Hot Exciton Dynamics 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 (PEA) 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 strongly confined to the lead halide layers with exciton binding energies &gt; 150 meV. We use x-ray diffraction and variable-temperature steady-state and time-resolved absorption and photoluminescence (PL) measurements to uncover the correlation between their structure and photophysical properties. (PEA)₂PbI₄ excitonic absorption and PL spectra at 15 K show splittings into regularly spaced resonances every 40-46 meV.¹ Anti-Stokes hot exciton PL is observed at the same energy as the optical absorption resonances. Replacing a single atom in the <i>para</i> position of the PEA-cation phenyl group increases its length and therefore the interlayer spacing, but leaves the cross-sectional area unchanged and results in structurally similar metal halide frameworks.² As the cation length increases, the absorption spectra broaden and blueshift, but the PL spectra remain invariant. Substitution in the <i>ortho</i> position with progressively larger cations increasingly distorts and strains the inorganic framework.³ Ortho substitutions change the number of and spacing between the discrete excitonic resonances and increase the hot exciton PL by &gt;10X. By correlating the atomic substitutions on the cation with changes in the excitonic structure, we show that the origin of the discrete excitonic resonances is consistent with a vibronic progression caused by strong exciton-phonon coupling to a phonon on the organic cation. We also show evidence of the structure-dependent formation of exciton polarons.⁴ The properties of 2DHPs can be tailored by the selection of the cation without directly modifying the inorganic framework.^5,6<br/><br/>(1) Straus, D. B.; Hurtado Parra, S.; Iotov, N.; Gebhardt, J.; Rappe, A. M.; Subotnik, J. E.; Kikkawa, J. M.; Kagan, C. R. Direct Observation of Electron–Phonon Coupling and Slow Vibrational Relaxation in Organic–Inorganic Hybrid Perovskites. <i>J. Am. Chem. Soc.</i> <b>2016</b>, <i>138</i> (42), 13798–13801. https://doi.org/10.1021/jacs.6b08175.<br/><br/>(2) Straus, D. B.; Iotov, N.; Gau, M. R.; Zhao, Q.; Carroll, P. J.; Kagan, C. R. Longer Cations Increase Energetic Disorder in Excitonic 2D Hybrid Perovskites. <i>J. Phys. Chem. Lett.</i> <b>2019</b>, <i>10</i> (6), 1198–1205. https://doi.org/10.1021/acs.jpclett.9b00247.<br/><br/>(3) Straus, D. B.; Hurtado Parra, S.; Iotov, N.; Zhao, Q.; Gau, M. R.; Carroll, P. J.; Kikkawa, J. M.; Kagan, C. R. Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification. <i>ACS Nano</i> <b>2020</b>, <i>14</i> (3), 3621–3629. https://doi.org/10.1021/acsnano.0c00037.<br/><br/>(4) Hurtado Parra, S.; Straus, D. B.; Fichera, B. T.; Iotov, N.; Kagan, C. R.; Kikkawa, J. M. Large Exciton Polaron Formation in 2D Hybrid Perovskites via Time-Resolved Photoluminescence. <i>ACS Nano</i> <b>2022</b>, <i>16</i> (12), 21259–21265. https://doi.org/10.1021/acsnano.2c09256.<br/><br/>(5) Straus, D. B.; Kagan, C. R. Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties. <i>J. Phys. Chem. Lett.</i> <b>2018</b>, <i>9</i> (6). https://doi.org/10.1021/acs.jpclett.8b00201.<br/><br/>(6) Straus, D. B.; Kagan, C. R. Photophysics of Two-Dimensional Semiconducting Organic–Inorganic Metal-Halide Perovskites. <i>Annu. Rev. Phys. Chem.</i> <b>2022</b>, <i>73</i> (1), 403–428. https://doi.org/10.1146/annurev-physchem-082820-015402.