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 > 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 para 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 ortho 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 >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

(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. J. Am. Chem. Soc. 2016, 138 (42), 13798–13801. https://doi.org/10.1021/jacs.6b08175.

(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. J. Phys. Chem. Lett. 2019, 10 (6), 1198–1205. https://doi.org/10.1021/acs.jpclett.9b00247.

(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. ACS Nano 2020, 14 (3), 3621–3629. https://doi.org/10.1021/acsnano.0c00037.

(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. ACS Nano 2022, 16 (12), 21259–21265. https://doi.org/10.1021/acsnano.2c09256.

(5) Straus, D. B.; Kagan, C. R. Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties. J. Phys. Chem. Lett. 2018, 9 (6). https://doi.org/10.1021/acs.jpclett.8b00201.

(6) Straus, D. B.; Kagan, C. R. Photophysics of Two-Dimensional Semiconducting Organic–Inorganic Metal-Halide Perovskites. Annu. Rev. Phys. Chem. 2022, 73 (1), 403–428. https://doi.org/10.1146/annurev-physchem-082820-015402.