Breakthrough in colloidal quantum dot films leads to record efficiency for next-generation solar cells

Jul 29, 2012

Researchers from the University of Toronto and King Abdullah University of Science & Technology (KAUST) have made a breakthrough in the development of colloidal quantum dot (CQD) films, leading to the most efficient CQD solar cell ever. Their work is featured in a letter published in Nature Nanotechnology.

The researchers, led by U of T Engineering Professor Ted Sargent, created a solar cell out of inexpensive materials that was certified at a world-record 7.0% efficiency.

"Previously, quantum dot have been limited by the large internal surface areas of the nanoparticles in the film, which made extracting electricity difficult," said Dr. Susanna Thon, a lead co-author of the paper. "Our breakthrough was to use a combination of organic and inorganic chemistry to completely cover all of the exposed surfaces."

Quantum dots are semiconductors only a few nanometres in size and can be used to harvest electricity from the entire solar spectrum – including both visible and invisible wavelengths. Unlike current slow and expensive semiconductor growth techniques, CQD films can be created quickly and at low cost, similar to paint or ink. This research paves the way for solar cells that can be fabricated on flexible substrates in the same way newspapers are rapidly printed in mass quantities.

The U of T cell represents a 37% increase in efficiency over the previous certified record. In order to improve efficiency, the researchers needed a way to both reduce the number of "traps" for electrons associated with poor surface quality while simultaneously ensuring their films were very dense to absorb as much light as possible. The solution was a so-called "hybrid passivation" scheme.

"By introducing small chlorine atoms immediately after synthesizing the dots, we're able to patch the previously unreachable nooks and crannies that lead to electron traps," explained doctoral student and lead co-author Alex Ip. "We follow that by using short organic linkers to bind quantum dots in the film closer together."

Work led by Professor Aram Amassian of KAUST showed that the organic ligand exchange was necessary to achieve the densest film.

"The KAUST group used state-of-the-art synchrotron methods with sub-nanometer resolution to discern the structure of the films and prove that the hybrid passivation method led to the densest films with the closest-packed nanoparticles," stated Professor Amassian.

The advance opens up many avenues for further research and improvement of device efficiencies, which could contribute to a bright future with reliable, low cost solar energy.

According to Professor Sargent, "Our world urgently needs innovative, cost-effective ways to convert the sun's abundant energy into usable electricity. This work shows that the abundant materials interfaces inside colloidal quantum dots can be mastered in a robust manner, proving that low cost and steadily-improving efficiencies can be combined."

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CapitalismPrevails
3.7 / 5 (3) Jul 29, 2012
7% doesn't seem like a lot. Although aren't quantum dots cable of absorbing infrared light and ambient light? So shouldn't they offer more consistent power compared to silicon panels?
PhotonX
5 / 5 (4) Jul 29, 2012
CQD films can be created quickly and at low cost, similar to paint or ink. This research paves the way for solar cells that can be fabricated on flexible substrates in the same way newspapers are rapidly printed in mass quantities.
Sweet! Now, is it in any way possible of actually using newspaper presses to print these things? There's an awful lot of newspapers falling on hard times in the Internet age. Sure would be nice to see at least some of these come back to life. I know, not likely to happen compared to just building new specialized presses, but recycling the old ones has a certain appeal to me. Maybe they could just throw in a page of solar cells in the Sunday edition?
Jimee
5 / 5 (2) Jul 29, 2012
The key to me seemed to be the words "inexpensive materials."
h20dr
5 / 5 (2) Jul 29, 2012
The discoveries in this discipline are growing by leaps and bounds. That's why I haven't invested in this technology for personal use. I keep thinking that if I do then it's only a short time before someone comes out with something better and my investment isn't as attractive from a ROI point of view. Kinda like other technologies such as computers, iPads and iPhones... Buyers remorse. I guess that's part of why Solyndra and other companies are failing- besides the mismanagement and economic supply side government meddling aspect that is...
h20dr
5 / 5 (2) Jul 29, 2012
I wish someone with technical savvy related to this field would shine some light on what this really means compared to existing solar cell efficiencies of panels that are mass produced for residential use and put it into layman's terms for my small brain to understand.
Thanks in advance!
h20dr
5 / 5 (2) Jul 30, 2012
7% doesn't seem like a lot. Although aren't quantum dots cable of absorbing infrared light and ambient light? So shouldn't they offer more consistent power compared to silicon panels?


