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Novel process enriches solar cell efficiency

Optimising colloidal quantum dot growth leads to significant improvements in capturing a broader range of the solar spectrum more effectively
In a paper published in the journal Nano Letters, a group of researchers from the University of Toronto describes a new technique to improve efficiency in what are called colloidal quantum dot photovoltaics.

It's a technology that already promises inexpensive and more efficient solar cell technology.

But researchers, led by Ted Sargent, say such devices could be even more effective if they could better harness the infrared portion of the sun’s spectrum, which is responsible for half of the sun’s power that reaches the Earth.



Ted Sargent in his lab

The solution has an unwieldy name: spectrally tuned, solution-processed plasmonic nanoparticles. These particles, researchers say, provide unprecedented control over light’s propagation and absorption.

The new technique developed by Sargent’s group shows a possible 35 percent increase in the technology’s efficiency in the near-infrared spectral region, says co-author Susanna Thon.

Overall, this could translate to an 11 percent solar power conversion efficiency increase, she says, making quantum dot photovoltaics even more attractive as an alternative to current solar cell technologies.

“There are two advantages to colloidal quantum dots,” Thon says. “First, they’re much cheaper, so they reduce the cost of electricity generation measured in cost per watt of power. But the main advantage is that by simply changing the size of the quantum dot, you can change its light-absorption spectrum.

"Changing the size is very easy, and this size-tunability is a property shared by plasmonic materials: by changing the size of the plasmonic particles, we were able to overlap the absorption and scattering spectra of these two key classes of nanomaterials.”

Sargent’s group achieved the increased efficiency by embedding gold nanoshells directly into the quantum dot absorber film. Gold is not usually thought of as an economical material but researchers say lower-cost metals can be used to implement the same concept proved by Thon and her co-workers.

The current research provides a proof of principle, says Thon.



“People have tried to do similar work but the problem has always been that the metal they use also absorbs some light and doesn't contribute to the photocurrent - so it's just lost light.” More work needs to be done, she adds.

“We want to achieve more optimisation, and we’re also interested in looking at cheaper metals to build a better cell. We’d also like to better target where photons are absorbed in the cell - this is important photovoltaics because you want to absorb as many photons as you can as close to the charge collecting electrode as you possibly can.”



The research is also important because it shows the potential of tuning nanomaterial properties to achieve a certain goal, says Paul Weiss, Director of the California NanoSystems Institute at the University of California, Los Angeles.

"This work is a great example of fulfilling the promise of nanoscience and nanotechnology,” Weiss says. “By developing the means to tune the properties of nanomaterials, Sargent and his co-workers have been able to make significant improvements in an important device function, namely capturing a broader range of the solar spectrum more effectively.”

Further details of this study have been published in the paper " Jointly Tuned Plasmonic–Excitonic Photovoltaics Using Nanoshells," by Daniel Paz-Soldan et al in Nano Letters (2013). 

DOI: 10.1021/nl304604y

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