Bulky organic molecules like ethanedithiol (yellow and blue spheres) are often used as a passivating layer on colloidal quantum dots made of lead sulphide (red and green spheres) in solar cells (A). By replacing these with halogen ions (B, blue spheres) a team at the University of Toronto has improved electron transport between the quantum dots and increased the efficiency of the solar cells to record levels.
By Tyler Irving
Posted November 2011
Six per cent may not sound like a big number, but for Ted Sargent, Canada Research Chair in Nanotechnology at the University of Toronto, it’s meaningful. It represents the power conversion efficiency of his group’s colloidal quantum dot (CQD) solar cells — the highest ever reported for this technology.
Traditional silicon photovoltaics are fabricated as a single rigid crystalline layer. By contrast, CQD technology is based on nanoparticles of semiconducting materials (in this case, lead sulphide) that can be spin-coated or sprayed on a substrate surface, including ones that are lumpy or flexible. Because the size of the particles is on the same scale as the wavelengths of light, researchers can tune them to absorb whatever wavelength they like by making them slightly bigger or smaller. This past July, Sargent’s group published a paper in Nature Photonics reporting the firstever tandem CQD solar cell, which absorbed sunlight from two different frequencies using two different sizes of CQD particles.
Their latest breakthrough concerns the passivation layer, a coating that surrounds each nanoparticle and holds them together in a matrix. Traditionally, this layer was composed of bulky organic compounds like ethanedithiol. But in their latest paper, published in Nature Materials, the group was able to replace this compound with inorganic compounds: ions of bromine, chlorine and iodine. This effectively shrunk the passivation layer to the thickness of a single atom, which greatly improved the transport of electrons through the quantum dot layer. It led to the six per cent efficiency, beating the previous record of 5.1 per cent, also set by Sargent’s group.
In the future, it is hoped that CQD technology will lead to highly efficient solar cells that take less energy to produce, are flexible and absorb more of the sun’s energy than silicon, which is limited by its inherent absorption spectrum. Sargent admits that the group has more work to do in order to meet the 10 per cent efficiency that’s considered the target for CQD solar cell commercialization, or the 14 to 18 per cent achievable with silicon. But he’s confident that within the next decade or so, CQD will come into its own. “This shows that inorganic passivation strategies can be extremely effective and there’s no reason to believe that we’ve come anywhere close to what’s possible,” he says. “I think we’ve just scratched the surface.”
Photo Credit: Jiang Tang
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