Ultrafast Microscope Images Material’s Molecular Structure and Dynamics

Perovskites, a family of organic-inorganic hybrid materials, are efficient at converting light to electricity and relatively easy to make. They absorb certain colors of the visible spectrum effectively and can be layered with other materials, such as silicon, that absorb wavelengths the perovskites cannot capture.

But the low photostability of perovskites makes it difficult to improve their performance. Perovskite solar cells can be made from many different combinations of chemical compositions and prepared under various conditions. It is hard to predict how these factors will affect the structure and performance of the perovskite cell.

Many complex materials found in semiconductors, displays, and quantum and biomedical applications present the same challenge. To better understand how to improve these materials, scientists need to be able to visualize the material’s dynamics at the subatomic, atomic, and molecular levels.
A research team at the University of Colorado-Boulder (CU Boulder) developed a microscope for spatio-spectral-temporal ultrafast nanoimaging of the structural characteristics of materials. The ultrafast microscope enables researchers to directly image the role of molecular order, disorder, and local crystallinity in the optical and electronic properties of materials.

The researchers used the microscope to perform combined ground- and ultrafast excited-state IR nanoimaging of a metal halide perovskite that is a promising candidate for tandem solar cells, photocatalysis, and optoelectronic applications.

Ultrafast nanoimaging of structure and dynamics in a perovskite quantum material also used for photovoltaic applications. Different femtosecond laser pulses are used to excite and measure the material. They are focused to the nanoscale with an ultrasharp metallic tip. The photoexcited electrons and coupled changes of the lattice structure (so-called polarons, red ellipses) are diagnosed spectroscopically with simultaneous ultrahigh spatial and temporal resolution. Courtesy of Branden Esses.

The microscope is equipped with a metal-coated nanotip that is positioned within a nanometer of the perovskite layer, then hit with a sequence of ultrashort laser pulses. The first pulse excites the electrons in the material in the visible, and subsequent pulses in the IR capture the movement and interaction of the electrons and molecules in the material over time. The nanotip functions like an antenna for the laser light, focusing the laser to the nanoscale.

The researchers scanned the nanotip across the perovskite layer, creating an image of the material pixel by pixel. Each image was the equivalent of one movie frame, due to the temporal differences in the laser pulses.

To reduce noise, the researchers used optical amplification techniques and developed a method to modulate the laser beams. “If you shine a light on this very tiny tip, the light that comes back is very weak since it only interacts with very few electrons or molecules,” researcher Branden Esses said. “It’s so weak that you need special techniques to detect it.”

According to researcher Roland Wilcken, controlling the way the light is focused at the nanometer scale and how it is emitted and detected is essential to achieving the contrast and signal necessary to make an ultrafast movie of the material.

The researchers captured ultrahigh-resolution images of atomic and molecular movement in the perovskite at the femtosecond scale and measured atomic motion in the molecules with very high precision. The photoexcited electrons and coupled changes of the lattice structure (i.e., polarons) were diagnosed spectroscopically with ultrahigh spatiotemporal resolution, enabling the researchers to better understand the perovskite’s structure and composition and its performance as a photovoltaic material.

The team’s findings suggest that the more disorder in the material, the better the photovoltaic performance. “In contrast to conventional semiconductors, it seems that more structural disorder gives rise to more stable photogenerated electrons in hybrid perovskites,” professor Markus Raschke said.

According to Raschke, there is limited knowledge of the processes that occur after sunlight is absorbed by photovoltaic materials, and how the excited electrons move in the material without being dispersed.

“We like to say that we’re making ultrafast movies,” he said. “For the first time, we can actually sort this out, because we can record spatial, temporal, and spectral dimensions simultaneously in this microscope.”

The team expects the ultrafast microscope to have a significant impact on the ability of material scientists to improve the performance of new semiconductor and quantum materials for computing, energy, and medical applications.

“This is a way to examine the material properties on a very elementary level, so that in the future we’ll be able to design materials with certain properties in a more directed way,” professor Sean Shaheen said.

“We’re able to say, ‘We know we prefer this kind of structure, which results in, for example, longer-lived electronic excitations as linked to photovoltaic performance,’ and then we’re able to inform our material synthesis partners to help make them,” Esses said.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.ads3706).