First time physicists observe fleeting “polaron” quasiparticles

UNITED STATES (VOP TODAY NEWS) — Polarons are important nanoscale phenomena: a transitional configuration between electrons and atoms (known as quasiparticles) that exists for only a trillionth of a second.

These configurations have unique characteristics that can help us understand some of the mysterious properties of the materials within which they form – and this is the first time scientists have observed them.

Polarons have been measured in lead hybrid perovskites, a new generation of solar cell materials that promise improved conversion rates over silicon panels that are mostly used today. Scientists hope that observations of polarons will tell us to some extent how perovskites convert sunlight into electricity so well.

To find polarons, scientists focused light on single crystals of lead hybrid perovskites, observing with a giant X-ray free electron laser called the Linac Coherent Light Source (LCLS), capable of imaging materials at the smallest scales in the shortest time frame, up to trillionths of a second (or picoseconds).

“When you place a charge in a material, directing light at it, as in a solar cell, electrons are released and those free electrons begin to move through the material,” says physicist Burak Guzelturk of Argonne National Laboratory. U.S. Department of Energy.

“Soon they are surrounded and enveloped in a kind of bubble of local distortion – a polaron – that moves with them. Some scientists have argued that this bubble protects electrons from scattering by defects in the material and helps explain why they travel so efficiently towards the solar cell’s contact to flow out as electricity.”

As promising as perovskites are as a material for solar panels, it is not entirely clear why: they have many defects that should limit the rate of current flow through them, and they are known to be fragile and unstable.

Polarons, in fact, are short-term distortions of the atomic lattice structure of a material, and have been shown to displace approximately 10 layers of atoms outward. The distortion increased the distance between surrounding atoms by about 50 times – up to 5 billionths of a meter – in tens of picoseconds.

In a sense, a material behaves like a solid and a liquid at the same time.

“These materials have exploded into solar energy research because of their high efficiency and low cost, but people still argue about why they work,” says materials scientist Aaron Lindenberg of Stanford University.

“The idea that polarons can be used has been around for several years, but our experiments are the first to directly observe the formation of these local distortions, including their size, shape and how they evolve.”

While perovskites are already being used in solar energy, often in combination with silicon, they have their own problems – while we’ve seen significant efficiency gains from these materials, they are supposed to be capable of more.

However, the researchers behind the polaron discovery are keen to emphasize that they have not yet answered all the questions related to these quasiparticles, and there is still much to be learned about their effects on perovskites and other materials.

“While this experiment shows as directly as possible that these objects do exist, it does not show how they affect the efficiency of a solar cell,” says Lindenberg. “More work remains to be done to understand how these processes affect material properties.”


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