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New CU Boulder research harnesses the power of an ultrafast microscope to study molecular movement in space and time

The interactions in photovoltaic materials that convert light into electricity happens in femtoseconds. How fast is that? One femtosecond is a quadrillionth of a second­­. To put that in perspective, the difference between a second and a femtosecond is comparable to the difference between the second right now and 32 million years ago.

Subatomic particles like electrons move within atoms, and atoms move within molecules, in femtoseconds. This speed has long presented challenges for researchers working to make more efficient, cost-effective and sustainable photovoltaic materials, including solar cells. Imaging materials on the nanoscale with high enough spatial resolution to uncover the fundamental physical processes poses an additional challenge.

Understanding how, where and when electrons move, and how their movement depends on the molecular structure of these materials, is key to honing them or developing better ones.

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Ultrafast nano-imaging 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 photo-excited electrons and coupled changes of the lattice structure (so called polarons, red ellipses) are diagnosed spectroscopically with simultaneous ultrahigh spatial and temporal resolution.��(Illustration: Branden Esses)

Building on more than five years of research developing a unique ultrafast microscope that can make real-time “movies” of electron and molecular motion in materials, a team of �������� scientists the results of significant innovations in ultrafast nanoimaging, visualizing matter at its elementary atomic and molecular level.

The research team, led by Markus Raschke, professor of physics and JILA fellow, applied the ultrafast nanoimaging techniques they developed to novel perovskite materials. Perovskites are a family of organic-inorganic hybrid materials that are efficient at converting light to electricity, generally stable and relatively easy to make.

Working with a thin perovskite layer, the researchers directed ultrashort laser pulses onto tiny metallic tips positioned above the perovskite layer. The tip functions like an antenna for the laser light and focuses it to a spot much smaller than what is possible in conventional microscopes. The tip is then scanned across the perovskite layer, creating an image pixel by pixel. Each image provides one frame of a movie as the different laser pulses are varied in time.

The movie also has “color,” albeit in the infrared and invisible to the human eye but where the molecules and electrons respond. Through different wavelengths of light, the researchers can follow both the electron and molecular motion and their coupling, which is what controls the photovoltaic efficiency in perovskites.

This milestone not only helps them better understand the missing links between the perovskite’s crystal structure and composition and its performance as a photovoltaic material but also led to the surprising discovery that more disorder seems to facilitate better photovoltaic performance.

“We like to say that we’re making ultrafast movies,” Raschke says, adding that there have long been many unknowns about the elementary processes after sunlight gets absorbed in photovoltaic materials and how the excited electrons move in them without being dispersed, but “for the first time, we can actually sort this out because we can record spatial, temporal and spectral dimensions simultaneously in this microscope.”

Molecules as spectators of how the electrons move

In recent years, much research has focused on perovskites, particularly in the quest to create more efficient and sustainable solar cells. These materials absorb certain colors of the visible spectrum of sunlight effectively and can be layered with other materials, such as silicon, that catch additional wavelengths of light the perovskite does not absorb.

“(Perovskites) are easy to fabricate and have a very high solar cell efficiency, and can be applied as a very thin film,” explains Roland Wilcken, first author on the new paper and a post-doctoral researcher in Raschke’s research group. “But the problem with this material is it has relatively low photostability.”

Improving the material’s performance is no easy feat. There’s a large possible combination of chemical compositions and preparation conditions of perovskite solar cells, which affect their structure, performance and stability in ways that are difficult to predict. This is a challenge also faced by many other complex materials used for semiconductors, quantum materials, displays or in biomedical applications.

This is where the ultrafast microscope helps the researchers gain the spatial and temporal information needed to optimize the material—and in turn—find a good compromise between stability and performance.

Building the ultrafast microscope was a challenge, explained Branden Esses, a physics graduate student and research contributor. The team used nanotips, coated in a platinum alloy or gold, which are brought within nanometers of the perovskite layer, then hit with a sequence of laser pulses.

The first pulse excites the electrons in the visible, and subsequent pulses in the infrared watch how the electrons and molecules interact and move in time,�� Esses says, adding that “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; it’s so weak that you need special techniques to detect it.”

So, they developed a special method, modulating the light beams and using optical-amplification techniques to reduce noise and background to isolate the desired information.��

Both how “the light is focused at the nanometer scale with the tips and how it is emitted and detected was essential to get enough contrast and signal to make these ultrafast movies of the material,” Wilcken explains.

And thanks to the ultrafast microscope technology, researchers are able to capture ultra-high-resolution images of femtosecond movement, measuring atomic motion in the molecules with very high precision. A particular feature of this development is the ability to resolve the dynamics of the molecular vibrations as a spectator of how the material responds to the photoexcited electrons.

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Building better and functional materials from the bottom up

“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,” explains Sean Shaheen, a professor of electrical, computer and energy engineering who provided the material sample and collaborated on the research.

“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 adds.

One of the surprising results of the work is that “in contrast to conventional semiconductors it seems that more structural disorder gives rise to more stable photogenerated electrons in hybrid perovskites,” Raschke explains. With the ultrafast microscope it became possible for the first time “to directly image the role of molecular order, disorder and local crystallinity on the optical and electronic properties of materials in general.”

This discovery is expected to have a profound impact on material science for advancing the performance of novel semiconductor and quantum materials for computing, energy and medical applications.

The instrument development was supported by the National Science Foundation, through����an NSF Science and Technology Center for which Raschke serves as co-principal investigator.

Roland Wilcken, Branden Esses, Rachith Nithyananda Kumar, Luaren Hurley, Sean Shaheen and Markus Raschke contributed to this research.

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