Researchers fine-tune the properties of layered perovskites

Scientists in Italy have devised a way to fine-tune the optical emission and robustness of a new set of Ruddlesden'Popper metal'halide layered perovskites. It is shown that the type of molecule regulates the number of hydrogen bonds that it forms with the edge'sharing [PbBr₆]^4' octahedra layers, leading to strong differences in the material emission and tunability of the color coordinates, from deep'blue to pure'white. In addition, the emission intensity strongly depends on the length of the molecules, thereby providing an additional parameter to optimize their emission efficiency.

The combined experimental and computational study provides a detailed understanding of the impact of lattice distortions, compositional defects, and the anisotropic crystal structure on the emission of such layered materials.

There are myriad possible ways to make layered perovskites, with different chemical elements for the organic and inorganic parts. I their recent study, Milena P. Arciniegas from Istituto Italiano di Tecnologia in Genoa and her colleagues have started working on a manual for various perovskite compositions. They used lead and bromine for the inorganic layer, and tried out various amines (a class of compounds based on nitrogen, hydrogen and carbon) for the organic part. 'It is like a millefoglie cake,' Arciniegas says. 'The crust layers keep the cake together, and the cream layers give the flavor.' In this case, the cream is the organic layer, and the flavor is the color of the light emission.

Most known bromine-based perovskites emit blue light, but the researchers thought that by tweaking the way hydrogen atoms in the amines bind to the semiconducting part, it would be possible to get other colors from the same material. 'We worked with computational scientists and asked them to help us predict how a change in the anchor site would impact the light emission, and how the length of the molecule would impact the robustness and efficiency of the material,' Arciniegas explains.

They narrowed the search to three families of amines with different hydrogen bonds, and used the results of the simulation to guide laboratory experiments. Eventually, they managed to create a new set of layered perovskites with blue, yellow and white emission. The set was made at room temperature, using relatively simple molecules and environmentally friendly solvents ' which bodes well for future manufacturing.

'This is an initial step, but it shows a method to make the design of these materials more systematic, and the library of possible compounds to use is huge,' Arciniegas explains. Other organic molecules, for example, would change the way the material conducts electricity, or would make it foldable.

The team plans to explore other materials for the organic part, and to try to replace lead with other non-toxic elements for the inorganic one, while preserving efficiency.