Synthesis of Chiral Two-Dimensional Perovskites with Bis-Ammonium Cations for Circular Polarized Light Detection (2DCPL)

• Applicant: Dr. Lucas Scalon
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Synthesis and Properties of Functional Materials ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558721159

Two-dimensional (2D) perovskites are formed by introducing bulky ammonium-based organic molecules (A’) into the perovskite structure. Mono- and bis-ammonium molecules can be used. The former leads to the Ruddlesden-Popper (RP) phase with the chemical formula A’2An-1PbnX3n+1, while the latter leads to the Dion-Jacobson (DJ) phase, defined by the chemical formula chemical formula A’An-1PbnX3n+1. Here, A is a small ammonium-based organic molecule (formamidinium or methylammonium), X is a halide (Cl, Br, I) and n is the number of [PbX6]4- octahedra connected by the corners in the perpendicular direction. It is noteworthy that the number of A’ cations per chemical formula differs between the RP and DJ phases, because the latter is formed by an organic molecule with two ammonium groups to interact with the lead halide octahedra. In addition to changing the dimensionality, the organic cation can functionalize the 2D perovskites to achieve different optoelectronic properties. For example, the use of chiral A’ cations in low-dimensional perovskites has demonstrated excellent chiroptical and spin-dependent charge transport properties, which are attractive for applications in circularly polarized light (CPL) photodetectors and light-emitting diodes. Most of the research on chiral perovskites has focused on RP-type 2D structures, which feature a mono-ammonium molecule with only one chiral carbon, e.g. methylbenzylammonium (MBA). The introduction of chiral bis-ammonium molecules capable of yielding DJ-type 2D perovskites has not been reported. The stabilization of bulky chiral bis-ammonium molecules within the inorganic network of the perovskite is challenging. This is because the ammonium group in these chiral molecules is typically attached to a secondary chiral carbon, resulting in a significant steric hindrance for the molecule to interact with the inorganic network of the perovskite. In this Project, we aim to demonstrate for the first time the synthesis of DJ-type chiral 2D perovskites. To obtain a pure phase of this material, it is necessary to find ways to overcome the intrinsic steric hindrance in the chiral bis-ammonium organic molecule. We will address this issue by optimizing film deposition conditions, using solvent mixtures capable of retarding perovskite crystallization, and adding additives that help stabilize the 2D phase. This approach can improve the crystal orientation and reduce the formation of impurities, which benefits the chiroptical response of the system. The optimized chiral DJ-type 2D perovskite will be used as an active layer in a CPL photodetector. The results obtained here can shed light on the development of a new class of chiral perovskites and enable the design of novel chiral bis-ammonium cations.

• DFG Programme: WBP Position