Polarons are important nanoscale phenomena: a transient configuration between electrons and atoms (known as quasiparticles) that exist only trillionths of a second.
These configurations have unique characteristics that can help us understand some of the mysterious behaviors of the materials that form them within – 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 panels primarily used today. Scientists hope that Polaron observations will tell us somehow how perovskites turn sunlight into electricity so well.
To find the polarons, scientists trained light on single crystals of lead hybrid perovskites, viewing with a giant X-ray-free electron laser called the Linac Coherent Light Source (LCLS) – capable of imaging materials on the smallest scale in the shortest time. to trillionths of a second (or picoseconds).
(Greg Stewart / SLAC National Accelerator Laboratory)
Above: illustration of polarons in lead hybrid perovskite.
“When you charge a material by hitting it with light, like what happens in a solar cell, electrons are released and those free electrons start to move around the material,” says physicist Burak Guzelturk of Argonne National Laboratory. by the US Department of Energy.
Soon they are surrounded and engulfed by a bubble of local distortion – the polaron – that travels with them. Some people have claimed that this bubble protects electrons from scattering defects in the material, and helps explain why they travel so efficiently. to the contact of the solar cell to flow out as electricity. “
As promising as perovskites are as a material for solar panels, it’s not entirely clear why: they have many defects that should limit how well the current can flow through them, and they are notoriously fragile and unstable. Polarons may offer some answers.
These polarons are essentially momentary displacements of the atomic lattice structure of the material, and have been shown to shift outwardly around 10 layers of atoms. The deformation has increased the distance between the surrounding atoms about 50 times – up to 5 billionths of a meter – over tens of picoseconds.
The minute distortions or bubbles were larger than scientists expected, made possible by 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 the field of solar energy research by storm because of their high efficiency and low cost, but people are still debating why they work,” said materials scientist Aaron Lindenberg of Stanford University.
“The idea that polarons 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 deformations, including their size, shape and how they evolve.”
While perovskites are already used in solar energy production, often in conjunction with silicon, they are not without challenges – while we have seen major efficiencies with these materials, they are believed to be capable of even more.
As the years go by, scientists continue to overcome hurdles that have kept solar panel efficiency lower than it should be, and with our increasing reliance on solar farms, improvements of even a few percentage points can make a big difference.
However, the researchers behind the Polaron discovery would like to emphasize that they have not yet answered all the questions surrounding these quasi-particles – and there is much more to learn about their impact on perovskites and other materials.
“Although 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. “More needs to be done to understand how these processes affect the properties of these materials.”
The research is published in Natural materials.