Among the group of materials called perovskites, one of the most fascinating is a material that is capable of changing solar light to electricity as efficiently as present-day silicon solar cells available in the market. This material could also be produced much more easily and cost-effectively.
But there is one major issue—among the four potential atomic phases, or configurations, assumed by this unique material, three phases are efficient but unstable at room temperatures and in normal environments, and they rapidly return to the fourth phase, which is completely impractical for solar applications.
Researchers from Stanford University and the SLAC National Accelerator Laboratory of the Department of Energy (DOE) have now identified a new solution—the useless material version can simply be placed in a diamond anvil cell and squeezed at high temperatures. This process pushes the atomic structure of the material into an efficient configuration and maintains it that way, even in relatively moist air and at room temperatures.
The team has explained the study results in the Nature Communications journal.
This is the first study to use pressure to control this stability, and it really opens up a lot of possibilities. Now that we’ve found this optimal way to prepare the material, there’s potential for scaling it up for industrial production, and for using this same approach to manipulate other perovskite phases.
Yu Lin, Staff Scientist, SLAC National Accelerator Laboratory
Lin is also an investigator at the Stanford Institute for Materials and Energy Sciences.
A Search for Stability
The name “perovskites” came from a natural mineral that has the same atomic structure. In this example, the team investigated a lead halide perovskite that is a mixture of cesium, lead, and iodine. One material phase, called the yellow phase, lacks a real perovskite structure and, hence, cannot be utilized in solar cells.
But investigators discovered some time ago that if the phase is processed in specific ways, it converts to a black perovskite phase that is highly efficient at changing solar light into electricity.
This has made it highly sought after and the focus of a lot of research.
Wendy Mao Professor and Study Co-Author, Stanford Stanford Institute for Materials and Energy Sciences
Regrettably, such black phases are structurally unstable and are inclined to rapidly slump back into the useless configuration. Besides this, the black phases only work with high efficiency at elevated temperatures, added Mao, and scientists will need to resolve both those issues before the phases can be applied to practical devices.
Previous attempts have been made to stabilize these black phases with temperature, strain, or chemistry, but in moisture-free surroundings, that does not reflect the real-world conditions in which the solar cells work. The new study integrated both temperature and pressure in more realistic working surroundings.
Pressure and Heat do the Trick
In association with collaborators in the Stanford research teams of Mao and Professor Hemamala Karunadasa, Lin and Feng Ke—a postdoctoral researcher—developed a setup, in which yellow phase crystals were squeezed between the diamond tips in the so-called diamond anvil cell. Keeping the pressure still on, the researchers heated the yellow phase crystals to 450 °C and subsequently cooled them down.
Under a suitable combination of temperature and pressure, the crystals changed from yellow to a black color and continued to remain in the black phase after the release of pressure, added the team.
The crystals, however, were resistant to deterioration from moist air and continued to remain efficient and stable at room temperatures for 10 to 30 days or beyond.
Analysis with many techniques, including X-rays, validated the shift in the crystal structure of the material, and estimations made by SIMES theorists Chunjing Jia and Thomas Devereaux offers a better understanding of how the pressure altered the structure and maintained the black phase.
The pressure, which was required to change the crystals black and maintain them that way, was about 1,000 to 6,000 times atmospheric pressure, added Lin—that is, about a tenth of the pressures regularly utilized in the synthetic diamond sector.
Therefore, one of the objectives for additional studies will be to transfer what the investigators have understood from their diamond anvil cell experiments to industry and upgrade the process to bring it within the manufacturing realm.
Other SIMES investigators were Wendy Mao and Hemamala Karunadasa. The study was party carried out at the Advanced Photon Source at the Argonne National Laboratory and the Advanced Light Source at Lawrence Berkeley National Laboratory. The study also used resources of the National Energy Research Scientific Computing Center (NERSC). All three are DOE Office of Science user facilities.
The DOE Office of Science provided major funding for the study.
Ke, F., et al. (2021) Preserving a robust CsPbI3 perovskite phase via pressure-directed octahedral tilt. Nature Communications. doi.org/10.1038/s41467-020-20745-5.