Study of promising photovoltaic material leads to discovery of a new state of matter
May
26, 2006 -- Researchers at McGill University have gained new insight into the
workings of perovskites, a semiconductor material that shows great promise for
making high-efficiency, low-cost solar cells and a range of other optical and
electronic devices.
Perovskites have drawn attention over
the past decade because of their ability to act as semiconductors even when
there are defects in the material’s crystal structure. This makes perovskites
special because getting most other semiconductors to work well requires
stringent and costly manufacturing techniques to produce crystals that are as defect-free
as possible. In what amounts to the discovery of a new state of matter, the
McGill team has made a step forward in unlocking the mystery of how perovskites
pull off this trick.
“Historically, people have been using
bulk semiconductors that are perfect crystals. And now, all of a sudden, this
imperfect, soft crystal starts to work for semiconductor applications, from
photovoltaics to LEDs,” explains senior author Patanjali Kambhampati,
an associate professor in the Department of Chemistry at McGill. “That's the
starting point for our research: how can something that’s defective work in a
perfect way?”
Quantum dots, but not as we know them
In a paper published May 26 in Physical
Review Research, the researchers reveal that a phenomenon known as quantum
confinement occurs within bulk perovskite crystals. Until now, quantum
confinement had only been observed in particles a few nanometres in size – the
quantum dots of flatscreen TV fame being one much-vaunted example. When
particles are this small, their physical dimensions constrain the movement of
electrons in a way that gives the particles distinctly different properties
from larger pieces of the same material – properties that can be fine-tuned to
produce useful effects such as the emission of light in precise colours.
Using a technique known as
state-resolved pump/probe spectroscopy, the researchers have shown a similar
type of confinement occurs in bulk caesium lead bromide perovskite crystals. In
other words, their experiments have uncovered quantum dot-like behaviour taking
place in pieces of perovskite significantly larger than quantum dots.
Surprising result leads to unexpected
discovery
The work builds on earlier research which
established that perovskites, while appearing to be a solid substance to the
naked eye, have certain characteristics more commonly associated with liquids.
At the heart of this liquid-solid
duality is an atomic lattice able to distort in response to the
presence of free electrons. Kambhampati draws a comparison to a trampoline
absorbing the impact of a rock thrown into its centre. Just as the trampoline
will eventually bring the rock to a standstill, the distortion of the
perovskite crystal lattice – a phenomenon known as polaron formation – is
understood to have a stabilizing effect on the electron.
While the trampoline analogy would
suggest a gradual dissipation of energy consistent with a system moving from an
excited state back to a more stable one, the pump/probe spectroscopy data in
fact revealed the opposite. To the researchers’ surprise, their measurements
showed an overall increase in energy in the aftermath of polaron formation.
“The fact that the energy was raised
shows a new quantum mechanical effect, quantum confinement like a quantum dot,”
Kambhampati says, explaining that, at the size scale of electrons, the rock in
the trampoline is an exciton, the bound pairing of an electron with the space
it leaves behind when it is in an excited state.
“What the polaron does is confine
everything into a spatially well-defined area. One of the things our group was
able to show is that the polaron mixes with an exciton to form what looks like
a quantum dot. In a sense, it’s like a liquid quantum dot, which is something
we call a quantum drop. We hope that exploring the behavior of these quantum
drops will give rise to a better understanding of how to engineer
defect-tolerant optoelectronic materials.”
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