Argonne X-Rays Light The Way To More Stable Solar Cell Materials

By tracking the movement of ions in perovskites, scientists hope to discover how to prevent solar cell degradation in a new kind of solar cell

Using the bright X-rays of the Advanced Photon Source and a custom-built characterization platform, scientists have traced the ion movements inside perovskites, a potential material for new solar energy harvesting devices.

Solar farms are gaining an increasing foothold in combating climate change, and scientists are on the hunt for new materials to make solar panels even more efficient. Increasing that efficiency means studying light-detecting and light-emitting materials, which form the basis of the study of optoelectronics. One class of these new materials is called perovskites.

Researchers using the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, recently published a study revealing the way ions move within different perovskite crystals under ultraviolet radiation (UV). Scientists are interested in testing material stability under UV because it can significantly degrade solar cell performance, sometimes by more than 50%, after extended exposure.

Electricity is harvested from light when it collides with a solar cell, knocking electrons out of their bonds and allowing them to circulate and move around. However, instability in perovskites means that iodine, for example, leaves the system as iodine gas, creating a vacancy for ions to migrate towards, causing defects that make the system cease functioning. Researchers hope to improve perovskite stability to achieve a solar cell lifetime of twenty to thirty years, making them more useful to industry.

“Perovskite has a lot of potential for solar cells, and also for use in LED displays. At Argonne, we hope to use the powerful X-ray beams to decode perovskites’ mysteries and uncover potential pathways to conquer their stability hurdles,” said Argonne materials scientist Yanqi (Grace) Luo, the lead author on the paper.

Perovskites’ potential use in solar cells is high on the list for the next solar energy breakthrough in climate resilience. Commercial multicrystalline silicon solar cells can convert 10-15% of the sun’s energy to electricity. But perovskites, since initial experiments in 2009, have increased that to as much as 26%, outperforming many other types of solar cells. To enhance perovskite solar energy conversion, scientists improve material stability through innovative compositions and structural engineering. By altering halide ratios adding these ions in varying sizes or amounts, scientists can change favorably perovskites’ nature and use.

Since the light-harvesting characteristics of these hybrid perovskites are unstable and easily altered, extra care and a specially designed scientific setup were required to study them. Some microscopes can record only a snapshot, providing a certain set of information about a sample at the instant of measurement. Instruments at the APS can record and provide data on conditions of the sample during the entire observation, meaning researchers studying nanoscience can witness changes as they occur.

Luo’s team has shown that by using a sophisticated technique called nanoprobe X-ray fluorescence (nano-XRF), they can directly capture the movement of halide atoms in perovskite materials before damaging them.

“This is a new platform to see precisely, at the nanoscale, what happens when the experimental materials are in operation,” said Argonne physicist Luxi Li, another author of the study.

The perovskite samples used by the team were lab-created low-dimensional or 2D materials. They consist of thin sheet of perovskites that are neatly sandwiched between two layers of bulky organic molecules. The researchers first performed nano-XRF measurements on the 2D crystals by collecting high-resolution elemental maps of atoms inside the materials. Then the researchers shined the same nano-focused X-ray probe to measure the atomic structure via X-ray absorption spectroscopy (XAS). The nano-XRF and XAS captured the halide redistribution and structural reduction in these 2D crystals under continuous UV irradiation, respectively. These findings provide new insights into understanding the degradation mechanisms in these material systems.

With the newly constructed XRF platform, the researchers added a few specialized optics and sensors that allowed them to carefully adjust the brightness of light and detect X-ray-excited optical photons during scans. The results indicated that lower dimensional perovskites show a clear link between stability and dimensionality. Substituting certain elements of the material and protecting the material with sheets of organic molecules provide a potential route to increase the stability of perovskite-based photovoltaics.

So, what’s next for perovskites? The MIT Technology Review selected perovskite solar as one of it’s top 10 breakthrough technologies for 2024, and the DOE continues to sponsor perovskite research. Currently, Luo and her team are exploring other approaches to limit the degree of halide redistribution to enhance material stability. And when the APS upgrade is complete, researchers like Luo and Li will be ready to pummel perovskites with yet more powerful x-rays.

“With the APS Upgrade coming up, it should allow us to better understand the behavior and working principles in energy materials on various time scales,” Luo said.

The upgraded APS is expected to be online by spring 2024, at which time the APS will feature X-ray beams that are up to 500 times brighter. With Luo’s characterization platform, the team will get a brighter and better picture of how to continue advancing perovskite potential.

About the Advanced Photon Source
The U. S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory is one of the world’s most productive X-ray light source facilities. The APS provides high-brightness X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. These X-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce over 2,000 publications detailing impactful discoveries, and solve more vital biological protein structures than users of any other X-ray light source research facility. APS scientists and engineers innovate technology that is at the heart of advancing accelerator and light-source operations. This includes the insertion devices that produce extreme-brightness X-rays prized by researchers, lenses that focus the X-rays down to a few nanometers, instrumentation that maximizes the way the X-rays interact with samples being studied, and software that gathers and manages the massive quantity of data resulting from discovery research at the APS.

This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation’s first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America’s scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.

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