"Who would have expected that an electrical insulator could
be used to improve solar energy conversion?"
DREXEL UNIVERSITY
Designers of solar cells may soon be setting their sights
higher, as a discovery by a team of researchers has revealed a class of
materials that could be better at converting sunlight into energy than those
currently being used in solar arrays.
Their research shows how a material can be used to extract power
from a small portion of the sunlight spectrum with a conversion efficiency that
is above its theoretical maximum -- a value called the Shockley-Queisser limit.
This finding, which could lead to more power-efficient solar
cells, was seeded in a near-half-century old discovery by Russian physicist
Vladimir M. Fridkin, a visiting professor of physics at Drexel, who is also
known as one of the innovators behind the photocopier.
The team, which includes scientists from Drexel University, the Shubnikov Institute of Crystallography of the Russian Academy of Sciences, the University of Pennsylvania and the U. S. Naval Research Laboratory recently published its findings in the journal Nature Photonics.
Their article "Power conversion
efficiency exceeding the Shockley-Queisser limit in a ferroelectric insulator," explains how they were able to use a barium
titanate crystal to convert sunlight into electric power much more efficiently
than the Shockley-Queisser limit would dictate for a material that absorbs
almost no light in the visible spectrum -- only ultraviolet.
A phenomenon that is the foundation for the new findings was
observed by Fridkin, who is one of the principal co-authors of the paper, some
47 years ago, when he discovered a physical mechanism for converting light into
electrical power -- one that differs from the method currently employed in
solar cells.
The mechanism relies on collecting "hot" electrons,
those that carry additional energy in a photovoltaic material when excited by
sunlight, before they lose their energy. And though it has received relatively
little attention until recently, the so-called "bulk photovoltaic
effect," might now be the key to revolutionizing our use of solar energy.
The Limits of Solar Energy
Solar energy conversion has been limited thus far due to solar
cell design and electrochemical characteristics inherent to the materials used
to make them.
"In a conventional solar cell -- made with a semiconductor
-- absorption of sunlight occurs at an interface between two regions, one
containing an excess of negative-charge carriers, called electrons, and the
other containing an excess of positive-charge carriers, called holes,"
said Alessia Polemi, a research professor in Drexel's College of Engineering
and one of the co-authors of the paper.
In order to generate electron-hole pairs at the interface, which
is necessary to have an electric current, the sunlight's photons must excite
the electrons to a level of energy that enables them to vacate the valence band
and move into the conduction band -- the difference in energy levels between
these two bands is referred to as the "band gap."
This means that in photovoltaic materials, not all of the
available solar spectrum can be converted into electrical power. And for
sunlight photon energies that are higher than the band gap, the excited
electrons will lose it excess energy as heat, rather than converting it to
electric current. This process further reduces the amount of power can be
extracted from a solar cell.
"The light-induced carriers generate a voltage, and their
flow constitutes a current. Practical solar cells produce power, which is the
product of current and voltage," Polemi said. "This voltage, and
therefore the power that can be obtained, is also limited by the band
gap."
But, as Fridkin discovered in 1969 -- and the team validates
with this research -- this limitation is not universal, which means solar cells
can be improved.
New Life for an Old Theory
When Fridkin and his colleagues at the Institute of
Crystallography in Moscow observed an unusually high photovoltage while
studying the ferroelectric antimony sulfide iodide -- a material that did not
have any junction separating the carriers -- he posited that crystal symmetry
could be the origin for its remarkable photovoltaic properties.
He later explained how this "bulk photovoltaic
effect," which is very weak, involves the transport of photo-generated hot
electrons in a particular direction without collisions, which cause cooling of
the electrons.
This is significant because the limit on solar power conversion
from the Shockley-Queisser theory is based on the assumption that all of this
excess energy is lost -- wasted as heat. But the team's discovery shows that
not all of the excess energy of hot electrons is lost, and that the energy can,
in fact, be extracted as power before thermalizing.
"The main result -- exceeding [the energy gap-specific]
Shockley-Queisser [power efficiency limit] using a small fraction of the solar
spectrum -- is caused by two mechanisms," Fridkin said.
"The first is the bulk photovoltaic effect involving hot
carriers and second is the strong screening field, which leads to impact
ionization and multiplication of these carriers, increasing the quantum
yield."
Impact ionization, which leads to carrier multiplication, can be
likened to an array of dominoes in which each domino represents a bound
electron. When a photon interacts with an electron, it excites the electron,
which, when subject to the strong field, accelerates and 'ionizes' or liberates
other bound electrons in its path, each of which, in turn, also accelerates and
triggers the release of others. This process continues successively -- like
setting off multiple domino cascades with a single tipped tile -- amounting to
a much greater current.
This second mechanism, the screening field, is an electric field
is present in all ferroelectric materials. But with the nanoscale electrode
used to collect the current in a solar cell, the field is enhanced, and this
has the beneficial effect of promoting impact ionization and carrier
multiplication.
Following the domino analogy, the field drives the cascade
effect, ensuring that it continues from one domino to the next.
"This result is very promising for high efficiency solar
cells based on application of ferroelectrics having an energy gap in the higher
intensity region of the solar spectrum," Fridkin said.
Building Toward a Breakthrough
"Who would have expected that an electrical insulator could
be used to improve solar energy conversion?" said Jonathan E. Spanier, a
professor of materials science, physics and electrical engineering at Drexel
and one of the principal authors of the study.
"Barium titanate absorbs less than a tenth of the spectrum
of the sun. But our device converts incident power 50 percent more efficiently
than the theoretical limit for a conventional solar cell constructed using this
material or a material of the same energy gap."
This breakthrough builds on research conducted several years ago
by Andrew M. Rappe, Blanchard Professor of Chemistry and of Materials Science
& Engineering at the University of Pennsylvania, one of the principal
authors, and Steve M. Young, also a co-author on the new report.
Rappe and
Young showed how bulk photovoltaic currents could be calculated -- which led
Spanier and collaborators to investigate if higher power conversion efficiency
could be attained in ferroelectrics.
"There are many exciting reports utilizing nanoscale
materials or phenomena for improving solar energy conversion," Spanier
said. "Professor Fridkin appreciated decades ago that the bulk
photovoltaic effect enables free electrons that are generated by light and have
excess energy to travel in a particular direction before they cool or
'thermalize'--and lose their excess energy to vibrations of the crystal
lattice."
Rappe was also responsible for connecting Spanier to Fridkin in
2015, a collaboration that set in motion the research now detailed in Nature
Photonics -- a validation of Fridkin's decades-old vision.
"Vladimir is internationally renowned for his pioneering
contributions to the field of electroxerography, having built the first working
photocopier in the world," Rappe said. "He then became a leader in
ferroelectricity and piezoelectricity, and preeminent in understanding light
interactions with ferroelectrics.
Fridkin explained how, in crystals that lack inversion symmetry,
photo-excited electrons acquire asymmetry in their momenta. This, in turn,
causes them to move in one direction instead of the opposite direction. It is
amazing that the same person who discovered these bulk photovoltaic effects
nearly 50 years ago is now helping to harness them for practical use in
nanomaterials."
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The research was supported by the U.S. Army Research Office, the
Office of Naval Research, the U. S. Department of Energy, and the National
Science Foundation.