Perovskite Prodding Reveals Structural Dynamics

A Rice University research team, with help from the Department of Energy’s SLAC National Accelerator Laboratory, has directly measured the structural dynamics in 2D perovskites. The team’s investigation into the light-induced physical behavior of perovskites could lead to a better understanding of this material, which is used in solar cells, photodetectors, photocatalysts, LEDs, quantum emitters, and other applications.

The researchers’ goal was to learn how the atoms in a perovskite lattice rearranged themselves when a hot carrier — a short-lived, high-energy charge carrier with either a positive or negative charge — was created in the middle of a lattice. According to professor Aditya Mohite, the ability to harvest the energy of hot carriers could make light-to-energy conversion devices, like those powered by perovskites, more efficient than thermodynamics.

Researcher Wenbin Li (left) and professor Aditya Mohite. Courtesy of Jeff Fitlow/Rice University.

“Studies have shown that hot carriers in perovskite can live up to 10 to 100 times longer than in classical semiconductors,” Mohite said. “However, the mechanisms and design principles for the energy transfer and how they interact with the lattice are not understood.”

Mohite said that, unlike classical semiconductors, when a perovskite is exposed to stimuli like electric fields, the electrons and electron “holes” that are generated tend to couple to the lattice in strong, unusual ways. The researchers aimed to visualize these interactions directly.

To observe how the perovskite structure responded when light was shined on it at very fast timescales, the team turned to SLAC for help. SLAC’s mega-electron-volt ultrafast electron diffraction (MeV-UED) facility is one of the few places in the world with pulsed lasers capable of creating the electron-hole plasma in perovskites needed to reveal how the lattice structure changed in less than a billionth of a second in response to a hot carrier.

The researchers used ultrafast electron diffraction to visualize the reorganization of perovskite lattices in real time.

At the SLAC facility, the Rice team directly observed the ultrafast coupling between charge carriers and lattice degrees of freedom and monitor the evolution of ultrafast electron diffraction intensity in perovskite structures following above-bandgap, high-density photoexcitation.

“The way this experiment works is that you shoot a laser through the material and then you send an electron beam that goes past it at a very short time delay,” Mohite said. “You start to see exactly what you would in a TEM (transmission electron microscope) image. With the high-energy electrons at SLAC, you can see diffraction patterns from thicker samples, and that allows you to monitor what happens to those electrons and holes and how they interact with the lattice.”

The researchers interpreted the before-and-after diffraction patterns produced by the experiments at SLAC to show how the lattice changed. They found that after the lattice was excited, it relaxed and straightened in as little as 1 ps.

Researcher Hao Zhang. Courtesy of Rice University.

“There’s a subtle tilting of the perovskite octahedra, which triggers this transient lattice reorganization towards a higher symmetric phase,” researcher Hao Zhang said. Correlated ultrafast spectroscopy showed that the creation of a dense electron-hole plasma triggered the relaxation of lattice distortion at shorter timescales by modulating the crystal’s cohesive energy. By demonstrating that the perovskite lattice could become less distorted in response to light, the researchers introduced a potential way to tune the interactions between perovskite lattices and light.

However, this effect depended on the type of structure and the type of organic cation that was used as the spacer between the perovskite’s semiconducting layers, researcher Wenbin Li said. The researchers could alter the interaction between the carrier gas and the lattice by tailoring the rigidity of the 2D perovskite, which they did by choosing an appropriate organic spacer layer. By substituting or subtly changing organic cations, the researchers could dial the lattice’s rigidity up or down to modify the material’s response to light.

The experiments further showed that a perovskite’s heat-transfer properties could be changed by tuning its lattice.

Mohite said that when electrons are excited at a high energy level, the expectation is that they will lose their energy to the lattice. “Some of that energy is converted to whatever process you want, but a lot of it is lost as heat, which shows in the diffraction pattern as a loss in intensity,” he said.

“The lattice is getting more energy from thermal energy. That’s the classical effect, which is expected,” Mohite said. “But because we can now know exactly what’s happening in every direction of the crystal lattice, we see the lattice starts to get more crystalline or ordered. And that’s totally counterintuitive.

Insight into the way excited perovskites handle heat is an added benefit of the research, Mohite believes.

“As we make devices smaller and smaller, one of the biggest challenges from a microelectronics perspective is heat management,” he said. “Understanding this heat generation and how it’s being transported through materials is important.

“When people talk about stacking devices, they need to be able to extract heat very fast. As we move to new technologies that consume less power and generate less heat, these types of measurements will allow us to directly probe how heat is flowing.”

The research was published in Nature Physics (www.doi.org/10.1038/s41567-022-01903-6).