate toward the singularity, slowing down as they approach but never reaching the singularity. As a result, energy builds up at these points and the SP resonance modes are continuous.

Theoretically, the singularities in these sharp-cornered metal nanoparticles could greatly increase the light absorption and efficiency of solar cells and other devices. However, in reality, such perfectly sharp corners are nearly impossible to fabricate.

Now in a new study, Pendry, Luo, Dang Yuan Lei, and Stefan Maier, all from Imperial College London, have investigated just how sharp the nanoparticles’ corners need to be to have a continuous SP spectrum and provide an increase in light absorption. Surprisingly, they found that some nanostructures with blunt corners, as long as they obey certain other parameters, can provide the same large field enhancement and increased light harvesting efficiency as sharp-cornered nanostructures. The study is published in a recent issue of .

In the study, the researchers theoretically analyzed how rounding off the corners of a crescent moon-shaped nanostructure alters its optical properties. While some previous studies have also analyzed the optical properties of other blunt-edged nanostructures, they have not used a systematic strategy like the scientists used here. The new analytical model, which is based on transformation optics, applies to a wide variety of blunt plasmonic nanostructures such as wedges and cylinders. The advantage of having a general model is that it may enable researchers to more easily design light-harvesting devices in the future.

“I think the greatest significance of our work is that it presents a systematic strategy to analytically deal with the effect of edge rounding,” Luo told PhysOrg.com. “The approach itself is very general; therefore it can be used to study a variety of nanoparticles with sharp geometrical features, and to facilitate efficient modeling and fast optimization of plasmonic nanostructures.”

As the scientists explained, increasing the edge bluntness generally decreases the number of SP modes exponentially. However, here they found that adjusting the crescent thickness as well as the crescent tip angle could make a nanostructure’s light absorbing properties nearly independent of its tip bluntness. The robustness holds for 2D nanostructures that are smaller than 100 nanometers in diameter. As Luo explained, this finding could greatly improve the light-to-electricity conversion process in solar cells.

“A solar cell is an electrical device that converts the energy of light into electricity,” he said. “However, the wavelength of light in free space is usually much larger than that of electrons. Therefore, the conversion process often requires gathering light on the micron-sized scale of the wavelength and concentrating it to nanoscale active centers where the energy of photons can be efficiently converted into electrical energy. And the nanostructures designed with our approach can achieve this light harvesting effect over a very broad frequency band.

“Of course, apart from light harvesting, the of is also related to some other parameters (such as the recombination and resistive losses), which are not considered in our study. But as the general analytic model proposed in our paper enables us a profound understanding and accurate estimation of the optical properties of different nanostructures, we anticipate that it could assist engineers in their design of solar cell nanoparticles.”

Some other applications of the study could include Raman scattering, single-molecule detection, ultrafast nonlinearity, and flammable gas detection, among others. Such applications will benefit from the new approach’s capability to efficiently harvest and concentrate light energy into deep-subwavelength hot spots and to achieve significant field enhancement.

In the future, the scientists plan to extend the approach to 3D, as 3D blunt structures are easier to engineer and more suitable for practical use. Another goal is to account for the retardation effect, which could extend the theory to larger than 100 nanometers.

Explore further: Color hologram uses plasmonic nanoparticles to store large amounts of information

More information: Yu Luo, et al. “Broadband Light Harvesting Nanostructures Robust to Edge Bluntness.” Physical Review Letters 108, 023901 (2012). DOI: 10.1103/PhysRevLett.108.023901

Journal reference: Physical Review Letters search and more info website

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Sonhouse
not rated yet Jan 24, 2012
When do we see these things in actual PV's? With real gain numbers?
CapitalismPrevails
1 / 5 (1) Jan 24, 2012
LOL, it seems like there's been a solar power breakthrough everyday of the week now.
Onno
not rated yet Jan 24, 2012
Can somebody explain what is right about this statementin the article:"The nanoparticles scatter incoming light in different directions, which allows the solar cells to absorb more light than they otherwise would."
kaasinees
1 / 5 (1) Jan 24, 2012
PVs are a dead end. Its more efficient to turn light to infrared and use that.
quaillrun
not rated yet Jan 24, 2012
It would seem that when they can make a cell that uses the entire electromagnetic spectrum and not just visible light we might have something useful.
samarth
not rated yet Jan 24, 2012
@Onno : The problem in general solar cell(specially thin film) is that light is not absorbed in the material in single pass. or in other way the light need more time to be in material to be absorbed (or more material to be absorbed), Now by scattering light into different directions will increase the path length of light traveled into the material. Also another important thing in using nanoparticles is that Scattering cross section is much much bigger then the physical cross section. In other we can say that they scatter light by factor of many time then their original area.
samarth
not rated yet Jan 24, 2012
""SP resonance modes are continuous."" If that is really possible in real world, It can defiantly change the Solar cell world
SmarterThanYou
1 / 5 (1) Jan 27, 2012
Who cares? As anyone who actually works in the PV INDUSTRY (as opposed to academia) knows, monocrystalline and polycrystalline Si wafer PV has gotten so inexpensive, that the dominant cost of modules using them is the balance of plant, not the PV. What that means is that even if someone came up with a "free" PV technology in the 20-30% efficiency range, it won't result in modules significantly less expensive than present day monocrystalline and polycrystalline Si wafer PV modules. In other words, all of the so-called economic justification for plasmonic and thin film PV development is moot. (Besides the fact that the literature is full of bad "plasmonic" simulations and short of any of those simulations being experimentally verified.)
rawa1
not rated yet Jan 27, 2012
The nano-structured surfaces have serious drawback in lowered life-time of solar panels, because they're more prone to the photo-oxidation and degradation.