Researchers at Argonne National Laboratory and Purdue University recently reported an effort to prevent the degradation of perovskite solar cells by tracking the movement of ions in perovskites.
The team used X-rays from the Advanced Photon Source (APS) laboratory and a specially crafted characterization platform to reveal how ions move within different perovskite crystals under ultraviolet (UV) radiation. Scientists are interested in testing material stability under ultraviolet light, which significantly degrades the performance of solar cells, sometimes by more than 50% after prolonged exposure.
When light interacts with solar cells, the light knocks electrons out of the chemical bonds and allows them to cycle and move. However, the instability of perovskites means that iodine ions leave the system as iodine gas, creating an ionic vacancy that causes defects to stop the battery. The researchers hope to improve the stability of perovskites so that solar cells can last for 20 to 30 years, allowing them to be used industrially.
Perovskites have great potential in solar cells and can also be used in LED displays. At Argonne National Laboratory, we hope to use the powerful X-ray beam to decode the mysteries of perovskites and discover potential ways to overcome their stability issues," said Yanqi (Grace) Luo, lead author of the **.
To improve the efficiency of perovskite solar energy, scientists have improved the stability of the material through innovative compositional and structural engineering. By changing the halide ratio, adding ions in different sizes or quantities, scientists can effectively change the properties and uses of perovskites.
Since the light-trapping properties of hybrid perovskites are unstable and easily altered, they require extra care and specially designed scientific devices to study them. Some microscopes can only record snapshots, providing specific information about the sample at the moment of measurement. APS's instruments can record and provide data on the condition of the sample throughout the observation process, which means that researchers studying nanoscience can witness changes occur.
Luo's team has demonstrated that by using a technique called nanoprobe X-ray fluorescence (nano-XRF), they can directly capture the movement of halide atoms before destroying perovskite materials. "This is a new platform that allows you to see exactly what happens to experimental materials at the nanoscale as they are run," said Luxi Li, a physicist at Argonne and another author of the study.
The perovskite samples used by the team are lab-made low- or two-dimensional materials consisting of thin sheets of perovskite neatly sandwiched between two layers of organic molecules. The researchers first performed nano-XRF measurements of the 2D crystal by collecting high-resolution elemental maps of the atoms inside the material. The researchers then used the same nano-focused X-ray probe to measure the atomic structure by X-ray absorption spectroscopy (XAS). NanoXRF and XAS capture halide redistribution and structural changes in these two-dimensional crystals under continuous UV irradiation, respectively. These findings provide new insights into the mechanisms of degradation in these material systems.
With the newly constructed XRF platform, the researchers added specialized optics and sensors that allowed them to carefully adjust the brightness of the light and detect X-ray-excited optical photons during the scanning process. The results show that low-dimensional perovskites show a clear relationship between stability and dimensionality. Replacing certain elements of the material with organic molecular layers and protecting the material provides a potential avenue to improve the stability of perovskite photovoltaic cells.
Currently, Luo and her team are exploring other ways to limit the degree of redistribution of halides to improve the stability of the material. When the APS upgrade is complete, researchers like LUO and LI will be ready to hit perovskites with more powerful X-rays. "With the advent of the APS upgrade, it should allow us to better understand how energy materials behave and work on various time scales," Luo said.
The upgraded APS is expected to come online in the spring of 2024 and will be equipped with X-ray beams that are up to 500 times brighter.