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

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.publishe

Benzimidazolium Lead Halide Perovskites : Effects of Anion Substitution and Dimensionality on the Bandgap

We present the synthesis and structural characterization of a series of benzimidazolium-based lead halide perovskites including (C7H7N2)2PbCl4, (C7H7N2)2PbBr4, (C7H7N2)2PbI4, and (C7H7N2)PbI3, which serves as a platform to investigate the change in optical properties as a function of the halide and the dimensionality of the inorganic sublattice. The structural similarity of the layered systems with A2MX4 stoichiometry was verified by single-crystal X-ray diffraction and solid- state NMR spectroscopy. The optical properties were analyzed by absorption and photoluminescence (PL) measurements, confirming the decrease in bandgap when exchanging the chloride with its higher homologues Br and I. In addition, comparison of the layered compound (C7H7N2)2PbI4 with the known compound (C7H7N2)PbI3 reveals an increase in bandgap on going from a 2D to a dimensionally reduced 1D topology. Besides, significant differences in the PL spectra of the halides are observed, which likely are due to recombination of self-trapped excitons stabilized through lattice distortions or permanent lattice defects in the compounds featuring broad PL emission bands.publishe

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

The intrinsic moisture 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 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 toward 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