Polarons are important nanoscale phenomena: a transient configuration between electrons and atoms (known as quasiparticles) that exists only for trillions of seconds.
These configurations have unique features that can help us understand some of the mysterious behaviors of the materials they form, and scientists have just observed them for the first time.
Polarons were measured in hybrid lead perovskites, next-generation solar cell materials that promise to increase conversion rates beyond the silicon panels that are primarily used today. Scientists hope that polaron observations will help us explain exactly how perovskites convert sunlight into electricity so well.
To find the polarons, the scientists trained light on simple crystals of lead hybrid perovskites, observing with a giant X-ray free electron laser called Linac Coherent Light Source (LCLS), capable of imagining materials in the smallest stairs for the shortest time. up to trillions of seconds (or picoseconds).
(Greg Stewart / SLAC National Accelerator Laboratory)
At the top: illustration of polarons in lead hybrid perovskite.
“When a charge is placed on a material by striking it with light, like what happens in a solar cell, the electrons are released and those free electrons begin to move through the material,” says physicist Burak Guzelturk of the National Laboratory. d’Aronne. by the U.S. Department of Energy.
“They are soon surrounded and surrounded by a kind of local distortion bubble (the polaron) that travels with them. Some people have argued that this bubble protects electrons from the scattering of defects in the material and helps explain why they travel as efficient at the contact of the solar cell to flow as electricity “.
As promising as perovskites as solar panel material, it is not entirely clear why: they have many defects that should limit the flow of current through them and are remarkably fragile and unstable. Polarons may offer some answers.
These polarons are essentially brief traveling distortions of the structure of the material’s atomic lattice and were shown to move about 10 layers of atoms outward. The distortion increased the spacing of the surrounding atoms about 50 times (up to 5 billion parts per meter) for tens of seconds.
The small distortions or bubbles were larger than the scientists expected, letting themselves be moved by the flexible, soft atomic lattice structure of the hybrid perovskite. The material behaves in some way as a solid and a liquid at the same time.
“These materials have taken the field of solar energy research by storm because of their high efficiency and low cost, but people are still debating why they work,” says materials scientist Aaron Lindenberg of the University. of Stanford.
“The idea that polar bears may be involved has been around for a number of years, but our experiments are the first to directly observe the formation of these local distortions, including their size, shape, and how they evolve.”
Although perovskites are already used in the production of solar energy, often in combination with silicon, they are not exempt from their challenges; although we have seen significant efficiency gains with these materials, they are supposed to be able to make more.
As the years go by, scientists continue to overcome the obstacles that have kept the efficiency of solar panels lower than they should be, and with our reliance on solar farms, improvements of even a few percentage points can make a big difference.
However, the researchers behind the discovery of polaron want to emphasize that they have not yet answered all the questions around these quasiparticles, and that there is much more to know about their impacts on perovskites and other materials.
“While this experiment demonstrates as directly as possible that these objects actually exist, it does not show how they contribute to the efficiency of a solar cell,” says Lindenberg. “There is still more work to be done to understand how these processes affect the properties of these materials.”
The research has been published in Materials of nature.