More Consistent and Efficient? But when the lights go out storage is still an issue.
h20dr
5 / 5 (1) Jul 30, 2012
"Maybe they could just throw in a page of solar cells in the Sunday edition?"

THAT would be cool, lol.
lbuz
not rated yet Jul 30, 2012
"An amorphous silicon solar cell having a conversion efficiency of 10.50 percent. This paper appears in: Electron Device Letters, IEEE, Date of Publication: Aug 1984 Author(s): Yamazaki, S. et. al."
Note the date of publication; 1984. But consumer level amorphous silicon systems (the most widely installed technology)are only 3-4% efficient. What's up with that? What's up is that 'efficiency' is only one of the many metrics that quantify the practical and economical performance of a given system. The 10.5% efficient technology referenced in the paper noted above in this comment was obviously not widely adopted, probably for practical considerations involving fabrication costs, life-cycle performance and/or any number of other potential show stoppers. Thermodynamic efficiency, physical stability, amount of incident light absorbed/reflected, transmission losses in charge transport, tough problems, difficult to quantify and address in 9 paragraphs. Keep salt handy when consuming summaries
DonaldJLucas
5 / 5 (1) Jul 30, 2012
Solar cells come in a number of varieties and the last time that I checked (about a year ago), the best solar cell efficiencies were running around 23%. The quantum dot cells are a really big deal because the technique is very low cost. Yes, the surface areas will have to be about 3.5 times larger than traditional solar cells, but they won't cost an arm and a leg, either.

I tested one manufacturer's low cost printable cell and they were getting around 12 watts out of a sheet that was 3ft x 6ft on a bright day. I don't know the efficiency that this manufacturer achieved, but a 2ft x 4ft traditional solar panel achieves around 240W.

The application space that I find really exciting for very low cost printing technology type cells are in merging solar cells with glass office tower panels and other building exterior construction methods.
DonaldJLucas
5 / 5 (1) Jul 30, 2012
Amorphous silicon cells are the lower cost cells that have become popular of late. Ibuz' 10.5% number is probably about right. The 23% cells that I referred to in my previous post are the very expensive monocrystalline silicon cells, not amorphous. But even the lower cost of amorphous silicon cells should still be much more expensive to produce than the quantum dot technology.
lbuz
not rated yet Jul 30, 2012
Right, agreed DL, I picked AS as a handy low end benchmark in response to the previous question and was egregiously off on the 3%, wish it were a typo. But I think that when considering the utility of new tech its wise to take a broader view than any single metric. As you say this is a very promising avenue with many potential applications, but with inherent limitations as well. Time will tell. Apologies for the misstatement.
ab3a
not rated yet Jul 30, 2012
OK, so the solar cells have lots of ways to get cheaper. However, there are a couple notable issues: First, it is not just the cost of the cells, it is the cost of installation that makes a difference too. Do not overlook the fact that chasing dollars per watt doesn't help much if you need acres of this stuff to power just one house.
antialias_physorg
3 / 5 (2) Jul 30, 2012
it is not just the cost of the cells, it is the cost of installation that makes a difference too

It really depends on whether you even need a separate installation. Today that is the case. But there are already companies on the market that sell PV shingles (where the additional installation is much less). The addition of PV there has its cost in the manufacturing process so these aren't cheap. Having a cheap way to add PV to virtually anything could be a game changer. (Think windows, wallpaint, shingles, large stretches of fabric that could cover entire fields when not in use (e.g. during the wintertime) or even on lakes, etc. )

Do not overlook the fact that chasing dollars per watt doesn't help much if you need acres of this stuff

With houses and consumer electronics getting more energy efficient, that, too, will change. Over here (germany) electricity use per person is already dropping. Per person we use, on average, about 750W. That's doable with PV.