Mohite: 'The next frontier in light-to-energy conversion devices is harvesting hot carriers'

In an effort to maximize the potential of perovskite-based materials, including solar cells, researchers from Rice University have achieved a breakthrough in visualizing the precise movements of atoms within perovskites.

According to a study published in "Natural Physics," Rice University researchers have advanced their knowledge of the structural dynamics under light-induced excitation in 2D perovskites, according to a Jan. 23 news release.

“The next frontier in light-to-energy conversion devices is harvesting hot carriers,” Aditya Mohite, an associate professor of chemical and biomolecular engineering at Rice University and a corresponding author of the study, said in the news release. “Studies have shown that hot carriers in perovskite can live up to 10-100 times longer than in classical semiconductors. However, the mechanisms and design principles for the energy transfer and how they interact with the lattice are not understood.”

Mohite led the team, which focused on harnessing hot carriers in perovskites, according to the news release. The hot carriers are short-lived, high-energy charge carriers that, when harvested efficiently, may facilitate light-harvesting devices in exceeding the limits of thermodynamic efficiency. Those mechanisms, along with design principles for energy transfer and interaction with the lattice structure in perovskites, have remained a mystery to scientists.

“So there was a fundamental physics question,” Mohite said in the news release. “Can we visualize these interactions? Can we see how the structure is actually responding at very fast timescales as you put light onto this material?”

In an effort to answer many key questions, Mohite’s team worked with researchers at the SLAC National Accelerator Laboratory, and they used ultrafast electron diffraction create a visualization of the lattice when a hot carrier is generated within the perovskite material in real time, the release reported.

Electric fields stimulate soft semiconductors like perovskites, which demonstrate interesting behaviors. The researchers found electrons and holes sparked in the materials often couple with the lattice in unique and robust ways, setting them apart from typical materials and semiconductors, according to the release.

The team set out witness and comprehend the interactions by examining how the lattice structure responds to light on very fast timescales. The researchers were able to attain their goal by using SLAC's mega-electron-volt ultrafast electron diffraction (MeV-UED) facility, equipped with pulsed lasers that can create an electron-hole plasma in perovskites, the release reported.

The plasma lets researchers observe the quick lattice reorganization happening in less than a billionth of a second in response to a hot carrier, the release said.

During these experiments, Mohite’s team was able to witness the lattice after being enticed by light and undergoing a key straightening effect as quickly as one picosecond, according to the release. They found a minor tilting of the perovskite octahedra kickstarted a reworking of transient lattice into a higher symmetric phase.

The researchers may demonstrate the distortion of a perovskite lattice could be tuned in response to light, and with shifted organic cations within the perovskite structure they can alter the lattice rigidity, which controls how it interacts with light, the release said.

Mohite’s team also found heat is generated and handled internally in excited perovskites, and knowledge of this generation and the corresponding transport properties of the materials is key for creating microelectronics and improving heat management in advanced technologies, the release noted, and this could lead to the development of new technologies going forward.

Support was received form the U.S. Department of Energy, the Office of Naval Research, the Robert A. Welch Foundation and the Academic Institute of France, further demonstrating the collaborative effort of the researchers, the release reported.