Exploring azetidinium as the A-site in organic-inorganic hybrid perovskites

Abstract

Organic inorganic hybrid perovskites (OIHPs) have appealing optoelectronic properties. As sample preparation is relatively straightforward, there are opportunities to investigate novel compositions and structures. This thesis concerns the synthesis, structure and optoelectrical properties of OIHPs with azetidinium as an A-site cation.

Azetidinium [(CH₂)₃NH₂⁺, Az] is a four-member ring ammonium, and its size is calculated to be promising as an A-cation for OIHPs. A stable 6H hexagonal perovskite AzPbBr₃ was synthesised and analysed, and its bandgap was determined to be 2.81 eV. On cooling, AzPbBr₃ undergoes a symmetry lowering distortion which was identified by variable temperature PXRD and dielectric spectroscopy. An anisotropic change in lattice parameters on cooling marked a phase transition likely driven by the Pb⋯Pb repulsion in the face sharing octahedra.

Compositional and structural analyses were performed on precipitation synthesised and mechanosynthesised OIHPs Az₁₋ₓFAₓPbBr₃ and Az₁₋ₓMAₓPbBr₃ (0 ≤ x ≤ 1). For samples obtained from precipitation synthesis, the actual FA% or MA% in the precipitate was found to be less than the nominal composition in the reaction solution. No such mismatch was found for mechanosynthesised samples. PXRD indicated partial solid solution formation for Az-rich and MA- or FA-rich compositions, separated by an intermediate two-phase region. The result suggests the extent of the solid solution of halide perovskites is dependent only on the average A-cation size; the size mismatch is less of an influence. This is in contrast to solid solution formation observed in oxide perovskites.

A tuneable bandgap was achieved ranging from 2.00 eV (AzPbI₃) to 3.41 eV (AzPbCl₃) for the mixed halide perovskite AzPbBr₃₋ₓXₓ (X = Cl or I, 0 ≤ x ≤ 3). The structural analyses revealed a complete 6H solid solution for AzPbBr₃₋ₓClₓ in comparison to the structural progression from 6H, 4H to 9R polytypes, when varying the halide composition from Br (x = 0) to I (x = 3) in AzPbBr₃₋ₓIₓ. A linear variation in unit cell volume as a function of anion average radius was observed not only within the solid solution of each polytype (following Vegard’s law) but continuously across all three polytypes.

Preliminary results on the synthesis and structural analysis indicate that Az₂PbBr₄ adopts the 𝘯 = 1 Ruddlesden-Popper structure while azetidinium bismuth bromide has a 1D chain structure. Detailed structural and optical analysis are planned in future projects.