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Polarons are important nanoscale phenomena: a transient configuration between electrons and atoms (called quasiparticles) that only exist for trillionths of a second.
These configurations have unique characteristics that can help us understand some of the mysterious behaviors of the materials they form inside – and scientists have just observed them for the first time.
Polarons have been measured in hybrid lead perovskites, next-generation solar cell materials that promise to increase conversion rates beyond the silicon panels that are mainly used today. Scientists hope that polaron observations will help tell us exactly how perovskites transform sunlight into electricity so well.
To find the polarons, the scientists dragged light onto single crystals of lead hybrid perovskites, looking with a giant x-ray free electron laser called the Linac Coherent Light Source (LCLS) – capable of imaging materials at the smallest scales over the shortest times, down to trillionths of a second (or picoseconds).
(Greg Stewart / SLAC National Accelerator Laboratory)
Above: Illustration of polarons in the lead hybrid perovskite.
“When you put a charge in a material by hitting it with light, like what happens in a solar cell, electrons are released and these free electrons start to move around the material,” says physicist Burak Guzelturk of the Argonne National Laboratory, by the US Department of Energy.
“Soon they are surrounded and engulfed by a kind of local warp bubble – the polaron – that accompanies them. Some people have argued that this bubble protects electrons from scattering defects in the material and helps explain why they travel so efficiently on contact with the solar cell to flow as electricity. “
As promising as perovskites are as a solar panel material, it’s not entirely clear why – they have many flaws that should limit how current can pass through them, and they’re notoriously brittle and unstable. Polarons might offer some answers.
These polarons are essentially brief shifting distortions of the material’s atomic lattice structure and have moved about 10 atom layers outward. The distortion increased the spacing of surrounding atoms by about 50 times – at 5 billionths of a meter – over tens of picoseconds.
The tiny distortions or bubbles were larger than scientists expected, allowed to move by the flexible and supple atomic lattice structure of the hybrid perovskite. The material behaves in a certain way 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 are still wondering why they work,” says materials scientist Aaron Lindenberg of the ‘Stanford University.
“The idea that polarons may be involved 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 evolution.
While perovskites are already used in solar power generation, often in combination with silicon, they are not without challenges – while we have seen major efficiency gains from these materials, they are believed to be capable of ‘even more.
Over the years, scientists continue to overcome the hurdles that have kept solar panel efficiency below what they should be, and with our growing reliance on solar farms, improvements in the if just a few percentage points can make a big difference.
However, the researchers behind the discovery of the polaron are keen to point out that they have yet to answer all questions about these quasiparticles – and there is much more to learn about their impacts on perovskites and other materials. .
“While this experiment shows as directly as possible that these objects really exist, it does not show how they contribute to the efficiency of a solar cell,” says Lindenberg. “There is still work to be done to understand how these processes affect the properties of these materials.”
The research was published in Materials from nature.
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