Toward Fluorinated Spacers for MAPI-Derived Hybrid Perovskites : Synthesis, Characterization and Phase Transitions of (FC₂H₄NH₃)₂PbCl₄

Abstract

The intrinsic water sensitivity of the hybrid perovskite methylammonium lead iodide (MAPI) calls for new synthetic strategies to enhance moisture resistance and thus, long-term stability. Here, we combine two strategies – (i) transitioning from 3D to 2D hybrid perovskites by inserting larger A-site cations as spacers and (ii) using fluorinated linkers to enhance the hydrophobicity of the material – and identify two new hybrid perovskite-type compounds, (FC2H4NH3)2PbCl4 and (FC2H4NH3)PbBr3 • DMF, carrying 2-fluoroethylammonium (FC2H4NH3)+ as a promising organic cation for the synthesis of moisture resistant hybrid perovskites. (FC2H4NH3)2PbCl4 features a two-dimensional structure and pronounced long-term stability as confirmed by single-crystal and powder X-ray diffraction. The observed reversible phase transitions at 87 °C and 107 °C investigated with thermal analysis, temperature-dependent powder X-ray diffraction measurements and 1H, 13C and 207Pb solid-state NMR spectroscopy can be assigned to changes in the inorganic lead chloride and organic sublattices, respectively, both having clearly observable fingerprints in the solid-state NMR spectra. DFT calculations trace the origin of the observed severe distortion of the inorganic sublattice in (FC2H4NH3)2PbCl4 back to structural features including the formation of hydrogen bonds. The optical properties of (FC2H4NH3)2PbCl4 were characterized by optical absorption spectroscopy and time-resolved photoluminescence measurements with a view towards the interaction between the organic and inorganic sublattices. The broad photoluminescence spectrum as well as specific absorption characteristics are assigned to exciton self-trapping due to a strong coupling of the excited states to lattice distortions.