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Perovskite crystals appear to dance when they get excited

In today’s world of carbon emission reductions and clean energy production, more and more people are turning to solar cells to generate electricity from the sun’s light. All photovoltaic solar cells rely on semiconductors to turn the energy from light into electricity; light excites electrons in the semiconductor material, which flow into conducting electrodes and produce a current of electricity. However, the large silicon crystals used as semiconductors in conventional solar panels require an expensive, multi-step manufacturing process that requires a lot of energy.

In the search for an alternative semiconductor material, scientists have turned to perovskites that can be used to form semiconductors with similar properties to silicon. Perovskite solar cells can be manufactured using simple techniques, for a fraction of the cost and energy. In 2012, researchers first discovered how to make a stable, thin-film perovskite solar cell with a light photon-to-electron conversion efficiency of over 10 percent. This has evolved since then, and the record for sunlight-to-electrical power conversion efficiency using perovskite now stands at over 25 percent. 

Perovskites are layered materials that have well-ordered crystal lattices. They are highly efficient harvesters of light that are being explored for use as photodetectors, photocatalysts, light-emitting diodes, and quantum emitters, apart from their use in solar cells. And yet, the ways in which atoms in the layers of perovskite rearrange themselves to enable the conversion of energy is not fully understood.

“The next frontier in light-to-energy conversion devices is harvesting hot carriers,” said study co-author Aditya Mohite of Rice University. “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.”

Hot carriers are short-lived, high-energy charge carriers, either electrons for negative charges or electron “holes” for positive charges, which have gained very high kinetic energy. Being able to harvest their energy would allow light-harvesting devices to “surpass thermodynamic efficiency,” said Mohite, an associate professor of chemical and biomolecular engineering in Rice’s George R. Brown School of Engineering.

Mohite and three members of his research group – senior scientist Jean-Christophe Blancon and graduate students Hao Zhang and Wenbin Li – worked with colleagues at the Stanford Linear Accelerator Center’s  National Accelerator Laboratory to see how atoms in a perovskite lattice rearranged themselves when a hot carrier was created in their midst. They visualized lattice reorganization in real time using ultrafast electron diffraction.

“Whenever you expose these soft semiconductors to stimuli like electric fields, interesting things happen,” Mohite said. “When you generate electrons and holes, they tend to couple to the lattice in unusual and really strong ways, which is not the case for classical materials and semiconductors.”

“So there was a fundamental physics question. 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?”

The answer was yes, but only with a strong input. 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 that was needed to reveal how the lattice structure changed in less than a billionth of a second in response to the introduction of a hot carrier.

“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 explained. “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 experiments at SLAC produced before-and-after diffraction patterns that Mohite’s team interpreted to show how the lattice changed. They found that after the lattice was excited by light, it relaxed and literally straightened up in as little as one picosecond, or one-trillionth of a second. The analysis revealed that the change in structure resulted from a strong interaction between the electron-hole plasma and the perovskite lattice. This triggered the relaxation of the lattice and can be likened to seeing atoms dancing around in response to excitation. 

“There’s a subtle tilting of the perovskite octahedral, which triggers this transient lattice reorganization towards a higher symmetric phase,” said Zhang.

This breakthrough in visualization supports the researchers’ efforts to squeeze every possible drop of utility out of perovskite-based materials, including solar cells, a long-standing project that only recently yielded an advance to make the devices far more durable

By demonstrating that a perovskite lattice can suddenly become less distorted in response to light, the research showed it should be possible to tune how perovskite lattices interact with light, and it suggested a way to accomplish the tuning. Li said that this effect is very dependent on the type of structure and type of organic spacer cation.

There are many recipes for making perovskites, but all contain organic cations, an ingredient that acts as a spacer between the materials’ semiconducting layers. By substituting or subtly changing organic cations, researchers could tailor lattice rigidity, dialing it up or down to alter how the material responds to light, Li explained.

Mohite said the experiments also show that tuning a perovskite’s lattice alters its heat-transfer properties. “What is generally expected is that when you excite electrons at a very high energy level, they lose their energy to the lattice,” he said. “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.”

“The lattice is getting more energy from thermal energy. That’s the classical effect, which is expected, and is well-known as the Debye-Waller factor. 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.” A better understanding of how excited perovskites handle heat is a bonus of the research, noted Mohite.

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

“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.”

By Alison Bosman, Staff Writer

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