Researchers at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, working with colleagues at Penn State University, have uncovered surprising dynamics in the behavior of excited electrons in a prototypical perovskite material. Using ultrafast electron microscopy, the team was able to capture nanoscale structural changes that occur within picoseconds of excitation, revealing that hot electrons can transiently rearrange atoms. These insights, reported in Nature Communications, could have implications for the design and optimization of next-generation optoelectronic materials and devices such as solar cells, sensors, and light-emitting diodes (LEDs).

Perovskites, a class of materials that adopt a specific crystal structure, have recently emerged as promising candidates for various optoelectronic applications due to their excellent light-absorbing properties and relatively low cost. However, a fundamental understanding of how these materials respond to light excitation is still lacking, hindering further improvements and practical applications.

In this study, the researchers used a state-of-the-art ultrafast electron microscope, housed at Brookhaven Lab's Center for Functional Nanomaterials (CFN), to capture structural changes in individual cesium lead bromide (CsPbBr3) perovskite nanocrystals upon ultrafast light excitation. The unique design of the CFN's microscope allowed the team to record high-resolution images at a temporal resolution of only a few picoseconds.

The results revealed that within a few picoseconds after the nanocrystals absorbed light, their crystal lattice—normally distorted due to the arrangement of atoms within—underwent a transformation, becoming more symmetrical. This unexpected straightening of the lattice was attributed to the movement of highly energetic or "hot" electrons, which transiently redistributed within the nanocrystals.

Lead author Ming-Chang Chen, a Brookhaven Lab scientist, provided insight into the experimental results: "We found that the lattice rearrangement is tightly linked to the relaxation dynamics of hot electrons, which are the key carriers of energy in photovoltaic and optoelectronic devices. By controlling these ultrafast processes, we could improve the efficiency of these devices."

The observed lattice straightening could have important implications for understanding the light-driven properties and performance of perovskites. For example, in solar cells, the transient lattice changes could affect the movement and separation of charge carriers, influencing the cell's ability to convert light into electricity.

"Our findings open new avenues for exploring and controlling the properties of perovskites at the nanoscale," added corresponding author James M. Kikkawa, a physicist in Brookhaven Lab's Condensed Matter Physics and Materials Science Department. "By manipulating these ultrafast processes, we can potentially improve the efficiency and performance of perovskite-based devices for a range of applications."

The research team plans to further investigate these ultrafast dynamics in different perovskite materials and explore potential strategies to manipulate and harness them for practical applications.