Correlating Structure and Function in Two-Dimensional Organic-Inorganic Hybrid Perovskites

Two-dimensional, organic-inorganic hybrid perovskites (2DHPs) are stoichiometric compounds composed of alternating sheets of corner-sharing, metal-halide octahedra and organoammonium cationic layers. We study 2DHPs containing single lead iodide layers separated by intervening substituted, phenethylammonium cations with the chemical structure (x-PEA)₂PbI₄, where x=F, Cl, Br, or CH₃. These 2DHPs form type-I heterojunctions in which excitons and carriers are confined to the lead halide layers and strong quantum and dielectric confinement effects establish exciton binding energies that exceed 150 meV. We use powder and single-crystal x-ray diffraction studies, in combination with variable-temperature, steady-state and time-resolved absorption and photoluminescence (PL) measurements to uncover the correlation between their structure and photophysical properties. 15 K spectra of unsubstituted (PEA)₂PbI₄ show splitting of the excitonic absorption and PL into regularly spaced resonances every 40-46 meV. In addition, anti-Stokes hot exciton PL is observed at the same energy as optical absorption resonances. Substitution on the organic cations, for example in the para position, increases the length of the organic cation and therefore the interlayer spacing, but leaves the cross-sectional area for the organic cation unchanged and results in structurally similar metal halide frameworks. As the length of the cation increases in the 2DHP, the absorption spectra broaden and blueshift, but the PL spectra remain invariant. We show that the excitonic structure is consistent with strong electron-phonon coupling and slow vibrational relaxation originating from the softness of the metal halide framework and the organic cations.