Researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University used an x-ray laser to watch and directly measure the formation of polarons for the very first time. Polarons are distortions in the atomic lattice in a material that forms around a moving electron in a few trillionths of a second and then disappear quickly. Despite their short duration, they do affect a material’s behavior. They could be why solar panels made with lead hybrid perovskites can reach high efficiencies in the lab.
Researchers say that material has taken solar energy research by storm because of its high efficiency and low cost. Despite the performance, researchers argue about why they work. Some have theorized that polarons could be involved for several years. Still, the new experiment is the first to directly observe the formation of those distortions, including their size, shape, and how they evolve.
Perovskites are crystalline materials that take their name from the mineral perovskite. Researchers began to incorporate the crystalline material into solar cells about ten years ago, steadily increasing the efficiency of solar cells. Interestingly, the number of defects within perovskite components should inhibit the flow of the current, but it doesn’t.
There are challenges in working with the material. Despite being efficient and easy to make, they’re highly unstable, breakdown when exposed to air, and contain lead that has to be kept out of the environment. Researchers on this study use the lab’s Linac Coherent Light Source, a powerful x-ray free-electron laser able to image materials in near-atomic detail. It’s also able to capture atomic motions occurring in millions of a billionth of a second.
The team looked at single crystals of the material synthesized by one of the researchers from Stanford. During the study, the team hit a sample of the material with light from an optical laser and then used the x-ray laser to observe how the material responded over tens of trillionths of a second. The observations revealed polaronic distortions that start on a microscopic scale about the size of the spacing between atoms in a solid and rapidly expand outward in all directions to a diameter of about 5 billionths of a meter. That action nudged about ten layers of atoms slightly outward over a roughly spherical area over a span of picoseconds.