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This mean an 11 percent solar power conversion efficiency increase overall

A Canadian research team has used a certain type of nanoparticle to increase the efficiency of solar technology.

Ted Sargent, study leader and a professor at the University of Toronto's Engineering Department, along with Dr. Susanna Thon, has improved the efficiency of colloidal quantum dot photovoltaics through the use of plasmonic nanoparticles.

While colloidal quantum dot photovoltaics offer a lot of potential for large-area, low-cost solar power, they're not quite as efficient in the infrared area of the sun's spectrum.

To address this, the team used plasmonic nanoparticles that are spectrally tuned, meaning that they offer control over light absorption. The gold nanoshells were embedded directly into the quantum dot absorber film to do so.

The team added that gold is not the only material that can be used for this, since it isn't the most economical. Lower-cost materials can be used as well.

Using this technique, the team saw a 35 percent increase in efficiency in the near-infrared spectral region. This mean an 11 percent solar power conversion efficiency increase overall.

"There are two advantages to colloidal quantum dots," Thon said. "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."

From here, the team plans to look into cheaper metals to build a better cell.

Source: Science Daily

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Gaps in information.
By Sam07 on 3/11/2013 4:59:07 PM , Rating: 2
So the maximum conversion efficiency (CE) for traditional Si solar cells is 32%. So then what would be the maximum CE for these new quantum dot cells? Or is that unknown because no one knows how tightly one can pack in the dots per given area?

RE: Gaps in information.
By 3DoubleD on 3/12/2013 10:18:41 AM , Rating: 4
The theoretical maximum efficiency of solar cells are generally well known. If you know the bandgap of your material, you can calculate the maximum efficiency using a detailed balance approach. The famous example is the Shockley–Queisser limit, which was done for silicon solar cells, which found the limiting efficiency is ~31% depending on your illumination conditions.

The quantum dot solar cells are a more difficult to predict. For one, there is no fixed bandgap, since they can tune this by changing the quantum dot diameters. This is a good thing, as this degree of freedom allows for further optimization, but it adds complexity. Now we are dealing with a tandem solar cell configuration.

Since they are able to "infinitely tune" the quantum dot bandgaps to absorb radiation across the solar spectrum, the theoretical maximum efficiency could approach 87% efficiency; however, this would require a perfect quantum efficiency, which is extremely difficult to approach in this system.

To achieve a perfect quantum efficiency, every electron-hole pair that is produced must escape the quantum dots before they recombine. In many ways, this goes against the nature of quantum dots, which make excellent recombination centers. After the electrons and holes escape, they must conduct through the matrix material (the stuff between the dots), which will be a lossy process as well. Add in additional losses at tunnel junctions, imperfect current matching, and reflective losses, ect., and the maximum efficiency in practice will be much less.

Still, if the $/Watt turns out to be favorable it could become widely used. If not, it will be limited to niche applications where it's unique packaging or form factor give it a unique advantage.

Lastly, this technology is also really interesting for photodetectors, especially as a replacement for camera sensors. With these quantum dot films, you can achieve a 100% front illuminated pixel designs (no shadowing losses) and true RBG pixels. Low level light performance would greatly increase compared to current front and back illuminated designs.

RE: Gaps in information.
By Sam07 on 3/12/2013 3:20:58 PM , Rating: 2
Thank you for that excellent reply! So is the amorphous nature of quantum dots the reason for the much higher theoretical efficiency of 87%? That would completely obsolete traditional Si solar cells considering that any gains in Si past the Shockley-Queisser limit involve intricate magnification techniques that would not be easily applied into a commercial setting. Assuming of course that quantum dots can indeed to tuned to reach it's theoretical maximum!

As for cost, from what I remember in school, quantum dots could be printed using already existing inkjet printers adapted for suitable use. That would mean that QD solar cells could be produced using the extremely cheap thin film method which could produce large sheets of solar panels relatively quickly and inexpensively.

At any rate, thank you for all the fantastic info! You answered my question well and then some!

RE: Gaps in information.
By 3DoubleD on 3/12/2013 7:53:06 PM , Rating: 2
I'm happy I was able to answer it.

I wouldn't go so far as to call the quantum dots amorphous. They are "quasi-crystals" in that their atomic structure has long range order... over their small diameter. The fact that they can easily produce these crystals in a test tube by chemistry, chemically modify the surface, and electrically connect the quantum dots to a matrix material that is both conductive and allows for the separation of the electron-hole pairs is what makes this work. It is entirely different than any other kind of solar cell out there, which are usually crystalline films. It is really more similar to organic solar cells in principle, but using inorganic semiconductors instead.

Now while I said the theoretical maximum could be up to 87%, I wouldn't expect it to get anywhere near that. If they could get 30% though (especially in a production cell), that would be massively impressive.

The process they use to make these does lend itself to cheap manufacturing methods like you said, so we'll see if they can scale it up!

RE: Gaps in information.
By Sam07 on 3/13/2013 4:35:23 PM , Rating: 2
That's interesting, although since quantum dots are able to be infinitely tuned I wouldn't be surprised if QDs can be tuned for the entire spectrum of sunlight to absorb as much energy as possible. Heck, one could even use a multi-layered approach where one layer is tuned for say infrared and visible light while the next layer is tuned for ultra violet and visible light. The materials science is really the meaty and interesting part of this research in that the potential applications are truly limitless. :)

However, do you think that you may be a tad conservative about the practical limits QD solar cells? Commercialized solar panels can reach ~22% CE for polycrystalline panels and ~15% for the much cheaper thin film variety, or roughly 67% and 50% of the Shockley-Queisser limit respectively. So just carrying over the ratios, if researchers can reach even 50% of the theoretical limit that would translate to a CE of 44%!

Considering that a square meter of sunlight that reaches the Earth's surface contains 1100 watts of energy, a one square meter panel could harness 484 watts of energy on a sunny day! Also consider that the roof size of an average middle class home is 223 square meters and if you can allocate 75% for panel coverage then you can collect almost 81kW of energy just in one sunny day! Plus, seeing as how that's much more energy than what most people use in a day, you could make a fortune selling energy back to the utility company! Being a personal energy tycoon sounds rather appealing, wouldn't you say? :)

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