Tag Archives: Susanna Thon

University of Toronto’s Ted Sargent and his colloidal quantum dots make news again

Ted Sargent at the University of Toronto is one of the most consistent communicators, in Canada, about nanoscale research. His work is focused on solar panels/cells and colloidal quantum dots and according to a Mar. 7, 2013 news release on EurekAlert, there have been some new developments,

A new technique developed by U of T Engineering Professor Ted Sargent and his research group could lead to significantly more efficient solar cells, according to a recent paper published in the journal Nano Letters.

The paper, “Jointly-tuned plasmonic-excitonic photovoltaics using nanoshells,” describes a new technique to improve efficiency in colloidal quantum dot photovoltaics, a technology which already promises inexpensive, more efficient solar cell technology. Quantum dot photovoltaics offers the potential for low-cost, large-area solar power – however these devices are not yet highly efficient in the infrared portion of the sun’s spectrum, which is responsible for half of the sun’s power that reaches the Earth.

The solution? Spectrally tuned, solution-processed plasmonic nanoparticles. These particles, the researchers say, provide unprecedented control over light’s propagation and absorption.

The new technique developed by Sargent’s group shows a possible 35 per cent increase in the technology’s efficiency in the near-infrared spectral region, says co-author Dr. Susanna Thon. Overall, this could translate to an 11 per cent solar power conversion efficiency increase, she says, making quantum dot photovoltaics even more attractive as an alternative to current solar cell technologies.

The University of Toronto Mar. 7, 2013 news release written by Terry Lavender, which is the original of the one on EurekAlert, goes on to explain the interest in colloidal quantum dots and to describe the new technique,

“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.

It’s exciting work and a 35% increase in efficiency sounds great, although the base efficiency isn’t mentioned. If your base is one and you increase it to two, you have a 100% increase. As I noted in my July 30, 2012 posting about the team’s last breakthrough which showed a 37% increase in efficiency for their technique but actually worked out to a 7% increase for solar cell efficiency,

I think the excitement over 7% indicates just how much hard work the researchers have accomplished to achieve this efficiency. It reminds me of reading about the early development of electricity (Power struggles; Scientific authority and the creation of practical electricity before Edison by Michael Brian Schiffer)  where accomplishments we would now consider minuscule built careers.

These increases  may be small but they are important not only for the development of solar cells but also as an illustration of how scientific breakthroughs are often a series of small steps and of the infinite patience exercised by researchers.

Colloidal quantum dot film from the University of Toronto and KAUST certified world’s most efficient

In my Sept. 20, 2011 posting, I featured an item about Ted Sargent ‘s (University of Toronto, Canada) work on colloidal quantum dot films. These films have now been certified as the world’s most efficient. There seems to be a lot of excitement given that these films have achieved a 7% efficiency rating. From the July 30, 2012 news item by Will Soutter on Azonano,

A team of scientists from the King Abdullah University of Science & Technology (KAUST) and University of Toronto (U of T) headed by Ted Sargent, an U of T Engineering Professor, has achieved a significant progress in the advancement of colloidal quantum dot (CQD) films, which in turn results in a CQD solar cell with an unprecedented efficiency of 7%.

The July 30, 2012 news release from the University of Toronto provides more detail,

“Previously, quantum dot solar cells 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.”

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.

I think the excitement over 7% indicates just how much hard work the researchers have accomplished to achieve this efficiency. It reminds me of reading about the early development of electricity (Power struggles; Scientific authority and the creation of practical electricity before Edison by Michael Brian Schiffer)  where accomplishments we would now consider minuscule built careers.