Polarons are important nanoscale phenomena: a transient configuration between electrons and atoms (known as quasi-particles) 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 have been measured in hybrid lead perovskites, state-of-the-art solar cell materials that promise to increase conversion rates beyond the silicon panels that are used primarily today. Scientists hope that the polar observations will somehow help to tell us exactly how perovskites turn sunlight into electricity so well.
To find the polarons, scientists trained light on single crystals of hybrid lead perovskites, tracking them with a giant free-electron X-ray laser called Linac Coherent Light Source (LCLS) – capable of imaging materials at the smallest scale in the shortest periods of time to trillions of seconds (or picoseconds).
(Greg Stewart / SLAC National Acceleration Laboratory)
Above: Illustration of lead hybrid perovskite polarons.
“When you put a charge into a material by hitting it with light, like what happens in a solar cell, the electrons are released and those free electrons start moving around the material,” says physicist Burak Guzelturk of the National Laboratory. Argonne, by the US Department of Energy.
“They are soon surrounded and swallowed by a kind of local distortion bubble – the polaron – that travels with them. Some people have argued that this bubble protects electrons from the spread of defects in the material and helps explain why they travel so efficiently in contact with the solar cell to flow as electricity. “
As promising as perovskites are as a solar panel material, it is not at all clear why: they have a lot of defects that should limit how well the current can flow through them and are notoriously fragile and unstable. Polarons could provide some answers.
These polarons are essentially short travel distortions of the atomic lattice structure of the material and have been shown to move around 10 layers of atoms outward. The distortion increased the distance between the atoms by about 50 times – to 5 billion meters – over tens of picoseconds.
The tiny distortions or bubbles were larger than scientists expected, being allowed to move through the flexible and soft atomic lattice structure of the hybrid perovskite. The material behaves in a certain way as a solid and a liquid at the same time.
“These materials have taken the field of solar research by storm because of their high efficiency and low cost, but people are still arguing why they work,” says materials scientist Aaron Lindenberg of Stanford University.
“The idea that polarones could be involved has been around for several years, but our experiments are the first to directly observe the formation of these local distortions, including the size, shape and way they evolve.”
While perovskites are already used in the production of solar energy, often in combination with silicon, they are not without challenges – while we have seen major efficiency gains from these materials, they are supposed to be able to do even more.
As the years go by, scientists continue to overcome the obstacles that have kept the efficiency of solar panels smaller than they should be, and as our dependence on solar farms increases, improvements of only a few percentage points can make a big difference.
However, the researchers behind the discovery of polaron are keen to point out that they have not yet answered all the questions surrounding these quasi-particles – and much remains to be learned about their impact on perovskites and other materials.
“While this experiment shows as directly as possible that these objects do 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 for nature.