These materials are exceptionally good at turning sunlight into electrical current in solar cells.
From:
SLAC National Accelerator Laboratory
By Glennda Chui
January 4, 2021 -- Polarons are
fleeting distortions in a material’s atomic lattice that form around a moving
electron in a few trillionths of a second, then quickly disappear. As ephemeral
as they are, they affect a material’s behavior, and may even be the reason that
solar cells made with lead hybrid perovskites achieve extraordinarily high
efficiencies in the lab.
Now scientists at the Department of
Energy’s SLAC National Accelerator Laboratory and Stanford University have used
the lab’s X-ray laser to watch and directly measure the formation of polarons
for the first time. They reported their findings in Nature Materials
today.
“These materials have taken the field of
solar energy research by storm because of their high efficiencies and low cost,
but people still argue about why they work,” said Aaron Lindenberg, an
investigator with the Stanford Institute for Materials and Energy Sciences
(SIMES) at SLAC and associate professor at Stanford who led the research.
“The idea that polarons may be involved
has been around for a number of years,” he said. “But our experiments are the
first to directly observe the formation of these local distortions, including
their size, shape and how they evolve.”
Exciting, complex and hard to understand
Perovskites are crystalline materials
named after the mineral perovskite, which has a similar atomic structure.
Scientists started to incorporate them into solar cells about a decade ago, and
the efficiency of those cells at converting sunlight to energy has steadily
increased, despite the fact that their perovskite components have a lot of
defects that should inhibit the flow of current.
These materials are famously complex and
hard to understand, Lindenberg said. While scientists find them exciting
because they are both efficient and easy to make, raising the possibility that
they could make solar cells cheaper than today’s silicon cells, they are also
highly unstable, break down when exposed to air and contain lead that has to be
kept out of the environment.
Previous studies at SLAC have delved
into the nature of perovskites with an “electron camera” or with X-ray beams.
Among other things, they revealed that light
whirls atoms around in perovskites, and they also measured
the lifetimes
of acoustic phonons – sound waves – that carry heat through the
materials.
For this study, Lindenberg’s team used
the lab’s Linac
Coherent Light Source (LCLS), a powerful X-ray free-electron laser
that can image materials in near-atomic detail and capture atomic motions
occurring in millionths of a billionth of a second. They looked at single
crystals of the material synthesized by Associate Professor Hemamala Karunadasa’s
group at Stanford.
They hit a small sample of the material
with light from an optical laser and then used the X-ray laser to observe how
the material responded over the course of tens of trillionths of a second.
Expanding bubbles of distortion
“When you put a charge into a material
by hitting it with light, like what happens in a solar cell, electrons are
liberated, and those free electrons start to move around the material,” said
Burak Guzelturk, a scientist at DOE’s Argonne National Laboratory who was a
postdoctoral researcher at Stanford at the time of the experiments.
“Soon they are surrounded and engulfed
by a sort of bubble of local distortion – the polaron – that travels along with
them,” he said. “Some people have argued that this ‘bubble’ protects electrons
from scattering off defects in the material, and helps explain why they travel
so efficiently to the solar cell’s contact to flow out as electricity.”
The hybrid perovskite lattice structure
is flexible and soft – like “a strange combination of a solid and a liquid at
the same time,” as Lindenberg puts it – and this is what allows polarons to
form and grow.
Their observations revealed that
polaronic distortions start very small – on the scale of a few angstroms,
about the spacing between atoms in a solid – and rapidly expand outward in all
directions to a diameter of about 5 billionths of a meter, which is about a
50-fold increase. This nudges about 10 layers of atoms slightly outward within
a roughly spherical area over the course of tens of picoseconds, or trillionths
of a second.
“This distortion is actually quite
large, something we had not known before,” Lindenberg said. “That’s something
totally unexpected.”
He added, “While this experiment shows
as directly as possible that these objects really do exist, it doesn’t show how
they contribute to the efficiency of a solar cell. There’s still further work
to be done to understand how these processes affect the properties of these
materials.”
LCLS is a DOE Office of Science user
facility. Lindenberg is also an investigator with the Stanford PULSE Institute,
which like SIMES is a joint institute of SLAC and Stanford. Scientists from the
University of Cambridge in the U.K.; Aarhus University in Denmark; and
Paderborn University and the Technical University of Munich in Germany also
contributed to this study. Major funding came from the DOE Office of
Science.
Citation: Burak Guzelturk et al., Nature
Materials, 4 January 2021 (10.1038/s41563-020-00865-5)
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