Not just for solar cells

Materials based upon perovskites have great potential to become the backbone of the next generation of solar cells. But research carried out by the Adolphe Merkle Institute’s Smart Energy Materials group demonstrates other possible uses.

For several years, perovskites have been touted as the next significant development in photovoltaics, being more efficient for power generation than silicon-based solar cells. Researchers have slowly developed these perovskite cells to the point where they can be produced by pilot plants and brought onto the commercial market. However, these materials have other potential applications.

Prof. Jovana Milic, head of AMI’s Smart Energy Materials group, has investigated two variants of layered (2D) perovskites known as Dion–Jacobson and Ruddlesden–Popper phases. Each is made of thin layers of organic and perovskite material stacked on each other, the differences between the two being mainly due to how the stacking is assembled. These crystalline yet soft materials are found to be more stable than conventional 3D perovskite materials under environmental conditions, which has attracted considerable research interest for their use in stabilizing other perovskite materials.

Rather than apply these materials in solar cells, Milic and her collaborators exposed them to varying levels of mild pressure. Their investigations showed that the perovskites underwent mechanochromic effects when the pressure was applied - in other words, they changed color. This takes place because the pressure changes the structural properties of the layers. This, in turn, affects how the material interacts with light, leading to different hues. The researchers also found that they could control the color changes by adjusting the amount of pressure that they applied to the material. The effect is also reversible, with the perovskites returning to their initial state once pressure is removed, demonstrating their potential for various applications.

“This reversibility of the mechanochromic response of layered hybrid perovskites is of interest to their use as model systems for elucidating structure-property relationships in hybrid materials,” explains Milic. “It is also relevant for the long-term stability of flexible perovskite devices, and it points toward the use of mechanophores in the development of smart materials and pressure sensors.”

For example, these materials could be used in sensors that change color in response to pressure or in materials that can switch between different colors depending on the conditions they are exposed to. Their application is facilitated by the ease of solution processing onto various substrates for functional devices. Moreover, their responsiveness to mild pressure in the range comparable to levels of strain caused by internal structural rearrangements opens the perspectives for their use as scaffolds to assemble other functional dynamic materials in the future.

Reference: Muscarella, L. A.; Dučinskas, A.; Dankl, M.; Andrzejewski, M.; Casati, N. P. M.; Rothlisberger, U.; Maier, J.; Graetzel, M.; Ehrler, B.; Milić, J. V. Reversible Pressure-Dependent Mechanochromism of Dion–Jacobson and Ruddlesden–Popper Layered Hybrid Perovskites. Advanced Materials 2022, 34 (17), 2108720.