Physicists observe fleeting “Polaron” quasiparticles for the first time

Polarons Important nanoscale phenomena: transient formation between electrons and atoms (known as Quasiparticles) Only exist for fractions of a trillionth of a second.

These formations have unique properties that can help us understand some of the mysterious behaviors of the substances that form within them – and scientists have just noticed them for the first time.

Polarons were measured in the lead-perovskite hybrid material, a next-generation solar cell material that promises to increase conversion rates well beyond the silicon panels primarily used today. Scientists hope Polaron’s observations will go some way to tell us exactly how perovskites convert sunlight into electricity so well.

To find polarons, the scientists trained light on single crystals of perovskite hybrid lead, and watched a giant X-ray laser without an electron called Linac coherent light source (LCLS) – Capable of imaging materials at the smallest scales in the shortest times, down to a trillionth of a second (or Picoseconds).

(Greg Stewart / SLAC National Accelerator Laboratory)

Above: an illustration of polarons in perovskite lead hybrid.

“When you place a charge in a substance by hitting it with light, such as what happens in a solar cell, the electrons are released, and those free electrons begin to move around the substance,” Physicist Burak Guzleturk says From the Argonne National Laboratory, operated by the US Department of Energy.

“Soon it became surrounded and engulfed by a kind of local distortion bubble – the polaron – that travels with it. Some people have argued that this bubble protects electrons from scattering defects in the material, and helps explain why it travels efficiently to the solar cell connection to flow as electricity.”

Although perovskite is promising as a material for solar panels, it is not entirely clear why: they have many disadvantages that should limit the extent to which current flows through them, and they are known to be fragile and unstable. Polarons may offer some answers.

These electrodes are essentially short travel deformations of the atomic network structure of matter, and have been shown to move around 10 layers of atoms outwards. The distortion increased the spacing of the surrounding atoms by about 50 times – to 5 parts per billionth of a meter – over tens of picoseconds.

The deformations or microscopic bubbles were larger than the scientists expected, and they were allowed to move through the flexible and soft atomic lattice structure of the hybrid perovskite. The substance behaves in some ways as a solid and a liquid at the same time.

“These materials have taken the solar energy research field by storm due to their high efficiency and low cost, but people are still arguing about why they work,” Says material scientist Aaron Lindenberg From 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 anomalies, including their size, shape, and how they develop.”

While perovskites are already used in the production of solar energy, often with silicon, it is not without its challenges – while we have seen significant efficiency gains from these materials, it is presumed to be capable of more.

As the years go by, scientists continue to overcome the hurdles that kept solar panels ’efficiency below it should be, and as we increasingly rely on solar farms, improvements of even a few percentage points can make a huge difference.

However, the researchers who discovered Polaron are careful to emphasize that they haven’t answered all of the questions about these quasi-particles yet – and there is a lot to learn about their effect on perovskites and other materials.

“While this experiment shows as directly as possible that these things actually exist, it does not show how they contribute to the efficiency of the solar cell,” Lindenberg says. “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 Nature materials.