On a clear night, Kaitlyn VanSant will be able to watch her work whiz by.
Knowing the success of her project, however, will have to wait until her
tiny, temporary addition to the International Space Station returns to
Earth.
"My family and I have definitely been looking up at night more frequently,"
said VanSant, who earned her doctorate from the Colorado School of Mines in
materials science last year. Now a postdoctoral researcher with NASA,
VanSant holds a unique collaborative appointment at the National Renewable
Energy Laboratory (NREL).
The pairing of NREL and NASA continues a long-standing alliance between
solar power and space. Specialized photovoltaic (PV) panels turned to the
sun have been used to generate electricity for Mars rovers and space probes,
but the manufacturing costs of these high-efficiency solar cells are too
high for use on Earth. Researchers at NREL are testing ways to bring those
costs down for terrestrial applications and transforming how PV technologies
could work in space as well.
The latest test will evaluate the potential use of perovskite solar cells in
space and assess the durability of materials used in those cells. VanSant
worked with Ahmad Kirmani, Joey Luther, Severin Habisreutinger, Rosie
Bramante, Dave Ostrowski, Brian Wieliczka, and Bill Nemeth at NREL to
prepare the perovskite cells and materials. Eight of these samples are
scheduled to launch to the space station in August and another set of 25
samples will be launched in the spring of 2022. The samples, each of which
are a square inch in size, are part of the Materials International Space
Station Experiment (MISSE) program and will be fastened to the outside of
the orbiting platform.
The International Space Station (ISS) serves as an orbiting research
laboratory and observatory that conducts scientific experiments in a range
of fields that include astronomy, physics and materials science, to name
just a few.
"We get to prove very nascent technologies in such a way that we don't fool
ourselves by simulating the space environment on the ground in a vacuum
chamber, for example," said Timothy Peshek, an electrical engineer in the
photovoltaics group at NASA Glenn Research Center in Cleveland and VanSant's
postdoctoral adviser. "This is the real-world operation."
With approval in hand to return PV experiments to the space station, Peshek
put out calls for researchers who might want to take part. Adele Tamboli, a
researcher in the Materials Physics research group at NREL, welcomed the
opportunity, and introduced Peshek to VanSant.
"Partnering with the National Renewable Energy Laboratory just made a lot of
sense," said Peshek, himself a former post-doctoral researcher at NREL.
"They had the facilities and abilities ready to go on day one."
Solar power on Earth tends to be generated from silicon modules. Other PV
technologies, such as those used in space, rely on materials from the III
and V columns of the periodic table and are dubbed III-V cells. Scientists
have experimented with stacking a III-V cell atop a layer of silicon to
increase the efficiency of capturing sunlight to convert to electricity. By
itself, the most efficient silicon solar cell is about 26%, when measured
under the typical terrestrial solar spectrum. (The solar spectrum is
different on Earth and in space.)
Tamboli was among the research group that set records in 2017 for III-V
cells on silicon, including a triple-junction cell with an efficiency of
35.9%. She, along with VanSant and staff scientist Emily Warren, would later
propose that these types of cells could find a use to power satellites in a
low Earth orbit. Before that could happen, the cells had to be tested in the
extreme conditions of space.
If the moon is a harsh mistress, space itself can be equally cruel.
Equipment is subjected to extreme swings in temperatures and bombarded by
solar radiation. When the ISS moves behind the Earth and away from the sun,
the temperature plummets to 250 degrees below zero Fahrenheit. Emerging into
sunlight spikes the temperature to 250 degrees above zero.
"That's harsh," Peshek said. "That's a pretty brutal environment."
"Radiation damage is a factor," said Warren. "Our record cell was gallium
arsenide on silicon, and the one that we sent up is actually gallium indium
phosphide on silicon. That was because we know that those materials would be
more radiation tolerant."
SpaceX's cargo re-supply spacecraft carried NREL's III-V-on-silicon solar
cell to the ISS in March 2020. VanSant, whose Ph.D. research centered on
III-V-on-silicon tandem solar cells, worked with Michelle Young and John
Geisz at NREL to fabricate the prototype cell for the MISSE project, and
watched a broadcast of the rocket launch carrying it into space.
