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Excitation-Energy-Dependent Carrier-Phonon Coupling in Two-Dimensional Ruddlesden–Popper Perovskites
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DOI：10.1021/acs.jpcc.6c02225.png)
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Abstract
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Understanding how excess excitation energy governs hot exciton relaxation is crucial for the fundamental photophysics of two-dimensional (2D) perovskites. Using femtosecond transient absorption (TA) spectroscopy, we investigate how spacer cation chemistry and excitation energy jointly influence carrier-phonon coupling in 2D Ruddlesden–Popper (RP) perovskites. Hot excitons generated by above-band-gap excitation relax to the band edge excitonic state within ∼0.3 ps, independent of spacer cations within the arylamine and alkylamine series examined, reflecting the dominant role of the inorganic framework. The transient decay traces exhibit coherent oscillations arising from the carrier-phonon coupling. Above-band-gap photoexcitation with excess energy (380 and 330 nm) generates hot excitons in quasi-continuum electronic states, from which both a low-frequency (∼35 cm–1) I–Pb–I bending mode and another higher-frequency (∼145 cm–1) Pb–I stretching mode are coherently accessible, whereas near-band edge excitation (490 nm) generates excitons closer to the lowest excitonic states that coherently access only the low-frequency bending mode. The high-frequency stretching mode shows a rapid dephasing possibly via phonon–phonon scattering, whereas the low-frequency bending mode shows relatively longer dephasing time, and remains coupled to the excitonic state beyond the completion of the hot exciton cooling. These results suggest that excitation energy determines the initially prepared electronic state and its relaxation pathway, which in turn governs the phonon modes that are coherently accessed during the ultrafast relaxation of the hot excitons in quantum-confined 2D perovskites.
Keywords:
hot excitons
carrier-phonon coupling
two-dimensional perovskites
femtosecond transient absorption
excitation energy
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