13 August 2012
Scientists at the University of Toronto, Canada, and the King Abdullah University of Science & Technology (KAUST) in Saudi Arabia collaboratively have created a colloidal quantum dot (CQD) thin-film solar cell with a certified world-record efficiency of 7%. The 37% increase over the previous certified world record was made possible by a technical advance called “hybrid passivation scheme”.
The breakthrough could further pave the way for inexpensive, fast mass production of thin-film solar technologies. “Our work shows that colloidal quantum dots — a light and versatile material — continue to advance rapidly, offering the possibility of creating solar cells on flexible substrates, using ‘roll-to-roll’ manufacturing in the same way that newspapers are printed in mass quantities,” says Alex Ip, PhD candidate at the University of Toronto and a member of the Sargent group. Professor Edward Sargent led this research work.
This record-breaking headway in solar technology would not have been possible without the breakthrough in CQD film design accredited to the Saudi-Canadian team. The efficiency of previous quantum dot solar cells had been limited by trap states in which electrons can get stuck, reducing electricity output. The biggest challenge here was to eliminate these electronic traps in imperfect CQD surfaces. “We developed a process by which these sites are largely passivated in solution during the growth phase of the nanocrystals,” explains Ip, who, along with postdoctoral fellow Susanna Thon, is a lead co-author of the letter “Hybrid passivated colloidal quantum dot solids” published in Nature Nanotechnology. “By using small chlorine atoms in the solution phase, we are able to passivate the surface sites, where bulky organic ligands would be unable to reach. CQD devices require not only high quality quantum dots, but a high density of nanocrystals for good electrical transport. Thus, our solid-state exchange to short bidentate linkers was used to pack the quantum dots closely.”
Ip considers the research a true team effort between his university and KAUST. “We developed our hybrid passivation scheme with the goal of improving the quality of our quantum dot surfaces within a dense film. We showed, using electronic spectroscopy techniques at the University of Toronto, that our films had a low number of electron traps,” he explains. “The KAUST group used synchrotron X-ray scattering methods with sub-nanometer resolution to verify the structure of the films and prove that our hybrid passivation method led to the densest films with the closest-packed nanoparticles.”
His team is wasting no time resting on their world record laurels. “It turns out that there are many organic and inorganic materials out there that could be used in a hybrid passivation scheme, so we are exploring the options with the goal of further reducing electron traps in our films for higher efficiency,” Ip reports about their plans. “We are also interested in using layers of different-sized quantum dots to make a multijunction solar cell that could absorb even more of the solar spectrum.”
In fact, the group has their eyes on the maximum efficiency of 49% for triple-junction cells. The CQD photovoltaics expert says, “Since quantum dots exhibit simple tuning of absorption properties by altering the nanocrystal diameter, they are prime candidates for producing multijunction solar cells to absorb the entire solar spectrum.” They are now investigating other candidate materials for their hybrid passivation approach to improve the electrical and optical properties of the films and are optimising device architectures for better power conversion efficiencies.
Written by Sandra Henderson, Research Editor, Solar Novus Today
, PV Cells & Modules
, Organic PV
, Research - Universities
, Quantum Dots
, Middle East