Hot Exciton Dynamics in Two-Dimensional Organic-Inorganic Hybrid Perovskites
Invited
Abstract
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) 2 PbI 4 , where x=F, Cl,
Br, or CH 3 . 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) 2 PbI 4 excitonic
absorption and PL spectra at 15 K show splittings into regularly spaced resonances every 40-46 meV.
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. 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. 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. The
properties of 2DHPs can be tailored by the selection of the cation without directly modifying the
inorganic framework.
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) 2 PbI 4 , where x=F, Cl,
Br, or CH 3 . 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) 2 PbI 4 excitonic
absorption and PL spectra at 15 K show splittings into regularly spaced resonances every 40-46 meV.
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. 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. 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. The
properties of 2DHPs can be tailored by the selection of the cation without directly modifying the
inorganic framework.
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Presenters
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Cherie R Kagan
University of Pennsylvania
Authors
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Cherie R Kagan
University of Pennsylvania