HYBRID ORGANIC-INORGANIC HALIDE PEROVSKITE SINGLE CRYSTALS DIRECTED BY PHOSPHONIUM CATION-TEMPLATES

Abstract

Inorganic/organic hybrid perovskites have emerged as synthetic materials that exhibit promising applications in optoelectronic devices. There is a wealth in choice of organic cations that can be used to create hybrid structures with different shape, size, polarizability, and electronegativity. Although perovskites with ammonium cations have been previously reported, a perovskite family with phosphonium cations has been less investigated. This study reports the synthesis, crystal, and characterization of the novel low-dimensional phosphonium-base organic-inorganic hybrid single crystal materials.

First, the phosphonium bromide salts were reacted with the lead bromides and iodides towards creating a perovskite crystal. Further reactions of phosphonium salts with the transition metal halides were made to discover if variant perovskite structures will be obtained. On the basis of the single-crystal X-ray diffraction results carried out at room temperature, the reaction of lead(II) bromide and iodide with various phosphonium salts produced a charge-balanced network of PbBr3 and PbI3– fragments connecting in a face-sharing octahedral motif. However, the reaction of lead(II) chloride and lead(II) iodide with tetraphenylphosphonium bromide (TPPB = C24H20PBr) produced a [Pb3(Cl/Br)8]2– and a [Pb2I6]2– that is surrounded by (C24H20P+) molecules (organic part), featuring a one-dimensional (1D) double chines and zero-dimensional (0D) structure, respectively. We note the increased cation size causes greater separation of charges,

which tears apart the linked metal anion chains and lowers dimensionality; a separate effect was the formation of a new plumbate ion [Pb3(Cl/Br)8]2– where a Pb-phenyl interaction from a cation replaces an expected Pb-halogen bond.

These phosphonium-templated haloplumbates were found to possess semiconducting properties, as suggested by the results of UV-Vis. Four novel crystal structures were isolated from reactions of tetraphenylphosphonium salt with zinc(II), cadmium(II), cobalt(II) and copper(II) chloride, each of them producing a unique geometry. Reacting TPPB with ZnCl2, (C24H20P)[ZnCl3(C3H7NO)] results in one (C24H20P+) cation and one [ZnCl3(DMF)] anion (DMF is dimethylformamide). In the anion, the zinc atom is tetrahedrally coordinated by a DMF ligand via the O atom and by three terminal Cl atoms. However, a different structure forms using TPPB with CdCl2, in which the CdX4 ion has a mixture of Cl and Br in a disordered state. As a result of the larger TPP cation, the TPP cation was found to break the -Cu-Cu-Cu-Cu- infinite chain and form dimers in which the smaller Cl atoms are bridging in the middle and the Br atoms are occupying the outer position. Photoluminescence investigation for (C24H20P)2Pb3(Cl/Br)8, zinc and cadmium components reveals a strong emission in the green region at 310 and 350 nm excitation wavelengths, respectively.