Effect of Halide Tuning on the Structural, Dielectric, and Optical Properties of Two-Dimensional 2-Chloroethylammonium Lead Halides

Layered hybrid organic-inorganic lead halides have gained a lot of attention for optoelectronic applications. A notable subset within this category is perovskites comprising halogenated amines since they may exhibit reduced band gap or polar order. We synthesized three compounds comprising 2-chloroethylammonium (CEA⁺) cations, with the chemical formula CEA₂PbX₄ (X = Cl, Br, I). X-ray diffraction studies show that at room temperature (RT), CEA₂PbBr₄ and CEA₂PbI₄ crystallize in Pbnm symmetry, with ordered CEA⁺ cations. CEA₂PbBr₄ and CEA₂PbI₄ undergo one structural phase transition (PT) into a disordered Pmnm phase near 315 and 360 K, respectively. CEA₂PbCl₄ shows a different packing of CEA⁺ with the organic chains oriented perpendicularly to the perovskite layers. It undergoes two PTs at 332 and 203 K from the high-temperature (HT) disordered I4/mmm phase to the partially ordered intermediate Pbnm phase and completely ordered low-temperature (LT) phase of the unknown space group. All compounds emit photoluminescence (PL): orange, yellow-green, and yellow for CEA₂PbI₄, CEA₂PbCl₄, and CEA₂PbBr₄, respectively, and bromide exhibits a very high quantum efficiency of 48%. Overall, our findings show that halide engineering strongly modulates hydrogen and halogen bonding strength, affecting the structural arrangement of building units, molecular dynamics, and thus optoelectronic properties.