Blue
quantum dot solution glowing in a vial in a lab.
Courtesy: Canadian Light Source
There are
many things quantum dots could do, but the most obvious place they could change
our lives is to make the colors on our TVs and screens more pristine. Research
using the Canadian Light Source (CLS) at the University of Saskatchewan is
helping to bring this technology closer to our living rooms.
Quantum
dots are nanocrystals that glow, a property that scientists have been working
with to develop next-generation LEDs. When a quantum dot glows, it creates very
pure light in a precise wavelength of red, blue or green. Conventional LEDs,
found in our TV screens today, produce white light that is filtered to achieve
desired colors, a process that leads to less bright and muddier colors.
Until now,
blue-glowing quantum dots, which are crucial for creating a full range of
color, have proved particularly challenging for researchers to develop.
However, University of Toronto (U of T) researcher Dr. Yitong Dong and
collaborators have made a huge leap in blue quantum dot fluorescence, results
they recently published in Nature Nanotechnology.
"The
idea is that if you have a blue LED, you have everything. We can always down
convert the light from blue to green and red," says Dong. "Let's say
you have green, then you cannot use this lower-energy light to make blue."
The team's breakthrough has led to quantum dots that produce green light at an external quantum efficiency (EQE) of 22% and blue at 12.3%. The theoretical maximum efficiency is not far off at 25%, and this is the first blue perovskite LED reported as achieving an EQE higher than 10%.
Yitong
Dong, a researcher with the University of Toronto.
Courtesy: Canadian Light
Source
The
Science
Dong has
been working in the field of quantum dots for two years in Dr. Edward Sargent's
research group at the U of T. This astonishing increase in efficiency took
time, an unusual production approach, and overcoming several scientific hurdles
to achieve.
CLS
techniques, particularly GIWAXS on the HXMA beamline, allowed the researchers
to verify the structures achieved in their quantum dot films. This validated
their results and helped clarify what the structural changes achieve in terms
of LED performance.
"The
CLS was very helpful. GIWAXS is a fascinating technique," says Dong.
The first challenge was uniformity, important to ensuring a clear blue color and to prevent the LED from moving towards producing green light.
Quantum
dot LED light while operating.
Courtesy: Canadian Light Source
Next, the
team needed to tackle the charge injection needed to excite the dots into
luminescence. Since the crystals are not very stable, they need stabilizing
molecules to act as scaffolding and support them. These are typically long
molecule chains, with up to 18 carbon-non-conductive molecules at the surface,
making it hard to get the energy to produce light.
"We
used a special surface structure to stabilize the quantum dot. Compared to the
films made with long chain molecules capped quantum dots, our film has 100
times higher conductivity, sometimes even 1000 times higher."
This
remarkable performance is a key benchmark in bringing these nanocrystal LEDs to
market. However, stability remains an issue and quantum dot LEDs suffer from
short lifetimes. Dong is excited about the potential for the field and adds,
"I like photons, these are interesting materials, and, well, these glowing
crystals are just beautiful."