Rare Quasi-Particle Observed For The First Time Inside A Next-Gen Energy Material

An artist's illustration of polarons in an atomic lattice. Image Credit: Greg Stewart/SLAC National Accelerator Laboratory

Lead hybrid perovskites are a very exciting material due to their potential for being a highly efficient solar cell, but the reason why this next-gen material is so good is not exactly clear. A hypothesis placed the merit on a curious quasi-particle known as a polaron. Researchers have now reported the first observation of polarons in these materials.

A polaron is a quasi-particle – an interaction that behaves just like a particle but it isn’t one. This polaron could be seen as a fleeting distortion in the structured lattice of a solid material, caused by the interaction between an electron and the atoms around it.

As reported in Nature Materials, scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have used a lab X-ray laser to study these polarons like never before. They were able to watch them and study their formation in lead hybrid perovskites for the first time.

“These materials have taken the field of solar energy research by storm because of their high efficiencies and low cost, but people still argue about why they work,” senior author and research leader Professor Aaron Lindenberg said in a statement.

“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 distortions, including their size, shape and how they evolve.”

The X-ray laser allows researchers to measure the motion of the atoms happening in millionths of a billionth of a second. The material is hit with an optical laser, liberating some electrons which begin to move across the lattice.

As the electron moves, the polaron comes into “existence”. It is a distortion that rapidly expands. The team saw it form at less than half a billionth of a meter and then becoming 50 times larger. The polaron actually nudges about ten layers of atoms in about tens trillionths of a second.

As this animation shows, polaronic distortions start very small and rapidly expand outward in all directions to a diameter of about 5 billionths of a meter, which is about a 50-fold increase. This nudges about 10 layers of atoms slightly outward within a roughly spherical area over the course of tens of picoseconds, or trillionths of a second. (Greg Stewart/SLAC National Accelerator Laboratory)
As this animation shows, polaronic distortions start very small and rapidly expand outward in all directions, nudging 10 layers of atoms slightly outward within a roughly spherical area over the course of tens trillionths of a second. Image Credit: Greg Stewart/SLAC National Accelerator Laboratory

“This distortion is actually quite large, something we had not known before,” Lindenberg said. “That’s something totally unexpected. While this experiment shows as directly as possible that these objects really do exist, it doesn’t show how they contribute to the efficiency of a solar cell. There’s still further work to be done to understand how these processes affect the properties of these materials.”

Perovskites are a bit of a puzzle. Researchers have been adding them to solar cells for about a decade, leading to improved efficiency in converting sunlight into electricity. But they are also full of defects which should be impeding the flow of electricity.

The material has many hurdles to overcome before it can substitute silicon solar cells, but studies like this are crucial to understanding it, and maybe even find something better.  

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