Researchers
from the University of Toronto (U of T) and King Abdullah University of
Science & Technology (KAUST) have made a breakthrough in the
development of colloidal quantum dot (CQD) films, leading to the most
efficient CQD solar cell ever. Their work is featured in a letter
published in Nature Nanotechnology (“Hybrid passivated colloidal quantum
dot solids”).

The
researchers, led by U of T Engineering Professor Ted Sargent, created a
solar cell out of inexpensive materials that was certified at a
world-record 7.0% efficiency.

“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.”

Quantum
dots are semiconductors only a few nanometers in size and can be used
to harvest electricity from the entire solar spectrum—including both
visible and invisible wavelengths.

Unlike
current slow and expensive semiconductor growth techniques, CQD films
can be created quickly and at low cost, similar to paint or ink. This
research paves the way for solar cells that can be fabricated on
flexible substrates in the same way newspapers are rapidly printed in
mass quantities.

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.

The
advance opens up many avenues for further research and improvement of
device efficiencies, which could contribute to a bright future with
reliable, low cost solar energy.

According
to Professor Sargent, “Our world urgently needs innovative,
cost-effective ways to convert the sun’s abundant energy into usable
electricity. 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.”

Source: University of Toronto