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Nanoscale solar cells absorb 10 times more energy than previously thought possible

Schematic diagram of a thin film organic solar cell shows the top layer, a patterned, roughened scattering layer, in green, the organic thin film layer where light is trapped and electrical current is generated, in red, while the film that is sandwiched between two layers helps keep light contained within the thin film (Image: PNAS)
Schematic diagram of a thin film organic solar cell shows the top layer, a patterned, roughened scattering layer, in green, the organic thin film layer where light is trapped and electrical current is generated, in red, while the film that is sandwiched between two layers helps keep light contained within the thin film (Image: PNAS)
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Graduate student Aaswath Raman, Associate Professor Shanhui Fan, and post doctoral fellow Zongfu Yu (Image: L.A. Cicero)
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Graduate student Aaswath Raman, Associate Professor Shanhui Fan, and post doctoral fellow Zongfu Yu (Image: L.A. Cicero)
Schematic diagram of a thin film organic solar cell shows the top layer, a patterned, roughened scattering layer, in green, the organic thin film layer where light is trapped and electrical current is generated, in red, while the film that is sandwiched between two layers helps keep light contained within the thin film (Image: PNAS)
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Schematic diagram of a thin film organic solar cell shows the top layer, a patterned, roughened scattering layer, in green, the organic thin film layer where light is trapped and electrical current is generated, in red, while the film that is sandwiched between two layers helps keep light contained within the thin film (Image: PNAS)

Research has already shown that at the nanoscale, chemistry is different and the same is apparently true for light, which Engineers at Stanford University say behaves differently at scales of around a nanometer. By creating solar cells thinner than the wavelengths of light the engineers say it is possible to trap the photons inside the solar cell for longer, increasing the chance they can get absorbed, thereby increasing the efficiency of the solar cell. In this way, they calculate that by properly configuring the thicknesses of several thin layers of films, an organic polymer thin film could absorb as much as 10 times more energy from sunlight than predicted by conventional theory.

The key to overcoming the theoretical limit lies in holding sunlight in the grip of the solar cell long enough to squeeze the maximum amount of energy from it, using a technique called “light trapping.” Light trapping has been used for several decades with silicon solar cells and is done by roughening the surface of the silicon to cause incoming light to bounce around inside the cell for a while after it penetrates, rather than reflecting right back out as it does off a mirror. But over the years, no matter how much researchers tinkered with the technique, they couldn't boost the efficiency of typical "macroscale" silicon cells beyond a certain amount.

Duality of light

Light has a dual nature, sometimes behaving as a particle and other times as a wave of energy. As a wave, visible light has a wavelength of around 400 to 700 nanometers and Shanhui Fan, associate professor of electrical engineering, and postdoctoral researcher Zongfu Yu decided to explore whether the conventional limit on light trapping held true at such a nanoscale. They found that, even at the wavelength of visible light, the theoretical limit held true but when they began investigating the behavior of light inside a material substantially smaller than the wavelength of light, it became evident that light could be confined for a longer time, increasing energy absorption beyond the conventional limit at the macroscale.

"The amount of benefit of nanoscale confinement we have shown here really is surprising," said Yu. "Overcoming the conventional limit opens a new door to designing highly efficient solar cells."

Yu determined through numerical simulations that the most effective structure for capitalizing on the benefits of nanoscale confinement was a combination of several different types of layers around an organic thin film.

Constructing the nanoscale solar cell

He sandwiched the organic thin film between two layers of material – called "cladding" layers – that acted as confining layers once the light passed through the upper one into the thin film. Atop the upper cladding layer, he placed a patterned rough-surfaced layer designed to send the incoming light off in different directions as it entered the thin film. By varying the parameters of the different layers, he was able to achieve a 12-fold increase in the absorption of light within the thin film, compared to the macroscale limit.As well as offering greater efficiency, nanoscale solar cells offer savings in material costs, as the organic polymer films and other materials used are less expensive than silicon and the quantities required for the cells are much smaller. The organic materials also have the advantage of being manufactured in chemical reactions in solution, rather than needing high-temperature or vacuum processing, as is required for silicon manufacture.

Yu is the lead author of the paper describing the Stanford team's work, which was published online this week by Proceedings of the National Academy of Sciences.

12 comments
Aradoth
Try and tell us oil is still a better solution now Koch brothers, try and sell America your lies greedy falsifications. I dare you!
froginapot
We have an over 10 times more energy in solar power but we currently can get over 20%, that would make it 200%??? I love this idea and I applaud everyone doing this, I just don\'t understand the 10 times idea.
Muraculous
Show me a operating panel.
voluntaryist
I went to PNAS to read the full article on Nanoscale solar cells and found nothing. This could be the biggest break through in photovoltaic physics in 40 years. If this can be mass produced we can all be energy independent, e.g., good bye utilities and oil companies.
Ed
Yet another article about solar energy breakthrough, and nothing ever comes from it from a consumer standpoint!
Ronald Cooper
why can\'t they trap light in the edge of the layer, slanting the layers so that the edges of the material face slightly up, then the light would bounce many times back and forth between layers before escaping. Sounds promising, but I\'m with Ed on this one, they say great things and they produce nothing for the public and in a few years it will be replaced by the next great (non producing ) idea that will require five to ten years of research and developement funding to perfect.
csbrudy
Put the ideas in a few different time capsules and place them in Germany somewhere. When the oil companies try to squelch this one, the information will then be available. Why? Because the Germans won\'t let the oil companies squelch this one.
mhenriday
The first silicon solar cells, produced in 1941, had an energy conversion efficiency of less than 1 %. Present best efforts boast conversion efficiencies of over 25 %. The claim that this research «produce[s] nothing for the public» simply does not correspond to the facts of the matter.... Henri
Jeff Chernoff
No need to dare the Koch brothers...you can be sure they\'re moving on the patent holder already: Stanford University. Watch for the new Koch Arena at Stanford, coming soon.
Doug Batchelor
Full text here ... http://arxiv.org/ftp/arxiv/papers/1004/1004.2902.pdf