Physicists Observe Fleeting ‘Polaron’ Quasiparticles For The First Time

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Polarons are important nanoscale phenomena: a transient configuration between electrons and atoms (so-called quasiparticles) that only exist for trillionths of a second.

These configurations have unique properties that can help us understand some of the mysterious behaviors of the materials in which they are formed – and scientists have just observed them for the first time.

Polarons have been measured in lead hybrid perovskites, next-generation solar cell materials that promise to increase conversion rates beyond the silicon plates primarily used today. Scientists hope polaron observations will help tell us exactly how perovskites convert sunlight into electricity so well.

To find the polarons, the scientists trained light on single crystals of lead hybrid perovskites and observed them with a giant X-ray free-electron laser called the Linac Coherent Light Source (LCLS), which can image materials on a very small scale in a very short time . down to trillionths of a second (or picoseconds).

(Greg Stewart / SLAC National Accelerator Laboratory)

Above: Representation of polarons in lead hybrid perovskite.

“When you add a charge to a material by hitting it with light, as you do in a solar cell, electrons are released and these free electrons start to move around the material,” says Argonne National physicist Burak Guzelturk Laboratory from the US Department of Energy.

“Soon they are surrounded and engulfed by some sort of local distortion bubble – the polaron – that moves with them. Some people have argued that this bubble protects electrons from scattering from material defects and explains why they move so efficiently on contact with the solar cell to flow out as electricity. “

As promising as perovskites are as a solar panel material, it is not entirely clear why: They have many defects that should limit the flow of current through them, and they are notoriously fragile and unstable. Polarons might offer some answers.

These polarons are essentially short distortions of the atomic lattice structure of the material and have been shown to shift outward by 10 layers of atoms. The distortion increased the distance between the surrounding atoms by 50 times over ten picoseconds – to 5 billionths of a meter.

The tiny distortions or bubbles were larger than scientists expected and were able to move through the flexible and soft atomic lattice structure of the hybrid perovskite. In a sense, the material behaves like a solid and a liquid at the same time.

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

“The idea that polarons might be involved has been around for a few years, but our experiments are the first to directly observe the formation of these local distortions, including their size, shape, and evolution.”

While perovskites are already used in solar power generation, often in combination with silicon, they are not without their challenges – although we have made significant efficiency gains with these materials, it is believed that they can do even more.

Over the years, scientists continue to overcome hurdles that have kept solar panels less efficient than they should be, and as our reliance on solar farms increases, improvements of just a few percentage points can make a big difference.

However, the researchers behind the Polaron discovery want to stress that they have not yet answered all of the questions about these quasiparticles – and there is much more to be learned about their effects on perovskites and other materials.

“While this experiment shows as directly as possible that these objects actually exist, it doesn’t show how they contribute to the efficiency of a solar cell,” says Lindenberg. “There is still a lot to be done to understand how these processes affect the properties of these materials.”

The research was published in Nature Materials.

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