Researchers from North Carolina State University, Pennsylvania State University and University of North Carolina at Chapel Hill have found that channeling ions into defined pathways in perovskite materials improves the stability and operational performance of perovskite solar cells.
The team's recent study presented a multiscale diffusion framework that describes vacancy-mediated halide diffusion in polycrystalline metal halide perovskites, differentiating fast grain boundary diffusivity from volume diffusivity that is two to four orders of magnitude slower.
Perovskite materials have -- to date -- not been able to sustain long term operational stability in perovskite solar cells. Perovskites are ionic materials, and when a voltage is applied to a perovskite, it causes ions to migrate through the material. These migrating ions are believed to contribute to chemical and structural changes in the material that ultimately make the materials inefficient and unstable.
"We have not found a way to prevent ions from moving through perovskite materials, but we have found that it is possible to steer these ions into a safe conduit that does not impair the material's structural integrity or performance," Aram Amassian, corresponding author of a paper, said. "It's a big step forward."
The safe conduit, in this case, is a grain boundary. Perovskite materials are multi-crystalline materials. That means that when you are "growing" a perovskite, the material forms as a series of crystals -- or "grains" -- that are flush with each other. These grains are responsible for absorbing light and generating the charges responsible for the electrical current. Each of those grains has the same crystalline structure, but the grains may be oriented in slightly different directions. The area where the grains touch is called a grain boundary.
"What we've found is that grains are better protected from impairment when the ions move predominantly along the grain boundary," says first author and co-corresponding author Masoud Ghasemi, a former postdoctoral researcher at NC State who is now a postdoctoral researcher at Penn State. "Coupling this with what is already known about perovskite materials, it's clear that problems start when grain boundaries are weak, which makes it easier for ions to move into the grains themselves. Designing stronger grain boundaries that protect the grains is essential to block migrating ions and other harmful species like oxygen from entering the grains, mitigating problematic chemical and structural changes in the material."
"This is an important insight, because there are established techniques we can use to engineer perovskite materials and their grain boundaries; we can now make use of these approaches to protect the grains," Amassian says. "We demonstrate how those techniques strengthen grain boundaries in this paper. In short, we now know what needs to be done to make far more stable perovskites."
The work may also inform the development of more efficient energy storage technologies.
"This work advances our fundamental understanding of how ions move through any crystalline material that can carry charge, not just halide perovskites," Amassian says. "We're excited to talk to colleagues who work on energy storage about how this may inform the engineering of faster ion conductors."