"I watched it with my two daughters," VanSant said. "They got a real kick
out of it. I mean, you can't really watch a space launch without just being
completely fascinated. Nobody can be blasé about a space launch."
The prototype spent 10 months affixed to the exterior of the ISS before
being returned to Earth in January.
"The post-flight analysis of the cell gives us the opportunity to study how
we want to evolve the design and to improve it for performance and to see
whether it's realistic that this could be a technology for providing power
in space," VanSant said.
Now she is playing a waiting game for the perovskite solar cells and
materials, which are expected to spend six months on the ISS. The process is
not a straight shot into space. After NREL, the cells are shipped to
Alphaspace, a Houston company that prepares the samples for operation on the
MISSE platform and arranges the launch of the experiment aboard a SpaceX
flight.
Perovskite solar cells are grown using a mixture of chemicals, and notable
for a rapid improvement in how efficiently they are able to harness sunlight
for energy. Ongoing experimentation involves readying perovskite cells for
commercial use. The early perovskite cells degraded too quickly. Progress
has been made but there is still work to do.
"It's a real interesting problem," Peshek said, "because these cells are
notorious for having degradation problems. But the reason they degrade is
because of moisture and oxygen. We don't have to worry about that in space."
Earth-bound experiments conducted in radiation test facilities demonstrate
perovskite solar cells are surprisingly tolerant to radiation, said Joseph
Luther, a senior scientist at NREL, co-adviser on the project, and an expert
in perovskite technology. "They are very thin, and so that helps a lot. Most
of the radiation just goes right through them. Silicon, relative to
perovskites, is hundreds of times thicker. It's also very cheap due to the
production scale and is awesome for terrestrial PV applications, but in
space it's so thick that when radiation is impinging on the surface it gets
absorbed and it damages the cell, causing problems."
Lightweight perovskite solar cells would fit with NASA's ongoing mission to
reduce the price for putting a payload into orbit, from about $10,000 per
pound today to hundreds of dollars a pound within a quarter-century.
"We're very interested in trying to match the efficiency of the III-V solar
cells, but do it in an extremely lightweight cell design," Luther said.
"Perovskites can be deposited on plastics or metal foils and things like
that, which are comparatively lightweight."
The efficiency of the solar cells was measured before leaving NREL and will
be measured again upon their return. Both the cells and the component
materials of the cells will also be characterized before and after flight,
with imaging expertise provide by Steve Johnston. How well the perovskite
cells and materials survived their trip will be immediately apparent.
Lyndsey McMillon-Brown, a research engineer at NASA Glenn Research Center
and principal investigator on the effort to bring working with Peshek on
bringing perovskites to space, said a color change offers the first clue.
"The desirable phase for a perovskite solar cell is a black phase," she
said. "The film is jet black. However, when these things degrade, they turn
into a yellowy mustard color. So we're hoping to see black films upon their
return."
The lessons learned from the time the perovskites spend in space could help
with the technology terrestrially. "Some of the things that we're facing in
space are extreme, like extreme temperature cycling, extreme UV exposure,
but when you're here on Earth you still have UV exposure and you still have
temperature cycling," McMillon-Brown said. "It's just not as rapid and
frequent. We're still thinking that our lessons learned and our findings
will apply and help make perovskites more marketable and gain a bigger
commercial market share here on Earth, too."
While waiting for the return of the perovskite samples, VanSant receives a
regular reminder of the ongoing work. She signed up for text notifications
about when the ISS is visible overhead. When the time is right and her 7-
and 9-year-old daughters are awake, they try to spot the space station.
"In addition to watching the ISS go by in the night sky, we have also
watched NASA's video footage from cameras outside the ISS that show the
Earth passing by as the ISS orbits," VanSant said. "The launch of these
cells has been a great reminder to look up into the night sky, but also an
opportunity to see things from a completely different perspective."
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