Quantum dots are more efficient when packed tightly together. Image: Ted Sargent |
Researchers from the University of Toronto
(U of T), King Abdullah University of Science & Technology (KAUST), and Pennsylvania State
University (Penn State)
have created the most efficient colloidal quantum dot (CQD) solar cell ever.
The discovery is reported in Nature Materials.
Quantum dots are nano-scale semiconductors that capture
light and convert it into electrical energy. Because of their small scale, the
dots can be sprayed onto flexible surfaces, including plastics. This enables
the production of solar cells that are less expensive than the existing
silicon-based version.
“We figured out how to shrink the wrappers that encapsulate
quantum dots down to the smallest imaginable size—a mere layer of atoms,”
states Professor Ted Sargent, corresponding author on the work and holder of
the Canada Research Chair in Nanotechnology at U of T.
A crucial challenge for the field has been striking a
balance between convenience and performance. The ideal design is one that
tightly packs the quantum dots together. The greater the distance between
quantum dots, the lower the efficiency.
Until now, quantum dots have been capped with organic
molecules that separate the nanoparticles by a nanometer. On the nanoscale,
that is a long distance for electrons to travel.
To solve this problem, the researchers utilized inorganic
ligands, sub-nanometer-sized atoms that bind to the surfaces of the quantum
dots and take up less space. The combination of close packing and charge trap
elimination enabled electrons to move rapidly and smoothly through the solar
cells, thus providing record efficiency.
“We wrapped a single layer of atoms around each particle.
This allowed us to pack well-passivated quantum dots into a dense solid,”
explains Jiang Tang, PhD, the first author of the paper who conducted the
research while a post-doctoral fellow in The Edward S. Rogers Department of
Electrical and Computer Engineering at U of T.
“Our team at Penn State proved that we could remove charge
traps—locations where electrons get stuck—while still packing the quantum dots
closely together,” says Professor John Asbury of Penn State, a coauthor of the
work.
“At KAUST, we used visualization methods with sub-nanometer
resolution and accuracy to investigate the structure and composition of the passivated
quantum dots,” states co-author Professor Aram Amassian of KAUST in Saudi Arabia. “We proved that the inorganic passivants were tightly correlated with the
location of the quantum dots and that it was the chemical passivation, rather
than nanocrystal ordering, that led to the remarkable colloidal quantum dot
solar cell performance,” he adds.
“It is very impressive that the team was able to make solar
cells with power conversion efficiency up to 6% from quantum dots,” states Professor
Michael McGehee of Stanford
University, an expert in
solution-processed organic solar cells. “There is a lot of surface area in
these films that could have dangling bonds which would hinder the performance
of solar cells by creating traps states.”
The team’s quantum dots had the highest electrical currents
and the highest overall power conversion efficiency ever seen in CQD solar
cells. The performance results were certified by an external laboratory, Newport, which is
accredited by the U.S. National Renewable Energy Laboratory.
“This work proves the power of inorganic ligands in building
practical devices,” states Professor Dmitri Talapin of The University of
Chicago, a pioneer in inorganic ligands and materials chemistry. “This new
surface chemistry provides the path toward both efficient and stable quantum
dot solar cells. It should also impact other electronic and optoelectronic
devices that utilize colloidal nanocrystals. Advantages of the all-inorganic
approach include vastly improved electronic transport and a path to long-term
stability.”
As a result of the potential of this research discovery, a
technology licensing agreement has been signed by U of T and KAUST, brokered by
MaRS Innovations (MI), which will enable the global commercialization of this
new technology.
“The world—and the marketplace—need solar innovations that
break the existing compromise between performance and cost. Through the
partnership between U of T, MI, and KAUST, we are poised to translate exciting
research into tangible innovations that can be commercialized,” says Sargent.
