This artist’s conception depicts the sudden contraction and elongation experienced by the unit cell of the ferroelectric material lead titanate as an intense pulse of violet light hits it. This atomic-scale wiggle represents the first step in the photovoltaic response that light produces in this and related materials. Illustration by Gregory M. Stewart/SLAC National Accelerator Laboratory |
A
surprising atomic-scale wiggle underlies the way a special class of
materials reacts to light, according to research that may lead to new
devices for harvesting solar energy.
For
decades, scientists have known that some ferroelectric
materials—materials that possess a stable electrical polarization
switchable by an external electric field—are also photovoltaic: They
produce an electric voltage when exposed to light, just as solar cells
do. But it was not clear how the light induced voltages in these
materials.
Such
insight is very useful to researchers hoping to design ferroelectrics
with improved photovoltaic properties for use in solar cells and other
applications, such as sensors and ultrafast optical switches for data
and telecommunications networks. Several possible mechanisms have been
proposed, with many open questions still remaining.
Now, in research published last week in Physical Review Letters,
scientists led by Aaron Lindenberg of SLAC’s Stanford Institute for
Materials and Energy Science and the Stanford Materials Science and
Engineering Department, along with graduate student Dan Daranciang, have
determined first-hand what is going on: Stop-action X-ray snapshots of a
ferroelectric nanolayer showed that the height of its basic building
block, called a unit cell, contracted in response to bright light and
then rebounded to become even longer than it was to begin with.
The
entire in-and-out atomic-scale wiggle took just 10 trillionths of a
second, yet it indicated the mechanisms responsible for the materials’
photovoltaic effect. “What we saw was unanticipated,” Lindenberg said.
“It was amazing to see such dramatic structural changes, which we showed
were caused by light-induced electrical currents in the ferroelectric
material.”
The
telling X-ray images were taken at the X-ray Pump Probe instrument of
SLAC’s Linac Coherent Light Source (LCLS), which hit the ferroelectric
samples with a stunningly rapid one-two punch of violet laser light (40
quadrillionths of a second long) and X-rays (60 quadrillionths of a
second long). The researchers analyzed information from thousands of
images to determine the photovoltaic mechanism.
The
fact that ferroelectric materials produce much higher voltages than
conventional silicon-based materials makes them an attractive option for
making solar cells, Lindenberg said. But their very low
light-conversion efficiency has precluded commercial applications. Now
that researchers understand the underlying mechanism, he said, they can
more effectively create ferroelectric materials that are more suitable
for photovoltaic applications.
