Over the years, this blog has reported on the work of Edward H. Sargent, and his research team at the University of Toronto, in employing colloidal quantum dots (CQD) for photovoltaics (PVs).
Last year, Sargent’s work, which had been backed by funding from the King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia, managed to develop a CQD multijunction PV that had a “graded recombination layer” that served as an interface between the visible and infrared junction, passing electrons between the two layers.
While being able to harvest both visible and invisible light with PVs was impressive, the PVs had comparatively low conversion efficiencies of 4.2 percent, which was significantly lower than the 5 percent levels that were the state of the art at the time for CQD multijunction PVs. Even with Sargent’s team reaching 6 percent with their CQD multijunction PVs later in the year, it seemed there was still room for improvement, especially if you consider theoretical levels for CQDs of 42 percent.
Now Sargent and his team—along with both financial and research support from his Saudi backers at KAUST—have pushed the conversion efficiencies of these devices up to what they claim is a record-breaking 7 percent efficiency.
The research, which was published in the journal Nature Nanotechnology in a paper titled “Hybrid passivated colloidal quantum dot solids” (available without a subscription), looked at the problem of the high number of electron traps in the surfaces of CQD films. They determined that a hybrid passivation scheme could dramatically improve the combination of surface passivation and film density and thereby reduce the trap densities.
The "hybrid passivation scheme” consisted of introducing chlorine atoms to the quantum dots immediately after synthesizing them. This made it possible to coat areas on CQD films that had been unreachable before.
“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 lead co-author Susanna Thon, in a University of Toronto press release. “Our breakthrough was to use a combination of organic and inorganic chemistry to completely cover all of the exposed surfaces.” Thon is a post-doctoral fellow at the university.
The researchers believe that this latest 37-percent improvement in conversion efficiency for the CQD PVs represents a sign of things to come.
As Sargent further notes in the press release: “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."
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.