Nature of Exciton-Lattice Coupling and the Role of Organic Cation Substitution in Two-Dimensional Derivatives of Metal Halide Perovskites

A critical element of the photo-physics of metal halide perovskites (MHP) and their derivatives stems from the strong electron–phonon coupling arising from their ionic character and from the convoluted dynamics of the hybrid organic–inorganic lattice. Here, we investigate the nature and consequences of such strong lattice interactions on the exciton dynamics in prototypical two-dimensional Ruddlesden-Popper metal halides (RPMH), which are the 2D derivatives of MHPs. We have previously experimentally identified that the primary excitations in these material systems are exciton-polarons[1], where the polaronic coupling is intrinsically built into the exciton wavefunction and fundamentally drives the thermalization, recombination and dephasing dynamics[2,3]. In an attempt to underpin the role of the organic cation in these polaronic interactions, we perform a comprehensive study on a halogenated phenylethylammonium lead iodide perovskite [(X-PEA)₂PbI₄] to compare with our previous studies on phenylethylammonium lead iodide perovskites [(PEA)₂PbI₄]. The choice of the library of the cation ensures minimum reorganization of the lattice and enables us to explore unambiguously the changes governed primarily by the organic-inorganic framework. Analysis of the low-temperature linear optical spectra suggests negligible disruption in the electronic structure due to the substitution. Resonance Raman spectra obtained through impulsive vibrational spectroscopy however indicate significant change in the phonon characteristics. We observe that the lowest energy optical phonon mode shows a systematic blue shift upon substitution. We further analyze this through density functional theory calculations and identify the changes in the degrees of freedom within the inorganic metal halide layer induced by the chemical modification of the organic layer. Importantly, despite the changes in the phonon structure, exciton-phonon scattering, which is mediated by a relatively wide distribution of phonon modes in the 2-10 meV energy range remains largely intact. We also present a detailed analysis of coherent nonlinear spectral lineshapes measured via two-dimensional spectroscopy[4] and comment on the modifications in the many-body excitonic interactions induced by the organic cation substitution.<br/><br/>[1] A. R. Srimath Kandada and C. Silva, Exciton polarons in two-dimensional hybrid metal-halide perovskites, Journal of Physical Chemistry Letters, 11, 3173-3184 (2020).<br/>[2] A. R. Srimath Kandada, H. Li, E. R. Bittner and C. Silva-Acuna, Homogeneous optical linewidths in hybrid Ruddlesden-Popper metal halides can be only measured using nonlinear spectroscopy, Journal of Physical Chemistry C, 126, 5378 (2022).<br/>[3] F. Thouin, D. Cortecchia, A. Petrozza, A. R. Srimath Kandada and C. Silva, Enhanced screening and spectral diversity in many-body elastic scattering of excitons in two-dimensional hybrid metal-halide perovskites, Phys. Rev. Research., 1, 032032(R) (2019).<br/>[4] H. Li, S. A. Shah, A. R. Srimath Kandada, C. Silvia-Acuna, E. R. Bittner, The optical signatures of stochastic processes in many-body exciton scattering, Annual Review of Physical Chemistry, 74, 20.1 - 20.26 (2023).